Semaphorin 3A is required for guidance of olfactory axons in mice.

Semaphorin 3A (Sema3A) is a membrane-associated secreted protein that has chemorepulsive properties for neuropilin-1 (npn-1)- expressing axons. Although mice lacking the Sema3A protein display skeletal abnormalities and heart defects, most axonal projections in the CNS develop normally. We show here that Sema3A is expressed in the lamina propria surrounding the olfactory epithelium (OE) and by ensheathing cells in the nerve layer of the ventral olfactory bulb (OB) throughout development. Subsets of sensory neurons expressing npn-1 are distributed throughout the OE and extend fibers to the developing OB. In wild-type mice, npn-1-positive (npn-1(+)) axons extend to lateral targets in the rostral OB and medial targets in the caudal OB, avoiding regions expressing Sema3A. In Sema3A homozygous mutant mice, many npn-1(+) axons are misrouted into and through the ventral nerve layer, beginning as early as embryonic day 13 and continuing at least until birth. At postnatal day 0, npn-1(+) glomeruli are atypically located in the ventral OB of Sema3A(-/-) mice, indicating that aberrant axon trajectories are not corrected during development and that connections are made in inappropriate target regions. In addition, subsets of OCAM(+) axons that normally project to the ventrolateral OB and some lactosamine-containing glycan(+) axons that normally target the ventral OB are also misrouted in Sema3A mutants. These observations indicate that Sema3A expression by ensheathing cells plays an important role in guiding olfactory axons into specific compartments of the OB.