Ion Channel Structure and Function

Last post on electrophysiology!

Describe the structure and function of voltage-gated ion channels

Voltage-gated ion channels, as the name suggests, are ion channels that open in response to a specific voltage. Voltage-gated channels are made up of several subunits- usually four α₁-subunits that make up the pore, as well as a few other auxiliary subunits. In K⁺ channels, the auxiliary subunits consist of four cytoplasmic β-subunits. In Na⁺ channels, these are two transmembrane β-subunits. In Ca²⁺ channels, there is an extracellular α₂-subunit, a cytoplasmic β-subunit, and transmembrane γ and δ subunits. These auxiliary subunits may help modulate the gating activity of the channel, but we're still not 100% sure.

Since the α₁-subunits are probably the most important, we're going to focus on them. α₁-subunits have six transmembrane domains, imaginatively named S1 through to S6. S4 also serves as a "voltage sensing domain." Between S5 and S6 lies a P (pore) domain, which contains the selectivity filter (i.e. the thing that lets only the ion you want pass) as well as binding sites for other molecules.

Now let's have a look at how these channels work! As I just mentioned, the S4 region senses changes in voltage. That is because S4 domains are made up of largely positive residues, such as arginine, which are usually balanced out by negative charges on a neighbouring helix. When the cell is depolarised (more positive), however, the positive charge inside the cell repels the helix, pushing it around and up (like a screw). This movement also moves the S6 helices, thus opening the channel.

As the ability to sense voltage is clearly pretty important to a voltage-gated channel, it makes sense that this domain has been well conserved throughout evolution. Rats, fruit flies and electric eels share a lot of the same amino acid residues in their S4 regions.

The other key part of the ion channel is the pore region- i.e. the S5-P-S6 portion. In fact, Streptomyces bacteria only have this portion- they don't have the voltage sensing region. The P region, as mentioned above, contains a selectivity filter. This filter is located near the top of the pore and is quite short and narrow, minimising the distance that the ion in question requires in order to interact with the channel. It is lined with residues that will attract the ion in question (e.g. negative charges to attract positive ions), and the spacing is such that it is only energetically favourable for the right size of ion. Just below the selectivity filter is a water-filled cavity and some charged helix dipoles (negatively charged in potassium channels). As ions of the same charge pass through the channel, the repulsion between ions helps propel them through the channel more rapidly.

So far, what I've said applies mainly to K⁺ channels. Other ion channels are similar, but there are some differences. For example, in Na⁺ and Ca²⁺ channels, the four α₁-subunits are actually joined end-to-end to make a giant α subunit with four domains. Furthermore, each of these domains is slightly different- i.e. these are heterotetramers, not monotetramers like K⁺ channels. As I mentioned in a previous post, Na⁺ channels have inactivation gates, which are located between the third and fourth domains.

Understand the similarities and differences between different classes of ion channels

There are several different classes of ion channels. We're going to focus mainly on K⁺ channels for now.

Delayed outward rectifiers

Delayed outward rectifiers, as their name suggests, are delayed in opening and cause potassium to move outwards. The outward current rises steeply at positive voltages (i.e. as the voltage goes up, flow of positive ions out of the cell also goes up). This is probably where the "rectifier" part comes from.

Transient outward rectifiers

Transient currents, also known as A-type currents, are activated and inactivated over a relatively short time period. They tend to be activated when the membrane potential is very negative, such as during hyperpolarisation.

Ca²⁺-activated K⁺ currents

There doesn't seem to be anything in the lecture about these, so... moving on, I guess?

Inward rectifiers

Inward rectifiers, just like the potassium channels of Streptomyces, only has the S5-P-S6 part. As their name suggests, they control current going into, but not out of the cell. They do this with the help of Mg²⁺. When the inside of the cell is positive, Mg²⁺ is pushed towards the edge of the cell, where it blocks the pore of the ion channel. This prevents intracellular K⁺ from leaving, but allows it to enter from the outside. This is important for preventing excessive loss of K⁺ during repeated and/or lengthy action potentials.

(Note: When the inside of the cell is not positive, Mg²⁺ isn't blocking the channel and thus K⁺ will simply travel in the direction of the concentration gradient during this time. This will become more important when we start talking about smooth muscle.)

Become familiar with the impact of genetic mutations on ion channel function and how this impact membrane potentials and cell function

Ion channels, as hopefully you've realised over the past couple of weeks, are pretty important. Hence, genetic mutations can cause a range of problems, from pain disorders to long QT syndrome (which I'll talk about in one of my posts for PHYL3002). Here are some examples of ion channel problems:

Lambert-Eaton Syndrome

Lambert-Eaton Syndrome is an autoimmune disorder in which antibodies are produced against the S5-S6 region of voltage-gated Ca²⁺ channels. This decreases Ca²⁺ influx, which in turn decreases the amount of ACh released. As ACh is important for muscle contraction, this causes muscle weakness. It mainly affects proximal limb muscles, which can make it difficult to climb stairs, but can also affect respiratory muscles. Lambert-Eaton Syndrome can be treated in three main ways: by decreasing the breakdown of ACh (by using drugs such as pyridostigmine), increasing calcium influx (via 3,4-diaminopyridine) or by using immunosuppressants.

Myotonia

Myotonia is slow or relaxed relaxation after contraction. This manifests as difficulty in releasing grip on tools and so on. Myotonia is sometimes aggravated by cold and vigorous exercise.

There are several different causes of myotonia. Myotonia congenita results from mutations in the CLCN1 gene, causing reduced conductance of chloride. As chloride can't move around and balance out the positive charges, this causes repetitive firing of action potentials. Potassium-aggravated myotonia, or PAM, results from slow inactivation of certain sodium channels, also causing a chain of action potentials after the stimulation stops.

Hyperkalemic Periodic Paralysis (HyperPP)

HyperPP results from a large, persistent Na⁺ current. This causes the cell to lose excitability, resuting in paralysis.

Describe the effect of disruption to ion concentration gradients on membrane potential and cell function

When extracellular calcium is high, sodium channels become more likely to open at higher voltages than usual. Essentially, this means that if you want sodium channels to open, you'll need to depolarise the cell more than usual. This also means that a muscle, for instance, will become less excitable and weaker.

On the other hand, when extracellular calcium is low, muscles can become hyper-excitable and twitch spontaneously.