Molecular defects of NADH-ubiquinone oxidoreductase (complex I) in mitochondrial diseases

Shilajit attenuates behavioral symptoms of chronic fatigue syndrome by modulating the hypothalamic-pituitary-adrenal axis and mitochondrial bioenergetics in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Shilajit has been used as a rejuvenator for ages in Indian ancient traditional medicine and has been validated for a number of pharmacological activities.

AIM OF THE STUDY

The effect of processed shilajit which was standardized to dibenzo-α-pyrones (DBPs;0.43% w/w), DBP-chromoproteins (DCPs; 20.45% w/w) and fulvic acids (56.75% w/w) was evaluated in a rat model of chronic fatigue syndrome (CFS). The mitochondrial bioenergetics and the activity of hypothalamus-pituitary-adrenal (HPA) axis were evaluated for the plausible mechanism of action of shilajit.

MATERIALS AND METHODS

CFS was induced by forcing the rats to swim for 15mins for 21 consecutive days. The rats were treated with shilajit (25, 50 and 100mg/kg) for 21 days before exposure to stress procedure. The behavioral consequence of CFS was measured in terms of immobility and the climbing period. The post-CFS anxiety level was assessed by elevated plus maze (EPM) test. Plasma corticosterone and adrenal gland weight were estimated as indices of HPA axis activity. Analysis of mitochondrial complex chain enzymes (Complex I, II, IV and V) and mitochondrial membrane potential (MMP) in prefrontal cortex (PFC) were performed to evaluate the mitochondrial bioenergetics and integrity respectively.

RESULTS

Shilajit reversed the CFS-induced increase in immobility period and decrease in climbing behavior as well as attenuated anxiety in the EPM test. Shilajit reversed CFS-induced decrease in plasma corticosterone level and loss of adrenal gland weight indicating modulation of HPA axis. Shilajit prevented CFS-induced mitochondrial dysfunction by stabilizing the complex enzyme activities and the loss of MMP. Shilajit reversed CFS-induced mitochondrial oxidative stress in terms of NO concentration and, LPO, SOD and catalase activities.

CONCLUSION

The results indicate that shilajit mitigates the effects of CFS in this model possibly through the modulation of HPA axis and preservation of mitochondrial function and integrity. The reversal of CFS-induced behavioral symptoms and mitochondrial bioenergetics by shilajit indicates mitochondria as a potential target for treatment of CFS.

Targeting mitochondria for neuroprotection in Parkinson's disease

SIGNIFICANCE

Several genetic causes of familial Parkinson's disease (PD) have now been identified and include mutations of genes encoding mitochondrial proteins. Mitochondrial complex I toxins can induce dopaminergic cell death and produce a parkinsonian state. Importantly, defects of mitochondrial function have been identified in postmortem substantia nigra from pathologically proven cases of PD.

RECENT ADVANCES

These observations provide compelling evidence to support the notion that mitochondria play an important role in the pathogenesis of PD. Thus, targeting mitochondrial function to delay or prevent neuronal cell death would represent a logical means to modify the course of this disease. Several attempts have already been made in this respect, and have been tested in clinical trial.

CRITICAL ISSUES

To date, there is no unequivocal evidence for an effective intervention to slow the disease. However, several novel mitochondrial targets are now emerging, including the potential to manipulate the mitochondrial pool to maintain function via biogenesis and mitophagy.

FUTURE DIRECTIONS

This development in drug targets needs to be supported by a parallel improvement in clinical trial design to be able to detect a neuroprotective or disease-modifying effect over a reasonable time scale.

Mitochondrial pathology in Parkinson's disease

The last 25 years have witnessed remarkable advances in our understanding of the etiology and pathogenesis of Parkinson's disease. The ability to undertake detailed biochemical analyses of the Parkinson's disease postmortem brain enabled the identification of defects of mitochondrial and free-radical metabolism. The discovery of the first gene mutation for Parkinson's disease, in alpha-synuclein, ushered in the genetic era for the disease and the subsequent finding of several gene mutations causing parkinsonism, 15 at the time of writing. Technological advances both in sequencing technology and software analysis have allowed association studies of sufficiently large size accurately to describe genes conferring an increased risk for Parkinson's disease. What has been so surprising is the convergence of these 2 separate disciplines (biochemistry and genetics) in terms of reinforcing the importance of the same pathways (ie, mitochondrial dysfunction and free-radical metabolism). Other pathways are also important in pathogenesis, including protein turnover, inflammation, and post-translational modification, particularly protein phosphorylation and ubiquitination. However, even these additional pathways overlap with each other and with those of mitochondrial dysfunction and oxidative stress. This review explores these concepts with particular relevance to mitochondrial involvement.

Mitochondrial contribution to Parkinson's disease pathogenesis

The identification of the etiologies and pathogenesis of Parkinson's disease (PD) should play an important role in enabling the development of novel treatment strategies to prevent or slow the progression of the disease. The last few years have seen enormous progress in this respect. Abnormalities of mitochondrial function and increased free radical mediated damage were described in post mortem PD brain before the first gene mutations causing familial PD were published. Several genetic causes are now known to induce loss of dopaminergic cells and parkinsonism, and study of the mechanisms by which these mutations produce this effect has provided important insights into the pathogenesis of PD and confirmed mitochondrial dysfunction and oxidative stress pathways as central to PD pathogenesis. Abnormalities of protein metabolism including protein mis-folding and aggregation are also crucial to the pathology of PD. Genetic causes of PD have specifically highlighted the importance of mitochondrial dysfunction to PD: PINK1, parkin, DJ-1 and most recently alpha-synuclein proteins have been shown to localise to mitochondria and influence function. The turnover of mitochondria by autophagy (mitophagy) has also become a focus of attention. This review summarises recent discoveries in the contribution of mitochondrial abnormalities to PD etiology and pathogenesis.

Neuroprotection in Parkinson's disease

A major focus in Parkinson's disease (PD) research is to produce drugs or other interventions that can slow or stop clinical progression. This should include an effect on both motor and non-motor symptoms and so target dopaminergic and non-dopaminergic pathways. It is logical to assume that the best chance of developing such therapies will be based on forming a better understanding of the aetiology and pathogenesis of PD and to identify critical molecular targets. There have been great advances in finding different genetic causes and risk factors for PD, but less so in the discovery of environmental contributions. The separate genetic causes still share common pathways to cell dysfunction and death, and these interconnect at several levels. Despite the major advances in genetics and PD pathogenesis, we still do not have good models of PD that can be used with confidence to accurately predict the effect of drugs on disease progression. Clinical trial design and study population selection are also areas that represent significant challenges to testing any putative neuro-protective agent. Several drugs have attracted attention as potential neuroprotective agents in PD. There are numerous studies demonstrating beneficial effects in the laboratory, but clinical efficacy for neuroprotection remains unproven.

Complex I: inhibitors, inhibition and neurodegeneration

Complex I is the first protein component of the mitochondrial respiratory chain and as such plays a crucial role in ATP production and mitochondrial function in general. Mitochondrial dysfunction has been identified in a number of neurodegenerative diseases. In some of these the mitochondrial abnormality is primary and in others secondary. Mitochondrial toxins are capable of producing relatively selective neuronal cell death and have been used to produce models of human neurodegenerative diseases e.g. 1-methyl 4-phenyl 1,2,3,6 tetrahydropyridine (MPTP) for Parkinson's disease, and 3-nitropropionic acid for Huntington's disease. Annonacin, an ingredient of local soursop, is a Complex I inhibitor and has been incriminated as the cause of a parkinsonian tauopathy disorder in Guadeloupe. A systematic analysis has identified several environmentally available potent lipophilic Complex I inhibitors that can induce neuronal cell death in striatal cultures and somatodendritic redistribution of tau protein. It is possible that these compounds may contribute to the pathogenesis of neurodegenerative disorders, although further work must be done to confirm their potential participation in pathogenesis.

Riboflavin enhances the assembly of mitochondrial cytochrome c oxidase in C. elegans NADH-ubiquinone oxidoreductase mutants

Mitochondrial respiratory chain dysfunction is responsible for a large variety of early and late-onset diseases. NADH-ubiquinone oxidoreductase (complex I) defects constitute the most commonly observed mitochondrial disorders. We have generated Caenorhabditis elegans strains with mutations in the 51 kDa active site subunit of complex I. These strains exhibit decreased NADH-dependent respiration and lactic acidosis, hallmark features of complex I deficiency. Surprisingly, the mutants display a significant decrease in the amount and activity of cytochrome c oxidase (complex IV). The metabolic and reproductive fitness of the mutants is markedly improved by riboflavin. In this study, we have examined how the assembly and activity of complexes I and IV are affected by riboflavin. Our results reveal that the mutations result in variable steady-state levels of different complex I subunits and in a significant reduction in the amount of COXI subunit. Using native gel electrophoresis, we detected assembly intermediates for both complexes I and IV. Riboflavin promotes the assembly of both complexes, resulting in increased catalytic activities. We propose that one primary pathogenic mechanism of some complex I mutations is to destabilize complex IV. Enhancing complex I assembly with riboflavin results in the added benefit of partially reversing the complex IV deficit.

Childhood encephalopathies and myopathies: a prospective study in a defined population to assess the frequency of mitochondrial disorders

OBJECTIVES

To assess the frequency of mitochondrial abnormalities in muscle histology, defects in respiratory chain enzyme activities, and mutations in mitochondrial DNA (mtDNA) in children with unexplained psychomotor retardation in the population of Northern Finland.

BACKGROUND

The frequency of mitochondrial diseases among patients with childhood encephalopathies and myopathies is not known. Frequencies are difficult to estimate because the clinical presentation of these disorders is variable.

METHODS

A total of 116 consecutive patients with undefined encephalopathies and myopathies were enrolled during a 7-year period in a hospital serving as the only neurologic unit for a pediatric population of 97 609 and as the only tertiary level neurologic unit for a pediatric population of 48 873. Biochemical and morphologic investigations were performed on muscle biopsy material, including oximetric and spectrophotometric analyses of oxidative phosphorylation, histochemistry, electron microscopy, and molecular analysis of mtDNA.

RESULTS

Ultrastructural changes in the mitochondria were the most common finding in the muscle biopsies (71%). Ragged-red fibers were found in 4 cases. An oxidative phosphorylation defect was found in 26 children (28%), complex I (n = 15) and complex IV (n = 13) defects being the most common. Fifteen percent of patients (n = 17/116) with unexplained encephalomyopathy or myopathy had a probable mitochondrial disease. Common pathogenic mutations were found in the mtDNA of only 1 patient (.9%).

CONCLUSIONS

The common known mutations in mtDNA are rarely causes of childhood encephalomyopathies, which is in contrast to the considerable frequency of the common MELAS mutation observed among adults in the same geographical area. Biochemically and morphologically verified mitochondrial disorders were nevertheless common among the children, making the analysis of a muscle biopsy very important for clinical diagnostic purposes.

Nuclear DNA origin of mitochondrial complex I deficiency in fatal infantile lactic acidosis evidenced by transnuclear complementation of cultured fibroblasts

We have studied complex I (NADH-ubiquinone reductase) defects of the mitochondrial respiratory chain in 2 infants who died in the neonatal period from 2 different neurological forms of severe neonatal lactic acidosis. Specific and marked decrease in complex I activity was documented in muscle, liver, and cultured skin fibroblasts. Biochemical characterization and study of the genetic origin of this defect were performed using cultured fibroblasts. Immunodetection of 6 nuclear DNA-encoded (20, 23, 24, 30, 49, and 51 kDa) and 1 mitochondrial DNA-encoded (ND1) complex I subunits in fibroblast mitochondria revealed 2 distinct patterns. In 1 patient, complex I contained reduced amounts of the 24- and 51-kDa subunits and normal amounts of all the other investigated subunits. In the second patient, amounts of all the investigated subunits were severely decreased. The data suggest partial or extensive impairment of complex I assembly in both patients. Cell fusion experiments between 143B206 rho degrees cells, fully depleted of mitochondrial DNA, and fibroblasts from both patients led to phenotypic complementation of the complex I defects in mitochondria of the resulting cybrid cells. These results indicate that the complex I defects in the 2 reported cases are due to nuclear gene mutations.

Quinone specificity of complex I

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

The 24-kDa subunit of the bovine mitochondrial NADH:ubiquinone oxidoreductase is a G protein

Based on the results obtained from GTP overlay assay, immunoprecipitation, two dimensional electrophoresis and radiolabeled GTP binding, we provide evidence that the bona fide subunit of Complex I, the long known 24 kDa protein is a G protein. Bacterially expressed 24 kDa protein with additional N-terminal methionine and alanine residues or naturally expressed truncated isoform fail to bind GTP suggesting that secondary modification/ processed N-terminal end is necessary for GTP binding. Competitive inhibition of binding of radiolabeled GTP to electroblotted 24 kDa protein with unlabelled nucleotides showed that the protein binds GTP and GDP with high affinity in presence of Mg2+, and has decreased to very low affinity for ITP, CTP, GMP and UTP. A comparative binding of [gamma-35S]-GTP to Complex I and 24 kDa protein (electroblotted) suggests that the GTP binding in the native Complex is solely due to 24 kDa protein. Further, four fold difference in the binding affinities between native Complex I and 24 kDa protein (electroblotted) as seen by Scatchard analysis of the binding data indicates that protein undergoes structural rearrangement in Complex I bound form, that presumably triggers divalent cation dependent GTPase activity in native complex. We were unable to detect the effect of GTP/ GDP on the ubiquinone/ferricyanide reductase activity. Since the subunit is found missing in tissues affected by mitochondrial respiratory chain diseases, we presume that the subunit has regulatory role in the Complex I function in the electron transport chain.

Metabolic myopathies

Disorders of glycogen, lipid or mitochondrial metabolism may cause two main clinical syndromes, namely (1) progressive weakness (eg, acid maltase, debrancher enzyme, and brancher enzyme deficiencies among the glycogenoses; long- and very-long-chain acyl-CoA dehydrogenase (LCAD, VLCAD), and trifunctional enzyme deficiencies among the fatty acid oxidation (FAO) defects; and mitochondrial enzyme deficiencies) or (2) acute, recurrent, reversible muscle dysfunction with exercise intolerance and acute muscle breakdown or myoglobinuria (with or without cramps) (eg, phosphorylase (PPL), phosphorylase b kinase (PBK), phosphofructokinase (PFK), phosphoglycerate kinase (PGK), phosphoglycerate mutase (PGAM), and lactate dehydrogenase (LDH) among the glycogenoses and carnitine palmitoyltransferase II (CPT II) deficiency among the disorders of FAO or (3) both (eg, PPL, PBK, PFK among the glycogenoses; LCAD, VLCAD, short-chain L-3-hydroxyacyl-CoA dehydrogenase (SCHAD), and trifunctional enzyme deficiencies among the FAO defects; and multiple mitochondrial DNA (mtDNA) deletions). Myoadenylate deaminase deficiency, a purine nucleotide cycle defect, is somewhat controversial and is characterized by exercise-related cramps leading rarely to myoglobinuria.

Steady-state kinetics of the reduction of coenzyme Q analogs by complex I (NADH:ubiquinone oxidoreductase) in bovine heart mitochondria and submitochondrial particles

The reduction kinetics of coenzyme Q (CoQ, ubiquinone) by NADH:ubiquinone oxidoreductase (complex I, EC 1.6.99.3) was investigated in bovine heart mitochondrial membranes using water-soluble homologs and analogs of the endogenous ubiquinone acceptor CoQ10 [the lower homologs from CoQ0 to CoQ3, the 6-pentyl (PB) and 6-decyl (DB) analogs, and duroquinone]. By far the best substrates in bovine heart submitochondrial particles are CoQ1 and PB. The kinetics of NADH-CoQ reductase was investigated in detail using CoQ1 and PB as acceptors. The kinetic pattern follows a ping-pong mechanism; the Km for CoQ1 is in the range of 20 microM but is reversibly increased to 60 microM by extraction of the endogenous CoQ10. The increased Km in CoQ10-depleted membranes indicates that endogenous ubiquinone not only does not exert significant product inhibition but rather is required for the appropriate structure of the acceptor site. The much lower Vmax with CoQ2 but not with DB as acceptor, associated with an almost identical Km, suggests that the sites for endogenous ubiquinone bind 6-isoprenyl- and 6-alkylubiquinones with similar affinity, but the mode of electron transfer is less efficient with CoQ2. The Kmin (kcat/Km) for CoQ1 is 4 orders of magnitude lower than the bimolecular collisional constant calculated from fluorescence quenching of membrane probes; moreover, the activation energy calculated from Arrhenius plots of kmin is much higher than that of the collisional quenching constants. These observations strongly suggest that the interaction of the exogenous quinones with the enzyme is not diffusion-controlled. Contrary to other systems, in bovine submitochondrial particles, CoQ1 usually appears to be able to support a rate approaching that of endogenous CoQ10, as shown by application of the "pool equation" [Kröger, A., & Klingenberg, M. (1973) Eur. J. Biochem. 39, 313-323] relating the rate of ubiquinone reduction to the rate of ubiquinol oxidation and the overall rate through the ubiquinone pool.

Molecular cloning and characterization of the active human mitochondrial NADH:ubiquinone oxidoreductase 24-kDa gene (NDUFV2) and its pseudogene

Two distinct loci for the 24-kDa subunit of the mitochondrial NADH:ubiquinone oxidoreductase (complex I of the respiratory chain) were detected in the human genome: a transcribed gene from chromosome 18 and an inactive locus on chromosome 19. Cosmid clones containing the functional gene (NDUFV2) and the pseudogene (NDUFV2P1) were isolated. The NDUFV2 gene spans approximately 20 kb and contains 8 exons. Refined mapping of both NDUFV2 genes by FISH resulted in an assignment of the NDUFV2 gene to 18p11.2-p11.31 and of the NDUFV2P1 gene to 19q13.3-qter. The nucleotide sequence of the NDUFV2P1 pseudogene differs from the cDNA sequence by the lack of the methionine initiator codon, an additional 165 bp of the first intron sequence, and a 1-nucleotide deletion.

Multiple deficiencies of mitochondrial DNA- and nuclear-encoded subunits of respiratory NADH dehydrogenase detected with peptide- and subunit-specific antibodies in mitochondrial myopathies

Antibodies have been raised against synthetic peptides corresponding to several computer-predicted epitopes of three mtDNA-encoded subunits, ND4, ND5 and ND6, of the human respiratory chain NADH dehydrogenase (Complex I). Antibodies were characterized by a sensitive immunoblotting assay using proteins from human skeletal muscle mitochondria and by immunoprecipitation of radio-labeled HeLa cell mitochondrial translation products. Only antibodies against two of six selected peptides of the ND4 subunit, i.e., the C-terminal peptide and an internal peptide close to the C-terminus, reacted in both assays with the subunit. Antibodies raised against an internal peptide close to the N-terminus of the ND5 subunit and antibodies raised against an internal epitope of the ND6 subunit also reacted in both the immunoblotting and immunoprecipitation assays. The antibodies described above and other Complex I subunit- or holoenzyme-specific antibodies were used to investigate the subunit deficiencies of the respiratory NADH dehydrogenase in the skeletal muscle of patients affected by mitochondrial myopathies associated with Complex I defects. The reduction in enzyme activity correlated in an immunoblot assay with a decrease of four mtDNA-encoded subunits of the enzyme, as well as with a decrease of other subunits of Complex I encoded in the nDNA. The present work provides the first evidence of a decrease in NADH dehydrogenase subunits encoded in the mitochondrial genome in myopathy patients.

Systemic 3-nitropropionic acid: behavioral deficits and striatal damage in adult rats

Previous animal studies have demonstrated that systemic administration of 3-nitropropionic acid (3-NP) leads to neuropathological changes similar to those seen in Huntington's disease (HD). Recently, we reported hypoactivity in 6- and 10-week old rats treated with systemic 3-NP (IP, 10 mg/kg/day) once every 4 days for 28 days. Although these behavioral results seem to differ from the observed hyperactivity in most excitotoxic models of HD, 3-NP may provide a better model of juvenile onset and advanced HD. In the present study, older rats were similarly treated with 3-NP to further characterize the reported age dependency of striatal neuronal death caused by 3-NP. Hypoactivity was observed in 14- and 28-week old rats with the latter demonstrating more profound features. The present study also provided the first direct evidence of a 3-NP effect on passive avoidance behavior. Experimental and control animals showed no significant difference in daytime acquisition and retention of a passive avoidance task. However, when the retention tests were conducted during the night time (in contrast to previous daytime tests), 3-NP-treated animals exhibited significant retention deficits. In addition, the neuropathological effects of 3-NP were determined by Nissl, AChE and NADPH-diaphorase histochemistry. Metabolic activity was studied using cytochrome oxidase activity as an index. Results revealed striatal glial infiltration, loss of intrinsic striatal cholinergic neurons, but some sparing of large AChE positive neurons, minimal damage of NADPH-diaphorase-containing neurons, and very slight, if any, alterations in cytochrome oxidase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin-responsive complex I deficiency in a myopathic patient with increased activity of the terminal respiratory chain and lactic acidosis

An 11-year-old girl with exercise intolerance, fatiguability from early childhood, had high blood lactate levels. Histochemistry showed increased activity of succinate dehydrogenase at the periphery of the muscle fibres, whereas aggregates of mitochondria were seen by electron microscopy. Biochemical investigation of isolated mitochondria and homogenate from muscle showed evidence of a severe complex I deficiency. In contrast, succinate dehydrogenase, complex II+III and complex IV were increased in activity. Therapy with biotin, riboflavin, nicotinamide, carnitine and amino acids resulted in an improvement of her endurance. 31P NMR spectroscopy of her forearm muscle showed a decreased ratio of phosphocreatine (PCr) over ATP. After exercise the PCr recovery rate was 26% of the average rate in 20 healthy untrained controls. When the therapy was suspended the PCr/ATP ratio at rest decreased from 2.60 to 2.34, and the PCr recovery rate after exercise decreased to 21% of the average control rate. The therapy was reinstituted but only riboflavin and carnitine were given. The PCr/ATP ratio increased to 2.60 and the PCr recovery rate increased to 32% of the control rate. Improvement of the energy metabolism in patients with defects in the oxidative phosphorylation may add to the quality of life; 31P NMR spectroscopy can measure these improvements.

N-methyl-4-phenylpyridinium (MPP+) potentiates the killing of cultured hepatocytes by catecholamines

The role of catecholamines in the toxicity of MPTP (N-methyl-4-phenyl- 1,2,3,6-tetrahydropyridine) was explored. The killing of cultured hepatocytes by dopamine and 6-hydroxydopamine was enhanced following inhibition of glutathione reductase by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), a manipulation known to sensitize such cells to an oxidative stress. The participation of activated oxygen species in the cell injury under such circumstances was shown by the ability of catalase and the ferric iron chelator deferoxamine to protect the hepatocytes. The toxicity of catecholamines was also potentiated by the mitochondrial site I (NADH dehydrogenase) inhibitor rotenone. MPP+ (N-methyl-4-phenyl-pyridinium), the putative toxic metabolite of MPTP is also a site I inhibitor. Incubation of hepatocytes with MPP+ similarly potentiated the toxicity of 6-hydroxydopamine, dopamine, and norepinephrine under conditions where MPP+ alone or catecholamines alone did not kill cells. Hepatocytes that had accumulated dopamine from the medium were killed by a subsequent exposure to MPP+ in the absence of a catecholamine in the medium. Hepatocytes that had not been pretreated with dopamine were not affected by the subsequent exposure to MPP+. These data indicated that catecholamines render hepatocytes more susceptible to the toxicity of MPP+ and suggest that the presence of catecholamines in specific neurons in the brain may be related to the selective neurotoxicity of MPTP.

The expanding clinical spectrum of mitochondrial diseases

The mitochondrion is the only extranuclear organelle containing DNA (mtDNA). As such, genetically determined mitochondrial diseases may result from a molecular defect involving the mitochondrial or the nuclear genome. The first is characterized by maternal inheritance and the second by Mendelian inheritance. Ragged-red fibers (RRF) are commonly seen with primary lesions of mtDNA, but this association is not invariant. Conversely, RRF are seldom associated with primary lesions of nuclear DNA. Large-scale rearrangements (deletions and insertions) and point mutations of mtDNA are commonly associated with RRF and lactic acidosis, e.g. Kearns-Sayre syndrome (KSS) (major large-scale rearrangements), Pearson syndrome (large-scale rearrangements), myoclonus epilepsy with RRF (MERRF) (point mutation affecting tRNA(lys) gene), mitochondrial myopathy, lactic acidosis, and stroke-like episodes (MELAS) (two point mutations affecting tRNA(leu)(UUR) gene) and a maternally-inherited myopathy with cardiac involvement (MIMyCa) (point mutation affecting tRNA(leu)(UUR) gene). However, RRF and lactic acidosis are absent in Leber hereditary optic neuropathy (LHON) (one point mutation affecting ND4 gene, two point mutations affecting ND1 gene, and one point mutation affecting the apocytochrome b subunit of complex III), and the condition associated with maternally inherited sensory neuropathy (N), ataxia (A), retinitis pigmentosa (RP), developmental delay, dementia, seizures, and limb weakness (NARP) (point mutation affecting ATPase subunit 6 gene). The point mutations in MELAS, MIMyCa, and MERRF, and the large-scale mtDNA rearrangements in KSS and Pearson syndrome have a broader biochemical impact since these molecular defects involve the translational sequence of mitochondrial protein synthesis. The nuclear defects involving mitochondrial function generally are not associated with RRF. The biochemical classification of mitochondrial diseases principally catalogues these nuclear defects. This classification divides mitochondrial diseases into five categories. Primary and secondary deficiencies of carnitine are examples of a substrate transport defect. A lipid storage myopathy is often present. Disturbances of pyruvate or fatty acid metabolism are examples of substrate utilization defects. Only four defects of the Krebs cycle are known: fumarase deficiency, dihydrolipoyl dehydrogenase deficiency, alpha-ketoglutarate dehydrogenase deficiency, and combined defects of muscle succinate dehydrogenase and aconitase. Luft disease is the singular example of a defect in oxidation-phosphorylation coupling. Defects of respiratory chain function are manifold. Two clinical syndromes predominate, one involving limb weakness, and the other primarily affecting brain function. Leigh syndrome may result from different enzyme defects, most notably pyruvate dehydrogenase complex deficiency, cytochrome c oxidase deficiency, complex I deficiency, and complex V deficiency associated with the recently described NARP point mutation. A new group of mitochondrial diseases has emerged.(ABSTRACT TRUNCATED AT 400 WORDS)

Mitochondrial function in neurodegeneration and ageing

The mitochondrial respiratory chain and oxidative phosphorylation system are responsible for the production of ATP by aerobic metabolism. Defects of the respiratory chain are increasingly recognised as important causes of human disease, and neurodegenerative disorders in particular. This article will seek to review the clinical and biochemical effects of respiratory chain defects, and summarise what is known about the molecular mechanisms that underlie them. Increasing age is also associated with a decline in mitochondrial function. The biochemical correlates of this dysfunction and the possible molecular defects that may cause it will also be reviewed.

One-step immunoaffinity purification of complex I subunits from beef heart mitochondria

Polypeptides of beef heart mitochondrial complex I were isolated from 15 mg of solubilized beef heart mitochondria using antibodies immobilized on an agarose chromatography column. The preparation was examined by SDS electrophoresis and Western blotting using affinity-purified antibodies to complex I and compared to beef heart complex I purified according to the conventional method of Hatefi and Rieske. There was a high degree of homology between the two preparations as judged by SDS-polyacrylamide electrophoresis and by immunoblotting with seven affinity-purified antibodies to various complex I subunits. This method could be applied to the preparation of complex I subunits from small samples such as human muscle biopsy specimens.

Does impairment of energy metabolism result in excitotoxic neuronal death in neurodegenerative illnesses?

The etiology of nerve cell death in neuronal degenerative disease is unknown, but it has been hypothesized that excitotoxic mechanisms may play a role. Such mechanisms may play a role in diseases such as Huntington's disease, Parkinson's disease, amyotropic lateral sclerosis, and Alzheimer's disease. In these illnesses, the slowly evolving neuronal death is unlikely to be due to a sudden release of glutamate, such as occurs in ischemia. One possibility, however, is that a defect in mitochondrial energy metabolism could secondarily lead to slow excitotoxic neuronal death, by making neurons more vulnerable to endogenous glutamate. With reduced oxidative metabolism and partial cell membrane depolarization, voltage-dependent N-methyl-D-aspartate (NMDA) receptor ion channels would be more easily activated. In addition, several other processes involved in buffering intracellular calcium may be impaired. Recent studies in experimental animals showed that mitochondrial toxins can result in a pattern of neuronal degeneration closely resembling that seen in Huntington's disease, which can be blocked with NMDA antagonists. NMDA antagonists also block neuronal degeneration induced by 1-methyl-4-phenylpyridium, which has been implicated in experimental models of Parkinson's disease. The delayed onset of neurodegenerative illnesses could be related to the progressive impairment of mitochondrial oxidative phosphorylation, which accompanies normal aging. If defective mitochondrial energy metabolism plays a role in cell death in neurodegenerative disorders, potential therapeutic strategies would be to use excitatory amino acid antagonists or agents to bypass bioenergetic defects.

Epilepsy in a mitochondrial disorder

In a large family with maternally inherited mitochondrial disease, a mild defect in the NADH-ubiquinone oxidoreductase step (complex 1) in the respiratory chain was found. Epilepsy was seen in nine (22%) of the 37 family members. Five of them, belonging to one branch of the family, had myoclonus epilepsy and EEG abnormalities consistent with this. The remaining four patients, belonging to other branches of the family tree, had partial epilepsy. Neurological symptoms also varied in different parts of the family. Possible explanations for the differences in phenotypic expressions are discussed.

Determination of the cDNA sequence for the human mitochondrial 75-kDa Fe-S protein of NADH-coenzyme Q reductase

A human-hepatoma cDNA lambda gt11 expression library was probed with an antibody to holoenzyme complex I (NADH-CoQ reductase) of the respiratory chain. One of the 30 antibody positive clones was purified to homogeneity, amplified by the polymerase chain reaction (PCR), subcloned and sequenced. It proved to be highly similar to the cDNA sequence for the bovine 75-kDa Fe--S protein. Using the sequence obtained from this library, both sense and antisense oligonucleotides were constructed and used to probe a human kidney cDNA library using PCR amplification with oligonucleotides that flank the polylinker region of the lambda phage. Two further cDNA clones were obtained which overlapped and covered the entire cDNA sequence of 2526 bp. The encoded protein of 727 amino acids has 21 amino acids that differ from the bovine-protein sequence. Northern blot analysis of mRNA from fibroblasts of complex-I deficient patients revealed no abnormalities. We show that this Fe--S protein has significant similarity with (a) the gamma chain of the hydrogen hydrogenase of Alcaligenes eutrophus and (b) the A chain of the formate dehydrogenase of Methanobacterium formicum.

Genetic biochemical and pathophysiological characterization of a familial mitochondrial encephalomyopathy (MERRF)

Myoclonic epilepsy with ragged-red fibers (MERRF) syndrome is a neuromuscular disorder characterized by mitochondrial myopathy and progressive myoclonus epilepsy. A heteroplasmic A to G transition mutation in the mitochondrial encoded tRNA(Lys) gene at nucleotide pair 8344 has been suggested to be linked to the MERRF-syndrome. We have investigated biochemically and histochemically muscle biopsies and studied the mitochondrial genomes of hair, blood and muscle tissue of a family including three cases of MERRF-syndrome as well as unaffected relatives within the maternal lineage. Sequence analysis of the mtDNAs, performed after amplification by the polymerase chain reaction (PCR), confirmed the A to G transition mutation in the tRNA(Lys) gene at position 8344. The additional point mutation at nucleotide pair 750 in the 12 S rRNA gene, which was also found by Shoffner et al. (1990), however, was absent in all investigated tissues. Quantitative analysis of the percentage of mutated mtDNA by mispairing PCR (Seibel et al., 1990) revealed variable contents in different tissues and individuals, including unaffected family members. Mitochondrial protein synthesis in cultured fibroblasts from MERRF patients revealed diminished incorporation of 35S-methionine into lysine-containing peptides.

Affinity chromatography isolation of human cytochrome oxidase and small-scale Western immunoblot probing of the enzyme complex in mitochondrial cytopathy patients

Isolation of human cytochrome oxidase by a one-step affinity chromatography procedure on a Sepharose 4B-ferrocytochrome c matrix following solubilization with the nonionic detergent laurylmaltoside yields an enzyme isolate of adequate purity for producing polyclonal antisera. Such an antiserum produced a distinctive immunoreactive profile in Western immunoblot studies to that reported using the enzyme isolated with ionic detergents. A sensitive and highly reproducible Western immunoblotting method is described for probing mitochondrial fractions prepared from small frozen skeletal muscle biopsies with an antiserum against the human placenta cytochrome oxidase. Application of this method to mitochondrial cytopathy patients with partial cytochrome oxidase deficiency shows that the detected subunits are synthesized in these patients.

Fatal infantile mitochondrial cardiomyopathy and myopathy with heterogeneous tissue expression of combined respiratory chain deficiencies

A 5-month-old boy died of progressive heart failure that started at the age of 3 months. Autopsy revealed a mitochondrial cardiomyopathy and a mitochondrial myopathy of the limb muscle and diaphragm. Cytochemically random defects of cytochrome c oxidase were visualized by light and electron microscopy in the diaphragm and especially the heart muscle, the limb muscle showing a diffuse attenuation whereas the liver and kidneys reacted normally. The activities of NADH-dehydrogenase (complex I) and cytochrome c oxidase (complex IV) were severely diminished (20% residual activity of controls) in the skeletal and heart muscle. In the heart, succinate cytochrome c reductase (complex II/III) was additionally decreased to the same degree. Loss of cytochrome c oxidase activity was based on a reduction of both mitochondrial and nuclear derived subunits in the heart and diaphragm as revealed by immunohistochemical analysis, whereas the limb muscle showed a normal immunoreactive protein content. The results illustrate heterogeneous tissue expression of respiratory chain enzyme defects and demonstrate that a cardiomyopathy may be the leading presentation of a mitochondrial disorder in early infancy.

Purpuric cutaneous manifestations in mitochondrial encephalomyopathy

A six-month-old boy with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome developed repeated crops of purpuric macules on his soles and palms, which were histologically identified as hemorrhage into the dermis without inflammatory infiltrates. Transmission electron microscopy of the skin eruptions revealed various stages of endothelial degeneration in the dermal capillaries associated with consequent extravasation of erythrocytes. The degenerative change was characterized by swollen and vacuolated mitochondria which showed disintegration of their cristae. These morphological changes in the mitochondria of the endothelial cells resembled those seen in skeletal muscle fibers. Similar changes were also noted in other tissues of the skin, such as the axons of myelinated peripheral nerves and some of the keratinocytes in the epidermis. Although these fine structural features are difficult to differentiate from artifacts, abnormal mitochondria could result in functional disturbance particularly in the tissues that require relatively high kinetics, and thus contribute the symptoms of myopathy, encephalopathy, acidosis and stroke-like episodes.

Neurological disease and mitochondrial genes

Mitochondria contain 2-10 copies of a small, double-stranded, circular DNA molecule that is exclusively maternally transmitted. Until recently, the only function of mitochondrial DNA that had any possible significance for clinicians was the fact that the mutation conferring chloramphenicol resistance occurs in one of the mitochondrial ribosomal RNA genes. It is now clear that major deletions and point mutations of mitochondrial DNA cause human diseases, chiefly mitochondrial myopathies and encephalopathies, and Leber's hereditary optic neuropathy.

Functional respiratory chain studies in subjects with chronic progressive external ophthalmoplegia and large heteroplasmic mitochondrial DNA deletions

The functional consequences of large heteroplasmic mtDNA deletions were investigated in a group of 6 patients with chronic progressive external ophthalmoplegia (CPEO) syndromes. State III respiration rates corrected for age were low with site I and II substrates in all cases and cytochrome oxidase activity was depressed. The severity of impairment varied and is consistent with inclusion of a variable percentage of non-functioning mitochondria (with deleted mtDNA) in the pellet. Western blot studies with a holocomplex antibody battery revealed no abnormalities in subunit content of complexes III and IV. A deficiency of several complex I subunits in 3 cases suggests that abnormal nuclear-mitochondrial regulation of complex I assembly may follow large mtDNA deletions.

Deletions of mitochondrial DNA in Kearns-Sayre syndrome and ocular myopathies: genetic, biochemical and morphological studies

Genetic, biochemical and morphological investigations were conducted on skeletal muscle mitochondria from 6 cases of ocular myopathy: 4 cases with Kearns-Sayre syndrome (KSS) and 2 with chronic progressive external ophthalmoplegia. All of these 6 cases showed mitochondrial DNA (mtDNA) deletions in addition to normal sized DNA in the quadriceps muscle. The deletions ranging from 3 to 8 kbp were also mapped between nucleotides 5500 and 16000 by Southern blot. The deleted genes encoded for some subunits of complexes I, IV, V and 5-10 tRNAS. The boundaries of the deletions have been sequenced in three patients. Five patients had mitochondrial respiratory chain deficiency in complex I as shown by the low oxygen consumption in isolated mitochondria using three NAD(+)-linked substrates. Mitochondria with an abnormal ultrastructure were also observed in 2 cases. A good relationship between the cytochrome c oxidase deficiency and the amount of deleted mtDNA was shown in our present investigations.

Isolated and combined deficiencies of NADH dehydrogenase (complex I) in muscle tissue of children with mitochondrial myopathies

We describe eight children with complex I deficiency, four of them with an isolated, the other four with an additional deficiency of complex IV. Clinical, chemical and morphological findings were compared from patients with isolated and combined deficiency. In both groups, the age of onset of symptoms was between the 1st day and the 4th month of life. Clinical and biochemical heterogeneity were observed. We found no correlation between residual activity of complex I in muscle, blood lactate level, and severity of clinical symptoms. Newborns presenting with severe lactic acidosis and children with later onset myopathy were seen in both groups. The group with combined complex I deficiency showed a more severe clinical course. By light microscopy ragged red fibres were only found in two patients with combined deficiency. However, by electron microscopy structural alterations of the mitochondria were observed in six out of seven muscle specimens.

A case of mitochondrial myopathy, lactic acidosis and complex I deficiency

A 34-year-old man affected by exercise intolerance, mild proximal weakness and severe lactic acidosis is described. Muscle biopsy revealed mitochondrial abnormalities and an increase of cytochrome c oxidase histochemical reaction. Biochemical investigations on isolated muscle mitochondria as well as polarographic studies revealed a mitochondrial NADH-CoQ reductase (complex I) deficiency. Mitochondrial dysfunction was confirmed by 31P nuclear magnetic resonance spectroscopy. Immunological investigation showed a generalized reduction of all complex I polypeptides. Genetic analysis did not reveal mitochondrial DNA deletions. The biochemical defect was not present in the patient's muscle tissue culture. Metabolic measurements and functional evaluation showed a reduced mechanical efficiency during exercise.

Oxidative phosphorylation in human muscle in patients with ocular myopathy and after general anaesthesia

The fuel preference of human muscle mitochondria has been given. Substrates which are oxidized with low velocity cannot be used to detect defects in oxidative phosphorylation. After general anaesthesia, the oxygen uptake with the different substrates is much lower than after local analgesia. The latter was therefore used in the subsequent study. In 15 out of 18 patients with ocular myopathy, defects in oxidative phosphorylation could be detected in isolated muscle mitochondria prepared from freshly biopsied tissue. Measurement of the activity of segments of the respiratory chain in homogenate from frozen muscle showed no, or minor defects. In two of these patients showing exercise intolerance, decreased oxidation of NAD(+)-linked substrates and apparently normal mitochondrial DNA, further study revealed deficiency of pyruvate dehydrogenase in a girl with ptosis and a high Km of complex I for NADH in a man. Both patients responded to vitamin therapy.

The molecular pathology of respiratory-chain dysfunction in human mitochondrial myopathies

Some of the different molecular pathologies of respiratory-chain dysfunction in human mitochondrial myopathies will be reviewed in relation to the findings in 58 cases. Deletions of mitochondrial DNA were identified in 21 cases [36%]. There was some correlation between the sites of the deletion and the mitochondrial biochemistry in patients with defects of Complex I but not in cases with more extensive loss of respiratory chain activity. Complex I and Complex IV polypeptides were usually normal in deleted cases. Non-deleted cases, however, often showed specific subunit deficiencies which involved the products of both nuclear and mitochondrial genes. Immunoblots of respiratory-chain polypeptides in one case pointed to defective translocation of the Rieske precursor from the cytosol into the mitochondria. The pathogenic role of circulating autoantibodies to specific matrix proteins and the nature of the target antigens in two patients with mitochondrial encephalomyopathies and respiratory-chain dysfunction will also be discussed.

A mitochondrial myopathy in an infant with lactic acidosis

We describe a girl with mitochondrial myopathy, who presented with general muscle weakness, muscle hypotonia and motor retardation. The level of blood lactate and pyruvate was consistently increased. Enzymatic studies showed impairment of NADH-dehydrogenase activity (complex I of the respiratory chain) in skeletal muscle. Electron-microscopy of a muscle biopsy showed abnormalities of a mitochondrial myopathy. The girl, now aged 30 months, has been treated with riboflavine (vitamin B2) since the age of 14 months, and lactate and pyruvate levels have decreased to normal. The patient still shows mild muscle hypotonia and weakness, but good motor progress and normal cognitive development.

Mitochondrial complex I deficiency in Parkinson's disease

The structure and function of mitochondrial respiratory-chain enzyme proteins were studied postmortem in the substantia nigra of nine patients with Parkinson's disease and nine matched controls. Total protein and mitochondrial mass were similar in the two groups. NADH-ubiquinone reductase (Complex I) and NADH cytochrome c reductase activities were significantly reduced, whereas succinate cytochrome c reductase activity was normal. These results indicated a specific defect of Complex I activity in the substantia nigra of patients with Parkinson's disease. This biochemical defect is the same as that produced in animal models of parkinsonism by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and adds further support to the proposition that Parkinson's disease may be due to an environmental toxin with action(s) similar to those of MPTP.

Impaired mitochondrial beta-oxidation in a patient with an abnormality of the respiratory chain. Studies in skeletal muscle mitochondria

Defects of complex I of the mitochondrial respiratory chain are important causes of neurological disease. We report studies that demonstrate a severe deficiency of complex I activity with less severe abnormalities of complexes III and IV (less than 5, 63, and 30% of control values, respectively) in a skeletal muscle mitochondrial fraction from a 22-yr-old female with weakness, lactic acidemia, and the deposition of intramuscular neutral lipid. The observation that lipid accumulates in this and other patients with complex I deficiency suggests impaired mitochondrial fatty acid oxidation. To investigate this mechanism we have shown impaired flux through beta-oxidation [( U-14C]hexadecanoate oxidation was 66% of control rate) and accumulation of specific acyl-CoA ester intermediates. The changes in fatty acid metabolism in complex I deficiency are secondary to the reduced state within the mitochondrial matrix with low NAD+/NADH ratios.

Mitochondrial myopathies: clinical and biochemical features of 30 patients with major deletions of muscle mitochondrial DNA

Analysis of mitochondrial DNA (mtDNA) in muscle and blood from 72 patients with mitochondrial myopathy showed that 30 had major deletions of a variable proportion of muscle mtDNA. All of these 30 patients presented with progressive external ophthalmoplegia and limb weakness, and 8 had the additional features of the Kearns-Sayre syndrome. Of the 42 patients without detectable muscle mtDNA deletions, 10 had progressive external ophthalmoplegia and limb weakness, 2 had the Kearns-Sayre syndrome, 11 had limb weakness without extraocular involvement, and 19 had multisystem disorders predominantly affecting the central nervous system. Only 2 patients with mtDNA deletions had clinically affected relatives, compared with 10 of those without deletions. In the 4 patients with polarographic defects exclusively involving complex I (NADH coenzyme Q reductase), the deleted protein-coding genes were confined to those for complex I subunits. Thirteen other patients with apparently identical deletions had variable clinical and biochemical features. Immunoblots of complex I polypeptides from patients with deletions were either indistinguishable from controls or showed only a mild generalized decrease in all identifiable subunits.