Mouse and human mitochondrial nucleoid--detailed structure in relation to function

The independent mitochondrial genetic information is organized in so-called mitochondrial nucleoids that, in vertebrates, typically contain 5-7 genetic units. The total number of nucleoids per cell is several hundred in cultured cells. Mitochondrial nucleoids, similarly to the whole mitochondrial network, have recently been successfully and extensively visualized using fluorescent and confocal microscopy. In the present work, we show high-resolution micrographs of mouse and human mitochondrial nucleoids obtained by transmission electron microscopy. Position in the mitochondria, size, general appearance and other properties of the human nucleoids appear the same as those of mouse nucleoids, and all observations are also in full agreement with the results obtained in different laboratories using different approaches. Most of nucleoids are located inside mitochondrial tubes. However, we show directly that certain part of the nucleoids close to inner membrane is bound to the complex of molecules that crosscut both, the inner and the outer mitochondrial membranes. Nucleoids in cells starving for serum are mostly more dense than those in dividing cells. We discuss the position, appearance and other properties of the nucleoids in relation to functional stage. Other electron-dense structures inside mitochondria that could be erroneously considered to be mitochondrial nucleoids are also described.

Leishmania major ascorbate peroxidase overexpression protects cells against reactive oxygen species-mediated cardiolipin oxidation

Heme peroxidases are a class of multifunctional redox-active proteins found in all organisms. We recently cloned, expressed, and characterized an ascorbate peroxidase from Leishmania major (LmAPX) that was capable of detoxifying hydrogen peroxide. Localization studies using green fluorescent protein fusions revealed that LmAPX was localized within the mitochondria by its N-terminal signal sequence. Subcellular fractionation analysis of the cell homogenate by the Percoll density-gradient method and subsequent Western blot analysis with anti-LmAPX antibody further confirmed the mitochondrial localization of mature LmAPX. Submitochondrial fractionation analysis showed that the mature enzyme (~3.6 kDa shorter than the theoretical value of the whole gene) was present in the intermembrane space side of the inner membrane. Moreover, expression of the LmAPX gene was increased by treatment with exogenous H(2)O(2), indicating that LmAPX was induced by oxidative stress. To investigate the biological role of LmAPX we generated Leishmania cells overexpressing LmAPX in the mitochondria. Flow-cytometric analysis, thin-layer chromatography, and IC(50) measurements suggested that overexpression of LmAPX caused depletion of the mitochondrial ROS burden and conferred a protection against mitochondrial cardiolipin oxidation and increased tolerance to H(2)O(2). These results suggest that the single-copy LmAPX gene plays a protective role against oxidative damage.

Mitochondrial glycerol-3-P acyltransferase 1 is most active in outer mitochondrial membrane but not in mitochondrial associated vesicles (MAV)

Glycerol 3-phosphate acyltransferase-1 (GPAT1), catalyzes the committed step in phospholipid and triacylglycerol synthesis. Because both GPAT1 and carnitine-palmitoyltransferase 1 are located on the outer mitochondrial membrane (OMM) it has been suggested that their reciprocal regulation controls acyl-CoA metabolism at the OMM. To determine whether GPAT1, like carnitine-palmitoyltransferase 1, is enriched in both mitochondrial contact sites and OMM, and to correlate protein location and enzymatic function, we used Percoll and sucrose gradient fractionation of rat liver to obtain submitochondrial fractions. Most GPAT1 protein was present in a vesicular membrane fraction associated with mitochondria (MAV) but GPAT specific activity in this fraction was low. In contrast, highest GPAT1 specific activity was present in purified mitochondria. Contact sites from crude mitochondria, which contained markers for both endoplasmic reticulum (ER) and mitochondria, also showed high expression of GPAT1 protein but low specific activity, whereas contact sites isolated from purified mitochondria lacked ER markers and expressed highly active GPAT1. To determine how GPAT1 is targeted to mitochondria, recombinant protein was synthesized in vitro and its incorporation into crude and purified mitochondria was assayed. GPAT1 was rapidly incorporated into mitochondria, but not into microsomes. Incorporation was ATP-driven, and lack of GPAT1 removal by alkali and a chaotropic agent showed that GPAT1 had become an integral membrane protein after incorporation. These results demonstrate that two pools of GPAT1 are present in rat liver mitochondria: an active one, located in OMM and a less active one, located in membranes (ER-contact sites and mitochondrial associated vesicles) associated with both mitochondria and ER.

Mutagenicity of diesel exhaust particles mediated by cell-particle interaction in mammalian cells

Diesel exhaust particle (DEP) has been identified as a class 2A human carcinogen and closely related to the increased incidence of respiratory allergy, cardiopulmonary morbidity and mortality, and risk of lung cancer. However, the molecular mechanisms of DEP mutagenicity/carcinogenicity are still largely unknown. In the present study, we focused on the mutagenicity of DEPs in human-hamster hybrid (A(L)) cells and evaluated the role of cell-particle interaction in mediating mutagenic process. We found that DEPs formed micron-sized aggregates in the medium and located mainly in large cytoplasmic vacuoles of cells by 24h treatment. The cellular granularity was increased by DEP treatment in a dose-dependent manner. DEPs resulted in a dose-dependent increase of mutation yield at CD59 locus in A(L) cells, while inflicting minimal cytotoxicity. There was a more than two-fold increase of mutation yield at CD59 locus in A(L) cells exposed to DEPs at a dose of 50mug/ml. Such induction was significantly reduced by concurrent treatment with phagocytosis inhibitors, cytochalasin B and ammonium chloride (p<0.05). These results provided direct evidence that DEPs was mutagenic in mammalian cells and that cell-particle interaction played an essential role in the process.

The peripheral stalk of the mitochondrial ATP synthase

The peripheral stalk of F-ATPases is an essential component of these enzymes. It extends from the membrane distal point of the F1 catalytic domain along the surface of the F1 domain with subunit a in the membrane domain. Then, it reaches down some 45 A to the membrane surface, and traverses the membrane, where it is associated with the a-subunit. Its role is to act as a stator to hold the catalytic alpha3beta3 subcomplex and the a-subunit static relative to the rotary element of the enzyme, which consists of the c-ring in the membrane and the attached central stalk. The central stalk extends up about 45 A from the membrane surface and then penetrates into the alpha3beta3 subcomplex along its central axis. The mitochondrial peripheral stalk is an assembly of single copies of the oligomycin sensitivity conferral protein (the OSCP) and subunits b, d and F6. In the F-ATPase in Escherichia coli, its composition is simpler, and it consists of a single copy of the delta-subunit with two copies of subunit b. In some bacteria and in chloroplasts, the two copies of subunit b are replaced by single copies of the related proteins b and b' (known as subunits I and II in chloroplasts). As summarized in this review, considerable progress has been made towards establishing the structure and biophysical properties of the peripheral stalk in both the mitochondrial and bacterial enzymes. However, key issues are unresolved, and so our understanding of the role of the peripheral stalk and the mechanism of synthesis of ATP are incomplete.

Structure and dynamics of the mitochondrial inner membrane cristae

Three-dimensional images of mitochondria provided by electron tomography reveal that the micro-compartments (cristae) defined by the inner membrane are connected to the periphery of this membrane by narrow tubular junctions, which likely restrict diffusion. The tomograms also strongly suggest that inner membrane topology represents a balance between membrane fusion and fission processes. The hypothesis being developed is that inner membrane topology is a regulated property of mitochondria. This review summarizes the evidence about how inner membrane shape influences mitochondrial function and, conversely, what is known about the factors that determine this membrane's topology.

Evaluation of copper toxicity in isolated human peripheral blood mononuclear cells and it's attenuation by zinc: ex vivo

Copper and zinc act as a cofactor of over 300 mammalian proteins. Both have same electronic configuration therefore they are antagonist at higher individual concentration. The present study was designed with the aim to investigate the mechanisms pertaining to toxic effects of copper on human peripheral blood mononuclear cells (PBMCs) and to evaluate the cytoprotective effect of zinc on copper-induced cytotoxicity. The copper uptake into PBMCs was progressively increased with increasing concentration of metal in the growth medium. However, no significant effect on copper uptake was observed in the presence of zinc. Cell proliferation rate was decreased with increasing copper concentration. Interestingly, the proliferation rate of zinc treated PBMCs remained nearly the same as that of control cells. LD(50) of copper (115 microM) was increased six times (710 microM) in presence of zinc for PBMCs. At higher concentrations of copper (> 100 microM) decrease level of GSH was noticed. Increased levels of metallothionein in PBMCs were observed in response to zinc. DNA fragmentation studies also showed that copper produced DNA fragmentation at LD(50) (115 microM). Subsequently, zinc showed protection against DNA fragmentation caused by copper. Cell structure of PBMCs at LD(50) (115 microM copper) showed membrane bound cystic spaces and mitochondria having disrupted cristae and few myelin figures. In presence of zinc at LD(50) of copper (115 microM) cells showed improvement in mitochondrial structure and membrane bound cystic spaces. Taken together, the results of our study demonstrates that zinc play an important role in prevention of copper toxicity in peripheral blood mononuclear cells.

Alternative translation initiation generates a novel isoform of insulin-degrading enzyme targeted to mitochondria

IDE (insulin-degrading enzyme) is a widely expressed zinc-metallopeptidase that has been shown to regulate both cerebral amyloid beta-peptide and plasma insulin levels in vivo. Genetic linkage and allelic association have been reported between the IDE gene locus and both late-onset Alzheimer's disease and Type II diabetes mellitus, suggesting that altered IDE function may contribute to some cases of these highly prevalent disorders. Despite the potentially great importance of this peptidase to health and disease, many fundamental aspects of IDE biology remain unresolved. Here we identify a previously undescribed mitochondrial isoform of IDE generated by translation at an in-frame initiation codon 123 nucleotides upstream of the canonical translation start site, which results in the addition of a 41-amino-acid N-terminal mitochondrial targeting sequence. Fusion of this sequence to the N-terminus of green fluorescent protein directed this normally cytosolic protein to mitochondria, and full-length IDE constructs containing this sequence were also directed to mitochondria, as revealed by immuno-electron microscopy. Endogenous IDE protein was detected in purified mitochondria, where it was protected from digestion by trypsin and migrated at a size consistent with the predicted removal of the N-terminal targeting sequence upon transport into the mitochondrion. Functionally, we provide evidence that IDE can degrade cleaved mitochondrial targeting sequences. Our results identify new mechanisms regulating the subcellular localization of IDE and suggest previously unrecognized roles for IDE within mitochondria.

Mitochondrial localization of cellular prion protein (PrPC) invokes neuronal apoptosis in aged transgenic mice overexpressing PrPC

Recent studies suggest that the disease isoform of prion protein (PrPSc) is non-neurotoxic in the absence of cellular isoform of prion protein (PrPC), indicating that PrPC may participate directly in the neurodegenerative damage by itself. Meanwhile, transgenic mice harboring a high-copy-number of wild-type mouse (Mo) PrPC develop a spontaneous neurological dysfunction in an age-dependent manner, even without inoculation of PrPSc and thus, investigations of these aged transgenic mice may lead to the understanding how PrPC participate in the neurotoxic property of PrP. Here we demonstrate mitochondria-mediated neuronal apoptosis in aged transgenic mice overexpressing wild-type MoPrPC (Tg(MoPrP)4053/FVB). The aged mice exhibited an aberrant mitochondrial localization of PrPC concomitant with decreased proteasomal activity, while younger littermates did not. Such aberrant mitochondrial localization was accompanied by decreased mitochondrial manganese superoxide dismutase (Mn-SOD) activity, cytochrome c release into the cytosol, caspase-3 activation, and DNA fragmentation, most predominantly in hippocampal neuronal cells. Following cell culture studies confirmed that decrease in the proteasomal activity is fundamental for the PrPC-related, mitochondria-mediated apoptosis. Hence, the neurotoxic property of PrPC could be explained by the mitochondria-mediated neuronal apoptosis, at least in part.

Localization of an alkyl-acetyl-glycerol-CDP-choline: cholinephosphotransferase activity in submitochondrial fractions of Tetrahymena pyriformis

Tetrahymena pyriformis contains platelet-activating factor (PAF) as a minor lipid, which is biosynthesized de novo. A dithiothreitol-insensitive CDP-choline:cholinephosphotransferase (AAG-CPT), which utilizes alkyl-acetyl-glycerol as a substrate, had been detected in both the mitochondrial and microsomal fractions of the protozoan. In the present report, localization of this enzyme in submitochondrial fractions was studied. Cell fractionation was evaluated with enzyme and morphological markers. In this respect, succinate dehydrogenase, NADPH:cytochrome c reductase, glucose-6-phosphatase, alkaline phosphatase, monoaminoxidase, and cytochrome c oxidase activities were investigated. In the presence of antimycin A, mitochondrial activity of NADPH-cytochrome c reductase, was increased, while the microsomal one was reduced. Cardiolipin was distributed in the inner mitochondrial membrane. Alkaline phosphatase was found exclusively in the cytosol of the protozoan. The main portion of the dithiothreitol-insensitive AAG-CPT was localized in the inner mitochondrial membrane. Our data indicate that mitochondria are able to produce PAF, which might be associated with their function.

Hyperglycaemia and intramitochondrial glycogen granules in the brain of mice. Ultrastructural study

The mechanism of cytotoxic effects of hyperglycaemia on the brain has not yet been explained and the proposed hypotheses are not fully convincing. In the present study, we aimed to assess the effect of high doses of glucose on the ultrastructure of the mice brain. The results, which are in agreement with the literature data, show that the administration of a single high dose of glucose, as well as its chronic application, leads to accumulation of glycogen granules in the cytoplasm of astrocytes. A new observation is the detection of glycogen granules in the ultrastructurally changed mitochondria of astrocytes as well as in the mitochondria of some synapses. Our hypothesis assumes that excess of glucose may cause an increase in the vulnerability of the brain mitochondria. This in turn may enable glucose and cytoplasmic enzymes to penetrate into the mitochondria and they therein synthesise glycogen. Mitochondrial dysfunction may in turn lead to neurodegeneration by apoptotic process.

[Electron microscope study of haemolymph cells of Decticus verrucivorus (Orthoptera, Tettigoniidae) in larva and imago stages]

The haemolymph of larvae and imago stages of Decticus verrucirus was studied with electron and light microscope. PAS-positive and PAS-negative granules were detected in haemocytes. On the electronograms, granulocytes were recognized as the only type of haemocytes. In the cytoplasm of granulocytes, granules of two types were found: those of mitochondrial origin, and originating from the Golgi apparatus, respectively. The discharge of a secret is realized by the merocrine way. Four stages of granulocyte development have been distinguished: 1) granule formation and organelle development, 2) granule formation and accumulation, 3) active secretion, and 4) cell destruction.

Novel mitochondrial creatine transport activity. Implications for intracellular creatine compartments and bioenergetics

Immunoblotting of isolated mitochondria from rat heart, liver, kidney, and brain with antibodies made against N- and C-terminal peptide sequences of the creatine transporter, together with in situ immunofluorescence staining and immunogold electron microscopy of adult rat myocardium, revealed two highly related polypeptides with molecular masses of approximately 70 and approximately 55 kDa in mitochondria. These polypeptides were localized by immunoblotting of inner and outer mitochondrial membrane fractions, as well as by immunogold labeling in the mitochondrial inner membrane. In addition, a novel creatine uptake via a mitochondrial creatine transport activity was demonstrated by [(14)C]creatine uptake studies with isolated mitochondria from rat liver, heart, and kidney showing a saturable low affinity creatine transporter, which was largely inhibited in a concentration-dependent manner by the sulfhydryl-modifying reagent NEM, as well as by the addition of the above anti-creatine transporter antibodies to partially permeabilized mitochondria. Mitochondrial creatine transport was to a significant part dependent on the energetic state of mitochondria and was inhibited by arginine, and to some extent also by lysine, but not by other creatine analogues and related compounds. The existence of an active creatine uptake mechanism in mitochondria indicates that not only creatine kinase isoenzymes, but also creatine transporters and thus a certain proportion of the creatine kinase substrates, might be subcellularly compartmentalized. Our data suggest that mitochondria, shown here to possess creatine transport activity, may harbor such a creatine/phosphocreatine pool.

Immuno-electron localization of DNA in chondriolites of Saccharomyces cerevisiae mitochondria

Under electron microscope, the matrix of sectioned mitochondria exhibits ribosomes and an oval, electron-transparent zone which is devoid of ribosomes and is named chondriolite. Fine fibers or clumps of an electron-dense material appeared in this zone after several fixation and contrasting steps and were identified with mitochondrial DNA by cytologists. To verify this assumption, we labeled DNA by a monoclonal antibody and a secondary antibody coupled to immunogold. The label was observed in the nucleus and in the chondriolite zone of sectioned mitochondria. Because the ultrastructure of chondriolites resembles that of nucleoids of prokaryotes, we suggest the term mitochondrial nucleoid for the zone of mitochondrial matrix devoid of ribosomes and containing DNA.

Fine structural modifications of liver, pancreas and brown adipose tissue mitochondria from hibernating, arousing and euthermic dormice

An ultrastructural and morphometric study was performed on mitochondria of euthermic, hibernating and arousing hazel dormice (Muscardinus avellanarius), in order to investigate possible modifications during the seasonal cycle. Hepatocytes, pancreatic acinar cells and brown adipocytes were considered. Our results demonstrated that: (1) the general morphology of mitochondria of all cell types shows slight modifications during the seasonal cycle; (2) mitochondrial size and inner membrane length significantly increase from euthermia to hibernation and decrease upon arousal in all cell types; (3) mitochondrial matrix granules drastically increase in number during hibernation and decrease upon arousal in hepatocytes and pancreatic acinar cells, whereas they do not change in brown adipocytes. These structural modifications are probably related to the changes in cellular energy needs during the euthermia-hibernation-arousal cycle.

The many shapes of mitochondrial membranes

The roles of mitochondria in cell death and in aging have generated much excitement in recent years. At the same time, however, a quiet revolution in our thinking about mitochondrial ultrastructure has begun. This revolution started with the use of vital dyes and of green fluorescent protein fusion proteins, showing that mitochondria are very dynamic structures that constantly move, divide and fuse throughout the life of a cell. More recently, some of the first proteins contributing to these various processes have been discovered. Our view of the internal structures of mitochondria has also changed. Three-dimensional reconstructions obtained with high voltage electron microscopy show that cristae are often connected to the mitochondrial inner membrane by thin tubules. These new insights are brought to bear on the wealth of data collected by conventional electron microscopic analysis.

Dynamics of the mitochondrial reticulum in live cells using Fourier imaging correlation spectroscopy and digital video microscopy

We report detailed studies of the dynamics of the mitochondrial reticulum in live cells using two independent experimental techniques: Fourier imaging correlation spectroscopy and digital video fluorescence microscopy. When both methods are used to study the same system, it is possible to directly compare measurements of preaveraged statistical dynamical quantities with their microscopic counterparts. This approach allows the underlying mechanism of the observed rates to be determined. Our results indicate that the dynamics of the reticulum structure is composed of two independent contributions, each important on very different time and length scales. During short time intervals (1-15 sec), local regions of the reticulum primarily undergo constrained thermally activated motion. During long time intervals (>15 sec), local regions of the reticulum undergo long-range "jump" motions that are associated with the action of cytoskeletal filaments. Although the frequency of the jumps depend on the physiological state of the cells, the average jump distance ( approximately 0.8 microm) is unaffected by metabolic activity. During short time intervals, the dynamics appear to be spatially heterogeneous, whereas the cumulative effect of the infrequent jumps leads to the appearance of diffusive motion in the limit of long time intervals.

Ultrastructural studies on the effect of heat shock treatment on larval salivary gland cells of Drosophila auraria

In this study, the effect of heat shock treatment on Drosophila auraria late 3rd instar larval salivary glands was examined. Heat shock treatment was applied on whole animals and on isolated salivary glands. The fine structural changes were examined using transmission electron microscopy, after a temperature rise from normal (25 +/- 1degreesC) to 37 degreesC or 40 degreesC for various periods of time. The AcPace histochemical technique was used to demonstrate the acid phosphatase activity on lysosomal structures and x-ray microanalysis to determine the elemental composition of intramitochondrial granules. Our results indicate that the extent of heat shock damage on salivary gland cells depends on the heat shock intensity (temperature and duration). Three main changes were observed after heat shock treatment: a) appearance of lysosomal structures; b) alteration in the mitochondrial morphology and appearance of intramitochondrial granules and c) morphological alterations of secretory granules. Vesiculation of the Golgi complex and dilation of the rough endoplasmic reticulum were often seen. Irregular structures of unknown function were observed in the cytoplasm, which are referred to as x-structures. Rectangular secretory granules were observed in some cases, for the first time in a Drosophila species. These results are discussed in correlation with the heat shock effect on larval salivary glands of Drosophila.

Properties and submitochondrial localization of pig and rat renal phosphate-activated glutaminase

Two pools of phosphate-activated glutaminase (PAG) were separated from pig and rat renal mitochondria. The partition of enzyme activity corresponded with that of the immunoreactivity and also with the postembedding immunogold labeling of PAG, which was associated partly with the inner membrane and partly with the matrix. The outer membrane was not labeled. PAG in intact mitochondria showed enzymatic characteristics that were similar to that of the membrane fraction and also mimicked that of the polymerized form of purified pig renal PAG. PAG in the soluble fraction showed properties similar to that of the monomeric form of purified enzyme. It is indicated that the pool of PAG localized inside the inner mitochondrial membrane is dormant due to the presence of high concentrations of the inhibitor glutamate. Thus the enzymatically active PAG is assumed to be localized on the outer face of the inner mitochondrial membrane. The activity of this pool of PAG appears to be regulated by compounds in the cytosol, of which glutamate may be most important.

The malonyl-CoA-sensitive form of carnitine palmitoyltransferase is not localized exclusively in the outer membrane of rat liver mitochondria

The data used to support the idea that malonyl-coenzyme A (CoA)-sensitive carnitine palmitoyltransferase (CPT-I) is localized on the outer mitochondrial membrane are based on harsh techniques that disrupt mitochondrial physiology. We have turned to the use of the French press, which produces a shearing force that denudes mitochondria of their outer membrane without the physiologically disruptive effects characteristic of phosphate swelling. Our results indicate that the mitoplasts contain just 15-19% of the outer membrane marker enzyme activity while retaining 85% of the total CPT activity and 50% of both CPT-I, as well as long-chain acyl-CoA synthase activity, the latter two supposed outer membrane enzymes. These mitoplasts were shown by electron microscopy to have the configuration of mitochondria that merely have been divested of their outer membranes. Carnitine-dependent fatty acid oxidation was retained in the mitoplasts, showing that they were physiologically intact. Moreover, protein immunoblotting analysis showed that CPT-I, as well as the inner CPT-II, was localized in the mitoplast fraction. The outer membrane fraction, which consisted of membrane "ghosts," contained most (50-60%) of marker enzyme activity, monoamine oxidase-B and porin proteins, but only about 27-29% CPT-I activity. Because CPT-I and long-chain acyl-CoA synthetase appear to be associated with both inner and outer membranes, we postulate that these enzymes reside in contact sites, which represent a melding of both limiting membranes.

Bcl-2 is located predominantly in the inner membrane and crista of mitochondria in rat liver

Bcl-2 is now recognized as a potent inhibitor of apoptotic cell death. It has been reported that Bcl-2 is located in the mitochondria, endoplasmic reticulum, and nuclear membrane in some cell lines, and it is not expressed in normal human and rat liver. An earlier study showed that Bcl-2 is an inner mitochondrial membrane protein. On the contrary, the following investigations using immunoelectron microscopy demonstrated that Bcl-2 resides predominantly in the mitochondrial outer membrane. In this study, using a cryo-sectioning immunogold labeling technique and immunoblotting, we carefully determined the subcellular localization of Bcl-2. Here we report that Bcl-2 is expressed in normal rat liver, and it is located predominantly in the inner membrane and crista rather than in the outer membrane of mitochondria.

Calcium distribution in high-pressure frozen bone cells by electron energy loss spectroscopy and electron spectroscopic imaging

Subcellular localization of total calcium requires tissue processing that preserves the chemical composition of the samples and a highly sensitive microanalytical technique. In this study rat fetal bone samples were submitted to high-pressure freezing and freeze substitution. Ultrastructural preservation was good in the superficial sections: osteoblasts near the bone mineral had clearly defined plasma and nuclear membranes, dense mitochondria, and numerous ribosomes. Electron energy loss spectroscopy allowed high-resolution calcium-sensitive images to be obtained using ionization edge loss electrons. In biological samples, the Ca-L2,3 signal is superimposed on the carbon edge and artifacts may result from thickness and scattering effects. Therefore the relative thickness was established for each area analyzed (t/lambda <0.5). Background was subtracted using the three-images method, allowing high resolution calcium-sensitive images of intramitochondrial granules and of intracellular compartments, and semiquantitative data from the granules to be obtained. Calcium maps were confirmed by spectra collected on defined areas of the images and the shape of the net Ca-L2,3 edges was compared to the characteristic Ca-L2,3 edge of bone crystals. These procedures will provide new information about total calcium localization in bone cells and the possibility of examining the distribution of other elements.

Analysis of the functional domain of the rat liver mitochondrial import receptor Tom20

Tom20 is an outer mitochondrial membrane protein and functions as a component of the import receptor complex for the cytoplasmically synthesized mitochondrial precursor proteins. It consists of the N-terminal membrane-anchor segment, the tetratricopeptide repeat (TPR) motif, a charged amino acids-rich linker segment between the membrane anchor and the TPR motif, and the C-terminal acidic amino acid cluster. To assess the functional significance of these segments in mammalian Tom20, we cloned rat Tom20 and expressed mutant rat Tom20 proteins in Deltatom20 yeast cells and examined their ability to complement the defects of respiration-driven growth and mitochondrial protein import. Tom20N69, a mutant consisting of the membrane anchor and the linker segments, was targeted to mitochondria and complemented the growth and import defects as efficiently as wild-type Tom20, whereas a mutant lacking the linker segment did not. In vitro protein import into mitochondria isolated from the complemented yeast cells revealed that the precursor targeted to yeast Tom70 was efficiently imported into the mitochondria via rat Tom20N69. Thus the linker segment is essential for the function of rat Tom20, whereas the TPR motif and the C-terminal acidic amino acids are not.

Adsorptive immobilization of submitochondrial particles on concanavalin A Sepharose-4B

Submitochondrial particles (SMPs) prepared from beef liver mitochondria were immobilized on concanavalin A Sepharose-4B (Con A-Sepharose). The process of immobilization was optimized by choosing an appropriate buffer system containing Mn2+ and Ca2+. Adsorption of SMPs on Con A-Sepharose was found to be reversible process, nonelectrostatic in nature, and responsive to the presence of methyl alpha-d-glucopyranose and alpha-d-mannose. The involvement of membrane glycoproteins in the adsorption process was thus demonstrated. Further analysis of the data obtained on competition of binding by sugar molecules provided competition constants reflecting the potency of inhibition by each individual carbohydrate. Positive-cooperative interactions for binding to the matrix were observed especially at high concentrations of SMPs. The immobilized preparations were used successfully in continuous catalytic transformations involving succinate-cytochrome c reductase (SCR) and enzyme complex of the inner-mitochondrial membrane. Best results were obtained when such operations were carried out at 37 degrees C.

Effect of reversible ischemia on the activity of the mitochondrial ATPase: relationship to ischemic preconditioning

The mitochondrial ATPase enzyme accounts for roughly 35-50% of the overall energy demand that leads to ATP depletion under conditions of severe myocardial ischemia. In larger mammalian hearts, this energy squandering action of the ATPase is modulated by an endogenous inhibitor protein. The present studies were undertaken to characterize the time course of inhibition of the mitochondrial ATPase in canine myocardium under conditions of severe regional ischemia in vivo. In addition, we determined if the energy sparing effects of ischemic preconditioning (PC) can be explained by persistent inhibition of the mitochondrial ATPase enzyme. The circumflex coronary artery was ligated for 1.5 min (n = 4), 5 min (n = 6), or 15 min (n = 5). In a separate group (n = 7), hearts were preconditioned by four 5-min periods of ischemia each followed by 5 min of reperfusion. Sub-mitochondrial particles were prepared from the sub-endocardial zone of the ischemic and non-ischemic regions and were assayed for oligomycin-sensitive ATPase activity. ATPase activity was reduced to about 79% at 1.5 min and to approximately 55% at 5 and 15 min of ischemia, relative to non-ischemic tissue from the same heart. The rate of HEP utilization slowed concurrently with the development of ATPase inhibition. In preconditioned myocardium, ATPase activity was not significantly different from control myocardium from the same heart. We conclude that the early inhibition of the mitochondrial ATPase activity slows the utilization of high energy phosphate and thereby serves as an important endogenous cardioprotective mechanism. Nevertheless, altered activity of the ATPase is not the explanation of the energy sparing effect of ischemic preconditioning.

Direct analysis and significance of cardiolipin transverse distribution in mitochondrial inner membranes

The distribution of cardiolipin across the inner mitochondrial membrane was directly determined by using the ability of the fluorescent dye 10-N-nonyl-3,6-bis(dimethylamino)acridine (10-N-nonyl acridine orange) to form dimers when it interacts with the diacidic phospholipid. Two independent methods were employed: (a) a spectrophotometric measurement of 10-N-nonyl acridine orange binding to isolated rat liver mitochondria, mitoplasts and inside-out submitochondrial particles, and (b) a flow-cytometric analysis of specific red fluorescence, emitted when two dye molecules are bound to one membrane cardiolipin; the stoichiometry of 10-N-nonyl acridine orange binding to phosphatidylserine and phosphatidylinositol, 1 mol dye/mol phospholipid, prevented dye dimerisation and subsequent red-fluorescence appearance. 57% total cardiolipin was present in the outer leaflets of inner membranes of isolated organelles, a distribution confirmed by saturation measurements for mitoplasts and inside-out submitochondrial particles. The same asymmetry was directly observed in situ with mitochondrial membranes of quiescent L1210 cells, and with mitochondrial membranes of respiring yeasts. Nevertheless, alterations in ATP synthesis and inhibition of mitochondrial protein synthesis revealed that cardiolipin distribution was apparently tightly correlated with mitochondrial membrane assembly and activity.

Involvement of mitochondrial contact sites in the subcellular compartmentalization of phospholipid biosynthetic enzymes

The concerted synthesis of phospholipids derived from serine involving two microsomal enzymes (phosphatidylserine synthase and phosphatidylethanolamine N-methyltransferase) and a mitochondrial one (phosphatidylserine decarboxylase) occurs in reconstituted cell-free systems. Subfractionation of crude mitochondria after swelling and separating on a sucrose density gradient resulted in the isolation of two contact site-enriched fractions from total outer membranes and inner membranes, respectively. Estimation of marker enzyme activities shows a high recovery of glucose-6-phosphate phosphatase (a marker for the endoplasmic reticulum) associated with contact site-enriched fractions. Accordingly, the linked synthesis of phosphatidylserine, phosphatidylethanolamine, and at a lesser extent phosphatidylcholine can occur. This biosynthetic pathway was absent from purified contact site-enriched fractions correlative with the absence of glucose-6-phosphate phosphatase activity. Reconstitution experiments, including contact site-enriched fractions incubated with endoplasmic reticulum-rich fraction, led to the restoration of the linked synthesis of phospholipids, thereby demonstrating that a reversible association between these two fractions can occur. These functional interactions between the endoplasmic reticulum and mitochondria are confirmed at the ultrastructural level using either chemical or physical fixation before resin embedding. These results show that the interorganelle trafficking of lipids may involve only highly specialized microdomains of both membranes, thereby allowing the maintenance of a specific lipid composition and distribution within membranes.

Investigation of the subcellular distribution of the bcl-2 oncoprotein: residence in the nuclear envelope, endoplasmic reticulum, and outer mitochondrial membranes

A multidisciplinary approach was taken to investigate the intracellular locations of the 26-kDa integral membrane protein encoded by the bcl-2 gene. Subcellular fractionation analysis of a t(14;18)-containing lymphoma cell line revealed the presence of Bcl-2 protein in nuclear, heavy-membrane, and light-membrane fractions but not in cytosol. Sedimentation of heavy-membrane fractions in Nycodenz and Percoll continuous gradients demonstrated comigration of p26-Bcl-2 with mitochondrial but not other organelle-associated proteins. Fractionation of light-membrane fractions using discontinuous sucrose-gradients revealed association of Bcl-2 protein primarily with lighter-density microsomes (smooth endoplasmic reticulum) as opposed to heavy-density microsomes (rough endoplasmic reticulum). Immune microscopy studies using laser-scanning microscopy, pre- and postembedding electron microscopic methods, and six different anti-Bcl-2 antibodies demonstrated Bcl-2 immunoreactivity in the nuclear envelope and outer mitochondrial membrane in a patchy distribution. Furthermore, anti-Bcl-2 antibody immunoreactivity generally appeared to directly overlie the nuclear envelope in high magnification electron microscopic studies, reminiscent of nuclear pore complexes. Addition of in vitro translated p26-Bcl-2 to isolated translocation-competent mitochondria revealed transmembrane domain-dependent association of Bcl-2 protein with mitochondria but provided no evidence for import into a protease-resistant compartment, consistent with immunomicroscopic localization to the outer mitochondrial membrane. Taken together, the findings demonstrate that p26-Bcl-2 resides primarily in the nuclear envelope, endoplasmic reticulum, and outer mitochondrial membrane in a nonuniform distribution suggestive of participation in protein complexes perhaps involved in some aspect of transport.

Reaction of bulk protons with a mitochondrial inner membrane preparation: time-resolved measurements and their analysis

The laser-induced proton pulse technique [Gutman, M. (1986) Methods Enzymol. 127, 522-538] was applied on suspensions of submitochondrial vesicles, and the exchange of protons between the bulk and the mitochondrial membranes was measured in the time-resolved domain with a submicrosecond resolution. The protons were discharged by photoexcitation of pyranine (8-hydroxypyrene-1,3,6-trisulfonate) by a short laser pulse, and the reprotonation of the pyranine anion was monitored at 457.8 nm. In parallel, the protonation of the membrane was followed at 496.5 nm, looking at the transient absorbance of fluorescein, covalently attached to the M side of the membrane. The analysis of the relaxation dynamics was carried out by a simulation procedure that reconstructs the observed dynamics of the two chromophores. The analysis revealed the presence of the membrane indigenous buffering moieties. The low-pK buffer (pK 4.1) was present in a quantity of 100 +/- 20 nmol/mg of protein, and its kinetics indicate that it appears in multianionic clusters bearing a negative electric charge. The medium-pK buffer (pK 6.9) was present in a larger quantity (200 +/- 20 nmol/mg), and its kinetic parameter indicated clustering into positively charged domains. Both types of indigenous buffer reacted with the proton and pyranine anion in unhindered diffusion-controlled reactions. On the other hand, the exchange of protons between the indigenous buffer moieties was rather slow. No evidence was found for the presence of sites capable of retaining a proton, secluded from the bulk, for a time frame longer than 100 microseconds as required by the models of localized proton gradient.

Enhanced enzymatic degradation of radical damaged mitochondrial membrane components

The location of a protein (soluble or membrane-bound) influences the extent of oxidative damage caused by free radicals. It has been established that after radical attack, soluble proteins can become more susceptible to hydrolysis by individual proteinases than native proteins. We have now examined the hydrolytic susceptibility following radical attack of a protein that is located within a membrane environment, mitochondrial monoamine oxidase (MAO). After exposure to oxygen radicals generated by gamma irradiation, hydrolysis of sub-mitochondrial particles (SMP) containing MAO was increased in three respects. First, the generation of small fragments of MAO by the proteinases elastase and trypsin, was enhanced. Second, the generation by these enzymes and by phospholipase A2 of non-sedimentable membrane fragments containing MAO was also increased. Third, autolysis of SMP was enhanced. Hence, proteins located within membranes may become more susceptible to enzymatic degradation following oxidative damage.

Structure of paracrystalline arrays on outer membranes of rat-liver and rat-heart mitochondria

Crystalline arrays are induced in outer membranes of rat-liver and rat-heart mitochondria by phosphotungstate and silicotungstate. The basic structure of the arrays has been determined by correlation averaging of electron microscopic images of side views of tubular arrays and en face views of planar arrays. The arrays consist of rows of bilobed projecting subunits and are similar (in lattice parameters and projected subunit dimensions) to periodic arrays of ion transport ATPases, e.g., arrays of Ca(2+)-ATPase induced by vanadate in sarcoplasmic reticulum. Hexokinase-labeled colloidal gold particles do not specifically decorate the arrays, suggesting that the hexokinase receptor (VDAC channel) is not a component of the arrays.

Ultrastructural aspects and amino acid composition of the purified inner and outer membranes of human liver mitochondria as compared to rat liver mitochondria

1. The mitochondria isolated from human or rat liver were fractionated into submitochondrial particles and purified inner and outer membrane. According to different marker enzymes the inner membranes were enriched about 5-6-fold and the outer membranes about 12-14-fold. The electron microscopical appearance of the membranes was that expected on the basis of enzymic characterization. 2. A comparison of the average amino acid composition of the membrane proteins from the two types of mitochondria has been made. In the case of submitochondrial particles there were statistically significant differences between the human and rat hydrolysates for only five amino acids. Analysing the purified mitochondrial membranes there were significant differences between the two species for nine amino acids in the case of outer membranes and for 12 amino acids in the case of inner membranes. 3. With one exception all amino acids that were increased or decreased in the outer membrane exhibited a similar trend in the inner membrane of human compared with rat liver mitochondria. It appears that liver mitochondrial membranes have a species-dependent pattern of amino acid composition of their proteins.

X-ray diffraction by crystals of beef heart ubiquinol: cytochrome c oxidoreductase

Beef heart mitochondrial ubiquinol:cytochrome c oxidoreductase has been crystallized in the shape of hexagonal bipyramids. At present the crystals diffract X-rays to 4.7 A. From preliminary analysis the diffraction pattern appears to be consistent with space group P6(1)22 or P6(5)22 and with unit cell parameters a = b = 212 A and c = 352 A.

Identification of a GTP-binding protein in the contact sites between inner and outer mitochondrial membranes

Whilst investigating whether GTP hydrolysis may be required for the import of preproteins into mitochondria we have found that a GTP-binding protein is located at the contact sites between mitochondrial inner and outer membranes. When mitochondrial outer membranes purified from rat liver were UV-irradiated in the presence of [alpha-32P]GTP, a 52 kDa protein was radiolabelled, whereas [alpha-32P]ATP did not label this protein. GTP-binding proteins were also labelled in the cytosolic and microsomal fractions, but the 52 kDa protein was concentrated in mitochondrial membranes and was the only protein specifically labelled by GTP in these membranes. Fractionation of mitochondrial membrane vesicles into outer membranes, inner membranes and contact sites between outer and inner membranes showed that the GTP-binding activity was highly enriched in contact sites, the location at which preprotein import is believed to occur. A protein of almost identical size was also found to be labelled in mitochondria from yeast.

Isolation of highly purified lysosomes from rat liver: identification of electron carrier components on lysosomal membranes

Using Percoll density gradient centrifugation after treatment of the postnuclear supernatant (PNS) with 1 mM Ca2+ to swell and lighten mitochondria, we isolated highly purified lysosomes (dextranosomes) in high yield (25%) from the livers of rats to which dextran had been administered. The lysosomal fraction obtained by this method was enriched more than 100-fold in N-acetyl-beta-glucosaminidase and arylsulfatase and 40-fold in acid phosphatase and beta-glucosidase. Electron microscopic examination and measurement of marker enzyme activity for various subcellular organella indicated that the lysosomal fraction was essentially free from contamination by other organella. Flavins, ubiquinones, and hemochromes were found on lysosomal membranes and investigated. The FAD and ubiquinone-9 contents of the purified lysosomal membranes were 0.118 and 6.93 nmol/mg of protein, respectively. Hemochromes in lysosomes showed spectra similar to that of a b-type cytochrome, with the alpha-peak at 562 nm and the gamma-peak at 436 nm.

Development of mammalian endothermic metabolism: quantitative changes in tissue mitochondria

The development of energy metabolism of mammalian tissues was assessed in the tammar wallaby Macropus eugenii by the measurement of mitochondrial parameters in the liver, heart, kidney, and brain. Tissues taken from wallabies (n = 27) ranging from 10-day-old pouch young (weighing approximately 4 g) to adults (averaging 6.2 kg) were weighed and fixed, and mitochondrial volume and mitochondrial membrane surface area (MMSA) were determined by quantitative electron microscopy techniques. Developmental changes in these parameters were analyzed chronologically and allometrically. Relative growth rates of all four tissues decreased during development. Liver and heart showed constant allometric growth throughout development, whereas kidney and brain showed biphasic allometric growth. Tissue metabolic intensity assessed by MMSA (m2/cm3 tissue) was constant in liver, showed a threefold increase in brain during pouch life, showed a fourfold increase in the heart between 100 and 200 days of age, and showed a twofold increase in the kidney at the end of pouch life. In all tissues, adult levels of tissue metabolic capacity were present at pouch exit. In all four tissues, total MMSAs were at "reptilian" levels at birth and gradually increased to "mammalian" levels. Each tissue exhibited a different developmental timetable. When the total MMSAs for all four tissues were summed there was a similar pattern of allometric development between summed MMSA and whole animal metabolic rate.

Contribution of renal medullary mitochondrial density to urinary concentrating ability in mammals

In mammals, the length of the loops of Henle increases with increasing body size without a concomitant rise in urinary concentrating ability. Because mass-specific metabolic rate falls with increasing body mass, this study sought to determine the extent to which this decline in metabolic rate could explain the low urinary concentrating ability of large mammals with long loops of Henle. Mitochondrial ultrastructural parameters were measured in the medullary thick ascending limbs (mTALs) of a series of nine mammalian genera ranging in body mass from 0.011 kg (bats) to approximately 400 kg (horses). The volume of mitochondria as a percent of mTAL cellular volume declined with increasing body mass (Mb-0.056). Inner mitochondrial membrane area per volume of mitochondrion also declined with increasing body mass (Mb-0.034), as did basolateral membrane area per unit mTAL cellular volume (Mb-0.075). Thus, not only do mitochondria occupy more volume of mTAL cells of smaller mammals, but those mitochondria are also more densely packed with cristae. Inner mitochondrial membrane area per unit volume of mTAL cell cytoplasm scaled as Mb-0.092. The decline in inner mitochondrial membrane area and basolateral membrane area per volume of mTAL cell may explain at least in part the relationship between body mass and renal concentrating ability in mammals of different sizes.

Subcellular distribution of imidazoline-guanidinium-receptive sites in human and rabbit liver. Major localization to the mitochondrial outer membrane

Imidazoline-guanidinium-receptive site (IGRS) is a membrane protein that, even if recognized by a series of imidazoline and guanidinium alpha 2-adrenergic compounds, is insensitive to catecholamine and physically distinct from alpha 2 receptors (Parini, A., Coupry, I., Graham, R. M., Uzielli, I., Atlas, D., and Lanier, S. M. (1989) J. Biol. Chem. 264, 11874-11878). In the present report, we defined the subcellular localization of IGRS by performing binding studies with the imidazoline radioligand [3H]idazoxan. Binding studies on subcellular fractions of homogenates from human and rabbit liver showed a significant increase in [3H]idazoxan binding in a membrane fraction enriched in cytochrome oxidase activity, a specific marker for mitochondria. The enrichment in [3H]idazoxan binding sites correlates closely with cytochrome oxidase activity in the nuclear, mitochondrial, plasma membrane, microsomal, and soluble fractions (r = 0.966, p less than 0.002) but not with the specific markers for other cell compartments, suggesting a major localization of IGRS in mitochondria. Separation of inner and outer mitochondrial membranes by digitonin treatment showed that [3H]idazoxan binding correlates positively with monoamine oxidase (r = 0.960) and negatively with cytochrome oxidase (r = -0.950) activities. In addition, in highly purified preparations of outer mitochondrial membranes obtained by hypotonic shock, [3H]idazoxan binding activity was 12.5-fold enriched with respect to intact mitochondria. Taken together, these data show, for the first time, that IGRS in human and rabbit liver are mainly associated with the outer mitochondrial membranes. This demonstration of the major mitochondrial localization of IGRS will facilitate the characterization of its functional activity in liver.

Identification of a mitochondrial receptor complex required for recognition and membrane insertion of precursor proteins

The mitochondrial import receptors MOM19 and MOM72 form a complex with two other proteins of the mitochondrial outer membrane, MOM38 and MOM22. This receptor complex is involved in recognition, membrane insertion and translocation of precursor proteins with MOM38 constituting (at least part of) the general insertion site GIP.

Improved methods to isolate and subfractionate rat liver mitochondria. Lipid composition of the inner and outer membrane

Rat liver mitochondria were isolated by a combination of differential and Percoll gradient centrifugation, resulting in a highly pure and intact preparation, as assessed by marker enzyme analysis, latency of cytochrome-c oxidase, respiratory control index and electron microscopy. Two different methods were compared for the separation of inner and outer membranes. In the swell-shrink-sonicate procedure glycerol was included resulting in the isolation of one outer membrane and two inner membrane fractions of high purity. Using digitonin a highly selective and gradual solubilization of the outer membrane could be accomplished. Analysis of the phospholipid composition of the intact mitochondria and all subfractions showed that the inner membrane was virtually devoid of phosphatidylinositol and -serine, while the outer membrane contained 23% of the total mitochondrial cardiolipin, which did not originate from inner membrane contamination and therefore is a true component of the outer membrane.

Intermitochondrial junctions in a subpopulation of peripheral blood lymphocytes from healthy subjects

This paper reports the observation of intermitochondrial junctions (IMJ) with a periodicity of 15.4 +/- 0.65 nm in peripheral blood lymphocytes of healthy subjects. The percentage of IMJ-containing cells appears constant (about 10% of examined lymphocytes) and is independent of the delay of fixation. Immunogold staining reveals that lymphocytes with IMJ exhibit a T-phenotype. IMJ have been reported in other types of tissues but, to our knowledge, have not been previously described in the blood cell. The signification of these structures is discussed.

Protein import into mitochondria: ATP-dependent protein translocation activity in a submitochondrial fraction enriched in membrane contact sites and specific proteins

To identify the membrane regions through which yeast mitochondria import proteins from the cytoplasm, we have tagged these regions with two different partly translocated precursor proteins. One of these was bound to the mitochondrial surface of ATP-depleted mitochondria and could subsequently be chased into mitochondria upon addition of ATP. The other intermediate was irreversibly stuck across both mitochondrial membranes at protein import sites. Upon subfraction of the mitochondria, both intermediates cofractionated with membrane vesicles whose buoyant density was between that of inner and outer membranes. When these vesicles were prepared from mitochondria containing the chaseable intermediate, they internalized it upon addition of ATP. A non-hydrolyzable ATP analogue was inactive. This vesicle fraction contained closed, right-side-out inner membrane vesicles attached to leaky outer membrane vesicles. The vesicles contained the mitochondrial binding sites for cytoplasmic ribosomes and contained several mitochondrial proteins that were enriched relative to markers of inner or outer membranes. By immunoelectron microscopy, two of these proteins were concentrated at sites where mitochondrial inner and outer membranes are closely apposed. We conclude that these vesicles contain contact sites between the two mitochondrial membranes, that these sites are the entry point for proteins into mitochondria, and that the isolated vesicles are still translocation competent.