Acute respiratory distress syndrome (ARDS), an inflammatory condition with high mortality rates, is common in severe COVID-19, whose risk is reduced by metformin rather than other anti-diabetic medications. Detecting of inflammasome assembly in post-mortem COVID-19 lungs, we asked whether and how metformin inhibits inflammasome activation while exerting its anti-inflammatory effect. We show that metformin inhibited NLRP3 inflammasome activation and interleukin (IL)-1β production in cultured and alveolar macrophages along with inflammasome-independent IL-6 secretion, thus attenuating lipopolysaccharide (LPS)- and SARS-CoV-2-induced ARDS. By targeting electron transport chain complex 1 and independently of AMP-activated protein kinase (AMPK) or NF-κB, metformin blocked LPS-induced and ATP-dependent mitochondrial (mt) DNA synthesis and generation of oxidized mtDNA, an NLRP3 ligand. Myeloid-specific ablation of LPS-induced cytidine monophosphate kinase 2 (CMPK2), which is rate limiting for mtDNA synthesis, reduced ARDS severity without a direct effect on IL-6. Thus, inhibition of ATP and mtDNA synthesis is sufficient for ARDS amelioration.

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Defective mitophagy linked to dysfunction in the proteins Parkin and PTEN-induced putative kinase 1 (PINK1) is implicated in the pathogenesis of Parkinson's disease. Although the mechanism by which Parkin mediates mitophagy in a PINK1-dependent manner is becoming clearer, the triggers for this mitophagy pathway remain elusive. Reactive oxygen species (ROS) have been suggested as such triggers, but this proposal remains controversial because ROS scavengers fail to retard mitophagy. Here we demonstrate that the role of ROS in mitophagy has been underappreciated as a result of the inefficiency of ROS scavengers to control ROS bursts after high-dose treatment with carbonyl cyanide -chlorophenylhydrazone. Supporting this, combinatorial treatment with-acetyl-l-cysteine and catalase substantially inhibited the ROS upsurge and PINK1-dependent Parkin translocation to mitochondria in response to carbonyl cyanide -chlorophenylhydrazone treatment. In addition to the chemical mitophagy inducer, overexpression of voltage-dependent anion channel 1 (VDAC1) induced Parkin translocation to mitochondria, presumably by stimulating ROS generation. Similarly, combined-acetyl-l-cysteine and catalase treatment also suppressed VDAC1-induced redistribution of Parkin. Alongside these observations, we also found that the elevated protein level of PINK1 was not necessary for Parkin translocation to mitochondria. Thus, our data suggest that ROS may act as a trigger for the induction of Parkin/PINK1-dependent mitophagy. In addition, our study casts doubt on the importance of protein quantity of PINK1 in the recruitment of Parkin to mitochondria.

Mitochondrial quality control is essential for cellular homeostasis and accumulating evidence show that mitochondria can be selectively targeted for autophagic degradation (mitophagy). Mitophagy allows for the removal of dysfunctional mitochondria, which is highly implicated in energetically demanding cells including muscle and nerve cells. However, despite the well characterization of PINK1/Parkin route of mitophagy, mechanisms concerning PINK1/Parkin‐independent mitophagy are still poorly understood. With imaging approaches including structure‐illumination microscopy (SR‐SIM), here we demonstrate that the autophagy protein Syntaxin 17 (STX17), initiates mitophagy upon the depletion of outer mitochondrial membrane protein Fis1. Using mass spectrometry analysis, we identify STX17 interacts with Fis1, which preferentially gatekeeps the dynamic shuffling of STX17 between ER and mitochondria. Loss of Fis1 results in the accumulation of STX17 on mitochondria and mitochondria associated membranes (MAM), exposing its N‐terminus to assemble and self‐oligomerize for mitophagy. Mitochondrial STX17 interacts with ATG14 and further recruits core autophagy proteins hierarchically to form mitophagosomes, followed by Rab7‐dependent mitophagosome‐lysosome fusion. In addition, our results reveal that Fis1 loss impairs mitochondrial metabolic function, and potentially sensitizes mitochondria to STX17‐mediated mitochondrial engulfment within autophagosomes, which is directly initiated through canonical autophagy machinery, closely linking non‐selective macroautophagy and mitochondria. Our findings uncover a novel PINK1/Parkin‐independent mitophagy mechanism, in which outer mitochondrial membrane protein Fis1 gates the elimination of mitochondria. Support or Funding Information This work is financially supported by grants Tier 2 MOE and NUS, Singapore to Y.‐C.L. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Most cellular stress responses converge on the mitochondria. Consequently, the mitochondria must rapidly respond to maintain cellular homeostasis and physiological demands by fine-tuning a plethora of mitochondria-associated processes. The outer mitochondrial membrane (OMM) proteins are central to mediating mitochondrial dynamics, coupled with continuous fission and fusion. These OMM proteins also have vital roles in controlling mitochondrial quality and serving as mitophagic receptors for autophagosome enclosure during mitophagy. Mitochondrial fission segregates impaired mitochondria in smaller sizes from the mother mitochondria and may favor mitophagy for eliminating damaged mitochondria. Conversely, mitochondrial fusion mixes dysfunctional mitochondria with healthy ones to repair the damage by diluting the impaired components and consequently prevents mitochondrial clearance via mitophagy. Despite extensive research efforts into deciphering the interplay between fission-fusion and mitophagy, it is still not clear whether mitochondrial fission essentially precedes mitophagy. In this review, we summarize recent breakthroughs concerning OMM research, and dissect the functions of these proteins in mitophagy from their traditional roles in fission-fusion dynamics, in response to distinct context, at the intersection of the OMM platform. These insights into the OMM proteins in mechanistic researches would lead to new aspects of mitochondrial quality control and better understanding of mitochondrial homeostasis intimately tied to pathological impacts.