Increased vascular smooth muscle contractility in TRPC6-/- mice

Transient receptor potential cation channels in normal and dystrophic mdx muscle

To investigate the defective calcium regulation of dystrophin-deficient muscle fibres we studied gene expression and localization of non-voltage gated cation channels in normal and mdx mouse skeletal muscle. We found TRPC3, TRPC6, TRPV4, TRPM4 and TRPM7 to be the most abundant isoforms. Immunofluorescent staining of muscle cross-sections with antibodies against TRP proteins showed sarcolemmal localization of TRPC6 and TRPM7, both, for mdx and control. TRPV4 was found only in a fraction of fibres at the sarcolemma and around myonuclei, while TRPC3 staining revealed intracellular patches, preferentially in mdx muscle. Transcripts of low abundance coding for TRPC5, TRPA1 and TRPM1 channels were increased in mdx skeletal muscle at certain stages. The increased Ca(2+)-influx into dystrophin-deficient mdx fibres cannot be explained by increased gene expression of major TRP channels. However, a constant TRP channel expression in combination with the well described weaker Ca(2+)-handling system of mdx fibres may indicate an imbalance between Ca(2+)-influx and cellular Ca(2+)-control.

Elucidation of a TRPC6-TRPC5 channel cascade that restricts endothelial cell movement

Canonical transient receptor potential (TRPC) channels are opened by classical signal transduction events initiated by receptor activation or depletion of intracellular calcium stores. Here, we report a novel mechanism for opening TRPC channels in which TRPC6 activation initiates a cascade resulting in TRPC5 translocation. When endothelial cells (ECs) are incubated in lysophosphatidylcholine (lysoPC), rapid translocation of TRPC6 initiates calcium influx that results in externalization of TRPC5. Activation of this TRPC6-5 cascade causes a prolonged increase in intracellular calcium concentration ([Ca(2+)](i)) that inhibits EC movement. When TRPC5 is down-regulated with siRNA, the lysoPC-induced rise in [Ca(2+)](i) is shortened and the inhibition of EC migration is lessened. When TRPC6 is down-regulated or EC from TRPC6(-/-) mice are studied, lysoPC has minimal effect on [Ca(2+)](i) and EC migration. In addition, TRPC5 is not externalized in response to lysoPC, supporting the dependence of TRPC5 translocation on the opening of TRPC6 channels. Activation of this novel TRPC channel cascade by lysoPC, resulting in the inhibition of EC migration, could adversely impact on EC healing in atherosclerotic arteries where lysoPC is abundant.

Diverse properties of store-operated TRPC channels activated by protein kinase C in vascular myocytes

In vascular smooth muscle, store-operated channels (SOCs) contribute to many physiological functions including vasoconstriction and cell growth and proliferation. In the present work we compared the properties of SOCs in freshly dispersed myocytes from rabbit coronary and mesenteric arteries and portal vein. Cyclopiazonic acid (CPA)-induced whole-cell SOC currents were sixfold greater at negative membrane potentials and displayed markedly different rectification properties and reversal potentials in coronary compared to mesenteric artery myocytes. Single channel studies showed that endothelin-1, CPA and the cell-permeant Ca(2+) chelator BAPTA-AM activated the same 2.6 pS SOC in coronary artery. In 1.5 mM [Ca(2+)](o) the unitary conductance of SOCs was significantly greater in coronary than in mesenteric artery. Moreover in 0 mM [Ca(2+)](o) the conductance of SOCs in coronary artery was unaltered whereas the conductance of SOCs in mesenteric artery was increased fourfold. In coronary artery SOCs were inhibited by the protein kinase C (PKC) inhibitor chelerythrine and activated by the phorbol ester phorbol 12,13-dibutyrate (PDBu), the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) and a catalytic subunit of PKC. These data infer an important role for PKC in activation of SOCs in coronary artery similar to mesenteric artery and portal vein. Anti-TRPC1 and -TRPC5 antibodies inhibited SOCs in coronary and mesenteric arteries and portal vein but anti-TRPC6 blocked SOCs only in coronary artery and anti-TRPC7 blocked SOCs only in portal vein. Immunoprecipitation showed associations between TRPC1 and TRPC5 in all preparations but between TRPC5 and TRPC6 only in coronary artery and between TRPC5 and TRPC7 only in portal vein. Finally, flufenamic acid increased SOC activity in coronary artery but inhibited SOCs in mesenteric artery and portal vein myocytes. These data provide strong evidence that vascular myocytes express diverse SOC isoforms, which are likely to be composed of different TRPC proteins and have different physiological functions.

Genome-wide high-density SNP-based linkage analysis of infantile hypertrophic pyloric stenosis identifies loci on chromosomes 11q14-q22 and Xq23

Infantile hypertrophic pyloric stenosis (IHPS) has an incidence of 1-8 per 1000 live births and is inherited as a complex sex-modified multifactorial trait with a striking male preponderance. Syndromic and monogenic forms exist, and two loci have been identified. Infants present with vomiting due to gastric-outlet obstruction caused by hypertrophy of the smooth muscle of the pylorus. A genome-wide SNP-based high-density linkage scan was carried out on 81 IHPS pedigrees. Nonparametric and parametric linkage analysis identified loci on chromosomes 11q14-q22 (Z(max) = 3.9, p < 0.0001; HLOD(max) = 3.4, alpha = 0.34) and Xq23 (Z(max) = 4.3, p < 0.00001; HLOD(max) = 4.8, alpha = 0.56). The two linked chromosomal regions each harbor functional candidate genes that are members of the canonical transient receptor potential (TRPC) family of ion channels and have a potential role in smooth-muscle control and hypertrophy.

Expression of trpC1 and trpC6 orthologs in zebrafish

Transient receptor potential (TRP) genes encode subunits that form cation-selective ion channels in a variety of organisms and cell types. TRP channels serve diverse functions ranging from thermal, tactile, taste, and osmolar sensing to fluid flow sensing. TRPC1 and TRPC6 belong to the TRPC subfamily, members of which are thought to contribute to several cellular events such as regulated migration of neuronal dendrites, contractile responses of smooth muscle cells and maintenance of the structural integrity of kidney podocytes. Pathogenic roles have been suggested for TRPC1 in asthma and chronic obstructive pulmonary disease, and TRPC6 dysfunction was recently linked to proteinuric kidney disease. To explore the potential roles for TRPC channels in zebrafish organ function, we cloned zebrafish trpC1 and trpC6 cDNAs, and investigated their expression during zebrafish development. We detected trpC1 expression in the head, in cells surrounding the outflow tract of the heart, and in the ganglion cells as well as the inner nuclear layer of the eye. trpC6 expression was detected in the head, pectoral fins, aortic endothelial cells, and gastrointestinal smooth muscle cells. Our results point to roles of TRPC channels in several tissues during zebrafish development, and suggest that the zebrafish may be a suitable model system to study the pathophysiology of TRPC1 and TRPC6 in specific cell types.

Molecular basis of the mammalian pressure-sensitive ion channels: focus on vascular mechanotransduction

Mechano-gated ion channels are implicated in a variety of neurosensory functions ranging from touch sensitivity to hearing. In the heart, rhythm disturbance subsequent to mechanical effects is also associated with the activation of stretch-sensitive ion channels. Arterial autoregulation in response to hemodynamic stimuli, a vital process required for protection against hypertension-induced injury, is similarly dependent on the activity of force-sensitive ion channels. Seminal work in prokaryotes and invertebrates, including the nematode Caenorhabditis elegans and the fruit fly drosophila, greatly helped to identify the molecular basis of volume regulation, hearing and touch sensitivity. In mammals, more recent findings have indicated that members of several structural family of ion channels, namely the transient receptor potential (TRP) channels, the amiloride-sensitive ENaC/ASIC channels and the potassium channels K2P and Kir are involved in cellular mechanotransduction. In the present review, we will focus on the molecular and functional properties of these channel subunits and will emphasize on their role in the pressure-dependent arterial myogenic constriction and the flow-mediated vasodilation.

Transient receptor potential (TRP) channels, vascular tone and autoregulation of cerebral blood flow

Members of the transient receptor potential (TRP) channel superfamily are present in vascular smooth muscle cells and play important roles in the regulation of vascular contractility. The TRPC3 and TRPC6 channels are activated by stimulation of several excitatory receptors in vascular smooth muscle cells. Activation of these channels leads to myocyte depolarization, which stimulates Ca2+ entry via voltage-dependent Ca2+ channels (VDCC), leading to vasoconstriction. The TRPV4 channels in arterial myocytes are activated by epoxyeicosatrienoic acids, and activation of the channels enhances Ca2+ spark and transient Ca2+-sensitive K+ channel activity, thereby hyperpolarizing and relaxing vascular smooth muscle cells. The TRPC6 and TRPM4 channels are activated by mechanical stimulation of cerebral artery myocytes. Subsequent depolarization and activation of VDCC Ca2+ entry is directly linked to the development of myogenic tone in vitro and to autoregulation of cerebral blood flow in vivo. These findings imply a fundamental importance of TRP channels in the regulation of vascular smooth muscle tone and suggest that TRP channels could be important targets for drug therapy under conditions in which vascular contractility is disturbed (e.g. hypertension, stroke, vasospasm).

TRPC6 contributes to the Ca(2+) leak of human erythrocytes

Human erythrocytes express cation channels which contribute to the background leak of Ca(2+), Na(+) and K(+). Excessive activation of these channels upon energy depletion, osmotic shock, Cl(-) depletion, or oxidative stress triggers suicidal death of erythrocytes (eryptosis), characterized by cell-shrinkage and exposure of phosphatidylserine at the cell surface. Eryptotic cells are supposed to be cleared from circulating blood. The present study aimed to identify the cation channels. RT-PCR revealed mRNA encoding the non-selective cation channel TRPC6 in erythroid progenitor cells. Western blotting indicated expression of TRPC6 protein in erythrocytes from man and wildtype mice but not from TRPC6(-/-) mice. According to flow-cytometry, Ca(2+) entry into human ghosts prepared by hemolysis in EGTA-buffered solution containing the Ca(2+) indicator Fluo3/AM was inhibited by the reducing agent dithiothreitol and the erythrocyte cation channel blockers ethylisopropylamiloride and amiloride. Loading of the ghosts with antibodies against TRPC6 or TRPC3/6/7 but neither with antibodies against TRPM2 or TRPC3 nor antibodies pre-adsorbed with the immunizing peptides inhibited ghost Ca(2+) entry. Moreover, free Ca(2+) concentration, cell-shrinkage, and phospholipid scrambling were significantly lower in Cl(-)-depleted TRPC6(-/-) erythrocytes than in wildtype mouse erythrocytes. In conclusion, human and mouse erythrocytes express TRPC6 cation channels which participate in cation leak and Ca(2+)-induced suicidal death.

TRP channels and mechanosensory transduction: insights into the arterial myogenic response

Mechano-gated ion channels are implicated in a variety of key physiological functions ranging from touch sensitivity to arterial pressure regulation. Seminal work in prokaryotes and invertebrates provided strong evidence for the role of specific ion channels in volume regulation, touch sensitivity, or hearing, specifically the mechanosensitive channel subunits of large and small conductances (MscL and MscS), the mechanosensory channel subunits (MEC) and the transient receptor potential channel subunits (TRP). In mammals, recent studies further indicate that members of the TRP channel family may also be considered as possible candidate mechanosensors responding to either tension, flow, or changes in cell volume. However, contradictory results have challenged whether these TRP channels, including TRPC1 and TRPC6, are directly activated by mechanical stimulation. In the present review, we will focus on the mechanosensory function of TRP channels, discuss whether a direct or indirect mechanism is at play, and focus on the proposed role for these channels in the arterial myogenic response to changes in intraluminal pressure.

Angiotensin II-stimulated Ca2+ entry mechanisms in afferent arterioles: role of transient receptor potential canonical channels and reverse Na+/Ca2+ exchange

In afferent arterioles, the signaling events that lead to an increase in cytosolic Ca2+ concentration ([Ca2+]i) and initiation of vascular contraction are increasingly being delineated. We have recently studied angiotensin II (ANG II)-mediated effects on sarcoplasmic reticulum (SR) mobilization of Ca2+ and the role of superoxide and cyclic adenosine diphosphoribose in these processes. In the current study we investigated the participation of transient receptor potential canonical channels (TRPC) and a Na+/Ca2+ exchanger (NCX) in Ca2+ entry mechanisms. Afferent arterioles, isolated with the magnetized polystyrene bead method, were loaded with fura-2 to measure [Ca2+]i ratiometrically. We observed that the Ca2+-dependent chloride channel blocker niflumic acid (10 and 50 μ M) affects neither the peak nor plateau [Ca2+]i response to ANG II. Arterioles were pretreated with ryanodine (100 μM) and TMB-8 to block SR mobilization via the ryanodine receptor and inositol trisphosphate receptor, respectively. The peak [Ca2+]i response to ANG II was reduced by 40%. Addition of 2-aminoethoxydiphenyl borane to block TRPC-mediated Ca2+ entry inhibited the peak [Ca2+]i ANG II response by 80% and the plateau by 74%. Flufenamic acid (FFA; 50 μM), which stimulates TRPC6, caused a sustained increase of [Ca2+]i of 146 nM. This response was unaffected by diltiazem or nifedipine. KB-R7943 (at the low concentration of 10 μM) inhibits reverse (but not forward) mode NCX. KB-R7943 decreased the peak [Ca2+]i response to ANG II by 48% and to FFA by 38%. We conclude that TRPC6 and reverse-mode NCX may be important Ca2+ entry pathways in afferent arterioles.

Association of transient receptor potential canonical type 3 (TRPC3) channel transcripts with proinflammatory cytokines

We investigated whether expression of non-selective cation channels of the transient receptor potential canonical (TRPC) channel family are associated with proinflammatory cytokines in monocytes. Using quantitative RT-PCR we studied the expression of TRPC3, interleukin-1beta (IL-1beta), and tumor necrosis factor-alpha (TNF-alpha) in monocytes from 15 patients with essential hypertension and 16 age- and sex-matched normotensive control subjects. We observed an approximately 8-fold increase of TRPC3 transcripts in monocytes from patients with essential hypertension compared to normotensive control subjects (p<0.05). We found an approximately 3-fold increase of IL-1beta, and an approximately 9-fold increase of TNF-alpha in patients with essential hypertension compared to normotensive control subjects (each p<0.05). We observed a significant correlation between TRPC3 transcripts with systolic blood pressure, expression of IL-1beta, and TNF-alpha. Using quantitative RT-PCR we observed an association of TRPC3 transcripts and proinflammatory cytokines in monocytes.

Characterization of agonist-induced vasoconstriction in mouse pulmonary artery

In recent years, transgenic mouse models have been developed to examine the underlying cellular and molecular mechanisms of lung disease and pulmonary vascular disease, such as asthma, pulmonary thromboembolic disease, and pulmonary hypertension. However, there has not been systematic characterization of the basic physiological pulmonary vascular reactivity in normal and transgenic mice. This represents an intellectual "gap", since it is important to characterize basic murine pulmonary vascular reactivity in response to various contractile and relaxant factors to which the pulmonary vasculature is exposed under physiological conditions. The present study evaluates excitation- and pharmacomechanical-contraction coupling in pulmonary arteries (PA) isolated from wild-type BALB/c mice. We demonstrate that both pharmaco- and electromechanical coupling mechanisms exist in mice PA. These arteries are also reactive to stimulation by alpha(1)-adrenergic agonists, serotonin, endothelin-1, vasopressin, and U-46619 (a thromboxane A(2) analog). We conclude that the basic vascular responsiveness of mouse PA is similar to those observed in PA of other species, including rat, pig, and human, albeit on a different scale and to varying amplitudes.

Attenuation of store-operated Ca2+ current impairs salivary gland fluid secretion in TRPC1(-/-) mice

Agonist-induced Ca(2+) entry via store-operated Ca(2+) (SOC) channels is suggested to regulate a wide variety of cellular functions, including salivary gland fluid secretion. However, the molecular components of these channels and their physiological function(s) are largely unknown. Here we report that attenuation of SOC current underlies salivary gland dysfunction in mice lacking transient receptor potential 1 (TRPC1). Neurotransmitter-regulated salivary gland fluid secretion in TRPC1-deficient TRPC1(-/-) mice was severely decreased (by 70%). Further, agonist- and thapsigargin-stimulated SOC channel activity was significantly reduced in salivary gland acinar cells isolated from TRPC1(-/-) mice. Deletion of TRPC1 also eliminated sustained Ca(2+)-dependent potassium channel activity, which depends on Ca(2+) entry and is required for fluid secretion. Expression of key proteins involved in fluid secretion and Ca(2+) signaling, including STIM1 and other TRPC channels, was not altered. Together, these data demonstrate that reduced SOC entry accounts for the severe loss of salivary gland fluid secretion in TRPC1(-/-) mice. Thus, TRPC1 is a critical component of the SOC channel in salivary gland acinar cells and is essential for neurotransmitter-regulation of fluid secretion.

Revisiting TRPC1 and TRPC6 mechanosensitivity

This article addresses whether TRPC1 or TRPC6 is an essential component of a mammalian stretch-activated mechano-sensitive Ca(2+) permeable cation channel (MscCa). We have transiently expressed TRPC1 and TRPC6 in African green monkey kidney (COS) or Chinese hamster ovary (CHO) cells and monitored the activity of the stretch-activated channels using a fast pressure clamp system. Although both TRPC1 and TRPC6 are highly expressed at the protein level, the amplitude of the mechano-sensitive current is not significantly altered by overexpression of these subunits. In conclusion, although several TRPC channel members, including TRPC1 and TRPC6, have been recently proposed to form MscCa in vertebrate cells, the functional expression of these TRPC subunits in heterologous systems remains problematic.

TRP channels

The TRP (Transient Receptor Potential) superfamily of cation channels is remarkable in that it displays greater diversity in activation mechanisms and selectivities than any other group of ion channels. The domain organizations of some TRP proteins are also unusual, as they consist of linked channel and enzyme domains. A unifying theme in this group is that TRP proteins play critical roles in sensory physiology, which include contributions to vision, taste, olfaction, hearing, touch, and thermo- and osmosensation. In addition, TRP channels enable individual cells to sense changes in their local environment. Many TRP channels are activated by a variety of different stimuli and function as signal integrators. The TRP superfamily is divided into seven subfamilies: the five group 1 TRPs (TRPC, TRPV, TRPM, TRPN, and TRPA) and two group 2 subfamilies (TRPP and TRPML). TRP channels are important for human health as mutations in at least four TRP channels underlie disease.

TrpC3 regulates hypertrophy-associated gene expression without affecting myocyte beating or cell size

Pathological cardiac hypertrophy is associated with an increased risk of heart failure and cardiovascular mortality. Calcium (Ca²⁺) -regulated gene expression is essential for the induction of hypertrophy, but it is not known how myocytes distinguish between the Ca²⁺ signals that regulate contraction and those that lead to cardiac hypertrophy. We used in vitro neonatal rat ventricular myocytes to perform an RNA interference (RNAi) screen for ion channels that mediate Ca²⁺-dependent gene expression in response to hypertrophic stimuli. We identified several ion channels that are linked to hypertrophic gene expression, including transient receptor potential C3 (TrpC3). RNAi-mediated knockdown of TrpC3 decreases expression of hypertrophy-associated genes such as the A- and B-type natriuretic peptides (ANP and BNP) in response to numerous hypertrophic stimuli, while TrpC3 overexpression increases BNP expression. Furthermore, stimuli that induce hypertrophy dramatically increase TrpC3 mRNA levels. Importantly, whereas TrpC3-knockdown strongly reduces gene expression associated with hypertrophy, it has a negligible effect on cell size and on myocyte beating. These results suggest that Ca²⁺ influx through TrpC3 channels increases transcription of genes associated with hypertrophy but does not regulate the signaling pathways that control cell size or contraction. Thus TrpC3 may represent an important therapeutic target for the treatment of cardiac hypertrophy and heart failure.

Downregulation of TRPC6 protein expression by high glucose, a possible mechanism for the impaired Ca2+ signaling in glomerular mesangial cells in diabetes

The present study was performed to investigate whether transient receptor potential (TRPC)6 participated in Ca(2+) signaling of glomerular mesangial cells (MCs) and expression of this protein was altered in diabetes. Western blots and real-time PCR were used to evaluate the expression level of TRPC6 protein and mRNA, respectively. Cell-attached patch-clamp and fura-2 fluorescence measurements were utilized to assess angiotensin II (ANG II)-stimulated membrane currents and Ca(2+) responses in MCs. In cultured human MCs, high glucose significantly reduced expression of TRPC6 protein, but there was no effect on either TRPC1 or TRPC3. The high glucose-induced effect on TRPC6 was time and dose dependent with the maximum effect observed on day 7 and at 30 mM glucose, respectively. In glomeruli isolated from streptozotocin-induced diabetic rats, TRPC6, but not TRPC1, was markedly reduced compared with the glomeruli of control rats. Furthermore, TRPC6 mRNA in MCs was also significantly decreased by high glucose as early as 1 day after treatment with maximal reduction on day 4. Patch-clamp experiments showed that ANG II-stimulated membrane currents in MCs were significantly attenuated or enhanced by knockdown or overexpression of TRPC6, respectively. Fura-2 fluorescence measurements revealed that the ANG II-induced Ca(2+) influxes were markedly inhibited in MCs with TRPC6 knockdown, reminiscent of the impaired Ca(2+) entry in response to ANG II in high glucose-treated MCs. These results suggest that the TRPC6 protein expression in MCs was downregulated by high glucose and the deficiency of TRPC6 protein might contribute to the impaired Ca(2+) signaling of MCs seen in diabetes.

Galpha12/13-mediated up-regulation of TRPC6 negatively regulates endothelin-1-induced cardiac myofibroblast formation and collagen synthesis through nuclear factor of activated T cells activation

Sustained elevation of [Ca(2+)](i) has been implicated in many cellular events. We previously reported that alpha subunits of G(12) family G proteins (Galpha(12/13)) participate in sustained Ca(2+) influx required for the activation of nuclear factor of activated T cells (NFAT), a Ca(2+)-responsive transcriptional factor, in rat neonatal cardiac fibroblasts. Here, we demonstrate that Galpha(12/13)-mediated up-regulation of canonical transient receptor potential 6 (TRPC6) channels participates in sustained Ca(2+) influx and NFAT activation by endothelin (ET)-1 treatment. Expression of constitutively active Galpha(12) or Galpha(13) increased the expression of TRPC6 proteins and basal Ca(2+) influx activity. The treatment with ET-1 increased TRPC6 protein levels through Galpha(12/13), reactive oxygen species, and c-Jun N-terminal kinase (JNK)-dependent pathways. NFAT is activated by sustained increase in [Ca(2+)](i) through up-regulated TRPC6. A Galpha(12/13)-inhibitory polypeptide derived from the regulator of the G-protein signaling domain of p115-Rho guanine nucleotide exchange factor and a JNK inhibitor, SP600125, suppressed the ET-1-induced increase in expression of marker proteins of myofibroblast formation through a Galpha(12/13)-reactive oxygen species-JNK pathway. The ET-1-induced myofibroblast formation was suppressed by overexpression of TRPC6 and CA NFAT, whereas it was enhanced by TRPC6 small interfering RNAs and cyclosporine A. These results suggest two opposite roles of Galpha(12/13) in cardiac fibroblasts. First, Galpha(12/13) mediate ET-1-induced myofibroblast formation. Second, Galpha(12/13) mediate TRPC6 up-regulation and NFAT activation that negatively regulates ET-1-induced myofibroblast formation. Furthermore, TRPC6 mediates hypertrophic responses in cardiac myocytes but suppresses fibrotic responses in cardiac fibroblasts. Thus, TRPC6 mediates opposite responses in cardiac myocytes and fibroblasts.

TRP channels in hypertension

Pulmonary and systemic arterial hypertension are associated with profound alterations in Ca(2+) homeostasis and smooth muscle cell proliferation. A novel class of non-selective cation channels, the transient receptor potential (TRP) channels, have emerged at the forefront of research into hypertensive disease states. TRP channels are identified as molecular correlates for receptor-operated and store-operated cation channels in the vasculature. Over 10 TRP isoforms are identified at the mRNA and protein expression levels in the vasculature. Current research implicates upregulation of specific TRP isoforms to be associated with increased Ca(2+) influx, characteristic of vasoconstriction and vascular smooth muscle cell proliferation. TRP channels are implicated as Ca(2+) entry pathways in pulmonary hypertension and essential hypertension. Caveolae have recently emerged as membrane microdomains in which TRP channels may be co-localized with the endoplasmic reticulum in both smooth muscle and endothelial cells. Such enhanced expression and function of TRP channels and their localization in caveolae in pathophysiological hypertensive disease states highlights their importance as potential targets for pharmacological intervention.

In vivo TRPC functions in the cardiopulmonary vasculature

Cardiovascular diseases are the leading cause of death in the industrialized countries. The cardiovascular system includes the systemic blood circulation, the heart and the pulmonary circulation providing sufficient blood flow and oxygen to peripheral tissues and organs according to their metabolic demand. This review focuses on three major cell types of the cardiovascular system: myocytes of the heart as well as smooth muscle cells and endothelial cells from the systemic and pulmonary circulation. Ion channels initiate and regulate contraction in all three cell types, and the identification of their genes has significantly improved our knowledge of signal transduction pathways in these cells. Among the ion channels expressed in smooth muscle cells, cation channels of the TRPC family allow for the entry of Na(+) and Ca(2+). Physiological functions of TRPC1, TRPC3, TRPC4, TRPC5, TRPC6 and TRPC7 in the cardiovascular system, dissected by down-regulating channel activity in isolated tissues or by the analysis of gene-deficient mouse models, are reviewed. Possible functional roles and physiological regulation of TRPCs as homomeric or heteromeric channels in these cell types are discussed. Moreover, TRP channels may also be responsible for pathophysiological processes of the cardiovascular system like hypertension as well as cardiac hypertrophy and increased endothelial permeability.

Pressure-induced and store-operated cation influx in vascular smooth muscle cells is independent of TRPC1

Among the classical transient receptor potential (TRPC) subfamily, TRPC1 is described as a mechanosensitive and store-operated channel proposed to be activated by hypoosmotic cell swelling and positive pipette pressure as well as regulated by the filling status of intracellular Ca(2+) stores. However, evidence for a physiological role of TRPC1 may most compellingly be obtained by the analysis of a TRPC1-deficient mouse model. Therefore, we have developed and analyzed TRPC1(-/-) mice. Pressure-induced constriction of cerebral arteries was not impaired in TRPC1(-/-) mice. Smooth muscle cells from cerebral arteries activated by hypoosmotic swelling and positive pipette pressure showed no significant differences in cation currents compared to wild-type cells. Moreover, smooth muscle cells of TRPC1(-/-) mice isolated from thoracic aortas and cerebral arteries showed no change in store-operated cation influx induced by thapsigargin, inositol-1,4,5 trisphosphate, and cyclopiazonic acid compared to cells from wild-type mice. In contrast to these results, small interference RNAs decreasing the expression of stromal interaction molecule 1 (STIM1) inhibited thapsigargin-induced store-operated cation influx, demonstrating that STIM1 and TRPC1 are mutually independent. These findings also imply that, as opposed to current concepts, TRPC1 is not an obligatory component of store-operated and stretch-activated ion channel complexes in vascular smooth muscle cells.

Homer proteins in Ca2+ signaling by excitable and non-excitable cells

Homers are scaffolding proteins that bind Ca(2+) signaling proteins in cellular microdomains. The Homers participate in targeting and localization of Ca(2+) signaling proteins in signaling complexes. However, recent work showed that the Homers are not passive scaffolding proteins, but rather they regulate the activity of several proteins within the Ca(2+) signaling complex in an isoform-specific manner. Homer2 increases the GAP activity of RGS proteins and PLCbeta that accelerate the GTPase activity of Galpha subunits. Homer1 gates the activity of TRPC channels, controls the rates of their translocation and retrieval from the plasma membrane and mediates the conformational coupling between TRPC channels and IP(3)Rs. Homer1 stimulates the activity of the cardiac and neuronal L-type Ca(2+) channels Ca(v)1.2 and Ca(v)1.3. Homer1 also mediates the communication between the cardiac and smooth muscle ryanodine receptor RyR2 and Ca(v)1.2 to regulate E-C coupling. In many cases the Homers function as a buffer to reduce the intensity of Ca(2+) signaling and create a negative bias that can be reversed by the immediate early gene form of Homer1. Hence, the Homers should be viewed as the buffers of Ca(2+) signaling that ensure a high spatial and temporal fidelity of the Ca(2+) signaling and activation of downstream effects.

TRP channels in mechanosensation: direct or indirect activation?

Ion channels of the transient receptor potential (TRP) superfamily are involved in a wide variety of neural signalling processes, most prominently in sensory receptor cells. They are essential for mechanosensation in systems ranging from fruitfly hearing, to nematode touch, to mouse mechanical pain. However, it is unclear in many instances whether a TRP channel directly transduces the mechanical stimulus or is part of a downstream signalling pathway. Here, we propose criteria for establishing direct mechanical activation of ion channels and review these criteria in a number of mechanosensory systems in which TRP channels are involved.

Hormonal signaling and signal pathway crosstalk in the control of myometrial calcium dynamics

Understanding the basis for the control of myometrial contractant and relaxant signaling pathways is important to understanding how to manage myometrial contractions. Signaling pathways are influenced by the level of expression of the signals and signal pathway components, the location of these components in the appropriate subcellular environment, and covalent modification. Crosstalk between these pathways regulates the effectiveness of signal transduction and represents an important way by which hormones can regulate phenotype. This review deals primarily with signaling pathways that control Ca2+ entry and intracellular release, as well as the interplay between these pathways.

Organization and function of TRPC channelosomes

TRPC proteins constitute a family of conserved Ca2+-permeable cation channels which are activated in response to agonist-stimulated PIP2 hydrolysis. These channels were initially proposed to be components of the store-operated calcium entry channel (SOC). Subsequent studies have provided substantial evidence that some TRPCs contribute to SOC activity. TRPC proteins have also been shown to form agonist-stimulated calcium entry channels that are not store-operated but are likely regulated by PIP2 or diacylglycerol. Further, and consistent with the presently available data, selective homomeric or heteromeric interactions between TRPC monomers generate distinct agonist-stimulated cation permeable channels. We suggest that interaction between TRPC monomers, as well as the association of these channels with accessory proteins, determines their mode of regulation as well as their cellular localization and function. Currently identified accessory proteins include key Ca2+ signaling proteins as well as proteins involved in vesicle trafficking, cytoskeletal interactions, and scaffolding. Studies reported until now demonstrate that TRPC proteins are segregated into specific Ca2+ signaling complexes which can generate spatially and temporally controlled [Ca2+]i signals. Thus, the functional organization of TRPC channelosomes dictates not only their regulation by extracellular stimuli but also serves as a platform to coordinate specific downstream cellular functions that are regulated as a consequence of Ca2+ entry. This review will focus on the accessory proteins of TRPC channels and discuss the functional implications of TRPC channelosomes and their assembly in microdomains.

STIM1 heteromultimerizes TRPC channels to determine their function as store-operated channels

Stromal interacting molecule 1 (STIM1) is a Ca(2+) sensor that conveys the Ca(2+) load of the endoplasmic reticulum to store-operated channels (SOCs) at the plasma membrane. Here, we report that STIM1 binds TRPC1, TRPC4 and TRPC5 and determines their function as SOCs. Inhibition of STIM1 function inhibits activation of TRPC5 by receptor stimulation, but not by La(3+), suggesting that STIM1 is obligatory for activation of TRPC channels by agonists, but STIM1 is not essential for channel function. Through a distinct mechanism, STIM1 also regulates TRPC3 and TRPC6. STIM1 does not bind TRPC3 and TRPC6, and regulates their function indirectly by mediating the heteromultimerization of TRPC3 with TRPC1 and TRPC6 with TRPC4. TRPC7 is not regulated by STIM1. We propose a new definition of SOCs, as channels that are regulated by STIM1 and require the store depletion-mediated clustering of STIM1. By this definition, all TRPC channels, except TRPC7, function as SOCs.

Beta1 (KCNMB1) subunits mediate lithocholate activation of large-conductance Ca2+-activated K+ channels and dilation in small, resistance-size arteries

Among the nongenomic effects of steroids, control of vasomotion has received increasing attention. Lithocholate (LC) and other physiologically relevant cholane-derived steroids cause vasodilation, yet the molecular targets and mechanisms underlying this action remain largely unknown. We demonstrate that LC (45 microM) reversibly increases the diameter of pressurized resistance cerebral arteries by approximately 10%, which would result in approximately 30% increase in cerebral blood flow. LC action is independent of endothelial integrity, prevented by 55 nM iberiotoxin, and unmodified by 0.8 mM 4-aminopyridine, indicating that LC causes vasodilation via myocyte BK channels. Indeed, LC activates BK channels in isolated myocytes through a destabilization of channel long-closed states without modifying unitary conductance. LC channel activation occurs within a wide voltage range and at Ca2+ concentrations reached in the myocyte at rest and during contraction. Channel accessory beta1 subunits, which are predominant in smooth muscle, are necessary for LC to modify channel activity. In contrast, beta4 subunits, which are predominant in neuronal tissues, fail to evoke LC sensitivity. LC activation of cbv1+beta1 and native BK channels display identical characteristics, including EC50 (46 microM) and Emax (approximately 300 microM) values, strongly suggesting that the cbv1+beta1 complex is necessary and sufficient to evoke LC action. Finally, intact arteries from beta1 subunit knockout mice fail to relax in response to LC, although they are able to respond to other vasodilators. This study pinpoints the BK beta1 subunit as the molecule that senses LC, which results in myocyte BK channel activation and, thus, endothelial-independent relaxation of small, resistance-size arteries.

TRPCs as MS Channels

This chapter reviews recent evidence indicating that canonical or classical transient receptor potential (TRPC) channels are directly or indirectly mechanosensitive (MS) and can therefore be designated as mechano-operated channels (MOCs). The MS functions of TRPCs may be mechanistically related to their better known functions as store-operated and receptor-operated channels (SOCs and ROCs). Mechanical forces may be conveyed to TRPC channels through the "conformational coupling" mechanism that transmits information regarding the status of internal Ca(2+) stores. All TRPCs are regulated by receptors coupled to phospholipases that are themselves MS and can regulate channels via lipidic second messengers. Accordingly, there may be several nonexclusive mechanisms by which mechanical forces may regulate TRPC channels, including direct sensitivity to bilayer mechanics, physical coupling to internal membranes and/or cytoskeletal proteins, and sensitivity to lipidic second messengers generated by MS enzymes. Various strategies that can be used for separating out different MS-gating mechanisms and their possible role in specific TRPCs are discussed.

Increased store-operated and 1-oleoyl-2-acetyl-sn-glycerol-induced calcium influx in monocytes is mediated by transient receptor potential canonical channels in human essential hypertension

OBJECTIVE

Activation of nonselective cation channels of the transient receptor potential canonical (TRPC) family has been associated with hypertension. Whether store-operated channels, which are activated after depletion of intracellular stores, or second-messenger-operated channels, which are activated by 1-oleoyl-2-acetyl-sn-glycerol, are affected in essential hypertension is presently unknown.

METHODS

Using a polymerase chain reaction, an in-cell western assay and the fluorescent dye technique we studied TRPC3, TRPC5, and TRPC6 expression and store-operated and 1-oleoyl-2-acetyl-sn-glycerol-induced calcium influx into human monocytes in 19 patients with essential hypertension and in 17 age-matched and sex-matched normotensive control individuals.

RESULTS

We observed a significantly increased expression of TRPC3 and TRPC5, but not TRPC6, in essential hypertension. Store-operated calcium influx was significantly elevated in essential hypertension. Store-operated calcium influx was reduced by the inhibitor 2-aminoethoxydiphenylborane, specific TRPC3 and TRPC5 knockdown, but not TRPC6 knockdown using gene silencing by RNA interference. 1-Oleoyl-2-acetyl-sn-glycerol-induced calcium influx and barium influx were also significantly elevated in essential hypertension. The 1-oleoyl-2-acetyl-sn-glycerol-induced cation influx was reduced by TRPC3 and TRPC5 knockdown.

CONCLUSION

We demonstrated an increased TRPC3 and TRPC5 expression and a subsequently increased store-operated calcium influx and increased 1-oleoyl-2-acetyl-sn-glycerol-induced cation influx in monocytes of patients with essential hypertension. This increased activation of monocytes through TRPC channels in patients with essential hypertension may promote vascular disease in these patients.

Integration of phosphoinositide- and calmodulin-mediated regulation of TRPC6

Multiple TRP channels are regulated by phosphoinositides (PIs). However, it is not known whether PIs bind directly to TRP channels. Furthermore, the mechanisms through which PIs regulate TRP channels are obscure. To analyze the role of PI/TRP interactions, we used a biochemical approach, focusing on TRPC6. TRPC6 bound directly to PIs, and with highest potency to phosphatidylinositol 3,4,5-trisphosphate (PIP(3)). We found that PIP(3) binding disrupted the association of calmodulin (CaM) with TRPC6. We identified the PIP(3)-binding site and found that mutations that increased or decreased the affinity of the PIP(3)/TRPC6 interaction enhanced or reduced the TRPC6-dependent current, respectively. PI-mediated disruption of CaM binding appears to be a theme that applies to other TRP channels, such as TRPV1, as well as to the voltage-gated channels KCNQ1 and Ca(v)1.2. We propose that regulation of CaM binding by PIs provides a mode for integration of channel regulation by Ca(2+) and PIs.

TRPC6 and FSGS: the latest TRP channelopathy

Focal and segmental glomerulosclerosis (FSGS) is a common cause of nephrotic syndrome in children and adults throughout the world. In the past 50 years, significant advances have been made in the identification and characterization of familial forms of nephrotic syndrome and FSGS. Resultant to these pursuits, several podocyte structural proteins such as nephrin, podocin, alpha-actinin 4 (ACTN4), and CD2-associated protein (CD2AP) have emerged to provide critical insight into the pathogenesis of hereditary nephrotic syndromes. The latest advance in familial FSGS has been the discovery of a mutant form of canonical transient receptor potential cation channel 6 (TRPC6), which causes an increase in calcium transients and essentially a gain of function in this cation channel located on the podocyte cell membrane. The TRP ion channel family is a diverse group of cation channels united by a common primary structure which contains six membrane-spanning domains, with both carboxy and amino termini located intracellularly. TRP channels are unique in their ability to activate independently of membrane depolarization. TRPC6 channels have been shown to be activated via phospholipase C stimulation. The mechanisms by which mutant TRPC6 causes an increase in intracellular calcium and leads to glomerulosclerosis are unknown. Mutant TRPC6 may affect critical interactions with the aforementioned podocyte structural proteins, leading to abnormalities in the slit diaphragm or podocyte foot processes. Mutant TRPC6 may also amplify injurious signals mediated by Ang II, a common final pathway of podocyte apoptosis in various mammalian species. Current evidence also suggests that blocking TRPC6 channels may be of therapeutic benefit in idiopathic FSGS, a disease with a generally poor prognosis. Preliminary experiments reveal the commonly used immunosuppressive agent FK-506 can inhibit TRPC6 activity in vivo. This creates the exciting possibility that blocking TRPC6 channels within the podocyte may translate into long-lasting clinical benefits in patients with FSGS.

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.

TRPC channels: interacting proteins

TRP channels, in particular the TRPC and TRPV subfamilies, have emerged as important constituents of the receptor-activated Ca2+ influx mechanism triggered by hormones, growth factors, and neurotransmitters through activation ofphospholipase C (PLC). Several TRPC channels are also activated by passive depletion of endoplasmic reticulum (ER) Ca2+. Although in several studies the native TRP channels faithfully reproduce the respective recombinant channels, more often the properties of Ca2+ entry and/or the store-operated current are strikingly different from that of the TRP channels expressed in the same cells. The present review aims to discuss this disparity in the context of interaction of TRPC channels with auxiliary proteins that may alter the permeation and regulation of TRPC channels.

TRP channels in platelet function

Ca2+ entry forms an essential component of platelet activation; however, the mechanisms associated with this process are not understood. Ca2+ entry upon receptor activation occurs as a consequence of intracellular store depletion (referred to as store-operated Ca2+ entry or SOCE), a direct action of second messengers on cation entry channels or the direct occupancy of a ligand-gated P2(Xi) receptor. The molecular identity of the SOCE channel has yet to be established. Transient receptor potential (TRP) proteins are candidate cation entry channels and are classified into a number of closely related subfamilies including TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin) and TRPML (mucolipins). From the TRPC family, platelets have been shown to express TRPC6 and TRPC1, and are likely to express other TRPC and other TRP members. TRPC6 is suggested to be involved with receptor-activated, diacyl-glycerol-mediated cation entry. TRPC1 has been suggested to be involved with SOCE, though many of the suggested mechanisms remain controversial. As no single TRP channel has the properties described for SOCE in platelets, it is likely that it is composed of a heteromeric association of TRP and related subunits, some of which may be present in intracellular compartments in the resting cell.

Biological functions of TRPs unravelled by spontaneous mutations and transgenic animals

The identification of the biological functions of TRP (transient receptor potential) proteins requires genetic approaches because a selective TRP channel pharmacology to unravel the roles of TRPs is not available so far for most TRPs. A survey is therefore presented of transgenic animal models carrying mutations in TRP genes, as well as of those TRP genes that when mutated result in human disease; the chromosomal locations of TRP channel genes in the human and mouse are also presented.

An introduction on TRP channels

The transient receptor potential (TRP) ion channels are named after the role of the channels in Drosophila phototransduction. Mammalian TRP channel subunit proteins are encoded by at least 28 genes. TRP cation channels display an extraordinary assortment of selectivities and activation mechanisms, some of which represent previously unrecognized modes of regulating ion channels. In addition, the biological roles of TRP channels appear to be equally diverse and range from roles in thermosensation and pain perception to Ca2+ and Mg2+ absorption, endothelial permeability, smooth muscle proliferation and gender-specific behaviour.

Phospholipase C-coupled receptors and activation of TRPC channels

The canonical transient receptor potential (TRPC) cation channels are mammalian homologs of the photoreceptor channel TRP in Drosophila melanogaster. All seven TRPCs (TRPC1 through TRPC7) can be activated through Gq/11 receptors or receptor tyrosine kinase (RTK) by mechanisms downstream of phospholipase C. The last decade saw a rapidly growing interest in understanding the role of TRPC channels in calcium entry pathways as well as in understanding the signal(s) responsible for TRPC activation. TRPC channels have been proposed to be activated by a variety of signals including store depletion, membrane lipids, and vesicular insertion into the plasma membrane. Here we discuss recent developments in the mode of activation as well as the pharmacological and electrophysiological properties of this important and ubiquitous family of cation channels.

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.

TRPC channels: integrators of multiple cellular signals

TRPC channels are ubiquitously expressed among cell types and mediate signals in response to phospholipase C (PLC)-coupled receptors. TRPC channels function as integrators of multiple signals resulting from receptor-induced PLC activation, which catalyzes the breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) to produce inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG). InsP3 depletes Ca2+ stores and TRPC3 channels can be activated by store-depletion. InsP3 also activates the InsP3 receptor, which may undergo direct interactions with the TRPC3 channel, perhaps mediating store-dependence. The other PLC product, DAG, has a direct non-PKC-dependent activating role on TRPC3 channels likely by direct binding. DAG also has profound effects on the TRPC3 channel through PKC. Thus PKC is a powerful inhibitor of most TRPC channels and DAG is a dual regulator of the TRPC3 channel. PLC-mediated DAG results in rapid channel opening followed later by a slower DAG-induced PKC-mediated deactivation of the channel. The decreased level of PIP2 from PLC activation also has an important modifying action on TRPC3 channels. Thus, the TRPC3 channel and PLCgamma form an intermolecular PH domain that has high specificity for binding PIP2. This interaction allows the channel to be retained within the plasma membrane, a further operational control factor for TRPC3. As nonselective cation channels, TRPC channel opening results in the entry of both Na+ and Ca2+ ions. Thus, while they may mediate Ca2+ entry signals, TRPC channels are also powerful modifiers of membrane potential.

Classical transient receptor potential channel 6 (TRPC6) is essential for hypoxic pulmonary vasoconstriction and alveolar gas exchange

Regional alveolar hypoxia causes local vasoconstriction in the lung, shifting blood flow from hypoxic to normoxic areas, thereby maintaining gas exchange. This mechanism is known as hypoxic pulmonary vasoconstriction (HPV). Disturbances in HPV can cause life-threatening hypoxemia whereas chronic hypoxia triggers lung vascular remodeling and pulmonary hypertension. The signaling cascade of this vitally important mechanism is still unresolved. Using transient receptor potential channel 6 (TRPC6)-deficient mice, we show that this channel is a key regulator of acute HPV as this regulatory mechanism was absent in TRPC6(-/-) mice whereas the pulmonary vasoconstrictor response to the thromboxane mimetic U46619 was unchanged. Accordingly, induction of regional hypoventilation resulted in severe arterial hypoxemia in TRPC6(-/-) but not in WT mice. This effect was mirrored by a lack of hypoxia-induced cation influx and currents in smooth-muscle cells from precapillary pulmonary arteries (PASMC) of TRPC6(-/-) mice. In both WT and TRPC6(-/-) PASMC hypoxia caused diacylglycerol (DAG) accumulation. DAG seems to exert its action via TRPC6, as DAG kinase inhibition provoked a cation influx only in WT but not in TRPC6(-/-) PASMC. Notably, chronic hypoxia-induced pulmonary hypertension was independent of TRPC6 activity. We conclude that TRPC6 plays a unique and indispensable role in acute hypoxic pulmonary vasoconstriction. Manipulation of TRPC6 function may thus offer a therapeutic strategy for the control of pulmonary hemodynamics and gas exchange.

TRPpathies

Many human diseases are caused by mutations in ion channels. Dissecting the pathogenesis of these 'channelopathies' has yielded important insights into the regulation of vital biological processes by ions and has become a productive tool of modern ion channel biology. One of the best examples of a synergism between the clinical and basic science aspects of a modern biological topic is cystic fibrosis. Not only did the identification of the ion channel mutated in cystic fibrosis pinpoint the root cause of this disease, but it also has significantly advanced our understanding of basic biological processes as diverse as protein folding and epithelial fluid and electrolyte secretion. The list of confirmed 'channelopathies' is growing and several members of the TRP family of ion channels have been implicated in human diseases such as mucolipidosis type IV (MLIV), autosomal dominant polycystic kidney disease (ADPKD), familial focal segmental glomerulosclerosis (FSG), hypomagnesemia with secondary hypocalcaemia (HSH), and several forms of cancer. Analysing pathogenesis of the diseases linked to TRP dysregulation provides an exciting means of identifying novel functions of TRP channels.

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.