Axonal wiring of guanylate cyclase-D-expressing olfactory neurons is dependent on neuropilin 2 and semaphorin 3F

The olfactory system of the mouse includes several subsystems that project axons from the olfactory epithelium to the olfactory bulb. Among these is a subset of neurons that do not express the canonical pathway of olfactory signal transduction, but express guanylate cyclase-D (GC-D). These GC-D-positive (GC-D⁺) neurons are not known to express odorant receptors. Axons of GC-D⁺ neurons project to the necklace glomeruli, which reside between the main and accessory olfactory bulbs. To label the subset of necklace glomeruli that receive axonal input from GC-D⁺ neurons, we generated two strains of mice with targeted mutations in the GC-D gene (Gucy2d). These mice co-express GC-D with an axonal marker, tau--galactosidase or tauGFP, by virtue of a bicistronic strategy that leaves the coding region of the Gucy2d gene intact. With these strains, the patterns of axonal projections of GC-D⁺ neurons to necklace glomeruli can be visualized in whole mounts. We show that deficiency of one of the neuropilin 2 ligands of the class III semaphorin family, Sema3f, but not Sema3b, phenocopies the loss of neuropilin 2 (Nrp2) for axonal wiring of GC-D⁺ neurons. Some glomeruli homogeneously innervated by axons of GC-D⁺ neurons form ectopically within the glomerular layer, across wide areas of the main olfactory bulb. Similarly, axonal wiring of some vomeronasal sensory neurons is perturbed by a deficiency of Nrp2 or Sema3f, but not Sema3b or Sema3c. Our findings provide genetic evidence for a Nrp2-Sema3f interaction as a determinant of the wiring of axons of GC-D⁺ neurons into the unusual configuration of necklace glomeruli.