Kirrel3 is required for the coalescence of vomeronasal sensory neuron axons into glomeruli and for male-male aggression

The accessory olfactory system controls social and sexual interactions in mice that are crucial for survival. Vomeronasal sensory neurons (VSNs) form synapses with dendrites of second order neurons in glomeruli of the accessory olfactory bulb (AOB). Axons of VSNs expressing the same vomeronasal receptor coalesce into multiple glomeruli within spatially conserved regions of the AOB. Here we examine the role of the Kirrel family of transmembrane proteins in the coalescence of VSN axons within the AOB. We find that Kirrel2 and Kirrel3 are differentially expressed in subpopulations of VSNs and that their expression is regulated by activity. Although Kirrel3 expression is not required for early axonal guidance events, such as fasciculation of the vomeronasal tract and segregation of apical and basal VSN axons in the AOB, it is necessary for proper coalescence of axons into glomeruli. Ablation of Kirrel3 expression results in disorganization of the glomerular layer of the posterior AOB and formation of fewer, larger glomeruli. Furthermore, Kirrel3(-/-) mice display a loss of male-male aggression in a resident-intruder assay. Taken together, our results indicate that differential expression of Kirrels on vomeronasal axons generates a molecular code that dictates their proper coalescence into glomeruli within the AOB.

Robo-2 controls the segregation of a portion of basal vomeronasal sensory neuron axons to the posterior region of the accessory olfactory bulb

The ability of sensory systems to detect and process information from the environment relies on the elaboration of precise connections between sensory neurons in the periphery and second order neurons in the CNS. In mice, the accessory olfactory system is thought to regulate a wide variety of social and sexual behaviors. The expression of the Slit receptors Robo-1 and Robo-2 in vomeronasal sensory neurons (VSNs) suggests they may direct the stereotypic targeting of their axons to the accessory olfactory bulb (AOB). Here, we have examined the roles of Robo-1 and Robo-2 in the formation of connections by VSN axons within the AOB. While Robo-1 is not necessary for the segregation of VSN axons within the anterior and posterior regions of the AOB, Robo-2 is required for the targeting of some basal VSN axons to the posterior region of the AOB but is dispensable for the fasciculation of VSN axons. Furthermore, the specific ablation of Robo-2 expression in VSNs leads to mistargeting of a portion of basal VSN axons to the anterior region of the AOB, indicating that Robo-2 expression is required on projecting VSN axons. Together, these results identify Robo-2 as a receptor that controls the targeting of basal VSN axons to the posterior AOB.