The permeability transition pore opening in intact mitochondria and submitochondrial particles

The mitochondrial permeability transition: a current perspective on its identity and role in ischaemia/reperfusion injury

The mitochondrial permeability transition pore (MPTP) is a non-specific pore that opens in the inner mitochondrial membrane (IMM) when matrix [Ca(2+)] is high, especially when accompanied by oxidative stress, high [Pi] and adenine nucleotide depletion. Such conditions occur during ischaemia and subsequent reperfusion, when MPTP opening is known to occur and cause irreversible damage to the heart. Matrix cyclophilin D facilitates MPTP opening and is the target of its inhibition by cyclosporin A that is cardioprotective. Less certainty exists over the composition of the pore itself, with structural and/or regulatory roles proposed for the adenine nucleotide translocase, the phosphate carrier and the FoF1 ATP synthase. Here we critically review the supporting data for the role of each and suggest that they may interact with each other through their bound cardiolipin to form the ATP synthasome. We propose that under conditions favouring MPTP opening, calcium-triggered conformational changes in these proteins may perturb the interface between them generating the pore. Proteins associated with the outer mitochondrial membrane (OMM), such as members of the Bcl-2 family and hexokinase (HK), whilst not directly involved in pore formation, may regulate MPTP opening through interactions between OMM and IMM proteins at "contact sites". Recent evidence suggests that cardioprotective protocols such as preconditioning inhibit MPTP opening at reperfusion by preventing the loss of mitochondrial bound HK2 that stabilises these contact sites. Contact site breakage both sensitises the MPTP to [Ca(2+)] and facilitates cytochrome c loss from the intermembrane space leading to greater ROS production and further MPTP opening. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

Release of NO by a nitrosyl complex upon activation by the mitochondrial reducing power

The reaction of trans-[Ru(NH(3))(4)P(OEt)(3)NO](3+) and mitochondria was investigated through differential pulse polarography and fluorimetry. The nitrosyl complex undergoes one-electron reduction centered on the NO ligand site. The reaction between the mitochondrial reductor and trans-[Ru(NH(3))(4)P(OEt)(3)NO](3+) exhibits a second order specific rate constant calculated as k=2 x 10(1) M(-1) s(-1). The reduced species, trans-[Ru(NH(3))(4)P(OEt)(3)NO](2+), quickly releases NO, yielding trans-[Ru(NH(3))(4)P(OEt)(3)H(2)O](2+). The low toxicities of both trans-[Ru(NH(3))(4)P(OEt)(3)(NO)](2+) and trans-[Ru(NH(3))(4)P(OEt)(3)H(2)O](2+) and its ability to release NO after reductive activation in a biological medium make the nitrosyl compound a useful model of a hypotensive drug.

Coexistence of translocated cytochrome c and nitrated protein in neurons of the rat cerebral cortex after oxygen and glucose deprivation

Changes in the distribution of immunoreactive cytochrome c and protein nitration were studied in the rat cerebral cortex after oxygen and glucose deprivation by bright field, confocal and electron microscopy. In control cerebral cortex, nitrotyrosine immunoreactivity indicating protein nitration was found mostly in the neuronal nuclear region, with only a small amount distributed in the cytosol, whereas cytochrome c immunoreactivity was found at the inner membrane and in the intermembrane space of the mitochondria. During the recovery phase after oxygen and glucose deprivation, cytochrome c immunoreactivity was released from the intermembrane space of swollen mitochondria into the surrounding cytosol. The cytosol now also displayed nitrotyrosine immunoreactivity, which had diminished in the nuclear region. Both immunoreactivities were dispersed throughout the soma and processes of the cortical neurons. These changes were largely prevented by the administration of cyclosporin A, which inhibits both the mitochondrial permeability transition and the neuronal isoform of nitric oxide synthase while blocking the induction of the inducible isoform. Ischemia/reperfusion injury increases the production of nitric oxide, reactive oxygen species and intracellular factors that damage the mitochondria and liberate apoptotic factors. We suggest that translocation of cytochrome c from the mitochondria to the cytosol, which has been shown to precede the mitochondrial permeability transition, could result from peroxynitrite-mediated nitration. This phenomenon is attenuated by cyclosporin A administration, suggesting a neuroprotective role for this agent.

Reduced mitochondrial membrane potential and altered responsiveness of a mitochondrial membrane megachannel in p53-induced senescence

There is accumulating evidence that mitochondrial membrane potential (DeltaPsi(M)) is reduced in aged cells. In addition, a decrease of DeltaPsi(M) has been shown to be an early event in many forms of apoptosis. Here we use a mitochondrial potentiometric dye with in situ laser scanning confocal microscopic (LSCM) imaging to demonstrate that DeltaPsi(M) is dramatically decreased in both the p53-overexpressing, senescent EJ tumor cells and in pre-apoptotic PC12 cells compared to controls. Treatment with cyclosporin A (CSA), which facilitates closure of the mitochondrial permeability transition pore (PTP), was able to reverse the decrease in DeltaPsi(M) in pre-apoptotic PC12 cells but not in the senescent EJ-p53 cells. The capacity to prevent dissipation of DeltaPsi(M) in response to agents that facilitate PTP closure may differentiate cells entering apoptosis from those participating in senescence. Therefore, regulation of the closure of the mitochondrial PTP in the presence of decreased DeltaPsi(M) may be a decisional checkpoint in distinguishing between growth arrest pathways.

Glucose protection from MPP+-induced apoptosis depends on mitochondrial membrane potential and ATP synthase

MPP+ inhibits mitochondrial complex I and alpha-ketoglutarate dehydrogenase causing necrosis or apoptosis of catecholaminergic neurons. Low glucose levels or glycolytic blockade has been shown to potentiate MPP+ toxicity. We found that MPP+ caused concentration-dependent apoptosis of neuronally differentiated PC12 cells and that glucose, but not pyruvate, supplementation reduced apoptosis. Oligomycin concentrations sufficient to inhibit ATP synthase blocked the decreased apoptosis afforded by glucose supplementation. Laser-scanning confocal microscope imaging of chloromethyl-tetramethylrosamine methyl ester fluorescence to estimate DeltaPsiM showed that MPP+ and atractyloside reduced DeltaPsiM, while cyclosporin A (CSA) and glucose supplementation reversed decreases in DeltaPsiM caused by MPP+. Oligomycin blocked the effect of glucose supplementation on DeltaPsiM. These findings show that (i) MPP+-induced and atractyloside-induced apoptosis are associated with reduced DeltaPsiM; (ii) CSA maintains DeltaPsiM and reduces MPP+-induced apoptosis; and (iii) glucose supplementation maintains DeltaPsiM, likely by glycolytic ATP-dependent proton pumping at ATP synthase and reduces MPP+-induced apoptosis.

Mitochondrial swelling and oxygen consumption during respiratory state 4 induced by phospholipase A2 isoforms isolated from the South American rattlesnake (Crotalus durissus terrificus) venom

The non-covalent interaction between two molecular entities namely, phospholipase A2 and crotapotin, results in the main toxin, crotoxin, present in the venom of the South American rattlesnake Crotalus durissus terrificus. High performance liquid chromatography has enabled us the isolation of three phospholipase A2 isoforms (F1, F2 and F3), characterized through denaturing and non-denaturing polyacrylamide gel electrophoresis and also through the N-terminal amino acid sequence analysis. The effect of each purified phospholipase A2 isoform on isolated rat liver mitochondria was determined through mitochondrial swelling and O2 consumption during respiratory state 4. F1 showed a dose-dependent stimulation of O2 consumption while F2 and F3 caused stimulation only at low doses and inhibition at high amounts. These effects were completely suppressed by the presence of 0.1% bovine serum albumin or 0.5 mM EGTA in the incubation medium. Taking the mitochondrial swelling as an activity parameter, all of them presented the same behaviour at different intensities, leading to permeabilization of the mitochondrial membrane. In this case, addition of EGTA prevented it whereas bovine serum albumin was ineffective, indicating that the lipid microenvironment was affected. These results suggest that free fatty acids are directly responsible for the observed effects induced by phospholipase A2 isoforms on oxygen consumption experiments. The protection conferred by cyclosporin-A on swelling induced by the isoforms, when present in low concentrations, may suggest that cyclosporin-A binds to a mitochondrial membrane site protecting the membrane against the phospholipase A2 attack.