Lack of an endothelial store-operated Ca2+ current impairs agonist-dependent vasorelaxation in TRP4-/- mice

Isoform-specific inhibition of TRPC4 channel by phosphatidylinositol 4,5-bisphosphate

Full-length transient receptor potential (TRP) cation channel TRPC4alpha and shorter TRPC4beta lacking 84 amino acids in the cytosolic C terminus are expressed in smooth muscle and endothelial cells where they regulate membrane potential and Ca(2+) influx. In common with other "classical" TRPCs, TRPC4 is activated by G(q)/phospholipase C-coupled receptors, but the underlying mechanism remains elusive. Little is also known about any isoform-specific channel regulation. Here we show that TRPC4alpha but not TRPC4beta was strongly inhibited by intracellularly applied phosphatidylinositol 4,5-bisphosphate (PIP(2)). In contrast, several other phosphoinositides (PI), including PI(3,4)P(2), PI(3,5)P(2), and PI(3,4,5)P(3), had no effect or even potentiated TRPC4alpha indicating that PIP(2) inhibits TRPC4alpha in a highly selective manner. We show that PIP(2) binds to the C terminus of TRPC4alpha but not that of TRPC4beta in vitro. Its inhibitory action was dependent on the association of TRPC4alpha with actin cytoskeleton as it was prevented by cytochalasin D treatment or by the deletion of the C-terminal PDZ-binding motif (Thr-Thr-Arg-Leu) that links TRPC4 to F-actin through the sodium-hydrogen exchanger regulatory factor and ezrin. PIP(2) breakdown appears to be a required step in TRPC4alpha channel activation as PIP(2) depletion alone was insufficient for channel opening, which additionally required Ca(2+) and pertussis toxin-sensitive G(i/o) proteins. Thus, TRPC4 channels integrate a variety of G-protein-dependent stimuli, including a PIP(2)/cytoskeleton dependence reminiscent of the TRPC4-like muscarinic agonist-activated cation channels in ileal myocytes.

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.

The spectrin cytoskeleton influences the surface expression and activation of human transient receptor potential channel 4 channels

Despite over a decade of research, only recently have the mechanisms governing transient receptor potential channel (TRPC) channel function begun to emerge, with an essential role for accessory proteins in this process. We previously identified a tyrosine phosphorylation event as critical in the plasma membrane translocation and activation of hTRPC4 channels following epidermal growth factor (EGF) receptor activation. To further characterize the signaling events underlying this process, a yeast-two hybrid screen was performed on the C terminus of hTRPC4. The intracellular C-terminal region from proline 686 to leucine 977 was used to screen a human brain cDNA library. Two members of the spectrin family, alphaII- and betaV-spectrin, were identified as binding partners. The interaction of hTRPC4 with alphaII-spectrin and betaV-spectrin was confirmed by glutathione S-transferase pulldown and co-immunoprecipitation experiments. Deletion analysis identified amino acids 730-758 of hTRPC4 as critical for the interaction with this region located within a coiled-coil domain, juxtaposing the Ca(2+)/calmodulin- and IP(3)R-binding region (CIRB domain). This region is deleted in the proposed deltahTRPC4 splice variant form, which failed to undergo both EGF-induced membrane insertion and activation, providing a genetic mechanism for regulating channel activity. We also demonstrate that the exocytotic insertion and activation of hTRPC4 following EGF application is accompanied by dissociation from alphaII-spectrin. Furthermore, depletion of alphaII-spectrin by small interference RNA reduces the basal surface expression of alphahTRPC4 and prevents the enhanced membrane insertion in response to EGF application. Importantly, depletion of alphaII-spectrin did not affect the expression of the delta variant. Taken together, these results demonstrate that a direct interaction between hTRPC4 and the spectrin cytoskeleton is involved in the regulation of hTRPC4 surface expression and activation.

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.

Novel mechanism of endothelin-1-induced vasospasm after subarachnoid hemorrhage

Cerebral vasospasm is a major cause of morbidity and mortality after aneurysmal subarachnoid hemorrhage (SAH). It is a sustained constriction of the cerebral arteries that can be reduced by endothelin (ET) receptor antagonists. Voltage-gated Ca(2+) channel antagonists such as nimodipine are relatively less effective. Endothelin-1 is not increased enough after SAH to directly cause the constriction, so we sought alternate mechanisms by which ET-1 might mediate vasospasm. Vasospasm was created in dogs, and the smooth muscle cells were studied molecularly, electrophysiologically, and by isometric tension. During vasospasm, ET-1, 10 nmol/L, induced a nonselective cation current carried by Ca(2+) in 64% of cells compared with in only 7% of control cells. Nimodipine and 2-aminoethoxydiphenylborate (a specific antagonist of store-operated channels) had no effect, whereas SKF96365 (a nonspecific antagonist of nonselective cation channels) decreased this current in vasospastic smooth muscle cells. Transient receptor potential (TRP) proteins may mediate entry of Ca(2+) through nonselective cationic pathways. We tested their role by incubating smooth muscle cells with anti-TRPC1 or TRPC4, both of which blocked ET-1-induced currents in SAH cells. Anti-TRPC5 had no effect. Anti-TRPC1 also inhibited ET-1 contraction of SAH arteries in vitro. Quantitative polymerase chain reaction and Western blotting of seven TRPC isoforms found increased expression of TRPC4 and a novel splice variant of TRPC1 and increased protein expression of TRPC4 and TRPC1. Taken together, the results support a novel mechanism whereby ET-1 significantly increases Ca(2+) influx mediated by TRPC1 and TRPC4 or their heteromers in smooth muscle cells, which promotes development of vasospasm after SAH.

Store-operated calcium entry in vascular smooth muscle

In non-excitable cells, activation of G-protein-coupled phospholipase C (PLC)-linked receptors causes the release of Ca(2+) from intracellular stores, which is followed by transmembrane Ca(2+) entry. This Ca(2+) entry underlies a small and sustained phase of the cellular [Ca(2+)](i) increases and is important for several cellular functions including gene expression, secretion and cell proliferation. This form of transmembrane Ca(2+) entry is supported by agonist-activated Ca(2+)-permeable ion channels that are activated by store depletion and is referred to as store-operated Ca(2+) entry (SOCE) and represents a major pathway for agonist-induced Ca(2+) entry. In excitable cells such as smooth muscle cells, Ca(2+) entry mechanisms responsible for sustained cellular activation are normally considered to be mediated via either voltage-operated or receptor-operated Ca(2+) channels. Although SOCE occurs following agonist activation of smooth muscle, this was thought to be more important in replenishing Ca(2+) stores rather than acting as a source of activator Ca(2+) for the contractile process. This review summarizes our current knowledge of SOCE as a regulator of vascular smooth muscle tone and discusses its possible role in the cardiovascular function and disease. We propose a possible hypothesis for its activation and suggest that SOCE may represent a novel target for pharmacological therapeutic intervention.

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.

Regulation of endothelial permeability by Src kinase signaling: vascular leakage versus transcellular transport of drugs and macromolecules

An important function of the endothelium is to regulate the transport of liquid and solutes across the semi-permeable vascular endothelial barrier. Two cellular pathways have been identified controlling endothelial barrier function. The normally restrictive paracellular pathway, which can become "leaky" during inflammation when gaps are induced between endothelial cells at the level of adherens and tight junctional complexes, and the transcellular pathway, which transports plasma proteins the size of albumin via transcytosis in vesicle carriers originating from cell surface caveolae. During non-inflammatory conditions, caveolae-mediated transport may be the primary mechanism of vascular permeability regulation of fluid phase molecules as well as lipids, hormones, and peptides that bind avidly to albumin. Src family protein tyrosine kinases have been implicated in the upstream signaling pathways that lead to endothelial hyperpermeability through both the paracellular and transcellular pathways. Endothelial barrier dysfunction not only affects vascular homeostasis and cell metabolism, but also governs drug delivery to underlying cells and tissues. In this review of the field, we discuss the current understanding of Src signaling in regulating paracellular and transcellular endothelial permeability pathways and effects on endogenous macromolecule and drug delivery.

TRPC1: The link between functionally distinct store-operated calcium channels

Although store-operated calcium entry (SOCE) was identified more that two decades ago, understanding the molecular mechanisms that regulate and mediate this process continue to pose a major challenge to investigators in this field. Thus, there has been major focus on determining which of the models proposed for this mechanism is valid and conclusively establishing the components of the store-operated calcium (SOC) channel(s). The transient receptor potential canonical (TRPC) proteins have been suggested as candidate components of the elusive store-operated Ca(2+) entry channel. While all TRPCs are activated in response to agonist-stimulated phosphatidylinositol 4,5, bisphosphate (PIP(2)) hydrolysis, only some display store-dependent regulation. TRPC1 is currently the strongest candidate component of SOC and is shown to contribute to SOCE in many cell types. Heteromeric interactions of TRPC1 with other TRPCs generate diverse SOC channels. Recent studies have revealed novel components of SOCE, namely the stromal interacting molecule (STIM) and Orai proteins. While STIM1 has been suggested to be the ER-Ca(2+) sensor protein relaying the signal to the plasma membrane for activation of SOCE, Orai1 is reported to be the pore-forming component of CRAC channel that mediates SOCE in T-lymphocytes and other hematopoetic cells. Several studies now demonstrate that TRPC1 also associates with STIM1 suggesting that SOC and CRAC channels are regulated by similar molecular components. Interestingly, TRPC1 is also associated with Orai1 and a TRPC1-Orai1-STIM1 ternary complex contributes to SOC channel function. This review will focus on the diverse SOC channels formed by TRPC1 and the suggestion that TRPC1 might serve as a molecular link that determines their regulation by store-depletion.

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.

Multiple activation mechanisms of store-operated TRPC channels in smooth muscle cells

Store-operated channels (SOCs) are plasma membrane Ca2+-permeable cation channels which are activated by agents that deplete intracellular Ca2+ stores. In smooth muscle SOCs are involved in contraction, gene expression, cell growth and proliferation. Single channel recording has demonstrated that SOCs with different biophysical properties are expressed in smooth muscle indicating diverse molecular identities. Moreover it is apparent that several gating mechanisms including calmodulin, protein kinase C and lysophospholipids are involved in SOC activation. Evidence is accumulating that TRPC proteins are important components of SOCs in smooth muscle. More recently Orai and STIM proteins have been proposed to underlie the well-described calcium-release-activated current (ICRAC) in non-excitable cells but at present there is little information on the role of Orai and STIM proteins in smooth muscle. In addition it is likely that different TRPC subunits coassemble as heterotetrameric structures to form smooth muscle SOCs. In this brief review we summarize the diverse properties and gating mechanisms of SOCs in smooth muscle. We propose that the heterogeneity of the properties of these conductances in smooth muscle results from the formation of heterotetrameric TRPC structures in different smooth muscle preparations.

A mathematical model of plasma membrane electrophysiology and calcium dynamics in vascular endothelial cells

Vascular endothelial cells (ECs) modulate smooth muscle cell (SMC) contractility, assisting in vascular tone regulation. Cytosolic Ca2+ concentration ([Ca2+]i) and membrane potential ( Vm) play important roles in this process by controlling EC-dependent vasoactive signals and intercellular communication. The present mathematical model integrates plasmalemma electrophysiology and Ca2+ dynamics to investigate EC responses to different stimuli and the controversial relationship between [Ca2+]i and Vm. The model contains descriptions for the intracellular balance of major ionic species and the release of Ca2+ from intracellular stores. It also expands previous formulations by including more detailed transmembrane current descriptions. The model reproduces Vm responses to volume-regulated anion channel (VRAC) blockers and extracellular K+ concentration ([K+]o) challenges, predicting 1) that Vm changes upon VRAC blockade are [K+]o dependent and 2) a biphasic response of Vm to increasing [K+]o. Simulations of agonist-induced Ca2+ mobilization replicate experiments under control and Vm hyperpolarization blockade conditions. They show that peak [Ca2+]i is governed by store Ca2+ release while Ca2+ influx (and consequently Vm) impacts more the resting and plateau [Ca2+]i. The Vm sensitivity of rest and plateau [Ca2+]i is dictated by a [Ca2+]i “buffering” system capable of masking the Vm-dependent transmembrane Ca2+ influx. The model predicts plasma membrane Ca2+-ATPase and Ca2+ permeability as main players in this process. The heterogeneous Vm impact on [Ca2+]i may elucidate conflicting reports on how Vm influences EC Ca2+. The present study forms the basis for the development of multicellular EC-SMC models that can assist in understanding vascular autoregulation in health and disease.

TRPC channels as STIM1-regulated store-operated channels

Receptor-activated Ca(2+) influx is mediated largely by store-operated channels (SOCs). TRPC channels mediate a significant portion of the receptor-activated Ca(2+) influx. However, whether any of the TRPC channels function as a SOC remains controversial. Our understanding of the regulation of TRPC channels and their function as SOCs is being reshaped with the discovery of the role of STIM1 in the regulation of Ca(2+) influx channels. The findings that STIM1 is an ER resident Ca(2+) binding protein that regulates SOCs allow an expanded and molecular definition of SOCs. SOCs can be considered as channels that are regulated by STIM1 and require the clustering of STIM1 in response to depletion of the ER Ca(2+) stores and its translocation towards the plasma membrane. TRPC1 and other TRPC channels fulfill these criteria. STIM1 binds to TRPC1, TRPC2, TRPC4 and TRPC5 but not to TRPC3, TRPC6 and TRPC7, and STIM1 regulates TRPC1 channel activity. Structure-function analysis reveals that the C-terminus of STIM1 contains the binding and gating function of STIM1. The ERM domain of STIM1 binds to TRPC channels and a lysine-rich region participates in the gating of SOCs and TRPC1. Knock-down of STIM1 by siRNA and prevention of its translocation to the plasma membrane inhibit the activity of native SOCs and TRPC1. These findings support the conclusion that TRPC1 is a SOC. Similar studies with other TRPC channels demonstrate their regulation by STIM1 and indicate that all TRPC channels, except TRPC7, function as SOCs.

Characteristics of the ATP-induced Ca2+-entry pathway in the t-BBEC 117 cell line derived from bovine brain endothelial cells

ATP-receptor (P2Y) stimulation induced sustained Ca2+-entry, which was essential for the enhanced cell-proliferation in t-BBEC117, an immortalized cell-line derived from bovine brain endothelial cells. Application of Ca2+ following store-depletion with thapsigargin in Ca2+-free solution induced Ca2+-entry through store-operated channels (SOCs). Ca2+-entry induced by ATP or 1-oleoyl-2-acetyl-sn-glycerol (OAG) together with Ca2+ was significantly larger than that by Ca2+ alone, suggesting the involvement of receptor-operated channels (ROCs) in the Ca2+-entry. Results obtained using pharmacological tools suggest that the contribution of Ca2+ sources to ATP-induced [Ca2+]i rise in t-BBEC117 is estimated as approximately 2:1:2 for Ca2+-release and Ca2+-entry though SOCs and ROCs, respectively.

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.

Small- and intermediate-conductance Ca2+-activated K+ channels directly control agonist-evoked nitric oxide synthesis in human vascular endothelial cells

The contribution of small-conductance (SK(Ca)) and intermediate-conductance Ca(2+)-activated K(+) (IK(Ca)) channels to the generation of nitric oxide (NO) by Ca(2+)-mobilizing stimuli was investigated in human umbilical vein endothelial cells (HUVECs) by combining single-cell microfluorimetry with perforated patch-clamp recordings to monitor agonist-evoked NO synthesis, cytosolic Ca(2+) transients, and membrane hyperpolarization in real time. ATP or histamine evoked reproducible elevations in NO synthesis and cytosolic Ca(2+), as judged by 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) and fluo-3 fluorescence, respectively, that were tightly associated with membrane hyperpolarizations. Whereas evoked NO synthesis was unaffected by either tetraethylammonium (10 mmol/l) or BaCl(2) (50 micromol/l) + ouabain (100 micromol/l), depleting intracellular Ca(2+) stores by thapsigargin or removing external Ca(2+) inhibited NO production, as did exposure to high (80 mmol/l) external KCl. Importantly, apamin and charybdotoxin (ChTx)/ triarylmethane (TRAM)-34, selective blockers SK(Ca) and IK(Ca) channels, respectively, abolished both stimulated NO synthesis and membrane hyperpolarization and decreased evoked Ca(2+) transients. Apamin and TRAM-34 also inhibited an agonist-induced outwardly rectifying current characteristic of SK(Ca) and IK(Ca) channels. Under voltage-clamp control, we further observed that the magnitude of agonist-induced NO production varied directly with the degree of membrane hyperpolarization. Mechanistically, our data indicate that SK(Ca) and IK(Ca) channel-mediated hyperpolarization represents a critical early event in agonist-evoked NO production by regulating the influx of Ca(2+) responsible for endothelial NO synthase activation. Moreover, it appears that the primary role of agonist-induced release of intracellular Ca(2+) stores is to trigger the opening of both K(Ca) channels along with Ca(2+) entry channels at the plasma membrane. Finally, the observed inhibition of stimulated NO synthesis by apamin and ChTx/TRAM-34 demonstrates that SK(Ca) and IK(Ca) channels are essential for NO-mediated vasorelaxation.

N-(p-Amylcinnamoyl)anthranilic Acid (ACA): A Phospholipase A2 Inhibitor and TRP Channel Blocker

Phospholipase A(2) enzymes display a superfamily of structurally different enzymes classified in at least nine subfamilies by biochemical and structural properties. N-(p-amylcinnamoyl)anthranilic acid commonly referred to as ACA is often used as a broad-spectrum inhibitor for the characterization of phospholipase A(2)-mediated pathways. Compounds like ACA and ACA-like structures have been described to block the receptor-induced release of arachidonic acid and subsequent signaling cascades in the pancreas and the cardiovascular system. We showed that ACA directly blocks several transient receptor potential (TRP) channels (TRPC6, TRPM2, TRP and TRPM8). With respect to the published data of ACA in the phospholipase A(2) field, the finding that ACA blocks diacylglycerol-activated TRP channels is of specific interest as it offers the opportunity to interfere with receptor-induced calcium-dependent signaling processes in platelets and vascular smooth muscle cells. Overall, N-phenylcinnamides, as a new pharmaceutical lead structure, form the first class of synthetic TRP channel blockers and represent a promising start for the development of small organic TRP channel-specific blockers.

Genetic evidence supporting caveolae microdomain regulation of calcium entry in endothelial cells

Various cellular signals initiate calcium entry into cells, and there is evidence that lipid rafts and caveolae may concentrate proteins that regulate transmembrane calcium fluxes. Here, using mice deficient in caveolin-1 (Cav-1) and Cav-1 knock-out reconstituted with endothelium-specific Cav-1, we show that Cav-1 is essential for calcium entry in endothelial cells and governs the localization and protein-protein interactions between transient receptor channels C4 and C1. Thus, Cav-1 is required for calcium entry in vascular endothelial cells and perhaps other specialized cell types containing caveolae.

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.

TRP channels in endothelial function and dysfunction

Endothelial cells produce various factors that regulate vascular tone, vascular permeability, angiogenesis, and inflammatory responses. The dysfunction of endothelial cells is believed to be the major culprit in various cardiovascular diseases, including hypertension, atherosclerosis, heart and renal failure, coronary syndrome, thrombosis, and diabetes. Endothelial cells express multiple transient receptor potential (TRP) channel isoforms, the activity of which serves to modulate cytosolic Ca(2+) levels ([Ca(2+)](i)) and regulate membrane potential, both of which affect various physiological processes. The malfunction and dysregulation of TRP channels is associated with endothelial dysfunction, which is reflected by decreased nitric oxide (NO) bioavailability, inappropriate regulation of vascular smooth muscle tonicity, endothelial barrier dysfunction, increased oxidative damage, impaired anti-thrombogenic properties, and perturbed angiogenic competence. Evidence suggests that dysregulation of TRPC4 and -C1 results in vascular endothelial barrier dysfunction; malfunction of TRPP1 and -P2 impairs endothelial NO synthase; the reduced expression or activity of TRPC4 and -V1 impairs agonist-induced vascular relaxation; the decreased activity of TRPV4 reduces flow-induced vascular responses; and the activity of TRPC3 and -C4 is associated with oxidative stress-induced endothelial damage. In this review, we present a comprehensive summary of the literature on the role of TRP channels in endothelial cells, with an emphasis on endothelial dysfunction.

Orai proteins interact with TRPC channels and confer responsiveness to store depletion

The TRPC (C-type transient receptor potential) class of ion channels has been hypothesized to participate in store-operated Ca(2+) entry (SOCE). Recently, however, STIM1 and Orai1 proteins have been proposed to form SOCE channels. Whether TRPCs participate in SOCE that is dependent on or regulated by Orai has not been explored. Here we show that Orai1 physically interacts with the N and C termini of TRPC3 and TRPC6, and that in cells overexpressing either TRPC3 or TRPC6 in a store-depletion insensitive manner, these TRPCs become sensitive to store depletion upon expression of an exogenous Orai. Thus, Orai-1, -2, and -3 enhanced thapsigargin-induced calcium entry by 50-150% in cells stably overexpressing either TRPC3 or TRPC6. Orai1 expression had no significant effect on endogenous, thapsigargin-induced calcium entry in wild-type cells (HEK-293, COS1), in HEK cells expressing a thapsigargin-sensitive variant of TRPC3 (TRPC3a), or in HEK cells overexpressing another membrane protein, V1aR. Single-channel cation currents present in membrane patches of TRPC3-overexpressing cells were suppressed by expression of Orai1. We propose that Orai proteins by interacting with TRPCs act as regulatory subunits that confer STIM1-mediated store depletion sensitivity to these channels.

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.

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.

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.

Ionic channels formed by TRPC4

TRPC4 (transient receptor potential canonical 4) is a member of the TRPC sub-family and, within this sub-family, TRPC4 is most closely related to TRPC5. A number of splice variants of TRPC4 have been identified, whereby TRPC4alpha and TRPC4beta appear to be the most abundant isoforms in various species. TRPC4alpha comprises six transmembrane segments and the N- and C-termini are located intracellularly. Additionally, TRPC4alpha shares other structural features with members of the TRPC sub-group, including ankyrin-like repeats, coiled-coil regions and binding sites for calmodulin and IP3 receptors. Three calmodulin-binding domains have been identified in the C-terminus of TRPC4alpha. TRPC4beta lack 84 amino acids in the C-terminus, which correspond to the last two calmodulin-binding sites of TRPCalpha. The first and last calmodulin-binding domains of TRPC4alpha overlap with binding sites for the N- and C-termini of IP3 receptors. The ionic channels formed by TRPC4 appear to be Ca(2+)-permeable, although there is a considerably discrepancy in the degree of Ca2+ selectivity. Studies with mice lacking TRPC4 (TRPC4(-/-)) suggest an important role for TRPC4 in supporting Ca2+ entry. The defect in Ca2+ entry in TRPC4(-/-) mice appears to be associated with a reduction of the vasorelaxation of arteries, vascular permeability in the lung and neurotransmitter release from thalamic dendrites.

Regulation of cation channels in cardiac and smooth muscle cells by intracellular magnesium

Magnesium regulates various ion channels in many tissues, including those of the cardiovascular system. General mechanisms by which intracellular Mg(2+) (Mg(i)(2+)) regulates channels are presented. These involve either a direct interaction with the channel, or an indirect modification of channel function via other proteins, such as enzymes or G proteins, or via membrane surface charges and phospholipids. To provide an insight into the role of Mg(i)(2+) in the cardiovascular system, effects of Mg(i)(2+) on major channels in cardiac and smooth muscle cells and the underlying mechanisms are then reviewed. Although Mg(i)(2+) concentrations are known to be stable, conditions under which they may change exist, such as following stimulation of beta-adrenergic receptors and of insulin receptors, or during pathophysiological conditions such as ischemia, heart failure or hypertension. Modifications of cardiovascular electrical or mechanical function, possibly resulting in arrhythmias or hypertension, may result from such changes of Mg(i)(2+) and their effects on cation channels.

Calcium signaling in non-excitable cells: Ca2+ release and influx are independent events linked to two plasma membrane Ca2+ entry channels

The regulatory mechanism of Ca2+ influx into the cytosol from the extracellular space in non-excitable cells is not clear. The "capacitative calcium entry" (CCE) hypothesis suggested that Ca2+ influx is triggered by the IP(3)-mediated emptying of the intracellular Ca2+ stores. However, there is no clear evidence for CCE and its mechanism remains elusive. In the present work, we have provided the reported evidences to show that inhibition of IP(3)-dependent Ca2+ release does not affect Ca2+ influx, and the experimental protocols used to demonstrate CCE can stimulate Ca2+ influx by means other than emptying of the Ca2+ stores. In addition, we have presented the reports showing that IP(3)-mediated Ca2+ release is linked to a Ca2+ entry from the extracellular space, which does not increase cytosolic [Ca2+] prior to Ca2+ release. Based on these and other reports, we have provided a model of Ca2+ signaling in non-excitable cells, in which IP(3)-mediated emptying of the intracellular Ca2+ store triggers entry of Ca2+ directly into the store, through a plasma membrane TRPC channel. Thus, emptying and direct refilling of the Ca2+ stores are repeated in the presence of IP(3), giving rise to the transient phase of oscillatory Ca2+ release. Direct Ca2+ entry into the store is regulated by its filling status in a negative and positive manner through a Ca2+ -binding protein and Stim1/Orai complex, respectively. The sustained phase of Ca2+ influx is triggered by diacylglycerol (DAG) through the activation of another TRPC channel, independent of Ca2+ release. The plasma membrane IP(3) receptor (IP(3)R) plays an essential role in Ca2+ influx, by interacting with the DAG-activated TRPC, without the requirement of binding to IP(3).

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.

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.

TRPC, cGMP-dependent protein kinases and cytosolic Ca2+

Ca2+, nitric oxide (NO), and protein kinase G (PKG) are important signaling molecules that play pivotal roles in many physiological processes such as vascular tone control, platelet activation, and synaptic plasticity. TRPC channels allow Ca2+ influx, thus contributing to the production of NO, which subsequently stimulates PKG. It has been demonstrated that PKG can phosphorylate human TRPC3 at Thr-11 and Ser-263 and that this phosphorylation inactivates TRPC3. These two PKG phosphorylation sites, Thr-11 and Ser-263 in human TRPC3, are conserved in other members of the TRPC3/6/7 subfamily, suggesting that PKG may also phosphorylate TRPC6 and TRPC7. In addition, protein kinase C (PKC) also inactivates TRPC3, partly through activating PKG. The PKG-mediated inhibition of TRPC channels may provide a feedback control for the fine tuning of [Ca2+]i levels and protect the cells from the detrimental effects of excessive [Ca2+]i and/or NO.

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.

Interaction between store-operated and arachidonate-activated calcium entry

A ubiquitous pathway for cellular Ca(2+) influx involves 'store-operated channels' that respond to depletion of intracellular Ca(2+) pools via an as yet unknown mechanism. Due to its wide-spread expression, store-operated Ca(2+) entry (SOCE) has been considered a principal route for Ca(2+) influx. However, recent evidence has suggested that alternative pathways, activated for example by lipid metabolites, are responsible for physiological Ca(2+) influx. It is not clear if these messenger-activated Ca(2+) entry routes exist in all cells and what interaction they have with SOCE. In the present study we demonstrate that HEK-293 cells and Saos-2 cells express an arachidonic acid (AA)-activated Ca(2+) influx pathway that is distinct from SOCE on the basis of sensitivity to pharmacological blockers and depletion of cellular cholesterol. We examined the functional interaction between SOCE and the arachidonate-triggered Ca(2+) influx (denoted non-SOCE). Both Ca(2+) entry routes could underlie substantial long-lasting Ca(2+) elevations. However, the two pathways could not operate simultaneously. With cells that had an on-going SOCE response, addition of arachidonate gave two profound effects. Firstly, it rapidly inhibited SOCE. Secondly, the mode of Ca(2+) influx switched to the non-SOCE mechanism. Addition of arachidonate to naïve cells resulted in rapid activation of the non-SOCE pathway. However, this Ca(2+) entry route was very slowly engaged if the SOCE pathway was already operative. These data indicate that the SOCE and arachidonate-activated non-SOCE pathways interact in an inhibitory manner. We probed the plausible mechanisms by which these two pathways may communicate.

Know thy neighbor: a survey of diseases and complex syndromes that map to chromosomal regions encoding TRP channels

On the basis of their ever-expanding roles, not only in sensory signaling but also in a plethora of other, often Ca(2+)-mediated actions in cell and whole body homeostasis, it is suggested that mutations in TRP channel genes not only cause disease states but also contribute in more subtle ways to simple and complex diseases. A survey is therefore presented of diseases and syndromes that map to one or multiple chromosomal loci containing TRP channel genes. A visual map of the chromosomal locations of TRP channel genes in man and mouse is also presented.

Calcium signalling in the endothelium

Elevations in cytosolic Ca2+ concentration are the usual initial response of endothelial cells to hormonal and chemical transmitters and to changes in physical parameters, and many endothelial functions are dependent upon changes in Ca2+ signals produced. Endothelial cell Ca2+ signalling shares similar features with other electrically non-excitable cell types, but has features unique to endothelial cells. This chapter discusses the major components of endothelial cell Ca2+ signalling.

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.

Emerging perspectives in store-operated Ca2+ entry: Roles of Orai, Stim and TRP

Depletion of intracellular Ca2+ stores induces Ca2+ influx across the plasma membrane through store-operated channels (SOCs). This store-operated Ca2+ influx is important for the replenishment of the Ca2+ stores, and is also involved in many signaling processes by virtue of the ability of intracellular Ca2+ to act as a second messenger. For many years, the molecular identities of particular SOCs, as well as the signaling mechanisms by which these channels are activated, have been elusive. Recently, however, the mammalian proteins STIM1 and Orai1 were shown to be necessary for the activation of store-operated Ca2+ entry in a variety of mammalian cells. Here we present molecular, pharmacological, and electrophysiological properties of SOCs, with particular focus on the roles that STIM1 and Orai1 may play in the signaling processes that regulate various pathways of store-operated entry.

Mechanism and functional significance of TRPC channel multimerization

Ca(2+) signaling regulates many important physiological events within a diverse set of living organisms. In particular, sustained Ca(2+) signals play an important role in controlling cell proliferation, cell differentiation and the activation of immune cells. Two key elements for the generation of sustained Ca(2+) signals are store-operated and receptor-operated Ca(2+) channels that are activated downstream of phospholipase C (PLC) stimulation, in response to G-protein-coupled receptor or growth factor receptor stimulation. One goal of this review is to help clarify the role of canonical transient receptor potential (TRPC) proteins in the formation of native store-operated and native receptor-operated channels. Toward that end, data from studies of endogenous TRPC proteins will be reviewed in detail to highlight the strong case for the involvement of certain TRPC proteins in the formation of one subtype of store-operated channel, which exhibits a low Ca(2+)-selectivity, in contrast to the high Ca(2+)-selectivity exhibited by the CRAC subtype of store-operated channel. A second goal of this review is to highlight the growing body of evidence indicating that native store-operated and native receptor-operated channels are formed by the heteromultimerization of TRPC subunits. Furthermore, evidence will be provided to argue that some TRPC proteins are able to form multiple channel types.

Functional organization of TRPC-Ca2+ channels and regulation of calcium microdomains

TRP family of proteins are components of unique cation channels that are activated in response to diverse stimuli ranging from growth factor and neurotransmitter stimulation of plasma membrane receptors to a variety of chemical and sensory signals. This review will focus on members of the TRPC sub-family (TRPC1-TRPC7) which currently appear to be the strongest candidates for the enigmatic Ca(2+) influx channels that are activated in response to stimulation of plasma membrane receptors which result in phosphatidyl inositol-(4,5)-bisphosphate (PIP(2)) hydrolysis, generation of IP(3) and DAG, and IP(3)-induced Ca(2+) release from the intracellular Ca(2+) store via inositol trisphosphate receptor (IP(3)R). Homomeric or selective heteromeric interactions between TRPC monomers generate distinct channels that contribute to store-operated as well as store-independent Ca(2+) entry mechanisms. The former is regulated by the emptying/refilling of internal Ca(2+) store(s) while the latter depends on PIP(2) hydrolysis (due to changes in PIP(2) per se or an increase in diacylglycerol, DAG). Although the exact physiological function of TRPC channels and how they are regulated has not yet been conclusively established, it is clear that a variety of cellular functions are controlled by Ca(2+) entry via these channels. Thus, it is critical to understand how cells coordinate the regulation of diverse TRPC channels to elicit specific physiological functions. It is now well established that segregation of TRPC channels mediated by interactions with signaling and scaffolding proteins, determines their localization and regulation in functionally distinct cellular domains. Furthermore, both protein and lipid components of intracellular and plasma membranes contribute to the organization of these microdomains. Such organization serves as a platform for the generation of spatially and temporally dictated [Ca(2+)](i) signals which are critical for precise control of downstream cellular functions.

Nitric oxide activates TRP channels by cysteine S-nitrosylation

Transient receptor potential (TRP) proteins form plasma-membrane cation channels that act as sensors for diverse cellular stimuli. Here, we report a novel activation mechanism mediated by cysteine S-nitrosylation in TRP channels. Recombinant TRPC1, TRPC4, TRPC5, TRPV1, TRPV3 and TRPV4 of the TRPC and TRPV families, which are commonly classified as receptor-activated channels and thermosensor channels, induce entry of Ca(2+) into cells in response to nitric oxide (NO). Labeling and functional assays using cysteine mutants, together with membrane sidedness in activating reactive disulfides, show that cytoplasmically accessible Cys553 and nearby Cys558 are nitrosylation sites mediating NO sensitivity in TRPC5. The responsive TRP proteins have conserved cysteines on the same N-terminal side of the pore region. Notably, nitrosylation of native TRPC5 upon G protein-coupled ATP receptor stimulation elicits entry of Ca(2+) into endothelial cells. These findings reveal the structural motif for the NO-sensitive activation gate in TRP channels and indicate that NO sensors are a new functional category of cellular receptors extending over different TRP families.

Interaction between TRPC channel subunits in endothelial cells

Transient Receptor Potential Canonical (TRPC) proteins have been identified in mammals as a family of plasma membrane calcium-permeable channels activated by different kinds of stimuli in several cell types. We have studied TRPC subunit expression in bovine aortic endothelial (BAE-1) cells, where stimulation with basic fibroblast growth factor (bFGF), a potent angiogenetic factor, induces calcium entry carried at least partially by TRPC1 channels. By means of a RT-PCR approach, we have found that, in addition to TRPC1, only TRPC4 is expressed, both at the mRNA and protein level, as confirmed by immunoblotting and immunocytochemical analysis. Because functional TRPC channels are formed by assembly of four subunits in either homo- or heterotetrameric structures, we have carried out immunoprecipitation experiments and showed that TRPC1 and TRPC4 interact to form heteromers in these cells, independently from culture conditions (high or low percent of fetal calf serum, stimulation with bFGF). Moreover, the data show that TRPC subunits are not tyrosine-phosphorylated after bFGF stimulation and they do not co-immunoprecipitate with the type 1 FGF receptor. These results suggest that BAE-1 cells are a suitable model to study function and regulation of endogenous TRPC1/TRPC4 heteromers.

Canonical transient receptor potential channels in disease: targets for novel drug therapy?

The canonical transient receptor potential (TRPC) channels constitute one of the three major families within the large transient receptor potential (TRP) superfamily. TRPC channels are the closest mammalian homologues of Drosophila TRP, the light-activated channel in Drosophila photoreceptor cells. All TRPC channels (TRPC1-7) are activated via phospholipase-C-coupled receptors and were, therefore, proposed to encode elusive native receptor-activated cation channels in many cell types. A physiological role has been established for all of the known TRPC channels, including the control of vascular tone (TRPC1, TRPC4 and TRPC6) or lymphocyte activation, which is essential for immune competence (TRPC1 and TRPC3). The emergence of TRPC channels in controlling a variety of biological functions offers new and promising targets for drug development.

Vascular physiology of a Ca2+ mobilizing second messenger - cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is a novel Ca(2+) mobilizing second messenger, which is capable of inducing Ca(2+) release from the sarcoplasmic reticulum (SR) via activation of ryanodine receptors (RyR) in vascular cells. This signaling nucleotide has also been reported to participate in generation or modulation of intracellular Ca(2+) sparks, Ca(2+) waves or oscillations, Ca(2+)- induced Ca(2+) release (CICR) and spontaneous transient outward currents (STOCs) in vascular smooth muscle cells (VSMCs). With respect to the role of cADPR-mediated signaling in mediation of vascular responses to different stimuli, there is accumulating evidence showing that cADPR is importantly involved in the Ca(2+) response of vascular endothelial cells (ECs) and VSMCs to various chemical factors such as vasoactive agonists acetylcholine, oxotremorine, endothelin, and physical stimuli such as stretch, electrical depolarization and sheer stress. This cADPR-RyR-mediated Ca(2+) signaling is now recognized as a fundamental mechanism regulating vascular function. Here we reviewed the literature regarding this cADPR signaling pathway in vascular cells with a major focus on the production of cADPR and its physiological roles in the control of vascular tone and vasomotor response. We also summarized some publish results that unveil the underlying mechanisms mediating the actions of cADPR in vascular cells. Given the importance of Ca(2+) in the regulation of vascular function, the results summarized in this brief review will provide new insights into vascular physiology and circulatory regulation.

Transient receptor potential channels in essential hypertension

OBJECTIVE

The role of nonselective cation channels of the transient receptor potential channel (TRPC) family in essential hypertension has not yet been investigated.

METHODS

We studied TRPCs in 51 patients with essential hypertension and 51 age-matched and sex-matched normotensive control subjects. Calcium and gadolinium influx into human monocytes was determined using the fluorescent dye technique. TRPC expression was measured using reverse transcriptase-polymerase chain reaction and in-cell western assay. Gene silencing by small interfering RNA for specific TRPC knockdown was also performed.

RESULTS

We observed an increased gadolinium/calcium-influx ratio through TRPC in essential hypertensive patients compared with normotensive control subjects [cation influx ratio (mean +/- SEM), 125 +/- 14 versus 80 +/- 7%; each n = 51; P < 0.01], due to an increase of gadolinium influx in hypertensive patients compared with normotensive control subjects (48 +/- 4 versus 36 +/- 3%; each n = 51; P < 0.05). We observed a significant increase of TRPC3 and TRPC5 protein expression in essential hypertensive patients compared with normotensive control subjects (normalized TRPC3 expression, 3.21 +/- 0.59 versus 1.36 +/- 0.07; each n = 20; P < 0.01; normalized TRPC5 expression, 2.10 +/- 0.28 versus 1.40 +/- 0.52; each n = 12; P < 0.05). We used small interfering RNA for knockdown of TRPC5. The thereby reduced channel expression caused a significant attenuation of calcium and gadolinium influx.

CONCLUSION

This study points to an important role of TRPCs in essential hypertension.