Nervous system development requires billions of neurons to migrate to the appropriate locations in the brain and grow nerve fibers (axons) that connect to other nerve cells in an intricate network.
Growth cones, structures in the tips of growing axons, are responsible for steering axons in the right direction, guided by a complex set of signals from cells they encounter along the way. Some signals lure the axons to extend and grow in a particular direction; others are inhibitory, making the axon turn away or stop growing.
In two papers in the April 21 Neuron, researchers from Children's Hospital Boston reveal important insights into how inhibitory cues affect the growth cone, and identify possible targets within axons that could be blocked to overcome this inhibition. Such intervention could possibly enable damaged axons to regenerate (normally impossible in a mature nervous system) and ultimately restore nerve function.
It's been known that cells synthesize an inhibitory protein called ephrin, which binds to a receptor called Eph on the axon's growth cone. But how this triggers the axon to change course or stop growing has been a mystery.
"Very little has been known about the inner workings of the cell that govern axon guidance," says Michael Greenberg, PhD, Director of the Neurobiology Program at Children's and senior author on both studies. "These studies begin to give insight into how the various steps of axon guidance are controlled."
The first paper found that when ephrin binds to Eph receptors on the axon, it activates a protein called Vav2 in the cell's growth cone. Activation of Vav2 induces the cell to engulf the ephrin-Eph complex, breaking the bond between the two and repelling the axon, causing it to turn away. When mice were genetically modified to lack Vav2 and the related Vav3, thereby eliminating this repellent signal, the mice had abnormal axon projections and defects in neural circuitry formation.
The second paper demonstrates the role of a protein called ephexin1 in axon guidance. By itself, ephexin 1 promotes axon growth; neurons from mice genetically modified to lack ephexin1 had significantly shorter axons. But when ephrin is present and binds to Eph receptors, ephexin1 is chemically modified, causing it to alter the cell's cytoskeleton, or internal scaffolding. This alteration makes the growth cone collapse, steering the axon in a new direction or halting its growth. In chicken motor neurons whose ephexin1 was inactivated, the axons grew into the hind limb prematurely, indicating faulty axon guidance.
"Understanding these pathways could help in understanding the process of nerve regeneration," says Greenberg, who is also Professor of Neurology and Neurobiology at Harvard Medical School. "The mechanisms we've uncovered could provide opportunities for the development of therapies for spinal cord injury, targeting ephexin and possibly Vav," he speculates, "but much more needs to be known about how ephexin, Vav and other proteins work together to coordinate axon guidance."