Genetically engineered models with alterations in cardiac membrane calcium-handling proteins

Identification of hub genes and potential ceRNA networks of diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM), a common complication of diabetes, is defined as ventricular dysfunction in the absence of underlying heart disease. Noncoding RNAs (ncRNAs), including long noncoding RNAs (lncRNAs) and microRNAs (miRNAs), play a crucial role in the development of DCM. Weighted Gene Co-Expression Network Analysis (WGCNA) was used to identify key modules in DCM-related pathways. DCM-related miRNA-mRNA network and DCM-related ceRNA network were constructed by miRNA-seq to identify hub genes in these modules. We identified five hub genes that are associated with the onset of DCM, including Troponin C1 (Tnnc1), Phospholamban (Pln), Fatty acid binding proteins 3 (Fabp3), Popeye domain containing 2 (Popdc2), and Tripartite Motif-containing Protein 63 (Trim63). miRNAs that target the hub genes were mainly involved in TGF-β and Wnt signaling pathways. GO BP enrichment analysis found these miRNAs were involved in the signaling of TGF-β and glucose homeostasis. Q-PCR results found the gene expressions of Pln, Fabp3, Trim63, Tnnc1, and Popdc2 were significantly increased in DCM. Our study identified five hub genes (Tnnc1, Pln, Fabp3, Popdc2, Trim63) whose associated ceRNA networks are responsible for the onset of DCM.

Adenoviral βARKct Cardiac Gene Transfer Ameliorates Postresuscitation Myocardial Injury in a Porcine Model of Cardiac Arrest

OBJECTIVE

The aim of the study was to determine whether the inhibition of the G-protein-coupled receptor kinase 2 by adenoviral βARKct cardiac gene transfer can ameliorate postresuscitation myocardial injury in pigs with cardiac arrest (CA) and explore the mechanism of myocardial protection.

METHODS

Male landrace domestic pigs were randomized into the sham group (anesthetized and instrumented, but ventricular fibrillation was not induced) (n = 4), control group (ventricular fibrillation 8 min, n = 8), and βARKct group (ventricular fibrillation 8 min, n = 8). Hemodynamic parameters were monitored continuously. Blood samples were collected at baseline, 30 min, 2 h, 4 h, and 6 h after the return of spontaneous circulation (ROSC). Left ventricular ejection fraction was assessed by echocardiography at baseline and 6 h after ROSC. These animals were euthanized, and the cardiac tissue was removed for analysis at 6 h after ROSC.

RESULTS

Compared with those in the sham group, left ventricular +dp/dtmax, -dp/dtmax, cardiac output (CO), and ejection fraction (EF) in the control group and the βARKct group were significantly decreased at 6 h after the restoration of spontaneous circulation. However, the βARKct treatment produced better left ventricular +dp/dtmax, -dp/dtmax, CO, and EF after ROSC. The βARKct treatment also produced lower serum cardiac troponin I, CK-MB, and lactate after ROSC. Furthermore, the adenoviral βARKct gene transfer significantly increased β1 adrenergic receptors, SERCA2a, RyR2 levels, and decreased GRK2 levels compared to control.

CONCLUSIONS

The inhibition of GRK2 by adenoviral βARKct cardiac gene transfer can ameliorate postresuscitation myocardial injury through beneficial effects on restoring the sarcoplasmic reticulum Ca-handling proteins expression and upregulating the β1-adrenergic receptor level after cardiac arrest.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Sex Differences in Cardiac Mitochondria in the New Zealand Obese Mouse

Background: Obesity is a risk factor for diseases including type 2 diabetes mellitus (T2DM) and cardiovascular disorders. Diabetes itself contributes to cardiac damage. Thus, studying cardiovascular events and establishing therapeutic intervention in the period of type T2DM onset and manifestation are of highest importance. Mitochondrial dysfunction is one of the pathophysiological mechanisms leading to impaired cardiac function. Methods: An adequate animal model for studying pathophysiology of T2DM is the New Zealand Obese (NZO) mouse. These mice were maintained on a high-fat diet (HFD) without carbohydrates for 13 weeks followed by 4 week HFD with carbohydrates. NZO mice developed severe obesity and only male mice developed manifest T2DM. We determined cardiac phenotypes and mitochondrial function as well as cardiomyocyte signaling in this model. Results: The development of an obese phenotype and T2DM in male mice was accompanied by an impaired systolic function as judged by echocardiography and MyH6/7 expression. Moreover, the mitochondrial function only in male NZO hearts was significantly reduced and ERK1/2 and AMPK protein levels were altered. Conclusions: This is the first report demonstrating that the cardiac phenotype in male diabetic NZO mice is associated with impaired cardiac energy function and signaling events.

Review of novel therapeutic targets for improving heart failure treatment based on experimental and clinical studies

Heart failure (HF) is a major public health priority due to its epidemiological transition and the world's aging population. HF is typified by continuous loss of contractile function with reduced, normal, or preserved ejection fraction, elevated vascular resistance, fluid and autonomic imbalance, and ventricular dilatation. Despite considerable advances in the treatment of HF over the past few decades, mortality remains substantial. Pharmacological treatments including β-blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and aldosterone antagonists have been proven to prolong the survival of patients with HF. However, there are still instances where patients remain symptomatic, despite optimal use of existing therapeutic agents. This understanding that patients with chronic HF progress into advanced stages despite receiving optimal treatment has increased the quest for alternatives, exploring the roles of additional pathways that contribute to the development and progression of HF. Several pharmacological targets associated with pathogenesis of HF have been identified and novel therapies have emerged. In this work, we review recent evidence from proposed mechanisms to the outcomes of experimental and clinical studies of the novel pharmacological agents that have emerged for the treatment of HF.

In vivo model with targeted cAMP biosensor reveals changes in receptor-microdomain communication in cardiac disease

3',5'-cyclic adenosine monophosphate (cAMP) is an ubiquitous second messenger that regulates physiological functions by acting in distinct subcellular microdomains. Although several targeted cAMP biosensors are developed and used in single cells, it is unclear whether such biosensors can be successfully applied in vivo, especially in the context of disease. Here, we describe a transgenic mouse model expressing a targeted cAMP sensor and analyse microdomain-specific second messenger dynamics in the vicinity of the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA). We demonstrate the biocompatibility of this targeted sensor and its potential for real-time monitoring of compartmentalized cAMP signalling in adult cardiomyocytes isolated from a healthy mouse heart and from an in vivo cardiac disease model. In particular, we uncover the existence of a phosphodiesterase-dependent receptor-microdomain communication, which is affected in hypertrophy, resulting in reduced β-adrenergic receptor-cAMP signalling to SERCA.

SERCA-2a is involved in the right ventricular function following myocardial infarction in rats

AIMS

Right ventricular (RV) function is considered an independent predictor of mortality and development of heart failure (HF) in patients with left ventricle dysfunction following myocardial infarction (MI). The functional and molecular mechanisms that may explain the RV dysfunction are still poorly understood. Our study was conducted to investigate RV contractility and the myocardium protein involved in the calcium handling following MI in rats.

MAIN METHODS

MI was surgically induced in male Wistar rats to create transmural infarctions involving 40-60% of the left ventricle surface. Infarcted rats were divided into two groups: those that presented classical signs of congestive heart failure (HF group) and those that did not (INF group), and compared to control animals (Sham). RV contractility was studied using isometric contraction in isolated strips and isovolumetric pressure in isolated heart.

KEY FINDINGS

Inotropic responses in RV strips were preserved in the INF group but were reduced in the HF group (3.75 mM Ca(2+) treatment: Sham = 163 ± 18; INF = 148 ± 19; HF = 68 ± 11 g/g*; *p < 0.05; 5 × 10(-5) M isoproterenol: Sham = 151 ± 15, INF = 134 ± 17, HF = 52 ± 7 g/g*; *p < 0.05). An increase in SERCA-2a protein expression in the RV was observed in the INF group but not in the HF group, which could explain the preserved inotropic response in these animals.

SIGNIFICANCE

Increased SERCA-2a protein expression may play a role in the preservation of RV function post-MI. Therefore, therapeutic strategies that attempt to increase SERCA protein expression levels may be useful for the treatment of HF.

From stem cells to cardiomyocytes: the role of forces in cardiac maturation, aging, and disease

Stem cell differentiation into a variety of lineages is known to involve signaling from the extracellular niche, including from the physical properties of that environment. What regulates stem cell responses to these cues is there ability to activate different mechanotransductive pathways. Here, we will review the structures and pathways that regulate stem cell commitment to a cardiomyocyte lineage, specifically examining proteins within muscle sarcomeres, costameres, and intercalated discs. Proteins within these structures stretch, inducing a change in their phosphorylated state or in their localization to initiate different signals. We will also put these changes in the context of stem cell differentiation into cardiomyocytes, their subsequent formation of the chambered heart, and explore negative signaling that occurs during disease.

Development and characterization of a novel fluorescent indicator protein PMCA4-GCaMP2 in cardiomyocytes

Isoform 4 of the plasma membrane calcium/calmodulin dependent ATPase (PMCA4) has recently emerged as an important regulator of several key pathophysiological processes in the heart, such as contractility and hypertrophy. However, direct monitoring of PMCA4 activity and assessment of calcium dynamics in its vicinity in cardiomyocytes are difficult due to the lack of molecular tools. In this study, we developed novel calcium fluorescent indicators by fusing the GCaMP2 calcium sensor to the N-terminus of PMCA4 to generate the PMCA4-GCaMP2 fusion molecule. We also identified a novel specific inhibitor of PMCA4, which might be useful for studying the role of this molecule in cardiomyocytes and other cell types. Using an adenoviral system we successfully expressed PMCA4-GCaMP2 in both neonatal and adult rat cardiomyocytes. This fusion molecule was correctly targeted to the plasma membrane and co-localised with caveolin-3. It could monitor signal oscillations in electrically stimulated cardiomyocytes. The PMCA4-GCaMP2 generated a higher signal amplitude and faster signal decay rate compared to a mutant inactive PMCA4(mut)GCaMP2 fusion protein, in electrically stimulated neonatal and adult rat cardiomyocytes. A small molecule library screen enabled us to identify a novel selective inhibitor for PMCA4, which we found to reduce signal amplitude of PMCA4-GCaMP2 and prolong the time of signal decay (Tau) to a level comparable with the signal generated by PMCA4(mut)GCaMP2. In addition, PMCA4-GCaMP2 but not the mutant form produced an enhanced signal in response to β-adrenergic stimulation. Together, the PMCA4-GCaMP2 and PMCA4(mut)GCaMP2 demonstrate calcium dynamics in the vicinity of the pump under active or inactive conditions, respectively. In summary, the PMCA4-GCaMP2 together with the novel specific inhibitor provides new means with which to monitor calcium dynamics in the vicinity of a calcium transporter in cardiomyocytes and may become a useful tool to further study the biological functions of PMCA4. In addition, similar approaches could be useful for studying the activity of other calcium transporters during excitation-contraction coupling in the heart.

Novel therapies in acute and chronic heart failure

Despite past advances in the pharmacological management of heart failure, the prognosis of these patients remains poor, and for many, treatment options remain unsatisfactory. Additionally, the treatments and clinical outcomes of patients with acute decompensated heart failure have not changed substantially over the past few decades. Consequently, there is a critical need for new drugs that can improve clinical outcomes. In the setting of acute heart failure, new inotrops such as cardiac myosin activators and new vasodilators such as relaxin have been developed. For chronic heart failure with reduced ejection fraction, there are several new approaches that target multiple pathophysiological mechanism including novel blockers of the renin-angiotensin-aldosterone system (direct renin inhibitors, dual-acting inhibitors of the angiotensin II receptor and neprilysin, aldosterone synthase inhibitors), ryanodine receptor stabilizers, and SERCA activators. Heart failure with preserved ejection fraction represents a substantial therapeutic problem as no therapy has been demonstrated to improve symptoms or outcomes in this condition. Newer treatment strategies target specific structural and functional abnormalities that lead to increased myocardial stiffness. Dicarbonyl-breaking compounds reverse advanced glycation-induced cross-linking of collagen and improve the compliance of aged and/or diabetic myocardium. Modulation of titin-dependent passive tension can be achieved via phosphorylation of a unique sequence on the extensible region of the protein. This review describes the pathophysiological basis, mechanism of action, and available clinical efficacy data of drugs that are currently under development. Finally, new therapies for the treatment of heart failure complications, such as pulmonary hypertension and anemia, are discussed.

A family of microRNAs encoded by myosin genes governs myosin expression and muscle performance

Myosin is the primary regulator of muscle strength and contractility. Here we show that three myosin genes, Myh6, Myh7, and Myh7b, encode related intronic microRNAs (miRNAs), which, in turn, control muscle myosin content, myofiber identity, and muscle performance. Within the adult heart, the Myh6 gene, encoding a fast myosin, coexpresses miR-208a, which regulates the expression of two slow myosins and their intronic miRNAs, Myh7/miR-208b and Myh7b/miR-499, respectively. miR-208b and miR-499 play redundant roles in the specification of muscle fiber identity by activating slow and repressing fast myofiber gene programs. The actions of these miRNAs are mediated in part by a collection of transcriptional repressors of slow myofiber genes. These findings reveal that myosin genes not only encode the major contractile proteins of muscle, but act more broadly to influence muscle function by encoding a network of intronic miRNAs that control muscle gene expression and performance.

Elucidating the role of reversible protein phosphorylation in sepsis-induced myocardial dysfunction

Mortality in children with sepsis is most often related to diminished cardiac output with cardiovascular collapse, resulting in impaired oxygen delivery and, ultimately, end-organ failure. Although cardiovascular "collapse" is commonly observed in individuals with septic shock, the hemodynamic causes of this differ greatly. In children, intrinsic myocardial dysfunction is most commonly present, whereas the systemic vascular resistance is typically high. This pattern is distinct from adults with sepsis where the principal hemodynamic profile shows elevated cardiac output, but substantially reduced systemic vascular resistance. Various studies support the concept that myocardial dysfunction, as occurs in pediatric septic patients, is due to intrinsic abnormalities in cardiomyocyte function and is not related to hypoperfusion as a result of low systemic vascular resistance. Importantly, when examined more closely, data from adults with septic shock also reveal that intrinsic myocardial dysfunction may play a larger role than previously appreciated. As a result, cardiovascular support, especially in pediatric sepsis, requires a treatment strategy directed at the underlying mechanism(s) responsible for this dysfunction. Thus, it is imperative to gain a better understanding of the myocardial derangements that occur during sepsis to identify targets that will ultimately influence the management of children with septic shock and favorably alter the associated mortality. We hypothesize that key signaling pathways that control myocardial calcium flux, regulated to key kinases and phosphatases, influence myocyte contractility in sepsis. Thus, we review the data relevant to the sepsis-induced intracellular alterations in calcium flux in the cardiomyocyte, with an emphasis on changes in the phosphorylation state of the contractile proteins regulated by the balance between kinases and phosphatases. We believe that therapies modulating the activity of these key proteins may provide an improvement in calcium handling and myocardial contractility and alter the clinical outcomes in sepsis.

Modelling sarcoplasmic reticulum calcium ATPase and its regulation in cardiac myocytes

When developing large-scale mathematical models of physiology, some reduction in complexity is necessarily required to maintain computational efficiency. A prime example of such an intricate cell is the cardiac myocyte. For the predictive power of the cardiomyocyte models, it is vital to accurately describe the calcium transport mechanisms, since they essentially link the electrical activation to contractility. The removal of calcium from the cytoplasm takes place mainly by the Na(+)/Ca(2+) exchanger, and the sarcoplasmic reticulum Ca(2+) ATPase (SERCA). In the present study, we review the properties of SERCA, its frequency-dependent and beta-adrenergic regulation, and the approaches of mathematical modelling that have been used to investigate its function. Furthermore, we present novel theoretical considerations that might prove useful for the elucidation of the role of SERCA in cardiac function, achieving a reduction in model complexity, but at the same time retaining the central aspects of its function. Our results indicate that to faithfully predict the physiological properties of SERCA, we should take into account the calcium-buffering effect and reversible function of the pump. This 'uncomplicated' modelling approach could be useful to other similar transport mechanisms as well.

Ca2+ clearance and contractility in vascular smooth muscle: evidence from gene-altered murine models

The central importance of calcium clearance proteins, and their regulators, in the modulation of myocardial contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) has long been established. Key players identified include the Na(+)-Ca(2+) exchanger, the Na(+)-K(+) ATPase, the sarco(endo)plasmic reticulum Ca(2+)-ATPase and associated phospholamban. Gene-targeted and transgenic murine models have been critical in the elucidation of their function. The study of these proteins in the regulation of contractile parameters in vascular smooth muscle, on the other hand, is less well studied. More recently, gene-targeted and transgenic models have expanded our knowledge of Ca(2+) clearance proteins and their role in both tonic and phasic smooth muscle contractility. In this review, we will briefly treat the mechanisms which underlie Ca(2+) clearance in smooth muscle. These will be addressed in light of studies using gene-modified mouse models, the results of which will be compared and contrasted with those in the cardiomyocyte. The recently identified human mutations in phospholamban, which lead to dilated cardiomyopathy, are also present in vascular and other smooth muscle. Given the importance of these Ca(2+) clearance systems to modulation of smooth muscle, it is likely that mutations will also lead to smooth muscle pathology.

Calcium in the heart: when it's good, it's very very good, but when it's bad, it's horrid

Ca(2+) increases in the heart control both contraction and transcription. To accommodate a short-term increased cardiovascular demand, neurohormonal modulators acting on the cardiac pacemaker and individual myocytes induce an increase in frequency and magnitude of myocyte contraction respectively. Prolonged, enhanced function results in hypertrophic growth of the heart, which is initially also associated with greater Ca(2+) signals and cardiac contraction. As a result of disease, however, hypertrophy progresses to a decompensated state and Ca(2+) signalling capacity and cardiac output are reduced. Here, the role that Ca(2+) plays in the induction of hypertrophy as well as the impact that cardiac hypertrophy and failure has on Ca(2+) fluxes will be discussed.

Targeting myocardial beta-adrenergic receptor signaling and calcium cycling for heart failure gene therapy

Heart failure (HF) is a leading cause of morbidity and mortality in Western countries and projections reveal that HF incidence in the coming years will rise significantly because of an aging population. Pharmacologic therapy has considerably improved HF treatment during the last 2 decades, but fails to rescue failing myocardium and to increase global cardiac function. Therefore, novel therapeutic approaches to target the underlying molecular defects of ventricular dysfunction and to increase the outcome of patients in HF are needed. Failing myocardium generally exhibits distinct changes in beta-adrenergic receptor (betaAR) signaling and intracellular Ca2+-handling providing opportunities for research. Recent advances in transgenic and gene therapy techniques have presented novel therapeutic strategies to alter myocardial function and to target both betaAR signaling and Ca2+-cycling. In this review, we will discuss functional alterations of the betaAR system and Ca2+-handling in HF as well as corresponding therapeutic strategies. We will then focus on recent in vivo gene therapy strategies using the targeted inhibition of the betaAR kinase (betaARK1 or GRK2) and the restoration of S100A1 protein expression to support the injured heart and to reverse or prevent HF.

Control of stress-dependent cardiac growth and gene expression by a microRNA

The heart responds to diverse forms of stress by hypertrophic growth accompanied by fibrosis and eventual diminution of contractility, which results from down-regulation of alpha-myosin heavy chain (alphaMHC) and up-regulation of betaMHC, the primary contractile proteins of the heart. We found that a cardiac-specific microRNA (miR-208) encoded by an intron of the alphaMHC gene is required for cardiomyocyte hypertrophy, fibrosis, and expression of betaMHC in response to stress and hypothyroidism. Thus, the alphaMHC gene, in addition to encoding a major cardiac contractile protein, regulates cardiac growth and gene expression in response to stress and hormonal signaling through miR-208.

The GAR-3 muscarinic receptor cooperates with calcium signals to regulate muscle contraction in the Caenorhabditis elegans pharynx

Muscarinic acetylcholine receptors regulate the activity of neurons and muscle cells through G-protein-coupled cascades. Here, we identify a pathway through which the GAR-3 muscarinic receptor regulates both membrane potential and excitation-contraction coupling in the Caenorhabditis elegans pharyngeal muscle. GAR-3 signaling is enhanced in worms overexpressing gar-3 or lacking GPB-2, a G-protein beta-subunit involved in RGS-mediated inhibition of G(o)alpha- and G(q)alpha-linked pathways. High levels of signaling through GAR-3 inhibit pharyngeal muscle relaxation and impair feeding--but do not block muscle repolarization--when worms are exposed to arecoline, a muscarinic agonist. Loss of gar-3 function results in shortened action potentials and brief muscle contractions in the pharyngeal terminal bulb. High levels of calcium entry through voltage-gated channels also impair terminal bulb relaxation and sensitize worms to the toxic effects of arecoline. Mutation of gar-3 reverses this sensitivity, suggesting that GAR-3 regulates calcium influx or calcium-dependent processes. Because the effects of GAR-3 signaling on membrane depolarization and muscle contraction can be separated, we conclude that GAR-3 regulates multiple calcium-dependent processes in the C. elegans pharyngeal muscle.

Impact of beta-myosin heavy chain expression on cardiac function during stress

OBJECTIVES

In failing mouse and human hearts, a shift in myosin heavy chain (MHC) isoform content from alpha to beta can occur. However, the impact of this phenomenon on disease progression is not well understood. Therefore, using transgenic (TG) mice, we tested how a pre-existing shift from alpha- to beta-MHC affects cardiac function under chronic mechanical or pharmacologic cardiovascular stress.

BACKGROUND

Expression of beta-MHC is considered to be energetically favorable, but this might be offset by depressed cardiac function.

METHODS

Transgenic mice with near-complete replacement of the normally predominant alpha- with beta-MHC were subjected to cardiac stress.

RESULTS

At baseline, TG mice show moderately reduced cardiac contractile function but are otherwise healthy with normal ventricular morphology. After four weeks of swimming, both TG and non-TG animals showed a 20% increase in left ventricular (LV)/body weight ratios. The TG hearts displayed mildly greater end-diastolic and end-systolic LV diameters than nontransgenic hearts after training, but no signs of LV failure were observed. However, chronic stimulation with isoproterenol resulted in augmented LV hypertrophy with signs of LV decompensation in TG mice. Furthermore, in a post-infarction failure model, TG hearts displayed accelerated LV dilation and a faster decline of shortening fraction.

CONCLUSIONS

Expression of beta-MHC appears to be disadvantageous to the mice under severe cardiovascular stress, implying that the alpha-->beta-MHC isoform shift observed in cardiac disease may be a maladaptive response.

Effects of Phospholipids on the Oligomeric State of Phospholamban of the Cardiac Sarcoplasmic Reticulum

BACKGROUND

Phospholamban is a reversible inhibitor of the Ca(2+)-ATPase of the cardiac sarcoplasmic reticulum (SR) and contributes to the regulation of heart muscle contractility. Because only the monomeric form, not the pentameric form, of phospholamban inhibits the Ca2+-pumping activity of the SR, it is important to understand the dynamic equilibrium between these 2 forms.

METHODS AND RESULTS

The effects of various detergents and phospholipids on the oligomeric state of phospholamban were examined. Among the detergents examined, 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS) stabilized the pentameric form whereas octylglucoside was an effective inducer of dissociation into the monomer. On the basis of these properties of the 2 detergents, the effects of various phospholipids on the oligomeric state of phospholamban with the use of mixed micelles containing octylglucoside or CHAPS were examined. Among the 8 phospholipids examined, phosphatidic acid was an effective inducer of pentamer dissociation, whereas the other phospholipids exhibited pentamer-stabilizing activity. Lysophosphatidylcholine, lysophosphatidylethanolamine, and phosphatidylglycerol were highly effective stabilizers of the pentamer.

CONCLUSIONS

The phospholipids in the SR membrane are important determinants of the equilibrium between the monomeric and pentameric forms of this protein. The results suggest that the metabolism of phospholipids in the cardiac SR may contribute to regulation of heart muscle contractility by shifting this equilibrium.

Molecular properties of cardiac tail-anchored membrane protein SLMAP are consistent with structural role in arrangement of excitation-contraction coupling apparatus

The spatial arrangement of the cell-surface membranes (sarcolemma and transverse tubules) and internal membranes of the sarcoplasmic reticulum relative to the myofibril is critical for effective excitation-contraction (E-C) coupling in cardiac myocytes; however, the molecular determinants of this order remain to be defined. Here, we ascribe molecular and cellular properties to the coiled-coil, tail-anchored sarcolemmal membrane-associated protein (SLMAP) that are consistent with a potential role in organizing the E-C coupling apparatus of the cardiomyocyte. The expression of SLMAP was developmentally regulated and its localization was distinctly apparent at the level of the membranes involved in regulating the E-C coupling mechanism. Several SLMAP isoforms were expressed in the cardiac myocyte with unique COOH-terminal membrane anchors that could target this molecule to distinct subcellular membranes. Protein interaction analysis indicated that SLMAPs could self assemble and bind myosin in cardiac muscle. The cardiac-specific expression of SLMAP isoforms that can be targeted to distinct subcellular membranes, self assemble, and interact with the myofibril suggests a potential role for this molecule in the structural arrangement of the E-C coupling apparatus.

Cardiac-specific overexpression of sarcolipin inhibits sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA2a) activity and impairs cardiac function in mice

Sarcolipin (SLN) inhibits the cardiac sarco(endo)plasmic reticulum Ca(2+) ATPase (SERCA2a) by direct binding and is superinhibitory if it binds through phospholamban (PLN). To determine whether overexpression of SLN in the heart might impair cardiac function, transgenic (TG) mice were generated with cardiac-specific overexpression of NF-SLN (SLN tagged at its N terminus with the FLAG epitope). The level of NF-SLN expression (the NF-SLN/PLN expression ratio) was equivalent to that which induces profound superinhibition when coexpressed with PLN and SERCA2a in HEK-293 cells. In TG hearts, the apparent affinity of SERCA2a for Ca(2+) was decreased compared with non-TG littermate control hearts. Invasive hemodynamic and echocardiographic analyses revealed impaired cardiac contractility and ventricular hypertrophy in TG mice. Basal PLN phosphorylation was reduced. In isolated papillary muscle subjected to isometric tension, peak amplitudes of Ca(2+) transients and peak tensions were reduced, whereas decay times of Ca(2+) transients and relaxation times of tension were increased in TG mice. Isoproterenol largely restored contractility in papillary muscle and stimulated PLN phosphorylation to wild-type levels in intact hearts. No compensatory changes in expression of SERCA2a, PLN, ryanodine receptor, and calsequestrin were observed in TG hearts. Coimmunoprecipitation indicated that overexpressed NF-SLN was bound to both SERCA2a and PLN, forming a ternary complex. These data suggest that NF-SLN overexpression inhibits SERCA2a through stabilization of SERCA2a-PLN interaction in the absence of PLN phosphorylation and through the inhibition of PLN phosphorylation. Inhibition of SERCA2a impairs contractility and calcium cycling, but responsiveness to beta-adrenergic agonists may prevent progression to heart failure.

Models of Dilated Cardiomyopathy in Small Animals and Novel Positive Inotropic Therapies

Several randomized clinical trials of vesnarinone and milrinone in patients with heart failure left disappointing results in the 1990s. Thereafter, use of positive inotropic agents has been avoided. Exceptions are the use of digitalis glycosides to treat mild-moderate heart failure and the intravenous administration of catecholamines and phosphodiesterase inhibitors in patients with acute and/or refractory heart failure. It is not, however, exactly known whether chronic enhancement of cardiac contractility indeed has harmful effects, besides increased risk of arrhythmia and mortality. We investigated the potential chronic benefit of positive inotropic modification to treat progressive cardiomyopathy and associated heart failure using a genetic complementation strategy of muscle lim-protein and phospholamban (PLN) double mutagenesis in the mouse and found clear evidence of positive effects. Subsequent somatic modification of PLN function via gene transfer with recombinant adeno-associated virus vectors in small animal models of dilated cardiomyopathy further supported the chronic benefit of enhanced cardiac function achieved in an beta-adrenergic stimulus-independent manner. This study examines current small animal models of dilated cardiomyopathy and recent multiple attempts to use these models as novel gene-based inotropic therapies.

N -Acetylcysteine Prevents the Deleterious Effect of Tumor Necrosis Factor-α on Calcium Transients and Contraction in Adult Rat Cardiomyocytes

Background— The negative effect of tumor necrosis factor-α (TNF-α) on heart contraction, which is mediated by sphingosine, is a major component in heart failure. Because the cellular level of glutathione may limit sphingosine production via the inhibition of the Mg-dependent neutral sphingomyelinase (N-SMase), we hypothesized that cardiac glutathione status might determine the negative contractile response to TNF-α.

Methods and Results— We examined the effects of TNF-α in isolated cardiomyocytes obtained from control rats or rats that were given the glutathione precursor N -acetylcysteine (NAC, 100 mg IP per animal). In cardiomyocytes obtained from control rats, 25 ng/mL TNF-α increased reactive oxygen species generation and N-SMase activity (500% and 34% over basal, respectively) and decreased the amplitude of [Ca 2+ ] i in response to electrical stimulation (22% below basal). NAC treatment increased cardiac glutathione content by 42%. In cardiomyocytes obtained from NAC-treated rats, 25 ng/mL TNF-α had no effect on reactive oxygen species production or N-SMase activity but increased the amplitude of [Ca 2+ ] i transients and contraction in response to electrical stimulation by 40% to 50% over basal after 20 minutes. This was associated with a hastened relaxation (20% reduction in t 1/2 compared with basal) and an increased phosphorylation of both Ser 16 - and Thr 17 -phospholamban residues (260% and 115% of maximal isoproterenol effect, respectively).

Conclusions— It is concluded that cardiac glutathione status, by controlling N-SMase activation, determines the severity of the adverse effects of TNF-α on heart contraction. Glutathione supplementation may therefore provide therapeutic benefits for vulnerable hearts.

Role of dual-site phospholamban phosphorylation in the stunned heart: insights from phospholamban site-specific mutants

Phosphorylation of phospholamban (PLB) at Ser16 (protein kinase A site) and at Thr17 [Ca2+/calmodulin kinase II (CaMKII) site] increases sarcoplasmic reticulum Ca2+ uptake and myocardial contractility and relaxation. In perfused rat hearts submitted to ischemia-reperfusion, we previously showed an ischemia-induced Ser16 phosphorylation that was dependent on beta-adrenergic stimulation and an ischemia and reperfusion-induced Thr17 phosphorylation that was dependent on Ca2+ influx. To elucidate the relationship between these two PLB phosphorylation sites and postischemic mechanical recovery, rat hearts were submitted to ischemia-reperfusion in the absence and presence of the CaMKII inhibitor KN-93 (1 microM) or the beta-adrenergic blocker dl-propranolol (1 microM). KN-93 diminished the reperfusion-induced Thr17 phosphorylation and depressed the recovery of contraction and relaxation after ischemia. dl-Propranolol decreased the ischemia-induced Ser16 phosphorylation but failed to modify the contractile recovery. To obtain further insights into the functional role of the two PLB phosphorylation sites in postischemic mechanical recovery, transgenic mice expressing wild-type PLB (PLB-WT) or PLB mutants in which either Thr17 or Ser16 were replaced by Ala (PLB-T17A and PLB-S16A, respectively) into the PLB-null background were used. Both PLB mutants showed a lower contractile recovery than PLB-WT. However, this recovery was significantly impaired all along reperfusion in PLB-T17A, whereas it was depressed only at the beginning of reperfusion in PLB-S16A. Moreover, the recovery of relaxation was delayed in PLB-T17A, whereas it did not change in PLB-S16A, compared with PLB-WT. These findings indicate that, although both PLB phosphorylation sites are involved in the mechanical recovery after ischemia, Thr17 appears to play a major role.

Gene therapy for the treatment of heart failure--calcium signaling

The knowledge of molecular mechanisms indicated in cardiac dysfunction has increased dramatically over the last decade and yields considerable potential for new treatment options in heart failure. Alterations in intracellular calcium signaling play a crucial role in the pathophysiology of heart failure, and in recent years, somatic gene transfer has been identified as an important tool to help understand the relative contribution of specific calcium-handling proteins in heart failure. This article reviews recent advances in gene delivery techniques aimed at global myocardial transfection and discusses molecular therapeutic targets identified within intracellular calcium signaling pathways in heart failure.

Mutation of the phospholamban promoter associated with hypertrophic cardiomyopathy

Phospholamban is an endogenous inhibitor of sarcoplasmic reticulum calcium ATPase and plays a prime role in cardiac contractility and relaxation. Phospholamban may be a candidate gene responsible for cardiomyopathy. We investigated genome sequence of phospholamban in patients with cardiomyopathy. PCR-based direct sequence was performed for the promoter region and the whole coding region of phospholamban in 87 hypertrophic, 10 dilated, and 2 restricted cardiomyopathic patients. We found a heterozygous single nucleotide transition from A to G at -77-bp upstream of the transcription start site in the phospholamban promoter region of one patient with familial hypertrophic cardiomyopathy. This nucleotide change was not found in 296 control subjects. Using neonatal rat cardiomyocytes, the mutation, -77A-->G, increased the phospholamban promoter activity. No nucleotide change in the phospholamban coding region was found in 99 patients with cardiomyopathy. We suspect that the mutation plays an important role in the development of hypertrophic cardiomyopathy.

Is depressed myocyte contractility centrally involved in heart failure?

This review examines the evidence for and against the hypothesis that abnormalities in cardiac contractility initiate the heart failure syndrome and drive its progression. There is substantial evidence that the contractility of failing human hearts is depressed and that abnormalities of basal Ca2+ regulation and adrenergic regulation of Ca2+ signaling are responsible. The cellular and molecular defects that cause depressed myocyte contractility are not well established but seem to culminate in abnormal sarcoplasmic reticulum uptake, storage, and release. There are also strong links between Ca2+ regulation, Ca2+ signaling pathways, hypertrophy, and heart failure that need to be more clearly delineated. There is not substantial direct evidence for a causative role for depressed contractility in the initiation and progression of human heart failure, and some studies show that heart failure can occur without depressed myocyte contractility. Stronger support for a causal role for depressed contractility in the initiation of heart failure comes from animal studies where maintaining or improving contractility can prevent heart failure. Recent clinical studies in humans also support the idea that beneficial heart failure treatments, such as beta-adrenergic antagonists, involve improved contractility. Current or previously used heart failure treatments that increase contractility, primarily by increasing cAMP, have generally increased mortality. Novel heart failure therapies that increase or maintain contractility or adrenergic signaling by selectively modulating specific molecules have produced promising results in animal experiments. How to reliably implement these potentially beneficial inotropic therapies in humans without introducing negative side effects is the major unanswered question in this field.

Ablation of PLB exacerbates ischemic injury to a lesser extent in female than male mice: protective role of NO

Recent studies suggest a role for phospholamban phosphorylation during ischemia and reperfusion. The role of phospholamban in ischemia was studied by subjecting hearts from male and female wild-type (MWT/FWT) and phospholamban-knockout (MKO/FKO) mice to 20 min of ischemia-40 min of reperfusion while (31)P NMR spectra were acquired. ATP and pH values fell lower during ischemia, and postischemic contractility was less, in MKO and FKO versus WT hearts. After shorter ischemia (15 min), recoveries of contraction, ATP, and pH were greater in FKO than MKO hearts. To examine the role of nitric oxide (NO) synthases (NOS) in the protection in FKO versus MKO hearts, we utilized 1 microM l-NAME, a NOS inhibitor, or 100 microM S-nitroso-N-acetylpenicillamine (SNAP), an NO donor. Recoveries of function, ATP, and pH were less in l-NAME-treated FKO than untreated FKO hearts and greater in SNAP-treated MKO than untreated MKO hearts. In conclusion, phospholamban ablation increased ischemic injury in both males and females; however, female hearts were less susceptible than male hearts. Protection in females was decreased by a NOS inhibitor and mimicked in males by an NO donor, implying that protection was NOS mediated.

Comparable levels of Ca-ATPase inhibition by phospholamban in slow-twitch skeletal and cardiac sarcoplasmic reticulum

Alterations in expression levels of phospholamban (PLB) relative to the sarcoplasmic reticulum (SR) Ca-ATPase have been suggested to underlie defects of calcium regulation in the failing heart and other cardiac pathologies. To understand how variation in PLB expression relative to that of the Ca-ATPase can modulate calcium transport, we have investigated the inhibition of the Ca-ATPase by PLB in native SR membranes from slow-twitch skeletal and cardiac muscle and in reconstituted proteoliposomes. Quantitative immunoblotting in combination with affinity-purified protein standards was used to measure protein concentrations of PLB and of the Ca-ATPase. Functional inhibition of the Ca-ATPase was determined from both the calcium concentrations for half-maximal activation (Ca(1/2)) and the shift in the calcium concentrations following release of PLB inhibition (i.e., (Delta)Ca(1/2)) by incubation with monoclonal antibodies against PLB, which are equivalent to phosphorylation of PLB by cAMP-dependent protein kinase. We report that equivalent levels of PLB inhibition and antibody-induced activation ((Delta)Ca(1/2) = 0.25 +/- 0.02 microM) are observed in SR membranes from slow-twitch skeletal and cardiac muscle, where molar stoichiometries of PLB expressed per Ca-ATPase vary, respectively, from 0.9 +/- 0.1 to 4.1 +/- 0.8. Similar levels of inhibition to those observed in isolated SR vesicles were observed using reconstituted proteoliposomes following co-reconstitution of affinity-purified Ca-ATPase with PLB. These results indicate that total expression levels of one PLB per Ca-ATPase result in full inhibition of the Ca-ATPase and, based on the measured K(D) (140 +/- 30 microM), suggests one PLB complexed with two Ca-ATPase molecules is sufficient for full inhibition of activity. Therefore, the excess PLB expressed in the heart over that required for inhibition suggests a capability for graded responses of the Ca-ATPase activity to endogenous kinases and phosphatases that modulate the level of phosphorylation necessary to relieve inhibition of the Ca-ATPase by PLB.

Ryanodine receptor calcium release channels

The ryanodine receptors (RyRs) are a family of Ca2+ release channels found on intracellular Ca2+ storage/release organelles. The RyR channels are ubiquitously expressed in many types of cells and participate in a variety of important Ca2+ signaling phenomena (neurotransmission, secretion, etc.). In striated muscle, the RyR channels represent the primary pathway for Ca2+ release during the excitation-contraction coupling process. In general, the signals that activate the RyR channels are known (e.g., sarcolemmal Ca2+ influx or depolarization), but the specific mechanisms involved are still being debated. The signals that modulate and/or turn off the RyR channels remain ambiguous and the mechanisms involved unclear. Over the last decade, studies of RyR-mediated Ca2+ release have taken many forms and have steadily advanced our knowledge. This robust field, however, is not without controversial ideas and contradictory results. Controversies surrounding the complex Ca2+ regulation of single RyR channels receive particular attention here. In addition, a large body of information is synthesized into a focused perspective of single RyR channel function. The present status of the single RyR channel field and its likely future directions are also discussed.

Beta-adrenoceptor agonists stimulate endothelial nitric oxide synthase in rat urinary bladder urothelial cells

We have investigated the intracellular signaling mechanisms underlying the release of nitric oxide (NO) evoked by beta-adrenoceptor (AR) agonists in urinary bladder strips and cultured bladder urothelial cells from adult rats. Reverse transcription-PCR revealed that inducible NO synthase and endothelial NOS but not neuronal NOS genes were expressed in urothelial cells. NO release from both urothelial cells and bladder strips was decreased (37-42%) in the absence of extracellular Ca2+ (100 microm EGTA) and was ablated after incubation with BAPTA-AM (5 microm) or caffeine (10 mm), indicating that the NO production is mediated in part by intracellular calcium stores. NO release was reduced (18-24%) by nifedipine (10 microm) and potentiated (29-32%) by incubation with the Ca2+ channel opener BAYK8644 (1-10 microm). In addition, beta-AR-evoked NO release (isoproterenol; dobutamine; terbutaline; 10(-9) to 10(-5) m) was blocked by the NOS inhibitors N(G)-nitro-L-arginine methyl ester (30 microm) or N(G)-monomethyl-L-arginine (50 microm), by beta-adrenoceptor antagonists (propranol, beta1/beta2; atenolol, beta1; ICI 118551; beta2; 100 microm), or by the calmodulin antagonist trifluoperazine (50 microm). Incubating cells with the nonhydrolyzable GTP analog GTPgammaS (1 microm) or the membrane-permeant cAMP analog dibutyryl-cAMP (10-100 microm) directly evoked NO release. Forskolin (10 microm) or the phosphodiesterase IBMX (50 microm) enhanced (39-42%) agonist-evoked NO release. These results indicate that beta-adrenoceptor stimulation activates the adenylate cyclase pathway in bladder epithelial cells and initiates an increase in intracellular Ca2+ that triggers NO production and release. These findings are considered in light of recent reports that urothelial cells may exhibit a number of "neuron-like" properties, including the expression of receptors/ion channels similar to those found in sensory neurons.

Cardiac remodeling and atrial fibrillation in transgenic mice overexpressing junctin

Junctin is a 26-kDa integral membrane protein, colocalized with the ryanodine receptor (RyR) and calsequestrin at the junctional sarcoplasmic reticulum (SR) membrane in cardiac and skeletal muscles. To elucidate the functional role of junctin in heart, transgenic (TG) mice overexpressing canine junctin (24-29 folds) under the control of mouse a-myosin heavy chain promoter were generated. Overexpression of the junctin in mouse heart was associated with heart enlargements, bradycardia, atrial fibrillation, and increased fibrosis. Many ultrastructural alterations were observed in TG atria. The junctional SR cisternae facing transverse-tubules contained a dense matrix of calsequestrin in TG heart. According to echocardiography, TG mice showed enlarged left ventricles, dilated right atriums, and ventricles with paradoxical septal motion and impaired left ventricular systolic function. Overexpression of junctin led to down-regulation of triadin and RyR but to up-regulation of dihydropyridine receptor. The L-type Ca2+ current density and action potential durations increased, which could be the cause for the bradycardia in TG heart. This study provides an important example of pathogenesis leading to substantial cardiac remodeling and atrial fibrillation, which was caused by overexpression of junctin in heart.

Three novel sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 3 isoforms. Expression, regulation, and function of the membranes of the SERCA3 family

Sarco/endoplasmic reticulum Ca2+-ATPases (SERCAs) pump Ca2+ into the endoplasmic reticulum. Recently, three human SERCA3 (h3a-c) proteins and a previously unknown rat SERCA3 (r3b/c) mRNA have been described. Here, we (i) document two novel human SERCA3 splice variants h3d and h3e, (ii) provide data for the expression and mechanisms regulating the expression of all known SERCA3 variants (r3a, r3b/c, and h3a-e), and (iii) show functional characteristics of the SERCA3 isoforms. h3d and h3e are issued from the insertion of an additional penultimate exon 22 resulting in different carboxyl termini for these variants. Distinct distribution patterns of the SERCA3 gene products were observed in a series of cell lines of hematopoietic, epithelial, embryonic origin, and several cancerous types, as well as in panels of rat and human tissues. Hypertension and protein kinase C, calcineurin, or retinoic acid receptor signaling pathways were found to differently control rat and human splice variant expression, respectively. Stable overexpression of each variant was performed in human embryonic kidney 293 cells, and the SERCA3 isoforms were fully characterized. All SERCA3 isoforms were found to pump Ca2+ with similar affinities. However, they modulated the cytosolic Ca2+ concentration ([Ca2+]c) and the endoplasmic reticulum Ca2+ content ([Ca2+]er) in different manners. A newly generated polyclonal antibody and a pan-SERCA3 antibody proved the endogenous expression of the three novel SERCA3 proteins, h3d, h3e, and r3b/c. All these data suggest that the SERCA3 gene products have a more widespread role in cellular Ca2+ signaling than previously appreciated.

Conditional mutation of the ErbB2 (HER2) receptor in cardiomyocytes leads to dilated cardiomyopathy

The ErbB2 (Her2) proto-oncogene encodes a receptor tyrosine kinase, which is frequently amplified and overexpressed in human tumors. ErbB2 provides the target for a novel and effective antibody-based therapy (Trastuzumab/Herceptin) used for the treatment of mammary carcinomas. However, cardiomyopathies develop in a proportion of patients treated with Trastuzumab, and the incidence of such complications is increased by combination with standard chemotherapy. Gene ablation studies have previously demonstrated that the ErbB2 receptor, together with its coreceptor ErbB4 and the ligand Neuregulin-1, are essential for normal development of the heart ventricle. We use here Cre-loxP technology to mutate ErbB2 specifically in ventricular cardiomyocytes. Conditional mutant mice develop a severe dilated cardiomyopathy, with signs of cardiac dysfunction generally appearing by the second postnatal month. We infer that signaling from the ErbB2 receptor, which is enriched in T-tubules in cardiomyocytes, is crucial for adult heart function. Conditional ErbB2 mutant mice provide a model of dilated cardiomyopathy. In particular, they will allow a rigorous assessment of the role of ErbB2 in the heart and provide insight into the molecular mechanisms that underlie the adverse effects of anti-ErbB2 antibodies.

Impaired cardiac contractility response to hemodynamic stress in S100A1-deficient mice

Ca(2+) signaling plays a central role in cardiac contractility and adaptation to increased hemodynamic demand. We have generated mice with a targeted deletion of the S100A1 gene coding for the major cardiac isoform of the large multigenic S100 family of EF hand Ca(2+)-binding proteins. S100A1(-/-) mice have normal cardiac function under baseline conditions but have significantly reduced contraction rate and relaxation rate responses to beta-adrenergic stimulation that are associated with a reduced Ca(2+) sensitivity. In S100A1(-/-) mice, basal left-ventricular contractility deteriorated following 3-week pressure overload by thoracic aorta constriction despite a normal adaptive hypertrophy. Surprisingly, heterozygotes also had an impaired response to acute beta-adrenergic stimulation but maintained normal contractility in response to chronic pressure overload that coincided with S100A1 upregulation to wild-type levels. In contrast to other genetic models with impaired cardiac contractility, loss of S100A1 did not lead to cardiac hypertrophy or dilation in aged mice. The data demonstrate that high S100A1 protein levels are essential for the cardiac reserve and adaptation to acute and chronic hemodynamic stress in vivo.

Structure and orientation of sarcolipin in lipid environments

Sarcolipin (SLN) is a 31 amino acid integral membrane protein that regulates Ca-ATPase activity in skeletal muscle. Here, we report the three-dimensional structure and topology of synthetic SLN in lipid environments, as determined by solution and solid-state NMR spectroscopy. 2D solution NMR experiments were performed on SLN solubilized in sodium dodecyl sulfate (SDS) micelles. We found that SLN adopts a highly defined alpha-helical conformation from F9 through R27, with a backbone RMSD of 0.65 A and a side chain RMSD of 1.66 A. The N-terminus (M1 through L8) and the C-terminus (S28 through Y31) are mostly unstructured. The orientation of the SLN was determined using one-dimensional (15)N NMR solid-state spectroscopy. The protein was incorporated into phospholipid bilayers prepared from a mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine. The (15)N chemical shift solid-state spectra from selectively labeled SLN samples indicate that SLN orients perpendicularly to the plane of the membrane bilayers. These results support the proposed mechanism of Ca-ATPase regulation of SLN via protein-protein intramembranous interactions between the highly conserved transmembrane domains of the Ca-ATPase and the conserved transmembrane domain of SLN.

Ryanodine receptor dysfunction in hearts of streptozotocin-induced diabetic rats

Studies have shown that evoked calcium release from sarcoplasmic reticulum is compromised in diabetic rat hearts. The present study was undertaken to determine whether this decrease might be ascribed to a reduction in expression and/or alteration in function of ryanodine receptor (RyR2) and whether changes could be minimized with insulin treatment. Hearts were isolated from 4- and 6-week streptozotocin (STZ)-induced diabetic, 4-week diabetic/2-week insulin-treated, and age-matched control rats. RyR2 mRNA and protein levels were determined using reverse transcription-polymerase chain reactions and polyacrylamide gel electrophoresis, respectively, whereas the functional integrity of RyR2 was assessed from their ability to bind [3H]ryanodine. RyR2 protein was unchanged with up to 6 weeks of untreated STZ-induced diabetes. Two weeks of insulin treatment initiated after 4 weeks of diabetes increased RyR2 mRNA levels by 42% and RyR2 protein levels by 45 to 61%. At equivalent amounts, RyR2 protein from 4-week STZ-induced diabetic rat hearts bound 9% less [3H]ryanodine than age-matched control rats (74.1 +/- 3.9 versus 67.4 +/- 3.4 fmol/microg RyR2), whereas that from 6-week STZ-diabetic rats bound 36% less than control rats (47.9 +/- 4.8 versus 74.2 +/- 4.5 fmol/microg RyR2, p < 0.05). RyR2 from insulin-treated animals bound significantly less [3H]ryanodine than control rats (65.2 +/- 4.9 fmol/microg RyR2, p < 0.05). Apparent affinity of ryanodine for RyR2 was similar among all groups (K(d) approximately 1.04 +/- 0.08 nM). Because expression did not change significantly but ryanodine binding decreased, these data suggest that the functional integrity of RyR2 is compromised in diabetic rat hearts, and these changes can be attenuated with 2 weeks of insulin treatment.

Molecular genetics of Ca(2+) stores and intracellular Ca(2+) signalling

An increasing number of studies based on recombinant cells and on mouse models that express an altered repertoire of some of the key components of the intracellular Ca(2+) release stores are becoming available as a result of molecular genetics techniques. Information from these studies, together with results from studies of human diseases caused by mutations in genes that encode proteins of the intracellular Ca(2+) stores, are providing a significant advancement in understanding the interactive nature of the molecular machinery that underlies intracellular Ca(2+) signalling and how the different components of the Ca(2+) stores contribute to the regulation of cellular functions.

Overexpression of sarcolemmal calcium pump attenuates induction of cardiac gene expression in response to ET-1

The function of the plasma membrane calmodulin-dependent calcium ATPase (PMCA) in myocardium is unknown. PMCA is localized in caveolae, 50- to 100-nm membrane invaginations, which also contain receptors for endothelin-1 (ET-1) and various other ligands. PMCA has been suggested to play a role in regulation of caveolar signal transduction. We studied the effects of the hypertrophic agonist ET-1 and increased coronary perfusion pressure on cardiac synthesis of B-type natriuretic peptide (BNP) in transgenic rats overexpressing the human PMCA 4CI in isolated perfused heart preparation. ET-1 infusion for 2 h increased BNP mRNA levels twofold in left ventricles (LV) of nontransgenic rats, whereas no increase was noted in PMCA rat hearts. Similar responses were seen in adrenomedullin and c-fos mRNA levels, and in immunoreactive BNP secretion. Increased mechanical load produced by elevated perfusion pressure induced similar 1.5- to 1.6-fold increases in LV BNP mRNA in both nontransgenic and PMCA rat hearts. These results show that cardiac overexpression of PMCA attenuates ET-1-stimulated early induction of cardiac gene expression, suggesting that PMCA may modulate myocardial growth responses.

Gene transfer: manipulating and monitoring function in cells and tissues

1. The ectopic expression of genes has proven to be an extremely valuable tool for biologists. The most widely used systems involve electrically or chemically mediated transfer of genes to immortalized cell lines and, at the other end of the spectrum, transgenic animal models. As would be expected, there are compromises to be made when using either of these broad approaches. Immortalized cell lines have limited "physiological relevance" and transgenic approaches are costly and out of the reach of many laboratories. There is also significant time required for the de novo generation of a transgenic animal. 2. As a viable alternative to these approaches, we describe the use of recombinant adenovirus and Sindbis virus to deliver genes to cells and tissues. 3. We exemplify this approach with studies from our laboratories: (i) an investigation of Ca2+ handling deficits in cardiac myocytes of hypertrophied hearts using infection with recombinant adenovirus encoding either green fluorescent protein (GFP) or the sarcoplasmic/endoplasmic reticulum calcium-ATPase (Serca2a); (ii) a study of the mechanism of macrophage/microglial migration by infection of embryonic phagocytes with a GFP-encoding virus and coculture with brain slices to then track the movement of labelled cells; and (iii) we are also exploiting the natural tropism of the Sindbis virus to label neurons in hippocampal brain slices in culture to resolve high-resolution structure and to map neuronal connectivity. 4. Further development of these approaches should open new avenues of investigation for the study of physiology in a range of cells and tissues.

Human coxsackie-adenovirus receptor is colocalized with integrins alpha(v)beta(3) and alpha(v)beta(5) on the cardiomyocyte sarcolemma and upregulated in dilated cardiomyopathy: implications for cardiotropic viral infections

BACKGROUND

The coxsackievirus and adenovirus receptor (CAR) was identified as a common cellular receptor for both viruses, but its biological and pathogenic relevance is uncertain. Knowledge of CAR localization in the human cardiovascular system is limited but important with respect to CAR-dependent viral infections and gene transfer using CAR-dependent viral vectors.

METHODS AND RESULTS

Explanted failing hearts from 13 patients (8 with dilated cardiomyopathy [DCM] and 5 with other heart diseases [non-DCM]) and normal donor hearts (n=7) were investigated for the expression levels and subcellular localization of CAR and the adenovirus coreceptors alpha(v)beta(3) and alpha(v)beta(5) integrins. CAR immunoreactivity was very low in normal and non-DCM hearts, whereas strong CAR signals occurred at the intercalated discs and sarcolemma in 5 of the 8 DCM hearts (62.5%); these strong signals colocalized with both integrins. In all hearts, CAR was detectable in subendothelial layers of the vessel wall, but not on the luminal endothelial surface, and on interstitial cells. Human CAR (hCAR) expressed in rat cardiomyocytes was targeted to cell-cell contacts, which resembled CAR localization in DCM hearts and resulted in 15-fold increased adenovirus uptake.

CONCLUSIONS

Low hCAR abundance may render normal human myocardium resistant to CAR-dependent viruses, whereas re-expression of hCAR, such as that observed in DCM, may be a key determinant of cardiac susceptibility to viral infections. Asymmetric expression of hCAR in the vessel wall may be an important determinant of adenovirus tropism in humans. hCAR subcellular localization in human myocardium and hCAR targeting to cell-cell contacts in cardiomyocyte cultures suggest that hCAR may play a role in cell-cell contact formation.