The research goals of the Section of Molecular Neuroscience are to define the molecular mechanisms underlying the development and function of mammalian chemosensory systems. Research efforts this past year have been directed towards establishing functional assays for bitter taste receptors and identifying and characterizing novel genes selectively expressed in taste receptor cells. To characterize the functional properties of members of the human T2R family of bitter receptors, we have developed an in vitro reconstitution assay for these receptors. For this assay, baculoviral expression vectors were constructed with each of the 23 human T2Rs. Western analyses with cellular membranes isolated from insect cells infected with these baculoviruses indicate that the membranes are highly enriched in bitter receptors. Using these membrane fractions, we are able to functionally reconstitute receptor activity by the addition of purified G proteins. We are currently screening orphan members of the hT2R family with a panel of 60 bitter-tasting compounds to uncover ligand-receptor interactions. Thus far, we have identified four novel receptor-ligand interactions including receptors for the bitter-tasting compounds aristolochic acid, denatonium, and chloroquine. Using these defined receptor/ligand pairs, we are now performing quantitative assessments of the ligand binding properties and the G protein selectivities of these receptors. This studies will provide insight into the both the specificities of bitter receptors and the signal transduction pathways they activate. This past year we also characterized a novel family 2B G-protein-coupled receptor, GPR113, that is specifically expressed in taste receptor cells. The 2B family, a new subgroup of G-protein-coupled receptors, consists of about 30 members of which only three have been functionally characterized. GPR113 displays little sequence homology to known sweet and bitter taste receptors and is the first of its class shown to be selectively expressed in a subset of taste receptor cells. The long extracellular domain of GPR113 contains a hormone-binding domain, characteristically seen in the well-studied peptide hormone binding G-protein-coupled receptors, suggesting one ligand for GPR113 is a peptide. As a putative peptide receptor, GPR113 could function to recognize an endogenous neuropeptide that could, for example, act to modulate taste sensitivities. Alternatively, GPR113 might act as a taste receptor for one of the many ingested peptides that are known to elicit taste sensations.
