Comparison of functional properties of the Ca2+-activated cation channels TRPM4 and TRPM5 from mice

The transduction channel TRPM5 is gated by intracellular calcium in taste cells

Bitter, sweet, and umami tastants are detected by G-protein-coupled receptors that signal through a common second-messenger cascade involving gustducin, phospholipase C beta2, and the transient receptor potential M5 (TRPM5) ion channel. The mechanism by which phosphoinositide signaling activates TRPM5 has been studied in heterologous cell types with contradictory results. To resolve this issue and understand the role of TRPM5 in taste signaling, we took advantage of mice in which the TRPM5 promoter drives expression of green fluorescent protein and mice that carry a targeted deletion of the TRPM5 gene to unequivocally identify TRPM5-dependent currents in taste receptor cells. Our results show that brief elevation of intracellular inositol trisphosphate or Ca2+ is sufficient to gate TRPM5-dependent currents in intact taste cells, but only intracellular Ca2+ is able to activate TRPM5-dependent currents in excised patches. Detailed study in excised patches showed that TRPM5 forms a nonselective cation channel that is half-activated by 8 microM Ca2+ and that desensitizes in response to prolonged exposure to intracellular Ca2+. In addition to channels encoded by the TRPM5 gene, we found that taste cells have a second type of Ca2+-activated nonselective cation channel that is less sensitive to intracellular Ca2+. These data constrain proposed models for taste transduction and suggest a link between receptor signaling and membrane potential in taste cells.

Inspiratory bursts in the preBötzinger complex depend on a calcium-activated non-specific cation current linked to glutamate receptors in neonatal mice

Inspiratory neurons of the preBötzinger complex (preBötC) form local excitatory networks and display 10-30 mV transient depolarizations, dubbed inspiratory drive potentials, with superimposed spiking. AMPA receptors are critical for rhythmogenesis under normal conditions in vitro but whether other postsynaptic mechanisms contribute to drive potential generation remains unknown. We examined synaptic and intrinsic membrane properties that generate inspiratory drive potentials in preBötC neurons using neonatal mouse medullary slice preparations that generate respiratory rhythm. We found that NMDA receptors, group I metabotropic glutamate receptors (mGluRs), but not group II mGluRs, contributed to inspiratory drive potentials. Subtype 1 of the group I mGluR family (mGluR1) probably regulates a K+ channel, whereas mGluR5 operates via an inositol 1,4,5-trisphosphate (IP3) receptor-dependent mechanism to augment drive potential generation. We tested for and verified the presence of a Ca2+-activated non-specific cation current (I(CAN)) in preBötC neurons. We also found that high concentrations of intracellular BAPTA, a high-affinity Ca2+ chelator, and the I(CAN) antagonist flufenamic acid (FFA) decreased the magnitude of drive potentials. We conclude that I(CAN) underlies robust inspiratory drive potentials in preBötC neurons, and is only fully evoked by ionotropic and metabotropic glutamatergic synaptic inputs, i.e. by network activity.

Non-selective cation channels, transient receptor potential channels and ischemic stroke

Several pathways to neural cell death are involved in ischemic stroke, and all require monovalent or divalent cation influx, implicating non-selective cation (NC) channels. NC channels are also likely to be involved in the dysfunction of vascular endothelial cells that leads to formation of edema following cerebral ischemia. Two newly described NC channels have emerged as potential participants in ischemic stroke, the acid sensing ion channel (ASIC), and the sulfonylurea receptor-1 (SUR1)-regulated NC(Ca-ATP) channel. Non-specific blockers of NC channels, including pinokalant (LOE 908 MS) and rimonabant (SR141716A), have beneficial effects in rodent models of ischemic stroke. Evidence is accumulating that NC channels formed by members of the transient receptor potential (TRP) family are also up-regulated in ischemic stroke and may play a direct role in calcium-mediated neuronal death. The nascent field of NC channels, including TRP channels, in ischemic stroke is poised to provide novel mechanistic insights and therapeutic strategies for this often devastating human condition.

Transient receptor potential cation channels in disease

The transient receptor potential (TRP) superfamily consists of a large number of cation channels that are mostly permeable to both monovalent and divalent cations. The 28 mammalian TRP channels can be subdivided into six main subfamilies: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and the TRPA (ankyrin) groups. TRP channels are expressed in almost every tissue and cell type and play an important role in the regulation of various cell functions. Currently, significant scientific effort is being devoted to understanding the physiology of TRP channels and their relationship to human diseases. At this point, only a few channelopathies in which defects in TRP genes are the direct cause of cellular dysfunction have been identified. In addition, mapping of TRP genes to susceptible chromosome regions (e.g., translocations, breakpoint intervals, increased frequency of polymorphisms) has been considered suggestive of the involvement of these channels in hereditary diseases. Moreover, strong indications of the involvement of TRP channels in several diseases come from correlations between levels of channel expression and disease symptoms. Finally, TRP channels are involved in some systemic diseases due to their role as targets for irritants, inflammation products, and xenobiotic toxins. The analysis of transgenic models allows further extrapolations of TRP channel deficiency to human physiology and disease. In this review, we provide an overview of the impact of TRP channels on the pathogenesis of several diseases and identify several TRPs for which a causal pathogenic role might be anticipated.

Protein kinase C regulates vascular myogenic tone through activation of TRPM4

Myogenic vasoconstriction results from pressure-induced vascular smooth muscle cell depolarization and Ca(2+) influx via voltage-dependent Ca(2+) channels, a process that is significantly attenuated by inhibition of protein kinase C (PKC). It was recently reported that the melastatin transient receptor potential (TRP) channel TRPM4 is a critical mediator of pressure-induced smooth muscle depolarization and constriction in cerebral arteries. Interestingly, PKC activity enhances the activation of cloned TRPM4 channels expressed in cultured cells by increasing sensitivity of the channel to intracellular Ca(2+). Thus we postulated that PKC-dependent activation of TRPM4 might be a critical mediator of vascular myogenic tone. We report here that PKC inhibition attenuated pressure-induced constriction of cerebral vessels and that stimulation of PKC activity with phorbol 12-myristate 13-acetate (PMA) enhanced the development of myogenic tone. In freshly isolated cerebral artery myocytes, we identified a Ca(2+)-dependent, rapidly inactivating, outwardly rectifying, iberiotoxin-insensitive cation current with properties similar to those of expressed TRPM4 channels. Stimulation of PKC activity with PMA increased the intracellular Ca(2+) sensitivity of this current in vascular smooth muscle cells. To validate TRPM4 as a target of PKC regulation, antisense technology was used to suppress TRPM4 expression in isolated cerebral arteries. Under these conditions, the magnitude of TRPM4-like currents was diminished in cells from arteries treated with antisense oligonucleotides compared with controls, identifying TRPM4 as the molecular entity responsible for the PKC-activated current. Furthermore, the extent of PKC-induced smooth muscle cell depolarization and vasoconstriction was significantly decreased in arteries treated with TRPM4 antisense oligonucleotides compared with controls. We conclude that PKC-dependent regulation of TRPM4 activity contributes to the control of cerebral artery myogenic tone.

Afferent neurotransmission mediated by hemichannels in mammalian taste cells

In mammalian taste buds, ionotropic P2X receptors operate in gustatory nerve endings to mediate afferent inputs. Thus, ATP secretion represents a key aspect of taste transduction. Here, we characterized individual vallate taste cells electrophysiologically and assayed their secretion of ATP with a biosensor. Among electrophysiologically distinguishable taste cells, a population was found that released ATP in a manner that was Ca(2+) independent but voltage-dependent. Data from physiological and pharmacological experiments suggested that ATP was released from taste cells via specific channels, likely to be connexin or pannexin hemichannels. A small fraction of ATP-secreting taste cells responded to bitter compounds, indicating that they express taste receptors, their G-protein-coupled and downstream transduction elements. Single cell RT-PCR revealed that ATP-secreting taste cells expressed gustducin, TRPM5, PLCbeta2, multiple connexins and pannexin 1. Altogether, our data indicate that tastant-responsive taste cells release the neurotransmitter ATP via a non-exocytotic mechanism dependent upon the generation of an action potential.

TRPM5 and taste transduction

TRPM5 is a cation channel that it is essential for transduction of bitter, sweet and umami tastes. Signaling of these tastes involves the activation of G protein-coupled receptors that stimulate phospholipase C (PLC) beta2, leading to the breakdown of phosphatidylinositol bisphosphate (PIP2) into diacylglycerol (DAG) and inositol trisphosphate (IP3), and release of Ca2+ from intracellular stores. TRPM5 forms a nonselective cation channel that is directly activated by Ca2+ and it is likely to be the downstream target of this signaling cascade. Therefore, study of TRPM5 promises to provide insight into fundamental mechanisms of taste transduction. This review highlights recent work on the mechanisms of activation of the TRPM5 channel. The mouse TRPM5 gene encodes a protein of 1,158 amino acids that is proposed to have six transmembrane domains and to function as a tetramer. TRPM5 is structurally most closely related to the Ca(2+)-activated channel TRPM4 and it is more distantly related to the cold-activated channel TRPM8. In patch clamp recordings, TRPM5 channels are activated by micromolar concentrations of Ca2+ and are permeable to monovalent but not divalent cations. TRPM5 channel activity is strongly regulated by voltage, phosphoinositides and temperature, and is blocked by acid pH. Study of TRPM4 and TRPM8, which show similar modes of regulation, has yielded insights into possible structural domains of TRPM5. Understanding the structural basis for TRPM5 function will ultimately allow the design of pharmaceuticals to enhance or interfere with taste sensations.

Insights into TRPM4 function, regulation and physiological role

In the current review we will summarise data from the recent literature describing molecular and functional properties of TRPM4. Together with TRPM5, these channels are up till now the only molecular candidates for a class of non-selective, Ca(2+)-impermeable cation channels which are activated by elevated Ca2+ levels in the cytosol. Apart from intracellular Ca2+, TRPM4 activation is also dependent on membrane potential. Additionally, channel activity is modulated by ATP, phosphatidylinositol bisphosphate (PiP2), protein kinase C (PKC) phosphorylation and heat. The molecular determinants for channel activation, permeation and modulation are increasingly being clarified, and will be discussed here in detail. The physiological role of Ca(2+)-activated non-selective cation channels is unclear, especially in the absence of gene-specific knock-out mice, but evidence indicates a role as a regulator of membrane potential, and thus the driving force for Ca2+ entry from the extracellular medium.

TRPM2

TRPM2 is a cation channel enabling influx of Na+ and Ca2+, leading to depolarization and increases in the cytosolic Ca2+ concentration ([Ca2+]i). It is widely expressed, e.g. in many neurons, blood cells and the endocrine pancreas. Channel gating is induced by ADP-ribose (ADPR) that binds to a Nudix box motif in the cytosolic C-terminus of the channel. Endogenous ADPR concentrations in leucocytes are sufficiently high to activate TRPM2 in the presence of an increased [Ca2+]i but probably not at resting [Ca2+]i. Another channel activator is oxidative stress, especially hydrogen peroxide (H2O2) that may act through ADPR after ADPR polymers have been formed by poly(ADP-ribose) polymerases (PARPs) and hydolysed by glycohydrolases. H2O2-stimulated TRPM2 channels essentially contribute to insulin secretion in pancreatic beta-cells and alloxan-induced diabetes mellitus. Inhibition of TRPM2 channels may be achieved by channel blockers such as flufenamic acid or the anti-fungal agents clotrimazole or econazole. Selective blockers of TRPM2 are not yet available; those would be valuable for a characterization of biological roles of TRPM2 in various tissues and as potential drugs directed against oxidative cell damage, reperfusion injury or leucocyte activation. Activation of TRPM2 may be prevented by anti-oxidants, PARP inhibitors and glycohydrolase inhibitors. In future, binding of ADPR to the Nudix box may be targeted. In light of the wide-spread expression and growing list of cellular functions of TRPM2, useful therapeutic applications are expected for future drugs that block TRPM2 channels or inhibit their activation.

C-type natriuretic peptide activates a non-selective cation current in acutely isolated rat cardiac fibroblasts via natriuretic peptide C receptor-mediated signalling

In the heart, fibroblasts play an essential role in the deposition of the extracellular matrix and they also secrete a number of hormonal factors. Although natriuretic peptides, including C-type natriuretic peptide (CNP) and brain natriuretic peptide, have antifibrotic effects on cardiac fibroblasts, the effects of CNP on fibroblast electrophysiology have not been examined. In this study, acutely isolated ventricular fibroblasts from the adult rat were used to measure the effects of CNP (2 x 10(-8) M) under whole-cell voltage-clamp conditions. CNP, as well as the natriuretic peptide C receptor (NPR-C) agonist cANF (2 x 10(-8) M), significantly increased an outwardly rectifying non-selective cation current (NSCC). This current has a reversal potential near 0 mV. Activation of this NSCC by cANF was abolished by pre-treating fibroblasts with pertussis toxin, indicating the involvement of G(i) proteins. The cANF-activated NSCC was inhibited by the compounds Gd(3+), SKF 96365 and 2-aminoethoxydiphenyl borate. Quantitative RT-PCR analysis of mRNA from rat ventricular fibroblasts revealed the expression of several transient receptor potential (TRP) channel transcripts. Additional electrophysiological analysis showed that U73122, a phospholipase C antagonist, inhibited the cANF-activated NSCC. Furthermore, the effects of CNP and cANF were mimicked by the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG), independently of protein kinase C activity. These are defining characteristics of specific TRPC channels. More detailed molecular analysis confirmed the expression of full-length TRPC2, TRPC3 and TRPC5 transcripts. These data indicate that CNP, acting via the NPR-C receptor, activates a NSCC that is at least partially carried by TRPC channels in cardiac fibroblasts.

Calcium-Sensing Receptor Stimulation Induces Nonselective Cation Channel Activation in Breast Cancer Cells

The calcium-sensing receptor (CaR) is expressed in epithelial ducts of both normal human breast and breast cancer tissue, as well as in the MCF-7 cell line as assessed by immunohistochemistry and Western blot analysis. However, to date, there are no data regarding the transduction pathways of CaR in breast cancer cells. In this study, we show that a CaR agonist, spermine, and increased extracellular Ca(2+) ([Ca(2+)](o)) sequentially activate two inward currents at -80 mV. The first was highly permeable to Ca(2+) and inhibited by 2-aminophenyl borate (2-APB). In contrast, the second was more sensitive to Na(+) and Li(+) than to Ca(2+) and insensitive to 2-APB. Furthermore, intracellular dialysis with high Mg(2+), flufenamic acid or amiloride perfusion was without any effect on the second current. Both currents were inhibited by La(3+). Calcium imaging recordings showed that both [Ca(2+)](o) and spermine induced an increase in intracellular calcium ([Ca(2+)](i)) and that removal of extracellular Ca(2+) or perfusion of 2-APB caused a decline in [Ca(2+)](i). It is well known that stimulation of CaR by an increase in [Ca(2+)](o) or with spermine is associated with activation of phospholipase C (PLC). Inhibition of PLC reduced the [Ca(2+)](o)-stimulated [Ca(2+)](i) increase. Lastly, reverse-transcriptase polymerase chain reaction showed that MCF-7 cells expressed canonical transient receptor potential (TRPCs) channels. Our results suggest that, in MCF-7 cells, CaR is functionally coupled to Ca(2+)-permeable cationic TRPCs, for which TRPC1 and TRPC6 are the most likely candidates for the highly selective Ca(2+) current. Moreover, the pharmacology of the second Na(+) current excludes the involvement of the more selective Na(+) transient receptor potential melastatin (TRPM4 and TRPM5) and the classical epithelial Na(+ )channels.

Functional expression of the TRPM4 cationic current in ventricular cardiomyocytes from spontaneously hypertensive rats

Cardiac hypertrophy is associated with electrophysiological modifications, including modification of action potential shape that can give rise to arrhythmias. We report here a higher detection of a calcium-activated nonselective cation current in cardiomyocytes of spontaneously hypertensive rats (SHRs), a model of hypertension and heart hypertrophy when compared with Wistar-Kyoto (WKY) rat, its normotensive equivalent. Freshly isolated cells from the left ventricles of 3- to 6-month-old WKY rats or SHRs were used for patch-clamp recordings. In inside-out patches, the channel presented a linear conductance of 25+/-0.5 pS, did not discriminate Na(+) over K(+), and was not permeable to Ca(2+). Open probability was increased by depolarization and a rise in [Ca(2+)](i) (dissociation constant=10+/-5.4 micromol/L) but reduced by 0.5 mmol/L [ATP](i), 10 micromol/L glibenclamide, or flufenamic acid (IC(50)=5.5+/-1.7 micromol/L). Thus, it owns the fingerprint of the TRPM4 current. Although rarely detected in WKY cardiomyocytes, the current was present in >50% of patches from SHR cardiomyocytes. Moreover, by performing RT-PCR from ventricular samples, we observed that TRPM4 mRNA detection was higher in SHRs than in WKY rats. We propose that a TRPM4 current is expressed in ventricular cardiomyocytes from SHRs. According to its properties, this channel may contribute to the transient inward current implicated in delayed-after-depolarizations observed during [Ca(2+)] overload of cardiomyocytes.

A road map for TR(I)Ps

The development of our knowledge on the structure, molecular regulation, and cell function on transient receptor potential (TRP) channels has been growing dramatically during the last few years. Many meetings in the past and upcoming events are now focused on TRP channels as general sensor molecules in cell physiology. However, most of the scientists in the field still feel that we are just beginning to understand these truly remarkable proteins, called TRPs, and there is still a long way to go from structure via molecular regulation to cell and organ function. This generally accepted but exciting view about the long road to the understanding of TRPs dominated all presentations given at the 2006 Minerva-Gentner Symposium on TRP channels and calcium signalling, which was held in Eilat, Israel, and was excellently organized by Baruch Minke (Jerusalem, Israel) and supported by Veit Flockerzi (Homburg, Germany).

An introduction to TRP channels

The aim of this review is to provide a basic framework for understanding the function of mammalian transient receptor potential (TRP) channels, particularly as they have been elucidated in heterologous expression systems. Mammalian TRP channel proteins form six-transmembrane (6-TM) cation-permeable channels that may be grouped into six subfamilies on the basis of amino acid sequence homology (TRPC, TRPV, TRPM, TRPA, TRPP, and TRPML). Selected functional properties of TRP channels from each subfamily are summarized in this review. Although a single defining characteristic of TRP channel function has not yet emerged, TRP channels may be generally described as calcium-permeable cation channels with polymodal activation properties. By integrating multiple concomitant stimuli and coupling their activity to downstream cellular signal amplification via calcium permeation and membrane depolarization, TRP channels appear well adapted to function in cellular sensation. Our review of recent literature implicating TRP channels in neuronal growth cone steering suggests that TRPs may function more widely in cellular guidance and chemotaxis. The TRP channel gene family and its nomenclature, the encoded proteins and alternatively spliced variants, and the rapidly expanding pharmacology of TRP channels are summarized in online supplemental material.

From cardiac cation channels to the molecular dissection of the transient receptor potential channel TRPM4

In 2006, we celebrate not only the milestone paper on the patch-clamp technique but also the publication of the first single-channel measurements in cardiac cells revealing a Ca(2+)-activated, nonselective cation channel. Considerable effort has been undertaken since this time to identify molecular candidates for this class of cation channels that can be found in a variety of tissues. Recent work has shown that this channel is very likely TRPM4, a member of the TRPM ion channel family. The current review links the epochal Colquhoun et al. paper to the detailed molecular knowledge and structure function aspects of this TRP channel. It will be shown that TRPM4 is a Ca(2+)- and voltage-activated channel, which is dramatically modulated by the phospholipid phosphatidyl inositol bisphosphate (PIP(2)) and belongs to the heat-activated thermoTRPs. A functional hallmark of TRPM4, as for several TRP channels, is a dramatic shift of its voltage dependence towards negative, physiologically meaningful potentials.

A "sample-and-hold" pulse-counting integrator as a mechanism for graded memory underlying sensorimotor adaptation

The mechanisms behind the induction of cellular correlates of memory by sensory input and their contribution to meaningful behavioral changes are largely unknown. We previously reported a graded memory in the form of sensorimotor adaptation in the electromotor output of electric fish. Here we show that the mechanism for this adaptation is a synaptically induced long-lasting shift in intrinsic neuronal excitability. This mechanism rapidly integrates hundreds of spikes in a second, or gradually integrates the same number of spikes delivered over tens of minutes. Thus, this mechanism appears immune to frequency-dependent fluctuations in input and operates as a simple pulse counter over a wide range of time scales, enabling it to transduce graded sensory information into a graded memory and a corresponding change in the behavioral output. This adaptation is based on an NMDA receptor-mediated change in intrinsic excitability of the postsynaptic neurons involving the Ca2+-dependent activation of TRP channels.

Effects of intracellular pH and Ca2+ on the activity of stretch-sensitive cation channels in leech neurons

The effects of intracellular pH and calcium on the activity of the leech mechanosensitive cation channels have been studied. These channels exhibited two activity modes denoted as spike-like (SL) and multiconductance (MC). In the absence of mechanical stimulation, acidification of the intracellular side of membrane patches from 7.2 to 6.2 reversibly increased the mean channel open time as well as the opening frequency in the SL mode. Channels in MC mode were activated by a pH(i) reduction from 7.2 to 6.2, but were inhibited at pH(i) 5.5. Unlike MC mode, SL mode was strongly activated by intracellular Ca(2+). Fura-2 imaging experiments showed that intracellular calcium was induced to increase by hypotonic cell swelling. The major component of this response did not require extracellular calcium. A component of the swelling-induced calcium response was sensitive to blockers of stretch-sensitive cation channels. The results indicate that the two activity modes of mechanosensitive channels of leech neurons respond differently to changes of intracellular pH and calcium. The sensitivity of the channel to micromolar concentrations of internal free calcium, along with its permeability to this ion, is consistent with a role in the amplification of mechanically induced Ca(2+) signals in leech neurons.

PI3-kinase promotes TRPV2 activity independently of channel translocation to the plasma membrane

Cellular or chemical activators for most transient receptor potential channels of the vanilloid subfamily (TRPV) have been identified in recent years. A remarkable exception to this is TRPV2, for which cellular events leading to channel activation are still a matter of debate. Diverse stimuli such as extreme heat or phosphatidylinositol-3 kinase (PI3-kinase) regulated membrane insertion have been shown to promote TRPV2 channel activity. However, some of these results have proved difficult to reproduce and may underlie different gating mechanisms depending on the cell type in which TRPV2 channels are expressed. Here, we show that expression of recombinant TRPV2 can induce cytotoxicity that is directly related to channel activity since it can be prevented by introducing a charge substitution in the pore-forming domain of the channel, or by reducing extracellular calcium. In stably transfected cells, TRPV2 expression results in an outwardly rectifying current that can be recorded at all potentials, and in an increase of resting intracellular calcium concentration that can be partly prevented by serum starvation. Using cytotoxicity as a read-out of channel activity and direct measurements of cell surface expression of TRPV2, we show that inhibition of the PI3-kinase decreases TRPV2 channel activity but does not affect the trafficking of the channel to the plasma membrane. It is concluded that PI3-kinase induces or modulates the activity of recombinant TRPV2 channels; in contrast to the previously proposed mechanism, activation of TRPV2 channels by PI3-kinase is not due to channel translocation to the plasma membrane.

The Ca2+-activated cation channel TRPM4 is regulated by phosphatidylinositol 4,5-biphosphate

Transient receptor potential (TRP) channel, melastatin subfamily (TRPM)4 is a Ca2+-activated monovalent cation channel that depolarizes the plasma membrane and thereby modulates Ca2+ influx through Ca2+-permeable pathways. A typical feature of TRPM4 is its rapid desensitization to intracellular Ca2+ ([Ca2+]i). Here we show that phosphatidylinositol 4,5-biphosphate (PIP2) counteracts desensitization to [Ca2+]i in inside-out patches and rundown of TRPM4 currents in whole-cell patch-clamp experiments. PIP2 shifted the voltage dependence of TRPM4 activation towards negative potentials and increased the channel's Ca2+ sensitivity 100-fold. Conversely, activation of the phospholipase C (PLC)-coupled M1 muscarinic receptor or pharmacological depletion of cellular PIP2 potently inhibited currents through TRPM4. Neutralization of basic residues in a C-terminal pleckstrin homology (PH) domain accelerated TRPM4 current desensitization and strongly attenuated the effect of PIP2, whereas mutations to the C-terminal TRP box and TRP domain had no effect on the PIP2 sensitivity. Our data demonstrate that PIP2 is a strong positive modulator of TRPM4, and implicate the C-terminal PH domain in PIP2 action. PLC-mediated PIP2 breakdown may constitute a physiologically important brake on TRPM4 activity.

Heat activation of TRPM5 underlies thermal sensitivity of sweet taste

TRPM5, a cation channel of the TRP superfamily, is highly expressed in taste buds of the tongue, where it has a key role in the perception of sweet, umami and bitter tastes. Activation of TRPM5 occurs downstream of the activation of G-protein-coupled taste receptors and is proposed to generate a depolarizing potential in the taste receptor cells. Factors that modulate TRPM5 activity are therefore expected to influence taste. Here we show that TRPM5 is a highly temperature-sensitive, heat-activated channel: inward TRPM5 currents increase steeply at temperatures between 15 and 35 degrees C. TRPM4, a close homologue of TRPM5, shows similar temperature sensitivity. Heat activation is due to a temperature-dependent shift of the activation curve, in analogy to other thermosensitive TRP channels. Moreover, we show that increasing temperature between 15 and 35 degrees C markedly enhances the gustatory nerve response to sweet compounds in wild-type but not in Trpm5 knockout mice. The strong temperature sensitivity of TRPM5 may underlie known effects of temperature on perceived taste in humans, including enhanced sweetness perception at high temperatures and 'thermal taste', the phenomenon whereby heating or cooling of the tongue evoke sensations of taste in the absence of tastants.

The Channel Kinases TRPM6 and TRPM7 Are Functionally Nonredundant

TRPM7 and its closest homologue, TRPM6, are the only known fusions of an ion channel pore with a kinase domain. Deletion of TRPM7 in DT40 B-lymphocytes causes growth arrest, Mg(2+) deficiency, and cell death within 24-48 h. Amazingly, in analogy to TRPM6-deficient patients who can live a normal life if provided with a Mg(2+)-rich diet, TRPM7-deficient DT40 B-lymphocytes show wild type cell growth if supplied with 5-10 mm Mg(2+) concentrations in their extracellular medium. Here we have investigated the functional relationship between TRPM6 and TRPM7. We show that TRPM7 deficiency in DT40 cells cannot be complemented by heterologously expressed TRPM6. Nevertheless, both channels can influence each other's biological activity. Our data demonstrate that TRPM6 requires TRPM7 for surface expression in HEK-293 cells and also that TRPM6 is capable of cross-phosphorylating TRPM7 as assessed using a phosphothreonine-specific antibody but not vice versa. TRPM6 and TRPM7 coexpression studies in DT40 B-cells indicate that TRPM6 can modulate TRPM7 function. In conclusion, although TRPM6 and TRPM7 are closely related and deficiency in either one of these molecules severely affects Mg(2+) homeostasis regulation, TRPM6 and TRPM7 do not appear to be functionally redundant but rather two unique and essential components of vertebrate ion homeostasis regulation.

Phosphatidylinositol 4,5-bisphosphate rescues TRPM4 channels from desensitization

TRPM4 is a Ca(2+)-activated nonselective cation channel that regulates membrane potential in response to intracellular Ca(2+) signaling. In lymphocytes it plays an essential role in shaping the pattern of intracellular Ca(2+) oscillations that lead to cytokine secretion. To better understand its role in this and other physiological processes, we investigated mechanisms by which TRPM4 is regulated. TRPM4 was expressed in ChoK1 cells, and currents were measured in excised patches. Under these conditions, TRPM4 currents were activated by micromolar concentrations of cytoplasmic Ca(2+) and progressively desensitized. Here we show that desensitization can be explained by a loss of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) from the channels. Poly-l-lysine, a PI(4,5)P(2) scavenger, caused rapid desensitization, whereas MgATP, at concentrations that activate lipid kinases, promoted recovery of TRPM4 currents. Application of exogenous PI(4,5)P(2) to the intracellular surface of the patch restored the properties of TRPM4 currents. Our results suggest that PI(4,5)P(2) acts to uncouple channel opening from changes in the transmembrane potential, allowing current activation at physiological voltages. These data argue that hydrolysis of PI(4,5)P(2) underlies desensitization of TRPM4 and support the idea that PI(4,5)P(2) is a general regulator for the gating of TRPM ion channels.

TRP channels: a TR(I)P through a world of multifunctional cation channels

The "transient receptor potential" (TRP) family of ion channels comprises more than 50 cation-permeable channels expressed from yeast to man. On the basis of structural homology, the TRP family can be subdivided in to seven main subfamilies: the TRPC ('Canonical') group, the TRPV ('Vanilloid') group, the TRPM ('Melastatin') group, the TRPP ('Polycystin'), the TRPML ('Mucolipin'), the TRPA ('Ankyrin') and the TRPN ('NOMP') family. The cloning and characterization of members of this cation channel family has exploded during recent years, leading to a plethora of data concerning TRPs in a variety of cell types, tissues and species. This paper briefly reviews the TRP superfamily and the basic properties of its many members as a reader's guide in this Special Issue. Hopefully, a better understanding of TRP channel physiology will provide important insight into the relationship between TRP channel dysfunction and human diseases.

The mammalian melastatin-related transient receptor potential cation channels: an overview

The mammalian melastatin-related transient receptor potential (TRPM) subfamily contains eight members. TRPM proteins, consisting of six putative transmembrane domains and intracellular N and C termini, form monovalent-permeable cation channels with variable selectivity for Ca(2+), Mg(2+) and other divalent cations. Some functions are linked to their individual cation selectivity: the highly divalent-permeable cation channels TRPM6 and TRPM7 are involved in the control of Mg(2+) influx, whereas the Ca(2+)-impermeable channels TRPM4 and TRPM5 modulate cellular Ca(2+) entry by determining the membrane potential. TRPM2, TRPM3 and TRPM8 mediate a direct influx of Ca(2+) in response to specific stimuli. Electrophysiological properties of the founding member, melastatin (TRPM1), are unexplored. The individual TRPM members are activated by different stimuli, including voltage, Ca(2+), temperature, cell swelling, lipid compounds and other endogenous or exogenous ligands. This review summarizes molecular features, activation mechanisms, biophysical properties and modulators of TRPM channels.

Gating of TRP channels: a voltage connection?

TRP channels represent the main pathways for cation influx in non-excitable cells. Although TRP channels were for a long time considered to be voltage independent, several TRP channels now appear to be weakly voltage dependent with an activation curve extending mainly into the non-physiological positive voltage range. In connection with this voltage dependence, there is now abundant evidence that physical stimuli, such as temperature (TRPV1, TRPM8, TRPV3), or the binding of various ligands (TRPV1, TRPV3, TRPM8, TRPM4), shift this voltage dependence towards physiologically relevant potentials, a mechanism that may represent the main functional hallmark of these TRP channels. This review discusses some features of voltage-dependent gating of TRPV1, TRPM4 and TRPM8. A thermodynamic principle is elaborated, which predicts that the small gating charge of TRP channels is a crucial factor for the large voltage shifts induced by various stimuli. Some structural considerations will be given indicating that, although the voltage sensor is not yet known, the C-terminus may substantially change the voltage dependence of these channels.

The selectivity filter of the cation channel TRPM4

Transient receptor potential channel melastatin subfamily (TRPM) 4 and its close homologue, TRPM5, are the only two members of the large transient receptor potential superfamily of cation channels that are impermeable to Ca(2+). In this study, we located the TRPM4 selectivity filter and investigated possible structural elements that render it Ca(2+)-impermeable. Based on homology with known cation channel pores, we identified an acidic stretch of six amino acids in the loop between transmembrane helices TM5 and TM6 ((981)EDMDVA(986)) as a potential selectivity filter. Substitution of this six-amino acid stretch with the selectivity filter of TRPV6 (TIIDGP) resulted in a functional channel that combined the gating hallmarks of TRPM4 (activation by Ca(2+), voltage dependence) with TRPV6-like sensitivity to block by extracellular Ca(2+) and Mg(2+) as well as Ca(2+) permeation. Neutralization of Glu(981) resulted in a channel with normal permeability properties but a strongly reduced sensitivity to block by intracellular spermine. Neutralization of Asp(982) yielded a functional channel that exhibited extremely fast desensitization (tau < 5 s), possibly indicating destabilization of the pore. Neutralization of Asp(984) resulted in a non-functional channel with a dominant negative phenotype when coexpressed with wild type TRPM4. Combined neutralization of all three acidic residues resulted in a functional channel whose voltage dependence was shifted toward very positive potentials. Substitution of Gln(977) by a glutamate, the corresponding residue in divalent cation-permeable TRPM channels, altered the monovalent cation permeability sequence and resulted in a pore with moderate Ca(2+) permeability. Our findings delineate the selectivity filter of TRPM channels and provide the first insight into the molecular basis of monovalent cation selectivity.