Cytochrome c oxidase subunit VIIa isoforms. Characterization and expression of bovine cDNAs

Deficiency of COX7RP, a mitochondrial supercomplex assembly promoting factor, lowers blood glucose level in mice

Mitochondria are essential organelles to efficiently produce ATP by ATP-synthase, which uses a proton-gradient generated by respiratory chain complexes. We previously demonstrated that COX7RP/COX7A2L/SCAF1 is a key molecule that promotes respiratory supercomplex assembly and regulates energy generation. The contribution of COX7RP to metabolic homeostasis, however, remains to be clarified. In the present study, we showed a metabolic phenotype of Cox7rp knockout (Cox7rpKO) mice, which exhibit lower blood glucose levels after insulin or pyruvate injection. Notably, ATP synthesis rate was reduced in Cox7rpKO mice liver, in accordance with decreased percentages of complex III subunit RISP and complex IV subunit COX1 involved in I + III + IV supercomplex fraction. The present findings suggest that COX7RP-mediated mitochondrial respiration plays crucial roles in the regulation of glucose homeostasis and its impairment will lead to the pathophysiology of metabolic states.

COX7AR is a Stress-inducible Mitochondrial COX Subunit that Promotes Breast Cancer Malignancy

Cytochrome c oxidase (COX), the terminal enzyme of the mitochondrial respiratory chain, plays a key role in regulating mitochondrial energy production and cell survival. COX subunit VIIa polypeptide 2-like protein (COX7AR) is a novel COX subunit that was recently found to be involved in mitochondrial supercomplex assembly and mitochondrial respiration activity. Here, we report that COX7AR is expressed in high energy-demanding tissues, such as brain, heart, liver, and aggressive forms of human breast cancer cells. Under cellular stress that stimulates energy metabolism, COX7AR is induced and incorporated into the mitochondrial COX complex. Functionally, COX7AR promotes cellular energy production in human mammary epithelial cells. Gain- and loss-of-function analysis demonstrates that COX7AR is required for human breast cancer cells to maintain higher rates of proliferation, clone formation, and invasion. In summary, our study revealed that COX7AR is a stress-inducible mitochondrial COX subunit that facilitates human breast cancer malignancy. These findings have important implications in the understanding and treatment of human breast cancer and the diseases associated with mitochondrial energy metabolism.

The brown and brite adipocyte marker Cox7a1 is not required for non-shivering thermogenesis in mice

The cytochrome c oxidase subunit isoform Cox7a1 is highly abundant in skeletal muscle and heart and influences enzyme activity in these tissues characterised by high oxidative capacity. We identified Cox7a1, well-known as brown adipocyte marker gene, as a cold-responsive protein of brown adipose tissue. We hypothesised a mechanistic relationship between cytochrome c oxidase activity and Cox7a1 protein levels affecting the oxidative capacity of brown adipose tissue and thus non-shivering thermogenesis. We subjected wildtype and Cox7a1 knockout mice to different temperature regimens and tested characteristics of brown adipose tissue activation. Cytochrome c oxidase activity, uncoupling protein 1 expression and maximal norepinephrine-induced heat production were gradually increased during cold-acclimation, but unaffected by Cox7a1 knockout. Moreover, the abundance of uncoupling protein 1 competent brite cells in white adipose tissue was not influenced by presence or absence of Cox7a1. Skin temperature in the interscapular region of neonates was lower in uncoupling protein 1 knockout pups employed as a positive control, but not in Cox7a1 knockout pups. Body mass gain and glucose tolerance did not differ between wildtype and Cox7a1 knockout mice fed with high fat or control diet. We conclude that brown adipose tissue function in mice does not require the presence of Cox7a1.

A stabilizing factor for mitochondrial respiratory supercomplex assembly regulates energy metabolism in muscle

The mitochondrial respiratory chain is essential for oxidative phosphorylation and comprises multiple complexes, including cytochrome c oxidase, assembled in macromolecular supercomplexes. Little is known about factors that contribute to supercomplex organization. Here we identify COX7RP as a factor that promotes supercomplex assembly. Cox7rp-knockout mice exhibit decreased muscular activity and heat production failure in the cold due to reduced COX activity. In contrast, COX7RP-transgenic mice exhibit increased exercise performance with increased cytochrome c oxidase activity. Two-dimensional blue native electrophoresis reveals that COX7RP is a key molecule that promotes assembly of the III2/IVn supercomplex with complex I. Our study identified COX7RP as a protein that functions in I/III2/IVn supercomplex assembly and is required for full activity of mitochondrial respiration.

Cytochrome c oxidase dysfunction in oxidative stress

Cytochrome c oxidase (CcO) is the terminal oxidase of the mitochondrial electron transport chain. This bigenomic enzyme in mammals contains 13 subunits of which the 3 catalytic subunits are encoded by the mitochondrial genes. The remaining 10 subunits with suspected roles in the regulation, and/or assembly, are coded by the nuclear genome. The enzyme contains two heme groups (heme a and a3) and two Cu(2+) centers (Cu(2+) A and Cu(2+) B) as catalytic centers and handles more than 90% of molecular O(2) respired by the mammalian cells and tissues. CcO is a highly regulated enzyme which is believed to be the pacesetter for mitochondrial oxidative metabolism and ATP synthesis. The structure and function of the enzyme are affected in a wide variety of diseases including cancer, neurodegenerative diseases, myocardial ischemia/reperfusion, bone and skeletal diseases, and diabetes. Despite handling a high O(2) load the role of CcO in the production of reactive oxygen species still remains a subject of debate. However, a volume of evidence suggests that CcO dysfunction is invariably associated with increased mitochondrial reactive oxygen species production and cellular toxicity. In this paper we review the literature on mechanisms of multimodal regulation of CcO activity by a wide spectrum of physiological and pathological factors. We also review an array of literature on the direct or indirect roles of CcO in reactive oxygen species production.

Cytochrome c oxidase: evolution of control via nuclear subunit addition

According to theory, present eukaryotic cells originated from a beneficial association between two free-living cells. Due to this endosymbiotic event the pre-eukaryotic cell gained access to oxidative phosphorylation (OXPHOS), which produces more than 15 times as much ATP as glycolysis. Because cellular ATP needs fluctuate and OXPHOS both requires and produces entities that can be toxic for eukaryotic cells such as ROS or NADH, we propose that the success of endosymbiosis has largely depended on the regulation of endosymbiont OXPHOS. Several studies have presented cytochrome c oxidase as a key regulator of OXPHOS; for example, COX is the only complex of mammalian OXPHOS with known tissue-specific isoforms of nuclear encoded subunits. We here discuss current knowledge about the origin of nuclear encoded subunits and the appearance of different isozymes promoted by tissue and cellular environments such as hypoxia. We also review evidence for recent selective pressure acting on COX among vertebrates, particularly in primate lineages, and discuss the unique pattern of co-evolution between the nuclear and mitochondrial genomes. Finally, even though the addition of nuclear encoded subunits was a major event in eukaryotic COX evolution, this does not lead to emergence of a more efficient COX, as might be expected from an anthropocentric point of view, for the "higher" organism possessing large brains and muscles. The main function of these subunits appears to be "only" to control the activity of the mitochondrial subunits. We propose that this control function is an as yet under appreciated key point of evolution. Moreover, the importance of regulating energy supply may have caused the addition of subunits encoded by the nucleus in a process comparable to a "domestication scenario" such that the host tends to control more and more tightly the ancestral activity of COX performed by the mtDNA encoded subunits.

Identification of the mitochondrial glutamate transporter. Bacterial expression, reconstitution, functional characterization, and tissue distribution of two human isoforms

The mitochondrial carriers are a family of transport proteins in the inner membranes of mitochondria. They shuttle substrates, metabolites, and cofactors through this membrane and connect cytoplasm functions with others in the matrix. Glutamate is co-transported with H(+) (or exchanged for OH(-)), but no protein has ever been associated with this activity. Two human expressed sequence tags encode proteins of 323 and 315 amino acids with 63% identity that are related to the aspartate-glutamate carrier, a member of the carrier family. They have been overexpressed in Escherichia coli and reconstituted into phospholipid vesicles. Their transport properties demonstrate that the two proteins are isoforms of the glutamate/H(+) symporter described in the past in whole mitochondria. Isoform 1 is expressed at higher levels than isoform 2 in all the tissues except in brain, where the two isoforms are expressed at comparable levels. The differences in expression levels and kinetic parameters of the two isoforms suggest that isoform 2 matches the basic requirement of all tissues especially with respect to amino acid degradation, and isoform 1 becomes operative to accommodate higher demands associated with specific metabolic functions such as ureogenesis.

Genomic organization and promoter regulation of human cytochrome c oxidase subunit VII heart/muscle isoform (COX7AH)

We have isolated and characterized the human gene (COX7AH) for the contractile muscle isoform of cytochrome c oxidase (COX) subunit VIIa. This subunit is one of the 10 nuclear encoded subunits of the 13-subunit holoenzyme that carries out the terminal step in the electron transport chain. Using transient transfection assays, we have located a 5'-flanking region sufficient to direct high level, skeletal myotube-specific reporter gene expression. This 792 bp basal promoter, which contains the single transcription start but no canonical TATA or CCAAT boxes, contains one MEF2 site, three E boxes, and an Sp1 site that show binding to their cognate factors, and are all required for full expression. Mutation and transactivation analysis suggest that there is functional interaction between these binding sites.

Characterization of the murine gene for subunit VIIaL of cytochrome c oxidase

Mammalian cytochrome c oxidase consists of thirteen subunits, ten encoded by the nuclear genome and three by the mitochondrial DNA. In several species, two isoforms have been isolated for nuclear-encoded subunits VIa, VIIa and VIII: an ubiquitous L (liver) form and a heart- and skeletal-muscle specific H form. The gene for murine cytochrome c oxidase subunit VIIa-L (Cox7aL) and its promoter region were isolated, sequenced and analysed. The coding region is split in four exons spanning 4.1 kbp and the promoter carries potential binding sites for Sp1, NRF1 and NRF2 transcription factors. Transcriptional activity of the promoter in reporter assays suggested an ubiquitous expression in mouse tissues.

Mitochondrial energy metabolism is regulated via nuclear-coded subunits of cytochrome c oxidase

A new mechanism on regulation of mitochondrial energy metabolism is proposed on the basis of reversible control of respiration by the intramitochondrial ATP/ADP ratio and slip of proton pumping (decreased H+/e- stoichiometry) in cytochrome c oxidase (COX) at high proton motive force delta p. cAMP-dependent phosphorylation of COX switches on and Ca2+-dependent dephosphorylation switches off the allosteric ATP-inhibition of COX (nucleotides bind to subunit IV). Control of respiration via phosphorylated COX by the ATP/ADP ratio keeps delta p (mainly delta psi(m)) low. Hormone induced Ca2+-dependent dephosphorylation results in loss of ATP-inhibition, increase of respiration and delta p with consequent slip in proton pumping. Slip in COX increases the free energy of reaction, resulting in increased rates of respiration, thermogenesis and ATP-synthesis. Increased delta psi(m) stimulates production of reactive oxygen species (ROS), mutations of mitochondrial DNA and accelerates aging. Slip of proton pumping without dephosphorylation and increase of delta p is found permanently in the liver-type isozyme of COX (subunit VIaL) and at high intramitochondrial ATP/ADP ratios in the heart-type isozyme (subunit VIaH). High substrate pressure (sigmoidal v/s kinetics), palmitate and 3,5-diiodothyronine (binding to subunit Va) increase also delta p, ROS production and slip but without dephosphorylation of COX.

Isolation and sequence of the human cytochrome c oxidase subunit VIIaL gene

The gene for human cytochrome c oxidase subunit VIIa liver isoform (COX7AL) was isolated and its sequence determined and analyzed. The three introns of the gene are considerably larger than those of the heart isoform of subunit VIIa (COX7AH), but the position of the introns relative to the cDNA sequences is homologous between the two genes. Comparison with other isolated COX7AL genes suggests that the promoter region binding motifs for transcription factors have evolved along with the coding region. In fibroblasts cultured originally from a Leigh's disease patient, a shortened COX7AL cDNA was identified by RT-PCR, consisting of exon I joined to exon IV, omitting exons II and III. No mutation could be identified in COX7AL of the patient, suggesting that the shortened cDNA is due to an alteration of the genome during cell culture. A surprising transcription of COX7AH was observed in cultured fibroblasts, suggesting a potential utility of these cells for study of its gene expression.

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.

Expression in Escherichia coli, functional characterization, and tissue distribution of isoforms A and B of the phosphate carrier from bovine mitochondria

The two isoforms of the mammalian mitochondrial phosphate carrier (PiC), A and B, differing in the sequence near the N terminus, arise from alternative splicing of a primary transcript of the PiC gene (Dolce, V., Iacobazzi, V., Palmieri, F., and Walker, J. E. (1994) J. Biol. Chem. 269, 10451-10460). To date, the PiC isoforms A and B have not been studied at the protein level. To explore the tissue-distribution and the potential functional differences between the two isoforms, polyclonal site-directed antibodies specific for PiC-A and PiC-B were raised, and the two bovine isoforms were obtained by expression in Escherichia coli and reconstituted into phospholipid vesicles. Western blot analysis demonstrated that isoform A is present in high amounts in heart, skeletal muscle, and diaphragm mitochondria, whereas isoform B is present in the mitochondria of all tissues examined. Heart and liver bovine mitochondria contained 69 and 0 pmol of PiC-A/mg of protein, and 10 and 8 pmol of PiC-B/mg of protein, respectively. In the reconstituted system the pure recombinant isoforms A and B both catalyzed the two known modes of transport (Pi/Pi antiport and Pi/H+ symport) and exhibited similar properties of substrate specificity and inhibitor sensitivity. However, they strongly differed in their kinetic parameters. The transport affinities of isoform B for phosphate and arsenate were found to be 3-fold lower than those of isoform A. Furthermore, the maximum transport rate of isoform B is about 3-fold higher than that of isoform A. These results support the hypothesis that the sequence divergence between PiC-A and PiC-B may have functional significance in determining the affinity and the translocation rate of the substrate through the PiC molecule.

Transcriptional regulation of mammalian cytochrome c oxidase genes

The cytochrome c oxidase (COX) holoenzyme is a 13-subunit complex that carries out the terminal step in the electron transport chain. Three of the subunits, which contain the electron transfer function, are coded by mitochondrial DNA and the other ten subunits by nuclear DNA. Since the holoenzyme contains equivalent amounts of each subunit, we and others have examined transcriptional regulation of COX nuclear subunits to explore whether there is a common basis for co-regulation. Each gene is seen to have a unique pattern of recognition by regulatory factors; although some factors bind to more than one gene, not all COX genes seem to be regulated by the same set of factors. Current information about the COX promoters that have been examined is summarized, and the relation of promoter regulation to coordinate gene expression is discussed.

Tissue-specific expression and mapping of the Cox7ah gene in mouse

We have isolated and examined the gene for the heart isoform of cytochrome c oxidase subunit VIIa (COX VIIa-H) in mouse, an isoform gene previously thought to be lacking in rodents. Interspecies amino acid comparisons indicate that mouse COX VIIa-H protein displays 82.5 and 70.9% identity with the bovine and human heart isoforms of COX VIIa, but only 53.7% identity with the paralogous mouse liver isoform (COX VIIa-L). Expression in adult mouse tissues is limited to heart and skeletal muscle, as found in other species. In the early mouse embryo, Cox7al was the exclusive isoform expressed and Cox7ah mRNA was not detectable until day 17 postcoitum. That the mouse Cox7ah gene characterized in this study is orthologous to the human COX7AH gene was also suggested by its mapping to mouse chromosome 7, to a conserved region syntenic with the human chromosome location of COX7AH, 19q13.1. As a result, all three COX heart isoform genes in mouse group to chromosome 7. Interestingly, mapping of the mouse Cox7al to chromosome 9 suggests a new syntenic region between the mouse and the human genomes.

Matrix-assisted laser desorption/ionization mass spectrometry analysis and thiol-group determination of isoforms of bovine cytochrome c oxidase, a hydrophobic multisubunit membrane protein

Matrix-assisted laser desorption/ionization-mass spectra (MALDI-MS) are obtained from entire bovine heart (H) and liver (L) cytochrome c oxidase membrane protein complexes. Molecular masses of most of the subunits are in excellent agreement with the published sequences. Some corrections are necessary for the nuclear coded subunit IX, which is N-acetylated, and X, with a corrected C-terminal peptide sequence. The mass values of two of the three tissue-specific subunits (VIII-L and X-L) are not in agreement with the DNA-deduced sequences and have been corrected by protein sequencing. For the investigation of the cysteine status 7-diethyl-amino-3-(4'-maleimidylphenyl)-4-methylcoumarin proved to be an excellent site-specific reagent. MALDI-MS with the SH-reacted enzyme indicates disulfide bridges only in subunit VII and a distorted tetrahedral S coordination of the zinc in subunit VI.

Isolation of estrogen-responsive genes with a CpG island library

In order to isolate novel estrogen-responsive genes, we utilized a CpG island library in which the regulatory regions of genes are enriched. CpG islands were screened for the ability to bind to a recombinant estrogen receptor protein with a genomic binding site (GBS) cloning method. Six CpG islands were selected, and they contained perfect, imperfect, and/or multiple half-palindromic estrogen-responsive elements (EREs). Northern blot analysis of various human cells showed that all these genomic fragments hybridized to specific mRNAs, suggesting that the genes associated with these EREs might be transcribed in human cells. Then cDNAs associated with two of them, EB1 and EB9, were isolated from libraries of human placenta and MCF-7 cells derived from a human breast cancer, respectively. Both transcripts were increased by estrogen in MCF-7 cells. The increase is inhibited by actinomycin D but not by cycloheximide, indicating that no protein synthesis is required for the up-regulation. The cDNA associated with EB1 encodes a 114-amino-acid protein similar to the cytochrome c oxidase subunit VIIa, named COX7RP (cytochrome c oxidase subunit VII-related protein). The cDNA associated with EB9 is homologous only to an express sequence tag and was named EBAG9 (estrogen receptor-binding fragment-associated gene 9). The palindromic ERE of EB1 is located in an intron of COX7RP, and that of EB9 is in the 5' upstream region of the cDNA. Both EREs had significant estrogen-dependent enhancer activities in a chloramphenicol acetyltransferase assay, when they were inserted into the 5' upstream region of the chicken beta-globin promoter. We therefore propose that the CpG-GBS method described here for isolation of the DNA binding site from the CpG island library would be useful for identification of novel target genes of certain transcription factors.

Genomic sequence and organization of the human gene for cytochrome c oxidase subunit (COX7A1) VIIa-M

Cytochrome c oxidase (COX, EC 1.9.3.1), the last component of the mitochondrial electron transfer chain, is built up by 13 polypeptides; 3 of them are encoded by the mitochondrial genome while the 10 smaller subunits are encoded by the nuclear genome. Several nuclear-encoded subunits occur in two different tissue-specific isoforms, a constitutive "L"-form and an "M"-form specific for skeletal and heart muscle. In this article, we describe the genomic sequence and organization of the human gene for COX subunit VIIa-M (COX7A1) located on chromosome 19q13.1 and compare it to its bovine homologue. The coding region of the gene extends over 1.45 kb of genomic sequence, organized in four exons. Intron-exon boundaries are well conserved between cattle and humans. Although it is a gene for a tissue-specific isoform, it has some features of a housekeeping gene: it is located in a CpG island, like its bovine homologue, and no TATA or CCAAT boxes were found in the 5' flanking sequence. Southern hybridization of COX7A1 to human genomic DNA revealed no pseudogenes. Putative binding sites for ubiquitous transcription factors like Sp1 and specific expression in skeletal as well as in heart muscle have been found. In contrast to the bovine gene, the human gene contains putative binding sites for nuclear respiratory factor 2 (NRF-2), which is implicated in the activation of other respiratory enzymes. Therefore, the human and the bovine genes, although well conserved in their coding regions, could differ in the tissue-specific regulation of gene expression. Knowledge of the gene structure will facilitate the analysis of the involvement of subunit VIIa in mitochondrial myopathies and may provide clues to the function of this subunit in a multicomponent enzyme.

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.

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.

Tissue-specific expression of the two isoforms of the mitochondrial phosphate carrier in bovine tissues

Comparison of the sequence of the human mitochondrial phosphate carrier (PiC) gene with cDNA clones characterised from a human heart cDNA library suggested the existence of two isoforms of the PiC, which were generated by alternative splicing of exon IIIA or exon IIIB and which differed in 13 amino acids [Dolce et al. (1994) J. Biol. Chem. 269, 10451]. In this work the expression of isoforms A and B of the PiC was investigated in different bovine tissues by Northern blot analysis using two probes that are specific for bovine exon IIIA and exon IIIB, respectively. Isoform A is highly expressed in heart and skeletal muscle. Isoform B is ubiquitously expressed in all tissues that were examined, although at different levels. The tissue-specific expression pattern of the two PiC isoforms is similar to that reported for the isoforms of several mitochondrial proteins required for energy production.

Mitochondrial dysfunction after fetal alcohol exposure

Specific mitochondrial enzyme activities and mRNA levels were measured in the heart, brain, and liver tissues of a group of 1-day-old neonatal rats whose mothers were alcohol-fed during pregnancy and compared with a control group. The results show a significant decrease in mitochondrial ATP synthase activity in both the brain and liver, as well as a decrease in complex III activity in the liver of rats exposed to alcohol. Other mitochondrial enzymes activities (e.g., citrate synthase, cytochrome c oxidase, and complex I), as well as specific mitochondrial transcript levels, were not significantly affected. Heart mitochondrial enzyme activities were not significantly affected. These data reveal that a tissue-specific response occurs after fetal exposure to alcohol and may explain some of the cellular events occurring in fetal alcohol syndrome resulting in abnormal growth and neurological development.

Identification of an additional member of the cytochrome c oxidase subunit VIIa family of proteins

We report the cloning, nucleotide sequence, evolutionary analysis, and intracellular localization of SIG81, a silica-induced cDNA from mouse macrophages. The cDNA encodes a 111-amino acid protein with extensive sequence identity with members of the mammalian cytochrome c oxidase subunit VIIa (COX7a) family. A human SIG81 sequence >80% identical with the mouse cDNA was deducted from homologous sequences in the human expressed tags data base. The deduced aminoterminal region shows features common to mitochondrial targeting sequences. A phylogenetic analysis of the carboxyl-terminal domain homologous to COX7a identifies SIG81 as a divergent member of the family with an ancient origin. Southern blot analysis showed that the mouse genome contains two to three copies of the SIG81 gene. Northern blot analysis revealed that the SIG81 transcript is approximately 1 kb and expressed in every tissue tested, with higher levels of expression observed in kidney and liver. Antibodies raised against a glutathione S-transferase SIG81 fusion protein detected a 13.5-kDa protein that co-fractionates with mitochondrial localized enzymatic activity. Taken together, our data suggest that SIG81 is a novel member of the COX7a family that is constitutively expressed in mouse cells.

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.

Heart mitochondria response to alcohol is different than brain and liver

Specific mitochondrial enzyme activities, mitochondrial DNA copy number, and mRNA levels were measured in heart, brain, and liver tissues of a group of alcohol-fed rats and compared with a control group. The results show a significant increase in mitochondrial enzyme activities (citrate synthase, complex IV, complex III, complex I, and complex V), as well as an increase in mitochondrial DNA in the cardiac tissue of the alcohol-fed animals. These data are indicative of an increase in mitochondrial number in the cardiac tissue that may occur as the result of an adaptive response to the alcoholic insult. However, in the liver and brain of the alcohol-treated rat, specific mitochondrial activities were decreased, in particular, complex III and ATP synthase, whereas levels of other mitochondrial enzymes (e.g., citrate synthase, specific mitochondrial transcripts, and mitochondrial DNA levels) do not seem to be affected. These data suggest that a tissue-specific response to alcohol exists that may have a common molecular mechanism in brain and liver, but is different in the heart.

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.

The mRNA-binding protein COLBP is glutamate dehydrogenase

Expression of the liver-type isopeptides of cytochrome c oxidase is regulated post-transcriptionally. An RNA-binding activity has been found in only those cells where the liver-type subunits are detected. This binding protein, termed COLBP, recognises sequences or structures within the 3'-untranslated regions of transcripts encoding these liver-type isopeptides and has been implicated in the modulation of mRNA expression. We now show by subcellular fractionation, immunocompetition, UV-crosslinking and shift-Western studies that the metabolic enzyme glutamate dehydrogenase, previously reported as being able to bind RNA, is the cytochrome c oxidase transcript-binding protein, COLBP.

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.

Cloning, sequence analysis, and expression of a mouse cDNA encoding cytochrome c oxidase subunit VIa liver isoform

A cDNA encoding cytochrome c oxidase (COX) subunit VIa liver isoform (COX6aL) was isolated from a Mus musculus library and sequenced. The protein translated from the nucleotide sequence contains a presequence and is 91% identical to the human cognate sequence over the processed polypeptide region. Northern analysis shows the expression of COX6aL is developmentally regulated in heart, being about equally transcribed with the heart isoform (COX6aH) in 18-day embryos but consisting of less than 25% in adult heart.

Structure and chromosomal location of the bovine gene for the heart muscle isoform of cytochrome c oxidase subunit VIII

We have isolated the bovine COX8H gene for the heart/muscle isoform of cytochrome c oxidase (COX) subunit VIII from a library of bovine genomic DNA cloned into lambda EMBL3. Primer extension assays on bovine heart mRNA mapped the 5' ends of COX8H transcripts to a CA dinucleotide 62-bp upstream from the ATG codon. The gene thus spans 1565-bp and comprises two exons and one large intron of 1227 bp. Exon 1 encodes the 5' untranslated region, a 24-amino acid presequence, and the first 13 amino acids of the mature COX VIII-H protein. Exon 2 encodes the remainder of the cDNA: amino acids 14 to 46 plus the 66-bp 3' untranslated region. The exon-intron boundaries matched the consensus splice junction sequences. Two protein polymorphisms were seen: an Ala/Val polymorphism at position -6 in the presequence and the previously noted Lys/Arg polymorphism at residue 7 of the mature protein. A TaqI polymorphism occurs in the intron. The COX8H gene was mapped by bovine x rodent somatic cell hybrid mapping panels to bovine (BTA) Chromosome (Chr) 25 with 100% concordancy. BTA 25 is conserved relative to the long arm of human (HSA) Chr 11, which contains COX8, the gene for the single human COX VIII subunit that is homologous to the liver isoform.

Developmental regulation of tissue-specific isoforms of subunit VIa of beef cytochrome c oxidase

The switching of the subunit VIa isoforms of cytochrome c oxidase has been followed in heart tissue during bovine development both by transcript levels and in terms of the incorporation of L- (liver) and H- (heart) polypeptides into mitochondria. In early fetuses, e.g., 60-days development, there are high levels of VIaL transcript and high levels of the VIaL polypeptide incorporated into mitochondria. In late fetuses (after 200 days), the levels of VIaL transcript are still high, with less but still significant amounts of VIaL polypeptide present in comparison to adult heart in which the amount of this isoform is negligible. As the proportion of VIaL transcript is reduced, the proportion of VIaH transcript increases along with the amount of the VIaH isoform in mitochondria. These data indicate isoform switching during late fetal development. The presence of COLBP (cytochrome oxidase liver isoform binding protein) (Preiss, T. and Lightowlers, R.N. (1993) J. Biol. Chem. 268, 10659-10667) was examined at different developmental stages. COLBP binding activity was observed in hearts of late fetuses but not found in adult heart tissue, providing a correlation between the presence of this factor and the presence of the VIaL polypeptide in mitochondria.

The tissue-specific RNA-binding protein COLBP is differentially regulated during myogenesis

Cytochrome c oxidase L-form transcript-binding protein (COLBP) activity parallels the tissue-specific mRNA expression of bovine cytochrome c oxidase liver isopeptides. A similar RNA-binding activity is found in human myoblast and liver Hep G2 cell homogenates. Human COLBP activity, however, is not present in myotubes or adult skeletal muscle. It is proposed that COLBP down-regulation during muscle cell differentiation may underlie oxidase isoform switching during myogenesis.

Structural organization and evolution of the liver isoform gene for bovine cytochrome c oxidase subunit VIIa

Subunit VIIa of mammalian cytochrome c oxidase is one of three nuclear-encoded subunits that exhibit isoforms, existing predominantly as an H-form in cardiac and skeletal muscle tissues and as an L-form in others. We have isolated and characterized the L-isoform gene (COX7aL). It is 5.4 kb long, consists of four exons, and is located at a CpG island. Sp1 sites and an NRF1 site are located in an approximately 100-bp region immediately upstream of the gene. Comparison of the sequence and organization with the previously described H-isoform gene shows identical intron-exon organizations, with the first intron of both isoform genes splitting the presequence coding region almost identically. These results suggest that the isoform genes arose by duplication from a common ancestor prior to the mammalian radiation and that the ancestor already contained the presequences. In addition, four processed pseudogenes of the L-type have been isolated and characterized, one of which (COX7aLP1) contains no deletions, insertions, or frame-shifts and can encode a precursor protein of 83 amino acids. Construction of a phylogenetic tree employing extant COX7aL cDNA and bovine pseudogene sequences suggests that the expressed bovine gene and COX7aLP1 arose from a gene duplication event 4.6-6.8 Mya.

Expression of human cytochrome c oxidase subunits during fetal development

Expression of human cytochrome c oxidase (COX) subunits was examined at fetal (20-28 weeks) and adult state by Northern blot hybridization with mRNA from liver, heart, skeletal muscle, and intestine. The data were related to COX and citrate synthase activities and to immunodetected COX subunits (II/III, IV, VIIaH). In liver little changes of COX transcripts are observed from fetal to adult state. In contrast, in heart and skeletal muscle all transcripts of COX subunits increase between 2-20-fold, when related to the amount of 28S rRNA. In fetal heart and skeletal muscle the relative amounts of the liver-type transcripts of subunit VIa were 30% and 25% from total VIa transcripts (VIaL+VIaH), respectively, but decrease to only 2-5% at adult state. The liver-type transcripts of subunit VIIa occur to 50% in fetal heart and skeletal muscle, which remained unchanged in adult heart and decrease to 5-8% in adult skeletal muscle. The results clearly indicate a switch of gene expression in heart and skeletal muscle during development, from the liver type to the heart/muscle type of subunit VIa (and partly VIIa).

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.

Tissue-specific expression and chromosome assignment of genes specifying two isoforms of subunit VIIa of human cytochrome c oxidase

Subunit VIIa of mammalian cytochrome c oxidase (COX; EC 1.9.3.1) exists in at least two isoforms, one present in all tissue types ('liver' isoform; COX VIIa-L) and the other specific for cardiac and skeletal muscle (COX VIIa-M). We have isolated a full-length cDNA encoding human COX VIIa-M. The deduced polypeptide represents the human ortholog of COX VIIa-M, as it shares 78% identity with bovine COX VIIa-M, but only 63% identity with human COX VIIa-L. Northern-blot analysis of primate tissues demonstrated that COXVIIa-M mRNA is present only in muscle tissues; in contrast, the COXVIIa-L mRNA is present in both muscle and nonmuscle tissues. Southern-blot hybridization of human-rodent cell hybrid genomic DNA indicates that the COXVIIa-M gene maps to a single locus on chromosome 19, designated COX7AM. In contrast, COXVIIa-L cDNA probes hybridized to fragments from two COX7AL loci, on chromosomes 4 and 14.

Differential expression of genes specifying two isoforms of subunit VIa of human cytochrome c oxidase

Subunit VIa of mammalian cytochrome c oxidase (COX; EC 1.9.3.1) exists in two isoforms, one present ubiquitously ('liver' isoform; COX VIa-L) and the other present only in cardiac and skeletal muscle (COX VIa-M). We have now isolated a full-length cDNA specifying human COX VIa-M. The deduced mature COX VIa-M polypeptide is 62% identical to the human COX VIa-L isoform, but is approximately 80% identical to the bovine and rat COX VIa-M isoforms, suggesting that the two COX VIa isoform-encoding genes arose prior to the mammalian radiation. Transcriptional analysis showed a tissue-specific pattern: whereas COXVIa-L is transcribed ubiquitously, COXVIa-M is transcribed only in heart and skeletal muscle. The cDNA specifying COX VIa-M is a prime candidate for use in investigations of Mendelian-inherited COX deficiencies with primary involvement of muscle.

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.