Identification of agonists for a group of human odorant receptors

Abstract

Olfaction plays a critical role in several aspects of the human life. Odorants are detected by hundreds of odorant receptors (ORs) which belong to the superfamily of G protein-coupled receptors. These receptors are expressed in the olfactory sensory neurons of the nose. The information provided by the activation of different combinations of ORs in the nose is transmitted to the brain, leading to odorant perception and emotional and behavioral responses. There are ~400 intact human ORs, and to date only a small percentage of these receptors (~10%) have known agonists. The determination of the specificity of the human ORs will contribute to a better understanding of how odorants are discriminated by the olfactory system. In this work, we aimed to identify human specific ORs, that is, ORs that are present in humans but absent from other species, and their corresponding agonists. To do this, we first selected 22 OR gene sequences from the human genome with no counterparts in the mouse, rat or dog genomes. Then we used a heterologous expression system to screen a subset of these human ORs against a panel of odorants of biological relevance, including foodborne aroma volatiles. We found that different types of odorants are able to activate some of these previously uncharacterized human ORs.

Keywords: GPCRs; functional screening; heterologous expression; human odorant receptor; human olfactory receptor; odorant perception; odorants; orphan receptors.

Figure 1

Selection of the human odorant receptors. The nucleotide sequences of the functional human ORs were aligned against the chimpanzee, dog, mouse and rat genomes. Odorant receptor sequences that were present in humans but not in the other species were selected. Gray regions of the bars correspond to the number of human ORs that show no similarity to the ORs in the other species. Only two odorant receptor sequences are present in the human genome but not in the other four species (chimp., dog, mouse and rat), and 25 odorant receptor sequences are present in the human and chimpanzee genomes but not in the dog, mouse, and rat genomes (dog and mouse and rat).

Figure 2

Functional expression of mOR S6 in the heterologous screening system. Responses of the mouse odorant receptor mOR S6 to nonanedioic acid were tested using the heterologous screening. Three different amounts of the mORS6 expression vector were used as indicated (25, 50, and 100 ng). Tested odorant concentrations were 100, 200, and 400 μM. SEAP activity is expressed in arbitrary fluorescence units.

Figure 3

Response profiles of the human ORs to odorants. Dose response curves of the human ORs expressed in the heterologous expression system to the indicated odorants. SEAP activity was normalized as a percentage of the maximum response across a set of ORs. X-axis is the concentration of odorants in log Molar. Error bars represent s.e.m. over two replicates.

Figure 4

Relationships among the selected human ORs. Phylogenetic tree of the selected human OR amino acid sequences. The chemical structures of some of the agonists identified in this study are shown next to the corresponding responsive ORs.