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

The mitochondrial permeability transition pore: a mystery solved?

The permeability transition (PT) denotes an increase of the mitochondrial inner membrane permeability to solutes with molecular masses up to about 1500 Da. It is presumed to be mediated by opening of a channel, the permeability transition pore (PTP), whose molecular nature remains a mystery. Here I briefly review the history of the PTP, discuss existing models, and present our new results indicating that reconstituted dimers of the F_OF₁ ATP synthase form a channel with properties identical to those of the mitochondrial megachannel (MMC), the electrophysiological equivalent of the PTP. Open questions remain, but there is now promise that the PTP can be studied by genetic methods to solve the large number of outstanding problems.

IF1 limits the apoptotic-signalling cascade by preventing mitochondrial remodelling

Mitochondrial structure has a central role both in energy conversion and in the regulation of cell death. We have previously shown that IF1 protects cells from necrotic cell death and supports cristae structure by promoting the oligomerisation of the F1Fo-ATPsynthase. As IF1 is upregulated in a large proportion of human cancers, we have here explored its contribution to the progression of apoptosis and report that an increased expression of IF1, relative to the F1Fo-ATPsynthase, protects cells from apoptotic death. We show that IF1 expression serves as a checkpoint for the release of Cytochrome c (Cyt c) and hence the completion of the apoptotic program. We show that the progression of apoptosis engages an amplification pathway mediated by: (i) Cyt c-dependent release of ER Ca(2+), (ii) Ca(2+)-dependent recruitment of the GTPase Dynamin-related protein 1 (Drp1), (iii) Bax insertion into the outer mitochondrial membrane and (iv) further release of Cyt c. This pathway is accelerated by suppression of IF1 and delayed by its overexpression. IF1 overexpression is associated with the preservation of mitochondrial morphology and ultrastructure, consistent with a central role for IF1 as a determinant of the inner membrane architecture and with the role of mitochondrial ultrastructure in the regulation of Cyt c release. These data suggest that IF1 is an antiapoptotic and potentially tumorigenic factor and may be a valuable predictor of responsiveness to chemotherapy.

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.

Mitoplasticity: adaptation biology of the mitochondrion to the cellular redox state in physiology and carcinogenesis

Adaptation and transformation biology of the mitochondrion to redox status is an emerging domain of physiology and pathophysiology. Mitochondrial adaptations occur in response to accidental changes in cellular energy demand or supply while mitochondrial transformations are a part of greater program of cell metamorphosis. The possible role of mitochondrial adaptations and transformations in pathogenesis remains unexplored, and it has become critical to decipher the stimuli and the underlying molecular pathways. Immediate activation of mitochondrial function was described during acute exercise, respiratory chain injury, Endoplasmic Reticulum stress, genotoxic stress, or environmental toxic insults. Delayed adaptations of mitochondrial form, composition, and functions were evidenced for persistent changes in redox status as observed in endurance training, in fibroblasts grown in presence of respiratory chain inhibitors or in absence of glucose, in the smooth muscle of patients with severe asthma, or in the skeletal muscle of patients with a mitochondrial disease. Besides, mitochondrial transformations were observed in the course of human cell differentiation, during immune response activation, or in cells undergoing carcinogenesis. Little is known on the signals and downstream pathways that govern mitochondrial adaptations and transformations. Few adaptative loops, including redox sensors, kinases, and transcription factors were deciphered, but their implication in physiology and pathology remains elusive. Mitoplasticity could play a protective role against aging, diabetes, cancer, or neurodegenerative diseases. Research on adaptation and transformation could allow the design of innovative therapies, notably in cancer.

F1-ATPase of Escherichia coli: the ε- inhibited state forms after ATP hydrolysis, is distinct from the ADP-inhibited state, and responds dynamically to catalytic site ligands

F1-ATPase is the catalytic complex of rotary nanomotor ATP synthases. Bacterial ATP synthases can be autoinhibited by the C-terminal domain of subunit ε, which partially inserts into the enzyme's central rotor cavity to block functional subunit rotation. Using a kinetic, optical assay of F1·ε binding and dissociation, we show that formation of the extended, inhibitory conformation of ε (εX) initiates after ATP hydrolysis at the catalytic dwell step. Prehydrolysis conditions prevent formation of the εX state, and post-hydrolysis conditions stabilize it. We also show that ε inhibition and ADP inhibition are distinct, competing processes that can follow the catalytic dwell. We show that the N-terminal domain of ε is responsible for initial binding to F1 and provides most of the binding energy. Without the C-terminal domain, partial inhibition by the ε N-terminal domain is due to enhanced ADP inhibition. The rapid effects of catalytic site ligands on conformational changes of F1-bound ε suggest dynamic conformational and rotational mobility in F1 that is paused near the catalytic dwell position.

Immediate early response gene X-1, a potential prognostic biomarker in cancers

INTRODUCTION

The immediate early response gene X-1 (IEX-1) plays a pivotal role in the regulation of cell apoptosis, proliferation, differentiation and metabolism. Deregulation of IEX-1 expression has been confirmed in multiple cancers in humans, in association with either poor or better prognosis depending on the type and progression stages of the cancer.

AREAS COVERED

This review summarizes clinical studies of altered IEX-1 expression in ovarian, pancreatic, blood, breast and colorectal cancers, lymphoma and myeloma. The authors also outline the current understandings of the complex functions of IEX-1 gained from studies with animal models and tumor cell lines so as to help us comprehend the significance of the clinical findings.

EXPERT OPINION

IEX-1 holds great promise to be a valuable biomarker, either alone or in combination with other genes, for monitoring progression of some cancers. IEX-1 expression is highly sensitive to environmental cues and distinct between normal and cancer cells. However, use of IEX-1 as a biomarker remains a significant challenge because too little is understood about the mechanism underlying the diverse activities of IEX-1 and a standardized clinical assay for IEX-1 detection and validation of clinical results across different studies are still critically lacking.

Regulation of mitochondrial morphogenesis by annexin A6

Mitochondrial homeostasis is critical in meeting cellular energy demands, shaping calcium signals and determining susceptibility to apoptosis. Here we report a role for anxA6 in the regulation of mitochondrial morphogenesis, and show that in cells lacking anxA6 mitochondria are fragmented, respiration is impaired and mitochondrial membrane potential is reduced. In fibroblasts from AnxA6^−/− mice, mitochondrial Ca²⁺ uptake is reduced and cytosolic Ca²⁺ transients are elevated. These observations led us to investigate possible interactions between anxA6 and proteins with roles in mitochondrial fusion and fission. We found that anxA6 associates with Drp1 and that mitochondrial fragmentation in AnxA6^−/− fibroblasts was prevented by the Drp1 inhibitor mdivi-1. In normal cells elevation of intracellular Ca²⁺ disrupted the interaction between anxA6 and Drp1, displacing anxA6 to the plasma membrane and promoting mitochondrial fission. Our results suggest that anxA6 inhibits Drp1 activity, and that Ca²⁺-binding to anxA6 relieves this inhibition to permit Drp1-mediated mitochondrial fission.

Comparative analysis of cytosolic and mitochondrial ATP synthesis in embryonic and postnatal hippocampal neuronal cultures

ATP in neurons is commonly believed to be synthesized mostly by mitochondria via oxidative phosphorylation. Neuronal mitochondria have been studied primarily in culture, i.e., in neurons isolated either from embryos or from neonatal pups. Although it is generally assumed that both embryonic and postnatal cultured neurons derive their ATP from mitochondrial oxidative phosphorylation, this has never been tested experimentally. We expressed the FRET-based ATP sensor AT1.03 in cultured hippocampal neurons isolated either from E17 to E18 rat embryos or from P1 to P2 rat pups and monitored [ATP]c simultaneously with mitochondrial membrane potential (ΔΨm; TMRM) and NAD(P)H autofluorescence. In embryonic neurons, transient glucose deprivation induced a near-complete decrease in [ATP]c, which was partially reversible and was accelerated by inhibition of glycolysis with 2-deoxyglucose. In the absence of glucose, pyruvate did not cause any significant increase in [ATP]c in 84% of embryonic neurons, and inhibition of mitochondrial ATP synthase with oligomycin failed to decrease [ATP]c. Moreover, ΔΨm was significantly reduced by oligomycin, indicating that mitochondria acted as consumers rather than producers of ATP in embryonic neurons. In sharp contrast, in postnatal neurons pyruvate added during glucose deprivation significantly increased [ATP]c (by 54 ± 8%), whereas oligomycin induced a sharp decline in [ATP]c and increased ΔΨm. These signs of oxidative phosphorylation were observed in all tested P1-P2 neurons. Measurement of ΔΨm with the potential-sensitive probe JC-1 revealed that neuronal mitochondrial membrane potential was significantly reduced in embryonic cultures compared to the postnatal ones, possibly due to increased proton permeability of inner mitochondrial membrane. We conclude that, in embryonic, but not postnatal neuronal cultures, ATP synthesis is predominantly glycolytic and the oxidative phosphorylation-mediated synthesis of ATP by mitochondrial F1Fo-ATPase is insignificant.

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.

Mitochondrial Atpif1 regulates haem synthesis in developing erythroblasts

Defects in the availability of haem substrates or the catalytic activity of the terminal enzyme in haem biosynthesis, ferrochelatase (Fech), impair haem synthesis and thus cause human congenital anaemias. The interdependent functions of regulators of mitochondrial homeostasis and enzymes responsible for haem synthesis are largely unknown. To investigate this we used zebrafish genetic screens and cloned mitochondrial ATPase inhibitory factor 1 (atpif1) from a zebrafish mutant with profound anaemia, pinotage (pnt (tq209)). Here we describe a direct mechanism establishing that Atpif1 regulates the catalytic efficiency of vertebrate Fech to synthesize haem. The loss of Atpif1 impairs haemoglobin synthesis in zebrafish, mouse and human haematopoietic models as a consequence of diminished Fech activity and elevated mitochondrial pH. To understand the relationship between mitochondrial pH, redox potential, [2Fe-2S] clusters and Fech activity, we used genetic complementation studies of Fech constructs with or without [2Fe-2S] clusters in pnt, as well as pharmacological agents modulating mitochondrial pH and redox potential. The presence of [2Fe-2S] cluster renders vertebrate Fech vulnerable to perturbations in Atpif1-regulated mitochondrial pH and redox potential. Therefore, Atpif1 deficiency reduces the efficiency of vertebrate Fech to synthesize haem, resulting in anaemia. The identification of mitochondrial Atpif1 as a regulator of haem synthesis advances our understanding of the mechanisms regulating mitochondrial haem homeostasis and red blood cell development. An ATPIF1 deficiency may contribute to important human diseases, such as congenital sideroblastic anaemias and mitochondriopathies.

Delving deeper: MCL-1's contributions to normal and cancer biology

BCL-2 molecules are regulators of programmed cell death and defects in this pathway contribute to human diseases. One family member, MCL-1, is unique because its expression is tightly regulated and it is essential for promoting the survival of myriad cellular lineages. Additionally, MCL-1 promotes the maintenance of normal mitochondrial morphology and energy production. Dissection of these functions revealed recently that they depend on separate mitochondrial sublocalizations. MCL-1's antiapoptotic activity is restricted to the outer mitochondrial membrane (OMM), whereas its function in mitochondrial physiology requires localization to the matrix. These findings provide an attractive model for how MCL-1's diverse functions may contribute to normal cell homeostasis and function. MCL-1 is highly amplified in human cancer, suggesting that these functions may contribute to malignant cell growth and evasion of apoptosis.

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 ROS production and subsequent ERK phosphorylation are necessary for temperature preconditioning of isolated ventricular myocytes

Hypothermia and hypothermic preconditioning are known to be profoundly cardioprotective, but the molecular mechanisms of this protection have not been fully explained. In this study, temperature preconditioning (16 °C) was found to be cardioprotective in isolated adult rat ventricular myocytes, enhancing contractile recovery and preventing calcium dysregulation after oxidative stress. Hypothermic preconditioning preserved mitochondrial function by delaying the pathological opening of the mitochondrial permeability transition pore (mPTP), whereas transient mPTP flickering remained unaltered. For the first time, reactive oxygen species (ROS) from the mitochondria are shown to be released exclusively during the hypothermic episodes of the temperature-preconditioning protocol. Using a mitochondrially targeted ROS biosensor, ROS release was shown during the brief bursts to 16 °C of temperature preconditioning. The ROS scavenger N-(2-mercaptopropionyl) glycine attenuated ROS accumulation during temperature preconditioning, abolishing the protective delay in mPTP opening. Temperature preconditioning induces ROS-dependant phosphorylation of the prosurvival kinase extracellular signal-regulated kinase (ERK)1/2. ERK1/2 activation was shown to be downstream of ROS release, as the presence of a ROS scavenger during temperature preconditioning completely blocked ERK1/2 activation. The cardioprotective effects of temperature preconditioning on mPTP opening were completely lost by inhibiting ERK1/2 activation. Thus, mitochondrial ROS release and ERK1/2 activation are both necessary to signal the cardioprotective effects of temperature preconditioning in cardiac myocytes.

HtrA2 deficiency causes mitochondrial uncoupling through the F₁F₀-ATP synthase and consequent ATP depletion

Loss of the mitochondrial protease HtrA2 (Omi) in mice leads to mitochondrial dysfunction, neurodegeneration and premature death, but the mechanism underlying this pathology remains unclear. Using primary cultures from wild-type and HtrA2-knockout mice, we find that HtrA2 deficiency significantly reduces mitochondrial membrane potential in a range of cell types. This depolarisation was found to result from mitochondrial uncoupling, as mitochondrial respiration was increased in HtrA2-deficient cells and respiratory control ratio was dramatically reduced. HtrA2-knockout cells exhibit increased proton translocation through the ATP synthase, in combination with decreased ATP production and truncation of the F1 α-subunit, suggesting the ATP synthase as the source of the proton leak. Uncoupling in the HtrA2-deficient mice is accompanied by altered breathing pattern and, on a cellular level, ATP depletion and vulnerability to chemical ischaemia. We propose that this vulnerability may ultimately cause the neurodegeneration observed in these mice.

The autophagy-associated factors DRAM1 and p62 regulate cell migration and invasion in glioblastoma stem cells

The aggressiveness of glioblastoma multiforme (GBM) is defined by local invasion and resistance to therapy. Within established GBM, a subpopulation of tumor-initiating cells with stem-like properties (GBM stem cells, GSCs) is believed to underlie resistance to therapy. The metabolic pathway autophagy has been implicated in the regulation of survival in GBM. However, the status of autophagy in GBM and its role in the cancer stem cell fraction is currently unclear. We found that a number of autophagy regulators are highly expressed in GBM tumors carrying a mesenchymal signature, which defines aggressiveness and invasion, and are associated with components of the MAPK pathway. This autophagy signature included the autophagy-associated genes DRAM1 and SQSTM1, which encode a key regulator of selective autophagy, p62. High levels of DRAM1 were associated with shorter overall survival in GBM patients. In GSCs, DRAM1 and SQSTM1 expression correlated with activation of MAPK and expression of the mesenchymal marker c-MET. DRAM1 knockdown decreased p62 localization to autophagosomes and its autophagy-mediated degradation, thus suggesting a role for DRAM1 in p62-mediated autophagy. In contrast, autophagy induced by starvation or inhibition of mTOR/PI-3K was not affected by either DRAM1 or p62 downregulation. Functionally, DRAM1 and p62 regulate cell motility and invasion in GSCs. This was associated with alterations of energy metabolism, in particular reduced ATP and lactate levels. Taken together, these findings shed new light on the role of autophagy in GBM and reveal a novel function of the autophagy regulators DRAM1 and p62 in control of migration/invasion in cancer stem cells.

Assessing actual contribution of IF1, inhibitor of mitochondrial FoF1, to ATP homeostasis, cell growth, mitochondrial morphology, and cell viability

F(o)F(1)-ATP synthase (F(o)F(1)) synthesizes ATP in mitochondria coupled with proton flow driven by the proton motive force (pmf) across membranes. It has been known that isolated IF1, an evolutionarily well conserved mitochondrial protein, can inhibit the ATP hydrolysis activity of F(o)F(1). Here, we generated HeLa cells with permanent IF1 knockdown (IF1-KD cells) and compared their energy metabolism with control cells. Under optimum growth conditions, IF1-KD cells have lower cellular ATP levels and generate a higher pmf and more reactive oxygen species. Nonetheless, IF1-KD cells and control cells show the same rates of cell growth, glucose consumption, and mitochondrial ATP synthesis. Furthermore, contrary to previous reports, the morphology of mitochondria in IF1-KD cells appears to be normal. When cells encounter sudden dissipation of pmf, the cytoplasmic ATP level in IF1-KD cells drops immediately (~1 min), whereas it remains unchanged in the control cells, indicating occurrence of futile ATP hydrolysis by F(o)F(1) in the absence of IF1. The lowered ATP level in IF1-KD cells then recovers gradually (~10 min) to the original level by consuming more glucose than control cells. The viability of IF1-KD cells and control cells is the same in the absence of pmf. Thus, IF1 contributes to ATP homeostasis, but its deficiency does not affect the growth and survival of HeLa cells. Only when cells are exposed to chemical ischemia (no glycolysis and no respiration) or high concentrations of reactive oxygen species does IF1 exhibit its ability to alleviate cell injury.

The mitochondrial ATPase inhibitory factor 1 triggers a ROS-mediated retrograde prosurvival and proliferative response

Recent findings indicate that prevalent human carcinomas overexpress the mitochondrial ATPase Inhibitory Factor 1 (IF1). Overexpression of IF1 inhibits the synthase activity of the mitochondrial H(+)-ATP synthase and plays a crucial role in metabolic adaptation of cancer cells to enhanced aerobic glycolysis. Herein, we demonstrate that IF1 overexpression in colon cancer cells triggers mitochondrial hyperpolarization and the subsequent production of superoxide radical, a reactive oxygen species (ROS). ROS are required to promote the transcriptional activation of the NFκB pathway via phosphorylation-dependent IκBα degradation. Activation of NFκB results in a cellular adaptive response that includes proliferation and Bcl-xL mediated resistance to drug-induced cell death. Quenching the mitochondrial production of ROS prevents the activation of NFκB and abolishes the IF1-mediated cellular adaptive response. Overall, our findings provide evidence linking the activity of a mitochondrial protein with retrograde signaling to the nucleus to promote cellular proliferation and survival.

Mitochondrial ATP synthase: architecture, function and pathology

Human mitochondrial (mt) ATP synthase, or complex V consists of two functional domains: F(1), situated in the mitochondrial matrix, and F(o), located in the inner mitochondrial membrane. Complex V uses the energy created by the proton electrochemical gradient to phosphorylate ADP to ATP. This review covers the architecture, function and assembly of complex V. The role of complex V di-and oligomerization and its relation with mitochondrial morphology is discussed. Finally, pathology related to complex V deficiency and current therapeutic strategies are highlighted. Despite the huge progress in this research field over the past decades, questions remain to be answered regarding the structure of subunits, the function of the rotary nanomotor at a molecular level, and the human complex V assembly process. The elucidation of more nuclear genetic defects will guide physio(patho)logical studies, paving the way for future therapeutic interventions.

Glucose-modulated mitochondria adaptation in tumor cells: a focus on ATP synthase and inhibitor Factor 1

Warburg's hypothesis has been challenged by a number of studies showing that oxidative phosphorylation is repressed in some tumors, rather than being inactive per se. Thus, treatments able to shift energy metabolism by activating mitochondrial pathways have been suggested as an intriguing basis for the optimization of antitumor strategies. In this study, HepG2 hepatocarcinoma cells were cultivated with different metabolic substrates under conditions mimicking "positive" (activation/biogenesis) or "negative" (silencing) mitochondrial adaptation. In addition to the expected up-regulation of mitochondrial biogenesis, glucose deprivation caused an increase in phosphorylating respiration and a rise in the expression levels of the ATP synthase β subunit and Inhibitor Factor 1 (IF1). Hyperglycemia, on the other hand, led to a markedly decreased level of the transcriptional coactivator PGC-α suggesting down-regulation of mitochondrial biogenesis, although no change in mitochondrial mass and no impairment of phosphorylating respiration were observed. Moreover, a reduction in mitochondrial networking and in ATP synthase dimer stability was produced. No effect on β-ATP synthase expression was elicited. Notably, hyperglycemia caused an increase in IF1 expression levels, but it did not alter the amount of IF1 associated with ATP synthase. These results point to a new role of IF1 in relation to high glucose utilization by tumor cells, in addition to its well known effect upon mitochondrial ATP synthase regulation.

Cell metabolism affects selective vulnerability in PINK1-associated Parkinson's disease

Mitochondrial dysfunction plays a primary role in the pathogenesis of Parkinson's disease (PD), particularly in autosomal recessive forms of the disease caused by mutations encoding PINK1. Although mitochondrial pathology can be demonstrated in many cell types, it is neurons that bear the brunt of cell death in PD. We studied the mitochondrial physiology of neurons and muscle cells with loss of function of the nuclear encoded mitochondrial protein PINK1. PINK1 is widely expressed in many types of tissues, but deficiency selectively induces death in neurons. We report here that the same genetic defect results in opposing phenotypes in different cell types, depending on the metabolic properties of the cell. Thus, PINK1-deficient myocytes exhibit high basal mitochondrial membrane potential (Δψm), whereas PINK1-deficient neurons have been shown to exhibit a low Δψm. PINK1 deficiency induces impaired respiration in both cell types, with a concomitant increase in glycolytic activity. We demonstrate that the high glycolytic capacity in myocytes compared with neurons enables them to produce more ATP and, therefore, compensates for the metabolic defects induced by PINK1 deficiency. Furthermore, the high Δψm generated in PINK1 knockout (KO) muscle mitochondria enables them to buffer cytosolic Ca(2+) fluxes, rendering them resistant to Ca(2+) stress effectively. Conversely, PINK1 KO neurons were previously shown to develop mitochondrial Ca(2+) overload and Ca(2+)-induced mitochondrial depolarisation. Prevention of Ca(2+) dysregulation in myocytes might therefore account for the sparing of these cells in PD.

UCP2 regulates energy metabolism and differentiation potential of human pluripotent stem cells

It has been assumed, based largely on morphologic evidence, that human pluripotent stem cells (hPSCs) contain underdeveloped, bioenergetically inactive mitochondria. In contrast, differentiated cells harbour a branched mitochondrial network with oxidative phosphorylation as the main energy source. A role for mitochondria in hPSC bioenergetics and in cell differentiation therefore remains uncertain. Here, we show that hPSCs have functional respiratory complexes that are able to consume O(2) at maximal capacity. Despite this, ATP generation in hPSCs is mainly by glycolysis and ATP is consumed by the F(1)F(0) ATP synthase to partially maintain hPSC mitochondrial membrane potential and cell viability. Uncoupling protein 2 (UCP2) plays a regulating role in hPSC energy metabolism by preventing mitochondrial glucose oxidation and facilitating glycolysis via a substrate shunting mechanism. With early differentiation, hPSC proliferation slows, energy metabolism decreases, and UCP2 is repressed, resulting in decreased glycolysis and maintained or increased mitochondrial glucose oxidation. Ectopic UCP2 expression perturbs this metabolic transition and impairs hPSC differentiation. Overall, hPSCs contain active mitochondria and require UCP2 repression for full differentiation potential.

Bioenergetic consequences of PINK1 mutations in Parkinson disease

Background

Mutations of the gene for PTEN-induced kinase 1 (PINK1) are a cause of familial Parkinson's disease (PD). PINK1 protein has been localised to mitochondria and PINK1 gene knockout models exhibit abnormal mitochondrial function. The purpose of this study was to determine whether cells derived from PD patients with a range of PINK1 mutations demonstrate similar defects of mitochondrial function, whether the nature and severity of the abnormalities vary between mutations and correlate with clinical features.

Methodology

We investigated mitochondrial bioenergetics in live fibroblasts from PINK1 mutation patients using single cell techniques. We found that fibroblasts from PINK1 mutation patients had significant defects of bioenergetics including reduced mitochondrial membrane potential, altered redox state, a respiratory deficiency that was determined by substrate availability, and enhanced sensitivity to calcium stimulation and associated mitochondrial permeability pore opening. There was an increase in the basal rate of free radical production in the mutant cells. The pattern and severity of abnormality varied between different mutations, and the less severe defects in these cells were associated with later age of onset of PD.

Conclusions

The results provide insight into the molecular pathology of PINK1 mutations in PD and also confirm the critical role of substrate availability in determining the biochemical phenotype – thereby offering the potential for novel therapeutic strategies to circumvent these abnormalities.

A mitochondrial-focused genetic interaction map reveals a scaffold-like complex required for inner membrane organization in mitochondria

Statement MITO-MAP, a high-density genetic interaction map in budding yeast, identifies a mitochondrial inner membrane–associated complex that promotes normal mitochondrial membrane organization and morphology.

To broadly explore mitochondrial structure and function as well as the communication of mitochondria with other cellular pathways, we constructed a quantitative, high-density genetic interaction map (the MITO-MAP) in Saccharomyces cerevisiae. The MITO-MAP provides a comprehensive view of mitochondrial function including insights into the activity of uncharacterized mitochondrial proteins and the functional connection between mitochondria and the ER. The MITO-MAP also reveals a large inner membrane–associated complex, which we term MitOS for mitochondrial organizing structure, comprised of Fcj1/Mitofilin, a conserved inner membrane protein, and five additional components. MitOS physically and functionally interacts with both outer and inner membrane components and localizes to extended structures that wrap around the inner membrane. We show that MitOS acts in concert with ATP synthase dimers to organize the inner membrane and promote normal mitochondrial morphology. We propose that MitOS acts as a conserved mitochondrial skeletal structure that differentiates regions of the inner membrane to establish the normal internal architecture of mitochondria.

Mitochondrial adenine nucleotide transport and cardioprotection

Mitochondria are highly metabolically active cell organelles that not only act as the powerhouse of the cell by supplying energy through ATP production, but also play a destructive role by initiating cell death pathways. Growing evidence recognizes that mitochondrial dysfunction is one of the major causes of cardiovascular disease. Under de-energized conditions, slowing of adenine nucleotide transport in and out of the mitochondria significantly attenuates myocardial ischemia-reperfusion injury. The purpose of this review is to elaborate on and update the mechanistic pathways which may explain how altered adenine nucleotide transport can influence cardiovascular function. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

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.

Atypical cristae morphology of human syncytiotrophoblast mitochondria: role for complex V

Mitochondrial complexes I, III(2), and IV from human cytotrophoblast and syncytiotrophoblast associate to form supercomplexes or respirasomes, with the following stoichiometries: I(1):(III(2))(1) and I(1):(III(2))(1-2):IV(1-4). The content of respirasomes was similar in both cell types after isolating mitochondria. However, syncytiotrophoblast mitochondria possess low levels of dimeric complex V and do not have orthodox cristae morphology. In contrast, cytotrophoblast mitochondria show normal cristae morphology and a higher content of ATP synthase dimer. Consistent with the dimerizing role of the ATPase inhibitory protein (IF(1)) (García, J. J., Morales-Ríos, E., Cortés-Hernandez, P., and Rodríguez-Zavala, J. S. (2006) Biochemistry 45, 12695-12703), higher relative amounts of IF(1) were observed in cytotrophoblast when compared with syncytiotrophoblast mitochondria. Therefore, there is a correlation between dimerization of complex V, IF(1) expression, and the morphology of mitochondrial cristae in human placental mitochondria. The possible relationship between cristae architecture and the physiological function of the syncytiotrophoblast mitochondria is discussed.

Mitochondrial consumption of cytosolic ATP: not so fast

In various pathologic circumstances depolarized mitochondria are thought to precipitate cell death by avidly consuming cytosolic ATP. However, for as long as the inner mitochondrial membrane remains intact the reversal potentials of the adenine nucleotide translocase (ANT) and that of F(0)-F(1) ATP synthase are strategically positioned so that they oppose import of cytosolic ATP into the matrix of respiration-impaired mitochondria. This arrangement also seems to protect against a hysteretic consumption of cytosolic ATP accumulating in the mitochondrial matrix, in view of the depolarization caused by inhibition of F(0)-F(1) ATP synthase by the endogenous protein IF1, yielding fast ANT reversal rates.

Modulation of F0F1-ATP synthase activity by cyclophilin D regulates matrix adenine nucleotide levels

Cyclophilin D was recently shown to bind to and decrease the activity of F(0)F(1)-ATP synthase in submitochondrial particles and permeabilized mitochondria [Giorgio V et al. (2009) J Biol Chem, 284, 33982-33988]. Cyclophilin D binding decreased both ATP synthesis and hydrolysis rates. In the present study, we reaffirm these findings by demonstrating that, in intact mouse liver mitochondria energized by ATP, the absence of cyclophilin D or the presence of cyclosporin A led to a decrease in the extent of uncoupler-induced depolarization. Accordingly, in substrate-energized mitochondria, an increase in F(0)F(1)-ATP synthase activity mediated by a relief of inhibition by cyclophilin D was evident in the form of slightly increased respiration rates during arsenolysis. However, the modulation of F(0)F(1)-ATP synthase by cyclophilin D did not increase the adenine nucleotide translocase (ANT)-mediated ATP efflux rate in energized mitochondria or the ATP influx rate in de-energized mitochondria. The lack of an effect of cyclophilin D on the ANT-mediated adenine nucleotide exchange rate was attributed to the ∼ 2.2-fold lower flux control coefficient of the F(0)F(1)-ATP synthase than that of ANT, as deduced from measurements of adenine nucleotide flux rates in intact mitochondria. These findings were further supported by a recent kinetic model of the mitochondrial phosphorylation system, suggesting that an ∼ 30% change in F(0)F(1)-ATP synthase activity in fully energized or fully de-energized mitochondria affects the ADP-ATP exchange rate mediated by the ANT in the range 1.38-1.7%. We conclude that, in mitochondria exhibiting intact inner membranes, the absence of cyclophilin D or the inhibition of its binding to F(0)F(1)-ATP synthase by cyclosporin A will affect only matrix adenine nucleotides levels.

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.

Trans fat diet causes decreased brood size and shortened lifespan in Caenorhabditis elegans delta-6-desaturase mutant fat-3

Trans-fatty acids (TFAs) enter the diet through industrial processes and can cause adverse human health effects. The present study was aimed to examine the effects of dietary cis- and trans-fatty acids on the model organism Caenorhabditis elegans. Cis- or trans-18:1n9 triglycerides (25 μM) caused no apparent changes in the numbers of viable progeny of wild-type N2 animals. However, in fat-3 mutants lacking delta-6-desaturase, the trans-isomer caused modest decreases in lifespan and progeny after three generations. Long-chain polyunsaturated fatty acids (PUFA) profiles were significantly altered in fat-3 mutants compared to wild type but were not altered after exposure to dietary cis- or trans-18:1n9. Genome-wide expression analysis of fat-3 mutants revealed hundreds of changes. Several genes involved in fat metabolism (acs-2, fat-7, mdt-15) were significantly increased by cis- or trans-18:1n9 without discrimination between isomers. These results provide support for the hypothesis that dietary trans fats are detrimental to development and aging.

Roles of mitochondria in human disease

The chapters throughout this volume illustrate the many contributions of mitochondria to the maintenance of normal cell and tissue function, experienced as the health of the individual. Mitochondria are essential for maintaining aspects of physiology as fundamental as cellular energy balance, the modulation of calcium signalling, in defining cellular redox balance, and they house significant biosynthetic pathways. Mitochondrial numbers and volume within cells are regulated and have an impact on their functional roles, while, especially in the CNS (central nervous system), mitochondrial trafficking is critical to ensure the cellular distribution and strategic localization of mitochondria, presumably driven by local energy demand. Maintenance of a healthy mitochondrial population involves a complex system of quality control, involving degrading misfolded proteins, while damaged mitochondria are renewed by fusion or removed by autophagy. It seems evident that mechanisms that impair any of these processes will impair mitochondrial function and cell signalling pathways, leading to disordered cell function which manifests as disease. As gatekeepers of cell life and cell death, mitochondria regulate both apoptotic and necrotic cell death, and so at its most extreme, disturbances involving these pathways may trigger untimely cell death. Conversely, the lack of appropriate cell death can lead to inappropriate tissue growth and development of tumours, which are also characterized by altered mitochondrial metabolism. The centrality of mitochondrial dysfunction to a surprisingly wide range of major human diseases is slowly becoming recognized, bringing with it the prospect of novel therapeutic approaches to treat a multitude of unpleasant and pervasive diseases.

Oxidative phosphorylation in cancer cells

Evidence suggests that mitochondrial metabolism may play a key role in controlling cancer cells life and proliferation. Recent evidence also indicates how the altered contribution of these organelles to metabolism and the resistance of cancer mitochondria against apoptosis-associated permeabilization are closely related. The hallmarks of cancer growth, increased glycolysis and lactate production in tumours, have raised attention due to recent observations suggesting a wide spectrum of oxidative phosphorylation deficit and decreased availability of ATP associated with malignancies and tumour cell expansion. More specifically, alteration in signal transduction pathways directly affects mitochondrial proteins playing critical roles in controlling the membrane potential as UCP2 and components of both MPTP and oxphos complexes, or in controlling cells life and death as the Bcl-2 proteins family. Moreover, since mitochondrial bioenergetics and dynamics, are also involved in processes of cells life and death, proper regulation of these mitochondrial functions is crucial for tumours to grow. Therefore a better understanding of the key pathophysiological differences between mitochondria in cancer cells and in their non-cancer surrounding tissue is crucial to the finding of tools interfering with these peculiar tumour mitochondrial functions and will disclose novel approaches for the prevention and treatment of malignant diseases. Here, we review the peculiarity of tumour mitochondrial bioenergetics and the mode it is linked to the cell metabolism, providing a short overview of the evidence accumulated so far, but highlighting the more recent advances.

Structure of dimeric F1F0-ATP synthase

The structure of the dimeric ATP synthase from yeast mitochondria was analyzed by transmission electron microscopy and single particle image analysis. In addition to the previously reported side views of the dimer, top view and intermediate projections served to resolve the arrangement of the rotary c(10) ring and the other stator subunits at the F(0)-F(0) dimeric interface. A three-dimensional reconstruction of the complex was calculated from a data set of 9960 molecular images at a resolution of 27 Å. The structural model of the dimeric ATP synthase shows the two monomers arranged at an angle of ∼45°, consistent with our earlier analysis of the ATP synthase from bovine heart mitochondria (Minauro-Sanmiguel, F., Wilkens, S., and Garcia, J. J. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 12356-12358). In the ATP synthase dimer, the two peripheral stalks are located near the F(1)-F(1) interface but are turned away from each other so that they are not in contact. Based on the three-dimensional reconstruction, a model of how dimeric ATP synthase assembles to form the higher order oligomeric structures that are required for mitochondrial cristae biogenesis is discussed.

Up-regulation of the ATPase inhibitory factor 1 (IF1) of the mitochondrial H+-ATP synthase in human tumors mediates the metabolic shift of cancer cells to a Warburg phenotype

The H(+)-ATP synthase is a reversible engine of mitochondria that synthesizes or hydrolyzes ATP upon changes in cell physiology. ATP synthase dysfunction is involved in the onset and progression of diverse human pathologies. During ischemia, the ATP hydrolytic activity of the enzyme is inhibited by the ATPase inhibitory factor 1 (IF1). The expression of IF1 in human tissues and its participation in the development of human pathology are unknown. Here, we have developed monoclonal antibodies against human IF1 and determined its expression in paired normal and tumor biopsies of human carcinomas. We show that the relative mitochondrial content of IF1 increases significantly in carcinomas, suggesting the participation of IF1 in oncogenesis. The expression of IF1 varies significantly in cancer cell lines. To investigate the functional activity of IF1 in cancer, we have manipulated its cellular content. Overexpression of IF1 or of its pH-insensitive H49K mutant in cells that express low levels of IF1 triggers the up-regulation of aerobic glycolysis and the inhibition of oxidative phosphorylation with concurrent mitochondrial hyperpolarization. Treatment of the cells with the H(+)-ATP synthase inhibitor oligomycin mimicked the effects of IF1 overexpression. Conversely, small interfering RNA-mediated silencing of IF1 in cells that express high levels of IF1 promotes the down-regulation of aerobic glycolysis and the increase in oxidative phosphorylation. Overall, these findings support that the mitochondrial content of IF1 controls the activity of oxidative phosphorylation mediating the shift of cancer cells to an enhanced aerobic glycolysis, thus supporting an oncogenic role for the de-regulated expression of IF1 in cancer.

Cyclophilin D in mitochondrial pathophysiology

Cyclophilins are a family of peptidyl-prolyl cis-trans isomerases whose enzymatic activity can be inhibited by cyclosporin A. Sixteen cyclophilins have been identified in humans, and cyclophilin D is a unique isoform that is imported into the mitochondrial matrix. Here we shall (i) review the best characterized functions of cyclophilin D in mitochondria, i.e. regulation of the permeability transition pore, an inner membrane channel that plays an important role in the execution of cell death; (ii) highlight new regulatory interactions that are emerging in the literature, including the modulation of the mitochondrial F1FO ATP synthase through an interaction with the lateral stalk of the enzyme complex; and (iii) discuss diseases where cyclophilin D plays a pathogenetic role that makes it a suitable target for pharmacologic intervention.

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).

Mitochondrial DNA mutations affect calcium handling in differentiated neurons

Mutations in the mitochondrial genome are associated with a wide range of neurological symptoms, but many aspects of the basic neuronal pathology are not understood. One candidate mechanism, given the well-established role of mitochondria in calcium buffering, is a deficit in neuronal calcium homoeostasis. We therefore examined calcium responses in the neurons derived from various ‘cybrid’ embryonic stem cell lines carrying different mitochondrial DNA mutations. Brief (∼50 ms), focal glutamatergic stimuli induced a transient rise in intracellular calcium concentration, which was visualized by bulk loading the cells with the calcium dye, Oregon Green BAPTA-1. Calcium entered the neurons through N-methyl-d-aspartic acid and voltage-gated calcium channels, as has been described in many other neuronal classes. Intriguingly, while mitochondrial mutations did not affect the calcium transient in response to single glutamatergic stimuli, they did alter the responses to repeated stimuli, with each successive calcium transient decaying ever more slowly in mitochondrial mutant cell lines. A train of stimuli thus caused intracellular calcium in these cells to be significantly elevated for many tens of seconds. These results suggest that calcium-handling deficits are likely to contribute to the pathological phenotype seen in patients with mitochondrial DNA mutations.

The ups and downs of mitochondrial calcium signalling in the heart

Regulation of intramitochondrial free calcium ([Ca2+]m) is critical in both physiological and pathological functioning of the heart. The full extent and importance of the role of [Ca2+]m is becoming apparent as evidenced by the increasing interest and work in this area over the last two decades. However, controversies remain, such as the existence of beat-to-beat mitochondrial Ca2+ transients; the role of [Ca2+]m in modulating whole-cell Ca2+ signalling; whether or not an increase in [Ca2+]m is essential to couple ATP supply and demand; and the role of [Ca2+]m in cell death by both necrosis and apoptosis, especially in formation of the mitochondrial permeability transition pore. The role of [Ca2+]m in heart failure is an area that has also recently been highlighted. [Ca2+]m can now be measured reasonably specifically in intact cells and hearts thanks to developments in fluorescent indicators and targeted proteins and more sensitive imaging technology. This has revealed interactions of the mitochondrial Ca2+ transporters with those of the sarcolemma and sarcoplasmic reticulum, and has gone a long way to bringing the mitochondrial Ca2+ transporters to the forefront of cardiac research. Mitochondrial Ca2+ uptake occurs via the ruthenium red sensitive Ca2+ uniporter (mCU), and efflux via an Na+/Ca2+ exchanger (mNCX). The purification and cloning of the transporters, and development of more specific inhibitors, would produce a step-change in our understanding of the role of these apparently critical but still elusive proteins. In this article we will summarise the key physiological roles of [Ca2+]m in ATP production and cell Ca2+ signalling in both adult and neonatal hearts, as well as highlighting some of the controversies in these areas. We will also briefly discuss recent ideas on the interactions of nitric oxide with [Ca2+]m.

Assembly and oligomerization of human ATP synthase lacking mitochondrial subunits a and A6L

Here we study ATP synthase from human rho0 (rho zero) cells by clear native electrophoresis (CNE or CN-PAGE) and show that ATP synthase is almost fully assembled in spite of the absence of subunits a and A6L. This identifies subunits a and A6L as two of the last subunits to complete the ATP synthase assembly. Minor amounts of dimeric and even tetrameric forms of the large assembly intermediate were preserved under the conditions of CNE, suggesting that it associated further into higher order structures in the mitochondrial membrane. This result was reminiscent to the reduced amounts of dimeric and tetrameric ATP synthase from yeast null mutants of subunits e and g detected by CNE. The dimer/oligomer-stabilizing effects of subunits e/g and a/A6L seem additive in human and yeast cells. The mature IF1 inhibitor was specifically bound to the dimeric/oligomeric forms of ATP synthase and not to the monomer. Conversely, nonprocessed pre-IF1 still containing the mitochondrial targeting sequence was selectively bound to the monomeric assembly intermediate in rho0 cells and not to the dimeric form. This supports previous suggestions that IF1 plays an important role in the dimerization/oligomerization of mammalian ATP synthase and in the regulation of mitochondrial structure and function.

Inorganic polyphosphate and energy metabolism in mammalian cells

Inorganic polyphosphate (poly P) is a polymer made from as few as 10 to several hundred phosphate molecules linked by phosphoanhydride bonds similar to ATP. Poly P is ubiquitous in all mammalian organisms, where it plays multiple physiological roles. The metabolism of poly P in mammalian organisms is not well understood. We have examined the mechanism of poly P production and the role of this polymer in cell energy metabolism. Poly P levels in mitochondria and intact cells were estimated using a fluorescent molecular probe, 4',6-diamidino-2-phenylindole. Poly P levels were dependent on the metabolic state of the mitochondria. Poly P levels were increased by substrates of respiration and in turn reduced by mitochondrial inhibitor (rotenone) or an uncoupler (carbonyl cyanide p-trifluoromethoxyphenylhydrazone). Oligomycin, an inhibitor of mitochondrial ATP-synthase, blocked the production of poly P. Enzymatic depletion of poly P from cells significantly altered the rate of ATP metabolism. We propose the existence of a feedback mechanism where poly P production and cell energy metabolism regulate each other.

Clonal characterization of rat muscle satellite cells: proliferation, metabolism and differentiation define an intrinsic heterogeneity

Satellite cells (SCs) represent a distinct lineage of myogenic progenitors responsible for the postnatal growth, repair and maintenance of skeletal muscle. Distinguished on the basis of their unique position in mature skeletal muscle, SCs were considered unipotent stem cells with the ability of generating a unique specialized phenotype. Subsequently, it was demonstrated in mice that opposite differentiation towards osteogenic and adipogenic pathways was also possible. Even though the pool of SCs is accepted as the major, and possibly the only, source of myonuclei in postnatal muscle, it is likely that SCs are not all multipotent stem cells and evidences for diversities within the myogenic compartment have been described both in vitro and in vivo. Here, by isolating single fibers from rat flexor digitorum brevis (FDB) muscle we were able to identify and clonally characterize two main subpopulations of SCs: the low proliferative clones (LPC) present in major proportion (approximately 75%) and the high proliferative clones (HPC), present instead in minor amount (approximately 25%). LPC spontaneously generate myotubes whilst HPC differentiate into adipocytes even though they may skip the adipogenic program if co-cultured with LPC. LPC and HPC differ also for mitochondrial membrane potential (DeltaPsi(m)), ATP balance and Reactive Oxygen Species (ROS) generation underlying diversities in metabolism that precede differentiation. Notably, SCs heterogeneity is retained in vivo. SCs may therefore be comprised of two distinct, though not irreversibly committed, populations of cells distinguishable for prominent differences in basal biological features such as proliferation, metabolism and differentiation. By these means, novel insights on SCs heterogeneity are provided and evidences for biological readouts potentially relevant for diagnostic purposes described.

Cyclophilin D modulates mitochondrial F0F1-ATP synthase by interacting with the lateral stalk of the complex

Blue native gel electrophoresis purification and immunoprecipitation of F(0)F(1)-ATP synthase from bovine heart mitochondria revealed that cyclophilin (CyP) D associates to the complex. Treatment of intact mitochondria with the membrane-permeable bifunctional reagent dimethyl 3,3-dithiobis-propionimidate (DTBP) cross-linked CyPD with the lateral stalk of ATP synthase, whereas no interactions with F(1) sector subunits, the ATP synthase natural inhibitor protein IF1, and the ATP/ADP carrier were observed. The ATP synthase-CyPD interactions have functional consequences on enzyme catalysis and are modulated by phosphate (increased CyPD binding and decreased enzyme activity) and cyclosporin (Cs) A (decreased CyPD binding and increased enzyme activity). Treatment of MgATP submitochondrial particles or intact mitochondria with CsA displaced CyPD from membranes and activated both hydrolysis and synthesis of ATP sustained by the enzyme. No effect of CsA was detected in CyPD-null mitochondria, which displayed a higher specific activity of the ATP synthase than wild-type mitochondria. Modulation by CyPD binding appears to be independent of IF1, whose association to ATP synthase was not affected by CsA treatment. These findings demonstrate that CyPD association to the lateral stalk of ATP synthase modulates the activity of the complex.

Mitochondria as ATP consumers in cellular pathology

ATP provided by oxidative phosphorylation supports highly complex and energetically expensive cellular processes. Yet, in several pathological settings, mitochondria could revert to ATP consumption, aggravating an existing cellular pathology. Here we review (i) the pathological conditions leading to ATP hydrolysis by the reverse operation of the mitochondrial F(o)F(1)-ATPase, (ii) molecular and thermodynamic factors influencing the directionality of the F(o)F(1)-ATPase, (iii) the role of the adenine nucleotide translocase as the intermediary adenine nucleotide flux pathway between the cytosol and the mitochondrial matrix when mitochondria become ATP consumers, (iv) the role of the permeability transition pore in bypassing the ANT, thereby allowing the flux of ATP directly to the hydrolyzing F(o)F(1)-ATPase, (v) the impact of the permeability transition pore on glycolytic ATP production, and (vi) endogenous and exogenous interventions for limiting ATP hydrolysis by the mitochondrial F(o)F(1)-ATPase.

The changing shape of mitochondrial apoptosis

Mitochondria are key organelles in conversion of energy, regulation of cellular signaling and amplification of programmed cell death. The anatomy of the organelle matches this functional versatility in complexity and is modulated by the concerted action of proteins that impinge on its fusion-fission equilibrium. A growing body of evidence implicates changes in mitochondrial shape in the progression of apoptosis and, therefore, proteins governing such changes are likely candidates for involvement in pathogenetic mechanisms in neurodegeneration and cancer. Here, we discuss the recent advancements in our knowledge about the machinery that regulates mitochondrial shape and on the role of molecular mechanisms controlling mitochondrial morphology during cell death.

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.

PINK1 function in health and disease

The role of mitochondria in sporadic Parkinson's disease (PD) has been debated for a little over 20 years since the description of complex I deficiency in the substantia nigra pars compacta (SNpc) of PD patients. However, the identification of recessive pathogenic mutations in the pink1 gene in familial PD cases firmly re-ignited interest in the pathophysiology of mitochondria in PD. PINK1 is a putative mitochondrial serine/threonine kinase, which protects cells against oxidative stress induced apoptosis. The mechanism by which this is achieved and the effect of the pathogenic mutations has been an area of intensive research over the past five years. Significant progress has been made and, in this review, we summarize the physiological roles that have been assigned to PINK1 and the potential mechanisms behind pathogenesis.