Exitron splicing of odor receptor genes in <i>Drosophila</i>.

Abstract

Proper expression of odor receptor genes is critical for the function of olfactory systems. In this study, we identified exitrons (exonic introns) in four of the 39 <i>Odorant receptor</i> (<i>Or</i>) genes expressed in the <i>Drosophila</i> antenna. Exitrons are sequences that can be spliced out from within a protein-coding exon, thereby altering the encoded protein. We focused on <i>Or88a</i>, which encodes a pheromone receptor, and found that exitron splicing of <i>Or88a</i> is conserved across five <i>Drosophila</i> species over 20 My of evolution. The exitron was spliced out in 15% of <i>Or88a</i> transcripts. Removal of this exitron creates a non-coding RNA rather than an RNA that encodes a stable protein. Our results suggest the hypothesis that in the case of <i>Or88a</i>, exitron splicing could act in neuronal modulation by decreasing the level of functional <i>Or</i> transcripts. Activation of <i>Or88a</i>-expressing olfactory receptor neurons via either optogenetics or pheromone stimulation increased the level of exitron-spliced transcripts, with optogenetic activation leading to a 14-fold increase. A fifth <i>Or</i> can also undergo an alternative splicing event that eliminates most of the canonical open reading frame. Besides these cases of alternative splicing, we found alternative polyadenylation of four <i>Ors</i>, and exposure of Or67c to its ligand ethyl lactate in the antenna downregulated all of its 3' isoforms. Our study reveals mechanisms by which neuronal activity could be modulated via regulation of the levels of <i>Or</i> isoforms.