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

Therapeutics of bitter plants from Northeast region of India and their pharmacological and phytochemical perspectives

Natural resources have been used for food and medicine since the beginning of human civilization, and they have always been a low-cost, easily accessible source for individuals. Northeast region of India (NER) represents a significant portion of India's flora and fauna. Marginality, fragility, inaccessibility, ethnicity, and cultural diversity thrived in the region, resulting in the richest reservoir of genetic variation of bioresources. Several bitter plants are used by the locals as both food and medicine to treat a variety of diseases. These medicinal plants are an excellent source of chemically diverse biologically active phytometabolites. There have been few efforts to raise awareness about health benefits of bitter plant resources abound in this region that may provides opportunities for their sustainable utilization. Understanding the structural features of plant derived bitterants in relationship with specific bitter receptor will provide research prospects to identify biomolecules with health benefits. In this context the present review is intended to deliver phyto-pharmacological aspects of bitter plant resources of NER together with detailed understanding of possible association between plant derived phytometabolites as bitter agonists with extraoral bitter receptors.

Bitter taste receptors as sensors of gut luminal contents

Taste is important in the selection of food and is orchestrated by a group of distinct receptors, the taste G protein-coupled receptors (GPCRs). Taste 1 receptors (Tas1rs in mice and TAS1Rs in humans; also known as T1Rs) detect sweet and umami tastes, and taste 2 receptors (Tas2rs in mice and TAS2Rs in humans; also known as T2Rs) detect bitterness. These receptors are also expressed in extraoral sites, including the gastrointestinal mucosa. Tas2rs/TAS2Rs have gained interest as potential targets to prevent or treat metabolic disorders. These bitter taste receptors are expressed in functionally distinct types of gastrointestinal mucosal cells, including enteroendocrine cells, which, upon stimulation, increase intracellular Ca2+ and release signalling molecules that regulate gut chemosensory processes critical for digestion and absorption of nutrients, for neutralization and expulsion of harmful substances, and for metabolic regulation. Expression of Tas2rs/TAS2Rs in gut mucosa is upregulated by high-fat diets, and intraluminal bitter 'tastants' affect gastrointestinal functions and ingestive behaviour through local and gut-brain axis signalling. Tas2rs/TAS2Rs are also found in Paneth and goblet cells, which release antimicrobial peptides and glycoproteins, and in tuft cells, which trigger type 2 immune response against parasites, thus providing a direct line of defence against pathogens. This Review will focus on gut Tas2r/TAS2R distribution, signalling and regulation in enteroendocrine cells, supporting their role as chemosensors of luminal content that serve distinct functions as regulators of body homeostasis and immune response.

Long term Coptidis Rhizoma intake induce gastrointestinal emptying inhibition and colon barrier weaken via bitter taste receptors activation in mice

BACKGROUND

Coptidis Rhizoma, a classic bitter traditional Chinese medicine, can lead to digestive dysfunction when long-term use according to traditional experience. Bitter taste receptors have been found to regulate gastrointestinal smooth muscle contraction. Coptidis Rhizoma alkaloids are potential agonists for bitter taste receptors, but whether they can induce gastrointestinal dysfunction via bitter taste receptors is not clear.

PURPOSE

The purpose of this study is to elucidate whether long-term Coptidis Rhizoma decoction/berberine intake can affect gastrointestinal function via bitter taste receptors.

METHODS

Firstly, mice were orally administered Coptidis Rhizoma decoction (or berberine) for 8 weeks, then their appetite, gastrointestinal emptying function, colon barrier function, and gut microbiota homeostasis were evaluated. Subsequently, isolated intestine, molecular docking, calcium release, and immunofluorescence co-localization experiments were applied to explore the mechanism of Coptidis Rhizoma decoction (or berberine) inhibition effects on gastrointestinal motility. Finally, transmembrane resistance, scratch assay, tight junction and cytoskeletal protein immunofluorescence staining were conducted to verify that the bitter taste receptor is the target for Coptidis Rhizoma decoction (or berberine) to damage the colon barrier function.

RESULT

Long-term Coptidis Rhizoma decoction (or berberine) intake can reduce appetite, inhibit gastrointestinal contractions, disrupt bacterial balance and colon barrier function in mice. Further mechanistic studies have shown that the alkaloids of Coptidis Rhizoma are agonists for bitter taste receptors, which can promote α-gustducin binding to CHRM3 by activating bitter taste receptors, finally inhibiting gastrointestinal smooth muscle contraction. In addition, Coptidis Rhizoma decoction (or berberine) can activate bitter taste receptors and its downstream pathways PKCβ/RhoA/ROCK1/MLC-2, reshape skeletal proteins, downregulate tight junction protein expression, and ultimately disrupt colon barrier function.

CONCLUSIONS

Long term Coptidis Rhizoma intake induce gastrointestinal emptying inhibition and colon barrier weaken via bitter taste receptor activation in mice.

International Union of Basic and Clinical Pharmacology. CXVII: Taste 2 receptors-Structures, functions, activators, and blockers

For most vertebrates, bitter perception plays a critical role in the detection of potentially harmful substances in food items. The detection of bitter compounds is facilitated by specialized receptors located in the taste buds of the oral cavity. This work focuses on these receptors, including their sensitivities, structure-function relationships, agonists, and antagonists. The existence of numerous bitter taste receptor variants in the human population and the fact that several of them profoundly affect individual perceptions of bitter tastes are discussed as well. Moreover, the identification of bitter taste receptors in numerous tissues outside the oral cavity and their multiple proposed roles in these tissues are described briefly. Although this work is mainly focused on human bitter taste receptors, it is imperative to compare human bitter taste with bitter taste of other animals to understand which forces might have shaped the evolution of bitter taste receptors and their functions and to distinguish apparently typical human features from rather general ones. For readers who are not very familiar with the gustatory system, short descriptions of taste anatomy, signal transduction, and oral bitter taste receptor expression are included in the beginning of this article. SIGNIFICANCE STATEMENT: Apart from their role as sensors for potentially harmful substances in the oral cavity, the numerous additional roles of bitter taste receptors in tissues outside the gustatory system have recently received much attention. For careful assessment of their functions inside and outside the taste system, a solid knowledge of the specific and general pharmacological features of these receptors and the growing toolbox available for studying them is imperative and provided in this work.

The Remarkable Diversity of Vertebrate Bitter Taste Receptors: Recent Advances in Genomic and Functional Studies

Bitter taste perception is crucial for animal survival. By detecting potentially harmful substances, such as plant secondary metabolites, as bitter, animals can avoid ingesting toxic compounds. In vertebrates, this function is mediated by taste receptors type 2 (T2Rs), a family of G protein-coupled receptors (GPCRs) expressed on taste buds. Given their vital roles, T2Rs have undergone significant selective pressures throughout vertebrate evolution, leading to frequent gene duplications and deletions, functional changes, and intrapopulation differentiation across various lineages. Recent advancements in genomic and functional research have uncovered the repertoires and functions of bitter taste receptors in a wide range of vertebrate species, shedding light on their evolution in relation to dietary habits and other ecological factors. This review summarizes recent research on bitter taste receptors and explores the mechanisms driving the diversity of these receptors from the perspective of vertebrate ecology and evolution.

Identification of Sweetness Preference-Related Single-Nucleotide Polymorphisms for Polygenic Risk Scores Associated with Obesity

Our study aimed to identify sweetness preference-associated single-nucleotide polymorphisms (SNPs), characterize the related genetic loci, and develop SNP-based polygenic risk scores (PRS) to analyze their associations with obesity. For genotyping, we utilized a pooled genome-wide association study (GWAS) dataset of 18,499 females and 10,878 males. We conducted genome-wide association analyses, functional annotation, and employed the weighted method to calculate the levels of PRS from 677 sweetness preference-related SNPs. We used Cox proportional hazards modeling with time-varying covariates to estimate age-adjusted and multivariable hazard ratios (HRs) and 95% confidence intervals (CIs) for obesity incidence. We also tested the correlation between PRS and environmental factors, including smoking and dietary components, on obesity. Our results showed that in males, the TT genotype of rs4861982 significantly increased obesity risk compared to the GG genotype in the Health Professionals Follow-up Study (HPFS) cohort (HR = 1.565; 95% CI, 1.122-2.184; p = 0.008) and in the pooled analysis (HR = 1.259; 95% CI, 1.030-1.540; p = 0.025). Protein tyrosine phosphatase receptor type O (PTPRO) was identified as strongly associated with sweetness preference, indicating a positive correlation between sweetness preference and obesity risk. Moreover, each 10 pack-year increment in smoking was significantly associated with an increased risk of obesity in the HPFS cohort (HR = 1.024; 95% CI, 1.000-1.048) in males but not in females. In conclusion, significant associations between rs4861982, sweetness preference, and obesity were identified, particularly among males, where environmental factors like smoking are also correlated with obesity risk.

The Growing Complexity of Human Bitter Taste Perception

Human bitter perception is important for the identification of potentially harmful substances in food. For quite some years, research focused on the identification of activators for ∼25 human bitter taste receptors. The discovery of antagonists as well as increasing knowledge about agonists of different efficacies has substantially added to the intricacy of bitter taste perception. This article seeks to raise awareness for an underestimated new level of complexity when compound mixtures or even whole food items are assessed for their bitter taste.

The Acute Effect of Hydroxychloroquine Sulfate on Hunger, the Plasma Concentration of Orexigenic Peptides and Hedonic Food Intake: A Pilot Study

The direct infusion of bitter solutions in the gastrointestinal tract can reduce the secretion of orexigenic hormones and influence appetite and food intake. We aimed to explore whether oral ingestion of the bitter tastant hydroxychloroquine sulfate can exert similar effects. Ten lean adult women were included in this double-blind, randomized, two-visit, crossover study. After an overnight fast, each volunteer received film-coated tablets containing 400 mg of hydroxychloroquine sulfate (Plaquenil®) or placebo. Plasma-ghrelin, -motilin, -insulin and blood-glucose concentrations were determined every 10 min before and 30 min after feeding; appetite was scored every 10 min. Hunger scores were investigated with a special interest 50-60 min after the ingestion of hydroxychloroquine sulfate, right before a rewarding chocolate milkshake was offered to drink ad libitum. Compared with the placebo, hydroxychloroquine sulfate tended to reduce hunger at the time of interest (p = 0.10). No effect was found upon subsequent milkshake intake. Motilin plasma concentrations were unaltered, but acyl-ghrelin plasma concentrations decreased after the ingestion of hydroxychloroquine sulfate (t = 40-50; p < 0.05). These data suggest that the oral intake of hydroxychloroquine sulfate tablets reduces subjective hunger via a ghrelin-dependent mechanism but does not affect motilin release, hedonic food intake or insulin levels in healthy women.

Microbiota-Dependent Upregulation of Bitter Taste Receptor Subtypes in the Mouse Large Intestine in High-Fat Diet-Induced Obesity

Bitter taste receptors (Tas2rs in mice) detect bitterness, a warning signal for toxins and poisons, and are expressed in enteroendocrine cells. We tested the hypothesis that Tas2r138 and Tas2r116 mRNAs are modulated by microbiota alterations induced by a long-term high-fat diet (HFD) and antibiotics (ABX) (ampicillin and neomycin) administered in drinking water. Cecum and colon specimens and luminal contents were collected from C57BL/6 female and male mice for qRT-PCR and microbial luminal 16S sequencing. HFD with/without ABX significantly increased body weight and fat mass at 4, 6, and 8 weeks. Tas2r138 and Tas2r116 mRNAs were significantly increased in mice fed HFD for 8 weeks vs. normal diet, and this increase was prevented by ABX. There was a distinct microbiota separation in each experimental group and significant changes in the composition and diversity of microbiome in mice fed a HFD with/without ABX. Tas2r mRNA expression in HFD was associated with several genera, particularly with Akkermansia, a Gram-negative mucus-resident bacterium. These studies indicate that luminal bacterial composition is affected by sex, diet, and ABX and support a microbial dependent upregulation of Tas2rs in HFD-induced obesity, suggesting an adaptive host response to specific diet-induced dysbiosis.

Bitter taste receptors along the gastrointestinal tract: comparison between humans and rodents

For decades bitter taste receptors (TAS2R) were thought to be located only in the mouth and to serve as sensors for nutrients and harmful substances. However, in recent years Tas2r have also been reported in extraoral tissues such as the skin, the lungs, and the intestine, where their function is still uncertain. To better understand the physiological role of these receptors, in this paper we focused on the intestine, an organ in which their activation may be similar to the receptors found in the mouth. We compare the relative presence of these receptors along the gastrointestinal tract in three main species of biomedical research (mice, rats and humans) using sequence homology. Current data from studies of rodents are scarce and while more data are available in humans, they are still deficient. Our results indicate, unexpectedly, that the reported expression profiles do not always coincide between species even if the receptors are orthologs. This may be due not only to evolutionary divergence of the species but also to their adaptation to different dietary patterns. Further studies are needed in order to develop an integrated vision of these receptors and their physiological functionality along the gastrointestinal tract.

Physiological activation of human and mouse bitter taste receptors by bile acids

Beside the oral cavity, bitter taste receptors are expressed in several non-gustatory tissues. Whether extra-oral bitter taste receptors function as sensors for endogenous agonists is unknown. To address this question, we devised functional experiments combined with molecular modeling approaches to investigate human and mouse receptors using a variety of bile acids as candidate agonists. We show that five human and six mouse receptors are responsive to an array of bile acids. Moreover, their activation threshold concentrations match published data of bile acid concentrations in human body fluids, suggesting a putative physiological activation of non-gustatory bitter receptors. We conclude that these receptors could serve as sensors for endogenous bile acid levels. These results also indicate that bitter receptor evolution may not be driven solely by foodstuff or xenobiotic stimuli, but also depend on endogenous ligands. The determined bitter receptor activation profiles of bile acids now enable detailed physiological model studies.

The gut odorant receptor and taste receptor make sense of dietary components: A focus on gut hormone secretion

Odorant receptors (ORs) and taste receptors (TRs) are expressed primarily in the nose and tongue in which they transduce electrical signals to the brain. Advances in deciphering the dietary component-sensing mechanisms in the nose and tongue prompted research on the role of gut chemosensory cells. Acting as the pivotal interface between the body and dietary cues, gut cells "smell" and "taste" dietary components and metabolites by taking advantage of chemoreceptors-ORs and TRs, to maintain physiological homeostasis. Here, we reviewed this novel field, highlighting the latest discoveries pertinent to gut ORs and TRs responding to dietary components, their impacts on gut hormone secretion, and the mechanisms involved. Recent studies indicate that gut cells sense dietary components including fatty acid, carbohydrate, and phytochemical by activating relevant ORs, thereby modulating GLP-1, PYY, CCK, and 5-HT secretion. Similarly, gut sweet, umami, and bitter receptors can regulate the gut hormone secretion and maintain homeostasis in response to dietary components. A deeper understanding of the favorable influence of dietary components on gut hormone secretion via gut ORs and TRs, coupled with the facts that gut hormones are involved in diverse physiological or pathophysiological phenomena, may ultimately lead to a promising treatment for various human diseases.

Overlapping activation pattern of bitter taste receptors affect sensory adaptation and food perception

The composition of menus and the sequence of foodstuffs consumed during a meal underlies elaborate rules. However, the molecular foundations for the observed taste- and pleasure-raising effects of complex menus are obscure. The molecular identification and characterization of taste receptors can help to gain insight into the complex interrelationships of food items and beverages during meals. In our study, we quantified important bitter compounds in chicory and chicory-based surrogate coffee and used them to identify responsive bitter taste receptors. The two receptors, TAS2R43 and TAS2R46, are exquisitely sensitive to lactucin, lactucopicrin, and 11β,13-dihydrolactucin. Sensory testing demonstrated a profound influence of the sequence of consumption of chicory, surrogate coffee, and roasted coffee on the perceived bitterness by human volunteers. These findings pave the way for a molecular understanding of some of the mixture effects underlying empirical meal compositions.

Bitter taste sensitivity in domestic dogs (Canis familiaris) and its relevance to bitter deterrents of ingestion

As the most favoured animal companion of humans, dogs occupy a unique place in society. Understanding the senses of the dog can bring benefits to both the dogs themselves and their owners. In the case of bitter taste, research may provide useful information on sensitivity to, and acceptance of, diets containing bitter tasting materials. It may also help to protect dogs from the accidental ingestion of toxic substances, as in some instances bitter tasting additives are used as deterrents to ingestion. In this study we examined the receptive range of dog bitter taste receptors (Tas2rs). We found that orthologous dog and human receptors do not always share the same receptive ranges using in vitro assays. One bitter chemical often used as a deterrent, denatonium benzoate, is only moderately active against dTas2r4, and is almost completely inactive against other dog Tas2rs, including dTas2r10, a highly sensitive receptor in humans. We substituted amino acids to create chimeric dog-human versions of the Tas2r10 receptor and found the ECL2 region partly determined denatonium sensitivity. We further confirmed the reduced sensitivity of dogs to this compound in vivo. A concentration of 100μM (44.7ppm) denatonium benzoate was effective as a deterrent to dog ingestion in a two-bottle choice test indicating higher concentrations may increase efficacy for dogs. These data can inform the choice and concentration of bitter deterrents added to toxic substances to help reduce the occurrence of accidental dog poisonings.

Extra-Oral Taste Receptors-Function, Disease, and Perspectives

Taste perception is crucial for the critical evaluation of food constituents in human and other vertebrates. The five basic taste qualities salty, sour, sweet, umami (in humans mainly the taste of L-glutamic acid) and bitter provide important information on the energy content, the concentration of electrolytes and the presence of potentially harmful components in food items. Detection of the various taste stimuli is facilitated by specialized receptor proteins that are expressed in taste buds distributed on the tongue and the oral cavity. Whereas, salty and sour receptors represent ion channels, the receptors for sweet, umami and bitter belong to the G protein-coupled receptor superfamily. In particular, the G protein-coupled taste receptors have been located in a growing number of tissues outside the oral cavity, where they mediate important processes. This article will provide a brief introduction into the human taste perception, the corresponding receptive molecules and their signal transduction. Then, we will focus on taste receptors in the gastrointestinal tract, which participate in a variety of processes including the regulation of metabolic functions, hunger/satiety regulation as well as in digestion and pathogen defense reactions. These important non-gustatory functions suggest that complex selective forces have contributed to shape taste receptors during evolution.

Tuft Cells and Their Role in Intestinal Diseases

The interests in intestinal epithelial tuft cells, their basic physiology, involvement in immune responses and relevance for gut diseases, have increased dramatically over the last fifteen years. A key discovery in 2016 of their close connection to helminthic and protozoan infection has further spurred the exploration of these rare chemosensory epithelial cells. Although very sparse in number, tuft cells are now known as important sentinels in the gastrointestinal tract as they monitor intestinal content using succinate as well as sweet and bitter taste receptors. Upon stimulation, tuft cells secrete a broad palette of effector molecules, including interleukin-25, prostaglandin E2 and D2, cysteinyl leukotriene C4, acetylcholine, thymic stromal lymphopoietin, and β-endorphins, some of which with immunomodulatory functions. Tuft cells have proven indispensable in anti-helminthic and anti-protozoan immunity. Most studies on tuft cells are based on murine experiments using double cortin-like kinase 1 (DCLK1) as a marker, while human intestinal tuft cells can be identified by their expression of the cyclooxygenase-1 enzyme. So far, only few studies have examined tuft cells in humans and their relation to gut disease. Here, we present an updated view on intestinal epithelial tuft cells, their physiology, immunological hub function, and their involvement in human disease. We close with a discussion on how tuft cells may have potential therapeutic value in a clinical context.

Functional and genomic comparative study of the bitter taste receptor family TAS2R: Insight into the role of human TAS2R5

Bitterness is perceived in humans by 25 subtypes of bitter taste receptors (hTAS2R) that range from broadly tuned to more narrowly tuned receptors. hTAS2R5 is one of the most narrowly tuned bitter taste receptors in humans. In this study, we review the literature on this receptor and show there is no consensus about its role. We then compare the possible role of hTAS2R5 with that of the proteins of the TAS2R family in rat, mouse, and pig. A phylogenetic tree of all mammalian TAS2R domain-containing proteins showed that human hTAS2R5 has no ortholog in pig, mouse, or rat genomes. By comparing the agonists that are common to hTAS2R5 and other members of the family, we observed that hTAS2R39 is the receptor that shares most agonists with hTAS2R5. In mouse, some of these agonists activate mTas2r105 and mTas2r144, which are distant paralogs of hTAS2R5. mTas2r144 seems to be the receptor that is most similar to hTAS2R5 because they are both activated by the same agonists and have affinities in the same range of values. Then, we can conclude that hTAS2R5 has a unique functional specificity in humans as it is activated by selective agonists and that its closest functional homolog in mouse is the phylogenetically distant mTas2r144.

Sensation of dietary nutrients by gut taste receptors and its mechanisms

Nutrients sensing is crucial for fundamental metabolism and physiological functions, and it is also an essential component for maintaining body homeostasis. Traditionally, basic taste receptors exist in oral cavity to sense sour, sweet, bitter, umami, salty and et al. Recent studies indicate that gut can sense the composition of nutrients by activating relevant taste receptors, thereby exerting specific direct or indirect effects. Gut taste receptors, also named as intestinal nutrition receptors, including at least bitter, sweet and umami receptors, have been considered to be activated by certain nutrients and participate in important intestinal physiological activities such as eating behavior, intestinal motility, nutrient absorption and metabolism. Additionally, gut taste receptors can regulate appetite and body weight, as well as maintain homeostasis via targeting hormone secretion or regulating the gut microbiota. On the other hand, malfunction of gut taste receptors may lead to digestive disorders, and then result in obesity, type 2 diabetes and gastrointestinal diseases. At present, researchers have confirmed that the brain-gut axis may play indispensable roles in these diseases via the secretion of brain-gut peptides, but the mechanism is still not clear. In this review, we summarize the current observation of knowledge in gut taste systems in order to shed light on revealing their important nutritional functions and promoting clinical implications.

Iso-α-acids in Nonalcoholic and Alcoholic Beer Stimulate Growth of Neuron-like SH-SY5Y Cells and Neuroepithelial Stem Cells

With the increasing popularity of nonalcoholic beer, the association between beer drinking and alcohol intake is lost. In the present study, we show that nonalcoholic beer can stimulate the expansion of neuron-like cell lines and neuroepithelial stem cells in culture, yielding an effect comparable to that of alcoholic beer. One ingredient in beer is hops, which is derived from the flower of hop plants. The female flower contains humulones, which are transformed into iso-α-acids during wort boiling and give beer its bitter taste. In this study, we tested the effects of these iso-α-acids and/or alcohol on the proliferation of neuron-like cells and neuroepithelial stem cells in culture. Iso-α-acids enhanced cell expansion, showing a bimodal dose-response curve with peaks around 2-30 nM and 2-5 μM, of which nanomolar concentrations are relevant in beer drinking. The more lipophilic trans-iso-α-acids, found to a greater extent in beer foam, are even more potent. Our results indicate that iso-α-acids, acting via peroxisome proliferator-activated receptors could be responsible for the observed effects. Altogether, our results indicate that nonalcoholic beer with ingredients such as iso-α-acids stimulate the proliferation of neuroepithelial stem cells.

Human intestinal bitter taste receptors regulate innate immune responses and metabolic regulators in obesity

Bitter taste receptors (taste 2 receptors, TAS2Rs) serve as warning sensors in the lingual system against the ingestion of potentially poisonous food. Here, we investigated the functional role of TAS2Rs in the human gut and focused on their potential to trigger an additional host defense pathway in the intestine. Human jejunal crypts, especially those from individuals with obesity, responded to bitter agonists by inducing the release of antimicrobial peptides (α-defensin 5 and regenerating islet–derived protein 3 α [REG3A]) but also regulated the expression of other innate immune factors (mucins, chemokines) that affected E. coli growth. We found that the effect of aloin on E. coli growth and on the release of the mucus glycoprotein CLCA1, identified via proteomics, was affected by TAS2R43 deletion polymorphisms and thus confirmed a role for TAS2R43. RNA-Seq revealed that denatonium benzoate induced an NRF2-mediated nutrient stress response and an unfolded protein response that increased the expression of the mitokine GDF15 but also ADM2 and LDLR, genes that are involved in anorectic signaling and lipid homeostasis. In conclusion, TAS2Rs in the intestine constitute a promising target for treating diseases that involve disturbances in the innate immune system and body weight control. TAS2R polymorphisms may be valuable genetic markers to predict therapeutic responses.

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.

Bitter Taste Receptors (T2Rs) are Sentinels that Coordinate Metabolic and Immunological Defense Responses

In addition to being responsible for bitter taste, type 2 taste receptors (T2Rs) regulate endocrine, behavioral, and immunological responses. T2R agonists include indicators of incoming threats to metabolic homeostasis, pathogens, and irritants. This review will provide an overview of T2R-regulated processes throughout the body that function defensively. We propose a broader definition of T2Rs as chemosensory sentinels that monitor toxic, metabolic, and infectious threats and initiate defensive responses.

Expression of TAS2R14 in the intestinal endocrine cells of non-human primates

BACKGROUND

Recent studies have demonstrated that genes related to bitter taste receptors (TAS2Rs) on various chromosomes are expressed in extra-oral organs of various animals. The bitter taste receptor TAS2R14 is conserved among primate species and shows broad ligand sensitivity. Mice have a number of orthologues to primate TAS2R14 located in tandem on chromosome 16; however, their expression patterns are not unique.

OBJECTIVE

We characterized the expression of TAS2R14 in various cell types in the intestines of the rhesus macaque and evaluated its role in hormone production in the gut.

METHODS

TAS2R14 expression was examined in the intestines of rhesus macaques, a common non-human primate model, by RT-qPCR and immunohistochemical staining.

RESULTS

Mean expression levels of TAS2R14 in the duodenum, ileum, and colon were similar to each other and were lower than those in circumvallate papillae. An immunohistochemical analysis revealed TAS2R14 immunoreactivity in enteroendocrine cells positive for cholecystokinin, serotonin, and the G protein GNAT3.

CONCLUSION

These results suggest that primate TAS2R14 is broadly expressed in the intestine, mainly in enteroendocrine cells, and promotes gut hormone secretion in response to bitter stimuli.

NIH Workshop Report: sensory nutrition and disease

In November 2019, the NIH held the "Sensory Nutrition and Disease" workshop to challenge multidisciplinary researchers working at the interface of sensory science, food science, psychology, neuroscience, nutrition, and health sciences to explore how chemosensation influences dietary choice and health. This report summarizes deliberations of the workshop, as well as follow-up discussion in the wake of the current pandemic. Three topics were addressed: A) the need to optimize human chemosensory testing and assessment, B) the plasticity of chemosensory systems, and C) the interplay of chemosensory signals, cognitive signals, dietary intake, and metabolism. Several ways to advance sensory nutrition research emerged from the workshop: 1) refining methods to measure chemosensation in large cohort studies and validating measures that reflect perception of complex chemosensations relevant to dietary choice; 2) characterizing interindividual differences in chemosensory function and how they affect ingestive behaviors, health, and disease risk; 3) defining circuit-level organization and function that link and interact with gustatory, olfactory, homeostatic, visceral, and cognitive systems; and 4) discovering new ligands for chemosensory receptors (e.g., those produced by the microbiome) and cataloging cell types expressing these receptors. Several of these priorities were made more urgent by the current pandemic because infection with sudden acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the ensuing coronavirus disease of 2019 has direct short- and perhaps long-term effects on flavor perception. There is increasing evidence of functional interactions between the chemosensory and nutritional sciences. Better characterization of this interface is expected to yield insights to promote health, mitigate disease risk, and guide nutrition policy.

Effects of Flavanols on Enteroendocrine Secretion

Some beneficial effects of grape seed proanthocyanidin extract (GSPE) can be explained by the modulation of enterohormone secretion. As GSPE comprises a combination of different molecules, the pure compounds that cause these effects need to be elucidated. The enterohormones and chemoreceptors present in the gastrointestinal tract differ between species, so if humans are to gain beneficial effects, species closer to humans-and humans themselves-must be used. We demonstrate that 100 mg/L of GSPE stimulates peptide YY (PYY) release, but not glucagon-like peptide 1 (GLP-1) release in the human colon. We used a pig ex vivo system that differentiates between apical and basolateral intestinal sides to analyse how apical stimulation with GSPE and its pure compounds affects the gastrointestinal tract. In pigs, apical GSPE treatment stimulates the basolateral release of PYY in the duodenum and colon and that of GLP-1 in the ascending, but not the descending colon. In the duodenum, luminal stimulation with procyanidin dimer B2 increased PYY secretion, but not CCK secretion, while catechin monomers (catechin/epicatechin) significantly increased CCK release, but not PYY release. The differential effects of GSPE and its pure compounds on enterohormone release at the same intestinal segment suggest that they act through chemosensors located apically and unevenly distributed along the gastrointestinal tract.

Gastrointestinal taste receptors: could tastants become drugs?

PURPOSE OF REVIEW

Numerous studies have pointed to profound nongustatory roles of tastants and the corresponding taste receptors expressed in the alimentary canal in the modulation of digestive and metabolic functions. Already in early reports, the intriguing possibility to use tastants as drug-like effectors for the treatment of metabolic diseases was raised. With this review, focusing on the most recent literature, we intend to question how close we meanwhile came to the initial promise - the use of tastants as medicines.

RECENT FINDINGS

Although the enormous complexity and experimental variability of studies investigating the effects of tastants on physiological functions still has not revealed a common fundament from which subsequent therapeutic measures could be designed, more and more evidence is mounting on an involvement of taste receptors and taste signaling molecules in the maintenance and fine regulation of gastrointestinal functions and immunity.

SUMMARY

Although the initial goal - using tastants to treat metabolic disorders - has, by far, not been reached, numerous promising findings suggest that dietary interventions could be devised to support conventional therapies in the future.

Distinct Cell Types With the Bitter Receptor Tas2r126 in Different Compartments of the Stomach

Cells expressing bitter taste receptors (T2Rs or Tas2rs) in extraoral tissues are considered to be chemosensory cells mediating protective responses to potentially harmful or even antiinflammatory or antimicrobial compounds. In a previous study the activity of the Tas2R143/Tas2R135/Tas2r126 cluster promoter in the stomach was monitored using a Cre-reporter mouse line. Reporter gene expression and Tas2r126 mRNA were found in brush cells located at the distal wall of the gastric groove. In this study, we explored whether brush cells and epithelial cells of the stomach in fact contain the Tas2r126 receptor protein. Using immunohistochemistry, we demonstrate the presence of Tas2r126 immunoreactivity in different cell populations in the glandular stomach, in a subset of brush cells at the gastric groove and in unique glandular units as well as in certain enteroendocrine cells. In brush cells at the gastric groove, a strong immunofluorescence signal for the Tas2r126 receptor was observed at the most apical region of the cells, i.e., the microvillar tuft. In addition, we found a high density of Tas2r126-positive brush cells in the unique glandular units. These invaginations are located distally to the groove, open directly into the furrow and are enwrapped by smoothelin-immunoreactive muscles. In the corpus, Tas2r126 immunoreactivity was found in histamine-producing ECL cells and in ghrelin-producing X/A-like cells, the main enteroendcrine cells of this compartment. In the antrum, Tas2r126 labeling was observed in serotonin-storing EC cells and ghrelin cells, both representing only minor populations of enteroendocrine cells in this compartment. In conclusion, our data provide evidence for the presence of the Tas2r126 receptor protein in distinct cell types in the epithelium lining the mouse stomach which render the stomach responsive to agonists for bitter receptors.

Extra-oral bitter taste receptors: New targets against obesity?

Taste perception on the tongue is essential to help us to identify nutritious or potential toxic food substances. Emerging evidence has demonstrated the expression and function of bitter taste receptors (TAS2Rs) in a wide range of extra-oral tissues. In particular, TAS2Rs in gastrointestinal enteroendocrine cells control the secretion of appetite regulating gut hormones and influence hunger and food intake. Furthermore, these effects may be reinforced by the presence of TAS2Rs on intestinal smooth muscle cells, adipocytes and the brain. This review summarises how activation of extra-oral TAS2Rs can influence appetite and body weight control and how obesity impacts the expression and function of TAS2Rs. Region-selective targeting of bitter taste receptors may be promising targets for the treatment of obesity.

Expression of Bitter Taste Receptors in the Intestinal Cells of Non-Human Primates

(1) Background: Recent studies have investigated the expression of taste-related genes in the organs of various animals, including humans; however, data for additional taxa are needed to facilitate comparative analyses within and among species. (2) Methods: We investigated the expression of taste-related genes in the intestines of rhesus macaques, the non-human primates most commonly used in experimental models. (3) Results: Based on RNAseq and qRT-PCR, genes encoding bitter taste receptors and the G-protein gustducin were expressed in the gut of rhesus macaques. RNAscope analysis showed that one of the bitter receptors, TAS2R38, was expressed in some cells in the small intestine, and immunohistochemical analysis revealed the presence of T2R38-positive cells in the villi of the intestines. (4) Conclusions: These results suggest that bitter receptors are expressed in the gut of rhesus macaques, supporting the use of macaques as a model for studies of human taste, including gut analyses.

Association between taste perception and adiposity in overweight or obese older subjects with metabolic syndrome and identification of novel taste-related genes

BACKGROUND

The relation between taste perception, diet, and adiposity remains controversial. Additionally, there is a lack of knowledge on the polymorphisms influencing taste given the scarcity of genome-wide association studies (GWASs) published.

OBJECTIVES

We studied the relation between perception of the basic tastes, i.e., sweet, salty, bitter, sour, and umami (separately and jointly in a "taste score"), and anthropometric measurements in older subjects with metabolic syndrome (MetS). GWASs were undertaken to identify genes associated with basic tastes and their score.

METHODS

Taste perception was cross-sectionally determined by challenging subjects (381 older individuals with MetS) with solutions (5 concentrations) of the basic tastes with the use of standard prototypical tastants (phenylthiocarbamide and 6-n-propylthiouracil, NaCl, sucrose, monopotassium glutamate, and citric acid, for bitter, salt, sweet, umami, and sour, respectively). Taste perception intensities were expressed on a scale. A total taste score was derived.

RESULTS

The total taste score was inversely associated with body weight, body mass index, and waist circumference (P < 0.05). Subjects having a total taste score higher than or equal to the median (11 points for concentration V) were less likely to be classified as obese than subjects below the median (OR: 0.36; 95% CI: 0.22, 0.59; P < 0.001). Associations were similar, albeit less strong, for some taste qualities. In the GWASs, the highest associations were for bitter taste (rs1726866-TAS2R38, with P = 7.74 × 10-18 for phenylthiocarbamide and P = 3.96 × 10-19 for 6-n-propylthiouracil). For other tastes, several single-nucleotide polymorphisms (SNPs) exceeded the P threshold of 1 × 10-5. However, the top-ranked SNPs independently explained a low percentage of taste variability, hence their use as single proxies for the association between taste perception and adiposity is limited.

CONCLUSIONS

We found a strong inverse association between greater taste perception and body weight, body mass index, and waist circumference in older subjects with MetS and identified some taste-related SNPs. It would be advantageous to identify additional genetic proxies for taste and to develop polygenic scores. Data used in this study were derived from the clinical trial PREDIMED PLUS at baseline, registered at http://www.isrctn.com as ISRCTN89898870.

A role for taste receptors in (neuro)endocrinology?

The sense of taste is positioned at the forefront when it comes to the interaction of our body with foodborne chemicals. However, the role of our taste system, and in particular its associated taste receptors, is not limited to driving food preferences leading to ingestion or rejection before other organs take over responsibility for nutrient digestion, absorption and metabolic regulation. Taste sensory elements do much more. On the one hand, extra-oral taste receptors from the brain to the gut continue to sense nutrients and noxious substances after ingestion and, on the other hand, the nutritional state feeds back on the taste system. This intricate regulatory network is orchestrated by endocrine factors that are secreted in response to taste receptor signalling and, in turn regulate the taste receptor cells themselves. The present review summarises current knowledge on the endocrine regulation of the taste perceptual system and the release of hunger/satiety regulating factors by gastrointestinal taste receptors. Furthermore, the regulation of blood glucose levels via the activation of pancreatic sweet taste receptors and subsequent insulin secretion, as well as the influence of bitter compounds on thyroid hormone release, is addressed. Finally, the central effects of tastants are discussed briefly.

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.

Intestinal bitter taste receptor activation alters hormone secretion and imparts metabolic benefits

OBJECTIVES

Extracts of the hops plant have been shown to reduce weight and insulin resistance in rodents and humans, but elucidation of the mechanisms responsible for these benefits has been hindered by the use of heterogeneous hops-derived mixtures. Because hop extracts are used as flavoring agents for their bitter properties, we hypothesized that bitter taste receptors (Tas2rs) could be mediating their beneficial effects in metabolic disease. Studies have shown that exposure of cultured enteroendocrine cells to bitter tastants can stimulate release of hormones, including glucagon-like peptide 1 (GLP-1). These findings have led to the suggestion that activation of Tas2rs may be of benefit in diabetes, but this tenet has not been tested. Here, we have assessed the ability of a pure derivative of a hops isohumulone with anti-diabetic properties, KDT501, to signal through Tas2rs. We have further used this compound as a tool to systematically assess the impact of bitter taste receptor activation in obesity-diabetes.

METHODS

KDT501 was tested in a panel of bitter taste receptor signaling assays. Diet-induced obese mice (DIO) were dosed orally with KDT501 and acute effects on glucose homeostasis determined. A wide range of metabolic parameters were evaluated in DIO mice chronically treated with KDT501 to establish the full impact of activating gut bitter taste signaling.

RESULTS

We show that KDT501 signals through Tas2r108, one of 35 mouse Tas2rs. In DIO mice, acute treatment stimulated GLP-1 secretion and enhanced glucose tolerance. Chronic treatment caused weight and fat mass loss, increased energy expenditure, enhanced glucose tolerance and insulin sensitivity, normalized plasma lipids, and induced broad suppression of inflammatory markers. Chronic KDT501 treatment altered enteroendocrine hormone levels and bile acid homeostasis and stimulated sustained GLP-1 release. Combined treatment with a dipeptidyl peptidase IV inhibitor amplified the incretin-based benefits of this pure isohumulone.

CONCLUSIONS

Activation of Tas2r108 in the gut results in a remodeling of enteroendocrine hormone release and bile acid metabolism that ameliorates multiple features of metabolic syndrome. Targeting extraoral bitter taste receptors may be useful in metabolic disease.

Role of Intestinal Bitter Sensing in Enteroendocrine Hormone Secretion and Metabolic Control

The gastrointestinal tract stores ingested nutrients in the stomach which are then delivered to the small intestine at a controlled rate to optimize their digestion and absorption. The interaction of nutrients with the small and large intestine generates feedback that slows gastric emptying, induces satiation, and reduces postprandial glycemic excursions. The mechanisms underlying these nutrient-gut interactions are complex; it has only recently been appreciated that the gut has the capacity to detect intraluminal contents in much the same way as the tongue, via activation of specific G-protein-coupled receptors, and that ensuing signaling mechanisms modulate the release of an array of gut hormones that influence gastrointestinal motility, appetite and glycemia. Interestingly, evidence from preclinical models supports a functional link between intestinal bitter taste receptor (BTRs) and gastrointestinal hormone secretion, and the outcomes of recent studies indicate that stimulation of intestinal BTRs may be used to modulate gastrointestinal function, to diminish energy intake and limit postprandial blood glucose excursions in humans. This review summarizes current evidence about the expression and function of intestinal BTRs in relation to enteroendocrine hormone release and discusses the clinical implications of this pathway for the management of obesity and type 2 diabetes.