Evolution of bitter taste receptors in humans and apes

Bitter taste perception is crucial for the survival of organismsbecause it enables them to avoid the ingestion of potentiallyharmful substances. Bitter taste receptors are encoded by agene family that in humans has been shown to contain 25 putativelyfunctional genes and 8 pseudogenes and in mouse 33 putativelyfunctional genes and 3 pseudogenes. Lineage-specific expansionsof bitter taste receptors have taken place in both mouse andhuman, but very little is known about the evolution of thesereceptors in primates. We report the analysis of the almostcomplete repertoires of bitter taste receptor genes in human,great apes, and two Old World monkeys. As a group, these genesseem to be under little selective constraint compared with olfactoryreceptors and other genes in the studied species. However, incontrast to the olfactory receptor gene repertoire, where humanshave a higher proportion of pseudogenes than apes, there isno evidence that the rate of loss of bitter taste receptor genesvaries among humans and apes.     

A regulatory gene network related to the porcine umami taste receptor (TAS1R1/TAS1R3)

Taste perception plays an important role in the mediation of food choices in mammals. The first porcine taste receptor genes identified, sequenced and characterized, TAS1R1 and TAS1R3, were related to the dimeric receptor for umami taste. However, little is known about their regulatory network. The objective of this study was to unfold the genetic network involved in porcine umami taste perception. We performed a meta‐analysis of 20 gene expression studies spanning 480 porcine microarray chips and screened 328 taste‐related genes by selective mining steps among the available 12 320 genes. A porcine umami taste‐specific regulatory network was constructed based on the normalized coexpression data of the 328 genes across 27 tissues. From the network, we revealed the ‘taste module’ and identified a coexpression cluster for the umami taste according to the first connector with the TAS1R1/TAS1R3 genes. Our findings identify several taste‐related regulatory genes and extend previous genetic background of porcine umami taste.     

Computational Studies of Ligand-Receptor Interactions in Bitter Taste Receptors

Phenylthiocarbamide tastes intensely bitter to some individuals, but others find it completely tasteless. Recently, it was suggested that phenylthiocarbamide elicits bitter taste by interacting with a human G protein-coupled receptor (hTAS2R38) encoded by the PTC gene. The phenylthiocarbamide nontaster trait was linked to three single nucleotide polymorphisms occurring in the PTC gene. Using the crystal structure of bovine rhodopsin as template, we generated the 3D structure of hTAS2R38 bitter taste receptor. We were able to map on the receptor structure the amino acids affected by the genetic polymorphisms and to propose molecular functions for two of them that explained the emergence of the nontaster trait. We used molecular docking simulations to find that phenylthiocarbamide exhibited a higher affinity for the target receptor than the structurally similar molecule 6-n-propylthiouracil, in line with recent experimental studies. A 3D model was constructed for the hTAS2R16 bitter taste receptor as well, by applying the same protocol. We found that the recently published experimental ligand binding affinity data for this receptor correlated well with the binding scores obtained from our molecular docking calculations.     

Evaluation of the bitterness of green tea catechins by a cell-based assay with the human bitter taste receptor hTAS2R39

Catechins have a broad range of physiological functions and act as the main taste ingredient of green tea. Although catechins show a strong bitterness, the bitter taste receptor for catechins has not been fully understood. The objective of this study was to identify the receptor for the major green tea catechins such as (−)-epicatechin (EC), (−)-epicatechin gallate (ECg), (−)-epigallocatechin (EGC), and (−)-epigallocatechin gallate (EGCg). By the cell-based assay using cultured cells expressing human bitter taste receptor, a clear response of hTAS2R39-expressing cells was observed to 300 μM of either ECg or EGCg, which elicit a strong bitterness in humans. The response of hTAS2R39-expressing cells to ECg was the strongest among the tested catechins, followed by EGCg. Because the cellular response to EC and EGC is much weaker than those of ECg and EGCg, galloyl groups was strongly supposed to be involved in the bitter intensity. This finding is similar to the observations of taste intensity obtained from a human sensory study. Our results suggest the participation of hTAS2R39 in the detection of catechins in humans, indicating the possibility that bitterness of tea catechins can be evaluated by using cells expressing hTAS2R39.     

Biochemical Modulation of NMDA Receptors: Role in Conditioned Taste Aversion

Glutamate neurotransmission plays a crucial role in a variety of functions in the central nervous system, including learning and memory. However, little is known about the mechanisms underlying this process in mammals because of the scarceness of experimental models that permit correlation of behavioral and biochemical changes occurring during the different stages of learning and the retrieval of the acquired information. One model that has been useful to study these mechanisms is conditioned taste aversion (CTA), a paradigm in which animals learn to avoid new tastes when they are associated with gastrointestinal malaise. Glutamate receptors of the N-methyl-D-aspartate (NMDA) type appear to be necessary in this process, because blockade of this receptor prevents CTA. Phosphorylation of the main subunits of the NMDA receptor is a well-established biochemical mechanism for the modulation of the receptor response. Such modulation seems to be involved in CTA, because inhibitors of protein kinase C (PKC) block CTA acquisition and because the exposure to an unfamiliar taste results in an increased phosphorylation of tyrosine and serine residues of the NR2B subunit of the receptor in the insular cortex, the cerebral region where gustatory and visceral information converge. In this work we review these mechanisms of NMDA receptor modulation in CTA.     

Molecular mechanisms underlying the reception and transmission of sour taste information

Taste enables organisms to determine the properties of ingested substances by conveying information regarding the five basic taste modalities: sweet, salty, sour, bitter, and umami. The sweet, salty, and umami taste modalities convey the carbohydrate, electrolyte, and glutamate content of food, indicating its desirability and stimulating appetitive responses. The sour and bitter modalities convey the acidity of food and the presence of potential toxins, respectively, stimulating aversive responses to such tastes. In recent years, the receptors mediating sweet, bitter, and umami tastes have been identified as members of the T1R and T2R G-protein-coupled receptor families; however, the molecular mechanisms underlying sour taste detection have yet to be clearly elucidated. This review covers the molecular mechanisms proposed to mediate the detection and transmission of sour stimuli, focusing on polycystic kidney disease 1-like 3 (Pkd1l3), Pkd2l1, and carbonic anhydrase 4 (Car4).     

昆虫味觉感受机制研究进展

很多昆虫具有极其灵敏的味觉感受系统, 在其取食选择、 交配和产卵等过程中起重要作用。相对于昆虫的嗅觉机制, 对昆虫味觉感受机制的研究较少。传统的味觉感受研究主要集中在味觉感器外部形态、 味觉电生理和行为学上。近年来随着分子遗传学、 生物信息学和神经生物学技术的应用, 昆虫味觉的研究不断深入, 主要体现在下列两方面： （1）味觉受体方面, 通过分子生物信息学等手段获得了多种昆虫的味觉受体, 不同种昆虫之间受体数目差异较大, 不同受体之间氨基酸的相似性较低。通常, 根据味觉受体配体物质的性质可以把味觉受体分为取食抑制素受体和取食刺激素受体两大类。（2）味觉神经元的投射及味觉编码机制方面, 多个研究表明昆虫外围味觉神经元在中枢神经系统中的投射部位为咽下神经节和后脑, 但是不同性质的受体神经元投射的具体位置有所不同。本文对昆虫味觉感器和神经元的基本特征, 味觉受体的进化、 表达和功能, 味觉神经元在中枢神经系统中的投射, 味觉神经元的编码机制及味觉可塑性等进行了综述。     

Glutamate-induced cobalt uptake elicited by kainate receptors in rat taste bud cells

Glutamate-induced cobalt uptake reveals non-N-methyl-D-aspartate (non-NMDA) glutamate receptors (GluRs) in rat taste bud cells. However, it is not known which type of non-NMDA glutamate receptors is involved. We used a cobalt staining technique combined with pharmacological tests for kainate or alpha-amino-3-hydroxy-5-methyl-isoxazole-propionic acid (AMPA) receptors and/or immunohistochemistry against subunits of GluRs to examine the presence of non-NMDA receptors in rat foliate tastebud cells. Cobalt uptake into taste cells was elicited by treating taste buds with glutamate, kainate or SYM 2081, a kainate receptor agonist. Treating taste buds with AMPA or fluorowillardiine did not stimulate significant cobalt uptake. Moreover, 6-cyano-7-nitro-quinoxaline-2, 3-dione significantly reduced cobalt staining elicited by glutamate or kainate receptor agonists, but SYM 2206, an AMPA receptor antagonist, did not. Immunohistochemistry against subunits of GluRs reveals GluR6 and KA1-like immunoreactivity. Moreover, most glutamate-induced cobalt-stained cells showed GluR6 and KA1-like immunoreactivity. These results suggest that glutamate-induced cobalt uptake in taste cells occurs mainly via kainate type GluRs.     

Constitutively active mutant gives novel insights into the mechanism of bitter taste receptor activation

The human bitter taste receptors (T2Rs) belong to the G-protein coupled receptor (GPCR) superfamily. T2Rs share little homology with the large subfamily of Class A G-protein coupled receptors, and their mechanisms of activation are poorly understood. Guided by biochemical and molecular approaches, we identified two conserved amino acids Gly281·?? and Ser285?·?? present on transmembrane (TM) helices, TM1 and TM7, which might play important roles in T2R activation. Previously, it was shown that naturally occurring Gly511·?? mutations in the dim light receptor, rhodopsin, cause autosomal dominant retinitis pigmentosa, with the mutants severely defective in signal transduction. We mutated Gly281·?? and Ser285?·?? in T2R4 to G28A, G28L, S285A, S285T, and S285P, and carried out pharmacological characterization of the mutants. No major changes in signaling were observed upon mutation of Gly281·?? in T2R4. Interestingly, S285A mutant displayed agonist-independent activity (approximately threefold over basal wild-type T2R4 or S285T or S285P). We propose that Ser285?·?? stabilizes the inactive state of T2R4 by a network of hydrogen-bonds connecting important residues on TM1-TM2-TM7. We compare and contrast this hydrogen-bond network with that present in rhodopsin. Thus far, S285A is the first constitutively active T2R mutant reported, and gives novel insights into T2R activation.     

Modulation of taste sensitivity by GLP-1 signaling

In many sensory systems, stimulus sensitivity is dynamically modulated through mechanisms of peripheral adaptation, efferent input, or hormonal action. In this way, responses to sensory stimuli can be optimized in the context of both the environment and the physiological state of the animal. Although the gustatory system critically influences food preference, food intake and metabolic homeostasis, the mechanisms for modulating taste sensitivity are poorly understood. In this study, we report that glucagon-like peptide-1 (GLP-1) signaling in taste buds modulates taste sensitivity in behaving mice. We find that GLP-1 is produced in two distinct subsets of mammalian taste cells, while the GLP-1 receptor is expressed on adjacent intragemmal afferent nerve fibers. GLP-1 receptor knockout mice show dramatically reduced taste responses to sweeteners in behavioral assays, indicating that GLP-1 signaling normally acts to maintain or enhance sweet taste sensitivity. A modest increase in citric acid taste sensitivity in these knockout mice suggests GLP-1 signaling may modulate sour taste, as well. Together, these findings suggest a novel paracrine mechanism for the regulation of taste function.     

Identification of novel genes for bitter taste receptors in sheep (Ovis aries)

Genetic studies on taste sensitivity, and bitter taste receptors (T2R) in particular, are an essential tool to understand ingestive behavior and its relation to variations of nutritional status occurring in ruminants. In the present study, we conducted a data-mining search to identify T2R candidates in sheep by comparison with the described T2R in cattle and using recently available ovine genome. In sheep, we identified eight orthologs of cattle genes: T2R16, T2R10B, T2R12, T2R3, T2R4, T2R67, T2R13 and T2R5. The in silico predicted genes were then confirmed by PCR and DNA sequencing. The sequencing results showed a 99% to 100% similarity with the in silico predicted sequence. Moreover, we address the chromosomal distribution and compare, in homology and phylogenetic terms, the obtained genes with the known T2R in human, mouse, dog, cattle, horse and pig. The eight novel genes identified map either to ovine chromosome 3 or 4. The phylogenetic data suggest a clustering by receptor type rather than by species for some of the receptors. From the species analyzed, we observed a clear proximity between the two ruminant species, sheep and cattle, in contrast with lower similarities obtained for the comparison of sheep with other mammals. Although further studies are needed to identify the complete T2R repertoire in domestic sheep, our data represent a first step for genetic studies on this field.     

Laryngeal chemosensory clusters

The expression of molecules involved in the transductory cascade of the sense of taste (TRs, alpha-gustducin, PLCbeta2, IP3R3) has been described in lingual taste buds or in solitary chemoreceptor cells located in different organs. At the laryngeal inlet, immunocytochemical staining at the light and electron microscope levels revealed that alpha-gustducin and PLCbeta2 are mainly localized in chemosensory clusters (CCs), which are multicellular organizations differing from taste buds, being generally composed of two or three chemoreceptor cells. Compared with lingual taste buds, CCs are lower in height and smaller in diameter. In laryngeal CCs, immunocytochemistry using the two antibodies identified a similar cell type which appears rather unlike the alpha-gustducin-immunoreactive (IR) and PLCbeta2-IR cells visible in lingual taste buds. The laryngeal IR cells are shorter than the lingual ones, with poorly developed basal processes and their apical process is shorter and thicker. Some cells show a flask-like shape due to the presence of a large body and the absence of basal processes. CCs lack pores and their delimitation from the surrounding epithelium is poorly evident. The demonstration of the existence of CCs strengthens the hypothesis of a phylogenetic link between gustatory and solitary chemosensory cells.     

Taste receptor cells express voltage-dependent potassium channels in a cell age-specific manner

Two voltage-dependent potassium channels, KCNQ1 and KCNH2, are expressed in the taste buds and were identified as strong candidates involved in the repolarization of taste receptor cells expressing phospholipase C-beta2 and TRPM5 (beta2/M5-TRCs). In cell type-specific expression, KCNQ1 was expressed in most taste bud cells, including beta2/M5-TRCs, whereas KCNH2 was expressed in a subset of beta2/M5-TRCs with no correlation with their taste modality, such as sweet or bitter taste reception. Expression of KCNH2 was restricted to young beta2/M5-TRCs. These results suggest that taste bud cells other than beta2/M5-TRCs are depolarized by some stimuli and also that beta2/M5-TRCs have cell age-dependent molecular mechanisms of repolarization.     

Denatonium as a bitter taste receptor agonist damages jejunal epithelial cells of yellow-feathered chickens via inducing apoptosis

The sense of bitter taste is critical for chickens to acquire and select feeds. It is important to understand the roles and mechanisms of bitter taste transduction in chickens. Denatonium is extensively used as a bitter taste receptor agonist to activate bitter taste receptors in recent studies. The objective of this study was to investigate the physiological effects and the potential molecular mechanisms of dietary exposure to a strong bitter taste receptor agonist on the jejunal epithelial cells of yellow-feathered chickens. A total of 240 yellow-feathered chickens were divided into four treatments receiving a normal diet (Control), a low-dose denatonium treatment (Control + 5 mg/kg denatonium), a middle-dose denatonium treatment (Control + 20 mg/kg denatonium) and a high-dose denatonium treatment (Control + 100 mg/kg denatonium) for 56 days, respectively. The results showed that dietary denatonium reduced (P < 0.05) the growth performance of chickens. High-dose denatonium damaged the morphology of the jejunal epithelium and decreased (P < 0.05) the activities of Ca²⁺-ATPase, sucrase and maltase after 56 days of exposure. Meanwhile, high-dose denatonium increased (P < 0.05) mRNA expressions of bitter taste receptors, which resulted in enhanced apoptosis in jejunal epithelial cells after 56 days of exposure. Furthermore, middle-dose and high-dose denatonium exhibited increased (P < 0.05) mRNA level of claudin 2 and decreased (P < 0.05) mRNA level of occludin after 28 days of exposure. Only high-dose denatonium decreased (P < 0.05) mRNA level of occludin after 56 days of exposure. In conclusion, denatonium manifested deleterious effects on the jejunum of chickens in a dose–effect manner via damaging the morphology of the jejunal epithelium, and inducing apoptosis associated with bitter taste receptors. Our data suggest that bitter-tasting feed additives may have side effects on the growth and development of intestines in chickens.     

Differential covariation in taste responsiveness to bitter stimuli in rats

Variation exists in the sensitivity of individual rodents and humans to different bitter tastants. An absence of uniform correlation in responsiveness to different bitter substances across individuals within a species suggests heterogeneity in the mechanisms underlying stimulus processing within this taste modality. Here, we examined taste responsiveness of individual rats to three bitter compounds (quinine hydrochloride, denatonium benzoate, and cycloheximide) in short-term lick tests to determine the magnitude of covariation among responses to these stimuli and infer commonalities in their receptor and neural mechanisms. Rats were tested with a given pair of bitter stimuli during three sessions comprising randomized trial blocks of six concentrations of each stimulus + deionized water. Psychophysical functions were generated for individual rats for respective stimulus pairs, and concentrations of each stimulus that produced equivalent lick suppression relative to water were correlated across animals. Behavioral taste responsiveness to quinine hydrochloride strongly covaried with responsiveness to denatonium benzoate (r = +0.82). Lick responsiveness to quinine was less robustly correlated with that to cycloheximide (r = +0.44), and denatonium and cycloheximide responses failed to correlate. These results imply substantial overlap in the bitter taste coding mechanisms for quinine and denatonium but some degree of independence in the mechanisms responsible for gustatory processing of cycloheximide. More generally, these data reinforce the notion that bitter taste processing is not a homogeneous event.     

The oral lipid sensor GPR120 is not indispensable for the orosensory detection of dietary lipids in mice

Implication of the long-chain fatty acid (LCFA) receptor GPR120, also termed free fatty acid receptor 4, in the taste-guided preference for lipids is a matter of debate. To further unravel the role of GPR120 in the “taste of fat”, the present study was conducted on GPR120-null mice and their wild-type littermates. Using a combination of morphological [i.e., immunohistochemical staining of circumvallate papillae (CVP)], behavioral (i.e., two-bottle preference tests, licking tests and conditioned taste aversion) and functional studies [i.e., calcium imaging in freshly isolated taste bud cells (TBCs)], we show that absence of GPR120 in the oral cavity was not associated with changes in i) gross anatomy of CVP, ii) LCFA-mediated increases in intracellular calcium levels ([Ca²⁺]_i), iii) preference for oily and LCFA solutions and iv) conditioned avoidance of LCFA solutions. In contrast, the rise in [Ca²⁺]_i triggered by grifolic acid, a specific GPR120 agonist, was dramatically curtailed when the GPR120 gene was lacking. Taken together, these data demonstrate that activation of lingual GPR120 and preference for fat are not connected, suggesting that GPR120 expressed in TBCs is not absolutely required for oral fat detection in mice     

The history of sweet taste: not exactly a piece of cake

Understanding the molecular bases of sweet taste is of crucial importance not only in biotechnology but also for its medical implications, since an increasing number of people is affected by food-related diseases like, diabetes, hyperlipemia, caries, that are more or less directly linked to the secondary effects of sugar intake. Despite the interest paid to the field, it is only through the recent identification and functional expression of the receptor for sweet taste that new perspectives have been opened, drastically changing our approach to the development of new sweeteners. We shall give an overview of the field starting from the early days up to discussing the newest developments. After a review of early models of the active site, the mechanisms of interaction of small and macromolecular sweet molecules will be examined in the light of accurate modeling of the sweet taste receptor. The analysis of the homology models of all possible dimers allowed by combinations of the human T1R2 and T1R3 sequences of the sweet receptor and the closed (A) and open (B) conformations of the mGluR1 glutamate receptor shows that only 'type B' sites, either T1R2(B) and T1R3(B), can host the majority of small molecular weight sweeteners. Simultaneous binding to the A and B sites is not possible with two large sweeteners but is possible with a small molecule in site A and a large one in site B. This observation accounted for the first time for the peculiar phenomenon of synergy between some sweeteners. In addition to these two sites, the models showed an external binding site that can host sweet proteins.     

The good taste of peptides

The taste of peptides is seldom one of the most relevant issues when one considers the many important biological functions of this class of molecules. However, peptides generally do have a taste, covering essentially the entire range of established taste modalities: sweet, bitter, umami, sour and salty. The last two modalities cannot be attributed to peptides as such because they are due to the presence of charged terminals and/or charged side chains, thus reflecting only the zwitterionic nature of these compounds and/or the nature of some side chains but not the electronic and/or conformational features of a specific peptide. The other three tastes, that is, sweet, umami and bitter, are represented by different families of peptides. This review describes the main peptides with a sweet, umami or bitter taste and their relationship with food acceptance or rejection. Particular emphasis will be given to the sweet taste modality, owing to the practical and scientific relevance of aspartame, the well‐known sweetener, and to the theoretical importance of sweet proteins, the most potent peptide sweet molecules. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.     

Fat taste in humans: Sources of within- and between-subject variability

Non-esterified fatty acids (NEFA) are reportedly detectable through taste mechanisms in the human oral cavity. However, wide variability has been observed in NEFA taste sensitivity between and within subjects as well as across research groups. Some of this variability may be due to the hydrophobic nature of the NEFA and the methods used to make stimuli emulsions. As NEFA are poorly soluble in water, emulsification is necessary for delivery of stimuli to taste receptors. However, properties of emulsions may also be detected by somatosensory cues complicating attribution of sensory findings to taste. Additionally, learning (improved test performance) has been observed when using traditional tests for measuring sensitivity to NEFA, which may contribute greatly to within-subject variability if not standardized. Factors such as sex, diet, and BMI have been proposed to affect NEFA taste sensitivity, but the degree to which these individual factors influence NEFA detection thresholds remains to be fully established. Improved knowledge of stimulus properties and individual sensory capabilities will be needed to further evaluate the posited taste component to human oral fat detection. Progress in this area should facilitate the translation of findings on how NEFA taste may contribute to or reflect food choice and chronic disease risk.     

Two members of the TRPP family of ion channels, Pkd1l3 and Pkd2l1, are co-expressed in a subset of taste receptor cells

Taste receptors cells are responsible for detecting a wide variety of chemical stimuli. Several molecules including both G protein coupled receptors and ion channels have been shown to be involved in the detection and transduction of tastants. We report on the expression of two members of the transient receptor potential (TRP) family of ion channels, PKD1L3 and PKD2L1, in taste receptor cells. Both of these channels belong to the larger polycystic kidney disease (PKD or TRPP) subfamily of TRP channels, members of which have been demonstrated to be non-selective cation channels and permeable to both Na(+) and Ca(2+). Pkd1l3 and Pkd2l1 are co-expressed in a select subset of taste receptor cells and therefore may, like other PKD channels, function as a heteromer. We found the taste receptor cells expressing Pkd1l3 and Pkd2l1 to be distinct from those that express components of sweet, bitter and umami signal transduction pathways. These results provide the first evidence for a role of TRPP channels in taste receptor cell function.