Influence of Ca2+ on the isolation from rat brain mitochondria of a fraction enriched of boundary membrane contact sites

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.

High-affinity peripheral benzodiazepine receptor ligand, PK11195, regulates protein phosphorylation in rat brain mitochondria under control of Ca(2+)

The effects of PK11195, a high-affinity peripheral benzodiazepine receptor (PBR) ligand, on protein phosphorylation in isolated purified rat brain mitochondria were investigated. The isoquinoline carboxamide ligand of PBR, PK11195, but not the benzodiazepine ligand Ro5-4864, in the nanomolar concentration range strongly increased the phosphorylation of 3.5 and 17 kDa polypeptides. The effect of PK11195 was seen in the presence of elevated Ca(2+) levels (3 x 10(-7) to 10(-6) m), but not at very low Ca(2+) levels (10(-8) to 3 x 10(-8) m). This indicates that PBR involves Ca(2+) as a second messenger in the regulation of protein phosphorylation. Staurosporine, an inhibitor of protein kinase activity was able to suppress the PK11195-promoted protein phosphorylation. When the permeability transition pore (PTP) was opened by threshold Ca(2+) load, phosphorylation of the 3.5-kDa polypeptide was diminished, but strong phosphorylation of the 43-kDa protein was revealed. The 43-kDa protein appears to be a PTP-specific phosphoprotein. If PTP was opened, PK11195 did not increase the phosphorylation of the 3.5 and 17-kDa proteins but suppressed the phosphorylation of the PTP-specific 43-kDa phosphoprotein. The ability of PK11195 to increase the protein phosphorylation, which was lost under Ca(2+)-induced PTP opening, was restored again in the presence of calmidazolium, an antagonist of calmodulin and inhibitor of protein phosphatase PP2B. These results show a tight interaction of PBR with the PTP complex in rat brain mitochondria. In conclusion, a novel function of PBR in brain mitochondria has been revealed, and the PBR-mediated protein phosphorylation has to be considered an important element of the PBR-associated signal transducing cascades in mitochondria and cells.

Characterization of the N-acetylaspartate biosynthetic enzyme from rat brain

Aspartate N-acetyltransferase (Asp-NAT; EC 2.3.1.17) activity was found in highly purified intact mitochondria prepared by Percoll gradient centrifugation as well as in the three subfractions obtained after the sucrose density gradient centrifugation of Percoll purified mitochondria; citrate synthase was used as a marker enzyme for mitochondria. The proportion of recoverable activities of Asp-NAT and citrate synthase were comparable in mitochondrial and synaptosomal fractions but not in the fraction containing myelin. Asp-NAT was solubilized from the pellet of the rat brain homogenate (26 000 g for 1 h) for the recovery of maximum activity and partially purified using three protein separation methods: DEAE anion exchange chromatography, continuous elution native gel electrophoresis and size-exclusion high performance liquid chromatography. Asp-NAT activity and the optical density pattern of the eluted protein from size-exclusion column indicated a single large protein (approximately 670 kDa), which on sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed at least 10 bands indicative of an enzyme complex. This seemingly multi-subunit complex Asp-NAT was stable towards ionic perturbations but vulnerable to hydrophobic perturbation; almost 95% of activity was lost after 10 mm 3-[(3-cholamidopropyl)dimethylammonia]-1-propanesulfonate (CHAPS) treatment followed by size-exclusion chromatography. Asp-NAT showed an order of magnitude difference in Km between l-aspartate (l-Asp, approximately 0.5 mm) and acetyl CoA (approximately 0.05 mm). Asp-NAT showed high specificity towards l-Asp with 3% or less activity towards l-Glu, l-Asn, l-Gln and Asp-Glu. A model on the integral involvement of NAA synthesis in the energetics of neuronal mitochondria is proposed.

Bcl-xL overexpression blocks bax-mediated mitochondrial contact site formation and apoptosis in rod photoreceptors of lead-exposed mice

Photoreceptor apoptosis and resultant visual deficits occur in humans and animals with inherited and disease-, injury-, and chemical-induced retinal degeneration. A clinically relevant mouse model of progressive rod photoreceptor-selective apoptosis was produced by low-level developmental lead exposure and studied in combination with transgenic mice overexpressing Bcl-x(L) only in the photoreceptors. A multiparametric analysis of rod apoptosis and mitochondrial structure-function was performed. Mitochondrial cristae topography and connectivity, matrix volume, and contact sites were examined by using 3D electron tomography. Lead-induced rod-selective apoptosis was accompanied by rod Ca(2+) overload, rhodopsin loss, translocation of Bax from the cytosol to the mitochondria, decreased rod mitochondrial respiration and membrane potential, mitochondrial cytochrome c release, caspase-3 activation, and an increase in the number of mitochondrial contact sites. These effects occurred without mitochondrial matrix swelling, outer membrane rupture, caspase-8 activation, or Bid cleavage. Bcl-x(L) overexpression completely blocked all apoptotic events, except Ca(2+) overload, and maintained normal rod mitochondrial function throughout adulthood. This study presents images of mitochondrial contact sites in an in vivo apoptosis model and shows that Bcl-x(L) overexpression blocks increased contact sites and apoptosis. These findings extend our in vitro retinal studies with Pb(2+) and Ca(2+) and suggest that developmental lead exposure produced rod-selective apoptosis without mitochondrial swelling by translocating cytosolic Bax to the mitochondria, which likely sensitized the Pb(2+) and Ca(2+) overloaded rod mitochondria to release cytochrome c. These results have relevance for therapies in a wide variety of progressive retinal and neuronal degenerations where Ca(2+) overload, lead exposure, andor mitochondrial dysfunction occur.

Model of the outer membrane potential generation by the inner membrane of mitochondria

Voltage-dependent anion channels in the outer mitochondrial membrane are strongly regulated by electrical potential. In this work, one of the possible mechanisms of the outer membrane potential generation is proposed. We suggest that the inner membrane potential may be divided on two resistances in series, the resistance of the contact sites between the inner and outer membranes and the resistance of the voltage-dependent anion channels localized beyond the contacts in the outer membrane. The main principle of the proposed mechanism is illustrated by simplified electric and kinetic models. Computational behavior of the kinetic model shows a restriction of the steady-state metabolite flux through the mitochondrial membranes at relatively high concentration of the external ADP. The flux restriction was caused by a decrease of the voltage across the contact sites and by an increase in the outer membrane potential (up to +60 mV) leading to the closure of the voltage-dependent anion channels localized beyond the contact sites. This mechanism suggests that the outer membrane potential may arrest ATP release through the outer membrane beyond the contact sites, thus tightly coordinating mitochondrial metabolism and aerobic glycolysis in tumor and normal proliferating cells.

Distribution of protein disulphide isomerase in rat liver mitochondria

Here we report the localization of protein disulphide isomerase (PDI) in the mitochondrial compartments, comparing it with that of thioredoxin reductase. The latter enzyme is present mostly in the matrix, whereas PDI is located at the level of the outer membrane. We characterize the different submitochondrial fractions with specific marker enzymes. PDI, whether isolated from whole mitochondria or from purified outer membranes, exhibits the same electrophoretic mobility, indicating identical molecular masses. Moreover, immunoblot analysis with monoclonal anti-PDI antibody shows immunoreactivity only with the microsomal PDI, indicating the specificity of the mitochondrial isoform. The significance of these findings is discussed with reference to the potential role of PDI and thioredoxin reductase in regulating the mitochondrial functions dependent on the thiol-disulphide transition.

Perspectives on the mitochondrial multiple conductance channel

A multiple conductance channel (MCC) with a peak conductance of over 1 nS is recorded from mitoplasts (mitochondria with the inner membrane exposed) using patch-clamp techniques. MCC shares many general characteristics with other intracellular megachannels, many of which are weakly selective, voltage-dependent, and calcium sensitive. A role in protein import is suggested by the transient blockade of MCC by peptides responsible for targeting mitochondrial precursor proteins. MCC is compared with the peptide-sensitive channel of the outer membrane because of similarities in targeting peptide blockade. The pharmacology and regulation of MCC by physiological effectors are reviewed and compared with the properties of the pore hypothesized to be responsible for the mitochondrial inner membrane permeability transition.

Trace amounts of Triton X-100 modify the inhibitor sensitivity of the mitochondrial porin

Transport properties of mitochondrial porin were investigated on the basis of changes in the activity of hexokinase utilizing external ATP. Production of glucose 6-phosphate is inhibited by polyanion both in intact brain mitochondria and in contact point vesicles. Hexokinase activity is restored by solubilization of the enzyme by high ionic strength or 0.5-1% Triton X-100. In very low concentrations (0.001-0.005%) Triton does not mobilize hexokinase from its binding sites but it is able to release polyanion-inhibition completely. This finding provides an explanation for the discrepancy observed in the transport properties of porin when studied 'in situ' or in artificial lipid membranes.

Distribution of phospholipid hydroperoxide glutathione peroxidase (PHGPx) in rat testis mitochondria

The distribution of phospholipid hydroperoxide glutathione peroxidase (PHGPx) in isolated rat testis mitochondria was investigated, using a reverse sucrose density gradient centrifugation procedure for the separation of the inner and outer membranes and the contact sites between the two membranes. The results indicate that PHGPx is largely localized in the contact sites fraction. This finding might therefore suggest that the enzyme has more than just an antioxidant function.

Mitochondrial benzodiazepine receptor linked to inner membrane ion channels by nanomolar actions of ligands

The mitochrondrial benzodiazepine receptor (mBzR) binds a subset of benzodiazepines and isoquinoline carboxamides with nanomolar affinity and consists of the voltage-dependent anion channel, the adenine nucleotide translocator, and an 18-kDa protein. The effect of ligands of the mBzR on two inner mitochondrial membrane channel activities was determined with patch-clamp techniques. The relative inhibitory potencies of the drugs resemble their binding affinities for the mBzR. Ro5-4864 and protoporphyrin IX inhibit activity of the multiple conductance channel (MCC) and the mitochondrial centum-picosiemen (mCtS) channel activities at nanomolar concentrations. PK11195 inhibits mCtS activity at similar levels. Higher concentrations of protoporphyrin IX induce MCC but possibly not mCtS activity. Clonazepam, which has low affinity for mBzR, is at least 500 times less potent at both channel activities. Ro15-1788, which also has a low mBzR affinity, inhibits MCC at very high concentrations (16 microM). The findings indicate an association of these two channel activities with the proteins forming the mBzR complex and are consistent with an interaction of inner and outer membrane channels.

Channels in mitochondrial membranes: knowns, unknowns, and prospects for the future

Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.

Phospholipid import into mitochondria: possible regulation mediated through lipid polymorphism

We previously demonstrated that the translocation of microsomal phosphatidylserine to the inner mitochondrial membrane occurs via contact sites before decarboxylation. According to the specific lipid composition of contact sites, we investigated lipid polymorphism as a possible regulation mechanism of phospholipid import into mitochondria. Phosphatidylserine import into mitochondria is increased in the presence of calcium, under conditions where non bilayer lipid-structures can be induced in cardiolipin-containing membranes. The results are discussed in terms of structural as well as functional domains heterogeneity within contact sites.

Isolation of the mitochondrial benzodiazepine receptor: association with the voltage-dependent anion channel and the adenine nucleotide carrier

The mitochondrial benzodiazepine receptor (mBzR) has been solubilized with retention of reversible ligand binding, and the associated subunits were characterized. mBzR comprises immunologically distinct protein subunits of 18-, 30-, and 32-kDa. The 18-kDa protein is labeled by the isoquinoline carboxamide mBzR ligand [3H]PK14105, whereas the 30- and 32-kDa subunits are labeled by the benzodiazepine (Bz) ligands [3H]flunitrazepam and [3H]AHN-086. Selective antibodies and reagents identify the 32- and 30-kDa proteins as the voltage-dependent anion channel (VDAC) and the adenine nucleotide carrier (ADC), respectively. While isoquinoline carboxamide and Bz ligands target different subunits, they interact allosterically, as the binding of Bz and isoquinoline carboxamide ligands is mutually competitive at low nanomolar concentrations. Moreover, eosin-5-maleimide and mercuric chloride inhibit [3H]PK11195 binding to the intact receptor via sulfhydryl groups that are present in ADC. VDAC and ADC, outer and inner mitochondrial membrane channel proteins, respectively, together with the 18-kDa subunit, may comprise mBzR at functionally important transport sites at the junction of two mitochondrial membranes.

High-conductance pathways in mitochondrial membranes

The outer and inner membranes of mitochondria have recently been studied with the patch clamp technique. What has emerged is still an ill-defined picture for either membrane, primarily for the wide range of conductances found. Interestingly, however, a few conductances (in the range of 10-80 pS) seem to be ubiquitously distributed. Parallel studies in situ and in reconstituted systems have allowed the assignment to distinct membrane locations of some conductances, whose physiological role is, however, not yet elucidated.

Modulation of inner mitochondrial membrane channel activity

Three classes of inner mitochondrial membrane (IMM) channel activities have been defined by direct measurement of conductance levels in membranes with patch clamp techniques in 150 mM KCl. The "107 pS activity" is slightly anion selective and voltage dependent (open with matrix positive potentials). "Multiple conductance channel" (MCC) activity includes several levels from about 40 to over 1000 pS and can be activated by voltage or Ca2+. MCC may be responsible for the Ca(2+)-induced permeability transition observed with mitochondrial suspensions. A "low conductance channel" (LCC) is activated by alkaline pH and inhibited by Mg2+. LCC has a unit conductance of about 15 pS and may correspond to the inner membrane anion channel, IMAC, which was proposed from the results obtained from suspension studies. All of the IMM channels defined thus far appear to be highly regulated and have a low open probability under physiological conditions. A summary of what is known about IMM channel regulation and pharmacology is presented and possible physiological roles of these channels are discussed.

The mitochondrial benzodiazepine receptor: evidence for association with the voltage-dependent anion channel (VDAC)

Specific, high-affinity receptors for numerous drugs have recently been localized to mitochondrial membrane proteins. This review discusses the association of the mitochondrial receptor for benzodiazepines (mBzR) with the voltage-dependent anion channel (VDAC), indicating a possible auxiliary role for VDAC as a putative drug binding protein. The proposed subunit composition of the purified mBzR complex isolated from rat kidney mitochondria includes VDAC, which functions as a recognition site for benzodiazepines (e.g., flunitrazepam), the adenine nucleotide carrier (ADC), and an 18 kDa outer membrane protein identified by covalent labelling with the mBzR antagonists isoquinoline carboxamides (e.g., PK14105).

Distribution of glutathione peroxidases and glutathione reductase in rat brain mitochondria

The distribution of glutathione reductase (GR), glutathione peroxidase (GPx) and phospholipid hydroperoxide glutathione peroxidase (PHGPx) in isolated rat brain mitochondria was investigated, using a fractionation procedure for the separation of inner and outer membranes, contact sites between the two membranes and a soluble fraction mainly originating from the mitochondrial matrix. The data indicate that GR and GPx are concentrated in the soluble fraction, with a minor portion of the two enzymes being associated with the contact sites. PHGPx is localized largely in the inner membrane. The possible functional significance of these findings is discussed.

Calcium modulation of mitochondrial inner membrane channel activity

Protocols were defined that enable patch-clamp studies of the approximately 107 pS voltage dependent channel and a class of activity we refer to as MCC (multiconductance channel) which is characterized by multiple levels and transitions as high as 1 to 1.5 nS. If free calcium was kept at 10(-7) M or lower during mitochondrial isolation, no activity was observed at low voltage (+/- 60 mV). If free calcium levels were higher, MCC activity was observed in about 96% of the patches. The observation of approximately 107 pS channel was enhanced from 2% to 68% of patches by washing isolated mitoplasts (mitochondria stripped of outer membrane) with EGTA. Increasing matrix calcium from 10(-9) to 10(-5) M decreased the probability of opening for the MCC and approximately 107 pS activities.

Hydrogen peroxide production by monoamine oxidase in isolated rat-brain mitochondria: its effect on glutathione levels and Ca2+ efflux

H2O2 production and accumulation during incubation of isolated rat-brain mitochondria with substrates of monoamine oxidase A and B were investigated. All substrates gave rise to an accumulation of H2O2 which was inhibited by malate + pyruvate or isocitrate, consistent with a need for mitochondrial NADPH to maintain glutathione in the reduced state. However, in the absence of these additions the level of reduced glutathione decreased only by about 30%, indicating that only a fraction of the mitochondrial glutathione pool was accessible to the glutathione peroxidase and glutathione reductase activities responsible for the continuous removal of H2O2 generated by monoamine oxidase. The H2O2 accumulation was also inhibited by externally added reduced glutathione or NADPH but not NADH. External NADPH was oxidized by added oxidized glutathione but not alpha-ketoglutarate + NH4+. These results suggest that the removal of H2O2 generated by monoamine oxidase proceeds by way of special fractions of glutathione peroxidase and glutathione reductase that are located in the intermembrane space of mitochondria in such a way that they can react with both intra- and extra-mitochondrial glutathione and NADPH, possibly at the contact sites between the inner and outer mitochondrial membranes. Evidence is also presented that H2O2 generated by monoamine oxidase enhances Ca2+ release from mitochondria and may thus function as a regulator of mitochondrial Ca2+ efflux.