Comparison of SERCA1 and SERCA2a expressed in COS-1 cells and cardiac myocytes

Activation of the cannabinoid type 2 (CB2) receptor improves cardiac contractile performance in fish, Brycon amazonicus

In addition to their well-known classical effects, cannabinoid CB1 and CB2 receptors have also been involvement in both deleterious and protective actions on the heart under various pathological conditions. While the potential therapeutic applications of the endocannabinoid system in the context of cardiovascular function are indeed a viable prospect, significant debate exists within the literature regarding whether CB1, CB2, or a combination of both receptors exert a favorable influence on cardiac function. Hence, the aim of this study was to investigate the effects of CB1 + CB2 or CB2 agonists on cardiac excitation-contraction (E-C) coupling, utilizing fish (Brycon amazonicus) as an experimental model. The CB2 agonist elicited marked positive inotropic and lusitropic responses in isolated ventricular myocardium, induced cyclic adenosine 3',5'-monophosphate (cAMP) production, and upregulated critical Ca2+ handling proteins, such as sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and Na+/Ca2+ exchanger (NCX). Our current study demonstrated, for the first time, that CB2 receptor activation-induced effects improved the efficiency of Ca2+ cycling, excitation-contraction coupling (E-C coupling), and cardiac performance in under physiological conditions. Hence, CB2 receptors could be considered a potential therapeutic target for modulating cardiac contractile dysfunctions.

Roles of ATP and SERCA in the Regulation of Calcium Turnover in Unloaded Skeletal Muscles: Current View and Future Directions

A decrease in skeletal muscle contractile activity or its complete cessation (muscle unloading or disuse) leads to muscle fibers’ atrophy and to alterations in muscle performance. These changes negatively affect the quality of life of people who, for one reason or another, are forced to face a limitation of physical activity. One of the key regulatory events leading to the muscle disuse-induced changes is an impairment of calcium homeostasis, which leads to the excessive accumulation of calcium ions in the sarcoplasm. This review aimed to analyze the triggering mechanisms of calcium homeostasis impairment (including those associated with the accumulation of high-energy phosphates) under various types of muscle unloading. Here we proposed a hypothesis about the regulatory mechanisms of SERCA and IP3 receptors activity during muscle unloading, and about the contribution of these mechanisms to the excessive calcium ion myoplasmic accumulation and gene transcription regulation via excitation–transcription coupling.

The Sarcoplasmic Reticulum of Skeletal Muscle Cells: A Labyrinth of Membrane Contact Sites

The sarcoplasmic reticulum of skeletal muscle cells is a highly ordered structure consisting of an intricate network of tubules and cisternae specialized for regulating Ca2+ homeostasis in the context of muscle contraction. The sarcoplasmic reticulum contains several proteins, some of which support Ca2+ storage and release, while others regulate the formation and maintenance of this highly convoluted organelle and mediate the interaction with other components of the muscle fiber. In this review, some of the main issues concerning the biology of the sarcoplasmic reticulum will be described and discussed; particular attention will be addressed to the structure and function of the two domains of the sarcoplasmic reticulum supporting the excitation-contraction coupling and Ca2+-uptake mechanisms.

Moving human muscle physiology research forward: an evaluation of fiber type-specific protein research methodologies

Human skeletal muscle is a heterogeneous tissue composed of multiple fiber types that express unique contractile and metabolic properties. While analysis of mixed fiber samples predominates and holds value, increasing attention has been directed toward studying proteins segregated by fiber type, a methodological distinction termed “fiber type-specific.” Fiber type-specific protein studies have the advantage of uncovering key molecular effects that are often missed in mixed fiber homogenate studies but also require greater time and resource-intensive methods, particularly when applied to human muscle. This review summarizes and compares current methods used for fiber type-specific protein analysis, highlighting their advantages and disadvantages for human muscle studies, in addition to recent advances in these techniques. These methods can be grouped into three categories based on the initial processing of the tissue: 1) muscle-specific fiber homogenates, 2) cross sections of fiber bundles, and 3) isolated single fibers, with various subtechniques for performing fiber type identification and protein quantification. The relative implementation for each unique methodological approach is analyzed from 83 fiber type-specific studies of proteins in live human muscle found in the literature to date. These studies have investigated several proteins involved in a wide range of cellular functions that are important to muscle tissue. The second half of this review summarizes key findings from this ensemble of fiber type-specific human protein studies. We highlight examples of where this analytical approach has helped to improve understanding of important physiological topics such as insulin sensitivity, muscle hypertrophy, muscle fatigue, and adaptation to different exercise programs.

Effects of acute warming on cardiac and myotomal sarco(endo)plasmic reticulum ATPase (SERCA) of thermally acclimated brown trout (Salmo trutta)

At high temperatures, ventricular beating rate collapses and depresses cardiac output in fish. The role of sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) in thermal tolerance of ventricular function was examined in brown trout (Salmo trutta) by measuring heart SERCA and comparing it to that of the dorsolateral myotomal muscle. Activity of SERCA was measured from crude homogenates of cold-acclimated (+ 3 °C, c.a.) and warm-acclimated (+ 13 °C, w.a.) brown trout as cyclopiazonic acid (20 µM) sensitive Ca²⁺-ATPase between + 3 and + 33 °C. Activity of the heart SERCA was significantly higher in c.a. than w.a. trout and increased strongly between + 3 and + 23 °C with linear Arrhenius plots but started to plateau between + 23 and + 33 °C in both acclimation groups. The rate of thermal inactivation of the heart SERCA at + 35 °C was similar in c.a. and w.a. fish. Activity of the muscle SERCA was less temperature dependent and more heat resistant than that of the heart SERCA and showed linear Arrhenius plots between + 3 and + 33 °C in both c.a. and w.a. fish. SERCA activity of the c.a. muscle was slightly higher than that of w.a. muscle. The rate of thermal inactivation at + 40 °C was similar for both c.a. and w.a. muscle SERCA at + 40 °C. Although the heart SERCA is more sensitive to high temperatures than the muscle SERCA, it is unlikely to be a limiting factor for heart rate, because its heat tolerance, unlike that of the ventricular beating rate, was not changed by temperature acclimation.

The effects of buthionine sulfoximine treatment on diaphragm contractility and SERCA pump function in adult and middle aged rats

This study examined the effects of 10 days of buthionine sulfoximine (BSO) treatment on in vitro contractility and sarcoplasmic reticulum calcium pump (SERCA) expression and function in adult (AD; 6–8 months old) and middle aged (MA; 14–17 months old) rat diaphragm in both the basal state and following fatiguing stimulation. BSO treatment reduced the cellular concentrations of free glutathione (GSH) by >95% and oxidized glutathione (GSSG) by >80% in both age cohorts. GSH content in AD Control diaphragm was 32% higher (P < 0.01) than in MA Control, with no differences in GSSG. The ratio of GSH:GSSG, an indicator of cellular oxidative state, was 34.6 ± 7.4 in MA Control, 52.5 ± 10.1 in AD Control, 10.6 ± 1.7 in MA BSO, and 9.5 ± 1.1 in AD BSO (BSO vs. Control, P < 0.05). Several findings suggest that the effects of BSO treatment are age dependent. AD BSO diaphragm had 26% higher twitch and 28% higher tetanic force (both P < 0.05) than AD Controls, whereas no significant difference existed between the two MA groups. In contrast to our previous work on BSO-treated AD rats, BSO treatment did not influence maximal SERCA ATPase activity in MA rat diaphragm, nor did SERCA2a expression increase in BSO-treated MA diaphragm. Biotinylated iodoacetamide binding to SERCA1a, a specific marker of free cysteine residues, was reduced by 35% (P < 0.05) in AD Control diaphragm following fatiguing stimulation, but was not reduced in any other group. Collectively, these results suggest an important role for redox regulation in both contractility and SERCA function which is influenced by aging.

The Effect of SERCA1b Silencing on the Differentiation and Calcium Homeostasis of C2C12 Skeletal Muscle Cells

The sarcoplasmic/endoplasmic reticulum Ca2+ATPases (SERCAs) are the main Ca2+ pumps which decrease the intracellular Ca2+ level by reaccumulating Ca2+ into the sarcoplasmic reticulum. The neonatal SERCA1b is the major Ca2+ pump in myotubes and young muscle fibers. To understand its role during skeletal muscle differentiation its synthesis has been interfered with specific shRNA sequence. Stably transfected clones showing significantly decreased SERCA1b expression (cloneC1) were selected for experiments. The expression of the regulatory proteins of skeletal muscle differentiation was examined either by Western-blot at the protein level for MyoD, STIM1, calsequestrin (CSQ), and calcineurin (CaN) or by RT-PCR for myostatin and MCIP1.4. Quantitative analysis revealed significant alterations in CSQ, STIM1, and CaN expression in cloneC1 as compared to control cells. To examine the functional consequences of the decreased expression of SERCA1b, repeated Ca2+-transients were evoked by applications of 120 mM KCl. The significantly higher [Ca2+]i measured at the 20th and 40th seconds after the beginning of KCl application (112±3 and 110±3 nM vs. 150±7 and 135±5 nM, in control and in cloneC1 cells, respectively) indicated a decreased Ca2+-uptake capability which was quantified by extracting the maximal pump rate (454±41 μM/s vs. 144±24 μM/s, in control and in cloneC1 cells). Furthermore, the rate of calcium release from the SR (610±60 vs. 377±64 μM/s) and the amount of calcium released (843±75 μM vs. 576±80 μM) were also significantly suppressed. These changes were also accompanied by a reduced activity of CaN in cells with decreased SERCA1b. In parallel, cloneC1 cells showed inhibited cell proliferation and decreased myotube nuclear numbers. Moreover, while cyclosporineA treatment suppressed the proliferation of parental cultures it had no effect on cloneC1 cells. SERCA1b is thus considered to play an essential role in the regulation of [Ca2+]i and its ab ovo gene silencing results in decreased skeletal muscle differentiation.

A structural mechanism for calcium transporter headpiece closure

To characterize the conformational dynamics of sarcoplasmic reticulum (SR) calcium pump (SERCA) we performed molecular dynamics simulations beginning with several different high-resolution structures. We quantified differences in structural disorder and dynamics for an open conformation of SERCA versus closed structures and observed that dynamic motions of SERCA cytoplasmic domains decreased with decreasing domain–domain separation distance. The results are useful for interpretation of recent intramolecular Förster resonance energy transfer (FRET) distance measurements obtained for SERCA fused to fluorescent protein tags. Those previous physical measurements revealed several discrete structural substates and suggested open conformations of SERCA are more dynamic than compact conformations. The present simulations support this hypothesis and provide additional details of SERCA molecular mechanisms. Specifically, all-atoms simulations revealed large-scale translational and rotational motions of the SERCA N-domain relative to the A- and P-domains during the transition from an open to a closed headpiece conformation over the course of a 400 ns trajectory. The open-to-closed structural transition was accompanied by a disorder-to-order transition mediated by an initial interaction of an N-domain loop (Nβ5-β6, residues 426–436) with residues 133–139 of the A-domain. Mutation of three negatively charged N-domain loop residues abolished the disorder-to-order transition and prevented the initial domain–domain interaction and subsequent closure of the cytoplasmic headpiece. Coarse-grained molecular dynamics simulations were in harmony with all-atoms simulations and physical measurements and revealed a close communication between fluorescent protein tags and the domain to which they were fused. The data indicate that previous intramolecular FRET distance measurements report SERCA structure changes with high fidelity and suggest a structural mechanism that facilitates the closure of the SERCA cytoplasmic headpiece.

Sarcoplasmic reticulum Ca2+ uptake and leak properties, and SERCA isoform expression, in type I and type II fibres of human skeletal muscle

The Ca(2+) uptake properties of the sarcoplasmic reticulum (SR) were compared between type I and type II fibres of vastus lateralis muscle of young healthy adults. Individual mechanically skinned muscle fibres were exposed to solutions with the free [Ca(2+)] heavily buffered in the pCa range (-log10[Ca(2+)]) 7.3-6.0 for set times and the amount of net SR Ca(2+) accumulation determined from the force response elicited upon emptying the SR of all Ca(2+). Western blotting was used to determine fibre type and the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) isoform present in every fibre examined. Type I fibres contained only SERCA2 and displayed half-maximal Ca(2+) uptake rate at ∼pCa 6.8, whereas type II fibres contained only SERCA1 and displayed half-maximal Ca(2+) uptake rate at ∼pCa 6.6. Maximal Ca(2+) uptake rate was ∼0.18 and ∼0.21 mmol Ca(2+) (l fibre)(-1) s(-1) in type I and type II fibres, respectively, in good accord with previously measured SR ATPase activity. Increasing free [Mg(2+)] from 1 to 3 mM had no significant effect on the net Ca(2+) uptake rate at pCa 6.0, indicating that there was little or no calcium-induced calcium release occurring through the Ca(2+) release channels during uptake in either fibre type. Ca(2+) leakage from the SR at pCa 8.5, which is thought to occur at least in part through the SERCA, was ∼2-fold lower in type II fibres than in type I fibres, and was little affected by the presence of ADP, in marked contrast to the larger SR Ca(2+) leak observed in rat muscle fibres under the same conditions. The higher affinity of Ca(2+) uptake in the type I human fibres can account for the higher relative level of SR Ca(2+) loading observed in type I compared to type II fibres, and the SR Ca(2+) leakage characteristics of the human fibres suggest that the SERCAs are regulated differently from those in rat and contribute comparatively less to resting metabolic rate.

Endoplasmic reticulum Ca(2+) handling in excitable cells in health and disease

The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle capable of integrating multiple extracellular and internal signals and generating adaptive cellular responses. It plays fundamental roles in protein synthesis and folding and in cellular responses to metabolic and proteotoxic stress. In addition, the ER stores and releases Ca(2+) in sophisticated scenarios that regulate a range of processes in excitable cells throughout the body, including muscle contraction and relaxation, endocrine regulation of metabolism, learning and memory, and cell death. One or more Ca(2+) ATPases and two types of ER membrane Ca(2+) channels (inositol trisphosphate and ryanodine receptors) are the major proteins involved in ER Ca(2+) uptake and release, respectively. There are also direct and indirect interactions of ER Ca(2+) stores with plasma membrane and mitochondrial Ca(2+)-regulating systems. Pharmacological agents that selectively modify ER Ca(2+) release or uptake have enabled studies that revealed many different physiological roles for ER Ca(2+) signaling. Several inherited diseases are caused by mutations in ER Ca(2+)-regulating proteins, and perturbed ER Ca(2+) homeostasis is implicated in a range of acquired disorders. Preclinical investigations suggest a therapeutic potential for use of agents that target ER Ca(2+) handling systems of excitable cells in disorders ranging from cardiac arrhythmias and skeletal muscle myopathies to Alzheimer disease.

Silencing calcineurin A subunit reduces SERCA2 expression in cardiac myocytes

Resting intracellular Ca(2+) can be raised, in neonatal rat cardiac myocytes, by exposure to very low concentration of thapsigargin (TG). Such a Ca(2+) rise yields calcineurin (CN) activation demonstrated by increased expression of transfected luciferase cDNA under control of nuclear factor of activated T-cells (NFAT) promoter and increased translocation of NFAT to nuclei. We found that exposure of cardiac myocytes to TG is followed by increase of sarcroplasmic reticulum Ca(2+) transport ATPase (SERCA2) expression, which is further increased when CN inactivation by CAMKII (calmodulin-dependent kinase) is prevented with KN93 (CAMKII inhibitor). On the other hand, SERCA2 expression is reduced by CN inhibition with cyclosporine. We have now induced calcineurin A (CNA) α- or β-subunit gene silencing with small interfering RNA (siRNA) and observed strong interference with expression of SERCA2, both in control myocytes and following exposure to TG. Such interference is also obtained following NFAT displacement from CN with 9,10-dihydro-9,10[1',2']-benzenoanthracene-1,4-dione (INCA-6). We have also observed analogous effects on expression of phospholamban (PLB) and Na(+)/Ca(2+) exchanger (NCX). Pertinent to these findings, we have identified, by in-silico analysis, NFAT binding sites in SERCA2, PLB, and NCX1 promoters. Our experiments indicate that activation of the calcineurin-NFAT pathway by rise of resting cytosolic Ca(2+) elevates transcription/expression of SERCA2, PLB, and NCX1, providing a homeostatic mechanism for long-term control of cytosolic Ca(2+).

Regulation of myocardial SERCA2a expression in ventricular hypertrophy and heart failure

Diminished contractility of the hypertrophic cardiomyocyte is a principal determinant of ventricular dysfunction in chronic heart failure. Reduction of activity of the sarcoplasmic/endoplasmic reticulum calcium ion (Ca2+)-ATPase (SERCA2a), underlies many of the effects of overload-induced hypertrophy on cardiomyocyte performance, and it may be critical in the progression of compensatory hypertrophy to heart failure. This review shall focus on the transcriptional regulation of SERCA2a expression as the primary cause of decreased SERCA2a activity in heart failure. Furthermore, the relevance for SERCA2a expression of signal transduction routes involved in pathologic hypertrophy and the possible therapeutic implications, shall be addressed.

Effects of thapsigargin and phenylephrine on calcineurin and protein kinase C signaling functions in cardiac myocytes

Neonatal rat cardiac myocytes were exposed to 10 nM thapsigargin (TG) or 20 muM phenylephrine (PE) to compare resulting alterations of Ca(2+) homeostasis. Either treatment results in resting cytosolic [Ca(2+)] rise and reduction of Ca(2+) signals in myocytes following electrical stimuli. In fact, ATP-dependent Ca(2+) transport is reduced due to catalytic inhibition of sarcoplasmic reticulum ATPase (SERCA2) by TG or reduction of SERCA2 protein expression by PE. A marked rise of nuclear factor of activated T cells (NFAT)-dependent expression of transfected luciferase cDNA is produced by TG or PE, which is dependent on increased NFAT dephosphorylation by activated calcineurin and reduced phosphorylation by inactivated glycogen synthase kinase 3beta. Expression of SERCA2 (inactivated) protein is increased following exposure to TG, whereas no hypertrophy is produced. On the contrary, SERCA2 expression is reduced, despite high CN activity, following protein kinase C (PKC) activation by PE (or phorbol 12-myristate 13-acetate) under conditions producing myocyte hypertrophy. Both effects of TG and PE are dependent on NFAT dephosphorylation by CN, as demonstrated by CN inhibition with cyclosporine (CsA). However, the hypertrophy program triggered by PKC activation bypasses SERCA2 transcription and expression due to competitive recruitment of NFAT and/or other transcriptional factors. A similar dependence on CN activation, but relative reduction under conditions of PKC activation, involves transcription and expression of the Na(+)/Ca(2+) exchanger-1. On the other hand, significant upregulation of transient receptor potential channel proteins is noted following PKC activation. The observed alterations of Ca(2+) homeostasis may contribute to development of contractile failure.

Limited functional and metabolic improvements in hypertrophic and healthy rat heart overexpressing the skeletal muscle isoform of SERCA1 by adenoviral gene transfer in vivo

Adenoviral gene transfer of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2a to the hypertrophic heart in vivo has been consistently reported to lead to enhanced myocardial contractility. It is unknown if the faster skeletal muscle isoform, SERCA1, expressed in the whole heart in early failure, leads to similar improvements and whether metabolic requirements are maintained during an adrenergic challenge. In this study, Ad.cmv.SERCA1 was delivered in vivo to aortic banded and sham-operated Sprague-Dawley rat hearts. The total SERCA content increased 34%. At 48-72 h posttransfer, echocardiograms were acquired, hearts were excised and retrograded perfused, and hemodynamics were measured parallel to NMR measures of the phosphocreatine (PCr)-to-ATP ratio (PCr/ATP) and energy substrate selection at basal and high workloads (isoproterenol). In the Langendorff mode, the rate-pressure product was enhanced 27% with SERCA1 in hypertrophic hearts and 10% in shams. The adrenergic response to isoproterenol was significantly potentiated in both groups with SERCA1. 31P NMR analysis of PCr/ATP revealed that the ratio remained low in the hypertrophic group with SERCA1 overexpression and was not further compromised with adrenergic challenge. 13C NMR analysis revealed fat and carbohydrate oxidation were unaffected at basal with SERCA1 expression; however, there was a shift from fats to carbohydrates at higher workloads with SERCA1 in both groups. Transport of NADH-reducing equivalents into the mitochondria via the alpha-ketoglutamate-malate transporter was not affected by either SERCA1 overexpression or adrenergic challenge in both groups. Echocardiograms revealed an important distinction between in vivo versus ex vivo data. In contrast to previous SERCA2a studies, the echocardiogram data revealed that SERCA1 expression compromised function (fractional shortening) in the hypertrophic group. Shams were unaffected. While our ex vivo findings support much of the earlier cardiomyocyte and transgenic data, the in vivo data challenge previous reports of improved cardiac function in heart failure models after SERCA intervention.

Expression of SERCA isoform with faster Ca2+ transport properties improves postischemic cardiac function and Ca2+ handling and decreases myocardial infarction

Myocardial ischemia-reperfusion (I/R) injury is associated with contractile dysfunction, arrhythmias, and myocyte death. Intracellular Ca(2+) overload with reduced activity of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) is a critical mechanism of this injury. Although upregulation of SERCA function is well documented to improve postischemic cardiac function, there are conflicting reports where pharmacological inhibition of SERCA improved postischemic function. SERCA2a is the primary cardiac isoform regulating intracellular Ca(2+) homeostasis; however, SERCA1a has been shown to substitute SERCA2a with faster Ca(2+) transport kinetics. Therefore, to further address this issue and to evaluate whether SERCA1a expression could improve postischemic cardiac function and myocardial salvage, in vitro and in vivo myocardial I/R studies were performed on SERCA1a transgenic (SERCA1a(+/+)) and nontransgenic (NTG) mice. Langendorff-perfused hearts were subjected to 30 min of global ischemia followed by reperfusion. Baseline preischemic coronary flow and left ventricular developed pressure were significantly greater in SERCA1a(+/+) mice compared with NTG mice. Independent of reperfusion-induced oxidative stress, SERCA1a(+/+) hearts demonstrated greatly improved postischemic (45 min) contractile recovery with less persistent arrhythmias compared with NTG hearts. Morphometry showed better-preserved myocardial structure with less infarction, and electron microscopy demonstrated better-preserved myofibrillar and mitochondrial ultrastructure in SERCA1a(+/+) hearts. Importantly, intraischemic Ca(2+) levels were significantly lower in SERCA1a(+/+) hearts. The cardioprotective effect of SERCA1a was also observed during in vivo regional I/R with reduced myocardial infarct size after 24 h of reperfusion. Thus SERCA1a(+/+) hearts were markedly protected against I/R injury, suggesting that expression of SERCA 1a isoform reduces postischemic Ca(2+) overload and thus provides potent myocardial protection.

Phenylephrine hypertrophy, Ca2+-ATPase (SERCA2), and Ca2+ signaling in neonatal rat cardiac myocytes

We endeavored to use a basic and well-controlled experimental system to characterize the extent and time sequence of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) involvement in the development of cardiac hypertrophy, including transcription, protein expression, Ca(2+) transport, and cytoplasmic Ca(2+) signaling. To this end, hypertrophy of neonatal rat cardiac myocytes in culture was obtained after adrenergic activation with phenylephrine (PE). Micrographic assessment of myocyte size, rise of [(14)C]phenylalanine incorporation and total protein expression, and increased transcription of atrial natriuretic factor demonstrated unambiguously the occurrence of hypertrophy. An early and prominent feature of hypertrophy was a reduction of the SERCA2 transcript, as determined by RT-PCR with reference to a stable marker such as glyceraldehyde-3-phosphate dehydrogenase. Reduction of Ca(2+)-ATPase protein levels and Ca(2+) transport activity to approximately 50% of control values followed with some delay, evidently as a consequence of a primary effect on transcription. Cytosolic Ca(2+) signaling kinetics, measured with a Ca(2+)-sensitive dye after electrical stimuli, were significantly altered in hypertrophic myocytes. However, the effect of PE hypertrophy on cytosolic Ca(2+) signaling kinetics was less prominent than observed in myocytes subjected to drastic SERCA2 downregulation with small interfering RNA or inhibition with thapsigargin (10 nM). We conclude that SERCA2 undergoes significant downregulation after hypertrophic stimuli, possibly due to lack of SERCA gene involvement by the hypertrophy transcriptional program. The consequence of SERCA2 downregulation on Ca(2+) signaling is partially compensated by alternate Ca(2+) transport mechanisms. These alterations may contribute to a gradual onset of functional failure in long-term hypertrophy.

SERCA overexpression reduces hydroxyl radical injury in murine myocardium

Hydroxyl radicals (·OH) are involved in the pathogenesis of ischemia-reperfusion injury and are observed in clinical situations, including acute heart failure, stroke, and myocardial infarction. Acute transient exposure to ·OH causes an intracellular Ca2+overload and leads to impaired contractility. We investigated whether upregulation of sarcoplasmic reticulum Ca2+-ATPase function (SERCA) can attenuate ·OH-induced dysfunction. Small, contracting right ventricular papillary muscles from wild-type (WT) SERCA1a-overexpressing (transgenic, TG) and SERCA2a heterogeneous knockout (HET) mice were directly exposed to ·OH. This brief 2-min exposure led to a transient elevation of diastolic force (Fdia) and depression of developed force (Fdev). In WT mice, Fdiaincreased to 485 ± 49% and Fdevdecreased to 11 ± 3%. In sharp contrast, in TG mice Fdiaincreased only to 241 ± 17%, whereas Fdevdecreased only to 51 ± 5% ( P < 0.05 vs. WT). In HET mice, Fdiarose more than WT (to 597 ± 20%, P < 0.05), whereas Fdevwas reduced in a similar amount. After ∼45 min after ·OH exposure, a new steady state was reached: Fdevreturned to 37 ± 6% and 32 ± 6%, whereas Fdiacame back to 238 ± 28% and 292 ± 17% in WT and HET mice, respectively. In contrast, the sustained dysfunction was significantly less in TG mice: Fdiaand Fdevreturned to 144 ± 20% and 67 ± 6%, respectively. Before exposure to ·OH, there is decrease in phospholamban (PLB) phosphorylation at Ser16 (pPLBSer16) and PLB phosphorylation at Thr17 (pPLBThr17) in TG mice and an increase in pPLBSer16 and pPLBThr17 in HET mice versus WT. After exposure to ·OH there is decrease in pPLBSer16 in WT, TG, and HET mice but no significant change in the level of pPLBThr17 in any group. The results indicate that SERCA overexpression can reduce the ·OH-induced contractile dysfunction in murine myocardium, whereas a reduced SR Ca2+-ATPase activity aggravates this injury. Loss of pPLB levels at Ser16 likely amplifies the differences observed in injury response.

Doxycycline inducible expression of SERCA2a improves calcium handling and reverts cardiac dysfunction in pressure overload-induced cardiac hypertrophy

Delayed cardiac relaxation in failing hearts has been attributed to reduced activity and/or expression of sarco(endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a). Although constitutive overexpression of SERCA2a has proven effective in preventing cardiac dysfunction, it is unclear whether increasing SERCA2a expression in hearts with preexisting hypertrophy will be therapeutic. To test this hypothesis, we generated a binary transgenic (BTG) system that allows tetracycline-inducible, cardiac-specific SERCA2a expression. In this system (tet-on SERCA2a), a FLAG-tagged SERCA2a transgene is expressed in the presence of doxycycline (Dox) but not in the absence of Dox (2.3-fold more mRNA, 45% more SERCA2a protein). Calcium transients measured in isolated cardiac myocytes from nonbanded Dox-treated BTG mice showed an accelerated calcium decline and an increased systolic Ca2+ peak. Sarcoplasmic reticulum (SR) calcium loading was increased by 45% in BTG mice. In the presence of pressure overload (aortic banding), echocardiographic analysis revealed that expression of SERCA2a-FLAG caused an improvement in fractional shortening. SERCA2a-FLAG expression alleviated the resultant cardiac dysfunction. This was illustrated by an increase in the rate of decline of the calcium transient. Cell shortening and SR calcium loading were also improved in cardiac myocytes isolated from banded BTG mice after SERCA2a overexpression. In conclusion, we generated a novel transgenic mouse that conditionally overexpresses SERCA2a. This model is suitable for both long- and short-term studies of the effects of controlled SERCA2a expression on cardiac function. In addition, inducible overexpression of SERCA2a improved cardiac function and calcium handling in mice with established contractile dysfunction.

Cell-specific expression of SERCA, the exogenous Ca2+transport ATPase, in cardiac myocytes

We evaluated various constructs to obtain cell-specific expression of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) gene in cardiac myocytes after cDNA transfer by means of transfections or infections with adenovirus vectors. Expression of exogenous enhanced green fluorescent protein (EGFP) and SERCA genes was studied in cultured chicken embryo and neonatal rat cardiac myocytes, skeletal and smooth muscle cells, fibroblasts, and hepatocytes. Whereas the cytomegalovirus (CMV) promoter yielded high levels of protein expression in all cells studied, cardiac troponin T (cTnT) promoter segments demonstrated high specificity for cardiac myocytes. Their efficiency for protein expression was lower than that of the CMV promoter, but higher than that of cardiac myosin light chain or β-myosin heavy chain promoter segments. A double virus system for Cre-dependent expression under control of the CMV promoter and Cre expression under control of a cardiac-specific promoter yielded high protein levels in cardiac myocytes, but only partial cell specificity due to significant Cre expression in hepatocytes. Specific intracellular targeting of gene products was demonstrated in situ by specific immunostaining of exogenous SERCA1 and endogenous SERCA2 and comparative fluorescence microscopy. The -374 cTnT promoter segment was the most advantageous of the promoters studied, producing cell-specific SERCA expression and a definite increase over endogenous Ca2+-ATPase activity as well as faster removal of cytosolic calcium after membrane excitation. We conclude that analysis of promoter efficiency and cell specificity is of definite advantage when cell-specific expression of exogenous SERCA is wanted in cardiac myocytes after cDNA delivery to mixed cell populations.

Altered dose response to beta-agonists in SERCA1a-expressing hearts ex vivo and in vivo

In this study we evaluated the contractile characteristics of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)1a-expressing hearts ex vivo and in vivo and in particular their response to beta-adrenergic stimulation. Analysis of isolated, work-performing hearts revealed that transgenic (TG) hearts develop much higher maximal rates of contraction and relaxation than wild-type (WT) hearts. Addition of isoproterenol only moderately increased the maximal rate of relaxation (+20%) but did not increase contractility or decrease relaxation time in TG hearts. Perfusion with varied buffer Ca(2+) concentrations indicated an altered dose response to Ca(2+). In vivo TG hearts displayed fairly higher maximal rates of contraction (+ 25%) but unchanged relaxation parameters and a blunted but significant response to dobutamine. Our study also shows that the phospholamban (PLB) level was decreased (-40%) and its phosphorylation status modified in TG hearts. This study clearly demonstrates that increases in SERCA protein level alter the beta-adrenergic response and affect the phosphorylation of PLB. Interestingly, the overall cardiac function in the live animal is only slightly enhanced, suggesting that (neuro)hormonal regulations may play an important role in controlling in vivo heart function.

Dual gene therapy with SERCA1 and Kir2.1 abbreviates excitation without suppressing contractility

Heart failure is characterized by depressed contractility and delayed repolarization. The latter feature predisposes the failing heart to ventricular arrhythmias and represents a logical target for gene therapy. Unfortunately, unopposed correction of the delay in repolarization will decrease the time available for calcium cycling during each heartbeat, potentially aggravating the depression of contractility. Here we describe the development and application of a novel gene therapy strategy designed to abbreviate excitation without depressing contraction. The calcium ATPase SERCA1 was coexpressed with the potassium channel Kir2.1 in guinea pig hearts. Myocytes from the hearts had bigger calcium transients and shorter action potentials. In vivo, repolarization was abbreviated, but contractile function remained unimpaired. Dual gene therapy of the sort described here can be generalized to exploit opposing or synergistic therapeutic principles to achieve a tailored phenotype.

Exogenous Ca2+-ATPase isoform effects on Ca2+ transients of embryonic chicken and neonatal rat cardiac myocytes

1. Sarco-endoplasmic reticulum Ca2+-ATPase from fast skeletal (SERCA1) or cardiac muscle (SERCA2a) was expressed in embryonic chicken and neonatal rat cardiac myocytes by adenovirus vectors, with c-myc tags on both constructs to compare expression and distinguish exogenous from endogenous SERCA2a in myocytes. 2. Expression of the two isoforms was similar (approximately 3-fold higher than endogenous SERCA). However, SERCA1 activity was 2-fold greater than SERCA2a activity, due to intrinsic differences in turnover rates. Activation of both exogenous SERCA isoforms by Ca2+ was displaced to slightly lower [Ca2+], suggesting that the overexpressed isoforms were independent of phospholamban. In fact, phospholamban and calsequestrin expression were unchanged. 3. Decay time constants of cytosolic Ca2+ transients from cells overexpressing SERCA1 were reduced by 30-40 % and half-widths by 10-15 % compared to controls. SERCA2a overexpression produced much less acceleration of transients in chick than in rat, and less acceleration than SERCA1 overexpression in either species. There was no significant change in resting [Ca2+], peak amplitudes, or in the amount of Ca2+ releasable by caffeine from overexpression of either SERCA isoform. However, the amplitudes of the transients increased with SERCA1 overexpression when pacing frequency limited refilling of the sarcoplasmic reticulum. 4. It is concluded that total SERCA transport velocity has a primary effect on the decay phase of transients. Transport velocity is affected by SERCA isoform turnover rate, temperature, and/or SERCA copy number.