Basal Activity of Odorant Receptors Directs Coarse Targeting of Olfactory Neuron Axons Using Gs, But Not Golf

In the mouse olfactory system, odorant receptors (ORs) play an instructive role in axonal projection of olfactory sensory neurons (OSNs). We have previously reported that OR-derived cAMP signals regulate the transcription of axon guidance molecules such as Neuropilin-1 (Nrp1) and Semaphorin-3A (Sema3A), thereby directing anterior-posterior (A-P) targeting of OSN axons. It was unclear, however, how cAMP signals are generated in an OR-specific manner during the initial stage of olfactory map formation. What we found is that A-P targeting is regulated by receptor basal activity. G-protein coupled receptors (GPCRs) including ORs are known to possess two different conformations, active and inactive, and generate unique levels of basal activity by interchanging between the two in the absence of agonists. We analyzed transgenic mice expressing basal-activity mutants of b2-adrenergic receptor (b2-AR) for receptor-instructed OSN projection. b2-AR mutants that altered basal activity, but not agonist-induced activity, changed transcription levels of A-P targeting molecules, thus, causing shifts in glomerular locations. These mutants, however, did not affect glomerular segregation molecules, e.g., Kirrel2 and Kirrel3, whose expression is regulated by stimulus-driven neuronal activity. Subsequent knockout experiments demonstrated that G_s, rather than G_olf, is essential for A-P targeting. In a reconstituted system using HEK293 ells, ORs generated basal activity with G_s, but not with G_olf.  We conclude that OR molecules require G_s to generate basal activity and thereby regulate A-P targeting of OSN axons. Our study has clarified a long-standing question in olfactory research, namely, what defines the identity of OSNs in the OR-instructed axonal projection.