Purification and liposomal reconstitution of permeability transition pore complex

The mitochondrial permeability transition pore in AD 2016: An update

Over the past 30years the mitochondrial permeability transition - the permeabilization of the inner mitochondrial membrane due to the opening of a wide pore - has progressed from being considered a curious artifact induced in isolated mitochondria by Ca(2+) and phosphate to a key cell-death-inducing process in several major pathologies. Its relevance is by now universally acknowledged and a pharmacology targeting the phenomenon is being developed. The molecular nature of the pore remains to this day uncertain, but progress has recently been made with the identification of the FOF1 ATP synthase as the probable proteic substrate. Researchers sharing this conviction are however divided into two camps: these believing that only the ATP synthase dimers or oligomers can form the pore, presumably in the contact region between monomers, and those who consider that the ring-forming c subunits in the FO sector actually constitute the walls of the pore. The latest development is the emergence of a new candidate: Spastic Paraplegia 7 (SPG7), a mitochondrial AAA-type membrane protease which forms a 6-stave barrel. This review summarizes recent developments of research on the pathophysiological relevance and on the molecular nature of the mitochondrial permeability transition pore. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

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.

Physiological roles of the permeability transition pore

Mitochondria are implicated in many important cellular functions covering the whole life cycle from mitochondrial biogenesis to cell death. Mitochondrial homeostasis is tightly regulated, and mitochondrial dysfunction is frequently associated with severe human pathologies (eg, cardiovascular diseases, cancer, and neurodegeneration). The permeability transition pore (PTP) is an unselective voltage-dependent mitochondrial channel. Despite the extensive use of electrophysiology, biochemistry, pharmacology, and genetic invalidation in mice, the molecular identity of PTP is still unknown. Nevertheless, PTP is central to mitochondrial vital functions and can play a lethal role in many pathophysiological conditions. This review recapitulates the current knowledge of the various modes of conductance of the PTP channel and discusses their implication in the physiological roles of PTP and their regulation. Based on its involvement in normal physiology and human pathology, a better understanding of this channel and its roles remains a major goal for basic scientists and clinicians.

GD3-7-aldehyde is an apoptosis inducer and interacts with adenine nucleotide translocase

We prepared GD3-7-aldehyde (GD3-7) and determined its apoptotic potential. GD3-7 proved to be more efficient to induce pro-apoptotic mitochondrial alterations than GD3 when tested on mouse liver mitochondria. GD3-7-induced mitochondrial swelling and depolarization was blocked by cyclosporin A (CsA) supporting a critical role of the permeability transition pore complex (PTPC) during GD3-7-mediated apoptosis. In contrast to GD3, GD3-7 was able to induce channel formation in proteoliposomes containing adenine nucleotide translocase (ANT). This suggests that ANT is the molecular target of GD3-7. Using a specific antiserum, GD3-7 was detected in the lipid extract of the myeloid tumor cell line HL-60 after apoptosis induction, but not in living cells. Therefore, GD3-7 might be a novel mediator of PTPC-dependent apoptosis in cancer cells.

GAPDH, a novel regulator of the pro-apoptotic mitochondrial membrane permeabilization

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a pleiotropic enzyme that is overexpressed in apoptosis and in several human chronic pathologies. Here, we report that the protein accumulates in mitochondria during apoptosis, and induces the pro-apoptotic mitochondrial membrane permeabilization, a decisive event of the intrinsic pathway of apoptosis. GAPDH was localized by immunogold labeling and identified by matrix-assisted laser desorption/ionization-time of flight and nano liquid chromatography mass spectroscopy/mass spectroscopy in the mitochondrion of various tissues and origins. In isolated mitochondria, GAPDH can be imported and interact with the voltage-dependent anion channel (VDAC1), but not the adenine nucleotide translocase (ANT). The protein mediates a cyclosporin A-inhibitable permeability transition, characterized by a loss of the inner transmembrane potential, matrix swelling, permeabilization of the inner mitochondrial membrane and the release of two pro-apoptotic proteins, cytochrome c and apoptosis-inducing factor (AIF). This novel function of GAPDH might have implications for the understanding of mitochondrial biology, oncogenesis and apoptosis.

The permeability transition pore complex in cancer cell death

The permeability transition pore (PTP) is a multi-protein complex at contact sites of the inner with the outer mitochondrial membrane. Research over the past years has led to the concept that the PTP occupies a central role in cell death induction. Numerous apoptosis signals convert this protein aggregate into an unspecific pore, thus activating mitochondria for the cellular self-destruction process. Here, we describe the evidence for this and the various approaches being undertaken to elucidate its subunit composition and mode of regulation. In particular, we review data that indicate a role of specific PTP subunits for apoptosis inhibition during tumorigenesis.

Therapeutic peptides: Targeting the mitochondrion to modulate apoptosis

For many years, medical drug discovery has extensively exploited peptides as lead compounds. Currently, novel structures of therapeutic peptides are derived from active pre-existing peptides or from high-throughput screening, and optimized following a rational drug design approach. Molecules of interest may prove their ability to influence the disease outcome in animal models and must respond to a set of criteria based on toxicity studies, ease of administration, the cost of their synthesis, and logistic for clinical use to validate it as a good candidate in a therapeutic perspective. This applies to the potential use of peptides to target one central intracellular organelle, the mitochondrion, to modulate (i.e. activate or prevent) apoptosis. Putative mitochondrial protein targets and the strategies already elaborated to correct the defects linked to these proteins (overexpression, inactivation, mutation..., etc.) are described, and recent advances that led or may lead to the conception of therapeutic peptides via a specific action on these mitochondrial targets in the future are discussed.

Inhibition of adenine nucleotide translocator pore function and protection against apoptosis in vivo by an HIV protease inhibitor

Inhibitors of HIV protease have been shown to have antiapoptotic effects in vitro, yet whether these effects are seen in vivo remains controversial. In this study, we have evaluated the impact of the HIV protease inhibitor (PI) nelfinavir, boosted with ritonavir, in models of nonviral disease associated with excessive apoptosis. In mice with Fas-induced fatal hepatitis, Staphylococcal enterotoxin B-induced shock, and middle cerebral artery occlusion-induced stroke, we demonstrate that PIs significantly reduce apoptosis and improve histology, function, and/or behavioral recovery in each of these models. Further, we demonstrate that both in vitro and in vivo, PIs block apoptosis through the preservation of mitochondrial integrity and that in vitro PIs act to prevent pore function of the adenine nucleotide translocator (ANT) subunit of the mitochondrial permeability transition pore complex.

Dynamic evolution of the adenine nucleotide translocase interactome during chemotherapy-induced apoptosis

The mitochondrial permeability transition pore complex (PTPC) is involved in the control of the mitochondrial membrane permeabilization during apoptosis, necrosis and autophagy. Indeed, the adenine nucleotide translocator (ANT) and the voltage-dependent anion channel (VDAC), two major components of PTPC, are the targets of a variety of proapoptotic inducers. Using co-immunoprecipitation and proteomic analysis, we identified some of the interacting partners of ANT in several normal tissues and human cancer cell lines. During chemotherapy-induced apoptosis, some of these interactions were constant (e.g. ANT-VDAC), whereas others changed strongly concomitantly with the dissipation of the mitochondrial transmembrane potential and until nuclear degradation occurred (e.g. Bax, Bcl-2, subunits of the respiratory chain, a subunit of the phosphatase PP2A, phospholipase PLC beta 4 and IP3 receptor). In addition, a glutathione-S-transferase (GST) interacts with ANT in normal tissue, in colon carcinoma cells and in vitro. This interaction is lost during apoptosis induction, suggesting that GST behaves as an endogenous repressor of PTPC and ANT pore opening. Thus, ANT is connected to mitochondrial proteins as well as to proteins from other organelles such as the endoplasmic reticulum forming a dynamic polyprotein complex. Changes within this ANT interactome coordinate the lethal response of cells to apoptosis induction.

The chemopreventive agent N-(4-hydroxyphenyl)retinamide induces apoptosis through a mitochondrial pathway regulated by proteins from the Bcl-2 family

N-(4-hydroxyphenyl)retinamide (4-HPR, fenretinide) is a potent chemopreventive agent whose effect has been suggested to involve apoptosis induction. 4-HPR induces a loss of the mitochondrial transmembrane potential and the mitochondrial release of cytochrome c before caspase activation. Inhibition of mitochondrial membrane permeabilization (MMP) by transfection with Bcl-2 or the Cytomegalovirus UL37 gene product vMIA prevented caspase activation and cell death. In contrast to other retinoid derivatives, 4-HPR has no direct MMP-inducing effects when added to isolated mitochondria or when added to proteoliposomes containing the MMP-regulatory permeability transition pore complex (PTPC). Moreover, although reactive oxygen species (ROS) overproduction appears to be instrumental for 4-HPR-induced MMP and apoptosis, inhibition of the NF-kappaB or p53-mediated signal transduction pathways failed to modulate 4-HPR-induced apoptosis. 4-HPR was found to cause an antioxidant-inhibitable conformational change of both Bax and Bak, leading to the exposure of their N-termini and to the mitochondrial relocalization of Bax. Cells with a Bax(-/-) Bak(-/-) genotype were resistant against the 4-HPR-induced MMP, overproduction of ROS and cell death. Altogether, these data indicate that 4-HPR induces MMP through an ROS-mediated pathway that involves the obligatory contribution of the proapopotic Bcl-2 family members Bax and/or Bak.

Nitric oxide and differential effects of ATP on mitochondrial permeability transition

The mitochondrial permeability transition pore (PTP) undergoes a calcium-dependent transition (MPT) that disrupts membrane potential and releases apoptogenic proteins. Because PTP opening is enhanced by oxidation of thiols at the so-called "S-site," we hypothesized that nitrogen monoxide (NO*) could enhance the open probability of the PTP, e.g., by S-nitrosylation or S-thiolation. At low NO donor concentrations (1 to 20 microM), PTP opening in succinate-energized liver mitochondria at nonlimiting calcium was delayed or unaffected, while it was accelerated by NO donors at 20 to 100 microM. At low donor concentrations, PTP opening was facilitated twofold by adenosine triphosphate (ATP), which normally delays PTP opening. Among NO donors, the oxatriazole GEA 3162, with an activation constant (Ka) of 1.9 microM at 500 microM ATP was more effective at enhancing pore transition than SIN-1 or SNAP. NO donor effects were superseded by diamide, which induces disulfide formation, but independent of SH-adduct formation by alkylation. NO-related changes in PTP function were accompanied by protein mixed disulfide formation, inhibited by dithiothreitol (DTT), and reversed by DTT after donor addition. PTP opening was stimulated in the presence of ATP by L-arginine-dependent NO production, i.e., mitochondrial NOS activity. ATP-facilitated pore opening was sensitive to atractyloside and depended on nucleotide interactions but not on hydrolysis, because specific nonhydrolyzable ATP analogs accelerated pore opening. These data indicate NO can influence pore transition by oxidation of thiols that produce conformational changes governing the ATP interaction at the adenine nucleotide transporter.

The adenine nucleotide translocator in apoptosis

Alteration of mitochondrial membrane permeability is a central mechanism leading invariably to cell death, which results, at least in part, from the opening of the permeability transition pore complex (PTPC). Indeed, extended PTPC opening is sufficient to trigger an increase in mitochondrial membrane permeability and apoptosis. Among the various PTPC components, the adenine nucleotide translocator (ANT) appears to act as a bi-functional protein which, on the one hand, contributes to a crucial step of aerobic energy metabolism, the ADP/ATP translocation, and on the other hand, can be converted into a pro-apoptotic pore under the control of onco- and anti-oncoproteins from the Bax/Bcl-2 family. In this review, we will discuss recent advances in the cooperation between ANT and Bax/Bcl-2 family members, the multiplicity of agents affecting ANT pore function and the putative role of ANT isoforms in apoptosis control.

Bid acts on the permeability transition pore complex to induce apoptosis

Similar to most if not all pro-apoptotic members of the Bcl-2 family, Bid (and its truncated product t-Bid) triggers cell death via mitochondrial membrane permeabilization (MMP). This effect can be monitored in intact cells, upon microinjection of recombinant Bid protein into the cytoplasm, as well as in purified mitochondria, upon addition of Bid protein. Here we show that Bid-induced MMP can be inhibited, both in cells and in the cell-free system, by three pharmacological inhibitors of the permeability transition pore complex (PTPC), namely cyclosporin A, N-methyl-4-Val-cyclosporin A, and bongkrekic acid (a ligand of the adenine nucleotide translocase, ANT, one of the PTPC components). Bid effects on synthetic membranes were studied either in proteoliposomes or in synthetic bilayers subjected to electrophysiological measurements. Full length Bid preferentially permeabilizes membranes and induces the formation of large conductance channels at neutral pH, when added to liposomes or bilayers containing both purified ANT and Bax, yet has no or little effect combined with ANT or Bax alone. t-Bid acts on membranes containing ANT alone with the same efficiency as on those containing both ANT and Bax. These results suggest that the proapoptotic effects of Bid are mediated, at least in part, by its functional interaction with ANT, one of the major components of PTPC.