Skeletal Muscle Disease

In this post, we'll be talking about possibly one of the more common skeletal muscle diseases: duchenne muscular dystrophy (DMD), which I've also spoken about here and here.

What is duchenne muscular dystrophy (DMD)?

DMD is an X-linked disease in which a protein called dystrophin is deficient. Dystrophin connects the cytoskeleton to membrane receptors, which in turn anchor the muscle fibre to the extracellular matrix. The absence of this simple protein can lead to severe muscular problems that start off with difficulties in walking and lead to death due to respiratory problems or heart failure.

Increased Ca²⁺ influx in muscular dystrophy

In DMD, muscle fibres have chronically elevated calcium which can occur for several reasons. The main reasons are related to the fragility of the membrane in DMD and the altered activity of calcium ion channels, particularly stretch-gated ion channels.

The membrane of DMD muscle fibres is more fragile and more permeable, as has been determined by studies looking at hypo-osmotic shock. In hypo-osmotic shock, the cells are placed in a medium with a lower concentration of solutes/ higher concentration of water. This causes water to rush into the cell, causing the cell to lyse. DMD fibres are more susceptible to hypo-osmotic shock, suggesting that the membrane is more fragile and allows more water (and possibly more calcium as well) to rush in.

The lack of dystrophin, and thus the lack of "tethering" to the ECM, affects the mechanosensing of the cell. This means that channels that rely on mechanosensing, such as stretch-gated ion channels, are more active. In vitro, blocking stretch-gated ion channels has been shown to reduce calcium permeability in DMD fibres.

What's the problem with high intracellular calcium? Well, calcium can activate certain calcium-activated enzymes, such as calpains, which also made a cameo appearance here. It's no surprise, then, that calpain inhibitors, such as leupeptin, have delayed muscle degeneration in mice. Another consequence of elevated calcium is that the calcium can enter the mitochondria and upregulate oxidative phosphorylation (a topic of a later post). Oxidative phosphorylation produces some reactive oxygen species (ROS), which are usually fine, except in cases of chronically elevated calcium (like in DMD). Both calpain and ROS can cause damage to the membrane and cytoskeleton, causing an even greater influx of calcium, creating a vicious cycle where muscles become progressively more damaged over time.

Inflammation in muscular dystrophy

Inflammation is usually a good thing, as it helps us to fight disease and repair damage to our bodies. In chronic inflammation, as happens during muscular dystrophy and many other illnesses, can be a real problem.

I've probably spoken quite a bit about inflammation back when I was doing MICR360 in Canada, but here's a quick overview. Inflammation starts when there is some damage. Neutrophils come in to mop up the mess by releasing ROS and proteases and doing a bit of phagocytosis to remove the debris. Later on the monocytes come along through the blood, turning into macrophages when they enter the tissue. These also release NO and proteases and do some phagocytosis, but they also release cytokines, chemokines and growth factors. These growth factors can help with muscle repair.

To determine whether inflammation caused muscle weakness, a highly inflammatory polysaccharide called carrageenan was injected (presumably not into humans, though the slides don't say which model was used for this experiment). Carrageenan decreased maximum specific force and shifted the force-frequency relationship curve to the right, so that higher frequencies were required for the same amount of force.

Carrageenan was also found to increase the levels of pro-inflammatory cytokines, such as IL-1β, IL-6 and TNFα. TNFα is particularly interesting as it is thought that it might contribute to the damage caused by eccentric contraction (contractions in which the muscle actually lengthens- yes, that's a thing 0_o). Experiments in mdx mice (a DMD mouse model) have found that mdx mice experience a greater loss of force after eccentric contractions as compared to control, but this loss of force could be attenuated by anti-TNF antibodies. Histological sections also show less necrosis in mice treated with the antibody as compared to mice injected with a control antibody. As TNFα levels are also high in humans with DMD, TNFα may also play a role in human DMD.