Making scent of the presence and local translation of odorant receptor mRNAs in olfactory axons

Cell type-dependent axonal localization of translational regulators and mRNA in mouse peripheral olfactory neurons

Local protein synthesis in mature axons may play a role in synaptic plasticity, axonal arborization, or functional diversity of the circuit. To gain insight into this question, we investigated the axonal localization of translational regulators and associated mRNAs in five parallel olfactory circuits, four in the main olfactory bulb and one in the accessory olfactory bulb. Axons in all four main olfactory bulb circuits exhibited axonal localization of Fragile X granules (FXGs), structures that comprise ribosomes, mRNA, and RNA binding proteins including Fragile X mental retardation protein (FMRP) and the related protein FXR2P. In contrast, FXGs were not seen in axons innervating the accessory olfactory bulb. Similarly, axons innervating the main olfactory bulb, but not the accessory olfactory bulb, contained the FXG-associated mRNA Omp (olfactory marker protein). This differential localization was not explained by circuit-dependent differences in expression of FXG components or Omp, suggesting that other factors must regulate their axonal transport. The specificity of this transport was highlighted by the absence from olfactory axons of the calmodulin transcript Calm1, which is highly expressed in peripheral olfactory neurons at levels equivalent to Omp. Regulation of axonal translation by FMRP may shape the structure and function of the axonal arbor in mature sensory neurons in the main olfactory system but not in the accessory olfactory system.

Mammalian olfactory receptors: molecular mechanisms of odorant detection, 3D-modeling, and structure-activity relationships

This chapter describes the main characteristics of olfactory receptor (OR) genes of vertebrates, including generation of this large multigenic family and pseudogenization. OR genes are compared in relation to evolution and among species. OR gene structure and selection of a given gene for expression in an olfactory sensory neuron (OSN) are tackled. The specificities of OR proteins, their expression, and their function are presented. The expression of OR proteins in locations other than the nasal cavity is regulated by different mechanisms, and ORs display various additional functions. A conventional olfactory signal transduction cascade is observed in OSNs, but individual ORs can also mediate different signaling pathways, through the involvement of other molecular partners and depending on the odorant ligand encountered. ORs are engaged in constitutive dimers. Ligand binding induces conformational changes in the ORs that regulate their level of activity depending on odorant dose. When present, odorant binding proteins induce an allosteric modulation of OR activity. Since no 3D structure of an OR has been yet resolved, modeling has to be performed using the closest G-protein-coupled receptor 3D structures available, to facilitate virtual ligand screening using the models. The study of odorant binding modes and affinities may infer best-bet OR ligands, to be subsequently checked experimentally. The relationship between spatial and steric features of odorants and their activity in terms of perceived odor quality are also fields of research that development of computing tools may enhance.

Homotypic and heterotypic adhesion induced by odorant receptors and the β2-adrenergic receptor

In the mouse olfactory system regulated expression of a large family of G Protein-Coupled Receptors (GPCRs), the Odorant Receptors (ORs), provides each sensory neuron with a single OR identity. In the wiring of the olfactory sensory neuron projections, a complex axon sorting process ensures the segregation of >1,000 subpopulations of axons of the same OR identity into homogeneously innervated glomeruli. ORs are critical determinants in axon sorting, and their presence on olfactory axons raises the intriguing possibility that they may participate in axonal wiring through direct or indirect trans-interactions mediating adhesion or repulsion between axons. In the present work, we used a biophysical assay to test the capacity of ORs to induce adhesion of cell doublets overexpressing these receptors. We also tested the β2 Adrenergic Receptor, a non-OR GPCR known to recapitulate the functions of ORs in olfactory axon sorting. We report here the first evidence for homo- and heterotypic adhesion between cells overexpressing the ORs MOR256-17 or M71, supporting the hypothesis that ORs may contribute to olfactory axon sorting by mediating differential adhesion between axons.