Loss of function of Sco1 and its interaction with cytochrome c oxidase

Targeting cuproplasia and cuproptosis in cancer

Copper, an essential trace element that exists in oxidized and reduced forms, has pivotal roles in a variety of biological processes, including redox chemistry, enzymatic reactions, mitochondrial respiration, iron metabolism, autophagy and immune modulation; maintaining copper homeostasis is crucial as both its deficiency and its excess are deleterious. Dysregulated copper metabolism has a dual role in tumorigenesis and cancer therapy. Specifically, cuproplasia describes copper-dependent cell growth and proliferation, including hyperplasia, metaplasia and neoplasia, whereas cuproptosis refers to a mitochondrial pathway of cell death triggered by excessive copper exposure and subsequent proteotoxic stress (although complex interactions between cuproptosis and other cell death mechanisms, such as ferroptosis, are likely and remain enigmatic). In this Review, we summarize advances in our understanding of copper metabolism, the molecular machineries underlying cuproplasia and cuproptosis, and their potential targeting for cancer therapy. These new findings advance the rapidly expanding field of translational cancer research focused on metal compounds.

More than Just Bread and Wine: Using Yeast to Understand Inherited Cytochrome Oxidase Deficiencies in Humans

Inherited defects in cytochrome c oxidase (COX) are associated with a substantial subset of diseases adversely affecting the structure and function of the mitochondrial respiratory chain. This multi-subunit enzyme consists of 14 subunits and numerous cofactors, and it requires the function of some 30 proteins to assemble. COX assembly was first shown to be the primary defect in the majority of COX deficiencies 36 years ago. Over the last three decades, most COX assembly genes have been identified in the yeast Saccharomyces cerevisiae, and studies in yeast have proven instrumental in testing the impact of mutations identified in patients with a specific COX deficiency. The advent of accessible genome-wide sequencing capabilities has led to more patient mutations being identified, with the subsequent identification of several new COX assembly factors. However, the lack of genotype-phenotype correlations and the large number of genes involved in generating a functional COX mean that functional studies must be undertaken to assign a genetic variant as being causal. In this review, we provide a brief overview of the use of yeast as a model system and briefly compare the COX assembly process in yeast and humans. We focus primarily on the studies in yeast that have allowed us to both identify new COX assembly factors and to demonstrate the pathogenicity of a subset of the mutations that have been identified in patients with inherited defects in COX. We conclude with an overview of the areas in which studies in yeast are likely to continue to contribute to progress in understanding disease arising from inherited COX deficiencies.

Cytochrome c oxidase deficiency detection in human fibroblasts using scanning electrochemical microscopy

Cytochrome c oxidase deficiency (COXD) is an inherited disorder characterized by the absence or mutation in the genes encoding for the cytochrome c oxidase protein (COX). COX deficiency results in severe muscle weakness, heart, liver, and kidney disorders, as well as brain damage in infants and adolescents, leading to death in many cases. With no cure for this disorder, finding an efficient, inexpensive, and early means of diagnosis is essential to minimize symptoms and long-term disabilities. Furthermore, muscle biopsy, the traditional detection method, is invasive, expensive, and time-consuming. This study demonstrates the applicability of scanning electrochemical microscopy to quantify COX activity in living human fibroblast cells. Taking advantage of the interaction between the redox mediator N, N, N', N'-tetramethyl-para-phenylene-diamine, and COX, the enzymatic activity was successfully quantified by monitoring current changes using a platinum microelectrode and determining the apparent heterogeneous rate constant k0 using numerical modeling. This study provides a foundation for developing a diagnostic method for detecting COXD in infants, which has the potential to increase treatment effectiveness and improve the quality of life of affected individuals.

Hepatic depletion of nucleolar protein mDEF causes excessive mitochondrial copper accumulation associated with p53 and NRF1 activation

Copper is an essential component in the mitochondrial respiratory chain complex IV (cytochrome c oxidases). However, whether any nucleolar factor(s) is(are) involved in regulating the mitochondrial copper homeostasis remains unclear. The nucleolar localized Def-Capn3 protein degradation pathway cleaves target proteins, including p53, in both zebrafish and human nucleoli. Here, we report that hepatic depletion of mDEF in mice causes an excessive copper accumulation in the mitochondria. We find that mDEF-depleted hepatocytes show an exclusion of CAPN3 from the nucleoli and accumulate p53 and NRF1 proteins in the nucleoli. Furthermore, we find that NRF1 is a CAPN3 substrate. Elevated p53 and NRF1 enhances the expression of Sco2 and Cox genes, respectively, to allow more copper acquirement in the mDef^loxp/loxp, Alb:Cre mitochondria. Our findings reveal that the mDEF-CAPN3 pathway serves as a novel mechanism for regulating the mitochondrial copper homeostasis through targeting its substrates p53 and NRF1.

Mitochondrial copper in human genetic disorders

Copper is an essential micronutrient that serves as a cofactor for enzymes involved in diverse physiological processes, including mitochondrial energy generation. Copper enters cells through a dedicated copper transporter and is distributed to intracellular cuproenzymes by copper chaperones. Mitochondria are critical copper-utilizing organelles that harbor an essential cuproenzyme cytochrome c oxidase, which powers energy production. Mutations in copper transporters and chaperones that perturb mitochondrial copper homeostasis result in fatal genetic disorders. Recent studies have uncovered the therapeutic potential of elesclomol, a copper ionophore, for the treatment of copper deficiency disorders such as Menkes disease. Here we review the role of copper in mitochondrial energy metabolism in the context of human diseases and highlight the recent developments in copper therapeutics.

Differential requirements for mitochondrial electron transport chain components in the adult murine liver

Mitochondrial electron transport chain (ETC) dysfunction due to mutations in the nuclear or mitochondrial genome is a common cause of metabolic disease in humans and displays striking tissue specificity depending on the affected gene. The mechanisms underlying tissue-specific phenotypes are not understood. Complex I (cI) is classically considered the entry point for electrons into the ETC, and in vitro experiments indicate that cI is required for basal respiration and maintenance of the NAD+/NADH ratio, an indicator of cellular redox status. This finding has largely not been tested in vivo. Here, we report that mitochondrial complex I is dispensable for homeostasis of the adult mouse liver; animals with hepatocyte-specific loss of cI function display no overt phenotypes or signs of liver damage, and maintain liver function, redox and oxygen status. Further analysis of cI-deficient livers did not reveal significant proteomic or metabolic changes, indicating little to no compensation is required in the setting of complex I loss. In contrast, complex IV (cIV) dysfunction in adult hepatocytes results in decreased liver function, impaired oxygen handling, steatosis, and liver damage, accompanied by significant metabolomic and proteomic perturbations. Our results support a model whereby complex I loss is tolerated in the mouse liver because hepatocytes use alternative electron donors to fuel the mitochondrial ETC.

Echocardiography and MALDI-TOF Identification of Myosin-Binding Protein C3 A74T Gene Mutations Involved Healthy and Mutated Bengal Cats

This study aimed to identify the potential peptide candidates and expected proteins associated with MYBPC3-A74T gene mutations in Bengal cats and determine if peptidome profiles differ between healthy controls and cats with MYBPC3-A74T gene mutations. All animals were evaluated using echocardiography. DNA was isolated and followed by the screening test of MYBPC3 gene mutation. The MALDI-TOF mass spectrometry was conducted for analyzing the targeted peptide and protein patterns. The expected protein candidates were searched for within the NCBI database. Our results demonstrated that the MYBPC3-A74T gene mutation was dominant in Bengal cats but not in domestic shorthair cats. Correlations between baseline characteristics and echocardiographic parameters were discovered in Bengal cats. Mass spectrometry profiles of the candidate proteins were suspected to accompany the cat with the MYBPC3-A74T gene mutation, involving integral protein–membrane, organization of nucleus, DNA replication, and ATP-binding protein. Therefore, MYBPC3-A74T gene mutations occur frequently in Bengal cat populations. The high incidence of homozygotes for the mutation supports the causal nature of the MYBPC3-A74T mutation. In addition, peptidomics analysis was established for the first time under this condition to promise a complementary technique for the future clinical diagnosis of the MYBPC3-A74T mutation associated with physiological variables and cardiac morphology in cats.

Dynamic Mitochondrial Proteome Under Polyamines Treatment in Cardiac Aging

Age-related alteration of mitochondria causes impaired cardiac function, along with cellular and molecular changes. Polyamines can extend the life span in mice. However, whether polyamines can affect the dynamic mitochondrial proteome, thereby preventing age-related changes in cardiac function and cardiac aging, remains unclear. In this study, we found that spermine (Spm) and spermidine (Spd) injection for 6 weeks could prevent 24-month-old rats heart dysfunction, improve mitochondrial function, and downregulate apoptosis. Using iTRAQ tools, we identify 75 mitochondrial proteins of statistically significant alteration in aging hearts, which mainly participate in important mitochondrial physiological activity, such as metabolism, translation, transport, apoptosis, and oxidative phosphorylation. Moreover, four proteins of differential expression, pyruvate dehydrogenase kinase (PDK4), trifunctional enzyme subunit alpha (HADHA), nicotinamide nucleotide transhydrogenase (NNT), and Annexin6, which were significantly associated with heart aging, were validated by Western blotting. In vitro, we further demonstrated polyamines could retard cardiomyocytes aging through downregulating the expression of PDK4 and thereby inhibiting cell apoptosis. In summary, the distinct mitochondrial proteins identified in this study suggested some candidates involved in the anti-aging of the heart after polyamines treatment, and PDK4 may provide molecular clues for polyamines to inhibit apoptosis and thus retard aging-induced cardiac dysfunction.

An Emerging Role of Defective Copper Metabolism in Heart Disease

Copper is an essential trace metal element that significantly affects human physiology and pathology by regulating various important biological processes, including mitochondrial oxidative phosphorylation, iron mobilization, connective tissue crosslinking, antioxidant defense, melanin synthesis, blood clotting, and neuron peptide maturation. Increasing lines of evidence obtained from studies of cell culture, animals, and human genetics have demonstrated that dysregulation of copper metabolism causes heart disease, which is the leading cause of mortality in the US. Defects of copper homeostasis caused by perturbed regulation of copper chaperones or copper transporters or by copper deficiency resulted in various types of heart disease, including cardiac hypertrophy, heart failure, ischemic heart disease, and diabetes mellitus cardiomyopathy. This review aims to provide a timely summary of the effects of defective copper homeostasis on heart disease and discuss potential underlying molecular mechanisms.

Tackling Dysfunction of Mitochondrial Bioenergetics in the Brain

Oxidative phosphorylation (OxPhos) is the basic function of mitochondria, although the landscape of mitochondrial functions is continuously growing to include more aspects of cellular homeostasis. Thanks to the application of -omics technologies to the study of the OxPhos system, novel features emerge from the cataloging of novel proteins as mitochondrial thus adding details to the mitochondrial proteome and defining novel metabolic cellular interrelations, especially in the human brain. We focussed on the diversity of bioenergetics demand and different aspects of mitochondrial structure, functions, and dysfunction in the brain. Definition such as 'mitoexome', 'mitoproteome' and 'mitointeractome' have entered the field of 'mitochondrial medicine'. In this context, we reviewed several genetic defects that hamper the last step of aerobic metabolism, mostly involving the nervous tissue as one of the most prominent energy-dependent tissues and, as consequence, as a primary target of mitochondrial dysfunction. The dual genetic origin of the OxPhos complexes is one of the reasons for the complexity of the genotype-phenotype correlation when facing human diseases associated with mitochondrial defects. Such complexity clinically manifests with extremely heterogeneous symptoms, ranging from organ-specific to multisystemic dysfunction with different clinical courses. Finally, we briefly discuss the future directions of the multi-omics study of human brain disorders.

Blackout in the powerhouse: clinical phenotypes associated with defects in the assembly of OXPHOS complexes and the mitoribosome

Mitochondria produce the bulk of the energy used by almost all eukaryotic cells through oxidative phosphorylation (OXPHOS) which occurs on the four complexes of the respiratory chain and the F₁–F₀ ATPase. Mitochondrial diseases are a heterogenous group of conditions affecting OXPHOS, either directly through mutation of genes encoding subunits of OXPHOS complexes, or indirectly through mutations in genes encoding proteins supporting this process. These include proteins that promote assembly of the OXPHOS complexes, the post-translational modification of subunits, insertion of cofactors or indeed subunit synthesis. The latter is important for all 13 of the proteins encoded by human mitochondrial DNA, which are synthesised on mitochondrial ribosomes. Together the five OXPHOS complexes and the mitochondrial ribosome are comprised of more than 160 subunits and many more proteins support their biogenesis. Mutations in both nuclear and mitochondrial genes encoding these proteins have been reported to cause mitochondrial disease, many leading to defective complex assembly with the severity of the assembly defect reflecting the severity of the disease. This review aims to act as an interface between the clinical and basic research underpinning our knowledge of OXPHOS complex and ribosome assembly, and the dysfunction of this process in mitochondrial disease.

Mitochondrial disorders of the OXPHOS system

Mitochondrial disorders are among the most frequent inborn errors of metabolism, their primary cause being the dysfunction of the oxidative phosphorylation system (OXPHOS). OXPHOS is composed of the electron transport chain (ETC), formed by four multimeric enzymes and two mobile electron carriers, plus an ATP synthase [also called complex V (cV)]. The ETC performs the redox reactions involved in cellular respiration while generating the proton motive force used by cV to synthesize ATP. OXPHOS biogenesis involves multiple steps, starting from the expression of genes encoded in physically separated genomes, namely the mitochondrial and nuclear DNA, to the coordinated assembly of components and cofactors building each individual complex and eventually the supercomplexes. The genetic cause underlying around half of the diagnosed mitochondrial disease cases is currently known. Many of these cases result from pathogenic variants in genes encoding structural subunits or additional factors directly involved in the assembly of the ETC complexes. Here, we review the historical and most recent findings concerning the clinical phenotypes and the molecular pathological mechanisms underlying this particular group of disorders.

Human Mitochondrial Pathologies of the Respiratory Chain and ATP Synthase: Contributions from Studies of Saccharomyces cerevisiae

The ease with which the unicellular yeast Saccharomyces cerevisiae can be manipulated genetically and biochemically has established this organism as a good model for the study of human mitochondrial diseases. The combined use of biochemical and molecular genetic tools has been instrumental in elucidating the functions of numerous yeast nuclear gene products with human homologs that affect a large number of metabolic and biological processes, including those housed in mitochondria. These include structural and catalytic subunits of enzymes and protein factors that impinge on the biogenesis of the respiratory chain. This article will review what is currently known about the genetics and clinical phenotypes of mitochondrial diseases of the respiratory chain and ATP synthase, with special emphasis on the contribution of information gained from pet mutants with mutations in nuclear genes that impair mitochondrial respiration. Our intent is to provide the yeast mitochondrial specialist with basic knowledge of human mitochondrial pathologies and the human specialist with information on how genes that directly and indirectly affect respiration were identified and characterized in yeast.

COA6 Is Structurally Tuned to Function as a Thiol-Disulfide Oxidoreductase in Copper Delivery to Mitochondrial Cytochrome c Oxidase

In eukaryotes, cellular respiration is driven by mitochondrial cytochrome c oxidase (CcO), an enzyme complex that requires copper cofactors for its catalytic activity. Insertion of copper into its catalytically active subunits, including COX2, is a complex process that requires metallochaperones and redox proteins including SCO1, SCO2, and COA6, a recently discovered protein whose molecular function is unknown. To uncover the molecular mechanism by which COA6 and SCO proteins mediate copper delivery to COX2, we have solved the solution structure of COA6, which reveals a coiled-coil-helix-coiled-coil-helix domain typical of redox-active proteins found in the mitochondrial inter-membrane space. Accordingly, we demonstrate that COA6 can reduce the copper-coordinating disulfides of its client proteins, SCO1 and COX2, allowing for copper binding. Finally, our determination of the interaction surfaces and reduction potentials of COA6 and its client proteins provides a mechanism of how metallochaperone and disulfide reductase activities are coordinated to deliver copper to CcO.

Soma et al. reports the solution structure of cytochrome c oxidase assembly factor COA6 and establishes that it functions as a thiol-disulfide oxidoreductase in a relay system that delivers copper to COX2, a copper-containing subunit of the mitochondrial cytochrome c oxidase.

Molecular Perspectives of Mitochondrial Adaptations and Their Role in Cardiac Proteostasis

Mitochondria are the key to properly functioning energy generation in the metabolically demanding cardiomyocytes and thus essential to healthy heart contractility on a beat-to-beat basis. Mitochondria being the central organelle for cellular metabolism and signaling in the heart, its dysfunction leads to cardiovascular disease. The healthy mitochondrial functioning critical to maintaining cardiomyocyte viability and contractility is accomplished by adaptive changes in the dynamics, biogenesis, and degradation of the mitochondria to ensure cellular proteostasis. Recent compelling evidence suggests that the classical protein quality control system in cardiomyocytes is also under constant mitochondrial control, either directly or indirectly. Impairment of cytosolic protein quality control may affect the position of the mitochondria in relation to other organelles, as well as mitochondrial morphology and function, and could also activate mitochondrial proteostasis. Despite a growing interest in the mitochondrial quality control system, very little information is available about the molecular function of mitochondria in cardiac proteostasis. In this review, we bring together current understanding of the adaptations and role of the mitochondria in cardiac proteostasis and describe the adaptive/maladaptive changes observed in the mitochondrial network required to maintain proteomic integrity. We also highlight the key mitochondrial signaling pathways activated in response to proteotoxic stress as a cellular mechanism to protect the heart from proteotoxicity. A deeper understanding of the molecular mechanisms of mitochondrial adaptations and their role in cardiac proteostasis will help to develop future therapeutics to protect the heart from cardiovascular diseases.

Mitochondrial disease disrupts hepatic allostasis and lowers the threshold for immune-mediated liver toxicity

Objective

In individuals with mitochondrial disease, respiratory viral infection can result in metabolic decompensation with mitochondrial hepatopathy. Here, we used a mouse model of liver-specific Complex IV deficiency to study hepatic allostasis during respiratory viral infection.

Methods

Mice with hepatic cytochrome c oxidase deficiency (LivCox10^−/−) were infected with aerosolized influenza, A/PR/8 (PR8), and euthanized on day five after infection following three days of symptoms. This time course is marked by a peak in inflammatory cytokines and mimics the timing of a common clinical scenario in which caregivers may first attempt to manage the illness at home before seeking medical attention. Metabolic decompensation and mitochondrial hepatopathy in mice were characterized by serum hepatic testing, histology, electron microscopy, biochemistry, metabolomics, and bioenergetic profiling.

Results

Following influenza infection, LivCox10^−/− mice displayed marked liver disease including hepatitis, enlarged mitochondria with cristae loss, and hepatic steatosis. This pathophysiology was associated with viremia. Primary hepatocytes from LivCox10^−/− mice cocultured with WT Kupffer cells in the presence of PR8 showed enhanced lipid accumulation. Treatment of hepatocytes with recombinant TNFα implicated Kupffer cell-derived TNFα as a precipitant of steatosis in LivCox10^−/− mice. Eliminating Kupffer cells or blocking TNFα in vivo during influenza infection mitigated the steatosis and mitochondrial morphologic changes.

Conclusions

Taken together, our data shift the narrative of metabolic decompensation in mitochondrial hepatopathy beyond the bioenergetic costs of infection to include an underlying susceptibility to immune-mediated damage. Moreover, our work suggests that immune modulation during metabolic decompensation in mitochondrial disease represents a future viable treatment strategy needing further exploration.

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Influenza infection leads to worsening mitochondrial function and steatohepatitis in a model of mitochondrial hepatopathy.

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Kupffer cells may mediate this damage by the uptake of influenza virus and the secretion of TNFa.

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Hepatocytes affected by mitochondrial disease have a lower threshold for immune mediated toxicity by TNFa.

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Modulating the immune response leads to an improvement in the phenotype.

Modular biogenesis of mitochondrial respiratory complexes

Mitochondrial function relies on the activity of oxidative phosphorylation to synthesise ATP and generate an electrochemical gradient across the inner mitochondrial membrane. These coupled processes are mediated by five multi-subunit complexes that reside in this inner membrane. These complexes are the product of both nuclear and mitochondrial gene products. Defects in the function or assembly of these complexes can lead to mitochondrial diseases due to deficits in energy production and mitochondrial functions. Appropriate biogenesis and function are mediated by a complex number of assembly factors that promote maturation of specific complex subunits to form the active oxidative phosphorylation complex. The understanding of the biogenesis of each complex has been informed by studies in both simple eukaryotes such as Saccharomyces cerevisiae and human patients with mitochondrial diseases. These studies reveal each complex assembles through a pathway using specific subunits and assembly factors to form kinetically distinct but related assembly modules. The current understanding of these complexes has embraced the revolutions in genomics and proteomics to further our knowledge on the impact of mitochondrial biology in genetics, medicine, and evolution.

Structural and functional characterization of the mitochondrial complex IV assembly factor Coa6

Assembly factors play key roles in the biogenesis of many multi-subunit protein complexes regulating their stability, activity, and the incorporation of essential cofactors. The human assembly factor Coa6 participates in the biogenesis of the Cu_A site in complex IV (cytochrome c oxidase, COX). Patients with mutations in Coa6 suffer from mitochondrial disease due to complex IV deficiency. Here, we present the crystal structures of human Coa6 and the pathogenic ^W59CCoa6-mutant protein. These structures show that Coa6 has a 3-helical bundle structure, with the first 2 helices tethered by disulfide bonds, one of which likely provides the copper-binding site. Disulfide-mediated oligomerization of the ^W59CCoa6 protein provides a structural explanation for the loss-of-function mutation.

Identification of Surf1 as an assembly factor of the cytochrome bc1-aa3 supercomplex of Actinobacteria

Respiration in aerobic Actinobacteria involves a cytochrome bc₁-aa₃ supercomplex with a diheme cytochrome c₁, first isolated from Corynebacterium glutamicum. Synthesis of a functional cytochrome c oxidase requires incorporation of Cu_A, Cu_B, heme a, and heme a₃. In contrast to eukaryotes and α-proteobacteria, this process is poorly understood in Actinobacteria. Here, we analyzed the role of a Surf1 homolog of C. glutamicum in the formation of a functional bc₁-aa₃ supercomplex. Deletion of the surf1 gene (cg2460) in C. glutamicum caused a growth defect and cytochrome spectra revealed reduced levels of cytochrome c and a and an increased level of cytochrome d. Membranes of the Δsurf1 strain had lost the ability to oxidize the artificial electron donor N,N,N',N'-tetramethyl-p-phenylenediamine, suggesting that Surf1 is essential for the formation of a functional cytochrome aa₃ oxidase. In contrast to the wild type, a bc₁-aa₃ supercomplex could not be purified from solubilized membranes of the Δsurf1 mutant. A transcriptome comparison revealed that the genes of the SigC regulon including those for cytochrome bd oxidase were upregulated in the Δsurf1 strain as well as the copper deprivation-inducible gene ctiP. Complementation studies showed that the Surf1 homologs of Corynebacterium diphtheriae, Mycobacterium smegmatis and Mycobacterium tuberculosis could at least partially abolish the growth defect of the C. glutamicum Δsurf1 mutant, suggesting that Surf1 is a conserved assembly factor for actinobacterial cytochrome aa₃ oxidase.

Modulating effect of tiron on the capability of mitochondrial oxidative phosphorylation in the brain of rats exposed to radiation or manganese toxicity

The brain is an important organ rich in mitochondria and more susceptible to oxidative stress. Tiron (sodium 4,5-dihydroxybenzene-1,3-disulfonate) is a potent antioxidant. This study aims to evaluate the effect of tiron on the impairment of brain mitochondria induced by exposure to radiation or manganese (Mn) toxicity. We assessed the capability of oxidative phosphorylation (OXPHOS) through determination of mitochondrial redox state, the activity of electron transport chain (ETC), and Krebs cycle as well as the level of adenosine triphosphate (ATP) production. Rats were exposed to 7 Gy of γ-rays or injected i.p. with manganese chloride (100 mg/kg), then treated with tiron (471 mg/kg) for 7 days. The results showed that tiron treatment revealed positive modulation on the mitochondrial redox state manifested by a marked decrease of hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and total nitrate/nitrite (NO_x) associated with a significant increase in total antioxidant capacity (TAC), glutathione (GSH) content, manganese superoxide dismutase (MnSOD), and glutathione peroxidase (GPx) activities. Moreover, tiron can increase the activity of ETC through preventing the depletion in the activity of mitochondrial complexes (I, II, III, and IV), an elevation of coenzyme Q10 (CoQ10) and cytochrome c (Cyt-c) levels. Additionally, tiron showed a noticeable increase in mitochondrial aconitase (mt-aconitase) activity as the major component of Krebs cycle to maintain a high level of ATP production. Tiron also can restore mitochondrial metal homeostasis through positive changes in the levels of calcium (Ca), iron (Fe), Mn, and copper (Cu). It can be concluded that tiron may be used as a good mitigating agent to attenuate the harmful effects on the brain through the inhibition of mitochondrial injury post-exposure to radiation or Mn toxicity.

Mitochondrial Disease Caused by a Novel Homozygous Mutation (Gly106del) in the SCO1 Gene

The cytochrome C oxidase assembly protein SCO1 gene encodes a mitochondrial protein essential for the mammalian energy metabolism. Only three pedigrees of SCO1mutations have thus far been reported. They all presented with lactate acidosis and encephalopathy. Two had hepatopathy and hypotonia, and the other presented with intrauterine growth retardation and hypertrophic cardiomyopathy leading to cardiac failure. Mitochondrial disease may manifest in neonates, but early diagnosis has so far been difficult. Here, we present a novel mutation in the SCO1 gene: in-frame deletion (Gly106del)with a different phenotype without encephalopathy, hepatopathy, hypotonia, or cardiac involvement. Within the first 2 h the girl developed hypoglycemia and severe chronic lactate acidosis. Because of the improved technique in whole exome sequencing, an early diagnosis was made when the girl was only 9 days old, which enabled the prediction of prognosis as well as level of treatment. She died at 1 month of age.

Human diseases associated with defects in assembly of OXPHOS complexes

The structural biogenesis and functional proficiency of the multiheteromeric complexes forming the mitochondrial oxidative phosphorylation system (OXPHOS) require the concerted action of a number of chaperones and other assembly factors, most of which are specific for each complex. Mutations in a large number of these assembly factors are responsible for mitochondrial disorders, in most cases of infantile onset, typically characterized by biochemical defects of single specific complexes. In fact, pathogenic mutations in complex-specific assembly factors outnumber, in many cases, the repertoire of mutations found in structural subunits of specific complexes. The identification of patients with specific defects in assembly factors has provided an important contribution to the nosological characterization of mitochondrial disorders, and has also been a crucial means to identify a huge number of these proteins in humans, which play an essential role in mitochondrial bioenergetics. The wide use of next generation sequencing (NGS) has led to and will allow the identifcation of additional components of the assembly machinery of individual complexes, mutations of which are responsible for human disorders. The functional studies on patients' specimens, together with the creation and characterization of in vivo models, are fundamental to better understand the mechanisms of each of them. A new chapter in this field will be, in the near future, the discovery of mechanisms and actions underlying the formation of supercomplexes, molecular structures formed by the physical, and possibly functional, interaction of some of the individual respiratory complexes, particularly complex I (CI), III (CIII), and IV (CIV).

Mitochondrial dysfunction and its role in tissue-specific cellular stress

Mitochondrial bioenergetics require the coordination of two different and independent genomes. Mutations in either genome will affect mitochondrial functionality and produce different sources of cellular stress. Depending on the kind of defect and stress, different tissues and organs will be affected, leading to diverse pathological conditions. There is no curative therapy for mitochondrial diseases, nevertheless, there are strategies described that fight the various stress forms caused by the malfunctioning organelles. Here, we will revise the main kinds of stress generated by mutations in mitochondrial genes and outline several ways of fighting this stress.

The mitochondrial metallochaperone SCO1 maintains CTR1 at the plasma membrane to preserve copper homeostasis in the murine heart

SCO1 is a ubiquitously expressed, mitochondrial protein with essential roles in cytochrome c oxidase (COX) assembly and the regulation of copper homeostasis. SCO1 patients present with severe forms of early onset disease, and ultimately succumb from liver, heart or brain failure. However, the inherent susceptibility of these tissues to SCO1 mutations and the clinical heterogeneity observed across SCO1 pedigrees remain poorly understood phenomena. To further address this issue, we generated Sco1hrt/hrt and Sco1stm/stm mice in which Sco1 was specifically deleted in heart and striated muscle, respectively. Lethality was observed in both models due to a combined COX and copper deficiency that resulted in a dilated cardiomyopathy. Left ventricular dilation and loss of heart function was preceded by a temporal decrease in COX activity and copper levels in the longer-lived Sco1stm/stm mice. Interestingly, the reduction in copper content of Sco1stm/stm cardiomyocytes was due to the mislocalisation of CTR1, the high affinity transporter that imports copper into the cell. CTR1 was similarly mislocalized to the cytosol in the heart of knockin mice carrying a homozygous G115S substitution in Sco1, which in humans causes a hypertrophic cardiomyopathy. Our current findings in the heart are in marked contrast to our prior observations in the liver, where Sco1 deletion results in a near complete absence of CTR1 protein. These data collectively argue that mutations perturbing SCO1 function have tissue-specific consequences for the machinery that ultimately governs copper homeostasis, and further establish the importance of aberrant mitochondrial signaling to the etiology of copper handling disorders.

LACE1 interacts with p53 and mediates its mitochondrial translocation and apoptosis

p53 is a major cellular tumor suppressor that in addition to its nuclear, transcription-dependent activity is also known to function extranuclearly. Cellular stressors such as reactive oxygen species can promote translocation of p53 into mitochondria where it acts to protect mitochondrial genome or trigger cell death via transcription-independent manner. Here we report that the mammalian homologue of yeast mitochondrial Afg1 ATPase (LACE1) promotes translocation of p53 into mitochondria. We further show that LACE1 exhibits significant pro-apoptotic activity, which is dependent on p53, and that the protein is required for normal mitochondrial respiratory function. LACE1 physically interacts with p53 and is necessary for mitomycin c-induced translocation of p53 into mitochondria. Conversely, increased expression of LACE1 partitions p53 to mitochondria, causes reduction in nuclear p53 content and induces apoptosis. Thus, LACE1 mediates mitochondrial translocation of p53 and its transcription-independent apoptosis.

The mitochondrial TMEM177 associates with COX20 during COX2 biogenesis

The three mitochondrial-encoded proteins, COX1, COX2, and COX3, form the core of the cytochrome c oxidase. Upon synthesis, COX2 engages with COX20 in the inner mitochondrial membrane, a scaffold protein that recruits metallochaperones for copper delivery to the CuA-Site of COX2. Here we identified the human protein, TMEM177 as a constituent of the COX20 interaction network. Loss or increase in the amount of TMEM177 affects COX20 abundance leading to reduced or increased COX20 levels respectively. TMEM177 associates with newly synthesized COX2 and SCO2 in a COX20-dependent manner. Our data shows that by unbalancing the amount of TMEM177, newly synthesized COX2 accumulates in a COX20-associated state. We conclude that TMEM177 promotes assembly of COX2 at the level of CuA-site formation.

COX16 promotes COX2 metallation and assembly during respiratory complex IV biogenesis

Cytochrome c oxidase of the mitochondrial oxidative phosphorylation system reduces molecular oxygen with redox equivalent-derived electrons. The conserved mitochondrial-encoded COX1- and COX2-subunits are the heme- and copper-center containing core subunits that catalyze water formation. COX1 and COX2 initially follow independent biogenesis pathways creating assembly modules with subunit-specific, chaperone-like assembly factors that assist in redox centers formation. Here, we find that COX16, a protein required for cytochrome c oxidase assembly, interacts specifically with newly synthesized COX2 and its copper center-forming metallochaperones SCO1, SCO2, and COA6. The recruitment of SCO1 to the COX2-module is COX16- dependent and patient-mimicking mutations in SCO1 affect interaction with COX16. These findings implicate COX16 in Cu_A-site formation. Surprisingly, COX16 is also found in COX1-containing assembly intermediates and COX2 recruitment to COX1. We conclude that COX16 participates in merging the COX1 and COX2 assembly lines.

The mitochondrion: a central architect of copper homeostasis

All known eukaryotes require copper for their development and survival. The essentiality of copper reflects its widespread use as a co-factor in conserved enzymes that catalyze biochemical reactions critical to energy production, free radical detoxification, collagen deposition, neurotransmitter biosynthesis and iron homeostasis. However, the prioritized use of copper poses an organism with a considerable challenge because, in its unbound form, copper can potentiate free radical production and displace iron-sulphur clusters to disrupt protein function. Protective mechanisms therefore evolved to mitigate this challenge and tightly regulate the acquisition, trafficking and storage of copper such that the metal ion is rarely found in its free form in the cell. Findings by a number of groups over the last ten years emphasize that this regulatory framework forms the foundation of a system that is capable of monitoring copper status and reprioritizing copper usage at both the cellular and systemic levels of organization. While the identification of relevant molecular mechanisms and signaling pathways has proven to be difficult and remains a barrier to our full understanding of the regulation of copper homeostasis, mounting evidence points to the mitochondrion as a pivotal hub in this regard in both healthy and diseased states. Here, we review our current understanding of copper handling pathways contained within the organelle and consider plausible mechanisms that may serve to functionally couple their activity to that of other cellular copper handling machinery to maintain copper homeostasis.

Building the CuA site of cytochrome c oxidase: A complicated, redox-dependent process driven by a surprisingly large complement of accessory proteins

Cytochrome c oxidase (COX) was initially purified more than 70 years ago. A tremendous amount of insight into its structure and function has since been gleaned from biochemical, biophysical, genetic, and molecular studies. As a result, we now appreciate that COX relies on its redox-active metal centers (heme a and a₃, Cu_A and Cu_B) to reduce oxygen and pump protons in a reaction essential for most eukaryotic life. Questions persist, however, about how individual structural subunits are assembled into a functional holoenzyme. Here, we focus on what is known and what remains to be learned about the accessory proteins that facilitate Cu_A site maturation.

Tissue- and Condition-Specific Isoforms of Mammalian Cytochrome c Oxidase Subunits: From Function to Human Disease

Cytochrome c oxidase (COX) is the terminal enzyme of the electron transport chain and catalyzes the transfer of electrons from cytochrome c to oxygen. COX consists of 14 subunits, three and eleven encoded, respectively, by the mitochondrial and nuclear DNA. Tissue- and condition-specific isoforms have only been reported for COX but not for the other oxidative phosphorylation complexes, suggesting a fundamental requirement to fine-tune and regulate the essentially irreversible reaction catalyzed by COX. This article briefly discusses the assembly of COX in mammals and then reviews the functions of the six nuclear-encoded COX subunits that are expressed as isoforms in specialized tissues including those of the liver, heart and skeletal muscle, lung, and testes: COX IV-1, COX IV-2, NDUFA4, NDUFA4L2, COX VIaL, COX VIaH, COX VIb-1, COX VIb-2, COX VIIaH, COX VIIaL, COX VIIaR, COX VIIIH/L, and COX VIII-3. We propose a model in which the isoforms mediate the interconnected regulation of COX by (1) adjusting basal enzyme activity to mitochondrial capacity of a given tissue; (2) allosteric regulation to adjust energy production to need; (3) altering proton pumping efficiency under certain conditions, contributing to thermogenesis; (4) providing a platform for tissue-specific signaling; (5) stabilizing the COX dimer; and (6) modulating supercomplex formation.

Cytochrome c Oxidase Biogenesis and Metallochaperone Interactions: Steps in the Assembly Pathway of a Bacterial Complex

Biogenesis of mitochondrial cytochrome c oxidase (COX) is a complex process involving the coordinate expression and assembly of numerous subunits (SU) of dual genetic origin. Moreover, several auxiliary factors are required to recruit and insert the redox-active metal compounds, which in most cases are buried in their protein scaffold deep inside the membrane. Here we used a combination of gel electrophoresis and pull-down assay techniques in conjunction with immunostaining as well as complexome profiling to identify and analyze the composition of assembly intermediates in solubilized membranes of the bacterium Paracoccus denitrificans. Our results show that the central SUI passes through at least three intermediate complexes with distinct subunit and cofactor composition before formation of the holoenzyme and its subsequent integration into supercomplexes. We propose a model for COX biogenesis in which maturation of newly translated COX SUI is initially assisted by CtaG, a chaperone implicated in CuB site metallation, followed by the interaction with the heme chaperone Surf1c to populate the redox-active metal-heme centers in SUI. Only then the remaining smaller subunits are recruited to form the mature enzyme which ultimately associates with respiratory complexes I and III into supercomplexes.

Mitochondrial disease genes COA6, COX6B and SCO2 have overlapping roles in COX2 biogenesis

Biogenesis of cytochrome c oxidase (CcO), the terminal enzyme of the mitochondrial respiratory chain, is a complex process facilitated by several assembly factors. Pathogenic mutations were recently reported in one such assembly factor, COA6, and our previous work linked Coa6 function to mitochondrial copper metabolism and expression of Cox2, a copper-containing subunit of CcO. However, the precise role of Coa6 in Cox2 biogenesis remained unknown. Here we show that yeast Coa6 is an orthologue of human COA6, and like Cox2, is regulated by copper availability, further implicating it in copper delivery to Cox2. In order to place Coa6 in the Cox2 copper delivery pathway, we performed a comprehensive genetic epistasis analysis in the yeast Saccharomyces cerevisiae and found that simultaneous deletion of Coa6 and Sco2, a mitochondrial copper metallochaperone, or Coa6 and Cox12/COX6B, a structural subunit of CcO, completely abrogates Cox2 biogenesis. Unlike Coa6 deficient cells, copper supplementation fails to rescue Cox2 levels of these double mutants. Overexpression of Cox12 or Sco proteins partially rescues the coa6Δ phenotype, suggesting their overlapping but non-redundant roles in copper delivery to Cox2. These genetic data are strongly corroborated by biochemical studies demonstrating physical interactions between Coa6, Cox2, Cox12 and Sco proteins. Furthermore, we show that patient mutations in Coa6 disrupt Coa6-Cox2 interaction, providing the biochemical basis for disease pathogenesis. Taken together, these results place COA6 in the copper delivery pathway to CcO and, surprisingly, link it to a previously unidentified function of CcO subunit Cox12 in Cox2 biogenesis.

MITRAC7 Acts as a COX1-Specific Chaperone and Reveals a Checkpoint during Cytochrome c Oxidase Assembly

Cytochrome c oxidase, the terminal enzyme of the respiratory chain, is assembled from mitochondria- and nuclear-encoded subunits. The MITRAC complex represents the central assembly intermediate during this process as it receives imported subunits and regulates mitochondrial translation of COX1 mRNA. The molecular processes that promote and regulate the progression of assembly downstream of MITRAC are still unknown. Here, we identify MITRAC7 as a constituent of a late form of MITRAC and as a COX1-specific chaperone. MITRAC7 is required for cytochrome c oxidase biogenesis. Surprisingly, loss of MITRAC7 or an increase in its amount causes selective cytochrome c oxidase deficiency in human cells. We demonstrate that increased MITRAC7 levels stabilize and trap COX1 in MITRAC, blocking progression in the assembly process. In contrast, MITRAC7 deficiency leads to turnover of newly synthesized COX1. Accordingly, MITRAC7 affects the biogenesis pathway by stabilizing newly synthesized COX1 in assembly intermediates, concomitantly preventing turnover.

Cooperation between COA6 and SCO2 in COX2 maturation during cytochrome c oxidase assembly links two mitochondrial cardiomyopathies

Three mitochondria-encoded subunits form the catalytic core of cytochrome c oxidase, the terminal enzyme of the respiratory chain. COX1 and COX2 contain heme and copper redox centers, which are integrated during assembly of the enzyme. Defects in this process lead to an enzyme deficiency and manifest as mitochondrial disorders in humans. Here we demonstrate that COA6 is specifically required for COX2 biogenesis. Absence of COA6 leads to fast turnover of newly synthesized COX2 and a concomitant reduction in cytochrome c oxidase levels. COA6 interacts transiently with the copper-containing catalytic domain of newly synthesized COX2. Interestingly, similar to the copper metallochaperone SCO2, loss of COA6 causes cardiomyopathy in humans. We show that COA6 and SCO2 interact and that corresponding pathogenic mutations in each protein affect complex formation. Our analyses define COA6 as a constituent of the mitochondrial copper relay system, linking defects in COX2 metallation to cardiac cytochrome c oxidase deficiency.

Cardiac deficiency of single cytochrome oxidase assembly factor scox induces p53-dependent apoptosis in a Drosophila cardiomyopathy model

The heart is a muscle with high energy demands. Hence, most patients with mitochondrial disease produced by defects in the oxidative phosphorylation (OXPHOS) system are susceptible to cardiac involvement. The presentation of mitochondrial cardiomyopathy includes hypertrophic, dilated and left ventricular noncompaction, but the molecular mechanisms involved in cardiac impairment are unknown. One of the most frequent OXPHOS defects in humans frequently associated with cardiomyopathy is cytochrome c oxidase (COX) deficiency caused by mutations in COX assembly factors such as Sco1 and Sco2. To investigate the molecular mechanisms that underlie the cardiomyopathy associated with Sco deficiency, we have heart specifically interfered scox expression, the single Drosophila Sco orthologue. Cardiac-specific knockdown of scox reduces fly lifespan, and it severely compromises heart function and structure, producing dilated cardiomyopathy. Cardiomyocytes with low levels of scox have a significant reduction in COX activity and they undergo a metabolic switch from OXPHOS to glycolysis, mimicking the clinical features found in patients harbouring Sco mutations. The major cardiac defects observed are produced by a significant increase in apoptosis, which is dp53-dependent. Genetic and molecular evidence strongly suggest that dp53 is directly involved in the development of the cardiomyopathy induced by scox deficiency. Remarkably, apoptosis is enhanced in the muscle and liver of Sco2 knock-out mice, clearly suggesting that cell death is a key feature of the COX deficiencies produced by mutations in Sco genes in humans.

The Mitochondrial Metallochaperone SCO1 Is Required to Sustain Expression of the High-Affinity Copper Transporter CTR1 and Preserve Copper Homeostasis

Human SCO1 fulfills essential roles in cytochrome c oxidase (COX) assembly and the regulation of copper (Cu) homeostasis, yet it remains unclear why pathogenic mutations in this gene cause such clinically heterogeneous forms of disease. Here, we establish a Sco1 mouse model of human disease and show that ablation of Sco1 expression in the liver is lethal owing to severe COX and Cu deficiencies. We further demonstrate that the Cu deficiency is explained by a functional connection between SCO1 and CTR1, the high-affinity transporter that imports Cu into the cell. CTR1 is rapidly degraded in the absence of SCO1 protein, and we show that its levels are restored in Sco1^-/- mouse embryonic fibroblasts upon inhibition of the proteasome. These data suggest that mitochondrial signaling through SCO1 provides a post-translational mechanism to regulate CTR1-dependent Cu import into the cell, and they further underpin the importance of mitochondria in cellular Cu homeostasis.

Mitochondrial Diseases Part I: mouse models of OXPHOS deficiencies caused by defects in respiratory complex subunits or assembly factors

Mitochondrial disorders are the most common inborn errors of metabolism affecting the oxidative phosphorylation system (OXPHOS). Because of the poor knowledge of the pathogenic mechanisms, a cure for these disorders is still unavailable and all the treatments currently in use are supportive more than curative. Therefore, in the past decade a great variety of mouse models have been developed to assess the in vivo function of several mitochondrial proteins involved in human diseases. Due to the genetic and physiological similarity to humans, mice represent reliable models to study the pathogenic mechanisms of mitochondrial disorders and are precious to test new therapeutic approaches. Here we summarize the features of several mouse models of mitochondrial diseases directly related to defects in subunits of the OXPHOS complexes or in assembly factors. We discuss how these models recapitulate many human conditions and how they have contributed to the understanding of mitochondrial function in health and disease.

Deciphering protein-protein interactions during the biogenesis of cytochrome c oxidase from Paracoccus denitrificans

Biogenesis of the mitochondrial cytochrome c oxidase (COX) is a complex process due to its numerous subunits encoded by two genomes, as well as the localization of redox centers deep within the membrane. Here, we have assessed the biogenesis of the homologous aa₃-type oxidase of the soil bacterium Paracoccus denitrificans. First, protein partners were analyzed using various membrane solubilization strategies to show interactions between COX and CtaG, a chaperone implicated in CuB site metallation. Using an unbiased MS approach after immunological pull-down from untreated or cross-linked membranes, we then extend our view towards a hypothetical 'biogenesis complex' by identifying two further metal-inserting chaperones, Surf1c and Sco, together with enzymes catalyzing heme a synthesis. Our study also tentatively supports previous speculation regarding the existence of a predominantly co-translational mechanism for cofactor insertion during COX biogenesis.

Mutations in COA6 cause cytochrome c oxidase deficiency and neonatal hypertrophic cardiomyopathy

COA6/C1ORF31 is involved in cytochrome c oxidase (complex IV) biogenesis. We present a new pathogenic COA6 variant detected in a patient with neonatal hypertrophic cardiomyopathy and isolated complex IV deficiency. For the first time, clinical details about a COA6-deficient patient are given and patient fibroblasts are functionally characterized: COA6 protein is undetectable and steady-state levels of complex IV and several of its subunits are reduced. The monomeric COX1 assembly intermediate accumulates. Using pulse-chase experiments, we demonstrate an increased turnover of mitochondrial encoded complex IV subunits. Although monomeric complex IV is decreased in patient fibroblasts, the CI/CIII2 /CIVn -supercomplexes remain unaffected. Copper supplementation shows a partial rescue of complex IV deficiency in patient fibroblasts. We conclude that COA6 is required for complex IV subunit stability. Furthermore, the proposed role in the copper delivery pathway to complex IV subunits is substantiated and a therapeutic lead for COA6-deficient patients is provided.

Protein chaperones mediating copper insertion into the CuA site of the aa3-type cytochrome c oxidase of Paracoccus denitrificans

The biogenesis of the mitochondrial cytochrome c oxidase is a complex process involving the stepwise assembly of its multiple subunits encoded by two genetic systems. Moreover, several chaperones are required to recruit and insert the redox-active metal centers into subunits I and II, two a-type hemes and a total of three copper ions, two of which form the CuA center located in a hydrophilic domain of subunit II. The copper-binding Sco protein(s) have been implicated with the metallation of this site in various model organisms. Here we analyze the role of the two Sco homologues termed ScoA and ScoB, along with two other copper chaperones, on the biogenesis of the cytochrome c oxidase in the bacterium Paracoccus denitrificans by deleting each of the four genes individually or pairwise, followed by assessing the functionality of the assembled oxidase both in intact membranes and in the purified enzyme complex. Copper starvation leads to a drastic decrease of oxidase activity in membranes from strains involving the scoB deletion. This loss is shown to be of dual origin, (i) a severe drop in steady-state oxidase levels in membranes, and (ii) a diminished enzymatic activity of the remaining oxidase complex, traced back to a lower copper content, specifically in the CuA site of the enzyme. Neither of the other proteins addressed here, ScoA or the two PCu proteins, exhibit a direct effect on the metallation of the CuA site in P. denitrificans, but are discussed as potential interaction partners of ScoB.

Large copy number variations in combination with point mutations in the TYMP and SCO2 genes found in two patients with mitochondrial disorders

Mitochondrial disorders are caused by defects in mitochondrial or nuclear DNA. Although the existence of large deletions in mitochondrial DNA (mtDNA) is well known, deletions affecting whole genes are not commonly described in patients with mitochondrial disorders. Based on the results of whole-genome analyses, copy number variations (CNVs) occur frequently in the human genome and may overlap with many genes associated with clinical phenotypes. We report the discovery of two large heterozygous CNVs on 22q13.33 in two patients with mitochondrial disorders. The first patient harboured a novel point mutation c.667G>A (p.D223N) in the SCO2 gene in combination with a paternally inherited 87-kb deletion. As hypertrophic cardiomyopathy (HCMP) was not documented in the patient, this observation prompted us to compare his clinical features with all 44 reported SCO2 patients in the literature. Surprisingly, the review shows that HCMP was present in only about 50% of the SCO2 patients with non-neonatal onset. In the second patient, who had mitochondrial neurogastrointestinal encephalopathy (MNGIE), a maternally inherited 175-kb deletion and the paternally inherited point mutation c.261G>T (p.E87D) in the TYMP gene were identified.

Gastrointestinal and hepatic manifestations of mitochondrial disorders

Inherited defects of oxidative phosphorylation lead to heterogeneous, often multisystem, mitochondrial diseases. This review highlights those mitochondrial syndromes with prominent gastrointestinal and hepatic symptoms, categorised according to underlying disease mechanism. Mitochondrial encephalopathies with major gastrointestinal involvement include mitochondrial neurogastrointestinal encephalopathy and ethylmalonic encephalopathy, which are each associated with highly specific clinical and metabolic profiles. Mitochondrial hepatopathies are most frequently caused by defects of mitochondrial DNA maintenance and expression. Although mitochondrial disorders are notorious for extreme clinical, biochemical and genetic heterogeneity, there are some pathognomonic clinical and metabolic clues that suggest a specific diagnosis, and these are highlighted. An approach to diagnosis of these complex disorders is presented, together with a genetic classification, including mitochondrial DNA disorders and nuclear-encoded defects of mitochondrial DNA maintenance and translation, OXPHOS complex assembly and mitochondrial membrane lipids. Finally, supportive and experimental therapeutic options for these currently incurable diseases are reviewed, including liver transplantation, allogeneic haematopoietic stem cell transplantation and gene therapy.

hCOA3 stabilizes cytochrome c oxidase 1 (COX1) and promotes cytochrome c oxidase assembly in human mitochondria

Cytochrome c oxidase (COX) or complex IV of the mitochondrial respiratory chain plays a fundamental role in energy production of aerobic cells. In humans, COX deficiency is the most frequent cause of mitochondrial encephalomyopathies. Human COX is composed of 13 subunits of dual genetic origin, whose assembly requires an increasing number of nuclear-encoded accessory proteins known as assembly factors. Here, we have identified and characterized human CCDC56, an 11.7-kDa mitochondrial transmembrane protein, as a new factor essential for COX biogenesis. CCDC56 shares sequence similarity with the yeast COX assembly factor Coa3 and was termed hCOA3. hCOA3-silenced cells display a severe COX functional alteration owing to a decreased stability of newly synthesized COX1 and an impairment in the holoenzyme assembly process. We show that hCOA3 physically interacts with both the mitochondrial translation machinery and COX structural subunits. We conclude that hCOA3 stabilizes COX1 co-translationally and promotes its assembly with COX partner subunits. Finally, our results identify hCOA3 as a new candidate when screening for genes responsible for mitochondrial diseases associated with COX deficiency.

COX19 mediates the transduction of a mitochondrial redox signal from SCO1 that regulates ATP7A-mediated cellular copper efflux

The study of patient tissues and cell lines shows that SCO1 and SCO2 function collaboratively to generate a redox-dependent signal that is transduced from mitochondria to the cytosol by COX19, where it is relayed to ATP7A to regulate the rate of copper efflux from the cell.

SCO1 and SCO2 are metallochaperones whose principal function is to add two copper ions to the catalytic core of cytochrome c oxidase (COX). However, affected tissues of SCO1 and SCO2 patients exhibit a combined deficiency in COX activity and total copper content, suggesting additional roles for these proteins in the regulation of cellular copper homeostasis. Here we show that both the redox state of the copper-binding cysteines of SCO1 and the abundance of SCO2 correlate with cellular copper content and that these relationships are perturbed by mutations in SCO1 or SCO2, producing a state of apparent copper overload. The copper deficiency in SCO patient fibroblasts is rescued by knockdown of ATP7A, a trans-Golgi, copper-transporting ATPase that traffics to the plasma membrane during copper overload to promote efflux. To investigate how a signal from SCO1 could be relayed to ATP7A, we examined the abundance and subcellular distribution of several soluble COX assembly factors. We found that COX19 partitions between mitochondria and the cytosol in a copper-dependent manner and that its knockdown partially rescues the copper deficiency in patient cells. These results demonstrate that COX19 is necessary for the transduction of a SCO1-dependent mitochondrial redox signal that regulates ATP7A-mediated cellular copper efflux.

SCO2 induces p53-mediated apoptosis by Thr845 phosphorylation of ASK-1 and dissociation of the ASK-1-Trx complex

p53 prevents cancer via cell cycle arrest, apoptosis, and the maintenance of genome stability. p53 also regulates energy-generating metabolic pathways such as oxidative phosphorylation (OXPHOS) and glycolysis via transcriptional regulation of SCO2 and TIGAR. SCO2, a cytochrome c oxidase assembly factor, is a metallochaperone which is involved in the biogenesis of cytochrome c oxidase subunit II. Here we have shown that SCO2 functions as an apoptotic protein in tumor xenografts, thus providing an alternative pathway for p53-mediated apoptosis. SCO2 increases the generation of reactive oxygen species (ROS) and induces dissociation of the protein complex between apoptosis signal-regulating kinase 1 (ASK-1) (mitogen-activated protein kinase kinase kinase [MAPKKK]) and its cellular inhibitor, the redox-active protein thioredoxin (Trx). Furthermore, SCO2 induces phosphorylation of ASK-1 at the Thr(845) residue, resulting in the activation of the ASK-1 kinase pathway. The phosphorylation of ASK-1 induces the activation of mitogen-activated protein kinase kinases 4 and 7 (MAP2K4/7) and MAP2K3/6, which switches the c-Jun N-terminal protein kinase (JNK)/p38-dependent apoptotic cascades in cancer cells. Exogenous addition of the SCO2 gene to hypoxic cancer cells and hypoxic tumors induces apoptosis and causes significant regression of tumor xenografts. We have thus discovered a novel apoptotic function of SCO2, which activates the ASK-1 kinase pathway in switching "on" an alternate mode of p53-mediated apoptosis. We propose that SCO2 might possess a novel tumor suppressor function via the ROS-ASK-1 kinase pathway and thus could be an important candidate for anticancer gene therapy.