The function of Semaphorin on the neural network formation in the olfactory system.

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Semaphorin 3A (Sema3A) repels growing olfactory axons that express Neuropilin-1, a receptor for Sema3A. The Sema3A-mediated axon guidance seems to be essential for the formation of the glomerular sensory map in the olfactory bulb. To understand whether and how Sema3A is involved in the sensory map formation, we examined the glomerular map in the olfactory bulb of adult Sema3A-deficient mice. In the wild-type mice Neuropilin-1-positive glomeruli form the lateral and medial bands and avoid the anteromedial and ventral regions of the olfactory bulb. In the Sema3A-deficient olfactory bulb Neuropilin-1-positive glomeruli spread over the entire olfactory bulb and we consistently found the ectopic arrangement of Neuropilin-1-positive glomeruli in the anteromedial and ventral regions. In addition, a specific subset of Neuropilin-1-negative and OCAM-positive glomeruli, especially those in the anteromedial region, disappeared from the mutant olfactory bulb. These results show a critical role for Sema3A in the spatial arrangement of glomeruli in the olfactory bulb. Optical imaging from the dorsal olfactory bulb showed that the distorted glomerular map conserved molecular-feature domains. However, the positions of the domains were shifted, which suggests a secondary rearrangement of the glomerular map in the Sema3A-deficient olfactory bulb.