Novel pharmacological treatments for heart failure

Genetic, clinical, molecular, and pathogenic aspects of the South Asian-specific polymorphic MYBPC3Δ25bp variant

Hypertrophic cardiomyopathy (HCM) is a cardiac genetic disease characterized by ventricular enlargement, diastolic dysfunction, and increased risk for sudden cardiac death. Sarcomeric genetic defects are the predominant known cause of HCM. In particular, mutations in the myosin-binding protein C gene (MYBPC3) are associated with ~ 40% of all HCM cases in which a genetic basis has been established. A decade ago, our group reported a 25-base pair deletion in intron 32 of MYBPC3 (MYBPC3Δ25bp) that is uniquely prevalent in South Asians and is associated with autosomal dominant cardiomyopathy. Although our studies suggest that this deletion results in left ventricular dysfunction, cardiomyopathies, and heart failure, the precise mechanism by which this variant predisposes to heart disease remains unclear. Increasingly appreciated, however, is the contribution of secondary risk factors, additional mutations, and lifestyle choices in augmenting or modifying the HCM phenotype in MYBPC3Δ25bp carriers. Therefore, the goal of this review article is to summarize the current research dedicated to understanding the molecular pathophysiology of HCM in South Asians with the MYBPC3Δ25bp variant. An emphasis is to review the latest techniques currently applied to explore the MYBPC3Δ25bp pathogenesis and to provide a foundation for developing new diagnostic strategies and advances in therapeutics.

Phosphorylation of cardiac myosin-binding protein-C contributes to calcium homeostasis

Cardiac myosin-binding protein-C (cMyBP-C) is highly phosphorylated under basal conditions. However, its phosphorylation level is decreased in individuals with heart failure. The necessity of cMyBP-C phosphorylation for proper contractile function is well-established, but the physiological and pathological consequences of decreased cMyBP-C phosphorylation in the heart are not clear. Herein, using intact adult cardiomyocytes from mouse models expressing phospho-ablated (AAA) and phosphomimetic (DDD) cMyBP-C as well as controls, we found that cMyBP-C dephosphorylation is sufficient to reduce contractile parameters and calcium kinetics associated with prolonged decay time of the calcium transient and increased diastolic calcium levels. Isoproterenol stimulation reversed the depressive contractile and Ca²⁺-kinetic parameters. Moreover, caffeine-induced calcium release yielded no difference between AAA/DDD and controls in calcium content of the sarcoplasmic reticulum. On the other hand, sodium-calcium exchanger function and phosphorylation levels of calcium-handling proteins were significantly decreased in AAA hearts compared with controls. Stress conditions caused increases in both spontaneous aftercontractions in AAA cardiomyocytes and the incidence of arrhythmias in vivo compared with the controls. Treatment with omecamtiv mecarbil, a positive cardiac inotropic drug, rescued the contractile deficit in AAA cardiomyocytes, but not the calcium-handling abnormalities. These findings indicate a cascade effect whereby cMyBP-C dephosphorylation causes contractile defects, which then lead to calcium-cycling abnormalities, resulting in aftercontractions and increased incidence of cardiac arrhythmias under stress conditions. We conclude that improvement of contractile deficits alone without improving calcium handling may be insufficient for effective management of heart failure.

Molecular basis of functional myogenic specification of Bona Fide multipotent adult cardiac stem cells

Ischemic Heart Disease (IHD) remains the developed world's number one killer. The improved survival from Acute Myocardial Infarction (AMI) and the progressive aging of western population brought to an increased incidence of chronic Heart Failure (HF), which assumed epidemic proportions nowadays. Except for heart transplantation, all treatments for HF should be considered palliative because none of the current therapies can reverse myocardial degeneration responsible for HF syndrome. To stop the HF epidemic will ultimately require protocols to reduce the progressive cardiomyocyte (CM) loss and to foster their regeneration. It is now generally accepted that mammalian CMs renew throughout life. However, this endogenous regenerative reservoir is insufficient to repair the extensive damage produced by AMI/IHD while the source and degree of CM turnover remains strongly disputed. Independent groups have convincingly shown that the adult myocardium harbors bona-fide tissue specific cardiac stem cells (CSCs). Unfortunately, recent reports have challenged the identity and the endogenous myogenic capacity of the c-kit expressing CSCs. This has hampered progress and unless this conflict is settled, clinical tests of repair/regenerative protocols are unlikely to provide convincing answers about their clinical potential. Here we review recent data that have eventually clarified the specific phenotypic identity of true multipotent CSCs. These cells when coaxed by embryonic cardiac morphogens undergo a precisely orchestrated myogenic commitment process robustly generating bona-fide functional cardiomyocytes. These data should set the path for the revival of further investigation untangling the regenerative biology of adult CSCs to harness their potential for HF prevention and treatment.

Deleterious effects of sugar and protective effects of starch on cardiac remodeling, contractile dysfunction, and mortality in response to pressure overload

Little is known about the effects of the composition of dietary carbohydrate on the development of left ventricular (LV) hypertrophy (LVH) and heart failure (HF) under conditions of pressure overload. The objective of this study was to determine the effect of carbohydrate composition on LVH, LV function, and mortality in a mouse model of chronic pressure overload. Male C57BL/6J mice of 6 wk of age ( n = 14–16 mice/group) underwent transverse aortic constriction (TAC) or sham surgery and were fed either standard chow (STD; 32% corn starch, 35% sucrose, 3% maltodextrin, and 10% fat expressed as a percent of the total energy), high-starch chow (58% corn starch, 12% maltodextrin, and 10% fat), or high-fructose chow (9% corn starch, 61% fructose, and 10% fat). After 16 wk of treatment, mice with TAC fed the STD or high-fructose diets exhibited increased LV mass, larger end-diastolic and end-systolic diameters, and decreased ejection fraction compared with sham. The high-starch diet, in contrast, prevented changes in LV dimensions and contractile function. Cardiac mRNA for myosin heavy chain-β was increased dramatically in the fructose-fed banded animals, as was mortality (54% compared with 8% and 29% in the starch and STD banded groups, respectively). In conclusion, a diet high in simple sugar was deleterious, resulting in the highest mortality and expression of molecular markers of cardiac dysfunction in TAC animals compared with sham, whereas a high-starch diet blunted mortality, increases in cardiac mass, and contractile dysfunction.

Emerging therapies for the management of decompensated heart failure: from bench to bedside

While pharmaceutical innovation has been highly successful in reducing mortality in chronic heart failure, this has not been matched by similar success in decompensated heart failure syndromes. Despite outstanding issues over definitions and end points, we argue in this paper that an unprecedented wealth of pharmacologic innovation may soon transform the management of these challenging patients. Agents that target contractility, such as cardiac myosin activators and novel adenosine triphosphate-dependent transmembrane sodium-potassium pump inhibitors, provide inotropic support without arrhythmogenic increases in cytosolic calcium or side effects of more traditional agents. Adenosine receptor blockade may improve glomerular filtration and diuresis by exerting a direct beneficial effect on glomerular blood flow while vasopressin antagonists promote free water excretion without compromising renal function and may simultaneously inhibit myocardial remodeling. Urodilatin, the renally synthesized isoform of atrial natriuretic peptide, may improve pulmonary congestion via vasodilation and enhanced diuresis. Finally, metabolic modulators such as perhexiline may optimize myocardial energy utilization by shifting adenosine triphosphate production from free fatty acids to glucose, a unique and conceptually appealing approach to the management of heart failure. These advances allow optimism not only for the advancement of our understanding and management of decompensated heart failure syndromes but for the translational research effort in heart failure biology in general.

Oxidative stress causes heart failure with impaired mitochondrial respiration

Elderly people insidiously manifest the symptoms of heart failure, such as dyspnea and/or physical disabilities in an age-dependent manner. Although previous studies suggested that oxidative stress plays a pathological role in the development of heart failure, no direct evidence has been documented so far. In order to investigate the pathological significance of oxidative stress in the heart, we generated heart/muscle-specific manganese superoxide dismutase-deficient mice. The mutant mice developed progressive congestive heart failure with specific molecular defects in mitochondrial respiration. In this paper, we showed for the first time that the oxidative stress caused specific morphological changes of mitochondria, excess formation of superoxide (O(2)(*)(-)), reduction of ATP, and transcriptional alterations of genes associated with heart failure in respect to cardiac contractility. Accordingly, administration of a superoxide dismutase mimetic significantly ameliorated the symptoms. These results implied that O(2)(*)(-) generated in mitochondria played a pivotal role in the development and progression of heart failure. We here present a bona fide model for human cardiac failure with oxidative stress valuable for therapeutic interventions.

Mental retardation genes in drosophila: New approaches to understanding and treating developmental brain disorders

Drosophila melanogaster is emerging as a valuable genetic model system for the study of mental retardation (MR). MR genes are remarkably similar between humans and fruit flies. Cognitive behavioral assays can detect reductions in learning and memory in flies with mutations in MR genes. Neuroanatomical methods, including some at single-neuron resolution, are helping to reveal the cellular bases of faulty brain development caused by MR gene mutations. Drosophila fragile X mental retardation 1 (dfmr1) is the fly counterpart of the human gene whose malfunction causes fragile X syndrome. Research on the fly gene is leading the field in molecular mechanisms of the gene product's biological function and in pharmacological rescue of brain and behavioral phenotypes. Future work holds the promise of using genetic pathway analysis and primary neuronal culture methods in Drosophila as tools for drug discovery for a wide range of MR and related disorders.

Myocardial substrate metabolism in the normal and failing heart

The alterations in myocardial energy substrate metabolism that occur in heart failure, and the causes and consequences of these abnormalities, are poorly understood. There is evidence to suggest that impaired substrate metabolism contributes to contractile dysfunction and to the progressive left ventricular remodeling that are characteristic of the heart failure state. The general concept that has recently emerged is that myocardial substrate selection is relatively normal during the early stages of heart failure; however, in the advanced stages there is a downregulation in fatty acid oxidation, increased glycolysis and glucose oxidation, reduced respiratory chain activity, and an impaired reserve for mitochondrial oxidative flux. This review discusses 1) the metabolic changes that occur in chronic heart failure, with emphasis on the mechanisms that regulate the changes in the expression of metabolic genes and the function of metabolic pathways; 2) the consequences of these metabolic changes on cardiac function; 3) the role of changes in myocardial substrate metabolism on ventricular remodeling and disease progression; and 4) the therapeutic potential of acute and long-term manipulation of cardiac substrate metabolism in heart failure.

Evolving concepts in left ventricular systolic and diastolic remodeling: implications for therapy

Left ventricular (LV) remodeling describes dynamic changes in ventricular size and shape that result from hemodynamic and metabolic insults to the failing heart. The remodeling hypothesis in heart failure asserts that LV remodeling is the central pathophysiologic lesion whereby alterations in cardiac structure are followed by impairment in function, with a wide range of genetic-environment interactions that determine the ultimate phenotype. Several therapeutic targets of LV remodeling have already been exploited (such as neurohormonal and cytokine activation). On the other hand, great efforts are still being made to understand the complex array of genetic and metabolic derangements. Nevertheless, we have realized that there is no single phenotypic change, protein expression, or signal-transduction pathway that is dominant in the process of cardiac remodeling. This implies that better characterization of this heterogeneous heart failure phenotype is desperately needed.