Increased permeability of mitochondria during Ca2+ release induced by t-butyl hydroperoxide or oxalacetate. the effect of ruthenium red

Increasing Ca2+ in photoreceptor mitochondria alters metabolites, accelerates photoresponse recovery, and reveals adaptations to mitochondrial stress

Photoreceptors are specialized neurons that rely on Ca²⁺ to regulate phototransduction and neurotransmission. Photoreceptor dysfunction and degeneration occur when intracellular Ca²⁺ homeostasis is disrupted. Ca²⁺ homeostasis is maintained partly by mitochondrial Ca²⁺ uptake through the mitochondrial Ca²⁺ uniporter (MCU), which can influence cytosolic Ca²⁺ signals, stimulate energy production, and trigger apoptosis. Here we discovered that zebrafish cone photoreceptors express unusually low levels of MCU. We expected that this would be important to prevent mitochondrial Ca²⁺ overload and consequent cone degeneration. To test this hypothesis, we generated a cone-specific model of MCU overexpression. Surprisingly, we found that cones tolerate MCU overexpression, surviving elevated mitochondrial Ca²⁺ and disruptions to mitochondrial ultrastructure until late adulthood. We exploited the survival of MCU overexpressing cones to additionally demonstrate that mitochondrial Ca²⁺ uptake alters the distributions of citric acid cycle intermediates and accelerates recovery kinetics of the cone response to light. Cones adapt to mitochondrial Ca²⁺ stress by decreasing MICU3, an enhancer of MCU-mediated Ca²⁺ uptake, and selectively transporting damaged mitochondria away from the ellipsoid toward the synapse. Our findings demonstrate how mitochondrial Ca²⁺ can influence physiological and metabolic processes in cones and highlight the remarkable ability of cone photoreceptors to adapt to mitochondrial stress.

Mitochondrial calcium transport and the redox nature of the calcium-induced membrane permeability transition

Mitochondria possess a Ca²⁺ transport system composed of separate Ca²⁺ influx and efflux pathways. Intramitochondrial Ca²⁺ concentrations regulate oxidative phosphorylation, required for cell function and survival, and mitochondrial redox balance, that participates in a myriad of signaling and damaging pathways. The interaction between Ca²⁺ accumulation and redox imbalance regulates opening and closing of a highly regulated inner membrane pore, the membrane permeability transition pore (PTP). In this review, we discuss the regulation of the PTP by mitochondrial oxidants, reactive nitrogen species, and the interactions between these species and other PTP inducers. In addition, we discuss the involvement of mitochondrial redox imbalance and PTP in metabolic conditions such as atherogenesis, diabetes, obesity and in mtDNA stability.

The mitochondrial permeability transition pore in AD 2016: An update

Over the past 30years the mitochondrial permeability transition - the permeabilization of the inner mitochondrial membrane due to the opening of a wide pore - has progressed from being considered a curious artifact induced in isolated mitochondria by Ca(2+) and phosphate to a key cell-death-inducing process in several major pathologies. Its relevance is by now universally acknowledged and a pharmacology targeting the phenomenon is being developed. The molecular nature of the pore remains to this day uncertain, but progress has recently been made with the identification of the FOF1 ATP synthase as the probable proteic substrate. Researchers sharing this conviction are however divided into two camps: these believing that only the ATP synthase dimers or oligomers can form the pore, presumably in the contact region between monomers, and those who consider that the ring-forming c subunits in the FO sector actually constitute the walls of the pore. The latest development is the emergence of a new candidate: Spastic Paraplegia 7 (SPG7), a mitochondrial AAA-type membrane protease which forms a 6-stave barrel. This review summarizes recent developments of research on the pathophysiological relevance and on the molecular nature of the mitochondrial permeability transition pore. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

A mathematical model of mitochondrial swelling

Background

The permeabilization of mitochondrial membranes is a decisive event in apoptosis or necrosis culminating in cell death. One fundamental mechanism by which such permeabilization events occur is the calcium-induced mitochondrial permeability transition. Upon Ca²⁺-uptake into mitochondria an increase in inner membrane permeability occurs by a yet unclear mechanism. This leads to a net water influx in the mitochondrial matrix, mitochondrial swelling, and finally the rupture of the outer membrane. Although already described more than thirty years ago, many unsolved questions surround this important biological phenomenon. Importantly, theoretical modeling of the mitochondrial permeability transition has only started recently and the existing mathematical models fail to characterize the swelling process throughout the whole time range.

Results

We propose here a new mathematical approach to the mitochondrial permeability transition introducing a specific delay equation and resulting in an optimized representation of mitochondrial swelling. Our new model is in accordance with the experimentally determined course of volume increase throughout the whole swelling process, including its initial lag phase as well as its termination. From this new model biological consequences can be deduced, such as the confirmation of a positive feedback of mitochondrial swelling which linearly depends on the Ca²⁺-concentration, or a negative exponential dependence of the average swelling time on the Ca²⁺-concentration. Finally, our model can show an initial shrinking phase of mitochondria, which is often observed experimentally before the actual swelling starts.

Conclusions

We present a model of the mitochondrial swelling kinetics. This model may be adapted and extended to diverse other inducing/inhibiting conditions or to mitochondria from other biological sources and thus may benefit a better understanding of the mitochondrial permeability transition.

Characteristics and possible functions of mitochondrial Ca(2+) transport mechanisms

Mitochondria produce around 92% of the ATP used in the typical animal cell by oxidative phosphorylation using energy from their electrochemical proton gradient. Intramitochondrial free Ca(2+) concentration ([Ca(2+)](m)) has been found to be an important component of control of the rate of this ATP production. In addition, [Ca(2+)](m) also controls the opening of a large pore in the inner mitochondrial membrane, the permeability transition pore (PTP), which plays a role in mitochondrial control of programmed cell death or apoptosis. Therefore, [Ca(2+)](m) can control whether the cell has sufficient ATP to fulfill its functions and survive or is condemned to death. Ca(2+) is also one of the most important second messengers within the cytosol, signaling changes in cellular response through Ca(2+) pulses or transients. Mitochondria can also sequester Ca(2+) from these transients so as to modify the shape of Ca(2+) signaling transients or control their location within the cell. All of this is controlled by the action of four or five mitochondrial Ca(2+) transport mechanisms and the PTP. The characteristics of these mechanisms of Ca(2+) transport and a discussion of how they might function are described in this paper.

Two critical factors affecting the release of mitochondrial cytochrome C as revealed by studies using N,N'-dicyclohexylcarbodiimide as an atypical inducer of permeability transition

N,N'-dicyclohexylcarbodiimide (DCCD) was earlier reported to have stimulatory effects on mitochondrial respiration and to induce mitochondrial swelling, when it was added to mitochondrial suspensions. These data seem to imply that DCCD caused the mitochondrial permeability transition (PT), but this possibility had never been investigated. In the present study, effects of DCCD on the mitochondrial structure and function were studied in detail. DCCD was found to induce mitochondrial PT in a cyclosporine A-insensitive manner. Electron microscopic analysis also supported the induction of the mitochondrial PT by DCCD. However, different from many other PT inducers, DCCD failed to cause massive release of mitochondrial cytochrome c. To understand the relationship between the induction of mitochondrial PT and the release of mitochondrial cytochrome c, we compared the actions of DCCD on mitochondrial structure and function with those of Ca2+, known as an ordinary PT inducer. As a result, two parameters considered to be critical for controlling the release of mitochondrial cytochrome c on the induction of PT were mitochondrial volume and the velocity of mitochondrial oxygen consumption.

Molecular basis of morphological changes in mitochondrial membrane accompanying induction of permeability transition, as revealed by immuno-electron microscopy

The mitochondrial inner membrane typically shows a condensed structure when examined by electron microscopy. However, this typical structure is known to disappear upon induction of the mitochondrial permeability transition (PT). This change in the appearance of the mitochondrial membrane structure that accompanies the induction of PT is thought to reflect changes in the permeability of inner mitochondrial membrane; however, its molecular basis has remained uncertain. In the present study, changes in membrane status were examined by immuno-electron microscopy using antibodies against the voltage-dependent anion channel (VDAC), beta-subunit of F1-ATPase (F1beta), and cytochrome c (cyt. c). In control mitochondria, antibody against VDAC was observed at the rim of the mitochondria, whereas antibodies against F1beta and cytochrome c bound these molecules inside of the mitochondria. However, in PT-induced mitochondria, all three antibodies were observed at the mitochondrial rim. These results strongly suggest that the inner mitochondrial membrane is shoved to the rim region of mitochondria upon induction of mitochondrial PT.

Morphology of mitochondrial permeability transition: morphometric volumetry in apoptotic cells

Here we report on the mitochondrial permeability transition (MPT), which refers to the morphology of mitochondria whose inner membrane has lost its selective permeability. In all types of apoptotic cells so far examined, we found outer mitochondrial membranes that had been ruptured. These mitochondria present a swollen matrix covered by an inner membrane herniating into the cytoplasm through the breached outer membrane. Similarly ruptured outer mitochondrial membranes have been reported in studies on mitochondrial fractions induced to undergo MPT, carried out by others. Our observations were made on five types of rat tissue cells and six different cultured cell lines in the early stages of apoptosis. Samples from the cell lines HL-60, HeLa, WEHI-164, and a special batch of PC-12 cells were subjected to various apoptogenic agents and analyzed morphometrically. Nonapoptotic companion cells with unaltered nuclear structure (CUNS) were also analyzed. The mitochondrial volume in microm(3) and the volume fraction of the cytoplasm occupied by mitochondria in cells with typical nuclear signs of apoptosis and also in CUNS were evaluated. The volume of the mitochondria with ruptured membrane represents at least 69% (47-89%) of the total mitochondrial volume of the apoptotic cells. Thus, a considerable fraction of the cellular mitochondrial mass is or was in the state of permeability transition and probably involved in enhancement of the apoptotic program. In all samples, a fraction of the cells with normal nuclei possessed mitochondria with breached outer membranes as described above. In these cells, MPT occurred before the appearance of the typical nuclear phenotype of the apoptotic cells.

Multiple effects of DiS-C3(5) on mitochondrial structure and function

3,3'-Dipropyl-2,2'-thiadicarbocyanine iodide [DiS-C(3)(5)], often used as a tracer dye to assess the mitochondrial membrane potential, was investigated in detail regarding its effects on the structure and function of isolated mitochondria. As reported previously, DiS-C(3)(5) had an inhibitory effect on NADH-driven mitochondrial electron transfer. On the contrary, in the presence of inorganic phosphate, DiS-C(3)(5) showed dose-dependent biphasic effects on mitochondria energized by succinate. At higher concentrations, such as 50 micro m, DiS-C(3)(5) accelerated mitochondrial oxygen consumption. Measurements of the permeability of DiS-C(3)(5)-treated mitochondrial membranes to poly(ethylene glycol) and analysis of mitochondrial configuration by transmission electron microscopy revealed that the accelerating effect of DiS-C(3)(5) on mitochondrial oxygen consumption reflects the induction of the mitochondrial permeability transition (PT). When the mitochondrial PT was induced by DiS-C(3)(5), release of mitochondrial cytochrome c was observed, as in the case of the PT induced by Ca(2+). On the contrary, at a low concentration such as 5 micro m, DiS-C(3)(5) showed an inhibitory effect on the latent oxygen consumption by mitochondria. This effect was shown to reflect inhibition of the PT induced by a low concentration of Ca(2+). Furthermore, in the absence of inorganic phosphate, DiS-C(3)(5) caused mitochondrial swelling. Under this condition, DiS-C(3)(5) caused changes in the membrane status of the mitochondria, but did not induce a release of mitochondrial cytochrome c.

A permeability transition in liposomes induced by the formation of Ca2+/palmitic acid complexes

Formation of palmitic acid/Ca(2+) (PA/Ca(2+)) complexes was suggested to play a key role in the non-classical permeability transition in mitochondria (NCPT), which seems to be involved in the PA-induced apoptosis of cardiomyocytes. Our previous studies of complexation of free fatty acids (FFA) with Ca(2+) showed that long-chain (C:16-C:22) saturated FFA had an affinity to Ca(2+), which was much higher than that of other FFA and lipids. The formation of FFA/Ca(2+) complexes in the black-lipid membrane (BLM) was demonstrated to induce a nonspecific ion permeability of the membrane. In the present work, we have found that binding of Ca(2+) to PA incorporated into the membrane of sulforhodamine B (SRB)-loaded liposomes results in an instant release of a part of SRB, with the quantity of SRB released depending on the concentration of PA and Ca(2+). The pH-optimum of this phenomenon, similar to that of PA/Ca(2+) complexation, is in the alkaline range. The same picture of SRB release has been revealed for stearic, but not for linoleic acid. Along with Ca(2+), some other bivalent cations (Ba(2+), Sr(2+), Mn(2+), Ni(2+), Co(2+)) also induce SRB release upon binding to PA-containing liposomes, while Mg(2+) turns out to be relatively ineffective. As revealed by fluorescence correlation spectroscopy, the apparent size of liposomes does not alter after the addition of PA, Ca(2+) or their combination. So it has been supposed that the cause of SRB release from liposomes is the formation of lipid pores. The effect of FFA/Ca(2+)-induced permeabilization of liposomal membranes has several analogies with NCPT, suggesting that both these phenomena are of similar nature.

Permeability transition-independent release of mitochondrial cytochrome c induced by valinomycin

To examine whether valinomycin induces a mitochondrial permeability transition (PT), we investigated its effects on mitochondrial functions under various conditions. The acceleration of mitochondrial respiration and swelling, induced by valinomycin, were found to be insensitive to inhibitors of the ordinary PT, indicating that valinomycin does not induce the ordinary PT. Results of experiments using mitochondria isolated from transgenic mice expressing human bcl-2 also supported this conclusion. Furthermore, evidence for induction of PT pores by valinomycin was not obtained by either electron microscopic analysis of mitochondrial configurations or by measurement of the permeability of the inner mitochondrial membrane by use of polyethylene glycol. However, valinomycin did induce a significant release of cytochrome c, and thus it may be a nice tool to study the processes of mitochondrial cytochrome c release.

Cytochrome c release occurs via Ca2+-dependent and Ca2+-independent mechanisms that are regulated by Bax

Release of cytochrome c from mitochondria is a key initiative step in the apoptotic process, although the mechanisms regulating this event remain elusive. In the present study, using isolated liver mitochondria, we demonstrate that cytochrome c release occurs via distinct mechanisms that are either Ca(2+)-dependent or Ca(2+)-independent. An increase in mitochondrial matrix Ca(2+) promotes the opening of the permeability transition (PT) pore and the release of cytochrome c, an effect that is significantly enhanced when these organelles are incubated in a reaction buffer that is based on a physiologically relevant concentration of K(+) (150 mm KCl) versus a buffer composed of mannitol/sucrose/Hepes. Moreover, low concentrations of Ca(2+) are sufficient to induce mitochondrial cytochrome c release without measurable manifestations of PT, though inhibitors of PT effectively prevent this release, indicating that the critical threshold for PT varies among mitochondria within a single population of these organelles. In contrast, Ca(2+)-independent cytochrome c release is induced by oligomeric Bax protein and occurs without mitochondrial swelling or the release of matrix proteins, although our data also indicate that Bax enhances permeability transition-induced cytochrome c release. Taken together, our results suggest that the intramitochondrial Ca(2+) concentration, as well as the reaction buffer composition, are key factors in determining the mode and amount of cytochrome c release. Finally, oligomeric Bax appears to be capable of stimulating cytochrome c release via both Ca(2+)-dependent and Ca(2+)-independent mechanisms.

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.

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.

Cationic uncouplers of oxidative phosphorylation are inducers of mitochondrial permeability transition

To determine whether cationic uncouplers of oxidative phosphorylation induce permeability transition in mitochondria, the effects of the divalent cationic sulfhydryl cross-linker copper-o-phenanthroline (Cu(OP)2) and the cyanine dye tri-S-C4(5) on rat liver mitochondria were examined. Like Ca2+, they accelerated mitochondrial respiration with succinate and induced mitochondrial swelling when inorganic phosphate (Pi) was present in the incubation medium. The acceleration of respiration and swelling were inhibited by the SH-reagent N-ethylmaleimide, and by the specific permeability transition inhibitor cyclosporin A (CsA). In addition, these cations, like Ca2+, induced release of ADP entrapped in the mitochondrial matrix space, and the morphological change of mitochondria induced by these cations was essentially the same as that induced by Ca2+. It is concluded that the uncoupling actions of Cu(OP)2 and tri-S-C4(5) are due to induction of permeability transition in the inner mitochondrial membrane.

Properties of a cyclosporin-insensitive permeability transition pore in yeast mitochondria

Yeast mitochondria (Saccharomyces cerevisiae) contain a permeability transition pore which is regulated differently than the pore in mammalian mitochondria. In a mannitol medium containing 10 mM Pi and ethanol (oxidizable substrate), yeast mitochondria accumulate large amounts of Ca2+ (>400 nmol/mg of protein) upon the addition of an electrophoretic Ca2+ ionophore (ETH129). Pore opening does not occur following Ca2+ uptake, even though ruthenium red-inhibited rat liver mitochondria undergo rapid pore opening under analogous conditions. However, a pore does arise in yeast mitochondria when Ca2+ and Pi are not present, as monitored by swelling, ultrastructure, and matrix solute release. Pore opening is slow unless a respiratory substrate is provided (ethanol or NADH) but also occurs rapidly in response to ATP (2 mM) when oligomycin is present. Pi and ADP inhibit pore opening (EC50 approximately 1 and 4 mM, respectively), however, cyclosporin A (7 microg/ml), oligomycin (20 microg/ml), or carboxyatractyloside (25 microM) have no effect. The pore arising during respiration is also inhibited by nigericin or uncoupler, indicating that an acidic matrix pH antagonizes the process. Pi also inhibits pore opening by lowering the matrix pH (Pi/OH- antiport). However, inhibition of the ATP-induced pore by Pi is seen in the presence of mersalyl, suggesting a second mechanism of action. Since pore induction by ATP is not sensitive to carboxyatractyloside, ATP appears to act at an external site and Pi may antagonize the interaction. Isoosmotic polyethylene glycol-induced contraction of yeast mitochondria swollen during respiration, or in the presence of ATP, is 50% effective at a solute size of 1.0-1.1 kDa. This suggests that the same pore is induced in both cases and is comparable in size with the permeability transition pore of heart and liver mitochondria.

Differential cytoprotection by glycine against oxidant damage to proximal tubule cells

Tert-butyl hydroperoxide (tBHP) injured freshly isolated proximal tubules in an Fe-dependent fashion that was ameliorated by a lipophilic antioxidant, diphenyl-p-phenylenediamine (DPPD), but was only minimally affected by glycine. Menadione-induced injury was Fe-independent and was unaffected by DPPD, but was strongly blocked by glycine. Fe was highly toxic when intracellular loading was facilitated by concomitant treatment with hydroxyquinoline (HQ). This toxicity was blocked by DPPD or chelating the Fe, but not by glycine. All of the lesions were characterized by severe depletion of glutathione and other soluble thiols. Menadione induced large increases in protein associated with the Triton-insoluble cytoskeleton and decreases in protein thiol content, consistent with extensive cross linking, but did not increase thiobarbituric acid reactive substances (TBARS). tBHP and HQ + Fe had either no effect or only moderate, delayed effects on cytoskeletal proteins, but induced substantial increases of TBARS. Glycine did not the alter changes in cytoskeletal proteins, thiols, or TBARS produced by any of the agents. Protection against tBHP toxicity by deferoxamine and DPPD was accompanied by substantial suppression of TBARS accumulation. Superimposition of hypoxia during tBHP exposure reduced TBARS accumulation and restored cytoprotective activity to glycine. Thus, in contrast to its consistently strong cytoprotection against a number of other insults, glycine is only variably cytoprotective against oxidant lesions in freshly isolated proximal tubules. Extensive oxidative crosslinking of proteins is compatible with maintenance of glycine cytoprotection against lethal membrane damage. Fe-induced injury to proximal tubules associated with lipid peroxidation as manifested by TBARS formation is a relatively glycine-insensitive insult.

Mitochondrial damage as a source of diseases and aging: a strategy of how to fight these

Some aspects of a defense against an oxidative stress are reviewed. All these aspects are focused on the necessity to defend mtDNA against damage. Protecting mechanisms involve the regulation of mitochondrial transport of nucleic acids, and the development of antioxygen defense as preventive measures. In the first case an exclusive role is supposed to play the mitochondrial benzodiazepine receptor and components, regulating the activity of its participants (mitochondrial porin and adenine nucleotide translocator). The possible transport of nucleic acids through Ca(2+)-dependent permeability transition pore, representing one of the functional states of mitochondrial benzodiazepine receptor, is put forth. Such mechanisms can also cover the genomic nuclear-mitochondrial exchange. The second aspect reviews the possible complex of measures to lower the harmful effect of oxygen. Among these measures are mild uncoupling, the opening of a permeability transition pore and cellular apoptosis as was recently suggested by Skulachev. Problems such as cellular aging and mitochondrial diseases, are discussed in light of the relevance to the problem of oxidative stress.

Permeability transition pore of the inner mitochondrial membrane can operate in two open states with different selectivities

Prooxidants induce release of Ca2+ from mitochondria through the giant solute pore in the mitochondrial inner membrane. However, under appropriate conditions prooxidants can induce Ca2+ release without inducing a nonspecific permeability change. Prooxidant-induced release of Ca2+ is selective. Presumably, this is the result of the operation of a permeability pathway for H+ coupled to the reversal of the Ca2+ uniporter, the latter generating the selectivity. The solute pore and prooxidant-induced Ca2+-specific pathways exhibit common sensitivities to a set of inhibitors and activators. It is proposed that the pore can operate in two open states: (1) permeable to H+ only and (2) permeable to solutes of M(r) < 1500. Under some conditions, prooxidants induce the H+-selective state which, in turn, collapses the inner membrane potential and permits selective loss of Ca2+ via the Ca2+ uniporter.

Influence of metabolic inhibitors on mitochondrial permeability transition and glutathione status

Treatment of isolated mitochondria with Ca2+ and inorganic phosphate (Pi) induces an inner membrane permeability that appears to be mediated through a cyclosporin A (CsA)-inhibitable Ca(2+)-dependent pore. Isolated mitochondria during inner membrane permeability undergo rapid efflux of matrix solutes such as glutathione as GSH and Ca2+, loss of coupled functions, and large amplitude swelling. Permeability transition without large amplitude swelling, a parameter often used to assess inner membrane permeability, has been observed. The addition of either oligomycin, antimycin, or sulfide to incubation buffer containing Ca2+ and Pi abolished large amplitude swelling of mitochondria. The GSH status during a Ca(2+)- and Pi-dependent mechanism of mitochondrial GSH release in isolated mitochondria was influenced significantly by metabolic inhibitors of the respiratory chain but did not prevent inner membrane permeability as demonstrated by the release of mitochondrial GSH and Ca2+. The release of GSH was inhibited by the addition of CsA, a potent inhibitor of permeability transition. Under these conditions we did not find GSSG; however, rapid oxidation of pyridine nucleotides and depletion of ATP and ADP with conversion to AMP occurred. The addition of CsA, prevented the oxidation of pyridine nucleotides and depletion of ATP and ADP. Since NADH and NADPH were extensively oxidized, protection against oxidative stress is reflected in maintenance of GSH and not observable lipid peroxidation. Evidence from transmission electron microscopy analysis, combined with the GSH release data, indicate that permeability transition can be observed in the absence of large amplitude swelling.

Ca(2+)-triggered membrane permeability transition in deenergized mitochondria from rat liver

The opening of the cyclosporin A-sensitive permeability transition pore (MTP) in deenergized mitochondria was induced only at millimolar Ca2+. Pretreatment of the mitochondria with 'inducers', such as duroquinone and phenylarsine oxide, allowed observing the pore opening at 0.01-0.1 mM Ca2+. Duroquinone caused a rapid (within 20 s) NAD(P)H oxidation which was followed by a slow (20 min) induction of the pore sensitive to low Ca2+. Phenylarsine oxide capable of cross-linking of vicinal SH-groups caused pore formation without the oxidation of NAD(P)H. The pore opening by both 'inducers' was prevented by N-ethylmaleimide. We propose that oxidation or cross-linking of critical dithiol(s) in membrane proteins increase the sensitivity of a putative 'Ca(2+)-sensor' that regulates the permeability transition pore opening.

Markers for vulnerability in acute porphyria. A hypothesis paper

Previously symptomatic and permanently asymptomatic carriers of a gene mutation for acute intermittent porphyria as well as matched controls were screened with regard to a series of variables of possible relevance to the development of porphyric symptoms. The basis for the study was a concept of acute porphyria as a condition of a permanent system overload of oxidative stress, with long term effects on hepatic and renal tissue, and with instances of periodic overload of free radicals giving rise to acute neurologic involvement. Leukocyte concentrations of manganese, calcium, iron and zinc, as well as erythrocyte calcium differed between the groups, acute intermittent porphyria gene carriers, irrespective of previous porphyric illness, showing significantly higher levels than the controls. Manganese was found to be the most discriminative component of all the 78 variables investigated, accounting for about 98 per cent of the variance between the groups. An increment, by a factor of four, in cellular manganese is suggestive of an increase, in acute intermittent porphyria, of a manganese associated enzyme, e.g. glutamine synthetase, pyruvate carboxylase or mitochondrial superoxide dismutase. The best fit into the model considered is provided by a theory focused on superoxide dismutase, induced in response to superoxide anion radical produced from aminolaevulinic acid. In porphyria gene carriers seemingly resistant to porphyric manifestations, an increase in potentially prooxidant cellular iron is matched by a proportional increment in manganese, i.e. presumably by a corresponding mitochondrial superoxide dismutase induction. This mechanism is not operative in porphyric individuals prone to development of neuropsychiatric symptoms. In acute intermittent porphyria with a history of porphyric illness there is a positive correlation between erythrocyte manganese and serum folate and a negative correlation between leukocyte ferrochelatase activity and serum cobalamin concentration. This may mirror a role of the cobalamin-folate system in the acute porphyric process.

Electrophysiologic alterations in the rabbit nodal cells induced by membrane lipid peroxidation

To investigate cellular electrophysiologic alterations due to lipid peroxidation of the cell membrane by free radicals as a possible cause of coronary reperfusion arrhythmias, we studied the effects of t-butyl hydroperoxide on the spontaneous action potential and membrane currents of the rabbit sinoatrial and atrioventricular node preparations (0.2 x 0.2 x 0.1 mm). 1-5 min of superfusion with t-butyl hydroperoxide (100-500 microM) caused a transient increase in the spontaneous firing frequency by 9%, accompanied by a 4% increase in the action potential amplitude and a 33% increase in the maximal rate of depolarization (P < 0.05, n = 6). t-Butyl hydroperoxide then gradually suppressed physiological automaticity, but induced abnormal repetitive firing due to early and delayed after-depolarizations. 15 min of superfusion with t-butyl hydroperoxide caused a complete standstill of nodal cells at a resting potential of -46 +/- 3 mV (n = 12). Such effects of t-butyl hydroperoxide on the spontaneous action potential were attenuated by pretreating the cells with butylated hydroxytoluene, a lipid peroxidation inhibitor. Voltage clamp experiments using double microelectrode methods revealed that t-butyl hydroperoxide transiently increased the Ca2+ current by 22% after 5 min of superfusion but subsequently reduced it to 46% of the control value after 15 min (P < 0.05, n = 6). Similar biphasic changes were observed in the delayed rectifying K+ current and hyperpolarization-activated inward current (n = 6). Background current was progressively increased without any change in its reversal potential (n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Transport of calcium by mitochondria

The identification of intramitochondrial free calcium ([Ca2+]m) as a primary metabolic mediator [see Hansford (this volume) and Gunter, T. E., Gunter, K. K., Sheu, S.-S., and Gavin, C. E. (1994) Am. J. Physiol. 267, C313-C339, for reviews] has emphasized the importance of understanding the characteristics of those mechanisms that control [Ca2+]m. In this review, we attempt to update the descriptions of the mechanisms that mediate the transport of Ca2+ across the mitochondrial inner membrane, emphasizing the energetics of each mechanism. New concepts within this field are reviewed and some older concepts are discussed more completely than in earlier reviews. The mathematical forms of the membrane potential dependence and concentration dependence of the uniporter are interpolated in such a way as to display the convenience of considering Vmax to be an explicit function of the membrane potential. Recent evidence for a transient rapid conductance state of the uniporter is discussed. New evidence concerning the energetics and stoichiometries of both Na(+)-dependent and Na(+)-independent efflux mechanisms is reviewed. Explicit mathematical expressions are used to describe the energetics of the system and the kinetics of transport via each Ca2+ transport mechanism.

Mitochondrial calcium transport: physiological and pathological relevance

Since the initiation of work on mitochondrial Ca2+ transport in the early 1960s, the relationship between experimental observations and physiological function has often seemed enigmatic. Why, for example, should an organelle dedicated to the crucial task of producing approximately 95% of the cell's ATP sequester Ca2+, sometimes in preference to phosphorylating ADP? Why should there be two separate efflux mechanisms, the Na+ independent and the Na+ dependent, both thought until recently to be driven exclusively either directly or indirectly by the energy of the pH gradient? Does intramitochondrial free Ca2+ concentration control metabolism? Is there evidence for any separate function of the mitochondrial Ca2+ transport mechanisms under pathological conditions? What is the relationship between mitochondrial Ca2+ transport, the mitochondrial membrane permeability transition, and irreversible cell damage under pathological conditions? First, we review what is known about control of metabolism, evidence for a role for intramitochondrial Ca2+ in control of metabolism, the cellular conditions under which mitochondria are exposed to Ca2+, characteristics of the mitochondrial Ca2+ transport mechanisms including the permeability transition, and evidence for and against mitochondrial Ca2+ uptake in vivo. Then the questions listed above and others are addressed from the perspective of the characteristics of the mechanisms of mitochondrial Ca2+ transport.

Further characterization of the events involved in mitochondrial Ca2+ release and pore formation by prooxidants

Addition of the prooxidant 3,5-dimethyl-N-acetyl-p-benzoquinone imine (3,5(Me)2NAPQI) to Ca(2+)-loaded mitochondria caused a rapid and extensive release of the sequestered Ca2+. Ca2+ release was accompanied by irreversible NAD(P)H oxidation and was followed by the release of adenine and pyridine nucleotides into the extramitochondrial medium; this is evidence of the opening of the pore in the inner mitochondrial membrane. Preincubation of the mitochondria with ADP, cyclosporin A (CSA), m-iodobenzylguanidine (MIBG) or Mg2+ inhibited the prooxidant-induced Ca2+ release and prevented pore-opening. When mitochondria were preincubated with ruthenium red, Ca2+ release was only minimally stimulated by 3,5(Me)2NAPQI. However, increasing the concentration of the prooxidant caused release of an increasing fraction of the sequestered Ca2+. Alternatively, increasing the intramitochondrial Ca2+ load resulted in a lowering of the concentration of 3,5(Me)2NAPQI required for near complete Ca2+ release to occur. In the presence of ruthenium red, 3,5(Me)2NAPQI-induced Ca2+ release was accompanied by irreversible pyridine nucleotide oxidation and followed by the release of nucleotides into the extramitochondrial medium, events which were prevented on preincubation with CSA. Similarly, the addition of CSA, ADP or MIBG during 3,5(Me)2NAPQI-induced Ca2+ release arrested further Ca2+ release. In addition to their inhibitory effect on the 3,5(Me)2NAPQI-induced Ca2+ release, CSA, ADP or MIBG also decreased the rate of the basal, ruthenium red-induced mitochondrial Ca2+ release by 45-70%. It is proposed that the basal, ruthenium red-induced and the prooxidant-induced mitochondrial Ca2+ release occur through a common component that is sensitive to inhibition by CSA, ADP and MIBG and that is involved in mitochondrial pore formation. Furthermore, 3,5(Me)2NAPQI-induced pore opening does not involve Ca(2+)-cycling, but rather involves a site(s) that is (are) synergistically activated by Ca2+ and the prooxidant.

Cholera toxin directly stimulates pregnenolone generation with increasing Ca2+ efflux in bovine adrenocortical mitochondria

The present experiments demonstrated that the holotoxin as well as the A- and B-subunits of cholera toxin were able to directly enhance pregnenolone synthesis when isolated intact mitochondria, prepared from bovine adrenocortical tissue, were incubated; they were not, however, able to enhance pregnenolone synthesis when the inner mitochondrial fraction was similarly incubated, suggesting that the conformational structure of mitochondria is very important for activation of cholesterol side-chain cleavage by cholera toxin. Data are also presented demonstrating that cholera toxin can enhance Ca2+ release from isolated mitochondria, while pertussis toxin could activate neither pregnenolone generation nor increase Ca2+ efflux from mitochondria. Thus it is suggested that cholera toxin may activate pregnenolone synthesis by regulating Ca2+ movement in mitochondria.

Mitochondrial inner membrane permeability changes induced by octadecadienoic acid hydroperoxide. Role of mitochondrial GSH pool

The effect of exogenous octadecadienoic acid hydroperoxide (HPODE) on the functional properties of inner membrane of isolated rat liver mitochondria, as evaluated by the measurement of the membrane potential (delta psi) has been studied. Very low concentrations of HPODE (1.5-4.5 nmol/mg prot.) do not modify the delta psi of control mitochondria appreciably while bringing about the drop of delta psi, in a concentration-dependent mode, in mitochondria with a GSH level diminished by approx. 60%. Mitochondrial GSH depletion was obtained by intraperitoneal administration of buthionine sulfoximine, a specific inhibitor of GSH synthesis, to rats. The presence in the incubation system of GSH-methyl ester which normalizes mitochondrial GSH, fully prevents any drop in levels of delta psi induced by HPODE. The same protective effect has been presented by EGTA, which chelates the available Ca2+. Neither an antioxidant nor a specific inhibitor of mitochondrial phospholipase A2 are able to prevent the HPODE effect. From the results obtained we can assume that HPODE itself, at the concentrations used here, induces permeability changes in the inner membrane, with the loss of coupled functions, when the GSH mitochondrial level is below a critical value.

Calcium- and phosphate-dependent release and loading of glutathione by liver mitochondria

The status of glutathione (GSH) was studied in isolated rat liver mitochondria under conditions which induce a permeability transition. This transition, which is inhibited by cyclosporin A (CyA), requires the presence of Ca2+ and an inducing agent such as near physiological levels (3 mM) of inorganic phosphate (Pi). The transition is characterized by an increased inner membrane permeability to some low molecular weight solutes and by large amplitude swelling under some experimental conditions. Addition of 70 microM Ca2+ and 3 mM Pi to mitochondria resulted in mitochondrial swelling and extensive release of GSH that was recovered in the extramitochondrial medium as GSH. Both swelling and the efflux of mitochondrial GSH were prevented by CyA. Incubation of mitochondria in the presence of Ca2+, Pi, and GSH followed by addition of CyA provided a mechanism to load mitochondria with exogenous GSH that was greater than the rate of uptake by untreated mitochondria. Thus, GSH efflux from mitochondria may occur under toxicological and pathological conditions in which mitochondria are exposed to elevated Ca2+ in the presence of near physiological concentrations of Pi through a nonspecific pore. Cyclical opening and closing of the pore could also provide a mechanism for uptake of GSH by mitochondria.

A comparison of phospholipid degradation by oxidation and hydrolysis during the mitochondrial permeability transition

The peroxidation and hydrolysis of mitochondrial phospholipids has been examined under conditions which are referable to induction of the permeability transition by t-butylhydroperoxide. Over a 30-min time course, the peroxide causes formation of 0.3 nmol/mg protein of malondialdehyde. This value is little effected by Ca2+, Sr2+, or Mn2+ but is increased approximately fivefold by Fe2+. The latter cation, but not the others, results in malondialdehyde formation in the absence of added peroxide. Partially oxidized phosphatidylethanolamine is present in normal mitochondria and is increased by approximately 50% following t-butylhydroperoxide treatment; however, the amounts observed are in the range of 0.4-0.6 mol% of total phosphatidylethanolamine. The minor degradation by peroxidation is in contrast to approximately 2.5 mol% degradation which occurs by hydrolysis. This degree of hydrolysis is accompanied by mitochondrial swelling and Mg2+ release, while a comparable level of peroxidation (malondialdehyde formation) is not. It is concluded that induction of the permeability transition by t-butylhydroperoxide does not represent damage to the membrane lipid phase caused by peroxidation. It is possible, however, that peroxidation accelerates the accumulation of phospholipid hydrolysis products and is thereby a factor which favors the transition.

Mechanisms by which mitochondria transport calcium

It has been firmly established that the rapid uptake of Ca2+ by mitochondria from a wide range of sources is mediated by a uniporter which permits transport of the ion down its electrochemical gradient. Several mechanisms of Ca2+ efflux from mitochondria have also been extensively discussed in the literature. Energized mitochondria must expend a significant amount of energy to transport Ca2+ against its electrochemical gradient from the matrix space to the external space. Two separate mechanisms have been found to mediate this outward transport: a Ca2+/nNa+ exchanger and a Na(+)-independent efflux mechanism. These efflux mechanisms are considered from the perspective of available energy. In addition, a reversible Ca2(+)-induced increase in inner membrane permeability can also occur. The induction of this permeability transition is characterized by swelling of the mitochondria, leakiness to small ions such as K+, Mg2+, and Ca2+, and loss of the mitochondrial membrane potential. It has been suggested that the permeability transition and its reversal may also function as a mitochondrial Ca2+ efflux mechanism under some conditions. The characteristics of each of these mechanisms are discussed, as well as their possible physiological functions.

Perturbation in liver mitochondrial Ca2+ homeostasis in experimental iron overload: a possible factor in cell injury

The functional state of isolated mitochondria and specifically the integrity of the inner membrane, were investigated in the liver of rats made siderotic by dietary supplementation with carbonyl iron. The concentration of iron in the hepatic tissue increased progressively up to nearly 40 days and reached a steady-state level. When the iron content reached a threshold value (higher than 90 nmol/mg protein) the occurrence of in vivo lipid peroxidation in the mitochondrial membrane was detected. This process did not result in gross alterations in the mitochondrial membrane, as indicated by electron microscopy, phosphorylative capability and membrane potential measurements. On the contrary, the induction of lipoperoxidative reaction appeared to be associated with the activation of Ca2+ release from mitochondria. This was shown to occur as a consequence of rather subtle modifications in the inner membrane structure via a specific efflux route, which appeared to be linked to the oxidation level of mitochondrial pyridine nucleotides. The induction of this Ca2+ release from iron-treated mitochondria resulted in enhancement of Ca2+ cycling, a process which dissipates energy to reaccumulate into mitochondria the released Ca2+. The perturbation in mitochondrial Ca2+ homeostasis reported here may be a factor in the onset of cell damage in this experimental model of hepatic iron overload.

Relationships between the NAD(P) redox state, fatty acid oxidation, and inner membrane permeability in rat liver mitochondria

Dysfunction of mitochondria after oxidation of endogenous NAD(P)H, especially after calcium accumulation, has been abundantly reported, but the causes of membrane perturbations did not receive a full explanation. In light of several additional observations reported in this study, we propose a general scheme which shows the sequential processes that are likely involved in the appearance of calcium-induced membrane leakiness. Addition of acetoacetate, oxaloacetate, or ketomalonate to rotenone-treated mitochondria led to a massive oxidation of both NADH and NADPH. Under these conditions, stimulation of fatty acid oxidation could be observed. This process was shown to be accompanied by a reduction of intramitochondrial NADP+. The reduction of NADP+ was inhibited by uncouplers, electron transfer inhibitors and N,N'-dicyclohexylcarbodiimide. It was thus probably catalyzed by the mitochondrial transhydrogenase. Oxidation of pyridine nucleotides in the presence of acetoacetate induced (i) a slight decrease in the number of sulfhydryl groups reactive with N-ethylmaleimide (but no change in the amount of intramitochondrial reduced glutathione) and (ii) modifications of the kinetics and the orientation of the ADP/ATP carrier. In the presence of calcium ions, acetoacetate-stimulated fatty acid oxidation promoted an extensive swelling of mitochondria. Uptake of calcium ions into the matrix was a critical factor for triggering the swelling. Thiols, if they were added at a sufficiently high concentration, suppressed the swelling. Also ligands of the ADP/ATP carrier which stabilized the m-state conformation of the protein, exerted an efficient protective action. Three essential interacting factors emerge from this study: (i) The crucial role of the ADP/ATP carrier orientation in promoting the calcium-induced membrane destabilization. More precisely, it has been shown that the ADP/ATP carrier adopts the c-state conformation (i.e., nucleotide binding site facing the cytoplasm) during fatty acid oxidation. (ii) The modification of a very small number of sulfhydryl groups of mitochondrial protein. These groups are probably in an oxidized state when the level of reduced pyridine nucleotides is low. (iii) The prevailing role of the transhydrogenase, the function of which is also intimately associated with fatty acid oxidation. After energization, transhydrogenase can hinder thiol oxidation and therefore partially protect the membrane structure.

Cyclosporin A-sensitive and insensitive mechanisms produce the permeability transition in mitochondria

Cyclosporin A is a potent inhibitor of the mitochondrial permeability transition, possibly by blocking an inner membrane pore through which solute movements occur [Broekemeier et al. (1989) J. Biol. Chem. 264, 7826-7830]. The inhibitory effect of cyclosporin, however, is transient. Trifluoperazine, at concentrations which inhibit the mitochondrial phospholipase A2, also produces a transient inhibition. When both inhibitors are used together, the inhibitory effect is long lasting. These findings suggest that the transition can be caused by two overlapping and/or interactive mechanisms, one dependent on an inner membrane pore and the other on phospholipase A2.

Release of mitochondrial matrix proteins through a Ca2+-requiring, cyclosporin-sensitive pathway

Induction of the inner membrane permeability transition, normally associated with the release of small molecules and ions from the mitochondrial matrix, also causes the release of matrix proteins. The release is linear with time and slow when compared to the time course of mitochondrial swelling. Transient induction of the high permeability state is reflected in transient release of proteins. Cyclosporin A (0.5 nmol/mg protein) or chelation of free Ca2+, which reverses the permeability transition, also block the subsequent release of protein even when added after extended preincubation. Possible mechanisms of protein release are discussed.

The mechanism of acute cytotoxicity of triethylphosphine gold(I) complexes. III. Chlorotriethylphosphine gold(I)-induced alterations in isolated rat liver mitochondrial function

Chlorotriethylphosphine gold(I) (TEPAu) is an organo-gold compound that has therapeutic activity in animal models of rheumatoid arthritis. Initial studies have suggested that TEPAu is a potent cytotoxic compound in vitro against a variety of cultured cell types and isolated hepatocytes. Mitochondrial dysfunction induced by this compound has been suggested as a primary biochemical alteration which may result in lethal cell injury in isolated hepatocytes. The purpose of this study was, therefore, to determine the mechanism of TEPAu-induced dysfunction of isolated rat liver mitochondria. TEPAu induced a rapid, concentration-related collapse of the mitochondrial inner membrane potential (EC50 = 24.7 +/- 2.5 microM) which was potentiated in Ca2+ loaded mitochondria (EC50 = 11.3 +/- 3.8 microM). TEPAu-induced collapse of the membrane potential was partially inhibited in the presence of ruthenium red or EGTA. TEPAu caused the rapid release of mitochondrially sequestered Ca2+ which was not inhibited by ruthenium red and, thus, was not via a reversal of the Ca2+ uniporter. TEPAu caused mitochondrial swelling, increased permeability of the inner membrane, and the oxidation/hydrolysis of endogenous mitochondrial pyridine nucleotides. Addition of exogenous ATP slightly reversed the effects of TEPAu on pyridine nucleotides. TEPAu-induced mitochondrial alterations were reversed or inhibited by exposure to the sulfhydryl reducing agent, dithiothreitol. Also, the TEPAu-induced collapse of the mitochondrial membrane potential was partially inhibited by dibucaine, a non-specific inhibitor of phospholipases. These data suggest that TEPAu-induced mitochondrial dysfunction is sulfhydryl dependent. TEPAu-induced mitochondrial dysfunction results in dissipation of the potential difference across the inner mitochondrial membrane which inhibits mitochondrial oxidative phosphorylation. The mechanism by which TEPAu induces the collapse of the membrane potential may be mediated by a sulfhydryl-dependent increase in permeability of the inner membrane to protons.

Mitochondrial damage as a mechanism of cell injury in the killing of cultured hepatocytes by tert-butyl hydroperoxide

The killing of cultured hepatocytes by tert-butyl hydroperoxide (TBHP) occurs by different mechanisms depending on the presence or absence of the antioxidant N,N'-diphenylphenylenediamine (DPPD). In either situation there is evidence of mitochondrial damage. The mitochondrial inner membrane potential is lost, a result determined by the release from the cells of the lipophilic cation [3H]triphenylmethylphosphonium (TPMP+). Deenergization of the mitochondria is accompanied by a loss of ATP. Oligomycin reduced ATP stores without release of TPMP+ or without effect on the viability of the hepatocytes over the same time course that TBHP killed the majority of the cells. Monensin, a H+/Na+ ionophore, potentiated the toxicity of tert-butyl hydroperoxide in the presence or absence of DPPD. By contrast, extracellular acidosis reduced the toxicity of tert-butyl hydroperoxide in the presence or absence of DPPD. Neither monensin nor extracellular acidosis affected the metabolism of tert-butyl hydroperoxide, the release of TPMP+, or the extent of the peroxidation of cellular lipids. These data document the presence of mitochondrial damage in hepatocytes intoxicated with TBHP in both the presence and absence of DPPD. Furthermore, the potentiation by monensin is readily explained by the proposal that mitochondrial deenergization is accompanied by an intracellular acidosis. Such acidosis tends to delay the development of lethal cell injury. The protective effect of extracellular acidosis supports this interpretation.