Expression of Trp3 determines sensitivity of capacitative Ca2+ entry to nitric oxide and mitochondrial Ca2+ handling: evidence for a role of Trp3 as a subunit of capacitative Ca2+ entry channels

TRPC channels: Structure, function, regulation and recent advances in small molecular probes

Transient receptor potential canonical (TRPC) channels constitute a group of receptor-operated calcium-permeable nonselective cation channels of the TRP superfamily. The seven mammalian TRPC members, which can be further divided into four subgroups (TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7) based on their amino acid sequences and functional similarities, contribute to a broad spectrum of cellular functions and physiological roles. Studies have revealed complexity of their regulation involving several components of the phospholipase C pathway, G_i and G_o proteins, and internal Ca²⁺ stores. Recent advances in cryogenic electron microscopy have provided several high-resolution structures of TRPC channels. Growing evidence demonstrates the involvement of TRPC channels in diseases, particularly the link between genetic mutations of TRPC6 and familial focal segmental glomerulosclerosis. Because TRPCs were discovered by the molecular identity first, their pharmacology had lagged behind. This is rapidly changing in recent years owning to great efforts from both academia and industry. A number of potent tool compounds from both synthetic and natural products that selective target different subtypes of TRPC channels have been discovered, including some preclinical drug candidates. This review will cover recent advancements in the understanding of TRPC channel regulation, structure, and discovery of novel TRPC small molecular probes over the past few years, with the goal of facilitating drug discovery for the study of TRPCs and therapeutic development.

Canonical Transient Receptor Potential Channel 3 Contributes to Febrile Seizure Inducing Neuronal Cell Death and Neuroinflammation

Febrile seizure (FS) counts as the most common seizures symptom in children undergoing recurrent seizures, posing a high risk to developing subsequent temporal lobe epilepsy. Canonical transient receptor potential channel (TRPC) members are identified as the FS-related genes in hyperthermia prone rats. However, the role of TRPC3 in hyperthermia-induced FS rats remains unclear. In the present study, we investigated whether TRPC3 functionally contributes to the development of FSs. Elevated TRPC3 mRNA and protein levels was detected in hyperthermia-induced FS rats and rat hippocampal neuron cells. The specific inhibitor of TRPC3, Pyr3, remarkably attenuated the susceptibility and severity of seizures, neuronal cell death, and neuroinflammation in FS rats. Conversely, NCX3 activation was apparently suppressed in rats subjected to recurrent FS and rat hippocampal neuron cells. The expression of NCX3 was up-regulated after TRPC3 inhibition in vivo and in vitro. Furthermore, an interaction between TRPC3 and NCX3 was detected by co-immunoprecipitation. Inhibition of TRPC3 suppressed intracellular Ca²⁺ levels in hyperthermia-treated hippocampal neuronal cells. In conclusion, our findings supported that TRPC3 functions as a critical regulator of seizure susceptibility and targeting TRPC3 may be a new therapeutic strategy for FS.

Cardiovascular and Hemostatic Disorders: SOCE in Cardiovascular Cells: Emerging Targets for Therapeutic Intervention

The discovery of the store-operated Ca²⁺ entry (SOCE) phenomenon is tightly associated with its recognition as a pathway of high (patho)physiological significance in the cardiovascular system. Early on, SOCE has been investigated primarily in non-excitable cell types, and the vascular endothelium received particular attention, while a role of SOCE in excitable cells, specifically cardiac myocytes and pacemakers, was initially ignored and remains largely enigmatic even to date. With the recent gain in knowledge on the molecular components of SOCE as well as their cellular organization within nanodomains, potential tissue/cell type-dependent heterogeneity of the SOCE machinery along with high specificity of linkage to downstream signaling pathways emerged for cardiovascular cells. The basis of precise decoding of cellular Ca²⁺ signals was recently uncovered to involve correct spatiotemporal organization of signaling components, and even minor disturbances in these assemblies trigger cardiovascular pathologies. With this chapter, we wish to provide an overview on current concepts of cellular organization of SOCE signaling complexes in cardiovascular cells with particular focus on the spatiotemporal aspects of coupling to downstream signaling and the potential disturbance of these mechanisms by pathogenic factors. The significance of these mechanistic concepts for the development of novel therapeutic strategies will be discussed.

Effects of protease-activated receptors (PARs) on intracellular calcium dynamics of acinar cells in rat lacrimal glands

Protease-activated receptors (PARs) represent a novel class of seven transmembrane domain G-protein coupled receptors, which are activated by proteolytic cleavage. PARs are present in a variety of cells and have been prominently implicated in the regulation of a number of vital functions. Here, lacrimal gland acinar cell responses to PAR activation were examined, with special reference to intracellular Ca(2+) concentration ([Ca(2+)]i) dynamics. In the present study, detection of acinar cell mRNA specific to known PAR subtypes was determined by reverse transcriptase polymerase chain reaction. Only PAR2 mRNA was detected in acinar cells of lacrimal glands. Both trypsin and a PAR2-activating peptide (PAR2-AP), SLIGRL-NH2, induced an increase in [Ca(2+)]i in acinar cells. The removal of extracellular Ca(2+) and the use of Ca(2+) channel blockers did not inhibit PAR2-AP-induced [Ca(2+)]i increases. Furthermore, U73122 and xestospongin C failed to inhibit PAR2-induced increases in [Ca(2+)]i. The origin of the calcium influx observed after activated PAR2-induced Ca(2+) release from intracellular Ca(2+) stores was also evaluated. The NO donor, GEA 3162, mimicked the effects of PAR2 in activating non-capacitative calcium entry (NCCE). However, both calyculin A (100 nM) and a low concentration of Gd(3+) (5 μM) did not completely block the PAR2-AP-induced increase in [Ca(2+)]i. These findings indicated that PAR2 activation resulted primarily in Ca(2+) mobilization from intracellular Ca(2+) stores and that PAR2-mediated [Ca(2+)]i changes were mainly independent of IP3. RT-PCR indicated that TRPC 1, 3 and 6, which play a role in CCE and NCCE, are expressed in acinar cells. We suggest that PAR2-AP differentially regulates both NCCE and CCE, predominantly NCCE. Finally, our results suggested that PAR2 may function as a key receptor in calcium-related cell homeostasis under pathophysiological conditions such as tissue injury or inflammation.

Novel pyrazole compounds for pharmacological discrimination between receptor-operated and store-operated Ca(2+) entry pathways

BACKGROUND AND PURPOSE

Pyrazole derivatives have recently been suggested as selective blockers of transient receptor potential cation (TRPC) channels but their ability to distinguish between the TRPC and Orai pore complexes is ill-defined. This study was designed to characterize a series of pyrazole derivatives in terms of TRPC/Orai selectivity and to delineate consequences of selective suppression of these pathways for mast cell activation.

EXPERIMENTAL APPROACH

Pyrazoles were generated by microwave-assisted synthesis and tested for effects on Ca(2+) entry by Fura-2 imaging and membrane currents by patch-clamp recording. Experiments were performed in HEK293 cells overexpressing TRPC3 and in RBL-2H3 mast cells, which express classical store-operated Ca(2+) entry mediated by Orai channels. The consequences of inhibitory effects on Ca(2+) signalling in RBL-2H3 cells were investigated at the level of both degranulation and nuclear factor of activated T-cells activation.

KEY RESULTS

Pyr3, a previously suggested selective inhibitor of TRPC3, inhibited Orai1- and TRPC3-mediated Ca(2+) entry and currents as well as mast cell activation with similar potency. By contrast, Pyr6 exhibited a 37-fold higher potency to inhibit Orai1-mediated Ca(2+) entry as compared with TRPC3-mediated Ca(2+) entry and potently suppressed mast cell activation. The novel pyrazole Pyr10 displayed substantial selectivity for TRPC3-mediated responses (18-fold) and the selective block of TRPC3 channels by Pyr10 barely affected mast cell activation.

CONCLUSIONS AND IMPLICATIONS

The pyrazole derivatives Pyr6 and Pyr10 are able to distinguish between TRPC and Orai-mediated Ca(2+) entry and may serve as useful tools for the analysis of cellular functions of the underlying Ca(2+) channels.

TRP channels: sensors and transducers of gasotransmitter signals

The transient receptor potential (trp) gene superfamily encodes cation channels that act as multimodal sensors for a wide variety of stimuli from outside and inside the cell. Upon sensing, they transduce electrical and Ca²⁺ signals via their cation channel activities. These functional features of TRP channels allow the body to react and adapt to different forms of environmental changes. Indeed, members of one class of TRP channels have emerged as sensors of gaseous messenger molecules that control various cellular processes. Nitric oxide (NO), a vasoactive gaseous molecule, regulates TRP channels directly via cysteine (Cys) S-nitrosylation or indirectly via cyclic GMP (cGMP)/protein kinase G (PKG)-dependent phosphorylation. Recent studies have revealed that changes in the availability of molecular oxygen (O₂) also control the activation of TRP channels. Anoxia induced by O₂-glucose deprivation and severe hypoxia (1% O₂) activates TRPM7 and TRPC6, respectively, whereas TRPA1 has recently been identified as a novel sensor of hyperoxia and mild hypoxia (15% O₂) in vagal and sensory neurons. TRPA1 also detects other gaseous molecules such as hydrogen sulfide (H₂S) and carbon dioxide (CO₂). In this review, we focus on how signaling by gaseous molecules is sensed and integrated by TRP channels.

Regulation of TRPM8 channel activity

Transient Receptor Potential Melastatin 8 (TRPM8) is a Ca(2+) permeable non-selective cation channel directly activated by cold temperatures and chemical agonists such as menthol. It is a well established sensor of environmental cold temperatures, found in peripheral sensory neurons, where its activation evokes depolarization and action potentials. The activity of TRPM8 is regulated by a number of cellular signaling pathways, most notably by phosphoinositides and the activation of phospholipase C. This review will summarize current knowledge on the physiological and pathophysiological roles of TRPM8 and its regulation by various intracellular messenger molecules and signaling pathways.

RANKL-mediated reactive oxygen species pathway that induces long lasting Ca2+ oscillations essential for osteoclastogenesis

RANKL (receptor activator of NF-kappaB ligand) induces osteoclastogenesis by activating multiple signaling pathways in osteoclast precursor cells, chief among which is induction of long lasting oscillations in the intracellular concentration of Ca(2+) ([Ca(2+)](i)). The [Ca(2+)](i) oscillations activate calcineurin, which activates the transcription factor NFATc1. The pathway by which RANKL induces [Ca(2+)](i) oscillations and osteoclastogenesis is poorly understood. Here we report the discovery of a novel pathway induced by RANKL to cause a long lasting increase in reactive oxygen species (ROS) and [Ca(2+)](i) oscillations that is essential for differentiation of bone marrow-derived monocytes into osteoclasts. The pathway includes RANKL-mediated stimulation of Rac1 to generate ROS, which stimulate phospholipase Cgamma1 to evoke [Ca(2+)](i) oscillations by stimulating Ca(2+) release from the inositol 1,4,5-trisphosphate pool and STIM1-regulated Ca(2+) influx. Induction and activation of the pathway is observed only after 24-h stimulation with RANKL and lasts for at least 3 days. The physiological role of the pathway is demonstrated in mice with deletion of the Peroxiredoxin II gene and results in a mark increase is ROS and, consequently, a decrease in bone density. Moreover, bone marrow-derived monocytes in PrxII(-/-) primary culture show increased ROS and spontaneous [Ca(2+)](i) oscillations. These findings identify the primary RANKL-stimulated pathway to trigger the late stages of osteoclastogenesis and regulate bone resorption.

Pharmacological profile of store-operated Ca(2+) entry in intrapulmonary artery smooth muscle cells

Store-operated Ca(2+) entry (SOCE) plays an important role in the contraction and proliferation of pulmonary artery smooth muscle cells (PASMCs). The aim of this study was to characterise the pharmacological properties of the SOCE pathway in freshly isolated PASMCs from rat lung and to determine whether this Ca(2+) entry pathway is sensitive to nitric oxide donor drugs. Following depletion of Ca(2+) from the sarcoplasmic reticulum, by treating cells with thapsigargin, re-addition of Ca(2+) produced an increase in cytosolic fluo-4 fluorescence that was sustained for the period that extracellular Ca(2+) was present. Thapsigargin also increased the rate of quench of fura-2 fluorescence, confirming that SOCE was activated. The SOCE pathway was not affected by nifedipine or verapamil; however, it was inhibited by the divalent cations Ni(2+) (10 microM) and Cd(2+) (10 microM) by 47+/-5% and 49+/-5% respectively. SOCE was also inhibited 42+/-5% by 2-aminoethoxydiphenyl borate (2-APB; 75 microM) and 58+/-4% by Gd(3+) (10 microM), although La(3+) (100 microM) had little effect. None of the NO donors examined, including sodium nitroprusside, glyceryl trinitrate, and 2-(N,N-diethylamino)-diazenolate-2-oxide had any effect on SOCE. Thus, the pulmonary vasorelaxation produced by NO does not involve direct inhibition of SOCE in PASMCs. Western blot and immunocytochemistry using antibodies directed against specific TRPC subunits detected the presence of TRPC1, 3, and 6 in pulmonary artery and the pharmacological profile of SOCE in PASMCs favours a role for TRPC1 in mediating the underlying channels that are activated by store depletion.

Pulmonary artery smooth muscle cells from normal subjects and IPAH patients show divergent cAMP-mediated effects on TRPC expression and capacitative Ca2+ entry

Pulmonary vascular remodeling due to overgrowth of pulmonary artery smooth muscle cells (PASMC) is a major cause for the elevated vascular resistance in patients with idiopathic pulmonary arterial hypertension (IPAH). Increased cytosolic Ca(2+) concentration, resulting from enhanced capacitative Ca(2+) entry (CCE) and upregulated transient receptor potential (TRP) channel expression, is involved in stimulating PASMC proliferation. The current study was designed to determine the impact of cAMP, a second messenger that we hypothesized would blunt aspects of PASMC activity, as a possible contributor to IPAH pathophysiology. Short-term (30 min) pretreatment with forskolin (FSK; 10 muM), a direct activator of adenylyl cyclase, in combination with the cyclic nucleotide phosphodiesterase inhibitor isobutylmethylxanthine (IBMX; 200 muM), attenuated CCE in PASMC from normal subjects, patients without pulmonary hypertension (NPH), and patients with IPAH. The FSK-mediated CCE inhibition was independent of protein kinase A (PKA), because the PKA inhibitor H89 negligibly affected the decrease in CCE produced by cAMP. By contrast, longer (4 h) treatment with FSK (with IBMX) attenuated CCE in normal and NPH PASMC but enhanced CCE in IPAH PASMC. This enhancement of CCE was abolished by PKA inhibition and associated with an upregulation of TRPC3. In addition, cAMP increased TRPC1 mRNA expression in IPAH (but not in normal or NPH) PASMC, an effect blunted by H89. Furthermore, iloprost, a prostacyclin analog that increases cAMP, downregulated TRPC3 expression in IPAH PASMC and FSK-mediated cAMP increase inhibited IPAH PASMC proliferation. Although a rapid rise in cellular cAMP decreases CCE by a PKA-independent mechanism, sustained cAMP increase inhibits CCE in normal and NPH PASMC but increases CCE via a PKA-dependent pathway in IPAH PASMC. The divergent effect of cAMP on CCE parallels effects on TRPC expression. The results suggest that the combined use of a PKA inhibitor and cAMP-elevating drugs may provide a novel approach for treatment of IPAH.

Different phospholipase-C-coupled receptors differentially regulate capacitative and non-capacitative Ca2+ entry in A7r5 cells

Several receptors, including those for AVP (Arg8-vasopressin) and 5-HT (5-hydroxytryptamine), share an ability to stimulate PLC (phospholipase C) and so production of IP3 (inositol 1,4,5-trisphosphate) and DAG (diacylglycerol) in A7r5 vascular smooth muscle cells. Our previous analysis of the effects of AVP on Ca2+ entry [Moneer, Dyer and Taylor (2003) Biochem. J. 370, 439-448] showed that arachidonic acid released from DAG stimulated NO synthase. NO then stimulated an NCCE (non-capacitative Ca2+ entry) pathway, and, via cGMP and protein kinase G, it inhibited CCE (capacitative Ca2+ entry). This reciprocal regulation ensured that, in the presence of AVP, all Ca2+ entry occurred via NCCE to be followed by a transient activation of CCE only when AVP was removed [Moneer and Taylor (2002) Biochem. J. 362, 13-21]. We confirm that, in the presence of AVP, all Ca2+ entry occurs via NCCE, but 5-HT, despite activating PLC and evoking release of Ca2+ from intracellular stores, stimulates Ca2+ entry only via CCE. We conclude that two PLC-coupled receptors differentially regulate CCE and NCCE. We also address evidence that, in some A7r5 cells lines, AVP fails either to stimulate NCCE or inhibit CCE [Brueggemann, Markun, Barakat, Chen and Byron (2005) Biochem. J. 388, 237-244]. Quantitative PCR analysis suggests that these cells predominantly express TRPC1 (transient receptor potential canonical 1), whereas cells in which AVP reciprocally regulates CCE and NCCE express a greater variety of TRPC subtypes (TRPC1=6>2>3).

TRPC3: a versatile transducer molecule that serves integration and diversification of cellular signals

Transient receptor potential (TRP) proteins have been recognized as sensors for a wide variety of external and internal signals involved in maintenance of cellular homeostasis and control of physiological functions. Evidence of a striking versatility in terms of signal integration and transduction has been reported for members of the canonical (or classical) TRP subfamily (TRPCs). TRPC species are cation channel subunits and emerge as multifunctional signal transduction molecules that are able to function as components of divergent signalplexes. Results obtained in heterologous expression systems suggest TRPC3 as a paradigm of multifunctional signal transduction by a cation channel protein. TRPC3 serves cellular Ca(2+) signaling by multiple mechanisms and may control a variety of distinct physiological functions. In this review, we summarize current knowledge on the properties and possible signaling partners of TRPC3, and discuss the role of TRPC3 channel proteins in cellular signaling networks.

Calcium channels, transporters and exchangers in placenta: a review

Calcium (Ca2+) entry in cells is crucial for development and physiology of virtually all cell types. It acts as an intracellular (second) messenger to regulate a diverse array of cellular functions, from cell division and differentiation to cell death. Among candidates for Ca2+ entry in cells are-voltage-dependant Ca2+ channels (VDCCs), transient receptor potential (TRP)-related Ca2+ channels and store-operated Ca2+ (SOC) channels. Plasma membrane Ca2+-ATPases (PMCA) and Na+/Ca2+ exchanger (NCX) are mainly responsible for Ca2+ extrusion. These different Ca2+channels/transporters and exchangers exhibit specific distribution and physiological properties. During pregnancy, the syncytiotrophoblast layer of the human placenta transfers as much as 30 g of Ca2+ from the mother to the fetus, especially in late gestation where Ca2+ transport through different channels must increase in response to the demands of accelerating bone mineralization of the fetus. The identification and characterization of the different Ca2+ channels/transporters and exchangers on the brush-border membrane (BBM) facing the maternal circulation, and the basal plasma membrane (BPM) facing the fetal circulation; placental membrane of the syncytiotrophoblasts have been the focus of numerous studies. This review discusses current views in this field regarding localization and functions during transcellular Ca2+ entry and extrusion from cells particularly in the placenta.

Role of Na+/Ca2+ exchange in regulating cytosolic Ca2+ in cultured human pulmonary artery smooth muscle cells

A rise in cytosolic Ca2+ concentration ([Ca2+]cyt) in pulmonary artery smooth muscle cells (PASMC) is an important stimulus for cell contraction, migration, and proliferation. Depletion of intracellular Ca2+ stores opens store-operated Ca2+ channels (SOC) and causes Ca2+ entry. Transient receptor potential (TRP) cation channels that are permeable to Na+ and Ca2+ are believed to form functional SOC. Because sarcolemmal Na+/Ca2+ exchanger has also been implicated in regulating [Ca2+]cyt, this study was designed to test the hypothesis that the Na+/Ca2+ exchanger (NCX) in cultured human PASMC is functionally involved in regulating [Ca2+]cyt by contributing to store depletion-mediated Ca2+ entry. RT-PCR and Western blot analyses revealed mRNA and protein expression for NCX1 and NCKX3 in cultured human PASMC. Removal of extracellular Na+, which switches the Na+/Ca2+ exchanger from the forward (Ca2+ exit) to reverse (Ca2+ entry) mode, significantly increased [Ca2+]cyt, whereas inhibition of the Na+/Ca2+ exchanger with KB-R7943 (10 microM) markedly attenuated the increase in [Ca2+]cyt via the reverse mode of Na+/Ca2+ exchange. Store depletion also induced a rise in [Ca2+]cyt via the reverse mode of Na+/Ca2+ exchange. Removal of extracellular Na+ or inhibition of the Na+/Ca2+ exchanger with KB-R7943 attenuated the store depletion-mediated Ca2+ entry. Furthermore, treatment of human PASMC with KB-R7943 also inhibited cell proliferation in the presence of serum and growth factors. These results suggest that NCX is functionally expressed in cultured human PASMC, that Ca2+ entry via the reverse mode of Na+/Ca2+ exchange contributes to store depletion-mediated increase in [Ca2+]cyt, and that blockade of the Na+/Ca2+ exchanger in its reverse mode may serve as a potential therapeutic approach for treatment of pulmonary hypertension.

Arachidonic acid regulates two Ca2+ entry pathways via nitric oxide

Several regulated Ca2+ entry pathways have been identified, with capacitative Ca2+ entry (CCE) being the most characterized. In the present study, we examined Ca2+ entry pathways regulated by arachidonic acid (AA) in mouse parotid acini. AA induced Ca2+ release from intracellular stores, and increased Ca2+ entry. AA inhibited thapsigargin (Tg)-induced CCE, whereas AA activated Ca2+ entry when CCE was blocked by gadolinium (Gd3+). AA-induced Ca2+ entry was associated with depletion of calcium from ryanodine-sensitive stores; both AA-induced Ca2+ release and Ca2+ entry were inhibited by tetracaine and the nitric oxide synthase (NOS) inhibitor, 7-nitroindazole (7-NI). The nitric oxide (NO) donor, 1,2,3,4-ox-triazolium,5-amino-3-(3,4-dichlorophenyl)-chloride (GEA 3162), but not 8-bromo-cGMP, mimicked the effects of AA in inhibiting CCE. Results suggest that AA acts via nitric acid to inhibit the CCE pathway that is selective for Ca2+, and to activate a second Ca2+ entry pathway that is dependent on depletion of Ca2+ from ryanodine-sensitive stores.

Vascular smooth muscle mitochondria at the cross roads of Ca2+ regulation

Mitochondria play an essential role in the regulation of vascular smooth muscle Ca(2+) signaling being simultaneously integrated in the regulation of ion channels and Ca(2+) transporters, oxygen radical production, metabolite recycling and intracellular redox potential. Mitochondria buffer Ca(2+) from cytoplasmic microdomains to alter the spatio-temporal pattern of Ca(2+) gradients following Ca(2+)-influx and Ca(2+)-release, and thus control site-specific, Ca(2+)-dependent ion channel activation and inactivation. The sub-cellular localization of mitochondria in conjunction with tissue-specific channel expression is fundamental to vascular heterogeneity. The mitochondrial electron transport chain recycles metabolic intermediates that modulate cellular redox potential and produces oxygen radicals in proportion to oxygen tension. Perturbation of specific complexes within the transport chain can affects NADH:NAD and ATP:ADP ratios and radical production, which can in turn influence second messenger metabolism, ion channel gating and Ca(2+)-transporter activity. Mitochondria thus provide the common ground for cross-talk between these regulatory systems that are mutually sensitive to one another. This cross-talk between signaling systems provides a means to render the physiological regulation of vascular tone responsive to complex stimulation by paracrine and endocrine factors, blood pressure and flow, tissue oxygenation and metabolic state.

Regulation of canonical transient receptor potential isoform 3 (TRPC3) channel by protein kinase G

Canonical transient receptor potential (TRPC) channels are Ca2+-permeable nonselective cation channels that are widely expressed in numerous cell types. Seven different members of TRPC channels have been isolated. The activity of these channels is regulated by the filling state of intracellular Ca2+ stores and/or diacylglycerol and/or Ca2+/calmodulin. However, no evidence is available as to whether TRPC channels are regulated by direct phosphorylation on the channels. In the present study, TRPC isoform 3 (TRPC3) gene was overexpressed in HEK293 cells that were stably transfected with protein kinase G (PKG). We found that the overexpressed TRPC3 mediated store-operated Ca2+ influx and that this type of Ca2+ influx was inhibited by cGMP. The inhibitory effect of cGMP was abolished by KT5823 or H8. Point mutations at two consensus PKG phosphorylation sites (T11A and S263Q) of TRPC3 channel markedly reduced the inhibitory effect of cGMP. In addition, TRPC3 proteins were purified from HEK293 cells that were transfected with either wild-type or mutant TRPC3 constructs, and in vitro PKG phosphorylation assay was carried out. It was found that wild-type TRPC3 could be directly phosphorylated by PKG in vitro and that the phosphorylation was abolished in the presence of KT5823. The phosphorylation signal was greatly reduced in mutant protein T11A or S263Q. Taken together, TRPC3 channels could be directly phosphorylated by PKG at position T11 and S263, and this phosphorylation abolished the store-operated Ca2+ influx mediated by TRPC3 channels in HEK293 cells.

Ca(2+) signaling by TRPC3 involves Na(+) entry and local coupling to the Na(+)/Ca(2+) exchanger

TRPC3 has been suggested as a key component of phospholipase C-dependent Ca(2+) signaling. Here we investigated the role of TRPC3-mediated Na(+) entry as a determinant of plasmalemmal Na(+)/Ca(2+) exchange. Ca(2+) signals generated by TRPC3 overexpression in HEK293 cells were found to be dependent on extracellular Na(+), in that carbachol-stimulated Ca(2+) entry into TRPC3 expressing cells was significantly suppressed when extracellular Na(+) was reduced to 5 mm. Moreover, KB-R9743 (5 microm) an inhibitor of the Na(+)/Ca(2+) exchanger (NCX) strongly suppressed TRPC3-mediated Ca(2+) entry but not TRPC3-mediated Na(+) currents. NCX1 immunoreactivity was detectable in HEK293 as well as in TRPC3-overexpressing HEK293 cells, and reduction of extracellular Na(+) after Na(+) loading with monensin resulted in significant rises in intracellular free Ca(2+) (Ca(2+)(i)) of HEK293 cells. Similar rises in Ca(2+)(i) were recorded in TRPC3-overexpressing cells upon the reduction of extracellular Na(+) subsequent to stimulation with carbachol. These increases in Ca(2+)(i) were associated with outward membrane currents at positive potentials and inhibited by KB-R7943 (5 microm), chelation of extracellular Ca(2+), or dominant negative suppression of TRPC3 channel function. This suggests that Ca(2+) entry into TRPC3-expressing cells involves reversed mode Na(+)/Ca(2+) exchange. Cell fractionation experiments demonstrated co-localization of TRPC3 and NCX1 in low density membrane fractions, and co-immunoprecipitation experiments provided evidence for association of TRPC3 and NCX1. Glutathione S-transferase pull-down experiments revealed that NCX1 interacts with the cytosolic C terminus of TRPC3. We suggest functional and physical interaction of nonselective TRPC cation channels with NCX proteins as a novel principle of TRPC-mediated Ca(2+) signaling.

TRPC3 mediates T-cell receptor-dependent calcium entry in human T-lymphocytes

Stimulation of the T-cell receptor (TCR) activates Ca2+ entry across the plasma membrane, which is a key triggering event for the T-cell-associated immune response. We show that TRPC3 channels are important for the TCR-dependent Ca2+ entry pathway. The TRPC3 gene was found to be damaged in human T-cell mutants defective in Ca2+ influx. Mutations of the TRPC3 gene were accompanied by changes of TRPC3 gene expression. Introduction of the complete human TRPC3 cDNA into those mutants rescued Ca2+ currents as well as TCR-dependent Ca2+ signals. Our data provide the initial step toward understanding the molecular nature of endogenous Ca2+ channels participating in T-cell activation and put forward TRPC3 as a new target for modulating the immune response.

Store-operated Ca2+ entry in first trimester and term human placenta

We have examined whether store-operated Ca2+ entry, a common pathway for Ca2+ entry in non-excitable tissue, is apparent in the syncytiotrophoblast of both first trimester and term human placenta. Expression of transient receptor potential (TRPC) homologues, a family of channels thought to be involved in store-operated Ca2+ entry, was also studied at the mRNA and protein levels. [Ca2+]i in syncytiotrophoblast of first trimester and term placental villous fragments was measured by microfluorimetry using the Ca2+-sensitive dye fura-2. Store-operated Ca2+ entry was stimulated using 1 microM thapsigargin in Ca(2+)-free Tyrode buffer (no added Ca2+ + 1 mM EGTA) followed by superfusion with control (Ca2+-containing) buffer. In term fragments, this protocol resulted in a rapid increase in [Ca2+]i, which was inhibited in the presence of 150 microM GdCl3, 200 microM NiCl2, 200 microM CoCl2 or 30 microM SKF96365 but was unaffected by addition of 10 microM nifedipine. It was not possible to stimulate such a rise in [Ca2+]i in first trimester fragments. Messenger RNA encoding TRPC1, TRPC3, TRPC4, TRPC5 and TRPC6 was identified in both first trimester and term placentas. From Western blotting, TRPC3 and TRPC6 proteins were detected in term, but not in first trimester, placentas, while TRPC1 protein was not detected. By immunocytochemistry, TRPC3 and TRPC4 were localised to cytotrophoblast cells in first trimester placentas and to the syncytiotrophoblast in term placentas. TRPC6 staining was present in the syncytiotrophoblast of both first trimester and term placenta, but the intensity was much greater in the latter. We propose that store-operated Ca2+ entry may be an important route for Ca2+ entry into the syncytiotrophoblast of term, but not first trimester placentas, and that in human placenta TRPC channels may underlie this entry mechanism.

Nitric oxide co-ordinates the activities of the capacitative and non-capacitative Ca2+-entry pathways regulated by vasopressin

In A7r5 vascular smooth muscle cells vasopressin, via arachidonic acid, regulates two Ca(2+)-entry pathways. Capacitative Ca(2+) entry (CCE), activated by empty Ca(2+) stores, is inhibited by arachidonic acid, and non-capacitative Ca(2+) entry (NCCE) is stimulated by it. This reciprocal regulation ensures that all Ca(2+) entry is via NCCE in the presence of vasopressin, while CCE mediates a transient Ca(2+) entry only after removal of vasopressin. We demonstrate that type III NO synthase (NOS III) is expressed in A7r5 cells and that NO inhibits CCE. Inhibition of CCE by vasopressin requires NOS III and the requirement lies downstream of arachidonic acid. Activation of soluble guanylate cyclase by NO and subsequent activation of protein kinase G are required for inhibition of CCE. Stimulation of NCCE by vasopressin also requires NOS III, but the stimulation is neither mimicked by cGMP nor blocked by inhibitors of soluble guanylate cyclase or protein kinase G. We conclude that arachidonic acid formed in response to vasopressin stimulates NOS III. NO then directly stimulates Ca(2+) entry through NCCE and, via protein kinase G, it inhibits CCE. The additional amplification provided by the involvement of guanylate cyclase and protein kinase G ensures that CCE will always be inhibited when vasopressin activates NCCE.

The cellular and molecular basis of store-operated calcium entry

The impact of calcium signalling on so many areas of cell biology reflects the crucial role of calcium signals in the control of diverse cellular functions. Despite the precision with which spatial and temporal details of calcium signals have been resolved, a fundamental aspect of the generation of calcium signals -- the activation of 'store-operated channels' (SOCs) -- remains a molecular and mechanistic mystery. Here we review new insights into the exchange of signals between the endoplasmic reticulum (ER) and plasma membrane that result in activation of calcium entry channels mediating crucial long-term calcium signals.

Multiple Trp isoforms implicated in capacitative calcium entry are expressed in human pregnant myometrium and myometrial cells

Capacitative Ca2+ entry plays a role in thapsigargin- and oxytocin-mediated increases in intracellular free Ca2+ in human myometrium. Members of the Trp protein family have been implicated in capacitative Ca2+ entry in a number of tissues. Pregnant human myometrium and the human myometrial cell line PHM1-41 expressed mRNA for hTrp1, hTrp3, hTrp4, hTrp6, and hTrp7. A number of known splice variants of hTrp1 and hTrp4 were expressed in these cells. In addition, novel splice variants for hTrp1 and hTrp3 were discovered. hTrp1gamma1 and hTrp1gamma2 contain insertions between previously described exons 9 and 10 that would alter reading frame and produce Trp proteins truncated in the membrane spanning region if expressed. The hTrp3 variant introduces sequence between exons 8 and 9 that would insert 16 amino acids in the C-terminal region of the protein upstream of the calmodulin and inositol 1,4,5-triphosphate receptor interaction domain. hTrp1, hTrp3, and hTrp4 proteins were detected in both pregnant human myometrial and PHM1-41 membranes; a weak band consistent with hTrp6 expression was detected in pregnant human myometrium. These data are consistent with the presence of proteins that could form putative capacitative Ca2+ channels in human myometrium. Control of the activity of these channels may be important for the control of uterine contractile activity.