Johns Hopkins scientists have discovered how one family of proteins repels growing nerves and keeps them properly on track during development. The finding, described in the Dec. 16 issue of Neuron, might provide a chance to overcome the proteins' later role in preventing regrowth of injured nerves, the researchers say.
The proteins, known as chondroitin sulfate proteoglycans (CSPGs), have long been known to prevent nerve regeneration after injury by recruiting a stew of other proteins and agents, but exactly what part of the mix keeps nerves from regrowing is unknown.
In studies of nerve growth in developing rats, the Hopkins scientists have linked CSPGs' no-growth effects to a protein called semaphorin 5A. The scientists, including David Kantor, an M.D./Ph.D. candidate, found that when CSPGs bind to semaphorin 5A, growing nerves are stopped in their tracks. Blocking this particular interaction freed the nerves to continue growing.
"CSPGs are a critical obstacle to nerve regeneration after injury, and without details about what's really happening, it's impossible to rationally intervene," says study leader Alex Kolodkin, Ph.D., professor of neuroscience in Johns Hopkins' Institute for Basic Biomedical Sciences. "We studied nerve growth, rather than re-growth, but our work provides a starting point for identifying more partners of CSPGs and for finding targets to try to counter these proteins' effects in nerve regeneration."
Semaphorins, including 5A, are a family of proteins that help direct growing nerves as they extend toward their eventual targets, largely by keeping nerves out of places they shouldn't be.
"These proteins are classic 'guidance cues' for nerves. There's nothing particularly fancy about what they do -- they bind to spots on the tip of the growing nerve, and the nerve doesn't continue going in that direction," says Kolodkin, whose lab studies semaphorins. "Scientists studying CSPGs' effects haven't really been considering classic guidance cues as CSPGs' key partners, but our study suggests they just might be."
When a nerve is damaged, large amounts of CSPG proteins accumulate at the site of injury. These proteins, in turn, draw in a host of other factors, including semaphorins. Others have shown that without CSPGs, damaged nerves growing in a dish can regenerate, a finding that suggests blocking CSPGs might permit the same in animals.
In experiments in laboratory dishes, Kantor simulated a particular step in the brain development of developing rats. During this step, specific nerves begin connecting between what will eventually be two sections of the brain.
Because the accurate extension of these nerves requires semaphorin 5A, Kantor was able to identify key molecules that interact with it. He found that CSPGs bind semaphorin 5A to prevent nerves from extending across a no-man's-land between the nerves' simulated origin and target. Preventing this interaction by adding an enzyme that destroys only CSPGs allowed nerves to penetrate that space when they shouldn't have.
Kantor also found that semaphorin 5A helps keep the bundle of growing nerve fibers together by interacting with a different family of proteins, those known as heparan sulfate proteoglygans (HSPGs). Other semaphorins also are known to have the apparent paradox of both encouraging growth and restricting it.
"Semaphorins' dual abilities likely stem in part from interactions with different partners, as we've seen here," says Kolodkin, whose team is now studying how semaphorin 5A's signal and binding partners change, and whether it also partners with CSPGs to suppress regeneration. "During development, the available partners change with time and place, helping a limited number of guidance cues accomplish a very complex task."
The Hopkins researchers were funded by the Johns Hopkins Medical Scientist Training Program, the Christopher Reeve Paralysis Foundation, and the National Institute of Neurological Disorders and Stroke.
Authors on the paper are Kantor, Kolodkin and Katherine Peer of Johns Hopkins; Onanong Chivatakarn and Roman Giger of the University of Rochester, NY; Stephen Oster and David Sretavan, University of California, San Francisco; Masaru Inatani and Yu Yamaguchi, The Burnham Institute, La Jolla, Calif.; and Michael Hansen and John Flanagan, Harvard Medical School.
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