Functional Analysis of a Subset of Canine Olfactory Receptor Genes

In this paper, we explored the level complexity of the combinatorial olfactory code that allows mammals with a repertoire of about thousand putatively active olfactory receptors encoded in their genomes to recognize and identify a much larger repertoire of odorant molecules. To that end, we cloned 38 canine OR genes belonging to the same OR gene family and transiently expressed them in a subclone of embryonic human kidney cells (HEK293) permanently expressing the G(olf) subunit. Using a Ca2þ imaging approach, we established for example that as many as 26 out of the 38 cloned OR elicited a Ca2þ response when exposed to octanal, whereas 10 responded to nonanal, other aldehydes providing intermediate responses. Altogether, these results demonstrated that the combinatorial code is quite complex in support to the highly developed sense of olfaction demonstrated by dogs. Olfaction is a complicated process beginning in the olfac-tory epithelium with the specific binding of volatile odorant molecules to dedicated olfactory receptors (ORs) expressed by olfactory sensory neurons (OSNs) (Buck 1996; Mombaerts 1999; Firestein 2001; Touhara 2002). OR genes constitute the largest known gene family—the olfactory subgenome. The