Detection of sweet and umami taste in the absence of taste receptor T1r3

Oral, post-oral and genetic interactions in sweet appetite

Inbred mouse strains differ in their preferences for sweeteners, due in part to variations in their T1R3 sweet taste receptor. Recent studies of sweet sensitive C57BL/6J (B6) and subsensitive 129P3/J (129) mice indicate that experiential and post-oral effects of sugar substantially modify sweetener preference. In fact, the strain difference in sucrose preference disappeared after the mice were given 23 h/day tests with sucrose at ascending concentrations (0.5-32%). Intragastric infusions of sucrose (16%) also conditioned increased preference for and absolute intake of flavored sweet solutions in B6 and 129 mice. An operant analysis of sweetener appetite revealed, unexpectedly, that sugar-experienced 129 mice respond more vigorously than B6 mice for 16% sucrose rewards. These findings indicate that experiential and nutritional factors can, to some degree, override genetic differences in peripheral taste sensitivity in determining food appetite.

Enhanced sucrose and Polycose preference in sweet “sensitive” (C57BL/6J) and “subsensitive” (129P3/J) mice after experience with these saccharides

Prior research with inbred mouse strains indicates that C57BL/6J (B6) mice display stronger preference and acceptance for various sweeteners than do 129P3/J (129) mice. Experiment 1 examined the extent to which this strain difference could be modified by repeated exposure to sucrose. Sucrose-naive 129 mice displayed weaker preferences than did B6 mice for 0.5% to 4% sucrose solutions during 23h/day sugar vs. water tests. Sucrose preference did not differ at 8-32% concentrations. Yet, when retested with sucrose, the 129 and B6 mice showed identical robust preferences (>90%) for 0.5-32% solutions. The strains also did not differ in sucrose preference in tests with descending sucrose concentrations (0.5-0.0625%). Sucrose-experienced 129 mice also showed enhanced preference for dilute saccharin solutions suggesting that their sweet taste responsivity was enhanced. Experiment 2 revealed that preference for dilute saccharin solutions was enhanced by prior saccharin experience in B6 but not 129 mice. Experiment 3 tested the strains with Polycose which has a palatable taste different from that of sucrose. Polycose-naive 129 mice displayed weaker preferences for dilute (0.5-4%) but not concentrated (8-32%) Polycose solutions relative to B6 mice. In the second test series Polycose preferences were nearly identical in the two strains. In Experiments 1 and 3, prior sucrose or Polycose experience also reduced or eliminated strain differences in saccharide acceptance (absolute intake) at higher but not lower concentrations. Thus, exposure to the oral and post-oral actions of sucrose and Polycose increased saccharide preference in B6 mice and even more in 129 mice so that the strain difference virtually disappeared. Whether the 129 mice responded to the taste or other properties (e.g., odor) of the dilute saccharide solutions is not certain but their gustatory sensitivity needs to be reconsidered.

Sucrose motivation in sweet "sensitive" (C57BL/6J) and "subsensitive" (129P3/J) mice measured by progressive ratio licking

As compared to C57BL/6J mice, 129P3/J mice show weaker preferences for and lower intakes of dilute sugar solutions. These differences have been attributed to genetic differences in their sweet taste receptor. The two mouse strains do not differ, however, in their intake of concentrated sugar solutions. The post-oral satiating effect of concentrated sugar solutions may mask strain differences in the avidity for these solutions. This hypothesis was investigated using fixed ratio (FR, low demand) and progressive ratio (PR, high demand) operant licking tests (22h/day) to measure sugar appetite. In Experiment 1, sucrose-experienced 129 mice licked less than did B6 mice for 4% but not for 16% sucrose in free access bottle tests and FR operant tests. Yet, in PR tests the 129 mice licked as much for 4% sucrose and more for 16% sucrose than did B6 mice. In Experiment 2, sucrose-naive 129 mice licked less than did B6 mice in FR and PR tests with 0.4% saccharin but the strains did not differ in PR licking in their first test with 16% sucrose. After they were given unconstrained bottle access to 16% sucrose for 3 days, the 129 mice now licked more than B6 mice in a second sucrose PR test. Thus, despite having a less sensitive sweet taste receptor, 129 mice are as much or more motivated to obtain sucrose than are B6 mice and appear to be more influenced by prior experience with sugar. This suggests that the strains differ in their central reward processing of sweet taste.

Sucrose and monosodium glutamate taste thresholds and discrimination ability of T1R3 knockout mice

Molecular and behavioral studies have identified heterodimers of the T1R family as receptors for detecting the tastes of sweet (T1R2 + T1R3) and umami (T1R1 + T1R3). However, behavioral studies have reported conflicting findings with T1R3 knockout (KO) mice. One study showed a complete or nearly complete loss of preference for sweet and umami substances by KO mice, whereas KO mice in another study showed only a partial reduction in preferences for sucrose and monosodium glutamate (MSG), the prototypical umami substance. The present experiments used psychophysical methods to assess how sensitive T1R1-KO mice are to sucrose and MSG and discrimination methods to determine if these mice could distinguish between the tastes of sucrose and MSG. Detection thresholds of T1R3-KO mice and wild-type (WT) C57Bl mice were nearly identical for sucrose and MSG. Mice of both genotypes were easily able to discriminate between the tastes of sucrose and MSG. When amiloride (a sodium channel blocker) was added to all solutions to reduce the taste of Na+, discrimination accuracy of both genotypes of mice decreased but more so for the T1R3-KO mice than the WT mice. However, even when the sodium taste of MSG was neutralized, both genotypes could still discriminate between the two substances well above chance performance. These results suggest that sucrose and MSG can be detected by taste receptors other than T1R2 + T1R3 and T1R1 + T1R3 and that the conflicts between the previous studies may have been due to the methodological limitations.

Umami responses in mouse taste cells indicate more than one receptor

A number of gustatory receptors have been proposed to underlie umami, the taste of L-glutamate, and certain other amino acids and nucleotides. However, the response profiles of these cloned receptors have not been validated against responses recorded from taste receptor cells that are the native detectors of umami taste. We investigated umami taste responses in mouse circumvallate taste buds in an intact slice preparation, using confocal calcium imaging. Approximately 5% of taste cells selectively responded to L-glutamate when it was focally applied to the apical chemosensitive tips of receptor cells. The concentration-response range for L-glutamate fell approximately within the physiologically relevant range for taste behavior in mice, namely 10 mm and above. Inosine monophosphate enhanced taste cell responses to L-glutamate, a characteristic feature of umami taste. Using pharmacological agents, ion substitution, and immunostaining, we showed that intracellular pathways downstream of receptor activation involve phospholipase C beta2. Each of the above features matches those predicted by studies of cloned and expressed receptors. However, the ligand specificity of each of the proposed umami receptors [taste metabotropic glutamate receptor 4, truncated metabotropic glutamate receptor 1, or taste receptor 1 (T1R1) and T1R3 dimers], taken alone, did not appear to explain the taste responses observed in mouse taste cells. Furthermore, umami responses were still observed in mutant mice lacking T1R3. A full explanation of umami taste transduction may involve novel combinations of the proposed receptors and/or as-yet-undiscovered taste receptors.

Taste recognition: food for thought

The ability to identify food that is nutrient-rich and avoid toxic substances is essential for an animal's survival. Although olfaction and vision contribute to food detection, the gustatory system acts as a final checkpoint control for food acceptance or rejection behavior. Recent studies with model organisms such as mice and Drosophila have identified candidate taste receptors and examined the logic of taste coding in the periphery. Despite differences in terms of gustatory anatomy and taste-receptor families, these gustatory systems share a basic organization that is different from other sensory systems. This review will summarize our current understanding of taste recognition in mammals and Drosophila, highlighting similarities and raising several as yet unanswered questions.

Trpm5 null mice respond to bitter, sweet, and umami compounds

Trpm5 is a calcium-activated cation channel expressed selectively in taste receptor cells. A previous study reported that mice with an internal deletion of Trpm5, lacking exons 15-19 encoding transmembrane segments 1-5, showed no taste-mediated responses to bitter, sweet, and umami compounds. We independently generated knockout mice null for Trpm5 protein expression due to deletion of Trpm5's promoter region and exons 1-4 (including the translation start site). We examined the taste-mediated responses of Trpm5 null mice and wild-type (WT) mice using three procedures: gustatory nerve recording [chorda tympani (CT) and glossopharyngeal (NG) nerves], initial lick responses, and 24-h two-bottle preference tests. With bitter compounds, the Trpm5 null mice showed reduced, but not abolished, avoidance (as indicated by licking responses and preference ratios higher than those of WT), a normal CT response, and a greatly diminished NG response. With sweet compounds, Trpm5 null mice showed no licking response, a diminished preference ratio, and absent or greatly reduced nerve responses. With umami compounds, Trpm5 null mice showed no licking response, a diminished preference ratio, a normal NG response, and a greatly diminished CT response. Our results demonstrate that the consequences of eliminating Trmp5 expression vary depending upon the taste quality and the lingual taste field examined. Thus, while Trpm5 is an important factor in many taste responses, its absence does not eliminate all taste responses. We conclude that Trpm5-dependent and Trpm5-independent pathways underlie bitter, sweet, and umami tastes.

The liaison of sweet and savory

The sense of taste provides humans with necessary information about the composition and quality of food. For humans, five basic tastes are readily distinguishable and include sweet, bitter, salty, sour, and savory (or umami). Although each of these qualities has individualized transduction pathways, sweet and umami tastes are believed to share a common receptor element, the T1R3 receptor subunit. The two G-protein-coupled heteromer receptors that comprise an umami stimulus receptor (T1R1-T1R3) and a sweetener receptor (T1R2-T1R3) constitute a potential link between these two qualities of perception. While the role of the individual monomers in each human heteromer has been examined in vitro, very little is known of the implication of this research for human perception, or specifically, how sweet and savory taste perceptions may be connected. Using a psychophysical approach, we demonstrate that lactisole, a potent sweetness inhibitor that binds in vitro to hT1R3, also inhibits a significant portion of the perception of umami taste from monosodium glutamate. Following the molecular logic put forward by Xu et al. (2004, Proc. Natl Acad. Sci. USA, 101, 14258-14263), our psychophysical data support the in vitro hypothesis that the shared T1R3 monomer moderates the activation of both T1R2 and T1R1 in humans and impairs suprathreshold perception, respectively, of sweetness and, to a lesser degree, umaminess in the presence of lactisole.

Drinking spout orifice size affects licking behavior in inbred mice

Using a lickometer, we assessed the effect of drinking spout orifice size on the licking behavior of inbred mice [C57BL/6J, SWR/J, 129P3/J and DBA/2J]. Animals licked from drinking spout sipper tubes that had what were defined as either a large (2.7 mm) or a small (1.5 mm) orifice. Mice took approximately twice as many licks from a stationary single small orifice drinking spout than when licking from a spout with a large orifice during separate 30-min sessions. However, their total intake volume was approximately the same. We calculated that mice received a mean of 0.55 muL per lick from the drinking tubes with a small orifice and a mean of 1.15 muL per lick from the drinking tubes with a large orifice. Thus, the animals appear to have regulated their fluid intake by proportionally adjusting their licking as a function of the lick volume. On average, this regulation occurred through modulation of the size of licking bursts and not their frequency. However, strain differences in compensation strategy were observed. When licking was restricted to a series of 5-s trials in a 30-min brief access test session, the smaller orifice size increased the range of responsiveness that was expressed. Mice increased their average licks per trial by 20% and took 60% more trials when licking from a spout with a small orifice. Interestingly, when the orifice size was quasi-randomly varied within a brief access session, licking was greater from large orifice drinking spouts, suggesting that water delivered from the two orifice sizes differs in its reinforcement efficacy. These findings demonstrate that drinking spout orifice size can significantly influence experimental outcomes in licking tests involving mice and care should be taken in controlling this variable in testing the effects of taste or other factors on ingestive behavior.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

Structure–Sweetness Relationship in Egg White Lysozyme: Role of Lysine and Arginine Residues on the Elicitation of Lysozyme Sweetness

Lysozyme is one of the sweet-tasting proteins. To clarify the structure-sweetness relationship and the basicity-sweetness relationship in lysozyme, we have generated lysozyme mutants with Pichia systems. Alanine substitution of lysine residues demonstrated that two out of six lysine residues, Lys13 and Lys96, are required for lysozyme sweetness, while the remaining four lysine residues do not play a significant role in the perception of sweetness. Arginine substitution of lysine residues revealed that the basicity, but not the shape, of the side chain plays a significant role in sweetness. Single alanine substitutions of arginine residues showed that three arginine residues, Arg14, Arg21, and Arg73, play significant roles in lysozyme sweetness, whereas Arg45, Arg68, Arg125 and chemical modification by 1,2-cyclohexanedione did not affect sweetness. From investigation of the charge-specific mutations, we found that the basicity of a broad surface region formed by five positively charged residues, Lys13, Lys96, Arg14, Arg21, and Arg73, is required for lysozyme sweetness. Differences in the threshold values among sweet-tasting proteins might be caused by the broadness and/or the density of charged residues on the protein surface.

Initial licking responses of mice to sweeteners: effects of tas1r3 polymorphisms

Recent studies have established that the T1R3 receptor plays a central role in the taste-mediated ingestive response to sweeteners by mice. First, transgenic mice lacking the gene for T1R3, Tas1r3, show dramatically reduced lick responsiveness to most sweeteners. Second, strains with the taster allele of Tas1r3 (T strains) are more sensitive to low sweetener concentrations than strains with the nontaster allele (NT strains) and consume greater quantities of low- to midrange concentrations of sweeteners during 24-h tests. We asked how Tas1r3 polymorphisms influence the initial licking responses of four T strains (FVB/NJ, SWR/J, SM/J, and C57BL/6J) and four NT strains (BALB/cJ, 129P3/J, DBA/2J, and C3H/HeJ) to two sweeteners (sucrose and SC-45647, an artificial sweetener). We used the initial licking response as a measure of the taste-mediated ingestive response because its brief duration minimizes the potential contribution of nontaste factors (e.g., negative and positive postingestive feedback). Further, we used two complimentary short-term intake tests (the brief-access taste test and a novel 1-min preference test) to reduce the possibility that our findings were an epiphenomenon of a specific testing procedure. In both tests, the T strains were more responsive than the NT strains to low concentrations of each sweetener. At higher concentrations, however, there was considerable overlap between the T and NT strains. In fact, the initial licking response of several NT strains was more vigorous than (or equivalent to) that of several T strains. There was also considerable variation among strains with the same Tas1r3 allele. We conclude that Tas1r3 polymorphisms contribute to strain differences in initial lick responsiveness to low but not high concentrations of sweeteners.

Identification of the cyclamate interaction site within the transmembrane domain of the human sweet taste receptor subunit T1R3

The artificial sweetener cyclamate tastes sweet to humans, but not to mice. When expressed in vitro, the human sweet receptor (a heterodimer of two taste receptor subunits: hT1R2 + hT1R3) responds to cyclamate, but the mouse receptor (mT1R2 + mT1R3) does not. Using mixed-species pairings of human and mouse sweet receptor subunits, we determined that responsiveness to cyclamate requires the human form of T1R3. Using chimeras, we determined that it is the transmembrane domain of hT1R3 that is required for the sweet receptor to respond to cyclamate. Using directed mutagenesis, we identified several amino acid residues within the transmembrane domain of T1R3 that determine differential responsiveness to cyclamate of the human versus mouse sweet receptors. Alanine-scanning mutagenesis of residues predicted to line a transmembrane domain binding pocket in hT1R3 identified six residues specifically involved in responsiveness to cyclamate. Using molecular modeling, we docked cyclamate within the transmembrane domain of T1R3. Our model predicts substantial overlap in the hT1R3 binding pockets for the agonist cyclamate and the inverse agonist lactisole. The transmembrane domain of T1R3 is likely to play a critical role in the interconversion of the sweet receptor from the ground state to the active state.

Expression of sweet taste receptors of the T1R family in the intestinal tract and enteroendocrine cells

The composition of the intestinal luminal content varies considerably with diet. It is important therefore that the intestinal epithelium senses and responds to these significant changes and regulates its functions accordingly. Although it is becoming evident that the gut epithelium senses and responds to luminal nutrients, little is known about the nature of the nutrient sensing molecule and the downstream cellular events. A prototype example is the modulation in the capacity of the gut to absorb monosaccharides via the intestinal luminal membrane Na(+)/glucose cotransporter, SGLT1. The experimental evidence suggests that luminal sugar is sensed by a glucose sensor residing on the luminal membrane of the gut epithelium and linked to a G-protein-coupled receptor, cAMP/PKA (protein kinase A) pathway, resulting ultimately in modulation of intestinal monosaccharide absorption. Here we report the expression, at mRNA and protein levels, of members of the T1R sweet taste receptors, and the alpha-subunit of the G-protein gustducin, in the small intestine and the enteroendocrine cell line, STC-1. In the small intestine, there is a highly coordinated expression of sweet taste receptors and gustducin, a G-protein implicated in intracellular taste signal transduction, throughout the gut. The potential involvement of these receptors in sugar sensing in the intestine will facilitate our understanding of intestinal nutrient sensing, with implications for better nutrition and health maintenance.

Male rats show an indifference-avoidance response for increasing concentrations of the artificial sweetener sucralose

Sucralose is a non-nutritive halogenated sucrose derivative that has been described by humans as tasting predominately sweet with little or no aftertaste. In this study we examined the preference for sucralose in adult male Sprague Dawley rats. A standard 24 hr two-bottle test was used to compare a wide range of sucralose concentrations (0.0003-10g/L; 0.8 μM-25 mM) with water. The rats did not prefer sucralose to water at low concentrations (0.0003-0.3 g/L) and avoided sucralose at high concentrations (1-10g/L). Although there are many similarities in the taste preference of humans, mice, and rats, these results suggest that male rats do not prefer sucralose and avoid it at high concentrations. An awareness of the potential species differences in preference testing for novel sweeteners is critical for the taste and nutritional research communities.

Teaching resources. Sensory systems: taste perception

This teaching resource provides lecture notes and slides for a class covering the human taste system and is part of the course "Cell signaling systems: a course for graduate students." The lecture begins with a discussion of five distinct qualities of taste and then proceeds to describe receptors and signaling mechanisms.

Bitter taste receptors for saccharin and acesulfame K

Weight-conscious subjects and diabetics use the sulfonyl amide sweeteners saccharin and acesulfame K to reduce their calorie and sugar intake. However, the intrinsic bitter aftertaste, which is caused by unknown mechanisms, limits the use of these sweeteners. Here, we show by functional expression experiments in human embryonic kidney cells that saccharin and acesulfame K activate two members of the human TAS2R family (hTAS2R43 and hTAS2R44) at concentrations known to stimulate bitter taste. These receptors are expressed in tongue taste papillae. Moreover, the sweet inhibitor lactisole did not block the responses of cells transfected with TAS2R43 and TAS2R44, whereas it did block the response of cells expressing the sweet taste receptor heteromer hTAS1R2-hTAS1R3. The two receptors were also activated by nanomolar concentrations of aristolochic acid, a purely bitter-tasting compound. Thus, hTAS2R43 and hTAS2R44 function as cognate bitter taste receptors and do not contribute to the sweet taste of saccharin and acesulfame K. Consistent with the in vitro data, cross-adaptation studies in human subjects also support the existence of common receptors for both sulfonyl amide sweeteners.

Mouse Strain Differences in Gurmarin-sensitivity of Sweet Taste Responses Are Not Associated with Polymorphisms of the Sweet Receptor Gene, Tas1r3

Gurmarin (Gur) is a peptide that selectively inhibits responses of the chorda tympani (CT) nerve to sweet compounds in rodents. In mice, the sweet-suppressing effect of Gur differs among strains. The inhibitory effect of Gur is clearly observed in C57BL/6 mice, but only slightly, if at all, in BALB/c mice. These two mouse strains possess different alleles of the sweet receptor gene, Sac (Tas1r3) (taster genotype for C57BL/6 and non-taster genotype for BALB/c mice), suggesting that polymorphisms in the gene may account for differential sensitivity to Gur. To investigate this possibility, we examined the effect of Gur in another Tas1r3 non-taster strain, 129 X 1/Sv mice. The results indicated that unlike non-taster BALB/c mice but similar to taster C57BL/6 mice, 129 X 1/Sv mice exhibited significant inhibition of CT responses to various sweet compounds by Gur. This suggests that the mouse strain difference in the Gur inhibition of sweet responses of the CT nerve may not be associated with polymorphisms of Tas1r3.

Dimerization of alpha1-adrenoceptors

Three distinct genes encode alpha(1)-adrenoceptors. Although homodimers of each subtype have been reported, certain but not all combinations of heterodimers of the alpha(1)-adrenoceptors appear to form. Key studies in this field are reviewed and the approaches that have been applied to monitoring the selectivity and the basis of alpha(1)-adrenoceptor dimerization are discussed.

Umami taste responses are mediated by alpha-transducin and alpha-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. Alpha-gustducin, a taste cell-expressed G-protein alpha subunit closely related to the alpha-transducins, is a key mediator of sweet and bitter tastes. Alpha-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 alpha-gustducin also mediates umami taste and whether rod alpha-transducin (alpha(t-rod)), which is also expressed in taste receptor cells, plays a role in any of the taste responses that remain in alpha-gustducin KO mice. Behavioral tests and taste nerve recordings of single and double KO mice lacking alpha-gustducin and/or alpha(t-rod) confirmed the involvement of alpha-gustducin in bitter (quinine and denatonium) and sweet (sucrose and SC45647) taste and demonstrated the involvement of alpha-gustducin in umami [monosodium glutamate (MSG), monopotassium glutamate (MPG), and inosine monophosphate (IMP)] taste as well. We found that alpha(t-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 alpha-gustducin and alpha(t-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.

Irradiation impairment of umami taste in patients with head and neck cancer

BACKGROUND

Previous studies have reported the alteration of the four basic tastes (sweet, salty, sour and bitter) in patients with head and neck cancer (HNC) during radiotherapy. However, there has been no investigation on the function of umami taste, a novel basic taste recognized recently, during head and neck irradiation.

OBJECTIVE

To investigate the recognition threshold of umami and the four basic tastes at different irradiation dose intervals during radiotherapy for head and neck cancer.

METHODS

In 30 patients with malignant neoplasm of head and neck, objective taste thresholds were examined by use of a whole-mouth method at pre-radiotherapy, irradiation doses at 15, 30, 45, and 60 Gy, respectively. Subjective taste loss and some distresses were recorded simultaneously.

RESULTS

Sweet, sour, salty, and bitter tastes showed temporarily and slightly increased thresholds during the treatment, but no statistical difference was found between the threshold at pre-radiotherapy and that at 15, 30, 45, and 60 Gy in any taste quality (all P > 0.05). Significantly impaired threshold of umami taste was revealed at 30 Gy (P < 0.05) and remained throughout the following treatment (at 45 and 60 Gy, both P < 0.01). Subjective taste impairment, appetite loss and satisfaction with the current state tended to deteriorate significantly during the irradiation. Mean body weight of the patients experienced a continuous loss, decreasing from an average of 60.4 kg before treatment to 57.3 kg at 60 Gy (P < 0.01). Scores of satisfaction with current state showed a significant correlation with umami taste thresholds (P = 0.035).

CONCLUSION

The clinical impairment pattern of umami taste is different from that of the other four basic tastes in HNC patients during radiotherapy. Impaired umami taste acuity plays an important role in impacting the quality of life of the patients irradiated to the head and neck.

Gustatory neural responses to umami taste stimuli in C57BL/6ByJ and 129P3/J mice

In long-term two-bottle tests, mice from the C57BL/6ByJ (B6) strain drink more monosodium L-glutamate (MSG) and inosine-5'-monophosphate (IMP) compared with mice from the 129P3/J (129) strain. The goal of this study was to assess the role of afferent gustatory input in these strain differences. We measured integrated responses of the mouse chorda tympani and glossopharyngeal nerves to lingual application of compounds that evoke umami taste in humans: MSG, monoammonium L-glutamate (NH(4) glutamate), IMP and guanosine-5'-monophosphate (GMP) and also to other taste stimuli. Chorda tympani responses to MSG and NH(4) glutamate were similar in B6 and 129 mice. Chorda tympani responses to IMP and GMP were lower in B6 than in 129 mice. Responses to umami stimuli in the glossopharyngeal nerve did not differ between the B6 and 129 strains. Responses to MSG, IMP and GMP were not affected by sodium present in these compounds because B6 and 129 mice had similar neural taste responses to NaCl. This study has demonstrated that the increased ingestive responses to the umami stimuli in B6 mice are accompanied by either unchanged or decreased neural responses to these stimuli. Lack of support for the role of the chorda tympani or glossopharyngeal nerves in the enhanced consumption of MSG and IMP by B6 mice suggests that it is due to some other factors. Although results of our previous study suggest that postingestive effects of MSG can affect its intake, contribution of other gustatory components (e.g. greater superficial petrosal nerve or central gustatory processing) to the strain differences in consumption of umami compounds also cannot be excluded. Strain differences in gustatory neural responses to nucleotides but not glutamate suggest that these compounds may activate distinct taste transduction mechanisms.

Sweet proteins--potential replacement for artificial low calorie sweeteners

Exponential growth in the number of patients suffering from diseases caused by the consumption of sugar has become a threat to mankind's health. Artificial low calorie sweeteners available in the market may have severe side effects. It takes time to figure out the long term side effects and by the time these are established, they are replaced by a new low calorie sweetener. Saccharine has been used for centuries to sweeten foods and beverages without calories or carbohydrate. It was also used on a large scale during the sugar shortage of the two world wars but was abandoned as soon as it was linked with development of bladder cancer. Naturally occurring sweet and taste modifying proteins are being seen as potential replacements for the currently available artificial low calorie sweeteners. Interaction aspects of sweet proteins and the human sweet taste receptor are being investigated.

Expression of the sweet receptor protein, T1R3, in the human liver and pancreas

The expression of T1R3, a taste receptor essential for the perception of sweetness and umami-taste, was examined by immunohistochemistry to determine whether and where it may be localized in the liver and pancreas. In the liver, both immunopositive and immunonegative reactions were detected; bile ducts and intercalated portions of the bile ductules were immunopositive to T1R3, while arterioles and venules were immunonegative in interlobular connective tissue. In the hepatic lobule, all other cells including liver cells (hepatocytes) and bile capillaries were immunonegative. In the pancreas, all endocrine portions of the pancreas were immunonegative to T1R3. Within the exocrine portions, immunopositive reactions were detected in excretory duct cells, intercalated cells, and centroacinar cells. In contrast, acinar cells were immunonegative, as were vessels, lymph capillaries, nerve fibers, and connective tissue cells in the exocrine portions. The restricted localization of T1R3 in the duct cells of the liver and pancreas in the present study may indicate that T1R3 is involved in monitoring changes in the makeup of bile and pancreatic juices in the hepatic and pancreatic duct systems.

Lactisole interacts with the transmembrane domains of human T1R3 to inhibit sweet taste

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.

Elucidation of mammalian bitter taste

A family of approximately 30 TAS2R bitter taste receptors has been identified in mammals. Their genes evolved through adaptive diversification and are linked to chromosomal loci known to influence bitter taste in mice and humans. The agonists for various TAS2Rs have been identified and all of them were established as bitter tastants. TAS2Rs are broadly tuned to detect multiple bitter substances, explaining, in part, how mammals can recognize numerous bitter compounds with a limited set of receptors. The TAS2Rs are expressed in a subset of taste receptor cells, which are distinct from those mediating responses to other taste qualities. However, cells devoted to the detection of sweet, umami, and bitter stimuli share common signal transduction components. Transgenic expression of a human TAS2R in sweet or bitter taste receptor-expressing cells of mice induced either strong attraction or aversion to the receptor's cognate bitter tastant. Thus, dedicated taste receptor cells appear to function as broadly tuned detectors for bitter substances and are wired to elicit aversive behavior.

Mutant G-protein-coupled receptors as a cause of human diseases

G-protein-coupled receptors (GPCR) are involved in directly and indirectly controlling an extraordinary variety of physiological functions. Their key roles in cellular communication have made them the target for more than 60% of all currently prescribed drugs. Mutations in GPCR can cause acquired and inherited diseases such as retinitis pigmentosa (RP), hypo- and hyperthyroidism, nephrogenic diabetes insipidus, several fertility disorders, and even carcinomas. To date, over 600 inactivating and almost 100 activating mutations in GPCR have been identified which are responsible for more than 30 different human diseases. The number of human disorders is expected to increase given the fact that over 160 GPCR have been targeted in mice. Herein, we summarize the current knowledge relevant to understanding the molecular basis of GPCR function, with primary emphasis on the mechanisms underlying GPCR malfunction responsible for different human diseases.

Divergence of T2R chemosensory receptor families in humans, bonobos, and chimpanzees

T2R (Tas2R) genes encode a family of G protein-coupled gustatory receptors, several involved in bitter taste perception. So far, few ligands for these receptors have been identified, and the specificity of most T2Rs is unclear. Differences between individual T2Rs result in altered taste perception in either specificity or sensitivity. All 33 human T2Rs are characterized by significant sequence homology. However, with a total of eight pseudogenes and >83 coding region single-nucleotide polymorphisms, the family displays broad diversity. The underlying variability of individual T2Rs might be the source for personalized taste perception. To test this hypothesis and also to identify T2Rs that possibly function beyond bitter taste, we compared all human T2R genes with those of the closely related primate species Pan paniscus (bonobo) and Pan troglodytes (chimpanzee). The differences identified range from large sequence alterations to nonsynonymous and synonymous changes of single base pairs. In contrast to olfactory receptors, no human-specific loss of the amount of functional genes was observed. Taken together, the results indicate ongoing evolutionary diversification of T2R receptors and a role for T2Rs in dietary adaptation and personalized food uptake.

Different functional roles of T1R subunits in the heteromeric taste receptors

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.

Taste perception and coding in Drosophila

BACKGROUND

Discrimination between edible and contaminated foods is crucial for the survival of animals. In Drosophila, a family of gustatory receptors (GRs) expressed in taste neurons is thought to mediate the recognition of sugars and bitter compounds, thereby controlling feeding behavior.

RESULTS

We have characterized in detail the expression of eight Gr genes in the labial palps, the fly's main taste organ. These genes fall into two distinct groups: seven of them, including Gr66a, are expressed in 22 or fewer taste neurons in each labial palp. Additional experiments show that many of these genes are coexpressed in partially overlapping sets of neurons. In contrast, Gr5a, which encodes a receptor for trehalose, is expressed in a distinct and larger set of taste neurons associated with most chemosensory sensilla, including taste pegs. Mapping the axonal targets of cells expressing Gr66a and Gr5a reveals distinct projection patterns for these two groups of neurons in the brain. Moreover, tetanus toxin-mediated inactivation of Gr66a- or Gr5a-expressing cells shows that these two sets of neurons mediate distinct taste modalities-the perception of bitter (caffeine) and sweet (trehalose) taste, respectively.

CONCLUSION

Discrimination between two taste modalities-sweet and bitter-requires specific sets of gustatory receptor neurons that express different Gr genes. Unlike the Drosophila olfactory system, where each neuron expresses a single olfactory receptor gene, taste neurons can express multiple receptors and do so in a complex Gr gene code that is unique for small sets of neurons.

The cysteine-rich region of T1R3 determines responses to intensely sweet proteins

A wide variety of chemically diverse compounds taste sweet, including natural sugars such as glucose, fructose, sucrose, and sugar alcohols, small molecule artificial sweeteners such as saccharin and acesulfame K, and proteins such as monellin and thaumatin. Brazzein, like monellin and thaumatin, is a naturally occurring plant protein that humans, apes, and Old World monkeys perceive as tasting sweet but that is not perceived as sweet by other species including New World monkeys, mouse, and rat. It has been shown that heterologous expression of T1R2 plus T1R3 together yields a receptor responsive to many of the above-mentioned sweet tasting ligands. We have determined that the molecular basis for species-specific sensitivity to brazzein sweetness depends on a site within the cysteine-rich region of human T1R3. Other mutations in this region of T1R3 affected receptor activity toward monellin, and in some cases, overall efficacy to multiple sweet compounds, implicating this region as a previously unrecognized important determinant of sweet receptor function.

Extracellular proton sensing of the rat gustatory cyclic nucleotide-gated channel

Elevations of the intracellular levels of cyclic nucleotides appear to cause the cation influx through gustatory cyclic nucleotide-gated (CNGgust) channels expressed in taste cells. Although changes in the oral pH may directly regulate the activity of the CNGgust channel, the mechanism of pH-dependent control of the channel is not understood. In the present study, we combined the whole-cell patch-clamp recording and the site-directed mutagenesis to investigate the effect of extracellular pH on the ion permeation through CNGgust channels expressed in HEK293 cells. Extracellular acidification strongly inhibited ion permeation through open CNGgust channels. Mutation of Glu(289) remarkably attenuated the pH-dependence of the channel, suggesting that Glu(289) in the pore-forming region is a major proton acceptor site. However, the mutant E289A-CNGgust channel possesses the other residual protonation/deprotonation site. The channel activity, tightly regulated by pH(o) and [cNMP](i), suggests the involvement of its pH(o)-dependent ion permeation in taste signal transduction events.

Genetics of human taste perception

Genetic approaches are rapidly yielding new information about our sense of taste. This information comes from both molecular studies of genes encoding taste receptors and other taste-signaling components, and from studies of inherited variation in taste abilities. Our understanding of bitter taste has advanced by combined information from discovery and study of the TAS2R family of taste receptor genes, hand in hand with genetic linkage and positional cloning studies, notably on the ability to taste phenylthiocarbamide (PTC). Sweet and umami tastes, mediated by TAS1R receptors, are becoming well-characterized at the molecular genetic level, and these taste classes are now targets for linkage, positional cloning, and genetic association strategies. Salty and sour tastes are still poorly characterized in genetic terms, and represent opportunities for the future.

Alcohol Activates a Sucrose-Responsive Gustatory Neural Pathway

A strong positive association exists between the ingestion of alcohol and sweet-tasting solutions. The neural mechanisms underlying this relationship are unknown, although recent data suggest that gustatory substrates are involved. Here, we examined the role of sweet taste receptors and central neural circuits for sugar taste in the gustatory processing of ethanol. Taste responses to ethanol (3, 5, 10, 15, 25, and 40% vol/vol) and stimuli of different taste qualities (e.g., sucrose, NaCl, HCl, and quinine-HCl) were recorded from neurons of the nucleus of the solitary tract in anesthetized rats prior to and after oral application of the sweet receptor blocker gurmarin. The magnitude of ethanol-evoked activity was compared between sucrose-responsive ( n = 21) and sucrose-unresponsive ( n = 20) neurons and the central neural representation of ethanol taste was explored using multivariate analysis. Ethanol produced robust concentration-dependent responses in sucrose-responsive neurons that were dramatically larger than those in sucrose-unresponsive cells. Gurmarin selectively and similarly inhibited ethanol and sucrose responses, leaving NaCl, HCl, and quinine responses unaltered. Across-neuron patterns of response to ethanol were most similar to those evoked by sucrose, becoming increasingly more so as the ethanol concentration was raised. Results implicate taste receptors for sucrose as candidate receptors for ethanol and reveal that alcohol and sugar taste are represented similarly by gustatory activity in the CNS. These findings have important implications for the sensory and reward properties of alcohol.

Allelic variation of the Tas1r3 taste receptor gene selectively affects behavioral and neural taste responses to sweeteners in the F2 hybrids between C57BL/6ByJ and 129P3/J mice

Recent studies have shown that the T1R3 receptor protein encoded by the Tas1r3 gene is involved in transduction of sweet taste. To assess ligand specificity of the T1R3 receptor, we analyzed the association of Tas1r3 allelic variants with taste responses in mice. In the F2 hybrids between the C57BL/6ByJ (B6) and 129P3/J (129) inbred mouse strains, we determined genotypes of markers on chromosome 4, where Tas1r3 resides, measured consumption of taste solutions presented in two-bottle preference tests, and recorded integrated responses of the chorda tympani gustatory nerve to lingual application of taste stimuli. For intakes and preferences, significant linkages to Tas1r3 were found for the sweeteners sucrose, saccharin, and D-phenylalanine but not glycine. For chorda tympani responses, significant linkages to Tas1r3 were found for the sweeteners sucrose, saccharin, D-phenylalanine, D-tryptophan, and SC-45647 but not glycine, L-proline, L-alanine, or L-glutamine. No linkages to distal chromosome 4 were detected for behavioral or neural responses to non-sweet quinine, citric acid, HCl, NaCl, KCl, monosodium glutamate, inosine 5'-monophosphate, or ammonium glutamate. These results demonstrate that allelic variation of the Tas1r3 gene affects gustatory neural and behavioral responses to some, but not all, sweeteners. This study describes the range of ligand sensitivity of the T1R3 receptor using an in vivo approach and, to our knowledge, is the first genetic mapping study of activity in gustatory nerves.

Polymorphisms in the taste receptor gene (Tas1r3) region are associated with saccharin preference in 30 mouse strains

The results of recent studies suggest that the mouse Sac (saccharin preference) locus is identical to the Tas1r3 (taste receptor) gene. The goal of this study was to identify Tas1r3 sequence variants associated with saccharin preference in a large number of inbred mouse strains. Initially, we sequenced approximately 6.7 kb of the Tas1r3 gene and its flanking regions from six inbred mouse strains with high and low saccharin preference, including the strains in which the Sac alleles were described originally (C57BL/6J, Sac(b); DBA/2J, Sac(d)). Of the 89 sequence variants detected among these six strains, eight polymorphic sites were significantly associated with preferences for 1.6 mm saccharin. Next, each of these eight variant sites were genotyped in 24 additional mouse strains. Analysis of the genotype-phenotype associations in all 30 strains showed the strongest association with saccharin preference at three sites: nucleotide (nt) -791 (3 bp insertion/deletion), nt +135 (Ser45Ser), and nt +179 (Ile60Thr). We measured Tas1r3 gene expression, transcript size, and T1R3 immunoreactivity in the taste tissue of two inbred mouse strains with different Tas1r3 haplotypes and saccharin preferences. The results of these experiments suggest that the polymorphisms associated with saccharin preference do not act by blocking gene expression, changing alternative splicing, or interfering with protein translation in taste tissue. The amino acid substitution (Ile60Thr) may influence the ability of the protein to form dimers or bind sweeteners. Here, we present data for future studies directed to experimentally confirm the function of these polymorphisms and highlight some of the difficulties of identifying specific DNA sequence variants that underlie quantitative trait loci.

Taste perception: cracking the code

Our sense of taste begins with taste buds and ends in the brain. Researchers are beginning to unravel the mechanisms and connections that lie in between

Regional expression patterns of taste receptors and gustducin in the mouse tongue

In order to understand differences in taste sensitivities of taste bud cells between the anterior and posterior part of tongue, it is important to analyze the regional expression patterns of genes related to taste signal transduction on the tongue. Here we examined the expression pattern of a taste receptor family, the T1r family, and gustducin in circumvallate and fungiform papillae of the mouse tongue using double-labeled in situ hybridization. Each member of the T1r family was expressed in both circumvallate and fungiform papillae with some differences in their expression patterns. The most striking difference between fungiform and circumvallate papillae was observed in their co-expression patterns of T1r2, T1r3, and gustducin. T1r2-positive cells in fungiform papillae co-expressed T1r3 and gustducin, whereas T1r2 and T1r3 double-positive cells in circumvallate papillae merely expressed gustducin. These results suggested that in fungiform papillae, gustducin might play a role in the sweet taste signal transduction cascade mediated by a sweet receptor based on the T1r2 and T1r3 combination, in fungiform papillae.

The receptors for mammalian sweet and umami taste

Sweet and umami (the taste of monosodium glutamate) are the main attractive taste modalities in humans. T1Rs are candidate mammalian taste receptors that combine to assemble two heteromeric G-protein-coupled receptor complexes: T1R1+3, an umami sensor, and T1R2+3, a sweet receptor. We now report the behavioral and physiological characterization of T1R1, T1R2, and T1R3 knockout mice. We demonstrate that sweet and umami taste are strictly dependent on T1R-receptors, and show that selective elimination of T1R-subunits differentially abolishes detection and perception of these two taste modalities. To examine the basis of sweet tastant recognition and coding, we engineered animals expressing either the human T1R2-receptor (hT1R2), or a modified opioid-receptor (RASSL) in sweet cells. Expression of hT1R2 in mice generates animals with humanized sweet taste preferences, while expression of RASSL drives strong attraction to a synthetic opiate, demonstrating that sweet cells trigger dedicated behavioral outputs, but their tastant selectivity is determined by the nature of the receptors.