Channels in the mitochondrial outer membrane: evidence from patch clamp studies

Ceramide channels and mitochondrial outer membrane permeability

Among the permeability pathways in the mitochondrial outer membrane (MOM), whose elucidation was pioneered by Kathleen Kinnally, there is one formed by the lipid, ceramide. Electron microscopic visualization shows that ceramide channels are large cylindrical structures of varying pore size, with a most frequent size of 10 nm in diameter, large enough to allow all soluble proteins to translocate between the cytosol and the mitochondrial intermembrane space. Similar results were obtained with electrophysiological measurements. Studies of the dynamics of the channels are consistent with a right cylinder. Ceramide channels form at mole fractions of ceramide that are found in the MOM early in the apoptotic process, before or at the time of protein release from mitochondria. That these channels are good candidates for the protein release pathway is supported by the fact that channel formation is inhibited by anti-apoptotic proteins and favored by Bax. Bcl-xL inhibits ceramide channel formation by binding to the apolar ceramide tails using its hydrophobic grove. Bax interaction with the polar regions of ceramide results in MOM permeabilization through synergy with ceramide. Evidence that ceramide channels actually function to favor apoptosis in vivo is supported by the expression of Bcl-xL containing point mutations in cells induced to undergo apoptosis. The Bcl-xL mutants inhibit differentially Bax and ceramide channels and thus tease apart, to some extent, these two modes of MOM permeabilization. Ceramide channels have the right properties and appropriate regulation to be key players in the induction of apoptosis.

In vitro reconstitution of eukaryotic ion channels using droplet interface bilayers

The ability to routinely study eukaryotic ion channels in a synthetic lipid environment would have a major impact on our understanding of how different lipids influence ion channel function. Here, we describe a straightforward, detergent-free method for the in vitro reconstitution of eukaryotic ion channels and ionotropic receptors into droplet interface bilayers and measure their electrical activity at both the macroscopic and single-channel level. We explore the general applicability of this method by reconstitution of channels from a wide range of sources including recombinant cell lines and native tissues, as well as preparations that are difficult to study by conventional methods including erythrocytes and mitochondria.

VDAC, a multi-functional mitochondrial protein regulating cell life and death

Research over the past decade has extended the prevailing view of the mitochondrion to include functions well beyond the generation of cellular energy. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organellar communication, aging, cell proliferation, diseases and cell death. Thus, mitochondria play a central role in the regulation of apoptosis (programmed cell death) and serve as the venue for cellular decisions leading to cell life or death. One of the mitochondrial proteins controlling cell life and death is the voltage-dependent anion channel (VDAC), also known as mitochondrial porin. VDAC, located in the mitochondrial outer membrane, functions as gatekeeper for the entry and exit of mitochondrial metabolites, thereby controlling cross-talk between mitochondria and the rest of the cell. VDAC is also a key player in mitochondria-mediated apoptosis. Thus, in addition to regulating the metabolic and energetic functions of mitochondria, VDAC appears to be a convergence point for a variety of cell survival and cell death signals mediated by its association with various ligands and proteins. In this article, we review what is known about the VDAC channel in terms of its structure, relevance to ATP rationing, Ca(2+) homeostasis, protection against oxidative stress, regulation of apoptosis, involvement in several diseases and its role in the action of different drugs. In light of our recent findings and the recently solved NMR- and crystallography-based 3D structures of VDAC1, the focus of this review will be on the central role of VDAC in cell life and death, addressing VDAC function in the regulation of mitochondria-mediated apoptosis with an emphasis on structure-function relations. Understanding structure-function relationships of VDAC is critical for deciphering how this channel can perform such a variety of functions, all important for cell life and death. This review also provides insight into the potential of VDAC1 as a rational target for new therapeutics.

Electrophysiology clarifies the megariddles of the mitochondrial permeability transition pore

After a brief review of the early history of mitochondrial electrophysiology, the contribution of this approach to the study of the mitochondrial permeability transition (MPT) is recapitulated. It has for example provided evidence for a dimeric nature of the MPT pore, allowed the distinction between two levels of control of its activity, and underscored the relevance of redox events for the phenomenon. Single-channel recording provides a means to finally solve the riddle of the biochemical entity underlying it by comparing the characteristics of the pore with those of channels formed by candidate molecules or complexes. The possibility that this entity may be the protein import machinery of the inner mitochondrial membrane is emphasized.

Reflections on VDAC as a voltage-gated channel and a mitochondrial regulator

There is excellent agreement between the electrophysiological properties and the structure of the mitochondrial outer membrane protein, VDAC, ex vivo. However, the inference that the well-defined canonical "open" state of the VDAC pore is the normal physiological state of the channel in vivo is being challenged by several lines of evidence. Knowing the atomic structure of the detergent solubilized protein, a long sought after goal, will not be sufficient to understand the functioning of this channel protein. In addition, detailed information about VDAC's topology in the outer membrane of intact mitochondria, and the structural changes that it undergoes in response to different stimuli in the cell will be needed to define its physiological functions and regulation.

Molecular participants in mitochondrial cell death channel formation during neuronal ischemia

Mitochondrial ion channels are involved in numerous cellular processes. Membrane pores and transporters regulate the influx and efflux of calcium, sodium, potassium, zinc and determine the membrane compartmentalization of numerous cytosolic metabolites. The permeability of the inner membrane to ions and solutes helps determine the membrane potential of the inner membrane, but the permeability of the outer membrane, controlled in part by VDAC and the BCL-2 family proteins, regulates the release of important signaling molecules that determine the onset of programmed cell death. BCL-2 family proteins have properties of ion channels and perform specialized physiological functions, for example, regulating the strength and pattern of synaptic transmission, in addition to their well known role in cell death. The ion channels of the inner and outer membranes may come together in a complex of proteins during programmed cell death, particularly during neuronal ischemia, where elevated levels of the divalents calcium and zinc activate inner membrane ion channel conductances. The variety of possible molecular participants within the ion channel complex may be matched only by the variety of different types of programmed cell death.

Zinc-dependent multi-conductance channel activity in mitochondria isolated from ischemic brain

Transient global ischemia is a neuronal insult that induces delayed cell death. A hallmark event in the early post-ischemic period is enhanced permeability of mitochondrial membranes. The precise mechanisms by which mitochondrial function is disrupted are, as yet, unclear. Here we show that global ischemia promotes alterations in mitochondrial membrane contact points, a rise in intramitochondrial Zn2+, and activation of large, multi-conductance channels in mitochondrial outer membranes by 1 h after insult. Mitochondrial channel activity was associated with enhanced protease activity and proteolytic cleavage of BCL-xL to generate its pro-death counterpart, deltaN-BCL-xL. The findings implicate deltaN-BCL-xL in large, multi-conductance channel activity. Consistent with this, large channel activity was mimicked by introduction of recombinant deltaN-BCL-xL to control mitochondria and blocked by introduction of a functional BCL-xL antibody to post-ischemic mitochondria via the patch pipette. Channel activity was also inhibited by nicotinamide adenine dinucleotide, indicative of a role for the voltage-dependent anion channel (VDAC) of the outer mitochondrial membrane. In vivo administration of the membrane-impermeant Zn2+ chelator CaEDTA before ischemia or in vitro application of the membrane-permeant Zn2+ chelator tetrakis-(2-pyridylmethyl) ethylenediamine attenuated channel activity, suggesting a requirement for Zn2+. These findings reveal a novel mechanism by which ischemic insults disrupt the functional integrity of the outer mitochondrial membrane and implicate deltaN-BCL-xL and VDAC in the large, Zn2+-dependent mitochondrial channels observed in post-ischemic hippocampal mitochondria.

Polyanions decelerate the kinetics of positively charged gramicidin channels as shown by sensitized photoinactivation

The effects of different anionic polymers on the kinetic properties of ionic channels formed by neutral gramicidin A (gA) and its positively charged analogs gramicidin-tris(2-aminoethyl)amine (gram-TAEA) and gramicidin-ethylenediamine (gram-EDA) in a bilayer lipid membrane were studied using a method of sensitized photoinactivation. The addition of Konig's polyanion caused substantial deceleration of the photoinactivation kinetics of gram-TAEA channels, which expose three positive charges to the aqueous phase at both sides of the membrane. In contrast, channels formed of gram-EDA, which exposes one positive charge, and neutral gA channels were insensitive to Konig's polyanion. The effect strongly depended on the nature of the polyanion added, namely: DNA, RNA, polyacrylic acid, and polyglutamic acid were inactive, whereas modified polyacrylic acid induced deceleration of the channel kinetics at high concentrations. In addition, DNA was able to prevent the action of Konig's polyanion. In single-channel experiments, the addition of Konig's polyanion resulted in the appearance of long-lived gram-TAEA channels. The deceleration of the gram-TAEA channel kinetics was ascribed to electrostatic interaction of the polyanion with gram-TAEA that reduces the mobility of gram-TAEA monomers and dimers in the membrane via clustering of channels.

The high-conductance channel of porin-less yeast mitochondria

Patch-clamp and planar bilayer experiments on porin-less yeast mitochondria have allowed the characterization of a cationic channel activated at matrix-side positive (unphysiological) potentials. In voltage-pulse experiments, inactivation was a faster process than activation and the time constant for inactivation was more steeply dependent on voltage than the one for activation. The channel exhibited various conductance states whose occupancy depended on the applied transmembrane potential. In bilayer experiments, the presence of the pCOx-IV leader peptide induced fast gating in a voltage-dependent manner. A comparison with previously described activities suggests that the pore may coincide with the peptide-sensitive channel (PSC) (Thieffry et al. (1988) EMBO J. 7, 1449-1454) as well as with two other activities (Dihanich et al. (1989) Eur. J. Biochem. 181, 703-708; Tedeschi et al. (1987) J. Membr. Biol. 97, 21-29) assigned to the mitochondrial outer membrane. The possible relationship of this channel to the mitochondrial megachannel is discussed.

Retarded diffusion of ADP in cardiomyocytes: possible role of mitochondrial outer membrane and creatine kinase in cellular regulation of oxidative phosphorylation

Possible reasons for retarded intracellular diffusion of ADP were investigated. The isolated skinned cardiac fibers were used to study apparent kinetic parameters for externally added ADP in control of mitochondrial respiration. Participation of myosin-ATPase in binding of ADP within cells as it was supposed earlier (Saks, V.A., Belikova, Yu.O. and Kuznetsov, A.V. (1991) Biochim. Biophys. Acta 1074, 302-311) was completely excluded, since myosin-deprived skinned cardiac fibers ('ghosts') displayed the same kinetic parameters as intact ones (Kmapp for ADP about 300 microM). Significantly lower apparent Km values were obtained for fibers with osmotically disrupted outer mitochondrial membrane (25-35 microM), which was close to that observed for isolated heart mitochondria. The data obtained are in favor of limitation of ADP movement via anion-selective low-conductance porine channels in the outer membrane of mitochondria. It is proposed that the permeability of this membrane is controlled by some unknown intracellular factor(s). In the presence of saturating concentrations of creatine (25 mM) the apparent Km for ADP significantly decreases due to coupling of creatine kinase and oxidative phosphorylation reactions in mitochondria. This coupling is not observed in KCl medium in which mitochondrial creatine kinase is detached from the membrane. It is concluded that in the cells in-vivo ADP movement between cytoplasm and intramitochondrial space is controlled by low-conductivity anion channels in the outer membrane. Thus, the mitochondrial creatine kinase reaction coupled to the adenine nucleotide translocase is an important mechanism in control of oxidative phosphorylation in vivo due to its ability to manifold amplify these very weak ADP signals from cytoplasm.

A characterization of cuprizone-induced giant mouse liver mitochondria

Cuprizone affects the liver of treated mice in a random manner, causing no appreciable change in some cases and inducing the formation of megamitochondria with altered properties in others. Lack of a full appreciation of this variability may be at the origin of some discrepancies in published work dealing with the properties of cuprizone mouse liver mitochondria (CMLM). CMLM from fully affected livers were remarkably labile and difficult to isolate in a coupled state by homogenization and centrifugation techniques. The integral respiratory chain proteins of CMLM were functionally normal, with the exception of succinic dehydrogenase which showed considerable inhibition. Coupled morphological and functional analysis provided evidence that these properties were independent of CMLM size, a matter which had remained doubtful thus far and bears on the validity of literature reports.

Power transmission along biological membranes

Hypothesis on long-distance power transmission along extended energy-transducing membranes (Skulachev, 1969, 1971, 1980), has been experimentally proven in four different systems, namely, (i) trichomes of filamentous cyanobacterium Phormidium uncinatum; (ii) filamentous mitochondria and mitochondrial network in fibroblasts; (iii) clusters of roundish heart muscle mitochondria interconnected with mitochondrial junctions; (iv) mixed animal cell cultures interconnected with gap junctions. In all cases, energy was shown to be transmitted in the form of a transmembrane electric potential difference. The transmission occurred for distances as long as several tens of micrometers. Since the (a) delta-muH-bearing cytoplasmic membrane of cyanobacteria and the inner mitochondrial membrane and (b) delta-muNa-bearing outer animal cell membrane were found to be competent in such an effect, one may assume that the power transmission is a fundamental function of extended membrane systems. This mechanism can be used at the intracellular level (mitochondrial) as well as at the supracellular level (cytoplasmic and outer cell membranes). Studies on the possible involvement of membranes in lateral transport of oxygen, ions, fatty acids and membrane proteins seem to hold good promise.