The nuclear-encoded 18 kDa (IP) AQDQ subunit of bovine heart complex I is phosphorylated by the mitochondrial cAMP-dependent protein kinase

A high copy suppressor screen identifies factors enhancing the allotopic production of subunit II of cytochrome c oxidase

Allotopic expression refers to the artificial relocation of an organellar gene to the nucleus. Subunit 2 (Cox2) of cytochrome c oxidase, a subunit with 2 transmembrane domains (TMS1 and TMS2) residing in the inner mitochondrial membrane with a Nout-Cout topology, is typically encoded in the mitochondrial cox2 gene. In the yeast Saccharomyces cerevisiae, the cox2 gene can be allotopically expressed in the nucleus, yielding a functional protein that restores respiratory growth to a Δcox2 null mutant. In addition to a mitochondrial targeting sequence followed by its natural 15-residue leader peptide, the cytosol synthesized Cox2 precursor must carry one or several amino acid substitutions that decrease the mean hydrophobicity of TMS1 and facilitate its import into the matrix by the TIM23 translocase. Here, using a yeast strain that contains a COX2W56R gene construct inserted in a nuclear chromosome, we searched for genes whose overexpression could facilitate import into mitochondria of the Cox2W56R precursor and increase respiratory growth of the corresponding mutant strain. A COX2W56R expressing strain was transformed with a multicopy plasmid genomic library, and transformants exhibiting enhanced respiratory growth on nonfermentable carbon sources were selected. We identified 3 genes whose overexpression facilitates the internalization of the Cox2W56R subunit into mitochondria, namely: TYE7, RAS2, and COX12. TYE7 encodes a transcriptional factor, RAS2, a GTP-binding protein, and COX12, a non-core subunit of cytochrome c oxidase. We discuss potential mechanisms by which the TYE7, RAS2, and COX12 gene products could facilitate the import and assembly of the Cox2W56R subunit produced allotopically.

Diesel exhaust particles alter mitochondrial bioenergetics and cAMP producing capacity in human bronchial epithelial cells

Introduction: Air pollution from diesel combustion is linked in part to the generation of diesel exhaust particles (DEP). DEP exposure induces various processes, including inflammation and oxidative stress, which ultimately contribute to a decline in lung function. Cyclic AMP (cAMP) signaling is critical for lung homeostasis. The impact of DEP on cAMP signaling is largely unknown. Methods: We exposed human bronchial epithelial (BEAS-2B) cells to DEP for 24-72 h and evaluated mitochondrial bioenergetics, markers of oxidative stress and inflammation and the components of cAMP signaling. Mitochondrial bioenergetics was measured at 72 h to capture the potential and accumulative effects of prolonged DEP exposure on mitochondrial function. Results: DEP profoundly altered mitochondrial morphology and network integrity, reduced both basal and ATP-linked respiration as well as the glycolytic capacity of mitochondria. DEP exposure increased gene expression of oxidative stress and inflammation markers such as interleukin-8 and interleukin-6. DEP significantly affected mRNA levels of exchange protein directly activated by cAMP-1 and -2 (Epac1, Epac2), appeared to increase Epac1 protein, but left phospho-PKA levels unhanged. DEP exposure increased A-kinase anchoring protein 1, β2-adrenoceptor and prostanoid E receptor subtype 4 mRNA levels. Interestingly, DEP decreased mRNA levels of adenylyl cyclase 9 and reduced cAMP levels stimulated by forskolin (AC activator), fenoterol (β2-AR agonist) or PGE2 (EPR agonist). Discussion: Our findings suggest that DEP induces mitochondrial dysfunction, a process accompanied by oxidative stress and inflammation, and broadly dampens cAMP signaling. These epithelial responses may contribute to lung dysfunction induced by air pollution exposure.

The pro-oncogenic protein IF1 does not contribute to the Warburg effect and is not regulated by PKA in cancer cells

The endogenous inhibitor of mitochondrial F1Fo-ATPase (ATP synthase), IF1, has been shown to exert pro-oncogenic actions, including reprogramming of cellular energy metabolism (Warburg effect). The latter action of IF1 has been reported to be hampered by its PKA-dependent phosphorylation, but both reprogramming of metabolism and PKA-dependent phosphorylation are intensely debated. To clarify these critical issues, we prepared stably IF1-silenced clones and compared their bioenergetics with that of the three parental IF1-expressing cancer cell lines. All functional parameters: respiration rate, ATP synthesis rate (OXPHOS), and mitochondrial membrane potential were similar in IF1-silenced and control cells, clearly indicating that IF1 cannot inhibit the ATP synthase in cancer cells when the enzyme works physiologically. Furthermore, all cell types exposed to PKA modulators and energized with NAD+-dependent substrates or succinate showed similar OXPHOS rate regardless of the presence or absence of IF1. Therefore, our results rule out that IF1 action is modulated by its PKA-dependent phosphorylated/dephosphorylated state. Notably, cells exposed to a negative PKA modulator and energized with NAD+-dependent substrates showed a significant decrease of the OXPHOS rate matching previously reported inactivation of complex I. Overall, this study definitively demonstrates that IF1 inhibits neither mitochondrial ATP synthase nor OXPHOS in normoxic cancer cells and does not contribute to the Warburg effect. Thus, currently the protection of cancer cells from severe hypoxia/anoxia and apoptosis remain the only unquestionable actions of IF1 as pro-oncogenic factor that may be exploited to develop therapeutic approaches.

Intraperitoneal Administration of Forskolin Reverses Motor Symptoms and Loss of Midbrain Dopamine Neurons in PINK1 Knockout Rats

BACKGROUND

Parkinson's disease (PD) is a relentless, chronic neurodegenerative disease characterized by the progressive loss of substantia nigra (SN) neurons that leads to the onset of motor and non-motor symptoms. Standard of care for PD consists of replenishing the loss of dopamine through oral administration of Levodopa; however, this treatment is not disease-modifying and often induces intolerable side effects. While the etiology that contributes to PD is largely unknown, emerging evidence in animal models suggests that a significant reduction in neuroprotective Protein Kinase A (PKA) signaling in the SN contributes to PD pathogenesis, suggesting that restoring PKA signaling in the midbrain may be a new anti-PD therapeutic alternative.

OBJECTIVE

We surmised that pharmacological activation of PKA via intraperitoneal administration of Forskolin exerts anti-PD effects in symptomatic PTEN-induced kinase 1 knockout (PINK1-KO), a bona fide in vivo model of PD.

METHODS

By using a beam balance and a grip strength analyzer, we show that Forskolin reverses motor symptoms and loss of hindlimb strength with long-lasting therapeutic effects (> 5 weeks) following the last dose.

RESULTS

In comparison, intraperitoneal treatment with Levodopa temporarily (24 h) reduces motor symptoms but unable to restore hindlimb strength in PINK1-KO rats. By using immunohistochemistry and an XF24e BioAnalyzer, Forskolin treatment reverses SN neurons loss, elevates brain energy production and restores PKA activity in SN in symptomatic PINK1-KO rats.

CONCLUSION

Overall, our collective in vivo data suggest that Forskolin is a promising disease-modifying therapeutic alternative for PD and is superior to Levodopa because it confers long-lasting therapeutic effects.

Accessory Subunits of the Matrix Arm of Mitochondrial Complex I with a Focus on Subunit NDUFS4 and Its Role in Complex I Function and Assembly

NADH:ubiquinone-oxidoreductase (complex I) is the largest membrane protein complex of the respiratory chain. Complex I couples electron transfer to vectorial proton translocation across the inner mitochondrial membrane. The L shaped structure of complex I is divided into a membrane arm and a matrix arm. Fourteen central subunits are conserved throughout species, while some 30 accessory subunits are typically found in eukaryotes. Complex I dysfunction is associated with mutations in the nuclear and mitochondrial genome, resulting in a broad spectrum of neuromuscular and neurodegenerative diseases. Accessory subunit NDUFS4 in the matrix arm is a hot spot for mutations causing Leigh or Leigh-like syndrome. In this review, we focus on accessory subunits of the matrix arm and discuss recent reports on the function of accessory subunit NDUFS4 and its interplay with NDUFS6, NDUFA12, and assembly factor NDUFAF2 in complex I assembly.

Mitochondrial Kinases and the Role of Mitochondrial Protein Phosphorylation in Health and Disease

The major role of mitochondria is to provide cells with energy, but no less important are their roles in responding to various stress factors and the metabolic changes and pathological processes that might occur inside and outside the cells. The post-translational modification of proteins is a fast and efficient way for cells to adapt to ever changing conditions. Phosphorylation is a post-translational modification that signals these changes and propagates these signals throughout the whole cell, but it also changes the structure, function and interaction of individual proteins. In this review, we summarize the influence of kinases, the proteins responsible for phosphorylation, on mitochondrial biogenesis under various cellular conditions. We focus on their role in keeping mitochondria fully functional in healthy cells and also on the changes in mitochondrial structure and function that occur in pathological processes arising from the phosphorylation of mitochondrial proteins.

The basics of mitochondrial cAMP signalling: Where, when, why

Cytosolic cAMP signalling in live cells has been extensively investigated in the past, while only in the last decade the existence of an intramitochondrial autonomous cAMP homeostatic system began to emerge. Thanks to the development of novel tools to investigate cAMP dynamics and cAMP/PKA-dependent phosphorylation within the matrix and in other mitochondrial compartments, it is now possible to address directly and in intact living cells a series of questions that until now could be addressed only by indirect approaches, in isolated organelles or through subcellular fractionation studies. In this contribution we discuss the mechanisms that regulate cAMP dynamics at the surface and inside mitochondria, and its crosstalk with organelle Ca²⁺ handling. We then address a series of still unsolved questions, such as the intramitochondrial localization of key elements of the cAMP signaling toolkit, e.g., adenylate cyclases, phosphodiesterases, protein kinase A (PKA) and Epac. Finally, we discuss the evidence for and against the existence of an intramitochondrial PKA pool and the functional role of cAMP increases within the organelle matrix.

Insight into the Interactome of Intramitochondrial PKA Using Biotinylation-Proximity Labeling

Mitochondria are fully integrated in cell signaling. Reversible phosphorylation is involved in adjusting mitochondrial physiology to the cellular needs. Protein kinase A (PKA) phosphorylates several substrates present at the external surface of mitochondria to maintain cellular homeostasis. However, few targets of PKA located inside the organelle are known. The aim of this work was to characterize the impact and the interactome of PKA located inside mitochondria. Our results show that the overexpression of intramitochondrial PKA decreases cellular respiration and increases superoxide levels. Using proximity-dependent biotinylation, followed by LC-MS/MS analysis and in silico phospho-site prediction, we identified 21 mitochondrial proteins potentially targeted by PKA. We confirmed the interaction of PKA with TIM44 using coimmunoprecipitation and observed that TIM44-S80 is a key residue for the interaction between the protein and the kinase. These findings provide insights into the interactome of intramitochondrial PKA and suggest new potential mechanisms in the regulation of mitochondrial functions.

Differential posttranslational modification of mitochondrial enzymes corresponds with metabolic suppression during hibernation

During hibernation, small mammals, including the 13-lined ground squirrel (Ictidomys tridecemlineatus), cycle between two distinct metabolic states: torpor, where metabolic rate is suppressed by >95% and body temperature falls to ~5°C, and interbout euthermia (IBE), where both metabolic rate and body temperature rapidly increase to euthermic levels. Suppression of whole animal metabolism during torpor is paralleled by rapid, reversible suppression of mitochondrial respiration. We hypothesized that these changes in mitochondrial metabolism are regulated by posttranslational modifications to mitochondrial proteins. Differential two-dimensional gel electrophoresis and two-dimensional blue-native PAGE revealed differences in the isoelectric point of several liver mitochondrial proteins between torpor and IBE. Quadrupole time-of-flight LC/MS and matrix-assisted laser desorption/ionization MS identified these as proteins involved in β-oxidation, the tricarboxylic acid cycle, reactive oxygen species detoxification, and the electron transport system (ETS). Immunoblots revealed that subunit 1 of ETS complex IV was acetylated during torpor but not IBE. Phosphoprotein staining revealed significantly greater phosphorylation of succinyl-CoA ligase and the flavoprotein subunit of ETS complex II in IBE than torpor. In addition, the 75-kDa subunit of ETS complex I was 1.5-fold more phosphorylated in torpor. In vitro treatment with alkaline phosphatase increased the maximal activity of complex I from liver mitochondria isolated from torpid, but not IBE, animals. By contrast, phosphatase treatment decreased complex II activity in IBE but not torpor. These findings suggest that the rapid changes in mitochondrial metabolism in hibernators are mediated by posttranslational modifications of key metabolic enzymes, perhaps by intramitochondrial kinases and deacetylases.

Mitochondrial Biogenesis and Mitochondrial Reactive Oxygen Species (ROS): A Complex Relationship Regulated by the cAMP/PKA Signaling Pathway

Mitochondrial biogenesis is a complex process. It requires the contribution of both the nuclear and the mitochondrial genomes and therefore cross talk between the nucleus and mitochondria. Cellular energy demand can vary by great length and it is now well known that one way to adjust adenosine triphosphate (ATP) synthesis to energy demand is through modulation of mitochondrial content in eukaryotes. The knowledge of actors and signals regulating mitochondrial biogenesis is thus of high importance. Here, we review the regulation of mitochondrial biogenesis both in yeast and in mammalian cells through mitochondrial reactive oxygen species.

Mitochondrial cAMP-PKA signaling: What do we really know?

Mitochondria are key organelles for cellular homeostasis. They generate the most part of ATP that is used by cells through oxidative phosphorylation. They also produce reactive oxygen species, neurotransmitters and other signaling molecules. They are important for calcium homeostasis and apoptosis. Considering the role of this organelle, it is not surprising that most mitochondrial dysfunctions are linked to the development of pathologies. Various mechanisms adjust mitochondrial activity according to physiological needs. The cAMP-PKA signaling emerged in recent years as a direct and powerful mean to regulate mitochondrial functions. Multiple evidence demonstrates that such pathway can be triggered from cytosol or directly within mitochondria. Notably, specific anchor proteins target PKA to mitochondria whereas enzymes necessary for generation and degradation of cAMP are found directly in these organelles. Mitochondrial PKA targets proteins localized in different compartments of mitochondria, and related to various functions. Alterations of mitochondrial cAMP-PKA signaling affect the development of several physiopathological conditions, including neurodegenerative diseases. It is however difficult to discriminate between the effects of cAMP-PKA signaling triggered from cytosol or directly in mitochondria. The specific roles of PKA localized in different mitochondrial compartments are also not completely understood. The aim of this work is to review the role of cAMP-PKA signaling in mitochondrial (patho)physiology.

Mitochondrial cAMP signaling

Cyclic adenosine 3, 5'-monophosphate (cAMP) is a ubiquitous second messenger regulating many biological processes, such as cell migration, differentiation, proliferation and apoptosis. cAMP signaling functions not only on the plasma membrane, but also in the nucleus and in organelles such as mitochondria. Mitochondrial cAMP signaling is an indispensable part of the cytoplasm-mitochondrion crosstalk that maintains mitochondrial homeostasis, regulates mitochondrial dynamics, and modulates cellular stress responses and other signaling pathways. Recently, the compartmentalization of mitochondrial cAMP signaling has attracted great attentions. This new input should be carefully taken into account when we interpret the findings of mitochondrial cAMP signaling. In this review, we summarize previous and recent progress in our understanding of mitochondrial cAMP signaling, including the components of the signaling cascade, and the function and regulation of this signaling pathway in different mitochondrial compartments.

Post-translational modifications in mitochondria: protein signaling in the powerhouse

There is an intimate interplay between cellular metabolism and the pathophysiology of disease. Mitochondria are essential to maintaining and regulating metabolic function of cells and organs. Mitochondrial dysfunction is implicated in diverse diseases, such as cardiovascular disease, diabetes and metabolic syndrome, neurodegeneration, cancer, and aging. Multiple reversible post-translational protein modifications are located in the mitochondria that are responsive to nutrient availability and redox conditions, and which can act in protein-protein interactions to modify diverse mitochondrial functions. Included in this are physiologic redox signaling via reactive oxygen and nitrogen species, phosphorylation, O-GlcNAcylation, acetylation, and succinylation, among others. With the advent of mass proteomic screening techniques, there has been a vast increase in the array of known mitochondrial post-translational modifications and their protein targets. The functional significance of these processes in disease etiology, and the pathologic response to their disruption, are still under investigation. However, many of these reversible modifications act as regulatory mechanisms in mitochondria and show promise for mitochondrial-targeted therapeutic strategies. This review addresses the current knowledge of post-translational processing and signaling mechanisms in mitochondria, and their implications in health and disease.

Mitochondrial phosphoproteomics of mammalian tissues

Mitochondria are essential for several biological processes including energy metabolism and cell survival. Accordingly, impaired mitochondrial function is involved in a wide range of human pathologies including diabetes, cancer, cardiovascular, and neurodegenerative diseases. Within the past decade a growing body of evidence indicates that reversible phosphorylation plays an important role in the regulation of a variety of mitochondrial processes as well as tissue-specific mitochondrial functions in mammals. The rapidly increasing number of mitochondrial phosphorylation sites and phosphoproteins identified is largely ascribed to recent advances in phosphoproteomic technologies such as fractionation, phosphopeptide enrichment, and high-sensitivity mass spectrometry. However, the functional importance and the specific kinases and phosphatases involved have yet to be determined for the majority of these mitochondrial phosphorylation sites. This review summarizes the progress in establishing the mammalian mitochondrial phosphoproteome and the technical challenges encountered while characterizing it, with a particular focus on large-scale phosphoproteomic studies of mitochondria from human skeletal muscle.

Intramitochondrial adenylyl cyclase controls the turnover of nuclear-encoded subunits and activity of mammalian complex I of the respiratory chain

In mammalian cells the nuclear-encoded subunits of complex I are imported into mitochondria, where they are assembled with mt-DNA encoded subunits in the complex, or exchanged with pre-existing copies in the complex. The present work shows that in fibroblast cultures inhibition by KH7 of cAMP production in the mitochondrial matrix by soluble adenylyl cyclase (sAC) results in decreased amounts of free non-incorporated nuclear-encoded NDUFS4, NDUFV2 and NDUFA9 subunits of the catalytic moiety and inhibition of the activity of complex I. Addition of permeant 8-Br-cAMP prevents this effect of KH7. KH7 inhibits accumulation in isolated rat-liver mitochondria and incorporation in complex I of "in vitro" produced, radiolabeled NDUFS4 and NDUFV2 subunits. 8-Br-cAMP prevents also this effect of KH7. Use of protease inhibitors shows that intramitochondrial cAMP exerts this positive effect on complex I by preventing digestion of nuclear-encoded subunits by mitochondrial protease(s), whose activity is promoted by KH7 and H89, an inhibitor of PKA.

Role of soluble adenylyl cyclase in mitochondria

The soluble adenylyl cyclase (sAC) catalyzes the conversion of ATP into cyclic AMP (cAMP). Recent studies have shed new light on the role of sAC localized in mitochondria and its product cAMP, which drives mitochondrial protein phosphorylation and regulation of the oxidative phosphorylation system and other metabolic enzymes, presumably through the activation of intra-mitochondrial PKA. In this review article, we summarize recent findings on mitochondrial sAC activation by bicarbonate (HCO(3)(-)) and calcium (Ca²⁺) and the effects on mitochondrial metabolism. We also discuss putative mechanisms whereby sAC-mediated mitochondrial protein phosphorylation regulates mitochondrial metabolism. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

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.

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.

Cellular and animal models for mitochondrial complex I deficiency: a focus on the NDUFS4 subunit

To allow the rational design of effective treatment strategies for human mitochondrial disorders, a proper understanding of their biochemical and pathophysiological aspects is required. The development and evaluation of these strategies require suitable model systems. In humans, inherited complex I (CI) deficiency is one of the most common deficiencies of the mitochondrial oxidative phosphorylation system. During the last decade, various cellular and animal models of CI deficiency have been presented involving mutations and/or deletion of the Ndufs4 gene, which encodes the NDUFS4 subunit of CI. In this review, we discuss these models and their validity for studying human CI deficiency.

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.

Src kinases are important regulators of mitochondrial functions

Mitochondria produce the most part of the energy used by the cells. This energetic production occurs through the oxidative phosphorylation (OXPHOS) process. Mitochondrial functions such as OXPHOS need to be tightly regulated to respect the needs of cells. Phosphorylation of mitochondrial proteins now appears as a major regulation pathway of mitochondrial functions. Several kinases and phosphatases are specifically targeted to mitochondria where they modulate mitochondrial functions. However, we still poorly understand the extent of tyrosine phosphorylation events on mitochondrial metabolism. Among the tyrosine-kinases observed in mitochondria, Src kinases emerge as key players. In the past years, several mitochondrial proteins were shown to be substrates of Src kinases. Notably, these kinases can impact greatly OXPHOS and apoptosis. Important regulators of Src kinases activity are also observed in mitochondria. The aim of this review is to summarize the recent findings on how overall mitochondrial tyrosine phosphorylation events and more specifically Src kinases can influence mitochondrial functions. The different mechanisms of Src kinases regulation and translocation into mitochondria will be also discussed. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Mouse models of mitochondrial complex I dysfunction

Diseases of the mitochondria generally affect cells with high-energy demand, and appear to most profoundly affect excitatory cells that have localized high energy requirements, such as neurons and cardiac and skeletal muscle cells. Complex I of the mammalian mitochondrial respiratory chain is a very large, 45 subunit enzyme, and functional deficiency of complex I is the most frequently observed cause of oxidative phosphorylation (OXPHOS) disorders. Impairment of complex I results in decreased cellular energy production and is responsible for a variety of human encephalopathies, myopathies and cardiomyopathies. Complex I deficiency may be caused by mutations in any of the seven mitochondrial or 38 nuclear genes that encode complex I subunits or by mutations in various other nuclear genes that affect complex I assembly or function. Mouse models that faithfully mimic human complex I disorders are needed to better understand the role of complex I in health and disease and for evaluation of potential therapies for mitochondrial diseases. In this review we discuss existing mouse models of mitochondrial complex I dysfunction, focusing on those with similarities to human mitochondrial disorders. We also discuss some of the noteworthy murine genetic models in which complex I genes are not disrupted, but complex I dysfunction is observed, along with some of the more popular chemical compounds that inhibit complex I function and are useful for modeling complex I deficiency in mice. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Novel mitochondrial DNA mutations responsible for maternally inherited nonsyndromic hearing loss

Some cases of maternally inherited isolated deafness are caused by mtDNA mutations, frequently following an exposure to aminoglycosides. Two mitochondrial genes have been clearly described as being affected by mutations responsible for this pathology: the ribosomal RNA 12S gene and the transfer RNA serine (UCN) gene. A previous study identified several candidate novel mtDNA mutations, localized in a variety of mitochondrial genes, found in patients with no previous treatment with aminoglycosides. Five of these candidate mutations are characterized in the present study. These mutations are localized in subunit ND1 of complex I of the respiratory chain (m.3388C>A [p.MT-ND1:Leu28Met]), the tRNA for Isoleucine (m.4295A>G), subunit COII of complex IV (m.8078G>A [p.MT-CO2:Val165Ile]), the tRNA of Serine 2 (AGU/C) (m.12236G>A), and Cytochrome B, subunit of complex III (m.15077G>A [p.MT-CYB:Glu111Lys]). Cybrid cell lines have been constructed for each of the studied mtDNA mutations and functional studies have been performed to assess the possible consequences of these mutations on mitochondrial bioenergetics. This study shows that a variety of mitochondrial genes, including protein-coding genes, can be responsible for nonsyndromic deafness, and that exposure to aminoglycosides is not required to develop the disease, giving new insights on the molecular bases of this pathology.

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.

Respiratory chain complex I, a main regulatory target of the cAMP/PKA pathway is defective in different human diseases

In mammals, complex I (NADH-ubiquinone oxidoreductase) of the mitochondrial respiratory chain has 31 supernumerary subunits in addition to the 14 conserved from prokaryotes to humans. Multiplicity of structural protein components, as well as of biogenesis factors, makes complex I a sensible pace-maker of mitochondrial respiration. The work reviewed here shows that the cAMP/PKA pathway regulates the biogenesis, assembly and catalytic activity of complex I and mitochondrial oxygen superoxide production. The structural, functional and regulatory complexity of complex I, renders it particularly vulnerable to genetic and sporadic pathological factors. Complex I dysfunction has, indeed, been found, to be associated with several human diseases. Knowledge of the pathogenetic mechanisms of these diseases can help to develop new therapeutic strategies.

Progesterone synthesis by human placental mitochondria is sensitive to PKA inhibition by H89

The transfer of cholesterol to mitochondria, which might involve the phosphorylation of proteins, is the rate-limiting step in human placental steroidogenesis. Protein kinase A (PKA) activity and its role in progesterone synthesis by human placental mitochondria were assessed in this study. The results showed that PKA and phosphotyrosine phosphatase D1 are associated with syncytiotrophoblast mitochondrial membrane by an anchoring kinase cAMP protein-121. The ³²P-labeled of four major proteins was analyzed. The specific inhibitor of PKA, H89, decreased progesterone synthesis in mitochondria while in mitochondrial steroidogenic contact sites protein-phosphorylation was diminished, suggesting that PKA plays a role in placental hormone synthesis. In isolated mitochondria, PKA activity was unaffected by the addition of cAMP suggesting a constant activity of this kinase in the syncytiotrophoblast. The presence of PKA and phosphotyrosine phosphatase D1 anchored to mitochondria by an anchoring kinase cAMP protein-121 indicated that syncytiotrophoblast mitochondria contain a full phosphorylation/dephosphorylation system.

Allosteric nucleotide-binding site in the mitochondrial NADH:ubiquinone oxidoreductase (respiratory complex I)

The rotenone-insensitive NADH:hexaammineruthenium III (HAR) oxidoreductase reactions catalyzed by bovine heart and Yarrowia lipolytica submitochondrial particles or purified bovine complex I are stimulated by ATP and other purine nucleotides. The soluble fraction of mammalian complex I (FP) and prokaryotic complex I homolog NDH-1 in Paracoccus denitrificans plasma membrane lack stimulation of their activities by ATP. The stimulation appears as a decrease in apparent K(m) values for NADH and HAR. Thus, the "accessory" subunits of eukaryotic complex I bear an allosteric ATP-binding site.

Nitric oxide and calcium participate in the fine regulation of mitochondrial biogenesis in follicular thyroid carcinoma cells

Members of the peroxisome proliferator-activated receptor γ coactivator-1 family (i.e. PGC-1α, PGC-1β, and the PGC-1-related coactivator (PRC)) are key regulators of mitochondrial biogenesis and function. These regulators serve as mediators between environmental or endogenous signals and the transcriptional machinery governing mitochondrial biogenesis. The FTC-133 and RO82 W-1 follicular thyroid carcinoma cell lines, which present significantly different numbers of mitochondria, metabolic mechanisms, and expression levels of PRC and PGC-1α, may employ retrograde signaling in response to respiratory dysfunction. Nitric oxide (NO) and calcium have been hypothesized to participate in this activity. We investigated the effects of the S-nitroso-N-acetyl-DL-penicillamine-NO donor, on the expression of genes involved in mitochondrial biogenesis and cellular metabolic functions in FTC-133 and RO82 W-1 cells by measuring lactate dehydrogenase and cytochrome c oxidase (COX) activities. We studied the action of ionomycin and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA/AM) (i.e. a calcium ionophore and a cytosolic calcium chelator) on whole genome expression and mitochondrial biogenesis in RO82 W-1 cells. COX activity and the dynamics of endoplasmic reticulum and mitochondrial networks were analyzed in regard to calcium-modulating treatments. In the FTC-133 and RO82 W-1 cells, the mitochondrial biogenesis induced by NO was mainly related to PRC expression as a retrograde mitochondrial signaling. Ionomycin diminished COX activity and negatively regulated PRC-mediated mitochondrial biogenesis in RO82 W-1 cells, whereas BAPTA/AM produced the opposite effects with a reorganization of the mitochondrial network. This is the first demonstration that NO and calcium regulate mitochondrial biogenesis through the PRC pathway in thyroid cell lines.

Cardiac mitochondria in heart failure: normal cardiolipin profile and increased threonine phosphorylation of complex IV

Mitochondrial dysfunction is a major contributor in heart failure (HF). We investigated whether the decrease in respirasome organization reported by us previously in cardiac mitochondria in HF is due to changes in the phospholipids of the mitochondrial inner membrane or modifications of the subunits of the electron transport chain (ETC) complexes. The contents of the main phospholipid species, including cardiolipin, as well as the molecular species of cardiolipin were unchanged in cardiac mitochondria in HF. Oxidized cardiolipin molecular species were not observed. In heart mitochondria isolated from HF, complex IV not incorporated into respirasomes exhibits increased threonine phosphorylation. Since HF is associated with increased adrenergic drive to cardiomyocytes, this increased protein phosphorylation might be explained by the involvement of cAMP-activated protein kinase. Does the preservation of cAMP-induced phosphorylation changes of mitochondrial proteins or the addition of exogenous cAMP have similar effects on oxidative phosphorylation? The usage of phosphatase inhibitors revealed a specific decrease in complex I-supported respiration with glutamate. In saponin-permeabilized cardiac fibers, pre-incubation with cAMP decreases oxidative phosphorylation due to a defect localized at complex IV of the ETC inter alia. We propose that phosphorylation of specific complex IV subunits decreases oxidative phosphorylation either by limiting the incorporation of complex IV in supercomplexes or by decreasing supercomplex stability.

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.

Mitochondria in heart failure

This review focuses on the evidence accumulated in humans and animal models to the effect that mitochondria are key players in the progression of heart failure (HF). Mitochondria are the primary source of energy in the form of adenosine triphosphate that fuels the contractile apparatus, and are thus essential for the pumping activity of the heart. We evaluate changes in mitochondrial morphology and alterations in the main components of mitochondrial energetics, such as substrate utilization and oxidative phosphorylation coupled with the level of respirasomes, in the context of their contribution to the chronic energy deficit and mechanical dysfunction in HF.

The mitochondrial proteome: a dynamic functional program in tissues and disease states

The nuclear DNA transcriptional programming of the mitochondria proteome varies dramatically between tissues depending on its functional requirements. This programming generally regulates all of the proteins associated with a metabolic or biosynthetic pathway associated with a given function, essentially regulating the maximum rate of the pathway while keeping the enzymes at the same molar ratio. This may permit the same regulatory mechanisms to function at low- and high-flux capacity situations. This alteration in total protein content results in rather dramatic changes in the mitochondria proteome between tissues. A tissues mitochondria proteome also changes with disease state, in Type 1 diabetes the liver mitochondrial proteome shifts to support ATP production, urea synthesis, and fatty acid oxidation. Acute flux regulation is modulated by numerous posttranslational events that also are highly variable between tissues. The most studied posttranslational modification is protein phosphorylation, which is found all of the complexes of oxidative phosphorylation and most of the major metabolic pathways. The functional significance of these modifications is currently a major area of research along with the kinase and phosphatase regulatory network. This near ubiquitous presence of protein phosphorylations, and other posttranslational events, in the matrix suggest that not all posttranslational events have functional significance. Screening methods are being introduced to detect the active or dynamic posttranslational sites to focus attention on sites that might provide insight into regulatory mechanisms.

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.

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.

Modulation of mitochondrial protein phosphorylation by soluble adenylyl cyclase ameliorates cytochrome oxidase defects

Phosphorylation of respiratory chain components has emerged as a mode of regulation of mitochondrial energy metabolism, but its mechanisms are still largely unexplored. A recently discovered intramitochondrial signalling pathway links CO₂ generated by the Krebs cycle with the respiratory chain, through the action of a mitochondrial soluble adenylyl cyclase (mt-sAC). Cytochrome oxidase (COX), whose deficiency causes a number of fatal metabolic disorders, is a key mitochondrial enzyme activated by mt-sAC. We have now discovered that the mt-sAC pathway modulates mitochondrial biogenesis in a reactive oxygen species dependent manner, in cultured cells and in animals with COX deficiency. We show that upregulation of mt-sAC normalizes reactive oxygen species production and mitochondrial biogenesis, thereby restoring mitochondrial function. This is the first example of manipulation of a mitochondrial signalling pathway to achieve a direct positive modulation of COX, with clear implications for the development of novel approaches to treat mitochondrial diseases.

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.

Cyclic AMP produced inside mitochondria regulates oxidative phosphorylation

Mitochondria constantly respond to changes in substrate availability and energy utilization to maintain cellular ATP supplies, and at the same time control reactive oxygen radical (ROS) production. Reversible phosphorylation of mitochondrial proteins has been proposed to play a fundamental role in metabolic homeostasis, but very little is known about the signaling pathways involved. We show here that protein kinase A (PKA) regulates ATP production by phosphorylation of mitochondrial proteins, including subunits of cytochrome c oxidase. The cyclic AMP (cAMP), which activates mitochondrial PKA, does not originate from cytoplasmic sources but is generated within mitochondria by the carbon dioxide/bicarbonate-regulated soluble adenylyl cyclase (sAC) in response to metabolically generated carbon dioxide. We demonstrate for the first time the existence of a CO(2)-HCO(3)(-)-sAC-cAMP-PKA (mito-sAC) signaling cascade wholly contained within mitochondria, which serves as a metabolic sensor modulating ATP generation and ROS production in response to nutrient availability.

Mitochondrial biogenesis and function in Arabidopsis

Mitochondria represent the powerhouse of cells through their synthesis of ATP. However, understanding the role of mitochondria in the growth and development of plants will rely on a much deeper appreciation of the complexity of this organelle. Arabidopsis research has provided clear identification of mitochondrial components, allowed wide-scale analysis of gene expression, and has aided reverse genetic manipulation to test the impact of mitochondrial component loss on plant function. Forward genetics in Arabidopsis has identified mitochondrial involvement in mutations with notable impacts on plant metabolism, growth and development. Here we consider the evidence for components involved in mitochondria biogenesis, metabolism and signalling to the nucleus.

Ischemic preconditioning targets the respiration of synaptic mitochondria via protein kinase C epsilon

In the brain, ischemic preconditioning (IPC) diminishes mitochondrial dysfunction after ischemia and confers neuroprotection. Activation of epsilon protein kinase C (epsilonPKC) has been proposed to be a key neuroprotective pathway during IPC. We tested the hypothesis that IPC increases the levels of epsilonPKC in synaptosomes from rat hippocampus, resulting in improved synaptic mitochondrial respiration. Preconditioning significantly increased the level of hippocampal synaptosomal epsilonPKC to 152% of sham-operated animals at 2 d of reperfusion, the time of peak neuroprotection. We tested the effect of epsilonPKC activation on hippocampal synaptic mitochondrial respiration 2 d after preconditioning. Treatment with the specific epsilonPKC activating peptide, tat-psiepsilonRACK (tat-psiepsilon-receptor for activated C kinase), increased the rate of oxygen consumption in the presence of substrates for complexes I, II, and IV to 157, 153, and 131% of control (tat peptide alone). In parallel, we found that epsilonPKC activation in synaptosomes from preconditioned animals resulted in altered levels of phosphorylated mitochondrial respiratory chain proteins: increased serine and tyrosine phosphorylation of 18 kDa subunit of complex I, decreased serine phosphorylation of FeS protein in complex III, increased threonine phosphorylation of COX IV (cytochrome oxidase IV), increased mitochondrial membrane potential, and decreased H2O2 production. In brief, ischemic preconditioning promoted significant increases in the level of synaptosomal epsilonPKC. Activation of epsilonPKC increased synaptosomal mitochondrial respiration and phosphorylation of mitochondrial respiratory chain proteins. We propose that, at 48 h of reperfusion after ischemic preconditioning, epsilonPKC is poised at synaptic mitochondria to respond to ischemia either by direct phosphorylation or activation of the epsilonPKC signaling pathway.

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.

Adenosine 3',5'-cyclic monophosphate (cAMP)-dependent phosphoregulation of mitochondrial complex I is inhibited by nucleoside reverse transcriptase inhibitors

Nucleoside analog reverse transcriptase inhibitors (NRTIs) are known to directly inhibit mitochondrial complex I activity as well as various mitochondrial kinases. Recent observations that complex I activity and superoxide production are modulated through cAMP-dependent phosphorylation suggests a mechanism through which NRTIs may affect mitochondrial respiration via kinase-dependent protein phosphorylation. In the current study, we examine the potential for NRTIs to inhibit the cAMP-dependent phosphorylation of complex I and the associated NADH:CoQ oxidoreductase activities and rates of superoxide production using HepG2 cells. Phosphoprotein staining of immunocaptured complex I revealed that 3'-azido-3'-deoxythymidine (AZT; 10 and 50 microM), AZT monophosphate (150 microM), and 2',3'-dideoxycytidine (ddC; 1 microM) prevented the phosphorylation of the NDUFB11 subunit of complex I. This was associated with a decrease in complex I activity with AZT and AZT monophosphate only. In the presence of succinate, superoxide production was increased with 2',3'-dideoxyinosine (ddI; 10 microM) and ddC (1 microM). In the presence of succinate+cAMP, AZT showed an inverse dose-dependent effect on superoxide production. None of the NRTIs examined inhibit PKA activity suggesting that the observed effects are due to a direct interaction with complex I. These data demonstrate a direct effect of NRTIs on cAMP-dependent regulation of mitochondrial bioenergetics independent of DNA polymerase-gamma activity; in the case of AZT, these observations may provide a mechanism for the observed long-term toxicity with this drug.

The phosphorylation pattern of bovine heart complex I subunits

The phosphoproteome of bovine heart complex I of the respiratory chain has been analysed with a procedure based on nondenaturing gel electrophoretic separation of complex I from small quantities of mitochondria samples, in-gel digestion, in combination with phosphopeptide enrichment by titanium dioxide and MS. The results, complemented by analyses of purified samples of complex I, showed phosphorylation of five subunits of the complex, 42 kDa (human gene NDUFA10), ESSS, B14.5a (human gene NDUFA7), B14.5b (human gene NDUFC2) and B16.6 (GRIM-19). MS also revealed the presence of phosphorylated programmed cell death protein 8(AIF) in native and purified samples of complex I analysed. The possible physiological relevance of these findings is discussed.

Phosphorylation of B14.5a subunit from bovine heart complex I identified by titanium dioxide selective enrichment and shotgun proteomics

Shotgun proteomics was used to study the steady phosphorylation state of NADH:ubiquinone oxidoreductase (complex I) subunits from bovine heart mitochondria. A total tryptic digestion of enzymatically active complex I was performed, and the resulting peptide mixture was subjected to phosphopeptide enrichment by the use of titanium dioxide (TiO2). The phosphopeptide-enriched fraction was separated and analyzed with nanoscale reverse-phase HPLC-ESI-MS/MS in single information-dependent acquisition. Hence two phosphorylated complex I subunits were detected: 42 kDa and B14.5a. Phosphorylation of 42-kDa subunit at Ser-59 has already been determined with fluorescent phosphoprotein-specific gel staining and mass spectrometry (Schilling, B., Aggeler, R., Schulenberg, B., Murray, J., Row, R. H., Capaldi, R. A., and Gibson, B. W. (2005) Mass spectrometric identification of novel phosphorylation site in subunit NDUFA10 of bovine mitochondrial complex I. FEBS Lett. 579, 2485-2490). In our work, this finding was confirmed using a non-gel-based approach. In addition, we report novel phosphorylation on B14.5a nuclear encoded subunit. We demonstrated evidence of the phosphorylation site at Ser-95 residue by collision-induced dissociation experiments on three different molecular ions of two tryptic phosphopeptides of B14.5a.

Shotgun proteomics for the characterization of subunit composition of mitochondrial complex I

Here we propose shotgun proteomics as an alternative method to gel-based bottom-up proteomic platform for the structural characterization of mitochondrial NADH:ubiquinone oxidoreductase (complex I). The approach is based on simultaneous identification of subunits after global digestion of the intact complex. Resulting mixture of tryptic peptides is purified, concentrated, separated and online analyzed using nano-scale reverse-phase nano-ESI-MS/MS in a single information dependent acquisition mode. The usefulness of the method is demonstrated in our work on the well described model system of complex I from bovine heart mitochondria. The shotgun method led to the identification and partial sequence characterization of 42 subunits representing more than 95% coverage of the complex. In particular, almost all nuclear (except MLRQ) and 5 mitochondria DNA encoded subunits (except ND4L and ND6) were identified. Furthermore, it was possible to identify 30 co-purified proteins of the inner mitochondrial membrane structurally not belonging to complex I. The method's efficiency is shown by comparing it to two classical 1 D gel-based strategies. Shotgun proteomics is less laborious, significantly faster and requires less sample material with minimal treatment, facilitating the screening for post-translational modifications and quantitative and qualitative differences of complex I subunits in genetic disorders.

Modulation of mitochondrial metabolic function by phorbol 12-myristate 13-acetate through increased mitochondrial translocation of protein kinase Calpha in C2C12 myocytes

Protein kinase C (PKC) agonists including phorbol 12-myristate 13-acetate (PMA) not only induce the redistribution of cytosolic PKC to various subcellular compartments but also activate the kinase domain of the protein. In the present study we have investigated the nature of mitochondrial PKC pool and its effects on mitochondrial function in cells treated with PMA. Treatment of C2C12 myoblasts, C6 glioma and COS7 cells with PMA resulted in a dramatic redistribution of intracellular PKCalpha pool, with large fraction of the protein pool sequestered in the mitochondrial compartment. We also observed mitochondrial PKCdelta accumulation in a cell restricted manner. The intramitochondrial localization was ascertained by using a combination of protection against protease treatment of isolated mitochondria and immunofluorescence microscopy. PMA-induced mitochondrial localization of PKCalpha was accompanied by increased mitochondrial PKC activity, altered cell morphology, disruption of mitochondrial membrane potential, decreased complex I and pyruvate dehydrogenase activities, and increased mitochondrial ROS production. All of these changes could be retarded by treatment with PKC inhibitors. These results show a direct role for PMA-mediated PKCalpha translocation to mitochondria in inducing mitochondrial toxicity.