Transient receptor potential 1 regulates capacitative Ca(2+) entry and Ca(2+) release from endoplasmic reticulum in B lymphocytes

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

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

A mutation in Orai1 causes immune deficiency by abrogating CRAC channel function

Antigen stimulation of immune cells triggers Ca2+ entry through Ca2+ release-activated Ca2+ (CRAC) channels, promoting the immune response to pathogens by activating the transcription factor NFAT. We have previously shown that cells from patients with one form of hereditary severe combined immune deficiency (SCID) syndrome are defective in store-operated Ca2+ entry and CRAC channel function. Here we identify the genetic defect in these patients, using a combination of two unbiased genome-wide approaches: a modified linkage analysis with single-nucleotide polymorphism arrays, and a Drosophila RNA interference screen designed to identify regulators of store-operated Ca2+ entry and NFAT nuclear import. Both approaches converged on a novel protein that we call Orai1, which contains four putative transmembrane segments. The SCID patients are homozygous for a single missense mutation in ORAI1, and expression of wild-type Orai1 in SCID T cells restores store-operated Ca2+ influx and the CRAC current (I(CRAC)). We propose that Orai1 is an essential component or regulator of the CRAC channel complex.

Dissociation of regulated trafficking of TRPC3 channels to the plasma membrane from their activation by phospholipase C

Regulated translocation of canonical transient receptor potential (TRPC) proteins to the plasma membrane has been proposed as a mechanism of their activation. By using total internal reflection fluorescence microscopy (TIRFM), we monitored green fluorescent protein-labeled TRPC3 (TRPC3-GFP) movement to the plasma membrane in HEK293 cells stably expressing this fusion protein. We observed no increase in TRPC3-GFP TIRFM in response to the muscarinic receptor agonist methacholine or the synthetic diacylglycerol, 1-oleoyl-2-acetyl-sn-glycerol, despite activation of TRPC3 by these agents. We did, however, observe a TIRFM response to epidermal growth factor (EGF). This TIRFM response to EGF was accompanied by increased Ba2+ entry and TRPC3 currents. However, 1-oleoyl-2-acetyl-sn-glycerol-induced TRPC3 activity was not increased. TIRFM also increased in response to Gd3+, a competitive inhibitor of TRPC3 channels. This may be indicative of constitutive trafficking of TRPC3, with Gd3+ acting to "trap" cycling TRPC3 molecules in the plasma membrane. Consistent with this interpretation, TRPC3-expressing cells exhibited large variance in membrane capacitance, and this variance was decreased by both Gd3+ and EGF. These results indicate the following: (i) trafficking of TRPC3 may play a role in regulating the concentration of channels in the plasma membrane but is not involved in activation through the phospholipase C pathway; (ii) TRPC3 undergoes constitutive cyclical trafficking in the plasma membrane, and the mechanism by which growth factors increase the number of plasma membrane channels may involve stabilizing them in the plasma membrane.

A severe defect in CRAC Ca2+ channel activation and altered K+ channel gating in T cells from immunodeficient patients

Engagement of the TCR triggers sustained Ca²⁺ entry through Ca²⁺ release-activated Ca²⁺ (CRAC) channels, which helps drive gene expression underlying the T cell response to pathogens. The identity and activation mechanism of CRAC channels at a molecular level are unknown. We have analyzed ion channel expression and function in T cells from SCID patients which display 1–2% of the normal level of Ca²⁺ influx and severely impaired T cell activation. The lack of Ca²⁺ influx is not due to deficient regulation of Ca²⁺ stores or expression of several genes implicated in controlling Ca²⁺ entry in lymphocytes (kcna3/Kv1.3, kcnn4/IKCa1, trpc1, trpc3, trpv6, stim1). Instead, electrophysiologic measurements show that the influx defect is due to a nearly complete absence of functional CRAC channels. The lack of CRAC channel activity is correlated with diminished voltage sensitivity and slowed activation kinetics of the voltage-dependent Kv1.3 channel. These results demonstrate that CRAC channels provide the major, if not sole, pathway for Ca²⁺ entry activated by the TCR in human T cells. They also offer evidence for a functional link between CRAC and Kv1.3 channels, and establish a model system for molecular genetic studies of the CRAC channel.

Protection of TRPC7 cation channels from calcium inhibition by closely associated SERCA pumps

Numerous studies have demonstrated that members of the transient receptor potential (TRP) superfamily of channels are involved in regulated Ca2+ entry. Additionally, most Ca2+-permeable channels are themselves regulated by Ca2+, often in complex ways. In the current study, we have investigated the regulation of TRPC7, a channel known to be potentially activated by both store-operated mechanisms and non-store-operated mechanisms involving diacylglycerols. Surprisingly, we found that activation of TRPC7 channels by diacylglycerol was blocked by the SERCA pump inhibitor thapsigargin. The structurally related channel, TRPC3, was similarly inhibited. This effect depended on extracellular calcium and on the driving force for Ca2+ entry. The inhibition is not due to calcium entry through store-operated channels but rather results from calcium entry through TRPC7 channels themselves. The effect of thapsigargin was prevented by inhibition of calmodulin and was mimicked by pharmacological disruption of the actin cytoskeleton. Our results suggest the presence of a novel mechanism involving negative regulation of TRPC channels by calcium entering through the channels. Under physiological conditions, this negative feedback by calcium is attenuated by the presence of closely associated SERCA pumps.

Calcium signaling, ion channels and more. The DT40 system as a model of vertebrate ion homeostasis and cell physiology

The DT40 B-lymphocyte cell line is a chicken bursal lymphocyte tumor cell line which grows rapidly, expresses a variety of types of constitutive and signal dependent ion transport systems., and supports the efficient use of stable and conditional genetic manipulations. Below, we review the use of DT40 cells in dissecting molecular mechanisms involved in Ca2+, Mg2+, and Zn2+ transport physiology. These studies highlight the flexibility and advantages the DT40 environment offers to investigators interested in the study of basic vertebrate ion transport physiology.

Ca2+ signaling microdomains:platforms for the assembly and regulation of TRPC channels

The transient receptor potential canonical family (TRPC1-TRPC7) of ion channel proteins, which are activated in response to agonist-stimulated phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] hydrolysis, are proposed components of the elusive store-operated Ca(2+) (SOC) channel. TRPC channels display distinct properties and interact to form homomeric or heteromeric channels that differ in their function and regulation. Although the exact function of TRPC channels and how they are regulated has not been established, increasing data suggest that they are localized and regulated within Ca(2+) signaling microdomains. TRPC channels contribute to store-operated and store-independent Ca(2+) entry mechanisms, both of which are activated by agonist-stimulated PtdIns(4,5)P(2) hydrolysis. Elucidation of how cells achieve specificity and precise temporal and spatial coordination of channel activation is crucial for understanding the molecular basis of agonist-mediated stimulation of Ca(2+) entry and identifying downstream physiological functions. This review will address the assembly and localization of TRPC channels and how these processes impact their function.

Evidence that TRPC1 contributes to calcium-induced differentiation of human keratinocytes

External calcium ion concentration is a major regulator of epidermal keratinocyte differentiation in vitro and probably also in vivo. Regulation of calcium-induced differentiation changes is proposed to occur via an external calcium-sensing, signaling pathway that utilizes increases in intracellular calcium ion concentration to activate differentiation-related gene expression. Calcium ion release from intracellular stores and calcium ion influx via store-operated calcium-permeable channels are key elements in this proposed signaling pathway; however, the channels involved have not yet been identified. The present report shows that human gingival keratinocytes (HGKs) also undergo calcium-induced differentiation in vitro as indicated by involucrin expression and morphological changes. Moreover, TRPC1, which functions as a store-operated calcium channel in a number of cell types, including epidermal keratinocytes, is expressed in both proliferating and differentiating HGKs. Transfection of HGKs with TRPC1 siRNA disrupted expression of TRPC1 mRNA and protein compared with transfection with scrambled TRPC1 siRNA. Cells with disrupted TRPC1 expression showed decreased calcium-induced differentiation as measured by involucrin expression or morphological changes, as well as decreased thapsigargin-induced calcium ion influx, and a decreased rate of store calcium release. These results indicate that TRPC1 is involved in calcium-induced differentiation of HGKs likely by supporting a store-operated calcium ion influx.

Induction of intracellular calcium elevation by Δ9-tetrahydrocannabinol in T cells involves TRPC1 channels

We have reported previously that Delta9-tetrahydrocannabinol (Delta9-THC) treatment of resting human and murine splenic T cells robustly elevated intracellular calcium ([Ca2+]i). The objective of the present investigation was to examine the putative role of [Ca2+]i store depletion and store-operated calcium (SOC) and receptor-operated cation (ROC) channels in the mechanism by which Delta9-THC increases [Ca2+]i in the cannabinoid-2 receptor-expressing human peripheral blood-acute lymphoid leukemia (HPB-ALL) human T cell line. By using the smooth endoplasmic reticulum Ca2+-ATPase pump inhibitor, thapsigargin, and the ryanodine receptor antagonist, 8-bromo-cyclic adenosine diphosphate ribose, we demonstrate that the Delta9-THC-mediated elevation in [Ca2+]i occurs independently of [Ca2+]i store depletion. Furthermore, the ROC channel inhibitor, SK&F 96365 was more efficacious at attenuating the Delta9-THC-mediated elevation in [Ca2+]i than SOC channel inhibitors, 2-aminoethoxydiphenyl borate and La3+. Recently, several members of the transient receptor potential canonical (TRPC) channel subfamily have been suggested to operate as SOC or ROC channels. In the present studies, treatment of HPB-ALL cells with 1-oleoyl-2-acetyl-sn-glycerol (OAG), a cell-permeant analog of diacylglycerol (DAG), which gates several members of the TRPC channel subfamily, rapidly elevated [Ca2+]i, as well as prevented a subsequent, additive elevation in [Ca2+]i by Delta9-THC, independent of protein kinase C. Reverse transcriptase-polymerase chain reaction analysis for TRPC1-7 showed that HPB-ALL cells express detectable mRNA levels of only TRPC1. Finally, small interference RNA knockdown of TRPC1 attenuated the Delta9-THC-mediated elevation of [Ca2+]i. Collectively, these results suggest that Delta9-THC-induced elevation in [Ca2+]i is attributable entirely to extracellular calcium influx, which is independent of [Ca2+]i store depletion, and is mediated, at least partially, through the DAG-sensitive TRPC1 channels.

Transient receptor potential protein subunit assembly and membrane distribution in human platelets

We have previously suggested that the human homologue of the Drosophila transient receptor potential protein, TRPC1, is involved in conducting store-operated Ca2+ entry (SOCE) in human platelets since an antibody raised against the pore-forming region of TRPC1 inhibited SOCE. Here we have investigated plasma membrane expression of TRPC1 in human platelets and have probed for the presence of other TRPC proteins in these cells. Biotinylation revealed the presence of TRPC1 in the plasma membrane of resting platelets. Surface expression was not detectibly changed following Ca2+ store depletion or stimulation with thrombin. Western blotting demonstrated the presence of TRPC1, TRPC3, TRPC4, TRPC5 and TRPC6 in platelet lysates. TRPC1, TRPC4 and TRPC5 coimmunoprecipitated, as did TRPC3 and TRPC6. TRPC1, TRPC4 and TRPC5 were associated with detergent-resistant platelet membranes, from which they were partially released when the cells were cholesterol-depleted using methyl-beta-cyclodextrin. The distributions of TRPC3 and TRPC6 between soluble and membrane fractions were not affected by methyl-beta-cyclodextrin treatment. These results suggest that TRPC1, TRPC4 and TRPC5 form a heteromultimer associated with platelet lipid raft domains, whereas TRPC3 and TRPC6 associate independently of lipid rafts.

The role of canonical transient receptor potential 7 in B-cell receptor-activated channels

Phospholipase C signaling stimulates Ca2+ entry across the plasma membrane through multiple mechanisms. Ca2+ store depletion stimulates store-operated Ca2+-selective channels, or alternatively, other phospholipase C-dependent events activate Ca2+-permeable non-selective cation channels. Transient receptor potential 7 (TRPC7) is a non-selective cation channel that can be activated by both mechanisms when ectopically expressed, but the regulation of native TRPC7 channels is not known. We knocked out TRPC7 in DT40 B-cells, which expresses both forms of Ca2+ entry. No difference in the store-operated current I(crac) was detected between TRPC7-/- and wild-type cells. Wild-type cells demonstrated nonstore-operated cation entry and currents in response to activation of the B-cell receptor or protease-activated receptor 2, intracellular dialysis with GTPgammaS, or application of the synthetic diacylglycerol oleyl-acetyl-glycerol. These responses were absent in TRPC7-/- cells but could be restored by transfection with human TRPC7. In conclusion, in B-lymphocytes, TRPC7 appeared to participate in the formation of ion channels that could be activated by phospholipase C-linked receptors. This represents the first demonstration of a physiological function for endogenous TRPC7 channels.

Physiological mechanisms of TRPC activation

TRPC (canonical transient receptor potential) channels are vertebrate homologs of the Drosophila photoreceptor channel, TRP. Considerable research has been brought to bear on the seven members of this family, especially with regard to their possible role in calcium entry. Unfortunately, the current literature presents a confusing picture, with different laboratories producing widely differing results and interpretations. It appears that ectopically expressed TRPC channels can be activated by phospholipase C products (generally, diacylglycerols), by stimulation of trafficking to the plasma membrane, or by depletion of intracellular Ca(2+) stores. Here, I discuss the possibility that these diverse experimental findings arise because TRPC channels can, under both experimental as well as physiological conditions, be activated in three distinct ways, possibly depending on their subunit composition and/or signaling complex environment. The TRPCs may be unique among ion-channel subunit families in being able to participate in the assembly and function of multiple types of physiologically important ion channels.

Ion channel clustering enhances weak electric field detection by neutrophils: apparent roles of SKF96365-sensitive cation channels and myeloperoxidase trafficking in cellular responses

We have tested Galvanovskis and Sandblom's prediction that ion channel clustering enhances weak electric field detection by cells as well as how the elicited signals couple to metabolic alterations. Electric field application was timed to coincide with certain known intracellular chemical oscillators (phase-matched conditions). Polarized, but not spherical, neutrophils labeled with anti-K(v)1.3, FL-DHP, and anti-TRP1, but not anti-T-type Ca(2+) channels, displayed clusters at the lamellipodium. Resonance energy transfer experiments showed that these channel pairs were in close proximity. Dose-field sensitivity studies of channel blockers suggested that K(+) and Ca(2+) channels participate in field detection, as judged by enhanced oscillatory NAD(P)H amplitudes. Further studies suggested that K(+) channel blockers act by reducing the neutrophil's membrane potential. Mibefradil and SKF93635, which block T-type Ca(2+) channels and SOCs, respectively, affected field detection at appropriate doses. Microfluorometry and high-speed imaging of indo-1-labeled neutrophils was used to examine Ca(2+) signaling. Electric fields enhanced Ca(2+) spike amplitude and triggered formation of a second traveling Ca(2+) wave. Mibefradil blocked Ca(2+) spikes and waves. Although 10 microM SKF96365 mimicked mibefradil, 7 microM SKF96365 specifically inhibited electric field-induced Ca(2+) signals, suggesting that one SKF96365-senstive site is influenced by electric fields. Although cells remained morphologically polarized, ion channel clusters at the lamellipodium and electric field sensitivity were inhibited by methyl-beta-cyclodextrin. As a result of phase-matched electric field application in the presence of ion channel clusters, myeloperoxidase (MPO) was found to traffic to the cell surface. As MPO participates in high amplitude metabolic oscillations, this suggests a link between the signaling apparatus and metabolic changes. Furthermore, electric field effects could be blocked by MPO inhibition or removal while certain electric field effects were mimicked by the addition of MPO to untreated cells. Therefore, channel clustering plays an important role in electric field detection and downstream responses of morphologically polarized neutrophils. In addition to providing new mechanistic insights concerning electric field interactions with cells, our work suggests novel methods to remotely manipulate physiological pathways.

Functional role of TRPC proteins in native systems: implications from knockout and knock-down studies

Available data on transient receptor potential channel (TRPC) protein functions indicate that these proteins represent essential constituents of agonist-activated and phospholipase C-dependent cation entry pathways in primary cells which contribute to the elevation of cytosolic Ca2+. In addition, a striking number of biological functions have already been assigned to the various TRPC proteins, including mechanosensing activity (TRPC1), chemotropic axon guidance (TRPC1 and TRPC3), pheromone sensing and the regulation of sexual and social behaviour (TRPC2), endothelial-dependent regulation of vascular tone, endothelial permeability and neurotransmitter release (TRPC4), axonal growth (TRPC5), modulation of smooth muscle tone in blood vessels and lung and regulation of podocyte structure and function in the kidney (TRPC6). The lack of compounds which specifically block or activate TRPC proteins impairs the analysis of TRPC function in primary cells. We therefore concentrate in this contribution on (i) studies of TRPC-deficient mouse lines, (ii) data obtained by gene-silencing approaches using antisense oligonucleotides or RNA interference, (iii) expression experiments employing dominant negative TRPC constructs, and (iv) recent data correlating mutations of TRPC genes associated with human disease.

TRPC1: store-operated channel and more

Transient receptor potential canonical 1 (TRPC1) is a transmembrane protein expressed in a range of vertebrate cells including smooth muscle, endothelium, neurones and salivary gland cells. It functions as an element of a mixed cationic Ca(2+)-permeable channel, probably commonly as part of a heterotetrameric assembly involving other related proteins such as TRPC5. Wide-ranging biological roles of TRPC1 are suggested, including regulation of smooth muscle and stem cell proliferation, endothelin-evoked arterial contraction, salivary gland secretion, endothelial permeability, glutamatergic neurotransmission, growth cone turning, neuroprotection, neuronal differentiation, lipid raft integrity and the nuclear factor of activated T-cell transcription factor. The mechanisms by which TRPC1 serves these functions are starting to emerge. At one level, it is apparent that TRPC1 is subcellularly compartmentalised, at least in part in cholesterol-rich caveolae closely associated with sub-plasmalemmal endoplasmic reticulum. At another level, TRPC1 is embedded in a protein complex that can include inositol trisphosphate receptor, homer, calmodulin, caveolin-1, FKBP25, I-mfa, MxA, GluR1alpha, bFGFR-1, G(q/11) protein, phospholipase C-beta/gamma, protein kinase C-alpha and RhoA. It is also apparent that TRPC1 responds to general stimuli-not only depletion of intracellular Ca(2+) stores, but also receptor activation, and membrane stretch. We are at the early stages of understanding of how these various signals and components integrate to form a functional channel, and this article provides a brief overview of current progress.

Multiple roles of calmodulin and other Ca(2+)-binding proteins in the functional regulation of TRP channels

Transient receptor potential channels (TRP) have emerged as cellular sensors of various internal and external cues. Generally, the activation of TRP canonical (TRPC) channels is triggered by the stimulation of phospholipase C; however, multiple factors are involved in the regulation of these channels. Among them, Ca(2+)-mediated feedback channel modulations are often mediated by calmodulin (CaM) and other Ca(2+)-binding proteins. In vitro binding studies have revealed multiple CaM-binding sites on TRPC proteins. Among them, a common CaM/inositol 1,4,5-trisphosphate receptor-binding site is found at the carboxyl terminus of every TRPC isoform. Additional non-conserved CaM-binding sites are present at the amino and carboxyl termini of several TRPC proteins. Likewise, multiple CaM-binding sites were found in other TRP proteins. These, together with the presence in close vicinity of the interaction sites for the related neuronal Ca(2+)-binding proteins, such as CaBP1, suggest a multitude of diverse intracellular Ca(2+)-dependent regulations of TRP channels. Functional studies have begun to reveal the unique roles of CaM and CaBP1 binding to several TRP channels. This review will focus on the CaM- and CaBP1-mediated regulations of TRPC channels. Related studies on TRPM and TRPV channels will also be highlighted.

2-Aminoethoxydiphenyl borate perturbs hormone-sensitive calcium stores and blocks store-operated calcium influx pathways independent of cytoskeletal disruption in human A549 lung cancer cells

Recent studies have identified novel actions for 2-aminoethoxydiphenyl borate (2-APB) in triggering calcium release and enhancing calcium influx induced by the depletion of intracellular calcium stores. In this study, we have examined the effects of 2-APB on the human lung adenocarcinoma A549 cell line, which we have previously shown displays a unique calcium influx response, when ER calcium stores are depleted by thapsigargin (TG) treatment. Here, we show that low concentrations of 2-APB failed to induce the rapid augmentation of TG-activated calcium influx previously reported for other cell types. We observed that store-operated calcium (SOC) channels in the A549 cell line exhibited short-term sensitivity to low doses of 2-APB, perhaps reflecting a delayed augmentation of SOC channel activity or the recruitment of 2-APB-insensitive SOC channels. In both intact and permeabilized cells, 2-APB effectively discharged a subset of A549 calcium pools corresponding to the hormone-sensitive intracellular calcium stores. The 2-APB-induced calcium release produced a long-lasting perturbation of the adenosine triphosphate (ATP)-releasable calcium pools, effectively uncoupling ATP-activated calcium release even, when stores are replenished with calcium. In contrast to previous reports, we found that disruption of either the actin or microtubule-based cytoskeleton failed to block the 2-APB-induced effects on calcium signaling in A549 cells. Our study describes novel cytoskeletal-independent effects of 2-APB on Ca2+-signaling pathways, revealing differentially sensitive Ca2+-influx pathways and long-term perturbation of hormone-sensitive Ca2+ stores.

STIM1, an essential and conserved component of store-operated Ca2+ channel function

Store-operated Ca2+ (SOC) channels regulate many cellular processes, but the underlying molecular components are not well defined. Using an RNA interference (RNAi)-based screen to identify genes that alter thapsigargin (TG)-dependent Ca2+ entry, we discovered a required and conserved role of Stim in SOC influx. RNAi-mediated knockdown of Stim in Drosophila S2 cells significantly reduced TG-dependent Ca2+ entry. Patch-clamp recording revealed nearly complete suppression of the Drosophila Ca2+ release-activated Ca2+ (CRAC) current that has biophysical characteristics similar to CRAC current in human T cells. Similarly, knockdown of the human homologue STIM1 significantly reduced CRAC channel activity in Jurkat T cells. RNAi-mediated knockdown of STIM1 inhibited TG- or agonist-dependent Ca2+ entry in HEK293 or SH-SY5Y cells. Conversely, overexpression of STIM1 in HEK293 cells modestly enhanced TG-induced Ca2+ entry. We propose that STIM1, a ubiquitously expressed protein that is conserved from Drosophila to mammalian cells, plays an essential role in SOC influx and may be a common component of SOC and CRAC channels.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

Phospholipase cgamma1 is required for activation of store-operated channels in human keratinocytes

Store-operated calcium entry depicts the movement of extracellular Ca2+ into cells through plasma membrane Ca2+ channels activated by depletion of intracellular Ca2+ stores. The members of the canonical subfamily of transient receptor potential channels (TRPC) have been implicated as the molecular bases for store-operated channels (SOC). Here we investigate the role of phospholipase C (PLC) in regulation of native SOC and the expression of endogenous TRPC in human epidermal keratinocytes. Calcium entry in response to store depletion with thapsigargin was reversibly blocked by 2-aminoethoxydiphenyl borane, an effective SOC inhibitor, and suppressed by the diacylglycerol analoge, 1-oleoyl-2-acetyl-sn-glycerol. Inhibition of PLC with U73122 or transfection of a PLCgamma1 antisense cDNA construct completely blocked SOC activity, indicating a requirement for PLC, especially PLCgamma1, in the activation of SOC. RT-PCR and immunoblotting analyses showed that TRPC1, TRPC3, TRPC4, TRPC5, and TRPC6 are expressed in keratinocytes. Knockdown of the level of endogenous TRPC1 or TRPC4 inhibited store-operated calcium entry, indicating they are part of the native SOC. Co-immunoprecipitation studies demonstrated that TRPC1, but not TRPC4, interacts with PLCgamma1 and the inositol 1,4,5-trisphosphate receptor (IP3R). The association of TRPC1 with PLCgamma1 and IP3R decreased in keratinocytes with higher intracellular Ca2+, coinciding with a downregulation in SOC activity. Our results indicate that the activation of SOC in keratinocytes depends, at least partly, on the interaction of TRPC with PLCgamma1 and IP3R.

The mammalian TRPC cation channels

Transient Receptor Potential-Canonical (TRPC) channels are mammalian homologs of Transient Receptor Potential (TRP), a Ca(2+)-permeable channel involved in the phospholipase C-regulated photoreceptor activation mechanism in Drosophila. The seven mammalian TRPCs constitute a family of channels which have been proposed to function as store-operated as well as second messenger-operated channels in a variety of cell types. TRPC channels, together with other more distantly related channel families, make up the larger TRP channel superfamily. This review summarizes recent findings on the structure, regulation and function of the apparently ubiquitous TRPC cation channels.

Distinct Calcium Channels Regulate Responses of Primary B Lymphocytes to B Cell Receptor Engagement and Mechanical Stimuli

Intracellular Ca(2+) plays a central role in controlling lymphocyte function. Nonetheless, critical gaps remain in our understanding of the mechanisms that regulate its concentration. Although Ca(2+)-release-activated calcium (CRAC) channels are the primary Ca(2+) entry pathways in T cells, additional pathways appear to be operative in B cells. Our efforts to delineate these pathways in primary murine B cells reveal that Ca(2+)-permeant nonselective cation channels (NSCCs) operate in a cooperative fashion with CRAC. Interestingly, these non-CRAC channels are selectively activated by mechanical stress, although the mechanism overlaps with BCR-activated pathways, suggesting that they may operate in concert to produce functionally diverse Ca(2+) signals. NSCCs also regulate the membrane potential, which activates integrin-dependent binding of B cells to extracellular matrix elements involved in their trafficking and localization within secondary lymphoid organs. Thus, CRAC and distinct Ca(2+) permeant NSCCs are differentially activated by the BCR and mechanical stimuli and regulate distinct aspects of B cell physiology.

Grb2 and the Non-T Cell Activation Linker NTAL Constitute a Ca2+-Regulating Signal Circuit in B Lymphocytes

Activation of the B cell antigen receptor triggers phosphorylation of cytoplasmic and transmembrane adaptor proteins such as SLP-65 and NTAL, respectively. Specific phosphoacceptor sites in SLP-65 serve as docking sites for Ca(2+)-mobilizing enzymes Btk and PLC-gamma2. Phosphorylated NTAL recruits the Grb2 linker, but downstream signaling cascades are unclear. We now show that receptor-induced tyrosine phosphorylation of NTAL and concomitant Grb2 complex formation critically modulate the Ca(2+) response without affecting SLP-65 and PLC-gamma2 phosphorylation. Grb2 turned out to play a negative regulatory role, which appears to be eliminated upon binding to NTAL. This allows for a sustained release of intracellular Ca(2+) and is mandatory for subsequent entry of Ca(2+) from extracellular sources. Thus, elevation of Ca(2+) is regulated by at least two signaling modules, the B cell-specific Ca(2+) initiation complex comprising SLP-65, Btk, and PLC-gamma2 and the more ubiquitously expressed NTAL/Grb2 complex, which acts as an amplifier by switching off inhibitory elements.

A store-operated calcium channel in Drosophila S2 cells

Using whole-cell recording in Drosophila S2 cells, we characterized a Ca(2+)-selective current that is activated by depletion of intracellular Ca2+ stores. Passive store depletion with a Ca(2+)-free pipette solution containing 12 mM BAPTA activated an inwardly rectifying Ca2+ current with a reversal potential >60 mV. Inward currents developed with a delay and reached a maximum of 20-50 pA at -110 mV. This current doubled in amplitude upon increasing external Ca2+ from 2 to 20 mM and was not affected by substitution of choline for Na+. A pipette solution containing approximately 300 nM free Ca2+ and 10 mM EGTA prevented spontaneous activation, but Ca2+ current activated promptly upon application of ionomycin or thapsigargin, or during dialysis with IP3. Isotonic substitution of 20 mM Ca2+ by test divalent cations revealed a selectivity sequence of Ba2+ > Sr2+ > Ca2+ >> Mg2+. Ba2+ and Sr2+ currents inactivated within seconds of exposure to zero-Ca2+ solution at a holding potential of 10 mV. Inactivation of Ba2+ and Sr2+ currents showed recovery during strong hyperpolarizing pulses. Noise analysis provided an estimate of unitary conductance values in 20 mM Ca2+ and Ba2+ of 36 and 420 fS, respectively. Upon removal of all external divalent ions, a transient monovalent current exhibited strong selectivity for Na+ over Cs+. The Ca2+ current was completely and reversibly blocked by Gd3+, with an IC50 value of approximately 50 nM, and was also blocked by 20 microM SKF 96365 and by 20 microM 2-APB. At concentrations between 5 and 14 microM, application of 2-APB increased the magnitude of Ca2+ currents. We conclude that S2 cells express store-operated Ca2+ channels with many of the same biophysical characteristics as CRAC channels in mammalian cells.

Glomerular structural and functional changes in a high-fat diet mouse model of early-stage Type 2 diabetes

AIMS/HYPOTHESIS

Type 2 diabetes often results in diabetic nephropathy, which is preceded by an elevated glomerular filtration rate (GFR). This study was designed to develop a mouse model of Type 2 diabetes and to elucidate the glomerular events in the early stages of diabetic nephropathy.

METHODS

Four-week-old mice were fed a normal or high-fat (42% of total calories from fat) diet, and body weight, blood glucose, insulin, leptin, lipids and GFR were monitored from 9 to 21 weeks or longer after the feeding programme. Mesangial cell dedifferentiation was accessed by alpha-smooth muscle actin staining. Glomerular hypertrophy was determined using image analysis with haematoxylin-eosin staining. Matrix deposition was determined by type IV collagen staining.

RESULTS

After 9 weeks, mice fed a high-fat diet weighed more than mice fed a normal diet (30.5+/-1.2 vs 22.3+/-0.5 g, p<0.05), and mice fed a high-fat diet were hyperinsulinaemic (283.9+/-69.7 vs 102.9+/-36.4 pmol/l, p<0.05), hyperglycaemic (8.0+/-0.6 vs 6.5+/-0.2 mmol/l, p<0.05) and their leptin levels were increased six-fold (1.48+/-0.45 vs 0.25+/-0.03 ng/ml, p<0.05). After 13 weeks, mice fed a high-fat diet showed hyperfiltration (GFR; 440+/-60 vs 210+/-10 microl/min, p<0.05). During the early stages of diabetic nephropathy, mesangial cell dedifferentiation was evident, shown by increased expression of alpha-smooth muscle actin in the glomeruli. After 9 weeks, mice fed a high-fat diet already demonstrated increased type IV collagen deposition. After 13 weeks, they developed enlarged glomerular tufts compared with those of their age-matched controls.

CONCLUSIONS/INTERPRETATION

The results of this study suggest that collagen IV deposition precedes the hyperfiltration and enlargement of glomeruli in early-stage diabetic nephropathy. Dedifferentiation of mesangial cells may be associated with collagen IV deposition.

Association of Immunophilins with Mammalian TRPC Channels

Drosophila photoreceptor channels TRP and TRPL are held in a large signalplex by the scaffolding protein, INAD. Immunophilin FKBP59, another member of the signalplex, binds to both INAD and TRPL. Mutation P702Q or P709Q in the highly conserved TRPL sequence (701)LPPPFNVLP(709), eliminates TRPL interaction with FKBP59. The first leucylprolyl (LP) dipeptide in this region is conserved in mammalian TRPC channel proteins. However, the second LP is changed to isoleucylprolyl (IP) in TRPC1, -C4, and -C5, and valylprolyl (VP) in TRPC3, -C6, and -C7. The purpose of the present study was to determine if mammalian FKBP12 or FKBP52 interact with TRPC channel proteins. Using TRPC-specific antibodies, immunoprecipitations from Sf9 cells individually co-expressing each of the TRPC proteins along with the immunophilins showed that TRPC3, -C6, and -C7 interact with FKBP12, whereas TRPC1, -C4, and -C5 interact with FKBP52. The binding of FKBP12 and FKBP52 was specific and could be displaced by the immunosuppressant drug FK506, at concentrations of 0.5 and 10 microm, respectively. To evaluate TRPC-immunophilin interactions in vivo, immunoprecipitations were performed using membrane lysates of rat cerebral cortex. FKBP12 co-immunoprecipitated with TRPC3, -C6, and -C7 from rat brain, whereas FKBP52 was found to associate with TRPC1, -C4, and -C5. The association of immunophilins with the TRPC channels in rat brain lysates could be displaced by FK506. Receptor-mediated activation of TRPC6, stably expressed in HEK cells, was significantly inhibited by FK506, which also disrupted interaction between TRPC6 and the endogenous immunophilin found in HEK cells. Pro to Gln mutations in the first LP dipeptide in the putative FKBP binding domain eliminated FKBP12 and FKBP52 interaction with TRPC3 and -C6, and TRPC1 and -C4, respectively. However, mutual swap of VP and IP in TRPC3 and TRPC5 did not alter the association or the selectivity of the channels for their respective immunophilin binding partner. These results suggest that immunophilins are TRPC channel accessory proteins that play an important role in the mechanism of channel activation following receptor stimulation.

Cytoskeletal reorganization internalizes multiple transient receptor potential channels and blocks calcium entry into human neutrophils

Store-operated calcium entry (SOCE) is required for polymorphonuclear neutrophil (PMN) activation in response to G protein-coupled agonists. Some immunocytes express proteins homologous to the Drosophila transient receptor potential gene (trp) calcium channel. TRP proteins assemble into heterotetrameric ion channels and are known to support SOCE in overexpression systems, but the evidence that TRP proteins support SOCE and are functionally important in wild-type cells remains indirect. We therefore studied the expression and function of TRP proteins in primary human PMN. TRPC1, TRPC3, TRPC4, and TRPC6 were all expressed as mRNA as well as membrane proteins. Immunofluorescence microscopy demonstrated localization of TRPC1, TRPC3, and TRPC4 to the PMN cell membrane and their internalization after cytoskeletal reorganization by calyculin A (CalyA). Either TRPC internalization by CalyA or treatment with the inositol triphosphate receptor inhibitor 2-aminoethoxydiphenyl borane resulted in the loss of PMN SOCE. Cytochalasin D (CytoD) disrupts actin filaments, thus preventing cytoskeletal reorganization, and pretreatment with CytoD rescued PMN SOCE from inhibition by CalyA. Comparative studies of CytoD and 2-aminoethoxydiphenyl borane inhibition of PMN cationic entry after thapsigargin or platelet-activating factor suggested that SOCE occurs through both calcium-specific and nonspecific pathways. Taken together, these studies suggest that the multiple TRPC proteins expressed by human PMN participate in the formation of at least two store-operated calcium channels that have differing ionic permeabilities and regulatory characteristics.

Novel Real-Time Sensors to Quantitatively Assess In Vivo Inositol 1,4,5-Trisphosphate Production in Intact Cells

Real-time observation of messenger molecules in individual intact cells is essential for physiological studies of signaling mechanisms. We have developed a novel inositol 1,4,5-trisphosphate (IP(3)) sensor based on the pleckstrin homology (PH) domain from phospholipase C (PLC) delta. The environmentally sensitive fluorophore 6-bromoacetyl-2-dimethyl-aminonaphtalene was conjugated to the genetically introduced cysteine at the mouth of the IP(3) binding pocket for enhanced IP(3) selectivity and for rapid and direct visualization of intracellular IP(3) > or = 0.5 microM as fluorescence emission decreased. The probe, tagged with arginine-rich sequences for efficient translocation into various cell types, revealed a major contribution of Ca2+ influx to PLC-mediated IP(3) production that boosts Ca2+ release from endoplasmic reticulum. Thus, our IP(3) probe was extremely effective to quantitatively assess real-time physiological IP(3) production via those pathways formed only in the intact cellular configuration.

Deficiency of polycystin‐2 reduces Ca2+channel activity and cell proliferation in ADPKD lymphoblastoid cells

Polycystin-2 (PC2), encoded by the PKD2 gene, mutated in 10-15% of autosomal-dominant polycystic kidney disease (ADPKD) patients, is a Ca2+-permeable cation channel present in kidney epithelia and other tissues. As PC2 was found expressed in B-lymphoblastoid cells (LCLs) and Ca2+ signaling pathways are important regulators of B cell function activities, we investigated whether PC2 plays some role in B-LCLs. In LCLs, PC2 was found mainly in ER membranes but ~8 times less than in kidney HEK293 cells. The same reductions were found in PKD2 and PKD1 RNA; thus, PKD genes maintained, in LCLs, the same reciprocal proportion as they do in kidney cells. In LCLs obtained from subjects carrying PKD2 mutations (PKD2-LCLs) and showing reduced PC2 levels, intracellular Ca2+ concentrations evoked by platelet-activating factor (PAF), were significantly lower than in non-PKD-LCLs. This reduction was also found in PKD1-LCLs but without PC2 reductions. Likewise, cell proliferation, which is controlled by Ca2+, was reduced in PKD2- and PKD1-LCLs. Moreover, in LCLs with PKD2 nonsense mutations, aminoglycoside antibiotics reduced the PC2 defect by promoting readthrough of stop codons. Therefore, PC2 and PC1 are functionally expressed in LCLs, which provide a model, easily obtainable from ADPKD patients, to study PKD gene expression and function.

Effect of flow and stretch on the [Ca2+]i response of principal and intercalated cells in cortical collecting duct

An acute increase in tubular fluid flow rate in the microperfused cortical collecting duct (CCD), associated with a approximately 20% increase in tubular diameter, leads to an increase in intracellular Ca2+ concentration ([Ca2+]i)in both principal and intercalated cells (Woda CB, Leite M Jr, Rohatgi R, and Satlin LM. Am J Physiol Renal Physiol 283: F437-F446, 2002). The apical cilium present in principal but not intercalated cells has been proposed to be a flow sensor. To determine whether flow across the cilium and/or epithelial stretch mediates the [Ca2+]i response, CCDs from New Zealand White rabbits were microperfused in vitro, split-open (to isolate the effect of flow across cilia), or occluded (to examine the effect of stretch and duration/magnitude of the flow impulse), and [Ca2+]i was measured using fura 2. In perfused and occluded CCDs, a rapid (<1 s) but not slow (>3 min) increase in luminal flow rate and/or circumferential stretch led to an approximately threefold increase in [Ca2+]i in both principal and intercalated cells within approximately 10 s. This response was mediated by external Ca2+ entry and inositol 1,4,5-trisphosphate-mediated release of cell Ca2+ stores. In split-open CCDs, an increase in superfusate flow led to an approximately twofold increase in [Ca2+]i in both cell types within approximately 30 s. These experimental findings are interpreted using mathematical models to predict the fluid stress on the apical membranes of the CCD and the forces and torques on and deformation of the cilia. We conclude that rapid increases in luminal flow rate and circumferential stretch, leading to shear or hydrodynamic impulses at the cilium or apical membrane, lead to increases in [Ca2+]i in both principal and intercalated cells.

Store-operated cation entry mediated by CD20 in membrane rafts

B cell activation requires sustained elevation of cytoplasmic free calcium, achieved by influx through store-operated calcium (SOC) channels. The molecular identity of these channels is not known. Ectopic expression of the raft-associated tetraspan protein CD20 in Chinese hamster ovary cells introduced a novel SOC entry pathway that was permeable to strontium as well as to calcium. The activity of this SOC pathway was abolished by deletion of a cytoplasmic sequence in CD20 essential for its efficient raft localization. Strontium-permeable SOC channels were detected in B cells, and B cell receptor-stimulated influx was significantly reduced by downregulation of CD20 expression using short interfering RNA and also by cholesterol depletion. This is the first evidence that raft-associated CD20 constitutes a component of a SOC entry pathway activated by the B cell receptor.

Inhibitor of myogenic family, a novel suppressor of store-operated currents through an interaction with TRPC1

Depletion of intracellular Ca2+ stores leads to the activation of Ca2+ inflow through store-operated Ca2+ channels. Although the identity of these channels is unknown, there is considerable evidence that the transient receptor potential channel 1 (TRPC1) participates in the formation of these channels. We show that TRPC1 physically interacts with the a-isoform of the inhibitor of the myogenic family (I-mfa), a known inhibitor of basic helix-loop-helix transcription factors, in vitro and in vivo. The interaction is mediated by the C-terminal cytoplasmic tail of TRPC1 and the C-terminal cysteine-rich domain of I-mfa. Using the whole cell configuration of the patch clamp technique, we show that ectopic expression of I-mfa in CHO-K1 cells reduces native store-activated Ca2+ currents, whereas knock-down of endogenous I-mfa in A431 cells by RNA interference enhances these currents. Pipette perfusion of purified recombinant I-mfa rescues the effect of I-mfa knock-down on store-operated conductance. Finally, cell dialysis with a monoclonal antibody specific to TRPC1 results in the suppression of store-activated conductance in cells lacking I-mfa, but not in I-mfa expressing cells. We propose that I-mfa functions as a molecular switch to suppress the store dependence of TRPC1.

Capacitative calcium entry in the nervous system

Capacitative calcium entry is a process whereby the depletion of Ca(2+) from intracellular stores (likely endoplasmic or sarcoplasmic reticulum) activates plasma membrane Ca(2+) channels. Current research has focused on identification of capacitative calcium entry channels and the mechanism by which Ca(2+) store depletion activates the channels. Leading candidates for the channels are members of the transient receptor potential (TRP) superfamily, although no single gene or gene product has been definitively proven to mediate capacitative calcium entry. The mechanism for activation of the channels is not known; proposals fall into two general categories, either a diffusible signal released from the Ca(2+) stores when their Ca(2+) levels become depleted, or a more direct protein-protein interaction between constituents of the endoplasmic reticulum and the plasma membrane channels. Capacitative calcium entry is a major mechanism for regulated Ca(2+) influx in non-excitable cells, but recent research has indicated that this pathway plays an important role in the function of neuronal cells, and may be important in a number of neuropathological conditions. This review will summarize some of these more recent findings regarding the role of capacitative calcium entry in normal and pathological processes in the nervous system.

Amplification of receptor signalling by Ca2+ entry-mediated translocation and activation of PLCgamma2 in B lymphocytes

In non-excitable cells, receptor-activated Ca2+ signalling comprises initial transient responses followed by a Ca2+ entry-dependent sustained and/or oscillatory phase. Here, we describe the molecular mechanism underlying the second phase linked to signal amplification. An in vivo inositol 1,4,5-trisphosphate (IP3) sensor revealed that in B lymphocytes, receptor-activated and store-operated Ca2+ entry greatly enhanced IP3 production, which terminated in phospholipase Cgamma2 (PLCgamma2)-deficient cells. Association between receptor-activated TRPC3 Ca2+ channels and PLCgamma2, which cooperate in potentiating Ca2+ responses, was demonstrated by co-immunoprecipitation. PLCgamma2-deficient cells displayed diminished Ca2+ entry-induced Ca2+ responses. However, this defect was canceled by suppressing IP3-induced Ca2+ release, implying that IP3 and IP3 receptors mediate the second Ca2+ phase. Furthermore, confocal visualization of PLCgamma2 mutants demonstrated that Ca2+ entry evoked a C2 domain-mediated PLCgamma2 translocation towards the plasma membrane in a lipase-independent manner to activate PLCgamma2. Strikingly, Ca2+ entry-activated PLCgamma2 maintained Ca2+ oscillation and extracellular signal-regulated kinase activation downstream of protein kinase C. We suggest that coupling of Ca2+ entry with PLCgamma2 translocation and activation controls the amplification and co-ordination of receptor signalling.

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.

Caveolin-1 contributes to assembly of store-operated Ca2+ influx channels by regulating plasma membrane localization of TRPC1

TRPC1, a component of store-operated Ca2+ entry (SOCE) channels, is assembled in a complex with caveolin-1 (Cav1) and key Ca2+ signaling proteins. This study examines the role of Cav1 in the function of TRPC1. TRPC1 and Cav1 were colocalized in the plasma membrane region of human submandibular gland and Madin-Darby canine kidney cells. Full-length Cav1 bound to both the N and C termini of TRPC1. Amino acids 271-349, which includes a Cav1 binding motif (amino acids 322-349), was identified as the Cav1 binding domain in the TRPC1 N terminus. Deletion of amino acids 271-349 or 322-349 prevented plasma membrane localization of TRPC1. Importantly, TRPC1Delta271-349 induced a dominant suppression of SOCE and was associated with wild-type TRPC1. Although the role of the C-terminal Cav1 binding domain is not known, its deletion did not affect localization of TRPC1 (Singh, B. B., Liu, X., and Ambudkar, I. S. (2000) J. Biol. Chem. 275, 36483-36486). Further, expression of a truncated Cav1 (Cav1Delta51-169), but not full-length Cav1, similarly disrupted plasma membrane localization of endogenously and exogenously expressed TRPC1 in human submandibular gland and Madin-Darby canine kidney cells. Cav1Delta51-169 also suppressed thapsigarginand carbachol-stimulated Ca2+ influx and increased the detergent solubility of TRPC1, although plasma membrane lipid raft domains were not disrupted. These data demonstrate that plasma membrane localization of TRPC1 depends on an interaction between its N terminus and Cav1. Thus, our data suggest that Cav1 has an important role in the assembly of SOCE channel(s).

Evidence that TRPC1 (transient receptor potential canonical 1) forms a Ca(2+)-permeable channel linked to the regulation of cell volume in liver cells obtained using small interfering RNA targeted against TRPC1

The TRPC1 (transient receptor potential canonical 1) protein, which is thought to encode a non-selective cation channel activated by store depletion and/or an intracellular messenger, is expressed in a number of non-excitable cells. However, the physiological functions of TRPC1 are not well understood. The aim of these studies was to investigate the function of TRPC1 in liver cells using small interfering RNA (siRNA) to ablate the TRPC1 protein. Treatment of H4-IIE liver cells with siRNA targeted against TRPC1 caused an approx. 50% decrease in expression of the human TRPC1 protein in cells transfected with cDNA encoding human TRPC1, and a 50% decrease in expression of the endogenous TRPC1 protein (assessed by Western blot and immunofluorescence). The decrease in endogenous TRPC1 protein in cells transfected with TRPC1 siRNA was associated with a greater increase in cell volume (compared with the increase observed in control cells) immediately after cells were placed in a hypotonic medium, and an enhanced regulatory cell volume decrease after exposure to hypotonic medium. Treatment with siRNA targeted against TRPC1 also led to a 25% inhibition of thapsigargin-stimulated Ca(2+) inflow, a 40% inhibition of ATP and maitotoxin-stimulated Ca(2+) inflow, and a 50% inhibition of maitotoxin-stimulated Mn(2+) inflow. The idea that, in liver cells, TRPC1 encodes a non-selective cation channel involved directly or indirectly in the regulation of cell volume is consistent with the results obtained.

The store-operated calcium entry pathways in human carcinoma A431 cells: functional properties and activation mechanisms

Activation of phospholipase C (PLC)-mediated signaling pathways in nonexcitable cells causes the release of Ca2+ from intracellular Ca2+ stores and activation of Ca2+ influx across the plasma membrane. Two types of Ca2+ channels, highly Ca2+-selective ICRAC and moderately Ca2+-selective ISOC, support store-operated Ca2+ entry process. In previous patch-clamp experiments with a human carcinoma A431 cell line we described store-operated Imin/ICRACL plasma membrane Ca2+ influx channels. In the present paper we use whole-cell and single-channel recordings to further characterize store-operated Ca2+ influx pathways in A431 cells. We discovered that (a) ICRAC and ISOC are present in A431 cells; (b) ICRAC currents are highly selective for divalent cations and fully activate within 150 s after initiation of Ca2+ store depletion; (c) ISOC currents are moderately selective for divalent cations (PBa/PCs = 14.5) and require at least 300 s for full activation; (d) ICRAC and ISOC currents are activated by PLC-coupled receptor agonists; (e) ISOC currents are supported by Imin/ICRACL channels that display 8.5-10 pS conductance for sodium; (f) ICRAC single channel conductance for sodium is estimated at 0.9 pS by the noise analysis; (g) Imin/ICRACL channels are activated in excised patches by an amino-terminal fragment of InsP3R1 (InsP3R1N); and (h) InsP3 binding to InsP3R1N is necessary for activation of Imin/ICRACL channels. Our findings provide novel information about store-operated Ca2+ influx pathways in A431 cells.

B cells as effectors

B cells act as immune effectors, primarily through antigen-specific clonal expansion and plasma-cell differentiation. B1 (CD5(+)) B cells and marginal zone B cells dominate T-cell independent humoral responses under the molecular control of activated dendritic cells. Helper T cell-regulated B-cell responses draw on follicular B cells as precursors and rely on qualitatively different patterns of immune synapse formation to regulate B-cell fate. These activities culminate in the germinal center reaction, during which somatic hypermutation and antigen-driven selection produce and preserve high-affinity plasma cells with extended longevity and memory B cells as the sensitized precursors for antigen recall.

TRPC1 store-operated cationic channel subunit

TRPC1 is a membrane protein that is highly conserved in mammals, amphibians and birds. It is widely expressed in cells throughout the body including in the heart and nervous system. Amino acid sequence analysis and over-expression studies indicate it is an ion channel that allows the transmembrane flux of small cations including sodium and calcium. In some cell types it is apparent that at least a fraction of TRPC1 exists in the plasma membrane. Inhibition of TRPC1 expression or block by TRPC1-specific antibody leads to attenuation of the plasma membrane calcium influx that occurs in response to depletion of calcium levels in sarcoplasmic or endoplasmic reticulum. TRPC1 would, therefore, seem to be a key subunit of store-operated channels (SOCs). TRPC1 is, nevertheless, unlikely to act alone. There is good evidence that it can heteromultimerise with the related proteins TRPC4, TRPC5 and polycystin-2; a tetrameric arrangement is envisaged, but not demonstrated. Like its relative in Drosophila, TRPC1 looks likely to function in a signalplex, a protein complex including inositol 1,4,5-triphosphate (IP(3)) receptor, plasma membrane calcium-ATPase, caveolin-1 and calmodulin. Its localisation in membranes is punctate and associated with functionally discrete calcium signals. TRPC1's function may not only be linked to SOCs but also to other cellular events including the nuclear translocation of the NFAT transcription factor. There is still much to be learned about this fundamental protein.

Transient receptor potential (TRP) channels as potential drug targets in respiratory disease

Calcium-permeable channels have traditionally been thought of as therapeutic targets in excitable cells. For instance, voltage-operated Ca2+ channels in neurones and smooth muscle cells for neurological and cardiovascular diseases although calcium-permeable channels are also functionally important in electrically non-excitable cells. In the lung, calcium channels play a pivotal role in the activation of all the cell types present, whether resident cells such as airway smooth muscle cells and macrophages or migratory cells such as neutrophils or lymphocytes.Previously, research in this area has been hindered by the lack of obvious molecular identity. More recently, the emergence of the transient receptor potential (TRP) cation family has yielded promising candidates which may underpin the different receptor-operated calcium influx pathways. The challenge now, is to ascribe function to the TRP channels expressed in each cell type as a first step in identifying which TRP channels may be potential drug targets for asthma and chronic obstructive pulmonary disease (COPD) (Fig. 1).

CRAC channels: activation, permeation, and the search for a molecular identity

The Ca2+ release-activated Ca2+ (CRAC) channel is a highly Ca2+-selective store-operated channel that is expressed in T lymphocytes, mast cells, and other hematopoietic cells. In T cells, CRAC channels are essential for generating the prolonged intracellular Ca2+ ([Ca2+](i)) elevation required for the expression of T-cell activation genes. Here we review recent work addressing CRAC channel regulation, pore properties, and the search for CRAC channel genes. Of the current models for CRAC current (I(CRAC)) activation, several new studies argue against a conformational coupling mechanism in which IP(3) receptors communicate store depletion to CRAC channels through direct physical interaction. The study of CRAC channels has been complicated by the fact that they lose activity in the absence of extracellular Ca2+. Attempts to maintain current size by removing intracellular Mg2+ have been found to unmask Mg2+-inhibited cation (MIC/MagNuM/TRPM7) channels, which have been mistaken in several studies for the CRAC channel. Recent studies under conditions that prevent MIC activation reveal that CRAC channels use high-affinity binding of Ca2+ in the pore to achieve high Ca2+ selectivity but have a surprisingly low conductance for both Ca2+ (approximately 10fS) and Na+ (approximately 0.2pS). Pore properties provide a unique fingerprint that provides a stringent test for potential CRAC channel genes and suggest models for the ion selectivity mechanism.

TRPC1 is required for functional store-operated Ca2+ channels. Role of acidic amino acid residues in the S5-S6 region

The exact role of TRPC1 in store-operated calcium influx channel (SOCC) function is not known. We have examined the effect of overexpression of full-length TRPC1, depletion of endogenous TRPC1, and expression of TRPC1 in which the proposed pore region (S5-S6, amino acids (aa) 557-620) was deleted or modified by site-directed mutagenesis on thapsigargin- and carbachol-stimulated SOCC activity in HSG cells. TRPC1 overexpression induced channel activity that was indistinguishable from the endogenous SOCC activity. Transfection with antisense hTRPC1 decreased SOCC activity although characteristics of SOCC-mediated current, I(SOC), were not altered. Expression of TRPC1 Delta 567-793, but not TRPC1 Delta 664-793, induced a similar decrease in SOCC activity. Furthermore, TRPC1 Delta 567-793 was co-immunoprecipitated with endogenous TRPC1. Simultaneous substitutions of seven acidic aa in the S5-S6 region (Asp --> Asn and Glu --> Gln) decreased SOCC-mediated Ca(2+), but not Na(+), current and induced a left shift in E(rev). Similar effects were induced by E576K or D581K, but not D581N or E615K, substitution. Furthermore, expressed TRPC1 proteins interacted with each other. Together, these data demonstrate that TRPC1 is required for generation of functional SOCC in HSG cells. We suggest that TRPC1 monomers co-assemble to form SOCC and that specific acidic aa residues in the proposed pore region of TRPC1 contribute to Ca(2+) influx.

mRNA distribution analysis of human TRPC family in CNS and peripheral tissues

The mammalian homologues of the Drosophila transient receptor potential (TRP) channel are plasma membrane proteins involved in the regulation of cellular Ca(2+) influx. These ion channels can be activated subsequent to either depletion of Ca(2+) from internal stores or through receptor-mediated processes. The mRNA expression patterns of several individual mammalian short transient receptor potential channels (TRPCs) have been described. Cross-comparisons between these data, however, are at best difficult predominantly due to the non-quantitative methods used. Furthermore there is limited data on the expression of TRPC family members in human tissues. In the present study we used a single technique, namely TaqMan real-time quantitative RT-PCR, to investigate the mRNA distribution of human TRPC1, TRPC3, TRPC4, TRPC5, TRPC6 and TRPC7 (hTRPCs) in discrete human brain areas, peripheral tissues as well as a panel of cell-lines. All hTRPCs studied were widely expressed within CNS and significant peripheral expression was often observed. Despite this, each channel exhibited a distinctive hallmark distribution profile. hTRPC1 was widely expressed in CNS and peripheral tissues, whereas hTRPC3 and hTRPC5 were predominantly expressed in tissues of CNS. hTRPC4 mRNA was detected in CNS and certain peripheral tissues such as bone, heart and prostate. hTRPC6 was homogeneously expressed throughout the CNS and peripheral tissues with the highest levels in placenta and lung. hTRPC7 mRNA was also broadly expressed in CNS as well as some peripheral tissues. The pattern of expression of the TRPCs was quite different in the various cell lines examined. TRPC3 and TRPC6 were selectively present in HEK-293 cells whilst TRPC1 was broadly distributed in the cell lines analyzed. In contrast TRPC4 and TRPC5 mRNAs were predominantly expressed in HK-2 and HEK-293 cell lines respectively. TRPC7 was selectively expressed in COS-1, COS-7 and HK-2 cell lines. These results show tissue- and cell-specific co-expression of multiple TRPC forms indicating widespread potential for formation of heteromeric channels. These data will be useful in the complex task of relating channel subunit composition to function in native cells.

Selective association of TRPC channel subunits in rat brain synaptosomes

TRPC genes encode a ubiquitous family of ion channel proteins responsible for Ca(2+) influx following stimulation of G-protein-coupled membrane receptors linked to phospholipase C. These channels may be localized to large multimeric signaling complexes via association with PDZ-containing scaffolding proteins. Based on sequence homology, the TRPC channel family can be divided into two major subgroups: TRPC1, -C4, and -C5 and TRPC3, -C6, and -C7. Although TRPC channels are thought to be tetramers, the actual subunit composition remains unknown. To determine subunit arrangement, individual TRPC channel pairs were heterologously expressed in Sf9 insect cells and immunoprecipitated using affinity-purified rabbit polyclonal antibodies specific for each channel subtype. Reciprocal co-immunoprecipitations showed that TRPC1, -C4, and -C5 co-associate and that TRPC3, -C6, and -C7 co-associate but that cross-association between the two major subgroups does not occur. Additionally, the interaction between each TRPC channel and the PDZ-containing protein, INAD (protein responsible for the inactivation-no-after-potential Drosophila mutant), was examined. TRPC1, -C4, and -C5 co-immunoprecipitated with INAD, whereas TRPC3, -C6, and -C7 did not. To define channel subunit interactions in vivo, immunoprecipitations were performed from isolated rat brain synaptosomal preparations. The results revealed that TRPC1, -C4, and -C5 co-associate and that TRPC3, -C6, and -C7 co-associate in both cortex and cerebellum but that cross-association between the two major subgroups does not occur. These results demonstrate that TRPC channels are present in nerve terminals and provide the first direct evidence for selective assembly of channel subunits in vivo.

Kaposi's sarcoma-associated herpesvirus mitochondrial K7 protein targets a cellular calcium-modulating cyclophilin ligand to modulate intracellular calcium concentration and inhibit apoptosis

On viral infection, infected cells can become the target of host immune responses or can go through a programmed cell death process, called apoptosis, as a defense mechanism to limit the ability of the virus to replicate. To prevent this, viruses have evolved elaborate mechanisms to subvert the apoptotic process. Here, we report the identification of a novel antiapoptotic K7 protein of Kaposi's sarcoma-associated herpesvirus (KSHV) which expresses during lytic replication. The KSHV K7 gene encodes a small mitochondrial membrane protein, and its expression efficiently inhibits apoptosis induced by a variety of apoptogenic agents. The yeast two-hybrid screen has demonstrated that K7 targets cellular calcium-modulating cyclophilin ligand (CAML), a protein that regulates the intracellular Ca(2+) concentration. Similar to CAML, K7 expression significantly enhances the kinetics and amplitudes of the increase in intracellular Ca(2+) concentration on apoptotic stimulus. Mutational analysis showed that K7 interaction with CAML is required for its function in the inhibition of apoptosis. This indicates that K7 targets cellular CAML to increase the cytosolic Ca(2+) response, which consequently protects cells from mitochondrial damage and apoptosis. This is a novel viral antiapoptosis strategy where the KSHV mitochondrial K7 protein targets a cellular Ca(2+)-modulating protein to confer resistance to apoptosis, which allows completion of the viral lytic replication and, eventually, maintenance of persistent infection in infected host.

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.