Molecular species of cardiolipin in relation to other mitochondrial phospholipids. Is there an acyl specificity of the interaction between cardiolipin and the ADP/ATP carrier?

Functional diversity among cardiolipin binding sites on the mitochondrial ADP/ATP carrier

Lipid-protein interactions play a multitude of essential roles in membrane homeostasis. Mitochondrial membranes have a unique lipid-protein environment that ensures bioenergetic efficiency. Cardiolipin (CL), the signature mitochondrial lipid, plays multiple roles in promoting oxidative phosphorylation (OXPHOS). In the inner mitochondrial membrane, the ADP/ATP carrier (AAC in yeast; adenine nucleotide translocator, ANT in mammals) exchanges ADP and ATP, enabling OXPHOS. AAC/ANT contains three tightly bound CLs, and these interactions are evolutionarily conserved. Here, we investigated the role of these buried CLs in AAC/ANT using a combination of biochemical approaches, native mass spectrometry, and molecular dynamics simulations. We introduced negatively charged mutations into each CL-binding site of yeast Aac2 and established experimentally that the mutations disrupted the CL interactions. While all mutations destabilized Aac2 tertiary structure, transport activity was impaired in a binding site-specific manner. Additionally, we determined that a disease-associated missense mutation in one CL-binding site in human ANT1 compromised its structure and transport activity, resulting in OXPHOS defects. Our findings highlight the conserved significance of CL in AAC/ANT structure and function, directly tied to specific lipid-protein interactions.

Layered mechanisms regulating the human mitochondrial NAD+ transporter SLC25A51

SLC25A51 is the primary mitochondrial NAD+ transporter in humans and controls many local reactions by mediating the influx of oxidized NAD+. Intriguingly, SLC25A51 lacks several key features compared with other members in the mitochondrial carrier family, thus its molecular mechanism has been unclear. A deeper understanding would shed light on the control of cellular respiration, the citric acid cycle, and free NAD+ concentrations in mammalian mitochondria. This review discusses recent insights into the transport mechanism of SLC25A51, and in the process highlights a multitiered regulation that governs NAD+ transport. The aspects regulating SLC25A51 import activity can be categorized as contributions from (1) structural characteristics of the transporter itself, (2) its microenvironment, and (3) distinctive properties of the transported ligand. These unique mechanisms further evoke compelling new ideas for modulating the activity of this transporter, as well as new mechanistic models for the mitochondrial carrier family.

Function-related asymmetry of the specific cardiolipin binding sites on the mitochondrial ADP/ATP carrier

The ADP/ATP carrier (AAC) transports matrix ATP and cytosolic ADP across the inner mitochondrial membrane (IMM). It is well known that cardiolipin (CL) plays an important role in regulating the function of AAC, yet the underlying mechanism still remains elusive. AAC is composed of three homologous domains, and three specific CL binding sites are located at the domain-domain interfaces near the matrix side. Here we report an in-depth investigation on the dynamic properties of the bound CL within the three specific sites through all-atom molecular dynamics simulations of up to 13 μs in total. Our results highlight the importance of the basic and polar residues in CL binding. The basic residues from the linker helix and/or the [Y/W/F][K/R]G motif enable the bound CL to form an intra-domain binding mode, and the canonical inter-domain binding mode only forms when these basic residues are occupied by an additional phospholipid. Of special significance, differences in the basic and polar residues lead to remarkable asymmetry among the three specific CL binding sites. We found that the bound CL at the interface of domains 2 and 3 predominantly adopts inter-domain binding mode, while CLs at the other two sites have much more intra-domain populations. This is consistent with the asymmetric crystal structure of the matrix state (m-state) AAC which implies an asymmetric transport mechanism. The dynamic equilibrium between the inter-domain and intra-domain binding modes observed in our simulations could be highly important for the bound CLs to adapt to the movements during state transitions.

Cardiolipin, Perhydroxyl Radicals, and Lipid Peroxidation in Mitochondrial Dysfunctions and Aging

Mitochondrial dysfunctions caused by oxidative stress are currently regarded as the main cause of aging. Accumulation of mutations and deletions of mtDNA is a hallmark of aging. So far, however, there is no evidence that most studied oxygen radicals are directly responsible for mutations of mtDNA. Oxidative damages to cardiolipin (CL) and phosphatidylethanolamine (PEA) are also hallmarks of oxidative stress, but the mechanisms of their damage remain obscure. CL is the only phospholipid present almost exclusively in the inner mitochondrial membrane (IMM) where it is responsible, together with PEA, for the maintenance of the superstructures of oxidative phosphorylation enzymes. CL has negative charges at the headgroups and due to specific localization at the negative curves of the IMM, it creates areas with the strong negative charge where local pH may be several units lower than in the surrounding bulk phases. At these sites with the higher acidity, the chance of protonation of the superoxide radical (O₂^•), generated by the respiratory chain, is much higher with the formation of the highly reactive hydrophobic perhydroxyl radical (HO₂^•). HO₂^• specifically reacts with the double bonds of polyunsaturated fatty acids (PUFA) initiating the isoprostane pathway of lipid peroxidation. Because HO₂^• is formed close to CL aggregates and PEA, it causes peroxidation of the linoleic acid in CL and also damages PEA. This causes disruption of the structural and functional integrity of the respirosomes and ATP synthase. We provide evidence that in elderly individuals with metabolic syndrome (MetS), fatty acids become the major substrates for production of ATP and this may increase several-fold generation of O₂^• and thus HO₂^•. We conclude that MetS accelerates aging and the mitochondrial dysfunctions are caused by the HO₂^•-induced direct oxidation of CL and the isoprostane pathway of lipid peroxidation (IPLP). The toxic products of IPLP damage not only PEA, but also mtDNA and OXPHOS proteins. This results in gradual disruption of the structural and functional integrity of mitochondria and cells.

The mitochondrial ADP/ATP carrier exists and functions as a monomer

For more than 40 years, the oligomeric state of members of the mitochondrial carrier family (SLC25) has been the subject of debate. Initially, the consensus was that they were dimeric, based on the application of a large number of different techniques. However, the structures of the mitochondrial ADP/ATP carrier, a member of the family, clearly demonstrated that its structural fold is monomeric, lacking a conserved dimerisation interface. A re-evaluation of previously published data, with the advantage of hindsight, concluded that technical errors were at the basis of the earlier dimer claims. Here, we revisit this topic, as new claims for the existence of dimers of the bovine ADP/ATP carrier have emerged using native mass spectrometry of mitochondrial membrane vesicles. However, the measured mass does not agree with previously published values, and a large number of post-translational modifications are proposed to account for the difference. Contrarily, these modifications are not observed in electron density maps of the bovine carrier. If they were present, they would interfere with the structure and function of the carrier, including inhibitor and substrate binding. Furthermore, the reported mass does not account for three tightly bound cardiolipin molecules, which are consistently observed in other studies and are important stabilising factors for the transport mechanism. The monomeric carrier has all of the required properties for a functional transporter and undergoes large conformational changes that are incompatible with a stable dimerisation interface. Thus, our view that the native mitochondrial ADP/ATP carrier exists and functions as a monomer remains unaltered.

Cardiolipin dynamics and binding to conserved residues in the mitochondrial ADP/ATP carrier

Cardiolipin in eukaryotes is found in the mitochondrial inner membrane, where it interacts with membrane proteins and, although not essential, is necessary for the optimal activity of a number of proteins. One of them is the mitochondrial ADP/ATP carrier, which imports ADP into the mitochondrion and exports ATP. In the crystal structures, cardiolipin is bound to three equivalent sites of the ADP/ATP carrier, but its role is unresolved. Conservation of residues at these cardiolipin binding sites across other members of the mitochondrial carrier superfamily indicates cardiolipin binding is likely to be important for the function of all mitochondrial carriers. Multiscale simulations were performed in a cardiolipin-containing membrane to investigate the dynamics of cardiolipin around the yeast and bovine ADP/ATP carriers in a lipid bilayer and the properties of the cardiolipin-binding sites. In coarse-grain simulations, cardiolipin molecules bound to the carriers for longer periods of time than phosphatidylcholine and phosphatidylethanolamine lipids—with timescales in the tens of microseconds. Three long-lived cardiolipin binding sites overlapped with those in the crystal structures of the carriers. Other shorter-lived cardiolipin interaction sites were identified in both membrane leaflets. However, the timescales of the interactions were of the same order as phosphatidylcholine and phosphatidylethanolamine, suggesting that these sites are not specific for cardiolipin binding. The calculation of lipid binding times and the overlap of the cardiolipin binding sites between the structures and simulations demonstrate the potential of multiscale simulations to investigate the dynamics and behavior of lipids interacting with membrane proteins.

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Coarse-grained models of AAC in mixed lipid bilayers were simulated.

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Three long-lived cardiolipin sites correspond to those in the crystal structures.

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No other long-lived binding sites were observed for cardiolipin or other phospholipids.

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Trimethylation of Lys-51 of AAC had no effect on cardiolipin interactions.

Cardiolipin plays an essential role in the formation of intracellular membranes in Escherichia coli

Mitochondria, chloroplasts and photosynthetic bacteria are characterized by the presence of complex and intricate membrane systems. In contrast, non-photosynthetic bacteria lack membrane structures within their cytoplasm. However, large scale over-production of some membrane proteins, such as the fumarate reductase, the mannitol permease MtlA, the glycerol acyl transferase PlsB, the chemotaxis receptor Tsr or the ATP synthase subunit b, can induce the proliferation of intra cellular membranes (ICMs) in the cytoplasm of Escherichia coli. These ICMs are particularly rich in cardiolipin (CL). Here, we have studied the effect of CL in the generation of these membranous structures. We have deleted the three genes (clsA, clsB and clsC) responsible of CL biosynthesis in E. coli and analysed the effect of these mutations by fluorescent and electron microscopy and by lipid mass spectrometry. We have found that CL is essential in the formation of non-lamellar structures in the cytoplasm of E. coli cells. These results could help to understand the structuration of membranes in E. coli and other membrane organelles, such as mitochondria and ER.

Cardiolipin profiles as a potential biomarker of mitochondrial health in diet-induced obese mice subjected to exercise, diet-restriction and ephedrine treatment

Cardiolipin (CL) is crucial for mitochondrial energy metabolism and structural integrity. Alterations in CL quantity or CL species have been associated with mitochondrial dysfunction in several pathological conditions and diseases, including mitochondrial dysfunction-related compound attrition and post-market withdrawal of promising drugs. Here we report alterations in the CL profiles in conjunction with morphology of soleus muscle (SM) and brown adipose tissue (BAT) in diet-induced obese (DIO) mice, subjected to ephedrine treatment (EPH: 200 mg kg(-1)  day(-1) orally), treadmill exercise (EX: 10 meters per min, 1 h per day), or dietary restriction (DR: 25% less of mean food consumed by the EX group) for 7 days. Mice from the DR and EPH groups had a significant decrease in percent body weight and reduced fat mass compared with DIO controls. Morphologic alterations in the BAT included brown adipocytes with reduced cytoplasmic lipid droplets and increased cytoplasmic eosinophilia in the EX, DR and EPH groups. Increased cytoplasmic eosinophilia in the BAT was ultrastructurally manifested by increased mitochondrial cristae, fenestration of mitochondrial cristae, increased electron density of mitochondrial matrix, and increased complexity of shape and elongation of mitochondria. Mitochondrial ultrastructural alterations in the SM of the EX and DR groups included increased mitochondrial cristae, cup-shaped mitochondria and mitochondrial degeneration. All four CL species (tri-linoleoyl-mono-docosahexaenoyl, tetralinoleoyl, tri-linoleoyl-mono-oleoyl, and di-linoleoyl-di-oleoyl) were increased in the BAT of the DR and EPH groups and in the SM of the EPH and EX groups. In conclusion, cardiolipin profiling supported standard methods for assessing mitochondrial biogenesis and health, and may serve as a potential marker of mitochondrial dysfunction in preclinical toxicity studies.

Cardiolipin content is involved in liver mitochondrial energy wasting associated with cancer-induced cachexia without the involvement of adenine nucleotide translocase

Cancer-induced cachexia describes the progressive skeletal muscle wasting associated with many cancers leading to shortened survival time in cancer patients. We previously reported that cardiolipin content and energy-wasting processes were both increased in liver mitochondria in a rat model of peritoneal carcinosis (PC)-induced cachexia. To increase the understanding of the cellular biology of cancer cachexia, we investigated the involvement of adenine nucleotide translocator (ANT) in mitochondrial energy-wasting processes in liver mitochondria of PC and pair-fed control rats and its interactions with cardiolipin in isolated liver mitochondria from healthy rats exposed to cardiolipin-enriched liposomes. We showed in this study that functional ANT content was decreased in liver mitochondria from PC rats but without any effects on the efficiency of ATP synthesis. Moreover, non-phosphorylating energy wasting was not affected by saturating concentrations of carboxyatractylate (CAT), a potent inhibitor of ANT, in liver mitochondria from PC rats. Decreased efficiency of ATP synthesis was found in normal liver mitochondria exposed to cardiolipin-enriched liposomes, with increased non-phosphorylating energy wasting, thus mimicking mitochondria from PC rats. However, the functional ANT content in these cardiolipin-enriched mitochondria was unchanged, although non-phosphorylating energy wasting was reduced by CAT-induced inhibition of ANT. Finally, non-phosphorylating energy wasting was increased in cardiolipin-enriched mitochondria with substrates for complexes 1 and 2, but not for complex 4. In conclusion, increased energy wasting measured in liver mitochondria from rats with cancer cachexia is dependent on cardiolipin but independent of ANT. Interactions between ANT and cardiolipin are modified when cancer cachexia occurs.

Dietary docosahexaenoic Acid (22:6) incorporates into cardiolipin at the expense of linoleic Acid (18:2): analysis and potential implications

Cardiolipin is a signature phospholipid of major functional significance in mitochondria. In heart mitochondria the fatty acid composition of cardiolipin is commonly viewed as highly regulated due to its high levels of linoleic acid (18:2n − 6) and the dominant presence of a 4×18:2 molecular species. However, analysis of data from a comprehensive compilation of studies reporting changes in fatty acid composition of cardiolipin in heart and liver mitochondria in response to dietary fat shows that, in heart the accrual of 18:2 into cardiolipin conforms strongly to its dietary availability at up to 20% of total dietary fatty acid and thereafter is regulated. In liver, no dietary conformer trend is apparent for 18:2 with regulated lower levels across the dietary range for 18:2. When 18:2 and docosahexaenoic acid (22:6n − 3) are present in the same diet, 22:6 is incorporated into cardiolipin of heart and liver at the expense of 18:2 when 22:6 is up to ~20% and 10% of total dietary fatty acid respectively. Changes in fatty acid composition in response to dietary fat are also compared for the two other main mitochondrial phospholipids, phosphatidylcholine and phosphatidylethanolamine, and the potential consequences of replacement of 18:2 with 22:6 in cardiolipin are discussed.

Cardiac and skeletal muscle defects in a mouse model of human Barth syndrome

Barth syndrome is an X-linked genetic disorder caused by mutations in the tafazzin (taz) gene and characterized by dilated cardiomyopathy, exercise intolerance, chronic fatigue, delayed growth, and neutropenia. Tafazzin is a mitochondrial transacylase required for cardiolipin remodeling. Although tafazzin function has been studied in non-mammalian model organisms, mammalian genetic loss of function approaches have not been used. We examined the consequences of tafazzin knockdown on sarcomeric mitochondria and cardiac function in mice. Tafazzin knockdown resulted in a dramatic decrease of tetralinoleoyl cardiolipin in cardiac and skeletal muscles and accumulation of monolysocardiolipins and cardiolipin molecular species with aberrant acyl groups. Electron microscopy revealed pathological changes in mitochondria, myofibrils, and mitochondrion-associated membranes in skeletal and cardiac muscles. Echocardiography and magnetic resonance imaging revealed severe cardiac abnormalities, including left ventricular dilation, left ventricular mass reduction, and depression of fractional shortening and ejection fraction in tafazzin-deficient mice. Tafazzin knockdown mice provide the first mammalian model system for Barth syndrome in which the pathophysiological relationships between altered content of mitochondrial phospholipids, ultrastructural abnormalities, myocardial and mitochondrial dysfunction, and clinical outcome can be completely investigated.

Reduced capacity of Ca²+ retention in liver as compared to kidney mitochondria. ADP requirement

Ca²+ loading in mitochondria promotes the opening of a non-selective transmembrane pathway. Permeability transition is also associated with the interaction of cyclophilin D at the internal surface of the non-specific transmembrane pore. This interaction is circumvented by cyclosporin A and ADP. Our results show that, in the absence of ADP, liver mitochondria were unable to retain Ca²+, they underwent a fast and large amplitude swelling, as well as a rapid collapse of the transmembrane potential. In contrast, in the absence of ADP, kidney mitochondria retained Ca²+, swelling did not occur, and the collapse of the membrane potential was delayed. Ca²+ efflux was reversed by the addition of ADP and cyclosporin A. Our findings indicate that the differences between liver and kidney mitochondria are due to the low association of cyclophilin D to the ADP/ATP carrier found in kidney mitochondria as compared to liver mitochondria.

Degradation of phospholipids by oxidative stress--exceptional significance of cardiolipin

The aim of this study was to investigate the effect of oxidative stress on mitochondrial phospholipids. In this context, this study investigated (i) the content of phosphatidylethanolamine (PE), phosphatidylcholine (PC) and cardiolipin (CL), (ii) the correlation of CL degradation with mitochondrial function and (iii) the correlation of CL degradation and CL oxidation. Oxidative stress induced by iron/ascorbate caused a dramatic decrease of these phospholipids, in which CL was the most sensitive phospholipid. Even moderate oxidative stress by hypoxia/reoxygenation caused a decrease in CL that was parallelled by a decrease in active respiration of isolated rat heart mitochondria. The relation between oxidative stress, CL degradation and CL oxidation was studied by in vitro treatment of commercially available CL with superoxide anion radicals and H2O2. The degradation of CL was mediated by H2O2 and required the presence of cytochrome c. Other peroxidases such as horse radish peroxidase and glutathione peroxidase had no effect. Cytochrome c in the presence of H2O2 caused CL oxidation. The data demonstrate that oxidative stress may cause degradation of phospholipids by oxidation, in particular CL; resulting in mitochondrial dysfunction.

Association of phosphatidic acid with the bovine mitochondrial ADP/ATP carrier

The beef heart adenine nucleotide carrier protein (Anc) of the inner mitochondrial membrane can be purified in a form stabilized by binding the inhibitor carboxyatractyloside. The protein is copurified with bound lipid. We show for the first time that phosphatidic acid, although a minor component, is one of the lipids bound to Anc. The short spin-lattice relaxation time found by (31)P magic angle spinning nuclear magnetic resonance (MAS/NMR) for phosphatidic acid indicates that it is tightly bound to the protein. However, this lipid also has a comparatively small chemical shift anisotropy, suggesting that it can undergo rapid reorientation in space. In contrast, most of the lipid bound to Anc shows anisotropic motion typical of a bilayer arrangement. The phosphatidic acid that is detected in the purified preparation of Anc is also shown to be present initially in the unfractionated mitochondria, prior to the isolation of Anc. In Triton-solubilized mitochondria, phosphatidic acid, cardiolipin, phosphatidylethanolamine, and phosphatidylcholine exhibit resonance lines in the static (31)P NMR spectra, but in the purified Anc, only the phosphatidylethanolamine and phosphatidylcholine can be detected by this method, even though the other lipids are still present. This demonstrates that the phosphatidic acid and cardiolipin are interacting with the Anc. The thermal denaturation of the Anc was determined by differential scanning calorimetry. The protein denatures at 74 degrees C both before and after the NMR studies with the same characteristics.

Cyclosporin A inhibits UV-radiation-induced membrane damage but is unable to inhibit carboxyatractyloside-induced permeability transition

This work was undertaken to gain further information on the chemical characteristics of the membrane entity involved in the formation of the nonspecific pore. Mitochondria were subjected to oxidative stress by exposure to UV radiation. The results indicate that ultraviolet C radiation induces structural modifications in the adenine nucleotide translocase that lead to membrane permeability transition. Membrane leakage was assessed by measuring mitochondrial Ca2+ transport, the transmembrane electric gradient, and mitochondrial swelling. UV-irradiated mitochondria were unable to retain matrix Ca2+ or to maintain a high level of membrane potential when Ca2+ was added; furthermore, UV-irradiated mitochondria underwent large amplitude swelling. Release of cytochrome c and formation of malondialdehyde, owing to lipid peroxidation, were also seen. Structural modifications of the translocase were revealed by an increase in the binding of the fluorescent probe eosin-5-maleimide to thiol residues of the ADP/ATP carrier. These modifications, taken together with findings indicating that cyclosporin resulted unable to inhibit carboxyatractyloside-induced permeability transition, prompted us to conclude that the translocase could constitute the nonspecific pore or at least be an important modulator of it.

Cardiolipin and mitochondrial carriers

Members of the mitochondrial carrier family interact with cardiolipin (CL) as evident from a variety of functional and structural effects. CL stabilises carrier proteins on isolation with detergents, with the P(i) carrier as the prime example. CL is required for transport in reconstituted vesicles, prime examples are the P(i)- and ADP/ATP carrier (AAC). CL binds to the AAC in a graded manner; 6 CL/AAC dimer bind tightly as measured on the (31)P NMR time scale. 2 additional CL/dimer bind reversibly and a fast exchanging envelope of phospholipids includes CL as measured on the ESR time scale. In the crystal structure of the CAT-AAC complex 3 CL bind to the periphery of the AAC in a three-fold pseudo-symmetry. The binding of CL is implicated to contribute lowering the high transition energy barriers in the AAC. Para-functions of the AAC, as in the mitochondrial pore transition (MPT) and in cell death are linked to the CL binding of the AAC. Ca(++) or oxidants can sequester or destroy AAC bound CL, rendering AAC labile, allowing pore formation and degradation. Thus AAC, by being vital for energy transfer, constitutes an Achilles heel in the eukaryotic cell. AAC together with CL is also engaged in respiratory supercomplexes. Different from AAC the similarly structured uncoupling protein (UCP1) has no tightly bound CL, but CL addition lowers affinity of the inhibitory nucleotide binding that may contribute to the physiological regulation of the uncoupling activity by ATP.

Cardiolipin, a critical determinant of mitochondrial carrier protein assembly and function

The ability of phospholipids to act as determinants of membrane protein structure and function is probably best exemplified by cardiolipin (CL), the signature phospholipid of mitochondria. Early efforts to reconstitute individual respiratory complexes and members of the mitochondrial carrier family, most notably the ADP/ATP carrier (AAC), often demonstrated the importance of CL. Over the past decade, the significance of CL in the organization of components of the electron transport chain into higher order assemblies, termed respiratory supercomplexes, has been established. Another protein required for oxidative phosphorylation, AAC, has received comparatively little attention likely stemming from the fact that AACs were thought to function in isolation as either homodimers or monomers. Recently however, AACs have been demonstrated to interact with the respiratory supercomplex, other members of the mitochondrial carrier family, and the TIM23 translocon. Interestingly, many if not all of these interactions depend on CL. As the paradigm for the mitochondrial carrier family, these discoveries with AAC suggest that other members of this large group of important proteins may be more gregarious than anticipated. Moreover, it is proposed that AAC and perhaps additional members of the mitochondrial carrier family might represent downstream targets of pathological states involving alterations in CL.

Determination of the molecular mass and dimensions of membrane proteins by size exclusion chromatography

Size exclusion chromatography is an established technique for the determination of hydrodynamic volumes of proteins or protein complexes. When applied to membrane proteins, the contribution of the detergent micelle, which is required to keep the protein soluble in the aqueous phase, needs to be determined to obtain accurate measurements for the protein. In a detergent series, in which the detergents differ only by the length of the alkyl chain, the contribution of the detergent micelle to the hydrodynamic volume is variable, whereas the contribution of the protein is constant. By using this approach, several parameters of membrane proteins can be estimated by extrapolation, such as the radius at the midpoint of the membrane, the average radius, the Stokes radius, and the excluded volume. The molecular mass of the protein can be determined by two independent measurements that arise from the behaviour of the free detergent micelle and protein-detergent micelle during size exclusion chromatography and the determination of the detergent-protein ratio. Determining the dimensions of protein-detergent micelles may facilitate membrane protein purification and crystallization by defining the accessibility of the protein surface.

The ADP and ATP transport in mitochondria and its carrier

Different from some more specialised short reviews, here a general although not encyclopaedic survey of the function, metabolic role, structure and mechanism of the ADP/ATP transport in mitochondria is presented. The obvious need for an "old fashioned" review comes from the gateway role in metabolism of the ATP transfer to the cytosol from mitochondria. Amidst the labours, 40 or more years ago, of unravelling the role of mitochondrial compartments and of the two membranes, the sequence of steps of how ATP arrives in the cytosol became a major issue. When the dust settled, a picture emerged where ATP is exported across the inner membrane in a 1:1 exchange against ADP and where the selection of ATP versus ADP is controlled by the high membrane potential at the inner membrane, thus uplifting the free energy of ATP in the cytosol over the mitochondrial matrix. Thus the disparate energy and redox states of the two major compartments are bridged by two membrane potential responsive carriers to enable their symbiosis in the eukaryotic cell. The advance to the molecular level by studying the binding of nucleotides and inhibitors was facilitated by the high level of carrier (AAC) binding sites in the mitochondrial membrane. A striking flexibility of nucleotide binding uncovered the reorientation of carrier sites between outer and inner face, assisted by the side specific high affinity inhibitors. The evidence of a single carrier site versus separate sites for substrate and inhibitors was expounded. In an ideal setting principles of transport catalysis were elucidated. The isolation of intact AAC as a first for any transporter enabled the reconstitution of transport for unravelling, independently of mitochondrial complications, the factors controlling the ADP/ATP exchange. Electrical currents measured with the reconstituted AAC demonstrated electrogenic translocation and charge shift of reorienting carrier sites. Aberrant or vital para-functions of AAC in basal uncoupling and in the mitochondrial pore transition were demonstrated in mitochondria and by patch clamp with reconstituted AAC. The first amino acid sequence of AAC and of any eukaryotic carrier furnished a 6-transmembrane helix folding model, and was the basis for mapping the structure by access studies with various probes, and for demonstrating the strong conformation changes demanded by the reorientation mechanism. Mutations served to elucidate the function of residues, including the particular sensitivity of ATP versus ADP transport to deletion of critical positive charge in AAC. After resisting for decades, at last the atomic crystal structure of the stabilised CAT-AAC complex emerged supporting the predicted principle fold of the AAC but showing unexpected features relevant to mechanism. Being a snapshot of an extreme abortive "c-state" the actual mechanism still remains a conjecture.

Oxidative lipidomics of gamma-irradiation-induced intestinal injury

Although gamma-irradiation-induced tissue injury has been associated with lipid peroxidation, the individual phospholipid molecular targets have not been identified. We employed oxidative lipidomics to qualitatively and quantitatively characterize phospholipid peroxidation in a radiosensitive tissue, the small intestine, of mice exposed to total body irradiation (TBI) (10 and 15 Gy). Using electrospray ionization mass spectrometry we found that the major classes of intestine phospholipids-phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol-included clusters with highly oxidizable molecular species containing docosahexaenoic fatty acid. Molecular species of cardiolipin were represented by only two major less oxidizable individual molecular species-tetralinoleoylcardiolipin and trilinoleoyl-mono-oleoylcardiolipin. Selective and robust oxidation of two anionic phospholipids-cardiolipin in mitochondria and phosphatidylserine outside of mitochondria-was observed 24 h after gamma-irradiation. MS analysis detected several TBI-induced molecular species of oxidized cardiolipin: (C(18:2))(3)(C(18:2)-OOH), (C(18:2))(2)(C(18:2)-OOH)(2), (C(18:2))(1)(C(18:2)-OOH)(3), and (C(18:2)-OOH)(4). The major molecular species involved in TBI-triggered peroxidation of phosphatidylserine included C(18:0)/C(22:6)-OOH, C(18:0)/C(22:5)-OOH, and C(18:0)/C(22:4)-OOH. More abundant phospholipids-phosphatidylcholine and phosphatidylethanolamine-did not reveal any oxidative stress responses despite the presence of highly oxidizable docosahexaenoic fatty acid residues in their molecular species. A marked activation of caspases 3/7 that was detected in the intestine of gamma-irradiated mice indicates the involvement of apoptotic cell death in the TBI injury. Given that oxidized molecular species of cardiolipin and phosphatidylserine accumulate during apoptosis of different cells in vitro we speculate that cardiolipin and phosphatidylserine oxidation products may be useful as potential biomarkers of gamma-irradiation-induced intestinal apoptosis in vivo and may represent a promising target for the discovery of new radioprotectors and radiosensitizers.

Cardiolipin synthesis for the assembly of bacterial and mitochondrial membranes

In this article, the formation of prokaryotic and eukaryotic cardiolipin is reviewed in light of its biological function. I begin with a detailed account of the structure of cardiolipin, its stereochemistry, and the resulting physical properties, and I present structural analogs of cardiolipin that occur in some organisms. Then I continue to discuss i) the de novo formation of cardiolipin, ii) its acyl remodeling, iii) the assembly of cardiolipin into biological membranes, and iv) the degradation of cardiolipin, which may be involved in apoptosis and mitochondrial fusion. Thus, this article covers the entire metabolic cycle of this unique phospholipid. It is shown that mitochondria produce cardiolipin species with a high degree of structural uniformity and molecular symmetry, among which there is often a dominant form with four identical acyl chains. The subsequent assembly of cardiolipin into functional membranes is largely unknown, but the analysis of crystal structures of membrane proteins has revealed a first glimpse into the underlying principles of cardiolipin-protein interactions. Disturbances of cardiolipin metabolism are crucial in the pathophysiology of human Barth syndrome and perhaps also play a role in diabetes and ischemic heart disease.

Isolation and characterization of lipids strictly associated to PSII complexes: Focus on cardiolipin structural and functional role

In this work, lipid extracts from spinach membrane fragments enriched in Photosystem II (PSII) and from spinach PSII dimers were analyzed, by means of Thin Layer Chromatography (TLC) and Electro-Spray Ionization Mass Spectrometry. Cardiolipin found in association with PSII was isolated and purified by preparative TLC, then characterized by mass and mass-mass analyses. Cardiolipin structures with four unsaturated C18 acyl chains and variable saturation degrees were evidenced. Structural and functional effects of different phospholipids on PSII complexes were investigated by Fluorescence, Resonance Light Scattering and Oxygen Evolution Rate measurements. An increment of PSII thermal stability was observed in the presence of cardiolipin and phosphatidylglycerol.

Oxidation-induced changes in human lens epithelial cells. 1. Phospholipids

Lipid compositional changes in lens epithelial cells (HLE B-3) grown in a hyperoxic atmosphere were studied to determine if oxidation could cause changes in the amount and type of phospholipid similar to those found in vivo with age and cataract. The phosphatidylcholines in HLE B-3 cells were 8 times more unsaturated than the sphingomyelins. Cell viability was the same for cells grown for up to 48 h in a normoxic or hyperoxic atmosphere. Lipid oxidation was about three times higher after growth in a hyperoxic atmosphere compared with cells grown in a normoxic atmosphere. The lack of change in the relative amount of sphingomyelin and the decrease in phosphatidylcholine coupled with the increase in lysophosphatidylcholine support the idea that similar mechanisms may be responsible for the lipid compositional changes in both lens epithelial and fiber cells. It is postulated that lipases eliminate oxidized unsaturated glycerolipids, leaving a membrane increasingly composed of more ordered and more saturated sphingolipids. Oxidative stress leads to changes in membrane composition that are consistent with those seen with age in human epithelial cells. Oxidation-induced epithelial phospholipid change is an area of research that has gone virtually unexplored in the human lens and could be relevant to all cell types and may be important to lens clarity.

Monolysocardiolipin in cultured fibroblasts is a sensitive and specific marker for Barth Syndrome

Barth Syndrome (BTHS) is an X-linked recessive disorder that results in abnormal metabolism of the mitochondrial phospholipid cardiolipin (CL). CLs are decreased and monolysocardiolipins (MLCLs), intermediates in CL metabolism, are increased in a variety of tissues. Measurement of decreased CL levels in skin fibroblasts has previously been proposed as a diagnostic test for BTHS. We investigated whether elevated MLCL is specific for BTHS and whether the MLCL-to-CL ratio is a more sensitive and specific marker for BTHS. We measured CLs and MLCLs in skin fibroblasts from 5 BTHS patients, 8 controls, and 14 patients with biochemical and clinical findings similar to those in BTHS (group D), using high performance liquid chromatography-mass spectrometry. Our results showed a clear decrease of CL in combination with a marked increase of MLCL in fibroblasts from BTHS patients when compared with controls. MLCL/CL ratios ranged from 0.03-0.12 in control fibroblasts and from 5.41-13.83 in BTHS fibroblasts. In group D, the MLCL/CL ratio range was 0.02-0.06. We therefore conclude that elevations of MLCLs are specific for BTHS and that the MLCL/CL ratio in fibroblasts is a better diagnostic marker than CL alone. We also report the finding of two novel mutations in the TAZ gene that cause BTHS.

Oxidation-induced changes in human lens epithelial cells 2. Mitochondria and the generation of reactive oxygen species

The relationships among reactive oxygen species (ROS) generation, lipid compositional changes, antioxidant power, and mitochondrial membrane potential were determined in a human lens epithelial cell line, HLE-B3. Cells grown in a hyperoxic atmosphere grew linearly for about 3 days, and then progressively died. Total antioxidant power and ROS generation increased by 50 and 43%, respectively, in cells grown in a hyperoxic atmosphere compared to those cultured in a normoxic atmosphere. By specifically uncoupling the mitochondrial proton gradient, we determined that the mitochondria are most likely the major source of ROS generation. ROS generation correlated inversely with mitochondrial membrane potential and the amount of cardiolipin, factors likely to contribute to loss of cell viability. Our results support the idea that hyperoxic damage to HLE-B3 cells derives from enhanced generation of ROS from the mitochondrial electron transport chain resulting in the oxidation of cardiolipin. With extended hyperoxic insult, the oxidants overwhelm the antioxidant defense system and eventually cell death ensues.

Role of cardiolipin alterations in mitochondrial dysfunction and disease

Cardiolipin (CL) is a structurally unique dimeric phospholipid localized in the inner mitochondrial membrane where it is required for optimal mitochondrial function. In addition to its role in maintaining membrane potential and architecture, CL is known to provide essential structural and functional support to several proteins involved in mitochondrial bioenergetics. A loss of CL content, alterations in its acyl chain composition, and/or CL peroxidation have been associated with mitochondrial dysfunction in multiple tissues in a variety of pathological conditions, including ischemia, hypothyroidism, aging, and heart failure. Recently, aberrations in CL metabolism have been implicated as a primary causative factor in the cardioskeletal myopathy known as Barth syndrome, underscoring an important role of CL in human health and disease. The purpose of this review is to provide an overview of evidence that has linked changes in the CL profile to mitochondrial dysfunction in various pathological conditions. In addition, a brief overview of CL function and biosynthesis, and a discussion of methods used to examine CL in biological tissues are provided.

New insights into the regulation of cardiolipin biosynthesis in yeast: implications for Barth syndrome

Recent studies have revealed an array of novel regulatory mechanisms involved in the biosynthesis and metabolism of the phospholipid cardiolipin (CL), the signature lipid of mitochondria. CL plays an important role in cellular and mitochondrial function due in part to its association with a large number of mitochondrial proteins, including many which are unable to function optimally in the absence of CL. New insights into the complexity of regulation of CL provide further evidence of its importance in mitochondrial and cellular function. The biosynthesis of CL in yeast occurs via three enzymatic steps localized in the mitochondrial inner membrane. Regulation of this process by general phospholipid cross-pathway control and factors affecting mitochondrial development has been previously established. In this review, novel regulatory mechanisms that control CL biosynthesis are discussed. A unique form of inositol-mediated regulation has been identified in the CL biosynthetic pathway, independent of the INO2-INO4-OPI1 regulatory circuit that controls general phospholipid biosynthesis. Inositol leads to decreased activity of phosphatidylglycerolphosphate (PGP) synthase, which catalyzes the committed step of CL synthesis. Reduced enzymatic activity does not result from alteration of expression of the structural gene, but is instead due to increased phosphorylation of the enzyme. This is the first demonstration of phosphorylation in response to inositol and may have significant implications in understanding the role of inositol in other cellular regulatory pathways. Additionally, synthesis of CL has been shown to be dependent on mitochondrial pH, coordinately controlled with synthesis of mitochondrial phosphatidylethanolamine (PE), and may be regulated by mitochondrial DNA absence sensitive factor (MIDAS). Further characterization of these regulatory mechanisms holds great potential for the identification of novel functions of CL in mitochondrial and cellular processes.

Interactions between phospholipids and NADH:ubiquinone oxidoreductase (complex I) from bovine mitochondria

NADH:ubiquinone oxidoreductase (complex I) from bovine heart mitochondria is a highly complicated, energy transducing, membrane-bound enzyme. It contains 46 different subunits and nine redox cofactors: a noncovalently bound flavin mononucleotide and eight iron-sulfur clusters. The mechanism of complex I is not known. Mechanistic studies using the bovine enzyme, a model for human complex I, have been precluded by the difficulty of preparing complex I which is pure, monodisperse, and fully catalytically active. Here, we describe and characterize a preparation of bovine complex I which fulfills all of these criteria. The catalytic activity is strongly dependent on the phospholipid content of the preparation, and three classes of phospholipid interactions with complex I have been identified. First, complex I contains tightly bound cardiolipin. Cardiolipin may be required for the structural integrity of the complex or play a functional role. Second, the catalytic activity is determined by the amounts of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) which are bound to the complex. They are more weakly bound than cardiolipin, exchange with PC and PE in solution, and can substitute for one another. However, their nontransitory loss leads to irreversible functional impairment. Third, phospholipids are also required in the assay buffer for the purified enzyme to exhibit its full activity. It is likely that they are required for solubilization and presentation of the hydrophobic ubiquinone substrate.

Cardiolipin metabolism and Barth Syndrome

Many advances have occurred in the field of Barth Syndrome biology in the 26 years since it was first described as an X-linked cardiomyopathy. Barth Syndrome is the first human disease recognized in which the primary causative factor is an alteration in cardiolipin remodeling. Cardiolipin is required for the optimal function of many proteins within the mitochondria, particularly in the respiratory chain and is involved in the mitochondrial-mediated apoptotic process. The appropriate content of cardiolipin appears to be critical for these functions. Cardiolipin is synthesized de novo in mitochondria and is rapidly remodeled to produce CL enriched in linoleic acid. The Barth Syndrome gene TAZ has been identified and expression of the gene yields proteins known as tafazzins. Mutations in TAZ result in a decrease in tetra-linoleoyl species of cardiolipin and an accumulation of monolysocardiolipin within cells from Barth Syndrome patients. Although the protein product of the TAZ gene shows sequence homology to the glycerolipid acyltransferase family of enzymes, its precise biochemical function remains to be elucidated. In this review we highlight some of the recent literature on cardiolipin metabolism and Barth Syndrome.

Thermodynamic and structural study of the main phospholipid components comprising the mitochondrial inner membrane

Cardiolipin (CL) is a phospholipid found in the energy-transducing membranes of bacteria and mitochondria and it is thought to be involved in relevant biological processes as apoptosis. In this work, the mixing properties of CL and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocoline (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) at the air-water interface, have been examined using the thermodynamic framework analysis of compression isotherms. Accordingly, the values of the Gibbs energy of mixing, the more stable monolayers assayed were: POPC:CL (0.6:0.4, mol:mol) and POPE:CL (0.8:0.2, mol:mol). The results reflect that attractive forces are the greatest contributors to the total interaction in these compositions. Supported planar bilayers (SPBs) with such compositions were examined using atomic force microscopy (AFM) at different temperatures. With the POPC:CL mixture, rounded and featureless SPBs were obtained at 4 degrees C and 24 degrees C. In contrast, the extension of the POPE:CL mixture revealed the existence of different lipid domains at 24 degrees C and 37 degrees C. Three lipid domains coexisted which can be distinguished by measuring the step height difference between the uncovered mica and the bilayer. While the low and intermediate domains were temperature dependent, the high domain was composition dependent. When cytochrome c (cyt c) was injected into the fluid cell, the protein showed a preferential adsorption onto the high domain of the POPC:CL. These results suggest that the high domain is mainly formed by CL.

Docosahexaenoic acid- and eicosapentaenoic acid-enriched cardiolipin in the Manila clam Ruditapes philippinarum

The FA composition of cardiolipin (CL) from the Manila clam Ruditapes philippinarum was investigated in whole body and individual organs. CL was isolated by HPLC and its chemical structure characterized using NMR spectroscopy. Two prominent FA, EPA and DHA, were found in approximately equal proportions, contributing together up to 73 mol% of the total FA. The FA composition of CL is presumed to reflect a specific synthesis pathway independent of diet and of total glycerophospholipid FA composition. To the best of our knowledge, this is the first time that a CL dominated by the two PUFA 22:6n-3 and 20:5n-3 has been characterized and described. This EPA + DHA specificity of the CL in the Manila clam is thought to reflect a functional and structural modification of mitochondrial membranes of this bivalve species compared with scallops, oysters, and mussels that possess a CL dominated by DHA. The FA composition and levels of CL differed little between separated organs, and the large pool of DHA and EPA was found fairly equally distributed in gills, mantle, foot, siphon, and muscle. However, whereas DHA and PUFA levels were most stable between organs, EPA and arachidonic acid were significantly more variable and seemed to be interrelated.

Molecular symmetry in mitochondrial cardiolipins

Cardiolipin is a unique mitochondrial phospholipid with an atypical fatty acid profile, but the significance of its acyl specificity has not been understood. We explored the enormous combinatorial diversity among cardiolipin species, which results from the presence of four fatty acids in each molecule, by integrated use of high-performance liquid chromatography, mass spectrometry, diacylglycerol species analysis, fatty acid analysis, and selective cleavage of fatty acids by phospholipase A2. The most abundant cardiolipin species from various organisms and tissues (human heart, human lymphoblasts, rat liver, Drosophila, sea urchin sperm, yeast, mung bean hypocotyls) contained only one or two types of fatty acids, which generated a high degree of structural uniformity and molecular symmetry. However, an exception was found in patients with Barth syndrome, in whom an acyltransferase deficiency led to loss of acyl selectivity and formation of multiple molecular species. These results suggest that restriction of the number of fatty acid species, rather than the selection of a particular kind of fatty acid, is the common theme of eukaryotic cardiolipins. This limits the structural diversity of the cardiolipin species and creates molecular symmetry with implications for the stereochemistry of cardiolipin.

Cytochrome c acts as a cardiolipin oxygenase required for release of proapoptotic factors

Programmed death (apoptosis) is turned on in damaged or unwanted cells to secure their clean and safe self-elimination. The initial apoptotic events are coordinated in mitochondria, whereby several proapoptotic factors, including cytochrome c, are released into the cytosol to trigger caspase cascades. The release mechanisms include interactions of B-cell/lymphoma 2 family proteins with a mitochondria-specific phospholipid, cardiolipin, to cause permeabilization of the outer mitochondrial membrane. Using oxidative lipidomics, we showed that cardiolipin is the only phospholipid in mitochondria that undergoes early oxidation during apoptosis. The oxidation is catalyzed by a cardiolipin-specific peroxidase activity of cardiolipin-bound cytochrome c. In a previously undescribed step in apoptosis, we showed that oxidized cardiolipin is required for the release of proapoptotic factors. These results provide insight into the role of reactive oxygen species in triggering the cell-death pathway and describe an early role for cytochrome c before caspase activation.

Monolysocardiolipins accumulate in Barth syndrome but do not lead to enhanced apoptosis

Barth syndrome (BTHS) is an X-linked recessive disorder that is biochemically characterized by low cellular levels of the mitochondrial phospholipid cardiolipin (CL). Previously, we discovered that the yeast disruptant of the TAZ ortholog in Saccharomyces cerevisiae not only displays CL deficiency but also accumulates monolysocardiolipins (MLCLs), which are intermediates in CL remodeling. Therefore, we set out to investigate whether MLCL accumulation also occurs in BTHS. Indeed, we observed MLCL accumulation in heart, muscle, lymphocytes, and cultured lymphoblasts of BTHS patients; however, only very low levels of these lysophospholipids were found in platelets and fibroblasts of these patients. Although the fatty acid composition of the MLCLs was different depending on the tissue source, it did parallel the fatty acid composition of the (remaining) CLs. The possible implications of these findings for the two reported CL remodeling mechanisms, transacylation and deacylation/reacylation, are discussed. Because MLCLs have been proposed to be involved in the initiation of apoptosome-mediated cell death by the sequestration of the proapoptotic protein (t)BH3-interacting domain death agonist (Bid) to the mitochondrial membrane, we used control and BTHS lymphoblasts to investigate whether the accumulation of MLCLs results in higher levels of apoptosis. We found no differences in susceptibility to death receptor-mediated apoptosis or in cellular distribution of Bid, cytochrome c, and other parameters, implying that MLCL accumulation does not lead to enhanced apoptosis in cultured BTHS lymphoblasts.

Valine 181 is critical for the nucleotide exchange activity of human mitochondrial ADP/ATP carriers in yeast

We isolated yeast Saccharomyces cerevisiae cells transformed with one of the three human adenine nucleotide carrier genes (HANC) that exhibited higher growth capacity than previously observed. The HANC genes were isolated from these clones, and we identified two independent mutations of HANC that led to replacement of valine 181 located in the fourth transmembrane segment by methionine or phenylalanine. Tolerance of this position toward substitution with various amino acids was systematically investigated, and since HANC/V181M was among the most efficient in growth complementation, it was more extensively studied. Here we show that increased growth capacities were associated with higher ADP/ATP exchange activities and not with higher human carrier amount in yeast mitochondria. These results are discussed in the light of the bovine Ancp structure, that shares more than 90% amino acid identity with Hancps, and its interaction with the lipid environment.

Neosynthesis of cardiolipin in Rhodobacter sphaeroides under osmotic stress

The phospholipid composition of Rhodobacter sphaeroides cells resuspended in various hypertonic solutions has been examined by thin-layer chromatography and ESI mass spectrometry. R. sphaeroides responds to hyperosmotic stress by increasing the amount of cardiolipin in the membranes; this phenomenon occurs in spheroplasts also. Cardiolipin increases quickly and continuously during the time when the cells are resuspended in hypertonic medium. The optimum of stimulation of the neosynthesis of cardiolipin during osmotic stress was found to be at external 1 osm. ESI-MS analyses allowed the identification of two different cardiolipins in R. sphaeroides: the tetravaccenylcardiolipin ([M - H](-), m/z 1456.9) and the trivaccenylmonopalmitoylcardiolipin ([M - H](-), m/z 1430.0).

Electrostatic interactions between model mitochondrial membranes

Lipids are very diverse in both their respective structures and functions; and cells exquisitely control membrane composition. One intriguing issue is the specific role of lipids in modulating the physical properties of membranes. Cardiolipin (CL) is a unique four-tailed, doubly negatively charged lipid found predominately within the inner mitochondrial membrane, and is thought to be influential in determining the inner mitochondrial membrane potential and permeability. To determine the role of cardiolipin in modulating the charge properties of membranes, this study investigated the electrostatic interactions between mixed cardiolipin and phosphatidylcholine bilayers as a function of cardiolipin concentration. For physiologically relevant concentrations of cardiolipin, the surface charge density of the membrane was found to increase linearly with increasing concentration of cardiolipin. However, only a fraction of the cardiolipin molecules predicted to carry a charge from pK-values were ionized. Clearly environmental factors, beyond that of pH, play a role in determining the charge of bilayers containing cardiolipin.

Lipids in membrane protein structures

This review describes the recent knowledge about tightly bound lipids in membrane protein structures and deduces general principles of the binding interactions. Bound lipids are grouped in annular, nonannular, and integral protein lipids. The importance of lipid binding for vertical positioning and tight integration of proteins in the membrane, for assembly and stabilization of oligomeric and multisubunit complexes, for supercomplexes, as well as their functional roles are pointed out. Lipid binding is stabilized by multiple noncovalent interactions from protein residues to lipid head groups and hydrophobic tails. Based on analysis of lipids with refined head groups in membrane protein structures, distinct motifs were identified for stabilizing interactions between the phosphodiester moieties and side chains of amino acid residues. Differences between binding at the electropositive and electronegative membrane side, as well as a preferential binding to the latter, are observed. A first attempt to identify lipid head group specific binding motifs is made. A newly identified cardiolipin binding site in the yeast cytochrome bc(1) complex is described. Assignment of unsaturated lipid chains and evolutionary aspects of lipid binding are discussed.

Phospholipid abnormalities in children with Barth syndrome

OBJECTIVES

We sought to identify characteristic lipid abnormalities in patients with Barth syndrome (BTHS) and to correlate the lipid profile to phenotype and genotype.

BACKGROUND

Barth syndrome typically includes cardiomyopathy, skeletal myopathy, neutropenia, growth retardation, and 3-methylglutaconic aciduria, and it is commonly associated with mutations in the tafazzin (TAZ) gene, whose products are homologous to phospholipid acyltransferases. However, clinical features of BTHS have also been found in patients with normal TAZ gene.

METHODS

We analyzed molecular species of phospholipids in left and right ventricle, skeletal muscle, platelets, lymphoblasts, and fibroblasts from 19 children with BTHS (positive TAZ mutation), 6 children with BTHS-like syndromes (wild-type TAZ), 4 children with isolated cardiomyopathy (wild-type TAZ), and various controls.

RESULTS

Cardiolipin, the specific lipid found only in mitochondria, was decreased in all tissues from BTHS patients, whereas concentrations of other phospholipids were normal. The molecular composition of cardiolipin was altered in all tissues from BTHS patients. The molecular compositions of phosphatidylcholine and phosphatidylethanolamine were altered in the heart. Cardiolipin abnormalities were only found in children with true BTHS, not in children with BTHS-like disease or with isolated cardiomyopathy. The degree of cardiolipin deficiency was tissue-specific but did not correlate with severity or specific phenotypic expression of BTHS.

CONCLUSIONS

Abnormal cardiolipin is a specific diagnostic marker of cardiomyopathies caused by TAZ mutations. These mutations lead to alterations in the fatty acid composition of several phospholipids, supporting the idea that TAZ encodes a human acyltransferase.

Remodeling of Cardiolipin by Phospholipid Transacylation

Mitochondrial cardiolipin (CL) contains unique fatty acid patterns, but it is not known how the characteristic molecular species of CL are formed. We found a novel reaction that transfers acyl groups from phosphatidylcholine or phosphatidylethanolamine to CL in mitochondria of rat liver and human lymphoblasts. Acyl transfer was stimulated by ADP, ATP, and ATP gamma S, but not by other nucleotides. Coenzyme A stimulated the reaction only in the absence of adenine nucleotides. Free fatty acids were not incorporated into CL under the same incubation condition. The transacylation required addition of exogenous CL or monolyso-CL, whereas dilyso-CL was not a substrate. Transacylase activity was decreased in lymphoblasts from patients with Barth syndrome (tafazzin deletion), and this was accompanied by drastic changes in the molecular composition of CL. In rat liver, where linoleic acid was the most abundant residue of CL, only linoleoyl groups were transferred into CL, but not oleoyl or arachidonoyl groups. We demonstrated complete remodeling of tetraoleoyl-CL to tetralinoleoyl-CL in rat liver mitochondria and identified the intermediates linoleoyl-trioleoyl-CL, dilinoleoyl-dioleoyl-CL, and trilinoleoyl-oleoyl-CL by high-performance liquid chromatography. The data suggest that CL is remodeled by acyl specific phospholipid transacylation and that tafazzin is an acyltransferase involved in this mechanism.

Regulation of phosphatidylglycerophosphate synthase by inositol in Saccharomyces cerevisiae is not at the level of PGS1 mRNA abundance

Phosphatidylglycerophosphate synthase catalyzes the committed step in the synthesis of the mitochondrial phospholipid cardiolipin. We showed previously that phosphatidylglycerophosphate synthase activity in Saccharomyces cerevisiae is increased in conditions favoring mitochondrial development and during growth in the absence of inositol. Interestingly, the regulatory effects of inositol were not altered in ino2, ino4, or opi1 mutants suggesting that regulation in response to inositol is not at the level of gene transcription. We report here that steady state mRNA levels of the PGS1 gene, which encodes phosphatidylglycerophosphate synthase, were not altered by inositol or choline. Growth in the presence of the inositol-depleting drug valproate led to an increase in phosphatidylglycerophosphate synthase activity unaccompanied by increased PGS1 mRNA. PGS1 mRNA abundance was not decreased in ino2 or ino4 mutants and was unaffected in an opi1 mutant. Therefore, regulation of phosphatidylglycerophosphate synthase by inositol is not mediated at the level of mRNA abundance and does not require the INO2-INO4-OPI1 regulatory circuit. PGS1 was increased in glycerol/ethanol compared with glucose media and was maximally expressed as cells entered the stationary phase. Deletion of the mitochondrial genome did not affect PGS1 expression. Thus, whereas inositol controls phosphatidylglycerophosphate synthase activity, regulation of PGS1 expression occurs primarily in response to mitochondrial development cues.

Citrate carrier activity and cardiolipin level in eel (Anguilla anguilla) liver mitochondria

The activity of the tricarboxylate (citrate) carrier has been assayed in intact liver mitochondria from yellow eel (Anguilla anguilla) and compared to that from rat. The eel-citrate carrier specific activity was approximately 1.7-fold higher than that assayed in rat-liver mitochondria. The content of the main mitochondrial phospholipids, phosphatidylethanolamine and phosphatidylcholine, did not show a significant difference between the two species, while in eel a higher cardiolipin level was observed. Fatty acid composition of eel-liver mitochondrial phospholipids was characterised by a large amount of unsaturated fatty acids, dominated by octadecaenoic acid (C(18:1) (n-9)) and docosahexaenoic acid (C(22:6) (n-3)). The cardiolipin fatty acid pattern of eel-liver mitochondria showed, with respect to the rat, a higher C(20:5) (n-3) and C(22:6) (n-3) content and a lower amount of C(18:2) (n-6) and C(20:4) (n-6). A noticeable activity of lipogenic enzymes was also detected in eel liver cytosol. The results of this study suggest that the remarkable activity of the citrate carrier in eel-liver mitochondria can most likely be ascribed to a considerable cardiolipin level. A covariance of citrate carrier and lipogenic enzyme activities was observed.

Decreased cardiolipin synthesis corresponds with cytochrome c release in palmitate-induced cardiomyocyte apoptosis

Apoptosis has been identified recently as a component of many cardiac pathologies. However, the potential triggers of programmed cell death in the heart and the involvement of specific metabolic pathway(s) are less well characterized. Detachment of cytochrome c from the mitochondrial inner membrane is a necessary first step for cytochrome c release into the cytosol and initiation of apoptosis. The saturated long chain fatty acid, palmitate, induces apoptosis in rat neonatal cardiomyocytes and diminishes content of the mitochondrial anionic phospholipid, cardiolipin. These changes are accompanied by 1) acyl chain saturation of phosphatidic acid and phosphatidylglycerol, 2) large increases in the levels of these two phospholipids, and 3) a decline in cardiolipin synthesis. Although cardiolipin synthase activity is unchanged, saturated phosphatidylglycerol is a poor substrate for this enzyme. Under these conditions, decreased cardiolipin synthesis and release of cytochrome c are directly and significantly correlated. The results suggest that phosphatidylglycerol saturation and subsequent decreases in cardiolipin affect the association of cytochrome c with the inner mitochondrial membrane, directly influencing the pathway to cytochrome c release and subsequent apoptosis.

Cardiolipins and mitochondrial proton-selective leakage

The proton-selective leak (State 4 respiratory rate) but not delta psi, in mitochondria from thyroid-sensitive tissues, responds to in vivo stimuli in unique correlation with changes in cardiolipins, saturated and mono-unsaturated (extended) fatty acyl contents, cardiolipins/phospholipids ratios, and/or membrane outer-sidedness. Liver mitochondrial State 4 respiration, basal in fasted rats, contributes little to resting metabolic rate in fed rats, where State 3 depresses delta psi. In a proposed model, an essential inner-membrane outer-surface proton antenna collects protons and donates them, via a water-shuttle, to transmembrane porters: transient water-molecule-chains between extended phospholipid acyls; protonophores, and uncoupling proteins. Only cardiolipin microdomains can donate, from an anomalously-dissociating phosphate group in each headgroup; unadapted cardiolipins have few conducting water chains. Thyroid states regulate each cardiolipin property, and are permissive, via the proton antenna, for proton leaks, including those through adapted and possibly constitutive BAT and ectopic uncoupling proteins. Slow leakage in liposomes may reflect insufficient cardiolipin proton antennas.

Specific cardiolipin binding interferes with labeling of sulfhydryl residues in the adenosine diphosphate/adenosine triphosphate carrier protein from beef heart mitochondria

The interaction of cardiolipin with the isolated ADP/ATP carrier protein from beef heart mitochondria has been studied by means of the unmasking of a single cysteinyl residue, Cys56, which accompanies the conformational transition of the protein [Leblanc, P., & Clauser, H, (1972) FEBS Lett. 23, 107-113]. The unmasking was monitored by using the static fluorescence of the sulfhydryl reagent N-(1-pyrenyl)maleimide (PYM). The rate of PYM binding that was observed after initiation of the conformational transition by ADP was drastically reduced in the presence of cardiolipin (CL). Phospholipids other than CL were much less effective. It can be shown that the conformational transition and the binding reaction are both affected by CL, although to varying extents. An enhancement of the rate of the ADP-dependent PYM binding was observed upon digestion of the protein bound phospholipid by phospholipase A2. The phospholipase treatment also led to an increased ADP-independent PYM binding, thus indicating that the ADP control of the carrier transition was gradually lost. The ADP control could be fully restored through the addition of CL, provided that the phospholipase incubation had been terminated after approximately 1 h. These results will be discussed in relation to an earlier report of tight cardiolipin binding [Beyer, K., & Klingenberg, M. (1985) Biochemistry 24, 3821-3826] and to current structural models of the ADP/ATP carrier protein.

Large-scale chromatographic purification of F1F0-ATPase and complex I from bovine heart mitochondria

A new chromatographic procedure has been developed for the isolation of F1F0-ATPase and NADH:ubiquinone oxidoreductase (complex I) from a single batch of bovine heart mitochondria. The method employed dodecyl beta-delta-maltoside, a monodisperse, homogeneous detergent in which many respiratory complexes exhibit high activity, for solubilization and subsequent purification by ammonium sulphate fractionation and column chromatography. A combination of anion-exchange, gel-filtration, and dye-ligand affinity chromatography was used to purify both complexes to homogeneity. The F1F0-ATPase preparation contains only the 16 known subunits of the enzyme. It has oligomycin-sensitive ATP hydrolysis activity and, as demonstrated elsewhere, when reconstituted into lipid vesicles it is capable of ATP-dependent proton pumping and of ATP synthesis driven by a proton gradient [Groth and Walker (1996) Biochem. J. 318, 351-357]. The complex I preparation contains all of the subunits identified in other preparations of the enzyme, and has rotenone-sensitive NADH:ubiquinone oxidoreductase and NADH:ferricyanide oxidoreductase activities. The procedure is rapid and reproducible, yielding 50-80 mg of purified F1F0-ATPase and 20-40 mg of purified complex I from 1 g of mitochondrial membranes. Both preparations are devoid of phospholipids, and gel filtration and dynamic light scattering experiments indicate that they are monodisperse. Therefore, the preparations fulfil important prerequisites for structural analysis.

Mitochondrial cardiolipin in diverse eukaryotes. Comparison of biosynthetic reactions and molecular acyl species

Cardiolipin, a unique dimeric phospholipid of bacteria and mitochondria, can be synthesized by two alternative pathways discovered in rat and Escherichia coli, respectively. In mitochondrial preparations from fungi (Saccharomyces cerevisiae, Neurospora crassa), higher plants (Phaseolus aureus), molluscs (Mytilus edulis) and mammals (rat liver, bovine adrenal gland), cardiolipin was synthesized from CDP-diacylglycerol and phosphatidylglycerol, suggesting a common eukaryotic mechanism of cardiolipin formation which is in contrast to the prokaryotic biosynthesis from two molecules of phosphatidylglycerol. All mitochondrial cardiolipin synthases were inhibited by lysophosphatidylglycerol, were insensitive to N-ethylmaleimide and required divalent cations, although they had different cation specificities. The molecular species of cardiolipin from rat liver, bovine heart, S. cerevisiae and N. crassa were analysed by high-performance liquid chromatography of the derivative 1,3-bis[3'-sn-phosphatidyl]-2-benzoyl-sn-glycerol dimethyl ester. Cardiolipins from these organisms contained mainly monounsaturated or diunsaturated chains with 16 or 18 carbon atoms, resulting in a relatively homogeneous distribution of double bonds and carbon numbers among the four acyl positions. About half of the molecular species were symmetrical, i.e. they combined two identical diacylglycerol moieties. In N. crassa, the same species pattern was found at growth temperatures of 25 degrees C and 37 degrees C. Tentative molecular models were created for the most abundant molecular species and subjected to energy minimization. Geometric data, derived from these models, suggested similarities in the gross structure of the major cardiolipin species from different sources.