Parkin does not prevent accelerated cardiac aging in mitochondrial DNA mutator mice

The E3 ubiquitin ligase Parkin plays an important role in regulating clearance of dysfunctional or unwanted mitochondria in tissues, including the heart. However, whether Parkin also functions to prevent cardiac aging by maintaining a healthy population of mitochondria is still unclear. Here, we have examined the role of Parkin in the context of mtDNA damage and myocardial aging using a mouse model carrying a proofreading defective mitochondrial DNA polymerase gamma (POLG). We observed both decreased Parkin protein levels and development of cardiac hypertrophy in POLG hearts with age; however, cardiac hypertrophy in POLG mice was neither rescued, nor worsened by cardiac specific overexpression or global deletion of Parkin, respectively. Unexpectedly, mitochondrial fitness did not substantially decline with age in POLG mice when compared to WT. We found that baseline mitophagy receptor-mediated mitochondrial turnover and biogenesis were enhanced in aged POLG hearts. We also observed the presence of megamitochondria in aged POLG hearts. Thus, these processes may limit the accumulation of dysfunctional mitochondria as well as the degree of cardiac functional impairment in the aging POLG heart. Overall, our results demonstrate that Parkin is dispensable for constitutive mitochondrial quality control in a mtDNA mutation model of cardiac aging.

BNIP3L/NIX and FUNDC1-mediated mitophagy is required for mitochondrial network remodeling during cardiac progenitor cell differentiation

Cell-based therapies represent a very promising strategy to repair and regenerate the injured heart to prevent progression to heart failure. To date, these therapies have had limited success due to a lack of survival and retention of the infused cells. Therefore, it is important to increase our understanding of the biology of these cells and utilize this information to enhance their survival and function in the injured heart. Mitochondria are critical for progenitor cell function and survival. Here, we demonstrate the importance of mitochondrial autophagy, or mitophagy, in the differentiation process in adult cardiac progenitor cells (CPCs). We found that mitophagy was rapidly induced upon initiation of differentiation in CPCs. We also found that mitophagy was mediated by mitophagy receptors, rather than the PINK1-PRKN/PARKIN pathway. Mitophagy mediated by BNIP3L/NIX and FUNDC1 was not involved in regulating progenitor cell fate determination, mitochondrial biogenesis, or reprogramming. Instead, mitophagy facilitated the CPCs to undergo proper mitochondrial network reorganization during differentiation. Abrogating BNIP3L- and FUNDC1-mediated mitophagy during differentiation led to sustained mitochondrial fission and formation of donut-shaped impaired mitochondria. It also resulted in increased susceptibility to cell death and failure to survive the infarcted heart. Finally, aging is associated with accumulation of mitochondrial DNA (mtDNA) damage in cells and we found that acquiring mtDNA mutations selectively disrupted the differentiation-activated mitophagy program in CPCs. These findings demonstrate the importance of BNIP3L- and FUNDC1-mediated mitophagy as a critical regulator of mitochondrial network formation during differentiation, as well as the consequences of accumulating mtDNA mutations. Abbreviations: Baf: bafilomycin A₁; BCL2L13: BCL2 like 13; BNIP3: BCL2 interacting protein 3; BNIP3L: BCL2 interacting protein 3 like; CPCs: cardiac progenitor cells; DM: differentiation media; DNM1L: dynamin 1 like; EPCs: endothelial progenitor cells; FCCP: carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone; FUNDC1: FUN14 domain containing 1; HSCs: hematopoietic stem cells; MAP1LC3B/LC3: microtubule-associated protein 1 light chain 3 beta; MFN1/2: mitofusin 1/2; MSCs: mesenchymal stem cells; mtDNA: mitochondrial DNA; OXPHOS: oxidative phosphorylation; PPARGC1A: PPARG coactivator 1 alpha; PHB2: prohibitin 2; POLG: DNA polymerase gamma, catalytic subunit; SQSTM1: sequestosome 1; TEM: transmission electron microscopy; TMRM: tetramethylrhodamine methyl ester.

Hypoxia Prevents Mitochondrial Dysfunction and Senescence in Human c-Kit+ Cardiac Progenitor Cells

Senescence-associated dysfunction deleteriously affects biological activities of human c-Kit⁺ cardiac progenitor cells (hCPCs), particularly under conditions of in vitro culture. In comparison, preservation of self-renewal and decreases in mitochondrial reactive oxygen species (ROS) are characteristics of murine CPCs in vivo that reside within hypoxic niches. Recapitulating hypoxic niche oxygen tension conditions of ∼1% O₂ in vitro for expansion of hCPCs rather than typical normoxic cell culture conditions (21% O₂ ) could provide significant improvement of functional and biological activities of hCPCs. hCPCs were isolated and expanded under permanent hypoxic (hCPC-1%) or normoxic (hCPC-21%) conditions from left ventricular tissue explants collected during left ventricular assist device implantation. hCPC-1% exhibit increased self-renewal and suppression of senescence characteristics relative to hCPC-21%. Oxidative stress contributed to higher susceptibility to apoptosis, as well as decreased mitochondrial function in hCPC-21%. Hypoxia prevented accumulation of dysfunctional mitochondria, supporting higher oxygen consumption rates and mitochondrial membrane potential. Mitochondrial ROS was an upstream mediator of senescence since treatment of hCPC-1% with mitochondrial inhibitor antimycin A recapitulated mitochondrial dysfunction and senescence observed in hCPC-21%. NAD⁺ /NADH ratio and autophagic flux, which are key factors for mitochondrial function, were higher in hCPC-1%, but hCPC-21% were highly dependent on BNIP3/NIX-mediated mitophagy to maintain mitochondrial function. Overall, results demonstrate that supraphysiological oxygen tension during in vitro expansion initiates a downward spiral of oxidative stress, mitochondrial dysfunction, and cellular energy imbalance culminating in early proliferation arrest of hCPCs. Senescence is inhibited by preventing ROS through hypoxic culture of hCPCs. Stem Cells 2019;37:555-567.

Evolving and Expanding the Roles of Mitophagy as a Homeostatic and Pathogenic Process

The central functions fulfilled by mitochondria as both energy generators essential for tissue homeostasis and gateways to programmed apoptotic and necrotic cell death mandate tight control over the quality and quantity of these ubiquitous endosymbiotic organelles. Mitophagy, the targeted engulfment and destruction of mitochondria by the cellular autophagy apparatus, has conventionally been considered as the mechanism primarily responsible for mitochondrial quality control. However, our understanding of how, why, and under what specific conditions mitophagy is activated has grown tremendously over the past decade. Evidence is accumulating that nonmitophagic mitochondrial quality control mechanisms are more important to maintaining normal tissue homeostasis whereas mitophagy is an acute tissue stress response. Moreover, previously unrecognized mitophagic regulation of mitochondrial quantity control, metabolic reprogramming, and cell differentiation suggests that the mechanisms linking genetic or acquired defects in mitophagy to neurodegenerative and cardiovascular diseases or cancer are more complex than simple failure of normal mitochondrial quality control. Here, we provide a comprehensive overview of mitophagy in cellular homeostasis and disease and examine the most revolutionary concepts in these areas. In this context, we discuss evidence that atypical mitophagy and nonmitophagic pathways play central roles in mitochondrial quality control, functioning that was previously considered to be the primary domain of mitophagy.

Abstract 283: Parkin Fails to Rescue Age-Dependent Cardiomyopathy in Mitochondrial DNA Mutator Mice

Temporal decline in mitochondrial function is widely considered to be a driver of cardiomyocyte aging, which in turn contributes to the prevalence of cardiovascular disease in the aging population. The accumulation of dysfunctional mitochondria appears to be a direct consequence of reduced autophagy and mitochondrial quality control in the aging heart. Parkin, an E3 ubiquitin ligase, plays a critical role in this process, marking dysfunctional mitochondria for degradation by autophagosomes; however, whether Parkin functions to prevent cardiac aging by maintaining a healthy population of mitochondria is still unclear. To examine the role of Parkin in the context of mtDNA damage and myocardial aging, we used a mouse model carrying a proofreading defective mitochondrial DNA polymerase gamma (POLG). We observed a significant decrease in Parkin protein levels in the hearts of aged (6 months) POLG mice, in spite of elevated Parkin mRNA. Seahorse analysis revealed a decrease in cardiac mitochondrial respiration in 6-month POLG mice. While cardiac structure and function were similar in both genotypes, POLG mice displayed modest but significant cardiac hypertrophy at this age. Next, we generated mice with concomitant Parkin deletion or cardiac specific Parkin overexpression with the mutant POLG. However, loss of Parkin did not exacerbate the accelerated cardiac aging phenotype observed in the POLG mice, and enhancing cardiac Parkin protein levels did not rescue the mitochondrial dysfunction or the cardiac hypertrophy observed in POLG mice up to 12 months of age. Surprisingly, we found that Parkin levels were reduced in POLG hearts, even with cardiac specific overexpression of Parkin. Translation of Parkin was unperturbed in 12-month POLGxParkin TG hearts, suggesting instead that Parkin protein stability is compromised in aged POLG hearts, a hypothesis we are currently testing. We also found both diminished protein ubiquitination and reduced degradation of Mfn1, a known Parkin substrate, in POLGxParkin TG hearts compared to Parkin TG hearts. These results provide new insights into mitophagy and aging, and suggest that Parkin plays a minor role in baseline mitochondrial maintenance and that overexpression of Parkin fails to prevent the cardiac aging process.

Balancing Autophagy for a Healthy Heart

Autophagy is a well-known intracellular degradation process involved in clearing damaged or unnecessary components in cells. Functional autophagy is important for cardiac homeostasis. Given this, it is not surprising that dysregulation of autophagy has been implicated in the aging process and in various cardiovascular diseases. Therefore, understanding the functional role of autophagy in the heart under various conditions and whether manipulation of the pathway has therapeutic benefits have been a major focus of many investigations in recent years. Although consensus exists that autophagy is a critical cellular quality control pathway in the heart, its role in disease remains controversial. Whether altered autophagy is protective or detrimental in the heart seems to depend on the context and the disease. Here, we review the latest insights into autophagy in cardiovascular homeostasis and disease and its role in disease development.

Isolation of Rab5-positive endosomes reveals a new mitochondrial degradation pathway utilized by BNIP3 and Parkin

Degradation of mitochondria is an important cellular quality control mechanism mediated by two distinct pathways: one involving Parkin-mediated ubiquitination and the other dependent on mitophagy receptors. It is known that mitochondria are degraded by the autophagy pathway; however, we recently reported that the small GTPase Rab5 and early endosomes also participate in Parkin-mediated mitochondrial clearance. Here, we have developed a protocol to isolate Rab5-positive vesicles from cells for proteomics analysis and provide additional data confirming that mitophagy regulators and mitochondrial proteins are present in these vesicles. We also demonstrate that the mitophagy receptor BNIP3 utilizes the Rab5-endosomal pathway to clear mitochondria in cells. These findings indicate that a redundancy exists in the downstream degradation pathways to ensure efficient mitochondrial clearance.

Decline in cellular function of aged mouse c-kit+ cardiac progenitor cells

KEY POINTS

While autologous stem cell-based therapies are currently being tested on elderly patients, there are limited data on the function of aged stem cells and in particular c-kit⁺ cardiac progenitor cells (CPCs). We isolated c-kit⁺ cells from young (3 months) and aged (24 months) C57BL/6 mice to compare their biological properties. Aged CPCs have increased senescence, decreased stemness and reduced capacity to proliferate or to differentiate following dexamethasone (Dex) treatment in vitro, as evidenced by lack of cardiac lineage gene upregulation. Aged CPCs fail to activate mitochondrial biogenesis and increase proteins involved in mitochondrial oxidative phosphorylation in response to Dex. Aged CPCs fail to upregulate paracrine factors that are potentially important for proliferation, survival and angiogenesis in response to Dex. The results highlight marked differences between young and aged CPCs, which may impact future design of autologous stem cell-based therapies.

ABSTRACT

Therapeutic use of c-kit⁺ cardiac progenitor cells (CPCs) is being evaluated for regenerative therapy in older patients with ischaemic heart failure. Our understanding of the biology of these CPCs has, however, largely come from studies of young cells and animal models. In the present study we examined characteristics of CPCs isolated from young (3 months) and aged (24 months) mice that could underlie the diverse outcomes reported for CPC-based therapeutics. We observed morphological differences and altered senescence indicated by increased senescence-associated markers β-galactosidase and p16 mRNA in aged CPCs. The aged CPCs also proliferated more slowly than their young counterparts and expressed lower levels of the stemness marker LIN28. We subsequently treated the cells with dexamethasone (Dex), routinely used to induce commitment in CPCs, for 7 days and analysed expression of cardiac lineage marker genes. While MEF2C, GATA4, GATA6 and PECAM mRNAs were significantly upregulated in response to Dex treatment in young CPCs, their expression was not increased in aged CPCs. Interestingly, Dex treatment of aged CPCs also failed to increase mitochondrial biogenesis and expression of the mitochondrial proteins Complex III and IV, consistent with a defect in mitochondria complex assembly in the aged CPCs. Dex-treated aged CPCs also had impaired ability to upregulate expression of paracrine factor genes and the conditioned media from these cells had reduced ability to induce angiogenesis in vitro. These findings could impact the design of future CPC-based therapeutic approaches for the treatment of older patients suffering from cardiac injury.

A Rab5 endosomal pathway mediates Parkin-dependent mitochondrial clearance

Damaged mitochondria pose a lethal threat to cells that necessitates their prompt removal. The currently recognized mechanism for disposal of mitochondria is autophagy, where damaged organelles are marked for disposal via ubiquitylation by Parkin. Here we report a novel pathway for mitochondrial elimination, in which these organelles undergo Parkin-dependent sequestration into Rab5-positive early endosomes via the ESCRT machinery. Following maturation, these endosomes deliver mitochondria to lysosomes for degradation. Although this endosomal pathway is activated by stressors that also activate mitochondrial autophagy, endosomal-mediated mitochondrial clearance is initiated before autophagy. The autophagy protein Beclin1 regulates activation of Rab5 and endosomal-mediated degradation of mitochondria, suggesting cross-talk between these two pathways. Abrogation of Rab5 function and the endosomal pathway results in the accumulation of stressed mitochondria and increases susceptibility to cell death in embryonic fibroblasts and cardiac myocytes. These data reveal a new mechanism for mitochondrial quality control mediated by Rab5 and early endosomes.

Abstract 375: MCL-1 Regulates Mitochondrial Dynamics and Mitophagy

The anti-apoptotic BCL-2 family protein Myeloid Cell Leukemia-1 (MCL-1) is essential for maintaining mitochondrial integrity and cardiac function in the adult heart. Recently, we reported that cardiac-specific deletion of MCL-1 in mice leads to rapid mitochondrial dysfunction, hypertrophy, and lethal cardiomyopathy. Surprisingly, MCL-1 ablation does not result in apoptotic cell death, but rather cells show signs of mitochondrial deterioration and necrotic cell death. This suggests that in addition to its anti-apoptotic role, MCL-1 has unidentified roles in maintaining mitochondrial function in cardiac myocytes. MCL-1 exists in two distinct locations in myocytes: one form is localized to the outer mitochondrial membrane (MCL-1 OM ) and a shorter cleaved form exists in the mitochondrial matrix (MCL-1 Matrix ). Interestingly, overexpression of MCL-1 WT or MCL-1 OM , but not MCL-1 Matrix , leads to translocation of Drp1 to mitochondria, which correlates with fission and perinuclear aggregation of mitochondria. We also found that Drp1 co-immunoprecipitates with MCL-1 in heart lysates and that MCL-1-deficient hearts lack mitochondrial Drp1. Additionally, the interaction between Drp1 and MCL-1 and mitochondrial fission increase in response to serum starvation in neonatal myocytes. This suggests that MCL-1 OM recruits Drp1 to mitochondria to induce fission. Since there is a strong link between mitochondrial fission and degradation, we examined the effect of MCL-1 on mitophagy. Parkin is an E3 ubiquitin ligase that plays an important role in mitophagy. MEFs, which lack detectable Parkin, do not efficiently clear their depolarized mitochondria in response to FCCP treatment. However, overexpression of Parkin or MCL-1 increased the efficiency of mitophagy in MEFs in response to FCCP treatment. MCL-1-mediated mitophagy is abrogated in autophagy-deficient Atg5 -/- MEFs, confirming that the clearance occurs via enhanced autophagy. Thus, our data suggest that MCL-1 OM has dual actions in coordinating Drp1-mediated fission and mitophagy, which allows for more efficient removal of damaged mitochondria.

Abstract 223: Parkin is Critical for the Clearance of Damaged Mitochondria via an Autophagy-independent Pathway

Functional mitochondria are essential for highly metabolic organs such as the heart. When mitochondria are damaged they can release pro-death factors and reactive oxygen species which in turn can result in cell death. The E3 ubiquitin ligase Parkin plays an important role in clearing damaged mitochondria via the autophagy pathway to protect cells against unnecessary cell death. Interestingly, we have found that Parkin can mediate clearance of damaged mitochondria via an autophagy-independent pathway. In fact, Parkin promotes clearance of depolarized mitochondria at the same rate in both wild-type (WT) and autophagy deficient Atg5-/- mouse embryonic fibroblasts (MEFs) in response to the mitochondrial uncoupler FCCP. We also found that Parkin-mediated ubiquitination is critical for this process as disease associated mutants of Parkin were incapable of inducing mitochondrial clearance in Atg5-/- MEFs. Upon further investigation, we observed a significant increase in the number of Rab5+ early- and Rab7+ late endosomes in both WT and Atg5-/- MEFs after depolarization of mitochondria with FCCP or valinomycin, indicating activation of the endosomal-lysosomal degradation pathway. We did not observe activation of the endosomal pathway after exposure to actinomycin D, an inhibitor RNA synthesis and activator of apoptosis, confirming that mitochondrial damage specifically activates the endosomal degradation pathway. We also observed activation of the endosomal pathway in neonatal myocytes in response to FCCP treatment or after exposure to simulated ischemia/reperfusion (sI/R). Overexpression of the dominant negative Rab5S34N significantly enhanced sI/R-mediated cell death, suggesting that this is a protective pathway activated by cells in response to stress. Moreover, Beclin1 is well known to regulate activation of autophagy. Here, we found that knockdown of Beclin1 inhibited both the number of Rab5+ early endosomes and their colocalization with mitochondria in response to either FCCP or sI/R in myocytes, suggesting that Beclin1 is a critical upstream regulator of the endosomal degradation pathway. Thus, our data suggest that Parkin mediates clearance of damaged mitochondria via both the autophagy and endosomal pathways in cells.

Abstract 45: The Dark Side of Parkin: The Toxic Effects of Enhanced Parkin Levels

Parkin is an E3 ubiquitin ligase that facilitates clearance of damaged mitochondria. We have previously shown that Parkin is important for cardiac adaptation to myocardial infarction and that loss of Parkin leads to accumulation of dysfunctional mitochondria. As Parkin is upregulated in response to cardiac stress, we sought to determine the effect of overexpression of Parkin in response to hemodynamic stress. Wild type (WT) and cardiac-specific Parkin transgenic (Parkin-TG) mice were subjected to trans-aortic constriction. Surprisingly, Parkin-TG mice rapidly developed hypertrophy, pulmonary edema, and ventricular dysfunction compared to WT mice. We observed no differences in either mitochondrial content or LC3 levels in Parkin-TG hearts, suggesting that the early progression to heart failure was not due to excessive mitophagy. Cardiac contractility is regulated by intracellular calcium and abnormalities in calcium homeostasis are associated with cardiac dysfunction. To investigate whether Parkin affects calcium homeostasis in the heart, we performed a subcellular fractionation on Parkin -/- , WT, and Parkin-TG hearts. We found that Parkin localizes to the sarcoplasmic reticulum where it ubiquitinates proteins under baseline conditions. Furthermore, Parkin-TG mice had increased levels Phospholamban (PLB), but did not alter SERCA, suggesting that Parkin may alter calcium homeostasis via PLB. In addition, loss of-function mutations in Parkin are found in patients with familial Parkinson’s disease (PD). Because Parkin is upregulated in response to stress, we overexpressed either WT Parkin or PD-associated Parkin mutants in HeLa cells to evaluate how elevated Parkin affects cell function. Interestingly, Parkin mutants, but not WT Parkin, caused rapid cell death when overexpressed in HeLa cells. Overexpressing Parkin mutants also triggered elevated autophagic activity, and cells with impaired autophagy were more susceptible to toxicity induced by overexpression of the Parkin mutants. This suggests that cells upregulate autophagy to protect against toxic Parkin mutants. Therefore, in addition to defects in mitophagy, upregulation of mutant Parkin in patients may also detrimentally contribute to cell death and development of disease.

Evaluating mitochondrial autophagy in the mouse heart

Mitochondrial autophagy plays an important role in mediating mitochondrial quality control. Evaluating the extent of mitochondrial autophagy is challenging in the adult heart in vivo. Keima is a fluorescent protein that emits different colored signals at acidic and neutral pHs. Keima targeted to mitochondria (Mito-Keima) is useful in evaluating the extent of mitochondrial autophagy in cardiomyocytes in vitro. In order to evaluate the level of mitochondrial autophagy in the heart in vivo, we generated adeno-associated virus (AAV) serotype 9 harboring either Mito-Keima or Lamp1-YFP. AAV9-Mito-Keima and AAV9-Lamp1-YFP were administered intravenously and mice were subjected to either forty-eight hours of fasting or normal chow. Thin slices of the heart prepared within cold PBS were subjected to confocal microscopic analyses. The acidic dots Mito-Keima elicited by 561nm excitation were co-localized with Lamp1-YFP dots (Pearson's correlation, 0.760, p<0.001), confirming that the acidic dots of Mito-Keima were localized in lysosomes. The area co-occupied by Mito-Keima puncta with 561nm excitation and Lamp1-YFP was significantly greater 48h after fasting. Electron microscopic analyses indicated that autophagosomes containing only mitochondria were observed in the heart after fasting. The mitochondrial DNA content and the level of COX1/GAPDH, indicators of mitochondrial mass, were significantly smaller in the fasting group than in the control group, consistent with the notion that lysosomal degradation of mitochondria is stimulated after fasting. In summary, the level of mitochondrial autophagy in the adult heart can be evaluated with intravenous injection of AAV-Mito-Keima and AAV-Lamp1-YFP and confocal microscopic analyses.

Staying young at heart: autophagy and adaptation to cardiac aging

Aging is a predominant risk factor for developing cardiovascular disease. Therefore, the cellular processes that contribute to aging are attractive targets for therapeutic interventions that can delay or prevent the development of age-related diseases. Our understanding of the underlying mechanisms that contribute to the decline in cell and tissue functions with age has greatly advanced over the past decade. Classical hallmarks of aging cells include increased levels of reactive oxygen species, DNA damage, accumulation of dysfunctional organelles, oxidized proteins and lipids. These all contribute to a progressive decline in the normal physiological function of the cell and to the onset of age-related conditions. A major cause of the aging process is progressive loss of cellular quality control. Autophagy is an important quality control pathway and is necessary to maintain cardiac homeostasis and to adapt to stress. A reduction in autophagy has been observed in a number of aging models and there is compelling evidence that enhanced autophagy delays aging and extends life span. Enhancing autophagy counteracts age-associated accumulation of protein aggregates and damaged organelles in cells. In this review, we discuss the functional role of autophagy in maintaining homeostasis in the heart, and how a decline is associated with accelerated cardiac aging. We also evaluate therapeutic approaches being researched in an effort to maintain a healthy young heart.

Harnessing the Power of Integrated Mitochondrial Biology and Physiology: A Special Report on the NHLBI Mitochondria in Heart Diseases Initiative

Mitochondrial biology is the sum of diverse phenomena from molecular profiles to physiological functions. A mechanistic understanding of mitochondria in disease development, and hence the future prospect of clinical translations, relies on a systems-level integration of expertise from multiple fields of investigation. Upon the successful conclusion of a recent National Institutes of Health, National Heart, Lung, and Blood Institute initiative on integrative mitochondrial biology in cardiovascular diseases, we reflect on the accomplishments made possible by this unique interdisciplinary collaboration effort and exciting new fronts on the study of these remarkable organelles.

Abstract 174: The BH3-Only Protein BNIP3 Induces Mitochondrial Clearance via Multiple Pathways

Autophagy plays an important role in cellular quality control and is responsible for removing protein aggregates and dysfunctional organelles. BNIP3 is an atypical BH3-only protein which is known to cause mitochondrial dysfunction and cell death in the myocardium. Interestingly, BNIP3 can also protect against cell death by promoting removal of dysfunctional mitochondria via autophagy (mitophagy). We have previously reported that BNIP3 is a potent inducer of mitophagy in cardiac myocytes and that BNIP3 contains an LC3 Interacting Region (LIR) that binds to LC3 on the autophagosome, tethering the mitochondrion to the autophagosome for engulfment. However, the molecular mechanism(s) underlying BNIP3-mediated mitophagy are still unclear. In this study, we discovered that BNIP3 can mediate mitochondrial clearance in cells even in the absence of a functional autophagy pathway. We found that overexpression of BNIP3 led to significant clearance of mitochondria in both wild type (WT) and autophagy deficient Atg5-/- MEFs. BNIP3 caused an increase in LC3II levels in WT MEFs, indicating increased formation of autophagosomes. In contrast, LC3II was undetectable in Atg5-/- MEFs. Furthermore, we found that BNIP3-mediated clearance in WT and Atg5-/- MEFs did not require the presence of Parkin, an E3 ubiquitin ligase which plays a critical role in clearing dysfunctional mitochondria in cells. Also, overexpression of Parkin did not enhance BNIP3-mediated mitochondrial clearance. When investigating activation of alternative cellular degradation pathways, we found that BNIP3 induced activation of the endosomal-lysosomal pathway in both WT and Atg5-/- MEFs. Mutating the LC3 binding site in BNIP3 did not interfere with the activation of the endosomal pathway and clearance of mitochondria in Atg5-/- MEFs. Thus, these findings suggest that BNIP3 can promote clearance of mitochondria via multiple pathways in cells. The role of autophagy in removing mitochondria is already well established and we are currently exploring the roles of the endosomal and alternative autophagy pathways in BNIP3-mediated mitochondrial clearance in myocytes.

Abstract 208: Parkin Contributes to the Development of Cardiac Hypertrophy in Response to Cardiac Pressure Overload

Parkin is an E3 ubiquitin ligase known to mediate mitochondrial clearance by marking damaged mitochondria for autophagy. Our lab has previously shown that Parkin is important for stress adaptation following myocardial infarction, and that loss of Parkin leads to accumulation of dysfunctional mitochondria. However, whether Parkin plays a role in cardiac adaptation to pressure overload is currently unknown. Here we investigated the functional importance of Parkin in cardiac hypertrophy and development of heart failure in response to hemodynamic stress. Wild type (WT), Parkin knock out (Parkin -/- ), and cardiac-specific Parkin transgenic (Parkin-TG) mice were subjected to trans-aortic constriction (TAC). Cardiac anatomy and function was evaluated by histology and echocardiography. Inflammation and hypertrophy gene expression profiles were assessed using qPCR and immunohistochemistry. We discovered that after 2 weeks of TAC, cardiac hypertrophy markers were not increased in hearts from Parkin -/- mice, and there was no increase in the heart weight to body weight ratio (HW/BW). However, after 8 weeks of TAC, Parkin -/- mice showed similar cardiac hypertrophy and loss of function as WT hearts. Parkin deficient hearts also displayed increased interstitial and perivascular fibrosis compared to WT hearts after 8 weeks of TAC. This suggests that there is a delay in activating the hypertrophy program in the absence of Parkin, and that lack of Parkin leads to excessive fibrosis. In contrast, Parkin-TG mice showed a rapid development of hypertrophy and progression to heart failure compared to WT mice. Interestingly, we observed no differences in either mitochondrial content or LC3 levels after two weeks of TAC in Parkin-TG hearts, suggesting that the rapid development of hypertrophy and early progression to heart failure was not due to excessive mitophagy. These data suggest that Parkin plays an important role in the activation of the cardiac hypertrophy program and that this function may be independent of its role in regulating mitophagy. Thus, this study provides novel insight into the functional importance of Parkin in the heart. Additional studies are needed to determine the mechanism of how Parkin regulates cardiac hypertrophy.

Abstract 243: The E3 Ubiquitin Ligase Parkin Mediates Clearance of Damaged Mitochondria via Two Distinct Pathways

Damaged mitochondria release reactive oxygen species and pro-death factors which can lead to loss of cardiac myocytes. To protect against such damage, myocytes have developed several mechanisms of quality control that act both on the protein and organelle levels. We have previously identified the E3 ubiquitin ligase Parkin as an important regulator of mitochondrial clearance via autophagy in the myocardium. Here, we report that Parkin can also mediate clearance of mitochondria via the endosomal-lysosomal pathway. We found that Parkin promotes clearance of damaged mitochondria in both wild type (WT) and autophagy-deficient Atg5 knockout mouse embryonic fibroblasts (MEFs) treated with the mitochondria uncoupler FCCP. Mitochondrial damage leads to rapid activation of the endosomal-lysosomal pathway in both WT and Atg5-/- MEFs. We further observed increased activation of Rab5, a protein involved in early endosome formation, in both WT and Atg5-/- MEFs after treatment with FCCP. In addition, we observed sequestration of damaged mitochondria in Rab5+ and Rab7+ early and late endosomes, respectively. Mitochondria also colocalized with Lamp2+ vesicles in Atg5-/- MEFs indicating that the mitochondria are ultimately being delivered to the lysosomes for degradation. Overexpression of Rab5S34N, a dominant negative of Rab5, reduces FCCP-mediated clearance and increases cell death in Atg5-/- MEFs. Pharmacological inhibition of the endosomal-lysosomal pathway also results in increased FCCP-mediated cell death. Furthermore, we confirmed that FCCP treatment or simulated ischemia reperfusion exposure induces Rab5 activation with subsequent mitochondrial sequestration in early endosomes in neonatal myocytes. Interestingly, the activation of Rab5 is abrogated in the presence of the mitochondrial targeted antioxidant Mito-Tempo, suggesting that mitochondrial ROS is involved in the activation the endosomal pathway. Mitochondrial clearance via this pathway is also dependent on Parkin, as FCCP treatment fails to activate Rab5 and induce mitochondrial clearance in both WT and Atg5-/- MEFS in the absence of Parkin. Thus, our data suggest that Parkin can mediate clearance of damaged mitochondria via two distinct pathways in cells.

Abstract 393: MCL-1 Promotes Survival and Influences Mitochondrial Dynamics in Cardiac Myocytes

The BCL-2 family proteins are important regulators of mitochondrial structure and integrity. MCL-1 is an anti-apoptotic BCL-2 protein that is highly expressed in the myocardium compared to the other anti-apoptotic proteins BCL-2 and BCL-X L. Recently, we reported that MCL-1 is essential for myocardial homeostasis. Cardiac-specific deletion of MCL-1 in mice led to rapid mitochondrial dysfunction, hypertrophy, and lethal cardiomyopathy. Surprisingly, MCL-1 deficient myocytes did not undergo apoptotic cell death. Instead, the cells displayed signs of mitochondrial deterioration and necrotic cell death, suggesting that MCL-1 has an additional role in maintaining mitochondrial function in cardiac myocytes. Similarly, deletion of MCL-1 in fibroblasts caused rapid mitochondrial fragmentation followed by cell death at 72 hours. Interestingly, the MCL-1 deficient fibroblasts retained cytochrome c in the mitochondria , confirming that the cells were not undergoing apoptotic cell death. We have also identified that MCL-1 localizes to the mitochondrial outer membrane (OM) and the matrix in the myocardium and that the two forms respond differently to stress. MCL-1 OM was rapidly degraded after myocardial infarction or fasting, whereas MCL-1 Matrix levels were maintained. Similarly, starvation of MEFs resulted in rapid degradation of MCL-1 OM , whereas MCL-1 Matrix showed delayed degradation. Treatment with the mitochondrial uncoupler FCCP led to rapid degradation of both forms. This suggests that the susceptibility to degradation is dependent on its localization and the nature of the stress. Our data also suggests that these two forms perform distinct functions in regulating mitochondrial morphology and survival. Overexpression of MCL-1 Matrix promoted mitochondrial fusion in fibroblasts under baseline conditions and protected cells against FCCP-mediated mitochondrial fission and clearance by autophagosomes. Thus, our data suggest that MCL-1 exists in two separate locations where it performs different functions. MCL-1 Matrix promotes mitochondrial fusion, which protects cells against excessive mitochondrial clearance during unfavorable conditions.

Abstract 11: Acquisition of Mitochondrial DNA Mutations Impairs Mitochondrial Function in Cardiac Progenitor Cells

Activation of cardiac progenitor cells (CPCs) are critical for effective repair in response to pathologic injury. Stem cell activation and commitment involve increased energy demand and mitochondrial biogenesis. We have previously shown that incubation of c-kit+ CPCs in differentiation medium led to expansion of the mitochondrial network and lineage commitment. CPC function is reduced with age but the underlying mechanism is still unclear. Mitochondria contain their own DNA (mtDNA) which accumulates mutations over time that can impair mitochondrial function. In this study, we investigated the effects of acquiring mtDNA mutations on CPC proliferation, survival, and differentiation. We utilized a mouse model in which a mutation in the mtDNA polymerase gamma (POLGm/m) leads to accumulation of mtDNA mutations, mitochondrial dysfunction, and accelerated aging. Isolated CPCs from hearts of 2-month old POLGm/m mice had reduced proliferation and were more susceptible to oxidative stress and chemotherapeutic agents compared to WT CPCs. Incubation in differentiation medium resulted in fewer lineage committed POLGm/m CPCs compared to WT. In addition, the POLGm/m CPCs failed to activate mitochondrial biogenesis and did not increase levels of proteins involved in mitochondrial oxidative phosphorylation. We measured mitochondrial respiration with the Seahorse XF Analyzer and found that POLGm/m CPCs had undetectable oxygen consumption but still generated similar amounts of ATP as WT CPCs. Interestingly, POLGm/m CPCs produced increased amounts of l-lactate and were more sensitive to 2-deoxyglucose treatment, suggesting that these cells rely on glycolysis for energy production. Both WT and POLGm/m CPCs downregulated expression of glycolytic enzymes during differentiation. However, POLGm/m CPCs failed to undergo the metabolic transition from glycolysis to OXPHOS, which led to activation of cell death during differentiation. These data demonstrate that mitochondria play a critical role in CPC function, and accumulation of mtDNA mutations impairs CPC function and reduces their repair potential.

Accumulation of Mitochondrial DNA Mutations Disrupts Cardiac Progenitor Cell Function and Reduces Survival

Transfer of cardiac progenitor cells (CPCs) improves cardiac function in heart failure patients. However, CPC function is reduced with age, limiting their regenerative potential. Aging is associated with numerous changes in cells including accumulation of mitochondrial DNA (mtDNA) mutations, but it is unknown how this impacts CPC function. Here, we demonstrate that acquisition of mtDNA mutations disrupts mitochondrial function, enhances mitophagy, and reduces the replicative and regenerative capacities of the CPCs. We show that activation of differentiation in CPCs is associated with expansion of the mitochondrial network and increased mitochondrial oxidative phosphorylation. Interestingly, mutant CPCs are deficient in mitochondrial respiration and rely on glycolysis for energy. In response to differentiation, these cells fail to activate mitochondrial respiration. This inability to meet the increased energy demand leads to activation of cell death. These findings demonstrate the consequences of accumulating mtDNA mutations and the importance of mtDNA integrity in CPC homeostasis and regenerative potential.

PINK1 Is Dispensable for Mitochondrial Recruitment of Parkin and Activation of Mitophagy in Cardiac Myocytes

Myocyte function and survival relies on the maintenance of a healthy population of mitochondria. The PINK1/Parkin pathway plays an important role in clearing defective mitochondria via autophagy in cells. However, how the PINK1/Parkin pathway regulates mitochondrial quality control and whether it coordinates with other mitophagy pathways are still unclear. Therefore, the objective of this study was to investigate the effect of PINK1-deficiency on mitochondrial quality control in myocytes. Using PINK1-deficient (PINK1-/-) mice, we found that Parkin is recruited to damaged cardiac mitochondria in hearts after treatment with the mitochondrial uncoupler FCCP or after a myocardial infarction even in the absence of PINK1. Parkin recruitment to depolarized mitochondria correlates with increased ubiquitination of mitochondrial proteins and activation of mitophagy in PINK1-/- myocytes. In addition, induction of mitophagy by the atypical BH3-only protein BNIP3 is unaffected by lack of PINK1. Overall, these data suggest that Parkin recruitment to depolarized cardiac mitochondria and subsequent activation of mitophagy is independent of PINK1. Moreover, alternative mechanisms of Parkin activation and pathways of mitophagy remain functional in PINK1-/- myocytes and could compensate for the PINK1 deficiency.

Unbreak my heart: targeting mitochondrial autophagy in diabetic cardiomyopathy

SIGNIFICANCE

Diabetes is strongly associated with increased incidence of heart disease and mortality due to development of diabetic cardiomyopathy. Even in the absence of cardiovascular disease, cardiomyopathy frequently arises in diabetic patients. Current treatment options for cardiomyopathy in diabetic patients are the same as for nondiabetic patients and do not address the causes underlying the loss of contractility.

RECENT ADVANCES

Although there are numerous distinctions between Type 1 and Type 2 diabetes, recent evidence suggests that the two disease states converge on mitochondria as an epicenter for cardiomyocyte damage.

CRITICAL ISSUES

Accumulation of dysfunctional mitochondria contributes to cardiac tissue injury in both acute and chronic conditions. Removal of damaged mitochondria by macroautophagy, termed "mitophagy," is critical for maintaining cardiomyocyte health and contractility both under normal conditions and during stress. However, very little is known about the involvement of mitophagy in the pathogenesis of diabetic cardiomyopathy. A growing interest in this topic has given rise to a wave of publications that aim at deciphering the status of autophagy and mitophagy in Type 1 and Type 2 diabetes.

FUTURE DIRECTIONS

This review summarizes these recent studies with the goal of drawing conclusions about the activation or suppression of autophagy and mitophagy in the diabetic heart. A better understanding of how autophagy and mitophagy are affected in the diabetic myocardium is still needed, as well as whether they can be targeted therapeutically.

Interdependence of Parkin-Mediated Mitophagy and Mitochondrial Fission in Adult Mouse Hearts

RATIONALE

The role of Parkin in hearts is unclear. Germ-line Parkin knockout mice have normal hearts, but Parkin is protective in cardiac ischemia. Parkin-mediated mitophagy is reportedly either irrelevant, or a major factor, in the lethal cardiomyopathy evoked by cardiac myocyte-specific interruption of dynamin-related protein 1 (Drp1)-mediated mitochondrial fission.

OBJECTIVE

To understand the role of Parkin-mediated mitophagy in normal and mitochondrial fission-defective adult mouse hearts.

METHODS AND RESULTS

Parkin mRNA and protein were present at low levels in normal mouse hearts, but were upregulated after cardiac myocyte-directed Drp1 gene deletion in adult mice. Alone, forced cardiac myocyte Parkin overexpression activated mitophagy without adverse effects. Likewise, cardiac myocyte-specific Parkin deletion evoked no adult cardiac phenotype, revealing no essential function for, and tolerance of, Parkin-mediated mitophagy in normal hearts. Concomitant conditional Parkin deletion with Drp1 ablation in adult mouse hearts prevented Parkin upregulation in mitochondria of fission-defective hearts, also increasing 6-week survival, improving ventricular ejection performance, mitigating adverse cardiac remodeling, and decreasing cardiac myocyte necrosis and replacement fibrosis. Underlying the Parkin knockout rescue was suppression of Drp1-induced hyper-mitophagy, assessed as ubiquitination of mitochondrial proteins and mitochondrial association of autophagosomal p62/sequestosome 1 (SQSTM1) and processed microtubule-associated protein 1 light chain 3 (LC3-II). Consequently, mitochondrial content of Drp1-deficient hearts was preserved. Parkin deletion did not alter characteristic mitochondrial enlargement of Drp1-deficient cardiac myocytes.

CONCLUSIONS

Parkin is rare in normal hearts and dispensable for constitutive mitophagic quality control. Ablating Drp1 in adult mouse cardiac myocytes not only interrupts mitochondrial fission, but also markedly upregulates Parkin, thus provoking mitophagic mitochondrial depletion that contributes to the lethal cardiomyopathy.