bims-mikwok Biomed News
on Mitochondrial quality control
Issue of 2021‒09‒19
seven papers selected by
Avinash N. Mukkala
University of Toronto

  1. J Biol Chem. 2021 Sep 13. pii: S0021-9258(21)00998-4. [Epub ahead of print] 101196
      Mitochondria undergo continuous cycles of fission and fusion to promote inheritance, regulate quality control, and mitigate organelle stress. More recently, this process of mitochondrial dynamics has been demonstrated to be highly sensitive to nutrient supply, ultimately conferring bioenergetic plasticity to the organelle. However, whether regulators of mitochondrial dynamics play a causative role in nutrient regulation remains unclear. In this study, we generated a cellular loss-of-function model for dynamin-related protein 1 (DRP1), the primary regulator of outer membrane mitochondrial fission. Loss of DRP1 (shDRP1) resulted in extensive ultrastructural and functional remodeling of mitochondria, characterized by pleomorphic enlargement, increased electron density of the matrix, and defective NADH and succinate oxidation. Despite increased mitochondrial size and volume, shDRP1 cells exhibited reduced cellular glucose uptake and mitochondrial fatty acid oxidation. Untargeted transcriptomic profiling revealed severe downregulation of genes required for cellular and mitochondrial calcium homeostasis, inhibition of ATP-stimulated calcium flux, and impaired substrate oxidation stimulated by calcium levels. The insights obtained herein suggest that DRP1 regulates fatty acid oxidation by altering whole-cell and mitochondrial calcium dynamics. These findings are relevant to the targetability of mitochondrial fission and have clinical relevance in the identification of treatments for fission-related pathologies such as hereditary neuropathies, inborn errors in metabolism, cancer, and chronic diseases.
    Keywords:  calcium signaling; dynamin-related protein 1; mitochondrial dynamics; skeletal muscle; β-oxidation
  2. Redox Biol. 2021 Sep 10. pii: S2213-2317(21)00284-6. [Epub ahead of print]46 102125
      Heme is an essential cofactor required for a plethora of cellular processes in eukaryotes. In metazoans the heme biosynthetic pathway is typically partitioned between the cytosol and mitochondria, with the first and final steps taking place in the mitochondrion. The pathway has been extensively studied and its biosynthetic enzymes structurally characterized to varying extents. Nevertheless, understanding of the regulation of heme synthesis and factors that influence this process in metazoans remains incomplete. Therefore, we investigated the molecular organization as well as the physical and genetic interactions of the terminal pathway enzyme, ferrochelatase (Hem15), in the yeast Saccharomyces cerevisiae. Biochemical and genetic analyses revealed dynamic association of Hem15 with Mic60, a core component of the mitochondrial contact site and cristae organizing system (MICOS). Loss of MICOS negatively impacts Hem15 activity, affects the size of the Hem15 high-mass complex, and results in accumulation of reactive and potentially toxic tetrapyrrole precursors that may cause oxidative damage. Restoring intermembrane connectivity in MICOS-deficient cells mitigates these cytotoxic effects. These data provide new insights into how heme biosynthetic machinery is organized and regulated, linking mitochondrial architecture-organizing factors to heme homeostasis.
    Keywords:  Ferrochelatase; Heme; MICOS; Mitochondria; Yeast
  3. Hepatology. 2021 Sep 12.
      BACKGROUND AND AIMS: Hepatic ischemia-reperfusion injury (IRI) is the leading cause of early post-transplantation organ failure, as mitochondrial respiration and ATP production are affected. Shortage of donors has extended liver donor criteria, including aged or steatotic livers, which are more susceptible to IRI. Given the lack of an effective treatment and the extensive transplantation waitlist, we aimed at characterizing the effects of an accelerated mitochondrial activity by silencing Methylation-controlled J protein (MCJ) in three pre-clinical models of IRI and liver regeneration, focusing on metabolically compromised animal models.APPROACH AND RESULTS: Wt, MCJ KO and Mcj silenced Wt mice were subjected to 70% Partial hepatectomy (Phx), prolonged IRI and 70% Phx with IRI. Old and mice with metabolic syndrome were also subjected to these procedures. Expression of MCJ, an endogenous negative regulator of mitochondrial respiration, increases in pre-clinical models of Phx with or without vascular occlusion, and in donors' livers. Mice lacking MCJ initiate liver regeneration 12h faster than WT, show reduced ischemic injury and increased survival. MCJ knockdown enables a mitochondrial adaptation that restores the bioenergetic supply for enhanced regeneration and prevents cell death after IRI. Mechanistically, increased ATP secretion facilitates the early activation of kupffer cells and production of TNF, IL-6 and HB-EGF accelerating the priming phase and the progression through G1/S transition during liver regeneration. Therapeutic silencing of MCJ in 15-month-old mice and in mice fed with a high fat-high fructose diet for 12 weeks improves mitochondrial respiration, reduces steatosis and overcomes regenerative limitations.
    CONCLUSIONS: Boosting mitochondrial activity by silencing MCJ could pave the way for a novel protective approach after major liver resection or IRI, specially in metabolically compromised, IRI susceptible organs.
    Keywords:  adenosine triphosphate; inflammation; ischemia-reperfusion injury; liver transplantation; non-alcoholic steatohepatitis
  4. J Vis Exp. 2021 Aug 27.
      Mitochondrial energetics is a central theme in animal biochemistry and physiology, with researchers using mitochondrial respiration as a metric to investigate metabolic capability. To obtain the measures of mitochondrial respiration, fresh biological samples must be used, and the entire laboratory procedure must be completed within approximately 2 h. Furthermore, multiple pieces of specialized equipment are required to perform these laboratory assays. This creates a challenge for measuring mitochondrial respiration in the tissues of wild animals living far from physiology laboratories as live tissue cannot be preserved for very long after collection in the field. Moreover, transporting live animals over long distances induces stress, which can alter mitochondrial energetics. This manuscript introduces the Auburn University (AU) MitoMobile, a mobile mitochondrial physiology laboratory that can be taken into the field and used on-site to measure mitochondrial metabolism in tissues collected from wild animals. The basic features of the mobile laboratory and the step-by-step methods for measuring isolated mitochondrial respiration rates are presented. Additionally, the data presented validate the success of outfitting the mobile mitochondrial physiology laboratory and making mitochondrial respiration measurements. The novelty of the mobile laboratory lies in the ability to drive to the field and perform mitochondrial measurements on the tissues of animals captured on site.
  5. Cell Rep. 2021 Sep 14. pii: S2211-1247(21)01151-7. [Epub ahead of print]36(11): 109704
      Histone variants are crucial regulators of chromatin structure and gene transcription, yet their functions within the brain remain largely unexplored. Here, we show that the H2A histone variant H2A.Z is essential for neuronal survival. Mice lacking H2A.Z in GABAergic neurons or Purkinje cells (PCs) present with a progressive cerebellar ataxia accompanied by widespread degeneration of PCs. Ablation of H2A.Z in other neuronal subtypes also triggers cell death. H2A.Z binds to the promoters of key nuclear-encoded mitochondrial genes to regulate their expression and promote organelle function. Bolstering mitochondrial activity genetically or by organelle transplant enhances the survival of H2A.Z-ablated neurons. Changes in bioenergetic status alter H2A.Z occupancy at the promoters of nuclear-encoded mitochondrial genes, an adaptive response essential for cell survival. Our results highlight that H2A.Z fulfills a key, conserved role in neuronal survival by acting as a transcriptional rheostat to regulate the expression of genes critical to mitochondrial function.
    Keywords:  H2A.Z; Purkinje cells; bioenergetics; cerebellar ataxia; epigenetics; histone variants; mitochondria; neurodegeneration; neurons
  6. Mitochondrion. 2021 Sep 08. pii: S1567-7249(21)00119-7. [Epub ahead of print]
      Mitochondrial biogenesis in the brain is impaired in various neurological disorders including traumatic brain injury (TBI). The long-lasting effects of TBI may be, in part, attributed to epigenetic mechanisms such as DNA methylation. However, the role of DNA methylation on regulatory elements of nuclear and mitochondrial genome in mitochondrial biogenesis is not known. We examined the epigenetic regulation of mitochondrial transcription factor A (TFAM), and further probed its implications in mitochondrial dysfunction in the hippocampus of rats subjected to repeated mild TBI (rMTBI) using weight drop injury paradigm. rMTBI-induced hypermethylation at TFAM promoter resulted in deficits in its protein levels in mitochondria after immediate (48 h) and protracted (30 d) time points. Further, rMTBI also caused hypomethylation of mitochondrial DNA (mtDNA) promoters (HSP1 and HSP2), which further culminated into low binding of TFAM. rMTBI-induced changes weakened mitochondrial biogenesis in terms of reduced mtDNA-encoded rRNA, mRNA, and protein levels leading to shortages of ATP. To verify the potential role of mtDNA methylation in rMTBI-induced persistent mitochondrial dysfunction, rMTBI-induced rats were treated with methionine, a methyl donor. Methionine treatment restored the methylation levels on HSP1 and HSP2 resulting in efficient binding of TFAM and normalized the rRNA, mRNA, and protein levels. These findings suggest the crucial role of DNA methylation at nuclear and mitochondrial promoter regions in mitochondrial gene expression and ATP activity in the hippocampus after rMTBI.
    Keywords:  DNA methylation; Heavy strand promoter; Mild traumatic brain injury; Mitochondrial biogenesis; Mitochondrial transcription factor A; mitochondrial DNA
  7. Ann Transl Med. 2021 Aug;9(16): 1295
      Background: Hepatic steatosis creates a significant risk of liver resection and transplantation and is extremely susceptible to ischemia/reperfusion (I/R) injury. Ischemic postconditioning (IPostC) has been shown to attenuate I/R injury in normal livers; however, its role in steatotic livers remains unknown. The current study sought to explore whether IPostC could attenuate normothermic I/R injury in rats with steatotic livers and to investigate potential protective measures.Methods: Hepatic steatosis was triggered in Wistar rats fed high-fat diets. The role of IPostC was detected in normal and steatotic livers with 30 min of ischemia and 6 h of reperfusion. Blood and liver tissues were collected to assess hepatocyte damage, lipid peroxidation, inflammatory factors, neutrophil accumulation, and adenosine triphosphate (ATP) content.
    Results: Compared to normal livers, steatotic livers were more susceptible to I/R damage, as evidenced by incremental concentrations of liver enzymes in the blood and more severe pathological changes in the liver. Hepatic I/R injury was significantly reduced by IPostC in both normal and steatotic livers. We further found that endogenous protective measures moderated lipid peroxidation, inflammatory cytokine expression and neutrophil accumulation, and reduced follow-up hepatic injury. The ATP content of steatotic livers was also significantly lower than that of Normal livers before and after I/R injury. IPostC greatly preserved the ATP content of normal and steatotic livers with I/R injury.
    Conclusions: IPostC appears to provide important protection against hepatic I/R injury in normal and steatotic livers under normothermic conditions. These data have important clinical implications for liver surgery and transplantation.
    Keywords:  Steatosis; ischemia reperfusion injury; ischemic postconditioning (IPostC); liver