Mitochondrial ATP synthase disorders: molecular mechanisms and the quest for curative therapeutic approaches

Leigh syndrome: Resolving the clinical and genetic heterogeneity paves the way for treatment options

Leigh syndrome is a progressive neurodegenerative disorder, affecting 1 in 40,000 live births. Most patients present with symptoms between the ages of three and twelve months, but adult onset Leigh syndrome has also been described. The disease course is characterized by a rapid deterioration of cognitive and motor functions, in most cases resulting in death due to respiratory failure. Despite the high genetic heterogeneity of Leigh syndrome, patients present with identical, symmetrical lesions in the basal ganglia or brainstem on MRI, while additional clinical manifestations and age of onset varies from case to case. To date, mutations in over 60 genes, both nuclear and mitochondrial DNA encoded, have been shown to cause Leigh syndrome, still explaining only half of all cases. In most patients, these mutations directly or indirectly affect the activity of the mitochondrial respiratory chain or pyruvate dehydrogenase complex. Exome sequencing has accelerated the discovery of new genes and pathways involved in Leigh syndrome, providing novel insights into the pathophysiological mechanisms. This is particularly important as no general curative treatment is available for this devastating disorder, although several recent studies imply that early treatment might be beneficial for some patients depending on the gene or process affected. Timely, gene-based personalized treatment may become an important strategy in rare, genetically heterogeneous disorders like Leigh syndrome, stressing the importance of early genetic diagnosis and identification of new genes/pathways. In this review, we provide a comprehensive overview of the most important clinical manifestations and genes/pathways involved in Leigh syndrome, and discuss the current state of therapeutic interventions in patients.

Biochemical investigation of a human pathogenic mutation in the nuclear ATP5E gene using yeast as a model

F₁F₀-ATP synthase is a key enzyme of the mitochondrial energetic metabolism responsible for the production of most cellular ATP in humans. Mayr et al. (2010) recently described a patient with a homozygote (Y12C) mutation in the nuclear gene ATP5E encoding the ε-subunit of ATP synthase. To better define how it affects ATP synthase, we have modeled this mutation in the yeast Saccharomyces cerevisiae. A yeast equivalent of this mutation (Y11C) had no significant effect on the growth of yeast on non-fermentable carbon sources (glycerol/ethanol or lactate), conditions under which the activity of the mitochondrial energy transducing system is absolutely essential. In addition, similar to what was observed in patient, this mutation in yeast has a minimal effect on the ATPase/synthase activities. On the contrary, this mutation which has been shown to have a strong impact on the assembly of the ATP synthase complex in humans, shows no significant impact on the assembly/stability of this complex in yeast, suggesting that biogenesis of this complex differs significantly.

Understanding structure, function, and mutations in the mitochondrial ATP synthase

The mitochondrial ATP synthase is a multimeric enzyme complex with an overall molecular weight of about 600,000 Da. The ATP synthase is a molecular motor composed of two separable parts: F₁ and F_o. The F₁ portion contains the catalytic sites for ATP synthesis and protrudes into the mitochondrial matrix. F_o forms a proton turbine that is embedded in the inner membrane and connected to the rotor of F₁. The flux of protons flowing down a potential gradient powers the rotation of the rotor driving the synthesis of ATP. Thus, the flow of protons though F_o is coupled to the synthesis of ATP. This review will discuss the structure/function relationship in the ATP synthase as determined by biochemical, crystallographic, and genetic studies. An emphasis will be placed on linking the structure/function relationship with understanding how disease causing mutations or putative single nucleotide polymorphisms (SNPs) in genes encoding the subunits of the ATP synthase, will affect the function of the enzyme and the health of the individual. The review will start by summarizing the current understanding of the subunit composition of the enzyme and the role of the subunits followed by a discussion on known mutations and their effect on the activity of the ATP synthase. The review will conclude with a summary of mutations in genes encoding subunits of the ATP synthase that are known to be responsible for human disease, and a brief discussion on SNPs.

Mitochondrial protein sorting as a therapeutic target for ATP synthase disorders

Mitochondrial diseases are systemic, prevalent and often fatal; yet treatments remain scarce. Identifying molecular intervention points that can be therapeutically targeted remains a major challenge, which we confronted via a screening assay we developed. Using yeast models of mitochondrial ATP synthase disorders, we screened a drug repurposing library, and applied genomic and biochemical techniques to identify pathways of interest. Here we demonstrate that modulating the sorting of nuclear-encoded proteins into mitochondria, mediated by the TIM23 complex, proves therapeutic in both yeast and patient-derived cells exhibiting ATP synthase deficiency. Targeting TIM23-dependent protein sorting improves an array of phenotypes associated with ATP synthase disorders, including biogenesis and activity of the oxidative phosphorylation machinery. Our study establishes mitochondrial protein sorting as an intervention point for ATP synthase disorders, and because of the central role of this pathway in mitochondrial biogenesis, it holds broad value for the treatment of mitochondrial diseases.

Comparison of whole mitochondrial genome sequences from two clades of the invasive ascidian, Didemnum vexillum

The mitochondria are the main source of cellular energy production and have an important role in development, fertility, and thermal limitations. Adaptive mitochondrial DNA mutations have the potential to be of great importance in determining aspects of the life history of an organism. Phylogenetic analyses of the globally invasive marine ascidian Didemnum vexillum using the mitochondrial cytochrome c oxidase 1 (COX1) coding region, revealed two distinct clades. Representatives of one clade (denoted by 'B') are geographically restricted to D. vexillum's native region (north-west Pacific Ocean, including Japan), whereas members of the other clade (denoted by 'A') have been introduced and become invasive in temperate coastal areas around the world. Persistence of clade B's restricted distribution may reflect it being inherently less invasive than clade A. To investigate this we sought to determine if the two clades differ significantly in other mitochondrial genes of functional significance, specifically, alterations in amino acids encoded in mitochondrial enzyme subunits. Differences in functional mitochondrial genes could indicate an increased ability for clade A colonies to tolerate a wider range of environmental temperature. Full mitochondrial genomic sequences from D. vexillum clades A and B were obtained and they predict significant sequence differences in genes encoding for enzymes involved in oxidative phosphorylation. Diversity levels were relatively high and showed divergence across almost all genes, with p-distance values between the two clades indicating recent divergence. Both clades showed an excess of rare variants, which is consistent with balancing selection or a recent population expansion. Results presented here will inform future research focusing on examining the functional properties of the corresponding mitochondrial respiration enzymes, of A and B clade enzymes. By comparing closely related taxa that have differing distributions it is possible to identify genes and phenotypes suited to particular environments. The examination of mitochondrial genotypes, and associated enzyme functioning, across populations may aid in our understanding of thermal tolerance and environmental adaptation.

The a subunit asymmetry dictates the two opposite rotation directions in the synthesis and hydrolysis of ATP by the mitochondrial ATP synthase

The main and best known role of the mitochondrial ATP synthase is to synthesize ATP by exploiting the transmembrane electrochemical gradient of protons and their downhill movement. However, under different conditions, the same enzyme can also switch to the opposite function of ATP hydrolysis and exploits its energy to pump protons against their gradient and energize the membrane. The change in functionality is linked to the change of direction of rotation of the two matched sectors of this unique complex, namely the hydrophilic F1, which performs the catalysis, and the hydrophobic membrane-embedded FO, which channels protons. Accordingly, viewed from the matrix side, ATP synthesis is driven by counterclockwise rotation and ATP hydrolysis by clockwise rotation of the FO rotor which is transmitted to F1. ATP dissipation through this mechanism features some diseases such as myocardial ischemia. Increasing evidence shoulders the hypothesis that the asymmetry of the a subunit of FO and particularly the steric arrangement of the two inner semi-channels for protons, play a key role in conferring to the coupled bi-functional complex the ability to reverse rotation by switching from ATP synthesis to ATP hydrolysis and vice versa. Accordingly, the conserved steric arrangement of the chiral a subunit of FO yields the same direction of rotation for all the ATP synthases. According to this hypothesis, the a subunit chirality imposes the direction of rotation of the rotor according to the proton gradient across the membrane. It seems likely that the direction of rotation of the membrane-embedded c-ring, which is adjacent to the a-subunit and acts as a rotor, may be under multiple control, being rotation essential to make the whole enzyme machinery work. However, the asymmetric features of the a subunit would make it the master regulator, thus directly determining which of the two functions, ATP production or ATP dissipation, will be performed. The handedness of a subunit should be considered in drug design to counteract tissue damage under all pathological conditions linked to functional impairment of ATP synthase.

The genetics of Leigh syndrome and its implications for clinical practice and risk management

Leigh syndrome, also referred to as subacute necrotizing encephalomyelopathy, is a severe, early-onset neurodegenerative disorder that is relentlessly progressive and devastating to both the patient and the patient’s family. Attributed to the ultimate failure of the mitochondrial respiratory chain, once it starts, the disease often results in the regression of both mental and motor skills, leading to disability and rapid progression to death. It is a mitochondrial disorder with both phenotypic and genetic heterogeneity. The cause of death is most often respiratory failure, but there are a whole host of complications, including refractory seizures, that may further complicate morbidity and mortality. The symptoms may develop slowly or with rapid progression, usually associated with age of onset. Although the disease is usually diagnosed within the first year of life, it is important to note that recent studies reveal phenotypic heterogeneity, with some patients having evidence of in utero presentation and others having adult-onset symptoms.

Socially responsive effects of brain oxidative metabolism on aggression

Despite ongoing high energetic demands, brains do not always use glucose and oxygen in a ratio that produces maximal ATP through oxidative phosphorylation. In some cases glucose consumption exceeds oxygen use despite adequate oxygen availability, a phenomenon known as aerobic glycolysis. Although metabolic plasticity seems essential for normal cognition, studying its functional significance has been challenging because few experimental systems link brain metabolic patterns to distinct behavioral states. Our recent transcriptomic analysis established a correlation between aggression and decreased whole-brain oxidative phosphorylation activity in the honey bee (Apis mellifera), suggesting that brain metabolic plasticity may modulate this naturally occurring behavior. Here we demonstrate that the relationship between brain metabolism and aggression is causal, conserved over evolutionary time, cell type-specific, and modulated by the social environment. Pharmacologically treating honey bees to inhibit complexes I or V in the oxidative phosphorylation pathway resulted in increased aggression. In addition, transgenic RNAi lines and genetic manipulation to knock down gene expression in complex I in fruit fly (Drosophila melanogaster) neurons resulted in increased aggression, but knockdown in glia had no effect. Finally, honey bee colony-level social manipulations that decrease individual aggression attenuated the effects of oxidative phosphorylation inhibition on aggression, demonstrating a specific effect of the social environment on brain function. Because decreased neuronal oxidative phosphorylation is usually associated with brain disease, these findings provide a powerful context for understanding brain metabolic plasticity and naturally occurring behavioral plasticity.

The INA complex facilitates assembly of the peripheral stalk of the mitochondrial F1Fo-ATP synthase

Mitochondrial F1Fo-ATP synthase generates the bulk of cellular ATP. This molecular machine assembles from nuclear- and mitochondria-encoded subunits. Whereas chaperones for formation of the matrix-exposed hexameric F1-ATPase core domain have been identified, insight into how the nuclear-encoded F1-domain assembles with the membrane-embedded Fo-region is lacking. Here we identified the INA complex (INAC) in the inner membrane of mitochondria as an assembly factor involved in this process. Ina22 and Ina17 are INAC constituents that physically associate with the F1-module and peripheral stalk, but not with the assembled F1Fo-ATP synthase. Our analyses show that loss of Ina22 and Ina17 specifically impairs formation of the peripheral stalk that connects the catalytic F1-module to the membrane embedded Fo-domain. We conclude that INAC represents a matrix-exposed inner membrane protein complex that facilitates peripheral stalk assembly and thus promotes a key step in the biogenesis of mitochondrial F1Fo-ATP synthase.

Assembly of the Escherichia coli FoF1 ATP synthase involves distinct subcomplex formation

The ATP synthase (FoF1) of Escherichia coli couples the translocation of protons across the cytoplasmic membrane by Fo to ATP synthesis or hydrolysis in F1. Whereas good knowledge of the nanostructure and the rotary mechanism of the ATP synthase is at hand, the assembly pathway of the 22 polypeptide chains present in a stoichiometry of ab2c10α3β3γδϵ has so far not received sufficient attention. In our studies, mutants that synthesize different sets of FoF1 subunits allowed the characterization of individually formed stable subcomplexes. Furthermore, the development of a time-delayed in vivo assembly system enabled the subsequent synthesis of particular missing subunits to allow the formation of functional ATP synthase complexes. These observations form the basis for a model that describes the assembly pathway of the E. coli ATP synthase from pre-formed subcomplexes, thereby avoiding membrane proton permeability by a concomitant assembly of the open H+-translocating unit within a coupled FoF1 complex.

Nuclear Genetic Defects of Mitochondrial ATP Synthase

Disorders of ATP synthase, the key enzyme of mitochondrial energy provision belong to the most severe metabolic diseases presenting as early-onset mitochondrial encephalo-cardiomyopathies. Up to now, mutations in four nuclear genes were associated with isolated deficiency of ATP synthase. Two of them, ATP5A1 and ATP5E encode enzyme’s structural subunits α and ε, respectively, while the other two ATPAF2 and TMEM70 encode specific ancillary factors that facilitate the biogenesis of ATP synthase. All these defects share a similar biochemical phenotype with pronounced decrease in the content of fully assembled and functional ATP synthase complex. However, substantial differences can be found in their frequency, molecular mechanism of pathogenesis, clinical manifestation as well as the course of the disease progression. While for TMEM70 the number of reported patients as well as spectrum of the mutations is steadily increasing, mutations in ATP5A1, ATP5E and ATPAF2 genes are very rare. Apparently, TMEM70 gene is highly prone to mutagenesis and this type of a rare mitochondrial disease has a rather frequent incidence. Here we present overview of individual reported cases of nuclear mutations in ATP synthase and discuss, how their analysis can improve our understanding of the enzyme biogenesis.

Defining the impact on yeast ATP synthase of two pathogenic human mitochondrial DNA mutations, T9185C and T9191C

Mutations in the human mitochondrial ATP6 gene encoding ATP synthase subunit a/6 (referred to as Atp6p in yeast) are at the base of neurodegenerative disorders like Neurogenic Ataxia and Retinitis Pigmentosa (NARP), Leigh syndrome (LS), Charcot-Marie-Tooth (CMT), and ataxia telangiectasia. In previous studies, using the yeast Saccharomyces cerevisiae as a model we were able to better define how several of these mutations impact the ATP synthase. Here we report the construction of yeast models of two other ATP6 pathogenic mutations, T9185C and T9191C. The first one was reported as conferring a mild, sometimes reversible, CMT clinical phenotype; the second one has been described in a patient presenting with severe LS. We found that an equivalent of the T9185C mutation partially impaired the functioning of yeast ATP synthase, with only a 30% deficit in mitochondrial ATP production. An equivalent of the mutation T9191C had much more severe effects, with a nearly complete block in yeast Atp6p assembly and an >95% drop in the rate of ATP synthesis. These findings provide a molecular basis for the relative severities of the diseases induced by T9185C and T9191C.

Subunit δ is the key player for assembly of the H(+)-translocating unit of Escherichia coli F(O)F1 ATP synthase

The ATP synthase (F(O)F1) of Escherichia coli couples the translocation of protons across the cytoplasmic membrane to the synthesis or hydrolysis of ATP. This nanomotor is composed of the rotor c10γε and the stator ab2α3β3δ. To study the assembly of this multimeric enzyme complex consisting of membrane-integral as well as peripheral hydrophilic subunits, we combined nearest neighbor analyses by intermolecular disulfide bond formation or purification of partially assembled F(O)F1 complexes by affinity chromatography with the use of mutants synthesizing different sets of F(O)F1 subunits. Together with a time-delayed in vivo assembly system, the results demonstrate that F(O)F1 is assembled in a modular way via subcomplexes, thereby preventing the formation of a functional H(+)-translocating unit as intermediate product. Surprisingly, during the biogenesis of F(O)F1, F1 subunit δ is the key player in generating stable F(O). Subunit δ serves as clamp between ab2 and c10α3β3γε and guarantees that the open H(+) channel is concomitantly assembled within coupled F(O)F1 to maintain the low membrane proton permeability essential for viability, a general prerequisite for the assembly of multimeric H(+)-translocating enzymes.

Time-delayed in vivo assembly of subunit a into preformed Escherichia coli FoF1 ATP synthase

Escherichia coli F(O)F(1) ATP synthase, a rotary nanomachine, is composed of eight different subunits in a α3β3γδεab2c10 stoichiometry. Whereas F(O)F(1) has been studied in detail with regard to its structure and function, much less is known about how this multisubunit enzyme complex is assembled. Single-subunit atp deletion mutants are known to be arrested in assembly, thus leading to formation of partially assembled subcomplexes. To determine whether those subcomplexes are preserved in a stable standby mode, a time-delayed in vivo assembly system was developed. To establish this approach, we targeted the time-delayed assembly of membrane-integrated subunit a into preformed F(O)F(1) lacking subunit a (F(O)F(1)-a) which is known to form stable subcomplexes in vitro. Two expression systems (araBADp and T7p-laco) were adjusted to provide compatible, mutually independent, and sufficiently stringent induction and repression regimens. In detail, all structural atp genes except atpB (encoding subunit a) were expressed under the control of araBADp and induced by arabinose. Following synthesis of F(O)F(1)-a during growth, expression was repressed by glucose/d-fucose, and degradation of atp mRNA controlled by real-time reverse transcription-PCR. A time-delayed expression of atpB under T7p-laco control was subsequently induced in trans by addition of isopropyl-β-d-thiogalactopyranoside. Formation of fully assembled, and functional, F(O)F(1) complexes was verified. This demonstrates that all subunits of F(O)F(1)-a remain in a stable preformed state capable to integrate subunit a as the last subunit. The results reveal that the approach presented here can be applied as a general method to study the assembly of heteromultimeric protein complexes in vivo.

Mitochondria as a source of reactive oxygen and nitrogen species: from molecular mechanisms to human health

Mitochondrially generated reactive oxygen species are involved in a myriad of signaling and damaging pathways in different tissues. In addition, mitochondria are an important target of reactive oxygen and nitrogen species. Here, we discuss basic mechanisms of mitochondrial oxidant generation and removal and the main factors affecting mitochondrial redox balance. We also discuss the interaction between mitochondrial reactive oxygen and nitrogen species, and the involvement of these oxidants in mitochondrial diseases, cancer, neurological, and cardiovascular disorders.

Defining the pathogenesis of human mtDNA mutations using a yeast model: the case of T8851C

More and more mutations are found in the mitochondrial DNA of various patients but ascertaining their pathogenesis is often difficult. Due to the conservation of mitochondrial function from yeast to humans, the unique ability of yeast to survive without production of ATP by oxidative phosphorylation, and the amenability of the yeast mitochondrial genome to site-directed mutagenesis, yeast is an excellent model for investigating the consequences of specific human mtDNA mutations. Here we report the construction of a yeast model of a point mutation (T8851C) in the mitochondrially-encoded subunit a/6 of the ATP synthase that has been associated with bilateral striatal lesions, a group of rare human neurological disorders characterized by symmetric degeneration of the corpus striatum. The biochemical consequences of this mutation are unknown. The T8851C yeast displayed a very slow growth phenotype on non-fermentable carbon sources, both at 28°C (the optimal temperature for yeast growth) and at 36°C. Mitochondria from T8851C yeast grown in galactose at 28°C showed a 60% deficit in ATP production. When grown at 36°C the rate of ATP synthesis was below 5% that of the wild-type, indicating that heat renders the mutation much more deleterious. At both growth temperatures, the mutant F(1)F(o) complex was correctly assembled but had only very weak ATPase activity (about 10% that of the control), both in mitochondria and after purification. These findings indicate that a block in the proton-translocating domain of the ATP synthase is the primary cause of the neurological disorder in the patients carrying the T8851C mutation. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

miR-127-5p targets the 3'UTR of human β-F1-ATPase mRNA and inhibits its translation

The mitochondrial H(+)-ATP synthase is a bottleneck component in the provision of metabolic energy by oxidative phosphorylation. The expression of its catalytic subunit (β-F1-ATPase) is stringently controlled at post-transcriptional levels during oncogenesis, the cell cycle and in development. Here we show that miR-127-5p targets the 3'UTR of β-F1-ATPase mRNA (β-mRNA) significantly reducing its translational efficiency without affecting β-mRNA abundance. Despite the reduced expression of β-F1-ATPase in most human carcinomas, we observed no expression of miR-127-5p in different human cancer cell lines, minimizing the potential role of miR-127-5p as a regulator of the bioenergetic activity of mitochondria in cancer. In contrast, miR-127-5p is highly over-expressed in the human fetal liver. Consistent with previous findings in the rat, the expression of β-F1-ATPase in the human liver also seems to be controlled at post-transcriptional levels during development, what might suggest a role for miR-127-5p in controlling β-mRNA translation and thus in defining the bioenergetic activity of human liver mitochondria. Moreover, immunolocalization techniques and subcellular fractionation experiments using different antibodies against β-F1-ATPase reveal that the ectopic expression of β-F1-ATPase at the cell surface of the hepatocytes and HepG2 cells is negligible or stands for scrutiny.

Mitochondrial ATP synthase: architecture, function and pathology

Human mitochondrial (mt) ATP synthase, or complex V consists of two functional domains: F(1), situated in the mitochondrial matrix, and F(o), located in the inner mitochondrial membrane. Complex V uses the energy created by the proton electrochemical gradient to phosphorylate ADP to ATP. This review covers the architecture, function and assembly of complex V. The role of complex V di-and oligomerization and its relation with mitochondrial morphology is discussed. Finally, pathology related to complex V deficiency and current therapeutic strategies are highlighted. Despite the huge progress in this research field over the past decades, questions remain to be answered regarding the structure of subunits, the function of the rotary nanomotor at a molecular level, and the human complex V assembly process. The elucidation of more nuclear genetic defects will guide physio(patho)logical studies, paving the way for future therapeutic interventions.

Molecular mechanisms of neonatal brain injury

Fetal/neonatal brain injury is an important cause of neurological disability. Hypoxia-ischemia and excitotoxicity are considered important insults, and, in spite of their acute nature, brain injury develops over a protracted time period during the primary, secondary, and tertiary phases. The concept that most of the injury develops with a delay after the insult makes it possible to provide effective neuroprotective treatment after the insult. Indeed, hypothermia applied within 6 hours after birth in neonatal encephalopathy reduces neurological disability in clinical trials. In order to develop the next generation of treatment, we need to know more about the pathophysiological mechanism during the secondary and tertiary phases of injury. We review some of the critical molecular events related to mitochondrial dysfunction and apoptosis during the secondary phase and report some recent evidence that intervention may be feasible also days-weeks after the insult.

The oxidative phosphorylation system in mammalian mitochondria

The chapter provides a review of the state of art of the oxidative phosphorylation system in mammalian mitochondria. The sections of the paper deal with: (i) the respiratory chain as a whole: redox centers of the chain and protonic coupling in oxidative phosphorylation (ii) atomic structure and functional mechanism of protonmotive complexes I, III, IV and V of the oxidative phosphorylation system (iii) biogenesis of oxidative phosphorylation complexes: mitochondrial import of nuclear encoded subunits, assembly of oxidative phosphorylation complexes, transcriptional factors controlling biogenesis of the complexes. This advanced knowledge of the structure, functional mechanism and biogenesis of the oxidative phosphorylation system provides a background to understand the pathological impact of genetic and acquired dysfunctions of mitochondrial oxidative phosphorylation.

Modes of metabolic compensation during mitochondrial disease using the Drosophila model of ATP6 dysfunction

Numerous mitochondrial DNA mutations cause mitochondrial encephalomyopathy: a collection of related diseases for which there exists no effective treatment. Mitochondrial encephalomyopathies are complex multisystem diseases that exhibit a relentless progression of severity, making them both difficult to treat and study. The pathogenic and compensatory metabolic changes that are associated with chronic mitochondrial dysfunction are not well understood. The Drosophila ATP6¹ mutant models human mitochondrial encephalomyopathy and allows the study of metabolic changes and compensation that occur throughout the lifetime of an affected animal. ATP6¹animals have a nearly complete loss of ATP synthase activity and an acute bioenergetic deficit when they are asymptomatic, but surprisingly we discovered no chronic bioenergetic deficit in these animals during their symptomatic period. Our data demonstrate dynamic metabolic compensatory mechanisms that sustain normal energy availability and activity despite chronic mitochondrial complex V dysfunction resulting from an endogenous mutation in the mitochondrial DNA. ATP6¹animals compensate for their loss of oxidative phosphorylation through increases in glycolytic flux, ketogenesis and Kreb's cycle activity early during pathogenesis. However, succinate dehydrogenase activity is reduced and mitochondrial supercomplex formation is severely disrupted contributing to the pathogenesis seen in ATP6¹ animals. These studies demonstrate the dynamic nature of metabolic compensatory mechanisms and emphasize the need for time course studies in tractable animal systems to elucidate disease pathogenesis and novel therapeutic avenues.

A yeast-based assay identifies drugs active against human mitochondrial disorders

Due to the lack of relevant animal models, development of effective treatments for human mitochondrial diseases has been limited. Here we establish a rapid, yeast-based assay to screen for drugs active against human inherited mitochondrial diseases affecting ATP synthase, in particular NARP (neuropathy, ataxia, and retinitis pigmentosa) syndrome. This method is based on the conservation of mitochondrial function from yeast to human, on the unique ability of yeast to survive without production of ATP by oxidative phosphorylation, and on the amenability of the yeast mitochondrial genome to site-directed mutagenesis. Our method identifies chlorhexidine by screening a chemical library and oleate through a candidate approach. We show that these molecules rescue a number of phenotypes resulting from mutations affecting ATP synthase in yeast. These compounds are also active on human cybrid cells derived from NARP patients. These results validate our method as an effective high-throughput screening approach to identify drugs active in the treatment of human ATP synthase disorders and suggest that this type of method could be applied to other mitochondrial diseases.

Hsp90 and mitochondrial proteases Yme1 and Yta10/12 participate in ATP synthase assembly in Saccharomyces cerevisiae

Hsc82 and Hsp82, the Hsp90 family proteins of yeast, are both required for fermentative growth at 37°C. Inactivation of either of the mitochondrial AAA proteases, Yme1 or Yta10/12, allows fermentative growth of hsc82∆ or hsp82∆ strains at 37°C. Genetic evidence indicates interaction of Hsc82/Hsp82 with the Yme1 and Yta10/Yta12 complexes in promoting F(1)F(o)-ATPase activity, with Hsc82 specifically required for F(1)-ATPase assembly. A previously reported mutation in Rpt3, one of the six ATPases of the proteasome, suppresses yme1∆ phenotypes and increases transcription of HSC82 but not HSP82. These genetic interactions describe a functional role for Hsp90 proteins in mitochondrial biogenesis.

Up-regulation of the ATPase inhibitory factor 1 (IF1) of the mitochondrial H+-ATP synthase in human tumors mediates the metabolic shift of cancer cells to a Warburg phenotype

The H(+)-ATP synthase is a reversible engine of mitochondria that synthesizes or hydrolyzes ATP upon changes in cell physiology. ATP synthase dysfunction is involved in the onset and progression of diverse human pathologies. During ischemia, the ATP hydrolytic activity of the enzyme is inhibited by the ATPase inhibitory factor 1 (IF1). The expression of IF1 in human tissues and its participation in the development of human pathology are unknown. Here, we have developed monoclonal antibodies against human IF1 and determined its expression in paired normal and tumor biopsies of human carcinomas. We show that the relative mitochondrial content of IF1 increases significantly in carcinomas, suggesting the participation of IF1 in oncogenesis. The expression of IF1 varies significantly in cancer cell lines. To investigate the functional activity of IF1 in cancer, we have manipulated its cellular content. Overexpression of IF1 or of its pH-insensitive H49K mutant in cells that express low levels of IF1 triggers the up-regulation of aerobic glycolysis and the inhibition of oxidative phosphorylation with concurrent mitochondrial hyperpolarization. Treatment of the cells with the H(+)-ATP synthase inhibitor oligomycin mimicked the effects of IF1 overexpression. Conversely, small interfering RNA-mediated silencing of IF1 in cells that express high levels of IF1 promotes the down-regulation of aerobic glycolysis and the increase in oxidative phosphorylation. Overall, these findings support that the mitochondrial content of IF1 controls the activity of oxidative phosphorylation mediating the shift of cancer cells to an enhanced aerobic glycolysis, thus supporting an oncogenic role for the de-regulated expression of IF1 in cancer.

Hereditary spastic paraplegia-like disorder due to a mitochondrial ATP6 gene point mutation

Hereditary spastic paraplegia refers to a genetically heterogeneous syndrome. We identified five members of a family suffering from a late-onset spastic paraplegia-like disorder, carrying the homoplasmic m.9176 T>C mutation in the mitochondrial ATP6 gene. The clinical severity of the disease observed in the family was correlated with the biochemical and assembly defects of the ATP synthase. The m.9176 T>C mutation has been previously associated to Leigh syndrome or familial bilateral striatal necrosis. Other factors such as modifying genes may be involved in the phenotypic expression of the disease. The family belongs to the mitochondrial haplogroup J, previously shown to play a role in modulating the phenotype of mitochondrial diseases and be associated with longevity. Moreover other nuclear modifying genes or environmental factors may contribute to the disease phenotype. This finding extends the genetic heterogeneity of the hereditary spastic paraplegia together with the clinical spectrum of mutations of the ATP6 gene.

Reactive oxygen species-mitochondria pathway involved in LYG-202-induced apoptosis in human hepatocellular carcinoma HepG(2) cells

Previously, we demonstrated that LYG-202, a newly synthesized flavonoid with a piperazine substitution, exhibited obvious antitumor activity in vivo and in vitro. The exact mechanism of this new compound remains unclear. In the present study, we examined the effects of LYG-202 on reactive oxygen species (ROS) production and the downstream signaling pathway in the apoptosis of human hepatocellular carcinoma HepG(2) cells. Pretreatment with NAC (N-acetylcysteine), a ROS production inhibitor, partly inhibited the apoptosis induced by LYG-202 via blocking the ROS generation. Further data revealed that LYG-202 induced ROS accumulation followed by a decrease in mitochondrial membrane potential (MMP), release of cytochrome c (Cyt c) and apoptosis-inducing factor (AIF) to cytosol, which induced apoptosis of the cells. Moreover, the mitogen-activated protein kinases (MAPK), the downstream effect of ROS accumulation including c-Jun N-terminal kinase (JNK) and p38 MAPK, could be activated by LYG-202. Taken together, the generation of ROS might play an important role in LYG-202-induced mitochondrial apoptosis pathway, which provided further support for LYG-202 as a novel anticancer therapeutic candidate.

F1F0-ATP synthases of alkaliphilic bacteria: lessons from their adaptations

This review focuses on the ATP synthases of alkaliphilic bacteria and, in particular, those that successfully overcome the bioenergetic challenges of achieving robust H+-coupled ATP synthesis at external pH values>10. At such pH values the protonmotive force, which is posited to provide the energetic driving force for ATP synthesis, is too low to account for the ATP synthesis observed. The protonmotive force is lowered at a very high pH by the need to maintain a cytoplasmic pH well below the pH outside, which results in an energetically adverse pH gradient. Several anticipated solutions to this bioenergetic conundrum have been ruled out. Although the transmembrane sodium motive force is high under alkaline conditions, respiratory alkaliphilic bacteria do not use Na+- instead of H+-coupled ATP synthases. Nor do they offset the adverse pH gradient with a compensatory increase in the transmembrane electrical potential component of the protonmotive force. Moreover, studies of ATP synthase rotors indicate that alkaliphiles cannot fully resolve the energetic problem by using an ATP synthase with a large number of c-subunits in the synthase rotor ring. Increased attention now focuses on delocalized gradients near the membrane surface and H+ transfers to ATP synthases via membrane-associated microcircuits between the H+ pumping complexes and synthases. Microcircuits likely depend upon proximity of pumps and synthases, specific membrane properties and specific adaptations of the participating enzyme complexes. ATP synthesis in alkaliphiles depends upon alkaliphile-specific adaptations of the ATP synthase and there is also evidence for alkaliphile-specific adaptations of respiratory chain components.

Inhibition of Respiratory Growth and Survival in Yeast by Dopamine and Counteraction with Ascorbate or Glutathione

Dopamine is a key monamine neurotransmitter, yet it can also exhibit toxicity to neuronal cells. There are suggestions that dopamine may be neurotoxic due to its propensity to induce the formation of reactive oxygen species, which may in turn adversely affect mitochondrial function and cell viability. In this study, the effects of dopamine or a dopamine reaction product on yeast growth and survival have been explored. Yeast is ideal for such a study because, unlike mammalian cells, yeast cells can be grown even when respiratory function is totally absent. Indeed, dopamine was found to be inhibitory to yeast growth in media where respiratory function was required and cytotoxic to yeast cells suspended in water. The inhibitory effects of dopamine were reduced greatly by the antioxidants ascorbate and glutathione, suggesting the involvement of reactive oxygen species in dopamine-mediated toxicity. It would appear that yeast may offer a convenient model to perform screens for further compounds that may provide protection against dopamine-mediated growth inhibition and toxicity.

Consequences of the pathogenic T9176C mutation of human mitochondrial DNA on yeast mitochondrial ATP synthase

Several human neurological disorders have been associated with various mutations affecting mitochondrial enzymes involved in cellular ATP production. One of these mutations, T9176C in the mitochondrial DNA (mtDNA), changes a highly conserved leucine residue into proline at position 217 of the mitochondrially encoded Atp6p (or a) subunit of the F1FO-ATP synthase. The consequences of this mutation on the mitochondrial ATP synthase are still poorly defined. To gain insight into the primary pathogenic mechanisms induced by T9176C, we have investigated the consequences of this mutation on the ATP synthase of yeast where Atp6p is also encoded by the mtDNA. In vitro, yeast atp6-T9176C mitochondria showed a 30% decrease in the rate of ATP synthesis. When forcing the F1FO complex to work in the reverse mode, i.e. F1-catalyzed hydrolysis of ATP coupled to proton transport out of the mitochondrial matrix, the mutant showed a normal proton-pumping activity and this activity was fully sensitive to oligomycin, an inhibitor of the ATP synthase proton channel. However, under conditions of maximal ATP hydrolytic activity, using non-osmotically protected mitochondria, the mutant ATPase activity was less efficiently inhibited by oligomycin (60% inhibition versus 85% for the wild type control). Blue Native Polyacrylamide Gel Electrophoresis analyses revealed that atp6-T9176C yeast accumulated rather good levels of fully assembled ATP synthase complexes. However, a number of sub-complexes (F1, Atp9p-ring, unassembled alpha-F1 subunits) could be detected as well, presumably because of a decreased stability of Atp6p within the ATP synthase. Although the oxidative phosphorylation capacity was reduced in atp6-T9176C yeast, the number of ATP molecules synthesized per electron transferred to oxygen was similar compared with wild type yeast. It can therefore be inferred that the coupling efficiency within the ATP synthase was mostly unaffected and that the T9176C mutation did not increase the proton permeability of the mitochondrial inner membrane.

Synthesis of cytochrome c oxidase subunit 1 is translationally downregulated in the absence of functional F1F0-ATP synthase

The mitochondrial F(1)F(0)-ATP synthase or ATPase is a key enzyme for aerobic energy production in eukaryotic cells. Mutations in ATPase structural and assembly genes are the primary cause of severe human encephalomyopathies, frequently associated with a pleiotropic decrease in cytochrome c oxidase (COX) activity. We have studied the structural and functional constraints underlying the COX defect using Saccharomyces cerevisiae genetic and pharmacological models of ATPase deficiency. In both yeast Deltaatp10 and oligomycin-treated wild type cells, COX assembly is selectively impaired in the absence of functional ATPase. The COX biogenesis defect does not involve a primary alteration in the expression of the COX subunits as previously suggested but in their maturation and/or assembly. Expression of COX subunit 1, however, is translationally regulated as in most bona fide COX assembly mutants. Additionally, the COX defect in oligomycin-inhibited ATPase-deficient yeast cells, but not in atp10 cells could be partially prevented by partially dissipating the mitochondrial membrane potential using the uncoupler CCCP. Similar results were obtained with oligomycin-treated and ATP12-deficient human fibroblasts respectively. Our findings imply that fully assembled ATPase and its proton pumping function are both required for COX biogenesis in yeast and mammalian cells through a mechanism independent of Cox1p synthesis.

Biochemical consequences in yeast of the human mitochondrial DNA 8993T>C mutation in the ATPase6 gene found in NARP/MILS patients

We have created and analyzed the properties of a yeast model of the human mitochondrial DNA T8993C mutation that has been associated with maternally-inherited Leigh syndrome and/or with neurogenic muscle weakness, ataxia and retinitis pigmentosa. This mutation changes a highly conserved leucine to proline in the Atp6p subunit of the ATP synthase, at position 156 in the human protein, position 183 in yeast. In vitro the yeast T8993C mitochondria showed a 40-50% decrease in the rate of ATP synthesis. The ATP-driven translocation of protons across the inner mitochondrial membrane was normal in the mutant and fully sensitive to oligomycin, an inhibitor of the ATP synthase proton channel. However under conditions of maximal ATP hydrolytic activity, using non-osmotically protected mitochondria, the mutant ATPase activity was poorly inhibited by oligomycin (by 40% versus 85% in wild type cells). These anomalies were attributed by BN-PAGE and mitochondrial protein synthesis analyses to a less efficient incorporation of Atp6p within the ATP synthase. Interestingly, the cytochrome c oxidase content was selectively decreased by 40-50% in T8993C yeast, apparently due to a reduced synthesis of its mitochondrially encoded Cox1p subunit. This observation further supports the existence of a control of cytochrome c oxidase expression by the ATP synthase in yeast mitochondria. Despite the ATPase deficiency, growth of the atp6-L183P mutant on respiratory substrates and the efficiency of oxidative phosphorylation were similar to that of wild type, indicating that the mutation did not affect the proton permeability of the mitochondrial inner membrane.