Protocol to separate small and large extracellular vesicles from mouse and human cardiac tissues

Extracellular vesicles (EVs) are secreted by cells under various conditions and can contribute to the disease progression in tissues. Here, we present a protocol to separate small and large EVs from mouse hearts and cardiac tissues collected from patients. We describe steps for utilizing enzymatic digestion for release of EVs from interstitial space followed by differential centrifugation and immunoaffinity purification. The isolated EVs can be used for various experiments to gain insight into their in vivo functions. For complete details on the use and execution of this protocol, please refer to Liang et al. (2023).1.

Cancer-cell-secreted extracellular vesicles target p53 to impair mitochondrial function in muscle

Skeletal muscle loss and weakness are associated with bad prognosis and poorer quality of life in cancer patients. Tumor-derived factors have been implicated in muscle dysregulation by inducing cachexia and apoptosis. Here, we show that extracellular vesicles secreted by breast cancer cells impair mitochondrial homeostasis and function in skeletal muscle, leading to decreased mitochondrial content and energy production and increased oxidative stress. Mechanistically, miR-122-5p in cancer-cell-secreted EVs is transferred to myocytes, where it targets the tumor suppressor TP53 to decrease the expression of TP53 target genes involved in mitochondrial regulation, including Tfam, Pgc-1α, Sco2, and 16S rRNA. Restoration of Tp53 in muscle abolishes mitochondrial myopathology in mice carrying breast tumors and partially rescues their impaired running capacity without significantly affecting muscle mass. We conclude that extracellular vesicles from breast cancer cells mediate skeletal muscle mitochondrial dysfunction in cancer and may contribute to muscle weakness in some cancer patients.

Mitochondria are secreted in extracellular vesicles when lysosomal function is impaired

Mitochondrial quality control is critical for cardiac homeostasis as these organelles are responsible for generating most of the energy needed to sustain contraction. Dysfunctional mitochondria are normally degraded via intracellular degradation pathways that converge on the lysosome. Here, we identified an alternative mechanism to eliminate mitochondria when lysosomal function is compromised. We show that lysosomal inhibition leads to increased secretion of mitochondria in large extracellular vesicles (EVs). The EVs are produced in multivesicular bodies, and their release is independent of autophagy. Deletion of the small GTPase Rab7 in cells or adult mouse heart leads to increased secretion of EVs containing ubiquitinated cargos, including intact mitochondria. The secreted EVs are captured by macrophages without activating inflammation. Hearts from aged mice or Danon disease patients have increased levels of secreted EVs containing mitochondria indicating activation of vesicular release during cardiac pathophysiology. Overall, these findings establish that mitochondria are eliminated in large EVs through the endosomal pathway when lysosomal degradation is inhibited.

Deciphering functional roles and interplay between Beclin1 and Beclin2 in autophagosome formation and mitophagy

The degradation of macromolecules and organelles by the process of autophagy is critical for cellular homeostasis and is often compromised during aging and disease. Beclin1 and Beclin2 are implicated in autophagy induction, and these homologs share a high degree of amino acid sequence similarity but have divergent N-terminal regions. Here, we investigated the functions of the Beclin homologs in regulating autophagy and mitophagy, a specialized form of autophagy that targets mitochondria. Both Beclin homologs contributed to autophagosome formation, but a mechanism of autophagosome formation independent of either Beclin homolog occurred in response to starvation or mitochondrial damage. Mitophagy was compromised only in Beclin1-deficient HeLa cells and mouse embryonic fibroblasts because of defective autophagosomal engulfment of mitochondria, and the function of Beclin1 in mitophagy required the phosphorylation of the conserved Ser15 residue by the kinase Ulk1. Mitochondria-ER-associated membranes (MAMs) are important sites of autophagosome formation during mitophagy, and Beclin1, but not Beclin2 or a Beclin1 mutant that could not be phosphorylated at Ser15, localized to MAMs during mitophagy. Our findings establish a regulatory role for Beclin1 in selective mitophagy by initiating autophagosome formation adjacent to mitochondria, a function facilitated by Ulk1-mediated phosphorylation of Ser15 in its distinct N-terminal region.

The role of mitochondrial fission in cardiovascular health and disease

Mitochondria are organelles involved in the regulation of various important cellular processes, ranging from ATP generation to immune activation. A healthy mitochondrial network is essential for cardiovascular function and adaptation to pathological stressors. Mitochondria undergo fission or fusion in response to various environmental cues, and these dynamic changes are vital for mitochondrial function and health. In particular, mitochondrial fission is closely coordinated with the cell cycle and is linked to changes in mitochondrial respiration and membrane permeability. Another key function of fission is the segregation of damaged mitochondrial components for degradation by mitochondrial autophagy (mitophagy). Mitochondrial fission is induced by the large GTPase dynamin-related protein 1 (DRP1) and is subject to sophisticated regulation. Activation requires various post-translational modifications of DRP1, actin polymerization and the involvement of other organelles such as the endoplasmic reticulum, Golgi apparatus and lysosomes. A decrease in mitochondrial fusion can also shift the balance towards mitochondrial fission. Although mitochondrial fission is necessary for cellular homeostasis, this process is often aberrantly activated in cardiovascular disease. Indeed, strong evidence exists that abnormal mitochondrial fission directly contributes to disease development. In this Review, we compare the physiological and pathophysiological roles of mitochondrial fission and discuss the therapeutic potential of preventing excessive mitochondrial fission in the heart and vasculature.

Abstract P3033: The Pathogenic R120G Mutant Of Alpha-B-crystallin Accumulates Within Cardiac Mitochondria

Alpha-B-crystallin (CryAB) is an abundant molecular chaperone comprising nearly 5% of total cardiac mass. While predominately cytosolic, some studies note mitochondrial translocation of CryAB during stress. The autosomal dominant R120G mutation leads to CryAB misfolding and protein aggregation resulting in cardiomyopathy. Although mitochondrial dysfunction has been reported in cardiac-specific CryAB R120G transgenic mice, it is unclear whether these effects are secondary to the formation of cytotoxic aggregates or directly relate to mitochondrial damage. Using proteinase K protection assays in isolated mitochondria from mouse hearts and embryonic fibroblasts (MEFs), we have discovered that wild-type (WT) CryAB is present in the mitochondrial matrix indicating that this protein likely functions as a chaperone within the organelle. Interestingly, however, compared to the WT protein, CryAB R120G was significantly more enriched in MEF mitochondria. Furthermore, treatment of mice with adeno-associated virus serotype 9 (AAV9) encoding CryAB R120G under the control of a cardiac-specific promoter resulted in time-dependent intramitochondrial CryAB R120G accumulation across 2 and 4-weeks post injection. These data are the first to indicate that misfolded CryAB R120G resides specifically within the mitochondrial matrix. Knockdown of the mitochondrial matrix protease LonP1 significantly increased intramitochondrial as well as total CryAB R120G protein levels, and sensitized MEFs to cell death indicating that Lonp1 functions to proteolytically process imported CryAB R120G . While this response may acutely preserve mitochondrial function, sustained accumulation of intramitochondrial CryAB R120G likely exhausts proteolytic machinery within the organelle leading to induction of mitochondrial autophagy (mitophagy). Indeed, mitochondrial respiration was unchanged in both AAV9-treated and homozygous CryAB R120G knock-in mice at early time-points, whereas mitophagy-deficient Parkin -/- mice displayed significantly exacerbated mitochondrial CryAB R120G protein accumulation. Altogether, these data indicate aberrant accumulation of misfolded proteins within cardiac mitochondria underlies the pathogenicity of CryAB R120G .

Mcl-1 Differentially Regulates Autophagy in Response to Changes in Energy Status and Mitochondrial Damage

Myeloid cell leukemia-1 (Mcl-1) is a unique antiapoptotic Bcl-2 member that is critical for mitochondrial homeostasis. Recent studies have demonstrated that Mcl-1′s functions extend beyond its traditional role in preventing apoptotic cell death. Specifically, data suggest that Mcl-1 plays a regulatory role in autophagy, an essential degradation pathway involved in recycling and eliminating dysfunctional organelles. Here, we investigated whether Mcl-1 regulates autophagy in the heart. We found that cardiac-specific overexpression of Mcl-1 had little effect on baseline autophagic activity but strongly suppressed starvation-induced autophagy. In contrast, Mcl-1 did not inhibit activation of autophagy during myocardial infarction or mitochondrial depolarization. Instead, overexpression of Mcl-1 increased the clearance of depolarized mitochondria by mitophagy independent of Parkin. The increase in mitophagy was partially mediated via Mcl-1′s LC3-interacting regions and mutation of these sites significantly reduced Mcl-1-mediated mitochondrial clearance. We also found that Mcl-1 interacted with the mitophagy receptor Bnip3 and that the interaction was increased in response to mitochondrial stress. Overall, these findings suggest that Mcl-1 suppresses nonselective autophagy during nutrient limiting conditions, whereas it enhances selective autophagy of dysfunctional mitochondria by functioning as a mitophagy receptor.

Aging is associated with a decline in Atg9b-mediated autophagosome formation and appearance of enlarged mitochondria in the heart

Advancing age is a major risk factor for developing heart disease, and the biological processes contributing to aging are currently under intense investigation. Autophagy is an important cellular quality control mechanism that is reduced in tissues with age but the molecular mechanisms underlying the age-associated defects in autophagy remain poorly characterized. Here, we have investigated how the autophagic process is altered in aged mouse hearts. We report that autophagic activity is reduced in aged hearts due to a reduction in autophagosome formation. Gene expression profile analysis to evaluate changes in autophagy regulators uncovered a reduction in Atg9b transcript and protein levels. Atg9 proteins are critical in delivering membrane to the growing autophagosome, and siRNA knockdown of Atg9b in cells confirmed a reduction in autophagosome formation. Autophagy is also the main pathway involved in eliminating dysfunctional mitochondria via a process known as mitophagy. The E3 ubiquitin ligase Parkin plays a key role in labeling mitochondria for mitophagy. We also found increased levels of Parkin-positive mitochondria in the aged hearts, an indication that they have been labeled for mitophagy. In contrast, Nrf1, a major transcriptional regulator of mitochondrial biogenesis, was significantly reduced in aged hearts. Additionally, our data showed reduced Drp1-mediated mitochondrial fission and formation of enlarged mitochondria in the aged heart. Overall, our findings suggest that cardiac aging is associated with reduced autophagosome number, decreased mitochondrial turnover, and formation of megamitochondria.

Mcl-1-mediated mitochondrial fission protects against stress but impairs cardiac adaptation to exercise

Myeloid cell leukemia-1 (Mcl-1) is a structurally and functionally unique anti-apoptotic Bcl-2 protein. While elevated levels of Mcl-1 contribute to tumor cell survival and drug resistance, loss of Mcl-1 in cardiac myocytes leads to rapid mitochondrial dysfunction and heart failure development. Although Mcl-1 is an anti-apoptotic protein, previous studies indicate that its functions extend beyond regulating apoptosis. Mcl-1 is localized to both the mitochondrial outer membrane and matrix. Here, we have identified that Mcl-1 in the outer mitochondrial membrane mediates mitochondrial fission, which is independent of its anti-apoptotic function. We demonstrate that Mcl-1 interacts with Drp1 to promote mitochondrial fission in response to various challenges known to perturb mitochondria morphology. Induction of fission by Mcl-1 reduces nutrient deprivation-induced cell death and the protection is independent of its BH3 domain. Finally, cardiac-specific overexpression of Mcl-1_OM, but not Mcl-1_Matrix, contributes to a shift in the balance towards fission and leads to reduced exercise capacity, suggesting that a pre-existing fragmented mitochondrial network leads to decreased ability to adapt to an acute increase in workload and energy demand. Overall, these findings highlight the importance of Mcl-1 in maintaining mitochondrial health in cells.

Mitochondrial Quality Control and Cellular Proteostasis: Two Sides of the Same Coin

Mitochondrial dysfunction is a hallmark of cardiac pathophysiology. Defects in mitochondrial performance disrupt contractile function, overwhelm myocytes with reactive oxygen species (ROS), and transform these cellular powerhouses into pro-death organelles. Thus, quality control (QC) pathways aimed at identifying and removing damaged mitochondrial proteins, components, or entire mitochondria are crucial processes in post-mitotic cells such as cardiac myocytes. Almost all of the mitochondrial proteins are encoded by the nuclear genome and the trafficking of these nuclear-encoded proteins necessitates significant cross-talk with the cytosolic protein QC machinery to ensure that only functional proteins are delivered to the mitochondria. Within the organelle, mitochondria contain their own protein QC system consisting of chaperones and proteases. This system represents another level of QC to promote mitochondrial protein folding and prevent aggregation. If this system is overwhelmed, a conserved transcriptional response known as the mitochondrial unfolded protein response is activated to increase the expression of proteins involved in restoring mitochondrial proteostasis. If the mitochondrion is beyond repair, the entire organelle must be removed before it becomes cytotoxic and causes cellular damage. Recent evidence has also uncovered mitochondria as participants in cytosolic protein QC where misfolded cytosolic proteins can be imported and degraded inside mitochondria. However, this process also places increased pressure on mitochondrial QC pathways to ensure that the imported proteins do not cause mitochondrial dysfunction. This review is focused on discussing the pathways involved in regulating mitochondrial QC and their relationship to cellular proteostasis and mitochondrial health in the heart.

Nuclear Parkin Activates the ERRα Transcriptional Program and Drives Widespread Changes in Gene Expression Following Hypoxia

Parkin is an E3 ubiquitin ligase well-known for facilitating clearance of damaged mitochondria by ubiquitinating proteins on the outer mitochondrial membrane. However, knowledge of Parkin's functions beyond mitophagy is still limited. Here, we demonstrate that Parkin has functions in the nucleus and that Parkinson's disease-associated Parkin mutants, ParkinR42P and ParkinG430D, are selectively excluded from the nucleus. Further, Parkin translocates to the nucleus in response to hypoxia which correlates with increased ubiquitination of nuclear proteins. The serine-threonine kinase PINK1 is responsible for recruiting Parkin to mitochondria, but translocation of Parkin to the nucleus occurs independently of PINK1. Transcriptomic analyses of HeLa cells overexpressing wild type or a nuclear-targeted Parkin revealed that during hypoxia, Parkin contributes to both increased and decreased transcription of genes involved in regulating multiple metabolic pathways. Furthermore, a proteomics screen comparing ubiquitinated proteins in hearts from Parkin-/- and Parkin transgenic mice identified the transcription factor estrogen-related receptor α (ERRα) as a potential Parkin target. Co-immunoprecipitation confirmed that nuclear-targeted Parkin interacts with and ubiquitinates ERRα. Further analysis uncovered that nuclear Parkin increases the transcriptional activity of ERRα. Overall, our study supports diverse roles for Parkin and demonstrates that nuclear Parkin regulates transcription of genes involved in multiple metabolic pathways.

The Aging Heart: Mitophagy at the Center of Rejuvenation

Aging is associated with structural and functional changes in the heart and is a major risk factor in developing cardiovascular disease. Many recent studies have focused on increasing our understanding of the basis of aging at the cellular and molecular levels in various tissues, including the heart. It is known that there is an age-related decline in cellular quality control pathways such as autophagy and mitophagy, which leads to accumulation of potentially harmful cellular components in cardiac myocytes. There is evidence that diminished autophagy and mitophagy accelerate the aging process, while enhancement preserves cardiac homeostasis and extends life span. Here, we review the current knowledge of autophagy and mitophagy in aging and discuss how age-associated alterations in these processes contribute to cardiac aging and age-related cardiovascular diseases.

Abstract 105: MCL-1 Promotes Drp1-Mediated Mitochondrial Fission as an Adaptive Response to Stress

The anti-apoptotic BCL-2 family member, Myeloid Cell Leukemia-1 (MCl-1), is highly expressed in myocardium and plays a critical role in maintaining mitochondrial homeostasis. We have previously found that cardiomyocyte-specific MCL-1 knockout mice develop rapid cardiac dysfunction and cardiomyopathy but show little activation of apoptotic cell death. Instead, loss of MCL-1 resulted in atypical mitochondrial morphology and function. This suggests that besides its anti-apoptotic role, MCL-1 may have broader functions in regulating mitochondrial dynamics and function. MCL-1 localizes to two different mitochondrial locations in myocytes. One form exists on the outer mitochondrial membrane (MCL-1 OM ) and a shorter cleaved form resides in the mitochondrial matrix (MCL-1 Matrix ). We found that overexpression of MCL-1 WT or MCL-1 OM , but not MCL-1 Matrix , induces fragmentation and perinuclear aggregation of the mitochondria in a Drp1-dependent manner. Mutating MCL-1’s BH3 domain (G198E D199A), which is required for MCL-1’s anti-apoptotic function, completely abrogates its ability to induce perinuclear aggregation. Interestingly, a MCL-1-BCL-2 chimera, in which MCL-1’s BH domains are replaced with those of BCL-2, is still able to induce perinuclear aggregation. This suggests that the presence of a functional BH3 domain is sufficient for induction of perinuclear aggregation. We confirmed that there is increased interaction between endogenous MCL-1 and Drp1 in response to a variety of fission-promoting stimuli, including glucose deprivation, hypoxia, and treatment with rotenone or FCCP. MCL-1 overexpression also protects against cell death in response to these stimuli, but this protection is abrogated when Drp1 is knocked down using siRNA. Additionally, Drp1 levels are significantly increased at the mitochondria in the hearts of MCL-1 OM transgenic mice, and this increase corresponds to an increased interaction between MCL-1 and Drp1 in these transgenic mice. Consistent with these findings, many of the mitochondria in these transgenic mice appear to be smaller than those from the WT mice, indicative of enhanced mitochondrial fission. Thus, our data suggests that MCL-1 functions as a positive regulator of Drp1-mediated mitochondrial fission.

Abstract 865: The E3 Ubiquitin Ligase Parkin Regulates Metabolism From the Nucleus

Parkin facilitates mitophagy by ubiquitinating depolarized mitochondria to label them for degradation. We have previously shown that Parkin is important for adaptation to myocardial infarction (MI) and that loss of Parkin leads to accumulation of dysfunctional mitochondria. Although Parkin is mainly studied for its role at the mitochondria, it is unclear whether Parkin also functions in other subcellular compartments. Here, we used immunofluorescence and subcellular fractionation to find that a portion of Parkin localizes to the nucleus under basal conditions in vitro and in vivo . Pathogenic Parkin mutants, however, are selectively excluded from the nucleus. To further examine conditions that induce Parkin nuclear enrichment, we subjected HeLa cells stably expressing YFP-Parkin to starvation or hypoxia. During nutrient-deprivation, Parkin rapidly exits the nucleus and accumulates in the cytosol. Conversely, exposure to hypoxia causes Parkin enrichment in the nucleus and depletion from the cytosol. To separate the roles of nuclear versus mitochondrial Parkin, we generated nuclear- and mitochondrial- targeted constructs: NLS-Parkin and Mito-Parkin. RNA-sequencing analysis revealed widespread transcriptional changes in response to targeting Parkin to the nucleus, particularly in metabolic processes. Through a non-biased proteomics screen, we identified the transcription factor estrogen related receptor α (ERRα) as a potential Parkin target. ERRα is highly expressed in the heart and promotes transcription of genes involved in mitochondrial biogenesis and metabolism. We therefore used co-immunoprecipitation and qPCR to determine whether Parkin regulates cellular energetics through ERRα. Indeed, we found that NLS-Parkin interacts with and ubiquitinates ERRα. Additionally, expressing NLS-Parkin increases protein levels of both endogenous and overexpressed ERRα and promotes transcription of ERRα-targets. These data indicate that Parkin-mediated ubiquitination of ERRα leads to its increased stability and activity resulting in increased transcription of genes involved in mitochondrial biogenesis and metabolism. This study provides insight into new ways Parkin may be targeted to enhance energetic function in the heart.

Abstract 166: MCL-1 Facilitates the Removal of Damaged Mitochondria via the Mitophagy Receptor BNIP3

Myeloid Cell Leukemia-1 (MCL-1) is an anti-apoptotic BCL-2 family protein that is necessary to maintain cardiac homeostasis in the adult heart. MCL-1 localizes to two distinct mitochondrial locations in myocytes, both on the outer mitochondrial membrane (MCL-1 OM ) and in the mitochondrial matrix (MCL-1 Matrix ). Our lab previously showed that cardiac-specific ablation of MCL-1 at both of these mitochondrial locations in mice led to severe contractile dysfunction and compromised mitochondrial function. Intriguingly, these defects were accompanied by signs of necrotic, rather than apoptotic, cell death. This indicates that MCL-1 has an alternate role in maintaining mitochondrial homeostasis in cardiac myocytes. Unexpectedly, we found that MCL-1 induces mitochondrial clearance in response to treatment with the chemical uncoupler FCCP in a Parkin-independent manner. Hypoxia is also known to induce mitochondrial clearance, and overexpression of MCL-1 further enhances hypoxia-mediated mitophagy. Fluorescence imaging identified MCL-1-positive mitochondria sequestered inside autophagosomes. MCL-1-mediated clearance is abrogated in autophagy deficient Atg5-/- cells, confirming that clearance is occurring via the autophagy pathway. Mutation of MCL-1’s BH3 domain (G198E D199A) does not affect its ability to induce clearance, suggesting that this role may be independent of its anti-apoptotic function. Also, replacing MCL-1’s BH domains with those of BCL-2, does not affect its ability to induce mitophagy. Next, we investigated whether MCL-1 functions as a mitophagy receptor and promotes removal of damaged mitochondria by binding to directly to LC3 through one or more of its three putative LC3-Interacting Region (LIR) motifs. Endogenous MCL-1 and LC3 co-immunoprecipitate in response to stress induced by FCCP. However, mutating each of MCL-1’s individual LIR motifs, as well as generating combined mutations in all three, does not affect MCL-1-mediated mitophagy. Instead, we found that MCL-1 interacts with the known mitophagy receptor BNIP3 both in vitro and in vivo . Thus, our data suggest that MCL-1 promotes elimination of dysfunctional mitochondria by positively regulating the mitophagy receptor BNIP3.

Abstract 549: Cardiac Aging is Associated With Impaired Mitophagy and Formation of Megamitochondria

Aging is associated with increased risk of developing cardiovascular disease. Currently, the molecular mechanisms contributing to the cardiac aging process and the resulting pathogenesis are unclear. Here, we have characterized changes in mitochondria and mitochondrial autophagy (mitophagy) in aging mouse hearts. To evaluate structural and functional changes that occur with cardiac aging, we compared young (4 months) and old (24 month) mice. We found that aging led to cardiac hypertrophy as characterized by increased left ventricular (LV) mass index and heart weight/body weight ratio as well as elevated myosin heavy chain 7 mRNA levels. Although echocardiography analysis showed no detectable changes in systolic function, the aged mice showed evidence of diastolic dysfunction as there was a significant decrease in the E/A ratio. We also found increased levels of fibrosis and inflammation in the aged hearts. Evaluation of autophagy in aged hearts revealed impaired autophagic flux, as indicated by accumulation of the autophagy substrate p62. Treatment of mice with the mTOR inhibitor Rapamycin, led to the expected increase in LC3II-positive autophagosomes in young hearts but not in the aged hearts. Parkin is an E3 ubiquitin ligase involved in labeling mitochondria for mitophagy by ubiquitinating proteins on the outer mitochondrial membrane. We found a significant increase in Parkin at the mitochondria in aged hearts which was accompanied with a significant increase in mitochondrial protein ubiquitination and p62 levels. However, LC3II protein levels in the mitochondrial fraction did not increase. This suggests that although mitochondria have been labeled for mitophagy, they are not eliminated due to decreased formation of autophagosomes. Finally, ultrastructural analysis of hearts showed the presence of enlarged mitochondria in aged hearts. Although we observed no changes in mitochondrial fusion proteins Mfn1/2, we found decreased activation of the mitochondrial fission protein Drp1. Overall, these data suggest that which might contribute to the development of age-related cardiac pathologies.

Mitochondria and autophagy in adult stem cells: proliferate or differentiate

Adult stem cells are undifferentiated cells that are found in many different tissues after development. They are responsible for regenerating and repairing tissues after injury, as well as replacing cells when needed. Adult stem cells maintain a delicate balance between self-renewal to prevent depletion of the stem cell pool and differentiation to continually replenish downstream lineages. The important role of mitochondria in generating energy, calcium storage and regulating cell death is well established. However, new research has linked mitochondria to stem cell maintenance and fate. In addition, efficient mitochondrial quality control is critical for stem cell homeostasis to ensure their long-term survival in tissues. In this review, we discuss the latest evidence linking mitochondrial function, remodeling and turnover via autophagy to regulation of adult stem cell self-renewal and differentiation.

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.