Summary Mitochondrial respiratory dysfunction is linked to the pathogenesis of multiple diseases including heart failure but the specific mechanisms for this link remain largely elusive. We modeled the impairment of mitochondrial respiration by inactivation of the Ndufs4 gene, a protein critical for Complex I (C-I) assembly, in the mouse heart (cKO). While C-I supported respiration decreased by >40%, the cKO mice maintained normal cardiac function in vivo and high-energy phosphate content in isolated perfused hearts. However, the cKO mice developed accelerated heart failure after pressure overload or repeated pregnancy. Decreased NAD+/NADH ratio by C-I deficiency inhibited Sirt3 activity, leading to increase in protein acetylation, and sensitization of the permeability transition in mitochondria (mPTP). NAD+ precursor supplementation to cKO mice partially normalized the NAD+/NADH ratio, protein acetylation and mPTP sensitivity. These findings describe a mechanism connecting mitochondrial dysfunction to the susceptibility to diseases and propose a potential therapeutic target.

Cytochrome c oxidase (COX) is hypothesized to be an important regulator of oxidative phosphorylation. However, no animal phenotypes have been described due to genetic defects in nuclear-encoded subunits of COX. We knocked down predicted homologues of COX IV and COX Va in the nematode Caenorhabditis elegans. Animals treated with W09C5.8 (COX IV) or Y37D8A.14 (COX Va) RNA interference had shortened lifespans and severe defects in mitochondrial respiratory chain function. Amount and activity of complex IV, as well as supercomplexes that included complex IV, were decreased in COXdeficient worms. The formation of supercomplex I:III was not dependent on COX. We found that COX deficiencies decreased intrinsic complex I enzymatic activity, as well as complex I-III enzymatic activity. However, overall amounts of complex I were not decreased in these animals. Surprisingly, intrinsic complex I enzymatic activity is dependent on the presence of complex IV, despite no overall decrease in the amount of complex I. Presumably the association of complex I with complex IV within the supercomplex I:III:IV enhances electron flow through complex I. Our results indicate that reduction of a single subunit within the electron transport chain can affect multiple enzymatic steps of electron transfer, including movement within a different protein complex. Patients presenting with multiple defects of electron transport may, in fact, harbor a single genetic defect.

Mitochondrial supercomplexes containing complexes I, III, and IV of the electron transport chain are now regarded as an established entity. Supercomplex I⅐III⅐IV has been theorized to improve respiratory chain function by allowing quinone channeling between complexes I and III. Here, we show that the role of the supercomplexes extends beyond channeling. Mutant analysis in Caenorhabditis elegans reveals that complex III affects supercomplex I⅐III⅐IV formation by acting as an assembly or stabilizing factor. Also, a complex III mtDNA mutation, ctb-1, inhibits complex I function by weakening the interaction of complex IV in supercomplex I⅐III⅐IV. Other complex III mutations inhibit complex I function either by decreasing the amount of complex I (isp-1), or decreasing the amount of complex I in its most active form, the I⅐III⅐IV supercomplex (isp-1;ctb-1). ctb-1 suppresses a nuclear encoded complex III defect, isp-1, without improving complex III function. Allosteric interactions involve all three complexes within the supercomplex and are necessary for maximal enzymatic activities.A solid-state model (1) of the mitochondrial respiratory chain within the mitochondrial membrane was proposed a half-century ago. In this model, the respiratory complexes are assembled into multicomplex structures, supercomplexes. Supercomplexes are capable of substrate channeling and thus facilitate transfer of electrons from one complex to the next (2). This is in contrast to the random collision model (3), which proposes that the complexes of the mitochondrial respiratory chain are embedded in the inner mitochondrial membrane as separate entities. Individual complexes are functionally connected to each other by the small, mobile electron carriers, coenzyme Q, and cytochrome c. The random collision model became more generally accepted as kinetic studies demonstrated homogenous pool behavior of coenzyme Q (4), rates of electron transfer that did not require substrate channeling, and the successful isolation of individual respiratory complexes that were enzymatically active (3, 5). However, supercomplex structures containing complexes I, III, and IV, have been investigated by blue native polyacrylamide gel electrophoresis (BN-PAGE), 3 sucrose gradient centrifugation, and single particle analysis (6, 7). A recent study by convincingly showed that supercomplexes are functional units capable of consuming oxygen when provided appropriate electron donors. They concluded that supercomplexes are in fact the functional respiratory unit of the mitochondrion in vivo. Schagger and Pfeiffer (9) demonstrated that supercomplex I⅐III⅐IV appears on blue native gels (BNGs) with increasing stoichiometries of complex IV. They termed these entities S0 -S4, where the numeral denotes the number of complex IVs within the supercomplex.Supercomplexes provide a logical explanation for combined deficiencies of the electron transport chain (ETC) observed in patients. It is estimated that ϳ30% of all ETC disorders involve multiple complexes (10). Combined I⅐IV deficie...

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

BACKGROUND Complex I of the electron transport chain (ETC) is a possible target of volatile anesthetics (VAs). Complex I enzymatic activities are inhibited by VAs, and dysfunction of complex I can lead to hypersensitivity to VAs in worms and in people. Mutant analysis in Caenorhabditis (C.) elegans suggests that VAs may specifically interfere with complex I function at the binding site for its substrate ubiquinone. We hypothesized that isoflurane inhibits electron transport by competing with ubiquinone for binding to complex I. METHODS Wildtype and mutant C. elegans were used to study the effects of isoflurane on isolated mitochondria. Enzymatic activities of the ETC were assayed and dose-response curves determined using established techniques. Two-dimensional native gels of mitochondrial proteins were performed after exposure of mitochondria to isoflurane. RESULTS Complex I is the most sensitive component of the ETC to isoflurane inhibition; however the proximal portion of complex I (the flavoprotein) is relatively insensitive to isoflurane. Isoflurane and quinone do not compete for a common binding site on complex I. The absolute rate of complex I enzymatic activity in vitro does not predict immobilization of the animal by isoflurane. Isoflurane had no measurable effect on stability of mitochondrial supercomplexes. Reduction of ubiquinone by complex I displayed positive cooperative kinetics not disrupted by isoflurane. CONCLUSIONS Isoflurane directly inhibits complex I at a site distal to the flavoprotein subcomplex. However, we have excluded our original hypothesis that isoflurane and ubiquinone compete for a common hydrophobic binding site on complex I. In addition, immobilization of the nematode by isoflurane is not due to limiting absolute amounts of complex I electron transport as measured in isolated mitochondria.

The processes that control aging remain poorly understood. We have exploited mutants in the nematode, C. elegans, that compromise mitochondrial function and scavenging of reactive oxygen species (ROS) to understand their relation to lifespan. We discovered unanticipated roles and interactions of the mitochondrial superoxide dismutases (mtSODs), SOD-2 and SOD-3. Both SODs localize to mitochondrial supercomplex I:III:IV. Loss of SOD-2 specifically: 1) decreases the activities of complexes I and II; complexes III, and IV remain normal, 2) increases the lifespan of animals with a complex I defect, but not the lifespan of animals with a complex II defect, and kills an animal with a complex III defect, 3) induces a presumed pro-inflammatory response. Knockdown of a molecule that may be a pro-inflammatory mediator very markedly extends lifespan and health of certain mitochondrial mutants. The relationship between the electron transport chain, ROS and lifespan is complex, and defects in mitochondrial function have specific interactions with ROS scavenging mechanisms. We conclude that mtSODs are embedded within the supercomplex I:III:IV, and stabilize or locally protect it from reactive oxygen species (ROS) damage. The results call for a change in the usual paradigm for the interaction of electron transport chain function, ROS release, scavenging and compensatory responses.

Complex I dysfunction is a common, heterogeneous cause of human mitochondrial disease having poorly understood pathogenesis. The extensive conservation of complex I composition between humans and Caenorhabditis elegans permits analysis of individual subunit contribution to mitochondrial functions at both the whole animal and mitochondrial levels. We provide the first experimentally-verified compilation of complex I composition in C. elegans, demonstrating 84% conservation with human complex I. Individual subunit contribution to mitochondrial respiratory capacity, holocomplex I assembly, and animal anesthetic behavior was studied in C. elegans by RNA interference-generated knockdown of nuclear genes encoding 28 complex I structural subunits and 2 assembly factors. Not all complex I subunits directly impact respiratory capacity. Subcomplex Iλ subunits along the electron transfer pathway specifically control whole animal anesthetic sensitivity and complex II upregulation, proportionate to their relative impairment of complex I-dependent oxidative capacity. Translational analysis of complex I dysfunction facilitates mechanistic understanding of individual gene contribution to mitochondrial disease. We demonstrate that functional consequences of complex I deficiency vary with the particular subunit that is defective.

Crude extract from the pericarp of the mangosteen (mangosteen extract [ME]) has exhibited several medicinal properties in both animal models and human cell lines. Interestingly, the cytotoxic activities were always observed in nonpolar fraction of the extract whereas the potent antioxidant was often found in polar fraction. Although it has been demonstrated that the polar fraction of ME exhibited the antioxidant activity, the safety of the polar fraction of ME has never been thoroughly investigated in humans. In this study, we investigated the safety of oral administration of the polar fraction of ME in 11 healthy Thai volunteers. During a 24-week period of the study, only minor and tolerable side effects were reported; no serious side effects were documented. Blood chemistry studies also showed no liver damage or kidney dysfunction in all subjects. We also demonstrated antioxidant property of the polar fraction of ME both in vitro and in vivo. Interestingly, oral administration of the polar fraction of ME enhanced the antioxidant capability of red blood cells and decreased oxidative damage to proteins within red blood cells and whole blood.