Store-operated Ca2+ entry uncoupled with ryanodine receptor and junctional membrane complex in heart muscle cells

Store-Operated Calcium Entry Increases Nuclear Calcium in Adult Rat Atrial and Ventricular Cardiomyocytes

Store-operated calcium entry (SOCE) in cardiomyocytes may be involved in cardiac remodeling, but the underlying mechanisms remain elusive. We hypothesized that SOCE may increase nuclear calcium, which alters gene expression via calcium/calmodulin-dependent enzyme signaling, and elucidated the underlying cellular mechanisms. An experimental protocol was established in isolated adult rat cardiomyocytes to elicit SOCE by re-addition of calcium following complete depletion of sarcoplasmic reticulum (SR) calcium and to quantify SOCE in relation to the electrically stimulated calcium transient (CaT) measured in the same cell before SR depletion. Using confocal imaging, calcium changes were recorded simultaneously in the cytosol and in the nucleus of the cell. In ventricular myocytes, SOCE was observed in the cytosol and nucleus amounting to ≈15% and ≈25% of the respective CaT. There was a linear correlation between the SOCE-mediated calcium increase in the cytosol and nucleus. Inhibitors of TRPC or Orai channels reduced SOCE by ≈33-67%, whereas detubulation did not. In atrial myocytes, SOCE with similar characteristics was observed in the cytosol and nucleus. However, the SOCE amplitudes in atrial myocytes were ≈two-fold larger than in ventricular myocytes, and this was associated with ≈1.4- to 3.6-fold larger expression of putative SOCE proteins (TRPC1, 3, 6, and STIM1) in atrial tissue. The results indicated that SOCE in atrial and ventricular myocytes is able to cause robust calcium increases in the nucleus and that both TRPC and Orai channels may contribute to SOCE in adult cardiomyocytes.

Mechanisms of adaptive hypertrophic cardiac remodeling in a large animal model of premature ventricular contraction-induced cardiomyopathy

Frequent premature ventricular contractions (PVCs) promoted eccentric cardiac hypertrophy and reduced ejection fraction (EF) in a large animal model of PVC-induced cardiomyopathy (PVC-CM), but the molecular mechanisms and markers of this hypertrophic remodeling remain unexplored. Healthy mongrel canines were implanted with pacemakers to deliver bigeminal PVCs (50% burden with 200-220 ms coupling interval). After 12 weeks, left ventricular (LV) free wall samples were studied from PVC-CM and Sham groups. In addition to reduced LV ejection fraction (LVEF), the PVC-CM group showed larger cardiac myocytes without evident ultrastructural alterations compared to the Sham group. Biochemical markers of pathological hypertrophy, such as store-operated Ca2+ entry, calcineurin/NFAT pathway, β-myosin heavy chain, and skeletal type α-actin were unaltered in the PVC-CM group. In contrast, pro-hypertrophic and antiapoptotic pathways including ERK1/2 and AKT/mTOR were activated and/or overexpressed in the PVC-CM group, which appeared counterbalanced by an overexpression of protein phosphatase 1 and a borderline elevation of the anti-hypertrophic factor atrial natriuretic peptide. Moreover, the potent angiogenic and pro-hypertrophic factor VEGF-A and its receptor VEGFR2 were significantly elevated in the PVC-CM group. In conclusion, a molecular program is in place to keep this structural remodeling associated with frequent PVCs as an adaptive pathological hypertrophy.

The SOCE Machinery: An Unbalanced Knowledge between Left and Right Ventricular Pathophysiology

Right ventricular failure (RVF) is the most important prognostic factor for morbidity and mortality in pulmonary arterial hypertension (PAH) or pulmonary hypertension (PH) caused by left heart diseases. However, right ventricle (RV) remodeling is understudied and not targeted by specific therapies. This can be partly explained by the lack of basic knowledge of RV remodeling. Since the physiology and hemodynamic function of the RV differ from those of the left ventricle (LV), the mechanisms of LV dysfunction cannot be generalized to that of the RV, albeit a knowledge of these being helpful to understanding RV remodeling and dysfunction. Store-operated Ca²⁺ entry (SOCE) has recently emerged to participate in the LV cardiomyocyte Ca²⁺ homeostasis and as a critical player in Ca²⁺ mishandling in a pathological context. In this paper, we highlight the current knowledge on the SOCE contribution to the LV and RV dysfunctions, as SOCE molecules are present in both compartments. he relative lack of studies on RV dysfunction indicates the necessity of further investigations, a significant challenge over the coming years.

Activation of STIM1/Orai1‑mediated SOCE in sepsis‑induced myocardial depression

Unbalanced Ca2⁺ homeostasis serves an essential role in the occurrence and development of septic myocardial injury. However, the mechanism of Ca2⁺ homeostasis in septic myocardial depression is poorly understood due to the complexity of Ca2⁺ transporters in excitable cells. It was therefore hypothesized that cardiac dysfunction, myocardial injury and cardiac apoptosis in septic myocardial depression are associated with elevated intracellular Ca2⁺ concentrations caused by stromal interaction molecule 1 (STIM1)/Orai calcium release‑activated calcium modulator 1 (Orai1)‑mediated store‑operated Ca2⁺ entry (SOCE). A septic myocardial depression model was established using the cecal ligation and puncture operation (CLP) in mice and was simulated in H9C2 cells via lipopolysaccharide (LPS) stimulation. Cardiac function, myocardial injury, cardiac apoptosis and the expression levels of Bax, Bcl‑2, STIM1 and Orai1 were quantified in vivo at 6, 12 and 24 h. Changes in the intracellular Ca2⁺ concentration, SOCE and the distribution of STIM1 were assessed in vitro within 6 h. The morphological changes of heart tissue were observed by hematoxylin‑eosin staining. Myocardial cellular apoptosis was determined by TUNEL method. The expression of Bax, Bcl‑2, STIM1 and Orai1 were visualized by western blot. Cytosolic calcium concentration and SOCE were evaluated by confocal microscopy. The results demonstrated that cardiac contractile function was significantly reduced at 6 h and morphological changes in cardiac tissues, as well as the myocardial apoptosis rate, were markedly increased at 6, 12 and 24 h following CLP. mRNA and protein expression levels of Bax/Bcl‑2 were significantly enhanced at 6 and 12 h and glycosylation of Orai1 in the myocardium of septic mice was significantly increased at 6 h following CLP. The intracellular Ca2⁺ concentration, SOCE, was significantly increased at 1‑2 h and the clustering and distribution of STIM1 were markedly changed in H9C2 cells at 1 and 2 h. These findings suggested that myocardial dysfunction, cardiac injury and myocardial depression may be related to increased intracellular Ca2⁺ concentration resulting from STIM1/Orai1‑mediated SOCE, which may provide a potential method to alleviate septic myocardial depression.

Manganese Dynamics in Mouse Brain After Systemic MnCl2 Administration for Activation-Induced Manganese-Enhanced MRI

Activation-induced manganese-enhanced MRI (AIM-MRI) is an attractive tool for non-invasively mapping whole brain activities. Manganese ions (Mn²⁺) enter and accumulate in active neurons via calcium channels. Mn²⁺ shortens the longitudinal relaxation time (T1) of H⁺, and the longitudinal relaxation rate R1 (1/T1) is proportional to Mn²⁺ concentration. Thus, AIM-MRI can map neural activities throughout the brain by assessing the R1 map. However, AIM-MRI is still not widely used, partially due to insufficient information regarding Mn²⁺ dynamics in the brain. To resolve this issue, we conducted a longitudinal study looking at manganese dynamics after systemic administration of MnCl₂ by AIM-MRI with quantitative analysis. In the ventricle, Mn²⁺ increased rapidly within 1 h, remained high for 3 h, and returned to near control levels by 24 h after administration. Microdialysis showed that extracellular Mn returned to control levels by 4 h after administration, indicating a high concentration of extracellular Mn²⁺ lasts at least about 3 h after administration. In the brain parenchyma, Mn²⁺ increased slowly, peaked 24–48 h after administration, and returned to control level by 5 days after a single administration and by 2 weeks after a double administration with a 24-h interval. These time courses suggest that AIM-MRI records neural activity 1–3 h after MnCl₂ administration, an appropriate timing of the MRI scan is in the range of 24–48 h following systemic administration, and at least an interval of 5 days or a couple of weeks for single or double administrations, respectively, is needed for a repeat AIM-MRI experiment.

SOCE in the cardiomyocyte: the secret is in the chambers

Store-operated Ca²⁺ entry (SOCE) is an ancient and ubiquitous Ca²⁺ signaling pathway that is present in virtually every cell type. Over the last two decades, many studies have implicated this non-voltage dependent Ca²⁺ entry pathway in cardiac physiology. The relevance of the SOCE pathway in cardiomyocytes is often questioned given the well-established role for excitation contraction coupling. In this review, we consider the evidence that STIM1 and SOCE contribute to Ca²⁺ dynamics in cardiomyocytes. We discuss the relevance of this pathway to cardiac growth in response to developmental and pathologic cues. We also address whether STIM1 contributes to Ca²⁺ store refilling that likely impacts cardiac pacemaking and arrhythmogenesis in cardiomyocytes.

Physiological and Pathological Relevance of Selective and Nonselective Ca2+ Channels in Skeletal and Cardiac Muscle

Contraction of the striated muscle is fundamental for human existence. The action of voluntary skeletal muscle enables activities such as breathing, establishing body posture, and diverse body movements. Additionally, highly precise motion empowers communication, artistic expression, and other activities that define everyday human life. The involuntary contraction of striated muscle is the core function of the heart and is essential for blood flow. Several ion channels are important in the transduction of action potentials to cytosolic Ca2+ signals that enable muscle contraction; however, other ion channels are involved in the progression of muscle pathologies that can impair normal life or threaten it. This chapter describes types of selective and nonselective Ca2+ permeable ion channels expressed in the striated muscle, their participation in different aspects of muscle excitation and contraction, and their relevance to the progression of some pathological states.

Targeting Orai1-Mediated Store-Operated Ca2+ Entry in Heart Failure

The archetypal store-operated Ca²⁺ channels (SOCs), Orai1, which are stimulated by the endo/sarcoplasmic reticulum (ER/SR) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) upon Ca²⁺ store depletion is traditionally viewed as instrumental for the function of non-excitable cells. In the recent years, expression and function of Orai1 have gained recognition in excitable cardiomyocytes, albeit controversial. Even if its cardiac physiological role in adult is still elusive and needs to be clarified, Orai1 contribution in cardiac diseases such as cardiac hypertrophy and heart failure (HF) is increasingly recognized. The present review surveys our current arising knowledge on the new role of Orai1 channels in the heart and debates on its participation to cardiac hypertrophy and HF.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Changes in rat myocardium contractility under subchronic intoxication with lead and cadmium salts administered alone or in combination

Subchronic intoxications induced in male rats by repeated intraperitoneal injections of lead acetate and cadmium chloride, administered either alone or in combination, are shown to affect the biochemical, cytological and morphometric parameters of blood, liver, heart and kidneys. The single twitch parameters of myocardial trabecular and papillary muscle preparations were measured in the isometric regime to identify changes in the heterometric (length-force) and chronoinotropic (frequency-force) contractility regulation systems. Differences in the responses of these systems in trabecules and papillary muscles to the above intoxications are shown. A number of myocardium mechanical characteristics changing in rats under the effect of a combined lead-cadmium intoxication and increased proportion of α-myosin heavy chains were observed to normalize fully or partially if such intoxication was induced against background administration of a proposed bioprotective complex. Based on the experimental results and literature data, some assumptions are suggested concerning the mechanisms of the cardiotoxic effects produced by lead and cadmium.

The Control of Diastolic Calcium in the Heart: Basic Mechanisms and Functional Implications

Normal cardiac function requires that intracellular Ca²⁺ concentration be reduced to low levels in diastole so that the ventricle can relax and refill with blood. Heart failure is often associated with impaired cardiac relaxation. Little, however, is known about how diastolic intracellular Ca²⁺ concentration is regulated. This article first discusses the reasons for this ignorance before reviewing the basic mechanisms that control diastolic intracellular Ca²⁺ concentration. It then considers how the control of systolic and diastolic intracellular Ca²⁺ concentration is intimately connected. Finally, it discusses the changes that occur in heart failure and how these may result in heart failure with preserved versus reduced ejection fraction.

Pathophysiological Significance of Store-Operated Calcium Entry in Cardiovascular and Skeletal Muscle Disorders and Angiogenesis

Store-Operated Ca²⁺ Entry (SOCE) is an important Ca²⁺ influx pathway expressed by several excitable and non-excitable cell types. SOCE is recognized as relevant signaling pathway not only for physiological process, but also for its involvement in different pathologies. In fact, independent studies demonstrated the implication of essential protein regulating SOCE, such as STIM, Orai and TRPCs, in different pathogenesis and cell disorders, including cardiovascular disease, muscular dystrophies and angiogenesis. Compelling evidence showed that dysregulation in the function and/or expression of isoforms of STIM, Orai or TRPC play pivotal roles in cardiac hypertrophy and heart failure, vascular remodeling and hypertension, skeletal myopathies, and angiogenesis. In this chapter, we summarized the current knowledge concerning the mechanisms underlying abnormal SOCE and its involvement in some diseases, as well as, we discussed the significance of STIM, Orai and TRPC isoforms as possible therapeutic targets for the treatment of angiogenesis, cardiovascular and skeletal muscle diseases.

Specific Upregulation of TRPC1 and TRPC5 Channels by Mineralocorticoid Pathway in Adult Rat Ventricular Cardiomyocytes

Whereas cardiac TRPC (transient receptor potential canonical) channels and the associated store-operated Ca²⁺ entry (SOCE) are abnormally elevated during cardiac hypertrophy and heart failure, the mechanism of this upregulation is not fully elucidated but might be related to the activation of the mineralocorticoid pathway. Using a combination of biochemical, Ca²⁺ imaging, and electrophysiological techniques, we determined the effect of 24-h aldosterone treatment on the TRPCs/Orai-dependent SOCE in adult rat ventricular cardiomyocytes (ARVMs). The 24-h aldosterone treatment (from 100 nM to 1 µM) enhanced depletion-induced Ca²⁺ entry in ARVMs, as assessed by a faster reduction of Fura-2 fluorescence decay upon the addition of Mn²⁺ and increased Fluo-4/AM fluorescence following Ca²⁺ store depletion. These effects were prevented by co-treatment with a specific mineralocorticoid receptor (MR) antagonist, RU-28318, and they are associated with the enhanced depletion-induced N-[4-[3,5-Bis(trifluoromethyl)-1H-pyrazol-1-yl]phenyl]-4-methyl-1,2,3-thiadiazole-5-carboxamide (BTP2)-sensitive macroscopic current recorded by patch-clamp experiments. Molecular screening by qRT-PCR and Western blot showed a specific upregulation of TRPC1, TRPC5, and STIM1 expression at the messenger RNA (mRNA) and protein levels upon 24-h aldosterone treatment of ARVMs, corroborated by immunostaining. Our study provides evidence that the mineralocorticoid pathway specifically promotes TRPC1/TRPC5-mediated SOCE in adult rat cardiomyocytes.

Store-Operated Calcium Channels Are Involved in Spontaneous Slow Calcium Oscillations in Striatal Neurons

The striatum plays an important role in linking cortical activity to basal ganglia output. Striatal neurons exhibit spontaneous slow Ca²⁺ oscillations that result from Ca²⁺ release from the endoplasmic reticulum (ER) induced by the mGluR5-IP3R signaling cascade. The maximum duration of a single oscillatory event is about 300 s. A major question arises as to how such a long-duration Ca²⁺ elevation is maintained. Store-operated calcium channels (SOCCs) are one of the calcium (Ca²⁺)-permeable ion channels. SOCCs are opened by activating the metabotropic glutamate receptor type 5 and inositol 1,4,5-trisphosphate receptor (mGluR5-IP3R) signal transduction cascade and are related to the pathophysiology of several neurological disorders. However, the functions of SOCCs in striatal neurons remain unclear. Here, we show that SOCCs exert a functional role in striatal GABAergic neurons. Depletion of calcium stores from the ER induced large, sustained calcium entry that was blocked by SKF96365, an inhibitor of SOCCs. Moreover, the application of SKF96365 greatly reduced the frequency of slow Ca²⁺ oscillations. The present results indicate that SOCCs contribute to Ca²⁺ signaling in striatal GABAergic neurons, including medium spiny projection neurons (MSNs) and GABAergic interneurons, through elevated Ca²⁺ due to spontaneous slow Ca²⁺ oscillations.

Changes in STIM Isoforms Expression and Gender-Specific Alterations in Orai Expression in Human Heart Failure

Store-operated calcium entry (SOCE) is one of regulatory mechanisms which regulates Ca2+ cycling in the heart. SOCE alterations in pathological conditions contribute to progression of heart failure and cardiac hypertrophy by multiple signaling pathways such as Cn/NFAT and CaMKII/MEF2. Several components mediating SOCE have been identified, such as STIM and Orai. Different isoforms of both Orai and STIM have been detected in animal studies, exhibiting distinct functional properties. This study is focused on the analysis of STIM and Orai isoforms expression in the end-stage human failing myocardium. Left ventricle samples isolated from 43 explanted hearts from patients undergoing heart transplant and from 5 healthy donor hearts were used to determine the mRNA levels of Orai1, Orai2 and Orai3, STIM1, STIM2 and STIM2.1 by qRT-PCR. The expression was further analyzed for connection with gender, related co-morbidities, pathoetiology, clinical data and biochemical parameters. We show that Orai1 expression is decreased by 30 % in failing myocardium, even though we detected no significant changes in expression of Orai2 or Orai3. Interestingly, this decrease in Orai1 was gender-specific and was present only in men, with no change in women. The ratio Orai1/Orai3 was significantly lower in males as well. The novel STIM2.1 isoform was detected both in healthy and failing human myocardium. In the end-stage heart failure, the expression of STIM2.1 was significantly decreased. The lower ratio of STIM2.1/STIM2 in failing hearts indicates a switch from SOCE-inhibiting STIM2.1 isoform to stimulatory STIM2.2. STIM1 mRNA levels were not significantly changed. These observed alterations in Orai and STIM expression were independent of functional heart parameters, clinical or biochemical patient characteristics. These results provide detailed insight into the alterations of SOCE regulation in human failing myocardium. Gender-specific change in Orai1 expression might represent a possible mechanism of cardioprotective effects of estrogens. The switch from STIM2.1 to STIM2.2 indicates an amplification of SOCE and could contribute to the hypertrophy development in the filing heart.

Expression of calcium release-activated and voltage-gated calcium channels genes in peripheral blood mononuclear cells is altered in pregnancy and in type 1 diabetes

Calcium (Ca2+) is an important ion in physiology and is found both outside and inside cells. The intracellular concentration of Ca2+ is tightly regulated as it is an intracellular signal molecule and can affect a variety of cellular processes. In immune cells Ca2+ has been shown to regulate e.g. gene transcription, cytokine secretion, proliferation and migration. Ca2+ can enter the cytoplasm either from intracellular stores or from outside the cells when Ca2+ permeable ion channels in the plasma membrane open. The Ca2+ release-activated (CRAC) channel is the most prominent Ca2+ ion channel in the plasma membrane. It is formed by ORAI1-3 and the channel is opened by the endoplasmic reticulum Ca2+ sensor proteins stromal interaction molecules (STIM) 1 and 2. Another group of Ca2+ channels in the plasma membrane are the voltage-gated Ca2+ (CaV) channels. We examined if a change in immunological tolerance is accompanied by altered ORAI, STIM and CaV gene expression in peripheral blood mononuclear cells (PBMCs) in pregnant women and in type 1 diabetic individuals. Our results show that in pregnancy and type 1 diabetes ORAI1-3 are up-regulated whereas STIM1 and 2 are down-regulated in pregnancy but only STIM2 in type 1 diabetes. Expression of L-, P/Q-, R- and T-type voltage-gated Ca2+ channels was detected in the PBMCs where the CaV2.3 gene was up-regulated in pregnancy and type 1 diabetes whereas the CaV 2.1 and CaV3.2 genes were up-regulated only in pregnancy and the CaV1.3 gene in type 1 diabetes. The results are consistent with that expression of ORAI, STIM and CaV genes correlate with a shift in immunological status of the individual in health, as during pregnancy, and in the autoimmune disease type 1 diabetes. Whether the changes are in general protective or in type 1 diabetes include some pathogenic components remains to be clarified.

SOCE and STIM1 signaling in the heart: Timing and location matter

Store operated Ca²⁺ entry (SOCE) is an ancient and ubiquitous Ca²⁺ signaling pathway discovered decades ago, but the function of SOCE in human physiology is only now being revealed. The relevance of this pathway to striated muscle was solidified with the description of skeletal myopathies that result from mutations in STIM1 and Orai1, the two SOCE components. Here, we consider the evidence for STIM1 and SOCE in cardiac muscle and the sinoatrial node. We highlight recent studies revealing a role for STIM1 in cardiac growth in response to developmental and pathologic cues. We also review the role of STIM1 in the regulation of SOCE and Ca²⁺ store refilling in a non-Orai dependent manner. Finally, we discuss the importance of this pathway in ventricular cardiomyocytes where SOCE contribute to developmental growth and in pacemaker cells where SOCE likely has a fundamental to generating the cardiac rhythm.

The Complex Role of Store Operated Calcium Entry Pathways and Related Proteins in the Function of Cardiac, Skeletal and Vascular Smooth Muscle Cells

Cardiac, skeletal, and smooth muscle cells shared the common feature of contraction in response to different stimuli. Agonist-induced muscle's contraction is triggered by a cytosolic free Ca²⁺ concentration increase due to a rapid Ca²⁺ release from intracellular stores and a transmembrane Ca²⁺ influx, mainly through L-type Ca²⁺ channels. Compelling evidences have demonstrated that Ca²⁺ might also enter through other cationic channels such as Store-Operated Ca²⁺ Channels (SOCCs), involved in several physiological functions and pathological conditions. The opening of SOCCs is regulated by the filling state of the intracellular Ca²⁺ store, the sarcoplasmic reticulum, which communicates to the plasma membrane channels through the Stromal Interaction Molecule 1/2 (STIM1/2) protein. In muscle cells, SOCCs can be mainly non-selective cation channels formed by Orai1 and other members of the Transient Receptor Potential-Canonical (TRPC) channels family, as well as highly selective Ca²⁺ Release-Activated Ca²⁺ (CRAC) channels, formed exclusively by subunits of Orai proteins likely organized in macromolecular complexes. This review summarizes the current knowledge of the complex role of Store Operated Calcium Entry (SOCE) pathways and related proteins in the function of cardiac, skeletal, and vascular smooth muscle cells.

The STIM1 inhibitor ML9 disrupts basal autophagy in cardiomyocytes by decreasing lysosome content

Stromal-interaction molecule 1 (STIM1)-mediated store-operated Ca²⁺ entry (SOCE) plays a key role in mediating cardiomyocyte hypertrophy, both in vitro and in vivo. Moreover, there is growing support for the contribution of SOCE to the Ca²⁺ overload associated with ischemia/reperfusion injury. Therefore, STIM1 inhibition is proposed as a novel target for controlling both hypertrophy and ischemia/reperfusion-induced Ca²⁺ overload. Our aim was to evaluate the effect of ML9, a STIM1 inhibitor, on cardiomyocyte viability. ML9 was found to induce cell death in cultured neonatal rat cardiomyocytes. Caspase-3 activation, apoptotic index and release of the necrosis marker lactate dehydrogenase to the extracellular medium were evaluated. ML9-induced cardiomyocyte death was not associated with increased intracellular ROS or decreased ATP levels. Moreover, treatment with ML9 significantly increased levels of the autophagy marker LC3-II, without altering Beclin1 or p62 protein levels. However, treatment with ML9 followed by bafilomycin-A1 did not produce further increases in LC3-II content. Furthermore, treatment with ML9 resulted in decreased LysoTracker® Green staining. Collectively, these data suggest that ML9-induced cardiomyocyte death is triggered by a ML9-dependent disruption of autophagic flux due to lysosomal dysfunction.

Calcium and Excitation-Contraction Coupling in the Heart

Cardiac contractility is regulated by changes in intracellular Ca concentration ([Ca²⁺]_i). Normal function requires that [Ca²⁺]_i be sufficiently high in systole and low in diastole. Much of the Ca needed for contraction comes from the sarcoplasmic reticulum and is released by the process of calcium-induced calcium release. The factors that regulate and fine-tune the initiation and termination of release are reviewed. The precise control of intracellular Ca cycling depends on the relationships between the various channels and pumps that are involved. We consider 2 aspects: (1) structural coupling: the transporters are organized within the dyad, linking the transverse tubule and sarcoplasmic reticulum and ensuring close proximity of Ca entry to sites of release. (2) Functional coupling: where the fluxes across all membranes must be balanced such that, in the steady state, Ca influx equals Ca efflux on every beat. The remainder of the review considers specific aspects of Ca signaling, including the role of Ca buffers, mitochondria, Ca leak, and regulation of diastolic [Ca²⁺]_i.

Store-operated calcium entry suppressed the TGF-β1/Smad3 signaling pathway in glomerular mesangial cells

Our previous study demonstrated that the abundance of extracellular matrix proteins was suppressed by store-operated Ca²⁺ entry (SOCE) in mesangial cells (MCs). The present study was conducted to investigate the underlying mechanism focused on the transforming growth factor-β1 (TGF-β1)/Smad3 pathway, a critical pathway for ECM expansion in diabetic kidneys. We hypothesized that SOCE suppressed ECM protein expression by inhibiting this pathway in MCs. In cultured human MCs, we observed that TGF-β1 (5 ng/ml for 15 h) significantly increased Smad3 phosphorylation, as evaluated by immunoblot. However, this response was markedly inhibited by thapsigargin (1 µM), a classical activator of store-operated Ca²⁺ channels. Consistently, both immunocytochemistry and immunoblot showed that TGF-β1 significantly increased nuclear translocation of Smad3, which was prevented by pretreatment with thapsigargin. Importantly, the thapsigargin effect was reversed by lanthanum (La³⁺; 5 µM) and GSK-7975A (10 µM), both of which are selective blockers of store-operated Ca²⁺ channels. Furthermore, knockdown of Orai1, the pore-forming subunit of the store-operated Ca²⁺ channels, significantly augmented TGF-β1-induced Smad3 phosphorylation. Overexpression of Orai1 augmented the inhibitory effect of thapsigargin on TGF-β1-induced phosphorylation of Smad3. In agreement with the data from cultured MCs, in vivo knockdown of Orai1 specific to MCs using a targeted nanoparticle small interfering RNA delivery system resulted in a marked increase in abundance of phosphorylated Smad3 and in nuclear translocation of Smad3 in the glomerulus of mice. Taken together, our results indicate that SOCE in MCs negatively regulates the TGF-β1/Smad3 signaling pathway.

Tissue Specificity: Store-Operated Ca2+ Entry in Cardiac Myocytes

Calcium (Ca²⁺) is a key regulator of cardiomyocyte contraction. The Ca²⁺ channels, pumps, and exchangers responsible for the cyclical cytosolic Ca²⁺ signals that underlie contraction are well known. In addition to those Ca²⁺ signaling components responsible for contraction, it has been proposed that cardiomyocytes express channels that promote the influx of Ca²⁺ from the extracellular milieu to the cytosol in response to depletion of intracellular Ca²⁺ stores. With non-excitable cells, this store-operated Ca²⁺ entry (SOCE) is usually easily demonstrated and is essential for prolonging cellular Ca²⁺ signaling and for refilling depleted Ca²⁺ stores. The role of SOCE in cardiomyocytes, however, is rather more elusive. While there is published evidence for increased Ca²⁺ influx into cardiomyocytes following Ca²⁺ store depletion, it has not been universally observed. Moreover, SOCE appears to be prominent in embryonic cardiomyocytes but declines with postnatal development. In contrast, there is overwhelming evidence that the molecular components of SOCE (e.g., STIM, Orai, and TRPC proteins) are expressed in cardiomyocytes from embryo to adult. Moreover, these proteins have been shown to contribute to disease conditions such as pathological hypertrophy, and reducing their expression can attenuate hypertrophic growth. It is plausible that SOCE might underlie Ca²⁺ influx into cardiomyocytes and may have important signaling functions perhaps by activating local Ca²⁺-sensitive processes. However, the STIM, Orai, and TRPC proteins appear to cooperate with multiple protein partners in signaling complexes. It is therefore possible that some of their signaling activities are not mediated by Ca²⁺ influx signals, but by protein-protein interactions.

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.

Conformation of ryanodine receptor-2 gates store-operated calcium entry in rat pulmonary arterial myocytes

AIMS

Store-operated Ca(2+) entry (SOCE) contributes to a multitude of physiological and pathophysiological functions in pulmonary vasculatures. SOCE attributable to inositol 1,4,5-trisphosphate receptor (InsP3R)-gated Ca(2+) store has been studied extensively, but the role of ryanodine receptor (RyR)-gated store in SOCE remains unclear. The present study aims to delineate the relationship between RyR-gated Ca(2+) stores and SOCE, and characterize the properties of RyR-gated Ca(2+) entry in pulmonary artery smooth muscle cells (PASMCs).

METHODS AND RESULTS

PASMCs were isolated from intralobar pulmonary arteries of male Wister rats. Application of the RyR1/2 agonist 4-chloro-m-cresol (4-CmC) activated robust Ca(2+) entry in PASMCs. It was blocked by Gd(3+) and the RyR2 modulator K201 but was unaffected by the RyR1/3 antagonist dantrolene and the InsP3R inhibitor xestospongin C, suggesting RyR2 is mainly involved in the process. siRNA knockdown of STIM1, TRPC1, and Orai1, or interruption of STIM1 translocation with ML-9 significantly attenuated the 4-CmC-induced SOCE, similar to SOCE induced by thapsigargin. However, depletion of RyR-gated store with caffeine failed to activate Ca(2+) entry. Inclusion of ryanodine, which itself did not cause Ca(2+) entry, uncovered caffeine-induced SOCE in a concentration-dependent manner, suggesting binding of ryanodine to RyR is permissive for the process. This Ca(2+) entry had the same molecular and pharmacological properties of 4-CmC-induced SOCE, and it persisted once activated even after caffeine washout. Measurement of Ca(2+) in sarcoplasmic reticulum (SR) showed that 4-CmC and caffeine application with or without ryanodine reduced SR Ca(2+) to similar extent, suggesting store-depletion was not the cause of the discrepancy. Moreover, caffeine/ryanodine and 4-CmC failed to initiate SOCE in cells transfected with the ryanodine-binding deficient mutant RyR2-I4827T.

CONCLUSIONS

RyR2-gated Ca(2+) store contributes to SOCE in PASMCs; however, store-depletion alone is insufficient but requires a specific RyR conformation modifiable by ryanodine binding to activate Ca(2+) entry.

Store-operated calcium entry and diabetic complications

Store-operated Ca(2+) entry (SOCE) is mediated by the store-operated Ca(2+) channel (SOC) that opens upon depletion of internal Ca(2+) stores following activation of G protein-coupled receptors or receptor tyrosine kinases. Over the past two decades, the physiological and pathological relevance of SOCE has been extensively studied. Recently, accumulating evidence suggests associations of altered SOCE with diabetic complications. This review focuses on the implication of SOCE as it pertains to various complications resulting from diabetes. We summarize recent findings by us and others on the involvement of abnormal SOCE in the development of diabetic complications, such as diabetic nephropathy and diabetic vasculopathy. The underlying mechanisms that mediate the diabetes-associated alterations of SOCE are also discussed. The SOCE pathway may be considered as a potential therapeutic target for diabetes-associated diseases.

STIM1-Ca2+ signaling modulates automaticity of the mouse sinoatrial node

Cardiac pacemaking is governed by specialized cardiomyocytes located in the sinoatrial node (SAN). SAN cells (SANCs) integrate voltage-gated currents from channels on the membrane surface (membrane clock) with rhythmic Ca(2+) release from internal Ca(2+) stores (Ca(2+) clock) to adjust heart rate to meet hemodynamic demand. Here, we report that stromal interaction molecule 1 (STIM1) and Orai1 channels, key components of store-operated Ca(2+) entry, are selectively expressed in SANCs. Cardiac-specific deletion of STIM1 in mice resulted in depletion of sarcoplasmic reticulum (SR) Ca(2+) stores of SANCs and led to SAN dysfunction, as was evident by a reduction in heart rate, sinus arrest, and an exaggerated autonomic response to cholinergic signaling. Moreover, STIM1 influenced SAN function by regulating ionic fluxes in SANCs, including activation of a store-operated Ca(2+) current, a reduction in L-type Ca(2+) current, and enhancing the activities of Na(+)/Ca(2+) exchanger. In conclusion, these studies reveal that STIM1 is a multifunctional regulator of Ca(2+) dynamics in SANCs that links SR Ca(2+) store content with electrical events occurring in the plasma membrane, thereby contributing to automaticity of the SAN.

Ca²⁺ microdomains organized by junctophilins

Excitable cells typically possess junctional membrane complexes (JMCs) constructed by the plasma membrane and the endo/sarcoplasmic reticulum (ER/SR) for channel crosstalk. These JMCs are termed triads in skeletal muscle, dyads in cardiac muscle, peripheral couplings in smooth and developing striated muscles, and subsurface cisterns in neurons. Junctophilin subtypes contribute to the formation and maintenance of JMCs by serving as a physical bridge between the plasma membrane and ER/SR membrane in different cell types. In muscle cells, junctophilin deficiency prevents JMC formation and functional crosstalk between cell-surface Ca²⁺ channels and ER/SR Ca²⁺ release channels. Human genetic mutations in junctophilin subtypes are linked to congenital hypertrophic cardiomyopathy and neurodegenerative diseases. Furthermore, growing evidence suggests that dysregulation of junctophilins induces pathological alterations in skeletal and cardiac muscle.

TRPC1, CaN and NFATC3 signaling pathway in the pathogenesis and progression of left ventricular hypertrophy in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) was used to study left ventricular hypertrophy (LVH) and its dynamic change after the interventions with Telmisartan and Amlodipine. The results showed that the expression of TRPC1, CaN and NFATC3 increased gradually with the pathogenesis and progression of LVH. Telmisartan reduced blood pressure and LVH, and down-regulated the expression of TRPC1, CaN and NFATC3 in left ventricle of SHR. Amlodipine reduced the blood pressure in SHR but had no impact on the hypertrophy and expression of above factors. Our data suggest that the pathogenesis and progression of LVH in SHR are related to upregulation of TRPC1, CaN and NFATC3 signaling pathway.

Neuroprotective effect of weak static magnetic fields in primary neuronal cultures

Low intensity static magnetic fields (SMFs) interact with various biological tissues including the CNS, thereby affecting key biological processes such as gene expression, cell proliferation and differentiation, as well as apoptosis. Previous studies describing the effect of SMFs on apoptotic cell death in several non-neuronal cell lines, emphasize the importance of such a potential modulation in the case of neurodegenerative disorders, where apoptosis constitutes a major route via which neurons degenerate and die. In this study, we examine the effect of SMFs on neuronal survival in primary cortical and hippocampal neurons that constitute a suitable experimental system for modeling the neurodegenerative state in vitro. We show that weak SMF exposure interferes with the apoptotic programing in rat primary cortical and hippocampal neurons, thereby providing protection against etoposide-induced apoptosis in a dose- and time-dependent manner. Primary cortical neurons exposed to SMF (50G) for 7days exhibited a 57.1±6.3% decrease in the percentage of cells undergoing apoptosis induced by etoposide (12μM), accompanied by a marked decrease in the expression of the pro-apoptotic markers: cleaved poly ADP ribose polymerase-1, cleaved caspase-3, active caspase-9 and the phospho-histone H2A variant (Ser139) by 41.0±5.0%, 81.2±5.0%, 72.9±6.4%, 42.75±2.9%, respectively, and by a 57.2±1.0% decrease in the extent of mitochondrial membrane potential collapse. Using the L-type voltage-gated Ca(2+) channel inhibitor nifedipine, which is selective to Ca(2+) influx through Cav1.2, we found that the anti-apoptotic effect of SMFs was mediated by Ca(2+) influx through these channels. Our findings demonstrating altered Ca(2+)-influx in response to thapsigargin stimulation in SMF-exposed cortical neurons, along with enhanced inhibition of KCl-induced Ca(2+)-influx through Cav1.2 channels and enhanced expression of Cav1.2 and Cav1.3 channels, allude to the involvement of voltage- and store-operated Ca(2+) channels in various aspects of the protective effect exerted by SMFs. These findings show the potential susceptibility of the CNS to weak SMF exposure and have implications for the design of novel strategies for the treatment and/or prevention of neurodegenerative diseases.

STIM1/Orai1-mediated SOCE: current perspectives and potential roles in cardiac function and pathology

Store-operated Ca²⁺ entry (SOCE) is critical for Ca²⁺ signaling in nonexcitable cells; however, its role in the regulation of cardiomyocyte Ca²⁺ homeostasis has only recently been investigated. The increased understanding of the role of stromal interaction molecule 1 (STIM1) in regulating SOCE combined with recent studies demonstrating the presence of STIM1 in cardiomyocytes provides support that this pathway co-exists in the heart with the more widely recognized Ca²⁺ handling pathways associated with excitation-contraction coupling. There is now substantial evidence that STIM1-mediated SOCE plays a key role in mediating cardiomyocyte hypertrophy, both in vitro and in vivo, and there is growing support for the contribution of SOCE to Ca²⁺ overload associated with ischemia/reperfusion injury. Here, we provide an overview of our current understanding of the molecular regulation of SOCE and discuss the evidence supporting the role of STIM1/Orai1-mediated SOCE in regulating cardiomyocyte function.

Emerging roles for native Orai Ca2+ channels in cardiovascular disease

Orai proteins form highly calcium (Ca(2+))-selective channels located in the plasma membrane of both nonexcitable and excitable cells, where they make important contributions to many cellular processes. The well-characterized Ca(2+) release-activated Ca(2+) current is mediated by Orai1 multimers and is activated, upon depletion of inositol 1,4,5-trisphosphate-sensitive stores, by direct interaction of Orai1 with the endoplasmic reticulum Ca(2+) sensor, stromal interaction molecule 1 (STIM1). This pathway is known as capacitative Ca(2+) entry or store-operated Ca(2+) entry. While most investigations have focused on STIM1 and Orai1 in their store-dependent mode, emerging evidence suggests that Orai1 and Orai3 heteromultimeric channels can form store-independent Ca(2+)-selective channels. The role of store-dependent and store-independent channels in excitation-transcription coupling and the pathological remodeling of the cardiovascular system are beginning to come forth. Recent evidence suggests that STIM/Orai-generated Ca(2+) signaling couples to gene transcription and subsequent phenotypic changes associated with the processes of cardiac and vascular remodeling. This short review will explore the contributions of native Orai channels to heart and vessel physiology and their role in cardiovascular diseases.

Calcium flux balance in the heart

This article reviews the consequences of the need for the cardiac cell to be in calcium flux balance in the steady state. We first discuss how this steady state condition affects the control of resting [Ca(2+)]i. The next section considers how sarcoplasmic reticulum (SR) Ca content is controlled by a feedback mechanism whereby changes of SR Ca affect the amplitude of the Ca transient and this, in turn, controls sarcolemmal Ca fluxes. Subsequent sections review the effects of altering the activity of individual Ca handling proteins. Increasing the activity of the SR Ca-ATPase (SERCA) increases both the amplitude and rate constant of decay of the systolic Ca transient. The Ca flux balance condition requires that this must be achieved with no change of Ca efflux placing constraints on the magnitude of change of amplitude and decay rate. We analyze the quantitative dependence of Ca transient amplitude and SR content on SERCA activity. Increasing the open probability of the RyR during systole is predicted to have no steady state effect on the amplitude of the systolic Ca transient. We discuss the effects of changing the amplitude of the L-type Ca current in the context of both triggering Ca release from the SR and loading the cell with calcium. These manoeuvres are considered in the context of the effects of β-adrenergic stimulation. Finally, we review calcium flux balance in the presence of Ca waves.

Modification of STIM1 by O-linked N-acetylglucosamine (O-GlcNAc) attenuates store-operated calcium entry in neonatal cardiomyocytes

Store-operated calcium entry (SOCE) is a major Ca(2+) signaling pathway responsible for regulating numerous transcriptional events. In cardiomyocytes SOCE has been shown to play an important role in regulating hypertrophic signaling pathways, including nuclear translocation of NFAT. Acute activation of pathways leading to O-GlcNAc synthesis have been shown to impair SOCE-mediated transcription and in diabetes, where O-GlcNAc levels are chronically elevated, cardiac hypertrophic signaling is also impaired. Therefore the goal of this study was to determine whether changes in cardiomyocyte O-GlcNAc levels impaired the function of STIM1, a widely recognized mediator of SOCE. We demonstrated that acute activation of SOCE in neonatal cardiomyocytes resulted in STIM1 puncta formation, which was inhibited in a dose-dependent manner by increasing O-GlcNAc synthesis with glucosamine or inhibiting O-GlcNAcase with thiamet-G. Glucosamine and thiamet-G also inhibited SOCE and were associated with increased O-GlcNAc modification of STIM1. These results suggest that activation of cardiomyocyte O-GlcNAcylation attenuates SOCE via STIM1 O-GlcNAcylation and that this may represent a new mechanism by which increased O-GlcNAc levels regulate Ca(2+)-mediated events in cardiomyocytes. Further, since SOCE is a fundamental mechanism underlying Ca(2+) signaling in most cells and tissues, it is possible that STIM1 represents a nexus linking protein O-GlcNAcylation with Ca(2+)-mediated transcription.

Temperature and RyR1 regulate the activation rate of store-operated Ca²+ entry current in myotubes

Store-operated calcium entry (SOCE) is an important Ca(2+) entry pathway in skeletal muscle. However, direct electrophysiological recording and full characterization of the underlying SOCE current in skeletal muscle cells (I(SkCRAC)) has not been reported. Here, we characterized the biophysical properties, pharmacological profile, and molecular identity of I(SkCRAC) in skeletal myotubes, as well as the regulation of its rate of activation by temperature and the type I ryanodine receptor (RyR1). I(SkCRAC) exhibited many hallmarks of Ca(2+) release activated Ca(2+) currents (I(CRAC)): store dependence, strong inward rectification, positive reversal potential, limited cesium permeability, and sensitivity to SOCE channel blockers. I(SkCRAC) was reduced by siRNA knockdown of stromal interaction molecule 1 and expression of dominant negative Orai1. Average I(SkCRAC) current density at -80mV was 1.00 ± 0.05 pA/pF. In the presence of 20 mM intracellular EGTA, I(SkCRAC) activation occurred over tens of seconds during repetitive depolarization at 0.5Hz and was inhibited by treatment with 100 μM ryanodine. The rate of SOCE activation was reduced threefold in myotubes from RyR1-null mice and increased 4.6-fold at physiological temperatures (35-37°C). These results show that I(SkCRAC) exhibits similar biophysical, pharmacological, and molecular properties as I(CRAC) in nonexcitable cells and its rate of activation during repetitive depolarization is strongly regulated by temperature and RyR1 activity.

Presence of store-operated Ca2+ entry in C57BL/6J mouse ventricular myocytes and its suppression by sevoflurane

BACKGROUND

Store-operated Ca(2+) entry (SOCE) has been implicated in various pathological conditions of the heart including ischaemia/reperfusion and ventricular hypertrophy. This study investigated the effects of sevoflurane on SOCE.

METHODS

Fluorescence imaging was performed on fluo-3- and mag-fluo-4-loaded mouse ventricular myocytes to measure the cytosolic and intraluminal sarcoplasmic reticulum (SR) Ca(2+) levels, respectively, using a confocal laser scanning microscope. Whole-cell membrane currents were recorded using the patch-clamp technique. Ventricular myocytes were exposed to thapsigargin and angiotensin II to deplete SR Ca(2+) stores and thereby activate SOCE.

RESULTS

The combined application of thapsigargin and angiotensin II to the Ca(2+)-free medium evoked a significant decrease in the SR Ca(2+) levels, which was followed by the elevation of cytosolic Ca(2+) and the development of cellular hypercontracture upon subsequent addition of extracellular Ca(2+). This cytosolic Ca(2+) elevation was inhibited by 2-aminoethoxydiphenyl borate but not by verapamil and KB-R7943, which indicates that SOCE was present in mouse ventricular myocytes. Sevoflurane concentration-dependently inhibited the SOCE-mediated Ca(2+) overload (IC(50) of 137 μM, which corresponds to 0.96%) with a significant reduction occurring at concentrations of ≥2%. Patch-clamp experiments revealed that the SOCE current was also concentration-dependently blocked by sevoflurane (IC(50) of 144 μM, which corresponds to 1.0%).

CONCLUSIONS

Sevoflurane at concentrations of ≥2% significantly inhibits the SOCE activity and prevents the resultant cellular Ca(2+) overload that leads to hypercontracture in ventricular myocytes. This inhibitory action may be involved in the cardioprotective effect of sevoflurane against Ca(2+) overload-mediated injury.

Loss of junctophilin-3 contributes to Huntington disease-like 2 pathogenesis

OBJECTIVE

Huntington disease-like 2 (HDL2) is a progressive, late onset autosomal dominant neurodegenerative disorder, with remarkable similarities to Huntington disease (HD). HDL2 is caused by a CTG/CAG repeat expansion. In the CTG orientation, the repeat is located within the alternatively spliced exon 2A of junctophilin-3 (JPH3), potentially encoding polyleucine and polyalanine, whereas on the strand antisense to JPH3, the repeat is in frame to encode polyglutamine. The JPH3 protein product serves to stabilize junctional membrane complexes and regulate neuronal calcium flux. We have previously demonstrated the potential pathogenic properties of JPH3 transcripts containing expanded CUG repeats. The aim of this study was to test the possibility that loss of JPH3 expression or expanded amino acid tracts also contribute to HDL2 pathogenesis.

METHODS

Transcripts from the HDL2 locus, and their protein products, were examined in HDL2, HD, and control frontal cortex. The effect of loss of Jph3 was examined in mice with partial or complete loss of Jph3.

RESULTS

Bidirectional transcription occurs at the HDL2 locus, although expression of antisense transcripts with expanded CAG repeats is limited. Protein products with expanded amino acid tracts were not detected in HDL2 brain. However, JPH3 transcripts and full-length JPH3 protein are decreased in HDL2 brain, and Jph3 hemizygous and null mice exhibit abnormal motor function.

INTERPRETATION

Our results suggest that the pathogenic mechanism of HDL2 is multifactorial, involving both a toxic gain of function of JPH3 RNA and a toxic loss of JPH3 expression.

Compartmentation of membrane processes and nucleotide dynamics in diffusion-restricted cardiac cell microenvironment

Orchestrated excitation-contraction coupling in heart muscle requires adequate spatial arrangement of systems responsible for ion movement and metabolite turnover. Co-localization of regulatory and transporting proteins into macromolecular complexes within an environment of microanatomical cell components raises intracellular diffusion barriers that hamper the mobility of metabolites and signaling molecules. Compared to substrate diffusion in the cytosol, diffusional restrictions underneath the sarcolemma are much larger and could impede ion and nucleotide movement by a factor of 10(3)-10(5). Diffusion barriers thus seclude metabolites within the submembrane space enabling rapid and vectorial effector targeting, yet hinder energy supply from the bulk cytosolic space implicating the necessity for a shunting transfer mechanism. Here, we address principles of membrane protein compartmentation, phosphotransfer enzyme-facilitated interdomain energy transfer, and nucleotide signal dynamics at the subsarcolemma-cytosol interface. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

STIM1-dependent store-operated Ca²⁺ entry is required for pathological cardiac hypertrophy

Alterations in intracellular Ca(2+) homeostasis are an important trigger of pathological cardiac remodeling; however, mechanisms governing context-dependent changes in Ca(2+) influx are poorly understood. Store-operated Ca(2+) entry (SOCE) is a major mechanism regulating Ca(2+) trafficking in numerous cell types, yet its prevalence in adult heart and possible role in physiology and disease are each unknown. The Ca(2+)-binding protein, stromal interaction molecule 1 (STIM1), is a Ca(2+) sensor in the sarcoplasmic reticulum (SR), capable of triggering SOCE by interacting with plasma membrane Ca(2+) channels. We report that SOCE is abundant and robust in neonatal cardiomyocytes; however, SOCE is absent from adult cardiomyocytes. Levels of STIM1 transcript and protein correlate with the amplitude of SOCE, and manipulation of STIM1 protein levels (via shRNA) or activity (via expression of constitutively active or dominant-negative mutants) reveals a critical role for STIM1 in activating SOCE in cardiac myocytes. In neonatal hearts a recently identified STIM1 splice variant (STIM1L) is predominant but diminishes with maturation, only to reemerge with agonist- or afterload-induced cardiac stress. To test for pathophysiological relevance, we evaluated both in vitro and in vivo models of cardiac hypertrophy, finding that STIM1 expression is re-activated by pathological stress to trigger significant SOCE-dependent Ca(2+) influx. STIM1 amplifies agonist-induced hypertrophy via activation of the calcineurin-NFAT pathway. Importantly, inhibition of STIM1 suppresses agonist-triggered hypertrophy, pointing to a requirement for SOCE in this remodeling response. Stress-triggered STIM1 re-expression, and consequent SOCE activation, are critical elements in the upstream, Ca(2+)-dependent control of pathological cardiac hypertrophy.

Store-operated calcium entry is present in HL-1 cardiomyocytes and contributes to resting calcium

Store-operated Ca(2+) entry (SOCE) has recently been shown to be of physiological and pathological importance in the heart, particularly during cardiac hypertrophy. However, measuring changes in intracellular Ca(2+) during SOCE is very difficult to study in adult primary cardiomyocytes. As a result there is a need for a stable and reliable in vitro model of SOCE which can be used to test cardiac drugs and investigate the role of SOCE in cardiac pathology. HL-1 cells are the only immortal cardiomyocyte cell line available that continuously divides and spontaneously contracts while maintaining phenotypic characteristics of the adult cardiomyocyte. To date the role of SOCE has not yet been investigated in the HL-1 cardiac cell line. We report for the first time that these cells expressed stromal interaction molecule 1 (STIM1) and the Ca(2+) release-activated Ca(2+) (CRAC) channel Orai1, which are essential components of the SOCE machinery. In addition, SOCE was tightly coupled to sarcoplasmic reticulum (SR)-Ca(2+) release in HL-1 cells, and such response was not impaired in the presence of voltage dependent Ca(2+) channels (L-type and T-type channels) or reverse mode Na(+)/Ca(2+) exchanger (NCX) inhibitors. We were able to abolish the SOCE response with known SOCE inhibitors (BTP-2 and SKF-96365) and by targeted knockdown of Orai1 with RNAi. In addition, knockdown of Orai1 resulted in lower baseline Ca(2+) and an attenuated response to thapsigargin (TG) and caffeine, indicating that SOCE may play a role in Ca(2+) homeostasis during unstressed conditions in cardiomyocytes. Currently, there is little knowledge about SOCE in cardiomyocytes, and the present results suggest that HL-1 cells will be of great utility in investigating the role of SOCE in the heart.

A role of canonical transient receptor potential 5 channel in neuronal differentiation from A2B5 neural progenitor cells

Store-operated Ca(2+) entry (SOCE) channels are the main pathway of Ca(2+) entry in non-excitable cells such as neural progenitor cells (NPCs). However, the role of SOCE channels has not been defined in the neuronal differentiation from NPCs. Here, we show that canonical transient receptor potential channel (TRPC) as SOCE channel influences the induction of the neuronal differentiation of A2B5(+) NPCs isolated from postnatal-12-day rat cerebrums. The amplitudes of SOCE were significantly higher in neural cells differentiated from proliferating A2B5(+) NPCs and applications of SOCE blockers, 2-aminoethoxy-diphenylborane (2-APB), and ruthenium red (RR), inhibited their rise of SOCE. Among TRPC subtypes (TRPC1-7), marked expression of TRPC5 and TRPC6 with turned-off TRPC1 expression was observed in neuronal cells differentiated from proliferating A2B5(+) NPCs. TRPC5 small interfering RNA (siRNA) blocked the neuronal differentiation from A2B5(+) NPCs and reduced the rise of SOCE. In contrast, TRPC6 siRNA had no significant effect on the neuronal differentiation from A2B5(+) NPCs. These results indicate that calcium regulation by TRPC5 would play a key role as a switch between proliferation and neuronal differentiation from NPCs.

Stress-induced switch in Numb isoforms enhances Notch-dependent expression of subtype-specific transient receptor potential channel

The Notch signaling pathway plays an essential role in the regulation of cell specification by controlling differentiation, proliferation, and apoptosis. Numb is an intrinsic regulator of the Notch pathway and exists in four alternative splice variants that differ in the length of their phosphotyrosine-binding domain (PTB) and proline-rich region domains. The physiological relevance of the existence of the Numb splice variants and their exact regulation are still poorly understood. We previously reported that Numb switches from isoforms containing the insertion in PTB to isoforms lacking this insertion in neuronal cells subjected to trophic factor withdrawal (TFW). The functional relevance of the TFW-induced switch in Numb isoforms is not known. Here we provide evidence that the TFW-induced switch in Numb isoforms regulates Notch signaling strength and Notch target gene expression. PC12 cells stably overexpressing Numb isoforms lacking the PTB insertion exhibited higher basal Notch activity and Notch-dependent transcription of the transient receptor potential channel 6 (TRPC6) when compared with those overexpressing Numb isoforms with the PTB insertion. The differential regulation of TRPC6 expression is correlated with perturbed calcium signaling and increased neuronal vulnerability to TFW-induced death. Pharmacological inhibition of the Notch pathway or knockdown of TRPC6 function ameliorates the adverse effects caused by the TFW-induced switch in Numb isoforms. Taken together, our results indicate that Notch and Numb interaction may influence the sensitivity of neuronal cells to injurious stimuli by modulating calcium-dependent apoptotic signaling cascades.

alpha1G-dependent T-type Ca2+ current antagonizes cardiac hypertrophy through a NOS3-dependent mechanism in mice

In noncontractile cells, increases in intracellular Ca2+ concentration serve as a second messenger to signal proliferation, differentiation, metabolism, motility, and cell death. Many of these Ca2+-dependent regulatory processes operate in cardiomyocytes, although it remains unclear how Ca2+ serves as a second messenger given the high Ca2+ concentrations that control contraction. T-type Ca2+ channels are reexpressed in adult ventricular myocytes during pathologic hypertrophy, although their physiologic function remains unknown. Here we generated cardiac-specific transgenic mice with inducible expression of alpha1G, which generates Cav3.1 current, to investigate whether this type of Ca2+ influx mechanism regulates the cardiac hypertrophic response. Unexpectedly, alpha1G transgenic mice showed no cardiac pathology despite large increases in Ca2+ influx, and they were even partially resistant to pressure overload-, isoproterenol-, and exercise-induced cardiac hypertrophy. Conversely, alpha1G-/- mice displayed enhanced hypertrophic responses following pressure overload or isoproterenol infusion. Enhanced hypertrophy and disease in alpha1G-/- mice was rescued with the alpha1G transgene, demonstrating a myocyte-autonomous requirement of alpha1G for protection. Mechanistically, alpha1G interacted with NOS3, which augmented cGMP-dependent protein kinase type I activity in alpha1G transgenic hearts after pressure overload. Further, the anti-hypertrophic effect of alpha1G overexpression was abrogated by a NOS3 inhibitor and by crossing the mice onto the Nos3-/- background. Thus, cardiac alpha1G reexpression and its associated pool of T-type Ca2+ antagonize cardiac hypertrophy through a NOS3-dependent signaling mechanism.

Plasma membrane Ca2+-ATPase isoform 4 antagonizes cardiac hypertrophy in association with calcineurin inhibition in rodents

How Ca2+-dependent signaling effectors are regulated in cardiomyocytes, given the extreme cytoplasmic Ca2+ concentration changes that underlie contraction, remains unknown. Cardiomyocyte plasma membrane Ca2+-ATPase (PMCA) extrudes Ca2+ but has little effect on excitation-contraction coupling, suggesting its potential role in controlling Ca2+-dependent signaling effectors such as calcineurin. We generated cardiac-specific inducible PMCA4b transgenic mice that displayed normal global Ca2+ transient and cellular contraction levels and reduced cardiac hypertrophy following transverse aortic constriction (TAC) or phenylephrine/Ang II infusion, but showed no reduction in exercise-induced hypertrophy. Transgenic mice were protected from decompensation and fibrosis following long-term TAC. The PMCA4b transgene reduced the hypertrophic augmentation associated with transient receptor potential canonical 3 channel overexpression, but not that associated with activated calcineurin. Furthermore, Pmca4 gene-targeted mice showed increased cardiac hypertrophy and heart failure events after TAC. Physical associations between PMCA4b and calcineurin were enhanced by TAC and by agonist stimulation of cultured neonatal cardiomyocytes. PMCA4b reduced calcineurin nuclear factor of activated T cell-luciferase activity after TAC and in cultured neonatal cardiomyocytes after agonist stimulation. PMCA4b overexpression inhibited cultured cardiomyocyte hypertrophy following agonist stimulation, but much less so in a Ca2+ pumping-deficient PMCA4b mutant. Thus, Pmca4b likely reduces the local Ca2+ signals involved in reactive cardiomyocyte hypertrophy via calcineurin regulation.

Ca2+ store determines gating of store operated calcium entry in mammalian skeletal muscle

This work describes the gating of the store operated calcium entry (SOCE) in adult mammalian skeletal muscle. Flexor digitorum brevis fibers (FDB) were isolated from adult mice and exposed to conditions to deplete the sarcoplasmic reticulum (SR). A transient SR depletion caused either by repetitive depolarizations, chlorocresol (CMC) or, cyclopiazonic acid (CPA) induced a bell shaped calcium entry that raised the [Ca(2+)](i) to a maximum of 27.09 +/- 4.35 nM from the resting value. The activation time to reach 10-90% of the maximum amplitude was 112 +/- 10 s (n = 22). On the other hand, any mechanism that caused a permanent SR depletion (like thapsigargin, continuous CPA, or continuous CMC) triggered a calcium entry pathway that lasted 325 +/- 23 s and raised the [Ca(2+)](i )to 129.50 +/- 13.05 nM from the resting level (n = 28). Then, a prolonged depletion triggered an increase in [Ca(2+)](i) to higher values and for a longer time than when the SR is transiently depleted (p < 0.001). Our results, in skeletal muscle, showed that calcium store depletion was the signal for SOCE activation and how the SR got depleted was not relevant. Also, we found that SOCE deactivation was not caused by [Ca(2+)](i) but by the SR content. Our results suggest that the SR calcium content plays an important role in SOCE gating in mammalian skeletal muscle and a calcium sensor is located inside the SR.

Sarcolemmal cation channels and exchangers modify the increase in intracellular calcium in cardiomyocytes on inhibiting Na+-K+-ATPase

Although inhibition of the sarcolemmal (SL) Na+-K+-ATPase is known to cause an increase in the intracellular concentration of Ca2+([Ca2+]i) by stimulating the SL Na+/Ca2+exchanger (NCX), the involvement of other SL sites in inducing this increase in [Ca2+]iis not fully understood. Isolated rat cardiomyocytes were treated with or without different agents that modify Ca2+movements by affecting various SL sites and were then exposed to ouabain. Ouabain was observed to increase the basal levels of both [Ca2+]iand intracellular Na+concentration ([Na+]i) as well as to augment the KCl-induced increases in both [Ca2+]iand [Na+]iin a concentration-dependent manner. The ouabain-induced changes in [Na+]iand [Ca2+]iwere attenuated by treatment with inhibitors of SL Na+/H+exchanger and SL Na+channels. Both the ouabain-induced increase in basal [Ca2+]iand augmentation of the KCl response were markedly decreased when cardiomyocytes were exposed to 0–10 mM Na+. Inhibitors of SL NCX depressed but decreasing extracellular Na+from 105–35 mM augmented the ouabain-induced increase in basal [Ca2+]iand the KCl response. Not only was the increase in [Ca2+]iby ouabain dependent on the extracellular Ca2+concentration, but it was also attenuated by inhibitors of SL L-type Ca2+channels and store-operated Ca2+channels (SOC). Unlike the SL L-type Ca2+-channel blocker, the blockers of SL Na+channel and SL SOC, when used in combination with SL NCX inhibitor, showed additive effects in reducing the ouabain-induced increase in basal [Ca2+]i. These results support the view that in addition to SL NCX, SL L-type Ca2+channels and SL SOC may be involved in raising [Ca2+]ion inhibition of the SL Na+-K+-ATPase by ouabain. Furthermore, both SL Na+/H+exchanger and Na+channels play a critical role in the ouabain-induced Ca2+increase in cardiomyocytes.

Mutation of junctophilin type 2 associated with hypertrophic cardiomyopathy

Junctophilin subtypes, designated as JPH1 approximately 4, are protein components of junctional complexes and play essential roles in cellular Ca2+ signaling in excitable cells. Knockout mice lacking the cardiac-type Jph2 die of embryonic cardiac arrest, and the mutant cardiac myocytes exhibit impaired formation of peripheral couplings and arrhythmic Ca2+ signaling caused by functional uncoupling between dihydropyridine and ryanodine receptor channels. Based on these observations, we hypothesized that mutations of JPH2 could cause human genetic cardiac diseases. Among 195 Japanese patients (148 index cases and 47 affected family members) with hypertrophic cardiomyopathy (HCM), two heterozygous nonsynonymous nucleotide transitions, G505S and R436C, were newly found in JPH2. When Fisher's exact test was used to compare index cases with HCM to unrelated Japanese healthy controls in the frequencies of mutant alleles, only the G505S mutation showed statistical significance (4/296 HCM patients and 0/472 control individuals, P=0.022). This result was still significant after Bonferroni's correction for multiple comparisons (P=0.044). To the best of our knowledge, this is the first report on JPH2 mutation associated with HCM.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

TRPC6 fulfills a calcineurin signaling circuit during pathologic cardiac remodeling

The heart responds to injury and chronic pressure overload by pathologic growth and remodeling, which frequently result in heart failure and sudden death. Calcium-dependent signaling pathways promote cardiac growth and associated changes in gene expression in response to stress. The calcium/calmodulin-dependent phosphatase calcineurin, which signals to nuclear factor of activated T cells (NFAT) transcription factors, serves as a transducer of calcium signals and is sufficient and necessary for pathologic cardiac hypertrophy and remodeling. Transient receptor potential (TRP) proteins regulate cation entry into cells in response to a variety of signals, and in skeletal muscle, expression of TRP cation channel, subfamily C, member 3 (TRPC3) is increased in response to neurostimulation and calcineurin signaling. Here we show that TRPC6 was upregulated in mouse hearts in response to activated calcineurin and pressure overload, as well as in failing human hearts. Two conserved NFAT consensus sites in the promoter of the TRPC6 gene conferred responsiveness to cardiac stress. Cardiac-specific overexpression of TRPC6 in transgenic mice resulted in heightened sensitivity to stress, a propensity for lethal cardiac growth and heart failure, and an increase in NFAT-dependent expression of beta-myosin heavy chain, a sensitive marker for pathologic hypertrophy. These findings implicate TRPC6 as a positive regulator of calcineurin-NFAT signaling and a key component of a calcium-dependent regulatory loop that drives pathologic cardiac remodeling.

Calcineurin-dependent cardiomyopathy is activated by TRPC in the adult mouse heart

The manner in which Ca2+-sensitive signaling proteins are activated in contracting cardiomyocytes is an intriguing theoretical problem given that the cytoplasm is continually bathed with systolic Ca2+ concentrations that should maximally activate most Ca2+-sensitive signaling kinases and phosphatases. Store-operated Ca2+ entry, partially attributed to transient receptor potential (TRP) proteins, can mediate activation of the Ca2+-sensitive phosphatase calcineurin in nonexcitable cells. Here we investigated the gain-of-function phenotype associated with TRPC3 expression in the mouse heart using transgenesis to examine the potential role of store-operated Ca2+ entry in regulating cardiac calcineurin activation and ensuing hypertrophy/myopathy. Adult myocytes isolated from TRPC3 transgenic mice showed abundant store-operated Ca2+ entry that was inhibited with SKF96365 but not verapamil or KB-R7943. Associated with this induction in store-operated Ca2+ entry, TRPC3 transgenic mice showed increased calcineurin-nuclear factor of activated T cells (NFAT) activation in vivo, cardiomyopathy, and increased hypertrophy after neuroendocrine agonist or pressure overload stimulation. The cardiomyopathic phenotype and increased hypertrophy after pressure overload stimulation were blocked by targeted disruption of the calcineurin Abeta gene. Thus, enhanced store-operated Ca2+ entry in the heart can regulate calcineurin-NFAT signaling in vivo, which could secondarily impact the hypertrophic response and cardiomyopathy.