Activation Of Bitter Taste Receptors Research Articles

Denatonium enhanced the tone of denuded rat aorta via bitter taste receptor and phosphodiesterase activation

Bitter taste receptors (Tas2rs) initiate a bitter taste signaling involving the activation of taste-specific G protein gustducin and phosphodiesterases (PDEs); it leads to the decrease of cytosolic level of cyclic adenosine monophosphate (cAMP) in taste cells. Recent studies have identified the expression of Tas2rs in a variety of non-lingual tissues including vascular smooth muscle (VSM), pulmonary smooth muscle and airway smooth muscle. The current study aims to determine the expression of Tas2rs and gustducin in rat aortic smooth muscle tissue and to investigate the effect of Tas2rs agonist denatonium on the tone of isolated denuded aorta rings. Here we reported the expression of six subtypes of Tas2r mRNA and the taste receptor-associated G proteins in endothelium-denuded aorta. Immunostaining experiments showed that the protein of gustducin expressed in vascular smooth muscle cells (VSMCs). Furthermore, denatonium increased the tone of freshly isolated denuded aorta rings in a concentration-dependent manner, and the potentiation effect of denatonium was blocked by a Tas2rs antagonist adenosine 5′-monophosphate (5′-AMP), by the cAMP-hydrolyzing PDE inhibitors, and by a cAMP-synthesizing enzyme activator forskolin, respectively. The blockade of Gβγ signaling did not have a negative impact on the denatonium-induced tonic contractions. These findings suggested that the functional Tas2rs and gustducin are expressed in rat aortic smooth muscle and that denatonium might increase the smooth muscle tone through a Tas2rs signaling pathway involving the activation of PDEs.

Intragastric administration of the bitter tastant quinine lowers the glycemic response to a nutrient drink without slowing gastric emptying in healthy men.

The rate of gastric emptying and the release of gastrointestinal (GI) hormones are major determinants of postprandial blood-glucose concentrations and energy intake. Preclinical studies suggest that activation of GI bitter-taste receptors potently stimulates GI hormones, including glucagon-like peptide-1 (GLP-1), and thus may reduce postprandial glucose and energy intake. We evaluated the effects of intragastric quinine on the glycemic response to, and the gastric emptying of, a mixed-nutrient drink and the effects on subsequent energy intake in healthy men. The study consisted of 2 parts: part A included 15 lean men, and part B included 12 lean men (aged 26 ± 2 yr). In each part, participants received, on 3 separate occasions, in double-blind, randomized fashion, intragastric quinine (275 or 600 mg) or control, 30 min before a mixed-nutrient drink (part A) or before a buffet meal (part B). In part A, plasma glucose, insulin, glucagon, and GLP-1 concentrations were measured at baseline, after quinine alone, and for 2 h following the drink. Gastric emptying of the drink was also measured. In part B, energy intake at the buffet meal was quantified. Quinine in 600 mg (Q600) and 275 mg (Q275) doses alone stimulated insulin modestly (P < 0.05). After the drink, Q600 and Q275 reduced plasma glucose and stimulated insulin (P < 0.05), Q275 stimulated GLP-1 (P < 0.05), and Q600 tended to stimulate GLP-1 (P = 0.066) and glucagon (P = 0.073) compared with control. Quinine did not affect gastric emptying of the drink or energy intake. In conclusion, in healthy men, intragastric quinine reduces postprandial blood glucose and stimulates insulin and GLP-1 but does not slow gastric emptying or reduce energy intake under our experimental conditions.

Reduced Salivary Mucin Binding and Glycosylation in Older Adults Influences Taste in an In Vitro Cell Model.

Background: Taste loss is a significant problem in older adults, affecting quality of life and nutrition. Altered salivary rheology and loss of mucin function may contribute to taste loss by reducing mucosal defences in the oral cavity, impairing sensitivity to oral stimulants. This study aimed to investigate the effects of salivary rheology on taste loss in ageing. Salivary mucin glycosylation and binding to the oral epithelium was investigated in older and younger adults. A cell-based model was utilised to consider the role of saliva in taste loss. Methods: Human subjects aged >60 years (n = 25) and 18–30 (n = 30) provided saliva samples which were analysed for viscosity, mucin composition and mucin binding to oral epithelial cells (TR146/MUC1). Oral epithelial cells (TR146/MUC1 and SCC090) provided models for taste receptor activation. Results: Reduced levels and sialylation of MUC7 were evident in saliva of older adults which may lead to reduced viscoelasticity, while viscosity is unaffected. Impaired muco-adhesion of saliva from older adults was also observed. Saliva from older adults facilitated the bitter taste receptor activation less well than saliva from younger adults. The causes of taste dysfunction in older adults are unknown, but this study supports a role of saliva in facilitating the activation of taste receptors.

Denatonium Benzoate-Induces Oxidative Stress in the Heart and Kidney of Chinese Fast Yellow Chickens by Regulating Apoptosis, Autophagy, Antioxidative Activities and Bitter Taste Receptor Gene Expressions.

Simple SummaryDenatonium benzoate is a strong bitter taste receptor agonist, extensively used for its activation of different cell pathways. Taste signals have been associated to food recognition and avoidance, and bitter taste provokes an aversive reaction and is assumed to protect chickens from consuming poisons and harmful toxic substances. The results of the study revealed that dietary supplementation with medium and high doses of denatonium benzoate damaged the epithelial cells of the heart and kidneys by inducing apoptosis and autophagy and reduced the growth of chickens, respectively. However, mRNA expressions of bitter taste receptors, downstream signaling effector genes, apoptosis-, autophagy- and antioxidant-related genes were higher on day 7, while these expressions were subsequently decreased on day-28 in the heart and kidney of Chinese Fast Yellow chickens in a dose-response manner.The sense of taste which tells us which prospective foods are nutritious, poisonous and harmful is essential for the life of the organisms. Denatonium benzoate (DB) is a bitter taste agonist known for its activation of bitter taste receptors in different cells. The aim of the current study was to investigate the mRNA expressions of bitter taste, downstream signaling effectors, apoptosis-, autophagy- and antioxidant-related genes and effector signaling pathways in the heart/kidney of chickens after DB dietary exposure. We randomly assigned 240, 1-day-old Chinese Fast Yellow chicks into four groups with five replicates of 12 chicks and studied them for 28 consecutive days. The dietary treatments consisted of basal diet and feed containing DB (5, 20 and 100 mg/kg). The results revealed that dietary DB impaired (p < 0.05) the growth performance of the chickens. Haemotoxylin and eosin staining and TUNEL assays confirmed that medium and high doses of DB damaged the epithelial cells of heart/kidney and induced apoptosis and autophagy. Remarkably, the results of RT-PCR and qRT-PCR indicated that different doses of DB gradually increased (p < 0.05) mRNA expressions of bitter taste, signaling effectors, apoptosis-, autophagy- and antioxidant- related genes on day 7 in a dose-response manner, while, these expressions were decreased (p < 0.05) subsequently by day-28 but exceptional higher (P < 0.05) expressions were observed in the high-dose DB groups of chickens. In conclusion, DB exerts adverse effects on the heart/kidney of chickens in a dose-response manner via damaging the epithelium of the heart/kidney by inducing apoptosis, autophagy associated with bitter taste and effector gene expressions. Correlation analyses for apoptosis/autophagy showed agonistic relationships. Our data provide a novel perspective for understanding the interaction of bitter taste, apoptosis, autophagy and antioxidative genes with bitter taste strong activators in the heart/kidney of chicken. These insights might help the feed industries and pave the way toward innovative directions in chicken husbandry.

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

ObjectivesExtracts of the hops plant have been shown to reduce weight and insulin resistance in rodents and humans, but elucidation of the mechanisms responsible for these benefits has been hindered by the use of heterogeneous hops-derived mixtures. Because hop extracts are used as flavoring agents for their bitter properties, we hypothesized that bitter taste receptors (Tas2rs) could be mediating their beneficial effects in metabolic disease. Studies have shown that exposure of cultured enteroendocrine cells to bitter tastants can stimulate release of hormones, including glucagon-like peptide 1 (GLP-1). These findings have led to the suggestion that activation of Tas2rs may be of benefit in diabetes, but this tenet has not been tested. Here, we have assessed the ability of a pure derivative of a hops isohumulone with anti-diabetic properties, KDT501, to signal through Tas2rs. We have further used this compound as a tool to systematically assess the impact of bitter taste receptor activation in obesity-diabetes. MethodsKDT501 was tested in a panel of bitter taste receptor signaling assays. Diet-induced obese mice (DIO) were dosed orally with KDT501 and acute effects on glucose homeostasis determined. A wide range of metabolic parameters were evaluated in DIO mice chronically treated with KDT501 to establish the full impact of activating gut bitter taste signaling. ResultsWe show that KDT501 signals through Tas2r108, one of 35 mouse Tas2rs. In DIO mice, acute treatment stimulated GLP-1 secretion and enhanced glucose tolerance. Chronic treatment caused weight and fat mass loss, increased energy expenditure, enhanced glucose tolerance and insulin sensitivity, normalized plasma lipids, and induced broad suppression of inflammatory markers. Chronic KDT501 treatment altered enteroendocrine hormone levels and bile acid homeostasis and stimulated sustained GLP-1 release. Combined treatment with a dipeptidyl peptidase IV inhibitor amplified the incretin-based benefits of this pure isohumulone. ConclusionsActivation of Tas2r108 in the gut results in a remodeling of enteroendocrine hormone release and bile acid metabolism that ameliorates multiple features of metabolic syndrome. Targeting extraoral bitter taste receptors may be useful in metabolic disease.

Berberine activates bitter taste responses of enteroendocrine STC-1 cells.

Glucagon-like peptide-1 (GLP-1) is involved in the regulation of insulin secretion and glucose homeostasis. GLP-1 release is stimulated when berberine interacts with a novel G protein family (TAS2Rs) in enteroendocrine cells. In this study, we used STC-1 cells and examined a marked increase in Ca2+ in response to various bitter compounds. Ca2+ responses to traditional Chinese medicine extracts, including berberine, phellodendrine and coptisine, in STC-1 cells were suppressed by the phospholipase C (PLC) inhibitor U-73122, suggesting the involvement of bitter taste receptors in changing the physiological status of enteroendocrine cells in a PLC-dependent manner. STC-1 cells showed berberine-up-regulated preproglucagon (GLP-1 precursor) mRNA and GLP-1 secretion. A QPCR analysis demonstrated that TAS2R38, a subtype of the bitter taste receptor, was associated with GLP-1 secretion. Berberine-mediated GLP-1 secretion was attenuated in response to small interfering RNA silencing of TAS2R38. The current studies demonstrated that Gα-gustducin co-localized with GLP-1 and Tas2r106 in the STC-1 cells. We further utilized inhibitors of PLC and TRPM5, which are known to participate in taste signal transduction, to investigate the underlying pathways mediated in berberine-induced GLP-1 secretion. Berberine-induced GLP-1 release from enteroendocrine cells is modulated in a PLC-dependent manner through a process involving the activation of bitter taste receptors. Together, our data demonstrated a berberine-mediated GLP-1 secretion pathway in mouse enteroendocrine cells that could be of therapeutic relevance to hyperglycemia and the role of bitter taste receptors in the function of the small intestine.

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.

Activation of Bitter Taste Receptors Has Distinct Modulatory Effects on Different Lung Afferent Signaling in Rats

We have reported recently that activation of bitter taste receptors (T2Rs) in lung nociceptors significantly augments capsaicin‐evoked TRPV1 responses, likely through PLC and PKC intracellular signaling pathway. Using whole‐cell patch‐clamp recordings, the present study was carried out to investigate how T2Rs activation may affect lung sensory neurons responding to different airway stimulations. Three airway irritants that are known to stimulate different pulmonary sensory receptors are selected: zinc, an agonist for TRPA1; low pH or acid, an activator for both acid‐sensing ion channels (ASICs) and TRPV1; and ATP, an agonist for P2X purinoceptors. Our results showed that pretreatment with T2Rs activator chloroquine (0.01, 0.1 and 1 mM, 90 s) concentration‐dependently potentiated zinc (30 μM, 3–16 s)‐evoked TRPA1 currents, and markedly inhibited ATP (0.3 or 1 μM, 4–6 s)‐evoked P2X currents, whereas affected acid (pH 5.5 and pH 6.5, 4–16 s)‐evoked inward currents differently: an inhibition for the slow inactivation ASIC‐like current, and a potentiation for the TRPV1‐like current as well as the fast‐activation and fast‐inactivation ASIC current. Surprisingly, our results also showed that none of the above modulations was completely abolished by the inhibition of PLC, PKC or PKA transduction pathway. In summary, our results demonstrated that T2Rs activation in lung afferents has distinct regulatory effects on various ion channels that are sensitive to different airway irritants.Support or Funding InformationSupported by funds from Navicent Health Foundation and MUSM

Activation of bitter taste receptors in pulmonary nociceptors sensitizes TRPV1 channels through the PLC and PKC signaling pathway.

Bitter taste receptors (T2Rs), a G protein-coupled receptor family capable of detecting numerous bitter-tasting compounds, have recently been shown to be expressed and play diverse roles in many extraoral tissues. Here we report the functional expression of T2Rs in rat pulmonary sensory neurons. In anesthetized spontaneously breathing rats, intratracheal instillation of T2R agonist chloroquine (10 mM, 0.1 ml) significantly augmented chemoreflexes evoked by right-atrial injection of capsaicin, a specific activator for transient receptor potential vanilloid receptor 1 (TRPV1), whereas intravenous infusion of chloroquine failed to significantly affect capsaicin-evoked reflexes. In patch-clamp recordings with isolated rat vagal pulmonary sensory neurons, pretreatment with chloroquine (1-1,000 µM, 90 s) concentration dependently potentiated capsaicin-induced TRPV1-mediated inward currents. Preincubating with diphenitol and denatonium (1 mM, 90 s), two other T2R activators, also enhanced capsaicin currents in these neurons but to a lesser extent. The sensitizing effect of chloroquine was effectively prevented by the phospholipase C inhibitor U73122 (1 µM) or by the protein kinase C inhibitor chelerythrine (10 µM). In summary, our study showed that activation of T2Rs augments capsaicin-evoked TRPV1 responses in rat pulmonary nociceptors through the phospholipase C and protein kinase C signaling pathway.

Activation of human bitter taste receptors by polymethoxylated flavonoids.

Tangeretin and nobiletin are polymethoxylated flavonoids in citrus peel. Both tangeretin and nobiletin are bitter; however, their bitterness has not been evaluated using human bitter taste receptors (hTAS2Rs). We screened 25 kinds of hTAS2Rs and found that hTAS2R14 and hTAS2R46 received both compounds.

Activation of bitter taste receptors (tas2rs) relaxes detrusor smooth muscle and suppresses overactive bladder symptoms.

Bitter taste receptors (TAS2Rs) are traditionally thought to be expressed exclusively on the taste buds of the tongue. However, accumulating evidence has indicated that this receptor family performs non-gustatory functions outside the mouth in addition to taste. Here, we examined the role of TAS2Rs in human and mouse detrusor smooth muscle (DSM). We showed that mRNA for various TAS2R subtypes was expressed in both human and mouse detrusor smooth muscle (DSM) at distinct levels. Chloroquine (CLQ), an agonist for TAS2Rs, concentration-dependently relaxed carbachol- and KCl-induced contractions of human DSM strips. Moreover, 100 μM of CLQ significantly inhibited spontaneous and electrical field stimulation (EFS)-induced contractions of human DSM strips. After a slight contraction, CLQ (1 mM) entirely relaxed carbachol-induced contraction of mouse DSM strips. Furthermore, denatonium and quinine concentration-dependently decreased carbachol-induced contractions of mouse DSM strips. Finally, we demonstrated that CLQ treatment significantly suppressed the overactive bladder (OAB) symptoms of mice with partial bladder outlet obstruction (PBOO). In conclusion, we for the first time provide evidence of the existence of TAS2Rs in the urinary DSM and demonstrate that TAS2Rs may represent a potential target for OAB. These findings open a new approach to develop drugs for OAB in the future.

Berberine induces GLP-1 secretion through activation of bitter taste receptor pathways

Our previous studies revealed that berberine-mediated GLP-1 secretion was a possible mechanism for berberine exerting good effects on hyperglycemia. This study was designed to ascertain whether berberine-induced secretion of GLP-1 was related with activation of bitter taste receptors expressed in gastrointestinal tract. Western blotting results showed that TAS2R38, a subtype of bitter taste receptor, was expressed on human enteroendocrine NCI-H716 cells. GLP-1 secretion induced by berberine from NCI-H716 cells was inhibited by incubation with anti-TAS2R38 antibody. We further performed gene silencing using siRNA to knockdown TAS2R38 from NCI-H716 cells, which showed that siRNA knockdown of the TAS2R38 reduced berberine-mediated GLP-1 secretion. We adopted inhibitors of PLC and TRPM5 known to be involved in bitter taste transduction to investigate the underlying pathways mediated in berberine-induced GLP-1 secretion. It was found that PLC inhibitor U73122 inhibited berberine-induced GLP-1 release in NCI-H716 cells, while TRPM5 blocker quinine failed to attenuate berberine-induced secretion of GLP-1. The present results demonstrated that berberine stimulated GLP-1 secretion via activation of gut-expressed bitter taste receptors in a PLC-dependent manner. Because berberine was found to be a ligand of bitter taste receptor, the results of present study may provide an explanation for some bitter taste substance obtain hypoglycemic effect.

The effect of MP29‐02* is mediated via bitter taste receptors (TAS2R)

Background Dymista (MP29-02*) is a novel intranasal formulation of azelastine hydrochloride (AZE) and fluticasone propionate (FP) in an advanced delivery system with a welldocumented effect on allergic rhinitis. Its somewhat bitter taste is probably derived from AZE, a potent histamineH1-receptor antagonist. The taste is sometimes looked upon as a disadvantage. However, it might be that AZE induces some of its beneficial effect through activation of bitter taste receptors. Recent evidence indicates that bitter taste receptors are present in human upper airway mucosa in regions associated with high airflow and particulate deposition. Consequently the MP29-02* effects might be due not only to its anti-histaminic, mast-cell stabilizing, anti-leukotriene, and anti-inflammatory properties but also as due to activation of bitter taste receptors.

Salicin from Willow Bark can Modulate Neurite Outgrowth in Human Neuroblastoma SH-SY5Y Cells.

Salicin from willow bark has been used throughout centuries in China and Europe for the treatment of pain, headache, and inflammatory conditions. Recently, it could be demonstrated that salicin binds and activates the bitter taste receptor TAS2R16. Studies on rodent tissues showed the general expression of bitter taste receptors (TAS2Rs) in rodent brain. Here, we demonstrate the expression of hTAS2R16 in human neuronal tissues and the neuroblastoma cell line SH-SY5Y. The functionality was analyzed in the neuroblastoma cell line SH-SY5Y after stimulation with salicin, a known TAS2R16 agonist. In this setting salicin induced in SH-SY5Y cells phosphorylation of ERK and CREB, the key transcription factor of neuronal differentiation. PD98059, an inhibitor of the ERK pathway, as well as probenecid, a TAS2R16 antagonist, inhibited receptor phosphorylation as well as neurite outgrowth. These data show that salicin might modulate neurite outgrowth by bitter taste receptor activation.

Mouth rinsing and ingestion of a bitter-tasting solution increases corticomotor excitability in male competitive cyclists.

Recently, we have shown that the combination of mouth rinsing and ingesting a bitter-tasting quinine solution immediately prior to the performance of a maximal 30-s cycling sprint significantly improves mean and peak power output. This ergogenic effect was proposed to be related to the activation of the corticomotor pathway by afferent taste signals originating from bitter taste receptors in the oral cavity. The aim of the present study was to use single-pulse transcranial magnetic stimulation to investigate whether mouth rinsing and ingestion of a bitter quinine solution increases corticomotor excitability. A series of 10 motor-evoked potentials (MEPs) were recorded from the relaxed first dorsal interosseus muscle in 16 male competitive cyclists immediately before and after they rinsed their mouth for 10 s and then ingested either a 2 mM bitter quinine solution or plain water. Mean MEP amplitude was significantly increased in response to quinine administration by 16% (p < 0.05), with no evidence of a time-dependent effect over the 10 pulses. Mean MEP amplitude also increased by 10% in response to water administration (p < 0.05), though this increase was significantly smaller than the response to quinine (p < 0.05). We conclude that the activation of bitter taste receptors in the oral cavity and upper gastrointestinal tract has the capacity to increase corticomotor excitability in male competitive cyclists.

Dextromethorphan mediated bitter taste receptor activation in the pulmonary circuit causes vasoconstriction.

Activation of bitter taste receptors (T2Rs) in human airway smooth muscle cells leads to muscle relaxation and bronchodilation. This finding led to our hypothesis that T2Rs are expressed in human pulmonary artery smooth muscle cells and might be involved in regulating the vascular tone. RT-PCR was performed to reveal the expression of T2Rs in human pulmonary artery smooth muscle cells. Of the 25 T2Rs, 21 were expressed in these cells. Functional characterization was done by calcium imaging after stimulating the cells with different bitter agonists. Increased calcium responses were observed with most of the agonists, the largest increase seen for dextromethorphan. Previously in site-directed mutational studies, we have characterized the response of T2R1 to dextromethorphan, therefore, T2R1 was selected for further analysis in this study. Knockdown with T2R1 specific shRNA decreased mRNA levels, protein levels and dextromethorphan-induced calcium responses in pulmonary artery smooth muscle cells by up to 50%. To analyze if T2Rs are involved in regulating the pulmonary vascular tone, ex vivo studies using pulmonary arterial and airway rings were pursued. Myographic studies using porcine pulmonary arterial and airway rings showed that stimulation with dextromethorphan led to contraction of the pulmonary arterial and relaxation of the airway rings. This study shows that dextromethorphan, acting through T2R1, causes vasoconstrictor responses in the pulmonary circuit and relaxation in the airways.

Denatonium induces secretion of glucagon-like peptide-1 through activation of bitter taste receptor pathways.

This study was designed to ascertain whether human enteroendocrine cells express bitter taste receptors, and whether activation of these receptors with bitter-tasting ligands induces secretion of glucagon-like peptide-1 (GLP-1) and peptide YY (PYY). We used human enteroendocrine NCI-H716 cells, isolated duodenal segments from mice, and whole mice as our experimental systems for investigating stimuli and mechanisms underlying GLP-1- and PYY-stimulated release. We measured hormone levels by ELISA and determined bitter taste receptor expression by real-time quantitative PCR. We adopted a pharmacological approach using inhibitors and enhancers of downstream signalling pathways known to be involved in bitter taste transduction in taste bud cells to investigate these pathways in NCI-H716 cells. Using a pharmacological approach, we identified signalling pathways triggered by the denatonium benzoate (DB)-activated bitter receptors. This involved activation of α-gustducin (Gαgust)-the specific G-protein subunit that is also present in taste bud cells-reduction of intracellular cAMP levels and enhancement of phospholipase C (PLC) activity, which ultimately led to increased intracellular calcium concentrations and hormone release. Gavage of DB, followed by gavage of glucose, to db/db mice stimulated GLP-1 and subsequent insulin secretion, leading to lower blood glucose levels. Our study demonstrates that activation of gut-expressed bitter taste receptors stimulates GLP-1 secretion in a PLC-dependent manner. In diabetic mice, DB (a ligand of bitter taste receptor cells), when given via gavage, lowers blood glucose levels in diabetic mice after oral glucose administration, through increased secretion of GLP-1.

Bitter Taste Receptor Activation by Flavonoids and Isoflavonoids: Modeled Structural Requirements for Activation of hTAS2R14 and hTAS2R39

Many flavonoids and isoflavonoids have an undesirable bitter taste, which hampers their use as food bioactives. The aim of this study was to investigate the effect of a large set of structurally similar (iso)flavonoids on the activation of bitter receptors hTAS2R14 and hTAS2R39 and to predict their structural requirements to activate these receptors. In total, 68 compounds activated hTAS2R14 and 70 compounds activated hTAS2R39, among which 58 ligands were overlapping. Their activation threshold values varied over a range of 3 log units between 0.12 and 500 μM. Ligand-based 2D-fingerprint and 3D-pharmacophore models were created to detect structure-activity relationships. The 2D models demonstrated excellent predictive power in identifying bitter (iso)flavonoids and discrimination from inactive ones. The structural characteristics for an (iso)flavonoid to activate hTAS2R14 (or hTAS2R39) were determined by 3D-pharmacophore models to be composed of two (or three) hydrogen bond donor sites, one hydrogen bond acceptor site, and two aromatic ring structures, of which one had to be hydrophobic. The additional hydrogen bond donor feature for hTAS2R39 ligands indicated the possible presence of another complementary acceptor site in the binding pocket, compared to hTAS2R14. Hydrophobic interaction of the aromatic feature with the binding site might be of higher importance in hTAS2R14 than in hTAS2R39. Together, this might explain why OH-rich compounds showed different behaviors on the two bitter receptors. The combination of in vitro data and different in silico methods created a good insight in activation of hTAS2R14 and hTAS2R39 by (iso)flavonoids and provided a powerful tool in the prediction of their potential bitterness. By understanding the "bitter motif", introduction of bitter taste in functional foods enriched in (iso)flavonoid bioactives might be avoided.

How Sweet It Is: Intestinal Sweet Taste Receptors in Type 2 Diabetes

In humans, there are a number of taste receptors on the tongue. These include G protein–coupled receptors (GPRs) for sweet, bitter, and umami (savory) tastes and ion channel–based receptors for salt and sour. In recent years, there has been increasing interest in the distribution and function of these receptors elsewhere in the body (1). This has led to interesting and occasionally surprising findings, some with potential therapeutic implications (e.g., activation of bitter taste receptors in the airway’s smooth muscle causes bronchodilation, suggesting a novel target for asthma therapy) (2). Of particular interest in the area of diabetes and obesity is the role of sweet taste receptors (STRs) in the intestine (3,4). The upper gastrointestinal tract is well-endowed with taste and fat receptors, with sweet taste being detected, as elsewhere, by a heterodimer of the taste 1 receptor (T1R) family, T1R2/T1R3. These receptors have been localized to intestinal brush and enteroendocrine cells, and are coupled with α-gustducin as the α-subunit of the G protein (Fig. 1). They recognize sugars, d-amino acids, sweet proteins, and artificial sweeteners. Of importance to a possible role in the incretin response, these receptors are colocalized with glucagon-like peptide 1 (GLP-1) and L cells containing peptide YY (PYY) and K cells containing glucose-dependent insulinotropic polypeptide (GIP) (3). Incidentally, fatty acid responsive GPRs are also coupled to GLP-1 release, but are found predominantly …

The third intracellular loop plays a critical role in bitter taste receptor activation

Bitter taste receptors (T2Rs) belong to the superfamily of G protein-coupled receptors (GPCRs). T2Rs are chemosensory receptors with important therapeutic potential. In humans, bitter taste is perceived by 25 T2Rs, which are distinct from the well-studied Class A GPCRs. The activation mechanism of T2Rs is poorly understood and none of the structure–function studies are focused on the role of the important third intracellular loop (ICL3). T2Rs have a unique signature sequence at the cytoplasmic end of fifth transmembrane helix (TM5), a highly conserved LxxSL motif. Here, we pursue an alanine scan mutagenesis of the ICL3 of T2R4 and characterize the functionality of 23 alanine mutants. We identify four mutants, H214A, Q216A, V234A and M237A, that exhibit constitutive activity. To our surprise, the H214A mutant showed very high constitutive activity over wild type T2R4. Interestingly, His214 is highly conserved (96%) in T2Rs and is present two amino acids below the LxxSL motif in TM5. Molecular modeling shows a dynamic network of interactions involving residues in TM5–ICL3–TM6 that restrain the movement of the helices. Changes in this network, as in the case of H214A, Q216A, V234A and M237A mutants, cause the receptor to adopt an active conformation. The conserved LxxSL motif in TM5 performs both structural and functional roles in this process. These results provide insight into the activation mechanism of T2Rs, and emphasize the unique functional role of ICL3 even within the GPCR subfamilies.