E3-targeted anti-TRPC5 antibody inhibits store-operated calcium entry in freshly isolated pial arterioles

Interactions, functions, and independence of plasma membrane STIM1 and TRPC1 in vascular smooth muscle cells

Stromal interaction molecule 1 (STIM1) is a predicted single membrane-spanning protein involved in store-operated calcium entry and interacting with ion channels including TRPC1. Here, we focus on endogenous STIM1 of modulated vascular smooth muscle cells, which exhibited a nonselective cationic current in response to store depletion despite strong buffering of intracellular calcium at the physiological concentration. STIM1 mRNA and protein were detected and suppressed by specific short interfering RNA. Calcium entry evoked by store depletion was partially inhibited by STIM1 short interfering RNA, whereas calcium release was unaffected. STIM1 short interfering RNA suppressed cell migration but not proliferation. Antibody that specifically bound STIM1 revealed constitutive extracellular N terminus of STIM1 and extracellular application of the antibody caused fast inhibition of the current evoked by store depletion. The antibody also inhibited calcium entry and cell migration but not proliferation. STIM1 interacted with TRPC1, and TRPC1 contributed partially to calcium entry and cationic current. However, the underlying processes could not be explained only by a STIM1-TRPC1 partnership because extracellular TRPC1 antibody suppressed cationic current only in a fraction of cells, TRPC1-containing channels were important for cell proliferation as well as migration, and cell surface localization studies revealed TRPC1 alone, as well as with STIM1. The data suggest a complex situation in which there is not only plasma membrane-spanning STIM1 that is important for cell migration and TRPC1-independent store-operated cationic current but also TRPC1-STIM1 interaction, a TRPC1-dependent component of store-operated current, and STIM1-independent TRPC1 linked to cell proliferation.

Ca2+ entry through TRPC1 channels contributes to intracellular Ca2+ dynamics and consequent glutamate release from rat astrocytes

Astrocytes can respond to a variety of stimuli by elevating their cytoplasmic Ca2+ concentration and can in turn release glutamate to signal adjacent neurons. The majority of this Ca2+ is derived from internal stores while a portion also comes from outside of the cell. Astrocytes use Ca2+ entry through store-operated Ca2+ channels to refill their internal stores. Therefore, we investigated what role this store-operated Ca2+ entry plays in astrocytic Ca2+ responses and subsequent glutamate release. Astrocytes express canonical transient receptor potential (TRPC) channels that have been implicated in mediating store-operated Ca2+ entry. Here, we show that astrocytes in culture and freshly isolated astrocytes from visual cortex express TRPC1, TRPC4, and TRPC5. Indirect immunocytochemistry reveals that these proteins are present throughout the cell; the predominant expression of functionally tested TRPC1, however, is on the plasma membrane. Labeling in freshly isolated astrocytes reveals changes in TRPC expression throughout development. Using an antibody against TRPC1 we were able to block the function of TRPC1 channels and determine their involvement in mechanically and agonist-evoked Ca2+ entry in cultured astrocytes. Blocking TRPC1 was also found to reduce mechanically induced Ca2+-dependent glutamate release. These data indicate that Ca2+ entry through TRPC1 channels contributes to Ca2+ signaling in astrocytes and the consequent glutamate release from these cells.

Production of a specific extracellular inhibitor of TRPM3 channels

BACKGROUND AND PURPOSE

Isoform-specific ion channel blockers are useful for target validation in drug discovery and can provide the basis for new therapeutic agents and aid in determination of physiological functions of ion channels. The aim of this study was to generate a specific blocker of human TRPM3 channels as a tool to help investigations of this member of the TRP cationic channel family.

EXPERIMENTAL APPROACH

A polyclonal antibody (TM3E3) was made to a conserved peptide of the third extracellular (E3) loop of TRPM3 and tested for binding and functional effect. Studies of channel activity were made by whole-cell planar patch-clamp and fura-2 intracellular Ca(2+) measurement.

KEY RESULTS

Ionic current mediated by TRPM3 was inhibited partially by TM3E3 over a period of 5-10 min. Ca(2+) entry in TRPM3-expressing cells was also partially inhibited by TM3E3 in a peptide-specific manner and independently of the type of agonist used to activate TRPM3. TM3E3 had no effect on TRPC5, TRPV4, TRPM2 or an endogenous ATP response.

CONCLUSIONS AND IMPLICATIONS

The data show the successful development of a specific TRPM3 inhibitor and give further confidence in E3 targeting as an approach to producing isoform-specific ion channel blockers.

Ca2+ handling is altered when arterial myocytes progress from a contractile to a proliferative phenotype in culture

Phenotypic modulation of vascular myocytes is important for vascular development and adaptation. A characteristic feature of this process is alteration in intracellular Ca(2+) handling, which is not completely understood. We studied mechanisms involved in functional changes of inositol 1,4,5-trisphosphate (IP(3))- and ryanodine (Ry)-sensitive Ca(2+) stores, store-operated Ca(2+) entry (SOCE), and receptor-operated Ca(2+) entry (ROCE) associated with arterial myocyte modulation from a contractile to a proliferative phenotype in culture. Proliferating, cultured myocytes from rat mesenteric artery have elevated resting cytosolic Ca(2+) levels and increased IP(3)-sensitive Ca(2+) store content. ATP- and cyclopiazonic acid [CPA; a sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor]-induced Ca(2+) transients in Ca(2+)-free medium are significantly larger in proliferating arterial smooth muscle cells (ASMCs) than in freshly dissociated myocytes, whereas caffeine (Caf)-induced Ca(2+) release is much smaller. Moreover, the Caf/Ry-sensitive store gradually loses sensitivity to Caf activation during cell culture. These changes can be explained by increased expression of all three IP(3) receptors and a switch from Ry receptor type II to type III expression during proliferation. SOCE, activated by depletion of the IP(3)/CPA-sensitive store, is greatly increased in proliferating ASMCs. Augmented SOCE and ROCE (activated by the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol) in proliferating myocytes can be attributed to upregulated expression of, respectively, transient receptor potential proteins TRPC1/4/5 and TRPC3/6. Moreover, stromal interacting molecule 1 (STIM1) and Orai proteins are upregulated in proliferating cells. Increased expression of IP(3) receptors, SERCA2b, TRPCs, Orai(s), and STIM1 in proliferating ASMCs suggests that these proteins play a critical role in an altered Ca(2+) handling that occurs during vascular growth and remodeling.

Differences in STIM1 and TRPC expression in proximal and distal pulmonary arterial smooth muscle are associated with differences in Ca2+ responses to hypoxia

Hypoxic pulmonary vasoconstriction (HPV) requires Ca(2+) influx through store-operated Ca(2+) channels (SOCC) in pulmonary arterial smooth muscle cells (PASMC) and is greater in distal than proximal pulmonary arteries (PA). SOCC may be composed of canonical transient receptor potential (TRPC) proteins and activated by stromal interacting molecule 1 (STIM1). To assess the possibility that HPV is greater in distal PA because store-operated Ca(2+) entry (SOCE) is greater in distal PASMC, we measured intracellular Ca(2+) concentration ([Ca(2+)](i)) and SOCE in primary cultures of PASMC using fluorescent microscopy and the Ca(2+)-sensitive dye fura 2. Both hypoxia (4% O(2)) and KCl (60 mM) increased [Ca(2+)](i). Responses to hypoxia, but not KCl, were greater in distal cells. We measured SOCE in PASMC perfused with Ca(2+)-free solutions containing cyclopiazonic acid to deplete Ca(2+) stores in sarcoplasmic reticulum and nifedipine to prevent Ca(2+) entry through L-type voltage-operated Ca(2+) channels. Under these conditions, the increase in [Ca(2+)](i) caused by restoration of extracellular Ca(2+) and the decrease in fura 2 fluorescence caused by Mn(2+) were greater in distal PASMC, indicating greater SOCE. Moreover, the increase in SOCE caused by hypoxia was also greater in distal cells. Real-time quantitative polymerase chain reaction analysis of PASMC and freshly isolated deendothelialized PA tissue demonstrated expression of STIM1 and five of seven known TRPC isoforms (TRPC1 > TRPC6 > TRPC4 >> TRPC3 approximately TRPC5). For both protein, as measured by Western blotting, and mRNA, expression of STIM1, TRPC1, TRPC6, and TRPC4 was greater in distal than proximal PASMC and PA. These results provide further support for the importance of SOCE in HPV and suggest that HPV is greater in distal than proximal PA because greater numbers and activation of SOCC in distal PASMC generate bigger increases in [Ca(2+)](i).

IP3 constricts cerebral arteries via IP3 receptor-mediated TRPC3 channel activation and independently of sarcoplasmic reticulum Ca2+ release

Vasoconstrictors that bind to phospholipase C-coupled receptors elevate inositol-1,4,5-trisphosphate (IP(3)). IP(3) is generally considered to elevate intracellular Ca(2+) concentration ([Ca(2+)](i)) in arterial myocytes and induce vasoconstriction via a single mechanism: by activating sarcoplasmic reticulum (SR)-localized IP(3) receptors, leading to intracellular Ca(2+) release. We show that IP(3) also stimulates vasoconstriction via a SR Ca(2+) release-independent mechanism. In isolated cerebral artery myocytes and arteries in which SR Ca(2+) was depleted to abolish Ca(2+) release (measured using D1ER, a fluorescence resonance energy transfer-based SR Ca(2+) indicator), IP(3) activated 15 pS sarcolemmal cation channels, generated a whole-cell cation current (I(Cat)) caused by Na(+) influx, induced membrane depolarization, elevated [Ca(2+)](i), and stimulated vasoconstriction. The IP(3)-induced I(Cat) and [Ca(2+)](i) elevation were attenuated by cation channel (Gd(3+), 2-APB) and IP(3) receptor (xestospongin C, heparin, 2-APB) blockers. TRPC3 (canonical transient receptor potential 3) channel knockdown with short hairpin RNA and diltiazem and nimodipine, voltage-dependent Ca(2+) channel blockers, reduced the SR Ca(2+) release-independent, IP(3)-induced [Ca(2+)](i) elevation and vasoconstriction. In pressurized arteries, SR Ca(2+) depletion did not alter IP(3)-induced constriction at 20 mm Hg but reduced IP(3)-induced constriction by approximately 39% at 60 mm Hg. [Ca(2+)](i) elevations and constrictions induced by endothelin-1, a phospholipase C-coupled receptor agonist, were both attenuated by TRPC3 knockdown and xestospongin C in SR Ca(2+)-depleted arteries. In summary, we describe a novel mechanism of IP(3)-induced vasoconstriction that does not occur as a result of SR Ca(2+) release but because of IP(3) receptor-dependent I(Cat) activation that requires TRPC3 channels. The resulting membrane depolarization activates voltage-dependent Ca(2+) channels, leading to a myocyte [Ca(2+)](i) elevation, and vasoconstriction.

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.

TRPC channel activation by extracellular thioredoxin

Mammalian homologues of Drosophila melanogaster transient receptor potential (TRP) are a large family of multimeric cation channels that act, or putatively act, as sensors of one or more chemical factor. Major research objectives are the identification of endogenous activators and the determination of cellular and tissue functions of these channels. Here we show the activation of TRPC5 (canonical TRP 5) homomultimeric and TRPC5-TRPC1 heteromultimeric channels by extracellular reduced thioredoxin, which acts by breaking a disulphide bridge in the predicted extracellular loop adjacent to the ion-selectivity filter of TRPC5. Thioredoxin is an endogenous redox protein with established intracellular functions, but it is also secreted and its extracellular targets are largely unknown. Particularly high extracellular concentrations of thioredoxin are apparent in rheumatoid arthritis, an inflammatory joint disease that disables millions of people worldwide. We show that TRPC5 and TRPC1 are expressed in secretory fibroblast-like synoviocytes from patients with rheumatoid arthritis, that endogenous TRPC5-TRPC1 channels of the cells are activated by reduced thioredoxin, and that blockade of the channels enhances secretory activity and prevents the suppression of secretion by thioredoxin. The data indicate the presence of a previously unrecognized ion-channel activation mechanism that couples extracellular thioredoxin to cell function.

Ion channel switching and activation in smooth-muscle cells of occlusive vascular diseases

Blood vessels are essential for animal life, allowing flow of oxygen and nutrients to tissues and removal of waste products. Consequently, inappropriate remodelling of blood vessels, resulting in occlusion, can lead to disabling or catastrophic events: heart attacks, strokes and claudication. An important cell type of remodelling is the VSMC (vascular smooth-muscle cell), a fascinating cell that contributes significantly to occlusive vascular diseases by virtue of its ability to 'modulate' to a cell that no longer contracts and arranges radially in the medial layer of the vessel wall but migrates, invades, proliferates and adopts phenotypes of other cells. An intriguing aspect of modulation is switching to different ion transport systems. Initial events include loss of the Ca(V)1.2 (L-type voltage-gated calcium) channel and gain of the K(Ca)3.1 (IKCa) potassium channel, which putatively occur to enable membrane hyperpolarization that increases rather than decreases a type of calcium entry coupled with cell cycle activity, cell proliferation and cell migration. This type of calcium entry is related to store- and receptor-operated calcium entry phenomena, which, in VSMCs, are contributed to by TRPC [TRP (transient receptor potential) canonical] channel subunits. Instead of being voltage-gated, these channels are chemically gated - importantly, by key phospholipid factors of vascular development and disease. This brief review focuses on the hypothesis that the transition to a modulated cell may require a switch from predominantly voltage- to predominantly lipid-sensing ion channels.

Discrete influx events refill depleted Ca2+ stores in a chick retinal neuron

The depletion of ER Ca2+ stores, following the release of Ca2+ during intracellular signalling, triggers the Ca2+ entry across the plasma membrane known as store-operated calcium entry (SOCE). We show here that brief, local [Ca2+]i increases (motes) in the thin dendrites of cultured retinal amacrine cells derived from chick embryos represent the Ca2+ entry events of SOCE and are initiated by sphingosine-1-phosphate (S1P), a sphingolipid with multiple cellular signalling roles. Externally applied S1P elicits motes but not through a G protein-coupled membrane receptor. The endogenous precursor to S1P, sphingosine, also elicits motes but its action is suppressed by dimethylsphingosine (DMS), an inhibitor of sphingosine phosphorylation. DMS also suppresses motes induced by store depletion and retards the refilling of depleted stores. These effects are reversed by exogenously applied S1P. In these neurons formation of S1P is a step in the SOCE pathway that promotes Ca2+ entry in the form of motes.

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.

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.

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.

Bipolar phospholipid sensing by TRPC5 calcium channel

TRPC5 [TRP (transient receptor potential) canonical (or classical) 5] is a widely expressed mammalian homologue of Drosophila TRP, forming a calcium- and sodium-permeable channel in the plasma membrane either as a homomultimer or heteromultimer with other proteins (e.g. TRPC1). Although several factors are known to stimulate the channel, understanding of its endogenous activators and functions is limited. This paper provides a brief and focused review of our latest findings that show that TRPC5 is a sensor of important signalling phospholipids, including lysophosphatidylcholine and sphingosine 1-phosphate, acting extracellularly or intracellularly. Underlying mechanisms of action and biological relevance are discussed.

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.

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.