Intracellular Ca ²⁺ and the phospholipid PIP ₂ regulate the taste transduction ion channel TRPM5

Abstract

The transduction of taste is a fundamental process that allows animals to discriminate nutritious from noxious substances. Three taste modalities, bitter, sweet, and amino acid, are mediated by G protein-coupled receptors that signal through a common transduction cascade: activation of phospholipase C ␤2, leading to a breakdown of phosphatidylinositol-4,5-bisphosphate (PIP2) into diacylglycerol and inositol 1,4,5-trisphosphate, which causes release of Ca 2؉ from intracellular stores. The ion channel, TRPM5, is an essential component of this cascade; however, the mechanism by which it is activated is not known. Here we show that heterologously expressed TRPM5 forms a cation channel that is directly activated by micromolar concentrations of intracellular Ca 2؉ (K1͞2 ‫؍‬ 21 M). Sustained exposure to Ca 2؉ desensitizes TRPM5 channels, but PIP2 reverses desensitization, partially restoring channel activity. Whole-cell TRPM5 currents can be activated by intracellular Ca 2؉ and show strong outward rectification because of voltage-sensitive gating of the channels. TRPM5 channels are nonselective among monovalent cations and not detectably permeable to divalent cations. We propose that the regulation of TRPM5 by Ca 2؉ mediates sensory activation in the taste system. I on channels in the transient receptor potential (TRP) family conduct second messenger-gated currents that play diverse roles in cellular physiology. Many are known to mediate vertebrate and invertebrate sensory transduction, including visual transduction (1), pheromone detection (2-4), thermal reception (5-8), and mechanoreception (9, 10). TRP channels can be divided into three classes: TRPC, TRPV, and TRPM (11). Eight vertebrate TRP channels fall into the TRPM class on the basis of structural similarity to the founding member (melastatin) (12). Some of these channels are activated by varied stimuli, including ADP-ribose (TRPM2) (13,14), Ca 2ϩ (TRPM4) (15), and cold or menthol (TRPM8) (6, 7), whereas others are constitutively active (TRPM7) (16)(17)(18). Recently, a member of this family, TRPM5, was implicated in vertebrate taste transduction (19,20); however, the mechanisms by which it is gated are not known.The transduction of bitter, sweet, and amino acid tastes is mediated by G protein-coupled receptors that are distinct for each modality (reviewed in refs. 21 and 22). Although a variety of mechanisms have been proposed for taste transduction, recent evidence indicates that all three modalities use elements of a common pathway (20, 23); receptors signal through a heterotrimeric G protein to phospholipase C ␤2 (PLC␤2), which breaks down phosphatidylinositol 4,5-bisphosphate (PIP 2 ) into inositol 1,4,5-trisphosphate (IP 3 ) and diacylglycerol (DAG), and IP 3 opens Ca 2ϩ -release channels to elevate intracellular Ca 2ϩ . Strong support for this pathway comes from a study of mice that carry a targeted deletion of PLC␤2 and are completely unresponsive to bitter, sweet, and amino acid tastes (20). In addition, it is known that taste cells respond to bitter a...

Olfactory neurons expressing transient receptor potential channel M5 (TRPM5) are involved in sensing semiochemicals

“…Could these two channels interact directly? TRPM5 is a Ca 2ϩ -activated channel (40,41), whereas, under physiological conditions, the current flowing through CNGA2 is made up mostly of Ca 2ϩ (42). Intriguingly, the finding that immunoreactivity for TRPM5 is found in small spots with an average diameter of 69 nm suggests that these channels may be clustered in small microdomains.…”

Taste perception, associated hormonal modulation, and nutrient intake

“…36,37 Activation of these G protein-coupled receptors results in the digestion of plasma membrane phospholipids into diacylglycerol and inositol triphosphate, which in turn mobilizes intracellular calcium. [38][39][40] Increased intracellular calcium generated in response to tastant binding initiates membrane depolarization and the release of ATP via Calcium homeostasis modulator 1 (CALHM1) ion channels. 41,42 Type II cells also express voltage-gated sodium and potassium channels that mediate the secretion of ATP as a function of action potential firing rate.…”

The Mg2+ and Mg2+-Nucleotide-Regulated Channel-Kinase TRPM7

“…Clapham and colleagues (2002) suggested that receptormediated PIP 2 is required to maintain TRPM7 activity, and its breakdown by receptormediated stimulation of phospholipase C (PLC) is responsible for the inhibition TRPM7 they observed when stimulating cells with agonists that couple to PLC. Although several ion channels, including members of the TRPM family [TRPM4 (Zhang et al 2005;Nilius et al 2006), TRPM5 (Liu and Liman 2003), and TRPM8 (Liu and Qin 2005;Rohacs et al 2005)] are subject to PIP 2 -mediated regulation, the case for TRPM7 has not been made without ambiguity. Most of the experiments presented in support of PIP 2 regulation were performed in cells that overexpress TRPM7 or with receptor agonists that can couple to PLC (or both).…”