The ADP and ATP transport in mitochondria and its carrier

Functional expression and characterisation of membrane transport proteins

Membrane transporters set the framework organising the complexity of plant metabolism in cells, tissues and organisms. Their substrate specificity and controlled activity in different cells is a crucial part for plant metabolism to run pathways in concert. Transport proteins catalyse the uptake and exchange of ions, substrates, intermediates, products and cofactors across membranes. Given the large number of metabolites, a wide spectrum of transporters is required. The vast majority of in silico annotated membrane transporters in plant genomes, however, has not yet been functionally characterised. Hence, to understand the metabolic network as a whole, it is important to understand how transporters connect and control the metabolic pathways of plant cells. Heterologous expression and in vitro activity studies of recombinant transport proteins have highly improved their functional analysis in the last two decades. This review provides a comprehensive overview of the recent advances in membrane protein expression and functional characterisation using various host systems and transport assays.

Mitochondrial network energetics in the heart

At the core of eukaryotic aerobic life, mitochondrial function like 'hubs' in the web of energetic and redox processes in cells. In the heart, these networks-extending beyond the complex connectivity of biochemical circuit diagrams and apparent morphology-exhibit collective dynamics spanning several spatiotemporal levels of organization, from the cell, to the tissue, and the organ. The network function of mitochondria, i.e., mitochondrial network energetics, represents an advantageous behavior. Its coordinated action, under normal physiology, provides robustness despite failure in a few nodes, and improves energy supply toward a swiftly changing demand. Extensive diffuse loops, encompassing mitochondrial-cytoplasmic reaction/transport networks, control and regulate energy supply and demand in the heart. Under severe energy crises, the network behavior of mitochondria and associated glycolytic and other metabolic networks collapse, thereby triggering fatal arrhythmias.

Cardiac mitochondrial matrix and respiratory complex protein phosphorylation

It has become appreciated over the last several years that protein phosphorylation within the cardiac mitochondrial matrix and respiratory complexes is extensive. Given the importance of oxidative phosphorylation and the balance of energy metabolism in the heart, the potential regulatory effect of these classical signaling events on mitochondrial function is of interest. However, the functional impact of protein phosphorylation and the kinase/phosphatase system responsible for it are relatively unknown. Exceptions include the well-characterized pyruvate dehydrogenase and branched chain α-ketoacid dehydrogenase regulatory system. The first task of this review is to update the current status of protein phosphorylation detection primarily in the matrix and evaluate evidence linking these events with enzymatic function or protein processing. To manage the scope of this effort, we have focused on the pathways involved in energy metabolism. The high sensitivity of modern methods of detecting protein phosphorylation and the low specificity of many kinases suggests that detection of protein phosphorylation sites without information on the mole fraction of phosphorylation is difficult to interpret, especially in metabolic enzymes, and is likely irrelevant to function. However, several systems including protein translocation, adenine nucleotide translocase, cytochrome c, and complex IV protein phosphorylation have been well correlated with enzymatic function along with the classical dehydrogenase systems. The second task is to review the current understanding of the kinase/phosphatase system within the matrix. Though it is clear that protein phosphorylation occurs within the matrix, based on (32)P incorporation and quantitative mass spectrometry measures, the kinase/phosphatase system responsible for this process is ill-defined. An argument is presented that remnants of the much more labile bacterial protein phosphoryl transfer system may be present in the matrix and that the evaluation of this possibility will require the application of approaches developed for bacterial cell signaling to the mitochondria.

Impaired transport of nucleotides in a mitochondrial carrier explains severe human genetic diseases

The mitochondrial ADP/ATP carrier (AAC) is a prominent actor in the energetic regulation of the cell, importing ADP into the mitochondria and exporting ATP toward the cytoplasm. Severe genetic diseases have been ascribed to specific mutations in this membrane protein. How minute, well-localized modifications of the transporter impact the function of the mitochondria remains, however, largely unclear. Here, for the first time, the relationship between all documented pathological mutations of the AAC and its transport properties is established. Activity measurements combined synergistically with molecular-dynamics simulations demonstrate how all documented pathological mutations alter the binding affinity and the translocation kinetics of the nucleotides. Throwing a bridge between the pathologies and their molecular origins, these results reveal two distinct mechanisms responsible for AAC-related genetic disorders, wherein the mutations either modulate the association of the nucleotides to the carrier by modifying its electrostatic signature or reduce its conformational plasticity.

Absence of Ca2+-induced mitochondrial permeability transition but presence of bongkrekate-sensitive nucleotide exchange in C. crangon and P. serratus

Mitochondria from the embryos of brine shrimp (Artemia franciscana) do not undergo Ca²⁺-induced permeability transition in the presence of a profound Ca²⁺ uptake capacity. Furthermore, this crustacean is the only organism known to exhibit bongkrekate-insensitive mitochondrial adenine nucleotide exchange, prompting the conjecture that refractoriness to bongkrekate and absence of Ca²⁺-induced permeability transition are somehow related phenomena. Here we report that mitochondria isolated from two other crustaceans, brown shrimp (Crangon crangon) and common prawn (Palaemon serratus) exhibited bongkrekate-sensitive mitochondrial adenine nucleotide transport, but lacked a Ca²⁺-induced permeability transition. Ca²⁺ uptake capacity was robust in the absence of adenine nucleotides in both crustaceans, unaffected by either bongkrekate or cyclosporin A. Transmission electron microscopy images of Ca²⁺-loaded mitochondria showed needle-like formations of electron-dense material strikingly similar to those observed in mitochondria from the hepatopancreas of blue crab (Callinectes sapidus) and the embryos of Artemia franciscana. Alignment analysis of the partial coding sequences of the adenine nucleotide translocase (ANT) expressed in Crangon crangon and Palaemon serratus versus the complete sequence expressed in Artemia franciscana reappraised the possibility of the 208-214 amino acid region for conferring sensitivity to bongkrekate. However, our findings suggest that the ability to undergo Ca²⁺-induced mitochondrial permeability transition and the sensitivity of adenine nucleotide translocase to bongkrekate are not necessarily related phenomena.

Inhibition of the Mitochondrial Permeability Transition for Cytoprotection: Direct versus Indirect Mechanisms

Mitochondria are fascinating organelles, which fulfill multiple cellular functions, as diverse as energy production, fatty acid β oxidation, reactive oxygen species (ROS) production and detoxification, and cell death regulation. The coordination of these functions relies on autonomous mitochondrial processes as well as on sustained cross-talk with other organelles and/or the cytosol. Therefore, this implies a tight regulation of mitochondrial functions to ensure cell homeostasis. In many diseases (e.g., cancer, cardiopathies, nonalcoholic fatty liver diseases, and neurodegenerative diseases), mitochondria can receive harmful signals, dysfunction and then, participate to pathogenesis. They can undergo either a decrease of their bioenergetic function or a process called mitochondrial permeability transition (MPT) that can coordinate cell death execution. Many studies present evidence that protection of mitochondria limits disease progression and severity. Here, we will review recent strategies to preserve mitochondrial functions via direct or indirect mechanisms of MPT inhibition. Thus, several mitochondrial proteins may be considered for cytoprotective-targeted therapies.

The interaction between cell adhesion molecule L1, matrix metalloproteinase 14, and adenine nucleotide translocator at the plasma membrane regulates L1-mediated neurite outgrowth of murine cerebellar neurons

We have identified the adenine nucleotide translocator (ANT) isoforms ANT1 and ANT2 that are present in the plasma membrane of mouse cerebellar neurons as novel binding partners of the cell adhesion molecule L1. The direct interaction between ANT and L1 is mediated by sites within the fibronectin type III domains of L1 and the first and third extracellular loops of the ANT proteins. We also show that L1 interacts with the ANT binding partner matrix metalloprotease 14 (MMP14) and that the ANT proteins bind directly to the L1 interaction partner glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Moreover, we provide evidence that the functional interplay between L1, ANT proteins, MMP14, and GAPDH at the plasma membrane mediates L1-induced neurite outgrowth of cerebellar neurons. Disruption of this interplay by ANT inhibitors, ANT-derived synthetic peptides, and/or function-blocking MMP14 and ANT antibodies leads to alterations in L1-dependent neurite outgrowth. Stimulation of L1-mediated signaling in cerebellar neurons triggers transient ATP secretion via ANT proteins and leads to transient src family-dependent tyrosine phosphorylation of L1, ANT1, ANT2, and MMP14. Thus, our results indicate that plasma membrane-localized ANT1 and ANT2 regulate L1-mediated neurite outgrowth in conjunction with MMP14.

Transgenic overexpression of the adenine nucleotide translocase 1 protects cardiomyocytes against TGFβ1-induced apoptosis by stabilization of the mitochondrial permeability transition pore

AIMS

Since adenine nucleotide translocase 1 (ANT1) overexpression improved cardiac function in rats with activated renin-angiotensin system (RAS) and angiotensin II is known to enhance transforming growth factor β (TGFβ) signaling in cardiomyocytes, we assumed that ANT1 might modulate the classical TGFβ/SMAD pathway. We therefore investigated whether the cardioprotective effect of ANT1 overexpression suppresses TGFβ(1)-induced apoptosis, whether mitochondrial permeability transition pore (MPTP) regulation is involved, and SMAD signaling pathway is affected.

METHODS AND RESULTS

Ventricular cardiomyocytes isolated from wild-type (WT) and ANT1 transgenic rats were treated with the apoptosis-inducing agent TGFβ(1) (1 ng/ml). TGFβ(1) treatment of WT cells enhanced the number of apoptotic cells by 31.8 ± 11.7% (p<0.01 vs. WT) measured by chromatin condensation. Apoptosis was blocked by 1μM cyclosporine A and by ANT1 overexpression. The protecting effect of ANT1 overexpression on TGFβ(1)-induced apoptosis was verified by reduced caspase 3/7 activity and increased Bcl-2 expression. In addition, TGFβ(1) decreased mitochondrial membrane potential as measured by JC-1 staining by 18.0 ± 3.7% in WT cardiomyocytes, but only by 7.2 ± 2.8% (p<0.05 vs. WT) in ANT1 cardiomyocytes. Cyclosporine A also attenuated the decline in mitochondrial membrane potential under TGFβ(1) in WT cardiomyocytes. Determination of MPTP opening by Calcein assay in isolated cardiomyocytes and calcium retention assay in isolated mitochondria revealed a reduced open probability of MPTP after ANT1 overexpression. In addition to the effects of ANT1 on MPTP opening we investigated if ANT1 may interfere with the classical TGFβ signaling pathway. Interestingly, ANT1-transgenic cardiomyocytes expressed less TGFβ receptor II than WT cells. However, SMAD2 phosphorylation was already enhanced without TGFβ(1) stimulation in these cells. Although no additional increase in SMAD2 phosphorylation was detectable after TGFβ(1) treatment, SMAD signaling was still responsive to TGFβ(1) indicated by an upregulation of SMAD7, a TGFβ(1) target protein.

CONCLUSION

Heart-specific overexpression of ANT1 leads to a reduced apoptotic response to TGFβ(1) by preservation of the mitochondrial membrane potential, resistance to MPTP opening and altered TGFβ signaling.

A critical tyrosine residue determines the uncoupling protein-like activity of the yeast mitochondrial oxaloacetate carrier

The mitochondrial Oac (oxaloacetate carrier) found in some fungi and plants catalyses the uptake of oxaloacetate, malonate and sulfate. Despite their sequence similarity, transport specificity varies considerably between Oacs. Indeed, whereas ScOac (Saccharomyces cerevisiae Oac) is a specific anion-proton symporter, the YlOac (Yarrowia lipolytica Oac) has the added ability to transport protons, behaving as a UCP (uncoupling protein). Significantly, we identified two amino acid changes at the matrix gate of YlOac and ScOac, tyrosine to phenylalanine and methionine to leucine. We studied the role of these amino acids by expressing both wild-type and specifically mutated Oacs in an Oac-null S. cerevisiae strain. No phenotype could be associated with the methionine to leucine substitution, whereas UCP-like activity was dependent on the presence of the tyrosine residue normally expressed in the YlOac, i.e. Tyr-ScOac mediated proton transport, whereas Phe-YlOac lost its protonophoric activity. These findings indicate that the UCP-like activity of YlOac is determined by the tyrosine residue at position 146.

Immune and stress response 'cross-talk' in the Drosophila Malpighian tubule

The success of insects is in large part due to their ability to survive environmental stress, including heat, cold, and dehydration. Insects are also exposed to infection, osmotic or oxidative stress, and to xenobiotics or toxins. The molecular mechanisms of stress sensing and response have been widely investigated in mammalian cell lines, and the area of stress research is now so vast to be beyond the scope of a single review article. However, the mechanisms by which stress inputs to the organism are sensed and integrated at the tissue and cellular level are less well understood. Increasingly, common molecular events between immune and other stress responses are observed in vivo; and much of this work stems of efforts in insect molecular science and physiology. We describe here the current knowledge in the area of immune and stress signalling and response at the level of the organism, tissue and cell, focussing on a key epithelial tissue in insects, the Malpighian tubule, and drawing together the known pathways that modulate responses to different stress insults. The tubules are critical for insect survival and are increasingly implicated in responses to multiple and distinct stress inputs. Importantly, as tubule function is central to survival, they are potentially key targets for insect control, which will be facilitated by increased understanding of the complexities of stress signalling in the organism.

The mechanistic basis for noncompetitive ibogaine inhibition of serotonin and dopamine transporters

Ibogaine, a hallucinogenic alkaloid proposed as a treatment for opiate withdrawal, has been shown to inhibit serotonin transporter (SERT) noncompetitively, in contrast to all other known inhibitors, which are competitive with substrate. Ibogaine binding to SERT increases accessibility in the permeation pathway connecting the substrate-binding site with the cytoplasm. Because of the structural similarity between ibogaine and serotonin, it had been suggested that ibogaine binds to the substrate site of SERT. The results presented here show that ibogaine binds to a distinct site, accessible from the cell exterior, to inhibit both serotonin transport and serotonin-induced ionic currents. Ibogaine noncompetitively inhibited transport by both SERT and the homologous dopamine transporter (DAT). Ibogaine blocked substrate-induced currents also in DAT and increased accessibility of the DAT cytoplasmic permeation pathway. When present on the cell exterior, ibogaine inhibited SERT substrate-induced currents, but not when it was introduced into the cytoplasm through the patch electrode. Similar to noncompetitive transport inhibition, the current block was not reversed by increasing substrate concentration. The kinetics of inhibitor binding and dissociation, as determined by their effect on SERT currents, indicated that ibogaine does not inhibit by forming a long-lived complex with SERT, but rather binds directly to the transporter in an inward-open conformation. A kinetic model for transport describing the noncompetitive action of ibogaine and the competitive action of cocaine accounts well for the results of the present study.

Plasma membrane potential oscillations in insulin secreting Ins-1 832/13 cells do not require glycolysis and are not initiated by fluctuations in mitochondrial bioenergetics

Oscillations in plasma membrane potential play a central role in glucose-induced insulin secretion from pancreatic β-cells and related insulinoma cell lines. We have employed a novel fluorescent plasma membrane potential (Δψ(p)) indicator in combination with indicators of cytoplasmic free Ca(2+) ([Ca(2+)](c)), mitochondrial membrane potential (Δψ(m)), matrix ATP concentration, and NAD(P)H fluorescence to investigate the role of mitochondria in the generation of plasma membrane potential oscillations in clonal INS-1 832/13 β-cells. Elevated glucose caused oscillations in plasma membrane potential and cytoplasmic free Ca(2+) concentration over the same concentration range required for insulin release, although considerable cell-to-cell heterogeneity was observed. Exogenous pyruvate was as effective as glucose in inducing oscillations, both in the presence and absence of 2.8 mM glucose. Increased glucose and pyruvate each produced a concentration-dependent mitochondrial hyperpolarization. The causal relationships between pairs of parameters (Δψ(p) and [Ca(2+)](c), Δψ(p) and NAD(P)H, matrix ATP and [Ca(2+)](c), and Δψ(m) and [Ca(2+)](c)) were investigated at single cell level. It is concluded that, in these β-cells, depolarizing oscillations in Δψ(p) are not initiated by mitochondrial bioenergetic changes. Instead, regardless of substrate, it appears that the mitochondria may simply be required to exceed a critical bioenergetic threshold to allow release of insulin. Once this threshold is exceeded, an autonomous Δψ(p) oscillatory mechanism is initiated.

Cytosolic calcium coordinates mitochondrial energy metabolism with presynaptic activity

Most neurons fire in bursts, imposing episodic energy demands, but how these demands are coordinated with oxidative phosphorylation is still unknown. Here, using fluorescence imaging techniques on presynaptic termini of Drosophila motor neurons (MNs), we show that mitochondrial matrix pH (pHm), inner membrane potential (Δψm), and NAD(P)H levels ([NAD(P)H]m) increase within seconds of nerve stimulation. The elevations of pHm, Δψm, and [NAD(P)H]m indicate an increased capacity for ATP production. Elevations in pHm were blocked by manipulations that blocked mitochondrial Ca2+ uptake, including replacement of extracellular Ca2+ with Sr2+ and application of either tetraphenylphosphonium chloride or KB-R7943, indicating that it is Ca2+ that stimulates presynaptic mitochondrial energy metabolism. To place this phenomenon within the context of endogenous neuronal activity, the firing rates of a number of individually identified MNs were determined during fictive locomotion. Surprisingly, although endogenous firing rates are significantly different, there was little difference in presynaptic cytosolic Ca2+ levels ([Ca2+]c) between MNs when each fires at its endogenous rate. The average [Ca2+]c level (329±11 nM) was slightly above the average Ca2+ affinity of the mitochondria (281±13 nM). In summary, we show that when MNs fire at endogenous rates, [Ca2+]c is driven into a range where mitochondria rapidly acquire Ca2+. As we also show that Ca2+ stimulates presynaptic mitochondrial energy metabolism, we conclude that [Ca2+]c levels play an integral role in coordinating mitochondrial energy metabolism with presynaptic activity in Drosophila MNs.

Uncoupling protein-4 (UCP4) increases ATP supply by interacting with mitochondrial Complex II in neuroblastoma cells

Mitochondrial uncoupling protein-4 (UCP4) protects against Complex I deficiency as induced by 1-methyl-4-phenylpyridinium (MPP(+)), but how UCP4 affects mitochondrial function is unclear. Here we investigated how UCP4 affects mitochondrial bioenergetics in SH-SY5Y cells. Cells stably overexpressing UCP4 exhibited higher oxygen consumption (10.1%, p<0.01), with 20% greater proton leak than vector controls (p<0.01). Increased ATP supply was observed in UCP4-overexpressing cells compared to controls (p<0.05). Although state 4 and state 3 respiration rates of UCP4-overexpressing and control cells were similar, Complex II activity in UCP4-overexpressing cells was 30% higher (p<0.05), associated with protein binding between UCP4 and Complex II, but not that of either Complex I or IV. Mitochondrial ADP consumption by succinate-induced respiration was 26% higher in UCP4-overexpressing cells, with 20% higher ADP:O ratio (p<0.05). ADP/ATP exchange rate was not altered by UCP4 overexpression, as shown by unchanged mitochondrial ADP uptake activity. UCP4 overexpression retained normal mitochondrial morphology in situ, with similar mitochondrial membrane potential compared to controls. Our findings elucidate how UCP4 overexpression increases ATP synthesis by specifically interacting with Complex II. This highlights a unique role of UCP4 as a potential regulatory target to modulate mitochondrial Complex II and ATP output in preserving existing neurons against energy crisis.

Yeast mitochondrial interactosome model: metabolon membrane proteins complex involved in the channeling of ADP/ATP

The existence of a mitochondrial interactosome (MI) has been currently well established in mammalian cells but the exact composition of this super-complex is not precisely known, and its organization seems to be different from that in yeast. One major difference is the absence of mitochondrial creatine kinase (MtCK) in yeast, unlike that described in the organization model of MI, especially in cardiac, skeletal muscle and brain cells. The aim of this review is to provide a detailed description of different partner proteins involved in the synergistic ADP/ATP transport across the mitochondrial membranes in the yeast Saccharomyces cerevisiae and to propose a new mitochondrial interactosome model. The ADP/ATP (Aacp) and inorganic phosphate (PiC) carriers as well as the VDAC (or mitochondrial porin) catalyze the import and export of ADP, ATP and Pi across the mitochondrial membranes. Aacp and PiC, which appear to be associated with the ATP synthase, consist of two nanomotors (F₀, F₁) under specific conditions and form ATP synthasome. Identification and characterization of such a complex were described for the first time by Pedersen and co-workers in 2003.

Brown adipose tissue mitochondria: modulation by GDP and fatty acids depends on the respiratory substrates

The UCP1 [first UCP (uncoupling protein)] that is found in the mitochondria of brown adipocytes [BAT (brown adipose tissue)] regulates the heat production, a process linked to non-shivering thermogenesis. The activity of UCP1 is modulated by GDP and fatty acids. In this report, we demonstrate that respiration and heat released by BAT mitochondria vary depending on the respiratory substrate utilized and the coupling state of the mitochondria. It has already been established that, in the presence of pyruvate/malate, BAT mitochondria are coupled by faf-BSA (fatty-acid-free BSA) and GDP, leading to an increase in ATP synthesis and mitochondrial membrane potential along with simultaneous decreases in both the rates of respiration and heat production. Oleate restores the uncoupled state, inhibiting ATP synthesis and increasing the rates of both respiration and heat production. We now show that in the presence of succinate: (i) the rates of uncoupled mitochondria respiration and heat production are five times slower than in the presence of pyruvate/malate; (ii) faf-BSA and GDP accelerate heat and respiration as a result and, in coupled mitochondria, these two rates are accelerated compared with pyruvate/malate; (iii) in spite of the differences in respiration and heat production noted with the two substrates, the membrane potential and the ATP synthesized were the same; and (iv) oleate promoted a decrease in heat production and respiration in coupled mitochondria, an effect different from that observed using pyruvate/malate. These effects are not related to the production of ROS (reactive oxygen species). We suggest that succinate could stimulate a new route to heat production in BAT mitochondria.

Substrate specificity of the two mitochondrial ornithine carriers can be swapped by single mutation in substrate binding site

Mitochondrial carriers are a large family of proteins that transport specific metabolites across the inner mitochondrial membrane. Sequence and structure analysis has indicated that these transporters have substrate binding sites in a similar location of the central cavity consisting of three major contact points. Here we have characterized mutations of the proposed substrate binding site in the human ornithine carriers ORC1 and ORC2 by carrying out transport assays with a set of different substrates. The different substrate specificities of the two isoforms, which share 87% identical amino acids, were essentially swapped by exchanging a single residue located at position 179 that is arginine in ORC1 and glutamine in ORC2. Altogether the substrate specificity changes demonstrate that Arg-179 and Glu-180 of contact point II bind the C(α) carboxylate and amino group of the substrates, respectively. Residue Glu-77 of contact point I most likely interacts with the terminal amino group of the substrate side chain. Furthermore, it is likely that all three contact points are involved in the substrate-induced conformational changes required for substrate translocation because Arg-179 is probably connected with Arg-275 of contact point III through Trp-224 by cation-π interactions. Mutations at position 179 also affected the turnover number of the ornithine carrier severely, implying that substrate binding to residue 179 is a rate-limiting step of the catalytic transport cycle. Given that Arg-179 is located in the vicinity of the matrix gate, it is concluded that it is a key residue in the opening of the carrier to the matrix side.

Phylogenetic Analysis of the Thylakoid ATP/ADP Carrier Reveals New Insights into Its Function Restricted to Green Plants

ATP is the common energy currency of cellular metabolism in all living organisms. Most of them synthesize ATP in the cytosol or on the mitochondrial inner membrane, whereas land plants, algae, and cyanobacteria also produce it on the thylakoid membrane during the light-dependent reactions of photosynthesis. From the site of synthesis, ATP is transported to the site of utilization via intracellular membrane transporters. One major type of ATP transporters is represented by the mitochondrial ADP/ATP carrier family. Here we review a recently characterized member, namely the thylakoid ATP/ADP carrier from Arabidopsis thaliana (AtTAAC). Thus far, no orthologs of this carrier have been characterized in other organisms, although similar sequences can be recognized in many sequenced genomes. Protein Sequence database searches and phylogenetic analyses indicate the absence of TAAC in cyanobacteria and its appearance early in the evolution of photosynthetic eukaryotes. The TAAC clade is composed of carriers found in land plants and some green algae, but no proteins from other photosynthetic taxa, such as red algae, brown algae, and diatoms. This implies that TAAC-like sequences arose only once before the divergence of green algae and land plants. Based on these findings, it is proposed that TAAC may have evolved in response to the need of a new activity in higher photosynthetic eukaryotes. This activity may provide the energy to drive reactions during biogenesis and turnover of photosynthetic complexes, which are heterogeneously distributed in a thylakoid membrane system composed of appressed and non-appressed regions.

The plant mitochondrial carrier family: functional and evolutionary aspects

Mitochondria play a key role in respiration and energy production and are involved in multiple eukaryotic but also in several plant specific metabolic pathways. Solute carriers in the inner mitochondrial membrane connect the internal metabolism with that of the surrounding cell. Because of their common basic structure, these transport proteins affiliate to the mitochondrial carrier family (MCF). Generally, MCF proteins consist of six membrane spanning helices, exhibit typical conserved domains and appear as homodimers in the native membrane. Although structurally related, MCF proteins catalyze the specific transport of various substrates, such as nucleotides, amino acids, dicarboxylates, cofactors, phosphate or H(+). Recent investigations identified MCF proteins also in several other cellular compartments and therefore their localization and physiological function is not only restricted to mitochondria. MCF proteins are a characteristic feature of eukaryotes and bacterial genomes lack corresponding sequences. Therefore, the evolutionary origin of MCF proteins is most likely associated with the establishment of mitochondria. It is not clear whether the host cell, the symbiont, or the chimerical organism invented the ancient MCF sequence. Here, we try to explain the establishment of different MCF proteins and focus on the characteristics of members from plants, in particular from Arabidopsis thaliana.

Introduction to mitochondrial oxidative phosphorylation

The basic mechanism of ATP synthesis in the mitochondria by oxidative phosphorylation (OxPhos) was revealed in the second half of the twentieth century. The OxPhos complexes I-V have been analyzed concerning their subunit composition, genes, and X-ray structures. This book presents new developments regarding the morphology, biogenesis, gene evolution, heat, and reactive oxygen species (ROS) generation in mitochondria, as well as the structure and supercomplex formation of OxPhos complexes. In addition, multiple mitochondrial diseases based on mutations of nuclear-encoded genes have been identified. Little is known, however, of the regulation of OxPhos according to the variable cellular demands of ATP. In particular, the functions of the supernumerary (nuclear-encoded) subunits of mitochondrial OxPhos complexes, which are mostly absent in bacteria, remain largely unknown, although the corresponding and conserved core subunits exhibit the same catalytic activity. Identification of regulatory pathways modulating OxPhos activity, by subunit isoform expression, by allosteric interaction with ATP/ADP, by reversible phosphorylation of protein subunits, or by supercomplex formation, will help to understand the role of mitochondria in the many degenerative diseases, mostly based on ROS formation in mitochondria and/or insufficient energy production.

Modulation of the mitochondrial permeability transition by cyclophilin D: moving closer to F(0)-F(1) ATP synthase?

Cyclophilin D was recently shown to mask an inhibitory site of the mitochondrial permeability transition pore (PTP) for phosphate, and to constitutively bind F(0)-F(1) ATP synthase resulting in the slowing of ATP synthesis and hydrolysis rates, thus regulating matrix adenine nucleotide levels. Here we review the striking similarities of the factors affecting the threshold for PTP induction, to those affecting binding of phosphate to formerly proposed sides on F(1)-ATPase affecting ATP hydrolytic activity, including critical arginine residues, matrix pH, [Mg(2+)], adenine nucleotides and proton motive force. Based on these similarities, we scrutinize the hypothesis that in depolarized mitochondria exhibiting reversal of F(0)-F(1) ATP synthase operation, the genetic ablation of cyclophilin D or its inhibition by cyclosporin A results in accelerated proton pumping by ATP hydrolysis, opposing a further decrease in membrane potential and promoting high matrix phosphate levels, both negatively affecting the probability of PTP opening.

Production of UCP1 a membrane protein from the inner mitochondrial membrane using the cell free expression system in the presence of a fluorinated surfactant

Structural studies of membrane protein are still challenging due to several severe bottlenecks, the first being the overproduction of well-folded proteins. Several expression systems are often explored in parallel to fulfil this task, or alternately prokaryotic analogues are considered. Although, mitochondrial carriers play key roles in several metabolic pathways, only the structure of the ADP/ATP carrier purified from bovine heart mitochondria was determined so far. More generally, characterisations at the molecular level are restricted to ADP/ATP carrier or the uncoupling protein UCP1, another member of the mitochondrial carrier family, which is abundant in brown adipose tissues. Indeed, mitochondrial carriers have no prokaryotic homologues and very few efficient expression systems were described so far for these proteins. We succeeded in producing UCP1 using a cell free expression system based on E. coli extracts, in quantities that are compatible with structural approaches. The protein was synthesised in the presence of a fluorinated surfactant, which maintains the protein in a soluble form. Further biochemical and biophysical analysis such as size exclusion chromatography, circular dichroism and thermal stability, of the purified protein showed that the protein is non-aggregated, monodisperse and well-folded.

Nitroalkenes confer acute cardioprotection via adenine nucleotide translocase 1

Electrophilic nitrated lipids (nitroalkenes) are emerging as an important class of protective cardiovascular signaling molecules. Although species such as nitro-linoleate (LNO(2)) and nitro-oleate can confer acute protection against cardiac ischemic injury, their mechanism of action is unclear. Mild uncoupling of mitochondria is known to be cardioprotective, and adenine nucleotide translocase 1 (ANT1) is a key mediator of mitochondrial uncoupling. ANT1 also contains redox-sensitive cysteines that may be targets for modification by nitroalkenes. Therefore, in this study we tested the hypothesis that nitroalkenes directly modify ANT1 and that nitroalkene-mediated cardioprotection requires ANT1. Using biotin-tagged LNO(2) infused into intact perfused hearts, we obtained mass spectrometric (MALDI-TOF-TOF) evidence for direct modification (nitroalkylation) of ANT1 on cysteine 57. Furthermore, in a cell model of ischemia-reperfusion injury, siRNA knockdown of ANT1 inhibited the cardioprotective effect of LNO(2). Although the molecular mechanism linking ANT1-Cys(57) nitroalkylation and uncoupling is not yet known, these data suggest that ANT1-mediated uncoupling may be a mechanism for nitroalkene-induced cardioprotection.

Nerolidol effects on mitochondrial and cellular energetics

In the present work, we evaluated the potential toxic effects of nerolidol, a sesquiterpenoid common in plants essential oils, both on mitochondrial and cellular energetics. Samples of enriched natural extracts of nerolidol (a racemic mixture of cis and trans isomers) were tested on rat liver mitochondria and a decrease in phosphorylative system was observed but not in the mitochondrial respiratory chain activity, which reflects a direct effect on F1-ATPase. Hence, respiratory control ratio was also decreased. Cellular ATP/ADP levels were significantly decreased in a concentration-dependent manner, possibly due to the direct effect of nerolidol on F(0)F(1)-ATPsynthase. Nerolidol stimulates respiratory activity probably due to an unspecific effect, since it does not show any protonophoric effect. Furthermore, we observed that mitochondrial permeability transition was delayed in the presence of nerolidol, possibly due to its antioxidant activity and because this compound decreases mitochondrial transmembrane electric potential. Our results also show that, in human hepatocellular liver carcinoma cell line (HepG2), nerolidol both induces cell death and arrests cell growth, probably related with the observed lower bioenergetic efficiency.

Mapping multiple potential ATP binding sites on the matrix side of the bovine ADP/ATP carrier by the combined use of MD simulation and docking

The mitochondrial adenosine diphosphate/adenosine triphosphate (ADP/ATP) carrier-AAC-was crystallized in complex with its specific inhibitor carboxyatractyloside (CATR). The protein consists of a six-transmembrane helix bundle that defines the nucleotide translocation pathway, which is closed towards the matrix side due to sharp kinks in the odd-numbered helices. In this paper, we describe the interaction between the matrix side of the AAC transporter and the ATP(4-) molecule using carrier structures obtained through classical molecular dynamics simulation (MD) and a protein-ligand docking procedure. Fifteen structures were extracted from a previously published MD trajectory through clustering analysis, and 50 docking runs were carried out for each carrier conformation, for a total of 750 runs ("MD docking"). The results were compared to those from 750 docking runs performed on the X-ray structure ("X docking"). The docking procedure indicated the presence of a single interaction site in the X-ray structure that was conserved in the structures extracted from the MD trajectory. MD docking showed the presence of a second binding site that was not found in the X docking. The interaction strategy between the AAC transporter and the ATP(4-) molecule was analyzed by investigating the composition and 3D arrangement of the interaction pockets, together with the orientations of the substrate inside them. A relationship between sequence repeats and the ATP(4-) binding sites in the AAC carrier structure is proposed.

Mass spectrometry-based quantification of myocardial protein adducts with acrolein in an in vivo model of oxidative stress

Acrolein (ACR) exposure leads to the formation of protein-ACR adducts. Protein modification by ACR has been associated with various chronic diseases including cardiovascular and neurodegenerative diseases. Here, we report an analytical strategy that enables the quantification of Michael-type protein adducts of ACR in mitochondrial proteome samples using liquid chromatography in combination with tandem mass spectrometry and selected ion monitoring (LC-MS/MS SRM) analysis. Our approach combines site-specific identification and relative quantification at the peptide level of protein-ACR adducts in relation to the unmodified protein thiol pool. Treatment of 3-month-old rats with CCl(4) , an established in vivo model of acute oxidative stress, resulted in significant increases in the ratios of distinct ACR-adducted peptides to the corresponding unmodified thiol-peptides obtained from proteins that were isolated from cardiac mitochondria. The mitochondrial proteins that were found adducted by ACR were malate dehydrogenase, NADH dehydrogenase [ubiquinone] flavoprotein 1, cytochrome c oxidase subunit VIb isoform 1, ATP synthase d chain, and ADP/ATP translocase 1. The findings indicate that protein modification by ACR has potential value as an index of mitochondrial oxidative stress.

Adenine nucleotide transport in plants: much more than a mitochondrial issue

Adenine nucleotides play a vital role in plant metabolism and physiology, essentially representing the major energy currency of the cell. Heterotrophic cells regenerate most of the ATP in mitochondria, whereas autotrophic cells also possess chloroplasts, representing a second powerhouse for ATP regeneration. Even though the synthesis of these nucleotides is restricted to a few locations, their use is nearly ubiquitous across the cell and thereby highly efficient systems are required to transport these molecules into and out of different compartments. Here, we discuss the location, biochemical characterization and evolution of corresponding transport systems in plants. We include recent scientific findings concerning organellar transporters from plants and algae and also focus on the physiological importance of adenine nucleotide exchange in these cells.

Identification and characterization of interactions between abscisic acid and mitochondrial adenine nucleotide translocators

ABA (abscisic acid) is a plant hormone involved in important processes including development and stress responses. Recent reports have identified a number of plant ABA receptors and transporters, highlighting novel mechanisms of ABA action. In the present paper we describe application of a chemical proteomics approach leading to the identification of mitochondrial ANTs (adenine nucleotide translocators) as ABA-interacting proteins. Initial in vitro studies confirmed inhibition of ANT-dependent ATP translocation by ABA. Further analysis demonstrated ANT-dependent uptake of ABA into both recombinant Arabidopsis thaliana ANT2-containing proteoliposomes and native isolated spinach mitochondria; the latter with a Km of 3.5 μM and a Vmax of 2.5 nmol/min per g of protein. ATP was found to inhibit ANT-dependent ABA translocation. Specificity profiles highlight the possibility of mechanistic differences in translocation of ABA and ATP. Finally, ABA was shown to stimulate ATPase activity in spinach mitochondrial extracts. ABA concentrations in plant cells are estimated to reach the low micromolar range during stress responses, supporting potential physiological relevance of these in vitro findings. Overall, the present in vitro work suggests the possibility of as yet uncharacterized mechanisms of ABA action in planta related to inhibition of mitochondrial ATP translocation and functional localization of ABA in the mitochondrial matrix.

Yeast ADP/ATP carrier isoform 2: conformational dynamics and role of the RRRMMM signature sequence methionines

The mitochondrial ADP/ATP carrier, or Ancp, is a member of the mitochondrial carrier family responsible for exchanging ADP and ATP across the mitochondrial inner membrane. ADP/ATP transport involves Ancp switching between two conformational states. These can be analyzed using specific inhibitors, carboxyatractyloside (CATR) and bongkrekic acid (BA). The high resolution three-dimensional structure of bovine Anc1p (bAnc1p), as a CATR-carrier complex, has been solved. However, because the structure of the BA-carrier complex has not yet been determined, the detailed mechanism of transport remains unknown. Recently, sample processing for hydrogen/deuterium exchange experiments coupled to mass spectrometry was improved, providing novel insights into bAnc1p conformational transitions due to inhibitor binding. In this work we performed both hydrogen/deuterium exchange-mass spectrometry experiments and genetic manipulations. Because these are very difficult to apply with bovine Anc1p, we used Saccharomyces cerevisiae Anc isoform 2 (ScAnc2p). Significant differences in solvent accessibility were observed throughout the amino acid sequence for ScAnc2p complexed to either CATR or BA. Interestingly, in detergent solution, the conformational dynamics of ScAnc2p were dissimilar to those of bAnc1p, in particular for the upper half of the cavity, toward the intermembrane space, and the m2 loop, which is thought to be easily accessible to the solvent from the matrix in bAnc1p. Our study then focused on the methionyl residues of the Ancp signature sequence, RRRMMM. All our results indicate that the methionine cluster is involved in the ADP/ATP transport mechanism and confirm that the Ancp cavity is a highly dynamic structure.

Studies of the role of tubulin beta II isotype in regulation of mitochondrial respiration in intracellular energetic units in cardiac cells

The aim of this study was to investigate the possible role of tubulin βII, a cytoskeletal protein, in regulation of mitochondrial oxidative phosphorylation and energy fluxes in heart cells. This isotype of tubulin is closely associated with mitochondria and co-expressed with mitochondrial creatine kinase (MtCK). It can be rapidly removed by mild proteolytic treatment of permeabilized cardiomyocytes in the absence of stimulatory effect of cytochrome c, that demonstrating the intactness of the outer mitochondrial membrane. Contrary to isolated mitochondria, in permeabilized cardiomyocytes (in situ mitochondria) the addition of pyruvate kinase (PK) and phosphoenolpyruvate (PEP) in the presence of creatine had no effect on the rate of respiration controlled by activated MtCK, showing limited permeability of voltage-dependent anion channel (VDAC) in mitochondrial outer membrane (MOM) for ADP regenerated by MtCK. Under normal conditions, this effect can be considered as one of the most sensitive tests of the intactness of cardiomyocytes and controlled permeability of MOM for adenine nucleotides. However, proteolytic treatment of permeabilized cardiomyocytes with trypsin, by removing mitochondrial βII tubulin, induces high sensitivity of MtCK-regulated respiration to PK-PEP, significantly changes its kinetics and the affinity to exogenous ADP. MtCK coupled to ATP synthasome and to VDAC controlled by tubulin βII provides functional compartmentation of ATP in mitochondria and energy channeling into cytoplasm via phosphotransfer network. Therefore, direct transfer of mitochondrially produced ATP to sites of its utilization is largely avoided under physiological conditions, but may occur in pathology when mitochondria are damaged. This article is part of a Special Issue entitled ''Local Signaling in Myocytes''.

Intracellular Energetic Units regulate metabolism in cardiac cells

This review describes developments in historical perspective as well as recent results of investigations of cellular mechanisms of regulation of energy fluxes and mitochondrial respiration by cardiac work - the metabolic aspect of the Frank-Starling law of the heart. A Systems Biology solution to this problem needs the integration of physiological and biochemical mechanisms that take into account intracellular interactions of mitochondria with other cellular systems, in particular with cytoskeleton components. Recent data show that different tubulin isotypes are involved in the regular arrangement exhibited by mitochondria and ATP-consuming systems into Intracellular Energetic Units (ICEUs). Beta II tubulin association with the mitochondrial outer membrane, when co-expressed with mitochondrial creatine kinase (MtCK) specifically limits the permeability of voltage-dependent anion channel for adenine nucleotides. In the MtCK reaction this interaction changes the regulatory kinetics of respiration through a decrease in the affinity for adenine nucleotides and an increase in the affinity for creatine. Metabolic Control Analysis of the coupled MtCK-ATP Synthasome in permeabilized cardiomyocytes showed a significant increase in flux control by steps involved in ADP recycling. Mathematical modeling of compartmentalized energy transfer represented by ICEUs shows that cyclic changes in local ADP, Pi, phosphocreatine and creatine concentrations during contraction cycle represent effective metabolic feedback signals when amplified in the coupled non-equilibrium MtCK-ATP Synthasome reactions in mitochondria. This mechanism explains the regulation of respiration on beat to beat basis during workload changes under conditions of metabolic stability. This article is part of a Special Issue entitled "Local Signaling in Myocytes."

The roles of phosphate and the phosphate carrier in the mitochondrial permeability transition pore

Phosphate activation of the mitochondrial permeability transition pore (MPTP) opening is well-documented and could involve the phosphate carrier (PiC) that we have proposed is the pore's cyclophilin-D binding component. However, others have reported that following CyP-D ablation Pi inhibits MPTP opening while cyclosporine-A (CsA) inhibits MPTP opening only when Pi is present. Here we demonstrate that Pi activates MPTP opening under all energised and de-energised conditions tested while CsA inhibits pore opening whether or not Pi is present. Using siRNA in HeLa cells we could reduce PiC expression by 65–80% but this inhibited neither mitochondrial calcium accumulation nor MPTP opening.

Identification of an Arabidopsis plasma membrane-located ATP transporter important for anther development

ATP acts as an extracellular signal molecule in plants. However, the nature of the mechanisms that export this compound into the apoplast are under debate. We identified the protein PM-ANT1 as a candidate transporter able to mediate ATP export. PM-ANT1 joins the mitochondrial carrier family, lacks an N-terminal amino acid extension required for organelle localization, and locates to the plasma membrane. Recombinant PM-ANT1 transports ATP, and the gene is substantially expressed in mature pollen grains. Artificial microRNA (amiRNA) mutants show reduced silique length and less seeds per silique but increased seed weight associated with unchanged pollen viability. Anthers from amiRNA mutants exhibited a normal early development, but stomium breakage is inhibited, leading to impaired anther dehiscence. This results in reduced self-pollination and thus decreased fertilization efficiency. amiRNA pollen grains showed increased intracellular ATP levels but decreased extracellular ATP levels. The latter effects are in line with transport properties of recombinant PM-ANT1, supporting in planta that functional PM-ANT1 resides in the plasma membrane and concur with the PM-ANT1 expression pattern. We assume that PM-ANT1 contributes to ATP export during pollen maturation. ATP export may serve as an extracellular signal required for anther dehiscence and is a novel factor critical for pollination and autogamy.

Functional expression of human adenine nucleotide translocase 4 in Saccharomyces cerevisiae

The adenine nucleotide translocase (ANT) mediates the exchange of ADP and ATP across the inner mitochondrial membrane. The human genome encodes multiple ANT isoforms that are expressed in a tissue-specific manner. Recently a novel germ cell-specific member of the ANT family, ANT4 (SLC25A31) was identified. Although it is known that targeted depletion of ANT4 in mice resulted in male infertility, the functional biochemical differences between ANT4 and other somatic ANT isoforms remain undetermined. To gain insight into ANT4, we expressed human ANT4 (hANT4) in yeast mitochondria. Unlike the somatic ANT proteins, expression of hANT4 failed to complement an AAC-deficient yeast strain for growth on media requiring mitochondrial respiration. Moreover, overexpression of hANT4 from a multi-copy plasmid interfered with optimal yeast growth. However, mutation of specific amino acids of hANT4 improved yeast mitochondrial expression and supported growth of the AAC-deficient yeast on non-fermentable carbon sources. The mutations affected amino acids predicted to interact with phospholipids, suggesting the importance of lipid interactions for function of this protein. Each mutant hANT4 and the somatic hANTs exhibited similar ADP/ATP exchange kinetics. These data define common and distinct biochemical characteristics of ANT4 in comparison to ANT1, 2 and 3 providing a basis for study of its unique adaptation to germ cells.

Adenine nucleotide transporters in organelles: novel genes and functions

In eukaryotes, cellular energy in the form of ATP is produced in the cytosol via glycolysis or in the mitochondria via oxidative phosphorylation and, in photosynthetic organisms, in the chloroplast via photophosphorylation. Transport of adenine nucleotides among cell compartments is essential and is performed mainly by members of the mitochondrial carrier family, among which the ADP/ATP carriers are the best known. This work reviews the carriers that transport adenine nucleotides into the organelles of eukaryotic cells together with their possible functions. We focus on novel mechanisms of adenine nucleotide transport, including mitochondrial carriers found in organelles such as peroxisomes, plastids, or endoplasmic reticulum and also mitochondrial carriers found in the mitochondrial remnants of many eukaryotic parasites of interest. The extensive repertoire of adenine nucleotide carriers highlights an amazing variety of new possible functions of adenine nucleotide transport across eukaryotic organelles.

Evolution, structure and function of mitochondrial carriers: a review with new insights

The mitochondrial carriers (MC) constitute a large family (MCF) of inner membrane transporters displaying different substrate specificities, patterns of gene expression and even non-mitochondrial organelle localization. In Arabidopsis thaliana 58 genes encode these six trans-membrane domain proteins. The number in other sequenced plant genomes varies from 37 to 125, thus being larger than that of Saccharomyces cerevisiae and comparable with that of Homo sapiens. In addition to displaying highly similar secondary structures, the proteins of the MCF can be subdivided into subfamilies on the basis of substrate specificity and the presence of specific symmetry-related amino acid triplets. We assessed the predictive power of these triplets by comparing predictions with experimentally determined data for Arabidopsis MCs, and applied these predictions to the not yet functionally characterized mitochondrial carriers of the grass, Brachypodium distachyon, and the alga, Ostreococcus lucimarinus. We additionally studied evolutionary aspects of the plant MCF by comparing sequence data of the Arabidopsis MCF with those of Saccharomyces cerevisiae and Homo sapiens, then with those of Brachypodium distachyon and Ostreococcus lucimarinus, employing intra- and inter-genome comparisons. Finally, we discussed the importance of the approaches of global gene expression analysis and in vivo characterizations in order to address the relevance of these vital carrier proteins.

The role of transporters in supplying energy to plant plastids

The energy status of plant cells strongly depends on the energy metabolism in chloroplasts and mitochondria, which are capable of generating ATP either by photosynthetic or oxidative phosphorylation, respectively. Another energy-rich metabolite inside plastids is the glycolytic intermediate phosphoenolpyruvate (PEP). However, chloroplasts and most non-green plastids lack the ability to generate PEP via a complete glycolytic pathway. Hence, PEP import mediated by the plastidic PEP/phosphate translocator or PEP provided by the plastidic enolase are vital for plant growth and development. In contrast to chloroplasts, metabolism in non-green plastids (amyloplasts) of starch-storing tissues strongly depends on both the import of ATP mediated by the plastidic nucleotide transporter NTT and of carbon (glucose 6-phosphate, Glc6P) mediated by the plastidic Glc6P/phosphate translocator (GPT). Both transporters have been shown to co-limit starch biosynthesis in potato plants. In addition, non-photosynthetic plastids as well as chloroplasts during the night rely on the import of energy in the form of ATP via the NTT. During energy starvation such as prolonged darkness, chloroplasts strongly depend on the supply of ATP which can be provided by lipid respiration, a process involving chloroplasts, peroxisomes, and mitochondria and the transport of intermediates, i.e. fatty acids, ATP, citrate, and oxaloacetate across their membranes. The role of transporters involved in the provision of energy-rich metabolites and in pathways supplying plastids with metabolic energy is summarized here.

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.

Effects of a high-fat diet on energy metabolism and ROS production in rat liver

BACKGROUND & AIMS

A high-fat diet affects liver metabolism, leading to steatosis, a complex disorder related to insulin resistance and mitochondrial alterations. Steatosis is still poorly understood since diverse effects have been reported, depending on the different experimental models used.

METHODS

We hereby report the effects of an 8 week high-fat diet on liver energy metabolism in a rat model, investigated in both isolated mitochondria and hepatocytes.

RESULTS

Liver mass was unchanged but lipid content and composition were markedly affected. State-3 mitochondrial oxidative phosphorylation was inhibited, contrasting with unaffected cytochrome content. Oxidative phosphorylation stoichiometry was unaffected, as were ATPase and adenine nucleotide translocator proteins and mRNAs. Mitochondrial acylcarnitine-related H(2)O(2) production was substantially higher and the mitochondrial quinone pool was smaller and more reduced. Cellular consequences of these mitochondrial alterations were investigated in perifused, freshly isolated hepatocytes. Ketogenesis and fatty acid-dependent respiration were lower, indicating a lower β-oxidation rate contrasting with higher RNA contents of CD36, FABP, CPT-1, and AcylCoA dehydrogenases. Concomitantly, the cellular redox state was more reduced in the mitochondrial matrix but more oxidized in the cytosol: these opposing changes are in agreement with a significantly higher in situ mitochondrial proton motive force.

CONCLUSIONS

A high-fat diet results in both a decrease in mitochondrial quinone pool and a profound modification in mitochondrial lipid composition. These changes appear to play a key role in the resulting inhibition of fatty acid oxidation and of mitochondrial oxidative-phosphorylation associated with an increased mitochondrial ROS production. Mitochondrial quinone pool could have prospects as a crucial event, potentially leading to interesting therapeutic perspectives.

Molecular recognition of ADP over ATP in aqueous solution by a polyammonium receptor containing a pyrimidine residue

The synergistic action of the different binding groups of the polyfunctional HL receptor leads to the recognition of ADP over ATP in water, with a selectivity coefficient of up to 116, a phenomenon which is unprecedented in the context of synthetic receptors in water. The recognition is mostly due to the good matching between H(3)L(2+) and the protonated forms of ADP.

A distinct sequence in the adenine nucleotide translocase from Artemia franciscana embryos is associated with insensitivity to bongkrekate and atypical effects of adenine nucleotides on Ca2+ uptake and sequestration

Mitochondria isolated from embryos of the crustacean Artemia franciscana lack the Ca(2+)-induced permeability transition pore. Although the composition of the pore described in mammalian mitochondria is unknown, the impacts of several effectors of the adenine nucleotide translocase (ANT) on pore opening are firmly established. Notably, ADP, ATP and bongkrekate delay, whereas carboxyatractyloside hastens, Ca(2+)-induced pore opening. Here, we report that adenine nucleotides decreased, whereas carboxyatractyloside increased, Ca(2+) uptake capacity in mitochondria isolated from Artemia embryos. Bongkrekate had no effect on either Ca(2+) uptake or ADP-ATP exchange rate. Transmission electron microscopy imaging of Ca(2+)-loaded Artemia mitochondria showed needle-like formations of electron-dense material in the absence of adenine nucleotides, and dot-like formations in the presence of adenine nucleotides or Mg(2+). Energy-filtered transmission electron microscopy showed the material to be rich in calcium and phosphorus. Sequencing of the Artemia mRNA coding for ANT revealed that it transcribes a protein with a stretch of amino acids in the 198-225 region with 48-56% similarity to those from other species, including the deletion of three amino acids in positions 211, 212 and 219. Mitochondria isolated from the liver of Xenopus laevis, in which the ANT shows similarity to that in Artemia except for the 198-225 amino acid region, demonstrated a Ca(2+)-induced bongkrekate-sensitive permeability transition pore, allowing the suggestion that this region of ANT may contain the binding site for bongkrekate.

Isolation and functional assessment of mitochondria from small amounts of mouse brain tissue

Recent discoveries have brought mitochondria functions in focus of the neuroscience research community and greatly stimulated the demand for approaches to study mitochondria dysfunction in neurodegenerative diseases. Many mouse disease models have been generated, but studying mitochondria isolated from individual mouse brain regions is a challenge because of small amount of the available brain tissue. Conventional techniques for isolation and purification of mitochondria from mouse brain subregions, such as ventral midbrain, hippocampus, or striatum, require pooling brain tissue from six to nine animals for a single mitochondrial preparation. Working with pooled tissue significantly decreases the quality of data because of the time required to dissect several brains. It also greatly increases the labor intensity and the cost of experiments as several animals are required per single data point. We describe a method for isolation of brain mitochondria from mouse striata or other 7-12 mg brain samples. The method utilizes a refrigerated table-top microtube centrifuge, and produces research grade quality mitochondria in amounts sufficient for performing multiple enzymatic and functional assays, thereby eliminating the necessity for pooling mouse brain tissue. We also include a method of measuring ADP-ATP exchange rate as a function of mitochondrial membrane potential (ΔΨm) in small amounts of isolated mitochondria, adapted to a plate reader format.

Mitochondrial metabolite transport

The flux of a variety of metabolites, nucleotides and coenzymes across the inner membrane of mitochondria is catalysed by a nuclear-coded superfamily of secondary transport proteins called MCs (mitochondrial carriers). The importance of MCs is demonstrated by their wide distribution in all eukaryotes, their role in numerous metabolic pathways and cell functions, and the identification of several diseases caused by alterations of their genes. MCs can easily be recognized in databases thanks to their striking sequence features. Until now, 22 MC subfamilies, which are well conserved throughout evolution, have been functionally characterized, mainly by transport assays upon heterologous gene expression, purification and reconstitution into liposomes. Given the significant sequence conservation, it is thought that all MCs use the same basic transport mechanism, although they exhibit different modes of transport and driving forces and their substrates vary in nature and size. Based on substrate specificity, sequence conservation and carrier homology models, progress has recently been made in understanding the transport mechanism of MCs by new insights concerning the existence of a substrate-binding site in the carrier cavity, of cytosolic and matrix gates and conserved proline and glycine residues in each of the six transmembrane alpha-helices. These structural properties are believed to play an important role in the conformational changes required for substrate translocation.

Mitochondrial free [Ca2+] increases during ATP/ADP antiport and ADP phosphorylation: exploration of mechanisms

ADP influx and ADP phosphorylation may alter mitochondrial free [Ca2+] ([Ca2+](m)) and consequently mitochondrial bioenergetics by several postulated mechanisms. We tested how [Ca2+](m) is affected by H2PO4(-) (P(i)), Mg2+, calcium uniporter activity, matrix volume changes, and the bioenergetic state. We measured [Ca2+](m), membrane potential, redox state, matrix volume, pH(m), and O2 consumption in guinea pig heart mitochondria with or without ruthenium red, carboxyatractyloside, or oligomycin, and at several levels of Mg2+ and P(i). Energized mitochondria showed a dose-dependent increase in [Ca2+](m) after adding CaCl2 equivalent to 20, 114, and 485 nM extramatrix free [Ca2+] ([Ca2+](e)); this uptake was attenuated at higher buffer Mg2+. Adding ADP transiently increased [Ca2+](m) up to twofold. The ADP effect on increasing [Ca2+](m) could be partially attributed to matrix contraction, but was little affected by ruthenium red or changes in Mg2+ or P(i). Oligomycin largely reduced the increase in [Ca2+](m) by ADP compared to control, and [Ca2+](m) did not return to baseline. Carboxyatractyloside prevented the ADP-induced [Ca2+](m) increase. Adding CaCl2 had no effect on bioenergetics, except for a small increase in state 2 and state 4 respiration at 485 nM [Ca2+](e). These data suggest that matrix ADP influx and subsequent phosphorylation increase [Ca2+](m) largely due to the interaction of matrix Ca2+ with ATP, ADP, P(i), and cation buffering proteins in the matrix.

Adenine nucleotide translocase family: four isoforms for apoptosis modulation in cancer

Mitochondria have important functions in mammalian cells as the energy powerhouse and integrators of the mitochondrial pathway of apoptosis. The adenine nucleotide translocase (ANT) is a family of proteins involved in cell death pathways that perform distinctly opposite functions to regulate cell fate decisions. On the one hand, ANT catalyzes the adenosine triphosphate export from the mitochondrial matrix to the intermembrane space with the concomitant import of ADP from the intermembrane space to the matrix. On the other hand, during periods of stress, ANT could function as a lethal pore and trigger the process of mitochondrial membrane permeabilization, which leads irreversibly to cell death. In human, ANT is encoded by four homologous genes, whose expression is not only tissue specific, but also varies according to the pathophysiological state of the cell. Recent evidence revealed a differential role of the ANT isoforms in apoptosis and a deregulation of their expression in cancer. In this review, we introduce the current knowledge of ANT in apoptosis and cancer cells and propose a novel classification of ANT isoforms.