Mitochondrial Adenine Nucleotide Translocase Research Articles

Adenine nucleotide translocase 2 (Ant2) is required for individualization of spermatogenesis of Drosophila melanogaster.

Successful completion of spermatogenesis is crucial for the perpetuation of the species. In Drosophila, spermatid individualization, a process involving changes in mitochondrial structure and function is critical to produce functional mature sperm. Ant2, encoding a mitochondrial adenine nucleotide translocase, is highly expressed in male testes and plays a role in energy metabolism in the mitochondria. However, its molecular function remains unclear. Here, we identified an important role of Ant2 in spermatid individualization. In Ant2 knockdown testes, spermatid individualization complexes composed of F-actin cones exhibited a diffuse distribution, and mature sperms were absent in the seminal vesicle, thus leading to male sterility. The most striking effects in Ant2-knockdown spermatids were decrease in tubulin polyglycylation and disruption of proper mitochondria derivatives function. Excessive apoptotic cells were also observed in Ant2-knockdown testes. To further investigate the phenotype of Ant2 knockdown in testes at the molecular level, complementary transcriptome and proteome analyses were performed. At the mRNA level, 868 differentially expressed genes were identified, of which 229 genes were upregulated and 639 were downregulated induced via Ant2 knockdown. iTRAQ-labeling proteome analysis revealed 350 differentially expressed proteins, of which 117 proteins were upregulated and 233 were downregulated. The expression of glutathione transferase (GstD5, GstE5, GstE8, and GstD3), proteins involved in reproduction were significantly regulated at both the mRNA and protein levels. These results indicate that Ant2 is crucial for spermatid maturation by affecting mitochondrial morphogenesis.

FA Sliding as the Mechanism for the ANT1-Mediated Fatty Acid Anion Transport in Lipid Bilayers.

Mitochondrial adenine nucleotide translocase (ANT) exchanges ADP for ATP to maintain energy production in the cell. Its protonophoric function in the presence of long-chain fatty acids (FA) is also recognized. Our previous results imply that proton/FA transport can be best described with the FA cycling model, in which protonated FA transports the proton to the mitochondrial matrix. The mechanism by which ANT1 transports FA anions back to the intermembrane space remains unclear. Using a combined approach involving measurements of the current through the planar lipid bilayers reconstituted with ANT1, site-directed mutagenesis and molecular dynamics simulations, we show that the FA anion is first attracted by positively charged arginines or lysines on the matrix side of ANT1 before moving along the positively charged protein-lipid interface and binding to R79, where it is protonated. We show that R79 is also critical for the competitive binding of ANT1 substrates (ADP and ATP) and inhibitors (carboxyatractyloside and bongkrekic acid). The binding sites are well conserved in mitochondrial SLC25 members, suggesting a general mechanism for transporting FA anions across the inner mitochondrial membrane.

Inhibition of mitochondrial translocase SLC25A5 and histone deacetylation is an effective combination therapy in neuroblastoma.

The mitochondrion is a gatekeeper of apoptotic processes, and mediates drug resistance to several chemotherapy agents used to treat cancer. Neuroblastoma is a common solid cancer in young children with poor clinical outcomes following conventional chemotherapy. We sought druggable mitochondrial protein targets in neuroblastoma cells. Among mitochondria-associated gene targets, we found that high expression of the mitochondrial adenine nucleotide translocase 2 (SLC25A5/ANT2), was a strong predictor of poor neuroblastoma patient prognosis and contributed to a more malignant phenotype in pre-clinical models. Inhibiting this transporter with PENAO reduced cell viability in a panel of neuroblastoma cell lines in a TP53-status-dependant manner. We identified the histone deacetylase inhibitor, suberanilohydroxamic acid (SAHA), as the most effective drug in clinical use against mutant TP53 neuroblastoma cells. SAHA and PENAO synergistically reduced cell viability, and induced apoptosis, in neuroblastoma cells independent of TP53-status. The SAHA and PENAO drug combination significantly delayed tumour progression in pre-clinical neuroblastoma mouse models, suggesting that these clinically advanced inhibitors may be effective in treating the disease.

Abstract 11029: Bioenergetic Independent Stress Signaling in the Heart Due to Mitochondrial Protein Import Stress

Mitochondrial diseases are some of the most common genetically inherited disorders, yet the prognosis for affected patients has not drastically improved in recent years. Most therapies are supportive as the underlying biochemical derangements in these diseases are incompletely understood. Mitochondrial functions rely on the import of nuclear-encoded proteins, and it has recently been appreciated that defective import of mitochondrial proteins can result in the cytosolic accumulation of these preproteins. The accumulation of these preproteins results in cellular toxicity independently of any changes to the mitochondrial bioenergetic function, raising the possibility that mitochondrial protein import stress may be a yet uncharacterized pathway that links mitochondrial dysfunction with pathology. Here, we model this cytosolic mitochondrial precursor overaccumulation stress (mPOS) in mice through overexpression of the inner mitochondrial membrane protein Adenine Nucleotide Translocase 1 (ANT1). The goal of this study is to determine whether ANT1 can induce mPOS and tissue remodeling prior to the onset of cardiac bioenergetic defects. Using isolated cardiac mitochondria from these mice, we detected a moderate decrease in the levels of Complex I, II ,and IV. By measuring oxygen consumption in these mitochondria from 2-month-old animals, we found only a mild reduction in complex II based state 3 and 4 respirations. No change in complex I-based state 3 and 4 respirations was observed. Using RNAseq and western blot analysis, we find the robust activation of the integrated stress response (ISR). Transcriptomic data further demonstrates a strong upregulation of transcripts involved with 1-carbon metabolism, the amino acid starvation response, and proteolysis. Using echocardiographic evaluation of heart function on anesthetized mice at 6 months old, we find no significant alteration in systolic function. Together these data support the idea that mPOS may be a potent trigger of the ISR in a bioenergetics-independent manner. Whether activation of the ISR can alter cardiac function later in life is being evaluated.

The mechanism of fatty acid anion transport across the inner mitochondrial membrane by the adenine nucleotide translocase

The flip-flop of phospholipids and fatty acids across cellular membranes is mediated by lipid flippases and scramblases. It is described by the credit-card model in which the lipid head is transported through a membrane-spanning hydrophilic groove, while the lipid tail remains in the membrane1. We have previously shown that the mitochondrial adenine nucleotide translocase (ANT) transports fatty acid anions (FA-) according to the fatty acid cycling model2. However, the absence of such a groove indicates a substantially different FA- transport mechanism, which was not resolved until now.

Loss of Mitochondrial Adenine Nucleotide Translocase (ANT1) Regulates Cellular Senescence and Protects against Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is a destructive lung disease resulting in alveolar tissue loss, airway remodeling and inflammation with cigarette smoke (CS) being a major causative factor. Mitochondrial dysfunction can lead to changes in cell fate implicated in COPD. Cellular senescence, a state of cell cycle arrest, results in abnormal responses in lung epithelial cells. An important protein in mitochondrial metabolism and cell fate is adenine nucleotide translocase (ANT), a mitochondrial ATP transporter. Our studies suggest that levels of ANT are altered in COPD. We hypothesize that loss of ANTs results in increased cellular senescence thus altering COPD pathogenesis and airway dysfunction. Methods Utilizing siRNA knockdown of SLC25A4 (ANT1) and SLC25A5 (ANT2) in human bronchial epithelial cells, Beas-2bs, we assessed cellular senescence (p53, p21, p16) via western blot, qt-PCR and B-galactocidase analysis. Cells were also exposed to CS extract. Wildtype (WT) and ant1-null mice were exposed to CS for 2 and 6mo or air control with lung histology and morphometry analysis. We performed single cell RNA sequencing on lungs from 2mo air and CS-exposed mice. Single-cell library preparation was performed using 10X Genomics 5prime v1 reagents to analyze 5000 cells/mouse (n=3 mice/group). Alignment was performed using Cell Ranger and standard workflow analysis using R package Seurat v3.2. Cell clusters were generated using unsupervised UMAP. Pathway analysis on differential gene expression lists by cell type was performed using Ingenuity Pathway Analysis. Results Knockdown of ANT1 or ANT2 in Beas-2b cells results in an increase in cell senescence proteins p16, p21, p53 and beta-galactosidase staining. Surprisingly, ant1-null mice exposed to 6months of CS exposure are protected from alveolar destruction (fig.1), airway remodeling and chronic inflammation with fewer macrophages. Single cell RNA sequencing of WT lung cells demonstrates that CS (2 mo) causes a significant increase in genes associated with senescence in alveolar type 1 epithelial cells (AT1, p-value 1.0x10-6) with dysregulation of mitochondrial-related gene pathways when compared to air controls: mitochondrial dysfunction, upregulation of Eif2 signaling and NRF2-mediated oxidative stress, and downregulation in sirtuin signaling. Alveolar type 2 cells (AT2) from WT-smoked mice show downregulation of Eif2 signaling, unfolded protein response, NRF2-mediated oxidative stress, and sirtuin signaling. Expression of slc25a4 (ant1) is upregulated in AT2 cells in WT-smoked mice (p-value 1.7x10-17). Mitochondrial and cell fate pathways were significantly altered in AT1 and AT2 cells from WT-smoked mice but not from ant1-null mice, suggesting a key role in lung epithelial responses in COPD. Conclusions ANTs alter cell senescence in alveolar epithelial cells. An In vivo mouse model shows that ant1 is upregulated in AT2 cells after CS and loss of ant1 is protective against CS lung injury. Ant1 regulates epithelial cell fate, mitochondrial function and CS-related cell injury, providing a potential therapeutic avenue for COPD.

193 - Antioxidant activity of calcium-independent phospholipase A2γ in brain mitochondria

Mitochondrial calcium-independent phospholipase A2γ (iPLA2γ) belongs to a family of enzymes that participate in cellular signaling by simultaneously producing free fatty acids and lysophospholipids. We have shown previously that the activity of iPLA2γ is increased following the addition of oxidants, such as tert-butyl hydroperoxide (tBHP) or H2O2. We have also shown that iPLA2γ-liberated fatty acids induce H+ transport mediated by mitochondrial uncoupling proteins and mitochondrial adenine nucleotide translocase (ANT) in heart, lung and spleen mitochondria, leading to a decrease in the mitochondrial protonmotive force and subsequent decrease in mitochondrial superoxide production [1,2]. Here we tested the hypothesis that iPLA2γ serves similar antioxidant function in brain. Using brain mitochondria isolated from wild-type (WT) mice, we found that low micromolar concentrations of tBHP induced increase in respiration and a parallel decrease in mitochondrial superoxide and H2O2 production. These effects of tBHP were fully prevented by bovine serum albumin and inhibited by R-bromoenol lactone, a selective inhibitor of iPLA2γ, and were absent in brain mitochondria isolated from iPLA2γ-KO mice. Moreover, the tBHP-induced effects were fully inhibited by carboxyatractyloside, indicating the participation of ANT. In addition, we observed decrease in mRNA levels of selected antioxidant enzymes and increase in lipid peroxidation products in brain tissues homogenates of iPLA2γ-KO mice compared to the WT controls. Our results support the hypothesis of an antioxidant role of iPLA2γ in brain, consistent with iPLA2γ-dependent, ANT-mediated attenuation of mitochondrial superoxide production. Supported by LTAUSA17174

471 - Modulation of Mitochondrial Adenine Nucleotide Translocase (ANT) regulation with ageing

471 - Modulation of Mitochondrial Adenine Nucleotide Translocase (ANT) regulation with ageing

Bongkrekic Acid-a Review of a Lesser-Known Mitochondrial Toxin.

Bongkrekic acid (BA) has a unique mechanism of toxicity among the mitochondrial toxins: it inhibits adenine nucleotide translocase (ANT) rather than the electron transport chain. Bongkrekic acid is produced by the bacterium Burkholderia gladioli pathovar cocovenenans (B. cocovenenans) which has been implicated in outbreaks of food-borne illness involving coconut- and corn-based products in Indonesia and China. Our objective was to summarize what is known about the epidemiology, exposure sources, toxicokinetics, pathophysiology, clinical presentation, and diagnosis and treatment of human BA poisoning. We searched MEDLINE (1946 to present), EMBASE (1947 to present), SCOPUS, The Indonesia Publication Index ( http://id.portalgaruda.org/ ), ToxNet, book chapters, Google searches, Pro-MED alerts, and references from previously published journal articles. We identified a total of 109 references which were reviewed. Of those, 29 (26%) had relevant information and were included. Bongkrekic acid is a heat-stable, highly unsaturated tricarboxylic fatty acid with a molecular weight of 486kDa. Outbreaks have been reported from Indonesia, China, and more recently in Mozambique. Very little is known about the toxicokinetics of BA. Bongkrekic acid produces its toxic effects by inhibiting mitochondrial (ANT). ANT can also alter cellular apoptosis. Signs and symptoms in humans are similar to the clinical findings from other mitochondrial poisons, but they vary in severity and time course. Management of patients is symptomatic and supportive. Bongkrekic acid is a mitochondrial ANT toxin and is reported primarily in outbreaks of food-borne poisoning involving coconut and corn. It should be considered in outbreaks of food-borne illness when signs and symptoms manifest involving the liver, brain, and kidneys and when coconut- or corn-based foods are implicated.

Modulation of mitochondrial adenine nucleotide translocase (ANT) regulation with ageing

Modulation of mitochondrial adenine nucleotide translocase (ANT) regulation with ageing

ANT1-mediated fatty acid-induced uncoupling as a target for improving myocellular insulin sensitivity.

Aims/hypothesisDissipating energy via mitochondrial uncoupling has been suggested to contribute to enhanced insulin sensitivity. We hypothesised that skeletal muscle mitochondria of endurance-trained athletes have increased sensitivity for fatty acid (FA)-induced uncoupling, which is driven by the mitochondrial protein adenine nucleotide translocase 1 (ANT1).MethodsCapacity for FA-induced uncoupling was measured in endurance-trained male athletes (T) and sedentary young men (UT) in an observational study and also in isolated skeletal muscle mitochondria from Zucker diabetic fatty (ZDF) rats and C2C12 myotubes following small interfering RNA (siRNA)-mediated gene silencing of ANT1. Thus, fuelled by glutamate/succinate (fibres) or pyruvate (mitochondria and myotubes) and in the presence of oligomycin to block ATP synthesis, increasing levels of oleate (fibres) or palmitate (mitochondria and myotubes) were automatically titrated while respiration was monitored. Insulin sensitivity was measured by hyperinsulinaemic–euglycaemic clamp in humans and via insulin-stimulated glucose uptake in myotubes.ResultsSkeletal muscle from the T group displayed increased sensitivity to FA-induced uncoupling (p = 0.011) compared with muscle from the UT group, and this was associated with elevated insulin sensitivity (p = 0.034). ANT1 expression was increased in T (p = 0.013). Mitochondria from ZDF rats displayed decreased sensitivity for FA-induced uncoupling (p = 0.008). This difference disappeared in the presence of the adenine nucleotide translocator inhibitor carboxyatractyloside. Partial knockdown of ANT1 in C2C12 myotubes decreased sensitivity to the FA-induced uncoupling (p = 0.008) and insulin-stimulated glucose uptake (p = 0.025) compared with controls.Conclusions/interpretationIncreased sensitivity to FA-induced uncoupling is associated with enhanced insulin sensitivity and is affected by ANT1 activity in skeletal muscle. FA-induced mitochondrial uncoupling may help to preserve insulin sensitivity in the face of a high supply of FAs.Trial registrationwww.trialregister.nl NTR2002

Changes in the expression of the human adenine nucleotide translocase isoforms condition cellular metabolic/proliferative status.

Human cells express four mitochondrial adenine nucleotide translocase (hANT) isoforms that are tissue-specific and developmentally regulated. hANT1 is mainly expressed in terminally differentiated muscle cells; hANT2 is growth-regulated and is upregulated in highly glycolytic and proliferative cells; and hANT3 is considered to be ubiquitous and non-specifically regulated. Here, we studied how the expression of hANT isoforms is regulated by proliferation and in response to metabolic stimuli, and examined the metabolic consequences of their silencing and overexpression. In HeLa and HepG2 cells, expression of hANT3 was upregulated by shifting metabolism towards oxidation or by slowed growth associated with contact inhibition or growth-factor deprivation, indicating that hANT3 expression is highly regulated. Under these conditions, changes in hANT2 mRNA expression were not observed in either HeLa or HepG2 cells, whereas in SGBS preadipocytes (which, unlike HeLa and HepG2 cells, are growth-arrest-sensitive cells), hANT2 mRNA levels decreased. Additionally, overexpression of hANT2 promoted cell growth and glycolysis, whereas silencing of hANT3 decreased cellular ATP levels, limited cell growth and induced a stress-like response. Thus, cancer cells require both hANT2 and hANT3, depending on their proliferation status: hANT2 when proliferation rates are high, and hANT3 when proliferation slows.

Two Adenine Nucleotide Translocase Paralogues Involved in Cell Proliferation and Spermatogenesis in the Silkworm Bombyx mori

Mitochondrial adenine nucleotide translocase (ANT) specifically acts in ADP/ATP exchange through the mitochondrial inner membrane. This transporter protein thereby plays a significant role in energy metabolism in eukaryotic cells. Most mammals have four paralogous ANT genes (ANT1-4) and utilize these paralogues in different types of cells. The fourth paralogue of ANT (ANT4) is present only in mammals and reptiles and is exclusively expressed in testicular germ cells where it is required for meiotic progression in the spermatocytes. Here, we report that silkworms harbor two ANT paralogues, the homeostatic paralogue (BmANTI1) and the testis-specific paralogue (BmANTI2). The BmANTI2 protein has an N-terminal extension in which the positions of lysine residues in the amino acid sequence are distributed as in human ANT4. An expression analysis showed that BmANTI2 transcripts were restricted to the testis, suggesting the protein has a role in the progression of spermatogenesis. By contrast, BmANTI1 was expressed in all tissues tested, suggesting it has an important role in homeostasis. We also observed that cultured silkworm cells required BmANTI1 for proliferation. The ANTI1 protein of the lepidopteran Plutella xylostella (PxANTI1), but not those of other insect species (or PxANTI2), restored cell proliferation in BmANTI1-knockdown cells suggesting that ANTI1 has similar energy metabolism functions across the Lepidoptera. Our results suggest that BmANTI2 is evolutionarily divergent from BmANTI1 and has developed a specific role in spermatogenesis similar to that of mammalian ANT4.

184 - Dehydrosilybin and Quercetin Interact with the Mitochondrial Adenine Nucleotide Translocase to Induce a Weak Acid - Mediated H+ Conductance

184 - Dehydrosilybin and Quercetin Interact with the Mitochondrial Adenine Nucleotide Translocase to Induce a Weak Acid - Mediated H+ Conductance

Designed benzothiadiazole fluorophores for selective mitochondrial imaging and dynamics.

A series of new rationale designed 2,1,3-benzothiadiazole (BTD) fluorescent derivatives has been synthesized and applied for cellular selective staining of cancer cells in cell-imaging experiments. Four new synthesized BTD derivatives showed only poor or reasonable cellular selection, but with excellent fluorescence intensity and almost no background signal emitting at the blue or green channels. The knowledge gained by analysing their molecular architecture, however, allowed the planning and synthesis of a fluorescent BTD, which was then successfully tested and showed superior mitochondrial selection with outstanding results in bioimaging experiments in living cells. The new marker (named Splendor) was then compared with the commercially available MitoTracker Red (also through co-staining experiments) and showed far better mitochondrial selection, fluorescence intensity and chemical stability. Mitochondrial imaging and tracking (dynamic changes) was possible using Splendor during the whole cellular division cycle. DFT calculations were performed to offer insights into the origin of the chemical- and photostability of BTD derivatives. In addition, molecular docking calculations hint at a potential molecular target for the BTD derivatives in the mitochondrial protein adenine nucleotide translocase, which may explain the mitochondrial selectivity of Splendor versus the other four BTD derivatives.

The Drosophila effector caspase Dcp-1 regulates mitochondrial dynamics and autophagic flux via SesB.

Increasing evidence reveals that a subset of proteins participates in both the autophagy and apoptosis pathways, and this intersection is important in normal physiological contexts and in pathological settings. In this paper, we show that the Drosophila effector caspase, Drosophila caspase 1 (Dcp-1), localizes within mitochondria and regulates mitochondrial morphology and autophagic flux. Loss of Dcp-1 led to mitochondrial elongation, increased levels of the mitochondrial adenine nucleotide translocase stress-sensitive B (SesB), increased adenosine triphosphate (ATP), and a reduction in autophagic flux. Moreover, we find that SesB suppresses autophagic flux during midoogenesis, identifying a novel negative regulator of autophagy. Reduced SesB activity or depletion of ATP by oligomycin A could rescue the autophagic defect in Dcp-1 loss-of-function flies, demonstrating that Dcp-1 promotes autophagy by negatively regulating SesB and ATP levels. Furthermore, we find that pro-Dcp-1 interacts with SesB in a nonproteolytic manner to regulate its stability. These data reveal a new mitochondrial-associated molecular link between nonapoptotic caspase function and autophagy regulation in vivo.

Mitochondrially targeted nitro‐linoleate: a new tool for the study of cardioprotection

Cardiac ischaemia-reperfusion (IR) injury remains a significant clinical problem with limited treatment options available. We previously showed that cardioprotection against IR injury by nitro-fatty acids, such as nitro-linoleate (LNO2 ), involves covalent modification of mitochondrial adenine nucleotide translocase 1 (ANT1). Thus, it was hypothesized that conjugation of LNO2 to the mitochondriotropic triphenylphosphonium (TPP(+) ) moiety would enhance its protective properties. TPP(+) -LNO2 was synthesized from aminopropyl-TPP(+) and LNO2 , and characterized by direct infusion MS/MS. Its effects were assayed in primary cultures of cardiomyocytes from adult C57BL/6 mice and in mitochondria from these cells, exposed to simulated IR (SIR) conditions (oxygen and metabolite deprivation for 1h followed by normal conditions for 1h) by measuring viability by LDH release and exclusion of Trypan blue. Nitro-alkylated mitochondrial proteins were also measured by Western blots, using antibodies to TPP(+) . TPP(+) -LNO2 protected cardiomyocytes from SIR injury more potently than the parent compound LNO2 . In addition, TPP(+) -LNO2 modified mitochondrial proteins, including ANT1, in a manner sensitive to the mitochondrial uncoupler carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP) and the ANT1 inhibitor carboxyatractyloside. Similar protein nitro-alkylation was obtained in cells and in isolated mitochondria, indicating the cell membrane was not a significant barrier to TPP(+) -LNO2 . Together, these results emphasize the importance of ANT1 as a target for the protective effects of LNO2 , and suggest that TPP(+) -conjugated electrophilic lipid compounds may yield novel tools for the investigation of cardioprotection.

The antiestrogen 4-hydroxytamoxifen protects against isotretinoin-induced permeability transition and bioenergetic dysfunction of liver mitochondria: comparison with tamoxifen

The combination of isotretinoin (13-cis-retinoic acid) with antiestrogens seems to be a promising strategy for cancer chemotherapy. The aim of the study was to evaluate the effects of isotretinoin alone or in combination with 4-hydroxytamoxifen (OHTAM) and with its prodrug tamoxifen (TAM), on the functions of rat liver mitochondria, i.e., mitochondrial permeability transition (MPT), bioenergetic functions and adenine nucleotide translocase (ANT). Isotretinoin (5 nmol/mg protein) induced the Ca²⁺-dependent MPT pore opening in mitochondria energized with succinate, which was prevented by OHTAM, cyclosporine A, TAM and ANT ligands. When mitochondria were energized with glutamate/malate and in the absence of added Ca²⁺ isotretinoin decreased the state 3 respiration, the ATP levels, the active ANT content and increased the lag phase of the phosphorylation cycle, demonstrating that isotretinoin decreased the mitochondrial phosphorylation efficiency. These changes of isotretinoin in bioenergetic parameters were not significant in the presence of succinate. The effects of isotretinoin at 5 nmol/mg protein on the Ca²⁺-dependent MPT and phosphorylative efficacy may be related with interactions with the ANT. Above 10 nmol/mg protein isotretinoin strongly diminished the active ANT content, decreased the Δψ, inhibited the complex I and induced proton leak through the Fo fraction of complex V. The combination of OHTAM with isotretinoin only induced significant changes in the energy production systems at concentrations ≥5 nmol isotretinoin/mg protein. Therefore, our results suggest that isotretinoin-associated liver toxicity is possibly related with mitochondrial dysfunctions and that the combination with OHTAM may contribute to decrease its toxicity.

Mitochondrial Proteins and Molecular Interaction with Cardiolipin at Atomistic Level

Functional coupling between mitochondrial creatine kinase (MtCK) and adenine nucleotide translocase (ANT) can play an important role in determining energy transfer pathways in the cell. MtCK, which is an octameric enzyme, catalyzes the conversion of creatine (Cr) and consumes adenosine triphosphate (ATP) to create phosphocreatine (PCr) and adenosine diphosphate (ADP). Mitochondrial CK is located partly in contact sites of outer and inner mitochondrial membranes and partly in the cristae region, but always firmly attached to the inner membrane and in the vicinity of transmembrane ANTs. It is shown that MtCK binds electrostatically to the negatively charged cardiolipins (CL) of the inner membrane. CL is a kind of diphosphatidylglycerol lipid and is found almost exclusively in the inner mitochondrial membrane where it is essential for keeping spatially close and functional CK, ANT and other respiratory complexes. The formation of contact sites of CK-ANT by CL patches is a dynamic process and important to understand at atomistic level, which is the main goal of this work.The molecular dynamics simulation technique is used to study the atomistic details of the functional role of CL close to mitochondrial CK and ANT complexes. The thermodynamics-based coarse-grained force-field, called MARTINI (version 2.0), has been used together with the GROMACS molecular dynamics package (version 4.5.4) for a model of a patch of the mitochondrial inner membrane containing several transmembrane ANTs, and a single CK above the membrane. The membrane model consists of three major type of lipids ­ phosphatidylcholine, phosphatidylethanolamine and cardiolipin in roughly 2:1:1 molar ratio.

Characterization of the Intracellular Distribution of Adenine Nucleotide Translocase (ANT) in Drosophila Indirect Flight Muscles

Background: The high power output necessary for insect flight has driven the evolution of muscles with large myofibrils (primary energy consumers) and abundant mitochondria (primary energy suppliers). The intricate functional interrelationship between these two organelles remains largely unknown despite its fundamental importance in understanding insect flight bioenergetics. Unlike vertebrate muscle that relies on a phosphagen (creatine phosphate/creatine kinase) system to regulate high energy phosphate flux, insect flight muscle has been reported to lack mitochondrial arginine kinase (analogous to creatine kinase), a key enzyme that enables intracellular energy transport. Creatine kinase is known to interact with mitochondrial adenine nucleotide translocase (ANT) in the transfer of ADP and ATP into and out of the mitochondria. Results: Here, we use quantitative immunogold transmission electron microscopy to show that in Drosophila melanogaster indirect flight muscles (IFM), ANT is present in the mitochondria as well as throughout the myofibril. To confirm this unexpected result, we created a transgenic line that expresses a chimeric GFP-ANT protein and used an anti-GFP antibody to determine the intracellular distribution of the fusion protein in the IFM. Similar to results obtained with anti-ANT, the fusion GFP-ANT protein is detected in myofibrils and mitochondria. We confirmed the absence of arginine kinase from IFM mitochondria and show that its sarcomeric (i.e., intramyofibrillar) distribution is similar to that of ANT. Conclusions: These results raise the possibility that direct channeling of nucleotides between mitochondria and myofibrils is assisted by an ANT protein thereby circumventing the need for a phosphagen shuttle in the IFM. The myofibrillar ANT may represent a unique adaptation in the muscles that require efficient exchange of nucleotides between mitochondria and myofibrils.