Respiratory chain supercomplexes set the threshold for respiration defects in human mtDNA mutant cybrids

Understanding coenzyme Q

Coenzyme Q (CoQ), also known as ubiquinone, comprises a benzoquinone head group and a long isoprenoid side chain. It is thus extremely hydrophobic and resides in membranes. It is best known for its complex function as an electron transporter in the mitochondrial electron transport chain (ETC) but is also required for several other crucial cellular processes. In fact, CoQ appears to be central to the entire redox balance of the cell. Remarkably, its structure and therefore its properties have not changed from bacteria to vertebrates. In metazoans, it is synthesized in all cells and is found in most, and maybe all, biological membranes. CoQ is also known as a nutritional supplement, mostly because of its involvement with antioxidant defenses. However, whether there is any health benefit from oral consumption of CoQ is not well established. Here we review the function of CoQ as a redox-active molecule in the ETC and other enzymatic systems, its role as a prooxidant in reactive oxygen species generation, and its separate involvement in antioxidant mechanisms. We also review CoQ biosynthesis, which is particularly complex because of its extreme hydrophobicity, as well as the biological consequences of primary and secondary CoQ deficiency, including in human patients. Primary CoQ deficiency is a rare inborn condition due to mutation in CoQ biosynthetic genes. Secondary CoQ deficiency is much more common, as it accompanies a variety of pathological conditions, including mitochondrial disorders as well as aging. In this context, we discuss the importance, but also the great difficulty, of alleviating CoQ deficiency by CoQ supplementation.

A ketogenic diet can mitigate SARS-CoV-2 induced systemic reprogramming and inflammation

The ketogenic diet (KD) has demonstrated benefits in numerous clinical studies and animal models of disease in modulating the immune response and promoting a systemic anti-inflammatory state. Here we investigate the effects of a KD on systemic toxicity in mice following SARS-CoV-2 infection. Our data indicate that under KD, SARS-CoV-2 reduces weight loss with overall improved animal survival. Muted multi-organ transcriptional reprogramming and metabolism rewiring suggest that a KD initiates and mitigates systemic changes induced by the virus. We observed reduced metalloproteases and increased inflammatory homeostatic protein transcription in the heart, with decreased serum pro-inflammatory cytokines (i.e., TNF-α, IL-15, IL-22, G-CSF, M-CSF, MCP-1), metabolic markers of inflammation (i.e., kynurenine/tryptophane ratio), and inflammatory prostaglandins, indicative of reduced systemic inflammation in animals infected under a KD. Taken together, these data suggest that a KD can alter the transcriptional and metabolic response in animals following SARS-CoV-2 infection with improved mice health, reduced inflammation, and restored amino acid, nucleotide, lipid, and energy currency metabolism.

Translation termination in human mitochondria - substrate specificity of mitochondrial release factors

Mitochondria are the essential players in eukaryotic ATP production by oxidative phosphorylation, which relies on the maintenance and accurate expression of the mitochondrial genome. Even though the basic principles of translation are conserved due to the descendance from a bacterial ancestor, some deviations regarding translation factors as well as mRNA characteristics and the applied genetic code are present in human mitochondria. Together, these features are certain challenges during translation the mitochondrion has to handle. Here, we discuss the current knowledge regarding mitochondrial translation focusing on the termination process and the associated quality control mechanisms. We describe how mtRF1a resembles bacterial RF1 mechanistically and summarize in vitro and recent in vivo data leading to the conclusion of mtRF1a being the major mitochondrial release factor. On the other hand, we discuss the ongoing debate about the function of the second codon-dependent mitochondrial release factor mtRF1 regarding its role as a specialized termination factor. Finally, we link defects in mitochondrial translation termination to the activation of mitochondrial rescue mechanisms highlighting the importance of ribosome-associated quality control for sufficient respiratory function and therefore for human health.

Methods to Evaluate Changes in Mitochondrial Structure and Function in Cancer

Mitochondria are regulators of key cellular processes, including energy production and redox homeostasis. Mitochondrial dysfunction is associated with various human diseases, including cancer. Importantly, both structural and functional changes can alter mitochondrial function. Morphologic and quantifiable changes in mitochondria can affect their function and contribute to disease. Structural mitochondrial changes include alterations in cristae morphology, mitochondrial DNA integrity and quantity, and dynamics, such as fission and fusion. Functional parameters related to mitochondrial biology include the production of reactive oxygen species, bioenergetic capacity, calcium retention, and membrane potential. Although these parameters can occur independently of one another, changes in mitochondrial structure and function are often interrelated. Thus, evaluating changes in both mitochondrial structure and function is crucial to understanding the molecular events involved in disease onset and progression. This review focuses on the relationship between alterations in mitochondrial structure and function and cancer, with a particular emphasis on gynecologic malignancies. Selecting methods with tractable parameters may be critical to identifying and targeting mitochondria-related therapeutic options. Methods to measure changes in mitochondrial structure and function, with the associated benefits and limitations, are summarized.

Cooperative assembly of the mitochondrial respiratory chain

Deep understanding of the pathophysiological role of the mitochondrial respiratory chain (MRC) relies on a well-grounded model explaining how its biogenesis is regulated. The lack of a consistent framework to clarify the modes and mechanisms governing the assembly of the MRC complexes and supercomplexes (SCs) works against progress in the field. The plasticity model was postulated as an attempt to explain the coexistence of mammalian MRC complexes as individual entities and associated in SC species. However, mounting data accumulated throughout the years question the universal validity of the plasticity model as originally proposed. Instead, as we argue here, a cooperative assembly model provides a much better explanation to the phenomena observed when studying MRC biogenesis in physiological and pathological settings.

Human mtRF1 terminates COX1 translation and its ablation induces mitochondrial ribosome-associated quality control

Translation termination requires release factors that read a STOP codon in the decoding center and subsequently facilitate the hydrolysis of the nascent peptide chain from the peptidyl tRNA within the ribosome. In human mitochondria eleven open reading frames terminate in the standard UAA or UAG STOP codon, which can be recognized by mtRF1a, the proposed major mitochondrial release factor. However, two transcripts encoding for COX1 and ND6 terminate in the non-conventional AGA or AGG codon, respectively. How translation termination is achieved in these two cases is not known. We address this long-standing open question by showing that the non-canonical release factor mtRF1 is a specialized release factor that triggers COX1 translation termination, while mtRF1a terminates the majority of other mitochondrial translation events including the non-canonical ND6. Loss of mtRF1 leads to isolated COX deficiency and activates the mitochondrial ribosome-associated quality control accompanied by the degradation of COX1 mRNA to prevent an overload of the ribosome rescue system. Taken together, these results establish the role of mtRF1 in mitochondrial translation, which had been a mystery for decades, and lead to a comprehensive picture of translation termination in human mitochondria.

Coordination of metal center biogenesis in human cytochrome c oxidase

Mitochondrial cytochrome c oxidase (CcO) or respiratory chain complex IV is a heme aa₃-copper oxygen reductase containing metal centers essential for holo-complex biogenesis and enzymatic function that are assembled by subunit-specific metallochaperones. The enzyme has two copper sites located in the catalytic core subunits. The COX1 subunit harbors the Cu_B site that tightly associates with heme a₃ while the COX2 subunit contains the binuclear Cu_A site. Here, we report that in human cells the CcO copper chaperones form macromolecular assemblies and cooperate with several twin CX₉C proteins to control heme a biosynthesis and coordinate copper transfer sequentially to the Cu_A and Cu_B sites. These data on CcO illustrate a mechanism that regulates the biogenesis of macromolecular enzymatic assemblies with several catalytic metal redox centers and prevents the accumulation of cytotoxic reactive assembly intermediates.

Cytochrome c Interaction with Cardiolipin Plays a Key Role in Cell Apoptosis: Implications for Human Diseases

In the cell cytochrome, c performs different functions depending on the environment in which it acts; therefore, it has been classified as a multifunction protein. When anchored to the outer side of the inner mitochondrial membrane, native cytochrome c acts as a Schweitzer-StennerSchweitzer-Stenner that transfers electrons from cytochrome c reductase to cytochrome c oxidase in the respiratory chain. On the other hand, to interact with cardiolipin (one of the phospholipids making up the mitochondrial membrane) and form the cytochrome c/cardiolipin complex in the apoptotic process, the protein reorganizes its structure into a non-native state characterized by different asymmetry. The formation of the cytochrome c/cardiolipin complex is a fundamental step of the apoptotic pathway, since the structural rearrangement induces peroxidase activity in cytochrome c, the subsequent permeabilization of the membrane, and the release of the free protein into the cytoplasm, where cytochrome c activates the apoptotic process. Apoptosis is closely related to the pathogenesis of neoplastic, neurodegenerative and cardiovascular diseases; in this contest, the biosynthesis and remodeling of cardiolipin are crucial for the regulation of the apoptotic process. Since the role of cytochrome c as a promoter of apoptosis strictly depends on the non-native conformation(s) that the protein acquires when bound to the cardiolipin and such event leads to cytochrome c traslocation into the cytosol, the structural and functional properties of the cytochrome c/cardiolipin complex in cell fate will be the focus of the present review.

Regulation and functional role of the electron transport chain supercomplexes

Mitochondria are one of the most exhaustively investigated organelles in the cell and most attention has been paid to the components of the mitochondrial electron transport chain (ETC) in the last 100 years. The ETC collects electrons from NADH or FADH2 and transfers them through a series of electron carriers within multiprotein respiratory complexes (complex I to IV) to oxygen, therefore generating an electrochemical gradient that can be used by the F1-F0-ATP synthase (also named complex V) in the mitochondrial inner membrane to synthesize ATP. The organization and function of the ETC is a continuous source of surprises. One of the latest is the discovery that the respiratory complexes can assemble to form a variety of larger structures called super-complexes (SCs). This opened an unexpected level of complexity in this well-known and fundamental biological process. This review will focus on the current evidence for the formation of different SCs and will explore how they modulate the ETC organization according to the metabolic state. Since the field is rapidly growing, we also comment on the experimental techniques used to describe these SC and hope that this overview may inspire new technologies that will help to advance the field.

Distinct Roles of Mitochondrial HIGD1A and HIGD2A in Respiratory Complex and Supercomplex Biogenesis

The mitochondrial respiratory chain enzymes are organized as individual complexes and supercomplexes, whose biogenesis remains to be fully understood. To disclose the role of the human Hypoxia Inducible Gene Domain family proteins HIGD1A and HIGD2A in these processes, we generate and characterize HIGD-knockout (KO) cell lines. We show that HIGD2A controls and coordinates the modular assembly of isolated and supercomplexed complex IV (CIV) by acting on the COX3 assembly module. In contrast, HIGD1A regulates CIII and CIII-containing supercomplex biogenesis by supporting the incorporation of UQCRFS1. HIGD1A also clusters with COX4-1 and COX5A CIV subunits and, when overexpressed, suppresses the CIV biogenesis defect of HIGD2A-KO cells. We conclude that HIGD1A and HIGD2A have both independent and overlapping functions in the biogenesis of respiratory complexes and supercomplexes. Our data illuminate the existence of multiple pathways to assemble these structures by dynamic HIGD-mediated CIV biogenesis, potentially to adapt to changing environmental and nutritional conditions.

Molecular and Supramolecular Structure of the Mitochondrial Oxidative Phosphorylation System: Implications for Pathology

Under aerobic conditions, mitochondrial oxidative phosphorylation (OXPHOS) converts the energy released by nutrient oxidation into ATP, the currency of living organisms. The whole biochemical machinery is hosted by the inner mitochondrial membrane (mtIM) where the protonmotive force built by respiratory complexes, dynamically assembled as super-complexes, allows the F1FO-ATP synthase to make ATP from ADP + Pi. Recently mitochondria emerged not only as cell powerhouses, but also as signaling hubs by way of reactive oxygen species (ROS) production. However, when ROS removal systems and/or OXPHOS constituents are defective, the physiological ROS generation can cause ROS imbalance and oxidative stress, which in turn damages cell components. Moreover, the morphology of mitochondria rules cell fate and the formation of the mitochondrial permeability transition pore in the mtIM, which, most likely with the F1FO-ATP synthase contribution, permeabilizes mitochondria and leads to cell death. As the multiple mitochondrial functions are mutually interconnected, changes in protein composition by mutations or in supercomplex assembly and/or in membrane structures often generate a dysfunctional cascade and lead to life-incompatible diseases or severe syndromes. The known structural/functional changes in mitochondrial proteins and structures, which impact mitochondrial bioenergetics because of an impaired or defective energy transduction system, here reviewed, constitute the main biochemical damage in a variety of genetic and age-related diseases.

Hypoxia Promotes Mitochondrial Complex I Abundance via HIF-1α in Complex III and Complex IV Eficient Cells

Murine fibroblasts deficient in mitochondria respiratory complexes III (CIII) and IV (CIV) produced by either the ablation of Uqcrfs1 (encoding for Rieske iron sulfur protein, RISP) or Cox10 (encoding for protoheme IX farnesyltransferase, COX10) genes, respectively, showed a pleiotropic effect in complex I (CI). Exposure to 1-5% oxygen increased the levels of CI in both RISP and COX10 KO fibroblasts. De novo assembly of the respiratory complexes occurred at a faster rate and to higher levels in 1% oxygen compared to normoxia in both RISP and COX10 KO fibroblasts. Hypoxia did not affect the levels of assembly of CIII in the COX10 KO fibroblasts nor abrogated the genetic defect impairing CIV assembly. Mitochondrial signaling involving reactive oxygen species (ROS) has been implicated as necessary for HIF-1α stabilization in hypoxia. We did not observe increased ROS production in hypoxia. Exposure to low oxygen levels stabilized HIF-1α and increased CI levels in RISP and COX10 KO fibroblasts. Knockdown of HIF-1α during hypoxic conditions abrogated the beneficial effect of hypoxia on the stability/assembly of CI. These findings demonstrate that oxygen and HIF-1α regulate the assembly of respiratory complexes.

Altered Expression Ratio of Actin-Binding Gelsolin Isoforms Is a Novel Hallmark of Mitochondrial OXPHOS Dysfunction

Mitochondrial oxidative phosphorylation (OXPHOS) defects are the primary cause of inborn errors of energy metabolism. Despite considerable progress on their genetic basis, their global pathophysiological consequences remain undefined. Previous studies reported that OXPHOS dysfunction associated with complex III deficiency exacerbated the expression and mitochondrial location of cytoskeletal gelsolin (GSN) to promote cell survival responses. In humans, besides the cytosolic isoform, GSN presents a plasma isoform secreted to extracellular environments. We analyzed the interplay between both GSN isoforms in human cellular and clinical models of OXPHOS dysfunction. Regardless of its pathogenic origin, OXPHOS dysfunction induced the physiological upregulation of cytosolic GSN in the mitochondria (mGSN), in parallel with a significant downregulation of plasma GSN (pGSN) levels. Consequently, significantly high mGSN-to-pGSN ratios were associated with OXPHOS deficiency both in human cells and blood. In contrast, control cells subjected to hydrogen peroxide or staurosporine treatments showed no correlation between oxidative stress or cell death induction and the altered levels and subcellular location of GSN isoforms, suggesting their specificity for OXPHOS dysfunction. In conclusion, a high mitochondrial-to-plasma GSN ratio represents a useful cellular indicator of OXPHOS defects, with potential use for future research of a wide range of clinical conditions with mitochondrial involvement.

Multiple pathways coordinate assembly of human mitochondrial complex IV and stabilization of respiratory supercomplexes

Mitochondrial respiratory chain complexes I, III, and IV can associate into larger structures termed supercomplexes or respirasomes, thereby generating structural interdependences among the individual complexes yet to be understood. In patients, nonsense mutations in complex IV subunit genes cause severe encephalomyopathies randomly associated with pleiotropic complex I defects. Using complexome profiling and biochemical analyses, we have explored the structural rearrangements of the respiratory chain in human cell lines depleted of the catalytic complex IV subunit COX1 or COX2. In the absence of a functional complex IV holoenzyme, several supercomplex I+III2 species coexist, which differ in their content of COX subunits and COX7A2L/HIGD2A assembly factors. The incorporation of an atypical COX1-HIGD2A submodule attenuates supercomplex I+III2 turnover rate, indicating an unexpected molecular adaptation for supercomplexes stabilization that relies on the presence of COX1 independently of holo-complex IV formation. Our data set the basis for complex I structural dependence on complex IV, revealing the co-existence of alternative pathways for the biogenesis of "supercomplex-associated" versus individual complex IV, which could determine physiological adaptations under different stress and disease scenarios.

Mitochondria in cancer

The rediscovery and reinterpretation of the Warburg effect in the year 2000 occulted for almost a decade the key functions exerted by mitochondria in cancer cells. Until recent times, the scientific community indeed focused on constitutive glycolysis as a hallmark of cancer cells, which it is not, largely ignoring the contribution of mitochondria to the malignancy of oxidative and glycolytic cancer cells, being Warburgian or merely adapted to hypoxia. In this review, we highlight that mitochondria are not only powerhouses in some cancer cells, but also dynamic regulators of life, death, proliferation, motion and stemness in other types of cancer cells. Similar to the cells that host them, mitochondria are capable to adapt to tumoral conditions, and probably to evolve to ‘oncogenic mitochondria' capable of transferring malignant capacities to recipient cells. In the wider quest of metabolic modulators of cancer, treatments have already been identified targeting mitochondria in cancer cells, but the field is still in infancy.

Mitochondrial respiratory chain composition and organization in response to changing oxygen levels

Mitochondria are the major consumer of oxygen in eukaryotic cells, owing to the requirement of oxygen to generate ATP through the mitochondrial respiratory chain (MRC) and the oxidative phosphorylation system (OXPHOS). This aerobic energy transduction is more efficient than anaerobic processes such as glycolysis. Hypoxia, a condition in which environmental or intracellular oxygen levels are below the standard range, triggers an adaptive signaling pathway within the cell. When oxygen concentrations are low, hypoxia-inducible factors (HIFs) become stabilized and activated to mount a transcriptional response that triggers modulation of cellular metabolism to adjust to hypoxic conditions. Mitochondrial aerobic metabolism is one of the main targets of the hypoxic response to regulate its functioning and efficiency in the presence of decreased oxygen levels. During evolution, eukaryotic cells and tissues have increased the plasticity of their mitochondrial OXPHOS system to cope with metabolic needs in different oxygen contexts. In mammalian mitochondria, two factors contribute to this plasticity. First, several subunits of the multimeric MRC complexes I and IV exist in multiple tissue-specific and condition-specific isoforms. Second, the MRC enzymes can coexist organized as individual entities or forming supramolecular structures known as supercomplexes, perhaps in a dynamic manner to respond to environmental conditions and cellular metabolic demands. In this review, we will summarize the information currently available on oxygen-related changes in MRC composition and organization and will discuss gaps of knowledge and research opportunities in the field.

Cardiac Function is not Susceptible to Moderate Disassembly of Mitochondrial Respiratory Supercomplexes

Mitochondrial respiratory chain supercomplexes (RCS), particularly, the respirasome, which contains complexes I, III, and IV, have been suggested to participate in facilitating electron transport, reducing the production of reactive oxygen species (ROS), and maintaining the structural integrity of individual electron transport chain (ETC) complexes. Disassembly of the RCS has been observed in Barth syndrome, neurodegenerative and cardiovascular diseases, diabetes mellitus, and aging. However, the physiological role of RCS in high energy-demanding tissues such as the heart remains unknown. This study elucidates the relationship between RCS assembly and cardiac function. Adult male Sprague Dawley rats underwent Langendorff retrograde perfusion in the presence and absence of ethanol, isopropanol, or rotenone (an ETC complex I inhibitor). We found that ethanol had no effects on cardiac function, whereas rotenone reduced heart contractility, which was not recovered when rotenone was excluded from the perfusion medium. Blue native polyacrylamide gel electrophoresis revealed significant reductions of respirasome levels in ethanol- or rotenone-treated groups compared to the control group. In addition, rotenone significantly increased while ethanol had no effect on mitochondrial ROS production. In isolated intact mitochondria in vitro, ethanol did not affect respirasome assembly; however, acetaldehyde, a byproduct of ethanol metabolism, induced dissociation of respirasome. Isopropanol, a secondary alcohol which was used as an alternative compound, had effects similar to ethanol on heart function, respirasome levels, and ROS production. In conclusion, ethanol and isopropanol reduced respirasome levels without any noticeable effect on cardiac parameters, and cardiac function is not susceptible to moderate reductions of RCS.

Mitochondrial Supercomplexes: Physiological Organization and Dysregulation in Age-Related Neurodegenerative Disorders

Several studies suggest that the assembly of mitochondrial respiratory complexes into structures known as supercomplexes (SCs) may increase the efficiency of the electron transport chain, reducing the rate of production of reactive oxygen species. Therefore, the study of the (dis)assembly of SCs may be relevant for the understanding of mitochondrial dysfunction reported in brain aging and major neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). Here we briefly reviewed the biogenesis and structural properties of SCs, the impact of mtDNA mutations and mitochondrial dynamics on SCs assembly, the role of lipids on stabilization of SCs and the methodological limitations for the study of SCs. More specifically, we summarized what is known about mitochondrial dysfunction and SCs organization and activity in aging, AD and PD. We focused on the critical variables to take into account when postmortem tissues are used to study the (dis)assembly of SCs. Since few works have been performed to study SCs in AD and PD, the impact of SCs dysfunction on the alteration of brain energetics in these diseases remains poorly understood. The convergence of future progress in the study of SCs structure at high resolution and the refinement of animal models of AD and PD, as well as the use of iPSC-based and somatic cell-derived neurons, will be critical in understanding the biological relevance of the structural remodeling of SCs.

Oncogenic pathways and the electron transport chain: a dangeROS liaison

Driver mutations in oncogenic pathways, rewiring of cellular metabolism and altered ROS homoeostasis are intimately connected hallmarks of cancer. Electrons derived from different metabolic processes are channelled into the mitochondrial electron transport chain (ETC) to fuel the oxidative phosphorylation process. Electrons leaking from the ETC can prematurely react with oxygen, resulting in the generation of reactive oxygen species (ROS). Several signalling pathways are affected by ROS, which act as second messengers controlling cell proliferation and survival. On the other hand, oncogenic pathways hijack the ETC, enhancing its ROS-producing capacity by increasing electron flow or by impinging on the structure and organisation of the ETC. In this review, we focus on the ETC as a source of ROS and its modulation by oncogenic pathways, which generates a vicious cycle that resets ROS levels to a higher homoeostatic set point, sustaining the cancer cell phenotype.

Fine-tuning of the respiratory complexes stability and supercomplexes assembly in cells defective of complex III

The respiratory complexes are organized in supramolecular assemblies called supercomplexes thought to optimize cellular metabolism under physiological and pathological conditions. In this study, we used genetically and biochemically well characterized cells bearing the pathogenic microdeletion m.15,649-15,666 (ΔI300-P305) in MT-CYB gene, to investigate the effects of an assembly-hampered CIII on the re-organization of supercomplexes. First, we found that this mutation also affects the stability of both CI and CIV, and evidences the occurrence of a preferential structural interaction between CI and CIII₂, yielding a small amount of active CI+CIII₂ supercomplex. Indeed, a residual CI+CIII combined redox activity, and a low but detectable ATP synthesis driven by CI substrates are detectable, suggesting that the assembly of CIII into the CI+CIII₂ supercomplex mitigates the detrimental effects of MT-CYB deletion. Second, measurements of oxygen consumption and ATP synthesis driven by NADH-linked and FADH₂-linked substrates alone, or in combination, indicate a common ubiquinone pool for the two respiratory pathways. Finally, we report that prolonged incubation with rotenone enhances the amount of CI and CIII₂, but reduces CIV assembly. Conversely, the antioxidant N-acetylcysteine increases CIII₂ and CIV₂ and partially restores respirasome formation. Accordingly, after NAC treatment, the rate of ATP synthesis increases by two-fold compared with untreated cell, while the succinate level, which is enhanced by the homoplasmic mutation, markedly decreases. Overall, our findings show that fine-tuning the supercomplexes stability improves the energetic efficiency of cells with the MT-CYB microdeletion.

Role of ectopically expressed mtDNA encoded cytochrome c oxidase subunit I (MT-COI) in tumorigenesis

Somatic mutations within mitochondrial DNA (mtDNA) encoded cytochrome c oxidase subunit I (MT-CO1 or MT-COI) are frequent in various cancer types. In addition, perturbation from orchestrated expression of mitochondrial DNA encoded genes is also associated with complex disorders, including cancer. Since codon bias and the mitochondrial translation system restricts functional characterization of over-expressed wild type or mutant mitochondrial DNA encoded genes, the codon optimization and artificial synthesis of entire MT-CO1 allowed us to over-express the wild type and one of its deleterious mutants into the mitochondria of the transfected cells. Ectopically expressed MT-CO1 was observed to efficiently express and localized to mitochondria but showed high level of aggregation under denaturing condition. Over-expression of wild type or mutant variant of MT-CO1 promoted anchorage dependent and independent proliferation potential in in-vitro experiments and introduced the cancer cell metabolic phenotype of high glucose uptake and lactate release. Reactive oxygen species generated in cells over-expressing MT-CO1 variants acted as key effectors mediating differential expression of apoptosis and DNA damage pathway related genes. High ROS generated also down-regulated the expression of global regulators of gene expression, DNMT3A and DNMT3B. The down-regulated expression of DNMTs co-related with differential methylation of the CpG islands in the promoter region of a select set of studied genes, in a manner to promote pro-cancerous phenotype. Apart from assigning the mechanistic role to the MT-CO1 variants and their perturbed expression in cancer development, the present study provides novel insights into the functional role of somatic mutations within MT-CO1 promoting cancer phenotype.

Mitochondria, OxPhos, and neurodegeneration: cells are not just running out of gas

Mitochondrial respiratory deficiencies have been observed in numerous neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. For decades, these reductions in oxidative phosphorylation (OxPhos) have been presumed to trigger an overall bioenergetic crisis in the neuron, resulting in cell death. While the connection between respiratory defects and neuronal death has never been proven, this hypothesis has been supported by the detection of nonspecific mitochondrial DNA mutations in these disorders. These findings led to the notion that mitochondrial respiratory defects could be initiators of these common neurodegenerative disorders, instead of being consequences of a prior insult, a theory we believe to be misconstrued. Herein, we review the roots of this mitochondrial hypothesis and offer a new perspective wherein mitochondria are analyzed not only from the OxPhos point of view, but also as a complex organelle residing at the epicenter of many metabolic pathways.

Three-dimensional electron microscopy techniques for unravelling mitochondrial dysfunction in heart failure and identification of new pharmacological targets

A hallmark of heart failure is mitochondrial dysfunction leading to a bioenergetics imbalance in the myocardium. Consequently, there is much interest in targeting mitochondrial abnormalities to attenuate the pathogenesis of heart failure. This review discusses (i) how electron microscopy (EM) techniques have been fundamental for the current understanding of mitochondrial structure–function, (ii) the paradigm shift in resolutions now achievable by 3‐D EM techniques due to the introduction of direct detection devices and phase plate technology, and (iii) the application of EM for unravelling mitochondrial pathological remodelling in heart failure. We further consider the tremendous potential of multi‐scale EM techniques for the development of therapeutics, structure‐based ligand design and for delineating how a drug elicits nanostructural effects at the molecular, organelle and cellular levels. In conclusion, 3‐D EM techniques have entered a new era of structural biology and are poised to play a pivotal role in discovering new therapies targeting mitochondria for treating heart failure.

Linked Articles

This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc

The Organization of Mitochondrial Supercomplexes is Modulated by Oxidative Stress In Vivo in Mouse Models of Mitochondrial Encephalopathy

We examine the effect of oxidative stress on the stability of mitochondrial respiratory complexes and their association into supercomplexes (SCs) in the neuron-specific Rieske iron sulfur protein (RISP) and COX10 knockout (KO) mice. Previously we reported that these two models display different grades of oxidative stress in distinct brain regions. Using blue native gel electrophoresis, we observed a redistribution of the architecture of SCs in KO mice. Brain regions with moderate levels of oxidative stress (cingulate cortex of both COX10 and RISP KO and hippocampus of the RISP KO) showed a significant increase in the levels of high molecular weight (HMW) SCs. High levels of oxidative stress in the piriform cortex of the RISP KO negatively impacted the stability of CI, CIII and SCs. Treatment of the RISP KO with the mitochondrial targeted antioxidant mitoTEMPO preserved the stability of respiratory complexes and formation of SCs in the piriform cortex and increased the levels of glutathione peroxidase. These results suggest that mild to moderate levels of oxidative stress can modulate SCs into a more favorable architecture of HMW SCs to cope with rising levels of free radicals and cover the energetic needs.

Mitochondrial Respiratory Chain Complexes

Mitochondria are the power stations of the eukaryotic cell, using the energy released by the oxidation of glucose and other sugars to produce ATP. Electrons are transferred from NADH, produced in the citric acid cycle in the mitochondrial matrix, to oxygen by a series of large protein complexes in the inner mitochondrial membrane, which create a transmembrane electrochemical gradient by pumping protons across the membrane. The flow of protons back into the matrix via a proton channel in the ATP synthase leads to conformational changes in the nucleotide binding pockets and the formation of ATP. The three proton pumping complexes of the electron transfer chain are NADH-ubiquinone oxidoreductase or complex I, ubiquinone-cytochrome c oxidoreductase or complex III, and cytochrome c oxidase or complex IV. Succinate dehydrogenase or complex II does not pump protons, but contributes reduced ubiquinone. The structures of complex II, III and IV were determined by x-ray crystallography several decades ago, but complex I and ATP synthase have only recently started to reveal their secrets by advances in x-ray crystallography and cryo-electron microscopy. The complexes I, III and IV occur to a certain extent as supercomplexes in the membrane, the so-called respirasomes. Several hypotheses exist about their function. Recent cryo-electron microscopy structures show the architecture of the respirasome with near-atomic detail. ATP synthase occurs as dimers in the inner mitochondrial membrane, which by their curvature are responsible for the folding of the membrane into cristae and thus for the huge increase in available surface that makes mitochondria the efficient energy plants of the eukaryotic cell.

Complex II phosphorylation is triggered by unbalanced redox homeostasis in cells lacking complex III

A marked stimulation of complex II enzymatic activity was detected in cybrids bearing a homoplasmic MTCYB microdeletion causing disruption of both the activity and the assembly of complex III, but not in cybrids harbouring another MTCYB mutation affecting only the complex III activity. Moreover, complex II stimulation was associated with SDHA subunit tyrosine phosphorylation. Despite the lack of detectable hydrogen peroxide production, up-regulation of the levels of mitochondrial antioxidant defenses revealed a significant redox unbalance. This effect was also supported by the finding that treatment with N-acetylcysteine dampened the complex II stimulation, SDHA subunit tyrosine phosphorylation, and levels of antioxidant enzymes. In the absence of complex III, the cellular amount of succinate, but not fumarate, was markedly increased, indicating that enhanced activity of complex II is hampered due to the blockage of respiratory electron flow. Thus, we propose that complex II phosphorylation and stimulation of its activity represent a molecular mechanism triggered by perturbation of mitochondrial redox homeostasis due to severe dysfunction of respiratory complexes. Depending on the site and nature of the damage, complex II stimulation can either bypass the energetic deficit as an efficient compensatory mechanism, or be ineffectual, leaving cells to rely on glycolysis for survival.

Association between ROS production, swelling and the respirasome integrity in cardiac mitochondria

Although mitochondrial Ca²⁺ overload and ROS production play a critical role in mitochondria-mediated cell death, a cause-effect relationship between them remains elusive. This study elucidated the crosstalk between mitochondrial swelling, ROS production, and electron transfer chain (ETC) supercomplexes in rat heart mitochondria in response to Ca²⁺ and tert-butyl hydroperoxide (TBH), a lipid-soluble organic peroxide. Results showed that ROS production induced by TBH was significantly increased in the presence of Ca²⁺ in a dose-dependent manner. TBH markedly inhibited the state 3 respiration rate with no effect on the mitochondrial swelling. Ca²⁺ exerted a slight effect on mitochondrial respiration that was greatly aggravated by TBH. Analysis of supercomplexes revealed a minor difference in the presence of TBH and/or Ca²⁺. However, incubation of mitochondria in the presence of high Ca²⁺ (1 mM) or inhibitors of ETC complexes (rotenone and antimycin A) induced disintegration of the main supercomplex, respirasome. Thus, PTP-dependent swelling of mitochondria solely depends on Ca²⁺ but not ROS. TBH has no effect on the respirasome while Ca²⁺ induces disintegration of the supercomplex only at a high concentration. Intactness of individual ETC complexes I and III is important for maintenance of the structural integrity of the respirasome.

RLR-mediated antiviral innate immunity requires oxidative phosphorylation activity

Mitochondria act as a platform for antiviral innate immunity, and the immune system depends on activation of the retinoic acid-inducible gene I (RIG-I)-like receptors (RLR) signaling pathway via an adaptor molecule, mitochondrial antiviral signaling. We report that RLR-mediated antiviral innate immunity requires oxidative phosphorylation (OXPHOS) activity, a prominent physiologic function of mitochondria. Cells lacking mitochondrial DNA or mutant cells with respiratory defects exhibited severely impaired virus-induced induction of interferons and proinflammatory cytokines. Recovery of the OXPHOS activity in these mutants, however, re-established RLR-mediated signal transduction. Using in vivo approaches, we found that mice with OXPHOS defects were highly susceptible to viral infection and exhibited significant lung inflammation. Studies to elucidate the molecular mechanism of OXPHOS-coupled immune activity revealed that optic atrophy 1, a mediator of mitochondrial fusion, contributes to regulate the antiviral immune response. Our findings provide evidence for functional coordination between RLR-mediated antiviral innate immunity and the mitochondrial energy-generating system in mammals.

Cyclophilin D over-expression increases mitochondrial complex III activity and accelerates supercomplex formation

Cyclophilin D (CyPD), a mitochondrial matrix protein, has been widely studied for its role in mitochondrial-mediated cell death. Unexpectedly, we previously discovered that overexpression of CyPD in a stable cell line, increased mitochondrial membrane potentials and enhanced cell survival under conditions of oxidative stress. Here, we investigated the underlying mechanisms responsible for these findings. Spectrophotometric measurements in isolated mitochondria revealed that overexpression of CyPD in HEK293 cells increased respiratory chain activity, but only for Complex III (CIII). Acute treatment of mitochondria with the immumosupressant cyclosporine A did not affect CIII activity. Expression levels of the CIII subunits cytochrome b and Rieske-FeS were elevated in HEK293 cells overexpressing CyPD. However, CIII activity was still significantly higher compared to control mitochondria, even when normalized by protein expression. Blue native gel electrophoresis and Western blot assays revealed a molecular interaction of CyPD with CIII and increased levels of supercomplexes in mitochondrial protein extracts. Radiolabeled protein synthesis in mitochondria showed that CIII assembly and formation of supercomplexes containing CIII were significantly faster when CyPD was overexpressed. Taken together, these data indicate that CyPD regulates mitochondrial metabolism, and likely cell survival, by promoting more efficient electrons flow through the respiratory chain via increased supercomplex formation.

Functional asymmetry and electron flow in the bovine respirasome

Respirasomes are macromolecular assemblies of the respiratory chain complexes I, III and IV in the inner mitochondrial membrane. We determined the structure of supercomplex I₁III₂IV₁ from bovine heart mitochondria by cryo-EM at 9 Å resolution. Most protein-protein contacts between complex I, III and IV in the membrane are mediated by supernumerary subunits. Of the two Rieske iron-sulfur cluster domains in the complex III dimer, one is resolved, indicating that this domain is immobile and unable to transfer electrons. The central position of the active complex III monomer between complex I and IV in the respirasome is optimal for accepting reduced quinone from complex I over a short diffusion distance of 11 nm, and delivering reduced cytochrome c to complex IV. The functional asymmetry of complex III provides strong evidence for directed electron flow from complex I to complex IV through the active complex III monomer in the mammalian supercomplex.

DOI:http://dx.doi.org/10.7554/eLife.21290.001

Analysis of Mitochondrial Respiratory Chain Supercomplexes Using Blue Native Polyacrylamide Gel Electrophoresis (BN-PAGE)

Mitochondria are cellular organelles that harvest energy in the form of ATP through a process termed oxidative phosphorylation (OXPHOS), which occurs via the protein complexes of the electron transport chain (ETC). In recent years it has become unequivocally clear that mitochondrial complexes of the ETC are not static entities in the inner mitochondrial membrane. These complexes are dynamic and in mammals they aggregate in different stoichiometric combinations to form supercomplexes (SCs) or respirasomes. It has been proposed that the net respiration is more efficient via SCs than via isolated complexes. However, it still needs to be determined whether the activity of a particular SC is associated with a disease etiology. Here we describe a simplified method to visualize and assess in-gel activity of SCs and the individual complexes with good resolution using blue native polyacrylamide gel electrophoresis (BN-PAGE).

Loss of mitochondrial DNA-encoded protein ND1 results in disruption of complex I biogenesis during early stages of assembly

Mitochondrial complex I (NADH:ubiquinone oxidoreductase) must be assembled precisely from 45 protein subunits for it to function correctly. One of its mitochondrial DNA (mtDNA) encoded subunits, ND1, is incorporated during the early stages of complex I assembly. However, little is known about how mutations in ND1 affect this assembly process. We found that in human 143B cybrid cells carrying a homoplasmic MT-ND1 mutation, ND1 protein could not be translated. As a result, the early stages of complex I assembly were disrupted, with mature complex I undetectable and complex I-linked respiration severely reduced to 2.0% of control levels. Interestingly, complex IV (ferrocytochrome c:oxygen oxidoreductase) steady-state levels were also reduced to 40.3%, possibly due to its diminished stability in the absence of respiratory supercomplex formation. This was in comparison with 143B cybrid controls (that contained wild-type mtDNA on the same nuclear background), which exhibited normal complex I, complex IV, and supercomplex assembly. We conclude that the loss of ND1 stalls complex I assembly during the early stages of its biogenesis, which not only results in the loss of mature complex I but also disrupts the stability of complex IV and the respiratory supercomplex to cause mitochondrial dysfunction.-Lim, S. C., Hroudová, J., Van Bergen, N. J., Lopez Sanchez, M. I. G., Trounce, I. A., McKenzie, M. Loss of mitochondrial DNA-encoded protein ND1 results in disruption of complex I biogenesis during early stages of assembly.

The mammalian homologue of yeast Afg1 ATPase (lactation elevated 1) mediates degradation of nuclear-encoded complex IV subunits

Mitochondrial protein homeostasis is crucial for cellular function and integrity and is therefore maintained by several classes of proteins possessing chaperone and/or proteolytic activities. In the present study, we focused on characterization of LACE1 (lactation elevated 1) function in mitochondrial protein homeostasis. LACE1 is the human homologue of yeast mitochondrial Afg1 (ATPase family gene 1) ATPase, a member of the SEC18-NSF, PAS1, CDC48-VCP, TBP family. Yeast Afg1 was shown to mediate degradation of mitochondrially encoded complex IV subunits, and, on the basis of its similarity to CDC48 (p97/VCP), it was suggested to facilitate extraction of polytopic membrane proteins. We show that LACE1, which is a mitochondrial integral membrane protein, exists as part of three complexes of approximately 140, 400 and 500 kDa and is essential for maintenance of fused mitochondrial reticulum and lamellar cristae morphology. We demonstrate that LACE1 mediates degradation of nuclear-encoded complex IV subunits COX4 (cytochrome c oxidase 4), COX5A and COX6A, and is required for normal activity of complexes III and IV of the respiratory chain. Using affinity purification of LACE1-FLAG expressed in a LACE1-knockdown background, we show that the protein interacts physically with COX4 and COX5A subunits of complex IV and with mitochondrial inner-membrane protease YME1L. Finally, we demonstrate by ectopic expression of both K142A Walker A and E214Q Walker B mutants, that an intact ATPase domain is essential for LACE1-mediated degradation of nuclear-encoded complex IV subunits. Thus the present study establishes LACE1 as a novel factor with a crucial role in mitochondrial protein homeostasis.

Supramolecular Organization of Respiratory Complexes

Since the discovery of the existence of superassemblies between mitochondrial respiratory complexes, such superassemblies have been the object of a passionate debate. It is accepted that respiratory supercomplexes are structures that occur in vivo, although which superstructures are naturally occurring and what could be their functional role remain open questions. The main difficulty is to make compatible the existence of superassemblies with the corpus of data that drove the field to abandon the early understanding of the physical arrangement of the mitochondrial respiratory chain as a compact physical entity (the solid model). This review provides a nonexhaustive overview of the evolution of our understanding of the structural organization of the electron transport chain from the original idea of a compact organization to a view of freely moving complexes connected by electron carriers. Today supercomplexes are viewed not as a revival of the old solid model but rather as a refined revision of the fluid model, which incorporates a new layer of structural and functional complexity.

Distribution and dynamics of OXPHOS complexes in the bacterial cytoplasmic membrane

Oxidative phosphorylation (OXPHOS) is an essential process for most living organisms mostly sustained by protein complexes embedded in the cell membrane. In order to thrive, cells need to quickly respond to changes in the metabolic demand or in their environment. An overview of the strategies that can be employed by bacterial cells to adjust the OXPHOS outcome is provided. Regulation at the level of gene expression can only provide a means to adjust the OXPHOS outcome to long-term trends in the environment. In addition, the actual view is that bioenergetic membranes are highly compartmentalized structures. This review discusses what is known about the spatial organization of OXPHOS complexes and the timescales at which they occur. As exemplified with the commensal gut bacterium Escherichia coli, three levels of spatial organization are at play: supercomplexes, membrane microdomains and polar assemblies. This review provides a particular focus on whether dynamic spatial organization can fine-tune the OXPHOS through the definition of specialized functional membrane microdomains. Putative mechanisms responsible for spatio-temporal regulation of the OXPHOS complexes are discussed. This article is part of a Special Issue entitled Organization and dynamics of bioenergetic systems in bacteria, edited by Conrad Mullineaux.

Oxyphil cell metaplasia in the parathyroids is characterized by somatic mitochondrial DNA mutations in NADH dehydrogenase genes and cytochrome c oxidase activity-impairing genes

Oxyphil cell transformation of epithelial cells due to the accumulation of mitochondria occurs often during cellular aging. To understand the pathogenic mechanisms, we studied mitochondrial DNA (mtDNA) alterations in the three cell types of the parathyroids using multiplex real-time PCR and next-generation sequencing. mtDNA was analyzed from cytochrome c oxidase (COX)-positive and COX-negative areas of 19 parathyroids. Mitochondria-rich pre-oxyphil/oxyphil cells were more prone to develop COX defects than the mitochondria-poor clear chief cells (P < 0.001). mtDNA increased approximately 2.5-fold from clear chief to oxyphil cells. In COX deficiency, the increase was even more pronounced, and COX-negative oxyphil cells had approximately two times more mtDNA than COX-positive oxyphil cells (P < 0.001), illustrating the influence of COX deficiency on mtDNA biosynthesis, probably as a consequence of insufficient ATP synthesis. Next-generation sequencing revealed a broad spectrum of putative pathogenic mtDNA point mutations affecting NADH dehydrogenase and COX genes as well as regulatory elements of mtDNA. NADH dehydrogenase gene mutations preferentially accumulated in COX-positive pre-oxyphil/oxyphil cells and, therefore, could be essential for inducing oxyphil cell transformation by increasing mtDNA/mitochondrial biogenesis. In contrast, COX-negative cells predominantly harbored mutations in the MT-CO1 and MT-CO3 genes and in regulatory mtDNA elements, but only rarely NADH dehydrogenase mutations. Thus, multiple hits in NADH dehydrogenase and COX activity-impairing genes represent the molecular basis of oxyphil cell transformation in the parathyroids.

The Interplay Between Respiratory Supercomplexes and ROS in Aging

SIGNIFICANCE

The molecular mechanism of aging is still vigorously debated, although a general consensus exists that mitochondria are significantly involved in this process. However, the previously postulated role of mitochondrial-derived reactive oxygen species (ROS) as the damaging agents inducing functional loss in aging has fallen out of favor in the recent past. In this review, we critically examine the role of ROS in aging in the light of recent advances on the relationship between mitochondrial structure and function.

RECENT ADVANCES

The functional mitochondrial respiratory chain is now recognized as a reflection of the dynamic association of respiratory complexes in the form of supercomplexes (SCs). Besides providing kinetic advantage (channeling), SCs control ROS generation by the respiratory chain, thus providing a means to regulate ROS levels in the cell. Depending on their concentration, these ROS are either physiological signals essential for the life of the cell or toxic species that damage cell structure and functions.

CRITICAL ISSUES

We propose that under physiological conditions the dynamic nature of SCs reversibly controls the generation of ROS as signals involved in mitochondrial-nuclear communication. During aging, there is a progressive loss of control of ROS generation so that their production is irreversibly enhanced, inducing a vicious circle in which signaling is altered and structural damage takes place.

FUTURE DIRECTIONS

A better understanding on the forces affecting SC association would allow the manipulation of ROS generation, directing these species to their physiological signaling role.

A method aimed at assessing the functional consequences of the supramolecular organization of the respiratory electron transfer chain by time-resolved studies

A steadily increasing number of physiological, biochemical, and structural studies have provided a growing support to the notion that the respiratory electron transfer chain may contain supra-molecular edifices made of the assembly of some, if not all, of its individual links. This structure, usually referred to as the solid state model-in comparison to the liquid state model in which the electron transfer reactions between the membrane bound enzymes are diffusion controlled-is seen as conferring specific kinetic properties to the chain and thus as being highly relevant from a functional point of view. Although the assumption that structural changes are mirrored by functional adjustment is undoubtedly legitimate, experimental evidences supporting it remain scarce. Here we review a recent methodological development aimed at tackling the functional relevance of the supramolecular organization of the respiratory electron transfer chain in intact cells.

Respiratory supercomplexes: plasticity and implications

The plasticity model of the electron transport chain has slowly begun to replace both the liquid model of free complexes and the solid model of supercomplexes. The plasticity model predicts that respiratory complexes exist and function both as single complexes and as supercomplexes. The advantages of this system is an electron transport train which is able to adapt to changes in its environment. This review will investigate the current body of work on supercomplexes including their assembly, regulation, and plasticity, and particularly their role in the generation of reactive oxygen species and aging.

Coenzyme q and the respiratory chain: coenzyme q pool and mitochondrial supercomplexes

Two alternative models of organization of the mitochondrial electron transport chain (mETC) have been alternatively favored or questioned by the accumulation evidences of different sources, the solid model or the random collision model. Both agree in the number of respiratory complexes (I-IV) that participate in the mETC, but while the random collision model proposes that Complexes I-IV do not interact physically and that electrons are transferred between them by coenzyme Q and cytochrome c, the solid model proposes that all complexes super-assemble in the so-called respirasome. Recently, the plasticity model has been developed to incorporate the solid and the random collision model as extreme situations of a dynamic organization, allowing super-assembly free movement of the respiratory complexes. In this review, we evaluate the supporting evidences of each model and the implications of the super-assembly in the physiological role of coenzyme Q.

Expression of alternative oxidase in Drosophila ameliorates diverse phenotypes due to cytochrome oxidase deficiency

Mitochondrial dysfunction is a significant factor in human disease, ranging from systemic disorders of childhood to cardiomyopathy, ischaemia and neurodegeneration. Cytochrome oxidase, the terminal enzyme of the mitochondrial respiratory chain, is a frequent target. Lower eukaryotes possess alternative respiratory-chain enzymes that provide non-proton-translocating bypasses for respiratory complexes I (single-subunit reduced nicotinamide adenine dinucleotide dehydrogenases, e.g. Ndi1 from yeast) or III + IV [alternative oxidase (AOX)], under conditions of respiratory stress or overload. In previous studies, it was shown that transfer of yeast Ndi1 or Ciona intestinalis AOX to Drosophila was able to overcome the lethality produced by toxins or partial knockdown of complex I or IV. Here, we show that AOX can provide a complete or substantial rescue of a range of phenotypes induced by global or tissue-specific knockdown of different cIV subunits, including integral subunits required for catalysis, as well as peripheral subunits required for multimerization and assembly. AOX was also able to overcome the pupal lethality produced by muscle-specific knockdown of subunit CoVb, although the rescued flies were short lived and had a motility defect. cIV knockdown in neurons was not lethal during development but produced a rapidly progressing locomotor and seizure-sensitivity phenotype, which was substantially alleviated by AOX. Expression of Ndi1 exacerbated the neuronal phenotype produced by cIV knockdown. Ndi1 expressed in place of essential cI subunits produced a distinct residual phenotype of delayed development, bang sensitivity and male sterility. These findings confirm the potential utility of alternative respiratory chain enzymes as tools to combat mitochondrial disease, while indicating important limitations thereof.

Genetic variability of respiratory complex abundance, organization and activity in mouse brain

Mitochondrial dysfunction is implicated in the etiology and pathogenesis of numerous human disorders involving tissues with high energy demand. Murine models are widely used to elucidate genetic determinants of phenotypes relevant to human disease, with recent studies of C57BL/6J (B6), DBA/2J (D2) and B6xD2 populations implicating naturally occurring genetic variation in mitochondrial function/dysfunction. Using blue native polyacrylamide gel electrophoresis, immunoblots and in-gel activity analyses of complexes I, II, III, IV and V, our studies are the first to assess abundance, organization and catalytic activity of mitochondrial respiratory complexes and supercomplexes in mouse brain. Remarkable strain differences in supercomplex assembly and associated activity are evident, without differences in individual complexes I, II, III or IV. Supercomplexes I1 III2 IV2-3 exhibit robust complex III immunoreactivity and activities of complexes I and IV in D2, but with little detected in B6 for I1 III2 IV2 , and I1 III2 IV3 is not detected in B6. I1 III2 IV1 and I1 III2 are abundant and catalytically active in both strains, but significantly more so in B6. Furthermore, while supercomplex III2 IV1 is abundant in D2, none is detected in B6. In aggregate, these results indicate a shift toward more highly assembled supercomplexes in D2. Respiratory supercomplexes are thought to increase electron flow efficiency and individual complex stability, and to reduce electron leak and generation of reactive oxygen species. Our results provide a framework to begin assessing the role of respiratory complex suprastructure in genetic vulnerability and treatment for a wide variety of mitochondrial-related disorders.

Mitochondrial respiratory supercomplex association limits production of reactive oxygen species from complex I

AIMS

The mitochondrial respiratory chain is recognized today to be arranged in supramolecular assemblies (supercomplexes). Besides conferring a kinetic advantage (substrate channeling) and being required for the assembly and stability of Complex I, indirect considerations support the view that supercomplexes may also prevent excessive formation of reactive oxygen species (ROS) from the respiratory chain. In the present study, we have directly addressed this issue by testing the ROS generation by Complex I in two experimental systems in which the supramolecular organization of the respiratory assemblies is impaired by: (i) treatment either of bovine heart mitochondria or liposome-reconstituted supercomplex I-III with dodecyl maltoside; (ii) reconstitution of Complexes I and III at high phospholipids to protein ratio.

RESULTS

The results of our investigation provide experimental evidence that the production of ROS is strongly increased in either model, supporting the view that disruption or prevention of the association between Complex I and Complex III by different means enhances the generation of superoxide from Complex I.

INNOVATION

Dissociation of supercomplexes may link oxidative stress and energy failure in a vicious circle.

CONCLUSION

Our findings support a central role of mitochondrial supramolecular structure in the development of the aging process and in the etiology and pathogenesis of most major chronic diseases.

Respiratory supercomplexes: structure, function and assembly

The mitochondrial respiratory chain consists of 5 enzyme complexes that are responsible for ATP generation. The paradigm of the electron transport chain as discrete enzymes diffused in the inner mitochondrial membrane has been replaced by the solid state supercomplex model wherein the respiratory complexes associate with each other to form supramolecular complexes. Defects in these supercomplexes, which have been shown to be functionally active and required for forming stable respiratory complexes, have been associated with many genetic and neurodegenerative disorders demonstrating their biomedical significance. In this review, we will summarize the functional and structural significance of supercomplexes and provide a comprehensive review of their assembly and the assembly factors currently known to play a role in this process.

A role of mitochondrial complex II defects in genetic models of Huntington's disease expressing N-terminal fragments of mutant huntingtin

Huntington's disease (HD) is a neurodegenerative disorder caused by an abnormal expansion of a CAG repeat encoding a polyglutamine tract in the huntingtin (Htt) protein. The mutation leads to neuronal death through mechanisms which are still unknown. One hypothesis is that mitochondrial defects may play a key role. In support of this, the activity of mitochondrial complex II (C-II) is preferentially reduced in the striatum of HD patients. Here, we studied C-II expression in different genetic models of HD expressing N-terminal fragments of mutant Htt (mHtt). Western blot analysis showed that the expression of the 30 kDa Iron–Sulfur (Ip) subunit of C-II was significantly reduced in the striatum of the R6/1 transgenic mice, while the levels of the FAD containing catalytic 70 kDa subunit (Fp) were not significantly changed. Blue native gel analysis showed that the assembly of C-II in mitochondria was altered early in N171-82Q transgenic mice. Early loco-regional reduction in C-II activity and Ip protein expression was also demonstrated in a rat model of HD using intrastriatal injection of lentiviral vectors encoding mHtt. Infection of the rat striatum with a lentiviral vector coding the C-II Ip or Fp subunits induced a significant overexpression of these proteins that led to significant neuroprotection of striatal neurons against mHtt neurotoxicity. These results obtained in vivo support the hypothesis that structural and functional alterations of C-II induced by mHtt may play a critical role in the degeneration of striatal neurons in HD and that mitochondrial-targeted therapies may be useful in its treatment.

Barth syndrome: cellular compensation of mitochondrial dysfunction and apoptosis inhibition due to changes in cardiolipin remodeling linked to tafazzin (TAZ) gene mutation

Cardiolipin is a mitochondrion-specific phospholipid that stabilizes the assembly of respiratory chain complexes, favoring full-yield operation. It also mediates key steps in apoptosis. In Barth syndrome, an X chromosome-linked cardiomyopathy caused by tafazzin mutations, cardiolipins display acyl chain modifications and are present at abnormally low concentrations, whereas monolysocardiolipin accumulates. Using immortalized lymphoblasts from Barth syndrome patients, we showed that the production of abnormal cardiolipin led to mitochondrial alterations. Indeed, the lack of normal cardiolipin led to changes in electron transport chain stability, resulting in cellular defects. We found a destabilization of the supercomplex (respirasome) I+III2+IVn but also decreased amounts of individual complexes I and IV and supercomplexes I+III and III+IV. No changes were observed in the amounts of individual complex III and complex II. We also found decreased levels of complex V. This complex is not part of the supercomplex suggesting that cardiolipin is required not only for the association/stabilization of the complexes into supercomplexes but also for the modulation of the amount of individual respiratory chain complexes. However, these alterations were compensated by an increase in mitochondrial mass, as demonstrated by electron microscopy and measurements of citrate synthase activity. We suggest that this compensatory increase in mitochondrial content prevents a decrease in mitochondrial respiration and ATP synthesis in the cells. We also show, by extensive flow cytometry analysis, that the type II apoptosis pathway was blocked at the mitochondrial level and that the mitochondria of patients with Barth syndrome cannot bind active caspase-8. Signal transduction is thus blocked before any mitochondrial event can occur. Remarkably, basal levels of superoxide anion production were slightly higher in patients' cells than in control cells as previously evidenced via an increased protein carbonylation in the taz1Δ mutant in the yeast. This may be deleterious to cells in the long term. The consequences of mitochondrial dysfunction and alterations to apoptosis signal transduction are considered in light of the potential for the development of future treatments.

The cytochrome b p.278Y>C mutation causative of a multisystem disorder enhances superoxide production and alters supramolecular interactions of respiratory chain complexes

Cytochrome b is the only mtDNA-encoded subunit of the mitochondrial complex III (CIII), the functional bottleneck of the respiratory chain. Previously, the human cytochrome b missense mutation m.15579A>G, which substitutes the Tyr 278 with Cys (p.278Y>C), was identified in a patient with severe exercise intolerance and multisystem manifestations. In this study, we characterized the biochemical properties of cybrids carrying this mutation and report that the homoplasmic p.278Y>C mutation caused a dramatic reduction in the CIII activity and in CIII-driven mitochondrial ATP synthesis. However, the CI, CI + CIII and CII + CIII activities and the rate of ATP synthesis driven by the CI or CII substrate were only partially reduced or unaffected. Consistent with these findings, mutated cybrids maintained the mitochondrial membrane potential in the presence of oligomycin, indicating that it originated from the respiratory electron transport chain. The p.278Y>C mutation enhanced superoxide production, as indicated by direct measurements in mitochondria and by the imbalance of glutathione homeostasis in intact cybrids. Remarkably, although the assembly of CI or CIII was not affected, the examination of respiratory supercomplexes revealed that the amounts of CIII dimer and III2IV1 were reduced, whereas those of I1III2IVn slightly increased. We therefore suggest that the deleterious effects of p.278Y>C mutation on cytochrome b are palliated when CIII is assembled into the supercomplexes I1III2IVn, in contrast to when it is found alone. These findings underline the importance of supramolecular interactions between complexes for maintaining a basal respiratory chain activity and shed light to the molecular basis of disease manifestations associated with this mutation.

Relevance of mitochondrial genetics and metabolism in cancer development

Cancer cells are characterized in general by a decrease of mitochondrial respiration and oxidative phosphorylation, together with a strong enhancement of glycolysis, the so-called Warburg effect. The decrease of mitochondrial activity in cancer cells may have multiple reasons, related either to the input of reducing equivalents to the electron transfer chain or to direct alterations of the mitochondrial respiratory complexes. In some cases, the depression of respiratory activity is clearly the consequence of disruptive mitochondrial DNA (mtDNA) mutations and leads as a consequence to enhanced generation of reactive oxygen species (ROS). By acting both as mutagens and cellular mitogens, ROS may contribute directly to cancer progression. On the basis of our experimental evidence, we suggest a deep implication of the supercomplex organization of the respiratory chain as a missing link between oxidative stress, energy failure, and tumorigenesis. We speculate that under conditions of oxidative stress, a dissociation of mitochondrial supercomplexes occurs, with destabilization of complex I and secondary enhanced generation of ROS, thus leading to a vicious circle amplifying mitochondrial dysfunction. An excellent model to dissect the role of pathogenic, disassembling mtDNA mutations in tumor progression and their contribution to the metabolic reprogramming of cancer cells (glycolysis vs. respiration) is provided by an often underdiagnosed subset of tumors, namely, the oncocytomas, characterized by disruptive mutations of mtDNA, especially of complex I subunits. Such mutations almost completely abolish complex I activity, which slows down the Krebs cycle, favoring a high ratio of α-ketoglutarate/succinate and consequent destabilization of hypoxia inducible factor 1α (HIF1α). On the other hand, if complex I is partially defective, the levels of NAD(+) may be sufficient to implement the Krebs cycle with higher levels of intermediates that stabilize HIF1α, thus favoring tumor malignancy. The threshold model we propose, based on the population-like dynamics of mitochondrial genetics (heteroplasmy vs. homoplasmy), implies that below threshold complex I is present and functioning correctly, thus favoring tumor growth, whereas above threshold, when complex I is not assembled, tumor growth is arrested. We have therefore termed "oncojanus" the mtDNA genes whose disruptive mutations have such a double-edged effect.