A Drosophila model of mitochondrial disease caused by a complex I mutation that uncouples proton pumping from electron transfer

Mutations affecting mitochondrial complex I, a multi-subunit assembly that couples electron transfer to proton pumping, are the most frequent cause of heritable mitochondrial diseases. However, the mechanisms by which complex I dysfunction results in disease remain unclear. Here, we describe a Drosophila model of complex I deficiency caused by a homoplasmic mutation in the mitochondrial-DNA-encoded NADH dehydrogenase subunit 2 (ND2) gene. We show that ND2 mutants exhibit phenotypes that resemble symptoms of mitochondrial disease, including shortened lifespan, progressive neurodegeneration, diminished neural mitochondrial membrane potential and lower levels of neural ATP. Our biochemical studies of ND2 mutants reveal that complex I is unable to efficiently couple electron transfer to proton pumping. Thus, our study provides evidence that the ND2 subunit participates directly in the proton pumping mechanism of complex I. Together, our findings support the model that diminished respiratory chain activity, and consequent energy deficiency, are responsible for the pathogenesis of complex-I-associated neurodegeneration.KEY WORDS: Mitochondria, Drosophila, Mitochondrial disease, Respiratory chain, Leigh syndrome, Neurodegeneration     

Cardiac Metabolic Pathways Affected in the Mouse Model of Barth Syndrome

Cardiolipin (CL) is a mitochondrial phospholipid essential for electron transport chain (ETC) integrity. CL-deficiency in humans is caused by mutations in the tafazzin (Taz) gene and results in a multisystem pediatric disorder, Barth syndrome (BTHS). It has been reported that tafazzin deficiency destabilizes mitochondrial respiratory chain complexes and affects supercomplex assembly. The aim of this study was to investigate the impact of Taz-knockdown on the mitochondrial proteomic landscape and metabolic processes, such as stability of respiratory chain supercomplexes and their interactions with fatty acid oxidation enzymes in cardiac muscle. Proteomic analysis demonstrated reduction of several polypeptides of the mitochondrial respiratory chain, including Rieske and cytochrome c1 subunits of complex III, NADH dehydrogenase alpha subunit 5 of complex I and the catalytic core-forming subunit of F₀F₁-ATP synthase. Taz gene knockdown resulted in upregulation of enzymes of folate and amino acid metabolic pathways in heart mitochondria, demonstrating that Taz-deficiency causes substantive metabolic remodeling in cardiac muscle. Mitochondrial respiratory chain supercomplexes are destabilized in CL-depleted mitochondria from Taz knockdown hearts resulting in disruption of the interactions between ETC and the fatty acid oxidation enzymes, very long-chain acyl-CoA dehydrogenase and long-chain 3-hydroxyacyl-CoA dehydrogenase, potentially affecting the metabolic channeling of reducing equivalents between these two metabolic pathways. Mitochondria-bound myoglobin was significantly reduced in Taz-knockdown hearts, potentially disrupting intracellular oxygen delivery to the oxidative phosphorylation system. Our results identify the critical pathways affected by the Taz-deficiency in mitochondria and establish a future framework for development of therapeutic options for BTHS.     

Assembly defects induce oxidative stress in inherited mitochondrial complex I deficiency

Complex I (CI) deficiency is the most common respiratory chain defect representing more than 30% of mitochondrial diseases. CI is an L-shaped multi-subunit complex with a peripheral arm protruding into the mitochondrial matrix and a membrane arm. CI sequentially assembled into main assembly intermediates: the P (pumping), Q (Quinone) and N (NADH dehydrogenase) modules. In this study, we analyzed 11 fibroblast cell lines derived from patients with inherited CI deficiency resulting from mutations in the nuclear or mitochondrial DNA and impacting these different modules. In patient cells carrying a mutation located in the matrix arm of CI, blue native-polyacrylamide gel electrophoresis (BN-PAGE) revealed a significant reduction of fully assembled CI enzyme and an accumulation of intermediates of the N module. In these cell lines with an assembly defect, NADH dehydrogenase activity was partly functional, even though CI was not fully assembled. We further demonstrated that this functional N module was responsible for ROS production through the reduced flavin mononucleotide. Due to the assembly defect, the FMN site was not re-oxidized leading to a significant oxidative stress in cell lines with an assembly defect. These findings not only highlight the relationship between CI assembly and oxidative stress, but also show the suitability of BN-PAGE analysis in evaluating the consequences of CI dysfunction. Moreover, these data suggest that the use of antioxidants may be particularly relevant for patients displaying a CI assembly defect.     

Mutations in NDUFAF3 (C3ORF60), Encoding an NDUFAF4 (C6ORF66)-Interacting Complex I Assembly Protein, Cause Fatal Neonatal Mitochondrial Disease

Mitochondrial complex I deficiency is the most prevalent and least understood disorder of the oxidative phosphorylation system. The genetic cause of many cases of isolated complex I deficiency is unknown because of insufficient understanding of the complex I assembly process and the factors involved. We performed homozygosity mapping and gene sequencing to identify the genetic defect in five complex I-deficient patients from three different families. All patients harbored mutations in the NDUFAF3 (C3ORF60) gene, of which the pathogenic nature was assessed by NDUFAF3-GFP baculovirus complementation in fibroblasts. We found that NDUFAF3 is a genuine mitochondrial complex I assembly protein that interacts with complex I subunits. Furthermore, we show that NDUFAF3 tightly interacts with NDUFAF4 (C6ORF66), a protein previously implicated in complex I deficiency. Additional gene conservation analysis links NDUFAF3 to bacterial-membrane-insertion gene cluster SecF/SecD/YajC and to C8ORF38, also implicated in complex I deficiency. These data not only show that NDUFAF3 mutations cause complex I deficiency but also relate different complex I disease genes by the close cooperation of their encoded proteins during the assembly process.     

Redirection of the NADH oxidation pathway in Torulopsis glabrata leads to an enhanced pyruvate production

This study aimed at increasing the pyruvate productivity of a multi-vitamin auxotrophic yeast Torulopsis glabrata by redirecting NADH oxidation from adenosine triphosphate (ATP)-production pathway (oxidative phosphorylation pathway) to non-ATP production pathway (fermentative pathway). Two respiratory-deficient mutants, RD-17 and RD-18, were screened and selected after ethidium bromide (EtBr) mutagenesis of the parent strain T. glabrata CCTCC M202019. Compared with the parent strain, cytochrome aa ₃ and b in electron transfer chain (ETC) of RD-18 and cytochrome b in RD-17 were disrupted. As a consequence, the activities of key ETC enzymes of the mutant RD-18, including F₀F₁-ATP synthase, complex I, complex I + III, complex II + III, and complex IV, decreased by 22.2, 41.6, 53.1, 23.6, and 84.7%, respectively. With the deficiency of cytochromes in ETC, a large amount of excessive cytosolic NADH was accumulated, which hampered the further increase of the glycolytic flux. An exogenous electron acceptor, acetaldehyde, was added to the strain RD-18 culture to oxidize the excessive NADH. Compared with the parent strain, the concentration of pyruvate and the glucose consumption rate of strain RD-18 were increased by 26.5 and 17.6%, respectively, upon addition of 2.1 mM of acetaldehyde. The strategy for increasing the glycolytic flux in T. glabrata by redirecting the NADH oxidation pathway may provide an alternative approach to enhance the glycolytic flux in yeast.     

An RDH‐Plin2 axis modulates lipid droplet size by antagonizing Bmm lipase

The size of lipid droplets varies greatly in vivo and is determined by both intrinsic and extrinsic factors. From an RNAi screen in Drosophila, we found that knocking down subunits of COP9 signalosome (CSN) results in enlarged lipid droplets under high‐fat, but not normal, conditions. We identified CG2064, a retinol dehydrogenase (RDH) homolog, as the proteasomal degradation target of CSN in regulating lipid droplet size. RDH/CG2064 interacts with the lipid droplet‐resident protein Plin2 and the RDH/CG2064‐Plin2 axis acts to reduce the overall level and lipid droplet localization of Bmm/ATGL lipase. This axis is important for larval survival under prolonged starvation. Thus, we discovered an RDH‐Plin2 axis modulates lipid droplet size.     

Drosophila SMN complex proteins Gemin2, Gemin3, and Gemin5 are components of U bodies

Uridine-rich small nuclear ribonucleoproteins (U snRNPs) play key roles in pre-mRNA processing in the nucleus. The assembly of most U snRNPs takes place in the cytoplasm and is facilitated by the survival motor neuron (SMN) complex. Discrete cytoplasmic RNA granules called U bodies have been proposed to be specific sites for snRNP assembly because they contain U snRNPs and SMN. U bodies invariably associate with P bodies, which are involved in mRNA decay and translational control. However, it remains unknown whether other SMN complex proteins also localise to U bodies. In Drosophila there are four SMN complex proteins, namely SMN, Gemin2/CG10419, Gemin3 and Gemin5/Rigor mortis. Drosophila Gemin3 was originally identified as the Drosophila orthologue of human and yeast Dhh1, a component of P bodies. Through an in silico analysis of the DEAD-box RNA helicases we confirmed that Gemin3 is the bona fide Drosophila orthologue of vertebrate Gemin3 whereas the Drosophila orthologue of Dhh1 is Me31B. We then made use of the Drosophila egg chamber as a model system to study the subcellular distribution of the Gemin proteins as well as Me31B. Our cytological investigations show that Gemin2, Gemin3 and Gemin5 colocalise with SMN in U bodies. Although they are excluded from P bodies, as components of U bodies, Gemin2, Gemin3 and Gemin5 are consistently found associated with P bodies, wherein Me31B resides. In addition to a role in snRNP biogenesis, SMN complexes residing in U bodies may also be involved in mRNP assembly and/or transport.     

Partial Purification and Characterization of Complex I, NADH:Ubiquinone Reductase, from the Inner Membrane of Beetroot Mitochondria

A NADH dehydrogenase was isolated from an inner membrane-enriched fraction of beetroot mitochondria (Beta vulgaris L.) by solubilization with sodium deoxycholate and purified using gel filtration and affinity chromatography. The NADH dehydrogenase preparation contained a minor ATPase contamination. Beetroot mitochondria were chosen as the isolation material for purifying the enzymes responsible for oxidizing matrix NADH due to the absence of the externally facing NADH dehydrogenase in the variety we have used. The purified NADH dehydrogenase complex catalyzed the reduction of various electron acceptors with NADH as the electron donor, was not sensitive to rotenone inhibition, and had a slow NADPH-ubiquinone 5 reductase activity. The isolated complex contained 14 major polypeptides. It was concluded that the dehydrogenase represented a form of the plant mitochondrial complex I and not the internally facing rotenone-insensitive NADH dehydrogenase found in plant mitochondria because of its complex structure, its cross-reactivity with antisera raised against bovine heart mitochondrial complex I, and the similarity of its kinetics and inhibitor responses to rotenone-sensitive NADH oxidation by beetroot submitochondrial particles.     

Functional Dissection of the Drosophila melanogaster Condensin Subunit Cap-G Reveals Its Exclusive Association with Condensin I

The heteropentameric condensin complexes have been shown to participate in mitotic chromosome condensation and to be required for unperturbed chromatid segregation in nuclear divisions. Vertebrates have two condensin complexes, condensin I and condensin II, which contain the same structural maintenance of chromosomes (SMC) subunits SMC2 and SMC4, but differ in their composition of non–SMC subunits. While a clear biochemical and functional distinction between condensin I and condensin II has been established in vertebrates, the situation in Drosophila melanogaster is less defined. Since Drosophila lacks a clear homolog for the condensin II–specific subunit Cap-G2, the condensin I subunit Cap-G has been hypothesized to be part of both complexes. In vivo microscopy revealed that a functional Cap-G-EGFP variant shows a distinct nuclear enrichment during interphase, which is reminiscent of condensin II localization in vertebrates and contrasts with the cytoplasmic enrichment observed for the other EGFP-fused condensin I subunits. However, we show that this nuclear localization is dispensable for Cap-G chromatin association, for its assembly into the condensin I complex and, importantly, for development into a viable and fertile adult animal. Immunoprecipitation analyses and complex formation studies provide evidence that Cap-G does not associate with condensin II–specific subunits, while it can be readily detected in complexes with condensin I–specific proteins in vitro and in vivo. Mass-spectrometric analyses of proteins associated with the condensin II–specific subunit Cap-H2 not only fail to identify Cap-G but also the other known condensin II–specific homolog Cap-D3. As condensin II–specific subunits are also not found associated with SMC2, our results question the existence of a soluble condensin II complex in Drosophila.     

Adipose deficiency and aberrant autophagy in a Drosophila model of MPS VII is corrected by pharmacological stimulators of mTOR

Mucopolysaccharidosis type VII (MPS VII) is a recessively inherited lysosomal storage disorder caused due to β-glucuronidase (β-GUS) enzyme deficiency. Prominent clinical symptoms include hydrops fetalis, musculoskeletal deformities, neurodegeneration and hepatosplenomegaly leading to premature death in most cases. Apart from these, MPS VII is also characterized as adipose storage deficiency disorder although the underlying mechanism of this lean phenotype in the patients or β-GUS-deficient mice still remains a mystery. We addressed this issue using our recently developed Drosophila model of MPS VII (the CG2135^-/- fly), which also exhibited a significant loss of body fat. We report here that the lean phenotype of the CG2135^?/? larvae is due to fewer number of adipocytes, smaller lipid droplets and reduced adipogenesis. Our data further revealed that there is an abnormal accumulation of autophagosomes in the CG2135^?/? larvae due to autophagosome-lysosome fusion defect. Decreased lysosome-mediated turnover also led to attenuated mTOR activity in the CG2135^?/? larvae. Interestingly, treatment of the CG2135^?/? larvae with mTOR stimulators, 3BDO or glucose, led to the restoration of mTOR activity with simultaneous correction of the autophagy defect and adipose storage deficiency. Our finding thus established a hitherto unknown mechanistic link between autophagy dysfunction, mTOR downregulation and reduced adiposity in MPS VII.     

The topology of superoxide production by complex III and glycerol 3-phosphate dehydrogenase in Drosophila mitochondria

The topology of superoxide generation by sn-glycerol 3-phosphate dehydrogenase and complex III in intact Drosophila mitochondria was studied using aconitase inactivation to measure superoxide production in the matrix, and hydrogen peroxide formation in the presence of superoxide dismutase to measure superoxide production from both sides of the membrane. Aconitase inactivation was calibrated using the known rate of matrix superoxide production from complex I. Glycerol phosphate dehydrogenase generated superoxide about equally to each side of the membrane, whereas centre o of complex III in the presence of antimycin A generated superoxide about 30% on the cytosolic side and 70% on the matrix side.     

C6ORF66 is an assembly factor of mitochondrial complex I

Homozygosity mapping was performed in five patients from a consanguineous family who presented with infantile mitochondrial encephalomyopathy attributed to isolated NADH:ubiquinone oxidoreductase (complex I) deficiency. This resulted in the identification of a missense mutation in a conserved residue of the C6ORF66 gene, which encodes a 20.2 kDa mitochondrial protein. The mutation was also detected in a patient who presented with antenatal cardiomyopathy. In muscle of two patients, the levels of the C6ORF66 protein and of the fully assembled complex I were markedly reduced. Transfection of the patients' fibroblasts with wild-type C6ORF66 cDNA restored complex I activity. These data suggest that C6ORF66 is an assembly factor of complex I. Interestingly, the C6ORF66 gene product was previously shown to promote breast cancer cell invasiveness.     

Combination of Hypomorphic Mutations of the Drosophila Homologues of Aryl Hydrocarbon Receptor and Nucleosome Assembly Protein Family Genes Disrupts Morphogenesis,Memory and Detoxification

Aryl hydrocarbon receptor is essential for biological responses to endogenous and exogenous toxins in mammals. Its Drosophila homolog spineless plays an important role in fly morphogenesis. We have previously shown that during morphogenesis spineless genetically interacts with CG5017 gene, which encodes a nucleosome assembly factor and may affect cognitive function of the fly. We now demonstrate synergistic interactions of spineless and CG5017 in pathways controlling oxidative stress response and long-term memory formation in Drosophila melanogaster. Oxidative stress was induced by low doses of X-ray irradiation of flies carrying hypomorphic mutation of spineless, mutation of CG5017, and their combination. To determine the sensitivity of these mutants to pharmacological modifiers of the irradiation effect, we irradiated flies growing on standard medium supplemented by radiosensitizer furazidin and radioprotector serotonin. The effects of irradiation were investigated by analyzing leg and antenna morphological structures and by using real-time PCR to measure mRNA expression levels for spineless, Cyp6g1 and Gst-theta genes. We also examined long-term memory in these mutants using conditioned courtship suppression paradigm. Our results show that the interaction of spineless and CG5017 is important for regulation of morphogenesis, long-term memory formation, and detoxification during oxidative stress. Since spineless and CG5017 are evolutionary conserved, these results must be considered when evaluating the risk of combining similar mutations in other organisms, including humans.     

Measuring the functionality of the mitochondrial pumping complexes with multi-wavelength spectroscopy

The proton pumps of the mitochondrial electron transport chain (ETC) convert redox energy into the proton motive force (ΔP), which is subsequently used by the ATP synthase to regenerate ATP. The limited available redox free energy requires the proton pumps to operate close to equilibrium in order to maintain a high ΔP, which in turn is needed to maintain a high phosphorylation potential. Current biochemical assays measure complex activities far from equilibrium and so shed little light on their function under physiological conditions. Here we combine absorption spectroscopy of the ETC hemes, NADH fluorescence spectroscopy and oxygen consumption to simultaneously measure the redox potential of the intermediate redox pools, the components of ΔP and the electron flux in RAW 264.7 mouse macrophages. We confirm that complex I and III operate near equilibrium and quantify the linear relationship between flux and disequilibrium as a metric of their function under physiological conditions. In addition, we quantify the dependence of complex IV turnover on ΔP and the redox potential of cytochrome c to determine the complex IV driving force and find that the turnover is proportional to this driving force. This form of quantification is a more relevant metric of ETC function than standard biochemical assays and can be used to study the effect of mutations in either mitochondrial or nuclear genome affecting mitochondrial function, post-translation changes, different subunit isoforms, as well as the effect of pharmaceuticals on ETC function.     

Evidence for a role for the putative <Emphasis Type="Italic">Drosophila</Emphasis> hGRX1 orthologue in copper homeostasis

Glutaredoxins are a family of small molecular weight proteins that have a central role in cellular redox regulation. Human GRX1 (hGRX1) has also been shown to play an integral role in copper homeostasis by regulating the redox activity of the metalated sites of copper chaperones such as ATOX1 and SOD1, and the copper efflux proteins ATP7A and ATP7B. To further elucidate the role of hGRX1 in copper homeostasis, we examined the impact of RNA interference-mediated knockdown of CG6852, a putative Drosophila orthologue of hGRX1. CG6852 shares ~41 % amino acid identity with hGRX1 and key functional domains including the metal-binding CXXC motif are conserved between the two proteins. Knockdown of CG6852 in the adult midline caused a thoracic cleft and reduced scutellum, phenotypes that were exacerbated by additional knockdown of copper uptake transporters Ctr1A and Ctr1B. Knockdown of CG6852 in the adult eye enhanced a copper-deficiency phenotype caused by Ctr1A knockdown while ubiquitous knockdown of CG6852 resulted a mild systemic copper deficiency. Therefore we conclude that CG6852 is a putative orthologue of hGRX1 and may play an important role in Drosophila copper homeostasis.     

线粒体呼吸链复合体Ⅰ

线粒体呼吸链复合体Ⅰ（简称复合体Ⅰ）是呼吸链电子传递的起始复合体,作为电子传递过程的限速酶,复合体Ⅰ的分子量远大于其余的四个呼吸链复合体。复合体Ⅰ相关的疾病发生除了与40余个复合体Ⅰ组成亚基的突变相关外,还同参与其组装的多个组装因子存在密切联系。该文对复合体I的结构以及参与调控复合体Ⅰ组装的各类组装因子进行了综述,旨在为全面了解复合体Ⅰ相关疾病的发生提供具体参考。     

Comparison of the Kinetic Behavior toward Pyridine Nucleotides of NAD-Linked Dehydrogenases from Plant Mitochondria

In this article we compare the kinetic behavior toward pyridine nucleotides (NAD⁺, NADH) of NAD⁺-malic enzyme, pyruvate dehydrogenase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and glycine decarboxylase extracted from pea (Pisum sativum) leaf and potato (Solanum tuberosum) tuber mitochondria. NADH competitively inhibited all the studied dehydrogenases when NAD⁺ was the varied substrate. However, the NAD⁺-linked malic enzyme exhibited the weakest affinity for NAD⁺ and the lowest sensitivity for NADH. It is suggested that NAD⁺-linked malic enzyme, when fully activated, is able to raise the matricial NADH level up to the required concentration to fully engage the rotenone-resistant internal NADH-dehydrogenase, whose affinity for NADH is weaker than complex I.     

Mutations in the gene encoding C8orf38 block complex I assembly by inhibiting production of the mitochondria-encoded subunit ND1

The assembly of complex I (NADH-ubiquinone oxidoreductase) is a complicated process, requiring the integration of 45 subunits encoded by both nuclear and mitochondrial DNAs into a structure of approximately 1 MDa. A number of “assembly factors” that aid complex I biogenesis have recently been described, including C8orf38. This protein was identified as an assembly factor by its evolutionary conservation in organisms containing complex I and by a C8orf38 mutation in a patient presenting with Leigh syndrome and isolated complex I deficiency. In this report, we have undertaken the characterization of C8orf38 and its role in complex I assembly. Analysis of mitochondria from fibroblasts of a patient harboring a C8orf38 mutation showed almost undetectable levels of steady-state complex I and defective biogenesis of the mtDNA-encoded subunit ND1. Complementation with wild-type C8orf38 restored the levels of both ND1 and complex I, confirming the C8orf38 mutation as the cause of the complex I defect in the patient. In the absence of ND1 in patient cells, early- and mid-stage intermediate complexes were still formed; however, assembly of late-stage intermediates was impaired, indicating a convergence point in the assembly process. While C8orf38 appears to behave at a step in complex I biogenesis similar to that of the assembly factor C20orf7, complementation studies showed that both proteins are required for ND1 synthesis/stabilization. We conclude that C8orf38 is a crucial factor required for the translation and/or integration of ND1 into an early-stage assembly intermediate and that mutation of C8orf38 disrupts the initial stages of complex I biogenesis.     

Biphasic modulation of the mitochondrial electron transport chain in myocardial ischemia and reperfusion

Mitochondrial electron transport chain (ETC) is the major source of reactive oxygen species during myocardial ischemia-reperfusion (I/R) injury. Ischemic defect and reperfusion-induced injury to ETC are critical in the disease pathogenesis of postischemic heart. The properties of ETC were investigated in an isolated heart model of global I/R. Rat hearts were subjected to ischemia for 30 min followed by reperfusion for 1 h. Studies of mitochondrial function indicated a biphasic modulation of electron transfer activity (ETA) and ETC protein expression during I/R. Analysis of ETAs in the isolated mitochondria indicated that complexes I, II, III, and IV activities were diminished after 30 min of ischemia but increased upon restoration of flow. Immunoblotting analysis and ultrastructural analysis with transmission electron microscopy further revealed marked downregulation of ETC in the ischemic heart and then upregulation of ETC upon reperfusion. No significant difference in the mRNA expression level of ETC was detected between ischemic and postischemic hearts. However, reperfusion-induced ETC biosynthesis in myocardium can be inhibited by cycloheximide, indicating the involvement of translational control. Immunoblotting analysis of tissue homogenates revealed a similar profile in peroxisome proliferator-activated receptor-γ coactivator-1α expression, suggesting its essential role as an upstream regulator in controlling ETC biosynthesis during I/R. Significant impairment caused by ischemic and postischemic injury was observed in the complexes I- III. Analysis of NADH ferricyanide reductase activity indicated that injury of flavoprotein subcomplex accounts for 50% decline of intact complex I activity from ischemic heart. Taken together, our findings provide a new insight into the molecular mechanism of I/R-induced mitochondrial dysfunction.