The phosphorylation pattern of bovine heart complex I subunits

OXPHOS capacity is diminished and the phosphorylation system inhibited during diapause in an extremophile, embryos of Artemia franciscana

Diapause exhibited by embryos of Artemia franciscana is accompanied by severe arrest of respiration. A large fraction of this depression is attributable to downregulation of trehalose catabolism that ultimately restricts fuel to mitochondria. This study now extends knowledge on the mechanism by revealing metabolic depression is heightened by inhibitions within mitochondria. Compared with that in embryo lysates during post-diapause, oxidative phosphorylation (OXPHOS) capacity P is depressed during diapause when either NADH-linked substrates (pyruvate and malate) for electron transfer (electron transfer capacity, E) through respiratory Complex I or the Complex II substrate succinate are used. When pyruvate, malate and succinate were combined, respiratory inhibition by the phosphorylation system in diapause lysates was discovered as judged by P/E flux control ratios (two-way ANOVA; F1,24=38.78; P<0.0001). Inhibition was eliminated as the diapause extract was diluted (significant interaction term; F2,24=9.866; P=0.0007), consistent with the presence of a diffusible inhibitor. One candidate is long-chain acyl-CoA esters known to inhibit the adenine nucleotide translocator. Addition of oleoyl-CoA to post-diapause lysates markedly decreased the P/E ratio to 0.40±0.07 (mean±s.d.; P=0.002) compared with 0.79±0.11 without oleoyl-CoA. Oleoyl-CoA inhibits the phosphorylation system and may be responsible for the depressed P/E in lysates from diapause embryos. With isolated mitochondria, depression of P/E by oleoyl-CoA was fully reversed by addition of l-carnitine (control versus recovery with l-carnitine, P=0.338), which facilitates oleoyl-CoA transport into the matrix and elimination by β-oxidation. In conclusion, severe metabolic arrest during diapause promoted by restricting glycolytic carbon to mitochondria is reinforced by depression of OXPHOS capacity and the phosphorylation system.

The Mysterious Multitude: Structural Perspective on the Accessory Subunits of Respiratory Complex I

Complex I (CI) is the largest protein complex in the mitochondrial oxidative phosphorylation electron transport chain of the inner mitochondrial membrane and plays a key role in the transport of electrons from reduced substrates to molecular oxygen. CI is composed of 14 core subunits that are conserved across species and an increasing number of accessory subunits from bacteria to mammals. The fact that adding accessory subunits incurs costs of protein production and import suggests that these subunits play important physiological roles. Accordingly, knockout studies have demonstrated that accessory subunits are essential for CI assembly and function. Furthermore, clinical studies have shown that amino acid substitutions in accessory subunits lead to several debilitating and fatal CI deficiencies. Nevertheless, the specific roles of CI’s accessory subunits have remained mysterious. In this review, we explore the possible roles of each of mammalian CI’s 31 accessory subunits by integrating recent high-resolution CI structures with knockout, assembly, and clinical studies. Thus, we develop a framework of experimentally testable hypotheses for the function of the accessory subunits. We believe that this framework will provide inroads towards the complete understanding of mitochondrial CI physiology and help to develop strategies for the treatment of CI deficiencies.

Prevention of mitochondrial impairment by inhibition of protein phosphatase 1 activity in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease caused by progressive loss of motor neurons (MNs) and subsequent muscle weakness. These pathological features are associated with numerous cellular changes, including alteration in mitochondrial morphology and function. However, the molecular mechanisms associating mitochondrial structure with ALS pathology are poorly understood. In this study, we found that Dynamin-related protein 1 (Drp1) was dephosphorylated in several ALS models, including those with SOD1 and TDP-43 mutations, and the dephosphorylation was mediated by the pathological induction of protein phosphatase 1 (PP1) activity in these models. Suppression of the PP1-Drp1 cascade effectively prevented ALS-related symptoms, including mitochondrial fragmentation, mitochondrial complex I impairment, axonal degeneration, and cell death, in primary neuronal culture models, iPSC-derived human MNs, and zebrafish models in vivo. These results suggest that modulation of PP1-Drp1 activity may be a therapeutic target for multiple pathological features of ALS.

Insights from Drosophila on mitochondrial complex I

NADH:ubiquinone oxidoreductase, more commonly referred to as mitochondrial complex I (CI), is the largest discrete enzyme of the oxidative phosphorylation system (OXPHOS). It is localized to the mitochondrial inner membrane. CI oxidizes NADH generated from the tricarboxylic acid cycle to NAD+, in a series of redox reactions that culminates in the reduction of ubiquinone, and the transport of protons from the matrix across the inner membrane to the intermembrane space. The resulting proton-motive force is consumed by ATP synthase to generate ATP, or harnessed to transport ions, metabolites and proteins into the mitochondrion. CI is also a major source of reactive oxygen species. Accordingly, impaired CI function has been associated with a host of chronic metabolic and degenerative disorders such as diabetes, cardiomyopathy, Parkinson's disease (PD) and Leigh syndrome. Studies on Drosophila have contributed to our understanding of the multiple roles of CI in bioenergetics and organismal physiology. Here, we explore and discuss some of the studies on Drosophila that have informed our understanding of this complex and conclude with some of the open questions about CI that can be resolved by studies on Drosophila.

Traditional and novel tools to probe the mitochondrial metabolism in health and disease

Mitochondrial metabolism links energy production to other essential cellular processes such as signaling, cellular differentiation, and apoptosis. In addition to producing adenosine triphosphate (ATP) as an energy source, mitochondria are responsible for the synthesis of a myriad of important metabolites and cofactors such as tetrahydrofolate, α-ketoacids, steroids, aminolevulinic acid, biotin, lipoic acid, acetyl-CoA, iron-sulfur clusters, heme, and ubiquinone. Furthermore, mitochondria and their metabolism have been implicated in aging and several human diseases, including inherited mitochondrial disorders, cardiac dysfunction, heart failure, neurodegenerative diseases, diabetes, and cancer. Therefore, there is great interest in understanding mitochondrial metabolism and the complex relationship it has with other cellular processes. A large number of studies on mitochondrial metabolism have been conducted in the last 50 years, taking a broad range of approaches. In this review, we summarize and discuss the most commonly used tools that have been used to study different aspects of the metabolism of mitochondria: ranging from dyes that monitor changes in the mitochondrial membrane potential and pharmacological tools to study respiration or ATP synthesis, to more modern tools such as genetically encoded biosensors and trans-omic approaches enabled by recent advances in mass spectrometry, computation, and other technologies. These tools have allowed the large number of studies that have shaped our current understanding of mitochondrial metabolism. WIREs Syst Biol Med 2017, 9:e1373. doi: 10.1002/wsbm.1373 For further resources related to this article, please visit the WIREs website.

GRIM-19: A master regulator of cytokine induced tumor suppression, metastasis and energy metabolism

Cytokines induce cell proliferation or growth suppression depending on the context. It is increasingly becoming clear that success of standard radiotherapy and/or chemotherapeutics to eradicate solid tumors is dependent on IFN signaling. In this review we discuss the molecular mechanisms of tumor growth suppression by a gene product isolated in our laboratory using a genome-wide expression knock-down strategy. Gene associated with retinoid-IFN-induced mortality -19 (GRIM-19) functions as non-canonical tumor suppressor by antagonizing oncoproteins. As a component of mitochondrial respiratory chain, GRIM-19 influences the degree of "Warburg effect" in cancer cells as many advanced and/or aggressive tumors show severely down-regulated GRIM-19 levels. In addition, GRIM-19 appears to regulate innate and acquired immune responses in mouse models. Thus, GRIM-19 is positioned at nodes that favor cell protection and/or prevent aberrant cell growth.

Atomic structure of the entire mammalian mitochondrial complex I

Mitochondrial complex I (also known as NADH:ubiquinone oxidoreductase) contributes to cellular energy production by transferring electrons from NADH to ubiquinone coupled to proton translocation across the membrane. It is the largest protein assembly of the respiratory chain with a total mass of 970 kilodaltons. Here we present a nearly complete atomic structure of ovine (Ovis aries) mitochondrial complex I at 3.9 Å resolution, solved by cryo-electron microscopy with cross-linking and mass-spectrometry mapping experiments. All 14 conserved core subunits and 31 mitochondria-specific supernumerary subunits are resolved within the L-shaped molecule. The hydrophilic matrix arm comprises flavin mononucleotide and 8 iron-sulfur clusters involved in electron transfer, and the membrane arm contains 78 transmembrane helices, mostly contributed by antiporter-like subunits involved in proton translocation. Supernumerary subunits form an interlinked, stabilizing shell around the conserved core. Tightly bound lipids (including cardiolipins) further stabilize interactions between the hydrophobic subunits. Subunits with possible regulatory roles contain additional cofactors, NADPH and two phosphopantetheine molecules, which are shown to be involved in inter-subunit interactions. We observe two different conformations of the complex, which may be related to the conformationally driven coupling mechanism and to the active-deactive transition of the enzyme. Our structure provides insight into the mechanism, assembly, maturation and dysfunction of mitochondrial complex I, and allows detailed molecular analysis of disease-causing mutations.

Mitochondrial Oxidative Phosphorylation System (OXPHOS) Deficits in Schizophrenia: Possible Interactions with Cellular Processes

Mitochondria are key players in the generation and regulation of cellular bioenergetics, producing the majority of adenosine triphosphate molecules by the oxidative phosphorylation system (OXPHOS). Linked to numerous signaling pathways and cellular functions, mitochondria, and OXPHOS in particular, are involved in neuronal development, connectivity, plasticity, and differentiation. Impairments in a variety of mitochondrial functions have been described in different general and psychiatric disorders, including schizophrenia (SCZ), a severe, chronic, debilitating illness that heavily affects the lives of patients and their families. This article reviews findings emphasizing the role of OXPHOS in the pathophysiology of SCZ. Evidence accumulated during the past few decades from imaging, transcriptomic, proteomic, and metabolomic studies points at OXPHOS deficit involvement in SCZ. Abnormalities have been reported in high-energy phosphates generated by the OXPHOS, in the activity of its complexes and gene expression, primarily of complex I (CoI). In addition, cellular signaling such as cAMP/protein kinase A (PKA) and Ca(+2), neuronal development, connectivity, and plasticity have been linked to OXPHOS function and are reported to be impaired in SCZ. Finally, CoI has been shown as a site of interaction for both dopamine (DA) and antipsychotic drugs, further substantiating its role in the pathology of SCZ. Understanding the role of mitochondria and the OXPHOS in particular may encourage new insights into the pathophysiology and etiology of this debilitating disorder.

The Effect of Aging on Mitochondrial Complex I and the Extent of Oxidative Stress in the Rat Brain Cortex

One of the characteristic features of the aging is dysfunction of mitochondria. Its role in the regulation of metabolism and apoptosis suggests a possible link between these cellular processes. This study investigates the relationship of respiratory complex I with aging-related oxidative stress in the cerebral mitochondria. Deterioration of complex I seen in senescent (26-months old) mitochondria was accompanied by decline in total thiol group content, increase of HNE and HNE-protein adducts as well as decreased content of complex I subunits, GRIM-19 and NDUFV2. On the other hand, decline of complex I might be related with the mitochondrial apoptosis through increased Bax/Bcl-2 cascade in 15-month old animal brains. Higher amount of Bcl-2, Bcl-xL with the lower content of GRIM-19 could maintain to some extent elevated oxidative stress in mitochondria as seen in the senescent group. In the cortical M1 region increased presence of TUNEL+ cells and more than 20-times higher density of Fluoro-Jade C+ cells in 26-months old was observed, suggesting significant neurodegenerative effect of aging in the neuronal cells. Our study supports a scenario in which the age-related decline of complex I might sensitize neurons to the action of death agonists, such as Bax through lipid and protein oxidative stimuli in mitochondria. Although aging is associated with oxidative stress, these changes did not increase progressively with age, as similar extent of lesions was observed in oxidative stress markers of the both aged groups.

Bacterial Electron Transfer Chains Primed by Proteomics

Electron transport phosphorylation is the central mechanism for most prokaryotic species to harvest energy released in the respiration of their substrates as ATP. Microorganisms have evolved incredible variations on this principle, most of these we perhaps do not know, considering that only a fraction of the microbial richness is known. Besides these variations, microbial species may show substantial versatility in using respiratory systems. In connection herewith, regulatory mechanisms control the expression of these respiratory enzyme systems and their assembly at the translational and posttranslational levels, to optimally accommodate changes in the supply of their energy substrates. Here, we present an overview of methods and techniques from the field of proteomics to explore bacterial electron transfer chains and their regulation at levels ranging from the whole organism down to the Ångstrom scales of protein structures. From the survey of the literature on this subject, it is concluded that proteomics, indeed, has substantially contributed to our comprehending of bacterial respiratory mechanisms, often in elegant combinations with genetic and biochemical approaches. However, we also note that advanced proteomics offers a wealth of opportunities, which have not been exploited at all, or at best underexploited in hypothesis-driving and hypothesis-driven research on bacterial bioenergetics. Examples obtained from the related area of mitochondrial oxidative phosphorylation research, where the application of advanced proteomics is more common, may illustrate these opportunities.

Post-Translational Modifications of Cardiac Mitochondrial Proteins in Cardiovascular Disease: Not Lost in Translation

Protein post-translational modifications (PTMs) are crucial in regulating cellular biology by playing key roles in processes such as the rapid on and off switching of signaling network and the regulation of enzymatic activities without affecting gene expressions. PTMs lead to conformational changes in the tertiary structure of protein and resultant regulation of protein function such as activation, inhibition, or signaling roles. PTMs such as phosphorylation, acetylation, and S-nitrosylation of specific sites in proteins have key roles in regulation of mitochondrial functions, thereby contributing to the progression to heart failure. Despite the extensive study of PTMs in mitochondrial proteins much remains unclear. Further research is yet to be undertaken to elucidate how changes in the proteins may lead to cardiovascular and metabolic disease progression in particular. We aimed to summarize the various types of PTMs that occur in mitochondrial proteins, which might be associated with heart failure. This study will increase the understanding of cardiovascular diseases through PTM.

Pivotal Importance of STAT3 in Protecting the Heart from Acute and Chronic Stress: New Advancement and Unresolved Issues

The transcription factor, signal transducer and activator of transcription 3 (STAT3), has been implicated in protecting the heart from acute ischemic injury under both basal conditions and as a crucial component of pre- and post-conditioning protocols. A number of anti-oxidant and antiapoptotic genes are upregulated by STAT3 via canonical means involving phosphorylation on Y705 and S727, although other incompletely defined posttranslational modifications are involved. In addition, STAT3 is now known to be present in cardiac mitochondria and to exert actions that regulate the electron transport chain, reactive oxygen species production, and mitochondrial permeability transition pore opening. These non-canonical actions of STAT3 are enhanced by S727 phosphorylation. The molecular basis for the mitochondrial actions of STAT3 is poorly understood, but STAT3 is known to interact with a critical subunit of complex I and to regulate complex I function. Dysfunctional complex I has been implicated in ischemic injury, heart failure, and the aging process. Evidence also indicates that STAT3 is protective to the heart under chronic stress conditions, including hypertension, pregnancy, and advanced age. Paradoxically, the accumulation of unphosphorylated STAT3 (U-STAT3) in the nucleus has been suggested to drive pathological cardiac hypertrophy and inflammation via non-canonical gene expression, perhaps involving a distinct acetylation profile. U-STAT3 may also regulate chromatin stability. Our understanding of how the non-canonical genomic and mitochondrial actions of STAT3 in the heart are regulated and coordinated with the canonical actions of STAT3 is rudimentary. Here, we present an overview of what is currently known about the pleotropic actions of STAT3 in the heart in order to highlight controversies and unresolved issues.

Energy metabolism and cytochrome oxidase activity: linking metabolism to gene expression

Modification of mitochondrial content demands the synthesis of hundreds of proteins encoded by nuclear and mitochondrial genomes. The responsibility for coordination of this process falls to nuclear-encoded master regulators of transcription. DNA-binding proteins and coactivators integrate information from energy-sensing pathways and hormones to alter mitochondrial gene expression. In mammals, the signaling cascade for mitochondrial biogenesis can be described as follows: hormonal signals and energetic information are sensed by protein-modifying enzymes that in turn regulate the post-translational modification of transcription factors. Once activated, transcription-factor complexes form on promoter elements of many of the nuclear-encoded mitochondrial genes, recruiting proteins that remodel chromatin and initiate transcription. One master regulator in mammals, PGC-1α, is well studied because of its role in determining the metabolic phenotype of muscles, but also due to its importance in mitochondria-related metabolic diseases. However, relatively little is known about the role of this pathway in other vertebrates. These uncertainties raise broader questions about the evolutionary origins of the pathway and its role in generating the diversity in muscle metabolic phenotypes seen in nature.

Neuroglobin gene therapy prevents optic atrophy and preserves durably visual function in Harlequin mice

Neuroglobin (NGB) is considered as an endogenous neuroprotective molecule against stroke, since the protein alleviates the adverse effects of hypoxic and ischemic insults. We previously demonstrated the functional link between NGB and mitochondria since it is required for respiratory chain function. Thus, here, we evaluated the relevance of this effect in the Harlequin (Hq) mouse strain, which exhibits retinal ganglion cell (RGC) loss and optic atrophy due to a respiratory chain complex I (CI) defect. A twofold decrease of NGB amounts was observed in Hq retinas. We constructed a recombinant adeno-associated virus which combines to the mouse NGB open reading frame, its 5' and 3'UTR, for guarantying mRNA stability and translation capacity. The vector was administrated intravitreally to Hq mice and NGB expression was stable for up to 7 months without negative effect on retinal architecture or function. On the contrary, RGCs and their axons were substantially preserved from degeneration; consequently, CI activity in optic nerves was protected conferring improvements in vision. Hence, we established that NGB prevents respiratory chain impairment, therefore, protecting visual function otherwise compromised by mitochondrial energetic failure.

Accessory subunits of mitochondrial complex I

Mitochondrial complex I has a molecular mass of almost 1 MDa and comprises more than 40 polypeptides. Fourteen central subunits harbour the bioenergetic core functions. We are only beginning to understand the significance of the numerous accessory subunits. The present review addresses the role of accessory subunits for assembly, stability and regulation of complex I and for cellular functions not directly associated with redox-linked proton translocation.

Unraveling the phosphoproteome dynamics in mammal mitochondria from a network perspective

With mitochondrion garnering more attention for its inextricable involvement in pathophysiological conditions, it seems imperative to understand the means by which the molecular pathways harbored in this organelle are regulated. Protein phosphorylation has been considered a central event in cellular signaling and, more recently, in the modulation of mitochondrial activity. Efforts have been made to understand the molecular mechanisms by which protein phosphorylation regulates mitochondrial signaling. With the advances in mass-spectrometry-based proteomics, there is a substantial hope and expectation in the increased knowledge of protein phosphorylation profile and its mode of regulation. On the basis of phosphorylation profiles, attempts have been made to disclose the kinases involved and how they control the molecular processes in mitochondria and, consequently, the cellular outcomes. Still, few studies have focused on mitochondrial phosphoproteome profiling, particularly in diseases. The present study reviews current data on protein phosphorylation profiling in mitochondria, the potential kinases involved and how pathophysiological conditions modulate the mitochondrial phosphoproteome. To integrate data from distinct research papers, we performed network analysis, with bioinformatic tools like Cytoscape, String, and PANTHER taking into consideration variables such as tissue specificity, biological processes, molecular functions, and pathophysiological conditions. For instance, data retrieved from these analyses evidence some homology in the mitochondrial phosphoproteome among liver and heart, with proteins from transport and oxidative phosphorylation clusters particularly susceptible to phosphorylation. A distinct profile was noticed for adipocytes, with proteins form metabolic processes, namely, triglycerides metabolism, as the main targets of phosphorylation. Regarding disease conditions, more phosphorylated proteins were observed in diabetics with some distinct phosphoproteins identified in type 2 prediabetic states and early type 2 diabetes mellitus. Heart-failure-related phosphorylated proteins are in much lower amount and are mainly involved in transport and metabolism. Nevertheless, technical considerations related to mitochondria isolation and protein separation should be considered in data comparison among different proteomic studies. Data from the present review will certainly open new perspectives of protein phosphorylation in mitochondria and will help to envisage future studies targeting the underlying regulatory mechanisms.

Post-translational modifications near the quinone binding site of mammalian complex I

Complex I (NADH:ubiquinone oxidoreductase) in mammalian mitochondria is an L-shaped assembly of 44 protein subunits with one arm buried in the inner membrane of the mitochondrion and the orthogonal arm protruding about 100 Å into the matrix. The protruding arm contains the binding sites for NADH, the primary acceptor of electrons flavin mononucleotide (FMN), and a chain of seven iron-sulfur clusters that carries the electrons one at a time from FMN to a coenzyme Q molecule bound in the vicinity of the junction between the two arms. In the structure of the closely related bacterial enzyme from Thermus thermophilus, the quinone is thought to bind in a tunnel that spans the interface between the two arms, with the quinone head group close to the terminal iron-sulfur cluster, N2. The tail of the bound quinone is thought to extend from the tunnel into the lipid bilayer. In the mammalian enzyme, it is likely that this tunnel involves three of the subunits of the complex, ND1, PSST, and the 49-kDa subunit. An arginine residue in the 49-kDa subunit is symmetrically dimethylated on the ω-N^G and ω-N^G′ nitrogen atoms of the guanidino group and is likely to be close to cluster N2 and to influence its properties. Another arginine residue in the PSST subunit is hydroxylated and probably lies near to the quinone. Both modifications are conserved in mammalian enzymes, and the former is additionally conserved in Pichia pastoris and Paracoccus denitrificans, suggesting that they are functionally significant.

cAMP and mitochondria

Phosphorylation of mitochondrial proteins has emerged as a major regulatory mechanism for metabolic adaptation. cAMP signaling and PKA phosphorylation of mitochondrial proteins have just started to be investigated, and the presence of cAMP-generating enzymes and PKA inside mitochondria is still controversial. Here, we discuss the role of cAMP in regulating mitochondrial bioenergetics through protein phosphorylation and the evidence for soluble adenylyl cyclase as the source of cAMP inside mitochondria.

Complex I deficiencies in neurological disorders

Complex I is the point of entry in the mitochondrial electron transport chain for NADH reducing equivalents, and it behaves as a regulatable pacemaker of respiratory ATP production in human cells. Defects in complex I are associated with several human neurological disorders, including primary mitochondrial diseases, Parkinson disease (PD), and Down syndrome, and understanding the activity and regulation of complex I may reveal aspects of the underlying pathogenic mechanisms. Complex I is regulated by cyclic AMP (cAMP) and the protein kinase A (PKA) signal transduction pathway, and elucidating the role of the cAMP/PKA system in regulating complex I and oxygen free radical production provides new perspectives for devising therapeutic strategies for neurological diseases.

AIF, reactive oxygen species, and neurodegeneration: a "complex" problem

Apoptosis-inducing factor (AIF) is a flavin-binding mitochondrial intermembrane space protein that is implicated in diverse but intertwined processes that include maintenance of electron transport chain function, reactive oxygen species regulation, cell death, and neurodegeneration. In acute brain injury, AIF acquires a pro-death role upon translocation from the mitochondria to the nucleus, where it initiates chromatin condensation and large-scale DNA fragmentation. Although harlequin mice exhibiting an 80-90% global reduction in AIF protein are resistant to numerous forms of acute brain injury, they paradoxically undergo slow, progressive neurodegeneration beginning at three months of age. Brain deterioration, accompanied by markers of oxidative stress, is most pronounced in the cerebellum and retina, although it also occurs in the cortex, striatum, and thalamus. Loss of an AIF pro-survival function linked to assembly or stabilization of electron transport chain complex I underlies chronic neurodegeneration. To date, most studies of neurodegeneration have failed to adequately separate the relative importance of the mitochondrial and nuclear functions of AIF in determining the extent of injury, or whether oxidative stress plays a causative role. This review explores the complicated relationship among AIF, complex I, and the regulation of mitochondrial reactive oxygen species levels. It also discusses the controversial role of complex I deficiency in Parkinson's disease, and what can be learned from the AIF- and complex I-depleted harlequin mouse.

Mitochondria: A mirror into cellular dysfunction in heart disease

Cardiovascular (CV) disease is the single most significant cause of morbidity and mortality worldwide. The emerging global impact of CV disease means that the goals of early diagnosis and a wider range of treatment options are now increasingly pertinent. As such, there is a greater need to understand the molecular mechanisms involved and potential targets for intervention. Mitochondrial function is important for physiological maintenance of the cell, and when this function is altered, the cell can begin to suffer. Given the broad range and significant impacts of the cellular processes regulated by the mitochondria, it becomes important to understand the roles of the proteins associated with this organelle. Proteomic investigations of the mitochondria are hampered by the intrinsic properties of the organelle, including hydrophobic mitochondrial membranes; high proportion of basic proteins (pI greater than 8.0); and the relative dynamic range issues of the mitochondria. For these reasons, many proteomic studies investigate the mitochondria as a discrete subproteome. Once this has been achieved, the alterations that result in functional changes with CV disease can be observed. Those alterations that lead to changes in mitochondrial function, signaling and morphology, which have significant implications for the cardiomyocyte in the development of CV disease, are discussed.

Mitochondrial proteomic analysis reveals deficiencies in oxygen utilization in medullary thick ascending limb of Henle in the Dahl salt-sensitive rat

The renal medullary thick ascending limb (mTAL) of the Dahl salt-sensitive (SS) rat is the site of enhanced NaCl reabsorption and excess superoxide production. In the present studies we isolated mitochondria from mTAL of SS and salt-resistant control strain SS.13(BN) rats on 0.4 and 8% salt diet for 7 days and performed a proteomic analysis. Purity of mTAL and mitochondria isolations exceeded 93.6 and 55%, respectively. Using LC/MS spectral analysis techniques we identified 96 mitochondrial proteins in four biological mTAL mitochondria samples, run in duplicate, as defined by proteins with a false discovery rate <5% and scan count ≥2. Seven of these 96 proteins, including IDH2, ACADM, SCOT, Hsp60, ATPA, EFTu, and VDAC2 were differentially expressed between the two rat strains. Oxygen consumption and high-resolution respirometry analyses showed that mTAL cells and the mitochondria in the outer medulla of SS rats fed high-salt diet exhibited lower rates of oxygen utilization compared with those from SS.13(BN) rats. These studies advance the conventional proteomic paradigm of focusing exclusively upon whole tissue homogenates to a focus upon a single cell type and specific subcellular organelle. The results reveal the importance of a largely unexplored role for deficiencies of mTAL mitochondrial metabolism and oxygen utilization in salt-induced hypertension and renal medullary oxidative stress.

The oxidative phosphorylation system in mammalian mitochondria

The chapter provides a review of the state of art of the oxidative phosphorylation system in mammalian mitochondria. The sections of the paper deal with: (i) the respiratory chain as a whole: redox centers of the chain and protonic coupling in oxidative phosphorylation (ii) atomic structure and functional mechanism of protonmotive complexes I, III, IV and V of the oxidative phosphorylation system (iii) biogenesis of oxidative phosphorylation complexes: mitochondrial import of nuclear encoded subunits, assembly of oxidative phosphorylation complexes, transcriptional factors controlling biogenesis of the complexes. This advanced knowledge of the structure, functional mechanism and biogenesis of the oxidative phosphorylation system provides a background to understand the pathological impact of genetic and acquired dysfunctions of mitochondrial oxidative phosphorylation.

Mitochondrial protein phosphorylation as a regulatory modality: implications for mitochondrial dysfunction in heart failure

Phosphorylation of mitochondrial proteins has been recognized for decades, and the regulation of pyruvate- and branched-chain α-ketoacid dehydrogenases by an atypical kinase/phosphatase cascade is well established. More recently, the development of new mass spectrometry-based technologies has led to the discovery of many novel phosphorylation sites on a variety of mitochondrial targets. The evidence suggests that the major classes of kinase and several phosphatases may be present at the mitochondrial outer membrane, intermembrane space, inner membrane, and matrix, but many questions remain to be answered as to the location, timing, and reversibility of these phosphorylation events and whether they are functionally relevant. The authors review phosphorylation as a mitochondrial regulatory strategy and highlight its possible role in the pathophysiology of cardiac hypertrophy and failure.

Divide and conquer: the application of organelle proteomics to heart failure

Chronic heart failure is a worldwide cause of mortality and morbidity and is the final outcome of a number of different etiologies. This reflects both the complexity of the disease and our incomplete understanding of its underlying molecular mechanisms. One experimental approach to address this is to study subcellular organelles and how their functions are activated and synchronized under physiological and pathological conditions. In this review, we discuss the application of proteomic technologies to organelles and how this has deepened our perception of the cellular proteome and its alterations with heart failure. The use of proteomics to monitor protein quantity and posttranslational modifications has revealed a highly intricate and sophisticated level of protein regulation. Posttranslational modifications have the potential to regulate organelle function and interplay most likely by targeting both structural and signaling proteins throughout the cell, ultimately coordinating their responses. The potentials and limitations of existing proteomic technologies are also discussed emphasizing that the development of novel methods will enhance our ability to further investigate organelles and decode intracellular communication.

cAMP/Ca2+ response element-binding protein plays a central role in the biogenesis of respiratory chain proteins in mammalian cells

In mammalian cells, promotion of mitochondrial biogenesis by various agents involves cAMP and Ca(2+)-mediated signal transduction pathways. Recruitment of these pathways results in phosphorylation by cAMP and Ca(2+)-dependent protein kinases of cAMP/Ca(2+) response element-binding protein (CREB). Phosphorylation of CREB, bound to transcriptional complexes of target genes, activates a down-stream cascade of transcriptional complexes, which involve in sequence, the nuclear factors TORCs, PGC-1, NRF1 and NRF2, and the mitochondrial factor mitochondrial transcriptional factor A. CREB also binds directly to the D-loop of mitochondrial DNA and activates its expression. Activation of this network of transcriptional complexes results in concerted promotion of the expression of nuclear and mitochondrial genes encoding subunits of oxidative phosphorylation complexes.

Apoptogenic factors released from mitochondria

When cells kill themselves, they usually do so by activating mechanisms that have evolved specifically for that purpose. These mechanisms, which are broadly conserved throughout the metazoa, involve two processes: activation in the cytosol of latent cysteine proteases (termed caspases), and disruption of mitochondrial functions. These processes are linked in a number of different ways. While active caspases can cleave proteins in the mitochondrial outer membrane, and cleave and thereby activate certain pro-apoptotic members of the Bcl-2 family, proteins released from the mitochondria can trigger caspase activation and antagonise IAP family proteins. This review will focus on the pro-apoptotic molecules that are released from the mitochondria of cells endeavouring to kill themselves. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Mammalian mitochondrial complex I: biogenesis, regulation, and reactive oxygen species generation

Virtually every mammalian cell contains mitochondria. These double-membrane organelles continuously change shape and position and contain the complete metabolic machinery for the oxidative conversion of pyruvate, fatty acids, and amino acids into ATP. Mitochondria are crucially involved in cellular Ca2+ and redox homeostasis and apoptosis induction. Maintenance of mitochondrial function and integrity requires an inside-negative potential difference across the mitochondrial inner membrane. This potential is sustained by the electron-transport chain (ETC). NADH:ubiquinone oxidoreductase or complex I (CI), the first and largest protein complex of the ETC, couples the oxidation of NADH to the reduction of ubiquinone. During this process, electrons can escape from CI and react with ambient oxygen to produce superoxide and derived reactive oxygen species (ROS). Depending on the balance between their production and removal by antioxidant systems, ROS may function as signaling molecules or induce damage to a variety of biomolecules or both. The latter ultimately leads to a loss of mitochondrial and cellular function and integrity. In this review, we discuss (a) the role of CI in mitochondrial functioning; (b) the composition, structure, and biogenesis of CI; (c) regulation of CI function; (d) the role of CI in ROS generation; and (e) adaptive responses to CI deficiency.

Phosphorylation pattern of the NDUFS4 subunit of complex I of the mammalian respiratory chain

The NDUFS4 subunit of complex I of the mammalian respiratory chain has a fully conserved carboxy-terminus with a canonical RVSTK phosphorylation site. Immunochemical analysis with specific antibodies shows that the serine in this site of the protein is natively present in complex I in both the phosphorylated and non-phosphorylated state. Two-dimensional IEF/SDS-PAGE electrophoresis, (32)P labelling and immunodetection show that "in vitro" PKA phosphorylates the serine in the C-terminus of the NDUFS4 subunit in isolated bovine complex I. (32)P labelling and TLC phosphoaminoacid mapping show that PKA phosphorylates serine and threonine residues in the purified heterologous human NDUFS4 protein.

Structure and organization of mitochondrial respiratory complexes: a new understanding of an old subject

The enzymatic complexes of the mitochondrial respiratory chain have been extensively investigated in their structural and functional properties. A clear distinction is possible today between three complexes in which the difference in redox potential allows proton translocation (complexes I, III, and IV) and those having the mere function to convey electrons to the respiratory chain. We also have a clearer understanding of the structure and function of most respiratory complexes, of their biogenesis and regulation, and of their capacity to generate reactive oxygen species. Past investigations led to the conclusion that the complexes are randomly dispersed and functionally connected by diffusion of smaller redox components, coenzyme Q and cytochrome c. More-recent investigations by native gel electrophoresis and single-particle image processing showed the existence of supramolecular associations. Flux-control analysis demonstrated that complexes I and III in mammals and I, III, and IV in plants kinetically behave as single units, suggesting the existence of substrate channeling. This review discusses conditions affecting the formation of supercomplexes that, besides kinetic advantage, have a role in the stability and assembly of the individual complexes and in preventing excess oxygen radical formation. Disruption of supercomplex organization may lead to functional derangements responsible for pathologic changes.

Cytokine-induced tumor suppressors: a GRIM story

Cytokines belonging to the IFN family are potent growth suppressors. In a number of clinical and preclinical studies, vitamin A and its derivatives like retinoic acid (RA) have been shown to exert synergistic growth-suppressive effects on several tumor cells. We have employed a genome-wide expression-knockout approach to identify the genes critical for IFN/RA-induced growth suppression. A number of novel genes associated with Retinoid-Interferon-induced Mortality (GRIM) were isolated. In this review, we will describe the molecular mechanisms of actions of one, GRIM-19, which participates in multiple pathways for exerting growth control and/or cell death. This protein is emerging as a new tumor suppressor. In addition, GRIM-19 appears to participate in innate immune responses as its activity is modulated by several viruses and bacteria. Thus, GRIMs seem to couple with multiple biological responses by acting at critical nodes.

cAMP-dependent protein kinase regulates post-translational processing and expression of complex I subunits in mammalian cells

Work is presented on the role of cAMP-dependent protein phosphorylation in post-translational processing and biosynthesis of complex I subunits in mammalian cell cultures. PKA-mediated phosphorylation of the NDUFS4 subunit of complex I promotes in cell cultures in vivo import/maturation in mitochondria of the precursor of this protein. The import promotion appears to be associated with the observed cAMP-dependent stimulation of the catalytic activity of complex I. These effects of PKA are counteracted by activation of protein phosphatase(s). PKA and the transcription factor CREB play a critical role in the biosynthesis of complex I subunits. CREB phosphorylation, by PKA and/or CaMKs, activates at nuclear and mitochondrial level a transcriptional regulatory cascade which promotes the concerted expression of nuclear and mitochondrial encoded subunits of complex I and other respiratory chain proteins.

Dynamics of the Dictyostelium discoideum mitochondrial proteome during vegetative growth, starvation and early stages of development

In this study, a quantitative comparative proteomics approach has been used to analyze the Dictyostelium discoideum mitochondrial proteome variations during vegetative growth, starvation and the early stages of development. Application of 2-D DIGE technology allowed the detection of around 2000 protein spots on each 2-D gel with 180 proteins exhibiting significant changes in their expression level. In total, 96 proteins (51 unique and 45 redundant) were unambiguously identified. We show that the D. discoideum mitochondrial proteome adaptations mainly affect energy metabolism enzymes (the Krebs cycle, anaplerotic pathways, the oxidative phosphorylation system and energy dissipation), proteins involved in developmental and signaling processes as well as in protein biosynthesis and fate. The most striking observations were the opposite regulation of expression of citrate synthase and aconitase and the very large variation in the expression of the alternative oxidase that highlighted the importance of citrate and alternative oxidase in the physiology of the development of D. discoideum. Mitochondrial energy states measured in vivo with MitoTracker Orange CM Ros showed an increase in mitochondrial membrane polarization during D. discoideum starvation and starvation-induced development.

A survey of the Arabidopsis thaliana mitochondrial phosphoproteome

Plant mitochondria play central roles in cellular energy production, metabolism and stress responses. Recent phosphoproteomic studies in mammalian and yeast mitochondria have presented evidence indicating that protein phosphorylation is a likely regulatory mechanism across a broad range of important mitochondrial processes. This study investigated protein phosphorylation in purified mitochondria from cell suspensions of the model plant Arabidopsis thaliana using affinity enrichment and proteomic tools. Eighteen putative phosphoproteins consisting of mitochondrial metabolic enzymes, HSPs, a protease and several proteins of unknown function were detected on 2-DE separations of Arabidopsis mitochondrial proteins and affinity-enriched phosphoproteins using the Pro-Q Diamond phospho-specific in-gel dye. Comparisons with mitochondrial phosphoproteomes of yeast and mouse indicate that these three species share few validated phosphoproteins. Phosphorylation sites for seven of the eighteen mitochondrial proteins were characterized by titanium dioxide enrichment and MS/MS. In the process, 71 phosphopeptides from Arabidopsis proteins which are not present in mitochondria but found as contaminants in various types of mitochondrial preparations were also identified, indicating the low level of phosphorylation of mitochondrial components compared with other cellular components in Arabidopsis. Information gained from this study provides a better understanding of protein phosphorylation at both the subcellular and the cellular level in Arabidopsis.

Redox signaling and protein phosphorylation in mitochondria: progress and prospects

As we learn more about the factors that govern cardiac mitochondrial bioenergetics, fission and fusion, as well as the triggers of apoptotic and necrotic cell death, there is growing appreciation that these dynamic processes are finely-tuned by equally dynamic post-translational modification of proteins in and around the mitochondrion. In this minireview, we discuss the evidence that S-nitrosylation, glutathionylation and phosphorylation of mitochondrial proteins have important bioenergetic consequences. A full accounting of these targets, and the functional impact of their modifications, will be necessary to determine the extent to which these processes underlie ischemia/reperfusion injury, cardioprotection by pre/post-conditioning, and the pathogenesis of heart failure.

CD247 can bind SHC1, no matter if CD247 is phosphorylated

On T cell receptor (TCR) stimulation, src homology 2 domain-containing transforming protein C1 (SHC1) had been found to bind the tyrosine-phosphorylated CD247 chain of the receptor via its src homology 2 (SH2) domain, delivering signals that control T cell development and activation. However, how the phosphorylation of CD247 led to the instant binding has not been characterized clearly. To study the binding process in detail, we simulated and compared the interaction processes of the SH2 domain with CD247 and phosphorylated CD247, respectively. Unexpectedly, the simulation revealed that SHC1 can also bind the nonphosphorylated CD247 peptide, which was further validated to be a weak binding by affinity pull-down experiment. The molecular dynamics (MD) simulation also revealed that the CD247 peptide formed a folding conformation with its Leu209 inserted into the hydrophobic binding pocket in SHC1. And on phosphorylation, it was the electrostatic attraction between the CD247 Tyr(P)206 and the SHC1 Tyr(P)-binding pocket that destroyed the folding conformation of the nonphosphorylated CD247 and, aided by the electrostatic attraction between SHC1 and the Asp203 of CD247, led to the extended conformation of the phosphorylated CD247 binding to SHC1 strongly. The results suggest that nonphosphorylated CD247 can recruit SHC1 in advance to prepare for the instant needs for SHC1 on TCR stimulation. In view of the ubiquity of phosphorylation in protein interaction regulation, we think this study also exemplified the usefulness of MD in more interactome research involving phosphorylation.

Use of (32)P to study dynamics of the mitochondrial phosphoproteome

Protein phosphorylation is a well-characterized regulatory mechanism in the cytosol, but remains poorly defined in the mitochondrion. In this study, we characterized the use of (32)P-labeling to monitor the turnover of protein phosphorylation in the heart and liver mitochondria matrix. The (32)P labeling technique was compared and contrasted to Phos-tag protein phosphorylation fluorescent stain and 2D isoelectric focusing. Of the 64 proteins identified by MS spectroscopy in the Phos-Tag gels, over 20 proteins were correlated with (32)P labeling. The high sensitivity of (32)P incorporation detected proteins well below the mass spectrometry and even 2D gel protein detection limits. Phosphate-chase experiments revealed both turnover and phosphate associated protein pool size alterations dependent on initial incubation conditions. Extensive weak phosphate/phosphate metabolite interactions were observed using nondisruptive native gels, providing a novel approach to screen for potential allosteric interactions of phosphate metabolites with matrix proteins. We confirmed the phosphate associations in Complexes V and I due to their critical role in oxidative phosphorylation and to validate the 2D methods. These complexes were isolated by immunocapture, after (32)P labeling in the intact mitochondria, and revealed (32)P-incorporation for the alpha, beta, gamma, OSCP, and d subunits in Complex V and the 75, 51, 42, 23, and 13a kDa subunits in Complex I. These results demonstrate that a dynamic and extensive mitochondrial matrix phosphoproteome exists in heart and liver.

New extension of the Mitchell Theory for oxidative phosphorylation in mitochondria of living organisms

The Mitchell Theory implies the proton motive force Deltap across the inner mitochondrial membrane as the energy-rich intermediate of oxidative phosphorylation. Deltap is composed mainly of an electrical (DeltaPsi(m)) and a chemical part (DeltapH) and generated by the respiratory chain complexes I, III and IV. It is consumed mostly by the ATP synthase (complex V) to produce ATP. The free energy of electron transport within the proton pumps is sufficient to generate Deltap of about 240 mV. The proton permeability of biological membranes, however, increases exponentially above 130 mV leading to a waste of energy at high values (DeltaPsi(m)>140 mV). In addition, at DeltaPsi(m)>140 mV, the production of the superoxide radical anion O(2)(-) at complexes I, II and III increases exponentially with increasing DeltaPsi(m). O(2)(-) and its neutral product H(2)O(2) (=ROS, reactive oxygen species) induce oxidative stress which participates in aging and in the generation of degenerative diseases. Here we describe a new mechanism which acts independently of the Mitchell Theory and keeps DeltaPsi(m) at low values through feedback inhibition of complex IV (cytochrome c oxidase) at high ATP/ADP ratios, thus preventing the formation of ROS and maintaining high efficiency of oxidative phosphorylation.

Transformation by retroviral vectors of bone marrow-derived mesenchymal cells induces mitochondria-dependent cAMP-sensitive reactive oxygen species production

Retroviral vectors are used in human gene therapy trials to stably introduce therapeutic genes in the genome of patients' cells. Their applicability, however, is frustrated by the limited viability of transformed cells and/or by risks linked to selection of oncogene-mutated clones. The reasons for these drawbacks are not yet completely understood. In this study, we show that LXSN-NeoR gene/interleukin-7-engineered mesenchymal stromal cells exhibited a marked enhancement of reactive oxygen species production compared with untransfected cells. This effect resulted to be independent on the product of the gene carried by the retroviral vehicle as it was reproducible in cells transfected with the empty vector alone. Stable transfection of mesenchymal stromal cells with the different retroviral vectors pBabe-puro and PINCO-puro and the lentiviral vector pSico PGK-puro caused similar redox imbalance, unveiling a phenomenon of more general impact. The enhanced production of reactive oxygen species over the basal level was attributable to mitochondrial dysfunction and brought back to altered activity of the NADH-CoQ oxidoreductase (complex I) of the respiratory chain. The oxidative stress in transfected mesenchymal stem cells was completely reversed by treatment with a cAMP analog, thus pointing to alteration in the protein kinase A-dependent signaling pathway of the host cell. Transfection of mesenchymal stromal cells with a PINCO-parental vector harboring the green fluorescent protein gene as selection marker in place of the puromycin-resistance gene resulted in no alteration of the redox phenotype. These novel findings provide insights and caveats to the applicability of cell- or gene-based therapies and indicate possible intervention to improve them. Disclosure of potential conflicts of interest is found at the end of this article.

Phosphoproteome analysis of isoflurane-protected heart mitochondria: phosphorylation of adenine nucleotide translocator-1 on Tyr194 regulates mitochondrial function

AIMS

Reversible phosphorylation of mitochondrial proteins is essential in the regulation of respiratory function, energy metabolism, and mitochondrion-mediated cell death. We hypothesized that mitochondrial protein phosphorylation plays a critical role in cardioprotection during pre and postconditioning, two of the most efficient anti-ischaemic therapies.

METHODS AND RESULTS

Using phosphoproteomic approaches, we investigated the profiles of phosphorylated proteins in Wistar rat heart mitochondria protected by pharmacological pre and postconditioning elicited by isoflurane. Sixty-one spots were detected by two-dimensional blue-native gel electrophoresis-coupled Western blotting using a phospho-Ser/Thr/Tyr-specific antibody, and 45 of these spots were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Eleven protein spots related to oxidative phosphorylation, energy metabolism, chaperone, and carrier functions exhibited significant changes in their phosphorylation state when protected mitochondria were compared with unprotected. Using a phosphopeptide enrichment protocol followed by liquid chromatography-MS/MS, 26 potential phosphorylation sites were identified in 19 proteins. Among these, a novel phosphorylation site was detected in adenine nucleotide translocator-1 (ANT1) at residue Tyr(194). Changes in ANT phosphorylation between protected and unprotected mitochondria were confirmed by immunoprecipitation. The biological significance of ANT phosphorylation at Tyr(194) was further tested with site-directed mutagenesis in yeast. Substitution of Tyr(194) with Phe, mimicking the non-phosphorylated state, resulted in the inhibition of yeast growth on non-fermentable carbon sources, implying a critical role of phosphorylation at this residue in regulating ANT function and cellular respiration.

CONCLUSIONS

Our analysis emphasizes the regulatory functions of the phosphoproteome in heart mitochondria and reveals a novel, potential link between bioenergetics and cardioprotection.

Mammalian complex I: a regulable and vulnerable pacemaker in mitochondrial respiratory function

In this paper the regulatory features of complex I of mammalian and human mitochondria are reviewed. In a variety of mitotic cell-line cultures, activation in vivo of the cAMP cascade, or direct addition of cAMP, promotes the NADH-ubiquinone oxidoreductase activity of complex I and lower the cellular level of ROS. These effects of cAMP are found to be associated with PKA-mediated serine phosphorylation in the conserved C-terminus of the subunit of complex I encoded by the nuclear gene NDUFS4. PKA mediated phosphorylation of this Ser in the C-terminus of the protein promotes its mitochondrial import and maturation. Mass-spectrometry analysis of the phosphorylation pattern of complex I subunits is also reviewed.

Complex I specific increase in superoxide formation and respiration rate by PrP-null mouse brain mitochondria

An imbalance in free radical production and removal is considered by many to be an important factor in the etiology of many degenerative diseases. Since mitochondria are a major source of free radicals, we have examined mitochondrial free radical production in relation to oxidative phosphorylation in PrP-null mice. Quantitative electron paramagnetic resonance spectroscopy revealed up to a 70% increase in superoxide production from Complex I of submitochondrial particles prepared from PrP-null mice. This was accompanied by elevated respiratory capacity through Complex I without any discernible alteration in respiratory efficiency. These differences are associated with changes in superoxide dismutase levels and defects in mitochondrial morphology, confirming previously reported results. Our results demonstrate a clear difference in free radical production and oxygen consumption by mitochondrial Complex I between PrP-null mice and wild-type controls, pointing to Complex I as a potential target for pathological change, suggesting similarities between prion-related and other neurodegenerative diseases.