Store-Operated Ca2+ Entry

Restoration of calcium release synchrony: A novel target for heart failure and ventricular arrhythmia

Myocardial calcium (Ca2+) signaling plays a crucial role in contractile function and membrane electrophysiology. An abnormal myocardial Ca2+ transient is linked to heart failure and ventricular arrhythmias. At the subcellular level, the synchronous release of Ca2+ sparks from sarcoplasmic Ca2+ release units determines the configuration and amplitude of the global Ca2+ transient. This narrative review evaluates the role of aberrant Ca2+ release synchrony in the pathophysiology of cardiomyopathies and ventricular arrhythmias. The potential therapeutic benefits of restoration of Ca2+ release synchrony in heart failure and ventricular arrhythmias are also discussed.

The endoplasmic reticulum stress and unfolded protein response in Alzheimer's disease: a calcium dyshomeostasis perspective

Protein misfolding is prominent in early cellular pathology of Alzheimer's disease (AD), implicating pathophysiological significance of endoplasmic reticulum stress/unfolded protein response (ER stress/UPR) and highlighting it as a target for drug development. Experimental data from animal AD models and observations on human specimens are, however, inconsistent. ER stress and associated UPR are readily observed in in vitro AD cellular models and in some AD model animals. In the human brain, components and markers of ER stress as well as UPR transducers are observed at Braak stages III-VI associated with severe neuropathology and neuronal death. The picture, however, is further complicated by the brain region- and cell type-specificity of the AD-related pathology. Terms 'disturbed' or 'non-canonical' ER stress/UPR were used to describe the discrepancies between experimental data and the classic ER stress/UPR cascade. Here we discuss possible 'disturbing' or 'interfering' factors which may modify ER stress/UPR in the early AD pathogenesis. We focus on the dysregulation of the ER Ca²⁺ homeostasis, store-operated Ca²⁺ entry, and the interaction between the ER and mitochondria. We suggest that a detailed study of the CNS cell type-specific alterations of Ca²⁺ homeostasis in early AD may deepen our understanding of AD-related dysproteostasis.

Relevance of stromal interaction molecule 1 (STIM1) in experimental and human stroke

Stroke represents a main cause of death and permanent disability worldwide. In the attempt to develop targeted preventive and therapeutic strategies, several efforts were performed over the last decades to identify the specific molecular abnormalities preceding cerebral ischemia and neuronal death. In this regard, mitochondrial dysfunction, autophagy, and intracellular calcium homeostasis appear important contributors to stroke development, as underscored by recent pre-clinical evidence. Intracellular calcium (Ca²⁺) homeostasis is regulated, among other mechanisms, by the calcium sensor stromal interaction molecule 1 (STIM1) and calcium release-activated calcium modulator (ORAI) members, which mediate the store-operated Ca²⁺ entry (SOCE). The activity of SOCE is deregulated in animal models of ischemic stroke, leading to ischemic injury exacerbation. We found a different pattern of expression of few SOCE components, dependent from a STIM1 mutation, in cerebral endothelial cells isolated from the stroke-prone spontaneously hypertensive rat (SHRSP), compared to the stroke-resistant (SHRSR) strain, suggesting a potential involvement of this mechanism into the stroke predisposition of SHRSP. In this article, we discuss the relevant role of STIM1 in experimental stroke, as highlighted by the current literature and by our recent experimental findings, and the available evidence in the human disease. We also provide a glance on future perspectives and clinical implications of STIM1.

Expanding the clinical and genetic spectrum of pathogenic variants in STIM1

INTRODUCTION/AIMS

Stromal interaction molecule 1 (STIM1) is a reticular Ca²⁺ sensor composed of a luminal and a cytosolic domain. Autosomal dominant mutations in STIM1 cause tubular aggregate myopathy and Stormorken syndrome or its variant York platelet syndrome. In this study we aimed to expand the features related to new variants in STIM1.

METHODS

We performed a cross-sectional study of individuals harboring monoallelic STIM1 variants recruited at five tertiary centers involved in a study of inherited myopathies analyzed with a multigene-targeted panel.

RESULTS

We identified seven individuals (age range, 26-57 years) harboring variants in STIM1, including five novel changes: three located in the EF-hand domain, one in the sterile α motif (SAM) domain, and one in the cytoplasmatic region of the protein. Functional evaluation of the pathogenic variants using a heterologous expression system and measuring store-operated calcium entry demonstrated their causative role and suggested a link of new variants with the clinical phenotype. Muscle contractures, found in three individuals, showed variability in body distribution and in the number of joints involved. Three patients showed cardiac and respiratory involvement. Short stature, hyposplenism, sensorineural hearing loss, hypothyroidism, and Gilbert syndrome were variably observed among the patients. Laboratory tests revealed hyperCKemia in six patients, thrombocytopenia in two patients, and hypocalcemia in one patient. Muscle biopsy showed the presence of tubular aggregates in three patients, type I fiber atrophy in one patient, and nonspecific myopathic changes in two patients.

DISCUSSION

Our clinical, histological, and molecular data expand the genetic and clinical spectrum of STIM1-related diseases.

Pyridostigmine improves cardiac function and rhythmicity through RyR2 stabilization and inhibition of STIM1-mediated calcium entry in heart failure

Heart failure (HF) is characterized by asymmetrical autonomic balance. Treatments to restore parasympathetic activity in human heart failure trials have shown beneficial effects. However, mechanisms of parasympathetic-mediated improvement in cardiac function remain unclear. The present study examined the effects and underpinning mechanisms of chronic treatment with the cholinesterase inhibitor, pyridostigmine (PYR), in pressure overload HF induced by transverse aortic constriction (TAC) in mice. TAC mice exhibited characteristic adverse structural (left ventricular hypertrophy) and functional remodelling (reduced ejection fraction, altered myocyte calcium (Ca) handling, increased arrhythmogenesis) with enhanced predisposition to arrhythmogenic aberrant sarcoplasmic reticulum (SR) Ca release, cardiac ryanodine receptor (RyR2) hyper-phosphorylation and up-regulated store-operated Ca entry (SOCE). PYR treatment resulted in improved cardiac contractile performance and rhythmic activity relative to untreated TAC mice. Chronic PYR treatment inhibited altered intracellular Ca handling by alleviating aberrant Ca release and diminishing pathologically enhanced SOCE in TAC myocytes. At the molecular level, these PYR-induced changes in Ca handling were associated with reductions of pathologically enhanced phosphorylation of RyR2 serine-2814 and STIM1 expression in HF myocytes. These results suggest that chronic cholinergic augmentation alleviates HF via normalization of both canonical RyR2-mediated SR Ca release and non-canonical hypertrophic Ca signaling via STIM1-dependent SOCE.

Expression of canonical transient receptor potential channels in U-2 OS and MNNG-HOS osteosarcoma cell lines

In U-2 OS and MNNG-HOS osteosarcoma cells, small interfering RNA-mediated knockdown of the angiotensin-(1-7) receptor, Mas, increases cell proliferation. Whether alterations in canonical transient receptor potential channels (TRPC) expression contribute to this effect is not clear. In the present study, a basic description of TRPC subtype expression in osteosarcoma cell lines was provided. The pharmacological modulators of the angiotensin-(1-7) receptor, Mas, AVE0991 (agonist), or D-Ala⁷-Ang-(1-7) (antagonist) were applied to elucidate a possible role of Mas in the regulation of TRPC mRNA levels. The contribution of other G-protein coupled receptors (GPCR) or receptor tyrosine kinases to TRCP expression was studied by applying the selective pharmacological blockers of either PI3 kinase or MEK/Erk1/2 signaling, Ly294002 and PD98059. AVE0991 and D-Ala⁷-Ang-(1-7) exhibited no or marginal effects on TRPC mRNA expression. Ly294002 provoked a 9.6- and 5.9-fold increase in the amounts of TRPC5 mRNA in MNNG-HOS and U-2 OS cells, respectively. Additionally, Ly294002 increased TRPC6 mRNA levels; however, it had no effect on TRPCs 1, 3 and 4. Administration of PD98059 increased the amounts of TRPC6 and TRPC4 ~2-fold. In conclusion, the present study demonstrated that Mas-dependent alterations in osteosarcoma cell line proliferation were not mediated by any changes in TRPC subtype gene expression. The data shows in principle, and consistent with the literature, that the signaling pathways examined can regulate the expression of TRPCs at the mRNA level. Therefore, direct and signaling pathway-specific pharmacological targeting of TRPC subtypes may represent an option for improving the treatment of osteosarcoma.

A luminal EF-hand mutation in STIM1 in mice causes the clinical hallmarks of tubular aggregate myopathy

STIM and ORAI proteins play a fundamental role in calcium signaling, allowing for calcium influx through the plasma membrane upon depletion of intracellular stores, in a process known as store-operated Ca²⁺ entry. Point mutations that lead to gain-of-function activity of either STIM1 or ORAI1 are responsible for a cluster of ultra-rare syndromes characterized by motor disturbances and platelet dysfunction. The prevalence of these disorders is at present unknown. In this study, we describe the generation and characterization of a knock-in mouse model (KI-STIM1^I115F) that bears a clinically relevant mutation located in one of the two calcium-sensing EF-hand motifs of STIM1. The mouse colony is viable and fertile. Myotubes from these mice show an increased store-operated Ca²⁺ entry, as predicted. This most likely causes the dystrophic muscle phenotype observed, which worsens with age. Such histological features are not accompanied by a significant increase in creatine kinase. However, animals have significantly worse performance in rotarod and treadmill tests, showing increased susceptibility to fatigue, in analogy to the human disease. The mice also show increased bleeding time and thrombocytopenia, as well as an unexpected defect in the myeloid lineage and in natural killer cells. The present model, together with recently described models bearing the R304W mutation (located on the coiled-coil domain in the cytosolic side of STIM1), represents an ideal platform to characterize the disorder and test therapeutic strategies for patients with STIM1 mutations, currently without therapeutic solutions.

This article has an associated First Person interview with Celia Cordero-Sanchez, co-first author of the paper.

Summary: We describe a mouse model (KI-STIM1^I115F) that displays the clinical hallmarks of tubular aggregate myopathy. This model provides a new opportunity to characterize the disorder and test novel therapeutic strategies.

Enhancement of Cardiac Store Operated Calcium Entry (SOCE) within Novel Intercalated Disk Microdomains in Arrhythmic Disease

Store-operated Ca²⁺ entry (SOCE), a major Ca²⁺ signaling mechanism in non-myocyte cells, has recently emerged as a component of Ca²⁺ signaling in cardiac myocytes. Though it has been reported to play a role in cardiac arrhythmias and to be upregulated in cardiac disease, little is known about the fundamental properties of cardiac SOCE, its structural underpinnings or effector targets. An even greater question is how SOCE interacts with canonical excitation-contraction coupling (ECC). We undertook a multiscale structural and functional investigation of SOCE in cardiac myocytes from healthy mice (wild type; WT) and from a genetic murine model of arrhythmic disease (catecholaminergic ventricular tachycardia; CPVT). Here we provide the first demonstration of local, transient Ca²⁺ entry (LoCE) events, which comprise cardiac SOCE. Although infrequent in WT myocytes, LoCEs occurred with greater frequency and amplitude in CPVT myocytes. CPVT myocytes also evidenced characteristic arrhythmogenic spontaneous Ca²⁺ waves under cholinergic stress, which were effectively prevented by SOCE inhibition. In a surprising finding, we report that both LoCEs and their underlying protein machinery are concentrated at the intercalated disk (ID). Therefore, localization of cardiac SOCE in the ID compartment has important implications for SOCE-mediated signaling, arrhythmogenesis and intercellular mechanical and electrical coupling in health and disease.

Hepatitis B virus PreS2-mutant large surface antigen activates store-operated calcium entry and promotes chromosome instability

Hepatitis B virus (HBV) is a driver of hepatocellular carcinoma, and two viral products, X and large surface antigen (LHBS), are viral oncoproteins. During chronic viral infection, immune-escape mutants on the preS2 region of LHBS (preS2-LHBS) are gain-of-function mutations that are linked to preneoplastic ground glass hepatocytes (GGHs) and early disease onset of hepatocellular carcinoma. Here, we show that preS2-LHBS provoked calcium release from the endoplasmic reticulum (ER) and triggered stored-operated calcium entry (SOCE). The activation of SOCE increased ER and plasma membrane (PM) connections, which was linked by ER- resident stromal interaction molecule-1 (STIM1) protein and PM-resident calcium release- activated calcium modulator 1 (Orai1). Persistent activation of SOCE induced centrosome overduplication, aberrant multipolar division, chromosome aneuploidy, anchorage-independent growth, and xenograft tumorigenesis in hepatocytes expressing preS2- LHBS. Chemical inhibitions of SOCE machinery and silencing of STIM1 significantly reduced centrosome numbers, multipolar division, and xenograft tumorigenesis induced by preS2-LHBS. These results provide the first mechanistic link between calcium homeostasis and chromosome instability in hepatocytes carrying preS2-LHBS. Therefore, persistent activation of SOCE represents a novel pathological mechanism in HBV-mediated hepatocarcinogenesis.

Stromal Interaction Molecule 1 (STIM1) Regulates ATP-sensitive Potassium (KATP) and Store-operated Ca2+ Channels in MIN6 β-Cells

Stromal interaction molecule 1 (STIM1) regulates store-operated Ca²⁺ entry (SOCE) and other ion channels either as an endoplasmic reticulum Ca²⁺-sensing protein or when present in the plasma membrane. However, the role of STIM1 in insulin-secreting β-cells is unresolved. We report that lowering expression of STIM1, the gene that encodes STIM1, in insulin-secreting MIN6 β-cells with RNA interference inhibits SOCE and ATP-sensitive K⁺ (K_ATP) channel activation. The effects of STIM1 knockdown were reversed by transduction of MIN6 cells with an adenovirus gene shuttle vector that expressed human STIM1 Immunoprecipitation studies revealed that STIM1 binds to nucleotide binding fold-1 (NBF1) of the sulfonylurea receptor 1 (SUR1) subunit of the K_ATP channel. Binding of STIM1 to SUR1 was enhanced by poly-lysine. Our data indicate that SOCE and K_ATP channel activity are regulated by STIM1. This suggests that STIM1 is a multifunctional signaling effector that participates in the control of membrane excitability and Ca²⁺ signaling events in β-cells.

Overexpression of STIM1 in neurons in mouse brain improves contextual learning and impairs long-term depression

STIM1 is an endoplasmic reticulum calcium sensor that is involved in several processes in neurons, including store-operated calcium entry. STIM1 also inhibits voltage-gated calcium channels, such as Ca_v1.2 and Ca_v3.1, and is thus considered a multifunctional protein. The aim of this work was to investigate the ways in which transgenic neuronal overexpression of STIM1 in FVB/NJ mice affects animal behavior and the electrophysiological properties of neurons in acute hippocampal slices. We overexpressed STIM1 from the Thy1.2 promoter and verified neuronal expression by quantitative reverse-transcription polymerase chain reaction, Western blot, and immunohistochemistry. Mature primary hippocampal cultures expressed STIM1 but exhibited no changes in calcium homeostasis. Basal synaptic transmission efficiency and short-term plasticity were comparable in slices that were isolated from transgenic mice, similarly as the magnitude of long-term potentiation. However, long-term depression that was induced by the glutamate receptor 1/5 agonist (S)-3,5-dihydroxyphenylglycine was impaired in STIM1 slices. Interestingly, transgenic mice exhibited a decrease in anxiety-like behavior and improvements in contextual learning. In summary, our data indicate that STIM1 overexpression in neurons in the brain perturbs metabotropic glutamate receptor signaling, leading to impairments in long-term depression and alterations in animal behavior. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

ADP-Induced Ca2+ Signaling and Proliferation of Rat Ventricular Myofibroblasts Depend on Phospholipase C-Linked TRP Channels Activation Within Lipid Rafts

Nucleotides released during heart injury affect myocardium electrophysiology and remodeling through P2 purinoceptors activation in cardiac myofibroblasts. ATP and UTP endorse [Ca²⁺ ]_i accumulation and growth of DDR-2/α-SMA-expressing myofibroblasts from adult rat ventricles via P2Y₄ and P2Y₂ receptors activation, respectively. Ventricular myofibroblasts also express ADP-sensitive P2Y₁ , P2Y₁₂ , and P2Y₁₃ receptors as demonstrated by immunofluorescence confocal microscopy and western blot analysis, but little information exists on ADP effects in these cells. ADP (0.003-3 mM) and its stable analogue, ADPßS (100 μM), caused fast [Ca²⁺ ]_i transients originated from thapsigargin-sensitive internal stores, which partially declined to a plateau sustained by capacitative Ca²⁺ entry through transient receptor potential (TRP) channels inhibited by 2-APB (50 μM) and flufenamic acid (100 μM). Hydrophobic interactions between G_q/11 -coupled P2Y purinoceptors and TRP channels were suggested by prevention of the ADP-induced [Ca²⁺ ]_i plateau following PIP₂ depletion with LiCl (10 mM) and cholesterol removal from lipid rafts with methyl-ß-cyclodextrin (2 mM). ADP [Ca²⁺ ]_i transients were insensitive to P2Y₁ , P2Y₁₂ , and P2Y₁₃ receptor antagonists, MRS2179 (10μM), AR-C66096 (0.1 μM), and MRS2211 (10μM), respectively, but were attenuated by suramin and reactive blue-2 (100 μM) which also blocked P2Y₄ receptors activation by UTP. Cardiac myofibroblasts growth and type I collagen production were favored upon activation of MRS2179-sensitive P2Y₁ receptors with ADP or ADPßS (30 μM). In conclusion, ADP exerts a dual role on ventricular myofibroblasts: [Ca²⁺ ]_i transients are mediated by fast-desensitizing P2Y₄ receptors, whereas the pro-fibrotic effect of ADP involves the P2Y₁ receptor activation. Data also show that ADP-induced capacitative Ca²⁺ influx depends on phospholipase C-linked TRP channels opening in lipid raft microdomains. J. Cell. Physiol. 232: 1511-1526, 2017. © 2016 Wiley Periodicals, Inc.

Aristoyunnolin H attenuates extracellular matrix secretion in cardiac fibroblasts by inhibiting calcium influx

Aristoyunnolin H is a novel aristophyllene sesquiterpenoid isolated from the traditional Chinese medicine Aristolochia yunnanensis Franch. The present research was designed to explore the anti-fibrotic effects of aristoyunnolin H in adult rat cardiac fibroblasts (CFs) stimulated with angiotensin II (Ang II). Western blot analysis data showed that aristoyunnolin H reduced the upregulation of fibronectin (FN), connective tissue growth factor and collagen I(Col I) production induced by Ang II in CFs. By studying the dynamic intracellular changes of Ca²⁺, we further found that while aristoyunnolin H relieved the calcium influx, it has no effect on intracellular calcium store release. Meanwhile, aristoyunnolin H also inhibited the Ang II-stimulated phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II. In conclusion, aristoyunnolin H may attenuate extracellular matrix secretion in vitro by inhibiting Ang II-induced calcium signaling.

Orai1 and TRPC1 Proteins Co-localize with CaV1.2 Channels to Form a Signal Complex in Vascular Smooth Muscle Cells

Voltage-dependent Ca_V1.2 L-type Ca²⁺ channels (LTCC) are the main route for calcium entry in vascular smooth muscle cells (VSMC). Several studies have also determined the relevant role of store-operated Ca²⁺ channels (SOCC) in vascular tone regulation. Nevertheless, the role of Orai1- and TRPC1-dependent SOCC in vascular tone regulation and their possible interaction with Ca_V1.2 are still unknown. The current study sought to characterize the co-activation of SOCC and LTCC upon stimulation by agonists, and to determine the possible crosstalk between Orai1, TRPC1, and Ca_V1.2. Aorta rings and isolated VSMC obtained from wild type or smooth muscle-selective conditional Ca_V1.2 knock-out (Ca_V1.2^KO) mice were used to study vascular contractility, intracellular Ca²⁺ mobilization, and distribution of ion channels. We found that serotonin (5-HT) or store depletion with thapsigargin (TG) enhanced intracellular free Ca²⁺ concentration ([Ca²⁺]_i) and stimulated aorta contraction. These responses were sensitive to LTCC and SOCC inhibitors. Also, 5-HT- and TG-induced responses were significantly attenuated in Ca_V1.2^KO mice. Furthermore, hyperpolarization induced with cromakalim or valinomycin significantly reduced both 5-HT and TG responses, whereas these responses were enhanced with LTCC agonist Bay-K-8644. Interestingly, in situ proximity ligation assay revealed that Ca_V1.2 interacts with Orai1 and TRPC1 in untreated VSMC. These interactions enhanced significantly after stimulation of cells with 5-HT and TG. Therefore, these data indicate for the first time a functional interaction between Orai1, TRPC1, and Ca_V1.2 channels in VSMC, confirming that upon agonist stimulation, vessel contraction involves Ca²⁺ entry due to co-activation of Orai1- and TRPC1-dependent SOCC and LTCC.

Transient Receptor Potential Canonical (TRPC)/Orai1-dependent Store-operated Ca2+ Channels: NEW TARGETS OF ALDOSTERONE IN CARDIOMYOCYTES

Store-operated Ca(2+) entry (SOCE) has emerged as an important mechanism in cardiac pathology. However, the signals that up-regulate SOCE in the heart remain unexplored. Clinical trials have emphasized the beneficial role of mineralocorticoid receptor (MR) signaling blockade in heart failure and associated arrhythmias. Accumulated evidence suggests that the mineralocorticoid hormone aldosterone, through activation of its receptor, MR, might be a key regulator of Ca(2+) influx in cardiomyocytes. We thus assessed whether and how SOCE involving transient receptor potential canonical (TRPC) and Orai1 channels are regulated by aldosterone/MR in neonatal rat ventricular cardiomyocytes. Molecular screening using qRT-PCR and Western blotting demonstrated that aldosterone treatment for 24 h specifically increased the mRNA and/or protein levels of Orai1, TRPC1, -C4, -C5, and stromal interaction molecule 1 through MR activation. These effects were correlated with a specific enhancement of SOCE activities sensitive to store-operated channel inhibitors (SKF-96365 and BTP2) and to a potent Orai1 blocker (S66) and were prevented by TRPC1, -C4, and Orai1 dominant negative mutants or TRPC5 siRNA. A mechanistic approach showed that up-regulation of serum- and glucocorticoid-regulated kinase 1 mRNA expression by aldosterone is involved in enhanced SOCE. Functionally, 24-h aldosterone-enhanced SOCE is associated with increased diastolic [Ca(2+)]i, which is blunted by store-operated channel inhibitors. Our study provides the first evidence that aldosterone promotes TRPC1-, -C4-, -C5-, and Orai1-mediated SOCE in cardiomyocytes through an MR and serum- and glucocorticoid-regulated kinase 1 pathway.

Molecular mechanisms of STIM/Orai communication

Ca²⁺ entry into the cell via store-operated Ca²⁺ release-activated Ca²⁺ (CRAC) channels triggers diverse signaling cascades that affect cellular processes like cell growth, gene regulation, secretion, and cell death. These store-operated Ca²⁺ channels open after depletion of intracellular Ca²⁺ stores, and their main features are fully reconstituted by the two molecular key players: the stromal interaction molecule (STIM) and Orai. STIM represents an endoplasmic reticulum-located Ca²⁺ sensor, while Orai forms a highly Ca²⁺-selective ion channel in the plasma membrane. Functional as well as mutagenesis studies together with structural insights about STIM and Orai proteins provide a molecular picture of the interplay of these two key players in the CRAC signaling cascade. This review focuses on the main experimental advances in the understanding of the STIM1-Orai choreography, thereby establishing a portrait of key mechanistic steps in the CRAC channel signaling cascade. The focus is on the activation of the STIM proteins, the subsequent coupling of STIM1 to Orai1, and the consequent structural rearrangements that gate the Orai channels into the open state to allow Ca²⁺ permeation into the cell.

A stromal interaction molecule 1 variant up-regulates matrix metalloproteinase-2 expression by strengthening nucleoplasmic Ca2+ signaling

Very recent studies hold promise to reveal the role of stromal interaction molecule 1 (STIM1) in non-store-operated Ca2+ entry. Here we showed that in contrast to cytoplasmic membrane redistribution as previously noted, human umbilical vein endothelial STIM1 with a T-to-C nucleotide transition resulting in an amino acid substitution of leucine by proline in the signal peptide sequence translocated to perinuclear membrane upon intracellular Ca2+ depletion, amplified nucleoplasmic Ca2+ signaling through ryanodine receptor-dependent pathway, and enhanced the subsequent cAMP responsive element binding protein activity, matrix metalloproteinase-2 (MMP-2) gene expression, and endothelial tube forming. The abundance of mutated STIM1 and the MMP-2 expression were higher in native human umbilical vein endothelial cells of patients with gestational hypertension than controls and were significantly correlated with blood pressure. These findings broaden our understanding about structure-function bias of STIM1 and offer unique insights into its application in nucleoplasmic Ca2+, MMP-2 expression, endothelial dysfunction, and pathophysiological mechanism(s) of gestational hypertension.

Phospholipase A2 as a Molecular Determinant of Store-Operated Calcium Entry

Activation of phospholipases A2 (PLA2) leads to the generation of biologically active lipid products that can affect numerous cellular events. Ca(2+)-independent PLA2 (iPLA2), also called group VI phospholipase A2, is one of the main types forming the superfamily of PLA2. Beside of its role in phospholipid remodeling, iPLA2 has been involved in intracellular Ca(2+) homeostasis regulation. Several studies proposed iPLA2 as an essential molecular player of store operated Ca(2+) entry (SOCE) in a large number of excitable and non-excitable cells. iPLA2 activation releases lysophosphatidyl products, which were suggested as agonists of store operated calcium channels (SOCC) and other TRP channels. Herein, we will review the important role of iPLA2 on the intracellular Ca(2+) handling focusing on its role in SOCE regulation and its implication in physiological and/or pathological processes.

Modulation of Calcium Entry by Mitochondria

The role of mitochondria in intracellular Ca(2+) signaling relies mainly in its capacity to take up Ca(2+) from the cytosol and thus modulate the cytosolic [Ca(2+)]. Because of the low Ca(2+)-affinity of the mitochondrial Ca(2+)-uptake system, this organelle appears specially adapted to take up Ca(2+) from local high-Ca(2+) microdomains and not from the bulk cytosol. Mitochondria would then act as local Ca(2+) buffers in cellular regions where high-Ca(2+) microdomains form, that is, mainly close to the cytosolic mouth of Ca(2+) channels, both in the plasma membrane and in the endoplasmic reticulum (ER). One of the first targets proposed already in the 1990s to be regulated in this way by mitochondria were the store-operated Ca(2+) channels (SOCE). Mitochondria, by taking up Ca(2+) from the region around the cytosolic mouth of the SOCE channels, would prevent its slow Ca(2+)-dependent inactivation, thus keeping them active for longer. Since then, evidence for this mechanism has accumulated mainly in immunitary cells, where mitochondria actually move towards the immune synapse during T cell activation. However, in many other cell types the available data indicate that the close apposition between plasma and ER membranes occurring during SOCE activation precludes mitochondria from getting close to the Ca(2+)-entry sites. Alternative pathways for mitochondrial modulation of SOCE, both Ca(2+)-dependent and Ca(2+)-independent, have also been proposed, but further work will be required to elucidate the actual mechanisms at work. Hopefully, the recent knowledge of the molecular nature of the mitochondrial Ca(2+) uniporter will allow soon more precise studies on this matter.

Effects of stromal interaction molecule 1 or Orai1 overexpression on the associated proteins and permeability of podocytes

AIM

The present study was conducted to determine whether two important signalling molecules of store-operated channel (SOC), stromal interaction molecule 1 (STIM1) and Orai1, were involved in glomerular podocyte injury. We explored the effects of STIM1/Orai1 overexpression on podocyte associated proteins and cell permeability.

METHODS

The expression of STIM1 and Orai1 were examined in the renal cortex of adriamycin-induced nephropathy mice by real-time RT-PCR. The recombinant plasmid of STIM1/Orai1, identified by restriction enzyme digestion and PCR, was transfected into MPC5 cells via lipofectamine 2000. The transfecting efficiency was observed by a fluorescence microscope. RT-PCR and Western blotting were used to evaluate the expression levels of STIM1, Orai 1 and some podocyte-associated molecules in the transfected MPC5 cells. In addition, we examined the diffusion of FITC-dextran across the podocyte monolayer to investigate whether STIM1/Orai1 overexpression could affect cell permeability.

RESULTS

We found that the mRNA levels of STIM1 and Orai1 were increased in adriamycin-induced nephropathy mice. STIM1/Orai1 overexpression significantly decreased the expression of podocin and CD2-associated protein (CD2AP), whereas it increased the expression of α-actinin-4. The permeability was significantly increased in the STIM1/Orai1 overexpression group.

CONCLUSION

Our findings suggested that STIM1/Orai1 overexpression could affect the cell permeability and the expression of partial podocyte-associated proteins, which may ultimately result in podocyte injury.

Roles of store-operated Ca2+ channels in regulating cell cycling and migration of human cardiac c-kit+ progenitor cells

Cardiac c-kit(+) progenitor cells are important for maintaining cardiac homeostasis and can potentially contribute to myocardial repair. However, cellular physiology of human cardiac c-kit(+) progenitor cells is not well understood. The present study investigates the functional store-operated Ca(2+) entry (SOCE) channels and the potential role in regulating cell cycling and migration using confocal microscopy, RT-PCR, Western blot, coimmunoprecipitation, cell proliferation, and migration assays. We found that SOCE channels mediated Ca(2+) influx, and TRPC1, STIM1, and Orai1 were involved in the formation of SOCE channels in human cardiac c-kit(+) progenitor cells. Silencing TRPC1, STIM1, or Orai1 with the corresponding siRNA significantly reduced the Ca(2+) signaling through SOCE channels, decreased cell proliferation and migration, and reduced expression of cyclin D1, cyclin E, and/or p-Akt. Our results demonstrate the novel information that Ca(2+) signaling through SOCE channels regulates cell cycling and migration via activating cyclin D1, cyclin E, and/or p-Akt in human cardiac c-kit(+) cells.

FKBP25 and FKBP38 regulate non-capacitative calcium entry through TRPC6

Non-capacitative calcium entry (NCCE) contributes to cell activation in response to the occupation of G protein-coupled membrane receptors. Thrombin administration to platelets evokes the synthesis of diacylglycerol downstream of PAR receptor activation. Diacylglycerol evokes NCCE through activating TRPC3 and TRPC6 in human platelets. Although it is known that immunophilins interact with TRPCs, the role of immunophilins in the regulation of NCCE remains unknown. Platelet incubation with FK506, an immunophilin antagonist, reduced OAG-evoked NCCE in a concentration-dependent manner, an effect that was independent on the inactivation of calcineurin (CaN). FK506 was unable to reduce NCCE evoked by OAG in platelets from TRPC6-/- mice. In HEK-293 cells overexpressing TRPC6, currents through TRPC6 were altered in the presence of FK506. We have found interaction between FKBP38 and other FKBPs, like FKBP25, FKBP12, and FKBP52 that were not affected by FK506, as well as with calmodulin (CaM). FK506 modified the pattern of association between FKBP25 and TRPCs as well as impaired OAG-evoked TRPC3 and TRPC6 coupling in both human and mouse platelets. By performing biotinylation experiments we have elucidated that FKBP25 and FKBP38 might be found at different cellular location, the plasma membrane and the already described intracellular locations. Finally, FKBP25 and FKBP38 silencing significantly inhibits OAG-evoked NCCE in MEG-01 and HEK293 cells, while overexpression of FKBP38 does not modify NCCE in HEK293 cells. All together, these findings provide strong evidence for a role of immunophilins, including FKBP25 and FKBP38, in NCCE mediated by TRPC6.

Differential regulation of calcium signalling pathways by components of Piper methysticum ('Awa)

Kava is a soporific, anxiolytic and relaxant in widespread ritual and recreational use throughout the Pacific. Traditional uses of kava by indigenous Pacific Island peoples reflect a complex pharmacopeia, centered on GABA-ergic effects of the well-characterized kavalactones. However, peripheral effects of kava suggest active components other than the CNS-targeted kavalactones. We have previously shown that immunocytes exhibit calcium mobilization in response to traditionally prepared kava extracts, and that the kavalactones do not induce these calcium responses. Here, we characterize the complex calcium-mobilizing activity of traditionally prepared and partially HPLC-purified kava extracts, noting induction of both calcium entry and store release pathways. Kava components activate intracellular store depletion of thapsigargin-sensitive and -insensitive stores that are coupled to the calcium release activated (CRAC) current, and cause calcium entry through non-store-operated pathways. Together with the pepper-like potency reported by kava users, these studies lead us to hypothesize that kava extracts contain one or more ligands for the transient receptor potential (TRP) family of ion channels. Indeed, TRP-like conductances are observed in kava-treated cells under patch clamp. Thus TRP-mediated cellular effects may be responsible for some of the reported pharmacology of kava.

Mitochondrial Ca²⁺ uniporter is critical for store-operated Ca²⁺ entry-dependent breast cancer cell migration

Metastasis of cancer cells is a complicated multistep process requiring extensive and continuous cytosolic calcium modulation. Mitochondrial Ca(2+) uniporter (MCU), a regulator of mitochondrial Ca(2+) uptake, has been implicated in energy metabolism and various cellular signaling processes. However, whether MCU contributes to cancer cell migration has not been established. Here we examined the expression of MCU mRNA in the Oncomine database and found that MCU is correlated to metastasis and invasive breast cancer. MCU inhibition by ruthenium red (RuR) or MCU silencing by siRNA abolished serum-induced migration in MDA-MB-231 breast cancer cells and reduced serum- or thapsigargin (TG)-induced store-operated Ca2+ entry (SOCE). Serum-induced migrations in MDA-MB-231 cells were blocked by SOCE inhibitors. Our results demonstrate that MCU plays a critical role in breast cancer cell migration by regulating SOCE.

New insights into pharmacological tools to TR(i)P cancer up

The aim of this review is to address the recent advances regarding the use of pharmacological agents to target transient receptor potential (TRP) channels in cancer and their potential application in therapeutics. Physiologically, TRP channels are responsible for cation entry (Ca(2+) , Na(+) , Mg(2+) ) in many mammalian cells and regulate a large number of cellular functions. However, dysfunction in channel expression and/or activity can be linked to human diseases like cancer. Indeed, there is growing evidence that TRP channel expression is altered in cancer tissues in comparison with normal ones. Moreover, these proteins are involved in many cancerous processes, including cell proliferation, apoptosis, migration and invasion, as well as resistance to chemotherapy. Among the TRP superfamily, TRPC, TRPV, TRPM and TRPA1 have been shown to play a role in many cancer types, including breast, digestive, gliomal, head and neck, lung and prostate cancers. Pharmacological modulators are used to characterize the functional implications of TRP channels in whole-cell membrane currents, resting membrane potential regulation and intracellular Ca(2+) signalling. Moreover, pharmacological modulation of TRP activity in cancer cells is systematically linked to the effect on cancerous processes (proliferation, survival, migration, invasion, sensitivity to chemotherapeutic drugs). Here we describe the effects of such TRP modulators on TRP activity and cancer cell phenotype. Furthermore, the potency and specificity of these agents will be discussed, as well as the development of new strategies for targeting TRP channels in cancer.

Remodeling of channel-forming ORAI proteins determines an oncogenic switch in prostate cancer

ORAI family channels have emerged as important players in malignant transformation, yet the way in which they reprogram cancer cells remains elusive. Here we show that the relative expression levels of ORAI proteins in prostate cancer are different from that in noncancerous tissue. By mimicking ORAI protein remodeling observed in primary tumors, we demonstrate in in vitro models that enhanced ORAI3 expression favors heteromerization with ORAI1 to form a novel channel. These channels support store-independent Ca(2+) entry, thereby promoting cell proliferation and a smaller number of functional homomeric ORAI1-based store-operated channels, which are important in supporting susceptibility to apoptosis. Thus, our findings highlight disrupted dynamic equilibrium of channel-forming proteins as an oncogenic mechanism.

Urotensin-II promotes vascular smooth muscle cell proliferation through store-operated calcium entry and EGFR transactivation

AIMS

Urotensin-II (UII) is a vasoactive peptide that promotes vascular smooth muscle cells (VSMCs) proliferation and is involved in the pathogenesis of atherosclerosis, restenosis, and vascular remodelling. This study aimed to determine the role of calcium (Ca(2+))-dependent signalling and alternative signalling pathways in UII-evoked VSMCs proliferation focusing on store-operated Ca(2+) entry (SOCE) and epithelium growth factor receptor (EGFR) transactivation.

METHODS AND RESULTS

We used primary cultures of VSMCs isolated from Wistar rat aorta to investigate the effects of UII on intracellular Ca(2+) mobilization, and proliferation determined by the 5-bromo-2-deoxyuridine (BrdU) assay. We found that UII enhanced intracellular Ca(2+) concentration ([Ca(2+)]i) which was significantly reduced by classical SOCE inhibitors and by knockdown of essential components of the SOCE such as stromal interaction molecule 1 (STIM1), Orai1, or TRPC1. Moreover, UII activated a Gd(3+)-sensitive current with similar features of the Ca(2+) release-activated Ca(2+) current (ICRAC). Additionally, UII stimulated VSMCs proliferation and Ca(2+)/cAMP response element-binding protein (CREB) activation through the SOCE pathway that involved STIM1, Orai1, and TRPC1. Co-immunoprecipitation experiments showed that UII promoted the association between Orai1 and STIM1, and between Orai1 and TRPC1. Moreover, we determined that EGFR transactivation, extracellular signal-regulated kinase (ERK) and Ca(2+)/calmodulin-dependent kinase (CaMK) signalling pathways were involved in both UII-mediated Ca(2+) influx, CREB activation and VSMCs proliferation.

CONCLUSION

Our data show for the first time that UII-induced VSMCs proliferation and CREB activation requires a complex signalling pathway that involves on the one hand SOCE mediated by STIM1, Orai1, and TRPC1, and on the other hand EGFR, ERK, and CaMK activation.

In silico investigation of the short QT syndrome, using human ventricle models incorporating electromechanical coupling

INTRODUCTION

Genetic forms of the Short QT Syndrome (SQTS) arise due to cardiac ion channel mutations leading to accelerated ventricular repolarization, arrhythmias and sudden cardiac death. Results from experimental and simulation studies suggest that changes to refractoriness and tissue vulnerability produce a substrate favorable to re-entry. Potential electromechanical consequences of the SQTS are less well-understood. The aim of this study was to utilize electromechanically coupled human ventricle models to explore electromechanical consequences of the SQTS.

METHODS AND RESULTS

The Rice et al. mechanical model was coupled to the ten Tusscher et al. ventricular cell model. Previously validated K(+) channel formulations for SQT variants 1 and 3 were incorporated. Functional effects of the SQTS mutations on [Ca(2+)] i transients, sarcomere length shortening and contractile force at the single cell level were evaluated with and without the consideration of stretch-activated channel current (I sac). Without I sac, at a stimulation frequency of 1Hz, the SQTS mutations produced dramatic reductions in the amplitude of [Ca(2+)] i transients, sarcomere length shortening and contractile force. When I sac was incorporated, there was a considerable attenuation of the effects of SQTS-associated action potential shortening on Ca(2+) transients, sarcomere shortening and contractile force. Single cell models were then incorporated into 3D human ventricular tissue models. The timing of maximum deformation was delayed in the SQTS setting compared to control.

CONCLUSION

The incorporation of I sac appears to be an important consideration in modeling functional effects of SQT 1 and 3 mutations on cardiac electro-mechanical coupling. Whilst there is little evidence of profoundly impaired cardiac contractile function in SQTS patients, our 3D simulations correlate qualitatively with reported evidence for dissociation between ventricular repolarization and the end of mechanical systole.

Structure of the neuromuscular junction: function and cooperative mechanisms in the synapse

As an overview of the structure of the neuromuscular junction, three items are described focusing on cooperative mechanisms involving the synapse and leading to muscle contraction: (1) presynaptic acetylcholine release regulated by vesicle cycling (exocytosis and endocytosis); the fast-mode of endocytosis requires a large influx of external Ca(2+) and is promoted by the activation of G protein-coupled receptors and receptor tyrosine kinases; (2) postsynaptic acetylcholine receptor clustering mediated by the muscle-specific, Dok7-stimulated tyrosine kinase (MuSK) through two signaling mechanisms: one via agrin-Lrp4-MuSK (Ig1/2 domains) and the second via Wnt-MuSK (Frizzled-like cysteine-rich domain)-adaptor Dishevelled; Wnts/MuSK and Lrp4 direct a retrograde signal to presynaptic differentiation; (3) muscle contractile machinery regulated by Ca(2+) -release and Ca(2+) -influx channels, including the depolarization-activated ryanodine receptor-1 and the receptor- and/or store-operated transient receptor potential canonical. The first mechanism is dysfunctional in Lambert-Eaton myasthenic syndrome, the second in anti-acetylcholine receptor-negative myasthenia gravis (MG), and the third in thymoma-associated MG.

The unique contribution of ion channels to platelet and megakaryocyte function

Ion channels are transmembrane proteins that play ubiquitous roles in cellular homeostasis and activation. In addition to their recognized role in the regulation of ionic permeability and thus membrane potential, some channel proteins possess intrinsic kinase activity, directly interact with integrins or are permeable to molecules up to ≈1000 Da. The small size and anuclear nature of the platelet has often hindered progress in understanding the role of specific ion channels in hemostasis, thrombosis and other platelet-dependent events. However, with the aid of transgenic mice and 'surrogate' patch clamp recordings from primary megakaryocytes, important unique contributions to platelet function have been identified for several classes of ion channel. Examples include ATP-gated P2X1 channels, Orai1 store-operated Ca2+ channels, voltage-gated Kv1.3 channels, AMPA and kainate glutamate receptors and connexin gap junction channels. Furthermore, evidence exists that some ion channels, such as NMDA glutamate receptors, contribute to megakaryocyte development. This review examines the evidence for expression of a range of ion channels in the platelet and its progenitor cell, and highlights the distinct roles that these proteins may play in health and disease.