Genetic labeling of Tas1r1 and Tas2r131 taste receptor cells in mice

Genetic mutation of Tas2r104/Tas2r105/Tas2r114 cluster leads to a loss of taste perception to denatonium benzoate and cucurbitacin B

BACKGROUND

Bitter taste receptors (Tas2rs) are generally considered to sense various bitter compounds to escape the intake of toxic substances. Bitter taste receptors have been found to widely express in extraoral tissues and have important physiological functions outside the gustatory system in vivo.

METHODS

To investigate the physiological functions of the bitter taste receptor cluster Tas2r106/Tas2r104/Tas2r105/Tas2r114 in lingual and extraoral tissues, multiple Tas2rs mutant mice and Gnat3 were produced using CRISPR/Cas9 gene-editing technique. A mixture containing Cas9 and sgRNA mRNAs for Tas2rs and Gnat3 gene was microinjected into the cytoplasm of the zygotes. Then, T7EN1 assays and sequencing were used to screen genetic mutation at the target sites in founder mice. Quantitative real-time polymerase chain reaction (qRT-PCR) and immunostaining were used to study the expression level of taste signaling cascade and bitter taste receptor in taste buds. Perception to taste substance was also studied using two-bottle preference tests.

RESULTS

We successfully produced several Tas2rs and Gnat3 mutant mice using the CRISPR/Cas9 technique. Immunostaining results showed that the expression of GNAT3 and PLCB2 was not altered in Tas2rs mutant mice. But qRT-PCR results revealed the changed expression profile of mTas2rs gene in taste buds of these mutant mice. With two-bottle preference tests, these mutant mice eliminate responses to cycloheximide due to genetic mutation of Tas2r105. In addition, these mutant mice showed a loss of taste perception to quinine dihydrochloride, denatonium benzoate, and cucurbitacin B (CuB). Gnat3-mediated taste receptor and its signal pathway contribute to CuB perception.

CONCLUSIONS

These findings implied that these mutant mice would be a valuable means to understand the biological functions of TAS2Rs in extraoral tissues and investigate bitter compound-induced responses mediated by these TAS2Rs in many extraoral tissues.

Bitter taste receptors in the reproductive system: Function and therapeutic implications

Type 2 taste receptors (TAS2Rs), traditionally known for their role in bitter taste perception, are present in diverse reproductive tissues of both sexes. This review explores our current understanding of TAS2R functions with a particular focus on reproductive health. In males, TAS2Rs are believed to play potential roles in processes such as sperm chemotaxis and male fertility. Genetic insights from mouse models and human polymorphism studies provide some evidence for their contribution to male infertility. In female reproduction, it is speculated that TAS2Rs influence the ovarian milieu, shaping the functions of granulosa and cumulus cells and their interactions with oocytes. In the uterus, TAS2Rs contribute to uterine relaxation and hold potential as therapeutic targets for preventing preterm birth. In the placenta, they are proposed to function as vigilant sentinels, responding to infection and potentially modulating mechanisms of fetal protection. In the cervix and vagina, their analogous functions to those in other extraoral tissues suggest a potential role in infection defense. In addition, TAS2Rs exhibit altered expression patterns that profoundly affect cancer cell proliferation and apoptosis in reproductive cancers. Notably, TAS2R agonists show promise in inducing apoptosis and overcoming chemoresistance in these malignancies. Despite these advances, challenges remain, including a lack of genetic and functional studies. The application of techniques such as single-cell RNA sequencing and clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated endonuclease 9 gene editing could provide deeper insights into TAS2Rs in reproduction, paving the way for novel therapeutic strategies for reproductive disorders.

Anterior and Posterior Tongue Regions and Taste Papillae: Distinct Roles and Regulatory Mechanisms with an Emphasis on Hedgehog Signaling and Antagonism

Sensory receptors across the entire tongue are engaged during eating. However, the tongue has distinctive regions with taste (fungiform and circumvallate) and non-taste (filiform) organs that are composed of specialized epithelia, connective tissues, and innervation. The tissue regions and papillae are adapted in form and function for taste and somatosensation associated with eating. It follows that homeostasis and regeneration of distinctive papillae and taste buds with particular functional roles require tailored molecular pathways. Nonetheless, in the chemosensory field, generalizations are often made between mechanisms that regulate anterior tongue fungiform and posterior circumvallate taste papillae, without a clear distinction that highlights the singular taste cell types and receptors in the papillae. We compare and contrast signaling regulation in the tongue and emphasize the Hedgehog pathway and antagonists as prime examples of signaling differences in anterior and posterior taste and non-taste papillae. Only with more attention to the roles and regulatory signals for different taste cells in distinct tongue regions can optimal treatments for taste dysfunctions be designed. In summary, if tissues are studied from one tongue region only, with associated specialized gustatory and non-gustatory organs, an incomplete and potentially misleading picture will emerge of how lingual sensory systems are involved in eating and altered in disease.

Selective integration of diverse taste inputs within a single taste modality

A fundamental question in sensory processing is how different channels of sensory input are processed to regulate behavior. Different input channels may converge onto common downstream pathways to drive the same behaviors, or they may activate separate pathways to regulate distinct behaviors. We investigated this question in the Drosophila bitter taste system, which contains diverse bitter-sensing cells residing in different taste organs. First, we optogenetically activated subsets of bitter neurons within each organ. These subsets elicited broad and highly overlapping behavioral effects, suggesting that they converge onto common downstream pathways, but we also observed behavioral differences that argue for biased convergence. Consistent with these results, transsynaptic tracing revealed that bitter neurons in different organs connect to overlapping downstream pathways with biased connectivity. We investigated taste processing in one type of downstream bitter neuron that projects to the higher brain. These neurons integrate input from multiple organs and regulate specific taste-related behaviors. We then traced downstream circuits, providing the first glimpse into taste processing in the higher brain. Together, these results reveal that different bitter inputs are selectively integrated early in the circuit, enabling the pooling of information, while the circuit then diverges into multiple pathways that may have different roles.

An update on extra-oral bitter taste receptors

Bitter taste-sensing type 2 receptors (TAS2Rs or T2Rs), belonging to the subgroup of family A G-protein coupled receptors (GPCRs), are of crucial importance in the perception of bitterness. Although in the first instance, TAS2Rs were considered to be exclusively distributed in the apical microvilli of taste bud cells, numerous studies have detected these sensory receptor proteins in several extra-oral tissues, such as in pancreatic or ovarian tissues, as well as in their corresponding malignancies. Critical points of extra-oral TAS2Rs biology, such as their structure, roles, signaling transduction pathways, extensive mutational polymorphism, and molecular evolution, have been currently broadly studied. The TAS2R cascade, for instance, has been recently considered to be a pivotal modulator of a number of (patho)physiological processes, including adipogenesis or carcinogenesis. The latest advances in taste receptor biology further raise the possibility of utilizing TAS2Rs as a therapeutic target or as an informative index to predict treatment responses in various disorders. Thus, the focus of this review is to provide an update on the expression and molecular basis of TAS2Rs functions in distinct extra-oral tissues in health and disease. We shall also discuss the therapeutic potential of novel TAS2Rs targets, which are appealing due to their ligand selectivity, expression pattern, or pharmacological profiles.

Segregated Expression of ENaC Subunits in Taste Cells

Salt taste is one of the 5 basic taste qualities. Depending on the concentration, table salt is perceived either as appetitive or aversive, suggesting the contribution of several mechanisms to salt taste, distinguishable by their sensitivity to the epithelial sodium channel (ENaC) blocker amiloride. A taste-specific knockout of the α-subunit of the ENaC revealed the relevance of this polypeptide for low-salt transduction, whereas the response to other taste qualities remained normal. The fully functional ENaC is composed of α-, β-, and γ-subunits. In taste tissue, however, the precise constitution of the channel and the cell population responsible for detecting table salt remain uncertain. In order to examine the cells and subunits building the ENaC, we generated mice carrying modified alleles allowing the synthesis of green and red fluorescent proteins in cells expressing the α- and β-subunit, respectively. Fluorescence signals were detected in all types of taste papillae and in taste buds of the soft palate and naso-incisor duct. However, the lingual expression patterns of the reporters differed depending on tongue topography. Additionally, immunohistochemistry for the γ-subunit of the ENaC revealed a lack of overlap between all potential subunits. The data suggest that amiloride-sensitive recognition of table salt is unlikely to depend on the classical ENaCs formed by α-, β-, and γ-subunits and ask for a careful investigation of the channel composition.

Mouse Mandibular Retromolar Taste Buds Associated With a Mucus Salivary Gland

We have characterized a recently rediscovered chemosensory structure at the rear of the mandibular mucosa in the mouse oral cavity originally reported in the 1980s. This consists of unorganized taste buds, not contained within troughs, associated with the ducts of an underlying minor salivary gland. Using whole-mount preparations of transgenic mice expressing green fluorescent protein under the promoter of taste-signaling-specific genes, we determined that the structure contains taste bud clusters and salivary gland orifices at the rear of each mandible, distal to the last molar and anterior to the ascending ramus. Immunohistochemical analysis shows in the retromolar taste buds expression of the taste receptors Tas2R131 and T1R3 and taste cascade molecules TrpM5, PLCβ2, and GNAT3, consistent with type II taste cells, and expression of GAD1, consistent with type III taste cells. Furthermore, the neuronal marker, calcitonin gene-related peptide, in retromolar mucosa tissue wrapping around TrpM5+ taste buds was observed. RT-PCR showed that retromolar taste buds express all 3 mouse tas1r genes, 28 of the 35 tas2r genes, and taste transduction signaling genes gnat3, plcb2, and trpm5, making the retromolar taste buds similar to other lingual and palate taste buds. Finally, histochemistry demonstrated that the mandibular retromolar secretory gland is a minor salivary gland of mucous type. The mandibular retromolar taste structure may thus play a role in taste sensation and represent a potential novel pharmacological target for taste disorders.

The Mystery of "Metal Mouth" in Chemotherapy

Of all the oral sensations that are experienced, "metallic" is one that is rarely reported in healthy participants. So why, then, do chemotherapy patients so frequently report that "metallic" sensations overpower and interfere with their enjoyment of food and drink? This side-effect of chemotherapy-often referred to (e.g., by patients) as "metal mouth"-can adversely affect their appetite, resulting in weight loss, which potentially endangers (or at the very least slows) their recovery. The etiology of "metal mouth" is poorly understood, and current management strategies are largely unevidenced. As a result, patients continue to suffer as a result of this poorly understood phenomenon. Here, we provide our perspective on the issue, outlining the evidence for a range of possible etiologies, and highlighting key research questions. We explore the evidence for "metallic" as a putative taste, and whether "metal mouth" might therefore be a form of phantageusia, perhaps similar to already-described "release-of-inhibition" phenomena. We comment on the possibility that "metal mouth" may simply be a direct effect of chemotherapy drugs. We present the novel theory that "metal mouth" may be linked to chemotherapy-induced sensitization of TRPV1. Finally, we discuss the evidence for retronasal olfaction of lipid oxidation products in the etiology of "metal mouth." This article seeks principally to guide much-needed future research which will hopefully one day provide a basis for the development of novel supportive therapies for future generations of patients undergoing chemotherapy.

Rapid Throughput Concentration-Response Analysis of Human Taste Discrimination

Human taste threshold measurements often are used to infer tastant receptor functionality. However, taste thresholds can be influenced by receptor-independent variables. Examination of the full range of taste-active concentrations by taste discrimination has been hampered by logistics of testing multiple concentrations in replicate with human subjects. We developed an automated rapid throughput operant methodology for taste discrimination and applied it to concentration-response analysis of human taste. Tastant solutions (200 µl) drawn from a 96-well plate and self-administered to the tongue served as discriminative stimuli for money-reinforced responses on a touch-sensitive display. Robust concentration-response functions for "basic taste" stimuli were established, with particular focus on agonists of the taste 1 receptor member 2-taste 1 receptor member 3 heterodimer receptor (TAS1R2/R3). With a training cue of 100 mM sucrose, EC50 values of 56, 79, and 310 µM and 40 mM were obtained for rebaudioside A, sucralose, acesulfame potassium, and sucrose, respectively. Changing the sucrose training cue to 300 mM had no impact, but changing to 30 mM resulted in slight leftward shifts in potencies. A signal detection method also was used to determine values of d', a probabilistic value for discriminability, which indicated that 5 mM was near the limits of detection for sucrose. With repeated testing, both EC50 values and 5 mM sucrose d' values were established for each individual subject. The results showed little correspondence between threshold sensitivities and EC50 values for sucrose. We conclude that concentration-response analysis of taste discrimination provides a more reliable means of inferring receptor function than measurement of discriminability at the lowest detectable tastant concentrations. SIGNIFICANCE STATEMENT: Many inferences about human tastant receptor functionality have been made from taste threshold measurements, which can be influenced by variables unrelated to receptors. We herein report a new methodology that enables rigorous concentration-response analysis of human taste discrimination and its use toward quantitative characterization of tastant agonist activity. Our data suggest that taste discrimination concentration-response functions are a more reliable reflection of underlying receptor activity than threshold measures obtained at the lowest detectable tastant concentrations.

Regulation of immune responses by tuft cells

Tuft cells are rare, secretory epithelial cells that generated scant immunological interest until contemporaneous reports in 2016 linked tuft cells with type 2 immunity in the small intestine. Tuft cells have the capacity to produce an unusual spectrum of biological effector molecules, including IL-25, eicosanoids implicated in allergy (such as cysteinyl leukotrienes and prostaglandin D₂) and the neurotransmitter acetylcholine. In most cases, the extracellular signals controlling tuft cell effector function are unknown, but signal transduction is thought to proceed via canonical, G protein-coupled receptor-dependent pathways involving components of the signalling pathway used by type II taste bud cells to sense sweet, bitter and umami compounds. Tuft cells are ideally positioned as chemosensory sentinels that can detect and relay information from diverse luminal substances via what appear to be stereotyped outputs to initiate both positive and aversive responses through populations of immune and neuronal cells. Despite recent insights, numerous questions remain regarding tuft cell lineage, diversity and effector mechanisms and how tuft cells interface with the immunological niche in the tissues where they reside.

Expression of Taste Receptor 2 Subtypes in Human Testis and Sperm

Taste receptors (TASRs) are expressed not only in the oral cavity but also throughout the body, thus suggesting that they may play different roles in organ systems beyond the tongue. Recent studies showed the expression of several TASRs in mammalian testis and sperm, indicating an involvement of these receptors in male gametogenesis and fertility. This notion is supported by an impaired reproductive phenotype of mouse carrying targeted deletion of taste receptor genes, as well as by a significant correlation between human semen parameters and specific polymorphisms of taste receptor genes. To better understand the biological and thus clinical significance of these receptors for human reproduction, we analyzed the expression of several members of the TAS2Rs family of bitter receptors in human testis and in ejaculated sperm before and after in vitro selection and capacitation. Our results provide evidence for the expression of TAS2R genes, with TAS2R14 being the most expressed bitter receptor subtype in both testis tissue and sperm cells, respectively. In addition, it was observed that in vitro capacitation significantly affects both the expression and the subcellular localization of these receptors in isolated spermatozoa. Interestingly, α-gustducin and α-transducin, two Gα subunits expressed in taste buds on the tongue, are also expressed in human spermatozoa; moreover, a subcellular redistribution of both G protein α-subunits to different sub-compartments of sperm was registered upon in vitro capacitation. Finally, we shed light on the possible downstream transduction pathway initiated upon taste receptor activation in the male reproductive system. Performing ultrasensitive droplets digital PCR assays to quantify RNA copy numbers of a distinct gene, we found a significant correlation between the expression of TAS2Rs and TRPM5 (r = 0.87), the cation channel involved in bitter but also sweet and umami taste transduction in taste buds on the tongue. Even if further studies are needed to clarify the precise functional role of taste receptors for successful reproduction, the presented findings significantly extend our knowledge of the biological role of TAS2Rs for human male fertility.

Infection by the parasitic helminth Trichinella spiralis activates a Tas2r-mediated signaling pathway in intestinal tuft cells

Intestinal tuft cells are sentinels monitoring the luminal contents and play a critical role in type 2 immunity. In this work, Trichinella spiralis excretion–secretion and extract were shown to directly induce interleukin 25 (IL-25) release from the intestinal villi, evoke calcium responses in tuft cells, and activate Tas2r bitter-taste receptors, whereas the bitter compound salicin was shown to activate and induce tuft cells to release IL-25. Gα-gustducin/Gβ1γ13 and/or Gαo/Gβ1γ13, Plcβ2, Ip₃r2, and Trpm5 comprise the signal transduction pathways that tuft cells utilize to initiate type 2 immune responses. Potentiation of Trpm5 by a natural sweet compound, stevioside, can enhance the tuft cell–ILC2 circuit’s activity, indicating that modulating these signaling components can help devise new means of combating parasites.

The parasitic helminth Trichinella spiralis, which poses a serious health risk to animals and humans, can be found worldwide. Recent findings indicate that a rare type of gut epithelial cell, tuft cells, can detect the helminth, triggering type 2 immune responses. However, the underlying molecular mechanisms remain to be fully understood. Here we show that both excretory–secretory products (E–S) and extract of T. spiralis can stimulate the release of the cytokine interleukin 25 (IL-25) from the mouse small intestinal villi and evoke calcium responses from tuft cells in the intestinal organoids, which can be blocked by a bitter-taste receptor inhibitor, allyl isothiocyanate. Heterologously expressed mouse Tas2r bitter-taste receptors, the expression of which is augmented during tuft-cell hyperplasia, can respond to the E–S and extract as well as to the bitter compound salicin whereas salicin in turn can induce IL-25 release from tuft cells. Furthermore, abolishment of the G-protein γ13 subunit, application of the inhibitors for G-protein αo/i, Gβγ subunits, and phospholipase Cβ2 dramatically reduces the IL-25 release. Finally, tuft cells are found to utilize the inositol triphosphate receptor type 2 (Ip₃r2) to regulate cytosolic calcium and thus Trpm5 activity, while potentiation of Trpm5 by a sweet-tasting compound, stevioside, enhances tuft cell IL-25 release and hyperplasia in vivo. Taken together, T. spiralis infection activates a signaling pathway in intestinal tuft cells similar to that of taste-bud cells, but with some key differences, to initiate type 2 immunity.

Taste Receptors: New Players in Sperm Biology

Taste receptors were first described as sensory receptors located on the tongue, where they are expressed in small clusters of specialized epithelial cells. However, more studies were published in recent years pointing to an expression of these proteins not only in the oral cavity but throughout the body and thus to a physiological role beyond the tongue. The recent observation that taste receptors and components of the coupled taste transduction cascade are also expressed during the different phases of spermatogenesis as well as in mature spermatozoa from mouse to humans and the overlap between the ligand spectrum of taste receptors with compounds in the male and female reproductive organs makes it reasonable to assume that sperm "taste" these different cues in their natural microenvironments. This assumption is assisted by the recent observations of a reproductive phenotype of different mouse lines carrying a targeted deletion of a taste receptor gene as well as the finding of a significant correlation between human male infertility and some polymorphisms in taste receptors genes. In this review, we depict recent findings on the role of taste receptors in male fertility, especially focusing on their possible involvement in mechanisms underlying spermatogenesis and post testicular sperm maturation. We also highlight the impact of genetic deletions of taste receptors, as well as their polymorphisms on male reproduction.

The Chemosensory Receptor Repertoire of a True Shark Is Dominated by a Single Olfactory Receptor Family

Throughout the animal kingdom chemical senses are one of the primary means by which organisms make sense of their environment. To achieve perception of complex chemosensory stimuli large repertoires of olfactory and gustatory receptors are employed in bony vertebrates, which are characterized by high evolutionary dynamics in receptor repertoire size and composition. However, little is known about their evolution in earlier diverging vertebrates such as cartilaginous fish, which include sharks, skates, rays, and chimeras. Recently, the olfactory repertoire of a chimera, elephant shark, was found to be curiously reduced in odorant receptor number. Elephant sharks rely heavily on electroreception to localize prey; thus, it is unclear how representative their chemosensory receptor repertoire sizes would be for cartilaginous fishes in general. Here, we have mined the genome of a true shark, Scyliorhinus canicula (catshark) for olfactory and gustatory receptors, and have performed a thorough phylogenetic study to shed light on the evolution of chemosensory receptors in cartilaginous fish. We report the presence of several gustatory receptors of the TAS1R family in catshark and elephant shark, whereas TAS2R receptors are absent. The catshark olfactory repertoire is dominated by V2R receptors, with 5–8 receptors in the other three families (OR, ORA, TAAR). Species-specific expansions are mostly limited to the V2R family. Overall, the catshark chemosensory receptor repertoires are generally similar in size to those of elephant shark, if somewhat larger, showing similar evolutionary tendencies across over 400 Myr of separate evolution between catshark and elephant shark.

A Pharmacological Perspective on the Study of Taste

The study of taste has been guided throughout much of its history by the conceptual framework of psychophysics, where the focus was on quantification of the subjective experience of the taste sensations. By the mid-20th century, data from physiologic studies had accumulated sufficiently to assemble a model for the function of receptors that must mediate the initial stimulus of tastant molecules in contact with the tongue. But the study of taste as a receptor-mediated event did not gain momentum until decades later when the actual receptor proteins and attendant signaling mechanisms were identified and localized to the highly specialized taste-responsive cells of the tongue. With those discoveries a new opportunity to examine taste as a function of receptor activity has come into focus. Pharmacology is the science designed specifically for the experimental interrogation and quantitative characterization of receptor function at all levels of inquiry from molecules to behavior. This review covers the history of some of the major concepts that have shaped thinking and experimental approaches to taste, the seminal discoveries that have led to elucidation of receptors for taste, and how applying principles of receptor pharmacology can enhance understanding of the mechanisms of taste physiology and perception.

Olfactory, Taste, and Photo Sensory Receptors in Non-sensory Organs: It Just Makes Sense

Sensory receptors that detect and respond to light, taste, and smell primarily belong to the G-protein-coupled receptor (GPCR) superfamily. In addition to their established roles in the nose, tongue, and eyes, these sensory GPCRs have been found in many ‘non-sensory' organs where they respond to different physicochemical stimuli, initiating signaling cascades in these extrasensory systems. For example, taste receptors in the airway, and photoreceptors in vascular smooth muscle cells, both cause smooth muscle relaxation when activated. In addition, olfactory receptors are present within the vascular system, where they play roles in angiogenesis as well as in modulating vascular tone. By better understanding the physiological and pathophysiological roles of sensory receptors in non-sensory organs, novel therapeutic agents can be developed targeting these receptors, ultimately leading to treatments for pathological conditions and potential cures for various disease states.

Tuft Cells-Systemically Dispersed Sensory Epithelia Integrating Immune and Neural Circuitry

Tuft cells-rare solitary chemosensory cells in mucosal epithelia-are undergoing intense scientific scrutiny fueled by recent discovery of unsuspected connections to type 2 immunity. These cells constitute a conduit by which ligands from the external space are sensed via taste-like signaling pathways to generate outputs unique among epithelial cells: the cytokine IL-25, eicosanoids associated with allergic immunity, and the neurotransmitter acetylcholine. The classic type II taste cell transcription factor POU2F3 is lineage defining, suggesting a conceptualization of these cells as widely distributed environmental sensors with effector functions interfacing type 2 immunity and neural circuits. Increasingly refined single-cell analytics have revealed diversity among tuft cells that extends from nasal epithelia and type II taste cells to ex-Aire-expressing medullary thymic cells and small-intestine cells that mediate tissue remodeling in response to colonizing helminths and protists.

Tolerance of bitter stimuli and attenuation/accumulation of their bitterness in humans

Some components of bitterness make key flavor contributions to promote the palatability of foods, whereas other components are recognized as aversive signals to avoid consuming harmful substances. These contradictory behaviors suggest that humans tolerate tastes of bitterants based on certain criteria. Here, we investigated human taste tolerance and sensory cues leading to diverse taste tolerance of bitter compounds. Tolerance of eight bitter compounds, which are typically contained in foods, was evaluated by measuring detection and rejection thresholds. The results revealed that the level of tolerance of each compound was variable, and some compounds showed an acceptable concentration regarding the suprathreshold intensity. Tolerance did not depend on the nutritive value or attenuation and accumulation characteristics of bitterness and bitter taste receptors. These results suggest that the criteria controlling tolerance of bitter compounds may be derived from a complex relationship between the taste quality and cognitive process.

Tas2r125 functions as the main receptor for detecting bitterness of tea catechins in the oral cavity of mice

We attempted to identify mouse bitter taste receptors, Tas2rs, that respond to tea catechins. Among representative tea catechins, avoidance behavior of mice to (-)-epicatechin gallate (ECg) was the strongest, followed by (-)-epigallocatechin gallate (EGCg). Therefore, we measured ECg response using Tas2rs-expressing cells. Among the 35 members of Tas2r family, Tas2r108, 110, 113, 125, and 144 responded to ECg. Among these receptors, Tas2r113 and 125 also responded to EGCg. Because the response profiles of Tas2r125 were consistent with the results of the behavior assays, it was considered that Tas2r125 functions as the main receptor for detecting bitterness of tea catechins in the oral cavity. To determine the involvement of Tas2rs in the physiological action of catechins, mRNA expression of 5 Tas2rs was investigated in various tissues. Because mRNA expression of Tas2r108 was observed in some tissues including the gastrointestinal tract, it may be envisaged that Tas2r108 plays a part in exerting the physiological action of ECg. Tas2r125 expression was not observed in any of the tested tissues except the circumvallate papillae. Therefore, Tas2r125 was considered to mainly function in the events of catechin reception in the oral cavity.

Taste of phytocompounds: A better predictor for ethnopharmacological activities of medicinal plants than the phytochemical class?

ETHNOPHARMACOLOGICAL RELEVANCE

Understanding the patterns that shape traditional medical knowledge is essential for accelerating ethnopharmacological progress. According to Ayurveda, medicinal plants that belong to different taxa, but which have similar taste, may display similar (ethno)pharmacological activities (EPAs) (Bhishagratna, 1998; Sharma and Dash, 2006).

AIM OF THE STUDY

To understand the patterns that govern the distribution of herbal EPAs in Ayurveda and to evaluate the potential concordance between chemical class or taste of the constituent phytocompounds and EPAs.

MATERIAL AND METHODS

A mixed database (PhytoMolecularTasteDB) was constructed for Ayurvedic medicinal plants by integrating modern data (medicinal plant composition, phytochemical taste) with traditional data (ethnopharmacological activities of plant). PhytoMolecularTasteDB contains 431 Ayurvedic medicinal plants, 94 EPAs, 223 chemical classes of phytocompounds and 438 herbal tastants. Potential global or individual associations between chemical classes/taste of the phytoconstituents and EPAs were statistically analyzed.

RESULTS

There was no global statistical correlation between the various chemical classes of phytocompounds and EPAs, although there were several individual correlations. The results suggest the existence of a global statistical correlation (besides several individual correlations) between the plant "molecular taste" (various taste-based classes of phytocompounds) and EPAs.

CONCLUSIONS

These results suggest that phytochemical taste may be more relevant than chemical class for EPAs prediction.

Taste receptor polymorphisms and male infertility

STUDY QUESTION

Are polymorphisms of taste receptor genes associated with male infertility?

SUMMARY ANSWER

This study has showed the associations between three single nucleotide polymorphisms (SNPs) in taste receptors genes (TASR) and male infertility.

WHAT IS KNOWN ALREADY

Recent studies showed the expression of taste receptors in the testis and in spermatozoa, suggesting their possible role in infertility. The vast genetic variability in taste genes results in a large degree of diversity in various human phenotypes.

STUDY DESIGN, SIZE, DURATION

In this study, we genotyped 19 SNPs in 12 taste related genes in a total of 494 Caucasian male patients undergoing semen evaluation at the Centre of Couple Sterility of the Siena University Hospital. Consecutive patients were enrolled during infertility investigations from October 2014 to February 2016.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Median age of the patients was 36 years (18-58) and 141 were smokers. Genotyping was performed using the allele-specific PCR. The statistical analysis was carried out using generalized linear model (GLM) to explore the association between age, smoking, the genetic polymorphisms and sperm parameters.

MAIN RESULTS AND THE ROLE OF CHANCE

We observed that the homozygous carriers of the (G) allele of the TAS2R14-rs3741843 polymorphism showed a decreased sperm progressive motility compared to heterozygotes and (A) homozygotes (P = 0.003). Moreover, the homozygous carriers of the (T) allele of the TAS2R3-rs11763979 SNP showed fewer normal acrosome compared with the heterozygous and the homozygous carriers of the (G) allele (P = 0.002). Multiple comparisons correction was applied and the Bonferroni-corrected critical P-value was = 0.003.

LIMITATIONS, REASONS FOR CAUTION

The analysis is restricted to SNPs within genes and to men of Caucasian ancestry.

WIDER IMPLICATIONS OF THE FINDINGS

In silico analyses strongly point towards a functional effect of the two SNPs: TAS2R14-rs3741843 regulates TAS2R43 expression, a gene that is involved in cilia motility and therefore could influences sperm mobility; the (T) allele of TAS2R3-rs11763979 increases the expression of the WEE2 antisense RNA one gene (WEE2-AS1). According to Genotype-Tissue Expression (GTEx) project the WEE2 gene is expressed in the testes where presumably it has the role of down regulating meiotic cell division. It is plausible to hypothesize that the WEE2-AS1 increased expression may down regulate WEE2 which in turn can alter the natural timing of sperm maturation increasing the number of abnormal sperm cells.

STUDY FUNDING/COMPETING INTEREST(S)

None.

Tastant-Evoked Arc Expression in the Nucleus of the Solitary Tract and Nodose/Petrosal Ganglion of the Mouse Is Specific for Bitter Compounds

Despite long and intense research, some fundamental questions regarding representation of taste information in the brain still remain unanswered. This might in part be due to shortcomings of the established methods that limit the researcher either to thorough characterization of few elements or to analyze the response of the entirety of neurons to only one stimulus. To overcome these restrictions, we evaluate the use of the immediate early gene Arc as a neuronal activity marker in the early neural structures of the taste pathway, the nodose/petrosal ganglion (NPG) and the nucleus of the solitary tract (NTS). Responses of NPG and NTS neurons were limited to substances that taste bitter to humans and are avoided by mice. Arc-expressing cells were concentrated in the rostromedial part of the dorsal NTS suggesting a role in gustatory processing. The use of Arc as a neuronal activity marker has several advantages, primarily the possibility to analyze the response of large numbers of neurons while using more than one stimulus makes Arc an interesting new tool for research in the early stages of taste processing.

Characterization of mouse chorda tympani responses evoked by stimulation of anterior or posterior fungiform taste papillae

Different gustatory papilla types vary in their locations on the tongue. Distinctions have often made between types, but variation within fungiform papillae has seldom been explored. Here, regional differences in fungiform papillae were investigated by flowing solutions selectively over either an anterior fungiform (AF, tongue tip) or a posterior fungiform (PF, middle third) region as taste-evoked activity was measured in the chorda tympani nerve of C57BL/6J (B6) mice. Significantly larger responses were evoked by NaCl applied to the AF than PF region, and the ENaC blocker amiloride reduced the NaCl response size only for the former. Umami synergy, based on co-presenting MSG and IMP, was larger for the AF than PF region. The regions did not differ in response size to sour chemicals, but responses to l-lysine, l-arginine, sucrose, and tetrasodium pyrophosphate were larger for the AF than PF region. Thus, fungiform papillae on the tongue tip differed from those found further back in their transduction mechanisms for salty and umami compounds. Gustatory sensitivity also showed regional variation, albeit with a complex relationship to palatability and taste quality. Overall, the data support a regional organization for the mouse tongue, with different functional zones for the anterior, middle, and posterior thirds.

Gli3 is a negative regulator of Tas1r3-expressing taste cells

Mouse taste receptor cells survive from 3-24 days, necessitating their regeneration throughout adulthood. In anterior tongue, sonic hedgehog (SHH), released by a subpopulation of basal taste cells, regulates transcription factors Gli2 and Gli3 in stem cells to control taste cell regeneration. Using single-cell RNA-Seq we found that Gli3 is highly expressed in Tas1r3-expressing taste receptor cells and Lgr5+ taste stem cells in posterior tongue. By PCR and immunohistochemistry we found that Gli3 was expressed in taste buds in all taste fields. Conditional knockout mice lacking Gli3 in the posterior tongue (Gli3CKO) had larger taste buds containing more taste cells than did control wild-type (Gli3WT) mice. In comparison to wild-type mice, Gli3CKO mice had more Lgr5+ and Tas1r3+ cells, but fewer type III cells. Similar changes were observed ex vivo in Gli3CKO taste organoids cultured from Lgr5+ taste stem cells. Further, the expression of several taste marker and Gli3 target genes was altered in Gli3CKO mice and/or organoids. Mirroring these changes, Gli3CKO mice had increased lick responses to sweet and umami stimuli, decreased lick responses to bitter and sour taste stimuli, and increased glossopharyngeal taste nerve responses to sweet and bitter compounds. Our results indicate that Gli3 is a suppressor of stem cell proliferation that affects the number and function of mature taste cells, especially Tas1r3+ cells, in adult posterior tongue. Our findings shed light on the role of the Shh pathway in adult taste cell regeneration and may help devise strategies for treating taste distortions from chemotherapy and aging.

Expression profiling of Tas2r genes reveals a complex pattern along the mouse GI tract and the presence of Tas2r131 in a subset of intestinal Paneth cells

The chemical variability of the intestinal lumen requires the presence of molecular receptors detecting the various substances naturally occurring in the diet and as a result of the activity of the microbiota. Despite their early discovery, intestinal bitter taste receptors (Tas2r) have not yet been assigned an unambiguous physiological function. Recently, using a CRE-recombinant approach we showed that the Tas2r131 gene is expressed in a subset of mucin-producing goblet cells in the colon of mice. Moreover, we also demonstrated that the expression of the Tas2r131 locus is not restricted to this region. In the present study we aimed at characterizing the presence of positive cells also in other gastrointestinal regions. Our results show that Tas2r131+ cells appear in the jejunum and the ileum, and are absent from the stomach and the duodenum. We identified the positive cells as a subpopulation of deep-crypt Paneth cells in the ileum, strengthening the notion of a defensive role for Tas2rs in the gut. To get a broader perspective on the expression of bitter taste receptors in the alimentary canal, we quantified the expression of all 35 Tas2r genes along the gastrointestinal tract by qRT-PCR. We discovered that the number and expression level of Tas2r genes profoundly vary along the alimentary canal, with the stomach and the colon expressing the largest subsets.

Genetic Labeling of Car4-expressing Cells Reveals Subpopulations of Type III Taste Cells

Carbonic anhydrases form an enzyme family of 16 members, which reversibly catalyze the hydration of carbon dioxide to bicarbonate and protons. In lung, kidney, and brain, presence of carbonic anhydrases is associated with protons and bicarbonate transport in capillary endothelium of lung, reabsorption of bicarbonate in proximal renal tubules, and extracellular buffering. In contrast, their role in taste is less clear. Recently, carbonic anhydrase IV expression was detected in sour-sensing presynaptic taste cells and was associated with the taste of carbonation, yet the precise role and cell population remained uncertain. To examine the role of carbonic anhydrase 4-expressing cells in taste reception, we generated a mouse strain carrying a modified allele of the carbonic anhydrase 4 gene in which the coding region of the red fluorescent protein monomeric Cherry is attached to that of carbonic anhydrase 4 via an internal ribosome entry site. Monomeric Cherry fluorescence was detected in lingual papillae as well as taste buds of soft palate and naso-incisor duct. However, expression patterns on the tongue differ between posterior and fungiform papillae. Whereas monomeric Cherry auto-fluorescence was almost always co-localized with presynaptic cell markers aromatic L-amino-acid decarboxylase, synaptosomal-associated protein 25 or glutamic acid decarboxylase 67 in fungiform papillae and taste buds of palate and naso-incisor duct, monomeric Cherry-positive cells in posterior tongue papillae represent only a subpopulation of presynaptic cells. We conclude that this model is well suited for detailed investigation into the role of carbonic anhydrase in gustation and other processes.

Members of Bitter Taste Receptor Cluster Tas2r143/Tas2r135/Tas2r126 Are Expressed in the Epithelium of Murine Airways and Other Non-gustatory Tissues

The mouse bitter taste receptors Tas2r143, Tas2r135, and Tas2r126 are encoded by genes that cluster on chromosome 6 and have been suggested to be expressed under common regulatory elements. Previous studies indicated that the Tas2r143/Tas2r135/Tas2r126 cluster is expressed in the heart, but other organs had not been systematically analyzed. In order to investigate the expression of this bitter taste receptor gene cluster in non-gustatory tissues, we generated a BAC (bacterial artificial chromosome) based transgenic mouse line, expressing CreERT2 under the control of the Tas2r143 promoter. After crossing this line with a mouse line expressing EGFP after Cre-mediated recombination, we were able to validate the Tas2r143-CreERT2 transgenic mouse line and monitor the expression of Tas2r143. EGFP-positive cells, indicating expression of members of the cluster, were found in about 47% of taste buds, and could also be found in several other organs. A population of EGFP-positive cells was identified in thymic epithelial cells, in the lamina propria of the intestine and in vascular smooth muscle cells of cardiac blood vessels. EGFP-positive cells were also identified in the epithelium of organs readily exposed to pathogens including lower airways, the gastrointestinal tract, urethra, vagina, and cervix. With respect to the function of cells expressing this bitter taste receptor cluster, RNA-seq analysis in EGFP-positive cells isolated from the epithelium of trachea and stomach showed expression of genes related to innate immunity. These data further support the concept that bitter taste receptors serve functions outside the gustatory system.

Amino acid sensing in hypothalamic tanycytes via umami taste receptors

Objective

Hypothalamic tanycytes are glial cells that line the wall of the third ventricle and contact the cerebrospinal fluid (CSF). While they are known to detect glucose in the CSF we now show that tanycytes also detect amino acids, important nutrients that signal satiety.

Methods

Ca²⁺ imaging and ATP biosensing were used to detect tanycyte responses to l-amino acids. The downstream pathway of the responses was determined using ATP receptor antagonists and channel blockers. The receptors were characterized using mice lacking the Tas1r1 gene, as well as an mGluR4 receptor antagonist.

Results

Amino acids such as Arg, Lys, and Ala evoke Ca²⁺ signals in tanycytes and evoke the release of ATP via pannexin 1 and CalHM1, which amplifies the signal via a P2 receptor dependent mechanism. Tanycytes from mice lacking the Tas1r1 gene had diminished responses to lysine and arginine but not alanine. Antagonists of mGluR4 greatly reduced the responses to alanine and lysine.

Conclusion

Two receptors previously implicated in taste cells, the Tas1r1/Tas1r3 heterodimer and mGluR4, contribute to the detection of a range of amino acids by tanycytes in CSF.

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Hypothalamic tanycytes can detect amino acids in cerebrospinal fluid.

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The mechanism is taste receptor-dependent.

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Tas1r1/Tas1r3 mediates responses to l-arginine and l-lysine.

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mGluR4 mediates responses to l-alanine and partially those of l-lysine.

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ATP release from tanycytes evoked by amino acids reaches into the arcuate nucleus.

Extraoral Taste Receptor Discovery: New Light on Ayurvedic Pharmacology

More and more research studies are revealing unexpectedly important roles of taste for health and pathogenesis of various diseases. Only recently it has been shown that taste receptors have many extraoral locations (e.g., stomach, intestines, liver, pancreas, respiratory system, heart, brain, kidney, urinary bladder, pancreas, adipose tissue, testis, and ovary), being part of a large diffuse chemosensory system. The functional implications of these taste receptors widely dispersed in various organs or tissues shed a new light on several concepts used in ayurvedic pharmacology (dravyaguna vijnana), such as taste (rasa), postdigestive effect (vipaka), qualities (guna), and energetic nature (virya). This review summarizes the significance of extraoral taste receptors and transient receptor potential (TRP) channels for ayurvedic pharmacology, as well as the biological activities of various types of phytochemical tastants from an ayurvedic perspective. The relative importance of taste (rasa), postdigestive effect (vipaka), and energetic nature (virya) as ethnopharmacological descriptors within Ayurveda boundaries will also be discussed.

A sweet taste receptor-dependent mechanism of glucosensing in hypothalamic tanycytes

Hypothalamic tanycytes are glial‐like glucosensitive cells that contact the cerebrospinal fluid of the third ventricle, and send processes into the hypothalamic nuclei that control food intake and body weight. The mechanism of tanycyte glucosensing remains undetermined. While tanycytes express the components associated with the glucosensing of the pancreatic β cell, they respond to nonmetabolisable glucose analogues via an ATP receptor‐dependent mechanism. Here, we show that tanycytes in rodents respond to non‐nutritive sweeteners known to be ligands of the sweet taste (Tas1r2/Tas1r3) receptor. The initial sweet tastant‐evoked response, which requires the presence of extracellular Ca²⁺, leads to release of ATP and a larger propagating Ca²⁺ response mediated by P2Y1 receptors. In Tas1r2 null mice the proportion of glucose nonresponsive tanycytes was greatly increased in these mice, but a subset of tanycytes retained an undiminished sensitivity to glucose. Our data demonstrate that the sweet taste receptor mediates glucosensing in about 60% of glucosensitive tanycytes while the remaining 40% of glucosensitive tanycytes use some other, as yet unknown mechanism.

Extraoral bitter taste receptors in health and disease

Bitter taste receptors (TAS2Rs or T2Rs) belong to the superfamily of seven-transmembrane G protein-coupled receptors, which are the targets of >50% of drugs currently on the market. Canonically, T2Rs are located in taste buds of the tongue, where they initiate bitter taste perception. However, accumulating evidence indicates that T2Rs are widely expressed throughout the body and mediate diverse nontasting roles through various specialized mechanisms. It has also become apparent that T2Rs and their polymorphisms are associated with human disorders. In this review, we summarize the physiological and pathophysiological roles that extraoral T2Rs play in processes as diverse as innate immunity and reproduction, and the major challenges in this emerging field.

How taste works: cells, receptors and gustatory perception

The sensitivity of taste in mammals varies due to quantitative and qualitative differences in the structure of the taste perception organs. Gustatory perception is made possible by the peripheral chemosensory organs, i.e., the taste buds, which are distributed in the epithelium of the taste papillae of the palate, tongue, epiglottis, throat and larynx. Each taste bud consists of a community of ~100 cells that process and integrate taste information with metabolic needs. Mammalian taste buds are contained in circumvallate, fungiform and foliate papillae and react to sweet, salty, sour, bitter and umami stimuli. The sensitivity of the taste buds for individual taste stimuli varies extensively and depends on the type of papillae and the part of the oral cavity in which they are located. There are at least three different cell types found in mammalian taste buds: type I cells, receptor (type II) cells and presynaptic (type III) cells. This review focuses on the biophysiological mechanisms of action of the various taste stimuli in humans. Currently, the best-characterized proteins are the receptors (GPCR). In addition, the activation of bitter, sweet and umami tastes are relatively well known, but the activation of salty and sour tastes has yet to be clearly explained.

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.

Comprehensive Analysis of Mouse Bitter Taste Receptors Reveals Different Molecular Receptive Ranges for Orthologous Receptors in Mice and Humans

One key to animal survival is the detection and avoidance of potentially harmful compounds by their bitter taste. Variable numbers of taste 2 receptor genes expressed in the gustatory end organs enable bony vertebrates (Euteleostomi) to recognize numerous bitter chemicals. It is believed that the receptive ranges of bitter taste receptor repertoires match the profiles of bitter chemicals that the species encounter in their diets. Human and mouse genomes contain pairs of orthologous bitter receptor genes that have been conserved throughout evolution. Moreover, expansions in both lineages generated species-specific sets of bitter taste receptor genes. It is assumed that the orthologous bitter taste receptor genes mediate the recognition of bitter toxins relevant for both species, whereas the lineage-specific receptors enable the detection of substances differently encountered by mice and humans. By challenging 34 mouse bitter taste receptors with 128 prototypical bitter substances in a heterologous expression system, we identified cognate compounds for 21 receptors, 19 of which were previously orphan receptors. We have demonstrated that mouse taste 2 receptors, like their human counterparts, vary greatly in their breadth of tuning, ranging from very broadly to extremely narrowly tuned receptors. However, when compared with humans, mice possess fewer broadly tuned receptors and an elevated number of narrowly tuned receptors, supporting the idea that a large receptor repertoire is the basis for the evolution of specialized receptors. Moreover, we have demonstrated that sequence-orthologous bitter taste receptors have distinct agonist profiles. Species-specific gene expansions have enabled further diversification of bitter substance recognition spectra.

The bitter truth about bitter taste receptors: beyond sensing bitter in the oral cavity

The bitter taste receptor (TAS2R)-family of G-protein-coupled receptors has been identified on the tongue as detectors of bitter taste over a decade ago. In the last few years, they have been discovered in an ever growing number of extra-oral tissues, including the airways, the gut, the brain and even the testis. In tissues that contact the exterior, protective functions for TAS2Rs have been proposed, in analogy to their function on the tongue as toxicity detector. However, TAS2Rs have also been found in internal organs, suggesting other roles for these receptors, perhaps involving as yet unidentified endogenous ligands. The current review gives an overview of the different proposed functions for TAS2Rs in tissues other than the oral cavity; from appetite regulation to the treatment of asthma, regulation of gastrointestinal motility and control of airway innate immunity.

The Role of Bitter and Sweet Taste Receptors in Upper Airway Immunity

Over the past several years, taste receptors have emerged as key players in the regulation of innate immune defenses in the mammalian respiratory tract. Several cell types in the airway, including ciliated epithelial cells, solitary chemosensory cells, and bronchial smooth muscle cells, all display chemoresponsive properties that utilize taste receptors. A variety of bitter products secreted by microbes are detected with resultant downstream inflammation, increased mucous clearance, antimicrobial peptide secretion, and direct bacterial killing. Genetic variation of bitter taste receptors also appears to play a role in the susceptibility to infection in respiratory disease states, including that of chronic rhinosinusitis. Ongoing taste receptor research may yield new therapeutics that harness innate immune defenses in the respiratory tract and may offer alternatives to antibiotic treatment. The present review discusses taste receptor-protective responses and analyzes the role these receptors play in mediating airway immune function.

Rubemamine and Rubescenamine, Two Naturally Occurring N-Cinnamoyl Phenethylamines with Umami-Taste-Modulating Properties

Sensory screening of a series of naturally occurring N-cinnamoyl derivatives of substituted phenethylamines revealed that rubemamine (9, from Chenopodium album) and rubescenamine (10, from Zanthoxylum rubsecens) elicit strong intrinsic umami taste in water at 50 and 10 ppm, respectively. Sensory tests in glutamate- and nucleotide-containing bases showed that the compounds influence the whole flavor profile of savory formulations. Both rubemamine (9) and rubescenamine (10) at 10-100 ppm dose-dependently positively modulated the umami taste of MSG (0.17-0.22%) up to threefold. Among the investigated amides, only rubemamine (9) and rubescenamine (10) are able to directly activate the TAS1R1-TAS1R3 umami taste receptor. Moreover, both compounds also synergistically modulated the activation of TAS1R1-TAS1R3 by MSG. Most remarkably, rubemamine (9) was able to further positively modulate the IMP-enhanced TAS1R1-TAS1R3 response to MSG ∼ 1.8-fold. Finally, armatamide (11), zanthosinamide (13), and dioxamine (14), which lack intrinsic umami taste in vivo and direct receptor response in vitro, also positively modulated receptor activation by MSG about twofold and the IMP-enhanced MSG-induced TAS1R1-TAS1R3 responses approximately by 50%. In sensory experiments, dioxamine (14) at 25 ppm in combination with 0.17% MSG exhibited a sensory equivalent to 0.37% MSG.

Cre-Mediated Recombination in Tas2r131 Cells-A Unique Way to Explore Bitter Taste Receptor Function Inside and Outside of the Taste System

The type 2 taste receptors (Tas2rs) comprise a large family of G protein-coupled receptors that recognize compounds bitter to humans and aversive to vertebrates. Tas2rs are expressed in both gustatory and nongustatory tissues, however, identification and functional analyses of T2R-expressing cells have been difficult in most tissues. To overcome these limitations and to be able to manipulate Tas2r-expressing cells in vivo, we used gene-targeting to generate a Tas2r131-specific Cre knock-in mouse strain. We then employed a binary genetic approach to characterize Cre-mediated recombination in these animals and to investigate Tas2r131 expression during postnatal development. We demonstrate that a Cre-activated fluorescent reporter reliably visualizes Tas2r131-cells in gustatory tissue. We show that the onset of Tas2r131 as well as of α-Gustducin expression is initiated at different developmental stages depending on the type of taste bud. Furthermore, the number of Tas2r131- and α-Gustducin-expressing cells increased during postnatal development. Our results demonstrate that the Tas2r131-expressing cells constitute a subpopulation of α-Gustducin positive cells at all stages. We detected Tas2r131-expressing cells in several nongustatory tissues including lung, trachea, ovary, ganglia, and brain. Thus, the Tas2r131-Cre strain will help to dissect the functional role of Tas2r131 cells in both gustatory and nongustatory tissues in the future.

Transsynaptic Tracing from Taste Receptor Cells Reveals Local Taste Receptor Gene Expression in Gustatory Ganglia and Brain

Taste perception begins in the oral cavity by interactions of taste stimuli with specific receptors. Specific subsets of taste receptor cells (TRCs) are activated upon tastant stimulation and transmit taste signals to afferent nerve fibers and ultimately to the brain. How specific TRCs impinge on the innervating nerves and how the activation of a subset of TRCs leads to the discrimination of tastants of different qualities and intensities is incompletely understood. To investigate the organization of taste circuits, we used gene targeting to express the transsynaptic tracer barley lectin (BL) in the gustatory system of mice. Because TRCs are not synaptically connected with the afferent nerve fibers, we first analyzed tracer production and transfer within the taste buds (TBs). Surprisingly, we found that BL is laterally transferred across all cell types in TBs of mice expressing the tracer under control of the endogenous Tas1r1 and Tas2r131 promotor, respectively. Furthermore, although we detected the BL tracer in both ganglia and brain, we also found local low-level Tas1r1 and Tas2r131 gene, and thus tracer expression in these tissues. Finally, we identified the Tas1r1 and Tas2r131-expressing cells in the peripheral and CNS using a binary genetic approach. Together, our data demonstrate that genetic transsynaptic tracing from bitter and umami receptor cells does not selectively label taste-specific neuronal circuits and reveal local taste receptor gene expression in the gustatory ganglia and the brain.

SIGNIFICANCE STATEMENT

Previous papers described the organization of taste pathways in mice expressing a transsynaptic tracer from transgenes in bitter or sweet/umami-sensing taste receptor cells. However, reported results differ dramatically regarding the numbers of synapses crossed and the reduction of signal intensity after each transfer step. Nevertheless, all groups claimed this approach appropriate for quality-specific visualization of taste pathways. In the present study, we demonstrate that genetic transsynaptic tracing originating from umami and bitter taste receptor cells does not selectively label taste quality-specific neuronal circuits due to lateral transfer of the tracer in the taste bud and taste receptor expression in sensory ganglia and brain. Moreover, we visualized for the first time taste receptor-expressing cells in the PNS and CNS.

The generation of knock-in mice expressing fluorescently tagged galanin receptors 1 and 2

The neuropeptide galanin has diverse roles in the central and peripheral nervous systems, by activating the G protein-coupled receptors Gal1, Gal2 and the less studied Gal3 (GalR1-3 gene products). There is a wealth of data on expression of Gal1-3 at the mRNA level, but not at the protein level due to the lack of specificity of currently available antibodies. Here we report the generation of knock-in mice expressing Gal1 or Gal2 receptor fluorescently tagged at the C-terminus with, respectively, mCherry or hrGFP (humanized Renilla green fluorescent protein). In dorsal root ganglia (DRG) neurons expressing the highest levels of Gal1-mCherry, localization to the somatic cell membrane was detected by live-cell fluorescence and immunohistochemistry, and that fluorescence decreased upon addition of galanin. In spinal cord, abundant Gal1-mCherry immunoreactive processes were detected in the superficial layers of the dorsal horn, and highly expressing intrinsic neurons of the lamina III/IV border showed both somatic cell membrane localization and outward transport of receptor from the cell body, detected as puncta within cell processes. In brain, high levels of Gal1-mCherry immunofluorescence were detected within thalamus, hypothalamus and amygdala, with a high density of nerve endings in the external zone of the median eminence, and regions with lesser immunoreactivity included the dorsal raphe nucleus. Gal2-hrGFP mRNA was detected in DRG, but live-cell fluorescence was at the limits of detection, drawing attention to both the much lower mRNA expression than to Gal1 in mice and the previously unrecognized potential for translational control by upstream open reading frames (uORFs).

Using Animal Models to Determine the Role of Gustatory Neural Input in the Control of Ingestive Behavior and the Maintenance of Body Weight

INTRODUCTION

Decades of research have suggested that nutritional intake contributes to the development of human disease, mainly by influencing the development of obesity and obesity-related conditions. A relatively large body of research indicates that functional variation in human taste perception can influence nutritional intake as well as body mass accumulation. However, there are a considerable number of studies that suggest that no link between these variables actually exists. These discrepancies in the literature likely result from the confounding influence of a variety of other, uncontrolled, factors that can influence ingestive behavior.

STRATEGY

In this review, the use of controlled animal experimentation to alleviate at least some of these issues related to the lack of control of experimental variables is discussed. Specific examples of the use of some of these techniques are examined.

DISCUSSION AND CONCLUSIONS

The review will close with some specific suggestions aimed at strengthening the link between gustatory neural input and its putative influence on ingestive behaviors and the maintenance of body weight.

Cholinergic chemosensory cells of the thymic medulla express the bitter receptor Tas2r131

The thymus is the site of T cell maturation which includes positive selection in the cortex and negative selection in the medulla. Acetylcholine is locally produced in the thymus and cholinergic signaling influences the T cell development. We recently described a distinct subset of medullary epithelial cells in the murine thymus which express the acetylcholine-synthesizing enzyme choline acetyltransferase (ChAT) and components of the canonical taste transduction cascade, i.e. transient receptor potential melastatin-like subtype 5 channel (TRPM5), phospholipase Cβ(2), and Gα-gustducin. Such a chemical phenotype is characteristic for chemosensory cells of mucosal surfaces which utilize bitter receptors for detection of potentially hazardous compounds and cholinergic signaling to initiate avoidance reflexes. We here demonstrate mRNA expression of bitter receptors Tas2r105, Tas2r108, and Tas2r131 in the murine thymus. Using a Tas2r131-tauGFP reporter mouse we localized the expression of this receptor to cholinergic cells expressing the downstream elements of the taste transduction pathway. These cells are distinct from the medullary thymic epithelial cells which promiscuously express tissue-restricted self-antigens during the process of negative selection, since double-labeling immunofluorescence showed no colocalization of autoimmune regulator (AIRE), the key mediator of negative selection, and TRPM5. These data demonstrate the presence of bitter taste-sensing signaling in cholinergic epithelial cells in the thymic medulla and opens a discussion as to what is the physiological role of this pathway.

Bitter taste receptor mTas2r105 is expressed in small intestinal villus and crypts

The small intestine is the most important digestion and absorption organ in the body. Taste receptors and taste signal transduction cascades were detected in a variety of non-lingual tissues including testis, kidney, nasal cavity, lung, heart and gastrointestinal (GI) tract. Though the expression of bitter taste receptors and taste signal transduction cascades has been reported in the gut for a decade, the evidence revealing the expression of Tas2rs in the gut remain unbelievable. Here, the amplification of 35 bitter taste receptors from small intestine cDNA revealed that all transcripts are present in duodenum, jejunum and ileum, except Tas2r117. In addition, Tas2Rs and taste-related signaling transduction cascades are also observed in mouse small intestine including duodenum, jejunum and ileum by RT-PCR and Western Blot. On the other hand, three types of transgenic system were used to investigate the expression of the bitter taste receptor Tas2r105 in mouse intestine (Tas2r105-GFP/Cre, Tas2r105-GFP/Cre-DTA and Tas2r105-GFP/Cre-LacZ). With the bitter taste receptor mTas2r105 transgenic mice, the expression of mTas2r105 is showed in the villus and crypts of small intestine. mTas2r105 positive cells are also observed at the connective tissue of villus. DTA expression in mTas2r105 + cells completely ablate the expression of mTas2r105 in intestinal epithelia, but did not ablate mTas1r3 expression in intestine epithelia. LacZ staining further reveals that bitter taste receptor mTas2r105 is expressed in crypt base cells.

A Binary Genetic Approach to Characterize TRPM5 Cells in Mice

Transient receptor potential channel subfamily M member 5 (TRPM5) is an important downstream signaling component in a subset of taste receptor cells making it a potential target for taste modulation. Interestingly, TRPM5 has been detected in extra-oral tissues; however, the function of extra-gustatory TRPM5-expressing cells is less well understood. To facilitate visualization and manipulation of TRPM5-expressing cells in mice, we generated a Cre knock-in TRPM5 allele by homologous recombination. We then used the novel TRPM5-IRES-Cre mouse strain to report TRPM5 expression by activating a τGFP transgene. To confirm faithful coexpression of τGFP and TRPM5 we generated and validated a new anti-TRPM5 antiserum enabling us to analyze acute TRPM5 protein expression. τGFP cells were found in taste bud cells of the vallate, foliate, and fungiform papillae as well as in the palate. We also detected TRPM5 expression in several other tissues such as in the septal organ of Masera. Interestingly, in the olfactory epithelium of adult mice acute TRPM5 expression was detected in only one (short microvillar cells) of two cell populations previously reported to express TRPM5. The TRPM5-IC mouse strain described here represents a novel genetic tool and will facilitate the study and tissue-specific manipulation of TRPM5-expressing cells in vivo.

Gustatory responses of the mouse chorda tympani nerve vary based on region of tongue stimulation

Different parts of the mouth vary in their taste responsiveness and gustatory transduction components. However, there have been few attempts to consider regional variation among areas innervated by a single nerve branch or containing only one type of gustatory papilla. Here, we examined whether taste-elicited responses of a single nerve, the chorda tympani (CT), depend on where taste solutions are delivered on the tongue in mice. In experiment 1, multiunit CT responses to NaCl and sucrose were larger if sapid taste solutions were applied to the tongue tip, which contains the anterior-most fungiform papillae, than if they were flowed over fungiform and foliate papillae on the posterior tongue. Further, the epithelial sodium channel (ENaC) blocker amiloride suppressed NaCl responses to a greater degree for the tongue tip. In experiment 2, CT nerve responses were compared between the tongue tip and a region further back that contained only fungiform papillae. NaCl and sucrose solutions applied to posterior fungiform papillae produced smaller responses than did those elicited by the same taste stimuli applied to anterior fungiform papillae on the tongue tip. Amiloride suppressed the response to NaCl delivered to the anterior fungiform but not posterior fungiform papillae. These results indicate that the CT response is tongue-region dependent in the mouse. Furthermore, the spatial location of a fungiform papilla provides important information about its properties, such as whether sodium taste transduction is mediated by amiloride-sensitive ENaCs.

Fluorescent knock-in mice to decipher the physiopathological role of G protein-coupled receptors

G protein-coupled receptors (GPCRs) modulate most physiological functions but are also critically involved in numerous pathological states. Approximately a third of marketed drugs target GPCRs, which places this family of receptors in the main arena of pharmacological pre-clinical and clinical research. The complexity of GPCR function demands comprehensive appraisal in native environment to collect in-depth knowledge of receptor physiopathological roles and assess the potential of therapeutic molecules. Identifying neurons expressing endogenous GPCRs is therefore essential to locate them within functional circuits whereas GPCR visualization with subcellular resolution is required to get insight into agonist-induced trafficking. Both remain frequently poorly investigated because direct visualization of endogenous receptors is often hampered by the lack of appropriate tools. Also, monitoring intracellular trafficking requires real-time visualization to gather in-depth knowledge. In this context, knock-in mice expressing a fluorescent protein or a fluorescent version of a GPCR under the control of the endogenous promoter not only help to decipher neuroanatomical circuits but also enable real-time monitoring with subcellular resolution thus providing invaluable information on their trafficking in response to a physiological or a pharmacological challenge. This review will present the animal models and discuss their contribution to the understanding of the physiopathological role of GPCRs. We will also address the drawbacks associated with this methodological approach and browse future directions.