Emerging beneficial roles of sirtuins in heart failure

Sirtuins are a highly conserved family of histone/protein deacetylases whose activity can prolong the lifespan of model organisms such as yeast, worms and flies. In mammalian cells, seven sirtuins (SIRT1-7) modulate distinct metabolic and stress-response pathways, SIRT1 and SIRT3 having been most extensively investigated in the cardiovascular system. SIRT1 and SIRT3 are mainly located in the nuclei and mitochondria, respectively. They participate in biological functions related to development of heart failure, including regulation of energy production, oxidative stress, intracellular signaling, angiogenesis, autophagy and cell death/survival. Emerging evidence indicates that the two sirtuins play protective roles in failing hearts. Here, we summarize current knowledge of sirtuin functions in the heart and discuss its translation into therapy for heart failure.     

Mitochondrial signaling in apoptosis: Mitochondrial daggers to the breaking heart

Mitochondria respond to signals from the cytoplasmic environment, including energy needs, substrate and oxygen availability, ion fluxes, as well as kinase and proteolytic signals. They integrate these complex, often conflicting signals with a coordinated response, whether for growth, hibernation, or apoptosis. In this review we consider the panoply of signals that impact the mitochondria, and focus particularly on the apoptotic pathways in which the mitochondria stand at the crossroads.     

Mitochondrial dysfunction in heart failure

Heart failure (HF) is a complex chronic clinical syndrome. Energy deficit is considered to be a key contributor to the development of both cardiac and skeletal myopathy. In HF, several components of cardiac and skeletal muscle bioenergetics are altered, such as oxygen availability, substrate oxidation, mitochondrial ATP production, and ATP transfer to the contractile apparatus via the creatine kinase shuttle. This review focuses on alterations in mitochondrial biogenesis and respirasome organization, substrate oxidation coupled with ATP synthesis in the context of their contribution to the chronic energy deficit, and mechanical dysfunction of the cardiac and skeletal muscle in HF. We conclude that HF is associated with decreased mitochondrial biogenesis and function in both heart and skeletal muscle, supporting the concept of a systemic mitochondrial cytopathy. The sites of mitochondrial defects are located within the electron transport and phosphorylation apparatus and differ with the etiology and progression of HF in the two mitochondrial populations (subsarcolemmal and interfibrillar) of cardiac and skeletal muscle. The roles of adrenergic stimulation, the renin–angiotensin system, and cytokines are evaluated as factors responsible for the systemic energy deficit. We propose a cyclic AMP-mediated mechanism by which increased adrenergic stimulation contributes to the mitochondrial dysfunction.     

Mitochondrial dynamics in heart failure

Mitochondria have been widely studied for their critical role in cellular metabolism, energy production, and cell death. New developments in research on mitochondria derived from studies in yeast have led to the discovery of entirely new mitochondrial processes that have implications for mitochondrial function in heart failure. Recent studies have identified that maintaining normal mitochondrial morphology and function depends on the dynamic balance of mitochondrial fusion and fission (division). Mitochondrial fusion and fission are constant ongoing processes, which are essential for the maintenance of normal mitochondrial function. Studies in heart failure have been limited but suggest a possible reduction in mitochondrial fusion. As mitochondrial fusion and fission have important links to apoptosis, a key mechanism of loss of cardiac myocytes in heart failure, there are many implications for both heart failure research and treatment.     

Mitochondrial oxygen tension within the heart

By using a newly developed optical technique which enables non-invasive measurement of mitochondrial oxygenation (mitoPO₂) in the intact heart, we addressed three long-standing oxygenation questions in cardiac physiology: 1) what is mitoPO₂ within the in vivo heart?, 2) is mitoPO₂ heterogeneously distributed?, and 3) how does mitoPO₂ of the isolated Langendorff-perfused heart compare with that in the in vivo working heart? Following calibration and validation studies of the optical technique in isolated cardiomyocytes, mitochondria and intact hearts, we show that in the in vivo condition mean mitoPO₂ was 35 ± 5 mm Hg. The mitoPO₂ was highly heterogeneous, with the largest fraction (26%) of mitochondria having a mitoPO₂ between 10 and 20 mm Hg, and 10% between 0 and 10 mm Hg. Hypoxic ventilation (10% oxygen) increased the fraction of mitochondria in the 0–10 mm Hg range to 45%, whereas hyperoxic ventilation (100% oxygen) had no major effect on mitoPO₂. For Langendorff-perfused rat hearts, mean mitoPO₂ was 29 ± 5 mm Hg with the largest fraction of mitochondria (30%) having a mitoPO₂ between 0 and 10 mm Hg. Only in the maximally vasodilated condition, did the isolated heart compare with the in vivo heart (11% of mitochondria between 0 and 10 mm Hg). These data indicate 1) that the mean oxygen tension at the level of the mitochondria within the heart in vivo is higher than generally considered, 2) that mitoPO₂ is considerably heterogeneous, and 3) that mitoPO₂ of the classic buffer-perfused Langendorff heart is shifted to lower values as compared to the in vivo heart.     

Mitochondrial DNA deletions and the aging heart

Mitochondrial DNA (mtDNA) mutations appear to be associated with a wide spectrum of human disorders and proposed to be a potential contributor of aging. However, in an age-dependent increase of the common 4977 bp deletion of human mtDNA still many unanswered questions remain. Comparing mtDNA copy levels in different tissues revealed that cardiac muscle had the highest, while the cortex cerebelli showed the lowest copy number of mtDNA in every donor. Intriguingly, mtDNA copy number showed no changes during aging. In heart tissue, the amount of 4977 bp mtDNA deletion increased in an age-dependent manner showing significant differences at the age of 40 years and older (p<0.005). In vitro studies analyzing human normal cells transfected with telomerase (BJ-T) revealed that oxidative stress (OS)--a well accepted promoter of aging--induced 4977 bp deletion and point mutations as demonstrated by real-time PCR and DHPLC analysis. Interestingly, OS induced apoptosis only in transformed human fibroblasts by activation of the intrinsic (mitochondrial-mediated) signalling pathway as indicated by morphological damage of mitochondria, DNA laddering and increase of the Bax/Bcl-2 ratio. In conclusion, in heart tissue, the amount of the 4977 bp deletion increased in an age-dependent manner and it was more detectable after the 4th decade of life, although there was some scatter in the data. Since, apoptosis was induced by the mitochondria-mediated pathway only in transformed cells, the role for apoptosis in normal tissue of the aging heart remains unclear.     

Mitochondrial calcium and the regulation of metabolism in the heart

Consumption of adenosine triphosphate (ATP) by the heart can change dramatically as the energetic demands increase from a period of rest to strenuous activity. Mitochondrial ATP production is central to this metabolic response since the heart relies largely on oxidative phosphorylation as its source of intracellular ATP. Significant evidence has been acquired indicating that Ca^2 + plays a critical role in regulating ATP production by the mitochondria. Here the evidence that the Ca^2 + concentration in the mitochondrial matrix ([Ca^2 +]_m) plays a pivotal role in regulating ATP production by the mitochondria is critically reviewed and aspects of this process that are under current active investigation are highlighted. Importantly, current quantitative information on the bidirectional Ca^2 + movement across the inner mitochondrial membrane (IMM) is examined in two parts. First, we review how Ca^2 + influx into the mitochondrial matrix depends on the mitochondrial Ca^2 + channel (i.e., the mitochondrial calcium uniporter or MCU). This discussion includes how the MCU open probability (P_O) depends on the cytosolic Ca^2 + concentration ([Ca^2 +]_i) and on the mitochondrial membrane potential (ΔΨ_m). Second, we discuss how steady-state [Ca^2 +]_m is determined by the dynamic balance between this MCU-based Ca^2 + influx and mitochondrial Na⁺/Ca^2 + exchanger (NCLX) based Ca^2 + efflux. These steady-state [Ca^2 +]_m levels are suggested to regulate the metabolic energy supply due to Ca^2 +-dependent regulation of mitochondrial enzymes of the tricarboxylic acid cycle (TCA), the proteins of the electron transport chain (ETC), and the F₁F₀ ATP synthase itself. We conclude by discussing the roles played by [Ca^2 +]_m in influencing mitochondrial responses under pathological conditions. This article is part of a Special Issue entitled "Mitochondria: From BasicMitochondrial Biology to Cardiovascular Disease."     

Role of sirtuins in ischemia-reperfusion injury

Ischemia-reperfusion injury(IRI)remains an unresolved and complicated situation in clinical practice,especially in the case of organ transplantation.Several factors contribute to its complexity;the depletion of energy during ischemia and the induction of oxidative stress during reperfusion initiate a cascade of pathways that lead to cell death and finally to severe organ injury.Recently,the sirtuin family of nicotinamide adenine dinucleotide-dependent deacetylases has gained increasing attention from researchers,due to their involvement in the modulation of a wide variety of cellular functions.There are seven mammalian sirtuins and,among them,the nuclear/cytoplasmic sirtuin 1(SIRT1)and the mitochondrial sirtuin 3(SIRT3)are ubiquitously expressed in many tissue types.Sirtuins are known to play major roles in protecting against cellular stress and in controlling metabolic pathways,which are key processes during IRI.In this review,we mainly focus on SIRT1 and SIRT3 and examine their role in modulating pathways against energy depletion during ischemia and their involvement in oxidative stress,apoptosis,microcirculatory stress and inflammation during reperfusion.We present evidence of the beneficial effects of sirtuins against IRI and emphasize the importance of developing new strategies by enhancing their action.     

Mitochondrial respiration following acute hypoxia in the perfused rat heart

Mitochondria isolated from tissues of hypoxic animals have increased respiratory capacity (State 3 respiration) when assayed in vitro at ambient oxygen tensions. The present study utilized the isolated perfused rat heart to determine whether or not this change could be produced in the absence of the neural and hormonal changes that accompany hypoxia in vivo. Following 10-min single pass retrograde perfusion with normoxic Krebs-Henseleit buffer (PO2 greater than or equal to 600 mmHg), perfusion was continued for up to 15 min with either normoxic or hypoxic buffer (PO2 less than or equal to 150 mmHg). After 10 min of hypoxic perfusion State 3 respiration of the mitochondria from the hypoxic hearts was 13 to 15% higher (P less than or equal to 0.05) than that of normoxic hearts when assayed with either glutamate/malate or succinate as substrate but was unchanged when TMPD and ascorbate was the substrate. Succinate-supported State 4 respiration of the hypoxic mitochondria also showed a small (10%) but significant (P less than or equal to 0.05) increase. These changes were not abolished by preperfusing the heart with propranolol (10(-7), 10(-6), or 10(-5) M) indicating that the response was not attributable to release of local stores of catecholamines. Respiratory control and ADP/O ratios as well as contents of cytochrome c and aa3 of the mitochondria from the hypoxic hearts were similar to those of normoxic hearts indicating that the mitochondria remained intact and tightly coupled. We concluded that the hypoxia-induced increase in mitochondrial State 3 respiration, while independent of neural and hormonal influences from the body requires an intracellular event, since they cannot be reproduced by subjecting isolated mitochondria to hypoxia in vitro.     

Mitochondrial dynamics and cell death in heart failure

The highly regulated processes of mitochondrial fusion (joining), fission (division) and trafficking, collectively called mitochondrial dynamics, determine cell-type specific morphology, intracellular distribution and activity of these critical organelles. Mitochondria are critical for cardiac function, while their structural and functional abnormalities contribute to several common cardiovascular diseases, including heart failure (HF). The tightly balanced mitochondrial fusion and fission determine number, morphology and activity of these multifunctional organelles. Although the intracellular architecture of mature cardiomyocytes greatly restricts mitochondrial dynamics, this process occurs in the adult human heart. Fusion and fission modulate multiple mitochondrial functions, ranging from energy and reactive oxygen species production to Ca²⁺ homeostasis and cell death, allowing the heart to respond properly to body demands. Tightly controlled balance between fusion and fission is of utmost importance in the high energy-demanding cardiomyocytes. A shift toward fission leads to mitochondrial fragmentation, while a shift toward fusion results in the formation of enlarged mitochondria and in the fusion of damaged mitochondria with healthy organelles. Mfn1, Mfn2 and OPA1 constitute the core machinery promoting mitochondrial fusion, whereas Drp1, Fis1, Mff and MiD49/51 are the core components of fission machinery. Growing evidence suggests that fusion/fission factors in adult cardiomyocytes play essential noncanonical roles in cardiac development, Ca²⁺ signaling, mitochondrial quality control and cell death. Impairment of this complex circuit causes cardiomyocyte dysfunction and death contributing to heart injury culminating in HF. Pharmacological targeting of components of this intricate network may be a novel therapeutic modality for HF treatment.     

Mitochondrial involvement in myocyte death and heart failure

As the heart is an energy-demanding organ, impaired cardiac energy metabolism and mitochondrial function have been inexorably linked to cardiac dysfunction. There is a growing recognition that mitochondrial dysfunction contributes to impaired myocardial energetics and increased oxidative stress in cardiomyopathies, cardiac ischemic damage and heart failure (HF), and mitochondrial permeability transition pore opening has been reported a critical trigger of myocyte death and myocardial remodeling. It is well established that mitochondria play pivotal roles in intracellular signaling in both cell death as well as in cardioprotective pathways. Moreover, recent studies have shown that defects in mitochondrial dynamics affecting biogenesis and turnover are linked to cardiac senescence and HF. Accordingly, there has been an increasing interest in targeting mitochondria for HF therapy. This article reviews the background and recent evidence of mitochondrial involvement in several types of cell death (apoptosis, necrosis and autophagy) occurring in HF. In addition, potential strategies for targeting mitochondria are examined, and their utility in HF therapy considered.     

Mitochondrial tolerance to stress impaired in failing heart

Mitochondrial integrity is critical in the maintenance of bioenergetic homeostasis of the myocardium, with oxidative or metabolic challenge to mitochondria precipitating cell injury. In heart failure, where cardiac cells are exposed to elevated stress, mitochondrial vulnerability could contribute to the disease state. However, the mitochondrial response to stress is yet to be established in heart failure. Here, mitochondrial function and structure was evaluated prior and following stress using a transgenic (TG) model of heart failure, generated by cardiac overexpression of the cytokine TNFalpha. Compared to the wild type, mitochondria from TG failing hearts demonstrated impaired oxidative phosphorylation, mitochondrial DNA damage, reduced mitochondrial creatine kinase activity, abnormal calcium handling, and altered ultrastructure. Under anoxia/reoxygenation or calcium stress, mitochondria from failing hearts suffered exacerbated energetic failure with pronounced cytochrome c release. Thus, mitochondria from TNFalpha-TG failing hearts demonstrate structural and functional abnormalities, with reduced tolerance to stress manifested by impaired bioenergetics and increased susceptibility to injury. This abnormal vulnerability to stress underscores the impact of mitochondrial dysfunction in the pathobiology of heart failure.