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

Orai1/STIMs modulators in pulmonary vascular diseases

Calcium (Ca2+) is a secondary messenger that regulates various cellular processes. However, Ca2+ mishandling could lead to pathological conditions. Orai1 is a Ca2+channel contributing to the store-operated calcium entry (SOCE) and plays a critical role in Ca2+ homeostasis in several cell types. Dysregulation of Orai1 contributed to severe combined immune deficiency syndrome, some cancers, pulmonary arterial hypertension (PAH), and other cardiorespiratory diseases. During its activation process, Orai1 is mainly regulated by stromal interacting molecule (STIM) proteins, especially STIM1; however, many other regulatory partners have also been recently described. Increasing knowledge about these regulatory partners provides a better view of the downstream signalling pathways of SOCE and offers an excellent opportunity to decipher Orai1 dysregulation in these diseases. These proteins participate in other cellular functions, making them attractive therapeutic targets. This review mainly focuses on Orai1 regulatory partners in the physiological and pathological conditions of the pulmonary circulation and inflammation.

STIM2 variants regulate Orai1/TRPC1/TRPC4-mediated store-operated Ca2+ entry and mitochondrial Ca2+ homeostasis in cardiomyocytes

The stromal interaction molecules (STIMs) are the sarcoplasmic reticulum (SR) Ca2+ sensors that trigger store-operated Ca2+ entry (SOCE) in a variety of cell types. While STIM1 isoform has been the focus of the research in cardiac pathophysiology, the function of the homolog STIM2 remains unknown. Using Ca2+ imaging and patch-clamp techniques, we showed that knockdown (KD) of STIM2 by siRNAs increased SOCE and the ISOC current in neonatal rat ventricular cardiomyocytes (NRVMs). Within this cardiomyocyte model, we identified the transcript expression of Stim2.1 and Stim2.2 splice variants, with predominance for Stim2.2. Using conventional and super-resolution confocal microscopy (STED), we found that exogenous STIM2.1 and STIM2.2 formed pre-clusters with a reticular organization at rest. Following SR Ca2+ store depletion, some STIM2.1 and STIM2.2 clusters were translocated to SR-plasma membrane (PM) junctions and co-localized with Orai1. The overexpression strategy revealed that STIM2.1 suppressed Orai1-mediated SOCE and the ISOC current while STIM2.2 enhanced SOCE. STIM2.2-enhanced SOCE was also dependent on TRPC1 and TRPC4. Even if STIM2 KD or splice variants overexpression did not affect cytosolic Ca2+ cycling, we observed, using Rhod-2/AM Ca2+ imaging, that Orai1 inhibition or STIM2.1 overexpression abolished the mitochondrial Ca2+ (mCa2+) uptake, as opposed to STIM2 KD. We also found that STIM2 was present in the mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) by interacting with the inositol trisphosphate receptors (IP3Rs), voltage-dependent anion channel (VDAC), mitochondrial Ca2+ uniporter (MCU), and mitofusin-2 (MNF2). Our results suggested that, in NRVMs, STIM2.1 constitutes the predominant functional variant that negatively regulates Orai1-generated SOCE. It participates in the control of mCa2+ uptake capacity possibly via the STIM2-IP3Rs-VDAC-MCU and MNF2 complex.

Activation mechanisms and structural dynamics of STIM proteins

The family of stromal interaction molecules (STIM) includes two widely expressed single-pass endoplasmic reticulum (ER) transmembrane proteins and additional splice variants that act as precise ER-luminal Ca2+ sensors. STIM proteins mainly function as one of the two essential components of the so-called Ca2+ release-activated Ca2+ (CRAC) channel. The second CRAC channel component is constituted by pore-forming Orai proteins in the plasma membrane. STIM and Orai physically interact with each other to enable CRAC channel opening, which is a critical prerequisite for various downstream signalling pathways such as gene transcription or proliferation. Their activation commonly requires the emptying of the intracellular ER Ca2+ store. Using their Ca2+ sensing capabilities, STIM proteins confer this Ca2+ content-dependent signal to Orai, thereby linking Ca2+ store depletion to CRAC channel opening. Here we review the conformational dynamics occurring along the entire STIM protein upon store depletion, involving the transition from the quiescent, compactly folded structure into an active, extended state, modulation by a variety of accessory components in the cell as well as the impairment of individual steps of the STIM activation cascade associated with disease.

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.

Role of STIM1 in the Regulation of Cardiac Energy Substrate Preference

The heart requires a variety of energy substrates to maintain proper contractile function. Glucose and long-chain fatty acids (FA) are the major cardiac metabolic substrates under physiological conditions. Upon stress, a shift of cardiac substrate preference toward either glucose or FA is associated with cardiac diseases. For example, in pressure-overloaded hypertrophic hearts, there is a long-lasting substrate shift toward glucose, while in hearts with diabetic cardiomyopathy, the fuel is switched toward FA. Stromal interaction molecule 1 (STIM1), a well-established calcium (Ca2+) sensor of endoplasmic reticulum (ER) Ca2+ store, is increasingly recognized as a critical player in mediating both cardiac hypertrophy and diabetic cardiomyopathy. However, the cause-effect relationship between STIM1 and glucose/FA metabolism and the possible mechanisms by which STIM1 is involved in these cardiac metabolic diseases are poorly understood. In this review, we first discussed STIM1-dependent signaling in cardiomyocytes and metabolic changes in cardiac hypertrophy and diabetic cardiomyopathy. Second, we provided examples of the involvement of STIM1 in energy metabolism to discuss the emerging role of STIM1 in the regulation of energy substrate preference in metabolic cardiac diseases and speculated the corresponding underlying molecular mechanisms of the crosstalk between STIM1 and cardiac energy substrate preference. Finally, we briefly discussed and presented future perspectives on the possibility of targeting STIM1 to rescue cardiac metabolic diseases. Taken together, STIM1 emerges as a key player in regulating cardiac energy substrate preference, and revealing the underlying molecular mechanisms by which STIM1 mediates cardiac energy metabolism could be helpful to find novel targets to prevent or treat cardiac metabolic diseases.

STIM1 ablation impairs exercise-induced physiological cardiac hypertrophy and dysregulates autophagy in mouse hearts

Cardiac stromal interaction molecule 1 (STIM1), a key mediator of store-operated Ca2+ entry (SOCE), is a known determinant of cardiomyocyte pathological growth in hypertrophic cardiomyopathy. We examined the role of STIM1 and SOCE in response to exercise-dependent physiological hypertrophy. Wild-type (WT) mice subjected to exercise training (WT-Ex) showed a significant increase in exercise capacity and heart weight compared with sedentary (WT-Sed) mice. Moreover, myocytes from WT-Ex hearts displayed an increase in length, but not width, compared with WT-Sed myocytes. Conversely, exercised cardiac-specific STIM1 knock-out mice (cSTIM1KO-Ex), although displaying significant increase in heart weight and cardiac dilation, evidenced no changes in myocyte size and displayed a decreased exercise capacity, impaired cardiac function, and premature death compared with sedentary cardiac-specific STIM1 knock-out mice (cSTIM1KO-Sed). Confocal Ca2+ imaging demonstrated enhanced SOCE in WT-Ex myocytes compared with WT-Sed myocytes with no measurable SOCE detected in cSTIM1KO myocytes. Exercise training induced a significant increase in cardiac phospho-Akt Ser473 in WT mice but not in cSTIM1KO mice. No differences were observed in phosphorylation of mammalian target of rapamycin (mTOR) and glycogen synthase kinase (GSK) in exercised versus sedentary cSTIM1KO mice hearts. cSTIM1KO-Sed mice showed increased basal MAPK phosphorylation compared with WT-Sed that was not altered by exercise training. Finally, histological analysis revealed exercise resulted in increased autophagy in cSTIM1KO but not in WT myocytes. Taken together, our results suggest that adaptive cardiac hypertrophy in response to exercise training involves STIM1-mediated SOCE. Our results demonstrate that STIM1 is involved in and essential for the myocyte longitudinal growth and mTOR activation in response to endurance exercise training.NEW & NOTEWORTHY Store-operated Ca2+ entry (SOCE) has been implicated in pathological cardiac hypertrophy; however, its role in physiological hypertrophy is unknown. Here we report that SOCE is also essential for physiological cardiac hypertrophy and functional adaptations in response to endurance exercise. These adaptations were associated with activation of AKT/mTOR pathway and curtailed cardiac autophagy and degeneration. Thus, SOCE is a common mechanism and an important bifurcation point for signaling paths involved in physiological and pathological hypertrophy.

Blocking Store-Operated Ca2+ Entry to Protect HL-1 Cardiomyocytes from Epirubicin-Induced Cardiotoxicity

Epirubicin (EPI) is one of the most widely used anthracycline chemotherapy drugs, yet its cardiotoxicity severely limits its clinical application. Altered intracellular Ca2+ homeostasis has been shown to contribute to EPI-induced cell death and hypertrophy in the heart. While store-operated Ca2+ entry (SOCE) has recently been linked with cardiac hypertrophy and heart failure, its role in EPI-induced cardiotoxicity remains unknown. Using a publicly available RNA-seq dataset of human iPSC-derived cardiomyocytes, gene analysis showed that cells treated with 2 µM EPI for 48 h had significantly reduced expression of SOCE machinery genes, e.g., Orai1, Orai3, TRPC3, TRPC4, Stim1, and Stim2. Using HL-1, a cardiomyocyte cell line derived from adult mouse atria, and Fura-2, a ratiometric Ca2+ fluorescent dye, this study confirmed that SOCE was indeed significantly reduced in HL-1 cells treated with EPI for 6 h or longer. However, HL-1 cells presented increased SOCE as well as increased reactive oxygen species (ROS) production at 30 min after EPI treatment. EPI-induced apoptosis was evidenced by disruption of F-actin and increased cleavage of caspase-3 protein. The HL-1 cells that survived to 24 h after EPI treatment demonstrated enlarged cell sizes, up-regulated expression of brain natriuretic peptide (a hypertrophy marker), and increased NFAT4 nuclear translocation. Treatment by BTP2, a known SOCE blocker, decreased the initial EPI-enhanced SOCE, rescued HL-1 cells from EPI-induced apoptosis, and reduced NFAT4 nuclear translocation and hypertrophy. This study suggests that EPI may affect SOCE in two phases: the initial enhancement phase and the following cell compensatory reduction phase. Administration of a SOCE blocker at the initial enhancement phase may protect cardiomyocytes from EPI-induced toxicity and hypertrophy.

Store-operated Ca2+ channel signaling: Novel mechanism for podocyte injury in kidney disease

Studies over the last decade have markedly broadened our understanding of store-operated Ca2+ channels (SOCs) and their roles in kidney diseases and podocyte dysfunction. Podocytes are terminally differentiated glomerular visceral epithelial cells which are tightly attached to the glomerular capillary basement membrane. Podocytes and their unique foot processes (pedicels) constitute the outer layer of the glomerular filtration membrane and the final barrier preventing filtration of albumin and other plasma proteins. Diabetic nephropathy and other renal diseases are associated with podocyte injury and proteinuria. Recent evidence demonstrates a pivotal role of store-operated Ca2+ entry (SOCE) in maintaining structural and functional integrity of podocytes. This article reviews the current knowledge of SOCE and its contributions to podocyte physiology. Recent studies of the contributions of SOC dysfunction to podocyte injury in both cell culture and animal models are discussed, including work in our laboratory. Several downstream signaling pathways mediating SOC function in podocytes also are examined. Understanding the pivotal roles of SOC in podocyte health and disease is essential, as SOCE-activated signaling pathways are potential treatment targets for podocyte injury-related kidney diseases.

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.

Cardiac Hypertrophy: From Pathophysiological Mechanisms to Heart Failure Development

Cardiac hypertrophy develops in response to increased workload to reduce ventricular wall stress and maintain function and efficiency. Pathological hypertrophy can be adaptive at the beginning. However, if the stimulus persists, it may progress to ventricular chamber dilatation, contractile dysfunction, and heart failure, resulting in poorer outcome and increased social burden. The main pathophysiological mechanisms of pathological hypertrophy are cell death, fibrosis, mitochondrial dysfunction, dysregulation of Ca 2 + -handling proteins, metabolic changes, fetal gene expression reactivation, impaired protein and mitochondrial quality control, altered sarcomere structure, and inadequate angiogenesis. Diabetic cardiomyopathy is a condition in which cardiac pathological hypertrophy mainly develop due to insulin resistance and subsequent hyperglycaemia, associated with altered fatty acid metabolism, altered calcium homeostasis and inflammation. In this review, we summarize the underlying molecular mechanisms of pathological hypertrophy development and progression, which can be applied in the development of future novel therapeutic strategies in both reversal and prevention.

Trans-cinnamaldehyde protects against phenylephrine-induced cardiomyocyte hypertrophy through the CaMKII/ERK pathway

BACKGROUND

Trans-cinnamaldehyde (TCA) is one of the main pharmaceutical ingredients of Cinnamomum cassia Presl, which has been shown to have therapeutic effects on a variety of cardiovascular diseases. This study was carried out to characterize and reveal the underlying mechanisms of the protective effects of TCA against cardiac hypertrophy.

METHODS

We used phenylephrine (PE) to induce cardiac hypertrophy and treated with TCA in vivo and in vitro. In neonatal rat cardiomyocytes (NRCMs), RNA sequencing and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were carried out to identify potential pathways of TCA. Then, the phosphorylation and nuclear localization of calcium/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-related kinase (ERK) were detected. In adult mouse cardiomyocytes (AMCMs), calcium transients, calcium sparks, sarcomere shortening and the phosphorylation of several key proteins for calcium handling were evaluated. For mouse in vivo experiments, cardiac hypertrophy was evaluated by assessing morphological changes, echocardiographic parameters, and the expression of hypertrophic genes and proteins.

RESULTS

TCA suppressed PE-induced cardiac hypertrophy and the phosphorylation and nuclear localization of CaMKII and ERK in NRCMs. Our data also demonstrate that TCA blocked the hyperphosphorylation of ryanodine receptor type 2 (RyR2) and phospholamban (PLN) and restored Ca2+ handling and sarcomere shortening in AMCMs. Moreover, our data revealed that TCA alleviated PE-induced cardiac hypertrophy in adult mice and downregulated the phosphorylation of CaMKII and ERK.

CONCLUSION

TCA has a protective effect against PE-induced cardiac hypertrophy that may be associated with the inhibition of the CaMKII/ERK pathway.

STIM and Orai Mediated Regulation of Calcium Signaling in Age-Related Diseases

Tight spatiotemporal regulation of intracellular Ca²⁺ plays a critical role in regulating diverse cellular functions including cell survival, metabolism, and transcription. As a result, eukaryotic cells have developed a wide variety of mechanisms for controlling Ca²⁺ influx and efflux across the plasma membrane as well as Ca²⁺ release and uptake from intracellular stores. The STIM and Orai protein families comprising of STIM1, STIM2, Orai1, Orai2, and Orai3, are evolutionarily highly conserved proteins that are core components of all mammalian Ca²⁺ signaling systems. STIM1 and Orai1 are considered key players in the regulation of Store Operated Calcium Entry (SOCE), where release of Ca²⁺ from intracellular stores such as the Endoplasmic/Sarcoplasmic reticulum (ER/SR) triggers Ca²⁺ influx across the plasma membrane. SOCE, which has been widely characterized in non-excitable cells, plays a central role in Ca²⁺-dependent transcriptional regulation. In addition to their role in Ca²⁺ signaling, STIM1 and Orai1 have been shown to contribute to the regulation of metabolism and mitochondrial function. STIM and Orai proteins are also subject to redox modifications, which influence their activities. Considering their ubiquitous expression, there has been increasing interest in the roles of STIM and Orai proteins in excitable cells such as neurons and myocytes. While controversy remains as to the importance of SOCE in excitable cells, STIM1 and Orai1 are essential for cellular homeostasis and their disruption is linked to various diseases associated with aging such as cardiovascular disease and neurodegeneration. The recent identification of splice variants for most STIM and Orai isoforms while complicating our understanding of their function, may also provide insight into some of the current contradictions on their roles. Therefore, the goal of this review is to describe our current understanding of the molecular regulation of STIM and Orai proteins and their roles in normal physiology and diseases of aging, with a particular focus on heart disease and neurodegeneration.

STIM1-Orai1 interaction mediated calcium influx activation contributes to cardiac contractility of insulin-resistant rats

PURPOSE

Metabolic syndrome (MetS) became a tremendous public health burden in the last decades. Store-operated calcium entry (SOCE) is a unique mechanism that causes a calcium influx, which is triggered by calcium store depletion. MetS-induced alterations in cardiac calcium signaling, especially in SOCE are still unclear. Therefore, we aim to examine the possible role of SOCE and its components (STIM1 and Orai1) in the MetS-induced cardiac remodeling.

METHODS

We used male, adult (12 weeks) Wistar albino rats (n = 20). Animals were randomly divided into two groups which were: control (C) and MetS. We gave 33% sucrose solution to animals instead of water for 24 weeks to establish MetS model. In the end, papillary muscle function was evaluated, and various electrophysiological analyses were made in isolated cardiomyocytes. Additionally, STIM1 and Orai1 protein and mRNA expressions were analyzed.

RESULTS

We observed a deterioration in contractility in MetS animals and demonstrated the contribution of SOCE by applying a SOCE inhibitor (BTP2). Calcium spark frequency was increased while its amplitude was decreasing in MetS hearts, which was reversed after SOCE inhibition. The amplitude of transient calcium changes in the MetS group was decreased, and it decreased further BTP2 application. Both protein and mRNA levels of STIM1 and Orai1 were increased significantly in MetS hearts.

CONCLUSION

Current data indicate the significant contribution of SOCE to cardiac calcium handling in the MetS model. We think MetS-induced SOCE activation is a compensation mechanism that is required for the continuum of proper cardiac functioning, although the activation can also cause cardiac hypertrophy.

Cardiomyocyte stromal interaction molecule 1 is a key regulator of Ca2+ -dependent kinase and phosphatase activity in the mouse heart

Stromal interaction molecule 1 (STIM1) is a major regulator of store-operated calcium entry in non-excitable cells. Recent studies have suggested that STIM1 plays a role in pathological hypertrophy; however, the physiological role of STIM1 in the heart is not well understood. We have shown that mice with a cardiomyocyte deletion of STIM1 (cr STIM1-/- ) develop ER stress, mitochondrial, and metabolic abnormalities, and dilated cardiomyopathy. However, the specific signaling pathways and kinases regulated by STIM1 are largely unknown. Therefore, we used a discovery-based kinomics approach to identify kinases differentially regulated by STIM1. Twelve-week male control and cr STIM1-/- mice were injected with saline or phenylephrine (PE, 15 mg/kg, s.c, 15 min), and hearts obtained for analysis of the Serine/threonine kinome. Primary analysis was performed using BioNavigator 6.0 (PamGene), using scoring from the Kinexus PhosphoNET database and GeneGo network modeling, and confirmed using standard immunoblotting. Kinomics revealed significantly lower PKG and protein kinase C (PKC) signaling in the hearts of the cr STIM1-/- in comparison to control hearts, confirmed by immunoblotting for the calcium-dependent PKC isoform PKCα and its downstream target MARCKS. Similar reductions in cr STIM1-/- hearts were found for the kinases: MEK1/2, AMPK, and PDPK1, and in the activity of the Ca2+ -dependent phosphatase, calcineurin. Electrocardiogram analysis also revealed that cr STIM1-/- mice have significantly lower HR and prolonged QT interval. In conclusion, we have shown several calcium-dependent kinases and phosphatases are regulated by STIM1 in the adult mouse heart. This has important implications in understanding how STIM1 contributes to the regulation of cardiac physiology and pathophysiology.

Analysis of Drosophila cardiac hypertrophy by microcomputerized tomography for genetic dissection of heart growth mechanisms

Heart failure is often preceded by pathological cardiac hypertrophy, a thickening of the heart musculature driven by complex gene regulatory and signaling processes. The Drosophila heart has great potential as a genetic model for deciphering the underlying mechanisms of cardiac hypertrophy. However, current methods for evaluating hypertrophy of the Drosophila heart are laborious and difficult to carry out reproducibly. Here, we demonstrate that microcomputerized tomography (microCT) is an accessible, highly reproducible method for nondestructive, quantitative analysis of Drosophila heart morphology and size. To validate our microCT approach for analyzing Drosophila cardiac hypertrophy, we show that expression of constitutively active Ras (Ras85D^V12), previously shown to cause hypertrophy of the fly heart, results in significant thickening of both adult and larval heart walls when measured from microCT images. We then show using microCT analysis that genetic upregulation of store-operated Ca²⁺ entry (SOCE) driven by expression of constitutively active Stim (Stim^CA) or Orai (Orai^CA) proteins also results in significant hypertrophy of the Drosophila heart, through a process that specifically depends on Orai Ca²⁺ influx channels. Intravital imaging of heart contractility revealed significantly reduced end-diastolic and end-systolic dimensions in Stim^CA- and Orai^CA-expressing hearts, consistent with the hypertrophic phenotype. These results demonstrate that increased SOCE activity is an important driver of hypertrophic cardiomyocyte growth, and demonstrate how microCT analysis combined with tractable genetic tools in Drosophila can be used to delineate molecular signaling processes that underlie cardiac hypertrophy and heart failure.NEW & NOTEWORTHY Genetic analysis of Drosophila cardiac hypertrophy holds immense potential for the discovery of new therapeutic targets to prevent and treat heart failure. This potential has been hindered by a lack of rapid and effective methods for analyzing heart size in flies. Here, we demonstrate that analysis of the Drosophila heart with microcomputerized tomography yields accurate and highly reproducible heart size measurements that can be used to analyze heart growth and cardiac hypertrophy in Drosophila.

Trans-cinnamaldehyde suppresses microtubule detyrosination and alleviates cardiac hypertrophy

BACKGROUND

Trans-cinnamaldehyde (TCA) is a main compound of Cinnamomum cassia, used in traditional Chinese medicine to treat many ailments. Increasing evidence has demonstrated the therapeutic effects of TCA in cardiovascular diseases.

PURPOSE

The present study aimed to determine whether TCA exerts antihypertrophic effects in vitro and in vivo and to elucidate the underlying mechanisms of these effects.

METHODS

Neonatal rat cardiac myocytes (NRCMs) and adult mouse cardiac myocytes (AMCMs) were treated with 50 μΜ phenylephrine (PE) for 48 h. Tubulin detyrosination, store-operated Ca²⁺ entry (SOCE), stromal interaction molecule-1 (STIM1)/Orai1 translocation, and calcineurin/nuclear factor of activated T-cells (NFAT) signaling pathways were analyzed in NRCMs. Meanwhile, tubulin detyrosination, junctophilin-2, T-tubule distribution pattern, Ca²⁺ handling, and sarcomere shortening were observed in AMCMs. Male C57BL/6 mice were stimulated with PE (70 mg/kg per day) with or without TCA treatment for 2 weeks. Cardiac hypertrophy and tubulin detyrosination were also assessed.

RESULTS

TCA was confirmed to alleviate cardiac hypertrophy induced by PE stimulation in vitro and in vivo. PE-induced cardiac hypertrophy was associated with excessive tubulin detyrosination and overexpression of vasohibin 1 (VASH1) and small vasohibin binding protein (SVBP), two key proteins responsible for tubulin detyrosination. These effects were largely blocked by TCA administration. PE treatment also enhanced SOCE with massive translocation of STIM1 and Orai1, Ca²⁺ mishandling, reduced sarcomere shortening, junctophilin-2, and T-tubule redistribution, all of which were significantly ameliorated by TCA administration.

CONCLUSION

Our study indicated that the therapeutic effects of TCA against cardiac hypertrophy may be associated with its ability to reduce tubulin detyrosination.

Enhanced Store-Operated Ca2+ Signal of Small Intestinal Smooth Muscle Cells Accelerates Small Bowel Transit Speed in Type 1 Diabetic Mouse

Aims: The underlying mechanism of diabetic enteropathy, a common complication of type 1 diabetes, remains unclear. Store-operated Ca²⁺ entry (SOCE) is a ubiquitous type of Ca²⁺ influx involved in various cellular functions. Here, we show that SOCE-related stromal interaction molecule 1 (STIM1) and Orai1 participate in inappropriate cellular Ca²⁺ homeostasis, augmenting agonist-induced small intestinal smooth muscle contraction and small bowel transit speed in a mouse model of type 1 diabetes.

Methods and Results: We used small interfering (si)RNA to suppress STIM1 and Orai1 proteins, and employed intracellular Ca²⁺, small intestinal contraction and intestinal transit speed measurement to investigate the functional change. We found that SOCE activity and Orai1 and STIM1 expression levels of small intestinal smooth muscle were significantly increased in cells cultured in high glucose medium or in diabetic mice. Gastrointestinal transit speed and SOCE-mediated contractions were markedly increased in diabetic mice; Knocking down Orai1 or STIM1 with siRNA rescued both alterations in diabetic mice. However, the Orai1-large conductance Ca²⁺-activated K⁺ (BK_Ca) channel interaction was decreased in diabetic mice, and suppressing Orai1 expression or inhibiting the BK_Ca channel increased agonist-induced small intestinal contractions in normal mice.

Conclusion: We concluded that the increased SOCE caused by excessive STIM1 and Orai1 expression and decreased Orai1-BK_Ca interaction augmented small intestinal smooth muscle contraction and accelerated small bowel transit speed in diabetic mice. This finding demonstrates a pathological role for SOCE in diabetic enteropathy and provides a potential therapeutic target for diabetic enteropathy.

Mapping the Proximity Interaction Network of STIM1 Reveals New Mechanisms of Cytoskeletal Regulation

Stromal interaction molecule 1 (STIM1) resides primarily in the sarco/endoplasmic reticulum, where it senses intraluminal Ca2+ levels and activates Orai channels on the plasma membrane to initiate Ca2+ influx. We have previously shown that STIM1 is involved in the dynamic remodeling of the actin cytoskeleton. However, the downstream effectors of STIM1 that lead to cytoskeletal remodeling are not known. The proximity-labeling technique (BioID) can capture weak and transient protein-protein interactions, including proteins that reside in the close vicinity of the bait, but that may not be direct binders. Hence, in the present study, we investigated the STIM1 interactome using the BioID technique. A promiscuous biotin ligase was fused to the cytoplasmic C-terminus of STIM1 and was stably expressed in a mouse embryonic fibroblast (MEF) cell line. Screening of biotinylated proteins identified several high confidence targets. Here, we report Gelsolin (GSN) as a new member of the STIM1 interactome. GSN is a Ca2+-dependent actin-severing protein that promotes actin filament assembly and disassembly. Results were validated using knockdown approaches and immunostaining. We tested our results in neonatal cardiomyocytes where STIM1 overexpression induced altered actin dynamics and cytoskeletal instability. This is the first time that BioID assay was used to investigate the STIM1 interactome. Our work highlights the role of STIM1/GSN in the structure and function of the cytoskeleton.

The interplay between mitochondria and store-operated Ca2+ entry: Emerging insights into cardiac diseases

Store‐operated Ca²⁺ entry (SOCE) machinery, including Orai channels, TRPCs, and STIM1, is key to cellular calcium homeostasis. The following characteristics of mitochondria are involved in the physiological and pathological regulation of cells: mitochondria mediate calcium uptake through calcium uniporters; mitochondria are regulated by mitochondrial dynamic related proteins (OPA1, MFN1/2, and DRP1) and form mitochondrial networks through continuous fission and fusion; mitochondria supply NADH to the electron transport chain through the Krebs cycle to produce ATP; under stress, mitochondria will produce excessive reactive oxygen species to regulate mitochondria‐endoplasmic reticulum interactions and the related signalling pathways. Both SOCE and mitochondria play critical roles in mediating cardiac hypertrophy, diabetic cardiomyopathy, and cardiac ischaemia‐reperfusion injury. All the mitochondrial characteristics mentioned above are determinants of SOCE activity, and vice versa. Ca²⁺ signalling dictates the reciprocal regulation between mitochondria and SOCE under the specific pathological conditions of cardiomyocytes. The coupling of mitochondria and SOCE is essential for various pathophysiological processes in the heart. Herein, we review the research focussing on the reciprocal regulation between mitochondria and SOCE and provide potential interplay patterns in cardiac diseases.

Cardiomyocyte Stim1 Deficiency Exacerbates Doxorubicin Cardiotoxicity by Magnification of Endoplasmic Reticulum Stress

INTRODUCTION

Doxorubicin (Dox) is an effective anticancer agent; however, its cardiotoxicity remains a challenge. Dysfunction of intracellular calcium ion (Ca2+) is implicated in the process of Dox-induced cardiomyocyte apoptosis. Although store-operated Ca2+ entry (SOCE) is suggested to be responsible for Ca2+ entry in cardiomyocytes, the direct role of store-operated Ca2+ channels in Dox-related cardiomyocyte apoptosis is unknown.

MATERIALS AND METHODS

Cardiomyocyte Stim1-specific knockout or overexpression mice were treated with Dox. Cardiomyocytes were pretreated with Stim1 adenovirus or siRNA followed by Dox incubation in vitro. Cardiac function and underlying mechanisms echocardiography were assessed via immunofluorescence, flow cytometry, real-time PCR, Western blotting and immunoprecipitation.

RESULTS

We observed the inhibition of Stim1 expression, association of Stim1 to Orai1 or Trpc1, and SOCE in Dox-treated mouse myocardium and cardiomyocytes. Orai1 and Trpc1 expression remained unchanged. Cardiomyocyte-specific deficiency of Stim1 exacerbated Dox-induced cardiac dysfunction and myocardial apoptosis. However, specific overexpression of Stim1 in the myocardium was associated with amelioration of cardiac dysfunction and myocardial apoptosis. In vitro, STIM1 knockdown potentiated Dox-induced AC16 human cardiomyocyte apoptosis. This apoptosis was attenuated by STIM1 upregulation. Moreover, STIM1 downregulation enhanced Dox-induced endoplasmic reticulum (ER) stress in cardiomyocytes. In contrast, STIM1 overexpression inhibited the activation of the above molecular markers of ER stress. Immunoprecipitation assay showed that STIM1 interacted with GRP78 in cardiomyocytes. This interaction was attenuated in response to Dox treatment.

CONCLUSION

Our data demonstrate that cardiomyocyte STIM1 binding to GRP78 ameliorates Dox cardiotoxicity by inhibiting pro-apoptotic ER stress.

Amelioration of circadian disruption and calcium-handling protein defects by choline alleviates cardiac remodeling in abdominal aorta coarctation rats

The key pathophysiological process leading to heart failure is cardiac remodeling, a term referring to cardiac hypertrophy, fibrosis, and apoptosis. We explored circadian rhythm disruption and calcium dyshomeostasis in cardiac remodeling and investigated the cardioprotective effect of choline. The experiments were conducted using a model of cardiac remodeling by abdominal aorta coarctation (AAC) in Sprague-Dawley rats. In vitro cardiomyocyte remodeling was induced by exposing neonatal rat cardiomyocytes to angiotensin II. The circadian rhythms of the transcript levels of the seven major components of the mammalian clock (Bmal1, Clock, Rev-erbα, Per1/2, and Cry1/2) were altered in AAC rat hearts during a normal 24 h light/dark cycle. AAC also upregulated the levels of proteins that mediate store-operated Ca2+ entry/receptor-operated Ca2+ entry (stromal interaction molecule 1 [STIM1], Orai1, and transient receptor potential canonical 6 [TRPC6]) in rat hearts. Moreover, choline ameliorated circadian rhythm disruption, reduced the upregulated protein levels of STIM1, Orai1, and TRPC6, and alleviated cardiac dysfunction and remodeling (evidenced by attenuated cardiac hypertrophy, fibrosis, and apoptosis) in AAC rats. In vitro analyses showed that choline ameliorated calcium overload, downregulated STIM1, Orai1, and TRPC6, and inhibited thapsigargin-induced store-operated Ca2+ entry and 1-oleoyl-2-acetyl-sn-glycerol-induced receptor-operated Ca2+ entry in angiotensin II-treated cardiomyocytes. In conclusion, choline attenuated AAC-induced cardiac remodeling and cardiac dysfunction, which was related to amelioration of circadian rhythm disruption and attenuation of calcium-handling protein defects. Modulation of vagal activity by choline targeting the circadian rhythm and calcium homeostasis may have therapeutic potential for cardiac remodeling and heart failure.

Ang II Promotes Cardiac Autophagy and Hypertrophy via Orai1/STIM1

The pathophysiology of cardiac hypertrophy is complex and multifactorial. Both the store-operated Ca²⁺ entry (SOCE) and excessive autophagy are the major causative factors for pathological cardiac hypertrophy. However, it is unclear whether these two causative factors are interdependent. In this study, we examined the functional role of SOCE and Orai1 in angiotensin II (Ang II)-induced autophagy and hypertrophy using in vitro neonatal rat cardiomyocytes (NRCMs) and in vivo mouse model, respectively. We show that YM-58483 or SKF-96365 mediated pharmacological inhibition of SOCE, or silencing of Orai1 with Orail-siRNA inhibited Ang II-induced cardiomyocyte autophagy both in vitro and in vivo. Also, the knockdown of Orai1 attenuated Ang II-induced pathological cardiac hypertrophy. Together, these data suggest that Ang II promotes excessive cardiomyocyte autophagy through SOCE/Orai1 which can be the prime contributing factors in cardiac hypertrophy.

Cardiac-Specific Deletion of Orai3 Leads to Severe Dilated Cardiomyopathy and Heart Failure in Mice

Background Orai3 is a mammalian-specific member of the Orai family (Orai1‒3) and a component of the store-operated Ca2+ entry channels. There is little understanding of the role of Orai channels in cardiomyocytes, and its role in cardiac function remains unexplored. Thus, we developed mice lacking Orai1 and Orai3 to address their role in cardiac homeostasis. Methods and Results We generated constitutive and inducible cardiomyocyte-specific Orai3 knockout (Orai3cKO) mice. Constitutive Orai3-loss led to ventricular dysfunction progressing to dilated cardiomyopathy and heart failure. Orai3cKO mice subjected to pressure overload developed a fulminant dilated cardiomyopathy with rapid heart failure onset, characterized by interstitial fibrosis and apoptosis. Ultrastructural analysis of Orai3-deficient cardiomyocytes showed abnormal M- and Z-line morphology. The greater density of condensed mitochondria in Orai3-deficient cardiomyocytes was associated with the upregulation of DRP1 (dynamin-related protein 1). Cardiomyocytes isolated from Orai3cKO mice exhibited profoundly altered myocardial Ca2+ cycling and changes in the expression of critical proteins involved in the Ca2+ clearance mechanisms. Upregulation of TRPC6 (transient receptor potential canonical type 6) channels was associated with upregulation of the RCAN1 (regulator of calcineurin 1), indicating the activation of the calcineurin signaling pathway in Orai3cKO mice. A more dramatic cardiac phenotype emerged when Orai3 was removed in adult mice using a tamoxifen-inducible Orai3cKO mouse. The removal of Orai1 from adult cardiomyocytes did not change the phenotype of tamoxifen-inducible Orai3cKO mice. Conclusions Our results identify a critical role for Orai3 in the heart. We provide evidence that Orai3-mediated Ca2+ signaling is required for maintaining sarcomere integrity and proper mitochondrial function in adult mammalian cardiomyocytes.

TRIC-A regulates intracellular Ca2+ homeostasis in cardiomyocytes

Trimeric intracellular cation (TRIC) channels have been identified as monovalent cation channels that are located in the ER/SR membrane. Two isoforms discovered in mammals are TRIC-A (TMEM38a) and TRIC-B (TMEM38b). TRIC-B ubiquitously expresses in all tissues, and TRIC-B-/- mice is lethal at the neonatal stage. TRIC-A mainly expresses in excitable cells. TRIC-A-/- mice survive normally but show abnormal SR Ca2+ handling in both skeletal and cardiac muscle cells. Importantly, TRIC-A mutations have been identified in human patients with stress-induced arrhythmia. In the past decade, important discoveries have been made to understand the structure and function of TRIC channels, especially its role in regulating intracellular Ca2+ homeostasis. In this review article, we focus on the potential roles of TRIC-A in regulating cardiac function, particularly its effects on intracellular Ca2+ signaling of cardiomyocytes and discuss the current knowledge gaps.

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.

MiR-223-3p in Cardiovascular Diseases: A Biomarker and Potential Therapeutic Target

Cardiovascular diseases, involving vasculopathy, cardiac dysfunction, or circulatory disturbance, have become the major cause of death globally and brought heavy social burdens. The complexity and diversity of the pathogenic factors add difficulties to diagnosis and treatment, as well as lead to poor prognosis of these diseases. MicroRNAs are short non-coding RNAs to modulate gene expression through directly binding to the 3′-untranslated regions of mRNAs of target genes and thereby to downregulate the protein levels post-transcriptionally. The multiple regulatory effects of microRNAs have been investigated extensively in cardiovascular diseases. MiR-223-3p, expressed in multiple cells such as macrophages, platelets, hepatocytes, and cardiomyocytes to modulate their cellular activities through targeting a variety of genes, is involved in the pathological progression of many cardiovascular diseases. It participates in regulation of several crucial signaling pathways such as phosphatidylinositol 3-kinase/protein kinase B, insulin-like growth factor 1, nuclear factor kappa B, mitogen-activated protein kinase, NOD-like receptor family pyrin domain containing 3 inflammasome, and ribosomal protein S6 kinase B1/hypoxia inducible factor 1 α pathways to affect cell proliferation, migration, apoptosis, hypertrophy, and polarization, as well as electrophysiology, resulting in dysfunction of cardiovascular system. Here, in this review, we will discuss the role of miR-223-3p in cardiovascular diseases, involving its verified targets, influenced signaling pathways, and regulation of cell function. In addition, the potential of miR-223-3p as therapeutic target and biomarker for diagnosis and prediction of cardiovascular diseases will be further discussed, providing clues for clinicians.

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.

STIM Proteins: An Ever-Expanding Family

Stromal interaction molecules (STIM) are a distinct class of ubiquitously expressed single-pass transmembrane proteins in the endoplasmic reticulum (ER) membrane. Together with Orai ion channels in the plasma membrane (PM), they form the molecular basis of the calcium release-activated calcium (CRAC) channel. An intracellular signaling pathway known as store-operated calcium entry (SOCE) is critically dependent on the CRAC channel. The SOCE pathway is activated by the ligand-induced depletion of the ER calcium store. STIM proteins, acting as calcium sensors, subsequently sense this depletion and activate Orai ion channels via direct physical interaction to allow the influx of calcium ions for store refilling and downstream signaling processes. This review article is dedicated to the latest advances in the field of STIM proteins. New results of ongoing investigations based on the recently published functional data as well as structural data from nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations are reported and complemented with a discussion of the latest developments in the research of STIM protein isoforms and their differential functions in regulating SOCE.

New role of TRPM4 channel in the cardiac excitation-contraction coupling in response to physiological and pathological hypertrophy in mouse

The transient receptor potential Melastatin 4 (TRPM4) channel is a calcium-activated non-selective cation channel expressed widely. In the heart, using a knock-out mouse model, the TRPM4 channel has been shown to be involved in multiple processes, including β-adrenergic regulation, cardiac conduction, action potential duration and hypertrophic adaptations. This channel was recently shown to be involved in stress-induced cardiac arrhythmias in a mouse model overexpressing TRPM4 in ventricular cardiomyocytes. However, the link between TRPM4 channel expression in ventricular cardiomyocytes, the hypertrophic response to stress and/or cellular arrhythmias has yet to be elucidated. In this present study, we induced pathological hypertrophy in response to myocardial infarction using a mouse model of Trpm4 gene invalidation, and demonstrate that TRPM4 is essential for survival. We also demonstrate that the TRPM4 is required to activate both the Akt and Calcineurin pathways. Finally, using two hypertrophy models, either a physiological response to endurance training or a pathological response to myocardial infarction, we show that TRPM4 plays a role in regulating transient calcium amplitudes and leads to the development of cellular arrhythmias potentially in cooperation with the Sodium-calcium exchange (NCX). Here, we report two functions of the TRPM4 channel: first its role in adaptive hypertrophy, and second its association with NCX could mediate transient calcium amplitudes which trigger cellular arrhythmias.

The mitochondrial K-ATP channel opener diazoxide upregulates STIM1 and Orai1 via ROS and the MAPK pathway in adult rat cardiomyocytes

Background

Openers of mitochondrial adenosine triphosphate-dependent potassium (mKATP) channels like diazoxide increase reactive oxygen species (ROS) production in cardiac cells and reduce Ca²⁺ elevations produced by ischemia–reperfusion, protecting the heart from damage. In this study we tested the hypothesis that opening mKATP channels regulates expression of the major components of store-operated Ca²⁺ entry (SOCE) STIM1 and Orai1.

Results

Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and western blot experiments showed that diazoxide increased expression of STIM1 and Orai1 at the mRNA and protein levels, respectively, in adult rat cardiomyocytes. Immunofluorescence analyses revealed that diazoxide also disrupted the striated distribution pattern of STIM1. These effects were prevented by the ROS scavenger N-acetyl cysteine (NAC), the mKATP channel antagonist 5-hydroxydecanoate (5-HD), or the protein synthesis inhibitor cycloheximide (CHX). Confocal microscopy revealed that diazoxide also led to nuclear translocation of the transcription factors c-Fos and NFκB, which was also blocked by NAC or 5-HD. Finally, the MAPK pathway inhibitor UO126 attenuated diazoxide-induced upregulation of STIM1 and Orai1 expression.

Conclusions

Our results suggest that opening mitochondrial potassium ATP channels with diazoxide upregulates the expression of STIM1 and Orai1 by de novo synthesis by a mechanism that involves NFkB, c-Fos, and ROS via MAPK/ERK signaling.

The effects of macrophages on cardiomyocyte calcium-handling function using in vitro culture models

Following myocardial infarction (MI), myocardial inflammation plays a crucial role in the pathogenesis of MI injury and macrophages are among the key cells activated during the initial phases of the host response regulating the healing process. While macrophages have emerged as attractive effectors in tissue injury and repair, the contribution of macrophages on cardiac cell function and survival is not fully understood due to complexity of the in vivo inflammatory microenvironment. Understanding the key cells involved and how they communicate with one another is of paramount importance for the development of effective clinical treatments. Here, novel in vitro myocardial inflammation models were developed to examine how both direct and indirect interactions with polarized macrophage subsets present in the post-MI microenvironment affect cardiomyocyte function. The indirect model using conditioned medium from polarized macrophage subsets allowed examination of the effects of macrophage-derived factors on stem cell-derived cardiomyocyte function for up to 3 days. The results from the indirect model demonstrated that pro-inflammatory macrophage-derived factors led to a significant downregulation of cardiac troponin T (cTnT) and sarcoplasmic/endoplasmic reticulum calcium ATPase (Serca2) gene expression. It also demonstrated that inhibition of macrophage-secreted matricellular protein, osteopontin (OPN), led to a significant decrease in cardiomyocyte store-operated calcium entry (SOCE). In the direct model, stem cell-derived cardiomyocytes were co-cultured with polarized macrophage subsets for up to 3 days. It was demonstrated that anti-inflammatory macrophages significantly increased cardiomyocyte Ca²⁺ fractional release while macrophages independent of their subtypes led to significant downregulation of SOCE response in cardiomyocytes. This study describes simplified and controlled in vitro myocardial inflammation models, which allow examination of potential beneficial and deleterious effects of macrophages on cardiomyocytes and vise versa. This can lead to our improved understanding of the inflammatory microenvironment post-MI, otherwise difficult to directly investigate in vivo or by using currently available in vitro models.

Cardiomyocyte-Specific Deletion of Orai1 Reveals Its Protective Role in Angiotensin-II-Induced Pathological Cardiac Remodeling

Pathological cardiac remodeling correlates with chronic neurohumoral stimulation and abnormal Ca²⁺ signaling in cardiomyocytes. Store-operated calcium entry (SOCE) has been described in adult and neonatal murine cardiomyocytes, and Orai1 proteins act as crucial ion-conducting constituents of this calcium entry pathway that can be engaged not only by passive Ca²⁺ store depletion but also by neurohumoral stimuli such as angiotensin-II. In this study, we, therefore, analyzed the consequences of Orai1 deletion for cardiomyocyte hypertrophy in neonatal and adult cardiomyocytes as well as for other features of pathological cardiac remodeling including cardiac contractile function in vivo. Cellular hypertrophy induced by angiotensin-II in embryonic cardiomyocytes from Orai1-deficient mice was blunted in comparison to cells from litter-matched control mice. Due to lethality of mice with ubiquitous Orai1 deficiency and to selectively analyze the role of Orai1 in adult cardiomyocytes, we generated a cardiomyocyte-specific and temporally inducible Orai1 knockout mouse line (Orai1^CM–KO). Analysis of cardiac contractility by pressure-volume loops under basal conditions and of cardiac histology did not reveal differences between Orai1^CM–KO mice and controls. Moreover, deletion of Orai1 in cardiomyocytes in adult mice did not protect them from angiotensin-II-induced cardiac remodeling, but cardiomyocyte cross-sectional area and cardiac fibrosis were enhanced. These alterations in the absence of Orai1 go along with blunted angiotensin-II-induced upregulation of the expression of Myoz2 and a lack of rise in angiotensin-II-induced STIM1 and Orai3 expression. In contrast to embryonic cardiomyocytes, where Orai1 contributes to the development of cellular hypertrophy, the results obtained from deletion of Orai1 in the adult myocardium reveal a protective function of Orai1 against the development of angiotensin-II-induced cardiac remodeling, possibly involving signaling via Orai3/STIM1-calcineurin-NFAT related pathways.

Suppression of store-operated calcium entry causes dilated cardiomyopathy of the Drosophila heart

Store-operated Ca2+ entry (SOCE) is an essential Ca2+ signaling mechanism present in most animal cells. SOCE refers to Ca2+ influx that is activated by depletion of sarco/endoplasmic reticulum (S/ER) Ca2+ stores. The main components of SOCE are STIM and Orai. STIM proteins function as S/ER Ca2+ sensors, and upon S/ER Ca2+ depletion STIM rearranges to S/ER-plasma membrane junctions and activates Orai Ca2+ influx channels. Studies have implicated SOCE in cardiac hypertrophy pathogenesis, but SOCE's role in normal heart physiology remains poorly understood. We therefore analyzed heart-specific SOCE function in Drosophila, a powerful animal model of cardiac physiology. We show that heart-specific suppression of Stim and Orai in larvae and adults resulted in reduced contractility consistent with dilated cardiomyopathy. Myofibers were also highly disorganized in Stim and Orai RNAi hearts, reflecting possible decompensation or upregulated stress signaling. Furthermore, we show that reduced heart function due to SOCE suppression adversely affected animal viability, as heart specific Stim and Orai RNAi animals exhibited significant delays in post-embryonic development and adults died earlier than controls. Collectively, our results demonstrate that SOCE is essential for physiological heart function, and establish Drosophila as an important model for understanding the role of SOCE in cardiac pathophysiology.

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.

TRPC Channels in Cardiac Plasticity

The heart flexibly changes its structure in response to changing environments and oxygen/nutrition demands of the body. Increased and decreased mechanical loading induces hypertrophy and atrophy of cardiomyocytes, respectively. In physiological conditions, these structural changes of the heart are reversible. However, chronic stresses such as hypertension or cancer cachexia cause irreversible remodeling of the heart, leading to heart failure. Accumulating evidence indicates that calcium dyshomeostasis and aberrant reactive oxygen species production cause pathological heart remodeling. Canonical transient receptor potential (TRPC) is a nonselective cation channel subfamily whose multimodal activation or modulation of channel activity play important roles in a plethora of cellular physiology. Roles of TRPC channels in cardiac physiology have been reported in pathological cardiac remodeling. In this review, we summarize recent findings regarding the importance of TRPC channels in flexible cardiac remodeling (i.e., cardiac plasticity) in response to environmental stresses and discuss questions that should be addressed in the near future.

Progerin in muscle leads to thermogenic and metabolic defects via impaired calcium homeostasis

Mutations in lamin A (LMNA) are responsible for a variety of human dystrophic and metabolic diseases. Here, we created a mouse model in which progerin, the lamin A mutant protein that causes Hutchinson–Gilford progeria syndrome (HGPS), can be inducibly overexpressed. Muscle‐specific overexpression of progerin was sufficient to induce muscular dystrophy and alter whole‐body energy expenditure, leading to premature death. Intriguingly, sarcolipin (Sln), an endoplasmic reticulum (ER)‐associated protein involved in heat production, is upregulated in progerin‐expressing and Lmna knockout (Lmna^−/−) skeletal muscle. The depletion of Sln accelerated the early death of Lmna^−/− mice. An examination at the molecular level revealed that progerin recruits Sln and Calnexin to the nuclear periphery. Furthermore, progerin‐expressing myoblasts presented enhanced store‐operated Ca²⁺ entry, as well as increased co‐localization of STIM1 and ORAI1. These findings suggest that progerin dysregulates calcium homeostasis through an interaction with a subset of ER‐associated proteins, resulting in thermogenic and metabolic abnormalities.

The Pathogenic Role of Very Low Density Lipoprotein on Atrial Remodeling in the Metabolic Syndrome

Atrial fibrillation (AF) is the most common persistent arrhythmia, and can lead to systemic thromboembolism and heart failure. Aging and metabolic syndrome (MetS) are major risks for AF. One of the most important manifestations of MetS is dyslipidemia, but its correlation with AF is ambiguous in clinical observational studies. Although there is a paradoxical relationship between fasting cholesterol and AF incidence, the benefit from lipid lowering therapy in reduction of AF is significant. Here, we reviewed the health burden from AF and MetS, the association between two disease entities, and the metabolism of triglyceride, which is elevated in MetS. We also reviewed scientific evidence for the mechanistic links between very low density lipoproteins (VLDL), which primarily carry circulatory triglyceride, to atrial cardiomyopathy and development of AF. The effects of VLDL to atria suggesting pathogenic to atrial cardiomyopathy and AF include excess lipid accumulation, direct cytotoxicity, abbreviated action potentials, disturbed calcium regulation, delayed conduction velocities, modulated gap junctions, and sarcomere protein derangements. The electrical remodeling and structural changes in concert promote development of atrial cardiomyopathy in MetS and ultimately lead to vulnerability to AF. As VLDL plays a major role in lipid metabolism after meals (rather than fasting state), further human studies that focus on the effects/correlation of postprandial lipids to atrial remodeling are required to determine whether VLDL-targeted therapy can reduce MetS-related AF. On the basis of our scientific evidence, we propose a pivotal role of VLDL in MetS-related atrial cardiomyopathy and vulnerability to AF.

Pharmacological Preconditioning Using Diazoxide Regulates Store-Operated Ca2 + Channels in Adult Rat Cardiomyocytes

Voltage-dependent Ca²⁺ channels and store-operated Ca²⁺ channels (SOCs) are the major routes of Ca²⁺ entry into mammalian cells. Previously, we reported that pharmacological preconditioning (PPC) leads to a decrease in the amplitude of L-type calcium channel current in the heart. In this study, we examined PPC-associated changes in SOC function. We measured adult cardiomyocyte membrane currents using the whole-cell patch-clamp technique, and we evaluated reactive oxygen species (ROS) production and intracellular Ca²⁺ levels in cardiomyocytes using fluorescent probes. Diazoxide (Dzx) and thapsigargin (Tg) were used to induce PPC and to deplete internal stores of Ca²⁺, respectively. Ca²⁺ store depletion generated inward currents with strong rectification, which were suppressed by the SOC blocker GSK-7975-A. These currents were completely abolished by PPC, an effect that could be countered with 5-hydroxydecanoate (5-HD; a selective mitochondrial ATP-sensitive K⁺ channel blocker), an intracellular mitochondrial energizing solution, or Ni²⁺ [a blocker of sodium-calcium exchanger (NCX)]. Buffering of ROS and intracellular Ca²⁺ also prevented PPC effects on SOC currents. Refilling of intracellular stores was largely suppressed by PPC, as determined by measuring intracellular Ca²⁺ with a fluorescent Ca²⁺ indicator. These results indicate that influx of Ca²⁺ through SOCs is inhibited by their ROS and Ca²⁺-dependent inactivation during PPC and that NCX is a likely source of PPC-inactivating Ca²⁺. We further showed that NCX associates with Orai1. Down-regulation of SOCs by PPC may play a role in cardioprotection following ischemia-reperfusion.

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.

Reciprocality Between Estrogen Biology and Calcium Signaling in the Cardiovascular System

17β-Estradiol (E₂) is the main estrogenic hormone in the body and exerts many cardiovascular protective effects. Via three receptors known to date, including estrogen receptors α (ERα) and β (ERβ) and the G protein-coupled estrogen receptor 1 (GPER, aka GPR30), E₂ regulates numerous calcium-dependent activities in cardiovascular tissues. Nevertheless, effects of E₂ and its receptors on components of the calcium signaling machinery (CSM), the underlying mechanisms, and the linked functional impact are only beginning to be elucidated. A picture is emerging of the reciprocality between estrogen biology and Ca²⁺ signaling. Therein, E₂ and GPER, via both E₂-dependent and E₂-independent actions, moderate Ca²⁺-dependent activities; in turn, ERα and GPER are regulated by Ca²⁺ at the receptor level and downstream signaling via a feedforward loop. This article reviews current understanding of the effects of E₂ and its receptors on the cardiovascular CSM and vice versa with a focus on mechanisms and combined functional impact. An overview of the main CSM components in cardiovascular tissues will be first provided, followed by a brief review of estrogen receptors and their Ca²⁺-dependent regulation. The effects of estrogenic agonists to stimulate acute Ca²⁺ signals will then be reviewed. Subsequently, E₂-dependent and E₂-independent effects of GPER on components of the Ca²⁺ signals triggered by other stimuli will be discussed. Finally, a case study will illustrate how the many mechanisms are coordinated to moderate Ca²⁺-dependent activities in the cardiovascular system.

Molecular Basis and Regulation of Store-Operated Calcium Entry

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous mechanism for Ca²⁺ influx in mammalian cells with important physiological implications. Since the discovery of SOCE more than three decades ago, the mechanism that communicates the information about the amount of Ca²⁺ accumulated in the intracellular Ca²⁺ stores to the plasma membrane channels and the nature of these channels have been matters of intense investigation and debate. The stromal interaction molecule-1 (STIM1) has been identified as the Ca²⁺ sensor of the intracellular Ca²⁺ compartments that activates the store-operated channels. STIM1 regulates two types of store-dependent channels: the Ca²⁺ release-activated Ca²⁺ (CRAC) channels, formed by Orai1 subunits, that conduct the highly Ca²⁺ selective current I _CRAC and the cation permeable store-operated Ca²⁺ (SOC) channels, which consist of Orai1 and TRPC1 proteins and conduct the non-selective current I _SOC. While the crystal structure of Drosophila CRAC channel has already been solved, the architecture of the SOC channels still remains unclear. The dynamic interaction of STIM1 with the store-operated channels is modulated by a number of proteins that either support the formation of the functional STIM1-channel complex or protect the cell against Ca²⁺ overload.

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.

Cardiomyocyte-Specific STIM1 (Stromal Interaction Molecule 1) Depletion in the Adult Heart Promotes the Development of Arrhythmogenic Discordant Alternans

BACKGROUND

STIM1 (stromal interaction molecule 1) is a calcium (Ca²⁺) sensor that regulates cardiac hypertrophy by triggering store-operated Ca²⁺ entry. Because STIM1 binding to phospholamban increases sarcoplasmic reticulum Ca²⁺ load independent of store-operated Ca²⁺ entry, we hypothesized that it controls electrophysiological function and arrhythmias in the adult heart.

METHODS

Inducible myocyte-restricted STIM1-KD (STIM1 knockdown) was achieved in adult mice using an αMHC (α-myosin heavy chain)-MerCreMer system. Mechanical and electrophysiological properties were examined using echocardiography in vivo and optical action potential (AP) mapping ex vivo in tamoxifen-induced STIM1^flox/flox-Cre^tg/- (STIM1-KD) and littermate controls for STIM1^flox/flox (referred to as STIM1-Ctl) and for Cre^tg/- without STIM deletion (referred to as Cre-Ctl).

RESULTS

STIM1-KD mice (N=23) exhibited poor survival compared with STIM1-Ctl (N=22) and Cre-Ctl (N=11) with >50% mortality after only 8-days of cardiomyocyte-restricted STIM1-KD. STIM1-KD but not STIM1-Ctl or Cre-Ctl hearts exhibited a proclivity for arrhythmic behavior, ranging from frequent ectopy to pacing-induced ventricular tachycardia/ventricular fibrillation (VT/VF). Examination of the electrophysiological substrate revealed decreased conduction velocity and increased AP duration (APD) heterogeneity in STIM1-KD. These features, however, were comparable in VT/VF(+) and VT/VF(-) hearts. We also uncovered a marked increase in the magnitude of APD alternans during rapid pacing, and the emergence of a spatially discordant alternans profile in STIM1-KD hearts. Unlike conduction velocity slowing and APD heterogeneity, the magnitude of APD alternans was greater (by 80%, P<0.05) in VT/VF(+) versus VT/VF(-) STIM1-KD hearts. Detailed phase mapping during the initial beats of VT/VF identified one or more rotors that were localized along the nodal line separating out-of-phase alternans regions.

CONCLUSIONS

In an adult murine model with inducible and myocyte-specific STIM1 depletion, we demonstrate for the first time the regulation of spatially discordant alternans by STIM1. Early mortality in STIM1-KD mice is likely related to enhanced susceptibility to VT/VF secondary to discordant APD alternans.

Glucocorticoid stimulation increases cardiac contractility by SGK1-dependent SOCE-activation in rat cardiac myocytes

Aims

Glucocorticoid (GC) stimulation has been shown to increase cardiac contractility by elevated intracellular [Ca] but the sources for Ca entry are unclear. This study aims to determine the role of store-operated Ca entry (SOCE) for GC-mediated inotropy.

Methods and results

Dexamethasone (Dex) pretreatment significantly increased cardiac contractile force ex vivo in Langendorff-perfused Sprague-Dawley rat hearts (2 mg/kg BW i.p. Dex 24 h prior to experiment). Moreover, Ca transient amplitude as well as fractional shortening were significantly enhanced in Fura-2-loaded isolated rat ventricular myocytes exposed to Dex (1 mg/mL Dex, 24 h). Interestingly, these Dex-dependent effects could be abolished in the presence of SOCE-inhibitors SKF-96356 (SKF, 2 μM) and BTP2 (5 μM). Ca transient kinetics (time to peak, decay time) were not affected by SOCE stimulation. Direct SOCE measurements revealed a negligible magnitude in untreated myocytes but a dramatic increase in SOCE upon Dex-pretreatment. Importantly, the Dex-dependent stimulation of SOCE could be blocked by inhibition of serum and glucocorticoid-regulated kinase 1 (SGK1) using EMD638683 (EMD, 50 μM). Dex preincubation also resulted in increased mRNA expression of proteins involved in SOCE (stromal interaction molecule 2, STIM2, and transient receptor potential cation channels 3/6, TRPC 3/6), which were also prevented in the presence of EMD.

Conclusion

Short-term GC-stimulation with Dex improves cardiac contractility by a SOCE-dependent mechanism, which appears to involve increased SGK1-dependent expression of the SOCE-related proteins. Since Ca transient kinetics were unaffected, SOCE appears to influence Ca cycling more by an integrated response across multiple cardiac cycles but not on a beat-to-beat basis.

Diastolic Stress Testing Along the Heart Failure Continuum

PURPOSE OF REVIEW

This review summarizes recent developments highlighting the clinical utility of diastolic stress testing along the heart failure continuum.

RECENT FINDINGS

Invasive hemodynamic assessment of cardiac filling pressures during physiological stress is the gold-standard technique for unmasking diastolic dysfunction. Non-invasive surrogate techniques, such as Doppler ultrasound, have shown excellent agreement with invasive approaches and are now recommended by the American Society of Echocardiography and the European Association of Cardiovascular Imaging. While cycle exercise is often advocated, recent evidence supports the use of isometric handgrip as a viable alternative stressor. Diastolic stress testing is a powerful tool to enhance detection of diastolic dysfunction, is able to differentiate between cardiac and non-cardiac pathology, and should be incorporated into routine clinical assessment.

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.

Very Low-Density Lipoproteins of Metabolic Syndrome Modulates STIM1, Suppresses Store-Operated Calcium Entry, and Deranges Myofilament Proteins in Atrial Myocytes

Individuals with metabolic syndrome (MetS) are at high risk for atrial myopathy and atrial fibrillation. Very low-density lipoproteins (VLDLs) of MetS (MetS-VLDLs) are cytotoxic to atrial myocytes in vivo and in vitro. The calcineurin-nuclear factor of activated T-cells (NFAT) pathway, which is regulated by stromal interaction molecule 1 (STIM1)/ calcium release-activated calcium channel protein 1 (Orai1)-mediated store-operated Ca²⁺ entry (SOCE), is a pivotal mediator of adaptive cardiac hypertrophy. We hypothesized that MetS-VLDLs could affect SOCE and the calcineurin-NFAT pathway. Normal-VLDL and MetS-VLDL samples were isolated from the peripheral blood of healthy volunteers and individuals with MetS. VLDLs were applied to HL-1 atrial myocytes for 18 h and were also injected into wild-type C57BL/6 male mouse tails three times per week for six weeks. After the sarcoplasmic reticulum (SR) Ca²⁺ store was depleted, SOCE was triggered upon reperfusion with 1.8 mM of Ca²⁺. SOCE was attenuated by MetS-VLDLs, along with reduced transcriptional and membranous expression of STIM1 (P = 0.025), and enhanced modification of O-GlcNAcylation on STIM1 protein, while Orai1 was unaltered. The nuclear translocation and activity of calcineurin were both reduced (P < 0.05), along with the alteration of myofilament proteins in atrial tissues. These changes were absent in normal-VLDL-treated cells. Our results demonstrated that MetS-VLDLs suppressed SOCE by modulating STIM1 at the transcriptional, translational, and post-translational levels, resulting in the inhibition of the calcineurin-NFAT pathway, which resulted in the alteration of myofilament protein expression and sarcomere derangement in atrial tissues. These findings may help explain atrial myopathy in MetS. We suggest a therapeutic target on VLDLs to prevent atrial fibrillation, especially for individuals with MetS.