In vivo adenoviral transfer of sorcin reverses cardiac contractile abnormalities of diabetic cardiomyopathy

Sorcin in Cancer Development and Chemotherapeutic Drug Resistance

SOluble Resistance-related Calcium-binding proteIN (sorcin) earned its name due to its co-amplification with ABCB1 in multidrug-resistant cells. Initially thought to be an accidental consequence of this co-amplification, recent research indicates that sorcin plays a more active role as an oncoprotein, significantly impacting multidrug resistance (MDR). Sorcin is a highly expressed calcium-binding protein, often overproduced in human tumors and multidrug-resistant cancers, and is a promising novel MDR marker. In tumors, sorcin levels inversely correlate with both patient response to chemotherapy and overall prognosis. Multidrug-resistant cell lines consistently exhibit higher sorcin expression compared to their parental counterparts. Furthermore, sorcin overexpression via gene transfection enhances drug resistance to various chemotherapeutic drugs across numerous cancer lines. Conversely, silencing sorcin expression reverses drug resistance in many cell lines. Sorcin participates in several mechanisms of MDR, including drug efflux, drug sequestering, cell death inhibition, gene amplification, epithelial-to-mesenchymal transition, angiogenesis, and metastasis. The present review focuses on the structure and function of sorcin, on sorcin's role in cancer and drug resistance, and on the approaches aimed at targeting sorcin.

A plasma membrane-associated form of the androgen receptor enhances nuclear androgen signaling in osteoblasts and prostate cancer cells

Androgen binding to the androgen receptor (AR) in the cytoplasm induces the AR to translocate to the nucleus, where it regulates the expression of target genes. Here, we found that androgens rapidly activated a plasma membrane-associated signaling node that enhanced nuclear AR functions. In murine primary osteoblasts, dihydrotestosterone (DHT) binding to a membrane-associated form of AR stimulated plasma membrane-associated protein kinase G type 2 (PKG2), leading to the activation of multiple kinases, including ERK. Phosphorylation of AR at Ser515 by ERK increased the nuclear accumulation and binding of AR to the promoter of Ctnnb1, which encodes the transcription factor β-catenin. In male mouse osteoblasts and human prostate cancer cells, DHT induced the expression of Ctnnb1 and CTNN1B, respectively, as well as β-catenin target genes, stimulating the proliferation, survival, and differentiation of osteoblasts and the proliferation of prostate cancer cells in a PKG2-dependent fashion. Because β-catenin is a master regulator of skeletal homeostasis, these results explain the reported male-specific osteoporotic phenotype of mice lacking PKG2 in osteoblasts and imply that PKG2-dependent AR signaling is essential for maintaining bone mass in vivo. Our results suggest that widely used pharmacological PKG activators, such as sildenafil, could be beneficial for male and estrogen-deficient female patients with osteoporosis but detrimental in patients with prostate cancer.

Knowledge domain and emerging trends in diabetic cardiomyopathy: A scientometric review based on CiteSpace analysis

Objective

To review the literature related to diabetic cardiomyopathy (DCM), and investigate research hotspots and development trends of this field in the relevant studies based on CiteSpace software of text mining and visualization in scientific literature.

Methods

The relevant literature from the last 20 years was retrieved from the Web of Science (WoS) Core Collection database. After manual selection, each document record includes title, authors, year, organization, abstract, keywords, citation, descriptors, and identifiers. We imported the downloaded data into CiteSpace V (version 5.8.R2) to draw the knowledge map and conduct cooperative network analysis, cluster analysis, burst keyword analysis, and co-citation analysis.

Results

After manual screening, there were 3,547 relevant pieces of literature published in the last 18 years (from 2004 to 2021), including 2,935 articles and reviews, which contained 15,533 references, and the number was increasing year by year. The publications of DCM were dedicated by 778 authors of 512 institutions in 116 countries. The People's Republic of China dominated this field (1,117), followed by the USA (768) and Canada (176). In general, most articles were published with a focus on “oxidative stress,” “heart failure,” “diabetic cardiomyopathy,” “dysfunction,” “cardiomyopathy,” “expression,” “heart,” “mechanism,” and “insulin resistance.” Then, 10 main clusters were generated with a modularity Q of 0.6442 and a weighted mean silhouette of 0.8325 by the log-likelihood ratio (LLR) algorithm, including #0 heart failure, #1 perfused heart, #2 metabolic disease, #3 protective effect, #4 diabetic patient, #5 cardiac fibrosis, #6 vascular complication, #7 mitochondrial dynamics, #8 sarcoplasmic reticulum, and #9 zinc supplementation. The top five references with the strongest citation bursts include “Boudina and Abel”, “Jia et al.”, “Fang et al.”, “Poornima et al.”, and “Aneja et al.”.

Conclusion

The global field of DCM has expanded in the last 20 years. The People's Republic of China contributes the most. However, there is little cooperation among authors and institutions. Overall, this bibliometric study identified the hotspots in DCM research, including “stress state,” “energy metabolism,” “autophagy,” “apoptosis,” “inflammation,” “fibrosis,” “PPAR,” etc. Thus, further research focuses on these topics that may be more helpful to identify, prevent DCM and improve prophylaxis strategies to bring benefit to patients in the near future.

Constitutive protein kinase G activation exacerbates stress-induced cardiomyopathy

BACKGROUND AND PURPOSE

Heart failure is associated with high morbidity and mortality, and new therapeutic targets are needed. Preclinical data suggest that pharmacological activation of protein kinase G (PKG) can reduce maladaptive ventricular remodelling and cardiac dysfunction in the stressed heart. However, clinical trial results have been mixed and the effects of long-term PKG activation in the heart are unknown.

EXPERIMENTAL APPROACH

We characterized the cardiac phenotype of mice carrying a heterozygous knock-in mutation of PKG1 (Prkg1^R177Q/+ ), which causes constitutive, cGMP-independent activation of the kinase. We examined isolated cardiac myocytes and intact mice, the latter after stress induced by surgical transaortic constriction or angiotensin II (Ang II) infusion.

KEY RESULTS

Cardiac myocytes from Prkg1^R177Q/+ mice showed altered phosphorylation of sarcomeric proteins and reduced contractility in response to electrical stimulation, compared to cells from wild type mice. Under basal conditions, young PKG1^R177Q/+ mice exhibited no obvious cardiac abnormalities, but aging animals developed mild increases in cardiac fibrosis. In response to angiotensin II infusion or fixed pressure overload induced by transaortic constriction, young PKG^R177Q/+ mice exhibited excessive hypertrophic remodelling with increased fibrosis and myocyte apoptosis, leading to increased left ventricular dilation and dysfunction compared to wild type litter mates.

CONCLUSION AND IMPLICATIONS

Long-term PKG1 activation in mice may be harmful to the heart, especially in the presence of pressure overload and neurohumoral stress.

LINKED ARTICLES

This article is part of a themed issue on cGMP Signalling in Cell Growth and Survival. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.11/issuetoc.

Calcium Release Channels: Structure and Function of IP3 Receptors and Ryanodine Receptors

Ca²⁺-release channels are giant membrane proteins that control the release of Ca²⁺ from the endoplasmic and sarcoplasmic reticulum. The two members, ryanodine receptors (RyRs) and inositol-1,4,5-trisphosphate Receptors (IP₃Rs), are evolutionarily related and are both activated by cytosolic Ca²⁺. They share a common architecture, but RyRs have evolved additional modules in the cytosolic region. Their massive size allows for the regulation by tens of proteins and small molecules, which can affect the opening and closing of the channels. In addition to Ca²⁺, other major triggers include IP₃ for the IP₃Rs, and depolarization of the plasma membrane for a particular RyR subtype. Their size has made them popular targets for study via electron microscopic methods, with current structures culminating near 3Å. The available structures have provided many new mechanistic insights int the binding of auxiliary proteins and small molecules, how these can regulate channel opening, and the mechanisms of disease-associated mutations. They also help scrutinize previously proposed binding sites, as some of these are now incompatible with the structures. Many questions remain around the structural effects of post-translational modifications, additional binding partners, and the higher-order complexes these channels can make in situ. This review summarizes our current knowledge about the structures of Ca²⁺-release channels and how this informs on their function.

Sorcin: a novel potential target in therapies of cancers

Soluble resistance-related calcium-binding protein (sorcin) is a member of the penta-EF-hand protein family. Sorcin is widely distributed in normal human tissues, such as the brain, heart, lymphocytes, kidneys, breast and skin. Findings suggest that sorcin is associated with the regulation of calcium homeostasis, cell cycle and vesicle trafficking. It has been reported that many types of non-neoplastic diseases such as diabetes, viral infection, infertility, and nervous system diseases were affected by the expression of sorcin. One of the main issues is the role of sorcin in neoplastic diseases. Research proved that sorcin can be found to overexpress in cells of several cancers, particularly in the case of multidrug-resistant cancers. Additionally, the researchers proposed that the expression of sorcin was significantly associated with the foundation of multidrug resistance (MDR). All the findings mentioned above emphasized the importance of studying sorcin. This review mainly includes the following aspects: functions of sorcin, role in non-neoplastic and neoplastic diseases, and research related to drugs. To sum up, sorcin is a potential novel target to be studied to deal with MDR.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

Restoring mitochondrial calcium uniporter expression in diabetic mouse heart improves mitochondrial calcium handling and cardiac function

Diabetes mellitus is a growing health care problem, resulting in significant cardiovascular morbidity and mortality. Diabetes also increases the risk for heart failure (HF) and decreased cardiac myocyte function, which are linked to changes in cardiac mitochondrial energy metabolism. The free mitochondrial calcium level ([Ca²⁺] _m ) is fundamental in activating the mitochondrial respiratory chain complexes and ATP production and is also known to regulate pyruvate dehydrogenase complex (PDC) activity. The mitochondrial calcium uniporter (MCU) complex (MCUC) plays a major role in mediating mitochondrial Ca²⁺ import, and its expression and function therefore have a marked impact on cardiac myocyte metabolism and function. Here, we investigated MCU's role in mitochondrial Ca²⁺ handling, mitochondrial function, glucose oxidation, and cardiac function in the heart of diabetic mice. We found that diabetic mouse hearts exhibit altered expression of MCU and MCUC members and a resulting decrease in [Ca²⁺] _m , mitochondrial Ca²⁺ uptake, mitochondrial energetic function, and cardiac function. Adeno-associated virus-based normalization of MCU levels in these hearts restored mitochondrial Ca²⁺ handling, reduced PDC phosphorylation levels, and increased PDC activity. These changes were associated with cardiac metabolic reprogramming toward normal physiological glucose oxidation. This reprogramming likely contributed to the restoration of both cardiac myocyte and heart function to nondiabetic levels without any observed detrimental effects. These findings support the hypothesis that abnormal mitochondrial Ca²⁺ handling and its negative consequences can be ameliorated in diabetes by restoring MCU levels via adeno-associated virus-based MCU transgene expression.

Sorcin ablation plus β-adrenergic stimulation generate an arrhythmogenic substrate in mouse ventricular myocytes

Sorcin, a penta-EF hand Ca²⁺-binding protein expressed in cardiomyocytes, is known to interact with ryanodine receptors and other Ca²⁺ regulatory proteins. To investigate sorcin's influence on cardiac excitation-contraction coupling and its role in the development of cardiac malfunctions, we generated a sorcin knockout (KO) mouse model. Sorcin KO mice presented ventricular arrhythmia and sudden death when challenged by acute stress induced by isoproterenol plus caffeine. Chronic stress, which was induced by transverse aortic constriction, significantly decreased the survival rate of sorcin KO mice. Under isoproterenol stimulation, spontaneous Ca²⁺ release events were frequently observed in sorcin KO cardiomyocytes. Sorcin KO hearts of adult, but not young mice developed overexpression of L-type Ca²⁺ channel and Na⁺-Ca²⁺ exchanger, which enhanced I_Ca and I_NCX. Consequently, spontaneous Ca²⁺ release events in sorcin KO cardiomyocytes were more likely to induce arrhythmogenic delayed afterdepolarizations. Our study demonstrates sorcin deficiency may trigger cardiac ventricular arrhythmias due to Ca²⁺ disturbances, and evidences the critical role of sorcin in maintaining Ca²⁺ homeostasis, especially during the adrenergic response of the heart.

Expression of the mitochondrial calcium uniporter in cardiac myocytes improves impaired mitochondrial calcium handling and metabolism in simulated hyperglycemia

Diabetic cardiomyopathy is associated with metabolic changes, including decreased glucose oxidation (Gox) and increased fatty acid oxidation (FAox), which result in cardiac energetic deficiency. Diabetic hyperglycemia is a pathophysiological mechanism that triggers multiple maladaptive phenomena. The mitochondrial Ca²⁺ uniporter (MCU) is the channel responsible for Ca²⁺ uptake in mitochondria, and free mitochondrial Ca²⁺ concentration ([Ca²⁺]_m) regulates mitochondrial metabolism. Experiments with cardiac myocytes (CM) exposed to simulated hyperglycemia revealed reduced [Ca²⁺]_m and MCU protein levels. Therefore, we investigated whether returning [Ca²⁺]_m to normal levels in CM by MCU expression could lead to normalization of Gox and FAox with no detrimental effects. Mouse neonatal CM were exposed for 72 h to normal glucose [5.5 mM glucose + 19.5 mM mannitol (NG)], high glucose [25 mM glucose (HG)], or HG + adenoviral MCU expression. Gox and FAox, [Ca²⁺]_m, MCU levels, pyruvate dehydrogenase (PDH) activity, oxidative stress, mitochondrial membrane potential, and apoptosis were assessed. [Ca²⁺]_m and MCU protein levels were reduced after 72 h of HG. Gox was decreased and FAox was increased in HG, PDH activity was decreased, phosphorylated PDH levels were increased, and mitochondrial membrane potential was reduced. MCU expression returned these parameters toward NG levels. Moreover, increased oxidative stress and apoptosis were reduced in HG by MCU expression. We also observed reduced MCU protein levels and [Ca²⁺]_m in hearts from type 1 diabetic mice. Thus we conclude that HG-induced metabolic alterations can be reversed by restoration of MCU levels, resulting in return of [Ca²⁺]_m to normal levels.

Deletion of thioredoxin-interacting protein improves cardiac inotropic reserve in the streptozotocin-induced diabetic heart

Although the precise pathogenesis of diabetic cardiac damage remains unclear, potential mechanisms include increased oxidative stress, autonomic nervous dysfunction, and altered cardiac metabolism. Thioredoxin-interacting protein (Txnip) was initially identified as an inhibitor of the antioxidant thioredoxin but is now recognized as a member of the arrestin superfamily of adaptor proteins that classically regulate G protein-coupled receptor signaling. Here we show that Txnip plays a key role in diabetic cardiomyopathy. High glucose levels induced Txnip expression in rat cardiomyocytes in vitro and in the myocardium of streptozotocin-induced diabetic mice in vivo. While hyperglycemia did not induce cardiac dysfunction at baseline, β-adrenergic challenge revealed a blunted myocardial inotropic response in diabetic animals (24-wk-old male and female C57BL/6;129Sv mice). Interestingly, diabetic mice with cardiomyocyte-specific deletion of Txnip retained a greater cardiac response to β-adrenergic stimulation than wild-type mice. This benefit in Txnip-knockout hearts was not related to the level of thioredoxin activity or oxidative stress. Unlike the β-arrestins, Txnip did not interact with β-adrenergic receptors to desensitize downstream signaling. However, our proteomic and functional analyses demonstrated that Txnip inhibits glucose transport through direct binding to glucose transporter 1 (GLUT1). An ex vivo analysis of perfused hearts further demonstrated that the enhanced functional reserve afforded by deletion of Txnip was associated with myocardial glucose utilization during β-adrenergic stimulation. These data provide novel evidence that hyperglycemia-induced Txnip is responsible for impaired cardiac inotropic reserve by direct regulation of insulin-independent glucose uptake through GLUT1 and plays a role in the development of diabetic cardiomyopathy.

Sorcin Links Pancreatic β-Cell Lipotoxicity to ER Ca2+ Stores

Preserving β-cell function during the development of obesity and insulin resistance would limit the worldwide epidemic of type 2 diabetes. Endoplasmic reticulum (ER) calcium (Ca(2+)) depletion induced by saturated free fatty acids and cytokines causes β-cell ER stress and apoptosis, but the molecular mechanisms behind these phenomena are still poorly understood. Here, we demonstrate that palmitate-induced sorcin downregulation and subsequent increases in glucose-6-phosphatase catalytic subunit-2 (G6PC2) levels contribute to lipotoxicity. Sorcin is a calcium sensor protein involved in maintaining ER Ca(2+) by inhibiting ryanodine receptor activity and playing a role in terminating Ca(2+)-induced Ca(2+) release. G6PC2, a genome-wide association study gene associated with fasting blood glucose, is a negative regulator of glucose-stimulated insulin secretion (GSIS). High-fat feeding in mice and chronic exposure of human islets to palmitate decreases endogenous sorcin expression while levels of G6PC2 mRNA increase. Sorcin-null mice are glucose intolerant, with markedly impaired GSIS and increased expression of G6pc2 Under high-fat diet, mice overexpressing sorcin in the β-cell display improved glucose tolerance, fasting blood glucose, and GSIS, whereas G6PC2 levels are decreased and cytosolic and ER Ca(2+) are increased in transgenic islets. Sorcin may thus provide a target for intervention in type 2 diabetes.

Sorcin, a calcium binding protein involved in the multidrug resistance mechanisms in cancer cells

Sorcin is a penta-EF hand calcium binding protein, which participates in the regulation of calcium homeostasis in cells. Sorcin regulates calcium channels and exchangers located at the plasma membrane and at the endo/sarcoplasmic reticulum (ER/SR), and allows high levels of calcium in the ER to be maintained, preventing ER stress and possibly, the unfolded protein response. Sorcin is highly expressed in the heart and in the brain, and overexpressed in many cancer cells. Sorcin gene is in the same amplicon as other genes involved in the resistance to chemotherapeutics in cancer cells (multi-drug resistance, MDR) such as ABCB4 and ABCB1; its overexpression results in increased drug resistance to a number of chemotherapeutic agents, and inhibition of sorcin expression by sorcin-targeting RNA interference leads to reversal of drug resistance. Sorcin is increasingly considered a useful marker of MDR and may represent a therapeutic target for reversing tumor multidrug resistance.

Calcium signaling in diabetic cardiomyocytes

Diabetes mellitus is one of the most common medical conditions. It is associated to medical complications in numerous organs and tissues, of which the heart is one of the most important and most prevalent organs affected by this disease. In fact, cardiovascular complications are the most common cause of death among diabetic patients. At the end of the 19th century, the weakness of the heart in diabetes was noted as part of the general muscular weakness that exists in that disease. However, it was only in the eighties that diabetic cardiomyopathy was recognized, which comprises structural and functional abnormalities in the myocardium in diabetic patients even in the absence of coronary artery disease or hypertension. This disorder has been associated with both type 1 and type 2 diabetes, and is characterized by early-onset diastolic dysfunction and late-onset systolic dysfunction, in which alteration in Ca(2+) signaling is of major importance, since it controls not only contraction, but also excitability (and therefore is involved in rhythmic disorder), enzymatic activity, and gene transcription. Here we attempt to give a brief overview of Ca(2+) fluxes alteration reported on diabetes, and provide some new data on differential modulation of Ca(2+) handling alteration in males and females type 2 diabetic mice to promote further research. Due to space limitations, we apologize for those authors whose important work is not cited.

Genetic maneuvers to ameliorate ventricular function in heart failure: therapeutic potential and future implications

Gene therapy to treat heart failure has evolved into a growing field of investigation yielding remarkable results in preclinical models. Whether these results will persist in clinical trials remains to be seen. However, researchers still face a number of obstacles that need to be overcome before this treatment can be employed effectively. Efforts are required to identify better vectors with minimal side effects and maximal efficiency and durability. There is also a need to develop less invasive and more effective techniques to deliver these vectors. This review will discuss different methods to achieve these goals, the various pathologic mechanisms that have been targeted so far and those with strong potential for use in the future.

Sorcin modulates mitochondrial Ca(2+) handling and reduces apoptosis in neonatal rat cardiac myocytes

Sorcin localizes in cellular membranes and has been demonstrated to modulate cytosolic Ca(2+) handling in cardiac myocytes. Sorcin also localizes in mitochondria; however, the effect of sorcin on mitochondrial Ca(2+) handling is unknown. Using mitochondrial pericam, we measured mitochondrial Ca(2+) concentration and fluxes in intact neonatal cardiac myocytes overexpressing sorcin. Our results showed that sorcin increases basal and caffeine-stimulated mitochondrial Ca(2+) concentration. This effect was associated with faster Ca(2+) uptake and release. The effect of sorcin was specific for mitochondria, since similar results were obtained with digitonin-permeabilized cells, where cytosolic Ca(2+) flux was disrupted. Furthermore, mitochondria of cardiac myocytes in which sorcin was overexpressed were more Ca(2+)-tolerant. Experiments analyzing apoptotic signaling demonstrated that sorcin prevented 2-deoxyglucose-induced cytochrome c release. Furthermore, sorcin prevented hyperglycemia-induced cytochrome c release and caspase activation. In contrast, antisense sorcin induced caspase-3 activation. Thus, sorcin antiapoptotic properties may be due to modulation of mitochondrial Ca(2+) handling in cardiac myocytes.

Roles of Ca2+ ions in the control of ChREBP nuclear translocation

Carbohydrate-responsive element binding protein (ChREBP (MLXIPL)) is emerging as an important mediator of glucotoxity both in the liver and in the pancreatic β-cells. Although the regulation of its nuclear translocation and transcriptional activation by glucose has been the subject of intensive research, it is still not fully understood. We have recently uncovered a novel mechanism in the excitable pancreatic β-cell where ChREBP interacts with sorcin, a penta-EF-hand Ca(2)(+)-binding protein, and is sequestered in the cytosol at low glucose concentrations. Upon stimulation with glucose and activation of Ca(2)(+) influx, or application of ATP as an intracellular Ca(2)(+)-mobilising agent, ChREBP rapidly translocates to the nucleus. In sorcin-silenced cells, ChREBP is constitutively present in the nucleus, and both glucose and Ca(2)(+) are ineffective in stimulating further ChREBP nuclear shuttling. Whether an active Ca(2)(+)-sorcin element of ChREBP activation also exists in non-excitable cells is discussed.

Glucose-induced nuclear shuttling of ChREBP is mediated by sorcin and Ca(2+) ions in pancreatic β-cells

Carbohydrate-responsive element-binding protein (ChREBP) is a regulator of pancreatic β-cell gene expression and an important mediator of glucotoxicity. Glucose increases the activity and nuclear localization of ChREBP by still ill-defined mechanisms. Here we reveal, using both MIN6 and primary mouse β-cells, a unique mechanism behind ChREBP nuclear translocation. At low glucose concentrations, ChREBP interacts with sorcin, a penta EF hand Ca(2+) binding protein, and is sequestered in the cytosol. Sorcin overexpression inhibits ChREBP nuclear accumulation at high glucose and reduced the activity of L-type pyruvate kinase (L-PK) and TxNIP promoters, two well-characterized ChREBP target genes. Sorcin inactivation by RNA interference increases ChREBP nuclear localization and in vivo binding to the L-PK promoter at low glucose concentrations. Ca(2+) influx was essential for this process since Ca(2+) chelation with EGTA, or pharmacological inhibition with diazoxide and nifedipine, blocked the effects of glucose. Conversely, mobilization of intracellular Ca(2+) with ATP caused the nuclear accumulation of ChREBP. Finally, sorcin silencing inhibited ATP-induced increases in intracellular Ca(2+) and glucose-stimulated insulin secretion. We therefore conclude that sorcin retains ChREBP in the cytosol at low glucose concentrations and may act as a Ca(2+) sensor for glucose-induced nuclear translocation and the activation of ChREBP-dependent genes.

Inhibition of nitric oxide synthase uncoupling by sepiapterin improves left ventricular function in streptozotocin-induced diabetic mice

1. Uncoupling of nitric oxide synthase (NOS) has been implicated in the pathogenesis of left ventricular (LV) dysfunction in diabetes mellitus. In the present study, we investigated the role of NOS uncoupling in oxidative/nitrosative stress and LV dysfunction in the diabetic mouse heart. 2. Diabetes was induced in wild-type (WT), endothelial (e) NOS knockout (eNOS(-/-)), inducible (i) NOS knockout (iNOS(-/-)) and neuronal (n) NOS knockout (nNOS(-/-)) mice by streptozotocin (STZ) treatment. 3. In the diabetic heart, iNOS, but not eNOS or nNOS, expression was increased. Levels of malondialdehyde (MDA), 4-hydroxy-noneal (HNE) and nitrotyrosine (NT), as markers of oxidative/nitrosative stress, were increased in the diabetic mouse heart, but the increase in oxidative/nitrosative stress was significantly repressed in the iNOS(-/-) diabetic mouse heart. Levels of nitrite and nitrate (NO(x)), as an index of nitric oxide, bioavailability were significantly decreased in the iNOS(-/-) diabetic mouse heart. 4. Oral administration of sepiapterin (10 mg/kg per day), a precursor of tetrahydrobiopterin (BH(4)), significantly increased BH(4) and the BH(4)/BH(2) ratio in diabetic mouse heart. Similarly, sepiapterin inhibited the formation of HNE, MDA and NT in diabetic hearts from all three genotypes, but the increase in NO(x) following sepiapterin treatment was significantly attenuated in the iNOS(-/-) diabetic mouse heart. Percentage fractional shortening (FS), evaluated by echocardiography, decreased significantly in all genotypes of diabetic mice. Sepiapterin significantly increased percentage FS in diabetic mice, except in iNOS(-/-) mice. 5. These results suggest that sepiapterin inhibits uncoupling of NOS and improves LV function presumably by increasing iNOS-derived nitric oxide in the diabetic heart.

Molecular cardiology in translation: gene, cell and chemical-based experimental therapeutics for the failing heart

Acquired and inherited diseases of the heart represent a major health care issue in this country and throughout the World. Clinical medicine has made important advancements in the past quarter century to enable several effective treatment regimes for cardiac patients. Nevertheless, it is apparent that even with the best care, current treatment strategies and therapeutics are inadequate for treating heart disease, leaving it arguably the most pressing health issue today. In this context it is important to seek new approaches to redress the functional deficits in failing myocardium. This review focuses on several recent gene, cell and chemical-based experimental therapeutics currently being developed in the laboratory for potential translation to patient care. For example, new advances in bio-sensing inducible gene expression systems offer the potential for designer cardio-protective proteins to be expressed only during hypoxia/ischemia in the heart. Stem cells continue to offer the promise of cardiac repair, and some recent advances are discussed here. In addition, discovery and applications of synthetic polymers are presented as a chemical-based strategy for acute and chronic treatment of diseased and failing cardiac tissue. Collectively, these approaches serve as the front lines in basic biomedical research, with an eye toward translation of these findings to clinically meaningful applications in cardiac disease.

Ca(2+)-binding proteins in dogs with heart failure: effects of cardiac contractility modulation electrical signals

BACKGROUND

In dogs with heart failure (HF), chronic therapy with cardiac contractility modulation (CCM) electrical signals delivered to left ventricular (LV) muscle during the absolute refractory period improves LV function. This study examined the effects of CCM therapy on the expression of calcium (Ca(2+))-binding proteins (CBPs) in dogs with HF.

METHODS AND RESULTS

Studies were performed in LV tissue from seven CCM-treated HF dogs, seven untreated HF dogs, and six normal (NL) dogs. mRNA expression of S100A1, sorcin, presenillin-1 (PS1), PS2, histidine-rich Ca(2+)-binding protein (HRC), and 18S ribosomal RNA (18S), a housekeeping gene, was measured using RT-PCR. Protein levels of CBPs and calsequestrin (CSQ) were determined by Western blotting. No difference was observed in the expression of 18S and CSQ among study groups. Compared with NL, the expression of S100A1, sorcin, and HRC was decreased, whereas the expression of PS2 was increased in untreated HF dogs. CCM therapy normalized the expression of S100A1, sorcin, and PS2 but not of HRC. No change was seen in the expression of PS1 among study groups.

CONCLUSION

CCM therapy restores LV expression of S100A1, PS2, and sorcin. Normalization of CBPs may partly contribute to improved LV function in HF following CCM therapy.

Feasibility of in vivo pressure measurement using a pressure-tip catheter via transventricular puncture

Pressure-tip catheters (PTCs) are used to evaluate ventricular mechanics during surgical repair of congenital heart disease in children. Studies in infants require miniaturized sensors. We compared the safety and accuracy of a 2-Fr ultraminiature PTC with a 5-Fr PTC. In 10 piglets (weight 19-22 kg), a 5-Fr PTC was inserted through a 3-mm apical puncture with a #11 blade. A 20-gauge angiocatheter was inserted using a separate site. A 2-Fr PTC was threaded through the angiocatheter lumen. The angiocatheter was withdrawn, leaving the 2-Fr PTC within the left ventricle (LV). Left ventricular pressure (LVP) changes were measured during three inferior vena caval occlusions. Reliability coefficients demonstrated correlation between the 2-Fr PTC and 5-Fr PTC for LV end-diastolic pressure (0.90-0.95), peak LVP (0.92-0.99), and the maximal (0.87-0.93) and minimal (0.89-0.94) first derivatives of LVP. Bland-Altman analysis demonstrated agreement for all variables. Blood loss was trivial with pressure manipulation and catheter placement and removal. Pressure measurements using the 2-Fr PTC were accurate and comparable with those from the 5-Fr PTC. Transventricular placement of a 2-Fr PTC is feasible and should allow evaluation of ventricular mechanics during surgical repair of congenital heart disease.

Diabetes-related defects in sarcoplasmic Ca2+ release are prevented by inactivation of G(alpha)11 and G(alpha)q in murine cardiomyocytes

Neurohumoral stimulation of Gq-coupled receptors has been proposed as a central mechanism in the pathogenesis of diabetic heart disease. The resulting contractile dysfunction is closely related to abnormal intracellular Ca(2+) handling with functional defects of the sarcoplasmic reticulum (SR). The present study was therefore designed to determine the role of G(q)-protein signaling via G(alpha)(11) and G(alpha)(q) in diabetes for the induction of functional and structural changes in the Ca(2+) release complex of the SR. An experimental type 1-diabetes was induced in wild type, G(alpha)(11) knockout, and G(alpha)(11/q)-knockout mice by injection of streptozotocin. Cardiac morphology and function was assessed in vivo by echocardiography. SR Ca(2+) leak was tested in vitro based on a (45)Ca(2+) assay and protein densities as well as gene expression of ryanodine receptor (RyR2), FKBP12.6, sorcin, and annexin A7 were analyzed by immunoblot and RT-PCR. In wild type animals 8 weeks of diabetes resulted in cardiac hypertrophy and SR Ca(2+) leak was increased. In addition, diabetic wild type animals showed reduced protein levels of FKBP12.6 and annexin A7. In G(alpha)(11)- and G(alpha)(11/q)-knockout animals, however, SR Ca(2+) release and cardiac phenotype remained unchanged upon induction of diabetes. Densities of the proteins that we presently analyzed were also unaltered in G(alpha)(11)-knockout mice. G(alpha)(11/q)-knockout animals even showed increased expression of sorcin and annexin A7. Thus, based on the present study we suggest a signaling pathway via the G(q)-proteins, G(alpha)(11) and G(alpha)(q), that could link increased neurohumoral stimulation in diabetes with defective RyR2 channel function by regulating protein expression of FKBP12.6, annexin A7, and sorcin.

Activation of the cardiac Na(+)-Ca(2+) exchanger by sorcin via the interaction of the respective Ca(2+)-binding domains

Sorcin is a penta-EF-hand protein that interacts with intracellular target proteins after Ca²⁺ binding. The sarcolemmal Na⁺/Ca²⁺ exchanger (NCX1) may be an important sorcin target in cardiac muscle. In this study, RNAi knockdown of sorcin, purified sorcin or sorcin variants was employed in parallel measurements of: (i) NCX activity in isolated rabbit cardiomyocytes using electrophysiological techniques and (ii) sorcin binding to the NCX1 calcium binding domains (CBD1 and (iii) using surface plasmon resonance and gel overlay techniques. Sorcin is activated by Ca²⁺ binding to the EF3 and EF2 regions, which are connected by the D helix. To investigate the importance of this region in the interaction with NCX1, three variants were examined: W105G and W99G, mutated respectively near EF3 and EF2, and E124A that does not bind Ca²⁺ due to a mutation at EF3. Downregulation of sorcin decreased and supplementation with wt sorcin (3 μM) increased NCX activity in isolated cardiomyocytes. The relative stimulatory effects of the sorcin variants were: W105G > wt sorcin > Sorcin Calcium Binding Domain (SCBD) > W99G > E124A. Sorcin binding to both CBD1 and 2 was observed. In the presence of 50 µM Ca²⁺, the interaction with CBD1 followed the order W105G > SCBD > wt sorcin > W99G > E124A. In sorcin, the interacting surface can be mapped on the C-terminal Ca²⁺-binding domain in the D helix region comprising W99. The fast association/dissociation rates that characterize the interaction of sorcin with CBD1 and 2 may permit complex formation/dissociation during an excitation/contraction cycle.

Rescue of familial cardiomyopathies by modifications at the level of sarcomere and Ca2+ fluxes

Cardiomyopathies are a heterogeneous group of diseases of the myocardium associated with mechanical and/or electrical dysfunction that frequently show inappropriate ventricular hypertrophy or dilation. Current data suggest that numerous mutations in several genes can cause cardiomyopathies, and the severity of their phenotypes is also influenced by modifier genes. Two major types of inherited cardiomyopathies include familial hypertrophic cardiomyopathy (FHC) and dilated cardiomyopathy (DCM). FHC typically involves increased myofilament Ca(2+) sensitivity associated with diastolic dysfunction, whereas DCM often results in decreased myofilament Ca(2+) sensitivity and systolic dysfunction. Besides alterations in myofilament Ca(2+) sensitivity, alterations in the levels of Ca(2+)-handling proteins have also been described in both diseases. Recent work in animal models has attempted to rescue FHC and DCM via modifications at the myofilament level, altering Ca(2+) homeostasis by targeting Ca(2+)-handling proteins, such as the sarcoplasmic reticulum ATPase and phospholamban, or by interfering with the products of different modifiers genes. Although attempts to rescue cardiomyopathies in animal models have shown great promise, further studies are needed to validate these strategies in order to provide more effective and specific treatments.

Rodent models of diabetic cardiomyopathy

Diabetic cardiomyopathy increases the risk of heart failure in individuals with diabetes, independently of co-existing coronary artery disease and hypertension. The underlying mechanisms for this cardiac complication are incompletely understood. Research on rodent models of type 1 and type 2 diabetes, and the use of genetic engineering techniques in mice, have greatly advanced our understanding of the molecular mechanisms responsible for human diabetic cardiomyopathy. The adaptation of experimental techniques for the investigation of cardiac physiology in mice now allows comprehensive characterization of these models. The focus of the present review will be to discuss selected rodent models that have proven to be useful in studying the underlying mechanisms of human diabetic cardiomyopathy, and to provide an overview of the characteristics of these models for the growing number of investigators who seek to understand the pathology of diabetes-related heart disease.

Alterations in mitochondrial function and cytosolic calcium induced by hyperglycemia are restored by mitochondrial transcription factor A in cardiomyocytes

Mitochondrial transcription factor A (TFAM) is essential for mitochondrial DNA transcription and replication. TFAM transcriptional activity is decreased in diabetic cardiomyopathy; however, the functional implications are unknown. We hypothesized that a reduced TFAM activity may be responsible for some of the alterations caused by hyperglycemia. Therefore, we investigated the effect of TFAM overexpression on hyperglycemia-induced cytosolic calcium handling and mitochondrial abnormalities. Neonatal rat cardiomyocytes were exposed to high glucose (30 mM) for 48 h, and we examined whether TFAM overexpression, by protecting mitochondrial DNA, could reestablish calcium fluxes and mitochondrial alterations toward normal. Our results shown that TFAM overexpression increased to more than twofold mitochondria copy number in cells treated either with normal (5.5 mM) or high glucose. ATP content was reduced by 30% and mitochondrial calcium decreased by 40% after high glucose. TFAM overexpression returned these parameters to even higher than control values. Calcium transients were prolonged by 70% after high glucose, which was associated with diminished sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a and cytochrome-c oxidase subunit 1 expression. These parameters were returned to control values after TFAM overexpression. High glucose-induced protein oxidation was reduced by TFAM overexpression, indicating a reduction of the high glucose-induced oxidative stress. In addition, we found that TFAM activity can be modulated by O-linked beta-N-acetylglucosamine glycosylation. In conclusion, TFAM overexpression protected cell function against the damage induced by high glucose in cardiomyocytes.

Conditional increase in SERCA2a protein is able to reverse contractile dysfunction and abnormal calcium flux in established diabetic cardiomyopathy

Diabetic cardiomyopathy is characterized by reduced cardiac contractility independent of vascular disease. A contributor to contractile dysfunction in the diabetic heart is impaired sarcoplasmic reticulum function with reduced sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) pump activity, leading to disturbed intracellular calcium handling. It is currently unclear whether increasing SERCA2a activity in hearts with existing diabetic cardiomyopathy could still improve calcium flux and contractile performance. To test this hypothesis, we generated a cardiac-specific tetracycline-inducible double transgenic mouse, which allows for doxycycline (DOX)-based inducible SERCA2a expression in which DOX exposure turns on SERCA2a expression. Isolated cardiomyocytes and Langendorff perfused hearts from streptozotocin-induced diabetic mice were studied. Our results show that total SERCA2a protein levels were decreased in the diabetic mice by 60% compared with control. SERCA2a increased above control values in the diabetic mice after DOX. Dysfunctional contractility in the diabetic cardiomyocyte was restored to normal by induction of SERCA2a expression. Calcium transients from diabetic cardiomyocytes showed a delayed rate of diastolic calcium decay of 66%, which was reverted toward normal after SERCA2a expression induced by DOX. Global cardiac function assessed in the diabetic perfused heart showed diminished left ventricular pressure, rate of contraction, and relaxation. These parameters were returned to control values by SERCA2a expression. In conclusion, we have used mice allowing for inducible expression of SERCA2a and could demonstrate that increased expression of SERCA2a leads to improved cardiac function in mice with an already established diabetic cardiomyopathy in absence of detrimental effects.

Parvalbumin isoforms for enhancing cardiac diastolic function

Diastolic heart failure (DHF), characterized by depressed myocardial relaxation performance and poor ventricular filling, is a distinct form of heart failure accounting for nearly half of the heart failure patients with otherwise normal systolic performance. Defective intracellular calcium (Ca2+) cycling is an important mechanism underlying impaired relaxation in DHF. Recently, genetic manipulation of Ca2+ handling proteins in cardiac myocytes has been explored for its potential therapeutic application in DHF. Specifically, ectopic expression of the skeletal muscle Ca2+ binding protein parvalbumin (Parv) has been shown to accelerate myocardial relaxation in vitro and in vivo. Parv acts as a unique "delayed" Ca2+ buffer during diastole by promoting Ca2+ transient decay and sequestration and corrects diastolic dysfunction in an energy-independent manner. This brief review summarizes the rationale and development of Parv gene transfer approaches for DHF, and in particular, discusses the divergent effects of Parv isoforms on cardiac myocyte Ca2+ handling and contractile function with the long-range goal of alleviating diastolic dysfunction in DHF.

Diabetic Cardiomyopathy in OVE26 Mice Shows Mitochondrial ROS Production and Divergence Between In Vivo and In Vitro Contractility

Many diabetic patients suffer from a cardiomyopathy that cannot be explained solely by poor coronary perfusion. This cardiomyopathy may be due to either organ-based damage like fibrosis, or to direct damage to cardiomyocytes. Mitochondrial-derived reactive oxygen species (ROS) have been proposed to contribute to this cardiomyopathy. To address these questions, we used the OVE26 mouse model of severe type 1 diabetes to measure contractility in isolated cardiomyocytes by edge detection and in vivo with echocardiography. We also assessed the source of ROS generation using both a general and a mitochondrial specific indicator. When contractility was assayed in freshly isolated myocytes, contraction was much stronger in control myocytes. However, contractility of normal myocytes became weaker during 24 hours of in vitro culture. In contrast, contractility of diabetic OVE26 myocytes remains stable during culture. Echocardiography revealed normal or hyperdynamic function in OVE26 hearts under basal conditions but with a sharply reduced response to isoproterenol, a beta-adrenergic agonist. For ROS generation, we found that ROS production in diabetic myocytes was elevated after exposure to either high glucose or angiotensin II (AngII). Superoxide detection with the mitochondrial sensor MitoSOX Red confirmed that mitochondria are a major source of ROS generation in diabetic myocytes. These results show that contractile deficits in OVE26 diabetic hearts are due primarily to cardiomyocyte impairment and that ROS from mitochondria are a cause of that impairment.

Adeno-associated virus-mediated expression of thyroid hormone receptor isoforms-alpha1 and -beta1 improves contractile function in pressure overload-induced cardiac hypertrophy

Pressure overload-induced cardiac hypertrophy leads to decreased contractile performance, frequently progressing to heart failure. Cardiac hypertrophy and heart failure can be accompanied by the so-called sick thyroid syndrome, resulting in decreased serum T(3) levels along with decreased expression of thyroid hormone receptors (TRalpha1 and TRbeta1) and sarco(endo)plasmic reticulum Ca-ATPase (SERCA). Because the binding of T(3) occupied receptors to the thyroid response elements in the SERCA promotor can increase gene expression, we wanted to determine whether increasing TR expression in the hypertrophied heart could also improve SERCA expression and cardiac function. Mice subjected to aortic constriction to generate pressure overload-induced hypertrophy were also subjected to gene therapy using adeno-associated virus (AAV) expressing either TRalpha1 or TRbeta1, with LacZ expressing AAV serving as control. After 8 wk of aortic constriction, a similar degree of hypertrophy was observed in all three groups; however, mice treated with TRalpha1 or TRbeta1 showed improved contractile function. Administration of a physiological dose of T(3) increased serum T(3) levels only into the lower range of normal. This T(3) dose, with or without AAV TR treatment, did not result in any significant increase in contractile performance. Calcium transients measured in isolated myocytes also exhibited an enhanced rate of decay associated with TRalpha1 or TRbeta1 treatment. Western blot analysis showed increased SERCA expression in the TRalpha1- or TRbeta1-treated groups relative to the LacZ-treated control group. These results demonstrate that increasing TR expression in the hypertrophied heart is associated with an improvement in contractile function and increased SERCA expression.

Early myocardial dysfunction in streptozotocin-induced diabetic mice: a study using in vivo magnetic resonance imaging (MRI)

Background

Diabetes is associated with a cardiomyopathy that is independent of coronary artery disease or hypertension. In the present study we used in vivo magnetic resonance imaging (MRI) and echocardiographic techniques to examine and characterize early changes in myocardial function in a mouse model of type 1 diabetes.

Methods

Diabetes was induced in 8-week old C57BL/6 mice with two intraperitoneal injections of streptozotocin. The blood glucose levels were maintained at 19–25 mmol/l using intermittent low dosages of long acting insulin glargine. MRI and echocardiography were performed at 4 weeks of diabetes (age of 12 weeks) in diabetic mice and age-matched controls.

Results

After 4 weeks of hyperglycemia one marker of mitochondrial function, NADH oxidase activity, was decreased to 50% of control animals. MRI studies of diabetic mice at 4 weeks demonstrated significant deficits in myocardial morphology and functionality including: a decreased left ventricular (LV) wall thickness, an increased LV end-systolic diameter and volume, a diminished LV ejection fraction and cardiac output, a decreased LV circumferential shortening, and decreased LV peak ejection and filling rates. M-mode echocardiographic and Doppler flow studies of diabetic mice at 4 weeks showed a decreased wall thickening and increased E/A ratio, supporting both systolic and diastolic dysfunction.

Conclusion

Our study demonstrates that MRI interrogation can identify the onset of diabetic cardiomyopathy in mice with its impaired functional capacity and altered morphology. The MRI technique will lend itself to repetitive study of early changes in cardiac function in small animal models of diabetic cardiomyopathy.

A novel endothelin receptor antagonist CPU0213 improves diabetic cardiac insufficiency attributed to up-regulation of the expression of FKBP12.6, SERCA2a, and PLB in rats

The depressed sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a) and Ca2+-release channels (ryanodine receptor RyR2) are involved in the diabetic cardiomyopathy. However, an implication of a down-regulation of FK506-binding protein or calstabin-2 (FKBP12.6) is undefined. It was hypothesized that the down-regulation of FKBP12.6 and SERCA2a of the intracellular calcium handling system is closely related to an up-regulated endothelin (ET) system. An ET receptor antagonist CPU0213 is newly discovered and expected to ameliorate cardiac insufficiency which is mediated by the depressed FKBP12.6 and SERCA2a in diabetic rat heart. Diabetes was developed in male Sprague-Dawley rats 8 weeks after an injection of streptozotocin (60 mg/kg IP), and CPU0213 was instituted 30 mg/kg, SC in the last 4 weeks. The assessment of the cardiac function, cardiac calcium handling proteins, endothelin system, and redox enzyme system were conducted. The compromised cardiac function in diabetic rats was accompanied by a significant down-regulation of expression of FKBP12.6 as well as SERCA2a and phospholamban. These were closely linked with an increased ET-1 and up-regulation of endothelin converting enzyme, PropreET1, and inducible nitric oxide synthase mRNA in diabetic cardiomyopathy. After 4-week treatment, CPU0213 was capable to attenuate completely the down-regulated FKBP12.6 and SERCA2a, and up-regulated ET system in association with a recovery of the cardiac insufficiency of diabetic cardiomyopathy.

Sorcin modulation of Ca2+ sparks in rat vascular smooth muscle cells

Spontaneous, local Ca(2+) release events or Ca(2+) sparks by ryanodine receptors (RyRs) are important determinants of vascular tone and arteriolar resistance, but the mechanisms that modulate their properties in smooth muscle are poorly understood. Sorcin, a Ca(2+)-binding protein that associates with cardiac RyRs and quickly stops Ca(2+) release in the heart, provides a potential mechanism to modulate Ca(2+) sparks in vascular smooth muscle, but little is known about the functional role of sorcin in this tissue. In this work, we characterized the expression and intracellular location of sorcin in aorta and cerebral artery and gained mechanistic insights into its functional role as a modulator of Ca(2+) sparks. Sorcin is present in endothelial and smooth muscle cells, as assessed by immunocytochemical and Western blot analyses. Smooth muscle sorcin translocates from cytosolic to membranous compartments in a Ca(2+)-dependent manner and associates with RyRs, as shown by coimmunoprecipitation and immunostaining experiments. Ca(2+) sparks recorded in saponin-permeabilized vascular myocytes have increased frequency, duration and spatial spread but reduced amplitude with respect to Ca(2+) sparks in intact cells, suggesting that permeabilization disrupts the normal organization of RyRs and releases diffusible substances that control Ca(2+) spark properties. Perfusion of 2 mum sorcin onto permeabilized myocytes reduced the amplitude, duration and spatial spread of Ca(2+) sparks, demonstrating that sorcin effectively regulates Ca(2+) signalling in vascular smooth muscle. Together with a dense distribution in the perimeter of the cell along a pool of RyRs, these properties make sorcin a viable candidate to modulate vascular tone in smooth muscle.

A potyvirus-based gene vector allows producing active human S-COMT and animal GFP, but not human sorcin, in vector-infected plants

Potato virus A (PVA), a potyvirus with a (+)ssRNA genome translated to a large polyprotein, was engineered and used as a gene vector for expression of heterologous proteins in plants. Foreign genes including jellyfish GFP (Aequorea victoria) encoding the green fluorescent protein (GFP, 27 kDa) and the genes of human origin (Homo sapiens) encoding a soluble resistance-related calcium-binding protein (sorcin, 22 kDa) and the catechol-O-methyltransferase (S-COMT; 25 kDa) were cloned between the cistrons for the viral replicase and coat protein (CP). The inserts caused no adverse effects on viral infectivity and virulence, and the inserted sequences remained intact in progeny viruses in the systemically infected leaves. The heterologous proteins were released from the viral polyprotein following cleavage by the main viral proteinase, NIa, at engineered proteolytic processing sites flanking the insert. Active GFP, as indicated by green fluorescence, and S-COMT with high levels of enzymatic activity were produced. In contrast, no sorcin was detected despite the expected equimolar amounts of the foreign and viral proteins being expressed as a polyprotein. These data reveal inherent differences between heterologous proteins in their suitability for production in plants.

Regulation of the sarcoplasmic reticulum Ca2+-ATPase expression in the hypertrophic and failing heartThis paper is part of a series in the Journal's “Made in Canada” section. The paper has undergone peer review

The sarcoplasmic reticulum (SR) plays a central role in the contraction and relaxation coupling in the myocardium. The SR Ca2+-ATPase (SERCA2) transports Ca2+ inside the SR lumen during relaxation of the cardiac myocyte. It is well known that diminished contractility of the hypertrophic cardiac myocyte is the main factor of ventricular dysfunction in the failing heart. A key feature of the failing heart is a decreased content and activity of SERCA2, which is the cause of some of the physiological defects observed in the hypertrophic cardiomyocyte performance that are important during transition of compensated hypertrophy to heart failure. In this review different possible mechanisms responsible for decreased transcriptional regulation of the SERCA2 gene are examined, which appear to be the primary cause for decreased SERCA2 expression in heart failure. The experimental evidence suggests that several signalling pathways are involved in the downregulation of SERCA2 expression in the hypertrophic and failing cardiomyocyte. Therapeutic upregulation of SERCA2 expression using replication deficient adenoviral expression vectors, pharmacological interventions using thyroid hormone analogues, β-adrenergic receptor antagonists, and novel metabolically active compounds are currently under investigation for the treatment of uncompensated cardiac hypertrophy and heart failure.

The antidiabetic agent rosiglitazone upregulates SERCA2 and enhances TNF-alpha- and LPS-induced NF-kappaB-dependent transcription and TNF-alpha-induced IL-6 secretion in ventricular myocytes

Positive hemodynamic effects of the antidiabetic agent rosiglitazone on perfused whole hearts have recently been described, but the mechanisms regulating these effects are not well understood. This study reports the effects of rosiglitazone on calcium regulation in isolated neonatal rat ventricular myocytes by measurement of Ca2+ transient decay rates and SERCA2 gene expression, and shows that rosiglitazone enhances known cardioprotective signaling pathways. Myocyte treatment with 10 micromol/L rosiglitazone accelerated Ca2+ transient decay rates by approximately 30%, enhanced SERCA2 mRNA levels by approximately 1.5-fold and SERCA2 production by approximately 3-fold. Rosiglitazone treatment (1, 5, and 10 micromol/L) also led to a dose-dependent increase (approximately 1.2-1.5-fold) in SERCA2 promoter activity. Comparable levels of cardiac SERCA overexpression have been associated with physiologically relevant and compensatory effects in vivo. These data link thiazolidinedione-induced improvement in cardiac myocyte function to an upregulation of SERCA2 gene expression. Since NF-kappaB-dependent pathways, including the upregulation of IL-6 secretion, were shown to protect neonatal rat ventricular myocytes from apoptosis upon TNFalpha stimulation, additional experiments were designed to determine whether rosiglitazone enhances TNFalpha-induced NF-kappaB-dependent transcription and IL-6 secretion. Because the endotoxin stress response in ventricular myocytes involves the upregulation of TNFalpha, and the activation of NF-kappaB, the effects of rosiglitazone on lipopolysaccharide-induced NF-kappaB-dependent transcription were also investigated. Treatment of neonatal rat ventricular myocytes with 10 micromol/L rosiglitazone enhanced TNF-alpha- and lipopolysaccharide-induced NF-kappaB-dependent transcription by approximately 1.8- and approximately 1.4-fold respectively, and TNF-alpha-induced IL-6 secretion by n1.5-fold. Rosiglitazone had no significant effects on basal levels of NF-kappaB-dependent transcription and IL-6 secretion. Thus, cardioprotective effects of rosiglitazone may be partly mediated by NF-kappaB.

Adenovirus-mediated overexpression of O-GlcNAcase improves contractile function in the diabetic heart

To examine whether excessive protein O-GlcNAcylation plays a role in the dysfunction of the diabetic heart, we delivered adenovirus expressing O-GlcNAcase (Adv-GCA) into the myocardium of STZ-induced diabetic mice. Our results indicated that excessive cellular O-GlcNAcylation exists in the diabetic heart, and that in vivo GCA overexpression reduces overall cellular O-GlcNAcylation. Myocytes isolated from diabetic hearts receiving Adv-GCA exhibited improved calcium transients with a significantly shortened T(decay) (P<0.01) and increased sarcoplasmic reticulum Ca2+ load (P<0.01). These myocytes also demonstrated improved contractility including a significant increase in +dL/dt and -dL/dt and greater fractional shortening as measured by edge detection (P<0.01). In isolated perfused hearts, developed pressure and -dP/dt were significantly improved in diabetic hearts receiving Adv-GCA (P<0.05). These hearts also exhibited a 40% increase in SERCA2a expression. Phospholamban protein expression was reduced 50%, but the phosphorylated form was increased 2-fold in the diabetic hearts receiving Adv-GCA. We conclude that excess O-GlcNAcylation in the diabetic heart contributes to cardiac dysfunction, and reducing this excess cellular O-GlcNAcylation has beneficial effects on calcium handling and diabetic cardiac function.

Sorcin interacts with sarcoplasmic reticulum Ca(2+)-ATPase and modulates excitation-contraction coupling in the heart

Sorcin is a 21.6-kDa Ca(2+) binding protein of the penta-EF hand family. Several studies have shown that sorcin modulates multiple proteins involved in excitation-contraction (E-C) coupling in the heart, such as the cardiac ryanodine receptor (RyR2), L-type Ca(2+) channel, and Na(+)-Ca(2+) exchanger, while it has also been shown to be phosphorylated by cAMP-dependent protein kinase (PKA). To elucidate the effects of sorcin and its PKA-dependent regulation on E-C coupling in the heart, we identified the PKA-phosphorylation site of sorcin, and found that serine178 was preferentially phosphorylated by PKA and dephosphorylated by protein phosphatase-1. Isoproterenol allowed sorcin to translocate to the sarcoplasmic reticulum (SR). In addition, adenovirus-mediated overexpression of sorcin in adult rat cardiomyocytes significantly increased both the rate of decay of the Ca(2+) transient and the SR Ca(2+) load. An assay of oxalate-facilitated Ca(2+) uptake showed that recombinant sorcin increased Ca(2+) uptake in a dose-dependent manner. These data suggest that sorcin activates the Ca(2+)-uptake function in the SR. In UM-X7. 1 cardiomyopathic hamster hearts, the relative amount of sorcin was significantly increased in the SR fraction, whereas it was significantly decreased in whole-heart homogenates. In failing hearts, PKA-phosphorylated sorcin was markedly increased, as assessed using a back-phosphorylation assay with immunoprecipitated sorcin. Our results suggest that sorcin activates Ca(2+)-ATPase-mediated Ca(2+) uptake and restores SR Ca(2+) content, and may play critical roles in compensatory mechanisms in both Ca(2+) homeostasis and cardiac dysfunction in failing hearts.

Increased leakage of sarcoplasmic reticulum Ca2+ contributes to abnormal myocyte Ca2+ handling and shortening in sepsis*

OBJECTIVE

Changes in cardiac function due to sepsis have been widely reported. However, the underlying mechanisms remain poorly understood. In the mammalian heart, myocyte function and intracellular calcium homeostasis are closely coupled. In this study we tested the hypothesis that alterations in cardiac calcium homeostasis due to sepsis underlie the observed myocyte dysfunction.

DESIGN

Randomized prospective animal study.

SETTING

Research laboratory.

SUBJECTS

Male Sprague-Dawley rats weighing 250-275 g.

INTERVENTIONS

We induced sepsis by cecal ligation and puncture in the rat, which mimics the type of infection caused by perforation of the intestine in humans.

MEASUREMENTS AND RESULTS

Forty-eight hours after cecal ligation and puncture, isolated cardiac ventricular cardiomyocytes demonstrated a 57% decreased peak systolic [Ca]. The time constant of the Ca transient increased 71% and 57% in myocytes obtained 24 hrs and 48 hrs after cecal ligation and puncture, respectively. The average shortening of cardiomyocytes 48 hrs after cecal ligation and puncture was significantly decreased. To investigate the cellular mechanisms of altered Ca transients and myocyte shortening, we measured Ca sparks, the spontaneous local Ca release events in cardiomyocytes at resting states. The Ca spark frequency progressively increased in myocytes 24 hrs and 48 hrs after cecal ligation and puncture. The total activity of sparks also increased compared with sham-operated animals. The overall leakage of sarcoplasmic reticulum Ca in resting states was increased in sepsis and resulted in reduced sarcoplasmic reticulum Ca content.

CONCLUSIONS

Abnormal Ca leakage from the sarcoplasmic reticulum contributes significantly to the depressed myocyte shortening in sepsis. In the future, modalities that prevent this Ca leakage may prove beneficial in the treatment of sepsis-induced myocyte shortening.

Gene therapy targeted at calcium handling as an approach to the treatment of heart failure

Chronic congestive heart failure primarily of ischemic origin remains a leading cause of morbidity and mortality in the United States and other leading countries. The current main stream of therapy is, however, palliative and uses a complex regimen of drugs, the actions of which are not understood completely. On the other hand, unfavorable remodeling after cardiac injuries of multiple causes has been thought to lead to cardiac contractile dysfunction in heart failure, and a body of scientific evidence points to a central role of intrinsic defects in intracellular calcium handling in cardiomyocytes that arise from the distorted functions of several key regulatory molecules on plasma membrane or sarcoplasmic reticulum (SR), a muscle-specific intracellular membrane complex that stores calcium at high concentration. Accordingly, the initial appetite to use gene transfer strategies to modulate calcium regulatory proteins was to validate molecular targets for the development of new pharmaceuticals; however, remarkable therapeutic efficacies found in an initial series of studies using various heart failure animal models immediately promoted us to seek ways to directly apply gene transfer to cure clinical heart failure. The first part of this article reviews our up-to-date knowledge of various functional components to regulate calcium handling in cardiomyocytes, including beta-adrenergic receptor, L-type calcium channel, ryanodine receptor (RyR) and its associated proteins, sarco-endoplasmic reticulum calcium ATPase (SERCA), and phospholamban (PLN), and their abnormalities in failing hearts. A series of new somatic gene transfer attempts targeting calcium handling in cardiomyocytes are discussed thereafter.

Decreased sarcoplasmic reticulum activity and contractility in diabetic db/db mouse heart

Although it is known that insulin-dependent (type 1) diabetes results in depressed contractile performance associated with diminished sarcoendoplasmic reticular Ca2+-ATPase (SERCA2a) activity, findings in insulin-resistant (type 2) diabetes suggest a less clear association. The db/db insulin-resistant mouse model exhibits decreased cardiac performance both in situ and in isolated ex vivo working hearts. In this study, contractile performance and calcium transients were measured in Langendorff-perfused hearts and isolated cardiac myocytes. Diabetic (db/db) mouse hearts demonstrated decreased rates of contraction, relaxation, and pressure development. Calcium transients from isolated myocytes revealed significantly lower diastolic and systolic levels of calcium in diabetic hearts. Furthermore, the decay rate of the calcium transient was significantly reduced in diabetic myocytes, suggesting a diminished capacity for cytosolic calcium removal not associated with a change in sodium-calcium exchanger activity. Calcium leakage from the sarcoplasmic reticulum (SR) measured using tetracaine was significantly increased in diabetic myocytes. Western blot analysis indicated only a small decrease in SERCA2a expression in diabetic mice, but a large increase in phospholamban expression. Expression of the ryanodine receptor did not differ between groups. In conclusion, the decreased contractile function observed in the db/db diabetic mouse model appears to be related to decreased calcium handling by the SR.

Altered cardiac calcium handling in diabetes

Diabetes results in a cardiomyopathy characterized by reduced contractility that is primarily the result of changes in calcium handling within the myocyte. Because most of the calcium involved in excitation-contraction (EC) coupling is derived from the sarcoplasmic reticulum (SR), it is no surprise that many studies have found a reduction in sarco-endoplasmic reticulum calcium ATPase (SERCA) activity in the diabetic state. In this review, we outline the changes to SR calcium handling in the diabetic state and, through the use of transgenic mice and adenoviral gene therapy, we examine how SR function can be improved by the expression of various proteins that are directly and indirectly involved in calcium handling. Improving SERCA activity plays an important role in ameliorating the contractile phenotype associated with the diabetic state.