The IF(1) inhibitor protein of the mitochondrial F(1)F(0)-ATPase

Variability of Clinical Phenotypes Caused by Isolated Defects of Mitochondrial ATP Synthase

Disorders of ATP synthase, the key enzyme in mitochondrial energy supply, belong to the most severe metabolic diseases, manifesting as early-onset mitochondrial encephalo-cardiomyopathies. Since ATP synthase subunits are encoded by both mitochondrial and nuclear DNA, pathogenic variants can be found in either genome. In addition, the biogenesis of ATP synthase requires several assembly factors, some of which are also hotspots for pathogenic variants. While variants of MT-ATP6 and TMEM70 represent the most common cases of mitochondrial and nuclear DNA mutations respectively, the advent of next-generation sequencing has revealed new pathogenic variants in a number of structural genes and TMEM70, sometimes with truly peculiar genetics. Here we present a systematic review of the reported cases and discuss biochemical mechanisms, through which they are affecting ATP synthase. We explore how the knowledge of pathophysiology can improve our understanding of enzyme biogenesis and function. Keywords: Mitochondrial diseases o ATP synthase o Nuclear DNA o Mitochondrial DNA o TMEM70.

Peptides Targeting the IF1-ATP Synthase Complex Modulate the Permeability Transition Pore in Cancer HeLa Cells

The mitochondrial protein IF1 is upregulated in many tumors and acts as a pro-oncogenic protein through its interaction with the ATP synthase and the inhibition of apoptosis. We have recently characterized the molecular nature of the IF1-Oligomycin Sensitivity Conferring Protein (OSCP) subunit interaction; however, it remains to be determined whether this interaction could be targeted for novel anti-cancer therapeutic intervention. We generated mitochondria-targeting peptides to displace IF1 from the OSCP interaction. The use of one selective peptide led to displacement of the inhibitor IF1 from ATP synthase, as shown by immunoprecipitation. NMR spectroscopy analysis, aimed at clarifying whether these peptides were able to directly bind to the OSCP protein, identified a second peptide which showed affinity for the N-terminal region of this subunit overlapping the IF1 binding region. In situ treatment with the membrane-permeable derivatives of these peptides in HeLa cells, that are silenced for the IF1 inhibitor protein, showed significant inhibition in mitochondrial permeability transition and no effects on mitochondrial respiration. These peptides mimic the effects of the IF1 inhibitor protein in cancer HeLa cells and confirm that the IF1-OSCP interaction inhibits apoptosis. A third peptide was identified which counteracts the anti-apoptotic role of IF1, showing that OSCP is a promising target for anti-cancer therapies.

Unveiling OSCP as the potential therapeutic target for mitochondrial dysfunction-related diseases

Mitochondria are important organelles in cells responsible for energy production and regulation. Mitochondrial dysfunction has been implicated in the pathogenesis of many diseases. Oligomycin sensitivity-conferring protein (OSCP), a component of the inner mitochondrial membrane, has been studied for a long time. OSCP is a component of the F1Fo-ATP synthase in mitochondria and is closely related to the regulation of the mitochondrial permeability transition pore (mPTP). Studies have shown that OSCP plays an important role in cardiovascular disease, neurological disorders, and tumor development. This review summarizes the localization, structure, function, and regulatory mechanisms of OSCP and outlines its role in cardiovascular disease, neurological disease, and tumor development. In addition, this article reviews the research on the interaction between OSCP and mPTP. Finally, the article suggests future research directions, including further exploration of the mechanism of action of OSCP, the interaction between OSCP and other proteins and signaling pathways, and the development of new treatment strategies for mitochondrial dysfunction. In conclusion, in-depth research on OSCP will help to elucidate its importance in cell function and disease and provide new ideas for the treatment and prevention of related diseases.

Molecular mechanism on forcible ejection of ATPase inhibitory factor 1 from mitochondrial ATP synthase

IF₁ is a natural inhibitor protein for mitochondrial F_oF₁ ATP synthase that blocks catalysis and rotation of the F₁ by deeply inserting its N-terminal helices into F₁. A unique feature of IF₁ is condition-dependent inhibition; although IF₁ inhibits ATP hydrolysis by F₁, IF₁ inhibition is relieved under ATP synthesis conditions. To elucidate this condition-dependent inhibition mechanism, we have performed single-molecule manipulation experiments on IF₁-inhibited bovine mitochondrial F₁ (bMF₁). The results show that IF₁-inhibited F₁ is efficiently activated only when F₁ is rotated in the clockwise (ATP synthesis) direction, but not in the counterclockwise direction. The observed rotational-direction-dependent activation explains the condition-dependent mechanism of IF₁ inhibition. Investigation of mutant IF₁ with N-terminal truncations shows that the interaction with the γ subunit at the N-terminal regions is crucial for rotational-direction-dependent ejection, and the middle long helix is responsible for the inhibition of F₁.

The mitochondrial inhibitor IF1 binds to the ATP synthase OSCP subunit and protects cancer cells from apoptosis

The mitochondrial protein IF1 binds to the catalytic domain of the ATP synthase and inhibits ATP hydrolysis in ischemic tissues. Moreover, IF1 is overexpressed in many tumors and has been shown to act as a pro-oncogenic protein, although its mechanism of action is still debated. Here, we show that ATP5IF1 gene disruption in HeLa cells decreases colony formation in soft agar and tumor mass development in xenografts, underlining the role of IF1 in cancer. Notably, the lack of IF1 does not affect proliferation or oligomycin-sensitive mitochondrial respiration, but it sensitizes the cells to the opening of the permeability transition pore (PTP). Immunoprecipitation and proximity ligation analysis show that IF1 binds to the ATP synthase OSCP subunit in HeLa cells under oxidative phosphorylation conditions. The IF1-OSCP interaction is confirmed by NMR spectroscopy analysis of the recombinant soluble proteins. Overall, our results suggest that the IF1-OSCP interaction protects cancer cells from PTP-dependent apoptosis under normoxic conditions.

The F1Fo-ATPase inhibitor protein IF1 in pathophysiology

The endogenous inhibitor of ATP synthase is a protein of about 10 kDa, known as IF1 which binds to the catalytic domain of the enzyme during ATP hydrolysis. The main role of IF1 consists of limiting ATP dissipation under condition of severe oxygen deprivation or in the presence of dysfunctions of mitochondrial respiratory complexes, causing a collapse in mitochondrial membrane potential and therefore ATP hydrolysis. New roles of IF1 are emerging in the fields of cancer and neurodegeneration. Its high expression levels in tumor tissues have been associated with different roles favouring tumor formation, progression and evasion. Since discordant mechanisms of action have been proposed for IF1 in tumors, it is of the utmost importance to clarify them in the prospective of defining novel approaches for cancer therapy. Other IF1 functions, including its involvement in mitophagy, may be protective for neurodegenerative and aging-related diseases. In the present review we aim to clarify and discuss the emerging mechanisms in which IF1 is involved, providing a critical view of the discordant findings in the literature.

Identification of New Proteins and Potential Mitochondrial F1F0-ATPase Inhibitor Factor 1-Associated Mechanisms in Arabidopsis thaliana Using iTRAQ-Based Quantitative Proteomic Analysis

The mitochondrial synthesis of ATP makes a vital contribution to the growth and development of biological organisms, in which the enzyme mitochondrial F₁F₀-ATP synthase plays a pivotal role, in that it can either synthesize or hydrolyze cellular ATP. The finding of our previous study revealed that mitochondrial F₁F₀-ATPase inhibitor factor 1 (IF1) in Arabidopsis thaliana has a conserved function as an endogenous inhibitor affecting cellular energy status and plays an important role in plant growth and reproduction, particularly in fertility. In this study, to gain an insight into IF1-related traits, we performed isobaric tags for relative and absolute quantitation labeling analysis. In total, 67 of 4778 identified proteins were identified as differentially expressed proteins (DEPs; 59 up-regulated and 8 down-regulated) between wild-type and if1 mutant Arabidopsis thaliana seedlings. Gene ontology enrichment analysis revealed that these DEPs were the most significantly enriched in pathways such as “long-day photoperiodism, flowering,” “positive regulation of protein import into chloroplast stroma,” and “pollen sperm cell differentiation,” which are closely associated with reproductive development. Moreover, Kyoto Encyclopedia of Genes and Genomes enrichment analysis revealed that photosynthesis was the pathway most significantly enriched with DEPs. Collectively, our results revealed a global shift in protein abundance patterns corresponding to AtIF1 mutation, entailing changes in the abundance of multiple key proteins and metabolic processes, which will provide a valuable proteomic foundation for future studies.

The f subunit of human ATP synthase is essential for normal mitochondrial morphology and permeability transition

The f subunit is localized at the base of the ATP synthase peripheral stalk. Its function in the human enzyme is poorly characterized. Because full disruption of its ATP5J2 gene with the CRISPR-Cas9 strategy in the HAP1 human model has been shown to cause alterations in the amounts of other ATP synthase subunits, here we investigated the role of the f subunit in HeLa cells by regulating its levels through RNA interference. We confirm the role of the f subunit in ATP synthase dimer stability and observe that its downregulation per se does not alter the amounts of the other enzyme subunits or ATP synthase synthetic/hydrolytic activity. We show that downregulation of the f subunit causes abnormal crista organization and decreases permeability transition pore (PTP) size, whereas its re-expression in f subunit knockdown cells rescues mitochondrial morphology and PTP-dependent swelling.

The F1Fo-ATPase inhibitor, IF1, is a critical regulator of energy metabolism in cancer cells

In the last two decades, IF1, the endogenous inhibitor of the mitochondrial F1Fo-ATPase (ATP synthase) has assumed greater and ever greater interest since it has been found to be overexpressed in many cancers. At present, several findings indicate that IF1 is capable of playing a central role in cancer cells by promoting metabolic reprogramming, proliferation and resistance to cell death. However, the mechanism(s) at the basis of this pro-oncogenic action of IF1 remains elusive. Here, we recall the main features of the mechanism of the action of IF1 when the ATP synthase works in reverse, and discuss the experimental evidence that support its relevance in cancer cells. In particular, a clear pro-oncogenic action of IF1 is to avoid wasting of ATP when cancer cells are exposed to anoxia or near anoxia conditions, therefore favoring cell survival and tumor growth. However, more recently, various papers have described IF1 as an inhibitor of the ATP synthase when it is working physiologically (i.e. synthethizing ATP), and therefore reprogramming cell metabolism to aerobic glycolysis. In contrast, other studies excluded IF1 as an inhibitor of ATP synthase under normoxia, providing the basis for a hot debate. This review focuses on the role of IF1 as a modulator of the ATP synthase in normoxic cancer cells with the awareness that the knowledge of the molecular action of IF1 on the ATP synthase is crucial in unravelling the molecular mechanism(s) responsible for the pro-oncogenic role of IF1 in cancer and in developing related anticancer strategies.

Interface mobility between monomers in dimeric bovine ATP synthase participates in the ultrastructure of inner mitochondrial membranes

The ATP synthase complexes in mitochondria make the ATP required to sustain life by a rotary mechanism. Their membrane domains are embedded in the inner membranes of the organelle, and they dimerize via interactions between their membrane domains. The dimers form extensive chains along the tips of the cristae with the two rows of monomeric catalytic domains extending into the mitochondrial matrix at an angle to each other. Disruption of the interface between dimers by mutation affects the morphology of the cristae severely. By analysis of particles of purified dimeric bovine ATP synthase by cryo-electron microscopy, we have shown that the angle between the central rotatory axes of the monomeric complexes varies between ca. 76 and 95°. These particles represent active dimeric ATP synthase. Some angular variations arise directly from the catalytic mechanism of the enzyme, and others are independent of catalysis. The monomer-monomer interaction is mediated mainly by j subunits attached to the surface of wedge-shaped protein-lipid structures in the membrane domain of the complex, and the angular variation arises from rotational and translational changes in this interaction, and combinations of both. The structures also suggest how the dimeric ATP synthases might be interacting with each other to form the characteristic rows along the tips of the cristae via other interwedge contacts, molding themselves to the range of oligomeric arrangements observed by tomography of mitochondrial membranes, and at the same time allowing the ATP synthase to operate under the range of physiological conditions that influence the structure of the cristae.

Identification and characterization of Arabidopsis thaliana mitochondrial F1F0-ATPase inhibitor factor 1

Mitochondrial F1F0-ATP synthase (F1F0-ATPase) inhibitor factor 1 (IF1) has been extensively characterized as an endogenous inhibitor that prevents the hydrolysis of adenosine-5'-triphosphate (ATP) by mitochondrial ATPases in mammals and yeasts; however, IF1's functions in plants remain unclear. Here, a comprehensive bioinformatic analysis was performed to identify plant mitochondrial F1F0-ATPase IF1 orthologs. Plant IF1s contain a conserved F1F0-ATPase inhibitory domain, but lack the antiparallel α-helical coiled-coil structure compared with mammalian IF1s. A subcellular localization analysis in Arabidopsis thaliana revealed that AtIF1-green fluorescent protein was present only in mitochondria. Additionally, AtIF1 was widely expressed in diverse organs and intense β-glucuronidase staining was observed in reproductive tissues and germinating seeds. Compared with the wild-type and p35S:AtIF1-if1 etiolated seedlings, the ATP/ADP ratio was significantly lower in the AtIF1 T-DNA knockout seedlings (if1 mutant) growing under dark conditions, suggesting that AtIF1 can influence the energy state of cells. A significant reduction in seed yield and strong growth retardation under dark conditions were observed in the if1 mutant line. Furthermore, if1 plants exhibited a substantially decreased sensitivity to abscisic acid. Thus, the A. thaliana mitochondrial IF1, which is a conserved F1F0-ATPase inhibitor, is crucial for plant growth and responses to abscisic acid.

Kinetic analysis of ATP hydrolysis by complex V in four murine tissues: Towards an assay suitable for clinical diagnosis

Background

ATP synthase, the mitochondrial complex V, plays a major role in bioenergetics and its defects lead to severe diseases. Lack of a consensual protocol for the assay of complex V activity probably explains the under-representation of complex V defect among mitochondrial diseases. The aim of this work was to elaborate a fast, simple and reliable method to check the maximal complex V capacity in samples relevant to clinical diagnosis.

Methods

Using homogenates from four different murine organs, we tested the use of dodecylmaltoside, stability of the activity, linearity with protein amount, sensitivity to oligomycin and to exogenous inhibitory factor 1 (IF1), influence of freezing, and impact of mitochondrial purification.

Results

We obtained organ-dependent, reproducible and stable complex V specific activities, similar with fresh and frozen organs. Similar inhibition by oligomycin and exogenous IF1 demonstrated tight coupling between F₁ and F₀ domains. The Michaelis constant for MgATP had close values for all organs, in the 150–220 μM range. Complex V catalytic turnover rate, as measured in preparations solubilized in detergent using immunotitration and activity measurements, was more than three times higher in extracts from brain or muscle than in extracts from heart or liver. This tissue specificity suggested post-translational modifications. Concomitant measurement of respiratory activities showed only slightly different complex II/complex V ratio in the four organs. In contrast, complex I/complex V ratio differed in brain as compared to the three other organs because of a high complex I activity in brain. Mitochondria purification preserved these ratios, except for brain where selective degradation of complex I occurred. Therefore, mitochondrial purification could introduce a biased enzymatic evaluation.

Conclusion

Altogether, this work demonstrates that a reliable assay of complex V activity is perfectly possible with very small samples from frozen biopsies, which was confirmed using control and deficient human muscles.

Mitochondrial matrix pH acidifies during anoxia and is maintained by the F1Fo-ATPase in anoxia-tolerant painted turtle cortical neurons

The western painted turtle (Chrysemys picta bellii) can survive extended periods of anoxia via a series of mechanisms that serve to reduce its energetic needs. Central to these mechanisms is the response of mitochondria, which depolarize in response to anoxia in turtle pyramidal neurons due to an influx of K⁺. It is currently unknown how mitochondrial matrix pH is affected by this response and we hypothesized that matrix pH acidifies during anoxia due to increased K⁺/H⁺ exchanger activity. Inhibition of K⁺/H⁺ exchange via quinine led to a collapse of mitochondrial membrane potential (Ψ_m) during oxygenated conditions in turtle cortical neurons, as indicated by rhodamine‐123 fluorescence, and this occurred twice as quickly during anoxia which indicates an elevation in K⁺ conductance. Mitochondrial matrix pH acidified during anoxia, as indicated by SNARF‐1 fluorescence imaged via confocal microscopy, and further acidification occurred during anoxia when the F₁F_o‐ATPase was inhibited with oligomycin‐A, indicating that ΔpH collapse is prevented during anoxic conditions. Collectively, these results indicate that the mitochondrial proton electrochemical gradient is actively preserved during anoxia to prevent a collapse of Ψ_m and ΔpH.

F1F0-ATP Synthase Inhibitory Factor 1 in the Normal Pancreas and in Pancreatic Ductal Adenocarcinoma: Effects on Bioenergetics, Invasion and Proliferation

F1F0-ATP synthase inhibitory factor 1 (IF1) inhibits the reverse mode of F1F0-ATP synthase, and therefore protects cellular ATP content at the expense of accelerated loss of mitochondrial membrane potential (ΔΨm). There is considerable variability in IF1 expression and its influence on bioenergetics between different cell types. High levels of IF1 in a number of cancers have been linked to increased glycolysis, resistance to cell death, increased migration and proliferation. However, neither the expression nor role of IF1 in the normal pancreas or in pancreatic cancer has been characterized. In this study, we found that pancreatic ductal adenocarcinoma (PDAC) patients express higher levels of IF1 in cancerous cells than in pancreatic acinar cells (PACs). PDAC cell lines have a higher IF1 content and IF1/ATP synthase ratio than PACs. The observed differences are consistent with the ability of the respective cell types to maintain ΔΨm and ATP levels in conditions of chemical hypoxia. Acinar cells and PDAC cells preferentially express different IF1 isoforms. Both knockdown and knockout of IF1 in the PANC-1 pancreatic cancer cell line modified cellular bioenergetics and decreased migration, invasion and proliferation suggesting the putative importance of IF1 for PDAC growth and metastasis.

Two mutations in mitochondrial ATP6 gene of ATP synthase, related to human cancer, affect ROS, calcium homeostasis and mitochondrial permeability transition in yeast

The relevance of mitochondrial DNA (mtDNA) mutations in cancer process is still unknown. Since the mutagenesis of mitochondrial genome in mammals is not possible yet, we have exploited budding yeast S. cerevisiae as a model to study the effects of tumor-associated mutations in the mitochondrial MTATP6 gene, encoding subunit 6 of ATP synthase, on the energy metabolism. We previously reported that four mutations in this gene have a limited impact on the production of cellular energy. Here we show that two mutations, Atp6-P163S and Atp6-K90E (human MTATP6-P136S and MTATP6-K64E, found in prostate and thyroid cancer samples, respectively), increase sensitivity of yeast cells both to compounds inducing oxidative stress and to high concentrations of calcium ions in the medium, when Om45p, the component of porin complex in outer mitochondrial membrane (OM), was fused to GFP. In OM45-GFP background, these mutations affect the activation of yeast permeability transition pore (yPTP, also called YMUC, yeast mitochondrial unspecific channel) upon calcium induction. Moreover, we show that calcium addition to isolated mitochondria heavily induced the formation of ATP synthase dimers and oligomers, recently proposed to form the core of PTP, which was slower in the mutants. We show the genetic evidence for involvement of mitochondrial ATP synthase in calcium homeostasis and permeability transition in yeast. This paper is a first to show, although in yeast model organism, that mitochondrial ATP synthase mutations, which accumulate during carcinogenesis process, may be significant for cancer cell escape from apoptosis.

Serum inhibitory factor 1, high-density lipoprotein and cardiovascular diseases

PURPOSE OF REVIEW

The atheroprotective properties of HDL are supported by epidemiological and preclinical research. However, the results of interventional trials paradoxically indicate that drugs increasing HDL-cholesterol (HDL-C) do not reduce coronary artery disease (CAD) risk. Moreover, Mendelian randomization studies have shown no effect of HDL-C-modifying variants on CAD outcome. Thus, the protective effects of HDL particles are more governed by their functional status than their cholesterol content. In this context, any successful clinical exploitation of HDL will depend on the identification of HDL-related biomarkers, better than HDL-C level, for assessing cardiovascular risk and monitoring responses to treatment.

RECENT FINDINGS

Recent studies have enlightened the role of ecto-F1-ATPase as a cell surface receptor for apoA-I, the major apolipoprotein of HDL, involved in the important metabolic and vascular atheroprotective functions of HDL. In the light of these findings, the clinical relevance of ecto-F1-ATPase in humans has recently been supported by the identification of serum F1-ATPase inhibitor (IF1) as an independent determinant of HDL-C, CAD risk and cardiovascular mortality in CAD patients.

SUMMARY

Serum IF1 measurement might be used as a novel HDL-related biomarker to better stratify risk in high-risk populations or to determine pharmacotherapy.

Selective ATP hydrolysis inhibition in F1Fo ATP synthase enhances radiosensitivity in non-small-cell lung cancer cells (A549)

Background

F1Fo-ATP synthase (F1Fo-ATPase) is a reversibly rotary molecular machine whose dual functions of synthesizing or hydrolyzing ATP switch upon the condition of cell physiology. The robust ATP-hydrolyzing activity occurs in ischemia for maintaining the transmembrane proton motive force of mitochondria inner membrane, but the effect of F1Fo-ATPase on X-ray response of non-small-cell lung cancer (NSCLC) cells is unknown.

Methods and Findings

We studied whether ATP hydrolysis affected X-ray radiation induced cell death. NSCLC cells (A549) were pretreated with BTB06584 (BTB), an elective ATP hydrolysis inhibitor, followed by X-ray radiation. Cell viability and clonogenic survival were markedly decreased, clear indications of enhanced radiosensitivity through BTB incubation. Additionally, ATP5α1 was upregulated in parallel with elevated ATP hydrolytic activity after X-ray radiation, showing an increased mitochondrial membrane potential (ΔΨm). ATP hydrolysis inhibition led to collapse of ΔΨm suggesting ATP hydrolytic activity could enhance ΔΨm after X-ray radiation. Furthermore, we also demonstrated that apoptosis was pronounced with the prolonged collapse of ΔΨm due to hydrolysis inhibition by BTB incubation.

Conclusion

Overall, these findings supported that ATP hydrolysis inhibition could enhance the radiosensitivity in NSCLC cells (A549) after X-ray radiation, which was due to the collapse of ΔΨm.

Role for the ATPase inhibitory factor 1 in the environmental carcinogen-induced Warburg phenotype

Most tumors undergo metabolic reprogramming towards glycolysis, the so-called Warburg effect, to support growth and survival. Overexpression of IF1, the physiological inhibitor of the F0F1ATPase, has been related to this phenomenon and appears to be a relevant marker in cancer. Environmental contributions to cancer development are now widely accepted but little is known about the underlying intracellular mechanisms. Among the environmental pollutants humans are commonly exposed to, benzo[a]pyrene (B[a]P), the prototype molecule of polycyclic aromatic hydrocarbons (PAHs), is a well-known human carcinogen. Besides apoptotic signals, B[a]P can also induce survival signals in liver cells, both likely involved in cancer promotion. Our previous works showed that B[a]P elicited a Warburg-like effect, thus favoring cell survival. The present study aimed at further elucidating the molecular mechanisms involved in the B[a]P-induced metabolic reprogramming, by testing the possible involvement of IF1. We presently demonstrate, both in vitro and in vivo, that PAHs, especially B[a]P, strongly increase IF1 expression. Such an increase, which might rely on β2-adrenergic receptor activation, notably participates to the B[a]P-induced glycolytic shift and cell survival in liver cells. By identifying IF1 as a target of PAHs, this study provides new insights about how environmental factors may contribute to related carcinogenesis.

Impaired ADP channeling to mitochondria and elevated reactive oxygen species in hypertensive hearts

Systemic hypertension initially promotes a compensatory cardiac hypertrophy, yet it progresses to heart failure (HF), and energetic deficits appear to be central to this failure. However, the transfer of energy between the mitochondria and the myofibrils is not often considered as part of the energetic equation. We compared hearts from old spontaneously hypertensive rats (SHRs) and normotensive Wistar controls. SHR hearts showed a 35% depression in mitochondrial function, yet produced at least double the amount of reactive oxygen species (ROS) in all respiration states in left ventricular (LV) homogenates. To test the connectivity between mitochondria and myofibrils, respiration was further tested in situ with LV permeabilized fibers by addition of multiple substrates and ATP, which requires hydrolysis to mediate oxidative phosphorylation. By trapping ADP using a pyruvate kinase enzyme system, we tested ADP channeling towards mitochondria, and this suppressed respiration and elevated ROS production more in the SHR fibers. The ADP-trapped state was also less relieved on creatine addition, likely reflecting the 30% depression in total CK activity in the SHR heart fibers. Confocal imaging identified a 34% longer distance between the centers of myofibril to mitochondria in the SHR hearts, which increases transverse metabolite diffusion distances (e.g., for ATP, ADP, and creatine phosphate). We propose that impaired connectivity between mitochondria and myofibrils may contribute to elevated ROS production. Impaired energy exchange could be the result of ultrastructural changes that occur with hypertrophy in this model of hypertension.

The Dual Function of Reactive Oxygen/Nitrogen Species in Bioenergetics and Cell Death: The Role of ATP Synthase

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) targeting mitochondria are major causative factors in disease pathogenesis. The mitochondrial permeability transition pore (PTP) is a mega-channel modulated by calcium and ROS/RNS modifications and it has been described to play a crucial role in many pathophysiological events since prolonged channel opening causes cell death. The recent identification that dimers of ATP synthase form the PTP and the fact that posttranslational modifications caused by ROS/RNS also affect cellular bioenergetics through the modulation of ATP synthase catalysis reveal a dual function of these modifications in the cells. Here, we describe mitochondria as a major site of production and as a target of ROS/RNS and discuss the pathophysiological conditions in which oxidative and nitrosative modifications modulate the catalytic and pore-forming activities of ATP synthase.

F1FO ATP Synthase Is Expressed at the Surface of Embryonic Rat Heart-Derived H9c2 Cells and Is Affected by Cardiac-Like Differentiation

Taking advantage from the peculiar features of the embryonic rat heart-derived myoblast cell line H9c2, the present study is the first to provide evidence for the expression of F1FO ATP synthase and of ATPase Inhibitory Factor 1 (IF1) on the surface of cells of cardiac origin, together documenting that they were affected through cardiac-like differentiation. Subunits of both the catalytic F1 sector of the complex (ATP synthase-β) and of the peripheral stalk, responsible for the correct F1-FO assembly/coupling, (OSCP, b, F6) were detected by immunofluorescence, together with IF1. The expression of ATP synthase-β, ATP synthase-b and F6 were similar for parental and differentiated H9c2, while the levels of OSCP increased noticeably in differentiated cells, where the results of in situ Proximity Ligation Assay were consistent with OSCP interaction within ecto-F1FO complexes. An opposite trend was shown by IF1 whose ectopic expression appeared greater in the parental H9c2. Here, evidence for the IF1 interaction with ecto-F1FO complexes was provided. Functional analyses corroborate both sets of data. i) An F1FO ATP synthase contribution to the exATP production by differentiated cells suggests an augmented expression of holo-F1FO ATP synthase on plasma membrane, in line with the increase of OSCP expression and interaction considered as a requirement for favoring the F1-FO coupling. ii) The absence of exATP generation by the enzyme, and the finding that exATP hydrolysis was largely oligomycin-insensitive, are in line in parental cells with the deficit of OSCP and suggest the occurrence of sub-assemblies together evoking more regulation by IF1.

The Mitochondrial Permeability Transition Pore: Channel Formation by F-ATP Synthase, Integration in Signal Transduction, and Role in Pathophysiology

The mitochondrial permeability transition (PT) is a permeability increase of the inner mitochondrial membrane mediated by a channel, the permeability transition pore (PTP). After a brief historical introduction, we cover the key regulatory features of the PTP and provide a critical assessment of putative protein components that have been tested by genetic analysis. The discovery that under conditions of oxidative stress the F-ATP synthases of mammals, yeast, and Drosophila can be turned into Ca(2+)-dependent channels, whose electrophysiological properties match those of the corresponding PTPs, opens new perspectives to the field. We discuss structural and functional features of F-ATP synthases that may provide clues to its transition from an energy-conserving into an energy-dissipating device as well as recent advances on signal transduction to the PTP and on its role in cellular pathophysiology.

The compound BTB06584 is an IF1 -dependent selective inhibitor of the mitochondrial F1 Fo-ATPase

BACKGROUND AND PURPOSE

Ischaemia compromises mitochondrial respiration. Consequently, the mitochondrial F1 Fo-ATPsynthase reverses and acts as a proton-pumping ATPase, so maintaining the mitochondrial membrane potential (ΔΨm ), while accelerating ATP depletion and cell death. Here we have looked for a molecule that can selectively inhibit this activity without affecting ATP synthesis, preserve ATP and delay ischaemic cell death.

EXPERIMENTAL APPROACH

We developed a chemoinformatic screen based on the structure of BMS199264, which is reported to selectively inhibit F1 Fo-ATPase activity and which is cardioprotective. Results suggested the molecule BTB06584 (hereafter referred to as BTB). Fluorescence microscopy was used to study its effects on ΔΨm and on the rate of ATP consumption following inhibition of respiration in several cell types. The effect of BTB on oxygen (O2 ) consumption was explored and protective potential determined using ischaemia/reperfusion assays. We also investigated a potential mechanism of action through its interaction with inhibitor protein of F1 subunit (IF1 ), the endogenous inhibitor of the F1 Fo-ATPase.

KEY RESULTS

BTB inhibited F1 Fo-ATPase activity with no effect on ΔΨm or O2 consumption. ATP consumption was decreased following inhibition of respiration, and ischaemic cell death was reduced. BTB efficiency was increased by IF1 overexpression and reduced by silencing the protein. In addition, BTB rescued defective haemoglobin synthesis in zebrafish pinotage (pnt) mutants in which expression of the Atpif1a gene is lost.

CONCLUSIONS AND IMPLICATIONS

BTB may represent a valuable tool to selectively inhibit mitochondrial F1 Fo-ATPase activity without compromising ATP synthesis and to limit ischaemia-induced injury caused by reversal of the mitochondrial F1 Fo-ATPsynthase.

LC-MS proteomics analysis of the insulin/IGF-1-deficient Caenorhabditis elegans daf-2(e1370) mutant reveals extensive restructuring of intermediary metabolism

The insulin/IGF-1 receptor is a major known determinant of dauer formation, stress resistance, longevity, and metabolism in Caenorhabditis elegans. In the past, whole-genome transcript profiling was used extensively to study differential gene expression in response to reduced insulin/IGF-1 signaling, including the expression levels of metabolism-associated genes. Taking advantage of the recent developments in quantitative liquid chromatography mass spectrometry (LC-MS)-based proteomics, we profiled the proteomic changes that occur in response to activation of the DAF-16 transcription factor in the germline-less glp-4(bn2);daf-2(e1370) receptor mutant. Strikingly, the daf-2 profile suggests extensive reorganization of intermediary metabolism, characterized by the upregulation of many core intermediary metabolic pathways. These include glycolysis/gluconeogenesis, glycogenesis, pentose phosphate cycle, citric acid cycle, glyoxylate shunt, fatty acid β-oxidation, one-carbon metabolism, propionate and tyrosine catabolism, and complexes I, II, III, and V of the electron transport chain. Interestingly, we found simultaneous activation of reciprocally regulated metabolic pathways, which is indicative of spatiotemporal coordination of energy metabolism and/or extensive post-translational regulation of these enzymes. This restructuring of daf-2 metabolism is reminiscent to that of hypometabolic dauers, allowing the efficient and economical utilization of internal nutrient reserves and possibly also shunting metabolites through alternative energy-generating pathways to sustain longevity.

Are rod outer segment ATP-ase and ATP-synthase activity expression of the same protein?

Vertebrate retinal rod outer segments (OS) consist of a stack of disks surrounded by the plasma membrane, where phototransduction takes place. Energetic metabolism in rod OS remains obscure. Literature described a so-called Mg(2+)-dependent ATPase activity, while our previous results demonstrated the presence of oxidative phosphorylation (OXPHOS) in OS, sustained by an ATP synthetic activity. Here we propose that the OS ATPase and ATP synthase are the expression of the same protein, i.e., of F1Fo-ATP synthase. Imaging on bovine retinal sections showed that some OXPHOS proteins are expressed in the OS. Biochemical data on bovine purified rod OS, characterized for purity, show an ATP synthase activity, inhibited by classical F1Fo-ATP synthase inhibitors. Moreover, OS possess a pH-dependent ATP hydrolysis, inhibited by pH values below 7, suggestive of the functioning of the inhibitor of F1 (IF1) protein. WB confirmed the presence of IF1 in OS, substantiating the expression of F1Fo ATP synthase in OS. Data suggest that the OS F1Fo ATP synthase is able to hydrolyze or synthesize ATP, depending on in vitro or in vivo conditions and that the role of IF1 would be pivotal in the prevention of the reversal of ATP synthase in OS, for example during hypoxia, granting photoreceptor survival.

The serum- and glucocorticoid-induced protein kinase-1 (Sgk-1) mitochondria connection: identification of the IF-1 inhibitor of the F(1)F(0)-ATPase as a mitochondria-specific binding target and the stress-induced mitochondrial localization of endogenous Sgk-1

The expression, localization and activity of the serum- and glucocorticoid-induced protein kinase, Sgk-1, are regulated by multiple hormonal and environmental cues including cellular stress. Biochemical fractionation and indirect immunofluorescence demonstrated that sorbitol induced hyperosmotic stress stimulated expression and triggered the localization of endogenous Sgk-1 into the mitochondria of NMuMG mammary epithelial cells. The immunofluorescence pattern of endogenous Sgk-1 was similar to that of a green fluorescent linked fusion protein linked to the N-terminal Sgk-1 fragment that encodes the mitochondrial targeting signal. In the presence or absence of cellular stress, exogenously expressed wild type Sgk-1 efficiently compartmentalized into the mitochondria demonstrating the mitochondrial import machinery per se is not stressed regulated. Co-immunoprecipitation and GST-pull down assays identified the IF-1 mitochondrial matrix inhibitor of the F1F0-ATPase as a new Sgk-1 binding partner, which represents the first observed mitochondrial target of Sgk-1. The Sgk-1/IF-1 interaction requires the 122-176 amino acid region within the catalytic domain of Sgk-1 and is pH dependent, occurring at neutral pH but not at slightly acidic pH, which suggests that this interaction is dependent on mitochondrial integrity. An in vitro protein kinase assay showed that the F1F0-ATPase can be directly phosphorylated by Sgk-1. Taken together, our results suggest that stress-induced Sgk-1 localizes to the mitochondria, which permits access to physiologically appropriate mitochondrial interacting proteins and substrates, such as IF-1 and the F1F0-ATPase, as part of the cellular stressed induced program.

Proteomic analysis of F1F0-ATP synthase super-assembly in mitochondria of cardiomyoblasts undergoing differentiation to the cardiac lineage

Mitochondria are essential organelles with multiple functions, especially in energy metabolism. An increasing number of data highlighted their role for cellular differentiation processes. We investigated differences in ATP synthase supra-molecular organization occurring in H9c2 cardiomyoblasts in the course of cardiac-like differentiation, along with ATP synthase biogenesis and maturation of mitochondrial cristae morphology. Using BN-PAGE analysis combined with one-step mild detergent extraction from mitochondria, a significant increase in dimer/monomer ratio was observed, indicating a distinct rise in the stability of the enzyme super-assembly. Remarkably, sub-stoichiometric mean values for ATP synthase subunit e were determined in both parental and cardiac-like H9c2 by an MS-based quantitative proteomics approach. This indicates a similar high proportion of complex molecules lacking subunit e in both cell types, and suggests a minor contribution of this component in the observed changes. 2D BN-PAGE/immunoblotting analysis and MS/MS analysis on single BN-PAGE band showed that the amount of inhibitor protein IF1 bound within the ATP synthase complexes increased in cardiac-like H9c2 and appeared greater in the dimer. In concomitance, a consistent improvement of enzyme activity, measured as both ATP synthesis and ATP hydrolysis rate, was observed, despite the increase of bound IF1 evocative of a greater inhibitory effect on the enzyme ATPase activity. The results suggest i) a role for IF1 in promoting dimer stabilization and super-assembly in H9c2 with physiological IF1 expression levels, likely unveiled by the fact that the contacts through accessory subunit e appear to be partially destabilized, ii) a link between dimer stabilization and enzyme activation.

Mitochondrial F(0) F(1) -ATP synthase is a molecular target of 3-iodothyronamine, an endogenous metabolite of thyroid hormone

BACKGROUND AND PURPOSE

3-iodothyronamine (T1AM) is a metabolite of thyroid hormone acting as a signalling molecule via non-genomic effectors and can reach intracellular targets. Because of the importance of mitochondrial F(0) F(1) -ATP synthase as a drug target, here we evaluated interactions of T1AM with this enzyme.

EXPERIMENTAL APPROACH

Kinetic analyses were performed on F(0) F(1) -ATP synthase in sub-mitochondrial particles and soluble F(1) -ATPase. Activity assays and immunodetection of the inhibitor protein IF(1) were used and combined with molecular docking analyses. Effects of T1AM on H9c2 cardiomyocytes were measured by in situ respirometric analysis.

KEY RESULTS

T1AM was a non-competitive inhibitor of F(0) F(1) -ATP synthase whose binding was mutually exclusive with that of the inhibitors IF(1) and aurovertin B. Both kinetic and docking analyses were consistent with two different binding sites for T1AM. At low nanomolar concentrations, T1AM bound to a high-affinity region most likely located within the IF(1) binding site, causing IF(1) release. At higher concentrations, T1AM bound to a low affinity-region probably located within the aurovertin binding cavity and inhibited enzyme activity. Low nanomolar concentrations of T1AM increased ADP-stimulated mitochondrial respiration in cardiomyocytes, indicating activation of F(0) F(1) -ATP synthase consistent with displacement of endogenous IF(1,) , reinforcing the in vitro results.

CONCLUSIONS AND IMPLICATIONS

Effects of T1AM on F(0) F(1) -ATP synthase were twofold: IF(1) displacement and enzyme inhibition. By targeting F(0) F(1) -ATP synthase within mitochondria, T1AM might affect cell bioenergetics with a positive effect on mitochondrial energy production at low, endogenous, concentrations. T1AM putative binding locations overlapping with IF(1) and aurovertin binding sites are described.

Molecular Regulation of the Mitochondrial F(1)F(o)-ATPsynthase: Physiological and Pathological Significance of the Inhibitory Factor 1 (IF(1))

In mammals, the mitochondrial F₁F_o-ATPsynthase sets out the energy homeostasis by producing the bulk of cellular ATP. As for every enzyme, the laws of thermodynamics command it; however, it is privileged to have a dedicated molecular regulator that controls its rotation. This is the so-called ATPase Inhibitory Factor 1 (IF₁) that blocks its reversal to avoid the consumption of cellular ATP when the enzyme acts as an ATP hydrolase. Recent evidence has also demonstrated that IF₁ may control the alignment of the enzyme along the mitochondrial inner membrane, thus increasing the interest for the molecule. We conceived this review to outline the fundamental knowledge of the F₁F_o-ATPsynthase and link it to the molecular mechanisms by which IF₁ regulates its way of function, with the ultimate goal to highlight this as an important and possibly unique means to control this indispensable enzyme in both physiological and pathological settings.

Mitochondrial inhibitory factor 1 (IF1) is present in human serum and is positively correlated with HDL-cholesterol

BACKGROUND

Mitochondrial ATP synthase is expressed as a plasma membrane receptor for apolipoprotein A-I (apoA-I), the major protein component in High Density Lipoproteins (HDL). On hepatocytes, apoA-I binds to cell surface ATP synthase (namely ecto-F(1)-ATPase) and stimulates its ATPase activity, generating extracellular ADP. This production of extracellular ADP activates a P2Y(13)-mediated HDL endocytosis pathway. Conversely, exogenous IF1, classically known as a natural mitochondrial specific inhibitor of F(1)-ATPase activity, inhibits ecto-F(1)-ATPase activity and decreases HDL endocytosis by both human hepatocytes and perfused rat liver.

METHODOLOGY/PRINCIPAL FINDINGS

Since recent reports also described the presence of IF1 at the plasma membrane of different cell types, we investigated whether IF1 is present in the systemic circulation in humans. We first unambiguously detected IF1 in human serum by immunoprecipitation and mass spectrometry. We then set up a competitive ELISA assay in order to quantify its level in human serum. Analyses of IF1 levels in 100 normolipemic male subjects evidenced a normal distribution, with a median value of 0.49 µg/mL and a 95% confidence interval of 0.22-0.82 µg/mL. Correlations between IF1 levels and serum lipid levels demonstrated that serum IF1 levels are positively correlated with HDL-cholesterol and negatively with triglycerides (TG).

CONCLUSIONS/SIGNIFICANCE

Altogether, these data support the view that, in humans, circulating IF1 might affect HDL levels by inhibiting hepatic HDL uptake and also impact TG metabolism.

Mitochondrial bioenergetic profile and responses to metabolic inhibition in human hepatocarcinoma cell lines with distinct differentiation characteristics

The classical view of tumour cell bioenergetics has been recently revised. Then, the definition of the mitochondrial profile is considered of fundamental importance for the development of anti-cancer therapies, but it still needs to be clarified. We investigated two human hepatocellular carcinoma cell lines: the partially differentiated HepG2 and the undifferentiated JHH-6. High resolution respirometry revealed a marked impairment/uncoupling of OXPHOS in JHH-6 compared with HepG2, with the phosphorylation system limiting the capacity for electron transport much more in JHH-6. Blocking glycolysis or mitochondrial ATP synthase we demonstrated that in JHH-6 ATP synthase functions in reverse and consumes glycolytic ATP, thereby sustaining ΔΨm. A higher expression level of ATP synthase Inhibitor Factor 1 (IF1), a higher extent of IF1 bound to ATP synthase and a lower ATPase/synthase capacity were documented in JHH-6. Thus, here IF1 appears to down-regulate the reverse mode of ATPsynthase activity, thereby playing a crucial role in controlling energy waste and ΔΨm. These results, while confirming the over-expression of IF1 in cancer cells, are the first to indicate an inverse link between cell differentiation status and IF1 (expression level and regulatory function).

Characterization of Myelin Sheath F(o)F(1)-ATP synthase and its regulation by IF(1)

F(o)F(1)-ATP synthase is the nanomotor responsible for most of ATP synthesis in the cell. In physiological conditions, it carries out ATP synthesis thanks to a proton gradient generated by the respiratory chain in the inner mitochondrial membrane. We previously reported that isolated myelin vesicles (IMV) contain functional F(o)F(1)-ATP synthase and respiratory chain complexes and are able to conduct an aerobic metabolism, to support the axonal energy demand. In this study, by biochemical assay, Western Blot (WB) analysis and immunofluorescence microscopy, we characterized the IMV F(o)F(1)-ATP synthase. ATP synthase activity decreased in the presence of the specific inhibitors (olygomicin, DCCD, FCCP, valynomicin/nigericin) and respiratory chain inhibitors (antimycin A, KCN), suggesting a coupling of oxygen consumption and ATP synthesis. ATPase activity was inhibited in low pH conditions. WB and microscopy analyses of both IMV and optic nerves showed that the Inhibitor of F(1) (IF(1)), a small protein that binds the F(1) moiety in low pH when of oxygen supply is impaired, is expressed in myelin sheath. Data are discussed in terms of the role of IF(1) in the prevention of the reversal of ATP synthase in myelin sheath during central nervous system ischemic events. Overall, data are consistent with an energetic role of myelin sheath, and may shed light on the relationship among demyelination and axonal degeneration.

Modulation of the protein kinase Cdelta interaction with the "d" subunit of F1F0-ATP synthase in neonatal cardiac myocytes: development of cell-permeable, mitochondrially targeted inhibitor and facilitator peptides

The F(1)F(0)-ATP synthase provides approximately 90% of cardiac ATP, yet little is known regarding its regulation under normal or pathological conditions. Previously, we demonstrated that protein kinase Cdelta (PKCdelta) inhibits F(1)F(0) activity via an interaction with the "d" subunit of F(1)F(0)-ATP synthase (dF(1)F(0)) in neonatal cardiac myocytes (NCMs) (Nguyen, T., Ogbi, M., and Johnson, J. A. (2008) J. Biol. Chem. 283, 29831-29840). We have now identified a dF(1)F(0)-derived peptide (NH(2)-(2)AGRKLALKTIDWVSF(16)-COOH) that inhibits PKCdelta binding to dF(1)F(0) in overlay assays. We have also identified a second dF(1)F(0)-derived peptide (NH(2)-(111)RVREYEKQLEKIKNMI(126)-COOH) that facilitates PKCdelta binding to dF(1)F(0). Incubation of NCMs with versions of these peptides containing HIV-Tat protein transduction and mammalian mitochondrial targeting sequences resulted in their delivery into mitochondria. Preincubation of NCMs, with 10 nm extracellular concentrations of the mitochondrially targeted PKCdelta-dF(1)F(0) interaction inhibitor, decreased 100 nm 4beta-phorbol 12-myristate 13-acetate (4beta-PMA)-induced co-immunoprecipitation of PKCdelta with dF(1)F(0) by 50 +/- 15% and abolished the 30 nm 4beta-PMA-induced inhibition of F(1)F(0)-ATPase activity. A scrambled sequence (inactive) peptide, which contained HIV-Tat and mitochondrial targeting sequences, was without effect. In contrast, the cell-permeable, mitochondrially targeted PKCdelta-dF(1)F(0) facilitator peptide by itself induced the PKCdelta-dF(1)F(0) co-immunoprecipitation and inhibited F(1)F(0)-ATPase activity. In in vitro PKC add-back experiments, the PKCdelta-F(1)F(0) inhibitor blocked PKCdelta-mediated inhibition of F(1)F(0)-ATPase activity, whereas the facilitator induced inhibition. We have developed the first cell-permeable, mitochondrially targeted modulators of the PKCdelta-dF(1)F(0) interaction in NCMs. These novel peptides will improve our understanding of cardiac F(1)F(0) regulation and may have potential as therapeutics to attenuate cardiac injury.

Attenuation of the hypoxia-induced protein kinase Cdelta interaction with the 'd' subunit of F1Fo-ATP synthase in neonatal cardiac myocytes: implications for energy preservation and survival

The F1Fo-ATP synthase provides most of the heart's energy, yet events that alter its function during injury are poorly understood. Recently, we described a potent inhibitory effect on F1Fo-ATP synthase function mediated by the interaction of PKCdelta (protein kinase Cdelta) with dF1Fo ('d' subunit of the F1Fo-ATPase/ATP synthase). We have now developed novel peptide modulators which facilitate or inhibit the PKCdelta-dF1Fo interaction. These peptides include HIV-Tat (transactivator of transcription) protein transduction and mammalian mitochondrial-targeting sequences. Pre-incubation of NCMs (neonatal cardiac myocyte) with 10 nM extracellular concentrations of the mitochondrial-targeted PKCdelta-dF1Fo interaction inhibitor decreased Hx (hypoxia)-induced co-IP (co-immunoprecipitation) of PKCdelta with dF1Fo by 40+/-9%, abolished Hx-induced inhibition of F1Fo-ATPase activity, attenuated Hx-induced losses in F1Fo-derived ATP and protected against Hx- and reperfusion-induced cell death. A scrambled-sequence (inactive) peptide, which contained HIV-Tat and mitochondrial-targeting sequences, was without effect. In contrast, the cell-permeant mitochondrial-targeted PKCdelta-dF1Fo facilitator peptide, which we have shown previously to induce the PKCdelta-dF1Fo co-IP, was found to inhibit F1Fo-ATPase activity to an extent similar to that caused by Hx alone. The PKCdelta-dF1Fo facilitator peptide also decreased ATP levels by 72+/-18% under hypoxic conditions in the presence of glycolytic inhibition. None of the PKCdelta-dF1Fo modulatory peptides altered the inner mitochondrial membrane potential. Our studies provide the first evidence that disruption of the PKCdelta-dF1Fo interaction using cell-permeant mitochondrial-targeted peptides attenuates cardiac injury resulting from prolonged oxygen deprivation.

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.

The ectopic F(O)F(1) ATP synthase of rat liver is modulated in acute cholestasis by the inhibitor protein IF1

Rat liver plasma membranes contain F(O)F(1) complexes (ecto-F(O)F(1)) displaying a similar molecular weight to the mitochondrial F(O)F(1) ATP synthase, as evidenced by Blue Native PAGE. Their ATPase activity was stably reduced in short-term extra-hepatic cholestasis. Immunoblotting and immunoprecipitation analyses demonstrated that the reduction in activity was not due to a decreased expression of ecto-F(O)F(1) complexes, but to an increased level of an inhibitory protein, ecto-IF(1), bound to ecto-F(O)F(1). Since cholestasis down regulates the hepatic uptake of HDL-cholesterol, and ecto-F(O)F(1) has been shown to mediate SR-BI-independent hepatic uptake of HDL-cholesterol, these findings provide support to the hypothesis that ecto-F(O)F(1) contributes to the fine control of reverse cholesterol transport, in parallel with SR-BI. No activity change of the mitochondrial F(O)F(1) ATP synthase (m-F(O)F(1)), or any variation of its association with m-IF(1) was observed in cholestasis, indicating that ecto-IF(1) expression level is modulated independently from that of ecto-F(O)F(1), m-IF(1) and m-F(O)F(1).

IF(1): setting the pace of the F(1)F(o)-ATP synthase

When mitochondrial function is compromised and the mitochondrial membrane potential (Deltapsi(m)) falls below a threshold, the F(1)F(o)-ATP synthase can reverse, hydrolysing ATP to pump protons out of the mitochondrial matrix. Although this activity can deplete ATP and precipitate cell death, it is limited by the mitochondrial protein IF(1), an endogenous F(1)F(o)-ATPase inhibitor. IF(1), therefore, preserves ATP at the expense of Deltapsi(m). Despite a wealth of detailed knowledge on the biochemistry of the interaction of IF(1) and the F(1)F(o)-ATPase, little is known about its physiological activity. Emerging research suggests that IF(1) has a wider ranging impact on mitochondrial structure and function than previously thought.

Multiphoton imaging reveals differences in mitochondrial function between nephron segments

Mitochondrial dysfunction may play a role in the pathogenesis of several renal diseases. Although functional roles and metabolic demands differ among tubule segments, relatively little is known about the properties of mitochondria in different parts of the nephron. Clinically, the proximal tubule seems particularly vulnerable to mitochondrial toxicity. In this study, we used multiphoton imaging of live rat kidney slices to investigate differences in mitochondrial function along the nephron. The mitochondrial membrane potential was markedly higher in distal than proximal tubules. Inhibition of respiration rapidly collapsed the membrane potential in proximal tubules, but potential was better maintained in distal tubules. Inhibition of the F1F(o)-ATPase abolished this difference, suggesting that maintenance of potential via ATPase activity is more effective in distal than proximal tubules. Immunostaining revealed that the ratio of the expression of ATPase to IF1, an endogenous inhibitor of the mitochondrial ATPase, was lower in proximal tubules than in distal tubules. Production of reactive oxygen species was higher in proximal than distal cells, but inhibition of NADPH oxidase eliminated this difference. Glutathione levels were higher in proximal tubules. Overall, mitochondria in the proximal tubules were in a more oxidized state than those in the distal tubules. In summary, there are axial differences in mitochondrial function along the nephron, which may contribute to the pattern and pathophysiology of some forms of renal injury.

Mitochondria and energetic depression in cell pathophysiology

Mitochondrial dysfunction is a hallmark of almost all diseases. Acquired or inherited mutations of the mitochondrial genome DNA may give rise to mitochondrial diseases. Another class of disorders, in which mitochondrial impairments are initiated by extramitochondrial factors, includes neurodegenerative diseases and syndromes resulting from typical pathological processes, such as hypoxia/ischemia, inflammation, intoxications, and carcinogenesis. Both classes of diseases lead to cellular energetic depression (CED), which is characterized by decreased cytosolic phosphorylation potential that suppresses the cell's ability to do work and control the intracellular Ca(2+) homeostasis and its redox state. If progressing, CED leads to cell death, whose type is linked to the functional status of the mitochondria. In the case of limited deterioration, when some amounts of ATP can still be generated due to oxidative phosphorylation (OXPHOS), mitochondria launch the apoptotic cell death program by release of cytochrome c. Following pronounced CED, cytoplasmic ATP levels fall below the thresholds required for processing the ATP-dependent apoptotic cascade and the cell dies from necrosis. Both types of death can be grouped together as a mitochondrial cell death (MCD). However, there exist multiple adaptive reactions aimed at protecting cells against CED. In this context, a metabolic shift characterized by suppression of OXPHOS combined with activation of aerobic glycolysis as the main pathway for ATP synthesis (Warburg effect) is of central importance. Whereas this type of adaptation is sufficiently effective to avoid CED and to control the cellular redox state, thereby ensuring the cell survival, it also favors the avoidance of apoptotic cell death. This scenario may underlie uncontrolled cellular proliferation and growth, eventually resulting in carcinogenesis.

IF1, the endogenous regulator of the F(1)F(o)-ATPsynthase, defines mitochondrial volume fraction in HeLa cells by regulating autophagy

The protein IF1 limits mitochondrial ATP consumption when mitochondrial respiration is impaired by inhibiting the 'reverse' activity of the F(1)F(o)-ATPsynthase. We have found that IF1 also increases F(1)F(o)-ATPsynthase activity in respiring mitochondria, promoting its dimerization and increasing the density of mitochondrial cristae. We also noted that IF1 overexpression was associated with an increase in mitochondrial volume fraction that was conversely reduced when IF1 was knocked down using small interfering RNA (siRNA). The volume change did not correlate with the level of transcription factors involved in mitochondrial biogenesis. However, autophagy was dramatically increased in the IF1siRNA treated cells (-IF1), assessed by quantifying LC3-GFP translocation to autophagosomes, whilst levels of autophagy were low in IF1 overexpressing cells (+IF1). The increase in LC3-GFP labelled autophagosomes in -IF1 cells was prevented by the superoxide dismutase mimetic, manganese (III) tetrakis (4-benzoic acid) porphyrin (MnTBAP). An increase in the basal rate of generation of reactive oxygen species (ROS) in -IF1 cells was demonstrated using the fluorescent probe dihydroethidium (DHE). Thus, IF1 appears to limit mitochondrial ROS generation, limiting autophagy which is increased by IF1 knockdown. Variations in IF1 expression level may therefore play a significant role in defining both resting rates of ROS generation and cellular mitochondrial content.

IEX-1 targets mitochondrial F1Fo-ATPase inhibitor for degradation

IEX-1 (Immediate Early response gene X-1) is a stress-inducible gene. It suppresses production of reactive oxygen species (ROS) and protects cells from apoptosis induced by a wide range of stimuli, but the underlying mechanism is not known. This study reveals that IEX-1 targets the mitochondrial F1Fo-ATPase Inhibitor (IF1) for degradation, resulting in acceleration of ATP hydrolysis, concomitant with reduction in ROS production. A prominent role for IF1 degradation in the function of IEX-1 was corroborated by siRNA-mediated gene silencing of IF1 that recapitulated the effects of IEX-1 on ATP hydrolysis and ROS production. Moreover, progressive C-terminal truncation studies demonstrated that IEX-1 interacted with the C terminus of IF1 and the interaction might render IF1 prone to degradation by an as yet unidentified mitochondrial protease. In support of a physiological importance of IEX-1 in the modulation of IF1 expression, gene-targeted deletion of IEX-1 stabilized IF1 and reduced mitochondrial F1Fo-ATPase activity in vivo. The altered activity of the F1Fo enzyme may account for a metabolic switch from oxidative phosphorylation toward glycolysis in IEX-1 deficient cells. Thus, IEX-1 deficient cells were more susceptible to glucose deprivation than wild type counterparts and displayed increased glucose uptake and lactate production in hypoxic conditions. The cells were also relatively refractory to oligomycin-mediated inhibition of ATP production. The studies offer novel insights into the primary role of IEX-1 in regulating a balance between energy provision and ROS production.

Matching cellular metabolic supply and demand in energy-stressed animals

Certain environmental stressors can impair cellular ATP production to the point of harming or even killing an animal. Some exceptional animals employ strategies that maintain the balance between ATP production and consumption, allowing them to tolerate prolonged exposure to stressors such as hypoxia and anoxia. Anoxia- and hypoxia-tolerant animals reduce ATP consumption by ion-motive ATPases while concomitant reductions in passive ion flux reduce the demand for ion pumping and maintain transmembrane ion gradients. Reductions in gene transcription and protein turnover decrease ATP demand in hibernating and hypoxia-tolerant animals. Proton leak uncouples mitochondrial substrate oxidation from ATP synthesis and accounts for a considerable proportion of cellular energy demand, but there is little evidence that the proton permeability of inner mitochondrial membranes decreases in animals that tolerate energy stress. Indeed in some cases proton leak increases, possibly reducing reactive oxygen species production. Because substrate oxidation is important to the control of cellular metabolism, the downregulation of ATP supply pathways contributes significantly to metabolic suppression under energy stress. Mechanisms that coordinate the downregulation of both ATP supply and demand pathways include AMP kinase and ATP-sensitive ion channels. Strategies employed by animals tolerant to one energy stress often convey "cross-tolerance" to completely different stresses.

Pharmacological profile of the selective mitochondrial F1F0 ATP hydrolase inhibitor BMS-199264 in myocardial ischemia

The mitochondrial F1F0 ATP synthase is responsible for the majority of ATP production in mammals and does this through a rotary catalytic mechanism. Studies show that the F1F0 ATP synthase can switch to an ATP hydrolase, and this occurs under conditions seen during myocardial ischemia. This ATP hydrolysis causes wasting of ATP that does not produce work. The degree of ATP inefficiently hydrolyzed during ischemia may be as high as 50-90% of the total. A naturally occurring, reversible inhibitor (IF-1) of the hydrolase activity is in the mitochondria, and it has a pH optimum of 6.8. Based on studies with the nonselective (inhibit both synthase and hydrolase activity) inhibitors aurovertin B and oligomycin B reduce the rate of ATP depletion during ischemia, showing that IF-1 does not completely block hydrolase activity. Oligomycin and aurovertin cannot be used for treating myocardial ischemia as they will reduce ATP production in healthy tissue. We generated a focused structure-activity relationship, and several compounds were identified that selectively inhibited the F1F0 ATP hydrolase activity while having no effect on synthase function. One compound, BMS-199264 had no effect on F1F0 ATP synthase function in submitochondrial particles while inhibiting hydrolase function, unlike oligomycin that inhibits both. BMS-199264 selectively inhibited ATP decline during ischemia while not affecting ATP production in normoxic and reperfused hearts. BMS-191264 also reduced cardiac necrosis and enhanced the recovery of contractile function following reperfusion. These data also suggest that the reversal of the synthase and hydrolase activities is not merely a chemical reaction run in reverse.

Delta protein kinase C interacts with the d subunit of the F1F0 ATPase in neonatal cardiac myocytes exposed to hypoxia or phorbol ester. Implications for F1F0 ATPase regulation

Mitochondrial protein kinase C isozymes have been reported to mediate both cardiac ischemic preconditioning and ischemia/reperfusion injury. In addition, cardiac preconditioning improves the recovery of ATP levels after ischemia/reperfusion injury. We have, therefore, evaluated protein kinase C modulation of the F(1)F(0) ATPase in neonatal cardiac myocytes. Exposure of cells to 3 or 100 nM 4beta-phorbol 12-myristate-13-acetate induced co-immunoprecipitation of delta protein kinase C (but not alpha, epsilon, or zeta protein kinase C) with the d subunit of the F(1)F(0) ATPase. This co-immunoprecipitation correlated with 40+/-3% and 72+/-9% inhibitions of oligomycin-sensitive F(1)F(0) ATPase activity, respectively. We observed prominent expression of delta protein kinase C in cardiac myocyte mitochondria, which was enhanced following a 4-h hypoxia exposure. In contrast, hypoxia decreased mitochondrial zetaPKC levels by 85+/-1%. Following 4 h of hypoxia, F(1)F(0) ATPase activity was inhibited by 75+/-9% and delta protein kinase C co-immunoprecipitated with the d subunit of F(1)F(0) ATPase. In vitro incubation of protein kinase C with F(1)F(0) ATPase enhanced F(1)F(0) activity in the absence of protein kinase C activators and inhibited it in the presence of activators. Recombinant delta protein kinase C also inhibited F(1)F(0) ATPase activity. Protein kinase C overlay assays revealed delta protein kinase C binding to the d subunit of F(1)F(0) ATPase, which was modulated by diacylglycerol, phosphatidylserine, and cardiolipin. Our results suggest a novel regulation of the F(1)F(0) ATPase by the delta protein kinase C isozyme.

Energetic signalling in the control of mitochondrial F1F0 ATP synthase activity in health and disease

The mitochondrial F1F0 ATP synthase is a critical enzyme that works by coupling the proton motive force generated by the electron transport chain via proton transfer through the F0 or proton-pore forming domain of this enzyme to release ATP from the catalytic F1 domain. This enzyme is regulated by calcium, ADP, and inorganic phosphate as well as increased transcription through several pathways. This enzyme is also an ATP hydrolase under ischemic conditions. This "inefficient" hydrolysis of ATP consumes 90% of ATP consumed during ischemia as shown with non-selective ATPase inhibitors oligomycin and Aurovertin B. A benzopyran analog, BMS-199264, selectively inhibits F1F0 ATP hydrolase activity with no effect on ATP synthase activity. BMS-199264 had no effect on ATP before ischemia, but reduced the decline in ATP during ischemia. Selective hydrolase inhibition seen with the small molecule BMS-199264 suggests a conformational change in the F1F0 ATPase enzyme when switching from synthase to hydrolase activity.

Regulation of mitochondrial structure and function by the F1Fo-ATPase inhibitor protein, IF1

When mitochondrial respiration is compromised, the F(1)F(o)-ATP synthase reverses and consumes ATP, serving to maintain the mitochondrial membrane potential (Delta psi(m)). This process is mitigated by IF(1). As little is known of the cell biology of IF(1), we have investigated the functional consequences of varying IF(1) expression. We report that, (1) during inhibition of respiration, IF(1) conserves ATP at the expense of Delta psi(m); (2) overexpression of IF(1) is protective against ischemic injury; (3) relative IF(1) expression level varies between tissues and cell types and dictates the response to inhibition of mitochondrial respiration; (4) the density of mitochondrial cristae is increased by IF(1) overexpression and decreased by IF(1) suppression; and (5) IF(1) overexpression increases the formation of dimeric ATP synthase complexes and increases F(1)F(o)-ATP synthase activity. Thus, IF(1) regulates mitochondrial function and structure under both physiological and pathological conditions.

Regulatory mechanisms of proton-translocating F(O)F (1)-ATP synthase

H(+)-F(O)F(1)-ATP synthase catalyzes synthesis of ATP from ADP and inorganic phosphate using the energy of transmembrane electrochemical potential difference of proton (deltamu(H)(+). The enzyme can also generate this potential difference by working as an ATP-driven proton pump. Several regulatory mechanisms are known to suppress the ATPase activity of F(O)F(1): 1. Non-competitive inhibition by MgADP, a feature shared by F(O)F(1) from bacteria, chloroplasts and mitochondria 2. Inhibition by subunit epsilon in chloroplast and bacterial enzyme 3. Inhibition upon oxidation of two cysteines in subunit gamma in chloroplast F(O)F(1) 4. Inhibition by an additional regulatory protein (IF(1)) in mitochondrial enzyme In this review we summarize the information available on these regulatory mechanisms and discuss possible interplay between them.