Can a Single Subunit Yeast NADH Dehydrogenase (Ndi1) Remedy Diseases Caused by Respiratory Complex I Defects?

Effect of levan polysaccharide on chronological aging in the yeast Saccharomyces cerevisiae

Levan is a fructose-based biopolymer with diverse applications in the medicinal, pharmaceutical, and food industries. However, despite its extensive biological and pharmacological actions, including antioxidant, anti-inflammatory, and antidiabetic properties, research on its anti-aging potential is limited. This study explored levan's impact on the chronological lifespan (CLS) of yeast Saccharomyces cerevisiae for the first time. The results show that levan treatment significantly extended the CLS of wild-type (WT) yeast by preventing the accumulation of oxidative stress markers (reactive oxygen species, malondialdehyde, and protein carbonyl content) and ameliorating apoptotic features such as reduced mitochondrial membrane potential, loss of plasma membrane integrity, and externalization of phosphatidylserine. By day 40 of the CLS, a significant increase in yeast viability of 6.8 % (p < 0.01), 11.9 % (p < 0.01), and 20.8 % (p < 0.01) was observed at 0.25, 0.5, and 1 mg/mL of levan concentrations, respectively, compared to control (0 %). This study's results indicate that levan treatment substantially modulates the expression of genes involved in the TORC1/Sch9 pathway. Moreover, levan treatment significantly extended the CLS of yeast antioxidant-deficient mutant sod2Δ and antiapoptotic gene-deficient mutant pep4Δ. Levan also extended the CLS of signaling pathway gene-deficient mutants such as pkh2Δ, rim15Δ, atg1, and ras2Δ, while not affecting the CLS of tor1Δ and sch9Δ.

Activation of Oxidative Stress Response in Cancer Generates a Druggable Dependency on Exogenous Non-essential Amino Acids

Rewiring of metabolic pathways is a hallmark of tumorigenesis as cancer cells acquire novel nutrient dependencies to support oncogenic growth. A major genetic subtype of lung adenocarcinoma with KEAP1/NRF2 mutations, which activates the endogenous oxidative stress response, undergoes significant metabolic rewiring to support enhanced antioxidant production. We demonstrate that cancers with high antioxidant capacity exhibit a general dependency on exogenous non-essential amino acids (NEAAs) that is driven by the Nrf2-dependent secretion of glutamate through system x_c^- (XCT), which limits intracellular glutamate pools that are required for NEAA synthesis. This dependency can be therapeutically targeted by dietary restriction or enzymatic depletion of individual NEAAs. Importantly, limiting endogenous glutamate levels by glutaminase inhibition can sensitize tumors without alterations in the Keap1/Nrf2 pathway to dietary restriction of NEAAs. Our findings identify a metabolic strategy to therapeutically target cancers with genetic or pharmacologic activation of the Nrf2 antioxidant response pathway by restricting exogenous sources of NEAAs.

Codon optimization is an essential parameter for the efficient allotopic expression of mtDNA genes

Mutations in mitochondrial DNA can be inherited or occur de novo leading to several debilitating myopathies with no curative option and few or no effective treatments. Allotopic expression of recoded mitochondrial genes from the nucleus has potential as a gene therapy strategy for such conditions, however progress in this field has been hampered by technical challenges. Here we employed codon optimization as a tool to re-engineer the protein-coding genes of the human mitochondrial genome for robust, efficient expression from the nucleus. All 13 codon-optimized constructs exhibited substantially higher protein expression than minimally-recoded genes when expressed transiently, and steady-state mRNA levels for optimized gene constructs were 5-180 fold enriched over recoded versions in stably-selected wildtype cells. Eight of thirteen mitochondria-encoded oxidative phosphorylation (OxPhos) proteins maintained protein expression following stable selection, with mitochondrial localization of expression products. We also assessed the utility of this strategy in rescuing mitochondrial disease cell models and found the rescue capacity of allotopic expression constructs to be gene specific. Allotopic expression of codon optimized ATP8 in disease models could restore protein levels and respiratory function, however, rescue of the pathogenic phenotype for another gene, ND1 was only partially successful. These results imply that though codon-optimization alone is not sufficient for functional allotopic expression of most mitochondrial genes, it is an essential consideration in their design.

Alternative NADH dehydrogenase extends lifespan and increases resistance to xenobiotics in Drosophila

Mitochondrial alternative NADH dehydrogenase (aNDH) was found to extend lifespan when expressed in the fruit fly. We have found that fruit flies expressing aNDH from Ciona intestinalis (NDX) had 17–71% lifespan prolongation on media with different protein-tocarbohydrate ratios except NDX-expressing males that had 19% shorter lifespan than controls on a high protein diet. NDX-expressing flies were more resistant to organic xenobiotics, 2,4-dichlorophenoxyacetic acid and alloxan, and inorganic toxicant potassium iodate, and partially to sodium molybdate treatments. On the other hand, NDX-expressing flies were more sensitive to catechol and sodium chromate. Enzymatic analysis showed that NDX-expressing males had higher glucose 6-phosphate dehydrogenase activity, whilst both sexes showed increased glutathione S-transferase activity.

Differential Effects of Yeast NADH Dehydrogenase (Ndi1) Expression on Mitochondrial Function and Inclusion Formation in a Cell Culture Model of Sporadic Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder that exhibits aberrant protein aggregation and mitochondrial dysfunction. Ndi1, the yeast mitochondrial NADH dehydrogenase (complex I) enzyme, is a single subunit, internal matrix-facing protein. Previous studies have shown that Ndi1 expression leads to improved mitochondrial function in models of complex I-mediated mitochondrial dysfunction. The trans-mitochondrial cybrid cell model of PD was created by fusing mitochondrial DNA-depleted SH-SY5Y cells with platelets from a sporadic PD patient. PD cybrid cells reproduce the mitochondrial dysfunction observed in a patient's brain and periphery and form intracellular, cybrid Lewy bodies comparable to Lewy bodies in PD brain. To improve mitochondrial function and alter the formation of protein aggregates, Ndi1 was expressed in PD cybrid cells and parent SH-SY5Y cells. We observed a dramatic increase in mitochondrial respiration, increased mitochondrial gene expression, and increased PGC-1α gene expression in PD cybrid cells expressing Ndi1. Total cellular aggregated protein content was decreased but Ndi1 expression was insufficient to prevent cybrid Lewy body formation. Ndi1 expression leads to improved mitochondrial function and biogenesis signaling, both processes that could improve neuron survival during disease. However, other aspects of PD pathology such as cybrid Lewy body formation were not reduced. Consequently, resolution of mitochondrial dysfunction alone may not be sufficient to overcome other aspects of PD-related cellular pathology.

Studying Huntington's Disease in Yeast: From Mechanisms to Pharmacological Approaches

Huntington's disease (HD) is a neurodegenerative disorder that leads to progressive neuronal loss, provoking impaired motor control, cognitive decline, and dementia. So far, HD remains incurable, and available drugs are effective only for symptomatic management. HD is caused by a mutant form of the huntingtin protein, which harbors an elongated polyglutamine domain and is highly prone to aggregation. However, many aspects underlying the cytotoxicity of mutant huntingtin (mHTT) remain elusive, hindering the efficient development of applicable interventions to counteract HD. An important strategy to obtain molecular insights into human disorders in general is the use of eukaryotic model organisms, which are easy to genetically manipulate and display a high degree of conservation regarding disease-relevant cellular processes. The budding yeast Saccharomyces cerevisiae has a long-standing and successful history in modeling a plethora of human maladies and has recently emerged as an effective tool to study neurodegenerative disorders, including HD. Here, we summarize some of the most important contributions of yeast to HD research, specifically concerning the elucidation of mechanistic features of mHTT cytotoxicity and the potential of yeast as a platform to screen for pharmacological agents against HD.

Being right on Q: shaping eukaryotic evolution

Reactive oxygen species (ROS) formation by mitochondria is an incompletely understood eukaryotic process. I proposed a kinetic model [BioEssays (2011) 33, 88–94] in which the ratio between electrons entering the respiratory chain via FADH₂ or NADH (the F/N ratio) is a crucial determinant of ROS formation. During glucose breakdown, the ratio is low, while during fatty acid breakdown, the ratio is high (the longer the fatty acid, the higher is the ratio), leading to higher ROS levels. Thus, breakdown of (very-long-chain) fatty acids should occur without generating extra FADH₂ in mitochondria. This explains peroxisome evolution. A potential ROS increase could also explain the absence of fatty acid oxidation in long-lived cells (neurons) as well as other eukaryotic adaptations, such as dynamic supercomplex formation. Effective combinations of metabolic pathways from the host and the endosymbiont (mitochondrion) allowed larger varieties of substrates (with different F/N ratios) to be oxidized, but high F/N ratios increase ROS formation. This might have led to carnitine shuttles, uncoupling proteins, and multiple antioxidant mechanisms, especially linked to fatty acid oxidation [BioEssays (2014) 36, 634–643]. Recent data regarding peroxisome evolution and their relationships with mitochondria, ROS formation by Complex I during ischaemia/reperfusion injury, and supercomplex formation adjustment to F/N ratios strongly support the model. I will further discuss the model in the light of experimental findings regarding mitochondrial ROS formation.

Scope and limitations of yeast as a model organism for studying human tissue-specific pathways

Background

Budding yeast, S. cerevisiae, has been used extensively as a model organism for studying cellular processes in evolutionarily distant species, including humans. However, different human tissues, while inheriting a similar genetic code, exhibit distinct anatomical and physiological properties. Specific biochemical processes and associated biomolecules that differentiate various tissues are not completely understood, neither is the extent to which a unicellular organism, such as yeast, can be used to model these processes within each tissue.

Results

We present a novel framework to systematically quantify the suitability of yeast as a model organism for different human tissues. To this end, we develop a computational method for dissecting the global human interactome into tissue-specific cellular networks. By individually aligning these networks with the yeast interactome, we simultaneously partition the functional space of human genes, and their corresponding pathways, based on their conservation both across species and among different tissues. Finally, we couple our framework with a novel statistical model to assess the conservation of tissue-specific pathways and infer the overall similarity of each tissue with yeast. We further study each of these subspaces in detail, and shed light on their unique biological roles in the human tissues.

Conclusions

Our framework provides a novel tool that can be used to assess the suitability of the yeast model for studying tissue-specific physiology and pathophysiology in humans. Many complex disorders are driven by a coupling of housekeeping (universally expressed in all tissues) and tissue-selective (expressed only in specific tissues) dysregulated pathways. While tissue-selective genes are significantly associated with the onset and development of a number of tissue-specific pathologies, we show that the human-specific subset has even higher association. Consequently, they provide excellent candidates as drug targets for therapeutic interventions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0253-0) contains supplementary material, which is available to authorized users.

Type-II NADH:quinone oxidoreductase from Staphylococcus aureus has two distinct binding sites and is rate limited by quinone reduction

A prerequisite for any rational drug design strategy is understanding the mode of protein-ligand interaction. This motivated us to explore protein-substrate interaction in Type-II NADH:quinone oxidoreductase (NDH-2) from Staphylococcus aureus, a worldwide problem in clinical medicine due to its multiple drug resistant forms. NDHs-2 are involved in respiratory chains and recognized as suitable targets for novel antimicrobial therapies, as these are the only enzymes with NADH:quinone oxidoreductase activity expressed in many pathogenic organisms. We obtained crystal and solution structures of NDH-2 from S. aureus, showing that it is a dimer in solution. We report fast kinetic analyses of the protein and detected a charge-transfer complex formed between NAD(+) and the reduced flavin, which is dissociated by the quinone. We observed that the quinone reduction is the rate limiting step and also the only half-reaction affected by the presence of HQNO, an inhibitor. We analyzed protein-substrate interactions by fluorescence and STD-NMR spectroscopies, which indicate that NADH and the quinone bind to different sites. In summary, our combined results show the presence of distinct binding sites for the two substrates, identified quinone reduction as the rate limiting step and indicate the establishment of a NAD(+)-protein complex, which is released by the quinone.

Caenorhabditis elegans expressing the Saccharomyces cerevisiae NADH alternative dehydrogenase Ndi1p, as a tool to identify new genes involved in complex I related diseases

Isolated complex I deficiencies are one of the most commonly observed biochemical features in patients suffering from mitochondrial disorders. In the majority of these clinical cases the molecular bases of the diseases remain unknown suggesting the involvement of unidentified factors that are critical for complex I function. The Saccharomyces cerevisiae NDI1 gene, encoding the mitochondrial internal NADH dehydrogenase was previously shown to complement a complex I deficient strain in Caenorhabditis elegans with notable improvements in reproduction and whole organism respiration. These features indicate that Ndi1p can functionally integrate the respiratory chain, allowing complex I deficiency complementation. Taking into account the Ndi1p ability to bypass complex I, we evaluate the possibility to extend the range of defects/mutations causing complex I deficiencies that can be alleviated by NDI1 expression. We report here that NDI1 expressing animals unexpectedly exhibit a slightly shortened lifespan, a reduction in the progeny, and a depletion of the mitochondrial genome. However, Ndi1p is expressed and targeted to the mitochondria as a functional protein that confers rotenone resistance to those animals without affecting their respiration rate and ATP content. We show that the severe embryonic lethality level caused by the RNAi knockdowns of complex I structural subunit encoding genes (e.g., NDUFV1, NDUFS1, NDUFS6, NDUFS8, or GRIM-19 human orthologs) in wild type animals is significantly reduced in the Ndi1p expressing worm. All together these results open up the perspective to identify new genes involved in complex I function, assembly, or regulation by screening an RNAi library of genes leading to embryonic lethality that should be rescued by NDI1 expression.

Mitochondrial disease associated with complex I (NADH-CoQ oxidoreductase) deficiency

Mitochondrial diseases due to a reduced capacity for oxidative phosphorylation were first identified more than 20 years ago, and their incidence is now recognized to be quite significant. In a large proportion of cases the problem can be traced to a complex I (NADH-CoQ oxidoreductase) deficiency (Phenotype MIM #252010). Because the complex consists of 44 subunits, there are many potential targets for pathogenic mutations, both on the nuclear and mitochondrial genomes. Surprisingly, however, almost half of the complex I deficiencies are due to defects in as yet unidentified genes that encode proteins other than the structural proteins of the complex. This review attempts to summarize what we know about the molecular basis of complex I deficiencies: mutations in the known structural genes, and mutations in an increasing number of genes encoding "assembly factors", that is, proteins required for the biogenesis of a functional complex I that are not found in the final complex I. More such genes must be identified before definitive genetic counselling can be applied in all cases of affected families.

Engineering the alternative oxidase gene to better understand and counteract mitochondrial defects: state of the art and perspectives

Mitochondrial disorders are nowadays recognized as impinging on most areas of medicine. They include specific and widespread organ involvement, including both tissue degeneration and tumour formation. Despite the spectacular progresses made in the identification of their underlying molecular basis, effective therapy remains a distant goal. Our still rudimentary understanding of the pathophysiological mechanisms by which these diseases arise constitutes an obstacle to developing any rational treatments. In this context, the idea of using a heterologous gene, encoding a supplemental oxidase otherwise absent from mammals, potentially bypassing the defective portion of the respiratory chain, was proposed more than 10 years ago. The recent progress made in the expression of the alternative oxidase in a wide range of biological systems and disease conditions reveals great potential benefit, considering the broad impact of mitochondrial diseases. This review addresses the state of the art and the perspectives that can be now envisaged by using this strategy.

Structural insight into the type-II mitochondrial NADH dehydrogenases

The single-component type-II NADH dehydrogenases (NDH-2s) serve as alternatives to the multisubunit respiratory complex I (type-I NADH dehydrogenase (NDH-1), also called NADH:ubiquinone oxidoreductase; EC 1.6.5.3) in catalysing electron transfer from NADH to ubiquinone in the mitochondrial respiratory chain. The yeast NDH-2 (Ndi1) oxidizes NADH on the matrix side and reduces ubiquinone to maintain mitochondrial NADH/NAD(+) homeostasis. Ndi1 is a potential therapeutic agent for human diseases caused by complex I defects, particularly Parkinson's disease, because its expression restores the mitochondrial activity in animals with complex I deficiency. NDH-2s in pathogenic microorganisms are viable targets for new antibiotics. Here we solve the crystal structures of Ndi1 in its substrate-free, NADH-, ubiquinone- and NADH-ubiquinone-bound states, to help understand the catalytic mechanism of NDH-2s. We find that Ndi1 homodimerization through its carboxy-terminal domain is critical for its catalytic activity and membrane targeting. The structures reveal two ubiquinone-binding sites (UQ(I) and UQ(II)) in Ndi1. NADH and UQ(I) can bind to Ndi1 simultaneously to form a substrate-protein complex. We propose that UQ(I) interacts with FAD to act as an intermediate for electron transfer, and that NADH transfers electrons through this FAD-UQ(I) complex to UQ(II). Together our data reveal the regulatory and catalytic mechanisms of Ndi1 and may facilitate the development or targeting of NDH-2s for potential therapeutic applications.

Kluyveromyces lactis: a suitable yeast model to study cellular defense mechanisms against hypoxia-induced oxidative stress

Studies about hypoxia-induced oxidative stress in human health disorders take advantage from the use of unicellular eukaryote models. A widely extended model is the fermentative yeast Saccharomyces cerevisiae. In this paper, we describe an overview of the molecular mechanisms induced by a decrease in oxygen availability and their interrelationship with the oxidative stress response in yeast. We focus on the differential characteristics between S. cerevisiae and the respiratory yeast Kluyveromyces lactis, a complementary emerging model, in reference to multicellular eukaryotes.

No immune responses by the expression of the yeast Ndi1 protein in rats

Background

The rotenone-insensitive internal NADH-quinone oxidoreductase from yeast, Ndi1, has been shown to work as a replacement molecule for complex I in the respiratory chain of mammalian mitochondria. In the so-called transkingdom gene therapy, one major concern is the fact that the yeast protein is foreign in mammals. Long term expression of Ndi1 observed in rodents with no apparent damage to the target tissue was indicative of no action by the host's immune system.

Methodology/Principal Findings

In the present study, we examined rat skeletal muscles expressing Ndi1 for possible signs of inflammatory or immune response. In parallel, we carried out delivery of the GFP gene using the same viral vector that was used for the NDI1 gene. The tissues were subjected to H&E staining and immunohistochemical analyses using antibodies specific for markers, CD11b, CD3, CD4, and CD8. The data showed no detectable signs of an immune response with the tissues expressing Ndi1. In contrast, mild but distinctive positive reactions were observed in the tissues expressing GFP. This clear difference most likely comes from the difference in the location of the expressed protein. Ndi1 was localized to the mitochondria whereas GFP was in the cytosol.

Conclusions/Significance

We demonstrated that Ndi1 expression did not trigger any inflammatory or immune response in rats. These results push forward the Ndi1-based molecular therapy and also expand the possibility of using foreign proteins that are directed to subcellular organelle such as mitochondria.

Mitochondrial transfection for studying organellar DNA repair, genome maintenance and aging

Maintenance of the mitochondrial genome is a major challenge for cells, particularly as they begin to age. Although it is established that organelles possess regular DNA repair pathways, many aspects of these complex processes and of their regulation remain to be investigated. Mitochondrial transfection of isolated organelles and in whole cells with customized DNA synthesized to contain defined lesions has wide prospects for deciphering repair mechanisms in a physiological context. We document here the strategies currently developed to transfer DNA of interest into mitochondria. Methodologies with isolated mitochondria claim to exploit the protein import pathway or the natural competence of the organelles, to permeate the membranes or to use conjugal transfer from bacteria. Besides biolistics, which remains restricted to yeast and Chlamydomonas reinhardtii, nanocarriers or fusion proteins have been explored as methods to target custom DNA into mitochondria in intact cells. In further approaches, whole mitochondria have been transferred into recipient cells. Repair failure or error-prone repair leads to mutations which potentially could be rescued by allotopic expression of proteins. The relevance of the different approaches for the analysis of mitochondrial DNA repair mechanisms and of aging is discussed.

Reaction mechanism of single subunit NADH-ubiquinone oxidoreductase (Ndi1) from Saccharomyces cerevisiae: evidence for a ternary complex mechanism

The flavoprotein rotenone-insensitive internal NADH-ubiquinone (UQ) oxidoreductase (Ndi1) is a member of the respiratory chain in Saccharomyces cerevisiae. We reported previously that bound UQ in Ndi1 plays a key role in preventing the generation of reactive oxygen species. Here, to elucidate this mechanism, we investigated biochemical properties of Ndi1 and its mutants in which highly conserved amino acid residues (presumably involved in NADH and/or UQ binding sites) were replaced. We found that wild-type Ndi1 formed a stable charge transfer (CT) complex (around 740 nm) with NADH, but not with NADPH, under anaerobic conditions. The intensity of the CT absorption band was significantly increased by the presence of bound UQ or externally added n-decylbenzoquinone. Interestingly, however, when Ndi1 was exposed to air, the CT band transiently reached the same maximum level regardless of the presence of UQ. This suggests that Ndi1 forms a ternary complex with NADH and UQ, but the role of UQ in withdrawing an electron can be substitutable with oxygen. Proteinase K digestion analysis showed that NADH (but not NADPH) binding induces conformational changes in Ndi1. The kinetic study of wild-type and mutant Ndi1 indicated that there is no overlap between NADH and UQ binding sites. Moreover, we found that the bound UQ can reversibly dissociate from Ndi1 and is thus replaceable with other quinones in the membrane. Taken together, unlike other NAD(P)H-UQ oxidoreductases, the Ndi1 reaction proceeds through a ternary complex (not a ping-pong) mechanism. The bound UQ keeps oxygen away from the reduced flavin.

Protective Role of rAAV-NDI1, Serotype 5, in an Acute MPTP Mouse Parkinson's Model

Defects in mitochondrial proton-translocating NADH-quinone oxidoreductase (complex I) have been implicated in a number of acquired and hereditary diseases including Leigh's syndrome and more recently Parkinson's disease. A limited number of strategies have been attempted to repair the damaged complex I with little or no success. We have recently shown that the non-proton-pumping, internal NADH-ubiquinone oxidoreductase (Ndi1) from Saccharomyces cerevisiae (baker's yeast) can be successfully inserted into the mitochondria of mice and rats, and the enzyme was found to be fully active. Using recombinant adenoassociated virus vectors (serotype 5) carrying our NDI1 gene, we were able to express the Ndi1 protein in the substantia nigra (SN) of C57BL/6 mice with an expression period of two months. The results show that the AAV serotype 5 was highly efficient in expressing Ndi1 in the SN, when compared to a previous model using serotype 2, which led to nearly 100% protection when using an acute MPTP model. It is conceivable that the AAV-serotype5 carrying the NDI1 gene is a powerful tool for proof-of-concept study to demonstrate complex I defects as the causable factor in diseases of the brain.

Successful amelioration of mitochondrial optic neuropathy using the yeast NDI1 gene in a rat animal model

Background

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disorder with point mutations in mitochondrial DNA which result in loss of vision in young adults. The majority of mutations reported to date are within the genes encoding the subunits of the mitochondrial NADH-quinone oxidoreductase, complex I. Establishment of animal models of LHON should help elucidate mechanism of the disease and could be utilized for possible development of therapeutic strategies.

Methodology/Principal Findings

We established a rat model which involves injection of rotenone-loaded microspheres into the optic layer of the rat superior colliculus. The animals exhibited the most common features of LHON. Visual loss was observed within 2 weeks of rotenone administration with no apparent effect on retinal ganglion cells. Death of retinal ganglion cells occurred at a later stage. Using our rat model, we investigated the effect of the yeast alternative NADH dehydrogenase, Ndi1. We were able to achieve efficient expression of the Ndi1 protein in the mitochondria of all regions of retinal ganglion cells and axons by delivering the NDI1 gene into the optical layer of the superior colliculus. Remarkably, even after the vision of the rats was severely impaired, treatment of the animals with the NDI1 gene led to a complete restoration of the vision to the normal level. Control groups that received either empty vector or the GFP gene had no effects.

Conclusions/Significance

The present study reports successful manifestation of LHON-like symptoms in rats and demonstrates the potential of the NDI1 gene therapy on mitochondrial optic neuropathies. Our results indicate a window of opportunity for the gene therapy to be applied successfully after the onset of the disease symptoms.

Neuroprotective effect of long-term NDI1 gene expression in a chronic mouse model of Parkinson disorder

Previously, we showed that the internal rotenone-insensitive nicotinamide adenine dinucleotide (NADH)-quinone oxidoreductase (NDI1) gene from Saccharomyces cerevisiae (baker's yeast) can be successfully inserted into the mitochondria of mice and rats and the expressed enzyme was found to be fully functional. In this study, we investigated the ability of the Ndi1 enzyme to protect the dopaminergic neurons in a chronic mouse model of Parkinson disorder. After expression of the NDI1 gene in the unilateral substantia nigra of male C57BL/6 mice for 8 months, a chronic Parkinsonian model was created by administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) with probenecid and evaluated using neurochemical and behavioral responses 1-4 weeks post-MPTP/probenecid injection. We showed that expression of Ndi1 was able to significantly prevent the loss of dopamine and tyrosine hydroxylase as well as the dopaminergic transporters in the striatum of the chronic Parkinsonian mice. Behavioral assessment based on a methamphetamine-induced rotation test and spontaneous swing test further supported neurological preservation in the NDI1-treated Parkinsonian mice. The data presented in this study demonstrate a protective effect of the NDI1 gene in dopaminergic neurons, suggesting its therapeutic potential for Parkinson-like disorders.

Respiratory chain alternative enzymes as tools to better understand and counteract respiratory chain deficiencies in human cells and animals

Mitochondrial respiratory chain defects are now recognized to underlie a large number of human diseases with a spectacular variety in their phenotypic presentations. Despite progress made in the elucidation of their molecular basis, these diseases remain essentially untreatable. To date, most strategies to counteract these diseases, either in vitro or in vivo have proven unsuccessful. In humans, the respiratory chain lacks several redox active proteins long known in many micro-organisms, as well as in plants, and, as found recently, even in some metazoans. These alternative enzymes, e.g. the cyanide-insensitive alternative oxidase and the internal rotenone-insensitive NADH dehydrogenase, confer a significant flexibility to the respiratory chain, allowing it to overcome potential constraints exerted by the cell phosphorylation potential or by environmental xenobiotics. In plants, these alternative enzymes, activated by a subset of keto-acids, including pyruvate, are essentially engaged under highly reducing conditions. Because these are conditions observed in patients with respiratory chain dysfunction, we made the hypothesis that expression of these proteins might be of benefit in such situations. The observation that a functional alternative oxidase from Ciona intestinalis could be expressed in mammalian cells without obvious detrimental effect has provided a basis to develop a research programme to test the hypothesis, within an ad hoc international consortium, that this paper aims to describe. Combining research on human cells, flies and mice, the project aims, firstly, to verify that expressing these alternative enzymes is physiologically benign, useful as a tool to delineate the mechanisms of respiratory chain dysfunction and, finally, test their potential therapeutic benefit.

Parkinson's disease and mitochondrial complex I: a perspective on the Ndi1 therapy

Mitochondrial impairment has been collecting more and more attention as a contributing factor to the etiology of Parkinson's disease. Above all, the NADH-quinone oxidoreductase, complex I, of the respiratory chain seems to be most culpable. Complex I dysfunction is translated to an increased production of reactive oxygen species and a decreased energy supply. In the brain, the dopaminergic neurons are one of the most susceptible cells. Their death is directly linked to the disease apparition. Developing an effective gene therapy is challenged by harmful actions of reactive oxygen species. To overcome this problem a therapeutic candidate must be able to restore the NADH-quinone oxidoreductase activity regardless of how complex I is impaired. Here we discuss the potency of the yeast alternative NADH dehydrogenase, the Ndi1 protein, to reinstate the mitochondrial respiratory chain compensating for disabled complex I and the benefit Ndi1 brings toward retardation of Parkinson's disease.

Mitochondrial bioenergetics and redox state are unaltered in Trypanosoma cruzi isolates with compromised mitochondrial complex I subunit genes

In trypanosomatids the involvement of mitochondrial complex I in NADH oxidation has long been debated. Here, we took advantage of natural Trypanosoma cruzi mutants which present conspicuous deletions in ND4, ND5 and ND7 genes coding for complex I subunits to further investigate its functionality. Mitochondrial bioenergetics of wild type and complex I mutants showed no significant differences in oxygen consumption or respiratory control ratios in the presence of NADH-linked substrates or FADH(2)-generating succinate. No correlation could be established between mitochondrial membrane potentials and ND deletions. Since release of reactive oxygen species occurs at complex I, we measured mitochondrial H(2)O(2) formation induced by different substrates. Significant differences not associated to ND deletions were observed among the parasite isolates, demonstrating that these mutations are not important for the control of oxidant production. Our data support the notion that complex I has a limited function in T. cruzi.

Restoration of electron transport without proton pumping in mammalian mitochondria

We have restored the CoQ oxidative capacity of mouse mtDNA-less cells (rho degrees cells) by transforming them with the alternative oxidase Aox of Emericella nidulans. Cotransforming rho degrees cells with the NADH dehydrogenase of Saccharomyces cerevisiae, Ndi1 and Aox recovered the NADH DH/CoQ reductase and the CoQ oxidase activities. CoQ oxidation by AOX reduces the dependence of rho degrees cells on pyruvate and uridine. Coexpression of AOX and NDI1 further improves the recycling of NAD(+). Therefore, 2 single-protein enzymes restore the electron transport in mammalian mitochondria substituting >80 nuclear DNA-encoded and 11 mtDNA-encoded proteins. Because those enzymes do not pump protons, we were able to split electron transport and proton pumping (ATP synthesis) and inquire which of the metabolic deficiencies associated with the loss of oxidative phosphorylation should be attributed to each of the 2 processes.

NADPH oxidase has a directional response to shear stress

Vessel regions with predilection to atherosclerosis have negative wall shear stress due to flow reversal. The flow reversal causes the production of superoxides (O(2)(-)), which scavenge nitric oxide (NO), leading to a decrease in NO bioavailability and endothelial dysfunction. Here, we implicate NADPH oxidase as the primary source of O(2)(-) during full flow reversal. Nitrite production and the degree of vasodilation were measured in 46 porcine common femoral arteries in an ex vivo system. Nitrite production and vasodilation were determined before and after the inhibition of NADPH oxidase, xanthine oxidase, or mitochondrial oxidase. NADPH oxidase inhibition with gp91ds-tat or apocynin restored nitrite production and vasodilation during reverse flow. Xanthine oxidase inhibition increased nitrite production at the highest flow rate, whereas mitochondrial oxidase inhibition had no effect. These findings suggest that the NADPH oxidase system can respond to directional changes of flow and is activated to generate O(2)(-) during reverse flow in a dose-dependent fashion. These findings have important clinical implications for oxidative balance and NO bioavailability in regions of flow reversal in a normal and compromised cardiovascular system.

Mechanism of cell death caused by complex I defects in a rat dopaminergic cell line

Defects in the proton-translocating NADH-quinone oxidoreductase (complex I) of mammalian mitochondria are linked to neurodegenerative disorders. The mechanism leading to cell death elicited by complex I deficiency remains elusive. We have shown that expression of a rotenone-insensitive yeast NADH-quinone oxidoreductase (Ndi1) can rescue mammalian cells from complex I dysfunction. By using the Ndi1 enzyme, we have investigated the key events in the process of cell death using a rat dopaminergic cell line, PC12. We found that complex I inhibition provokes the following events: 1) activation of specific kinase pathways; 2) release of mitochondrial proapoptotic factors, apoptosis inducing factor, and endonuclease G. AS601245, a kinase inhibitor, exhibited significant protection against these apoptotic events. The traditional caspase pathway does not seems to be involved because caspase 3 activation was not observed. Our data suggest that overproduction of reactive oxygen species (ROS) caused by complex I inhibition is responsible for triggering the kinase activation, for the release of the proapoptotic factors, and then for cell death. Nearly perfect prevention of apoptotic cell death by Ndi1 agrees with our earlier observation that the presence of Ndi1 diminishes rotenone-induced ROS generation from complex I. In fact, this study demonstrated that Ndi1 keeps the redox potential high even in the presence of rotenone. Under these conditions, ROS formation by complex I is known to be minimal. Possible use of our cellular model is discussed with regard to development of therapeutic strategies for neurodegenerative diseases caused by complex I defects.

Adresser du matériel allogène dans le compartiment mitochondrial

Mitochondrial disorders can not be ignored anymore in most medical areas. They include specific and widespread organ involvement, with tissue degeneration or tumor formation, being the target of numerous viruses, e.g. the HIV. Primary or secondary actors, mitochondrial dysfunctions are also supposedly playing a role in the ageing process. Despite the progresses made in the identification of their molecular bases, nearly all remains to be done as regards therapy. Research dealing with mitochondrial physiology and pathology has a long history in France and is thus not a surprise if four French teams, coming from these fundamental domains, are involved in the challenge to find ways to fight these diseases. The directions described are working tracks which promise to be long and full of pitfalls. Being original, they share a part of risk and uncertainty, but they are also with great potential with high stakes if considering the impact of these diseases.

The mitochondrial pathway in yeast apoptosis

Mitochondria are not only important for the energetic status of the cell, but are also the fatal organelles deciding about cellular life and death. Complex mitochondrial features decisive for cell death execution in mammals are present and functional in yeast: AIF and cytochrome c release to the cytosol, mitochondrial fragmentation as well as mitochondrial hyperpolarisation followed by an oxidative burst, and breakdown of mitochondrial membrane potential. The easy accessibility of mitochondrial manipulations such as repression of respiration by growing yeast on glucose or deletion of mitochondrial DNA (rho(0)) on the one hand and the unique ability of yeast cells to grow on non-fermentable carbon sources by switching on mitochondrial respiration on the other hand have made yeast an excellent tool to delineate the necessity for mitochondria in cell death execution. Yeast research indicates that the connection between mitochondria and apoptosis is intricate, as abrogation of mitochondrial function can be either deleterious or beneficial for the cell depending on the specific context of the death scenario. Surprisingly, mitochondrion dependent yeast apoptosis currently helps to understand the aetiology (or the complex biology) of lethal cytoskeletal alterations, ageing and neurodegeneration. For example, mutation of mitochondrial superoxide dismutase or CDC48/VCP mutations, both implicated in several neurodegenerative disorders, are associated with mitochondrial impairment and apoptosis in yeast.

Yeast NDI1 improves oxidative phosphorylation capacity and increases protection against oxidative stress and cell death in cells carrying a Leber's hereditary optic neuropathy mutation

G11778A in the subunit ND4 gene of NADH dehydrogenase complex is the most common primary mutation found in Leber's hereditary optic neuropathy (LHON) patients. The NDI1 gene, which encodes the internal NADH-quinone oxidoreductase in Saccharomyces cerevisiae, was introduced into the nuclear genome of a mitochondrial defective human cell line, Le1.3.1, carrying the G11778A mutation. In transformant cell lines, LeNDI1-1 and -2, total and complex I-dependent respiration were fully restored and largely resistant to complex I inhibitor, rotenone, indicating a dominant role of NDI1 in the transfer of electrons in the host cells. Whereas the original mutant Le1.3.1 cell grows poorly in medium containing galactose, the transformants have a fully restored growth capacity in galactose medium, although the ATP production was not totally recovered. Furthermore, the increased oxidative stress in the cells carrying the G11778A mutation was alleviated in transformants, demonstrated by a decreased reactive oxygen species (ROS) level. Finally, transformants were also shown to be desensitized to induction to apoptosis and also exhibit greater resistance to paraquat-induced cell death. It is concluded that the yeast NDI1 enzyme can improve the oxidative phosphorylation capacity in cells carrying the G11778A mutation and protect the cells from oxidative stress and cell death.

Roles of bound quinone in the single subunit NADH-quinone oxidoreductase (Ndi1) from Saccharomyces cerevisiae

To understand the biochemical basis for the function of the rotenone-insensitive internal NADH-quinone (Q) oxidoreductase (Ndi1), we have overexpressed mature Ndi1 in Escherichia coli membranes. The Ndi1 purified from the membranes contained one FAD and showed enzymatic activities comparable with the original Ndi1 isolated from Saccharomyces cerevisiae. When extracted with Triton X-100, the isolated Ndi1 did not contain Q. The Q-bound form was easily reconstituted by incubation of the Q-free Ndi1 enzyme with ubiquinone-6. We compared the properties of Q-bound Ndi1 enzyme with those of Q-free Ndi1 enzyme, with higher activity found in the Q-bound enzyme. Although both are inhibited by low concentrations of AC0-11 (IC(50) = 0.2 microm), the inhibitory mode of AC0-11 on Q-bound Ndi1 was distinct from that of Q-free Ndi1. The bound Q was slowly released from Ndi1 by treatment with NADH or dithionite under anaerobic conditions. This release of Q was prevented when Ndi1 was kept in the reduced state by NADH. When Ndi1 was incorporated into bovine heart submitochondrial particles, the Q-bound form, but not the Q-free form, established the NADH-linked respiratory activity, which was insensitive to piericidin A but inhibited by KCN. Furthermore, Ndi1 produces H(2)O(2) as isolated regardless of the presence of bound Q, and this H(2)O(2) was eliminated when the Q-bound Ndi1, but not the Q-free Ndi1, was incorporated into submitochondrial particles. The data suggest that Ndi1 bears at least two distinct Q sites: one for bound Q and the other for catalytic Q.