Beyond the tasty morsel: the underlying molecular mechanisms for nutrient detection
Transduction of gustatory stimuli in receptor (Type II) cells.
As stated above, sweet, umami, and bitter compounds each activate different taste GPCRs that are expressed in discrete sets of receptor cells. For instance, receptor cells that express members of the T2R family of GPCRs sense bitter compounds (
Chandrashekar et al., 2000). In different mammals, 20–35 separate genes encode members of the T2R family. These taste receptors exhibit heterogeneous molecular receptive ranges: some are narrowly tuned to 2–4 bitter-tasting compounds, whereas others are promiscuously activated by numerous ligands (
Meyerhof et al., 2010). On the basis of in situ hybridizations with mixed probes on rodent taste buds, the T2Rs were reported either to be expressed as overlapping subsets of mRNAs (
Matsunami et al., 2000) or coexpressed in a single population of taste cells (
Adler et al., 2000). More recently, detailed analyses on human taste buds confirm that different bitter-responsive taste cells express subsets of 4–11 of the T2Rs in partially overlapping fashion (
Behrens et al., 2007). This observation is important insofar as it provides a molecular basis for discriminating between different bitter compounds. Bitter-sensing taste cells are known to functionally discriminate among bitter compounds (
Caicedo and Roper, 2001). This pattern of T2R expression, along with polymorphisms across the gene family, is thought to allow humans and animals to detect the enormous range of potentially toxic bitter compounds found in nature (
Drayna, 2005).
Receptor cells expressing the heterodimer T1R2+T1R3 respond to sugars, synthetic sweeteners, and sweet-tasting proteins such as monellin and brazzein (
Nelson et al., 2001;
Jiang et al., 2004;
Xu et al., 2004). Although the persistence of sensitivity to some sugars in mice lacking T1R3 suggests that additional receptors for sweet may exist (
Damak et al., 2003), candidate receptors have yet to be identified.
A third class of receptor cells expresses the heterodimeric GPCR, T1R1+T1R3, which responds to umami stimuli, particularly the combination of
l-glutamate and GMP/IMP, compounds that accumulate in many foods after hydrolysis of proteins and NTPs (
Li et al., 2002;
Nelson et al., 2002). Nevertheless, robust physiological responses and behavioral preference for umami tastants persist in mice in which T1R3 is knocked out, suggesting that additional taste receptors may contribute to umami detection (
Damak et al., 2003;
Maruyama et al., 2006;
Yasumatsu et al., 2009). Functional responses to various umami tastants occur in distinct subsets of cells within taste buds (
Maruyama et al., 2006) and neural responses show similarly heterogeneous patterns (
Yoshida et al., 2009b), observations that further suggest that umami taste is complex, and likely mediated through multiple types of taste receptors. In summary, although the T1R1+T1R3 dimer clearly acts as an umami receptor, additional GPCRs may play complementary roles. Candidates for additional umami receptors include a taste-specific variant or other isoforms of G protein–coupled glutamate receptors expressed in taste buds (
Chaudhari et al., 2000;
Li et al., 2002;
Nelson et al., 2002;
San Gabriel et al., 2009).
The T1Rs are dimeric Class III GPCRs, with large N-terminal extracellular domains (
Max et al., 2001). This domain forms a Venus Flytrap structure as in other family members. T1Rs also possess a multitude of additional ligand-binding sites on the exterior faces of the flytrap, in the linker, and perhaps even in the plane of the membrane (
Cui et al., 2006;
Temussi, 2009). In contrast, T2Rs resemble Class I GPCRs with binding sites in the transmembrane helices, in keeping with the nonpolar nature of many bitter ligands (
Floriano et al., 2006).
When they bind taste molecules, taste GPCRs activate heterotrimeric GTP-binding proteins (). For example, the bitter receptors (T2Rs) are coexpressed with and activate the taste-selective Gα subunit, α-gustducin, and the closely related α-transducin (
Ruiz-Avila et al., 1995). Taste receptors that include T1R3 may couple to Gα14 and other Gα subunits (
Tizzano et al., 2008). Despite this apparent selectivity of taste GPCRs for Gα subunits, the principal pathway for taste transduction appears to be via Gβγ, including Gγ13 and Gβ1 or Gβ3 (
Huang et al., 1999). Upon ligand binding, the Gβγ subunits are freed from the taste GPCR and interact functionally with a phospholipase, PLCβ2, an unusual isoform that is activated by Gβγ rather than the more common Gαq family subunits (
Rössler et al., 1998). Knocking out PLCβ2 severely diminishes, but does not eliminate taste sensitivity (
Zhang et al., 2003;
Dotson et al., 2005). PLCβ2 stimulates the synthesis of IP
3, which opens IP
3R3 ion channels on the endoplasmic reticulum, releasing Ca
2+ into the cytosol of receptor cells (
Simon et al., 2006;
Roper, 2007). The elevated intracellular Ca
2+ appears to have two targets in the plasma membrane: a taste-selective cation channel, TRPM5, and a gap junction hemichannel, both found in receptor cells (
Pérez et al., 2002;
Huang et al., 2007). The Ca
2+-dependent opening of TRPM5 produces a depolarizing generator potential in receptor cells (
Liu and Liman, 2003). If sufficiently large, generator potentials evoke action potentials in receptor cells. The two signals elicited by tastants: strong depolarization and increased cytoplasmic Ca
2+, are integrated by gap junction hemichannels. The outcome of this convergence is that the taste bud transmitter, ATP, and possibly other molecules, are secreted through the hemichannel pores into the extracellular space surrounding the activated receptor cell (, yellow cell; and ;
Huang et al., 2007;
Romanov et al., 2007;
Huang and Roper, 2010).
Although most researchers agree that ATP release occurs through a plasma membrane hemichannel, whether these channels are formed of pannexin (Panx) or connexin (Cx) subunits is not fully resolved. Panx1 is robustly expressed in receptor cells, whereas several Cx subunits are expressed at more modest levels (
Huang et al., 2007;
Romanov et al., 2007). Although there may be gap junctions presumably formed of connexins between cells in mammalian taste buds (
Yoshii, 2005), such junctions would not be expected to secrete ATP into extracellular spaces. A principal argument for Cx hemichannels in taste cells was based on the blocking action of certain isoform-selective mimetic peptides. However, the specificity of such peptides has recently been called into question (
Wang et al., 2007). Finally, Panx1 hemichannels are gated open by elevated cytoplasmic Ca
2+ and/or membrane depolarization (
Locovei et al., 2006). ATP release from taste cells similarly is mediated by both Ca
2+ and voltage (
Huang and Roper, 2010). In contrast, Cx hemichannels usually open only in the absence of extracellular Ca
2+ and typically are blocked by elevated cytoplasmic Ca
2+. Further, Panx1-selective antagonists block taste-evoked ATP secretion (
Huang et al., 2007;
Dando and Roper, 2009). Thus, the weight of the evidence strongly favors ATP release through Panx1 hemichannels in receptor cells. Nevertheless, the ideal test to resolve this question, namely testing ATP release from taste cells from Panx1 or Cx knockout mice, has yet to be reported.
and Stephen D. Roper
This article has been corrected. See J Cell Biol. 2010 October 18; 191(2): 429.
