Mitophagy is an evolutionarily conserved cellular process to remove dysfunctional or superfluous mitochondria, thus fine-tuning mitochondrial number and preserving energy metabolism. In this Review, we survey recent advances towards elucidating the molecular mechanisms that mediate mitochondrial elimination and the signalling pathways that govern mitophagy. We consider the contributions of mitophagy in physiological and pathological contexts and discuss emerging findings, highlighting the potential value of mitophagy modulation in therapeutic intervention.

Mechanisms of mitophagy in cellular homeostasis, physiology and pathology.

Mitochondrial fusion and structure depend on the dynamin-like GTPase OPA1, whose activity is regulated by proteolytic processing. Constitutive OPA1 cleavage by YME1L and OMA1 at two distinct sites leads to the accumulation of both long and short forms of OPA1 and maintains mitochondrial fusion. Stress-induced OPA1 processing by OMA1 converts OPA1 completely into short isoforms, inhibits fusion, and triggers mitochondrial fragmentation. Here, we have analyzed the function of different OPA1 forms in cells lacking YME1L, OMA1, or both. Unexpectedly, deletion of Oma1 restored mitochondrial tubulation, cristae morphogenesis, and apoptotic resistance in cells lacking YME1L. Long OPA1 forms were sufficient to mediate mitochondrial fusion in these cells. Expression of short OPA1 forms promoted mitochondrial fragmentation, which indicates that they are associated with fission. Consistently, GTPase-inactive, short OPA1 forms partially colocalize with ER–mitochondria contact sites and the mitochondrial fission machinery. Thus, OPA1 processing is dispensable for fusion but coordinates the dynamic behavior of mitochondria and is crucial for mitochondrial integrity and quality control.

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The i-AAA protease YME1L and OMA1 cleave OPA1 to balance mitochondrial fusion and fission

Mitochondria are highly dynamic and undergo constant fusion and fission that are essential for maintaining physiological functions of cells. Although dysfunction of mitochondria has been implicated in tumorigenesis, little is known about the roles of mitochondrial dynamics in metastasis, the major cause of cancer death. In the present study, we found a marked upregulation of mitochondrial fission protein dynamin-related protein 1 (Drp1) expression in human invasive breast carcinoma and metastases to lymph nodes. Compared with non-metastatic breast cancer cells, mitochondria also were more fragmented in metastatic breast cancer cells that express higher levels of total and active Drp1 and less mitochondrial fusion protein 1 (Mfn1). Silencing Drp1 or overexpression of Mfn1 resulted in mitochondria elongation or clusters, respectively, and significantly suppressed metastatic abilities of breast cancer cells. In contrast, silencing Mfn proteins led to mitochondrial fragmentation and enhanced metastatic abilities of breast cancer cells. Interestingly, these manipulations of mitochondrial dynamics altered the subcellular distribution of mitochondria in breast cancer cells. For example, silencing Drp1 or overexpression of Mfn1 inhibited lamellipodia formation, a key step for cancer metastasis, and suppressed chemoattractant-induced recruitment of mitochondria to lamellipodial regions. Conversely, silencing Mfn proteins resulted in more cell spreading and lamellipodia formation, causing accumulation of more mitochondria in lamellipodia regions. More importantly, treatment with a mitochondrial uncoupling agent or adenosine triphosphate synthesis inhibitor reduced lamellipodia formation and decreased breast cancer cell migration and invasion, suggesting a functional importance of mitochondria in breast cancer metastasis. Together, our findings show a new role and mechanism for regulation of cancer cell migration and invasion by mitochondrial dynamics. Thus targeting dysregulated Drp1-dependent mitochondrial fission may provide a novel strategy for suppressing breast cancer metastasis.

Mitochondrial dynamics regulates migration and invasion of breast cancer cells.

Degradation of mitochondria via a selective form of autophagy, named mitophagy, is a fundamental mechanism conserved from yeast to humans that regulates mitochondrial quality and quantity control. Mitophagy is promoted via specific mitochondrial outer membrane receptors, or ubiquitin molecules conjugated to proteins on the mitochondrial surface leading to the formation of autophagosomes surrounding mitochondria. Mitophagy-mediated elimination of mitochondria plays an important role in many processes including early embryonic development, cell differentiation, inflammation, and apoptosis. Recent advances in analyzing mitophagy in vivo also reveal high rates of steady-state mitochondrial turnover in diverse cell types, highlighting the intracellular housekeeping role of mitophagy. Defects in mitophagy are associated with various pathological conditions such as neurodegeneration, heart failure, cancer, and aging, further underscoring the biological relevance. Here, we review our current molecular understanding of mitophagy, and its physiological implications, and discuss how multiple mitophagy pathways coordinately modulate mitochondrial fitness and populations.

https://www.embopress.org/doi/pdf/10.15252/embj.2020104705

Molecular mechanisms and physiological functions of mitophagy.

Mitochondrial quality control systems are essential for the maintenance of functional mitochondria. At the organelle level, they include mitochondrial biogenesis, fusion and fission, to compensate for mitochondrial function, and mitophagy, for degrading damaged mitochondria. Specifically, in mitophagy, the target mitochondria are recognized by the autophagosomes and delivered to the lysosome for degradation. In this review, we describe the mechanisms of mitophagy and the factors that play an important role in this process. In particular, we focus on the roles of mitophagy adapters and receptors in the recognition of damaged mitochondria by autophagosomes. In addition, we also address a functional association of mitophagy with mitochondrial dynamics through the interaction of mitophagy adaptor and receptor proteins with mitochondrial fusion and fission proteins.

A Molecular Approach to Mitophagy and Mitochondrial Dynamics.

Mitochondrial dysfunction underlies many prevalent diseases including heart disease arising from acute ischemia/reperfusion (I/R) injury. Here, we demonstrate that mitophagy, which selectively removes damaged or unwanted mitochondria, regulated mitochondrial quality and quantity in vivo. Hypoxia induced extensive mitochondrial degradation in a FUNDC1-dependent manner in platelets, and this was blocked by in vivo administration of a cell-penetrating peptide encompassing the LIR motif of FUNDC1 only in wild-type mice. Genetic ablation of Fundc1 impaired mitochondrial quality and increased mitochondrial mass in platelets and rendered the platelets insensitive to hypoxia and the peptide. Moreover, hypoxic mitophagy in platelets protected the heart from worsening of I/R injury. This represents a new mechanism of the hypoxic preconditioning effect which reduces I/R injury. Our results demonstrate a critical role of mitophagy in mitochondrial quality control and platelet activation, and suggest that manipulation of mitophagy by hypoxia or pharmacological approaches may be a novel strategy for cardioprotection.

/pdf/hypoxic-mitophagy-regulates-mitochondrial-quality-and-j6vyay0b12.pdf

Hypoxic mitophagy regulates mitochondrial quality and platelet activation and determines severity of I/R heart injury

Mitofusins (Mfns) mediate the fusion of mitochondrial membranes. However, little is known about how Mfns are regulated to control mitochondrial fusion, which is a multistep process requiring tethering and docking of the outer membranes of two mitochondria. In this study, we report that guanine nucleotide binding protein-β subunit 2 (Gβ2), a WD40 repeats protein and a member of the β-subunits of the heterotrimeric G proteins, has a crucial function in mitochondrial fusion. Gβ2 was found to be enriched on the surface of mitochondria and interacted with mitofusin 1 (Mfn1) specifically. Gβ2 also regulated the mobility of Mfn1 on the surface of the mitochondrial membrane and affected the mitochondrial fusion. Depletion of endogenous Gβ2 resulted in mitochondrial fragmentation, which could be rescued by exogenous Gβ2. These findings have thus uncovered a novel role of Gβ2 in regulating mitochondrial fusion through its interaction with Mfn1.

/pdf/g-protein-b2-subunit-interacts-with-mitofusin-1-to-regulate-3tf6hpd9jq.pdf

G-protein β2 subunit interacts with mitofusin 1 to regulate mitochondrial fusion.

Mitochondria are highly-dynamic organelles, but it is challenging to monitor quantitatively their dynamics in a living cell. Here we developed a novel approach to determine the global occurrence of mitochondrial fission and fusion events in living human epithelial cells (Hela) and mouse embryonic fibroblast cells (MEF). Distinct patterns of sequential events including fusion followed by fission (Fu-Fi), the so-called "kiss and run'' model previously described, fission followed by fusion (Fi-Fu), fusion followed by fusion (Fu-Fu), and fission followed by fission (Fi-Fi) were observed concurrently. The paired events appeared in high frequencies with short lifetimes and large sizes of individual mitochondria, as compared to those for unpaired events. The high frequencies of paired events were found to be biologically significant. The presence of membrane uncoupler CCCP enhanced the frequency of paired events (from both Fu-Fi and Fi-Fu patterns) with a reduced mitochondrial size. Knock-out of mitofusin protein Mfn1 increased the frequency of fission with increased lifetime of unpaired events whereas deletion of both Mfn1 and Mfn2 resulted in an instable dynamics. These results indicated that the paired events were dominant but unpaired events were not negligible, which provided a new insight into mitochondrial dynamics. In addition to kiss and run model of action, our data suggest that, from a global visualization over an entire cell, multiple patterns of action appeared in mitochondrial fusion and fission.

/pdf/multi-patterned-dynamics-of-mitochondrial-fission-and-fusion-19mjeuohcr.pdf

Chuanmao Zhang

Papers

Hypoxic mitophagy regulates mitochondrial quality and platelet activation and determines severity of I/R heart injury

G-protein β2 subunit interacts with mitofusin 1 to regulate mitochondrial fusion.

Multi-patterned dynamics of mitochondrial fission and fusion in a living cell.