Extracellular Acid Block and Acid-enhanced Inactivation of the Ca2+-activated Cation Channel TRPM5 Involve Residues in the S3-S4 and S5-S6 Extracellular Domains

Abstract

TRPM5, a member of the superfamily of transient receptor potential ion channels, is essential for the detection of bitter, sweet, and amino acid tastes. In heterologous cell types it forms a nonselective cation channel that is activated by intracellular Ca 2؉ . TRPM5 is likely to be part of the taste transduction cascade, and regulators of TRPM5 are likely to affect taste sensation. In this report we show that TRPM5, but not the related channel TRPM4b, is potently blocked by extracellular acidification. External acidification has two effects, a fast reversible block of the current (IC 50 pH ‫؍‬ 6.2) and a slower irreversible enhancement of current inactivation. Mutation of a single Glu residue in the S3-S4 linker and a His residue in the pore region each reduced sensitivity of TRPM5 currents to fast acid block (IC 50 pH ‫؍‬ 5.8 for both), and the double mutant was nearly insensitive to acidic pH (IC 50 pH ‫؍‬ 5.0). Prolonged exposure to acidic pH enhanced inactivation of TRPM5 currents, and mutant channels that were less sensitive to acid block were also less sensitive to acid-enhanced inactivation, suggesting an intimate association between the two processes. These processes are, however, distinct because the pore mutant H896N, which has normal sensitivity to acid block, shows significant recovery from acid-enhanced inactivation. These data show that extracellular acidification acts through specific residues on TRPM5 to block conduction through two distinct but related mechanisms and suggest a possible interaction between extracellular pH and activation and adaptation of bitter, sweet, and amino acid taste transduction.All animals have evolved mechanisms by which they sense chemicals in the environment and discriminate the nutritious from the noxious. In mammals, taste cells signal the presence of at least five distinct types of chemicals that give us the perceptions of bitter, sour, sweet, salty, and amino acid tastes. Two of these modalities, salty and sour, detect ions (Na ϩ and H ϩ , respectively), and both modalities may be mediated directly by ion channels on the apical surface of taste cells (1). The other three modalities are mediated by distinct G proteincoupled receptors (2-4) that signal through a common transduction cascade, for which both phospholipase C ␤2 and the ion channel TRPM5 are essential elements (5). It is likely that upon binding of taste molecules, sweet, bitter, and amino acid taste receptors initiate a signaling cascade that activates phospholipase C ␤2, leading to the breakdown of phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate and diacylglycerol and the release of Ca 2ϩ from intracellular stores (6, 7). TRPM5 may be the final element of this cascade, converting the biochemical signal transduction pathway into an electrical signal sent to the brain (5, 8). The essential role of TRPM5 in multiple taste pathways suggests that chemicals that modify its activity could have profound effects on taste sensation.TRPM5 belongs to the transient receptor potential (TRP) 1 fa...

Hypomagnesemia with Secondary Hypocalcemia due to a Missense Mutation in the Putative Pore-forming Region of TRPM6

“…Therefore, we took advantage of the structural prediction of this segment and mapped P1017 to the putative pore helix, a conserved element of the pore structure in many ion channels (35). Remarkably, the three-dimensional model of the TRPM6 S5-S6 region is well compatible with the recent structural prediction of the equivalent region of a more distantly related family member, TRPM4 (43,44). Moreover, the model-based structural analysis of the S5-S6 segment of TRPM6 suggested particular amino acids to be involved in folding of the putative pore loop, which, like in other channels (35), are assumed to be critical determinants of ion selectivity.…”

The Transduction Channel TRPM5 Is Gated by Intracellular Calcium in Taste Cells

“…Despite its significance for taste, the properties of TRPM5 channels in taste cells have not been described previously, and any information about how the channels are regulated has been obtained solely from the study of heterologously expressed channels (Perez et al, 2002;Hofmann et al, 2003;Liu and Liman, 2003;Prawitt et al, 2003;Zhang et al, 2003;Liu et al, 2005;Talavera et al, 2005). In this report, we characterized native TRPM5 channels using transgenic animals that allowed us to identify those taste cells that normally express TRPM5 and to compare currents in these cells in the presence and absence of an intact TRPM5 gene.…”

Autonomous regulation of free Ca2+ concentrations in isolated plant cell nuclei: A mathematical analysis

“…Proton concentration is known to have a profound influence on a variety of biological processes and especially on ion channel, transporter activities, and the calcium-binding capacity of calcium-binding proteins. Acidification of the extracellular medium reduces the conductivity of many voltage-gated and ligand-gated channels [56][57][58][59]. This is the case particularly for L-type calcium channels [56,60] and sodium channels [61].…”