bims-mitmed Biomed News
on Mitochondrial medicine
Issue of 2023‒02‒19
twenty-two papers selected by
Dario Brunetti
Fondazione IRCCS Istituto Neurologico

  1. J Cell Biol. 2023 Apr 03. pii: e202204021. [Epub ahead of print]222(4):
      Mitochondria play critical roles in cellular metabolism and to maintain their integrity, they are regulated by several quality control pathways, including mitophagy. During BNIP3/BNIP3L-dependent receptor-mediated mitophagy, mitochondria are selectively targeted for degradation by the direct recruitment of the autophagy protein LC3. BNIP3 and/or BNIP3L are upregulated situationally, for example during hypoxia and developmentally during erythrocyte maturation. However, it is not well understood how they are spatially regulated within the mitochondrial network to locally trigger mitophagy. Here, we find that the poorly characterized mitochondrial protein TMEM11 forms a complex with BNIP3 and BNIP3L and co-enriches at sites of mitophagosome formation. We find that mitophagy is hyper-active in the absence of TMEM11 during both normoxia and hypoxia-mimetic conditions due to an increase in BNIP3/BNIP3L mitophagy sites, supporting a model that TMEM11 spatially restricts mitophagosome formation.
  2. Hum Mol Genet. 2023 Feb 16. pii: ddad031. [Epub ahead of print]
      SURF1 deficiency (OMIM # 220110) causes Leigh syndrome (LS, OMIM # 256000), a mitochondrial disorder typified by stress-induced metabolic strokes, neurodevelopmental regression, and progressive multisystem dysfunction. Here, we describe two novel surf1-/- zebrafish knockout models generated by CRISPR/Cas9 technology. While gross larval morphology, fertility, and survival into adulthood appeared unaffected, surf1-/- mutants manifested adult-onset ocular anomalies and decreased swimming activity, and classical biochemical hallmarks of human SURF1 disease, including reduced complex IV expression and enzymatic activity and increased tissue lactate. surf1-/- larvae also demonstrated oxidative stress and stressor hypersensitivity to the complex IV inhibitor, azide, which exacerbated their complex IV deficiency, reduced supercomplex formation, and induced acute neurodegeneration typical of LS including brain death, impaired neuromuscular responses, reduced swimming activity, and absent heartrate. Remarkably, prophylactic treatment of surf1-/- larvae with either cysteamine bitartrate or N-acetylcysteine, but not other antioxidants, significantly improved animal resiliency to stressor-induced brain death, swimming and neuromuscular dysfunction, and loss of heartbeat. Mechanistic analyses demonstrated cysteamine bitartrate pretreatment did not improve complex IV deficiency, ATP deficiency, or increased tissue lactate but did reduce oxidative stress and restore glutathione balance in surf1-/- animals. Overall, two novel surf1-/- zebrafish models recapitulate the gross neurodegenerative and biochemical hallmarks of LS, including azide stressor hypersensitivity that was associated with glutathione deficiency and ameliorated by cysteamine bitartrate or N-acetylcysteine therapy.
  3. Elife. 2023 Feb 17. pii: e75825. [Epub ahead of print]12
      Mitochondrial dysfunction caused by aberrant Complex I assembly and reduced activity of the electron transport chain is pathogenic in many genetic and age-related diseases. Mice missing the Complex I subunit NADH dehydrogenase [ubiquinone] iron-sulfur protein 4 (NDUFS4) are a leading mammalian model of severe mitochondrial disease that exhibit many characteristic symptoms of Leigh Syndrome including oxidative stress, neuroinflammation, brain lesions, and premature death. NDUFS4 knockout mice have decreased expression of nearly every Complex I subunit. As Complex I normally contains at least 8 iron-sulfur clusters and more than 25 iron atoms, we asked whether a deficiency of Complex I may lead to iron perturbations thereby accelerating disease progression. Consistent with this, iron supplementation accelerates symptoms of brain degeneration in these mice while iron restriction delays the onset of these symptoms, reduces neuroinflammation, and increases survival. NDUFS4 knockout mice display signs of iron overload in the liver including increased expression of hepcidin, and show changes in iron responsive element-regulated proteins consistent with increased cellular iron that were prevented by iron restriction. These results suggest that perturbed iron homeostasis may contribute to pathology in Leigh Syndrome and possibly other mitochondrial disorders.
    Keywords:  biochemistry; chemical biology; medicine; mouse
  4. Dev Neurobiol. 2023 Feb 16.
      Mutations in CHCHD10 and CHCHD2, encoding two paralogous mitochondrial proteins, have been identified in cases of amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTD), and Parkinson's disease (PD). Their role in disease is unclear, though both have been linked to mitochondrial respiration and mitochondrial stress responses. Here we investigated the biological roles of these proteins during vertebrate development using knockout (KO) models in zebrafish. We demonstrate that loss of either or both proteins leads to motor impairment, reduced survival and compromised neuromuscular junction (NMJ) integrity in larval zebrafish. Compensation by Chchd10 was observed in the chchd2-/- model, but not by Chchd2 in the chchd10 -/- model. The assembly of mitochondrial respiratory chain Complex I was impaired in chchd10 -/- and chchd2 -/- zebrafish larvae, but unexpectedly not in a double chchd10 -/- & chchd2 -/- model, suggesting that reduced mitochondrial Complex I cannot be solely responsible for the observed phenotypes, which are generally more severe in the double KO. We observed transcriptional activation markers of the mitochondrial integrated stress response (mt-ISR) in the double chchd10 -/- and chchd2 -/- KO model, suggesting that this pathway is involved in the restoration of Complex I assembly in our double KO model. The data presented here, demonstrates that the Complex I assembly defect in our single KO models arises independently of the mt-ISR. Furthermore, this study provides evidence that both proteins are required for normal vertebrate development. This article is protected by copyright. All rights reserved.
    Keywords:  CHCHD10; CHCHD2; amyotrophic lateral sclerosis; mitochondria; zebrafish
  5. Biochim Biophys Acta Gen Subj. 2023 Feb 13. pii: S0304-4165(23)00026-0. [Epub ahead of print] 130328
      Human MPV17, an evolutionarily conserved mitochondrial inner-membrane channel protein, accounts for the tissue-specific mitochondrial DNA depletion syndrome. However, the precise molecular function of the MPV17 protein is still elusive. Previous studies showed that the mitochondrial morphology and cristae organization are severely disrupted in the MPV17 knockout cells from yeast, zebrafish, and mammalian tissues. As mitochondrial cristae morphology is strictly regulated by the membrane phospholipids composition, we measured mitochondrial membrane phospholipids (PLs) levels in yeast Saccharomyces cerevisiae MPV17 ortholog, SYM1 (Stress-inducible Yeast MPV17) deleted cells. We found that Sym1 knockout decreases the mitochondrial membrane PLs, phosphatidyl ethanolamine (PE), and inhibits respiratory growth at 37 ̊C on rich media. Both the oxygen consumption rate and the steady state expressions of mitochondrial complex II and super-complexes are compromised. Apart from mitochondrial PE defect a significant depletion of mitochondrial phosphatidyl-choline (PC) was noticed in the sym1∆ cells grown on synthetic media at both 30 ̊C and 37 ̊C temperatures. Surprisingly, exogenous supplementation of methylglyoxal (MG), an intrinsic side product of glycolysis, rescues the respiratory growth of Sym1 deficient yeast cells. Using a combination of molecular biology and lipid biochemistry, we uncovered that MG simultaneously restores both the mitochondrial PE/PC levels and the respiration by enhancing cytosolic NAD-dependent glycerol-3-phosphate dehydrogenase 1 (Gpd1) enzymatic activity. Further, MG is incapable to restore respiratory growth of the sym1∆gpd1∆ double knockout cells. Thus, our work provides Gpd1 activation as a novel strategy for combating Sym1 deficiency and PC/PE defects.
    Keywords:  Glycerol-3-phosphate dehydrogenase 1; Methylglyoxal; Mitochondrial respiratory chain; Phospholipids; SYM1
  6. Front Neurosci. 2023 ;17 930422
      Introduction: Friedreich's ataxia (FRDA) is an inherited recessive neurodegenerative disorder caused by a homozygous guanine-adenine-adenine (GAA) repeat expansion within intron 1 of the FXN gene, which encodes the essential mitochondrial protein frataxin. There is still no effective therapy for FRDA, therefore the development of optimal cell and animal models of the disease is one of the priorities for preclinical therapeutic testing.Methods: We obtained the latest FRDA humanized mouse model that was generated on the basis of our previous YG8sR, by Jackson laboratory [YG8JR, Fxn null:YG8s(GAA) > 800]. We characterized the behavioral, cellular, molecular and epigenetics properties of the YG8JR model, which has the largest GAA repeat sizes compared to all the current FRDA mouse models.
    Results: We found statistically significant behavioral deficits, together with reduced levels of frataxin mRNA and protein, and aconitase activity in YG8JR mice compared with control Y47JR mice. YG8JR mice exhibit intergenerational GAA repeat instability by the analysis of parent and offspring tissue samples. Somatic GAA repeat instability was also detected in individual brain and cerebellum tissue samples. In addition, increased DNA methylation of CpG U13 was identified in FXN GAA repeat region in the brain, cerebellum, and heart tissues. Furthermore, we show decreased histone H3K9 acetylation and increased H3K9 methylation of YG8JR cerebellum tissues within the FXN gene, upstream and downstream of the GAA repeat region compared to Y47JR controls.
    Discussion: These studies provide a detailed characterization of the GAA repeat expansion-based YG8JR transgenic mouse models that will help investigations of FRDA disease mechanisms and therapy.
    Keywords:  FRDA; FXN; Friedreich’s ataxia; GAA repeat; Y47JR; YG8JR; frataxin; mouse model
  7. Br J Pharmacol. 2023 Feb 15.
      Mitochondria and mitochondria-mediated signaling pathways are known to control synaptic signaling as well as long-lasting changes in neuronal structure and function. Mitochondrial impairment is linked to synaptic dysfunction in normal aging and age-associated neurodegenerative ailments including Parkinson's disease (PD) and Alzheimer's disease (AD). Both proteolysis and mitophagy perform a major role in neuroprotection by maintaining a healthy mitochondrial population during aging. Mitophagy, a highly evolutionarily conserved cellular process, helps in the clearance of damaged mitochondria and thereby maintains the mitochondrial and metabolic balance, energy supply, neuronal survival, and neuronal health. Besides the maintenance of brain homeostasis, hippocampal mitophagy also helps in synapse formation, axonal development, dopamine release, and long-term depression. In contrast, defective mitophagy contributes to aging and age-related neurodegeneration by promoting the accumulation of damaged mitochondria leading to cellular dysfunction. Exercise, stress management, maintaining healthy mitochondrial dynamics, and administering natural or synthetic pharmacological compounds are some of the strategies used for neuroprotection during aging and age-related neurological diseases. The current review discusses the impact of defective mitophagy in aging and age-associated neurodegenerative conditions, the underlying molecular pathways, and potential therapies based on recently elucidated mitophagy-inducing strategies.
    Keywords:  Aging; Mitochondrial dysfunction; Mitophagy; Neurodegeneration; Pharmacological compounds; Therapeutic interventions
  8. Nat Commun. 2023 Feb 16. 14(1): 874
      Expanding mitochondrial base editing tools with broad sequence compatibility is of high need for both research and therapeutic purposes. In this study, we identify a DddA homolog from Simiaoa sunii (Ddd_Ss) which can efficiently deaminate cytosine in DC context in double-stranded DNA (dsDNA). We successfully develop Ddd_Ss-derived cytosine base editors (DdCBE_Ss) and introduce mutations at multiple mitochondrial DNA (mtDNA) loci including disease-associated mtDNA mutations in previously inaccessible GC context. Finally, by introducing a single amino acid substitution from Ddd_Ss, we successfully improve the activity and sequence compatibility of DdCBE derived from DddA of Burkholderia cenocepacia (DdCBE_Bc). Our study expands mtDNA editing tool boxes and provides resources for further screening and engineering dsDNA base editors for biological and therapeutic applications.
  9. Cell Rep. 2023 Feb 15. pii: S2211-1247(23)00126-2. [Epub ahead of print]42(2): 112115
      Mitochondria are vital organelles that require sophisticated homeostatic mechanisms for maintenance. Intercellular transfer of damaged mitochondria is a recently identified strategy broadly used to improve cellular health and viability. Here, we investigate mitochondrial homeostasis in the vertebrate cone photoreceptor, the specialized neuron that initiates our daytime and color vision. We find a generalizable response to mitochondrial stress that leads to loss of cristae, displacement of damaged mitochondria from their normal cellular location, initiation of degradation, and transfer to Müller glia cells, a key non-neuronal support cell in the retina. Our findings show transmitophagy from cones to Müller glia as a response to mitochondrial damage. Intercellular transfer of damaged mitochondria represents an outsourcing mechanism that photoreceptors use to support their specialized function.
    Keywords:  CP: Neuroscience; Müller glia; mitochondria; mitophagy; photoreceptor; retina; zebrafish
  10. Acta Myol. 2022 ;41(4): 201-206
      Objective: Mitofusin 2 (MFN2) is a mitochondrial outer membrane protein that serves primarily as a mitochondrial fusion protein but has additional functions including the tethering of mitochondrial-endoplasmic reticulum membranes, movement of mitochondria along axons, and control of the quality of mitochondria. Intriguingly, MFN2 has been referred to play a role in regulating cell proliferation in several cell types such that it acts as a tumour suppressor role in some forms of cancer. Previously, we found that fibroblasts derived from a Charcot-Marie-Tooth disease type 2A (CMT2A) patient with a mutation in the GTPase domain of MFN2 exhibit increased proliferation and decreased autophagy.Methods: Primary fibroblasts from a young patient affected by CMT2A harbouring c.650G > T/p.Cys217Phe mutation in the MFN2 gene were evaluated versus a healthy control to measure the proliferation rate by growth curves analysis and to assess the phosphorylation of protein kinase B (AKT) at Ser473 in response to different doses of torin1, a selective catalytic ATP-competitive mammalian target of rapamycin complex (mTOR) inhibitor, by immunoblot analysis.
    Results: Herein, we demonstrated that the mammalian target of rapamycin complex 2 (mTORC2) is highly activated in the CMT2AMFN2 fibroblasts to promote cell growth via the AKT(Ser473) phosphorylation-mediated signalling. We report that torin1 restores CMT2AMFN2 fibroblasts' growth rate in a dose-dependent manner by decreasing AKT(Ser473) phosphorylation.
    Conclusions: Overall, our study provides evidence for mTORC2, as a novel molecular target that lies upstream of AKT to restore the cell proliferation rate in CMT2A fibroblasts.
    Keywords:  AKT; Charcot-Marie-Tooth type 2A2; cell proliferation; mTORC2; mitofusin2
  11. Elife. 2023 Feb 17. pii: e83395. [Epub ahead of print]12
      Accumulation of somatic mutations in the mitochondrial genome (mtDNA) has long been proposed as a possible mechanism of mitochondrial and tissue dysfunction that occurs during aging. A thorough characterization of age-associated mtDNA somatic mutations has been hampered by the limited ability to detect low frequency mutations. Here, we used Duplex Sequencing on eight tissues of an aged mouse cohort to detect >89,000 independent somatic mtDNA mutations and show significant tissue-specific increases during aging across all tissues examined which did not correlate with mitochondrial content and tissue function. G→A/C→T substitutions, indicative of replication errors and/or cytidine deamination, were the predominant mutation type across all tissues and increased with age, whereas G→T/C→A substitutions, indicative of oxidative damage, were the second most common mutation type, but did not increase with age regardless of tissue. We also show that clonal expansions of mtDNA mutations with age is tissue and mutation type dependent. Unexpectedly, mutations associated with oxidative damage rarely formed clones in any tissue and were significantly reduced in the hearts and kidneys of aged mice treated at late age with Elamipretide or nicotinamide mononucleotide. Thus, the lack of accumulation of oxidative damage-linked mutations with age suggests a life-long dynamic clearance of either the oxidative lesions or mtDNA genomes harboring oxidative damage.
    Keywords:  genetics; genomics; mouse
  12. Nat Biomed Eng. 2023 Feb 16.
      The monogenic nature of Huntington's disease (HD) and other neurodegenerative diseases caused by the expansion of glutamine-encoding CAG repeats makes them particularly amenable to gene therapy. Here we show the feasibility of replacing expanded CAG repeats in the mutant HTT allele with a normal CAG repeat in genetically engineered pigs mimicking the selective neurodegeneration seen in patients with HD. A single intracranial or intravenous injection of adeno-associated virus encoding for Cas9, a single-guide RNA targeting the HTT gene, and donor DNA containing the normal CAG repeat led to the depletion of mutant HTT in the animals and to substantial reductions in the dysregulated expression and neurotoxicity of mutant HTT and in neurological symptoms. Our findings support the further translational development of virally delivered Cas9-based gene therapies for the treatment of genetic neurodegenerative diseases.
  13. Semin Cell Dev Biol. 2023 Feb 13. pii: S1084-9521(23)00030-7. [Epub ahead of print]
      Mitochondria are multifaceted organelles, with such functions as the production of cellular energy to the regulation of cell death. However, mitochondria incur various sources of damage from the accumulation of reactive oxygen species and DNA mutations that can impact the protein folding environment and impair their function. Since mitochondrial dysfunction is often associated with reductions in organismal fitness and possibly disease, cells must have safeguards in place to protect mitochondrial function and promote recovery during times of stress. The mitochondrial unfolded protein response (UPRmt) is a transcriptional adaptation that promotes mitochondrial repair to aid in cell survival during stress. While the earlier discoveries into the regulation of the UPRmt stemmed from studies using mammalian cell culture, much of our understanding about this stress response has been bestowed to us by the model organism Caenorhabditis elegans. Indeed, the facile but powerful genetics of this relatively simple nematode has uncovered multiple regulators of the UPRmt, as well as several physiological roles of this stress response. In this review, we will summarize these major advancements originating from studies using C. elegans.
    Keywords:  Aging; C. elegans; Host-pathogen interactions; Infection; Mitochondria; Mitochondrial UPR; Stress response
  14. Gene Ther. 2023 Feb 14.
      Most Friedreich ataxia (FRDA) cases are caused by the elongation of the GAA repeat (GAAr) sequence in the first intron of the FXN gene, leading to a decrease of the frataxin protein expression. Deletion of this GAAr with CRISPR/Cas9 technology leads to an increase in frataxin expression in vitro. We are therefore aiming to develop FRDA treatment based on the deletion of GAAr with CRISPR/Cas9 technology using a single AAV expressing a small Cas9 (CjCas9) and two single guide RNAs (sgRNAs) targeting the FXN gene. This AAV was intraperitoneally administrated to YG8sR (250-300 GAAr) and to YG8-800 (800 GAAr) mice. DNA and RNA were extracted from different organs a month later. PCR amplification of part of intron 1 of the FXN gene detected some GAAr deletion in some cells in heart and liver of both mouse models, but the editing rate was not sufficient to cause an increase in frataxin mRNA in the heart. However, the correlation observed between the editing rate and the distribution of AAV suggests a possible therapy based on the removal of the GAAr with a better delivery tool of the CRISPR/Cas9 system.
  15. Fertil Steril. 2023 Feb 12. pii: S0015-0282(23)00136-X. [Epub ahead of print]
      OBJECTIVE: To gain insights into the technical feasibility of maternal spindle transfer (MST) applied in the context of repeated IVF failures for treatment of idiopathic infertility.DESIGN: Prospective pilot study (trial registration number: ISRCTN11455145).
    SUBJECTS: Twenty-five infertile couples with multiple previous unsuccessful IVF cycles (range 3-11), no previous pregnancy and no history of mtDNA disease participated. The study focused on women <40 years, with previous IVF attempts characterized by a pattern of low fertilization rates and/or impaired embryo development. Couples with severe male-factor infertility were not eligible. Oocyte donors with previous successful IVF outcomes were matched with patients according to standard practice.
    INTERVENTION: MST was performed by transferring metaphase II spindles from the patients' oocytes into previously enucleated donor oocytes, followed by intracytoplasmic sperm injection, in vitro embryo culture, blastocyst biopsy and vitrification. Only euploid blastocysts were considered for embryo transfer.
    MAIN OUTCOME MEASURES: Outcome measures included oocyte fertilization, blastocyst development, clinical pregnancy and live birth, incidence of mitochondrial carryover and potential mtDNA reversal, as well as general health of the children born.
    RESULTS: 28 MST cycles produced 6 children (19 embryo transfers, 7 clinical pregnancies). Paediatric follow-up of the children, performed at intervals from birth to 12-24 months of age, revealed their development to be unremarkable. DNA-fingerprinting confirmed that nuclear DNA of MST children was inherited from both parents, without any contribution from the oocyte donor. For five of the children, mtDNA was derived almost exclusively (>99%) from the donor. However, one child, who had similarly low mtDNA carryover (0.8%) at the blastocyst stage, showed an increase in the maternal mtDNA haplotype, accounting for 30-60% of the total at birth.
    CONCLUSION: This pilot study provides the first insights into the feasibility of applying MST for patients with idiopathic infertility and repeated IVF failures. Reconstructed oocytes produced embryos capable of implanting, developing to term and producing apparently healthy newborns/babies. However, claims concerning the efficacy of MST with respect to infertility treatment would be premature considering the limitations of this study. Importantly, mtDNA reversal was detected in one child born following MST, a finding with possible implications for mitochondrial replacement therapies.
    Keywords:  infertility; maternal spindle transfer; mitochondrial replacement therapies; mtDNA reversal
  16. J Intensive Med. 2022 Apr;2(2): 78-88
      Immunometabolism is a dynamic process involving the interplay of metabolism and immune response in health and diseases. Increasing evidence suggests that impaired immunometabolism contributes to infectious and inflammatory diseases. In particular, the mitochondrial enzyme aconitate decarboxylase 1 (ACOD1, best known as immunoresponsive gene 1 [IRG1]) is upregulated under various inflammatory conditions and serves as a pivotal regulator of immunometabolism involved in itaconate production, macrophage polarization, inflammasome activation, and oxidative stress. Consequently, the activation of the ACOD1 pathway is implicated in regulating the pathogenic process of sepsis and septic shock, which are part of a clinical syndrome of life-threatening organ failure caused by a dysregulated host response to pathogen infection. In this review, we discuss the latest research advances in ACOD1 expression and function, with particular attention to how the ACOD1-itaconate pathway affects infection and sterile inflammation diseases. These new insights may give us a deeper understanding of the role of immunometabolism in innate immunity.
    Keywords:  Aconitate decarboxylase 1 (ACOD1); Disease; Inflammation; Metabolism; Sepsis
  17. EMBO Rep. 2023 Feb 16. e55548
      Mechanisms underlying the depletion of NAD+ and accumulation of reactive oxygen species (ROS) in aging and age-related disorders remain poorly defined. We show that reverse electron transfer (RET) at mitochondrial complex I, which causes increased ROS production and NAD+ to NADH conversion and thus lowered NAD+ /NADH ratio, is active during aging. Genetic or pharmacological inhibition of RET decreases ROS production and increases NAD+ /NADH ratio, extending the lifespan of normal flies. The lifespan-extending effect of RET inhibition is dependent on NAD+ -dependent Sirtuin, highlighting the importance of NAD+ /NADH rebalance, and on longevity-associated Foxo and autophagy pathways. RET and RET-induced ROS and NAD+ /NADH ratio changes are prominent in human induced pluripotent stem cell (iPSC) model and fly models of Alzheimer's disease (AD). Genetic or pharmacological inhibition of RET prevents the accumulation of faulty translation products resulting from inadequate ribosome-mediated quality control, rescues relevant disease phenotypes, and extends the lifespan of Drosophila and mouse AD models. Deregulated RET is therefore a conserved feature of aging, and inhibition of RET may open new therapeutic opportunities in the context of aging and age-related diseases including AD.
    Keywords:  Alzheimer's disease; NAD+/NADH ratio; lifespan; mitochondrial complex I; reverse electron transport
  18. Antioxid Redox Signal. 2023 Feb 15.
      SIGNIFICANCE: Mitochondrial (mt) reticulum network in the cell possesses amazing ultramorphology of parallel lamellar cristae, formed by the invaginated mitochondrial inner membrane (IMM). Its non-invaginated part, the inner boundary membrane (IBM) forms a cylindrical sandwich with the outer mitochondrial membrane (OMM). Crista membranes (CM) meet IBM at crista junctions (CJs) of mt cristae organizing system (MICOS) complexes connected to OMM SAM. Cristae dimensions, shape and CJs have characteristic patterns for different metabolic regimes, physiological, and pathological situations.RECENT ADVANCES: Cristae-shaping proteins were characterized, namely rows of ATP-synthase dimers forming the crista lamella edges, MICOS subunits, OPA1 isoforms and MGM1 filaments, prohibitins and others. Detailed cristae ultramorphology changes were imaged by focused-ion beam/scanning electron microscopy. Dynamics of crista lamellae and mobile CJs were demonstrated by nanoscopy in living cells. With tBID-induced apoptosis a single entirely fused cristae reticulum was observed in a mitochondrial spheroid.
    CRITICAL ISSUES: The mobility and composition of MICOS, OPA1, and ATP-synthase dimeric rows regulated by posttranslational modifications might be exclusively responsible for cristae morphology changes, but ion fluxes across CM and resulting osmotic forces might be also involved. Inevitably, cristae ultramorphology should reflect also mitochondrial redox homeostasis, but details are unknown. Disordered cristae typically reflect higher superoxide formation.
    FUTURE DIRECTIONS: To link redox homeostasis to cristae ultramorphology and define markers, recent progress will help in uncovering mechanisms involved in proton-coupled electron transfer via the respiratory chain and in regulation of cristae architecture, leading to structural determination of superoxide formation sites and cristae ultramorphology changes in diseases.
  19. Nat Commun. 2023 Feb 16. 14(1): 871
      Bacteria can inhibit the growth of other bacteria by injecting effectors using a type VI secretion system (T6SS). T6SS effectors can also be injected into eukaryotic cells to facilitate bacterial survival, often by targeting the cytoskeleton. Here, we show that the trans-kingdom antimicrobial T6SS effector VgrG4 from Klebsiella pneumoniae triggers the fragmentation of the mitochondrial network. VgrG4 colocalizes with the endoplasmic reticulum (ER) protein mitofusin 2. VgrG4 induces the transfer of Ca2+ from the ER to the mitochondria, activating Drp1 (a regulator of mitochondrial fission) thus leading to mitochondrial network fragmentation. Ca2+ elevation also induces the activation of the innate immunity receptor NLRX1 to produce reactive oxygen species (ROS). NLRX1-induced ROS limits NF-κB activation by modulating the degradation of the NF-κB inhibitor IκBα. The degradation of IκBα is triggered by the ubiquitin ligase SCFβ-TrCP, which requires the modification of the cullin-1 subunit by NEDD8. VgrG4 abrogates the NEDDylation of cullin-1 by inactivation of Ubc12, the NEDD8-conjugating enzyme. Our work provides an example of T6SS manipulation of eukaryotic cells via alteration of the mitochondria.