Lateral phase separations and structural integrity of the inner membrane of rat-liver mitochondria. Effect of compression. Implications in the centrifugation of these organelles

Effect of centrifugation on in vitro survival of fresh diluted canine spermatozoa

Prostatic fluid is unsuitable for preserving dog semen at 4 degrees C and exerts harmful effects upon the spermatozoa during the freezing process. Centrifugation immediately after sperm collection is a common method to remove prostatic admixture. In the present study, dog semen, diluted to 25 x 10(6)/ml, was exposed for 5 min to four different centrifugation speeds (180 x g, 720 x g, 1620 x g and 2880 x g) to determine subsequent sperm losses in the supernatant and to assess sperm survival over time. Using 180 x g as centrifugation speed, 8.9% of the sperm cells was lost upon supematant removal. Using 720 x g, 1620 x g or 2880 x g, sperm losses were lower, 2.3, 0.4 and 0.006%, respectively. After centrifugation, the sperm pellet was rediluted in egg-yolk-Tris extender, cooled and stored for 3 days at 4 degrees C. Motility, progressive motility, membrane integrity and sperm morphology were assessed daily. Acrosomal status was assessed after 3 days of storage. The only functional parameter which was influenced by centrifugation speed was membrane integrity as evaluated by means of SYBR14-PI staining: significantly more dead and moribund sperm cells were found after centrifugation at 1620 x g and 2880 x g after 48 and 72 h of storage at 4 degrees C. When higher initial sperm concentrations (50 x 10(6), 75 x 10(6) or 100 x 10(6)/ml) were evaluated for sperm losses, less than 2.3% of the initial total sperm cells was lost at lower centrifugation speeds. We conclude that centrifuging dog sperm for 5 min at 720 x g is the best strategy to remove prostatic fluid because the loss of sperm cells is acceptable and the functional parameters of the spermatozoa are well preserved, even after 3 days of storage.

Effects of methylcyclodextrin on lysosomes

The cholesterol complexing agent methyl-cyclodextrin (MCD) provides an efficient mean for the removal of cholesterol from biological membranes. In order to study the effects of this agent on the lysosomal membrane in situ, we treated HepG2 cells with MCD and studied the effects of this treatment on lysosomes in isolated fractions. We found that lysosomes prepared from treated cells are more sensitive to various membrane perturbing treatments such as: incubation of lysosomes in isotonic glucose, in hypotonic sucrose or in the presence of the lytic agent glycyl-L-phenylalanine 2-naphthylamide. The lysosomal membrane is also less resistant to increased hydrostatic pressure. Centrifugation methods were used to analyse the effect of MCD on lysosomes. Isopycnic centrifugation in sucrose density gradients demonstrates that the drug induces a reversible density increase of the lysosomes. Our study indicates that extracellularly added MCD can modify the properties of the lysosomal membrane in living cells. It suggests that MCD could be an effective tool to modulate the physical properties of lysosomes within intact cells and to monitor the cellular responses to such modifications.

Poly(ethylene glycol)-induced and temperature-dependent phase separation in fluid binary phospholipid membranes

Exclusion of the strongly hygroscopic polymer, poly(ethylene glycol) (PEG), from the surface of phosphatidylcholine liposomes results in an osmotic imbalance between the hydration layer of the liposome surface and the bulk polymer solution, thus causing a partial dehydration of the phospholipid polar headgroups. PEG (average molecular weight of 6000 and in concentrations ranging from 5 to 20%, w/w) was added to the outside of large unilamellar liposomes (LUVs). This leads to, in addition to the dehydration of the outer monolayer, an osmotically driven water outflow and shrinkage of liposomes. Under these conditions phase separation of the fluorescent lipid 1-palmitoyl-2[6-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PPDPC) embedded in various phosphatidylcholine matrices was observed, evident as an increase in the excimer-to-monomer fluorescence intensity ratio (IE/IM). Enhanced segregation of the fluorescent lipid was seen upon increasing and equal concentrations of PEG both inside and outside of the LUVs, revealing that osmotic gradient across the membrane is not required, and phase separation results from the dehydration of the lipid. Importantly, phase separation of PPDPC could be induced by PEG also in binary mixtures with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), for which temperature-induced phase segregation of the fluorescent lipid below Tm was otherwise not achieved. In the different lipid matrices the segregation of PPDPC caused by PEG was abolished above characteristic temperatures T0 well above their respective main phase transition temperatures Tm. For 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), DMPC, SOPC, and POPC, T0 was observed at approximately 50, 32, 24, and 20 degrees C, respectively. Notably, the observed phase separation of PPDPC cannot be accounted for the 1 degree C increase in Tm for DMPC or for the increase by 0.5 degrees C for DPPC observed in the presence of 20% (w/w) PEG. At a given PEG concentration maximal increase in IE/IM (correlating to the extent of segregation of PPDPC in the different lipid matrices) decreased in the sequence 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC) > DPPC > DMPC > SOPC > POPC, whereas no evidence for phase separation in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) LUV was observed (Lehtonen and Kinnunen, 1994, Biophys. J. 66: 1981-1990). Our results indicate that PEG-induced dehydration of liposomal membranes provides the driving force for the segregation of the pyrene lipid. In brief, phase separation of PPDPC from the matrix lipid could be attributed to the diminishing effective size of the phosphatidylcholine polar headgroup resulting from its partial dehydration by PEG. This in turn would allow for enhanced van der Waals interactions between the acyl chains of the matrix lipid, which then caused the exclusion of PPDPC due to the perturbing bulky pyrene moiety. Phase separation in DMPC/PPDPC liposomes was abolished by the inclusion of 25 mol % cholesterol and to a lesser extent by epicholesterol.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Cytotoxicity and effect of glycyl-D-phenylalanine-2-naphthylamide on lysosomes

Glycyl-D-phenylalanine-2-naphthylamide (Gly-D-Phe-2-NNap) is a cytotoxic agent as exemplified by its effect on Vero cells in culture. This effect is inhibited to some extent by nigericin. On the other hand, Gly-D-Phe-2-NNap induces an increase of free activity of N-acetylglucosaminidase when incubated with a mitochondrial fraction of rat liver at pH 7.5. The phenomenon is inhibited by chloroquine, NH4Cl and nigericin, substances that are known to increase the intralysosomal pH. The latency of enzymes located in other subcellular structures - mitochondria, peroxisomes and endoplasmic reticulum - is not affected by Gly-D-Phe-2-NNap. Moreover, that compound does not cause a release of FITC-Dextran present in endosomes. Apparently Gly-D-Phe-2-NNap is a specific lytic agent for lysosomes. It is proposed that the molecule behaves like a lysosomotropic substance that is able to attack the lysosomal membrane from the interior of the organelle. Its cytotoxic properties could be explained by its effect on lysosomes.

A freeze-etch study of the effects of extracellular freezing on cellular membranes of wheat

Seedlings of Triticum aestivum L. cv. Lennox were grown in different environments to obtain different hardiness. Pieces of laminae and leaf bases were slowly cooled to sub-zero temperatures and the damage caused was assessed by an ion-leakage method. Comparable pieces of tissue were slowly cooled to temperatures between 2° and-14°C and were then freeze-fixed and freeze-etched. Membranes generally retained their lamellar structures indicated by the abundance of typical membrane fracture faces in all treatments, and some membrane fracture faces had patches which lacked the usual scattering of intramembranous particles (IMP). These IMP-free areas were present in the plasma membrane of tissues given a damaging freezing treatment, but were absent from the plasma membrane of room-temperature controls, of supercooled tissues, and of tissues given a non-damaging freezing treatment. The frequency of IMP-free areas and the proportion of the plasma membrane affected increased with increasing damage. In the most damaged tissue (79% damage; leaf bases exposed to-8°C), 20% of the plasma membrane was IMP-free. The frequencies of IMP at a distance from the IMP-free areas were unaffected by freezing treatments. There was a patchy distribution of IMP in other membranes (nuclear envelope, tonoplast, thylakoids, chloroplast envelope), but only in the nuclear envelope did it appear possible that their occurrence coincided with damage. The IMP-free areas of several membranes were sometimes associated together in stacks. Such membranes lay both to the outside and inside of the plasma membrane, indicating that at least some of the adjacent membrane fragments arose as a result of membrane reorganization induced by the damaging treatment. Occasional views of folded IMP-free plasma membrane tended to confirm this conclusion. The following hypothesis is advanced to explain the damage induced by extracellular freezing. Areas of plasma membrane become free of IMP, probably as a result of the freezing-induced cellular dehydration. The lipids in these IMP-free patches may be in the fluid rather than the gel phase. The formation of these IMP-free patches, especially in the plasma membrane, initiates or involves proliferation and possibly fusion of membranes, and during or following this process, the cells become leaky.

Effect of hypnorm, chloralosane and pentobarbital on the ultrastructure of the inner membrane of rat heart mitochondria

Rat heart mitochondria were isolated from four groups of animals treated in a different way. The animals of the first group were killed after decapitation (D-group) without previous anaesthesia. The three other groups of animals were anaesthetised with different anaesthetics. The second group (N-group) was anaesthetised with nembutal (sodium pentobarbital), the third group with chloralosane (C-group) and the fourth group with hypnorm (H-group). From these three anaesthetics only nembutal is known to interact with mitochondria. After retrograde perfusion and excision of the heart, mitochondria were prepared from the ventricles by standard methods. After freeze-fracturing the mitochondrial suspension, the intramembrane particle dimension and density on both fracture faces of the inner mitochondrial membrane were measured. The intramembrane particle diameter on the P-face of the inner membrane of the N-group mitochondria was significantly different from D-, C- and H-group mitochondria. Also the density and diameter of the intramembrane particles on the mitochondrial inner membrane of D-group mitochondria compared to C- and H-group mitochondria were significantly different at the 95% level of confidence. Between C- and H-group mitochondria no differences of these parameters were observed. From these results it is clear that, depending on the pretreatment of the animals, a different substructure of the inner membrane of heart mitochondria is obtained.

Changes in freeze-fractured mitochondrial membranes correlated to their energetic state. Dynamic interactions of the boundary membranes

Structural changes of mitochondria in correlation to their energetic state have been observed as matrix expansion and condensation. In this communication we describe a morphological correlation in freeze-fractured mitochondrial membranes which is also dependent on the metabolic state of the organelle: the frequency by which the fracture plane following the inner or outer boundary membrane deviates by jumping from one membrane to the other is higher in phosphorylating mitochondria when compared to freshly isolated or energized mitochondria. These deflections of the fracture plane occur mostly in minimal, short steps showing close apposition of the two boundary membranes. We therefore conclude that the observed change in morphological appearance is produced by a change in interactions between the inner and outer membranes correlated to the different functional states of the inner membrane.

Lipid domain structures in biological membranes

Phase separation represents a possibility for segregation of lipidic membrane components into structurally distinct domains. Freeze-fracture electronmicroscopy is a useful method for detection of lipid domains. Indications of a possible domain-nature of structures are a regular pattern within a separated area, a regular outline of such an area and a local modulation of curvature (evagination or invagination). Candidates for domain structures in biological membranes are smooth particle-free areas and arrays of regularly arranged particles. The interpretation of the particle-free areas is more reliable than that of the arrays with regularly arranged particles. Phase separation in biological membranes can be induced experimentally by lowering the temperature, but physiologically the isothermically induced domains are more important. Factors in control of isothermic domain formation are divalent cations, proteins, cholesterol etc. Suggestions on the biological relevance of domain formation concern mainly their role in the mechanism of membrane fusion, but domains in form of transient or stable membrane structures seem to occur also otherwise and disturbances in domain formation or artificially induced domains can be suitable for pathological alterations.

Structure of the outer mitochondrial membrane: analysis of X-ray diffraction from the plant membrane

X-ray diffraction from centrifugally oriented specimens of plant outer mitochondrial membranes suggests that these membrane contain prominent in-plane subunits. The short lamellar repeat which these specimens display (as low as 5.1 nm) points to a predominantly internal localization of the protein components of these membranes. The simplest model for the putative in-plane subunit consistent with autocorrelation analysis of the normal-indicence diffraction data consists of two concentric rings of electron density with diameters of (approx.) 2 and 4 nm. These rings could represent the planar projections of concentric cylindrical shells, aligned normal to the membrane surface.

Pressure-induced inactivation of sarcoplasmic reticulum adenosine triphosphatase during high-speed centrifugation

Sarcoplasmic reticulum vesicles were found to be highly sensitive to high-speed centrifugation in metal-deprived mediums at low temperature (4 degrees C). The irreversible modifications induced were easily detected from observation of the environment-sensitive spectrum of an iodoacetamide spin-label bound to the ATPase. Centrifugation also resulted in vesicle aggregation and inhibition of calcium transport, ATPase activity, and phosphoenzyme formation. These denaturation-like phenomena were prevented in the presence of sucrose, or by nucleotide binding, or, again, by cation binding to the ATPase high-affinity calcium binding sites and were only present when centrifugation was performed at low temperature. The crucial parameter during this process was found to be the hydrostatic pressure which developed in the centrifuge tube. SR vesicles exposed to 800 bars in a pressure bomb displayed the same features. It is suggested that irreversible denaturation takes place after one or both of the two following well-documented effects of pressure: a rise in the lipid order/disorder transition temperature or dissociation of the oligomeric structure of the calcium pump.

Effect of high pressure on the heat activation in vivo of trehalase in the spores of Phycomyces blakesleeanus

The effect of pressure on the heat activation in vivo of trehalase in the spores of Phycomyces blakesleeanus has been investigated in order to obtain information about the molecular mechanism of the activation. For a protein conformational change directly induced in the enzyme by the heat treatment an upward shift with about 2-6 K/1000 atm (1.013 X 10(5) kPa) is to be expected in the moderate high-pressure region. On the other hand, for a phospholipid phase transition causing the activation, a continuous upward shift with about 20 K/1000 atm is to be expected. For trehalase activation we find a continuous upward shift of the activation temperature with about 5-9 K/1000 atm. The denaturation of trehalase, which occurs at slightly higher temperatures, is influenced by pressure completely as expected for a protein conformational change. The application of high pressure during spore heat activation makes it possibe to break the dormancy of the spores without concomitant activation of trehalase.

Imipramine and lipid phase transition in inner mitochondrial membrane

As ascertained by freeze-fracture electron microscopy, imipramine prevents lateral phase separation from taking place in inner mitochondrial membranes at sub-zero temperatures. Electron spin resonance (ESR) measurements performed on mitochondrial membranes labeled with the N-oxyl-4',4'-dimethyloxazolidine derivative of 16-ketostearic acid, show that the spin probe motion is markedly inhibited below 0 degree C and that 5 mM imipramine attenuates the temperature effect. These results are explained by supposing that imipramine is able to decrease the transition temperature of the inner mitochondrial membrane lipids as it does for simple lipid systems.

Permeability of mitochondria to sucrose induced by hydrostatic pressure

When subjected to increasing pressure at 0 degree C, rat liver mitochondria become permeable to sucrose, causing them to swell and their outer membrane to rupture. Afterwards they are lysed and their matrix content is released into the medium. This permeation to sucrose may be prevented to some extent by increasing the temperature at which compression is carried out. 0.75 mM imipramine protects mitochondria against lysis caused by hydrostatic pressure, but does not oppose their permeation to sucrose nor the swelling resulting from the compression. At this concentration, the drug does not exhibit a significant effect on the lateral phase separations which take place in the inner mitochondrial membrane under pressure. The mitochondria of rat fetal liver (21 days), kidney and Morris hepatoma 16 become permeable to sucrose when they are subjected to compression; under these conditions, lateral phase separations occur in their inner membrane. Contrary to liver mitochondria, the former do not undergo lysis. Taking into account both present and previous results, events leading to mitochondrial membrane deterioration by hydrostatic pressure may be summarized in the following way. Pressure first leads to a phase transition of the membrane lipids, thus causing a permeation to sucrose; as a result the mitochondria swell because they have absorbed osmotic water. The membrane lipids freeze increasingly as the pressure increases; the inner membrane becomes fragile and finally, in the case of the adult liver organelles, can no longer resist the swelling. All these events can be avoided by increasing the temperature; imipramine only prevents inner membrane lysis.

Electro-mechanical properties of human erythrocyte membranes: the pressure-dependence of potassium permeability

Electrical breakdown of cell membranes is interpreted in terms of an electro-mechanical model. It postulates for certain finite membrane areas that the actual membrane thickness depends on the voltage across the membrane and the applied pressure. The magnitude of the membrane compression depends both on the dielectric constant and the compressive, elastic modulus transverse to the membrane plane. The theory predicts the existence of a critical absolute hydrostatic pressure at which the intrinsic membrane potential is sufficiently high to induce "mechanical" breakdown of the membrane. The theoretically expected value for the critical pressure depends on the assumption made both for the pressure-dependence of the elastic modulus of the membrane and of the intrinsic membrane potential. It is shown that the critical pressure is expected at about 65 M Pa. The prediction of a critical pressure could be verified by subjecting human erythrocytes to high pressures (up to 100 M Pa) in a hyperbaric chamber. The net potassium efflux in dependence on pressure was used as an criterion for breakdown. Whereas the potassium net efflux was linearly dependent on pressure up to 60 M Pa, a significant increase in potassium permeability was observed towards higher pressure in agreement with the theory. The increase in the net potassium efflux above 60 M Pa was reversible, as indicated by measurements in which the same erythrocyte sample was subjected to several consecutive pressure pulses. Temperature changes in the erythrocyte suspension during compression and decompression were so small (less than 2 degrees C) that they could not account for the observed effects.

Heat activation of Phycomyces blakesleeanus spores: theromdynamics and effect of alcohols, furfural, and high pressure

The thermodynamic parameters for the heat activation of the sporangiospores of Phycomyces blakesleeanus were determined. For the apparent activation enthalpy (DeltaH(#)) a value of 1,151 kJ/mol was found, whereas a value of 3,644 J./ degrees K.mol was calculated for the apparent activation entropy (DeltaS(#)). n-Alcohols (from methanol to octanol), phenethyl alcohol, and furfural lowered the activation temperature of P. blakesleeanus spores. The heat resistance of the spores was lowered concomitantly. The effect of the alcohols was a linear function of the concentration in the range that could be applied. When the log of the concentration needed to produce an equal shift of the activation temperature was plotted for each alochol against the log of the octanol/water partition coefficient, a straight line was obtained. The free energy of adsorption of the n-alcohols to their active sites was calculated to be -2,487 J/mol of CH(2) groups. Although still inconclusive, this points toward an involvement of protein in the activation process. The effect of phenethyl alcohol was similar to the effect of n-alcohols, but furfural produced a greater shift than would be expected from the value of its partition coefficient. When the heat activation of the spores was performed under high pressure, the activation temperature was raised by 2 to 4 degrees K/1,000 atm. However, with pressures higher than 1,000 atm (1.013 x 10(5) kPa) the activation temperature was lowered until the pressure became lethal (more than 2,500 atm). It is known that membrane phase transition temperatures are shifted upward by about 20 degrees K/1,000 atm and that protein conformational changes are shifted upward by 2 to 6 degrees K/1,000 atm. Consequently, heat activation of fungal spores seems to be triggered by a protein conformational change and not by a membrane phase transition. Activation volumes of -54.1 cm(3)/mol at 38 degrees C and -79.3 cm(2)/mol at 40 degrees C were found for the lowering effect of high pressure on the heat activation temperature.

Particle segregation in chromaffin granule membranes by forced physical contact

Bovine chromaffin granules were exposed to different isotonic non-ionic and ionic solutions (sucrose; Ca2+- and Mg2+-free phosphate-buffered saline; Tris-HCl + NaCl; Ca2+- and Mg2+-free phosphate-buffered saline + sucrose; Tris-HCl + sucrose) at pH 7 and then frozen either in suspension or as firm pellets. Freezing was performed without prefixation or antifreeze treatments either by 'standard' techniques (approx. 1 mm3 suspended or pelleted material on gold specimen supports dipped into liquid Freon) or with increased cooling rates by spraying suspensions into liquid propane ('spray-freezing'). Regardless of the freezing method, membrane-intercalated particles were always randomly distributed when chromaffin granules were frozen in suspension. In contrast, forced physical contact between granules produced by centrifugation (12000 X g, 25 min) provoked dispersal of membrane-intercalated particles, but only in the presence of ions. Sucrose or EDTA in an ionic environment had no inhibitory effect. The following conclusions are derived: (1) Even below the reported phase transition region particle clustering is possible. (2) Chromaffin granule membranes are not liable to thermotropic segregation of membrane-intercalated particles. (3) Although the low freezing rates of 'standard' freezing techniques produce large-scale segregation artefacts (by which suspended chromaffin granules are pushed together within the segregated solute) this does not result in intramembraneous particle segregation. (4) Forced physical contact produces a Ca2+-independent particle segregation, but only when repulsive electrostatic forces of membrane components are partially screened in an ionic environment. (5) This does not invalidate results obtained by others, showing Ca2+-mediated chromaffin granules agglomeration and segregation of membrane-intercalated particles, but it might indicate the occurrence of another, not directly Ca2+-dependent particle segregation mechanism in a prefusional stage of close membrane-to-membrane contact during exocytosis.