Mitochondrial precursor signal peptide induces a unique permeability transition and release of cytochrome c from liver and brain mitochondria

Amyloid β, α-synuclein and the c subunit of the ATP synthase: Can these peptides reveal an amyloidogenic pathway of the permeability transition pore?

Mitochondrial Permeability Transition (PT) is a phenomenon of increased permeability of the inner mitochondrial membrane in response to high levels of Ca²⁺ and/or reactive oxygen species (ROS) in the matrix. PT occurs upon the opening of a pore, namely the permeability transition pore (PTP), which dissipates the membrane potential uncoupling the respiratory chain. mPT activation and PTP formation can occur through multiple molecular pathways. The specific focus of this review is to discuss the possible molecular mechanisms of PTP that involve the participation of mitochondrially targeted amyloid peptides Aβ, α-synuclein and c subunit of the ATP synthase (ATPase). As activators of PTP, amyloid peptides are uniquely different from other activators because they are capable of forming channels in lipid bilayers. This property rises the possibility that in this permeabilization pathway the formation of the channel involves the direct participation of peptides, making it uniquely different from other PTP induction mechanisms. In this pathway, a critical step of PTP activation involves the import of amyloidogenic peptides from the cytosol into the matrix. In the matrix these peptides, which would fold into α-helical structure in native conditions, interact with cyclophilin D (CypD) and upon stimulation by elevated ROS and/or the Ca²⁺ spontaneously misfold into β-sheet ion conducting pores, causing PTP opening.

Sevoflurane postconditioning protects against myocardial ischemia/reperfusion injury by restoring autophagic flux via an NO-dependent mechanism

Volatile anesthetics improve postischemic cardiac function and reduce infarction even when administered for only a brief time at the onset of reperfusion. A recent study showed that sevoflurane postconditioning (SPC) attenuated myocardial reperfusion injury, but the underlying mechanisms remain unclear. In this study, we examined the effects of sevoflurane on nitric oxide (NO) release and autophagic flux during the myocardial ischemia/reperfusion (I/R) injury in rats in vivo and ex vivo. Male rats were subjected to 30 min ischemia and 2 h reperfusion in the presence or absence of sevoflurane (1.0 minimum alveolar concentration) during the first 15 min of reperfusion. We found that SPC significantly improved hemodynamic performance after reperfusion, alleviated postischemic myocardial infarction, reduced nicotinamide adenine dinucleotide content loss, and cytochrome c release in heart tissues. Furthermore, SPC significantly increased the phosphorylation of endothelial nitric oxide synthase (NOS) and neuronal nitric oxide synthase, and elevated myocardial NOS activity and NO production. All these effects were abolished by treatment with an NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg, i.v.). We also observed myocardial I/R-induced accumulation of autophagosomes in heart tissues, as evidenced by increased ratios of microtubule-associated protein 1 light chain 3 II/I, up-regulation of Beclin 1 and P62, and reduced lysosome-associated membrane protein-2 expression. SPC significantly attenuated I/R-impaired autophagic flux, which were blocked by L-NAME. Moreover, pretreatment with the autophagic flux blocker chloroquine (10 mg/kg, i.p.) increased autophagosome accumulation in SPC-treated heart following I/R and blocked SPC-induced cardioprotection. The same results were also observed in a rat model of myocardial I/R injury ex vivo, suggesting that SPC protects rat hearts against myocardial reperfusion injury by restoring I/R-impaired autophagic flux via an NO-dependent mechanism.

The still uncertain identity of the channel-forming unit(s) of the mitochondrial permeability transition pore

Mitochondria from different organisms can undergo a sudden process of inner membrane unselective leakiness to molecules known as the mitochondrial permeability transition (MPT). This process has been studied for nearly four decades and several proteins have been claimed to constitute, or at least regulate the usually inactive pore responsible for this transition. However, no protein candidate proposed as the actual pore-forming unit has passed rigorous gain- or loss-of-function genetic tests. Here we review evidence for -and against- putative channel-forming components of the MPT pore. We conclude that the structure of the MPT pore still remains largely undefined and suggest that future studies should follow established technical considerations to unambiguously consolidate the channel forming constituent(s) of the MPT pore.

In silico analysis and experimental validation of lipoprotein and novel Tat signal peptides processing in Anabaena sp. PCC7120

Signal peptide (SP) plays a pivotal role in protein translocation. Lipoprotein- and twin arginine translocase (Tat) dependent signal peptides were studied in All3087, a homolog of competence protein of Synechocystis PCC6803 and in two putative alkaline phosphatases (ALPs, Alr2234 and Alr4976), respectively. In silico analysis of All3087 is shown to possess the characteristics feature of competence proteins such as helix-hairpin-helix, N and C-terminal HKD endonuclease domain, calcium binding domain and N-terminal lipoprotein signal peptide. The SP recognition-cleavage site in All3087 was predicted (AIA-AC) using SignalP while further in-depth analysis using Pred-Lipo and WebLogo analysis for consensus sequence showed it as IAA-C. Activities of putative ALPs were confirmed by heterologous overexpression, activity assessment and zymogram analysis. ALP activity in Anabaena remains cell bound in log-phase, but during late log/stationary phase, an enhanced ALP activity was detected in extracellular milieu. The enhancement of ALP activity during stationary phase was not only due to inorganic phosphate limitation but also contributed by the presence of novel bipartite Tat-SP. The Tat signal transported the folded active ALPs to the membrane, followed by anchoring into the membrane and successive cleavage enabling transportation of the ALPs to the extracellular milieu, because of bipartite architecture and processing of transit Tat-SP.

Rapid, specific, no-wash, far-red fluorogen activation in subcellular compartments by targeted fluorogen activating proteins

Live cell imaging requires bright photostable dyes that can target intracellular organelles and proteins with high specificity in a no-wash protocol. Organic dyes possess the desired photochemical properties and can be covalently linked to various protein tags. The currently available fluorogenic dyes are in the green/yellow range where there is high cellular autofluorescence and the near-infrared (NIR) dyes need to be washed out. Protein-mediated activation of far-red fluorogenic dyes has the potential to address these challenges because the cell-permeant dye is small and nonfluorescent until bound to its activating protein, and this binding is rapid. In this study, three single chain variable fragment (scFv)-derived fluorogen activating proteins (FAPs), which activate far-red emitting fluorogens, were evaluated for targeting, brightness, and photostability in the cytosol, nucleus, mitochondria, peroxisomes, and endoplasmic reticulum with a cell-permeant malachite green analog in cultured mammalian cells. Efficient labeling was achieved within 20–30 min for each protein upon the addition of nM concentrations of dye, producing a signal that colocalized significantly with a linked mCerulean3 (mCer3) fluorescent protein and organelle specific dyes but showed divergent photostability and brightness properties dependent on the FAP. These FAPs and the ester of malachite green dye (MGe) can be used as specific, rapid, and wash-free labels for intracellular sites in live cells with far-red excitation and emission properties, useful in a variety of multicolor experiments.

The mitochondrial unselective channel in Saccharomyces cerevisiae

Opening of the mitochondrial permeability transition (MPT) pore mediates the increase in the unselective permeability to ions and small molecules across the inner mitochondrial membrane. MPT results from the opening of channels of unknown identity in mitochondria from plants, animals and yeast. However, the effectors and conditions required for MPT to occur in different species are remarkably disparate. Here we critically review previous and recent findings concerning the mitochondrial unselective channel of the yeast Saccharomyces cerevisiae to determine if it can be considered a counterpart of the mammalian MPT pore.

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.

Bid-induced mitochondrial membrane permeabilization waves propagated by local reactive oxygen species (ROS) signaling

Bid-induced mitochondrial membrane permeabilization and cytochrome c release are central to apoptosis. It remains a mystery how tiny amounts of Bid synchronize the function of a large number of discrete organelles, particularly in mitochondria-rich cells. Looking at cell populations, the rate and lag time of the Bid-induced permeabilization are dose-dependent, but even very low doses lead eventually to complete cytochrome c release. By contrast, individual mitochondria display relatively rapid and uniform kinetics, indicating that the dose dependence seen in populations is due to a spreading of individual events in time. We report that Bid-induced permeabilization and cytochrome c release regularly demonstrate a wave-like pattern, propagating through a cell at a constant velocity without dissipation. Such waves do not depend on caspase activation or permeability transition pore opening. However, reactive oxygen species (ROS) scavengers suppressed the coordination of cytochrome c release and also inhibited Bid-induced cell death, whereas both superoxide and hydrogen peroxide sensitized mitochondria to Bid-induced permeabilization. Thus, Bid engages a ROS-dependent, local intermitochondrial potentiation mechanism that amplifies the apoptotic signal as a wave.

Glucose metabolism determines resistance of cancer cells to bioenergetic crisis after cytochrome-c release

How can cancer cells survive the consequences of cyt-c release? Huber et al provide a quantitative analysis of the protective role of enhanced glucose utilization in cancer cells and investigate the impact of cell-to-cell heterogeneity in mitochondrial bioenergetics.

How can cancer cells survive the consequences of cyt-c release? Huber et al provide a quantitative analysis of the protective role of enhanced glucose utilization in cancer cells and investigate the impact of cell-to-cell heterogeneity in mitochondrial bioenergetics.

We combine ordinary differential equations based computational modelling, single-cell microscopy and in biochemistry assays to provide the first integrated system study to portray the bioenergetic crisis in cell populations subsequent to cytochrome-c (cyt-c) release; a hallmark during chemotherapeutically induced cell death.

We experimentally identified a cell-to-cell heterogeneity in the dynamics of the ATP synthase subsequent to cyt-c release, which the model explained by variations in (i) accessible cytochrome-c after release and (ii) the cell's glycolytic capacity.

Our model predicted, and single-cell imaging confirmed, that high (increasing) glucose in media was able to sustain (repolarise) ΔΨm in HeLa cervical cancer and MCF-7 breast cancer cells, suggesting that they might recover from bioenergetic crisis upon elevation of glucose. However, no significant repolarisation was found in non-transformed human epithelial CRL-1807 cells. Therefore, this mechanism may provide cancer cells with a competitive advantage to evade cell death induced by anticancer drugs or other stress conditions when compared with non-transformed cells.

How can cells cope with a bioenergetic crisis? In particular, how can cancer cells survive the bioenergetic consequences of cyt-c release that are often induced by chemotherapeutic agents, and that lead to depolarisation of the mitochondrial membrane potential ΔΨ_m, result in loss of ionic homeostasis and induce cell death? Is there an inherent population heterogeneity that can lead to a non-synchronous response to above cell death stimuli, thereby aggravating treatment and contributing to clinical relapse? Do cancer cells have a competitive advantage to non-transformed cells in averting such a bioenergetic crisis after cyt-c release. We have investigated these questions in our study, which we regard as the first rigorous system study of single-cell bioenergetics subsequent to cyt-c release and one that bridges single-cell microscopy, in vitro analysis with ordinary differential equations (ODE) based modelling of bioenergetics pathways in the mitochondria and the cytosol.

Several laboratories have so far investigated cyt-c release experimentally (Slee et al, 1999; Atlante et al, 2000; Goldstein et al, 2000; Luetjens et al, 2001; Plas et al, 2001; Waterhouse et al, 2001; Ricci et al, 2003; Colell et al, 2007; Dussmann et al, 2003a, 2003b) and isolated mitochondria (Chinopoulos and Adam-Vizi, 2009; Kushnareva et al, 2002; Kushnareva et al, 2001). However, the cause and mechanistic of several key findings remain elusive and need a system level understanding of post-cyt-c release bioenergetic and its potential cross-talk to apoptosis signalling.

Ricci et al (2003) have identified that the cell death-inducing protease caspase-3, which get activated upon cyt-c release, can further impair mitochondrial function by cleaving and deactivating respiratory complexes I and II. We addressed the question of how such a mechanism could potentiate a bioenergetic crisis. To do so, we first assembled our ODE-based model by integrating approaches from metabolic modelling (Beard, 2005; Beard and Qian, 2007; Dash and Beard, 2008) and calibrated the model to literature data that describe bioenergetic state variables (mitochondrial membrane potential ΔΨ_m, mitochondrial transmembrane membrane ΔpH, respiration ratio between respiring and resting state mitochondria). By remodelling cyt-c release as observed experimentally and integrating it into our model as input, the single-cell model was able to correctly describe the kinetics of ΔΨ_m depolarisation and allowed its quantification. Moreover, it suggested that an additional complex I/II cleavage may further impair respiration and depolarise ΔΨ_m by approximately further 10%.

It was further reported that ATP synthase reversal, a change of direction in the ATP-producing enzyme that leads to pumping of protons from the mitochondrial matrix into the intramembrane space, can stabilise ΔΨ_m. By a remnant ΔΨ_m polarisation, cycling of Na⁺, Ca²⁺, K⁺, Cl⁻ ions and protons across the mitochondrial and the plasma membranes is preserved, and ionic homeostasis can be maintained (Nicholls and Budd, 2000; Dussmann et al, 2003a; Chinopoulos and Adam-Vizi, 2009; Garedew et al, 2010). Our model confirmed that ATP synthase activity was reversed 10 min after onset of cyt-c release, predicted that ATP synthase reversal consumed ATP and that glycolysis was required and sufficient to provide the necessary ATP to sustain this reversal. Reverting back to our single-cell HeLa system, we confirmed the presence of ATP synthase reversal. However, reversal was only present in 20% of cells, 65% of cells showed no detectable reaction and even 15% maintained ATP synthase in forward direction.

To explain this cell-to-cell heterogeneity, we modelled that a cyt-c fraction remains accessible by respiratory complexes and for respiration subsequent to release, which we denoted as ‘respiration-accessible cyt-c'. Our model suggested that small variations in such levels can sufficiently explain the experimentally detected population heterogeneity in the direction and amount of ATP synthase proton flux (Figure 6AB). Variations in respiration-accessible cyt-c may arise from incomplete mitochondrial release. Such incomplete release has been associated with failure of cristae remodelling in the absence of the BH3-only family member BID or the intramitochondrial protein OPA1 (Frezza et al, 2006; Scorrano et al, 2002).

As the model identified glycolysis as necessary for sustaining ATP synthase reversal, we next investigated cells cultured in a medium that contained Na-pyruvate instead of glucose and which consequently were not able to perform glycolysis. We found that such cell populations had a significantly higher fraction of cells that maintained ATP synthase in forward mode consistent with our model predictions. The common influence of respiration-accessible cyt-c and the cell's ability to perform glycolysis is summarised in Figure 7A.

Because glycolysis was able to influence ATP synthase proton pumping, which can affect ΔΨ_m levels, we investigating the effect of higher glucose in single cells. Our model predicted that an increase in glucose utilisation that generates higher cytosolic ATP levels is able to stabilise and repolarise ΔΨ_m and after release. This mechanism is independent from ATP synthase direction. For cells that have ATP synthase in reverse mode, elevated ATP leads to increased proton efflux from the matrix, cell populations that maintain ATP synthase in forward mode achieve a similar result through a reduction of proton influx at increased ATP. In both cases, the proton gradient along the inner membrane, and therefore ΔΨ_m, is increased as a consequence of ATP elevation. The mechanism is depicted in Figure 7B.

We confirmed our model predictions that high glucose was able to stabilise (cells maintained in high-glucose media) and/or to repolarise (cells where glucose was added subsequent to release) ΔΨ_m (Figure 6). While a similar recovery was also present in MCF7 breast cancer cell lines, no significant effect of elevated glucose was found in non-transformed CRL-1807 cells. In conjunction with an impairment of caspase-dependent cell death observed in many human cancers, cancer cells may use this mechanism, and this mechanism may provide cancer cells with a competitive advantage to evade cell death induced by anticancer drugs or other stress conditions when compared with non-transformed cells.

Many anticancer drugs activate caspases via the mitochondrial apoptosis pathway. Activation of this pathway triggers a concomitant bioenergetic crisis caused by the release of cytochrome-c (cyt-c). Cancer cells are able to evade these processes by altering metabolic and caspase activation pathways. In this study, we provide the first integrated system study of mitochondrial bioenergetics and apoptosis signalling and examine the role of mitochondrial cyt-c release in these events. In accordance with single-cell experiments, our model showed that loss of cyt-c decreased mitochondrial respiration by 95% and depolarised mitochondrial membrane potential ΔΨ_m from −142 to −88 mV, with active caspase-3 potentiating this decrease. ATP synthase was reversed under such conditions, consuming ATP and stabilising ΔΨ_m. However, the direction and level of ATP synthase activity showed significant heterogeneity in individual cancer cells, which the model explained by variations in (i) accessible cyt-c after release and (ii) the cell's glycolytic capacity. Our results provide a quantitative and mechanistic explanation for the protective role of enhanced glucose utilisation for cancer cells to avert the otherwise lethal bioenergetic crisis associated with apoptosis initiation.

Amphipathic tail-anchoring peptide and Bcl-2 homology domain-3 (BH3) peptides from Bcl-2 family proteins induce apoptosis through different mechanisms

Bcl-2 homology domain-3 (BH3) peptides are potent cancer therapeutic reagents that target regulators of apoptotic cell death in cancer cells. However, their cytotoxic effects are affected by different expression levels of Bcl-2 family proteins. We recently found that the amphipathic tail-anchoring peptide (ATAP) from Bfl-1, a bifunctional Bcl-2 family member, produced strong pro-apoptotic activity by permeabilizing the mitochondrial outer membrane. Here, we test whether the activity of ATAP requires other cellular factors and whether ATAP has an advantage over the BH3 peptides in targeting cancer cells. Confocal microscopic imaging illustrates specific targeting of ATAP to mitochondria, whereas BH3 peptides show diffuse patterns of cytosolic distribution. Although the pro-apoptotic activities of BH3 peptides are largely inhibited by either overexpression of anti-apoptotic Bcl-2 or Bcl-xL or nullification of pro-apoptotic Bax and Bak in cells, the pro-apoptotic function of ATAP is not affected by these cellular factors. Reconstitution of synthetic ATAP into liposomal membranes results in release of fluorescent molecules of the size of cytochrome c from the liposomes, suggesting that the membrane permeabilizing activity of ATAP does not require additional protein factors. Because ATAP can target to the mitochondrial membrane and its pro-apoptotic activity does not depend on the content of Bcl-2 family proteins, it represents a promising candidate for anti-cancer drugs that can potentially overcome the intrinsic apoptosis-resistant nature of cancer cells.

Extracellular signal-regulated kinase activation mediates mitochondrial dysfunction and necrosis induced by hydrogen peroxide in renal proximal tubular cells

Although tubular necrosis in acute renal failure is associated with excessive production of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), the mechanism of ROS-induced cell necrosis remains poorly understood. In this study, we examined the role of the extracellular signaling-regulated kinase (ERK) pathway in H2O2-induced necrosis of renal proximal tubular cells (RPTC) in primary culture. Exposure of 60 to 70% confluent RPTC to 1 mM H2O2 for 3 h resulted in 44% necrotic cell death, as measured by trypan blue uptake, and inactivation of mitogen-activated protein kinase kinase (MEK), the upstream activator of ERK, by either 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene (U0126) or 2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD98059) or overexpression of dominant-negative mutant of MEK1, inhibited cell death. In contrast, overexpression of active MEK1 enhanced H2O2-induced cell death. H2O2 treatment led to the loss of mitochondrial membrane potential (MMP) in RPTC, which was decreased by U0126 and PD98059. Furthermore, inhibition of the MEK/ERK pathway decreased oxidant-mediated ERK1/2 activation and mitochondrial swelling in isolated renal cortex mitochondria. However, treatment with cyclosporin A (CsA), a mitochondrial permeability transition blocker, did not suppress RPTC necrotic cell death, loss of MMP, and mitochondrial swelling. We suggest that ERK is a critical mediator of mitochondrial dysfunction and necrotic cell death of renal epithelial cells following oxidant injury. Oxidant-induced necrotic cell death was mediated by a CsA-insensitive loss of MMP that is regulated by the ERK pathway.

The tail-anchoring domain of Bfl1 and HCCS1 targets mitochondrial membrane permeability to induce apoptosis

Many Bcl2 family proteins target intracellular membranes by their C-terminal tail-anchor domain. Bfl1 is a bi-functional Bcl2 family protein with both anti- and pro-apoptotic activities and contains an amphipathic tail-anchoring peptide (ATAP; residues 147-175) with unique properties. Here we show that ATAP targets specifically to mitochondria, and induces caspase-dependent apoptosis that does not require Bax or Bak. Mutagenesis studies revealed that lysine residues flanking the ATAP sequence are involved in targeting of the peptide to the mitochondrial membrane, and charged residues that contribute to the amphipathic nature of ATAP are critical for its pro-apoptotic function. The ATAP sequence is present in another tumor suppressor gene, HCCS1, which contains an additional mitochondria-targeting signal (MTS) close to the ATAP. We propose that both ATAP and MTS could be used as therapeutic peptides to induce cell death in the treatment of cancer cells.

Tim17p regulates the twin pore structure and voltage gating of the mitochondrial protein import complex TIM23

The TIM23 complex mediates import of preproteins into mitochondria, but little is known of the mechanistic properties of this translocase. Here patch clamping reconstituted inner membranes allowed for first time insights into the structure and function of the preprotein translocase. Our findings indicate that the TIM23 channel has "twin pores" (two equal sized pores that cooperatively gate) thereby strikingly resembling TOM, the translocase of the outer membrane. Tim17p and Tim23p are homologues, but their functions differ. Tim23p acts as receptor for preproteins and may largely constitute the preprotein-conducting passageway. Conversely depletion of Tim17p induces a collapse of the twin pores into a single pore, whereas N terminus deletion or C terminus truncation results in variable sized pores that cooperatively gate. Further analysis of Tim17p mutants indicates that the N terminus is vital for both voltage sensing and protein sorting. These results suggest that although Tim23p is the main structural unit of the pore Tim17p is required for twin pore structure and provides the voltage gate for the TIM23 channel.

Design of manganese porphyrin modified with mitochondrial signal peptide for a new antioxidant

A new design of antioxidant, consisting of manganese (Mn) porphyrin and signal peptide for mitochondrial targeting, is reported. The resulting Mn-porphyrin-oligopeptide conjugate exhibited significant superoxide dismutase (SOD) activity and decomposed peroxynitrite (ONOO-). The new antioxidant caused the swelling of isolated mitochondria. By using the pH-sensitive drug carrier for intracellular delivery, furthermore, the new conjugate recovered the viability of lipopolysaccharide (LPS)-stimulated macrophage RAW 264.7 cells. These results suggest that the Mn-porphyrin modified with signal peptide for mitochondrial targeting is promising for a new class of antioxidants.

A role for mitochondrial aquaporins in cellular life-and-death decisions?

Mitochondria dominate the process of life-and-death decisions of the cell. Continuous generation of ATP is essential for cell sustenance, but, on the other hand, mitochondria play a central role in the orchestra of events that lead to apoptotic cell death. Changes of mitochondrial volume contribute to the modulation of physiological mitochondrial function, and several ion permeability pathways located in the inner mitochondrial membrane have been implicated in the mediation of physiological swelling-contraction reactions, such as the K+ cycle. However, the channels and transporters involved in these processes have not yet been identified. Osmotic swelling is also one of the fundamental characteristics exhibited by mitochondria in pathological situations, which activates downstream cascades, culminating in apoptosis. The permeability transition pore has long been postulated to be the primary mediator for water movement in mitochondrial swelling during cell death, but its molecular identity remains obscure. Inevitably, accumulating evidence shows that mitochondrial swelling induced by apoptotic stimuli can also occur independently of permeability transition pore activation. Recently, a novel mechanism for osmotic swelling of mitochondria has been described. Aquaporin-8 and -9 channels have been identified in the inner mitochondrial membrane of various tissues, including the kidney, liver, and brain, where they may mediate water transport associated with physiological volume changes, contribute to the transport of metabolic substrates, and/or participate in osmotic swelling induced by apoptotic stimuli. Hence, the recent discovery that aquaporins are expressed in mitochondria opens up new areas of investigation in health and disease.

Protective roles of CNS mitochondria

Mitochondria benefit their host cells by generating ATP, detoxifying oxygen, maintaining cellular redox potential, and detoxifying reactive oxygen species and xenobiotics. These beneficial roles are in stark contrast to mitochondrial participation in both necrotic and apoptotic degenerative pathways. However, cellular stresses do not always result in deleterious mitochondrial changes. Decreases in the calcium sensitivity of the permeability transition may be initial mitochondrial responses to stress that act to preserve mitochondrial function and prolong normal functioning of the host cell.

The powerhouse takes control of the cell: is the mitochondrial permeability transition a viable therapeutic target against neuronal dysfunction and death?

Stroke and neurodegenerative disease exert an increasing large toll on human health at the levels both of the individual and of society. As an example of each, in the United States, stroke is the major single cause of overall morbidity and mortality, and the financial costs of Alzheimer's disease alone dwarfs the entire federal medical research budget. It has been long recognized that mitochondrial energy production is essential for the second to second functions of the central nervous system (CNS), and that severe mitochondrial impairment is incompatible with normal cerebral function. The last decade, however, has brought a growing understanding that mitochondria play an even greater role than previously suspected. Increased understanding of the role of mitochondria in antioxidant defense and calcium homeostasis further solidified the importance of mitochondria in CNS function--just as increased understanding of mitochondrial roles in calcium-mediated toxicity and production of reactive species further exemplified the Janus role of mitochondria--as mediators of CNS dysfunction. Perhaps most unexpected, however, was the evidence that mitochondria serve as the dominant integrators, checkpoints, and amplifiers of the cell death signals in the CNS. The mechanism of propagation of cell death cascades by mitochondria remains controversial. In this review, we focus on the evidence that supports the involvement of an event termed the mitochondrial permeability transition that (i) occurs (patho)physiologically; (ii) occurs in the CNS, and; (iii) is a potential target for pharmaceutical intervention against CNS dysfunction, injury, and cell loss resulting from stroke, trauma, and neurodegenerative disease.

Calpain I induces cleavage and release of apoptosis-inducing factor from isolated mitochondria

The translocation of apoptosis-inducing factor (AIF) from mitochondria to the nucleus has been implicated in the mechanism of glutamate excitotoxicity in cortical neurons and has been observed in vivo following acute rodent brain injuries. However, the mechanism and time course of AIF redistribution to the nucleus is highly controversial. Because elevated intracellular calcium is one of the most ubiquitous features of neuronal cell death, this study tested the hypothesis that cleavage of AIF by the calcium-activated protease calpain mediates its release from mitochondria. Both precursor and mature forms of recombinant AIF were cleaved near the amino terminus by calpain I in vitro. Mitochondrial outer membrane permeabilization by truncated Bid induced cytochrome c release from isolated liver or brain mitochondria but only induced AIF release in the presence of active calpain. Enzymatic inhibition of calpain by calpeptin precluded AIF release, demonstrating that proteolytic activity was required for release. Calpeptin and the mitochondrial permeability transition pore antagonist cyclosporin A also inhibited calcium-induced AIF release from mouse liver mitochondria, implicating the involvement of an endogenous mitochondrial calpain in release of AIF during permeability transition. Cleavage of AIF directly decreased its association with pure lipid vesicles of mitochondrial inner membrane composition. Taken together, these results define a novel mechanism of AIF release involving calpain processing and identify a potential molecular checkpoint for cytoprotective interventions.

Mitochondrial permeability transitions: how many doors to the house?

The inner mitochondrial membrane is famously impermeable to solutes not provided with a specific carrier. When this impermeability is lost, either in a developmental context or under stress, the consequences for the cell can be far-reaching. Permeabilization of isolated mitochondria, studied since the early days of the field, is often discussed as if it were a biochemically well-defined phenomenon, occurring by a unique mechanism. On the contrary, evidence has been accumulating that it may be the common outcome of several distinct processes, involving different proteins or protein complexes, depending on circumstances. A clear definition of this putative variety is a prerequisite for an understanding of mitochondrial permeabilization within cells, of its roles in the life of organisms, and of the possibilities for pharmacological intervention.

Electrophysiological approaches to the study of protein translocation in mitochondria

Electrophysiological techniques have been integral to our understanding of protein translocation across various membranes, and, in particular, the mitochondrial inner and outer membranes. Descriptions of various methodologies (for example, patch clamp, planar bilayers, and tip dip, and their past and potential contributions) are detailed within. The activity of protein import channels of native mitochondrial inner and outer membranes can be studied by directly patch clamping mitochondria and mitoplasts (mitochondria stripped of their outer membrane by French pressing) from various genetically manipulated strains of yeast and mammalian tissue cultured cells. The channel activities of TOM, TIM23, and TIM22 complexes are compared with those reconstituted in proteoliposomes and with those of the recombinant proteins Tom40p, Tim23p, and Tim22p, which play major roles in protein translocation. Studies of the mechanism(s) and the role of channels in protein translocation in mitochondria are prototypes, as the same principles are likely followed in all biological membranes including the endoplasmic reticulum and chloroplasts. The ability to apply electrophysiological techniques to these channels is now allowing investigations into the role of mitochondria in diverse fields such as neurotransmitter release, long-term potentiation, and apoptosis.

Mitochondrial response to calcium in the developing brain

Developmental differences in mitochondrial content and metabolic enzyme activities have been defined, but less is understood about the responses of brain mitochondria to stressful stimuli during development. Cerebral mitochondrial response to high Ca(2+) loads after brain injury is a critical determinant of neuronal outcome. Brain mitochondria isolated from 16-18-day-old rats had lower maximal, respiration-dependent Ca(2+) uptake capacity than brain mitochondria isolated from adult rats in the presence of ATP at both a pH of 7.0 and 6.5. However, in the absence of ATP, immature brain mitochondria exhibited greater Ca(2+) uptake capacity at pH 7.0 and 6.5, indicating a greater resistance of immature brain mitochondria to Ca(2+)-induced dysfunction under conditions relevant to those that exist during acute ischemic and traumatic brain injury. Acidosis reduced the maximal Ca(2+) uptake capacity in both immature and adult brain mitochondria. Cytochrome c was released from both immature and adult brain mitochondria in response to Ca(2+) exposure, but was not affected by cyclosporin A, an inhibitor of the mitochondrial membrane permeability transition. Developmental changes in mitochondrial response to Ca(2+) loads may have important implications in the pathobiology of brain injury to the developing brain.

Some amphiphilic cations block the mitochondrial apoptosis-induced channel, MAC

The mitochondrial apoptosis-induced channel (MAC) forms in the outer membrane of mitochondria early in apoptosis and this activity is altered by physiological levels of cytochrome c. While cyclosporine A and lidocaine have no effect, dibucaine induces a fast blockade of MAC with an IC(50) of 39 microM. In contrast, the IC(50) for propranolol and trifluoperazine are 52 and 0.9 microM, respectively, and these drugs likely destabilize the open state of MAC. These agents, and others not yet identified, should be valuable tools in the study of apoptosis. Profiling MAC's pharmacology may generate novel therapeutic regimes for disease.

Cellular Deflagellation

Deciliation, also known as deflagellation, flagellar autotomy, flagellar excision, or flagellar shedding, refers to the process whereby eukaryotic cells shed their cilia or flagella, often in response to stress. Used for many decades as a tool for scientists interested in the structure, function, and genesis of cilia, deciliation itself is a process worthy of scientific investigation. Deciliation has numerous direct medical implications, but more profoundly, intriguing relationships between deciliation, ciliogenesis, and the cell cycle indicate that understanding the mechanism of deciliation will contribute to a deeper understanding of broad aspects of cell biology. This review provides a critical examination of diverse data bearing on this problem. It also highlights current deficiencies in our understanding of the mechanism of deciliation.

Inhibition of Bax-induced cytochrome c release from neural cell and brain mitochondria by dibucaine and propranolol

BH3 (Bcl-2 homology 3)-only proteins of the Bcl-2 family activate Bax or Bak during apoptosis to promote the release of pro-death factors sequestered in the mitochondrial intermembrane space. Previous results demonstrated that a synthetic BH3 peptide mimics the ability of the BH3-only protein Bid to promote Bax insertion and cytochrome c (cyt c) release from neural cell mitochondria. However, the BH3 peptide was deficient in promoting cyt c release from mitochondria without associated Bax, such as adult rat brain mitochondria. This study tested the hypothesis that the amphiphilic membrane-active cationic drugs dibucaine and propranolol block BH3 peptide-initiated cyt c efflux by preventing the integration of Bax into the mitochondrial outer membrane. BH3 peptide-initiated release of cyt c from GT1-7 neural cell mitochondria was inhibited by dibucaine and propranolol at concentrations of 100-300 microm. Recombinant Bax (100 nm) alone did not release cyt c from adult rat brain mitochondria; however, when BH3 peptide or caspase-8 cleaved Bid (cBid) was added, robust cyt c release was achieved that was inhibited completely by 200 microm dibucaine or propranolol. These drugs at similar concentrations also inhibited release of entrapped 10 kDa dextrans from protein-free liposomes treated with Bax and cBid. Contrary to the hypothesis that dibucaine and propranolol act by inhibiting the insertion of Bax into the mitochondrial outer membrane, membrane insertion of Bax was not inhibited in mitochondria or liposomes, indicating a mechanism of drug action downstream from this event. These results suggest that dibucaine and propranolol inhibit Bax-induced permeability changes through a direct interaction with the lipid membrane and present a novel target for the development of neuroprotective, antiapoptotic therapeutics.

Postnatal brain development and neural cell differentiation modulate mitochondrial Bax and BH3 peptide-induced cytochrome c release

Bax mediates cytochrome c release and apoptosis during neurodevelopment. Brain mitochondria that were isolated from 8-day, 17-day, and adult rats displayed decreasing levels of mitochondrial Bax. The amount of cytochrome c released from brain mitochondria by a peptide containing the BH3 cell death domain decreased with increasing age. However, approximately 60% of cytochrome c in adult brain mitochondria could be released by the BH3 peptide in the presence of exogenous human recombinant Bax. Mitochondrial Bax was downregulated in PC12S neural cells differentiated with nerve growth factor, and mitochondria isolated from these cells demonstrated decreased sensitivity to BH3-peptide-induced cytochrome c release. These results demonstrate that immature brain mitochondria and mitochondria from undifferentiated neural cells are particularly sensitive to cytochrome c release mediated by endogenous Bax and a BH3 death domain peptide. Postnatal developmental changes in mitochondrial Bax levels may contribute to the increased susceptibility of neurons to pathological apoptosis in immature animals.

Nitric oxide inhibits capacitative Ca2+ entry by suppression of mitochondrial Ca2+ handling

1. Nitric oxide (NO) is a key modulator of cellular Ca(2+) signalling and a determinant of mitochondrial function. Here, we demonstrate that NO governs capacitative Ca(2+) entry (CCE) into HEK293 cells by impairment of mitochondrial Ca(2+) handling. 2. Authentic NO as well as the NO donors 1-[2-(carboxylato)pyrrolidin-1-yl]diazem-1-ium-1,2-diolate (ProliNO) and 2-(N,N-diethylamino)-diazenolate-2-oxide (DEANO) suppressed CCE activated by thapsigargin (TG)-induced store depletion. Threshold concentrations for inhibition of CCE by ProliNO and DEANO were 0.3 and 1 micro M, respectively. 3. NO-induced inhibition of CCE was not mimicked by peroxynitrite (100 micro M), the peroxynitrite donor 3-morpholino-sydnonimine (SIN-1, 100 micro M) or 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP, 1 mM). In addition, the guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazole[4,3-a] quinoxalin-1-one (ODQ, 30 micro M) failed to antagonize the inhibitory action of NO on CCE. 4. DEANO (1-10 micro M) suppressed mitochondrial respiration as evident from inhibition of cellular oxygen consumption. Experiments using fluorescent dyes to monitor mitochondrial membrane potential and mitochondrial Ca(2+) levels, respectively, indicated that DEANO (10 micro M) depolarized mitochondria and suppressed mitochondrial Ca(2+) sequestration. The inhibitory effect of DEANO on Ca(2+) uptake into mitochondria was confirmed by recording mitochondrial Ca(2+) during agonist stimulation in HEK293 cells expressing ratiometric-pericam in mitochondria. 5. DEANO (10 micro M) failed to inhibit Ba(2+) entry into TG-stimulated cells when extracellular Ca(2+) was buffered below 1 micro M, while clear inhibition of Ba(2+) entry into store depleted cells was observed when extracellular Ca(2+) levels were above 10 micro M. Moreover, buffering of intracellular Ca(2+) by use of N,N'-[1,2-ethanediylbis(oxy-2,1-phenylene)] bis [N-[25-[(acetyloxy) methoxy]-2-oxoethyl]]-, bis[(acetyloxy)methyl] ester (BAPTA/AM) eliminated inhibition of CCE by NO, indicating that the observed inhibitory effects are based on an intracellular, Ca(2+) dependent-regulatory process. 6. Our data demonstrate that NO effectively inhibits CCE cells by cGMP-independent suppression of mitochondrial function. We suggest disruption of local Ca(2+) handling by mitochondria as a key mechanism of NO induced suppression of CCE.

Mitochondrial alterations induced by the p13II protein of human T-cell leukemia virus type 1. Critical role of arginine residues

Human T-cell leukemia virus type 1 encodes a number of "accessory" proteins of unclear function; one of these proteins, p13(II), is targeted to mitochondria and disrupts mitochondrial morphology. The present study was undertaken to unravel the function of p13(II) through (i) determination of its submitochondrial localization and sequences required to alter mitochondrial morphology and (ii) an assessment of the biophysical and biological properties of synthetic peptides spanning residues 9-41 (p13(9-41)), which include the amphipathic mitochondrial-targeting sequence of the protein. p13(9-41) folded into an alpha helix in micellar environments. Fractionation and immunogold labeling indicated that full-length p13(II) accumulates in the inner mitochondrial membrane. p13(9-41) induced energy-dependent swelling of isolated mitochondria by increasing inner membrane permeability to small cations (Na(+), K(+)) and released Ca(2+) from Ca(2+)-preloaded mitochondria. These effects as well as the ability of full-length p13(II) to alter mitochondrial morphology in cells required the presence of four arginines, forming the charged face of the targeting signal. The mitochondrial effects of p13(9-41) were insensitive to cyclosporin A, suggesting that full-length p13(II) might alter mitochondrial permeability through a permeability transition pore-independent mechanism, thus distinguishing it from the mitochondrial proteins Vpr and X of human immunodeficiency virus type 1 and hepatitis B virus, respectively.

BH3 death domain peptide induces cell type-selective mitochondrial outer membrane permeability

The BH3 domain is essential for the release of cytochrome c from mitochondria by pro-apoptotic Bcl-2 family proteins during apoptosis. This study tested the hypothesis that a Bax peptide that includes the BH3 domain can permeabilize the mitochondrial outer membrane and release cytochrome c in the absence of a permeability transition at the mitochondrial inner membrane. BH3 peptide (0.1-60 microm) released cytochrome c from mitochondria in the presence of physiological concentrations of ions in a cell type-selective manner, whereas a BH3 peptide with a single amino acid substitution was ineffective. The release of cytochrome c by BH3 peptide correlated with the presence of endogenous Bax at the mitochondria and its integral membrane insertion. Cytochrome c release was accompanied by adenylate kinase release, was not associated with mitochondrial swelling or substantial loss of electrical potential across the inner membrane, and was unaffected by inhibitors of the permeability transition pore. Cytochrome c release was, however, inhibited by Bcl-2. Although energy-coupled respiration was inhibited after the release of cytochrome c, mitochondria maintained membrane potential in the presence of ATP due to the reversal of the ATP synthase. Overall, results support the hypothesis that BH3 peptide releases cytochrome c by a Bax-dependent process that is independent of the mitochondrial permeability transition pore but regulated by Bcl-2.