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Umami taste transduction mechanisms^1,2,3,4

¹ From the Department of Biomedical Sciences, Colorado State University, Fort Collins, CO, and the Rocky Mountain Taste and Smell Center, University of Colorado Health Sciences Center, Aurora, CO.

² Presented at the "100th Anniversary Symposium of Umami Discovery: The Roles of Glutamate in Taste, Gastrointestinal Function, Metabolism, and Physiology," held in Tokyo, Japan, September 10–13, 2008.

³ Supported in part by NIH grant NIDCD R0100766 and the Ajinomoto Amino Acid Research Program (Ajinomoto is a manufacturer of food and amino acids including glutamate).

⁴ Address correspondence to SC Kinnamon, Department of Biomedical Sciences, Neuroscience Division, Colorado State University, Fort Collins, CO 80523. E-mail: sue.kinnamon{at}ucdenver.edu.

L-Glutamate elicits the umami taste sensation, now recognized as a fifth distinct taste quality. A characteristic feature of umami taste is its potentiation by 5'-ribonucleotides such as guanosine-5'-monophosphate and inosine 5'-monophosphate, which also elicit the umami taste on their own. Recent data suggest that multiple G protein–coupled receptors contribute to umami taste. This review will focus on events downstream of the umami taste receptors. Ligand binding leads to Gβ activation of phospholipase C β2, which produces the second messengers inositol trisphosphate and diacylglycerol. Inositol trisphosphate binds to the type III inositol trisphosphate receptor, which causes the release of Ca²⁺ from intracellular stores and Ca²⁺-dependent activation of a monovalent-selective cation channel, TRPM5. TRPM5 is believed to depolarize taste cells, which leads to the release of ATP, which activates ionotropic purinergic receptors on gustatory afferent nerve fibers. This model is supported by knockout of the relevant signaling effectors as well as physiologic studies of isolated taste cells. Concomitant with the molecular studies, physiologic studies show that L-glutamate elicits increases in intracellular Ca²⁺ in isolated taste cells and that the source of the Ca²⁺ is release from intracellular stores. Both G gustducin and G transducin are involved in umami signaling, because the knockout of either subunit compromises responses to umami stimuli. Both -gustducin and -transducin activate phosphodiesterases to decrease intracellular cAMP. The target of cAMP in umami transduction is not known, but membrane-permeant analogs of cAMP antagonize electrophysiologic responses to umami stimuli in isolated taste cells, which suggests that cAMP may have a modulatory role in umami signaling.

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