Analysis of COX2 mutants reveals cytochrome oxidase subassemblies in yeast

Cytochrome oxidase catalyses the reduction of oxygen to water. The mitochondrial enzyme contains up to 13 subunits, 11 in yeast, of which three, Cox1p, Cox2p and Cox3p, are mitochondrially encoded. The assembly pathway of this complex is still poorly understood. Its study in yeast has been so far impeded by the rapid turnover of unassembled subunits of the enzyme. In the present study, immunoblot analysis of blue native gels of yeast wild-type and Cox2p mutants revealed five cytochrome oxidase complexes or subcomplexes: a, b, c, d and f; a is likely to be the fully assembled enzyme; b lacks Cox6ap; d contains Cox7p and/or Cox7ap; f represents unassembled Cox1p; and c, observed only in the Cox2p mutants, contains Cox1p, Cox3p, Cox5p and Cox6p and lacks the other subunits. The identification of these novel cytochrome oxidase subcomplexes should encourage the reexamination of other yeast mutants.

Mutant torsinA, which causes early-onset primary torsion dystonia, is redistributed to membranous structures enriched in vesicular monoamine transporter in cultured human SH-SY5Y cells

A single GAG deletion in the DYT1 gene causes primary early-onset, generalized torsion dystonia. The DYT1 protein product, torsinA, belongs to the AAA+ family of proteins. When overexpressed, wild-type torsinA localizes mainly to the endoplasmic reticulum, whereas the mutant forms inclusions of unclear biogenetic origin. In this study, overexpressed wild-type torsinA in human neuroblastoma (SH-SY5Y) cell lines was distributed throughout the cell body and colocalized with a marker for the endoplasmic reticulum, confirming it is an endoplasmic reticulum protein. However, mutant torsinA showed perinuclear staining and formed distinct globular inclusions, which did not colocalize with endoplasmic reticulum markers. Immunoelectron microscopy of the mutant torsinA inclusions revealed membrane whorls staining for torsinA, as well as labeling of lamellae, isolated bilayers, and perinuclear membranes. This finding shows that mutant torsinA redistributes to specific membranous structures, which may represent different stages of maturation of the intracellular inclusions. The mutant torsinA-containing bodies were immunoreactive for vesicular monoamine transporter 2 (VMAT2). VMAT2 expression is important for the exocytosis of bioactive monoamines in neurons. Abnormal processing, transport, or entrapment of VMAT2 within the mutant torsinA membranous inclusions, therefore, may affect cellular dopamine release, providing a potential pathogenic mechanism for the DYT1-dependent dystonia.

Assessment of the significance of mitochondrial DNA damage by chemotherapeutic agents

The pathways which are activated following damage to nuclear DNA in cancer cells are well understood. There is evidence that treatment with several chemotherapeutic agents may result in damage to mitochondrial DNA. This study investigated the contribution of mitochondrial DNA to cytotoxicity of DNA-interactive agents. To understand the significance of drug interactions with mitochondrial DNA, we investigated A549 non-small cell lung cancer cell lines and their rho0 derivatives in which mitochondrial DNA has been eradicated. The parental cell line showed increased sensitivity to the anthracycline daunorubicin when compared with the A549 rho0 line. In addition, the A549 rho0 line was resistant to the rhodacyanine derivative, MKT-077, which has been shown to interact with mitochondrial DNA. Southern blotting demonstrated that MKT-077 mediated damage to mitochondrial but not nuclear DNA. Restoration of mitochondrial DNA by formation of cybrids restored sensitivity to these agents. The mitochondrial DNA damage, following treatment of A549 rho0 cells with MKT-077, resulted in G2 arrest which was not mediated by expression of p53. Mitochondrial DNA is a critical target for MKT-077 and daunorubicin, and is a potential target for novel chemotherapeutic agents.

Clinical, biochemical and morphological features of hepatocerebral syndrome with mitochondrial DNA depletion due to deoxyguanosine kinase deficiency

BACKGROUND/AIMS

The aim of this study was to delineate the specific clinical, biological and liver morphological alterations of the hepatocerebral syndrome due to alterations in the deoxyguanosine kinase gene, a rare and severe form of mitochondrial DNA depletion syndrome.

METHODS

We report seven cases from three unrelated families with the same mutation in the deoxyguanosine kinase gene.

RESULTS

All the patients presented in the first weeks of life with hepatomegaly and progressive liver failure that led to death few months later. Major psychomotor delay and multidirectional nystagmus were reported shortly after onset of the disease. Severe hyperlactacidaemia was constant. Histological examination of the liver disclosed a multifocal injury of hepatocytes with irregular foamy steatosis, cholestasis, and fibrosis, associated with different degrees of hepatosiderosis and glycogen depletion. Liver respiratory chain activities were abnormal in all analysed patients and the amount of liver mitochondrial DNA was severely decreased. An identical homozygous 4bp GATT duplication was identified in the deoxyguanosine kinase gene of all the cases.

CONCLUSIONS

These patients, together with patients reported in the literature, permit to delineate the specific features of the hepatocerebral form of mitochondrial DNA depletion syndrome and to differentiate them from other causes of neonatal liver failure.

Analysis of the trinucleotide CAG repeat from the DNA polymerase γ gene (POLG) in patients with Parkinson's disease

The human gene for the catalytic subunit of the mitochondrial DNA (mtDNA) polymerase (POLG) contains a trinucleotide CAG repeat encoding a polyglutamine tract near the amino-terminus of the protein. Expansions of similar polyglutamine-encoding CAG microsatellite repeats in other genes are known to cause neurodegenerative disorders. As mitochondrial dysfunction has been implicated in the aetiology of Parkinson's disease, we determined the POLG CAG repeat length in DNA samples extracted from 22 idiopathic Parkinson's disease patients and 31 control subjects. The distribution of the POLG CAG repeat length in the control samples matched the distribution reported for control samples by others. Comparison between the CAG repeat length distribution of control and Parkinson's disease samples revealed no evidence of either germ line or somatic POLG CAG repeat instability in Parkinson's disease patients. Our results rule out POLG CAG repeat instability as a common pathogenic mechanism in idiopathic Parkinson's disease.

Cytochrome c Oxidase Subassemblies in Fibroblast Cultures from Patients Carrying Mutations in COX10, SCO1, or SURF1

Cytochrome c oxidase contains two redox-active copper centers (Cu(A) and Cu(B)) and two redox-active heme A moieties. Assembly of the enzyme relies on several assembly factors in addition to the constituent subunits and prosthetic groups. We studied fibroblast cultures from patients carrying mutations in the assembly factors COX10, SCO1, or SURF1. COX10 is involved in heme A biosynthesis. SCO1 is required for formation of the Cu(A) center. The function of SURF1 is unknown. Immunoblot analysis of native gels demonstrated severely decreased levels of holoenzyme in the patient cultures compared with controls. In addition, the blots revealed the presence of five subassemblies: three subassemblies involving the core subunit MTCO1 but apparently no other subunits; a subassembly containing subunits MTCO1, COX4, and COX5A; and a subassembly containing at least subunits MTCO1, MTCO2, MTCO3, COX4, and COX5A. As some of the subassemblies correspond to known assembly intermediates of human cytochrome c oxidase, we think that these subassemblies are probably assembly intermediates that accumulate in patient cells. The MTCO1.COX4.COX5A subassembly was not detected in COX10-deficient cells, which suggests that heme A incorporation into MTCO1 occurs prior to association of MTCO1 with COX4 and COX5A. SCO1-deficient cells contained accumulated levels of the MTCO1.COX4.COX5A subassembly, suggesting that MTCO2 associates with the MTCO1.COX4.COX5A subassembly after the Cu(A) center of MTCO2 is formed. Assembly in SURF1-deficient cells appears to stall at the same stage as in SCO1-deficient cells, pointing to a role for SURF1 in promoting the association of MTCO2 with the MTCO1.COX4.COX5A subassembly.

Replication of mitochondrial DNA occurs throughout the mitochondria of cultured human cells

Replication of mitochondrial DNA (mtDNA) is dependent on nuclear-encoded factors. It has been proposed that this reliance may exert spatial restrictions on the sites of mtDNA replication within the cytoplasm, as a previous study only detected mtDNA synthesis in perinuclear mitochondria. We have studied mtDNA replication in situ in a variety of human cell cultures labeled with 5-bromo-2'-deoxyuridine. In contrast to what has been reported, mtDNA synthesis was detected at multiple sites throughout the mitochondrial network following short pulses with bromodeoxyuridine. Although no bromodeoxyuridine incorporation was observed in anuclear platelets, incorporation into mtDNA of fibroblasts that had been enucleated 2 h prior to labeling was readily detectable. Blotting experiments indicated that the bromodeoxyuridine incorporation into mtDNA observed in situ represents replication of the entire mtDNA molecule. The studies also showed that replication of mtDNA occurred at any stage of the cell cycle in proliferating cells and continued in postmitotic cells, although at a lower level. These results demonstrate that mtDNA replication is not restricted to mitochondria in the proximity of the nucleus and imply that all components of the replication machinery are available at sufficient levels throughout the mitochondrial network to permit mtDNA replication throughout the cytoplasm.

Mitochondrial DNA depletion can be prevented by dGMP and dAMP supplementation in a resting culture of deoxyguanosine kinase-deficient fibroblasts

Deoxyguanosine kinase is a constitutively expressed, mitochondrial enzyme of the deoxyribonucleoside salvage pathway. Deficiency of deoxyguanosine kinase causes early-onset, hepatocerebral mitochondrial DNA (mtDNA) depletion syndrome. To clarify the molecular mechanism of the disease, a skin fibroblast culture was studied from a patient carrying a homozygous nonsense mutation in the gene for deoxyguanosine kinase. In situ examination of DNA synthesis demonstrated that, although mtDNA synthesis is cell cycle independent in control fibroblasts, mtDNA synthesis occurs mainly during the S-phase in deoxyguanosine kinase-deficient cells. Consistent with this observation, it was found that the mtDNA content of exponentially growing, deoxyguanosine kinase-deficient cells is only mildly affected. When cycling is inhibited by serum-deprivation and cells are in a resting state, however, the mtDNA content drops considerably in deoxyguanosine kinase-deficient cells, yet remains stable in control fibroblasts. The decline in mtDNA content in resting, deoxyguanosine kinase-deficient cells can be prevented by dGMP and dAMP supplementation, providing conclusive evidence that substrate limitation triggers mtDNA depletion in deoxyguanosine kinase-deficient cells.

A novel mutation in the deoxyguanosine kinase gene causing depletion of mitochondrial DNA

Recently, a homozygous single-nucleotide deletion in exon 2 of the deoxyguanosine kinase gene (DGUOK) was identified as the disease-causing mutation in 3 apparently unrelated Israeli-Druze families with depleted hepatocerebral mitochondrial DNA. We have discovered a novel homozygous nonsense mutation in exon 3 of DGUOK (313C-->T) from a patient born to nonconsanguineous German parents. This finding shows that mutations in DGUOK causing mitochondrial DNA depletion are not confined to a single ethnic group.

A comprehensive survey of mutations in the OPA1 gene in patients with autosomal dominant optic atrophy

PURPOSE

To characterize the spectrum of mutations in the OPA1 gene in a large international panel of patients with autosomal dominant optic atrophy (adOA), to improve understanding of the range of functional deficits attributable to sequence variants in this gene, and to assess any genotype-phenotype correlations.

METHODS

All 28 coding exons of OPA1, intron-exon splice sites, 273 bp 5' to exon 1, and two intronic regions with putative function were screened in 94 apparently unrelated white patients of European origin with adOA by single-strand conformational polymorphism (SSCP)-heteroduplex analysis and direct sequencing. Clinical data were collated, and putative mutations were tested for segregation in the respective families by SSCP analysis or direct sequencing and in 100 control chromosomes. Further characterization of selected splice site mutations was performed by RT-PCR of patient leukocyte RNA. Staining of mitochondria in leukocytes of patients and control subjects was undertaken to assess gross differences in morphology and cellular distribution.

RESULTS

Twenty different mutations were detected, of which 14 were novel disease mutations (missense, nonsense, deletion-frameshift, and splice site alterations) and six were known mutations. Mutations were found in 44 (47%) of the 94 families included in the study. Ten new polymorphisms in the OPA1 gene were also identified. Mutations occur throughout the gene, with three clusters emerging: in the mitochondrial leader, in the highly conserved guanosine triphosphate (GTP)-binding domain, and in the -COOH terminus. Examination of leukocyte mitochondria from two unrelated patients with splice site mutations in OPA1 revealed no abnormalities of morphology or cellular distribution when compared with control individuals.

CONCLUSIONS

This study describes 14 novel mutations in the OPA1 gene in patients with adOA, bringing the total number so far reported to 54. It is likely that many cases of adOA are due to mutations outside the coding region of OPA1 or to large-scale rearrangements. Evaluation of the mutation spectrum indicates more than one pathophysiological mechanism for adOA. Preliminary data suggests that phenotype-genotype correlation is complex, implying a role for other genetic modifying or environmental factors. No evidence was found of pathologic changes in leukocyte mitochondria of patients with adOA.

Mutations of cytochrome c oxidase subunits 1 and 3 in Saccharomyces cerevisiae: assembly defect and compensation

Eukaryotic cytochrome oxidases are composed of up to 13 subunits, of which three, subunits 1, 2 and 3, are mitochondrially encoded. In this study, yeast mutants were used to investigate the role of subunits 1 and 3 domains on the enzyme assembly. Mutation S203L in subunit 3 which abolished the respiratory growth, decreased cytochrome oxidase content, as measured by optical spectroscopy and immunodetection. Secondary mutations in subunits 1 and 3 restored (partly) the enzyme level. Two reversions reintroduced residues with a hydroxyl group at the primary mutation site (S203T) or in a subunit 3 transmembrane helix close to subunit 1 (G104S). These residues may be involved in hydrogen bonding which strengthen subunits 1-3 interaction. Two other reversions (A224V and Q137K) are located in P-side loops in subunit 1, which may be involved in the enzyme assembly. A mutation in residue A224 has been reported in a family presenting with encephalomyopathy. Surprisingly, the introduction of the 'human' mutation A224S and of a more drastic change A224F had no effect on the yeast enzyme. This might be explained by differences in local folding in the two enzymes.

Depletion of mitochondrial DNA in the liver of an infant with neonatal giant cell hepatitis

A boy presented with lactic acidosis, hepatomegaly, hypoglycemia, generalised icterus, and muscle hypotonia in the first weeks of life. At the age of 2 months, neonatal giant cell hepatitis was diagnosed by light microscopy. Electron microscopy of the liver revealed an accumulation of abnormal mitochondria and steatosis. Skeletal muscle was normal on both light and electron microscopy. At the age of 5 months, the patient died of liver failure. Biochemical studies of the respiratory chain enzymes in muscle showed that cytochrome-c oxidase (complex IV) and succinate-cytochrome-c oxidoreductase (complex II + III) activities were (just) below the control range. When related to citrate synthase activity, however, complex IV and complex II + III activities were normal. Complex I activity was within the control range. The content of mitochondrial DNA (mtDNA) was severely reduced in the liver (17% to 18% of control values). Ultracytochemistry and immunocytochemistry of cytochrome-c oxidase demonstrated a mosaic pattern of normal and defective liver cells. In defective cells, a reduced amount of the mtDNA-encoded subunits II-III and the nuclear DNA-encoded subunits Vab was found. Cells of the biliary system were spared. Immunohistochemistry of mtDNA replication factors revealed normal expression of DNA polymerase gamma. The mitochondrial single-stranded binding protein (mtSSB) was absent in some abnormal hepatocytes, whereas the mitochondrial transcription factor A (mtTFA) was deficient in all abnormal hepatocytes. In conclusion, depletion of mtDNA may present as giant cell hepatitis. mtTFA and to a lesser degree mtSSB are reduced in mtDNA depletion of the liver and may, therefore, be of pathogenetic importance. The primary defect, however, is still unknown.

Diagnostic value of succinate ubiquinone reductase activity in the identification of patients with mitochondrial DNA depletion

Mitochondrial DNA (mtDNA) depletion syndrome (McKusick 251880) is characterized by a progressive quantitative loss of mtDNA resulting in severe mitochondrial dysfunction. A diagnosis of mtDNA depletion can only be confirmed after Southern blot analysis of affected tissue. Only a limited number of centres have the facilities to offer this service, and this is frequently on an irregular basis. There is therefore a need for a test that can refine sample selection as well as complementing the molecular analysis. In this study we compared the activities of the nuclear-encoded succinate ubiquinone reductase (complex II) to the activities of the combined mitochondrial and nuclear-encoded mitochondrial electron transport chain (ETC) complexes; NADH:ubiquinone reductase (complex I), ubiquinol-cytochrome-c reductase (complex III), and cytochrome-c oxidase (complex IV), in skeletal muscle biopsies from 7 patients with confirmed mtDNA depletion. In one patient there was no evidence of an ETC defect. However, the remaining 6 patients exhibited reduced complex I and IV activities. Five of these patients also displayed reduced complex II-III (succinate:cytochrome-c reductase) activity. Individual measurement of complex II and complex III activities demonstrated normal levels of complex II activity compared to complex III, which was reduced in the 5 biopsies assayed. These findings suggest a possible diagnostic value for the detection of normal levels of complex II activity in conjunction with reduced complex I, III and IV activity in the identification of likely candidates for mtDNA depletion syndrome

A mutant mitochondrial respiratory chain assembly protein causes complex III deficiency in patients with tubulopathy, encephalopathy and liver failure

Complex III (CIII; ubiquinol cytochrome c reductase of the mitochondrial respiratory chain) catalyzes electron transfer from succinate and nicotinamide adenine dinucleotide-linked dehydrogenases to cytochrome c. CIII is made up of 11 subunits, of which all but one (cytochrome b) are encoded by nuclear DNA. CIII deficiencies are rare and manifest heterogeneous clinical presentations. Although pathogenic mutations in the gene encoding mitochondrial cytochrome b have been described, mutations in the nuclear-DNA-encoded subunits have not been reported. Involvement of various genes has been indicated in assembly of yeast CIII (refs. 8-11). So far only one such gene, BCS1L, has been identified in human. BCS1L represents, therefore, an obvious candidate gene in CIII deficiency. Here, we report BCS1L mutations in six patients, from four unrelated families and presenting neonatal proximal tubulopathy, hepatic involvement and encephalopathy. Complementation study in yeast confirmed the deleterious effect of these mutations. Mutation of BCS1L would seem to be a frequent cause of CIII deficiency, as one-third of our patients have BCS1L mutations.

Immunological phenotyping of fibroblast cultures from patients with a mitochondrial respiratory chain deficit

Conventional approaches to the diagnosis of mitochondrial respiratory chain diseases, using enzyme assays and histochemistry, are laborious and give limited information concerning the genetic basis of a deficiency. We have evaluated the diagnostic value of 12 monoclonal antibodies to subunits of the four respiratory chain enzyme complexes and F(1)F(0)-ATP synthase. Antibodies were used in immunological studies with skin fibroblast cultures derived from patients with diverse mitochondrial diseases, including patients in which the disease was caused by a nuclear genetic defect and patients known to harbor a heteroplasmic mutation in a mitochondrial tRNA gene. Immunoblotting experiments permitted the identification of specific enzyme assembly deficits and immunocytochemical studies provided clues regarding the genetic origin of the disease. The immunological findings were in agreement with the biochemical and genetic data of the patients. Our study demonstrates that characterization of the fibroblast cultures with the monoclonal antibodies provides a convenient technique to complement biochemical assays and histochemistry in the diagnosis of mitochondrial respiratory chain disorders.

Assembly of cytochrome c oxidase: what can we learn from patients with cytochrome c oxidase deficiency?

Cytochrome c oxidase is an intricate metalloprotein that transfers electrons from cytochrome c to oxygen in the last step of the mitochondrial respiratory chain. It uses the free energy of this reaction to sustain a transmembrane electrochemical gradient of protons. Site-directed mutagenesis studies of bacterial terminal oxidases and the recent availability of refined crystal structures of the enzyme are rapidly expanding the understanding of the coupling mechanism between electron transfer and proton translocation. In contrast, relatively little is known about the assembly pathway of cytochrome c oxidase. Studies in yeast have indicated that assembly is dependent on numerous proteins in addition to the structural subunits and prosthetic groups. Human homologues of a number of these assembly factors have been identified and some are now known to be involved in disease. To dissect the assembly pathway of cytochrome c oxidase, we are characterizing tissues and cell cultures derived from patients with genetically defined cytochrome c oxidase deficiency, using biochemical, biophysical and immunological techniques. These studies have allowed us to identify some of the steps of the assembly process.

A novel mutation in SURF1 causes skipping of exon 8 in a patient with cytochrome c oxidase-deficient leigh syndrome and hypertrichosis

Leigh syndrome is a rare pediatric neurodegenerative disorder attributed to impaired mitochondrial energy metabolism. Mutations in SURF1 have been described in several patients with Leigh syndrome associated with cytochrome c oxidase deficiency. We report a new 18-bp deletion (821del18), spanning the splice donor junction of exon 8 of SURF1, in an infant presenting with cytochrome c oxidase-deficient Leigh syndrome and hypertrichosis. cDNA sequencing demonstrated that this deletion results in a messenger lacking exon 8. RT-PCR experiments suggested a rapid degradation of the aberrant mRNA species from the 5'-end.

A novel frameshift mutation of the mtDNA COIII gene leads to impaired assembly of cytochrome c oxidase in a patient affected by Leigh-like syndrome

We report on a novel frameshift mutation in the mtDNA gene encoding cytochrome c oxidase (COX) subunit III. The proband is an 11-year-old girl with a negative family history and an apparently healthy younger brother. Since 4 years of age, she has developed a progressive spastic paraparesis associated with ophthalmoparesis and moderate mental retardation. The presence of severe lactic acidosis and Leigh-like lesions of putamina prompted us to perform muscle and skin biopsies. In both, a profound, isolated defect of COX was found by histochemical and biochemical assays. Sequence analysis of muscle mtDNA resulted in the identification of a virtually homoplasmic frameshift mutation in the COIII gene, due to the insertion of an extra C at nucleotide position 9537 of mtDNA. Although the 9537C(ins) does not impair transcription of COIII, no full-length COX III protein was detected in mtDNA translation assays in vivo. Western blot analysis of two-dimensional blue-native electrophoresis showed a reduction of specific crossreacting material and the accumulation of early-assembly intermediates of COX, whereas the fully assembled complex was absent. One of these intermediates had an electrophoretic mobility different from those seen in controls, suggesting the presence of a qualitative abnormality of COX assembly. Immunostaining with specific antibodies failed to detect the presence of several smaller subunits in the complex lacking COX III, in spite of the demonstration that these subunits were present in the crude mitochondrial fraction of patient's cultured fibroblasts. Taken together, the data indicate a role for COX III in the incorporation and maintenance of smaller COX subunits within the complex.

Expression of mutant alpha-synuclein causes increased susceptibility to dopamine toxicity

Mutations of the alpha-synuclein gene have been identified in autosomal dominant Parkinson's disease (PD). Transgenic mice overexpressing wild-type human alpha-synuclein develop motor impairments, intraneuronal inclusions and loss of dopaminergic terminals in the striatum. To study the mechanism of action through which mutant alpha-synuclein toxicity is mediated, we have generated stable, inducible cell models expressing wild-type or PD-associated mutant (G209A) alpha-synuclein in human-derived HEK293 cells. Increased expression of either wild-type or mutant alpha-synuclein resulted in the formation of cytoplasmic aggregates which were associated with the vesicular (including monoaminergic) compartment. Expression of mutant alpha-synuclein induced a significant increase in sensitivity to dopamine toxicity compared with the wild-type protein expression. These results provide an explanation for the preferential dopaminergic neuronal degeneration seen in both the PD G209A mutant alpha-synuclein families and suggest that similar mechanisms may underlie or contribute to cell death in sporadic PD.

Mitochondrial dysfunction in congenital nephrotic syndrome

The molecular mechanisms maintaining the kidney glomerular filtration barrier remain poorly understood. Recent evidence suggests that mitochondrial dysfunction is a characteristic feature of kidney glomeruli in congenital nephrotic syndrome of the Finnish type (CNF). Here we searched for detailed functional evidence of mitochondrial lesion in CNF kidneys. We used histochemical and immunohistochemical methods, quantitative measurement of mitochondrial DNA, and superoxide production to characterize the mitochondrial function. The results unequivocally show down-regulation of mitochondria-encoded respiratory chain components, whereas the respective nuclearly encoded subunits were close to normal. These results give detailed evidence of distinct mitochondrial dysfunction and of the resulting abnormal production of reactive oxygen species in CNF and suggest a critical role for mitochondria in maintaining the glomerular permeability barrier.

A mutation in the human heme A:farnesyltransferase gene (COX10 ) causes cytochrome c oxidase deficiency

Cytochrome c oxidase (COX) defects are found in a clinically and genetically heterogeneous group of mitochondrial disorders. To date, mutations in only two nuclear genes causing COX deficiency have been described. We report here a genetic linkage study of a consanguineous family with an isolated COX defect and subsequent identification of a mutation in a third nuclear gene causing a deficiency of the enzyme. A genome-wide search for homozygosity allowed us to map the disease gene to chromosome 17p13.1-q11.1 (Z (max)= 2.46; theta = 0.00 at the locus D17S799). This region encompasses two genes, SCO1 and COX10, encoding proteins involved in COX assembly. Mutation analysis followed by a complementation study in yeast permitted us to ascribe the COX deficiency to a homozygous missense mutation in the COX10 gene. This gene encodes heme A:farnesyltransferase, which catalyzes the first step in the conversion of protoheme to the heme A prosthetic groups of the enzyme. All three nuclear genes now linked to isolated COX deficiency are involved in the maturation and assembly of COX, emphasizing the major role of such genes in COX pathology.

Cytochrome oxidase immunohistochemistry: clues for genetic mechanisms

Cytochrome c oxidase (COX) is encoded by three mitochondrial and nine nuclear genes. COX deficiency is genetically heterogeneous but current diagnostic methods cannot easily distinguish between mitochondrial and nuclear defects. We hypothesized that there may be differential expression of COX subunits depending on the underlying mutation. COX subunit expression was investigated in five patients with known mtDNA mutations. Severe and selective reduction of mtDNA-encoded COX subunits I and II was consistently observed in all these patients and was restricted to COX-deficient fibres. Immunostaining of nuclear-encoded subunits COX IV and Va was normal, whilst subunit VIc, also nuclear-encoded, was decreased. Twelve of 36 additional patients with histochemically defined COX deficiency also had this pattern of staining, suggesting that they had mtDNA defects. Clinical features in this group were heterogeneous, including infantile encephalopathy, multisystem disease, cardiomyopathy and childhood-onset isolated myopathy. The remaining patients did not have the same pattern of immunostaining. Fourteen had reduced staining of all subunits, whilst 10 had normal staining of all subunits despite reduced enzyme activity. Patients with COX deficiency secondary to mtDNA mutations have a specific pattern of subunit loss, but the majority of children with COX deficiency do not have this pattern of subunit loss and are likely to have nuclear gene defects.

A missense mutation of cytochrome oxidase subunit II causes defective assembly and myopathy

We report the first missense mutation in the mtDNA gene for subunit II of cytochrome c oxidase (COX). The mutation was identified in a 14-year-old boy with a proximal myopathy and lactic acidosis. Muscle histochemistry and mitochondrial respiratory-chain enzymology demonstrated a marked reduction in COX activity. Immunohistochemistry and immunoblot analyses with COX subunit-specific monoclonal antibodies showed a pattern suggestive of a primary mtDNA defect, most likely involving CO II, for COX subunit II (COX II). mtDNA-sequence analysis demonstrated a novel heteroplasmic T-->A transversion at nucleotide position 7,671 in CO II. This mutation changes a methionine to a lysine residue in the middle of the first N-terminal membrane-spanning region of COX II. The immunoblot studies demonstrated a severe reduction in cross-reactivity, not only for COX II but also for the mtDNA-encoded subunit COX III and for nuclear-encoded subunits Vb, VIa, VIb, and VIc. Steady-state levels of the mtDNA-encoded subunit COX I showed a mild reduction, but spectrophotometric analysis revealed a dramatic decrease in COX I-associated heme a3 levels. These observations suggest that, in the COX protein, a structural association of COX II with COX I is necessary to stabilize the binding of heme a3 to COX I.

Biochemical, genetic and immunoblot analyses of 17 patients with an isolated cytochrome c oxidase deficiency

Mitochondrial respiratory chain defects involving cytochrome c oxidase (COX) are found in a clinically heterogeneous group of diseases, yet the molecular basis of these disorders have been determined in only a limited number of cases. Here, we report the clinical, biochemical and molecular findings in 17 patients who all had isolated COX deficiency and expressed the defect in cultured skin fibroblasts. Immunoblot analysis of mitochondrial fractions with nine subunit specific monoclonal antibodies revealed that in most patients, including in a patient with a novel mutation in the SURF1 gene, steady-state levels of all investigated COX subunits were decreased. Distinct subunit expression patterns were found, however, in different patients. The severity of the enzymatic defect matched the decrease in immunoreactive material in these patients, suggesting that the remnant enzyme activity reflects the amount of remaining holo-enzyme. Four patients presented with a clear defect of COX activity but had near normal levels of COX subunits. An increased affinity for cytochrome c was observed in one of these patients. Our findings indicate a genetic heterogeneity of COX deficiencies and are suggestive of a prominent involvement of nuclear genes acting on the assembly and maintenance of cytochrome c oxidase.

Mitochondrial DNA depletion syndrome is expressed in amniotic fluid cell cultures

Mitochondrial DNA depletion syndrome is an autosomal inherited disease associated with grossly reduced cellular levels of mitochondrial DNA in infancy. Most patients are born after a full and uncomplicated pregnancy, are normal at birth, but develop symptoms in the early neonatal period. These observations have led to the suggestion that the patients have a defect affecting the control of mitochondrial DNA copy number after birth. Using immunocytochemical techniques, we demonstrated that the disease is already expressed in amniotic fluid cells. Detection of mitochondrial DNA depletion in these fetal cells indicates that the defect may already be expressed early in embryological development.

Altered gene expression and functions of mitochondria in human nephrotic syndrome

The molecular basis of glomerular permselectivity remains largely unknown. The congenital nephrotic syndrome of the Finnish type (CNF) characterized by massive proteinuria already present but without extrarenal symptoms is a unique human disease model of pure proteinuria. In search of genes and pathophysiologic mechanisms associated with proteinuria, we used differential display-PCR to identify differences in gene expression between glomeruli from CNF and control kidneys. A distinctly underexpressed PCR product of the CNF kidneys showed over 98% identity with a mitochondrially encoded cytochrome c oxidase (COX I). Using a full-length COX I cDNA probe, we verified down-regulation of COX I mRNA to 1/4 of normal kidney values on Northern blots. In addition, transcripts of other mitochondrially encoded respiratory chain complexes showed a similar down-regulation whereas the respective nuclearly encoded complexes were expressed at comparable levels. Additional studies using histochemical, immunohistochemical, in situ hybridization, RT-PCR, and biochemical and electron microscopic methods all showed a mitochondrial involvement in the diseased kidneys but not in extrarenal blood vessels. As a secondary sign of mitochondrial dysfunction, excess lipid peroxidation products were found in glomerular structures in CNF samples. Our data suggest that mitochondrial dysfunction occurs in the kidneys of patients with CNF, with subsequent lipid peroxidation at the glomerular basement membrane. Our additional studies have revealed similar down-regulation of mitochondrial functions in experimental models of proteinuria. Thus, mitochondrial dysfunction may be a crucial pathophysiologic factor in this symptom.

The mitochondrial genome: structure, transcription, translation and replication

Mitochondria play a central role in cellular energy provision. The organelles contain their own genome with a modified genetic code. The mammalian mitochondrial genome is transmitted exclusively through the female germ line. The human mitochondrial DNA (mtDNA) is a double-stranded, circular molecule of 16569 bp and contains 37 genes coding for two rRNAs, 22 tRNAs and 13 polypeptides. The mtDNA-encoded polypeptides are all subunits of enzyme complexes of the oxidative phosphorylation system. Mitochondria are not self-supporting entities but rely heavily for their functions on imported nuclear gene products. The basic mechanisms of mitochondrial gene expression have been solved. Cis-acting mtDNA sequences have been characterised by sequence comparisons, mapping studies and mutation analysis both in vitro and in patients harbouring mtDNA mutations. Characterisation of trans-acting factors has proven more difficult but several key enzymes involved in mtDNA replication, transcription and protein synthesis have now been biochemically identified and some have been cloned. These studies revealed that, although some factors may have an additional function elsewhere in the cell, most are unique to mitochondria. It is expected that cell cultures of patients with mitochondrial diseases will increasingly be used to address fundamental questions about mtDNA expression.

Decreased brain protein levels of cytochrome oxidase subunits in Alzheimer's disease and in hereditary spinocerebellar ataxia disorders: a nonspecific change?

Controversy exists as to the clinical importance, cause, and disease specificity of the cytochrome oxidase (CO) activity reduction observed in some patients with Alzheimer's disease (AD). Although it is assumed that the enzyme is present in normal amount in AD, no direct measurements of specific CO protein subunits have been conducted. We measured protein levels of CO subunits encoded by mitochondrial (COX I, COX II) and nuclear (COX IV, COX VIc) DNA in autopsied brain of patients with AD whom we previously reported had decreased cerebral cortical CO activity. To assess disease specificity, groups of patients with spinocerebellar ataxia type I and Friedreich's ataxia were also included. As compared with the controls, mean protein concentrations of all four CO subunits were significantly decreased (-19 to -47%) in temporal and parietal cortices in the AD group but were not significantly reduced (-12 to -17%) in occipital cortex. The magnitude of the reduction in protein levels of the CO subunits encoded by mitochondrial DNA (-42 to -47%) generally exceeded that encoded by nuclear DNA (-19 to -43%). In the spinocerebellar ataxia disorders, COX I and COX II levels were significantly decreased in cerebellar cortex (-22 to -32%) but were normal or close to normal in cerebral cortex, an area relatively unaffected by neurodegeneration. We conclude that protein levels of mitochondrial- and nuclear-encoded CO subunits are moderately reduced in degenerating but not in relatively spared brain areas in AD and that the decrease is not specific to this disorder. The simplest explanation for our findings is that CO is decreased in human brain disorders as a secondary event in brain areas having reduced neuronal activity or neuronal/synaptic elements consequent to the primary neurodegenerative process.

Biochemical abnormalities and excitotoxicity in Huntington's disease brain

The physiological role of huntingtin and the mechanisms by which the expanded CAG repeat in ITI5 and its polyglutamine stretch in mutant huntingtin induce Huntington's disease (HD) are unknown. Several techniques have now demonstrated abnormal metabolism in HD brain; direct measurement of respiratory chain enzyme activities has shown severe deficiency of complex II/III and a milder defect of complex IV. We confirm that these abnormalities appear to be confined to the striatum within the HD brain. Analysis of complex II/III activity in HD fibroblasts was normal, despite expression of mutant huntingtin. Although glyceraldehyde 3-phosphate dehydrogenase (a huntingtin binding protein) activity was normal in all areas studied, aconitase activity was decreased to 8% in HD caudate, 27% in putamen, and 52% in cerebral cortex, but normal in HD cerebellum and fibroblasts. We have demonstrated that although complexes II and III are those parts of the respiratory chain most vulnerable to inhibition in the presence of a nitric oxide (NO*) generator, aconitase activity was even more sensitive to inhibition. The pattern of these enzyme deficiencies and their parallel to the anatomical distribution of HD pathology support an important role for NO* and excitotoxicity in HD pathogenesis. Furthermore, based on the biochemical defects we have described, we suggest that NO* generation produces a graded response, with aconitase inhibition followed by complex II/III inhibition and the initiation of a self-amplifying cycle of free radical generation and aconitase inhibition, which results in severe ATP depletion. We propose that these events are important in determining neuronal cell death and are critical steps in the pathogenesis of HD.

SCID mice containing muscle with human mitochondrial DNA mutations. An animal model for mitochondrial DNA defects

Defects of the mitochondrial genome are important causes of disease. Despite major advances in our investigation of patients, there is no effective therapy. Progress in this area is limited by the absence of any animal models in which we can evaluate treatment. To develop such a model we have injected human myoblasts into the tibialis anterior of SCID mice after inducing necrosis. After injection of normal human myoblasts, regenerating fibers expressed human beta-spectrin, confirming they were derived from fusion of human myoblasts. The stability of the muscle fibers was inferred by demonstrating the formation of motor end plates on the regenerating fibers. In addition, we show the presence of human cytochrome c oxidase subunit II, which is encoded by the mitochondrial genome, in the regenerated fibers. After injection of human myoblasts containing either the A8344G or the T8993C heteroplasmic mitochondrial DNA mutations, human beta-spectrin positive fibers were found to contain the mutation at a similar level to the injected myoblasts. These studies highlight the potential value of this model for the study of mitochondrial DNA defects.

Mitochondria in the etiology and pathogenesis of Parkinson's disease

Mitochondria play a critical role in cellular energy metabolism. The identification of a respiratory chain defect in Parkinson's disease (PD) provides not only a direct link with toxin models of parkinsonism but also insight into the mechanisms involved in etiology and pathogenesis. The presence of the complex I deficiency in PD substantia nigra and platelets suggests the involvement of a systemic cause. Genomic transplantation studies have been undertaken that involve the transfer to a novel nuclear background of mitochondrial DNA (mtDNA) from PD patients with a complex I defect, followed by both mixed and clonal expansion of the resulting cybrids. The mixed cybrids with the PD mtDNA expressed the complex I defect present in the original PD donor platelets. Clonal expansion of one such mixed cybrid culture produced a spectrum of clones with complex I and complex IV activities, ranging from severe deficiency to normal range, a pattern typical of a heteroplasmic mtDNA mutation. Histochemical, immunohistochemical, and functional assessments of delta psi(m) all showed a pattern in the PD clones typical of that produced by a mtDNA mutation. Patients with focal dystonia and a platelet complex I defect were used as disease controls for the cybrid studies. The mitochondrial abnormality was eradicated by transfer of dystonia mtDNA to a control nuclear background in both mixed and clonal cybrids, with no evidence of clonal heterogeneity. These results help to validate our findings in the PD patients and suggest that the complex I deficiency in dystonia is not due to an abnormality of mtDNA. We hypothesize that the mtDNA defect alone may be the cause of PD in a proportion of patients and may contribute to pathogenesis in others. Identification of the mtDNA genotype responsible for PD may allow the testing of neuroprotective strategies in appropriate patients.

Mitochondrial DNA transmission of the mitochondrial defect in Parkinson's disease

Several groups have identified mitochondrial complex I deficiency in Parkinson's disease (PD) substantia nigra and in platelets. A search for any mitochondrial DNA (mtDNA) mutation underlying this defect has not yet produced any consistent result. We have made use of a mtDNA-less (p0) cell line to determine if the complex I deficiency follows the genomic transplantation of platelet mtDNA. From a preselected group of PD patients with low platelet complex I activity, 7 patients were used for detailed study. All 7 patients were used for mixed cybrid analysis and demonstrated a selective 25% deficiency of complex I activity. Individual clonal analysis of A549 p0/PD platelet fusion cybrids from 1 of the patients expressed combined complex I and IV deficiencies with 25% and 20% decreased activities in the PD clones, respectively. Histocytochemical, immunocytochemical, and cellular functional imaging studies of these clones showed the cells within the clones were heterogeneous with respect to cytochrome c oxidase (COX) function, COX I content, and mitochondrial respiratory chain activity. These results are in agreement with a previous study and support the proposition that an mtDNA abnormality may underlie the mitochondrial defect in at least a proportion of PD patients. This p0 technology may serve as a means to identify the subgroup of PD patients in whom an mtDNA defect may contribute to development of the disease.

Assembly of cytochrome-c oxidase in cultured human cells

The assembly of cytochrome-c oxidase was studied in human cells cultured in the presence of inhibitors of mitochondrial or cytosolic protein synthesis. Mitochondrial fractions were resolved using two-dimensional PAGE (blue native PAGE and tricine/SDS/PAGE) and subsequent western blots were developed with monoclonal antibodies against specific subunits of cytochrome-c oxidase. Proteins were also visualized using metabolic labeling followed by two-dimensional electrophoresis and fluorography. These techniques allowed identification of two assembly intermediates of cytochrome-c oxidase. Assembly of the 13 subunits of cytochrome-c oxidase starts with the association of subunit I with subunit IV. Then a larger subcomplex is formed, lacking only subunits VIa and either VIIa or VIIb.

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.

Cytochrome c oxidase subunit I microdeletion in a patient with motor neuron disease

An out-of-frame mutation of the mitochondrial DNA-encoded subunit I of cytochrome c oxidase (COX) was discovered during investigation of a severe isolated muscle COX deficiency in a patient with motor neuron-like degeneration. The mutation is a heteroplasmic 5-bp microdeletion located in the 5' end of the COI gene, leading to premature termination of the corresponding translation product. Western blot analysis, immunohistochemistry, and single-fiber polymerase chain reaction demonstrated a tight correlation between COX defect, COX I expression, and percentage of mutation. COX subunits II, III, and IV were decreased as well, suggesting a defective assembly of COX holoenzyme. The mutation was associated with a clinical phenotype unusual for a mitochondrial disorder, that is, an isolated motor neuron disease (MND) with some atypical findings, including early onset, preferential involvement of the upper motor neuron, and increased cerebrospinal fluid protein content. MND may arise from impaired scavenging and overproduction of free oxygen radicals, a by-product of oxidative phosphorylation (OXPHOS). Our observation suggests that OXPHOS impairment could play a role in the pathogenesis of some MND cases.

Expression of mtDNA and nDNA encoded respiratory chain proteins in chemically and genetically-derived Rho0 human fibroblasts: a comparison of subunit proteins in normal fibroblasts treated with ethidium bromide and fibroblasts from a patient with mtDNA depletion syndrome

Although much progress has been made in identifying genetic defects associated with mitochondrial diseases, the protein expression patterns of most disorders are poorly understood. Here we use immunochemical techniques to describe subunit expression patterns of respiratory chain enzyme complexes II (succinate dehydrogenase: SD) and IV (cytochrome c oxidase: COX) in cultured cells lacking mtDNA (Rho0 cells) derived either chemically by exposure of normal cells to ethidium bromide, or genetically in cells derived from a patient with mtDNA depletion syndrome. Both control cells and early passage patient-derived cells express a normal complement of SD and COX subunit proteins. Ethidium bromide treatment of normal cells and in vitro cell proliferation of patient-derived cells caused both populations to acquire identical Rho0 phenotypes. As expected, they lack mtDNA-encoded subunits COX-I and COX-II. In contrast, nDNA-encoded subunits are affected differentially, with some (COX-VIc) lacking and others (COX-IV, COX-Va, SD 30 and SD 70) maintained at somewhat reduced levels. We suggest that the differential stability of nDNA-encoded subunits in the absence of intact enzyme complexes is due to the ability of some, but not all, subunits to associate as partial complexes in the absence of mtDNA-encoded subunits.

Molecular mechanisms in mitochondrial DNA depletion syndrome

Depletion of mitochondrial DNA (mtDNA) appears to be an important cause of mitochondrial dysfunction in neonates and infants. We have identified another child in whom depletion of mtDNA was demonstrated in liver and serial skeletal muscle biopsies. A primary myoblast culture from the patient initially showed normal levels of mtDNA, but there was a progressive loss of mtDNA in later cell passages and clonal myoblast cell cultures, similar to that observed in the skeletal muscle tissue of the patient. Thus, these clonal myoblast cultures provide an in vitro model of the in vivo mtDNA dynamics. The levels of mitochondrial mRNAs for subunits I and II of cytochrome c oxidase declined with declining mtDNA levels, but the fall in mitochondrial transcript levels lagged behind that of the mtDNA levels. Levels of cytochrome c oxidase subunit I and II polypeptides, however, declined ahead of declining mtDNA levels. Immunocytochemistry showed that between individual cells of the clonal myoblast cultures, the expression of the mitochondrially encoded subunit I of cytochrome c oxidase was heterogeneous, suggesting variable levels of mtDNA. Transfer of patient mitochondria with residual mtDNA levels to control cells devoid of mtDNA (rho0 cells) led to restoration of mtDNA levels and, hence, suggests a nuclear involvement in the depletion.

Human cytochrome c oxidase: structure, function, and deficiency

As the terminal component of the mitochondrial respiratory chain, cytochrome c oxidase plays a vital role in cellular energy transformation. Human cytochrome c oxidase is composed of 13 subunits. The three major subunits form the catalytic core and are encoded by mitochondrial DNA (mtDNA). The remaining subunits are nuclear-encoded. The primary sequence is known for all human subunits and the crystal structure of bovine heart cytochrome c oxidase has recently been reported. However, despite this wealth of structural information, the role of the nuclear encoded subunits is still poorly understood. Yeast cytochrome c oxidase is a close model of its human counterpart and provides a means of studying the effects of mutations on the assembly, structure, stability and function of the enzyme complex. Defects in cytochrome c oxidase function are found in a clinically heterogeneous group of disorders. The molecular defects that underlie these diseases may arise from mutations of either mitochondrial or the nuclear genomes or both. A significant number of cytochrome c oxidase deficiencies, often associated with other respiratory chain enzyme defects, are attributed to mutations of mtDNA. Mutations of mtDNA appear, nonetheless, uncommon in early childhood. Pedigree analysis and cell fusion experiments have demonstrated a nuclear involvement in some infantile cases but a specific genomic lesion has not yet been reported. Detailed analyses of the many steps involved in the biogenesis of cytochrome c oxidase, often pioneered in yeast, offer several starting points for further molecular characterizations of cytochrome c oxidase deficiencies observed in clinical practice.

Subunit specific monoclonal antibodies show different steady-state levels of various cytochrome-c oxidase subunits in chronic progressive external ophthalmoplegia

Monoclonal antibodies recognizing the mitochondrially encoded subunits I and II, and the nuclear-encoded subunits IV, Va, Vb and VIc of human cytochrome-c oxidase were generated. These antibodies are highly specific and allow the assessment of subunit steady-state levels in crude cell extracts and tissue sections. In the experimental human cell line 143B206, which is devoid of mitochondrial DNA, immunovisualization with the antibodies revealed that the nuclear-encoded subunits IV and Va were present in amounts close to that of the parental cell line despite the absence of the mitochondrially encoded subunits. In contrast, the nuclear-encoded subunits Vb and VIc were severely reduced in cell line 143B206, suggesting that unassembled nuclear-encoded subunits are degraded at different rates. In skeletal muscle sections of a patient with chronic progressive external ophthalmoplegia known to harbor the 'common deletion' in a subpopulation of her mitochondrial DNA, most cytochrome-c oxidase activity negative fibers had greatly reduced levels of subunits I, II, Va, Vb and VIc of cytochrome-c oxidase. The steady-state level of subunit IV, however, was less affected. This was particularly evident in cytochrome-c oxidase activity negative fibers with accumulated mitochondria ('ragged-red' fibers) where immunodetection with anti-subunit IV resulted in intense staining. The data presented in this paper demonstrate that the battery of monoclonal antibodies can be employed for diagnostic purposes to analyze steady-state levels of mitochondrially and nuclear-encoded subunits of cytochrome-c oxidase.

Regulation of cytochrome c oxidase by interaction of ATP at two binding sites, one on subunit VIa

Cytochrome c oxidase isolated from a wild-type yeast strain and a mutant in which the gene for subunit VIa had been disrupted were used to study the interaction of adenine nucleotides with the enzyme complex. At low ionic strength (25 mM potassium phosphate), in the absence of nucleotides, the cytochrome c oxidase activity of the mutant enzyme lacking subunit VIa was higher than that of the wild-type enzyme. Increasing concentrations of ATP, in the physiological range, enhanced the cytochrome c oxidase activity of the mutant much more than the activity of the wild-type strain, whereas ADP, in the same concentration range, had no significant effect on the activity of the cytochrome c oxidase of either strain. These results indicate an interaction of ATP with subunit VIa in the wild-type enzyme that prevents the stimulation of the activity observed in the mutant enzyme. The stimulation of the mutant enzyme implies the presence of a second ATP binding site on the enzyme. Quantitative titrations with the fluorescent adenine nucleotide analogues 2'(or 3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP) and 2'(or 3')-O-(2,4,6-trinitrophenyl)adenosine 5'-diphosphate (TNP-ADP) confirmed the presence of two binding sites for adenine nucleotides per monomer of wild-type cytochrome c oxidase and one binding site per monomer of mutant enzyme. Covalent photolabeling of yeast cytochrome c oxidase with radioactive 2-azido-ATP further confirmed the presence of an ATP binding site on subunit VIa.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue distribution of cytochrome c oxidase isoforms in mammals. Characterization with monoclonal and polyclonal antibodies

Monoclonal and polyclonal antibodies specific to the two isoforms of subunit VIa of bovine cytochrome c oxidase were generated and used to study the tissue distribution of this subunit pair in beef, human and rat. The so-called H-(heart) form was found exclusively in heart and skeletal muscle, whereas the so-called L-(liver) form was the only isoform present in brain, kidney, liver and smooth muscle. Little or no L-form was detected in skeletal muscle. In bovine heart no subunit VIa-L was detected, while in human heart the subunit VIa-H and VIa-L isoforms were present in roughly equal proportions. These results imply that, in humans, the deficiency of a subunit VIa isoform may have a different effect on the physiology of heart then on the physiology of skeletal muscle.

Regulation of the expression of mitochondrial proteins: relationship between mtDNA copy number and cytochrome-c oxidase activity in human cells and tissues

The relationship between the relative amounts of nuclear and mitochondrial genes for cytochrome-c oxidase subunits and their transcripts and cytochrome-c oxidase activity was investigated in several human tissues and cell lines to get more insight into the regulation of the expression of this mitochondrial enzyme complex. The results show: (1) a wide range of mtDNA copy numbers; (2) constant ratios between the steady-state levels of the transcripts for the various cytochrome-c oxidase subunits, and (3) large variations in cytochrome-c oxidase activity in different tissues and cell lines that could not be related to the differences in mtDNA copy number. We conclude that the transcription of genes for both mitochondrial and nuclear cytochrome-c oxidase subunits is regulated coordinatedly, but also that the mtDNA copy number plays a minor role in determining differences in cytochrome-c oxidase activity between different cell and tissue types.

Subunit VIa of yeast cytochrome c oxidase is not necessary for assembly of the enzyme complex but modulates the enzyme activity. Isolation and characterization of the nuclear-coded gene

COX13, the nuclear gene for cytochrome c oxidase subunit VIa of Saccharomyces cerevisiae, has been isolated in two steps. First, the partial amino acid sequence information of the subunit was used to design two degenerate oligodeoxynucleotide primers to amplify part of the gene in a polymerase chain reaction. Next, the amplified product was used to screen a yeast genomic library in order to obtain the entire gene and its flanking sequences. COX13 is present as a single copy gene per haploid genome. Alignment of the N-terminal sequence of mature, subunit VIa with the amino acid sequence deduced from the DNA sequence indicates that subunit VIa is synthesized as a precursor comprised of a leader sequence of 9 amino acid residues and a mature polypeptide of 120 amino acid residues. The mature polypeptide shares 34% identical amino acid residues with the human subunit isoform VIa-L. Sequence analysis of the 3'-flanking region of COX13 revealed that the gene is located 599 base pairs downstream of CDC55, a gene which has been mapped to the left arm of chromosome VII. Null mutants of COX13, generated by gene replacement, showed a slightly reduced growth rate on nonfermentable carbon sources. Heme spectra and analysis of immunopurified cytochrome c oxidase from a null strain demonstrated that the enzyme is fully assembled without subunit VIa. At low ionic strength, cytochrome c oxidase missing subunit VIa was more active, whereas at high ionic strength, it was less active than the enzyme complex in which subunit VIa was present. In addition, distinct effects of ATP on the activity of the null and wild type enzyme were found. The results suggest that ATP interacts specifically with subunit VIa and thereby modulates the cytochrome c oxidase activity.

Purification of yeast cytochrome c oxidase with a subunit composition resembling the mammalian enzyme

Yeast cytochrome c oxidase has been isolated by ion exchange chromatography using lauryl maltoside (n-dodecyl beta-D-maltoside) as the solubilizing detergent. The enzyme prepared in this way has a heme aa3 concentration of 8-9 nmol/mg of protein and a turnover number in the range of 180-210 s-1 at pH 6.2 in 0.01% lauryl maltoside at 20 degrees C. Yeast cytochrome c oxidase prepared by any of several previously published methods which use Triton X-100 contains nine subunits. The enzyme isolated in lauryl maltoside contains these same nine different polypeptides and three others, including homologues of subunits VIa and VIb of the mammalian enzyme.

Steady-state transcript levels of cytochrome c oxidase genes during human myogenesis indicate subunit switching of subunit VIa and co-expression of subunit VIIa isoforms

Steady-state levels of the mitochondrial rRNAs, of mRNAs for mitochondrially and nuclear-encoded subunits of cytochrome c oxidase and for the beta subunit of ATP synthase were assessed by Northern blot hybridizations during the in vitro differentiation of human myoblasts. Transcript levels of the so-called liver-type form of subunit VIa of cytochrome c oxidase diminished during the course of differentiation, while transcription of the so-called heart-type form was induced. Transcripts for the liver-type form and for the heart-type form of subunit VIIa of cytochrome c oxidase were detected in all myogenic cultures; the levels of the heart-type form progressively increased during the course of differentiation. The levels of the other transcripts studied did not change substantially. The results suggest subunit switching of subunit VIa and co-expression of subunit VIIa isoforms during myogenesis. The differential changes in mRNA levels of the heart-type subunits VIa and VIIa and the differential changes in mRNA levels of the liver-type subunits VIa and VIIa demonstrate that different transcriptional regulation mechanisms are present for both heart-type genes as well as for both liver-type genes.