Functions of human bitter taste receptors depend on N-glycosylation

Potential of Bitter Medicinal Plants: A Review of Flavor Physiology

The function of the sense of taste is usually confined to the ability to perceive the flavor of food to assess and use the nutrients necessary for healthy survival and to discard those that may be harmful, toxic, or unpleasant. It is almost unanimously agreed that the perception of bitter taste prevents the consumption of toxins from plants, decaying foods, and drugs. Forty years ago, while practicing medicine in a rural area of the Colombian Amazon, I had an unexpected encounter with the Inga Indians. I faced the challenge of accepting that their traditional medicine was effective and that the medicinal plants they used had a real therapeutic effect. Wanting to follow a process of learning about medicinal plants on their terms, I found that, for them, the taste of plants is a primary and fundamental key to understanding their functioning. One of the most exciting results was discovering the therapeutic value of bitter plants. The present review aims to understand whether there is any scientific support for this hypothesis from the traditional world. Can the taste of plants explain their possible therapeutic benefit? In the last 20 years, we have made novel advances in the knowledge of the physiology of taste. Our purpose will be to explore these scientific advances to determine if the bitter taste of medicinal plants benefits human health.

Bitter flavors and bitter compounds in foods: identification, perception, and reduction techniques

Bitterness is one of the five basic tastes generally considered undesirable. The widespread presence of bitter compounds can negatively affect the palatability of foods. The classification and sensory evaluation of bitter compounds have been the focus in recent research. However, the rigorous identification of bitter tastes and further studies to effectively mask or remove them have not been thoroughly evaluated. The present paper focuses on identification of bitter compounds in foods, structural-based activation of bitter receptors, and strategies to reduce bitter compounds in foods. It also discusses the roles of metabolomics and virtual screening analysis in bitter taste. The identification of bitter compounds has seen greater success through metabolomics with multivariate statistical analysis compared to conventional chromatography, HPLC, LC-MS, and NMR techniques. However, to avoid false positives, sensory recognition should be combined. Bitter perception involves the structural activation of bitter taste receptors (TAS2Rs). Only 25 human TAS2Rs have been identified as responsible for recognizing numerous bitter compounds, showcasing their high structural diversity to bitter agonists. Thus, reducing bitterness can be achieved through several methods. Traditionally, the removal or degradation of bitter substances has been used for debittering, while the masking of bitterness presents a new effective approach to improving food flavor. Future research in food bitterness should focus on identifying unknown bitter compounds in food, elucidating the mechanisms of activation of different receptors, and developing debittering techniques based on the entire food matrix.

Recent advances in taste transduction mechanism, analysis methods and strategies employed to improve the taste of taste peptides

Taste peptides are oligopeptides that enhance both aroma and taste of food, and they are classified into five categories based on their taste characteristics: salty, sour, umami, sweet, bitter, and kokumi peptide. Recently, taste peptides have attracted the attention of several fields of research in food science and commercial applications. However, research on taste receptors of taste peptides and their taste transduction mechanisms are not clearly understood and we present a comprehensive review about these topics here. This review covers the aspects of taste peptides perceived by their receptors in taste cells, the proposed transduction pathway, as well as structural features of taste peptides. Apart from traditional methods, molecular docking, peptidomic analysis, cell and animal models and taste bud biosensors can be used to explore the taste mechanism of taste peptides. Furthermore, synergistic effect, Maillard reaction, structural modifications and changing external environment are employed to improve the taste of taste peptides. Consequently, we discussed the current challenges and future trends in taste peptide research. Based on the summarized developments, taste peptides derived from food proteins potentially appear to be important taste substances. Their applications meet the principles of "safe, nutritious and sustainable" in food development.

Human Gingival Fibroblasts as a Novel Cell Model Describing the Association between Bitter Taste Thresholds and Interleukin-6 Release

Human gingival fibroblast cells (HGF-1 cells) present an important cell model to investigate the gingiva's response to inflammatory stimuli such as lipopolysaccharides from Porphyromonas gingivalis (Pg-LPS). Recently, we demonstrated trans-resveratrol to repress the Pg-LPS evoked release of the pro-inflammatory cytokine interleukin-6 (IL-6) via involvement of bitter taste sensing receptor TAS2R50 in HGF-1 cells. Since HGF-1 cells express most of the known 25 TAS2Rs, we hypothesized an association between a compound's bitter taste threshold and its repressing effect on the Pg-LPS evoked IL-6 release by HGF-1 cells. To verify our hypothesis, 11 compounds were selected from the chemical bitter space and subjected to the HGF-1 cell assay, spanning a concentration range between 0.1 μM and 50 mM. In the first set of experiments, the specific role of TAS2R50 was excluded by results from structurally diverse TAS2R agonists and antagonists and by means of a molecular docking approach. In the second set of experiments, the HGF-1 cell response was used to establish a linear association between a compound's effective concentration to repress the Pg-LPS evoked IL-6 release by 25% and its bitter taste threshold concentration published in the literature. The Pearson correlation coefficient revealed for this linear association was R² = 0.60 (p < 0.01), exceeding respective data for the test compounds from a well-established native cell model, the HGT-1 cells, with R² = 0.153 (p = 0.263). In conclusion, we provide a predictive model for bitter tasting compounds with a potential to act as anti-inflammatory substances.

Anti-HIV drugs lopinavir/ritonavir activate bitter taste receptors

Lopinavir and ritonavir (LPV/r) are the primary anti-human immunodeficiency virus (HIV) drugs recommended by the World Health Organization for treating children aged 3 years and above who are infected with the HIV. These drugs are typically available in liquid formulations to aid in dosing for children who cannot swallow tablets. However, the strong bitter taste associated with these medications can be a significant obstacle to adherence, particularly in young children, and can jeopardize the effectiveness of the treatment. Studies have shown that poor palatability can affect the survival rate of HIV-infected children. Therefore, developing more child-friendly protease inhibitor formulations, particularly those with improved taste, is critical for children with HIV. The molecular mechanism by which lopinavir and ritonavir activate bitter taste receptors, TAS2Rs, is not yet clear. In this study, we utilized a calcium mobilization assay to characterize the activation of bitter taste receptors by lopinavir and ritonavir. We discovered that lopinavir activates TAS2R1 and TAS2R13, while ritonavir activates TAS2R1, TAS2R8, TAS2R13, and TAS2R14. The development of bitter taste blockers that target these receptors with a safe profile would be highly desirable in eliminating the unpleasant bitter taste of these anti-HIV drugs.

Construction of a bioluminescence-based assay for bitter taste receptors (TAS2Rs)

In humans, a family of 25 bitter taste receptors (TAS2Rs) mediates bitter taste perception. A common approach to characterize bitter causative agents involves expressing TAS2Rs and the appropriate signal transducers in heterologous cell systems, and monitoring changes in the intracellular free calcium levels upon ligand exposure using a fluorescence-based modality, which typically suffers from a low signal window, and is susceptible to interference by autofluorescence, therefore prohibiting its application to screening of plant or food extracts, which are likely to contain autofluorescent compounds. The aim of this study is to develop and validate a bioluminescence-based intracellular calcium release assay for TAS2Rs that has a better assay performance than a fluorescence-based assay. Furthermore, the bioluminescence-based assay enabled the evaluation of TAS2R agonists within an autofluorescent matrix, highlighting its potential utility in the assessment of the bitterness-inducing properties of plant or food fractions by the food industry. Additionally, improvement to the bioluminescence-based assay for some TAS2Rs was achieved by altering their N-terminal signal sequences, leading to signal window enhancement. Altogether, the bioluminescence-based TAS2R assay can be used to perform functional studies of TAS2Rs, evaluate TAS2R-modulating properties of autofluorescent samples, and facilitate the discovery of compounds that can function as promising bitter taste modulators.

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.

Activation of specific bitter taste receptors by olive oil phenolics and secoiridoids

Extra-virgin olive oil (EVOO) is a critical component of the Mediterranean diet, which has been found beneficial to human health. Bitterness is often positively associated with the presence of phenolic compounds in EVOO. There are twenty-five bitter taste receptors (TAS2Rs) in humans, each of which responds to specific bitter tastants. The identity of phenolic compounds and the bitter taste receptors they stimulate remain unknown. In this study, we isolated 12 phenolic and secoiridoid compounds from the olive fruit and the oil extracted from it, and tested their ability to stimulate bitter taste receptor activity, using a calcium mobilization functional assay. Our results showed that seven out of twelve studied compounds activated TAS2R8, and five of them activated TAS2R1, TAS2R8, and TAS2R14. The phenolic compounds oleuropein aglycon and ligstroside aglycon were the most potent bitter tastants in olive oil. TAS2R1 and TAS2R8 were the major bitter taste receptors activated most potently by these phenolic compounds. The results obtained here could be utilized to predict and control the bitterness of olive oil based on the concentration of specific bitter phenolics produced during the milling process of olives.

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.

A Bitter Taste in Your Heart

The human genome contains ∼29 bitter taste receptors (T2Rs), which are responsible for detecting thousands of bitter ligands, including toxic and aversive compounds. This sentinel function varies between individuals and is underpinned by naturally occurring T2R polymorphisms, which have also been associated with disease. Recent studies have reported the expression of T2Rs and their downstream signaling components within non-gustatory tissues, including the heart. Though the precise role of T2Rs in the heart remains unclear, evidence points toward a role in cardiac contractility and overall vascular tone. In this review, we summarize the extra-oral expression of T2Rs, focusing on evidence for expression in heart; we speculate on the range of potential ligands that may activate them; we define the possible signaling pathways they activate; and we argue that their discovery in heart predicts an, as yet, unappreciated cardiac physiology.

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.

The Effect of Calcium Buffering and Calcium Sensor Type on the Sensitivity of an Array-Based Bitter Receptor Screening Assay

The genetically encoded calcium sensor protein Cameleon YC3.6 has previously been applied for functional G protein-coupled receptor screening using receptor cell arrays. However, different types of sensors are available, with a wide range in [Ca2+] sensitivity, Hill coefficients, calcium binding domains, and fluorophores, which could potentially improve the performance of the assay. Here, we compared the responses of 3 structurally different calcium sensor proteins (Cameleon YC3.6, Nano140, and Twitch2B) simultaneously, on a single chip, at different cytosolic expression levels and in combination with 2 different bitter receptors, TAS2R8 and TAS2R14. Sensor concentrations were modified by varying the amount of calcium sensor DNA that was printed on the DNA arrays prior to reverse transfection. We found that ~2-fold lower concentrations of calcium sensor protein, by transfecting 4 times less sensor-coding DNA, resulted in more sensitive bitter responses. The best results were obtained with Twitch2B, where, relative to YC3.6 at the default DNA concentration, a 4-fold lower DNA concentration increased sensitivity 60-fold and signal strength 5- to 10-fold. Next, we compared the performance of YC3.6 and Twitch2B against an array with 11 different bitter taste receptors. We observed a 2- to 8-fold increase in sensitivity using Twitch2B compared with YC3.6. The bitter receptor arrays contained 300 spots and could be exposed to a series of 18 injections within 1 h resulting in 5400 measurements. These optimized sensor conditions provide a basis for enhancing receptomics calcium assays for receptors with poor Ca2+ signaling and will benefit future high-throughput receptomics experiments.

Chemosensory bitter taste receptors (T2Rs) are activated by multiple antibiotics

Many medications including antibiotics taste bitter. The potency of these antibiotics on the 25 bitter taste receptors (T2Rs) in humans remains poorly understood. Here we characterize by sensory and structure-function analyses how antibiotics frequently used to treat airway infections in cystic fibrosis activate multiple human T2Rs. The potency of the broad-spectrum antibiotics, tobramycin, levofloxacin, and azithromycin on the highly expressed T2Rs in airways, T2R4, T2R14, and T2R20 was pursued. The amino acids and structural features of T2R4, T2R14, and T2R20 important for antibiotic binding were characterized by mutational analysis in heterologous cell-based assays. Strikingly, extracellular loop 2 in T2Rs performs a key function in binding to antibiotics with contribution from residues in transmembrane helices. Our results suggest that different antibiotics activate multiple T2Rs with different potencies. An understanding of the nonantibiotic and physiologic effects mediated through T2Rs on the host cells is much needed.-Jaggupilli, A., Singh, N., De Jesus, V. C., Gounni, M. S., Dhanaraj, P., Chelikani, P. Chemosensory bitter taste receptors (T2Rs) are activated by multiple antibiotics.

Independent Evolution of Strychnine Recognition by Bitter Taste Receptor Subtypes

The 25 human bitter taste receptors (hT2Rs) recognize thousands of structurally and chemically diverse bitter substances. The binding modes of human bitter taste receptors hT2R10 and hT2R46, which are responsible for strychnine recognition, were previously established using site-directed mutagenesis, functional assays, and molecular modeling. Here we construct a phylogenetic tree and reconstruct ancestral sequences of the T2R10 and T2R46 clades. We next analyze the binding sites in view of experimental data to predict their ability to recognize strychnine. This analysis suggests that the common ancestor of hT2R10 and hT2R46 is unlikely to bind strychnine in the same mode as either of its two descendants. Estimation of relative divergence times shows that hT2R10 evolved earlier than hT2R46. Strychnine recognition was likely acquired first by the earliest common ancestor of the T2R10 clade before the separation of primates from other mammals, and was highly conserved within the clade. It was probably independently acquired by the common ancestor of T2R43-47 before the homo-ape speciation, lost in most T2Rs within this clade, but enhanced in the hT2R46 after humans diverged from the rest of primates. Our findings suggest hypothetical strychnine T2R receptors in several species, and serve as an experimental guide for further study. Improved understanding of how bitter taste receptors acquire the ability to be activated by particular ligands is valuable for the development of sensors for bitterness and for potential toxicity.

Plasticity of the ligand binding pocket in the bitter taste receptor T2R7

Bitter taste receptors (T2Rs) are a group of 25 G protein-coupled receptors (GPCRs) in humans. The cognate agonists and the mechanism of ligand binding to the majority of the T2Rs remain unknown. Here we report the first structure-function analysis of T2R7 and study the ability of this receptor to bind to different agonists by site-directed mutagenesis. Screening of ligands for T2R7 in calcium based assays lead to the identification of novel compounds that activate this receptor. Quinine, diphenidol, dextromethorphan and diphenhydramine showed substantial activation of T2R7. Interestingly, these bitter compounds showed different pharmacological characteristics. To investigate the structural features in T2R7 that might contribute to the observed differences in agonist specificities, molecular model guided ligand docking and site-directed mutagenesis was pursued. Amino acids D65, D86, W89, N167, T169, W170, S181, T255 and E271 in the ligand-binding pocket were replaced and the mutants characterized pharmacologically. Our results suggest D86, S181 and W170 present on the extracellular side of transmembrane 3 (TM3), TM5 and in extracellular loop 2 (ECL2) are essential for agonist binding in T2R7. Mutations of these amino acids lead to loss-of-function. We also identified gain-of-function residues that are agonist specific. These results suggest that agonists bind at an extracellular site rather than deep within the TM core involving residues present in both ECL2 and TM helices in T2R7. Similar to majority of the Class A GPCRs, ECL2 in T2R7 plays a significant role in agonist binding and activation.

Bitter substances from plants used in traditional Chinese medicine exert biased activation of human bitter taste receptors

The number and variety of bitter compounds originating from plants are vast. Whereas some bitter chemicals are toxic and should not be ingested, other compounds exhibit health beneficial effects, which is manifest in the cross-cultural believe that the bitterness of medicine is correlated with the desired medicinal activity. The bitter taste receptors in the oral cavity serve as sensors for bitter compounds and, as they are expressed in numerous extraoral tissues throughout the body, may also be responsible for some physiological effects exerted by bitter compounds. Chinese herbal medicine uses bitter herbs since ancient times for the treatment of various diseases; however, the routes by which these herbs modify physiology are frequently not well understood. We therefore screened 26 bitter substances extracted from medical herbs for the activation of the 25 human bitter taste receptors. We identified six receptors activated by in total 17 different bitter compounds. Interestingly, we observed a bias in bitter taste receptor activation with 10 newly identified agonists for the broadly tuned receptor TAS2R46, seven agonists activating the TAS2R14 and two compounds activating narrowly tuned receptors, suggesting that these receptors play dominant roles in the evaluation and perhaps physiological activities of Chinese herbal medicines.

How taste works: cells, receptors and gustatory perception

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

Probing the Binding Pocket of the Broadly Tuned Human Bitter Taste Receptor TAS2R14 by Chemical Modification of Cognate Agonists

Sensing potentially harmful bitter substances in the oral cavity is achieved by a group of (˜) 25 receptors, named TAS2Rs, which are expressed in specialized sensory cells and recognize individual but overlapping sets of bitter compounds. The receptors differ in their tuning breadths ranging from narrowly to broadly tuned receptors. One of the most broadly tuned human bitter taste receptors is the TAS2R14 recognizing an enormous variety of chemically diverse synthetic and natural bitter compounds, including numerous medicinal drugs. This suggests that this receptor possesses a large readily accessible ligand binding pocket. To allow probing the accessibility and size of the ligand binding pocket, we chemically modified cognate agonists and tested receptor responses in functional assays. The addition of large functional groups to agonists was usually possible without abolishing agonistic activity. The newly synthesized agonist derivatives were modeled in the binding site of the receptor, providing comparison to the mother substances and rationalization of the in vitro activities of this series of compounds.

Comparing Class A GPCRs to bitter taste receptors: Structural motifs, ligand interactions and agonist-to-antagonist ratios

G protein-coupled receptors (GPCRs) are seven transmembrane (TM) proteins that play a key role in human physiology. The GPCR superfamily comprises about 800 members, classified into several classes, with rhodopsin-like Class A being the largest and most studied thus far. A huge component of the human repertoire consists of the chemosensory GPCRs, including ∼400 odorant receptors, 25 bitter taste receptors (TAS2Rs), which are thought to guard the organism from consuming poisons, and sweet and umami TAS1R heteromers, which indicate the nutritive value of food. The location of the binding site of TAS2Rs is similar to that of Class A GPCRs. However, most of the known bitter ligands are agonists, with only a few antagonists documented thus far. The agonist-to-antagonist ratios of Class A GPCRs vary, but in general are much lower than for TAS2Rs. For a set of well-studied GPCRs, a gradual change in agonists-to-antagonists ratios is observed when comparing low (10 μM)- and high (10 nM)-affinity ligand sets from ChEMBL and the DrugBank set of drugs. This shift reflects pharmaceutical bias toward the therapeutically desirable pharmacology for each of these GPCRs, while the 10 μM sets possibly represent the native tendency of the receptors toward either agonists or antagonists. Analyzing ligand-GPCR interactions in 56 X-ray structures representative of currently available structural data, we find that the N-terminus, TM1 and TM2 are more involved in binding of antagonists than of agonists. On the other hand, ECL2 tends to be more involved in binding of agonists. This is of interest, since TAS2Rs harbor variations on the typical Class A sequence motifs, including the absence of the ECL2-TM3 disulfide bridge. This suggests an alternative mode of regulation of conformational states for TAS2Rs, with potentially less stabilized inactive state. The comparison of TAS2Rs and Class A GPCRs structural features and the pharmacology of the their ligands highlights the intricacies of GPCR architecture and provides a framework for rational design of new ligands.

Receptor Polymorphism and Genomic Structure Interact to Shape Bitter Taste Perception

The ability to taste bitterness evolved to safeguard most animals, including humans, against potentially toxic substances, thereby leading to food rejection. Nonetheless, bitter perception is subject to individual variations due to the presence of genetic functional polymorphisms in bitter taste receptor (TAS2R) genes, such as the long-known association between genetic polymorphisms in TAS2R38 and bitter taste perception of phenylthiocarbamide. Yet, due to overlaps in specificities across receptors, such associations with a single TAS2R locus are uncommon. Therefore, to investigate more complex associations, we examined taste responses to six structurally diverse compounds (absinthin, amarogentin, cascarillin, grosheimin, quassin, and quinine) in a sample of the Caucasian population. By sequencing all bitter receptor loci, inferring long-range haplotypes, mapping their effects on phenotype variation, and characterizing functionally causal allelic variants, we deciphered at the molecular level how a subjects’ genotype for the whole-family of TAS2R genes shapes variation in bitter taste perception. Within each haplotype block implicated in phenotypic variation, we provided evidence for at least one locus harboring functional polymorphic alleles, e.g. one locus for sensitivity to amarogentin, one of the most bitter natural compounds known, and two loci for sensitivity to grosheimin, one of the bitter compounds of artichoke. Our analyses revealed also, besides simple associations, complex associations of bitterness sensitivity across TAS2R loci. Indeed, even if several putative loci harbored both high- and low-sensitivity alleles, phenotypic variation depended on linkage between these alleles. When sensitive alleles for bitter compounds were maintained in the same linkage phase, genetically driven perceptual differences were obvious, e.g. for grosheimin. On the contrary, when sensitive alleles were in opposite phase, only weak genotype-phenotype associations were seen, e.g. for absinthin, the bitter principle of the beverage absinth. These findings illustrate the extent to which genetic influences on taste are complex, yet arise from both receptor activation patterns and linkage structure among receptor genes.

Human bitter taste is believed to protect us from the ingestion of poisonous substances, thereby shaping food rejections. Bitter perception differs, however, across individuals, due to genetic variations in the ~25 bitter taste receptor (TAS2R) genes. A famous example known since the 1930s is the inherited bitter taste sensitivity to phenylthiocarbamide, which is associated with genetic polymorphisms in a single TAS2R gene. Yet, such simple receptor-substance associations do not reflect the full complexity of bitter perception, since individual bitter substances frequently activate several TAS2Rs. Here, we provide an in-depth analysis of the genetic variability influencing human bitter taste. While each study subject carried a different set of genetic polymorphisms, we found that most variations reside in just six blocks, each harboring only one to five haplotypes. Thus, besides simple associations between taste and TAS2R gene polymorphisms, we revealed complex associations dependent on linkage between several high- and low-sensitivity alleles. Indeed, subjects carried either sensitive or insensitive alleles for receptors sensitive to grosheimin, a bitter compound in artichoke, or at least one sensitive allele for receptors specific for absinthin, the bitter principle of absinth. In short, simple associations and complex genomic linkage determine sensitivity to selected dietary bitter compounds.

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

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

The pharmacology of bitter taste receptors and their role in human airways

The receptors involved in bitter taste perception (bitter taste receptors--T2Rs) constitute a family of G-protein-coupled receptors, of which around 29 subtypes have been identified in humans. T2R expression was initially thought to be confined to the oral cavity but has recently been described in a range of other tissues (such as the heart, gut, nasal cavity and lungs) and cell types (chemosensory, smooth muscle, endothelial, epithelial and inflammatory cells). Although it is still not clear whether endogenous T2R agonists exist, the T2R receptors recognize many natural and synthetic compounds, such as the acyl-homoserine lactones produced by bacteria, caffeine, chloroquine, and erythromycin. In the upper airways, T2Rs are involved in neurogenic inflammation and bacterial clearance. Their known effects in the lungs are exerted at three different levels. Firstly, T2R agonists increase the beating frequency of cilia on epithelial cells. Secondly, the T2Rs induce bronchial smooth muscle cells to relax. Thirdly, the T2R receptors expressed on immune cells (such as macrophages and mast cells) modulate production of pro-inflammatory mediators. Furthermore, T2R agonists are effective in inhibiting lung inflammation or smooth muscle contraction in ex vivo and asthma animal models, and are known to be involved in bacterial killing in the nasal cavity and enhancing lung function in humans. This review focuses on the pharmacology and physiological functions of T2R receptors in the upper and lower airways. It presents recently acquired knowledge suggesting that T2Rs may become valuable drug targets in the treatment of diseases such as asthma and chronic rhinosinusitis.

Oligomerization of sweet and bitter taste receptors

The superfamily of G protein-coupled receptors (GPCRs) mediates numerous physiological processes, including neurotransmission, cell differentiation and metabolism, and sensory perception. In recent years, it became evident that these receptors might function not only as monomeric receptors but also as homo- or heteromeric receptor complexes. The family of TAS1R taste receptors are prominent examples of GPCR dimerization as they act as obligate functional heteromers: TAS1R1 and TAS1R3 combine to form an umami taste receptor, while the combination of TAS1R2 and TAS1R3 is a sweet taste receptor. So far, TAS2Rs, a second family of ~25 taste receptors in humans that mediates responses to bitter compounds, have been shown to function on their own, but if they do so as receptor monomers or as homomeric receptors still remains unknown. Using two different experimental approaches, we have recently shown that TAS2Rs can indeed form both homomeric and heteromeric receptor complexes. The employed techniques, coimmunoprecipitations and bioluminescence resonance energy transfer (BRET), are based on different principles and complement each other well and therefore provided compelling evidences for TAS2R oligomerization. Furthermore, we have adapted the protocols to include a number of controls and for higher throughput to accommodate the investigation of a large number of receptors and receptor combinations. Here, we present the protocols in detail.

Genetic variations in taste receptors are associated with chronic rhinosinusitis: a replication study

BACKGROUND

Recent evidence implicates polymorphisms of the bitter taste receptor TAS2R38 as defining characteristics in respiratory innate defense that may contribute to the complex genetic and environmental interactions predisposing to chronic rhinosinusitis (CRS). The purpose of this study was to (1) verify whether identified polymorphisms associated with respiratory infection in taste receptors replicate within our existing population of patients with CRS and (2) identify other taste receptors potentially associated with CRS.

METHODS

Pooling-based genomewide association studies (pGWAS) were previously performed on 2 populations of Canadian CRS patients (genetics of chronic rhinosinusitis 1, refractory CRS [GCRS1]; and genetics of chronic rhinosinusitis 2, CRS with nasal polyposis [GCRS2]) using the Illumina HumanHap 1-M chip. The pGWAS data were screened for polymorphisms in taste receptor genes. Single-nucleotide polymorphisms (SNPs) were considered replicated when the allele frequency differences were ≥10% in cases compared to controls.

RESULTS

The previously identified TAS2R38 coding SNP rs10246939 (I296V) was associated with CRS in both populations. The difference in allele frequency in cases compared to control subjects was 11% in GCRS1 and 15% in GCRS2. In addition, 3 previously undescribed missense variants were associated with CRS in our populations: 1 in the TAS2R13 gene (rs1015443), and the others in the TAS2R49 gene (rs12226920, rs12226919).

CONCLUSION

This study replicates previous work which showed that the coding SNP rs10246939 in the TAS2R38 gene is associated with CRS. Moreover, the results suggest that other taste receptors may be implicated in CRS. Further studies using individual genotyping and sequencing, and functional studies will provide more information about the implication of these genetic variants in CRS.

A novel human receptor involved in bitter tastant detection identified using Dictyostelium discoideum

Detection of substances tasting bitter to humans occurs in diverse organisms including the social amoeba Dictyostelium discoideum. To establish a molecular mechanism for bitter tastant detection in Dictyostelium, we screened a mutant library for resistance to a commonly used bitter standard, phenylthiourea. This approach identified a G-protein-coupled receptor mutant, grlJ(-), which showed a significantly increased tolerance to phenylthiourea in growth, survival and movement. This mutant was not resistant to a structurally dissimilar potent bitter tastant, denatonium benzoate, suggesting it is not a target for at least one other bitter tastant. Analysis of the cell-signalling pathway involved in the detection of phenylthiourea showed dependence upon heterotrimeric G protein and phosphatidylinositol 3-kinase activity, suggesting that this signalling pathway is responsible for the cellular effects of phenylthiourea. This is further supported by a phenylthiourea-dependent block in the transient cAMP-induced production of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) in wild-type but not grlJ(-) cells. Finally, we have identified an uncharacterized human protein γ-aminobutyric acid (GABA) type B receptor subunit 1 isoform with weak homology to GrlJ that restored grlJ(-) sensitivity to phenylthiourea in cell movement and PIP3 regulation. Our results thus identify a novel pathway for the detection of the standard bitter tastant phenylthiourea in Dictyostelium and implicate a poorly characterized human protein in phenylthiourea-dependent cell responses.

Extrasensory perception: odorant and taste receptors beyond the nose and mouth

G protein-coupled receptors (GPCRs) represent the largest family of transmembrane receptors and are prime therapeutic targets. The odorant and taste receptors account for over half of the GPCR repertoire, yet they are generally excluded from large-scale, drug candidate analyses. Accumulating molecular evidence indicates that the odorant and taste receptors are widely expressed throughout the body and functional beyond the oronasal cavity - with roles including nutrient sensing, autophagy, muscle regeneration, regulation of gut motility, protective airway reflexes, bronchodilation, and respiratory disease. Given this expanding array of actions, the restricted perception of these GPCRs as mere mediators of smell and taste is outdated. Moreover, delineation of the precise actions of odorant and taste GPCRs continues to be hampered by the relative paucity of selective and specific experimental tools, as well as the lack of defined receptor pharmacology. In this review, we summarize the evidence for expression and function of odorant and taste receptors in tissues beyond the nose and mouth, and we highlight their broad potential in physiology and pathophysiology.

Coarse-grained/molecular mechanics of the TAS2R38 bitter taste receptor: experimentally-validated detailed structural prediction of agonist binding

Bitter molecules in humans are detected by ∼25 G protein-coupled receptors (GPCRs). The lack of atomic resolution structure for any of them is complicating an in depth understanding of the molecular mechanisms underlying bitter taste perception. Here, we investigate the molecular determinants of the interaction of the TAS2R38 bitter taste receptor with its agonists phenylthiocarbamide (PTC) and propylthiouracil (PROP). We use the recently developed hybrid Molecular Mechanics/Coarse Grained (MM/CG) method tailored specifically for GPCRs. The method, through an extensive exploration of the conformational space in the binding pocket, allows the identification of several residues important for agonist binding that would have been very difficult to capture from the standard bioinformatics/docking approach. Our calculations suggest that both agonists bind to Asn103, Phe197, Phe264 and Trp201, whilst they do not interact with the so-called extra cellular loop 2, involved in cis-retinal binding in the GPCR rhodopsin. These predictions are consistent with data sets based on more than 20 site-directed mutagenesis and functional calcium imaging experiments of TAS2R38. The method could be readily used for other GPCRs for which experimental information is currently lacking.

Homology model-assisted elucidation of binding sites in GPCRs

G protein-coupled receptors (GPCRs) are important mediators of cell signaling and a major family of drug targets. Despite recent breakthroughs, experimental elucidation of GPCR structures remains a formidable challenge. Homology modeling of 3D structures of GPCRs provides a practical tool for elucidating the structural determinants governing the interactions of these important receptors with their ligands. The working model of the binding site can then be used for virtual screening of additional ligands that may fit this site, for determining and comparing specificity profiles of related receptors, and for structure-based design of agonists and antagonists. The current review presents the protocol and enumerates the steps for modeling and validating the residues involved in ligand binding. The main stages include (a) modeling the receptor structure using an automated fragment-based approach, (b) predicting potential binding pockets, (c) docking known binders, (d) analyzing predicted interactions and comparing with positions that have been shown to bind ligands in other receptors, (e) validating the structural model by mutagenesis.

Immunohistochemical detection of TAS2R38 protein in human taste cells

The sense of taste plays an important role in the evaluation of the nutrient composition of consumed food. Bitter taste in particular is believed to serve a warning function against the ingestion of poisonous substances. In the past years enormous progress was made in the characterization of bitter taste receptors, including their gene expression patterns, pharmacological features and presumed physiological roles in gustatory as well as in non-gustatory tissues. However, due to a lack in TAS2R-specifc antibodies the localization of receptor proteins within gustatory tissues has never been analyzed. In the present study we have screened a panel of commercially available antisera raised against human bitter taste receptors by immunocytochemical experiments. One of these antisera was found to be highly specific for the human bitter taste receptor TAS2R38. We further demonstrate that this antibody is able to detect heterologously expressed TAS2R38 protein on Western blots. The antiserum is, however, not able to interfere significantly with TAS2R38 function in cell based calcium imaging analyses. Most importantly, we were able to demonstrate the presence of TAS2R38 protein in human gustatory papillae. Using double immunofluorescence we show that TAS2R38-positive cells form a subpopulation of PLCbeta2 expressing cells. On a subcellular level the localization of this bitter taste receptor is neither restricted to the cell surface nor particularly enriched at the level of the microvilli protruding into the pore region of the taste buds, but rather evenly distributed over the entire cell body.

Binding of PF2 lectin from Olneya tesota to gut proteins of Zabrotes subfasciatus larvae associated with the insecticidal mechanism

Zabrotes subfasciatus (Boheman) is the main pest of common beans ( Phaselous vulgaris ). Wild legume seeds from Olneya tesota contain a lectin, PF2, that shows insecticidal activities against this insect. The binding of PF2 to midgut glycoproteins of 20-day-old larvae was evaluated using PF2 affinity chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the proteins retained on the gel revealed several putative glycoproteins, ranging in mass from 17 to 97 kDa. Subsequent protein digestion and analysis by liquid chromatography-tandem mass spectrometry (LC-MS/MS) provided amino acid fragments that identified an α-tubulin, cytochrome c oxidase subunit I, an odorant receptor, and a lysozyme from available insect sequence databases. The potential of these proteins to serve as part of the mechanisms involved in the insecticidal activity of PF2 to Z. subfasciatus is discussed.

Functional characterization of an allatotropin receptor expressed in the corpora allata of mosquitoes

Allatotropin is an insect neuropeptide with pleiotropic actions on a variety of different tissues. In the present work we describe the identification, cloning and functional and molecular characterization of an Aedes aegypti allatotropin receptor (AeATr) and provide a detailed quantitative study of the expression of the AeATr gene in the adult mosquito. Analysis of the tissue distribution of AeATr mRNA in adult female revealed high transcript levels in the nervous system (brain, abdominal, thoracic and ventral ganglia), corpora allata-corpora cardiaca complex and ovary. The receptor is also expressed in heart, hindgut and male testis and accessory glands. Separation of the corpora allata (CA) and corpora cardiaca followed by analysis of gene expression in the isolated glands revealed expression of the AeATr primarily in the CA. In the female CA, the AeATr mRNA levels were low in the early pupae, started increasing 6h before adult eclosion and reached a maximum 24h after female emergence. Blood feeding resulted in a decrease in transcript levels. The pattern of changes of AeATr mRNA resembles the changes in JH biosynthesis. Fluorometric Imaging Plate Reader recordings of calcium transients in HEK293 cells expressing the AeATr showed a selective response to A. aegypti allatotropin stimulation in the low nanomolar concentration range. Our studies suggest that the AeATr play a role in the regulation of JH synthesis in mosquitoes.

G protein-coupled receptors in human fat taste perception

In contrast to carbohydrates and proteins, which are detected by specialized taste receptors in the forms of their respective building blocks, sugars, and L-amino acids, the third macronutrient, lipids, has until now not been associated with gustatory receptors. Instead, the recognition of fat stimuli was believed to rely mostly on textural, olfactory, and postingestive cues. During the recent years, however, research done mainly in rodent models revealed an additional gustatory component for the detection of long-chain fatty acids (LCFAs), the main taste-activating component of lipids. Concomitantly, a number of candidate fat taste receptors were proposed to be involved in rodent's gustatory fatty acid perception. Compared with rodent models, much less is known about human fat taste. In order to investigate the ability of the human gustatory system to respond to fat components, we performed sensory experiments with fatty acids of different chain lengths and derivatives thereof. We found that our panelists discriminated a "fatty" and an irritant "scratchy" taste component, with the "fatty" percept restricted to LCFAs. Using functional calcium-imaging experiments with the human orthologs of mouse candidate fat receptors belonging to the G protein-coupled receptor family, we correlated human sensory data with receptor properties characterized in vitro. We demonstrated that the pharmacological activation profile of human GPR40 and GPR120, 2 LCFA-specific receptors associated with gustatory fat perception in rodents, is inconsistent with the "scratchy" sensation of human subjects and more consistent with the percept described as "fatty." Expression analysis of GPR40 and GPR120 in human gustatory tissues revealed that, while the GPR40 gene is not expressed, GPR120 is detected in gustatory and nongustatory epithelia. On a cellular level, we found GPR120 mRNA and protein in taste buds as well as in the surrounding epithelial cells. We conclude that GPR120 may indeed participate in human gustatory fatty acid perception.

Plant-insect interactions: what can we learn from plant lectins?

Many plant lectins have high anti-insect potential. Although the effects of most lectins are only moderately influencing development or population growth of the insect, some lectins have strong insecticidal properties. In addition, some studies report a deterrent activity towards feeding and oviposition behavior. Transmission of plant lectins to the next trophic level has been investigated for several tritrophic interactions. Effects of lectins with different sugar specificities can vary substantially with the insect species under investigation and with the experimental setup. Lectin binding in the insect is an essential step in exerting a toxic effect. Attempts have been made to study the interactions of lectins in several insect tissues and to identify lectin-binding receptors. Ingested lectins generally bind to parts of the insect gut. Furthermore, some lectins such as the Galanthus nivalus agglutinin (GNA) cross the gut epithelium into the hemolymph and other tissues. Recently, several candidate lectin-binding receptors have been isolated from midgut extracts. To date little is known about the exact mechanism for insecticidal activity of plant lectins. However, insect glycobiology is an emerging research field and the recent technological advances in the analysis of lectin carbohydrate specificities and insect glycobiology will certainly lead to new insights in the interactions between plant lectins and insects, and to a better understanding of the molecular mechanisms involved.

Glycoconjugate residues in a subpopulation of feline taste cells

There have been a number of studies which have categorized cells of feline taste buds: Types I, II, III and IV; however, few studies have examined whether feline taste bud cell types differ from each other histochemically. The goal of the present study is to figure out what kinds of glycoconjugates correspond to the four different types of cells in the taste bud. We have detected glycochains by lectin histochemistry. We have also identified Types II and III by immunohistochemistry. Then, we combined lectin histochemistry and immunohistochemistry to determine which types of cells have which glycochains. In addition, we have compared these reactions in different papillae in the oral cavity: circumvallate papillae, fungiform papillae and epiglottises. Our results demonstrated that glycoconjugates showed a variety of distributions among cells in these papillae, although immunopositive reactions of the proteins involved in the taste transduction showed similar distributions in the taste buds in these papillae. Amongst all, N-acetyllactosamine was the most prominently detected glycoconjugate residue in a subpopulation of Type II (receptor) cells and Type III (pre-synaptic) cells. Our findings suggest that 1) Different localization of glycol-residues in taste buds might be owing to the possibility that different types of cells need different types of glycoconjugates, possibly for the function of cells in the taste buds, and 2) N-acetyllactosamine might play some roles in taste sensation perception and their transfer by Type II and III cells.

Role of asparagine-linked glycosylation in cell surface expression and function of the human adrenocorticotropin receptor (melanocortin 2 receptor) in 293/FRT cells

Asparagine-linked glycosylation (N-glycosylation) of G protein-coupled receptors may be necessary for functions ranging from agonist binding, folding, maturation, stability, and internalization. Human melanocortin 2 receptor (MC2R) possesses putative N-glycosylation sites in its N-terminal extracellular domain; however, to date, the role of MC2R N-glycosylation has yet to be investigated. The objective of the present study is to examine whether N-glycosylation is essential or not for cell surface expression and cAMP production in native and MC2R accessory protein (MRAP alpha, -beta, or -dCT)-expressing cells using 293/FRT transfected with Myc-MC2R. Western blot analyses performed with or without endoglycosidase H, peptide:N-glycosidase F or tunicamycin treatments and site-directed mutagenesis revealed that MC2R was glycosylated in the N-terminal domain at its two putative N-glycosylation sites (Asn(12)-Asn(13)-Thr(14) and Asn(17)-Asn(18)-Ser(19)). In the absence of human MRAP coexpression, N-glycosylation of at least one of the two sites was necessary for MC2R cell surface expression. However, when MRAP was present, cell surface expression of MC2R mutants was either rescued entirely with the N17-18Q (QQNN) and N12-13Q (NNQQ) mutants or partially with the unglycosylated N12-13, 17-18Q (QQQQ) mutant. Functional and expression analyses revealed a discrepancy between wild-type (WT) and QQQQ cell surface receptor levels and maximal cAMP production with a 4-fold increase in EC(50) values. Taken together, these results indicate that the absence of MC2R N-glycosylation abrogates to a large extent MC2R cell surface expression in the absence of MRAPs, whereas when MC2R is N-glycosylated, it can be expressed at the plasma membrane without MRAP assistance.

Bitter taste receptors and their cells

The molecular basis of human bitter taste perception is an area of intense research. Only 25 G protein-coupled receptors belonging to the hTAS2R gene family face the challenge to detect thousands of structurally different bitter compounds, most of which are plant metabolites. Since many natural bitter compounds are highly toxic, whereas others are part of our daily diets, bitter taste was crucial during evolution and still most likely affects our food selection. The article presented here addresses biosynthesis, functional analyses of TAS2Rs and TAS2R variants, as well as gustatory expression of hTAS2R genes.