bims-mitdyn Biomed News
on Mitochondrial dynamics: mechanisms
Issue of 2021‒12‒05
twelve papers selected by
Edmond Chan
Queen’s University, School of Medicine

  1. Mol Cell. 2021 Nov 19. pii: S1097-2765(21)00954-0. [Epub ahead of print]
      Most mitochondrial proteins are translated in the cytosol and imported into mitochondria. Mutations in the mitochondrial protein import machinery cause human pathologies. However, a lack of suitable tools to measure protein uptake across the mitochondrial proteome has prevented the identification of specific proteins affected by import perturbation. Here, we introduce mePRODmt, a pulsed-SILAC based proteomics approach that includes a booster signal to increase the sensitivity for mitochondrial proteins selectively, enabling global dynamic analysis of endogenous mitochondrial protein uptake in cells. We applied mePRODmt to determine protein uptake kinetics and examined how inhibitors of mitochondrial import machineries affect protein uptake. Monitoring changes in translation and uptake upon mitochondrial membrane depolarization revealed that protein uptake was extensively modulated by the import and translation machineries via activation of the integrated stress response. Strikingly, uptake changes were not uniform, with subsets of proteins being unaffected or decreased due to changes in translation or import capacity.
    Keywords:  SILAC; TMT; disease; integrated stress response; mitochondria; protein translocation; proteomics; proteostasis; respiratory chain complexes; translation
  2. Cell Metab. 2021 Nov 24. pii: S1550-4131(21)00531-3. [Epub ahead of print]
      Uncoupling protein 1 (UCP1) is a major regulator of brown and beige adipocyte energy expenditure and metabolic homeostasis. However, the widely employed UCP1 loss-of-function model has recently been shown to have a severe deficiency in the entire electron transport chain of thermogenic fat. As such, the role of UCP1 in metabolic regulation in vivo remains unclear. We recently identified cysteine-253 as a regulatory site on UCP1 that elevates protein activity upon covalent modification. Here, we examine the physiological importance of this site through the generation of a UCP1 cysteine-253-null (UCP1 C253A) mouse, a precise genetic model for selective disruption of UCP1 in vivo. UCP1 C253A mice exhibit significantly compromised thermogenic responses in both males and females but display no measurable effect on fat accumulation in an obesogenic environment. Unexpectedly, we find that a lack of C253 results in adipose tissue redox stress, which drives substantial immune cell infiltration and systemic inflammatory pathology in adipose tissues and liver of male, but not female, mice. Elevation of systemic estrogen reverses this male-specific pathology, providing a basis for protection from inflammation due to loss of UCP1 C253 in females. Together, our results establish the UCP1 C253 activation site as a regulator of acute thermogenesis and sex-dependent tissue inflammation.
    Keywords:  UCP1; cysteine; inflammation; metabolism; reactive oxygen species; sex differences
  3. J Cell Sci. 2021 Dec 02. pii: jcs.259254. [Epub ahead of print]
      Endoplasmic reticulum stress (ERS) occurs when cellular demand for protein folding exceeds the capacity of the organelle. Adaptation and cell survival in response to ERS requires a critical contribution by mitochondria and peroxisomes. During ERS response, mitochondrial respiration increases to ameliorate reactive oxygen species (ROS) accumulation; we now show in yeast that peroxisome abundance also increases to promote an adaptive response. In pox1▵ cells, defective in peroxisomal ß oxidation of fatty acids, respiratory response to ERS is impaired, and ROS accrues. However, respiratory response to ERS is rescued, and ROS production is mitigated in pox1▵ cells by overexpression of Mpc1, the mitochondrial pyruvate carrier that provides another source of acetyl CoA to fuel the TCA cycle and oxidative phosphorylation. Using proteomics, select mitochondrial proteins were identified that undergo upregulation by ERS to remodel respiratory machinery. Several peroxisome-based proteins were also increased, corroborating the peroxisomal role in ERS adaptation. Finally, ERS stimulates assembly of respiratory complexes into higher order supercomplexes, underlying increased electron transfer efficiency. Our results highlight peroxisomal and mitochondrial support for ERS adaptation to favor cell survival.
    Keywords:  Endoplasmic reticulum; Mitochondria; Stress survival
  4. Commun Biol. 2021 Dec 02. 4(1): 1350
      Proteostasis is a challenge for cellular organisms, as all known protein synthesis machineries are error-prone. Here we show by cell fractionation and microscopy studies that misfolded proteins formed in the endoplasmic reticulum can become associated with and partly transported into mitochondria, resulting in impaired mitochondrial function. Blocking the endoplasmic reticulum-mitochondria encounter structure (ERMES), but not the mitochondrial sorting and assembly machinery (SAM) or the mitochondrial surveillance pathway components Msp1 and Vms1, abrogated mitochondrial sequestration of ER-misfolded proteins. We term this mitochondria-associated proteostatic mechanism for ER-misfolded proteins ERAMS (ER-associated mitochondrial sequestration). We testify to the relevance of this pathway by using mutant α-1-antitrypsin as an example of a human disease-related misfolded ER protein, and we hypothesize that ERAMS plays a role in pathological features such as mitochondrial dysfunction.
  5. J Cell Sci. 2021 Dec 03. pii: jcs.253591. [Epub ahead of print]
      Smaug is a conserved translational regulator that binds numerous mRNAs, including nuclear transcripts that encode mitochondrial enzymes. Smaug orthologs form cytosolic membrane-less organelles (MLOs) in several organisms and cell types. We have performed single-molecule FISH assays that revealed that SDHB and UQCRC1 mRNAs associate with Smaug1 bodies in U2OS cells. Loss of function of Smaug1 and Smaug2 affected both mitochondrial respiration and morphology of the mitochondrial network. Phenotype rescue by Smaug1 transfection depends on the presence of its RNA binding domain. Moreover, we identified specific Smaug1 domains involved in MLO formation, and found that impaired Smaug1 MLO condensation correlates with mitochondrial defects. Mitochondrial Complex I inhibition by rotenone -but not strong mitochondrial uncoupling by CCCP- rapidly induced Smaug1 MLOs dissolution. Metformin and rapamycin elicited similar effects, which were blocked by pharmacological inhibition of AMPK. Finally, we found that Smaug1 MLO dissolution weakens the interaction with target mRNAs, thus enabling their release. We propose that mitochondrial respiration and the AMPK/mTOR balance controls the condensation and dissolution of Smaug1 MLOs, thus regulating nuclear mRNAs that encode key mitochondrial proteins.
    Keywords:  AMPK; Membrane-less organelles; Metformin; Mitochondria; Processing bodies; Smaug; Uqcrc1
  6. Open Biol. 2021 Dec;11(12): 210238
      Mitochondria are complex organelles with two membranes. Their architecture is determined by characteristic folds of the inner membrane, termed cristae. Recent studies in yeast and other organisms led to the identification of four major pathways that cooperate to shape cristae membranes. These include dimer formation of the mitochondrial ATP synthase, assembly of the mitochondrial contact site and cristae organizing system (MICOS), inner membrane remodelling by a dynamin-related GTPase (Mgm1/OPA1), and modulation of the mitochondrial lipid composition. In this review, we describe the function of the evolutionarily conserved machineries involved in mitochondrial cristae biogenesis with a focus on yeast and present current models to explain how their coordinated activities establish mitochondrial membrane architecture.
    Keywords:  ATP synthase; MICOS; Mgm1; Saccharomyces cerevisiae; cristae; mitochondrial lipids
  7. J Cell Sci. 2021 Dec 03. pii: jcs.258910. [Epub ahead of print]
      Apoptosis is an important cellular response to viral infection. In current study, we identified activating molecule in Beclin1-regulated autophagy protein 1 (AMBRA1) as a positive regulator of apoptosis triggered by dsRNA. Depletion of AMBRA1 by gene editing significantly reduced dsRNA-induced apoptosis, which was largely restored by trans-complementation of AMBRA1. Mechanistically, AMBRA1 interacts with mitochondrial antiviral-signaling protein (MAVS), a key mitochondrial adaptor in the apoptosis pathway induced by dsRNA and viral infection. Further Co-IP analysis demonstrated that the mitochondrial localization of MAVS was essential for their interaction. The impact of AMBRA1 on dsRNA-induced apoptosis relied on the presence of MAVS and caspase-8. AMBRA1 was involved in the stabilization of MAVS through preventing its proteasomal degradation induced by dsRNA. Consistently, AMBRA1 upregulated the apoptosis induced by Semliki Forest virus infection. Taken together, our work illustrated a role of AMBRA1 in the virus-induced apoptosis through interacting with and stabilizing MAVS.
    Keywords:  AMBRA1; Apoptosis; MAVS; Virus; dsRNA
  8. STAR Protoc. 2021 Dec 17. 2(4): 100952
      Cell-Specific Mitochondria Affinity Purification (CS-MAP) enables isolation and purification of intact mitochondria from individual cell types of Caenorhabditis elegans. The approach is based on the cell-specific expression of a recombinant hemagglutinin (HA)-tag fused to the TOMM-20 protein that decorates the surface of mitochondria, thereby allowing their immunomagnetic purification. This protocol describes the CS-MAP procedure performed on large populations of animals. The purified mitochondria are suitable for subsequent nucleic acid, protein, and functional analyses. For complete details on the use and execution of this protocol, please refer to Ahier et al. (2018, 2021).
    Keywords:  Cell Biology; Cell separation/fractionation; Metabolism; Model Organisms; Molecular Biology; Protein Biochemistry; Protein expression and purification
  9. Nat Commun. 2021 Dec 03. 12(1): 7056
      Mitochondrial defects are implicated in multiple diseases and aging. Exercise training is an accessible, inexpensive therapeutic intervention that can improve mitochondrial bioenergetics and quality of life. By combining multiple omics techniques with biochemical and in silico normalisation, we removed the bias arising from the training-induced increase in mitochondrial content to unearth an intricate and previously undemonstrated network of differentially prioritised mitochondrial adaptations. We show that changes in hundreds of transcripts, proteins, and lipids are not stoichiometrically linked to the overall increase in mitochondrial content. Our findings suggest enhancing electron flow to oxidative phosphorylation (OXPHOS) is more important to improve ATP generation than increasing the abundance of the OXPHOS machinery, and do not support the hypothesis that training-induced supercomplex formation enhances mitochondrial bioenergetics. Our study provides an analytical approach allowing unbiased and in-depth investigations of training-induced mitochondrial adaptations, challenging our current understanding, and calling for careful reinterpretation of previous findings.
  10. Nat Rev Genet. 2021 Dec 02.
      Mitochondria are subject to unique genetic control by both nuclear DNA and their own genome, mitochondrial DNA (mtDNA), of which each mitochondrion contains multiple copies. In humans, mutations in mtDNA can lead to devastating, heritable, multi-system diseases that display different tissue-specific presentation at any stage of life. Despite rapid advances in nuclear genome engineering, for years, mammalian mtDNA has remained resistant to genetic manipulation, hampering our ability to understand the mechanisms that underpin mitochondrial disease. Recent developments in the genetic modification of mammalian mtDNA raise the possibility of using genome editing technologies, such as programmable nucleases and base editors, for the treatment of hereditary mitochondrial disease.
  11. Mol Cell Oncol. 2021 ;8(5): 1984162
      Autophagy is a central recycling process, and it plays a complex role in cancer. We discovered that when autophagy is blocked, cancer cells compensate by increasing mitochondrial-derived vesicles. However, there are many unanswered questions remaining, particularly in the context of the dual roles of autophagy in cancer.
    Keywords:  Autophagy; cancer; mitochondria; mitochondrial derived vesicles; mitophagy