Accumulation Of Dysfunctional Mitochondria Research Articles

Crosstalk between Mitochondrial Ca2+ Uptake and Autophagy in Skeletal Muscle.

Autophagy is responsible for the maintenance of skeletal muscle homeostasis, thanks to the removal of aberrant and dysfunctional macromolecules and organelles. During fasting, increased autophagy ensures the maintenance of the amino acid pool required for energy production. The activity of the mitochondrial Ca2+ uniporter (MCU), the highly selective channel responsible for mitochondrial Ca2+ uptake, controls skeletal muscle size, force, and nutrient utilization. Thus, both autophagy and mitochondrial Ca2+ accumulation play a pivotal role to maintain muscle homeostasis and to sustain muscle function. Here, we address whether, in skeletal muscle, mitochondrial Ca2+ uptake and autophagy are mutually related. Muscle-restricted MCU silencing partially inhibits the autophagy flux. Moreover, skeletal muscle-specific deletion of the essential autophagy gene Atg7, known to cause the accumulation of dysfunctional mitochondria, drastically reduces mitochondrial Ca2+ accumulation. Thus, a vicious cycle takes place, in which reduced MCU activity hampers the autophagic flux, and loss of autophagy further impairs mitochondrial Ca2+ signaling.

PINK1 import regulation at a crossroad of mitochondrial fate: the molecular mechanisms of PINK1 import.

PTEN-induced kinase 1 (PINK1) is a mitochondrial kinase whose activity is tightly regulated by the mitochondrial health status. In response to mitochondrial damage, activated PINK1 can promote mitophagy, an autophagic elimination of damaged mitochondria, by cooperating with Parkin ubiquitin ligase. Loss-of-function of PINK1/Parkin-mediated mitophagy results in the accumulation of dysfunctional mitochondria, which could be one aetiology of Parkinson's disease (PD). Within step-by-step signalling cascades of PINK1/Parkin-mediated mitophagy, mitochondrial damage-dependent PINK1 kinase activation is a critical step to trigger the mitophagy signal. Recent investigation of this process reveals that this stress-dependent PINK1 kinase activation is achieved by its regulated import into different mitochondrial compartments. Thus, PINK1 import regulation stands at an important crossroad to determine the mitochondrial fate-'keep' or 'remove'? In this review, we will summarize how the PINK1 import is regulated in a mitochondrial health status-dependent manner and how this process could be pharmacologically modulated to activate the PINK1/Parkin pathway.

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.

Inhibition of lysosomal function enhances inflammatory gene expression, induce oxidative stress and apoptosis in human chondrocytes

Purpose: Lysosomes are the major catabolic organelle of the cells and are essential for the clearance of autophagosomes. Loss of lysosomal function with age has been associated with mitochondrial dysfunction, oxidative stress and apoptosis of the cells. However, the role of lysosomes in chondrocytes homeostasis under pathological conditions has not been explored. Here, we used Chloroquine (CQ) and Bafilomycin A1 (Baf) to inhibit lysosomal function and investigated its consequences on chondrocyte homeostasis and viability in vitro. Methods: Human articular chondrocytes and cartilage explants were prepared from OA knee cartilage obtained from donors undergoing knee arthroplasty at the SUMMA Health Barberton Hospital. Human normal cartilage was obtained from NDRI. The expression of lysosomal associated membrane protein 1 (LAMP1) in normal and OA cartilage was determined by immunohistochemistry. Primary human OA chondrocytes or cartilage explants were maintained in complete DMEM/F12 and treated with CQ or Baf for 24 h or indicated time in DMEM/F12 medium without serum. Induction of apoptosis in chondrocytes treated with CQ or Baf was determined by measuring the activity of LDH released in the culture supernatant, caspase 3/7-luciferase reporter assay and TUNEL staining. The level of IL-1β and IL-6 was determined by TaqMan assays. The level of reactive oxygen species (ROS) was determined by DHR123 staining followed by flow cytometry and MitoSOX Red staining followed by confocal microscopy. The loss of mitochondrial membrane potential was determined by JC-1 dye staining and ATP levels were determined using a kit. The accumulation of dysfunctional mitochondria was determined by staining with MitoTracker Deep Red followed by flow cytometry. The signaling pathway activation was investigated by immunoblot analysis of NFκB and MAPK pathways and its involvement was confirmed by using small molecule inhibitors of the induced pathway. The role of ROS in CQ and Baf induced expression of IL-1β and IL-6 was determined by using anti-oxidants, MitoTempo and Trolox. Results: The expression of LAMP1 was highly downregulated in OA cartilage compared to normal cartilage. CQ and Baf significantly induced apoptosis of primary human OA chondrocytes as determined by LDH activity assay, caspase3/7 luciferase reporter assay and TUNEL staining. Similar results were obtained with mouse chondrocytes. Human OA cartilage explants as well as mouse femoral head cartilage explant culture treated with CQ or Baf showed significant increase in LDH release and TUNEL staining. The CQ and Baf induced inhibition of lysosomal function resulted in the loss of mitochondrial membrane potential, excessive levels of ROS, lower levels of ATP and the accumulation of dysfunctional mitochondria in the treated chondrocytes. Treatment of primary human OA chondrocytes with CQ or Baf significantly increased the gene and protein expression of IL-1β and IL-6. Investigation of the signaling pathways showed no role of NFκB signaling in the CQ and Baf induced upregulation of IL-1β and IL-6 expression. However, the JNK-MAPK, but not ERK- or p38-MAPK pathway, was activated in response to Baf treatment. Inhibition of JNK activation by using a specific inhibitor abrogated the upregulation of IL-1β and IL-6 expression in Baf treated human chondrocytes. The use of anti-oxidants, MitoTempo and Trolox also suppressed the CQ and Baf induced expression of IL-1β and IL-6. JNK inhibition and treatment with MitoTempo or Trolox failed to suppress apoptosis suggesting that this was independent of ROS levels and JNK pathway in chondrocytes with inhibited lysosomal function. Conclusions: We demonstrate here that inhibition of lysosomal function promotes inflammation, oxidative stress and chondrocyte apoptosis through JNK activation. Our data suggest that protecting lysosomal function may be a therapeutic target for OA management.

Salinomycin: Anti-tumor activity in a pre-clinical colorectal cancer model.

ObjectivesSalinomycin is a polyether antibiotic with selective activity against human cancer stem cells. The impact of salinomycin on patient-derived primary human colorectal cancer cells has not been investigated so far. Thus, here we aimed to investigate the activity of salinomycin against tumor initiating cells isolated from patients with colorectal cancer.MethodsPrimary tumor-initiating cells (TIC) isolated from human patients with colorectal liver metastases or from human primary colon carcinoma were exposed to salinomycin and compared to treatment with 5-FU and oxaliplatin. TICs were injected subcutaneously into NOD/SCID mice to induce a patient-derived mouse xenograft model of colorectal cancer. Animals were treated either with salinomycin, FOLFOX regimen, or salinomycin and FOLFOX. Human colorectal cancer cells were used to delineate an underlying molecular mechanism of salinomycin in this tumor entity.ResultsApplying TICs isolated from human patients with colorectal liver metastases or from human primary colon carcinoma, we demonstrated that salinomycin exerts increased antiproliferative activity compared to 5-fluorouracil and oxaliplatin treatment. Consistently, salinomycin alone or in combination with FOLFOX exerts superior antitumor activity compared to FOLFOX therapy in a patient-derived mouse xenograft model of colorectal cancer. Salinomycin induces apoptosis of human colorectal cancer cells, accompanied by accumulation of dysfunctional mitochondria and reactive oxygen species. These effects are associated with expressional down-regulation of superoxide dismutase-1 (SOD1) in response to salinomycin treatment.ConclusionCollectively, the results of this pre-clinical study indicate that salinomycin alone or in combination with 5-fluorouracil and oxaliplatin exerts increased antitumoral activity compared to common chemotherapy.

ALZHEIMER’S DISEASE: A CONSEQUENCE OF IMPAIRED MITOPHAGY?

Neurodegenerative disorders involve complexities of the pathologies, characterized by the progressive loss of neuronal viability, leading to severe physical and cognitive impairments in affected patients. These disorders although may differ in clinical outcomes yet they share common features such as aggregation of neurotoxic metabolites and perturbed cellular and neuronal homeostasis. Mitochondrion is an indispensable organelle for neuronal survival, and its role and place have become critical in research arena of aging and neurodegenaration. The accumulation of damaged mitochondria has been linked to normal aging and multiple age-related disorders including Alzheimer’s disease (AD). Survival and proper function of mitochondria depend on several attributes that include mitochondrial biogenesis and fusion and fission. Mitophagy is an utmost requirement for degradation and removal of damaged mitochondria where the target mitochondria are identified by the autophagosomes and delivered to the lysosome for degradation. Mitophagy plays important roles in mitochondrial homeostasis, neuroprotection, and resistance to neurodegeneration. AD besides other characteristic features involves mitochondrial dysfunctional, bioenergetic deficit, and altered mitophagy. The autophagy/lysosome pathway that removes damaged mitochondria (mitophagy) is compromised in AD, resulting in the accumulation of dysfunctional mitochondria that contribute to synaptic dysfunction and cognitive deficits by triggering Aβ and Tau accumulation through increases in oxidative damage and cellular energy deficits. The present work reviews the various implications of mitophagy in relevance to the pathology of AD.

FUN14 Domain-Containing 1-Mediated Mitophagy Suppresses Hepatocarcinogenesis by Inhibition of Inflammasome Activation in Mice.

Mitochondria lie at the heart of innate immunity, and aberrant mitochondrial activity contributes to immune activation and chronic inflammatory diseases, including liver cancers. Mitophagy is a selective process for removing dysfunctional mitochondria. The link between mitophagy and inflammation in tumorigenesis remains largely unexplored. We observed that FUN14 domain-containing 1 (FUNDC1), a previously characterized mitophagy receptor, accumulates in most human hepatocellular carcinomas (HCCs), and we thus explored the role of FUNDC1-mediated mitophagy in HCC initiation and progression in a mouse model in which HCC is induced by the chemical carcinogen, diethylnitrosamine (DEN). We showed that specific knockout of FUNDC1 in hepatocytes promotes the initiation and progression of DEN-induced HCC, whereas FUNDC1 transgenic hepatocytes protect against development of HCC. Hepatocyte-specific FUNDC1 ablation results in the accumulation of dysfunctional mitochondria and triggers a cascade of events involving inflammasome activation and hyperactivation of Janus kinase/signal transducer and activator of transcription signaling. Specifically, cytosolic mitochondrial DNA (mtDNA) release and caspase-1 activation are increased in FUNDC1-depleted hepatocytes. This subsequently results in the elevated release of proinflammatory cytokines, such as interleukin-1β (IL1β) and hyperproliferation of hepatocytes. Conclusion: Our results suggest that FUNDC1 suppresses HCC initiation by reducing inflammasome activation and inflammatory responses in hepatocytes, whereas up-regulation of FUNDC1 expression at the late stage of tumor development may benefit tumor growth. Our study thus describes a mechanistic link between mitophagic modulation of inflammatory response and tumorigenesis, and further implies that FUNDC1-mediated mitophagy and its related inflammatory response may represent a therapeutic target for liver cancer.

Disease‐associated tau impairs mitophagy by inhibiting Parkin translocation to mitochondria

Accumulation of the protein tau characterises Alzheimer's disease and other tauopathies, including familial forms of frontotemporal dementia (FTD) that carry pathogenic tau mutations. Another hallmark feature of these diseases is the accumulation of dysfunctional mitochondria. Although disease‐associated tau is known to impair several aspects of mitochondrial function, it is still unclear whether it also directly impinges on mitochondrial quality control, specifically Parkin‐dependent mitophagy. Using the mito‐QC mitophagy reporter, we found that both human wild‐type (hTau) and FTD mutant tau (hP301L) inhibited mitophagy in neuroblastoma cells, by reducing mitochondrial translocation of Parkin. In the Caenorhabditis elegans nervous system, hTau expression reduced mitophagy, whereas hP301L expression completely inhibited it. These effects were not due to changes in the mitochondrial membrane potential or the cytoskeleton, as tau specifically impaired Parkin recruitment to defective mitochondria by sequestering it in the cytosol. This sequestration was mediated by aberrant interactions of Parkin with the projection domain of tau. As mitochondria are dysfunctional in neurodegenerative conditions, these data suggest a vicious cycle, with tau also inhibiting the degradation of damaged mitochondria.

Mitochondrial quality control and neurodegenerative diseases.

Mitochondria homeostasis is sustained by the mitochondrial quality control (MQC) system, which is crucial for cellular health, especially in the maintenance of functional mitochondria. A healthy mitochondria network is essential for life as it regulates cellular metabolism processes, particularly ATP production. Mitochondrial dynamics and mitophagy are two highly integrated processes in MQC system that determines whether damaged mitochondria will be repaired or degraded. Neurons are highly differentiated cells which demand high energy consumption. Therefore, compromised MQC processes and the accumulation of dysfunctional mitochondria may be the main cause of neuronal death and lead to neurodegeneration. Here, we focus on the inseparable relationship of mitochondria dynamics and mitophagy and how their dysfunction may lead to neurodegenerative diseases.

VAMP7 Regulates Autophagosome Formation by Supporting Atg9a Functions in Pancreatic β-Cells From Male Mice.

Dysfunctional mitochondria are observed in β-cells of diabetic patients, which are eventually removed by autophagy. Vesicle-associated membrane protein (VAMP)7, a vesicular SNARE protein, regulates autophagosome formation to maintain mitochondrial homeostasis and control insulin secretion in pancreatic β-cells. However, its molecular mechanism is largely unknown. In this study, we investigated the molecular mechanism of VAMP7-dependent autophagosome formation using VAMP7-deficient β-cells and β-cell-derived Min6 cells. VAMP7 localized in autophagy-related (Atg)9a-resident vesicles of recycling endosomes (REs), which contributed to autophagosome formation, and it interacted with Hrb, Syntaxin16, and SNAP-47. Hrb recruited VAMP7 and Atg9a from the plasma membrane to REs. Syntaxin16 and SNAP-47 mediated autophagosome formation at a step later than the proper localization of VAMP7 to Atg9a-resident vesicles. Knockdown of Hrb, Syntaxin16, and SNAP-47 resulted in defective autophagosome formation, accumulation of dysfunctional mitochondria, and impairment of glucose-stimulated insulin secretion. Our data indicate that VAMP7 and Atg9a are initially recruited to REs to organize VAMP7 and Atg9a-resident vesicles in an Hrb-dependent manner. Additionally, VAMP7 forms a SNARE complex with Syntaxin16 and SNAP-47, which may cause fusions of Atg9a-resident vesicles during autophagosome formation. Thus, VAMP7 participates in autophagosome formation by supporting Atg9a functions that contribute to maintenance of mitochondrial quality.

Abstract 468: Mitochondrial Dysfunction and Senescence of Human Cardiac Progenitor Cells Are Prevented by Hypoxic Culture

Rationale: Therapeutic potential of c-Kit + cardiac progenitor cells (CPCs) is modest in autologous cell therapy clinical trials and could benefit from optimization to maximize functional activity. Early proliferation arrest limits expansion potential of human CPCs in vitro . In contrast, self-renewal is preserved and mitochondrial reactive oxidative species (mtROS) minimized within the CPC hypoxic niche in vivo . Expansion under oxygen tension comparable to the hypoxic niche may therefore improve CPC reparative function. Hypothesis: Senescence of CPCs derived from heart failure patients is suppressed by dampening mtROS through long-term hypoxic culture. Methods and Results: Environmental oxygen was maintained with constant hypoxia (1% O 2 ; H-CPCs) or atmospheric oxygen tension (20 - 21% O 2 ; N-CPCs) for isolation and expansion of CPCs from patients undergoing left ventricular assist device implantation. A 4-fold increase in clonogenesis and 73% fewer senescence-associated β-Gal-positive cells were found in H-CPCs. Oxidative stress and DNA damage were also reduced with 80% lower malondialdehyde content and significantly fewer nuclear γ-H2AX foci in H-CPCs. Accumulation of dysfunctional mitochondria was revealed in N-CPCs through functional metabolic analyses. Basal (-59%), maximal (-50%), and ATP production-coupled (-62%) oxygen consumption rates were significantly lower in N-CPCs, despite upregulation of electron transport chain subunits and 72% greater mitochondrial DNA content than H-CPCs. MtROS was elevated, and large mitochondria with swollen morphology were found by TEM in N-CPCs, confirming mitochondrial damage. Key indicators of mitochondrial function including NAD + /NADH ratio and autophagic flux were preserved in H-CPCs. Conclusions: Development of mitochondrial dysfunction and senescence are consequences of normoxic CPC expansion. CPC senescence is delayed by preserving mitochondrial health and limiting oxidative damage through hypoxic culture.

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.

The effect of aging and calorie restriction on mitochondrial morphology and dynamic in oxidative and glycolytic skeletal muscles

With aging, skeletal muscles progressively loose mass and function, a biological process known as sarcopenia. Recent data indicate that accumulation of mitochondrial dysfunction plays a major role in the development of sarcopenia and also suggested that alterations in mitophagy and mitochondrial dynamics/morphology may represent mechanisms causing this accumulation of dysfunctional mitochondria in the aged muscles. One most efficient lifestyle and non‐pharmacological intervention to attenuate sarcopenia in rodents is calorie restriction (CR). The hypothesis currently put forward to explain the protective and “anti‐aging” effects of caloric restriction is that CR would attenuate the effects of aging on mitochondrial function. However, the specific effects of CR on many aspects of mitochondrial biology, especially in the aged muscle, remain unclear or unknown. In particular, the effects of aging and CR on skeletal muscle mitochondrial morphology and dynamics, which are known to impact mitochondrial function, remain largely under investigated. The objectives of the present study were (i) to investigate the effects of aging on mitochondrial morphology and dynamic in glycolytic and oxidative skeletal muscles, define whether CR can attenuate the effects of aging on mitochondrial morphology and (iii) determine the effects of CR and aging on the muscular phenotype.Three groups of male Sprague Dawley rats were studied: 1‐adult (9‐month‐old) ad‐libitum fed (AL) (A‐AL); 2‐old (21‐month‐old) AL fed (O‐AL) and; 3‐old (21‐month‐old) calorie restricted (CR; 40% calorie restricted for 13 months; O‐CR) rats. The morphology of SubSarcolemmal (SS) and InterMyoFibrillar (IMF) mitochondria was assessed using a 2‐dimensional transmission electron microscopy approach (TEM) in the oxidative soleus (SOL) and glycolytic white gastrocnemius (WG) muscles. Westernblot analyses were performed to assess the contents of proteins regulating mitochondrial dynamics (fusion protein: Mfn2; Fission proteins: Drp1 and Fis1) in the SOL and WG.CR attenuated the impact of aging on the muscle mass to body weight ratio (a marker of sarcopenia). The SOL of O‐AL rats displayed fragmented SS and IMF mitochondria vs A‐AL rats. CR attenuated this aging‐related fragmentation of SS and IMF mitochondria in the SOL. The WG of O‐AL rats displayed enlarged SS mitochondria and more complex and branched IMF mitochondria vs their younger counterparts. In the WG, CR did not attenuate the effects of aging on mitochondrial morphology. In the SOL, the contents of Drp1 and Fis1 were higher in O‐AL vs A‐AL rats. When compared to the SOL of Y‐AL rats, O‐CR rats displayed higher contents of both Drp1 and Mfn2. In the WG, the content of Mfn2 and Drp1 was higher in O‐AL vs A‐AL rats. No difference between O‐CR and A‐AL rats were observed for proteins regulating mitochondrial dynamics.Taken altogether, our results indicate that the effects of aging and calorie restriction on mitochondrial morphology and dynamic are complex and muscle‐specific.Support or Funding InformationThis work was funded by grants from the Natural Sciences and Engineering Council of Canada (NSERC; #RGPIN‐2014‐04668), the Quebec Research Network on Aging and the FQRS (FRQS‐35184) awarded to Gilles Gouspillou. Jean‐Philippe Leduc‐Gaudet is supported by a CIHR Vanier Doctoral Scholarship.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Mitophagy in Parkinson's Disease: Pathogenic and Therapeutic Implications.

Neurons affected in Parkinson’s disease (PD) experience mitochondrial dysfunction and bioenergetic deficits that occur early and promote the disease-related α-synucleinopathy. Emerging findings suggest that the autophagy-lysosome pathway, which removes damaged mitochondria (mitophagy), is also compromised in PD and results in the accumulation of dysfunctional mitochondria. Studies using genetic-modulated or toxin-induced animal and cellular models as well as postmortem human tissue indicate that impaired mitophagy might be a critical factor in the pathogenesis of synaptic dysfunction and the aggregation of misfolded proteins, which in turn impairs mitochondrial homeostasis. Interventions that stimulate mitophagy to maintain mitochondrial health might, therefore, be used as an approach to delay the neurodegenerative processes in PD.

Beyond Mitophagy: The Diversity and Complexity of Parkin Function.

Parkin is an E3 ubiquitin ligase that mediates mitochondrial autophagy, or mitophagy, in multiple cell types. Although discovered in the context of Parkinson disease, Parkin is also an important regulator of mitophagy in the heart. In addition, while most current work is focused on the role of Parkin in mitophagy, accumulating evidence suggests that Parkin also impacts cellular physiology and function through additional processes. This Viewpoint discusses current controversies on the functional role of Parkin-mediated mitophagy and emerging evidence that Parkin regulates several additional pathways. Ubiquitylation is a post-translational modification that involves covalently attaching ubiquitin to target proteins.1 The process of ubquitylation requires a series of events including ubiquitin activation, conjugation, and ligation, catalyzed by E1, E2, and E3 enzymes, respectively. Parkin is one of >500 E3 ubiquitin ligases encoded by the human genome that selectively bestows ubiquitin molecules onto particular proteins. One of Parkin’s most widely appreciated roles is the ubiquitylation of proteins on the membrane of damaged mitochondria to designate them for clearance via mitophagy. Mitophagy is a selective form of macroautophagy that traffics damaged mitochondria to lysosomes for degradation and serves as a critical mechanism in maintaining the health of the mitochondrial collective in a cell. Although it is clear that mutations that impair Parkin’s E3 ligase function contribute to the attrition of dopaminergic neurons through cell death in Parkinson disease,2 the underlying mechanisms are still not fully understood and there is no direct evidence that a defect in mitophagy is the underlying cause of Parkinson disease. The assumption that faulty mitophagy is responsible for Parkinson disease stems from observations that defects in mitochondrial morphology and function are key features of Parkinson disease2 and that loss of Parkin in Drosophila leads to mitochondrial dysfunction and muscle degeneration.3 When combined with the seminal work …

The age-associated loss of ischemic preconditioning in the kidney is accompanied by mitochondrial dysfunction, increased protein acetylation and decreased autophagy

In young rats, ischemic preconditioning (IPC), which consists of 4 cycles of ischemia and reperfusion alleviated kidney injury caused by 40-min ischemia. However,old rats lost their ability to protect the ischemic kidney by IPC. A similar aged phenotype was demonstrated in 6-month-old OXYS rats having signs of premature aging. In the kidney of old and OXYS rats, the levels of acetylated nuclear proteins were higher than in young rats, however, unlike in young rats, acetylation levels in old and OXYS rats were further increased after IPC. In contrast to Wistar rats, age-matched OXYS demonstrated no increase in lysosome abundance and LC3 content in the kidney after ischemia/reperfusion. The kidney LC3 levels were also lower in OXYS, even under basal conditions, and mitochondrial PINK1 and ubiquitin levels were higher, suggesting impaired mitophagy. The kidney mitochondria from old rats contained a population with diminished membrane potential and this fraction was expanded by IPC. Apparently, oxidative changes with aging result in the appearance of malfunctioning renal mitochondria due to a low efficiency of autophagy. Elevated protein acetylation might be a hallmark of aging which is associated with a decreased autophagy, accumulation of dysfunctional mitochondria, and loss of protection against ischemia by IPC.

Nix restores mitophagy and mitochondrial function to protect against PINK1/Parkin-related Parkinson\u2019s disease

Therapeutic targets are needed to develop neuroprotective treatments for Parkinson’s disease (PD). Mitophagy, the selective autophagic elimination of dysfunctional mitochondria, is essential for the maintenance of mitochondrial integrity and is predominantly regulated by the PINK1/Parkin-mediated pathway. Loss of function mutations in Parkin and PINK1 cause an accumulation of dysfunctional mitochondria, leading to nigral neurodegeneration and early-onset PD with a high penetrance rate. We previously identified an asymptomatic homozygous Parkin mutation carrier who had not developed PD by her eighth decade despite the loss of functional Parkin. Here we discover a putative mechanism that protects her against PD. In contrast to Parkin-related PD patient-derived cells, the asymptomatic carrier cells show preserved mitochondrial function and mitophagy which is mediated by mitochondrial receptor Nip3-like protein X (Nix). Nix-mediated mitophagy was not affected by PINK1 knockdown. Both genetic and pharmacological induction of Nix restores mitophagy in PINK1- and Parkin-related PD patient cell lines, confirming its ability to induce mitophagy in the absence of PINK1/Parkin-mediated pathway. Moreover, Nix over-expression improves mitochondrial ATP production in these patient cells. Our results demonstrate that Nix can serve as an alternative mediator of mitophagy to maintain mitochondrial turnover, identifying Nix as a promising target for neuroprotective treatment in PINK1/Parkin-related PD.

VAMP7 Regulates Autophagy to Maintain Mitochondrial Homeostasis and to Control Insulin Secretion in Pancreatic β-Cells.

VAMP7 is a SNARE protein that mediates specific membrane fusions in intracellular trafficking and was recently reported to regulate autophagosome formation. However, its function in pancreatic β-cells is largely unknown. To elucidate the physiological role of VAMP7 in β-cells, we generated pancreatic β-cell-specific VAMP7 knockout (Vamp7(flox/Y);Cre) mice. VAMP7 deletion impaired glucose-stimulated ATP production and insulin secretion, though VAMP7 was not localized to insulin granules. VAMP7-deficient β-cells showed defective autophagosome formation and reduced mitochondrial function. p62/SQSTM1, a marker protein for defective autophagy, was selectively accumulated on mitochondria in VAMP7-deficient β-cells. These findings suggest that accumulation of dysfunctional mitochondria that are degraded by autophagy caused impairment of glucose-stimulated ATP production and insulin secretion in Vamp7(flox/Y);Cre β-cells. Feeding a high-fat diet to Vamp7(flox/Y);Cre mice exacerbated mitochondrial dysfunction, further decreased ATP production and insulin secretion, and consequently induced glucose intolerance. Moreover, we found upregulated VAMP7 expression in wild-type mice fed a high-fat diet and in db/db mice, a model for diabetes. Thus our data indicate that VAMP7 regulates autophagy to maintain mitochondrial quality and insulin secretion in response to pathological stress in β-cells.

Atg7 Knockdown Augments Concanavalin A-Induced Acute Hepatitis through an ROS-Mediated p38/MAPK Pathway.

Concanavalin A (ConA), a T-cell mitogen that induces acute autoimmune hepatitis, is widely used to model pathophysiological processes of human acute autoimmune liver disease. Although autophagy has been extensively studied in the past decade, little is known about its molecular mechanism underlying the regulation of ConA-induced acute hepatitis. In this study, we used a Cre-conditional atg7 KO mouse to investigate the effects of Atg7-associated autophagy on ConA-induced murine hepatitis. Our results demonstrated that atg7 deficiency in mice enhanced macrophage activation and increased pro-inflammatory cytokines upon ConA stimulation. Atg7 silencing resulted in accumulation of dysfunctional mitochondria, disruption of reactive oxygen species (ROS) degradation, and increase in pro-inflammatory cytokines in Raw264.7 cells. p38/MAPK and NF-κB levels were increased upon ConA induction due to Atg7 deficiency. Blocking ROS production inhibited ConA-induced p38/IκB phosphorylation and subsequent intracellular inflammatory responses. Hence, this study demonstrated that atg7 knockout in mice or Atg7 knockdown in cell culture augmented ConA-induced acute hepatitis and related cellular malfunction, indicating protective effects of Atg7 on regulating mitochondrial ROS via a p38/MAPK-mediated pathway. Collectively, our findings reveal that autophagy may attenuate macrophage-mediated inflammatory response to ConA and may be the potential therapeutic targets for acute liver injury.

MTOR kinase inhibitor pp242 causes mitophagy terminated by apoptotic cell death in E1A-Ras transformed cells

mTOR is a critical target for controlling cell cycle progression, senescence and cell death in mammalian cancer cells. Here we studied the role of mTOR-dependent autophagy in implementating the antiprolifrative effect of mTORC1-specific inhibitor rapamycin and ATP-competitive mTOR kinase inhibitor pp242. We carried out a comprehensive analysis of pp242- and rapamycin-induced autophagy in ERas tumor cells. Rapamycin exerts cytostatic effect on ERas tumor cells, thus causing a temporary and reversible cell cycle arrest, activation of non-selective autophagy not accompanied by cell death. The rapamycin-treated cells are able to continue proliferation after drug removal. The ATP-competitive mTORC1/mTORC2 kinase inhibitor pp242 is highly cytotoxic by suppressing the function of mTORC1-4EBP1 axis and mTORC1-dependent phosphorylation of mTORC1 target--ULK1-Ser757 (Atg1). In contrast to rapamycin, pp242 activates the selective autophagy targeting mitochondria (mitophagy). The pp242-induced mitophagy is accompanied by accumulation of LC3 and conversion of LC3-I form to LC3-II. However reduced degradation of p62/SQSTM indicates abnormal flux of autophagic process. According to transmission electron microscopy data, short-term pp242-treated ERas cells exhibit numerous heavily damaged mitochondria, which are included in single membrane-bound autophagic/autolysophagic vacuoles (mitophagy). Despite the lack of typical for apoptosis features, ERas-treated cells with induced mitophagy revealed the activation of caspase 3, 9 and nucleosomal DNA fragmentation. Thus, pp242 activates autophagy with suppressed later stages, leading to impaired recycling and accumulation of dysfunctional mitochondria and cell death. Better understanding of how autophagy determines the fate of a cell--survival or cell death, can help to development of new strategy for cancer therapy.