Critical role for transient receptor potential channel TRPM4 in myogenic constriction of cerebral arteries

Transient receptor potential channels as drug targets

The mammalian transient receptor potential (TRP) superfamily of ion channels consists of voltage-independent, non-selective cation channels that are expressed in excitable and non-excitable cells. The biologic roles of TRP channels are diverse and include vascular tone, thermosensation, irritant stimuli sensing and flow sensing in the kidney. TRP channels are a relatively new target in therapeutic drug discovery. During the past few years, pharmaceutical companies have focused their discovery efforts on developing TRP channel modulators with potential therapeutic value. This review focuses on the potential therapeutic benefits of drugs targeting TRP ion channels.

Increased IgE-dependent mast cell activation and anaphylactic responses in mice lacking the calcium-activated nonselective cation channel TRPM4

Mast cells are key effector cells in allergic reactions. Aggregation of the receptor FcepsilonRI in mast cells triggers the influx of calcium (Ca(2+)) and the release of inflammatory mediators. Here we show that transient receptor potential TRPM4 proteins acted as calcium-activated nonselective cation channels and critically determined the driving force for Ca(2+) influx in mast cells. Trpm4(-/-) bone marrow-derived mast cells had more Ca(2+) entry than did TRPM4(+/+) cells after FcepsilonRI stimulation. Consequently, Trpm4(-/-) bone marrow-derived mast cells had augmented degranulation and released more histamine, leukotrienes and tumor necrosis factor. Trpm4(-/-) mice had a more severe IgE-mediated acute passive cutaneous anaphylactic response, whereas late-phase passive cutaneous anaphylaxis was not affected. Our results establish the physiological function of TRPM4 channels as critical regulators of Ca(2+) entry in mast cells.

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.

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.

Characteristics of myogenic tone in the rat ophthalmic artery

The pressure-induced constriction in the rat ophthalmic artery was characterized. Ophthalmic arteries were isolated, cannulated in an arteriograph, and pressurized. Arteries developed 25% constriction at 70 mmHg of intraluminal pressure. Arteries maintained almost similar diameter over the range of pressures 50–210 mmHg, and forced dilatation was observed at pressures >210 mmHg. Denudation of endothelium increased the sensitivity of arteries to pressure-induced constriction, and significantly higher myogenic tone was observed in the pressure range of 10–100 mmHg. Indomethacin and cyclooxygenase-2 inhibition by SC-236 decreased myogenic tone, whereas cyclooxygenase-1 inhibition by SC-560 potentiated myogenic tone in a lower concentration range and decreased at a higher concentration. Pressure-induced constriction was completely blocked by 1 μM nifedipine. Phospholipase C inhibition by 6 μM U-73122 decreased myogenic tone by 39%, whereas PKC inhibitor GF-109203X (3 μM) had no effect. Constriction to phenylephrine was significantly decreased by U-73122 (1 μM) and GF-109203X (3 μM) at an intraluminal pressure of 10 mmHg. Rho-kinase inhibition by Y-27632 (30 μM) and HA-1077 (30 μM) decreased myogenic tone by 75% and 73%, respectively, and 1 μM Y-27632 significantly decreased myogenic tone developed in response to graded increases in pressure. These results suggest that rat ophthalmic artery has an efficient pressure-dependent autoregulatory function that is modulated by endothelium. Contribution of phospholipase C-activation to myogenic tone is minimal, whereas Rho-kinase activation plays a predominant role in the myogenic reactivity in this artery.

Transient Receptor Potential Channels in Mechanosensing and Cell Volume Regulation

Transient receptor potential (TRP) channels are unique cellular sensors responding to a wide variety of extra- and intracellular signals, including mechanical and osmotic stress. In recent years, TRP channels from multiple subfamilies have been added to the list of mechano- and/or osmosensitive channels, and it is becoming increasingly apparent that Ca(2+) influx via TRP channels plays a crucial role in the response to mechanical and osmotic perturbations in a wide range of cell types. Although the events translating mechanical and osmotic stimuli into regulation of TRP channels are still incompletely understood, the specific mechanisms employed vary between different TRP isoforms, and probably include changes in the tension and/or curvature of the lipid bilayer, changes in the cortical cytoskeleton, and signaling events such as lipid metabolism and protein phosphorylation/dephosphorylation. This chapter describes candidate mechanosensitive channels from mammalian TRP subfamilies, discusses inherent and technical issues potentially confounding evaluation of mechano- and/or osmosensitivity, and presents methods relevant to the study of TRP channel regulation by mechanical and osmotic stimuli and involvement in cell volume regulation.

TRPM7 is a stretch- and swelling-activated cation channel involved in volume regulation in human epithelial cells

Stretch- and swelling-activated cation (SSAC) channels play essential roles not only in sensing and transducing external mechanical stresses but also in regulating cell volume in living cells. However, the molecular nature of the SSAC channel has not been clarified. In human epithelial HeLa cells, single-channel recordings in cell-attached and inside-out patches revealed expression of a Mg2+- and Gd3+-sensitive nonselective cation channel that is exquisitely sensitive to membrane stretch. Whole cell recordings revealed that the macroscopic cationic currents exhibit transient receptor potential (TRP) melastatin (TRPM)7-like properties such as outward rectification and sensitivity to Mg2+ and Gd3+. The whole cell cation current was augmented by osmotic cell swelling. RT-PCR and Western blotting demonstrated molecular expression of TRPM7 in HeLa cells. Treatment with small interfering RNA (siRNA) targeted against TRPM7 led to abolition of single stretch-activated cation channel currents and of swelling-activated, whole cell cation currents in HeLa cells. The silencing of TRPM7 by siRNA reduced the rate of cell volume recovery after osmotic swelling. A similar inhibition of regulatory volume decrease was also observed when extracellular Ca2+ was removed or Gd3+ was applied. It is thus concluded that TRPM7 represents the SSAC channel endogenously expressed in HeLa cells and that, by serving as a swelling-induced Ca2+ influx pathway, it plays an important role in cell volume regulation.

Membrane Stretch-Induced Activation of a TRPM4-Like Nonselective Cation Channel in Cerebral Artery Myocytes

Stretch-activated cation channels (SACs) have been observed in many types of smooth muscle cells. However, the molecular identity and activation mechanisms of SACs remain poorly understood. We report that TRPM4-like cation channels are activated by membrane stretch in rat cerebral artery myocytes (CAMs). Negative pressure (> or =20 mmHg, cell-attached mode) activated single channels (approximately 20 pS) in isolated CAMs. These channels were permeable to Na(+) and Cs(+) and inhibited by Gd(3+) (30 microM) and DIDS (100 microM). The effect of negative pressure was abolished by membrane excision, but subsequent application of Ca(2+) (>100 nM) to the intracellular side of the membrane restored single channel activity that was indistinguishable from SACs. Caffeine (5 mM), which depletes SR Ca(2+)-stores, first activated and then abolished SACs. Tetracaine (100 microM), a ryanodine receptor antagonist, inhibited SACs. Overexpression of hTRPM4B in HEK293 cells resulted in the appearance of cation channels that were activated by both negative pressure and Ca(2+) and which had very similar biophysical and pharmacological properties as compared with SACs in CAMs. These studies indicate that TRPM4-like channels in CAMs can be activated by membrane stretch, possibly through ryanodine receptor activation, and this may contribute to the depolarization and concomitant vasoconstriction of intact cerebral arteries following mechanical stimulation.

TRPC6

TRPC6 is a Ca(2+)-permeable non-selective cation channel expressed in brain, smooth muscle containing tissues and kidney, as well as in immune and blood cells. Channel homomers heterologously expressed have a characteristic doubly rectifying current-voltage relationship and are six times more permeable for Ca2+ than for Na+. In smooth muscle tissues, however, Na+ influx and activation of voltage-gated calcium channels by membrane depolarization rather than Ca2+ elevation by TRPC6 channels is the driving force for contraction. TRPC6 channels are directly activated by the second messenger diacylglycerol (DAG) and regulated by specific tyrosine or serine phosphorylation. Extracellular Ca2+ has inhibitory effects, while Ca2+/calmodulin acting from the intracellular side has potentiator effects on channel activity. Given its specific expression, TRPC6 is likely to play a number of physiological roles. Studies with TRPC6(-/-) mice suggest a role for the channel in the regulation of vascular and pulmonary smooth muscle contraction. TRPC6 was identified as an essential component of the slit diaphragm architecture of kidney podocytes. Other functions in immune and blood cells, as well as in brain and in smooth muscle-containing tissues such as stomach, colon and myometrium, remain elusive.

Myogenic vasoconstriction in mouse renal interlobar arteries: role of endogenous β and γENaC

Mechanosensitive ion channels are thought to initiate pressure-induced vasoconstriction, however, the molecular identity of these channels is unknown. Recent work from our laboratory suggests that members of the Degenerin/Epithelial Na+ Channel (DEG/ENaC) family may be components of the mechanosensitive ion channel complex in vascular smooth muscle (VSM); however, the specific DEG/ENaC proteins mediating myogenic constriction are unknown. The goal of this study is to determine if specific knockdown of β or γENaC, using dominant-negative (DN) or small-interference RNA (siRNA) molecules, inhibits pressure-induced vasoconstriction in mouse renal interlobar arteries. To address this goal, isolated arteries were transiently transfected with β or γENaC DN-cDNA or siRNA molecules. After 24 h, vessels were either 1) cannulated and pressurized for pressure-diameter response curves or 2) dissociated and immunolabeled to determine VSM cell endogenous ENaC protein expression. We found that transfection of βENaC DN-cDNA or siRNA suppresses β-, but not γENaC protein expression. Similarly, γENaC DN-cDNA or siRNA suppresses γ-, but not βENaC protein expression. In addition, transfection of β- or γENaC DN-cDNA or siRNA molecules inhibits pressure-induced vasoconstriction, but does not block agonist-induced vasoconstriction. Our results provide the first direct evidence that β and γENaC proteins are essential in mediating myogenic vasoconstriction in mouse renal interlobar arteries.

TRP channels: molecular diversity and physiological function

Calcium ions (Ca(2+)) are particularly important in cellular homeostasis and activity. To elicit physiologically relevant timing and spatial patterns of Ca(2+) signaling, ion channels in the surface of each cell precisely control Ca(2+) influx across the plasma membrane. A group of surface membrane ion channels called receptor-activated cation/Ca(2+) channels (RACCs) are activated by diverse cellular stimuli from the surrounding extracellular environment via receptors and other pathways such as heat, osmotic pressure, and mechanical and oxidative stress. An important clue to understanding the molecular mechanisms underlying the functional diversity of RACCs was first attained by molecular identification of the transient receptor potential (trp) protein (TRP), which mediates light-induced depolarization in Drosophila photoreceptor cells, and its homologues from various biological species. Recent studies have revealed that respective TRP channels are indeed activated by characteristic cellular stimuli. Furthermore, the involvement of TRP channels has been demonstrated in the signaling pathways essential for tissue-specific functions as well as ubiquitous biological responses, such as cell proliferation, differentiation, and death. These findings encourage the usage of TRP channels and their signalplexes as powerful tools for developing novel pharmaceutical targets.

Hyposmotic challenge inhibits inward rectifying K+ channels in cerebral arterial smooth muscle cells

This study sought to define whether inward rectifying K(+) (K(IR)) channels were modulated by vasoactive stimuli known to depolarize and constrict intact cerebral arteries. Using pressure myography and patch-clamp electrophysiology, initial experiments revealed a Ba(2+)-sensitive K(IR) current in cerebral arterial smooth muscle cells that was active over a physiological range of membrane potentials and whose inhibition led to arterial depolarization and constriction. Real-time PCR, Western blot, and immunohistochemical analyses established the expression of both K(IR)2.1 and K(IR)2.2 in cerebral arterial smooth muscle cells. Vasoconstrictor agonists known to depolarize and constrict rat cerebral arteries, including uridine triphosphate, U46619, and 5-HT, had no discernable effect on whole cell K(IR) activity. Control experiments confirmed that vasoconstrictor agonists could inhibit the voltage-dependent delayed rectifier K(+) (K(DR)) current. In contrast to these observations, a hyposmotic challenge that activates mechanosensitive ion channels elicited a rapid and sustained inhibition of the K(IR) but not the K(DR) current. The hyposmotic-induced inhibition of K(IR) was 1) mimicked by phorbol-12-myristate-13-acetate, a PKC agonist; and 2) inhibited by calphostin C, a PKC inhibitor. These findings suggest that, by modulating PKC, mechanical stimuli can regulate K(IR) activity and consequently the electrical and mechanical state of intact cerebral arteries. We propose that the mechanoregulation of K(IR) channels plays a role in the development of myogenic tone.

Role of Cav1.2 L-type Ca2+ channels in vascular tone: effects of nifedipine and Mg2+

Ca(2+) entry via L-type voltage-gated Ca(2+) channels (LVGCs) is a key factor in generating myogenic tone (MT), as dihydropyridines (DHPs) and other LVGC blockers, including Mg(2+), markedly reduce MT. Recent reports suggest, however, that elevated external Mg(2+) concentration and DHPs may also inhibit other Ca(2+)-entry pathways. Here, we explore the contribution of LVGCs to MT in intact, pressurized mesenteric small arteries using mutant mice (DHP(R/R)) expressing functional but DHP-insensitive Ca(v)1.2 channels. In wild-type (WT), but not DHP(R/R), mouse arteries, nifedipine (0.3-1.0 microM) markedly reduced MT and vasoconstriction induced by high external K(+) concentrations ([K(+)](o)), a measure of LVGC-mediated Ca(2+) entry. Blocking MT and high [K(+)](o)-induced vasoconstriction by <1 microM nifedipine in WT but not in DHP(R/R) arteries implies that Ca(2+) entry via Ca(v)1.2 LVGCs is obligatory for MT and that nifedipine inhibits MT exclusively by blocking LVGCs. We also examined the effects of Mg(2+) on MT and LVGCs. High external Mg(2+) concentration (10 mM) blocked MT, slowed the high [K(+)](o)-induced vasoconstrictions, and decreased their amplitude in WT and DHP(R/R) arteries. To verify that these effects of Mg(2+) are due to block of LVGCs, we characterized the effects of extracellular and intracellular Mg(2+) on LVGC currents in isolated mesenteric artery myocytes. DHP-sensitive LVGC currents are inhibited by both external and internal Mg(2+). The results indicate that Mg(2+) relaxes MT by inhibiting Ca(2+) influx through LVGCs. These data provide new information about the central role of Ca(v)1.2 LVGCs in generating and maintaining MT in mouse mesenteric small arteries.

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 steady-state electrochemical model of vascular smooth muscle cells

A model of the steady-state electrochemical response of vascular smooth muscle cells to external stimuli is presented, which accounts for K, Na, and Ca fluxes. The results of the model are broadly in accordance with experimental data 1), at various transmural pressures; 2), with channel and pump blockade; and 3), under manipulation of external ionic concentrations. The model exhibits dual stable states which sometimes coexist, and abrupt transitions between these states may account for nongraded responses in arteries as external potassium or pressure is varied. The simulations suggest that changes in the intracellular sodium concentration ([Na]i) often accompany smooth muscle responses. For example, [Na]i values vary threefold over the range of pressures from 10 to 100 mmHg.

Transient receptor potential channels in cardiovascular function and disease

Sustained elevation in the intracellular Ca2+ concentration via Ca2+ influx, which is activated by a variety of mechanisms, plays a central regulatory role for cardiovascular functions. Recent molecular biological research has disclosed an unexpectedly diverse array of Ca(2+-entry channel molecules involved in this Ca2+ influx. These include more than ten transient receptor potential (TRP) superfamily members such as TRPC1, TRPC3-6, TRPV1, TRPV2, TRPV4, TRPM4, TRPM7, and polycystin (TRPP2). Most of them appear to be multimodally activated or modulated and show relevant features to both acute hemodynamic control and long-term remodeling of the cardiovascular system, and many of them have been found to respond not only to receptor stimulation but also to various forms of stimuli. There is good evidence to implicate TRPC1 in neointimal hyperplasia after vascular injury via store-depletion-operated Ca2+ entry. TRPC6 likely contributes to receptor-operated and mechanosensitive Ca2+ mobilizations, being involved in vasoconstrictor and myogenic responses and pulmonary arterial proliferation and its associated disease (idiopathic pulmonary arterial hypertension). Considerable evidence has also been accumulated for unique involvement of TRPV1 in blood flow/pressure regulation via sensory vasoactive neuropeptide release. New lines of evidence suggest that TRPV2 may act as a Ca2+-overloading pathway associated with dystrophic cardiomyopathy, TRPV4 as a mediator of endothelium-dependent hyperpolarization, TRPM7 as a proproliferative vascular Mg2+ entry channel, and TRPP2 as a Ca2+-entry channel requisite for vascular integrity. This review attempts to provide an overview of the current knowledge on TRP proteins and discuss their possible roles in cardiovascular functions and diseases.

Tissue distribution profiles of the human TRPM cation channel family

Eight members of the TRP-melastatin (TRPM) subfamily have been identified, whose physiological functions and distribution are poorly characterized. Although tissue expression and distribution patterns have been reported for individual TRPM channels, comparisons between individual studies are not possible because of variations in analysis techniques and tissue selection. We report here a comparative analysis of the expression patterns of all of the human TRPM channels in selected peripheral tissues and the central nervous system (CNS) using two distinct but complimentary approaches: TaqMan and SYBR Green real-time quantitative reverse transcription polymerase chain reaction (RT-PCR). These techniques generated comparative distribution profiles and demonstrated tissue-specific co-expression of TRPM mRNA species, indicating significant potential for the formation of heteromeric channels. TRPM channels 2, 4, 5, 6, and 7 in contrast to 1, 3, and 8 are widely distributed in the CNS and periphery. The tissues demonstrating highest expression for individual family members were brain (TRPM1), brain and bone marrow (TRPM2), brain and pituitary (TRPM3), intestine and prostate (TRPM4), intestine, pancreas, and prostate (TRPM5), intestine and brain (TRPM6), heart, pituitary, bone, and adipose tissue (TRPM7), and prostate and liver (TRPM8). The data reported here will guide the elucidation of TRPM channel physiological functions.

Cation channels of the transient receptor potential superfamily: their role in physiological and pathophysiological processes of smooth muscle cells

Smooth muscle cells (SMC) are essential components of many tissues of the body. Ion channels regulate their membrane potential, the intracellular Ca(2+) concentration ([Ca(2+)](i)) and their contractility. Among the ion channels expressed in SMC cation channels of the transient receptor potential (TRP) superfamily allow the entry of Na(+), Ca(2+) and Mg(2+). Members of the TRP superfamily are essential constituents of tonically active channels (TAC), receptor-operated channels (ROC), store-operated channels (SOC) and stretch-activated channels (SAC). This review focusses on TRP channels (TRPC1, TRPC3, TRPC4, TRPC5, TRPC6, TRPC7, TRPV2, TRPV4, TRPM4, TRPM7, TRPP2) whose physiological functions in SMC were dissected by downregulating channel activity in isolated tissues or by the analysis of gene-deficient mouse models. Their possible functional role and physiological regulation as homomeric or heteromeric channels in SMC are discussed. Moreover, TRP channels may also be responsible for pathophysiological processes involving SMC-like airway hyperresponsiveness and pulmonary hypertension. Therefore, they present important drug targets for future pharmacological interventions.

TRPM4 controls insulin secretion in pancreatic beta-cells

TRPM4 is a calcium-activated non-selective cation channel that is widely expressed and proposed to be involved in cell depolarization. In excitable cells, TRPM4 may regulate calcium influx by causing the depolarization that drives the activation of voltage-dependent calcium channels. We here report that insulin-secreting cells of the rat pancreatic beta-cell line INS-1 natively express TRPM4 proteins and generate large depolarizing membrane currents in response to increased intracellular calcium. These currents exhibit the characteristics of TRPM4 and can be suppressed by expressing a dominant negative TRPM4 construct, resulting in significantly decreased insulin secretion in response to a glucose stimulus. Reduced insulin secretion was also observed with arginine vasopressin stimulation, a Gq-coupled receptor agonist in beta-cells. Moreover, the recruitment of TRPM4 currents was biphasic in both INS-1 cells as well as HEK-293 cells overexpressing TRPM4. The first phase is due to activation of TRPM4 channels localized within the plasma membrane followed by a slower secondary phase, which is caused by the recruitment of TRPM4-containing vesicles to the plasma membrane during exocytosis. The secondary phase can be observed during perfusion of cells with increasing [Ca(2+)](i), replicated with agonist stimulation, and coincides with an increase in cell capacitance, loss of FM1-43 dye, and vesicle fusion. Our data suggest that TRPM4 may play a key role in the control of membrane potential and electrical activity of electrically excitable secretory cells and the dynamic translocation of TRPM4 from a vesicular pool to the plasma membrane via Ca(2+)-dependent exocytosis may represent a key short- and midterm regulatory mechanism by which cells regulate electrical activity.

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.

Heteromultimeric TRPC6-TRPC7 channels contribute to arginine vasopressin-induced cation current of A7r5 vascular smooth muscle cells

The molecular identity of receptor-operated, nonselective cation channels (ROCs) of vascular smooth muscle (VSM) cells is not known for certain. Mammalian homologues of the Drosophila canonical transient receptor potential channels (TRPCs) are possible candidates. This study tested the hypothesis that heteromultimeric TRPC channels contribute to ROC current of A7r5 VSM cells activated by [Arg(8)]-vasopressin. A7r5 cells expressed transcripts encoding TRPC1, TRPC4beta, TRPC6, and TRPC7. TRPC4, TRPC6, and TRPC7 protein expression was confirmed by immunoblotting and association of TRPC6 with TRPC7, but not TRPC4beta, was detected by coimmunoprecipitation. The amplitude of arginine vasopressin (AVP)-induced ROC current was suppressed by dominant-negative mutant TRPC6 (TRPC6(DN)) but not TRPC5 (TRPC5(DN)) mutant subunit expression. These data indicate a role for TRPC6- and/or TRPC7-containing channels and rule a more complex subunit composition including TRPC1 and TRPC4. Increasing extracellular Ca(2+) concentration ([Ca(2+)](o)) from 0.05 to 1 mmol/L suppressed currents owing to native, TRPC7, and heteromultimeric TRPC6-TRPC7 channels, but not TRPC6 current, which was slightly enhanced. The relative changes in native and heteromultimeric TRPC6-TRPC7 current amplitudes for [Ca(2+)](o) between approximately 0.01 and 1 mmol/L were identical, but the changes in homomultimeric TRPC6 and TRPC7 currents were significantly less and greater, respectively, compared with the native channels. Taken together, the data provide biochemical and functional evidence supporting the view that heteromultimeric TRPC6-TRPC7 channels contribute to receptor-activated, nonselective cation channels of A7r5 VSM cells.

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 sphingosine-1-phosphate-activated calcium channel controlling vascular smooth muscle cell motility

In a screen of potential lipid regulators of transient receptor potential (TRP) channels, we identified sphingosine-1-phosphate (S1P) as an activator of TRPC5. We explored the relevance to vascular biology because S1P is a key cardiovascular signaling molecule. TRPC5 is expressed in smooth muscle cells of human vein along with TRPC1, which forms a complex with TRPC5. Importantly, S1P also activates the TRPC5-TRPC1 heteromultimeric channel. Because TRPC channels are linked to neuronal growth cone extension, we considered a related concept for smooth muscle. We find S1P stimulates smooth muscle cell motility, and that this is inhibited by E3-targeted anti-TRPC5 antibody. Ion permeation involving TRPC5 is crucial because S1P-evoked motility is also suppressed by the channel blocker 2-aminoethoxydiphenyl borate or a TRPC5 ion-pore mutant. S1P acts on TRPC5 via two mechanisms, one extracellular and one intracellular, consistent with its bipolar signaling functions. The extracellular effect appears to have a primary role in S1P-evoked cell motility. The data suggest S1P sensing by TRPC5 calcium channel is a mechanism contributing to vascular smooth muscle adaptation.

Functional expression of transient receptor potential melastatin- and vanilloid-related channels in pulmonary arterial and aortic smooth muscle

Transient receptor potential melastatin- (TRPM) and vanilloid-related (TRPV) channels are nonselective cation channels pertinent to diverse physiological functions. Multiple TRPM and TRPV channel subtypes have been identified and cloned in different tissues. However, their information in vascular tissue is scant. In this study, we sought to identify TRPM and TRPV channel subtypes expressed in rat deendothelialized intralobar pulmonary arteries (PAs) and aorta. With RT-PCR, mRNA of TRPM2, TRPM3, TRPM4, TRPM7, and TRPM8 of TRPM family and TRPV1, TRPV2, TRPV3, and TRPV4 of TRPV family were detected in both PAs and aorta. Quantitative real-time RT-PCR showed that TRPM8 and TRPV4 were the most abundantly expressed TRPM and TRPV subtypes, respectively. Moreover, Western blot analysis verified expression of TRPM2, TRPM8, TRPV1, and TRPV4 proteins in both types of vascular tissue. To examine the functional activities of these channels, we monitored intracellular Ca(2+) transients ([Ca(2+)](i)) in pulmonary arterial smooth muscle cells (PASMCs) and aortic smooth muscle cells (ASMCs). The TRPM8 agonist menthol (300 muM) and the TRPV4 agonist 4alpha-phorbol 12,13-didecanoate (1 muM) evoked significant increases in [Ca(2+)](i) in PASMCs and ASMCs. These Ca(2+) responses were abolished in the absence of extracellular Ca(2+) or the presence of 300 muM Ni(2+) but were unaffected by 1 muM nifedipine, suggesting Ca(2+) influx via nonselective cation channels. Hence, for the first time, our results indicate that multiple functional TRPM and TRPV channels are coexpressed in rat intralobar PAs and aorta. These novel Ca(2+) entry pathways may play important roles in the regulation of pulmonary and systemic circulation.

Mechanisms of Mechanotransduction

Essentially all organisms from bacteria to humans are mechanosensitive. Physical forces regulate a large array of physiological processes, and dysregulation of mechanical responses contributes to major human diseases. A survey of both specialized and widely expressed mechanosensitive systems suggests that physical forces provide a general means of altering protein conformation to generate signals. Specialized systems differ mainly in having acquired efficient mechanisms for transferring forces to the mechanotransducers.

TRPV4 Forms a Novel Ca 2+ Signaling Complex With Ryanodine Receptors and BK Ca Channels

Vasodilatory factors produced by the endothelium are critical for the maintenance of normal blood pressure and flow. We hypothesized that endothelial signals are transduced to underlying vascular smooth muscle by vanilloid transient receptor potential (TRPV) channels. TRPV4 message was detected in RNA from cerebral artery smooth muscle cells. In patch-clamp experiments using freshly isolated cerebral myocytes, outwardly rectifying whole-cell currents with properties consistent with those of expressed TRPV4 channels were evoked by the TRPV4 agonist 4alpha-phorbol 12,13-didecanoate (4alpha-PDD) (5 micromol/L) and the endothelium-derived arachidonic acid metabolite 11,12 epoxyeicosatrienoic acid (11,12 EET) (300 nmol/L). Using high-speed laser-scanning confocal microscopy, we found that 11,12 EET increased the frequency of unitary Ca2+ release events (Ca2+ sparks) via ryanodine receptors located on the sarcoplasmic reticulum of cerebral artery smooth muscle cells. EET-induced Ca2+ sparks activated nearby sarcolemmal large-conductance Ca2+-activated K+ (BKCa) channels, measured as an increase in the frequency of transient K+ currents (referred to as "spontaneous transient outward currents" [STOCs]). 11,12 EET-induced increases in Ca2+ spark and STOC frequency were inhibited by lowering external Ca2+ from 2 mmol/L to 10 micromol/L but not by voltage-dependent Ca2+ channel inhibitors, suggesting that these responses require extracellular Ca2+ influx via channels other than voltage-dependent Ca2+ channels. Antisense-mediated suppression of TRPV4 expression in intact cerebral arteries prevented 11,12 EET-induced smooth muscle hyperpolarization and vasodilation. Thus, we conclude that TRPV4 forms a novel Ca2+ signaling complex with ryanodine receptors and BKCa channels that elicits smooth muscle hyperpolarization and arterial dilation via Ca2+-induced Ca2+ release in response to an endothelial-derived factor.

Recent developments in vascular endothelial cell transient receptor potential channels

Among the 28 identified and unique mammalian TRP (transient receptor potential) channel isoforms, at least 19 are expressed in vascular endothelial cells. These channels appear to participate in a diverse range of vascular functions, including control of vascular tone, regulation of vascular permeability, mechanosensing, secretion, angiogenesis, endothelial cell proliferation, and endothelial cell apoptosis and death. Malfunction of these channels may result in disorders of the human cardiovascular system. All TRP channels, except for TRPM4 and TRPM5, are cation channels that allow Ca2+ influx. However, there is a daunting diversity in the mode of activation and regulation in each case. Specific TRP channels may be activated by different stimuli such as vasoactive agents, oxidative stress, mechanical stimuli, and heat. TRP channels may then transform these stimuli into changes in the cytosolic Ca2+, which are eventually coupled to various vascular responses. Evidence has been provided to suggest the involvement of at least the following TRP channels in vascular function: TRPC1, TRPC4, TRPC6, and TRPV1 in the control of vascular permeability; TRPC4, TRPV1, and TRPV4 in the regulation of vascular tone; TRPC4 in hypoxia-induced vascular remodeling; and TRPC3, TRPC4, and TRPM2 in oxidative stress-induced responses. However, in spite of the large body of data available, the functional role of many endothelial TRP channels is still poorly understood. Elucidating the mechanisms regulating the different endothelial TRP channels, and the associated development of drugs selectively to target the different isoforms, as a means to treat cardiovascular disease should, therefore, be a high priority.

Emerging functions of 10 types of TRP cationic channel in vascular smooth muscle

1. The influx of Ca2+, Mg2+ and Na+ and the efflux of K+ have central importance for the function and survival of vascular smooth muscle cells, but progress in understanding the influx/efflux pathways has been restricted by a lack of identification of the genes underlying many of the non-voltage-gated cationic channels. 2. The present review highlights evidence suggesting the genes are mammalian homologues of the Transient Receptor Potential (TRP) gene of the fruit-fly Drosophila. The weight of evidence supports roles for TRPC1, TRPP2/1 and TRPC6, but recent studies point also to TRPC3, TRPC4/5, TRPV2, TRPM4 and TRPM7. 3. Activity of these TRP channels is suggested to modulate contraction and sense changes in intracellular Ca2+ storage, G-protein-coupled receptor activation and osmotic stress. Roles in relation to myogenic tone, actions of vasoconstrictors substances, Mg2+ homeostasis and the vascular injury response are suggested. 4. Knowledge that TRP channels are relevant to vascular smooth muscle cells in both their contractile and proliferative phenotypes should pave the way for a better understanding of vascular biology and provide the basis for the discovery of a new set of therapeutic agents targeted to vascular disease.

Signal transduction pathways and gating mechanisms of native TRP-like cation channels in vascular myocytes

Activation of Ca2+-permeable non-selective cation channels produces an increase in excitability of vascular smooth muscle cells which has an important role in vasoconstriction. These channels are activated by various physiological stimuli including vasoconstrictor agents such as noradrenaline, depletion of internal Ca2+ stores and cell stretching. In addition cation channels have been shown to be constitutively active and these channels are thought to contribute to resting membrane conductance and basal Ca2+ influx in vascular myocytes. Recent evidence has suggested that transient receptor potential (TRP) proteins represent strong candidates for these channels in the vasculature. This review discusses proposed signal transduction pathways and gating mechanisms which link physiological stimuli to opening of cation channels in vascular myocytes. It is apparent that G-protein-coupled pathways linked to stimulation of phospholipase activity have a profound effect on regulating channel activity and that generation of diacylglycerol (DAG) is a central event in these signalling cascades with this triglyceride having a pivotal role in gating cation channels via both PKC-independent and -dependent mechanisms. Moreover phosphorylation processes produced by stimulation of protein kinases have been proposed to have an important role in regulating cation channel activity.

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.

Increased vascular smooth muscle contractility in TRPC6-/- mice

Among the TRPC subfamily of TRP (classical transient receptor potential) channels, TRPC3, -6, and -7 are gated by signal transduction pathways that activate C-type phospholipases as well as by direct exposure to diacylglycerols. Since TRPC6 is highly expressed in pulmonary and vascular smooth muscle cells, it represents a likely molecular candidate for receptor-operated cation entry. To define the physiological role of TRPC6, we have developed a TRPC6-deficient mouse model. These mice showed an elevated blood pressure and enhanced agonist-induced contractility of isolated aortic rings as well as cerebral arteries. Smooth muscle cells of TRPC6-deficient mice have higher basal cation entry, increased TRPC-carried cation currents, and more depolarized membrane potentials. This higher basal cation entry, however, was completely abolished by the expression of a TRPC3-specific small interference RNA in primary TRPC6(-)(/)(-) smooth muscle cells. Along these lines, the expression of TRPC3 in wild-type cells resulted in increased basal activity, while TRPC6 expression in TRPC6(-/-) smooth muscle cells reduced basal cation influx. These findings imply that constitutively active TRPC3-type channels, which are up-regulated in TRPC6-deficient smooth muscle cells, are not able to functionally replace TRPC6. Thus, TRPC6 has distinct nonredundant roles in the control of vascular smooth muscle tone.

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

TRP channels in disease

The mammalian TRP (transient receptor potential) family consists of six main subfamilies termed the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and TRPA (ankyrin) groups. These subfamilies encompass 28 ion channels that function as diverse cellular sensors. All of the channels are permeable to monovalent cations, and most are also permeable to Ca(2+). There are strong indications that TRP channels are involved in many diseases. At this point, four channelopathies have been identified in which a defect in a TRP channel-encoding gene is the direct cause of disease. TRPs are also involved in some systemic diseases because of their role as receptors for irritants, inflammation products, and xenobiotic toxins. Other indications of the involvement of TRPs in several diseases come from correlations between the levels of channel expression and disease symptoms or from the mapping of TRP-encoding genes to susceptible chromosome regions. Finally, the phenotypes of TRP knockout mice and other transgenic models allow a degree of extrapolation to human diseases. We present an overview of current knowledge about the role of TRP channels in human disease and highlight some TRP "suspects" for which a role in disease can be anticipated. An understanding of the genetics of disease may lead to the development of targeted new therapies.

Rhythmicity in arterial smooth muscle

Many arteries and arterioles exhibit rhythmical contractions which are synchronous over considerable distances. This vasomotion is likely to assist in tissue perfusion especially during periods of altered metabolism or perfusion pressure. While the mechanism underlying vascular rhythmicity has been investigated for many years, it has only been recently, with the advent of imaging techniques for visualizing intracellular calcium release, that significant advances have been made. These methods, when combined with mechanical and electrophysiological recordings, have demonstrated that the rhythm depends critically on calcium released from intracellular stores within the smooth muscle cells and on cell coupling via gap junctions to synchronize oscillations in calcium release amongst adjacent cells. While these factors are common to all vessels studied to date, the contribution of voltage-dependent channels and the endothelium varies amongst different vessels. The basic mechanism for rhythmical activity in arteries thus differs from its counterpart in non-vascular smooth muscle, where specific networks of pacemaker cells generate electrical potentials which drive activity within the otherwise quiescent muscle cells.

Functional characterization and physiological relevance of the TRPC3/6/7 subfamily of cation channels

The mammalian transient receptor potential (TRP) superfamily of cation channels can be divided into six major families. Among them, the "classical" or "canonical" TRPC family is most closely related to Drosophila TRP, the founding member of the superfamily. All seven channels of this family designated TRPC1-7 share the common property of activation through phospholipase C (PLC)-coupled receptors, but their gating by receptor- or store-operated mechanisms is still controversial. The TRPC3, 6, and 7 channels are 75% identical and are also gated by direct exposure to diacylglycerols (DAG). TRPC3, 6, and 7 interact physically and, upon coexpression, coassemble to form functional tetrameric channels. This review will focus on the TRPC3/6/7 subfamily and describe their functional properties and regulation as homomers obtained from overexpression studies in cell lines. It will also summarize their heteromultimerization potential in vitro and in vivo and will present preliminary data concerning their physiological functions analyzed in isolated tissues with downregulated channel activity and gene-deficient mouse models.

The mechanosensitive nature of TRPV channels

Transient receptor potential vanilloid (TRPV) channels are widely expressed in both sensory and nonsensory cells. Whereas the channels display a broad diversity to activation by chemical and physical stimuli, activation by mechanical stimuli is common to many members of this group in both lower and higher organisms. Genetic screening in Caenorhabditis elegans has demonstrated an essential role for two TRPV channels in sensory neurons. OSM-9 and OCR-2, for example, are essential for both osmosensory and mechanosensory (nose-touch) behaviors. Likewise, two Drosophila TRPV channels, NAN and IAV, have been shown to be critical for hearing by the mechanosensitive chordotonal organs located in the fly's antennae. The mechanosensitive nature of the channels appears to be conserved in higher organisms for some TRPV channels. Two vertebrate channels, TRPV2 and TRPV4, are sensitive to hypotonic cell swelling, shear stress/fluid flow (TRPV4), and membrane stretch (TRPV2). In the osmosensing neurons of the hypothalamus (circumventricular organs), TRPV4 appears to function as an osmoreceptor, or part of an osmoreceptor complex, in control of vasopressin release, whereas in inner ear hair cells and vascular baroreceptors a mechanosensory role is suggestive, but not demonstrated. Finally, in many nonsensory cells expressing TRPV4, such as vascular endothelial cells and renal tubular epithelial cells, the channel exhibits well-developed local mechanosensory transduction processes where both cell swelling and shear stress/fluid flow lead to channel activation. Hence, many TRPV channels, or combinations of TRPV channels, display a mechanosensitive nature that underlies multiple mechanosensitive processes from worms to mammals.

Function and pharmacology of TRPM cation channels

The physiological function and cellular role of some members of the TRPM family are poorly understood and still mysterious. Melastatin, the founding member of the TRPM group, is the most prominent example of the mysteries involved in understanding TRP channel function. Melastatin or TRPM1 was first cloned in 1998 and since then it has been suggested that it functions as a tumor suppressor protein in melanocytes. On the other hand, TRPM8 and TRPA1 have been described as cold receptors, TRPM4 and TRPM5 as calcium-activated nonselective cation channels, TRPM6 and TRPM7 as magnesium-permeable and magnesium-modulated cation channels, TRPM2 as an ADP-ribose-activated channel of macrophages, and TRPM3 as a hypo-osmolarity- and sphingosine-activated channel. There are many unsolved questions and many studies have to be performed to understand the overall function of the TRPM family. In addition to electrophysiological recordings and biochemical characterization, the use of compounds modulating TRPM channel function has often been helpful to study TRPM channels in a cellular context. Therefore, the review will summarize the known functions, activation mechanisms, and pharmacological modulations of the TRPM 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.