Discrete roles for secreted and transmembrane semaphorins in neuronal growth cone guidance in vivo

From the initial stages of axon outgrowth to the formation of a functioning synapse, neuronal growth cones continuously integrate and respond to multi ple guidance cues. To investigate the role of semaphorins in the establishm ent of appropriate axon trajectories, we have characterized a novel secrete d semaphorin in grasshopper, gSema 2a. Sema 2a is expressed in a gradient i n the developing limb bud epithelium during Ti pioneer axon outgrowth. We d emonstrate that Sema 2a acts as chemorepulsive guidance molecule critical f or axon fasciculation and for determining both the initial direction and su bsequent pathfinding events of the Ti axon projection. Interestingly, simul taneous perturbation of both secreted Sema 2a and transmembrane Sema I resu lts in a broader range and increased incidence of abnormal Ti pioneer axon phenotypes, indicating that different semaphorin family members can provide functionally distinct guidance information to the same growth cone in vivo .