Coordination of two enhancers drives expression of olfactory trace amine-associated receptors VII

Olfactory sensory neurons (OSNs) are functionally defined by their expression of a unique odorant receptor (OR). Mechanisms underlying singular OR expression are well studied, and involve a massive cross-chromosomal enhancer interaction network. Trace amine-associated receptors (TAARs) form a distinct family of olfactory receptors, and here we find that mechanisms regulating Taar gene choice display many unique features. The epigenetic signature of Taar genes in TAAR OSNs is different from that in OR OSNs. We further identify that two TAAR enhancers conserved across placental mammals are absolutely required for expression of the entire Taar gene repertoire. Deletion of either enhancer dramatically decreases the expression probabilities of different Taar genes, while deletion of both enhancers completely eliminates the TAAR OSN populations. In addition, both of the enhancers are sufficient to drive transgene expression in the partially overlapped TAAR OSNs. We also show that the TAAR enhancers operate in cis to regulate Taar gene expression. Our findings reveal a coordinated control of Taar gene choice in OSNs by two remote enhancers, and provide an excellent model to study molecular mechanisms underlying formation of an olfactory subsystem. In order to gain genetic access to TAAR-expressing OSNs, we generated Taar5-ires-Cre and Taar6-ires-Cre knockin mouse lines, in which Cre recombinase is co-transcribed with Taar5 and Taar6 gene, respectively. Each Cre line was crossed with Cre-dependent reporter lines, including lox-L10-GFP and lox-ZsGreen, to fluorescently label the specific population of TAAR OSNs. The GFP- or ZsGreen-negative cells were also sorted to serve as control cells, approximately 70-80% of which are composed of OR-expressing OSNs.