MouSensor: A Versatile Genetic Platform to Create Super Sniffer Mice for Studying Human Odor Coding

Abstract

Typically, ∼0.1% of the total number of olfactory sensory neurons (OSNs) in the main olfactory epithelium express the same odorant receptor (OR) in a singular fashion and their axons coalesce into homotypic glomeruli in the olfactory bulb. Here, we have dramatically increased the total number of OSNs expressing specific cloned OR coding sequences by multimerizing a 21-bp sequence encompassing the predicted homeodomain binding site sequence, TAATGA, known to be essential in OR gene choice. Singular gene choice is maintained in these "MouSensors." In vivo synaptopHluorin imaging of odor-induced responses by known M71 ligands shows functional glomerular activation in an M71 MouSensor. Moreover, a behavioral avoidance task demonstrates that specific odor detection thresholds are significantly decreased in multiple transgenic lines, expressing mouse or human ORs. We have developed a versatile platform to study gene choice and axon identity, to create biosensors with great translational potential, and to finally decode human olfaction.

Single olfactory receptors set odor detection thresholds

“…Correlative studies in a number of species have noted a parallel between increasing OSN number and higher sensitivity 42,43 . Likewise, experimental overexpression of two different ORs in mice (Olfr151 or OR1A1) increased sensitivity as measured using an aversion-based, two bottle preference task 24 . On the other hand, a more pronounced over-expression (>95% of all OSNs) of an OR (Olfr2) yielded mice that did not differ in their investigation time to the corresponding ligand, octanal 44 .…”

“…Therefore, we asked whether increasing the number of OSNs and glomeruli for the threshold-determining receptor TAAR4 can enhance behavioral sensitivity. We generated transgenic mice in which the number of OSNs that express TAAR4 is significantly increased by driving expression from an OR transgene (5x21-TAAR4Tg) that includes a gene choice-promoting enhancer 24 (Figure 6a). Several independent mouse lines showed over-expression of TAAR4 throughout the olfactory epithelium, and supernumerary glomeruli in the olfactory bulb (Figure 6a).…”

“…The OMP-GCaMP3 strain is available from The Jackson laboratory (Stock #029581). 5x21-TAAR4Tg : A transgene consisting of a 6.8 kb fragment of the M71 locus was modified by 1) insertion of a 5x homeodomain sequence 24 placed 490 bp upstream of the transcription start site, and 2) by replacing the M71 coding sequence with the coding sequence for mouse TAAR4 flanked by Asc I sites and preceded by a Kozak consensus sequence (GGCGCGCCACCATG). The coding sequence was followed by an internal ribosome entry site and tauCherry (a fusion of bovine tau and monomeric cherry fluorescent protein 51 .…”

Variation in olfactory neuron repertoires is genetically controlled and environmentally modulated

“…Instead, our data are consistent with a model where non-uniform probabilities of OR choice are instructed by genetic variation in both specific OR promoters and enhancers. Supporting this model, we identified many putative promoters for differentially represented ORs where genetic variation has altered the number of Olf1/Ebf1 (O/E) and homeodomain (HD) transcription factors binding sites between the mouse strains, both sequences known to influence the probability of OR choice (D'Hulst et al, 2016; Vassalli et al, 2011). Moreover, through analysis of F1 hybrids, we confirmed the finding that the probability of choice of OR genes linked to the H element are regulated in cis (Fuss et al, 2007), and extended this to over 800 additional OR genes distributed throughout the genome that show little to no evidence in support of trans -acting regulation.…”

“…However, in most cases, these OR coding genetic variants explain only a small proportion of the observed phenotypic variance (reviewed in Logan [2014]), suggesting that other factors contribute to individual differences in perception. Recently, it has been demonstrated that increasing the number of a particular OSN subtype in a mouse nose increases olfactory sensitivity to its ligand (D'Hulst et al, 2016). Therefore, the very different OSN repertoires present between strains of mice are likely to result in significant phenotypic variation in olfactory thresholds, and thus contribute to the individualization of olfaction.…”