Odorant receptors at the growth cone are coupled to localized cAMP and Ca ²⁺ increases

Abstract

A distinctive feature in the topographic organization of the olfactory system in mammals is the dual function of the odorant receptor (OR): it detects odors in the nasal epithelium and plays an instructive role in the axonal convergence of olfactory sensory neurons (OSN) into the olfactory bulb (OB). The latter function is supported by genetic experiments and by the expression of the OR not only on the cilia, but also on the axon termini of the OSN. The signaling pathway coupled to the OR on the cilia is well known and is recognized to involve cAMP and Ca 2؉ , whereas, until now, nothing was known on the functional characteristics of the OR on the axon termini-growth cone. Here, by analyzing the spatiotemporal dynamics of cAMP and Ca 2؉ in living OSN in vitro and in situ, we found that the OR at the growth cone is capable of binding odors and is coupled to cAMP synthesis and Ca 2؉ influx through cyclic nucleotide gated (CNG) channels. Furthermore we found that selective odor activation of the OR on the growth cone is followed by nuclear translocation of protein kinase A catalytic subunit. These results define the functional properties of the OR on the growth cone and suggest a potential role of OR activation in axonal convergence and sensory map formation.olfactory sensory neurons ͉ real-time imaging ͉ second messengers I n sensory systems, neurons in the peripheral sheet send their axons in precise loci of the CNS to create an internal representation of the external world. The spatial segregation of afferent inputs from primary sensory neurons provides a topographic map that defines the quality and the location of the stimulus. In the olfactory system, each OSN expresses only one OR gene out of a repertoire of approximately 1000 (1). OSNs expressing different ORs are randomly dispersed in the nasal epithelium. However, spatial order is achieved in the olfactory bulb (OB), where OSNs expressing the same odorant receptor converge with exquisite precision to form glomeruli both on the lateral and the medial side of each OB. Each odor is thus encoded by a specific spatial pattern of activated glomeruli. The OR, a G-protein-coupled receptor, upon binding of odorant ligands at the cilia, activates a specific G protein, Golf, that stimulates adenylyl cyclase, AC, to synthesize cAMP. The cAMP then directly activates cyclic nucleotide gated (CNG) channels, leading to action potential generation (2, 3).A unique feature in the topographic organization of the OB is the ''dual role'' of the OR. Although it is well established that the OR is involved in the transduction of chemical signals (odors) at the cilia level, a number of evidence suggests that this receptor plays also an instructive role in glomerular convergence of OSN axons to the bulb (4-6). The latter property is supported by genetic observations demonstrating that alterations of OR sequence perturb the sensory map (7-9) and by the demonstration that the OR is expressed not only at the cilia but also on the OSN axon termini (10, 11). The OR is not the only determ...

Evidence for Developmentally Regulated Local Translation of Odorant Receptor mRNAs in the Axons of Olfactory Sensory Neurons

“…Whether the association of odorant receptor mRNAs to polysomes leads to the synthesis of functional odorant receptor proteins properly addressed to the plasma membrane of the axons is currently unknown. Even though strong evidence for the presence of functional odorant receptors at growth cone/axon termini surface of olfactory sensory neurons was recently obtained ex vivo and in a hemihead preparation (Maritan et al, 2009), it remains unknown whether these odorant receptors have a local origin. Although the presence of a functional secretory machinery in mature axons is controversial, evidence for the local synthesis of several membrane proteins in developing axons has been reported (Brittis et al, 2002;Bi et al, 2006), and the presence of a functional equivalent of endoplasmic reticulum and Golgi in retinal ganglion cell growth cones has been documented (Merianda et al, 2009).…”

“…In light of the known functions of odorant receptors in the guidance of olfactory axons, it is tempting to speculate that their regulated local synthesis in axons may play a role in this process, by allowing them to acquire their odorant receptor identity as they extend into the bulb. It will be critical to determine the contribution of such a hypothetical mechanism in the glomerular array formation, with respect to the recently established role of the odorant receptor/Gs/cAMP cascade regulating the transcription of adhesion and guidance molecules involved in axon-axon interactions (Imai et al, 2006;Serizawa et al, 2006;Chesler et al, 2007;Maritan et al, 2009). Correlation between sensory neuron maturation indices (calculated from OMP/GAP43 mRNAs ratio in the olfactory epithelium; the value for untreated adult mice is 100%) and PMPs for the MOR253-5 (A), MOR10 (B), OMP (C), and ␣1b-Tubulin (D) mRNAs in the olfactory bulb in various physiological or experimental conditions: P4 (purple); untreated adults (blue); adults after dichlobenil treatment (7 d after the onset of dichlobenil treatment in red, 9 d in orange, 14 d in brown, 28 d in green).…”

Interplay among cGMP, cAMP, and Ca2+ in Living Olfactory Sensory Neurons In Vitro and In Vivo

“…Alternatively, or in addition, it may indicate that the basal activity of the GCs is very low. As to the effect of IBMX on cGMP level, this most likely depends on the cAMP rise induced by the drug (Maritan et al, 2009), followed by a rise in Ca 2ϩ and NOS activation (see below).…”

“…Upon activation of the odorant receptor (OR) expressed at the cilia (Menini, 1999) and the axon terminus (Maritan et al, 2009), a rise of cAMP and Ca 2ϩ is locally generated. Several studies have demonstrated that odor exposure promotes the synthesis of another second cyclic messenger, cGMP.…”

Odorant receptors regulate the final glomerular coalescence of olfactory sensory neuron axons

“…It was first recognized almost 20 y ago that substituting an OR-coding region with that of a different OR resulted in the glomerular convergence of axons at an ectopic location relative to that of the native ORs (2). This led to the suggestion that ORs have an instructive role in the extension and glomerular coalescence of OSN axons, most likely mediated by homophilic fasciculation (3)(4)(5).…”