Binding of 10-N-nonyl acridine orange to cardiolipin-deficient yeast cells: Implications for assay of cardiolipin

Polydatin Prevents Electron Transport Chain Dysfunction and ROS Overproduction Paralleled by an Improvement in Lipid Peroxidation and Cardiolipin Levels in Iron-Overloaded Rat Liver Mitochondria

Increased intramitochondrial free iron is a key feature of various liver diseases, leading to oxidative stress, mitochondrial dysfunction, and liver damage. Polydatin is a polyphenol with a hepatoprotective effect, which has been attributed to its ability to enhance mitochondrial oxidative metabolism and antioxidant defenses, thereby inhibiting reactive oxygen species (ROS) dependent cellular damage processes and liver diseases. However, it has not been explored whether polydatin is able to exert its effects by protecting the phospholipid cardiolipin against damage from excess iron. Cardiolipin maintains the integrity and function of electron transport chain (ETC) complexes and keeps cytochrome c bound to mitochondria, avoiding uncontrolled apoptosis. Therefore, the effect of polydatin on oxidative lipid damage, ETC activity, cytochrome levels, and ROS production was explored in iron-exposed rat liver mitochondria. Fe2+ increased lipid peroxidation, decreased cardiolipin and cytochromes c + c1 and aa3 levels, inhibited ETC complex activities, and dramatically increased ROS production. Preincubation with polydatin prevented all these effects to a variable degree. These results suggest that the hepatoprotective mechanism of polydatin involves the attenuation of free radical production by iron, which enhances cardiolipin levels by counteracting membrane lipid peroxidation. This prevents the loss of cytochromes, improves ETC function, and decreases mitochondrial ROS production.

Setting the curve: the biophysical properties of lipids in mitochondrial form and function

Mitochondrial membranes are defined by their diverse functions, complex geometries, and unique lipidomes. In the inner mitochondrial membrane, highly curved membrane folds known as cristae house the electron transport chain and are the primary sites of cellular energy production. The outer mitochondrial membrane is flat by contrast, but is critical for the initiation and mediation of processes key to mitochondrial physiology: mitophagy, interorganelle contacts, fission and fusion dynamics, and metabolite transport. While the lipid composition of both the inner mitochondrial membrane and outer mitochondrial membrane have been characterized across a variety of cell types, a mechanistic understanding for how individual lipid classes contribute to mitochondrial structure and function remains nebulous. In this review, we address the biophysical properties of mitochondrial lipids and their related functional roles. We highlight the intrinsic curvature of the bulk mitochondrial phospholipid pool, with an emphasis on the nuances surrounding the mitochondrially-synthesized cardiolipin. We also outline emerging questions about other lipid classes - ether lipids, and sterols - with potential roles in mitochondrial physiology. We propose that further investigation is warranted to elucidate the specific properties of these lipids and their influence on mitochondrial architecture and function.

CLiB - a novel cardiolipin-binder isolated via data-driven and in vitro screening

Cardiolipin, the mitochondria marker lipid, is crucially involved in stabilizing the inner mitochondrial membrane and is vital for the activity of mitochondrial proteins and protein complexes. Directly targeting cardiolipin by a chemical-biology approach and thereby altering the cellular concentration of "available" cardiolipin eventually allows to systematically study the dependence of cellular processes on cardiolipin availability. In the present study, physics-based coarse-grained free energy calculations allowed us to identify the physical and chemical properties indicative of cardiolipin selectivity and to apply these to screen a compound database for putative cardiolipin-binders. The membrane binding properties of the 22 most promising molecules identified in the in silico approach were screened in vitro, using model membrane systems finally resulting in the identification of a single molecule, CLiB (CardioLipin-Binder). CLiB clearly affects respiration of cardiolipin-containing intact bacterial cells as well as of isolated mitochondria. Thus, the structure and function of mitochondrial membranes and membrane proteins might be (indirectly) targeted and controlled by CLiB for basic research and, potentially, also for therapeutic purposes.

Data-driven discovery of cardiolipin-selective small molecules by computational active learning

Subtle variations in the lipid composition of mitochondrial membranes can have a profound impact on mitochondrial function. The inner mitochondrial membrane contains the phospholipid cardiolipin, which has been demonstrated to act as a biomarker for a number of diverse pathologies. Small molecule dyes capable of selectively partitioning into cardiolipin membranes enable visualization and quantification of the cardiolipin content. Here we present a data-driven approach that combines a deep learning-enabled active learning workflow with coarse-grained molecular dynamics simulations and alchemical free energy calculations to discover small organic compounds able to selectively permeate cardiolipin-containing membranes. By employing transferable coarse-grained models we efficiently navigate the all-atom design space corresponding to small organic molecules with molecular weight less than ≈500 Da. After direct simulation of only 0.42% of our coarse-grained search space we identify molecules with considerably increased levels of cardiolipin selectivity compared to a widely used cardiolipin probe 10-N-nonyl acridine orange. Our accumulated simulation data enables us to derive interpretable design rules linking coarse-grained structure to cardiolipin selectivity. The findings are corroborated by fluorescence anisotropy measurements of two compounds conforming to our defined design rules. Our findings highlight the potential of coarse-grained representations and multiscale modelling for materials discovery and design.

TTAPE-Me dye is not selective to cardiolipin and binds to common anionic phospholipids nonspecifically

Identification, visualization, and quantitation of cardiolipin (CL) in biological membranes is of great interest because of the important structural and physiological roles of this lipid. Selective fluorescent detection of CL using noncovalently bound fluorophore 1,1,2,2-tetrakis[4-(2-trimethylammonioethoxy)-phenylethene (TTAPE-Me) has been recently proposed. However, this dye was only tested on wild-type mitochondria or liposomes containing negligible amounts of other anionic lipids, such as phosphatidylglycerol (PG) and phosphatidylserine (PS). No clear preference of TTAPE-Me for binding to CL compared to PG and PS was found in our experiments on artificial liposomes, Escherichia coli inside-out vesicles, or Saccharomyces cerevisiae mitochondria in vitro or in situ, respectively. The shapes of the emission spectra for these anionic phospholipids were also found to be indistinguishable. Thus, TTAPE-Me is not suitable for detection, visualization, and localization of CL in the presence of other anionic lipids present in substantial physiological amounts. Our experiments and complementary molecular dynamics simulations suggest that fluorescence intensity of TTAPE-Me is regulated by dynamic equilibrium between emitting dye aggregates, stabilized by unspecific but thermodynamically favorable electrostatic interactions with anionic lipids, and nonemitting dye monomers. These results should be taken into consideration when interpreting past and future results of CL detection and localization studies with this probe in vitro and in vivo. Provided methodology emphasizes minimal experimental requirements, which should be considered as a guideline during the development of novel lipid-specific probes.

Mitochondrial Cristae Architecture and Functions: Lessons from Minimal Model Systems

Mitochondria are known as the powerhouse of eukaryotic cells. Energy production occurs in specific dynamic membrane invaginations in the inner mitochondrial membrane called cristae. Although the integrity of these structures is recognized as a key point for proper mitochondrial function, less is known about the mechanisms at the origin of their plasticity and organization, and how they can influence mitochondria function. Here, we review the studies which question the role of lipid membrane composition based mainly on minimal model systems.

Induction of P-glycoprotein expression and activity by Aconitum alkaloids: Implication for clinical drug-drug interactions

The Aconitum species, which mainly contain bioactive Aconitum alkaloids, are frequently administered concomitantly with other herbal medicines or chemical drugs in clinics. The potential risk of drug–drug interactions (DDIs) arising from co-administration of Aconitum alkaloids and other drugs against specific targets such as P-glycoprotein (P-gp) must be evaluated. This study focused on the effects of three representative Aconitum alkaloids: aconitine (AC), benzoylaconine (BAC), and aconine, on the expression and activity of P-gp. We observed that Aconitum alkaloids increased P-gp expression in LS174T and Caco-2 cells in the order AC > BAC > aconine. Nuclear receptors were involved in the induction of P-gp. AC and BAC increased the P-gp transport activity. Strikingly, intracellular ATP levels and mitochondrial mass also increased. Furthermore, exposure to AC decreased the toxicity of vincristine and doxorubicin towards the cells. In vivo, AC significantly up-regulated the P-gp protein levels in the jejunum, ileum, and colon of FVB mice, and protected them against acute AC toxicity. Taken together, the findings of our in vitro and in vivo experiments indicate that AC can induce P-gp expression, and that co-administration of AC with P-gp substrate drugs may cause DDIs. Our findings have important implications for Aconitum therapy in clinics.

Carbon Monoxide-Releasing Molecules Attenuate Postresuscitation Myocardial Injury and Protect Cardiac Mitochondrial Function by Reducing the Production of Mitochondrial Reactive Oxygen Species in a Rat Model of Cardiac Arrest

The objective of this study is to examine whether carbon monoxide-releasing molecules (CORMs) can decrease the generation of excessive reactive oxygen species (ROS) in cardiac mitochondria, thereby protecting against postresuscitation myocardial injury and cardiac mitochondrial dysfunction after resuscitation in a rat model of ventricular fibrillation (VF), and further investigated the underlying mechanism. Rats suffered 8 minutes of untreated VF and resuscitation and were randomized into the control group with vehicle infusion and the CORM group with CO-releasing molecule 2 (CORM2) treatment. Animals in the Sham group were instrumented without induced VF and resuscitation. Effects of CORM2 on cardiac function, myocardial oxidative stress, cardiac mitochondrial function, and mitochondrial ROS generation were assessed. Moreover, to further evaluate the direct effect of CORM2 on cardiac mitochondria isolated from resuscitated rats, we measured mitochondrial function and ROS generation when isolated cardiac mitochondria were directly incubated with different concentrations of (CORM2). Compared with the Sham group, the control and CORM groups demonstrated impaired cardiac function, increased myocardial injury, and aggravated mitochondrial damage. CORM2 improved cardiac performance and attenuated myocardial damage and oxidative stress in resuscitated rats. Additionally, animals with CORM2 treatment showed the decreased generation of cardiac mitochondrial ROS, alleviated mitochondrial injury, and preserved mitochondrial function and complex activities when compared with the control group. In isolated cardiac mitochondria incubated with CORM2, low concentrations of CORM2 (20 μmol/L) mildly uncoupled mitochondrial respiration, leading to reduced mitochondrial ROS production. In contrast, high concentrations of CORM2 (60 μmol/L) resulted in the reverse effect presumably due to its excessive uncoupling action. These findings suggest that CORM2 attenuates oxidative stress of the heart and improves cardiac function after resuscitation. The mechanism was probably that CO, the product of CORM2, reduces the production of cardiac mitochondrial ROS and thereby attenuates mitochondrial injury and dysfunction during the postresuscitation period, due to the transient uncoupling of mitochondrial respiration.

Localization of anionic phospholipids in Escherichia coli cells

Cardiolipin (CL) is an anionic phospholipid with a characteristically large curvature and is of growing interest for two primary reasons: (i) it binds to and regulates many peripheral membrane proteins in bacteria and mitochondria, and (ii) it is distributed asymmetrically in rod-shaped cells and is concentrated at the poles and division septum. Despite the growing number of studies of CL, its function in bacteria remains unknown. 10-N-Nonyl acridine orange (NAO) is widely used to image CL in bacteria and mitochondria, as its interaction with CL is reported to produce a characteristic red-shifted fluorescence emission. Using a suite of biophysical techniques, we quantitatively studied the interaction of NAO with anionic phospholipids under physiologically relevant conditions. We found that NAO is promiscuous in its binding and has photophysical properties that are largely insensitive to the structure of diverse anionic phospholipids to which it binds. Being unable to rely solely on NAO to characterize the localization of CL in Escherichia coli cells, we instead used quantitative fluorescence microscopy, mass spectrometry, and mutants deficient in specific classes of anionic phospholipids. We found CL and phosphatidylglycerol (PG) concentrated in the polar regions of E. coli cell membranes; depletion of CL by genetic approaches increased the concentration of PG at the poles. Previous studies suggested that some CL-binding proteins also have a high affinity for PG and display a pattern of cellular localization that is not influenced by depletion of CL. Framed within the context of these previous experiments, our results suggest that PG may play an essential role in bacterial physiology by maintaining the anionic character of polar membranes.

The mitochondrial rhomboid protease: its rise from obscurity to the pinnacle of disease-relevant genes

The Rhomboid proteases belong to a highly conserved family of proteins that are present in all branches of life. In Drosophila, the secretory pathway-localized rhomboid proteases are crucial for epidermal growth factor (EGF) signaling. The identification of a mitochondrial-localized rhomboid protease shed light on other functions of rhomboid proteases including the maintenance of mitochondrial morphology and the regulation of apoptosis. More recent work has revealed other functions of the mitochondrial rhomboid protease in mitochondrial and cellular biology, failure of which have been implicated in human diseases. In this review, we will summarize the current knowledge and disease relevance of the mitochondrial-localized rhomboid protease. This article is part of a Special Issue entitled: Intramembrane Proteases.

Simple enrichment and analysis of plasma lysophosphatidic acids

A simple and highly efficient technique for the analysis of lysophosphatidic acid (LPA) subspecies in human plasma is described. The streamlined sample preparation protocol furnishes the five major LPA subspecies with excellent recoveries. Extensive analysis of the enriched sample reveals only trace levels of other phospholipids. This level of purity not only improves MS analyses, but enables HPLC post-column detection in the visible region with a commercially available fluorescent phospholipids probe. Human plasma samples from different donors were analyzed using the above method and validated by LC-ESI/MS/MS.

The dynamics of the mitochondrial organelle as a potential therapeutic target

Mitochondria play a central role in cell fate after stressors such as ischemic brain injury. The convergence of intracellular signaling pathways on mitochondria and their release of critical factors are now recognized as a default conduit to cell death or survival. Besides the individual processes that converge on or emanate from mitochondria, a mitochondrial organellar response to changes in the cellular environment has recently been described. Whereas mitochondria have previously been perceived as a major center for cellular signaling, one can postulate that the organelle's dynamics themselves affect cell survival. This brief perspective review puts forward the concept that disruptions in mitochondrial dynamics--biogenesis, clearance, and fission/fusion events--may underlie neural diseases and thus could be targeted as neuroprotective strategies in the context of ischemic injury. To do so, we present a general overview of the current understanding of mitochondrial dynamics and regulation. We then review emerging studies that correlate mitochondrial biogenesis, mitophagy, and fission/fusion events with neurologic disease and recovery. An overview of the system as it is currently understood is presented, and current assessment strategies and their limitations are discussed.

Changes in mitochondrial surface charge mediate recruitment of signaling molecules during apoptosis

Electrostatic interactions with negative lipids contribute to the subcellular localization of polycationic proteins. In situ measurements using cytosolic probes of surface charge indicate that normal mitochondria are not noticeably electronegative. However, during apoptosis mitochondria accrue negative charge and acquire the ability to attract cationic proteins, including K-Ras. The marked increase in the surface charge of mitochondria occurs early in apoptosis, preceding depolarization of their inner membrane, cytochrome c release, and flipping of phosphatidylserine across the plasmalemma. Using novel biosensors, we determined that the increased electronegativity of the mitochondria coincided with and was likely attributable to increased exposure of cardiolipin, which is dianionic. Ectopic (over)expression of cardiolipin-binding proteins precluded the increase in surface charge and inhibited apoptosis, implying that mitochondrial exposure of negatively charged lipids is required for progression of programmed cell death.

Cardiolipin membrane domains in prokaryotes and eukaryotes

Cardiolipin (CL) plays a key role in dynamic organization of bacterial and mitochondrial membranes. CL forms membrane domains in bacterial cells, and these domains appear to participate in binding and functional regulation of multi-protein complexes involved in diverse cellular functions including cell division, energy metabolism, and membrane transport. Visualization of CL domains in bacterial cells by the fluorescent dye 10-N-nonyl acridine orange is critically reviewed. Possible mechanisms proposed for CL dynamic localization in bacterial cells are discussed. In the mitochondrial membrane CL is involved in organization of multi-subunit oxidative phosphorylation complexes and in their association into higher order supercomplexes. Evidence suggesting a possible role for CL in concert with ATP synthase oligomers in establishing mitochondrial cristae morphology is presented. Hypotheses on CL-dependent dynamic re-organization of the respiratory chain in response to changes in metabolic states and CL dynamic re-localization in mitochondria during the apoptotic response are briefly addressed.

Dynamics of mitochondrial structure during apoptosis and the enigma of Opa1

"The large scale remodeling of mitochondria during apoptosis is a necessary step for the complete release of cytochrome c" has been a tenet since 2002. However, more recent findings strongly indicate that the large-scale remodeling previously described actually takes place after the release of cytochrome c and in a caspase-dependent manner, bringing into question whether mitochondria remodeling is necessary. In a more recent article, however, it was shown that a much more subtle form of remodeling is taking place which is only observable by electron tomography. In the Bcl-2 inhibitable Bax/Bak-dependent intrinsic pathway of apoptosis, the release of cytochrome c from mitochondria is a consequence of two carefully coordinated events: formation of outer membrane pores and opening of crista junctions triggered by Opa1 oligomer disassembly, and both steps are necessary for the complete release of cytochrome c. We review the recent literature pertaining to the coordinated release of cytochrome c during cell death.

The microaerophilic flagellate, Trichomonas vaginalis, contains unusual acyl lipids but no detectable cardiolipin

Previous lipid analysis of trichomonads has led to controversy as to whether these hydrogenosome-containing organisms contain cardiolipin (CL), which is a characteristic component of mitochondria. Here we report a careful lipid analysis of the sexually transmitted protist Trichomonas vaginalis. Major lipids were phosphatidylethanolamine (42%) and phosphatidylcholine (20%) with lesser amounts of phosphatidylglycerol (PG) (12%) and non-polar components. Two unusual lipids, acyl-PG (8%) and ceramide phosphorylethanolamine (2%), were also significant components. The structures of these lipids were confirmed by tandem mass spectrometry following reverse-phase high-performance liquid chromatography. This is the first time ceramide phosphorylethanolamine has been reported in a trichomonad. In contrast, CL (diphosphatidylglycerol) could not be detected either by two-dimensional thin-layer chromatography or by mass spectrometry. These data are discussed in relation to the organism's phylogenetic origin as a parasite showing secondary adaptation to microaerobic conditions.

Mitochondria-targeted plastoquinone derivatives as tools to interrupt execution of the aging program. 1. Cationic plastoquinone derivatives: synthesis and in vitro studies

Synthesis of cationic plastoquinone derivatives (SkQs) containing positively charged phosphonium or rhodamine moieties connected to plastoquinone by decane or pentane linkers is described. It is shown that SkQs (i) easily penetrate through planar, mitochondrial, and outer cell membranes, (ii) at low (nanomolar) concentrations, posses strong antioxidant activity in aqueous solution, BLM, lipid micelles, liposomes, isolated mitochondria, and cells, (iii) at higher (micromolar) concentrations, show pronounced prooxidant activity, the "window" between anti- and prooxidant concentrations being very much larger than for MitoQ, a cationic ubiquinone derivative showing very much lower antioxidant activity and higher prooxidant activity, (iv) are reduced by the respiratory chain to SkQH2, the rate of oxidation of SkQH2 being lower than the rate of SkQ reduction, and (v) prevent oxidation of mitochondrial cardiolipin by OH*. In HeLa cells and human fibroblasts, SkQs operate as powerful inhibitors of the ROS-induced apoptosis and necrosis. For the two most active SkQs, namely SkQ1 and SkQR1, C(1/2) values for inhibition of the H2O2-induced apoptosis in fibroblasts appear to be as low as 1x10(-11) and 8x10(-13) M, respectively. SkQR1, a fluorescent representative of the SkQ family, specifically stains a single type of organelles in the living cell, i.e. energized mitochondria. Such specificity is explained by the fact that it is the mitochondrial matrix that is the only negatively-charged compartment inside the cell. Assuming that the Deltapsi values on the outer cell and inner mitochondrial membranes are about 60 and 180 mV, respectively, and taking into account distribution coefficient of SkQ1 between lipid and water (about 13,000 : 1), the SkQ1 concentration in the inner leaflet of the inner mitochondrial membrane should be 1.3x10(8) times higher than in the extracellular space. This explains the very high efficiency of such compounds in experiments on cell cultures. It is concluded that SkQs are rechargeable, mitochondria-targeted antioxidants of very high efficiency and specificity. Therefore, they might be used to effectively prevent ROS-induced oxidation of lipids and proteins in the inner mitochondrial membrane in vivo.

Critical age-related loss of cofactors of neuron cytochrome C oxidase reversed by estrogen

The mechanistic basis for the correlation between mitochondrial dysfunction and neurodegenerative disease is unclear, but evidence supports involvement of cytochrome C oxidase (CCO) deficits with age. Neurons isolated from the brains of 24 month and 9 month rats and cultured in common conditions provide a model of intrinsic neuronal aging. In situ CCO activity was decreased in 24 month neurons relative to 9 month neurons. Possible CCO-related deficits include holoenzyme activity, cofactor, and substrate. No difference was found between neurons from 24 month and 9 month rats in mitochondrial counts per neuron, CCO activity in submitochondrial particles, or basal respiration. Immunostaining for cytochrome C in individual mitochondria revealed an age-related deficit of this electron donor. 24 month neurons did not have adequate respiratory capacity to upregulate respiration after a glutamate stimulus, in spite of a two-fold upregulation of respiration seen in 9 month neurons. Respiration in 24 month neurons was inhibited by lower concentrations of potassium cyanide, suggesting a 50% deficit in functional enzyme in 24 month compared to 9 month neurons. In addition to cytochrome C, CCO requires cardiolipin to function. Staining with nonylacridine orange revealed an age-related deficit in cardiolipin. Treatment of 24 month neurons with 17-beta-estradiol restored cardiolipin levels (10 ng/mL) and upregulated respiration under glutamate stress (1 pg/mL). Attempts to induce mitochondrial turnover by neuronal multiplication also rejuvenated CCO activity in 24 month neurons. These data suggest cytochrome C and cardiolipin levels are deficient in 24 month neurons, preventing normal upregulation of respiration needed for oxidative phosphorylation in response to stress. Furthermore, the data suggest this deficit can be corrected with estrogen treatment.

Mechanism of the elevation in cardiolipin during HeLa cell entry into the S-phase of the human cell cycle

CL (cardiolipin) is a key phospholipid involved in ATP generation. Since progression through the cell cycle requires ATP we examined regulation of CL synthesis during S-phase in human cells and investigated whether CL or CL synthesis was required to support nucleotide synthesis in S-phase. HeLa cells were made quiescent by serum depletion for 24 h. Serum addition resulted in substantial stimulation of [methyl-3H]thymidine incorporation into cells compared with serum-starved cells by 8 h, confirming entry into the S-phase. CL mass was unaltered at 8 h, but increased 2-fold by 16 h post-serum addition compared with serum-starved cells. The reason for the increase in CL mass upon entry into S-phase was an increase in activity and expression of CL de novo biosynthetic and remodelling enzymes and this paralleled the increase in mitochondrial mass. CL de novo biosynthesis from D-[U-14C]glucose was elevated, and from [1,3-3H]glycerol reduced, upon serum addition to quiescent cells compared with controls and this was a result of differences in the selection of precursor pools at the level of uptake. Triascin C treatment inhibited CL synthesis from [1-14C]oleate but did not affect [methyl-3H]thymidine incorporation into HeLa cells upon serum addition to serum-starved cells. Barth Syndrome lymphoblasts, which exhibit reduced CL, showed similar [methyl-3H]thymidine incorporation into cells upon serum addition to serum-starved cells compared with cells from normal aged-matched controls. The results indicate that CL de novo biosynthesis is up-regulated via elevated activity and expression of CL biosynthetic genes and this accounted for the doubling of CL seen during S-phase; however, normal de novo CL biosynthesis or CL itself is not essential to support nucleotide synthesis during entry into S-phase of the human cell cycle.

An improved method for separating cardiolipin by HPLC

Herein we report an improved method to separate cardiolipin (Ptd(2)Gro) from tissue total lipid extracts using a biphasic solvent system combined with high performance liquid chromatography. This method uses a normal phase silica column and two mobile phases: mobile phase A that was n-hexane:2-propanol (3:2 by vol) and mobile phase B that was n-hexane:2-propanol:water (56.7:37.8:5.5 by vol). The initial solvent conditions were 95% A and 5% B, with a flow rate of 1.5 mL/min. The samples were from non-derivatized aliquots of liver, heart, or brain lipid extracts. The peak corresponding to Ptd(2)Gro appeared at 31 min, was well defined and did not overlap with neighboring peaks. The adjacent peak corresponded to ethanolamine glycerophospholipids and the remaining phospholipids were eluted in a single peak. The identity of the phospholipids separated by this method was verified by thin layer chromatography (TLC) and fatty acid analysis, which confirmed that the Ptd(2)Gro was well resolved from other phospholipids. This method is useful to separate and quantify Ptd(2)Gro from small tissue samples thereby avoiding the variability associated with TLC methods.

Mitochondrial translocation of alpha-synuclein is promoted by intracellular acidification

Mitochondrial dysfunction plays a central role in the selective vulnerability of dopaminergic neurons in Parkinson's disease (PD) and is influenced by both environmental and genetic factors. Expression of the PD protein alpha-synuclein or its familial mutants often sensitizes neurons to oxidative stress and to damage by mitochondrial toxins. This effect is thought to be indirect, since little evidence physically linking alpha-synuclein to mitochondria has been reported. Here, we show that the distribution of alpha-synuclein within neuronal and non-neuronal cells is dependent on intracellular pH. Cytosolic acidification induces translocation of alpha-synuclein from the cytosol onto the surface of mitochondria. Translocation occurs rapidly under artificially-induced low pH conditions and as a result of pH changes during oxidative or metabolic stress. Binding is likely facilitated by low pH-induced exposure of the mitochondria-specific lipid cardiolipin. These results imply a direct role for alpha-synuclein in mitochondrial physiology, especially under pathological conditions, and in principle, link alpha-synuclein to other PD genes in regulating mitochondrial homeostasis.

Targeting of mitochondria by 10-N-alkyl acridine orange analogues: role of alkyl chain length in determining cellular uptake and localization

10-N-Nonyl acridine orange (NAO) is used as a mitochondrial probe because of its high affinity for cardiolipin (CL). Targeting of NAO may also depend on mitochondrial membrane potential. As the nonyl group has been considered essential for targeting, a systematic study of alkyl chain length was undertaken; three analogues (10-methyl-, 10-hexyl-, and 10-hexadecyl-acridine orange) were synthesized and their properties studied in phospholipid monolayers and breast cancer cells. The shortest and longest alkyl chains reduced targeting, whereas the hexyl group was superior to the nonyl group, allowing very clear and specific targeting to mitochondria at concentrations of 20-100 nM, where no evidence of toxicity was apparent. Additional studies in wild-type and cardiolipin-deficient yeast cells suggested that cellular binding was not absolutely dependent upon cardiolipin.

Reduction in glutathione peroxidase 4 increases life span through increased sensitivity to apoptosis

Glutathione peroxidase 4 (Gpx4) is an antioxidant defense enzyme that plays an important role in detoxification of oxidative damage to membrane lipids. Because oxidative stress is proposed to play a causal role in aging, we compared the life spans of Gpx4 heterozygous knockout mice (Gpx4(+/-) mice) and wild-type mice (WT mice). To our surprise, the median life span of Gpx4(+/-) mice (1029 days) was significantly longer than that of WT mice (963 days) even though the expression of Gpx4 was reduced approximately 50% in all tissues of Gpx4(+/-) mice. Pathological analysis revealed that Gpx4(+/-) mice showed a delayed occurrence of fatal tumor lymphoma and a reduced severity of glomerulonephritis. Compared to WT mice, Gpx4(+/-) mice showed significantly increased sensitivity to oxidative stress-induced apoptosis. Our data indicate that lifelong reduction in Gpx4 increased life span and reduced/retarded age-related pathology most likely through alterations in sensitivity of tissues to apoptosis.

Selective early cardiolipin peroxidation after traumatic brain injury: an oxidative lipidomics analysis

OBJECTIVE

Enhanced lipid peroxidation is well established in traumatic brain injury. However, its molecular targets, identity of peroxidized phospholipid species, and their signaling role have not been deciphered.

METHODS

Using controlled cortical impact as a model of traumatic brain injury, we employed a newly developed oxidative lipidomics approach to qualitatively and quantitatively characterize the lipid peroxidation response.

RESULTS

Electrospray ionization and matrix-assisted laser desorption/ionization mass spectrometry analysis of rat cortical mitochondrial/synaptosomal fractions demonstrated the presence of highly oxidizable molecular species containing C(22:6) fatty acid residues in all major classes of phospholipids. However, the pattern of phospholipid oxidation at 3 hours after injury displayed a nonrandom character independent of abundance of oxidizable species and included only one mitochondria-specific phospholipid, cardiolipin (CL). This selective CL peroxidation was followed at 24 hours by peroxidation of other phospholipids, most prominently phosphatidylserine, but also phosphatidylcholine and phosphatidylethanolamine. CL oxidation preceded appearance of biomarkers of apoptosis (caspase-3 activation, terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling-positivity) and oxidative stress (loss of glutathione and ascorbate).

INTERPRETATION

The temporal sequence combined with the recently demonstrated role of CL hydroperoxides (CL-OOH) in in vitro models of apoptosis suggest that CL-OOH may be both a key in vivo trigger of apoptotic cell death and a therapeutic target in experimental traumatic brain injury.

Ethanol induced mitochondria injury and permeability transition pore opening: Role of mitochondria in alcoholic liver disease

AIM: To observe changes of mitochondria and investigate the effect of ethanol on mitochondrial perme-ability transition pore (PTP), mitochondrial membrane potential (MMP, ΔΨm) and intracellular calcium concentration in hepatocytes by establishing an animal model of alcoholic liver disease (ALD).

METHODS: Fourty adult male Wistar rats were randomly divided into two groups, the model group (20) was administered alcohol intragastrically plus an Oliver oil diet to establish an ALD model, and the control group (20) was given an equal amount of normal saline. The ultramicrostructural changes of mitochondria were observed under electron microscopy. Mitochondria of liver was extracted, and patency of PTP, mitochondrial membrane potential (ΔΨm), mitochondrial mass and intracellular calcium concentration of isolated hepacytes were detected by flow cytometry using rhodamine123 (Rh123), Nonyl-Acridine Orange and calcium fluorescent probe Fluo-3/AM, respectively.

RESULTS: Membrane and cristae were broken or disappeared in mitochondria in different shapes under electron microscopy. Some mitochondria showed U shape or megamitochondrion. In the model group, liver mitochondria PTP was broken, and mitochondria swelled, the absorbance at 450 nm, A540 decreased (0.0136 ± 0.0025 vs 0.0321 ± 0.0013, model vs control, P < 0.01); mitochondria transmembrane potential (239.4638 ±12.7263 vs 377.5850 ± 16.8119, P < 0.01) was lowered; mitochondrial mass (17.4350 ± 1.9880 vs 31.6738 ± 3.4930, P < 0.01); and [Ca²⁺]i was increased in liver cells (7.0020 ± 0.5008 vs 10.2050 ± 0.4701, P < 0.01).

CONCLUSION: Chronic alcohol intake might lead to broken mitochondria PTP, decreased mitochondria membrane potential and injury, and elevated intracellular Ca²⁺ production. Ethanol-induced chondriosome injury may be an important mechanism of alcoholic diseases.

Fluorescent determination of cardiolipin using 10-N-nonyl acridine orange

Cardiolipin (CL) plays an essential role as a marker for cell apoptosis. Quantitative detection of phospholipids (PLs) by UV absorbance is problematic due to the presence of few double bonds in the structure. Although 10-N-nonyl acridine orange (NAO) has been utilized for fluorescent visualization of liposomes and mitochondria through its interaction with CL, in this work, we have developed a specific fluorescent method for CL in solution using NAO. The interaction of sodium n-dodecyl sulfate (SDS), used to treat cells prior to lipid extraction, and other PLs found in cell membranes such as phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), phosphatidiylserine (PS), and sphingomyelin (SM) with NAO is investigated. The fluorescence intensity of the 0.5 microM NAO signal is strongly quenched by SDS below 25% methanol in water but with a methanol content above 50%, no quenching of NAO by SDS is observed. No fluorescence quenching of NAO with a 50% methanol/50% water solvent by the previously mentioned PLs or 4-20 microM cholesterol with the exception of PG at above 8 microM is noted. Using this 50% methanol/50% water solvent, the fluorescence signal due to the NAO-CL interaction is quite stable from 3 to at least 15 min. With excitation and emission wavelengths set at 518 and 530 nm, respectively, 20 microM NAO provides an inverse linear fluorescence response at 0.2-10 microM CL with a correlation coefficient of 0.9929. The detection limit is 0.2 microM and the limit of quantification is 0.6 microM. Structurally analogous acridine orange and phenosafranin dyes are less effective as fluorescent probes for CL. The CL in the whole cell and membrane samples is quantitatively determined by standard addition to range from 0.2 to 1.5 microM. The level of CL in cell membrane samples, previously subjected to staurosporine which initiates cell apoptosis, is increased but not significantly through use of the t-test.

Ca2+-induced phase separation in the membrane of palmitate-containing liposomes and its possible relation to membrane permeabilization

A Ca(2+)-induced phase separation of palmitic acid (PA) in the membrane of azolectin unilamellar liposomes has been demonstrated with the fluorescent membrane probe nonyl acridine orange (NAO). It has been shown that NAO, whose fluorescence in liposomal membranes is quenched in a concentration-dependent way, can be used to monitor changes in the volume of lipid phase. The incorporation of PA into NAO-labeled liposomes increased fluorescence corresponding to the expansion of membrane. After subsequent addition of Ca(2+), fluorescence decreased, which indicated separation of PA/Ca(2+) complexes into distinct membrane domains. The Ca(2+)-induced phase separation of PA was further studied in relation to membrane permeabilization caused by Ca(2+) in the PA-containing liposomes. A supposition was made that the mechanism of PA/Ca(2+)-induced membrane permeabilization relates to the initial stage of Ca(2+)-induced phase separation of PA and can be considered as formation of fast-tightening lipid pores due to chemotropic phase transition in the lipid bilayer.

Apoptotic interactions of cytochrome c: redox flirting with anionic phospholipids within and outside of mitochondria

Since the (re)discovery of cytochrome c (cyt c) in the early 1920s and subsequent detailed characterization of its structure and function in mitochondrial electron transport, it took over 70 years to realize that cyt c plays a different, not less universal role in programmed cell death, apoptosis, by interacting with several proteins and forming apoptosomes. Recently, two additional essential functions of cyt c in apoptosis have been discovered that are carried out via its interactions with anionic phospholipids: a mitochondria specific phospholipid, cardiolipin (CL), and plasma membrane phosphatidylserine (PS). Execution of apoptotic program in cells is accompanied by substantial and early mitochondrial production of reactive oxygen species (ROS). Because antioxidant enhancements protect cells against apoptosis, ROS production was viewed not as a meaningless side effect of mitochondrial disintegration but rather playing some - as yet unidentified - role in apoptosis. This conundrum has been resolved by establishing that mitochondria contain a pool of cyt c, which interacts with CL and acts as a CL oxygenase. The oxygenase is activated during apoptosis, utilizes generated ROS and causes selective oxidation of CL. The oxidized CL is required for the release of pro-apoptotic factors from mitochondria into the cytosol. This redox mechanism of cyt c is realized earlier than its other well-recognized functions in the formation of apoptosomes and caspase activation. In the cytosol, released cyt c interacts with another anionic phospholipid, PS, and catalyzes its oxidation in a similar oxygenase reaction. Peroxidized PS facilitates its externalization essential for the recognition and clearance of apoptotic cells by macrophages. Redox catalysis of plasma membrane PS oxidation constitutes an important redox-dependent function of cyt c in apoptosis and phagocytosis. Thus, cyt c acts as an anionic phospholipid specific oxygenase activated and required for the execution of essential stages of apoptosis. This review is focused on newly discovered redox mechanisms of complexes of cyt c with anionic phospholipids and their role in apoptotic pathways in health and disease.