Unlike people, fish can regrow damaged nerve fibers in their central nervous systems. Now a study may have found the reason: The creatures lack a protein called Nogo-A that prevents nerve regeneration in mammals.
Axons, or nerve fibers, are the transmission lines that conduct electrical signals throughout the body. The fibers are protected by sheaths of myelin, a fatty insulator that speeds the electrical impulses along. Damaged axons in the brain and spinal cord of mammals don't regenerate, and spinal cord injuries can therefore lead to permanent paralysis. Fish are luckier: They can regrow the axons in their central nervous system, but curiously this regeneration stops if their nerve endings come into contact with mammalian myelin.
Because a protein in mammalian myelin called Nogo-A is known to inhibit central nervous system axon growth in mammals, a team of researchers led by biologist Claudia Stürmer at the University of Konstanz in Germany wondered if fish might be missing this protein. When the researchers exposed goldfish axons to rat Nogo-A, the nerves stopped growing. Furthermore, a comparison of genomes between ten species of fish, including zebrafish and pufferfish, and humans revealed that fish lack the genetic information to make Nogo-A or a similar inhibitor. The team reports its findings in the August issue of Molecular Biology and Evolution.
The paper's careful study of fish phylogeny supports an existing notion that Nogo-A may be a recent evolutionary development that correlates with more complex nervous systems and more complex functions, says Stephen Strittmatter, a neurologist at Yale University in New Haven, Connecticut. "It's an important addition to our growing understanding of the role these inhibitors play," he says.