Molecular, biochemical and physiological studies on bitter taste receptors

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G protein-coupled receptors (GPCRs) constitute the largest and most diverse group of membrane proteins. GPCRs share a conserved seven-helix motif and activate cognate G-proteins in response to diverse extracellular stimuli including light, peptides, hormones and tastants. The sense of taste is a chemosensory system responsible for basic food appraisal and is mediated mostly by GPCRs. Humans are capable of detecting five basic tastes that are sweet, umami, bitter, sour and salt. While the signal transduction for sweet, umami and bitter tastes are through GPCRs, the sour and salt tastes are sensed by ion channels. In humans, around 30 GPCRs are known to mediate taste perception, with 25 of these sensing bitter taste alone. While sweet and umami taste perceptions are well studied, very little is known regarding the bitter taste perception. Thus far, the identities of the tastants and blockers (antagonists or inverse agonists) for a majority of the bitter taste receptors (T2Rs) are unknown, Indeed even for the T2Rs with known ligands, very little structure-function data is available and the molecular mechanisms of action of these receptors are not yet understood. Our approach will be to use a combination of molecular, biochemical and physiological techniques to unravel details of the distribution and mechanism of action of T2Rs. OBJECTIVES The applicant’s research program can be classified into two broad areas, the first dealing with the Class A GPCRs (Prostanoid receptors and Beta2-Adrenergic receptor) and the second focussed on non-Class A and nutritionally important GPCRs, the human taste receptors. The long-term objective of this NSERC program is to elucidate molecular mechanisms of vertebrate bitter taste signal transduction. We will address the following questions, why do certain T2Rs have broad substrate specificities than others? What are the molecular and/or structural features in the T2Rs that contribute to this broad specificity? In addition, we will attempt to elucidate the physiological roles of T2Rs expressed in extra-oral tissues. The short-term goals (next 5 years) are: 1) To pursue structure-function studies on T2Rs, characterize their desensitization mechanisms and identify post-translational modifications. 2) To discover novel ligands (bitter taste blockers) that can reduce the activity of these receptors. 3) To characterize the physiological role of T2Rs expressed in extra-oral tissues including the brain tissues, vasculature and airways. SIGNIFICANCE OF THE PROPOSED RESEARCH Taste is often used to describe sensations arising from the oral cavity. Bitter taste has evolved as a central warning signal against the ingestion of potentially toxic substances. The molecular events in the perception of bitter taste start with the binding of specific water-soluble molecules (tastants or ligands) to T2Rs, initiating an intracellular signalling cascade. This research proposal focuses on elucidating the molecular mechanisms of signal transduction by T2Rs. Knowledge about how T2Rs interact with their ligand will enable design of artificial tastants that enhance food flavor. Bitter taste blockers can be used to suppress unpleasantness and thereby increase palatability of health-promoting bitter foods, such as fruit and vegetable extracts. Furthermore, characterization of the physiological roles of T2Rs expressed in extra-oral tissues will spur additional basic science research on GPCR structure and function. The proposed research would train graduate students, who will gain expertise in a broad range of modern molecular and cell biological techniques.