Sweet and umami TAS1R receptors: from molecular recognition to physiological function

In the current issue of Acta Physiologica, Yasumatsu et al addressed whether oral perception of dietary lipids constitutes a discrete basic taste modality.1 The sense of taste helps the body discriminate between beneficial versus toxic and spoiled food to ensure survival by covering daily nutrient needs and preventing poisoning. Taste not only informs the body of whether food can be ingested, but also prepares the body for processing the incoming food.2 Until the early 21st century, four basic taste modalities were commonly accepted: sweet, bitter, salty and sour. Although first described in 1909, a fifth basic taste modality, umami, was finally recognized in 2002, when a receptor was identified in human and rodent taste buds. Umami, the taste of glutamate and nucleotides, is in protein rich foods such as soy and fish sauce, seaweed, tomatoes, meats and aged cheeses. While sweet, umami and low concentrations of salt are attractive to humans, and bitter, sour and high concentrations of salt are generally avoided, an acquired attraction to bitter and sour can develop in life (eg coffee, beer, lemon, sour candies). In the past 20 years, the taste of fat or "oleogustus" has been suggested as a sixth basic taste modality, but the qualification of taste bud-mediated fat detection as a true taste modality has remained a subject of debate. Taste is defined as the conscious perception of food compounds that bind to specific receptors located on the apical membrane of taste bud receptor cells in the oral cavity and oropharynx.2 Binding of the nutrients triggers an intracellular signalling cascade in taste receptor cells leading to the release of neurotransmitters onto gustatory afferent fibres of the chorda tympani and glossopharyngeal nerves. Individual taste cells generally express receptors for a single taste modality, ie sweet, bitter or umami, and the nerve fibres that innervate those cells respond best to stimuli of that modality.3 These highly tuned fibres provide a distinct pathway to the brain, which is thought to underlie the coding of that taste quality, unique and distinct from the others. Dietary lipids are detected orally via texture and smell, as well as in the gut via post-ingestive cues. However, when somaesthetic, olfactory and post-ingestive cues are minimized, rodents still prefer lipids over control solutions, and human subjects can detect small concentrations of lipids, suggesting a gustatory dimension to fat perception. Although dietary lipids are composed mostly of triglycerides, free fatty acids seem to be the cue responsible for the orosensory perception of lipids. The lingual lipase hydrolyses triglycerides into non-esterified fatty acids, thereby increasing fatty acid concentration in the saliva and leading to lower fat perception thresholds. Interestingly, transection of the glossopharyngeal and chorda tympani nerves results in a loss of the preference and of conditioned taste aversion (consumption associated with illness induced by injection of lithium chloride) for fatty acids in rodents. Further, stimulation of the tongue with free fatty acids elicits responses in the chorda tympani and glossopharyngeal, suggesting that taste receptors on the tongue mediate these responses.4 However, does the taste of fat represent a unique modality, or do fatty acids simply modulate the responses to other appetitive tastants such as sweet or umami stimuli? Here, Yasumatsu and collaborators aimed to uncover whether coding of oral fat perception is mediated by clusters of lipid-specific fibres, which would strengthen the concept of fat as a sixth primary taste quality. They used single fibre recordings of the chorda tympani nerve and behavioural assays to investigate whether fatty acids elicit a specific taste quality. Further, they investigated the role of GPR120, a G protein-coupled receptor expressed in mouse and human taste buds, in lingual fat perception.1 Although recording from single gustatory nerve fibres in mice is a challenging technique, they convincingly demonstrate for the first time that specific nerve fibres, so-called "F-type fibres", are dedicated to the transmission of fatty acid signals from taste buds to the brain in mice. Activation of F-type fibres upon lingual stimulation with lipids is significantly diminished, albeit not abolished, in GPR120 knockout mice, suggesting GPR120 mediates at least a portion of this response. Interestingly, they found that fatty acids also stimulate subsets of umami and sweet-best fibres, M-type and S-type fibres, respectively, and the response of these fibres is also lower in GPR120 knockout mice than controls. In addition, the authors report that conditioned taste aversion for fatty acids tended to be different in GPR120 knockout mice compared to controls, suggesting that GPR120 may partially mediate fat taste perception and eating behaviour. Furthermore, mice showed some generalization to umami taste stimuli upon conditioned taste aversion to lipids, consistent with the existence of fat/umami-best fibres, in addition to fat-best only fibres, suggesting perceptual similarities between fatty acids and umami taste. The work by Yasumatsu et al strengthens the argument for the existence of fat taste as a sixth primary taste modality, and their findings support the role of GPR120 as a fat taste receptor involved in transmission of the gustatory signal via fat-specific fibres. This work helps clarify the function of GPR120, which heretofore, had remained unclear and controversial. A taste receptor role for GPR120 was first suggested when gustatory nerve responses and preference for fatty acids were reportedly reduced in GPR120 knockout mice compared to controls.5 However, some subsequent studies failed to replicate these earlier data.6, 7 In addition, wild-type mice do not display preference for GPR120 specific agonists, and most human subjects do not perceive these agonists. Mechanistically, little is known about signal transduction downstream of GPR120 in taste bud cells. Fatty acids induce calcium responses in isolated mouse and human taste cells, but responses are unaffected in taste cells isolated from GPR120 knockout mice compared to those of control mice. Nonetheless, the calcium response to high concentrations of fatty acids is partially abolished when siRNA targeting GPR120 is transfected.8 As described above and reported in this issue of Acta Physiologica, elimination of GPR120 did not fully abolish the response of fat-best fibres, fat/sweet-best, or fat/umami-best fibres to oral stimulation with fatty acids, or conditioned taste aversion to fatty acids, or calcium responses to fatty acids, suggesting that other receptors also mediate fat taste and the regulation of eating behaviour. Several other potential fatty acid taste receptors have been identified in rodents: Kv1.5, CD36 and GPR40, but only the gustatory functions of CD36 have been studied extensively. CD36, a transmembrane glycoprotein with a high affinity for fatty acids (nmol/L), is expressed in taste buds in mice, rats and humans. While wild-type mice prefer fatty acids over a control solution, CD36 knockout mice lose the ability to detect the fatty acids, and unlike wild-type mice, do not show conditioned taste aversion to fatty acids.4 In humans, a single nucleotide polymorphism associated with lower CD36 expression has been identified in subjects whose sensitivity for fatty acid is diminished compared to control subjects, and lower oral lipid sensitivity may be associated with higher food intake and obesity in subjects carrying CD36 polymorphisms.9 In addition to regulating eating behaviour, lingual CD36 seems to impact the cephalic phase of digestion. Stimulation of the tongue with fatty acids in control mice, that had the oesophagus ligated to prevent entry of food into the stomach, still elicits the secretion of pancreatobiliary juice, thereby preparing the gut to digest the incoming dietary lipids. This cephalic phase response is abolished in CD36 knockout mice. Further, signal transduction in CD36+ taste cells involves a similar transduction cascade as for sweet, bitter and umami taste,3 including IP3-dependent release of intracellular calcium and release of transmitters to activate gustatory nerve fibres. Finally, activation of the first gustatory relay in the brainstem, the Nucleus Tractus Solitarius (NTS), following oral exposure to fatty acids is abolished in CD36 knockout mice, further supporting a role for lingual CD36 in the taste perception of dietary lipids.4 Much evidence points to CD36 being the main fat taste receptor while the exact function of GPR120 is still under debate due to fewer available and sometimes conflicting data. Stimulation of isolated human taste cells with a high concentration of fatty acids results in an elevated intracellular calcium response which is partially abolished when siRNA targeting CD36 is transfected, or in the presence of GPR120 siRNA, albeit the reduction is smaller than with CD36 siRNA.8 Interestingly, simultaneous siRNA knockdown of CD36 and GPR120 potentiates the reduction in the fatty acid-mediated calcium response observed in CD36-only or GPR120-only siRNA transfected cells,8 suggesting a cooperative effect of GPR120 with CD36.4 One possibility is that, rather than directly controlling fatty acid perception, GPR120 may instead modulate CD36-dependent gustatory fat sensitivity. Stimulation of taste tissues with fatty acids and GPR120 agonists triggers the secretion of glucagon-like peptide 1 (GLP1), which is coexpressed with GPR120 in murine taste bud cells. Genetic deletion of GLP1 receptor, which is expressed by gustatory nerve fibres, is associated with changes in sweet, umami and sour taste perception, as well as diminished fat perception, supporting the modulation of CD36 fat detection function by GPR120 (for more details, see Ref. [4]). In summary, substantial evidence in rodents and human subjects strongly supports the existence of a unique taste for fat, and Yasumatsu and collaborators' findings offer new evidence supporting that oral perception of fatty acid is a sixth basic taste quality separate from the historic five primary taste modalities. They demonstrate for the first time the existence of fat-best gustatory nerve fibres that respond distinctively to oral fatty acid stimulation. Comparable to sweet and umami taste, the existence of fat taste is significant for survival by detecting and helping digest high energy food and nutrients that are essential for energy, signalling and structural purposes. However, additional clues are necessary to fully recognize the existence of fat taste; for instance, it is still unclear whether fat taste is consciously perceived and identifiable by human subjects as easily as the other taste qualities. Yasumatsu and collaborators also aimed to address the role of GPR120 in fat taste, but the specific physiological function of lingual GPR120 requires further investigation. Although this set of data demonstrates a partial involvement of GPR120 in single fat-best gustatory nerve fibre responses to fatty acids, it also supports previous findings that GPR120 is not exclusively required for the gustatory nerve response and fat perception. Altogether, GPR120-dependent stimulation of subsets of sweet and umami fibres upon oral stimulation with fatty acids and the generalization of fatty acid conditioned taste aversion to umami may support a role for GPR120-mediated lipid detection as a modulator of fat, umami and sweet taste qualities as proposed previously,4 (Figure 1). Nota bene: only key original articles pertaining to GPR120 were cited, while the most up-to-date and detailed reviews on taste and fat taste were referenced to comply with the limitation on the number of citations. We invite the reader to refer to Ref. [2] and [3] to learn more about the biology of taste, and to Ref. [4] and [9] for more detailed information about fat taste, the functions of CD36 and discussions about the possible cooperation of CD36 and GPR120 in the mediation of fat taste. This work was supported by grants from the National Institute for Deafness and Other Communication Disorders to DG (R21DC016131) and to SCK (R01DC012555 and R01DC017679). We thank Drs. Linda Barlow and Tom Finger for helpful comments on the manuscript. Illustrations used in the figure are modified from Servier Medical Art licensed under a Creative Commons Attribution 3.0 Unported License (https://smart.servier.com/). None.

Recent identification of taste receptors and their downstream signaling molecules, expressed in taste receptor cells, led to the understanding of taste coding in the periphery. Ion channels appear to mediate detection of salty and sour taste. The sensations of sweet, umami and bitter taste are initiated by the interaction of sapid molecules with the G-protein-coupled receptors T1Rs and T2Rs. Mice lacking either PLCbeta2 or TRPM5 diminish behavioral and nerve responses to sweet, umami and bitter taste stimuli, suggesting that both receptor families converge on a common signaling pathway in the taste receptor cells. Nevertheless, separate populations of taste cells appear to be uniquely tuned to sweet, umami and bitter taste. Since PLCbeta2-deficient mice still respond to sour and salty stimuli, sour and salty taste are perceived independent of bitter, umami and sweet taste. In this review, the recent characterization of the cellular mechanisms underlying taste reception and perception, and of taste coding in the periphery will be discussed.

The detection of sweet-tasting compounds is mediated in large part by a heterodimeric receptor comprised of T1R2+T1R3. Lactisole, a broad-acting sweet antagonist, suppresses the sweet taste of sugars, protein sweeteners, and artificial sweeteners. Lactisole's inhibitory effect is specific to humans and other primates; lactisole does not affect responses to sweet compounds in rodents. By heterologously expressing interspecies combinations of T1R2+T1R3, we have determined that the target for lactisole's action is human T1R3. From studies with mouse/human chimeras of T1R3, we determined that the molecular basis for sensitivity to lactisole depends on only a few residues within the transmembrane region of human T1R3. Alanine substitution of residues in the transmembrane region of human T1R3 revealed 4 key residues required for sensitivity to lactisole. In our model of T1R3's seven transmembrane helices, lactisole is predicted to dock to a binding pocket within the transmembrane region that includes these 4 key residues.

Bitter, sweet and umami taste receptors and downstream signaling effectors: Expression in embryonic and growing chicken gastrointestinal tract

Overview: The completion of the Human Genome Project allowed the identification of a large family of proteins with a common motif of seven groups of 20–24 hydrophobic amino acids arranged as α-helices. Approximately 800 of these seven transmembrane (7TM) receptors have been identified of which over 300 are non-olfactory receptors (see Fredriksson et al., 2003; Lagerstrom and Schioth, 2008). Subdivision on the basis of sequence homology allows the definition of rhodopsin, secretin, adhesion, glutamate and Frizzled receptor families. NC-IUPHAR recognizes Classes A, B, and C, which equate to the rhodopsin, secretin, and glutamate receptor families. The nomenclature of 7TM receptors is commonly used interchangeably with G protein-coupled receptors (GPCR), although the former nomenclature recognises signalling of 7TM receptors through pathways not involving G proteins. For example, adiponectin and membrane progestin receptors have some sequence homology to 7TM receptors but signal independently of G proteins and appear to reside in membranes in an inverted fashion compared to conventional GPCR. Additionally, the NPR-C natriuretic peptide receptor (see Page S195) has a single transmembrane domain structure, but appears to couple to G proteins to generate cellular responses. The 300+ non-olfactory GPCR are the targets for the majority of drugs in clinical usage (Overington et al., 2006), although only a minority of these receptors are exploited therapeutically. Signalling through GPCR is enacted by the activation of heterotrimeric GTP-binding proteins (G proteins), made up of α, β and γ subunits, where the α and βγ subunits are responsible for signalling. The α subunit (tabulated below) allows definition of one series of signalling cascades and permits grouping of GPCRs to suggest common cellular, tissue and behavioural responses. Gβγ subunits (tabulated below) also are able to signal, in a manner independent of the Gα subunits. Recently, the concept of agonist bias, or functional selectivity, has arisen (see Kenakin and Miller, 2010), which suggests that particular agonists, or allosteric modulators, may be able to promote post-receptor signalling through one cascade at the expense of an alternative. This has complicated the scenario for classification of GPCR. For the purposes of the Guide to Receptors and Channels, ‘Principal transduction’ is limited to the predominant established Gα signalling. Gαs family:β1-adrenoceptors (see Page S26) in the heart couple principally through Gαs to activate adenylyl cyclase activity (see Page S288) and elevate intracellular cyclic AMP levels. This in turn leads to activation of protein kinase A (see Page S310) and the consequent phosphorylation and enhancement of function of voltage-gated calcium channels (Cav1.2, see Page S142). This, in turn leads to the observed action of noradrenaline (norepinephrine) or adrenaline (epinephrine) in increasing cardiac rate and force of contraction. The identification of other Gs-coupled GPCR in the heart would allow prediction of a similar effect on heart rate and force through the same mechanisms. In other tissues, Gs-coupled receptors would be predicted to evoke smooth muscle relaxation (e.g. β2-adrenoceptors in bronchioles, Page S26), enhance secretion (e.g. H2 histamine receptors in gastric parietal cells, Page S70), stimulate lipolysis in adipocytes (e.g. β3-adrenoceptors, Page S26) and inhibit platelet aggregation (e.g. IP prostanoid receptors, Page S97). Gαi family: M2 muscarinic acetylcholine receptors (see Page S20) in the heart couple via Gαi subunits to inhibit adenylyl cyclase activity (see Page S288). Vagal innervation targets these receptors, primarily in the atria, to counteract the effects of noradrenaline and adrenaline in the cardiac myocyte, leading to a reduction in heart rate and force of contraction. In addition, Gαi subunits and Gβγ subunits (see below) enhance potassium channel opening (KiR2.x, see Page S158). The ensuing hyperpolarization of the cardiac myocyte leads to a reduction in voltage-gated L-type calcium channel activity and a consequent inhibition of rate and force of cardiac contraction – the manifestation of vagal nerve stimulation. In other tissues, Gi-coupled receptors would be predicted to inhibit neurotransmitter release (e.g. µ opioid receptors, Page S88, on parasympathetic nerve terminals in the small intestine), inhibit lipolysis in adipocytes (e.g. A1 adenosine receptors, Page S22) and enhance platelet aggregation (e.g. P2Y12 receptors, Page S91). In the retina, transducin (αt) subunits allow coupling to a cyclic GMP-specific phosphodiesterase, PDE6 (see Page S290). This reduces cellular cyclic GMP levels leading to a reduction of currents through cyclic nucleotide-gated channels (CNG, Page S153) and subsequent decrease of the ‘dark’ current. Gαq family: M3 muscarinic acetylcholine receptors (see Page S20) in bronchial smooth muscle couple via Gαq/11 subunits to stimulate phospholipase C-β activity (see Page S302). This leads to an elevation of intracellular calcium ions through inositol 1,4,5-trisphosphate action at IP3 receptors (see Page S157), activation of protein kinase C (see Page S311) and the consequent smooth muscle contraction and reduced airway conductance. In other tissues, Gq-coupled receptor activation leads to increased platelet aggregation (e.g. P2Y1 receptors, Page S91). Lysophosphatidic acid receptors (see Page S76) and proteinase-activated receptors (see Page S95) are examples of GPCR which couple through multiple G protein families, including Gα12/13 leading to activation of a guanine nucleotide exchange factor, or GEF, for the Rho family of low molecular GTP-binding proteins (ENSFM00500000269651), the subsequent activation of Rho kinase (see Page S313) and regulation of the cytoskeleton, leading to cellular shape changes and/or migration. GNAQP1 is a pseudogene (ENSG00000214077). Gβγ subunits: although β and γ subunits are synthesised as separate entities, they are considered to generate a complex which is essentially biologically irreversible. Acylation and prenylation ensure an association with the plasma membrane, where Gβγ subunits may regulate ion channel activities, or recruit members of the G protein-coupled receptor kinase family, also known as β-adrenoceptor kinases (see Page S310). Phosphorylation of particular cytoplasmic serine/threonine residues of GPCR allows binding of β-arrestin (ENSFM00250000000572). These proteins act as scaffolding partners facilitating internalization of GPCR as a mechanism of desensitization, or coupling to alternative signalling pathways (e.g. MAP kinases, see Page S312). GNB1L (ENSG00000185838) and GNB2L1 (ENSG00000204628) are described as Gβ-like proteins on the basis of sequence homology. Four Gγ pseudogenes are defined in the human genome (GNG5P1, ENSG00000213536; GNG5P2, ENSG00000133136; GNG5P3, ENSG00000254949; GNG5P5, ENSG00000234590). Foord SM, Bonner TI, Neubig RR, Rosser EM, Pin JP, Davenport AP et al. (2005). International Union of Pharmacology. XLVI. G protein-coupled receptor list. Pharmacol Rev57: 279–288. Fredholm BB, Hokfelt T, Milligan G (2007). G-protein-coupled receptors: an update. Acta Physiol190: 3–7. Fredriksson R, Lagerstrom MC, Lundin LG, Schioth HB (2003). The G-protein-coupled receptors in the human genome form five main families. Phylogenetic analysis, paralogon groups, and fingerprints. Mol Pharmacol63: 1256–1272. Hill SJ, Williams C, May LT (2010). Insights into GPCR pharmacology from the measurement of changes in intracellular cyclic AMP; advantages and pitfalls of differing methodologies. Br J Pharmacol161: 1266–1275. Kenakin T (2010). Being mindful of seven-transmembrane receptor ‘guests’ when assessing agonist selectivity. Br J Pharmacol160: 1045–1047. Kenakin T, Miller LJ (2010). Seven transmembrane receptors as shapeshifting proteins: the impact of allosteric modulation and functional selectivity on new drug discovery. Pharmacol Rev62: 265–304. Kenakin TP (2009). Cellular assays as portals to seven-transmembrane receptor-based drug discovery. Nat Rev Drug Discov8: 617–626. Lagerstrom MC, Schioth HB (2008). Structural diversity of G protein-coupled receptors and significance for drug discovery. Nat Rev Drug Discov7: 339–357. Milligan G (2009). G protein-coupled receptor hetero-dimerization: contribution to pharmacology and function. Br J Pharmacol158: 5–14. Overington JP, Al-Lazikani B, Hopkins AL (2006). How many drug targets are there? Nat Rev Drug Discov5: 993–996. http://www.iuphar-db.org/ While the remainder of this section focuses on those GPCR for which there is substantial pharmacological information, or interest, listed below are a number of putative GPCR identified by IUPHAR (Foord et al., 2005), for which only preliminary evidence for an endogenous ligand has been published, or for which there exists a potential link to a disease, or disorder. The GPCR in the table below are all Class A, rhodopsin-like GPCR. In the set of tables below, putative GPCR with as-yet unidentified endogenous ligands are listed. Adhesion GPCRs: Adhesion GPCRs are structurally identified on the basis of a large extracellular region, similar to the Class B GPCR, but which is linked to the 7TM region by a ‘stalk’ motif containing a GPCR proteolytic site. The N-terminus often shares structural homology with proteins such as lectins and immunoglobulins, leading to the term adhesion GPCR (see Fredriksson et al., 2003; Yona et al., 2008). Whilst the taste of acid and salty foods appears to be sensed by regulation of ion channel activity, bitter, sweet and umami tastes are sensed by specialised GPCR. Two classes of taste GPCR have been identified, T1R and T2R, which are similar in sequence and structure to Class C and Class A GPCR, respectively. Activation of taste receptors appears to involve gustducin (Gαt3, see Page S6) and Gα14-mediated signalling, although the precise mechanisms remain obscure. Gene disruption studies suggest the involvement of PLCβ2 (Zhang et al., 2003), TRPM5 (Zhang et al., 2003) and IP3 (Hisatsune et al., 2007) receptors in post-receptor signalling of taste receptors. Although predominantly associated with the oral cavity, taste receptors are also located elsewhere, including further down the gastrointestinal system, in the lungs and in the brain. Sweet/Umami: T1R3 acts as an obligate partner in T1R1/T1R3 and T1R2/T1R3 heterodimers, which sense umami or sweet, respectively. T1R1/T1R3 heterodimers respond to L-glutamate and may be positively allosterically modulated by 5′-nucleoside monophosphates, such as GMP (Li et al., 2002). T1R2/T1R3 heterodimers respond to sugars, such as sucrose, and artificial sweeteners, such as saccharin (Nelson et al., 2001). Bitter: the composition and stoichiometry of bitter taste receptors is not yet established. Bitter receptors appear to separate into two groups, with very restricted ligand specificity or much broader responsiveness. For example, T2R5 responded to cycloheximide, but not 10 other bitter compounds (Chandrashekar et al., 2000), while T2R14 responded to at least eight different bitter tastants, including (-)-α-thujone and picrotoxinin (Behrens et al., 2004). Andres-Barquin PJ, Conte C (2004). Molecular basis of bitter taste: the T2R family of G protein-coupled receptors. Cell Biochem Biophys41: 99–112. Bachmanov AA, Beauchamp GK (2007). Taste receptor genes. Annu Rev Nutr27: 389–414. DeSimone JA, Lyall V (2006). Taste receptors in the gastrointestinal tract III. Salty and sour taste: sensing of sodium and protons by the tongue. Am J Physiol Gastrointest Liver Physiol291: G1005–G1010. Foord SM, Bonner TI, Neubig RR, Rosser EM, Pin JP, Davenport AP et al. (2005). International Union of Pharmacology. XLVI. G protein-coupled receptor list. Pharmacol Rev57: 279–288. Fredriksson R, Lagerstrom MC, Lundin LG, Schioth HB (2003). The G-protein-coupled receptors in the human genome form five main families. Phylogenetic analysis, paralogon groups, and fingerprints. Mol Pharmacol63: 1256–1272. Kinnamon SC (2011). Taste receptor signaling – from tongues to lungs. Acta Physiol (Oxf) in press. Lagerstrom MC, Schioth HB (2008). Structural diversity of G protein-coupled receptors and significance for drug discovery. Nat Rev Drug Discov7: 339–357. Meyerhof W (2005). Elucidation of mammalian bitter taste. Rev Physiol Biochem Pharmacol154: 37–72. Overington JP, Al-Lazikani B, Hopkins AL (2006). How many drug targets are there? Nat Rev Drug Discov5: 993–996. Renwick AG, Molinary SV (2010). Sweet-taste receptors, low-energy sweeteners, glucose absorption and insulin release. Br J Nutr104: 1415–1420. Rozengurt E (2006). Taste receptors in the gastrointestinal tract. I. Bitter taste receptors and α-gustducin in the mammalian gut. Am J Physiol Gastrointest Liver Physiol291: G171–G177. Rozengurt E, Sternini C (2007). Taste receptor signaling in the mammalian gut. Curr Opin Pharmacol7: 557–562. Temussi PA (2009). Sweet, bitter and umami receptors: a complex relationship. Trends Biochem Sci34: 296–302. Tepper BJ (2008). Nutritional implications of genetic taste variation: the role of PROP sensitivity and other taste phenotypes. Annu Rev Nutr28: 367–388. Yarmolinsky DA, Zuker CS, Ryba NJ (2009). Common sense about taste: from mammals to insects. Cell139: 234–244. Yona S, Lin HH, Siu WO, Gordon S, Stacey M (2008). Adhesion-GPCRs: emerging roles for novel receptors. Trends Biochem Sci33: 491–500. 5-HT receptors [nomenclature as agreed by NC-IUPHAR Subcommittee on 5-HT receptors (Hoyer et al., 1994) and subsequently revised (Hartig et al., 1996)] are, with the exception of the ionotropic 5-HT3 class, GPCR where the endogenous agonist is 5-HT. The diversity of metabotropic 5-HT receptors is increased by alternative splicing that produces isoforms of the 5-HT2A (non-functional), 5-HT2C (non-functional), 5-HT4, 5-HT6 (non-functional) and 5-HT7 receptors. Unique amongst the GPCRs, RNA editing produces 5-HT2C receptor isoforms that differ in function, such as efficiency and specificity of coupling to Gq/11and also pharmacology (reviewed by Bockaert et al., 2006; Werry et al., 2008). Most 5-HT receptors (except 5-ht1e and 5-ht5a/5b) play specific roles mediating functional responses in different tissues (reviewed by Villalón and Centurión, 2007; Ramage and Villalón, 2008). Tabulated pKi and KD values refer to binding to human 5-HT receptors unless indicated otherwise. Unreferenced values are extracted from the NC-IUPHAR database (http://www.iuphar-db.org). The nomenclature of 5-HT1B/5-HT1D receptors has been revised (Hartig et al., 1996). Only the non-rodent form of the receptor was previously called 5-HT1Dβ. The human 5-HT1B receptor (tabulated) displays a different pharmacology to the rodent forms of the receptor due to Thr335 of the human sequence being replaced by Asn in rodent receptors. NAS181 is a selective antagonist of the rodent 5-HT1B receptor. Fananserin and ketanserin bind with high affinity to dopamine D4 and histamine H1 receptors respectively, and ketanserin is a potent α1 adrenoceptor antagonist, in addition to blocking 5-HT2A receptors. The human 5-ht5A receptor has been claimed to couple to several signal transduction pathways when stably expressed in C6 glioma cells (Noda et al., 2003). The human orthologue of the mouse 5-ht5b receptor is non-functional due to interruption of the gene by stop codons. The 5-ht1e receptor appears not to have been cloned from mouse, or rat, impeding definition of its function. In addition to the receptors listed in the table, an ‘orphan’ receptor, unofficially termed 5-HT1P, has been described (Gershon, 1999). Abbreviations: 5-CT, 5-carboxamidotryptamine; 8-OH-DPAT, 8-hydroxy-2-(di-n-propylamino)tetralin; [N-methyl-3H3]AZ10419369, 5-methyl-8-(4-methyl-piperazin-1-yl)-4-oxo-4H-chromene-2-carboxylicacid (4-morpholin-4-yl-phenyl)-amide; BIMU8, (endo-N-8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-3-isopropyl-2-oxo-1H-benzimidazol-1-carboxamide hydrochloride; BRL15572, 3-[4-(3-chlorophenyl) piperazin-1-yl]-1,1,-diphenyl-2-propanol; BRL54443, 5-hydroxy-3-(1-methylpiperidin-4-yl)-1H-indole; BW723C86, 1-[5(2-thienylmethoxy)-1H-3-indolyl]propan-2-amine hydrochloride; CP94253: 3- (1,2,5,6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3, 2-b] pyridine; E6801, 6-chloro-N-(3-(2-dimethylamino)ethyl)-1H-indol-5-yl)imidazo[2,1-b]thiazole-5-sulfonamide; E55888, dimethyl-{2-[3-(1,3,5-trimethyl-1H-pyrazol-4-yl)-phenyl]-ethyl}-amine; EGIS-7625, 1-benzyl-4-[(2-nitro-4-methyl-5-amino)-phenyl]-piperazine; F13640, 3-chloro-4-fluoro-phenyl)-[4-fluoro-4-{[(5-methyl-pyperidin-2-ylmethyl)-amino]-methyl}piperidin-1-yl]-methanone ; F15599, 3-chloro-4-fluorophenyl-(4-fluoro-4-{[(5-methylpyrimidin-2-ylmethyl)-amino]-methyl}-piperidin-1-yl)-methanone; FR260010, (N-[3-(4-methyl-1H-imidazol-1-yl)phenyl]-5,6-dihydrobenzo[h]quinazolin-4-amine; GR55562, 3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide; GR113808, [1-2[(methylsuphonyl)amino]ethyl]-4-piperidinyl]methyl-1-methyl-1H-indole-3-carboxylate; GR125743, n-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-3-methyl-4-(4-pyrindinyl)benzamide; GTI, 5-hydroxytryptamine-5-O-carboxymethylglycyltyrosinamide; L694247, 2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl]-1H-indol-3yl] ethanamine; LY334370, 5-(4-flurobenzoyl)amino-3-(1-methylpiperidin-4-yl)-1H-indole fumarate; LY344864, N-[(6R) -6-dimethylamino-6,7,8,9-tetrahydro-5H-carbazo-3-yl]-4- fluorobenzamide; LY573144, 2,4,6-trifluoro-N-[6-[(1-methylpiperidin-4-yl)carbonyl]pyridin-2yl]benzamide; MDL100907, (+/-)2,3-dimethoxyphenyl-1-[2-(4-piperidine)-methanol]; NAD299, (R)-3-N,N-dicyclobutylamino-8-fluoro-[6-3H]-3,4-dihydro-2H-1-benzo pyran-5-carboxamide; NAS181, (R)-(+)-2-[[[3-(morpholinomethyl)-2H-chromen-8-yl]oxy]methyl] morpholine methane sulfonate; p-[18F]MPPF 4-(2′-methoxyphenyl)-1-[2′-(N-2″-pyridinyl)-p-fluorobenzamido]-ethyl piperazine; PNU109291, (S)-3,4-dihydro-1-[2-[4-(4-methoxyphenyl)-1-piperazinyl]ethyl]-N-methyl-1H-2-benzopyran-6-carboximide; RP62203, 2-[3-(4-(4-fluorophenyl)-piperazinyl)propyl]naphto[1,8-ca]isothiazole-1,1-dioxide; Ro600175, (S)-2-(6-chloro-5-fluroindol-1-yl)-1-methyethylamine; Ro630563, 4-amino-N-[2,6-bis(methylamino)pyridin-4-yl]benzenesulphonamide; RS57639, 4-amino-5-chloro-2-methoxy benzoic acid 1-(3-[2,3-dihydrobenzo[1,4]dioxin-6yl)-propyl]-piperidin-4yl methyl ester; RS67506, 1-(4-amino-5-chloro-2-methoxyphenyl)-3-[1-(2-methyl sulphonylamino)ethyl-4-piperidinyl]-1-propanone hydrochloride; RS100235, 1-(8-amino-7-chloro-1,4-benzodioxan-5-yl)-5-((3-(3,4-dimethoxyphenyl)prop-1-yl)piperidin-4-yl)propan-1-one; RS102221, 8-[5-(5-amino 2,4-dimethoxyphenyl) 5-oxopentyl]-1,3,8-triazaspiro[4,5]decane-2,4-dione; RS127445, (2-amino-4-(4-fluoronaphthyl-1-yl)-6-isopropylpyrimidine); SB204070, 1-butyl-4-piperidinylmethyl-8-amino-7-chloro-1-4-benzoioxan-5-carboxylate; SB207710, 1-butyl-4-piperidinylmethyl-8-amino-7-iodo-1,4-benzodioxan-5-carboxylate; SB224289,1′-methyl-5[[2′-methyl-4′-)5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl-2,3,6,7-tetrahydrospiro[furo[2,3-f]indole-3,4′-piperidine]oxalate; SB236057,1′-ethyl-5-(2′-methyl-4′ (5-methyl-1,3,4-oxadiazol-2-yl)biphenyl-4-carbonyl)-2,3,6,7-tetrahydrospiro[furo[2,3-f]indol3-3,4′-piperidine; SB242084, 6-chloro-5-methyl-1-[2-(2-methylpyridyl-3-oxy)-pyrid-5-yl carbamoyl] indoline; SB258585, 4-iodo-N-[4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-benzenesulphonamide; SB258719, (R)-3,N-dimethyl-N-[1-methyl-3-(4-methylpiperidin-1-yl)propyl]benzene sulphonamide; SB269970, (R)-3-(2-(2-(4-methylpiperidin-1-yl)ethyl)pyrrolidine-1-sulphonyl)phenol; SB271046, 5-chloro-N-(4-methoxy-3-piperazin-1-yl-phenyl)-3-methyl-2-benzothiophenesulphonamide; SB357134, N-(2,5-dibromo-3-flurophenyl)-4-methoxy-3-piperazin-1-ylbenzenesulphonamide; SB-399885, N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)benzenesulfonamide; SB656104, 6-((R)-2-[2-[4-(4-Chloro-phenoxy)-piperidin-1-yl]-ethyl]-pyrrolidine-1-sulphonyl)-1H-indole hydrochloride; SB699551, 3-cyclopentyl-N-[2-(dimethylamino)ethyl]-N-[(4′-{[(2-phenylethyl)amino]methyl}-4-biphenylyl)methyl]propanamide dihydrochloride; SB714786, 2-methyl-5-({2-[4-(8-quinolinylmethyl)-1-piperazinyl]ethyl}oxy)quinoline; UH301, 5-fluoro-8-hydroxy-2-(dipropylamino) tetralin; U92016A, (+)-R)-2-cyano-N,N-dipropyl-8-amino-6,7,8,9-tetrahydro-3H-benz[e]indole; WAY100635, N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridyl)-cyclohexanecarboxamide trichloride, WAY163909, (7bR, 10aR)-1,2,3,4,8,9,10,10a-octahydro-7bH-cyclopenta-[b][1,4]diazepino[6,7,1hi]indole; WAY-181187, 2-[1-(6-chloroimidazo[2,1-b]thiazol-5-ylsulfonyl)-1H-indol-3-yl]ethylamine Aloyo VJ, Berg KA, Spampinato U, Clarke WP, Harvey JA (2009). Current status of inverse agonism at serotonin2A (5-HT2A) and 5-HT2C receptors. Pharmacol Ther121: 160–173. Barnes NM, Sharp T (1999). A review of central 5-HT receptors and their function. Neuropharmacology38: 1083–1152. Berger M, Gray JA, Roth BL (2009). The expanded biology of serotonin. Annu Rev Med60: 355–366. Bockaert J, S, C, A, (2006). 5-HT metabotropic receptors: of their structure, and roles in Cell Bockaert J, S, A (2008). receptors: molecular pharmacology and (2006). for metabotropic 5-HT receptor Curr SJ, of receptor a genetic of Pharmacol R, M, (2010). of of and clinical Curr E, (2005). as receptor ligands – and Curr M (2007). 5-HT receptor regulation of neurotransmitter release. Pharmacol (1999). roles by in the of the Pharmacol U, Roth R, M, M (2010). The of 5-HT6 receptor ligands with a on Curr S, Villalón (2010). The of for the of on and receptors. Pharmacol Clarke et al. International Union of classification of receptors for Pharmacol of receptor nomenclature with the human classification of 5-HT1B and receptor Trends Pharmacol A, (2008). 5-HT6 receptor in the of a novel pharmacological to the of and Pharmacol JP, pharmacological and functional diversity of 5-HT receptors. Pharmacol Biochem TI, (2010). potential of 5-HT2C receptor (2008). A role for the and 5-HT6 receptors in and Trends Pharmacol R, (2009). drug to receptor Curr (2007). receptor Curr M (2004). receptors. Curr Drug M, E, R, (2011). 5-HT7 receptor and potential in central Pharmacol (2004). receptors. Curr Drug (2004). receptors. Curr Drug signalling by receptors. Biochem Ramage AG, Villalón (2008). and Trends Pharmacol (2008). and the gastrointestinal where Trends Pharmacol Villalón receptors: a to Trends Pharmacol (2004). 5-HT7 receptors. Curr Drug (2006). 5-ht5A receptors as a Pharmacol Villalón (2007). responses by a pharmacological on the and B, S, G (2010). of receptor a receptor with Curr (2010). in the of selective and receptor Curr Werry R, A (2008). RNA editing of the receptor and its effects on signalling, pharmacology and function. Pharmacol (2004). receptors. Curr Drug Overview: acetylcholine receptors as agreed by NC-IUPHAR on and are GPCR of the Class A, rhodopsin-like family where the endogenous agonist is In addition to the listed in the table, its structural and and have been described as selective of the receptor via binding in a from that by et al., 2006; et al., 2003; et al., May et al., 2007; et al., et al., et al., et al., are two allosteric on muscarinic receptors, one defined by binding and and the other and et al., 2002). are selective of acetylcholine binding and and at receptors, at M2 receptors, and at M3 receptors and and at receptors, and at receptors and et al., et al., et al., et al., et al., et al., et al., has also been to activate the receptor via an allosteric et al., et al., The allosteric for and on M2 receptors be by et al., 2003). is a selective agonist that appears to in a with the and an allosteric on the M2 muscarinic receptor et al., 2008). and are ligands that also selectivity for M2 receptors by binding to the and a allosteric et al., 2007; et al., is a described antagonist with selectivity for the receptor et al., and has been described as a selective agonist of the receptor et al., are pKi values for human receptors. and are with the of the (see et al., and Abbreviations:

Searching for the bitter truth: Insights into receptors for taste

Bitter taste receptor T2R7 and umami taste receptor subunit T1R1 are expressed highly in Vimentin-negative taste bud cells in chickens

Although the five basic taste qualities—sweet, sour, bitter, salty and umami—can be recognized by the respective gustatory system, interactions between these taste qualities are often experienced when food is consumed. Specifically, the umami taste has been investigated in terms of whether it enhances or reduces the other taste modalities. These studies, however, are based on individual perception and not on a molecular level. In this study we investigated umami-sweet taste interactions using umami compounds including monosodium glutamate (MSG), 5’-mononucleotides and glutamyl-dipeptides, glutamate-glutamate (Glu-Glu) and glutamate-aspartic acid (Glu-Asp), in human sweet taste receptor hT1R2/hT1R3-expressing cells. The sensitivity of sucrose to hT1R2/hT1R3 was significantly attenuated by MSG and umami active peptides but not by umami active nucleotides. Inhibition of sweet receptor activation by MSG and glutamyl peptides is obvious when sweet receptors are activated by sweeteners that target the extracellular domain (ECD) of T1R2, such as sucrose and acesulfame K, but not by cyclamate, which interact with the T1R3 transmembrane domain (TMD). Application of umami compounds with lactisole, inhibitory drugs that target T1R3, exerted a more severe inhibitory effect. The inhibition was also observed with F778A sweet receptor mutant, which have the defect in function of T1R3 TMD. These results suggest that umami peptides affect sweet taste receptors and this interaction prevents sweet receptor agonists from binding to the T1R2 ECD in an allosteric manner, not to the T1R3. This is the first report to define the interaction between umami and sweet taste receptors.

The taste system of animals is used to detect valuable nutrients and harmful compounds in foods. In humans and mice, sweet, bitter, salty, sour and umami tastes are considered the five basic taste qualities. Sweet and umami tastes are mediated by G-protein-coupled receptors, belonging to the T1R (taste receptor type1) family. This family consists of three members (T1R1, T1R2 and T1R3). They function as sweet or umami taste receptors by forming heterodimeric complexes, T1R1+T1R3 (umami) or T1R2+T1R3 (sweet). Receptors for each of the basic tastes are thought to be expressed exclusively in taste bud cells. Sweet (T1R2+T1R3-expressing) taste cells were thought to be segregated from umami (T1R1+T1R3-expressing) taste cells in taste buds. However, recent studies have revealed that a significant portion of taste cells in mice expressed all T1R subunits and responded to both sweet and umami compounds. This suggests that sweet and umami taste cells may not be segregated. Mice are able to discriminate between sweet and umami tastes, and both tastes contribute to behavioural preferences for sweet or umami compounds. There is growing evidence that T1R3 is also involved in behavioural avoidance of calcium tastes in mice, which implies that there may be a further population of T1R-expressing taste cells that mediate aversion to calcium taste. Therefore the simple view of detection and segregation of sweet and umami tastes by T1R-expressing taste cells, in mice, is now open to re-examination.

The T1R receptors, a family of taste-specific class C G protein-coupled receptors, mediate mammalian sweet and umami tastes. The structure-function relationships of T1R receptors remain largely unknown. In this study, we demonstrate the different functional roles of T1R extracellular and transmembrane domains in ligand recognition and G protein coupling. Similar to other family C G protein-coupled receptors, the N-terminal Venus flytrap domain of T1R2 is required for recognizing sweeteners, such as aspartame and neotame. The G protein coupling requires the transmembrane domain of T1R2. Surprisingly, the C-terminal transmembrane domain of T1R3 is required for recognizing sweetener cyclamate and sweet taste inhibitor lactisole. Because T1R3 is the common subunit in the sweet taste receptor and the umami taste receptor, we tested the interaction of lactisole and cyclamate with the umami taste receptor. Lactisole inhibits the activity of the human T1R1/T1R3 receptor, and, as predicted, blocked the umami taste of l-glutamate in human taste tests. Cyclamate does not activate the T1R1/T1R3 receptor by itself, but potentiates the receptor's response to l-glutamate. Taken together, these findings demonstrate the different functional roles of T1R3 and T1R2 and the presence of multiple ligand binding sites on the sweet taste receptor.

Heterotrimeric G protein-coupled receptors (GPCRs) comprise the largest receptor family in mammals and are responsible for the regulation of most physiological functions. Besides mediating the sensory modalities of olfaction and vision, GPCRs also transduce signals for three basic taste qualities of sweet, umami (savory taste), and bitter, as well as the flavor sensation kokumi. Taste GPCRs reside in specialised taste receptor cells (TRCs) within taste buds. Type I taste GPCRs (TAS1R) form heterodimeric complexes that function as sweet (TAS1R2/TAS1R3) or umami (TAS1R1/TAS1R3) taste receptors, whereas Type II are monomeric bitter taste receptors or kokumi/calcium-sensing receptors. Sweet, umami and kokumi receptors share structural similarities in containing multiple agonist binding sites with pronounced selectivity while most bitter receptors contain a single binding site that is broadly tuned to a diverse array of bitter ligands in a non-selective manner. Tastant binding to the receptor activates downstream secondary messenger pathways leading to depolarization and increased intracellular calcium in TRCs, that in turn innervate the gustatory cortex in the brain. Despite recent advances in our understanding of the relationship between agonist binding and the conformational changes required for receptor activation, several major challenges and questions remain in taste GPCR biology that are discussed in the present review. In recent years, intensive integrative approaches combining heterologous expression, mutagenesis and homology modeling have together provided insight regarding agonist binding site locations and molecular mechanisms of orthosteric and allosteric modulation. In addition, studies based on transgenic mice, utilizing either global or conditional knock out strategies have provided insights to taste receptor signal transduction mechanisms and their roles in physiology. However, the need for more functional studies in a physiological context is apparent and would be enhanced by a crystallized structure of taste receptors for a more complete picture of their pharmacological mechanisms.

Glucose, amino acids, and free fatty acids are critical nutrients participating in stimulating or regulating the hormone secretion of islets. These nutrients are believed to be metabolized by pancreatic endocrine cells to function. However, recent evidence suggests that taste receptors, which play key roles in the oral cavity to sense glucose (sweet taste), amino acids (umami taste), and free fatty acids (fatty taste), are expressed in pancreatic islet cells and may act to sense these nutrients to regulate pancreatic hormone secretion, including insulin and glucagon. Disorders in these taste receptor pathways in islets may contribute to the pathogenesis of diabetes, or it may influence hyperglycemia, disturbance in amino acid metabolism, or hyperlipidemia. In this review, we su mMarize the expression and hormone-regulating functions of sweet, umami, and fatty taste receptors acting as nutrient sensors in pancreatic islets in vitro and in vivo. We discuss the potential roles of these taste receptor-nutrient sensor pathways in islets targeted to develop therapeutic strategies for diabetes and related disease.

The tongue can distinguish between five different tastes via the taste receptors, which are G-protein coupled receptors (GPCRs). There are two classes of taste receptors, the TAS1 (T1) and TAS2 (T2) families, and the T1R1-T1R3 dimer senses the umami taste and T1R2-T1R3 senses the sweet taste. Recently, the taste receptors have also been found in the brain, lungs, intestine and pancreas, where they sense changes in the nutrient environment and respond through GPCR signalling. Given the importance of glucose and amino acid metabolism in the heart, we hypothesized that the sweet and umami taste receptors have an important function in the heart. Using a variety of technologies and disease states, we have identified that T1R1, T1R2 and T1R3 are expressed in the heart. More specifically, mass spectrometry of a dog model of dyssynchrony has shown the presence of T1R1, T1R3 and T1R2. RNA seq of human patients who received a Left Ventricular Assist device and those who did not also revealed the presence of T1R1 and T1R3. The expression of these proteins was also confirmed using Western blot. We further showed T1R2 and T1R3 protein is localized in the plasma membrane of the cardiomyocytes by immunofluorescence (colocalized with Na/K ATPase) and PM enrichment. When we compared the taste receptor protein levels in dilated cardiomyopathy (DCM) compared to donor heart tissue, we found that T1R2 was overexpressed in DCM, showing that taste receptors may be important in nutrient sensing in disease. Furthermore, when neonatal rat ventricular myocytes were treated with sweet and umami agonists (aspartame for the sweet taste receptor and monosodium glutamate for the umami receptor), they had increased calcium transients as shown by an increase in peak calcium. Cardiomyocytes treated with aspartame also showed a decrease in time to relax. We hypothesize that in the heart, sweet and umami receptors induce positive inotropy upon a change in nutrient environment.

Detection of tastes is critical for animals. Sweet, umami and bitter taste are mediated by G-protein-coupled receptors that are expressed in the taste receptor cells. TAS1Rs which belong to class C G-protein-coupled receptors form heterodimeric complexes to function as sweet (TAS1R2 + TAS1R3) or umami (TAS1R1 + TAS1R3) taste receptors. Umami taste is also considered to be mediated by mGluRs. TAS2Rs belong to class A G-protein-coupled receptors and are responsible for bitter taste. After activation of these receptors, their second messenger pathways lead to depolarization and intracellular calcium increase in taste receptor cells. Then, transmitter is released from taste receptor cells leading to activation of taste nerve fibers and taste information is sent to the central nervous system. Recent studies on heterologous expression system and molecular modeling lead to better understanding of binding site of TAS1Rs and TAS2Rs and molecular mechanisms for interaction between taste substances and these receptors. TAS1Rs and TAS2Rs have multiple and single binding sites for structurally diverse ligands, respectively. Sensitivities of these receptors are known to differ among individuals, strains, and species. In addition, some species abolish these receptors and signaling molecules. Here we focus on structure, function, signaling, polymorphism, and molecular evolution of the taste G-protein-coupled receptors.

Different Neural Processing of Umami and Salty Taste Determined by Umami Identification Ability Independent of Repeated Umami Exposure