Receptors and Transduction of Umami Taste Stimuli

L-glutamate and 5'-ribonucleotides, such as GMP and IMP, elicit the umami taste, also known as the fifth taste. This review will highlight recent advancements in our understanding of umami taste receptors and their downstream signaling effectors in taste receptor cells. Several G protein-coupled receptors that bind umami stimuli have been identified in taste buds, including the heterodimer T1R1/T1R3, truncated and brain forms of mGluR4 and mGluR1, brain mGluR2, and brain mGluR3. Further, ionotropic glutamate receptors are expressed in taste cells and may play a role in glutamate transduction or signaling between taste cells and/or nerve fibers. Knockout of T1R1 or T1R3 reduces, but does not eliminate, responses to umami stimuli, suggesting that multiple receptors contribute to umami taste. The signaling effectors downstream of umami G protein-coupled receptors involve G beta gamma activation of PLC beta 2 to elicit Ca2+ release from intracellular stores and activation of a cation channel, TRPM5. In fungiform and palatal taste buds, T1R1/T1R3 is co-expressed with G alpha gustducin and transducin, but the G alpha proteins involved in circumvallate taste buds have not been identified. In most taste fields, however, cAMP antagonizes responses to umami stimuli, suggesting that the G alpha subunit serves to modulate umami taste sensitivity.