Evolution of a Regulatory Enzyme: Cytochrome-c Oxidase (Complex IV)

Metabolic Connectivity and Hemodynamic-Metabolic Coherence of Human Prefrontal Cortex at Rest and Post Photobiomodulation Assessed by Dual-Channel Broadband NIRS

Billions of neurons in the human brain form neural networks with oscillation rhythms. Infra-slow oscillation (ISO) presents three main physiological sources: endogenic, neurogenic, and myogenic vasomotions. Having an in vivo methodology for the absolute quantification of ISO from the human brain can facilitate the detection of brain abnormalities in cerebral hemodynamic and metabolic activities. In this study, we introduced a novel measurement-plus-analysis framework for the non-invasive quantification of prefrontal ISO by (1) taking dual-channel broadband near infrared spectroscopy (bbNIRS) measurements from 12 healthy humans during a 6-min rest and 4-min post transcranial photobiomodulation (tPBM) and (2) performing wavelet transform coherence (WTC) analysis on the measured time series data. The WTC indexes (IC, between 0 and 1) enabled the assessment of ipsilateral hemodynamic-metabolic coherence and bilateral functional connectivity in each ISO band of the human prefrontal cortex. At rest, bilateral hemodynamic connectivity was consistent across the three ISO bands (IC ≅ 0.66), while bilateral metabolic connectivity was relatively weaker. For post-tPBM/sham comparison, our analyses revealed three key findings: 8-min, right-forehead, 1064-nm tPBM (1) enhanced the amplitude of metabolic oscillation bilaterally, (2) promoted the bilateral metabolic connectivity of neurogenic rhythm, and (3) made the main effect on endothelial cells, causing alteration of hemodynamic-metabolic coherence on each side of the prefrontal cortex.

Histochemical and immunohistochemical staining methods to visualize mitochondrial proteins and activity

We describe here reliable histochemical and immunohistochemical techniques to visualize mitochondria and respiratory chain dysfunction in tissue sections. These morphological methods have been widely used for years, and yet remain relevant to obtain insight into the pathogenesis of mitochondrial diseases. Today, mitochondrial medicine is changing rapidly and genetic information plays an increasing role in the diagnostic process, owing to advances in next-generation sequencing. However, tissue analysis and morphological categorization remain essential, especially when genetic abnormalities of unknown significance might complicate a diagnostic odyssey. Furthermore, tissue assessment is an essential step in laboratory investigation using animal or cell models, in order to assess the distribution, severity, and/or progression of the disease, and to evaluate the effects of possible treatments. Optimized and reproducible staining and imaging methodology are the key elements for accurate tissue assessment. When these methods are used properly and integrated with wisely chosen genetic and biochemical approaches, powerful information can be obtained about the structure and function of mitochondria in both animal model systems and human patients. While the described protocols refer to skeletal muscle and brain mitochondria, the methods described can be applied to any tissue type.

Metabolic Energy Capacity of Dopaminergic Grafts and the Implanted Striatum in Parkinsonian Nonhuman Primates as Visualized with Cytochrome Oxidase Histochemistry

Histochemistry for visualization of the mitochondrial enzyme cytochrome oxidase has been used to detect cellular and regional differences in brain energy metabolism. We have examined the pattern of cytochrome oxidase (CO) staining in grafts of embryonic ventral mesencephalic tissue, and in the implanted striatum, of MPTP-treated monkeys as one index of the functional activity of grafted tissue and its influence on the host brain. Four monkeys were selected for study based on interesting variations in dopamine (DA) neuron content of their bilateral grafts as demonstrated with tyrosine-hydroxylase (TH) immunocytochemistry. The results suggest that grafts rich in DA neurons increase the metabolic activity of the implanted striatum of DA-depleted monkeys, and that this improvement of local energy metabolism is greater in the vicinity of grafts containing greater numbers of DA neurons. In addition, the pattern of CO staining within tissue transplants indicates that DA neurons exhibit the highest rate of metabolic activity among all cell types contained in the ventral mesencephalic grafts, and that the transplants receive metabolically active innervation from outside or within the grafted tissue.

Destabilization of the Quaternary Structure of Bovine Heart Cytochrome c Oxidase upon Removal of Tightly Bound Cardiolipin

The quaternary structural stability of cardiolipin-containing (CcO(CL+)) versus CL-free cytochrome c oxidase (CcO(CL-)) was compared using structural perturbants as probes. Exposure to increasing concentrations of urea or guanidinium chloride causes sequential dissociation of five subunits from CcO(CL+) in the order VIa and VIb, followed by III and VIIa, and ultimately Vb. Removal of CL from CcO destabilizes the association of each of these five subunits with the core of CcO. Subunits VIa and VIb spontaneously dissociate from CcO(CL-) even in the absence of denaturant and are no longer present after purification of the CL-free 11-subunit complex by ion exchange chromatography. The other 11 subunits remain associated in a partially active complex, but the association of subunits III, VIIa, and Vb is weakened; i.e., the midpoints for the subunit dissociation curves are each shifted to a lower perturbant concentration (lower by 1.1-1.7 M urea; lower by 0.3-0.4 M GdmCl). This corresponds to a decrease of ∼9 kJ in the Gibbs free association energy for each of these subunits when CL is removed from CcO. With either CcO(CL+) or CcO(CL-), loss of enzymatic activity occurs coincident with dissociation of subunits III and VIIa. The loss of activity is irreversible, and reactivation of CcO(CL-) by exogenous CL occurs only if both subunits remain associated with the core of CcO. Inclusion of sulfate anions stabilizes the association of VIIa more than III, resulting in a slight separation of the urea-induced dissociation curves. In this case, activity loss correlates much better with dissociation of subunit VIIa than III. We conclude that (1) bound cardiolipin is an important stabilizing factor in the quaternary structure of CcO and (2) association of subunit VIIa (possibly together with subunit III) is critical for functional CL binding and full electron-transfer activity of CcO.

Light microscopic methods to visualize mitochondria on tissue sections

Mitochondria are cytoplasmic, double-membrane organelles, a main role of which is to synthesize ATP, the universal energy 'supply' of cells. In the last three decades, molecular genetic, biochemical, immunological and cell biological techniques have been applied in a coordinated fashion to unveil the pathogenesis of known mitochondrial disorders, as well as to explore the role of mitochondria in aging and neurodegenerative diseases. Once to be thought to be rare, it is now clear that mitochondrial dysfunction is an important cause of neurological and cardiac diseases, and age-related disorders such as cancer. Here, we review, illustrate, and provide updated protocols of two histochemical, and three immunohistochemical methods that in our opinion are the most reliable tools to visualize mitochondria on tissue sections from normal and disease specimens.

Increased levels of mitochondrial gene transcripts in the thermally selected rainbow trout (Oncorhynchus mykiss) strain during embryonic development

To investigate molecular mechanisms involved in thermal resistance of rainbow trout, Oncorhynchus mykiss, embryos from thermally selected strain in various developmental stages were treated at 22 degrees C for 30 min and subsequently developed at 12 degrees C using the Donaldson strain as a reference. The embryos were evaluated for their hatching rate along with the ratio of embryos having an abnormal appearance and subjected to mRNA arbitrarily primed reverse transcription-polymerase chain reaction (RAP RT-PCR). One of the genes dominantly expressed in the thermally selected strain (COX II) coded for cytochrome c oxidase subunit II. Northern blot analysis revealed that the accumulated levels of COX II transcripts were more abundant in embryos and unfertilized eggs from the thermally selected strain than those from the Donaldson strain. Furthermore, the differential expression patterns of the ATPase 6-8 gene were similar to those of the COX II gene, whereas the ATP synthase beta-subunit gene showed no significant differences between the two strains.

Cytochrome C oxidase and the regulation of oxidative phosphorylation

Life of higher organisms is essentially dependent on the efficient synthesis of ATP by oxidative phosphorylation in mitochondria. An important and as yet unsolved question of energy metabolism is how are the variable rates of ATP synthesis at maximal work load during exercise or mental work and at rest or during sleep regulated. This article reviews our present knowledge on the structure of bacterial and eukaryotic cytochrome c oxidases and correlates it with recent results on the regulatory functions of nuclear-coded subunits of the eukaryotic enzyme, which are absent from the bacterial enzyme. A new molecular hypothesis on the physiological regulation of oxidative phosphorylation is proposed, assuming a hormonally controlled dynamic equilibrium in vivo between two states of energy metabolism, a relaxed state with low ROS (reactive oxygen species) formation, and an excited state with elevated formation of ROS, which are known to accelerate aging and to cause degenerative diseases and cancer. The hypothesis is based on the allosteric ATP inhibition of cytochrome c oxidase at high intramitochondrial ATP/ADP ratios ("second mechanism of respiratory control"), which is switched on by cAMP-dependent phosphorylation and switched off by calcium-induced dephosphorylation of the enzyme.

Molecular evolution of aerobic energy metabolism in primates

As part of our goal to reconstruct human evolution at the DNA level, we have been examining changes in the biochemical machinery for aerobic energy metabolism. We find that protein subunits of two of the electron transfer complexes, complex III and complex IV, and cytochrome c, the protein carrier that connects them, have all undergone a period of rapid protein evolution in the anthropoid lineage that ultimately led to humans. Indeed, subunit IV of cytochrome c oxidase (COX; complex IV) provides one of the best examples of positively selected changes of any protein studied. The rate of subunit IV evolution accelerated in our catarrhine ancestors in the period between 40 to 18 million years ago and then decelerated in the descendant hominid lineages, a pattern of rate changes indicative of positive selection of adaptive changes followed by purifying selection acting against further changes. Besides clear evidence that adaptive evolution occurred for cytochrome c and subunits of complexes III (e.g., cytochrome c(1)) and IV (e.g., COX2 and COX4), modest rate accelerations in the lineage that led to humans are seen for other subunits of both complexes. In addition the contractile muscle-specific isoform of COX subunit VIII became a pseudogene in an anthropoid ancestor of humans but appears to be a functional gene in the nonanthropoid primates. These changes in the aerobic energy complexes coincide with the expansion of the energy-dependent neocortex during the emergence of the higher primates. Discovering the biochemical adaptations suggested by molecular evolutionary analysis will be an exciting challenge.

Molecular evolution of cytochrome c oxidase subunit I in primates: is there coevolution between mitochondrial and nuclear genomes?

Phylogenetic analyses carried out on cytochrome c oxidase (COX) subunit I mitochondrial genes from 14 primates representing the major branches of the order and four outgroup nonprimate eutherians revealed that transversions and amino acid replacements (i.e., the more slowly occurring sequence changes) contained lower levels of homoplasy and thus provided more accurate information on cladistic relationships than transitions (i.e., the more rapidly occurring sequence changes). Several amino acids, each with a high likelihood of functionality involving the binding of cytochrome c or interaction with COX VIII, have changed in Anthropoidea, the primate suborder grouping New World monkey, Old World monkey, ape, and human lineages. They are conserved in other mammalian lineages and in nonanthropoid primates. Maximum-likelihood ancestral COX I nucleotide sequences were determined utilizing a near most parsimonious branching arrangement for the primate sequences that was consistent with previously hypothesized primate cladistic relationships based on larger and more diverse data sets. Relative rate tests of COX I mitochondrial sequences showed an elevated nonsynonymous (N) substitution rate for anthropoid-nonanthropoid comparisons. This finding for the largest mitochondrial (mt) DNA-encoded subunit is consistent with previous observations of elevated nonsynonymous substitution/synonymous substitution (S) rates in primates for mt-encoded COX II and for the nuclear-encoded COX IV and COX VIIa-H. Other COX-related proteins, including cytochrome c and cytochrome b, also show elevated amino acid replacement rates or N/S during similar time frames, suggesting that this group of interacting genes is likely to have coevolved during primate evolution.

Activities of mitochondrial oxidative phosphorylation enzymes in cultured amniocytes

Amniocytes represent a population of foetal cells that can be used for prenatal diagnosis in families with suspected mitochondrial oxidative phosphorylation (OXPHOS) defects. In this paper, we present a complex protocol for evaluation of the function of mitochondrial OXPHOS enzymes in cultured amniocytes using three independent and complementary methods: (a) spectrophotometry as a tool for determination of the capacities of mitochondrial respiratory-chain enzymes (NADH ubiquinone oxidoreductase, succinate- and glycerophosphate cytochrome c reductase, cytochrome c oxidase and citrate synthase); (b) polarography as a tool for the evaluation of mitochondrial OXPHOS enzyme functions in situ using digitonin-permeabilised amniocytes (rotenone-sensitive oxidation of pyruvate+malate, antimycin A-sensitive oxidation of succinate, KCN-sensitive oxidation of cytochrome c, ADP-activated substrate oxidation) and (c) cytofluorometric determination of tetramethyl rhodamine methyl ester (TMRM) fluorescence in digitonin-permeabilised amniocytes as a sensitive way to determine the mitochondrial membrane potential under steady-state conditions (state 4 with succinate). These protocols are presented together with reference control values using 9-22 independent cultures of amniocytes.

Human COX6A1 gene: promoter analysis, cDNA isolation and expression in the monkey brain

The human COX6A1 gene encodes the ubiquitous isoform of cytochrome c oxidase (COX) subunit VIa (VIa-L), and is located in a CpG island on chromosome 12q24.2. We compared the COX6A1 gene with the published cDNA and several ESTs and concluded that subunit COX VIa-L is synthesized as a preprotein, as are other COX subunits. The same transcription start sites were identified by primer extension analysis of human brain and lymphoblastoid RNA. Analysis of the COX6A1 promoter revealed several conserved sequence elements found in other COX genes, namely binding sites for nuclear respiratory factor 1 (NRF-1), nuclear respiratory factor 2/GA binding protein (NRF-2/GABP), and ying-yang protein 1 (YY1). These conserved elements were shown to bind nuclear proteins from HeLa nuclear extracts. COX6A1 cDNA was isolated from a human brain cDNA library, and the sequence was identical to that of human liver. The expression of this gene was demonstrated by in-situ hybridization in monkey brain sections with our human brain cDNA. Monocular impulse blockade in adult monkeys induced a downregulation of COX6A1 expression in deprived visual neurons, suggesting that this subunit gene is regulated by neuronal activity.

Turkey cytochrome c oxidase contains subunit VIa of the liver type associated with low efficiency of energy transduction

Cytochrome c oxidase was isolated from turkey liver, heart and breast skeletal muscle and separated by SDS/PAGE. The N-terminal amino-acid sequence of subunit VIa from all tissues and internal sequences from the skeletal muscle enzyme show homology to the mammalian liver-type subunit VIaL, which was verified by isolation and sequencing of the cDNA of turkey subunit VIa. No cDNA corresponding to subunit VIaH (mammalian heart-type) could be found by RACE-PCR with mRNA from all turkey tissues. Measurement of proton translocation with the reconstituted enzymes from turkey liver and heart revealed H+/e- ratios below 0.5 that were independent of the intraliposomal ATP/ADP ratio, as previously found with the bovine liver enzyme. Under identical conditions, the bovine heart enzyme revealed H+/e- ratios of 0.85 at low and 0.48 at high intraliposomal ATP/ADP ratios. The results suggest that in birds the lower H+/e-ratio of cytochrome c oxidase participates in elevated resting metabolic rate and thermogenesis.

A second mechanism of respiratory control

According to the chemosmotic hypothesis, ATP is synthesized in mitochondria, bacteria and chloroplasts via the proton motive force delta p, the energy-rich intermediate of electron transport and photosynthetic phosphorylation. The general applicability of the chemosmotic hypothesis, however, was disputed until present. In particular the relationship between the rate of respiration and delta p in mitochondria was found variable, depending on the experimental conditions. Recently, a new mechanism of respiratory control was found, based on binding of ATP or ADP to subunit IV of cytochrome c oxidase, which is independent of delta p and could explain many previous results contradicting the chemosmotic hypothesis.

Structural organization and transcription regulation of nuclear genes encoding the mammalian cytochrome c oxidase complex

Cytochrome c Oxidase (COX) is the terminal component of the bacterial as well as the mitochondrial respiratory chain complex that catalyzes the conversion of redox energy to ATP. In eukaryotes, the oligomeric enzyme is bound to mitochondrial innermembrane with subunits ranging from 7 to 13. Thus, its biosynthesis involves a coordinate interplay between nuclear and mitochondrial genomes. The largest subunits, I, II, and III, which represent the catalytic core of the enzyme, are encoded by the mitochondrial DNA and are synthesized within the mitochondria. The rest of the smaller subunits implicated in the regulatory function are encoded on the nuclear DNA and imported into mitochondria following their synthesis in the cytosol. Some of the nuclear coded subunits are expressed in tissue and developmental specific isologs. The ubiquitous subunits IV, Va, Vb, VIb, VIc, VIIb, VIIc, and VIII (L) are detected in all the tissues, although the mRNA levels for the individual subunits vary in different tissues. The tissue specific isologs VIa (H), VIIa (H), and VIII (H) are exclusive to heart and skeletal muscle. cDNA sequence analysis of nuclear coded subunits reveals 60 to 90% conservation among species both at the amino acid and nucleotide level, with the exception of subunit VIII, which exhibits 40 to 80% interspecies homology. Functional genes for COX subunits IV, Vb, VIa 'L' & 'H', VIIa 'L' & 'H', VIIc and VIII (H) from different mammalian species and their 5' flanking putative promoter regions have been sequenced and extensively characterized. The size of the genes range from 2 to 10 kb in length. Although the number of introns and exons are identical between different species for a given gene, the size varies across the species. A majority of COX genes investigated, with the exception of muscle-specific COXVIII(H) gene, lack the canonical 'TATAA' sequence and contain GC-rich sequences at the immediate upstream region of transcription start site(s). In this respect, the promoter structure of COX genes resemble those of many house-keeping genes. The ubiquitous COX genes show extensive 5' heterogeneity with multiple transcription initiation sites that bind to both general as well as specialized transcription factors such as YY1 and GABP (NRF2/ets). The transcription activity of the promoter in most of the ubiquitous genes is regulated by factors binding to the 5' upstream Sp1, NRF1, GABP (NRF2), and YY1 sites. Additionally, the murine COXVb promoter contains a negative regulatory region that encompasses the binding motifs with partial or full consensus to YY1, GTG, CArG, and ets. Interestingly, the muscle-specific COX genes contain a number of striated muscle-specific regulatory motifs such as E box, CArG, and MEF2 at the proximal promoter regions. While the regulation of COXVIa (H) gene involves factors binding to both MEF2 and E box in a skeletal muscle-specific fashion, the COXVIII (H) gene is regulated by factors binding to two tandomly duplicated E boxes in both skeletal and cardiac myocytes. The cardiac-specific factor has been suggested to be a novel bHLH protein. Mammalian COX genes provide a valuable system to study mechanisms of coordinated regulation of nuclear and mitochondrial genes. The presence of conserved sequence motifs common to several of the nuclear genes, which encode mitochondrial proteins, suggest a possible regulatory function by common physiological factors like heme/O2/carbon source. Thus, a well-orchestrated regulatory control and cross talks between the nuclear and mitochondrial genomes in response to changes in the mitochondrial metabolic conditions are key factors in the overall regulation of mitochondrial biogenesis.

Downregulation of oxidative phosphorylation in Alzheimer disease: loss of cytochrome oxidase subunit mRNA in the hippocampus and entorhinal cortex

Messenger RNA (mRNA) for cytochrome oxidase subunit II (COX II) was localized by in situ hybridization in the entorhinal cortex and hippocampal formation of postmortem brain tissue from normal human subjects and from patients with Alzheimer disease (AD). In the control entorhinal cortex, COX II mRNA was detected mainly in neuronal cell bodies of layers II and IV. In control hippocampal formation, highest levels were localized in neuronal cell bodies of the dentate gyrus and the CA3 and CA1 regions, neurons that are involved in the major input and output pathways of the hippocampal formation. In AD brain, COX II mRNA was markedly reduced in the entorhinal cortex and the hippocampal formation compared with control brain. In the AD hippocampal formation, reductions were in regions severely affected by AD pathology as well as in regions that were relatively spared. These results are consistent with the hypothesis that reduced mitochondrial energy metabolism reflects loss of neuronal connections in AD.

Human xenomitochondrial cybrids. Cellular models of mitochondrial complex I deficiency

The subunits forming the mitochondrial oxidative phosphorylation system are coded by both nuclear and mitochondrial genes. Recently, we attempted to introduce mtDNA from non-human apes into a human cell line lacking mtDNA (rho degrees), and succeeded in producing human-common chimpanzee, human-pigmy chimpanzee, and human-gorilla xenomitochondrial cybrids (HXC). Here, we present a comprehensive characterization of oxidative phosphorylation function in these cells. Mitochondrial complexes II, III, IV, and V had activities indistinguishable from parental human or non-human primate cells. In contrast, a complex I deficiency was observed in all HXC. Kinetic studies of complex I using decylubiquinone or NADH as limiting substrates showed that the Vmax was decreased in HXC by approximately 40%, and the Km for the NADH was significantly increased (3-fold, p < 0.001). Rotenone inhibition studies of intact cell respiration and pyruvate-malate oxidation in permeabilized cells showed that 3 nM rotenone produced a mild effect in control cells (0-10% inhibition) but produced a marked inhibition of HXC respiration (50-75%). Immunoblotting analyses of three subunits of complex I (ND1, 75 and 49 kDa) showed that their relative amounts were not significantly altered in HXC cells. These results establish HXC as cellular models of complex I deficiency in humans and underscore the importance of nuclear and mitochondrial genomes co-evolution in optimizing oxidative phosphorylation function.

Variations in the subunit content and catalytic activity of the cytochrome c oxidase complex from different tissues and different cardiac compartments

The composition and activity of cytochrome c oxidase (COX) was studied in mitochondria from rat liver, brain, kidney and heart and also in different compartments of the bovine heart to see whether any correlation exists between known oxidative capacity and COX activity. Immunoblot analysis showed that the levels of ubiquitously expressed subunits IV and Vb are about 8-12-fold lower in liver mitochondria as compared to the heart, kidney and brain. The heart enzyme with higher abundance of COX IV and Vb showed lower turnover number (495) while the liver enzyme with lower abundance of these subunits exhibited higher turnover number of 750. In support of the immunoblot results, immunohistochemical analysis of heart and kidney tissue sections showed an intense staining with the COX Vb antibody as compared to the liver sections. COX Vb antibody stained certain tubular regions of the kidney more intensely than the other regions suggesting region specific variation in the subunit level. Bovine heart compartments showed variation in subunit levels and also differed in the kinetic parameters of COX. The right atrium contained relatively more Vb protein, while the left ventricle contained higher level of subunit VIa. COX from both the ventricles showed high Km for cytochrome c (23-37 microM) as compared to the atrial COX (Km 8-15 microM). These results suggest a correlation between tissue specific oxidative capacity/work load and changes in subunit composition and associated changes in the activity of COX complex. More important, our results suggest variations based on the oxidative load of cell types within a tissue.

Hashimoto thyroiditis is associated with defects of cytochrome-c oxidase in oxyphil Askanazy cells and with the common deletion (4,977) of mitochondrial DNA

The activity of cytochrome-c oxidase, the terminal enzyme of the respiratory chain (complex IV), was studied at the ultrastructural level in a case of Hashimoto thyroiditis. Cytochrome-c oxidase showed a heterogeneous reaction pattern in oxyphil cells, with scattered foci of oxyphil cells lacking cytochrome-c oxidase staining. In most of the cells the defect involved all the mitochondria, but there were also oxyphil cells with a heterogeneous mitochondrial population characterized by an intracellular coexistence of mitochondria with either intact cytochrome-c oxidase or lacking activity. Immunocytochemistry further disclosed loss of mitochondrially and nuclearly encoded subunits of the enzyme. Molecular genetic analysis of mitochondrial DNA (mtDNA) revealed the presence of the 4977 base pair deletion ("common deletion") of mtDNA (8,482-13,459) in the affected areas but not in normal thyroid tissue of the patient. The amount of deleted mtDNA varied between 2 and 8% of total mtDNA. The results demonstrate that oxyphil cell change in Hashimoto thyroiditis is associated with functional and molecular genetic defects of the respiratory chain.

Cloning, characterization, and chromosomal localization of human liver form cytochrome c oxidase subunit VIa related genes

The chromosomal location of human cytochrome c oxidase (COX) subunit VIa Liver (VIa-L) isoform related sequences has been determined by a combination of in situ hybridization and analysis of human-hamster somatic cell hybrid panels. COX VIa-L related sequences were present on chromosomes 6 and 12. It has been verified that at least two COX VIa-L genes are on chromosome 6, one of which is a pseudogene. In total, four COX VIa-L related sequences have been cloned and their nucleotide sequences analyzed. At least three of the sequences represent pseudogenes; their relatedness to the COX VIa-L cDNA is discussed.

Structural organization and promoter analysis of the bovine cytochrome c oxidase subunit VIIc gene. A functional role for YY1

Cytochrome c oxidase (COX) subunit VIIc is one of the nuclear encoded subunits of the 13-subunit holoenzyme that carries out the terminal step in the electron transport chain. We have isolated the gene for this subunit, previously shown to be ubiquitously expressed from a single copy gene in the genome, and show that 167 base pairs of DNA surrounding the transcriptional start site contain the minimal promoter of this gene. This basal promoter contains two YY1 sites and at least one site for NRF-2, which show binding to their cognate factors. Mutation of both YY1 sites eliminates most of the promoter activity. Mutation at the upstream YY1 site significantly reduces the efficiency of transcript initiation at the major start site and thus plays the dominant role in COX7C regulation. COX7C is, thus, the second nuclear gene of COX that is regulated by YY1, suggesting that it is a third common factor, along with NRF-1 and NRF-2, to be associated with COX gene regulation.

Activity-dependent regulation of cytochrome b gene expression in monkey visual cortex

The recent demonstration that certain mitochondrial subunits of cytochrome oxidase (CO) are regulated by neuronal activity has stimulated interest in the molecular processes that coordinate nuclear and mitochondrial gene expression following synaptic stimulation. We have studied the constitutive expression and activity-guided regulation of cytochrome b (cyt b), a gene that is encoded by mitochondrial DNA and that was cloned by subtractive hybridization from the lateral geniculate nucleus in the monkey. We have found cyt b mRNA expression in monkey striate cortex to be similar to that of CO activity with regard to the laminar profile and the presence of blobs in the supragranular layers. Layers 2/3, 4C, and 6 contained large numbers of stained cells, many of which were judged to be excitatory neurons, because they showed a Zif268-immunopositive nucleus. We have also found that removal of functional activity reduced cyt b mRNA content in area V1. Columns of reduced cyt b staining were visible after 3 days and were especially striking after 6 days of monocular deprivation. After 3 months of deprivation, the columns lost their contrast and became blurred. Our principal finding, that neuronal activity regulates cyt b levels, suggests that expression of a mitochondrial gene can be affected in a manner similar to that of several known nuclear genes. The differences in cyt b mRNA levels and CO activity after long-term deprivation suggests that some form of differential control is exerted on cyt b. Cyt b expression, therefore, may be used as a marker of altered mitochondrial transcription that is guided by the metabolic demands of active neurons.

Brain cytochrome oxidase subunit complementary DNAs: isolation, subcloning, sequencing, light and electron microscopic in situ hybridization of transcripts, and regulation by neuronal activity

The goal of the present study was to isolate, for the first time, cytochrome oxidase subunit genes from murine brain complementary DNA library and to characterize the expression of these genes from mitochondrial and nuclear sources at both light and electron microscopic levels. Brain subunit III (mitochondrial) shared 100% identity with that of murine L cells. Subunit VIa (nuclear) was known to have tissue-specific isoforms in other species: the ubiquitous liver isoform and the heart/muscle isoform. Our brain subunit VIa shared 93% homology with that of the rat liver and 100% identity with the recently reported murine liver isoform, which is only 62% identical to that of the rat heart isoform. In situ hybridization with riboprobes revealed messenger RNA labelling that was similar, though not identical, to that of cytochrome oxidase histochemistry. Monocular enucleation in adult mice induced a significant down-regulation of both subunit messages in the contralateral lateral geniculate nucleus. However, the decrease in subunit III messenger RNAs surpassed that of subunit VIa at all time periods examined, suggesting that mitochondrial gene expression is more tightly regulated by neuronal activity than that of nuclear ones. At the electron microscopic level, subunit III messenger RNA was localized to the mitochondrial compartment in both cell bodies and processes, while that of nuclear-encoded subunit VIa was present exclusively in the extramitochondrial compartment of somata and not of dendrites or axons. Surprisingly, the message was primarily associated with the rough endoplasmic reticulum, suggesting a novel pathway for its synthesis and trafficking. Our results indicate that the unique properties of neurons impose special requirements for subunits of a single mitochondrial enzyme with dual genomic origins. At sites of high energy demands (such as postsynaptic dendrites and some axon terminals), mitochondrial-encoded cytochrome oxidase subunits can be locally transcribed and translated, and they provide the framework for the subsequent importation and incorporation of nuclear-encoded subunits, which are strictly synthesized in the cell bodies. Dynamic local energy needs are met when subunits from the two genomic sources are assembled to form functional holoenzymes.

Quantitative decrease of human cytochrome c oxidase during development: evidences for a post-transcriptional regulation

In an earlier study, we showed that cytochrome c oxidase activity, measured in mitochondria isolated from human muscular biopsies, decreased steadily and substantially between the age of four years and adulthood (P < 0.05), whereas complexes I and III activity remained constant. The present study investigates a number of possible causes for this change in activity: although there is a drop in the apparent Vmax, neither the apparent enzyme Km, nor the cellular mtDNA concentration shows any variations over the studied period. Steady-state concentrations of mitochondrial gene transcripts (CO I. CO II, CO III, but also 12S, cytochrome b, or ND4) increase within this age group, indicating an overall increase in mitochondrial genome expression. Concentrations of transcripts of nuclear genes CO IV, CO Vb, and CO VIaH likewise show an increase, albeit less marked. On the other hand, heme aa3 levels and concentrations of mitochondrial (CO II) or nuclear (CO IV, CO VIIaH) subunits, estimated using specific antibodies, correlate closely with enzymatic activity and show a parallel decrease between 4 and 20 years. The observed decrease in complex IV activity is thus quantitative, and subject to post-transcriptional and/or post-translational regulation.

Sequence of the cDNA for the liver/non-muscle isoform of mouse cytochrome-c oxidase subunit VIII

We have isolated and sequenced the cDNA for the liver (L) or non-muscle isoform of mouse cytochrome-c oxidase subunit VIII (COX VIII-L). Comparison of deduced COX VIII-L protein sequences from three mammalian species indicated that the human gene has sustained more amino acid replacement substitutions than either the mouse or the cow. The most highly conserved regions of this subunit are the N-terminal presequence and the C-terminal domain of the mature protein.

Mitochondrial- and nuclear-encoded subunits of cytochrome oxidase in neurons: differences in compartmental distribution, correlation with enzyme activity, and regulation by neuronal activity

Cytochrome oxidase (CO), a mitochondrial energy-generating enzyme, contains both mitochondrial- and nuclear-encoded subunits. In neurons, local levels of CO activity vary among different neuronal compartments, reflecting local demands for energy. The goals of the present study were to determine if compartmental distribution of CO subunit proteins from the two genomes was correlated with local CO activity, and if their expression was regulated proportionately in neurons. The subcellular distributions of mitochondrial-encoded CO III and nuclear-encoded CO Vb proteins were quantitatively analyzed in mouse cerebellar sections subjected to postembedding immunocytochemistry. Local levels of subunit proteins were also compared to local CO activity, as revealed by CO cytochemistry. In order to study the regulation of subunit protein expression, we assessed changes in immunoreactivity of the two CO subunits as well as changes in CO activity in mouse superior colliculus after 1 to 7 days of monocular enucleation. We found that immunoreaction product for both CO III and CO Vb existed almost exclusively in mitochondria, but their compartmental distributions were different. CO III was nonhomogeneously distributed among different neuronal compartments, where its local level was positively correlated with that of CO activity. In contrast, the subcellular distribution of CO Vb was relatively uniform and did not bear a direct relationship with that of CO activity. Moreover, the two subunit proteins were disproportionately regulated by neuronal activity. CO III and CO activity exhibited parallel decreases after the deprivation of afferent input, and their changes were earlier and to a greater degree than that of CO Vb proteins. Thus, the present findings indicate that the local expression and/or distribution of CO subunit proteins from the two genomes may involve different regulatory mechanisms in neurons. Our data also suggest that the activity-dependent regulation of mitochondrial-encoded CO subunits is likely to play a major role in controlling the local levels of CO content and its activity.

Biochemical analysis of fibroblasts from patients with cytochrome c oxidase-associated Leigh syndrome

Cultured skin fibroblasts from four patients with Leigh syndrome and cytochrome c oxidase deficiency were studied. Mitochondrial DNA (mtDNA) analysis excluded large-scale deletions and known point mutations associated with Leigh syndrome. The COX activities were reduced to 18-44% of healthy probands, when measured in the presence of laurylmaltoside. COX activity from patients was shown to be more temperature sensitive than COX activity from control cells. In order to determine the subunit composition of COX immunoblotting studies were performed using mono- and polyclonal antibodies to distinct subunits. A monoclonal antibody to subunit IV crossreacted with two unknown proteins of higher apparent molecular weight in mitochondria from three patients, but not in mitochondria from control and the fourth patient. Quantification of immunoreactivity revealed a decrease of subunits II/III and IV parallel to the determined enzyme activity. In contrast, a variable amount of subunit VIIa (and/or VIIb) was found in mitochondria from different patients. The results indicate a defective COX holoenzyme complex in patients with Leigh syndrome and suggest different molecular origins of the defect.

Defects of the respiratory chain in oxyphil and chief cells of the normal parathyroid and in hyperfunction

Immunohistochemical detection of complex HIII (ubiquinone- cytochrome-c-oxidoreductase) and complex IV (cytochrome-c-oxidase) of the respiratory chain was performed in parathyroids of 164 humans with normal renal function (group I) and in 55 patients with chronic renal insufficiency (group II) obtained at autopsy. In group I, 33 of the 164 cases showed defects of the respiratory chain (20%). Eighty-five percent of the defects occurred in advanced age (> 50 years). In group II, 39 of 55 cases (70%) had defects, and about 70% of the defects occurred after age 50. In both groups, more than 80% of the defects were localized in oxyphil cell nodules. However, not every oxyphil nodule was involved. In group I, selective defects of complex IV predominated and were found in 47 of 86 defects (55%). Combined defects of complexes III and IV were present in 25 of 86 defects (29%). In contrast, in group II combined defects predominated and were found in 45% (107 of 240 defects), whereas single defects of complex IV existed in 38% (93 of 240 defects). The frequency of selective defects of complex III was about 16% to 17% in both groups. In situ hybridization and PCR studies for the detection of the common deletion (4.977 base pairs) and of various point mutations of mitochondrial of (m)DNA revealed no consistent molecular genetic abnormalities. A point mutation in the tRNALeu(UUR) at nucleotide (nt) 3.260 was found in only one probe. The results show that defects of the respiratory chain occur already in normal parathyroids, most probably during cell aging, especially in oxyphil cells and at a higher rate in hyperfunction. The high predominance of respiratory chain defects in oxyphil cells and their random distribution favors mutations of mtDNA as a possible cause of oxyphilic cell transformation and of the respiratory chain defects. However, the mutations of mtDNA in the parathyroids are apparently different from those in other ageing tissues.

Cytochrome c oxidase subunit VIIa liver isoform. Characterization and identification of promoter elements in the bovine gene

Cytochrome c oxidase subunit VIIa is specified by two nuclear genes, one (COX7AH) producing a heart/muscle-specific isoform and the other (COX7AL) a form expressed in all tissues. We have isolated both genes to examine their transcriptional regulation. Here, we characterize the core promoter of COX7AL and show that a 92-base pair region flanking the 5'-end promotes most of the activity of this gene. The 92-bp basal promoter contains sites for the nuclear respiratory factors NRF-1 and NRF-2, which have been shown to contribute to the transcription of a number of nuclear genes involved in mitochondrial respiratory activity, and also at least four Sp1 motifs. We show that both the NRF-1 and NRF-2 binding sites are functional in COX7AL and present evidence suggesting that interaction between the NRF-1 site and an upstream element contributes to expression.

Disorders of the electron transport chain

Defects in a pathway as complex as the electron transport chain cause a variety of clinical abnormalities, which vary from fatal lactic acidosis in infancy to mild muscle disease in adults. The primary defect may reside in the nucleus or the mitochondrial genome. Until relatively recently, biochemical assays were the definitive means of establishing a defect of the electron transport chain. However, identification of mtDNA abnormalities allows defects to be defined more precisely and in a number of cases provides an easier (more reliable) means of investigation. Despite advances in this field, disorders of the electron transport chain still remain underdiagnosed. This review attempts to provide a general outline of the biochemistry and molecular genetics associated with these disorders and some of the factors involved in establishing a diagnosis in those patients with a suspected defect of the electron transport chain.

Cloning and characterization of senC, a gene involved in both aerobic respiration and photosynthesis gene expression in Rhodobacter capsulatus

The purple nonsulfur photosynthetic eubacterium Rhodobacter capsulatus is a versatile organism that can obtain cellular energy by several means, including the capture of light energy for photosynthesis as well as the use of light-independent respiration, in which molecular oxygen serves as a terminal electron acceptor. In this study, we have identified and characterized a novel gene, senC, mutations in which affect respiration as well as the induction of photosynthesis gene expression. The protein coded by senC exhibits 33% sequence identity to the yeast nucleus-encoded protein SCO1, which is thought to be a mitochondrion-associated cytochrome c oxidase assembly factor. Like yeast SCO1, SenC is required for optimal cytochrome c oxidase activity in aerobically grown R. capsulatus cells. We further show that senC is required for maximal induction from the puf and puh operons, which encode the structural polypeptides of the light-harvesting and reaction center complexes.

Structural characterization and regulatory element analysis of the heart isoform of cytochrome c oxidase VIa

In order to investigate the mechanism(s) governing the striated muscle-specific expression of cytochrome c oxidase VIaH we have characterized the murine gene and analyzed its transcriptional regulatory elements in skeletal myogenic cell lines. The gene is single copy, spans 689 base pairs (bp), and is comprised of three exons. The 5'-ends of transcripts from the gene are heterogeneous, but the most abundant transcript includes a 5'-untranslated region of 30 nucleotides. When fused to the luciferase reporter gene, the 3.5-kilobase 5'-flanking region of the gene directed the expression of the heterologous protein selectively in differentiated Sol8 cells and transgenic mice, recapitulating the pattern of expression of the endogenous gene. Deletion analysis identified a 300-bp fragment sufficient to direct the myotube-specific expression of luciferase in Sol8 cells. The region lacks an apparent TATA element, and sequence motifs predicted to bind NRF-1, NRF-2, ox-box, or PPAR factors known to regulate other nuclear genes encoding mitochondrial proteins are not evident. Mutational analysis, however, identified two cis-elements necessary for the high level expression of the reporter protein: a MEF2 consensus element at -90 to -81 bp and an E-box element at -147 to -142 bp. Additional E-box motifs at closely located positions were mutated without loss of transcriptional activity. The dependence of transcriptional activation of cytochrome c oxidase VIaH on cis-elements similar to those found in contractile protein genes suggests that the striated muscle-specific expression is coregulated by mechanisms that control the lineage-specific expression of several contractile and cytosolic proteins.

Isolation and characterization of the functional gene encoding bovine cytochrome c oxidase subunit IV

The structure and expression of the gene (COX4) encoding bovine cytochrome c oxidase subunit IV (COX IV) was studied in order to identify conserved DNA sequence elements involved in the control of mammalian nuclear respiratory genes. The functional bovine COX4 gene consists of five exons and four introns and is similar in organization to rat and mouse COX4. The domain encoded by exon 3 is the most highly conserved among the three species, suggesting it may encode a key functional domain of COX IV. Transcription of bovine COX4 begins at multiple sites, as has been seen previously for rat and mouse COX4 and other TATA-less genes. Comparative analysis of bovine, rat and mouse COX4 promoters identified multiple binding sites for the regulatory proteins Sp1 and GABP (NRF-2). The varied arrangements of multiple Sp1 and GABP sites in mammalian COX4 promoters suggests flexibility in the positioning of regulatory factors in controlling COX4 expression.

Structure at 2.8 A resolution of cytochrome c oxidase from Paracoccus denitrificans

The crystal structure at 2.8 A resolution of the four protein subunits containing cytochrome c oxidase from the soil bacterium Paracoccus denitrificans, complexed with antibody Fv fragment, is described. Subunit I contains 12 membrane-spanning, primarily helical segments and binds haem a and the haem a3-copper B binuclear centre where molecular oxygen is reduced to water. Two proton transfer pathways, one for protons consumed in water formation and one for 'proton pumping', could be identified. Mechanisms for proton pumping are discussed.

Kinetics and mechanism for the binding of HCN to cytochrome c oxidase

The kinetics of cyanide binding to cytochrome c oxidase were systematically studied as a function of [HCN], [oxidase], pH, ionic strength, temperature, type and concentration of solubilizing detergent, and monomer-dimer content of oxidase. On the basis of these results a minimum reaction mechanism is proposed in which the spectrally visible rapid and slow cyanide binding reactions are two consecutive first-order reactions, not parallel reactions with different conformers of cytochrome c oxidase. The fast reaction (k'obs) follows saturation type kinetics to form an HCN complex that subsequently undergoes a slow reaction (k'obs). The fast k'obs reaction is independent of ionic strength but is strongly dependent upon pH. Two pK values were evaluated from the bell-shaped rate versus pH profile; one is due to an ionizable group on the protein (pKa = 7.45), while the other is that of HCN (pKHCN = 9.15). Therefore, oxidase is reactive toward HCN only when the group on the protein is unprotonated. The slow k'obs reaction is not a reaction of oxidase with either CN- or HCN; in fact, the product formed by the fast k'obs reaction, the oxidase-HCN complex, still undergoes the slow k" process even if all of the excess KCN is removed. The apparent rate constant of the slower phase (k"obs) is independent of all the variations done in this study, and it probably corresponds to either a slow conformational change in the protein or a change in ligand coordination at one of the metal centers after HCN binds to the bimetallic center of oxidase. Based upon the bell-shaped pH dependence of the fast phase and the pH independence of the slow phase, the mechanism also predicts that a single conformer of cytochrome c oxidase can exhibit either monophasic or biphasic cyanide binding kinetics depending upon the pH. At either very low or very high pH, the two rates become comparable in magnitude, which makes the reaction appear to be monophasic even though both reactions still occur. The amount of monomeric or dimeric oxidase only slightly affects the magnitude of k'obs and k"obs values, and both processes are clearly present in both types of oxidase.

Disproportionate regulation of nuclear- and mitochondrial-encoded cytochrome oxidase subunit proteins by functional activity in neurons

Cytochrome oxidase is the terminal enzyme in the mitochondrial respiratory chain engaged in oxidative metabolism and energy production. In mammals, the holoenzyme is composed of 13 subunits encoded by both nuclear and mitochondrial genomes. The goal of the present study was to compare the effect of afferent impulse blockade on the expression of these two genomes at the subunit protein level. It also aimed to determine the correlation between the level of cytochrome oxidase activity and the relative amount of subunit proteins. Relative enzyme activity was analysed histochemically, and relative amounts of subunits IV (nuclear-encoded) and II/III (mitochondrial-derived) proteins were obtained immunohistochemically by anti-subunit IV and anti-subunit II/III antibodies in the lateral geniculate nucleus and the primary visual cortex of adult monkeys. In the normal visual centers, similar staining patterns were found for all three markers. After three and seven days of tetrodotoxin treatment, levels of enzyme activity and subunit proteins declined disproportionately in the deprived laminae of the visual center. Densitometric analysis indicates that changes in enzyme activity and subunit IV proteins were significantly greater than those of subunit II/III proteins (P < 0.01). The finding that nuclear and mitochondrial genomes are disproportionately regulated at subunit protein levels by neuronal activity implies that the two genomes operate under different regulatory mechanisms. Changes in subunit IV paralleled most closely those of cytochrome oxidase activity (coefficient of determination r2 = 0.95). This suggests that nuclear-derived subunit IV protein may play a pivotal role in controlling cytochrome oxidase holoenzyme activity.

Regulation of mitochondrial energy generation in health and disease

In mammalian cytochrome c oxidase (COX) three of the ten nuclear coded subunits (VIa, VIIa, VIII) occur in tissue-specific isoforms. The isoform distribution, however, varies in liver and heart of different species. Subunit VIII is different in liver and heart of bovine, dog, rat and chicken, but identical in human (liver-type) on one hand, and sheep, rabbit and rainbow trout (heart-type) on the other hand, as determined by N-terminal sequencing. Two moles of trinitrophenyl-ATP bind to monomeric COX from bovine heart and one to COX from bovine liver with dissociation equilibrium constant (Kd) values of about 3 microM. One binding site at the heart enzyme is blocked by a monoclonal antibody to subunit VIa-H. ATP (and/or ADP) interact with COX at two or three high-affinity binding sites, as shown by titration of the spectral changes of COX. Isolated COX from bovine heart was reconstituted with variable intraliposomal ATP/ADP ratios. By measuring the RCR (respiratory control ratio) and RCRVal (related to the valinomycin-respiration), which is a direct measure of the H+/e(-)-stoichiometry (Wilson and Prochaska, Arch. Biochem. Biophys. 282 (1990) 413-420), almost complete inhibition of the proton pump activity of COX by high intraliposomal ATP concentrations was found. The vectorial of protons for the formation of water, however, appears to be unaffected by nucleotides. This regulatory mechanism is assumed to have physiological significance for thermogenesis in muscle at rest. COX of fibroblasts from patients suffering from Leigh's syndrome, which is associated with a decreased COX activity, are suggested to have an incompletely assembled enzyme complex. This suggestion is further corroborated by the higher temperature-sensitivity of the enzyme when compared with COX from normal control fibroblasts. Defective regulation of COX via nuclear coded subunits is also proposed to cause mitochondrial diseases.

Sequence of the cDNA for the heart/muscle isoform of mouse cytochrome c oxidase subunit VIII

We have isolated and sequenced cDNAs for the heart/muscle (H) isoform of mouse cytochrome c oxidase subunit VIII (COX VIII-H). The deduced protein sequence enables us to compare the heart/muscle COX VIII isoforms from several species and to determine that the most highly conserved region of this subunit is the C-terminal domain.

Expression of cytochrome c oxidase during growth and development of Dictyostelium

In the slime mold Dictyostelium discoideum, the subunit composition of cytochrome c oxidase depends on oxygen that inversely regulates the concentrations of two alternative isoforms of the smallest enzyme subunit (Schiavo, G., and Bisson, R. (1989) J. Biol. Chem. 264, 7129-7134). In order to investigate their role in the Dictyostelium life cycle, the expression of the oxidase subunits was monitored during cell growth and development. The results obtained demonstrate that exponentially growing amoebae respond rapidly and precisely to hypoxia by switching the expression of the two isoforms and also by increasing the levels of the mRNAs of the different oxidase subunits in a highly coordinated process. During normal development the "hypoxic" subunit is not synthesized, but its level of expression appears to parallel the sensitivity to oxygen of development, rising steeply below 10% oxygen when the differentiation program is virtually blocked. Under these conditions, the expression of the alternative subunit isoform is essentially oxygen-insensitive. These findings suggest that the physiological relevance of the subunit switching concerns primarily the vegetative phase of growth, possibly as part of a more general mechanism evolved in order to evade conditions that do not allow development. Taken together, the data obtained offer an intriguing example of the fine control exerted on the expression of a key respiratory enzyme in a strictly aerobic organism.

Structure and function of a molecular machine: cytochrome c oxidase

Cytochrome c is responsible for over 90% of the dioxygen consumption in the living cell and contributes to the build-up of a proton electrochemical gradient derived by the vectorial transfer of electrons between cytochrome c and molecular oxygen. The metal ions found in cytochrome oxidases play a crucial role in these processes and have been extensively studied. In this review we present and discuss some of the relevant spectroscopic and kinetic properties of the prosthetic groups of cytochrome c oxidase.

Immunohistochemical analysis of muscle cytochrome c oxidase deficiency in children

Despite the demonstration of a clear biochemical defect, the genetic alterations causing childhood forms of cytochrome c oxidase (COX) deficiency remain unknown. The double genetic origin (nuclear and mitochondrial DNA), and the complexity of COX enzyme structure and regulation, indicate the need for genetic investigations of the molecular structure of individual COX subunits. In the present study a new monoclonal antibody, which reacts exclusively with heart-type human COX subunit VIIa (VIIa-H), and other monoclonal antibodies against human COX subunits, were used in the immunohistochemical analysis of skeletal muscle from children with different forms of mitochondrial myopathy with COX deficiency. By immunohistochemical investigation a normal reaction was seen with antibodies to COX subunits IV, Va+Vb, and VIa+VIc in all four cases, and in two cases with antibodies to COX VIIa-H and VIIa+VIIb. In muscle from a fatal infantile case with cardiac and skeletal muscle involvement, no immunohistochemical reaction was seen with the monoclonal antibody against the tissue-specific subunit VIIa-H. In muscle from an 11-year-old boy with exclusive muscular symptoms and signs, immunohistological reactions were absent with COX subunit VIIa-H and COX subunits VIIa+VIIb, and slightly decreased with COX subunit II, thus demonstrating a different molecular mechanism in each case. It is concluded that the molecular basis of COX deficiency in childhood may vary greatly between patients.

Mitochondrial biogenesis in early mouse embryos: expression of the mRNAs for subunits IV, Vb, and VIIc of cytochrome c oxidase and subunit 9 (P1) of H(+)-ATP synthase

The mouse egg contains about 90,000 mitochondria which undergo a buildup of mitochondrial cristae and increase in respiratory activity during cleavage. The mitochondrial DNA does not replicate during preimplantation development but is transcribed actively from the two-cell stage onward (Pikó and Taylor, 1987: Dev Biol 123:364-374). To gain further insight into mitochondrial biogenesis, we have now determined the steady state amounts of the mRNAs for the cytochrome c oxidase (COX) subunits IV, Vb and VIIc and the H(+)-ATPase subunit 9 (P1) (all encoded by nuclear genes) in slot hybridization experiments of total RNA from oocytes and early embryos. All four mRNAs showed a similar developmental pattern of prevalence, characterized by a steady decline in mRNA copy numbers from the late growth-phase oocyte through the two-cell embryo, and an about 30-fold rise during cleavage through the blastocyst stage. However, the ATPase subunit 9 (P1) mRNA was about three times more prevalent in cleavage-stage embryos than the COX mRNAs. A similar pattern was obtained previously for the mitochondrial-encoded COX I and II mRNAs, but the latter accumulate at a 30-50-fold excess over the nuclear-encoded COX subunit mRNAs during the cleavage stages. The results suggest a coordinated activation and transcription of the mitochondrial and nuclear genes for the components of the respiratory apparatus beginning with the two-cell stage. It is estimated that new respiratory chains are produced at a rate of 50-100 chains hr-1/mitochondrion in the early blastocyst, accounting for 3.5-7% of the total protein synthetic activity at this stage.(ABSTRACT TRUNCATED AT 250 WORDS)