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Regulation of the expression of odorant receptor genes in mouse

Final Report Summary - ORGENECHOICE (Regulation of the expression of odorant receptor genes in mouse)

The discovery of mammalian odorant receptor (OR) genes by Linda Buck and Richard Axel (Cell, 1991) and their subsequent research were recognized by the Nobel Prize in Physiology or Medicine in 2004. This proposal deals with the regulation of the expression of OR genes in the mouse. OR genes form the largest family in the mouse genome: ~1,100 genes with an intact coding sequence, plus a few hundred pseudogenes. The genes are spread over ~40 loci in the mouse genome, with most loci containing a cluster of OR genes at an average distance of 25 kilobase. There are eight OR genes that are "solitary": they do not belong to a cluster. OR genes encode polypeptides with a seven-transmembrane structure, which is a hallmark of G-protein coupled receptors. Each olfactory sensory neuron (OSN) in the main olfactory epithelium of the mouse expresses transcripts of just one OR gene (one neuron-one receptor rule), and from one allele (monoallelic expression). A given OR gene is expressed in a restricted part of the main olfactory epithelium, which is often called a "zone". The expression of one allele of one OR gene by a single OSN is typically referred to as "OR gene choice". Its mechanisms are not well understood.

We have here undertaken a genetic approach to OR gene choice in mice. We have introduced and developed the concept of probability of OR gene choice: among the 1,100 OR genes, there is a huge difference in OSN population size, i.e. in the probability with which a gene is expressed. We have shown that a regulatory element, called the P element, regulates the probability of choice for several OR genes in the adjacent cluster - but not the level of RNA transcripts per OSN, once a gene is chosen for expression (Khan et al., 2011).

We have confirmed and extending that the prototype G-protein coupled receptor, the β2-adrenergic receptor, can substitute for an OR in all its known properties including enabling signal transduction by a chemical applied in solution and the coalescence of axons of OSNs expressing this OR into a glomerulus (Omura et al., 2014). By applying the NanoString gene expression platform, we have described the temporal dynamics of OR gene expression during the aging process (Khan et al., 2013). We have also characterized the subset of OR genes that are coexpressed in OSN that express a "knockout" OR locus, ∆OR (Fuss et al., 2013).

We have identified an entirely novel neuronal cell type in the mouse main olfactory epithelium, which does not express any OR gene. These cells express Trpc2 and Gucy1b2 (Omura and Mombaerts, 2014, 2015). This discovery is particularly exciting as for 15 years Trpc2 was believed not to be expressed in the mouse main olfactory epithelium. Our findings have implications for the interpretation of the striking behavioral phenotypes of Trpc2 knockout mice.