Molecular Bases of Odor Discrimination: Reconstitution of Olfactory Receptors that Recognize Overlapping Sets of Odorants

TLDR

The vertebrate olfactory system discriminates a wide variety of odorants by relaying coded information from olfactory sensory neurons in the epithelium to cortical areas, and recent studies show that this first step is mediated by roughly 1,000 distinct olfactory receptors, the largest family of G‑protein‑coupled receptors. The study aimed to identify odor‑responsive mouse olfactory neurons and clone the corresponding receptor gene using Ca²⁺ imaging and single‑cell reverse transcription‑PCR. The authors employed Ca²⁺ imaging and single‑cell RT‑PCR to isolate responsive neurons and clone the receptor gene. Cloned receptors expressed in heterologous cells demonstrated that structurally related olfactory receptors recognize overlapping odorants with distinct affinities, supporting a receptor code in which odor identities are specified by unique combinations of receptors whose receptive ranges vary with concentration and whose coupling to Gαolf in HEK293 cells drives odorant‑dependent cAMP increases, thereby showing that odor discrimination is governed by receptor receptive‑range differences.

Abstract

The vertebrate olfactory system discriminates a wide variety of odorants by relaying coded information from olfactory sensory neurons in the olfactory epithelium to olfactory cortical areas of the brain. Recent studies have shown that the first step in odor discrimination is mediated by approximately 1000 distinct olfactory receptors, which comprise the largest family of G-protein-coupled receptors. In the present study, we used Ca(2+) imaging and single-cell reverse transcription-PCR techniques to identify mouse olfactory neurons responding to an odorant and subsequently to clone a receptor gene from the responsive cell. The functionally cloned receptors were expressed in heterologous systems, demonstrating that structurally related olfactory receptors recognized overlapping sets of odorants with distinct affinities and specificities. Our results provide direct evidence for the existence of a receptor code in which the identities of different odorants are specified by distinct combinations of odorant receptors that possess unique molecular receptive ranges. We further demonstrate that the receptor code for an odorant changes with odorant concentration. Finally, we show that odorant receptors in human embryonic kidney 293 cells couple to stimulatory G-proteins such as Galphaolf, resulting in odorant-dependent increases in cAMP. Odor discrimination is thus determined by differences in the receptive ranges of the odorant receptors that together encode specific odorant molecules.