Regulation of the permeability transition pore, a voltage-dependent mitochondrial channel inhibited by cyclosporin A

And all of a sudden it's over: mitochondrial outer-membrane permeabilization in apoptosis

Identification of pro-apoptotic activities for a variety of proteins normally resident in the mitochondrial inter-membrane space has substantiated the role of mitochondria as integral to the apoptotic process. Cytochrome c is involved in apoptosome formation and caspase activation, SMAC/Diablo deregulates the inhibitor of apoptosis proteins, apoptosis-inducing factor may play a role in chromatin condensation and release of other proteins such as adenylate kinase may adversely affect cellular metabolism and contribute to the death of a cell if the downstream apoptotic pathway is blocked. It is still unclear how these proteins are released from the mitochondria. Recent advances in our knowledge of mitochondrial outer-membrane permeabilization and the consequences of this event on mitochondria will be discussed.

The effect of aliphatic, naphthenic, and aromatic hydrocarbons on production of reactive oxygen species and reactive nitrogen species in rat brain synaptosome fraction: the involvement of calcium, nitric oxide synthase, mitochondria, and phospholipase A11Abbreviations: BIM, bisindolylmaleimide; [Ca2+]i, concentration of intracellular calcium; ChAT, cholin acetyltransferase; CSA, cyclosporin A; DCF, 2′,7′-dichlorofluorescein; H2DCF-DA, 2′,7′-dichlorodihydrofluorescin diacetate; DEDA, dimethyleicosadienoic acid; ERK, extracellular signal-regulated kinases; Fura-2 AM, 5-oxazolecarboxylic acid, 2-(6-(bis(2-((acetyloxy)methoxy)-2-oxoethyl)amino)-5-(2-(bis(2-((acetyloxy)methoxy)-2oxoethyl)amino)-5-methylphenoxy)ethoxy)-2-benzofuranyl)-, (acetyloxy) methyl ester; GABA-T, gamma-aminobutyric acid transaminase; HBSS, Hanks’ balanced salt solution; La3+, lanthanum; MAPK, mitogen-activated protein kinase; MeHg, methyl mercury; MEK, extracellular signal-regulated protein kinase; MeOH, methanol; MTP, mitochondrial permeability transition pore; L-NAME, Nω-nitro-l-arginine methyl ester; NO·, nitrogen oxide; NOS, NO· synthase; O2·−, superoxide; PLA2, phospholipase A2; PKC, protein kinase C; RNS, reactive nitrogen species; ROS, reactive oxygen species; SOD, superoxide dismutase; TMB, 1,2,4-trimethylbenzene; TMCH, 1,2,4-trimethylcyclohexane; and U73122, 1-(6-[17beta-3-methoxyestra- 1,3,5(10)-trien- 17-yl]-aminohexyl)- 1H-pyrrole-2,5-dione

This study investigated the effects of C7 and C9 aliphatic (n-heptane, n-nonane), naphthenic (methylcyclohexane, 1,2,4-trimethylcyclohexane (TMCH)) and aromatic (toluene, 1,2,4-trimethylbenzene (TMB)) hydrocarbons on the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in rat brain synaptosome fraction. Methyl mercury (MeHg) was included as a positive control. Exposure of the synaptosomes to the hydrocarbons produced a concentration-dependent linear increase in the formation of the fluorescence of 2',7'-dichlorofluorescein (DCF) as a measure of the production of ROS and RNS. Formation of RNS was demonstrated by preincubation of the synaptosome fraction with the neuronal nitric oxide synthase (nNOS) inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME), which reduced the MeHg and TMCH-stimulated fluorescence by 51% and 65%, respectively. The naphthenic hydrocarbon TMCH showed the strongest potential for ROS and RNS formation in rat brain synaptosomes, followed by TMB, toluene, n-nonane, n-heptane, and methylcyclohexane, respectively. TMCH was selected for mechanistic studies of the formation of ROS. Both MeHg and TMCH induced an increase in intracellular calcium concentration [Ca(2+)]i as measured with Fura-2. Blockade of voltage-dependent Ca(2+) channels with lanthanum prior to stimulation with MeHg and TMCH led to a reduction in the ROS/RNS formation of 72% and 70%, respectively. Furthermore, addition of cyclosporin A (CSA), a blocker of the mitochondrial permeability transition pore (MTP), lowered both the MeHg and TMCH-elevated DCF fluorescence by 72% and 59%. Preincubation of the synaptosome fraction with the protein tyrosine kinase inhibitor genistein lowered the MeHg and TMCH-stimulated fluorescence by 85% and 91%, respectively. Addition of the extracellular signal-regulated protein kinase (MEK)-1 and -2 inhibitor U0126 reduced the fluorescence stimulated by MeHg and TMCH by 62% and 63%. Furthermore, the protein kinase C inhibitor bisindolylmaleimide reduced the fluorescence stimulated by MeHg and TMCH by 52% and 56%. The compound 1-(6-[17beta-3-methoxyestra- 1,3,5(10)-trien- 17-yl]-aminohexyl)-1H-pyrrole-2,5-dione (U73122), which inhibits phospholipase C, was shown to decrease the ROS and RNS formation induced by MeHg and TMCH by 49% and 64%, respectively. The phospholipase A2 (PLA2) inhibitor 7,7-dimethyl eicosadienoic acid (DEDA) reduced fluorescence in response to MeHg and TMCH by 49% and 54%. Simultaneous addition of L-NAME, CSA, and DEDA to the synaptosome fraction totally abolished the DCF fluorescence. In conclusion, C7 and C9 aliphatic, naphthenic, and aromatic hydrocarbons stimulated formation of ROS and RNS in rat brain synaptosomes. The naphthenic hydrocarbon TMCH stimulated formation of ROS and RNS in the synaptosomes through Ca(2+)-dependent activation of PLA2 and nNOS, and through increased transition permeability of the MTP. Exposure of humans to the naphthenic hydrocarbon TMCH may stimulate formation of free radicals in the brain, which may be a key factor leading to neurotoxicity.

Protective effect of carvedilol on chenodeoxycholate induction of the permeability transition pore

Intracellular accumulation of toxic, hydrophobic bile acids has been proposed as one of the putative final common pathways leading to cholestatic liver injury. Furthermore, bile acids have been proposed as a causative factor for hepatic cardiomyopathy. Hepatic tissue concentrations of chenodeoxycholic acid (CDCA) during cholestasis are greater than those of other toxic bile acids. In the presence of calcium and phosphate, CDCA induced the permeability transition pore (PTP) in freshly isolated rat liver mitochondria. In this study, we evaluated the effects of carvedilol, a multirole cardioprotective compound, on CDCA-induced PTP. Mitochondrial membrane potential, osmotic swelling, and calcium fluxes were monitored. CDCA-induced PTP, characterized by membrane depolarization, release of matrix calcium, and osmotic swelling, was prevented by carvedilol. Under the same conditions, its hydroxylated analog BM-910228 did not reveal any protective effect. This finding reinforces carvedilol's therapeutic interest, because it may potentially prevent mitochondrial dysfunction associated with cardiomyopathy in the pathophysiology of cholestatic liver disease

Arachidonic acid causes cell death through the mitochondrial permeability transition. Implications for tumor necrosis factor-alpha aopototic signaling

We have investigated the effects of arachidonic and palmitic acids in isolated rat liver mitochondria and in rat hepatoma MH1C1 cells. We show that both compounds induce the mitochondrial permeability transition (PT). At variance from palmitic acid, however, arachidonic acid causes a PT at concentrations that do not cause PT-independent depolarization or respiratory inhibition, suggesting a specific effect on the PT pore. When added to intact MH1C1 cells, arachidonic acid but not palmitic acid caused a mitochondrial PT in situ that was accompanied by cytochrome c release and rapidly followed by cell death. All these effects of arachidonic acid could be prevented by cyclosporin A but not by the phospholipase A(2) inhibitor aristolochic acid. In contrast, tumor necrosis factor alpha caused phospholipid hydrolysis, induction of the PT, cytochrome c release, and cell death that could be inhibited by both cyclosporin A and aristolochic acid. These findings suggest that arachidonic acid produced by cytosolic phospholipase A(2) may be a mediator of tumor necrosis factor alpha cytotoxicity in situ through induction of the mitochondrial PT.

The mitochondrial permeability transition, release of cytochrome c and cell death. Correlation with the duration of pore openings in situ

We investigated the relationship between opening of the permeability transition pore (PTP), mitochondrial depolarization, cytochrome c release, and occurrence of cell death in rat hepatoma MH1C1 cells. Treatment with arachidonic acid or induces PTP opening in situ with similar kinetics, as assessed by the calcein loading-Co(2+) quenching technique (Petronilli, V., Miotto, G., Canton, M., Colonna, R., Bernardi, P., and Di Lisa, F. (1999) Biophys. J. 76, 725-734). Yet depolarization, as assessed from the changes of mitochondrial tetramethylrhodamine methyl ester (TMRM) fluorescence, is rapid and extensive with arachidonic acid and slow and partial with. Cyclosporin A-inhibitable release of cytochrome c and cell death correlate with the changes of TMRM fluorescence but not with those of calcein fluorescence. Since pore opening must be accompanied by depolarization, we conclude that short PTP openings are detected only by trapped calcein and may have little impact on cell viability, while changes of TMRM distribution require longer PTP openings, which cause release of cytochrome c and may result in cell death. Modulation of the open time appears to be the key element in determining the outcome of stimuli that converge on the PTP.

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.

Pro-caspase-8 is predominantly localized in mitochondria and released into cytoplasm upon apoptotic stimulation

The recruitment and cleavage of pro-caspase-8 to produce the active form of caspase-8 is a critical biochemical event in death receptor-mediated apoptosis. However, the source of pro-caspase-8 available for activation by apoptotic triggers is unknown. In human fibroblasts and mouse clonal striatal cells, confocal microscopy revealed that pro-caspase-8 immunofluorescence was colocalized with cytochrome c in mitochondria and was also distributed diffusely in some nuclei. Biochemical analysis of subcellular fractions indicated that pro-caspase-8 was enriched in mitochondria and in nuclei. Pro-caspase-8 was found in the intermembrane space, inner membrane, and matrix of mitochondria after limited digestion of mitochondrial fractions, and this distribution was confirmed by immunogold electron microscopy. Pro-caspase-8 and cytochrome c were released from isolated mitochondria that were treated with an inhibitor of the ADP/ATP carrier atractyloside, which opens the mitochondria permeability transition pore. Release was blocked by the mitochondria permeability transition pore inhibitor cyclosporin A (CsA). After clonal striatal cells were exposed for 6 h to an apoptotic inducer tumor necrosis factor-alpha (TNF-alpha), mitochondria immunoreactive for cytochrome c and pro-caspase-8 became clustered at perinuclear sites. Pro-caspase-8 and cytochrome c levels decreased in mitochondrial fractions and increased, along with pro-caspase-8 cleavage products, in the cytoplasm of the TNF-alpha-treated striatal cells. CsA blocked the TNF-alpha-induced release of pro-caspase 8 but not cytochrome c. Internucleosomal DNA fragmentation started at 6 h and peaked 12 h after TNF-alpha treatment. These results suggest that pro-caspase-8 is predominantly localized in mitochondria and is released upon apoptotic stimulation through a CsA-sensitive mechanism.

Cyclosporin A targets involved in protection against glutamate excitotoxicity

The toxicity of glutamate in neuronal cultures has been attributed in part to a mitochondrial dysfunction involving the permeability transition pore. The participation of the permeability transition pore in this process has been pharmacologically demonstrated by the use of cyclosporin A, which inhibits pore opening by interaction with mitochondrial cyclophilin and, thus, prevents cell death and upstream events. Since cyclosporin A also acts on calcineurin, we have investigated which of the targets of cyclosporin A was responsible for the inhibition of glutamate-excitotoxicity in cerebrocortical primary neuronal cultures. Reactive oxygen species production and early (30 min to 2 h) drop in ATP levels are initial events in glutamate excitotoxicity taking place before neuronal death. Cyclosporin A did not inhibit reactive oxygen species production, but reduced the drop in ATP levels and subsequent neuronal death. However, cyclosporin derivatives that do not bind to calcineurin had smaller effect on survival than cyclosporin A, (regardless of whether they were able to bind cyclophilin), indicating that cyclosporin A protects against glutamate toxicity also through calcineurin-related mechanisms. Consistent with this view, ATP loss appears to result from nitric oxide synthase (NOS) activation (including calcineurin-dependent dephosphorylation) and nitric oxide (NO)/peroxinitrite-dependent increase in poly (ADP-ribose) polymerase activity, since it was reduced by inhibitors of these activities. Collectively, these results suggest that cyclosporin A exerts its protective effects through calcineurin-dependent and independent mechanisms.

Palmitate-induced apoptosis in cardiomyocytes is mediated through alterations in mitochondria: prevention by cyclosporin A

Palmitate, a C16 fatty acid found in high concentrations in the blood in acute myocardial infarction, induces apoptotic cell death. To more completely define the nature and mechanism underlying palmitate-induced cell death, cardiomyocytes were cultured from embryonic chick heart and were treated with palmitate. Concentration-dependent loss of cell viability was established by loss of the ability of palmitate-treated cells to exclude propidium iodide (PI), metabolize 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) and retain fluorescein diacetate (FDA). Dual staining with PI and FDA and subsequent analysis by FACS established that palmitate-induced cell death was predominantly necrosis whereas apoptosis occurred in 13% of all dead cells. The low proportion of palmitate-induced apoptosis was confirmed by evaluation of the DNA content or PI fluorescent staining of the DNA of permeabilized cardiomyocytes. A critical role for mitochondria in the pathogenesis of palmitate-induced cell death was demonstrated, for the first time, based on palmitate-induced reduction of mitochondrial activity as assessed by the mitochondrial-selective dye chloromethyl-X-Rosamine and the presence of a greater amount of the mitochondrial marker cytochrome C in the cytosol of palmitate-treated cardiomyocytes than in control cells. Further, cyclosporin that inhibits the development of mitochondrial transition pores blocked palmitate-induced alteration in mitochondrial function and palmitate-induced cell death. We further demonstrated the selectivity of cyclosporin A for the prevention of apoptotic cell death in the heart as there was no alteration in necrotic cell death produced by palmitate with cyclosporin pretreatment. Our data demonstrate the nature of palmitate-induced cell death in cardiomyocytes (both apoptotic and necrotic), propose a mitochondrial basis for its pathogenesis and show that cyclosporin A prevents palmitate-induced apoptotic cardiomyocyte cell death.

Mitochondrial membrane potential and the permeability transition in excitotoxicity

Acute neuronal injury caused by activation of glutamate receptors in neurons, or excitotoxicity, can be triggered by the activation of N-methyl-D-aspartate receptors and the entry of large amounts of Ca2+. Recent studies have suggested that mitochondria have a critical role in the excitotoxicity injury mechanism. Mitochondria accumulate large amounts of Ca2+ following glutamate stimulation, and also generate reactive oxygen species. Moreover, the prevention of mitochondrial Ca2+ accumulation protects neurons from injury. The target for the actions of Ca2+ in the mitochondrial matrix has not yet been established. The permeability transition pore has the characteristics of a mechanism that is well suited to mediate neuronal injury. However, evidence for activation of the permeability transition pore in intact neurons is rather indirect, and these data suffer from some ambiguities that make it difficult to conclude that permeability transition is a critical contributor to mitochondrially mediated neuronal injury.

Mitochondrial transport of cations: channels, exchangers, and permeability transition

This review provides a selective history of how studies of mitochondrial cation transport (K+, Na+, Ca2+) developed in relation to the major themes of research in bioenergetics. It then covers in some detail specific transport pathways for these cations, and it introduces and discusses open problems about their nature and physiological function, particularly in relation to volume regulation and Ca2+ homeostasis. The review should provide the basic elements needed to understand both earlier mitochondrial literature and current problems associated with mitochondrial transport of cations and hopefully will foster new interest in the molecular definition of mitochondrial cation channels and exchangers as well as their roles in cell physiology.

Pivotal role of mitochondrial calcium uptake in neural cell apoptosis and necrosis

Perturbed cellular calcium homeostasis has been implicated in both apoptosis and necrosis, but the role of altered mitochondrial calcium handling in the cell death process is unclear. The temporal ordering of changes in cytoplasmic ([Ca2+]C) and intramitochondrial ([Ca2+]M) calcium levels in relation to mitochondrial reactive oxygen species (ROS) accumulation and membrane depolarization (MD) was examined in cultured neural cells exposed to either an apoptotic (staurosporine; STS) or a necrotic (the toxic aldehyde 4-hydroxynonenal; HNE) insult. STS and HNE each induced an early increase of [Ca2+]C followed by delayed increase of [Ca2+]M. Overexpression of Bcl-2 blocked the elevation of [Ca2+]M and the MD in cells exposed to STS but not in cells exposed to HNE. The cytoplasmic calcium chelator BAPTA-AM and the inhibitor of mitochondrial calcium uptake ruthenium red prevented both apoptosis and necrosis. STS and HNE each induced mitochondrial ROS accumulation and MD, which followed the increase of [Ca2+]M. Cyclosporin A prevented both apoptosis and necrosis, indicating critical roles for MD in both forms of cell death. Caspase activation occurred only in cells undergoing apoptosis and preceded increased [Ca2+]M. Collectively, these findings suggest that mitochondrial calcium overload is a critical event in both apoptotic and necrotic cell death.

Mitochondrial permeability transition and swelling can occur reversibly without inducing cell death in intact human cells

Severe disruption of mitochondrial function is generally considered to provide a powerful trigger for apoptosis in mammalian cells. We report here that intact cells may undergo the mitochondrial permeability transition and mitochondria swell in a fully reversible manner, without inducing cell death. Cultured human osteosarcoma cells (143B TK-) stained with JC-1, MitoTracker dyes, or calcein plus Co2+ were imaged by confocal microscopy to visualize changes of mitochondrial membrane potential (DeltaPsim), morphology, and permeability transition, respectively, during treatment with a protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Cells rapidly exhibited mitochondrial permeability transition and swelling after addition of CCCP, but the swelling subsided within hours, leaving mitochondria that appeared in punctate form, not filamentous as before CCCP treatment. Cyclosporin A impeded the permeability transition and swelling, although complete inhibition was not observed. Cells survived the dissipation of DeltaPsim by CCCP for up to 6 h without developing any obvious cell damage or signs of apoptosis. With the restoration of DeltaPsim after removal of CCCP (following 6 h of CCCP treatment), permeability transition pores were closed. These results suggest that none of the following events represent a point of no return in the process of apoptotic cell death: loss of DeltaPsim, mitochondrial permeability transition, or mitochondrial swelling.

Mitochondrial permeability transition during hypothermic to normothermic reperfusion in rat liver demonstrated by the protective effect of cyclosporin A

The purpose of this study was to test the hypothesis that mitochondrial permeability transition might be implicated in mitochondrial and intact organ dysfunctions associated with damage induced by reperfusion after cold ischaemia. Energetic metabolism was assessed continuously by 31P-NMR on a model system of isolated perfused rat liver; mitochondria were extracted from the livers and studied by using top-down control analysis. During the temperature transition from hypothermic to normothermic perfusion (from 4 to 37 degrees C) the ATP content of the perfused organ fell rapidly, and top-down metabolic control analysis of damaged mitochondria revealed a specific control pattern characterized by a dysfunction of the phosphorylation subsystem leading to a decreased response to cellular ATP demand. Both dysfunctions were fully prevented by cyclosporin A, a specific inhibitor of the mitochondrial transition pore (MTP). These results strongly suggest the involvement of the opening of MTP in vivo during the transition to normothermia on rat liver mitochondrial function and organ energetics.

Engagement of CD99 induces apoptosis through a calcineurin-independent pathway in Ewing's sarcoma cells

Programmed cell death (PCD) is a prominent feature of the development of the immune and nervous systems. In both systems, widespread PCD occurs in primitive progenitor cells during development. In this study, we demonstrated that Ewing's sarcoma (ES) cells, undifferentiated neural precursors, underwent apoptosis upon engagement of CD99 with anti-CD99 monoclonal antibody. Apoptosis via CD99 occurred only in the undifferentiated state of ES cells, but not in differentiated ES cells. CD99-induced apoptosis in ES cells appeared to require de novo synthesis of RNA and protein as well as caspase activation. Cyclosporin A, known to be a potent inhibitor of both calcineurin activation and mitochondrial permeability transition pore opening, inhibited CD99-mediated apoptosis, whereas FK-506, a specific calcineurin inhibitor, did not, indicating the induction of CD99-mediated apoptosis through a calcineurin-independent pathway. Furthermore, the dying cells displayed the reduction of mitochondrial transmembrane potential (delta psi m). These results suggest that CD99 engagement induce CsA-inhibitable mitochondrial permeability transition pore opening, followed by a reduction of delta psi m and caspase activation, thereby leading to apoptosis. Based on these results, we suggest the possible involvement of CD99 in the apoptotic processes that occur during nervous system development and also its application in immunotherapeutic trials for ES cases.

HIV-1 protein Tat induces apoptosis of hippocampal neurons by a mechanism involving caspase activation, calcium overload, and oxidative stress

Patients infected with HIV-1 often exhibit cognitive deficits that are related to progressive neuronal degeneration and cell death. The protein Tat, which is released from HIV-1-infected cells, was recently shown to be toxic toward cultured neurons. We now report that Tat induces apoptosis in cultured embryonic rat hippocampal neurons. Tat induced caspase activation, and the caspase inhibitor zVAD-fmk prevented Tat-induced neuronal death. Tat induced a progressive elevation of cytoplasmic-free calcium levels, which was followed by mitochondrial calcium uptake and generation of mitochondrial-reactive oxygen species (ROS). The intracellular calcium chelator BAPTA-AM and the inhibitor of mitochondrial calcium uptake ruthenium red protected neurons against Tat-induced apoptosis. zVAD-fmk suppressed Tat-induced increases of cytoplasmic calcium levels and mitochondrial ROS accumulation, indicating roles for caspases in the perturbed calcium homeostasis and oxidative stress induced by Tat. An inhibitor of nitric oxide synthase, and the peroxynitrite scavenger uric acid, protected neurons against Tat-induced apoptosis, indicating requirements for nitric oxide production and peroxynitrite formation in the cell death process. Finally, Tat caused a delayed and progressive mitochondrial membrane depolarization, and cyclosporin A prevented Tat-induced apoptosis, suggesting an important role for mitochondrial membrane permeability transition in Tat-induced apoptosis. Collectively, our data demonstrate that Tat can induce neuronal apoptosis by a mechanism involving disruption of calcium homeostasis, caspase activation, and mitochondrial calcium uptake and ROS accumulation. Agents that interupt this apoptotic cascade may prove beneficial in preventing neuronal degeneration and associated dementia in AIDS patients.

Tamoxifen inhibits induction of the mitochondrial permeability transition by Ca2+ and inorganic phosphate

Tamoxifen (TAM) is a synthetic, nonsteroidal antiestrogenic agent that is widely prescribed in the treatment of estrogen-dependent neoplasias, including breast cancer. The mechanism of action has yet to be defined, but likely is independent of estrogen receptor binding. In light of its high lipophilicity and peroxyl radical scavenging activities, we hypothesized that TAM might be an effective inhibitor of the mitochondrial permeability transition (MPT), which is widely implicated in the mechanisms of chemical-induced tissue injury and apoptosis. The MPT was induced in vitro by incubating freshly isolated rat liver mitochondria in 1 mM Pi with increasing concentrations of calcium. Induction of the MPT was characterized by the calcium-dependent depolarization of mitochondrial membrane potential, release of matrix calcium, and large amplitude swelling. Membrane potential and calcium release were measured with ion-selective electrodes; mitochondrial swelling was monitored spectrophotometrically. Preincubation with either cyclosporine A or TAM prevented, in a dose-dependent manner, the calcium-induced MPT. TAM also inhibited the calcium-induced release of matrix glutathione. TAM caused a time-dependent reversal of both the calcium-induced membrane depolarization and calcium release, suggesting that the effect was on the permeability transition pore and not due to inhibition of the mitochondrial calcium uniport. The results suggest that TAM mimics cyclosporine A to inhibit induction of the MPT and that this activity is not related to the antioxidant properties of TAM.

Mitochondria as regulators of apoptosis: doubt no more

Scientific revolution [1] implies a transformation of the world view in which a dominant paradigm is substituted by a new one, one which furnishes an ameliorated comprehension of facts, as well as an advantage for the design of informative experiments. Apoptosis research has recently experienced a change from a paradigm in which the nucleus determined the apoptotic process to a paradigm in which mitochondria constitute the center of death control. Several pieces of evidence imply mitochondria in the process of apoptosis. Kinetic data indicate that mitochondria undergo major changes in membrane integrity before classical signs of apoptosis become manifest. These changes concern both the inner and the outer mitochondrial membranes, leading to a disruption of the inner transmembrane potential (DeltaPsim) and the release of intermembrane proteins through the outer membrane. Cell-free systems of apoptosis demonstrate that mitochondrial products are rate limiting for the activation of caspases and endonucleases in cell extracts. Functional studies indicate that drug-enforced opening or closing of the mitochondrial megachannel (also called permeability transition pore) can induce or prevent apoptosis. The anti-apoptotic oncoprotein Bcl-2 acts on mitochondria to stabilize membrane integrity and to prevent opening of the megachannel. These observations are compatible with a three-step model of apoptosis: a premitochondrial phase during which signal transduction cascades or damage pathways are activated; a mitochondrial phase, during which mitochondrial membrane function is lost; and a post-mitochondrial phase, during which proteins released from mitochondria cause the activation of catabolic proteases and nucleases. The implication of mitochondria in apoptosis has important consequences for the understanding of the normal physiology of apoptosis, its deregulation in cancer and degenerative diseases, and the development of novel cytotoxic and cytoprotective drugs.

Elucidating the molecular mechanism of the permeability transition pore and its role in reperfusion injury of the heart

First, we present a summary of the evidence for our model of the molecular mechanism of the permeability transition (MPT). Our proposal is that the MPT occurs as a result of the binding of mitochondrial cyclophilin (CyP-D) to the adenine nucleotide translocase (ANT) in the inner mitochondrial membrane. This binding is enhanced by thiol modification of the ANT caused by oxidative stress or other thiol reagents. CyP-D binding enhances the ability of the ANT to undergo a conformational change triggered by Ca2+. Binding of ADP or ATP to a matrix site of the ANT antagonises this effect of Ca2+; modification of other ANT thiol groups inhibits ADP binding and sensitises the MPT to [Ca2+]. Increased membrane potential changes the ANT conformation to enhance ATP binding and hence inhibit the MPT. Our most recent data shows that a fusion protein of CyP-D and glutathione-S-transferase immobilised to Sepharose specifically binds the ANT from Triton-solubilised inner mitochondrial membranes in a cyclosporin A (CsA) sensitive manner. Second we summarise the evidence for the MPT being a major factor in the transition from reversible to irreversible injury during reperfusion of a heart following a period of ischaemia. We describe how in the perfused heart [3H]deoxyglucose entrapment within mitochondria can be used to measure the opening of MPT pore in situ. During ischaemia pore opening does not occur, but significant opening does occur during reperfusion, and recovery of the heart is dependent on subsequent pore closure. Pore opening is inhibited by the presence in the perfusion medium of pyruvate and the anaesthetic propofol which both protect the heart from reperfusion injury. Third we discuss how the MPT may be involved in determining whether cell death occurs by necrosis (extensive pore opening and ATP depletion) or apoptosis (transient pore opening with maintenance of ATP).

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

Some compounds are known to induce solute-nonselective permeability of the inner mitochondrial membrane (IMM) in Ca2+-loaded mitochondria. Existing data suggest that this process, following the opening of a mitochondrial permeability transition pore, is preceded by different solute-selective permeable states of IMM. At pH 7, for instance, the K0.5 for Ca2+-induced pore opening is 16 microM, a value 80-fold above a therapeutically relevant shift of intracellular Ca2+ during ischemia in vivo. The present work shows that in the absence of Ca2+, phenylarsine oxide and tetraalkyl thiuram disulfides (TDs) are able to induce a complex sequence of IMM permeability changes. At first, these agents activated an electrogenic K+ influx into the mitochondria. This K+-specific pathway had K0.5 = 35 mM for K+ and was inhibited by bromsulfalein with Ki = 2.5 microM. The inhibitors of mitochondrial KATP channel, ATP and glibenclamide, did not inhibit K+ transport via this pathway. Moreover, 50 microM glibenclamide induced by itself K+ influx into the mitochondria. After the increase in K+ permeability of IMM, mitochondria become increasingly permeable to protons. Mechanisms of H+ leak and nonselective permeability increase could also be different depending on the type of mitochondrial permeability transition (MPT) inducer. Thus, permeabilization of mitochondria induced by phenylarsine oxide was fully prevented by ADP and/or cyclosporin A, whereas TD-induced membrane alterations were insensitive toward these inhibitors. It is suggested that MPT in vivo leading to irreversible apoptosis is irrelevant in reversible ischemia/reperfusion injury.

Increased sensitivity to mitochondrial toxin-induced apoptosis in neural cells expressing mutant presenilin-1 is linked to perturbed calcium homeostasis and enhanced oxyradical production

Many cases of autosomal dominant early onset Alzheimer's disease (AD) result from mutations in the gene encoding presenilin-1 (PS-1). PS-1 is an integral membrane protein expressed ubiquitously in neurons throughout the brain in which it is located primarily in endoplasmic reticulum (ER). Although the pathogenic mechanism of PS-1 mutations is unknown, recent findings suggest that PS mutations render neurons vulnerable to apoptosis. Because increasing evidence indicates that mitochondrial alterations contribute to neuronal death in AD, we tested the hypothesis that PS-1 mutations sensitize neurons to mitochondrial failure. PC12 cell lines expressing a PS-1 mutation (L286V) exhibited increased sensitivity to apoptosis induced by 3-nitropropionic acid (3-NP) and malonate, inhibitors of succinate dehydrogenase, compared with control cell lines and lines overexpressing wild-type PS-1. The apoptosis-enhancing action of mutant PS-1 was prevented by antioxidants (propyl gallate and glutathione), zVAD-fmk, and cyclosporin A, indicating requirements of reactive oxygen species (ROS), caspases, and mitochondrial permeability transition in the cell death process. 3-NP induced a rapid elevation of [Ca2+]i, which was followed by caspase activation, accumulation of ROS, and decreases in mitochondrial reducing potential and transmembrane potential in cells expressing mutant PS-1. The calcium chelator BAPTA AM and agents that block calcium release from ER and influx through voltage-dependent channels prevented mitochondrial ROS accumulation and membrane depolarization and apoptosis. Our data suggest that by perturbing subcellular calcium homeostasis presenilin mutations sensitize neurons to mitochondria-based forms of apoptosis that involve oxidative stress.

The role of intracellular Na+ and mitochondria in buffering of kainate-induced intracellular free Ca2+ changes in rat forebrain neurones

1. We have examined the mechanisms by which cultured central neurones from embryonic rat brain buffer intracellular Ca2+ loads following kainate receptor activation using fluorescent indicators of [Ca2+]i and [Na+]i. 2. Stimulation of cultured forebrain neurones with 100 microM kainate produced a rapid increase in [Ca2+]i that displayed a variable rate of recovery. Kainate also increased [Na+]i with a response that was slightly slower in onset and markedly slower in recovery. 3. The recovery of [Ca2+]i to baseline was not very sensitive to the [Na+]i. The magnitude of the increase in [Na+]i in response to kainate did not correlate well with the [Ca2+]i recovery time, and experimental manipulations that altered [Na+]i did not have a large impact on the rate of recovery of [Ca2+]i. 4. The recovery of [Ca2+]i to baseline was accelerated by the mitochondrial Na+-Ca2+ exchange inhibitor CGP-37157, suggesting that the recovery rate is influenced by release of Ca2+ from a mitochondrial pool and also that variation in the recovery rate is related to the extent of mitochondrial Ca2+ loading. Kainate did not alter the mitochondrial membrane potential. 5. These studies reveal that mitochondria have a central role in buffering neuronal [Ca2+]i changes mediated by non-N-methyl-D-aspartate (NMDA) glutamate receptors, and that the variation in recovery times following kainate receptor activation reflects a variable degree of mitochondrial Ca2+ loading. However, unlike NMDA receptor-mediated Ca2+ loads, kainate receptor activation has minimal effects on mitochondrial function.

Mitochondrial manganese superoxide dismutase prevents neural apoptosis and reduces ischemic brain injury: suppression of peroxynitrite production, lipid peroxidation, and mitochondrial dysfunction

Oxidative stress is implicated in neuronal apoptosis that occurs in physiological settings and in neurodegenerative disorders. Superoxide anion radical, produced during mitochondrial respiration, is involved in the generation of several potentially damaging reactive oxygen species including peroxynitrite. To examine directly the role of superoxide and peroxynitrite in neuronal apoptosis, we generated neural cell lines and transgenic mice that overexpress human mitochondrial manganese superoxide dismutase (MnSOD). In cultured pheochromocytoma PC6 cells, overexpression of mitochondria-localized MnSOD prevented apoptosis induced by Fe2+, amyloid beta-peptide (Abeta), and nitric oxide-generating agents. Accumulations of peroxynitrite, nitrated proteins, and the membrane lipid peroxidation product 4-hydroxynonenal (HNE) after exposure to the apoptotic insults were markedly attenuated in cells expressing MnSOD. Glutathione peroxidase activity levels were increased in cells overexpressing MnSOD, suggesting a compensatory response to increased H2O2 levels. The peroxynitrite scavenger uric acid and the antioxidants propyl gallate and glutathione prevented apoptosis induced by each apoptotic insult, suggesting central roles for peroxynitrite and membrane lipid peroxidation in oxidative stress-induced apoptosis. Apoptotic insults decreased mitochondrial transmembrane potential and energy charge in control cells but not in cells overexpressing MnSOD, and cyclosporin A and caspase inhibitors protected cells against apoptosis, demonstrating roles for mitochondrial alterations and caspase activation in the apoptotic process. Membrane lipid peroxidation, protein nitration, and neuronal death after focal cerebral ischemia were significantly reduced in transgenic mice overexpressing human MnSOD. The data suggest that mitochondrial superoxide accumulation and consequent peroxynitrite production and mitochondrial dysfunction play pivotal roles in neuronal apoptosis induced by diverse insults in cell culture and in vivo.

The mitochondrial permeability transition

This review summarizes recent work on the regulation of the permeability transition pore, a cyclosporin A-sensitive mitochondrial channel that may play a role in intracellular calcium homeostasis and in a variety of forms of cell death. The basic bioenergetics aspects of pore modulation are discussed, with some emphasis on the links between oxidative stress and pore dysregulation as a potential cause of mitochondrial dysfunction that may be relevant to cell injury.

Nitric oxide induces apoptosis via triggering mitochondrial permeability transition

Nitric oxide (NO) induces apoptosis in thymocytes, peripheral T cells, myeloid cells and neurons. Here we show that NO is highly efficient in inducing mitochondrial permeability transition, thereby causing the liberation of apoptogenic factors from mitochondria which can induce nuclear apoptosis (DNA condensation and DNA fragmentation) in isolated nuclei in vitro. In intact thymocytes, NO triggers disruption of the mitochondrial transmembrane potential, followed by hypergeneration of reactive oxygen species, exposure of phosphatidyl serine on the outer plasma membrane leaflet, and nuclear apoptosis. Inhibitors of mitochondrial permeability transition such as bongkrekic acid and a cyclophilin D-binding cyclosporin A derivative, N-methyl-Val-4-cyclosporin A, prevent the mitochondrial as well as all post-mitochondrial signs of apoptosis induced by NO including nuclear DNA fragmentation and exposure of phosphatidylserine residues on the cell surface. These findings indicate that NO can cause apoptosis via triggering of permeability transition.

Release of cytochrome c from liver mitochondria during permeability transition

The mitochondrial permeability transition (PT) follows opening of megachannels in the inner membrane and may be part of a programmed death pathway. Recently a role for cytochrome c in programmed cell death has been proposed, although its relationship to PT has not been been determined. We studied the release of cytochrome c from liver mitochondria undergoing PT. Well-coupled mitochondria treated with 5 mM atractyloside (ATR) or 100 microM calcium chloride were found to undergo PT and release cytochrome c into the incubation buffer within 5 minutes. Control mitochondria and mitochondria treated with the uncoupler FCCP did not undergo PT or release cytochrome c at 5 or 15 minutes. PT induced by ATR could be prevented by pretreatment with 10 microM cyclosporin A. Mitochondria incubated with ATR or calcium caused a 20-30% decrease in electron transfer rate via cytochrome c and cytochrome c oxidase. We conclude that cytochrome c release is an early event during mitochondrial PT, and is sufficient to decrease electron transfer through the terminal electron transport components of the mitochondrion.

Oxidative stress, thiol reagents, and membrane potential modulate the mitochondrial permeability transition by affecting nucleotide binding to the adenine nucleotide translocase

Stimulation of the mitochondrial permeability transition (MPT) in de-energized mitochondria by phenylarsine oxide (PheArs) is greater than that by diamide and t-butylhydroperoxide (TBH), yet the increase in CyP binding to the inner mitochondrial membrane (Connern, C. P. and Halestrap, A. P. (1994) Biochem. J. 302, 321-324) is less. From a range of nucleotides tested only ADP, deoxy-ADP, and ATP inhibited the MPT. ADP inhibition involved two sites with Ki values of about 1 and 25 microM which were independent of [Ca2+] and CyP binding. Carboxyatractyloside (CAT) abolished the high affinity site. Following pretreatment of mitochondria with TBH or diamide, the Ki for ADP increased to 50-100 microM, whereas pretreatment with PheArs or eosin maleimide increased the Ki to >500 microM; only one inhibitory site was observed in both cases. Eosin maleimide is known to attack Cys159 of the adenine nucleotide translocase (ANT) in a CAT-sensitive manner (Majima, E., Shinohara, Y., Yamaguchi, N., Hong, Y. M., and Terada, H. (1994) Biochemistry 33, 9530-9536), and here we demonstrate CAT-sensitive binding of the ANT to a PheArs affinity column. In adenine nucleotide-depleted mitochondria, no stimulation of the MPT by uncoupler was observed in the presence or absence of thiol reagents, suggesting that membrane potential may inhibit the MPT by increasing adenine nucleotide binding through an effect on the ANT conformation. We conclude that CsA and ADP inhibit pore opening in distinct ways, CsA by displacing bound CyP and ADP by binding to the ANT. Both mechanisms act to decrease the Ca2+ sensitivity of the pore. Thiol reagents and oxidative stress may modify two thiol groups on the ANT and thus stimulate pore opening by both means.

Respiratory defect as an early event in preservation-reoxygenation injury of endothelial cells

Characterization of preservation injury in endothelial cells has been primarily accomplished by measurement of cell viability. To analyze early events and cellular mechanisms of preservation-reoxygenation injury, we developed high-resolution respirometry for the study of mitochondrial function in endothelial cells, to provide a quantitative marker for sublethal stress. Cultured human umbilical vein endothelial cells were stored for 4 and 8 hr at 4 degrees C under an atmosphere of 95% N2 and 5% CO2 in University of Wisconsin (UW) and histidine-tryptophan-ketoglutarate (HTK) solutions. Respiration of suspended cells, measured after reoxygenation in growth medium at 37 degrees C, was significantly reduced in all treatments in comparison to controls not subjected to cold preservation. In contrast, trypan blue staining was unchanged after 4 hr of preservation and was significant only after 8 hr. After 8 hr of cold storage in UW and HTK solutions, respiration was 64+/-5% and 49+/-6%, respectively, of controls (46.5+/-3.3 pmol O2 x s(-1 x 10(-6) cells), indicating significantly better protection by UW solution than HTK solution. A titration regimen with substrate (succinate), uncoupler (carbonyl cyanide p-trifluoromethoxyphenylhydrazone), and inhibitors of complexes I and III (rotenone and antimycin A) resulted in identical respiratory response patterns in all treatments. The plasma membrane remained impermeable to succinate. Inner mitochondrial membrane function was preserved as indicated by a constant relative increase of respiration after uncoupling. These results demonstrate that loss of catalytic capacity for respiration constitutes an early event in preservation-reoxygenation injury, whereas membrane damage is not a primary defect. Respirometric evaluation of sublethal cell injury and localization of cell damage may provide selective guidelines for further optimization of strategies in organ preservation.

Relation between accumulation of phospholipase A2 reaction products and Ca2+ release in isolated liver mitochondria

A Ca(2+)-dependent stimulation of mitochondrial phospholipase A2 is often assumed to play a role in mitochondrial Ca2+ release. We sought to clarify this relation by measuring Ca2+ transport and determining phospholipase A2 reaction products from the same sample of isolated, incubated rat liver mitochondria. When mitochondria had accumulated and spontaneously released again Ca2+, most probably by membrane permeability transition, there was no increase of phospholipase A2 reaction products. However, when the incubation was continued after Ca2+ release, significant increases of the content of lysophosphatidylcholine and unesterified fatty acids could be seen. Quinacrine, an inhibitor of phospholipase A2 activity, prevented Ca2+ release and p-hydroxymercuribenzoic acid, an inhibitor of lysophospholipid reesterification, induced a fast release of Ca2+ from isolated mitochondria. Such effects are usually taken as indirect evidence for a participation of phospholipase A2 in mitochondrial Ca2+ release, but analysis of the mitochondrial lipids revealed that no significant changes of the mass of phospholipase A2 reaction products had occurred. These experiments suggest that the accumulation of phospholipase A2 reaction products in mitochondria is the consequence rather than the cause of the membrane permeability transition. Exogenous phospholipase A2 products, lysophosphatidylcholine and arachidonic acid, induced mitochondrial Ca2+ release after a time lag, which decreased with aging of the mitochondrial preparation. The amount of lysophosphatidylcholine taken up by the mitochondria from the incubation medium during these experiments was measured and compared to the amount of lysophosphatidylcholine produced endogenously by mitochondrial phospholipase A2. From these data it appears likely that the amount of lysophosphatidylcholine generated in the mitochondria after the permeability transition is sufficient to sustain the permeable state. An accumulation of mitochondrially generated phospholipase A2 reaction products after the permeability transition could thus be a decisive factor for the limited reversibility of the membrane permeability transition.

Mitochondrial deenergization underlies neuronal calcium overload following a prolonged glutamate challenge

The purpose of our work was to study the relationship between glutamate (GLU)-induced mitochondrial depolarization and deterioration of neuronal Ca2+ homeostasis following a prolonged GLU challenge. The experiments were performed on cultured rat cerebellar granule cells using the fluorescent probes, rhodamine 123 and fura-2. All the cells, in which 100 microM GLU (10 microM glycine, 0 Mg2+) induced only relatively slight mitochondrial depolarization (1.1-1.3-fold increase in rhodamine 123 fluorescence), retained their ability to recover [Ca2+]i following a prolonged GLU challenge. In contrast, the cells in which GLU treatment induced pronounced mitochondrial depolarization (2-4-fold increase in rhodamine 123 fluorescence), exhibited a high Ca2+ plateau in the post-glutamate period. Application of 3-5 mM NaCN or 0.25-1 microM FCCP during this Ca2+ plateau phase usually failed to produce a further noticeable increase in [Ca2+]i. Regression analysis revealed a good correlation (r2 = 0.88 +/- 0.03, n = 19) between the increase in the percentage of rhodamine 123 fluorescence and the post-glutamate [Ca2+]i. Collectively, the results obtained led us to conclude that the GLU-induced neuronal Ca2+ overload was due to the collapse of the mitochondrial potential and subsequent ATP depletion.

A cluster of cytoplasmic histidine residues specifies pH dependence of the AE2 plasma membrane anion exchanger

Intracellular pH is maintained by a dynamic equilibrium balancing the opposing forces of proton loading and proton extrusion. By providing an efflux pathway for base, anion exchangers constitute a key component of the plasma membrane proton-loading machinery. The data in this paper identify a histidine-rich sequence within the cytoplasmic domain of the nonerythroid anion exchanger, AE2, that serves as an intracellular pH "sensor" that modulates anion exchange activity within the physiological range of cytoplasmic pH. These data reveal an interaction between the two major domains of the anion exchanger and suggest a novel substrate feedback mechanism by which intracellular protons directly control the activity of an acid-loading plasma membrane ion transporter.

The permeability transition pore. Control points of a cyclosporin A-sensitive mitochondrial channel involved in cell death

The permeability transition pore (MTP) is a high conductance channel of the mitochondrial inner membrane inhibited by cyclosporin A. While the physiological role of the MTP has not been clarified yet, it is becoming clear that this channel plays an important role in the pathways leading to cell death. The recent demonstrations that the MTP is controlled by the membrane potential, that a variety of physiological and pathological effectors can modulate the threshold voltage at which pore opening occurs, and that surface potential may contribute to pore modulation provide a useful framework to describe the mechanistic aspects of pore function in isolated mitochondria. Here we (i) briefly review the key features of pore regulation, and report our recent progress on the role of oxidants and mitochondrial cyclophilin; and (ii) elaborate on how MTP regulation by cellular pathophysiological effectors (such as cytosolic [Ca2+] transients, oxidative stress, and changes in the concentration of polyamines, nitric oxide, and metabolites of both the sphingomyelin and phospholipase A2 pathways) might take place in vivo. Further definition of the MTP checkpoints should help in the design of specific modulators, and offers great promise for the development of new conceptual and pharmacological tools aimed at therapeutic intervention in pathological conditions where pore opening is a critical event.

Inhibitors of permeability transition interfere with the disruption of the mitochondrial transmembrane potential during apoptosis

In a number of experimental systems, the early stage of the apoptotic process, i.e. the stage which precedes nuclear disintegration, is characterized by the breakdown of the inner mitochondrial transmembrane potential (delta psi m). Here we address the question as to whether mitochondrial permeability transition (PT) pores may account for the delta psi m dissipation in lymphocyte apoptosis. Drugs known for their PT-inhibitory potential (bongkrekic acid, cyclosporin A, and the non-immunosuppressive cyclosporin A analogue N-methyl-Val-4-cyclosporin A) are capable of preventing the apoptotic delta psi m disruption. Moreover, pharmacological modulation of PT-mediated delta psi m dissipation can prevent apoptosis. Thus, while suppressing the delta psi m disruption, bongkrekic acid also inhibits the apoptotic chromatinolysis. In conclusion, these data are compatible with the hypothesis that apoptotic delta psi m disruption is mediated by the formation of PT pores and that PT-mediated delta psi m disruption is a critical event of the apoptotic cascade.

The permeability transition pore as a mitochondrial calcium release channel: a critical appraisal

Mitochondria from a variety of sources possess an inner membrane channel, the permeability transition pore. The pore is a voltage-dependent channel, activated by matrix Ca2+ and inhibited by matrix H+, which can be blocked by cyclosporin A, presumably after binding to mitochondrial cyclophilin. The physiological function of the permeability transition pore remains unknown. Here we evaluate its potential role as a fast Ca2+ release channel involved in mitochondrial and cellular Ca2+ homeostasis. We (i) discuss the theoretical and experimental reasons why mitochondria need a fast, inducible Ca2+ release channel; (ii) analyze the striking analogies between the mitochondrial permeability transition pore and the sarcoplasmic reticulum ryanodine receptor-Ca2+ release channel; (iii) argue that the permeability transition pore can act as a selective release channel for Ca2+ despite its apparent lack of selectivity for the transported species in vitro; and (iv) discuss the importance of mitochondria in cellular Ca2+ homeostasis, and how disruption of this function could impinge upon cell viability, particularly under conditions of oxidative stress.

4-Hydroxyhexenal is a potent inducer of the mitochondrial permeability transition

Mitochondria undergo at least two types of structural alteration in response to various physiological and pathophysiological stimuli. One type is nonreversible and is associated with mitochondrial lysis. The second is reversible and appears to be associated with calcium-mediated activation of a specific inner mitochondrial membrane channel. The mechanisms underlying the induction of this second alteration, termed a mitochondrial permeability transition (PT), have been the subject of a great deal of recent research. Using rat liver mitochondria, our data demonstrate that calcium-mediated PT induction can be affected by the lipid peroxidation byproducts 4-hydroxynonenal and 4-hydroxyhexenal (HHE). 4-Hydroxynonenal appears inactive at concentrations <1 micromole but displays both stimulatory and inhibitory effects as part of a biphasic dose response between approximately 1 and 200 micromole. In contrast, HHE consistently enhances calcium-mediated induction of the PT, even at femtomolar concentrations. The exquisite specificity and sensitivity of HHE led to further studies to examine the nature of this induction. Studies showing that HHE-mediated induction could be prevented by cyclosporin A confirmed PT involvement. Further studies showed that induction was dependent on both calcium and electron transport chain function. Pretreatment of the HHE with glutathione also prevented PT induction, but simultaneous addition of the thiol reagents dithiothreitol or glutathione, which often prevents PT induction, was ineffective, attesting to the effectiveness of HHE as an inducer. Together, these data provide a possible mechanistic explanation for the previously observed effects of lipid peroxidation on PT induction.

Interactions of cyclophilin with the mitochondrial inner membrane and regulation of the permeability transition pore, and cyclosporin A-sensitive channel

Mammalian mitochondria possess an inner membrane channel, the permeability transition pore (MTP), which can be inhibited by nanomolar concentrations of cyclosporin (CS) A. The molecular basis for MTP inhibition by CSA remains unclear. Mitochondria also possess a matrix cyclophilin (CyP) with a unique N-terminal sequence (CyP-M). To test the hypothesis that it interacts with the MTP, we have studied the interactions of CyP-M with rat liver mitochondria by Western blotting with a specific antibody against its unique N terminus. Although sonication in isotonic sucrose at pH 7.4 refraction sediments with submitochondrial particles at 150,000 x g. We show that the interactions of this CyP-M pool with submitochondrial particles are disrupted (i) by the addition of CSA, which inhibits the pore, but not of CSH, which does not, and (ii) by acidic pH condition, which also leads to selective inhibition of the MTP; furthermore, we show that the effect of acidic pH on CyP-M fully prevents the inhibitory effect of H+ on the MTP (Nicolli, A., Petronilli, V., and Bernardi, P. (1993) Biochemistry 32, 4461-4465). These data suggest that CyP-M inhibition by CSA and protons may be due to unbinding of CyP-M from its putative binding site on the MTP. A role for CyP-M in MTP regulation is also supported by a study with a series of CSA derivatives with graded affinity for CyP. We show that with each derivative the isomerase activity of CyP-M purified to homogeneity is similar to that displayed at inhibition of MTP opening, CyP-M (but not CyP-A) and decreased efficiency at MTP inhibition is obtained by substitution in position 8 while a 4-substituted, nonimmunosuppressive derivative is a as effective as the native CSA molecule, indicating that calcineurin is not involved in MTP inhibition by CSA.

Effect of cyclosporin A on energy coupling in pea stem mitochondria

Effect of cyclosporin A on energy coupling in pea stem mitochondria is studied. It is found that incubation of mitochondria with 100 nM FCCP and/or CAtr, oligomycin, CaCl2, palmitate and ADP results, after some lag phase, in a collapse of delta psi generated by succinate oxidation in the presence of rotenone. Cyclosporin A (0.2-0.8 nmol/mg mitochondrial protein) markedly increases the lag phase. The cyclosporin A effect requires dithioerythritol to be added to the isolated medium. Metabisulphite fails to substitute for dithioerythritol. The relationships between these effects and cyclosporin A-sensitive mitochondrial permeability transition in animal mitochondria are discussed.

Sequential reduction of mitochondrial transmembrane potential and generation of reactive oxygen species in early programmed cell death

Programmed cell death (PCD) is a physiological process commonly defined by alterations in nuclear morphology (apoptosis) and/or characteristic stepwise degradation of chromosomal DNA occurring before cytolysis. However, determined characteristics of PCD such as loss in mitochondrial reductase activity or cytolysis can be induced in enucleated cells, indicating cytoplasmic PCD control. Here we report a sequential disregulation of mitochondrial function that precedes cell shrinkage and nuclear fragmentation. A first cyclosporin A-inhibitable step of ongoing PCD is characterized by a reduction of mitochondrial transmembrane potential, as determined by specific fluorochromes (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine++ + iodide; 3,3'dihexyloxacarbocyanine iodide). Cytofluorometrically purified cells with reduced mitochondrial transmembrane potential are initially incapable of oxidizing hydroethidine (HE) into ethidium. Upon short-term in vitro culture, such cells acquire the capacity of HE oxidation, thus revealing a second step of PCD marked by mitochondrial generation of reactive oxygen species (ROS). This step can be selectively inhibited by rotenone and ruthenium red yet is not affected by cyclosporin A. Finally, cells reduce their volume, a step that is delayed by radical scavengers, indicating the implication of ROS in the apoptotic process. This sequence of alterations accompanying early PCD is found in very different models of apoptosis induction: glucocorticoid-induced death of lymphocytes, activation-induced PCD of T cell hybridomas, and tumor necrosis factor-induced death of U937 cells. Transfection with the antiapoptotic protooncogene Bcl-2 simultaneously inhibits mitochondrial alterations and apoptotic cell death triggered by steroids or ceramide. In vivo injection of fluorochromes such as 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine iodide; 3,3'dihexyloxacarbocyanine iodide; or HE allows for the detection of cells that are programmed for death but still lack nuclear DNA fragmentation. In particular, assessment of mitochondrial ROS generation provides an accurate picture of PCD-mediated lymphocyte depletion. In conclusion, alterations of mitochondrial function constitute an important feature of early PCD.

Recent progress on regulation of the mitochondrial permeability transition pore; a cyclosporin-sensitive pore in the inner mitochondrial membrane

The mitochondrial permeability transition pore allows solutes with a m.w. approximately less than 1500 to equilibrate across the inner membrane. A closed pore is favored by cyclosporin A acting at a high-affinity site, which may be the matrix space cylophilin isozyme. Early results obtained with cyclosporin A analogs and metabolites support this hypothesis. Inhibition by cyclosporin does not appear to require inhibition of calcineurin activity; however, it may relate to inhibition of cyclophilin peptide bond isomerase activity. The permeability transition pore is strongly regulated by both the membrane potential (delta psi) and delta pH components of the mitochondrial protonmotive force. A voltage sensor which is influenced by the disulfide/sulhydryl state of vicinal sulfhydryls is proposed to render pore opening sensitive to delta psi. Early results indicate that this sensor is also responsive to membrane surface potential and/or to surface potential gradients. Histidine residues located on the matrix side of the inner membrane render the pore responsive to delta pH. The pore is also regulated by several ions and metabolites which act at sites that are interactive. There are many analogies between the systems which regulate the permeability transition pore and the NMDA receptor channel. These suggest structural similarities and that the permeability transition pore belongs to the family of ligand gated ion channels.

Aneurysm of the azygos pericallosal artery. One case

The Authors describe a case of aneurysmal rupture of the azygos pericallosal artery, a variant of the anterior cerebral artery. The association between aneurysm and this anatomical anomaly is of interest because of its rarity and clinical-therapeutic implications.