Involvement of T1R3 in calcium-magnesium taste

Role of taste receptors in salty taste perception of minerals and amino acids and developments in salt reduction strategies: A review

Salt (sodium chloride) plays a key role in maintaining the textural, microbiological, and sensorial aspects of the foods. However high dietary salt intake in the population has led to a series of health problems. Currently manufacturers are under pressure to reduce the sodium levels in foods without compromising the consumer experience. Because of the clean salty taste produced by sodium chloride, it has been challenging for the food industry to develop a suitable salt substitute. Studies have shown that different components within a food matrix can influence the perception of saltiness. This review aims to comprehend the potential synergistic effect of compounds such as minerals and amino acids on the perception of saltiness and covers the mechanism of perception where relevant to taste resulting from sodium ions and other metallic ions (such as K, Mg, Ca), as well as various amino acids and their derivatives. Finally, the review summarizes various salt reduction strategies explored by researchers, government organizations and food industry, including the potential use of plant-based extracts.

Taste coding of heavy metal ion-induced avoidance in Drosophila

Increasing pollution of heavy metals poses great risks to animals globally. Their survival likely relies on an ability to detect and avoid harmful heavy metal ions (HMIs). Currently, little is known about the neural mechanisms of HMI detection. Here, we show that Drosophila and related species of Drosophilidae actively avoid toxic HMIs at micromolar concentrations. The high sensitivity to HMIs is biologically relevant. Particularly, their sensitivity to cadmium is as high as that to the most bitter substance, denatonium. Detection of HMIs in food requires Gr66a ⁺ gustatory neurons but is independent of bitter-taste receptors. In these neurons, the ionotropic receptors IR76b, IR25a, and IR7a are required for the perception of heavy metals. Furthermore, IR47a mediates the activation of a distinct group of non-Gr66a ⁺ gustatory neurons elicited by HMIs. Together, our findings reveal a surprising taste quality represented by noxious metal ions.

Interkingdom Detection of Bacterial Quorum-Sensing Molecules by Mammalian Taste Receptors

Bitter and sweet taste G protein-coupled receptors (known as T2Rs and T1Rs, respectively) were originally identified in type II taste cells on the tongue, where they signal perception of bitter and sweet tastes, respectively. Over the past ~15 years, taste receptors have been identified in cells all over the body, demonstrating a more general chemosensory role beyond taste. Bitter and sweet taste receptors regulate gut epithelial function, pancreatic β cell secretion, thyroid hormone secretion, adipocyte function, and many other processes. Emerging data from a variety of tissues suggest that taste receptors are also used by mammalian cells to "eavesdrop" on bacterial communications. These receptors are activated by several quorum-sensing molecules, including acyl-homoserine lactones and quinolones from Gram-negative bacteria such as Pseudomonas aeruginosa, competence stimulating peptides from Streptococcus mutans, and D-amino acids from Staphylococcus aureus. Taste receptors are an arm of immune surveillance similar to Toll-like receptors and other pattern recognition receptors. Because they are activated by quorum-sensing molecules, taste receptors report information about microbial population density based on the chemical composition of the extracellular environment. This review summarizes current knowledge of bacterial activation of taste receptors and identifies important questions remaining in this field.

Oral expressions and functional analyses of the extracellular calcium-sensing receptor (CaSR) in chicken

In vertebrates, the extracellular calcium-sensing receptor (CaSR) plays a key role in calcium homeostasis by sensing slight changes in extracellular Ca²⁺. CaSR is also expressed in mammals including rodent taste cells and is involved in sensing kokumi, a rich, savory quality that enhances the intensities of salty, sweet, and umami tastes. In this study, we focused on chicken CaSR (cCaSR) since calcium is an essential nutrient that is necessary for making eggshell and for the extremely rapid initial growth of bones. First we confirmed that cCaSR is expressed in taste cells. Next we cloned the cCaSR gene from kidney and transiently transfected human embryonic kidney 293 T (HEK293T) cells with the recombinant cCaSR, or empty vector and looked for the agonists and allosteric modulators (including kokumi substances) of cCaSR by Ca²⁺ imaging. We found that cCaSR was activated by extracellular Ca²⁺ and Mg²⁺ in a dose dependent manner. Several L-amino acids and kokumi substances such as glutathione enhanced the response of cCaSR. In addition, NPS2143 as a negative allosteric modulator of human CaSR negatively modulated the response of cCaSR. These results suggest that cCaSR can sense extracellular Ca²⁺ and Mg²⁺ as well as positive and negative allosteric modulators. Taken together, the results imply that CaSR might be a multifunctional receptor for calcium, amino acids, and kokumi substances in chicken. The present finding that functional CaSR is expressed in the chicken oral tissues will allow us to further elucidate the physiological role of CaSR in the chickens' taste sense, and to create new feeds that will contribute to the poultry industry.

Dysgeusia: A review in the context of COVID-19

Background

Taste disorders in general, and dysgeusia in particular, are relatively common disorders that may be a sign of a more complex acute or chronic medical condition. During the COVID-19 pandemic, taste disorders have found their way into the realm of general as well as specialty dentistry, with significance in screening for patients who potentially may have the virus.

Types of Studies Reviewed

The authors searched electronic databases (PubMed, Embase, Web of Science, Google Scholar) for studies focused on dysgeusia, ageusia, and other taste disorders and their relationship to local and systemic causes.

Results

The authors found pertinent literature explaining the normal physiology of taste sensation, proposals for suggested new tastes, presence of gustatory receptors in remote tissues of the body, and etiology and pathophysiology of taste disorders, in addition to the valuable knowledge gained about gustatory disorders in the context of COVID-19. Along with olfactory disorders, taste disorders are one of the earliest suggestive symptoms of COVID-19 infection.

Conclusions

Gustatory disorders are the result of local or systemic etiology or both. Newer taste sensations, such as calcium and fat tastes, have been discovered, as well as taste receptors that are remote from the oropharyngeal area. Literature published during the COVID-19 pandemic to date reinforces the significance of early detection of potential patients with COVID-19 by means of screening for recent-onset taste disorders.

Practical Implications

Timely screening and identification of potential gustatory disorders are paramount for the dental care practitioner to aid in the early diagnosis of COVID-19 and other serious systemic disorders.

Understanding the evolution of nutritive taste in animals: Insights from biological stoichiometry and nutritional geometry

A major conceptual gap in taste biology is the lack of a general framework for understanding the evolution of different taste modalities among animal species. We turn to two complementary nutritional frameworks, biological stoichiometry theory and nutritional geometry, to develop hypotheses for the evolution of different taste modalities in animals. We describe how the attractive tastes of Na-, Ca-, P-, N-, and C-containing compounds are consistent with principles of both frameworks based on their shared focus on nutritional imbalances and consumer homeostasis. Specifically, we suggest that the evolution of multiple nutritive taste modalities can be predicted by identifying individual elements that are typically more concentrated in the tissues of animals than plants. Additionally, we discuss how consumer homeostasis can inform our understanding of why some taste compounds (i.e., Na, Ca, and P salts) can be either attractive or aversive depending on concentration. We also discuss how these complementary frameworks can help to explain the evolutionary history of different taste modalities and improve our understanding of the mechanisms that lead to loss of taste capabilities in some animal lineages. The ideas presented here will stimulate research that bridges the fields of evolutionary biology, sensory biology, and ecology.

Preference for dietary fat: From detection to disease

Recent advances in the field of taste physiology have clarified the role of different basic taste modalities and their implications in health and disease and proposed emphatically that there might be a distinct cue for oro-sensory detection of dietary long-chain fatty acids (LCFAs). Hence, fat taste can be categorized as a taste modality. During mastication, LCFAs activate tongue lipid sensors like CD36 and GPR120 triggering identical signaling pathways as the basic taste qualities do; however, the physico-chemical perception of fat is not as distinct as sweet or bitter or other taste sensations. The question arises whether "fat taste" is a basic or "alimentary" taste. There is compelling evidence that fat-rich dietary intervention modulates fat taste perception where an increase or a decrease in lipid contents in the diet results, respectively, in downregulation or upregulation of fat taste sensitivity. Evidently, a decrease in oro-sensory detection of LCFAs leads to high fat intake and, consequently, to obesity. In this article, we discuss recent relevant advances made in the field of fat taste physiology with regard to dietary fat preference and lipid sensors that can be the target of anti-obesity strategies.

Metal Ions Activate the Human Taste Receptor TAS2R7

Divalent and trivalent salts exhibit a complex taste profile. They are perceived as being astringent/drying, sour, bitter, and metallic. We hypothesized that human bitter-taste receptors may mediate some taste attributes of these salts. Using a cell-based functional assay, we found that TAS2R7 responds to a broad range of divalent and trivalent salts, including zinc, calcium, magnesium, copper, manganese, and aluminum, but not to potassium, suggesting TAS2R7 may act as a metal cation receptor mediating bitterness of divalent and trivalent salts. Molecular modeling and mutagenesis analysis identified 2 residues, H943.37 and E2647.32, in TAS2R7 that appear to be responsible for the interaction of TAS2R7 with metallic ions. Taste receptors are found in both oral and extraoral tissues. The responsiveness of TAS2R7 to various mineral salts suggests it may act as a broad sensor, similar to the calcium-sensing receptor, for biologically relevant metal cations in both oral and extraoral tissues.

Dried bonito dashi: Contributions of mineral salts and organic acids to the taste of dashi

Dried bonito dashi is often used in Japanese cuisine with a number of documented positive health effects. Its major taste is thought to be umami, elicited by inosine 5'-monophosphate (IMP) and L-amino acids. Previously we found that lactic acid, a major component of dried bonito dashi, enhanced the contribution of many of these amino acids to the taste of dried bonito dashi, and reduced the contribution of other amino acids. In addition to amino acids, dried bonito dashi also has a significant mineral salt component. The present study used conditioned taste aversion methods with mice (all had compromised olfactory systems) to compare the taste qualities of dried bonito dashi with four salts (NaCl, KCl, CaCl₂ and MgCl₂), with and without lactic acid or citric acid. A conditioned taste aversion to 25% dried bonitio dashi generalized significantly to NaCl and KCl, with or without 0.9% lactic acid added but not when citric acid was added. Generalization of the CTA to dried bonito dashi was much stronger to the divalent salts, but when either lactic acid or citric acid was added, this aversion was eliminated. These results suggest that these salts contribute to the complex taste of dried bonito dashi and that both organic acids appear able to modify the tastes of divalent salts.

Nutrient sensing, taste and feed intake in avian species

The anatomical structure and function of beaks, bills and tongue together with the mechanics of deglutition in birds have contributed to the development of a taste system denuded of macrostructures visible to the human naked eye. Studies in chickens and other birds have revealed that the avian taste system consists of taste buds not clustered in papillae and located mainly (60 %) in the upper palate hidden in the crevasses of the salivary ducts. That explains the long delay in the understanding of the avian taste system. However, recent studies reported 767 taste buds in the oral cavity of the chicken. Chickens appear to have an acute sense of taste allowing for the discrimination of dietary amino acids, fatty acids, sugars, quinine, Ca and salt among others. However, chickens and other birds have small repertoires of bitter taste receptors (T2R) and are missing the T1R2 (related to sweet taste in mammals). Thus, T1R2-independent mechanisms of glucose sensing might be particularly relevant in chickens. The chicken umami receptor (T1R1/T1R3) responds to amino acids such as alanine and serine (known to stimulate the umami receptor in rodents and fish). Recently, the avian nutrient chemosensory system has been found in the gastrointestinal tract and hypothalamus related to the enteroendocrine system which mediates the gut-brain dialogue relevant to the control of feed intake. Overall, the understanding of the avian taste system provides novel and robust tools to improve avian nutrition.

CALHM3 Is Essential for Rapid Ion Channel-Mediated Purinergic Neurotransmission of GPCR-Mediated Tastes

Binding of sweet, umami, and bitter tastants to G protein-coupled receptors (GPCRs) in apical membranes of type II taste bud cells (TBCs) triggers action potentials that activate a voltage-gated nonselective ion channel to release ATP to gustatory nerves mediating taste perception. Although calcium homeostasis modulator 1 (CALHM1) is necessary for ATP release, the molecular identification of the channel complex that provides the conductive ATP-release mechanism suitable for action potential-dependent neurotransmission remains to be determined. Here we show that CALHM3 interacts with CALHM1 as a pore-forming subunit in a CALHM1/CALHM3 hexameric channel, endowing it with fast voltage-activated gating identical to that of the ATP-release channel in vivo. Calhm3 is co-expressed with Calhm1 exclusively in type II TBCs, and its genetic deletion abolishes taste-evoked ATP release from taste buds and GPCR-mediated taste perception. Thus, CALHM3, together with CALHM1, is essential to form the fast voltage-gated ATP-release channel in type II TBCs required for GPCR-mediated tastes.

The Different Facets of Extracellular Calcium Sensors: Old and New Concepts in Calcium-Sensing Receptor Signalling and Pharmacology

The current interest of the scientific community for research in the field of calcium sensing in general and on the calcium-sensing Receptor (CaR) in particular is demonstrated by the still increasing number of papers published on this topic. The extracellular calcium-sensing receptor is the best-known G-protein-coupled receptor (GPCR) able to sense external Ca2+ changes. Widely recognized as a fundamental player in systemic Ca2+ homeostasis, the CaR is ubiquitously expressed in the human body where it activates multiple signalling pathways. In this review, old and new notions regarding the mechanisms by which extracellular Ca2+ microdomains are created and the tools available to measure them are analyzed. After a survey of the main signalling pathways triggered by the CaR, a special attention is reserved for the emerging concepts regarding CaR function in the heart, CaR trafficking and pharmacology. Finally, an overview on other Ca2+ sensors is provided.

Taste receptors in the upper airway

Taste receptors were named for their originally-identified expression on the tongue and role in the sensation of taste (gustation). They are now known to be involved in many chemosensory processes outside the tongue. Expression of the receptors for bitter, sweet, and umami was recently identified in many organs, including the brain, airway, gastrointestinal tract, and reproductive systems. We do not yet know the full roles of these receptors in all of these tissues, nor do we know all of the endogenous ligands that activate them. However, taste receptors are emerging as potentially important therapeutic targets. Moreover, they may mediate some off target effects of drugs, as many medications in common clinical use are known to be bitter. The focus of this review is on recent basic and clinical data describing the expression of bitter (T2R) and sweet (T1R) receptors in the airway and their activation by secreted bacterial compounds. These receptors play important roles in innate immune nitric oxide production and antimicrobial peptide secretion, and may be useful targets for stimulating immune responses in the upper respiratory tract via topical therapies. Moreover, genetic variation in these receptors may play a role in the differential susceptibility of patients to certain types of respiratory infections as well as to differential outcomes in patients with chronic rhinosinusitis (CRS). CRS is a syndrome of chronic upper respiratory infection and inflammation and has a significant detrimental impact on patient quality of life. CRS treatment accounts for approximately 20% of adult antibiotic prescriptions and is thus a large driver of the public health crisis of antibiotic resistance. Taste receptors represent a novel class of therapeutic target to potentially stimulate endogenous immune responses and treat CRS patients without conventional antibiotics.

Phosphorus Taste Involves T1R2 and T1R3

Rodents consume solutions of phosphates and pyrophosphates in preference to water. Recently, we found that the preference for trisodium pyrophosphate (Na3HP2O7) was greater in T1R3 knockout (KO) mice than wild-type (WT) controls, suggesting that T1R3 is a pyrophosphate detector. We now show that this heightened Na3HP2O7 preference of T1R3 KO mice extends to disodium phosphate (Na2HPO4), disodium and tetrasodium pyrophosphate (Na2H2PO4 and Na4H2PO4), a tripolyphosphate (Na5P3O10), a non-sodium phosphate [(NH4)2HPO4], and a non-sodium pyrophosphate (K4P2O7) but not to non-P salts with large anions (sodium gluconate, acetate, or propionate). Licking rates for Na3HP2O7 are higher in T1R2 KO mice than WT controls; Na3HP2O7 preference scores are increased even more in T1R2 KO mice and T1R2+T1R3 double KO mice than in T1R3 KO mice; preference scores for Na3HP2O7 are normal in T1R1 KO mice. These results implicate each subunit of the T1R2+T1R3 dimer in the behavioral response to P-containing taste compounds.

Detection of maltodextrin and its discrimination from sucrose are independent of the T1R2 + T1R3 heterodimer

Maltodextrins, such as Maltrin and Polycose, are glucose polymer mixtures of varying chain lengths that are palatable to rodents. Although glucose and other sugars activate the T1R2 + T1R3 "sweet" taste receptor, recent evidence from T1R2- or T1R3-knockout (KO) mice suggests that maltodextrins, despite their glucose polymer composition, activate a separate receptor mechanism to generate a taste percept qualitatively distinguishable from that of sweeteners. However, explicit discrimination of maltodextrins from prototypical sweeteners has not yet been psychophysically tested in any murine model. Therefore, mice lacking T1R2 + T1R3 and wild-type controls were tested in a two-response taste discrimination task to determine whether maltodextrins are 1) detectable when both receptor subunits are absent and 2) perceptually distinct from that of sucrose irrespective of viscosity, intensity, and hedonics. Most KO mice displayed similar Polycose sensitivity as controls. However, some KO mice were only sensitive to the higher Polycose concentrations, implicating potential allelic variation in the putative polysaccharide receptor or downstream pathways unmasked by the absence of T1R2 + T1R3. Varied Maltrin and sucrose concentrations of approximately matched viscosities were then presented to render the oral somatosensory features, intensity, and hedonic value of the solutions irrelevant. Although both genotypes competently discriminated Maltrin from sucrose, performance was apparently driven by the different orosensory percepts of the two stimuli in control mice and the presence of a Maltrin but not sucrose orosensory cue in KO mice. These data support the proposed presence of an orosensory receptor mechanism that gives rise to a qualitatively distinguishable sensation from that of sucrose.

Taste buds: cells, signals and synapses

The past decade has witnessed a consolidation and refinement of the extraordinary progress made in taste research. This Review describes recent advances in our understanding of taste receptors, taste buds, and the connections between taste buds and sensory afferent fibres. The article discusses new findings regarding the cellular mechanisms for detecting tastes, new data on the transmitters involved in taste processing and new studies that address longstanding arguments about taste coding.

Altered learning, memory, and social behavior in type 1 taste receptor subunit 3 knock-out mice are associated with neuronal dysfunction

The type 1 taste receptor member 3 (T1R3) is a G protein-coupled receptor involved in sweet-taste perception. Besides the tongue, the T1R3 receptor is highly expressed in brain areas implicated in cognition, including the hippocampus and cortex. As cognitive decline is often preceded by significant metabolic or endocrinological dysfunctions regulated by the sweet-taste perception system, we hypothesized that a disruption of the sweet-taste perception in the brain could have a key role in the development of cognitive dysfunction. To assess the importance of the sweet-taste receptors in the brain, we conducted transcriptomic and proteomic analyses of cortical and hippocampal tissues isolated from T1R3 knock-out (T1R3KO) mice. The effect of an impaired sweet-taste perception system on cognition functions were examined by analyzing synaptic integrity and performing animal behavior on T1R3KO mice. Although T1R3KO mice did not present a metabolically disrupted phenotype, bioinformatic interpretation of the high-dimensionality data indicated a strong neurodegenerative signature associated with significant alterations in pathways involved in neuritogenesis, dendritic growth, and synaptogenesis. Furthermore, a significantly reduced dendritic spine density was observed in T1R3KO mice together with alterations in learning and memory functions as well as sociability deficits. Taken together our data suggest that the sweet-taste receptor system plays an important neurotrophic role in the extralingual central nervous tissue that underpins synaptic function, memory acquisition, and social behavior.

Taste information derived from T1R-expressing taste cells in mice

The taste system of animals is used to detect valuable nutrients and harmful compounds in foods. In humans and mice, sweet, bitter, salty, sour and umami tastes are considered the five basic taste qualities. Sweet and umami tastes are mediated by G-protein-coupled receptors, belonging to the T1R (taste receptor type 1) family. This family consists of three members (T1R1, T1R2 and T1R3). They function as sweet or umami taste receptors by forming heterodimeric complexes, T1R1+T1R3 (umami) or T1R2+T1R3 (sweet). Receptors for each of the basic tastes are thought to be expressed exclusively in taste bud cells. Sweet (T1R2+T1R3-expressing) taste cells were thought to be segregated from umami (T1R1+T1R3-expressing) taste cells in taste buds. However, recent studies have revealed that a significant portion of taste cells in mice expressed all T1R subunits and responded to both sweet and umami compounds. This suggests that sweet and umami taste cells may not be segregated. Mice are able to discriminate between sweet and umami tastes, and both tastes contribute to behavioural preferences for sweet or umami compounds. There is growing evidence that T1R3 is also involved in behavioural avoidance of calcium tastes in mice, which implies that there may be a further population of T1R-expressing taste cells that mediate aversion to calcium taste. Therefore the simple view of detection and segregation of sweet and umami tastes by T1R-expressing taste cells, in mice, is now open to re-examination.

Maltodextrin Acceptance and Preference in Eight Mouse Strains

Rodents are strongly attracted to the taste(s) of maltodextrins. A first step toward discovery of the underlying genes involves identifying phenotypic differences among inbred strains of mice. To do this, we used 5-s brief-access tests and 48-h 2-bottle choice tests to survey the avidity for the maltodextrin, Maltrin M040, of mice from 8 inbred strains (129S1/SvImJ, A/J, CAST/EiJ, C57BL/6J, NOD/ShiLTJ, NZO/HlLtJ, PWK/PhJ, and WSB/EiJ). In brief-access tests, the CAST and PWK strains licked significantly less maltodextrin than equivalent concentrations of sucrose, whereas the other strains generally licked the 2 carbohydrates equally. Similarly, in 2-bottle choice tests, the CAST and PWK strains drank less 4% maltodextrin than 4% sucrose, whereas the other strains had similar intakes of these 2 solutions; the CAST and PWK strains did not differ from the C57, NOD, or NZO strains in 4% sucrose intake. In sum, we have identified strain variation in maltodextrin perception that is distinct from variation in sucrose perception. The phenotypic variation characterized here will aid in identifying genes responsible for maltodextrin acceptance. Our results identify C57 × PWK mice or NZO × CAST mice as informative crosses to produce segregating hybrids that will expose quantitative trait loci underlying maltodextrin acceptance and preference.

Multiple loss-of-function variants of taste receptors in modern humans

Despite recent advances in the knowledge of interindividual taste differences, the underlying genetic backgrounds have remained to be fully elucidated. Much of the taste variation among different mammalian species can be explained by pseudogenization of taste receptors. Here I investigated whether the most recent disruptions of taste receptor genes segregate with their intact forms in modern humans by analyzing 14 ethnically diverse populations. The results revealed an unprecedented prevalence of 25 segregating loss-of-function (LoF) taste receptor variants, identifying one of the most pronounced cases of functional population diversity in the human genome. LoF variant frequency in taste receptors (2.10%) was considerably higher than the overall LoF frequency in human genome (0.16%). In particular, molecular evolutionary rates of candidate sour (14.7%) and bitter (1.8%) receptors were far higher in humans than those of sweet (0.02%), salty (0.05%), and umami (0.17%) receptors compared with other carnivorous mammals, although not all of the taste receptors were identified. Many LoF variants are population-specific, some of which arose even after population differentiation, not before divergence of the modern and archaic human. I conclude that modern humans might have been losing some sour and bitter receptor genes because of high-frequency LoF variants.

Taste receptors in innate immunity

Taste receptors were first identified on the tongue, where they initiate a signaling pathway that communicates information to the brain about the nutrient content or potential toxicity of ingested foods. However, recent research has shown that taste receptors are also expressed in a myriad of other tissues, from the airway and gastrointestinal epithelia to the pancreas and brain. The functions of many of these extraoral taste receptors remain unknown, but emerging evidence suggests that bitter and sweet taste receptors in the airway are important sentinels of innate immunity. This review discusses taste receptor signaling, focusing on the G-protein-coupled receptors that detect bitter, sweet, and savory tastes, followed by an overview of extraoral taste receptors and in-depth discussion of studies demonstrating the roles of taste receptors in airway innate immunity. Future research on extraoral taste receptors has significant potential for identification of novel immune mechanisms and insights into host-pathogen interactions.

Heightened avidity for trisodium pyrophosphate in mice lacking Tas1r3

Laboratory rats and mice prefer some concentrations of tri- and tetrasodium pyrophosphate (Na3HP2O7 and Na4P2O7) to water, but how they detect pyrophosphates is unknown. Here, we assessed whether T1R3 is involved. We found that relative to wild-type littermate controls, Tas1r3 knockout mice had stronger preferences for 5.6-56mM Na3HP2O7 in 2-bottle choice tests, and they licked more 17.8-56mM Na3HP2O7 in brief-access tests. We hypothesize that pyrophosphate taste in the intact mouse involves 2 receptors: T1R3 to produce a hedonically negative signal and an unknown G protein-coupled receptor to produce a hedonically positive signal; in Tas1r3 knockout mice, the hedonically negative signal produced by T1R3 is absent, leading to a heightened avidity for pyrophosphate.

Emerging roles of the extracellular calcium-sensing receptor in nutrient sensing: control of taste modulation and intestinal hormone secretion

The extracellular Ca-sensing receptor (CaSR) is a sensor for a number of key nutrients within the body, including Ca ions (Ca²⁺) and L-amino acids. The CaSR is expressed in a number of specialised cells within the gastrointestinal (GI) tract, and much work has been done to examine CaSR's role as a nutrient sensor in this system. This review article examines two emerging roles for the CaSR within the GI tract--as a mediator of kokumi taste modulation in taste cells and as a regulator of dietary hormone release in response to L-amino acids in the intestine.

Macronutrient selection by seven inbred mouse strains and three taste-related knockout strains

Many animals thrive when given a choice of separate sources of macronutrients. How they do this is unknown. Here, we report some studies comparing the spontaneous choices between carbohydrate- and fat-containing food sources of seven inbred mouse strains (B6, BTBR, CBA, JF1, NZW, PWD and PWK) and three mouse models with genetic ablation of taste transduction components (T1R3, ITPR3 and CALHM1). For 8days, each mouse could choose between sources of carbohydrate (CHO-P; sucrose-cornstarch) and fat (Fat-P; vegetable shortening) with each source also containing protein (casein). We found that the B6 and PWK strains markedly preferred the CHO-P diet to the Fat-P diet, the BTBR and JF1 strains markedly preferred the Fat-P diet to the CHO-P diet, and the CBA, NZW and PWD strains showed equal intakes of the two diets (by weight). Relative to their WT littermates, ITPR3 and CALHM1 KO mice had elevated Fat-P preferences but T1R3 KO mice did not. There were differences among strains in adaption to the diet choice and there were differences in response between males and females on some days. These results demonstrate the diverse responses to macronutrients of inbred mice and they point to the involvement of chemosensory detectors (but not sweetness) as contributors to macronutrient selection.

Peripheral coding of taste

Five canonical tastes, bitter, sweet, umami (amino acid), salty, and sour (acid), are detected by animals as diverse as fruit flies and humans, consistent with a near-universal drive to consume fundamental nutrients and to avoid toxins or other harmful compounds. Surprisingly, despite this strong conservation of basic taste qualities between vertebrates and invertebrates, the receptors and signaling mechanisms that mediate taste in each are highly divergent. The identification over the last two decades of receptors and other molecules that mediate taste has led to stunning advances in our understanding of the basic mechanisms of transduction and coding of information by the gustatory systems of vertebrates and invertebrates. In this Review, we discuss recent advances in taste research, mainly from the fly and mammalian systems, and we highlight principles that are common across species, despite stark differences in receptor types.

Genetics of taste receptors

Taste receptors function as one of the interfaces between internal and external milieus. Taste receptors for sweet and umami (T1R [taste receptor, type 1]), bitter (T2R [taste receptor, type 2]), and salty (ENaC [epithelial sodium channel]) have been discovered in the recent years, but transduction mechanisms of sour taste and ENaC-independent salt taste are still poorly understood. In addition to these five main taste qualities, the taste system detects such noncanonical "tastes" as water, fat, and complex carbohydrates, but their reception mechanisms require further research. Variations in taste receptor genes between and within vertebrate species contribute to individual and species differences in taste-related behaviors. These variations are shaped by evolutionary forces and reflect species adaptations to their chemical environments and feeding ecology. Principles of drug discovery can be applied to taste receptors as targets in order to develop novel taste compounds to satisfy demand in better artificial sweeteners, enhancers of sugar and sodium taste, and blockers of bitterness of food ingredients and oral medications.

Out of the bottleneck: the Diversity Outcross and Collaborative Cross mouse populations in behavioral genetics research

The historical origins of classical laboratory mouse strains have led to a relatively limited range of genetic and phenotypic variation, particularly for the study of behavior. Many recent efforts have resulted in improved diversity and precision of mouse genetic resources for behavioral research, including the Collaborative Cross and Diversity Outcross population. These two populations, derived from an eight way cross of common and wild-derived strains, have high precision and allelic diversity. Behavioral variation in the population is expanded, both qualitatively and quantitatively. Variation that had once been canalized among the various inbred lines has been made amenable to genetic dissection. The genetic attributes of these complementary populations, along with advances in genetic and genomic technologies, makes a systems genetic analyses of behavior more readily tractable, enabling discovery of a greater range of neurobiological phenomena underlying behavioral variation.

Influence of cross-fostering on preference for calcium chloride in C57BL/6J and PWK/PhJ mice

We investigated whether maternal influences during the suckling period alter the avidity for calcium, using as models mice from the calcium-preferring PWK/PhJ strain and the calcium-avoiding C57BL/6J strain. We found that milk collected from PWK/PhJ dams had higher calcium concentrations than did milk collected from C57BL/6J dams. Despite this, cross-strain fostering had no effect on adult calcium preferences relative to mice of the same strain that were within-strain fostered or not fostered. Our results indicate that calcium avoidance by C57BL/6J mice and acceptance by PWK/PhJ mice are unaffected by maternal environment during the suckling period.

Taste dysfunction in BTBR mice due to a mutation of Itpr3, the inositol triphosphate receptor 3 gene

The BTBR T+ tf/J (BTBR) mouse strain is indifferent to exemplars of sweet, Polycose, umami, bitter, and calcium tastes, which share in common transduction by G protein-coupled receptors (GPCRs). To investigate the genetic basis for this taste dysfunction, we screened 610 BTBR×NZW/LacJ F2 hybrids, identified a potent QTL on chromosome 17, and isolated this in a congenic strain. Mice carrying the BTBR/BTBR haplotype in the 0.8-Mb (21-gene) congenic region were indifferent to sweet, Polycose, umami, bitter, and calcium tastes. To assess the contribution of a likely causative culprit, Itpr3, the inositol triphosphate receptor 3 gene, we produced and tested Itpr3 knockout mice. These were also indifferent to GPCR-mediated taste compounds. Sequencing the BTBR form of Itpr3 revealed a unique 12 bp deletion in Exon 23 (Chr 17: 27238069; Build 37). We conclude that a spontaneous mutation of Itpr3 in a progenitor of the BTBR strain produced a heretofore unrecognized dysfunction of GPCR-mediated taste transduction.

T1R3: a human calcium taste receptor

Many animals can detect the taste of calcium but it is unclear how or whether humans have this ability. We show here that calcium activates hTAS1R3-transfected HEK293 cells and that this response is attenuated by lactisole, an inhibitor of hT1R3. Moreover, trained volunteers report that lactisole reduces the calcium intensity of calcium lactate. Thus, humans can detect calcium by taste, T1R3 is a receptor responsible for this, and lactisole can reduce the taste perception of calcium by acting on T1R3.

Kokumi substances, enhancers of basic tastes, induce responses in calcium-sensing receptor expressing taste cells

Recently, we reported that calcium-sensing receptor (CaSR) is a receptor for kokumi substances, which enhance the intensities of salty, sweet and umami tastes. Furthermore, we found that several γ-glutamyl peptides, which are CaSR agonists, are kokumi substances. In this study, we elucidated the receptor cells for kokumi substances, and their physiological properties. For this purpose, we used Calcium Green-1 loaded mouse taste cells in lingual tissue slices and confocal microscopy. Kokumi substances, applied focally around taste pores, induced an increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) in a subset of taste cells. These responses were inhibited by pretreatment with the CaSR inhibitor, NPS2143. However, the kokumi substance-induced responses did not require extracellular Ca(2+). CaSR-expressing taste cells are a different subset of cells from the T1R3-expressing umami or sweet taste receptor cells. These observations indicate that CaSR-expressing taste cells are the primary detectors of kokumi substances, and that they are an independent population from the influenced basic taste receptor cells, at least in the case of sweet and umami.

Recombinant expression, in vitro refolding, and biophysical characterization of the N-terminal domain of T1R3 taste receptor

The sweet taste receptor is a heterodimeric receptor composed of the T1R2 and T1R3 subunits, while T1R1 and T1R3 assemble to form the umami taste receptor. T1R receptors belong to the family of class C G-protein coupled receptors (GPCRs). In addition to a transmembrane heptahelical domain, class C GPCRs have a large extracellular N-terminal domain (NTD), which is the primary ligand-binding site. The T1R2 and T1R1 subunits have been shown to be responsible for ligand binding, via their NTDs. However, little is known about the contribution of T1R3-NTD to receptor functions. To enable biophysical characterization, we overexpressed the human NTD of T1R3 (hT1R3-NTD) using Escherichia coli in the form of inclusion bodies. Using a fractional factorial screen coupled to a functional assay, conditions were determined for the refolding of hT1R3-NTD. Far-UV circular dichroism spectroscopic studies revealed that hT1R3-NTD was well refolded. Using size-exclusion chromatography, we found that the refolded protein behaves as a dimer. Ligand binding quantified by tryptophan fluorescence quenching and microcalorimetry showed that hT1R3-NTD is functional and capable of binding sucralose with an affinity in the millimolar range. This study also provides a strategy to produce functional hT1R3-NTD by heterologous expression in E. coli; this is a prerequisite for structural determination and functional analysis of ligand-binding regions of other class C GPCRs.

Detecting sweet and umami tastes in the gastrointestinal tract

Information about nutrients is a critical part of food selection in living creatures. Each animal species has developed its own way to safely seek and obtain the foods necessary for them to survive and propagate. Necessarily, humans and other vertebrates have developed special chemosensory organs such as taste and olfactory organs. Much attention, recently, has been given to the gastrointestinal (GI) tract as another chemosensory organ. Although the GI tract had been considered to be solely for digestion and absorption of foods and nutrients, researchers have recently found taste-signalling elements, including receptors, in this tissue. Further studies have revealed that taste cells in the oral cavity and taste-like cells in the GI tract appear to share common characteristics. Major receptors to detect umami, sweet and bitter are found in the GI tract, and it is now proposed that taste-like cells reside in the GI tract to sense nutrients and help maintain homeostasis. In this review, we summarize recent findings of chemoreception especially through sweet and umami sensors in the GI tract. In addition, the possibility of purinergic transmission from taste-like cells in the GI tract to vagus nerves is discussed.

Chorda tympani nerve modulates the rat's avoidance of calcium chloride

Calcium intake depends on orosensory factors, implying the presence of a mechanism for calcium detection in the mouth. To better understand how information about oral calcium is conveyed to the brain, we examined the effects of chorda tympani nerve transection on calcium chloride (CaCl(2)) taste preferences and thresholds in male Wistar rats. The rats were given bilateral transections of the chorda tympani nerve (CTX) or control surgery. After recovery, they received 48-h two-bottle tests with an ascending concentration series of CaCl(2). Whereas control rats avoided CaCl(2) at concentrations of 0.1mM and higher, rats with CTX were indifferent to CaCl(2) concentrations up to 10mM. Rats with CTX had significantly higher preference scores for 0.316 and 3.16 mM CaCl(2) than did control rats. The results imply that the chorda tympani nerve is required for the normal avoidance of CaCl(2) solution.

Sweet taste receptor gene variation and aspartame taste in primates and other species

Aspartame is a sweetener added to foods and beverages as a low-calorie sugar replacement. Unlike sugars, which are apparently perceived as sweet and desirable by a range of mammals, the ability to taste aspartame varies, with humans, apes, and Old World monkeys perceiving aspartame as sweet but not other primate species. To investigate whether the ability to perceive the sweetness of aspartame correlates with variations in the DNA sequence of the genes encoding sweet taste receptor proteins, T1R2 and T1R3, we sequenced these genes in 9 aspartame taster and nontaster primate species. We then compared these sequences with sequences of their orthologs in 4 other nontasters species. We identified 9 variant sites in the gene encoding T1R2 and 32 variant sites in the gene encoding T1R3 that distinguish aspartame tasters and nontasters. Molecular docking of aspartame to computer-generated models of the T1R2 + T1R3 receptor dimer suggests that species variation at a secondary, allosteric binding site in the T1R2 protein is the most likely origin of differences in perception of the sweetness of aspartame. These results identified a previously unknown site of aspartame interaction with the sweet receptor and suggest that the ability to taste aspartame might have developed during evolution to exploit a specialized food niche.

Comparison of differences between PWD/PhJ and C57BL/6J mice in calcium solution preferences and chorda tympani nerve responses

We used the C57BL/6J (B6) and PWD/PhJ (PWD) mouse strains to investigate the controls of calcium intake. Relative to the B6 strain, the PWD strain had higher preferences in two-bottle choice tests for CaCl(2), calcium lactate (CaLa), MgCl(2), citric acid and quinine hydrochloride, but not for sucrose, KCl or NaCl. We also measured taste-evoked chorda tympani (CT) nerve activity in response to oral application of these compounds. Electrophysiological results paralleled the preference test results, with larger responses in PWD than in B6 mice for those compounds that were more highly preferred for the former strain. The strain differences were especially large for tonic, rather than phasic, chorda tympani activity. These data establish the PWD strain as a "calcium-preferring" strain and suggest that differences between B6 and PWD mice in taste transduction or a related peripheral event contributes to the differences between the strains in preferences for calcium solutions.

Taste, visceral information and exocrine reflexes with glutamate through umami receptors

Chemical substances of foods drive the cognitive recognition of taste with the subsequent regulation of digestion in the gastrointestinal (GI) tract. Tastants like glutamate can bind to taste membrane receptors on the tip of specialized taste cells eliciting umami taste. In chemical-sensing cells diffused through the GI tract, glutamate induces functional changes. Most of the taste-like receptor-expressing cells from the stomach and intestine are neuroendocrine cells. The signaling molecules produced by these neuroendocrine cells either activate afferent nerve endings or release peptide hormones that can regulate neighboring cells in a paracrine fashion or travel through blood to their target receptor. Once afferent sensory fibers transfer the chemical information of the GI content to the central nervous system (CNS) facilitating the gut-brain signaling, the CNS regulates the GI through efferent cholinergic and noradrenergic fibers. Thus, this is a two-way extrinsic communication process. Glutamate within the lumen of the stomach stimulates afferent fibers and increases acid and pepsinogen release; whereas on the duodenum, glutamate increases the production of mucous to protect the mucosa against the incoming gastric acid. The effects of glutamate are believed to be mediated by G protein-coupled receptors expressed at the lumen of GI cells. The specific cell-type and molecular function of each of these receptors are not completely known. Here we will examine some of the glutamate receptors and their already understood role on GI function regulation.

Taste solution consumption by FHH-Chr nBN consomic rats

There has been extensive work to elucidate the behavioral and physiological mechanisms responsible for taste preferences of the rat but little attempt to delineate the underlying genetic architecture. Here, we exploit the FHH-Chr n(BN)/Mcwi consomic rat strain set to identify chromosomes carrying genes responsible for taste preferences. We screened the parental Fawn Hooded Hypertensive (FHH) and Brown Norway (BN) strains and 22 FHH-Chr n(BN) consomic strains, with 96-h 2-bottle tests, involving a choice between water and each of the following 16 solutions: 10 mM NaCl, 237 mM NaCl, 32 mM CaCl(2), 1 mM saccharin, 100 mM NH(4)Cl, 32 mM sucrose, 100 mM KCl, 4% ethanol, 1 mM HCl, 10 mM monosodium glutamate, 1 mM citric acid, 32 microM quinine hydrochloride, 1% corn oil, 32 microM denatonium, 1% Polycose, and 1 microM capsaicin. Depending on the taste solution involved, between 1 and 16 chromosomes were implicated in the response. Few of these chromosomes carried genes believed to mediate taste transduction in the mouse, and many chromosomes with no candidate taste genes were revealed. The genetic architecture of taste preferences is considerably more complex than has heretofore been acknowledged.

Functional expression of the extracellular-Ca2+-sensing receptor in mouse taste cells

Three types of morphologically and functionally distinct taste cells operate in the mammalian taste bud. We demonstrate here the expression of two G-protein-coupled receptors from the family C, CASR and GPRC6A, in the taste tissue and identify transcripts for both receptors in type I cells, no transcripts in type II cells and only CASR transcripts in type III cells, by using the SMART-PCR RNA amplification method at the level of individual taste cells. Type I taste cells responded to calcimimetic NPS R-568, a stereoselective CASR probe, with Ca(2+) transients, whereas type I and type II cells were not specifically responsive. Consistent with these findings, certain amino acids stimulated PLC-dependent Ca(2+) signaling in type III cells, but not in type I and type II cells, showing the following order of efficacies: Phe~Glu>Arg. Thus, CASR is coupled to Ca(2+) mobilization solely in type III cells. CASR was cloned from the circumvallate papilla into a pIRES2-EGFP plasmid and heterologously expressed in HEK-293 cells. The transfection with CASR enabled HEK-293 cells to generate Ca(2+) transients in response to the amino acids, of which, Phe was most potent. This observation and some other facts favor CASR as the predominant receptor subtype endowing type III cells with the ability to detect amino acids. Altogether, our results indicate that type III cells can serve a novel chemosensory function by expressing the polymodal receptor CASR. A role for CASR and GPRC6A in physiology of taste cells of the type I remains to be unveiled.

Genetics of taste and smell: poisons and pleasures

Eating is dangerous. While food contains nutrients and calories that animals need to produce heat and energy, it may also contain harmful parasites, bacteria, or chemicals. To guide food selection, the senses of taste and smell have evolved to alert us to the bitter taste of poisons and the sour taste and off-putting smell of spoiled foods. These sensory systems help people and animals to eat defensively, and they provide the brake that helps them avoid ingesting foods that are harmful. But choices about which foods to eat are motivated by more than avoiding the bad; they are also motivated by seeking the good, such as fat and sugar. However, just as not everyone is equally capable of sensing toxins in food, not everyone is equally enthusiastic about consuming high-fat, high-sugar foods. Genetic studies in humans and experimental animals strongly suggest that the liking of sugar and fat is influenced by genotype; likewise, the abilities to detect bitterness and the malodors of rotting food are highly variable among individuals. Understanding the exact genes and genetic differences that affect food intake may provide important clues in obesity treatment by allowing caregivers to tailor dietary recommendations to the chemosensory landscape of each person.

Involvement of the calcium-sensing receptor in human taste perception

By human sensory analyses, we found that various extracellular calcium-sensing receptor (CaSR) agonists enhance sweet, salty, and umami tastes, although they have no taste themselves. These characteristics are known as "kokumi taste" and often appear in traditional Japanese cuisine. Although GSH is a typical kokumi taste substance (taste enhancer), its mode of action is poorly understood. Here, we demonstrate how the kokumi taste is enhanced by the CaSR, a close relative of the class C G-protein-coupled receptors T1R1, T1R2, and T1R3 (sweet and umami receptors). We identified a large number of CaSR agonist gamma-glutamyl peptides, including GSH (gamma-Glu-Cys-Gly) and gamma-Glu-Val-Gly, and showed that these peptides elicit the kokumi taste. Further analyses revealed that some known CaSR agonists such as Ca(2+), protamine, polylysine, L-histidine, and cinacalcet (a calcium-mimetic drug) also elicit the kokumi taste and that the CaSR-specific antagonist, NPS-2143, significantly suppresses the kokumi taste. This is the first report indicating a distinct function of the CaSR in human taste perception.

Vegetable bitterness is related to calcium content

In the U.S. and Europe, most people do not consume the recommended amounts of either calcium or vegetables. We investigated whether there might be a connection; specifically, whether the taste of calcium in vegetables contributes to their bitterness and thus acceptability. We found a strong correlation between the calcium content of 24 vegetables, based on USDA Nutrient Database values, and bitterness, based on the average ratings of 35 people (r = 0.93). Correlations between the content of other nutrients and bitterness were lower and most were not statistically significant. To assess whether it is feasible that humans can detect calcium in vegetables we tested two animal models known to display a calcium appetite. Previous work indicates that calcium solutions are preferentially ingested by PWK/PhJ mice relative to C57BL/6J mice, and by rats deprived of dietary calcium relative to replete controls. In choice tests between collard greens, a high-calcium vegetable, and cabbage, a low-calcium vegetable, the calcium-favoring animals had higher preferences for collard greens than did controls. These observations raise the possibility that the taste of calcium contributes to the bitterness and thus acceptability of vegetables.

Molecular basis for amino acid sensing by family C G-protein-coupled receptors

Family C of human G-protein-coupled receptors (GPCRs) is constituted by eight metabotropic glutamate receptors, two gamma-aminobutyric acid type B (GABA(B1-2)) subunits forming the heterodimeric GABA(B) receptor, the calcium-sensing receptor, three taste1 receptors (T1R1-3), a promiscuous L-alpha;-amino acid receptor G-protein-coupled receptor family C, group 6, subtype A (GPRC6A) and seven orphan receptors. Aside from the orphan receptors, the family C GPCRs are dimeric receptors characterized by a large extracellular Venus flytrap domain which bind the endogenous agonists. Except from the GABA(B1-2) and T1R2-3 receptor, all receptors are either activated or positively modulated by amino acids. In this review, we outline mutational, biophysical and structural studies which have elucidated the interaction of the amino acids with the Venus flytrap domains, molecular mechanisms of receptor selectivity and the initial steps in receptor activation.

Sweet taste signaling functions as a hypothalamic glucose sensor

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 α-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.

Novel regulatory aspects of the extracellular Ca2+-sensing receptor, CaR

The capacity to sense and adapt to changes in environmental cues is of paramount importance for every living organism. From yeast to man, cells must be able to match cellular activities to growth environment and nutrient availability. Key to this process is the development of membrane-bound systems that can detect modifications in the extracellular environment and to translate these into biological responses. Evidence gathered over the last 15 years has demonstrated that many of these cell surface "sensors" belong to the G protein-coupled receptor superfamily. Crucial to our understanding of nutrient sensing in mammalian species has been the identification of the extracellular Ca(2+)/cation-sensing receptor, CaR. CaR was the first ion-sensing molecule identified in man and genetic studies in humans have revealed the importance of the CaR in mineral ion metabolism. Latter, it has become apparent that the CaR also plays an important role outside the Ca(2+) homeostatic system, as an integrator of multiple environmental signals for the regulation of many vital cellular processes, from cell-to-cell communication to secretion and cell survival/cell death. Recently, novel aspects of receptor function reveal an unexpected role for the CaR in the regulation of growth and development in utero.