Catalytic activity of complex I in cell lines that possess replacement mutations in the ND genes in Leber's hereditary optic neuropathy

Coenzyme Q10 trapping in mitochondrial complex I underlies Leber's hereditary optic neuropathy

How does a single amino acid mutation occurring in the blinding disease, Leber's hereditary optic neuropathy (LHON), impair electron shuttling in mitochondria? We investigated changes induced by the m.3460 G>A mutation in mitochondrial protein ND1 using the tools of Molecular Dynamics and Free Energy Perturbation simulations, with the goal of determining the mechanism by which this mutation affects mitochondrial function. A recent analysis suggested that the mutation's replacement of alanine A52 with a threonine perturbs the stability of a region where binding of the electron shuttling protein, Coenzyme Q10, occurs. We found two functionally opposing changes involving the role of Coenzyme Q10. The first showed that quantum electron transfer from the terminal Fe/S complex, N2, to the Coenzyme Q10 headgroup, docked in its binding pocket, is enhanced. However, this positive adjustment is overshadowed by our finding that the mobility of Coenzyme Q10 in its oxidized and reduced states, entering and exiting its binding pocket, is disrupted by the mutation in a manner that leads to conditions promoting the generation of reactive oxygen species. An increase in reactive oxygen species caused by the LHON mutation has been proposed to be responsible for this optic neuropathy.

A cell-based, infectious-free, platform to identify inhibitors of lassa virus ribonucleoprotein (vRNP) activity

The mammarenavirus Lassa (LASV) is highly prevalent in West Africa where it infects several hundred thousand individuals annually resulting in a high number of Lassa fever (LF) cases, a febrile disease associated with high morbidity and significant mortality. Mounting evidence indicates that the worldwide-distributed prototypic mammarenavirus lymphocytic choriomeningitis virus (LCMV) is a neglected human pathogen of clinical significance. There are not Food and Drug Administration (FDA) licensed vaccines and current anti-mammarenavirus therapy is limited to an off-label use of ribavirin that is only partially effective and can cause significant side effects. Therefore, there is an unmet need for novel antiviral drugs to combat LASV. This task would be facilitated by the implementation of high throughput screens (HTS) to identify inhibitors of the activity of the virus ribonucleoprotein (vRNP) responsible for directing virus RNA genome replication and gene transcription. The use of live LASV for this purpose is jeopardized by the requirement of biosafety level 4 (BSL4) containment. We have developed a virus-free cell platform, where expression levels of reporter genes serve as accurate surrogates of vRNP activity, to develop cell-based assays compatible with HTS to identify inhibitors of LASV and LCMV mammarenavirus vRNP activities.

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Generation of cell lines harboring functional viral ribonucleoprotein complexes (vRNP) of LCMV and LASV.

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LASV and LCMV vRNP-expressing cell-based screening assays for inhibitors of LASV and LCMV RNA replication and gene transcription.

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Ionophore antibiotics narasin and valinomycin are potent inhibitors of LCMV and LASV vRNPs.

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Inhibitors of mitochondrial electron transport complex (mETC) I and III are potent inhibitors of LCMV and LASV vRNPs.

Mubritinib Targets the Electron Transport Chain Complex I and Reveals the Landscape of OXPHOS Dependency in Acute Myeloid Leukemia

To identify therapeutic targets in acute myeloid leukemia (AML), we chemically interrogated 200 sequenced primary specimens. Mubritinib, a known ERBB2 inhibitor, elicited strong anti-leukemic effects in vitro and in vivo. In the context of AML, mubritinib functions through ubiquinone-dependent inhibition of electron transport chain (ETC) complex I activity. Resistance to mubritinib characterized normal CD34⁺ hematopoietic cells and chemotherapy-sensitive AMLs, which displayed transcriptomic hallmarks of hypoxia. Conversely, sensitivity correlated with mitochondrial function-related gene expression levels and characterized a large subset of chemotherapy-resistant AMLs with oxidative phosphorylation (OXPHOS) hyperactivity. Altogether, our work thus identifies an ETC complex I inhibitor and reveals the genetic landscape of OXPHOS dependency in AML.

Leber Hereditary Optic Neuropathy: A Mitochondrial Disease Unique in Many Ways

Leber hereditary optic neuropathy (LHON) was the first mitochondrial disease to be identified as being caused by mutations in the mitochondrial DNA (mtDNA). This disease has been studied extensively in the past two decades, particularly in Brazilian, Chinese and European populations; and many primary mutations have been reported. However, the disease is enigmatic with many unique features, and there still are several important questions to be resolved. The incomplete penetrance, the male-biased disease expression and the prevalence in young adults all defy a proper explanation. It has been reported that the development of LHON is affected by the interaction between mtDNA mutations, mtDNA haplogroup background, nuclear genes, environmental factors and epigenetics. Furthermore, with the help of new animal models for LHON that have been created in recent years, we are continuing to learn more about the mechanism of this disease. The stage has now been reached at which there is a good understanding of both the genetic basis of the disease and its epidemiology, but just how the blindness that follows from the death of cells in the optic nerve can be prevented remains to be a pharmacological challenge. In this chapter, we summarize the progress that has been made in various recent studies on LHON, focusing on the molecular pathogenic mechanisms, clinical features, biochemical effects, the pharmacology and its treatment.

Evidence for Detrimental Cross Interactions between Reactive Oxygen and Nitrogen Species in Leber's Hereditary Optic Neuropathy Cells

Here we have collected evidence suggesting that chronic changes in the NO homeostasis and the rise of reactive oxygen species bioavailability can contribute to cell dysfunction in Leber's hereditary optic neuropathy (LHON) patients. We report that peripheral blood mononuclear cells (PBMCs), derived from a female LHON patient with bilateral reduced vision and carrying the pathogenic mutation 11778/ND4, display increased levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS), as revealed by flow cytometry, fluorometric measurements of nitrite/nitrate, and 3-nitrotyrosine immunodetection. Moreover, viability assays with the tetrazolium dye MTT showed that lymphoblasts from the same patient are more sensitive to prolonged NO exposure, leading to cell death. Taken together these findings suggest that oxidative and nitrosative stress cooperatively play an important role in driving LHON pathology when excess NO remains available over time in the cell environment.

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.

LHON/MELAS overlap mutation in ND1 subunit of mitochondrial complex I affects ubiquinone binding as revealed by modeling in Escherichia coli NDH-1

Defects in complex I due to mutations in mitochondrial DNA are associated with clinical features ranging from single organ manifestation like Leber hereditary optic neuropathy (LHON) to multiorgan disorders like mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) syndrome. Specific mutations cause overlap syndromes combining several phenotypes, but the mechanisms of their biochemical effects are largely unknown. The m.3376G>A transition leading to p.E24K substitution in ND1 with LHON/MELAS phenotype was modeled here in a homologous position (NuoH-E36K) in the Escherichia coli enzyme and it almost totally abolished complex I activity. The more conservative mutation NuoH-E36Q resulted in higher apparent K(m) for ubiquinone and diminished inhibitor sensitivity. A NuoH homolog of the m.3865A>G transition, which has been found concomitantly in the overlap syndrome patient with the m.3376G>A, had only a minor effect. Consequences of a primary LHON-mutation m.3460G>A affecting the same extramembrane loop as the m.3376G>A substitution were also studied in the E. coli model and were found to be mild. The results indicate that the overlap syndrome-associated m.3376G>A transition in MTND1 is the pathogenic mutation and m.3865A>G transition has minor, if any, effect on presentation of the disease. The kinetic effects of the NuoH-E36Q mutation suggest its proximity to the putative ubiquinone binding domain in 49kD/PSST subunits. In all, m.3376G>A perturbs ubiquinone binding, a phenomenon found in LHON, and decreases the activity of fully assembled complex I as in MELAS.

Mitochondrial optic neuropathies - disease mechanisms and therapeutic strategies

Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.

The role of mitochondria in the radiation-induced bystander effect in human lymphoblastoid cells

Cells without intact mitochondrial DNA have been shown to lack the bystander effect, which is an energy-dependent process. We hypothesized that cells harboring mutations in mitochondrial genes responsible for ATP synthesis would show a decreased bystander effect compared to normal cells. Radiation-induced bystander effects were analyzed in two normal and four mitochondrial mutant human lymphoblastoid cells. Medium from previously irradiated cells (conditioned medium) was transferred to unirradiated cells from the respective cell lines and evaluated for the bystander effect using the cytokinesis-block micronucleus assay. Unlike normal cells that were used as a control, mitochondrial mutant cells neither generated nor responded to the bystander signals. The bystander effect was inhibited in normal cells by adding the mitochondrial inhibitors rotenone and oligomycin to the culture medium. Time-controlled blocking of the bystander effect by inhibitors was found to occur either for prolonged exposure to the inhibitor prior to irradiation with an immediate and subsequent removal of the inhibitors or immediate post-application of the inhibitor. Adding the inhibitors just prior to irradiation and removing them immediately after irradiation was uneventful. Fully functional mitochondrial metabolic capability may therefore be essential for the bystander effect.

Structure and organization of mitochondrial respiratory complexes: a new understanding of an old subject

The enzymatic complexes of the mitochondrial respiratory chain have been extensively investigated in their structural and functional properties. A clear distinction is possible today between three complexes in which the difference in redox potential allows proton translocation (complexes I, III, and IV) and those having the mere function to convey electrons to the respiratory chain. We also have a clearer understanding of the structure and function of most respiratory complexes, of their biogenesis and regulation, and of their capacity to generate reactive oxygen species. Past investigations led to the conclusion that the complexes are randomly dispersed and functionally connected by diffusion of smaller redox components, coenzyme Q and cytochrome c. More-recent investigations by native gel electrophoresis and single-particle image processing showed the existence of supramolecular associations. Flux-control analysis demonstrated that complexes I and III in mammals and I, III, and IV in plants kinetically behave as single units, suggesting the existence of substrate channeling. This review discusses conditions affecting the formation of supercomplexes that, besides kinetic advantage, have a role in the stability and assembly of the individual complexes and in preventing excess oxygen radical formation. Disruption of supercomplex organization may lead to functional derangements responsible for pathologic changes.

Chapter 9 Reliable assay for measuring complex I activity in human blood lymphocytes and skin fibroblasts

Complex I deficiency is probably the most common enzyme defect among the group of OXPHOS disorders. To evaluate a deficiency of complex I activity, biochemical measurements based on estimation of the mitochondrial rotenone-sensitive NADH: ubiquinone oxidoreductase activity are an important tool. Skeletal muscle is the most widely used tissue to examine complex I deficiency. However, obtaining a muscle biopsy requires an invasive surgical operation. It is much easier to obtain blood lymphocytes or skin fibroblasts, and, moreover, these cells can be expanded in number by standard techniques for extensive research on complex I. On the other hand, each of these cell types has disadvantages that hinder its measurement, such as the apparent low enzyme activity of lymphocytes and the highly contaminating nonmitochondrial NADH-quinone oxidoreductase activity of fibroblasts. This chapter describes a method to assay complex I activity reliably in a minute amount of either cell type.

Inherited mitochondrial optic neuropathies

Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (DOA) are the two most common inherited optic neuropathies and they result in significant visual morbidity among young adults. Both disorders are the result of mitochondrial dysfunction: LHON from primary mitochondrial DNA (mtDNA) mutations affecting the respiratory chain complexes; and the majority of DOA families have mutations in the OPA1 gene, which codes for an inner mitochondrial membrane protein critical for mtDNA maintenance and oxidative phosphorylation. Additional genetic and environmental factors modulate the penetrance of LHON, and the same is likely to be the case for DOA which has a markedly variable clinical phenotype. The selective vulnerability of retinal ganglion cells (RGCs) is a key pathological feature and understanding the fundamental mechanisms that underlie RGC loss in these disorders is a prerequisite for the development of effective therapeutic strategies which are currently limited.

Inhibition of mitochondrial function induces an integrated stress response in oligodendroglia

Maternal inheritance of a pathogenic point mutation within complex I of the mitochondrial genome causes Leber's hereditary optic neuropathy (LHON), resulting in the neurodegeneration and demyelination of the optic nerve. The integrated stress response (ISR), a signaling pathway that responds to various stresses by activating a common set of genes, has been linked to both mitochondrial defects and demyelinating diseases. Therefore, we wanted to determine whether mitochondrial dysfunction induced by complex I inhibition with rotenone can activate the ISR, specifically by the ER kinase PERK, in oligodendroglial cells. Our complex I-deficient oligodendroglial model reproduced similar biochemical defects as in LHON by decreasing ATP synthesis and ATP levels. The same doses of rotenone that reduced ATP production also induced dose-dependent increases in PERK and eIF2alpha phosphorylation as well as activated the ISR stress genes, ATF4 and CHOP. In addition, complex I inhibition at these same concentrations induced a PERK-dependent activation of the cell death kinase, JNK, and inhibited oligodendroglial proliferation. Taken together, our results demonstrate that activation of the ISR may be one example of mitochondrial retrograde signaling in response to complex I deficiency and we suggest that this response mechanism may be relevant to the pathophysiology of LHON.

Mitochondrial mutant cells are hypersensitive to ionizing radiation, phleomycin and mitomycin C

Mitochondrial DNA (mtDNA) is an important contributor to the ATP-generating oxidative phosphorylation complex. Single nucleotide mutations in mitochondrial genes involved in ATP synthesis result in a broad range of diseases. Leber optic atrophy and Leigh's syndrome are two such diseases arising from point mutations in the mitochondrial genome. Here, ionizing radiation, phleomycin and mitomycin C (MMC) were used to induce structural chromosomal aberrations in Leber's and Leigh's cells to investigate how these mitochondrial mutations affect the cell's DNA repair processes. Because of the energy deprivation that results from mitochondrial mutations, we hypothesized that these mutant cells would demonstrate hypersensitivity when exposed to oxidative and genotoxic stress and we also expected that these cells would not be able to repair nuclear DNA damage as efficiently as normal cells. As a consequence, these mutant cells are expected to show increased levels of DNA damage, longer cell cycle delays and increased levels of cell death. Following acute radiation exposure these mutant cells showed an increase in the number of chromosomal aberrations and decreased mitotic indices when compared with normal human lymphoblastoid cells with wild-type mtDNA. When exposed to phleomycin or MMC, the mitochondrial mutant cells again showed hypersensitivity and decreased mitotic indices compared to normal cells. These results suggest that Leber's and Leigh's cells have an impaired ability to cope with oxidative and genotoxic stress. These observations may help explain the role of ATP generation in understanding the enhanced sensitivity of mitochondrial mutant cells to cancer therapeutic agents and to adverse environmental exposure, suggesting that individuals with mtDNA mutations may be at a greater risk for cancer and other diseases that result from an accumulation of nuclear DNA damage.

Leber hereditary optic neuropathy mutations in the ND6 subunit of mitochondrial complex I affect ubiquinone reduction kinetics in a bacterial model of the enzyme

LHON (Leber hereditary optic neuropathy) is a maternally inherited disease that leads to sudden loss of central vision at a young age. There are three common primary LHON mutations, occurring at positions 3460, 11778 and 14484 in the human mtDNA (mitochondrial DNA), leading to amino acid substitutions in mitochondrial complex I subunits ND1, ND4 and ND6 respectively. We have now examined the effects of ND6 mutations on the function of complex I using the homologous NuoJ subunit of Escherichia coli NDH-1 (NADH:quinone oxidoreductase) as a model system. The assembly level of the NDH-1 mutants was assessed using electron transfer from deamino-NADH to the 'shortcut' electron acceptor HAR (hexammine ruthenium), whereas ubiquinone reductase activity was determined using DB (decylubiquinone) as a substrate. Mutant growth in minimal medium with malate as the main carbon source was used for initial screening of the efficiency of energy conservation by NDH-1. The results indicated that NuoJ-M64V, the equivalent of the common LHON mutation in ND6, had a mild effect on E. coli NDH-1 activity, while nearby mutations, particularly NuoJ-Y59F, NuoJ-V65G and NuoJ-M72V, severely impaired the DB reduction rate and cell growth on malate. NuoJ-Met64 and NuoJ-Met72 position mutants lowered the affinity of NDH-1 for DB and explicit C-type inhibitors, whereas NuoJ-Y59C displayed substrate inhibition by oxidized DB. The results are compatible with the notion that the ND6 subunit delineates the binding cavity of ubiquinone substrate, but does not directly take part in the catalytic reaction. How these changes in the enzyme's catalytic properties contribute to LHON pathogenesis is discussed.

A simplified and reliable assay for complex I in human blood lymphocytes

Complex I activity of the mitochondrial respiratory chain is difficult to measure in blood lymphocytes because of the limited access of substrates to the enzyme complex in these cells. The results of the present study show that permeabilization of human blood lymphocytes in the presence of protease inhibitors by three cycles of freeze-thawing enables reproducible detection of the rotenone-sensitive complex I activity. To that end, the water-soluble coenzyme Q(10) analogue CoQ(1) and a relatively high concentration of blood lymphocytes were combined in small quartz cuvettes so that the amount of blood needed for this assay remained low. The relationship between the initial rate of NADH oxidation by complex I and the protein concentration was quasi-linear. The fractional inhibition of the total NADH:CoQ(1) oxidoreductase by a saturating concentration of rotenone decreased sharply at CoQ(1) concentrations higher than 20 muM, which is indicative, but does not prove the involvement of a second CoQ(1) binding site at complex I. Since the present complex I assay requires only a small amount of blood, the functionality of this important respiratory chain complex can be assessed in an easy and reliable manner not only in adult patients but also in children suspected to have a mitochondrial disease.

The MELAS mutations 3946 and 3949 perturb the critical structure in a conserved loop of the ND1 subunit of mitochondrial complex I

The ND1 subunit gene of the mitochondrial NADH-ubiquinone oxidoreductase (complex I) is a hot spot for mutations causing Leber hereditary optic neuropathy and several mutations causing the mitochondrial encephalopathy, lactic acidosis and stroke-like episodes syndrome (MELAS). We have used Escherichia coli and Paracoccus denitrificans as model systems to study the effect of mutations 3946 and 3949, which change conserved residues in ND1 and cause MELAS. The vicinity of these mutations was also explored with a series of mutations in charged residues. The 3946 mutation results in E214K substitution in human ND1. Replacement of the equivalent residue in E. coli with lysine or glutamine detracted from enzyme assembly and the assembled enzyme was inactive. However, the equivalent E234Q mutant enzyme in P. denitrificans failed to assemble completely (or was rapidly degraded). Also the corresponding substitution with aspartate decreased the enzyme activity in P. denitrificans and E. coli. The 3949-equivalent substitution, Y229H in E. coli, lowered the catalytic activity by 30%. In addition, an activation of the enzyme during catalytic turnover was seen in this bacterial NDH-1, something that was even more pronounced in another mutant in the same loop, D213E. Several other mutations in this region decreased the enzyme activity. The studied MELAS mutations are situated in a matrix-side loop, which appears to be highly sensitive to structural perturbations. The results provide new information on the function of the region affected by the MELAS mutations 3946 and 3949 that is not obtainable from patient samples or current eukaryote models.

Electron Transport Complex I Is Required for CD8+T Cell Function

After Ag encounter, CD8+ T cells become activated and begin to proliferate. Early during infection, when Ag-specific effector CD8+ T cells are proliferating, producing cytokines, and lysing infected cells in vivo, their mitochondrial potential is increased. The purpose of the experiments presented here was to determine whether mitochondrial function was required for CD8+ T cell function. To block mitochondrial function, transgenic CD8+ T cells were incubated with increasing doses of rotenone, an inhibitor of electron transport complex I. Within minutes of T cell activation, rotenone incubation decreased the production of H(2)O(2), calcium flux, and ERK1/2 phosphorylation. Failure to undergo signal transduction resulted in a decrease in T cell division initiated by peptide-coated cells, CD3/CD28 Abs, and PMA/ionomycin stimulation. Decreased function following rotenone incubation was not restricted to naive cells, as effector and memory CD8+ T cells isolated directly ex vivo from lymphocytic choriomeningitis virus-infected mice displayed decreased production of IFN-gamma and TNF-alpha production after peptide stimulation. Furthermore, incubation with rotenone decreased degranulation of effector and memory cells, a critical step in the cytolysis of infected cells. These data suggest that electron transport complex I is required for CD8+ T cell signal transduction, proliferation, cytokine production, and degranulation.

New insights into structure and function of mitochondria and their role in aging and disease

This review covers some novel findings on mitochondrial biochemistry and discusses diseases due to mitochondrial DNA mutations as a model of the changes occurring during physiological aging. The random collision model of organization of the mitochondrial respiratory chain has been recently challenged on the basis of findings of supramolecular organization of respiratory chain complexes. The source of superoxide in Complex I is discussed on the basis of laboratory experiments using a series of specific inhibitors and is presumably iron sulfur center N2. Maternally inherited diseases due to mutations of structural genes in mitochondrial DNA are surveyed as a model of alterations mimicking those occurring during normal aging. The molecular defects in senescence are surveyed on the basis of the "Mitochondrial Theory of Aging", establishing mitochondrial DNA somatic mutations, caused by accumulation of oxygen radical damage, to be at the basis of cellular senescence. Mitochondrial production of reactive oxygen species increases with aging and mitochondrial DNA mutations and deletions accumulate and may be responsible for oxidative phosphorylation defects. Evidence is presented favoring the mitochondrial theory, with primary mitochondrial alterations, although the problem is made more complex by changes in the cross-talk between nuclear and mitochondrial DNA.

Bioenergetics shapes cellular death pathways in Leber's hereditary optic neuropathy: a model of mitochondrial neurodegeneration

Leber's hereditary optic neuropathy (LHON) was the first maternally inherited disease to be associated with point mutations in mitochondrial DNA and is now considered the most prevalent mitochondrial disorder. The pathology is characterized by selective loss of ganglion cells in the retina leading to central vision loss and optic atrophy, prevalently in young males. The pathogenic mtDNA point mutations for LHON affect complex I with the double effect of lowering the ATP synthesis driven by complex I substrates and increasing oxidative stress chronically. In this review, we first consider the biochemical changes associated with the proton-translocating NADH-quinone oxidoreductase of mitochondria in cybrid cells carrying the most common LHON mutations. However, the LHON cybrid bioenergetic dysfunction is essentially compensated under normal conditions, i.e. in glucose medium, but is unrevealed by stressful conditions such as growing cybrids in glucose free/galactose medium, which forces cells to rely only on respiratory chain for ATP synthesis. In fact, the second part of this review deals with the investigation of LHON cybrid death pathway in galactose medium. The parallel marked changes in antioxidant enzymes, during the time-course of galactose experiments, also reveal a relevant role played by oxidative stress. The LHON cybrid model sheds light on the complex interplay amongst the different levels of biochemical consequences deriving from complex I mutations in determining neurodegeneration in LHON, and suggests an unsuspected role of bioenergetics in shaping cell death pathways.

Bioenergetics of mitochondrial diseases associated with mtDNA mutations

This mini-review summarizes our present view of the biochemical alterations associated with mitochondrial DNA (mtDNA) point mutations. Mitochondrial cytopathies caused by mutations of mtDNA are well-known genetic and clinical entities, but the biochemical pathogenic mechanisms are often obscure. Leber's hereditary optic neuropathy (LHON) is due to three main mutations in genes for complex I subunits. Even if the catalytic activity of complex I is maintained except in cells carrying the 3460/ND1 mutation, in all cases there is a change in sensitivity to complex I inhibitors and an impairment of mitochondrial respiration, eliciting the possibility of generation of reactive oxygen species (ROS) by the complex. Neurogenic muscle weakness, Ataxia and Retinitis Pigmentosa (NARP), is due to a mutation in the ATPase-6 gene. In NARP patients ATP synthesis is strongly depressed to an extent proportional to the mutation load; nevertheless, ATP hydrolysis and ATP-driven proton translocation are not affected. It is suggested that the NARP mutation affects the ability of the enzyme to couple proton transport to ATP synthesis. A point mutation in subunit III of cytochrome c oxidase is accompanied by a syndrome resembling MELAS: however, no major biochemical defect is found, if we except an enhanced production of ROS. The mechanism of such enhancement is at present unknown. In this review, we draw attention to a few examples in which the overproduction of ROS might represent a common step in the induction of clinical phenotypes and/or in the progression of several human pathologies associated with mtDNA point mutations.

Functional roles of four conserved charged residues in the membrane domain subunit NuoA of the proton-translocating NADH-quinone oxidoreductase from Escherichia coli

The H(+)(Na(+))-translocating NADH-quinone (Q) oxidoreductase (NDH-1) of Escherichia coli is composed of 13 different subunits (NuoA-N). Subunit NuoA (ND3, Nqo7) is one of the seven membrane domain subunits that are considered to be involved in H(+)(Na(+)) translocation. We demonstrated that in the Paracoccus denitrificans NDH-1 subunit, Nqo7 (ND3) directly interacts with peripheral subunits Nqo6 (PSST) and Nqo4 (49 kDa) by using cross-linkers (Di Bernardo, S., and Yagi, T. (2001) FEBS Lett. 508, 385-388 and Kao, M.-C., Matsuno-Yagi, A., and Yagi, T. (2004) Biochemistry 43, 3750-3755). To investigate the structural and functional roles of conserved charged amino acid residues, a nuoA knock-out mutant and site-specific mutants K46A, E51A, D79N, D79A, E81Q, E81A, and D79N/E81Q were constructed by utilizing chromosomal DNA manipulation. In terms of immunochemical and NADH dehydrogenase activity-staining analyses, all site-specific mutants are similar to the wild type, suggesting that those NuoA site-specific mutations do not significantly affect the assembly of peripheral subunits in situ. In addition, site-specific mutants showed similar deamino-NADH-K(3)Fe(CN)(6) reductase activity to the wild type. The K46A mutation scarcely inhibited deamino-NADH-Q reductase activity. In contrast, E51A, D79A, D79N, E81A, and E81Q mutation partially suppressed deamino-NADH-Q reductase activity to 30, 90, 40, 40, and 50%, respectively. The double mutant D79N/E81Q almost completely lost the energy-transducing NDH-1 activities but did not display any loss of deamino-NADH-K(3)Fe(CN)(6) reductase activity. The possible functional roles of residues Asp-79 and Glu-81 were discussed.

Mitochondrial dysfunction as a cause of optic neuropathies

Mitochondria are increasingly recognized as central players in the life and death of cells and especially of neurons. The energy-dependence of retinal ganglion cells (RGC) and their axons, which form the optic nerve, is singularly skewed. In fact, while mitochondria are very abundant in the initial, unmyelinated part of the axons anterior to the lamina cribrosa, their number suddenly decreases as the myelin sheath begins more posteriorly. The vascular system also presents different blood-brain barrier properties anterior and posterior to the lamina, possibly reflecting the different metabolic needs of the optic nerve head (unmyelinated) and of the retrobulbar optic nerve (myelinated). Mitochondrial biogenesis occurs within the cellular somata of RGC in the retina. It needs the coordinated interaction of nuclear and mitochondrial genomes. Mitochondria are then transported down the axons and distributed where they are needed. These locations are along the unmyelinated portion of the nerve, under the nodes of Ranvier in the retrobulbar nerve, and at the synaptic terminals. Efficient transportation of mitochondria depends on multiple factors, including their own energy production, the integrity of the cytoskeleton and its protein components (tubulin, etc.), and adequate myelination of the axons. Any dysfunction of these systems may be of pathological relevance for optic neuropathies with primary or secondary involvement of mitochondria. Leber's hereditary optic neuropathy (LHON) is the paradigm of mitochondrial optic neuropathies where a primary role for mitochondrial dysfunction is certified by maternal inheritance and association with specific mutations in the mitochondrial DNA (mtDNA). Clinical phenocopies of this pathology are represented by the wide array of optic neuropathies associated with vitamin depletion, toxic exposures, alcohol and tobacco abuse, and use of certain drugs. Moreover, the recent identification of mutations in the nuclear gene OPA1 as the causative factor in dominant optic atrophy (DOA, Kjer's type) brought the unexpected finding that this gene encodes for a mitochondrial protein, suggesting that DOA and LHON may be linked by similar pathogenesis. Polymorphisms in this very same gene may be associated with normal tension glaucoma (NTG), which might be considered a genetically determined optic neuropathy that again shows similarities with both LHON and DOA. Exciting new developments come from first examples of mitochondrial optic neuropathies in animal models that are genetically determined or are the result of ingenious engineering of mitochondrial gene expression, or from biochemical manipulations of the respiratory complexes. Even more exciting is the first successful attempt to correct the LHON-related complex I dysfunction by the allotopic nuclear expression of the recoded mitochondrial gene. There is hope that the genetic complexities, biochemical dysfunctions, and integrated anatomical-physiological cellular relationships will soon be precisely delineated and that promising therapeutic and prophylactic strategies will be proposed.

The ubiquinone-binding site in NADH:ubiquinone oxidoreductase from Escherichia coli

An azido-ubiquinone derivative, 3-azido-2-methyl-5-methoxy[3H]-6-decyl-1,4-benzoquinone ([3H]azido-Q), was used to study the ubiquinone/protein interaction and to identify the ubiquinone-binding site in Escherichia coli NADH:ubiquinone oxidoreductase (complex I). The purified complex I showed no loss of activity after incubation with a 20-fold molar excess of [3H]azido-Q in the dark. Illumination of the incubated sample with long wavelength UV light for 10 min at 0 degrees C caused a 40% decrease of NADH:ubiquinone oxidoreductase activity. SDS-PAGE of the complex labeled with [3H]azido-Q followed by analysis of the radioactivity distribution among the subunits revealed that subunit NuoM was heavily labeled, suggesting that this protein houses the Q-binding site. When the [3H]azido-Q-labeled NuoM was purified from the labeled reductase by means of preparative SDS-PAGE, a 3-azido-2-methyl-5-methoxy-6-decyl-1,4-benzoquinone-linked peptide, with a retention time of 41.4 min, was obtained by high performance liquid chromatography of the protease K digest of the labeled subunit. This peptide had a partial NH2-terminal amino acid sequence of NH2-VMLIAILALV-, which corresponds to amino acid residues 184-193 of NuoM. The secondary structure prediction of NuoM using the Toppred hydropathy analysis showed that the Q-binding peptide overlaps with a proposed Q-binding motif located in the middle of the transmembrane helix 5 toward the cytoplasmic side of the membrane. Using the PHDhtm hydropathy plot, the labeled peptide is located in the transmembrane helix 4 toward the periplasmic side of the membrane.

The use of lymphocytes to screen for oxidative phosphorylation disorders

Biochemical analysis of oxidative phosphorylation (OXPHOS) disorders is traditionally carried out on muscle biopsies, cultured fibroblasts, and transformed lymphocytes. Here we present a new screening technique using lymphocytes to identify OXPHOS dysfunction and initially avoid an invasive diagnostic procedure. Lymphocytes represent an easily obtainable source of tissue that presents advantages over the use of fibroblasts or lymphoblast cell lines. The time delay in culturing skin fibroblasts and the interactions between cell transformation and mitochondrial activity are avoided in this methodology. The method requires a small amount of blood (<5 mL); can be completed in a few hours, and allows for repeated measurements. Our assay has been adapted from published methods utilizing cultured fibroblasts and transformed lymphocytes, and our data suggest that measurement of ATP synthesis in lymphocytes is an effective screening tool for diagnosing OXPHOS disorders. This method may also provide an objective tool for monitoring response to treatment and evaluating progression of disease.

Leber hereditary optic neuropathy

Leber hereditary optic neuropathy (LHON) is a mitochondrial genetic disease that preferentially causes blindness in young adult males, affecting about 1 in 25 000 of the British population. It is characterised by bilateral subacute loss of central vision owing to focal degeneration of the retinal ganglion cell layer and optic nerve. Over 95% of LHON cases are primarily the result of one of three mitochondrial DNA (mtDNA) point mutations, G3460A, G11778A, and T14484C, which all involve genes encoding complex I subunits of the respiratory chain. An intriguing feature of LHON is that only approximately 50% of males and approximately 10% of females who harbour a pathogenic mtDNA mutation actually develop the optic neuropathy. This marked incomplete penetrance and gender bias imply that additional mitochondrial and/or nuclear genetic factors must be modulating the phenotypic expression of LHON. It is also likely that environmental factors contribute to the onset of visual failure. However, these secondary precipitating factors remain poorly defined at present. In this review, we describe the natural history of this optic nerve disorder and highlight issues relating to clinical diagnosis, management, and genetic counselling. We also discuss the findings of recently published studies and the light they shed on the complex aetiology and pathophysiology of LHON.

The origin of cluster N2 of the energy-transducing NADH-quinone oxidoreductase: comparisons of phylogenetically related enzymes

NADH-quinone (Q) oxidoreductase is a large and complex redox proton pump, which utilizes the free energy derived from oxidation of NADH with lipophilic electron/proton carrier Q to translocate protons across the membrane to generate an electrochemical proton gradient. Although its molecular mechanism is largely unknown, recent biochemical, biophysical, and molecular biological studies have revealed that particular subunits and cofactors play an essential role in the energy-coupling reaction. Based on these latest experimental data, we exhaustively analyzed the sequence information available from evolutionarily related enzymes such as [NiFe] hydrogenases. We found significant and conserved sequence differences in the PSST/Nqo6/NuoB, 49kDa/Nqo4/NuoD, and ND1/Nqo8/NuoH subunit homologs between complex I/NDH-1 and [NiFe] hydrogenases. The alterations, especially in the postulated ligand motif for cluster N2 in the PSST/Nqo6/NuoB subunits, appear to be evolutionarily important in determining the physiological function of complex I/NDH-1. These observations led us to propose a hypothetical evolutionary scheme: during the course of evolution, drastic changes have occurred in the putative cluster N2 binding site in the PSST/Nqo6/NuoB subunit and the progenitors of complex I/NDH-1 have concurrently become to utilize a lipophilic electron/proton carrier such as Q as its physiological substrate. This scheme provides new insights into the structure and function relationship of complex I/NDH-1 and may help us understand its energy-coupling mechanism.

NADH dehydrogenases: from basic science to biomedicine

This review article is concerned with two on-going research projects in our laboratory, both of which are related to the study of the NADH dehydrogenase enzyme complexes in the respiratory chain. The goal of the first project is to decipher the structure and mechanism of action of the proton-translocating NADH-quinone oxidoreductase (NDH-1) from two bacteria, Paracoccus denitrificans and Thermus thermophilus HB-8. These microorganisms are of particular interest because of the close resemblance of the former (P. denitrificans) to a mammalian mitochondria, and because of the thermostability of the enzymes of the latter (T. thermophilus). The NDH-1 enzyme complex of these and other bacteria is composed of 13 to 14 unlike subunits and has a relatively simple structure relative to the mitochondrial proton-translocating NADH-quinone oxidoreductase (complex I), which is composed of at least 42 different subunits. Therefore, the bacterial NDH-I is believed to be a useful model for studying the mitochondrial complex I, which is understood to have the most intricate structure of all the membrane-associated enzyme complexes. Recently, the study of the NADH dehydrogenase complex has taken on new urgency as a result of reports that complex I defects are involved in many human mitochondrial diseases. Thus the goal of the second project is to develop possible gene therapies for mitochondrial diseases caused by complex I defects. This project involves attempting to repair complex I defects in the mammalian system using Saccharomyces cerevisiae NDI1 genes, which code for the internal, rotenone-insensitive NADH-quinone oxidoreductase. In this review, we will discuss our progress and the data generated by these two projects to date. In addition, background information and the significance of various approaches employed to pursue these research objectives will be described.

Toward a characterization of the connecting module of complex I

Complex I [NADH-ubiquinone oxidoreductase (complex I, EC 1.6.5.3)] couples electron transfer between NADH and ubiquinone to proton transport across the bacterial cytoplasmic membrane and the mitochondrial inner membrane. This sophisticated enzyme consists of three specialized modules: (1) a hydrophilic NADH-oxidizing module that constitutes the input machinery of the enzyme; (2) a hydrophobic module that anchors the enzyme in the membrane and must take part in proton transport; and (3) a connecting domain that links the two previous modules. Using the complex I of Rhodobacter capsulatus, we developed a genetic study of the structure and function of the connecting module. In the present review, we put together the salient results of these studies, with recent reports of the literature, to try and elucidate the structure of the connecting module and its potential role in the coupling process between electron and proton flux within complex I. From this overview, we conclude that the NUOB-NUOD dimer of the connecting module and a hydrophobic subunit such as NUOH must share a quinone-reduction site. The function of this site in the mechanism of complex I is discussed.

alpha-Tocopherol/lipid ratio in blood is decreased in patients with Leber's hereditary optic neuropathy and asymptomatic carriers of the 11778 mtDNA mutation

OBJECTIVES

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disease characterised by acute or subacute bilateral visual loss in young patients. The primary aetiological event is a mutation in the mitochondrial genome (mtDNA) affecting in most cases mtDNA-encoded subunits of the respiratory chain NADH: coenzyme Q oxidoreductase (complex I). The impaired function of complex I leads to a decline in mitochondrial energy production and enhances free radical generation.

METHODS

The concentrations of some non-enzymatic antioxidants (alpha-tocopherol, beta-carotene, lycopene, glutathione, free sulphydryl groups) and the lipid peroxides in the blood of patients with LHON, carriers with homoplasmic DNA mutation at 11 778, and controls were investigated using high performance liquid chromatography and spectrophotometric methods to assess the function of their antioxidant defence systems.

RESULTS

The alpha-tocopherol/cholesterol+ triglyceride ratio was significantly reduced (p<0.05) both in the patients and asymptomatic carriers. The concentrations of the other antioxidants and the lipid peroxides were not different from those of control subjects.

CONCLUSION

The low concentration of plasma alpha-tocopherol most probably reflects the consumption of the antioxidant by the affected tissues. Furthermore, it suggests that alpha-tocopherol may be the primary scavenger molecule against the free radicals induced by complex I deficiency.

Functional analysis of lymphoblast and cybrid mitochondria containing the 3460, 11778, or 14484 Leber's hereditary optic neuropathy mitochondrial DNA mutation

Leber's hereditary optic neuropathy (LHON) is a form of blindness caused by mitochondrial DNA (mtDNA) mutations in complex I genes. We report an extensive biochemical analysis of the mitochondrial defects in lymphoblasts and transmitochondrial cybrids harboring the three most common LHON mutations: 3460A, 11778A, and 14484C. Respiration studies revealed that the 3460A mutation reduced the maximal respiration rate 20-28%, the 11778A mutation 30-36%, and the 14484C mutation 10-15%. The respiration defects of the 3460A and 11778A mutations transferred in cybrid experiments linking these defects to the mtDNA. Complex I enzymatic assays revealed that the 3460A mutation resulted in a 79% reduction in specific activity and the 11778A mutation resulted in a 20% reduction, while the 14484C mutation did not affect the complex I activity. The enzyme defect of the 3460A mutation transferred with the mtDNA in cybrids. Overall, these data support the conclusion that the 3460A and 11778A mutants result in complex I defects and that the 14484C mutation causes a much milder biochemical defect. These studies represent the first direct comparison of oxidative phosphorylation defects among all of the primary LHON mtDNA mutations, thus permitting insight into the underlying pathophysiological mechanism of the disease.

Mutagenesis of three conserved Glu residues in a bacterial homologue of the ND1 subunit of complex I affects ubiquinone reduction kinetics but not inhibition by dicyclohexylcarbodiimide

Steady-state kinetics of the H(+)-translocating NADH:ubiquinone reductase (complex I) were analyzed in membrane samples from bovine mitochondria and the soil bacterium Paracoccus denitrificans. In both enzymes the calculated K(m) values, in the membrane lipid phase, for four different ubiquinone analogues were in the millimolar range. Both the structure and size of the hydrophobic side chain of the acceptor affected its affinity for complex I. The ND1 subunit of bovine complex I is a mitochondrially encoded protein that binds the inhibitor dicyclohexylcarbodiimide (DCCD) covalently [Yagi and Hatefi (1988) J. Biol. Chem. 263, 16150-16155]. The NQO8 subunit of P. denitrificans complex I is a homologue of ND1, and within it three conserved Glu residues that could bind DCCD, E158, E212, and E247, were changed to either Asp or Gln and in the case of E212 also to Val. The DCCD sensitivity of the resulting mutants was, however, unaffected by the mutations. On the other hand, the ubiquinone reductase activity of the mutants was altered, and the mutations changed the interactions of complex I with short-chain ubiquinones. The implications of the results for the location of the ubiquinone reduction site in this enzyme are discussed.

Mitochondrial disorders

The rate of advance of our understanding of mitochondrial pathology continues to accelerate. Trends in genotype-phenotype correlations in mitochondrial DNA mutations continue to be developed; the latest of these is the association of exercise intolerance with cytochrome b mutations and onset in infancy of multisystem disorders associated with cytochrome oxidase assembly defects. New models for mitochondrial disease are being developed. Drugs, toxins and deficiency of nuclear encoded proteins that are targeted at mitochondria are now recognized as important causes of secondary mitochondrial respiratory chain deficiency.

Application of the obligate aerobic yeast Yarrowia lipolytica as a eucaryotic model to analyse Leigh syndrome mutations in the complex I core subunits PSST and TYKY

We have used the obligate aerobic yeast Yarrowia lipolytica to reconstruct and analyse three missense mutations in the nuclear coded subunits homologous to bovine TYKY and PSST of mitochondrial complex I (proton translocating NADH:ubiquinone oxidoreductase) that have been shown to cause Leigh syndrome (MIM 25600), a severe progressive neurodegenerative disorder. While homozygosity for a V122M substitution in NDUFS7 (PSST) has been found in two siblings with neuropathologically proven Leigh syndrome (R. Triepels et al., Ann. Neurol. 45 (1999) 787), heterozygosity for a P79L and a R102H substitution in NDUFS8 (TYKY) has been found in another patient (J. Loeffen et al., Am. J. Hum. Genet. 63 (1998) 1598). Mitochondrial membranes from Y. lipolytica strains carrying any of the three point mutations exhibited similar complex I defects, with V(max) being reduced by about 50%. This suggests that complex I mutations that clinically present as Leigh syndrome may share common characteristics. In addition changes in the K(m) for n-decyl-ubiquinone and I(50) for hydrophobic complex I inhibitors were observed, which provides further evidence that not only the hydrophobic, mitochondrially coded subunits, but also some of the nuclear coded subunits of complex I are involved in its reaction with ubiquinone.

Tight control of respiration by NADH dehydrogenase ND5 subunit gene expression in mouse mitochondria

A mouse cell variant carrying in heteroplasmic form a nonsense mutation in the mitochondrial DNA-encoded ND5 subunit of the respiratory NADH dehydrogenase has been isolated and characterized. The derivation from this mutant of a large number of cell lines containing between 4 and 100% of the normal number of wild-type ND5 genes has allowed an analysis of the genetic and functional thresholds operating in mouse mitochondria. In wild-type cells, approximately 40% of the ND5 mRNA level was in excess of that required for ND5 subunit synthesis. However, in heteroplasmic cells, the functional mRNA level decreased in proportion to the number of wild-type ND5 genes over a 25-fold range, pointing to the lack of any compensatory increase in rate of transcription and/or stability of mRNA. Most strikingly, the highest ND5 synthesis rate was just sufficient to support the maximum NADH dehydrogenase-dependent respiration rate, with no upregulation of translation occurring with decreasing wild-type mRNA levels. These results indicate that, despite the large excess of genetic potential of the mammalian mitochondrial genome, respiration is tightly regulated by ND5 gene expression.

Functional consequences of the 3460-bp mitochondrial DNA mutation associated with Leber's hereditary optic neuropathy

Complex I is the largest of the mitochondrial respiratory chain proteins, and contains subunits encoded by both mitochondrial and nuclear genomes. Leber's hereditary optic neuropathy has been clearly linked to mutations of mitochondrial DNA complex I genes, and variable complex I functional defects have been reported. We have confirmed an approximate 60% defect in mitochondrial NADH CoQ1 reductase activity in cultured fibroblasts bearing the 3460-bp G to A mutation within the ND1 gene. However complex I-linked ATP synthesis was found to be normal in these fibroblasts. A 60% rotenone-induced decrease in complex I activity was shown to reduce ATP synthesis in normal fibroblasts, indicating that this level of complex I activity was below the threshold required to affect ATP synthesis. Although 3460 LHON mitochondria were less sensitive to rotenone inhibition, this did not explain the decreased complex I activity as the rotenone insensitive activity was not increased, nor did the inhibitor diphenyleneiodonium inhibit the NADH CoQ1 reductase activity to a greater extent. Decreased NADH cytochrome c reductase activity in cybrids homoplasmic for the 3460 LHON mtDNA mutation confirmed that the decrease in complex I activity was not specific to the assay used and was not caused by inhibitory effects of ubiquinone analogues used in the NADH CoQ1 reductase assay. These findings have important implications for our understanding of complex I dysfunction in the pathogenesis of 3460 Leber's hereditary optic neuropathy.

Human mitochondrial diseases: answering questions and questioning answers

Since the first identification in 1988 of pathogenic mitochondrial DNA (mtDNA) mutations, the mitochondrial diseases have emerged as a major clinical entity. The most striking feature of these disorders is their marked heterogeneity, which extends to their clinical, biochemical, and genetic characteristics. The major mitochondrial encephalomyopathies include MELAS (mitochondrial encephalopathy with lactic acidosis and stroke-like episodes), MERRF (myoclonic epilepsy with ragged red fibers), KSS/CPEO (Kearns-Sayre syndrome/chronic progressive external ophthalmoplegia), and NARP/MILS (neuropathy, ataxia, and retinitis pigmentosum/maternally inherited Leigh syndrome) and they typically present highly variable multisystem defects that usually involve abnormalities of skeletal muscle and/or the CNS. The primary emphasis here is to review recent investigations of these mitochondrial diseases from the standpoint of how the complexities of mitochondrial genetics and biogenesis might determine their varied features. In addition, the mitochondrial encephalomyopathies are compared and contrasted to Leber hereditary optic neuropathy, a mitochondrial disease in which the pathogenic mtDNA mutations produce a more uniform and focal neuropathology. All of these disorders involve, at some level, a mitochondrial respiratory chain dysfunction. Because mitochondrial genetics differs so strikingly from the Mendelian inheritance of chromosomes, recent research on the origin and subsequent segregation and transmission of mtDNA mutations is reviewed.

Analysis of the pathogenic human mitochondrial mutation ND1/3460, and mutations of strictly conserved residues in its vicinity, using the bacterium Paracoccus denitrificans

The human mitochondrial ND1/3460 mutation changes Ala52 to Thr in the ND1 subunit of Complex I, and causes Leber's hereditary optic neuropathy (LHON) [Huoponen et al. (1991) Am. J. Hum. Genet. 48, 1147]. We have used a bacterial counterpart of Complex I, NDH-1 from Paracoccus denitrificans, for studying the effect of mutations in the ND1 subunit on the enzymatic activity. The LHON mutation as well as several other mutations in strictly conserved amino acids in its vicinity were introduced into the NQO8 subunit of NDH-1, a bacterial homologue of ND1. The enzymatic activity of the mutants in the presence of hexammineruthenium (rotenone-insensitive) and ubiquinone-1 (rotenone-sensitive) were assayed. In addition, the kinetics of the interaction of selected mutant enzymes with ubiquinone-1, ubiquinone-2, and decylubiquinone was studied. The results suggest that the mutated residues play an important role in ubiquinone reduction by Complex I.

The nuoM arg368his mutation in NADH:ubiquinone oxidoreductase from Rhodobacter capsulatus: a model for the human nd4-11778 mtDNA mutation associated with Leber's hereditary optic neuropathy

Mutation at position 11778 in the nd4 gene of the human mitochondrial complex I is associated with Leber's hereditary optic neuropathy. Type I NADH:ubiquinone oxidoreductase of Rhodobacter capsulatus displays similar properties to complex I of the mitochondrial respiratory chain. The NUOM subunit of the bacterial enzyme is homologous to the ND4 subunit. Disruption of the nuoM gene led to a bacterial mutant exhibiting a defect in complex I activity and assembly. A nuoM-1103 point mutant reproducing the nd4-11778 mutation has been introduced in the R. capsulatus genome. This mutant showed a reduced ability to grow in a medium containing malate instead of lactate which indicated a clear impairment in oxidative phosphorylation capacity. NADH supported respiration of porous bacterial cells was significantly decreased in the nuoM-1103 mutant while no significant reduction could be observed in isolated bacterial membranes. As it has been observed in the case of the nd4-11778 mitochondrial mutation, proton-pump activity of the bacterial enzyme was not affected by the nuoM-1103 mutation. All these data which reproduce most of the biochemical features observed in patient mitochondria harboring the nd4-11778 mutation show that the R. capsulatus complex I might be used as a useful model to investigate mutations of the mitochondrial DNA which are associated with complex I deficiencies in human pathologies.

The influence of nuclear background on the biochemical expression of 3460 Leber's hereditary optic neuropathy

The role of mitochondrial DNA (mtDNA) mutations in the pathogenesis of Leber's hereditary optic neuropathy (LHON) has yet to be characterized. Several clinical features of the disease imply that nuclear genes might also be involved in its expression. We have confirmed the presence of a severe NADH:coenzyme Q1 reductase (complex I) defect in association with the A3460G mtDNA LHON mutation in cultured fibroblasts compared with age-matched controls. This defect was not seen in clonal fibroblasts with 0% mutant mtDNA developed from a heteroplasmic A3460G LHON subject, confirming the association between the A3460G mutation and the complex I defect. Cybrids prepared from the fusion of enucleated fibroblasts homoplasmic for the A3460G mutation with 206 (osteosarcoma) cells lacking mtDNA (p0) also had a severe deficiency of complex I activity. However, in A3460G LHON fusion cybrids containing a different nuclear background, A549 p0 (lung derived), this biochemical defect was not apparent in all the clones studied. These results suggest that the nuclear environment can influence the expression of the biochemical defect in LHON patients with the A3460G mutation.

Inhibitors of NADH-ubiquinone reductase: an overview

This article provides an updated overview of the plethora of complex I inhibitors. The inhibitors are presented within the broad categories of natural and commercial compounds and their potency is related to that of rotenone, the classical inhibitor of complex I. Among commercial products, particular attention is dedicated to inhibitors of pharmacological or toxicological relevance. The compounds that inhibit the NADH-ubiquinone reductase activity of complex I are classified according to three fundamental types of action on the basis of available evidence and recent insights: type A are antagonists of the ubiquinone substrate, type B displace the ubisemiquinone intermediate, and type C are antagonists of the ubiquinol product.

The complex I from Rhodobacter capsulatus

The NADH-ubiquinone oxidoreductase (type I NDH) of Rhodobacter capsulatus is a multisubunit enzyme encoded by the 14 genes of the nuo operon. This bacterial enzyme constitutes a valuable model for the characterization of the mitochondrial Complex I structure and enzymatic mechanism for the following reasons. (i) The mitochondria-encoded ND subunits are not readily accessible to genetic manipulation. In contrast, the equivalents of the mitochondrial ND1, ND2, ND4, ND4L, ND5 and ND6 genes can be easily mutated in R. capsulatus by homologous recombination. (ii) As illustrated in the case of ND1 gene, point mutations associated with human cytopathies can be reproduced and studied in this model system. (iii) The R. capsulatus model also allows the recombinant manipulations of iron-sulfur (Fe-S) subunits and the assignment of Fe-S clusters as illustrated in the case of the NUOI subunit (the equivalent of the mitochondrial TYKY subunit). (iv) Finally, like mitochondrial Complex I, the NADH-ubiquinone oxidoreductase of R. capsulatus is highly sensitive to the inhibitor piericidin-A which is considered to bind to or close to the quinone binding site(s) of Complex I. Therefore, isolation of R. capsulatus mutants resistant to piericidin-A represents a straightforward way to map the inhibitor binding sites and to try and define the location of quinone binding site(s) in the enzyme. These illustrations that describe the interest in the R. capsulatus NADH-ubiquinone oxidoreductase model for the general study of Complex I will be critically developed in the present review.

Quinone specificity of complex I

This review considers the interaction of Complex I with different redox acceptors, mainly homologs and analogs of the physiological acceptor, hydrophobic Coenzyme Q. After examining the physical properties of the different quinones and their efficacy in restoring mitochondrial respiration, a survey ensues of the advantages and drawbacks of the quinones commonly used in Complex I activity determination and of their kinetic properties. The available evidence is then displayed on structure-activity relationships of various quinone compounds in terms of electron transfer activity and proton translocation, and the present knowledge is discussed in terms of the nature of multiple quinone-binding sites in the Complex.