MOLECULAR BIOLOGY OF GLUTAMATE RECEPTORS IN TASTE BUDS

University Of Miami School Of Medicine, Coral Gables FL

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Abstract

The monosodium salt of L-glutamate (MSG) is a distinctive taste stimulus found naturally in protein-rich and other foods. A metabotropic receptor for glutamate, mGluR4, is expressed in rat taste buds and appears to function as a taste receptor for MSG. Ligands acting at mGluR4 mimic the taste of MSG. Recently, we have cloned a novel taste-specific form of mGluR4, that has a severely truncated glutamate-binding site. The receptor, taste-mGluR4, is activated only by high concentrations of glutamate, as expected for a taste receptor. We postulate that the altered binding site accounts for some of the unique features of MSG taste. Functional and molecular evidence also exists for ionotropic glutamate receptors (iGluRs) in taste cells. The goal of Project #1 is to use molecular techniques to address whether taste-mGluR4 alone is the taste receptor for MSG for whether activation of iGluRs also contributes to transduction. We will address this question by comparing the functional properties of cloned receptors from taste cells against the functions of taste cells themselves, to identify the key players in MSG taste transduction. Specifically, we will: 1. Test the hypothesis that taste-mGluR4 is a taste receptor for MSG. We will express taste-mGluR4 in CHO cells and measure its response to glutamate, GluR agonists and nucleotides that enhance MSG taste. We will also use Western blots and promoter analyses to test whether taste buds are able to express this novel GluR. 2. Determine if cAMP and/or cGMP are 2nd messengers in glutamate taste transduction as predicted in taste mGluR4 is a taste receptor for MSG. 3. Conduct detailed molecular and functional analyses of a novel iGluR- like sequence we have cloned from taste tissue. We will examine whether this receptor underlies an unusual glutamate-gated ionic conductance found in taste cells, and whether this receptor plays a role in MSG taste. We will also search for additional GluRs in taste tissue in order to achieve a comprehensive description of the taste transduction of MSG. The data from these molecular and cellular approaches will be closely coordinated with results from electrophysiological and behavioral analyses of the same questions, conducted from Projects #2 and #3.