Acidosis promotes the permeability transition in energized mitochondria: implications for reperfusion injury

A quinolinium-based dual-functional NIR fluorescent probe for the imaging of Aβ aggregation and mitochondrial pH

Mitochondria are the most important energy supply centers in the cell, the changes in function and structure are implicated in many diseases. Among them, Aβ peptide, one of the targets of Alzheimer's disease, is closely related to mitochondrial autophagy, during the process of mitochondrial autophagy, the mitochondrial matrix will undergo acidification and the pH will be obviously reduced. Herein, a quinolinium-based NIR fluorescent probe QM12 was rationally designed and synthesized for the simultaneous imaging of Aβ aggregates and mitochondrial pH with different emission readout. The probe QM12 exhibited excellent selective toward Aβ aggregates, and can also trace the real-time changes of mitochondrial pH, which could serve as a promising tool for the pathological study of Alzheimer's disease, especially the cross talk between different biomarkers of Alzheimer's disease.

Regulation of Mitochondrial Permeability Transition Pore Opening by Monovalent Cations in Liver Mitochondria

The opening of the permeability transition pore (PTP) in mitochondria is a key event in the initiation of cell death in various pathologic states, including ischemia/reperfusion. The activation of K⁺ transport into mitochondria protects cells from ischemia/reperfusion. However, the role of K⁺ transport in PTP regulation is unclear. Here, we studied the role of K⁺ and other monovalent cations in the regulation of the PTP opening in an in vitro model. The registration of the PTP opening, membrane potential, Ca²⁺-retention capacity, matrix pH, and K⁺ transport was performed using standard spectral and electrode techniques. We found that the presence of all cations tested in the medium (K⁺, Na⁺, choline⁺, and Li⁺) strongly stimulated the PTP opening compared with sucrose. Several possible reasons for this were examined: the effect of ionic strength, the influx of cations through selective and non-selective channels and exchangers, the suppression of Ca²⁺/H⁺ exchange, and the influx of anions. The data obtained indicate that the mechanism of PTP stimulation by cations includes the suppression of K⁺/H⁺ exchange and acidification of the matrix, which facilitates the influx of phosphate. Thus, the K⁺/H⁺ exchanger and the phosphate carrier together with selective K⁺ channels compose a PTP regulatory triad, which might operate in vivo.

Mitochondrial calcium and reactive oxygen species in cardiovascular disease

Cardiomyocytes are one of the most mitochondria-rich cell types in the body, with ∼30-40% of the cell volume being composed of mitochondria. Mitochondria are well established as the primary site of adenosine triphosphate (ATP) generation in a beating cardiomyocyte, generating up to 90% of its ATP. Mitochondria have many functions in the cell, which could contribute to susceptibility to and development of cardiovascular disease (CVD). Mitochondria are key players in cell metabolism, ATP production, reactive oxygen species (ROS) production, and cell death. Mitochondrial calcium (Ca2+) plays a critical role in many of these pathways, and thus the dynamics of mitochondrial Ca2+ are important in regulating mitochondrial processes. Alterations in these varied and in many cases interrelated functions play an important role in CVD. This review will focus on the interrelationship of mitochondrial energetics, Ca2+, and ROS and their roles in CVD. Recent insights into the regulation and dysregulation of these pathways have led to some novel therapeutic approaches.

Cellular and Mitochondrial NAD Homeostasis in Health and Disease

The mitochondrion has a unique position among other cellular organelles due to its dynamic properties and symbiotic nature, which is reflected in an active exchange of metabolites and cofactors between the rest of the intracellular compartments. The mitochondrial energy metabolism is greatly dependent on nicotinamide adenine dinucleotide (NAD) as a cofactor that is essential for both the activity of respiratory and TCA cycle enzymes. The NAD level is determined by the rate of NAD synthesis, the activity of NAD-consuming enzymes, and the exchange rate between the individual subcellular compartments. In this review, we discuss the NAD synthesis pathways, the NAD degradation enzymes, and NAD subcellular localization, as well as NAD transport mechanisms with a focus on mitochondria. Finally, the effect of the pathologic depletion of mitochondrial NAD pools on mitochondrial proteins' post-translational modifications and its role in neurodegeneration will be reviewed. Understanding the physiological constraints and mechanisms of NAD maintenance and the exchange between subcellular compartments is critical given NAD's broad effects and roles in health and disease.

Modulation and Pharmacology of the Mitochondrial Permeability Transition: A Journey from F-ATP Synthase to ANT

The permeability transition (PT) is an increased permeation of the inner mitochondrial membrane due to the opening of the PT pore (PTP), a Ca²⁺-activated high conductance channel involved in Ca²⁺ homeostasis and cell death. Alterations of the PTP have been associated with many pathological conditions and its targeting represents an incessant challenge in the field. Although the modulation of the PTP has been extensively explored, the lack of a clear picture of its molecular nature increases the degree of complexity for any target-based approach. Recent advances suggest the existence of at least two mitochondrial permeability pathways mediated by the F-ATP synthase and the ANT, although the exact molecular mechanism leading to channel formation remains elusive for both. A full comprehension of this to-pore conversion will help to assist in drug design and to develop pharmacological treatments for a fine-tuned PT regulation. Here, we will focus on regulatory mechanisms that impinge on the PTP and discuss the relevant literature of PTP targeting compounds with particular attention to F-ATP synthase and ANT.

Cerebral and myocardial mitochondrial injury differ in a rat model of cardiac arrest and cardiopulmonary resuscitation

Brain mitochondria are more sensitive to global ischemia compared to heart mitochondria. Complex I in the electron transport chain (ETC) is sensitive to ischemic injury and is a major control point of the rate of ADP stimulated oxygen consumption. The purpose of this study was to explore whether changes in cerebral and myocardial mitochondria differ after cardiac arrest. Animals were randomized into 4 groups (n = 6): 1) Sham 2) VF 3) VF+CPR 4) ROSC 1hr. Ventricular Fibrillation (VF) was induced through a guide wire advanced from the right jugular vein into the ventricle and untreated for 8 min. Resuscitation was attempted with a 4J defibrillation after 8 min of cardiopulmonary resuscitation (CPR). Brain mitochondria and cardiac mitochondrial subpopulations were isolated. Calcium retention capacity was measured to assess susceptibility to mitochondrial permeability transition pore opening. ADP stimulated oxygen consumption and ETC activity assays were performed. Brain mitochondria are far more sensitive to injury during cardiac arrest and resuscitation compared to cardiac mitochondria. Complex I is highly sensitive to injury in brain mitochondria. With markedly decreased calcium retention capacity, mitochondria contribute to cerebral reperfusion injury. Therapeutic preservation of cerebral mitochondrial activity and mitochondrial function during cardiac arrest may improve post-resuscitation neurologic function.

Influence of Microbial Metabolites on the Nonspecific Permeability of Mitochondrial Membranes under Conditions of Acidosis and Loading with Calcium and Iron Ions

Mitochondrial dysfunction is currently considered one of the main causes of multiple organ failure in chronic inflammation and sepsis. The participation of microbial metabolites in disorders of bioenergetic processes in mitochondria has been revealed, but their influence on the mitochondrial membrane permeability has not yet been studied. We tested the influence of various groups of microbial metabolites, including indolic and phenolic acids, trimethylamine-N-oxide (TMAO) and acetyl phosphate (AcP), on the nonspecific permeability of mitochondrial membranes under conditions of acidosis, imbalance of calcium ions and excess free iron, which are inherent in sepsis. Changes in the parameters of the calcium-induced opening of the mitochondrial permeability transition pore (MPTP) and iron-activated swelling of rat liver mitochondria were evaluated. The most active metabolites were indole-3-carboxylic acid (ICA) and benzoic acid (BA), which activated MPTP opening and swelling under all conditions. AcP showed the opposite effect on the induction of MPTP opening, increasing the threshold concentration of calcium by 1.5 times, while TMAO activated swelling only under acidification. All the redox-dependent effects of metabolites were suppressed by the lipid radical scavenger butyl-hydroxytoluene (BHT), which indicates the participation of these microbial metabolites in the activation of membrane lipid peroxidation. Thus, microbial metabolites can directly affect the nonspecific permeability of mitochondrial membranes, if conditions of acidosis, an imbalance of calcium ions and an excess of free iron are created in the pathological state.

Revealing Minor pH Changes of Mitochondria by a Highly Sensitive Molecular Fluorescent Probe

Mitochondrial pH is an important factor associated with cellular metabolism and pathological states. Thus, sensitively monitoring its minor change was essential. However, it was challengeable due to the lack of suitable probes. Here, a mitochondria-targeted probe (NIR-OH-1) was synthesized. Based on the protonation/deprotonation of the hydroxy group and the assistance of carboxyl group on NIR-OH-1 molecular structure, a dramatic NIR activated signal was generated for sensing pH. Probe NIR-OH-1 displayed a good photo-stability and reversibility and could detect pH change without interference by other biologically active species. Importantly, NIR-OH-1 had an appropriate pK_a value (7.77) and tiny acid-base transition range, which was allowed to map the small pH changes of cellular mitochondrial. Moreover, NIR-OH-1 was also successfully applied in real-time monitoring mitochondrial pH-related pathological events in living cells under different stimulation, demonstrating the prospect of its clinical application in accurate mitochondrial pH detection under related physiological and pathological conditions.

The effect of Ginsenoside Rg1 in hepatic ischemia reperfusion (I/R) injury ameliorates ischemia-reperfusion-induced liver injury by inhibiting apoptosis

Hepatic ischemia reperfusion (I/R) injury (HIRI) HIRI is a complex, multifactorial pathophysiological process and in liver surgery has been known to significantly affect disease prognosis, surgical success rates, and patient survival. Ginsenoside Rgl (Rgl) monomer is one of the main active ingredients of ginseng. Previous studies have demonstrated that Rgl exerts various pharmacological effects through several mechanisms including suppression of apoptosis-related proteins levels, downregulation of inflammatory mediators and as well as antioxidant, which effectively exerts an organ protective effect I/R-induced damage. However, the exact mechanisms of Rg1 on HIRI remain to be elucidated. In the present study, we investigated the protective effect of Rg1 on hepatic ischemia-reperfusion (I/R) injury (HIRI) and explored its underlying molecular mechanism. A rat warm I/R injury model in vivo and an oxygen-glucose deprivation/reperfusion (OGD/R)-treated BRL-3A cell model in vitro were established after pretreating with Rg1(20 mg/kg). The results showed that Rg1 reduced the levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). TUNEL staining showed that pretreated with Rg1 inhibited the apoptosis rate compared with the I/R group. Moreover, pretreated with Rg1 significantly reduced the expression of Cyt-C, Caspase-9 and Caspase-3 to inhibit the cell apoptosis. Flow cytometry analysis showed the MMP in the I/R group was significantly increased, whereas pretreated with Rg1 effectively stabilized the MMP compared with the I/R group. in vitro, the proliferation of BRL-3A cells was significantly decreased by the OGD/R treatment, while Rg1 effectively reversed this phenomenon. In addition, western blotting showed that the increase of Cyt-C, Caspase-9 and Caspase-3 was inhibited by H2O2. These observations suggest that Rg1 exerts the protective effect by inhibiting the CypD protein-mediated mitochondrial apoptotic pathway.

Role of NAD+-Modulated Mitochondrial Free Radical Generation in Mechanisms of Acute Brain Injury

It is commonly accepted that mitochondria represent a major source of free radicals following acute brain injury or during the progression of neurodegenerative diseases. The levels of reactive oxygen species (ROS) in cells are determined by two opposing mechanisms—the one that produces free radicals and the cellular antioxidant system that eliminates ROS. Thus, the balance between the rate of ROS production and the efficiency of the cellular detoxification process determines the levels of harmful reactive oxygen species. Consequently, increase in free radical levels can be a result of higher rates of ROS production or due to the inhibition of the enzymes that participate in the antioxidant mechanisms. The enzymes’ activity can be modulated by post-translational modifications that are commonly altered under pathologic conditions. In this review we will discuss the mechanisms of mitochondrial free radical production following ischemic insult, mechanisms that protect mitochondria against free radical damage, and the impact of post-ischemic nicotinamide adenine mononucleotide (NAD⁺) catabolism on mitochondrial protein acetylation that affects ROS generation and mitochondrial dynamics. We propose a mechanism of mitochondrial free radical generation due to a compromised mitochondrial antioxidant system caused by intra-mitochondrial NAD⁺ depletion. Finally, the interplay between different mechanisms of mitochondrial ROS generation and potential therapeutic approaches are reviewed.

Nicotinamide Mononucleotide: A Promising Molecule for Therapy of Diverse Diseases by Targeting NAD+ Metabolism

NAD+, a co-enzyme involved in a great deal of biochemical reactions, has been found to be a network node of diverse biological processes. In mammalian cells, NAD+ is synthetized, predominantly through NMN, to replenish the consumption by NADase participating in physiologic processes including DNA repair, metabolism, and cell death. Correspondingly, aberrant NAD+ metabolism is observed in many diseases. In this review, we discuss how the homeostasis of NAD+ is maintained in healthy condition and provide several age-related pathological examples related with NAD+ unbalance. The sirtuins family, whose functions are NAD-dependent, is also reviewed. Administration of NMN surprisingly demonstrated amelioration of the pathological conditions in some age-related disease mouse models. Further clinical trials have been launched to investigate the safety and benefits of NMN. The NAD+ production and consumption pathways including NMN are essential for more precise understanding and therapy of age-related pathological processes such as diabetes, ischemia–reperfusion injury, heart failure, Alzheimer’s disease, and retinal degeneration.

Role of Mitochondrial Calcium and the Permeability Transition Pore in Regulating Cell Death

Adult cardiomyocytes are postmitotic cells that undergo very limited cell division. Thus, cardiomyocyte death as occurs during myocardial infarction has very detrimental consequences for the heart. Mitochondria have emerged as an important regulator of cardiovascular health and disease. Mitochondria are well established as bioenergetic hubs for generating ATP but have also been shown to regulate cell death pathways. Indeed many of the same signals used to regulate metabolism and ATP production, such as calcium and reactive oxygen species, are also key regulators of mitochondrial cell death pathways. It is widely hypothesized that an increase in calcium and reactive oxygen species activate a large conductance channel in the inner mitochondrial membrane known as the PTP (permeability transition pore) and that opening of this pore leads to necroptosis, a regulated form of necrotic cell death. Strategies to reduce PTP opening either by inhibition of PTP or inhibiting the rise in mitochondrial calcium or reactive oxygen species that activate PTP have been proposed. A major limitation of inhibiting the PTP is the lack of knowledge about the identity of the protein(s) that form the PTP and how they are activated by calcium and reactive oxygen species. This review will critically evaluate the candidates for the pore-forming unit of the PTP and discuss recent data suggesting that assumption that the PTP is formed by a single molecular identity may need to be reconsidered.

NAD+ precursor modulates post-ischemic mitochondrial fragmentation and reactive oxygen species generation via SIRT3 dependent mechanisms

Global cerebral ischemia depletes brain tissue NAD⁺, an essential cofactor for mitochondrial and cellular metabolism, leading to bioenergetics failure and cell death. The post-ischemic NAD⁺ levels can be replenished by the administration of nicotinamide mononucleotide (NMN), which serves as a precursor for NAD⁺ synthesis. We have shown that NMN administration shows dramatic protection against ischemic brain damage and inhibits post-ischemic hippocampal mitochondrial fragmentation. To understand the mechanism of NMN-induced modulation of mitochondrial dynamics and neuroprotection we used our transgenic mouse models that express mitochondria targeted yellow fluorescent protein in neurons (mito-eYFP) and mice that carry knockout of mitochondrial NAD⁺-dependent deacetylase sirt3 gene (SIRT3KO). Following ischemic insult, the mitochondrial NAD⁺ levels were depleted leading to an increase in mitochondrial protein acetylation, high reactive oxygen species (ROS) production, and excessive mitochondrial fragmentation. Administration of a single dose of NMN normalized hippocampal mitochondria NAD⁺ pools, protein acetylation, and ROS levels. These changes were dependent on SIRT3 activity, which was confirmed using SIRT3KO mice. Ischemia induced increase in acetylation of the key mitochondrial antioxidant enzyme, superoxide dismutase 2 (SOD2) that resulted in inhibition of its activity. This was reversed after NMN treatment followed by reduction of ROS generation and suppression of mitochondrial fragmentation. Specifically, we found that the interaction of mitochondrial fission protein, pDrp1(S616), with neuronal mitochondria was inhibited in NMN treated ischemic mice. Our data thus provide a novel link between mitochondrial NAD⁺ metabolism, ROS production, and mitochondrial fragmentation. Using NMN to target these mechanisms could represent a new therapeutic approach for treatment of acute brain injury and neurodegenerative diseases.

From the Ca2+-activated F1FO-ATPase to the mitochondrial permeability transition pore: an overview

Based on recent advances on the Ca²⁺-activated F₁F_O-ATPase features, a novel multistep mechanism involving the mitochondrial F₁F_O complex in the formation and opening of the still enigmatic mitochondrial permeability transition pore (MPTP), is proposed. MPTP opening makes the inner mitochondrial membrane (IMM) permeable to ions and solutes and, through cascade events, addresses cell fate to death. Since MPTP forms when matrix Ca²⁺ concentration rises and ATP is hydrolyzed by the F₁F_O-ATPase, conformational changes, triggered by Ca²⁺ insertion in F₁, may be transmitted to F_O and locally modify the IMM curvature. These events would cause F₁F_O-ATPase dimer dissociation and MPTP opening.

Mitochondria as a target of cardioprotection in models of preconditioning

Over the recent years the view on mitochondria in the heart as a cellular powerhouse providing ATP supply needed to sustain contractile function, basal metabolic processes, and ionic homeostasis has changed radically. At present it is known that dysfunctions of these organelles are essential in the development of a large number of diseases, including cardiovascular diseases. Moreover, mitochondria are considered to be a very promising target of endogenous strategies that are essential in the protection of the myocardium from acute ischemia/reperfusion injury. These strategies including ischemic preconditioning, remote ischemic preconditioning as well as the acute phase of streptozotocin-induced diabetes mellitus, provide a similar effect of protection. Alterations observed in the functional and structural properties of heart mitochondria caused by short-term pathological impulses are associated with endogenous cardioprotective processes. It seems that the extent of mitochondrial membrane fluidization could be an active response mechanism to injury with a subtle effect on membrane-associated processes which further affect the environment of the whole organelle, thus inducing metabolic changes in the heart. In this review article, we provide an overview of endogenous protective mechanisms induced by hypoxic, pseudohypoxic and ischemic conditions with special consideration of the role of heart mitochondria in these processes.

Stimulation of mitochondrial ATP synthase activity - a new diazoxide-mediated mechanism of cardioprotection

Pharmacological preconditioning by diazoxide and a model of experimental streptozotocin-induced acute diabetes mellitus (STZ-DM) provided similar levels of cardioprotection assessed as limiting myocardial infarct size. The aim was to explore the possibility of existence of another in vitro mechanism, which could be contributory to cardioprotection mediated by diazoxide treatment. Mitochondrial membrane fluidity and ATP synthase activity in isolated heart mitochondria were determined under the influence of two factors, STZ-DM condition and treatment with diazoxide. Both factors independently increased the ATP synthase activity (p<0.05), as no interaction effect was observed upon the combination of STZ-DM with diazoxide. On the other hand, the mitochondrial membrane fluidity was significantly increased by STZ-DM only; no significant main effect for diazoxide was found. Based on the results from measurements of enzyme kinetics, we assume a direct interaction of diazoxide with the molecule of ATP synthase stimulated its activity by noncompetitive activation. Our present work revealed, for the first time, that cardioprotection induced by diazoxide may not be caused exclusively by mitochondrial K(ATP) opening, but presumably also by a direct interaction of diazoxide with ATP synthase, although the mechanisms for achieving this activation cannot be fully delineated.

The Mitochondrial Permeability Transition Pore: Channel Formation by F-ATP Synthase, Integration in Signal Transduction, and Role in Pathophysiology

The mitochondrial permeability transition (PT) is a permeability increase of the inner mitochondrial membrane mediated by a channel, the permeability transition pore (PTP). After a brief historical introduction, we cover the key regulatory features of the PTP and provide a critical assessment of putative protein components that have been tested by genetic analysis. The discovery that under conditions of oxidative stress the F-ATP synthases of mammals, yeast, and Drosophila can be turned into Ca(2+)-dependent channels, whose electrophysiological properties match those of the corresponding PTPs, opens new perspectives to the field. We discuss structural and functional features of F-ATP synthases that may provide clues to its transition from an energy-conserving into an energy-dissipating device as well as recent advances on signal transduction to the PTP and on its role in cellular pathophysiology.

From ATP to PTP and Back: A Dual Function for the Mitochondrial ATP Synthase

Mitochondria not only play a fundamental role in heart physiology but are also key effectors of dysfunction and death. This dual role assumes a new meaning after recent advances on the nature and regulation of the permeability transition pore, an inner membrane channel whose opening requires matrix Ca(2+) and is modulated by many effectors including reactive oxygen species, matrix cyclophilin D, Pi (inorganic phosphate), and matrix pH. The recent demonstration that the F-ATP synthase can reversibly undergo a Ca(2+)-dependent transition to form a channel that mediates the permeability transition opens new perspectives to the field. These findings demand a reassessment of the modifications of F-ATP synthase that take place in the heart under pathological conditions and of their potential role in determining the transition of F-ATP synthase from and energy-conserving into an energy-dissipating device.

Mitochondria-immobilized pH-sensitive off-on fluorescent probe

We report here a mitochondria-targetable pH-sensitive probe that allows for a quantitative measurement of mitochondrial pH changes, as well as the real-time monitoring of pH-related physiological effects in live cells. This system consists of a piperazine-linked naphthalimide as a fluorescence off–on signaling unit, a cationic triphenylphosphonium group for mitochondrial targeting, and a reactive benzyl chloride subunit for mitochondrial fixation. It operates well in a mitochondrial environment within whole cells and displays a desirable off–on fluorescence response to mitochondrial acidification. Moreover, this probe allows for the monitoring of impaired mitochondria undergoing mitophagic elimination as the result of nutrient starvation. It thus allows for the monitoring of the organelle-specific dynamics associated with the conversion between physiological and pathological states.

Cellular mechanisms against ischemia reperfusion injury induced by the use of anesthetic pharmacological agents

Ischemia-reperfusion (IR) cycle in the myocardium is associated with activation of an injurious cascade, thus leading to new myocardial challenges, which account for up to 50% of infarct size. Some evidence implicates reactive oxygen species (ROS) as a probable cause of myocardial injury in prooxidant clinical settings. Damage occurs during both ischemia and post-ischemic reperfusion in animal and human models. The mechanisms that contribute to this damage include the increase in cellular calcium (Ca(2+)) concentration and induction of ROS sources during reperfusion. Pharmacological preconditioning, which includes pharmacological strategies that counteract the ROS burst and Ca(2+) overload followed to IR cycle in the myocardium, could be effective in limiting injury. Currently widespread evidence supports the use of anesthetics agents as an important cardioprotective strategy that act at various levels such as metabotropic receptors, ion channels or mitochondrial level. Their administration before a prolonged ischemic episode is known as anesthetic preconditioning, whereas when given at the very onset of reperfusion, is termed anesthetic postconditioning. Both types of anesthetic conditioning reduce, albeit not to the same degree, the extent of myocardial injury. This review focuses on cellular and pathophysiological concepts on the myocardial damage induced by IR and how anesthetic pharmacological agents commonly used could attenuate the functional and structural effects induced by oxidative stress in cardiac tissue.

Phenoptosis in yeasts

The current view on phenoptosis and apoptosis as genetic programs aimed at eliminating potentially dangerous organisms and cells, respectively, is given. Special emphasis is placed on apoptosis (phenoptosis) in yeasts: intracellular defects and a plethora of external stimuli inducing apoptosis in yeasts; distinctive morphological and biochemical hallmarks accompanying apoptosis in yeasts; pro- and antiapoptotic factors involved in yeast apoptosis signaling; consecutive stages of apoptosis from external stimulus to the cell death; a prominent role of mitochondria and other organelles in yeast apoptosis; possible pathways for release of apoptotic factors from the intermembrane mitochondrial space into the cytosol are described. Using some concrete examples, the obvious physiological importance and expediency of altruistic death of yeast cells is shown. Poorly known aspects of yeast apoptosis and prospects for yeast apoptosis study are defined.

A novel neuroprotective strategy for ischemic stroke: transient mild acidosis treatment by CO2 inhalation at reperfusion

Acidosis is one of the key components in cerebral ischemic postconditioning that has emerged recently as an endogenous strategy for neuroprotection. We set out to test whether acidosis treatment at reperfusion can protect against cerebral ischemia/reperfusion injury. Adult male C57BL/6 J mice were subjected to 60-minute middle cerebral arterial occlusion followed by 24-hour reperfusion. Acidosis treatment by inhaling 10%, 20%, or 30% CO2 for 5 or 10 minutes at 5, 50, or 100 minutes after reperfusion was applied. Our results showed that inhaling 20% CO2 for 5 minutes at 5 minutes after reperfusion-induced optimal neuroprotection, as revealed by reduced infarct volume. Attenuating brain acidosis with NaHCO3 significantly compromised the acidosis or ischemic postconditioning-induced neuroprotection. Consistently, both acidosis-treated primary cultured cortical neurons and acute corticostriatal slices were more resistant to oxygen-glucose deprivation/reperfusion insult. In addition, acidosis inhibited ischemia/reperfusion-induced apoptosis, caspase-3 expression, cytochrome c release to cytoplasm, and mitochondrial permeability transition pore (mPTP) opening. The neuroprotection of acidosis was inhibited by the mPTP opener atractyloside both in vivo and in vitro. Taken together, these findings indicate that transient mild acidosis treatment at reperfusion protects against cerebral ischemia/reperfusion injury. This neuroprotection is likely achieved, at least partly, by inhibiting mPTP opening and mitochondria-dependent apoptosis.

Mitochondrial dysfunction and NAD(+) metabolism alterations in the pathophysiology of acute brain injury

Mitochondrial dysfunction is commonly believed to be one of the major players in mechanisms of brain injury. For several decades, pathologic mitochondrial calcium overload and associated opening of the mitochondrial permeability transition (MPT) pore were considered a detrimental factor causing mitochondrial damage and bioenergetics failure. Mitochondrial and cellular bioenergetic metabolism depends on the enzymatic reactions that require NAD(+) or its reduced form NADH as cofactors. Recently, it was shown that NAD(+) also has an important function as a substrate for several NAD(+) glycohydrolases whose overactivation can contribute to cell death mechanisms. Furthermore, downstream metabolites of NAD(+) catabolism can also adversely affect cell viability. In contrast to the negative effects of NAD(+)-catabolizing enzymes, enzymes that constitute the NAD(+) biosynthesis pathway possess neuroprotective properties. In the first part of this review, we discuss the role of MPT in acute brain injury and its role in mitochondrial NAD(+) metabolism. Next, we focus on individual NAD(+) glycohydrolases, both cytosolic and mitochondrial, and their role in NAD(+) catabolism and brain damage. Finally, we discuss the potential effects of downstream products of NAD(+) degradation and associated enzymes as well as the role of NAD(+) resynthesis enzymes as potential therapeutic targets.

Functional and pharmacological characteristics of permeability transition in isolated human heart mitochondria

The objective of the present study was to validate the presence and explore the characteristics of mitochondrial permeability transition (mPT) in isolated mitochondria from human heart tissue in order to investigate if previous findings in animal models of cardiac disorders are translatable to human disease. Mitochondria were rapidly isolated from fresh atrial tissue samples obtained from 14 patients undergoing Maze surgery due to atrial fibrillation. Human heart mitochondria exhibited typical mPT characteristics upon calcium overload such as swelling, evaluated by changes in light scattering, inhibition of respiration and loss of respiratory coupling. Swelling was a morphologically reversible event following transient calcium challenge. Calcium retention capacity (CRC), a quantitative measure of mPT sensitivity assayed by following extramitochondrial [Ca(2+)] and changes in respiration during a continuous calcium infusion, was significantly increased by cyclophilin D (CypD) inhibitors. The thiol-reactive oxidant phenylarsine oxide sensitized mitochondria to calcium-induced mPT. Release of the pro-apoptotic intermembrane protein cytochrome c was increased after, but not before, calcium discharge and respiratory inhibition in the CRC assay. From the present study, we conclude that adult viable heart mitochondria have a CypD- and oxidant-regulated mPT. The findings support that inhibition of mPT may be a relevant pharmacological target in human cardiac disease and may underlie the beneficial effect of cyclosporin A in reperfusion injury.

Extracellular brain pH with or without hypoxia is a marker of profound metabolic derangement and increased mortality after traumatic brain injury

Cerebral hypoxia and acidosis can follow traumatic brain injury (TBI) and are associated with increased mortality. This study aimed to evaluate a relationship between reduced pH(bt) and disturbances of cerebral metabolism. Prospective data from 56 patients with TBI, receiving microdialysis and Neurotrend monitoring, were analyzed. Four tissue states were defined based on pH(bt) and P(bt)O(2): 1--low P(bt)O(2)/pH(bt), 2--low pH(bt)/normal P(bt)O(2), 3--normal pH(bt)/low P(bt)O(2), and 4--normal pH(bt)/P(bt)O(2)). Microdialysis values were compared between the groups. The relationship between P(bt)O(2) and lactate/pyruvate (LP) ratio was evaluated at different pH(bt) levels. Proportional contribution of each state was evaluated against mortality. As compared with the state 4, the state 3 was not different, the state 2 exhibited higher levels of lactate, LP, and glucose and the state 1--higher LP and reduced glucose (P<0.001). A significant negative correlation between LP and P(bt)O(2) (rho=-0.159, P<0.001) was stronger at low pH(bt) (rho=-0.201, P<0.001) and nonsignificant at normal pH(bt) (P=0.993). The state 2 was a significant discriminator of mortality categories (P=0.031). Decreased pH(bt) is associated with impaired metabolism. Measuring pH(bt) with P(bt)O(2) is a more robust way of detecting metabolic derangements.

Mitochondrial pathways, permeability transition pore, and redox signaling in cardioprotection: therapeutic implications

Reperfusion therapy is the indispensable treatment of acute myocardial infarction (AMI) and must be applied as soon as possible to attenuate the ischemic insult. However, reperfusion is responsible for additional myocardial damage likely involving opening of the mitochondrial permeability transition pore (mPTP). A great part of reperfusion injury occurs during the first minute of reperfusion. The prolonged opening of mPTP is considered one of the endpoints of the cascade to myocardial damage, causing loss of cardiomyocyte function and viability. Opening of mPTP and the consequent oxidative stress due to reactive oxygen and nitrogen species (ROS/RNS) are considered among the major mechanisms of mitochondrial and myocardial dysfunction. Kinases and mitochondrial components constitute an intricate network of signaling molecules and mitochondrial proteins, which interact in response to stressors. Cardioprotective pathways are activated by stimuli such as preconditioning and postconditioning (PostC), obtained with brief intermittent ischemia or with pharmacological agents, which drastically reduce the lethal ischemia/reperfusion injury. The protective pathways converging on mitochondria may preserve their function. Protection involves kinases, adenosine triphosphate-dependent potassium channels, ROS signaling, and the mPTP modulation. Some clinical studies using ischemic PostC during angioplasty support its protective effects, and an interesting alternative is pharmacological PostC. In fact, the mPTP desensitizer, cyclosporine A, has been shown to induce appreciable protections in AMI patients. Several factors and comorbidities that might interfere with cardioprotective signaling are considered. Hence, treatments adapted to the characteristics of the patient (i.e., phenotype oriented) might be feasible in the future.

Cyclophilin-D inhibition in neuroprotection: dawn of a new era of mitochondrial medicine

Traumatic brain injury and ischemia can result in marked neuronal degeneration and residual impairment of cerebral function. However, no effective pharmacological treatment directed at tissues of the central nervous system (CNS) for acute intervention has been developed. The detailed pathophysiological cascade leading to -neurodegeneration in these conditions has not been elucidated, but cellular calcium overload and mitochondrial dysfunction have been implicated in a wide range of animal models involving degeneration of the CNS. In particular, activation of the calcium-induced mitochondrial permeability transition (mPT) is considered to be a major cause of cell death inferred by the broad and potent neuroprotective effects of -pharmacological inhibitors of mPT, especially modulators of cyclophilin activity and, more specifically, genetic inactivation of the mitochondrial cyclophilin, cyclophilin D. Reviewed are evidence and challenges that could bring on the dawning of mitochondrial medicine aimed at safeguarding energy supply following acute injury to the CNS.

Quercetin in hypoxia-induced oxidative stress: novel target for neuroprotection

Oxidative stress in the central nervous system is one of the key players for neurodegeneration. Thus, antioxidants could play important roles in treating several neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and aging-related brain disorders. This review is focused on the new developments in oxidative stress-induced neurodegeneration. Further, based on our own investigations, new roles of quercetin, an antioxidant compound in hypoxia and ischemia induced neuroprotection in relation to suppression of oxidative stress, improvement in behavioral function, reduction in infarct volume, brain swelling, and cellular injury in both in vivo and in vitro models are discussed. Our new findings clearly suggest that antioxidant compounds have potential role in therapeutic strategies to treat neurodegenerative diseases in clinical settings.

Intestinal epithelium is more susceptible to cytopathic injury and altered permeability than the lung epithelium in the context of acute sepsis

Mitochondrial morphology and function are altered in intestinal epithelia during endotoxemia. However, it is unclear whether mitochondrial abnormalities occur in lung epithelial cells during acute sepsis or whether mitochondrial dysfunction corresponds with altered epithelial barrier function. Thus, we hypothesized that the intestinal epithelium is more susceptible to mitochondrial injury than the lung epithelium during acute sepsis and that mitochondrial dysfunction precedes impaired barrier function. Using a resuscitated feline model of Escherichia coli-induced sepsis, lung and ileal tissues were harvested after 6 h for histological and mitochondrial ultrastructural analyses in septic (n = 6) and time-matched controls (n = 6). Human lung epithelial cells (HLEC) and Caco-2 monolayers (n = 5) were exposed to 'cytomix' (TNFα: 40 ng/ml, IL-1β: 20 ng/ml, IFNγ: 10 ng/ml) for 24-72 h, and measurements of transepithelial electrical resistance (TER), epithelial permeability and mitochondrial membrane potential (ΔΨ) were taken. Lung epithelial morphology, mitochondrial ultrastructure and pulmonary gas exchange were unaltered in septic animals compared to matching controls. While histologically intact, ileal epithelia demonstrated marked mitochondrial ultrastructural damage during sepsis. Caco-2 monolayers treated with cytomix showed a significant decrease in mitochondrial ΔΨ within 24 h, which was associated with a progressive reduction in TER and increased epithelial permeability over the subsequent 48 h. In contrast, mitochondrial ΔΨ and epithelial barrier functions were preserved in HLEC following cytomix. These findings indicate that intestinal epithelium is more susceptible to mitochondrial damage and dysfunction than the lung epithelium in the context of sepsis. Early alterations in mitochondrial function portend subsequent epithelial barrier dysfunction.

Ischemia/reperfusion injury and cardioprotective mechanisms: Role of mitochondria and reactive oxygen species

Reperfusion therapy must be applied as soon as possible to attenuate the ischemic insult of acute myocardial infarction (AMI). However reperfusion is responsible for additional myocardial damage, which likely involves opening of the mitochondrial permeability transition pore (mPTP). In reperfusion injury, mitochondrial damage is a determining factor in causing loss of cardiomyocyte function and viability. Major mechanisms of mitochondrial dysfunction include the long lasting opening of mPTPs and the oxidative stress resulting from formation of reactive oxygen species (ROS). Several signaling cardioprotective pathways are activated by stimuli such as preconditioning and postconditioning, obtained with brief intermittent ischemia or with pharmacological agents. These pathways converge on a common target, the mitochondria, to preserve their function after ischemia/reperfusion. The present review discusses the role of mitochondria in cardioprotection, especially the involvement of adenosine triphosphate-dependent potassium channels, ROS signaling, and the mPTP. Ischemic postconditioning has emerged as a new way to target the mitochondria, and to drastically reduce lethal reperfusion injury. Several clinical studies using ischemic postconditioning during angioplasty now support its protective effects, and an interesting alternative is pharmacological postconditioning. In fact ischemic postconditioning and the mPTP desensitizer, cyclosporine A, have been shown to induce comparable protection in AMI patients.

Mitochondrial injury and protection in ischemic pre- and postconditioning

Mitochondrial damage is a determining factor in causing loss of cardiomyocyte function and viability, yet a mild degree of mitochondrial dysfunction appears to underlie cardioprotection against injury caused by postischemic reperfusion. This review is focused on two major mechanisms of mitochondrial dysfunction, namely, oxidative stress and opening of the mitochondrial permeability transition pore. The formation of reactive oxygen species in mitochondria will be analyzed with regard to factors controlling mitochondrial permeability transition pore opening. Finally, these mitochondrial processes are analyzed with respect to cardioprotection afforded by ischemic pre- and postconditioning.

Pretreatment with the ciclosporin derivative NIM811 reduces delayed neuronal death in the hippocampus after transient forebrain ischaemia

OBJECTIVES

There have been several previous studies showing that ciclosporin, a ligand for cyclophilin D (CypD), reduces mitochondrial permeability transition (mPT) and ameliorates delayed neuronal death. NIM811 is a non-immunosuppressive ciclosporin derivative that also inhibits mPT, but has significantly less cytotoxicity than ciclosporin. Actually, in animal experiments, several investigators have reported that NIM811 ameliorates central nervous system disorders, such as traumatic brain injury, transient focal cerebral ischaemia and spinal cord injury. Therefore, we evaluated whether the ciclosporin derivative, NIM811 reduces mPT and ameliorates delayed neuronal death in the hippocampal CA1 sectors in mice when subjected to transient forebrain ischaemia.

METHODS

Male C57BL/6 mice were treated with 50 mg/kg ciclosporin, 10, 50 or 100 mg/kg NIM811 or phosphate-buffered saline. At 30 min post-injection, all mice were subjected to 20 min bilateral common carotid artery occlusion (BCCAO). To estimate delayed neuronal death, the sections were prepared for HE staining and terminal deoxynucleotidyl transferase-mediated dUTP end-labelling (TUNEL) staining at 72 h after 20 min BCCAO. Furthermore, using 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide (JC-1) staining technique, we evaluated whether NIM811 (1, 10, 100 or 1000 microm) inhibited mPT in the neurons exposed to 100 microm glutamate.

RESULTS

Both delayed neuronal injury and apoptosis in the hippocampal CA1 sectors were significantly ameliorated at 72 h after transient forebrain ischaemia in the mice treated with 100 mg/kg NIM811 or 50 mg/kg ciclosporin. The treatments with 100 microm and 1,000 microm NIM811 significantly inhibited the reduction of mitochondrial membrane potential in the neurons exposed to 100 microm glutamate.

CONCLUSIONS

These findings strongly suggest that NIM811 inhibits mPT and ameliorates delayed neuronal death in mice subjected to transient forebrain ischaemia.

Dynamic regulation of the mitochondrial proton gradient during cytosolic calcium elevations

Mitochondria extrude protons across their inner membrane to generate the mitochondrial membrane potential (ΔΨ_m) and pH gradient (ΔpH_m) that both power ATP synthesis. Mitochondrial uptake and efflux of many ions and metabolites are driven exclusively by ΔpH_m, whose in situ regulation is poorly characterized. Here, we report the first dynamic measurements of ΔpH_m in living cells, using a mitochondrially targeted, pH-sensitive YFP (SypHer) combined with a cytosolic pH indicator (5-(and 6)-carboxy-SNARF-1). The resting matrix pH (∼7.6) and ΔpH_m (∼0.45) of HeLa cells at 37 °C were lower than previously reported. Unexpectedly, mitochondrial pH and ΔpH_m decreased during cytosolic Ca²⁺ elevations. The drop in matrix pH was due to cytosolic acid generated by plasma membrane Ca²⁺-ATPases and transmitted to mitochondria by P_i/H⁺ symport and K⁺/H⁺ exchange, whereas the decrease in ΔpH_m reflected the low H⁺-buffering power of mitochondria (∼5 mm, pH 7.8) compared with the cytosol (∼20 mm, pH 7.4). Upon agonist washout and restoration of cytosolic Ca²⁺ and pH, mitochondria alkalinized and ΔpH_m increased. In permeabilized cells, a decrease in bath pH from 7.4 to 7.2 rapidly decreased mitochondrial pH, whereas the addition of 10 μm Ca²⁺ caused a delayed and smaller alkalinization. These findings indicate that the mitochondrial matrix pH and ΔpH_m are regulated by opposing Ca²⁺-dependent processes of stimulated mitochondrial respiration and cytosolic acidification.

Induction of permeability of the inner membrane of yeast mitochondria

The current view on apoptosis is given, with a special emphasis placed on apoptosis in yeasts. Induction of a nonspecific permeability transition pore (mPTP) in mammalian and yeast mitochondria is described, particularly in mitochondria from Yarrowia lipolytica and Dipodascus (Endomyces) magnusii yeasts, which are aerobes possessing the fully competent respiratory chain with all three points of energy conservation and well-structured mitochondria. They were examined for their ability to induce an elevated permeability transition of the inner mitochondrial membrane, being subjected to virtually all conditions known to induce the mPTP in animal mitochondria. Yeast mitochondria do not form Ca2+-dependent pores, neither the classical Ca2+/P(i)-dependent, cyclosporin A-sensitive pore even under de-energization of mitochondria or depletion of the intramitochondrial nucleotide pools, nor a pore induced in mammalian mitochondria upon concerted action of moderate Ca2+ concentrations (in the presence of the Ca2+ ionophore ETH129) and saturated fatty acids. No pore formation was found in yeast mitochondria in the presence of elevated phosphate concentrations at acidic pH values. It is concluded that the permeability transition in yeast mitochondria is not coupled with Ca2+ uptake and is differently regulated compared to the mPTP of animal mitochondria.

Cardioprotection by GSK-3 inhibition: role of enhanced glycogen synthesis and attenuation of calcium overload

AIMS

Glycogen synthase kinase-3 (GSK-3) is a multi-functional kinase that regulates signalling pathways affecting glycogen metabolism, protein synthesis, mitosis, and apoptosis. GSK-3 inhibition limits cardiac ischaemia-reperfusion (IR) injury, but mechanisms are not clearly defined. This study tested the hypothesis that acute GSK-3 inhibition stimulates glycogen synthesis, repartitions glucose away from glycolysis, reduces proton (H+) production from glucose metabolism, and attenuates intracellular Ca2+ (Ca2+(i)) overload.

METHODS AND RESULTS

In isolated perfused working rat hearts subjected to global ischaemia and reperfusion, the selective GSK-3 inhibitor, SB-216763 (SB, 3 micromol/L), when added either prior to ischaemia or at the onset of reperfusion, improved recovery of left-ventricular (LV) work. SB increased glycogen synthesis during reperfusion while glycolysis and H+ production were reduced. Rates of glucose and palmitate oxidation were improved by SB. Measurement of Ca2+(i) concentration by rapid acquisition indo-1 fluorescence imaging showed that SB, when added either prior to ischaemia or at the onset of reperfusion, reduced diastolic Ca2+(i) overload during reperfusion. In aerobic hearts depleted of glycogen by substrate-free perfusion to a level similar to that measured at the onset of reperfusion, SB accelerated glycogen synthesis and reduced glycolysis and H+ production independent of changes in LV work.

CONCLUSION

Our study indicates that reduction in H+ production by GSK-3 inhibition is an early and upstream event that lessens Ca2+(i) overload during ischaemia and early reperfusion independent of LV work which enhances the recovery of post-ischaemic LV function and that may ultimately contribute to previously observed reductions in cell death and infarction.

Increased potassium conductance of brain mitochondria induces resistance to permeability transition by enhancing matrix volume

Modulation of K(+) conductance of the inner mitochondrial membrane has been proposed to mediate preconditioning in ischemia-reperfusion injury. The mechanism is not entirely understood, but it has been linked to a decreased activation of mitochondrial permeability transition (mPT). In the present study K(+) channel activity was mimicked by picomolar concentrations of valinomycin. Isolated brain mitochondria were exposed to continuous infusions of calcium. Monitoring of extramitochondrial Ca(2+) and mitochondrial respiration provided a quantitative assay for mPT sensitivity by determining calcium retention capacity (CRC). Valinomycin and cyclophilin D inhibition separately and additively increased CRC. Comparable degrees of respiratory uncoupling induced by increased K(+) or H(+) conductance had opposite effects on mPT sensitivity. Protonophores dose-dependently decreased CRC, demonstrating that so-called mild uncoupling was not beneficial per se. The putative mitoK(ATP) channel opener diazoxide did not mimic the effect of valinomycin. An alkaline matrix pH was required for mitochondria to retain calcium, but increased K(+) conductance did not result in augmented DeltapH. The beneficial effect of valinomycin on CRC was not mediated by H(2)O(2)-induced protein kinase Cepsilon activation. Rather, increased K(+) conductance reduced H(2)O(2) generation during calcium infusion. Lowering the osmolarity of the buffer induced an increase in mitochondrial volume and improved CRC similar to valinomycin without inducing uncoupling or otherwise affecting respiration. We propose that increased potassium conductance in brain mitochondria may cause a direct physiological effect on matrix volume inducing resistance to pathological calcium challenges.

Induction of a non-specific permeability transition in mitochondria from Yarrowia lipolytica and Dipodascus (Endomyces) magnusii yeasts

In this study we used tightly-coupled mitochondria from Yarrowia lipolytica and Dipodascus (Endomyces) magnusii yeasts, possessing a respiratory chain with the usual three points of energy conservation. High-amplitude swelling and collapse of the membrane potential were used as parameters for demonstrating induction of the mitochondrial permeability transition due to opening of a pore (mPTP). Mitochondria from Y. lipolytica, lacking a natural mitochondrial Ca(2+) uptake pathway, and from D. magnusii, harboring a high-capacitive, regulated mitochondrial Ca(2+) transport system (Bazhenova et al. J Biol Chem 273:4372-4377, 1998a; Bazhenova et al. Biochim Biophys Acta 1371:96-100, 1998b; Deryabina and Zvyagilskaya Biochemistry (Moscow) 65:1352-1356, 2000; Deryabina et al. J Biol Chem 276:47801-47806, 2001) were very resistant to Ca(2+) overload. However, exposure of yeast mitochondria to 50-100 microM Ca(2+) in the presence of the Ca(2+) ionophore ETH129 induced collapse of the membrane potential, possibly due to activation of the fatty acid-dependent Ca(2+)/nH(+)-antiporter, with no classical mPTP induction. The absence of response in yeast mitochondria was not simply due to structural limitations, since large-amplitude swelling occurred in the presence of alamethicin, a hydrophobic, helical peptide, forming voltage-sensitive ion channels in lipid membranes. Ca(2+)- ETH129-induced activation of the Ca(2+)/H(+)-antiport system was inhibited and prevented by bovine serum albumin, and partially by inorganic phosphate and ATP. We subjected yeast mitochondria to other conditions known to induce the permeability transition in animal mitochondria, i.e., Ca(2+) overload (in the presence of ETH129) combined with palmitic acid (Mironova et al. J Bioenerg Biomembr 33:319-331, 2001; Sultan and Sokolove Arch Biochem Biophys 386:37-51, 2001), SH-reagents, carboxyatractyloside (an inhibitor of the ADP/ATP translocator), depletion of intramitochondrial adenine nucleotide pools, deenergization of mitochondria, and shifting to acidic pH values in the presence of high phosphate concentrations. None of the above-mentioned substances or conditions induced a mPTP-like pore. It is thus evident that the permeability transition in yeast mitochondria is not coupled with Ca(2+) uptake and is differently regulated compared to the mPTP of animal mitochondria.

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.

Cyclophilin D is expressed predominantly in mitochondria of gamma-aminobutyric acidergic interneurons

Brain mitochondria are relatively resistant to calcium-induced mitochondrial permeability transition (MPT), with heterogenic response to the insult. The cause for this heterogeneity is not clear, so we studied the distribution of a key regulator of the MPT, cyclophilin D (cypD), within the rat brain by using immunohistology and Western blotting. Motor and parietal cortex, hippocampus, striatum, substantia nigra, ventral tegmental area, septum, and mammillary nucleus displayed a strong immunoreactivity to cypD within specific subpopulation of neurons. The staining was punctate and intense, particularly in perinuclear regions of cells. Apart from neurons, a subpopulation of astrocytes and NG2-positive cells showed higher cypD immunoreactivity. Double staining of cypD with cytochrome oxidase confirmed the mitochondrial specificity of cypD immunoreactivity. The neurons with high levels of cypD also expressed glutamate decarboxylase (GAD) and the calcium binding protein parvalbumin or calbinding D-28k, identifying these cells as interneurons. Western blots confirmed our immunohistochemical findings, showing significantly higher levels of cypD in crude mitochondria of substantia nigra compared with cortex or striatum. Furthermore, nonsynaptic mitochondria representing mainly mitochondria from cell bodies of neurons and glia have about 16% higher levels of cypD compared with synaptic mitochondria that are localized in presynaptic buttons. These data suggest that the underlying factor of heterogenic response of isolated brain mitochondria to MPT-inducing insults can be the different expression levels of cypD, with mitochondria originated from interneurons as the most sensitive.

Amiloride derivatives induce apoptosis by depleting ER Ca(2+) stores in vascular endothelial cells

BACKGROUND AND PURPOSE

Amiloride derivatives are blockers of the Na(+)/H(+) exchanger (NHE) and at micromolar concentrations have protective effects on cardiac and brain ischaemia/reperfusion injury but at higher concentrations also induce apoptosis. Here, we aimed to elucidate the mechanism related to this cytotoxic action.

EXPERIMENTAL APPROACH

We quantified the expression of genes associated with endoplasmic reticulum (ER) stress and measured changes in luminal ER Ca(2+) concentration ([Ca(2+)](ER)) with a 'cameleon' indicator, D1ER.

KEY RESULTS

Amiloride derivatives induced apoptosis in vascular endothelial cells, an effect that increased at alkaline extracellular pH. The potency order for cytotoxicity was 5-(N,N-hexamethylene)-amiloride (HMA) > 5-(N-methyl-N-isobutyl) amiloride > 5-(N-ethyl-N-isopropyl) amiloride (EIPA) >> amiloride. HMA dose-dependently increased the transcription of the ER stress genes GADD153 and GADD34 and rapidly depleted [Ca(2+)](ER), mimicking the effects of the sarco/endoplasmic reticulum ATPase (SERCA) inhibitor thapsigargin. The NHE1-specific inhibitor HOE 694 inhibited NHE activity by 87% but did not alter [Ca(2+)](ER). The decrease in [Ca(2+)](ER) evoked by amiloride derivatives was also observed in HeLa cells and was mirrored by an increase in cytosolic Ca(2+) concentration.

CONCLUSIONS AND IMPLICATIONS

Amiloride derivatives disrupt ER and cytosolic Ca(2+) homeostasis by a mechanism unrelated to NHE inhibition, most likely by interfering with the activity of SERCA. We propose that ER Ca(2+) depletion and subsequent ER stress provide a rationale framework for the apoptotic effects of amiloride derivatives.

Targeting mitochondria for resuscitation from cardiac arrest

Reversal of cardiac arrest requires reestablishment of aerobic metabolism by reperfusion with oxygenated blood of tissues that have been ischemic for variable periods of time. However, reperfusion concomitantly activates a myriad of pathogenic mechanisms causing what is known as reperfusion injury. At the center of reperfusion injury are mitochondria, playing a critical role as effectors and targets of injury. Studies in animal models of ventricular fibrillation have shown that limiting myocardial cytosolic Na+ overload attenuates mitochondrial Ca2+ overload and maintains oxidative phosphorylation, which is the main bioenergetic function of mitochondria. This effect is associated with functional myocardial benefits such as preservation of myocardial compliance during chest compression and attenuation of myocardial dysfunction after return of spontaneous circulation. Additional studies in similar animal models of ventricular fibrillation have shown that mitochondrial injury leads to activation of the mitochondrial apoptotic pathway, characterized by the release of cytochrome c to the cytosol, reduction of caspase-9 levels, and activation of caspase-3 coincident with marked reduction in left ventricular function. Cytochrome c also "leaks" into the bloodstream attaining levels that are inversely proportional to survival. These data indicate that mitochondria play a key role during cardiac resuscitation by modulating energy metabolism and signaling apoptotic cascades and that targeting mitochondria could represent a promising strategy for cardiac resuscitation.

Probing the molecular mechanisms of neuronal degeneration: importance of mitochondrial dysfunction and calcineurin activation

Cerebral injury is a critical aspect of the management of patients in intensive care. Pathological conditions induced by cerebral ischemia, hypoxia, head trauma, and seizure activity can result in marked residual impairment of cerebral function. We have investigated the potential mechanisms leading to neuronal cell death in pathological conditions, with the aim of discovering therapeutic targets and methods to minimize neuronal damage resulting from insults directed at the central nervous system (CNS). Over the years, deeper understanding of the mechanisms of neuronal cell death has indeed evolved, enabling clinical critical care management to salvage neurons that are at the brink of degeneration and to support recovery of brain function. However, no substantial breakthrough has been achieved in the quest to develop effective pharmacological neuroprotective therapy directed at tissues of the CNS. The current situation is unacceptable, and preservation of function and protection of the brain from terminal impairment will be a vital medical issue in the twenty-first century. To achieve this goal, it is critical to clarify the key mechanisms leading to neuronal cell death. Here, we discuss the importance of the calcineurin/immunophilin signal transduction pathway and mitochondrial involvement in the detrimental chain of events leading to neuronal degeneration.