Nitric Oxide-cGMP-PKG Pathway Acts on Orai1 to Inhibit the Hypertrophy of Human Embryonic Stem Cell-Derived Cardiomyocytes.

Cardiac hypertrophy is an abnormal enlargement of heart muscle. It frequently results in congestive heart failure, which is a leading cause of human death. Previous studies demonstrated that the nitric oxide (NO), cyclic GMP (cGMP), and protein kinase G (PKG) signaling pathway can inhibit cardiac hypertrophy and thus improve cardiac function. However, the underlying mechanisms are not fully understood. Here, based on the human embryonic stem cell-derived cardiomyocyte (hESC-CM) model system, we showed that Orai1, the pore-forming subunit of store-operated Ca(2+) entry (SOCE), is the downstream effector of PKG. Treatment of hESC-CMs with an α-adrenoceptor agonist phenylephrine (PE) caused a marked hypertrophy, which was accompanied by an upregulation of Orai1. Moreover, suppression of Orai1 expression/activity using Orai1-siRNAs or a dominant-negative construct Orai1(G98A) inhibited the hypertrophy, suggesting that Orai1-mediated SOCE is indispensable for the PE-induced hypertrophy of hESC-CMs. In addition, the hypertrophy was inhibited by NO and cGMP via activating PKG. Importantly, substitution of Ala for Ser(34) in Orai1 abolished the antihypertrophic effects of NO, cGMP, and PKG. Furthermore, PKG could directly phosphorylate Orai1 at Ser(34) and thus prevent Orai1-mediated SOCE. Together, we conclude that NO, cGMP, and PKG inhibit the hypertrophy of hESC-CMs via PKG-mediated phosphorylation on Orai1-Ser-34. These results provide novel mechanistic insights into the action of cGMP-PKG-related antihypertrophic agents, such as NO donors and sildenafil.

Skeletal actin mRNA increases in the human heart during ontogenic development and is the major isoform of control and failing adult hearts.

Expression of the two sarcomeric actins, alpha-skeletal and alpha-cardiac, is regulated in the rodent heart in response to developmental, hormonal, and hemodynamic stimuli. Little is known in man, except that both isogenes were found to be coexpressed in three adult ventricles. In this report, we investigated the isoactin mRNA composition in ventricles from 21 control patients (4 fetal, 5 juvenile, 12 adult) and from 15 patients undergoing cardiac transplantation (5 idiopathic dilated cardiomyopathies, 5 ischemic myopathies with myocardial infarcts, 5 diverse etiologies) by two different and complementary techniques: RNA dot blot analysis with specific cDNA probes, and primer extensions with an oligonucleotide common to alpha-cardiac and alpha-skeletal actins. In the case of dot blot analysis, quantification of each isoform was performed by using as standards RNA transcripts obtained from cloned human alpha-actin sequences, and the total amount of sarcomeric actin mRNA was evaluated as a function of total poly(A+)RNA. We found that both isogenes are always coexpressed, and that the isoactin pattern changes during development. In utero and in neonatal hearts, alpha-skeletal actin mRNA represents less than or equal to 20% of sarcomeric actins, it increases to 48 +/- 6% during the first decade after birth and becomes the predominant isoform of adult hearts (60.4 +/- 8.5%). The 15 adult failing hearts exhibited the same isoactin pattern as the control ones (62.84 +/- 11.06%), and there was no difference in expression between patients with dilated cardiomyopathy or ischemic heart disease. These observations demonstrate that cardiac development in man, in contrast to rodent heart, is characterized by an up-regulation of the skeletal actin gene, the expression of which does not change in hypertrophied and failing hearts, and suggest that the actin and myosin heavy chain families are independently regulated in human heart.

Altered sarcoplasmic reticulum Ca2(+)-ATPase gene expression in the human ventricle during end-stage heart failure.

A decrease in the myocardial level of the mRNA encoding the Ca2(+)-ATPase of the sarcoplasmic reticulum (SR) has been recently reported during experimental cardiac hypertrophy and failure. To determine if such a deficit occurs in human end-stage heart failure, we compared the SR Ca2(+)-ATPase mRNA levels in left (LV) and right ventricular (RV) specimens from 13 patients undergoing cardiac transplantation (6 idiopathic dilated cardiomyopathies; 4 coronary artery diseases with myocardial infarctions; 3 diverse etiologies) with control heart samples using a rat cardiac SR Ca2(+)-ATPase cDNA probe. We observed a marked decrease in the mRNA for the Ca2(+)-ATPase relative to both the 18S ribosomal RNA and the myosin heavy chain mRNA in LV specimens of patients with heart failure compared to controls (-48%, P less than 0.01 and -47%, P less than 0.05, respectively). The LV ratio of Ca2(+)-ATPase mRNA to 18S RNA positively correlated with cardiac index (P less than 0.02). The RV ratio correlated negatively with systolic, diastolic and mean pulmonary arterial pressures (P less than 0.02, P less than 0.02, and P less than 0.01, respectively). We suggest that a decrease of the SR Ca2(+)-ATPase mRNA in the myocardium plays an important role in alterations of Ca2+ movements and myocardial relaxation reported during human end-stage heart failure.

Local control models of cardiac excitation-contraction coupling. A possible role for allosteric interactions between ryanodine receptors.

In cardiac muscle, release of activator calcium from the sarcoplasmic reticulum occurs by calcium- induced calcium release through ryanodine receptors (RyRs), which are clustered in a dense, regular, two-dimensional lattice array at the diad junction. We simulated numerically the stochastic dynamics of RyRs and L-type sarcolemmal calcium channels interacting via calcium nano-domains in the junctional cleft. Four putative RyR gating schemes based on single-channel measurements in lipid bilayers all failed to give stable excitation-contraction coupling, due either to insufficiently strong inactivation to terminate locally regenerative calcium-induced calcium release or insufficient cooperativity to discriminate against RyR activation by background calcium. If the ryanodine receptor was represented, instead, by a phenomenological four-state gating scheme, with channel opening resulting from simultaneous binding of two Ca2+ ions, and either calcium-dependent or activation-linked inactivation, the simulations gave a good semiquantitative accounting for the macroscopic features of excitation-contraction coupling. It was possible to restore stability to a model based on a bilayer-derived gating scheme, by introducing allosteric interactions between nearest-neighbor RyRs so as to stabilize the inactivated state and produce cooperativity among calcium binding sites on different RyRs. Such allosteric coupling between RyRs may be a function of the foot process and lattice array, explaining their conservation during evolution.

The molecular biology of heart failure.

In the mammalian heart, the development of cardiac hypertrophy is a common feature that normally precedes all forms of heart failure. This adaptive process involves molecular changes in the myocardium, including the altered expression of several genes encoding proteins for contraction and relaxation. The expression of myosin heavy chain (MHC) and sarcomeric alpha-actin messenger ribonucleic acid (mRNA) changes qualitatively during cardiac hypertrophy; however, their accumulations are not coordinated. Skeletal alpha-actin transcripts accumulate throughout the ventricles and earlier than beta-MHC transcripts, which accumulate primarily around large coronary vessels. Skeletal alpha-actin transcripts also "hyperaccumulate" relative to cardiac alpha-actin mRNA, whose expression does not change. Expression of MHC isomRNA shows an inverse relation; as beta-MHC accumulates, alpha-MHC decreases in abundance. From nuclear run-on assays, we present evidence that the accumulation of these gene products is at least under partial transcriptional control with developmental growth, suggesting that those changes that occur with hypertrophy and heart failure may be primarily transcriptionally regulated. The expression of the mRNA for the calcium-adenosine triphosphate (Ca(2+)-ATPase) of the sarcoplasmic reticulum changes quantitatively with cardiac hypertrophy without the reexpression of a different isoform. The relative mRNA and protein concentrations for this protein diminish with both cardiac hypertrophy and heart failure, a change that may partially explain the delayed relaxation rates seen in hypertrophied and failing hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

A distant upstream region of the rat multipartite Na(+)-Ca(2+) exchanger NCX1 gene promoter is sufficient to confer cardiac-specific expression.

The Na(+)-Ca(2+) exchanger (NCX) regulates intracellular calcium homeostasis. We report on an upstream region of the rat NCX1 multipartite promoter that is active in cardiac myocytes. Although inactive in most non-cardiac cell lines, its activity can be rescued by cotransfection with GATA-4 and -6, but not GATA-5 transcription factors. In transgenic mice and similar to endogenous NCX1 mRNA expression, the upstream promoter region directs uniform beta-galactosidase expression in cardiac myocytes from approximately 7.75dpc. In adult mouse hearts, promoter activity is, however, significantly reduced and heterogeneous, except in the conduction system (sinoatrial and atrioventricular node, atrioventricular bundles). The upstream NCX1 promoter region thus directs appropriate spatial and temporal control of cardiac expression throughout development.

Gene expression in cardiac hypertrophy.

Cardiac hypertrophy due to hemodynamic overload is a harbinger of morbidity and mortality in humans. The development of hypertrophy involves both quantitative and qualitative changes in gene expression that are thought to produce an enlarged organ more capable of meeting its new functional requirements. The genes are normal, but the way in which they are regulated is modified. Analysis of these changes and the mechanisms involved are essential if we are to understand the role that hypertrophy plays in the pathogenesis of heart failure.

Clenbuterol induces cardiac hypertrophy with normal functional, morphological and molecular features.

OBJECTIVE: Several pharmacological agents have been shown to produce 'physiological' or 'pathological' hypertrophy based on their functional characteristics. The aim of this study was to examine the features of cardiac hypertrophy induced by the selective beta 2-adrenergic agonist, clenbuterol. METHODS: Cardiac hypertrophy was induced in 7-week-old Sprague-Dawley rats by daily injections of clenbuterol for 3 weeks. Thyroxine and isoproterenol were also used to produce cardiac hypertrophy to serve as positive controls for physiological and pathological hypertrophy, respectively. Left ventricular function was determined using an isolated rat heart preparation. Ventricular samples were used for morphological examination while interstitial collagen was measured using high-pressure liquid chromatography. Expression of sarcoplasmic reticulum Ca(2+)-ATPase2a (SERCA2a) and phospholamban (PLB) were measured by dot blot analysis. RESULTS: Clenbuterol treatment induced 26% left ventricular hypertrophy. These hearts demonstrated normal systolic isovolumic parameters and diastolic (active relaxation and passive stiffness) function. In addition, left ventricular concentration of collagen and morphology was normal as were the expression of SERCA2a and PLB mRNA. CONCLUSION: These results suggest that clenbuterol-induced hypertrophy is 'physiological' in terms of its function, extracellular structure and gene expression.

Expressional analysis of the cardiac Na-Ca exchanger in rat development and senescence.

The cardiac Na-Ca exchanger (NCX) serves as the main calcium extrusion mechanism in heart muscle and is important in maintaining intracellular calcium homeostasis. The accumulations of NCX RNA and protein are known to be regulated in cardiac hypertrophy, by thyroid hormone and during postnatal development. In this study the temporal and spatial patterns of NCX mRNA and protein accumulations were examined, and nuclear run-on assays performed. NCX is highly expressed in late fetal and neonatal rat hearts, decreasing to adult levels by 20 days after birth for RNA (P < 0.05, fetal and 1 neonatal day old (1 ND) versus 20 day old (20 ND)). Maximal protein expression is seen in 19 embryonic day (ED) old hearts, and reaches adult levels sometime after 20 neonatal days. (P < 0.05, fetal versus adult). Spatially, NCX is homogenously expressed in early embryonic and fetal heart, followed by a decline after birth. The protein levels decline more slowly suggesting a long protein half-life. The lowest level of mRNA accumulation is seen in 6 and 18 month old animals (P < 0.05 for all time points before 10 neonatal days). In the 24 month old senescent rat, NCX transcripts are increased by almost 50% above that seen at 6 and 18 months (P < 0.05) but are not different from those at 15 neonatal days. Perinatal NCX expression is regulated transcriptionally: late fetal and neonatal hearts have high transcriptional activity but by 20 postnatal days, no detectable transcriptional activity can be demonstrated. Throughout development, at least five transcription start sites are used, and no significant difference in the 5' untranslated or 3' coding splice sites could be demonstrated, although several new cardiac splicing variants were identified. We also report the cloning of a 3.7 kb fragment containing the cardiac NCX1 promoter which is transcriptionally active in neonatal cardiomyocytes.

Sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) gene products are regulated post-transcriptionally during rat cardiac development.

OBJECTIVE: The Sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) plays a major role in the contraction-relaxation cycle and is responsible for transporting calcium into the lumen of the sarcoplasmic reticulum. This study was performed to determine if the increase in SERCA2 messenger RNA (mRNA) abundance during the perinatal period is regulated transcriptionally. METHODS: Transcriptional activity was determined by nuclear run-on assays and mRNA and protein abundances were determined during late fetal and early neonatal cardiac development in rat. RESULTS: From nuclear run-on assays, SERCA2 gene transcription at 17/18 embryonic days (139 +/- 41 parts per million (ppm), n = 7) did not differ from that at 20 neonatal days (139 +/- 37 ppm, n = 6) after birth. No increase in transcriptional activity could be demonstrated during the time frame examined. In contrast, both alpha and beta myosin heavy chains showed significant changes in measured transcriptional activity. SERCA2 mRNA normalized to 18S RNA levels are very low in the fetus (9.8 +/- 1.9 to 13.4 +/- 4.9 arbitrary units (A.U.) from 17/18 to 19/20 embryonic days) and significantly increase from birth (15 +/- 3.8 A.U.) to reach a maximum at 20 days of age (29.1 +/- 9.5 to 48.3 +/- 7.0 in 15 to 20 neonatal days rats respectively). Similarly, SR Ca(2+)-ATPase protein levels are less abundant in the fetus (0.82 +/- 0.08 to 1.13 +/- 0.13 A.U./microgram total protein) and reach a maximum at 15-20 neonatal days (3.08 +/- 0.58 to 2.98 +/- 0.17). Ca2+ uptake in the fetal heart is about one sixth the level seen in the adult, reaches the highest observed value at 5 days after birth (6.05 +/- 0.77 pmole Ca2+ per microgram/min) and remains relatively constant over the next 15 days. The activity increases even though phospholamban protein increases in abundance. CONCLUSIONS: Since the transcriptional activity of this gene is unchanged whereas the mRNA, protein abundance and activity increase, we conclude that the abundance of SERCA2 gene products is regulated primarily through post-transcriptional mechanisms during the perinatal period.

Characterization and expression of the rat heart sarcoplasmic reticulum Ca2+-ATPase mRNA.

Sarcoplasmic reticulum Ca2+-ATPase cDNA clones have been isolated from an adult rat heart cDNA library and the nucleotide sequence of the Ca2+-ATPase mRNA determined. The sequence has an open reading frame of 997 codons. It is identical to a cDNA isolated from a rat stomach cDNA library and 90% isologous to the rabbit and human slow/cardiac cDNAs. Nuclease S1 mapping analysis indicates that this sequence corresponds to the main Ca2+-ATPase mRNA present in heart and in slow skeletal muscle and that it is expressed in various proportions in smooth and non-muscle tissues, together with another isoform which differs from the cardiac form in the sequence of its 3'-end.

Discovering altered genomic expression patterns in heart: transcriptome determination by serial analysis of gene expression.

The development of cardiovascular diseases such as heart failure involve functional changes that are beneficial short-term, but may be fatal long-term. Current therapeutic approaches are tailored to limit progression of a disease and to maintain quality of life. At a molecular level, these disease processes involve quantitative and qualitative changes in gene expression. Although some changes in mRNA abundance may not have direct protein correlates, analysis of all the mRNAs present in a cell population (the cells transcriptome) has become a focal point of genomic research. The aim is to provide information about the dynamics of total genome expression in response to environmental changes and point to candidate genes responsible for the cascade of events that result in a disease state. One way of performing these analyses utilizes the technique of Serial Analysis of Gene Expression (SAGE). This method evaluates thousands of expressed transcripts both quantitatively and qualitatively in a single assay. In the first of two reviews on transcriptome analysis, we describe the current state of genomic research for determination of the transcriptome by Serial Analysis of Gene Expression, present the first limited SAGE analysis of rodent heart gene expression, and discuss how results generated with this approach can be applied to the study and treatment of cardiovascular diseases.

[Left ventricular hypertrophy: molecular aspects]. / Hypertrophie ventriculaire gauche: aspects moléculaires.

The expression of genes changes during left ventricular hypertrophy. These changes are known as mechanogenic transduction. With respect to the two main contractile proteins, myosin and actin, this is due to a differential expression of their respective multigenic families. Changes of isoform are uncoordinated in time and space. It is important to recognise that the genes themselves are normal and that it is their regulation which is disturbed. Recent data shows that, under normal conditions, the regulation occurs at the transcription stage. A fuller understanding of these mechanisms is necessary if we are to comprehend the pathogenesis of cardiac hypertrophy and failure.

Aging-associated changes in cardiac gene expression: large scale transcriptome analysis.

Aging and aging-related diseases are associated with altered patterns of gene expression, involving quantitative and qualitative changes in the abundance of specific transcripts. A complete and simultaneous analysis of gene expression should therefore lead to important insights into the transcriptional mechanisms underlying the aging process. Recently, we have employed high-throughput gene expression profiling to study transcriptional activity in heart. Two technologies, serial analysis of gene expression (SAGE) and gene expression arrays, allow rapid, large-scale expression profiling, which provides information about the dynamics of total gene expression with age and which can be employed to identify candidate genes that may serve as diagnostic and prognostic markers in age-associated cardiac diseases. The accompanying gene predictions from high-throughput gene expression profiling provide a starting point for understanding the function, the complexity of interactions, and the role of genes in promoting cellular/organismal phenotypes during senescence and disease. In this review we describe the current state of transcriptome profiling by SAGE and microarrays and discuss how results generated with these approaches in heart can be applied to the study of aging and the treatment of cardiovascular diseases.

[Molecular bases of cardiac aging]. / Bases moléculaires du vieillissement cardiaque.

The total volume occupied by myocytes in the heart decreases with age, and this is accompanied by a loss of myocytes, by hypertrophy and, to a lesser extent, by hyperplasia of the remaining myocytes. There are differences between ventricles: the right ventricular myocytes have a greater capacity for replication than those of the left ventricle. The expression of genes coding for the 2 principal contractile proteins (myosin and actin) is modified, and there are strong resemblances between the phenotype of the aged heart and that of an adult heart with haemodynamic overload.

Cardiac response to pressure overload in the rat: the selective alteration of in vitro directed RNA translation products.

As cardiac hypertrophy develops, total cardiac RNA and protein synthesis increase significantly. We have identified specific messenger RNAs that change in predominance with the induction of pressure-overload-stimulated cardiac hypertrophy. Total cardiac RNA was isolated from rats either undergoing cardiac hypertrophy secondary to subdiaphragmatic aortic constriction or subjected to sham surgery. The products translated in vitro were separated by two-dimensional gel electrophoresis and quantitated. The translation of four proteins decreased while the translation of four others increased in preparations from hypertrophied hearts compared with those from sham-treated rats. Two isoforms of creatine kinase M were translated in vitro. Only one of these isoforms decreased with cardiac hypertrophy, suggesting that the transcriptional or translational control for creatine kinase is much more complex than previously believed. Finally, since only eight of over 700 different translation products changed in relative predominance with cardiac hypertrophy, we conclude that the accumulation of existing RNA and protein products is the primary adaptive process responsible for cardiac hypertrophy.

Molecular aspects of myocardial differentiation.

The embryonic heart can pump blood in a single direction without one-way valves. With the development of molecular cell markers specific for contraction and relaxation, functional aspects of myocardial differentiation have been addressed through the use of in situ hybridization. In this study, we report how expression of the cardiac sarcoplasmic reticulum calcium-adenosine triphosphatase (SERCA2) and phospholamban (PLB) in the rat may partly explain why the embryonic atrium and ventricle function essentially as they do in the adult. SERCA2 is expressed in a craniocaudal gradient from as early as 10 embryonic days (ED) of development. PLB is first expressed at 12 ED but in a gradient essentially opposite to that seen for SERCA2. This spatial pattern of expression is maintained throughout much of fetal development. The spatial distribution of skeletal alpha-actin in the developing human heart indicates that alpha-actin isoform gradients or switching are not important in the establishment of unidirectional blood flow in the absence of valves, but it may serve as a marker for cardiac maturation.

Can exogenous stem cells be used in transplantation?

Today's most urgent problem in transplantation is the lack of suitable donor organs and tissues and as the population ages, demands for organs and tissue therapies will only increase. One alternative to organ transplantation is cell therapy whose aim is to replace, repair or enhance the biological function of damaged tissue or diseased organs. One goal of cellular transplantation thus has been to find a renewable source of cells that could be used in humans. Embryonic stem (ES) cells have the potential to proliferate in vitro in an undifferentiated and pluripotent state. Theoretically, ES cells are capable of unlimited proliferation in vitro. ES cells spontaneously differentiate into derivatives of all three primary germ layers: endoderm, ectoderm and mesoderm, hence providing cells in vitro which can theoretically be isolated and used for transplantation. Furthermore, these pluripotent stem cells can potentially be used to produce large numbers of cells that can be genetically modified in vitro. Once available, this source of cells may obviate some of the critical needs for organ transplantation. Murine ES cells have been extensively studied and all available evidence indicates that all aforementioned expectations are indeed fulfilled by ES cells. ES cells as well as embryonic germ cells have recently been isolated and maintained in culture. The recent descriptions of human ES cells portend the eventual use of allogeneic in vitro differentiated cells for human therapy. This goal, however, is fraught with obstacles. Our aim is first to review the recent advances made with murine ES cells and then to point out potentials and difficulties associated with the use of human ES cells for transplantation.