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The tastes of sugars (sweet) and glutamate (umami) are thought to be detected by T1r receptors expressed in taste cells. Molecular genetics and heterologous expression implicate T1r2 plus T1r3 as a sweet-responsive receptor, and T1r1 plus T1r3, as well as a truncated form of the type 4 metabotropic glutamate receptor (taste-mGluR4), as umami-responsive receptors. Here, we show that mice lacking T1r3 showed no preference for artificial sweeteners and had diminished but not abolished behavioral and nerve responses to sugars and umami compounds. These results indicate that T1r3-independent sweet- and umami-responsive receptors and/or pathways exist in taste cells.
The sac gene in mice is the major genetic determinant regulating behavioral and nerve responses to artificial sweeteners, such as saccharin, and to several sugars (1-6). Recently, the taste receptor T1r3 was identified as the sac gene product (7-12). Heterologously expressed T1rS appears not to function on its own. However, in combination with T1r2 it responds to many sweet compounds, and in combination with T1r1 it responds to glutamate and other umami compounds (11, 13, 14). To determine the role of T1r3 in vivo, we produced knockout (KO) mice lacking the entire T1r3 coding region by homologous recombination in C57BL6 (B6) embryonic stem (ES) cells and then injected the targeted stem cells into blastocysts (Fig. 1, A and B). T1r3 protein was absent in T1r3 KO mice, as demonstrated by indirect immunofluorescence (Fig. 1, C and D). The T1r3 KO mice were healthy and fertile with no obvious anatomical or behavioral abnormalities. The gross anatomy of the taste tissue and number of taste buds appeared normal in the T1r3 KO mice (Fig. 1F). Knocking out the T1r3 gene did not alter expression of T1r1 (15) or T1r2 (Fig. 1, E and F).
Behavioral tests (16) were conducted to examine the responses of T1r3 KO mice to tastants representing five taste qualities (sweet, bitter, salty, sour, and umami). In two-bottle preference tests, the T1r3 KO mice displayed indifference to sucrose and three artificial sweeteners (sucralose, acesulfame K, and SC45647) at concentrations that elicited maximal preference in B6 wild-type littermate controls (Fig. 2). At concentrations that were 5 to 10 times as high as those needed to elicit a strong preference in B6 wild-type mice, the T1r3 KO mice...