A role for the actin cytoskeleton in the initiation and maintenance of store-mediated calcium entry in human platelets. Evidence for conformational coupling

Functional relevance of the de novo coupling between hTRPC1 and type II IP3 receptor in store-operated Ca2+ entry in human platelets

Store-operated Ca2+ entry (SOCE), a major mechanism for Ca2+ entry in non-excitable cells, is regulated by the filling state of the intracellular Ca2+ stores. We have previously reported that a de novo conformational coupling between the type II IP3 receptor (IP3RII) and hTRPC1 channel occurs after depletion of the intracellular Ca2+ stores in human platelets, which might be involved in the activation of SOCE in these cells. Here we present for the first time direct evidence for the functional relevance of the coupling between hTRPC1 and IP3RII in SOCE in human platelets. Our data suggest that at least two pathways may contribute to SOCE in these cells. An early component, insensitive to cytochalasin D (Cyt D), is followed by a late component which is sensitive to Cyt D. Introduction of a peptide corresponding to IP3RII(317-334) (IP3BD-peptide(317-334)) in the cells by electrotransjection impairs the coupling between hTRPC1 and IP3RII but not the interaction between hTRPC1 and STIM1 induced by store depletion. Coimmunoprecipitation experiments indicated that endogenously expressed hTRPC1 interacts with the IP3BD-peptide(317-334). Electrotransjection of cells with IP3BD-peptide(317-334), significantly attenuated the late stage of Ca2+ and Mn2+ entry induced by 10 nM thapsigargin (TG) or 20 microM 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ), providing evidence for a functional role of the de novo coupling between hTRPC1 and IP3RII in the activation of SOCE in human platelets.

A key role for reverse Na+/Ca2+ exchange influenced by the actin cytoskeleton in store-operated Ca2+ entry in human platelets: Evidence against the de novo conformational coupling hypothesis

We have previously demonstrated a role for the reorganization of the actin cytoskeleton in store-operated calcium entry (SOCE) in human platelets and interpreted this as evidence for a de novo conformational coupling step in SOCE activation involving the type II IP(3) receptor and the platelet hTRPC1-containing store-operated channel (SOC). Here, we present evidence challenging this model. The actin polymerization inhibitors cytochalasin D or latrunculin A significantly reduced Ca2+ but not Mn2+ or Na+ entry into thapsigargin (TG)-treated platelets. Jasplakinolide, which induces actin polymerization, also inhibited Ca2+ but not Mn2+ or Na+ entry. However, an anti-hTRPC1 antibody inhibited TG-evoked entry of all three cations, indicating that they all permeate an hTRPC1-containing store-operated channel (SOC). These results indicate that the reorganization of the actin cytoskeleton is not involved in SOC activation. The inhibitors of the Na+/Ca2+ exchanger (NCX), KB-R7943 or SN-6, caused a dose-dependent inhibition of Ca2+ but not Mn2+ or Na+ entry into TG-treated platelets. The effects of the NCX inhibitors were not additive with those of actin polymerization inhibitors, suggesting a common point of action. These results indicate a role for two Ca2+ permeable pathways activated following Ca2+ store depletion in human platelets: A Ca2+-permeable, hTRPC1-containing SOC and reverse Na+/Ca2+ exchange, which is activated following Na+ entry through the SOC and requires a functional actin cytoskeleton.

Cell shape-dependent Control of Ca2+ influx and cell cycle progression in Swiss 3T3 fibroblasts

The ability of adherent cells such as fibroblasts to enter the cell cycle and progress to S phase is strictly dependent on the extent to which individual cells can attach to and spread on a substratum. Here we have used microengineered adhesive islands of 22 and 45 mum diameter surrounded by a nonadhesive substratum of polyhydroxyl methacrylate to accurately control the extent to which individual Swiss 3T3 fibroblasts may spread. The effect of cell shape on mitogen-evoked Ca2+ signaling events that accompany entry into the cell cycle was investigated. In unrestricted cells, the mitogens bombesin and fetal calf serum evoked a typical biphasic change in the cytoplasmic free Ca2+ concentration. However, when the spreading of individual cells was restricted, such that progression to S phase was substantially reduced, both bombesin and fetal calf serum caused a rapid transient rise in the cytoplasmic free Ca2+ concentration but failed to elicit the normal sustained influx of Ca2+ that follows Ca2+ release. As expected, restricting cell spreading led to the loss of actin stress fibers and the formation of a ring of cortical actin. Restricting cell shape did not appear to influence mitogen-receptor interactions, nor did it influence the presence of focal adhesions. Because Ca2+ signaling is an essential component of mitogen responses, these findings implicate Ca2+ influx as a necessary component of cell shape-dependent control of the cell cycle.

Dual role of tubulin-cytoskeleton in store-operated calcium entry in human platelets

Two mechanisms for store-operated Ca(2+) entry (SOCE) regulated by two independent Ca(2+) stores, the dense tubular system (DTS) and the acidic stores, have been described in platelets. We have previously suggested that coupling between the type II IP(3) receptor (IP(3)RII) and hTRPC1, involving reorganization of the actin microfilaments, play an important role in SOCE. However, the involvement of the tubulin microtubules, located beneath the plasma membrane, remains unclear. Here we show that the microtubule disrupting agent colchicine reduced Ca(2+) entry stimulated by low concentrations (0.1 U/mL) of thrombin, which activates SOCE mostly by depleting acidic Ca(2+)-store. Consistently, colchicine reduced SOCE activated by 2,5 di-(tertbutyl)-1,4-hydroquinone (TBHQ), which selectively depletes the acidic Ca(2+) stores. In contrast, colchicine enhanced SOCE mediated by depletion of the DTS, induced by high concentrations of thapsigargin (TG), which depletes both the acidic Ca(2+) stores and the DTS, the major releasable Ca(2+) store in platelets. These findings were confirmed by using Sr(2+) as a surrogate for Ca(2+) entry. Colchicine attenuated the coupling between IP(3)RII and hTRPC1 stimulated by thrombin while it enhanced that evoked by TG. Paclitaxel, which induces microtubular stabilization and polymerization, exerted the opposite effects on thrombin- and TG-evoked SOCE and coupling between IP(3)RII and hTRPC1 compared with colchicine. Neither colchicine nor paclitaxel altered the ability of platelets to extrude Ca(2+). These findings suggest that tubulin microtubules play a dual role in SOCE, acting as a barrier that prevents constitutive SOCE regulated by DTS, but also supporting SOCE mediated by the acidic Ca(2+) stores.

TCR/CD28-stimulated actin dynamics are required for NFAT1-mediated transcription of c-rel leading to CD28 response element activation

TCR/CD28 engagement triggers the initiation of a variety of signal transduction pathways that lead to changes in gene transcription. Although reorganization of the actin cytoskeleton is required for T cell activation, the molecular pathways controlled by the actin cytoskeleton are ill defined. To this end, we analyzed TCR/CD28-stimulated signaling pathways in cytochalasin D-treated T cells to determine the cytoskeletal requirements for T cell activation. Cytochalasin D treatment impaired T cell activation by causing a reduction in TCR/CD28-mediated calcium flux, and blocked activation of two regulatory elements within the IL-2 promoter, NFAT/AP-1 and CD28RE/AP. Treatment had no effect on signaling leading to the activation of either AP-1 or NF-kappaB. Significantly, we found that NFAT1 is required for optimal c-rel up-regulation in response to TCR/CD28 stimulation. In fact, NFAT1 could be detected bound at the c-rel promoter in response to TCR/CD28 stimulation, and targeting of NFAT1 using RNA interference in human CD4(+) T cells abrogated c-rel transcription. Overall, these findings establish that disrupting actin cytoskeletal dynamics impairs TCR/CD28-mediated calcium flux required for NFAT1-mediated c-rel transcription and, thus, activation of the CD28RE/AP.

The long and arduous road to CRAC

Store-operated calcium (SOC) entry is the major route of calcium influx in non-excitable cells, especially immune cells. The best characterized store-operated current, I(CRAC), is carried by calcium release activated calcium (CRAC) channels. The existence of the phenomenon of store-operated calcium influx was proposed almost two decades ago. However, in spite of rigorous research by many laboratories, the identity of the key molecules participating in the process has remained a mystery. In all these years, multiple different approaches have been adopted by countless researchers to identify the molecular players in this fundamental process. Along the way, many crucial discoveries have been made, some of which have been summarized here. The last couple of years have seen significant breakthroughs in the field-identification of STIM1 as the store Ca(2+) sensor and CRACM1 (Orai1) as the pore-forming subunit of the CRAC channel. The field is now actively engaged in deciphering the gating mechanism of CRAC channels. We summarize here the latest progress in this direction.

Phosphatidylinositol 4,5-bisphosphate enhances store-operated calcium entry through hTRPC6 channel in human platelets

Phosphatidylinositol 4,5-bisphosphate (PIP2) is a versatile regulator of TRP channels. We report that inclusion of a PIP2 analogue, PIP2 1,2-dioctanoyl, does not induce non-capacitative Ca2+ entry per se but enhanced Ca2+ entry stimulated either by thrombin or by selective depletion of the Ca2+ stores in platelets, the dense tubular system, using 10 nM TG, and the acidic stores, using 20 microM 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ). Reduction of PIP2 levels by blocking PIP2 resynthesis with Li+ or introducing a monoclonal anti-PIP2 antibody, or sequestering PIP2 using poly-lysine, attenuated Ca2+ entry induced by thrombin, TG and TBHQ, and reduced thrombin-evoked, but not TG- or TBHQ-induced, Ca2+ release from the stores. Incubation with the anti-hTRPC1 antibody did not alter the stimulation of Ca2+ entry by PIP2, whilst introduction of anti-hTRPC6 antibody directed towards the C-terminus of hTRPC6 reduced Ca2+ and Mn2+ entry induced by thrombin, TG or TBHQ, and abolished the stimulation of Ca2+ entry by PIP2. The anti-hTRPC6 antibody, but not the anti-hTRPC1 antibody or PIP2, reduced non-capacitative Ca2+ entry by the DAG analogue 1-oleoyl-2-acetyl-sn-glycerol. In summary, hTRPC6 plays a role both in store-operated and in non-capacitative Ca2+ entry. PIP2 enhances store-operated Ca2+ entry in human platelets, most probably by stimulation of hTRPC6 channels.

Disruption of the cortical actin cytoskeleton does not affect store operated Ca2+ channels in human T-cells

Lymphocyte signaling and activation leads to the influx of extracellular Ca(2+) via the activation of Ca(2+) release activated Ca(2+) (CRAC) channels in the plasma membrane. Activation of CRAC channels occurs following emptying of the endoplasmic reticulum intracellular Ca(2+) stores. One model to explain the coupling of store-emptying to CRAC activation is the secretion-like conformational coupling model. This model proposes that store depletion increases junctions between the endoplasmic reticulum and the plasma membrane in a manner that could be regulated by the cortical actin cytoskeleton. Here, we show that stabilization or depolymerization of the actin cytoskeleton failed to affect CRAC activation. We therefore conclude that rearrangement of the actin cytoskeleton is dispensable for store-operated Ca(2+) entry in T-cells.

Astrocytes refill intracellular Ca2+ stores in the absence of cytoplasmic [Ca2+] elevation: a functional rather than a structural ability

Capacitative Ca(2+) entry (CCE) is a phenomenon triggered by depletion of Ca(2+) content in intracellular stores (ICS). Data about this phenomenon in astrocytes are limited. We analyzed CCE in astrocytes by means of fura-2 based digital imaging. We found that in astrocytes CCE is not associated with an increase of cytosolic Ca(2+) concentration ([Ca(2+)](i)), although ICS are efficiently refilled. We used Mn(2+), thapsigargin and prolonged ATP exposure to show that CCE is not associated with cytosolic diffusion of Ca(2+) entering astrocytes. Our data suggest that the ion is being quickly sequestered in the ICS by the smooth endoplasmic reticulum Ca(2+)-ATP-ase (SERCA). Several experiments were carried out with the goal of failing the efficient uptake in the endoplasmic reticulum (ER). In fact, inhibition of SERCA activity, increased extracellular [Ca(2+)](i) or pharmacologic potentiation of CCE all caused [Ca(2+)](i) elevation during CCE, suggesting that the control of this phenomenon could have physiologic and pathological relevance. The molecular components involved in CCE have been proposed to be organized in a multi-molecular complex tethered by cytoskeleton components and arranged via a secretion coupling model. We show here that the efficient routing of Ca(2+) into the ICS in astrocytes is not affected by disruption of cytoskeleton organization or Golgi's function, but it is instead linked to the high efficiency of SERCA. We conclude that depleted ICS in astrocytes are efficiently refilled by CCE activation, although Ca(2+) influx is not accompanied by elevation of [Ca(2+)](i). This ability seems to be functional rather than structural in nature.

Differential involvement of thrombin receptors in Ca2+ release from two different intracellular stores in human platelets

Physiological agonists increase cytosolic free Ca2+ concentration to regulate a number of cellular processes. The platelet thrombin receptors, PAR (protease-activated receptor) 1 PAR-4 and GPIb-IX-V (glycoprotein Ib-IX-V) have been described as potential contributors of thrombin-induced platelet aggregation. Platelets present two separate Ca2+ stores, the DTS (dense tubular system) and acidic organelles, differentiated by the distinct sensitivity of their respective SERCAs (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPases) to TG (thapsigargin) and TBHQ [2,5-di-(tert-butyl)-1,4-hydroquinone]. However, the involvement of the thrombin receptors in Ca2+ release from each Ca2+ store remains unknown. Depletion of the DTS using ADP, which releases Ca2+ solely from the DTS, in combination with 10 nM TG, to selectively inhibit SERCA2 located on the DTS reduced Ca2+ release evoked by the PAR-1 agonist, SFLLRN, and the PAR-4 agonist, AYPGKF, by 80 and 50% respectively. Desensitization of PAR-1 and PAR-4 or pre-treatment with the PAR-1 and PAR-4 antagonists SCH 79797 and tcY-NH2 reduced Ca2+ mobilization induced by thrombin, and depletion of the DTS after desensitization or blockade of PAR-1 and PAR-4 had no significant effect on Ca2+ release stimulated by thrombin through the GPIb-IX-V receptor. Converse experiments showed that depletion of the acidic stores using TBHQ reduced Ca2+ release evoked by SFLLRN or AYPGKF, by 20 and 50% respectively, and abolished thrombin-stimulated Ca2+ release through the GPIb-IX-V receptor when PAR-1 and PAR-4 had been desensitized or blocked. Our results indicate that thrombin-induced activation of PAR-1 and PAR-4 evokes Ca2+ release from both Ca2+ stores, while activation of GPIb-IX-V by thrombin releases Ca2+ solely from the acidic compartments in human platelets.

Transient Receptor Potential Channels and Intracellular Signaling

The transient receptor potential (TRP) family of ion channels is composed of more than 50 functionally versatile cation-permeant ion channels expressed in most mammalian cell types. Considerable research has been brought to bear on the members of this family, especially with regard to their possible role as store-operated calcium channels, although studies have provided evidence that TRP channels exhibit a number of regulatory and functional aspects. Endogenous and transiently expressed TRP channels can be activated by different mechanisms grouped into four main categories: receptor-operated activation, store depletion-mediated activation, ligand-induced activation, and direct activation. This article reviews the biochemical characteristics of the different members of the TRP family and summarizes their involvement in a number of physiological events ranging from sensory transduction to development, which might help in understanding the relationship between TRP channel dysfunction and the development of several diseases.

Cinnamtannin B-1 from bay wood exhibits antiapoptotic effects in human platelets

Proanthocyanidins, such as cinnamtannin B-1, are polyphenolic compounds with antioxidant activity that induce apoptosis in a number of tumoral cells. We have now investigated the pro- or anti-apoptotic effects of cinnamtannin B-1 in human platelets. Platelet stimulation with thrombin induced cellular apoptosis, as detected by phosphatidylserine exposure and the activation of caspases-3 and -9. Pretreatment for 30 min with cinnamtannin B-1 impaired thrombin-induced apoptosis in platelets. Thrombin has been shown to induce H(2)O(2) generation in platelets, which induced similar apoptotic events than thrombin in these cells. Pretreatment with cinnamtannin B-1 reduced H(2)O(2)-induced phosphatidylserine exposure and caspase activation. Finally, platelet stimulation with thrombin induced translocation of caspases-3 and -9 to the cytoskeletal (Triton-insoluble) fraction, which is important for their activation and the development of apoptotic events. Pretreatment with cinnamtannin B-1 impaired translocation of caspases-3 and -9 to the cytoskeleton and, as a result, procaspases are accumulated in the Triton-soluble fraction. Our results provide evidence for the antiapoptotic actions of cinnamtannin B-1 in human platelets.

Calcium signalling in the endothelium

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

PAR-1-dependent pp60src activation is dependent on protein kinase C and increased [Ca2+]: evidence that pp60src does not regulate PAR-1-dependent Ca2+ entry in human platelets

BACKGROUND

The role of the tyrosine kinase pp60src in PAR-1-dependent Ca2+ entry was investigated in human platelets. pp60src plays a role in thapsigargin (TG)-evoked store-operated Ca2+ entry (SOCE), which is thought to be a major component of thrombin-evoked Ca2+ entry.

METHODS

pp60src tyr416 phosphorylation was used to assess pp60src activation. Fura-2-loaded platelets were used to monitor intracellular Ca2+ concentration ([Ca2+]i).

RESULTS

Activation of PAR-1 with the specific agonist SFLLRN increased pp60src activation within 10 s. This required phospholipase C (PLC) activity, Ca2+ release and a rise in intracellular Ca2+. PP2, an inhibitor of Src-family tyrosine kinases, inhibited SFLLRN-evoked Ca2+ entry, but also inhibited Ca2+ release and the extrusion of Ca2+ by the plasma membrane Ca2+ ATPase. Actin polymerization and conventional protein kinase C (cPKC) activity were required for TG- and SFLLRN-evoked pp60src activation. Although Gö6976, an inhibitor of cPKCs, inhibited TG-evoked SOCE, it had little effect on SFLLRN- or thrombin-evoked Ca2+ entry.

CONCLUSIONS

These data indicate that stimulation of PAR-1 leads to activation of pp60src in human platelets, through PLC and cPKC activation, Ca2+ release and actin polymerization. However, as PKC and actin polymerization are not needed for SFLLRN-evoked Ca2+ entry, we suggest that pp60src is also not required. The apparent inhibition of SFLLRN-evoked Ca2+ entry by PP2 is likely to be secondary to reduced Ca2+ release. These data argue against a contribution of this SOCE pathway to PAR-1-dependent Ca2+ entry.

Discovering the mechanism of capacitative calcium entry

This essay examines the historical significance of an APS classic paper that is freely available online:

Kwan CY, Takemura H, Obie JF, Thastrup O, and Putney JW Jr.

Effects of MeCh, thapsigargin, and La3+on plasmalemmal and intracellular Ca2+transport in lacrimal acinar cells. Am J Physiol Cell Physiol 258: C1006–C1015, 1990. ( http://ajpcell.physiology.org/cgi/reprint/258/6/C1006 )

Cinnamtannin B-1 from bay wood reduces abnormal intracellular Ca2+ homeostasis and platelet hyperaggregability in type 2 diabetes mellitus patients

Type 2 diabetes mellitus induces a number of cardiovascular disorders, including platelet hyperactivity and hyperaggregability, which is associated to an increased oxidant production and abnormal cytosolic Ca2+ mobilization. In the present study, we have investigated the effect of cinnamtannin B-1 obtained from bay wood on oxidants production, Ca2+ mobilization and aggregation in platelets from type 2 diabetic donors. Pretreatment of platelets with cinnamtannin B-1 reversed the enhanced oxidants production and Ca2+ mobilization, including Ca2+ entry, evoked by thapsigargin plus ionomycin or thrombin, observed in platelets from diabetic subjects, so that in the presence of cinnamtannin B-1 Ca2+ entry was similar in platelets from healthy and diabetic subjects. In addition, cinnamtannin B-1 reduced thrombin-induced aggregation in platelets from type 2 diabetic subjects. We conclude that cinnamtannin B-1 exerts an effective antioxidant action in platelets from patients with type 2 diabetes mellitus and reverses the enhanced Ca2+ mobilization and hyperaggregability.

Shedding of procoagulant microparticles from unstimulated platelets by integrin-mediated destabilization of actin cytoskeleton

Platelet activation by potent, Ca(2+)-mobilizing agonists results in shedding of microparticles that are active in coagulation. Here we show that platelets under storage produce procoagulant microparticles in the absence of agonist. Microparticle formation by resting platelets results from alphaIIbbeta3 signaling to destabilization of the actin cytoskeleton in the absence of calpain activation. Integrin-mediated spreading of platelets over fibrinogen similarly results in microparticle formation. After transfusion of stored platelet preparations to thrombocytopenic patients, the microparticles contribute to coagulant activity in vivo.

A role for the intracellular protease calpain in the activation of store-operated calcium entry in human platelets

Here, we report a novel role for the cysteine protease calpain in store-operated calcium entry. Several structurally and mechanistically unrelated inhibitors of calpain inhibited Ca2+ entry activated in human platelets by thapsigargin-evoked Ca2+ store depletion or the physiological agonist thrombin, whereas inhibitors of other cysteine proteases were without effect. The use of the cell-permeable fluorogenic calpain substrate 7-amino-4-chloromethylcoumarin, t-BOC-l-leucyl-l-methionine amide revealed rapid activation of calpain which was closely temporally correlated with Ca2+ store depletion even in the absence of a rise in cytosolic [Ca2+]. Calpain inhibition prevented the tyrosine phosphorylation of several proteins upon Ca2+ store depletion, suggesting that calpain may lie upstream of protein tyrosine phosphorylation that is known to be required for the activation of store-operated Ca2+ entry in human platelets. Earlier studies using calpain inhibitors may need reinterpretation in the light of this finding that calpain plays a role in the activation of physiological Ca2+ entry pathways.

Store-mediated calcium entry in pleural mesothelial cells

Store-mediated Ca2+ entry is thought as the main pathway for Ca2+ influx in non-excitable cells. Although a role for the actin cytoskeleton in store-mediated Ca2+ entry has been proposed in some cell types, the role of actin cytoskeleton in store-mediated Ca2+ entry is still a controversy. To address this question, the effects of cytoskeletal modifiers on store-mediated Ca2+ entry in pleural mesothelial cells were examined. Thapsigargin (1 microM) induced a sufficient signal for the activation of store-mediated Ca2+ entry in pleural mesothelial cells. In the absence of extracellular Ca2+, thapsigargin induced only a transient elevation of [Ca2+]i. Moreover, re-addition of Ca2+ increased the elevation of [Ca2+]i. Passive elevations in [Ca2+]i without thapsigargin, which is induced from Ca2+ containing solution switch to Ca2+ free solution and re-add Ca2+ containing solution, were not observed in pleural mesothelial cells. Thapsigargin-induced Ca2+ entry was still present after nifedipine (1 microM) treatment. However, SKF96365 (1 microM) blocked thapsigargin-induced Ca2+ entry. Mycalolide B (1 microM) completely disrupts actin cytoskeleton in pleural mesothelial cells, but thapsigargin-induced store-mediated Ca2+ entry was preserved. Jasplakinolide (3 microM) prevented thapsigargin-induced store-mediated Ca2+ entry. These results suggest that store-mediated Ca2+ entry in pleural mesothelial cells may be mediated by a recently proposed secretion-like coupling model for store-mediated Ca2+ entry.

Caspases 3 and 9 are translocated to the cytoskeleton and activated by thrombin in human platelets. Evidence for the involvement of PKC and the actin filament polymerization

Platelets express, among others, initiator caspase 9 and effector caspase 3. Upon activation by physiological agonists, calcium ionophores or under shear stress they might develop apoptotic events. Although it is well known that the cytoskeletal network plays a crucial role in apoptosis, the relationship between caspases 3 and 9 and the cytoskeleton is poorly understood. Here we demonstrate that the physiological agonist thrombin is able to induce activation of caspases 3 and 9 in human platelets and significantly increases the amount in the cytoskeleton of the active forms of both caspases and the procaspases 3 and 9. After stimulation with thrombin the amount of active caspases 3 and 9 in the cytosolic and cytoskeletal fractions were significantly reduced in Ro-31-8220-treated cells, which demonstrates that caspases activation and association with the cytoskeleton needs the contribution of PKC. Inhibition of actin polymerization by cytochalasin D inhibits translocation and activation of both caspases, suggesting that thrombin stimulates caspase 3 and 9 activation and association with the reorganizing actin cytoskeleton. Finally, our results show that inhibition of thrombin-induced caspase activation has no effect on their translocation to the cytoskeleton although impairment of thrombin-evoked caspase translocation has negative effects on caspase activity, suggesting that translocation to the cytoskeleton might be important for caspase activation by thrombin in human platelets.

Interaction of STIM1 with endogenously expressed human canonical TRP1 upon depletion of intracellular Ca2+ stores

STIM1 (stromal interaction molecule 1) has recently been proposed to communicate the intracellular Ca(2+) stores with the plasma membrane to mediate store-operated Ca(2+) entry. Here we describe for the first time that Ca(2+) store depletion stimulates rapid STIM1 surface expression and association with endogenously expressed human canonical TRP1 (hTRPC1) independently of rises in cytosolic free Ca(2+) concentration. These events require the support of the actin cytoskeleton in human platelets, as reported for the coupling between type II inositol 1,4,5-trisphosphate receptor in the Ca(2+) stores and hTRPC1 in the plasma membrane, which has been suggested to underlie the activation of store-operated Ca(2+) entry in these cells. Electrotransjection of cells with anti-STIM1 antibody, directed toward the N-terminal sequence that includes the Ca(2+)-binding region, prevented the migration of STIM1 toward the plasma membrane, the interaction between STIM1 and hTRPC1, the coupling between hTRPC1 and type II inositol 1,4,5-trisphosphate receptor, and reduced store-operated Ca(2+) entry. These findings provide evidence for a role of STIM1 in the activation of store-operated Ca(2+) entry probably acting as a Ca(2+) sensor.

On the activation mechanism of store-operated calcium channels

The development of the patch clamp technique has revolutionised our understanding of the life sciences. One area in which it has made an enormous contribution is cellular signalling. In many cell types, calcium influx across the plasma membrane is essential for the regulation of a wide range of critical physiological responses including secretion, gene transcription and cell growth. For many years the calcium influx pathways in non-excitable cells remained unknown, despite their importance in physiological and pathophysiological states. Very careful and insightful work by James Putney led to the formulation of the capacitative calcium entry (store-operated calcium influx) model, in which the process of emptying intracellular calcium stores resulted in the activation of calcium entry channels. Unequivocal evidence for this revolutionary model was provided by patch clamp studies carried out by Markus Hoth and Reinhold Penner, who demonstrated that store depletion activated a novel class of calcium channel called the CRAC channel. This review provides a historical perspective on the development of store-operated calcium influx and how patch clamping resolved a long-standing controversy in cell physiology. The review also discusses current ideas relating to how store emptying opens channels in the plasma membrane.

Intracellular Ca2+ homeostasis and aggregation in platelets are impaired by ethanol through the generation of H2O2 and oxidation of sulphydryl groups

The mechanisms involved in the effect of ethanol on Ca2+ entry and aggregability have been investigated in human platelets in order to shed new light on the pathogenesis of alcohol consumption. Ethanol (50 mM) induced H2O2 production in platelets by Ca2+-dependent and independent mechanisms. Ca2+ entry induced by ethanol was impaired by catalase. Ethanol reduced SOCE mediated by depletion of the 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ)-sensitive acidic stores but enhances SOCE regulated by the dense tubular system. This effect was abolished by treatment with catalase or the sulphydryl group reducing agent dithiotreitol (DTT). Similarly, the anti-aggregant effect of ethanol was prevented by platelet treatment with catalase or DTT. In conclusion we provide considerable evidence that ethanol alters Ca2+ entry and reduces thrombin-induced aggregation as a result of the generation of H2O2 and the oxidation of sulphydryl groups in human platelets.

Thrombin-induced caspases 3 and 9 translocation to the cytoskeleton is independent of changes in cytosolic calcium in human platelets

Apoptosis has been shown to be associated with changes in cytosolic free calcium concentration ([Ca(2+)](c)). Here we show that the agonist thrombin induces activation of caspases 9 and 3 and translocation of the caspase active forms and procaspases to the cytoskeleton in human platelets. Dimethyl-BAPTA loading did not affect thrombin-induced caspase 9 and 3 activation or translocation suggesting that these responses are independent of increases in [Ca(2+)](c). Treatment with thapsigargin plus ionomycin, to induce extensive Ca(2+) store depletion and subsequent increase in [Ca(2+)](c), stimulates caspase activation although it was unable to induce caspase translocation to the cytoskeleton. Similar results were observed in cells loaded with dimethyl-BAPTA, suggesting that activation of caspases 9 and 3 by thapsigargin plus ionomycin does not require rises in [Ca(2+)](c). These findings suggest that thrombin-induced caspase 9 and 3 activation and translocation are independent on rises in [Ca(2+)](c) but might require store depletion in human platelets.

Alterations in intracellular calcium homeostasis and platelet aggregation induced by ethanol

The in vitro effects of ethanol on intracellular Ca(2+) homeostasis and tyrosine phosphorylation have been investigated in human platelets in order to clarify the cellular mechanisms underlying its described anti-aggregant effects. Ethanol (1-50 mM) reduced, in a dose-dependent manner, the rate and amplitude of aggregation and attenuated the phosphotyrosine content both induced by 0.1U/ml of the physiological ligand, thrombin. Thrombin-induced Ca(2+) entry to the cytosol was significantly reduced, and capacitative Ca(2+) entry (CCE) significantly altered, by 50 mM ethanol, so that ethanol reduces CCE mediated by depletion of the 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ)-sensitive store but enhances CCE induced by the TBHQ-insensitive pool. In conclusion, we provide considerable evidence that ethanol reduces thrombin-induced aggregation, which is likely a result of a significant inhibition of Ca(2+) entry, as well as a reduction in the activity of protein tyrosine kinases.

A role for cofilin in the activation of store-operated calcium entry by de novo conformational coupling in human platelets

Store-operated Ca2+ entry (SOCE) is a major mechanism for Ca2+ influx in platelets and other cells. De novo conformational coupling between elements in the plasma membrane and Ca2+ stores, where the actin cytoskeleton plays an important regulatory role, has been proposed as the most likely mechanism to activate SOCE in platelets. Here we have examined for the first time changes in platelet F-actin levels on a subsecond time scale. Using stopped-flow fluorimetry and a quenched-flow approach, we provide evidence for the involvement of cofilin in actin filament reorganization and SOCE in platelets. Thrombin (0.1 U/mL) evoked an initial decrease in F-actin that commenced within 0.1 second and reached a minimum 0.9 second after stimulation, prior to the activation of SOCE. F-actin then increased, exceeding basal levels approximately 2.5 seconds after stimulation. Thrombin also induced cofilin dephosphorylation and activation, which paralleled the changes observed in F-actin, and rapid Btk activation. Inhibition of cofilin dephosphorylation by LFM-A13 resulted in the loss of net actin depolymerization and an increased delay in SOCE initiation. These results suggest that cofilin is important for the rapid actin remodeling necessary for the activation of SOCE in platelets through de novo conformational coupling.

Early caspase-3 activation independent of apoptosis is required for cellular function

A number of pro-apoptotic stimuli induce the activation of caspase-9, an initiator protease that activates executioner caspases, such as caspase-3, leading to the development of programmed cell death. Here we demonstrate that cell (platelets and pancreatic acinar cells) stimulation with agonists induces a bimodal activation of caspase-3. The early caspase-3 activation occurs within 1 min of stimulation and is independent on caspase-9 or mitochondrial cytochrome c release suggesting that is a non-apoptotic event. The ability of agonists to induce early activation of caspase-3 is similar to that observed for other physiological processes. Activation of caspase-3 by physiological concentrations of cellular agonists, including thrombin or CCK-8, is independent of rises in cytosolic calcium concentration but requires PKC activation, and is necessary for agonist-induced activation of the tyrosine kinases Btk and pp60src and for several cellular functions, including store-operated calcium entry, platelet aggregation, or pancreatic secretion. Thus, early activation of caspase-3 seems to be a non-apoptotic event required for cellular function.

Collaborative effect of SERCA and PMCA in cytosolic calcium homeostasis in human platelets

Intracellular free Ca2+ concentration ([Ca2+]c) is finely regulated by several mechanisms that either increase or reduce [Ca2+]c. Two different Ca2+ pumps have been described so far as the main mechanisms for Ca2+ removal from the cytosol, either by its sequestration into the stores, mediated by the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) or by Ca2+ extrusion to the extracellular medium, by the plasma membrane Ca2+-ATPase (PMCA). We have used inhibitors of these pumps to analyze their Ca2+ clearance efficacy in human platelets stimulated by the physiological agonist thrombin. Results demonstrate that, after platelet stimulation with thrombin, activation of SERCA precedes that of PMCA, although the ability of PMCA to remove Ca2+ from the cytosol last longer than that of SERCA. The efficacy of SERCA and PMCA removing Ca2+ from the cytosol is reduced when the concentration of thrombin increases. This phenomenon correlates with the greater increase in [Ca2+]c induced by higher concentrations of thrombin, which further confirms that SERCA and PMCA activities are regulated by [Ca2+]c.

A role for 5,6-epoxyeicosatrienoic acid in calcium entry by de novo conformational coupling in human platelets

A major pathway for Ca(2+) entry in non-excitable cells is activated following depletion of intracellular Ca(2+) stores. A de novo conformational coupling between elements in the plasma membrane (PM) and Ca(2+) stores has been proposed as the most likely mechanism to activate this capacitative Ca(2+) entry (CCE) in several cell types, including platelets. Here we report that a cytochrome P450 metabolite, 5,6-EET, might be a component of the de novo conformational coupling in human platelets. In these cells, 5,6-EET induces divalent cation entry without having any detectable effect on Ca(2+) store depletion. 5,6-EET-induced Ca(2+) entry was sensitive to the CCE blockers 2-APB, lanthanum, SKF-96365 and nickel and impaired by incubation with anti-hTRPC1 antibody. Ca(2+) entry stimulated by low concentrations of thapsigargin, which selectively depletes the dense tubular system and induces EET production, was impaired by the cytochrome P450 inhibitor 17-ODYA, which has no effect on CCE mediated by depletion of the acidic stores using 2,5-di-(tert-butyl)-1,4-hydroquinone. We have found that 5,6-EET-induced Ca(2+) entry requires basal levels of H(2)O(2), which might maintain a redox state favourable for this event. Finally, our results indicate that 5,6-EET induces the activation of tyrosine kinase proteins and the reorganization of the actin cytoskeleton, which might provide a support for the transport of portions of the Ca(2+) store towards the PM to facilitate de novo coupling between IP(3)R type II and hTRPC1 detected by coimmunoprecipitation. We propose that the involvement of 5,6-EET in TG-induced coupling between IP(3)R type II and hTRPC1 and subsequently CCE is compatible with the de novo conformational coupling in human platelets.

Inhibition by calyculin A and okadaic acid of the Ca(2+) release-activated Ca(2+) entry pathway in rat basophilic leukemia cells: evidence for regulation by type 1/2A serine/threonine phosphatase activity

Using a combination of fluorescence measurements of intracellular Ca(2+) ion concentration ([Ca(2+)](i)) and membrane potential we have investigated the sensitivity to serine/threonine phosphatase inhibition of Ca(2+) entry stimulated by activation of the Ca(2+) release-activated Ca(2+) (CRAC) entry pathway in rat basophilic leukemia cells. In both suspension and adherent cells, addition of the type 1/2A phosphatase inhibitor calyculin A, during activation of CRAC uptake, resulted in a fall in [Ca(2+)](i) to near preactivation levels. Pre-treatment with calyculin A abolished the component of the Ca(2+) rise associated with activation of CRAC uptake and inhibited Mn(2+) entry, consistent with a requirement of phosphatase activity for activation of the pathway. Depletion of intracellular Ca(2+) stores is accompanied by a large depolarisation which is absolutely dependent upon Ca(2+) entry via the CRAC uptake pathway. Application of calyculin A or okadaic acid, a structurally unrelated phosphatase antagonist inhibits this depolarisation. Taken in concert, these data demonstrate a marked sensitivity of the CRAC entry pathway to inhibition by calyculin A and okadaic acid.

Activation of the endothelial store-operated ISOC Ca2+ channel requires interaction of protein 4.1 with TRPC4

Store-operated calcium (SOC) entry represents the principal Ca2+ entry pathway into nonexcitable cells. Despite intensive investigation, mechanisms underlying activation of SOC entry have remained elusive. The endothelial ISOC channel is a Ca2+-selective SOC entry channel to which the transient receptor potential (TRP) proteins TRPC1 and TRPC4 contribute subunits. Activation of ISOC is specifically regulated by the spectrin-actin membrane skeleton; however, the nature of coupling between the ISOC channel and membrane skeleton is unknown. Here we demonstrate that protein 4.1 is an essential component of the ISOC channel gating mechanism. Protein 4.1 interacts with TRPC4 and the membrane skeleton. Deletion of the protein 4.1 binding domain on TRPC4 or peptide competition to the protein 4.1 binding domain prevents ISOC activation. These findings reveal that interaction of protein 4.1 with TRPC4 is required for activation of the endothelial ISOC channel.

CIF and other mysteries of the store-operated Ca2+-entry pathway

The molecular mechanism of the store-operated Ca2+-entry (SOCE) pathway remains one of the most intriguing and long lasting mysteries of Ca2+ signaling. The elusive calcium influx factor (CIF) that is produced upon depletion of Ca2+ stores has attracted growing attention, triggered by new discoveries that filled the gap in the chain of reactions leading to activation of store-operated channels and Ca2+ entry. Ca2+-independent phospholipase A2 emerged as a target of CIF, and a major determinant of the SOCE mechanism. Here, we present our viewpoint on CIF and conformational-coupling models of SOCE from a historical perspective, trying to resolve some of the problem areas, and summarizing our present knowledge on how depletion of intracellular Ca2+ stores signals to plasma membrane channels to open and provide Ca2+ influx that is required for many important physiological functions.

Inhibitors of actin polymerisation stimulate arachidonic acid release and 5-lipoxygenase activation by upregulation of Ca2+ mobilisation in polymorphonuclear leukocytes involving Src family kinases

Here, we show that actin polymerisation inhibitors such as latrunculin B (LB), and to a minor extent also cytochalasin D (Cyt D), enhance the release of arachidonic acid (AA) as well as nuclear translocation of 5-lipoxygenase (5-LO) and 5-LO product synthesis in human polymorphonuclear leukocytes (PMNL), challenged with thapsigargin (TG) or N-formyl-methionyl-leucyl-phenylalanine. The concentration-dependent effects of LB (EC50 approximately 200 nM) declined with prolonged preincubation (>3 min) prior TG and were barely detectable when PMNL were stimulated with Ca2+-ionophores. Investigation of the stimulatory mechanisms revealed that LB (or Cyt D) elicits Ca2+ mobilisation and potentiates stimulus-induced elevation of intracellular Ca2+, regardless of the nature of the stimulus. LB caused rapid but only moderate activation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK)2. The selective Src family kinase inhibitors PP2 and SU6656 blocked LB- or Cyt D-mediated Ca2+ mobilisation and suppressed the upregulatory effects on AA release and 5-LO product synthesis, without affecting AA metabolism evoked by ionophore alone. We conclude that in PMNL, inhibitors of actin polymerisation cause enhancement of intracellular Ca2+ levels through Src family kinase signaling, thereby facilitating stimulus-induced release of AA and 5-LO product formation.

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.

Two distinct Ca2+ compartments show differential sensitivity to thrombin, ADP and vasopressin in human platelets

Recent studies propose the existence of two distinct Ca2+ compartments in human platelets based on the expression of different SERCA isoforms with distinct sensitivity to thapsigargin and 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ). Using fura-2-loaded human platelets we have found that depletion of the TBHQ sensitive store reduces thrombin--but not ADP--or vasopressin (AVP)-induced Ca2+ release. Redistribution of cytosolic Ca2+ after thrombin stimulation resulted in overloading of the TBHQ-sensitive store. This phenomenon was not observed with ADP or AVP. We found that NAADP decreases the Ca2+ concentration into the stores in permeabilized platelets, which is prevented by depletion of the TBHQ-sensitive store. Nimodipine, an inhibitor of the NAADP receptor, reduced thrombin-induced Ca2+ release from the TBHQ-sensitive stores, without having any effect on the responses elicited by ADP or AVP. Finally, the phospholipase C inhibitor, U-73122, abolished ADP- and AVP-induced Ca2+ release, suggesting that their responses are entirely dependent on IP3 generation. In contrast, treatment with both U-73122 and nimodipine was required to abolish thrombin-induced Ca2+ release. We suggest that thrombin evokes Ca2+ release from TBHQ-sensitive and insensitive stores, which requires both NAADP and IP3, respectively, while ADP and AVP exert an IP3-dependent release of Ca2+ from the TBHQ-insensitive compartment in human platelets.

Ca2+-independent activation of Bruton's tyrosine kinase is required for store-mediated Ca2+ entry in human platelets

Store-mediated Ca(2+) entry (SMCE), which is rapidly activated by depletion of the intracellular Ca(2+) stores, is a major mechanism for Ca(2+) influx. Several studies have involved tyrosine kinases in the activation of SMCE, such as pp60(src), although at present those involved in the early activation steps are unknown. Here we report the involvement of Bruton's tyrosine kinase (Btk) in the early stages of SMCE in human platelets. Cell treatment with thrombin or thapsigargin (TG) plus ionomycin (Iono) results in rapid activation of Btk, which was independent of rise in intracellular Ca(2+) concentration ([Ca(2+)](i)) but dependent on H(2)O(2) generation. Platelet treatment with Btk inhibitors, LFM-A13 or terreic acid, significantly reduced TG+Iono- and thrombin-evoked SMCE. Btk was rapidly activated by addition of low concentrations of H(2)O(2), whose effect on Ca(2+) entry was prevented by Btk inhibitors. Our results indicate that pp60(src) and Btk co-immunoprecipitate after platelet stimulation with TG+Iono, thrombin or H(2)O(2). In addition, we have found that LFM-A13 impaired actin filament reorganization after store depletion and agonist-induced activation of pp60(src), while the inhibitor of pp60(src), a protein that requires actin reorganization for its activation, did not modify Btk activation, suggesting that Btk is upstream of pp60(src). We propose a role for Btk in the early steps of activation of SMCE in human platelets.

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.

Rearrangement of the endoplasmic reticulum and calcium transient formation: The computational approach

Experiments affecting calcium signaling often lead to changes in the calcium transient height. The present work is designed to approach this effect theoretically. Use of computational model let us to follow results of precisely designed changes in the endoplasmic reticulum distribution as a possible cause of cytoplasmic free calcium ion level. Obtained results suggest that indeed, rearrangement of the endoplasmic reticulum elements may be responsible for modulation of calcium signal's strength. We have also noticed that even if the endoplasmic reticulum concentration levels are local, the resulting changes in free calcium concentration are global and evenly distributed throughout the cell. The used mathematical method proved to be a powerful tool which made us understand the chemical dynamics of nonequilibrium processes of calcium transient formation. Presented data show how Ca2+ signal resulting from IP3 provoked release of calcium from the endoplasmic reticulum may depend on the cytoskeleton structure.

Calcium entry mediated by SOCs and TRP channels: variations and enigma

Ca(2+) signals in response to receptors mediate and control countless cellular functions ranging from short-term responses such as secretion and contraction to longer-term regulation of growth, cell division and apoptosis. The spatial and temporal details of Ca(2+) signals have been resolved with great precision in many cells. Ca(2+) signals activated by phospholipase C-coupled receptors have two components: Ca(2+) release from endoplasmic reticulum (ER) stores mediated by inositol 1,4,5-trisphosphate (InsP(3)) receptors, and Ca(2+) entry from outside the cell. The latter remains largely a molecular and mechanistic mystery. The activation of "store-operated" Ca(2+) channels is believed to account for the entry of Ca(2+). However, debate now focuses on how much of a contribution emptying of stores plays to the activation of Ca(2+) entry in response to physiological activation of receptors. Here we discuss recent information and ideas on the exchange of signals between the plasma membrane (PM) and ER that results in activation of Ca(2+) entry channels following receptor stimulation and/or store emptying.

The inositol 1,4,5-trisphosphate receptor regulates epidermal cell migration in Caenorhabditis elegans

Polarized migration and spreading of epithelial sheets is important during many processes in vivo, including embryogenesis and wound healing. However, the signaling pathways that regulate epithelial migrations are poorly understood. To identify molecular components that regulate the spreading of epithelial sheets, we performed a screen for mutations that perturb epidermal cell migration during embryogenesis in Caenorhabditis elegans. We identified one mutant (jc5) as a weak mutation in itr-1, which encodes the single inositol 1,4,5-trisphosphate receptor (ITR) in C. elegans. During the migration of the embryonic epidermis, jc5 embryos display defects including misdirected migration or premature cessation of migration. Cells that halt their migration have disorganized F-actin and display reduced filopodial protrusive activity at their leading edge. Furthermore, some filopodia formed by epidermal cells in itr-1(jc5) embryos exhibit abnormally long lifetimes. Pharmacological studies with the inositol 1,4,5-trisphosphate antagonist xestospongin C phenocopy these defects, confirming that ITR function is important for proper epidermal migration. Our results provide the first molecular evidence that movements of embryonic epithelial cell sheets can be controlled by ITRs and suggest that such regulation may be a widespread mechanism for coordinating epithelial cell movements during embryogenesis.

Store-operated Ca2+ entry: Vesicle fusion or reversible trafficking and de novo conformational coupling?

Store-operated Ca2+ entry (SOCE), a mechanism regulated by the filling state of the intracellular Ca2+ stores, is a major pathway for Ca2+ influx. Hypotheses to explain the communication between the Ca2+ stores and plasma membrane (PM) have considered both the existence of small messenger molecules, such as a Ca2+-influx factor (CIF), and both stable and de novo conformational coupling between proteins in the Ca2+ store and PM. Alternatively, a secretion-like coupling model based on vesicle fusion and channel insertion in the PM has been proposed, which shares some properties with the de novo conformational coupling model, such as the role of the actin cytoskeleton and soluble N-ethylmaleimide (NEM)-sensitive-factor attachment proteins receptor (SNARE) proteins. Here we review recent progress made in the characterization of the de novo conformational coupling and the secretion-like coupling models for SOCE. We pay particular attention into the involvement of SNARE proteins and the actin cytoskeleton in both SOCE models. SNAREs are recognized as proteins involved in exocytosis, participating in vesicle transport, membrane docking, and fusion. As with secretion, a role for the cortical actin network in Ca2+ entry has been demonstrated in a number of cell types. In resting cells, the cytoskeleton may prevent the interaction between the Ca2+ stores and the PM, or preventing fusion of vesicles containing Ca2+ channels with the PM. These are processes in which SNARE proteins might play a crucial role upon cell activation by directing a precise interaction between the membrane of the transported organelle and the PM.

Cleavage of SNAP-25 and VAMP-2 impairs store-operated Ca2+entry in mouse pancreatic acinar cells

We recently reported that store-operated Ca2+entry (SOCE) in nonexcitable cells is likely to be mediated by a reversible interaction between Ca2+channels in the plasma membrane and the endoplasmic reticulum, a mechanism known as “secretion-like coupling.” As for secretion, in this model the actin cytoskeleton plays a key regulatory role. In the present study we have explored the involvement of the secretory proteins synaptosome-associated protein (SNAP-25) and vesicle-associated membrane protein (VAMP) in SOCE in pancreatic acinar cells. Cleavage of SNAP-25 and VAMPs by treatment with botulinum toxin A (BoNT A) and tetanus toxin (TeTx), respectively, effectively inhibited amylase secretion stimulated by the physiological agonist CCK-8. BoNT A significantly reduced Ca2+entry induced by store depletion using thapsigargin or CCK-8. In addition, treatment with BoNT A once SOCE had been activated reduced Ca2+influx, indicating that SNAP-25 is needed for both the activation and maintenance of SOCE in pancreatic acinar cells. VAMP-2 and VAMP-3 are expressed in mouse pancreatic acinar cells. Both proteins associate with the cytoskeleton upon Ca2+store depletion, although only VAMP-2 seems to be sensitive to TeTx. Treatment of pancreatic acinar cells with TeTx reduced the activation of SOCE without affecting its maintenance. These findings support a role for SNAP-25 and VAMP-2 in the activation of SOCE in pancreatic acinar cells and show parallels between this process and secretion in a specialized secretory cell type.

Investigation into the relationship between calyculin A-mediated potentiation of NADPH oxidase activity and inhibition of store-operated uptake of calcium by human neutrophils

The primary objective of the current study was to investigate possible relationships between calyculin A (CA)-mediated potentiation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and inhibition of store-operated uptake of Ca2+ by chemoattractant-activated human neutrophils. Treatment of neutrophils with 100 nM CA, but not at lower concentrations (12.5-50 nM), prior to the addition of the N-formylated chemotactic tripeptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) (1 microM), both potentiated and prolonged the activity of NADPH oxidase which was accompanied by exaggerated membrane depolarisation, delayed and attenuated membrane repolarisation, and inhibition of store-operated Ca2+ influx. Inclusion of diphenylene iodonium chloride (DPI, 10 microM), an inhibitor of NADPH oxidase, antagonised the effects of CA on NADPH oxidase activity and the membrane repolarisation responses of FMLP-activated neutrophils, but failed to restore store-operated influx of Ca2+. Similarly, CA also inhibited store-operated influx of Ca2+ into FMLP-activated neutrophils from a patient with chronic granulomatous disease, a primary immunodeficiency disorder characterised by the absence of a functional NADPH oxidase. CA also inhibited the store-operated influx of Ca2+ into control neutrophils treated with 1 microM thapsigargin, a selective inhibitor of the endomembrane Ca2+-ATPase, which does not activate NADPH oxidase. Taken together, these observations demonstrate that augmentation of NADPH oxidase activity is not primarily involved in CA-mediated inhibition of the store-operated influx of Ca2+ into activated human neutrophils.

Stimulation of cellular free Ca2+ elevation and inhibition of store-operated Ca2+ entry by kazinol B in neutrophils

Kazinol B, a natural isoprenylated flavan, stimulated the [Ca(2+)](i) elevation in the presence or absence of Ca(2+) in the medium. Treatment with chymotrypsin or phorbol 12-myristate 13-acetate to shedding of L: -selectin had no effect on subsequent kazinol B-induced Ca(2+) response. Upon initial cyclopiazonic acid (CPA) treatment in the absence of external Ca(2+), the subsequent [Ca(2+)](i) rise followed by challenge with kazinol B was greatly diminished. The ryanodine receptor blockers, 8-bromo-cyclic ADP-ribose and ruthenium red did not affect kazinol B-evoked Ca(2+) release from internal stores. However, the inhibitors of sphingosine kinase, dimethylsphingosine, but not dihydrosphingosine, inhibited kazinol B-induced Ca(2+) release. Kazinol B-induced [Ca(2+)](i) rise was not affected by two nitric oxidase inhibitors, N-(3-aminomethyl)benzylacetamidine (1400W) and 7-nitroindazole, cytochalasin B and Na(+)-deprivation. This response was slightly attenuated by 2-aminoethyldiphenyl borate (2-APB), a D: -myo-inositol 1,4,5-trisphosphate (IP(3)) receptor blocker, and by genistein, a general tyrosine kinase inhibitor. However, the Ca(2+) response was greatly diminished by two actin filament reorganizers, calyculin A and jasplakinolide, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY 294002), an inhibitor of phosphoinositide 3-kinase, N-(3-aminomethyl)benzylacetamidine (SB 203580), the p38 mitogen-activated protein kinase inhibitor, 1-[6-[17beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U-73122), the inhibitor of phospholipase C-coupled processes, and by 0.3 mM La(3+) or Ni(2+). Kazinol B did not evoke any appreciable Ba(2+) and Sr(2+) entry into cells. The Ca(2+) entry blockers, 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole (SKF-96365), but not cis-N-(2-phenylcyclopentyl)azacyclotridec-1-en-2-amine (MDL-12,330A), inhibited a kazinol B-induced [Ca(2+)](i) rise. Kazinol B had no effect on the pharmacologically isolated plasma membrane Ca(2+)-ATPase activity. In a Ca(2+)-free medium, kazinol B inhibited the subsequent Ca(2+) addition, resulting in robust entry in CPA- and formyl peptide-activated cells. Kazinol B produced a concentration-dependent reduction in the mitochondrial membrane potential. These results indicate that kazinol B stimulates Ca(2+) release from internal Ca(2+) store, probably through the sphingosine 1-phosphate and IP(3) signaling, and activates external Ca(2+) influx mainly through a non-store-operated Ca(2+) entry (non-SOCE) pathway. Inhibition of SOCE by kazinol B is probably attributable to a break in the Ca(2+) driven force of mitochondria.

Sustained stimulation of platelet thrombin receptor is associated with tyrosine dephosphorylation of a novel p67 peptide in a manner regulated by extracellular calcium

Signaling pathways elicited by protease-activated receptor-1 (PAR-1) agonists, thrombin receptor-activating peptide (TRAP) and thrombin, are markedly different. Here we show that TRAP-induced disaggregation of platelets is a function of extracellular calcium. Chelation of calcium with EGTA after the onset of aggregation precluded subsequent destabilization of the aggregates in TRAP-stimulated platelets, whereas disaggregation was not observed in the platelets stimulated with thrombin. TRAP-induced disaggregation was independent of the activity of the calcium-dependent thiol protease, calpain. Inhibition of phosphoinositide 3-kinase activity provoked further destabilization of the platelet aggregates in the presence of calcium; however, EGTA attenuated this effect. Activation of protein kinase C (PKC) by phorbol ester prevented disaggregation of the TRAP-stimulated platelets independent of the extracellular calcium. Two proteins of relative mobilities 67 and 75 kD were found to be significantly dephosphorylated on tyrosine in calcium-pretreated platelets as compared to the EGTA-treated platelets following continued stimulation with either TRAP or thrombin for 15 min. Inhibition of phosphoinositide 3-kinase by two pharmacologically independent inhibitors also caused dephosphorylation of p67, which was completely abrogated by chelation of extracellular calcium. Platelet activation by phorbol ester was not associated with disaggregation, although dephosphorylation of p67 was induced under this condition. SHP-1, an abundant tyrosine phosphatase in platelets, co-migrated with the p67 protein and co-localized to the actin-based cytoskeleton of aggregated platelets; however, its identity with p67 was ruled out from immunoprecipitation studies.

Differentiation of answer of glioma C6 cells to SERCA pump inhibitors by actin disorganization

Capacitative calcium entry, usually evoked by receptor-ligand binding, may be also studied in the model system of calcium release after SERCA pump inhibition. We have previously found that disorganization of actin cytoskeleton has no effect on calcium influx into glioma C6 cells after thapsigargin administration [Biochem. Biophys. Res. Commun. 296 (2002) 484]. In the present work we show that the effect of other SERCA pump inhibitors depends on the endoplasmic reticulum distribution in a cell. Changing this distribution leads to changes in calcium release from ER stores. Intensity of calcium influx in the capacitative phase of cell answer does not depend on actin cytoskeleton state; however, administration of cytochalasin D significantly slows down signal build-up. While cyclopiazonic acid acts very similarly to thapsigargin, cytoskeleton disorganization leads to rise of calcium signal after administration of 2,5-di-(t-butyl)-1,4-benzohydroquinone. This effect may be caused by specific binding of this inhibitor to SERCA3 isoform of pump protein only.

A role for hTRPC1 and lipid raft domains in store-mediated calcium entry in human platelets

We have previously suggested that store-mediated Ca2+ entry (SMCE) in human platelets may be activated by a secretion-like coupling model, involving de novo coupling of the type II inositol 1,4,5-trisphosphate receptor (IP(3)RII) to the putative Ca2+ entry channel, hTRPC1. In other cells, hTRPC1 has been reported to be associated with cholesterol-rich lipid raft domains (LRDs) in the plasma membrane. Here we have shown that hTRPC1 is largely associated with detergent-resistant platelet membranes, from which it is partially released when the cells are depleted of cholesterol by treatment with methyl-beta-cyclodextrin (MBCD). MBCD treatment inhibited thapsigargin (TG)-evoked SMCE in a concentration-dependent manner, reducing it to 38.1+/-4.1% at a concentration of 10mM. Similarly, the Ca2+ entry evoked by thrombin (1unit/ml) was reduced to 48.2+/-4.5% of control following MBCD (10mM) treatment. Thrombin- and TG-evoked coupling between IP(3)RII and hTRPC1 was also reduced following cholesterol depletion. These results suggest that hTRPC1 is associated with LRDs in human platelets and that these domains are important for its participation in SMCE.

Two Pathways for Store-mediated Calcium Entry Differentially Dependent on the Actin Cytoskeleton in Human Platelets

A major pathway for stimulated Ca(2+) entry in non-excitable cells is activated following depletion of intracellular Ca(2+) stores. Secretion-like coupling between elements in the plasma membrane (PM) and Ca(2+) stores has been proposed as the most likely mechanism to activate this store-mediated Ca(2+) entry (SMCE) in several cell types. Here we identify two mechanisms for SMCE in human platelets activated by depletion of two independent Ca(2+) pools, which are differentially modulated by the actin cytoskeleton. Ca(2+) entry induced by depletion of a 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ)-sensitive pool is increased by disassembly of the actin cytoskeleton and that induced by a TBHQ-insensitive pool is reduced. Stabilization of the actin cytoskeleton prevented Ca(2+) entry by both mechanisms. We propose that the membrane-associated actin network prevents constitutive Ca(2+) entry via both pathways. Reorganization of the actin cytoskeleton permits the activation of Ca(2+) entry via both mechanisms, but only SMCE activated by the TBHQ-insensitive pool requires new actin polymerization, which may support membrane trafficking toward the PM.

Inositol 1,4,5-trisphosphate receptors as signal integrators

The inositol 1,4,5 trisphosphate (IP3) receptor (IP3R) is a Ca2+ release channel that responds to the second messenger IP3. Exquisite modulation of intracellular Ca2+ release via IP3Rs is achieved by the ability of IP3R to integrate signals from numerous small molecules and proteins including nucleotides, kinases, and phosphatases, as well as nonenzyme proteins. Because the ion conduction pore composes only approximately 5% of the IP3R, the great bulk of this large protein contains recognition sites for these substances. Through these regulatory mechanisms, IP3R modulates diverse cellular functions, which include, but are not limited to, contraction/excitation, secretion, gene expression, and cellular growth. We review the unique properties of the IP3R that facilitate cell-type and stimulus-dependent control of function, with special emphasis on protein-binding partners.