The mitochondrial permeability transition pore and the cardiac necrotic program

Role of ranolazine in heart failure: From cellular to clinic perspective

Ranolazine was approved by the US Food and Drug Administration as an antianginal drug in 2006, and has been used since in certain groups of patients with stable angina. The therapeutic action of ranolazine was initially attributed to inhibitory effects on fatty acids metabolism. As investigations went on, however, it developed that the main beneficial effects of ranolazine arise from its action on the late sodium current in the heart. Since late sodium currents were discovered to be involved in various heart pathologies such as ischemia, arrhythmias, systolic and diastolic dysfunctions, and all these conditions are associated with heart failure, ranolazine has in some way been tested either directly or indirectly on heart failure in numerous experimental and clinical studies. As the heart continuously remodels following any sort of severe injury, the inhibition by ranolazine of the underlying mechanisms of cardiac remodeling including ion disturbances, oxidative stress, inflammation, apoptosis, fibrosis, metabolic dysregulation, and neurohormonal impairment are discussed, along with unresolved issues. A projection of pathologies targeted by ranolazine from cellular level to clinical is provided in this review.

TRIC-A regulates intracellular Ca2+ homeostasis in cardiomyocytes

Trimeric intracellular cation (TRIC) channels have been identified as monovalent cation channels that are located in the ER/SR membrane. Two isoforms discovered in mammals are TRIC-A (TMEM38a) and TRIC-B (TMEM38b). TRIC-B ubiquitously expresses in all tissues, and TRIC-B-/- mice is lethal at the neonatal stage. TRIC-A mainly expresses in excitable cells. TRIC-A-/- mice survive normally but show abnormal SR Ca2+ handling in both skeletal and cardiac muscle cells. Importantly, TRIC-A mutations have been identified in human patients with stress-induced arrhythmia. In the past decade, important discoveries have been made to understand the structure and function of TRIC channels, especially its role in regulating intracellular Ca2+ homeostasis. In this review article, we focus on the potential roles of TRIC-A in regulating cardiac function, particularly its effects on intracellular Ca2+ signaling of cardiomyocytes and discuss the current knowledge gaps.

Chloride channel blocker IAA-94 increases myocardial infarction by reducing calcium retention capacity of the cardiac mitochondria

Indanyloxyacetic acid-94 (IAA-94), an intracellular chloride channel blocker, is shown to ablate cardioprotection rendered by ischemic preconditioning (IPC), N (6)-2-(4-aminophenyl) ethyladenosine or the PKC activator phorbol 12-myristate 13-acetate and cyclosporin A (CsA) in both ex-vivo and in-vivo ischemia-reperfusion (IR) injury. Thus signifying the role of the IAA-94 sensitive chloride channels in mediating cardio-protection upon IR injury. Although IAA-94 sensitive chloride currents are recorded in cardiac mitoplast, there is still a lack of understanding of the mechanism by which IAA-94 increases myocardial infarction (MI) by IR injury. Mitochondria are the key arbitrators of cell life and death pathways. Both oxidative stress and calcium overload in the mitochondria, elicit pathways resulting in the opening of mitochondrial permeability transition pore (mPTP) leading to cell death. Therefore, in this study we explored the role of IAA-94 in MI and in maintaining calcium retention capacity (CRC) of cardiac mitochondria after IR. IAA-94 inhibited the CRC of the isolated cardiac mitochondria in a concentration-dependent manner as measured spectrofluorimetrically using calcium green-5 N. Interestingly, IAA-94 did not change the mitochondrial membrane potential. Further, CsA a blocker of mPTP opening could not override the effect of IAA-94. We also showed for the first time that IAA-94 perfusion after ischemic event augments MI by reducing the CRC of mitochondria. To conclude, our results demonstrate that the mechanism of IAA-94 mediated cardio-deleterious effects is via modulating the mitochondria CRC, thereby playing a role in mPTP opening. These findings highlight new pharmacological targets, which can mediate cardioprotection from IR injury.

Upregulation of UCP2 Expression Protects against LPS-Induced Oxidative Stress and Apoptosis in Cardiomyocytes

Uncoupling protein 2 (UCP2) has a cardioprotective role under septic conditions, but the underlying mechanism remains unclear. This study aimed at investigating the effects of UCP2 on the oxidative stress and apoptosis of cardiomyocytes induced by lipopolysaccharide (LPS). First, LPS increased UCP2 expression in cardiomyocytes in a time-dependent manner. LPS increased the production of lactate dehydrogenase (LDH), reactive oxygen species (ROS), and malondialdehyde (MDA) and decreased the level of superoxide dismutase (SOD). However, UCP2 knockdown increased the LPS-induced cardiac injury and oxidative stress. In addition, LPS damaged the mitochondrial ultrastructure and led to the disruption of mitochondrial membrane potential (MMP), as well as the release of mitochondrial cytochrome c. UCP2 knockdown aggravated mitochondrial injury and the release of mitochondrial cytochrome c. LPS increased the protein levels of Bax and cleaved-caspase-3, decreased the protein level of Bcl-2, and upregulated the protein level of mitogen-activated protein kinase. However, upon UCP2 knockdown, the protein levels of Bax and cleaved-caspase-3 increased even further, and the protein level of Bcl-2 was further decreased. The protein level of phosphorylated p38 was also further enhanced. Thus, UCP2 protects against LPS-induced oxidative stress and apoptosis in cardiomyocytes.

Cyclophilin D, Somehow a Master Regulator of Mitochondrial Function

Cyclophilin D (CyPD) is an important mitochondrial chaperone protein whose mechanism of action remains a mystery. It is well known for regulating mitochondrial function and coupling of the electron transport chain and ATP synthesis by controlling the mitochondrial permeability transition pore (PTP), but more recent evidence suggests that it may regulate electron transport chain activity. Given its identification as a peptidyl-prolyl, cis-trans isomerase (PPIase), CyPD, is thought to be involved in mitochondrial protein folding, but very few reports demonstrate the presence of this activity. By contrast, CyPD may also perform a scaffolding function, as it binds to a number of important proteins in the mitochondrial matrix and inner mitochondrial membrane. From a clinical perspective, inhibiting CyPD to inhibit PTP opening protects against ischemia⁻reperfusion injury, making modulation of CyPD activity a potentially important therapeutic goal, but the lack of knowledge about the mechanisms of CyPD's actions remains problematic for such therapies. Thus, the important yet enigmatic nature of CyPD somehow makes it a master regulator, yet a troublemaker, for mitochondrial function.

The Mitochondrial Permeability Transition Pore and ATP Synthase

Mitochondrial ATP generation by oxidative phosphorylation combines the stepwise oxidation by the electron transport chain (ETC) of the reducing equivalents NADH and FADH₂ with the generation of ATP by the ATP synthase. Recent studies show that the ATP synthase is not only essential for the generation of ATP but may also contribute to the formation of the mitochondrial permeability transition pore (PTP). We present a model, in which the PTP is located within the c-subunit ring in the F_o subunit of the ATP synthase. Opening of the PTP was long associated with uncoupling of the ETC and the initiation of programmed cell death. More recently, it was shown that PTP opening may serve a physiologic role: it can transiently open to regulate mitochondrial signaling in mature cells, and it is open in the embryonic mouse heart. This review will discuss how the ATP synthase paradoxically lies at the center of both ATP generation and cell death.

Regulation of calcium release from the endoplasmic reticulum by the serine hydrolase ABHD2

The serine hydrolase inhibitors pyrrophenone and KT195 inhibit cell death induced by A23187 and H₂O₂ by blocking the release of calcium from the endoplasmic reticulum and mitochondrial calcium uptake. The effect of pyrrophenone and KT195 on these processes is not due to inhibition of their known targets, cytosolic phospholipase A₂ and α/β-hydrolase domain-containing (ABHD) 6, respectively, but represent off-target effects. To identify targets of KT195, fibroblasts were treated with KT195-alkyne to covalently label protein targets followed by click chemistry with biotin azide, enrichment on streptavidin beads and tryptic peptide analysis by mass spectrometry. Although several serine hydrolases were identified, α/β-hydrolase domain-containing 2 (ABHD2) was the only target in which both KT195 and pyrrophenone competed for binding to KT195-alkyne. ABHD2 is a serine hydrolase with a predicted transmembrane domain consistent with its pull-down from the membrane proteome. Subcellular fractionation showed localization of ABHD2 to the endoplasmic reticulum but not to mitochondria or mitochondrial-associated membranes. Knockdown of ABHD2 with shRNA attenuated calcium release from the endoplasmic reticulum, mitochondrial calcium uptake and cell death in fibroblasts stimulated with A23187. The results describe a novel mechanism for regulating calcium transfer from the endoplasmic reticulum to mitochondria that involves the serine hydrolase ABHD2.

A Bak-dependent mitochondrial amplification step contributes to Smac mimetic/glucocorticoid-induced necroptosis

Necroptosis is a form of programmed cell death that critically depends on RIP3 and MLKL. However, the contribution of mitochondria to necroptosis is still poorly understood. In the present study, we discovered that mitochondrial perturbations play a critical role in Smac mimetic/Dexamethasone (Dexa)-induced necroptosis independently of death receptor ligands. We demonstrate that the Smac mimetic BV6 and Dexa cooperate to trigger necroptotic cell death in acute lymphoblastic leukemia (ALL) cells that are deficient in caspase activation due to absent caspase-8 expression or pharmacological inhibition by the caspase inhibitor zVAD.fmk, since genetic silencing or pharmacological inhibition of RIP3 or MLKL significantly rescue BV6/Dexa-induced necroptosis. In addition, RIP3 or MLKL knockout mouse embryonic fibroblasts (MEFs) are protected from BV6/Dexa/zVAD.fmk-induced cell death. In contrast, antagonistic antibodies against the death receptor ligands TNFα, TRAIL or CD95 ligand fail to rescue BV6/Dexa-triggered cell death. Kinetic studies revealed that prior to cell death BV6/Dexa treatment causes hyperpolarization of the mitochondrial membrane potential (MMP) followed by loss of MMP, reactive oxygen species (ROS) production, Bak activation and disruption of mitochondrial respiration. Importantly, knockdown of Bak significantly reduces BV6/Dexa-induced loss of MMP and delays cell death, but not ROS production, whereas ROS scavengers attenuate Bak activation, indicating that ROS production occurs upstream of BV6/Dexa-mediated Bak activation. Consistently, BV6/Dexa treatment causes oxidative thiol modifications of Bak protein. Intriguingly, knockdown or knockout of RIP3 or MLKL protect ALL cells or MEFs from BV6/Dexa-induced ROS production, Bak activation, drop of MMP and disruption of mitochondrial respiration, demonstrating that these mitochondrial events depend on RIP3 and MLKL. Thus, mitochondria might serve as an amplification step in BV6/Dexa-induced necroptosis. These findings provide new insights into the role of mitochondrial dysfunctions during necroptosis and have important implications for the development of novel treatment approaches to overcome apoptosis resistance in ALL.

Off-target effect of the cPLA2α inhibitor pyrrophenone: Inhibition of calcium release from the endoplasmic reticulum

Cytosolic phospholipase A2α (cPLA2α) mediates agonist-induced release of arachidonic acid from membrane phospholipid for production of eicosanoids. The activation of cPLA2α involves increases in intracellular calcium, which binds to the C2 domain and promotes cPLA2α translocation from the cytosol to membrane to access substrate. The cell permeable pyrrolidine-containing cPLA2α inhibitors including pyrrophenone have been useful to understand cPLA2α function. Although this serine hydrolase inhibitor does not inhibit other PLA2s or downstream enzymes that metabolize arachidonic acid, we reported that it blocks increases in mitochondrial calcium and cell death in lung fibroblasts. In this study we used the calcium indicators G-CEPIA1er and CEPIA2mt to compare the effect of pyrrophenone in regulating calcium levels in the endoplasmic reticulum (ER) and mitochondria in response to A23187 and receptor stimulation. Pyrrophenone blocked calcium release from the ER and concomitant increases in mitochondrial calcium in response to stimulation by ATP, serum and A23187. In contrast, ER calcium release induced by the sarco/endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin was not blocked by pyrrophenone suggesting specificity for the calcium release pathway. As a consequence of blocking calcium mobilization, pyrrophenone inhibited serum-stimulated translocation of the cPLA2α C2 domain to Golgi. The ability of pyrrophenone to block ER calcium release is an off-target effect since it occurs in fibroblasts lacking cPLA2α. The results implicate a serine hydrolase in regulating ER calcium release and highlight the importance of careful dose-response studies with pyrrophenone to study cPLA2α function.

Cell death disguised: The mitochondrial permeability transition pore as the c-subunit of the F(1)F(O) ATP synthase

Ion transport across the mitochondrial inner and outer membranes is central to mitochondrial function, including regulation of oxidative phosphorylation and cell death. Although essential for ATP production by mitochondria, recent findings have confirmed that the c-subunit of the ATP synthase also houses a large conductance uncoupling channel, the mitochondrial permeability transition pore (mPTP), the persistent opening of which produces osmotic dysregulation of the inner mitochondrial membrane and cell death. This review will discuss recent advances in understanding the molecular components of mPTP, its regulatory mechanisms and how these contribute directly to its physiological as well as pathological roles.

Structural apelin analogues: mitochondrial ROS inhibition and cardiometabolic protection in myocardial ischaemia reperfusion injury

BACKGROUND AND PURPOSE

Mitochondria-derived oxidative stress is believed to be crucially involved in cardiac ischaemia reperfusion (I/R) injury, although currently no therapies exist that specifically target mitochondrial reactive oxygen species (ROS) production. The present study was designed to evaluate the potential effects of the structural analogues of apelin-12, an adipocyte-derived peptide, on mitochondrial ROS generation, cardiomyocyte apoptosis, and metabolic and functional recovery to myocardial I/R injury.

EXPERIMENTAL APPROACH

In cultured H9C2 cardiomyoblasts and adult cardiomyocytes, oxidative stress was induced by hypoxia reoxygenation. Isolated rat hearts were subjected to 35 min of global ischaemia and 30 min of reperfusion. Apelin-12, apelin-13 and structural apelin-12 analogues, AI and AII, were infused during 5 min prior to ischaemia.

KEY RESULTS

In cardiac cells, mitochondrial ROS production was inhibited by the structural analogues of apelin, AI and AII, in comparison with the natural peptides, apelin-12 and apelin-13. Treatment of cardiomyocytes with AI and AII decreased cell apoptosis concentration-dependently. In a rat model of I/R injury, pre-ischaemic infusion of AI and AII markedly reduced ROS formation in the myocardial effluent and attenuated cell membrane damage. Prevention of oxidative damage by AI and AII was associated with the improvement of functional and metabolic recovery after I/R in the heart.

CONCLUSIONS AND IMPLICATIONS

These data provide the evidence for the potential of the structural apelin analogues in selective reduction of mitochondrial ROS generation and myocardial apoptosis and form the basis for a promising therapeutic strategy in the treatment of oxidative stress-related heart disease.

Netrin-1 improves post-injury cardiac function in vivo via DCC/NO-dependent preservation of mitochondrial integrity, while attenuating autophagy

Reperfusion injury of the heart is a severe complication of angioplasty treatment of acute myocardial ischemia, for which no therapeutics are currently available. The present study aimed to identify whether and how a novel protein, netrin-1, induces cardioprotection in vivo during ischemia/reperfusion (I/R) injury. Wild type (WT) C57BL6/J mice were subjected to a 30 min coronary occlusion followed by a 24h reperfusion with vehicle (normal saline), netrin-1, UO126 (MEK1/2 inhibitor), PTIO (nitric oxide/NO scavenger), netrin-1/UO126 or netrin-1/PTIO intraventricularly. Some were injected of netrin-1 via tail vein. Netrin-1 at 5μg/kg induced a substantial reduction in infarct size (19.7 ± 5.0% from 41.3 ± 1.8% in the controls), and markedly improved cardiac function as measured by ejection fraction and fractional shortening from echocardiography. Experiments with mice deficient in netrin-1 receptor DCC (deleted in colorectal cancer, DCC+/-), or reperfusion with netrin-1/UO126 or netrin-1/PTIO, attenuated the protective effects of netrin-1, implicating intermediate roles of DCC, ERK1/2 and NO. Netrin-1 induced phosphorylation of ERK1/2 and eNOS was abolished in DCC+/-mice. Electron spin resonance (ESR) determination of NO production from isolated left ventricles demonstrated that netrin-1 improves NO bioavailability, which was attenuated by UO126 or in DCC+/-mice, suggesting upstream roles of DCC and ERK1/2 in NO production. Netrin-1 further reduced mitochondrial swelling and mitochondrial superoxide production, which was absent when co-treated with PTIO or UO126, or in DCC+/-mice, indicating critical roles of DCC, ERK1/2 and NO in preserving mitochondrial integrity. In a permanent coronary ligation model of myocardial infarction (MI) to assess post-MI remodeling, netrin-1 abolished the marked increase in autophagy. In summary, our data demonstrate robust cardioprotective effect of netrin-1 in vivo, as shown by reduced infarct size and improved cardiac function. Mechanistically, this protection is mediated by netrin-1 receptor DCC, and NO dependent preservation of mitochondria. This work clearly establishes a therapeutic potential of netrin-1 for acute treatment of MI, perhaps also for chronic post-MI remodeling. This article is part of a Special Issue entitled: Autophagy and protein quality control in cardiometabolic diseases.

The interplay between cell death signaling pathways in the heart

To date, one of the most intriguing and compelling concepts to impact contemporary cell biology is the notion that cell fate is "programmed" or genetically controlled. Indeed, the regulation of cell fate is crucial for embryonic development, and tissue homeostasis. Given the importance of removing damaged or irreversibly injured cells from the body, it is not surprising that defects in the regulatory mechanisms that govern cell death and/or survival more generally have been implicated in a number of human pathologies including cancer, neurodegenerative diseases, and cardiac failure. Several processes involved in the regulation of cell fate through apoptosis, necrosis, and autophagy are commonly linked through the actions of certain Bcl-2 proteins that act on the mitochondrion. For example, the Bcl-2 protein Beclin-1 is actively involved in the clearance of damaged mitochondria via mitophagy, while other Bcl-2 proteins such as Bax/Bak can initiate apoptosis or necrotic signaling pathways. The overlapping and redundant nature of these proteins highlights their evolutionary importance for regulating cardiac cell survival and death during normal and disease states. Here, we explore the interrelationship between these signaling pathways and the cellular effectors that influence cardiac cell fate.

Enhanced cell volume regulation: a key mechanism in local and remote ischemic preconditioning

We have previously shown that ischemic preconditioning (IPC) protection against necrosis in whole hearts and in both fresh and cultured cardiomyocytes, as well as the improved regulatory volume decrease to hypoosmotic swelling in cardiomyocytes, is abrogated through Cl− channel blockade, pointing to a role for enhanced cell volume regulation in IPC. To further define this cardioprotective mechanism, cultured rabbit ventricular cardiomyocytes were preconditioned either by 10-min simulated ischemia (SI) followed by 10-min simulated reperfusion (SR), by 10-min exposure/10-min washout of remote IPC (rIPC) plasma dialysate (from rabbits subjected to repetitive limb ischemia), or by adenoviral transfection with the constitutively active PKC-ε gene. These interventions were done before cardiomyocytes were subjected to either 60- or 75-min SI/60-min SR to assess cell necrosis (by trypan blue staining), 30-min SI to assess ischemic cell swelling, or 30-min hypoosmotic (200 mosM) stress to assess cell volume regulation. Necrosis after SI/SR and both SI- and hypoosmotic stress-induced swelling was reduced in preconditioned cardiomyocytes compared with control cardiomyocytes (neither preconditioned nor transfected). These effects on necrosis and cell swelling were blocked by either Cl− channel blockade or dominant negative knockdown of inwardly rectifying K+ channels with adenoviruses, suggesting that Cl− and K+ movements across the sarcolemma are critical for cell volume regulation and, thereby, cell survival under hypoxic/ischemic conditions. Our results define enhanced cell volume regulation as a key common mechanism of cardioprotection by preconditioning in cardiomyocytes.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Serine hydrolase inhibitors block necrotic cell death by preventing calcium overload of the mitochondria and permeability transition pore formation

Perturbation of calcium signaling that occurs during cell injury and disease, promotes cell death. In mouse lung fibroblasts A23187 triggered mitochondrial permeability transition pore (MPTP) formation, lactate dehydrogenase (LDH) release, and necrotic cell death that were blocked by cyclosporin A (CsA) and EGTA. LDH release temporally correlated with arachidonic acid release but did not involve cytosolic phospholipase A2α (cPLA2α) or calcium-independent PLA2. Surprisingly, release of arachidonic acid and LDH from cPLA2α-deficient fibroblasts was inhibited by the cPLA2α inhibitor pyrrophenone, and another serine hydrolase inhibitor KT195, by preventing mitochondrial calcium uptake. Inhibitors of calcium/calmodulin-dependent protein kinase II, a mitochondrial Ca(2+) uniporter (MCU) regulator, also prevented MPTP formation and arachidonic acid release induced by A23187 and H2O2. Pyrrophenone blocked MCU-mediated mitochondrial calcium uptake in permeabilized fibroblasts but not in isolated mitochondria. Unlike pyrrophenone, the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol and CsA blocked cell death and arachidonic acid release not by preventing mitochondrial calcium uptake but by inhibiting MPTP formation. In fibroblasts stimulated with thapsigargin, which induces MPTP formation by a direct effect on mitochondria, LDH and arachidonic acid release were blocked by CsA and 1-oleoyl-2-acetyl-sn-glycerol but not by pyrrophenone or EGTA. Therefore serine hydrolase inhibitors prevent necrotic cell death by blocking mitochondrial calcium uptake but not the enzyme releasing fatty acids that occurs by a novel pathway during MPTP formation. This work reveals the potential for development of small molecule cell-permeable serine hydrolase inhibitors that block MCU-mediated mitochondrial calcium overload, MPTP formation, and necrotic cell death.

Reduction of early reperfusion injury with the mitochondria-targeting peptide bendavia

We recently showed that Bendavia, a novel mitochondria-targeting peptide, reduced infarction and no-reflow across several experimental models. The purpose of this study was to determine the therapeutic timing and mechanism of action that underlie Bendavia's cytoprotective property. In rabbits exposed to in vivo ischemia/reperfusion (30/180 min), Bendavia administered 20 minutes prior to reperfusion (0.05 mg/kg/h, intravenously) reduced myocardial infarct size by ∼50% when administered for either 1 or 3 hours of reperfusion. However, when Bendavia perfusion began just 10 minutes after the onset of reperfusion, the protection against infarction and no-reflow was completely lost, indicating that the mechanism of protection is occurring early in reperfusion. Experiments in isolated mouse liver mitochondria found no discernible effect of Bendavia on blocking the permeability transition pore, and studies in isolated heart mitochondria showed no effect of Bendavia on respiratory rates. As Bendavia significantly lowered reactive oxygen species (ROS) levels in isolated heart mitochondria, the ROS-scavenging capacity of Bendavia was compared to well-known ROS scavengers using in vitro (cell-free) systems that enzymatically generate ROS. Across doses ranging from 1 nmol/L to 1 mmol/L, Bendavia showed no discernible ROS-scavenging properties, clearly differentiating itself from prototypical scavengers. In conclusion, Bendavia is a promising candidate to reduce cardiac injury when present at the onset of reperfusion but not after reperfusion has already commenced. Given that both infarction and no-reflow are related to increased cellular ROS, Bendavia's protective mechanism of action likely involves reduced ROS generation (as opposed to augmented scavenging) by endothelial and myocyte mitochondria.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Multiple interacting cell death mechanisms in the mediation of excitotoxicity and ischemic brain damage: a challenge for neuroprotection

There is currently no approved neuroprotective pharmacotherapy for acute conditions such as stroke and cerebral asphyxia. One of the reasons for this may be the multiplicity of cell death mechanisms, because inhibition of a particular mechanism leaves the brain vulnerable to alternative ones. It is therefore essential to understand the different cell death mechanisms and their interactions. We here review the multiple signaling pathways underlying each of the three main morphological types of cell death--apoptosis, autophagic cell death and necrosis--emphasizing their importance in the neuronal death that occurs during cerebral ischemia and hypoxia-ischemia, and we analyze the interactions between the different mechanisms. Finally, we discuss the implications of the multiplicity of cell death mechanisms for the design of neuroprotective strategies.

Enhanced cell-volume regulation in cyclosporin A cardioprotection

AIMS

Cyclosporin A (CsA) has been shown to protect against ischaemia/reperfusion injury presumably by its inhibition of mitochondrial permeability transition pore opening through cyclophilin D inhibition. We examine if CsA cardioprotection involves a cell-volume regulatory mechanism.

METHODS AND RESULTS

To address this issue, cultured rabbit cardiomyocytes were subjected to the following protocols: (i) cardiomyocytes were treated with 200 nM CsA either given for 10 min followed by 10 min of washout prior to 30 min hypo-osmotic stress (200 mOsm) or administered throughout 75 min simulated ischaemia/60 min simulated reperfusion. Cell necrosis and cell swelling were determined by trypan blue staining and cell-volume measurements, respectively; (ii) SPQ(6-methoxy-N-(3-sulfopropyl)quinolinium) dye loaded cardiomyocytes were treated with 200 nM CsA for 10 min followed by 10 min washout and intracellular Cl(-) concentration measured (Cl(-) efflux); (iii) 5,5',6,6'-tetrachloro-1,1',3,3'- tetraethylbenzimi-dazolylcarbocyanine iodide(JC-1) loaded cardiomyocytes were treated with 200 nM CsA to inhibit mitochondrial membrane potential (ΔΨm) dissipation (an index of mitochondria permeability transition pore opening) by either valinomycin (2 μM) or ischaemia/reperfusion injury. Cl(-) channels were blocked by indanyloxyacetic acid 94 (IAA-94, 50 μM). CsA not only significantly (P < 0.001) reduced the % of dead cells following simulated ischaemia/reperfusion but it also triggered an efflux of Cl(-), hence enhancing cardiomyocyte cell-volume regulatory response. CsA protection against cell necrosis and its effect on Cl(-) transport/volume regulation were all blocked by IAA-94. IAA-94 had no effect on ΔΨm.

CONCLUSION

These data indicate that CsA protects against cell necrosis at least in part by enhancing cardiomyocyte volume regulation, and not simply by inhibiting MPTP opening.

VDAC: old protein with new roles in diabetes

A decrease in capillary density due to an increase in endothelial cell apoptosis in the heart is implicated in cardiac ischemia in diabetes. The voltage-dependent anion channel (VDAC) plays a crucial role in the regulation of mitochondrial metabolic function and mitochondria-mediated apoptosis. This study is designed to examine the role of VDAC in coronary endothelial dysfunction in diabetes. Endothelial cells (ECs) were more apoptotic in diabetic left ventricle of diabetic mice and mouse coronary ECs (MCECs) isolated from diabetic mice exhibited significantly higher mitochondrial Ca(2+) concentration and VDAC protein levels than control MCECs. The expression of VDAC-short hairpin RNA (shRNA) not only decreased the resting mitochondrial Ca(2+) concentration but also attenuated mitochondrial Ca(2+) uptake in diabetic MCECs. Furthermore, the downregulation of VDAC in diabetic MCECs significantly decreased mitochondrial superoxide anion (O(2)(-)) production and the activity of the mitochondrial permeability transition pore (mPTP) opening (an indirect indicator of cell apoptosis) toward control levels. These data suggest that the increased VDAC level in diabetic MCECs is responsible for increased mitochondrial Ca(2+) concentration, mitochondrial O(2)(-) production, and mPTP opening activity. Normalizing VDAC protein level may help to decrease endothelial cell apoptosis, increase capillary density in the heart, and subsequently decrease the incidence of cardiac ischemia in diabetes.

Mitochondrial function in vascular endothelial cell in diabetes

Micro- and macrovascular complications are commonly seen in diabetic patients and endothelial dysfunction contributes to the development and progression of the complications. Abnormal functions in endothelial cells lead to the increase in vascular tension and atherosclerosis, followed by systemic hypertension as well as increased incidence of ischemia and stroke in diabetic patients. Mitochondria are organelles serving as a source of energy production and as regulators of cell survival (e.g., apoptosis and cell development) and ion homeostasis (e.g., H(+), Ca(2+)). Endothelial mitochondria are mainly responsible for generation of reactive oxygen species (ROS) and maintaining the Ca(2+) concentration in the cytosol. There is increasing evidence that mitochondrial morphological and functional changes are implicated in vascular endothelial dysfunction. Enhanced mitochondrial fission and/or attenuated fusion lead to mitochondrial fragmentation and disrupt the endothelial physiological function. Abnormal mitochondrial biogenesis and disturbance of mitochondrial autophagy increase the accumulation of damaged mitochondria, such as irreversibly depolarized or leaky mitochondria, and facilitate cell death. Augmented mitochondrial ROS production and Ca(2+) overload in mitochondria not only cause the maladaptive effect on the endothelial function, but also are potentially detrimental to cell survival. In this article, we review the physiological and pathophysiological role of mitochondria in endothelial function with special focus on diabetes.

Reduction of ischemia/reperfusion injury with bendavia, a mitochondria-targeting cytoprotective Peptide

Background

Manifestations of reperfusion injury include myocyte death leading to infarction, contractile dysfunction, and vascular injury characterized by the “no-reflow” phenomenon. Mitochondria-produced reactive oxygen species are believed to be centrally involved in each of these aspects of reperfusion injury, although currently no therapies reduce reperfusion injury by targeting mitochondria specifically.

Methods and Results

We investigated the cardioprotective effects of a mitochondria-targeted peptide, Bendavia (Stealth Peptides), across a spectrum of experimental cardiac ischemia/reperfusion models. Postischemic administration of Bendavia reduced infarct size in an in vivo sheep model by 15% (P=0.02) and in an ex vivo guinea pig model by 38% to 42% (P<0.05). In an in vivo rabbit model, the extent of coronary no-reflow was assessed with Thioflavin S staining and was significantly smaller in the Bendavia group for any given ischemic risk area than in the control group (P=0.0085). Myocardial uptake of Bendavia was ≈25% per minute, and uptake remained consistent throughout reperfusion. Postischemic recovery of cardiac hemodynamics was not influenced by Bendavia in any of the models studied. Isolated myocytes exposed to hypoxia/reoxygenation showed improved survival when treated with Bendavia. This protection appeared to be mediated by lowered reactive oxygen species–mediated cell death during reoxygenation, associated with sustainment of mitochondrial membrane potential in Bendavia-treated myocytes.

Conclusions

Postischemic administration of Bendavia protected against reperfusion injury in several distinct models of injury. These data suggest that Bendavia is a mitochondria-targeted therapy that reduces reperfusion injury by maintaining mitochondrial energetics and suppressing cellular reactive oxygen species levels. (J Am Heart Assoc. 2012;1:e001644 doi: 10.1161/JAHA.112.001644.)

Novel Ser/Thr protein phosphatases in cell death regulation

Cell death is regulated by a myriad of intracellular molecular pathways, with many involving protein phosphorylation and dephosphorylation. In this review, we will focus on Ser/Thr phosphatases-mediated regulation in cell apoptosis as well as on their potential roles in cell necrosis. The emerging functional importance of Ser/Thr protein phosphatases in cell death regulation adds new dimension to the signaling mechanisms of cellular function, physiology, and diseases.

The role of VDAC in cell death: friend or foe?

As the voltage-dependent anion channel (VDAC) forms the interface between mitochondria and the cytosol, its importance in metabolism is well understood. However, research on VDAC's role in cell death is a rapidly growing field, unfortunately with much confusing and contradictory results. The fact that VDAC plays a role in outer mitochondrial membrane permeabilization is undeniable, however, the mechanisms behind this remain very poorly understood. In this review, we will summarize the studies that show evidence of VDAC playing a role in cell death. To begin, we will discuss the evidence for and against VDAC's involvement in mitochondrial permeability transition (MPT) and attempt to clarify that VDAC is not an essential component of the MPT pore (MPTP). Next, we will evaluate the remaining literature on VDAC in cell death which can be divided into three models: proapoptotic agents escaping through VDAC, VDAC homo- or hetero-oligomerization, or VDAC closure resulting in outer mitochondrial membrane permeabilization through an unknown pathway. We will then discuss the growing list of modulators of VDAC activity that have been associated with induction/protection against cell death. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Ranolazine reduces Ca2+ overload and oxidative stress and improves mitochondrial integrity to protect against ischemia reperfusion injury in isolated hearts

Ranolazine is a clinically approved drug for treating cardiac ventricular dysrhythmias and angina. Its mechanism(s) of protection is not clearly understood but evidence points to blocking the late Na+ current that arises during ischemia, blocking mitochondrial complex I activity, or modulating mitochondrial metabolism. Here we tested the effect of ranolazine treatment before ischemia at the mitochondrial level in intact isolated hearts and in mitochondria isolated from hearts at different times of reperfusion. Left ventricular (LV) pressure (LVP), coronary flow (CF), and O2 metabolism were measured in guinea pig isolated hearts perfused with Krebs-Ringer's solution; mitochondrial (m) superoxide (O2·-), Ca2+, NADH/FAD (redox state), and cytosolic (c) Ca2+ were assessed on-line in the LV free wall by fluorescence spectrophotometry. Ranolazine (5 μM), infused for 1 min just before 30 min of global ischemia, itself did not change O2·-, cCa2+, mCa2+ or redox state. During late ischemia and reperfusion (IR) O2·- emission and m[Ca2+] increased less in the ranolazine group vs. the control group. Ranolazine decreased c[Ca2+] only during ischemia while NADH and FAD were not different during IR in the ranolazine vs. control groups. Throughout reperfusion LVP and CF were higher, and ventricular fibrillation was less frequent. Infarct size was smaller in the ranolazine group than in the control group. Mitochondria isolated from ranolazine-treated hearts had mild resistance to permeability transition pore (mPTP) opening and less cytochrome c release than control hearts. Ranolazine may provide functional protection of the heart during IR injury by reducing cCa2+ and mCa2+ loading secondary to its effect to block the late Na+ current. Subsequently it indirectly reduces O2·- emission, preserves bioenergetics, delays mPTP opening, and restricts loss of cytochrome c, thereby reducing necrosis and apoptosis.

Mitochondrial permeability transition in Ca(2+)-dependent apoptosis and necrosis

A variety of stimuli utilize an increase of cytosolic free Ca(2+) concentration as a second messenger to transmit signals, through Ca(2+) release from the endoplasmic reticulum or opening of plasma membrane Ca(2+) channels. Mitochondria contribute to the tight spatiotemporal control of this process by accumulating Ca(2+), thus shaping the return of cytosolic Ca(2+) to resting levels. The rise of mitochondrial matrix free Ca(2+) concentration stimulates oxidative metabolism; yet, in the presence of a variety of sensitizing factors of pathophysiological relevance, the matrix Ca(2+) increase can also lead to opening of the permeability transition pore (PTP), a high conductance inner membrane channel. While transient openings may serve the purpose of providing a fast Ca(2+) release mechanism, persistent PTP opening is followed by deregulated release of matrix Ca(2+), termination of oxidative phosphorylation, matrix swelling with inner membrane unfolding and eventually outer membrane rupture with release of apoptogenic proteins and cell death. Thus, a rise in mitochondrial Ca(2+) can convey both apoptotic and necrotic death signals by inducing opening of the PTP. Understanding the signalling networks that govern changes in mitochondrial free Ca(2+) concentration, their interplay with Ca(2+) signalling in other subcellular compartments, and regulation of PTP has important implications in the fine comprehension of the main biological routines of the cell and in disease pathogenesis.