Molecular defects in cardiac myofibrillar proteins due to thyroid hormone imbalance and diabetes

The heart very often becomes a victim of endocrine abnormalities such as thyroid hormone imbalance and insulin deficiency, which are manifested in a broad spectrum of cardiac dysfunction from mildly compromised function to severe heart failure. These functional changes in the heart are largely independent of alterations in the coronary arteries and instead reside at the level of cardiomyocytes. The status of cardiac function reflects the net of underlying subcellular modifications induced by an increase or decrease in thyroid hormone and insulin plasma levels. Changes in the contractile and regulatory proteins constitute molecular and structural alterations in myofibrillar assembly, called myofibrillar remodeling. These alterations may be adaptive or maladaptive with respect to the functional and metabolic demands on the heart as a consequence of the altered endocrine status in the body. There is a substantial body of information to indicate alterations in myofibrillar proteins including actin, myosin, tropomyosin, troponin, titin, desmin, and myosin-binding protein C in conditions such as hyperthyroidism, hypothyroidism, and diabetes. The present article is focussed on discussion how myofibrillar proteins are altered in response to thyroid hormone imbalance and lack of insulin or its responsiveness, and how their structural and functional changes explain the contractile defects in the heart.     

Absence of fatty acid transporter CD36 protects against Western-type diet-related cardiac dysfunction following pressure overload in mice

Cardiac patients often are obese and have hypertension, but in most studies these conditions are investigated separately. Here, we aimed at 1) elucidating the interaction of metabolic and mechanophysical stress in the development of cardiac dysfunction in mice and 2) preventing this interaction by ablation of the fatty acid transporter CD36. Male wild-type (WT) C57Bl/6 mice and CD36(-/-) mice received chow or Western-type diet (WTD) for 10 wk and then underwent a sham surgery or transverse aortic constriction (TAC) under anesthesia. After a 6-wk continuation of the diet, cardiac function, morphology, lipid profiles, and molecular parameters were assessed. WTD administration affected body and organ weights of WT and CD36(-/-) mice, but it affected only plasma glucose and insulin concentrations in WT mice. Cardiac lipid concentrations increased in WT mice receiving WTD, decreased in CD36(-/-) on chow, and remained unchanged in CD36(-/-) receiving WTD. TAC induced cardiac hypertrophy in WT mice on chow but did not affect cardiac function and cardiac lipid concentrations. WTD or CD36 ablation worsened the outcome of TAC. Ablation of CD36 protected against the WTD-related aggravation of cardiac functional and structural changes induced by TAC. In conclusion, cardiac dysfunction and remodeling worsen when the heart is exposed to two stresses, metabolic and mechanophysical, at the same time. CD36 ablation prevents the metabolic stress resulting from a WTD. Thus, metabolic conditions are a critical factor for the compromised heart and provide new targets for metabolic manipulation in cardioprotection.     

Sarcoplasmic reticulum Ca-ATPase-phospholamban interactions and dilated cardiomyopathy

Dilated cardiomyopathy is a disease of the heart muscle resulting from a diverse array of conditions that damages the heart and impairs myocardial function. Heart failure occurs when the heart is unable to pump blood at a rate which can accommodate the heart muscle's metabolic requirements. Several signaling pathways have been shown to be involved in the induction of cardiac disease and heart failure. Many of these pathways are linked to cardiac sarcoplasmic reticulum (SR) Ca cycling directly or indirectly. A large body of evidence points to the central role of abnormal Ca handling by SR proteins, Ca-ATPase pump (SERCA2a) and phospholamban (PLN), in pathophysiological heart conditions, compromising the contractile state of the cardiomyocytes. This review summarizes studies which highlight the key role of these two SR proteins in the regulation of cardiac function, the significance of SERCA2a-PLN interactions using transgenic approaches, and the recent discoveries of human PLN mutations leading to disease states. Finally, we will discuss extrapolation of experimental paradigms generated in animal models to the human condition.     

Imaging heart development using high-resolution episcopic microscopy

Development of the heart in vertebrate embryos is a complex process in which the organ is continually remodelled as chambers are formed, valves sculpted and connections established to the developing vascular system. Investigating the genetic programmes driving these changes and the environmental factors that may influence them is critical for our understanding of congenital heart disease. A recurrent challenge in this work is how to integrate studies as diverse as those of cardiac gene function and regulation with an appreciation of the localised interactions between cardiac tissues not to mention the manner in which both may be affected by cardiac function itself. Meeting this challenge requires an accurate way to analyse the changes in 3D morphology of the developing heart, which can be swift or protracted and both dramatic or subtle in consequence. Here we review the use of high-resolution episcopic microscopy as a simple and effective means to examine organ structure and one that allows modern computing methods pioneered by clinical imaging to be applied to the embryonic heart.     

Extracellular matrix, mechanotransduction and structural hierarchies in heart tissue engineering

The spatial and temporal scales of cardiac organogenesis and pathogenesis make engineering of artificial heart tissue a daunting challenge. The temporal scales range from nanosecond conformational changes responsible for ion channel opening to fibrillation which occurs over seconds and can lead to death. Spatial scales range from nanometre pore sizes in membrane channels and gap junctions to the metre length scale of the whole cardiovascular system in a living patient. Synchrony over these scales requires a hierarchy of control mechanisms that are governed by a single common principle: integration of structure and function. To ensure that the function of ion channels and contraction of muscle cells lead to changes in heart chamber volume, an elegant choreography of metabolic, electrical and mechanical events are executed by protein networks composed of extracellular matrix, transmembrane integrin receptors and cytoskeleton which are functionally connected across all size scales. These structural control networks are mechanoresponsive, and they process mechanical and chemical signals in a massively parallel fashion, while also serving as a bidirectional circuit for information flow. This review explores how these hierarchical structural networks regulate the form and function of living cells and tissues, as well as how microfabrication techniques can be used to probe this structural control mechanism that maintains metabolic supply, electrical activation and mechanical pumping of heart muscle. Through this process, we delineate various design principles that may be useful for engineering artificial heart tissue in the future.     

Intracellular angiotensin II disrupts chemical communication and impairs metabolic cooperation between cardiac myocytes

The influence of intracellular angiotensin II (Ang II) on the process of chemical communication and metabolic cooperation between cardiac cells is discussed. Emphasis is given to the influence of pathological conditions like heart failure, myocardial ischemia or hyperglycemia on the activation of the intracrine renin angiotensin aldosterone system (RAAS) and its consequence for the metabolic cooperation between heart cells. Furthermore, the influence of high glucose on the process of chemical communication was described as well as its implication for the failing and diabetic heart. The major conclusion is that the activation of the intracrine renin angiotensin induced by heart failure, hyperglycemia, aldosterone or myocardial ischemia generates metabolic imbalance in the heart with serious consequences for the cardiac function.     

ERRgamma directs and maintains the transition to oxidative metabolism in the postnatal heart

At birth, the heart undergoes a critical metabolic switch from a predominant dependence on carbohydrates during fetal life to a greater dependence on postnatal oxidative metabolism. This remains the principle metabolic state throughout life, although pathologic conditions such as heart failure and cardiac hypertrophy reactivate components of the fetal genetic program to increase carbohydrate utilization. Disruption of the ERRgamma gene (Esrrg), which is expressed at high levels in the fetal and postnatal mouse heart, blocks this switch, resulting in lactatemia, electrocardiographic abnormalities, and death during the first week of life. Genomic ChIP-on-chip and expression analysis identifies ERRgamma as both a direct and an indirect regulator of a nuclear-encoded mitochondrial genetic network that coordinates the postnatal metabolic transition. These findings reveal an unexpected and essential molecular genetic component of the oxidative metabolic gene program in the heart and highlight ERRgamma in the study of cardiac hypertrophy and failure.     

Assessment of Cardiac Function and Energetics in Isolated Mouse Hearts Using 31P NMR Spectroscopy

Bioengineered mouse models have become powerful research tools in determining causal relationships between molecular alterations and models of cardiovascular disease. Although molecular biology is necessary in identifying key changes in the signaling pathway, it is not a surrogate for functional significance. While physiology can provide answers to the question of function, combining physiology with biochemical assessment of metabolites in the intact, beating heart allows for a complete picture of cardiac function and energetics. For years, our laboratory has utilized isolated heart perfusions combined with nuclear magnetic resonance (NMR) spectroscopy to accomplish this task. Left ventricular function is assessed by Langendorff-mode isolated heart perfusions while cardiac energetics is measured by performing ³¹P magnetic resonance spectroscopy of the perfused hearts. With these techniques, indices of cardiac function in combination with levels of phosphocreatine and ATP can be measured simultaneously in beating hearts. Furthermore, these parameters can be monitored while physiologic or pathologic stressors are instituted. For example, ischemia/reperfusion or high workload challenge protocols can be adopted. The use of aortic banding or other models of cardiac pathology are apt as well. Regardless of the variants within the protocol, the functional and energetic significance of molecular modifications of transgenic mouse models can be adequately described, leading to new insights into the associated enzymatic and metabolic pathways. Therefore, ³¹P NMR spectroscopy in the isolated perfused heart is a valuable research technique in animal models of cardiovascular disease.     

Hemodynamic and metabolic responses of the working heart in relation to the oxygen carrying capacity of the perfusion medium

Hemodynamic and metabolic adaptations of isolated working heart perfused alternatively with normal or low oxygen carrying capacity medium were studied in an experimental model. A step change in arterial oxygen content (1.75 to 15.3 ml O2/100 ml) was followed by a decrease in coronary flow, an increase in aortic flow, external work, myocardial oxygen consumption and efficiency, respectively. Metabolic investigations (steady state values) showed the activities of both glycolysis and the Krebs cycle to increase with the oxygen carrying capacity of the perfusion medium. Within the limits of these aerobic conditions, most of the cardiac changes were reversible. The use of reconstituted blood provides physiological conditions of oxygenation, allows a dynamic equilibrium between oxygen supply and oxygen requirements and maintains a near physiological regulation between cardiac dynamic and metabolic functions. These conclusions stress the importance of optimal O2 carrying capacity of perfusion medium in metabolic studies on isolated working heart.     

Perfused heart studies to investigate lipid metabolism

The isolated perfused heart preparation is an invaluable model for investigating metabolism in a variety of physiological and pathological states. It avoids confounding systemic factors (e.g. endocrine, metabolic and work load changes) and permits simultaneous measurement of mechanical function. The ability to measure arteriovenous concentration differences across the myocardium and the coronary flow rate, together with the use of radiolabelled substrates, permits assessment of substrate assimilation and disposition of most potential energetic substrates. In the case of lipids, metabolism of non-esterified fatty acids has been extensively investigated in the perfused rat heart, but fatty acids may also be derived from circulating triacylglycerols (TAG) in lipoproteins [chylomicrons, very-low-density-lipoprotein (VLDL)]. TAG requires initial hydrolysis by the endothelial enzyme lipoprotein lipase and hence an intact heart preparation is vital to maintain tissue structural integrity. Chylomicron-TAG utilization and fate (oxidation, tissue-lipid deposition) in isolated working hearts has been studied using chylomicrons obtained from thoracic-duct catheters. However, lack of availability of sufficient quantities of VLDL has hindered examination of their cardiac utilization; the recent development of a technique to produce large quantities of radio-labelled rat VLDL has facilitated these studies and established that VLDL-TAG is an important metabolic substrate for working heart. Results relating to myocardial utilization of VLDL-TAG under varying physiological (lactation) and pathological (endotoxinaemia) conditions will be presented. The putative role of VLDL as a regulator of cardiac lipid metabolism will also be discussed.     

Growth plasticity of the embryonic and fetal heart

The developing mammalian heart responds to a variety of conditions, including changes in nutrient availability, blood oxygenation, hemodynamics, or tissue homeostasis, with impressive growth plasticity. This ensures the formation of a functional and normal sized organ by birth. During embryonic and fetal development the heart is exposed to various physiological and potentially pathological changes in the intrauterine environment which dramatically impact on normal cardiac function, tissue composition, and morphology. This paper summarizes the mechanisms employed by the embryonic and fetal heart to adapt to various intrauterine challenges to prevent or minimize postnatal consequences of impaired cardiac development. Future investigations of this growth plasticity might lead to new therapeutic strategies for the prevention of cardiac disease in postnatal life.     

Lipoprotein lipase mediated fatty acid delivery and its impact in diabetic cardiomyopathy

Although cardiovascular disease is the leading cause of diabetes-related death, its etiology is still not understood. The immediate change that occurs in the diabetic heart is altered energy metabolism where in the presence of impaired glucose uptake, glycolysis, and pyruvate oxidation, the heart switches to exclusively using fatty acids (FA) for energy supply. It does this by rapidly amplifying its lipoprotein lipase (LPL-a key enzyme, which hydrolyzes circulating lipoprotein-triglyceride to release FA) activity at the coronary lumen. An abnormally high capillary LPL could provide excess fats to the heart, leading to a number of metabolic, morphological, and mechanical changes, and eventually to cardiac disease. Unlike the initial response, chronic severe diabetes "turns off" LPL, this is also detrimental to cardiac function. In this review, we describe a number of post-translational mechanisms that influence LPL vesicle formation, actin cytoskeleton rearrangement, and transfer of LPL from cardiomyocytes to the vascular lumen to hydrolyze lipoprotein-triglyceride following diabetes. Appreciating the mechanism of how the heart regulates its LPL following diabetes should allow the identification of novel targets for therapeutic intervention, to prevent heart failure. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.     

Cellular Neuronal Control of Molluscan Heart

Bivalve and gastropod molluscs undergo large changes in externalenvironmental conditions, as well as in internal state. Cardiacresponses to these changing conditions have been recorded ina variety of species. There is a general tendency for heartrate, and presumably cardiac output, to increase in responseto situations that would increase the load on respiratory andexcretory systems. Changes in molluscan heart function in manycases appear not to be mediated directly by cardiac nerves,but rather by such indirect mechanisms as changes in blood constituentsor mechanical, hemodynamic effects on heart muscle. Three typesof cardiac response in Aplysia have been shown to be mediated,at least in part, by the heart regulator nerves. The neural circuits that regulate heart rate in Aplysia andin Helix have been partially described in cellular detail. InMercenaria, Aplysia and other molluscan species there is evidencethat cyclic adenosine monophosphate has a role in mediatingthe excitatory effects of serotonin on heart muscle. There appearsto be, in fact, a general tendency in the Aplysia nervous systemfor neurons that exert tonic, modulatory effects within neuralnetworks that control a variety of behaviors to use serotoninfor a transmitter. In each case there is evidence to suggestthat changes in cyclic adenosine monophosphate levels may mediatesome of the modulatory effects of serotonin.     

Caloric excess or restriction mediated modulation of metabolic enzyme acetylation-proposed effects on cardiac growth and function

Caloric excess has been postulated to disrupt cardiac function via (i) the generation of toxic intermediates, (ii) via protein glycosylation and (iii) through the generation of reactive oxygen species. It is now increasingly being recognized that the nutrient intermediates themselves may modulate metabolic pathways through the post-translational modifications of metabolic enzymes. In light of the high energy demand of the heart, these nutrient mediated modulations in metabolic pathway functioning may play an important role in cardiac function and in the capacity of the heart to adapt to biomechanical stressors. In this review the role of protein acetylation and deacetylation in the control of metabolic programs is explored. Although not extensively investigated directly in the heart, the emerging data support that these nutrient mediated post-translational regulatory events (i) modulate cardiac metabolic pathways, (ii) integrate nutrient flux mediated post-translational effects with cardiac function and (iii) may be important in the development of cardiac pathology. Areas of investigation that need to be explored are highlighted. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.     

Caloric excess or restriction mediated modulation of metabolic enzyme acetylation—proposed effects on cardiac growth and function

Caloric excess has been postulated to disrupt cardiac function via (i) the generation of toxic intermediates, (ii) via protein glycosylation and (iii) through the generation of reactive oxygen species. It is now increasingly being recognized that the nutrient intermediates themselves may modulate metabolic pathways through the post-translational modifications of metabolic enzymes. In light of the high energy demand of the heart, these nutrient mediated modulations in metabolic pathway functioning may play an important role in cardiac function and in the capacity of the heart to adapt to biomechanical stressors. In this review the role of protein acetylation and deacetylation in the control of metabolic programs is explored. Although not extensively investigated directly in the heart, the emerging data support that these nutrient mediated post-translational regulatory events (i) modulate cardiac metabolic pathways, (ii) integrate nutrient flux mediated post-translational effects with cardiac function and (iii) may be important in the development of cardiac pathology. Areas of investigation that need to be explored are highlighted. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.     

The role of adenosine in the isolatedRana ridibunda heart

Summary In an attempt to study the metabolic role of adenosine in the amphibian heart, we perfusedRana ridibunda hearts under conditions of decreased oxygen supply or increased oxygen demand and measured the rate of adenosine appearance as well as the concentrations of adenine nucleotides. Anoxia was associated with a significant increase in the myocardial and perfusate concentration of adenosine and its degradation products, inosine and hypoxanthine, while changes were also observed in the concentrations of adenine nucleotides and creatine phosphate. Furthermore, adenosine production inRana ridibunda hearts was enhanced under conditions of increased cardiac work induced by perfusion pressure elevation. Adenosine production was inversely proportional to the energy charge value calculated from the tissue content of adenine nucleotides under conditions of anoxia and increased heart work. The results are in accordance with the proposed role of adenosine as a physiological metabolic vasodilator in the coronary circulation of the mammalian heart and support the hypothesis that adenosine can be involved in regulating blood vessel resistance inRana ridibunda heart under conditions of low myocardial oxygen tension. Thus it appears that adenosine could act as a vasodilatory substance inRana ridibunda heart.     

An Anthelmintic Drug,Pyrvinium Pamoate,Thwarts Fibrosis and Ameliorates Myocardial Contractile Dysfunction in a Mouse Model of Myocardial Infarction

Metabolic adaptation to limited supplies of oxygen and nutrients plays a pivotal role in health and disease. Heart attack results from insufficient delivery of oxygen and nutrients to the heart, where cardiomyocytes die and cardiac fibroblasts proliferate – the latter causing scar formation, which impedes regeneration and impairs contractility of the heart. We postulated that cardiac fibroblasts survive metabolic stress by adapting their intracellular metabolism to low oxygen and nutrients, and impeding this metabolic adaptation would thwart their survival and facilitate the repair of scarred heart. Herein, we show that an anthelmintic drug, Pyrvinium pamoate, which has been previously shown to compromise cancer cell survival under glucose starvation condition, also disables cardiac fibroblast survival specifically under glucose deficient condition. Furthermore, Pyrvinium pamoate reduces scar formation and improves cardiac contractility in a mouse model of myocardial infarction. As Pyrvinium pamoate is an FDA-approved drug, our results suggest a therapeutic use of this or other related drugs to repair scarred heart and possibly other organs.