Semaphorin 5B is a novel inhibitory cue for corticofugal axons

Neuronal connectivity is generated by the precise guidance of neuronal growth cones in response to the spatiotemporal distribution of molecular guidance cues in the developing embryo. Here we show that the class 5 semaphorin, Semaphorin 5B, is expressed in regions of the cortex and subcortex flanking the projection of and avoided by descending cortical axons, suggesting a role as a repulsive guidance cue in the formation of the internal capsule. Axons from cortical explants cultured in vitro with Semaphorin 5B-expressing cells exhibited characteristic avoidance behaviors. In organotypic slices, ectopic Semaphorin 5B expression along the presumptive internal capsule was sufficient to cause cortical axons to avoid their normal trajectory, resulting in either stalling at the boundary of Semaphorin 5B or turning into inappropriate areas of the cortex. In contrast, thalamocortical axons were not inhibited either in vitro or in slice culture by ectopic Semaphorin 5B. To further examine the function of Semaphorin 5B in situ, we knocked down its expression in the ventricular zone (VZ) at the corticostriatal angle. We found that labeled cortical fibers aberrantly navigated into the VZ where Semaphorin 5B expression was reduced. We propose that Semaphorin 5B functions to prevent corticofugal axons from abnormally projecting into germinal regions as they project to their subcortical targets.