Familial cardiomyopathy with cataracts and lactic acidosis: a defect in complex I (NADH-dehydrogenase) of the mitochondria respiratory chain

Extending the phenotypic spectrum of Sengers syndrome: Congenital lactic acidosis with synthetic liver dysfunction

Sengers syndrome is a rare autosomal recessive mitochondrial disease characterized by lactic acidosis, hypertrophic cardiomyopathy and bilateral cataracts. We present here a case of neonatal demise, within the first day of life, who initially presented with severe lactic acidosis, with evidence of both chorioamnionitis and cardiogenic shock. Initial metabolic labs demonstrated a severe lactic acidosis prompting genetic testing which revealed a homozygous pathogenic variant for Sengers syndrome in AGK, c.979A >  T; p.K327^*. In addition to the canonical features of Sengers syndrome, our patient is the first reported case with liver dysfunction extending the phenotypic spectrum both in terms of severity and complications. This case also highlights the importance of maintaining a broad differential for congenital lactic acidosis.

Complex I and complex III inhibition specifically increase cytosolic hydrogen peroxide levels without inducing oxidative stress in HEK293 cells

Inhibitor studies with isolated mitochondria demonstrated that complex I (CI) and III (CIII) of the electron transport chain (ETC) can act as relevant sources of mitochondrial reactive oxygen species (ROS). Here we studied ROS generation and oxidative stress induction during chronic (24h) inhibition of CI and CIII using rotenone (ROT) and antimycin A (AA), respectively, in intact HEK293 cells. Both inhibitors stimulated oxidation of the ROS sensor hydroethidine (HEt) and increased mitochondrial NAD(P)H levels without major effects on cell viability. Integrated analysis of cells stably expressing cytosolic- or mitochondria-targeted variants of the reporter molecules HyPer (H2O2-sensitive and pH-sensitive) and SypHer (H2O2-insensitive and pH-sensitive), revealed that CI- and CIII inhibition increased cytosolic but not mitochondrial H2O2 levels. Total and mitochondria-specific lipid peroxidation was not increased in the inhibited cells as reported by the C11-BODIPY(581/591) and MitoPerOx biosensors. Also expression of the superoxide-detoxifying enzymes CuZnSOD (cytosolic) and MnSOD (mitochondrial) was not affected. Oxyblot analysis revealed that protein carbonylation was not stimulated by CI and CIII inhibition. Our findings suggest that chronic inhibition of CI and CIII: (i) increases the levels of HEt-oxidizing ROS and (ii) specifically elevates cytosolic but not mitochondrial H2O2 levels, (iii) does not induce oxidative stress or substantial cell death. We conclude that the increased ROS levels are below the stress-inducing level and might play a role in redox signaling.

Mitochondrial m-calpain opens the mitochondrial permeability transition pore in ischemia-reperfusion

BACKGROUND/OBJECTIVES

Opening of the mitochondrial permeability transition pore (mPTP) is involved in ischemia-reperfusion injury. Isoforms of Ca(2+)-activated cysteine proteases, calpains, are implicated in the development of myocardial infarction in ischemia-reperfusion. Growing evidence has revealed the presence of calpains in the mitochondria. We aimed to characterize mitochondrial calpains in the rat heart and to investigate the roles of calpains in mPTP opening after ischemia-reperfusion.

METHODS AND RESULTS

Western blotting analysis showed the expression of μ-calpain, m-calpain and calpain 10 in mitochondria isolated from male Sprague-Dawley rats, but casein zymography detected only m-calpain activity. Subcellular fractionation of mitochondria demonstrated the distribution of m-calpain to the matrix fraction. Addition of >500μM of Ca(2+) to isolated mitochondria induced mitochondrial swelling, reflecting mPTP opening, and calpain activation. Ca(2+)-induced mitochondrial swelling was inhibited partially by the calpain inhibitor calpeptin. These results support a partial contribution of calpain in the opening of the mPTP. The addition of Ca(2+) to the mitochondria induced inactivation of complex I of the electron transport chain, and cleavage of the ND6 complex I subunit, which were inhibited by calpeptin. Mitochondria isolated from rat hearts that underwent 30min of coronary occlusion followed by 30min of reperfusion showed activation of mitochondrial calpains, ND6 cleavage, complex I inactivation, and mPTP opening, which were inhibited by pretreatment with calpain inhibitor 1.

CONCLUSIONS

We demonstrated for the first time the presence of mitochondrial matrix m-calpain, and its contribution to complex I inactivation and mPTP opening after postischemic reperfusion in the rat heart.

Disorders of phospholipid metabolism: an emerging class of mitochondrial disease due to defects in nuclear genes

The human nuclear and mitochondrial genomes co-exist within each cell. While the mitochondrial genome encodes for a limited number of proteins, transfer RNAs, and ribosomal RNAs, the vast majority of mitochondrial proteins are encoded in the nuclear genome. Of the multitude of mitochondrial disorders known to date, only a fifth are maternally inherited. The recent characterization of the mitochondrial proteome therefore serves as an important step toward delineating the nosology of a large spectrum of phenotypically heterogeneous diseases. Following the identification of the first nuclear gene defect to underlie a mitochondrial disorder, a plenitude of genetic variants that provoke mitochondrial pathophysiology have been molecularly elucidated and classified into six categories that impact: (1) oxidative phosphorylation (subunits and assembly factors); (2) mitochondrial DNA maintenance and expression; (3) mitochondrial protein import and assembly; (4) mitochondrial quality control (chaperones and proteases); (5) iron–sulfur cluster homeostasis; and (6) mitochondrial dynamics (fission and fusion). Here, we propose that an additional class of genetic variant be included in the classification schema to acknowledge the role of genetic defects in phospholipid biosynthesis, remodeling, and metabolism in mitochondrial pathophysiology. This seventh class includes a small but notable group of nuclear-encoded proteins whose dysfunction impacts normal mitochondrial phospholipid metabolism. The resulting human disorders present with a diverse array of pathologic consequences that reflect the variety of functions that phospholipids have in mitochondria and highlight the important role of proper membrane homeostasis in mitochondrial biology.

Mitochondrial citrate synthase crystals: novel finding in Sengers syndrome caused by acylglycerol kinase (AGK) mutations

We report on two families with Sengers syndrome and mutations in the acylglycerol kinase gene (AGK). In the first family, two brothers presented with vascular strokes, lactic acidosis, cardiomyopathy and cataracts, abnormal muscle cell histopathology and mitochondrial function. One proband had very abnormal mitochondria with citrate synthase crystals visible in electron micrographs, associated with markedly high citrate synthase activity. Exome sequencing was used to identify mutations in the AGK gene in the index patient. Targeted sequencing confirmed the same homozygous mutation (c.3G>A, p.M1I) in the brother. The second family had four affected members, of which we examined two. They also presented with similar clinical symptoms, but no strokes. Postmortem heart and skeletal muscle tissues showed low complex I, III and IV activities in the heart, but normal in the muscle. Skin fibroblasts showed elevated lactate/pyruvate ratios and low complex I+III activity. Targeted sequencing led to identification of a homozygous c.979A>T, p.K327* mutation. AGK is located in the mitochondria and phosphorylates monoacylglycerol and diacylglycerol to lysophosphatidic acid and phosphatidic acid. Disruption of these signaling molecules affects the mitochondria's response to superoxide radicals, resulting in oxidative damage to mitochondrial DNA, lipids and proteins, and stimulation of cellular detoxification pathways. High levels of manganese superoxide dismutase protein were detected in all four affected individuals, consistent with increased free radical damage. Phosphatidic acid is also involved in the synthesis of phospholipids and its loss will result in changes to the lipid composition of the inner mitochondrial membrane. These effects manifest as cataract formation in the eye, respiratory chain dysfunction and cardiac hypertrophy in heart tissue. These two pedigrees confirm that mutation of AGK is responsible for the severe neonatal presentation of Sengers syndrome. The identification of citrate synthase precipitates by electron microscopy and the presence of vascular strokes in two siblings may expand the cellular and clinical phenotype of this disease.

Visual function, ocular motility and ocular characteristics in patients with mitochondrial complex I deficiency

PURPOSE

The aims of the present study were to investigate visual function, ocular motility and ocular characteristics in children and young adults with complex I deficiency.

MATERIAL AND METHODS

In a prospective study with longitudinal follow-up, the visual and ocular outcome in 13 patients with deficiency in complex I [nicotine-amide adenine dinucleotide (NADH) dehydrogenase] in the mitochondrial respiratory chain is presented. The patients were diagnosed during 1995-2007 and assessed during 1997-2009 at a median age of 12.8 years (range 3.1-23.4).

RESULTS

Twelve of 13 patients had visual impairment and/or ocular pathology. Four of 10 patients who co-operated in visual assessment had a best corrected decimal visual acuity of ≤ 0.5 in one or both eyes. Cataract surgery was performed in one patient and another patient showed retinal pigmentations and ptosis. Eleven patients demonstrated ocular motility problems, mainly saccade deficiencies. Five patients had optic atrophy (OA), which was bilateral in four patients. In four siblings, the OA showed a similarity to Leber's Hereditary Optic Neuropathy. These patients also had the 11778 G → A mutation in mitochondrial DNA. Only one patient had normal visual acuity and ocular outcome including refraction and visual fields. Follow-up time was median 3.0 years (range 0-11).

CONCLUSION

Visual impairment, ocular motility problems and OA are common in children and young adults with complex I deficiency and should prompt the paediatric ophthalmologist to consider mitochondrial disorders.

The production of reactive oxygen species by complex I

ROS (reactive oxygen species) are considered to be a major cause of cellular oxidative stress, linked to neuromuscular diseases and aging. Complex I (NADH:ubiquinone oxidoreductase) is one of the main contributors to superoxide production by mitochondria, and knowledge of its mechanism of O2 reduction is required for the formulation of causative connections between complex I defects and pathological effects. There is evidence for two distinct (but not mutually exclusive) sites of O2 reduction by complex I. Studies of the isolated enzyme largely support the participation of the reduced flavin mononucleotide in the active site for NADH oxidation, and this mechanism is supported in mitochondria by correlations between the NAD(P)+ potential and O2 reduction. In addition, studies of intact mitochondria or submitochondrial particles have suggested a mechanism involving the quinone-binding site, supported by observations during reverse electron transport and the use of 'Q-site' inhibitors. Here, we discuss extant data and models for O2 reduction by complex I. We compare results from the isolated enzyme with results from intact mitochondria, aiming to identify similarities and differences between them and progress towards combining them to form a single, unified picture.

Neuroradiologic findings in Sengers syndrome

Sengers syndrome is characterized by a constellation of congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy, and lactic acidosis. Two forms of the disease have been described: a fatal neonatal form, and a more benign form in which patients live into their second or third decades. With the exception of time to death, no findings have distinguished these two forms. We present 3 cases of neonatal Sengers syndrome with significant central nervous system involvement, a finding not previously described. We suggest that the fatal neonatal form of Sengers syndrome would be more accurately described as a mitochondrial encephalomyopathy. Cranial imaging may help distinguish the two types of this syndrome.

Mitochondrial structure and function in the untreated Jackson toxic milk (tx-j) mouse, a model for Wilson disease

Structural changes in hepatocellular mitochondria are characteristic of Wilson disease (WD). Features include variability in size and shape, increased density of matrix, discreet inclusions, and cystic dilatation of the cristae. We examined the functional basis for these mitochondrial changes in the toxic milk (tx-j) mouse model for WD. Its normal syngeic strain, C3H, served as control. Hepatic histology was near-normal in tx-j mice at 3-4-months-old and showed mild inflammation and steatosis at 6-months-old. Transmission electron microscopy showed typical mitochondrial abnormalities, specifically cystic dilatation of tips of cristae, in 3, 4, and 6-month-old tx-j mice and none in normal 3-month-old C3H mice. Citrate synthase (CS) activity was initially lower in tx-j mice than age-matched controls but increased over the first 6 months such that it was significantly greater at 5 and 6-months-old (p<0.003). No evidence for hepatic mtDNA depletion was found by long-PCR analysis. NB-PAGE showed preservation of all complexes in the oxidative-phosphorylation chain except complex IV which declined markedly from 5-months-old onwards. Hepatic complex IV activity was significantly decreased in 5-month-old tx-j mice (p<0.04). Expression of mitochondrial transfer factor A (TFAM) mRNA declined progressively in 6-8-month-old tx-j mice; immunodetectable protein levels declined in parallel. Expression of mtSSB mRNA was uniformly low in tx-j mice from 1-8-months-old. Levels of two mitochondrial antioxidant proteins capable of binding copper, thioredoxin-2 and peroxiredoxin-3, rose over the first 6 months of life. Mitochondrial changes occur early in WD and reflect complex, probably oxidative, injury.

Introduction of an additional pathway for lactate oxidation in the treatment of lactic acidosis and mitochondrial dysfunction in Caenorhabditis elegans

Mitochondrial dysfunction, with an estimated incidence of 1 in 5,000 births, is associated with a wide variety of multisystem degenerative diseases. Among the most prevalent forms of dysfunction are defects in the NADH:ubiquinone oxidoreductase (complex I). Caenorhabditis elegans strains with complex I mutations exhibit characteristic features of human mitochondrial disease including decreased rates of respiration and lactic acidosis. We hypothesized that introducing an additional pathway for the direct oxidation of lactate would be beneficial for energy metabolism. The yeast CYB2 gene encodes an L-lactate:cytochrome c oxidoreductase that oxidizes lactate, donates electrons directly into the mitochondrial respiratory chain, and supports lactate-dependent respiration. Cyb2p expression markedly increases lifespan, fertility, respiration rates, and ATP content in complex I-deficient animals. Our results indicate that metabolic imbalance leading to lactic acidosis and energy depletion are central mechanisms of pathogenesis in mitochondrial dysfunction and that introduction of an additional pathway for lactate oxidation should be considered as a treatment.

Are mitochondria critical in the pathogenesis of Alzheimer's disease?

This review summarizes recent findings that suggest a causal connection between mitochondrial abnormalities and sporadic Alzheimer's disease (AD). Genetic causes of AD are known only for a small proportion of familial AD patients, but for a majority of sporadic AD patients, genetic causal factors are still unknown. Currently, there are no early detectable biomarkers for sporadic AD, and there is a lack of understanding of the pathophysiology of the disease. Findings from recent genetic studies of AD pathogenesis suggest that mitochondrial defects may play an important role in sporadic AD progression, and that mitochondrial abnormalities and oxidative damage may play a significant role in the progression of familial AD. Findings from biochemical studies, in vitro studies, gene expression studies, and animal model studies of AD are reviewed, and the possible contribution of mitochondrial mutations to late-onset sporadic AD is discussed.

Biochemical and molecular diagnosis of mitochondrial respiratory chain disorders

Biochemical diagnosis of mitochondrial respiratory chain disorders requires caution to avoid misdiagnosis of secondary enzyme defects, and can be improved by the use of conservative diagnostic criteria. Pathogenic mutations causing mitochondrial disorders have now been identified in more than 30 mitochondrial DNA (mtDNA) genes encoding respiratory chain subunits, ribosomal- and t-RNAs. mtDNA mutations appear to be responsible for most adult patients with mitochondrial disease and approximately a quarter of paediatric patients. A family history suggesting maternal inheritance is the exception rather than the norm for children with mtDNA mutations, many of whom have de novo mutations. Prenatal diagnosis and pre-implantation genetic diagnosis can be offered to some women at risk of transmitting a mtDNA mutation, particularly those at lower recurrence risk. Mutations in more than 30 nuclear genes, including those encoding for respiratory chain subunits and assembly factors, have now been shown to cause mitochondrial disorders, creating difficulties in prioritising which genes should be studied by mutation analysis in individual patients. A number of approaches offer promise to guide the choice of candidate genes, including Blue Native-PAGE immunoblotting and microarray expression analysis.

Dietary oxysterols induce in vivo toxicity of coronary endothelial and smooth muscle cells

Dietary cholesterol oxidation products (COPs) were reported to exhibit in vitro toxicity toward vascular cells. The aim of this study was to determine whether dietary COPs induce in vivo toxicity toward coronary arteries and to evaluate their effect on the coronary reactivity. Golden Syrian hamsters were fed either a normolipidic diet or a hyperlipidic diet with or without a mixture of COPs (1.4 mg/kg/day). At the end of the feeding periods, cardiac mitochondria and cytosol were prepared to determine the subcellular distribution of cytochrome c. Oxidative phosphorylation was evaluated with glutamate, pyruvate or palmitoylcarnitine as a substrate. The main coronary artery was examined all along its length by transmission electron microscopy (TEM). Plasma sterol concentrations were determined. Furthermore, at the end of the 3-month feeding period, the hearts were perfused at constant pressure by the Langendorff method. The endothelium-dependent reactivity to acetylcholine was evaluated. The myocardial sterol concentration was also estimated. After a 15-day diet with dietary COPs, a release of cytochrome c into the cytosolic fraction of the whole heart occurred, which indicated apoptosis of one or several types of cardiac cells probably induced by excess circulating cholestanetriol. The morphological data obtained by TEM after three months of diet suggested that mainly vascular cells (endothelial and smooth muscle cells) were damaged by dietary COPs, whereas cardiomyocytes appeared healthy. Furthermore, the mitochondrial oxidation of palmitoylcarnitine was reduced and that of pyruvate was increased, suggesting some maintenance of energy metabolism. This strengthens the hypothesis of apoptosis. Several changes in coronary reactivity suggesting an increased NO production were observed. In conclusion, dietary COPs triggered in vivo apoptosis of coronary cells through the release of cytochrome c in the cytosol. This toxicity was counterbalanced by an increased endothelium-dependent dilation.

Regulation of NADH/CoQ oxidoreductase: do phosphorylation events affect activity?

We had previously suggested that phosphorylation of proteins by mitochondrial kinases regulate the activity of NADH/CoQ oxidoreductase. Initial data showed that pyruvate dehydrogenase kinase (PDK) and cAMP-dependent protein kinase A (PKA) phosphorylate mitochondrial membrane proteins. Upon phosphorylation with crude PDK, mitochondria appeared to be deficient in NADH/cytochrome c reductase activity associated with increased superoxide production. Conversely, phosphorylation by PKA resulted in increased NADH/cytochrome c reductase activity and decreased superoxide formation. Current data confirms PKA involvement in regulating Complex I activity through phosphorylation of an 18 kDa subunit. Beef heart NADH/ cytochrome c reductase activity increases to 150% of control upon incubation with PKA and ATP-gamma-S. We have cloned the four human isoforms of PDK and purified beef heart Complex I. Incubation of mitochondria with PDK isoforms and ATP did not alter Complex I activity or superoxide production. Radiolabeling of mitochondria and purified Complex I with PDK failed to reveal phosphorylated proteins.

Mitochondrial disease: mutations and mechanisms

The mitochondrial diseases encompass a diverse group of disorders that can exhibit various combinations of clinical features. Defects in mitochondrial DNA (mtDNA) have been associated with these diseases, and studies have been able to assign biochemical defects. Deficiencies in mitochondrial oxidative phosphorylation appear to be the main pathogenic factors, although recent studies suggest that other mechanisms are involved. Reactive oxygen species (ROS) generation has been implicated in a wide variety of neurodegenerative diseases, and mitochondrial ROS generation may be an important factor in mitochondrial disease pathogenesis. Altered apoptotic signaling as a consequence of defective mitochondrial function has also been observed in both in vitro and in vivo disease models. Our current understanding of the contribution of these various mechanisms to mitochondrial disease pathophysiology will be discussed.

Mitochondrial disease: mutations and mechanisms

The mitochondrial diseases encompass a diverse group of disorders that can exhibit various combinations of clinical features. Defects in mitochondrial DNA (mtDNA) have been associated with these diseases, and studies have been able to assign biochemical defects. Deficiencies in mitochondrial oxidative phosphorylation appear to be the main pathogenic factors, although recent studies suggest that other mechanisms are involved. Reactive oxygen species (ROS) generation has been implicated in a wide variety of neurodegenerative diseases, and mitochondrial ROS generation may be an important factor in mitochondrial disease pathogenesis. Altered apoptotic signaling as a consequence of defective mitochondrial function has also been observed in both in vitro and in vivo disease models. Our current understanding of the contribution of these various mechanisms to mitochondrial disease pathophysiology will be discussed.

Sengers disease: a rare association of hypertrophic cardiomyopathy and congenital cataracts

Hypertrophic cardiomyopathy is an uncommon childhood cardiac disease and can be primary or secondary. Several systemic diseases are known to be associated with this entity. Senger's disease is a mitochondrial disorder causing congenital cataracts lactic acidosis and skeletal and cardiac myopathy. Diagnosis should be kept in mind when routine neonatal eye screening reveals absent red reflex. The authors report a case of Sengers disease and discuss the underlying pathogenetic mechanisms.

Alterations in mitochondrial function in a mouse model of hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease characterized by varying degrees of ventricular hypertrophy and myofibrillar disarray. Mutations in cardiac contractile proteins cause HCM. However, there is an unexplained wide variability in the clinical phenotype, and it is likely that there are multiple contributing factors. Because mitochondrial dysfunction has been described in heart disease, we tested the hypothesis that mitochondrial dysfunction contributes to the varying HCM phenotypes. Mitochondrial function was assessed in two transgenic models of HCM: mice with a mutant myosin heavy chain gene (MyHC) or with a mutant cardiac troponin T (R92Q) gene. Despite mitochondrial ultrastructural abnormalities in both models, the rate of state 3 respiration was significantly decreased only in the mutant MyHC mice by approximately 23%. Notably, this decrease in state 3 respiration preceded hemodynamic dysfunction. The maximum activity of alpha-ketogutarate dehydrogenase as assayed in isolated disrupted mitochondria was decreased by 28% compared with isolated control mitochondria. In addition, complexes I and IV were decreased in mutant MyHC transgenic mice. Inhibition of beta-adrenergic receptor kinase, which is elevated in mutant MyHC mouse hearts, can prevent mitochondrial respiratory impairment in mutant MyHC mice. Thus our results suggest that mitochondria may contribute to the hemodynamic dysfunction seen in some forms of HCM and offer a plausible mechanism responsible for some of the heterogeneity of the disease phenotypes.

The eyes of mito-mouse: mouse models of mitochondrial disease

The recent creation of several mouse models of mitochondrial diseases has provided new insights into the understanding of human mitochondrial disorders. Whether these animals have clinical or histologic ophthalmologic abnormalities is of great interest given the high frequency of such abnormalities in humans with mitochondrial disorders. In this article, we describe the currently available mouse models for mitochondrial diseases with special emphasis on their ocular phenotype. These mouse models demonstrate multiple and varied ophthalmologic manifestations.

Electron transport chain defects in heart failure

In recent years, the possibility that disorders of cardiac metabolism play a role in the mechanisms that lead to ventricular dilatation and dysfunction in heart failure has attracted much attention. Electron transport chain is constituted by a series of multimeric protein complexes, located in the inner mitochondrial membranes, whose genes are distributed over both nuclear and mitochondrial DNA. Its normal function is essential to provide the energy for cardiac function. Many studies have described abnormalities in mitochondrial DNA genes encoding for electron transport chain (ETC) in dilated cardiomyopathies. In some cases, heart failure is one more or less relevant symptom among other multisystem manifestations characteristic of mitochondrial encephalomyopathies, being heart failure imputable to a primary mitochondrial disease. In the case of idiopathic dilated cardiomyopathies (IDC), many mitochondrial abnormalities have also been described using hystological, biochemical or molecular studies. The importance of such findings is under debate. The great variability in the mitochondrial abnormalities described has prompted the proposal that mitochondrial dysfunction could be a secondary phenomenon in IDC, and not a primary one. Among other possible explanations for such findings, the presence of an increased oxidative damage due to a free radical excess has been postulated. In this setting, the dysfunction of ETC could be a consequence, but also a cause of the presence of an increased free radical damage. Independently of its origin, ETC dysfunction may contribute to the persistence and worsening of heart failure. If this hypothesis, still to be proven, was certain, the modulation of cardiac metabolism could be an interesting approach to treat IDC. The precise mechanisms that lead to ventricular dilatation and dysfunction in heart failure are still nowadays poorly understood. Circumstances such as cytotoxic insults, viral infections, immune abnormalities, contractile protein defects, ischemic factors and familial conditions have been thoroughly investigated [1]. It is possible that several mechanisms combine to produce the clinical syndrome of heart failure. In recent years the possibility that disorders of energy metabolism, either isolated or in combination with the other aforementioned factors, may play a role in the development of heart failure in susceptible patients has attracted much attention. The present paper reviews the current knowledge on mitochondrial function in the failing myocardium. We restrain our discussion to heart failure where an impaired inotropic state leads to a weakened systolic contraction (i.e. the so-called systolic heart failure). Idiopathic dilated cardiomyopathy (IDC) is the prototype of the conditions under discussion. Other circumstances where a defect in myocardial contraction is due to a chronic excessive work load (i.e., hypertension, valvular or congenital heart diseases), and states in which the principal abnormality involves impaired relaxation of the ventricle (i.e. diastolic heart failure), as well as mitochondrial defects outside the electron transport chain (i.e., defects in Krebs cycle or beta-oxidation of fatty acids) are only approached circumstantially.

Control of oxygen free radical formation from mitochondrial complex I: roles for protein kinase A and pyruvate dehydrogenase kinase

Human NADH CoQ oxidoreductase is composed of a total of 43 subunits and has been demonstrated to be a major site for the production of superoxide by mitochondria. Incubation of rat heart mitochondria with ATP resulted in the phosphorylation of two mitochondrial membrane proteins, one with a M(r) of 6 kDa consistent with the NDUFA1 (MWFE), and one at 18kDa consistent with either NDUFS4 (AQDQ) or NDUFB7 (B18). Phosphorylation of both subunits was enhanced by cAMP derivatives and protein kinase A (PKA) and was inhibited by PKA inhibitors (PKAi). When mitochondrial membranes were incubated with pyruvate dehydrogenase kinase, phosphorylation of an 18kDa protein but not a 6kDa protein was observed. NADH cytochrome c reductase activity was decreased and superoxide production rates with NADH as substrate were increased. On the other hand, with protein kinase A-driven phosphorylation, NADH cytochrome c reductase was increased and superoxide production decreased. Overall there was a 4-fold variation in electron transport rates observable at the extremes of these phosphorylation events. This suggests that electron flow through complex I and the production of oxygen free radicals can be regulated by phosphorylation events. In light of these observations we discuss a potential model for the dual regulation of complex I and the production of oxygen free radicals by both PKA and PDH kinase.

Respiratory chain complex I deficiency

Oxidative phosphorylation disorders make a contribution of 1 per 10,000 live births in man, of which isolated complex I deficiency is frequently the cause. Complex I, or NADH:ubiquinone oxidoreductase, is the largest multi-protein enzyme complex of the mitochondrial electron transfer chain. In complex I deficiency, various clinical phenotypes have been recognized, often resulting in multi-system disorders with a fatal outcome at a young age. Recent advances in complex I deficiency, regarding clinical, biochemical, and molecular aspects are described. However, the genetic causes of about 60% of complex I deficiency remain unclear. As a consequence, further research will be needed to clarify the genetic defects in the remaining cases. Novel strategies in which interesting non-structural nuclear-encoded disease-causing genes may be found, as well as the molecular genetic composition of human complex I, are presented.

Mitochondria, oxygen free radicals, and apoptosis

Reactive oxygen species (ROS) generated by mitochondria are produced as by-products of normal oxidative metabolism. The fate of these species is governed by a number of factors that vary from tissue to tissue in mammals and may be involved in the pathogenesis of disease. Reactive oxygen species are also invoked as agents that are important in the processes which become active in cells undergoing apoptosis. Integration of knowledge surrounding these different aspects of ROS generation is difficult and reveals considerable gaps in our understanding.

The genetics and pathology of oxidative phosphorylation

The mitochondrial oxidative phosphorylation (OXPHOS) system is the final biochemical pathway in the production of ATP. The OXPHOS system consists of five multiprotein complexes, the individual subunits of which are encoded either by the mitochondrial or by the nuclear genome. Defects in the OXPHOS system result in devastating, mainly multisystem, diseases, and recent years have seen the description of the underlying genetic mutations in mitochondrial and nuclear genes. Advances in this arena have profited from progress in various genome projects, as well as improvements in our ability to create relevant animal models.

Functional, structural, and genetic mitochondrial abnormalities in myocardial diseases

Myocardial tissue is highly dependent on energy supplied by normal mitochondrial function. Therefore defects of energy production or utilization affect the heart in both syndromic and isolated disorders. Knowledge of the peculiar structural, functional, and genetic characteristics of mitochondria provides the basis for identification and classification of mitochondrial defects as well as for establishment of a diagnostic workup useful for related cardiac disorders. This review is therefore dedicated to the characteristics of normal mitochondria and the pathologic alterations of these organelles in various cardiovascular diseases.

Mitochondria, oxygen free radicals, disease and ageing

Superoxide is generated by the mitochondrial respiratory chain. The transformation of this superoxide into hydrogen peroxide and, under certain conditions, then into hydroxyl radicals is important in diseases where respiratory chain function is abnormal or where superoxide dismutase function is altered, as in amyotrophic lateral sclerosis. In addition, these reactive oxygen species can influence the ageing process through mechanisms involving mutagenesis of mtDNA or increased rates of shortening of telomeric DNA.

Superoxides from mitochondrial complex III: the role of manganese superoxide dismutase

In this report we show that ubiquinone cytochrome c reductase (complex III) from isolated rat heart mitochondria when inhibited with antimycin A, produces a large amount of superoxide as measured by the chemiluminescent probe coelenterazine. When mitochondria are inhibited with myxothiazol or stigmatellin, there is no detectable formation of superoxide. The antimycin A-sensitive free radical production can be dramatically reduced using either myxothiazol or stigmatellin. This suggests that the antimycin A-sensitive generation of superoxides originates primarily from the Q(o) semiubiquinone. When manganese superoxide dismutase depleted submitochondrial particles (SMP) were inhibited with myxothiazol or stigmatellin, a large superoxide signal was observed. These two inhibitors likely increase the concentration of the Q(i) semiquinone at the N center. The antimycin A-sensitive signal can, in the case of both the mitochondria and the SMP, be dissipated by the addition of copper zinc superoxide dismutase, suggesting that the measured coelenterazine signal was a result of superoxide production. Taken together, this data suggests that free radicals generated from the Q(i) species are more effectively eliminated by MnSOD in intact mitochondria.

Manganese superoxide dismutase levels are elevated in a proportion of amyotrophic lateral sclerosis patient cell lines

The most frequent genetic causes of amyotrophic lateral sclerosis (ALS) determined so far are mutations occurring in the gene for copper/zinc superoxide dismutase (CuZnSOD). The mechanism may involve inappropriate formation of hyroxyl radicals, peroxynitrite or malfunctioning of the SOD protein. We hypothesized that undiscovered genetic causes of sporadically occurring amyotrophic lateral sclerosis might be found in the mechanisms that create and destroy oxygen free radicals within the cell. After determining that there were no CuZnSOD mutations present, we measured superoxide production from mitochondria and manganese superoxide dismutase (MnSOD), glutathione peroxidase, NFkappaB, Bcl-2 and Bax by immunoblot. Of the ten sporadic patients we tested we found three patients with significantly increased concentrations of MnSOD. These patients also had lower levels of superoxide production from mitochondria and decreased expression of Bcl-2. No mutations were found in the cDNA sequence of either MnSOD in any of the sporadic patients. A patient with a CuZnSOD mutation (G82R) used as a positive control showed none of these abnormalities. The patients displaying the MnSOD aberrations showed no specific distinguishing features. This result suggests that the cause of ALS in a subgroup of ALS patients (30%) is genetic in origin and can be identified by these markers. The alteration in MnSOD and Bcl-2 are likely epiphenomena resulting from the primary genetic defect. It suggests also that the oxygen free radicals are part of the cause in this subgroup and that dysregulation of MnSOD or increased endogenous superoxide production might be responsible.

Isolated complex I deficiency in children: clinical, biochemical and genetic aspects

We retrospectively examined clinical and biochemical characteristics of 27 patients with isolated enzymatic complex I deficiency (established in cultured skin fibroblasts) in whom common pathogenic mtDNA point mutations and major rearrangements were absent. Clinical phenotypes present in this group are Leigh syndrome (n = 7), Leigh-like syndrome (n = 6), fatal infantile lactic acidosis (n = 3), neonatal cardiomyopathy with lactic acidosis (n = 3), macrocephaly with progressive leukodystrophy (n = 2), and a residual group of unspecified encephalomyopathy (n = 6) subdivided into progressive (n = 4) and stable (n = 2) variants. Isolated complex I deficiency is one of the most frequently observed disturbance of the OXPHOS system. Respiratory chain enzyme assays performed in cultured fibroblasts and skeletal muscle tissue in general reveal similar results, but for complete diagnostics we recommend enzyme measurements performed in at least two different tissues to minimize the possibility of overlooking the enzymatic diagnosis. Lactate levels in blood and CSF and cerebral CT/MRI studies are highly informative, although normal findings do not exclude complex I deficiency. With the discovery of mutations in nuclear encoded complex I subunits, adequate pre- and postnatal counseling becomes available. Finally, considering information currently available, isolated complex I deficiency in children seems to be caused in the majority by mutations in nuclear DNA.

The human NADH: ubiquinone oxidoreductase NDUFS5 (15 kDa) subunit: cDNA cloning, chromosomal localization, tissue distribution and the absence of mutations in isolated complex I-deficient patients

We have cloned the cDNA of the NDUFS5 subunit (15 kDa) of the human mitochondrial respiratory chain complex NADH: ubiquinone oxidoreductase (complex I). The open reading frame consists of 321 base-pairs, coding for 106 amino acids, with a calculated molecular mass of 12.5 kDa. There is an 81.0% identity with the bovine equivalent on cDNA level and 74.5% identity on amino acid basis. PCR analysis of rodent-human somatic cell hybrids revealed that the human NDUFS5 gene maps to chromosome 1. The NDUFS5 mRNA is expressed ubiquitously in human tissues, with a relative higher expression in human heart, skeletal muscle, liver, kidney and fetal heart. A mutation detection study of twenty isolated enzymatic complex I-deficient patients revealed no mutations, nor polymorphisms.

Myocardial aerobic metabolism is impaired in a cell culture model of cyanotic heart disease

A human pediatric cardiomyocyte cell culture model of chronic cyanosis was used to assess the effects of low oxygen tension on mitochondrial enzyme activity to address the postoperative increase in lactate and decreased ATP in the myocardium and the high incidence of low-output failure with restoration of normal oxygen tension, after technically successful corrective cardiac surgery. Chronically hypoxic cells (PO2 = 40 mmHg for 7 days) exhibited significantly reduced activities for pyruvate dehydrogenase, cytochrome-c oxidase, succinate cytochrome c reductase, succinate dehydrogenase, and citrate synthase. The activity of NADH-cytochrome c reductase was unaffected. Lactate production and the lactate-to-pyruvate ratio were significantly greater in hypoxic cardiomyocytes. Western and Northern analysis demonstrated a decrease in the levels of various mRNA and corresponding polypeptides in hypoxic cells. Thus hypoxia influences mitochondrial metabolism through acute and chronic adaptive mechanisms, reflecting allosteric (posttranscriptional) and transcriptional modulation. Transcriptional downregulation of key mitochondrial enzyme systems can explain the insufficient myocardial aerobic metabolism and low-output failure in children with cyanotic heart disease after cardiac surgery.

The role of manganese superoxide dismutase in health and disease

Manganese superoxide dismutase (MnSOD) is the mitochondrial enzyme that disposes of superoxide generated by respiratory chain activity. Of all electrons passing down the mitochondrial respiratory chain, 1-2% are diverted to form superoxide; thus production of hydrogen peroxide occurs at a constant rate due to MnSOD activity. Mice lacking MnSOD develop cardiomyopathy and basal ganglia lesions, have no lipid peroxidation products, but show destruction of enzymes with 4Fe-4S centres. Patients with complex I (NADH-CoQ oxidoreductase) deficiency show variable hyperinduction of MnSOD that is at least partially dependent on the extent of disturbance of redox state. This in turn appears to result in production of excess hydroxyl radicals, which are damaging to proteins, lipids and DNA. An alternative method of protection from oxygen radicals is employed by complex I-deficient cell types that do not induce MnSOD in that they show induction of the bcl-2 protein.

Liver failure associated with mitochondrial DNA depletion

BACKGROUND/AIMS

Liver failure in infancy can result from several disorders of the mitochondrial respiratory chain. In some patients, levels of mitochondrial DNA are markedly reduced, a phenomenon referred to as mitochondrial DNA depletion. To facilitate diagnosis of this condition, we have reviewed the clinical and pathological features in five patients with mitochondrial DNA depletion.

METHODS

Cases were identified by preparing Southern blots of DNA from muscle and liver, hybridising with appropriate probes and quantifying mitochondrial DNA relative to nuclear DNA.

RESULTS

All our patients with mitochondrial DNA depletion died of liver failure. Other problems included hypotonia, hypoglycaemia, neurological abnormalities (including Leigh syndrome) and cataracts. Liver histology showed geographic areas of fatty change, bile duct proliferation, collapse of liver architecture and fibrosis; some cells showed decreased cytochrome oxidase activity. Muscle from three patients showed mitochondrial proliferation, with loss of cytochrome oxidase activity in some fibres but not in others; in these cases, muscle mitochondrial DNA levels were less than 5% of the median control value. The remaining two patients (from a single pedigree) had normal muscle histology and histochemistry associated with less severe depletion of mitochondrial DNA in muscle.

CONCLUSIONS

Liver failure is common in patients with mitochondrial DNA depletion. Associated clinical features often include neuromuscular disease. Liver and muscle histology can be helpful in making the diagnosis. Mitochondrial DNA levels should be measured whenever liver failure is thought to have resulted from respiratory chain disease.

Metabolic and molecular bases of Menkes disease and occipital horn syndrome

Menkes disease and occipital horn syndrome (OHS) are related disorders of copper transport that involve abnormal neurodevelopment, connective tissue problems, and often premature death. Location of the gene responsible for these conditions on the X chromosome was indicated by pedigree analysis from the time of these syndromes' earliest descriptions. Characterization of an affected female with an X-autosomal translocation was used to identify the Menkes/OHS gene, which encodes a highly evolutionarily conserved, copper-transporting P-type ATPase. The gene normally is expressed in nearly all human tissues, and it localizes to the trans-Golgi network of cells. However, in over 70% of Menkes and OHS patients studied, expression of this gene has been demonstrated to be abnormal. Major gene deletions detectable by Southern blotting account for 15-20% of patients, and an interesting spectrum of other mutations is evident among 58 families whose precise molecular defects have been reported as of this writing. The center region of the gene seems particularly prone to mutation, and those that influence mRNA processing and splicing appear to be relatively common. Further advances in understanding the molecular and cell biological mechanisms involved in normal copper transport may ultimately yield new and better approaches to the management of these disorders.

The human mitochondrial elongation factor tu (EF-Tu) gene: cDNA sequence, genomic localization, genomic structure, and identification of a pseudogene

The human mitochondrial elongation factor Tu (EF-Tu) is nuclear-encoded and functions in the translational apparatus of mitochondria. The complete human EF-Tu cDNA sequence of 1677 base pairs (bp) with a 101 bp 5'-untranslated region, a 1368 bp coding region, and a 207 bp 3'-untranslated region, has been determined and updated. The predicted protein from this cDNA sequence is approximately 49.8 kDa in size and is composed of 455 amino acids (aa) with a putative N-terminal mitochondrial leader sequence of approximately 50 aa residues. The predicted amino acid sequence shows high similarity to other EF-Tu protein sequences from ox, yeast, and bacteria, and also shows limited similarity to human cystolic elongation factor 1 alpha. The complete size of this cDNA (1677 bp) obtained by cloning and sequencing was confirmed by Northern blot analysis, which showed a single transcript (mRNA) of approximately 1.7 kb in human liver. The genomic structure of this EF-Tu gene has been determined for the first time. This gene contains nine introns with a predicted size of approximately 3.6 kilobases (kb) and has been mapped to chromosome 16p11.2. In addition, an intronless pseudogene of approximately 1.7 kb with 92.6% nucleotide sequence similarity to the EF-Tu gene has also been identified and mapped to chromosome 17q11.2.

Increased plasma malondialdehyde associated with cerebellar structural defects

BACKGROUND

Malondialdehyde (MDA) in plasma is regarded as an indicator for increased lipid peroxidation.

METHOD

Measurements of MDA concentrations in plasma were compared among healthy children (n = 31), patients with neurological disorders or epileptic syndromes (n = 15), and children with pontocerebellar structural defects (n = 31), where the cause or genetic defect remained unknown.

RESULTS

In healthy children the median MDA value was 5.86 nmol/ml (mean (SD) value: 6.25 (1.97), range: 3.76-11.19). For the group with various neurological disorders or epilepsy, the values were similar with the median value at 5.66 nmol/ml (range 0.22-10.86). Compared with healthy controls and the neurological/ epileptic group, the 31 children with pontocerebellar structural defects had significantly increased MDA values with a median value at 11.29 nmol/ml (mean (SD) value: 11.62 (3.27), range 3.65-19.22).

IMPLICATION

These findings of increased plasma MDA in the majority of children with pontocerebellar structural defects of unknown origin raised the question whether increased lipid peroxidation leads to prenatal and postnatal pontocerebellar maldevelopment or degeneration.

Excessive formation of hydroxyl radicals and aldehydic lipid peroxidation products in cultured skin fibroblasts from patients with complex I deficiency

Previous studies suggest oxygen free radicals' involvement in the etiology of cardiomyopathy with cataracts. To investigate the role of free radicals in the pathogenesis of the cardiomyopathy with cataracts and complex I deficiency, fibroblasts from patients were assessed for hydroxyl radical formation and aldehydic lipid peroxidation products with and without redox active agents that increase free radicals. The rate of hydroxyl radical formation in patient cells was increased over 2-10-fold under basal conditions, and up to 20-fold after menadione or doxorubicin treatment compared with normal cells. We also found an overproduction of aldehydes in patient cells both under basal conditions and after treatment. Both hydroxyl radicals and toxic aldehydes such as hexanal, 4-hydroxynon-2-enal, and malondialdehyde were elevated in cells from patients with three types of complex I deficiency. In contrast, acyloins, the less toxic conjugated products of pyruvate and saturated aldehydes, were lower in the patient cells. Our data provide direct evidence for the first time that complex I deficiency is associated with excessive production of hydroxyl radicals and lipid peroxidation. The resultant damage may contribute to the early onset of cardiomyopathy and cataracts and death in early infancy in affected patients with this disease.

Mitochondrial complex I deficiency leads to increased production of superoxide radicals and induction of superoxide dismutase

Mitochondria were isolated from skin fibroblast cultures derived from healthy individuals (controls) and from a group patients with complex I (NADH-CoQ reductase) deficiency of the mitochondrial respiratory chain. The complex I deficient patients included those with fatal infantile lactic acidosis (FILA), cardiomyopathy with cataracts (CC), hepatopathy with tubulopathy (HT), Leigh's disease (LD), cataracts and developmental delay (CD), and lactic acidemia in the neonatal period followed by mild symptoms (MS). Production of superoxide radicals, on addition of NADH, were measured using the luminometric probe lucigenin with isolated fibroblast mitochondrial membranes. Superoxide production rates were highest with CD and decreased in the order CD >> MS > LD > control > HT > FILA = CC. The quantity of Mn-superoxide dismutase (MnSOD), as measured by ELISA techniques, however, was highest in CC and FILA and lowest in CD. Plots of MnSOD quantity versus superoxide production showed an inverse relationship for most conditions with complex I deficiency. We hypothesize that oxygen radical production is increased when complex I activity is compromised. However, the observed superoxide production rates are modulated by the variant induction of MnSOD which decreases the rates, sometimes below those seen in control fibroblast mitochondria. In turn, we show that the variant induction of MnSOD is most likely a function of the change in the redox state of the cell experienced rather than a result of the complex I defect per se.

NADH-coenzyme Q reductase (complex I) deficiency: heterogeneity in phenotype and biochemical findings

Twelve patient cell lines with biochemically proven complex I deficiency were compared for clinical presentation and outcome, together with their sensitivity to galactose and menadione toxicity. Each patient had elevated lactate to pyruvate ratios demonstrable in fibroblast cultures. Each patient also had decreased rotenone-sensitive NADH-cytochrome c reductase (complexes I and III) with normal succinate cytochrome c reductase (complexes II and III) and cytochrome oxidase (complex IV) activity in cultured skin fibroblasts, indicating a deficient NADH-coenzyme Q reductase (complex I) activity. The patients fell into five categories: severe neonatal lactic acidosis; Leigh disease; cardiomyopathy and cataracts; hepatopathy and tubulopathy; and mild symptoms with lactic acidaemia. Cell lines from 4 out of the 12 patients were susceptible to both galactose and menadione toxicity and 3 of these also displayed low levels of ATP synthesis in digitonin-permeabilized skin fibroblasts from a number of substrates. This study highlights the heterogeneity of complex I deficiency at the clinical and biochemical level.