Decreased expression of cardiac sarcoplasmic reticulum Ca(2+)-pump ATPase in congestive heart failure due to myocardial infarction

The involvement of soluble epoxide hydrolase in the development of cardiovascular diseases through epoxyeicosatrienoic acids

Arachidonic acid (AA) has three main metabolic pathways: the cycloxygenases (COXs) pathway, the lipoxygenases (LOXs) pathway, and the cytochrome P450s (CYPs) pathway. AA produces epoxyeicosatrienoic acids (EETs) through the CYPs pathway. EETs are very unstable in vivo and can be degraded in seconds to minutes. EETs have multiple degradation pathways, but are mainly degraded in the presence of soluble epoxide hydrolase (sEH). sEH is an enzyme of bifunctional nature, and current research focuses on the activity of its C-terminal epoxide hydrolase (sEH-H), which hydrolyzes the EETs to the corresponding inactive or low activity diol. Previous studies have reported that EETs have cardiovascular protective effects, and the activity of sEH-H plays a role by degrading EETs and inhibiting their protective effects. The activity of sEH-H plays a different role in different cells, such as inhibiting endothelial cell proliferation and migration, but promoting vascular smooth muscle cell proliferation and migration. Therefore, it is of interest whether the activity of sEH-H is involved in the initiation and progression of cardiovascular diseases by affecting the function of different cells through EETs.

Cardiac calcium regulation in human induced pluripotent stem cell cardiomyocytes: Implications for disease modeling and maturation

Human induced pluripotent stem cell cardiomyocytes (hiPSC-CMs) are based on ground-breaking technology that has significantly impacted cardiovascular research. They provide a renewable source of human cardiomyocytes for a variety of applications including in vitro disease modeling and drug toxicity testing. Cardiac calcium regulation plays a critical role in the cardiomyocyte and is often dysregulated in cardiovascular disease. Due to the limited availability of human cardiac tissue, calcium handling and its regulation have most commonly been studied in the context of animal models. hiPSC-CMs can provide unique insights into human physiology and pathophysiology, although a remaining limitation is the relative immaturity of these cells compared to adult cardiomyocytes Therefore, this field is rapidly developing techniques to improve the maturity of hiPSC-CMs, further establishing their place in cardiovascular research. This review briefly covers the basics of cardiomyocyte calcium cycling and hiPSC technology, and will provide a detailed description of our current understanding of calcium in hiPSC-CMs.

Force-frequency relationship and early relaxation kinetics are preserved upon sarcoplasmic blockade in human myocardium

In this study, we investigated the quantitative and qualitative role of the sarcoplasmic reticulum (SR) in the regulation of the force-frequency relationship (FFR). We blocked the function of SR with cyclopiazonic acid (CPA) and ryanodine and measured twitch kinetics and developed force at various stimulation frequencies in nonfailing and failing intact human right ventricular trabeculae. We found that developed forces are only slightly reduced upon SR blockade, while the positive FFR in nonfailing trabeculae and negative FFR in failing trabeculae were both preserved. The contraction kinetics (dF/dt, dF/dt/F, and time to peak), however, were significantly slower at all frequencies tested. Kinetics of first 50% of relaxation (RT50) was not affected by SR blockade. Kinetics of entire relaxation process (RT90) was overall slower at low frequencies, but not at high frequencies. From our findings, we conclude that the SR is not essential for FFR, and its role in regulation of FFR lies mostly in contraction kinetics. Unlike small rodents, human myocardium contractile function is near-normal in absence of a functional SR with little changes in contractile force, and with preservation with the main regulation of FFR.

The harmful effects of acute PM2.5 exposure to the heart and a novel preventive and therapeutic function of CEOs

Epidemiological researches have demonstrated the relationship between PM2.5 exposure and increased morbidity and mortality of cardiovascular injury. However, no effective therapeutic method was established. The purpose of this study is to investigate the effect of acute PM2.5 exposure on the mice heart tissue and explore the therapeutic effects of compound essential oils (CEOs) in this model. In this study, after mice were exposed to PM2.5 intratracheally, some obvious histopathological changes as well as some great alterations of proinflammatory cytokines were observed in the heart tissue. The imbalance of oxidative stress, the altered Ca2+ channel related proteins and the increased intracellular free Ca2+ were all involved in the heart impairment and would also be investigated in this model. The CEOs alleviated the heart impairment via its antioxidant effect rather than its anti-inflammatory function because our results revealed that oxidative stress related indicators were restored after CEOs administration. At the same time, increased concentration of intracellular free Ca2+ and ROS induced by PM2.5 were reduced after NAC (N-Acetyl-L-cysteine) administration. These data suggested that the acute PM2.5 exposure would damage heart tissue by inducing the inflammatory response, oxidative stress and intracellular free Ca2+ overload. PM2.5-induced oxidative stress probably increase intracellular free Ca2+ via RYR2 and SERCA2a. CEOs have the potential to be a novel effective and convenient therapeutic method to prevent and treat the acute heart impairment induced by PM2.5 via its antioxidant function.

Novel Insight Into the Epigenetic and Post-transcriptional Control of Cardiac Gene Expression by Thyroid Hormone

Thyroid hormone (TH) signaling is critically involved in the regulation of cardiovascular physiology. Even mild reductions of myocardial TH levels, as occur in hypothyroidism or low T3 state conditions, are thought to play a role in the progression of cardiac disorders. Due to recent advances in molecular mechanisms underlying TH action, it is now accepted that TH-dependent modulation of gene expression is achieved at multiple transcriptional and post-transcriptional levels and involves the cooperation of many processes. Among them, the epigenetic remodeling of chromatin structure and the interplay with non-coding RNA have emerged as novel TH-dependent pathways that add further degrees of complexity and broaden the network of genes controlled by TH signaling. Increasing experimental and clinical findings indicate that aberrant function of these regulatory mechanisms promotes the evolution of cardiac disorders such as post-ischemic injury, pathological hypertrophy, and heart failure, which may be reversed by the correction of the underlying TH dyshomeostasis. To encourage the clinical implementation of a TH replacement strategy in cardiac disease, here we discuss the crucial effect of epigenetic modifications and control of non-coding RNA in TH-dependent regulation of biological processes relevant for cardiac disease evolution.

EGR-mediated control of STIM expression and function

Ca²⁺ is a ubiquitous, dynamic and pluripotent second messenger with highly context-dependent roles in complex cellular processes such as differentiation, proliferation, and cell death. These Ca²⁺ signals are generated by Ca²⁺-permeable channels located on the plasma membrane (PM) and endoplasmic reticulum (ER) and shaped by PM- and ER-localized pumps and transporters. Differences in the expression of these Ca²⁺ homeostasis proteins contribute to cell and context-dependent differences in the spatiotemporal organization of Ca²⁺ signals and, ultimately, cell fate. This review focuses on the Early Growth Response (EGR) family of zinc finger transcription factors and their role in the transcriptional regulation of Stromal Interaction Molecule (STIM1), a critical regulator of Ca²⁺ entry in both excitable and non-excitable cells.

T-tubule remodelling disturbs localized β2-adrenergic signalling in rat ventricular myocytes during the progression of heart failure

Aims

Cardiomyocyte β2-adrenergic receptor (β2AR) cyclic adenosine monophosphate (cAMP) signalling is regulated by the receptors' subcellular location within transverse tubules (T-tubules), via interaction with structural and regulatory proteins, which form a signalosome. In chronic heart failure (HF), β2ARs redistribute from T-tubules to the cell surface, which disrupts functional signalosomes and leads to diffuse cAMP signalling. However, the functional consequences of structural changes upon β2AR-cAMP signalling during progression from hypertrophy to advanced HF are unknown.

Methods and results

Rat left ventricular myocytes were isolated at 4-, 8-, and 16-week post-myocardial infarction (MI), β2ARs were stimulated either via whole-cell perfusion or locally through the nanopipette of the scanning ion conductance microscope. cAMP release was measured via a Förster Resonance Energy Transfer-based sensor Epac2-camps. Confocal imaging of di-8-ANNEPS-stained cells and immunoblotting were used to determine structural alterations. At 4-week post-MI, T-tubule regularity, density and junctophilin-2 (JPH2) expression were significantly decreased. The amplitude of local β2AR-mediated cAMP in T-tubules was reduced and cAMP diffused throughout the cytosol instead of being locally confined. This was accompanied by partial caveolin-3 (Cav-3) dissociation from the membrane. At 8-week post-MI, the β2AR-mediated cAMP response was observed at the T-tubules and the sarcolemma (crest). Finally, at 16-week post-MI, the whole cell β2AR-mediated cAMP signal was depressed due to adenylate cyclase dysfunction, while overall Cav-3 levels were significantly increased and a substantial portion of Cav-3 dissociated into the cytosol. Overexpression of JPH2 in failing cells in vitro or AAV9.SERCA2a gene therapy in vivo did not improve β2AR-mediated signal compartmentation or reduce cAMP diffusion.

Conclusion

Although changes in T-tubule structure and β2AR-mediated cAMP signalling are significant even at 4-week post-MI, progression to the HF phenotype is not linear. At 8-week post-MI the loss of β2AR-mediated cAMP is temporarily reversed. Complete disorganization of β2AR-mediated cAMP signalling due to changes in functional receptor localization and cellular structure occurs at 16-week post-MI.

Dioxygen and Metabolism; Dangerous Liaisons in Cardiac Function and Disease

The heart must consume a significant amount of energy to sustain its contractile activity. Although the fuel demands are huge, the stock remains very low. Thus, in order to supply its daily needs, the heart must have amazing adaptive abilities, which are dependent on dioxygen availability. However, in myriad cardiovascular diseases, “fuel” depletion and hypoxia are common features, leading cardiomyocytes to favor low-dioxygen-consuming glycolysis rather than oxidation of fatty acids. This metabolic switch makes it challenging to distinguish causes from consequences in cardiac pathologies. Finally, despite the progress achieved in the past few decades, medical treatments have not improved substantially, either. In such a situation, it seems clear that much remains to be learned about cardiac diseases. Therefore, in this review, we will discuss how reconciling dioxygen availability and cardiac metabolic adaptations may contribute to develop full and innovative strategies from bench to bedside.

PM2.5 exposure in utero contributes to neonatal cardiac dysfunction in mice

OBJECTIVE

Exposure of fine particulate matter (PM_2.5) to pregnant dams has been shown to be strongly associated with adverse cardiovascular outcomes in offspring at adulthood, however, effects evident during neonatal periods are unclear. We designed this study to examine cardiac function of neonatal mice (14 days old) exposed to in utero PM_2.5.

METHODS

Pregnant FVB female mice were exposed either to filtered air (FA) or PM_2.5 at an average concentration of 91.78 μg/m³ for 6 h/day, 5 days/wk (similar to exposure in a large industrial area) throughout the gestation period (21 days). After birth, animals were analyzed at day 14 of life.

RESULTS

Fourteen day old mice exposed to PM_2.5 during the in utero period demonstrated decreased fractional shortening (%FS, 41.1 ± 1.2% FA, 33.7 ± 1.2% PM_2.5, p < 0.01) and LVEDd (2.87 ± 0.08 mm FA, 2.58 ± 0.07 mm PM_2.5, p < 0.05) compared to FA exposed mice. Contractile kinetics and calcium transients in isolated cardiomyocytes from PM_2.5 exposed mice illustrated reduced peak shortening (%PS, 16.7 ± 0.5% FA, 14.7 ± 0.4% PM_2.5, p < 0.01), negative contractile velocity (-dL/dT, -6.91 ± 0.3 μm/s FA, -5.46 ± 0.2 μm/s PM_2.5, p < 0.001), increased time to relaxation 90% (TR90, 0.07 ± 0.003 s FA, 0.08 ± 0.004 s PM_2.5, p < 0.05), decreased calcium transient amplitude (Δ340/380, 33.8 ± 3.4 FA, 29.5 ± 2.8 p.m._2.5) and slower fluorescence decay rate (τ, 0.72 ± 0.1 s FA, 1.16 ± 0.15 s PM_2.5, p < 0.05). Immunoblotting studies demonstrated alterations in expression of Ca²⁺ handling proteins- SERCA-2A, p-PLN, NCX and Ca_V1.2 in hearts of 14 day old in utero PM_2.5 exposed mice compared to FA exposed hearts.

CONCLUSION

PM_2.5 exposure during the critical in utero period adversely affects the developing mouse fetus leading to functional cardiac changes that were evident during the very early (14 days) stages of adolescence. These data demonstrated that exposure to PM_2.5 during the gestation period significantly impacts cardiovascular outcomes early in life.

Cardiac Calcium ATPase Dimerization Measured by Cross-Linking and Fluorescence Energy Transfer

The cardiac sarco/endoplasmic reticulum calcium ATPase (SERCA) establishes the intracellular calcium gradient across the sarcoplasmic reticulum membrane. It has been proposed that SERCA forms homooligomers that increase the catalytic rate of calcium transport. We investigated SERCA dimerization in rabbit left ventricular myocytes using a photoactivatable cross-linker. Western blotting of cross-linked SERCA revealed higher-molecular-weight species consistent with SERCA oligomerization. Fluorescence resonance energy transfer measurements in cells transiently transfected with fluorescently labeled SERCA2a revealed that SERCA readily forms homodimers. These dimers formed in the absence or presence of the SERCA regulatory partner, phospholamban (PLB) and were unaltered by PLB phosphorylation or changes in calcium or ATP. Fluorescence lifetime data are compatible with a model in which PLB interacts with a SERCA homodimer in a stoichiometry of 1:2. Together, these results suggest that SERCA forms constitutive homodimers in live cells and that dimer formation is not modulated by SERCA conformational poise, PLB binding, or PLB phosphorylation.

In Utero Particulate Matter Exposure Produces Heart Failure, Electrical Remodeling, and Epigenetic Changes at Adulthood

BACKGROUND

Particulate matter (PM; PM2.5 [PM with diameters of <2.5 μm]) exposure during development is strongly associated with adverse cardiovascular outcomes at adulthood. In the present study, we tested the hypothesis that in utero PM2.5 exposure alone could alter cardiac structure and function at adulthood.

METHODS AND RESULTS

Female FVB mice were exposed either to filtered air or PM2.5 at an average concentration of 73.61 μg/m3 for 6 h/day, 7 days/week throughout pregnancy. After birth, animals were analyzed at 12 weeks of age. Echocardiographic (n=9-10 mice/group) and pressure-volume loop analyses (n=5 mice/group) revealed reduced fractional shortening, increased left ventricular end-systolic and -diastolic diameters, reduced left ventricular posterior wall thickness, end-systolic elastance, contractile reserve (dP/dtmax/end-systolic volume), frequency-dependent acceleration of relaxation), and blunted contractile response to β-adrenergic stimulation in PM2.5-exposed mice. Isolated cardiomyocyte (n=4-5 mice/group) function illustrated reduced peak shortening, ±dL/dT, and prolonged action potential duration at 90% repolarization. Histological left ventricular analyses (n=3 mice/group) showed increased collagen deposition in in utero PM2.5-exposed mice at adulthood. Cardiac interleukin (IL)-6, IL-1ß, collagen-1, matrix metalloproteinase (MMP) 9, and MMP13 gene expressions were increased at birth in in utero PM2.5-exposed mice (n=4 mice/group). In adult hearts (n=5 mice/group), gene expressions of sirtuin (Sirt) 1 and Sirt2 were decreased, DNA methyltransferase (Dnmt) 1, Dnmt3a, and Dnmt3b were increased, and protein expression (n=6 mice/group) of Ca2+-ATPase, phosphorylated phospholamban, and Na+/Ca2+ exchanger were decreased.

CONCLUSIONS

In utero PM2.5 exposure triggers an acute inflammatory response, chronic matrix remodeling, and alterations in Ca2+ handling proteins, resulting in global adult cardiac dysfunction. These results also highlight the potential involvement of epigenetics in priming of adult cardiac disease.

Epigenetics of the failing heart

With the impressive advancement in high-throughput 'omics' technologies over the past two decades, epigenetic mechanisms have emerged as the regulatory interface between the genome and environmental factors. These mechanisms include DNA methylation, histone modifications, ATP-dependent chromatin remodeling and RNA-based mechanisms. Their highly interdependent and coordinated action modulates the chromatin structure controlling access of the transcription machinery and thereby regulating expression of target genes. Given the rather limited proliferative capability of human cardiomyocytes, epigenetic regulation appears to play a particularly important role in the myocardium. The highly dynamic nature of the epigenome allows the heart to adapt to environmental challenges and to respond quickly and properly to cardiac stress. It is now becoming evident that histone-modifying and chromatin-remodeling enzymes as well as numerous non-coding RNAs play critical roles in cardiac development and function, while their dysregulation contributes to the onset and development of pathological cardiac remodeling culminating in HF. This review focuses on up-to-date knowledge about the epigenetic mechanisms and highlights their emerging role in the healthy and failing heart. Uncovering the determinants of epigenetic regulation holds great promise to accelerate the development of successful new diagnostic and therapeutic strategies in human cardiac disease.

Effects of cyclopiazonic acid and dexamethasone on serotonin-induced calcium responses in vascular smooth muscle cells

We previously observed that sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA) blockade by cyclopiazonic acid (CPA) significantly potentiates serotonin (5-hydroxytryptamine (5-HT))-induced vascular contractions. Furthermore, 5-HT receptor antagonist methysergide partially inhibited CPA-potentiated 5-HT contractions. In the present study, we further investigated whether SERCA inhibition potentiates 5-HT-induced Ca(2+) responses along with attenuating the receptor antagonism by store-operated Ca(2+) (SOC) entry and protein kinase C (PKC)-mediated mechanisms. The effects of dexamethasone that was previously shown to induce SOC entry and enhance 5-HT responses were also tested. For this purpose, intracellular Ca(2+) levels were monitored in A7r5 embryonic rat vascular smooth muscle cells by spectrofluorometry using the fluorescent indicator fura-2. The results showed that CPA, although not dexamethasone, significantly potentiated 5-HT-induced Ca(2+) elevations. Ketanserin partially decreased 5-HT-induced and CPA-potentiated Ca(2+) elevations whereas both PKC inhibitor D-sphingosine and SOC entry blocker 2-aminoethoxydiphenyl borate (2-APB) abolished the remaining responses. The data suggests that diminished antagonistic effect on 5-HT-induced Ca(2+) elevations in the presence of SERCA inhibition is induced by SOC entry and PKC activation.

The Current and Future Landscape of SERCA Gene Therapy for Heart Failure: A Clinical Perspective

Gene therapy has been applied to cardiovascular disease for over 20 years but it is the application to heart failure that has generated recent interest in clinical trials. There is laboratory and early clinical evidence that delivery of sarcoplasmic reticulum calcium ATPase 2a (SERCA2a) gene therapy is beneficial for heart failure and this therapy could become the first positive inotrope with anti-arrhythmic properties. In this review we will discuss the rationale for SERCA2a gene therapy as a viable strategy in heart failure, review the published data, and discuss the ongoing clinical trials, before concluding with comments on the future challenges and potential for this therapy.

Gene therapy for heart failure

Heart failure is a major public health problem throughout the world and it is likely that its prevalence will continue to grow over the next several decades. Despite advances in the treatment of heart failure, morbidity and mortality remain unacceptably high. Gene transfer therapy provides a novel strategy for targeting abnormalities in cardiac cells that adversely affect cardiac function. New vectors for gene delivery, mainly adeno-associated viruses (AAVs) that are preferentially taken up by cardiomyocytes, can result in sustained transgene expression. The cardiac isoform of sarco(endo)plasmic reticulum Ca(2+)ATPase (SERCA2a) plays a major role in regulating calcium levels in cardiomyocytes. Abnormal calcium handling by the failing heart caused by a reduction in SERCA2a activity adversely affects both systolic and diastolic function. The Calcium Upregulation by Percutaneous Administration of Gene Therapy in Cardiac Disease (CUPID) study was a Phase 2a double-blind, randomized, placebo-controlled, dose-finding study that was performed in patients with advanced heart failure due to systolic dysfunction. Eligible patients received AAV/SERCA2a or placebo by direct antegrade infusion into the coronary circulation. At the end of 12 months, patients receiving high-dose therapy (i.e. 1×10(13) DNase Resistant Particles) had evidence of favorable changes in several clinically relevant domains compared to patients treated with placebo. There were no safety concerns at any dose of AAV/SERCA2a. Patients treated with AAV/SERCA2a exhibited a striking reduction in cardiovascular events that persisted through 36 months of follow-up compared to patients who received placebo. Transgene expression was detected in the myocardium of patients receiving AAV/SERCA2a gene therapy as long as 31 months after delivery. A second Phase 2b study, CUPID 2, designed to confirm this favorable effect on heart failure events, is currently underway with the results expected to be presented later in 2015. Additional studies using other vectors and targets are in planning or underway making gene transfer therapy one of the most exciting new approaches under development for treating heart failure.

Gene therapy in heart failure. SERCA2a as a therapeutic target

The treatment of heart failure (HF) may be entering a new era with clinical trials currently assessing the value of gene therapy as a novel therapeutic strategy. If these trials demonstrate efficacy then a new avenue of potential treatments could become available to the clinicians treating HF. In principle, gene therapy allows us to directly target the underlying molecular abnormalities seen in the failing myocyte. In this review we discuss the fundamentals of gene therapy and the challenges of delivering it to patients with HF. The molecular abnormalities underlying HF are discussed along with potential targets for gene therapy, focusing on SERCA2a. We discuss the laboratory and early clinical evidence for the benefit of SERCA2a gene therapy in HF. Finally, we discuss the ongoing clinical trials of SERCA2a gene therapy and possible future directions for this treatment.

Cyclopiazonic acid alters serotonin-induced responses in rat thoracic aorta

We previously showed that endothelin A (ETA) receptor antagonist BQ-123 partially inhibited cyclopiazonic acid (CPA)-enhanced endothelin-1 (ET-1)-induced contractions suggesting enhancement of ETA receptor internalization in caveolar structures by sarco/endoplasmic reticulum Ca+2 ATPase (SERCA) blockade. Since serotonin (5-Hydroxytryptamine, 5-HT) receptors are reported to be localized on caveolar membranes, we investigated whether SERCA inhibition affects 5-HT-induced responses and 5-HT receptor antagonism. For this purpose, vascular responses were measured in thoracic aorta segments from male Wistar albino rats using isolated tissue experiments. Data showed that CPA inhibits 5-HT- and PE-induced contractions in intact vessels while potentiating those in endothelium-denuded. Furthermore, non-selective 5-HT receptor blocker methysergide partially inhibited CPA-induced 5-HT contractions. However, α1-adrenergic receptor antagonist prazosin totally inhibited CPA-potentiated PE contractions. We suggest that SERCA inhibition results in 5-HT receptor internalization similar to ETA receptors possibly through protein kinase C activation by increased subsarcolemmal Ca2+ levels, eventually preventing 5-HT receptor antagonism.

CYP2J2-derived epoxyeicosatrienoic acids suppress endoplasmic reticulum stress in heart failure

Prolonged endoplasmic reticulum (ER) stress causes apoptosis and is associated with heart failure. Whether CYP2J2 and its arachidonic acid metabolites [epoxyeicosatrienoic acids (EETs)] have a protective influence on ER stress and heart failure has not been studied. Assays of myocardial samples from patients with end-stage heart failure showed evidence of ER stress. Chronic infusion of isoproterenol (ISO) or angiotensin II (AngII) by osmotic mini-pump induced cardiac hypertrophy and heart failure in mice as evaluated by hemodynamic measurements and echocardiography. Interestingly, transgenic (Tr) mice with cardiomyocyte-specific CYP2J2 expression were protected against heart failure compared with wild-type mice. ISO or AngII administration induced ER stress and apoptosis, and increased levels of intracellular Ca(2+). These phenotypes were abolished by CYP2J2 overexpression in vivo or exogenous EETs treatment of cardiomyocytes in vitro. ISO or AngII reduced sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA2a) expression in hearts or isolated cardiomyocytes; however, loss of SERCA2a expression was prevented in CYP2J2 Tr hearts in vivo or in cardiomyocytes treated with EETs in vitro. The reduction of SERCA2a activity was concomitant with increased oxidation of SERCA2a. EETs reversed SERCA2a oxidation through increased expression of antioxidant enzymes and reduced reactive oxygen species levels. Tempol, a membrane-permeable radical scavenger, similarly decreased oxidized SERCA2a levels, restored SERCA2a activity, and markedly reduced ER stress response in the mice treated with ISO. In conclusion, CYP2J2-derived EETs suppress ER stress response in the heart and protect against cardiac failure by maintaining intracellular Ca(2+) homeostasis and SERCA2a expression and activity.

Altered sarcoplasmic reticulum calcium cycling--targets for heart failure therapy

Cardiac myocyte function is dependent on the synchronized movements of Ca(2+) into and out of the cell, as well as between the cytosol and sarcoplasmic reticulum. These movements determine cardiac rhythm and regulate excitation-contraction coupling. Ca(2+) cycling is mediated by a number of critical Ca(2+)-handling proteins and transporters, such as L-type Ca(2+) channels (LTCCs) and sodium/calcium exchangers in the sarcolemma, and sarcoplasmic/endoplasmic reticulum calcium ATPase 2a (SERCA2a), ryanodine receptors, and cardiac phospholamban in the sarcoplasmic reticulum. The entry of Ca(2+) into the cytosol through LTCCs activates the release of Ca(2+) from the sarcoplasmic reticulum through ryanodine receptor channels and initiates myocyte contraction, whereas SERCA2a and cardiac phospholamban have a key role in sarcoplasmic reticulum Ca(2+) sequesteration and myocyte relaxation. Excitation-contraction coupling is regulated by phosphorylation of Ca(2+)-handling proteins. Abnormalities in sarcoplasmic reticulum Ca(2+) cycling are hallmarks of heart failure and contribute to the pathophysiology and progression of this disease. Correcting impaired intracellular Ca(2+) cycling is a promising new approach for the treatment of heart failure. Novel therapeutic strategies that enhance myocyte Ca(2+) homeostasis could prevent and reverse adverse cardiac remodeling and improve clinical outcomes in patients with heart failure.

Constitutively active MEK1 rescues cardiac dysfunction caused by overexpressed GSK-3α during aging and hemodynamic pressure overload

Expression of GSK-3α is increased in aging hearts and those subjected to hemodynamic overload. Overexpressed GSK-3α inhibits ERK and enhances pressure overload (PO)-induced cardiac dysfunction. We studied whether suppression of the MEK1/ERK pathway contributes to cardiac responses induced by overexpressed GSK-3α using constitutively active MEK1 (CA-MEK1)/GSK-3α bigenic mice (bigenic mice), which were obtained by crossing cardiac-specific GSK-3α transgenic mice (Tg-GSK) and cardiac-specific CA-MEK1 transgenic mice (Tg-MEK1). The suppression of ERK phosphorylation observed in Tg-GSK was eliminated in bigenic mice. At 12 mo, left ventricular (LV) weight/tibia length, LV weight/body weight, and cardiac myocyte size were significantly smaller in Tg-GSK than in nontransgenic mice (NTg), but were not significantly different between Tg-MEK1 and bigenic mice. The LV ejection fraction (LVEF), fractional shortening (FS), and change in pressure over time were significantly lower in Tg-GSK than in NTg, but were not significantly different between bigenic mice and Tg-MEK1. The increase in apoptosis in Tg-GSK was abolished in bigenic mice, although the increase in fibrosis was not. After PO, the decrease in cardiac hypertrophy and the enhancement of apoptosis seen in Tg-GSK were abrogated in bigenic mice. After PO, the LVEF and FS were significantly reduced in Tg-GSK compared with its sham, but not in NTg, Tg-MEK1, or bigenic mice compared with their respective shams. There was no significant difference in LVEF and FS between bigenic mice and Tg-MEK1 after PO. In conclusion, inhibition of the MEK1/ERK pathway mediates the hypertrophy suppression and cardiac dysfunction caused by GSK-3α overexpression in cardiac myocytes.

Hydralazine-induced promoter demethylation enhances sarcoplasmic reticulum Ca2+ -ATPase and calcium homeostasis in cardiac myocytes

Sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a) plays an essential role in Ca(2+) homeostasis and cardiac functions. The promoter region of SERCA2a has a high content of CpG islands; thus, epigenetic modification by inhibiting methylation can enhance SERCA2a expression in cardiomyocytes. Hydralazine, a drug frequently used in heart failure, is a potential DNA methylation inhibitor. We evaluated whether hydralazine can modulate Ca(2+) handling through an increase in SERCA2a expression via regulating methylation. We used indo-1 fluorescence, real-time RT-PCR, immunoblotting, and methylation-specific PCR to investigate intracellular Ca(2+), the expressions of RNA and protein, and methylation of SERCA2a in HL-1 cardiomyocytes with and without (control) the administration of hydralazine (1, 10, and 30 μM) for 72 h. Hydralazine (10 and 30 μM) increased the intracellular Ca(2+) transients and SR Ca(2+) contents. Hydralazine (10 and 30 μM) decreased methylation in the SERCA2a promoter region and increased the RNA and protein expressions of SERCA2a. Additionally, hydralazine (10 and 30 μM) decreased the expression of DNA methyltransferase 1. Moreover, treatment with hydralazine in isoproterenol-induced heart failure rats decreased the promoter methylation of SERCA2a and increased SERCA2a RNA expression. In conclusion, hydralazine-induced promoter demethylation may improve cardiac function through increasing SERCA2a and modulating calcium homeostasis in cardiomyocytes.

Gene therapy for heart failure

Despite the progress achieved in conventional treatment modalities, heart failure remains a major cause of mortality and morbidity. The identification of novel signaling pathways has provided a solid scientific rationale which has stimulated preclinical development of gene-based therapies for heart failure. Advances in somatic gene transfer technologies have been crucial to the advent of the first human clinical trials which are currently in progress. As these and other trials of gene transfer-based therapies are initiated, these approaches have generated excitement and hope for novel treatments for cardiovascular disease. In this review, we present a summary of advancements in construction of different vectors and methods of delivery that have been used for specific myocardial gene delivery. In addition, we will show results from studies focusing on the use of gene therapy to target heart failure mechanisms in animal models of cardiac dysfunction. Finally, we discuss the limited but highly promising results from clinical studies that have served as catalysts to translate preclinical achievements towards new treatment modalities for heart failure.

Ion channels, transporters, and pumps as targets for heart failure therapy

Congestive heart failure is a leading cause of morbidity and mortality. Congestive heart failure is marked by atrial and ventricular enlargements and reduced cardiac contractility and an association with an increased incidence of atrial and ventricular arrhythmias and sudden cardiac death. Dysfunctional ion channel function is one of the major underlying mechanisms of the reduced contractility and arrhythmias. In this review, we explore the utility of ion channels, transporters, and pumps as targets for the treatment of heart failure, focusing predominantly on the treatment for reduced contractility and arrhythmias.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

Long-term postinfarction melatonin administration alters the expression of DHPR, RyR2, SERCA2, and MT2 and elevates the ANP level in the rat left ventricle

We investigated the effect of 2 wk continuous postinfarction subcutaneous melatonin supply on the expression of the rat left ventricular (LV) dihydropyridine receptor (DHPR), ryanodine receptor (RyR(2)), and sarco-endoplasmic reticulum Ca(2+)-ATPase2 (SERCA2), as they are fundamental proteins in cardiac contractility. The levels of plasma and LV atrial (ANP) and brain natriuretic peptide and melatonin were also measured, as was the expression of LV MT(1) and MT(2) receptors and pineal arylalkylamine N-acetyltransferase. Myocardial infarction (MI) was induced by ligation of the left anterior descending coronary artery and vehicle or melatonin (4.5 mg/kg per day) was administered by subcutaneous osmotic pumps. Echocardiography, real-time quantitative reverse transcription-polymerase chain reaction, and western blotting were used to analyze the samples. Echocardiography revealed that MI induced serious systolic LV dysfunction. The expression of DHPR, RyR(2), and SERCA2 mRNAs was significantly lower in the LVs of melatonin-treated MI rats compared with vehicle-treated rats (P < 0.01 for DHPR and P < 0.05 for RyR(2) and SERCA2). Melatonin also elevated the amount of LV MT(2) receptors to 1.9-fold (P < 0.05) and the concentration of LV ANP to over fivefold (P < 0.05) compared with vehicle rats after MI. Therefore, the results suggest that melatonin may influence the cardiac contractility after MI by regulating the expression of DHPR, RyR(2), and SERCA2, and melatonin receptors, particularly MT(2)s, might contribute to the postinfarction cardioprotective actions of melatonin. Furthermore, the finding of the relationship between melatonin and ANP suggests a novel mechanism for melatonin in protecting the heart after MI.

Periostin induces proliferation of differentiated cardiomyocytes and promotes cardiac repair

Adult mammalian hearts respond to injury with scar formation and not with cardiomyocyte proliferation, the cellular basis of regeneration. Although cardiogenic progenitor cells may maintain myocardial turnover, they do not give rise to a robust regenerative response. Here we show that extracellular periostin induced reentry of differentiated mammalian cardiomyocytes into the cell cycle. Periostin stimulated mononucleated cardiomyocytes to go through the full mitotic cell cycle. Periostin activated alphaV, beta1, beta3 and beta5 integrins located in the cardiomyocyte cell membrane. Activation of phosphatidylinositol-3-OH kinase was required for periostin-induced reentry of cardiomyocytes into the cell cycle and was sufficient for cell-cycle reentry in the absence of periostin. After myocardial infarction, periostin-induced cardiomyocyte cell-cycle reentry and mitosis were associated with improved ventricular remodeling and myocardial function, reduced fibrosis and infarct size, and increased angiogenesis. Thus, periostin and the pathway that it regulates may provide a target for innovative strategies to treat heart failure.

Enhanced beta-adrenergic response in rat papillary muscle by inhibition of inducible nitric oxide synthase after myocardial infarction

AIM

Myocardial infarction (MI) induces a progressive ventricular remodelling leading to a contractility depression. During the acute phase of MI inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) production increases in the heart. The aim of this study was to investigate the role of iNOS in the left ventricular contractility at 3 days after MI.

METHODS

Wistar rats were divided into: sham operated (SHAM, n = 23), infarction (INF, n = 18); sham operated plus the iNOS inhibitor, S-methylisothiourea (SMT) 5 mg kg(-1) day(-1), i.p. treatment (SHAM-SMT, n = 26) and infarction plus SMT (INF-SMT, n = 22). Concentration-response curves for isoprenaline, Ca(2+) and frequency-force curve were studied in isolated papillary muscle from left ventricle.

RESULTS

After 3 days infarct area was similar between groups. SMT treatment reduced the time to peak tension during frequency-force curve in the infarct group (SHAM = 63 +/- 3; SHAM-SMT = 71 +/- 3; INF = 90 +/- 4; INF-SMT = 79 +/- 4 ms, P < 0.05) and increased the maximal response to isoprenaline (SHAM = 0.93 +/- 0.11; SHAM-SMT = 1.13 +/- 0.1; INF =0.84 +/- 0.16; INF-SMT = 1.49 +/- 0.15 g mm(-2), P < 0.05). The response to Ca(2+) was equally reduced in the INF and INF-SMT groups. SMT treatment did not change the reduced post-rest potentiation performed by INF group, but attenuated the plasma nitrite and nitrate (NOx) levels in the INF group without any haemodynamic effect.

CONCLUSION

These finding suggest that at 3 days after MI the iNOS modulates the isolated papillary muscle response to isoprenaline and its inhibition improves the beta-adrenergic inotropic responses.

Time course of changes in the expression of DHPR, RyR(2), and SERCA2 after myocardial infarction in the rat left ventricle

Postinfarction left ventricular remodeling leads to the functional decline of the left ventricle (LV). Since dihydropyridine receptor (DHPR), ryanodine receptor (RyR(2)), and sarco-endoplasmic reticulum (SR) Ca(2+)-ATPase2 (SERCA2a) play a major role in the contractility of the heart, the aim of our study was to evaluate the time course of changes in the expression of these proteins 1 day, 2 weeks and 4 weeks after myocardial infarction (MI). Myocardial infarction was produced by ligation of left anterior descending coronary artery of the rat. Transthoracic echocardiography was performed to characterize structural and functional changes after MI. To evaluate protein mRNA levels and the relative amount of proteins, real-time quantitative RT-PCR and Western blotting were used. LV ejection fraction and fractional shortening decreased significantly during the 4-week follow-up period (P < 0.001). Typical features of LV remodeling after MI were seen, with a decrease in anterior wall thickness (P < 0.001) and dilatation of the LV (P < 0.001). Expression of DHPR and RyR(2) mRNAs decreased and Serca2a mRNA tended to decrease 1 day after MI (P < 0.001, P < 0.01 and P = 0.06, respectively), followed by recovery of the expression during the next 4 weeks. In the infarcted hearts the quantities of SERCA2 proteins in the LV were significantly decreased at the time of 4 weeks. In conclusion, MI was associated with transient decrease in the expression of the DHPR and RyR(2) mRNAs and a reduced quantity of SERCA2 proteins in the LV. Since they have a key role in the contraction of the heart, changes in the expression of these proteins may be important regulators of LV systolic function after MI.

Sarcoplasmic-endoplasmic reticulum Ca2+-ATPase inhibition prevents endothelin A receptor antagonism in rat aorta

This study tested whether sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase regulates the ability of endothelin receptor antagonist to inhibit the endothelin-1 constriction. The endothelin A receptor antagonist BQ-123 (1 microM) completely relaxed constriction to 10 nM endothelin-1 in endothelium-denuded rat aorta. Challenge with cyclopiazonic acid (10 microM), a sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase inhibitor, during the plateau of endothelin-1 constriction enhanced the constriction by approximately 30%. BQ-123 relaxed the endothelin-1 plus cyclopiazonic acid constriction by only approximately 10%. In contrast, prazosin (1 microM), an alpha-adrenergic receptor antagonist, still completely relaxed the 0.3 muM phenylephrine constriction in the presence of cyclopiazonic acid. Verapamil relaxed the endothelin-1 plus cyclopiazonic acid constriction by approximately 30%, whereas Ni(2+) and 2-aminoethoxydiphenyl borate, nonselective cation channel and store-operated channel blockers, respectively, completely relaxed the constriction. These results suggest that lowered sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase activity selectively decreases the ability of endothelin receptor antagonist to inhibit the endothelin A receptor. The decreased antagonism may be related to the opening of store-operated channels and subsequent greater internalization of endothelin A receptor.

Cellular and molecular determinants of altered Ca2+ handling in the failing rabbit heart: primary defects in SR Ca2+ uptake and release mechanisms

Myocytes from the failing myocardium exhibit depressed and prolonged intracellular Ca(2+) concentration ([Ca(2+)](i)) transients that are, in part, responsible for contractile dysfunction and unstable repolarization. To better understand the molecular basis of the aberrant Ca(2+) handling in heart failure (HF), we studied the rabbit pacing tachycardia HF model. Induction of HF was associated with action potential (AP) duration prolongation that was especially pronounced at low stimulation frequencies. L-type calcium channel current (I(Ca,L)) density (-0.964 +/- 0.172 vs. -0.745 +/- 0.128 pA/pF at +10 mV) and Na(+)/Ca(2+) exchanger (NCX) currents (2.1 +/- 0.8 vs. 2.3 +/- 0.8 pA/pF at +30 mV) were not different in myocytes from control and failing hearts. The amplitude of peak [Ca(2+)](i) was depressed (at +10 mV, 0.72 +/- 0.07 and 0.56 +/- 0.04 microM in normal and failing hearts, respectively; P < 0.05), with slowed rates of decay and reduced Ca(2+) spark amplitudes (P < 0.0001) in myocytes isolated from failing vs. control hearts. Inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)2a revealed a greater reliance on NCX to remove cytosolic Ca(2+) in myocytes isolated from failing vs. control hearts (P < 0.05). mRNA levels of the alpha(1C)-subunit, ryanodine receptor (RyR), and NCX were unchanged from controls, while SERCA2a and phospholamban (PLB) were significantly downregulated in failing vs. control hearts (P < 0.05). alpha(1C) protein levels were unchanged, RyR, SERCA2a, and PLB were significantly downregulated (P < 0.05), while NCX protein was significantly upregulated (P < 0.05). These results support a prominent role for the sarcoplasmic reticulum (SR) in the pathogenesis of HF, in which abnormal SR Ca(2+) uptake and release synergistically contribute to the depressed [Ca(2+)](i) and the altered AP profile phenotype.

Insulin improves cardiomyocyte contractile function through enhancement of SERCA2a activity in simulated ischemia/reperfusion

AIM

Insulin exerts anti-apoptotic effects in both cardiomyocytes and coronary endothelial cells following ischemia/reperfusion (I/R) via the Akt-endothelial nitric oxide synthase survival signal pathway. This important insulin signaling might further contribute to the improvement of cardiac function after reperfusion. In this study, we tested the hypothesis that sarcoplasmic reticulum calcium-ATPase (SERCA2a) is involved in the insulin-induced improvement of cardiac contractile function following I/R.

METHODS

Ventricular myocytes were enzymatically isolated from adult SD rats. Simulated I/R was induced by perfusing cells with chemical anoxic solution for 15 min followed by reperfusion with Tyrode's solution with or without insulin for 30 min. Myocyte shortening and intracellular calcium transients were assessed and underlying mechanisms were investigated.

RESULTS

Reperfusion with insulin (10(-7) mol/L) significantly improved the recovery of contractile function (n=15-20 myocytes from 6-8 hearts, P<0.05), and increased calcium transients, as evidenced by the increased calcium [Ca2+] fluorescence ratio, shortened time to peak Ca2+ and time to 50% diastolic Ca2+, compared with those in cells reperfused with vehicle (P<0.05). In addition, Akt phosphorylation and SERCA2a activity were both increased in insulin-treated I/R cardiomyocytes, which were markedly inhibited by pretreatment of cells with a specific Akt inhibitor. Moreover, inhibition of Akt activity abolished insulin-induced positive contractile and calcium transients responses in I/R cardiomyocytes.

CONCLUSION

These data demonstrated for the first time that insulin improves the recovery of contractile function in simulated I/R cardiomyocytes in an Akt-dependent and SERCA2a-mediated fashion.

Subcellular remodeling as a viable target for the treatment of congestive heart failure

It is now well known that congestive heart failure (CHF) is invariably associated with cardiac hypertrophy, and changes in the shape and size of cardiomyocytes (cardiac remodeling) are considered to explain cardiac dysfunction in CHF. However, the mechanisms responsible for the transition of cardiac hypertrophy to heart failure are poorly understood. Several lines of evidence both from various experimental models of CHF and from patients with different types of CHF have indicated that the functions of different subcellular organelles such as extracellular matrix, sarcolemma, sarcoplasmic reticulum, myofibrils, mitochondria, and nucleus are defective. Subcellular abnormalities for protein contents, gene expression, and enzyme activities in the failing heart become evident as a consequence of prolonged hormonal imbalance, metabolic derangements, and cation maldistribution. In particular, the occurrence of oxidative stress, development of intracellular Ca2+ overload, activation of proteases and phospholipases, and alterations in cardiac gene expression result in changes in the biochemical composition, molecular structure, and function of different subcellular organelles (subcellular remodeling). Not only does subcellular remodeling appear to be intimately involved in the transition of cardiac hypertrophy to heart failure, the mismatching of the function of different subcellular organelles leads to the development of cardiac dysfunction. Although blockade of the renin-angiotensin system, sympathetic nervous system, and various other hormonal actions have been reported to produce beneficial effects on cardiac remodeling and heart dysfunction in CHF, the actions of various cardiac drugs on subcellular remodeling have not been examined extensively. Some recent studies have indicated that both the angiotensin-converting enzyme inhibitors and angiotensin receptor antagonists attenuate changes in sarcolemma, sarcoplasmic reticulum, and myofibril enzyme activities, protein contents, and gene expression, and partly improve cardiac function in the failing hearts. It is suggested that subcellular remodeling is an excellent target for the development of improved drug therapy for CHF. Furthermore, extensive studies should investigate the effects of different agents individually or in combination on reverse subcellular remodeling, cardiac remodeling, and cardiac dysfunction in various experimental models of CHF.

Control of protein expression through mRNA stability in calcium signalling

Specific sequences (cis-acting elements) in the 3'-untranslated region (UTR) of RNA, together with stabilizing and destabilizing proteins (trans-acting factors), determine the mRNA stability, and consequently, the level of expression of several proteins. Such interactions were discovered initially for short-lived mRNAs encoding cytokines and early genes like c-jun and c-myc. However, they may also determine the fate of more stable mRNAs in a tissue and disease-dependent manner. The interactions between the cis-acting elements and the trans-acting factors may also be modulated by Ca(2+) either directly or via a control of the phosphorylation status of the trans-acting factors. We focus initially on the basic concepts in mRNA stability with the trans-acting factors AUF1 (destabilizing) and HuR (stabilizing). Sarco/endoplasmic reticulum Ca(2+) pumps, SERCA2a (cardiac and slow twitch muscles) and SERCA2b (most cells including smooth muscle cells), are pivotal in Ca(2+) mobilization during signal transduction. SERCA2a and SERCA2b proteins are encoded by relatively stable mRNAs that contain cis-acting stability determinants in their 3'-regions. We present several pathways where 3'-UTR mediated mRNA decay is key to Ca(2+) signalling: SERCA2a and beta-adrenergic receptors in heart failure, renin-angiotensin system, and parathyroid hormones. Other examples discussed include cytokines vascular endothelial growth factor, endothelin and endothelial nitric oxide synthase. Roles of Ca(2+) and Ca(2+)-binding proteins in mRNA stability are also discussed. We anticipate that these novel modes of control of protein expression will form an emerging area of research that may explore the central role of Ca(2+) in cell function during development and in disease.

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.

Sarcoplasmic reticulum Ca2+ transport and gene expression in congestive heart failure are modified by imidapril treatment

This study was designed to test the hypothesis that blockade of the renin-angiotensin system improves cardiac function in congestive heart failure by preventing changes in gene expression of sarcoplasmic reticulum (SR) proteins. We employed rats with myocardial infarction (MI) to examine effects of an angiotensin-converting enzyme inhibitor, imidapril, on SR Ca2+ transport, protein content, and gene expression. Imidapril (1 mg·kg−1·day−1) was given for 4 wk starting 3 wk after coronary artery occlusion. Infarcted rats exhibited a fourfold increase in left ventricular end-diastolic pressure, whereas rates of pressure development and decay were decreased by 60 and 55%, respectively. SR Ca2+ uptake and Ca2+ pump ATPase, as well as Ca2+ release and ryanodine receptor binding activities, were depressed in the failing hearts; protein content and mRNA levels for Ca2+ pump ATPase, phospholamban, and ryanodine receptor were also decreased by ∼55–65%. Imidapril treatment of infarcted animals improved cardiac performance and attenuated alterations in SR Ca2+ pump and Ca2+ release activities. Changes in protein content and mRNA levels for SR Ca2+ pump ATPase, phospholamban, and ryanodine receptor were also prevented by imidapril treatment. Beneficial effects of imidapril on cardiac function and SR Ca2+ transport were not only seen at different intervals of MI but were also simulated by another angiotensin-converting enzyme inhibitor, enalapril, and an ANG II receptor antagonist, losartan. These results suggest that blockade of the renin-angiotensin system may increase the abundance of mRNA for SR proteins and, thus, may prevent the depression in SR Ca2+ transport and improve cardiac function in congestive heart failure due to MI.

Prior exercise training improves the outcome of acute myocardial infarction in the rat

OBJECTIVES

The aim of this research was to investigate the structural, functional, and molecular features of the remodeling heart in prior swim-trained infarcted rats.

BACKGROUND

Physical exercise training is a known protective factor against cardiovascular morbidity and mortality. The structural and molecular aspects underlying this protection in the remodeling heart have not been investigated.

METHODS

After seven weeks of swimming exercise training, rats underwent surgical ligation of the left coronary artery followed by a four-week sedentary period. Untrained control rats underwent the same surgical protocol. Left ventricular function was assessed by echocardiography four weeks after infarction, and hearts were sampled for histological and molecular analysis. Ribonucleic acid from the surviving left ventricle was analyzed by complementary deoxyribonucleic acid arrays followed by Northern blotting or quantitative reverse transcription polymerase chain reaction of selected messenger ribonucleic acids (mRNAs).

RESULTS

Scar area was 1.6-fold smaller (p = 0.0002), arteriolar density was 1.7-fold higher (p = 0.0002), and left ventricular shortening fraction was 1.9-fold higher (p = 0.003) in the exercise-trained compared with sedentary hearts. Eleven genes whose expression level varied by at least +/-1.5-fold distinguished the prior exercised rats from their sedentary counterparts. Compared with sedentary, the exercised hearts displayed 9- and 2.4-times lower levels of atrial natriuretic peptide and aldolase mRNA (p = 0.03 and 0.04, respectively), and a 2.7- and 1.9-fold higher abundance of cytochrome c-oxidase and fatty acid binding protein, respectively (p < 0.03, each).

CONCLUSIONS

Swimming exercise training before acute myocardial infarction reduces scar size, increases arteriole density, and manifests adaptation of stress- and energy-metabolism-related genes that may contribute to the improved heart function observed during remodeling.

Control of SERCA2a Ca2+ pump mRNA stability by nuclear proteins: role of domains in the 3′-untranslated region

Alternative splicing of the sarco/endoplasmic reticulum (SERCA2) Ca2+ pump transcript generates the two isoforms: SERCA2a in left ventricular myocytes (LVM) and SERCA2b in most tissues. Nuclear protein extracts from left ventricular myocytes can cause a decay of the 3'-region of the SERCA2a. To determine if all the domains in the 800 b SERCA2a 3'-end region (3344-4243) are equally stable, we examined in vitro decay of synthetically capped, polyadenylated overlapping RNA fragments 2A1-2A6 from the 3'-end region of SERCA2a. Whereas 2A1-2A5 RNAs were stable, the distal fragment 2A6 (4135-4243 b) decayed rapidly. Deleting the 2A6 sequence from the 800-b 3'-end region increased its stability. In mobility shift assays, 2A6 bound to protein(s) in the LVM nuclear extracts in a specific manner: unlabelled 2A6 or the 800 b 3'-region RNA competed for binding but poly A, poly U, and poly C RNA did not. Secondary structure analysis revealed three hairpin loops in 2A6. Experiments using small synthetic RNA fragments for competition with 2A6 binding to nuclear proteins were consistent with a model involving the three hairpin loops. Thus, the secondary structure of the distal domain of SERCA2a RNA may be important in regulating its stability.

Influence of long-term treatment of imidapril on mortality, cardiac function, and gene expression in congestive heart failure due to myocardial infarction

Although it is generally accepted that the efficacy of imidapril, an angiotensin-converting enzyme inhibitor, in congestive heart failure (CHF) is due to improvement of hemodynamic parameters, the significance of its effect on gene expression for sarcolemma (SL) and sarcoplasmic reticulum (SR) proteins has not been fully understood. In this study, we examined the effects of long-term treatment of imidapril on mortality, cardiac function, and gene expression for SL Na+/K+ATPase and Na+–Ca2+exchanger as well as SR Ca2+pump ATPase, Ca2+release channel (ryanodine receptor), phospholamban, and calsequestrin in CHF due to myocardial infarction. Heart failure subsequent to myocardial infarction was induced by occluding the left coronary artery in rats, and treatment with imidapril (1 mg·kg–1·day–1) was started orally at the end of 3 weeks after surgery and continued for 37 weeks. The animals were assessed hemody nam ically and the heart and lung were examined morphologically. Some hearts were immediately frozen at –70 °C for the isolation of RNA as well as SL and SR membranes. The mortality of imidapril-treated animals due to heart failure was 31% whereas that of the untreated heart failure group was 64%. Imidapril treatment improved cardiac performance, attenuated cardiac remodeling, and reduced morphological changes in the heart and lung. The depressed SL Na+/K+ATPase and increased SL Na+–Ca2+exchange activities as well as reduced SR Ca2+pump and SR Ca2+release activities in the failing hearts were partially prevented by imidapril. Although changes in gene expression for SL Na+/K+ATPase isoforms as well as Na+–Ca2+exchanger and SR phospholamban were attenuated by treatments with imidapril, no alterations in mRNA levels for SR Ca2+pump proteins and Ca2+release channels were seen in the untreated or treated rats with heart failure. These results suggest that the beneficial effects of imidapril in CHF may be due to improvements in cardiac performance and changes in SL gene expression.Key words: sarcolemmal Na+/K+ATPase, Na+–Ca2+exchange, sarcoplasmic reticulum, heart failure, ACE inhibition.

Partial prevention of changes in SR gene expression in congestive heart failure due to myocardial infarction by enalapril or losartan

Although activation of the renin-angiotensin system (RAS) is known to produce ventricular remodeling and congestive heart failure (CHF), its role in inducing changes in the sarcoplasmic reticulum (SR) protein and gene expression in CHF is not fully understood. In this study, CHF was induced in rats by ligation of the left coronary artery for 3 weeks and then the animals were treated orally with or without an angiotensin converting enzyme inhibitor, enalapril (10 mg/kg/day) or an angiotensin II receptor antagonist, losartan (20 mg/kg/day) for 4 weeks. Sham-operated animals were used as control. The animals were hemodynamically assessed and protein content as well as gene expression of SR Ca(2+)-release channel (ryanodine receptor, RYR), Ca(2+)-pump ATPase (SERCA2), phospholamban (PLB) and calsequestrin (CQS) were determined in the left ventricle (LV). The infarcted animals showed cardiac hypertrophy, lung congestion, depression in LV +dP/dt and -dP/dt, as well as increase in LV end diastolic pressure. Both protein content and mRNA levels for RYR, SERCA2 and PLB were decreased without any changes in CQS in the failing heart. These alterations in LV function as well as SR protein and gene expression in CHF were partially prevented by treatment with enalapril or losartan. The results suggest that partial improvement in LV function by enalapril and losartan treatments may be due to partial prevention of changes in SR protein and gene expression in CHF and that these effects may be due to blockade of the RAS.

TRANSCRIPTIONAL REGULATION OF CARDIAC SARCOPLASMIC RETICULUM CALCIUM-ATPase GENE DURING THE PROGRESSION OF SEPSIS

Changes in sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) gene expression in the rat heart during different phases of sepsis were studied. Sepsis was induced by cecal ligation and puncture (CLP). Septic rats were divided into two groups: the early hyperdynamic (9 h after CLP, early sepsis) and the late hypodynamic (18 h after CLP; late sepsis) groups. Western blot analyses reveal that SERCA2a protein level remained unaltered during early sepsis but was decreased by 59% during late sepsis. Northern blot analyses show that the steady-state level of SERCA2a mRNA stayed unchanged during the early phase but was decreased by 43% during the late phase of sepsis. Nuclear runoff assays show that the transcription rate of SERCA2a gene transcript remained unaffected during early sepsis but was decreased by 34% during late sepsis. The actinomycin D pulse-chase studies indicate that the half-life of SERCA2a mRNA was unaffected during the early and the late phases of sepsis. These findings demonstrate that during the early phase of sepsis, the protein level, the mRNA abundance, and the transcription rate of SERCA2a remained unaltered, whereas during the late phase of sepsis, the rate of transcription of SERCA2a was decreased, and the decreased transcription rate was associated with decreases in SERCA2a mRNA abundance and SERCA2a protein level in the rat heart. Based on these data, it is concluded that SERCA2a gene expression decreased during the late phase of sepsis in the rat heart and that the decreased expression was regulated at the transcriptional level.

Constitutive Cardiac Overexpression of Sarcoplasmic/Endoplasmic Reticulum Ca 2+ -ATPase Delays Myocardial Failure After Myocardial Infarction in Rats at a Cost of Increased Acute Arrhythmias

Background— Heart failure often complicates myocardial infarction (MI), and sarcoplasmic/endoplasmic reticulum Ca 2+ -ATPase (SERCA2a) is underexpressed in the failing myocardium. We examined the effect of preexisting cardiac SERCA2a protein overexpression on rat survival and left ventricular (LV) remodeling after MI.

Methods and Results— Baseline myocardial SERCA2a expression was 37% higher in transgenic (TG) rats than in their wild-type (WT) controls, consistent with enhanced myocardial function. The mortality rate of TG rats during the 24 hours after surgical MI was higher than that of WT rats (71% versus 35%, P <0.001), associated with a higher frequency of ventricular arrhythmias, and was normalized by lidocaine treatment. The increased acute-phase mortality in TG rats was not accompanied by increased 6-month mortality. Function of the noninfarcted myocardium, as assessed by tissue Doppler imaging, was higher in TG rats than in WT rats for up to 1 month after MI, a beneficial effect no longer observed at 3 months. LV remodeling and global function were similar in TG and WT rats. No difference in papillary muscle function was found at 6 months.

Conclusions— Constitutive cardiac SERCA2a overexpression has a transient beneficial effect on remote myocardium function in rat MI, with no improvement in LV global function or prevention of LV remodeling and failure. This benefit is associated with a higher risk of acute mortality, which is prevented by lidocaine treatment.

Mechanical load-dependent regulation of gene expression in monocrotaline-induced right ventricular hypertrophy in the rat

Rats treated with monocrotaline (MCT) develop pulmonary hypertension. Their right ventricles (RVs) exhibit severe pressure overload-induced hypertrophy, whereas the left ventricles (LVs) are normally loaded. In contrast, enhanced neuroendocrine stimulation during the transition to heart failure affects both ventricles. We assessed gene expression levels of Ca2+-regulating proteins in RVs and LVs of control and MCT rats in transition to heart failure to identify biomechanical load-regulated genes. In MCT RVs, both mRNA and protein levels of the Ca2+-ATPase of the sarcoplasmic/endoplasmic reticulum (SERCA2a) were reduced by 36% (P=0.001) and 17% (P=0.016), respectively, compared with control RVs. Phospholamban and ryanodine receptor mRNA levels likewise were reduced (by 27% [P=0.05] and 21% [P=0.011], respectively) in MCT RVs, whereas sarcolemmal Na+-Ca2+ exchanger expression was not altered. MCT LVs exhibited no significant expression changes compared with control LVs. Isometrically contracting MCT intact RV trabeculae showed enhanced baseline force development. Although control RV preparations exhibited a positive force-frequency relationship, MCT RVs showed a negative force-frequency relationship and blunted postrest potentiation. Contractile function of MCT LV trabeculae was normal. Maximum Ca2+-activated tension was enhanced by 64% in permeabilized RV MCT preparations (P=0.013). beta-Myosin heavy chain protein was upregulated in MCT RVs (P<0.001) but unaltered in MCT LVs. Degradation of troponin T was prominent in MCT RVs, a phenomenon not observed in the LV. Enhanced biomechanical load is necessary to induce the gene expression changes associated with the hypertrophic phenotype of the pressure-overloaded RV. Neuroendocrine factors, which equally affect both chambers, are not sufficient to alter the expression of Ca2+-cycling proteins.

Impaired beta-adrenergic response and decreased L-type calcium current of hypertrophied left ventricular myocytes in postinfarction heart failure

Infarct-induced heart failure is usually associated with cardiac hypertrophy and decreased -adrenergic responsiveness. However, conflicting results have been reported concerning the density of L-type calcium current (I Ca(L)), and the mechanisms underlying the decreased -adrenergic inotropic response. We determined I Ca(L) density, cytoplasmic calcium ([Ca2+]i) transients, and the effects of -adrenergic stimulation (isoproterenol) in a model of postinfarction heart failure in rats. Left ventricular myocytes were obtained by enzymatic digestion 8-10 weeks after infarction. Electrophysiological recordings were obtained using the patch-clamp technique. [Ca2+]i transients were investigated via fura-2 fluorescence. -Adrenergic receptor density was determined by [ H]-dihydroalprenolol binding to left ventricle homogenates. Postinfarction myocytes showed a significant 25% reduction in mean I Ca(L) density (5.7 0.28 vs 7.6 0.32 pA/pF) and a 19% reduction in mean peak [Ca2+]i transients (0.13 0.007 vs 0.16 0.009) compared to sham myocytes. The isoproterenol-stimulated increase in I Ca(L) was significantly smaller in postinfarction myocytes (Emax: 63.6 4.3 vs 123.3 0.9% in sham myocytes), but EC50 was not altered. The isoproterenol-stimulated peak amplitude of [Ca2+]i transients was also blunted in postinfarction myocytes. Adenylate cyclase activation through forskolin produced similar I Ca(L) increases in both groups. -Adrenergic receptor density was significantly reduced in homogenates from infarcted hearts (Bmax: 93.89 20.22 vs 271.5 31.43 fmol/mg protein in sham myocytes), while Kd values were similar. We conclude that postinfarction myocytes from large infarcts display reduced I Ca(L) density and peak [Ca2+]i transients. The response to -adrenergic stimulation was also reduced and was probably related to -adrenergic receptor down-regulation and not to changes in adenylate cyclase activity.

Effects of the AT1-receptor antagonist eprosartan on the progression of left ventricular dysfunction in dogs with heart failure

1. We examined the effects of eprosartan, an AT(1) receptor antagonist, on the progression of left ventricular (LV) dysfunction and remodelling in dogs with heart failure (HF) produced by intracoronary microembolizations (LV ejection fraction, EF 30 to 40%). 2. Dogs were randomized to 3 months of oral therapy with low-dose eprosartan (600 mg once daily, n=8), high-dose eprosartan (1200 mg once daily, n=8), or placebo (n=8). 3. In the placebo group, LV end-diastolic (EDV) and end-systolic (ESV) volumes increased after 3 months (68+/-7 vs 82+/-9 ml, P<0.004, 43+/-1 vs 58+/-7 ml, P<0.003, respectively), and EF decreased (37+/-1 vs 29+/-1%, P<0.001). In dogs treated with low-dose eprosartan, EF, EDV, and ESV remained unchanged over the course of therapy, whereas in dogs treated with high-dose eprosartan, EF increased (38+/-1 vs 42+/-1%, P<0.004) and ESV decreased (41+/-1 vs 37+/-1 ml, P<0.006), Eprosartan also decreased interstitial fibrosis and cardiomyocyte hypertrophy. 4. We conclude that eprosartan prevents progressive LV dysfunction and attenuates progressive LV remodelling in dogs with moderate HF and may be useful in treating patients with chronic HF.

Regulation of cardiac excitation-contraction coupling by action potential repolarization: role of the transient outward potassium current (I(to))

The cardiac action potential (AP) is critical for initiating and coordinating myocyte contraction. In particular, the early repolarization period of the AP (phase 1) strongly influences the time course and magnitude of the whole-cell intracellular Ca(2+) transient by modulating trans-sarcolemmal Ca(2+) influx through L-type Ca(2+) channels (I(Ca,L)) and Na-Ca exchangers (I(Ca,NCX)). The transient outward potassium current (I(to)) has kinetic properties that make it especially effective in modulating the trajectory of phase 1 repolarization and thereby cardiac excitation-contraction coupling (ECC). The magnitude of I(to) varies greatly during cardiac development, between different regions of the heart, and is invariably reduced as a result of heart disease, leading to corresponding variations in ECC. In this article, we review evidence supporting a modulatory role of I(to) in ECC through its influence on I(Ca,L), and possibly I(Ca,NCX). We also discuss differential effects of I(to) on ECC between different species, between different regions of the heart and in heart disease.

Sarcoplasmic reticulum Ca(2+) pump mRNA stability in cardiac and smooth muscle: role of the 3'-untranslated region

Stomach smooth muscle (SSM) and left ventricular muscle (LVM) express the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pump gene SERCA2. Alternative splicing yields two major isoforms, SERCA2a in LVM and slow twitch muscle and SERCA2b in SSM and most other tissues. The splices have different 3'-untranslated regions (UTR) and also encode proteins that differ slightly in their COOH-terminal domains. SERCA2 transcription rates are similar in the two tissues, yet LVM has a much higher level of SERCA2 mRNA than SSM. To understand the control of SERCA2 RNA expression, we inhibited transcription and showed that the half-life of SERCA2 mRNA is significantly longer (P < 0.05) in primary cultures of LVM cells than in SSM cells. Nuclear SERCA2 mRNA levels were also higher in LVM than in SSM. In vitro decay assays using synthetic RNA corresponding to the 3'-UTR of SERCA2a and -2b showed that nuclear extracts produced a faster decay of SERCA2 RNA than cytoplasmic extracts and that nuclear extracts produced a faster decay of SERCA2b than -2a. This was also true when the full-length native mRNA was used instead of the 3'-UTR RNA, and SERCA2b decay by cytoplasmic extracts was faster for LVM than for SSM. We propose that nuclear decay is an initial step in the control of SERCA2 RNA abundance and that this control is maintained or modulated in the cytoplasm. We discuss how these control mechanisms may be part of a control switch in cardiac development and pathophysiology.