bims-mitdyn Biomed News
on Mitochondrial dynamics: mechanisms
Issue of 2021‒05‒30
sixteen papers selected by
Edmond Chan
Queen’s University, School of Medicine

  1. Cell. 2021 May 27. pii: S0092-8674(21)00530-4. [Epub ahead of print]184(11): 2896-2910.e13
      Damaged mitochondria need to be cleared to maintain the quality of the mitochondrial pool. Here, we report mitocytosis, a migrasome-mediated mitochondrial quality-control process. We found that, upon exposure to mild mitochondrial stresses, damaged mitochondria are transported into migrasomes and subsequently disposed of from migrating cells. Mechanistically, mitocytosis requires positioning of damaged mitochondria at the cell periphery, which occurs because damaged mitochondria avoid binding to inward motor proteins. Functionally, mitocytosis plays an important role in maintaining mitochondrial quality. Enhanced mitocytosis protects cells from mitochondrial stressor-induced loss of mitochondrial membrane potential (MMP) and mitochondrial respiration; conversely, blocking mitocytosis causes loss of MMP and mitochondrial respiration under normal conditions. Physiologically, we demonstrate that mitocytosis is required for maintaining MMP and viability in neutrophils in vivo. We propose that mitocytosis is an important mitochondrial quality-control process in migrating cells, which couples mitochondrial homeostasis with cell migration.
    Keywords:  migrasome; mitochondrial quality control; mitochondrion; mitocytosis; mitosome
  2. Nat Metab. 2021 May;3(5): 618-635
      Macrophages generate mitochondrial reactive oxygen species and mitochondrial reactive electrophilic species as antimicrobials during Toll-like receptor (TLR)-dependent inflammatory responses. Whether mitochondrial stress caused by these molecules impacts macrophage function is unknown. Here, we demonstrate that both pharmacologically driven and lipopolysaccharide (LPS)-driven mitochondrial stress in macrophages triggers a stress response called mitohormesis. LPS-driven mitohormetic stress adaptations occur as macrophages transition from an LPS-responsive to LPS-tolerant state wherein stimulus-induced pro-inflammatory gene transcription is impaired, suggesting tolerance is a product of mitohormesis. Indeed, like LPS, hydroxyoestrogen-triggered mitohormesis suppresses mitochondrial oxidative metabolism and acetyl-CoA production needed for histone acetylation and pro-inflammatory gene transcription, and is sufficient to enforce an LPS-tolerant state. Thus, mitochondrial reactive oxygen species and mitochondrial reactive electrophilic species are TLR-dependent signalling molecules that trigger mitohormesis as a negative feedback mechanism to restrain inflammation via tolerance. Moreover, bypassing TLR signalling and pharmacologically triggering mitohormesis represents a new anti-inflammatory strategy that co-opts this stress response to impair epigenetic support of pro-inflammatory gene transcription by mitochondria.
  3. Elife. 2021 May 26. pii: e67624. [Epub ahead of print]10
      Dysfunction of the mitochondrial electron transport chain (mETC) is a major cause of human mitochondrial diseases. To identify determinants of mETC function, we screened a genome-wide human CRISPRi library under oxidative metabolic conditions with selective inhibition of mitochondrial Complex III and identified ovarian carcinoma immunoreactive antigen (OCIA) domain-containing protein 1 (OCIAD1) as a Complex III assembly factor. We find that OCIAD1 is an inner mitochondrial membrane protein that forms a complex with supramolecular prohibitin assemblies. Our data indicate that OCIAD1 is required for maintenance of normal steady-state levels of Complex III and the proteolytic processing of the catalytic subunit cytochrome c1 (CYC1). In OCIAD1 depleted mitochondria, unprocessed CYC1 is hemylated and incorporated into Complex III. We propose that OCIAD1 acts as an adaptor within prohibitin assemblies to stabilize and/or chaperone CYC1 and to facilitate its proteolytic processing by the IMMP2L protease.
    Keywords:  Complex III; cell biology; cytochrome c1; electron transport chain; human; mitochondria; prohibitin; protease
  4. J Cell Biol. 2021 Aug 02. pii: e202009092. [Epub ahead of print]220(8):
      Mitophagy is the degradation of surplus or damaged mitochondria by autophagy. In addition to programmed and stress-induced mitophagy, basal mitophagy processes exert organelle quality control. Here, we show that the sorting and assembly machinery (SAM) complex protein SAMM50 interacts directly with ATG8 family proteins and p62/SQSTM1 to act as a receptor for a basal mitophagy of components of the SAM and mitochondrial contact site and cristae organizing system (MICOS) complexes. SAMM50 regulates mitochondrial architecture by controlling formation and assembly of the MICOS complex decisive for normal cristae morphology and exerts quality control of MICOS components. To this end, SAMM50 recruits ATG8 family proteins through a canonical LIR motif and interacts with p62/SQSTM1 to mediate basal mitophagy of SAM and MICOS components. Upon metabolic switch to oxidative phosphorylation, SAMM50 and p62 cooperate to mediate efficient mitophagy.
  5. Sci Adv. 2021 May;pii: eabf0971. [Epub ahead of print]7(22):
      In response to disturbed mitochondrial gene expression and protein synthesis, an adaptive transcriptional response sharing a signature of the integrated stress response (ISR) is activated. We report an intricate interplay between three transcription factors regulating the mitochondrial stress response: CHOP, C/EBPβ, and ATF4. We show that CHOP acts as a rheostat that attenuates prolonged ISR, prevents unfavorable metabolic alterations, and postpones the onset of mitochondrial cardiomyopathy. Upon mitochondrial dysfunction, CHOP interaction with C/EBPβ is needed to adjust ATF4 levels, thus preventing overactivation of the ATF4-regulated transcriptional program. Failure of this interaction switches ISR from an acute to a chronic state, leading to early respiratory chain deficiency, energy crisis, and premature death. Therefore, contrary to its previously proposed role as a transcriptional activator of mitochondrial unfolded protein response, our results highlight a role of CHOP in the fine-tuning of mitochondrial ISR in mammals.
  6. JCI Insight. 2021 May 25. pii: 142801. [Epub ahead of print]
      ECSIT is a protein with roles in early development, activation of the transcription factor NFB and production of mitochondrial reactive oxygen species (mROS) that facilitates clearance of intracellular bacteria like Salmonella. ECSIT is also an important assembly factor for mitochondrial complex I. Unlike the murine form of Ecsit (mEcsit), we demonstrate here that human ECSIT (hECSIT) to be highly labile. In order to explore if the instability of hECSIT affects functions previously ascribed to its murine counterpart, we created a novel transgenic mouse in which the murine Ecsit gene is replaced by the human ECSIT gene. The humanised mouse has low levels of hECSIT protein in keeping with its intrinsic instability. Whereas low level expression of hECSIT was capable of fully compensating for mEcsit in its roles in early development and activation of the NFB pathway, macrophages from humanised mice showed impaired clearance of Salmonella that was associated with reduced production of mROS. Notably, severe cardiac hypertrophy manifested in ageing humanised mice leading to premature death. The cellular and molecular basis to this phenotype is delineated by showing that low levels of human ECSIT protein leads to marked reduction in assembly and activity of mitochondrial complex I with impaired oxidative phosphorylation and reduced production of ATP. Cardiac tissue from humanised hECSIT mice also shows reduced mitochondrial fusion and more fission but impaired clearance of fragmented mitochondria. A cardiomyocyte-intrinsic role for Ecsit in mitochondrial function and cardioprotection is also demonstrated. We also show that cardiac fibrosis and damage in humans correlates with low expression of human ECSIT. In summary, our findings identify a new role for ECSIT in cardioprotection whilst also generating a valuable new experimental model to study mitochondrial dysfunction and cardiac pathophysiology.
    Keywords:  Cardiology; Cardiovascular disease; Metabolism; Mitochondria
  7. Cell Rep. 2021 May 25. pii: S2211-1247(21)00525-8. [Epub ahead of print]35(8): 109180
      Mitochondrial respiratory complex subunits assemble in supercomplexes. Studies of supercomplexes have typically relied upon antibody-based quantification, often limited to a single subunit per respiratory complex. To provide a deeper insight into mitochondrial and supercomplex plasticity, we combine native electrophoresis and mass spectrometry to determine the supercomplexome of skeletal muscle from sedentary and exercise-trained mice. We quantify 422 mitochondrial proteins within 10 supercomplex bands in which we show the debated presence of complexes II and V. Exercise-induced mitochondrial biogenesis results in non-stoichiometric changes in subunits and incorporation into supercomplexes. We uncover the dynamics of supercomplex-related assembly proteins and mtDNA-encoded subunits after exercise. Furthermore, exercise affects the complexing of Lactb, an obesity-associated mitochondrial protein, and ubiquinone biosynthesis proteins. Knockdown of ubiquinone biosynthesis proteins leads to alterations in mitochondrial respiration. Our approach can be applied to broad biological systems. In this instance, comprehensively analyzing respiratory supercomplexes illuminates previously undetectable complexity in mitochondrial plasticity.
    Keywords:  complexome; exercise; mitochondrial respiratory complexes; mitochondrial supercomplexes; oxidative phosphorylation; protein complexes
  8. Nat Commun. 2021 May 28. 12(1): 3239
      The human mitochondrial AAA+ protein LONP1 is a critical quality control protease involved in regulating diverse aspects of mitochondrial biology including proteostasis, electron transport chain activity, and mitochondrial transcription. As such, genetic or aging-associated imbalances in LONP1 activity are implicated in pathologic mitochondrial dysfunction associated with numerous human diseases. Despite this importance, the molecular basis for LONP1-dependent proteolytic activity remains poorly defined. Here, we solved cryo-electron microscopy structures of human LONP1 to reveal the underlying molecular mechanisms governing substrate proteolysis. We show that, like bacterial Lon, human LONP1 adopts both an open and closed spiral staircase orientation dictated by the presence of substrate and nucleotide. Unlike bacterial Lon, human LONP1 contains a second spiral staircase within its ATPase domain that engages substrate as it is translocated toward the proteolytic chamber. Intriguingly, and in contrast to its bacterial ortholog, substrate binding within the central ATPase channel of LONP1 alone is insufficient to induce the activated conformation of the protease domains. To successfully induce the active protease conformation in substrate-bound LONP1, substrate binding within the protease active site is necessary, which we demonstrate by adding bortezomib, a peptidomimetic active site inhibitor of LONP1. These results suggest LONP1 can decouple ATPase and protease activities depending on whether AAA+ or both AAA+ and protease domains bind substrate. Importantly, our structures provide a molecular framework to define the critical importance of LONP1 in regulating mitochondrial proteostasis in health and disease.
  9. Cancer Res. 2021 May 28. pii: canres.0436.2021. [Epub ahead of print]
      Mitochondrial dynamics play vital roles in the tumorigenicity and malignancy of various types of cancers by promoting the tumor-initiating potential of cancer cells, suggesting that targeting crucial factors that drive mitochondrial dynamics may lead to promising anticancer therapies. In the current study, we report that overexpression of mitochondrial fission factor (MFF), which is upregulated significantly in liver cancer initiating cells (LCIC), promotes mitochondrial fission and enhances stemness and tumor-initiating capability in non-LCICs. MFF-induced mitochondrial fission evoked mitophagy and asymmetric stem cell division and promoted a metabolic shift from oxidative phosphorylation to glycolysis that decreased mitochondrial reactive oxygen species (ROS) production, which prevented ROS-mediated degradation of the pluripotency transcription factor OCT4. CRISPR affinity purification in situ of regulatory elements (CAPTURE) showed that T-Box transcription factor 19 (TBX19), which is overexpressed uniquely in LCICs compared to non-LCICs and liver progenitor cells, forms a complex with PRMT1 on the MFF promoter in LCICs, eliciting epigenetic histone H4R3me2a/H3K9ac-mediated transactivation of MFF. Targeting PRMT1 using furamidine, a selective pharmacological inhibitor, suppressed TBX19-induced mitochondrial fission, leading to a profound loss of self-renewal potential and tumor-initiating capacity of LCICs. These findings unveil a novel mechanism underlying mitochondrial fission-mediated cancer stemness and suggest that regulation of mitochondrial fission via inhibition of PRMT1 may be an attractive therapeutic option for liver cancer treatment.
  10. EMBO Mol Med. 2021 May 27. e14316
      Mitochondria exist as dynamic networks whose morphology is driven by the complex interplay between fission and fusion events. Failure to modulate these processes can be detrimental to human health as evidenced by dominantly inherited, pathogenic variants in OPA1, an effector enzyme of mitochondrial fusion, that lead to network fragmentation, cristae dysmorphology and impaired oxidative respiration, manifesting typically as isolated optic atrophy. However, a significant number of patients develop more severe, systemic phenotypes, although no genetic modifiers of OPA1-related disease have been identified to date. In this issue of EMBO Molecular Medicine, supervised machine learning algorithms underlie a novel tool that enables automated, high throughput and unbiased screening of changes in mitochondrial morphology measured using confocal microscopy. By coupling this approach with a bespoke siRNA library targeting the entire mitochondrial proteome, the work described by Cretin and colleagues yielded significant insight into mitochondrial biology, discovering 91 candidate genes whose endogenous depletion can remedy impaired mitochondrial dynamics caused by OPA1 deficiency.
  11. J Cell Sci. 2021 May 26. pii: jcs.253781. [Epub ahead of print]
      In Saccharomyces cerevisiae, the selective autophagic degradation of mitochondria, termed mitophagy, is critically regulated by the adapter protein, Atg32. Despite our knowledge about the molecular mechanisms by which Atg32 controls mitophagy, its physiological roles in yeast survival and fitness remains less clear. Here, we demonstrate a requirement for Atg32 in promoting spermidine production during respiratory growth and heat-induced mitochondrial stress. During respiratory growth, mitophagy-deficient yeast exhibit profound heat-stress induced defects in growth and viability due to impaired biosynthesis of spermidine and its biosynthetic precursor S-Adenosyl-Methionine (SAM). Moreover, spermidine production is crucial for the induction of cytoprotective nitric oxide (NO) during heat stress. Hence, the re-addition of spermidine to Atg32 mutant yeast is sufficient to both enhance NO production and restore respiratory growth during heat stress. Our findings uncover a previously unrecognized physiological role for yeast mitophagy in spermidine metabolism and illuminate new interconnections between mitophagy, polyamine biosynthesis and NO signaling.
    Keywords:  ATG32; Autophagy; Mitophagy; Nitric Oxide; S-Adenosyl-Methionine; Spermidine
  12. Nat Rev Cancer. 2021 May 27.
      Variation in the mitochondrial DNA (mtDNA) sequence is common in certain tumours. Two classes of cancer mtDNA variants can be identified: de novo mutations that act as 'inducers' of carcinogenesis and functional variants that act as 'adaptors', permitting cancer cells to thrive in different environments. These mtDNA variants have three origins: inherited variants, which run in families, somatic mutations arising within each cell or individual, and variants that are also associated with ancient mtDNA lineages (haplogroups) and are thought to permit adaptation to changing tissue or geographic environments. In addition to mtDNA sequence variation, mtDNA copy number and perhaps transfer of mtDNA sequences into the nucleus can contribute to certain cancers. Strong functional relevance of mtDNA variation has been demonstrated in oncocytoma and prostate cancer, while mtDNA variation has been reported in multiple other cancer types. Alterations in nuclear DNA-encoded mitochondrial genes have confirmed the importance of mitochondrial metabolism in cancer, affecting mitochondrial reactive oxygen species production, redox state and mitochondrial intermediates that act as substrates for chromatin-modifying enzymes. Hence, subtle changes in the mitochondrial genotype can have profound effects on the nucleus, as well as carcinogenesis and cancer progression.
  13. J Biol Chem. 2021 May 20. pii: S0021-9258(21)00612-8. [Epub ahead of print] 100816
      Mitochondrial tRNA 3'-end metabolism is critical for the formation of functional tRNAs. Deficient mitochondrial tRNA 3'-end metabolism is linked to an array of human diseases, including optic neuropathy, but their pathophysiology remains poorly understood. In this report, we investigated the molecular mechanism underlying the Leber's hereditary optic neuropathy (LHON)-associated tRNAAla 5587A>G mutation, which changes a highly conserved adenosine at position 73 (A73) to guanine (G73) on the 3'-end of the tRNA acceptor stem. The m.5587A>G mutation was identified in three Han Chinese families with suggested maternal inheritance of LHON. We hypothesized that the m.5877A>G mutation altered tRNAAla 3'-end metabolism and mitochondrial function. In vitro processing experiments showed that the m.5587A>G mutation impaired the 3'-end processing of tRNAAla precursors by RNase Z, and inhibited the addition of CCA by tRNA nucleotidyltransferase (TRNT1). Northern blot analysis revealed that the m.5587A>G mutation perturbed tRNAAla aminoacylation, as evidenced by decreased efficiency of aminoacylation and faster electrophoretic mobility of mutated tRNAAla in these cells. The impact of m.5587A>G mutation on tRNAAla function was further supported by increased melting temperature, conformational changes, and reduced levels of this tRNA. Failures in tRNAAla metabolism impaired mitochondrial translation, perturbed assembly and activity of oxidative phosphorylation complexes, diminished ATP production and membrane potential, and increased production of reactive oxygen species. These pleiotropic defects elevated apoptotic cell death and promoted mitophagy in cells carrying the m.5587A>G mutation, thereby contributing to visual impairment. Our findings may provide new insights into the pathophysiology of LHON arising from mitochondrial tRNA 3'-end metabolism deficiency.
    Keywords:  Leber’s hereditary optic neuropathy; Mitochondrial tRNA 3’-end metabolisms; apoptosis; autophagy; oxidative phosphorylation
  14. J Biol Chem. 2021 May 21. pii: S0021-9258(21)00623-2. [Epub ahead of print] 100825
      Normal contractile function of the heart depends on a constant and reliable production of ATP by cardiomyocytes. Dysregulation of cardiac energy metabolism can result in immature heart development and disrupt the ability of the adult myocardium to adapt to stress, potentially leading to heart failure. Further, restoration of abnormal mitochondrial function can have beneficial effects on cardiac dysfunction. Previously, we identified a novel protein termed Perm1 (PGC-1 and ERR induced regulator, muscle 1) that is enriched in skeletal and cardiac-muscle mitochondria and transcriptionally regulated by PGC-1 (Peroxisome proliferator-activated receptor gamma coactivator 1) and ERR (Estrogen-related receptor). The role of Perm1 in the heart is poorly understood and was studied here. We utilized cell culture, mouse models and human tissue, to study its expression and transcriptional control, as well as its role in transcription of other factors. Critically, we tested Perm1's role in cardiomyocyte mitochondrial function and its ability to protect myocytes from stress-induced damage. Our studies show Perm1 expression increases throughout mouse cardiogenesis, demonstrate that Perm1 interacts with PGC-1α and enhances activation of PGC-1 and ERR, increases mitochondrial DNA copy number, and augments oxidative capacity in cultured neonatal mouse cardiomyocytes. Moreover, we found that Perm1 reduced cellular damage produced as a result of hypoxia and reoxygenation-induced stress and mitigated cell death of cardiomyocytes. Taken together, our results show that Perm1 promotes mitochondrial biogenesis in mouse cardiomyocytes. Future studies can assess the potential of Perm1 to be used as a novel therapeutic to restore cardiac dysfunction induced by ischemic injury.
    Keywords:  Perm1; cardiomyocytes; mitochondrial biogenesis; oxidative metabolism
  15. Annu Rev Genomics Hum Genet. 2021 May 26.
      Mitochondria are unusual organelles in that they contain their own genomes, which are kept apart from the rest of the DNA in the cell. While mitochondrial DNA (mtDNA) is essential for respiration and most multicellular life, maintaining a genome outside the nucleus brings with it a number of challenges. Chief among these is preserving mtDNA genomic integrity from one generation to the next. In this review, we discuss what is known about negative (purifying) selection mechanisms that prevent deleterious mutations from accumulating in mtDNA in the germline. Throughout, we focus on the female germline, as it is the tissue through which mtDNA is inherited in most organisms and, therefore, the tissue that most profoundly shapes the genome. We discuss recent progress in uncovering the mechanisms of germline mtDNA selection, from humans to invertebrates. Expected final online publication date for the Annual Review of Genomics and Human Genetics, Volume 22 is August 2021. Please see for revised estimates.
  16. STAR Protoc. 2021 Jun 18. 2(2): 100533
      Mitochondrial metabolism is a critical mechanism that is deregulated in numerous retinal diseases. Here, we elaborate a protocol to quantify oxygen consumption rate as a measure of mitochondrial respiration directly from mouse retinal tissue pieces. Our procedure combines the use of Seahorse extracellular flux technology and ex vivo retinal tissue isolation and is robustly reproducible under different treatment conditions. This protocol allows direct assessment of mitochondrial function in response to drug treatments or genetic manipulation in mouse models. For complete details on the use and execution of this protocol, please refer to Shetty et al. (2020), Sardar Pasha et al. (2021), Kooragayala et al. (2015), and Joyal et al. (2016).
    Keywords:  Metabolism; Model Organisms; Neuroscience