bims-mikwok Biomed News
on Mitochondrial quality control
Issue of 2021‒02‒07
thirty-four papers selected by
Avinash N. Mukkala
University of Toronto

  1. Biol Chem. 2020 Nov 18. 402(1): 73-88
      Mitochondria are key players of cellular metabolism, Ca2+ homeostasis, and apoptosis. The functionality of mitochondria is tightly regulated, and dysfunctional mitochondria are removed via mitophagy, a specialized form of autophagy that is compromised in hereditary forms of Parkinson's disease. Through mitophagy, cells are able to cope with mitochondrial stress until the damage becomes too great, which leads to the activation of pro-apoptotic BCL-2 family proteins located on the outer mitochondrial membrane. Active pro-apoptotic BCL-2 proteins facilitate the release of cytochrome c from the mitochondrial intermembrane space (IMS) into the cytosol, committing the cell to apoptosis by activating a cascade of cysteinyl-aspartate specific proteases (caspases). We are only beginning to understand how the choice between mitophagy and the activation of caspases is determined on the mitochondrial surface. Intriguingly in neurons, caspase activation also plays a non-apoptotic role in synaptic plasticity. Here we review the current knowledge on the interplay between mitophagy and caspase activation with a special focus on the central nervous system.
    Keywords:  BCL-2 family; PINK1; Parkin; Parkinson’s disease; caspase-3; synaptic plasticity
  2. Cell Mol Life Sci. 2021 Feb 05.
      Mitochondrial quality control depends upon selective elimination of damaged mitochondria, replacement by mitochondrial biogenesis, redistribution of mitochondrial components across the network by fusion, and segregation of damaged mitochondria by fission prior to mitophagy. In this review, we focus on mitochondrial dynamics (fusion/fission), mitophagy, and other mechanisms supporting mitochondrial quality control including maintenance of mtDNA and the mitochondrial unfolded protein response, particularly in the context of the heart.
    Keywords:  Cardiac; Fission; Fusion; Mitochondria; Mitophagy
  3. FASEB J. 2021 Feb;35(2): e21361
      Bcl-2-associated athanogen-6 (BAG6) is a nucleocytoplasmic shuttling protein involved in protein quality control. We previously demonstrated that BAG6 is essential for autophagy by regulating the intracellular localization of the acetyltransferase EP300, and thus, modifying accessibility to its substrates (TP53 in the nucleus and autophagy-related proteins in the cytoplasm). Here, we investigated BAG6 localization and function in the cytoplasm. First, we demonstrated that BAG6 is localized in the mitochondria. Specifically, BAG6 is expressed in the mitochondrial matrix under basal conditions, and translocates to the outer mitochondrial membrane after mitochondrial depolarization with carbonyl cyanide m-chlorophenyl hydrazine, a mitochondrial uncoupler that induces mitophagy. In SW480 cells, the deletion of BAG6 expression abrogates its ability to induce mitophagy and PINK1 accumulation. On the reverse, its ectopic expression in LoVo colon cancer cells, which do not express endogenous BAG6, reduces the size of the mitochondria, induces mitophagy, leads to the activation of the PINK1/PARKIN pathway and to the phospho-ubiquitination of mitochondrial proteins. Finally, BAG6 contains two LIR (LC3-interacting Region) domains specifically found in receptors for selective autophagy and responsible for the interaction with LC3 and for autophagosome selectivity. Site-directed mutagenesis showed that BAG6 requires wild-type LIRs domains for its ability to stimulate mitophagy. In conclusion, we propose that BAG6 is a novel mitophagy receptor or adaptor that induces PINK1/PARKIN signaling and mitophagy in a LIR-dependent manner.
    Keywords:  BAG6; mitophagy; receptor; signaling
  4. Cell Rep. 2021 Feb 02. pii: S2211-1247(21)00002-4. [Epub ahead of print]34(5): 108689
      The epidermis regenerates continually to maintain a protective barrier at the body's surface composed of differentiating keratinocytes. Maturation of this stratified tissue requires that keratinocytes undergo wholesale organelle degradation upon reaching the outermost tissue layers to form compacted, anucleate cells. Through live imaging of organotypic cultures of human epidermis, we find that regulated breakdown of mitochondria is critical for epidermal development. Keratinocytes in the upper layers initiate mitochondrial fragmentation, depolarization, and acidification upon upregulating the mitochondrion-tethered autophagy receptor NIX. Depleting NIX compromises epidermal maturation and impairs mitochondrial elimination, whereas ectopic NIX expression accelerates keratinocyte differentiation and induces premature mitochondrial fragmentation via the guanosine triphosphatase (GTPase) DRP1. We further demonstrate that inhibiting DRP1 blocks NIX-mediated mitochondrial breakdown and disrupts epidermal development. Our findings establish mitochondrial degradation as a key step in terminal keratinocyte differentiation and define a pathway operating via the mitophagy receptor NIX in concert with DRP1 to drive epidermal morphogenesis.
    Keywords:  autophagy; cornification; differentiation; epidermis; epithelial morphogenesis; fission; keratinocyte; live microscopy; mitochondria; mitophagy
  5. Proc Natl Acad Sci U S A. 2021 Feb 09. pii: e2008778118. [Epub ahead of print]118(6):
      Human mitochondria contain their own genome, mitochondrial DNA, that is expressed in the mitochondrial matrix. This genome encodes 13 vital polypeptides that are components of the multisubunit complexes that couple oxidative phosphorylation (OXPHOS). The inner mitochondrial membrane that houses these complexes comprises the inner boundary membrane that runs parallel to the outer membrane, infoldings that form the cristae membranes, and the cristae junctions that separate the two. It is in these cristae membranes that the OXPHOS complexes have been shown to reside in various species. The majority of the OXPHOS subunits are nuclear-encoded and must therefore be imported from the cytosol through the outer membrane at contact sites with the inner boundary membrane. As the mitochondrially encoded components are also integral members of these complexes, where does protein synthesis occur? As transcription, mRNA processing, maturation, and at least part of the mitoribosome assembly process occur at the nucleoid and the spatially juxtaposed mitochondrial RNA granules, is protein synthesis also performed at the RNA granules close to these entities, or does it occur distal to these sites? We have adapted a click chemistry-based method coupled with stimulated emission depletion nanoscopy to address these questions. We report that, in human cells in culture, within the limits of our methodology, the majority of mitochondrial protein synthesis is detected at the cristae membranes and is spatially separated from the sites of RNA processing and maturation.
    Keywords:  click chemistry; human mitochondria; mitoribosomes; protein synthesis
  6. J Cell Sci. 2021 Feb 03. pii: jcs.256255. [Epub ahead of print]
      Mitophagy, the selective recycling of mitochondria through autophagy, is a crucial metabolic process induced by cellular stress, and defects are linked to aging, sarcopenia, and neurodegenerative diseases. To therapeutically target mitophagy, the fundamental in vivo dynamics and molecular mechanisms must be fully understood. Here, we generated mitophagy biosensor zebrafish lines expressing mitochondrially targeted, pH-sensitive, fluorescent probes mito-Keima and mito-EGFP-mCherry and used quantitative intravital imaging to illuminate mitophagy during physiological stresses-embryonic development, fasting and hypoxia. In fasted muscle, volumetric mitolysosome size analyses documented organelle stress-response dynamics, and time-lapse imaging revealed mitochondrial filaments undergo piecemeal fragmentation and recycling rather than the wholesale turnover observed in cultured cells. Hypoxia-inducible factor (Hif) pathway activation through physiological hypoxia or chemical or genetic modulation also provoked mitophagy. Intriguingly, mutation of a single mitophagy receptor bnip3 prevented this effect, whereas disruption of other putative hypoxia-associated mitophagy genes bnip3la (nix), fundc1, pink1 or prkn (Parkin) had no effect. This in vivo imaging study establishes fundamental dynamics of fasting-induced mitophagy and identifies bnip3 as the master regulator of Hif-induced mitophagy in vertebrate muscle.
    Keywords:  Autophagy; Fasting; Hypoxia; Lysosome; Mitochondria
  7. Eur Biophys J. 2021 Feb 02.
      Cellular membranes can adopt a plethora of complex and beautiful shapes, most of which are believed to have evolved for a particular physiological reason. The closely entangled relationship between membrane morphology and cellular physiology is strikingly seen in membrane trafficking pathways. During clathrin-mediated endocytosis, for example, over the course of a minute, a patch of the more or less flat plasma membrane is remodeled into a highly curved clathrin-coated vesicle. Such vesicles are internalized by the cell to degrade or recycle plasma membrane receptors or to take up extracellular ligands. Other, steadier, membrane morphologies can be observed in organellar membranes like the endoplasmic reticulum or mitochondria. In the case of mitochondria, which are double membrane-bound, ubiquitous organelles of eukaryotic cells, especially the mitochondrial inner membrane displays an intricated ultrastructure. It is highly folded and consequently has a much larger surface than the mitochondrial outer membrane. It can adopt different shapes in response to cellular demands and changes of the inner membrane morphology often accompany severe diseases, including neurodegenerative- and metabolic diseases and cancer. In recent years, progress was made in the identification of molecules that are important for the aforementioned membrane remodeling events. In this review, we will sum up recent results and discuss the main players of membrane remodeling processes that lead to the mitochondrial inner membrane ultrastructure and in clathrin-mediated endocytosis. We will compare differences and similarities between the molecular mechanisms that peripheral and integral membrane proteins use to deform membranes.
    Keywords:  Clathrin-mediated endocytosis; Membrane curvature; Membrane dynamics; Mitochondria; Mitochondrial morphology; Mitochondrial ultrastructure
  8. J Biol Chem. 2020 Apr 24. pii: S0021-9258(17)50290-2. [Epub ahead of print]295(17): 5588-5601
      Accumulating evidence suggests that brown adipose tissue (BAT) is a potential therapeutic target for managing obesity and related diseases. PGAM family member 5, mitochondrial serine/threonine protein phosphatase (PGAM5), is a protein phosphatase that resides in the mitochondria and regulates many biological processes, including cell death, mitophagy, and immune responses. Because BAT is a mitochondria-rich tissue, we have hypothesized that PGAM5 has a physiological function in BAT. We previously reported that PGAM5-knockout (KO) mice are resistant to severe metabolic stress. Importantly, lipid accumulation is suppressed in PGAM5-KO BAT, even under unstressed conditions, raising the possibility that PGAM5 deficiency stimulates lipid consumption. However, the mechanism underlying this observation is undetermined. Here, using an array of biochemical approaches, including quantitative RT-PCR, immunoblotting, and oxygen consumption assays, we show that PGAM5 negatively regulates energy expenditure in brown adipocytes. We found that PGAM5-KO brown adipocytes have an enhanced oxygen consumption rate and increased expression of uncoupling protein 1 (UCP1), a protein that increases energy consumption in the mitochondria. Mechanistically, we found that PGAM5 phosphatase activity and intramembrane cleavage are required for suppression of UCP1 activity. Furthermore, utilizing a genome-wide siRNA screen in HeLa cells to search for regulators of PGAM5 cleavage, we identified a set of candidate genes, including phosphatidylserine decarboxylase (PISD), which catalyzes the formation of phosphatidylethanolamine at the mitochondrial membrane. Taken together, these results indicate that PGAM5 suppresses mitochondrial energy expenditure by down-regulating UCP1 expression in brown adipocytes and that its phosphatase activity and intramembrane cleavage are required for UCP1 suppression.
    Keywords:  PGAM family member 5 mitochondrial serine/threonine protein phosphatase (PGAM5); adipocyte; brown adipocyte; brown adipose tissue; energy metabolism; intramembrane proteolysis; lipid metabolism; mitochondria; mitochondrial homeostasis; obesity; phosphatidylserine decarboxylase (PISD); protein phosphatase; uncoupling protein 1 (UCP1)
  9. Am J Transl Res. 2021 ;13(1): 223-233
      Skin flap ischemia-reperfusion (IR) injury is the key factor to the success rate of skin transplantation, the molecular mechanism of flap IR injury needs to be continuously explored to provide new ideas for its clinical treatment. G protein-coupled receptor kinase 2 (GRK2) was reported to be involved in regulating mitochondrial function, and mitochondria were essential in the process of flap IR. Thus, we aimed to investigate the function of GRK2 in flap ischemia-reperfusion injury and further explore the underlying mechanism. Sixty male C57BL/6 mice were randomly divided into four groups: sham, IR+sh-NC, IR+sh-GRK2 and IR+sh-GRK2+ dynamin-related GTPase 1 (Drp1). Flap function and mitochondrial function were determined after ischemia for 3 hours and reperfusion for 72 hours. Comparing with sham group, GRK2 was increased in flap after IR injury. Loss of GRK2 inhibited cell apoptosis and promoted cell proliferation of flap after IR injury. And deficiency of GRK2 promoted mitochondrial function in flap after IR injury. IR injury up-regulated Drp1 expression in flap, while sh-GRK2 down-regulated Drp1 expression. Furthermore, overexpression of Drp1 removed the protective effect of sh-GRK2. In conclusion, our study revealed that GRK2 deletion improved flap function and mitochondrial function by inhibiting Drp1 expression, which may provide a new insight for the clinical treatment of flap ischemia-reperfusion injury.
    Keywords:  Drp1; GRK2; ROS; Skin flap ischemia-reperfusion injury; mitochondrial function
  10. Life Sci. 2021 Feb 01. pii: S0024-3205(21)00124-7. [Epub ahead of print] 119139
      AIMS: Complicated mechanisms in cancer cells have been restricting the medicinal value of resveratrol (Res). The mechanisms by which Res exerts its anti-tumor activity in lung cancer cells have diverged among reports in recent years, whether cells choose to undergo autophagic cell death or apoptosis remains controversial. Yet, whether Res-induced autophagic cell death transforms into apoptosis is still unknown, and by which autophagy regulates programmed cell death is still undefined.MAIN METHODS: Here, A549 cells were treated with Res to investigate the mechanisms of autophagy and apoptosis using western blot, immunofluorescence staining for LC3B.
    KEY FINDINGS: Non-canonical autophagy was induced by Res-treatment in a Beclin-1- and ATG5-independent manner, with apoptosis being activated simultaneously. Autophagy induced by Res was activated by rapamycin with decreased apoptosis, suggesting that autophagy may serve as a protective pathway in cells. Mitophagy was found to be induced by Res using fluorescence co-localization of mitochondria with lysosomes. Subsequently, it was identified that mitophagy was mediated by LC3B/p62 interaction and could be inhibited by LC3B knockout and p62 knockdown following increased apoptosis.
    SIGNIFICANCE: In conclusion, the current results demonstrate that Res-induced non-canonical autophagy in A549 lung cancer cells with apoptosis activation simultaneously, while LC3B/p62-mediated mitophagy protects tumor cells against apoptosis, providing novel mechanisms about the critical role of mitophagy in regulating cell fate.
    Keywords:  A549; Apoptosis; Autophagy; LC3B; Mitophagy; Resveratrol; p62
  11. Biochim Biophys Acta Gen Subj. 2021 Feb 02. pii: S0304-4165(21)00017-9. [Epub ahead of print] 129858
      Mitochondria are dynamic organelles functioning in diverse reactions and processes such as energy metabolism, apoptosis, innate immunity, and aging, whose quality and quantity control is critical for cell homeostasis. Mitochondria-specific autophagy, termed mitophagy, is an evolutionarily conserved process that selectively degrades mitochondria via autophagy, thereby contributing to mitochondrial quality and quantity control. In the budding yeast Saccharomyces cerevisiae, the single-pass membrane protein Atg32 accumulates on the surface of mitochondria and recruit the autophagy machinery to initiate mitophagy. This catabolic process is elaborately regulated through transcriptional induction and post-translational modifications of Atg32. Notably, other factors acting in manifold pathways including protein N-terminal acetylation, phospholipid methylation, stress signaling, and endoplasmic reticulum-localized protein dephosphorylation and membrane protein insertion are also linked to mitophagy. Here we review recent discoveries of molecules regulating mitophagy in yeast.
    Keywords:  Atg32; Autophagy; Mitochondria; Mitophagy; Yeast
  12. Int J Biochem Cell Biol. 2021 Jan 30. pii: S1357-2725(21)00018-2. [Epub ahead of print] 105934
      Mitochondrial function is centrally involved in many cellular processes, such as energy production, metabolism of nucleotides, amino acids, and lipids, calcium buffering, and regulation of cell death. Multiple mechanisms are engaged under conditions of mitochondrial dysfunction to restore cellular and, subsequently, systemic functions. The mitochondrial unfolded protein response is a homeostatic mechanism that has attracted a lot of interest recently and has been described in several organisms, including humans. The mitochondrial unfolded protein response serves as a first-line-of-defence mechanism against stress to restore mitochondrial proteostasis and functions. Here, we discuss the canonical mechanisms via which the mitochondrial unfolded protein response is activated under stress and examine recent evidence that links the response with other processes that promote survival and the recovery of the mitochondrial network (i.e. the integrated stress response and mitophagy).
    Keywords:  UPR; disease; heart; mitochondria; stress
  13. CNS Neurosci Ther. 2021 Feb 03.
      Mitochondrial encephalomyopathies are disorders caused by mitochondrial and nuclear DNA mutations which affect the nervous and muscular systems. Current therapies for mitochondrial encephalomyopathies are inadequate and mostly palliative. However, stem cell-derived mitochondria transplantation has been demonstrated to play an key part in metabolic rescue, which offers great promise for mitochondrial encephalomyopathies. Here, we summarize the present status of stem cell therapy for mitochondrial encephalomyopathy and discuss mitochondrial transfer routes and the protection mechanisms of stem cells. We also identify and summarize future perspectives and challenges for the treatment of these intractable disorders based on the concept of mitochondrial transfer from stem cells.
    Keywords:  mitochondria; mitochondria quality control; mitochondrial dynamics; mitochondrial encephalomyopathy; stem cell; tunneling nanotube
  14. Front Cell Dev Biol. 2020 ;8 611938
      Maintenance of neuronal homeostasis is a challenging task, due to unique cellular organization and bioenergetic demands of post-mitotic neurons. It is increasingly appreciated that impairment of mitochondrial homeostasis represents an early sign of neuronal dysfunction that is common in both age-related neurodegenerative as well as in neurodevelopmental disorders. Mitochondrial selective autophagy, known as mitophagy, regulates mitochondrial number ensuring cellular adaptation in response to several intracellular and environmental stimuli. Mounting evidence underlines that deregulation of mitophagy levels has an instructive role in the process of neurodegeneration. Although mitophagy induction mediates the elimination of damaged mitochondria and confers neuroprotection, uncontrolled runaway mitophagy could reduce mitochondrial content overstressing the remaining organelles and eventually triggering neuronal cell death. Unveiling the molecular mechanisms of neuronal mitophagy and its intricate role in neuronal survival and cell death, will assist in the development of novel mitophagy modulators to promote cellular and organismal homeostasis in health and disease.
    Keywords:  aging; cell death; energy metabolism; homeostasis; mitochondria; mitophagy; neurodegeneration; neuroprotection
  15. Cell Rep. 2021 Feb 02. pii: S2211-1247(21)00036-X. [Epub ahead of print]34(5): 108723
      The metabolic changes controlling the stepwise differentiation of hematopoietic stem and progenitor cells (HSPCs) to mature erythrocytes are poorly understood. Here, we show that HSPC development to an erythroid-committed proerythroblast results in augmented glutaminolysis, generating alpha-ketoglutarate (αKG) and driving mitochondrial oxidative phosphorylation (OXPHOS). However, sequential late-stage erythropoiesis is dependent on decreasing αKG-driven OXPHOS, and we find that isocitrate dehydrogenase 1 (IDH1) plays a central role in this process. IDH1 downregulation augments mitochondrial oxidation of αKG and inhibits reticulocyte generation. Furthermore, IDH1 knockdown results in the generation of multinucleated erythroblasts, a morphological abnormality characteristic of myelodysplastic syndrome and congenital dyserythropoietic anemia. We identify vitamin C homeostasis as a critical regulator of ineffective erythropoiesis; oxidized ascorbate increases mitochondrial superoxide and significantly exacerbates the abnormal erythroblast phenotype of IDH1-downregulated progenitors, whereas vitamin C, scavenging reactive oxygen species (ROS) and reprogramming mitochondrial metabolism, rescues erythropoiesis. Thus, an IDH1-vitamin C crosstalk controls terminal steps of human erythroid differentiation.
    Keywords:  alpha-ketoglutarate; enucleation; erythropoiesis; hematopoietic stem and progenitor cell; human; isocitrate dehydrogenase; mitochondria; oxidative phosphorylation; redox stress; vitamin C
  16. Nature. 2021 Feb;590(7844): 57-66
      Mitochondria form dynamic networks in the cell that are balanced by the flux of iterative fusion and fission events of the organelles. It is now appreciated that mitochondrial fission also represents an end-point event in a signalling axis that allows cells to sense and respond to external cues. The fission process is orchestrated by membrane-associated adaptors, influenced by organellar and cytoskeletal interactions and ultimately executed by the dynamin-like GTPase DRP1. Here we invoke the framework of the 'mitochondrial divisome', which is conceptually and operationally similar to the bacterial cell-division machinery. We review the functional and regulatory aspects of the mitochondrial divisome and, within this framework, parse the core from the accessory machinery. In so doing, we transition from a phenomenological to a mechanistic understanding of the fission process.
  17. Pediatr Surg Int. 2021 Feb 06.
      PURPOSE: Necrotizing enterocolitis (NEC) is a severe neonatal gastrointestinal disease that can cause damage to remote organs. Previous studies have shown that inflammatory and oxidative injury occur in the liver during NEC. Mitochondrial DNA (mtDNA) plays an important role in hepatic injuries of many other diseases. We aimed to investigate the mechanism of mitochondrial dysfunction in hepatic oxidative injury during NEC.METHODS: NEC was induced in C57BL/6 mice (approval: 44032) by hypoxia, gavage feeding with hyperosmolar formula, and lipopolysaccharide administration from postnatal days 5 to 9 (n = 15). Two additional groups with hypoxia only (n = 10) and hypoxia and hyperosmolar formula (n = 13) were also examined. Breastfed pups were used as control (n = 15). Liver was harvested on postnatal day 9. Gene expressions of mtDNA markers cytochrome c oxidase subunit 3 (COX3), cytochrome b (CYTB) and NADH-ubiquinone oxidoreductase chain 1 (ND1) were measured by real-time qPCR. Mitochondrial morphology marker HSP60 and oxidative stress marker NRF2 were detected by immunofluorescence staining and compared between NEC and control. Data were presented as mean ± SD and compared using Student's t test; p < 0.05 was considered significant.
    RESULTS: Gene expression of mtDNA markers (COX3, CYTB, and ND1) were significantly decreased in the liver of NEC mice relative to control, hypoxia alone, and hypoxia with hyperosmolar formula. Immunofluorescence showed depletion of HSP60 indicating decreased mitochondria in NEC liver relative to control. Furthermore, a higher protein expression of NRF2 was observed indicating higher oxidative stress in NEC liver relative to control.
    CONCLUSIONS: Intestinal injury in experimental NEC leads to a systemic inflammatory response affecting the liver. Hepatic oxidative injury in NEC is characterized by decreased mitochondria and mtDNA depletion. This study provides insight into the mechanism of liver injury in NEC.
    Keywords:  Hepatic oxidative damage; Necrotizing enterocolitis (NEC); Reactive oxygen species
  18. Front Nutr. 2020 ;7 585484
      Sucralose is a non-caloric artificial sweetener widely used in processed foods that reportedly affects energy homeostasis through partially understood mechanisms. Mitochondria are organelles fundamental for cellular bioenergetics that are closely related to the development of metabolic diseases. Here, we addressed whether sucralose alters mitochondrial bioenergetics in the enterocyte cell line Caco-2. Sucralose exposure (0.5-50 mM for 3-24 h) increased cellular reductive power assessed through MTT assay, suggesting enhanced bioenergetics. Low doses of sucralose (0.5 and 5 mM) for 3 h stimulated mitochondrial respiration, measured through oxygraphy, and elevated mitochondrial transmembrane potential and cytoplasmic Ca2+, evaluated by fluorescence microscopy. Contrary to other cell types, the increase in mitochondrial respiration was insensitive to inhibition of mitochondrial Ca2+ uptake. These findings suggest that sucralose alters enterocyte energy homeostasis, contributing to its effects on organismal metabolism.
    Keywords:  Ca2+; artificial sweetener; metabolism; mitochondria; sucralose
  19. Cell Physiol Biochem. 2021 Feb 06. 55(1): 91-116
      BACKGROUND/AIMS: Signaling and metabolic perturbations contribute to dysregulated skeletal muscle protein homeostasis and secondary sarcopenia in response to a number of cellular stressors including ethanol exposure. Using an innovative multiomics-based curating of unbiased data, we identified molecular and metabolic therapeutic targets and experimentally validated restoration of protein homeostasis in an ethanol-fed mouse model of liver disease.METHODS: Studies were performed in ethanol-treated differentiated C2C12 myotubes and physiological relevance established in an ethanol-fed mouse model of alcohol-related liver disease (mALD) or pair-fed control C57BL/6 mice. Transcriptome and proteome from ethanol treated-myotubes and gastrocnemius muscle from mALD and pair-fed mice were analyzed to identify target pathways and molecules. Readouts including signaling responses and autophagy markers by immunoblots, mitochondrial oxidative function and free radical generation, and metabolic studies by gas chromatography-mass spectrometry and sarcopenic phenotype by imaging.
    RESULTS: Multiomics analyses showed that ethanol impaired skeletal muscle mTORC1 signaling, mitochondrial oxidative pathways, including intermediary metabolite regulatory genes, interleukin-6, and amino acid degradation pathways are β-hydroxymethyl-butyrate targets. Ethanol decreased mTORC1 signaling, increased autophagy flux, impaired mitochondrial oxidative function with decreased tricarboxylic acid cycle intermediary metabolites, ATP synthesis, protein synthesis and myotube diameter that were reversed by HMB. Consistently, skeletal muscle from mALD had decreased mTORC1 signaling, reduced fractional and total muscle protein synthesis rates, increased autophagy markers, lower intermediary metabolite concentrations, and lower muscle mass and fiber diameter that were reversed by β-hydroxymethyl-butyrate treatment.
    CONCLUSION: An innovative multiomics approach followed by experimental validation showed that β-hydroxymethyl-butyrate restores muscle protein homeostasis in liver disease.
    Keywords:  Autophagy; Mitochondria; Pathway-analyses; Protein synthesis; Proteomics; Transcriptomics
  20. Cardiovasc Res. 2021 Feb 01. pii: cvab034. [Epub ahead of print]
      AIMS: Proteostasis maintains protein homeostasis and participates in regulating critical cardiometabolic disease risk factors, including proprotein convertase subtilisin/kexin type 9 (PCSK9). Endoplasmic reticulum (ER) remodeling through release and incorporation of trafficking vesicles mediates protein secretion and degradation. We hypothesized that ER remodeling that drives mitochondrial fission participates in cardiometabolic proteostasis.METHODS AND RESULTS: We used in vitro and in vivo hepatocyte inhibition of a protein involved in mitochondrial fission, dynamin-related protein 1 (DRP1). Here, we show that DRP1 promotes remodeling of select ER microdomains by tethering vesicles at ER. A DRP1 inhibitor, mitochondrial division inhibitor 1 (mdivi-1) reduced ER localization of a DRP1 receptor, mitochondrial fission factor, suppressing ER remodeling-driven mitochondrial fission, autophagy, and increased mitochondrial calcium buffering and PCSK9 proteasomal degradation. DRP1 inhibition by CRISPR/Cas9 deletion or mdivi-1 alone or in combination with statin incubation in human hepatocytes and hepatocyte-specific Drp1-deficiency in mice reduced PCSK9 secretion (-78.5%). In HepG2 cells, mdivi-1 increased low-density lipoprotein receptor via c-Jun transcription and reduced PCSK9 mRNA levels via suppressed sterol regulatory binding protein-1c. Additionally, mdivi-1 reduced macrophage burden, oxidative stress, and advanced calcified atherosclerotic plaque in aortic roots of diabetic Apoe-deficient mice and inflammatory cytokine production in human macrophages.
    CONCLUSIONS: We propose a novel tethering function of DRP1 beyond its established fission function, with DRP1-mediated ER remodeling likely contributing to ER constriction of mitochondria that drives mitochondrial fission. We report DRP1-driven remodeling of select ER microdomains may critically regulate hepatic proteostasis and identify mdivi-1 as a novel small molecule PCSK9 inhibitor.
    TRANSLATIONAL PERSPECTIVE: PCSK9 is a critical protein participating in degradation of low-density lipoprotein receptor, a receptor involved in clearance of circulating low-density lipoprotein. Anti-PCSK9 therapies approved for clinical use are currently limited to antibody therapies. PCSK9 siRNA therapy is also showing promise in clinical trials, but small molecule PCSK9 inhibitors have proven difficult to develop. This study identifies a small molecule inhibitor of a mitochondrial fission protein, DRP1 in human hepatocytes and hepatocyte DRP1-deficiency in mice reduces PCSK9 secretion, providing initial proof-of-concept for novel small molecule PCSK9 inhibition.
  21. Cell Metab. 2021 Feb 02. pii: S1550-4131(21)00005-X. [Epub ahead of print]
      The haploinsufficiency of C9orf72 is implicated in the most common forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), but the full spectrum of C9orf72 functions remains to be established. Here, we report that C9orf72 is a mitochondrial inner-membrane-associated protein regulating cellular energy homeostasis via its critical role in the control of oxidative phosphorylation (OXPHOS). The translocation of C9orf72 from the cytosol to the inter-membrane space is mediated by the redox-sensitive AIFM1/CHCHD4 pathway. In mitochondria, C9orf72 specifically stabilizes translocase of inner mitochondrial membrane domain containing 1 (TIMMDC1), a crucial factor for the assembly of OXPHOS complex I. C9orf72 directly recruits the prohibitin complex to inhibit the m-AAA protease-dependent degradation of TIMMDC1. The mitochondrial complex I function is impaired in C9orf72-linked ALS/FTD patient-derived neurons. These results reveal a previously unknown function of C9orf72 in mitochondria and suggest that defective energy metabolism may underlie the pathogenesis of relevant diseases.
    Keywords:  ALS; C9orf72; FTD; OXPHOS; TIMMDC1; complex I; mitochondrial import; mitochondrion; neurodegeneration; oxidative phosphorylation
  22. Mol Metab. 2021 Jan 28. pii: S2212-8778(21)00013-2. [Epub ahead of print] 101173
      OBJECTIVE: Brown adipose tissue (BAT) thermogenesis offers the potential to improve metabolic health in mice and men. However, humans predominantly live under thermoneutral conditions, leading to BAT whitening - a reduction in BAT mitochondrial content and metabolic activity. Recent studies have established mitophagy as a major driver of mitochondrial degradation in the whitening of thermogenic brite/beige adipocytes, yet the pathways mediating mitochondrial breakdown in whitening of classical BAT remain largely elusive. The transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy belonging to the MIT family of transcription factors, is the only member of this family that is upregulated during whitening, pointing towards a role of TFEB in whitening-associated mitochondrial breakdown.METHODS: We generated brown adipocyte-specific TFEB knockout mice, and induced BAT whitening by thermoneutral housing. We characterized gene and protein expression patterns, BAT metabolic activity, systemic metabolism as well as mitochondrial localization using in vivo and in vitro approaches.
    RESULTS: Under conditions of low thermogenic activation, deletion of TFEB preserved mitochondrial mass independently of mitochondriogenesis in BAT and primary brown adipocytes. This did however not translate into elevated thermogenic capacity or protection from diet-induced obesity. Autophagosomal/lysosomal marker levels were altered in TFEB-deficient BAT and primary adipocytes, and lysosomal markers co-localized and co-purified with mitochondria in TFEB-deficient BAT, indicating trapping of mitochondria in late stages of mitophagy.
    CONCLUSION: We here identify TFEB as a driver of BAT whitening, mediating mitochondrial degradation via the autophagosomal and lysosomal machinery. This study provides proof of concept that interfering with the mitochondrial degradation machinery can increase mitochondrial mass in classical BAT under human-relevant conditions. It must however be considered that interfering with autophagy may result in accumulation of non-functional mitochondria. Future studies targeting earlier steps of mitophagy or target recognition are therefore warranted.
    Keywords:  TFEB; UCP1; brown adipose tissue; mitophagy; thermogenesis; whitening
  23. Sci Rep. 2021 Feb 03. 11(1): 2975
      We aim to determine the impact of an artificial liver support system (ALSS) treatment before liver transplantation (LT), and identify the prognostic factors and evaluate the predictive values of the current commonly used ACLF prognostic models for short-term prognosis after LT. Data from 166 patients who underwent LT with acute-on-chronic liver failure (ACLF) were retrospectively collected from January 2011 to December 2018 from the First Affiliated Hospital of Zhejiang University School of Medicine. Patients were divided into two groups depending on whether they received ALSS treatment pre-LT. In the observation group, liver function tests and prognostic scores were significantly lower after ALSS treatment, and the waiting time for a donor liver was significantly longer than that of the control group. Both intraoperative blood loss and period of postoperative ICU care were significantly lower; however, there were no significant differences between groups in terms of total postoperative hospital stays. Postoperative 4-week and 12-week survival rates in the observation group were significantly higher than those of the control group. Similar trends were also observed at 48 and 96 weeks, however, without significant difference. Multivariate Cox regression analysis of the risk factors related to prognosis showed that preoperative ALSS treatment, neutrophil-lymphocyte ratio, and intraoperative blood loss were independent predicting factors for 4-week survival rate after transplantation. ALSS treatment combined with LT in patients with HBV-related ACLF improved short-term survival. ALSS treatment pre-LT is an independent protective factor affecting the 4-week survival rate after LT.
  24. Front Cell Infect Microbiol. 2020 ;10 593805
      The mitochondrial network plays a critical role in the regulation of innate immune signaling and subsequent production of proinflammatory cytokines such as IFN-β and IL-1β. Dynamin-related protein 1 (DRP1) promotes mitochondrial fission and quality control to maintain cellular homeostasis during infection. However, mechanisms by which DRP1 and mitochondrial dynamics control innate immune signaling and the proinflammatory response are incompletely understood. Here we show that macrophage DRP1 is a positive regulator of TNF-α production during sterile inflammation or bacterial infection. Silencing macrophage DRP1 decreased mitochondrial fragmentation and TNF-α production upon stimulation with lipopolysaccharide (LPS) or methicillin-resistant Staphylococcus aureus (MRSA) infection. The defect in TNF-α induction could not be attributed to changes in gene expression. Instead, DRP1 was required for post-transcriptional control of TNF-α. In contrast, silencing DRP1 enhanced IL-6 and IL-1β production, indicating a distinct mechanism for DRP1-dependent TNF-α regulation. Our results highlight DRP1 as a key player in the macrophage pro-inflammatory response and point to its involvement in post-transcriptional control of TNF-α production.
    Keywords:  cellular stress; cytokine; inflammation; macrophage; mitochondria
  25. J Neuroimmunol. 2021 Jan 24. pii: S0165-5728(21)00023-0. [Epub ahead of print]353 577496
      Microglia-driven neuroinflammation contributes to neurodegenerative diseases. Mitochondrial phospholipid cardiolipin acts as a signaling molecule when released from damaged cells. We demonstrate that extracellular cardiolipin induces the secretion of monocyte chemoattractant protein-1 and interferon gamma-induced protein 10 by resting microglia while inhibiting secretion of cytokines by microglia stimulated with lipopolysaccharide, amyloid Aβ42 peptides, or α-synuclein. Extracellular cardiolipin also induces nitric oxide secretion by microglia-like cells and upregulates microglial phagocytosis. By using blocking antibodies, we determine that toll-like receptor 4 mediates the latter effect. Under physiological and pathological conditions characterized by cell death, extracellularly released cardiolipin may regulate immune responses of microglia.
    Keywords:  Alzheimer's disease; DAMPs; Neurodegeneration; Neuroinflammation; Neuroprotection; Parkinson's disease
  26. EMBO J. 2021 Feb 02. e105268
      Mitochondrial translation dysfunction is associated with neurodegenerative and cardiovascular diseases. Cells eliminate defective mitochondria by the lysosomal machinery via autophagy. The relationship between mitochondrial translation and lysosomal function is unknown. In this study, mitochondrial translation-deficient hearts from p32-knockout mice were found to exhibit enlarged lysosomes containing lipofuscin, suggesting impaired lysosome and autolysosome function. These mice also displayed autophagic abnormalities, such as p62 accumulation and LC3 localization around broken mitochondria. The expression of genes encoding for nicotinamide adenine dinucleotide (NAD+ ) biosynthetic enzymes-Nmnat3 and Nampt-and NAD+ levels were decreased, suggesting that NAD+ is essential for maintaining lysosomal acidification. Conversely, nicotinamide mononucleotide (NMN) administration or Nmnat3 overexpression rescued lysosomal acidification. Nmnat3 gene expression is suppressed by HIF1α, a transcription factor that is stabilized by mitochondrial translation dysfunction, suggesting that HIF1α-Nmnat3-mediated NAD+ production is important for lysosomal function. The glycolytic enzymes GAPDH and PGK1 were found associated with lysosomal vesicles, and NAD+ was required for ATP production around lysosomal vesicles. Thus, we conclude that NAD+ content affected by mitochondrial dysfunction is essential for lysosomal maintenance.
    Keywords:  GAPDH; NAD+; Nmnat3; lysosome; mitochondria
  27. J Biol Chem. 2020 May 22. pii: S0021-9258(17)50259-8. [Epub ahead of print]295(21): 7235-7248
      The mitochondrion of malaria parasites contains several clinically validated drug targets. Within Plasmodium spp., the causative agents of malaria, the mitochondrial DNA (mtDNA) is only 6 kb long, being the smallest mitochondrial genome among all eukaryotes. The mtDNA encodes only three proteins of the mitochondrial electron transport chain and ∼27 small, fragmented rRNA genes having lengths of 22-195 nucleotides. The rRNA fragments are thought to form a mitochondrial ribosome (mitoribosome), together with ribosomal proteins imported from the cytosol. The mitoribosome of Plasmodium falciparum is essential for maintenance of the mitochondrial membrane potential and parasite viability. However, the role of the mitoribosome in sustaining the metabolic status of the parasite mitochondrion remains unclear. The small ribosomal subunit in P. falciparum has 14 annotated mitoribosomal proteins, and employing a CRISPR/Cas9-based conditional knockdown tool, here we verified the location and tested the essentiality of three candidates (PfmtRPS12, PfmtRPS17, and PfmtRPS18). Using immuno-EM, we provide evidence that the P. falciparum mitoribosome is closely associated with the mitochondrial inner membrane. Upon knockdown of the mitoribosome, parasites became hypersensitive to inhibitors targeting mitochondrial Complex III (bc1), dihydroorotate dehydrogenase (DHOD), and the F1F0-ATP synthase complex. Furthermore, the mitoribosome knockdown blocked the pyrimidine biosynthesis pathway and reduced the cellular pool of pyrimidine nucleotides. These results suggest that disruption of the P. falciparum mitoribosome compromises the metabolic capacity of the mitochondrion, rendering the parasite hypersensitive to a panel of inhibitors that target mitochondrial functions.
    Keywords:  CRISPR/Cas; S12; S17; S18; malaria; metabolism; mitochondria; mitochondrial ribosome; mtDNA; plasmodium; ribosome
  28. J Hepatol. 2021 Jan 27. pii: S0168-8278(21)00040-4. [Epub ahead of print]
      BACKGROUND & AIM: Besides their physiological role in bile formation and fat digestion, bile acids (BAs) synthesized from cholesterol in hepatocytes act as signaling molecules that modulate hepatocellular carcinoma (HCC). Trafficking of cholesterol to mitochondria through steroidogenic acute regulatory protein 1 (STARD1) is the rate-limiting step in the alternative pathway of BAs generation, whose physiological relevance is not well understood. Moreover, the specific contribution of the STARD1-dependent BA synthesis pathway to HCC has not been explored.METHODS: STARD1 expression was analyzed in a cohort of human NASH derived HCC specimens. Experimental NASH-driven HCC models included MUP-uPA mice fed high fat high cholesterol diet (HFHC) and diethylnitrosamine (DEN) treatment in wild type (WT) mice fed HFHC. Molecular species of BAs and oxysterols were analyzed by mass spectrometry. Effects of NASH-derived BAs profile were investigated in tumor-initiated stem-like cells (TICs) and primary mouse hepatocytes (PMH).
    RESULTS: We show that patients with NASH-associated HCC exhibit increased hepatic expression of STARD1 and enhanced BAs pool. Using NASH-driven HCC models, STARD1 overexpression in WT mice increased liver tumor multiplicity, whereas hepatocyte-specific STARD1 deletion (Stard1ΔHep) in WT or MUP-uPA mice reduced tumor burden. These findings mirrored the levels of unconjugated primary BAs, β-muricholic acid and cholic acid, and their tauroconjugates in STARD1 overexpressing and Stard1ΔHep mice. Incubation of TICs or PMH with a mix of BAs mimicking this profile stimulated expression of genes involved in pluripotency, stemness and inflammation.
    CONCLUSIONS: We show a previously unrecognized role of STARD1 in HCC pathogenesis by promoting the synthesis of primary BAs through the mitochondrial pathway, whose products act in TICs to stimulate self-renewal, stemness and inflammation.
    Keywords:  Cholesterol; STARD1; bile acids; hepatocellular carcinoma; mitochondria; oxysterols
  29. Front Oncol. 2020 ;10 600113
      Ovarian cancer is the deadliest gynecological cancer in women, with a survival rate of less than 30% when the cancer has spread throughout the peritoneal cavity. Aggregation of cancer cells increases their viability and metastatic potential; however, there are limited studies that correlate these functional changes to specific phenotypic alterations. In this study, we investigated changes in mitochondrial morphology and dynamics during malignant transition using our MOSE cell model for progressive serous ovarian cancer. Mitochondrial morphology was changed with increasing malignancy from a filamentous network to single, enlarged organelles due to an imbalance of mitochondrial dynamic proteins (fusion: MFN1/OPA1, fission: DRP1/FIS1). These phenotypic alterations aided the adaptation to hypoxia through the promotion of autophagy and were accompanied by changes in the mitochondrial ultrastructure, mitochondrial membrane potential, and the regulation of reactive oxygen species (ROS) levels. The tumor-initiating cells increased mitochondrial fragmentation after aggregation and exposure to hypoxia that correlated well with our previously observed reduced growth and respiration in spheroids, suggesting that these alterations promote viability in non-permissive conditions. Our identification of such mitochondrial phenotypic changes in malignancy provides a model in which to identify targets for interventions aimed at suppressing metastases.
    Keywords:  fission; fragmentation; fusion; hypoxia; mitophagy; reactive oxygen species; spheroids; uncoupling protein
  30. Bioessays. 2021 Feb 04. e2000165
      It has been assumed that at the whole organismal level, the mitochondrial reactive oxygen species (ROS) production is proportional to the oxygen consumption. Recently, a number of researchers have challenged this assumption, based on the observation that the ROS production per unit oxygen consumed in the resting state of mitochondrial respiration is much higher than that in the active state. Here, we develop a simple model to investigate the validity of the assumption and the challenge of it. The model highlights the significance of the time budget that mitochondria operate in the different respiration states. The model suggests that under three physiologically possible conditions, the difference in ROS production per unit oxygen consumed between the respiration states does not upset the proportionality between the whole animal ROS production and oxygen consumption. The model also shows that mitochondrial uncoupling generally enhances the proportionality.
    Keywords:  mitochondria; respiration states; theoretical model; variation;  uncoupling
  31. Biomed Res Int. 2021 ;2021 8880179
      Regulated necrosis (necroptosis) is crucially involved in cardiac ischaemia-reperfusion injury (MIRI). The aim of our study is to investigate whether shock wave therapy (SWT) is capable of exerting protective effects by inhibiting necroptosis during myocardial ischaemia-reperfusion (I/R) injury and the possible role of autophagy in this process. We established a hypoxia/reoxygenation (H/R) model in vitro using HL-1 cells to simulate MIRI. MTS assays and LDH cytotoxicity assay were performed to measure cell viability and cell damage. Annexin V/PI staining was used to determine apoptosis and necrosis. Western blotting was performed to assess the changes in cell signaling pathways associated with autophagy, necroptosis, and apoptosis. Reactive oxygen species (ROS) production was detected using DHE staining. Autophagosome generation and degradation (autophagic flux) were analysed using GFP and RFP tandemly tagged LC3 (tfLC3). HL-1 cells were then transfected with p62/SQSTM1 siRNA in order to analyse its role in cardioprotection. Our results revealed that SWT increased cell viability in the H/R model and decreased receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and RIPK3 expression. ROS production was also inhibited by SWT. Moreover, SWT decreased Beclin1 expression and the ratio of LC3-II/LC3-I following H/R. Simultaneously, in the tfLC3 assay, the SWT provoked a decrease in the cumulative autophagosome abundance. siRNA-mediated knockdown of p62 attenuated H/R-induced necroptosis, and SWT did not exert additive effects. Taken together, SWT ameliorated H/R injury by inhibiting necroptosis. SWT also relieved the blockade of autophagic flux in response to H/R injury. The restoration of autophagic flux by SWT might contribute to its cardioprotective effect on necroptosis following H/R injury.
  32. Cell Calcium. 2021 Jan 23. pii: S0143-4160(21)00010-5. [Epub ahead of print]94 102356
      Voltage-dependent anion channel (VDAC), the most abundant mitochondrial outer membrane protein, is important for a variety of mitochondrial functions including metabolite exchange, calcium transport, and apoptosis. While VDAC's role in shuttling metabolites between the cytosol and mitochondria is well established, there is a growing interest in understanding the mechanisms of its regulation of mitochondrial calcium transport. Here we review the current literature on VDAC's role in calcium signaling, its biophysical properties, physiological function, and pathology focusing on its importance in cardiac diseases. We discuss the specific biophysical properties of the three VDAC isoforms in mammalian cells-VDAC 1, 2, and 3-in relationship to calcium transport and their distinct roles in cell physiology and disease. Highlighting the emerging evidence that cytosolic proteins interact with VDAC and regulate its calcium permeability, we advocate for continued investigation into the VDAC interactome at the contact sites between mitochondria and organelles and its role in mitochondrial calcium transport.
    Keywords:  Calcium signaling; Ion selectivity; Mitochondrial associated membrane; Mitochondrial outer membrane; Protein-protein interaction; Voltage gating; Voltage-dependent anion channel; α-synuclein
  33. Autophagy. 2021 Feb 02.
      Mitochondrial autophagy (mitophagy) selectively degrades mitochondria and plays an important role in mitochondrial homeostasis. In the yeast Saccharomyces cerevisiae, the phosphorylation of the mitophagy receptor Atg32 by casein kinase 2 is essential for mitophagy, whereas this phosphorylation is counteracted by the protein phosphatase Ppg1. Although Ppg1 functions cooperatively with the Far complex (Far3, Far7, Far8, Vps64/Far9, Far10 and Far11), their relationship and the underlying phosphoregulatory mechanism of Atg32 remain unclear. Our recent study revealed: (i) the Far complex plays its localization-dependent roles, regulation of mitophagy and target of rapamycin complex 2 (TORC2) signaling, via the mitochondria- and endoplasmic reticulum (ER)-localized Far complexes, respectively; (ii) Ppg1 and Far11 form a subcomplex, and Ppg1 activity is required to assemble the sub- and core-Far complexes; (iii) association and dissociation between the Far complex and Atg32 are crucial determinants for mitophagy regulation. Here, we summarize our findings and discuss unsolved issues.
    Keywords:  Atg32; Far complex; Ppg1; mitochondria; mitophagy; yeast
  34. Biomolecules. 2021 Jan 28. pii: 173. [Epub ahead of print]11(2):
      Recent studies undoubtedly show the importance of inter organellar connections to maintain cellular homeostasis. In normal physiological conditions or in the presence of cellular and environmental stress, each organelle responds alone or in coordination to maintain cellular function. The Endoplasmic reticulum (ER) and mitochondria are two important organelles with very specialized structural and functional properties. These two organelles are physically connected through very specialized proteins in the region called the mitochondria-associated ER membrane (MAM). The molecular foundation of this relationship is complex and involves not only ion homeostasis through the shuttling of calcium but also many structural and apoptotic proteins. IRE1alpha and PERK are known for their canonical function as an ER stress sensor controlling unfolded protein response during ER stress. The presence of these transmembrane proteins at the MAM indicates its potential involvement in other biological functions beyond ER stress signaling. Many recent studies have now focused on the non-canonical function of these sensors. In this review, we will focus on ER mitochondrial interdependence with special emphasis on the non-canonical role of ER stress sensors beyond ER stress.
    Keywords:  ER stress; endoplasmic reticulum; mitochondria associated membrane (MAM)