MEMBRANE JUNCTIONS IN THE INTERMEMBRANE SPACE OF MITOCHONDRIA FROM MAMMALIAN TISSUES

There have been several reports describing paracrystalline arrays in the intermembrane space of mitochondria. On closer inspection these structures appear to be junctions of two adjoining membranes. There are two types. They can be formed between the outer and inner mitochondrial membranes (designated outer-inner membrane junctions) or between two cristal membranes (intercristal membrane junctions). In rat heart, adjoining membranes appeared associated via a central dense midline approximately 30 Å wide. In rat kidney, the junction had a ladder-like appearance with electron-dense "bridges" approximately 80 Å wide, spaced 130 Å apart, connecting the adjoining membranes. We have investigated the conditions which favor the visualization of such structures in mitochondria. Heart mitochondria isolated rapidly from fresh tissue (within 30 min of death) contain membrane junctions in approximately 10–15% of the cross sections. This would indicate that the percentage of membrane junctions in the entire mitochondrion is far greater. Mitochondria isolated from heart tissue which was stored for 1 h at 0°–4°C showed an increased number of membrane junctions, so that 80% of the mitochondrial cross sections show membrane junctions. No membrane junctions are observed in mitochondria in rapidly fixed fresh tissue or in mitochondria isolated from tissue disrupted in fixative. Thus, the visualization of junctions in the intermembrane space of mitochondria appears to be dependent upon the storage of tissue after death. Membrane junctions can also be observed in mitochondria from other stored tissues such as skeletal muscle, kidney, and interstitial cells from large and small intestine. In each case, no such junctions are observed in these tissues when they are fixed immediately after removal from the animal. It would appear that most studies in the literature in which isolated mitochondria from tissues such as heart or kidney were used were carried out on mitochondria which contained membrane junctions. The presence of such structures does not significantly affect normal mitochondrial function in terms of respiratory control and oxidative phosphorylation.     

The inner membrane protein Mdm33 controls mitochondrial morphology in yeast

Mitochondrial distribution and morphology depend on MDM33, a Saccharomyces cerevisiae gene encoding a novel protein of the mitochondrial inner membrane. Cells lacking Mdm33 contain ring-shaped, mostly interconnected mitochondria, which are able to form large hollow spheres. On the ultrastructural level, these aberrant organelles display extremely elongated stretches of outer and inner membranes enclosing a very narrow matrix space. Dilated parts of Delta mdm33 mitochondria contain well-developed cristae. Overexpression of Mdm33 leads to growth arrest, aggregation of mitochondria, and generation of aberrant inner membrane structures, including septa, inner membrane fragments, and loss of inner membrane cristae. The MDM33 gene is required for the formation of net-like mitochondria in mutants lacking components of the outer membrane fission machinery, and mitochondrial fusion is required for the formation of extended ring-like mitochondria in cells lacking the MDM33 gene. The Mdm33 protein assembles into an oligomeric complex in the inner membrane where it performs homotypic protein-protein interactions. Our results indicate that Mdm33 plays a distinct role in the mitochondrial inner membrane to control mitochondrial morphology. We propose that Mdm33 is involved in fission of the mitochondrial inner membrane.     

Mitochondrial filaments and clusters as intracellular power-transmitting cables

Mitochondria exist in two interconverting forms; as small isolated particles, and as extended filaments, networks or clusters connected with intermitochondrial junctions. Extended mitochondria can represent electrically united systems, which can facilitate energy delivery from the cell periphery to the cell core and organize antioxidant defence of the cell interior when O2 is consumed by mitochondrial clusters near the the outer cell membrane, and protonic potential is transmitted to the cell core mitochondria to form ATP. As to small mitochondria, they might represent a transportable form of these organelles.     

THE INTRACELLULAR MEMBRANES OF BLASTOMYCES DERMATITIDIS

Edwards , George A., and Mercedes R. Edwards . (Div. Labs, and Research, N.Y.S. Dept. of Health, Albany.) The intracellular membranes of Blastomyces dermatitidis. Amer. Jour. Bot. 47 (8): 622–632. Illus. 1960.—The yeast cells of Blastomyces dermatitidis have been studied in thin sections with the electron microscope. The cell is multinucleate, and the nuclei are frequently interconnected by their outer limiting membranes. The cell is bordered by a cell wall and the plasma membrane, which may be seen in direct continuity with the nuclear envelope. The cytoplasm contains numerous mitochondria, many profiles of the endoplasmic reticulum, and few multivesicular bodies. The membranes of all the constant cellular components are interconnected. Mitochondria appear to be formed from any of several membrane systems. The micromorphology of the cell suggests efficiency of communication and cytoplasmic mobility.     

Cytochrome c sorption-desorption effects on the external NADH oxidation by mitochondria: experimental and computational study

The rupture of the outer mitochondrial membrane is known to be critical for cell death, but the mechanism, specifically its redox-signaling aspects, still needs to be studied in more detail. In this work, the external NADH oxidation by rat liver mitochondria was studied under the outer membrane rupture induced by the mitochondria hypotonic treatment or the inner membrane permeability transition. The saturation of the oxidation rate was observed as a function of mitochondrial protein concentration. This effect was shown to result from cytochrome c binding to the mitochondrial membranes. At a relatively high concentration of mitochondria, the oxidation rate was strongly activated by 4 mm Mg(2+) due to cytochrome c desorption from the membranes. A minimal kinetic model was developed to explain the main phenomena of the external NADH oxidation modulated by cytochrome c and Mg(2+) in mitochondria with the ruptured outer membrane. The computational behavior of the model closely agreed with the experimental data. We suggest that the redox state of the released cytochrome c, considered by other authors to be important for apoptosis, may strongly depend on its oxidation by the fraction of mitochondria with the ruptured outer membrane and on the cytoplasmic cytochrome c reductase activity.     

Ceramide synthesis in the endoplasmic reticulum can permeabilize mitochondria to proapoptotic proteins

Increased mitochondrial ceramide levels are associated with the initiation of apoptosis. There is evidence that ceramide is causal. Thus, the conversion of the precursor, dihydroceramide, to ceramide by the enzyme dihydroceramide desaturase may be important in preparing the cell for apoptosis. Ceramide can initiate apoptosis by permeabilizing the mitochondrial outer membrane to apoptosis-inducing proteins. However, the mitochondrion's ability to produce ceramide may be limited by its proteome. Here, we show that ceramide synthesized in isolated mammalian endoplasmic reticulum (ER) vesicles from either C8-dihydroceramide or sphingosine to produce long-chain ceramide can transfer to isolated mitochondria. The rate of transfer is consistent with a simple collision model. The transfer of the long-chain ceramide is faster than expected for an uncatalyzed process. Sufficient ceramide is transferred to permeabilize the outer membrane to cytochrome c and adenylate kinase. The mitochondria-associated membranes, ER-like membranes that are tightly associated with isolated mitochondria, can produce enough ceramide to permeabilize the outer membrane transiently. Thus, this ceramide exchange obviates the need for a complete ceramide de novo pathway in mitochondria to increase ceramide levels to the critical value required for functional changes, such as ceramide channel self-assembly followed by protein release.     

Targeting and insertion of nuclear-encoded preproteins into the mitochondrial outer membrane

Most mitochondrial proteins are synthesized in the cytosol as preproteins with a cleavable presequence and are delivered to the import receptors on the mitochondria by cytoplasmic import factors. The proteins are then imported to the intramitochondrial compartments by the import systems of the outer and inner membranes, TOM and TIM. Mitochondrial outer membrane proteins are synthesized without a cleavable presequence and most of them contain hydrophobic transmembrane domains, which, in conjunction with the flanking segments, function as the mitochondria import signals. Some of the proteins are inserted into the outer membrane by the TOM machinery; the import signal probably arrests further translocation and is released from the translocation channel to the lipid bilayer. The other proteins are inserted into the membrane by a novel pathway independent of the TOM machinery. This article reviews recent developments in the biogenesis of mitochondrial outer membrane proteins.     

大鼠心肌线粒体内、外膜磷脂动态结构的研究

我们以DPH为荧光探针.用毫微秒荧光分光光度计测定了大鼠心肌线粒体及线粒体内、外膜的动态微细结构;用HPLC分析了磷脂组成.实验结果提示.完整线粒体膜流动性主要反映了线粒体外膜的运动状态.线粒体内膜微粘度及磷脂分子摇动角大于外膜,扩散速率小于外膜.除去了蛋白质的线粒体内、外膜磷脂脂质体膜流动性无明显差异.提示线粒体内膜的高微粘度与膜中所含有的多量蛋白有关.     

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.     

Micro-managing the executioner: pathogen targeting of mitochondria

Eukaryotic cell viability is largely regulated at the level of mitochondria, with cell death executed by endogenous proteins that act to increase the permeability of the inner and/or outer membranes of these organelles. The gastric pathogen, Helicobacter pylori, can mimic this mechanism by producing the pro-apoptotic toxin, VacA, which was recently demonstrated to (i) localize to mitochondria within epithelial cells, (ii) rapidly transport into mitochondria in vitro, and (iii) induce changes consistent with permeabilization of mitochondrial membranes by a mechanism dependent on cellular entry and toxin membrane channel activity. The targeting of mitochondrial membranes is emerging as a strategy used by pathogenic microbes to control cell viability while circumventing upstream pathways and checkpoints of cell death.     

Effects of turnip crinkle virus infection on the structure and function of mitochondria and expression of stress proteins in turnips

We have investigated the effect of turnip crinkle virus (TCV) infection on mitochondrial structure and function in turnips ( Brassica rapa cultivar Just Right ). TCV infection resulted in plants with small, mottled leaves with severely crinkled edges, and in a 46% reduction in storage root mass. TCV infection resulted in specific vesicularization of mitochondrial outer membranes where TCV replication is thought to occur, with no apparent affect on other cellular membrane systems. Immunoblot analysis of mitochondrial proteins from storage roots indicated that the TCV p28 protein, which is essential for viral replication, was associated with mitochondria and that mitochondrial heat shock protein 70 and cpn60 levels increased upon TCV infection. Isolation of mitochondrial outer membranes further showed TCV p28 protein enrichment in the outer membrane as compared with total mitochondrial proteins or total cellular proteins. Analysis of mitochondrial electron transport chain activities indicated that TCV infection resulted in a 54% decrease in exogenous NADH-dependent oxygen uptake and a 8% decrease in succinate-dependent oxygen uptake. Together these results indicate that TCV infection induces a stress response in mitochondria and a reduction in the ability of mitochondria to supply adenosine 5'-triphosphate to the cell.     

Multiple partners can kiss-and-run: Bax transfers between multiple membranes and permeabilizes those primed by tBid

During apoptosis Bid and Bax are sufficient for mitochondrial outer membrane permeabilization, releasing pro-apoptotic proteins such as cytochrome c and Smac/Diablo into the cytoplasm. In most cells, both Bid and Bax are cytoplasmic but bind to mitochondrial outer membranes to exert pro-apoptotic functions. Binding to membranes is regulated by cleavage of Bid to truncated Bid (tBid), by conformation changes in tBid and Bax, and by interactions with other proteins. At least at the peripherally bound stage, binding is reversible. Therefore, regulation of apoptosis is closely linked with the interactions of tBid and Bax with mitochondria. Here we use fluorescence techniques and cell-free systems containing mitochondria or liposomes that faithfully mimic tBid/Bax-dependent membrane permeabilization to study the dynamic interactions of the proteins with membranes. We confirm that the binding of both proteins to the membrane is reversible by quantifying the binding affinity of proteins for the membrane. For Bax, both peripherally bound (inactive) and oligomerized (active) proteins migrate between membranes but much slower than and independent of tBid. When re-localized to a new membrane, Bax inserts into and permeabilizes it only if primed by an activator. In the case of tBid, the process of transfer is synergetic with Bax in the sense that tBid ‘runs'' faster if it has been ‘kissed'' by Bax. Furthermore, Mtch2 accelerates the re-localization of tBid at the mitochondria. In contrast, binding to Bcl-XL dramatically impedes tBid re-localization by lowering the off-rate threefold. Our results suggest that the transfer of activated tBid and Bax to different mitochondria is governed by dynamic equilibria and potentially contributes more than previously anticipated to the dissemination of the permeabilization signal within the cell.     

The cytoskeleton of cryofixed Purkinje cells of the chicken cerebellum

Summary Cerebella of 3- to 6-week-old chickens were cryofixed in a nitrogen-cooled propane jet, deep-etched and rotary-shadowed. The use of a brief perfusion of 0.32 M sucrose improved the quality of the cryofixation and allowed the study of the deeper layers of the cerebellar cortex. It is reported that the cytoskeleton of the Purkinje cells (PC) shows distinct domains and composition of filamentous structures in the different regions of the cell cytoplasm, such as the perikaryon, the cytoplasm of dendrites and the axoplasm. The perikaryon is occupied by a meshwork of fine filaments, 4–7 nm in diameter, that extends from the nuclear outer membrane to the cell membrane. In this zone the cell organelles (e.g., endoplasmic reticulum, mitochondria) adopt a circular arrangement around the nucleus. All structures are anchored by microfilaments to the cytoplasmic network. The dendrites show a dense cytoplasmic network including bundles of microtubules, neurofilaments and microfilaments. Numerous aggregated globular components are attached to this cytoskeleton. The cytoskeleton of the dendritic spines shows axially oriented 10-nm bundles of filaments, which are interconnected and anchored also to the cell membrane and the components of the agranular endoplasmic reticulum by cross-linkers. As described in peripheral nerves, the axoplasm of axons in the central nervous system exhibits predominantly neurofilaments and microtubules aligned along the axis of the neuntes in a three-dimensional arrangement and interconnected by cross-linker filaments and filamentous structures.     

Mitochondrial growth and division during the cell cycle in HeLa cells

The growth and division of mitochondria during the cell cycle was investigated by a morphometric analysis of electron micrographs of synchronized HeLa cells. The ratio of total outer membrane contour length to cytoplasmic area did not vary significantly during the cell cycle, implying a continuous growth of the mitochondrial outer membrane. The mean fraction of cytoplasmic area occupied by mitochondrial profiles was likewise found to remain constant, indicating that the increase in total mitochondrial volume per cell occurs continuously during interphase, in such a way that the mitochondrial complement occupies a constant fraction( approximately 10-11(percent)) of the volume of the cytoplasm. The mean area, outer membrane contour length, and axis ratio of the mitochondrial profiles also did not vary appreciably during the cell cycle; furthermore, the close similarity of the frequency distributions of these parameters for the six experimental time-points suggested a stable mitochondrial shape distribution. The constancy of both the mean mitochondrial profile area and the number of mitochondrial profiles per unit of cytoplasmic area was interpreted to indicate the continuous division of mitochondria at the level of the cell population. Furthermore, no evidence was found for the occurrence of synchronous mitochondrial growth and division within individual cells. Thus, it appears that, in HeLa cells, there is no fixed temporal relationship between the growth and division of mitochondria and the events of the cell cycle. A number of statistical methods were developed for the purpose of making numerical estimates of certain three-dimensional cellular and mitochondrial parameters. Mean cellular and cytoplasmic volumes were calculated for the six time-points; both exhibited a nonlinear, approx. twofold increase. A comparison of the axis ratio distributions of the mitochondrial profiles with theoretical distributions expected from random sectioning of bodies of various three-dimensional shapes allowed the derivation of an "average" mitochondrial shape. This, in turn, permitted calculations to be made which expressed the two-dimensional results in three-dimensional terms. Thus, the estimated values for the number of mitochondria per unit of cytoplasmic volume and for the mean mitochondrial volume were found to remain constant during the cell cycle, while the estimated number of mitochondria per cell increase approx. twofold in an essentially continuous manner.     

Biogenesis of mitochondrial outer membrane proteins

Mitochondria are surrounded by two distinct membranes: the outer and the inner membrane. The mitochondrial outer membrane mediates numerous interactions between the mitochondrial metabolic and genetic systems and the rest of the eukaryotic cell. Proteins of this membrane are nuclear-encoded and synthesized as precursor proteins in the cytosol. They are targeted to the mitochondria and inserted into their target membrane via various pathways. This review summarizes our current knowledge of the sorting signals for this specific targeting and describes the mechanisms by which the mitochondrial import machineries recognize precursor proteins, mediate their membrane integration and facilitate assembly into functional complexes.     

Transfer of phosphatidic acid between microsomal and mitochondrial outer and inner membranes

A protein fraction from rat liver cytoplasm, precipitable at 50-95% saturation of ammonium sulphate, binds phosphatidic acid from mitochondrial and microsomal membranes. Protein-bound phosphatidic acid was eluted from Sephadex G-75 in fractions corresponding to a molecular weight of about 10 000. No such binding was observed with mitochondrial soluble proteins, either total or precipitated with ammonium sulphate between 50 and 95% saturation. The transfer of phosphatidic acid from microsomes to mitochondria was increased by liver cytoplasmic proteins precipitable at 50-95% saturation of ammonium sulphate but not with mitochondrial soluble proteins. This increase by cytoplasmic proteins was pronounced in 200 mM sucrose but was negligible in 100 mM KCI where the spontaneous transfer was quite high. Cytoplasmic proteins stimulated the synthesis of cardiolipin and phosphatidylglycerol in mitochondria deprived of the outer membrane but not in intact mitochondria when phosphatidic acid was supplied either by microsomes or liposomes. It is suggested that the transfer of phosphatidic acid from the outer to the inner mitochondrial membrane is not mediated by transfer proteins but occurs either by direct contact of the membranes or as free diffusion through the aqueous phase.     

Deoxycholic acid modulates cell death signaling through changes in mitochondrial membrane properties

Cytotoxic bile acids, such as deoxycholic acid (DCA), are responsible for hepatocyte cell death during intrahepatic cholestasis. The mechanisms responsible for this effect are unclear, and recent studies conflict, pointing to either a modulation of plasma membrane structure or mitochondrial-mediated toxicity through perturbation of mitochondrial outer membrane (MOM) properties. We conducted a comprehensive comparative study of the impact of cytotoxic and cytoprotective bile acids on the membrane structure of different cellular compartments. We show that DCA increases the plasma membrane fluidity of hepatocytes to a minor extent, and that this effect is not correlated with the incidence of apoptosis. Additionally, plasma membrane fluidity recovers to normal values over time suggesting the presence of cellular compensatory mechanisms for this perturbation. Colocalization experiments in living cells confirmed the presence of bile acids within mitochondrial membranes. Experiments with active isolated mitochondria revealed that physiologically active concentrations of DCA change MOM order in a concentration- and time-dependent manner, and that these changes preceded the mitochondrial permeability transition. Importantly, these effects are not observed on liposomes mimicking MOM lipid composition, suggesting that DCA apoptotic activity depends on features of mitochondrial membranes that are absent in protein-free mimetic liposomes, such as the double-membrane structure, lipid asymmetry, or mitochondrial protein environment. In contrast, the mechanism of action of cytoprotective bile acids is likely not associated with changes in cellular membrane structure.     

Contact sites between the outer and inner membrane of mitochondria-role in protein transport

Many essential functions of mitochondrial metabolism have been studied in the past three decades in considerable depth: oxidative phosphorylation, catabolism of fatty acids, role in nitrogen metabolism, and amino acid metabolism. More recently, other aspects attracted much attention like protein translocation into mitochondria, inheritance of mitochondrial DNA, movement of mitochondria, their fusion and fission, and their involvement in apoptosis, ageing, cancer and other cellular processes. Together with these new views on the function of mitochondria, new ideas on the structure of mitochondria emerged. Here we will discuss the current knowledge about how the membranes of mitochondria are organized and how they interact. Interactions between components of the inner and the outer membrane are necessary for a number of central mitochondrial functions such as the channeling of metabolites, coordinated fusion and fission of mitochondria, and protein transport. Some of these interactions appear stable such as the so-called morphological contact sites; others are quite dynamic. Direct evidence that a certain protein is part of morphologically defined contact sites is lacking. Nevertheless, protein translocase complexes of the outer and the inner membrane exhibit stable interactions between the two membranes when precursor proteins are arrested during import into mitochondria. Finally, we discuss possible roles of cristae junctions, another morphologically defined membrane structure in mitochondria.     

Fine structural observations on nuclear maturation during spermiogenesis in Hydra littoralis

Cytodifferentiation during spermiogenesis in Hydra littoralis was studied at the fine structural level. Concentration of nuclear material as well as specific orientation of granular and filamentous nuclear elements are apparent in two regions of the early spermatid: where the nuclear envelope is in contact with mitochondrial membranes at one pole of the cell and at an opposite region where the nucleus is closely apposed to the plasma membrane. Ultimately the mass of condensed nuclear material becomes concentrated at the mitochondrial pole of the cell. Additional electron-dense material is extruded from the nucleus into a large vacuole which is in continuity with the nuclear membrane as well as associated with Golgi lamellae and vesicles. Eventually all residual cytoplasm is sloughed, leaving the nucleus, mitochondria, and flagellum. These observations are suggestive of nucleocytoplasmic interactions during development, especially influences of mitochondria and plasma membranes on chromatin condensation.