Today we are going to learn about analgesics, or pain killers. The name "analgesic" comes from the Greek word "algos," which means pain.
Pain pathways
To understand how to treat pain, it may help to understand how pain is caused in the first place. The first step is transduction, which occurs when free nerve endings are stimulated in some way. This stage can be blocked by NSAIDs, COX-2 inhibitors and topical local anaesthetics. The next stage is conduction or transmission, when the signal is sent down the nerve to the spinal cord. This can be blocked by epidurals and regional anaesthesia. The next step is modulation, when the sensory nerve from the body synapses with one going to the brain. This can be blocked by opioids, COX-2 inhibitors, ketamine, alpha-2-delta ligands and α2 agonists. The final step is perception, when the brain notices what's going on, and this can be blocked by opioids, COX-2 inhibitors and possibly paracetamol.
Most analgesics are based on salicylates (willow bark), which mainly work by inhibiting prostaglandin synthesis in the periphery, or on opioids, which mainly work directly on the brain. Analgesics can also be classed into opioids and non-opioids. The main non-opioids used are paracetamol and NSAIDs.
Paracetamol
Paracetamol has been around for a little while (since around 1894), and is useful for its analgesic and antipyretic (anti-fever) effects. It is also pretty safe unless you take too much of it (see here for an explanation as to why this is the case). Strangely enough, we don't actually know how it works, and there are several competing theories. One of the more popular ones suggests that paracetamol may interact with the serotonin pathway or cannabinoid receptors, but this is still very unclear.
NSAIDs
There are many different NSAIDs, but they all work by inhibiting cyclooxygenase. Cyclooxygenase, as mentioned here, can break down arachidonic acid into other stuff, like prostaglandins. Prostaglandins are responsible for inflammation and pain, but they are also responsible for supporting the kidneys, stomach and platelets, which explains some of the adverse side-effects of NSAIDs. Kidney effects include renal dysfunction and renal failure, stomach problems include GI bleeding and ulcers, and platelet effects include anti-platelet activity (which leads to increased blood loss, which is a real problem if you have stomach ulcers too). Hypersensitivity reactions, such as angioedema and bronchospasm, can also result. It's not all gloom and doom, however: NSAIDs have analgesic, antipyretic and anti-inflammatory actions, which basically one-ups paracetamol.
A newer class of drugs, called the coxibs, can cut down on some of the side-effects of NSAIDs. As mentioned here, NSAIDs inhibit both COX-1 and COX-2, whereas coxibs only inhibit COX-2 (which is the isoform of cyclooxygenase associated with pain, inflammation and fever). As such, coxibs have a lower risk of GI bleeding as opposed to regular NSAIDs, though some may have a slightly higher cardiovascular risk.
Finally, a quick note on aspirin, which was one of the first NSAIDs. Unlike other NSAIDs, it blocks cyclooxygenase irreversibly, inhibiting platelets for a long time. Aspirin is also unique in regards to its toxicity: children with viral infections can be at risk of Reyes syndrome if they take aspirin, which is basically liver and brain damage. As such, aspirin is not indicated for children. Also, if you want an analgesic and anti-inflammatory medication, you'd probably be better off taking another NSAID, like ibuprofen.
Opioids
Opioids are somewhat similar in structure to endorphins and enkephalins, which you might know as the natural "feel-good" chemicals. They bind to opioid receptors (!) which are G-protein coupled receptors that come in three classes. These classes are μ, κ and δ. Most opioids, such as morphine, work on the μ class. μ opioid receptors are mainly located in the brain stem and dorsal horn of the spinal cord, though there are some in the periphery as well. Aside from causing analgesia, activation of these receptors can cause other symptoms such as miosis (not to be confused with meiosis, miosis refers to shrinkage of the pupils), euphoria, respiratory depression, nausea, vomiting and so on.
Morphine is the gold standard when it comes to opioids. Drug dealers also like it because they can modify it a bit to make heroin, which gives you a rapid (but short-lasting) high, ensuring that their customers will keep coming back to get more. Another commonly-used opioid is oxycodone, which has no active metabolites, and acts on both μ and κ receptors. Methadone is also commonly used to treat addiction, but it has to be used carefully as it has a long half-life (and thus there is the risk of accumulation). Fentanyl is also handy as it can be administered transdermally, and thus you can give someone a fentanyl patch. One opioid to avoid is pethidine, as it is very addictive.
Constipation is the most common side effect of opioids (especially with codeine, which was actually originally developed to treat diarrhoea!). This can be treated with laxatives or by changing the mode of administration of the opioid. Another common symptom is nausea, which can be treated with anti-emetics or by changing up the opioid ("opioid rotation"). Other side effects include sedation, pruritis (itching) and hallucinations.
Monday, May 29, 2017
Mechanotransduction in muscle
Third post for mechanobiology!
Differentiation
First off, just a quick note about differentiation. The inner cell mass of the embryo has embryonic stem cells, which can differentiate into pretty much any cell of the body. Induced pluripotent stem cells can do the same. Throughout development, the ectoderm, mesoderm and endoderm form, along with slightly more mature cells, such as bone marrow-derived stem cells and adipose-derived stem cells (both from the mesoderm). These cells are more specific in what they will become later, but they are still fairly generalised (so if necessary, adipose-derived stem cells could become brain cells... I think). Eventually, these cells differentiate further into more and more mature cells.
Another quick note: there's a difference between progenitor cells and stem cells. Stem cells can become basically anything, whereas progenitor cells are pretty much destined to become only one type of mature cell (e.g. osteoblasts can become osteocytes).
Cardiac muscle
Cardiac muscle can be derived from cardiac stem cells, which in turn can be derived from bone marrow-derived stem cells or adipose stem cells, which in turn can be derived from embryonic stem cells (or induced pluripotent stem cells). Only a very small percentage of heart cells are cardiac stem cells, which is why the heart cannot repair itself after a myocardial infarction or other kinds of damage. Cardiac differentiation requires certain signalling pathways, such as the Wnt/β-catenin signalling pathway. When this pathway is knocked out, cardiac differentiation does not occur. (One marker that can tell you whether or not cardiac differentiation has occurred is Nkx2.5.)
Cardiac muscle, like every other kind of cell, has adhesions to the ECM and to other cells. Integrin and focal adhesion complexes are important for adhesions to the ECM, whereas N-cadherin and α/β cetanin are important for cell-cell adhesions. Cell-cell adhesions may be more important than cell-ECM adhesions in determining the stiffness of the cells.
When culturing cells in vitro, it is possible to detach them from the synthetic ECM without destroying cell-cell adhesions. This can be done by using a temperature-sensitive hydrogel which allows for spontaneous detaching when the temperature is decreased. "Sheets" of cells created in this manner can be stacked on top of each other (but only to a certain extent- if you stack too many sheets, the cells in the middle won't get enough nutrients).
Cardiac muscle, like every other kind of cell, has adhesions to the ECM and to other cells. Integrin and focal adhesion complexes are important for adhesions to the ECM, whereas N-cadherin and α/β cetanin are important for cell-cell adhesions. Cell-cell adhesions may be more important than cell-ECM adhesions in determining the stiffness of the cells.
When culturing cells in vitro, it is possible to detach them from the synthetic ECM without destroying cell-cell adhesions. This can be done by using a temperature-sensitive hydrogel which allows for spontaneous detaching when the temperature is decreased. "Sheets" of cells created in this manner can be stacked on top of each other (but only to a certain extent- if you stack too many sheets, the cells in the middle won't get enough nutrients).
Skeletal muscle
Skeletal muscle can be derived from satellite cells, bone marrow-derived stem cells, adipose-derived stem cells and embryonic stem cells. They are located under the basal lamina, which is located between the muscle cell and ECM, and are identified by the markers Pax3 and Pax7. When muscle fibres are damaged, they can differentiate into myoblasts, which can proliferate and migrate through the injury site. Myoblasts can then fuse and mature, regenerating the muscle fibre.
Integrin is not the only important protein involved in cell-ECM adhesions. Other important proteins are the dystroglycan complex, which binds to actin via dystrophin (which, as you may remember, is deficient in muscular dystrophy). Dystrobrevin may also be associated with the dystroglycan complex and dystrophin. Other important proteins involved in muscle fibre connections are N-cadherin and components of desmosomes (I swear I've written about these types of connections before, but can't find any posts. Oh well).
Since we're onto mechanobiology, we need to talk about mechanical forces that affect muscle differentiation and so forth. Striation of myotubes (which is due to the alignment of actin and myosin) depends on the underlying stiffness: stiffnesses of around 8-11kPa (usual muscle stiffness) result in the striation of muscle, whereas higher and lower stiffnesses do not. Mechanosensitive proteins are also involved in the maturation of satellite cells: in the beginning stages of maturation, YAP expression is increased and YAP translocates to the nucleus, but later on it is phosphorylated and translocates to the cytoplasm, where it is degraded.
Only one more lecture to go!
Skeletal muscle can be derived from satellite cells, bone marrow-derived stem cells, adipose-derived stem cells and embryonic stem cells. They are located under the basal lamina, which is located between the muscle cell and ECM, and are identified by the markers Pax3 and Pax7. When muscle fibres are damaged, they can differentiate into myoblasts, which can proliferate and migrate through the injury site. Myoblasts can then fuse and mature, regenerating the muscle fibre.
Integrin is not the only important protein involved in cell-ECM adhesions. Other important proteins are the dystroglycan complex, which binds to actin via dystrophin (which, as you may remember, is deficient in muscular dystrophy). Dystrobrevin may also be associated with the dystroglycan complex and dystrophin. Other important proteins involved in muscle fibre connections are N-cadherin and components of desmosomes (I swear I've written about these types of connections before, but can't find any posts. Oh well).
Since we're onto mechanobiology, we need to talk about mechanical forces that affect muscle differentiation and so forth. Striation of myotubes (which is due to the alignment of actin and myosin) depends on the underlying stiffness: stiffnesses of around 8-11kPa (usual muscle stiffness) result in the striation of muscle, whereas higher and lower stiffnesses do not. Mechanosensitive proteins are also involved in the maturation of satellite cells: in the beginning stages of maturation, YAP expression is increased and YAP translocates to the nucleus, but later on it is phosphorylated and translocates to the cytoplasm, where it is degraded.
Only one more lecture to go!
Friday, May 26, 2017
Parkinson's Disease
I've already written a bit about Parkinson's Disease here, but time to go into more detail I guess!
Symptoms
As mentioned in an earlier post, signs of Parkinson's Disease (PD) include resting tremor, bradykinesia, impaired postural reflexes and "cogwheel rigidity" (jerky movements when the muscle is passively stretched). Its onset is usually asymmetrical (i.e. one side of the body is affected before the other). Another diagnostic criterion for Parkinson's is responsiveness to levodopa, the dopamine precursor used as a treatment.
Pathology
Some of the pathological changes present in PD include the presence of Lewy bodies, dystrophic Lewy neurites, and loss of dopamine neurons in the substantia nigra. (Wikipedia tells me that Lewy bodies are abnormal aggregates of protein.) Lewy bodies and Lewy neurites form from the deposition of abnormal α-synuclein. Lewy bodies are not unique to PD, however: they occur in other disorders, and even healthy seniors may have them.
The neuronal pathways behind PD are a bit more complicated, especially since none of us have any neuroscience background. The lecturer's explanations made absolutely zero sense to me, so here's what I learned from Rang and Dale's Pharmacology:
First, let's talk about the extrapyramidal nervous system. There are several different components to this system, such as the substantia nigra (which is made up of the pars compact and pars reticulata), the corpus striatum, globus pallidus, subthalamic nucleus, thalamaus, and motor cortex. Dopaminergic neurons travel from the pars compacta (of the substantia nigra) to the corpus striatum, where they can have excitatory effects on GABAergic neurons going to the pars reticulata (also of the substantia nigra) or inhibitory effects on GABAergic neurons going to the globus pallidus.
First, I'm going to discuss the pars reticulata. GABAergic neurons are mostly inhibitory, so activation of the GABAergic neuron travelling from the corpus striatum to pars reticulata causes inhibition of the next neurons, which are GABAergic neurons going to the thalamus. Now, inhibiting an inhibitory neuron (disinhibition) causes activation, so glutamatergic (excitatory) neurons in the thalamus become activated. These neurons go to the motor cortex, stimulating movement.
But what about the other pathway? Inhibition of GABAergic neurons to the globus pallidus causes activation of the GABAergic neurons there. These GABAergic neurons go to the subthalamic nucleus, causing inhibition of glutamatergic neurons travelling to the pars reticulata. This also inhibits the GABAergic neurons travelling from the pars reticulata to the thalamus, thus removing the brakes on the glutamatergic neurons travelling to the motor cortex.
In Parkinson's Disease, the dopaminergic pathway between the pars compacta and corpus striatum is impaired in some way. This means that there is no disinhibition of GABAergic neurons travelling from the pars reticulata to the thalamus, which in turn means that the glutamatergic neurons to the motor cortex are inhibited (hence bradykinesia and difficulty in initiating movement).
Sorry if all of that was confusing. Here's a diagram from Rang and Dale's Pharmacology, Eighth Edition (p. 492), which might help you to visualise it all better:
Genetics
Most patients with PD (~95%) have sporadic PD, but the remaining 5% may have an inherited genetic disorder responsible for PD. Possible genes involved include α-syn, Parkin, DJ-1 and Pink-1.
MPTP, 6-OHDA, and Parkinson's
As discussed here, MPTP can damage dopaminergic neurons in a fairly roundabout manner. Therefore, it is often used to create animal models of Parkinson's. It has been found that you need to lose at least 80% of dopaminergic neurons in order to develop Parkinson's, which fits with Seeman's hypothesis stating that antagonising over 80% of dopaminergic neurons causes extrapyramidal side effects.
6-OHDA (6-hydroxydopamine) is another toxin that can destroy dopaminergic neurons. Interestingly enough, giving different drugs to 6-OHDA lesioned rats can make them turn in different directions. Giving amphetamine (which increases dopamine release) to these rats causes ipsiversive turning (i.e. towards the side of the lesion). Giving a direct D1 or D2 receptor agonist causes contraversive turning. (Unfortunately, I don't really understand why.)
Sinemet
Sinemet is a combination of L-DOPA (a dopamine precursor) and carbidopa (a peripherally selective aromatic L-amino acid decarboxylase inhibitor). Carbidopa blocks peripheral synthesis of dopamine so that high enough levels of L-DOPA can cross the blood-brain barrier. L-DOPA, being a precursor for dopamine, increases the production of dopamine. Sinemet is the gold-standard treatment for PD.
One of the problems with Sinemet is that it displays fluctuations in efficacy. This occurs for several reasons: firstly, it has a short half-life and secondly, after meals amino acids compete for transport across the blood-brain barrier, so meals can reduce the amount of L-DOPA getting into the brain. One of the other problems with Sinemet is that, being a dopamine agonist, it comes with a risk of psychosis.
Another problem with Sinemet is that it can result in the development of dyskinesia, called L-DOPA induced dyskinesia (LiD). It is still uncertain as to what causes LiD. Some people believe that LiD may be due to the pulsatile, rather than continuous, delivery of Sinemet. Therefore pumps and other methods of administration are being trialled. Others believe that LiD may be due to dopamine release by serotonergic neurons. L-DOPA may be taken up by serotonergic neurons, and since serotonergic neurons also have aromatic L-amino acid decarboxylase, they may also begin producing dopamine. Release of dopamine without feedback control may be responsible for dyskinesias.
Symptoms
As mentioned in an earlier post, signs of Parkinson's Disease (PD) include resting tremor, bradykinesia, impaired postural reflexes and "cogwheel rigidity" (jerky movements when the muscle is passively stretched). Its onset is usually asymmetrical (i.e. one side of the body is affected before the other). Another diagnostic criterion for Parkinson's is responsiveness to levodopa, the dopamine precursor used as a treatment.
Pathology
Some of the pathological changes present in PD include the presence of Lewy bodies, dystrophic Lewy neurites, and loss of dopamine neurons in the substantia nigra. (Wikipedia tells me that Lewy bodies are abnormal aggregates of protein.) Lewy bodies and Lewy neurites form from the deposition of abnormal α-synuclein. Lewy bodies are not unique to PD, however: they occur in other disorders, and even healthy seniors may have them.
The neuronal pathways behind PD are a bit more complicated, especially since none of us have any neuroscience background. The lecturer's explanations made absolutely zero sense to me, so here's what I learned from Rang and Dale's Pharmacology:
First, let's talk about the extrapyramidal nervous system. There are several different components to this system, such as the substantia nigra (which is made up of the pars compact and pars reticulata), the corpus striatum, globus pallidus, subthalamic nucleus, thalamaus, and motor cortex. Dopaminergic neurons travel from the pars compacta (of the substantia nigra) to the corpus striatum, where they can have excitatory effects on GABAergic neurons going to the pars reticulata (also of the substantia nigra) or inhibitory effects on GABAergic neurons going to the globus pallidus.
First, I'm going to discuss the pars reticulata. GABAergic neurons are mostly inhibitory, so activation of the GABAergic neuron travelling from the corpus striatum to pars reticulata causes inhibition of the next neurons, which are GABAergic neurons going to the thalamus. Now, inhibiting an inhibitory neuron (disinhibition) causes activation, so glutamatergic (excitatory) neurons in the thalamus become activated. These neurons go to the motor cortex, stimulating movement.
But what about the other pathway? Inhibition of GABAergic neurons to the globus pallidus causes activation of the GABAergic neurons there. These GABAergic neurons go to the subthalamic nucleus, causing inhibition of glutamatergic neurons travelling to the pars reticulata. This also inhibits the GABAergic neurons travelling from the pars reticulata to the thalamus, thus removing the brakes on the glutamatergic neurons travelling to the motor cortex.
In Parkinson's Disease, the dopaminergic pathway between the pars compacta and corpus striatum is impaired in some way. This means that there is no disinhibition of GABAergic neurons travelling from the pars reticulata to the thalamus, which in turn means that the glutamatergic neurons to the motor cortex are inhibited (hence bradykinesia and difficulty in initiating movement).
Sorry if all of that was confusing. Here's a diagram from Rang and Dale's Pharmacology, Eighth Edition (p. 492), which might help you to visualise it all better:
Genetics
Most patients with PD (~95%) have sporadic PD, but the remaining 5% may have an inherited genetic disorder responsible for PD. Possible genes involved include α-syn, Parkin, DJ-1 and Pink-1.
MPTP, 6-OHDA, and Parkinson's
As discussed here, MPTP can damage dopaminergic neurons in a fairly roundabout manner. Therefore, it is often used to create animal models of Parkinson's. It has been found that you need to lose at least 80% of dopaminergic neurons in order to develop Parkinson's, which fits with Seeman's hypothesis stating that antagonising over 80% of dopaminergic neurons causes extrapyramidal side effects.
6-OHDA (6-hydroxydopamine) is another toxin that can destroy dopaminergic neurons. Interestingly enough, giving different drugs to 6-OHDA lesioned rats can make them turn in different directions. Giving amphetamine (which increases dopamine release) to these rats causes ipsiversive turning (i.e. towards the side of the lesion). Giving a direct D1 or D2 receptor agonist causes contraversive turning. (Unfortunately, I don't really understand why.)
Sinemet
Sinemet is a combination of L-DOPA (a dopamine precursor) and carbidopa (a peripherally selective aromatic L-amino acid decarboxylase inhibitor). Carbidopa blocks peripheral synthesis of dopamine so that high enough levels of L-DOPA can cross the blood-brain barrier. L-DOPA, being a precursor for dopamine, increases the production of dopamine. Sinemet is the gold-standard treatment for PD.
One of the problems with Sinemet is that it displays fluctuations in efficacy. This occurs for several reasons: firstly, it has a short half-life and secondly, after meals amino acids compete for transport across the blood-brain barrier, so meals can reduce the amount of L-DOPA getting into the brain. One of the other problems with Sinemet is that, being a dopamine agonist, it comes with a risk of psychosis.
Another problem with Sinemet is that it can result in the development of dyskinesia, called L-DOPA induced dyskinesia (LiD). It is still uncertain as to what causes LiD. Some people believe that LiD may be due to the pulsatile, rather than continuous, delivery of Sinemet. Therefore pumps and other methods of administration are being trialled. Others believe that LiD may be due to dopamine release by serotonergic neurons. L-DOPA may be taken up by serotonergic neurons, and since serotonergic neurons also have aromatic L-amino acid decarboxylase, they may also begin producing dopamine. Release of dopamine without feedback control may be responsible for dyskinesias.
Vitamins C and K
Final post for BIOC3004- possibly also my last post for biochemistry! Wow...
Understand the outline of vitamin C biosynthesis
Vitamin C can be synthesised in some animals, but not in others (like us). Its synthesis begins with glucose 1-phosphate- a 6-membered ring that gets converted into a 5-membered ring further on in the process. Species that lack vitamin C are missing the final reaction in the vitamin C synthesis process.
Vitamin C (ascorbate) can act as a reductant (electron donor) in many reactions, and I'll explore what these processes are throughout this post. Restoration of reduced vitamin C (dehydroascorbate) to ascorbate can also convert reduced glutathione (GSH) into oxidised glutathione (GSSG) at the same time. (GSH and GSSG are also important regulators of oxidation/reduction potentials in cells.)
Vitamin C's role in hydroxyproline production and collagen formation
See previous post: Proteins- Modifications. Conversion of proline to hydroxyproline allows the three strands of collagen to pack closely together. Collagen triple-helices can be crosslinked further in a process requiring lysine and the enzyme lysyl oxidase.
Vitamin C in neurotransmitter synthesis
See previous post: Understanding Neurotransmitters in the CNS. The synthesis pathways involved in making neurotransmitters requires several cofactors, such as B6, Vitamin C, and SAM. Vitamin C reduces Cu2+ to Cu+, which is required for the action of dopamine β-hydroxylase (which converts dopamine to noradrenaline).
Know the daily requirements for vitamin C and disorders related to under supply.
Understand blood clotting and the indirect involvement of vitamin K
See previous posts:
Vitamin K is required to convert glutamate (Glu) to γ-carboxyglutamate (Gla) via Vitamin K dependent carboxylase. Gla, which binds Ca2+, is necessary for the activation of some of the zymogens (inactive enzymes) in the clotting pathway.
Understand that Vitamin K is chemically similar to vitamin A
Vitamin K has some similarities to vitamin A: both vitamins are fat-soluble, can be derived from plants or bacteria, and have a ring structure with an unsaturated lipid side chain. There are three types of vitamin K: vitamin K1, which is of plant origin; vitamin K2, which is of bacterial origin; and vitamin K3, which is a synthetic form lacking the side chain (only the ring part is required for its action).
Vitamin K dependent reactions
As I mentioned earlier, vitamin K is required to convert Glu into Gla. Throughout this process, vitamin K cycles through several different forms. When Glu is converted into Gla, vitamin K hydroquinone is converted into vitamin K 2,3-epoxide. Vitamin K 2,3-epoxide can be reduced via vitamin K epoxide reductase to form vitamin K quinone, which can be reduced again to re-form vitamin K hydroquinone. Quinones are important in processes such as the electron transport chain.
Vitamin K can also alter Glu in other proteins. One of these is osteocalcin, a protein found in osteoblasts. As osteocalcin is only found in osteoblasts, it is a useful marker for bone mineralisation and calcification. Matrix Gla protein can also regulate bone mineralisation and calcification, but it is not unique to osteoblasts.
Know the daily requirements for vitamin K and disorders related to under supply.
Understand the outline of vitamin C biosynthesis
Vitamin C can be synthesised in some animals, but not in others (like us). Its synthesis begins with glucose 1-phosphate- a 6-membered ring that gets converted into a 5-membered ring further on in the process. Species that lack vitamin C are missing the final reaction in the vitamin C synthesis process.
Vitamin C (ascorbate) can act as a reductant (electron donor) in many reactions, and I'll explore what these processes are throughout this post. Restoration of reduced vitamin C (dehydroascorbate) to ascorbate can also convert reduced glutathione (GSH) into oxidised glutathione (GSSG) at the same time. (GSH and GSSG are also important regulators of oxidation/reduction potentials in cells.)
Vitamin C's role in hydroxyproline production and collagen formation
See previous post: Proteins- Modifications. Conversion of proline to hydroxyproline allows the three strands of collagen to pack closely together. Collagen triple-helices can be crosslinked further in a process requiring lysine and the enzyme lysyl oxidase.
Vitamin C in neurotransmitter synthesis
See previous post: Understanding Neurotransmitters in the CNS. The synthesis pathways involved in making neurotransmitters requires several cofactors, such as B6, Vitamin C, and SAM. Vitamin C reduces Cu2+ to Cu+, which is required for the action of dopamine β-hydroxylase (which converts dopamine to noradrenaline).
Know the daily requirements for vitamin C and disorders related to under supply.
- Children: 35mg/day
- Teenagers: 40mg/day
- Adults: 45mg/day
- Pregnancy: 55mg/day
- Lactation: 80-85mg/day
Sources of vitamin C: capsicum, acerola (West Indian cherry), guava, litchi, oranges, lemons etc. It is readily absorbed and can aid in the absorption of iron.
The most well-known problem with vitamin C deficiency is scurvy. As vitamin C is important in collagen formation, the symptoms of vitamin C are mainly related to collagen breakdown, leading to gingival bleeding, loss of teeth, neuropathy and death. It was a large cause of death among sailors in the 18th century, but is relatively rare in developed countries now.
Understand blood clotting and the indirect involvement of vitamin K
See previous posts:
Vitamin K is required to convert glutamate (Glu) to γ-carboxyglutamate (Gla) via Vitamin K dependent carboxylase. Gla, which binds Ca2+, is necessary for the activation of some of the zymogens (inactive enzymes) in the clotting pathway.
Understand that Vitamin K is chemically similar to vitamin A
Vitamin K has some similarities to vitamin A: both vitamins are fat-soluble, can be derived from plants or bacteria, and have a ring structure with an unsaturated lipid side chain. There are three types of vitamin K: vitamin K1, which is of plant origin; vitamin K2, which is of bacterial origin; and vitamin K3, which is a synthetic form lacking the side chain (only the ring part is required for its action).
Vitamin K dependent reactions
As I mentioned earlier, vitamin K is required to convert Glu into Gla. Throughout this process, vitamin K cycles through several different forms. When Glu is converted into Gla, vitamin K hydroquinone is converted into vitamin K 2,3-epoxide. Vitamin K 2,3-epoxide can be reduced via vitamin K epoxide reductase to form vitamin K quinone, which can be reduced again to re-form vitamin K hydroquinone. Quinones are important in processes such as the electron transport chain.
Vitamin K can also alter Glu in other proteins. One of these is osteocalcin, a protein found in osteoblasts. As osteocalcin is only found in osteoblasts, it is a useful marker for bone mineralisation and calcification. Matrix Gla protein can also regulate bone mineralisation and calcification, but it is not unique to osteoblasts.
Know the daily requirements for vitamin K and disorders related to under supply.
- Children: 25-35 μg/day
- Teenagers: 45-55 μg/day
- Men: 70 μg/day
- Women: 60 μg/day
Vitamin K1 is found in parsley, green leafy vegetables, spices and broccoli, whereas vitamin K2 is found in chicken and prepared meats. Vitamin K is lost fairly rapidly so you do need to keep up your intake. Another issue with vitamin K is that very little of it is found in newborns, and not much is passed down through breast milk. In some babies, haemorrhagic disease (which comes with a risk of brain bleed) may be an issue due to low vitamin K, so vitamin K injections are routinely offered at birth. Vitamin K may also be deficient in patients who have a blockage in their bile ducts, as bile salts are required for vitamin K absorption.
Tuesday, May 23, 2017
Vitamins A and D
Second last post for biochemistry! That was quick...
Understand the example roles given for retinol, retinal and retinoic acid
Vitamin A, just like some of the other vitamins that I've spoken about, is actually a range of related molecules. These include retinol, retinal, retinoic acid and carotenoids. The main structure consists of a β-ionone ring with a chain made up of isoprenoid units.
Carotenoids are known for giving things colour. They are stored in fat (remember, vitamins A, D, E and K are all fat-soluble) where they give fat its yellow colour. Carotenoids can even give birds their colour. If you eat too many carotenoids, your skin can become more orange, as I'll mention again later. Plants have another use for carotenoids: they are associated with photosystems and therefore play a role in light transduction.
Retinal is important for vision. Retinal bound to opsin (a G-protein coupled receptor that makes up part of the retina) is called rhodopsin. When light hits rhodopsin, rhodopsin can change from the cis- to the trans- conformation. Trans-rhodopsin can activate a G-protein called transducin, releasing opsin at the same time. If retinal is deficient, this can lead to xerophthalmia (a fancy term that just means "blindness").
Retinoic acid (RA) can affect gene transcription. RA can bind to the retinoic acid receptor (RAR), which usually exists as a heterodimer with the retinoid receptor (RXR). Usually, the RAR/RXR combo are usually bound to a co-repressor that blocks the retinoic acid response element (RARE). When RA binds to RAR, however, RAR and RXR can "de-repress" RARE, allowing gene transcription. Retinoic acid is involved in many signalling pathways, including MAPK and chemokine signalling pathways. It is also involved in guiding differentiation during embryonic development: a retinoic acid gradient forms along the length of the embryo due to metabolism by CYP450, which in turn can lead to different outcomes. (Fun fact: while CYP450 can break down RA, it can be re-built by retinaldehyde dehydrogenase.)
Vitamin A in mucus, infection and malnutrition
Mucus
Retinoic acid is important for triggering the creation of mucus-secreting goblet cells. It does not affect existing goblet cells, but when it's time for those cells to be replaced, you need retinoic acid or bad shit will happen. That bad shit, by the way, is replacement of goblet cells with squamous cells, causing squamous metaplasia.
Infection and malnutrition
Infection can deplete vitamin A for several reasons. Firstly, when you're sick, you may eat less, so you might not get enough vitamin A. Secondly, absorption might be impaired due to your illness. Excretion may also be increased due to diarrhoea or in urine. The problem with this is that, as I just said, low levels of retinoic acid can result in replacement of goblet cells with squamous cells. Fewer goblet cells means less mucus, and mucus normally helps to protect us from infection. It's a bit of a vicious cycle.
Know the daily requirements for vitamin A and disorders related to under and over supply
Okay, you know the drill:
Understand the creation and transformation of active vitamin D, its role in calcium resorption
Vitamin D, unlike other vitamins, can be created by us as long as we have sunlight. The starting point for vitamin D synthesis is 7-dehydrocholesterol which, as mentioned here, is also used to produce cholesterol. There are different forms of vitamin D: plants mainly have D2, whereas we mainly have D3. 7-dehydrocholesterol can be converted into pre-vitamin D3 via UV light, and pre-vitamin D3 can be converted into vitamin D3 via warmth.
But wait! It's not done yet. Vitamin D3 needs to undergo further activation. It needs to be hydroxylated into calcidiol and then hydroxylated again to form calcitriol, which is the active form of vitamin D3.
Calcitriol helps to regulate calcium levels in the body, along with parathyroid hormone (PTH). When calcium intake is low, the parathyroid gland releases PTH, which stimulates 1-hydroxylase. PTH and calcitriol can then stimulate osteoclasts, increasing the resorption of bone. This combination of PTH and calcitriol can also increase calcium resorption by the kidneys.
Know the daily requirements for vitamin D and disorders related to under and over supply
Understand the example roles given for retinol, retinal and retinoic acid
Vitamin A, just like some of the other vitamins that I've spoken about, is actually a range of related molecules. These include retinol, retinal, retinoic acid and carotenoids. The main structure consists of a β-ionone ring with a chain made up of isoprenoid units.
Carotenoids are known for giving things colour. They are stored in fat (remember, vitamins A, D, E and K are all fat-soluble) where they give fat its yellow colour. Carotenoids can even give birds their colour. If you eat too many carotenoids, your skin can become more orange, as I'll mention again later. Plants have another use for carotenoids: they are associated with photosystems and therefore play a role in light transduction.
Retinal is important for vision. Retinal bound to opsin (a G-protein coupled receptor that makes up part of the retina) is called rhodopsin. When light hits rhodopsin, rhodopsin can change from the cis- to the trans- conformation. Trans-rhodopsin can activate a G-protein called transducin, releasing opsin at the same time. If retinal is deficient, this can lead to xerophthalmia (a fancy term that just means "blindness").
Retinoic acid (RA) can affect gene transcription. RA can bind to the retinoic acid receptor (RAR), which usually exists as a heterodimer with the retinoid receptor (RXR). Usually, the RAR/RXR combo are usually bound to a co-repressor that blocks the retinoic acid response element (RARE). When RA binds to RAR, however, RAR and RXR can "de-repress" RARE, allowing gene transcription. Retinoic acid is involved in many signalling pathways, including MAPK and chemokine signalling pathways. It is also involved in guiding differentiation during embryonic development: a retinoic acid gradient forms along the length of the embryo due to metabolism by CYP450, which in turn can lead to different outcomes. (Fun fact: while CYP450 can break down RA, it can be re-built by retinaldehyde dehydrogenase.)
Vitamin A in mucus, infection and malnutrition
Mucus
Retinoic acid is important for triggering the creation of mucus-secreting goblet cells. It does not affect existing goblet cells, but when it's time for those cells to be replaced, you need retinoic acid or bad shit will happen. That bad shit, by the way, is replacement of goblet cells with squamous cells, causing squamous metaplasia.
Infection and malnutrition
Infection can deplete vitamin A for several reasons. Firstly, when you're sick, you may eat less, so you might not get enough vitamin A. Secondly, absorption might be impaired due to your illness. Excretion may also be increased due to diarrhoea or in urine. The problem with this is that, as I just said, low levels of retinoic acid can result in replacement of goblet cells with squamous cells. Fewer goblet cells means less mucus, and mucus normally helps to protect us from infection. It's a bit of a vicious cycle.
Know the daily requirements for vitamin A and disorders related to under and over supply
Okay, you know the drill:
- Children: 300-400 μg/day
- Teenagers: (Males) 600-900 μg/day (Females) 600-700 μg/day
- Adults: (Males) 900 μg/day (Females) 700 μg/day
- Lactation: 1 100 μg/day
Understand the creation and transformation of active vitamin D, its role in calcium resorption
Vitamin D, unlike other vitamins, can be created by us as long as we have sunlight. The starting point for vitamin D synthesis is 7-dehydrocholesterol which, as mentioned here, is also used to produce cholesterol. There are different forms of vitamin D: plants mainly have D2, whereas we mainly have D3. 7-dehydrocholesterol can be converted into pre-vitamin D3 via UV light, and pre-vitamin D3 can be converted into vitamin D3 via warmth.
But wait! It's not done yet. Vitamin D3 needs to undergo further activation. It needs to be hydroxylated into calcidiol and then hydroxylated again to form calcitriol, which is the active form of vitamin D3.
Calcitriol helps to regulate calcium levels in the body, along with parathyroid hormone (PTH). When calcium intake is low, the parathyroid gland releases PTH, which stimulates 1-hydroxylase. PTH and calcitriol can then stimulate osteoclasts, increasing the resorption of bone. This combination of PTH and calcitriol can also increase calcium resorption by the kidneys.
Know the daily requirements for vitamin D and disorders related to under and over supply
- Under 50s: 5 μg/day
- 50-70 years old: 10 μg/day
- Over 70s: 15 μg/day
As I've stated earlier, Vitamin D is kind of unique in that the majority of our intake is not through food: we produce it when we are exposed to sunlight. We become less efficient at producing it as we age, which is why older adults have a higher vitamin D requirement than younger adults. Foods that contain vitamin D include cod liver oil, fish, mushrooms and fortified foods. Extra supplementation might be needed for people who don't get adequate sunlight. Oversupplementation can lead to calcium build-up in the heart, lungs and kidneys, which isn't good. Low vitamin D, on the other hand, can lead to rickets and osteomalacia (see here).
Another point of interest is that since vitamin D can be found associated with a transcription factor (RXR), it may have other roles. In fact, some researchers are looking at possible relationships between vitamin D and cancer.
Shock
Shock, in a nutshell, is inadequate peripheral perfusion that can lead to cell death and, well, death of the rest of the body if left untreated. Of course, there's more to say about shock than that: enough to fill up a whole 45-minute lecture, in fact!
Understand the different types and causes of shock
There are many different types of shock:
Here are the stages of haemorrhagic shock:
Discuss the compensatory mechanisms
Mean arterial pressure actually stays pretty constant until a fairly large amount of blood is lost (>20%), largely due to the baroreceptor reflex. The baroreceptors are maximally sensitive when mean arterial pressure is normal, which means they can easily detect even a small change in blood pressure. When blood pressure drops, the firing rate of the baroreceptors decreases. This is picked up by the nucleus tractus solitarius in the medulla, leading to sympathetic activation and parasympathetic inhibition.
Aside from the baroreceptor reflex, there are several other compensatory responses for hypovolemia. Peripheral chemoreceptors can play a role, especially when mean arterial pressure drops below 60mmHg- I suppose that less blood means less oxygen to go around. Chemoreceptor stimulation also activates the sympathetic nervous system. Due to sympathetic nervous system activation, the respiratory system may also be activated, which may enhance venous return (this all has to do with something called the "abdominothoracic pump"- maybe inflation of the lungs squishes the veins?). Yet another compensatory mechanism is an increase in circulating vasoconstrictors, such as ADH, aldosterone and catecholamines (i.e. adrenaline and noradrenaline).
Aaaaand I'm not done yet! When mean arterial pressure drops below 60mmHg, not only do peripheral chemoreceptors get activated, but the CNS ischaemic response may also be activated! The CNS ischaemic response is a very intense sympathetic response (from both sympathetic nerves and adrenal glands) that mainly exists to maintain perfusion to the brain.
Aaaaaaaand I'm still not done! Other compensatory mechanisms include redistribution of interstitial fluid (return of fluid back to the blood), stimulation of thirst (so that you drink more and get your blood volume back up) and haematopoiesis (via stimulation of EPO).
Unfortunately, in severe haemorrhage, these mechanisms may not be sufficient. A large drop in arterial pressure can create not one but two vicious cycles. In the first vicious cycle, a drop in arterial pressure causes a drop in coronary perfusion, which decreases inotropy, which decreases cardiac output, which decreases arterial pressure. In the second vicious cycle, a drop in arterial pressure decreases blood flow to the organs, which causes hypoxia, which causes release of vasodilating mediators as well as a phenomenon called "sympathetic escape" (desensitisation to sympathetic activity so that it doesn't cause as much constriction), which causes a decrease in arterial pressure.
Decompensated shock can cause cardiac failure, acidosis, CNS depression, an increase in capillary permeability, an increase in toxins (from dead cells) and blockage of small blood vessels (stagnation in blood flow can lead to clots). So, in short: you're f***ed.
Describe signs and symptoms
Signs and symptoms of shock include restlessness, anxiety, a decreased level of consciousness, dull eyes, rapid shallow respiration, nausea, vomiting, thirst, diminished urine output and usually some tachycardia. The skin may be pale, cool and clammy (in the case of hypovolemic and cardiogenic shock) or dry and flushed (in the case of septic, anaphylactic and neurogenic shock). In the case of anaphylactic shock, there may be other signs of an allergic reaction, such as hives, itching, wheezing and difficulty breathing.
Discuss treatment of shock
Treatment of shock generally comes down to treating the causes. For example, if shock is due to hypovolemia, then you need to restore blood volume by using crystalloids, colloids or blood transfusions. Furthermore, if there is bleeding, then you may need surgery to stop it. Using the same principle of "treat the cause," septic shock can be treated with antibiotics, cardiogenic shock can be treated with supports such as inotropic and chronotropic agents and anaphylactic shock can be treated with EpiPens. Vasoactive substances can be used to treat vasodilation and bicarbonate can be used to treat the acidosis that often occurs as a result of shock.
Understand the different types and causes of shock
There are many different types of shock:
- Hypovolemic shock: Shock due to loss of blood volume (could be due to trauma, burns, vomiting, diarrhoea and so on). I will be mainly discussing hypovolemic shock (particularly haemorrhagic shock) in this post.
- Neurogenic shock: Shock due to sudden dilation of the blood vessels, which in turn may be due to CNS damage and loss of autonomic nervous system signals to the vascular smooth muscle.
- Psychogenic shock: Shock in response to stress, pain or fright. Like neurogenic shock, there is sudden dilation of the blood vessels, but this is transient.
- Septic shock: Shock in response to a bacterial infection. Bacteria release exotoxins and endotoxins, which result in vasodilation and an increase in capillary permeability.
- Anaphylactic shock: Shock due to a severe allergic reaction. You can read more about anaphylaxis here.
- Cardiogenic shock: Shock due to some problem with the heart resulting in decreased stroke volume and decreased cardiac output. This may be due to left heart failure (which also causes pulmonary oedema) or right heart failure (which causes systemic oedema). See here for more information on heart failure.
- Obstructive shock: Shock due to obstruction of important blood vessels, causing a decrease in cardiac output. This can be due to cardiac tamponade (fluid filling the pericardial cavity), pneumothorax (air in the pleural cavity) or pulmonary embolism.
Here are the stages of haemorrhagic shock:
- Class I haemorrhage (loss of 0-15% of blood volume): Little tachycardia. Usually no significant change in blood pressure, pulse pressure or respiratory rate.
- Class II (15-30%): Elevated heart rate (>100bpm), tachypnea, decreased pulse pressure
- Class III (30-40%): Tachycardia, tachypnea, decreased systolic blood pressure, oliguria
- Class IV (>40%): Tachycardia, decreased systolic blood pressure, decreased pulse pressure, little (or no) urinary output. This stage is immediately life-threatening.
Discuss the compensatory mechanisms
Mean arterial pressure actually stays pretty constant until a fairly large amount of blood is lost (>20%), largely due to the baroreceptor reflex. The baroreceptors are maximally sensitive when mean arterial pressure is normal, which means they can easily detect even a small change in blood pressure. When blood pressure drops, the firing rate of the baroreceptors decreases. This is picked up by the nucleus tractus solitarius in the medulla, leading to sympathetic activation and parasympathetic inhibition.
Aside from the baroreceptor reflex, there are several other compensatory responses for hypovolemia. Peripheral chemoreceptors can play a role, especially when mean arterial pressure drops below 60mmHg- I suppose that less blood means less oxygen to go around. Chemoreceptor stimulation also activates the sympathetic nervous system. Due to sympathetic nervous system activation, the respiratory system may also be activated, which may enhance venous return (this all has to do with something called the "abdominothoracic pump"- maybe inflation of the lungs squishes the veins?). Yet another compensatory mechanism is an increase in circulating vasoconstrictors, such as ADH, aldosterone and catecholamines (i.e. adrenaline and noradrenaline).
Aaaaand I'm not done yet! When mean arterial pressure drops below 60mmHg, not only do peripheral chemoreceptors get activated, but the CNS ischaemic response may also be activated! The CNS ischaemic response is a very intense sympathetic response (from both sympathetic nerves and adrenal glands) that mainly exists to maintain perfusion to the brain.
Aaaaaaaand I'm still not done! Other compensatory mechanisms include redistribution of interstitial fluid (return of fluid back to the blood), stimulation of thirst (so that you drink more and get your blood volume back up) and haematopoiesis (via stimulation of EPO).
Unfortunately, in severe haemorrhage, these mechanisms may not be sufficient. A large drop in arterial pressure can create not one but two vicious cycles. In the first vicious cycle, a drop in arterial pressure causes a drop in coronary perfusion, which decreases inotropy, which decreases cardiac output, which decreases arterial pressure. In the second vicious cycle, a drop in arterial pressure decreases blood flow to the organs, which causes hypoxia, which causes release of vasodilating mediators as well as a phenomenon called "sympathetic escape" (desensitisation to sympathetic activity so that it doesn't cause as much constriction), which causes a decrease in arterial pressure.
Decompensated shock can cause cardiac failure, acidosis, CNS depression, an increase in capillary permeability, an increase in toxins (from dead cells) and blockage of small blood vessels (stagnation in blood flow can lead to clots). So, in short: you're f***ed.
Describe signs and symptoms
Signs and symptoms of shock include restlessness, anxiety, a decreased level of consciousness, dull eyes, rapid shallow respiration, nausea, vomiting, thirst, diminished urine output and usually some tachycardia. The skin may be pale, cool and clammy (in the case of hypovolemic and cardiogenic shock) or dry and flushed (in the case of septic, anaphylactic and neurogenic shock). In the case of anaphylactic shock, there may be other signs of an allergic reaction, such as hives, itching, wheezing and difficulty breathing.
Discuss treatment of shock
Treatment of shock generally comes down to treating the causes. For example, if shock is due to hypovolemia, then you need to restore blood volume by using crystalloids, colloids or blood transfusions. Furthermore, if there is bleeding, then you may need surgery to stop it. Using the same principle of "treat the cause," septic shock can be treated with antibiotics, cardiogenic shock can be treated with supports such as inotropic and chronotropic agents and anaphylactic shock can be treated with EpiPens. Vasoactive substances can be used to treat vasodilation and bicarbonate can be used to treat the acidosis that often occurs as a result of shock.
Stem Cell Mechanotransduction
This lecture had a lot more to do with general mechanobiology than with stem cells in particular, but oh well. For an introduction to mechanobiology, see here.
Heart cell differentiation
As mentioned here, stiffness clearly has an impact on the differentiation of cardiac cells (they turned into bone when transplanted into a stiffer region). In vitro, the type of hydrogel (synthetic ECM) used can also impact differentiation. Two different types of hydrogels: polyacrylamide (a static stiffness hydrogel) and hyaluronic acid (a dynamic stiffness hydrogel that becomes stiffer with time) were trialled. Differentiation was better on the hyaluronic acid gel.
Traction force
As also mentioned here, cells can exert force on the ECM by pulling on it. This is called traction force. As mentioned in that previous post, there is a chain of proteins connecting the ECM to the nucleus: ECM - integrin - focal adhesion complex (talin and vinculin) - actin/myosin - nucleus. This chain of proteins affects mechotransduction (like a "signalling pathway" for mechanical forces) which may affect traction force. Stimuli that affect traction force include shape, size and stiffness.
Myogenic differentiation
As another example of how stiffness can affect differentiation, adipose stem cells (ASCs) were cultured on a 10kPa hydrogel (10kPa is the approximate stiffness of muscle tissue). This was found to improve the differentiation of ASCs into myocytes. When certain aspects of the mechanotransduction pathway (like integrins) were "turned off" by using silencing RNA (siRNA), differentiation was markedly reduced.
Muscle-like stiffness also promoted the fusion of ASCs into myotubes. Don't get excited though: in a study done, only around 2% of ASCs actually fused into myotubes, but that's still a big improvement over previous trials that only showed 0.2% fusion. Fused myotubes remained fused when cultured on stiff tissues, but unfused cells were more likely to become bone when cultured on stiff tissues.
Durotaxis
As mentioned here, durotaxis is the phenomenon in which cells migrate from softer to stiffer tissue. This can be tested by using gradient hydrogels. One type of gradient is a linear gradient, in which the hydrogel gradually changes from soft to stiff along the length of the gel. Another type of gradient is the step gradient which strips of soft hydrogel alternate with strips of stiff hydrogel. An example of a gel that uses a step gradient is called "zebraxis." Cells cultured on zebraxis hydrogel have a tendency to congregate in the stiffer sections.
Stiffness vs. Pore Size
Stiffness, to some extent, depends on the amount of "stuff" in the ECM. In softer ECMs (and softer hydrogels), there are larger gaps between the "stuff," resulting in larger pore sizes. Conversely, in stiffer ECMs, there are smaller pore sizes. When proteins like collagen tether to hydrogels, the distance between tethering points also depends on the pore size: larger pores (and softer ECM) means a larger distance between tethering points, and smaller pores (and stiffer ECM) means a shorter distance between tethering points. It has been hypothesised that the length of the "tether" may be sensed by cells, rather than the actual stiffness of the ECM (but this is likely incorrect, as I'm about to explain.)
One way to test this in vitro is by varying the composition of the hydrogel. Polyacrylamide hydrogels are made up of two main components: acrylamide and bis-acrylamide. Bis-acrylamide acts as a cross-linker for acrylamide. When the proportions of acrylamide and bis-acrylamide are changed without changing the overall amount of molecules in the hydrogel, you can change the pore size without changing stiffness, allowing you to test the effect of pore size independent of stiffness. By testing this, it has been found that stiffness affects mechanotransduction independent of pore size.
Lamin-A
Lamin-A is a nuclear protein that scales with stiffness, i.e. there is more of it when tissues are stiffer. Interestingly enough, it may be involved in a feedback mechanism where stiff tissues maintain Lamin-A and the stiffness of the nucleus. Lamin-A, like other proteins, is synthesised in the cytoplasm. When the cell is under stress, it can be translocated to the nucleus and help in the assembly of nuclear lamins, but when the cell is more relaxed, it remains in the cytoplasm (or already assembled nuclear lamins may be transported back to the cytoplasm), where they can be degraded by proteases. (I think I'll need to double-check that I've got this part right, though. I was pretty tired during the lecture, and the slides aren't really helping.)
Other mechanosensors
Some other mechanosensors present in the cell include YAP/TAZ, talin and vinculin. YAP/TAZ are proteins that can localise in either the nucleus or the cytoplasm. When localised in the cytoplasm, differentiation into adipocytes, growth arrest and maybe even apoptosis can occur. When localised in the nucleus, proliferation and/or osteogenic differentiation may occur.
Talin and vinculin, as mentioned earlier, are components of the focal adhesion complex. When pulled, they can undergo conformational changes. When vinculin is bound to talin, it can reveal a cryptic binding site for MAPK (MAP kinase). (A "cryptic binding site" is basically a binding site on a protein that is normally hidden due to the conformation of the protein.)
Mechanomemory
One phenomenon that has been observed is that of "mechanomemory." Basically, if you culture cells on a very stiff surface such as glass before moving them to a softer hydrogel, they won't respond as well as cells that were on the soft hydrogel from the get-go. It's been thought that this is because the cells have "mechanomemory" of the stiffer hydrogel.
Heart cell differentiation
As mentioned here, stiffness clearly has an impact on the differentiation of cardiac cells (they turned into bone when transplanted into a stiffer region). In vitro, the type of hydrogel (synthetic ECM) used can also impact differentiation. Two different types of hydrogels: polyacrylamide (a static stiffness hydrogel) and hyaluronic acid (a dynamic stiffness hydrogel that becomes stiffer with time) were trialled. Differentiation was better on the hyaluronic acid gel.
Traction force
As also mentioned here, cells can exert force on the ECM by pulling on it. This is called traction force. As mentioned in that previous post, there is a chain of proteins connecting the ECM to the nucleus: ECM - integrin - focal adhesion complex (talin and vinculin) - actin/myosin - nucleus. This chain of proteins affects mechotransduction (like a "signalling pathway" for mechanical forces) which may affect traction force. Stimuli that affect traction force include shape, size and stiffness.
Myogenic differentiation
As another example of how stiffness can affect differentiation, adipose stem cells (ASCs) were cultured on a 10kPa hydrogel (10kPa is the approximate stiffness of muscle tissue). This was found to improve the differentiation of ASCs into myocytes. When certain aspects of the mechanotransduction pathway (like integrins) were "turned off" by using silencing RNA (siRNA), differentiation was markedly reduced.
Muscle-like stiffness also promoted the fusion of ASCs into myotubes. Don't get excited though: in a study done, only around 2% of ASCs actually fused into myotubes, but that's still a big improvement over previous trials that only showed 0.2% fusion. Fused myotubes remained fused when cultured on stiff tissues, but unfused cells were more likely to become bone when cultured on stiff tissues.
Durotaxis
As mentioned here, durotaxis is the phenomenon in which cells migrate from softer to stiffer tissue. This can be tested by using gradient hydrogels. One type of gradient is a linear gradient, in which the hydrogel gradually changes from soft to stiff along the length of the gel. Another type of gradient is the step gradient which strips of soft hydrogel alternate with strips of stiff hydrogel. An example of a gel that uses a step gradient is called "zebraxis." Cells cultured on zebraxis hydrogel have a tendency to congregate in the stiffer sections.
Stiffness vs. Pore Size
Stiffness, to some extent, depends on the amount of "stuff" in the ECM. In softer ECMs (and softer hydrogels), there are larger gaps between the "stuff," resulting in larger pore sizes. Conversely, in stiffer ECMs, there are smaller pore sizes. When proteins like collagen tether to hydrogels, the distance between tethering points also depends on the pore size: larger pores (and softer ECM) means a larger distance between tethering points, and smaller pores (and stiffer ECM) means a shorter distance between tethering points. It has been hypothesised that the length of the "tether" may be sensed by cells, rather than the actual stiffness of the ECM (but this is likely incorrect, as I'm about to explain.)
One way to test this in vitro is by varying the composition of the hydrogel. Polyacrylamide hydrogels are made up of two main components: acrylamide and bis-acrylamide. Bis-acrylamide acts as a cross-linker for acrylamide. When the proportions of acrylamide and bis-acrylamide are changed without changing the overall amount of molecules in the hydrogel, you can change the pore size without changing stiffness, allowing you to test the effect of pore size independent of stiffness. By testing this, it has been found that stiffness affects mechanotransduction independent of pore size.
Lamin-A
Lamin-A is a nuclear protein that scales with stiffness, i.e. there is more of it when tissues are stiffer. Interestingly enough, it may be involved in a feedback mechanism where stiff tissues maintain Lamin-A and the stiffness of the nucleus. Lamin-A, like other proteins, is synthesised in the cytoplasm. When the cell is under stress, it can be translocated to the nucleus and help in the assembly of nuclear lamins, but when the cell is more relaxed, it remains in the cytoplasm (or already assembled nuclear lamins may be transported back to the cytoplasm), where they can be degraded by proteases. (I think I'll need to double-check that I've got this part right, though. I was pretty tired during the lecture, and the slides aren't really helping.)
Other mechanosensors
Some other mechanosensors present in the cell include YAP/TAZ, talin and vinculin. YAP/TAZ are proteins that can localise in either the nucleus or the cytoplasm. When localised in the cytoplasm, differentiation into adipocytes, growth arrest and maybe even apoptosis can occur. When localised in the nucleus, proliferation and/or osteogenic differentiation may occur.
Talin and vinculin, as mentioned earlier, are components of the focal adhesion complex. When pulled, they can undergo conformational changes. When vinculin is bound to talin, it can reveal a cryptic binding site for MAPK (MAP kinase). (A "cryptic binding site" is basically a binding site on a protein that is normally hidden due to the conformation of the protein.)
Mechanomemory
One phenomenon that has been observed is that of "mechanomemory." Basically, if you culture cells on a very stiff surface such as glass before moving them to a softer hydrogel, they won't respond as well as cells that were on the soft hydrogel from the get-go. It's been thought that this is because the cells have "mechanomemory" of the stiffer hydrogel.
Monday, May 22, 2017
Airway Smooth Muscle
It's too late in the afternoon, I can't be bothered trying to figure out a nice introduction for this blog post. (Not like I ever do anyway.)
Describe the functional role of smooth muscle in the vascular, gastric, reproductive and urinary systems systems at a simple level.
See previous posts on smooth muscle in general:
List the possible functional roles of ASM
Explain the potential function of ASM in:
We are still uncertain about the functions of airway smooth muscle. It is activated during inspiration and has phasic contractions, but the function of this is unknown. Airway resistance is decreased when ASM is inhibited, but this doesn't appear to have any beneficial effect either. There are, however, a few hypotheses about the role of ASM:
Ventilation/perfusion matching
ASM may play a role in matching ventilation to perfusion. Airways with low blood perfusion have a lower CO2, and low CO2 has been found to increase contraction of airway smooth muscle. Contraction of airway smooth muscle may shunt air towards areas with better perfusion. The problem with this hypothesis is that smooth muscle contraction doesn't really increase until CO2 levels drop considerably.
Dead space regulation
Anatomical dead space, as mentioned here, here and here, consists of the conducting zone in the lungs where air passes through but is not exchanged. When we breathe in, the air that we breathe in is actually a mixture of fresh air and exhaled air from the last breath that's been sitting in the dead space. If dead space is decreased by bronchoconstriction, then the amount of rebreathed air in each breath will be reduced. The downside to this is that bronchoconstriction also increases resistance, making it harder to breathe. The optimum ratio of dead space to tidal volume is usually around 20%. It's possible that ASM can regulate the dead space volume to fit changes in tidal volume during exercise (where airways dilate, reducing resistance but increasing dead space) and other conditions.
Cough and airway stability
Even though coughing is mainly controlled by other muscles, such as the diaphragm, coughing also increases ASM tone and bronchoconstriction. While bronchoconstriction reduces air flow (the amount of volume flowing through in a given amount of time), it also increases flow velocity (which I think is the distance each of those air particles travels in a given amount of time). Increased flow velocity can increase the clearance of airways.
Another effect of an increase in ASM tone during cough is that it may also help to stabilise the airways. An increase in stiffness may reduce airway compression, which occurs during forced exhalation. (Forced exhalation occurs not only during coughing, but also crying, shouting, and so on.) A reduction in airway compression helps to keep the airways open. The overall effects of this, though, are yet to be determined.
Foetal breathing
Airway smooth muscle appears early on in foetal development. In both pig and human foetuses, airway smooth muscle shows peristaltic (i.e. contraction that moves along the length of the tube) contraction. It has been suggested that these peristaltic waves move the lung fluid and amniotic fluid into the periphery of the lung, where it is needed to help the lungs to grow. Therefore, it's possible that airway smooth muscle may have been very important in aiding lung growth in the foetus, but it may not be as useful in adults.
Describe the functional role of smooth muscle in the vascular, gastric, reproductive and urinary systems systems at a simple level.
- Arteries: Vasoconstriction, regulation of mean arterial pressure, distribution of cardiac output
- Veins: Venoconstriction, venous capacitance and cardiac filling
- Gastric: Propulsion of gut contents
- Reproductive: Contraction of the uterus pushes out the foetus
- Urinary: Contraction of bladder
- Excretory: Contraction of rectum
See previous posts on smooth muscle in general:
Mediators that activate airway smooth muscle: ACh, endothelin, LTC4 and PGF2
Mediators that inhibit activation of airway smooth muscle: NA, NO, VIP (vasoactive intestinal peptide) and PGE2
Also note that unlike vascular smooth muscle, airway smooth muscle receives input from both sympathetic and parasympathetic nerves. Airway smooth muscle can also use calcium from both inside and outside of the cell (which is apparently different to other smooth muscle, such as gastric smooth muscle).
List the possible functional roles of ASM
Explain the potential function of ASM in:
We are still uncertain about the functions of airway smooth muscle. It is activated during inspiration and has phasic contractions, but the function of this is unknown. Airway resistance is decreased when ASM is inhibited, but this doesn't appear to have any beneficial effect either. There are, however, a few hypotheses about the role of ASM:
Ventilation/perfusion matching
ASM may play a role in matching ventilation to perfusion. Airways with low blood perfusion have a lower CO2, and low CO2 has been found to increase contraction of airway smooth muscle. Contraction of airway smooth muscle may shunt air towards areas with better perfusion. The problem with this hypothesis is that smooth muscle contraction doesn't really increase until CO2 levels drop considerably.
Dead space regulation
Anatomical dead space, as mentioned here, here and here, consists of the conducting zone in the lungs where air passes through but is not exchanged. When we breathe in, the air that we breathe in is actually a mixture of fresh air and exhaled air from the last breath that's been sitting in the dead space. If dead space is decreased by bronchoconstriction, then the amount of rebreathed air in each breath will be reduced. The downside to this is that bronchoconstriction also increases resistance, making it harder to breathe. The optimum ratio of dead space to tidal volume is usually around 20%. It's possible that ASM can regulate the dead space volume to fit changes in tidal volume during exercise (where airways dilate, reducing resistance but increasing dead space) and other conditions.
Cough and airway stability
Even though coughing is mainly controlled by other muscles, such as the diaphragm, coughing also increases ASM tone and bronchoconstriction. While bronchoconstriction reduces air flow (the amount of volume flowing through in a given amount of time), it also increases flow velocity (which I think is the distance each of those air particles travels in a given amount of time). Increased flow velocity can increase the clearance of airways.
Another effect of an increase in ASM tone during cough is that it may also help to stabilise the airways. An increase in stiffness may reduce airway compression, which occurs during forced exhalation. (Forced exhalation occurs not only during coughing, but also crying, shouting, and so on.) A reduction in airway compression helps to keep the airways open. The overall effects of this, though, are yet to be determined.
Foetal breathing
Airway smooth muscle appears early on in foetal development. In both pig and human foetuses, airway smooth muscle shows peristaltic (i.e. contraction that moves along the length of the tube) contraction. It has been suggested that these peristaltic waves move the lung fluid and amniotic fluid into the periphery of the lung, where it is needed to help the lungs to grow. Therefore, it's possible that airway smooth muscle may have been very important in aiding lung growth in the foetus, but it may not be as useful in adults.
Antipsychotics and Schizophrenia
I'm not sure how easy (or not) it will be to write this post, given that this lecture went seriously overtime and the lecturer had to skip a good chunk of the slides. This guy has been seriously optimistic about how much he can cover in 45 minutes, but oh well.
Understand what Schizophrenia is
Schizophrenia is a mental illness with a range of symptoms. Here are the symptoms that are listed in the ICD-10 classification:
- Thought echo (repetitive thoughts), thought insertion (belief that others can insert thoughts into your mind), thought withdrawal (belief that others can remove thoughts from your mind) and/or thought broadcasting (belief that your thoughts may be transmitted to others)
- Delusions of control, influence or passivity, delusional perception
- Hallucinatory voices
- Culturally inappropriate delusions. These include persecutory delusions (belief that you are being persecuted), grandiose delusions (belief that you are super important in some way) and delusions of reference (belief that mundane things are very significant- for example, believing that a television commentator is talking directly to you). These do not include particular religious or political beliefs.
- I don't really know what this one means so I'm going to copy it word for word. "Persistent hallucinations in any modality, when accompanied either by fleeting or half-formed delusions without clear affective content, or by persistent over-valued ideas, or when occurring every day for weeks or months on end."
- Incoherent or irrelevant speech, neologisms (either making up words or using real words in a completely wrong way)
- Catatonic behaviour. I'm also not really sure how this is defined because it seems to encompass a wide range of things: excitement, posturing (a.k.a. "wavy flexibility," where you can move the person around in a random pose and they'll just hold it), negativism, mutism and stupor
EDIT: I found a good clip that explains catatonia - "Negative symptoms," i.e. things that a normal, healthy person would exhibit but a person with schizophrenia would not. These include apathy, paucity of speech, blunting or incongruity of emotional responses and social withdrawal
- Significant and consistent change in personal behaviour
Additionally, you need to rule out other things that could cause these symptoms, like brain disease, drugs and so on.
Schizophrenia can progress in many different ways. It can be continuous or episodic, progressive or stable. There may be incomplete or maybe even complete remission between episodes.
Be familiar with what is known (or perhaps unknown) about the pathology of schizophrenia
A long time ago, people thought that demonic possession or even masturbation were the causes of schizophrenia. Thankfully, we've moved past that, but we still don't really know what the true causes are. Between 1925-1967, scientists thought that they found some brain changes, but they soon changed their minds and decided that there were no changes that they could see. In the 90s, schizophrenia came to be associated with ventricular enlargement and decreased cortical volume, but later these changes were found to be associated more with antipsychotic use rather than the illness itself.
Be familiar with the Dopamine Theory of
Psychosis
Since all effective treatments for psychosis block dopamine D2 receptors, and drugs that increase dopamine transmission (like amphetamine) can induce psychosis, it makes sense that too much dopamine can cause psychosis, right? Furthermore, patients with schizophrenia, as well as patients deemed to be at ultra-high risk of developing schizophrenia ("ultra-high risk" in this case means that they have already developed some psychotic symptoms, but not full-blown schizophrenia), have increased synthesis and release of dopamine compared to healthy controls.
There are multiple pathways involved in the release of dopamine, including the mesolimbic and mesocortical pathways. Some scientists believe that an increase in dopamine transmission in the mesolimbic pathway is responsible for psychosis, whereas a decrease in dopamine transmission in the mesocortical pathway is responsible for the "negative" symptoms (such as apathy and social withdrawal).
Know the differences and similarities between
Typical and Atypical Neuroleptics
This part is going to be watered down, given that the lecturer didn't have time to cover it in detail.
First things first: neuroleptics, antipsychotics and major tranquillisers all refer to the same drugs.
The first neuroleptics developed are known as the "typical neuroleptics." They are all very effective D2 antagonists. The therapeutic effects are due to antagonising D2 in the mesolimbic system, whereas side effects are thought to be from D2 antagonism in the nigrostriatal system (part of the extrapyramidal motor system, so these side effects are also known as "extrapyramidal side effects"). And oh boy, the side effects do not sound pleasant:
- Parkinsonism (muscular rigidity, slow movement, tremor)
- Acute dystonic reactions (abnormal movements)
- Akathisia (restlessness)
- Neuroleptic malignant syndrome (high fever that may lead to death- pretty rare but obviously pretty nasty)
- Cardiovascular effects such as long QT, which may lead to heart failure
- Tardive dyskinesia (writhing movements, particularly of the feet, hands and/or tongue)
The next lot of neuroleptics are known as the "atypical neuroleptics." They generally have fewer extrapyramidal side effects as compared to the typical neuroleptics. The first atypical neuroleptic was clozapine, which was found to have greater therapeutic effects, even in treatment-resistant patients. Unfortunately, it also has really bad side effects, like agranulocytosis (which is potentially fatal), so now it is mainly used for treatment-resistant patients under close monitoring.
Clozapine, like the typical neuroleptics, acts mainly by D2 antagonism, though it has a lower affinity for these receptors. It can also antagonise other receptors, such as D1, D4, 5-HT2A, muscarinic, histamine and alpha-adrenergic receptors. Other atypical neuroleptics, such as olanzapine, respiridone and quetiapine, also have a decreased affinity for D2 receptors and can bind to other receptors, such as 5-HT2A. It is thought that 5-HT2A antagonism may help to reduce the risk of extrapyramidal side effects. One exception to the "D2 antagonism rule" is aripiprazole, which actually works by acting as a partial agonist of D2 receptors, which is weird but okay. I think the lecturer said something about why this works, but he was in a real hurry to move along to the next slide because we were already overtime by this point.
Atypical neuroleptics also have a lot of side effects. While neuroleptics other than clozapine don't have issues with agranulocytosis, obesity, type II diabetes and cardiovascular problems may all be problematic.
Comprehend Seeman’s Occupation of
dopamine D2 receptor hypothesis
Seeman developed a hypothesis that states that the best antipsychotic action occurs when D2 receptors are 65-80% occupied. When more than 80% of D2 receptors are occupied, extrapyramidal side effects may occur. The possible implication of this is that, if you reduced the dose of a typical neuroleptic so that less than 80% of D2 receptors are occupied, perhaps they would become equivalent to atypical neuroleptics.
Introduction to Mechanobiology
We're finally on to our last topic in this unit: cell signalling! More specifically, we are going to learn about mechanobiology, or how external mechanical forces affect cells.
Cells can form adhesions with other cells and with the extracellular matrix (ECM). These adhesions allow them to "feel" the stiffness and other mechanical properties of their surroundings. There are many consequences of their ability to "feel" the ECM, such as durotaxis (the tendency of cells to migrate from a softer ECM to stiffer ECM) and control of differentiation (stem cells cultured on softer ECM tend to be more likely to become brain or fat, whereas on stiffer ECMs they may become muscle or even bone). Cells can also exert force on the ECM by pulling on it.
The mechanical properties of the ECM are in part determined by its composition. ECM can consist of a variety of proteoglycans, such as aggrecan, as well as proteins, such as collagen and fibronectin. The composition of the ECM can differ from tissue to tissue and may also differ in certain conditions. Here are a few conditions that may affect the composition of the ECM (using epithelial tissue as an example):
Cells can form adhesions with other cells and with the extracellular matrix (ECM). These adhesions allow them to "feel" the stiffness and other mechanical properties of their surroundings. There are many consequences of their ability to "feel" the ECM, such as durotaxis (the tendency of cells to migrate from a softer ECM to stiffer ECM) and control of differentiation (stem cells cultured on softer ECM tend to be more likely to become brain or fat, whereas on stiffer ECMs they may become muscle or even bone). Cells can also exert force on the ECM by pulling on it.
The mechanical properties of the ECM are in part determined by its composition. ECM can consist of a variety of proteoglycans, such as aggrecan, as well as proteins, such as collagen and fibronectin. The composition of the ECM can differ from tissue to tissue and may also differ in certain conditions. Here are a few conditions that may affect the composition of the ECM (using epithelial tissue as an example):
- Ageing: Cell-cell junctions can become weak and the basement membrane (of epithelium) becomes thinner. There is increased cross-linking of ECM proteins, resulting in an increase in stiffness.
- Injury: During an injury, when endothelial cells are damaged, a clot fills the gap. Lots of collagen, mainly produced by myofibroblasts, surrounds the injury site. This also increases the stiffness at an injury site.
- Tumour: Epithelial cells become more mesenchymal. As mesenchymal cells don't care as much about their neighbours as epithelial cells, they may depart and start migrating through the matrix.
The properties of the ECM can affect the differentiation of stem cells. As I mentioned earlier, stiffness is one property that can affect stem cell differentiation. Shape can also affect differentiation: seeding cells onto small, circular sections of collagen is more likely to result in adipocytes, whereas seeding cells onto larger, square sections of collagen is more likely to result in osteocytes. Topography (i.e. the bumpiness of the ECM surface) also has an impact: smoother ECM did not appear to affect differentiation, whereas a rough ECM was more likely to produce osteocytes (bone cells).
The effects of ECM on cell differentiation may be the reason why stem cell therapies in myocardial infarction (heart attack) have so far been unsuccessful. Undifferentiated stem cells, when delivered to the site of the lesion, may even form bone due to the high stiffness in this area. Another option would be to differentiate stem cells before delivering them to the patients, but as yet getting cells to differentiate the way we want them to has still been a challenge. Understanding factors that govern cell differentiation (including stiffness) may help with this.
This lecture ended with a few slides on mechnotransduction, but we didn't get to go through all of them because time ran out. (We will go into them in more detail later on though.) For now, it's probably enough to know what connects the ECM to the nucleus, as these connections may ultimately result in gene transcription and some of the effects that we've explored so far (like stem cell differentiation). β-integrin connects the ECM to proteins such as talin and vinculin (which you might remember from one of my smooth muscle posts), which connect to actin, which connect through some other proteins (the slide says Nesprin 1, 2 and 3) to the nuclear membrane.
In the next post, we will be looking more at mechanotransduction in smooth muscle. Stay tuned!
In the next post, we will be looking more at mechanotransduction in smooth muscle. Stay tuned!
Friday, May 19, 2017
Alzheimer's Disease- Therapeutic Strategies
Cholinergic hypothesis
It has been found that patients with AD have deficits in cholinergic neurons, particularly in the nucleus basalis magnocellularis (a.k.a. basal nucleus of Meynert). The suggestion that a cholinergic deficit can impair memory has been backed up by other evidence: anticholinergics in young people can impair memory, and cholinergic stimulation can improve memory. Further research suggested that perhaps it's actually not so much memory that's affected, but instead selective attention. Nevertheless, enhancing selective attention may be helpful too.
Current therapies
Acetylcholinesterase inhibitors (AChEI)
Acetylcholinesterase inhibitors, as their name suggests, inhibit acetylcholinesterase (AChE). AChE normally breaks down ACh in the synapse, so when it is inhibited, there is more ACh bouncing around. The main AChEIs used are donepezil, galantamine and rivastigmine. (The very first AChEI to be used, tacrine, is no longer used due to safety concerns.) Despite being widely used, they are not particularly effective (at least not as effective as we would like them to be!) and can have adverse effects such as confusion, hallucinations, sudden changes in behaviour, nausea, and stomach pain.
Some AChEIs ("dual-binding AChEIs") can actually work through a second mechanism in which they bind to the peripheral anionic site (PAS) of AChE. Beta-amyloid can normally interact with the PAS, increasing aggregation of beta-amyloid. Therefore, blocking PAS also blocks aggregation of beta-amyloid.
Glutamate NMDA receptor inhibitors
Beta-amyloid can do some damage by binding to NMDA receptors (which normally bind glutamate), so if NMDA receptors are blocked, beta-amyloid can't do as much damage. Memantine is an NMDA-antagonist which is currently used to treat AD.
Combination treatments
Current combination treatments for AD include an AChEI and memantine. It is indicated for patients with moderate-to-severe AD.
Future potential treatments
CB1 receptors
CB1 receptors are cannabinoid receptors in the central nervous system. (CB2 receptors are mainly found in the peripheral nervous system.) CB1-receptor agonists have been found to impair memory, so CB1 antagonists such as rimonabant might help to improve it. Rat studies have found this to be the case, but I don't know whether or not this has also been found in humans.
Anti-inflammatories and anticholesterol drugs
AD lesions are often associated with activated microglia, suggesting inflammation. Beta-amyloid also activates the complement system. Furthermore, patients who regularly consume anti-inflammatory medication were less likely to develop AD. From this, it seems likely that targeting inflammation might help. Indeed, studies have found that NSAID use might have a somewhat protective effect.
As for cholesterol, studies in cells and animals have suggested that high cholesterol may increase levels of beta-amyloid. Hence, it seems like it would be a good idea to target cholesterol with drugs such as statins. Unfortunately, there is no evidence to show that this works.
Amyloid targeting treatments
Since amyloid plaques are a main feature of AD, γ-secretase and BACE1 (β-amyloid precursor protein cleaving enzyme 1) have also been suggested as potential targets. Unfortunately, targeting these enzymes may have some nasty side effects. Interestingly enough, it has been suggested that targeting both might actually be safe and effective.
Tau centred therapies
Since hyperphosphorylated tau is one of the hallmarks of AD, it makes sense to target it, right? The possibility of creating a vaccine against tau has been suggested, but while a potential vaccine (AADvac1) succeeded in a safety trial, it still needs to be tested further to establish safety and efficacy. It's likely that the blood-brain barrier will be an issue.
Another possible method of targeting tau is to use a tau aggregation inhibitor. Methylene blue (a.k.a. methylthion-inium chloride, or MTC) is a dye that can also inhibit the aggregation of tau. A new reduced form of methylene blue called LMTX has been tested in clinical trials, but has failed to show efficacy. Sigh...
Antioxidants
Various antioxidants have been trialled in the treatment of AD. Unfortunately, very few have shown any degree of success.
Phosphodiesterase inhibitors
Sildenafil (Viagra), which is a phosphodiesterase 5 inhibitor, can also enhance phosphorylation of CREB, which is involved in memory. In mice, Viagra improves synaptic function and memory by improving CREB phosphorylation. It was also associated with a long-lasting reduction in beta-amyloid levels. Sounds good, right? Unfortunately, of all of the trials done on Viagra to treat AD, the only study with published data shows no difference in AD symptoms. Ugh, back to the drawing board...
PPARγ agonists (Thiazolidinedione drugs)
In animals, thiazolidinediones (TZDs) have been shown to reduce neurodegeneration, improve cognition, inhibit neuroinflammation, facilitate beta-amyloid clearance, enhance mitochondrial function, improve synaptic plasticity and attenuate tau hyperphosphorylation. (Phew! That was a lot...) The two main TZDs that have been trialled are rosiglitazone and pioglitazone. Pioglitazone passed phase I safety, but no further data appears to have been published. Rosiglitazone has also been tested, but no data appears to have been published.
Nerve growth factors
In animals, nerve growth factor (NGF) and brain-derived nerve factor (BDNF) can stimulate cholinergic function and improve memory. Additionally, there may be other complex interactions between BDNF and beta-amyloid. NGF did okay in a phase I study, but results from phase II and III studies don't appear to have been published. As for BDNF, no clinical trials appear to have been done.
5-HT6 receptor antagonists
The rationale behind using drugs that target 5-HT6 (serotonin receptor, subtype 6) is still unclear. Studies in animals have found that 5-HT6 agonists and antagonists can improve learning and memory, which seems really contradictory. Patients with AD have been found to have reductions in 5-HT6 receptor density, but this seems unrelated to cognitive status. Another thing that has been found that 5-HT6 receptor blockade induces acetylcholine release. All in all, our knowledge of this receptor is still a bit hazy and unclear. There have been trials of 5-HT6 receptor antagonists in humans (such as SUVN-502), but no data as of yet.
α7 nicotine receptor agonists
Only one has been developed (EVP-6124), but no data.
tl;dr: the last 5 drugs that I talked about have little to no data to show for them.
It has been found that patients with AD have deficits in cholinergic neurons, particularly in the nucleus basalis magnocellularis (a.k.a. basal nucleus of Meynert). The suggestion that a cholinergic deficit can impair memory has been backed up by other evidence: anticholinergics in young people can impair memory, and cholinergic stimulation can improve memory. Further research suggested that perhaps it's actually not so much memory that's affected, but instead selective attention. Nevertheless, enhancing selective attention may be helpful too.
Current therapies
Acetylcholinesterase inhibitors (AChEI)
Acetylcholinesterase inhibitors, as their name suggests, inhibit acetylcholinesterase (AChE). AChE normally breaks down ACh in the synapse, so when it is inhibited, there is more ACh bouncing around. The main AChEIs used are donepezil, galantamine and rivastigmine. (The very first AChEI to be used, tacrine, is no longer used due to safety concerns.) Despite being widely used, they are not particularly effective (at least not as effective as we would like them to be!) and can have adverse effects such as confusion, hallucinations, sudden changes in behaviour, nausea, and stomach pain.
Some AChEIs ("dual-binding AChEIs") can actually work through a second mechanism in which they bind to the peripheral anionic site (PAS) of AChE. Beta-amyloid can normally interact with the PAS, increasing aggregation of beta-amyloid. Therefore, blocking PAS also blocks aggregation of beta-amyloid.
Glutamate NMDA receptor inhibitors
Beta-amyloid can do some damage by binding to NMDA receptors (which normally bind glutamate), so if NMDA receptors are blocked, beta-amyloid can't do as much damage. Memantine is an NMDA-antagonist which is currently used to treat AD.
Combination treatments
Current combination treatments for AD include an AChEI and memantine. It is indicated for patients with moderate-to-severe AD.
Future potential treatments
CB1 receptors
CB1 receptors are cannabinoid receptors in the central nervous system. (CB2 receptors are mainly found in the peripheral nervous system.) CB1-receptor agonists have been found to impair memory, so CB1 antagonists such as rimonabant might help to improve it. Rat studies have found this to be the case, but I don't know whether or not this has also been found in humans.
Anti-inflammatories and anticholesterol drugs
AD lesions are often associated with activated microglia, suggesting inflammation. Beta-amyloid also activates the complement system. Furthermore, patients who regularly consume anti-inflammatory medication were less likely to develop AD. From this, it seems likely that targeting inflammation might help. Indeed, studies have found that NSAID use might have a somewhat protective effect.
As for cholesterol, studies in cells and animals have suggested that high cholesterol may increase levels of beta-amyloid. Hence, it seems like it would be a good idea to target cholesterol with drugs such as statins. Unfortunately, there is no evidence to show that this works.
Amyloid targeting treatments
Since amyloid plaques are a main feature of AD, γ-secretase and BACE1 (β-amyloid precursor protein cleaving enzyme 1) have also been suggested as potential targets. Unfortunately, targeting these enzymes may have some nasty side effects. Interestingly enough, it has been suggested that targeting both might actually be safe and effective.
Tau centred therapies
Since hyperphosphorylated tau is one of the hallmarks of AD, it makes sense to target it, right? The possibility of creating a vaccine against tau has been suggested, but while a potential vaccine (AADvac1) succeeded in a safety trial, it still needs to be tested further to establish safety and efficacy. It's likely that the blood-brain barrier will be an issue.
Another possible method of targeting tau is to use a tau aggregation inhibitor. Methylene blue (a.k.a. methylthion-inium chloride, or MTC) is a dye that can also inhibit the aggregation of tau. A new reduced form of methylene blue called LMTX has been tested in clinical trials, but has failed to show efficacy. Sigh...
Antioxidants
Various antioxidants have been trialled in the treatment of AD. Unfortunately, very few have shown any degree of success.
Phosphodiesterase inhibitors
Sildenafil (Viagra), which is a phosphodiesterase 5 inhibitor, can also enhance phosphorylation of CREB, which is involved in memory. In mice, Viagra improves synaptic function and memory by improving CREB phosphorylation. It was also associated with a long-lasting reduction in beta-amyloid levels. Sounds good, right? Unfortunately, of all of the trials done on Viagra to treat AD, the only study with published data shows no difference in AD symptoms. Ugh, back to the drawing board...
PPARγ agonists (Thiazolidinedione drugs)
In animals, thiazolidinediones (TZDs) have been shown to reduce neurodegeneration, improve cognition, inhibit neuroinflammation, facilitate beta-amyloid clearance, enhance mitochondrial function, improve synaptic plasticity and attenuate tau hyperphosphorylation. (Phew! That was a lot...) The two main TZDs that have been trialled are rosiglitazone and pioglitazone. Pioglitazone passed phase I safety, but no further data appears to have been published. Rosiglitazone has also been tested, but no data appears to have been published.
Nerve growth factors
In animals, nerve growth factor (NGF) and brain-derived nerve factor (BDNF) can stimulate cholinergic function and improve memory. Additionally, there may be other complex interactions between BDNF and beta-amyloid. NGF did okay in a phase I study, but results from phase II and III studies don't appear to have been published. As for BDNF, no clinical trials appear to have been done.
5-HT6 receptor antagonists
The rationale behind using drugs that target 5-HT6 (serotonin receptor, subtype 6) is still unclear. Studies in animals have found that 5-HT6 agonists and antagonists can improve learning and memory, which seems really contradictory. Patients with AD have been found to have reductions in 5-HT6 receptor density, but this seems unrelated to cognitive status. Another thing that has been found that 5-HT6 receptor blockade induces acetylcholine release. All in all, our knowledge of this receptor is still a bit hazy and unclear. There have been trials of 5-HT6 receptor antagonists in humans (such as SUVN-502), but no data as of yet.
α7 nicotine receptor agonists
Only one has been developed (EVP-6124), but no data.
tl;dr: the last 5 drugs that I talked about have little to no data to show for them.
Effects of weightlessness on muscle and bone
Now we're going to talk about going to outer space! Yay!
Maintaining Muscle and Bone
When you work out, your muscles get stronger. The best type of exercise for this purpose is resistance exercise, which requires you to work against gravity. As I mentioned in a previous post, stretching and movement can activate MAPKs, which are important in growth and repair. Furthermore, exercise causes hormones such as IGF-1 to be released. (IGF-1, or insulin growth factor 1, is the most important hormone for muscle growth). The flipside of this is that if you don't have gravity to work against, these pathways are no longer activated and your muscles get weaker.
Bone mass can be determined by measuring bone mineral density (BMD), and just like muscle mass, it requires a constant load in order to be maintained. I've written about the regular processes involved in maintenance of bone here.
Space
Astronauts in spaceships and so forth are subject to "microgravity," or a feeling of weightlessness. This is because space vessels that orbit the Earth are essentially in constant freefall, giving that feeling of weightlessness. Unfortunately, this feeling of weightlessness means that muscles and bones lose their mass due to the reduced loads.
As already discussed, without the effects of gravity, muscles lose mass and therefore strength and speed as well. The muscles that are most significantly affected by space flight are postural "anti-gravity" muscles. In space, muscle fibres become smaller, and data from rats suggests that type I fibres may change phenotype to type II. Fibres may also be damaged and their contractile proteins moved out of alignment. Another effect of space flight on muscle is that muscles can become more prone to fatigue.
Without gravity, bones also lose mass, predisposing to osteoporosis. Just like with muscles, the bones that are most greatly affected are those that are "load bearing," like the legs and the lumbar spine. There is evidence to show that more calcium is lost in the urine during space flight as compared to before the flight. This suggests that bone loss may be due to excessive resorption by osteoclasts.
Preventing Muscle and Bone Loss
Initially, astronauts were given treadmills and bicycles to work out, but these machines do not provide the right loads required to prevent muscle wasting. Newer contraptions are being developed, such as bungee cords and "penguin suits" to provide resistance with every step the astronaut takes. So far, however, our efforts have been inadequate, so maybe we need to find better forms of resistance exercise in order to prevent muscle wastage. Other possible solutions include dietary supplementation with protein, and drug/hormone supplementation with steroids, IGF and so on. This latter suggestion is unlikely to be effective as there is no evidence to suggest that hormone levels change during space flight.
Bone loss is also a problem in space. If we can find better forms of resistance exercise, this might also help with bone loss. Another possibility are oral bisphosphonates, which inhibit osteoclasts and bone breakdown.
Aaaaand that concludes this section on skeletal muscle! Next up we'll be learning about mechanobiology and cell signalling!
Maintaining Muscle and Bone
When you work out, your muscles get stronger. The best type of exercise for this purpose is resistance exercise, which requires you to work against gravity. As I mentioned in a previous post, stretching and movement can activate MAPKs, which are important in growth and repair. Furthermore, exercise causes hormones such as IGF-1 to be released. (IGF-1, or insulin growth factor 1, is the most important hormone for muscle growth). The flipside of this is that if you don't have gravity to work against, these pathways are no longer activated and your muscles get weaker.
Bone mass can be determined by measuring bone mineral density (BMD), and just like muscle mass, it requires a constant load in order to be maintained. I've written about the regular processes involved in maintenance of bone here.
Space
Astronauts in spaceships and so forth are subject to "microgravity," or a feeling of weightlessness. This is because space vessels that orbit the Earth are essentially in constant freefall, giving that feeling of weightlessness. Unfortunately, this feeling of weightlessness means that muscles and bones lose their mass due to the reduced loads.
As already discussed, without the effects of gravity, muscles lose mass and therefore strength and speed as well. The muscles that are most significantly affected by space flight are postural "anti-gravity" muscles. In space, muscle fibres become smaller, and data from rats suggests that type I fibres may change phenotype to type II. Fibres may also be damaged and their contractile proteins moved out of alignment. Another effect of space flight on muscle is that muscles can become more prone to fatigue.
Without gravity, bones also lose mass, predisposing to osteoporosis. Just like with muscles, the bones that are most greatly affected are those that are "load bearing," like the legs and the lumbar spine. There is evidence to show that more calcium is lost in the urine during space flight as compared to before the flight. This suggests that bone loss may be due to excessive resorption by osteoclasts.
Preventing Muscle and Bone Loss
Initially, astronauts were given treadmills and bicycles to work out, but these machines do not provide the right loads required to prevent muscle wasting. Newer contraptions are being developed, such as bungee cords and "penguin suits" to provide resistance with every step the astronaut takes. So far, however, our efforts have been inadequate, so maybe we need to find better forms of resistance exercise in order to prevent muscle wastage. Other possible solutions include dietary supplementation with protein, and drug/hormone supplementation with steroids, IGF and so on. This latter suggestion is unlikely to be effective as there is no evidence to suggest that hormone levels change during space flight.
Bone loss is also a problem in space. If we can find better forms of resistance exercise, this might also help with bone loss. Another possibility are oral bisphosphonates, which inhibit osteoclasts and bone breakdown.
Aaaaand that concludes this section on skeletal muscle! Next up we'll be learning about mechanobiology and cell signalling!
Vitamins B12 (Cobalamins) and B6 (Pyridoxine)
Now we're onto our next lot of vitamins!
Understand the complexity of B12 biosynthesis
B12 is the largest and most complex vitamin, and therefore also has a relatively complex biosynthesis pathway... or rather pathways. B12 is synthesised in microbes in either an aerobic or an anaerobic pathway. These pathways also differ in when the metal ion (Co3+ in the case of B12) is inserted. In the aerobic pathway, Co3+ is inserted at the end, but in the anaerobic pathway, Co3+ is inserted earlier on.
The B12 biosynthesis pathways are complex and we aren't expected to understand them fully, but the lecturer did zoom in on a couple of enzymes (probably because he studies them :P). One of these enzymes is corrin reductase (CobR), which catalyses the conversion of Co(II) to Co(I). It is also known as a flavoprotein as it uses FAD to shuttle electrons around. The other enzyme mentioned was CbiH60, which helps in ring contraction (removal of one carbon from a ring, which is apparently really hard to do).
As mentioned here, B12 and heme are similar in that they both have tetrapyrrhole rings. They're not the only ones with this similar structure: many other molecules have tetrapyrrhole rings. Vitamin B12, coenzyme F430 (a Ni2+-containing coenzyme involved in methanogenesis), siroheme, heme and heme d1 all have tetrapyrrhole rings and can all be derived from precorrin-2. Precorrin-2, in turn, can be derived from uroporphyrinogen III, which can be derived from 5-ALA (mentioned here).
The roles of B12 in the cell
Remember that SAM pathway that I mentioned when talking about folate? Well, B12 is also involved in that pathway. It is involved in the conversion of homocysteine and methyl-H4 folate into methionine and H4 folate.
Methylmalonyl-CoA mutase is another enzyme that requires B12. It converts methylmalonyl-CoA into succinyl-CoA. Without this enzyme, methylmalonyl-CoA can build up, causing methylmalonic acidemia (MMA). Symptoms of MMA include vomiting, lethargy, profound ketoacidosis, hyperammonemia, pancytopenia and even death.
Methylmalonyl-CoA mutase is not the only enzyme that requires B12. Many enzymes require a B12 coenzyme such as adenosylcobalamin (AdoCbl) or methylcobalamin (MeCbl). AdoCbl-dependent enzymes include ribonucleotide reductase (the only enzyme that can produce deoxyribonucleotides), glutamine mutase (an enzyme that makes carbon backbones), α-methyleneglutarate mutase, isobutyryl-CoA mutase, amino mutases, dehydrases and deaminases. MeCbl-dependent enzymes include methionine synthase and methyltransferases.
B6: be aware of the different forms of B6 and some of the more important reactions which have B6 as a cofactor
B6 (pyridoxine) can come in many different forms, such as pyridoxal phosphate (PLP), pyridoxamine phosphate (PMP) and pyridoxic acid (PA). PA is excreted in urine, whereas PLP and PMP are cofactors that bind tightly to enzymes. PLP is sometimes considered a "prebiotic," or chemical present in very early life. PLP and PMP differ by one group (PLP has a carbonyl group, whereas PMP has an aminomethyl group), and can be interconverted.
B6 in amino acid biosynthesis
PMP, with its amino group, is essential in the biosynthesis of amino acids. PMP can combine with an amino acid side chain donor to form a ketimine, which can then become a quinoid intermediate and then an external aldimine. The external aldimine can then be broken down into an internal aldimine, releasing a shiny new L-amino acid in the process. The internal aldimine can then combine with glutamate to re-form PMP and α-ketoglutarate.
Know the daily requirements for B12, B6 and related disorders
B12 and B6, being vitamins, are not produced by us. Therefore, we have to get them from external sources.
B12 requirements
Understand the complexity of B12 biosynthesis
B12 is the largest and most complex vitamin, and therefore also has a relatively complex biosynthesis pathway... or rather pathways. B12 is synthesised in microbes in either an aerobic or an anaerobic pathway. These pathways also differ in when the metal ion (Co3+ in the case of B12) is inserted. In the aerobic pathway, Co3+ is inserted at the end, but in the anaerobic pathway, Co3+ is inserted earlier on.
The B12 biosynthesis pathways are complex and we aren't expected to understand them fully, but the lecturer did zoom in on a couple of enzymes (probably because he studies them :P). One of these enzymes is corrin reductase (CobR), which catalyses the conversion of Co(II) to Co(I). It is also known as a flavoprotein as it uses FAD to shuttle electrons around. The other enzyme mentioned was CbiH60, which helps in ring contraction (removal of one carbon from a ring, which is apparently really hard to do).
As mentioned here, B12 and heme are similar in that they both have tetrapyrrhole rings. They're not the only ones with this similar structure: many other molecules have tetrapyrrhole rings. Vitamin B12, coenzyme F430 (a Ni2+-containing coenzyme involved in methanogenesis), siroheme, heme and heme d1 all have tetrapyrrhole rings and can all be derived from precorrin-2. Precorrin-2, in turn, can be derived from uroporphyrinogen III, which can be derived from 5-ALA (mentioned here).
The roles of B12 in the cell
Remember that SAM pathway that I mentioned when talking about folate? Well, B12 is also involved in that pathway. It is involved in the conversion of homocysteine and methyl-H4 folate into methionine and H4 folate.
Methylmalonyl-CoA mutase is another enzyme that requires B12. It converts methylmalonyl-CoA into succinyl-CoA. Without this enzyme, methylmalonyl-CoA can build up, causing methylmalonic acidemia (MMA). Symptoms of MMA include vomiting, lethargy, profound ketoacidosis, hyperammonemia, pancytopenia and even death.
Methylmalonyl-CoA mutase is not the only enzyme that requires B12. Many enzymes require a B12 coenzyme such as adenosylcobalamin (AdoCbl) or methylcobalamin (MeCbl). AdoCbl-dependent enzymes include ribonucleotide reductase (the only enzyme that can produce deoxyribonucleotides), glutamine mutase (an enzyme that makes carbon backbones), α-methyleneglutarate mutase, isobutyryl-CoA mutase, amino mutases, dehydrases and deaminases. MeCbl-dependent enzymes include methionine synthase and methyltransferases.
B6: be aware of the different forms of B6 and some of the more important reactions which have B6 as a cofactor
B6 (pyridoxine) can come in many different forms, such as pyridoxal phosphate (PLP), pyridoxamine phosphate (PMP) and pyridoxic acid (PA). PA is excreted in urine, whereas PLP and PMP are cofactors that bind tightly to enzymes. PLP is sometimes considered a "prebiotic," or chemical present in very early life. PLP and PMP differ by one group (PLP has a carbonyl group, whereas PMP has an aminomethyl group), and can be interconverted.
B6 in amino acid biosynthesis
PMP, with its amino group, is essential in the biosynthesis of amino acids. PMP can combine with an amino acid side chain donor to form a ketimine, which can then become a quinoid intermediate and then an external aldimine. The external aldimine can then be broken down into an internal aldimine, releasing a shiny new L-amino acid in the process. The internal aldimine can then combine with glutamate to re-form PMP and α-ketoglutarate.
Know the daily requirements for B12, B6 and related disorders
B12 and B6, being vitamins, are not produced by us. Therefore, we have to get them from external sources.
B12 requirements
- Children: 1.0 μg/day
- Teenagers: 1.8-2.4 μg/day
- Adults: 2.4 μg/day
- Pregnancy: 2.6 μg/day
- Lactation: 2.8 μg/day
The main sources of B12 are meat, molluscs, fish and fortified cereals, so vegans may need supplementation.
B12 deficiency
Some of the causes of B12 deficiency are dietary insufficiency, pernicious anaemia and tapeworm infection. Pernicious anaemia is an autoimmune disease where parietal cells are destroyed. As mentioned here, parietal cells secrete intrinsic factor, which helps us to absorb B12. Therefore, without these cells, we can't get enough B12. Tapeworm infections can also affect our ability to absorb B12.
Symptoms of B12 deficiency are similar to symptoms of folate deficiency. This may be because B12 deficiency also causes a shortage of H4 folate. B12 deficiency also causes an insufficiency of succinyl-CoA, which is needed in the TCA cycle, and a build-up of methylmalonyl-CoA (remember, methylmalonyl-CoA mutase requires B12 to work properly). A build-up of methylmalonyl-CoA may result in methylmalonic acidaemia (MMA), particularly in babies. MMA can lead to mental retardation.
B6 requirements
B6 requirements
- Children: 0.5-0.6 mg/day
- Teenagers: 1.0-1.3 mg/day
- Adults: 1.3-1.7 mg/day (more in older adults, and more in men than in women)
B6 can be obtained from cereals, margarine, spices and yeast spreads.
B6 overdose
Too much B6 can be toxic and lead to fun symptoms like sensory neuropathy, dermatological lesions, photosensitivity and nausea.
B6 deficiency
B6 deficiency is rare, aside from in alcoholics due to their impaired absorption ability. B6 deficiency can lead to neurological issues, which may have something to do with GABA (an inhibitory neurotransmitter).
Comparative Physiology: Cardiovascular Systems
Understand the primary functions of the
vertebrate circulatory system
The circulatory system helps deliver all kinds of things all over the body. These include nutrients, wastes, immune system cells, heat, other regulatory molecules and so on. The circulatory system is much more efficient than using simple diffusion alone- if we relied on simple diffusion, it would take roughly 9.26 minutes for something like oxygen just to get through our skin! Imagine how long it would take then for something to diffuse to the middle of our bodies!
Compare the anatomy & efficiency of open & closed circulatory systems
Many invertebrates have an open circulatory system, in which there is no separation between blood and the interstitial fluid. They have a heart to pump things around, but the blood vessels quickly end in massive sinuses. Due to the lack of separation between blood and interstitial fluid, the circulating fluid in these invertebrates is known as "haemolymph." Invertebrates with an open circulatory system still need a certain amount of pressure in order to keep the fluid moving (despite also having a heart), and thus many of these creatures have a tough exoskeleton.
All vertebrates, as well as some invertebrates, have a closed circulatory system in which blood is combined to the vessels. However, solutes can diffuse into the interstitial fluid. This system requires more energy, as the vessels offer up quite a bit of resistance. On the other hand, it is much faster than an open circulatory system and has other advantages, such as conveying blood directly to the organs and its ability to "shunt" blood to different organs via constriction of vessels etc.
Understand the evolutionary changes in the vertebrate heart from primitive fish to amphibians, reptiles, birds & mammals in terms of structure & overall circulatory efficiency
Flatworm
The flatworm is probably the simplest in terms of getting its nutrients: it can be over a metre long, but is very flat (hence its name). Its "flatness" means that it can get everything it needs via simple diffusion alone.
Hagfish
The hagfish is a primitive eel-like fish. Its circulatory system is considered to be partially open: while it has closed blood vessels, it also has several sinuses. It has five hearts: a main heart that pumps to the gills and four accessory hearts. This is probably necessary because the gills offer up a lot of resistance which slows down the blood considerably. The four accessory hearts (but not the main heart) are under direct neural control and can increase their output under sympathetic stimulation. When their output increases, venous return to the main heart increases, increasing the cardiac output of the main heart.
Elasmobranchs and teleosts
Elasmobranchs and teleosts are kinds of fish. They have a single heart with four chambers, but the four chambers are in a row (kind of like the foetal human heart before it folds and divides), so it is considered to be 2-chambered. These four chambers are called the sinus venosus, atrium, ventricle and conus arteriosus (a.k.a. "bulbus cordis").
Lungfish
Lungfish are, well, fish with lungs. As opposed to hagfish, lungfish only have one heart. The atrium and ventricle of this heart are partly divided, but not entirely (i.e. the lungfish heart still only has two chambers). This heart pumps into five "branchial arteries," three of which pass through gills and two which do not. From here, blood can either go to the lungs or to the rest of the body. Two veins return blood to the heart: one from the lung and one from the body. There is some mixing of the oxygenated blood from the lungs and deoxygenated blood from the body, but this is relatively limited as there are spiral folds in the bulbus cordis that separate the blood.
Amphibians
Amphibians have a 3-chambered heart (two atria and one ventricle) as opposed to the two-chambered hearts of fish. Even though the ventricle is not fully divided, it has a functional division called the "dense trabeculation of the spongy myocardium" and a spiral fold in the conus arteriosus (just like the spiral folds in the bulbus cordis of lungfish).
The artery that leaves the amphibian heart splits into two: a pulmocutaneous artery that goes to the lungs and skin (which is also an accessory breathing organ in amphibians) and a systemic artery that goes to the rest of the body. The left atrium receives oxygenated blood from the lungs, whereas the right atrium receives a mixture of oxygenated blood from the skin and deoxygenated blood from the tissues (so there is some mixing of blood).
Reptiles (except for crocodiles because they're special)
Unlike amphibians, reptiles only use their lungs for gas exchange, so their circulatory system must be simpler too, right? Wrong.
The atria of reptiles are divided completely, so that part's easy. The ventricle, however? Not so easy. The ventricle of reptile hearts is divided incompletely into three chambers: the cavum venosum (CV), cavum arteriosum (CA) and cavum pulmonale (CP). These connect into two systemic arteries (the right and left systemic arch) and the pulmonary artery.
During ventricular systole, the pressure in the pulnonary artery is relatively low, allowing blood to flow in from the CP and CV. (The intercaval canal- the passage between the CA and CV- is still closed at this point.) As the pressure increases, the intercaval canal opens, allowing blood from the CA to enter the CV, while a muscular ridge forms between the CV and CP (preventing blood from flowing between these two sections). Blood from the CV then enters the systemic arches.
Well, that was confusing. Why are reptiles so confusing? Well, this confusing system allows reptiles to bypass the lungs, which is useful when diving. During diving, PA resistance increases, causing a right-to-left cardiac shunt (i.e. deoxygenated blood gets to flow to the rest of the body too). When they're back on land, they have a left-to-right shunt.
Crocodiles
Crocodiles are different to other reptiles in that they have a proper four-chambered heart. Just like in humans, the right atria receives deoxygenated blood from the body, whereas the left atria receives oxygenated blood from the lungs. Output from the ventricles is a bit more complicated: the right ventricle is connected to a pulmonary artery and to the left systemic arch, whereas the left ventricle is only connected to the right systemic arch. The two systemic arteries are connected by the foramen of Panizza. Usually, there is a higher pressure in the left systemic arch, which prevents the valve from the right ventricle to the left systemic arch from opening. However, when crocodiles are diving, pulmonary vasculature resistance increases, closing off the valve from the right ventricle to the pulmonary artery, which shunts blood from the right ventricle into the left systemic arch instead. This results in a right-to-left shunt. (Note that crocodiles can't have a left-to-right shunt as the valve from the left ventricle to the right systemic arch is pretty much always open.)
Birds
Birds also have a four-chambered heart. Oxygenated and deoxygenated blood are completely separated (unlike in crocodiles when they can mix during diving).
Mammals
I've already spoken a lot about humans, so not much to say here. Fun fact though: the sinus venosus, which I mentioned back when I wrote about elasmobranchs and teleosts, still remains in the mammalian heart: it's the sinoatrial node.
The circulatory system helps deliver all kinds of things all over the body. These include nutrients, wastes, immune system cells, heat, other regulatory molecules and so on. The circulatory system is much more efficient than using simple diffusion alone- if we relied on simple diffusion, it would take roughly 9.26 minutes for something like oxygen just to get through our skin! Imagine how long it would take then for something to diffuse to the middle of our bodies!
Compare the anatomy & efficiency of open & closed circulatory systems
Many invertebrates have an open circulatory system, in which there is no separation between blood and the interstitial fluid. They have a heart to pump things around, but the blood vessels quickly end in massive sinuses. Due to the lack of separation between blood and interstitial fluid, the circulating fluid in these invertebrates is known as "haemolymph." Invertebrates with an open circulatory system still need a certain amount of pressure in order to keep the fluid moving (despite also having a heart), and thus many of these creatures have a tough exoskeleton.
All vertebrates, as well as some invertebrates, have a closed circulatory system in which blood is combined to the vessels. However, solutes can diffuse into the interstitial fluid. This system requires more energy, as the vessels offer up quite a bit of resistance. On the other hand, it is much faster than an open circulatory system and has other advantages, such as conveying blood directly to the organs and its ability to "shunt" blood to different organs via constriction of vessels etc.
Understand the evolutionary changes in the vertebrate heart from primitive fish to amphibians, reptiles, birds & mammals in terms of structure & overall circulatory efficiency
Flatworm
The flatworm is probably the simplest in terms of getting its nutrients: it can be over a metre long, but is very flat (hence its name). Its "flatness" means that it can get everything it needs via simple diffusion alone.
Hagfish
The hagfish is a primitive eel-like fish. Its circulatory system is considered to be partially open: while it has closed blood vessels, it also has several sinuses. It has five hearts: a main heart that pumps to the gills and four accessory hearts. This is probably necessary because the gills offer up a lot of resistance which slows down the blood considerably. The four accessory hearts (but not the main heart) are under direct neural control and can increase their output under sympathetic stimulation. When their output increases, venous return to the main heart increases, increasing the cardiac output of the main heart.
Elasmobranchs and teleosts
Elasmobranchs and teleosts are kinds of fish. They have a single heart with four chambers, but the four chambers are in a row (kind of like the foetal human heart before it folds and divides), so it is considered to be 2-chambered. These four chambers are called the sinus venosus, atrium, ventricle and conus arteriosus (a.k.a. "bulbus cordis").
Lungfish
Lungfish are, well, fish with lungs. As opposed to hagfish, lungfish only have one heart. The atrium and ventricle of this heart are partly divided, but not entirely (i.e. the lungfish heart still only has two chambers). This heart pumps into five "branchial arteries," three of which pass through gills and two which do not. From here, blood can either go to the lungs or to the rest of the body. Two veins return blood to the heart: one from the lung and one from the body. There is some mixing of the oxygenated blood from the lungs and deoxygenated blood from the body, but this is relatively limited as there are spiral folds in the bulbus cordis that separate the blood.
Amphibians
Amphibians have a 3-chambered heart (two atria and one ventricle) as opposed to the two-chambered hearts of fish. Even though the ventricle is not fully divided, it has a functional division called the "dense trabeculation of the spongy myocardium" and a spiral fold in the conus arteriosus (just like the spiral folds in the bulbus cordis of lungfish).
The artery that leaves the amphibian heart splits into two: a pulmocutaneous artery that goes to the lungs and skin (which is also an accessory breathing organ in amphibians) and a systemic artery that goes to the rest of the body. The left atrium receives oxygenated blood from the lungs, whereas the right atrium receives a mixture of oxygenated blood from the skin and deoxygenated blood from the tissues (so there is some mixing of blood).
Reptiles (except for crocodiles because they're special)
Unlike amphibians, reptiles only use their lungs for gas exchange, so their circulatory system must be simpler too, right? Wrong.
The atria of reptiles are divided completely, so that part's easy. The ventricle, however? Not so easy. The ventricle of reptile hearts is divided incompletely into three chambers: the cavum venosum (CV), cavum arteriosum (CA) and cavum pulmonale (CP). These connect into two systemic arteries (the right and left systemic arch) and the pulmonary artery.
During ventricular systole, the pressure in the pulnonary artery is relatively low, allowing blood to flow in from the CP and CV. (The intercaval canal- the passage between the CA and CV- is still closed at this point.) As the pressure increases, the intercaval canal opens, allowing blood from the CA to enter the CV, while a muscular ridge forms between the CV and CP (preventing blood from flowing between these two sections). Blood from the CV then enters the systemic arches.
Well, that was confusing. Why are reptiles so confusing? Well, this confusing system allows reptiles to bypass the lungs, which is useful when diving. During diving, PA resistance increases, causing a right-to-left cardiac shunt (i.e. deoxygenated blood gets to flow to the rest of the body too). When they're back on land, they have a left-to-right shunt.
Crocodiles
Crocodiles are different to other reptiles in that they have a proper four-chambered heart. Just like in humans, the right atria receives deoxygenated blood from the body, whereas the left atria receives oxygenated blood from the lungs. Output from the ventricles is a bit more complicated: the right ventricle is connected to a pulmonary artery and to the left systemic arch, whereas the left ventricle is only connected to the right systemic arch. The two systemic arteries are connected by the foramen of Panizza. Usually, there is a higher pressure in the left systemic arch, which prevents the valve from the right ventricle to the left systemic arch from opening. However, when crocodiles are diving, pulmonary vasculature resistance increases, closing off the valve from the right ventricle to the pulmonary artery, which shunts blood from the right ventricle into the left systemic arch instead. This results in a right-to-left shunt. (Note that crocodiles can't have a left-to-right shunt as the valve from the left ventricle to the right systemic arch is pretty much always open.)
Birds
Birds also have a four-chambered heart. Oxygenated and deoxygenated blood are completely separated (unlike in crocodiles when they can mix during diving).
Mammals
I've already spoken a lot about humans, so not much to say here. Fun fact though: the sinus venosus, which I mentioned back when I wrote about elasmobranchs and teleosts, still remains in the mammalian heart: it's the sinoatrial node.
Comparative Physiology: Respiratory Systems
This week we're learning about comparative physiology, which is basically comparing the physiology of different animals. Enjoy!
Insects
Insects don't have a circulatory system as they have a highly branching tracheal system that can take air all the way to the cells. The tracheal system begins at one of many spiracles, which are little "air holes" on the outer surface (for lack of a better word) of the insect. These spiracles connect to tracheae, which branch out into tracheoles, and so on. Reliance on this system limits insect size. That being said, around 300 million years ago there was a giant dragonfly called Meganeura with a 70cm wingspan. It's been suggested that it was able to survive because atmospheric oxygen then was higher then than it is now (~30% as compared to ~21%).
Fish
As you probably know, fish have gills on the outside of their bodies, which allow them to breathe underwater. Gills are made up of highly vascularised thin parts known as lamellae, which create a very large surface area for gas exchange. Blood travels in the opposite direction to water, creating a "counter-current system" that is very efficient at absorbing oxygen. Despite the large surface area and high efficiency, lots of water needs to be moved past the gills in order for adequate oxygen to be obtained. Lamellae collapse when exposed to air, which is why fish can't breathe when out of water.
Amphibians
Amphibians have lungs, but they look quite different to ours. They are simple inverted bags and don't have as much folding as mammal lungs. Amphibians use "positive pressure" ventilation, which I think means that they mechanically pump something (the buccal cavity, which is like the mouth cavity of the frog) in order to move air around.
More "primitive" amphibians use 4-stroke ventilation which, as you may guess, has four steps. Steps 1 and 2 are for inspiration, whereas steps 3 and 4 are for expiration.
Reptiles
Reptiles have a "negative pressure" ventilation, just like us. However, most do not have a diaphragm which can contract in order to produce this "negative pressure." Instead, many have muscularised ribs that can do this job. Because of this, some reptiles can't walk and breathe at the same time. Crocodiles are a bit different: they do have a diaphragm-like structure called the diaphragmaticus, which connects the pelvic girdle to the liver (which is connected to the lungs). (Crocodile hearts are also a bit different to those of other reptiles, as I'll talk about in a later post.)
More primitive reptiles have "bag-like" lungs, just like amphibians. More modern ones, however, have folding in order to increase the surface area for gas exchange. Hence we're starting to get closer to the mammalian lung... yay I guess?
Oh and one more thing. Crocodiles and some lizards have something called "unidirectional airflow." Air coming down the trachea can go from the trachea into the dorsobronchi (an air sac), through some parabronchi (where gas exchange happens) and then into the ventrobronchi (another air sac) before leaving. In short, air goes around in one direction (rather than going down into the lungs and then back up the way it came). Birds also have something similar, as I'll talk about later.
Mammals
I won't talk too much about mammals, as I've already spoken quite a lot about the lungs of one mammal in particular (*cough*humans*cough*). They have "negative pressure" ventilation and alveolar lungs. One of the "cons" of the mammalian lung is that it has dead space, which is where air passes through but is not exchanged. There are two types of dead space: alveolar dead space and anatomic dead space. Alveolar dead space occurs when there is no blood flowing through the surrounding capillaries of an alveolus, whereas anatomic dead space occurs in the conducting zones of the lung where air passes through but is not exchanged.
Birds
Bird lungs are kind of weird in that they are calcified. They also have the parabronchial structure and unidirectional airflow that I talked about when talking about reptiles. They do not have a diaphragm but they do have some air sacs that help in pushing the air around. Their blood-gas barrier is very thin, making gas exchange efficient, while the calcification of the lung prevents tearing. Birds have a "cross-current" system where the blood runs perpendicular to the direction of airflow. This is not as efficient as the counter-current system in fish, but it's a lot more efficient than gas exchange without a fancy current system. In fact, they can more efficiently extract oxygen as the oxygen content of the air decreases. (They do this by taking big, slow breaths.)
There are two cycles in bird breathing. In cycle 1, air goes from outside to the posterior air sac during inspiration, and into the lung during exhalation. In cycle 2, air goes from the lung to the anterior air sac during inspiration, and leaves the body during expiration.
Air breathing
A random note on the issues with breathing air (as opposed to water, like fish). Even though air is usually not saturated with water vapour, gas exchange surfaces are moist and water is required in order for diffusion capacity to reach a suitable level. Additionally, inspired air is usually cooler than the body, so heating is also required. The other challenge is that the amount of water required to dissolve oxygen increases as temperature increases, so lots of water needs to be added. This water can be recuperated by decreasing the temperature of the air during expiration, allowing the water to condense out. Thankfully, we have a "nasal turbinate" system in our noses, which acts as a kind of "air-con."
"Arse breathing"
Basically what it says on the box. Some animals, like the Fitzroy River turtle, can breathe through their arses. Yay!
Insects
Insects don't have a circulatory system as they have a highly branching tracheal system that can take air all the way to the cells. The tracheal system begins at one of many spiracles, which are little "air holes" on the outer surface (for lack of a better word) of the insect. These spiracles connect to tracheae, which branch out into tracheoles, and so on. Reliance on this system limits insect size. That being said, around 300 million years ago there was a giant dragonfly called Meganeura with a 70cm wingspan. It's been suggested that it was able to survive because atmospheric oxygen then was higher then than it is now (~30% as compared to ~21%).
Fish
As you probably know, fish have gills on the outside of their bodies, which allow them to breathe underwater. Gills are made up of highly vascularised thin parts known as lamellae, which create a very large surface area for gas exchange. Blood travels in the opposite direction to water, creating a "counter-current system" that is very efficient at absorbing oxygen. Despite the large surface area and high efficiency, lots of water needs to be moved past the gills in order for adequate oxygen to be obtained. Lamellae collapse when exposed to air, which is why fish can't breathe when out of water.
Amphibians
Amphibians have lungs, but they look quite different to ours. They are simple inverted bags and don't have as much folding as mammal lungs. Amphibians use "positive pressure" ventilation, which I think means that they mechanically pump something (the buccal cavity, which is like the mouth cavity of the frog) in order to move air around.
More "primitive" amphibians use 4-stroke ventilation which, as you may guess, has four steps. Steps 1 and 2 are for inspiration, whereas steps 3 and 4 are for expiration.
- Nostrils open and buccal cavity expands, allowing air to enter the buccal cavity.
- Nostrils close, glottis opens and buccal cavity contracts, pushing air into the lungs.
- Lungs contract, pushing air back into the buccal cavity.
- Glottis closes, nostrils open and buccal cavity contracts, pushing air out.
More "modern" amphibians use 2-stroke ventilation:
- Buccal cavity expands. Air is drawn in from outside and from the lungs.
- Buccal cavity contracts. Air is forced out outside and into the lungs.
One of the downsides to 2-stroke ventilation is that there is some mixing of old and new air.
Reptiles
Reptiles have a "negative pressure" ventilation, just like us. However, most do not have a diaphragm which can contract in order to produce this "negative pressure." Instead, many have muscularised ribs that can do this job. Because of this, some reptiles can't walk and breathe at the same time. Crocodiles are a bit different: they do have a diaphragm-like structure called the diaphragmaticus, which connects the pelvic girdle to the liver (which is connected to the lungs). (Crocodile hearts are also a bit different to those of other reptiles, as I'll talk about in a later post.)
More primitive reptiles have "bag-like" lungs, just like amphibians. More modern ones, however, have folding in order to increase the surface area for gas exchange. Hence we're starting to get closer to the mammalian lung... yay I guess?
Oh and one more thing. Crocodiles and some lizards have something called "unidirectional airflow." Air coming down the trachea can go from the trachea into the dorsobronchi (an air sac), through some parabronchi (where gas exchange happens) and then into the ventrobronchi (another air sac) before leaving. In short, air goes around in one direction (rather than going down into the lungs and then back up the way it came). Birds also have something similar, as I'll talk about later.
Mammals
I won't talk too much about mammals, as I've already spoken quite a lot about the lungs of one mammal in particular (*cough*humans*cough*). They have "negative pressure" ventilation and alveolar lungs. One of the "cons" of the mammalian lung is that it has dead space, which is where air passes through but is not exchanged. There are two types of dead space: alveolar dead space and anatomic dead space. Alveolar dead space occurs when there is no blood flowing through the surrounding capillaries of an alveolus, whereas anatomic dead space occurs in the conducting zones of the lung where air passes through but is not exchanged.
Birds
Bird lungs are kind of weird in that they are calcified. They also have the parabronchial structure and unidirectional airflow that I talked about when talking about reptiles. They do not have a diaphragm but they do have some air sacs that help in pushing the air around. Their blood-gas barrier is very thin, making gas exchange efficient, while the calcification of the lung prevents tearing. Birds have a "cross-current" system where the blood runs perpendicular to the direction of airflow. This is not as efficient as the counter-current system in fish, but it's a lot more efficient than gas exchange without a fancy current system. In fact, they can more efficiently extract oxygen as the oxygen content of the air decreases. (They do this by taking big, slow breaths.)
There are two cycles in bird breathing. In cycle 1, air goes from outside to the posterior air sac during inspiration, and into the lung during exhalation. In cycle 2, air goes from the lung to the anterior air sac during inspiration, and leaves the body during expiration.
Air breathing
A random note on the issues with breathing air (as opposed to water, like fish). Even though air is usually not saturated with water vapour, gas exchange surfaces are moist and water is required in order for diffusion capacity to reach a suitable level. Additionally, inspired air is usually cooler than the body, so heating is also required. The other challenge is that the amount of water required to dissolve oxygen increases as temperature increases, so lots of water needs to be added. This water can be recuperated by decreasing the temperature of the air during expiration, allowing the water to condense out. Thankfully, we have a "nasal turbinate" system in our noses, which acts as a kind of "air-con."
"Arse breathing"
Basically what it says on the box. Some animals, like the Fitzroy River turtle, can breathe through their arses. Yay!
Thursday, May 18, 2017
Vitamin B9 (Folate)
This post is going to start off with a quick introduction to vitamins before diving into talking about folate (folic acid)!
Know the major vitamins and their basic deficiencies and overdoses
Vitamin is short for "vital amine"- they are vital because we can't synthesise them ourselves. Most vitamins are used in metabolic processes, but do not contribute energy (unlike carbs, fats and proteins).
As for the basic deficiencies and overdoses, I'm just going to link you straight to my lecturer's source: https://en.wikipedia.org/wiki/Vitamin
And yes, I just linked you to Wikipedia.
Know that vitamin B1, B2 and B3 are very important in basic cell function
Before going through B1, B2 and B3 specifically, I'm going to give a quick rundown of some of the more common B-vitamins and what they do.
Know the major vitamins and their basic deficiencies and overdoses
Vitamin is short for "vital amine"- they are vital because we can't synthesise them ourselves. Most vitamins are used in metabolic processes, but do not contribute energy (unlike carbs, fats and proteins).
As for the basic deficiencies and overdoses, I'm just going to link you straight to my lecturer's source: https://en.wikipedia.org/wiki/Vitamin
And yes, I just linked you to Wikipedia.
Know that vitamin B1, B2 and B3 are very important in basic cell function
Before going through B1, B2 and B3 specifically, I'm going to give a quick rundown of some of the more common B-vitamins and what they do.
- B1 (thiamine): Co-enzyme in catabolism of sugars and amino acids.
- B2 (riboflavin): Precursor of FAD and FMN.
- B3 (niacin): Precursor of NAD and NADP.
- B5 (pantothenic acid): Precursor of coenzyme A (CoASH).
- B6 (pyridoxine): Co-enzyme in many enzymatic reactions (such as transamination).
- B7 (biotin): Co-enzyme for carboxylase enzymes in fatty acid synthesis and gluconeogenesis.
- B9 (folic acid): I'll talk more about this one in the second half of this post.
- B12 (cobalamins): Co-enzymes involved in processes such as DNA synthesis, fatty acid metabolism and amino acid metabolism.
Now back to B1, B2 and B3! B1, B2 and B3 are precursors for three of the co-factors associated with pyruvate dehydrogenase. More specifically, B1 can become thiamin diphosphate, B2 can become FAD and B3 can become niacin. As B1, B2 and B3 are important, there are a range of symptoms associated with their deficiency:
- B1 deficiency: Beriberi, abnormal blood sugar, depression, fatigue, vomiting, GI disorders
- B2 deficiency: Light sensitivity, cracks/inflammation of the mouth, dizziness, insomnia
- B3 deficiency: Pellagra (dementia, death), nausea, vomiting, fatigue, dermatitis, loss of appetite, swollen red tongue. B3 deficiency may also increase the risk for some skin cancers.
Understand the forms of folate and its roles,
particularly 1-carbon metabolism and neural tube
development
Folate is also known as pteroyl-glutamate, as it consists of a folyl moiety with glutamate attached. We cannot synthesise it: it is formed in microbes from GTP. Folate can be found in many forms, such as folylpolyglutamates (a folyl moiety with 5-6 glutamates attached). Glutamate moieties can be added to folylpolyglutamates by folylpolyglutamate synthase (FPGS), and folate with only one glutamate can have that glutamate removed by glutamate carboxypeptidase 2 to form pteroate and glutamate. Folylpolyglutamates also serve as a storage form of folate in humans. (Google Chrome is giving me so many red squiggly lines right now...)
Folate is important in the transfer of methyl groups. To understand how that works, we need to meet SAM, or S-adenosyl-L-methionine. Remember how I said that methionine is a vital source of methyl groups? Well, S-adenosyl-L-methionine can donate its methyl group to something that needs it, resulting in S-adenosyl-L-homocysteine (SAH). This can be converted to L-homocysteine. In order to restore methionine and then S-adenosylmethionine, a methyl group must be returned. Methyl-H4 folate returns methyl groups to homocysteine to regenerate methionine, keeping the cycle going. Without this process, SAM would struggle to methylate all of the things that it normally does, including the DNA. (Yup, it methylates DNA, which affects gene transcription.)
Folate is also important in various stages of purine and pyrimidine synthesis. Methylene-H4 folate is a cofactor in both purine and pyrimidine synthesis, and 10-formyl H4 folate is important in purine synthesis. These pathways can be blocked by certain inhibitors such as 5-FU (5-fluorouracil) and MTX (methotrexate).
5-FU binds covalently to thymidylate synthase, blocking thymine production from dUMP. (This is one of the processes that methylene-H4 folate) is involved in.) Some dUMP can be shunted to dUTP, which can be used instead of dTTP, but can cause irreparable double-strand breaks.
Following the thymidylate synthase process, methylene-H4 folate is converted to H2 folate, which is then converted into H4 folate. This latter step (H2 folate to H4 folate) can be blocked by MTX. Low doses of this drug can actually be helpful in treating autoimmune diseases.
Folate is also known as pteroyl-glutamate, as it consists of a folyl moiety with glutamate attached. We cannot synthesise it: it is formed in microbes from GTP. Folate can be found in many forms, such as folylpolyglutamates (a folyl moiety with 5-6 glutamates attached). Glutamate moieties can be added to folylpolyglutamates by folylpolyglutamate synthase (FPGS), and folate with only one glutamate can have that glutamate removed by glutamate carboxypeptidase 2 to form pteroate and glutamate. Folylpolyglutamates also serve as a storage form of folate in humans. (Google Chrome is giving me so many red squiggly lines right now...)
Folate is important in the transfer of methyl groups. To understand how that works, we need to meet SAM, or S-adenosyl-L-methionine. Remember how I said that methionine is a vital source of methyl groups? Well, S-adenosyl-L-methionine can donate its methyl group to something that needs it, resulting in S-adenosyl-L-homocysteine (SAH). This can be converted to L-homocysteine. In order to restore methionine and then S-adenosylmethionine, a methyl group must be returned. Methyl-H4 folate returns methyl groups to homocysteine to regenerate methionine, keeping the cycle going. Without this process, SAM would struggle to methylate all of the things that it normally does, including the DNA. (Yup, it methylates DNA, which affects gene transcription.)
Folate is also important in various stages of purine and pyrimidine synthesis. Methylene-H4 folate is a cofactor in both purine and pyrimidine synthesis, and 10-formyl H4 folate is important in purine synthesis. These pathways can be blocked by certain inhibitors such as 5-FU (5-fluorouracil) and MTX (methotrexate).
5-FU binds covalently to thymidylate synthase, blocking thymine production from dUMP. (This is one of the processes that methylene-H4 folate) is involved in.) Some dUMP can be shunted to dUTP, which can be used instead of dTTP, but can cause irreparable double-strand breaks.
Following the thymidylate synthase process, methylene-H4 folate is converted to H2 folate, which is then converted into H4 folate. This latter step (H2 folate to H4 folate) can be blocked by MTX. Low doses of this drug can actually be helpful in treating autoimmune diseases.
Know the daily requirements for folate and disorders
related to under supply
The daily requirements for folate are as follows:
The daily requirements for folate are as follows:
- Children: 150-200 μg/day
- Teenagers: 300-400 μg/day
- Adults: 400 μg/day
- Pregnancy: 600 μg/day
- Lactation: 500 μg/day
Folate can be found in foods such as yeast spreads (vegemite!), fortified cereals and some beans. Flour is often fortified with folate, making folate deficiency rare in the Western diet, unless you're an alcoholic (alcoholism can cause malabsorption). If folate is deficient, its derivatives (methionine, nucleotides etc.) can be deficient too, and homocysteine can build up.
Perhaps the most well-known effect of folate deficiency is spina bifida, which is when the neural tube fails to close. I've blogged more about it here. Later in life, folate deficiency can cause megaloblastic anaemia, in which early erythroblasts do not divide, resulting in a deficiency of red blood cells. Hyperhomocysteinemia as a result of folate deficiency may also increase the risk of cardiovascular disease.
Perhaps the most well-known effect of folate deficiency is spina bifida, which is when the neural tube fails to close. I've blogged more about it here. Later in life, folate deficiency can cause megaloblastic anaemia, in which early erythroblasts do not divide, resulting in a deficiency of red blood cells. Hyperhomocysteinemia as a result of folate deficiency may also increase the risk of cardiovascular disease.
Why smoking is a problem
Aside from the obvious lung problems, smoking can also lower the levels of many vitamins. These include β-carotene (a precursor to vitamin A) and vitamins B6, B9, B12, C, D and E. As B9 is folic acid, and deficiencies of folic acid can lead to neural tube defects, smoking can also lead to neural tube defects. In fact, even exposing non-smoking pregnant women to secondhand smoke can increase the rates of neural tube defects.
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