Mitophagy for cardioprotection.

Mitochondrial function is maintained by several strictly coordinated mechanisms, collectively termed mitochondrial quality control mechanisms, including fusion and fission, degradation, and biogenesis. As the primary source of energy in cardiomyocytes, mitochondria are the central organelle for maintaining cardiac function. Since adult cardiomyocytes in humans rarely divide, the number of dysfunctional mitochondria cannot easily be diluted through cell division. Thus, efficient degradation of dysfunctional mitochondria is crucial to maintaining cellular function. Mitophagy, a mitochondria specific form of autophagy, is a major mechanism by which damaged or unnecessary mitochondria are targeted and eliminated. Mitophagy is active in cardiomyocytes at baseline and in response to stress, and plays an essential role in maintaining the quality of mitochondria in cardiomyocytes. Mitophagy is mediated through multiple mechanisms in the heart, and each of these mechanisms can partially compensate for the loss of another mechanism. However, insufficient levels of mitophagy eventually lead to mitochondrial dysfunction and the development of heart failure. In this review, we discuss the molecular mechanisms of mitophagy in the heart and the role of mitophagy in cardiac pathophysiology, with the focus on recent findings in the field.

The role of the Hippo pathway in autophagy in the heart.

The Hippo pathway, an evolutionarily conserved signalling mechanism, controls organ size and tumourigenesis. Increasing lines of evidence suggest that autophagy, an important mechanism of lysosome-mediated cellular degradation, is regulated by the Hippo pathway, which thereby profoundly affects cell growth and death responses in various cell types. In the heart, Mst1, an upstream component of the Hippo pathway, not only induces apoptosis but also inhibits autophagy through phosphorylation of Beclin 1. YAP/TAZ, transcription factor co-factors and the terminal effectors of the Hippo pathway, affect autophagy through transcriptional activation of TFEB, a master regulator of autophagy and lysosomal biogenesis. The cellular abundance of YAP is negatively regulated by autophagy and suppression of autophagy induces accumulation of YAP, which, in turn, acts as a feedback mechanism to induce autophagosome formation. Thus, the Hippo pathway and autophagy regulate each other, thereby profoundly affecting cardiomyocyte survival and death. This review discusses the interaction between the Hippo pathway and autophagy and its functional significance during stress conditions in the heart and the cardiomyocytes therein.

Molecular properties and regulation of NAD+ kinase (NADK).

Nicotinamide adenine dinucleotide (NAD+) kinase (NADK) phosphorylates NAD+, thereby producing nicotinamide adenine dinucleotide phosphate (NADP). Both NADK genes and the NADP(H)-producing mechanism are evolutionarily conserved among archaea, bacteria, plants and mammals. In mammals, NADK is activated by phosphorylation and protein-protein interaction. Recent studies conducted using genetically altered models validate the essential role of NADK in cellular redox homeostasis and metabolism in multicellular organisms. Here, we describe the evolutionary conservation, molecular properties, and signaling mechanisms and discuss the pathophysiological significance of NADK.

Mitochondrial Quality Control Mechanisms during Diabetic Cardiomyopathy.

One of the major complications of diabetes mellitus is diabetic cardiomyopathy. One of the mechanisms that initiates the irreversible deterioration of cardiac function in diabetic cardiomyopathy is mitochondrial dysfunction. Functionally impaired mitochondria result in greater levels of oxidative stress and lipotoxicity, both of which exacerbate mitochondrial damage. Mitochondrial health is constantly monitored by mitochondrial quality control mechanisms. Mitophagy selectively degrades damaged mitochondria, thereby maintaining the healthy pool of mitochondria and preserving myocardial function. Mitophagy in diabetic cardiomyopathy is mediated by multiple mechanisms in a time-dependent manner. Potential targets for the treatment of diabetic cardiomyopathy include increased oxidative stress, mitochondrial dynamics, and mitochondrial clearance. Thus, stimulation of mitophagy represents a promising strategy for the alleviation of diabetic cardiomyopathy.

The role of autophagic cell death in cardiac disease.

Cardiomyocytes undergo various forms of cell death during heart disease such as myocardial infarction and heart failure. Understanding the mechanisms of cell death in cardiomyocytes is one of the most fundamental issues in the treatment of heart failure. Among the several kinds of cell death mechanisms, this review will focus on autophagy-related cardiomyocyte cell death. Although autophagy plays an essential role in mediating cellular quality control mechanisms for cell survival, dysregulation of autophagy can cause cell death, referred to as autophagy-dependent cell death or type II programmed cell death. The recent discovery of autosis as a modality of autophagy-dependent cell death with unique morphological and biochemical features has allowed us to broaden our understanding of the mechanistic role of autophagy in cell death. Here, we discuss autophagy-dependent cardiomyocyte cell death, including autosis, in pathophysiological conditions of the heart.

Tfeb-Mediated Transcriptional Regulation of Autophagy Induces Autosis during Ischemia/Reperfusion in the Heart.

Autosis is a unique form of cell death with characteristic morphological and biochemical features caused by dysregulated autophagy. Autosis is observed in the heart during the late phase of ischemia/reperfusion (I/R), when marked accumulation of autophagosomes is induced. We previously showed that the excessive accumulation of autophagosomes promotes autosis in cardiomyocytes. Although the inhibition of autophagic flux via the upregulation of Rubicon induces the accumulation of autophagosomes during I/R, it appears that additional mechanisms exacerbating autophagosome accumulation are required for the induction of autosis. Here, we show that Tfeb contributes to the induction of autosis during the late phase of I/R in the heart. During myocardial reperfusion, Tfeb is activated and translocated into the nucleus, which in turn upregulates genes involved in autophagy and lysosomal function. The overexpression of Tfeb enhanced cardiomyocyte death induced by a high dose of TAT-Beclin 1, an effect that was inhibited by the downregulation of Atg7. Conversely, the knockdown of Tfeb attenuated high-dose TAT-Beclin1-induced death in cardiomyocytes. Although the downregulation of Tfeb in the heart significantly decreased the number of autophagic vacuoles and inhibited autosis during I/R, the activation of Tfeb activity via 3,4-dimethoxychalcone, an activator of Tfeb, aggravated myocardial injury during I/R. These findings suggest that Tfeb promotes cardiomyocyte autosis during the late phase of reperfusion in the heart.

The role of autophagy in death of cardiomyocytes.

Autophagy mediates cellular quality control mechanisms and energy homeostasis through lysosomal degradation. Autophagy is typically viewed as an adaptive process that allows cells to survive against stress, such as nutrient deprivation and hypoxia. However, autophagy also mediates cell death during development and in response to stress. Cell death accompanied by autophagy activation and accumulation of autophagosomes has been classified as type II programmed cell death. Compared to the wealth of knowledge regarding the adaptive role of autophagy, however, the molecular mechanisms through which autophagy induces cell death and its functional significance are poorly understood. Autophagy is activated excessively under some conditions, causing uncontrolled degradation of cellular materials and cell death. An imbalance between autophagosome formation and lysosomal degradation causes a massive accumulation of autophagosomes, which subsequently causes cellular dysfunction and death. Dysregulation of autophagy induces a unique form of cell death, termed autosis, with defined morphological and biochemical features distinct from other forms of programmed cell death, such as apoptosis and necrosis. In the heart, dysregulated autophagy induces death of cardiomyocytes and actively mediates cardiac injury and dysfunction in some conditions, including reperfusion injury, doxorubicin cardiomyopathy, and lysosomal storage disorders. The goal in this review is to introduce the concept of autophagic cell death and discuss its functional significance in various cardiac conditions.

The roles of the inhibitory autophagy regulator Rubicon in the heart: A new therapeutic target to prevent cardiac cell death.

Autophagy contributes to the maintenance of cardiac homeostasis. The level of autophagy is dynamically altered in heart disease. Although autophagy is a promising therapeutic target, only a few selective autophagy activator candidates have been reported thus far. Rubicon is one of the few endogenous negative regulators of autophagy and a potential target for autophagy-inducing therapeutics. Rubicon was initially identified as a component of the Class III PI3K complex, and it has multiple functions, not only in canonical autophagy but also in endosomal trafficking and inflammatory responses. This review summarizes the molecular action of Rubicon in canonical and noncanonical autophagy. We discuss the roles of Rubicon in cardiac stress and the therapeutic potential of Rubicon in cardiac diseases through its modulation of autophagy.

Autosis: A New Target to Prevent Cell Death.

Excessive autophagy induces a defined form of cell death called autosis, which is characterized by unique morphological features, including ballooning of perinuclear space and biochemical features, including sensitivity to cardiac glycosides. Autosis is observed during the late phase of reperfusion after a period of ischemia and contributes to myocardial injury. This review discusses unique features of autosis, the involvement of autosis in myocardial injury, and the molecular mechanism of autosis. Because autosis promotes myocardial injury under some conditions, a better understanding of autosis may lead to development of novel interventions to protect the heart against myocardial stress.

The role of Drp1 in mitophagy and cell death in the heart.

Maintenance of mitochondrial function and integrity is critical for normal cell survival, particularly in non-dividing cells with a high-energy demand such as cardiomyocytes. Well-coordinated quality control mechanisms in cardiomyocytes, involving mitochondrial biogenesis, mitochondrial dynamics-fission and fusion, and mitophagy, act to protect against mitochondrial dysfunction. Mitochondrial fission, which requires dynamin-related protein 1 (Drp1), is essential for segregation of damaged mitochondria for degradation. Alterations in this process have been linked to cardiomyocyte apoptosis and cardiomyopathy. In this review, we discuss the role of Drp1 in mitophagy and apoptosis in the context of cardiac pathology, including myocardial ischemia and heart failure.

Intermittent Fasting Reverses an Advanced Form of Cardiomyopathy.

See Article by Ma et al.

Endogenous muscle atrophy F-box is involved in the development of cardiac rupture after myocardial infarction.

Muscle atrophy F-box (MAFbx/atrogin-1), an E3 ubiquitin ligase, is a crucial mediator of skeletal muscle atrophy and cardiac hypertrophy in response to pressure overload and exercise. The role of MAFbx in the regulation of cardiac remodeling after myocardial infarction (MI) remains unclear. Permanent coronary ligation of the left coronary artery was performed on MAFbx knockout (KO) and wild-type (WT) mice and MAFbx expression in the WT mice was shown to be significantly increased in the left ventricles after MI. The mortality rate due to post-MI cardiac rupture was significantly decreased in MAFbx KO mice compared to that in the WT mice. DNA microarray and mRNA expression analyses revealed that the upregulation of genes involved in inflammatory processes and cell motility of leukocytes and neutrophils, including Mmp9, Il1b, Cxcl2, and Nlrp3, was significantly attenuated in MAFbx KO mice 1 day after MI. MAFbx downregulation inhibited nuclear factor-κB (Nfkb) activation after MI. Flow cytometry results demonstrated that the myocardial infiltration of neutrophils was suppressed in MAFbx KO mice 1 day after MI. Nlrp3 and Il1b protein levels were decreased in MAFbx KO mice compared with those in the WT mice. MAFbx downregulation significantly attenuated Tnfa-induced Cxcl2, Il1b, and Nlrp3 expression in cardiomyocytes. We conclude that MAFbx plays an important role in the mediation of excessive inflammation, including neutrophil infiltration, inflammasome formation, and production of proinflammatory cytokines through the activation of Nfkb, promoting cardiac rupture after MI.

The Role of Autophagy in the Heart.

Autophagy is an evolutionarily conserved mechanism by which cytoplasmic elements are degraded intracellularly. Autophagy has also emerged as a major regulator of cardiac homeostasis and function. Autophagy preserves cardiac structure and function under baseline conditions and is activated during stress, limiting damage under most conditions. It reduces injury and preserves cardiac function during ischemia. It also reduces chronic ischemic remodeling and mediates the cardiac adaptation to pressure overload by restricting misfolded protein accumulation, mitochondrial dysfunction, and oxidative stress. Impairment of autophagy is involved in the development of diabetes and aging-induced cardiac abnormalities. Autophagy defects contribute to the development of cardiac proteinopathy and doxorubicin-induced cardiomyopathy. However, massive activation of autophagy may be detrimental for the heart in certain stress conditions, such as reperfusion injury. In this review, we discuss recent evidence supporting the important role of autophagy and mitophagy in the regulation of cardiac homeostasis and adaptation to stress.

Metabolic reprogramming via PPARα signaling in cardiac hypertrophy and failure: From metabolomics to epigenetics.

Studies using omics-based approaches have advanced our knowledge of metabolic remodeling in cardiac hypertrophy and failure. Metabolomic analysis of the failing heart has revealed global changes in mitochondrial substrate metabolism. Peroxisome proliferator-activated receptor-α (PPARα) plays a critical role in synergistic regulation of cardiac metabolism through transcriptional control. Metabolic reprogramming via PPARα signaling in heart failure ultimately propagates into myocardial energetics. However, emerging evidence suggests that the expression level of PPARα per se does not always explain the energetic state in the heart. The transcriptional activities of PPARα are dynamic, yet highly coordinated. An additional level of complexity in the PPARα regulatory mechanism arises from its ability to interact with various partners, which ultimately determines the metabolic phenotype of the diseased heart. This review summarizes our current knowledge of the PPARα regulatory mechanisms in cardiac metabolism and the possible role of PPARα in epigenetic modifications in the diseased heart. In addition, we discuss how metabolomics can contribute to a better understanding of the role of PPARα in the progression of cardiac hypertrophy and failure.

Aging and Autophagy in the Heart.

The aging population is increasing in developed countries. Because the incidence of cardiac disease increases dramatically with age, it is important to understand the molecular mechanisms through which the heart becomes either more or less susceptible to stress. Cardiac aging is characterized by the presence of hypertrophy, fibrosis, and accumulation of misfolded proteins and dysfunctional mitochondria. Macroautophagy (hereafter referred to as autophagy) is a lysosome-dependent bulk degradation mechanism that is essential for intracellular protein and organelle quality control. Autophagy and autophagic flux are generally decreased in aging hearts, and murine autophagy loss-of-function models develop exacerbated cardiac dysfunction that is accompanied by the accumulation of misfolded proteins and dysfunctional organelles. On the contrary, stimulation of autophagy generally improves cardiac function in mouse models of protein aggregation by removing accumulated misfolded proteins, dysfunctional mitochondria, and damaged DNA, thereby improving the overall cellular environment and alleviating aging-associated pathology in the heart. Increasing lines of evidence suggest that autophagy is required for many mechanisms that mediate lifespan extension, such as caloric restriction, in various organisms. These results raise the exciting possibility that autophagy may play an important role in combating the adverse effects of aging in the heart. In this review, we discuss the role of autophagy in the heart during aging, how autophagy alleviates age-dependent changes in the heart, and how the level of autophagy in the aging heart can be restored.

Molecular mechanisms mediating mitochondrial dynamics and mitophagy and their functional roles in the cardiovascular system.

Mitochondria are essential organelles that produce the cellular energy source, ATP. Dysfunctional mitochondria are involved in the pathophysiology of heart disease, which is associated with reduced levels of ATP and excessive production of reactive oxygen species. Mitochondria are dynamic organelles that change their morphology through fission and fusion in order to maintain their function. Fusion connects neighboring depolarized mitochondria and mixes their contents to maintain membrane potential. In contrast, fission segregates damaged mitochondria from intact ones, where the damaged part of mitochondria is subjected to mitophagy whereas the intact part to fusion. It is generally believed that mitochondrial fusion is beneficial for the heart, especially under stress conditions, because it consolidates the mitochondria's ability to supply energy. However, both excessive fusion and insufficient fission disrupt the mitochondrial quality control mechanism and potentiate cell death. In this review, we discuss the role of mitochondrial dynamics and mitophagy in the heart and the cardiomyocytes therein, with a focus on their roles in cardiovascular disease. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

Muscle-specific RING finger 1 negatively regulates pathological cardiac hypertrophy through downregulation of calcineurin A.

BACKGROUND: Muscle-specific RING finger protein-1 (MuRF1) is an E3 ligase that inhibits cardiac hypertrophy. However, how MuRF1 regulates cardiac hypertrophy and function during pressure overload (PO) remains poorly understood. We investigated the role of endogenous MuRF1 in regulating cardiac hypertrophy in response to PO in vivo. METHODS AND RESULTS: Transverse aortic constriction (TAC) for 4 weeks significantly reduced expression of MuRF1 in the mouse heart. After 2 and 4 weeks of TAC, MuRF1 knockout (Murf1(-/-)) mice exhibited enhanced cardiac hypertrophy and left ventricular (LV) dysfunction compared with that of nontransgenic (NTg) mice. Histological analyses showed that Murf1(-/-) mice exhibited more severe fibrosis and apoptosis than NTg mice after TAC. TAC-induced increases in the activity of a nuclear factor of activated T cells (NFAT) luciferase reporter were significantly greater in Murf1(-/-) than in NTg mice. TAC-induced increases in calcineurin A (CnA) expression were also significantly enhanced in Murf1(-/-) compared with that in NTg mice. Coimmunoprecipitation assays showed that endogenous MuRF1 and CnA interact with one another. Polyubiquitination of CnA was attenuated in Murf1(-/-) mouse hearts at baseline and in response to TAC, and the protein stability of CnA was enhanced in cardiomyocytes, in which MuRF1 was downregulated in vitro. Furthermore, MuRF1 directly ubiquitinated CnA in vitro. Cardiac-specific overexpression of ZAKI-4ß, an endogenous inhibitor of CnA, significantly suppressed the enhancement of TAC-induced cardiac hypertrophy and dysfunction, as well as increases in cardiac fibrosis and apoptosis, in Murf1(-/-) mice. CONCLUSIONS: Endogenous MuRF1 negatively regulates cardiac hypertrophy and dysfunction in response to PO through inhibition of the calcineurin-NFAT pathway.