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Mitochondrial diseases are a diverse group of inborn disorders affecting cellular energy production by oxidative phosphorylation (OXPHOS) via the five (CI‐CV) mitochondrial respiratory chain (MRC) complexes. The sea squirt alternative oxidase (AOX) is able to bypass the distal part of the MRC and was shown to alleviate the consequences of CIII and CIV defects in several cellular and Drosophila models. In this issue of EMBO Molecular Medicine , Rajendran et al ( 2019 ) demonstrate the first proof of concept in mammals, by showing that AOX is capable to extend lifespan and prevent heart failure in a CIII deficient mouse model, raising the possibility of future human AOX bypass treatment. Graphical Abstract A. Saada discusses the proof‐of‐concept study by Rajendran et al (in this issue of EMBO Molecular Medicine ) that indicates that AOX bypass expands lifespan in a relevant genetic mouse model of a human mitochondrial disorder.

In bacteria, SOS is a global response to DNA damage, mediated by the recA-lexA genes, resulting in cell cycle arrest, DNA repair, and mutagenesis. Previously, we reported that Escherichia coli responds to DNA damage via another recA-lexA -mediated pathway resulting in programmed cell death (PCD). We called it apoptosis-like death (ALD) because it is characterized by membrane depolarization and DNA fragmentation, which are hallmarks of eukaryotic mitochondrial apoptosis. Here, we show that ALD is an extreme SOS response that occurs only under conditions of severe DNA damage. Furthermore, we found that ALD is characterized by additional hallmarks of eukaryotic mitochondrial apoptosis, including (i) rRNA degradation by the endoribonuclease YbeY, (ii) upregulation of a unique set of genes that we called e xtensive- d amage- i n duced ( Edin ) genes, (iii) a decrease in the activities of complexes I and II of the electron transport chain, and (iv) the formation of high levels of OH˙ through the Fenton reaction, eventually resulting in cell death. Our genetic and molecular studies on ALD provide additional insight for the evolution of mitochondria and the apoptotic pathway in eukaryotes. IMPORTANCE The SOS response is the first described and the most studied bacterial response to DNA damage. It is mediated by a set of two genes, recA-lexA , and it results in DNA repair and thereby in the survival of the bacterial culture. We have shown that Escherichia coli responds to DNA damage by an additional recA-lexA -mediated pathway resulting in an apoptosis-like death (ALD). Apoptosis is a mode of cell death that has previously been reported only in eukaryotes. We found that E. coli ALD is characterized by several hallmarks of eukaryotic mitochondrial apoptosis. Altogether, our results revealed that recA-lexA is a DNA damage response coordinator that permits two opposite responses: life, mediated by the SOS, and death, mediated by the ALD. The choice seems to be a function of the degree of DNA damage in the cell. The SOS response is the first described and the most studied bacterial response to DNA damage. It is mediated by a set of two genes, recA-lexA , and it results in DNA repair and thereby in the survival of the bacterial culture. We have shown that Escherichia coli responds to DNA damage by an additional recA-lexA -mediated pathway resulting in an apoptosis-like death (ALD). Apoptosis is a mode of cell death that has previously been reported only in eukaryotes. We found that E. coli ALD is characterized by several hallmarks of eukaryotic mitochondrial apoptosis. Altogether, our results revealed that recA-lexA is a DNA damage response coordinator that permits two opposite responses: life, mediated by the SOS, and death, mediated by the ALD. The choice seems to be a function of the degree of DNA damage in the cell.

The mitochondrial inner membrane possesses distinct subdomains including cristae, which are lamellar structures invaginated into the mitochondrial matrix and contain the respiratory complexes. Generation of inner membrane domains requires the complex interplay between the respiratory complexes, mitochondrial lipids and the recently identified mitochondrial contact site and cristae organizing system (MICOS) complex. Proper organization of the mitochondrial inner membrane has recently been shown to be important for respiratory function in yeast. Here we aimed at a molecular diagnosis in a brother and sister from a consanguineous family who presented with a neurodegenerative disorder accompanied by hyperlactatemia, 3-methylglutaconic aciduria, disturbed hepatocellular function with abnormal cristae morphology in liver and cerebellar and vermis atrophy, which suggest mitochondrial dysfunction. Using homozygosity mapping and exome sequencing the patients were found to be homozygous for the p.(Gly15Glufs*75) variant in the QIL1/MIC13 (C19orf70) gene. QIL1/MIC13 is a constituent of MICOS, a six subunit complex that helps to form and/or stabilize cristae junctions and determine the placement, distribution and number of cristae within mitochondria. In patient fibroblasts both MICOS subunits QIL1/MIC13 and MIC10 were absent whereas MIC60 was present in a comparable abundance to that of the control. We conclude that QIL1/MIC13 deficiency in human, is associated with disassembly of the MICOS complex, with the associated aberration of cristae morphology and mitochondrial respiratory dysfunction. 3-Methylglutaconic aciduria is associated with variants in genes encoding mitochondrial inner membrane organizing determinants, including TAZ, DNAJC19, SERAC1 and QIL1/MIC13.

Mitochondria are involved in many cellular processes and their main role is cellular energy production. They constantly undergo fission and fusion, and these counteracting processes are under strict balance. The cytosolic dynamin-related protein 1, Drp1, or dynamin-1-like protein (DNM1L) mediates mitochondrial and peroxisomal division. Defects in the DNM1L gene result in a complex neurodevelopmental disorder with heterogeneous symptoms affecting multiple organ systems. Currently there is no curative treatment available for this condition. We have previously described a patient with a de novo heterozygous c.1084G>A (p.G362S) DNM1L mutation and studied the effects of a small molecule, bezafibrate, on mitochondrial functions in this patient’s fibroblasts compared to controls. Bezafibrate normalized growth on glucose-free medium, as well as ATP production and oxygen consumption. It improved mitochondrial morphology in the patient’s fibroblasts, although causing a mild increase in ROS production at the same time. A human foreskin fibroblast cell line overexpressing the p.G362S mutation showed aberrant mitochondrial morphology, which normalized in the presence of bezafibrate. Further studies would be needed to show the consistency of the response to bezafibrate, possibly using fibroblasts from patients with different mutations in DNM1L, and this treatment should be confirmed in clinical trials. However, taking into account the favorable effects in our study, we suggest that bezafibrate could be offered as a treatment option for patients with certain DNM1L mutations.

Background Chanarin-Dorfman syndrome (CDS) is a multisystemic autosomal recessive rare disorder. CDS is caused by variants in the abhydrolase domain containing 5 (ABHD5) encoding gene ( CGI-58 ), which ultimately leads to excessive lipid storage, and therefore a high abundance of cellular lipid droplets (LDs). Although the molecular etiology of the disease was described many years ago, no treatment for CDS is currently available. Results To further characterize the molecular basis of the disease and to uncover new treatment avenues, we used skin fibroblasts originating from a young patient diagnosed with CDS due to a homozygous nonsense mutation. We show that dysfunctional ABHD5 does not only affect LDs, but also influences other metabolic-related organelles; the mitochondria and peroxisomes. Additionally, we found that expressing functional ABHD5 in CDS patient cells reduced LD number. Finally, we developed and applied a high content-based drug repurposing screen based on a collection of ∼2500 FDA approved compounds, yielding several compounds that affected LD total area and size. Conclusions Our findings enhance the understanding of the dysfunction underlying CDS and propose new avenues for the treatment of CDS patients.

Cytochrome-c-oxidase (COX) deficiency is a frequent cause of mitochondrial disease and is associated with a wide spectrum of clinical phenotypes. We studied mitochondrial function and biogenesis in fibroblasts derived from the Cohen (CDs) rat, an animal model of COX deficiency. COX activity in CDs-fibroblasts was 50% reduced compared to control rat fibroblasts (P<0.01). ROS-production in CDs fibroblasts increased, along with marked mitochondrial fragmentation and decreased mitochondrial membrane-potential, indicating mitochondrial dysfunction. Surprisingly, cellular ATP content, oxygen consumption rate (OCR) and the extracellular acidification rate (ECAR) were unchanged. To clarify the discrepancy between mitochondrial dysfunction and ATP production, we studied mitochondrial biogenesis and turnover. The content of mitochondria was higher in CDs-fibroblasts. Consistently, AMPK activity and the expression of NRF1-target genes, NRF2 and PGC1-α that mediate mitochondrial biogenesis were increased (P<0.01 vs control fibroblast). In CDs-fibrobalsts, the number of autophagosomes (LC3+ puncta) containing mitochondria in CDs fibroblasts was similar to that in control fibroblasts, suggesting that mitophagy was intact. Altogether, our findings demonstrate that mitochondrial dysfunction and oxidative stress are associated with an increase in mitochondrial biogenesis, resulting in preservation of ATP generation.

Mitochondria hold crucial importance in organs with high energy demand especially the heart. We investigated whether chronic kidney disease (CKD), which eventually culminates in cardiorenal syndrome, could affect cardiac mitochondria and assessed the potential involvement of angiotensin II (AngII) in the process. Male Lewis rats underwent 5/6 nephrectomy allowing CKD development for eight months or for eleven weeks. Short-term CKD rats were administered with AngII receptor blocker (ARB). Cardiac function was assessed by echocardiography and cardiac sections were evaluated for interstitial fibrosis and cardiomyocytes' hypertrophy. Electron microscopy was used to explore the spatial organization of the cardiomyocytes. Expression levels of mitochondrial content and activity markers were tested in order to delineate the underlying mechanisms for mitochondrial pathology in the CKD setting with or without ARB administration. CKD per-se resulted in induced cardiac interstitial fibrosis and cardiomyocytes' hypertrophy combined with a marked disruption of the mitochondrial structure. Moreover, CKD led to enhanced cytochrome C leakage to the cytosol and to enhanced PARP-1 cleavage which are associated with cellular apoptosis. ARB treatment did not improve kidney function but markedly reduced left ventricular mass, cardiomyocytes' hypertrophy and interstitial fibrosis. Interestingly, ARB administration improved the spatial organization of cardiac mitochondria and reduced their increased volume compared to untreated CKD animals. Nevertheless, ARB did not improve mitochondrial content, mitochondrial biogenesis or the respiratory enzyme activity. ARB mildly upregulated protein levels of mitochondrial fusion-related proteins. CKD results in cardiac pathological changes combined with mitochondrial damage and elevated apoptotic markers. We anticipate that the increased mitochondrial volume mainly represents mitochondrial swelling that occurs during the pathological process of cardiac hypertrophy. Chronic administration of ARB may improve the pathological appearance of the heart. Further recognition of the molecular pathways leading to mitochondrial insult and appropriate intervention is of crucial importance.

Background ATAD3A , a nuclear gene encoding the ATAD3A protein, has diverse roles in mitochondrial processes, encompassing mitochondrial dynamics, mitochondrial DNA maintenance, metabolic pathways and inter-organellar interactions. Pathogenic variants in this gene cause neurological diseases in humans with recognizable genotype-phenotype correlations. Yet, gaps in knowledge remain regarding the underlying pathogenesis. Methods To further investigate the gene function and its implication in health and disease, we utilized CRISPR/Cas9 genome editing to generate a knockout model of the zebrafish ortholog gene, atad3 . We characterized the phenotype of the null model, performed mitochondrial and functional tests, and compared the transcriptome of null embryos to their healthy siblings. Results Analysis of atad3 -null zebrafish embryos revealed microcephaly, small eyes, pericardial edema and musculature thinning, closely mirroring the human rare disease phenotype. Larvae exhibited delayed hatching and embryonic lethality by 13 days post-fertilization (dpf). Locomotor activity, ATP content, mitochondrial content, and mitochondrial activity were all reduced in the mutant embryos. Transcriptome analysis at 3 dpf via RNA-sequencing indicated decline in most mitochondrial pathways, accompanied by a global upregulation of cytosolic tRNA synthetases, presumably secondary to mitochondrial stress and possibly endoplasmic reticulum (ER)-stress. Differential expression of select genes was corroborated in fibroblasts from an affected individual. Conclusions The atad3 -null zebrafish model emerges as a reliable representation of human ATAD3A -associated disorders, with similarities in differentially expressed pathways and processes. Furthermore, our study underscores mitochondrial dysfunction as the primary underlying pathogenic mechanism in ATAD3A- associated disorders and identifies potential readouts for therapeutic studies.

Congenital deficiency of the mitochondrial respiratory chain complex I (CI) is a common defect of oxidative phosphorylation (OXPHOS). Despite major advances in the biochemical and molecular diagnostics and the deciphering of CI structure, function assembly and pathomechanism, there is currently no satisfactory cure for patients with mitochondrial complex I defects. Small molecules provide one feasible therapeutic option, however their use has not been systematically evaluated using a standardized experimental system. In order to evaluate potentially therapeutic compounds, we set up a relatively simple system measuring different parameters using only a small amount of patient's fibroblasts, in glucose free medium, where growth is highly OXPOS dependent. Ten different compounds were screened using fibroblasts derived from seven CI patients, harboring different mutations.5-Aminoimidazole-4-carboxamide ribotide (AICAR) was found to be the most beneficial compound improving growth and ATP content while decreasing ROS production. AICAR also increased mitochondrial biogenesis without altering mitochondrial membrane potential (Δψ). Fluorescence microscopy data supported increased mitochondrial biogenesis and activation of the AMP activated protein kinase (AMPK). Other compounds such as; bezafibrate and oltipraz were rated as favorable while polyphenolic phytochemicals (resverastrol, grape seed extract, genistein and epigallocatechin gallate) were found not significant or detrimental. Although the results have to be verified by more thorough investigation of additional OXPHOS parameters, preliminary rapid screening of potential therapeutic compounds in individual patient's fibroblasts could direct and advance personalized medical treatment.

Hemorrhage is the primary cause of preventable death in both military and civilian trauma cases, and the effective therapeutic options are limited. Activation of Protein Kinase C epsilon (PKC-ε) was shown to have a protective role in ischemia–reperfusion injury models. Thus, we evaluated the effects of a PKC-ε activator peptide in a swine model of controlled hemorrhagic shock. Controlled hemorrhage was induced in 25 Sus Domesticus female pigs by blood withdrawal. Fifteen animals were treated with PKC-ε activator peptide (3 mg/kg IM) five minutes following the initiation of hemorrhage, and 8 animals were bled without receiving the peptide. Two additional animals were treated with the peptide without having been bled for safety validation. Hemodynamic and biochemical parameters were monitored for 7 h, and mitochondrial function markers were analyzed and compared between groups. 73.3% of the pigs that received the peptide survived the hemorrhage until the end of the follow-up compared to only 25% of non-treated control animals. Kaplan–Meier survival analysis showed a significant difference between the groups ( p  = 0.044). This benefit was associated with a more favorable hemodynamic profile, including more stable blood pressure, heart rate and cardiac output, and a better acid–base balance. Mitochondrial analysis identified a significant increase in electron transport chain complex-I activity in the myocardium of treated animals compared with the controls ( p  = 0.033). In conclusion, Administration of PKC-ε activator is associated with improved survival, hemodynamic stability, and mitochondrial function in a porcine model of controlled hemorrhage.