ABSTRACT
Mitochondrial dysfunction is a key determinant of the development of cardiomyopathy in patients with obesity and diabetes. We recently reported that mitophagy is activated in the mouse heart during the chronic phase of high-fat diet (HFD) consumption, despite downregulation of general macroautophagy/autophagy. This form of mitophagy is mediated by a mechanism distinct from that of conventional autophagy and is termed alternative mitophagy. We here discuss the underlying mechanisms of alternative mitophagy and its functional significance in heart disease.
The size of the population suffering from obesity, defined by a body mass index >30, has tripled during the last four decades. Many obese patients develop heart problems characterized by an insufficient capacity of the heart to hold the blood that returns to the heart during relaxation, which often leads to lung congestion and decreased exercise tolerance, and eventually to reduced cardiac output, the hallmark of heart failure. Metabolic alteration caused by elevated fatty acid levels and insulin resistance induces mitochondrial dysfunction in cardiac cells. Because mitochondria are the powerhouses of cardiac cells, it is imperative for them to activate well-coordinated mechanisms to consistently maintain mitochondrial quality. Mitophagy is one of the most important mechanisms by which mitochondrial function is maintained in mice that consume a HFD, a model of obesity cardiomyopathy.
HFD consumption rapidly activates autophagy in the heart, where most of the LC3-positive autophagosomes colocalize with mitochondria. Mitophagy during the early phase of HFD consumption is mediated primarily through general autophagy, namely autophagy that is ATG7- and LC3-dependent and involves PRKN/parkin. However, this activation of autophagy does not last long, and it is completely inactivated by 20 weeks of HFD consumption. Surprisingly, mitophagy continues to be observed in the heart during the chronic phase of HFD consumption, even after conventional mechanisms of autophagy are fully downregulated or when ATG7 or PRKN is downregulated in genetically altered mouse models. Mitophagy during the chronic phase of HFD consumption is mediated by an alternative ULK1-RAB9-RIPK1/RIP1-DNM1L/DRP1-dependent mechanism [1]. This form of mitophagy resembles that previously identified by the Shimizu and Kanki groups and is functionally important, because genetic interventions to suppress alternative mitophagy exacerbate cardiomyopathy during the chronic phase of HFD consumption.
It has become increasingly clear that the maintenance of mitochondrial function is a key to preventing or delaying the development of heart failure in the stressed heart. Although mitophagy is rapidly activated in the heart in response to stress, it appears that the activation of mitophagy through conventional autophagy is almost always transient in the heart. Because mitochondria constitute more than 30% of the volume of heart cells, cellular mechanisms to support mitophagy through conventional pathways may be rapidly exhausted during stress. Alternatively, cellular signaling mechanisms that potently inhibit autophagy, such as those involving MST1 and MTOR, may prevent persistent activation of the conventional mechanism of autophagy. Interestingly, despite downregulation of the conventional mechanism of autophagy, evidenced by the complete loss of LC3-II in the mitochondrial fraction in the presence of lysosomal inhibitors, mitophagy continues to be observed in the heart. Complete dissociation of the time courses of conventional nonselective autophagy and mitophagy is also observed in hearts subjected to other forms of stress, including high blood pressure. Inhibition of autophagy during the chronic phase of diabetic cardiomyopathy is commonly observed in mice and humans. Thus, understanding how mitophagy is activated when conventional autophagy cannot be activated is quite important.
Because the ULK1-RAB9-RIPK1-DNM1L-dependent form of mitophagy is activated when conventional mitophagy is inactivated and with a delayed time course, it appears to be a compensatory mechanism. One mechanism promoting activation of this form of mitophagy is the TFE3-dependent transcription of RAB9. TFE3 is a member of the TFEB family of transcription factors. We speculate that downregulation of conventional autophagy and mitophagy enhances cellular stresses, including endoplasmic reticulum stress, which in turn induces activation of TFE3, thereby triggering alternative mitophagy. Although further experimentation is needed to test this hypothesis, it is possible that conventional and alternative mechanisms of mitophagy are functionally connected with one another.
Because RAB9-positive autophagosomes sequester depolarized mitochondria more than polarized ones, it is likely that alternative mitophagy degrades depolarized mitochondria. When alternative mitophagy is activated, a large protein complex containing ULK1, RAB9, RIPK1 and DNM1L, is formed and DNM1L is phosphorylated at Ser616. However, how damaged mitochondria are recognized and how autophagosomes surround them are unknown. We have shown that ULK1-induced phosphorylation of RAB9 at Ser179 is a key event for the formation of the large protein complex. Identifying additional components in this complex may help clarify the molecular mechanism of alternative autophagy.
Citation1We speculate that enhancing the activity of alternative mitophagy could prevent or even reverse mitochondrial dysfunction in patients suffering from obesity or diabetic cardiomyopathy. Because TAT-BECN1, a peptide that mobilizes endogenous BECN1, can promote mitophagy in atg7 knockout mice, BECN1 may be involved in alternative mitophagy and TAT- BECN1 may stimulate alternative mitophagy in the heart. Elucidation of the molecular mechanism of alternative mitophagy should increase the options available for better control of mitochondrial quality even when the conventional autophagy mechanism is inactivated in stressed hearts.
Acknowledgments
The author thanks Daniela Zablocki for critical reading of the manuscript.
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No potential conflict of interest was reported by the author(s).
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Reference
- Tong M, Saito T, Zhai P, et al. Alternative mitophagy protects the heart against obesity-associated cardiomyopathy. Circ Res. 2021;129(12):1105–1121.