Axon pruning and synaptic development: How are they per-plexin?

During the development of the nervous system, neurons must first migrate to their appropriate locations and then send out axons to make connections. Various environmental cues guide these migrating neurons and growing axons. After axons reach their target regions, neuronal contacts are created through the formation of synapses. Because excess axonal branches and synaptic contacts are often formed during early development, they are pruned or eliminated at later stages to create specific neuronal connections. Several groups of ligand-receptor pairs have been identified to regulate each of these cellular events. Evidence also indicates that these same molecules may be used in multiple developmental processes. Here, we discuss semaphorins and plexins, the largest family of axon guidance ligand-receptor pairs. Because the roles of semaphorins in neuronal migration and axonal repulsion have been extensively reviewed, we will focus on plexin receptors. We will discuss how semaphorin signals are specifically passed through these receptors into cells and how plexins mediate their newly identified roles in axon pruning and synaptic development.