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Humans perceive thousands of compounds as bitter. In sharp contrast, only approximately 25 taste 2 receptors (TAS2R) bitter taste receptors have been identified, raising the question as to how the vast array of bitter compounds can be detected by such a limited number of sensors. To address this issue, we have challenged 25 human taste 2 receptors (hTAS2Rs) with 104 natural or synthetic bitter chemicals in a heterologous expression system. Thirteen cognate bitter compounds for 5 orphan receptors and 64 new compounds for previously identified receptors were discovered. Whereas some receptors recognized only few agonists, others displayed moderate or extreme tuning broadness. Thus, 3 hTAS2Rs together were able to detect approximately 50% of the substances used. Conversely, though 63 bitter substances activated only 1-3 receptors, 19 compounds stimulated up to 15 hTAS2Rs. Our data suggest that the detection of the numerous bitter chemicals is related to the molecular receptive ranges of hTAS2Rs.

The capacity of the mammalian olfactory system to detect an enormous collection of different chemical compounds is based on a large repertoire of odorant receptors (ORs). A small group of these ORs, the OR37 family, is unique due to a variety of special features. Members of this subfamily are exclusively found in mammals, they share a high degree of sequence homology and are highly conserved during evolution. It is still elusive which odorants may activate these atypical receptors. We have reasoned that compounds from skin, hairs, or skin glands might be potential candidates. We have exposed mice to such compounds and monitored activation of glomeruli through the expression of the activity marker c-fos in juxtaglomerular cells surrounding ventrally positioned glomeruli in the olfactory bulb (OB). Employing this methodology it was found that stimulation with long-chain alkanes elicits activation in the ventral part of the OB, however, none of the OR37 glomeruli. Analyses of long-chain hydrocarbon compounds with different functional groups revealed that long-chain aliphatic aldehydes elicited an activation of defined OR37 glomeruli, each of them responding preferentially to an aldehyde with different chain lengths. These results indicate that OR37 receptors may be tuned to distinct fatty aldehydes with a significant degree of ligand specificity.

A family of 25 G protein-coupled receptors, TAS2Rs, mediates bitter taste in humans. Many of the members of this family are coexpressed in a subpopulation of taste receptor cells on the tongue, thereby allowing the possibility of receptor-receptor interactions with potential influences on their function. In this study, we used several experimental approaches to investigate whether TAS2Rs can form oligomers and if this has an effect on receptor function. Coimmunoprecipitations clearly demonstrated that TAS2Rs can physically interact in HEK293T cells. Further bioluminescence resonance energy transfer analysis of all 325 possible binary combinations of TAS2Rs established that the vast majority of TAS2R pairs form oligomers, both homomers and heteromers. Subsequent screenings of coexpressed bitter receptors with 104 different tastants did not reveal any heteromer-specific agonists. Additional studies also showed no obvious influence of TAS2R hetero-oligomerization on plasma membrane localization or pharmacological properties of the receptors. Thus, our results show that receptor oligomerization occurs between TAS2R bitter taste receptors; however, functional consequences of hetero-oligomerization were not obvious.

Bitter taste perception is innate and induces aversive reactions. Since numerous harmful compounds, including secondary plant metabolites, synthetic chemicals, inorganic ions and rancid fats, do taste bitter, this basic taste modality may be considered as a defence mechanism against the ingestion of potential poisons. For a complete understanding of this defence mechanism it is obligatory to identify and characterize the chemical detectors of the bitter compounds, which display the remarkable ability to recognize thousands of different chemicals. Screening of the genome data bases ultimately led to the discovery of a novel gene family of ∼40 members in mice and ∼30 in humans. Some of the genes identified by this approach are located within chromosomal loci associated with tasting various distinct bitter compounds (Adler et al., 2000; Matsunami et al., 2000; Bufe et al., 2002). Therefore, these genes encoding G-proteincoupled receptors, TAS2Rs (previously referred to as T2Rs or TRBs), have been suggested to represent bitter taste receptors. Several lines of independent evidence further support this assumption (Adler et al., 2000; Chandrashekar et al., 2000; Matsunami et al., 2000). First, the expression pattern of these genes on the rodent tongue was consistent with that of bitter taste receptors. Secondly, functional expression studies identified the bitter compound cycloheximide as an agonist for mTAS2R105. Thirdly, mice strains with impaired cycloheximide tasting have variant mTAS2R105 genes encoding receptors that are less responsive to cycloheximide. Fourthly, mTAS2R105 couples in vitro to α-gustducin (Chandrashekar et al., 2000), a G-protein α-subunit expressed in taste tissue that has amply been shown before to play a role in bitter taste transduction (Margolskee, 2002). Although this question has not been directly addressed, these investigations led to the impression of a narrow tuning of bitter taste receptors and raised the intriguing problem of how organisms that are equipped with a limited number of TAS2R genes are able to perceive numerous chemicals bitter. In the present report we address two questions that are important for the understanding of bitter taste in general and human bitter taste in particular. First, given the largely uncharacterized receptor repertoire, are all TAS2Rs true bitter taste receptors and, secondly, can their broad tuning explain how humans equipped with a fairly small number of TAS2R genes are able to perceive thousands of bitter compounds.