The mouse olfactory system, which detects numerous environmental chemical stimuli, is divided into subsystems. Sensory neurons in each subsystem within the olfactory epithelium have different receptor molecules and project their axons to different parts of the olfactory bulb. Now, on , Peter Mombaerts and colleagues report that semaphorin 3F (Sema3f) and its co-receptor neuropilin 2 (Nrp2) are involved in the axonal wiring of guanylate cyclase-D (GC-D+)-expressing sensory neurons. The axons of these neurons, which are not known to express odorant receptors, project to the necklace glomeruli, which sit between the main and accessory olfactory bulbs. To visualize the axonal projections of GC-D+ neurons, the researchers generated mice that co-express GC-D with labelled tau (an axonal marker). They show that in Sema3f-null (but not Sema3b-null)mice and Nrp2-null mice, GC-D+ neurons form ectopic glomeruli in the main olfactory bulb, as well as the necklace glomeruli. Thus,the researchers conclude, an Nrp2-Sema3f interaction helps to determine the axonal wiring of this relatively poorly studied olfactory subsystem.
