bims-mitdyn Biomed News
on Mitochondrial dynamics: mechanisms
Issue of 2021‒07‒04
twelve papers selected by
Edmond Chan
Queen’s University, School of Medicine

  1. PLoS Genet. 2021 Jul 02. 17(7): e1009664
      Mitochondrial defects can cause a variety of human diseases and protective mechanisms exist to maintain mitochondrial functionality. Imbalances in mitochondrial proteostasis trigger a transcriptional program, termed mitochondrial unfolded protein response (mtUPR). However, the temporal sequence of events in mtUPR is unclear and the consequences on mitochondrial protein import are controversial. Here, we have quantitatively analyzed all main import pathways into mitochondria after different time spans of mtUPR induction. Kinetic analyses reveal that protein import into all mitochondrial subcompartments strongly increases early upon mtUPR and that this is accompanied by rapid remodelling of the mitochondrial signature lipid cardiolipin. Genetic inactivation of cardiolipin synthesis precluded stimulation of protein import and compromised cellular fitness. At late stages of mtUPR upon sustained stress, mitochondrial protein import efficiency declined. Our work clarifies the enigma of protein import upon mtUPR and identifies sequential mtUPR stages, in which an early increase in protein biogenesis to restore mitochondrial proteostasis is followed by late stages characterized by a decrease in import capacity upon prolonged stress induction.
  2. Elife. 2021 Jun 30. pii: e65215. [Epub ahead of print]10
      The spatiotemporal distribution of mitochondria is crucial for precise ATP provision and calcium buffering required to support neuronal signaling. Fast-spiking GABAergic interneurons expressing parvalbumin (PV) have a high mitochondrial content reflecting their large energy utilization. The importance for correct trafficking and precise mitochondrial positioning remains poorly elucidated in inhibitory neurons. Miro1 is a Ca²⁺-sensing adaptor protein that links mitochondria to the trafficking apparatus, for their microtubule-dependent transport along axons and dendrites, in order to meet the metabolic and Ca2+-buffering requirements of the cell. Here, we explore the role of Miro1 in parvalbumin interneurons and how changes in mitochondrial trafficking could alter network activity in the mouse brain. By employing live and fixed imaging, we found that the impairments in Miro1-directed trafficking in PV+ interneurons altered their mitochondrial distribution and axonal arborization while PV+ interneuron mediated inhibition remained intact. These changes were accompanied by an increase in the ex vivo hippocampal γ-oscillation (30 - 80 Hz) frequency and promoted anxiolysis. Our findings show that precise regulation of mitochondrial dynamics in PV+ interneurons is crucial for proper neuronal signaling and network synchronization.
    Keywords:  cell biology; mouse; neuroscience
  3. Cell Rep. 2021 Jun 29. pii: S2211-1247(21)00697-5. [Epub ahead of print]35(13): 109321
      The major cap-binding protein eukaryotic translation initiation factor 4E (eIF4E), an ancient protein required for translation of all eukaryotic genomes, is a surprising yet potent oncogenic driver. The genetic interactions that maintain the oncogenic activity of this key translation factor remain unknown. In this study, we carry out a genome-wide CRISPRi screen wherein we identify more than 600 genetic interactions that sustain eIF4E oncogenic activity. Our data show that eIF4E controls the translation of Tfeb, a key executer of the autophagy response. This autophagy survival response is triggered by mitochondrial proteotoxic stress, which allows cancer cell survival. Our screen also reveals a functional interaction between eIF4E and a single anti-apoptotic factor, Bcl-xL, in tumor growth. Furthermore, we show that eIF4E and the exon-junction complex (EJC), which is involved in many steps of RNA metabolism, interact to control the migratory properties of cancer cells. Overall, we uncover several cancer-specific vulnerabilities that provide further resolution of the cancer translatome.
    Keywords:  Bcl-xL; CRISPRi; EJC; Tfeb; UPR(mt)-like stress response; autophagy; cancer; eIF4E; mitochondria; translation control
  4. EMBO J. 2021 Jun 30. e107913
      The formation of protein aggregates is a hallmark of neurodegenerative diseases. Observations on patient samples and model systems demonstrated links between aggregate formation and declining mitochondrial functionality, but causalities remain unclear. We used Saccharomyces cerevisiae to analyze how mitochondrial processes regulate the behavior of aggregation-prone polyQ protein derived from human huntingtin. Expression of Q97-GFP rapidly led to insoluble cytosolic aggregates and cell death. Although aggregation impaired mitochondrial respiration only slightly, it considerably interfered with the import of mitochondrial precursor proteins. Mutants in the import component Mia40 were hypersensitive to Q97-GFP, whereas Mia40 overexpression strongly suppressed the formation of toxic Q97-GFP aggregates both in yeast and in human cells. Based on these observations, we propose that the post-translational import of mitochondrial precursor proteins into mitochondria competes with aggregation-prone cytosolic proteins for chaperones and proteasome capacity. Mia40 regulates this competition as it has a rate-limiting role in mitochondrial protein import. Therefore, Mia40 is a dynamic regulator in mitochondrial biogenesis that can be exploited to stabilize cytosolic proteostasis.
    Keywords:  Mia40; huntingtin; mitochondria; protein aggregation; protein translocation
  5. Sci Adv. 2021 Jul;pii: eabf8631. [Epub ahead of print]7(27):
      We report a role for the mitochondrial single-stranded DNA binding protein (mtSSB) in regulating mitochondrial DNA (mtDNA) replication initiation in mammalian mitochondria. Transcription from the light-strand promoter (LSP) is required both for gene expression and for generating the RNA primers needed for initiation of mtDNA synthesis. In the absence of mtSSB, transcription from LSP is strongly up-regulated, but no replication primers are formed. Using deep sequencing in a mouse knockout model and biochemical reconstitution experiments with pure proteins, we find that mtSSB is necessary to restrict transcription initiation to optimize RNA primer formation at both origins of mtDNA replication. Last, we show that human pathological versions of mtSSB causing severe mitochondrial disease cannot efficiently support primer formation and initiation of mtDNA replication.
  6. iScience. 2021 Jun 25. 24(6): 102673
      Hydroxyurea (HU), an FDA-approved drug for treating sickle cell disease, is used as an antitumor drug alone and together with conventional chemotherapeutics or radiation therapy. HU is used primarily to treat myeloproliferative diseases because it inhibits the enzyme ribonucleotide reductase involved in DNA synthesis. The hydroxyl group in HU is considered critical for its antiproliferative and chemotherapeutic effects. Here, we substituted the hydroxyl group in HU with a triphenylphosphonium cation attached to an alkyl group with different chain lengths, forming a new class of mitochondria-targeted HU (Mito-HU). Elongating the alkyl side chain length increased the hydrophobicity of Mito-HUs, inhibition of oxidative phosphorylation, and antiproliferative effects in tumor cells. Both mitochondrial complex I- and complex III-induced oxygen consumption decreased with the increasing hydrophobicity of Mito-HUs. The more hydrophobic Mito-HUs also potently inhibited the monocytic myeloid-derived suppressor cells and suppressive neutrophils, and stimulated T cell response, implicating their potential antitumor immunomodulatory mechanism.
    Keywords:  Biological sciences; Drugs; Immunology; Organic chemistry
  7. iScience. 2021 Jun 25. 24(6): 102653
      Metabolic heterogeneity within the tumor microenvironment promotes cancer cell growth and immune suppression. We determined the impact of mitochondria-targeted complex I inhibitors (Mito-CI) in melanoma. Mito-CI decreased mitochondria complex I oxygen consumption, Akt-FOXO signaling, blocked cell cycle progression, melanoma cell proliferation and tumor progression in an immune competent model system. Immune depletion revealed roles for T cells in the antitumor effects of Mito-CI. While Mito-CI preferentially accumulated within and halted tumor cell proliferation, it also elevated infiltration of activated effector T cells and decreased myeloid-derived suppressor cells (MDSC) as well as tumor-associated macrophages (TAM) in melanoma tumors in vivo. Anti-proliferative doses of Mito-CI inhibited differentiation, viability, and the suppressive function of bone marrow-derived MDSC and increased proliferation-independent activation of T cells. These data indicate that targeted inhibition of complex I has synchronous effects that cumulatively inhibits melanoma growth and promotes immune remodeling.
    Keywords:  Cancer; Cell biology; Components of the immune system
  8. Biol Open. 2021 May 15. pii: bio058613. [Epub ahead of print]10(5):
      A dramatic rise of infections with antibiotic-resistant bacterial pathogens continues to challenge the healthcare field due to the lack of effective treatment regimes. As such, there is an urgent need to develop new antimicrobial agents that can combat these multidrug-resistant superbugs. Mitochondria are central regulators of metabolism and other cellular functions, including the regulation of innate immunity pathways involved in the defense against infection. The mitochondrial unfolded protein response (UPRmt) is a stress-activated pathway that mitigates mitochondrial dysfunction through the regulation of genes that promote recovery of the organelle. In the model organism Caenorhabditis elegans, the UPRmt also mediates an antibacterial defense program that combats pathogen infection, which promotes host survival. We sought to identify and characterize antimicrobial effectors that are regulated during the UPRmt. From our search, we discovered that the antimicrobial peptide CNC-4 is upregulated during this stress response. CNC-4 belongs to the caenacin family of antimicrobial peptides, which are predominantly found in nematodes and are known to have anti-fungal properties. Here, we find that CNC-4 also possesses potent antimicrobial activity against a spectrum of bacterial species and report on its characterization.
    Keywords:  Antimicrobial peptide; CNC-4; Caenacins; Innate immunity; Mitochondria; Mitochondrial UPR; Stress response
  9. Biol Open. 2021 Jun 15. pii: bio058553. [Epub ahead of print]10(6):
      Mitochondrial DNA (mtDNA) encodes gene products that are essential for oxidative phosphorylation. They organize as higher order nucleoid structures (mtNucleoids) that were shown to be critical for the maintenance of mtDNA stability and integrity. While mtNucleoid structures are associated with cellular health, how they change in situ under physiological maturation and aging requires further investigation. In this study, we investigated the mtNucleoid assembly at an ultrastructural level in situ using the TFAM-Apex2 Drosophila model. We found that smaller and more compact TFAM-nucleoids are populated in the mitochondria of indirect flight muscle of aged flies. Furthermore, mtDNA transcription and replication were cross-regulated in the mtTFB2-knockdown flies as in the mtRNAPol-knockdown flies that resulted in reductions in mtDNA copy numbers and nucleoid-associated TFAM. Overall, our study reveals that the modulation of TFAM-nucleoid structure under physiological aging, which is critically regulated by mtDNA content.
    Keywords:  Mitochondrial DNA; Mitochondrial RNA polymerase (mtRNAPol); Mitochondrial nucleoid; Mitochondrial transcription factor B2 (mtTFB2); Transcription factor A (TFAM)
  10. Biology (Basel). 2021 Jun 23. pii: 572. [Epub ahead of print]10(7):
      Mitochondria are highly dynamic organelles that interchange their contents mediated by fission and fusion. However, it has previously been shown that the mitochondria of cultured human epithelial cells exhibit a gradient in the relative abundance of several proteins, with the perinuclear mitochondria generally exhibiting a higher protein abundance than the peripheral mitochondria. The molecular mechanisms that are required for the establishment and the maintenance of such inner-cellular mitochondrial protein abundance gradients are unknown. We verified the existence of inner-cellular gradients in the abundance of clusters of the mitochondrial outer membrane protein Tom20 in the mitochondria of kidney epithelial cells from an African green monkey (Vero cells) using STED nanoscopy and confocal microscopy. We found that the Tom20 gradients are established immediately after cell division and require the presence of microtubules. Furthermore, the gradients are abrogated in hyperfused mitochondrial networks. Our results suggest that inner-cellular protein abundance gradients from the perinuclear to the peripheral mitochondria are established by the trafficking of individual mitochondria to their respective cellular destination.
    Keywords:  image analysis; inner-cellular heterogeneity; nanoscopy; protein distribution; super-resolution microscopy
  11. Biology (Basel). 2021 Jun 29. pii: 600. [Epub ahead of print]10(7):
      Mitochondria are double membrane-enclosed organelles performing important cellular and metabolic functions such as ATP generation, heme biogenesis, apoptosis, ROS production and calcium buffering. The mitochondrial inner membrane (IM) is folded into cristae membranes (CMs) of variable shapes using molecular players including the 'mitochondrial contact site and cristae organizing system' (MICOS) complex, the dynamin-like GTPase OPA1, the F1FO ATP synthase and cardiolipin. Aberrant cristae structures are associated with different disorders such as diabetes, neurodegeneration, cancer and hepato-encephalopathy. In this review, we provide an updated view on cristae biogenesis by focusing on novel roles of the MICOS complex in cristae dynamics and shaping of cristae. For over seven decades, cristae were considered as static structures. It was recently shown that cristae constantly undergo rapid dynamic remodeling events. Several studies have re-oriented our perception on the dynamic internal ambience of mitochondrial compartments. In addition, we discuss the recent literature which sheds light on the still poorly understood aspect of cristae biogenesis, focusing on the role of MICOS and its subunits. Overall, we provide an integrated and updated view on the relation between the biogenesis of cristae and the novel aspect of cristae dynamics.
    Keywords:  MICOS; cristae; cristae biogenesis; cristae dynamics; mitochondria
  12. Redox Biol. 2021 Jun 17. pii: S2213-2317(21)00206-8. [Epub ahead of print]45 102047
      The contribution of the Ubiquitin-Proteasome System (UPS) to mitophagy has been largely attributed to the E3 ubiquitin ligase Parkin. Here we show that in response to the oxidative stress associated with hypoxia or the hypoxia mimic CoCl2, the damaged and fragmented mitochondria are removed by Parkin-independent mitophagy. Mitochondria isolated from hypoxia or CoCl2-treated cells exhibited extensive ubiquitination, predominantly Lysine 48-linked and involves the degradation of key mitochondrial proteins such as the mitofusins MFN1/2, or the import channel component TOM20. Reflecting the critical role of mitochondrial protein degradation, proteasome inhibition blocked CoCl2-induced mitophagy. The five conserved ubiquitin-binding autophagy receptors (p62, NDP52, Optineurin, NBR1, TAX1BP1) were dispensable for the ensuing mitophagy, suggesting that the mitophagy step itself was independent of ubiquitination. Instead, the expression of two ubiquitin-independent mitophagy receptor proteins BNIP3 and NIX was induced by hypoxia or CoCl2-treatment followed by their recruitment to the oxidation-damaged mitochondria. By employing BNIP3/NIX double knockout and DRP1-null cell lines, we confirmed that mitochondrial clearance relies on DRP1-dependent mitochondrial fragmentation and BNIP3/NIX-mediated mitophagy. General antioxidants such as N-Acetyl Cysteine (NAC) or the mitochondria-specific Mitoquinone prevented HIF-1α stabilization, ameliorated hypoxia-related mitochondrial oxidative stress, and suppressed mitophagy. We conclude that the UPS and receptor-mediated autophagy converge to eliminate oxidation-damaged mitochondria.
    Keywords:  HIF-1α; Hypoxia; Mitochondria; Mitophagy; Oxidative stress; Proteasome; Ubiquitin