The expression of Cox17p in rodent tissues and cells

Molecular Regulation of Copper Homeostasis in the Male Gonad during the Process of Spermatogenesis

Owing to its redox properties, copper is a cofactor of enzymes that catalyze reactions in fundamental metabolic processes. However, copper-oxygen interaction, which is a source of toxic oxygen radicals generated by the Fenton reaction, makes copper a doubled-edged-sword in an oxygen environment. Among the microelements influencing male fertility, copper plays a special role because both copper deficiency and overload in the gonads worsen spermatozoa quality and disturb reproductive function in mammals. Male gametes are produced during spermatogenesis, a multi-step process that consumes large amounts of oxygen. Germ cells containing a high amount of unsaturated fatty acids in their membranes are particularly vulnerable to excess copper-mediated oxidative stress. In addition, an appropriate copper level is necessary to initiate meiosis in premeiotic germ cells. The balance between essential and toxic copper concentrations in germ cells at different stages of spermatogenesis and in Sertoli cells that support their development is handled by a network of copper importers, chaperones, recipient proteins, and exporters. Here, we describe coordinated regulation/functioning of copper-binding proteins expressed in germ and Sertoli cells with special emphasis on copper transporters, copper transporting ATPases, and SOD1, a copper-dependent antioxidant enzyme. These and other proteins assure copper bioavailability in germ cells and protection against copper toxicity.

Molecular mechanisms underlying copper homeostasis in Mammalian cells

Copper (Cu) is an essential metal for living organisms that utilize oxygen for respiration and is required as a cofactor of redox-regulating enzymes, such as superoxide dismutase, ceruloplasmin, lysyl oxidase, tyrosinase, and dopamine β-hydroxylase. However, the redox-active property of this metal may have toxic effects on cells due to the generation of harmful reactive oxygen species. Given these circumstances, it is said that cells have a dependable system for Cu homeostasis that efficiently distributes this essential metal to cuproenzymes, thereby preventing damage to proteins, nucleic acids, sugars, and lipids. In particular, influx, efflux, and intracellular distribution with maintenance of the oxidation state of Cu are strictly regulated. Several groups of Cu-regulating factors have been identified in mammalian cells, i.e., Cu transporters, Cu chaperones, Cu-binding proteins/peptides, and others. In this review, the features of the Cu-regulating factors are concisely examined in terms of molecular mechanisms underlying Cu homeostasis in cells.

Transcriptional regulation of copper metabolism genes in the liver of fetal and neonatal control and iron-deficient rats

Copper and iron metabolism have been known to interact for many years. We have previously shown, during pregnancy, that copper levels in the maternal liver rise as a consequence of iron deficiency, but that levels in the fetal liver decrease. In this paper, we measure expression of genes involved in copper metabolism in fetal and postnatal liver, to test whether alterations can explain this observation. Additionally, we study the extent to which gene expression changes in the latter stages of pregnancy and in the perinatal period. Ctr1 expression levels dropped to term, rising again thereafter. There was no difference in gene expression between control and iron deficient animals. Atox1 expression remained approximately stable until term, and then there was a rise to a maximum at about Day 8. Atp7a expression levels remained constant, except for a brief drop at term. Atp7b levels, in contrast, decreased from a maximum early in gestation to low levels in the term and post-natal livers. Ceruloplasmin expression appeared to be diametrically opposite to Atp7b. The other two metallochaperones showed the same pattern of expression as Atox1, with a decrease to term, a rise at Day 1, or a rise after birth followed by a brief decrease at about Day 3. None of the genes were significantly affected by iron deficiency, suggesting that changes in expression cannot explain the altered copper levels in the fetal and neonatal liver.

Links between copper and cholesterol in Alzheimer's disease

Altered copper homeostasis and hypercholesterolemia have been identified independently as risk factors for Alzheimer's disease (AD). Abnormal copper and cholesterol metabolism are implicated in the genesis of amyloid plaques and neurofibrillary tangles (NFT), which are two key pathological signatures of AD. Amyloidogenic processing of a sub-population of amyloid precursor protein (APP) that produces Aβ occurs in cholesterol-rich lipid rafts in copper deficient AD brains. Co-localization of Aβ and a paradoxical high concentration of copper in lipid rafts fosters the formation of neurotoxic Aβ:copper complexes. These complexes can catalytically oxidize cholesterol to generate H₂O₂, oxysterols and other lipid peroxidation products that accumulate in brains of AD cases and transgenic mouse models. Tau, the core protein component of NFTs, is sensitive to interactions with copper and cholesterol, which trigger a cascade of hyperphosphorylation and aggregation preceding the generation of NFTs. Here we present an overview of copper and cholesterol metabolism in the brain, and how their integrated failure contributes to development of AD.

Knockdown of human COX17 affects assembly and supramolecular organization of cytochrome c oxidase

Assembly of cytochrome c oxidase, the terminal enzyme of the mitochondrial respiratory chain, requires a concerted activity of a number of chaperones and factors for the insertion of subunits, accessory proteins, cofactors and prosthetic groups. It is now well accepted that the multienzyme complexes of the respiratory chain are organized in vivo as supramolecular functional structures, so-called supercomplexes. Here, we investigate the role of COX17 in the biogenesis of the respiratory chain in HeLa cells. In accordance with its predicted function as a copper chaperone and its role in formation of the binuclear copper centre of cytochrome c oxidase, COX17 siRNA knockdown affects activity and assembly of cytochrome c oxidase. While the abundance of cytochrome c oxidase dimers seems to be unaffected, blue native gel electrophoresis reveals the disappearance of COX-containing supercomplexes as an early response. We observe the accumulation of a novel approximately 150 kDa complex that contains Cox1, but not Cox2. This observation may indicate that the absence of Cox17 interferes with copper delivery to Cox2, but not to Cox1. We suggest that supercomplex formation is not simply due to assembly of completely assembled complexes. An interdependent assembly scenario for the formation of supercomplexes that rather requires the coordinated synthesis and association of individual complexes, is proposed.

Oxidative switches in functioning of mammalian copper chaperone Cox17

Cox17, a copper chaperone for cytochrome-c oxidase, is an essential and highly conserved protein in eukaryotic organisms. Yeast and mammalian Cox17 share six conserved cysteine residues, which are involved in complex redox reactions as well as in metal binding and transfer. Mammalian Cox17 exists in three oxidative states, each characterized by distinct metal-binding properties: fully reduced mammalian Cox17(0S-S) binds co-operatively to four Cu+; Cox17(2S-S), with two disulfide bridges, binds to one of either Cu+ or Zn2+; and Cox17(3S-S), with three disulfide bridges, does not bind to any metal ions. The E(m) (midpoint redox potential) values for two redox couples of Cox17, Cox17(3S-S)<-->Cox17(2S-S) (E(m1)) and Cox17(2S-S)<-->Cox17(0S-S) (E(m2)), were determined to be -197 mV and -340 mV respectively. The data indicate that an equilibrium exists in the cytosol between Cox17(0S-S) and Cox17(2S-S), which is slightly shifted towards Cox17(0S-S). In the IMS (mitochondrial intermembrane space), the equilibrium is shifted towards Cox17(2S-S), enabling retention of Cox17(2S-S) in the IMS and leading to the formation of a biologically competent form of the Cox17 protein, Cox17(2S-S), capable of copper transfer to the copper chaperone Sco1. XAS (X-ray absorption spectroscopy) determined that Cu4Cox17 contains a Cu4S6-type copper-thiolate cluster, which may provide safe storage of an excess of copper ions.

A selective requirement for copper-dependent activation of cytochrome c oxidase by Cox17p

Cox17p is cloned from yeast as a chaperone to deliver copper to the mitochondria of assembly for cytochrome c oxidase (CCO). In mammals, CCO is a key enzyme for cellular respiration and a defect in its function is associated with severe neonatal or infantile lactic acidosis and early death. Recently, we found that Cox17p is not only required for mitochondrial oxidative phosphorylation but also is essential for embryonic growth and development in COX17 gene-deficient mice. To investigate its biochemical features, recombinant human Cox17p was overexpressed and purified without a purification tag. It specifically binds Cu(I) at a molar copper content of 3.3+/-0.04 under reduced conditions and significantly activates the mitochondrial CCO in vitro. Although the Cu-Cox17p complex was maintained between pH values from 5.0 to 7.7, Cu was completely released from Cox17p at pH 8.0. An acute exposure of excess amount of copper ion to mouse cells resulted in a significant reduction of Cox17p mRNA expression, whereas copper starvation maintained the Cox17p transcription level. These results suggest that the stringent selectivity of Cox17p for copper is required for CCO activation, to prevent copper overload, or promote the supply of copper.

Mammalian copper chaperone Cox17p has an essential role in activation of cytochrome C oxidase and embryonic development

Cox17p is essential for the assembly of functional cytochrome c oxidase (CCO) and for delivery of copper ions to the mitochondrion for insertion into the enzyme in yeast. Although this small protein has already been cloned or purified from humans, mice, and pigs, the function of Cox17p in the mammalian system has not yet been elucidated. In vitro biochemical data for mammalian Cox17p indicate that the copper binds to the sequence -KPCCAC-. Although mouse embryos homozygous for COX17 disruption die between embryonic days E8.5 and E10, they develop normally until E6.5. This phenotype is strikingly similar to embryos of Ctr1(-/-), a cell surface copper transporter, in its lethality around the time of gastrulation. COX17-deficient embryos exhibit severe reductions in CCO activity at E6.5. Succinate dehydrogenase activity and immunoreactivities for anti-COX subunit antibodies were normal in the COX17(-/-) embryos, indicating that this defect was not caused by the deficiency of other complexes and/or subunits but was caused by impaired CCO activation by Cox17p. Since other copper chaperone (Atox1 and CCS)-deficient mice show a more moderate defect, the disruption of the COX17 locus causes the expression of only the phenotype of Ctr1(-/-). We found that the activity of lactate dehydrogenase was also normal in E6.5 embryos, implying that the activation of CCO by Cox17p may not be essential to the progress of embryogenesis before gastrulation.

Copper chaperones for cytochrome c oxidase and human disease

Biological processes in living cells are compartmentalized between lipid membranes. Integral membrane proteins often confer specific functions to these compartments and as such have a critical role in cellular metabolism and function. Cytochrome c oxidase is a macromolecular metalloprotein complex essential for the respiratory function of the cell. Elucidating the mechanisms of assembly of cytochrome c oxidase within the inner mitochondrial membrane represents a unique challenge for understanding metalloprotein biosynthesis. Elegant genetic experiments in yeast have defined several proteins required for copper delivery to cytochrome c oxidase. While the precise role of each of these proteins in copper incorporation remains unclear, recent studies have revealed that inherited mutations in two of these proteins can result in severe pathology in human infants in association with cytochrome c oxidase deficiency. Characterization of the molecular pathogenesis of these disorders offers new insights into the mechanisms of cellular copper metabolism and the role of these cytochrome c oxidase copper chaperones in human disease.

Mechanisms for storage of prolactin and growth hormone in secretory granules

There are three steps in the formation of secretory granules: aggregation of proteins to form the dense cores of granules, accumulation of appropriate membrane proteins necessary for function of the granules, and removal of extraneous membrane and inappropriate proteins by small vesicles. Formation of protein aggregates may be the initial step in this process, which is not well understood. Assays of aggregation of human prolactin and growth hormone in neuroendocrine cells indicate that acidic intracellular compartments are necessary, and Zn2+ and Cu2+ may facilitate aggregation through low affinity binding sites. There is more than one way to make proteins aggregate in solution; precipitates of human prolactin formed in "crowded" conditions most closely resemble what is likely to occur in cells. Understanding the properties of aggregates formed in cells may be important, as there are several examples of granules with different contents that function differently; human R183H-growth hormone, a mutant that causes autosomal dominant isolated growth hormone deficiency, also appears to be an example. Recognition of surface motifs on aggregates of proteins may be important to localize correctly membrane proteins necessary for function, an explanation for the means by which granule content may influence function.

Characterization and identification of promoter elements in the mouse COX17 gene

Cox17p, essential for the assembly of functional cytochrome c oxidase (CCO) in Saccharomyces cerevisiae, has been believed to deliver copper ions to the mitochondrion for insertion into the enzyme. We have recently isolated an approximately 20 kb genomic fragment of the mouse COX17. Reporter assay experiments have shown that most of the promoter activity was restricted to a 0.85 kb fragment flanking the first exon. Further intensive deletion and detailed mutation analysis suggested that the minimal essential region for transactivation was located at bases -155 to -70. This 5'-flanking region did not possess a TATA box, but contained putative Sp1, NRF-1 and NRF-2 binding sites. COX17 basal promoter activity was abrogated by site-directed mutagenesis of Sp1, NRF-1 and NRF-2 binding sites. Electrophoretic mobility shift assays with AtT-20 and NIH3T3 cell nuclear extract revealed that this region binds both a Sp1-like protein and NRF-1 transcription factors. These results indicated that Sp1, NRF-1 and NRF-2 are involved in basal transcription of the COX17 gene, similar to the transcription mechanism of other CCO-related genes.

Aggregation of human wild-type and H27A-prolactin in cells and in solution: roles of Zn(2+), Cu(2+), and pH

Aggregation of hormones is an important step in the formation of secretory granules that results in concentration of hormones. In transfected AtT20 cells, but not COS cells, Lubrol-insoluble aggregates of human prolactin (PRL) accumulated within 30 min after synthesis. Aggregation in AtT20 cells was reduced by incubation with 30 microM chloroquine, which neutralizes intracellular compartments, and was slowed by incubation with diethyldithiocarbamate, which chelates Cu(2+) and Zn(2+). H27A-PRL aggregated in AtT20 cells as well as wild-type PRL, indicating that a high affinity Zn(2+)-binding site is not necessary. In solution, purified recombinant human PRL was precipitated by 20 microM Cu(2+) or Zn(2+). In solution without polyethylene glycol there was no precipitation with acidic pH alone, precipitation with Zn(2+) was most effective at neutral pH, and the ratio of Zn(2+) to PRL was greater than 1 in the precipitate. In solution with polyethylene glycol, precipitation occurred with acidic pH, precipitation with Zn(2+) occurred effectively at acidic pH, and the ratio of Zn(2+) to PRL was less than 1. The aggregates obtained in polyethylene glycol are therefore better models for aggregates in cells. Unlike human PRL, aggregation of rat PRL has been shown to occur at neutral pH in cells and in solution, and therefore these two similar proteins form aggregates that are the cores of secretory granules in ways that are not completely identical.

Mitochondrial oxidative phosphorylation system assembly in man: recent achievements

The human oxidative phosphorylation system consists of five multi-subunit complexes of which the individual subunits, with the exception of complex II, are encoded either by mitochondrial or nuclear DNA. Consequently, a deficient enzyme activity of one or more of the complexes can be caused by mitochondrial or nuclear DNA mutations. In the past 5 years numerous mutations have been found in structural nuclear oxidative phosphorylation system genes. However, in a substantial number of patients with oxidative phosphorylation system complex deficiencies, despite extensive investigations, no mutations in the mitochondrial DNA or the structural nuclear genes have been found. Genetic defects in such patients are therefore suspected at the transcriptional, translational, post-translational level or in gene products involved in the assembly of the oxidative phosphorylation system. The latter is a complicated process, as the proteins encoded by the two genomes have to be brought together in a proper stoichiometric way to form five functional complexes. In the past year substantial progress in the knowledge of the human oxidative phosphorylation assembly process has been made. Several human assembly genes have been identified, and mutations in these genes responsible for human oxidative phosphorylation system complex-related diseases have been found. In this review, we summarize our current knowledge about human oxidative phosphorylation system assembly genes in health and disease.

Genomic structure of mouse copper chaperone, COX17

Coxl7p was first cloned as a cytoplasmic copper chaperone from yeast mutant and recent works suggested the existence of mammalian homologues. Previous report has shown that a gel filtration fraction of heart extract containing porcine Coxl7p peptide promoted the survival of NIH3T3 fibroblast cells. In the present study, we first cloned DNA fragments of the mouse COX17 gene. The mouse COX17 spans approximately 6kb and consists of three exons. It was mapped to the center of chromosome 16, using a radiation hybrid-mapping panel. The major transcription start site is 80 bp upstream of the ATG initiation codon as determined by rapid amplification of cDNA ends (5'-RACE) analysis. Two potential polyadenylation sites are 3233 and 3293 bp downstream of the termination codon, respectively. Transient transfection of reporter plasmids containing portions of the mouse COX17 5'-flanking region into AtT-20 and NIH3T3 cells allowed the localization of the essential promoter to a 0.8 kb region upstream of the transcription starting site. Furthermore, the transfected luciferase activity was much higher in AtT-20 than NIH3T3. According to sequence analysis of the approximately 0.8kb 5'-flanking region, GC rich segments including consensus sequences for binding of the transcription factor Sp1, but no TATA/CAAT boxes, exist in the region of the transcription start site. Besides the GC box, binding sites for NRF-1 and 2 known as specific transcription factors for COX subunits are also localized around the transcription starting site.