Keiko Yasumatsu

Heat activation of TRPM5 underlies thermal sensitivity of sweet taste

TRPM5, a cation channel of the TRP superfamily, is highly expressed in taste buds of the tongue, where it has a key role in the perception of sweet, umami and bitter tastes. Activation of TRPM5 occurs downstream of the activation of G-protein-coupled taste receptors and is proposed to generate a depolarizing potential in the taste receptor cells. Factors that modulate TRPM5 activity are therefore expected to influence taste. Here we show that TRPM5 is a highly temperature-sensitive, heat-activated channel: inward TRPM5 currents increase steeply at temperatures between 15 and 35 °C. TRPM4, a close homologue of TRPM5, shows similar temperature sensitivity. Heat activation is due to a temperature-dependent shift of the activation curve, in analogy to other thermosensitive TRP channels. Moreover, we show that increasing temperature between 15 and 35 °C markedly enhances the gustatory nerve response to sweet compounds in wild-type but not in Trpm5 knockout mice. The strong temperature sensitivity of TRPM5 may underlie known effects of temperature on perceived taste in humans, including enhanced sweetness perception at high temperatures and ‘thermal taste’, the phenomenon whereby heating or cooling of the tongue evoke sensations of taste in the absence of tastants.

Taste Preference for Fatty Acids Is Mediated by GPR40 and GPR120

The oral perception of fat has traditionally been considered to rely mainly on texture and olfaction, but recent findings suggest that taste may also play a role in the detection of long chain fatty acids. The two G-protein coupled receptors GPR40 (Ffar1) and GPR120 are activated by medium and long chain fatty acids. Here we show that GPR120 and GPR40 are expressed in the taste buds, mainly in type II and type I cells, respectively. Compared with wild-type mice, male and female GPR120 knock-out and GPR40 knock-out mice show a diminished preference for linoleic acid and oleic acid, and diminished taste nerve responses to several fatty acids. These results show that GPR40 and GPR120 mediate the taste of fatty acids.

Umami Taste Responses Are Mediated by α-Transducin and α-Gustducin

The sense of taste comprises at least five distinct qualities: sweet, bitter, sour, salty, and umami, the taste of glutamate. For bitter, sweet, and umami compounds, taste signaling is initiated by binding of tastants to G-protein-coupled receptors in specialized epithelial cells located in the taste buds, leading to the activation of signal transduction cascades. α-Gustducin, a taste cell-expressed G-protein α subunit closely related to the α-transducins, is a key mediator of sweet and bitter tastes. α-Gustducin knock-out (KO) mice have greatly diminished, but not entirely abolished, responses to many bitter and sweet compounds. We set out to determine whether α-gustducin also mediates umami taste and whether rod α-transducin (αt-rod), which is also expressed in taste receptor cells, plays a role in any of the taste responses that remain in α-gustducin KO mice. Behavioral tests and taste nerve recordings of single and double KO mice lacking α-gustducin and/or αt-rod confirmed the involvement of α-gustducin in bitter (quinine and denatonium) and sweet (sucrose and SC45647) taste and demonstrated the involvement ofα-gustducin in umami [monosodium glutamate (MSG), monopotassium glutamate (MPG), and inosine monophosphate (IMP)] taste as well. We found that αt-rod played no role in taste responses to the salty, bitter, and sweet compounds tested or to IMP but was involved in the umami taste of MSG and MPG. Umami detection involving α-gustducin and αt-rod occurs in anteriorly placed taste buds, however taste cells at the back of the tongue respond to umami compounds independently of these two G-protein subunits.

Endocannabinoids selectively enhance sweet taste

Endocannabinoids such as anandamide [N-arachidonoylethanolamine (AEA)] and 2-arachidonoyl glycerol (2-AG) are known orexigenic mediators that act via CB1 receptors in hypothalamus and limbic forebrain to induce appetite and stimulate food intake. Circulating endocannabinoid levels inversely correlate with plasma levels of leptin, an anorexigenic mediator that reduces food intake by acting on hypothalamic receptors. Recently, taste has been found to be a peripheral target of leptin. Leptin selectively suppresses sweet taste responses in wild-type mice but not in leptin receptor-deficient db/db mice. Here, we show that endocannabinoids oppose the action of leptin to act as enhancers of sweet taste. We found that administration of AEA or 2-AG increases gustatory nerve responses to sweeteners in a concentration-dependent manner without affecting responses to salty, sour, bitter, and umami compounds. The cannabinoids increase behavioral responses to sweet-bitter mixtures and electrophysiological responses of taste receptor cells to sweet compounds. Mice genetically lacking CB1 receptors show no enhancement by endocannnabinoids of sweet taste responses at cellular, nerve, or behavioral levels. In addition, the effects of endocannabinoids on sweet taste responses of taste cells are diminished by AM251, a CB1 receptor antagonist, but not by AM630, a CB2 receptor antagonist. Immunohistochemistry shows that CB1 receptors are expressed in type II taste cells that also express the T1r3 sweet taste receptor component. Taken together, these observations suggest that the taste organ is a peripheral target of endocannabinoids. Reciprocal regulation of peripheral sweet taste reception by endocannabinoids and leptin may contribute to their opposing actions on food intake and play an important role in regulating energy homeostasis.

Abnormal taste perception in mice lacking the type 3 inositol 1,4,5-trisphosphate receptor

Inositol 1,4,5-trisphosphate receptor (IP3R) is one of the important calcium channels expressed in the endoplasmic reticulum and has been shown to play crucial roles in various physiological phenomena. Type 3 IP3R is expressed in taste cells, but the physiological relevance of this receptor in taste perception in vivo is still unknown. Here, we show that mice lacking IP3R3 show abnormal behavioral and electrophysiological responses to sweet, umami, and bitter substances that trigger G-protein-coupled receptor activation. In contrast, responses to salty and acid tastes are largely normal in the mutant mice. We conclude that IP3R3 is a principal mediator of sweet, bitter, and umami taste perception and would be a missing molecule linking phospholipase C β2 to TRPM5 activation.

Sour taste responses in mice lacking PKD channels.

Background The polycystic kidney disease-like ion channel PKD2L1 and its associated partner PKD1L3 are potential candidates for sour taste receptors. PKD2L1 is expressed in type III taste cells that respond to sour stimuli and genetic elimination of cells expressing PKD2L1 substantially reduces chorda tympani nerve responses to sour taste stimuli. However, the contribution of PKD2L1 and PKD1L3 to sour taste responses remains unclear. Methodology/Principal Findings We made mice lacking PKD2L1 and/or PKD1L3 gene and investigated whole nerve responses to taste stimuli in the chorda tympani or the glossopharyngeal nerve and taste responses in type III taste cells. In mice lacking PKD2L1 gene, chorda tympani nerve responses to sour, but not sweet, salty, bitter, and umami tastants were reduced by 25–45% compared with those in wild type mice. In contrast, chorda tympani nerve responses in PKD1L3 knock-out mice and glossopharyngeal nerve responses in single- and double-knock-out mice were similar to those in wild type mice. Sour taste responses of type III fungiform taste cells (GAD67-expressing taste cells) were also reduced by 25–45% by elimination of PKD2L1. Conclusions/Significance These findings suggest that PKD2L1 partly contributes to sour taste responses in mice and that receptors other than PKDs would be involved in sour detection.

Discrimination of taste qualities among mouse fungiform taste bud cells

Multiple lines of evidence from molecular studies indicate that individual taste qualities are encoded by distinct taste receptor cells. In contrast, many physiological studies have found that a significant proportion of taste cells respond to multiple taste qualities. To reconcile this apparent discrepancy and to identify taste cells that underlie each taste quality, we investigated taste responses of individual mouse fungiform taste cells that express gustducin or GAD67, markers for specific types of taste cells. Type II taste cells respond to sweet, bitter or umami tastants, express taste receptors, gustducin and other transduction components. Type III cells possess putative sour taste receptors, and have well elaborated conventional synapses. Consistent with these findings we found that gustducin‐expressing Type II taste cells responded best to sweet (25/49), bitter (20/49) or umami (4/49) stimuli, while all GAD67 (Type III) taste cells examined (44/44) responded to sour stimuli and a portion of them showed multiple taste sensitivities, suggesting discrimination of each taste quality among taste bud cells. These results were largely consistent with those previously reported with circumvallate papillae taste cells. Bitter‐best taste cells responded to multiple bitter compounds such as quinine, denatonium and cyclohexamide. Three sour compounds, HCl, acetic acid and citric acid, elicited responses in sour‐best taste cells. These results suggest that taste cells may be capable of recognizing multiple taste compounds that elicit similar taste sensation. We did not find any NaCl‐best cells among the gustducin and GAD67 taste cells, raising the possibility that salt sensitive taste cells comprise a different population.

The taste transduction channel TRPM5 is a locus for bitter-sweet taste interactions

Ordinary gustatory experiences, which are usually evoked by taste mixtures, are determined by multiple interactions between different taste stimuli. The most studied model for these gustatory interactions is the suppression of the responses to sweeteners by the prototype bitter compound quinine. Here we report that TRPM5, a cation channel involved in sweet taste transduction, is inhibited by quinine (EC50=50 μM at —50 mV) owing to a decrease in the maximal whole‐cell TRPM5 conductance and an acceleration of channel closure. Notably, quinine inhibits the gustatory responses of sweet‐sensitive gustatory nerves in wild type (EC50=§1.6 mM) but not in Trpm5 knockout mice. Quinine induces a dose‐ and time‐dependent inhibition of TRPM5‐dependent responses of single sweet‐sensitive fibers to sucrose, according to the re stricted diffusion of the drug into the taste tissue. Quinidine, the stereoisomer of quinine, has similar effects on TRPM5 currents and on sweet‐induced gus tatory responses. In contrast, the chemically unrelated bitter compound denatonium benzoate has an § 100 fold weaker effect on TRPM5 currents and, accord ingly, at 10 mM it does not alter gustatory responses to sucrose. The inhibition of TRPM5 by bitter compounds constitutes the molecular basis of a novel mechanism of taste interactions, whereby the bitter tastant inhibits directly the sweet transduction pathway.—Talavera, K., Yasumatsu, K., Yoshida, R., Margolskee, R. F., Voets, T., Ninomiya, Y., Nilius, B. The taste transduction channel TRPM5 is a locus for bitter‐sweet taste inter actions. FASEBJ. 22, 1343–1355 (2008)

Multiple receptors underlie glutamate taste responses in mice

l-Glutamate is known to elicit a unique taste, umami, that is distinct from the tastes of sweet, salty, sour, and bitter. Recent molecular studies have identified several candidate receptors for umami in taste cells, such as the heterodimer T1R1/T1R3 and brain-expressed and taste-expressed type 1 and 4 metabotropic glutamate receptors (brain-mGluR1, brain-mGluR4, taste-mGluR1, and taste-mGluR4). However, the relative contributions of these receptors to umami taste reception remain to be elucidated. We critically discuss data from recent studies in which mouse taste cell, nerve fiber, and behavioral responses to umami stimuli were measured to evaluate whether receptors other than T1R1/T1R3 are involved in umami responses. We particularly emphasized studies of umami responses in T1R3 knockout (KO) mice and studies of potential effects of mGluR antagonists on taste responses. The results of these studies indicate the existence of substantial residual responses to umami compounds in the T1R3-KO model and a significant reduction of umami responsiveness after administration of mGluR antagonists. These findings thus provide evidence of the involvement of mGluRs in addition to T1R1/T1R3 in umami detection in mice and suggest that umami responses, at least in mice, may be mediated by multiple receptors.

Umami taste in mice uses multiple receptors and transduction pathways

Non‐technical summary  The distinctive umami taste elicited by l‐glutamate and some other amino acids is thought to be initiated by G‐protein‐coupled receptors, such as heteromers of taste receptor type 1, members 1 and 3, and metabotropic glutamate receptors 1 and 4. We demonstrate the existence of multiple types of glutamate‐sensitive gustatory nerve fibres and the contribution of multiple receptors and transduction pathways to umami taste. Such multiple systems for umami taste may differentially contribute to the behavioural preference for glutamate and discriminability of glutamate taste.