Transcription of the SERCA2 gene is decreased in pressure-overloaded hearts: A study using in vivo direct gene transfer into living myocardium

New insights into SERCA2a gene therapy in heart failure: pay attention to the negative effects of B-type natriuretic peptides

Sarcoplasmic/endoplasmic reticulum calcium ATPase 2a (SERCA2a) is a target of interest in gene therapy for heart failure with reduced ejection fraction (HFrEF). However, the results of an important clinical study, the Calcium Upregulation by Percutaneous Administration of Gene Therapy in Cardiac Disease (CUPID) trial, were controversial. Promising results were observed in the CUPID 1 trial, but the results of the CUPID 2 trial were negative. The factors that caused the controversial results remain unclear. Importantly, enrolled patients were required to have a higher plasma level of B-type natriuretic peptide (BNP) in the CUPID 2 trial. Moreover, BNP was shown to inhibit SERCA2a expression. Therefore, it is possible that high BNP levels interact with treatment effects of SERCA2a gene transfer and accordingly lead to negative results of CUPID 2 trial. From this point of view, effects of SERCA2a gene therapy should be explored in heart failure with preserved ejection fraction, which is characterised by lower BNP levels compared with HFrEF. In this review, we summarise the current knowledge of SERCA2a gene therapy for heart failure, analyse potential interaction between BNP levels and therapeutic effects of SERCA2a gene transfer and provide directions for future research to solve the identified problems.

Pressure mediated hypertrophy and mechanical stretch up-regulate expression of the long form of leptin receptor (ob-Rb) in rat cardiac myocytes

Background

Hyperleptinemia is known to participate in cardiac hypertrophy and hypertension, but the relationship between pressure overload and leptin is poorly understood. We therefore examined the expression of leptin (ob) and the leptin receptor (ob-R) in the pressure-overloaded rat heart. We also examined gene expressions in culture cardiac myocytes to clarify which hypertension-related stimulus induces these genes.

Results

Pressure overload was produced by ligation of the rat abdominal aorta, and ob and ob-R isoform mRNAs were measured using a real-time polymerase chain reaction (PCR). We also measured these gene expressions in neonatal rat cardiac myocytes treated with angiotensin II (ANGII), endothelin-1 (ET-1), or cyclic mechanical stretch. Leptin and the long form of the leptin receptor (ob-Rb) gene were significantly increased 4 weeks after banding, but expression of the short form of the leptin receptor (ob-Ra) was unchanged. ob-Rb protein expression was also detected by immunohistochemistry in hypertrophied cardiac myocytes after banding. Meanwhile, plasma leptin concentrations were not different between the control and banding groups. In cultured myocytes, ANGII and ET-1 increased only ob mRNA expression. However, mechanical stretch activated both ob and ob-Rb mRNA expression in a time-dependent manner, but ob-Ra mRNA was unchanged by any stress.

Conclusions

We first demonstrated that both pressure mediated hypertrophy and mechanical stretch up-regulate ob-Rb gene expression in heart and cardiac myocytes, which are thought to be important for leptin action in cardiac myocytes. These results suggest a new local mechanism by which leptin affects cardiac remodeling in pressure-overloaded hearts.

Early cardiac dysfunction is rescued by upregulation of SERCA2a pump activity in a rat model of metabolic syndrome

AIM

Various components of metabolic syndrome associate with cardiac intracellular calcium (Cai 2+) mishandling, a precipitating factor in the development of heart failure. We aimed to provide a thorough description of early stage Cai 2+-cycling alterations in the fructose-fed rat, an experimental model of the disorder, where insulin resistance, hypertension and dyslipidaemia act cooperatively on the heart.

METHOD

Rats were fed with fructose-rich chow. After 6 weeks, echocardiography was performed, which was followed by measurements of myocardial Cai 2+ transients recorded by Indo-1 surface fluorometry in isolated perfused hearts. Sarcoplasmic reticulum (SR) Ca(2+) -ATPase (SERCA2a) activity was assessed by administration of its inhibitor cyclopiazonic acid (CPA). Mathematical model analysis of Cai 2+ transients was used to estimate kinetic properties of SR Ca(2+) transporters. Protein levels of key Ca(2+) handling proteins were also measured.

RESULTS

Echocardiography showed signs of cardiac hypertrophy, but in vivo and ex vivo haemodynamic performance of fructose-fed rat hearts were unaltered. However, a decline in Ca(2+) sequestration capacity (-dCai 2+/dt and decay time of Cai 2+ transients) was observed. Model estimation showed decreased affinity for Ca(2+) (higher K(m) ) and elevated V(max) for SERCA2a. Diseased hearts were more vulnerable to CPA application. Fructose feeding caused elevation in SERCA2a and phosphorylated phospholamban (PLB) expression, while total PLB level remained unchanged.

CONCLUSION

In early stage, metabolic syndrome primarily disturbs SERCA2a function in the heart, but consequential haemodynamic dysfunction is prevented by upregulation of SERCA2a protein level and phosphorylation pathways regulating PLB. However, this compensated state is very vulnerable to a further decline in SERCA2a function.

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.

Pulmonary artery banding alters the expression of Ca2+ transport proteins in the right atrium in rabbits

Following pulmonary artery banding (PAB), the contractile function of right ventricle diminishes over time. Subsequently, the right atrium (RA) has to contract against a higher afterload, but it is unknown to what extent ventricular dysfunction has an effect on the atrial contractility. We hypothesized that right ventricular pressure overload may have an affect on atrial contractility and Ca(2+) transport protein expression. Therefore, we induced pressure overload of the right ventricle by PAB for 10 wk in rabbits and examined the changes in the expression of Ca(2+) transport proteins in the atrium. We demonstrate that PAB significantly decreased the expression of sarco(endo)plasmic reticulum Ca(2+)-ATPase (Serca) 2a while expression of Na(+)/Ca(2+) exchanger-1 was significantly upregulated in the RA but not in the left atria of rabbit hearts, indicating that pressure is the major trigger. A decrease in Serca2a expression was concomitant with a significant decrease in sarcolipin (SLN), possibly indicating a compensatory role of SLN. The decreased expression of SLN was unable to completely restore sarcoplasmic reticulum Ca(2+) uptake function of Serca2a. Functional contractile assessments in isolated trabeculae showed no difference between PAB- and sham-operated rabbits at 1 Hz but displayed an enhanced force development at higher frequencies and in the presence of isoproterenol, while twitch timing was unaffected. Our results indicate that right ventricular mechanical overload due to PAB affects the expression of the Ca(2+)-handling proteins in the RA in rabbits.

Prostaglandin F2alpha inhibits SERCA2 gene transcription through an induction of Egr-1 in cultured neonatal rat cardiac myocytes

Prostaglandin F(2alpha) (PGF(2alpha)) stimulates hypertrophic growth of neonatal rat cardiac myocytes, a feature of which includes downregulation of the Ca(2+)-ATPase (SERCA2), a major Ca(2+) transport protein in SR. The molecular mechanisms by which PGF(2alpha) inhibits SERCA2 gene expression remain unknown. We determined the cis-regulatory elements responsible for the regulation of the SERCA2 gene expression in cultured neonatal rat cardiac myocytes exposed to PGF(2alpha). The role of Egr-1 was evaluated by transient transfection of its expression vector and antisense oligonucleotide. Signaling pathways were determined by using the pharmacological inhibitors or cDNA expression plasmids coding for dominant negative forms of Ras and Rac. PGF(2alpha) reduced the SERCA2 mRNA levels in a time- and dose-dependent manner in cultured rat cardiac myocytes. Transient transfection analyses showed that PGF(2alpha) -responsive elements are located between -284 and -72 of the SERCA2 promoter, which contains G+C-rich sequences homologous to Sp1, Egr-1 and AP2-binding sites. PGF(2alpha) significantly increased Egr-1 expression, and overexpression of Egr-1 largely reduced the transcription of the SERCA2 gene. Egr-1 antisense oligonucleotides blocked the PGF(2alpha) -mediated decrease in SERCA2 mRNA expression. Furthermore, inhibitors for either genistein-sensitive tyrosine kinase or p38 MAPK, and dominant negative forms of either Ras or Rac, prevented PGF(2alpha) -induced repression of SERCA2 mRNA levels. These results suggest that Egr-1, as well as Ras, Rac, and p38 MAPK, plays a crucial role in the repression of SERCA2 gene expression during PGF(2alpha) -induced cardiac hypertrophy.

Increased FOG-2 in failing myocardium disrupts thyroid hormone-dependent SERCA2 gene transcription

Reduced expression of sarcoplasmic reticulum calcium ATPase (SERCA)2 and other genes in the adult cardiac gene program has raised consideration of an impaired responsiveness to thyroid hormone (T3) that develops in the advanced failing heart. Here, we show that human and murine cardiomyopathy hearts have increased expression of friend of GATA (FOG)-2, a cardiac nuclear hormone receptor corepressor protein. Cardiac-specific overexpression of FOG-2 in transgenic mice led to depressed cardiac function, activation of the fetal gene program, congestive heart failure, and early death. SERCA2 transcript and protein levels were reduced in FOG-2 transgenic hearts, and FOG-2 overexpression impaired T3-mediated SERCA2 expression in cultured cardiomyocytes. FOG-2 physically interacts with thyroid hormone receptor-alpha1 and abrogated even high levels of T3-mediated SERCA2 promoter activity. These results demonstrate that SERCA2 is an important target of FOG-2 and that increased FOG-2 expression may contribute to a decline in cardiac function in end-stage heart failure by impaired T3 signaling.

Pim-1 regulates cardiomyocyte survival downstream of Akt

The serine-threonine kinases Pim-1 and Akt regulate cellular proliferation and survival. Although Akt is known to be a crucial signaling protein in the myocardium, the role of Pim-1 has been overlooked. Pim-1 expression in the myocardium of mice decreased during postnatal development, re-emerged after acute pathological injury in mice and was increased in failing hearts of both mice and humans. Cardioprotective stimuli associated with Akt activation induced Pim-1 expression, but compensatory increases in Akt abundance and phosphorylation after pathological injury by infarction or pressure overload did not protect the myocardium in Pim-1-deficient mice. Transgenic expression of Pim-1 in the myocardium protected mice from infarction injury, and Pim-1 expression inhibited cardiomyocyte apoptosis with concomitant increases in Bcl-2 and Bcl-X(L) protein levels, as well as in Bad phosphorylation levels. Relative to nontransgenic controls, calcium dynamics were significantly enhanced in Pim-1-overexpressing transgenic hearts, associated with increased expression of SERCA2a, and were depressed in Pim-1-deficient hearts. Collectively, these data suggest that Pim-1 is a crucial facet of cardioprotection downstream of Akt.

Emerging drugs for acute and chronic heart failure: current and future developments

Heart failure continues to be a major public health issue. Although angiotensin-converting enzyme inhibitors and beta-adrenergic blockers have been broadly used as evidence-based therapies in heart failure, morbidity and mortality remains high. Furthermore, treatment for acute decompensated heart failure and diastolic heart failure (or 'heart failure with preserved ejection fraction') is far from perfect. This review provides a broad overview of some of the novel compounds under investigation for the treatment of heart failure. Novel strategies include drugs that aim to alleviate congestion and improve hemodynamics, drugs that preserve renal function, drugs that reduce arterial and myocardial stiffness, drugs that module myocardial contractility, drugs that affect metabolic and hormonal balance, and drugs that act on existing and novel physiologic targets.

Increased connective tissue growth factor relative to brain natriuretic peptide as a determinant of myocardial fibrosis

Excessive fibrosis contributes to an increase in left ventricular stiffness. The goal of the present study was to investigate the role of connective tissue growth factor (CCN2/CTGF), a profibrotic cytokine of the CCN (Cyr61, CTGF, and Nov) family, and its functional interactions with brain natriuretic peptide (BNP), an antifibrotic peptide, in the development of myocardial fibrosis and diastolic heart failure. Histological examination on endomyocardial biopsy samples from patients without systolic dysfunction revealed that the abundance of CTGF-immunopositive cardiac myocytes was correlated with the excessive interstitial fibrosis and a clinical history of acute pulmonary congestion. In a rat pressure overload cardiac hypertrophy model, CTGF mRNA levels and BNP mRNA were increased in proportion to one another in the myocardium. Interestingly, relative abundance of mRNA for CTGF compared with BNP was positively correlated with diastolic dysfunction, myocardial fibrosis area, and procollagen type 1 mRNA expression. Investigation with conditioned medium and subsequent neutralization experiments using primary cultured cells demonstrated that CTGF secreted by cardiac myocytes induced collagen production in cardiac fibroblasts. Further, G protein-coupled receptor ligands induced expression of the CTGF and BNP genes in cardiac myocytes, whereas aldosterone and transforming growth factor-beta preferentially induced expression of the CTGF gene. Finally, exogenous BNP prevented the production of CTGF in cardiac myocytes. These data suggest that a disproportionate increase in CTGF relative to BNP in cardiac myocytes plays a central role in the induction of excessive myocardial fibrosis and diastolic heart failure.

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.

Mechanical stress-dependent transcriptional regulation of sarcolipin gene in the rodent atrium

Sarcolipin, a homologue of phospholamban, regulates Ca2+ uptake through the interaction with sarcoplasmic reticulum Ca2+ ATPase (SERCA) and is predominantly expressed in the atrial muscle. Although the atrial chamber-specific expression of sarcolipin could be primarily regulated at the transcriptional level, the transcriptional regulation remains poorly understood. Since mechanical stress plays an important role in transcriptional regulation of a gene involved in cardiac hypertrophy and remodeling, we generated left-sided or right-sided pressure-overload models by transverse aortic constriction (TAC) in ddY mice or by monocrotaline administration in Wistar rats, respectively. TAC significantly decreased the expression of sarcolipin, SERCA2a, and phospholamban mRNAs in the left atrium (LA) than those in the right atrium (RA). By contrast, monocrotaline administration significantly decreased the expression of sarcolipin, SERCA2a, and phospholamban mRNAs in the RA than those in the LA. The two independent complementary experiments unequivocally demonstrated that mechanical stress down-regulates the transcription of the sarcolipin gene.

Phosphorylation and binding of AUF1 to the 3'-untranslated region of cardiomyocyte SERCA2a mRNA

Experimental animals and patients with cardiac hypertrophy and heart failure display abnormally slowed myocardial relaxation, which is associated with downregulation of sarco(endo)plasmic reticulum calcium ATPase 2a (SERCA2a), the cardiomyocyte sarcoplasmic reticulum Ca2+ pump. We previously showed that SERCA2a downregulation can be simulated in cultured neonatal rat ventricular myocytes (NRVM) by treatment with the hypertrophic agonist phorbol myristate acetate (PMA) or by overexpression of the novel protein kinase C (PKC) isoenzymes PKCdelta and PKCepsilon. PKC activation, in turn, decreased SERCA2a promoter activity and destabilized the SERCA2a mRNA. Here we demonstrate by using an RSV beta-galactosidase reporter system that a 609-nt fragment of the SERCA2a mRNA 3'-untranslated region (UTR), containing five adenylate-uridylate (AU)-rich regions, may be responsible for destabilizing the message following PMA treatment. UV cross-linking analysis demonstrated that several proteins found in the NRVM cell extracts bind to the 609-nt fragment. In addition, protein binding was transiently increased in response to PMA stimulation. 3'-UTR mRNA pull-down assays and Western blot analysis indicated that the AU binding protein AUF1 interacted with the SERCA2a 3'-UTR. AUF1 binding activity was predominantly found in the nuclear fraction, and PMA-induced AUF1 binding was associated with increased threonine phosphorylation of AUF1. These data suggest that the phosphorylation, binding, and location of AUF1 affect the posttranscriptional regulation of the SERCA2a message in NRVM.

Left ventricular SERCA2a gene down-regulation does not parallel ANP gene up-regulation during post-MI remodelling in rats

BACKGROUND

In most animal models of chronic hemodynamic overload of the left ventricle (LV) as well as in human end stage heart failure, the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a) mRNA levels are decreased in parallel with increased atrial natriuretic peptide (ANP) mRNA levels. The situation in the remote myocardium following myocardial infarction (MI) is unclear.

AIMS

(1) To examine SERCA2a mRNA levels in the non-infarcted LV myocardium of rats at the chronic stage of experimental MI and (2) To examine whether a negative linear correlation exists between SERCA2a and ANP mRNA levels in this model.

METHODS

Anesthetized adult male Wistar rats underwent left coronary artery ligation or sham operation. Three months later, the rats were divided into three groups: sham-operated rats (sham, n=21), HF-free rats with MI (non-failing (NF)-MI, n=29) and rats with both MI and HF (congestive heart failure (CHF)-MI, n=14). LV remodelling and function were assessed by echocardiography and hemodynamic measurements. SERCA2a and ANP mRNA levels were determined by Northern and dot blot analysis with specific cDNA probes.

RESULTS

LV SERCA2a mRNA levels varied markedly in sham-operated rats (0.9-1.8). Mean ANP mRNA level increased markedly and mean SERCA2a mRNA level decreased moderately in the remote myocardium. In some NF-MI rats, SERCA2a mRNA levels were higher than those in some sham controls. Whereas ANP mRNA levels correlated well with MI severity (r2=0.79, p<0.001), this was not the case for SERCA2a mRNA levels (r2=0.42, p<0.01). We found no negative correlation between ANP and SERCA2a mRNA levels.

CONCLUSION

SERCA2a gene down-regulation in the non-infarcted myocardium of rats with MI does not correlate with ANP gene up-regulation, suggesting that the two genes are not antithetically regulated.

PYK2 regulates SERCA2 gene expression in neonatal rat ventricular myocytes

The nonreceptor protein tyrosine kinase (PTK) proline-rich tyrosine kinase 2 (PYK2) has been implicated in cell signaling pathways involved in left ventricular hypertrophy and heart failure, but its exact role has not been elucidated. In this study, replication-defective adenoviruses (Adv) encoding green fluorescent protein (GFP)-tagged, wild-type (WT), and mutant forms of PYK2 were used to determine whether PYK2 overexpression activates MAPKs, and downregulates SERCA2 mRNA levels in neonatal rat ventricular myocytes (NRVM). PYK2 overexpression significantly decreased SERCA2 mRNA (as determined by Northern blot analysis and real-time RT-PCR) to 54 ± 4% of Adv-GFP-infected cells 48 h after Adv infection. Adv-encoding kinase-deficient (KD) and Y402F phosphorylation-deficient mutants of PYK2 also significantly reduced SERCA2 mRNA (WT>KD>Y402F). Conversely, the PTK inhibitor PP2 (which blocks PYK2 phosphorylation by Src-family PTKs) significantly increased SERCA2 mRNA levels. PYK2 overexpression had no effect on ERK1/2, but increased JNK1/2 and p38MAPKphosphorylation from fourfold to eightfold compared with GFP overexpression. Activation of both “stress-activated” protein kinase cascades appeared necessary to reduce SERCA2 mRNA levels. Adv-mediated overexpression of constitutively active (ca)MKK6 or caMKK7, which activated only p38MAPKor JNKs, respectively, was not sufficient, whereas combined infection with both Adv reduced SERCA2 mRNA levels to 45 ± 12% of control. WTPYK2 overexpression also significantly reduced SERCA2 promoter activity, as determined by transient transfection of a 3.8-kb SERCA2 promoter-luciferase construct. Thus a PYK2-dependent signaling cascade may have a role in abnormal cardiac Ca2+handling in left ventricular hypertrophy and heart failure via downregulation of SERCA2 gene transcription.

Phenotypes of SERCA and PMCA knockout mice

P-type Ca2+-ATPases of the sarco(endo)plasmic reticulum (SERCAs) and plasma membrane (PMCAs) are responsible for maintaining the Ca2+ gradients across cellular membranes that are required for regulation of Ca2+-mediated signaling and other biological processes. Gene-targeting studies of SERCA isoforms 1, 2, and 3 and PMCA isoforms 1, 2, and 4 have confirmed some of the general functions proposed for these pumps, such as a major role in excitation-contraction coupling for SERCA1 and SERCA2 and housekeeping functions for PMCA1 and SERCA2, but have also revealed some unexpected phenotypes. These include squamous cell cancer and plasticity in the regulation of Ca2+-mediated exocytosis in SERCA2 heterozygous mutant mice, modulation of Ca2+ signaling in SERCA3-deficient mice, deafness and balance disorders in PMCA2 null mice, and male infertility in PMCA4 null mice. These unique phenotypes provide new information about the cellular functions of these pumps, the requirement of their activities for higher order physiological processes, and the pathophysiological consequences of pump dysfunction.

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.

Isoenzyme-selective regulation of SERCA2 gene expression by protein kinase C in neonatal rat ventricular myocytes

Patients with cardiac hypertrophy and heart failure display abnormally slowed myocardial relaxation, which is associated with downregulation of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2) gene expression. We previously showed that SERCA2 downregulation can be simulated in cultured neonatal rat ventricular myocytes (NRVM) by treatment with the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA). However, NRVM express three different PMA-sensitive PKC isoenzymes (PKCalpha, PKCepsilon, and PKCdelta), which may be differentially regulated and have specific functions in the cardiomyocyte. Therefore, in this study we used adenoviral vectors encoding wild-type (wt) and kinase-defective, dominant negative (dn) mutant forms of PKCalpha, PKCepsilon, and PKCdelta to analyze their individual effects in regulating SERCA2 gene expression in NRVM. Overexpression of wtPKCepsilon and wtPKCdelta, but not wtPKCalpha, was sufficient to downregulate SERCA2 mRNA levels, as assessed by Northern blotting and quantitative, real-time RT-PCR (69 +/- 7 and 61 +/- 9% of control levels for wtPKCepsilon and wtPKCdelta, respectively; P < 0.05 for each adenovirus; n = 8 experiments). Conversely, overexpression of all three dnPKCs appeared to significantly increase SERCA2 mRNA levels (dnPKCdelta > dnPKCepsilon > dnPKCalpha). dnPKCdelta overexpression produced the largest increase (2.8 +/- 1.0-fold; n = 11 experiments). However, PMA treatment was still sufficient to downregulate SERCA2 mRNA levels despite overexpression of each dominant negative mutant. These data indicate that the novel PKC isoenzymes PKCepsilon and PKCdelta selectively regulate SERCA2 gene expression in cardiomyocytes but that neither PKC alone is necessary for this effect if the other novel PKC can be activated.

Gene knockout studies of Ca2+-transporting ATPases

The biochemical functions of intracellular and plasma membrane Ca2+-transporting ATPases in the control of cytosolic and organellar Ca2+ levels are well established, but the physiological roles of specific isoforms are less well understood. There appear to be three different types of Ca2+ pumps in mammalian tissues: the sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs), which sequester Ca2+ within the endoplasmic or sarcoplasmic reticulum, the plasma membrane Ca2+-ATPases (PMCAs), which extrude Ca2+ from the cell, and the putative secretory pathway Ca2+-ATPase (SPCA), the function of which is poorly understood. This review describes the results of recent analyses of mouse models with null mutations in the genes encoding SERCA and PMCA isoforms and genetic studies of SERCA and SPCA dysfunction in both humans and model organisms. These studies are yielding important insights regarding the physiological functions of individual Ca2+-transporting ATPases in vivo.

Function and regulation of sarcoplasmic reticulum Ca2+-ATPase: advances during the past decade and prospects for the coming decade

In cardiac muscle, the contraction-relaxation cycle is tightly controlled by the regulated release and uptake of intracellular Ca2+ between sarcoplasmic reticulum and cytoplasm. A major protein controlling Ca2+ cycling is Ca2+-ATPase (SERCA2a) located in the sarcoplasmic reticulum membrane. The function of SERCA2a protein is regulated by the phosphorylatable protein, phospholamban. Phosphorylation of phospholamban releases its inhibitory effect on SERCA2a through direct molecular interaction. Recently, mice whose SERCA2a function is increased (overexpression of the gene) or lost (knock out) were developed. These mice demonstrated that SERCA2a pump levels are a major determinant of cardiac muscle contractility and relaxation. These studies open the prospect that the overexpression of SERCA2a can correct cardiac dysfunction seen in heart failure. Advances in knowledge concerning the function and gene regulation of SERCA2a are discussed in this review.