Bitter taste receptors: Extraoral roles in pathophysiology

TAS2R5 and TAS2R38 are bitter taste receptors whose colonic expressions could play important roles in age-associated processes

Ageing disrupts how our bodies process nutrients, leading to deregulation of nutrient-sensing and increased inflammation. Dietary interventions can promote healthy ageing, which demonstrates the importance of both metabolism and the gastrointestinal tract for our health. Bitter taste receptors (TAS2R) present in the intestine are key members of metabolic regulation. TAS2R are involved in controlling enterohormonal secretion, detect phenolic compounds in our diet, and potentially have a great impact on the ageing process. Here, we aimed to analyze the potential role of intestinal TAS2R on the ageing process and establish potential impact of these receptors on the biomarkers. Healthy subjects were divided into two age cohorts: young (38.9±6) and aged (63.6±6). TAS2R expression was analyzed in the colon. Analyses of metabolomics and of phenolic markers were performed in plasma. Best discriminatory parameters were obtained using three machine-learning methods. Finally, Spearman's rank correlation was performed. The best separators of the age cohorts were docosahexaenoic acid and multiple lipoprotein fractions. Two TAS2R were also identified: TAS2R5 and TAS2R38. TAS2R5 correlated with multiple lipoprotein-derived fractions, inflammatory marker IL-6 and polyunsaturated fatty acids. TAS2R38 was much more selective, correlating with a few parameters, including membrane lipid sphingomyelin, ketone body acetone, and omega acids. Both TAS2R5 and TAS2R38 correlated with β-hydroxybutyrate. The parameters that correlated with TAS2R have known effects on the ageing process. This suggests that TAS2R5 and TAS2R38 are the bitter receptors most likely to play a role in the development and progress of ageing.

A deadly taste: linking bitter taste receptors and apoptosis

Humans can perceive five canonical tastes: salty, sour, umami, sweet, and bitter. These tastes are transmitted through the activation of ion channels and receptors. Bitter taste receptors (Taste Family 2 Receptors; T2Rs) are a sub-family of 25 G-protein coupled receptor (GPCR) isoforms that were first identified in type II taste bud cells. T2Rs are activated by a broad array of bitter agonists, which cause an increase in intracellular calcium (Ca2+) and a decrease in cyclic adenosine 3',5'-monophosphate (cAMP). Interestingly, T2Rs are expressed beyond the oral cavity, where they play diverse non-taste roles in cell physiology and disease. Here, we summarize the literature that explores the role of T2Rs in apoptosis. Activation of T2Rs with bitter agonists induces apoptosis in several cancers, the airway epithelia, smooth muscle, and more. In many of these tissues, T2R activation causes mitochondrial Ca2+ overload, a main driver of apoptosis. This response may be a result of T2R cellular localization, nuclear Ca2+ mobilization and/or a remnant of the established immunological roles of T2Rs in other cell types. T2R-induced apoptosis could be pharmacologically leveraged to treat diseases of altered cellular proliferation. Future work must explore additional extra-oral T2R-expressing tissues for apoptotic responses, develop methods for in-vivo studies, and discover high affinity bitter agonists for clinical application.

Optimized vector for functional expression of the human bitter taste receptor TAS2R14 in HEK293 cells

Bitter is one of the five basic taste qualities, along with salty, sour, sweet and umami, used by mammals to access the quality of their food and orient their eating behaviour. Bitter taste detection prevents the ingestion of food potentially contaminated by bitter-tasting toxins. Bitter taste perception is mediated by a family of G protein-coupled receptors (GPCRs) called TAS2Rs. Humans possess 25 TAS2Rs (human type II taste receptors), enabling the detection of thousands of chemically diverse bitter compounds. The identification of agonists/antagonists and molecular mechanisms that govern receptor-ligand interaction has been primarily achieved through functional expression of TAS2Rs in heterologous cells. However, TAS2R receptors, like many other GPCRs, suffer from marginal cell surface expression. In this study, we compared the functionality of 9 engineered chimeric receptors, focusing our experiments on TAS2R14, a broadly tuned receptor that recognizes over 151 identified compounds. Among the different tested signal peptides, rat somatostatin receptor subtype 3 results in higher potency of aristolochic acid-induced calcium signalling than other tested export tags, such as bovine rhodopsin, murine Igκ-chain or human mGluR5. The addition of a MAX sequence enhances both TAS2R14 potency and efficacy. We also confirm that the FLAG epitope, when located at the C-terminal, interferes less with the TAS2R14 functionality, enabling reliable evaluation of this receptor at the cell surface using immunohistochemistry. Finally, these observations are also confirmed for TAS2R14 and TAS1R2/TAS1R3 (the sweet taste receptor) stimulated by 12 bitter compounds and by sucralose and neotame, respectively.

Effects of chronic unpredictable mild stress-induced depression on bitter taste receptor expression in mice

OBJECTIVE

With the rapid increase in the pace of life, people are facing increasing pressures of all kinds, and depression has gradually become a serious psychological disorder in human society, strongly affecting normal social and physiological activities. Depression can disrupt an individual's taste perception and potentially result in taste disorders by affecting and altering taste receptors. This disruption can consequently impact their food preferences and overall eating experiences.

DESIGN

In this study, we used the chronic unpredictable mild stress (CUMS) method to establish a depression model in male C57BL/6 J mice and explored the changes in taste receptor expression in the lingual circumvallate papillae (CP) to elucidate the effects of depression on taste. After 6 weeks of CUMS, behavioral performance evaluations, such as forced swim, open field, and elevated plus maze tests, were conducted in depression model mice. A further two-bottle choice test was subsequently performed to determine the effect of depression on bitter taste, and the expression of bitter taste receptors in the lingual CP was detected via immunofluorescence staining.

RESULTS

In this study, we found for the first time that mice with CUMS-induced depression had decreased bitter taste sensitivity through a two-bottle choice test and demonstrated that the expression of T2r5, a receptor related to bitter taste perception, and the expression of secondary taste signaling proteins in the lingual CP were significantly decreased in mice exposed to CUMS, as determined via qRTPCR and immunofluorescence staining.

CONCLUSIONS

Our study highlights how CUMS influences the perception of bitterness in the peripheral taste system, potentially elucidating stress-induced changes in eating habits.

Activation of TAS2R4 signaling attenuates podocyte injury induced by high glucose

Bitter taste receptors (TAS2Rs) Tas2r108 gene possesses a high abundance in mouse kidney; however, the biological functions of Tas2r108 encoded receptor TAS2Rs member 4 (TAS2R4) are still unknown. In the present study, we found that mouse TAS2R4 (mTAS2R4) signaling was inactivated in chronic high glucose-stimulated mouse podocyte cell line MPC, evidenced by the decreased protein expressions of mTAS2R4 and phospholipase C β2 (PLCβ2), a key downstream molecule of mTAS2R4 signaling. Nonetheless, agonism of mTAS2R4 by quinine recovered mTAS2R4 and PLCβ2 levels, and increased podocyte cell viability as well as protein expressions of ZO-1 and nephrin, biomarkers of podocyte slit diaphragm, in high glucose-cultured MPC cells. However, blockage of mTAS2R4 signaling with mTAS2R4 blockers γ-aminobutyric acid and abscisic acid, a Gβγ inhibitor Gallein, or a PLCβ2 inhibitor U73122 all abolished the effects of quinine on NLRP3 inflammasome and p-NF-κB p65 as well as the functional podocyte proteins in MPC cells in a high glucose condition. Furthermore, knockdown of mTAS2R4 with lentivirus-carrying Tas2r108 shRNA also ablated the effect of quinine on the key molecules of the above inflammatory signalings and podocyte functions in high glucose-cultured MPC cells. In summary, we demonstrated that activation of TAS2R4 signaling alleviated the podocyte injury caused by chronic high glucose, and inhibition of NF-κB p65 and NLRP3 inflammasome mediated the protective effects of TAS2R4 activation on podocytes. Moreover, activation of TAS2R4 signaling could be an important strategy for prevention and treatment of diabetic kidney disease.

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.

Structure, function and drug discovery of GPCR signaling

G protein-coupled receptors (GPCRs) are versatile and vital proteins involved in a wide array of physiological processes and responses, such as sensory perception (e.g., vision, taste, and smell), immune response, hormone regulation, and neurotransmission. Their diverse and essential roles in the body make them a significant focus for pharmaceutical research and drug development. Currently, approximately 35% of marketed drugs directly target GPCRs, underscoring their prominence as therapeutic targets. Recent advances in structural biology have substantially deepened our understanding of GPCR activation mechanisms and interactions with G-protein and arrestin signaling pathways. This review offers an in-depth exploration of both traditional and recent methods in GPCR structure analysis. It presents structure-based insights into ligand recognition and receptor activation mechanisms and delves deeper into the mechanisms of canonical and noncanonical signaling pathways downstream of GPCRs. Furthermore, it highlights recent advancements in GPCR-related drug discovery and development. Particular emphasis is placed on GPCR selective drugs, allosteric and biased signaling, polyphamarcology, and antibody drugs. Our goal is to provide researchers with a thorough and updated understanding of GPCR structure determination, signaling pathway investigation, and drug development. This foundation aims to propel forward-thinking therapeutic approaches that target GPCRs, drawing upon the latest insights into GPCR ligand selectivity, activation, and biased signaling mechanisms.

A Bitter Taste Receptor as a Novel Molecular Target on Cancer-Associated Fibroblasts in Pancreatic Ductal Adenocarcinoma

Cancer-associated fibroblasts (CAFs) execute diverse and complex functions in cancer progression. While reprogramming the crosstalk between CAFs and cancer epithelial cells is a promising avenue to evade the adverse effects of stromal depletion, drugs are limited by their suboptimal pharmacokinetics and off-target effects. Thus, there is a need to elucidate CAF-selective cell surface markers that can improve drug delivery and efficacy. Here, functional proteomic pulldown with mass spectrometry was used to identify taste receptor type 2 member 9 (TAS2R9) as a CAF target. TAS2R9 target characterization included binding assays, immunofluorescence, flow cytometry, and database mining. Liposomes conjugated to a TAS2R9-specific peptide were generated, characterized, and compared to naked liposomes in a murine pancreatic xenograft model. Proof-of-concept drug delivery experiments demonstrate that TAS2R9-targeted liposomes bind with high specificity to TAS2R9 recombinant protein and exhibit stromal colocalization in a pancreatic cancer xenograft model. Furthermore, the delivery of a CXCR2 inhibitor by TAS2R9-targeted liposomes significantly reduced cancer cell proliferation and constrained tumor growth through the inhibition of the CXCL-CXCR2 axis. Taken together, TAS2R9 is a novel cell-surface CAF-selective target that can be leveraged to facilitate small-molecule drug delivery to CAFs, paving the way for new stromal therapies.

Functional Alteration and Differential Expression of the Bitter Taste Receptor T2R38 in Human Paranasal Sinus in Patients with Chronic Rhinosinusitis

The bitter taste receptors (T2Rs) expressed in human sinonasal mucosae are known to elicit innate immune responses involving the release of nitric oxide (NO). We investigated the expression and distribution of two T2Rs, T2R14 and T2R38, in patients with chronic rhinosinusitis (CRS) and correlated the results with fractional exhaled NO (FeNO) levels and genotype of the T2R38 gene (TAS2R38). Using the Japanese Epidemiological Survey of Refractory Eosinophilic Chronic Rhinosinusitis (JESREC) phenotypic criteria, we identified CRS patients as either eosinophilic (ECRS, n = 36) or non-eosinophilic (non-ECRS, n = 56) patients and compared these groups with 51 non-CRS subjects. Mucosal specimens from the ethmoid sinus, nasal polyps, and inferior turbinate were collected from all subjects, together with blood samples, for RT-PCR analysis, immunostaining, and single nucleotide polymorphism (SNP) typing. We observed significant downregulation of T2R38 mRNA levels in the ethmoid mucosa of non-ECRS patients and in the nasal polyps of ECRS patients. No significant differences in T2R14 or T2R38 mRNA levels were found among the inferior turbinate mucosae of the three groups. Positive T2R38 immunoreactivity was localized mainly in epithelial ciliated cells, whereas secretary goblet cells generally showed lack of staining. The patients in the non-ECRS group showed significantly lower oral and nasal FeNO levels compared with the control group. There was a trend towards higher CRS prevalence in the PAV/AVI and AVI/AVI genotype groups as compared to the PAV/PAV group. Our findings reveal complex but important roles of T2R38 function in ciliated cells associated with specific CRS phenotypes, suggesting the T2R38 pathway as a potential therapeutic target for promotion of endogenous defense mechanisms.

Association between Downstream Taste Signaling Genes, Oral Microbiome, and Severe Early Childhood Caries

Polymorphisms in taste receptor genes have been shown to play a role in early childhood caries (ECC), a multifactorial, biofilm-mediated disease. This study aimed to evaluate associations between severe-ECC (S-ECC), the oral microbiome, and variants in genes that encode components of the G protein-coupled receptor (GPCR) signaling cascade involved in taste sensation. A total of 176 children (88 caries-free; 88 with S-ECC) were recruited. Analyses of 16S and ITS1 rRNA microbial genes and seven (GNAQ, GNAS, GNAT3, GNAI2, RAC1, RALB, and PLCB2) human genes were pursued using next-generation sequencing. Regression analyses were performed to evaluate associations between genetic variants, S-ECC, and the supragingival plaque microbiome. Results suggest that PLCB2 rs2305645 (T), rs1869901 (G), and rs2305649 (G) alleles had a protective effect on S-ECC (rs2305645, odds ratio (OR) = 0.27 (95% confidence interval (CI): 0.14-0.51); rs1869901, OR = 0.34 (95% CI: 0.20-0.58); and rs2305649, OR = 0.43 (95% CI: 0.26-0.71)). Variants in GNAQ, GNAS, GNAT3, PLCB2, RALB, and RAC1 were associated with oral fungal and bacterial community composition. This study revealed that three loci at PLCB2 are significantly associated with S-ECC. Variants in multiple genes were associated with the composition of dental biofilm. These findings contribute to the current knowledge about the role of genetics in S-ECC.

Structural basis for strychnine activation of human bitter taste receptor TAS2R46

Taste sensing is a sophisticated chemosensory process, and bitter taste perception is mediated by type 2 taste receptors (TAS2Rs), or class T G protein-coupled receptors. Understanding the detailed molecular mechanisms behind taste sensation is hindered by a lack of experimental receptor structures. Here, we report the cryo-electron microscopy structures of human TAS2R46 complexed with chimeric mini-G protein gustducin, in both strychnine-bound and apo forms. Several features of TAS2R46 are disclosed, including distinct receptor structures that compare with known GPCRs, a new "toggle switch," activation-related motifs, and precoupling with mini-G protein gustducin. Furthermore, the dynamic extracellular and more-static intracellular parts of TAS2R46 suggest possible diverse ligand-recognition and activation processes. This study provides a basis for further exploration of other bitter taste receptors and their therapeutic applications.

Extraoral expression and characterization of bitter taste receptors in Astyanax mexicanus (Mexican tetra fish)

The chemical senses of olfaction and taste are well developed in fish and play a vital role in its various activities such as navigation, mate recognition, and food detection. The small teleost fish Astyanax mexicanus consists of interfertile river-dwelling and cave-dwelling populations, referred to as "surface fish" and "cavefish" respectively. An important anatomical feature of cavefish is the lack of eyes leading them to be referred to as blind fish and suggesting an enhanced functional role for other senses such as taste. In this study, we characterize the expression of bitter taste receptors (T2Rs or Tas2Rs) in A. mexicanus and investigate their functionality in a heterologous expression system. The genome database of A. mexicanus (ensemble and NCBI) showed 7 Tas2Rs, among these Tas2R1, Tas2R3, Tas2R4, and Tas2R114 are well characterized in humans and mice but not in A. mexicanus. Therefore, the 4 Tas2Rs were selected for further analysis and their expression in A. mexicanus was confirmed by in situ hybridization and RT-PCR in early developmental stages. These Tas2Rs are expressed in various oral and extraoral organs (liver, fins, jaws, and gills) in A. mexicanus, and Tas2R1 has maximum expression and is localized throughout the fish body. Using the heterologous expression of A. mexicanus T2Rs in HEK293T cells coupled with cell-based calcium mobilization assays, we show that A. mexicanus T2Rs are activated by commonly used fish food and known bitter agonists, including quinine. This study provides novel insights into the extraoral expression of T2Rs in A. mexicanus and suggests their importance in extraoral food detection.

Supplementing Citrus aurantium Flavonoid Extract in High-Fat Finishing Diets Improves Animal Behavior and Rumen Health and Modifies Rumen and Duodenum Epithelium Gene Expression in Holstein Bulls

One hundred and forty-six bulls (178.2 ± 6.64 kg BW and 146.0 ± 0.60 d of age) were randomly allocated to one of eight pens and assigned to control (C) or citrus flavonoid (BF) treatments (Citrus aurantium, Bioflavex CA, HTBA, S.L.U., Barcelona, Spain, 0.4 kg per ton of Bioflavex CA). At the finishing phase, the dietary fat content of the concentrate was increased (58 to 84 g/kg DM). Concentrate intake was recorded daily, and BW and animal behavior by visual scan, fortnightly. After 168 d, bulls were slaughtered, carcass data were recorded, and rumen and duodenum epithelium samples were collected. Performance data were not affected by treatment, except for the growing phase where concentrate intake (p < 0.05) was lesser in the BF compared with the C bulls. Agonistic and sexual behaviors were more frequent (p < 0.01) in the C than in the BF bulls. In the rumen epithelium, in contrast to duodenum, gene expression of some bitter taste receptors (7, 16, 39) and other genes related to behavior and inflammation was higher (p < 0.05) in the BF compared with the C bulls. Supplementing citrus flavonoids in high-fat finishing diets to Holstein bulls reduces growing concentrate consumption and improves animal welfare.

Landscape of Variability in Chemosensory Genes Associated With Dietary Preferences in Indian Population: Analysis of 1029 Indian Genomes

Perception and preferences for food and beverages determine dietary behaviour and health outcomes. Inherent differences in chemosensory genes, ethnicity, geo-climatic conditions, and sociocultural practices are other determinants. We aimed to study the variation landscape of chemosensory genes involved in perception of taste, texture, odour, temperature and burning sensations through analysis of 1,029 genomes of the IndiGen project and diverse continental populations. SNPs from 80 chemosensory genes were studied in whole genomes of 1,029 IndiGen samples and 2054 from the 1000 Genomes project. Population genetics approaches were used to infer ancestry of IndiGen individuals, gene divergence and extent of differentiation among studied populations. 137,760 SNPs including common and rare variants were identified in IndiGenomes with 62,950 novel (46%) and 48% shared with the 1,000 Genomes. Genes associated with olfaction harbored most SNPs followed by those associated with differences in perception of salt and pungent tastes. Across species, receptors for bitter taste were the most diverse compared to others. Three predominant ancestry groups within IndiGen were identified based on population structure analysis. We also identified 1,184 variants that exhibit differences in frequency of derived alleles and high population differentiation (F_ST ≥0.3) in Indian populations compared to European, East Asian and African populations. Examples include ADCY10, TRPV1, RGS6, OR7D4, ITPR3, OPRM1, TCF7L2, and RUNX1. This is a first of its kind of study on baseline variations in genes that could govern cuisine designs, dietary preferences and health outcomes. This would be of enormous utility in dietary recommendations for precision nutrition both at population and individual level.

Bitter taste receptor T2R38 is expressed on skin-infiltrating lymphocytes and regulates lymphocyte migration

Bitter taste receptors (T2Rs) are G protein-coupled receptors involved in the perception of bitter taste on the tongue. In humans, T2Rs have been found in several sites outside the oral cavity. Although T2R38 has been reported to be expressed on peripheral lymphocytes, it is poorly understood whether T2R38 plays immunological roles in inflammatory skin diseases such as atopic dermatitis (AD). Then, we first confirmed that T2R38 gene expression was higher in lesional skin of AD subjects than healthy controls. Furthermore, skin T2R38 expression levels were correlated with serum thymus and activation-regulated chemokine and IgE levels in AD patients. In lesional skin of AD, section staining revealed that CD3⁺ T cells in the dermis were T2R38 positive. In addition, flow cytometry analysis showed T2R38 expression in skin T cells. Migration assays using T2R38-transduced Jurkat T cell leukemia cells revealed that T2R38 agonists exerted a dose-dependent migration inhibitory effect. Moreover, skin tissue extracts, as well as supernatants of cultured HaCaT keratinocytes, caused T2R38-dependent migration inhibition, indicating that there should be an endogenous ligand for T2R38 in the skin epidermis. These findings implicate T2R38 as a migratory inhibitory receptor on the skin-infiltrating lymphocytes and as a therapeutic target for allergic/inflammatory skin diseases.

Bitter Taste Receptors in the Airway Cells Functions

G protein-coupled receptors (GPCRs) play a central role in regulating the functions of a diverse range of cell types in the airway. Taste 2 receptor (T2R) family of GPCRs is responsible for the transduction of bitter taste; however, recent studies have demonstrated that different subtypes of T2Rs and key components of T2R signaling are expressed in several extra-oral tissues including airways with many physiological roles. In the lung, expression of T2Rs has been confirmed in multiple airway cell types including airway smooth muscle (ASM) cells, various epithelial cell subtypes, and on both resident and migratory immune cells. Most importantly, activation of T2Rs with a variety of putative agonists elicits unique signaling in ASM and specialized airway epithelial cells resulting in the inhibition of ASM contraction and proliferation, promotion of ciliary motility, and innate immune response in chemosensory airway epithelial cells. Here we discuss the expression of T2Rs and the mechanistic basis of their function in the structural cells of the airways with some useful insights on immune cells in the context of allergic asthma and other upper airway inflammatory disorders. Emphasis on T2R biology and pharmacology in airway cells has an ulterior goal of exploiting T2Rs for therapeutic benefit in obstructive airway diseases.

Chemoinformatics View on Bitter Taste Receptor Agonists in Food

Food compounds with a bitter taste have a role in human health, both for their capability to influence food choice and preferences and for their possible systemic effect due to the modulation of extra-oral bitter taste receptors (TAS2Rs). Investigating the interaction of bitter food compounds with TAS2Rs is a key step to unravel their complex effects on health and to pave the way to rationally design new additives for food formulation or drugs. Here, we propose a collection of food bitter compounds, for which in vitro activity data against TAS2Rs are available. The patterns of TAS2R subtype-specific agonists were analyzed using scaffold decomposition and chemical space analysis, providing a detailed characterization of the associations between food bitter tastants and TAS2Rs.

The Role of Bitter Taste Receptors in Cancer: A Systematic Review

BACKGROUND

Since it is known that bitter taste receptors (TAS2Rs) are expressed and functionally active in various extra-oral cells, their genetic variability and functional response initiated by their activation have become of broader interest, including in the context of cancer.

METHODS

A systematic research was performed in PubMed and Google Scholar to identify relevant publications concerning the role of TAS2Rs in cancer.

RESULTS

While the findings on variations of TAS2R genotypes and phenotypes and their association to the risk of developing cancer are still inconclusive, gene expression analyses revealed that TAS2Rs are expressed and some of them are predominately downregulated in cancerous compared to non-cancerous cell lines and tissue samples. Additionally, receptor-specific, agonist-mediated activation induced various anti-cancer effects, such as decreased cell proliferation, migration, and invasion, as well as increased apoptosis. Furthermore, the overexpression of TAS2Rs resulted in a decreased tumour incidence in an in vivo study and TAS2R activation could even enhance the therapeutic effect of chemotherapeutics in vitro. Finally, higher expression levels of TAS2Rs in primary cancerous cells and tissues were associated with an improved prognosis in humans.

CONCLUSION

Since current evidence demonstrates a functional role of TAS2Rs in carcinogenesis, further studies should exploit their potential as (co-)targets of chemotherapeutics.

Bitter Taste Receptors Expression in Human Granulosa and Cumulus Cells: New Perspectives in Female Fertility

Bitter taste receptors (TAS2RS) expression is not restricted to the oral cavity and the presence of these receptors in the male reproductive system and sperm provides insights into their possible role in human reproduction. To elucidate the potential role of TAS2Rs in the female reproductive system, we investigated the expression and localization of bitter taste receptors and the components of signal transduction cascade involved in the pathway of taste receptors in somatic follicular cells obtained from women undergoing assisted reproductive techniques. We found that TAS2R genes are expressed in both cumulus (CCs) and granulosa (GCs) cells, with TAS2R14 being the most highly expressed bitter receptor subtype. Interestingly, a slight increase in the expression of TAS2R14 and TAS2R43 was shown in both GCs and CCs in young women (p < 0.05), while a negative correlation may be established between the number of oocytes collected at the pickup and the expression of TAS2R43. Regarding α-gustducin and α-transducin, two Gα subunits expressed in the taste buds on the tongue, we provide evidence for their expression in CCs and GCs, with α-gustducin showing two additional isoforms in GCs. Finally, we shed light on the possible downstream transduction pathway initiated by taste receptor activation in the female reproductive system. Our study, showing for the first time the expression of taste receptors in the somatic ovarian follicle cells, significantly extends the current knowledge of the biological role of TAS2Rs for human female fertility.

Association of Bitter Taste Receptor T2R38 Polymorphisms, Oral Microbiota, and Rheumatoid Arthritis

The association of taste genetics and the oral microbiome in autoimmune diseases such as rheumatoid arthritis (RA) has not been reported. We explored a novel oral mucosal innate immune pathway involving the bitter taste G protein-coupled receptor T2R38. This case–control study aimed to evaluate whether T2R38 polymorphisms associate with the buccal microbial composition in RA. Genomic DNA was obtained from buccal swabs of 35 RA patients and 64 non-RA controls. TAS2R38 genotypes were determined by Sanger sequencing. The buccal microbiome was assessed by Illumina MiSeq sequencing of the V4-16S rRNA gene. Bacterial community differences were analyzed with alpha and beta diversity measures. Linear discriminant analysis effect size identified taxa discriminating between RA versus non-RA and across TAS2R38 genotypes. TAS2R38 genotype frequency was similar between RA and non-RA controls (PAV/PAV; PAV/AVI; AVI/AVI: RA 42.9%; 45.7%; 11.4% versus controls 32.8%; 48.4%; 18.8%, chi-square (2, N = 99) = 2.1, p = 0.35). The relative abundance of Porphyromonas, among others, differed between RA and non-RA controls. The relative abundance of several bacterial species also differed across TAS2R38 genotypes. These findings suggest an association between T2R38 polymorphisms and RA buccal microbial composition. However, further research is needed to understand the impact of T2R38 in oral health and RA development.

Bitter Taste Receptor T2R14 Modulates Gram-Positive Bacterial Internalization and Survival in Gingival Epithelial Cells

Bitter-taste receptors (T2Rs) have emerged as key players in host–pathogen interactions and important modulators of oral innate immunity. Previously, we reported that T2R14 is expressed in gingival epithelial cells (GECs) and interacts with competence stimulating peptides (CSPs) secreted by the cariogenic Streptococcus mutans. The underlying mechanisms of the innate immune responses and physiological effects of T2R14 on Gram-positive bacteria are not well characterized. In this study, we examined the role of T2R14 in internalization and growth inhibitory effects on Gram-positive bacteria, namely Staphylococcus aureus and S. mutans. We utilized CRISPR-Cas9 T2R14 knockdown (KD) GECs as the study model to address these key physiological mechanisms. Our data reveal that the internalization of S. aureus is significantly decreased, while the internalization of S. mutans remains unaffected upon knockdown of T2R14 in GECs. Surprisingly, GECs primed with S. mutans CSP-1 resulted in an inhibition of growth for S. aureus, but not for S. mutans. The GECs infected with S. aureus induced T2R14-dependent human β-defensin-2 (hBD-2) secretion; however, S. mutans–infected GECs did not induce hBD-2 secretion, but induced T2R14 dependent IL-8 secretion. Interestingly, our results show that T2R14 KD affects the cytoskeletal reorganization in GECs, thereby inhibiting S. aureus internalization. Our study highlights the distinct mechanisms and a direct role of T2R14 in influencing physiological responses to Gram-positive bacteria in the oral cavity.

Possible Beneficial Actions of Caffeine in SARS-CoV-2

The COVID-19 pandemic has established an unparalleled necessity to rapidly find effective treatments for the illness; unfortunately, no specific treatment has been found yet. As this is a new emerging chaotic situation, already existing drugs have been suggested to ameliorate the infection of SARS-CoV-2. The consumption of caffeine has been suggested primarily because it improves exercise performance, reduces fatigue, and increases wakefulness and awareness. Caffeine has been proven to be an effective anti-inflammatory and immunomodulator. In airway smooth muscle, it has bronchodilator effects mainly due to its activity as a phosphodiesterase inhibitor and adenosine receptor antagonist. In addition, a recent published document has suggested the potential antiviral activity of this drug using in silico molecular dynamics and molecular docking; in this regard, caffeine might block the viral entrance into host cells by inhibiting the formation of a receptor-binding domain and the angiotensin-converting enzyme complex and, additionally, might reduce viral replication by the inhibition of the activity of 3-chymotrypsin-like proteases. Here, we discuss how caffeine through certain mechanisms of action could be beneficial in SARS-CoV-2. Nevertheless, further studies are required for validation through in vitro and in vivo models.

Bitter taste receptor T2R14 detects quorum sensing molecules from cariogenic Streptococcus mutans and mediates innate immune responses in gingival epithelial cells

Host-pathogen interactions play an important role in defining the outcome of a disease. Recent studies have shown that the bacterial quorum sensing molecules (QSM) can interact with host cell membrane proteins, mainly G protein-coupled receptors (GPCRs), and induce innate immune responses. However, few studies have examined QSM-GPCR interactions and their influence on oral innate immune responses. In this study, we examined the role of bitter taste receptor T2R14 in sensing competence stimulating peptides (CSPs) secreted by cariogenic bacterium Streptococcus mutans and in mediating innate immune responses in gingival epithelial cells (GECs). Transcriptomic and western blot analyses identify T2R14 to be highly expressed in GECs. Our data show that only CSP-1 from S. mutans induces robust intracellular calcium mobilization compared to CSP-2 and CSP-3. By using CRISPR-Cas9, we demonstrate that CSP-1 induced calcium signaling and secretion of cytokines CXCL-8/IL-8, TNF-α, and IL-6 is mediated through T2R14 in GECs. Interestingly, the NF-kB signaling activated by CSP-1 in GECs was independent of T2R14. CSP-1-primed GECs attracted differentiated HL-60 immune cells (dHL-60) and this effect was abolished in T2R14 knock down GECs and also in cells primed with T2R14 antagonist 6-Methoxyflavone (6-MF). Our findings identify S. mutans CSP-1 as a peptide ligand for the T2R family. Our study establishes a novel host-pathogen interaction between cariogenic S. mutans CSP-1 and T2R14 in GECs leading to an innate immune response. Collectively, these findings suggest T2Rs as potential therapeutic targets to modulate innate immune responses upon oral bacterial infections.

Pharmacology of T2R Mediated Host-Microbe Interactions

Bitter taste receptors (T2Rs) belong to the G protein-coupled receptor superfamily. Humans express 25 T2Rs that are known to detect several bitter compounds including bacterial quorum sensing molecules (QSM). Primarily found to be key receptors for bitter sensation T2Rs are known to play an important role in mediating innate immune responses in oral and extraoral tissues. Several studies have led to identification of Gram-negative and Gram-positive bacterial QSMs as agonists for T2Rs in airway epithelial cells and immune cells. However, the pharmacological characterization for many of the QSM-T2R interactions remains poorly defined. In this chapter, we discuss the extraoral roles including localization of T2Rs in extracellular vesicles, molecular pharmacology of QSM-T2R interactions, role of T2Rs in mediating innate immune responses, and some of the challenges in understanding T2R pharmacology.

Structure-Function Analyses of Human Bitter Taste Receptors-Where Do We Stand?

The finding that bitter taste receptors are expressed in numerous tissues outside the oral cavity and fulfill important roles in metabolic regulation, innate immunity and respiratory control, have made these receptors important targets for drug discovery. Efficient drug discovery depends heavily on detailed knowledge on structure-function-relationships of the target receptors. Unfortunately, experimental structures of bitter taste receptors are still lacking, and hence, the field relies mostly on structures obtained by molecular modeling combined with functional experiments and point mutageneses. The present article summarizes the current knowledge on the structure–function relationships of human bitter taste receptors. Although these receptors are difficult to express in heterologous systems and their homology with other G protein-coupled receptors is very low, detailed information are available at least for some of these receptors.

Denatonium as a Bitter Taste Receptor Agonist Modifies Transcriptomic Profile and Functions of Acute Myeloid Leukemia Cells

The contribution of cell-extrinsic factors in Acute Myeloid Leukemia (AML) generation and persistence has gained interest. Bitter taste receptors (TAS2Rs) are G protein-coupled receptors known for their primary role as a central warning signal to induce aversion toward noxious or harmful substances. Nevertheless, the increasing amount of evidence about their extra-oral localization has suggested a wider function in sensing microenvironment, also in cancer settings. In this study, we found that AML cells express functional TAS2Rs. We also highlighted a significant association between the modulation of some TAS2Rs and the poor-prognosis AML groups, i.e., TP53- and TET2-mutated, supporting a potential role of TAS2Rs in AML cell biology. Gene expression profile analysis showed that TAS2R activation with the prototypical agonist, denatonium benzoate, significantly modulated a number of genes involved in relevant AML cellular processes. Functional assay substantiated molecular data and indicated that denatonium reduced AML cell proliferation by inducing cell cycle arrest in G0/G1 phase or induced apoptosis via caspase cascade activation. Moreover, denatonium exposure impaired AML cell motility and migratory capacity, and inhibited cellular respiration by decreasing glucose uptake and oxidative phosphorylation. In conclusion, our results in AML cells expand the observation of cancer TAS2R expression to the setting of hematological neoplasms and shed light on a role of TAS2Rs in the extrinsic regulation of leukemia cell functions.

Clinical Role of Extraoral Bitter Taste Receptors

Humans can recognise five basic tastes: sweet, sour, salty, bitter and umami. Sour and salty substances are linked to ion channels, while sweet, bitter and umami flavours are transmitted through receptors linked to the G protein (G protein-coupled receptors; GPCRs). There are two main types of GPCRs that transmit information about sweet, umami and bitter tastes-the Tas1r and TAS2R families. There are about 25 functional TAS2R genes coding bitter taste receptor proteins. They are found not only in the mouth and throat, but also in the intestines, brain, bladder and lower and upper respiratory tract. The determination of their purpose in these locations has become an inspiration for much research. Their presence has also been confirmed in breast cancer cells, ovarian cancer cells and neuroblastoma, revealing a promising new oncological marker. Polymorphisms of TAS2R38 have been proven to have an influence on the course of chronic rhinosinusitis and upper airway defensive mechanisms. TAS2R receptors mediate the bronchodilatory effect in human airway smooth muscle, which may lead to the creation of another medicine group used in asthma or chronic obstructive pulmonary disease. The discovery that functionally compromised TAS2R receptors negatively impact glucose homeostasis has produced a new area of diabetes research. In this article, we would like to focus on what facts have been already established in the matter of extraoral TAS2R receptors in humans.

Anti-inflammatory Activity of Absinthin and Derivatives in Human Bronchoepithelial Cells

Bitter taste receptors (hTAS2R) are expressed ectopically in various tissues, raising the possibility of a pharmacological exploitation. This seems of particular relevance in airways, since hTAS2Rs are involved in the protection of the aerial tissues from infections and in bronchodilation. The bis-guaianolide absinthin (1), one of the most bitter compounds known, targets the hTAS2R46 bitter receptor. Absinthin (1), an unstable compound, readily turns into anabsinthin (2) with substantial retention of the bitter properties, and this compound was used as a starting material to explore the chemical space around the bis-guaianolide bitter pharmacophore. Capitalizing on the chemoselective opening of the allylic lactone ring, the esters 3 and 4, and the nor-azide 6 were prepared and assayed on human bronchoepithelial (BEAS-2B) cells expressing hTAS2R46. Anti-inflammatory activity was evaluated by measuring the expression of MUC5AC, iNOS, and cytokines, as well as the production of superoxide anion, qualifying the methyl ester 3 as the best candidate for additional studies.

Gustatory Function and the Uremic Toxin, Phosphate, Are Modulators of the Risk of Vascular Calcification among Patients with Chronic Kidney Disease: A Pilot Study

Patients with chronic kidney disease (CKD) have an increased risk of vascular calcification (VC), including aortic arch calcification (AAC). Few investigated the influence of gustatory function on the probability of having VC. We examined whether gustatory function results modulated the probability of having VC in patients with CKD. We prospectively enrolled adults with CKD (estimated glomerular filtration rate <60 mL/min/1.73 m²), with their AAC rated semi-quantitatively and gustatory function assessed by objective and subjective approaches. Multiple logistic regression was used to analyze the relationship between gustatory function results and AAC. Those with AAC had significantly better objective gustatory function in aggregate scores (p = 0.039) and categories (p = 0.022) and less defective bitter taste (p = 0.045) and scores (p = 0.037) than those without. Multiple regression analyses showed that higher aggregate scores (odds ratio (OR) 1.288, p = 0.032), or better gustatory function, and higher bitter taste scores (OR 2.558, p = 0.019) were each associated with a higher probability of having AAC among CKD patients; such an association was modulated by serum phosphate levels. In conclusion, better gustatory function was independently correlated with having AAC among CKD patients. A follow-up of VC severity may be an underrecognized component of care for CKD patients with a preserved gustatory function.

BitterDB: taste ligands and receptors database in 2019

BitterDB (http://bitterdb.agri.huji.ac.il) was introduced in 2012 as a central resource for information on bitter-tasting molecules and their receptors. The information in BitterDB is frequently used for choosing suitable ligands for experimental studies, for developing bitterness predictors, for analysis of receptors promiscuity and more. Here, we describe a major upgrade of the database, including significant increase in content as well as new features. BitterDB now holds over 1000 bitter molecules, up from the initial 550. When available, quantitative sensory data on bitterness intensity as well as toxicity information were added. For 270 molecules, at least one associated bitter taste receptor (T2R) is reported. The overall number of ligand-T2R associations is now close to 800. BitterDB was extended to several species: in addition to human, it now holds information on mouse, cat and chicken T2Rs, and the compounds that activate them. BitterDB now provides a unique platform for structure-based studies with high-quality homology models, known ligands, and for the human receptors also data from mutagenesis experiments, information on frequently occurring single nucleotide polymorphisms and links to expression levels in different tissues.

Class C GPCRs in the airway

Understanding and targeting of GPCRs remain a critical aspect of airway pharmacology and therapeutics for diseases such as asthma or COPD. Most attention has been on the large Class A GPCRs towards improved bronchodilation and blunting of remodeling. Better known in the central or peripheral nervous system, there is increasing evidence that Class C GPCRs which include metabotropic glutamate and GABA receptors, the calcium sensing receptor, sweet/umami taste receptors and a number of orphan receptors, can contribute to airway structure and function. In this review, we will summarize current state of knowledge regarding the pharmacology of Class C GPCRs, their expression and potential functions in the airways, and the application of pharmacological agents targeting this group in the context of airway diseases.

The bitterness of genitourinary infections: Properties, ligands of genitourinary bitter taste receptors and mechanisms linking taste sensing to inflammatory processes in the genitourinary tract

BACKGROUND

Though, first identified in the gastrointestinal tract, bitter taste receptors are now believed to be ubiquitously expressed in several regions of the body, including the respiratory tract, where they play a critical role in sensing and clearance of excess metabolic substrates, toxins, debris, and pathogens. More recently, bitter taste receptor expression has been reported in cells, tissues and organs of the genitourinary (GU) system, suggesting that these receptors may play an integral role in mediating inflammatory responses to microbial aggression in the GU tract. However, the mechanisms, linking bitter taste receptor sensing with inflammatory responses are not exactly clear. Here, I review recent data on the properties and ligands of bitter taste receptors and suggest mechanisms of bitter taste receptor signaling in the GU tract, and the molecular pathways that link taste sensing to inflammatory responses in GU tract.

METHOD

Computer-aided search was conducted in Scopus, PubMed, Web of Science and Google Scholar for relevant peer-reviewed articles published between 1990 and 2018, investigating the functional implication of bitter taste receptors in GU infections, using the following keywords: extra-oral bitter taste receptors, bitter taste receptors, GU bitter taste receptors, kidney OR renal OR ureteral OR urethral OR bladder OR detrusor smooth muscle OR testes OR spermatozoa OR prostate OR vaginal OR cervix OR ovarian OR endometrial OR myometrial OR placenta OR cutaneous bitter taste receptors. To identify research gaps on etiopathogenesis of GU infections/inflammation, additional search was conducted using the following keywords: GU inflammatory signaling, GU microbes, GU bacteria, GU virus, GU protozoa, GU microbial metabolites, and GU infection. The retrieved articles were filtered and further screened for relevance according to the aim of the study. A narrative review was performed for selected literatures.

RESULTS

Bitter taste receptors of the GU tract may constitute essential components of the pathogenetic mechanisms of GU infections/inflammation that are activated by microbial components, known as quorum sensing signal molecules. Based on accumulating evidences, indicating that taste receptors may signal downstream to activate inflammatory cascades, in addition to the nitric oxide-induced microbicidal effects produced upon taste receptor activation, it is suggested that the anti-inflammatory activities of bitter taste receptor stimulation are mediated via pathways involving the nuclear factor κB by downstream signaling of the metabolic and stress sensors, adenosine monophosphate-activated protein kinase and nicotinamide adenine dinucleotide-dependent silent mating type information regulation 2 homolog 1 (sirtuin 1), resulting to the synthesis of anti-inflammatory cytokines/chemokines, and antimicrobial factors, which ultimately, under normal conditions, leads to the elimination of microbial aggression.

CONCLUSIONS

GU bitter taste receptors may represent critical players in GU tract infections/inflammation. Bitter taste receptors may serve as important therapeutic target for treatment of a number of infectious diseases that affect the GU tract.

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.

Genetic Differences in Taste Receptors: Implications for the Food Industry

Inborn genetic differences in chemosensory receptors can lead to differences in perception and preference for foods and beverages. These differences can drive market segmentation for food products as well as contribute to nutritional status. This knowledge may be essential in the development of foods and beverages because the sensory profiles may not be experienced in the same way across individuals. Rather, distinct consumer groups may exist with different underlying genetic variations. Identifying genetic factors associated with individual variability can help better meet consumer needs through an enhanced understanding of perception and preferences. This review provides an overview of taste and chemesthetic sensations and their receptors, highlighting recent advances linking genetic variations in chemosensory genes to perception, food preference and intake, and health. With growing interest in personalized foods, this information is useful for both food product developers and nutrition health professionals alike.

Chemosensory bitter taste receptors T2R4 and T2R14 activation attenuates proliferation and migration of breast cancer cells

The emerging significance of the bitter taste receptors (T2Rs) role in the extraoral tissues alludes to their potential role in many pathophysiological conditions. The dysregulation of T2R expression and function in disease conditions has now been demonstrated in airways diseases, neurological disorders, and in some cancers. However, the role of T2Rs in the pathophysiology of breast cancer is unexplored thus far. Previously, we demonstrated differential expression of the 25 T2Rs in breast cancer (BC) cells. Based on our previous findings we selected two T2Rs, T2R4 and T2R14 for this work. The objective of the current study is to investigate the expression of T2R4 and T2R14 in BC clinical samples and to examine their physiological role using highly metastatic BC and non-cancerous cell lines. Using approaches, which involve receptor knockdown, pharmacological activation and biochemical assays we report that (i) T2R4 and T2R14 expression patterns are dissimilar, with decreased levels of T2R4 and increased levels of T2R14 in BC clinical samples compared to non-cancerous controls. (ii) Activation of T2Rs with their respective agonist elicited physiological responses in metastatic breast cancer cells, and no responses were seen in non-tumorigenic breast epithelial cells. (iii) Agonist activation of T2Rs (irrespective of T2R subtype) induced anti-proliferative, pro-apoptotic, and anti-migratory responses in highly metastatic breast cancer cells. Taken together, our findings demonstrate that the chemosensory T2R signaling network is involved in evoking physiological responses in the metastatic breast cancer cell line.

Bitter taste signaling mediated by Tas2r144 is down-regulated by 17β-estradiol and progesterone in the rat choroid plexus

The blood-cerebrospinal fluid barrier is constituted by choroid plexus epithelial cells (CPEC) that regulate molecular trafficking between the blood and the cerebrospinal fluid. We hypothesize that taste receptors expressed in CPEC monitor the composition of these body fluids in a sex hormone dependent way. Thus, we compared the expression of taste related genes in the choroid plexus of sham and ovariectomized female rats, and then studied the effect of 17β-estradiol and progesterone in their expression and function. We found that the bitter receptors Tas2r109, Tas2r144, and the taste-related genes Plcb2 and Trpm5 were down-regulated by ovarian hormones in vivo and ex vivo with functional implications. Knocking-down Tas2r144 with a specific siRNA in a CPEC line (Z310) effectively reduced the Ca²⁺ response to the bitter compound denatonium benzoate, in a similar manner to female sex hormones alone, suggesting that female sex hormones downregulated the responses of CPEC to chemical stimuli by reducing Tas2r144.

Highly conserved intracellular H208 residue influences agonist selectivity in bitter taste receptor T2R14

Bitter taste receptors (T2Rs) are a specialized class of cell membrane receptors of the G protein-coupled receptor family and perform a crucial role in chemosensation. The 25 T2Rs in humans are activated by structurally diverse ligands of plant, animal and microbial origin. The mechanisms of activation of these receptors are poorly understood. Therefore, identification of structural determinants of T2Rs that regulate its efficacy could be beneficial in understanding the molecular mechanisms of T2R activation. In this work, we characterized a highly conserved histidine (H208), present at TM5-ICL3 region of T2R14 and its role in agonist-induced T2R14 signaling. Surprisingly, mutation of the conserved H208 (H208A) did not result in increased basal activity of T2R14, in contrast to similar H206A mutation in T2R4 that showed constitutive or basal activity. However, H208A mutation in T2R14 resulted in an increase in agonist-induced efficacy for Flufenamic acid (FFA). Interestingly, H208A did not affect the potency of another T2R14 agonist Diphenhydramine (DPH). The H208R compensatory mutation showed FFA response similar to wild-type T2R14. Molecular modeling suggests a FFA-induced shift in TM3 and TM5 helices of H208A, which changes the network of interactions connecting TM5-ICL3-TM6. This report identifies a crucial residue on the intracellular surface of T2Rs that is involved in bitter ligand selectivity. It also highlights the varied roles carried out by some conserved residues in different T2Rs.

Responsiveness Expressions of Bitter Taste Receptors Against Denatonium Benzoate and Genistein in the Heart, Spleen, Lung, Kidney, and Bursa Fabricius of Chinese Fast Yellow Chicken

The present study was conducted to investigate the responsiveness expressions of ggTas2Rs against denatonium benzoate (DB) and genistein (GEN) in several organs of the Chinese Fast Yellow Chicken. A total of 300 one-day-old chicks that weighed an average of 32 g were randomly allocated into five groups with five replicates for 56 consecutive days. The dietary treatments consisted of basal diet, denatonium benzoate (5 mg/kg, 20 mg/kg, and 100 mg/kg), and genistein 25 mg/kg. The results of qRT-PCR indicated significantly (p < 0.05) high-level expressions in the heart, spleen, and lungs in the starter and grower stages except for in bursa Fabricius. The responsiveness expressions of ggTas2Rs against DB 100 mg/kg and GEN 25 mg/kg were highly dose-dependent in the heart, spleen, lungs, and kidneys in the starter and grower stages, but dose-independent in the bursa Fabricius in the finisher stage. The ggTas2Rs were highly expressed in lungs and the spleen, but lower in the bursa Fabricius among the organs. However, the organ growth performance significantly (p < 0.05) increased in the groups administered DB 5 mg/kg and GEN 25 mg/kg; meanwhile, the DB 20 mg/kg and DB 100 mg/kg treatments significantly reduced the growth of all the organs, respectively. These findings indicate that responsiveness expressions are dose-dependent, and bitterness sensitivity consequently decreases in aged chickens. Therefore, these findings may improve the production of new feedstuffs for chickens according to their growing stages.

Mechanistic role of plant-based bitter principles and bitterness prediction for natural product studies II: prediction tools and case studies

The first part of this chapter provides an overview of computer-based tools (algorithms, web servers, and software) for the prediction of bitterness in compounds. These tools all implement machine learning (ML) methods and are all freely accessible. For each tool, a brief description of the implemented method is provided, along with the training sets and the benchmarking results. In the second part, an attempt has been made to explain at the mechanistic level why some medicinal plants are bitter and how plants use bitter natural compounds, obtained through the biosynthetic process as important ingredients for adapting to the environment. A further exploration is made on the role of bitter natural products in the defense mechanism of plants against insect pest, herbivores, and other invaders. Case studies have focused on alkaloids, terpenoids, cyanogenic glucosides and phenolic derivatives.

Toward the Identification of Extra-Oral TAS2R Agonists as Drug Agents for Muscle Relaxation Therapies via Bioinformatics-Aided Screening of Bitter Compounds in Traditional Chinese Medicine

Significant advances have been made in the past decade in mapping the distributions and the physiological functions of extra-oral bitter taste receptors (TAS2Rs) in non-gustatory tissues. In particular, it has been found that TAS2Rs are expressed in various muscle tissues and activation of TAS2Rs can lead to muscle cell relaxation, which suggests that TAS2Rs may be important new targets in muscle relaxation therapy for various muscle-related diseases. So far, however, there is a lack of potent extra-oral TAS2R agonists that can be used as novel drug agents in muscle relaxation therapies. Interestingly, traditional Chinese medicine (TCM) often characterizes a drug’s property in terms of five distinct flavors (bitter, sweet, sour, salty, and pungent) according to its taste and function, and commonly regards “bitterness” as an intrinsic property of “good medicine.” In addition, many bitter flavored TCM are known in practice to cause muscle relaxation after long term use, and in lab experiments the compounds identified from some bitter flavored TCM do activate TAS2Rs and thus relax muscle cells. Therefore, it is highly possible to discover very useful extra-oral TAS2R agonists for muscle relaxation therapies among the abundant bitter compounds used in bitter flavored TCM. With this perspective, we reviewed in literature the distribution of TAS2Rs in different muscle systems with a focus on the map of bitter flavored TCM which can regulate muscle contractility and related functional chemical components. We also reviewed the recently established databases of TCM chemical components and the bioinformatics software which can be used for high-throughput screening and data mining of the chemical components associated with bitter flavored TCM. All together, we aim to present a knowledge-based approach and technological platform for identification or discovery of extra-oral TAS2R agonists that can be used as novel drug agents for muscle relaxation therapies through screening and evaluation of chemical compounds used in bitter flavored TCM.

Mechanistic role of plant-based bitter principles and bitterness prediction for natural product studies I: Database and methods

This chapter discusses the rationale behind the bitter sensation elicited by chemical compounds, focusing on natural products. Emphasis has been placed on a brief presentation of BitterDB (the database of bitter compounds), along with available methods for the prediction of bitterness in compounds. The fundamental basis for explaining bitterness has been provided, based on the structural features of human bitter taste receptors and have been used to shed light on the mechanistic role of a few out of the 25 known human taste receptors to provide the foundation for understanding how bitter compounds interact with their receptors. Some case studies of ligand-based prediction models based on 2D fingerprints and 3D pharmacophores, along with machine learning methods have been provided. The chapter closes with an attempt to establish the relationship between bitterness and toxicity.

Advanced Glycation End-Products Can Activate or Block Bitter Taste Receptors

Bitter taste receptors (T2Rs) are expressed in several tissues of the body and are involved in a variety of roles apart from bitter taste perception. Advanced glycation end-products (AGEs) are produced by glycation of amino acids in proteins. There are varying sources of AGEs, including dietary food products, as well as endogenous reactions within our body. Whether these AGEs are T2R ligands remains to be characterized. In this study, we selected two AGEs, namely, glyoxal-derived lysine dimer (GOLD) and carboxymethyllysine (CML), based on their predicted interaction with the well-studied T2R4, and its physiochemical properties. Results showed predicted binding affinities (K_d) for GOLD and CML towards T2R4 in the nM and μM range, respectively. Calcium mobilization assays showed that GOLD inhibited quinine activation of T2R4 with IC₅₀ 10.52 ± 4.7 μM, whilst CML was less effective with IC₅₀ 32.62 ± 9.5 μM. To characterize whether this antagonism was specific to quinine activated T2R4 or applicable to other T2Rs, we selected T2R14 and T2R20, which are expressed at significant levels in different human tissues. A similar effect of GOLD was observed with T2R14; and in contrast, GOLD and CML activated T2R20 with an EC₅₀ of 79.35 ± 29.16 μM and 65.31 ± 17.79 μM, respectively. In this study, we identified AGEs as novel T2R ligands that caused either activation or inhibition of different T2Rs.

Dual binding mode of "bitter sugars" to their human bitter taste receptor target

The 25 human bitter taste receptors (hTAS2Rs) are responsible for detecting bitter molecules present in food, and they also play several physiological and pathological roles in extraoral compartments. Therefore, understanding their ligand specificity is important both for food research and for pharmacological applications. Here we provide a molecular insight into the exquisite molecular recognition of bitter β-glycopyranosides by one of the members of this receptor subclass, hTAS2R16. Most of its agonists have in common the presence of a β-glycopyranose unit along with an extremely structurally diverse aglycon moiety. This poses the question of how hTAS2R16 can recognize such a large number of "bitter sugars". By means of hybrid molecular mechanics/coarse grained molecular dynamics simulations, here we show that the three hTAS2R16 agonists salicin, arbutin and phenyl-β-D-glucopyranoside interact with the receptor through a previously unrecognized dual binding mode. Such mechanism may offer a seamless way to fit different aglycons inside the binding cavity, while maintaining the sugar bound, similar to the strategy used by several carbohydrate-binding lectins. Our prediction is validated a posteriori by comparison with mutagenesis data and also rationalizes a wealth of structure-activity relationship data. Therefore, our findings not only provide a deeper molecular characterization of the binding determinants for the three ligands studied here, but also give insights applicable to other hTAS2R16 agonists. Together with our results for other hTAS2Rs, this study paves the way to improve our overall understanding of the structural determinants of ligand specificity in bitter taste receptors.

Understanding Ligand Binding to G-Protein Coupled Receptors Using Multiscale Simulations

Human G-protein coupled receptors (GPCRs) convey a wide variety of extracellular signals inside the cell and they are one of the main targets for pharmaceutical intervention. Rational drug design requires structural information on these receptors; however, the number of experimental structures is scarce. This gap can be filled by computational models, based on homology modeling and docking techniques. Nonetheless, the low sequence identity across GPCRs and the chemical diversity of their ligands may limit the quality of these models and hence refinement using molecular dynamics simulations is recommended. This is the case for olfactory and bitter taste receptors, which constitute the first and third largest GPCR groups and show sequence identities with the available GPCR templates below 20%. We have developed a molecular dynamics approach, based on the combination of molecular mechanics and coarse grained (MM/CG), tailored to study ligand binding in GPCRs. This approach has been applied so far to bitter taste receptor complexes, showing significant predictive power. The protein/ligand interactions observed in the simulations were consistent with extensive mutagenesis and functional data. Moreover, the simulations predicted several binding residues not previously tested, which were subsequently verified by carrying out additional experiments. Comparison of the simulations of two bitter taste receptors with different ligand selectivity also provided some insights into the binding determinants of bitter taste receptors. Although the MM/CG approach has been applied so far to a limited number of GPCR/ligand complexes, the excellent agreement of the computational models with the mutagenesis and functional data supports the applicability of this method to other GPCRs for which experimental structures are missing. This is particularly important for the challenging case of GPCRs with low sequence identity with available templates, for which molecular docking shows limited predictive power.

Multiscale simulations on human Frizzled and Taste2 GPCRs

Recently, molecular dynamics simulations, from all atom and coarse grained to hybrid methods bridging the two scales, have provided exciting functional insights into class F (Frizzled and Taste2) GPCRs (about 40 members in humans). Findings include: (i) The activation of one member of the Frizzled receptors (FZD4) involves a bending of transmembrane helix TM7 far larger than that in class A GPCRs. (ii) The affinity of an anticancer drug targeting another member (Smoothened receptor) decreases in a specific drug-resistant variant, because the mutation ultimately disrupts the binding cavity and affects TM6. (iii) A novel two-state recognition mechanism explains the very large agonist diversity for at least one member of the Taste2 GPCRs, hTAS2R46.

Beyond the Flavour: The Potential Druggability of Chemosensory G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) belong to the largest class of drug targets. Approximately half of the members of the human GPCR superfamily are chemosensory receptors, including odorant receptors (ORs), trace amine-associated receptors (TAARs), bitter taste receptors (TAS2Rs), sweet and umami taste receptors (TAS1Rs). Interestingly, these chemosensory GPCRs (csGPCRs) are expressed in several tissues of the body where they are supposed to play a role in biological functions other than chemosensation. Despite their abundance and physiological/pathological relevance, the druggability of csGPCRs has been suggested but not fully characterized. Here, we aim to explore the potential of targeting csGPCRs to treat diseases by reviewing the current knowledge of csGPCRs expressed throughout the body and by analysing the chemical space and the drug-likeness of flavour molecules.

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.

TAS2R38 Bitterness Receptor Genetic Variation and Risk of Gastrointestinal Neoplasm: A Meta-Analysis

Genetic variation in TAS2R38 bitterness taste receptor could alter the efficacy of molecular sensing, hence may be associated with cancer risk. Thus, we performed a meta-analysis to verify the association between the risk of gastrointestinal (GI) neoplasm and TAS2R38 genetic variation. Studies with TAS2R38 diplotype distribution and GI neoplasm phenotypes were searched from PubMed, EMBASE and SCOPUS, and five articles including eight studies were finally selected. The association between diplotype and neoplasm risk was estimated with summarized odds ratios (ORs) and 95% confidence intervals (CIs), applying of fixed- or random-effects models. The findings suggested TAS2R38 diplotype was not associated with GI neoplasms susceptibility [AVI vs. PAV: OR = 1.03 (95%CI: 0.97-1.09), AVI/PAV vs. PAV/PAV: OR = 1.05, (95%CI: 0.94-1.17), AVI/* vs. PAV/PAV: OR = 1.04 (95%CI: 0.94-1.16)]. Because of the presence of heterogeneity under the two genetic models (AVI/AVI vs. PAV/PAV and AVI/AVI vs. PAV/*), further subgroup analyses by ethnicity and neoplasm type were performed. However, results failed to show the neoplasm risk was altered by diplotype. In conclusion, the meta-analysis indicates that TAS2R38 diplotype minimally modified the GI neoplasm risk. Given the limited study size and resources, further well-designed and larger studies are required to validate the true effect of TAS2R38 polymorphisms on neoplasm risk.