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Snacking is an integral component of eating habits in young children that is often overlooked in nutrition research. While snacking is a substantial source of calories in preschoolers’ diets, there is limited knowledge about the factors that drive snacking patterns. The genetics of taste may help to better understand the snacking patterns of children. The rs1761667 single nucleotide polymorphism (SNP) in the CD36 gene has been linked to fat taste sensitivity, the rs35874116 SNP in the TAS1R2 gene has been related to sweet taste preference, and the rs713598 SNP in the TAS2R38 gene has been associated with aversion to bitter, green leafy vegetables. This study seeks to determine the cross-sectional associations between three taste receptor SNPs and snacking patterns among preschoolers in the Guelph Family Health Study. Preschoolers’ snack quality, quantity, and frequency were assessed using three-day food records and saliva was collected for SNP genotyping (n = 47). Children with the TT genotype in TAS1R2 consumed snacks with significantly more calories from sugar, and these snacks were consumed mostly in the evening. Total energy density of snacks was highest in the CC and CG genotypes compared to the GG genotype in TAS2R38, and also greater in the AA genotype in CD36 compared to G allele carriers, however this difference was not individually attributable to energy from fat, carbohydrates, sugar, or protein. Genetic variation in taste receptors may influence snacking patterns of preschoolers.

The T1R2 and T1R3 proteins are expressed in taste receptor cells and form a heterodimer binding with compounds described as sweet by humans. We examined whether Polycose taste might be mediated through this heterodimer by testing T1R2 knockout (KO) and T1R3 KO mice and their wild-type (WT) littermate controls in a series of brief-access taste tests (25-min sessions with 5-s trials). Sucrose, Na-saccharin, and Polycose were each tested for three consecutive sessions with order of presentation varied among subgroups in a Latin-Square manner. Both KO groups displayed blunted licking responses and initiated significantly fewer trials of sucrose and Na-saccharin across a range of concentrations. KO mice tested after Polycose exposure demonstrated some degree of concentration-dependent licking of sucrose, likely attributable to learning related to prior postingestive experience. These results are consistent with prior findings in the literature, implicating the T1R2+3 heterodimer as the principal taste receptor for sweet-tasting ligands, and also provide support for the potential of postingestive experience to influence responding in the KO mice. In contrast, T1R2 KO and T1R3 KO mice displayed concentration-dependent licking responses to Polycose that tracked those of their WT controls and in some cases licked midrange concentrations more; the number of Polycose trials initiated overall did not differ between KO and WT mice. Thus, the T1R2 and T1R3 proteins are individually unnecessary for normal concentration-dependent licking of Polycose to be expressed in a brief-access test. Whether at least one of these T1R protein subunits is necessary for normal Polycose responsiveness remains untested. Alternatively, there may be a novel taste receptor(s) that mediates polysaccharide taste.

Taste impairment is a complication of Diabetes mellitus and some studies have shown this taste disorder in diabetes. Diabetes can decrease the ability of individuals to detect and recognize sweet, salty and bitter tastes. alpha-Gustducin is a transducin-like G-protein selectively expressed in 20 - 30% of taste receptor cells, which has been shown to be involved in bitter, sweet and umami taste responses. The present study was performed to explore the protein and mRNA expression of alpha-gustducin in the taste buds of diabetic and control rat circumvallate papillae. Our results showed that the positive expression of alpha-gustducin in diabetic rat taste bud cells is higher than that in normal controls as shown by both immunohistochemistry and RT-PCR. There may be some variant of bitter, sweet or umami taste transduction during diabetes and that taste transduction variant may be one cause of diabetic taste impairment.

Calcium is an essential nutrient that induces a distinctive taste quality, but the sensing mechanism of calcium in the tongue is poorly understood. A recent study linked calcium to T1R3 receptor. Here, we propose another system for calcium taste involving the extracellular calcium-sensing receptor (CaSR). This G protein-coupled receptor that responds to calcium and magnesium cations is involved in calcium homeostasis regulating parathyroid and kidney functions. In this study, CaSR was found in isolated taste buds from rats and mice. It was expressed in a subset of cells in circumvallate and foliate papillae, with fewer cells in the fungiform papillae. This is the first evidence in mammals that locates CaSR in gustatory tissue and provides the basis for better understanding not only calcium taste but also the taste of multiple CaSR agonists.

Summary Glucagon-like peptide-1 (GLP-1), a 30-amino-acid peptide hormone, is a typical peptide of the brain–gut axis and can affect the metabolism of various tissues and organs. GLP-1 is secreted by intestinal L cells in response to nutrient ingestion. Some studies have shown that taste signaling elements were co-expressed in enteroendocrine cells of the small intestine, and in particular by L-cells. The present study was performed to explore the protein and mRNA expression of GLP-1 in the taste buds of rat circumvallate papillae and to try to determine the significance of its secretion. GLP-1 immunoreactivity was observed in spindle-shaped taste bud cells, with positive cells displaying a characteristic distribution of reaction product that was confined to the cytosol. Reverse transcription polymerase chain reaction (RT-PCR) assay showed that GLP-1 mRNA was expressed in circumvallate papillae. The expression of GLP-1 suggests that it may play an important role in the taste stimulation of nutrients and gut hormone secretion.

Glucagon-like peptide-1 (GLP-1), released from gut endocrine L cells in response to glucose, regulates appetite, insulin secretion, and gut motility. How glucose given orally, but not systemically, induces GLP-1 secretion is unknown. We show that human duodenal L cells express sweet taste receptors, the taste G protein gustducin, and several other taste transduction elements. Mouse intestinal L cells also express alpha-gustducin. Ingestion of glucose by alpha-gustducin null mice revealed deficiencies in secretion of GLP-1 and the regulation of plasma insulin and glucose. Isolated small bowel and intestinal villi from alpha-gustducin null mice showed markedly defective GLP-1 secretion in response to glucose. The human L cell line NCI-H716 expresses alpha-gustducin, taste receptors, and several other taste signaling elements. GLP-1 release from NCI-H716 cells was promoted by sugars and the noncaloric sweetener sucralose, and blocked by the sweet receptor antagonist lactisole or siRNA for alpha-gustducin. We conclude that L cells of the gut "taste" glucose through the same mechanisms used by taste cells of the tongue. Modulating GLP-1 secretion in gut "taste cells" may provide an important treatment for obesity, diabetes and abnormal gut motility.

Glutamate is one candidate for the neurotransmitters and/or neuromodulators involved in taste signaling in taste buds. Group II metabotropic glutamate receptors (mGluRs: mGluR2 and mGluR3) are known to function as presynaptic receptors that regulate the release of glutamate and/or other neurotransmitters in the central nervous system. Group II mGluRs are negatively linked to adenylyl cyclase through Galphai subunits and thereby reduce the turnover of cAMP. In rat taste tissues, a subset of adenylyl-cyclase-8-expressing taste cells coexpress the Galphai subunits gustducin and Galphai2. However, the expression patterns of group II mGluRs in rat taste tissues have not yet been elucidated. We have therefore examined the expression patterns of mGluR2, mGluR3, and gustducin in rat gustatory tissues. Reverse transcription/polymerase chain reaction assays have revealed that mGluR2 and mGluR3 mRNAs are expressed in the circumvallate papillae. In situ hybridization analyses have detected positive signals for mGluR2 and mGluR3 mRNAs only in the circumvallate taste buds. Among the fungiform, foliate, and circumvallate papillae, an antibody against mGluR2/3 labels a subset of taste bud cells and nerve fibers immediately beneath the taste lingual epithelium. Double-labeling experiments have demonstrated that mGluR2/3-positive cells coexpress gustducin. These results indicate that mGluR2 and mGluR3 are coupled to Galphai subunits and play roles in glutamate-mediated signaling in taste transductions.

In the soft palate, tongue, pharynx and larynx surrounding the oral region, taste buds are present, allowing the sensation of taste. On the tongue surface, 3 kinds of papillae are present: fungiform, foliate, and circumvallate. Approximately 5,000 taste buds cover the surface of the human tongue, with about 30% fungiform, 30% foliate and 40% circumvallate papillae. Each taste bud comprises 4 kinds of cells, namely high dark (type I), low light (type II), and intermediate (type III) cells in electron density and Merkel-like taste basal cells (type IV) located at a distance from taste pores. Type II cells sense taste stimuli and type III cells transmit taste signals to sensory afferent nerve fibers. However, type I and type IV cells are not considered to possess obvious taste functions. Synaptic interactions that mediate communication in taste cells provide signal outputs to primary afferent fibers. In the study of taste bud cells, molecular functional techniques using single cells have recently been applied. Serotonin (5-HT) plays a role in cell-to-cell transmission of taste signals. ATP fills the criterion of a neurotransmitter that activates receptors of taste nerve fibers. Findings on 5-HT and ATP suggest that various different transmitters and receptors are present in taste buds. However, no firm evidence for taste-evoked release from type III cells has been identified, except for 5-HT and ATP. These results suggest that different transmitters and receptors may not be present in taste buds. Accordingly, an understanding of how transmitters might function remains elusive.

Type 2 diabetes is closely associated with our daily diets and has become a global health problem with increasing number of patients. Maintaining energy homeostasis is essentially required for the treatment of diabetes. Energy metabolism starts with taking in a meal. Nutrients including amino acids, fatty acids and glucose in the digest have been shown to act on the neuroendocrine cells in the gastrointestinal (GI) tract, and thereby play important roles in energy homeostasis. Therefore, the GI tract is now recognized as a sensor system for nutrient signals. Taste receptor type 1 member 2 (T1R2) is known to function as a co-receptor with T1R3 to detect sweet chemicals in the taste buds. It has been proposed that the T1R2/T1R3 receptor complex acts as sweet sensor in the intestine, and plays a pivotal role in sensing sugars and maintaining glucose homeostasis through incretin secretion. To clarify the physiological roles of T1R2 in glucose homeostasis, T1r2-lacZ knock-in/knock-out mice were generated. We found lacZ gene expression in the GI tract where T1r3 expression has been reported. Interestingly, the T1r2-lacZ knock-in mice showed impaired glucose tolerance on oral glucose challenge but not on intraperitoneal injection. However, the fasting glucose level in T1r2-lacZ knock-in mice was comparable to that in wild type mice. These results suggest an important role of the sweet taste receptor system in the intestine when stimulated by glucose. Therefore, the roles of T1R2 will be presented and the mechanism for metabolic homeostasis will be discussed.

The aim of the study was to assess the relationship between sweet taste genes and dental caries prevalence in a large sample of adults. In addition, the association between sweet liking and sugar intake with dental caries was investigated. Caries was measured by the decayed, missing, filled teeth (DMFT) index in 647 Caucasian subjects (285 males and 362 females, aged 18–65 years), coming from six villages in northeastern Italy. Sweet liking was assessed using a 9-point scale, and the mean of the liking given by each individual to specific sweet food and beverages was used to create a sweet liking score. Simple sugar consumption was estimated by a dietary history interview, considering both added sugars and sugar present naturally in foods. Our study confirmed that polymorphisms in TAS1R2 and GLUT2 genes are related to DMFT index. In particular, GG homozygous individuals for rs3935570 in TAS1R2 gene (p value = 0.0117) and GG homozygous individuals for rs1499821 in GLUT2 gene (p value = 0.0273) showed higher DMFT levels compared to both heterozygous and homozygous for the alternative allele. Furthermore, while the relationship sugar intake–DMFT did not achieve statistical significance (p value = 0.075), a significant association was identified between sweet liking and DMFT (p value = 0.004), independent of other variables. Our study showed that sweet taste genetic factors contribute to caries prevalence and highlighted the role of sweet liking as a predictor of caries risk. Therefore, these results may open new perspectives for individual risk identification and implementation of target preventive strategies, such as identifying high-risk patients before caries development.

This is the first in a series of articles on presentations at the American Diabetes Association’s 66th Scientific Sessions, Washington, D.C., 9–13 June 2006. At a symposium at the 2006 American Diabetes Association (ADA) Scientific Sessions on the implications of taste perception, Robert F. Margolskee (New York, NY) discussed the molecular biology of taste. Multiple taste buds are contained within papillae of the tongue. Taste receptors are present in cells of the front, side, and posterior portion of the tongue, with different taste responses in different areas. The innervation of the taste buds involves the facial nerves and vagus. Each taste bud contains 50–100 taste receptor cells, specialized epithelial cells with neuron-like properties making contact with taste stimuli in the oral cavity, leading to activation of an intracellular cascade with subsequent neurotransmitter release. Two taste qualities, salty and sour, are mediated by microvilli in the superior portions of the taste receptor cells, with salty taste involving sodium-specific channels and sour taste involving a number of different mechanisms (either receptors or via blockade of apical potassium entry). The three other tastes are bitter, sweet, and umami (Japanese for “delicious,” the taste of monosodium glutamate), all mediated by G-protein receptors. Bitter taste receptors have 25–30 different structures coupled to a specific G-protein, while umami and sweet depend on a different family of taste receptors—T1R2 and T1R3 for sweet, and T1R1 and T1R3 (as well as other complex combinations) for the taste of glutamate. Both the sweet and umami receptors have large extracellular domains. Sweet taste genetics have been studied in inbred mice strains to determine the saccharine taste locus, with some strains tasting and others not tasting this. In humans, similar studies allowed identification of loci on chromosome 1p38.33 with a candidate gene identified for T1R3, which was previously thought to be …

Although domestic cats (Felis silvestris catus) possess an otherwise functional sense of taste, they, unlike most mammals, do not prefer and may be unable to detect the sweetness of sugars. One possible explanation for this behavior is that cats lack the sensory system to taste sugars and therefore are indifferent to them. Drawing on work in mice, demonstrating that alleles of sweet-receptor genes predict low sugar intake, we examined the possibility that genes involved in the initial transduction of sweet perception might account for the indifference to sweet-tasting foods by cats. We characterized the sweet-receptor genes of domestic cats as well as those of other members of the Felidae family of obligate carnivores, tiger and cheetah. Because the mammalian sweet-taste receptor is formed by the dimerization of two proteins (T1R2 and T1R3; gene symbols Tas1r2 and Tas1r3), we identified and sequenced both genes in the cat by screening a feline genomic BAC library and by performing PCR with degenerate primers on cat genomic DNA. Gene expression was assessed by RT-PCR of taste tissue, in situ hybridization, and immunohistochemistry. The cat Tas1r3 gene shows high sequence similarity with functional Tas1r3 genes of other species. Message from Tas1r3 was detected by RT-PCR of taste tissue. In situ hybridization and immunohistochemical studies demonstrate that Tas1r3 is expressed, as expected, in taste buds. However, the cat Tas1r2 gene shows a 247-base pair microdeletion in exon 3 and stop codons in exons 4 and 6. There was no evidence of detectable mRNA from cat Tas1r2 by RT-PCR or in situ hybridization, and no evidence of protein expression by immunohistochemistry. Tas1r2 in tiger and cheetah and in six healthy adult domestic cats all show the similar deletion and stop codons. We conclude that cat Tas1r3 is an apparently functional and expressed receptor but that cat Tas1r2 is an unexpressed pseudogene. A functional sweet-taste receptor heteromer cannot form, and thus the cat lacks the receptor likely necessary for detection of sweet stimuli. This molecular change was very likely an important event in the evolution of the cat's carnivorous behavior., Synopsis Although sweet sugars are ubiquitous in human foods, they are seldom added to cat food, and owners usually do not feed sweets to their cats. This is because, in contrast to most other mammals, both domestic cats and their wild cousins, the big cats, do not show a preference for and, most likely, cannot detect sweet-tasting compounds. Other than this sweet blindness, the cat's sense of taste is normal. The molecular mechanism for this unique behavior towards sweets was not known, until now. Sweet compounds, including sugars and artificial sweeteners, are recognized by a special taste bud receptor composed of the products of two genes. The authors found that in cats, one of these genes is not functional and is not expressed. (It is called a pseudogene.) Because the sweet receptor cannot be formed, the cat cannot taste sweet stimuli. During the evolution of the cats' strictly carnivorous behavior, selection to maintain a functional receptor was apparently relaxed. This research provides a molecular explanation for the common observation that the cat lives in a different sensory world than the cat owner.

The taste buds are composed of heterogeneous cell populations with diverse properties and at different stages of maturity. It is important to define the relationships between cell properties and cell maturity to understand the molecular events involved in intracellular taste signaling. In the present study, in situ hybridization analysis indicated that group IIA phospholipase A2 (PLA2-IIA) is expressed in a subset of taste bud cells. Immunohistochemical studies showed that PLA2-IIA was expressed in a subset of cells expressing phospholipase Cβ2, a molecule essential for taste signaling in taste receptor cells, and also that some PLA2-IIA-positive cells expressed gustducin (Ggust), a bitter-taste-signaling molecule. Although PLA2-IIA and Ggust were expressed at similar frequencies in taste buds, bromodeoxyuridine (BrdU) chase experiments indicated that the expression of Ggust began 2 days after BrdU injection, whereas the expression of PLA2-IIA commenced after 4 days. In addition, PLA2-IIA was coexpressed with SNAP-25, a synaptosomal-associated protein. These results indicated that PLA2-IIA is expressed in mature taste receptor cells that possess exocytotic machinery. J. Comp. Neurol. 494:876–886, 2006. © 2005 Wiley-Liss, Inc.

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 thein vitrodata, cross-adaptation studies in human subjects also support the existence of common receptors for both sulfonyl amide sweeteners.

The T1R receptors, a family of taste-specific class C G proteincoupled 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.

Extracellular nucleotides such as ATP are the signaling molecules which bind to membrane receptors (P2X ligand-gated ion channels and G-protein-coupled P2Y families). In the gustatory system, it is known that P2X receptors are expressed exclusively in nerve fibers innervating the taste buds. Also, P2Y receptors are suggested to play some important roles in taste signal transductions on the basis of the physiological studies. In the present study, we examined the expression patterns of P2Y1 receptor subtype by using reverse transcript polymerase chain reaction (RT-PCR), in situ hybridization, and immunohistochemistry. RT-PCR analyses showed that P2Y1 receptor mRNAs appeared in circumvallate papillae. P2Y1 receptor mRNA was detected in a subset of taste bud cells by in situ hybridization. By immunohistochemical analyses, P2Y1 receptor was detected in a subset of taste bud cells of fungiform, foliate, and circumvallate papillae. We showed that ATP induced a biphasic intracellular Ca2+ increase in taste cells by a Ca2+ imaging method. Furthermore, we showed by double-immunolabeling methods that P2Y1-expressing cells coexpressed both IP3R3 and SNAP-25. These results suggest that ATP may activate P2Y receptors resulting in Ca2+ release from internal stores via IP3R3. Since many SNAP-25-immunoreactive taste bud cells coexpressed P2Y1 immunoreactivity, it is suggested that P2Y1-expressing cells may possess synapses with afferent nerve fibers. The results of the present study suggest that P2Y1 receptor may play some roles in ATP-mediated signal transductions between taste bud cells and afferent taste fibers.

Acid-sensing ion channel-2a (ASIC2a) is an amiloride-blockable proton-gated cation channel, probably contributing to sour-taste detection in rat taste cells. To isolate another subtype of the sour-taste receptor, we screened a rat circumvallate papilla cDNA library and identified ASIC2b, an N-terminal splice variant of ASIC2a. Reverse transcription-PCR analyses confirmed the expression of ASIC2b transcripts in the circumvallate papilla and, moreover, demonstrated its expression in the foliate and fungiform papillae. Immunohistochemical analyses revealed that ASIC2b, as well as ASIC2a, was expressed in a subpopulation of taste cells in the circumvallate, foliate, and fungiform papillae, and some of the cells displayed both ASIC2a and ASIC2b immunoreactivities. Subsequent coimmunoprecipitation studies with circumvallate papillae extracts indicated that ASIC2b associated with ASIC2a to form assemblies and, together with our immunohistochemical findings, strongly suggested that both ASIC2 subunits formed heteromeric channels in taste cells in the circumvallate, foliate, and fungiform papillae. Oocyte electrophysiology demonstrated that the ASIC2a/ASIC2b channel generated maximal inward currents at a pH ofor =2.0, which is in agreement with the in vivo pH sensitivity of rat taste cells, and that the amiloride sensitivity of the heteromer decreased with decreasing pH and was almost completely abolished at a pH of 2.0. These findings provide persuasive explanations for the amiloride insensitivity of acid-induced responses of rat taste cells.

Sweet taste is a powerful factor influencing food acceptance. The peripheral taste response to sugar is mediated by the TAS1R2/TAS1R3 taste receptors. The aim of the study was to determine the relationship between TAS1R2 (rs35874116 or rs9701796) and/or TAS1R3 (rs307355) single nucleotide polymorphisms with dental caries experience in schoolchildren. A total of 184 schoolchildren aged between 7 and 12 years (101 girls, 83 boys) were included in the study. Genomic DNA was extracted from saliva samples and the genotypes were identified by qPCR. The genotype frequencies were as follows: 6.6% for homozygous wild type, 41.8% for heterozygous and 51.6% for homozygous polymorphic genotype carriers of TAS1R2 gene rs35874116; 27.8% for heterozygous and 72.2% for homozygous polymorphic genotype carriers of TAS1R2 gene rs9701796, and 83.1% for homozygous wild type and 16.9% for heterozygous genotype carriers of TAS1R3 gene rs307355 polymorphism. A significant association was observed between total caries experience (dft + DMFT - decayed filled primary teeth + decayed, missing and filled permanent teeth) and TAS1R2 rs35874116 (p = 0.008) and TAS1R3 rs307355 (p = 0.04) gene polymorphisms but not for TAS1R2 gene rs9701796 polymorphism. TAS1R3 gene rs307355 polymorphism has been found to be an independent risk factor for dental caries experience by logistic regression analysis and to have increased the risk of caries. Moderate caries experience (4-7 caries) was found to be associated with TAS1R3 rs307355 heterozygous genotype, whereas high-risk caries experience (>8 caries) was found to be associated with TAS1R2 rs35874116 homozygous polymorphic genotype.

The sense of taste helps humans to obtain information and form a picture of the world by recognizing chemicals in their environments. Over the past decade, large advances have been made in understanding the mechanisms of taste detection and mimicking its capability using artificial sensor devices. However, the detection capability of previous artificial taste sensors has been far inferior to that of animal tongues, in terms of its sensitivity and selectivity. Herein, we developed a bioelectronic tongue using heterodimeric human sweet taste receptors for the detection and discrimination of sweeteners with human-like performance, where single-walled carbon nanotube field-effect transistors were functionalized with nanovesicles containing human sweet taste receptors and used to detect the binding of sweeteners to the taste receptors. The receptors are heterodimeric G-protein-coupled receptors (GPCRs) composed of human taste receptor type 1 member 2 (hTAS1R2) and human taste receptor type 1 member 3 (hTAS1R3), which have multiple binding sites and allow a human tongue-like broad selectivity for the detection of sweeteners. This nanovesicle-based bioelectronic tongue can be a powerful tool for the detection of sweeteners as an alternative to labor-intensive and time-consuming cell-based assays and the sensory evaluation panels used in the food and beverage industry. Furthermore, this study also allows the artificial sensor to exam the functional activity of dimeric GPCRs.

In the gustatory system, the recognition of sugars, amino acids and bitter-tasting compounds is the function of specialized G protein-coupled receptors. Recently, two members of novel subfamily of G protein-coupled receptors were proposed to function as taste receptors based on their specific expression in taste receptor cells. Here, we report the identification of a third member, T1R3, of this family of receptors. T1R3 maps near the telomere of mouse chromosome 4 rendering it a candidate for the Sac locus, a primary determinant of sweet preference in mice. Consistent with its candidacy for the Sac locus, T1R3 displays taste receptor cell-specific expression. In addition, taster and non-taster strains of mouse harbor different alleles of T1R3.

Neural responses to sweet and bitter stimuli in the rat and mouse are compared to the expression of the molecular taste receptors, Tas1r2/Tas2rs. Integrated taste responses from the greater superficial petrosal nerve (GSP) innervating the soft palate (SP) and the chorda tympani (CT) nerve innervating the fungiform papillae (FF) were recorded in C57BL mice and SD rats. The sum of the phasic and tonic response magnitudes (SRM) was calculated by summating all relative mean responses to a concentration series of QHCl (10(-6)-10(-2)M) or Suc (10(-4)-1.0M). Molecular expression was analyzed by double-colored in situ hybridization for Gα-gustducin with Tas1r2 or Tas2rs in the SP and FF. The vast majority of cells expressing Tas1r2 or Tas2rs were included in Gα-gustducin-expressing cells in the SP of both species. Unexpectedly, a comparison between species revealed that the SRM from the GSP is not positively correlated with receptor expression in the SP. In the rat SP, the percentage of Tas2rs with Gα-gustducin (Tas2rs/gust, 65%) was twice larger than that for Tas1r2/gust (33%), while the SRM to Suc in the rat GSP was 1.5 times (tonic and phasic) larger than that to QHCl. In the mouse SP, the percentage of Tas2rs/gust (46%) was less than that in the rat and similar to that of Tas1r2/gust (40%). However, the SRM to QHCl in the mouse GSP was 2.4 (phasic) and 4.7 (tonic) times larger than to Suc. On the other hand, threshold to Suc in the rat GSP was 10(-3)M, one log unit lower than in mouse, and the threshold to QHCl in the mouse GSP was 10(-6)M, one log unit lower than in rat. These results suggest that the robust GSP response to Suc in rat and to QHCl in mouse likely do not depend upon a large number of taste cells expressing the taste receptors Tas1r2 for Suc or Tas2rs for QHCl, but upon a higher density of Tas1r2/Tas2rs within the respective taste cells of the two species.

Taste receptor cells located in taste buds respond to gustatory stimuli using a complex arrangement of receptor molecules, signaling cascades, and ion channels. To depolarize taste receptor cells, ionic stimuli, such as salts and acids (protons), interact directly with apically located ion channels, whereas the second messenger systems via specific taste receptors are important in transducing bitter, sweet, and umami stimuli. In this article, we report the isolation of a cDNA encoding a taste receptor for sourness from a rat circumvallate papilla cDNA library by combined homology and functional expression approaches. Taste receptors for other tastes are also mentioned.

P2X receptors have been suggested to play a role in the transduction of sensory signals such as pain and sound. In the present study, polyclonal antibodies against P2X1 to P2X6 receptors were used to localize P2X receptors in circumvallate and fungiform papillae of rats. Nerve fibres innervating the taste buds stained intensely with P2X3 receptor antibodies. P2X3 receptor-positive nerves were observed in the intra- and subgemmal regions. The nerve fibres were also stained with P2X2 receptor antibodies, but the intensity was much lower. The distribution of P2X2 receptor immunoreactivity overlaps with that of P2X3. These results suggest that ATP might be a neurotransmitter in taste reception cells in the taste buds, where it transducts the taste signals to the afferent taste nerves by activating P2X receptors at the synapses. This is the first experiment indicating such a role for ATP, although supplementary functional studies are required.

Taste represents a major form of sensory input in the animal kingdom. In mammals, taste perception begins with the recognition of tastant molecules by unknown membrane receptors localized on the apical surface of receptor cells of the tongue and palate epithelium. We report the cloning and characterization of two novel seven-transmembrane domain proteins expressed in topographically distinct subpopulations of taste receptor cells and taste buds. These proteins are specifically localized to the taste pore and are members of a new group of G protein-coupled receptors distantly related to putative mammalian pheromone receptors. We propose that these genes encode taste receptors.

Domestic cats (Felis silvestris catus) (herein referred to as “cats”) are neither attracted to, nor show avoidance of the taste of sweet carbohydrates and high-intensity sweeteners (1-3), yet they do show a preference for selected amino acids (4), and avoid stimuli that taste either bitter or very sour to humans (1,4). Consistent with this behavioral evidence, recordings from cat taste nerve fibers and from units of the geniculate ganglion innervating taste cells demonstrated responses to salty, sour, and bitter stimuli as well as to amino acids and nucleotides, but showed no response to sucrose and several other sugars (4-11). The sense of taste in cats appears similar to that of other mammals with the exception of an inability to taste sweet stimuli. Because only the sweet taste modality appears absent, we postulated that the defect in cats (and likely in other obligate carnivores of Felidae) lay at the receptor step, subtending this modality. The possible defects at the molecular level could range from a single to a few amino acid substitutions, such as is found between sweet “taster” and “nontaster” strains of mice (12-14), to more radical mechanisms, such as an unexpressed pseudogene. To distinguish among these possibilities, we identified the DNA sequences and examined the structures of the 2 known genes Tas1r2 and Tas1r3 that encode the sweet taste receptor heteromer T1R2/T1R3 in other mammals. We compared these with the sequence and structure of the same genes in dogs, humans, mice and rats, all species that respond to sweet stimuli.

The G-protein subunit α-gustducin, which is similar to rod transducin, has been implicated in the transduction of both sweet- and bitter-tasting substances. In rodents, there are differences in sensitivity to sweet and bitter stimuli in different populations of taste buds. Rat fungiform taste buds are more responsive to salts than to sweet stimuli, whereas those on the palate respond predominantly to sweet substances. In contrast, hamster fungiform taste buds are more sensitive to sweet-tasting stimuli. Taste buds in the vallate and foliate papillae of both species are sensitive to bitter compounds. These differences in sensitivity should be reflected in the numbers of gustducin-containing cells in different taste bud populations. We examined taste buds in the rat and hamster for immunoreactivity to an antibody against α-gustducin. Immunofluorescence of labeled taste cells was examined by confocal microscopy, and the cells were counted. Gustducin-positive cells were seen in all taste bud regions; they were spindle-shaped, with circular cross-sections and apical processes that extended to the taste pore. Cells with this characteristic shape in rat vallate taste buds are Type II (light) cells. In the rat, taste buds of the fungiform papillae had fewer gustducin-positive cells (3.1/taste bud) than those of other regions, including the posterior tongue and palate (>8.9/taste bud). Hamster fungiform taste buds contained twice as many gustducin-expressing cells (6.8/taste bud) as those of the rat. These data support the hypothesis that α-gustducin is involved in the transduction of both sweet- and bitter-tasting stimuli by mammalian taste receptor cells.

Several lines of evidence suggest that both sweet and bitter tastes are transduced via receptors coupled to heterotrimeric guanine-nucleotide-binding proteins (G proteins). Gustducin is a taste receptor cell (TRC)-specific G protein that is closely related to the transducins. Gustducin and rod transducin, which is also expressed in TRCs, have been proposed to couple bitter-responsive receptors to TRC-specific phosphodiesterases to regulate intracellular cyclic nucleotides. Here we investigate gustducin's role in taste transduction by generating and characterizing mice deficient in the gustducin alpha-subunit (alpha-gustducin). As predicted, the mutant mice showed reduced behavioural and electrophysiological responses to bitter compounds, whereas they were indistinguishable from wild-type controls in their responses to salty and sour stimuli. Unexpectedly, mutant mice also exhibited reduced behavioural and electrophysiological responses to sweet compounds. Our results suggest that gustducin is a principal mediator of both bitter and sweet signal transduction.

Salivary secretions modulate taste perception. Taste buds in the circumvallate and foliate papillae are bathed in secretions of unique lingual salivary glands, von Ebner's glands (VEG). We have identified a rat cDNA encoding a novel protein of 1290 amino acids, Ebnerin, that is specifically expressed in VEG and released onto the tongue surface along the apical region of taste buds in the clefts of circumvallate papillae. Ebnerin possesses a putative single transmembrane domain at the C terminus with 17 amino acids in the cytoplasmic area. The extracellular region of Ebnerin contains a number of repeated domains with homology to the scavenger receptor cysteine-rich domain and to a repeated domain of bone morphogenetic protein-I and other related proteins. Western blot analysis reveals that Ebnerin exists in particulate and soluble forms in VEG and is present in secretions from VEG. In situ hybridization and immunohistochemistry demonstrate that Ebnerin is located in secretory duct epithelial cells of VEG and is released onto the tongue surface along the apical region of taste buds in the clefts of circumvallate papillae. The unique structure and localization of Ebnerin suggest that it may function as a binding protein in saliva for the regulation of taste sensation.

Sweet state is a basic physiological sensation of humans and other mammals which is mediated by the broadly acting sweet taste receptor-the heterodimer of Tas1r2 (taste receptor type 1 member 2) and Tas1r3 (taste receptor type 1 member 3). Various sweeteners interact with either Tas1r2 or Tas1r3 and then activate the receptor. In this study, we cloned, expressed and functionally characterized the taste receptor Tas1r2 from a species of Old World monkeys, the rhesus monkey. Paired with the human TAS1R3, it was shown that the rhesus monkey Tas1r2 could respond to natural sugars, amino acids and their derivates. Furthermore, similar to human TAS1R2, rhesus monkey Tas1r2 could respond to artificial sweeteners and sweet-tasting proteins. However, the responses induced by rhesus monkey Tas1r2 could not be inhibited by the sweet inhibitor amiloride. Moreover, we found a species-dependent activation of the Tas1r2 monomeric receptors of human, rhesus monkey and squirrel monkey but not mouse by an intense sweetener perillartine. Molecular modeling and sequence analysis indicate that the receptor has the conserved domains and ligand-specific interactive residues, which have been identified in the characterized sweet taste receptors up to now. This is the first report of the functional characterization of sweet taste receptors from an Old World monkey species.

Synaptogyrins are conserved components of the exocytic apparatus and function as regulators of Ca(2+)-dependent exocytosis. The synaptogyrin family comprises three isoforms: two neuronal (synaptogyrin-1 and -3) and one ubiquitous (synaptogyrin-2) form. Although the expression patterns of the exocytic proteins synaptotagmin-1, SNAP-25, synaptobrevin-2 and synaptophysin have been elucidated in taste buds, the function and expression pattern of synaptogyrin-1 in rat gustatory tissues have not been determined. Therefore, we examined the expression patterns of synaptogyrin-1 and several cell-specific markers of type II and III cells in rat gustatory tissues. Reverse transcription/polymerase chain reaction assays and immunoblot analysis revealed the expression of synaptogyrin-1 mRNA and its protein in circumvallate papillae. In fungiform, foliate and circumvallate papillae, the antibody against synaptogyrin-1 immunolabeled a subset of taste bud cells and intra- and subgemmal nerve processes. Double-labeling experiments revealed the expression of synaptogyrin-1 in most taste cells immunoreactive for aromatic L-amino acid decarboxylase and the neural cell adhesion molecule. A subset of synaptogyrin-1-immunoreactive taste cells also expressed phospholipase Cβ2, gustducin, or sweet taste receptor (T1R2). In addition, most synaptogyrin-1-immunoreactive taste cells expressed synaptobrevin-2. These results suggest that synaptogyrin-1 plays a regulatory role in transmission at the synapses of type III cells and is involved in exocytic function with synaptobrevin-2 in a subset of type II cells in rat taste buds.

Sweet taste receptors may enhance glucose absorption.This study aimed to explore the cell biology of sweet taste receptors on glucose uptake.Artificial sweeteners increase glucose uptake via activating sweet taste receptors in the enterocyte to translocate GLUT2 to the apical membrane through the PLC βII pathway.Caco-2, RIE-1, and IEC-6 cells, starved from glucose for 1 h were pre-incubated with 10 mM acesulfame potassium (AceK). Glucose uptake was measured by incubating cells for 1 to 10 min with 0.5-50 mM glucose with or without U-73122, chelerythrine, and cytochalasin B.In Caco-2 and RIE-1 cells, 10 mM AceK increased glucose uptake by 20-30 % at glucose25 mM, but not in lesser glucose concentrations (10 mM), nor at 1 min or 10 min incubations. U-73122 (PLC βII inhibitor) inhibited uptake at glucose25 mM and for 5 min incubation; chelerythrine and cytochalasin B had similar effects. No effect occurred in IEC-6 cells. Activation of sweet taste receptors had no effect on glucose uptake in low (25 mM) glucose concentrations but increased uptake at greater concentrations (25 mM).Role of artificial sweeteners on glucose uptake appears to act in part by effects on the enterocyte itself.

Food preferences are the main factor driving food intake and choice. There are good reasons to suspect some genetic influence on food acceptance, not least because genetic factors are implicated in a number of factors that are likely to be related to food choice. In addition, some food dislikes show themselves early in life, before there is any evidence for aversive experiences. Although taste has been widely studied in regards of pure tastes such as bitter or sweet perception, the relationship between taste-related genes and food preferences has seldom been explored. In this work we investigated relationship of 37 taste-related genes with food preferences. The study was carried out during a scientific expedition through Caucasus and Central Asia (Silk Road) analyzing more than 400 samples from 5 different countries. A food preference questionnaire was administered to each participant and a DNA sample was obtained. Other information, such as age, sex, life style and anthropometrical measures, were also collected. We found significant associations with variants of: (1) TAS1R2 [Correction added after initial online publication on 27 Aug 2012. TAS1R3 was changed to TAS1R2.] gene and liking of Vodka (P= 1.6 × 10(-3)), white wine (P= 4.0 × 10(-4)) and lamb meat (P= 1.6 × 10(-3)); (2) PCLB2 gene and preference for Hot Tea (P= 8.0 × 10(-4)); (3) TPRV1 gene and beet liking (P= 3.8 × 10(-5)); and (4) ITPR3 gene and liking of both lamb meat (5.8 × 10(-4)) and sheep cheese (8.9×10(-4)). These findings give a new insight on a better understanding, of genetic factors influencing food preferences which is critical to the development of effective dietary interventions, especially for people that may be genetically not predisposed for liking specific nutrients.

Evidence in the literature supports the hypothesis that the T1R2+3 heterodimer binds to compounds that humans describe as sweet. Here, we assessed the necessity of the T1R2 and T1R3 subunits in the maintenance of normal taste sensitivity to carbohydrate stimuli. We trained and tested water-restricted T1R2 knockout (KO), T1R3 KO and their wild-type (WT) same-sex littermate controls in a two-response operant procedure to sample a fluid and differentially respond on the basis of whether the stimulus was water or a tastant. Correct responses were reinforced with water and incorrect responses were punished with a time-out. Testing was conducted with a modified descending method of limits procedure across daily 25-min sessions. Both KO groups displayed severely impaired performance and markedly decreased sensitivity when required to discriminate water from sucrose, glucose, or maltose. In contrast, when Polycose was tested, KO mice had normal EC50 values for their psychometric functions, with some slight, but significant, impairment in performance. Sensitivity to NaCl did not differ between these mice and their WT controls. Our findings support the view that the T1R2+3 heterodimer is the principal receptor that mediates taste detection of natural sweeteners, but not of all carbohydrate stimuli. The combined presence of T1R2 and T1R3 appears unnecessary for the maintenance of relatively normal sensitivity to Polycose, at least in this task. Some detectability of sugars at high concentrations might be mediated by the putative polysaccharide taste receptor, the remaining T1R subunit forming either a homodimer or heteromer with another protein(s), or nontaste orosensory cues.

The molecular mechanisms of the mammalian gustatory system have been examined in many studies using rodents as model organisms. In this study, we examined the mRNA expression of molecules involved in taste signal transduction in the fungiform papillae (FuP) and circumvallate papillae (CvP) of the rhesus macaque, Macaca mulatta, using in situ hybridization. TAS1R1, TAS1R2, TAS2Rs, and PKD1L3 were exclusively expressed in different subsets of taste receptor cells (TRCs) in the FuP and CvP. This finding suggests that TRCs sensing different basic taste modalities are mutually segregated in macaque taste buds. Individual TAS2Rs exhibited a variety of expression patterns in terms of the apparent level of expression and the number of TRCs expressing these genes, as in the case of human TAS2Rs. GNAT3, but not GNA14, was expressed in TRCs of FuP, whereas GNA14 was expressed in a small population of TRCs of CvP, which were distinct from GNAT3- or TAS1R2-positive TRCs. These results demonstrate similarities and differences between primates and rodents in the expression profiles of genes involved in taste signal transduction.

Mammalian sweet taste is primarily mediated by the type 1 taste receptor Tas1r2/Tas1r3, whereas Tas1r1/Tas1r3 act as the principal umami taste receptor. Bitter taste is mediated by a different group of G protein-coupled receptors, the Tas2rs, numbering 3 to ∼66, depending on the species. We showed previously that the behavioral indifference of cats toward sweet-tasting compounds can be explained by the pseudogenization of the Tas1r2 gene, which encodes the Tas1r2 receptor. To examine the generality of this finding, we sequenced the entire coding region of Tas1r2 from 12 species in the order Carnivora. Seven of these nonfeline species, all of which are exclusive meat eaters, also have independently pseudogenized Tas1r2 caused by ORF-disrupting mutations. Fittingly, the purifying selection pressure is markedly relaxed in these species with a pseudogenized Tas1r2. In behavioral tests, the Asian otter (defective Tas1r2) showed no preference for sweet compounds, but the spectacled bear (intact Tas1r2) did. In addition to the inactivation of Tas1r2, we found that sea lion Tas1r1 and Tas1r3 are also pseudogenized, consistent with their unique feeding behavior, which entails swallowing food whole without chewing. The extensive loss of Tas1r receptor function is not restricted to the sea lion: the bottlenose dolphin, which evolved independently from the sea lion but displays similar feeding behavior, also has all three Tas1rs inactivated, and may also lack functional bitter receptors. These data provide strong support for the view that loss of taste receptor function in mammals is widespread and directly related to feeding specializations.

In our article (1), we proposed that loss of taste receptor function in mammals is directly related to feeding specializations. This conclusion was based, in part, on our finding that loss of taste receptor function is widespread in species of the order Carnivora that feed exclusively on meat. Zhao and Zhang (2) believe that this proposed relationship is unwarranted and provide several examples that they argue undercut it. We argue that the examples they cite generally support rather than refute our conclusion. First, Zhao and Zhang (2) point out that two exclusive meat-eaters in our study have not (yet) lost functional sweet taste receptor genes ( Tas1r2 ). Our hypothesis is that loss of sweet taste … [↵][1]1To whom correspondence may be addressed. E-mail: pjiang{at}monell.org or beauchamp{at}monell.org. [1]: #xref-corresp-1-1

The heterodimer of Tas1R2 and Tas1R3 is a broadly acting sweet taste receptor, which mediates mammalian sweet taste toward natural and artificial sweeteners and sweet-tasting proteins. Perception of sweet taste is a species selective physiological process. For instance, artificial sweeteners aspartame and neotame taste sweet to humans, apes and Old World monkeys but not to New World monkeys and rodents. Although specific regions determining the activation of the receptors by these sweeteners have been identified, the molecular mechanism of species-dependent sweet taste remains elusive. Using human/squirrel monkey chimeras, mutagenesis and molecular modeling, we reveal that the different responses of mammalian species towards the artificial sweeteners aspartame and neotame are determined by the steric effect of a combination of a few residues in the ligand binding pocket. Residues S40 and D142 in the human Tas1R2, which correspond to residues T40 and E142 in the squirrel monkey Tas1R2, were found to be the critical residues for the species dependent difference in sweet taste. In addition, human Tas1R2 residue I67, which corresponds to S67 in squirrel monkey receptor, modulates the higher affinity of neotame than that of aspartame. Our studies not only shed light on the molecular mechanism of species dependent sweet taste toward artificial sweeteners, but also provide guidance for designing novel effective artificial sweet compounds.

Taste receptors and their downstream signaling molecules are activated by sugars and sweeteners in the gut and participate in the regulation of glucose transport into enterocytes. The glucose transporter families GLUT and SGLT are responsible for the absorption of glucose, GLUT4 and SGLT1 being expressed preferentially in T1R3-positive taste cells. However, the expression patterns of the other glucose transporters in mouse gustatory tissues have not yet been elucidated. Therefore, we have examined the expression patterns of the glucose transporters (GLUT1-4 and SGLT1-3) in mouse gustatory tissues. Reverse transcription/polymerase chain reaction assays have revealed that GLUT1, 3, and 4 and SGLT1 mRNAs are expressed in the circumvallate papillae. Immunohistochemical analysis has shown that SGLT1 is expressed in a subset of the epithelial cells: from the basal cell layer to the prickle cell layer and in intragemmal and extragemmal epithelium cells in the circumvallate, foliate, and fungiform papillae. GLUT1, GLUT3, and GLUT4 are expressed in the prickle cell layers and/or basal cell layers in these papillae. Moreover, GLUT1, but not GLUT3 or GLUT4, is expressed in a subset of intragemmal and extragemmal epithelium cells in these papillae. Double-labeling experiments have demonstrated that GLUT1-positive taste bud cells coexpress gustducin and inositol 1,4,5-triphosphate receptor type III. These results suggest that SGLT1 and GLUT1 play a role in glucose-sensing and/or transport in mouse taste buds.

Although the heteromeric combination of type 1 taste receptors 2 and 3 (T1r2 + T1r3) is well established as the major receptor for sugars and noncaloric sweeteners, there is also evidence of T1r-independent sweet taste in mice, particularly so for sugars. Before the molecular cloning of the T1rs, it had been proposed that sweet taste detection depended on ( a ) activation of sugar-gated cation channels and/or ( b ) sugar binding to G protein-coupled receptors to initiate second-messenger cascades. By either mechanism, sugars would elicit depolarization of sweet-responsive taste cells, which would transmit their signal to gustatory afferents. We examined the nature of T1r-independent sweet taste; our starting point was to determine if taste cells express glucose transporters (GLUTs) and metabolic sensors that serve as sugar sensors in other tissues. Using RT-PCR, quantitative PCR, in situ hybridization, and immunohistochemistry, we determined that several GLUTs (GLUT2, GLUT4, GLUT8, and GLUT9), a sodium–glucose cotransporter (SGLT1), and two components of the ATP-gated K + (K ATP ) metabolic sensor [sulfonylurea receptor (SUR) 1 and potassium inwardly rectifying channel (Kir) 6.1] were expressed selectively in taste cells. Consistent with a role in sweet taste, GLUT4, SGLT1, and SUR1 were expressed preferentially in T1r3-positive taste cells. Electrophysiological recording determined that nearly 20% of the total outward current of mouse fungiform taste cells was composed of K ATP channels. Because the overwhelming majority of T1r3-expressing taste cells also express SUR1, and vice versa, it is likely that K ATP channels constitute a major portion of K + channels in the T1r3 subset of taste cells. Taste cell-expressed glucose sensors and K ATP may serve as mediators of the T1r-independent sweet taste of sugars.

We have reported on the partial structures of a multigene family encoding GTP-binding protein (G protein)-coupled, seven-transmembrane receptors expressed in the tongue (Abe, K., Kusakabe, Y., Tanemura, K., Emori, Y., and Arai, S. (1993) FEBS Lett. 316, 253-256). Here we describe a full-length cDNA clone encoding a tongue cell-type specific receptor. The encoded protein consists of 312 amino acid residues. In overall structure, the protein is similar to known G protein-coupled, seven-transmembrane receptors such as an olfactory receptor (56% identity) but is significantly different in part, particularly in NH2-terminal extracellular and COOH-terminal cytoplasmic domain structures. Northern analysis showed that the mRNA for this protein is expressed only in the epithelium of the tongue, not in other organs. In situ hybridization experiments clearly indicated that the mRNA is expressed exclusively on the tongue apical surface, not on the reverse side of the tongue nor in its muscle layer. Expression was also detected in the taste buds and surrounding cellular tissues of the fungiform and circumvallate papillae. It is suggested that this gustatory receptor structurally related to olfactory receptors may be a candidate for a taste receptor.

Background Taste is an important determinant of food consumption, and genetic variations in the sweet taste receptor subunit TAS1R2 may contribute to interindividual variations in sugar consumption. Objective We determined whether Ser9Cys and Ile191Val variations in TAS1R2 were associated with differences in the consumption of sugars in 2 populations. Design Population 1 included 1037 diabetes-free young adults in whom we assessed dietary intake by using a 1-mo, 196-item food-frequency questionnaire. Population 2 consisted of 100 individuals with type 2 diabetes with dietary intakes assessed by using 2 sets of 3-d food records administered 2 wk apart. Dietary counseling was provided between food records 1 and 2. Dietary intakes between genotypes were compared by using analysis of covariance adjusted for potential confounders. Results In population 1, a significant Ile191Val × body mass index (BMI; in kg/m²) interaction was detected for the consumption of sugars, and the effect of genotype was significant only in individuals with a BMI ≥ 25 (n = 205). In comparison with individuals homozygous for the Ile allele, Val carriers consumed fewer sugars (122 ± 6 compared with 103 ± 6 g sugar/d, respectively; P = 0.01). Regression estimates that associated BMI with total sugar consumption by Ile/Ile and Val-carrier genotype intersected at a BMI of 23.5. In population 2, Val carriers also consumed less sugar than did individuals with the Ile/Ile genotype (99 ± 6 compared with 83 ± 6 g sugar/d, respectively; P = 0.04) on food record 2, and sugar was the only macronutrient that decreased significantly (-9 ± 4 g sugar/d, P = 0.02) in Val carriers who received dietary counseling. Conclusion Our findings show that a genetic variation in TAS1R2 affects habitual consumption of sugars and may contribute to interindividual differences in changing behaviors in response to dietary counseling.

Taste perception is an important component of an animal’s fitness. The identification of vertebrate taste receptor genes in the last decade has enabled molecular genetic studies of the evolution of taste perception in the context of the ecology and dietary preferences of organisms. Although such analyses have been conducted in a number of species for bitter taste receptors, a similar analysis of sweet taste receptors is lacking. Here, we survey the sole sweet taste–specific receptor gene Tas1r2 in 42 bat species that represent all major lineages of the order Chiroptera, one of the most diverse groups of mammals in terms of diet. We found that Tas1r2 is under strong purifying selection in the majority of the bats studied, with no significant difference in the strength of the selection between insect eaters and fruit eaters. However, Tas1r2 is a pseudogene in all three vampire bat species and the functional relaxation likely started in their common ancestor, probably due to the exclusive feeding of vampire bats on blood and their reliance on infrared sensors rather than taste perception to locate blood sources. Our survey of available genome sequences, together with previous reports, revealed additional losses of Tas1r2 in horse, cat, chicken, zebra finch, and western clawed frog, indicating that sweet perception is not as conserved as previously thought. Nonetheless, we found no common dietary pattern among the Tas1r2-lacking vertebrates, suggesting different causes for the losses of Tas1r2 in different species. The complexity of the ecological factors that impact the evolution of Tas1r2 calls for a better understanding of the physiological roles of sweet perception in different species.

We examined co-localization of vanilloid receptor (VR1) with sweet receptors T1R2, T1R3, or bitter receptor T2R6 in taste receptor cells of rat circumvallate papillae. Tissue sections of rat circumvallate papillae were doubly reacted with anti-VR1 antibodies and anti-T1R2, anti-T1R3 or anti-T2R6 antibodies, using double-immunofluorescence histochemistry technique. Localizations of VR1, T1Rs and T2R6 in the vallate taste cells containing α-gustducin were also examined. VR1 immunoreactivities (-ir) were observed in subsets of taste cells in the circumvallate papillae, and 96–99% of the vallate taste cells exhibiting T1R2-, T1R3- or T2R6-ir co-exhibited VR1-ir. Approximately half of T2R6-ir cells (~49%), and 50–58% of T1Rs-ir cells, co-exhibited α-gustducin-ir in the vallate taste buds. About 58% of VR1-ir cells in the vallate exhibited α-gustducin-ir as well. Results support the idea that capsaicin may interact with the transduction pathways of sweet and bitter taste stimuli, possibly in mediation of its receptor VR1 localized in taste receptor cells. Additionally, the partial co-localization of α-gustducin with VR1 suggests that a tentative modulatory function of capsaicin in sweet and bitter transductions in the rat circumvallate comprises of both α-gustducin-mediated and non-mediated transduction pathways.

l-Glutamate confers cognitive discrimination for umami taste (delicious or savory) and dietary information to the brain through the activation of G protein-coupled receptors in specialized taste receptor cells of the tongue. The taste heterologous receptor T1R1 plus T1R3 is not sufficient to detect umami taste in mice. The lack of T1R3 diminished but did not abolish nerve and behavioral responses in null mice that still contained umami-sensitive taste receptor cells. The remnant umami responses in T1R3 knockout mice indicate that there are also T1R3 independent receptors. Metabotropic glutamate receptor 1 (mGluR1), which is widely expressed throughout the central nervous system and regulates synaptic signaling, is another l-glutamate receptor candidate. It is found within taste buds, although the amount of l-glutamate in the perisynaptic region is in the order of micromol/L, whereas free dietary l-glutamate is in the mmol/L range. We reexamined the expression of one mGluR1 variant with a lower affinity for l-glutamate that is found in fungiform and circumvallate papillae. This taste mGluR1 receptor responds in vitro to the concentration of l-glutamate usually found in foodstuffs.

Brain glucosensing is essential for normal body glucose homeostasis and neuronal function. However, the exact signaling mechanisms involved in the neuronal sensing of extracellular glucose levels remain poorly understood. Of particular interest is the identification of candidate membrane molecular sensors that would allow neurons to change firing rates independently of intracellular glucose metabolism. Here we describe for the first time the expression of the taste receptor genes Tas1r1, Tas1r2 and Tas1r3, and their associated G-protein genes, in the mammalian brain. Neuronal expression of taste genes was detected in different nutrient-sensing forebrain regions, including the paraventricular and arcuate nuclei of the hypothalamus, the CA fields and dentate gyrus of the hippocampus, the habenula, and cortex. Expression was also observed in the intra-ventricular epithelial cells of the choroid plexus. These same regions were found to express the corresponding gene products that form the heterodimeric T1R2/T1R3 and T1R1/T1R3 sweet and l-amino acid taste G-protein coupled receptors, respectively, along with the taste G-protein alpha-gustducin. Moreover, in vivo studies in mice demonstrated that the hypothalamic expression of taste-related genes is regulated by the nutritional state of the animal, with food deprivation significantly increasing expression levels of Tas1r1 and Tas1r2 in hypothalamus, but not in cortex. Furthermore, exposing mouse hypothalamic cells to a low-glucose medium, while maintaining normal l-amino acid concentrations, specifically resulted in higher expression levels of the sweet-associated gene Tas1r2. This latter effect was reversed by adding the non-metabolizable artificial sweetener sucralose to the low-glucose medium, indicating that taste-like signaling in hypothalamic neurons does not require intracellular glucose oxidation. Taken together, our findings suggest that the heterodimeric G-protein coupled sweet receptor T1R2/T1R3 is a candidate membrane-bound brain glucosensor.

Human sweet taste perception is mediated by the heterodimeric G-protein coupled receptor complex encoded by the TAS1R2 and TAS1R3 genes [1-7]. The extent of variation in these genes has been recently characterized [8], but the functional consequences of such variation are unknown. In this study we report that two C/T single nucleotide polymorphisms located at positions −1572 (rs307355) and −1266 (rs35744813) upstream of the TAS1R3 coding sequence strongly correlate with human taste sensitivity to sucrose, and explain 16% of population variability in perception. Individuals who carry T alleles display reduced sensitivity to sucrose compared to those who carry C alleles at these nucleotide positions. Using a luciferase reporter assay, we demonstrate that the T allele of each SNP results in reduced promoter activity in comparison to the C alleles, consistent with the phenotype observed in humans. We also found that the TAS1R3 promoter region extending from position −1700 to −1000 harbors a novel composite cis-acting element that has a strong silencing effect on promoter activity. We conclude that the rs307355 and rs35744813 SNPs affect gene transcription by altering the function of this regulatory element. A worldwide population survey reveals that the T alleles of rs307355 and rs35744813 occur at lowest frequencies in European populations. We propose that inherited differences in TAS1R3 transcription account for a substantial fraction of worldwide differences in human sweet taste perception.

Epithelial sheets rich in taste buds and free of muscle tissue and von Ebner’s washing glands were isolated as a U-shaped cleft which surrounds the circumvallate (CV) papilla of the rat tongue. The sheet of CV tissue from one tongue (total protein content: 8–14 µg) was cut in two approximately equal parts which were incubated with permeant phosphodiesterase inhibitor (IBMX; 0.3 mM) and 0 or 150–600 mM sucrose. After 6 min of incubation, the sheets were washed, the cells permeabilized and their cyclic AMP (cAMP) content determined by radioimmunoassay. Paired estimates with tissue from the same animal showed a significant sucrose-induced cAMP production (range 5–20 fmol/µg protein at 600 mM sucrose). This increase in intracellular cAMP was linearly dependent on the sucrose concentration and was suppressed by about 65% when 50 mM of a competitive antagonist of sucrose (methyl 4,6-dichloro-4,6-dideoxy-α-D-galactopyranoside) was added to the sucrose solution. Epithelial sheets free of taste buds did not respond to either sucrose or the inhibitor. These results are in line with previous suggestions that cAMP may be a second messenger in the transduction of sweet taste in the rat.

Background Differences in sweet taste perception among species depend on structural variations of the sweet taste receptor. The commercially used isovanillyl sweetener neohesperidin dihydrochalcone activates the human but not the rat sweet receptor TAS1R2+TAS1R3. Analysis of interspecies combinations and chimeras of rat and human TAS1R2+TAS1R3 suggested that the heptahelical domain of human TAS1R3 is crucial for the activation of the sweet receptor by neohesperidin dihydrochalcone. Results By mutational analysis combined with functional studies and molecular modeling we identified a set of different amino acid residues within the heptahelical domain of human TAS1R3 that forms the neohesperidin dihydrochalcone binding pocket. Sixteen amino acid residues in the transmembrane domains 2 to 7 and one in the extracellular loop 2 of hTAS1R3 influenced the receptor's response to neohesperidin dihydrochalcone. Some of these seventeen residues are also part of the binding sites for the sweetener cyclamate or the sweet taste inhibitor lactisole. In line with this observation, lactisole inhibited activation of the sweet receptor by neohesperidin dihydrochalcone and cyclamate competitively, whereas receptor activation by aspartame, a sweetener known to bind to the N-terminal domain of TAS1R2, was allosterically inhibited. Seven of the amino acid positions crucial for activation of hTAS1R2+hTAS1R3 by neohesperidin dihydrochalcone are thought to play a role in the binding of allosteric modulators of other class C GPCRs, further supporting our model of the neohesperidin dihydrochalcone pharmacophore. Conclusion From our data we conclude that we identified the neohesperidin dihydrochalcone binding site at the human sweet taste receptor, which overlaps with those for the sweetener cyclamate and the sweet taste inhibitor lactisole. This readily delivers a molecular explanation of our finding that lactisole is a competitive inhibitor of the receptor activation by neohesperidin dihydrochalcone and cyclamate. Some of the amino acid positions crucial for activation of hTAS1R2+hTAS1R3 by neohesperidin dihydrochalcone are involved in the binding of allosteric modulators in other class C GPCRs, suggesting a general role of these amino acid positions in allosterism and pointing to a common architecture of the heptahelical domains of class C GPCRs.

In the vertebrate taste cell, heterotrimeric guanine nucleotide-binding proteins (G proteins) are involved in the transduction of both bitter and sweet taste stimulants. The bitter compound denatonium raises the intracellular Ca2+ concentration in rat taste cells, apparently via G protein-mediated increases in inositol trisphosphate. Sucrose causes a G protein-dependent generation of cAMP in rat taste bud membranes; elevation of cAMP levels leads to taste cell depolarization. To identify and characterize those proteins involved in the taste transduction process, we have cloned G protein alpha subunit (G alpha) cDNAs from rat taste cells. Using degenerate primers corresponding to conserved regions of G proteins, we used the polymerase chain reaction to amplify and clone taste cell G alpha cDNAs. Eight distinct G alpha cDNAs were isolated, cloned and sequenced from a taste cell library. Among these clones was alpha gustducin, a novel taste G alpha closely related to the transducins. In addition to alpha gustducin, we cloned rod and cone transducins from taste cells. This is the first identification of transducin expression outside photoreceptor cells. The primary sequence of alpha gustducin shows similarities to the transducins in the receptor interaction domain and the phosphodiesterase activation site. These sequence similarities suggest that gustducin and transducin regulate taste cell phosphodiesterase, probably in bitter taste transduction.