Attenuation of Obesity Cardiomyopathy by Ulk1/Rab9 Mediated Alternative Mitophagy

HomeCirculation ResearchVol. 129, No. 12Attenuation of Obesity Cardiomyopathy by Ulk1/Rab9 Mediated Alternative Mitophagy Free AccessEditorialPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessEditorialPDF/EPUBAttenuation of Obesity Cardiomyopathy by Ulk1/Rab9 Mediated Alternative Mitophagy Inna Rabinovich-Nikitin, Rachel C. Cogan and Lorrie A. Kirshenbaum Inna Rabinovich-NikitinInna Rabinovich-Nikitin Department of Physiology and Pathophysiology, The Institute of Cardiovascular Sciences, St. Boniface Hospital Albrechtsen Research Centre (I.R.-N., R.C.C., L.A.K.), Rady College of Medicine, Max Rady Faculty of Health Sciences, University of Manitoba, Winnipeg, Canada. , Rachel C. CoganRachel C. Cogan Department of Physiology and Pathophysiology, The Institute of Cardiovascular Sciences, St. Boniface Hospital Albrechtsen Research Centre (I.R.-N., R.C.C., L.A.K.), Rady College of Medicine, Max Rady Faculty of Health Sciences, University of Manitoba, Winnipeg, Canada. and Lorrie A. KirshenbaumLorrie A. Kirshenbaum Correspondence to: Lorrie A. Kirshenbaum, PhD, Institute of Cardiovascular Sciences, St. Boniface Hospital Albrechtsen Research Centre, Rm. 3016, 351 Taché Ave, Winnipeg, Manitoba, Canada R2H 2A6. Email E-mail Address: [email protected] https://orcid.org/0000-0002-9617-5803 Department of Physiology and Pathophysiology, The Institute of Cardiovascular Sciences, St. Boniface Hospital Albrechtsen Research Centre (I.R.-N., R.C.C., L.A.K.), Rady College of Medicine, Max Rady Faculty of Health Sciences, University of Manitoba, Winnipeg, Canada. Department of Pharmacology and Therapeutics (L.A.K.), Rady College of Medicine, Max Rady Faculty of Health Sciences, University of Manitoba, Winnipeg, Canada. Originally published2 Dec 2021https://doi.org/10.1161/CIRCRESAHA.121.320365Circulation Research. 2021;129:1122–1124This article is a commentary on the followingAlternative Mitophagy Protects the Heart Against Obesity-Associated CardiomyopathyArticle, see p 1105Obesity is defined as excessive accumulation of visceral and body fat and is a major risk factor for a variety of human diseases, including type-2 diabetes, metabolic syndrome, myocardial infarction, atherosclerosis, and cancer. Obesity is characterized by increased circulating lipids, insulin resistance, and elevated blood glucose levels. In the heart, obesity leads to structural and functional changes, which influence cardiac performance, hemodynamic loading, and cardiac output. The increased cardiac output is caused by higher stroke volume and ultimately results in ventricular dilation and hypertrophy. These obesity-related changes in cardiac function are classically defined as obesity cardiomyopathy and eventually lead to heart failure with preserved ejection fraction or in some cases to heart failure with reduced ejection fraction.1Cardiac myocytes require a continuous supply of ATP to sustain the heart’s ability to pump oxygenated blood to the body’s organs. This is achieved by the oxidation of glucose and fatty acids by the mitochondria. Indeed, cardiac myocytes are densely packed with mitochondria, which constitute ≈40% of the total myocyte volume.2 When mitochondria are damaged or become dysfunctional, they are cleared through a selective evolutionarily conserved lysosomal process, referred to as mitochondrial autophagy or mitophagy.3 Mitophagy can be mediated by the PINK1 (PTEN [phosphatase and tensin homolog]-induced kinase 1)/Parkin pathway. Depolarized mitochondria result in the stabilization and autophosphorylation of mitochondrial associated PINK1 on the outer mitochondrial membrane, which subsequently phosphorylates ubiquitin Ser65 leading to the recruitment and activation of Parkin. When Parkin is activated, it creates polyubiquitin chains on several different outer mitochondrial membrane proteins that trigger the formation of a mitochondrion encapsulated autophagosome that is subsequently targeted for lysosomal degradation.4,5 Receptor-mediated mitophagy is another form of mitophagy that appears to be independent of Parkin and removes damaged mitochondria by autophagy adapter proteins. In this mechanism, autophagy adapter proteins, such as LC3 (microtubule-associated protein 1 light chain 3), p62 (ubiquitin-binding protein p62), FUNDC1 (FUN14 domain containing 1), and Bnip3 (BCL2/adenovirus E1B 19 kDa protein-interacting protein 3)/Nix (BCL2/adenovirus E1B 19 kDa protein-interacting protein 3-like), target the mitochondria for removal by lysosomal regulated pathways.5,6 In 2019, another novel alternative mitophagy pathway involving the Ulk1 (unc-51 like kinase 1)/Rab9 (Ras-related protein Rab-9A) in the heart was identified by the Sadoshima laboratory.7,8 In this pathway, the initiation of mitophagy is mediated by phosphorylation of Rab9 by Ulk1 at serine 179. The Ulk1/Rab9 alternative mitophagy pathway is mediated by a protein complex consisting of Ulk1, Rab9, Ripk1 (receptor-interacting serine/threonine protein kinase 1), and Drp1 (dynamin-related protein 1). Generation of this complex allows the recruitment of trans-Golgi membranes associated with Rab9 on injured mitochondria via Ulk1 phosphorylation of Rab9 at S179 and the phosphorylation of Drp1 at S616 by Ripk1. This mode of alternative mitophagy was previously demonstrated as an important mitochondrial quality control mechanism in protecting the heart against ischemia-reperfusion injury.8Mitophagy plays an essential role in maintaining cardiac function during high-fat diet (HFD)–induced cardiomyopathy. A study by Tong et al9,10 showed that wild-type mice fed with HFD (60 kcal% fat), which simulated diabetic cardiomyopathy, exhibited a peak in cardiac autophagic flux after 6 weeks of HFD consumption, which declined in a time-dependent manner. Interestingly, while autophagy declined with time following HFD, mitophagy was induced 3 weeks after the beginning of the HFD consumption and lasted for at least 2 months after. The observation of increased mitophagy and decreased autophagy during the 2 months of HFD was consistent with cardiac hypertrophy and diastolic dysfunction in this diabetic cardiomyopathy model. Notably, the molecular mechanisms by which mitophagy was regulated in the HFD type II diabetic model involved Atg7 (autophagy related 7)” and Parkin. This was shown by studies demonstrating that downregulation of either Atg7 or Parkin exacerbates mitochondrial dysfunction and cardiac damage during HFD. Overall, this study highlighted that activation of mitophagy in the early stages of diabetic cardiomyopathy is not sufficient to maintain normal cardiac function and that Atg7 and Parkin are crucial regulators of mitochondrial turnover during diabetic cardiomyopathy.In a subsequent follow-up study by Tong et al,11 published in this issue of Circulation Research, the authors explored the role of mitophagy in the pathogenesis of obesity cardiomyopathy during long-term HFD consumption. By feeding transgenic mice harboring the mitophagy reporter gene Mito-Keima (Tg-Mito-Keima) with either a normal diet or an HFD for up to 24 weeks, the authors found that long-term HFD increased mitophagy levels as early as 3 weeks after the beginning of HFD consumption and further increased mitophagy in a time-dependent manner up to 20 weeks, which stayed stable thereafter up to 24 weeks. This observation coincided with increased autophagic flux that peaked at 6 weeks of HFD consumption and declined at 12 weeks of HFD consumption and was subsequently inhibited by 20 weeks. This interesting finding builds upon the previous observation by Tong et al9 by showing that while mitophagy can be activated for up to 20 weeks after the beginning of HFD, general autophagy declines at much earlier stage, demonstrating that autophagy clearance of mitochondria is mediated spatially and temporally by different mechanisms.Interestingly, activation of mitophagy in the hearts of mice fed with HFD was mediated by the Ulk1/Rab9 alternative mitophagy pathway. To examine the role of Ulk1/Rab9 alternative mitophagy in the development of obesity cardiomyopathy, cardiac-specific ulk1 knockout (ulk1 cKO) mice were crossed with mitophagy reporter mice Tg-Mito-Keima. The ulk1 cKO/ Tg-Mito-Keima mice were fed with either normal diet or HFD and demonstrated that levels of mitophagy were attenuated, suggesting that Ulk1 plays a crucial role in mediating mitophagy during long-term HFD consumption.This observation was further substantiated by the finding that Ulk1/Rab9-mediated autophagy was transcriptionally regulated by TFE3 (transcription factor binding to IGHM [immunoglobulin heavy constant mu] enhancer 3). TFE3 is a central mediator of autophagy and lysosomal biogenesis.12 Chromatin immunoprecipitation assays revealed that TFE3 was upregulated in the hearts of mice fed with HFD after 12 weeks of feeding and was bound to Rab9 promoter. In addition, TFE3 upregulation was shown to mediate induction of alternative mitophagy and maintain cardiac function during HFD consumption.Next, the authors examined whether stimulation of alternative mitophagy improves left ventricle (LV) dysfunction induced by HFD consumption for 24 weeks. Based on previous observation that overexpression of Rab9 induced Ulk1/Rab9 alternative mitophagy pathway,8 the authors fed HFD to transgenic mice overexpressing Rab9 in the heart (Tg-Rab9) and showed that gain of function by Rab9 overexpression augmented mitophagy, thereby protecting against cardiac hypertrophy and diastolic dysfunction induced by HFD consumption. This finding coincided with decreased number of depolarized mitochondria with disrupted cristae and lipid accumulation in the heart.Taken together, the study by Tong et al suggests that long-term HFD activates mitophagy, even when general autophagy is diminished. Furthermore, this study showed that mitophagy during HFD consumption is mediated mechanistically by Ulk1/Rab9 alternative mitophagy pathway, which is crucial for protecting against obesity cardiomyopathy (Figure).Download figureDownload PowerPointFigure. A scheme representing alternative mitophagy during obesity cardiomyopathy. Middle, Mice fed a high-fat diet (HFD) exhibited increased general autophagy levels 6 wk after the beginning of HFD consumption, which declined in a time-dependent manner. These mice also exhibited increased Ulk1 (unc-51 like kinase 1)/Rab9 alternative mitophagy 3 wk after the beginning of HFD consumption, which remained stable for 20 wk. Activation of Ulk1/Rab9 alternative mitophagy was not sufficient to protect the heart against cardiomyopathy. Left, Ulk1 KO mice fed with HFD had diminished Ulk1/Rab9 alternative mitophagy, which led to exacerbated cardiomyopathy. Right, Mice overexpressing Rab9 had higher levels of Ulk1/Rab9 alternative mitophagy, which attenuated the extent of cardiomyopathy. KO indicates knockout; OE, overexpression; and Rab9, Ras-related protein Rab-9A.While the study provides very important and convincing data for alternative mitophagy in protecting the heart during obesity cardiomyopathy, the study also raises some questions for new avenues of investigation. First, although mitophagy was shown to be crucial in preserving cardiac function during obesity cardiomyopathy, mitochondrial biogenesis markers were not significantly changing. If mitochondrial biogenesis is not activated following enhanced mitophagy, it remains unclear how restoration of mitophagy preserves cardiac function, in the absence of mitochondrial synthesis or fission and fusion processes. In addition, it remains unknown why autophagy was inhibited in mice at an early stage of HFD, while mitophagy was preserved in later stages of HFD consumption. Perhaps these differences can be attributed to distinct mechanisms that regulate autophagy, such as TFE3, which was shown to mediate mitophagy, versus TFEB that regulated autophagy presumably through enhanced lysosomal biogenesis in the early stages of HFD. Furthermore, it is also unclear how the alternative autophagy pathway intersects with mitochondrial-driven apoptosis and necrosis-regulated cell death pathways. Lastly, the current study examined the extent of autophagy and mitophagy up to 24 weeks of HFD consumption. It would be interesting to test whether mitophagy can be maintained over longer periods of time or whether autophagy can be restored in a later stage of HFD consumption and reverse the cardiac dysfunction. This is an important consideration for aging and senescence where autophagy and mitochondrial quality control declines with time.13 Whether the protective properties of alternative autophagy pathway induced by HFD decline with aging will require further investigation.Nevertheless, the studies presented by Tong et al further support the notion that alternative mitophagy pathway is crucial for maintaining quality control mechanisms and cardiac function during obesity cardiomyopathy.Article InformationSources of FundingThis work was supported by a Foundation grant to L.A. Kirshenbaum from the Canadian Institute for Health Research (CIHR); I. Rabinovich-Nikitin holds a postdoctoral fellowship from CIHR; L.A. Kirshenbaum holds a Canada Research Chair in Molecular Cardiology.Disclosures None.FootnotesThe opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.For Sources of Funding and Disclosures, see page 1124.Correspondence to: Lorrie A. Kirshenbaum, PhD, Institute of Cardiovascular Sciences, St. Boniface Hospital Albrechtsen Research Centre, Rm. 3016, 351 Taché Ave, Winnipeg, Manitoba, Canada R2H 2A6. Email [email protected]caReferences1. Alpert MA. Obesity cardiomyopathy: pathophysiology and evolution of the clinical syndrome.Am J Med Sci. 2001; 321:225–236. doi: 10.1097/00000441-200104000-00003CrossrefMedlineGoogle Scholar2. Li A, Gao M, Jiang W, Qin Y, Gong G. Mitochondrial dynamics in adult cardiomyocytes and heart diseases.Front Cell Dev Biol. 2020; 8:584800. doi: 10.3389/fcell.2020.584800CrossrefMedlineGoogle Scholar3. Mughal W, Dhingra R, Kirshenbaum LA. 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Circulation Research. 2021;129:1105-1121 December 3, 2021Vol 129, Issue 12 Advertisement Article InformationMetrics © 2021 American Heart Association, Inc.https://doi.org/10.1161/CIRCRESAHA.121.320365PMID: 34855463 Originally publishedDecember 2, 2021 Keywordsmitochondriadiet, high-fatcardiomyopathiesautophagyEditorialsobesitymitophagyPDF download Advertisement