Molecular receptive range variation among mouse odorant receptors for aliphatic carboxylic acids

Abstract

The ability of mammals to identify and distinguish among many thousands of different odorants suggests a combinatorial use of odorant receptors, with each receptor detecting multiple odorants and each odorant interacting with multiple receptors. Numerous receptors may be devoted to the sampling of particularly important regions of odor space. In this study, we explore the similarities and differences in the molecular receptive ranges of four mouse odorant receptors (MOR23-1, MOR31-4, MOR32-11 and MOR40-4), which have previously been identified as receptors for aliphatic carboxylic acids. Each receptor was expressed in Xenopus oocytes, along with Galpha(olf) and the cystic fibrosis transmembrane regulator to allow electrophysiological assay of receptor responses. We find that even though these receptors are relatively unrelated, there is extensive overlap among their receptive ranges. That is, these receptors sample a similar region of odor space. However, the receptive range of each receptor is unique. Thus, these receptors contribute to the depth of coverage of this small region of odor space. Such a group of receptors with overlapping, but distinct receptive ranges, may participate in making fine distinctions among complex mixtures of closely related odorant compounds.

Figure 1. Functionally accurate expression of MOR174-9 (mOR-EG) and MOR267-13 (MOR23) in Xenopus oocytes

A) Left trace, an oocyte expressing MOR174-9, Gα_olf and CFTR is challenged with 15 sec applications of 0.02% DMSO, 100μM butanol (B), eugenol (EG), −carvone (−C), vanillin (Van), Geraniol (GE), ethyl vanillin (EV) and 1mM IBMX. Right trace, a different oocyte expressing MOR174-9, Gα_olf and CFTR is challenged with 100μM +carvone (+C), guaiacol (Gu), eugenol (EG) and 1mM IBMX. Scale: 1μA, 10min. B) An oocyte expressing Gα_olf and CFTR, but no receptor, is challenged with 15 sec applications of 100μM eugenol (EG), vanillin (Van), ethyl vanillin (EV) and 1mM IBMX. Scale: 1μA, 10min. C) Responses of MOR174-9 expressing oocytes to eugenol, vanillin and ethyl vanillin (each at 100μM). Current responses to each ligand are normalized as a percent of the response of the same oocyte to 1 mM IBMX (mean ± SEM, n = 6). D) An oocyte expressing MOR267-13, Gα_olf and CFTR is challenged with 15 sec applications of 0.02% DMSO, 100μM butanol (B), 30μM lyral (Ly), a mix of 100μM −carvone and 100μM +carvone (100 +/− C), 100μM eugenol (EG), ethyl vanillin (EV), geraniol (GE) and 1mM IBMX. Scale: 0.5μA, 10min. E) Left trace, an oocyte expressing MOR267-13, Gα_olf and CFTR is challenged with a 15 sec application of 30μM lyral, followed by 1mM IBMX. Scale: 0.5μA, 10min. Right trace, an oocyte expressing Gα_olf and CFTR, but no receptor, is challenged with a 15 sec application of 30μM lyral, followed by 1mM IBMX. Scale: 0.5μA, 10min.

Figure 2. A ligand screen for MOR23-1

An oocyte expressing MOR23-1, RTP1, Gα_olf and CFTR is challenged with 15 sec applications of 35 ligands from our odorant panel (each at 100 μM). Each trace starts with an application of 100μM nonanoic acid, which serves as a normalization standard. All traces are from the same oocyte. Because the nonanol application occurred before the octanol response had ended, nonanol was retested at the end of the dicarboxylic acid screen (bottom trace). Scale bars: 0.5 μA, 10 min.

Figure 3. Molecular receptive range for MOR23-1 at several ligand concentrations

Oocytes expressing MOR23-1, RTP1, Gα_olf and CFTR were screened with a panel of 41 saturated, aliphatic primary alcohols, aldehydes, monocarboxylic acids, bromocarboxylic acids and dicarboxylic acids, ranging in length from 4 to 12 carbons (15 sec applications of 100 μM, as in Fig. 2). Compounds yielding responses at 100 μM were also screened at 30 μM, 10 μM and 3 μM. Values are the mean of results from 4-8 oocytes (SEM values may be found in Table 1). Flat squares at the base of the graph indicate tested compounds or concentrations that did not yield a response. Blank areas at the base of the graph indicate compounds or concentrations that were not tested.

Figure 4. Molecular receptive ranges for MOR31-4, MOR32-11 and MOR40-4

Oocytes expressing an MOR (MOR31-4 in panel A, MOR32-11 in panel B or MOR40-4 in panel C), Gα_olf and CFTR were screened with a panel of 41 saturated, aliphatic primary alcohols, aldehydes, monocarboxylic acids, bromocarboxylic acids and dicarboxylic acids, ranging in length from 4 to 12 carbons (15 sec applications of 100 μM as in Fig. 2). Values are the mean of results from 4-16 oocytes (SEM values may be found in Table 1). Flat squares at the base of the graph indicate tested compounds or concentrations that did not yield a response. Blank areas at the base of the graph indicate compounds or concentrations that were not tested.

Figure 5. Dose-response analysis for MOR23-1, MOR31-4, MOR32-11 and MOR40-4

A) Responses of MOR23-1 to a range of octanoic acid and octanal concentrations. Responses were normalized to 3 μM octanoic acid (see Experimental Procedures). Data are the mean ± SEM (n = 4-13). B) Responses of MOR31-4 to a range of dodecanoic acid concentrations. Responses were normalized to 3 μM dodecanoic acid (see Experimental Procedures). Data are the mean ± SEM (n = 5-13). C) Responses of MOR32-11 to a range of octanoic and nonanoic acid concentrations. Responses were normalized to 30 μM octanoic acid (see Experimental Procedures). Data are the mean ± SEM (n = 7-21). D) Responses of MOR40-4 to a range of undecanal concentrations. Responses were normalized to 30 μM undecanal (see Experimental Procedures). Data are the mean ± SEM (n = 5-14).