Gradual changes in permeability of inner mitochondrial membrane precede the mitochondrial permeability transition

Tissue-specific differences in Ca2+ sensitivity of the mitochondrial permeability transition pore (PTP). Experiments in male rat liver and heart

Permeability transition pore (PTP) opening dissipates ion and electron gradients across the internal mitochondrial membrane (IMM), including excess Ca2+ in the mitochondrial matrix. After opening, immediate PTP closure must follow to prevent outer membrane disruption, loss of cytochrome c, and eventual apoptosis. Flickering, defined as the rapid alternative opening/closing of PTP, has been reported in heart, which undergoes frequent, large variations in Ca2+. In contrast, in tissues that undergo depolarization events less often, such as the liver, PTP would not need to be as dynamic and thus these tissues would not be as resistant to stress. To evaluate this idea, it was decided to follow the reversibility of the permeability transition (PT) in isolated murine mitochondria from two different tissues: the very dynamic heart, and the liver, which suffers depolarizations less frequently. It was observed that in heart mitochondria PT remained reversible for longer periods and at higher Ca2+ loads than in liver mitochondria. In all cases, Ca2+ uptake was inhibited by ruthenium red and PT was delayed by Cyclosporine A. Characterization of this phenomenon included measuring the rate of oxygen consumption, organelle swelling and Ca2+ uptake and retention. Results strongly suggest that there are tissue-specific differences in PTP physiology, as it resists many more Ca2+ additions before opening in a highly active organ such as the heart than in an organ that seldom suffers Ca2+ loading, such as the liver.

Coupling/Uncoupling Reversibility in Isolated Mitochondria from Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae uses fermentation as the preferred pathway to obtain ATP and requires the respiratory chain to re-oxidize the NADH needed for activity of Glyceraldehyde-3-phosphate. This process is favored by uncoupling of oxidative phosphorylation (OxPhos), which is at least partially controlled by the mitochondrial unspecific pore (_ScMUC). When mitochondrial ATP synthesis is needed as in the diauxic phase or during mating, a large rise in Ca²⁺ concentration ([Ca²⁺]) closes _ScMUC, coupling OxPhos. In addition, _ScMUC opening/closing is mediated by the ATP/ADP ratio, which indicates cellular energy needs. Here, opening and closing of _ScMUC was evaluated in isolated mitochondria from S. cerevisiae at different incubation times and in the presence of different ATP/ADP ratios or varying [Ca²⁺]. Measurements of the rate of O₂ consumption, mitochondrial swelling, transmembrane potential and ROS generation were conducted. It was observed that _ScMUC opening was reversible, a high ATP/ADP ratio promoted opening and [Ca²⁺] closed _ScMUC even after several minutes of incubation in the open state. In the absence of ATP synthesis, closure of _ScMUC resulted in an increase in ROS.

Wss1 metalloprotease partners with Cdc48/Doa1 in processing genotoxic SUMO conjugates

Sumoylation during genotoxic stress regulates the composition of DNA repair complexes. The yeast metalloprotease Wss1 clears chromatin-bound sumoylated proteins. Wss1 and its mammalian analog, DVC1/Spartan, belong to minigluzincins family of proteases. Wss1 proteolytic activity is regulated by a cysteine switch mechanism activated by chemical stress and/or DNA binding. Wss1 is required for cell survival following UV irradiation, the smt3-331 mutation and Camptothecin-induced formation of covalent topoisomerase 1 complexes (Top1cc). Wss1 forms a SUMO-specific ternary complex with the AAA ATPase Cdc48 and an adaptor, Doa1. Upon DNA damage Wss1/Cdc48/Doa1 is recruited to sumoylated targets and catalyzes SUMO chain extension through a newly recognized SUMO ligase activity. Activation of Wss1 results in metalloprotease self-cleavage and proteolysis of associated proteins. In cells lacking Tdp1, clearance of topoisomerase covalent complexes becomes SUMO and Wss1-dependent. Upon genotoxic stress, Wss1 is vacuolar, suggesting a link between genotoxic stress and autophagy involving the Doa1 adapter.

Mitochondrial reactive oxygen species (ROS) and ROS-induced ROS release

Byproducts of normal mitochondrial metabolism and homeostasis include the buildup of potentially damaging levels of reactive oxygen species (ROS), Ca(2+), etc., which must be normalized. Evidence suggests that brief mitochondrial permeability transition pore (mPTP) openings play an important physiological role maintaining healthy mitochondria homeostasis. Adaptive and maladaptive responses to redox stress may involve mitochondrial channels such as mPTP and inner membrane anion channel (IMAC). Their activation causes intra- and intermitochondrial redox-environment changes leading to ROS release. This regenerative cycle of mitochondrial ROS formation and release was named ROS-induced ROS release (RIRR). Brief, reversible mPTP opening-associated ROS release apparently constitutes an adaptive housekeeping function by the timely release from mitochondria of accumulated potentially toxic levels of ROS (and Ca(2+)). At higher ROS levels, longer mPTP openings may release a ROS burst leading to destruction of mitochondria, and if propagated from mitochondrion to mitochondrion, of the cell itself. The destructive function of RIRR may serve a physiological role by removal of unwanted cells or damaged mitochondria, or cause the pathological elimination of vital and essential mitochondria and cells. The adaptive release of sufficient ROS into the vicinity of mitochondria may also activate local pools of redox-sensitive enzymes involved in protective signaling pathways that limit ischemic damage to mitochondria and cells in that area. Maladaptive mPTP- or IMAC-related RIRR may also be playing a role in aging. Because the mechanism of mitochondrial RIRR highlights the central role of mitochondria-formed ROS, we discuss all of the known ROS-producing sites (shown in vitro) and their relevance to the mitochondrial ROS production in vivo.

NO donor KMUP-1 improves hepatic ischemia-reperfusion and hypoxic cell injury by inhibiting oxidative stress and pro-inflammatory signaling

This study investigates whether KMUP-1 improves hepatic ischemia-reperfusion (I/R) and hypoxic cell injury via inhibiting Nox2- and reactive oxygen species (ROS)-mediated pro-inflammation. Rats underwent ischemia by occlusion of the portal vein and hepatic artery for 45 minutes. Reperfusion was allowed for 4 h. Serum was used for analysis of aspartate aminotransferase (AST) and alanine aminotransferase (ALT). DNA extracted from liver homogenate was analyzed by electrophoresis to observe the fragmentation. Lipid peroxidation (LPO) was evaluated by measuring thiobarbituric acid-reactive substances (TBARS). NO and ROS contents were measured using Griess reagent and 2′-7′-dichlorofluorescein, respectively. Proteins levels were visualized by Western blotting. Liver damage was observed under a microscope. Intravenous KMUP-1 (0.25, 0.5 and 1 mg/kg) reduced I/R-induced ALT and AST levels, DNA fragmentation, ROS and malondialdehyde (MDA) and restored the NO levels of I/R rats. KMUP-1 protected the liver architecture from worsening of damage and focal sinusoid congestion, increased endothelium NO synthase (eNOS), guanosine 3', 5'cyclic monophosphate (cGMP), protein kinase G (PKG) and the B-cell lymphoma 2/Bcl-2-associated X protein (Bcl-2/Bax) ratio, attenuated phosphodiesterase 5A (PDE-5A) and cleaved caspase-3 expression in I/R-liver. In hypoxic HepG2 cells, KMUP-1 increased cGMP/PKG, restored peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and decreased matrix metalloproteinases-9 (MMP-9), Rho kinase II (ROCK II), hypoxia-inducible factor-1alpha (HIF-1alpha) and vascular endothelium growth factor (VEGF). KMUP-1 protects liver from I/R-injury and hypoxic hepatocytes from apoptosis-associated free radical generation and pro-inflammation by restoring/increasing NO/cGMP/PPAR-gamma, reducing ROS/Nox2 and inhibiting ROCKII/MMP-9.

Breaking up the correlation between efficacy and toxicity for nonviral gene delivery

Nonviral nucleic acid delivery to cells and tissues is considered a standard tool in life science research. However, although an ideal delivery system should have high efficacy and minimal toxicity, existing materials fall short, most of them being either too toxic or little effective. We hypothesized that disulfide cross-linked low-molecular-weight (MW) linear poly(ethylene imine) (MW<4.6 kDa) would overcome this limitation. Investigations with these materials revealed that the extracellular high MW provided outstandingly high transfection efficacies (up to 69.62+/-4.18% in HEK cells). We confirmed that the intracellular reductive degradation produced mainly nontoxic fragments (cell survival 98.69+/-4.79%). When we compared the polymers in >1,400 individual experiments to seven commercial transfection reagents in seven different cell lines, we found highly superior transfection efficacies and substantially lower toxicities. This renders reductive degradation a highly promising tool for the design of new transfection materials.

The effect of the lipophilic cation lucigenin on mitochondria depends on the site of its reduction

The role of NAD(P)H-dependent oxidoreductases of the outer mitochondrial membrane (OMM) in the activation of lipophilic cationic dyes is poorly understood. In the present study we compared the rates of production of reactive oxygen species (ROS) and mitochondriotoxic effects of the redox-cycling lipophilic cationic dye lucigenin upon its activation by the respiratory chain and NAD(P)H-dependent oxidoreductases of the OMM. We found that, only in the presence of external NADH and NADPH, which are unable to penetrate the inner membrane, lucigenin stimulated a massive superoxide production and a fast permeabilization of mitochondrial membranes. The permeabilization was biphasic. The first, cyclosporin A-insensitive and Ca(2+)-independent phase was characterized by increased permeability of the inner mitochondrial membrane to solutes with molecular masses of <or=200 Da. The second phase was sensitive to the antagonists of the permeability transition pore (mPTP) and was characterized by permeability similar to that of mPTP (<or=1500 Da). A massive cytochrome c release was observed even at the first phase of permeability when the second phase was inhibited by mPTP antagonists. Whatever the site of lucigenin activation, antioxidants and scavengers of ROS that strongly decrease the ROS level were unable to delay or prevent the permeabilization of membranes, which casts doubt on the involvement of ROS in the regulation of permeability by redox-cycling lipophilic cations. Our results strongly support the idea that the NAD(P)H-dependent reductases of xenobiotics of the OMM can mediate the toxicity of cationic dyes.

Comparative kinetic analysis reveals that inducer-specific ion release precedes the mitochondrial permeability transition

Relationships among the multiple events that precede the mitochondrial membrane permeability transition (MPT) are not yet clearly understood. A combination of newly developed instrumental and computational approaches to this problem is described. The instrumental innovation is a high-resolution digital apparatus for the simultaneous, real-time measurement of four mitochondrial parameters as indicators of the respiration rate, membrane potential, calcium ion transport, and mitochondrial swelling. A computational approach is introduced that tracks the fraction of mitochondria that has undergone pore opening. This approach allows multiple comparisons on a single time scale. The validity of the computational approach for studying complex mitochondrial phenomena was evaluated with mitochondria undergoing an MPT induced by Ca(2+), phenylarsine oxide or alamethicin. Selective ion leaks were observed that precede the permeability transition and that are inducer specific. These results illustrate the occurrence of inducer-specific sequential changes associated with the induction of the permeability transition. Analysis of the temporal relationship among the multiple mitochondrial parameters of isolated mitochondria should provide insights into the mechanisms underlying these responses.

Menadione induces a low conductance state of the mitochondrial inner membrane sensitive to bongkrekic acid

When rat liver mitochondria are allowed to cycle Ca(2+) and are incubated in the presence of the pro-oxidant menadione, they undergo swelling, membrane potential (DeltaPsi) collapse, and ion release. These effects, which are inhibited by cyclosporin A (CsA), are fully consistent with the opening of the so-called permeability transition pore. However, when Ca(2+) cycling is abolished by EGTA, the mitochondria remain energized (DeltaPsi collapse and swelling are avoided), but Ca(2+) efflux, promoted by the chelating agent, is stimulated by menadione. This stimulation goes together with the release of Mg(2+), K(+), and adenine nucleotides (AdN) and is inhibited by bongkrekic acid (BKA). The effect of menadione is also characterized by biphasic NAD(P)H oxidation which becomes monophasic in the presence of BKA, CsA, or EGTA and by the oxidation of thiol groups not restrained by the above-mentioned inhibitors. These results suggest that BKA acts indirectly by preserving in the matrix a critical amount of AdN without modifying the monophasic oxidation of pyridine nucleotides by menadione. A critical number of thiol groups also seems to be involved in the phenomenon. Their oxidation most probably causes a conformational change on adenine nucleotide translocase with the opening of the "low-conductance state" of the mitochondrial permeability transition, resulting in ion permeability without DeltaPsi disruption and mitochondrial swelling.

Glibenclamide interferes with mitochondrial bioenergetics by inducing changes on membrane ion permeability

The interference of glibenclamide, an antidiabetic sulfonylurea, with mitochondrial bioenergetics was assessed on mitochondrial ion fluxes (H+, K+, and Cl-) by passive osmotic swelling of rat liver mitochondria in K-acetate, KNO3, and KCl media, by O2 consumption, and by mitochondrial transmembrane potential (Deltapsi). Glibenclamide did not permeabilize the inner mitochondrial membrane to H+, but induced permeabilization to Cl- by opening the inner mitochondrial anion channel (IMAC). Cl- influx induced by glibenclamide facilitates K+ entry into mitochondria, thus promoting a net Cl-/K+ cotransport, Deltapsi dissipation, and stimulation of state 4 respiration rate. It was concluded that glibenclamide interferes with mitochondrial bioenergetics of rat liver by permeabilizing the inner mitochondrial membrane to Cl- and promoting a net Cl-/K+ cotransport inside mitochondria, without significant changes on membrane permeabilization to H+.

Regulation and critical role of potassium homeostasis in apoptosis

Programmed cell death or apoptosis is broadly responsible for the normal homeostatic removal of cells and has been increasingly implicated in mediating pathological cell loss in many disease states. As the molecular mechanisms of apoptosis have been extensively investigated a critical role for ionic homeostasis in apoptosis has been recently endorsed. In contrast to the ionic mechanism of necrosis that involves Ca(2+) influx and intracellular Ca(2+) accumulation, compelling evidence now indicates that excessive K(+) efflux and intracellular K(+) depletion are key early steps in apoptosis. Physiological concentration of intracellular K(+) acts as a repressor of apoptotic effectors. A huge loss of cellular K(+), likely a common event in apoptosis of many cell types, may serve as a disaster signal allowing the execution of the suicide program by activating key events in the apoptotic cascade including caspase cleavage, cytochrome c release, and endonuclease activation. The pro-apoptotic disruption of K(+) homeostasis can be mediated by over-activated K(+) channels or ionotropic glutamate receptor channels, and most likely, accompanied by reduced K(+) uptake due to dysfunction of Na(+), K(+)-ATPase. Recent studies indicate that, in addition to the K(+) channels in the plasma membrane, mitochondrial K(+) channels and K(+) homeostasis also play important roles in apoptosis. Investigations on the K(+) regulation of apoptosis have provided a more comprehensive understanding of the apoptotic mechanism and may afford novel therapeutic strategies for apoptosis-related diseases.

Reciprocal effects between spermine and Mg2+ on their movements across the mitochondrial membrane

Mg(2+) competitively inhibits spermine transport in energized rat liver mitochondria (RLM) and exhibits a K(i) of 0.1mM on the initial rate and an I(50) of 0.6mM on total spermine accumulation after 20 min. Addition of 2mM Mg(2+) after spermine accumulation induces release of the polyamine. In view of the fact that spermine cycles across the inner membrane under physiological conditions, these results demonstrate that Mg(2+) inhibits spermine influx but does not affect the efflux pathway of the polyamine; the inhibitory effect occurs via an interaction with the specific site responsible for spermine transport. Instead, spermine inhibits Mg(2+) binding without affecting the rate of Mg(2+) transport, suggesting that both cations bind to the same site, which, however, is not used for Mg(2+) transport. Spermine also inhibits Mg(2+) efflux from RLM induced under conditions of the "low conductance state," a preliminary step preceding permeability transition pore opening.

Interaction of genistein with the mitochondrial electron transport chain results in opening of the membrane transition pore

Genistein, a natural isoflavone present in soybeans, is a potent agent in the prophylaxis and treatment of cancer. Addition of genistein to isolated rat liver mitochondria (RLM) induces swelling, loss of membrane potential and release of accumulated Ca2+. These changes are Ca2+-dependent and are prevented by cyclosporin A (CsA) and bongkrekic acid (BKA), two classical inhibitors of the mitochondrial permeability transition (MPT). Induction of the MPT by genistein is accompanied by oxidation of thiol groups and pyridine nucleotides. The reducing agent dithioerythritol and the alkylating agent N-ethylmaleimide (NEM) completely prevent the opening of the transition pore, thereby emphasizing that the effect of the isoflavone correlates with the mitochondrial redox state. Further analyses showed that genistein induces the MPT by the generation of reactive oxygen species (ROS) due to its interaction with the respiratory chain at the level of mitochondrial complex III.

Cyclosporin A-sensitive permeability transition pore is involved in Cd(2+)-induced dysfunction of isolated rat liver mitochondria: doubts no more

There is dose-dependent Cd(2+)-evoked swelling of isolated rat liver mitochondria energized by complex I, II, or IV respiratory substrates in sucrose medium in the absence of added Ca(2+) and P(i), which is prevented by Sr(2+). Permeability transition effectors (ADP, CsA, EGTA, RR, DTT, ATR, P(i), and Ca(2+)) affect in a corresponding way Cd(2+)-promoted membrane permeabilization in NH(4)NO(3), KCl, and sucrose media. Maximal depression of Cd(2+)-induced swelling is achieved by simultaneous addition of ADP, Mg(2+), and CsA that produces either synergistic (NH(4)NO(3)) or additive (KCl and sucrose media) action. Sustained activation by low [Cd(2+)] of mitochondrial basal respiration in KCl medium is observed both in the absence and in the presence of rotenone and/or oligomycin but only in the latter case (rotenone+oligomycin) CsA inhibits completely Cd(2+) activation of St 4 respiration and partially reverses DNP-uncoupled respiration depressed by cadmium. Cd(2+) effects are discussed in terms of comparison with those of Zn(2+) and PhAsO.

Mitochondrial injury by disulfiram: two different mechanisms of the mitochondrial permeability transition

Disulfiram (Ds), a clinically employed alcohol deterrent of the thiuram disulfide (TD) class of compounds, is known to cause hepatitis and neuropathies. Although this drug has been shown to inhibit different thiol-containing enzymes, the actual mechanism of Ds toxicity is not clear. We have previously demonstrated that Ds impairs the permeability of inner mitochondrial membrane (IMM) [Arch. Biochem. Biophys. 356 (1998) 46]. In this report, the effect of Ds and its structural analogue thiram (Th) on mitochondrial functions was studied in detail. We found that mitochondria metabolize TDs in a NAD(P)H- and GSH-dependent manner. At the concentration above characteristic threshold, TDs induced irreversible oxidation of NAD(P)H and glutathione (GSH) pools, collapse of transmembrane potential, and inhibition of oxidative phosphorylation. The presence of Ca(2+) and exhaustion of mitochondrial glutathione (GSH+GSSG) decreased the threshold concentration of TDs. Swelling of the mitochondria and leakage of non-transported fluorescent dye BCECF from the matrix indicated that TDs induced the mitochondrial permeability transition (MPT). Mitochondrial permeabilization by TDs involves two, apparently distinct mechanisms. In the presence of Ca(2+), TDs produced cylosporin A-sensitive swelling of mitochondria, which was inhibited by ADP and accelerated by carboxyatractyloside (CATR) and phosphate. In contrast, the swelling produced by TDs in the absence of Ca(2+) was not sensitive to cyclosporin A (CsA), ADP and CATR but was inhibited by phosphate. Titration with N-ethylmaleimide revealed that these two mechanisms involve different SH-groups and probably different transport proteins on the IMM. Our findings indicate that at pharmacologically relevant concentrations TDs may cause an irreversible mitochondrial injury as a result of induction of the MPT.

Cyclosporin A induces the opening of a potassium-selective channel in higher plant mitochondria

The immunosuppressive drug, cyclosporin A (CsA), induces the generation of a transmembrane electrical potential difference (deltapsi) in deenergized plant mitochondria incubated in sucrose-based media. Build up of deltapsi is prevented by external monovalent cations in the order K+ > Rb+ = Li+ > Na+, or by the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone, which also collapses the deltapsi generated by CsA. Entry of K+ into mitochondria can be monitored as swelling by incubating the organelles in a medium containing KCl to maintain constant osmolarity. This swelling is inhibited by ATP and stimulated by CsA or valinomycin. In addition, in mitochondria energized by succinate, KCl causes a dissipation of deltapsi, with sigmoidal kinetics, which is favored by CsA. Therefore, plant mitochondria appear to possess a K+ selective, voltage-dependent channel, which is opened by CsA, regulated by the redox state, and inhibited by nucleotides. The hypothetical roles of this new K+ATP channel are discussed in relation to its potential involvement in mitochondrial volume regulation, thermogenesis, apoptosis, and/or prevention of reactive oxygen species formation in plants.

Lipoic acid-derived amphiphiles for redox-controlled DNA delivery

BACKGROUND

Intracellular release of free DNA from the vector complex is one of the critical steps limiting the efficiency of non-viral gene delivery. The complex should be stable enough to prevent DNA degradation but it should be destabilized inside the cell to allow DNA release and transcription. Destabilization and degradation of synthetic vectors is also required to reduce their cytotoxicity and augment the life-time of transfected cells.

RESULTS

Here we describe new cationic amphiphiles made from the natural pro-vitamin, lipoic acid, that reversibly binds and releases DNA, depending on the redox state of the lipoate moieties. In the oxidized state these amphiphiles condense DNA into homogeneous spherical particles, which, upon reduction, swell into DNA toroids with subsequent release of free DNA. Complex reduction and DNA release can be induced by various thiols as well as enzymatically, by thioredoxin reductase. Transfection with amphiphile-DNA complexes in vitro shows a several fold increase of transgene expression compared with DOTAP, and can be further augmented by attachment of the nucleus-targeting peptide to the amphiphile. The increase of transfection efficiency results from GSH- and NAD(P)H-dependent complex reduction and release of free DNA inside the cells.

CONCLUSIONS

The present work demonstrates the principle of a redox-controlled gene delivery system that uses the reversibility of thiol-disulfide exchange reaction. Our data suggest that the efficiency of synthetic vectors can be augmented by their controlled destabilization inside the cells. Being formed from the natural non-toxic compound lipoic acid, these cationic amphiphiles provide a new promising class of synthetic vectors for gene delivery.

Plasma Membrane Potential in Thymocyte Apoptosis

Apoptosis is accompanied by major changes in ion compartmentalization and transmembrane potentials. Thymocyte apoptosis is characterized by an early dissipation of the mitochondrial transmembrane potential, with transient mitochondrial swelling and a subsequent loss of plasma membrane potential (ΔΨp) related to the loss of cytosolic K+, cellular shrinkage, and DNA fragmentation. Thus, a gross perturbation of ΔΨp occurs at the postmitochondrial stage of apoptosis. Unexpectedly, we found that blockade of plasma membrane K+ channels by tetrapentylammonium (TPA), which leads to a ΔΨp collapse, can prevent the thymocyte apoptosis induced by exposure to the glucocorticoid receptor agonist dexamethasone, the topoisomerase inhibitor etoposide, γ-irradiation, or ceramide. The TPA-mediated protective effect extends to all features of apoptosis, including dissipation of the mitochondrial transmembrane potential, loss of cytosolic K+, phosphatidylserine exposure on the cell surface, chromatin condensation, as well as caspase and endonuclease activation. In strict contrast, TPA is an ineffective inhibitor when cell death is induced by the potassium ionophore valinomycin, the specific mitochondrial benzodiazepine ligand PK11195, or by primary caspase activation by Fas/CD95 cross-linking. These results underline the importance of K+ channels for the regulation of some but not all pathways leading to thymocyte apoptosis.