Teaching Resource

Sensory Systems: Taste Perception

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Science's STKE  28 Jun 2005:
Vol. 2005, Issue 290, pp. tr20
DOI: 10.1126/stke.2902005tr20

Abstract

This Teaching Resource provides lecture notes and slides for a class covering the human taste system and is part of the course "Cell Signaling Systems: A Course for Graduate Students." The lecture begins with a discussion of five distinct qualities of taste and then proceeds to describe receptors and signaling mechanisms.

Lecture Notes

The sensation of taste is initiated by the interaction of tastants with receptors and ion channels in the apical microvilli of taste receptor cells (TRCs) found within taste buds in the oral cavity (1, 2). TRCs are specialized epithelial cells with many neuronal properties, including the ability to depolarize and to form synapses. Vertebrate taste is comprised of five distinct qualities: sweet, sour, bitter, salty, and umami (the taste of glutamate and other amino acids). Specific receptors, ion channels, and downstream signaling pathways underlie signal detection and signal transduction for each taste quality. Ion channels, such as ENaC (epithelial Na+ channel) and ASIC (acid-sensing ion channel), have been implicated in salty and sour taste, respectively. Sweet, bitter and umami taste detection and transduction depend on specific G protein–coupled receptors (GPCRs) and their coupled signaling pathways. Type 2 taste receptors (T2Rs) comprise a family of 25 to 30 GPCRs that underlie bitter taste detection (3, 4). Type 1 taste receptors (T1R1, T1R2, and T1R3) heterodimerize and have been implicated in sweet (T1R2 + T1R3) and umami (T1R1 + T1R3) (5, 6). Signaling elements downstream of the type 1 and type 2 taste receptors appear to have many elements in common: α-gustducin (a G protein α subunit) (7), Gβ3, Gγ13, PLC-β2 (phospholipase C-β2), IP3R3 (inositol trisphosphate receptor 3), and Trpm5 (a Ca2+-gated cation channel selectively expressed in TRCs).

The sweet taste receptor component T1R3 is the gene product of Sac, the major genetic determinant of sweet taste preference and sensitivity in inbred strains of mice (8). T1R3 is selectively expressed in TRCs, and polymorphisms of T1R3 underlie differential sensitivity to sweet compounds of taster versus nontaster strains of mice. T1R3 knockout mice are deficient in their responsiveness to natural and artificial sweeteners (9, 10). Sequence differences in T1R2 and T1R3 underlie differential responsiveness of humans versus rodents to various sweet compounds (11, 12). Humans find certain protein sweeteners (brazzein, monellin, and thaumatin) and small molecule sweeteners (aspartame and cyclamate) to be intensely sweet, whereas mice are indifferent to these compounds. A similar specificity has been observed with the sweet antagonist lactisole, which blocks the taste of all sweet compounds in humans, but has no effect on mice (12).

Heterologously expressed T1R2 + T1R3 channels are activated by diverse sweet compounds (5, 6, 11, 12). Expression of the human receptor (hT1R2 + hT1R3) recapitulates the human pattern of sweet responses. Expression of the mouse receptor (mT1R2 + mT1R3) reproduces the mouse pattern of response. Human/mouse mixtures (for example, hT1R2 + mT1R3) have been used to map the contributions of T1R2 and T1R3 to sweet detection. Human/mouse chimeras of these receptors have been used to identify those portions of each sweet receptor monomer involved in sweet ligand interactions. The cysteine-rich region of hT1R3 is required for sensitivity to the sweetness of brazzein, whereas the transmembrane domain of hT1R3 is required for sensitivity to aspartame, cyclamate, and lactisole. The heterodimeric sweet taste receptor employs multiple ligand binding sites to respond to diverse sweeteners.

Related Resources

Teaching Resource

R. Iyengar, M. Diverse-Pierluissi, D. Weinstein, L. A. Devi, Cell signaling systems: A course for graduate students. Sci. STKE 2005, tr3 (2005). [Abstract] [Syllabus]

Forum

A. Chan, R. Iyengar, S. A. Aaronson, A. J. Caplan, S. R. Salton, M.-M. Zhou, Principles of cell signaling and biological consequences. Sci. STKE (Forum, as seen June 2005), http://stke.sciencemag.org/cgi/forum-display/stkeforum;15?FORUM_ID=stkeforum;15. [Discussion]

Review

C. Montell, The TRP superfamily of cation channels. Sci. STKE 2005, re3 (2005). [Gloss] [Abstract] [Full Text]

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