Role of semaphorins in the adult nervous system.
In the developing nervous system, extending axons are directed towards their appropriate targets by a myriad of attractive and repulsive guidance cues. Work in the past decade has significantly advanced our understanding of these molecules and has made it increasingly clear that their function is not limited to the guidance of growing axons during embryogenesis. Axon guidance cues fulfill additional roles in angiogenesis, cell migration and the immune system, and often display sustained expression in adulthood. Here we focus on the semaphorin (Sema) family and review their proposed functions in the adult nervous system. Several semaphorin family members continue to be expressed in the adult brain and spinal cord, and increasing evidence indicates that their expression is regulated upon nervous system injury in rodents and in neuropathology in humans. The available evidence suggests that semaphorins might significantly contribute to the maintenance and stability of neuronal networks. Furthermore, semaphorins could play important roles in the regeneration, or failure thereof, of neuronal connections. In the future, genetic manipulation of semaphorins and their receptors in the adult intact and injured nervous system should provide a deeper insight into the mechanisms by which semaphorin signaling contributes to structural plasticity and regeneration in the adult brain.