MDA-supported research in Charcot-Marie-Tooth disease is focused on figuring out what goes wrong at the molecular level in CMT-affected axons or the myelin sheaths that surround them, rather than on attempting to fix the problem directly or preserving nerve function in spite of it. A central theme emerging from the last decade of research is that myelin and axons require constant signals from each other to stay functional.
Here is a look at work being done by three leading MDA-supported CMT researchers.
The axon and the myelin sheath need each other, says James Salzer, an MDA-supported professor of cell biology and neurology at New York University.
Myelin’s essential role in allowing for speedy conduction of nerve impulses and providing protection and insulation around axons is well-known, he says.
In addition, recent studies suggest that myelin provides sustaining signals to axons that help keep them intact.
Loss of the myelin sheath — demyelination — is first a problem for nerve conduction and eventually a problem for the health of the axon itself. The latter isn’t well understood and is an area of intense study, Salzer says.
But what interests him the most right now is another aspect of peripheral nerve function — namely, how the axon and the myelin sheath signal the myelin-making Schwann cells to either keep making myelin or to stop making it.
In many forms of CMT, Salzer suspects, the “make myelin” signals are disrupted, either because of abnormalities in the axon or in the myelin sheath. His lab is focused on trying to restore myelin production.
“If you don’t have an axon, the myelin sheath will break down,” Salzer says. “The current view is that, as the axon breaks down, it releases signals that tell the Schwann cell to break down its myelin sheath.” The Schwann cell seems to go back to a more primitive, undifferentiated state.
A similar process may happen if the myelin sheath is abnormal, as it is in many forms of CMT, including the most common form, CMT1A. It seems myelin proteins that are overproduced or abnormal can cause the same kind of shutdown in Schwann cells as occurs when the axon is defective.
The vast majority of CMT1A cases are caused by overproduction of a myelin protein called PMP22, because of the presence of an extra PMP22 gene. Only a small percentage of CMT1A cases are caused by a mutation in the PMP22 gene that causes an abnormal PMP22 protein to be made.
Salzer’s group is working on reducing production of PMP22 by targeting a molecular pathway known as mTOR.
“It’s a key nexus in the control of a process called protein translation,” he says, referring to how proteins are produced from the genetic material known as RNA. “That’s a path that I’m sure is going to become a robust area in CMT research over the next couple of years.”
But it’s not the only thing on his to-do list. “Rather than targeting the extra protein itself,” he says, “one could instead go to the consequences of the extra protein or the misfolded protein and target the signals that it induces.”
Stopping dedifferentiation signals and keeping Schwann cells differentiated, so that they’re in their myelin-making state, is an avenue Salzer plans to explore.
Like Salzer, Thien Nguyen has been thinking a lot about myelin and what it does for axons besides speeding up conduction.
Nguyen, a neurologist and neurophysiologist, is an assistant professor of neurology at Johns Hopkins University School of Medicine. He has MDA support to explore axonal protection as a strategy to treat CMT.
“Studies have shown that, even though there is slowing of the electrical signals when there is demyelination, patients actually do very well for many years,” Nguyen says. “But then eventually, after several years, the axons start dying, and that’s what leads to clinical [functional] deficits.”
Nguyen wanted to figure out how myelin protects and nourishes axons, as well as speeding nerve conduction.
He suspected there might be specific molecules in the myelin sheath that keep the axon alive, and that perhaps they could do so even when the sheath itself is unstable or improperly formed, as it is in type 1 CMT.
“The best place to look for such a molecule, we thought, is in the space between the myelin and the axon,” Nguyen says. And one of the most well-known molecules that sits there is myelin-associated glycoprotein, or MAG. Most of it is in the interface between the myelin and the axon, on the innermost coil of myelin.
“We figured that molecule would make perfect sense,” says Nguyen. “We asked, ‘If you lack this protein, is the axon more vulnerable to death?’ It turns out that yes, it is.”
More recently, Nguyen’s group has found that a protein called netrin 1 is located in the same myelin-axon interface as MAG and has many of the same functions. What’s particularly intriguing about netrin 1 is that it’s deficient in mice with a PMP22 mutation and a CMT1A-like disease.
“We think these myelin molecules — MAG, netrin 1 and possibly others — normally interact with axons,” Nguyen says, “and if that ability is interfered with, it will increase the vulnerability of the axon to injury and degeneration.
“Then we can extend that one additional step and ask, ‘If that’s the case, how about if we add them back? Are we able to reverse this process?’ That’s something we’ve been trying very hard to do.”
Meanwhile, he says, they’re also working on another approach — determining whether there’s a small piece of MAG and netrin 1 that’s the same and that sends protective signals to the axon. If that’s the case — and so far it seems to be — its small size would make it much more attractive as a candidate for drug development than a large protein would be.
Michael Granato, a professor of cell and developmental biology at the University of Pennsylvania, has MDA support to study degeneration and regeneration after damage to peripheral nerves in an animal that’s getting a lot of attention in scientific laboratories everywhere: the zebrafish.
Zebrafish have more in common with mammals, including humans, than most people think, Granato notes. But unlike other animals, they’re transparent, which is a huge advantage. Scientists can see what’s happening in structures like peripheral nerves by looking at them under a microscope while the fish is alive and swimming.
Granato says his research team has begun to revisit — with state-of-the art tools — many old assumptions about peripheral nerves and the cells with which they interact.
“Seeing is believing,” Granato says. “This not only pertains to what’s happening in the nerve that’s damaged, but also to other cell types. What’s happening to those? How do they interact with the peripheral nerves? This is really the basis of understanding what’s going on.”
|The peripheral nerves of zebrafish can be examined while they're moving.|
Two cell types that have been studied in connection with peripheral-nerve degeneration and regeneration are the myelin-making Schwann cells and the macrophages, cells made by the immune system. The word “macrophage” means “big eater,” and these cells gobble up debris from degenerating tissue in many different circumstances.
It’s long been assumed that macrophages go out to the damaged nerve some time after the damage has occurred, he says. But ongoing work in his lab studying nerve damage in the see-through zebrafish is focusing on the question of whether macrophages arrive even before the fibers start to break down.
Granato’s group is also using genetic tools to see how taking away macrophages would affect nerve regeneration in the zebrafish. If macrophage-supplied cleanup efforts had a positive effect on regeneration, attracting more macrophages to the site of an injury could be a therapeutic avenue. But if those efforts made matters worse, perhaps inhibiting macrophage recruitment would help.
Granato now has zebrafish with a mutation in the GARS gene, the cause of type 2D CMT. He’s testing the idea that nerve regeneration after injury is inhibited in these fish, and he believes figuring out the underlying molecular signals could ultimately be important for understanding and possibly treating some types of CMT.
Looking at something in real time in a model like the zebrafish “looks somewhat different” from what is seen in biopsy samples, he says. “We’re finding a lot of things that we hope will revise the literature regarding what people thought.”