Outer mitochondrial membrane permeabilization: an open-and-shut case?

Deciphering mitochondrial dysfunction: Pathophysiological mechanisms in vascular cognitive impairment

Vascular cognitive impairment (VCI) encompasses a range of cognitive deficits arising from vascular pathology. The pathophysiological mechanisms underlying VCI remain incompletely understood; however, chronic cerebral hypoperfusion (CCH) is widely acknowledged as a principal pathological contributor. Mitochondria, crucial for cellular energy production and intracellular signaling, can lead to numerous neurological impairments when dysfunctional. Recent evidence indicates that mitochondrial dysfunction-marked by oxidative stress, disturbed calcium homeostasis, compromised mitophagy, and anomalies in mitochondrial dynamics-plays a pivotal role in VCI pathogenesis. This review offers a detailed examination of the latest insights into mitochondrial dysfunction within the VCI context, focusing on both the origins and consequences of compromised mitochondrial health. It aims to lay a robust scientific groundwork for guiding the development and refinement of mitochondrial-targeted interventions for VCI.

Current Knowledge on the Role of Cardiolipin Remodeling in the Context of Lipid Oxidation and Barth Syndrome

Barth syndrome (BTHS, OMIM 302060) is a genetic disorder caused by variants of the TAFAZZIN gene (G 4.5, OMIM 300394). This debilitating disorder is characterized by cardio- and skeletal myopathy, exercise intolerance, and neutropenia. TAFAZZIN is a transacylase that catalyzes the second step in the cardiolipin (CL) remodeling pathway, preferentially converting saturated CL species into unsaturated CLs that are susceptible to oxidation. As a hallmark mitochondrial membrane lipid, CL has been shown to be essential in a myriad of pathways, including oxidative phosphorylation, the electron transport chain, intermediary metabolism, and intrinsic apoptosis. The pathological severity of BTHS varies substantially from one patient to another, even in individuals bearing the same TAFAZZIN variant. The physiological modifier(s) leading to this disparity, along with the exact molecular mechanism linking CL to the various pathologies, remain largely unknown. Elevated levels of reactive oxygen species (ROS) have been identified in numerous BTHS models, ranging from yeast to human cell lines, suggesting that cellular ROS accumulation may participate in the pathogenesis of BTHS. Although the exact mechanism of how oxidative stress leads to pathogenesis is unknown, it is likely that CL oxidation plays an important role. In this review, we outline what is known about CL oxidation and provide a new perspective linking the functional relevance of CL remodeling and oxidation to ROS mitigation in the context of BTHS.

Mitochondrial dysfunction: A potential target for Alzheimer's disease intervention and treatment

Alzheimer's disease (AD) is an irreversible neurodegenerative brain disorder which manifests as a progressive decline in cognitive function. Mitochondrial dysfunction plays a critical role in the early stages of AD, and advances the progression of this age-related neurodegenerative disorder. Therefore, it can be a potential target for interventions to treat AD. Several therapeutic strategies to target mitochondrial dysfunction have gained significant attention in the preclinical stage, but the clinical trials performed to date have shown little progress. Thus, we discuss the mechanisms and strategies of different therapeutic agents for targeting mitochondrial dysfunction in AD. We hope that this review will inspire and guide the development of efficient AD drugs in the future.

Polysaccharide from Angelica sinensis attenuates SNP-induced apoptosis in osteoarthritis chondrocytes by inducing autophagy via the ERK1/2 pathway

OBJECTIVE

Chondrocyte apoptosis plays a vital role in osteoarthritis (OA) progression. Angelica sinensis polysaccharide (ASP), a traditional Chinese medicine, possesses anti-inflammatory and anti-apoptotic properties in chondrocytes. This study aimed to determine the protective role of ASP on sodium nitroprusside (SNP)-induced chondrocyte apoptosis, and explore the underlying mechanism.

METHOD

Human primary chondrocytes isolated from the articular cartilage of OA patients were treated with SNP alone or in combination with different doses of ASP. Cell viability and apoptosis were assessed, and apoptosis-related proteins including Bcl-2 and Bax were detected. Autophagy levels were evaluated by light chain 3 (LC3) II immunofluorescence staining, mRFP-GFP-LC3 fluorescence localization, and western blot (LC3II, p62, Beclin-1, Atg5). Meanwhile, activation of the ERK 1/2 pathway was determined by western blot. The autophagy inhibitors, 3-methyladenine (3-MA), chloroquine (CQ), and a specific inhibitor of ERK1/2, SCH772984, were used to confirm the autophagic effect of ASP.

RESULTS

The results showed that SNP-induced chondrocyte apoptosis was significantly rescued by ASP, whereas ASP alone promoted chondrocyte proliferation. The anti-apoptotic effect of ASP was related to the enhanced autophagy and depended on the activation of the ERK1/2 pathway.

CONCLUSION

ASP markedly rescued SNP-induced apoptosis by activating ERK1/2-dependent autophagy in chondrocytes, and it made ASP as a potential therapeutic supplementation for OA treatment.

Electroacupuncture inhibits sodium nitroprusside‑mediated chondrocyte apoptosis through the mitochondrial pathway

In China, electroacupuncture (EA) is a therapeutic method that is extensively applied in the clinical treatment of osteoarthritis (OA); however, the underlying molecular mechanism remains unclear. Chondrocyte apoptosis may be observed in cartilage tissue in OA, and is often considered a key target for the treatment of this condition. Therefore, the present study aimed to determine the effects of EA on sodium nitroprusside (SNP)-induced chondrocyte apoptosis. Chondrocytes were obtained from the knee joints of Sprague Dawley rats by type II collagenase digestion. Following microscopic observation and authentication with type II collagen immunohistochemistry, articular cartilage cells were used in subsequent experiments. Using inverted phase contrast microscopy, DAPI staining and flow cytometry, it was revealed that chondrocytes treated with SNP became apoptotic, whereas EA inhibited SNP-induced chondrocyte apoptosis. Subsequently, JC-1 single staining, reverse transcription-quantitative polymerase chain reaction analysis, western blotting, colorimetric assays and immunofluorescence staining were performed for further investigation. The results demonstrated that, when compared with normal chondrocytes, the mitochondrial membrane potential of SNP-treated chondrocytes was markedly lowered, B-cell lymphoma 2 (Bcl-2) expression was reduced, and the expression levels of Bcl-2-associated X protein (Bax), cytochrome c, caspase-9 and caspase-3 were increased. Compared with in SNP-treated chondrocytes, the decrease in the mitochondrial membrane potential of chondrocytes treated with SNP and EA was smaller, Bcl-2 expression was increased, and the expression levels of Bax, cytochrome c, caspase-9 and caspase-3 were decreased following EA intervention. In conclusion, the present study demonstrated that EA modulated the mitochondrial pathway to suppress SNP-mediated chondrocyte apoptosis. Therefore, EA may be of value in the treatment of OA.

Mitochondrial swelling and incipient outer membrane rupture in preapoptotic and apoptotic cells

Outer mitochondrial membrane (OMM) rupture was first noted in isolated mitochondria in which the inner mitochondrial membrane (IMM) had lost its selective permeability. This phenomenon referred to as mitochondrial permeability transition (MPT) refers to a permeabilized inner membrane that originates a large swelling in the mitochondrial matrix, which distends the outer membrane until it ruptures. Here, we have expanded previous electron microscopic observations that in apoptotic cells, OMM rupture is not caused by a membrane stretching promoted by a markedly swollen matrix. It is shown that the widths of the ruptured regions of the OMM vary from 6 to 250 nm. Independent of the perforation size, herniation of the mitochondrial matrix appeared to have resulted in pushing the IMM through the perforation. A large, long focal herniation of the mitochondrial matrix, covered with the IMM, was associated with a rupture of the OMM that was as small as 6 nm. Contextually, the collapse of the selective permeability of the IMM may precede or follow the release of the mitochondrial proteins of the intermembrane space into the cytoplasm. When the MPT is a late event, exit of the intermembrane space proteins to the cytoplasm is unimpeded and occurs through channels that transverse the outer membrane, because so far, the inner membrane is impermeable. No channel within the outer membrane can expose to the cytoplasm a permeable inner membrane, because it would serve as a conduit for local herniation of the mitochondrial matrix. Anat Rec, 2012. © 2012 Wiley Periodicals, Inc.

Molecular mechanisms of apoptosis activation by heat shock in multidrug-resistant Chinese hamster cells

Multidrug resistance (MDR) is a major obstacle to the success of chemotherapy in cancer treatment and is associated with overexpression of P-glycoprotein. MDR cells, aside from resistance to chemotherapy, might also inhibit apoptosis at various levels in the death signaling pathways. Currently, hyperthermia is used in cancer treatment to sensitize tumor cells to radiation and/or chemotherapy. This study investigated the induction of death receptor and mitochondria-mediated signaling pathways of apoptosis by hyperthermia (41-43 degrees C) in MDR CHRC5 cells compared to drug-sensitive AuxB1 Chinese hamster ovary cells. In the receptor-mediated pathway, CHRC5 cells exhibited higher levels of c-FLIP and lower caspase 8 and caspase 10 activation in response to hyperthermia. In the mitochondria-mediated pathway of heat-induced apoptosis, CHRC5 cells showed higher mitochondrial levels of Bax and tBid, more pronounced mitochondrial membrane depolarization, and increased Apaf-1. Similar levels of caspase 3 activation and cleavage of caspase substrates occurred, showing that overall, CHRC5 cells are not resistant to hyperthermia-induced apoptosis compared to AuxB1 cells. This study reveals for the first time the molecular mechanisms of hyperthermia-induced apoptosis in MDR cells overexpressing P-glycoprotein. CHRC5 and AuxB1 cells showed similar clonogenic survival responses to heat, which implies that hyperthermia could be a promising strategy for eradicating MDR tumor cells in the clinic.

Apaf-1-deficient fog mouse cell apoptosis involves hypo-polarization of the mitochondrial inner membrane, ATP depletion and citrate accumulation

To explore how the intrinsic apoptosis pathway is controlled in the spontaneous fog (forebrain overgrowth) mutant mice with an Apaf1 splicing deficiency, we examined spleen and bone marrow cells from Apaf1(+/+) (+/+) and Apaf1(fog/fog) (fog/fog) mice for initiator caspase-9 activation by cellular stresses. When the mitochondrial inner membrane potential (Deltapsim) was disrupted by staurosporine, +/+ cells but not fog/fog cells activated caspase-9 to cause apoptosis, indicating the lack of apoptosome (apoptosis protease activating factor 1 (Apaf-1)/cytochrome c/(d)ATP/procaspase-9) function in fog/fog cells. However, when a marginal ( approximately 20%) decrease in Deltapsim was caused by hydrogen peroxide (0.1 mM), peroxynitritedonor 3-morpholinosydnonimine (0.1 mM) and UV-C irradiation (20 J/m(2)), both +/+ and fog/fog cells triggered procaspase-9 auto-processing and its downstream cascade activation. Supporting our previous results, procaspase-9 pre-existing in the mitochondria induced its auto-processing before the cytosolic caspase activation regardless of the genotypes. Cellular ATP concentration significantly decreased under the hypoactive Deltapsim condition. Furthermore, we detected accumulation of citrate, a kosmotrope known to facilitate procaspase-9 dimerization, probably due to a feedback control of the Krebs cycle by the electron transfer system. Thus, mitochondrial in situ caspase-9 activation may be caused by the major metabolic reactions in response to physiological stresses, which may represent a mode of Apaf-1-independent apoptosis hypothesized from recent genetic studies.

Interplay between bax, reactive oxygen species production, and cardiolipin oxidation during apoptosis

Bax/Bak activation and cardiolipin peroxidation are essential for cytochrome c release during apoptosis, yet, the link between them remains elusive. We report that sequence of events after exposure of mouse embryonic fibroblast (MEF) cells to actinomycin D followed the order: Bax translocation-->superoxide production-->cardiolipin peroxidation. Genetic ablation of Bax/Bak inhibited actinomycin D induced superoxide production and cardiolipin peroxidation. Rotenone caused robust superoxide generation but did not trigger cardiolipin peroxidation in Bax/Bak double knockout MEF cells. This suggests that, in addition to participating in ROS generation, Bax/Bak play another specific role in cardiolipin oxidation. In isolated mitochondria, recombinant Bax enhanced succinate induced cardiolipin oxidation and cytochrome c release. Mitochondrial peroxidase activity, likely involved in cardiolipin peroxidation, was enhanced upon incubation with recombinant Bax. Thus, cardiolipin peroxidation may be causatively and time-dependently related to Bax/Bak effects on ROS generation and peroxidase activation of cytochrome c.

The mitochondrial energy transduction system and the aging process

Aged mammalian tissues show a decreased capacity to produce ATP by oxidative phosphorylation due to dysfunctional mitochondria. The mitochondrial content of rat brain and liver is not reduced in aging and the impairment of mitochondrial function is due to decreased rates of electron transfer by the selectively diminished activities of complexes I and IV. Inner membrane H(+) impermeability and F(1)-ATP synthase activity are only slightly affected by aging. Dysfunctional mitochondria in aged rodents are characterized, besides decreased electron transfer and O(2) uptake, by an increased content of oxidation products of phospholipids, proteins and DNA, a decreased membrane potential, and increased size and fragility. Free radical-mediated oxidations are determining factors of mitochondrial dysfunction and turnover, cell apoptosis, tissue function, and lifespan. Inner membrane enzyme activities, such as those of complexes I and IV and mitochondrial nitric oxide synthase, decrease upon aging and afford aging markers. The activities of these three enzymes in mice brain are linearly correlated with neurological performance, as determined by the tightrope and the T-maze tests. The same enzymatic activities correlated positively with mice survival and negatively with the mitochondrial content of lipid and protein oxidation products. Conditions that increase survival, as vitamin E dietary supplementation, caloric restriction, high spontaneous neurological activity, and moderate physical exercise, ameliorate mitochondrial dysfunction in aged brain and liver. The pleiotropic signaling of mitochondrial H(2)O(2) and nitric oxide diffusion to the cytosol seems modified in aged animals and to contribute to the decreased mitochondrial biogenesis in old animals.

Sensitization for gamma-irradiation-induced apoptosis by second mitochondria-derived activator of caspase

Resistance to current treatment regimens, such as radiation therapy, remains a major concern in oncology and may be caused by defects in apoptosis programs. Because inhibitor of apoptosis proteins (IAPs), which are expressed at high levels in many tumors, block apoptosis at the core of the apoptotic machinery by inhibiting caspases, therapeutic modulation of IAPs could target a key control point in resistance. Here, we report for the first time that full-length or mature second mitochondria-derived activator of caspase (Smac), an inhibitor of IAPs, significantly enhanced gamma-irradiation-induced apoptosis and reduced clonogenic survival in neuroblastoma, glioblastoma, or pancreatic carcinoma cells. Notably, Smac had no effect on DNA damage/DNA repair, activation of nuclear factor-kappaB, up-regulation of p53 and p21 proteins, or cell cycle arrest following gamma-irradiation, indicating that Smac did not alter the initial damage and/or cellular stress response. Smac enhanced activation of caspase-2, caspase-3, caspase-8, and caspase-9, loss of mitochondrial membrane potential, and cytochrome c release on gamma-irradiation. Inhibition of caspases also blocked gamma-irradiation-induced mitochondrial perturbations, indicating that Smac facilitated caspase activation, which in turn triggered a mitochondrial amplification loop. Interestingly, mitochondrial perturbations were completely blocked by the broad-range caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone or the relatively selective caspase-2 inhibitor N-benzyloxycarbonyl-Val-Asp-Val-Ala-Asp-fluoromethylketone, whereas caspase-8 or caspase-3 inhibitors only inhibited the increased drop of mitochondrial membrane potential provided by Smac, suggesting that caspase-2 was acting upstream of mitochondria after gamma-irradiation. In conclusion, our findings provide evidence that targeting IAPs (e.g., by Smac agonists) is a promising strategy to enhance radiosensitivity in human cancers.

Cyclophilin D: knocking on death's door

Two recent genetic studies have identified a critical role for cyclophilin D, a component of the mitochondrial membrane permeability transition pore, in cell death induced by calcium, reactive oxygen species, and cardiac ischemia-reperfusion injury. Transgenic mice lacking cyclophilin D developed normally but showed reduced infarct size after coronary artery ligation and reperfusion. Cells from the knockout mice were resistant to death imposed by excess calcium and H2O2, but not to death from x-irradiation, staurosporine, tumor necrosis factor-alpha, or forced expression of proapoptotic proteins. These data raise questions about the relationship between apoptotic and necrotic cell death, and they also highlight cyclophilin D as a potential therapeutic target in myocardial infarction.

Mitochondrial Release of Pro-apoptotic Proteins

A key step in the initiation of apoptosis is the release from the mitochondrial intermembrane space of cytochrome c and other pro-apoptotic proteins such as Smac/DIABLO, Omi/HtrA2, apoptosis-inducing factor (AIF), and endonuclease G (EndoG). Discrepancies have arisen, however, as to whether all these proteins are released in different systems. Our results suggest that failure to observe cytochrome c release may be due to the use of different buffers because after permeabilization by caspase-8 cleaved human Bid (tBid), cytochrome c dissociation from mitochondria was highly dependent on ionic strength and required 50-80 mm KCl, NaCl, or LiCl. In addition, mitochondria isolated from apoptotic cells using low ionic strength buffer bound a greater proportion of endogenous cytochrome c. In contrast to cytochrome c, Smac/DIABLO and Omi/HtrA2 were released independent of ionic strength, and AIF and EndoG behaved as if they are exposed to the intermembrane space but tethered to or within the inner membrane. AIF and EndoG were also not released by active caspases, which suggests their involvement in apoptosis may be limited. In summary, whereas tBid permeabilizes the outer membrane to cytochrome c, Smac/DIABLO, and Omi/HtrA2, the release of cytochrome c during apoptosis will be underestimated unless sufficient ionic strength is maintained to overcome the electrostatic association of cytochrome c with membranes.

On the antioxidant mechanisms of Bcl-2: a retrospective of NF-kappaB signaling and oxidative stress

Antioxidant and prooxidant signaling pathways are emanating as major players in, and regulators of, cell death and apoptosis. Redox conception of the critical role of oxidative stress in determining cell fate is being established-a foundation that craves deeper than the basic understanding of physiochemical interactions to extend beyond that into the realms of deciphering the molecular codes implicated with apoptosis. The proto-oncogene Bcl-2 is no stranger being a major player and decoder in controlling apoptosis, ostensibly via the regulation of redox equilibrium and disequilibrium. One of those potential mechanisms exhibited by Bcl-2 is its ability to counteract the detrimental effects of cell damage caused by free radicals, thereby gaining its well-known property of being an antioxidant. But the question is: what are the molecular mechanisms involved with the antioxidant role of Bcl-2 in the face of cell damage and apoptosis? Currently, a stance is being upheld in that the Bcl-2 antioxidant efficacy should be weighed against its ability to manipulate transcriptional control, through the regulation of specific transcription factors. NF-kappaB is no doubt one of the best candidates when it comes to the arena of oxidative stress, inflammation, and apoptosis. Therein, current themes in the burgeoning antioxidant role of Bcl-2 are exposed within the context of transcriptional control of NF-kappaB, thereby holding potential avenues for alleviating therapeutic approaches in the regulation of apoptosis.

Styrene 7,8-oxide induces mitochondrial damage and oxidative stress in neurons

Styrene 7,8-oxide (SO) is the main metabolite of styrene, a neurotoxic compound used industrially. Neurons exposed to SO undergo apoptosis with characteristic features including chromatin rearrangements and caspase activation. We report that the execution phase of apoptosis induced by SO (0.3 mM) in SK-N-MC neurons is triggered by translocation of apoptogenic factors (e.g., cytochrome c) into the cytosol. In addition, mitochondria exhibit lower Ca2+ capacity and loss of mitochondrial membrane potential (DeltaPsi). Lipid peroxidation, measured as thiobarbituric acid reactive substances (TBARS), is increased after 12 h. Pre-treatment with the antioxidant MnTBAP (100 microM) prevents the decrease of Ca2+ capacity, cytochrome c release, activation of caspases, exposure of phosphatidylserine and cell death. Hence, the neurotoxic effects of SO are related to mitochondrial damage and oxidative stress.

In situ imaging of mitochondrial outer-membrane pores using atomic force microscopy

Here we describe a technique for imaging of the outer contours of the mitochondrial membrane using atomic force microscopy, subsequent to or during a toxic or metabolic challenge. Pore formation in both glucose-challenged and 1,3-dinitrobenzene (DNB)-challenged mitochondria was observed using this technique. Our approach enables quantification of individual mitochondrial membrane pore formations. With this work, we have produced some of the highest resolution images of the outer contours of the in situ mitochondrial membrane published to date. These are potentially the first images of the component protein clusters at the time of formation of the mitochondrial membrane transition pore in situ. With the current work, we have extended the application of atomic force microscopy of mitochondrial membranes to fluid imaging. We have also begun to correlate 3-D surface features of mitochondria dotted with open membrane pores with features previously viewed with electron microscopy (EM) of fixed sections.

Mitochondrial cytochrome c release may occur by volume-dependent mechanisms not involving permeability transition

The mechanisms regulating mitochondrial outer-membrane permeabilization and the release of cytochrome c during apoptosis remain controversial. In the present study, we show in an in vitro model system that the release of cytochrome c may occur via moderate modulation of mitochondrial volume, irrespective of the mechanism leading to the mitochondrial swelling. In contrast with mitochondrial permeability transition-dependent release of cytochrome c, in the present study mitochondria remain intact and functionally active.

Dimerization and Processing of Procaspase-9 by Redox Stress in Mitochondria

We studied the mechanism of intra-mitochondrial death initiator caspase-9 activation by a redox response, in which hydrogen peroxide (H(2)O(2)) caused a subtle decrease in the inner membrane potential (Deltapsim) with little evidence of cytochrome c release. Initiation of the intra-mitochondrial autocleavage of procaspase-9 preceded the onset of caspase cascade induction in the cytosol. Purified mitochondria demonstrated procaspase-9 processing and releasing abilities when exposed to H(2)O(2). Bcl-2 overexpression caused accumulation of the active form caspase-9 in the mitochondria, rendering the cells resistant to the redox stress. Intriguingly, disulfide-bonded dimers of autoprocessed caspase-9 were generated in the mitochondria in the pre-apoptotic phase. Using a substrate-analog inhibitor, dimer formation of procaspase-9 was also detectable inside the mitochondria. Furthermore, thiol reductant thioredoxin blocked the caspase-9 activation step and the cell death induction. Thus, redox stress-responsive thiol-disulfide converting reactions in the mitochondrion seemed to mediate procaspase-9 assembly that allows autoprocessing. This study offers an explanation for the recent observation that Apaf-1-null cells can execute apoptosis, which can be blocked by Bcl-2, and supports the proposition that the cytochrome c-Apaf-1-procaspase-9 complex functions in the caspase amplification rather than in its initiation.