A singular shark bitter taste receptor provides insights into the evolution of bitter taste perception.

Many animal and plant species synthesize toxic compounds as deterrent; thus, detection of these compounds is of vital importance to avoid their ingestion. Often, such compounds are recognized by taste 2 receptors that mediate bitter taste in humans. Until now, bitter taste receptors have only been found in bony vertebrates, where they occur as a large family already in coelacanth, a "living fossil" and the earliest-diverging extant lobe-finned fish. Here, we have revisited the evolutionary origin of taste 2 receptors (T2Rs) making use of a multitude of recently available cartilaginous fish genomes. We have identified a singular T2R in 12 cartilaginous fish species (9 sharks, 1 sawfish, and 2 skates), which represents a sister clade to all bony fish T2Rs. We have examined its ligands for two shark species, a catshark and a bamboo shark. The ligand repertoire of bamboo shark represents a subset of that of the catshark, with roughly similar thresholds. Amarogentin, one of the most bitter natural substances for humans, also elicited the highest signal amplitudes with both shark receptors. Other subsets of ligands are shared with basal bony fish T2Rs indicating an astonishing degree of functional conservation over nearly 500 mya of separate evolution. Both shark receptors respond to endogenous steroids as well as xenobiotic compounds, whereas separate receptors exist for xenobiotics both in early- and late-derived bony vertebrates (coelacanth, zebrafish, and human), consistent with the shark T2R reflecting the original ligand repertoire of the ancestral bitter taste receptor at the evolutionary origin of this family.

Membrane-bound chemoreception of bitter bile acids and peptides is mediated by the same subset of bitter taste receptors.

The vertebrate sense of taste allows rapid assessment of the nutritional quality and potential presence of harmful substances prior to ingestion. Among the five basic taste qualities, salty, sour, sweet, umami, and bitter, bitterness is associated with the presence of putative toxic substances and elicits rejection behaviors in a wide range of animals including humans. However, not all bitter substances are harmful, some are thought to be health-beneficial and nutritious. Among those compound classes that elicit a bitter taste although being non-toxic and partly even essential for humans are bitter peptides and L-amino acids. Using functional heterologous expression assays, we observed that the 5 dominant human bitter taste receptors responsive to bitter peptides and amino acids are activated by bile acids, which are notorious for their extreme bitterness. We further demonstrate that the cross-reactivity of bitter taste receptors for these two different compound classes is evolutionary conserved and can be traced back to the amphibian lineage. Moreover, we show that the cross-detection by some receptors relies on "structural mimicry" between the very bitter peptide L-Trp-Trp-Trp and bile acids, whereas other receptors exhibit a phylogenetic conservation of this trait. As some bile acid-sensitive bitter taste receptor genes fulfill dual-roles in gustatory and non-gustatory systems, we suggest that the phylogenetic conservation of the rather surprising cross-detection of the two substance classes could rely on a gene-sharing-like mechanism in which the non-gustatory function accounts for the bitter taste response to amino acids and peptides.

Pharmacology of TAS1R2/TAS1R3 Receptors and Sweet Taste.

The detection of energy-rich sweet food items has been important for our survival during evolution, however, in light of the changing lifestyles in industrialized and developing countries our natural sweet preference is causing considerable problems. Hence, it is even more important to understand how our sense of sweetness works, and perhaps even, how we may deceive it for our own benefit. This chapter summarizes current knowledge about sweet tastants and sweet taste modulators on the compound side as well as insights into the structure and function of the sweet taste receptor and the transduction of sweet signals. Moreover, methods to assess the activity of sweet substances in vivo and in vitro are compared and discussed.

Bitter taste receptors of the common vampire bat are functional and show conserved responses to metal ions in vitro.

The bitter taste sensation is important to warn mammals of the ingestion of potentially toxic food compounds. For mammals, whose nutrition relies on highly specific food sources, such as blood in the case of vampire bats, it is unknown if bitter sensing is involved in prey selection. By contrast to other bat species, vampire bats exhibit numerous bitter taste receptor pseudogenes, which could point to a decreased importance of bitter taste. However, electrophysiological and behavioural studies suggest the existence of functional bitter taste transmission. To determine the agonist spectra of the three bitter taste receptors that are conserved in all three vampire bat species, we investigated the in vitro activation of Desmodus rotundus T2R1, T2R4 and T2R7. Using a set of 57 natural and synthetic bitter compounds, we were able to identify agonists for all three receptors. Hence, we confirmed a persisting functionality and, consequently, a putative biological role of bitter taste receptors in vampire bats. Furthermore, the activation of the human TAS2R7 by metal ions is shown to be conserved in D. rotundus.

Rational design of agonists for bitter taste receptor TAS2R14: from modeling to bench and back.

Human bitter taste receptors (TAS2Rs) are a subfamily of 25 G protein-coupled receptors that mediate bitter taste perception. TAS2R14 is the most broadly tuned bitter taste receptor, recognizing a range of chemically diverse agonists with micromolar-range potency. The receptor is expressed in several extra-oral tissues and is suggested to have physiological roles related to innate immune responses, male fertility, and cancer. Higher potency ligands are needed to investigate TAS2R14 function and to modulate it for future clinical applications. Here, a structure-based modeling approach is described for the design of TAS2R14 agonists beginning from flufenamic acid, an approved non-steroidal anti-inflammatory analgesic that activates TAS2R14 at sub-micromolar concentrations. Structure-based molecular modeling was integrated with experimental data to design new TAS2R14 agonists. Subsequent chemical synthesis and in vitro profiling resulted in new TAS2R14 agonists with improved potency compared to the lead. The integrated approach provides a validated and refined structural model of ligand-TAS2R14 interactions and a general framework for structure-based discovery in the absence of closely related experimental structures.

Segregated Expression of ENaC Subunits in Taste Cells.

Salt taste is one of the 5 basic taste qualities. Depending on the concentration, table salt is perceived either as appetitive or aversive, suggesting the contribution of several mechanisms to salt taste, distinguishable by their sensitivity to the epithelial sodium channel (ENaC) blocker amiloride. A taste-specific knockout of the α-subunit of the ENaC revealed the relevance of this polypeptide for low-salt transduction, whereas the response to other taste qualities remained normal. The fully functional ENaC is composed of α-, ß-, and γ-subunits. In taste tissue, however, the precise constitution of the channel and the cell population responsible for detecting table salt remain uncertain. In order to examine the cells and subunits building the ENaC, we generated mice carrying modified alleles allowing the synthesis of green and red fluorescent proteins in cells expressing the α- and ß-subunit, respectively. Fluorescence signals were detected in all types of taste papillae and in taste buds of the soft palate and naso-incisor duct. However, the lingual expression patterns of the reporters differed depending on tongue topography. Additionally, immunohistochemistry for the γ-subunit of the ENaC revealed a lack of overlap between all potential subunits. The data suggest that amiloride-sensitive recognition of table salt is unlikely to depend on the classical ENaCs formed by α-, ß-, and γ-subunits and ask for a careful investigation of the channel composition.

Caffeine induces gastric acid secretion via bitter taste signaling in gastric parietal cells.

Caffeine, generally known as a stimulant of gastric acid secretion (GAS), is a bitter-tasting compound that activates several taste type 2 bitter receptors (TAS2Rs). TAS2Rs are expressed in the mouth and in several extraoral sites, e.g., in the gastrointestinal tract, in which their functional role still needs to be clarified. We hypothesized that caffeine evokes effects on GAS by activation of oral and gastric TAS2Rs and demonstrate that caffeine, when administered encapsulated, stimulates GAS, whereas oral administration of a caffeine solution delays GAS in healthy human subjects. Correlation analysis of data obtained from ingestion of the caffeine solution revealed an association between the magnitude of the GAS response and the perceived bitterness, suggesting a functional role of oral TAS2Rs in GAS. Expression of TAS2Rs, including cognate TAS2Rs for caffeine, was shown in human gastric epithelial cells of the corpus/fundus and in HGT-1 cells, a model for the study of GAS. In HGT-1 cells, various bitter compounds as well as caffeine stimulated proton secretion, whereby the caffeine-evoked effect was (i) shown to depend on one of its cognate receptor, TAS2R43, and adenylyl cyclase; and (ii) reduced by homoeriodictyol (HED), a known inhibitor of caffeine's bitter taste. This inhibitory effect of HED on caffeine-induced GAS was verified in healthy human subjects. These findings (i) demonstrate that bitter taste receptors in the stomach and the oral cavity are involved in the regulation of GAS and (ii) suggest that bitter tastants and bitter-masking compounds could be potentially useful therapeutics to regulate gastric pH.

Structure-Function Analyses of Human Bitter Taste Receptors-Where Do We Stand?

The finding that bitter taste receptors are expressed in numerous tissues outside the oral cavity and fulfill important roles in metabolic regulation, innate immunity and respiratory control, have made these receptors important targets for drug discovery. Efficient drug discovery depends heavily on detailed knowledge on structure-function-relationships of the target receptors. Unfortunately, experimental structures of bitter taste receptors are still lacking, and hence, the field relies mostly on structures obtained by molecular modeling combined with functional experiments and point mutageneses. The present article summarizes the current knowledge on the structure-function relationships of human bitter taste receptors. Although these receptors are difficult to express in heterologous systems and their homology with other G protein-coupled receptors is very low, detailed information are available at least for some of these receptors.

The human bitter taste receptor TAS2R7 facilitates the detection of bitter salts.

The human sense of taste is devoted to the analysis of the chemical composition of food prior to ingestion. Among the five basic taste qualities bitter taste perception is believed to avoid ingestion of potentially toxic substances. The receptors facilitating the detection of hundreds of chemically different bitter compounds belong to the taste 2 receptor (TAS2R) family, which are part of the G protein-coupled superfamily. Although the chemical classes of bitter compounds that have been identified as agonists of one of the 25 potentially functional human bitter taste receptors cover an enormous chemical space, one distinct group of bitter compounds, the bitter salts have not been assigned to any bitter taste receptor. To close this gap, we screened the entire human bitter taste receptor repertoire by functional calcium mobilization assays with the most famous bitter salt, magnesium sulfate, also known as Epsom salt. Although the profound pharmacological activity and the bitter taste of spring water containing magnesium sulfate has been known since 1697, the molecular basis for its taste has not been elucidated until now. Our screening resulted in the identification of a single receptor, the TAS2R7, responding to magnesium sulfate at concentrations humans perceive this salt as bitter. Subsequently, TAS2R7 was stimulated with other salts and it was found that this receptor also responds to manganese2+ and iron2+ ions, but not to potassium ions. Magnesium sulfate is known to exert a number of beneficial effects on the human body and thus, has been used as medicine against premature uterine contractions, as anti-arrhythmic drug and as laxative, however, magnesium sulfate overdosage can result in cardiac arrest and thus have fatal consequences. Therefore, it appears reasonable that nature placed TAS2R7 as sentinel for high concentrations of bitter salts on our tongues.

Expression profiling of Tas2r genes reveals a complex pattern along the mouse GI tract and the presence of Tas2r131 in a subset of intestinal Paneth cells.

The chemical variability of the intestinal lumen requires the presence of molecular receptors detecting the various substances naturally occurring in the diet and as a result of the activity of the microbiota. Despite their early discovery, intestinal bitter taste receptors (Tas2r) have not yet been assigned an unambiguous physiological function. Recently, using a CRE-recombinant approach we showed that the Tas2r131 gene is expressed in a subset of mucin-producing goblet cells in the colon of mice. Moreover, we also demonstrated that the expression of the Tas2r131 locus is not restricted to this region. In the present study we aimed at characterizing the presence of positive cells also in other gastrointestinal regions. Our results show that Tas2r131+ cells appear in the jejunum and the ileum, and are absent from the stomach and the duodenum. We identified the positive cells as a subpopulation of deep-crypt Paneth cells in the ileum, strengthening the notion of a defensive role for Tas2rs in the gut. To get a broader perspective on the expression of bitter taste receptors in the alimentary canal, we quantified the expression of all 35 Tas2r genes along the gastrointestinal tract by qRT-PCR. We discovered that the number and expression level of Tas2r genes profoundly vary along the alimentary canal, with the stomach and the colon expressing the largest subsets.

Beyond the Flavour: The Potential Druggability of Chemosensory G Protein-Coupled Receptors.

G protein-coupled receptors (GPCRs) belong to the largest class of drug targets. Approximately half of the members of the human GPCR superfamily are chemosensory receptors, including odorant receptors (ORs), trace amine-associated receptors (TAARs), bitter taste receptors (TAS2Rs), sweet and umami taste receptors (TAS1Rs). Interestingly, these chemosensory GPCRs (csGPCRs) are expressed in several tissues of the body where they are supposed to play a role in biological functions other than chemosensation. Despite their abundance and physiological/pathological relevance, the druggability of csGPCRs has been suggested but not fully characterized. Here, we aim to explore the potential of targeting csGPCRs to treat diseases by reviewing the current knowledge of csGPCRs expressed throughout the body and by analysing the chemical space and the drug-likeness of flavour molecules.

Probing the Evolutionary History of Human Bitter Taste Receptor Pseudogenes by Restoring Their Function.

Lineage-specific gene losses can be driven by selection or environmental adaptations. However, a lack of studies on the original function of species-specific pseudogenes leaves a gap in our understanding of their role in evolutionary histories. Pseudogenes are of particular relevance for taste perception genes, which encode for receptors that confer the ability to both identify nutritionally valuable substances and avoid potentially harmful substances. To explore the role of bitter taste pseudogenization events in human origins, we restored the open reading frames of the three human-specific pseudogenes and synthesized the reconstructed functional hTAS2R2, hTAS2R62 and hTAS2R64 receptors. We have identified ligands that differentially activate the human and chimpanzee forms of these receptors and several other human functional TAS2Rs. We show that these receptors are narrowly tuned, suggesting that bitter-taste sensitivities evolved independently in different species, and that these pseudogenization events occurred because of functional redundancy. The restoration of function of lineage-specific pseudogenes can aid in the reconstruction of their evolutionary history, and in understanding the forces that led to their pseudogenization.

Structure-Function Relationships of Olfactory and Taste Receptors.

The field of chemical senses has made major progress in understanding the cellular mechanisms of olfaction and taste in the past 2 decades. However, the molecular understanding of odor and taste recognition is still lagging far behind and will require solving multiple structures of the relevant full-length receptors in complex with native ligands to achieve this goal. However, the development of multiple complimentary strategies for the structure determination of G protein-coupled receptors (GPCRs) makes this goal realistic. The common conundrum of how multi-specific receptors that recognize a large number of different ligands results in a sensory perception in the brain will only be fully understood by a combination of high-resolution receptor structures and functional studies. This review discusses the first steps on this pathway, including biochemical and physiological assays, forward genetics approaches, molecular modeling, and the first steps towards the structural biology of olfactory and taste receptors.

Reengineering the ligand sensitivity of the broadly tuned human bitter taste receptor TAS2R14.

BACKGROUND: In humans, bitterness perception is mediated by ~25 bitter taste receptors present in the oral cavity. Among these receptors three, TAS2R10, TAS2R14 and TAS2R46, exhibit extraordinary wide agonist profiles and hence contribute disproportionally high to the perception of bitterness. Perhaps the most broadly tuned receptor is the TAS2R14, which may represent, because of its prominent expression in extraoral tissues, a receptor of particular importance for the physiological actions of bitter compounds beyond taste. METHODS: To investigate how the architecture and composition of the TAS2R14 binding pocket enables specific interactions with a complex array of chemically diverse bitter agonists, we carried out homology modeling and ligand docking experiments, subjected the receptor to point-mutagenesis of binding site residues and performed functional calcium mobilization assays. RESULTS: In total, 40 point-mutated receptor constructs were generated to investigate the contribution of 19 positions presumably located in the receptor's binding pocket to activation by 7 different TAS2R14 agonists. All investigated positions exhibited moderate to pronounced agonist selectivity. CONCLUSIONS: Since numerous modifications of the TAS2R14 binding pocket resulted in improved responses to individual agonists, we conclude that this bitter taste receptor might represent a suitable template for the engineering of the agonist profile of a chemoreceptive receptor. GENERAL SIGNIFICANCE: The detailed structure-function analysis of the highly promiscuous and widely expressed TAS2R14 suggests that this receptor must be considered as potentially frequent target for known and novel drugs including undesired off-effects.

Receptor Polymorphism and Genomic Structure Interact to Shape Bitter Taste Perception.

The ability to taste bitterness evolved to safeguard most animals, including humans, against potentially toxic substances, thereby leading to food rejection. Nonetheless, bitter perception is subject to individual variations due to the presence of genetic functional polymorphisms in bitter taste receptor (TAS2R) genes, such as the long-known association between genetic polymorphisms in TAS2R38 and bitter taste perception of phenylthiocarbamide. Yet, due to overlaps in specificities across receptors, such associations with a single TAS2R locus are uncommon. Therefore, to investigate more complex associations, we examined taste responses to six structurally diverse compounds (absinthin, amarogentin, cascarillin, grosheimin, quassin, and quinine) in a sample of the Caucasian population. By sequencing all bitter receptor loci, inferring long-range haplotypes, mapping their effects on phenotype variation, and characterizing functionally causal allelic variants, we deciphered at the molecular level how a subjects' genotype for the whole-family of TAS2R genes shapes variation in bitter taste perception. Within each haplotype block implicated in phenotypic variation, we provided evidence for at least one locus harboring functional polymorphic alleles, e.g. one locus for sensitivity to amarogentin, one of the most bitter natural compounds known, and two loci for sensitivity to grosheimin, one of the bitter compounds of artichoke. Our analyses revealed also, besides simple associations, complex associations of bitterness sensitivity across TAS2R loci. Indeed, even if several putative loci harbored both high- and low-sensitivity alleles, phenotypic variation depended on linkage between these alleles. When sensitive alleles for bitter compounds were maintained in the same linkage phase, genetically driven perceptual differences were obvious, e.g. for grosheimin. On the contrary, when sensitive alleles were in opposite phase, only weak genotype-phenotype associations were seen, e.g. for absinthin, the bitter principle of the beverage absinth. These findings illustrate the extent to which genetic influences on taste are complex, yet arise from both receptor activation patterns and linkage structure among receptor genes.

Comprehensive Analysis of Mouse Bitter Taste Receptors Reveals Different Molecular Receptive Ranges for Orthologous Receptors in Mice and Humans.

One key to animal survival is the detection and avoidance of potentially harmful compounds by their bitter taste. Variable numbers of taste 2 receptor genes expressed in the gustatory end organs enable bony vertebrates (Euteleostomi) to recognize numerous bitter chemicals. It is believed that the receptive ranges of bitter taste receptor repertoires match the profiles of bitter chemicals that the species encounter in their diets. Human and mouse genomes contain pairs of orthologous bitter receptor genes that have been conserved throughout evolution. Moreover, expansions in both lineages generated species-specific sets of bitter taste receptor genes. It is assumed that the orthologous bitter taste receptor genes mediate the recognition of bitter toxins relevant for both species, whereas the lineage-specific receptors enable the detection of substances differently encountered by mice and humans. By challenging 34 mouse bitter taste receptors with 128 prototypical bitter substances in a heterologous expression system, we identified cognate compounds for 21 receptors, 19 of which were previously orphan receptors. We have demonstrated that mouse taste 2 receptors, like their human counterparts, vary greatly in their breadth of tuning, ranging from very broadly to extremely narrowly tuned receptors. However, when compared with humans, mice possess fewer broadly tuned receptors and an elevated number of narrowly tuned receptors, supporting the idea that a large receptor repertoire is the basis for the evolution of specialized receptors. Moreover, we have demonstrated that sequence-orthologous bitter taste receptors have distinct agonist profiles. Species-specific gene expansions have enabled further diversification of bitter substance recognition spectra.

Genetic Labeling of Car4-expressing Cells Reveals Subpopulations of Type III Taste Cells.

Carbonic anhydrases form an enzyme family of 16 members, which reversibly catalyze the hydration of carbon dioxide to bicarbonate and protons. In lung, kidney, and brain, presence of carbonic anhydrases is associated with protons and bicarbonate transport in capillary endothelium of lung, reabsorption of bicarbonate in proximal renal tubules, and extracellular buffering. In contrast, their role in taste is less clear. Recently, carbonic anhydrase IV expression was detected in sour-sensing presynaptic taste cells and was associated with the taste of carbonation, yet the precise role and cell population remained uncertain. To examine the role of carbonic anhydrase 4-expressing cells in taste reception, we generated a mouse strain carrying a modified allele of the carbonic anhydrase 4 gene in which the coding region of the red fluorescent protein monomeric Cherry is attached to that of carbonic anhydrase 4 via an internal ribosome entry site. Monomeric Cherry fluorescence was detected in lingual papillae as well as taste buds of soft palate and naso-incisor duct. However, expression patterns on the tongue differ between posterior and fungiform papillae. Whereas monomeric Cherry auto-fluorescence was almost always co-localized with presynaptic cell markers aromatic L-amino-acid decarboxylase, synaptosomal-associated protein 25 or glutamic acid decarboxylase 67 in fungiform papillae and taste buds of palate and naso-incisor duct, monomeric Cherry-positive cells in posterior tongue papillae represent only a subpopulation of presynaptic cells. We conclude that this model is well suited for detailed investigation into the role of carbonic anhydrase in gustation and other processes.

From Cell to Beak: In-Vitro and In-Vivo Characterization of Chicken Bitter Taste Thresholds.

Bitter taste elicits an aversive reaction, and is believed to protect against consuming poisons. Bitter molecules are detected by the Tas2r family of G-protein-coupled receptors, with a species-dependent number of subtypes. Chickens demonstrate bitter taste sensitivity despite having only three bitter taste receptors-ggTas2r1, ggTas2r2 and ggTas2r7. This minimalistic bitter taste system in chickens was used to determine relationships between in-vitro (measured in heterologous systems) and in-vivo (behavioral) detection thresholds. ggTas2r-selective ligands, nicotine (ggTas2r1), caffeine (ggTas2r2), erythromycin and (+)-catechin (ggTas2r7), and the Tas2r-promiscuous ligand quinine (all three ggTas2rs) were studied. Ligands of the same receptor had different in-vivo:in-vitro ratios, and the ggTas2r-promiscuous ligand did not exhibit lower in-vivo:in-vitro ratios than ggTas2r-selective ligands. In-vivo thresholds were similar or up to two orders of magnitude higher than the in-vitro ones.

Copy Number Variation in TAS2R Bitter Taste Receptor Genes: Structure, Origin, and Population Genetics.

Bitter taste receptor genes (TAS2Rs) harbor extensive diversity, which is broadly distributed across human populations and strongly associated with taste response phenotypes. The majority of TAS2R variation is composed of single-nucleotide polymorphisms. However, 2 closely positioned loci at 12p13, TAS2R43 and -45, harbor high-frequency deletion (Δ) alleles in which genomic segments are absent, resulting in copy number variation (CNV). To resolve their chromosomal structure and organization, we generated maps using long-range contig alignments and local sequencing across the TAS2R43-45 region. These revealed that the deletion alleles (43Δ and 45Δ) are 37.8 and 32.2kb in length, respectively and span the complete coding region of each gene (~1kb) along with extensive up- and downstream flanking sequence, producing separate CNVs at the 2 loci. Comparisons with a chimpanzee genome, which contained intact homologs of TAS2R43, -45, and nearby TAS2Rs, indicated that the deletions evolved recently, through unequal recombination in a cluster of closely related loci. Population genetic analyses in 946 subjects from 52 worldwide populations revealed that copy number ranged from 0 to 2 at both TAS2R43 and TAS2R45, with 43Δ and 45Δ occurring at high global frequencies (0.33 and 0.18). Estimated recombination rates between the loci were low (ρ = 2.7×10(-4); r = 6.6×10(-9)) and linkage disequilibrium was high (D' = 1.0), consistent with their adjacent genomic positioning and recent origin. Geographic variation pointed to an African origin for the deletions. However, no signatures of natural selection were found in population structure or integrated haplotype scores spanning the region, suggesting that patterns of diversity at TAS2R43 and -45 are primarily due to genetic drift.

Bitter taste receptor research comes of age: from characterization to modulation of TAS2Rs.

The recognition of potentially harmful food components by the gustatory system is important for survival and well-being of vertebrates. The plethora of structurally diverse bitter substances present in nature is recognized by multiple bitter taste receptors belonging to the taste receptor 2 family (TAS2R) of heptahelical receptors resulting in a highly complex mechanism of bitterness perception. In particular, research on human bitter taste receptors allowed the characterization of the receptive range of most of the 25 TAS2Rs, which was a prerequisite for detailed experiments to elucidate the structure-function relationships of TAS2Rs and for the discovery of the first reasonably specific TAS2R antagonists. These new findings will be the focus of the present review.