Protein kinase Calpha phosphorylates the TRPC1 channel and regulates store-operated Ca2+ entry in endothelial cells

TRPC1 protein channel is major regulator of epidermal growth factor receptor signaling

TRP channels have been associated with cell proliferation and aggressiveness in several cancers. In particular, TRPC1 regulates cell proliferation and motility, two processes underlying cancer progression. We and others have described the mechanisms of TRPC1-dependent cell migration. However, the involvement of TRPC1 in cell proliferation remains unexplained. In this study, we show that siRNA-mediated TRPC1 depletion in non small cell lung carcinoma cell lines induced G(0)/G(1) cell cycle arrest resulting in dramatic decrease in cell growth. The expression of cyclins D1 and D3 was reduced after TRPC1 knockdown, pointing out the role of TRPC1 in G(1)/S transition. This was associated with a decreased phosphorylation and activation of EGFR and with a subsequent disruption of PI3K/Akt and MAPK downstream pathways. Stimulation of EGFR by its natural ligand, EGF, induced Ca(2+) release from the endoplasmic reticulum and Ca(2+) entry through TRPC1. Ca(2+) entry through TRPC1 conversely activated EGFR, suggesting that TRPC1 is a component of a Ca(2+)-dependent amplification of EGF-dependent cell proliferation.

TRP channels in normal and dystrophic skeletal muscle

TRP proteins constitute non-selective cation-permeable ion channels, most of which are permeable to Ca²⁺. In skeletal muscle, several isoforms of the TRPC (Canonical), TRPV (Vanilloid) and TRPM (Melastatin) subfamilies are expressed. In particular, TRPC1, C3 and C6, TRPV2 and V4, TRPM4 and TRPM7 have been consistently found in cultured myoblasts or in adult muscles. These channels seem to directly or indirectly respond to membrane stretch or to Ca²⁺ stores depletion; some isoforms might also constitute unregulated Ca²⁺ leak channels. Their function is largely unknown. TRPC1 and C3 have been involved in muscle development, in particular in myoblasts migration and differentiation. TRPC1 and V4 might allow a basal influx of Ca²⁺ at rest. Their lack has consequences on muscle fatigue. TRPV2 seems to be stretch-sensitive. It localizes mainly in intracellular pools at rest, and translocates to the plasma membrane upon IGF-1 stimulation. TRP channels seem to be involved in the pathophysiology of muscle disorders. In particular in Duchenne muscular dystrophy, the lack of the cytoskeletal protein dystrophin induces a disregulation of several ion channels leading to an abnormal influx of Ca²⁺. We discuss here, the possible involvement of TRP channels in this abnormal influx of Ca²⁺.

Integration of transient receptor potential canonical channels with lipids

Transient receptor potential canonical (TRPC) channels are the canonical (C) subset of the TRP proteins, which are widely expressed in mammalian cells. They are thought to be primarily involved in determining calcium and sodium entry and have wide-ranging functions that include regulation of cell proliferation, motility and contraction. The channels are modulated by a multiplicity of factors, putatively existing as integrators in the plasma membrane. This review considers the sensitivities of TRPC channels to lipids that include diacylglycerols, phosphatidylinositol bisphosphate, lysophospholipids, oxidized phospholipids, arachidonic acid and its metabolites, sphingosine-1-phosphate, cholesterol and some steroidal derivatives and other lipid factors such as gangliosides. Promiscuous and selective lipid sensing have been detected. There appear to be close working relationships with lipids of the phospholipase C and A₂ enzyme systems, which may enable integration with receptor signalling and membrane stretch. There are differences in the properties of each TRPC channel that are further complicated by TRPC heteromultimerization. The lipids modulate activity of the channels or insertion in the plasma membrane. Lipid microenvironments and intermediate sensing proteins have been described that include caveolae, G protein signalling, SEC14-like and spectrin-type domains 1 (SESTD1) and podocin. The data suggest that lipid sensing is an important aspect of TRPC channel biology enabling integration with other signalling systems.

Calcium and endothelium-mediated vasodilator signaling

Vascular tone refers to the balance between arterial constrictor and dilator activity. The mechanisms that underlie tone are critical for the control of haemodynamics and matching circulatory needs with metabolism, and thus alterations in tone are a primary factor for vascular disease etiology. The dynamic spatiotemporal control of intracellular Ca(2+) levels in arterial endothelial and smooth muscle cells facilitates the modulation of multiple vascular signaling pathways. Thus, control of Ca(2+) levels in these cells is integral for the maintenance of tone and blood flow, and intimately associated with both physiological and pathophysiological states. Hence, understanding the mechanisms that underlie the modulation of vascular Ca(2+) activity is critical for both fundamental knowledge of artery function, and for the development of targeted therapies. This brief review highlights the role of Ca(2+) signaling in vascular endothelial function, with a focus on contact-mediated vasodilator mechanisms associated with endothelium-derived hyperpolarization and the longitudinal conduction of responses over distance.

Transient receptor potential canonical channels in angiogenesis and axon guidance

Wiring of vascular and neural networks requires precise guidance of growing blood vessels and axons, respectively, to reach their targets during development. Both of the processes share common molecular signaling pathways. Transient receptor potential canonical (TRPC) channels are calcium-permeable cation channels and gated via receptor- or store-operated mechanisms. Recent studies have revealed the requirement of TRPC channels in mediating guidance cue-induced calcium influx and their essential roles in regulating axon navigation and angiogenesis. Dissecting TRPC functions in these physiological processes may provide therapeutic implications for suppressing pathological angiogenesis and improving nerve regeneration.

Baicalein, isolated from Scutellaria baicalensis, protects against endothelin-1-induced pulmonary artery smooth muscle cell proliferation via inhibition of TRPC1 channel expression

ETHNOPHARMACOLOGICAL RELEVANCE

We investigated the antiproliferative effects of baicalein, isolated from Scutellaria baicalensis (Huang-qin), on ET-1-mediated pulmonary artery smooth muscle cells (PASMCs) proliferation and the mechanisms underlying these effects.

MATERIALS AND METHODS

Intrapulmonary artery smooth muscle cells were isolated and cultured from female Sprague-Dawley rats and used during passages 3-6. The proliferation of PASMCs was quantified by cell counting and XTT assay. The protein expression of TRPC1 and PKCα were determined by western blotting. The cell cycle pattern was assayed by flow cytometry. The intracellular calcium concentrations ([Ca(2+)](i)) were measured using the fluorescent indicator fura-2-AM and flow cytometry.

RESULTS

Baicalein (0.3-3 μM) inhibited PASMCs proliferation, promoted cell cycle progression, enhanced [Ca(2+)](i) levels, increased capacitative Ca(2+) entry (CCE), upregulated the canonical transient receptor potential 1 (TRPC1) channel and membrane protein kinase Cα (PKCα) expression induced by ET-1 (0.1 μM). The PKC activator PMA (1 μM) reversed the inhibitory effects of baicalein on ET-1-induced upregulation of TRPC1 expression and S phase accumulation, while the PKC inhibitor chelerythrine (1 μM) potentiated baicalein-mediated G(2)/M phase arrest and TRPC1 channel inhibition.

CONCLUSION

Our findings suggest that baicalein protects against ET-1-induced PASMCs proliferation via modulation of the PKC-mediated TRPC channel.

TRPC1 proteins confer PKC and phosphoinositol activation on native heteromeric TRPC1/C5 channels in vascular smooth muscle: comparative study of wild-type and TRPC1-/- mice

Ca(2+)-permeable cation channels consisting of canonical transient receptor potential 1 (TRPC1) proteins mediate Ca(2+) influx pathways in vascular smooth muscle cells (VSMCs), which regulate physiological and pathological functions. We investigated properties conferred by TRPC1 proteins to native single TRPC channels in acutely isolated mesenteric artery VSMCs from wild-type (WT) and TRPC1-deficient (TRPC1(-/-)) mice using patch-clamp techniques. In WT VSMCs, the intracellular Ca(2+) store-depleting agents cyclopiazonic acid (CPA) and 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) both evoked channel currents, which had unitary conductances of ∼2 pS. In TRPC1(-/-) VSMCs, CPA-induced channel currents had 3 subconductance states of 14, 32, and 53 pS. Passive depletion of intracellular Ca(2+) stores activated whole-cell cation currents in WT but not TRPC1(-/-) VSMCs. Differential blocking actions of anti-TRPC antibodies and coimmunoprecipitation studies revealed that CPA induced heteromeric TRPC1/C5 channels in WT VSMCs and TRPC5 channels in TRPC1(-/-) VSMCs. CPA-evoked TRPC1/C5 channel activity was prevented by the protein kinase C (PKC) inhibitor chelerythrine. In addition, the PKC activator phorbol 12,13-dibutyrate (PDBu), a PKC catalytic subunit, and phosphatidylinositol-4,5-bisphosphate (PIP(2)) and phosphatidylinositol-3,4,5-trisphosphate (PIP(3)) activated TRPC1/C5 channel activity, which was prevented by chelerythrine. In contrast, CPA-evoked TRPC5 channel activity was potentiated by chelerythrine, and inhibited by PDBu, PIP(2), and PIP(3). TRPC5 channels in TRPC1(-/-) VSMCs were activated by increasing intracellular Ca(2+) concentrations ([Ca(2+)](i)), whereas increasing [Ca(2+)](i) had no effect in WT VSMCs. We conclude that agents that deplete intracellular Ca(2+) stores activate native heteromeric TRPC1/C5 channels in VSMCs, and that TRPC1 subunits are important in determining unitary conductance and conferring channel activation by PKC, PIP(2), and PIP(3).

Heteromeric TRPV4-C1 channels contribute to store-operated Ca(2+) entry in vascular endothelial cells

There is controversy as to whether TRP channels participate in mediating store-operated current (I(SOC)) and store-operated Ca(2+) entry (SOCE). Our recent study has demonstrated that TRPC1 forms heteromeric channels with TRPV4 in vascular endothelial cells and that Ca(2+) store depletion enhances the vesicle trafficking of heteromeric TRPV4-C1 channels, causing insertion of more channels into the plasma membrane in vascular endothelial cells. In the present study, we determined whether the enhanced TRPV4-C1 insertion to the plasma membrane could contribute to SOCE and I(SOC). We found that thapsigargin-induced SOCE was much lower in aortic endothelial cells derived from trpv4(-/-) or trpc1(-/-) knockout mice when compared to that of wild-type mice. In human umbilical vein endothelial cells (HUVECs), thapsigargin-induced SOCE was markedly reduced by knocking down the expression of TRPC1 and/or TRPV4 with respective siRNAs. Brefeldin A, a blocker of vesicular translocation, inhibited the SOCE. These results suggest that an enhanced vesicular trafficking of heteromeric TRPV4-C1 channels contributes to SOCE in vascular endothelial cells. Vascular tension studies suggest that such an enhanced trafficking of TRPV4-C1 channels may play a role in thapsigargin-induced vascular relaxation in rat small mesenteric arteries.

TRPC channels are involved in calcium-dependent migration and proliferation in immortalized GnRH neurons

Gonadotropin-releasing hormone (GnRH)-secreting neurons are key regulators of the reproductive behaviour in vertebrates. These neurons show a peculiar migratory pattern during embryonic development, and its perturbations have profound impact on fertility and other related functional aspects. Changes in the intracellular calcium concentration, [Ca(2+)](i), induced by different extracellular signals, play a central role in the control of neuronal migration, but the available knowledge regarding GnRH neurons is still limited. Our goal was to investigate mechanisms that may be involved in the Ca(2+) dependence of the migratory behaviour in these neurons. We focused on the "classical" Transient Receptor Potential (TRPC) subfamily of Ca(2+)-permeable cation channels, recently shown to be involved in other aspects of neuronal development. Using GN11 cells, immortalized early stage GnRH neurons, we set to investigate Ca(2+) signals under basal conditions and in the presence of a well-established motogen, fetal calf serum (FCS), and the effect of pharmacological TRPC agonists and antagonists on Ca(2+) oscillations, cell motility and proliferation. We have found that a subpopulation of GN11 cells shows spontaneous Ca(2+) transients and that this activity is increased in the presence of serum. Quantitative real-time PCR showed that transcripts of some TRPC members are expressed in GN11 cells. Interestingly, pharmacological experiments with inhibitors, SKF-96365, lanthanum, anti-TRPC1 antibody, and activators, 1-oleil 2-acetyl-sn-glycerol, of TRPCs suggested that the activation of these channels can account for both the basal Ca(2+) oscillations and the increased activity in the presence of FCS. Moreover, functional studies using the same pharmacological tools supported their involvement in the control of motility and proliferation. Thus, our data provide evidence for the involvement of Ca(2+)-permeable channels of the TRPC subfamily in the control of functional properties of neurosecretory cells and neuronal motility.

Amplification of EDHF-type vasodilatations in TRPC1-deficient mice

BACKGROUND AND PURPOSE

TRPC1 channels are expressed in the vasculature and are putative candidates for intracellular Ca(2+) handling. However, little is known about their role in endothelium-dependent vasodilatations including endothelium-derived hyperpolarizing factor (EDHF) vasodilatations, which require activation of Ca(2+) -activated K(+) channels (K(Ca)). To provide molecular information on the role of TRPC1 for K(Ca) function and the EDHF signalling complex, we examined endothelium-dependent and independent vasodilatations, K(Ca) currents and smooth muscle contractility in TRPC1-deficient mice (TRPC1-/-).

EXPERIMENTAL APPROACH

Vascular responses were studied using pressure/wire myography and intravital microscopy. We performed electrophysiological measurements, and confocal Ca(2+) imaging for studying K(Ca) channel functions and Ca(2+) sparks.

KEY RESULTS

TRPC1 deficiency in carotid arteries produced a twofold augmentation of TRAM-34- and UCL1684-sensitive EDHF-type vasodilatations and of endothelial hyperpolarization to acetylcholine. NO-mediated vasodilatations were unchanged. TRPC1-/- exhibited enhanced EDHF-type vasodilatations in resistance-sized arterioles in vivo associated with reduced spontaneous tone. Endothelial IK(Ca) /SK(Ca)-type K(Ca) currents, smooth muscle cell Ca(2+) sparks and associated BK(Ca)-mediated spontaneous transient outward currents were unchanged in TRPC1-/-. Smooth muscle contractility induced by receptor-operated Ca(2+) influx or Ca(2+) release and endothelium-independent vasodilatations were unaltered in TRPC1-/-. TRPC1-/- exhibited lower systolic blood pressure as determined by tail-cuff blood pressure measurements.

CONCLUSIONS AND IMPLICATIONS

Our data demonstrate that TRPC1 acts as a negative regulator of endothelial K(Ca) channel-dependent EDHF-type vasodilatations and thereby contributes to blood pressure regulation. Thus, we propose a specific role of TRPC1 in the EDHF-K(Ca) signalling complex and suggest that pharmacological inhibition of TRPC1, by enhancing EDHF vasodilatations, may be a novel strategy for lowering blood pressure.

Ca2+ paradox injury mediated through TRPC channels in mouse ventricular myocytes

BACKGROUND AND PURPOSE The Ca(2+) paradox is an important phenomenon associated with Ca(2+) overload-mediated cellular injury in myocardium. The present study was undertaken to elucidate molecular and cellular mechanisms for the development of the Ca(2+) paradox. EXPERIMENTAL APPROACH Fluorescence imaging was performed on fluo-3 loaded quiescent mouse ventricular myocytes using confocal laser scanning microscope. KEY RESULTS The Ca(2+) paradox was readily evoked by restoration of the extracellular Ca(2+) following 10-20 min of nominally Ca(2+)-free superfusion. The Ca(2+) paradox was significantly reduced by blockers of transient receptor potential canonical (TRPC) channels (2-aminoethoxydiphenyl borate, Gd(3+), La(3+)) and anti-TRPC1 antibody. The sarcoplasmic reticulum (SR) Ca(2+) content, assessed by caffeine application, gradually declined during Ca(2+)-free superfusion, which was further accelerated by metabolic inhibition. Block of SR Ca(2+) leak by tetracaine prevented Ca(2+) paradox. The Na(+) /Ca(2+) exchange (NCX) blocker KB-R7943 significantly inhibited Ca(2+) paradox when applied throughout superfusion period, but had little effect when added for a period of 3 min before and during Ca(2+) restoration. The SR Ca(2+) content was better preserved during Ca(2+) depletion by KB-R7943. Immunocytochemistry confirmed the expression of TRPC1, in addition to TRPC3 and TRPC4, in mouse ventricular myocytes. CONCLUSIONS AND IMPLICATIONS These results provide evidence that (i) the Ca(2+) paradox is primarily mediated by Ca(2+) entry through TRPC (probably TRPC1) channels that are presumably activated by SR Ca(2+) depletion; and (ii) reverse mode NCX contributes little to the Ca(2+) paradox, whereas inhibition of NCX during Ca(2+) depletion improves SR Ca(2+) loading, and is associated with reduced incidence of Ca(2+) paradox in mouse ventricular myocytes.

Thapsigargin activates Ca²+ entry both by store-dependent, STIM1/Orai1-mediated, and store-independent, TRPC3/PLC/PKC-mediated pathways in human endothelial cells

The ER Ca²+ sensor STIM1 and the Ca²+ channel Orai1 are key players in store-operated Ca²+ entry (SOCE). In addition, channels from the TRPC family were also shown to be engaged during SOCE, while their precise implication remains controversial. In this study, we investigated the molecular players involved in SOCE triggered by the SERCA pump inhibitor thapsigargin in an endothelial cell line, the EA.hy926. siRNA directed against STIM1 or Orai1 reduced Ca²+ entry by about 50-60%, showing that a large part of the entry is independent from these proteins. Blocking the PLC or the PKC pathway completely abolished thapsigargin-induced Ca²+ entry in cells depleted from STIM1 and/or Orai1. The phorbol ester PMA or the DAG analog OAG restored the Ca²+ entry inhibited by PLC blockers, showing an involvement of PLC/PKC pathway in SOCE. Using pharmacological inhibitors or siRNA revealed that the PKCeta is required for Ca²+ entry, and pharmacological inhibition of the tyrosine kinase Src also reduced Ca²+ entry. TRPC3 silencing diminished the entry by 45%, while the double STIM1/TRPC3 invalidation reduced Ca²+ entry by more than 85%. Hence, in EA.hy926 cells, TG-induced Ca²+ entry results from the activation of the STIM1/Orai1 machinery, and from the activation of TRPC3.

Chemotaxis of MDCK-F cells toward fibroblast growth factor-2 depends on transient receptor potential canonical channel 1

Movement toward the source of a chemoattractant gradient is a basic cellular property in health and disease. Enhanced migration during metastasis involves deregulated growth factor signaling. Growth factor stimulation and cell migration converge both on the important second messenger Ca(2+). To date, the molecular identification of Ca(2+) entry pathways activated by growth factors during chemotaxis is still an open issue. We investigated the involvement of the nonselective Ca(2+) channel TRPC1 (transient receptor potential canonical 1) in FGF-2 guided chemotaxis by means of time-lapse video microscopy and by functional Ca(2+) measurements. To specifically address TRPC1 function in transformed MDCK cells we altered the expression levels by siRNA or overexpression. We report that TRPC1 channels are required for the orientation of transformed MDCK cells in FGF-2 gradients because TRPC1 knockdown or pharmacological blockade prevented chemotaxis. Stimulation with FGF-2 triggered an immediate Ca(2+) influx via TRPC1 channels that depended on phospholipase C and phosphatidylinositol 3-kinase signaling. Impeding this Ca(2+) influx abolished chemotaxis toward FGF-2. This functional connection correlated with clustering of FGF receptors and TRPC1 channels as was observed by immunolabeling. These findings show the important interplay between growth factor signaling and Ca(2+) influx in chemotaxis.

Protein kinase D-mediated phosphorylation of polycystin-2 (TRPP2) is essential for its effects on cell growth and calcium channel activity

PKD2 is mutated in 15% of patients with autosomal dominant polycystic kidney disease. The PKD2 protein, polycystin-2 or TRPP2, is a nonselective Ca2+-permeable cation channel that has been shown to function at several locations, including primary cilia, basolateral membrane, and at the endoplasmic reticulum (ER). Nevertheless, the factors that regulate the channel activity of polycystin-2 are not well understood. Polycystin-2 has been shown previously to be regulated by phosphorylation at two serine residues (Ser812 and Ser76) with distinct functional consequences. Here, we report the identification of a previously unrecognized phosphorylation site within the polycystin-2 C terminus (Ser801), and we demonstrate that it is phosphorylated by protein kinase D. Phosphorylation at this site was significantly increased in response to serum and epidermal growth factor stimulation. In nonciliated Madin-Darby canine kidney I cells, inducible expression of polycystin-2 inhibited cell proliferation compared with wild-type cells. Mutagenesis at Ser801 abolished these effects and reduced ATP-stimulated Ca2+ release from ER stores. Finally, we show that a pathogenic mutation (S804N) within the consensus kinase recognition sequence abolished Ser801 phosphorylation. These results suggest that growth factor-stimulated, protein kinase D-mediated phosphorylation of polycystin-2 is essential for its ER channel function and links extracellular stimuli to its effects on cell growth and intracellular calcium regulation.

Molecular mechanisms underlying Ca2+-mediated motility of human pancreatic duct cells

We recently reported that transforming growth factor-β (TGF-β) induces an increase in cytosolic Ca(2+) ([Ca(2+)](cyt)) in pancreatic cancer cells, but the mechanisms by which TGF-β mediates [Ca(2+)](cyt) homeostasis in these cells are currently unknown. Transient receptor potential (TRP) channels and Na(+)/Ca(2+) exchangers (NCX) are plasma membrane proteins that play prominent roles in controlling [Ca(2+)](cyt) homeostasis in normal mammalian cells, but little is known regarding their roles in the regulation of [Ca(2+)](cyt) in pancreatic cancer cells and pancreatic cancer development. Expression and function of NCX1 and TRPC1 proteins were characterized in BxPc3 pancreatic cancer cells. TGF-β induced both intracellular Ca(2+) release and extracellular Ca(2+) entry in these cells; however, 2-aminoethoxydiphenyl borate [2-APB; a blocker for both inositol 1,4,5-trisphosphate (IP(3)) receptor and TRPC], LaCl(3) (a selective TRPC blocker), or KB-R7943 (a selective inhibitor for the Ca(2+) entry mode of NCX) markedly inhibited the TGF-β-induced increase in [Ca(2+)](cyt). 2-APB or KB-R7943 treatment was able to dose-dependently reverse membrane translocation of PKCα induced by TGF-β. Transfection with small interfering RNA (siRNA) against NCX1 almost completely abolished NCX1 expression in BxPc3 cells and also inhibited PKCα serine phosphorylation induced by TGF-β. Knockdown of NCX1 or TRPC1 by specific siRNA transfection reversed TGF-β-induced pancreatic cancer cell motility. Therefore, TGF-β induces Ca(2+) entry likely via TRPC1 and NCX1 and raises [Ca(2+)](cyt) in pancreatic cancer cells, which is essential for PKCα activation and subsequent tumor cell invasion. Our data suggest that TRPC1 and NCX1 may be among the potential therapeutic targets for pancreatic cancer.

TRP-ing up heart and vessels: canonical transient receptor potential channels and cardiovascular disease

Transient receptor potential channels are a large superfamily of non-selective and non-voltage-gated ion channels that convey signaling information linked to a broad range of sensory inputs. In the cardiovascular system, the canonical transient receptor potential (TRPC) family has been particularly found to play a role in vascular and cardiac disease, responding to neurohormonal and mechanical load stimulation. TRPC1, TRPC3, and TRPC6 are often upregulated in models of cardiovascular disease, and their inhibition ameliorates the associated pathophysiology. Studies in gene deletion models and overexpression models of wild-type and dominant-negative proteins supports a direct role of these channels, which likely act together as heterotetramers to influence signaling. Recent evidence has further revealed the importance of protein kinase G phosphorylation as a mechanism to suppress TRPC6 channel current and dependent signaling in vascular and cardiac myocytes. This suggests a novel mechanism underlying benefits of drugs such as sildenafil, a phosphodiesterase type 5 inhibitor, nitrates, and atrial natriuretic peptides. This review describes new evidence supporting a pathophysiologic role of these three TRPC channels, and the potential utility of inhibition strategies to treat cardiovascular disease.

Altered protein kinase C regulation of pulmonary endothelial store- and receptor-operated Ca2+ entry after chronic hypoxia

Chronic hypoxia (CH)-induced pulmonary hypertension is associated with decreased basal pulmonary artery endothelial cell (EC) Ca(2+), which correlates with reduced store-operated Ca(2+) (SOC) entry. Protein kinase C (PKC) attenuates SOC entry in ECs. Therefore, we hypothesized that PKC has a greater inhibitory effect on EC SOC and receptor-operated Ca(2+) entry after CH. To test this hypothesis, we assessed SOC in the presence or absence of the nonselective PKC inhibitor GF109203X [2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)maleimide] in freshly isolated, Fura-2-loaded ECs obtained from intrapulmonary arteries of control and CH rats (4 weeks at 0.5 atm). We found that SOC entry and 1-oleoyl-2-acetyl-sn-glycerol (OAG)- and ATP-induced Ca(2+) influx were attenuated in ECs from CH rats versus controls, and GF109203X restored SOC and OAG responses to the level of controls. In contrast, nonselective PKC inhibition with GF109203X or the selective PKC(epsilon) inhibitor myristoylated V1-2 attenuated ATP-induced Ca(2+) entry in ECs from control but not CH pulmonary arteries. ATP-induced Ca(2+) entry was also attenuated by the T-type voltage-gated Ca(2+) channel (VGCC) inhibitor mibefradil in control cells. Consistent with the presence of endothelial T-type VGCC, we observed depolarization-induced Ca(2+) influx in control cells that was inhibited by mibefradil. This response was largely absent in ECs from CH arteries. We conclude that CH enhances PKC-dependent inhibition of SOC- and OAG-induced Ca(2+) entry. Furthermore, these data suggest that CH may reduce the ATP-dependent Ca(2+) entry that is mediated, in part, by PKCepsilon and mibefradil-sensitive Ca(2+) channels in control cells.

Nocistatin excites rostral agranular insular cortex-periaqueductal gray projection neurons by enhancing transient receptor potential cation conductance via G(alphaq/11)-PLC-protein kinase C pathway

Rostral agranular insular cortex (RAIC) projects to periaqueductal gray (PAG) and inhibits spinal nociceptive transmission by activating PAG-rostral ventromedial medulla (RVM) descending antinociceptive circuitry. Despite being generated from the same precursor prepronociceptin, nocistatin (NST) and nociceptin/orphanin FQ (N/OFQ) produce supraspinal analgesic and hyperalgesic effects, respectively. Prepronociceptin is highly expressed in the RAIC. In the present study, we hypothesized that NST and N/OFQ modulate spinal pain transmission by regulating the activity of RAIC neurons projecting to ventrolateral PAG (RAIC-PAG). This hypothesis was tested by investigating electrophysiological effects of N/OFQ and NST on RAIC-PAG projection neurons in brain slice. Retrogradely labeled RAIC-PAG projection neurons are layer V pyramidal cells and express mRNA of vesicular glutamate transporter subtype 1, a marker for glutamatergic neurons. N/OFQ hyperpolarized 25% of RAIC-PAG pyramidal neurons by enhancing inwardly rectifying potassium conductance via pertussis toxin-sensitive G(alphai/o). In contrast, NST depolarized 33% of RAIC-PAG glutamatergic neurons by causing the opening of canonical transient receptor potential (TRPC) cation channels through G(alphaq/11)-phospholipase C-protein kinase C pathway. There were two separate populations of RAIC-PAG pyramidal neurons, one responding to NST and the other one to N/OFQ. Our results suggest that G(alphaq/11)-coupled NST receptor mediates NST excitation of RAIC-PAG glutamatergic neurons, which is expected to cause the supraspinal analgesia by enhancing the activity of RAIC-PAG-RVM antinociceptive pathway. Opposite effects of NST and N/OFQ on supraspinal pain regulation are likely to result from their opposing effects on RAIC-PAG pyramidal neurons.

Regulation of endothelial permeability via paracellular and transcellular transport pathways

The endothelium functions as a semipermeable barrier regulating tissue fluid homeostasis and transmigration of leukocytes and providing essential nutrients across the vessel wall. Transport of plasma proteins and solutes across the endothelium involves two different routes: one transcellular, via caveolae-mediated vesicular transport, and the other paracellular, through interendothelial junctions. The permeability of the endothelial barrier is an exquisitely regulated process in the resting state and in response to extracellular stimuli and mediators. The focus of this review is to provide a comprehensive overview of molecular and signaling mechanisms regulating endothelial barrier permeability with emphasis on the cross-talk between paracellular and transcellular transport pathways.

Transient receptor potential channels and vascular function

TRP (transient receptor potential) channels play important roles in the regulation of normal and pathological cellular function. In the vasculature, TRP channels are present both in ECs (endothelial cells) and vascular SMCs (smooth muscle cells) and contribute to vasomotor control mechanisms in most vascular beds. Vascular TRP channels are activated by various stimuli, such as mechanical perturbation, receptor activation and dietary molecules. Some of the specific roles of these channels in normal and impaired vascular function have emerged in recent years and include participation in vascular signalling processes, such as neurotransmission, hormonal signalling, NO production, myogenic tone and autoregulation of blood flow, thermoregulation, responses to oxidative stress and cellular proliferative activity. Current research is aimed at understanding the interactions of TRP channels with other vascular proteins and signalling mechanisms. These studies should reveal new targets for pharmacological therapy of vascular diseases, such as hypertension, ischaemia and vasospasm, and vascular proliferative states.

ROS-dependent signaling mechanisms for hypoxic Ca(2+) responses in pulmonary artery myocytes

Hypoxic exposure causes pulmonary vasoconstriction, which serves as a critical physiologic process that ensures regional alveolar ventilation and pulmonary perfusion in the lungs, but may become an essential pathologic factor leading to pulmonary hypertension. Although the molecular mechanisms underlying hypoxic pulmonary vasoconstriction and associated pulmonary hypertension are uncertain, increasing evidence indicates that hypoxia can result in a significant increase in intracellular reactive oxygen species concentration ([ROS](i)) through the mitochondrial electron-transport chain in pulmonary artery smooth muscle cells (PASMCs). The increased mitochondrial ROS subsequently activate protein kinase C-epsilon (PKCepsilon) and NADPH oxidase (Nox), providing positive mechanisms that further increase [ROS](i). ROS may directly cause extracellular Ca(2+) influx by inhibiting voltage-dependent K(+) (K(V)) channels and opening of store-operated Ca(2+) (SOC) channels, as well as intracellular Ca(2+) release by activating ryanodine receptors (RyRs), leading to an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) and associated contraction. In concert with ROS, PKCepsilon may also affect K(V) channels, SOC channels, and RyRs, contributing to hypoxic Ca(2+) and contractile responses in PASMCs.

CRAC channelopathies

Store-operated Ca2+ entry (SOCE) is an important Ca2+ influx pathway in many non-excitable and some excitable cells. It is regulated by the filling state of intracellular Ca2+ stores, notably the endoplasmic reticulum (ER). Reduction in [Ca2+]ER results in activation of plasma membrane Ca2+ channels that mediate sustained Ca2+ influx which is required for many cell functions as well as refilling of Ca2+ stores. The Ca2+ release activated Ca2+ (CRAC) channel is the best characterized SOC channel with well-defined electrophysiological properties. In recent years, the molecular components of the CRAC channel, long mysterious, have been defined. ORAI1 (or CRACM1) acts as the pore-forming subunit of the CRAC channel in the plasma membrane. Stromal interaction molecule (STIM) 1 is localized in the ER, senses [Ca2+]ER, and activates the CRAC channel upon store depletion by binding to ORAI1. Both proteins are widely expressed in many tissues in both human and mouse consistent with the widespread prevalence of SOCE and CRAC channel currents in many cells types. CRAC channelopathies in human patients with mutations in STIM1 and ORAI1 are characterized by abolished CRAC channel currents, lack of SOCE and-clinically-immunodeficiency, congenital myopathy, and anhydrotic ectodermal dysplasia. This article reviews the role of ORAI and STIM proteins for SOCE and CRAC channel function in a variety of cell types and tissues and compares the phenotypes of ORAI1 and STIM1-deficient human patients and mice with targeted deletion of Orai and Stim genes.

Involvement of STIM1 in the proteinase-activated receptor 1-mediated Ca2+ influx in vascular endothelial cells

Thrombin increases the cytosolic Ca(2+) concentrations and induces NO production by activating proteinase-activated receptor 1 (PAR(1)) in vascular endothelial cells. The store-operated Ca(2+) influx is a major Ca(2+) influx pathway in non-excitable cells including endothelial cells and it has been reported to play a role in the thrombin-induced Ca(2+) signaling in endothelial cells. Recent studies have identified stromal interaction molecule 1 (STIM1) to function as a sensor of the store site Ca(2+) content, thereby regulating the store-operated Ca(2+) influx. However, the functional role of STIM1 in the thrombin-induced Ca(2+) influx and NO production in endothelial cells still remains to be elucidated. Fura-2 and diaminorhodamine-4M fluorometry was utilized to evaluate the thrombin-induced changes in cytosolic Ca(2+) concentrations and NO production, respectively, in porcine aortic endothelial cells transfected with small interfering RNA (siRNA) targeted to STIM1. STIM1-targeted siRNA suppressed the STIM1 expression and the thapsigargin-induced Ca(2+) influx. The degree of suppression of the STIM1 expression correlated well to the degree of suppression of the Ca(2+) influx. The knockdown of STIM1 was associated with a substantial inhibition of the Ca(2+) influx and a partial reduction of the NO production induced by thrombin. The thrombin-induced Ca(2+) influx exhibited the similar sensitivity toward the Ca(2+) influx inhibitors to that seen with the thapsigargin-induced Ca(2+) influx. The present study provides the first evidence that STIM1 plays a critical role in the PAR(1)-mediated Ca(2+) influx and Ca(2+)-dependent component of the NO production in endothelial cells.

Protein kinase Calpha: disease regulator and therapeutic target

Protein kinase Cα (PKCα) is a member of the AGC (which includes PKD, PKG and PKC) family of serine/threonine protein kinases that is widely expressed in mammalian tissues. It is closely related in structure, function and regulation to other members of the protein kinase C family, but has specific functions within the tissues in which it is expressed. There is substantial recent evidence, from gene knockout studies in particular, that PKCα activity regulates cardiac contractility, atherogenesis, cancer and arterial thrombosis. Selective targeting of PKCα therefore has potential therapeutic value in a wide variety of disease states, although will be technically complicated by the ubiquitous expression and multiple functions of the molecule.

Glucose enhances expression of TRPC1 and calcium entry in endothelial cells

Hyperglycemia is a major risk factor for endothelial dysfunction and vascular disease, and in the current study, the link to glucose-induced abnormal intracellular Ca(2+) (Ca(i)(2+)) homeostasis was explored in bovine aortic endothelial cells in high glucose (HG; 25 mmol/l) versus low glucose (LG; 5.5 mmol/l; control). Transient receptor potential 1 (TRPC1) ion channel protein, but not TRPC3, TRPC4, or TRPC6 expression, was significantly increased in HG versus LG at 72 h. HG for 4, 24, and 72 h did not change basal Ca(i)(2+) or ATP-induced Ca(i)(2+) release; however, the amplitude of sustained Ca(i)(2+) was significantly increased at 24 and 72 h and reduced by low concentration of the putative, but nonspecific, TRPC blockers, gadolinium, SKF-96365, and 2-aminoethoxydiphenyl borate. Treatment with TRPC1 antisense significantly reduced TRPC1 protein expression and ATP-induced Ca(2+) entry in bovine aortic endothelial cells. Although the link between HG-induced changes in TRPC1 expression, enhanced Ca(2+) entry, and endothelial dysfunction require further study, the current data are suggestive that targeting these pathways may reduce the impact of HG on endothelial function.

Activation of native TRPC1/C5/C6 channels by endothelin-1 is mediated by both PIP3 and PIP2 in rabbit coronary artery myocytes

We investigate activation mechanisms of native TRPC1/C5/C6 channels (termed TRPC1 channels) by stimulation of endothelin-1 (ET-1) receptor subtypes in freshly dispersed rabbit coronary artery myocytes using single channel recording and immunoprecipitation techniques. ET-1 evoked non-selective cation channel currents with a unitary conductance of 2.6 pS which were not inhibited by either ET(A) or ET(B) receptor antagonists, respectively BQ-123 and BQ788, when administered separately. However, in the presence of both antagonists, ET-1-evoked channel activity was abolished indicating that both ET(A) and ET(B) receptor stimulation activate this conductance. Stimulation of both ET(A) and ET(B) receptors evoked channel activity which was inhibited by the protein kinase C (PKC) inhibitor chelerythrine and by anti-TRPC1 antibodies indicating that activation of both receptor subtypes causes TRPC1 channel activation by a PKC-dependent mechanism. ET(A) receptor-mediated TRPC1 channel activity was selectively inhibited by phosphoinositol-3-kinase (PI-3-kinase) inhibitors wortmannin (50 nM) and PI-828 and by antibodies raised against phosphoinositol-3,4,5-trisphosphate (PIP(3)), the product of PI-3-kinase-mediated phosphorylation of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Moreover, exogenous application of diC8-PIP(3) stimulated PKC-dependent TRPC1 channel activity. These results indicate that stimulation of ET(A) receptors evokes PKC-dependent TRPC1 channel activity through activation of PI-3-kinase and generation of PIP(3). In contrast, ET(B) receptor-mediated TRPC1 channel activity was inhibited by the PI-phospholipase C (PI-PLC) inhibitor U73122. 1-Oleoyl-2-acetyl-sn-glycerol (OAG), an analogue of diacylglycerol (DAG), which is a product of PI-PLC, also activated PKC-dependent TRPC1 channel activity. OAG-induced TRPC1 channel activity was inhibited by anti-phosphoinositol-4,5-bisphosphate (PIP(2)) antibodies and high concentrations of wortmannin (20 microM) which depleted tissue PIP(2) levels. In addition exogenous application of diC8-PIP(2) activated PKC-dependent TRPC1 channel activity. These data indicate that stimulation of ET(B) receptors evokes PKC-dependent TRPC1 activity through PI-PLC-mediated generation of DAG and requires a permissive role of PIP(2). In conclusion, we provide the first evidence that stimulation of ET(A) and ET(B) receptors activate native PKC-dependent TRPC1 channels through two distinct phospholipids pathways involving a novel action of PIP(3), in addition to PIP(2), in rabbit coronary artery myocytes.

Role of TRPC1 and NF-kappaB in mediating angiotensin II-induced Ca2+ entry and endothelial hyperpermeability

Endothelial dysfunction is associated with cardiovascular diseases. The Ca(2+) influx occurring via activation of plasmalemma Ca(2+) channels was shown to be critical in signaling the increase in endothelial permeability in response to a variety of permeability-increasing mediators. It has been reported that angiotensin II (AngII) could induce Ca(2+) signaling in some cells, and transient receptor potential canonical 1 (TRPC1) had an important role in this process. The objective of this study was to examine the mechanism of AngII-induced Ca(2+) entry and vascular endothelial hyperpermeability. Human umbilical vein endothelial cells (HUVECs) exposed to AngII exhibited dose-dependent increase in [Ca(2+)]i and endothelial permeability. Quantitative real-time RT-PCR and Western blotting showed that the level of TRPC1 expression had increased significantly at 12h and at 24h after treatment of HUEVCs with AngII. The expression of p65 was suppressed using an RNAi strategy. The results showed that the NF-kappaB signaling pathway and type-1 receptor of AngII was involved in AngII-induced TRPC1 upregulation. Moreover, knockdown of TRPC1 and NF-kappaB expression attenuates AngII-induced [Ca(2+)]i and endothelial permeability. NF-kappaB and TRPC1 have critical roles in AngII-induced Ca(2+) entry and endothelial permeability.

Intracellular Ca2+ stores modulate SOCCs and NMDA receptors via tyrosine kinases in rat hippocampal neurons

The regulation of intracellular Ca(2+) signalling by phosphorylation processes remains poorly defined, particularly with regards to tyrosine phosphorylation. Evidence from non-excitable cells implicates tyrosine phosphorylation in the activation of so-called store-operated Ca(2+) channels (SOCCs), but their involvement in neuronal Ca(2+) signalling is still elusive. In the present study, we determined the role of protein tyrosine kinases (PTKs) and tyrosine phosphatases (PTPs) in the coupling between intracellular Ca(2+) stores and SOCCs in neonatal rat hippocampal neurons by Fura-2 Ca(2+) imaging. An early Ca(2+) response from intracellular stores was triggered with thapsigargin, and followed by a secondary plasma membrane Ca(2+) response. This phase was blocked by the non-specific Ca(2+) channel blocker NiCl and the SOCC blocker, 2-aminoethoxydiphenyl borate (2-APB). Interestingly, two structurally distinct PTK inhibitors, genistein and AG126, also inhibited this secondary response. Application of the PTP inhibitor sodium orthovanadate (OV) also activated a sustained and tyrosine kinase dependent Ca(2+) response, blocked by NiCl and 2-APB. In addition, OV resulted in a Ca(2+) store dependent enhancement of NMDA responses, corresponding to, and occluding the signalling pathway for group I metabotropic glutamate receptors (mGluRs). This study provides first evidence for tyrosine based phospho-regulation of SOCCs and NMDA signalling in neurons.

TRPC1 and STIM1 mediate capacitative Ca2+ entry in mouse pulmonary arterial smooth muscle cells

Previous studies in pulmonary arterial smooth muscle cells (PASMCs) showed that the TRPC1 channel mediates capacitative Ca(2+) entry (CCE), but the molecular signal(s) that activate TRPC1 in PASMCs remains unknown. The aim of the present study was to determine if TRPC1 mediates CCE through activation of STIM1 protein in mouse PASMCs. In primary cultured mouse PASMCs loaded with fura-2, cyclopiazonic acid (CPA) caused a transient followed by a sustained rise in intracellular Ca(2+) concentration ([Ca(2+)](i)). The transient but not the sustained rise in [Ca(2+)](i) was partially inhibited by nifedipine. In addition, CPA increased the rate of Mn(2+) quench of fura-2 fluorescence that was inhibited by SKF 96365, Ni(2+), La(3+) and Gd(3+), exhibiting pharmacological properties characteristic of CCE. The nifedipine-insensitive sustained rise in [Ca(2+)](i) and the increase in Mn(2+) quench of fura-2 fluorescence caused by CPA were both inhibited in cells pretreated with antibody raised against an extracellular epitope of TRPC1. Moreover, STIM1 siRNA reduced the rise in [Ca(2+)](i) and Mn(2+) quench of fura-2 fluorescence caused by CPA, whereas overexpression of STIM1 resulted in a marked increase in these responses. RT-PCR revealed TRPC1 and STIM1 mRNAs, and Western blot analysis identified TRPC1 and STIM1 proteins in mouse PASMCs. Furthermore, TRPC1 was found to co-immunoprecipitate with STIM1, and the precipitation level of TRPC1 was increased in cells subjected to store depletion. Taken together, store depletion causes activation of voltage-operated Ca(2+) entry and CCE. These data provide direct evidence that CCE is mediated by TRPC1 channel through activation of STIM1 in mouse PASMCs.

TRPC channel lipid specificity and mechanisms of lipid regulation

TRPC channels are a subset of the transient receptor potential (TRP) proteins widely expressed in mammalian cells. They are thought to be primarily involved in determining calcium or sodium entry and have broad-ranging functions that include regulation of cell proliferation, motility and contraction. The channels do not respond to a single stimulator but rather are activated or modulated by a multiplicity of factors, potentially existing as integrators at the plasma membrane. This review considers the sensitivity of TRPCs to lipid factors, with focus on sensitivities to diacylglycerols, lysophospholipids, arachidonic acid and its metabolites, sphingosine-1-phosphate (S1P), cholesterol and derivatives, and other lipid factors such as gangliosides. Promiscuous and selective lipid-sensing are apparent. In many cases the lipids stimulate channel function or increase insertion of channels in the membrane. Both direct and indirect (receptor-dependent) lipid effects are evident. Although information is limited, the lipid profiles are consistent with TRPCs having close working relationships with phospholipase C and A2 enzymes. We need much more information about lipid-sensing by TRPCs if we are to fully appreciate its significance, but the available data suggest that lipid-sensing is a key, but not exclusive, aspect of TRPC biology.

Role of phosphoinositol 4,5-bisphosphate and diacylglycerol in regulating native TRPC channel proteins in vascular smooth muscle

Stimulation of receptor-operated (ROCs) and store-operated (SOCs) Ca(2+)-permeable cation channels by vasoconstrictors has many important physiological functions in vascular smooth muscle. The present review indicates that ROCs and SOCs with diverse properties in different blood vessels are likely to be explained by composition of different subunits from the canonical transient receptor potential (TRPC) family of cation channel proteins. In addition we illustrate that activation of native TRPC ROCs and SOCs involves different phospholipase-mediated transduction pathways linked to generation of diacylglycerol (DAG). Moreover we describe recent novel data showing that the endogenous phospholipid phosphoinositol 4,5-bisphosphate (PIP(2)) has profound and contrasting actions on TRPC ROCs and SOCs. Optimal activation of a native TRPC6 ROC by angiotensin II (Ang II) requires both depletion of PIP(2) and generation of DAG which leads to stimulation of TRPC6 via a PKC-independent mechanism. The data also indicate that PIP(2) has a marked constitutive inhibitory action of TRPC6 and DAG and PIP(2) are physiological antagonists on TRPC6 ROCs. In contrast PIP(2) stimulates TRPC1 SOCs and has an obligatory role in activation of these channels by store-depletion which requires PKC-dependent phosphorylation of TRPC1 proteins. Finally, we conclude that interactions between PIP(2) bound to TRPC proteins at rest, generation of DAG and PKC-dependent phosphorylation of TRPC proteins have a fundamental role in activation mechanisms of ROCs and SOCs in vascular smooth muscle.

Interaction between TRPC1/TRPC4 assembly and STIM1 contributes to store-operated Ca2+ entry in mesangial cells

Although Orai1 protein was recently identified as the component of CRAC channels in hematopoietic cells, store-operated channels (SOC) in other cell types may have a different molecular entity. Also, the activation mechanism of SOC remains unclear, in general. In the present study, we tested the hypothesis that TRPC1 and TRPC4 proteins were functional subunits of SOC in glomerular mesangial cells (MCs) and that STIM1 was required for the channel activation through interaction with the TRPC proteins. In cultured human MCs, cell-attached patch clamp and fura-2 fluorescence measurements showed that single knockdown of either TRPC1 or TRPC4 significantly attenuated thapsigargin-induced membrane currents and Ca2+ entry as well as Ang II-induced channel activity. Double knockdown of both TRPCs resulted in a comparable inhibition on store-operated Ca2+ entry with single knockdown of either TRPC. Consistent with our previous report, co-immunoprecipitation showed a physical interaction between TRPC1 and TRPC4. Furthermore, we found that knockdown of STIM1 using RNAi significantly reduced the thapsigargin-stimulated membrane currents. Co-immunoprecipitation showed that STIM1 interacted with TRPC4, but not TRPC1. In addition, simultaneous inhibition of STIM1 and TRPC1 resulted in a comparable effect on SOC with single inhibition of either one of them. Taken together, we conclude that in glomerular mesangial cells, the TRPC1/TRPC4 complexes constitute the functional subunits of SOC and that the interaction between STIM1 and TRPC4 may be the mechanism for the activation of the channels.

The angiopoietin-Tie2 system as a therapeutic target in sepsis and acute lung injury

BACKGROUND

Sepsis and acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) are life-threatening syndromes characterised by inflammation and increased vascular permeability. Amongst other factors, the angiopoietin-tyrosine kinase with immunoglobulin-like and EGF-like domains 2 (Tie2) system is involved.

OBJECTIVE

To explore whether the angiopoietin-Tie2 system provides suitable targets for the treatment of sepsis and ALI/ARDS.

METHODS

Original experimental and patient studies on angiopoietins and sepsis/endotoxemia, inflammation, lung injury, hyperpermeability, apoptosis, organ functions and vital outcomes were reviewed.

RESULTS/CONCLUSION

The angiopoietin-Tie2 system controls the responsiveness of the endothelium to inflammatory, hyperpermeability, apoptosis and vasoreactive stimuli. Angiopoietin-2 provokes inflammation and vascular hyperpermeability, while angiopoietin-1 has a protective effect. Targeted angiopoietin-2 inhibition with RNA aptamers or blocking antibodies is a potential anti-inflammatory and anti-vascular hyperpermeability strategy in the treatment of sepsis and ALI/ARDS.

Physiology and pathophysiology of canonical transient receptor potential channels

The existence of a mammalian family of TRPC ion channels, direct homologues of TRP, the visual transduction channel of flies, was discovered during 1995-1996 as a consequence of research into the mechanism by which the stimulation of the receptor-Gq-phospholipase Cbeta signaling pathway leads to sustained increases in intracellular calcium. Mammalian TRPs, TRPCs, turned out to be nonselective, calcium-permeable cation channels, which cause both a collapse of the cell's membrane potential and entry of calcium. The family comprises 7 members and is widely expressed. Many cells and tissues express between 3 and 4 of the 7 TRPCs. Despite their recent discovery, a wealth of information has accumulated, showing that TRPCs have widespread roles in almost all cells studied, including cells from excitable and nonexcitable tissues, such as the nervous and cardiovascular systems, the kidney and the liver, and cells from endothelia, epithelia, and the bone marrow compartment. Disruption of TRPC function is at the root of some familial diseases. More often, TRPCs are contributing risk factors in complex diseases. The present article reviews what has been uncovered about physiological roles of mammalian TRPC channels since the time of their discovery. This analysis reveals TRPCs as major and unsuspected gates of Ca(2+) entry that contribute, depending on context, to activation of transcription factors, apoptosis, vascular contractility, platelet activation, and cardiac hypertrophy, as well as to normal and abnormal cell proliferation. TRPCs emerge as targets for a thus far nonexistent field of pharmacological intervention that may ameliorate complex diseases.

Nocistatin and nociceptin exert opposite effects on the excitability of central amygdala nucleus-periaqueductal gray projection neurons

Central amygdala nucleus (CeA)-periaqueductal gray (PAG) pathway is the component of descending antinociceptive circuitry. Nociceptin/orphanin FQ (N/OFQ) and nocistatin (NST) produce supraspinal pronociceptive and antinociceptive effects, respectively. We hypothesized that opposite effects of N/OFQ and NST on supraspinal pain modulation result from their opposing effects on the excitability of CeA-PAG projection neurons. This hypothesis was tested by investigating electrophysiological effects of N/OFQ and NST on medial CeA neurons that project to PAG (CeA(M)-PAG). N/OFQ hyperpolarized CeA(M)-PAG projection neurons by enhancing inwardly rectifying potassium conductance. In contrast, NST depolarized CeA(M)-PAG neurons by causing the opening of TRPC cation channels via G(alphaq/11)-PLC-PKC pathway. CeA(M)-PAG neurons hyperpolarized by N/OFQ express CRF or neurotensin mRNA. NST-responsive CeA(M)-PAG neurons contain CRF or substance P mRNA. Our study provides the evidence that the molecular and cellular basis for opposite effects of N/OFQ and NST on supraspinal pain regulation is their opposing effects on the excitability of peptidergic CeA(M)-PAG neurons.

Obligatory role for phosphatidylinositol 4,5-bisphosphate in activation of native TRPC1 store-operated channels in vascular myocytes

In the present study the effect of phosphatidylinositol 4,5-bisphosphate (PIP₂) was studied on a native TRPC1 store-operated channel (SOC) in freshly dispersed rabbit portal vein myocytes. Application of diC8-PIP₂, a water soluble form of PIP₂, to quiescent inside-out patches evoked single channel currents with a unitary conductance of 1.9 pS. DiC8-PIP₂-evoked channel currents were inhibited by anti-TRPC1 antibodies and these characteristics are identical to SOCs evoked by cyclopiazonic acid (CPA) and BAPTA-AM. SOCs stimulated by CPA, BAPTA-AM and the phorbol ester phorbol 12,13-dibutyrate (PDBu) were reduced by anti-PIP₂ antibodies and by depletion of tissue PIP₂ levels by pre-treatment of preparations with wortmannin and LY294002. However, these reagents did not alter the ability of PIP₂ to activate SOCs in inside-out patches. Co-immunoprecipitation techniques demonstrated association between TRPC1 and PIP₂ at rest, which was greatly decreased by wortmannin and LY294002. Pre-treatment of cells with PDBu, which activates protein kinase C (PKC), augmented SOC activation by PIP₂ whereas the PKC inhibitor chelerythrine decreased SOC stimulation by PIP₂. Co-immunoprecipitation experiments provide evidence that PKC-dependent phosphorylation of TRPC1 occurs constitutively and was increased by CPA and PDBu but decreased by chelerythrine. These novel results show that PIP₂ can activate TRPC1 SOCs in native vascular myocytes and plays an important role in SOC activation by CPA, BAPTA-AM and PDBu. Moreover, the permissive role of PIP₂ in SOC activation requires PKC-dependent phosphorylation of TRPC1.

Stimulation of histamine H2 receptors activates TRPC3 channels through both phospholipase C and phospholipase D

Histamine plays an important role in many physiological functions; and a change in cytosolic Ca(2+) ([Ca(2+)](i)) may be an early signal in these processes. In the present study, we investigated the activation mechanism of TRPC3, the Canonical Transient Receptors Potential 3 Channels, by histamine via a non-capacitative Ca(2+) entry pathway. TRPC3 was transfected into HEK293 cells and the cells were treated with thapsigargin to deplete the intracellular Ca(2+) stores; re-addition of Ca(2+) initiated a capacitative Ca(2+) entry (CCE). A subsequent application of histamine evoked another Ca(2+) influx on top of the CCE signal only in the TRPC3-transfected HEK293 cells, indicating that histamine can activate TRPC3 via a non-capacitative Ca(2+) entry pathway (non-CCE). This histamine-induced non-CCE was abolished by cimitidine, a histamine H(2) receptors antagonist, but not by histamine H(1) receptor antagonists pyrilamine and diphenhydramine. KT5720, a protein kinase A (PKA) inhibitor, had no effect on the histamine-induced non-CCE. This histamine-induced non-CCE was partially reduced by U73122, a phospholipase C (PLC) inhibitor, and by butan-1-ol, a phospholipase D (PLD) inhibitor. When both PLC and PLD inhibitors were simultaneously applied, the non-CCE signal was completely abolished. Taken together, our results showed, for the first time, that histamine could activate TRPC3 via histamine H(2) receptors, and both PLC and PLD participated in this process.

Ca2+-independent PLA2 controls endothelial store-operated Ca2+ entry and vascular tone in intact aorta

During an agonist stimulation of endothelial cells, the sustained Ca2+ entry occurring through store-operated channels has been shown to significantly contribute to smooth muscle relaxation through the release of relaxing factors such as nitric oxide (NO). However, the mechanisms linking Ca2+ stores depletion to the opening of such channels are still elusive. We have used Ca2+ and tension measurements in intact aortic strips to investigate the role of the Ca2+-independent isoform of phospholipase A2 (iPLA2) in endothelial store-operated Ca2+ entry and endothelium-dependent relaxation of smooth muscle. We provide evidence that iPLA2 is involved in the activation of endothelial store-operated Ca2+ entry when Ca2+ stores are artificially depleted. We also show that the sustained store-operated Ca2+ entry occurring during physiological stimulation of endothelial cells with the circulating hormone ATP is due to iPLA2 activation and significantly contributes to the amplitude and duration of ATP-induced endothelium-dependent relaxation. Consistently, both iPLA2 metabolites arachidonic acid and lysophosphatidylcholine were found to stimulate Ca2+ entry in native endothelial cells. However, only the latter triggered endothelium-dependent relaxation through NO release, suggesting that lysophosphatidylcholine produced by iPLA2 upon Ca2+ stores depletion may act as an intracellular messenger that stimulates store-operated Ca2+ entry and subsequent NO production in endothelial cells. Finally, we found that ACh-induced endothelium relaxation also depends on iPLA2 activation, suggesting that the iPLA2-dependent control of endothelial store-operated Ca2+ entry is a key physiological mechanism regulating arterial tone.

Stim1 and Orai1 mediate CRAC currents and store-operated calcium entry important for endothelial cell proliferation

Recent breakthroughs in the store-operated calcium (Ca(2+)) entry (SOCE) pathway have identified Stim1 as the endoplasmic reticulum Ca(2+) sensor and Orai1 as the pore forming subunit of the highly Ca(2+)-selective CRAC channel expressed in hematopoietic cells. Previous studies, however, have suggested that endothelial cell (EC) SOCE is mediated by the nonselective canonical transient receptor potential channel (TRPC) family, TRPC1 or TRPC4. Here, we show that passive store depletion by thapsigargin or receptor activation by either thrombin or the vascular endothelial growth factor activates the same pathway in primary ECs with classical SOCE pharmacological features. ECs possess the archetypical Ca(2+) release-activated Ca(2+) current (I(CRAC)), albeit of a very small amplitude. Using a maneuver that amplifies currents in divalent-free bath solutions, we show that EC CRAC has similar characteristics to that recorded from rat basophilic leukemia cells, namely a similar time course of activation, sensitivity to 2-aminoethoxydiphenyl borate, and low concentrations of lanthanides, and large Na(+) currents displaying the typical depotentiation. RNA silencing of either Stim1 or Orai1 essentially abolished SOCE and I(CRAC) in ECs, which were rescued by ectopic expression of either Stim1 or Orai1, respectively. Surprisingly, knockdown of either TRPC1 or TRPC4 proteins had no effect on SOCE and I(CRAC). Ectopic expression of Stim1 in ECs increased their I(CRAC) to a size comparable to that in rat basophilic leukemia cells. Knockdown of Stim1, Stim2, or Orai1 inhibited EC proliferation and caused cell cycle arrest at S and G2/M phase, although Orai1 knockdown was more efficient than that of Stim proteins. These results are first to our knowledge to establish the requirement of Stim1/Orai1 in the endothelial SOCE pathway.

Thrombin-mediated increases in cytosolic [Ca2+] involve different mechanisms in human pulmonary artery smooth muscle and endothelial cells

Thrombin is a procoagulant inflammatory agonist that can disrupt the endothelium-lumen barrier in the lung by causing contraction of endothelial cells and promote pulmonary cell proliferation. Both contraction and proliferation require increases in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)). In this study, we compared the effect of thrombin on Ca(2+) signaling in human pulmonary artery smooth muscle (PASMC) and endothelial (PAEC) cells. Thrombin increased the [Ca(2+)](cyt) in both cell types; however, the transient response was significantly higher and recovered quicker in the PASMC, suggesting different mechanisms may contribute to thrombin-mediated increases in [Ca(2+)](cyt) in these cell types. Depletion of intracellular stores with cyclopiazonic acid (CPA) in the absence of extracellular Ca(2+) induced calcium transients representative of those observed in response to thrombin in both cell types. Interestingly, CPA pretreatment significantly attenuated thrombin-induced Ca(2+) release in PASMC; this attenuation was not apparent in PAEC, indicating that a PAEC-specific mechanism was targeted by thrombin. Treatment with a combination of CPA, caffeine, and ryanodine also failed to abolish the thrombin-induced Ca(2+) transient in PAEC. Notably, thrombin-induced receptor-mediated calcium influx was still observed in PASMC after CPA pretreatment in the presence of extracellular Ca(2+). Ca(2+) oscillations were triggered by thrombin in PASMC resulting from a balance of extracellular Ca(2+) influx and Ca(2+) reuptake by the sarcoplasmic reticulum. The data show that thrombin induces increases in intracellular calcium in PASMC and PAEC with a distinct CPA-, caffeine-, and ryanodine-insensitive release existing only in PAEC. Furthermore, a dynamic balance between Ca(2+) influx, intracellular Ca(2+) release, and reuptake underlie the Ca(2+) transients evoked by thrombin in some PASMC. Understanding of such mechanisms will provide an important insight into thrombin-mediated vascular injury during hypertension.

Regulation of blood-brain barrier permeability by transient receptor potential type C and type v calcium-permeable channels

OBJECTIVE

To identify plasma membrane ion channels mediating calcium influx at the blood-brain barrier in response to disrupting stimuli.

METHODS

We examined the expression and function of candidate transient receptor potential channels using reverse transcriptase polymerase chain recation, Fura-2 calcium imaging, and permeability assays.

RESULTS

Immortalized mouse brain microvessel endothelial cells expressed multiple transient receptor potential isoforms: transient receptor potential C1, C2, C4, and C7, M2, M3, M4, and M7, and V2 and V4. Similar profiles were observed in freshly isolated cerebral microvessels and primary cultured rat brain endothelial cells. Thrombin-stimulated calcium influx in brain endothelial cells was blocked by transient receptor potential C inhibitors. Transient receptor potential V activating stimuli also increased intracellular calcium. This increase was inhibited by a transient receptor potential V blocker or by removal of extracellular calcium. Barrier integrity was compromised by thrombin, hypo-osmolar stress, and PMA treatment. The increase in barrier permeability induced by transient receptor potential V activators was blocked by transient receptor potential V inhibition, while thrombin effects were inhibited by transient receptor potential C inhibitors.

CONCLUSIONS

These results demonstrate that transient receptor potential C and transient receptor potential V channels mediate calcium influx at the blood-brain barrier, and as a consequence, may modulate barrier integrity.

Regulation of endothelial junctional permeability

The endothelium is a semi-permeable barrier that regulates the flux of liquid and solutes, including plasma proteins, between the blood and surrounding tissue. The permeability of the vascular barrier can be modified in response to specific stimuli acting on endothelial cells. Transport across the endothelium can occur via two different pathways: through the endothelial cell (transcellular) or between adjacent cells, through interendothelial junctions (paracellular). This review focuses on the regulation of the paracellular pathway. The paracellular pathway is composed of adhesive junctions between endothelial cells, both tight junctions and adherens junctions. The actin cytoskeleton is bound to each junction and controls the integrity of each through actin remodeling. These interendothelial junctions can be disassembled or assembled to either increase or decrease paracellular permeability. Mediators, such as thrombin, TNF-alpha, and LPS, stimulate their respective receptor on endothelial cells to initiate signaling that increases cytosolic Ca2+ and activates myosin light chain kinase (MLCK), as well as monomeric GTPases RhoA, Rac1, and Cdc42. Ca2+ activation of MLCK and RhoA disrupts junctions, whereas Rac1 and Cdc42 promote junctional assembly. Increased endothelial permeability can be reversed with "barrier stabilizing agents," such as sphingosine-1-phosphate and cyclic adenosine monophosphate (cAMP). This review provides an overview of the mechanisms that regulate paracellular permeability.

TRPC1 binds to caveolin-3 and is regulated by Src kinase - role in Duchenne muscular dystrophy

Transient receptor potential canonical 1 (TRPC1), a widely expressed calcium (Ca(2+))-permeable channel, is potentially involved in the pathogenesis of Duchenne muscular dystrophy (DMD). Ca(2+) influx through stretch-activated channels, possibly formed by TRPC1, induces muscle-cell damage in the mdx mouse, an animal model of DMD. In this study, we showed that TRPC1, caveolin-3 and Src-kinase protein levels are increased in mdx muscle compared with wild type. TRPC1 and caveolin-3 colocalised and co-immunoprecipitated. Direct binding of TRPC1-CFP to caveolin-3-YFP was confirmed in C2 myoblasts by fluorescence energy resonance transfer (FRET). Caveolin-3-YFP targeted TRPC1-CFP to the plasma membrane. Hydrogen peroxide, a reactive oxygen species (ROS), increased Src activity and enhanced Ca(2+) influx, but only in C2 myoblasts co-expressing TRPC1 and caveolin-3. In mdx muscle, Tiron, a ROS scavenger, and PP2, a Src inhibitor, reduced stretch-induced Ca(2+) entry and increased force recovery. Because ROS production is increased in mdx/DMD, these results suggest that a ROS-Src-TRPC1/caveolin-3 pathway contributes to the pathogenesis of mdx/DMD.

Tricarbonyldichlororuthenium (II) dimer (CORM2) activates non-selective cation current in human endothelial cells independently of carbon monoxide releasing

Tricarbonyldichlororuthenium (II) dimer (CORM2) has been developed as carbon monoxide (CO) donor. We found that CORM2 activated a type of specific current which was distinct from the big-conductance Ca(2+)-activated K(+) current activated by CO in human umbilical vein endothelial cells (HUVECs). So the aim of the present study was to characterize the CORM2-induced current and to access the relation with CO releasing. CORM2 (100 microM) activated a kind of bi-directional current in HUVECs when the ramp protocol (holding potential 0 mV, from -120 mV to +120 mV) was applied. The current was not blocked by apamin, TRAM-34 and iberiotoxin, the small, intermediate and big-conductance Ca(2+) -activated K(+) channel blockers, and it was not sensitive to the pipette solution chelated with EGTA. CORM2 still activated the current when the chloride in the pipette solution was substituted by equal mol gluconic acid. Substitution of the sodium in the bath with choline significantly reduced the current activated by CORM2. The current was regarded as the non-selective cation current. The current showed slightly inward rectifier property and was not sensitive to Gd(3+) (100 microM), La(3+) (10 microM) or 2-aminoethoxydiphenyl borate (100 microM). CO (10 microM), CORM3 (100, 200 microM) and RuCl(3) (100 microM) were used as controls and showed no effect of the current activation. In conclusion, CORM2 activated the non-selective cation current in HUVECs independently of its CO releasing.