The mitochondrial permeability transition pore and its involvement in cell death and in disease pathogenesis

The mitochondrial permeability transition from yeast to mammals

Regulated permeability changes have been detected in mitochondria across species. We review here their key features, with the goal of assessing whether a "permeability transition" similar to that observed in higher eukaryotes is present in other species. The recent discoveries (i) that treatment with cyclosporin A (CsA) unmasks an inhibitory site for inorganic phosphate (Pi) [Basso, E., Petronilli, V., Forte, M.A. and Bernardi, P. (2008) Phosphate is essential for inhibition of the mitochondrial permeability transition pore by cyclosporin A and by cyclophilin D ablation. J. Biol. Chem. 283, 26307-26311], the classical inhibitor of the permeability transition of yeast and (ii) that under proper experimental conditions a matrix Ca(2+)-dependence can be demonstrated in yeast as well [Yamada, A., Yamamoto, T., Yoshimura, Y., Gouda, S., Kawashima, S., Yamazaki, N., Yamashita, K., Kataoka, M., Nagata, T., Terada, H., Pfeiffer, D.R. and Shinohara Y. (2009) Ca(2+)-induced permeability transition can be observed even in yeast mitochondria under optimized experimental conditions. Biochim. Biophys. Acta 1787, 1486-1491] suggest that the mitochondrial permeability transition has been conserved during evolution.

A bioenergetic model of the mitochondrial population undergoing permeability transition

Mitochondrial permeability transition (MPT) is a highly regulated complex phenomenon that is a type of ischemia/reperfusion injury that can lead to cell death and ultimately organ dysfunction. A novel population transition and detailed permeability transition pore regulation model were integrated with an existing bioenergetics model to describe MPT induction under a variety of conditions. The framework of the MPT induction model includes the potential states of the mitochondria (aggregated, orthodox and post-transition), their transitions from one state to another as well as their interaction with the extra-mitochondrial environment. The model encodes the three basic necessary conditions for MPT: a high calcium load, alkaline matrix pH and circumstances which favor de-energization. The MPT induction model was able to reproduce the expected bioenergetic trends observed in a population of mitochondria subjected to conditions that favor MPT. The model was corroborated and used to predict that MPT in an acidic environment is mitigated by an increase in activity of the mitochondrial potassium/hydrogen exchanger. The model was also used to present the beneficial impact of reducing the duration mitochondria spend in the orthodox state on preserving the extra-mitochondrial ATP levels. The model serves as a tool for investigators to use to understand the MPT induction phenomenon, explore alternative hypotheses for PTP regulation, as well as identify endogenous pharmacological targets and evaluate potential therapeutics for MPT mitigation.

Cyclophilin D in mitochondrial pathophysiology

Cyclophilins are a family of peptidyl-prolyl cis-trans isomerases whose enzymatic activity can be inhibited by cyclosporin A. Sixteen cyclophilins have been identified in humans, and cyclophilin D is a unique isoform that is imported into the mitochondrial matrix. Here we shall (i) review the best characterized functions of cyclophilin D in mitochondria, i.e. regulation of the permeability transition pore, an inner membrane channel that plays an important role in the execution of cell death; (ii) highlight new regulatory interactions that are emerging in the literature, including the modulation of the mitochondrial F1FO ATP synthase through an interaction with the lateral stalk of the enzyme complex; and (iii) discuss diseases where cyclophilin D plays a pathogenetic role that makes it a suitable target for pharmacologic intervention.

Quick assessment of cytotoxins effect on Daphnia magna using in vivo fluorescence microscopy

A novel approach to contaminant toxicity screening is proposed. The use of fluorescent microscopy with fluorescent dyes allows for assessing intoxication of Daphnia magna tissues, at various stages of exposure, to contaminants present in water. As shown, D. magna may not only be used as a test species in toxicity tests based on its lethality, but due to its translucency and application of fluorescent probes, separate steps of its intoxication and dying can be visualized. Using a variety of fluorescent probes, the present study also contributes to a better understanding of the toxicity mechanisms.

Mitochondria: the calcium connection

Calcium handling by mitochondria is a key feature in cell life. It is involved in energy production for cell activity, in buffering and shaping cytosolic calcium rises and also in determining cell fate by triggering or preventing apoptosis. Both mitochondria and the mechanisms involved in the control of calcium homeostasis have been extensively studied, but they still provide researchers with long-standing or even new challenges. Technical improvements in the tools employed for the investigation of calcium dynamics have been-and are still-opening new perspectives in this field, and more prominently for mitochondria. In this review we present a state-of-the-art toolkit for calcium measurements, with major emphasis on the advantages of genetically encoded indicators. These indicators can be efficiently and selectively targeted to specific cellular sub-compartments, allowing previously unavailable high-definition calcium dynamic studies. We also summarize the main features of cellular and, in more detail, mitochondrial calcium handling, especially focusing on the latest breakthroughs in the field, such as the recent direct characterization of the calcium microdomains that occur on the mitochondrial surface upon cellular stimulation. Additionally, we provide a major example of the key role played by calcium in patho-physiology by briefly describing the extensively reported-albeit highly controversial-alterations of calcium homeostasis in Alzheimer's disease, casting lights on the possible alterations in mitochondrial calcium handling in this pathology.

Postconditioning and protection from reperfusion injury: where do we stand? Position paper from the Working Group of Cellular Biology of the Heart of the European Society of Cardiology

Ischaemic postconditioning (brief periods of ischaemia alternating with brief periods of reflow applied at the onset of reperfusion following sustained ischaemia) effectively reduces myocardial infarct size in all species tested so far, including humans. Ischaemic postconditioning is a simple and safe manoeuvre, but because reperfusion injury is initiated within minutes of reflow, postconditioning must be applied at the onset of reperfusion. The mechanisms of protection by postconditioning include: formation and release of several autacoids and cytokines; maintained acidosis during early reperfusion; activation of protein kinases; preservation of mitochondrial function, most strikingly the attenuation of opening of the mitochondrial permeability transition pore (MPTP). Exogenous recruitment of some of the identified signalling steps can induce cardioprotection when applied at the time of reperfusion in animal experiments, but more recently cardioprotection was also observed in a proof-of-concept clinical trial. Indeed, studies in patients with an acute myocardial infarction showed a reduction of infarct size and improved left ventricular function when they underwent ischaemic postconditioning or pharmacological inhibition of MPTP opening during interventional reperfusion. Further animal studies and large-scale human studies are needed to determine whether patients with different co-morbidities and co-medications respond equally to protection by postconditioning. Also, our understanding of the underlying mechanisms must be improved to develop new therapeutic strategies to be applied at reperfusion with the ultimate aim of limiting the burden of ischaemic heart disease and potentially providing protection for other organs at risk of reperfusion injury, such as brain and kidney.

Autophagy in health and disease. 5. Mitophagy as a way of life

Our understanding of autophagy has expanded greatly in recent years, largely due to the identification of the many genes involved in the process and to the development of better methods to monitor the process, such as green fluorescent protein-LC3 to visualize autophagosomes in vivo. A number of groups have demonstrated a tight connection between autophagy and mitochondrial turnover. Mitochondrial quality control is the process whereby mitochondria undergo successive rounds of fusion and fission with a dynamic exchange of components to segregate functional and damaged elements. Removal of the mitochondrion that contains damaged components is accomplished via autophagy (mitophagy). Mitophagy also serves to eliminate the subset of mitochondria producing the most reactive oxygen species, and episodic removal of mitochondria will reduce the oxidative burden, thus linking the mitochondrial free radical theory of aging with longevity achieved through caloric restriction. Mitophagy must be balanced by biogenesis to meet tissue energy needs, but the system is tunable and highly dynamic. This process is of greatest importance in long-lived cells such as cardiomyocytes, neurons, and memory T cells. Autophagy is known to decrease with age, and the failure to maintain mitochondrial quality control through mitophagy may explain why the heart, brain, and components of the immune system are most vulnerable to dysfunction as organisms age.

Mitochondrial energetics and therapeutics

Mitochondrial dysfunction has been linked to a wide range of degenerative and metabolic diseases, cancer, and aging. All these clinical manifestations arise from the central role of bioenergetics in cell biology. Although genetic therapies are maturing as the rules of bioenergetic genetics are clarified, metabolic therapies have been ineffectual. This failure results from our limited appreciation of the role of bioenergetics as the interface between the environment and the cell. A systems approach, which, ironically, was first successfully applied over 80 years ago with the introduction of the ketogenic diet, is required. Analysis of the many ways that a shift from carbohydrate glycolytic metabolism to fatty acid and ketone oxidative metabolism may modulate metabolism, signal transduction pathways, and the epigenome gives us an appreciation of the ketogenic diet and the potential for bioenergetic therapeutics.

HTLV-1 p13, a small protein with a busy agenda

Human T-cell leukemia virus type 1 (HTLV-1) infection is characterized by life-long persistence of the virus in the host. While most infected individuals remain asymptomatic, 3-5% will eventually develop adult T-cell leukemia/lymphoma (ATLL) or tropical spastic paraparesis/HTLV-associated myelopathy (TSP/HAM) after a clinical latency that can span years (TSP/HAM) to decades (ATLL). The major oncogenic determinant among HTLV-1 proteins is the Tax transactivator, which influences the expression and function of a great number of cellular proteins, drives cell proliferation, reduces cell death, and induces genetic instability. The present review is focused on the current knowledge of p13, an HTLV-1 accessory protein targeted to the inner mitochondrial membrane and, under certain conditions, to the nucleus. In mitochondria, p13 produces an inward K+current that results in an increased production of ROS by mitochondria. These effects are linked to the protein's effects on cell turnover which include activation of primary T-cells and reduced proliferation/sensitization to death of tumor cells. Recent findings suggest that in the presence of Tax, p13 is subjected to ubiquitylation and partly targeted to the nucleus. Nuclear p13 binds Tax and inhibits its transcriptional activity. These findings suggest that the protein might exert distinct functions depending on its intracellular localization and influence both the turnover of infected cells and the balance between viral latency and productive infection.

Signal transduction to the permeability transition pore

The permeability transition pore (PTP) is an inner mitochondrial membrane channel that has been thoroughly characterized functionally, yet remains an elusive molecular entity. The best characterized PTP-regulatory component, cyclophilin (CyP) D, is a matrix protein that favors pore opening. CyP inhibitors, CyP-D null animals, and in situ PTP readouts have established the role of PTP as an effector mechanism of cell death, and the growing definition of PTP signalling mechanisms. This review briefly covers the functional features of the PTP and the role played by its dysregulation in disease pathogenesis. Recent progress on PTP modulation by kinase/phosphatase signal transduction is discussed, with specific emphasis on hexokinase and on the Akt-ERK-GSK3 axis, which might modulate the PTP through CyP-D phosphorylation.

Olesoxime (TRO19622): A Novel Mitochondrial-Targeted Neuroprotective Compound

Olesoxime (TRO19622) is a novel mitochondrial-targeted neuroprotective compound undergoing a pivotal clinical efficacy study in Amyotrophic Lateral Sclerosis (ALS) and also in development for Spinal Muscular Atrophy (SMA). It belongs to a new family of cholesterol-oximes identified for its survival-promoting activity on purified motor neurons deprived of neurotrophic factors. Olesoxime targets proteins of the outer mitochondrial membrane, concentrates at the mitochondria and prevents permeability transition pore opening mediated by, among other things, oxidative stress. Olesoxime has been shown to exert a potent neuroprotective effect in various in vitro and in vivo models. In particular olesoxime provided significant protection in experimental animal models of motor neuron disorders and more particularly ALS. Olesoxime is orally active, crosses the blood brain barrier, and is well tolerated. Collectively, its pharmacological properties designate olesoxime as a promising drug candidate for motor neuron diseases.

Cyclosporine A-induced nitration of tyrosine 34 MnSOD in endothelial cells: role of mitochondrial superoxide

AIMS

Cyclosporine A (CsA) has represented a fundamental therapeutic weapon in immunosuppression for the past three decades. However, its clinical use is not devoid of side effects, among which hypertension and vascular injury represent a major drawback. Endothelial cells are able to generate reactive oxygen and nitrogen species upon exposure to CsA, including formation of peroxynitrite. This may result in endothelial cell toxicity and increased tyrosine nitration. We have now studied the subcellular origin of superoxide formation in endothelial cells treated with CsA and the biochemical consequences for the function of mitochondrial enzymes.

METHODS AND RESULTS

By using electron spin resonance and endothelial cells lacking functional mitochondria, we showed that superoxide anion is generated in mitochondria. This was associated with an effect of CsA on bioenergetic parameters: increased mitochondrial membrane potential and inhibition of cellular respiration. In addition, CsA inhibited the activity of the mitochondrial enzymes aconitase and manganese superoxide dismutase (MnSOD). The use of murine lung endothelial cells deficient in endothelial nitric oxide synthase (eNOS) and NOS/peroxynitrite inhibitors allowed us to establish that the presence of eNOS and concomitant NO synthesis and peroxynitrite formation were essential for CsA induced nitration and inhibition of MnSOD activity. As the latter has been shown to become inactivated by nitration, we sought to identify this modification by mass spectrometry analysis. We found that CsA induced specific MnSOD tyrosine 34 nitration both in the recombinant protein and in endothelial cells overexpressing MnSOD.

CONCLUSION

We propose that CsA induced endothelial damage may be related to increased mitochondrial superoxide formation and subsequent peroxynitrite-dependent nitroxidative damage, specifically targeting MnSOD. The inactivation of this key antioxidant enzyme by tyrosine nitration represents a pathophysiological cellular mechanism contributing to self-perpetuation and amplification of CsA-related vascular toxicity.

Postconditioning: from experimental proof to clinical concept

The therapeutic strategies for acute myocardial infarction in the last decade have, among other therapeutic targets, focused on myocardial reperfusion injury, which accounts for a significant part of the final infarct size. Although several experiments in the last 20 years have reported that pharmacological interventions at reperfusion might reduce myocardial reperfusion injury, this could not be consistently confirmed in animal models or human studies. An alternative to chemical modifiers, postconditioning (brief repeated periods of ischemia applied at the onset of reperfusion) is the first method proven to be efficient in different animal models and to be confirmed in a recent human study. This simple method, applied in the first minute of reperfusion, reduces the final infarct size by 30–50%. This review will focus on the postconditioning technique and show how the data from different animal models and experimental settings have advanced our understanding of both the mechanisms and the definition of an accurate protocol that is easily applicable in human patients in the setting of acute myocardial infarction.

Mitochondria mediated cell death in diabetes

Mitochondrial dysfunction plays a role in the pathogenesis of a wide range of diseases that involve disordered cellular fuel metabolism and survival/death pathways, including neurodegenerative diseases, cancer and diabetes. Cytokine, virus recognition and cellular stress pathways converging on mitochondria cause apoptotic and/or necrotic cell death of beta-cells in type-1 diabetes. Moreover, since mitochondria generate crucial metabolic signals for glucose stimulated insulin secretion (GSIS), mitochondrial dysfunction underlies both the functional derangement of GSIS and (over-nutrition) stress-induced apoptotic/necrotic beta-cell death, hallmarks of type-2 diabetes. The apparently distinct mechanisms governing beta-cell life/death decisions during the development of diabetes provide a remarkable example where remote metabolic, immune and stress signalling meet with mitochondria mediated apoptotic/necrotic death pathways to determine the fate of the beta-cell. We summarize the main findings supporting such a pivotal role of mitochondria in beta-cell death in the context of current trends in diabetes research.

Activation of mitochondrial ERK protects cancer cells from death through inhibition of the permeability transition

We studied human cancer cell models in which we detected constitutive activation of ERK. A fraction of active ERK was found to be located in mitochondria in RWPE-2 cells, obtained by v-Ki-Ras transformation of the epithelial prostate RWPE-1 cell line; in metastatic prostate cancer DU145 cells; and in osteosarcoma SAOS-2 cells. All these tumor cells displayed marked resistance to death caused by apoptotic stimuli like arachidonic acid and the BH3 mimetic EM20-25, which cause cell death through the mitochondrial permeability transition pore (PTP). PTP desensitization and the ensuing resistance to cell death induced by arachidonic acid or EM20-25 could be ablated by inhibiting ERK with the drug PD98059 or with a selective ERK activation inhibitor peptide. ERK inhibition enhanced glycogen synthase kinase-3 (GSK-3)-dependent phosphorylation of the pore regulator cyclophilin D, whereas GSK-3 inhibition protected from PTP opening. Neither active ERK in mitochondria nor pore desensitization was observed in non-transformed RWPE-1 cells. Thus, in tumor cells mitochondrial ERK activation desensitizes the PTP through a signaling axis that involves GSK-3 and cyclophilin D, a finding that provides a mechanistic basis for increased resistance to apoptosis of neoplastic cells.

Not all mitochondrial carrier proteins support permeability transition pore formation: no involvement of uncoupling protein 1

The mPTP (mitochondrial permeability transition pore) is a non-specific channel that is formed in the mitochondrial inner membrane in response to several stimuli, including elevated levels of matrix calcium. The pore is proposed to be composed of the ANT (adenine nucleotide translocase), voltage-dependent anion channel and cyclophilin D. Knockout studies, however, have demonstrated that ANT is not essential for permeability transition, which has led to the proposal that other members of the mitochondrial carrier protein family may be able to play a similar function to ANT in pore formation. To investigate this possibility, we have studied the permeability transition properties of BAT (brown adipose tissue) mitochondria in which levels of the mitochondrial carrier protein, UCP1 (uncoupling protein 1), can exceed those of ANT. Using an improved spectroscopic assay, we have quantified mPTP formation in de-energized mitochondria from wild-type and Ucp1KO (Ucp1-knockout) mice and assessed the dependence of pore formation on UCP1. When correctly normalized for differences in mitochondrial morphology, we find that calcium-induced mPTP activity is the same in both types of mitochondria, with similar sensitivity to GDP (approximately 50% inhibited), although the portion sensitive to cyclosporin A is higher in mitochondria lacking UCP1 (approximately 80% inhibited, compared with approximately 60% in mitochondria containing UCP1). We conclude that UCP1 is not a component of the cyclosporin A-sensitive mPTP in BAT and that playing a role in mPTP formation is not a general characteristic of the mitochondrial carrier protein family but is, more likely, restricted to specific members including ANT.

Regulation of the mitochondrial permeability transition in kidney proximal tubules and its alteration during hypoxia-reoxygenation

Development of the mitochondrial permeability transition (MPT) can importantly contribute to lethal cell injury from both necrosis and apoptosis, but its role varies considerably with both the type of cell and type of injury, and it can be strongly opposed by the normally abundant endogenous metabolites ADP and Mg(2+). To better characterize the MPT in kidney proximal tubule cells and assess its contribution to injury to them, we have refined and validated approaches to follow the process in whole kidney proximal tubules and studied its regulation in normoxic tubules and after hypoxia-reoxygenation (H/R). Physiological levels of ADP and Mg(2+) greatly decreased sensitivity to the MPT. Inhibition of cyclophilin D by cyclosporine A (CsA) effectively opposed the MPT only in the presence of ADP and/or Mg(2+). Nonesterified fatty acids (NEFA) had a large role in the decreased resistance to the MPT seen after H/R irrespective of the available substrate or the presence of ADP, Mg(2+), or CsA, but removal of NEFA was less effective at restoring normal resistance to the MPT in the presence of electron transport complex I-dependent substrates than with succinate. The data indicate that the NEFA accumulation that occurs during both hypoxia in vitro and ischemic acute kidney injury in vivo is a critical sensitizing factor for the MPT that overcomes the antagonistic effect of endogenous metabolites and cyclophilin D inhibition, particularly in the presence of complex I-dependent substrates, which predominate in vivo.

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.

Adaptation to oxidative stress, chemoresistance, and cell survival

The discovery of some additional properties and functions of reactive oxygen species (ROS), beyond their toxic effects, provides a novel scenario for the molecular basis and cell regulation of several pathophysiologic processes. ROS are generated by redox-sensitive, prosurvival signaling pathways and function as second messengers in the transduction of several extracellular signals. A complex intracellular redox buffering network has developed to adapt and protect cells against the dangerous effects of oxidative stress. However, pathways involved in ROS-adaptive response may also play a critical role in protecting cells against cytotoxic effects of anticancer agents, thus supporting the hypothesis of a correlation between adaptation/resistance to oxidative stress and resistance to anticancer drugs. This review summarizes the main systems involved in the adaptive responses: an overview on the pathophysiologic relevance of mitochondria on redox-sensitive transcription factors and genes and main antioxidant networks in tumor cells is provided. One of the major aims is to highlight the adaptive mechanisms and their interplay in the intricate connection between oncogenic signaling, oxidative stress, and chemoresistance. Clarification of these mechanisms has tremendous application potential, in terms of developing novel molecular-targeted anticancer therapies and innovative strategies for rational combination of these agents with chemotherapeutic or tumor-specific biologic drugs.

Genome-wide RNAi screen identifies Letm1 as a mitochondrial Ca2+/H+ antiporter

Mitochondria are integral components of cellular calcium (Ca2+) signaling. Calcium stimulates mitochondrial adenosine 5'-triphosphate production, but can also initiate apoptosis. In turn, cytoplasmic Ca2+ concentrations are regulated by mitochondria. Although several transporter and ion-channel mechanisms have been measured in mitochondria, the molecules that govern Ca2+ movement across the inner mitochondrial membrane are unknown. We searched for genes that regulate mitochondrial Ca2+ and H+ concentrations using a genome-wide Drosophila RNA interference (RNAi) screen. The mammalian homolog of one Drosophila gene identified in the screen, Letm1, was found to specifically mediate coupled Ca2+/H+ exchange. RNAi knockdown, overexpression, and liposome reconstitution of the purified Letm1 protein demonstrate that Letm1 is a mitochondrial Ca2+/H+ antiporter.

Melatonin prevents oxidative stress-mediated mitochondrial permeability transition and death in skeletal muscle cells

Oxidative stress-induced mitochondrial dysfunction plays a crucial role in the pathogenesis of a wide range of diseases including muscle disorders. In this study, we demonstrate that melatonin readily rescued mitochondria from oxidative stress-induced dysfunction and effectively prevented subsequent apoptosis of primary muscle cultures prepared from C57BL/6J mice. In particular, melatonin (10(-4)-10(-6) m) fully prevented myotube death induced by tert-butylhydroperoxide (t-BHP; 10 microm-24 hr) as assessed by acid phosphatase, caspase-3 activities and cellular morphological changes. Using fluorescence imaging, we showed that the mitochondrial protection provided by melatonin was associated with an inhibition of t-BHP-induced reactive oxygen species generation. In line with this observation, melatonin prevented t-BHP-induced mitochondrial depolarization and mitochondrial permeability transition pore (PTP) opening. This was associated with a highly reduced environment as reflected by an increased glutathione content and an increased ability to maintain mitochondrial pyridine nucleotides and glutathione in a reduced state. Using isolated mitochondria, in a similar manner as cyclosporin A, melatonin (10(-8)-10(-6) m) desensitized the PTP to Ca(2+) and prevented t-BHP-induced mitochondrial swelling, pyridine nucleotide and glutathione oxidation. In conclusion, our findings suggest that inhibition of the PTP essentially contributes to the protective effect of melatonin against oxidative stress in myotubes.

Can Drug Safety be Predicted and Animal Experiments Reduced by Using Isolated Mitochondrial Fractions?

Mitochondrial toxicity has resulted in the withdrawal of several drugs from the market. One particular example is nefazodone, an anti-depressant withdrawn in the USA due to hepatoxicity caused by drug-induced mitochondrial dysfunction. Drug development and safety testing can involve the use of large numbers of laboratory animals, which, without a decisive pre-screening for mitochondrial toxicity, are often unable to pre-empt higher mortality rates in some patient groups. The use of isolated mitochondria as a screening tool for drug safety can decrease the number of laboratory animals used in pre-clinical studies, thus improving animal welfare and healthcare outcomes and costs. Novel techniques involving high-throughput methods can be used to investigate whether a molecule is a mitochondrial toxicant. Moreover, these screens are mechanistically-based, since the effects of the drug on oxidative phosphorylation, calcium homeostasis and mitochondrial genetics can be assessed. This review is intended to demonstrate that isolated mitochondrial fractions are suitable for predicting drug and general chemical safety in toxicological screenings, thus contributing to the refinement and reduction of animal use in laboratory research.

The VDAC2-BAK rheostat controls thymocyte survival

The proapoptotic proteins BAX and BAK constitute the mitochondrial apoptotic gateway that executes cellular demise after integrating death signals. The lethal BAK is kept in check by voltage-dependent anion channel 2 (VDAC2), a mammalian-restricted VDAC isoform. Here, we provide evidence showing a critical role for the VADC2-BAK complex in determining thymocyte survival in vivo. Genetic depletion of Vdac2 in the thymus resulted in excessive cell death and hypersensitivity to diverse death stimuli including engagement of the T cell receptor. These phenotypes were completely rescued by the concurrent deletion of Bak but not that of Bax. Thus, the VDAC2-BAK axis provides a mechanism that governs the homeostasis of thymocytes. Our study reveals a sophisticated built-in rheostat that likely fine-tunes immune competence to balance autoimmunity and immunodeficiency.

Role of cyclophilin D-dependent mitochondrial permeability transition in glutamate-induced calcium deregulation and excitotoxic neuronal death

In the present study we tested the hypothesis that the cyclophilin D-dependent (CyD) mitochondrial permeability transition (CyD-mPT) plays an important role in glutamate-triggered delayed calcium deregulation (DCD) and excitotoxic neuronal death. We used cultured cortical neurons from wild-type C57BL/6 and cyclophilin D-knockout mice (Ppif(-/-)). Induction of the mPT was identified by following the rapid secondary acidification of mitochondrial matrices monitored with mitochondrially targeted pH-sensitive yellow fluorescent protein. Suppression of the CyD-mPT due to genetic CyD ablation deferred DCD and mitochondrial depolarization, and increased the survival rate after exposure of neurons to 10 microM glutamate, but not to 100 microM glutamate. Ca(2+) influx into Ppif(-/-) neurons was not diminished in comparison with WT neurons judging by (45)Ca accumulation. In both types of neurons, 100 microM glutamate produced greater Ca(2+) influx than 10 microM glutamate. We hypothesize that greater Ca(2+) influx produced by higher glutamate rapidly triggered the CyD-independent mPT in both WT and Ppif(-/-) neurons equalizing their responses to supra-physiologic excitotoxic insults. In neurons exposed to moderate but pathophysiologically-relevant glutamate concentrations, an induction of the CyD-mPT appears to play an important role in mitochondrial injury contributing to DCD and cell death.

Regulation of colorectal cancer cell apoptosis by the n-3 polyunsaturated fatty acids Docosahexaenoic and Eicosapentaenoic

Several studies have suggested that the n-3 fatty acids Docosahexaenoic (DHA) and Eicosapentaenoic (EPA) have an important protective effect on colorectal cancer, and this could be at least partly due to their proapoptotic activity. It is unclear, however, how this phenomenon is triggered and what mechanisms are implicated. Here, we show that both DHA and EPA have an important proapoptotic effect on colorectal cancer cells with different molecular phenotypes but not in noncancerous cells. Apoptosis is caspase dependent, and both intrinsic and extrinsic pathways are implicated. The dimerization of Bax and Bak, the depolarization of the mitochondrial membrane, and the subsequent release of cytochrome c and Smac/Diablo to the cytosol evidence the activation of the intrinsic pathway. The implication of the extrinsic pathway is shown by the activation of caspase-8, along with the down-regulation of FLIP. The timing of caspase-8 activation, and the oligomerization of Bid with Bax, suggest a cross-talk with the intrinsic pathway. None of the death receptors that commonly initiate the extrinsic pathway: FAS, TNF-R1, and TRAIL-R2 are found to be responsible for triggering the apoptosis cascade induced by DHA and EPA. Neither PPARgamma nor cyclooxygenase-2, two likely candidates to regulate this process, play a significant role. Our findings suggest that the down-regulation of two key regulatory elements of the extrinsic and intrinsic pathways, FLIP and XIAP, respectively, is determinant in the induction of apoptosis by DHA and EPA. These fatty acids could potentially be useful adjuvant anticancer agents in combination with other chemotherapeutic elements.

Regulation and pharmacology of the mitochondrial permeability transition pore

The 'mitochondrial permeability transition', characterized by a sudden induced change of the inner mitochondrial membrane permeability for water as well as for small substances ( Collapse

Key Words

• adenine nucleotide translocator
• cyclophilin d
• mitochondrial voltage-dependent anion channel
• hexokinase
• creatine kinase
• mitochondrial peripheral benzodiazepine receptor
• bcl-2
• glycogen synthase kinase-3β

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MESH Headings

• Adenine Nucleotide Translocator 1/metabolism
• Animals
• Cardiovascular Agents/pharmacology
• Creatine Kinase, Mitochondrial Form/metabolism
• Peptidyl-Prolyl Isomerase F
• Cyclophilins/metabolism
• Hexokinase/metabolism
• Humans
• Lipid Metabolism
• Membrane Potential, Mitochondrial
• Mitochondria, Heart/drug effects
• Mitochondria, Heart/metabolism
• Mitochondrial Membrane Transport Proteins/chemistry
• Mitochondrial Membrane Transport Proteins/drug effects
• Mitochondrial Membrane Transport Proteins/genetics
• Mitochondrial Membrane Transport Proteins/metabolism
• Mitochondrial Permeability Transition Pore
• Myocardium/metabolism
• Phosphates/metabolism
• Receptors, GABA-A/metabolism
• Voltage-Dependent Anion Channels/metabolism

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Grants

• Intramural NIH HHS

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Affiliation(s)

Dmitry B. Zorov
Magdalena Juhaszova
Yael Yaniv
H. Bradley Nuss
Su Wang
Steven J. Sollott Laboratory of Cardiovascular Science, Gerontology Research Center, Box 13, Intramural Research Program, National Institute on Aging, NIH, 5600 Nathan Shock Drive, Baltimore, MD 21224-6825, USA

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Biapigenin modulates the activity of the adenine nucleotide translocator in isolated rat brain mitochondria

In this study, we investigated the effects of biapigenin, a biflavone present in the extracts of Hypericum perforatum, in rat brain mitochondrial bioenergetics and calcium homeostasis. We found that biapigenin significantly decreased adenosine diphosphate (ADP)-induced membrane depolarization and increased repolarization (by 68 and 37%, respectively). These effects were blocked by atractyloside and bongkrekic acid, but not oligomycin. In the presence of biapigenin, an ADP-stimulated state 3 respiration was still noticeable, which did not happen in the presence of adenine nucleotide translocator (ANT) inhibitors. Taking in consideration the relevance of the ANT in the modulation of the mitochondrial permeability transition pore (mPTP), mitochondrial calcium homeostasis was evaluated alone or in the presence of biapigenin. We found that biapigenin reduces mitochondrial calcium retention by increasing calcium efflux, an effect that was blocked by ADP plus oligomycin, an efficient blocker of the mPTP in brain mitochondria. Taken together, the results in this article suggest that biapigenin modulates mPTP opening, possibly by modulating ANT function, contributing for enhanced mitochondrial calcium efflux, thereby reducing calcium burden and contributing for neuroprotection against excitotoxicity.

Caspase-cleaved tau expression induces mitochondrial dysfunction in immortalized cortical neurons: implications for the pathogenesis of Alzheimer disease

In Alzheimer disease (AD) mitochondrial abnormalities occur early in the pathogenic process and likely play a significant role in disease progression. Tau is a microtubule-associated protein that is abnormally processed in AD, and a connection between tau pathology and mitochondrial impairment has been proposed. However, few studies have examined the relationship between pathological forms of tau and mitochondrial dysfunction. We recently demonstrated that inducible expression of tau truncated at Asp-421 to mimic caspase cleavage (T4C3) was toxic to immortalized cortical neurons compared with a full-length tau isoform (T4). In this study we investigated the effects of T4C3 on mitochondrial function. Expression of T4C3 induced mitochondrial fragmentation and elevated oxidative stress levels in comparison with T4-expressing cells. Thapsigargin treatment of T4 or T4C3 cells, which causes an increase in intracellular calcium levels, resulted in a significant decrease in mitochondrial potential and loss of mitochondrial membrane integrity in T4C3 cells when compared with cells expressing T4. The mitochondrial fragmentation and mitochondrial membrane damage were ameliorated in T4C3 cells by pretreatment with cyclosporine A or FK506, implicating the calcium-dependent phosphatase calcineurin in these pathogenic events. Increased calcineurin activity has been reported in AD brain, and thus, inhibition of this phosphatase may provide a therapeutic target for the treatment of AD.

Cardiolipin and mitochondrial carriers

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

Quercetin can act either as an inhibitor or an inducer of the mitochondrial permeability transition pore: A demonstration of the ambivalent redox character of polyphenols

The Ca(2+)- and oxidative stress-induced mitochondrial permeability transition (MPT) plays an important role in phenomena ranging from tissue damage upon infarction to muscle wasting in some forms of dystrophy. The process is due to the activation of a large pore in the inner mitochondrial membrane. Anti-oxidants are considered a preventive and remedial tool, and mitochondria-targeted redox-active compounds have been developed. Plant polyphenols are generally considered as anti-oxidants, and thus candidates to the role of mitochondria-protecting agents. In patch-clamp experiments, easily oxidizable polyphenols induced closure of the MPT channel. In swelling experiments with suspensions of mitochondria, high (20-50 microM) concentrations of quercetin, the most efficient inhibitor, promoted instead the onset of the MPT. Chelators of Fe(2+/3+) and Cu(+/2+) ions counteracted this effect. Fluorescent indicators of superoxide production confirmed that quercetin potentiates O(2)(*-) generation by isolated mitochondria and cultured cells. Since this was not affected by chelating Fe and Cu ions, the MPT-inducing effect can be ascribed to a "secondary", metal ion-catalyzed production of ROS. These results are a direct demonstration of the ambivalent redox character of polyphenols. Their mode of action in vivo cannot be taken for granted, but needs to be experimentally verified.

The cyclophilin inhibitor Debio 025 normalizes mitochondrial function, muscle apoptosis and ultrastructural defects in Col6a1-/- myopathic mice

BACKGROUND AND PURPOSE

We have investigated the therapeutic effects of the selective cyclophilin inhibitor D-MeAla(3)-EtVal(4)-cyclosporin (Debio 025) in myopathic Col6a1(-/-) mice, a model of muscular dystrophies due to defects of collagen VI.

EXPERIMENTAL APPROACH

We studied calcineurin activity based on NFAT translocation; T cell activation based on expression of CD69 and CD25; propensity to open the permeability transition pore in mitochondria and skeletal muscle fibres based on the ability to retain Ca(2+) and on membrane potential, respectively; muscle ultrastructure by electronmicroscopy; and apoptotic rates by terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling assays in Col6a1(-/-) mice before after treatment with Debio 025.

KEY RESULTS

Debio 025 did not inhibit calcineurin activity, yet it desensitizes the mitochondrial permeability transition pore in vivo. Treatment with Debio 025 prevented the mitochondrial dysfunction and normalized the apoptotic rates and ultrastructural lesions of myopathic Col6a1(-/-) mice.

CONCLUSIONS AND IMPLICATIONS

Desensitization of the mitochondrial permeability transition pore can be achieved by selective inhibition of matrix cyclophilin D without inhibition of calcineurin, resulting in an effective therapy of Col6a1(-/-) myopathic mice. These findings provide an important proof of principle that collagen VI muscular dystrophies can be treated with Debio 025. They represent an essential step towards an effective therapy for Ullrich Congenital Muscular Dystrophy and Bethlem Myopathy, because Debio 025 does not expose patients to the potentially harmful effects of immunosuppression.

GSK-3beta, a therapeutic target for cardiomyocyte protection

Glycogen synthase kinase-3beta (GSK-3beta) is a multifunctional Ser/Thr kinase that plays important roles in necrosis and apoptosis of cardiomyocytes. A major mechanism of cell necrosis is the opening of the mitochondrial permeability transition pore (mPTP), which consists of multiple protein subunits, including adenine nucleotide translocase (ANT). The threshold for mPTP opening is elevated by phosphorylation of GSK-3beta at Ser9, which reduces activity of this kinase. How inactivation of GSK-3beta suppresses mPTP opening has not been fully understood, but evidence to date suggests that preservation of hexokinase-II in the mPTP complex, inhibition of cyclophilin-D-ANT binding, inhibition of p53 and inhibition of ANT into the mitochondria are contributory. GSK-3beta phosphorylation is a step to which multiple protective signaling pathways converge, and thus GSK-3beta phosphorylation is crucial in cardioprotection of a variety of interventions against ischemia/reperfusion injury. Apoptosis of cardiomyocytes by pressure overload or ischemia/reperfusion is also suppressed by inactivation of GSK-3beta, in which reduced phosphorylation of p53, heat shock factor-1 and myeloid cell leukemia sequence-1 and inhibition of Bax translocation might be involved. Considering predominant roles of GSK-3beta in cardiomyocyte death, manipulation of this protein kinase is a promising strategy for myocardial protection in coronary artery disease and heart failure.

A CaPful of mechanisms regulating the mitochondrial permeability transition

Despite the lack of its molecular identification, the mitochondrial permeability transition pore (PTP) is a fascinating subject because of its important role in cell death. This holds especially true for cardiovascular diseases and in particular for ischemia-reperfusion injury, where research on PTP inhibition has been successfully translated from bench to clinical evidence of cardioprotection. In addition, recent reports extend the relevance of PTP to heart failure and atherosclerosis. This review summarizes the major factors involved in PTP control with specific emphasis on cardiovascular pathophysiology, and highlights recent findings on the pivotal role of inorganic phosphate as a mediator of the inhibitory effects of cyclosporin A and cyclophilin D ablation.

Mitochondrial abnormalities, energy deficit and oxidative stress are features of calpain 3 deficiency in skeletal muscle

Mutations in the non-lysosomal cysteine protease calpain-3 cause autosomal recessive limb girdle muscular dystrophy. Pathological mechanisms occurring in this disease have not yet been elucidated. Here, we report both morphological and biochemical evidence of mitochondrial abnormalities in calpain-3 knockout (C3KO) muscles, including irregular ultrastructure and distribution of mitochondria. The morphological abnormalities in C3KO muscles are associated with reduced in vivo mitochondrial ATP production as measured by (31)P magnetic resonance spectroscopy. Mitochondrial abnormalities in C3KO muscles also correlate with the presence of oxidative stress; increased protein modification by oxygen free radicals and an elevated concentration of the anti-oxidative enzyme Mn-superoxide dismutase were observed in C3KO muscles. Previously we identified a number of mitochondrial proteins involved in beta-oxidation of fatty acids as potential substrates for calpain-3. In order to determine if the mitochondrial abnormalities resulted from the loss of direct regulation of mitochondrial proteins by calpain-3, we validated the potential substrates that were identified in previous proteomic studies. This analysis showed that the beta-oxidation enzyme, VLCAD, is cleaved by calpain-3 in vitro, but we were not able to confirm that VLCAD is an in vivo substrate for calpain-3. However, the activity of VLCAD was decreased in C3KO mitochondrial fractions compared with wild type, a finding that likely reflects a general mitochondrial dysfunction. Taken together, these data suggest that mitochondrial abnormalities leading to oxidative stress and energy deficit are important pathological features of calpainopathy and possibly represent secondary effects of the absence of calpain-3.

Stimulation of glutamate receptors in cultured hippocampal neurons causes Ca2+-dependent mitochondrial contraction

Cultured hippocampal neurons expressing mitochondrially-targeted enhanced yellow fluorescent protein (mito-eYFP) were used to quantitatively examine mitochondrial remodelling in response to excitotoxic glutamate. Mitochondrial morphology was evaluated using laser spinning-disk confocal microscopy followed by calibrated image processing and 3D image rendering. Glutamate triggered an increase in cytosolic Ca(2+) and mitochondrial depolarization accompanied by Ca(2+)-dependent morphological transformation of neuronal mitochondria from "thread-like" to rounded structures. The quantitative analysis of the mitochondrial remodelling revealed that exposure to glutamate resulted in a decrease in mitochondrial volume and surface area concurrent with an increase in sphericity of the organelles. NIM811, an inhibitor of the mitochondrial permeability transition, attenuated the glutamate-induced sustained increase in cytosolic Ca(2+) and suppressed mitochondrial remodelling in the majority of affected neurons, but it did not rescue mitochondrial membrane potential. Shortening, fragmentation, and formation of circular mitochondria with decreased volume and surface area accompanied mitochondrial depolarization with FCCP. However, FCCP-induced morphological alterations appeared to be distinctly different from mitochondrial remodelling caused by glutamate. Moreover, FCCP prevented glutamate-induced mitochondrial remodelling suggesting an important role of Ca(2+) influx into mitochondria in the morphological alterations. Consistent with this, in saponin-permeabilized neurons, Ca(2+) caused mitochondrial remodelling which could be prevented by Ru(360).

Different redox states in malignant and nonmalignant esophageal epithelial cells and differential cytotoxic responses to bile acid and honokiol

Esophageal adenocarcinoma (EAC) is a highly lethal cancer in western countries. EAC cells are believed to develop from esophageal epithelial cells through complex transformation processes involving inflammation and oxidative stress. The purpose of this study was to compare the redox status of malignant and nonmalignant esophageal epithelial cells and to test their responses to bile acid-induced oxidative stress and to treatment with honokiol (HNK), a natural product with anticancer activity. We demonstrated that esophageal adenocarcinoma cells express significantly higher levels of antioxidant molecules and were resistant to reactive oxygen species (ROS) stress induced by bile acid, but were sensitive to the cytotoxic action of HNK. Mechanistic study showed that HNK caused cancer cell death by disruption of mitochondrial transmembrane potential and was correlated with cyclophilin D (CypD) expression. Inhibition of CypD by cyclosporin A or abrogation of its expression by siRNA significantly suppressed the cytotoxicity of HNK, suggesting that CypD may be a key molecule that mediates the cytotoxicity. Our study suggests that the high antioxidant capacity in EAC cells confers on them the ability to survive the oxidative microenvironment in the reflux esophagus, and that HNK is a promising compound to kill the transformed cells preferentially.

Lipid peroxidation, assessed with BODIPY-C11, increases after cryopreservation of stallion spermatozoa, is stallion-dependent and is related to apoptotic-like changes

Lipid peroxidation (LPO) of stallion spermatozoa was assessed in fresh semen and in samples of the same ejaculates after freezing and thawing. Particular attention was paid to individual differences in the susceptibility to LPO and its possible relationship with freezability. Innate levels of LPO were very low in fresh spermatozoa but increased after thawing, a change that was largely stallion-dependent. The level of LPO in fresh spermatozoa was not correlated with that of the thawed spermatozoa. Negative correlations existed between LPO and intact membranes post-thaw (r=-0.789, P<0.001), and also between LPO and spermatozoa with high mitochondrial membrane potential (Deltapsim) post-thaw (r=-0.689, P<0.001). LPO was also highly and significantly correlated with caspase activity. The correlation between caspase activity in ethidium positive cells and LPO was r=0.772, P<0.001. This LPO is unlikely to represent, per se, a sign of cryopreservation-induced injury, but it is apparently capable of triggering 'apoptotic-like changes' that could result in the sub-lethal cryodamage often seen among surviving spermatozoa.

A comparison of Zn2+- and Ca2+-triggered depolarization of liver mitochondria reveals no evidence of Zn2+-induced permeability transition

Intracellular Zn(2+) toxicity is associated with mitochondrial dysfunction. Zn(2+) depolarizes mitochondria in assays using isolated organelles as well as cultured cells. Some reports suggest that Zn(2+)-induced depolarization results from the opening of the mitochondrial permeability transition pore (mPTP). For a more detailed analysis of this relationship, we compared Zn(2+)-induced depolarization with the effects of Ca(2+) in single isolated rat liver mitochondria monitored with the potentiometric probe rhodamine 123. Consistent with previous work, we found that relatively low levels of Ca(2+) caused rapid, complete and irreversible loss of mitochondrial membrane potential, an effect that was diminished by classic inhibitors of mPT, including high Mg(2+), ADP and cyclosporine A. Zn(2+) also depolarized mitochondria, but only at relatively high concentrations. Furthermore Zn(2+)-induced depolarization was slower, partial and sometimes reversible, and was not affected by inhibitors of mPT. We also compared the effects of Ca(2+) and Zn(2+) in a calcein-retention assay. Consistent with the well-documented ability of Ca(2+) to induce mPT, we found that it caused rapid and substantial loss of matrix calcein. In contrast, calcein remained in Zn(2+)-treated mitochondria. Considered together, our results suggest that Ca(2+) and Zn(2+) depolarize mitochondria by considerably different mechanisms, that opening of the mPTP is not a direct consequence of Zn(2+)-induced depolarization, and that Zn(2+) is not a particularly potent mitochondrial inhibitor.

Interactions between the endoplasmic reticulum, mitochondria, plasma membrane and other subcellular organelles

Several recent works show structurally and functionally dynamic contacts between mitochondria, the plasma membrane, the endoplasmic reticulum, and other subcellular organelles. Many cellular processes require proper cooperation between the plasma membrane, the nucleus and subcellular vesicular/tubular networks such as mitochondria and the endoplasmic reticulum. It has been suggested that such contacts are crucial for the synthesis and intracellular transport of phospholipids as well as for intracellular Ca(2+) homeostasis, controlling fundamental processes like motility and contraction, secretion, cell growth, proliferation and apoptosis. Close contacts between smooth sub-domains of the endoplasmic reticulum and mitochondria have been shown to be required also for maintaining mitochondrial structure. The overall distance between the associating organelle membranes as quantified by electron microscopy is small enough to allow contact formation by proteins present on their surfaces, allowing and regulating their interactions. In this review we give a historical overview of studies on organelle interactions, and summarize the present knowledge and hypotheses concerning their regulation and (patho)physiological consequences.

Postconditioning inhibits mPTP opening independent of oxidative phosphorylation and membrane potential

Mitochondrial permeability transition pore (mPTP) inhibition plays a relevant role in postconditioning (PostC). Ischemia damages the electron transport chain, and the potential contribution of additional modifications in mitochondrial function caused by PostC remains unknown. We sought to determine which mitochondrial functions are involved in the inhibition of mPTP opening during the first minutes of reperfusion. Anesthetized New Zealand White rabbits underwent 30-min ischemia followed by 10-min reperfusion. At reperfusion, they received either no intervention (Control, C), PostC with 4 cycles of 1-min ischemia followed by 1-min reperfusion, or an IV injection of 5 mg/kg cyclosporine A (CsA: a powerful inhibitor of mPTP opening). Sham rabbits underwent no ischemia throughout the 40-min experiment. At the end of the 10-min reperfusion, mitochondria were isolated from the area at risk by differential centrifugations. Calcium retention capacity (CRC) and mitochondrial membrane potential (DeltaPsi(m)) were assessed by fluorimetry in subsarcolemmal (SSM) and interfibrillar (IFM) mitochondria. Oxidative phosphorylation was assessed using a Clark-type electrode, and oxidative stress via protein carbonylation by Western blotting. PostC and CsA treatments improved CRC when compared to the C group. Control, PostC and CsA mitochondria exhibited a comparable significant dissipation of DeltaPsi(m), together with a comparable significant decrease in state 3 and an increase in state 4 respiration, in both SSM and IFM. However, PostC but not CsA treatment reduced total heart oxidative stress. These data suggest that during the early minutes of reperfusion, PostC reduces oxidative stress and inhibits mPTP opening, independent of alteration of oxidative phosphorylation or of DeltaPsi(m).

Developmental shift of cyclophilin D contribution to hypoxic-ischemic brain injury

Cyclophilin D (CypD), a regulator of the mitochondrial membrane permeability transition pore (PTP), enhances Ca(2+)-induced mitochondrial permeabilization and cell death in the brain. However, the role of CypD in hypoxic-ischemic (HI) brain injury at different developmental ages is unknown. At postnatal day (P) 9 or P60, littermates of CypD-deficient [knock-out (KO)], wild-type (WT), and heterozygous mice were subjected to HI, and brain injury was evaluated 7 d after HI. CypD deficiency resulted in a significant reduction of HI brain injury at P60 but worsened injury at P9. After HI, caspase-dependent and -independent cell death pathways were more induced in P9 CypD KO mice than in WT controls, and apoptotic activation was minimal at P60. The PTP had a considerably higher induction threshold and lower sensitivity to cyclosporin A in neonatal versus adult mice. On the contrary, Bax inhibition markedly reduced caspase activation and brain injury in immature mice but was ineffective in the adult brain. Our findings suggest that CypD/PTP is critical for the development of brain injury in the adult, whereas Bax-dependent mechanisms prevail in the immature brain. The role of CypD in HI shifts from a predominantly prosurvival protein in the immature to a cell death mediator in the adult brain.