Mitochondria and GSK-3β in Cardioprotection Against Ischemia/Reperfusion Injury

The mitochondrion is a powerhouse of the cell, a platform of cell signaling and decision-maker of cell death, including death by ischemia/reperfusion. Ischemia shuts off ATP production by mitochondria, and cell viability is compromised by energy deficiency and build-up of cytotoxic metabolites during ischemia. Furthermore, the mitochondrial permeability transition pore (mPTP) is primed by ischemia to open upon reperfusion, leading to reperfusion-induced cell necrosis. mPTP opening can be suppressed by ischemic preconditioning (IPC) and other interventions that induce phosphorylation of GSK-3β. Activation of the mitochondrial ATP-sensitive K⁺ channel (mK_ATP channel) is an important signaling step in a trigger phase of IPC, which ultimately enhances GSK-3β phosphorylation upon reperfusion, and this channel functions as a mediator of cytoprotection as well. The mitochondrial Ca²⁺-activated K⁺ channel appears to play roles similar to those of the mK_ATP channel, though regulatory mechanisms of the channels are different. Phosphorylated GSK-3β inhibits mPTP opening presumably by multiple mechanisms, including preservation of hexokinase II in mPTP complex, prevention of interaction of cyclophilin-D with adenine nucleotide translocase, inhibition of p53 activation and attenuation of ATP hydrolysis during ischemia. However, cytoprotective signaling pathways to GSK-3β phosphorylation and other mPTP regulatory factors are modified by co-morbidities, including type 2 diabetes, and such modification makes the myocardium refractory to IPC and other cardioprotective agents. Regulatory mechanisms of mPTP, and their alterations by morbidities frequently associated with ischemic heart disease need to be further characterized for translation of mitochondrial and mPTP biology to the clinical arena.     

Mitofusin 2-containing mitochondrial-reticular microdomains direct rapid cardiomyocyte bioenergetic responses via interorganelle Ca2+ crosstalk

Rationale: Mitochondrial Ca(2+) uptake is essential for the bioenergetic feedback response through stimulation of Krebs cycle dehydrogenases. Close association of mitochondria to the sarcoplasmic reticulum (SR) may explain efficient mitochondrial Ca(2+) uptake despite low Ca(2+) affinity of the mitochondrial Ca(2+) uniporter. However, the existence of such mitochondrial Ca(2+) microdomains and their functional role are presently unresolved. Mitofusin (Mfn) 1 and 2 mediate mitochondrial outer membrane fusion, whereas Mfn2 but not Mfn1 tethers endoplasmic reticulum to mitochondria in noncardiac cells. Objective: To elucidate roles for Mfn1 and 2 in SR-mitochondrial tethering, Ca(2+) signaling, and bioenergetic regulation in cardiac myocytes. Methods and Results: Fruit fly heart tubes deficient of the Drosophila Mfn ortholog MARF had increased contraction-associated and caffeine-sensitive Ca(2+) release, suggesting a role for Mfn in SR Ca(2+) handling. Whereas cardiac-specific Mfn1 ablation had no effects on murine heart function or Ca(2+) cycling, Mfn2 deficiency decreased cardiomyocyte SR-mitochondrial contact length by 30% and reduced the content of SR-associated proteins in mitochondria-associated membranes. This was associated with decreased mitochondrial Ca(2+) uptake (despite unchanged mitochondrial membrane potential) but increased steady-state and caffeine-induced SR Ca(2+) release. Accordingly, Ca(2+)-induced stimulation of Krebs cycle dehydrogenases during β-adrenergic stimulation was hampered in Mfn2-KO but not Mfn1-KO myocytes, evidenced by oxidation of the redox states of NAD(P)H/NAD(P)(+) and FADH(2)/FAD. Conclusions: Physical tethering of SR and mitochondria via Mfn2 is essential for normal interorganelle Ca(2+) signaling in the myocardium, consistent with a requirement for SR-mitochondrial Ca(2+) signaling through microdomains in the cardiomyocyte bioenergetic feedback response to physiological stress.     

Bak regulates mitochondrial morphology and pathology during apoptosis by interacting with mitofusins

Mitochondrial injury, characterized by outer membrane permeabilization and consequent release of apoptogenic factors, is a key to apoptosis of mammalian cells. Bax and Bak, two multidomain Bcl-2 family proteins, provide a requisite gateway to mitochondrial injury. However it is unclear how Bax and Bak cooperate to provoke mitochondrial injury and whether their roles are redundant. Here, we have identified a unique role of Bak in mitochondrial fragmentation, a seemingly morphological event that contributes to mitochondrial injury during apoptosis. We show that mitochondrial fragmentation is attenuated in Bak-deficient mouse embryonic fibroblasts, baby mouse kidney cells, and, importantly, also in primary neurons isolated from brain cortex of Bak-deficient mice. In sharp contrast, Bax deficiency does not prevent mitochondrial fragmentation during apoptosis. Bcl-2 and Bcl-XL inhibit mitochondrial fragmentation, and their inhibitory effects depend on the presence of Bak. Reconstitution of Bak into Bax/Bak double-knockout cells restores mitochondrial fragmentation, whereas reconstitution of Bax is much less effective. Bak interacts with Mfn1 and Mfn2, two mitochondrial fusion proteins. During apoptosis, Bak dissociates from Mfn2 and enhances the association with Mfn1. Mutation of Bak in the BH3 domain prevents its dissociation from Mfn2 and diminishes its mitochondrial fragmentation activity. This study has uncovered a previously unrecognized function of Bak in the regulation of mitochondrial morphological dynamics during apoptosis. By this function, Bak may collaborate with Bax to permeabilize the outer membrane of mitochondria, unleashing the apoptotic cascade.     

Mitochondrial dynamics and cell death in heart failure

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

线粒体融合素2突变体对血管平滑肌细胞凋亡的影响

目的研究大鼠线粒体融合素2(mitofusin 2,Mfn2)基因蛋白激酶A(PKA)磷酸化位点的2种突变体对大鼠血管平滑肌细胞(VSMCs)凋亡及其相关的信号通路的影响。方法构建4种重组腺病毒,分别携带磷酸化位点突变为丙氨酸(重组1组)、Mfn2基因(重组2组)、突变为天冬酰胺(重组3组)和半乳糖苷酶基因(对照组),感染培养的VSMCs,另设未感染腺病毒的空白组。流式细胞术比较各组细胞的凋亡率,JC-1染色法检测线粒体膜电位变化,Western blot分析各组Mfn2蛋白的表达以及磷酸化蛋白激酶B(p-Akt)和活性半胱天冬酶9(caspase-9)表达。结果与空白组和对照组比较,重组1组、重组2组和重组3组Mfn蛋白显著增加(P0.01);重组1组和重组2组细胞凋亡率显著增强(P0.01);线粒体膜电位显著降低(P0.01);p-Akt表达水平显著降低(P0.01),活性caspase-9表达水平显著增高(P0.01);且重组1组作用较重组2组更明显(P0.01);而重组3组上述指标无显著差异(P0.05)。结论 Mfn2突变为丙氨酸的位点PKA通过Akt信号及线粒体途径诱导VSMCs凋亡的作用较Mfn2更明显,表明PKA磷酸化位点是调控Mfn2诱导VSMCs凋亡的重要功能位点。     

Expression of mitochondrial fusion–fission proteins during post-infarction remodeling: the effect of NHE-1 inhibition

Studies on the role of mitochondrial fission/fusion (MFF) proteins in the heart have been initiated recently due to their biological significance in cell metabolism. We hypothesized that the expression of MFF proteins is affected by post-infarction remodeling and in vitro cardiomyocyte hypertrophy, and serves as a target for the Na⁺/H⁺ exchanger 1 (NHE-1) inhibition. Post-infarction remodeling was induced in Sprague–Dawley rats by coronary artery ligation (CAL) while in vitro hypertrophy was induced in cardiomyocytes by phenylephrine (PE). Mitochondrial fission (Fis1, DRP1) and fusion (Mfn2, OPA1) proteins were analyzed in heart homogenates and cell lysates by Western blotting. Our results showed that 12 and 18 weeks after CAL, Fis1 increased by 80% (P < 0.01) and 31% (P < 0.05), and Mfn2 was reduced by 17% (P < 0.05) and 22% (P < 0.05), respectively. OPA1 was not changed at 12 weeks, although its expression decreased by 18% (P < 0.05) with 18 weeks of ligation. MFF proteins were also affected by PE-induced hypertrophy that was dependent on mitochondrial permeability transition pore opening and oxidative stress. The NHE-1-specific inhibitor EMD-87580 (EMD) attenuated changes in the expression of MFF proteins in both the models of hypertrophy. The effect of EMD was likely mediated, at least in part, through its direct action on mitochondria since Percoll-purified mitochondria and mitoplasts have been shown to contain NHE-1. Our study provides the first evidence linking cardiac hypertrophy with MFF proteins expression that was affected by NHE-1 inhibition, thus suggesting that MFF proteins might be a target for pharmacotherapy with anti-hypertrophic drugs.     

Notch-activated signaling cascade interacts with mitochondrial remodeling proteins to regulate cell survival

Survival of differentiated cells is one of several processes regulated by Notch activity, although the general principles underlying this function remain to be characterized. Here, we probe the mechanism underlying Notch-mediated survival, building on emerging evidence that apoptotic responses coordinated by specialized intermediates converge on mitochondria, identifying a core event in death pathways. The Bcl-2 family protein Bax is one such intermediate, which in a unifying response to diverse apoptotic stimuli nucleates multiprotein assemblies on mitochondria, committing cells to irrevocable damage. Using Bax as the prototype stimulus, we analyze Notch signaling for potential interactions with mitochondria, probe intrinsic properties of the Notch receptor, and describe key intermediates in the Notch-activated signaling cascade. Ligand-dependent processing was necessary to generate the Notch intracellular domain (NIC) although signaling was independent of canonical interactions with nuclear factors. Notably, antiapoptotic activity was recapitulated by NIC recombinants, localized outside the nucleus, and compromised by enforced nuclear sequestration. NIC signaled via the kinase Akt to prevent the loss of mitochondrial function, contiguity, and consequent nuclear damage, outcomes critically depend on mitochondrial remodeling proteins Mitofusins-(Mfn)-1 and 2. Thus, the NIC-Akt-Mfn signaling cascade identifies a pathway regulating cell-survival, independent of canonical functions associated with NIC activity.     

The Critical Role of Bioenergetics in Donor Cardiac Allograft Preservation

The traditional philosophy of ex vivo organ preservation has been to limit metabolic activity by storing organs in hypothermic, static conditions. This methodology cannot provide longevity of hearts for more than 4–6 h and is thereby insufficient to expand the number of available organs. Albeit at lower rate, the breakdown of ATP still occurs during hypothermia. Furthermore, cold static preservation does not prevent the permanent damage that occurs upon reperfusion known as ischemia-reperfusion (IR) injury. This damage is caused by increased reactive oxygen species (ROS) production in combination with mitochondrial permeability transition pore (mPTP) opening, highlighting the importance of mitochondria in ischemic storage. There has recently been a major paradigm shift in the field, with emerging research supporting changes in traditional storage approaches. Novel research suggests achieving metabolic homeostasis instead of attempting to limit metabolic activity which reduces IR injury and improves graft preservation. Maintaining high ATP levels and circumventing cold organ storage would be a much more sophisticated standard for organ storage and should be the focus of future research in organ preservation. Given the link between mPTP, Ca2⁺, and ROS, managing Ca2⁺ influx into the mitochondria during conditioning might be the next critical step towards preventing irreversible IR injury.     

Early protective effect of mitofusion 2 overexpression in STZ-induced diabetic rat kidney

Diabetic nephropathy (DN) is a serious complication of diabetes with a poorly defined etiology and limited treatment options. Early intervention is key to preventing the progression of DN. Mitofusin 2 (Mfn2) regulates mitochondrial morphology and signaling, and is involved in the pathogenesis of numerous diseases. Furthermore, Mfn2 is also closely associated with the development of diabetes, but its functional roles in the diabetic kidney remain unknown. This study investigated the effect of Mfn2 at an early stage of DN. Mfn2 was overexpressed by adenovirus-mediated gene transfer in streptozotocin-induced diabetic rats. Clinical parameters (proteinuria, albumin/creatinine ratio), pathological changes, ultra-microstructural changes in nephrons, expression of collagen IV and phosph-p38, ROS production, mitochondrial function, and apoptosis were evaluated and compared with diabetic rats expressing control levels of Mfn2. Endogenous Mfn2 expression decreased with time in DN. Compared to the blank transfection control group, overexpression of Mfn2 decreased kidney weight relative to body weight, reduced proteinuria and ACR, and improved pathological changes typical of the diabetic kidney, like enlargement of glomeruli, accumulation of ECM, and thickening of the basement membrane. In addition, Mfn2 overexpression inhibited activation of p38, and the accumulation of ROS; prevented mitochondrial dysfunction; and reduced the synthesis of collagen IV, but did not affect apoptosis of kidney cells. This study demonstrates that Mfn2 overexpression can attenuate pathological changes in the kidneys of diabetic rats. Further studies are needed to clarify the underlying mechanism of this protective function. Mfn2 might be a potential therapeutic target for the treatment of early stage DN.     

Neuroprotective effect of tanshinone IIA against neuropathic pain in diabetic rats through the Nrf2/ARE and NF-κB signaling pathways

The study aims to investigate whether tanshinone (TSN) IIA affects diabetic neuropathic pain (DNP) via the Nrf2/AR and NF-κB signaling pathways. Rats were randomly assigned into DNP group, TSN group (injected with TSN IIA), TSN + DRB group (injected with TSN IIA and 15 mg/kg Nrf2/ARE inhibitors), TSN + PDTC group (injected with TSN IIA and 60 mg/kg NF-κB inhibitors) or control group. The first four groups were successfully established as DNP models after injection of streptozotocin. The blood glucose level, mechanical withdrawal threshold (MWT), thermal withdrawal latency (TWL), nerve conduction velocity (NCV) and antioxidase level were detected. Transmission electron microscopy and toluidine blue staining were utilized to observe the sciatic nerve. RT-qPCR and western blot were used to measure expression levels of Nrf2/ARE and NF-κB signaling pathway-related genes. Blood glucose, malondialdehyde (MDA) and erythrocyte glutathione peroxidase (GSH-Px) levels as well as expression of Keap1 and NF-κB were increased in the TSN, TSN + DRB and TSN + PDTC groups compared to control group. Furthermore, the MWT, TWL, NVC, and superoxide dismutase (SOD) levels and expression of Nrf2, heme oxygenase 1 (HO-1) and inhibitory kappa B (IκB) decreased in the treatment groups. The TSN + DRB and TSN + PDTC groups showed similar trend when compared with the TSN group, while the opposite trend was observed in the TSN group when compared with the DNP group. Our study demonstrates that TSN IIA alleviates neuropathic pain by activating the Nrf2/ARE signaling pathway and inhibiting the NF-κB signaling pathway in diabetic rats.     

Prolactin receptor and proliferating/apoptotic cells in esophagus of the Mozambique tilapia (Oreochromis mossambicus) in fresh water and in seawater

We have previously shown that esophageal epithelium of a euryhaline goby displays elevated cell proliferation in freshwater (FW) fish, but undergo apoptosis during seawater (SW) acclimation. Prolactin (PRL) injection into the goby induced the cell proliferation, whereas cortisol treatment stimulated the cell proliferation and apoptosis [Takahashi, H., Sakamoto, T., Narita, K., 2006a. Cell proliferation and apoptosis in the anterior intestine of an amphibious, euryhaline mudskipper (Periophthalmus modestus). J. Comp. Physiol. B 176, 463-468, 2006). In the euryhaline tilapia (Oreochromis mossambicus), the dynamics of changes in cortisol-glucocorticoid receptors (GR) during acclimation to different salinities also suggests a role for glucocorticoid signaling in the esophageal cell turnover, but the mode of PRL action remains largely unclear. In the present study, we report on effects in the tilapia esophagus that result from changes in environmental salinity. Specifically, we assessed: (1) mRNA expression of PRL receptor (PRLR) using quantitative real-time RT-PCR; (2) esophageal cell proliferation and apoptosis, using immunohistochemistry of proliferating cells nuclear antigen (PCNA) and in situ nick end-labeling of genomic DNA (TUNEL); (3) the possible localization of immunoreactive PRLR on proliferating/apoptotic cells. Plasma PRL increased after FW acclimation; PRLR mRNA levels in the esophagus of FW fish were significantly higher than those in SW-acclimated fish. Cell proliferation was induced randomly throughout the esophageal epithelium after FW acclimation, while cell division and increased apoptosis were concentrated at the tips of esophageal epithelial folds in SW-acclimated fish. Immunoreactive PRLR appeared to be localized at proliferating cells and at certain apoptotic cells, whereas immunoreactive GR was observed over the whole epithelium including the apoptotic/proliferating cells. Thus, PRL appears to affect cell turnover directly in the esophageal epithelium of the euryhaline tilapia.     

PTEN-induced kinase 1-induced dynamin-related protein 1 Ser637 phosphorylation reduces mitochondrial fission and protects against intestinal ischemia reperfusion injury

BACKGROUND Intestinal ischemia reperfusion(I/R) occurs in various diseases, such as trauma and intestinal transplantation. Excessive reactive oxygen species(ROS)accumulation and subsequent apoptotic cell death in intestinal epithelia are important causes of I/R injury. PTEN-induced putative kinase 1(PINK1) and phosphorylation of dynamin-related protein 1(DRP1) are critical regulators of ROS and apoptosis. However, the correlation of PINK1 and DRP1 and their function in intestinal I/R injury have not been investigated. Thus, examining the PINK1/DRP1 pathway may help to identify a protective strategy and improve the patient prognosis.AIM To clarify the mechanism of the PINK1/DRP1 pathway in intestinal I/R injury.METHODS Male C57 BL/6 mice were used to generate an intestinal I/R model via superior mesenteric artery occlusion followed by reperfusion. Chiu's score was used to evaluate intestinal mucosa damage. The mitochondrial fission inhibitor mdivi-1 was administered by intraperitoneal injection. Caco-2 cells were incubated in vitro in hypoxia/reoxygenation conditions. Small interfering RNAs and overexpression plasmids were transfected to regulate PINK1 expression. The protein expression levels of PINK1, DRP1, p-DRP1 and cleaved caspase 3 were measured by Western blotting. Cell viability was evaluated using a Cell Counting Kit-8 assay and cell apoptosis was analyzed by TUNEL staining. Mitochondrial fission and ROS were tested by Mito Tracker and Mito SOX respectively.RESULTS Intestinal I/R and Caco-2 cell hypoxia/reoxygenation decreased the expression of PINK1 and p-DRP1 Ser637. Pretreatment with mdivi-1 inhibited mitochondrial fission, ROS generation, and apoptosis and ameliorated cell injury in intestinal I/R. Upon PINK1 knockdown or overexpression in vitro, we found that p-DRP1 Ser637 expression and DRP1 recruitment to the mitochondria were associated with PINK1. Furthermore, we verified the physical combination of PINK1 and pDRP1 Ser637.CONCLUSION PINK1 is correlated with mitochondrial fission and apoptosis by regulating DRP1 phosphorylation in intestinal I/R. These results suggest that the PINK1/DRP1 pathway is involved in intestinal I/R injury, and provide a new approach for prevention and treatment.     

Improvement of Functional Recovery of Donor Heart Following Cold Static Storage with Doxycycline Cardioplegia

Injury to the donor heart during cold preservation has a negative impact on graft survival before transplantation. This study aims to examine whether doxycycline, known as an MMP-2 inhibitor, has a positive effect on donor heart preservation via its antioxidant action when added to standard preservation solution. Hearts were obtained from 3-month-old male Wistar rats and randomly divided into three groups: hearts stored for 1 h at 4 °C (1) with doxycycline preservation solution (DOX cardioplegia) with low Ca²⁺; (2) with standard cardioplegia with low Ca²⁺; and (3) unstored hearts. All hearts were perfused in working mode, arrested at 37 °C, removed from the perfusion system, reattached in Langendorff perfusion system, and converted to working mode for 1 h. At the end of the storage period, hearts preserved in DOX cardioplegia had significantly less weight gain than those preserved in the standard cardioplegia. DOX cardioplegia-induced preservation resulted in significantly higher heart rates and better recovery quality during reperfusion in aortic flow compared to the standard cardioplegia group. Recovery in the left ventricular function and Lambeth Convention Arrhythmia scores during 1 h reperfusion were also significantly better in the DOX cardioplegia group. Biochemical data showed that DOX cardioplegia prevented an increase in MMP-2 activity and blocked apoptosis through increased activity of the pro-survival kinase Akt in the donor heart homogenates. DOX cardioplegia also led to a balanced oxidant/antioxidant level in the heart homogenates. This is the first study to report that cardioplegia solution containing doxycycline provides better cardioprotection via the preservation of heart function, through its role in controlling cellular redox status during static cold storage.     

Mst1敲除减轻高脂饮食诱发的心肌线粒体动力学紊乱

目的 探讨哺乳动物STE20样激酶1（Mst1）基因敲除是否可以减轻高脂饮食导致的心肌线粒体动力学紊乱并探究其机制。 方法 C57BL/6小鼠（WT）和Mst1基因敲除小鼠（Mst1-/-）(C57BL/6背景)随机分为正常饮食（Normal diet,ND）组和高脂饮食（High-fat diet,HFD）组,连续喂养16w后,检测各组小鼠体质量、心脏重量/胫骨长度和空腹血脂水平,超声心动图评估心脏功能,HE染色及透射电镜观察心肌肥厚和线粒体形态,心肌柠檬酸合酶(CS)活性和ATP含量评价线粒体功能,Western blot检测线粒体动力学相关蛋白的表达水平。 结果 与ND+WT组相比,HFD+WT组小鼠出现肥胖、高血脂、心肌肥厚和心脏舒张功能障碍,同时线粒体分裂增多、形态明显受损,心肌CS活性、ATP含量均降低,线粒体分裂蛋白Drp1、Fis1的表达明显上调（P<0.05）,融合蛋白Mfn2的表达明显下调（P<0.05）;与HFD+WT组相比,HFD+Mst1-/-组小鼠心脏舒张功能、线粒体形态与功能的损伤明显减轻,Drp1表达明显下调（P<0.05）,Mfn2表达明显上调（P<0.05）。 结论 Mst1基因敲除可以通过抑制线粒体分裂,促进线粒体融合,以维持线粒体动力学稳定,从而减轻脂毒性相关心肌损害。     

Mitochondrial permeability transition pore and postconditioning

Postconditioning has recently been described as a powerful cardioprotection that prevents lethal reperfusion injury. Growing evidence suggests that mitochondrial permeability transition may be a key event in postconditioning. This proposition arises from the complementary observations that: (1) conditions for the mitochondrial permeability transition pore (mPTP) opening are built up during early reperfusion, (2) mPTP opens at the time of reperfusion, (3) transgenic structural alteration of mPTP modifies its opening probability following ischemia-reperfusion, (4) mPTP plays a role in preconditioning, and (5) postconditioning attenuates lethal reperfusion injury. We review in this article current evidence for an important role of the mitochondrial transition pore in postconditioning.     

Drosophila Parkin requires PINK1 for mitochondrial translocation and ubiquitinates Mitofusin

Loss of the E3 ubiquitin ligase Parkin causes early onset Parkinson''s disease, a neurodegenerative disorder of unknown etiology. Parkin has been linked to multiple cellular processes including protein degradation, mitochondrial homeostasis, and autophagy; however, its precise role in pathogenesis is unclear. Recent evidence suggests that Parkin is recruited to damaged mitochondria, possibly affecting mitochondrial fission and/or fusion, to mediate their autophagic turnover. The precise mechanism of recruitment and the ubiquitination target are unclear. Here we show in Drosophila cells that PINK1 is required to recruit Parkin to dysfunctional mitochondria and promote their degradation. Furthermore, PINK1 and Parkin mediate the ubiquitination of the profusion factor Mfn on the outer surface of mitochondria. Loss of Drosophila PINK1 or parkin causes an increase in Mfn abundance in vivo and concomitant elongation of mitochondria. These findings provide a molecular mechanism by which the PINK1/Parkin pathway affects mitochondrial fission/fusion as suggested by previous genetic interaction studies. We hypothesize that Mfn ubiquitination may provide a mechanism by which terminally damaged mitochondria are labeled and sequestered for degradation by autophagy.