A punctured automobile tire that's self-sealing is nothing new. Neither is a self-fixing muscle cell. Those cells are forever repairing their own self-inflicted wounds.
"Constant muscle damage is a part of life," observed physiologist Kevin Campbell at the University of Iowa in Iowa City. He explained: "One of the ways you build muscle is when you exercise, which causes tears in those muscle cell membranes. Then the healing process produces an increased amount of healthy muscle. This is a normal, everyday process in the body. Lots of cells are under mechanical stress," he pointed out. "Mammalian muscles don't have a cell wall, as yeast does, for example. So mammalian membranes are more susceptible to damage, which is one of the normal processes.
"When a man or a mouse fails for some reason to repair its punctured muscle-cell membrane," Campbell continued, "muscular dystrophy [MD] ensues. This progressive disorder of impaired bodily movement exists in half a hundred distinct versions, of which Duchenne dystrophy is best known. Most of the others are fairly exotic." (See BioWorld Today, July 25, 2002.)
"Duchenne," Campbell went on, "occurs about once in 4,000 live male births. But two MD variants, limb-girdle muscular dystrophy and Miyoshi myopathy are 10 times less prevalent. They probably total 1 in 20,000, 1 in 50,000.
"Duchenne is X-linked recessive," Campbell noted, "so only males get that disorder. Miyoshi and limb-girdle are autosomal-recessive - meaning you have to have two genes, inherited one from mother, one from father, to have the disease. Also, females can have it, too."
Campbell is senior author of a paper in the current issue of Nature, dated May 8, 2003, titled "Defective membrane repair in dysferlin-deficient muscular dystrophy."
Fix-It Dysferlin Aids Muscle Membrane Repair
"The major finding in that Nature paper," he observed, "is that we've uncovered a novel protein mechanism whereby muscle cells are able to repair their membrane when wounded. Dysferlin was discovered previously by a group studying the genetics of MD. We discovered its function - a protein involved in muscle membrane repair. This molecule is missing in some forms of MD. Also we believe that once we fully understand this dysferlin protein and the other proteins involved, we may be able to use this repair mechanism to modify the pathogenic pathway in different forms of MD. Then maybe we'll be able to use it as a strategy to develop a treatment for the disorder."
Campbell and his co-authors chose limb-girdle and Miyoshi MD variants on which to conduct their in vivo mouse experiments. "We wanted to understand the dysferlin proteins, their functions. They also have a later-onset phenotype. The limb-girdle affects the girdle around your waist and the limb muscles around your shoulders. The Miyoshi myopathy affects the distal muscles, the bottom of your legs - more distal from the center of the body.
"Those two variants were active," he explained. "They were interesting to us because they are different phenotypes. One has limb-girdle muscles, the other, distal muscles. What surprised us was when the genetics were done by different groups. They found that dysferlins, which cause different types of MD, have the same genes. We wanted to figure out how this was occurring. It looked to us that it's the same mechanism in both forms of MD - a defect in the membrane-repair mechanism.
"We don't know all of the mechanisms but we believe that dysferlin is present in either membranes or vesicles in the cells. When a muscle cell gets damaged, calcium rushes in and causes these vesicles to come to the site of wound, to fuse and make a patch and heal the membrane. The patch hypothesis,'" he explained, "is that a group of these vesicles containing dysferlin come and fuse with the sarcolemma, the plasma membrane, and form a patch that heals the damage. A repair takes less than one minute.
"To generate our colony of knockout mice," Campbell recounted, "we made a mouse that lacked dysferlin. We manipulated the protein's gene so it couldn't produce dysferlin. We then studied muscle fibers and tissues from that KO mouse. Some of those experiments were done where we tried to damage the muscle fibers and looked at where the dysferlin went. In other experiments, we used a laser to disable the muscle fibers and watched the repair process. We put a fluorescent dye outside the cell and when we damaged the membrane we let the dye go into the cell as a visible marker.
"The overall results of our mouse experiments are detailed in our Nature paper. In brief, it demonstrates that these dysferlin-minus mice have a defect that renders them incapable of repairing their muscle membrane. And just like humans, they developed a muscular dystrophy that got progressively worse with age. However, treadmill tests revealed that the muscles of mice lacking dysferlin were not much more at risk of damage than the musculature of normal rodents."
Dysferlin Family Portends Other Ills, Such As Deafness
"In our present ongoing work," Campbell said, "we're trying to more fully understand this mechanism, identify other proteins that may play a role in it, and also be involved in MD. So there's a whole series of experiments for us to do. At the present time, there's no immediate human application of this work, but hopefully future experiments would lead to an application."
Campbell made a final point that "the process of cell membrane repair might be involved in other diseases than MD. We do know that there is a hearing-loss disorder related to a protein called otoferlin, which is related to dysferlin. We don't know the mechanism there either, but as this is a basic cellular process, it's possible," he concluded, "that other diseases that disrupt this process will in time be found."