Activation Of Bitter Taste Receptors Research Articles

Effects Of Dextromethorphan Mediated Bitter Taste Receptor Activation In Pulmonary Artery Smooth Muscle Cells

Activation of bitter taste receptors (T2Rs) in human airway smooth muscle cells lead to muscle relaxation and bronchodilation. However, the presence of T2Rs in human pulmonary artery smooth muscle cells (hPASMCs) has not been established. The previous finding in airway smooth muscle led to our hypothesis that T2Rs, if present in hPASMCs, might be involved in regulating the vascular tone. RT‐PCR analysis revealed the expression of multiple T2R transcripts in hPASMCs. Functional analysis showed that these cells responded to many bitter‐tasting compounds, by increasing intracellular calcium concentration, indicating that T2Rs in hPASMCs are functional. Since the response of T2R1 to dextromethorphan (DXM) has been characterized in our previous studies, T2R1 was selected for further analysis in this study. Knockdown with T2R1‐specific shRNA decreased mRNA levels and DXM‐induced calcium responses by up to 40%. ELISA data obtained from in vitro studies using hPASMC showed that DXM‐treatment led to increased levels of endothelin‐1, biomarker of systemic hypertension. To analyze if T2R1 is involved in regulating the pulmonary vascular tone, ex vivo studies using pulmonary arterial rings are being pursued. Furthermore, it remains to be analyzed if this effect of DXM in hPASMCs is T2R1 specific or due to the blockade of NMDA receptors.Supported by MHRC, NSERC and a New Investigator Award from HSFC.

Different Phenolic Compounds Activate Distinct Human Bitter Taste Receptors

Bitterness is a major sensory attribute of several common foods and beverages rich in polyphenol compounds. These compounds are reported as very important for health as chemopreventive compounds, but they are also known to taste bitter. In this work, the activation of the human bitter taste receptors, TAS2Rs, by six polyphenol compounds was analyzed. The compounds chosen are present in a wide range of plant-derived foods and beverages, namely, red wine, beer, tea, and chocolate. Pentagalloylglucose (PGG) is a hydrolyzable tannin, (-)-epicatechin is a precursor of condensed tannins, procyanidin dimer B3 and trimer C2 belong to the condensed tannins, and malvidin-3-glucoside and cyanidin-3-glucoside are anthocyanins. The results show that the different compounds activate different combinations of the ~25 TAS2Rs. (-)-Epicatechin activated three receptors, TAS2R4, TAS2R5, and TAS2R39, whereas only two receptors, TAS2R5 and TAS2R39, responded to PGG. In contrast, malvidin-3-glucoside and procyanidin trimer stimulated only one receptor, TAS2R7 and TAS2R5, respectively. Notably, tannins are the first natural agonists found for TAS2R5 that display high potency only toward this receptor. The catechol and/or galloyl groups appear to be important structural determinants that mediate the interaction of these polyphenolic compounds with TAS2R5. Overall, the EC(50) values obtained for the different compounds vary 100-fold, with the lowest values for PGG and malvidin-3-glucoside compounds, suggesting that they could be significant polyphenols responsible for the bitterness of fruits, vegetables, and derived products even if they are present in very low concentrations.

Constitutively active mutant gives novel insights into the mechanism of bitter taste receptor activation

The human bitter taste receptors (T2Rs) belong to the G-protein coupled receptor (GPCR) superfamily. T2Rs share little homology with the large subfamily of Class A G-protein coupled receptors, and their mechanisms of activation are poorly understood. Guided by biochemical and molecular approaches, we identified two conserved amino acids Gly28¹·⁴⁶ and Ser285⁷·⁴⁷ present on transmembrane (TM) helices, TM1 and TM7, which might play important roles in T2R activation. Previously, it was shown that naturally occurring Gly51¹·⁴⁶ mutations in the dim light receptor, rhodopsin, cause autosomal dominant retinitis pigmentosa, with the mutants severely defective in signal transduction. We mutated Gly28¹·⁴⁶ and Ser285⁷·⁴⁷ in T2R4 to G28A, G28L, S285A, S285T, and S285P, and carried out pharmacological characterization of the mutants. No major changes in signaling were observed upon mutation of Gly28¹·⁴⁶ in T2R4. Interestingly, S285A mutant displayed agonist-independent activity (approximately threefold over basal wild-type T2R4 or S285T or S285P). We propose that Ser285⁷·⁴⁷ stabilizes the inactive state of T2R4 by a network of hydrogen-bonds connecting important residues on TM1-TM2-TM7. We compare and contrast this hydrogen-bond network with that present in rhodopsin. Thus far, S285A is the first constitutively active T2R mutant reported, and gives novel insights into T2R activation.

Bitter taste receptors and α-gustducin regulate the secretion of ghrelin with functional effects on food intake and gastric emptying

Ghrelin is a hunger hormone with gastroprokinetic properties but the factors controlling ghrelin secretion from the stomach are unknown. Bitter taste receptors (T2R) and the gustatory G proteins, α-gustducin (gust) and α-transducin, are expressed in the gut and are involved in the chemosensation of nutrients. This study aimed to investigate whether T2R-agonists affect (i) ghrelin release via α-gustducin and (ii) food intake and gastric emptying via the release of ghrelin. The mouse stomach contains two ghrelin cell populations: cells containing octanoyl and desoctanoyl ghrelin, which were colocalized with α-gustducin and α-transducin, and cells staining for desoctanoyl ghrelin. Gavage of T2R-agonists increased plasma octanoyl ghrelin levels in WT mice but the effect was partially blunted in gust(-/-) mice. Intragastric administration of T2R-agonists increased food intake during the first 30 min in WT but not in gust(-/-) and ghrelin receptor knockout mice. This increase was accompanied by an increase in the mRNA expression of agouti-related peptide in the hypothalamus of WT but not of gust(-/-) mice. The temporary increase in food intake was followed by a prolonged decrease (next 4 h), which correlated with an inhibition of gastric emptying. The delay in emptying, which was partially counteracted by ghrelin, was not mediated by cholecystokinin and GLP-1 but involved a direct inhibitory effect of T2R-agonists on gastric contractility. This study is unique in providing functional evidence that activation of bitter taste receptors stimulates ghrelin secretion. Modulation of endogenous ghrelin levels by tastants may provide novel therapeutic applications for the treatment of weight -and gastrointestinal motility disorders.

Bitter treats for better breathing

A new study in mice and human tissue shows that activation of bitter taste receptors in the airways relaxes airway smooth muscle, resulting in bronchodilation (pages 1299–1304 ). Agonists of these receptors seem to be better than current β2-adrenergic bronchodilators in a mouse asthma model, suggesting a new way to ease breathing during asthma.

Insights into the Binding of Phenyltiocarbamide (PTC) Agonist to Its Target Human TAS2R38 Bitter Receptor

Humans' bitter taste perception is mediated by the hTAS2R subfamily of the G protein-coupled membrane receptors (GPCRs). Structural information on these receptors is currently limited. Here we identify residues involved in the binding of phenylthiocarbamide (PTC) and in receptor activation in one of the most widely studied hTAS2Rs (hTAS2R38) by means of structural bioinformatics and molecular docking. The predictions are validated by site-directed mutagenesis experiments that involve specific residues located in the putative binding site and trans-membrane (TM) helices 6 and 7 putatively involved in receptor activation. Based on our measurements, we suggest that (i) residue N103 participates actively in PTC binding, in line with previous computational studies. (ii) W99, M100 and S259 contribute to define the size and shape of the binding cavity. (iii) W99 and M100, along with F255 and V296, play a key role for receptor activation, providing insights on bitter taste receptor activation not emerging from the previously reported computational models.

Structural requirements of bitter taste receptor activation

An important question in taste research is how 25 receptors of the human TAS2R family detect thousands of structurally diverse compounds. An answer to this question may arise from the observation that TAS2Rs in general are broadly tuned to interact with numerous substances. Ultimately, interaction with chemically diverse agonists requires architectures of binding pockets tailored to combine flexibility with selectivity. The present study determines the structure of hTAS2R binding pockets. We focused on a subfamily of closely related hTAS2Rs exhibiting pronounced amino acid sequence identities but unique agonist activation spectra. The generation of chimeric and mutant receptors followed by calcium imaging analyses identified receptor regions and amino acid residues critical for activation of hTAS2R46, -R43, and -R31. We found that the carboxyl-terminal regions of the investigated receptors are crucial for agonist selectivity. Intriguingly, exchanging two residues located in transmembrane domain seven between hTAS2R46, activated by strychnine, and hTAS2R31, activated by aristolochic acid, was sufficient to invert agonist selectivity. Further mutagenesis revealed additional positions involved in agonist interaction. The transfer of functionally relevant amino acids identified in hTAS2R46 to the corresponding positions of hTAS2R43 and -R31 resulted in pharmacological properties indistinguishable from the parental hTAS2R46. In silico modeling of hTAS2R46 allowed us to visualize the putative mode of interaction between agonists and hTAS2Rs. Detailed structure-function analyses of hTAS2Rs may ultimately pave the way for the development of specific antagonists urgently needed for more sophisticated analyses of human bitter taste perception.

Artificial sweeteners and salts producing a metallic taste sensation activate TRPV1 receptors

Throughout the world many people use artificial sweeteners (AS) for the purpose of reducing caloric intake. The most prominently used of these molecules include saccharin, aspartame (Nutrasweet), acesulfame-K, and cyclamate. Despite the caloric advantage they provide, one key concern in their use is their aversive aftertaste that has been characterized on a sensory level as bitter and/or metallic. Recently, it has been shown that the activation of particular T2R bitter taste receptors is partially involved with the bitter aftertaste sensation of saccharin and acesulfame-K. To more fully understand the biology behind these phenomena we have addressed the question of whether AS could stimulate transient receptor potential vanilloid-1 (TRPV1) receptors, as these receptors are activated by a large range of structurally different chemicals. Moreover, TRPV1 receptors and/or their variants are found in taste receptor cells and in nerve terminals throughout the oral cavity. Hence, TRPV1 activation could be involved in the AS aftertaste or even contribute to the poorly understood metallic taste sensation. Using Ca(2+) imaging on TRPV1 receptors heterologously expressed in the human embryonic kidney (HEK) 293 cells and on dissociated primary sensory neurons, we find that in both systems, AS activate TRPV1 receptors, and, moreover, they sensitize these channels to acid and heat. We also found that TRPV1 receptors are activated by CuSO(4), ZnSO(4), and FeSO(4), three salts known to produce a metallic taste sensation. In summary, our results identify a novel group of compounds that activate TRPV1 and, consequently, provide a molecular mechanism that may account for off tastes of sweeteners and metallic tasting salts.