Search

In the mouse, most mature olfactory sensory neurons (OSNs) express one allele of one gene from the repertoire of ~1100 odorant receptor (OR) genes, which encode G-protein coupled receptors (GPCRs). Axons of OSNs that express a given OR coalesce into homogeneous glomeruli, which reside at conserved positions in the olfactory bulb. ORs are intimately involved in ensuring the expression of one OR per OSN and the coalescence of OSN axons into glomeruli. But the mechanisms whereby ORs accomplish these diverse functions remain poorly understood. An experimental approach that has been informative is to substitute an OR genetically with another GPCR that is normally not expressed in OSNs, in order to determine in which aspects this GPCR can serve as surrogate OR in mouse OSNs. Thus far only the β2-adrenergic receptor (β2AR, Ardb2) has been shown to be able to serve as surrogate OR in OSNs; the β2AR could substitute for the M71 OR in all aspects examined. Can other non-olfactory GPCRs function equally well as surrogate ORs in OSNs? Here, we have generated and characterized two novel gene-targeted mouse strains in which the mouse melanocortin 4 receptor (Mc4r) or the mouse dopamine receptor D1 (Drd1a) is coexpressed with tauGFP in OSNs that express the OR locus M71. These alleles and strains are abbreviated as Mc4r → M71-GFP and Drd1a → M71-GFP. We detected strong Mc4r or Drd1a immunoreactivity in axons and dendritic knobs and cilia of OSNs that express Mc4r or Drd1a from the M71 locus. These OSNs responded physiologically to cognate agonists for Mc4r (Ro27-3225) or Drd1a (SKF81297), and not to the M71 ligand acetophenone. Axons of OSNs expressing Mc4r → M71-GFP coalesced into glomeruli. Axons of OSNs expressing Drd1a → M71-GFP converged onto restricted areas of the olfactory bulb but did not coalesce into glomeruli. Thus, OR functions in OSNs can be substituted by Mc4r or Drd1a, but not as well as by β2AR. We attribute the weak performance of Drd1a as surrogate OR to poor OSN maturation.

The mouse olfactory system employs ~1100 G‐protein‐coupled odorant receptors (ORs). Each mature olfactory sensory neuron (OSN) is thought to express just one OR gene, and the expressed OR determines the odorant response properties of the OSN. The broadest odorant response profile thus far demonstrated in native mouse OSNs is for OSNs that express the OR gene SR1 (also known as Olfr124 and MOR256‐3). Here we showed that the odorant responsiveness of native mouse OSNs expressing the OR gene MOR256‐17 (also known as Olfr15 and OR3) is even broader than that of OSNs expressing SR1. We investigated the electrophysiological properties of green fluorescent protein (GFP)+ OSNs in a MOR256‐17‐IRES‐tauGFP gene‐targeted mouse strain, in parallel with GFP+ OSNs in the SR1‐IRES‐tauGFP gene‐targeted mouse strain that we previously reported. Of 35 single chemical compounds belonging to distinct structural classes, MOR256‐17+ OSNs responded to 31 chemicals, compared with 10 for SR1+ OSNs. The 10 compounds that activated SR1+ OSNs also activated MOR256‐17+ OSNs. Interestingly, MOR256‐17+ OSNs were activated by three amines (cyclohexylamine, isopenthylamine, and phenylethylamine) that are typically viewed as ligands for chemosensory neurons in the main olfactory epithelium that express trace amine‐associated receptor genes, a family of 15 genes encoding G‐protein‐coupled receptors unrelated in sequence to ORs. We did not observe differences in membrane properties, indicating that the differences in odorant response profiles between the two OSN populations were due to the expressed OR. MOR256‐17+ OSNs appear to be at one extreme of odorant responsiveness among populations of OSNs expressing distinct OR genes in the mouse.

In the mouse, mature olfactory sensory neurons (OSNs) express one allele of one of the similar to 1200 odorant receptor (OR) genes, which encode G-protein coupled receptors (GPCRs). Axons of OSNs that express the same OR coalesce into homogeneous glomeruli at conserved positions in the olfactory bulb. ORs are involved in OR gene choice and OSN axonal wiring, but the mechanisms remain poorly understood. One approach is to substitute an OR genetically with another GPCR, and to determine in which aspects this GPCR can serve as a surrogate OR under experimental conditions. Here, we characterize a novel gene-targeted mouse strain in which the mouse beta 2-adrenergic receptor (beta 2AR) is coexpressed with tauGFP in OSNs that choose the OR locus M71 for expression (beta 2AR -> M71-GFP). By crossing these mice with beta 2AR -> M71-lacZ gene-targeted mice, we find that differentially tagged beta 2AR -> M71 alleles are expressed monoallelically. The OR coding sequence is thus not required for monoallelic expression - the expression of one of the two alleles of a given OR gene in an OSN. We detect strong (beta 2AR immunoreactivity in dendritic cilia of (beta 2AR -> M71-GFP OSNs. These OSNs respond to the beta 2AR agonist isoproterenol in a dose-dependent manner. Axons of beta 2AR -> M71-GFP OSNs coalesce into homogeneous glomeruli, and beta 2AR immunoreactivity is detectable within these glomeruli. We do not find evidence for expression of endogenous beta 2AR in OSNs of wild-type mice, also not in M71-expressing OSNs, and we do not observe overt differences in the olfactory system of beta 2AR and beta 1AR knockout mice. Our findings corroborate the experimental value of the beta 2AR as a surrogate OR, including for the study of the mechanisms of monoallelic expression. (C) 2013 Elsevier Inc All rights reserved.

Olfactory signals play an important role so that breeding efforts are synchronized with appropriate social and environmental circumstances. In this context, the mammalian olfactory system is characterized by the existence of several olfactory subsystems that have evolved to process olfactory information. While the vomeronasal (or accessory) olfactory system is usually conceived as being involved in the processing of pheromonal signals due to its close connections with the reproductive hypothalamus, the main olfactory system is, by contrast, considered as a general analyzer of volatile chemosignals, especially those that are used for the social identification of conspecifics. In fact, several recent sets of experiments suggest that both the main and accessory olfactory systems have the ability to process partly overlapping pheromonal chemosignals and that both systems converge at a downstream level of pheromonal processing. As a consequence, both systems have the ability to support complimentary aspects in mate discrimination and sexual behavior. However, the relative roles played by these systems and their interactions are at present still far from being understood.