Fusion of secretory vesicles isolated from rat liver

Lipid vesicles and membrane fusion

Membrane fusion is essential for cell survival and has attracted a great deal of both theoretical and experimental interest. Fluorescence (de)quenching measurements were designed to distinguish between bilayermerging and vesicle-mixing. Theoretical studies and various microscopic and diffraction methods have elucidated the mechanism of membrane fusion. These have revealed that membrane proximity and high defect density in the adjacent bilayers are the only prerequisites for fusion. Intermediates, such as stalk or inverse micellar structures can, but need not, be involved in vesicle fusion. Nonlamellar phase creation is accompanied by massive membrane fusion although it is not a requirement for bilayer merging. Propensity for membrane fusion is increased by increasing the local membrane disorder as well by performing manipulations that bring bilayers closer together. Membrane rigidification and enlarged bilayer separation opposes this trend. Membrane fusion is promoted by defects created in the bilayer due to the vicinity of lipid phase transition, lateral phase separation or domain generation, high local membrane curvature, osmotic or electric stress in or on the membrane; the addition of amphiphats or macromolecules which insert themselves into the membrane, freezing or other mechanical membrane perturbation have similar effects. Lowering the water activity by the addition of water soluble polymers or by partial system dehydration invokes membrane aggregation and hence facilitates fusion; as does the membrane charge neutralization after proton or other ion binding to the lipids and intermembrane scaffolding by proteins or other macromolecules. The alignment of defect rich domains and polypeptides or protein binding is pluripotent: not only does it increase the number of proximal defects in the bilayers, it triggers the vesicle aggregation and is fusogenic. Exceptions are the bound molecules that create steric or electrical barriers between the membranes which prevent fusion. Membrane fusion can be non-leaky but it is very common to lose material from the vesicle interior during the later stages of membrane unification, that is, after a few hundred microseconds following the induction of fusion.

Hyperosmotic relaxation lysis of chromaffin granules is caused by interactions between the granular membrane and intragranular vesicles

Bovine chromaffin granules undergo irreversible structural changes during osmotic shrinkage in hypertonic sucrose and salt solutions, such that, on reexposure to isoosmotic conditions they do not regain their original morphology, but undergo lysis ('hyperosmotic relaxation lysis'). Irreversible alterations of granules were induced by hypertonic incubations lasting for as little as 1 min. Fluorescence and EPR membrane labelling experiments showed that hypertonicity did not induce membrane loss for instance by inwardly or outwardly directed pinching off of membrane material. The mean sizes of chromaffin granules as a function of increasing and subsequently decreasing osmotic pressure were measured by photon correlation spectroscopy; there was no significant difference in sizes of hyperosmotically pretreated granules as compared with controls. Freeze-fracture electron micrographs showed the formation of 'twins' and 'triplets' under hypertonic conditions. They also revealed intragranular vesicles of 50-200 nm in diameter in both hypertonically and isotonically suspended granules. 'Twin' and 'triplet' granules were formed by the attachment of intragranular vesicles to the granule membranes. We suggest that hyperosmotic relaxation lysis is caused by the fact that this adhesion partly prevents the granule membrane from reexpanding, thus, leading to its rupture.

Further characterization of dopamine release by permeabilized PC12 cells

Rat pheochromocytoma cells (PC12) permeabilized with staphylococcal alpha-toxin release [3H]dopamine after addition of micromolar Ca2+. This does not require additional Mg2+-ATP (in contrast to bovine adrenal medullary chromaffin cells). We also observed Ca2+-dependent [3H]-dopamine release from digitonin-permeabilized PC12 cells. Permeabilization with alpha-toxin or digitonin and stimulation of the cells were done consecutively to wash out endogenous Mg2+-ATP. During permeabilization, ATP was removed effectively from the cytoplasm by both agents but the cells released [3H]dopamine in response to micromolar Ca2+ alone. Replacement by chloride of glutamate, which could sustain mitochondrial ATP production in permeabilized cells, does not significantly alter catecholamine release induced by Ca2+. However, Mg2+ without ATP augments the Ca2+-induced release. The release was unaltered by thiol-, hydroxyl-, or calmodulin-interfering substances. Thus Mg2+-ATP, calmodulin, or proteins containing -SH or -OH groups are not necessary for exocytosis in permeabilized PC12 cells.

Processing and secretion in the neurohypophysis. Stability of isolated secretory vesicles and role of internal pH

A possible role of low pH in secretory vesicles for processing and secretion in the neurohypophysis was investigated. Subcellular fractionation of guinea-pig neural lobes revealed that a proton present in the membranes from this tissue could not be ascribed to secretory vesicles. However, a proton pump was found in coated microvesicles. Secretory vesicles isolated from rats and guinea pigs were stable under conditions known to lyse secretory vesicles from the adrenal medulla owing to the generation of a proton gradient. These results suggest that the internal pH of secretory vesicles from the neurohypophysis is closer to neutral than is the pH in chromaffin secretory vesicles. Processing of a neurophysin-glycopeptide intermediate from the biosynthesis of vasopressin in intact secretory vesicles incubated in vitro was activated by the addition of NH4Cl, known to increase the intravesicular pH. This activation of neurohormone processing was also apparent in isolated nerve endings incubated in the presence of NH4Cl, suggesting that NH4Cl can also be used to increase the intravesicular pH in intact nerve endings. However, NH4Cl did not affect the secretion of neurohormones, indicating that a low intravesicular pH is not important for exocytosis in the neurohypophysis. Our results indicate that a low pH generated during processing by mechanisms other than ATP-dependent proton transport may inhibit the processing enzymes, thereby preventing extensive breakdown of neurohormone precursors.

Gramicidin induced aggregation and size increase of phosphatidylcholine vesicles

To investigate the role of membrane proteins in the fusion process, linear hydrophobic polypeptide gramicidin was used as fusogenic agent in small unilamellar vesicles (SUV) constituted of saturated lecithins. It was found that gramicidin, externally added to a suspension of vesicles, induces a reversible vesicles aggregation. When incorporated into the bilayer, gramicidin induces increase in vesicle size. The vesicle size increase was monitored by column chromatography and transmission electron microscopy. The process of vesicle size increase occurs only when the lipid membrane is in the gel state. A maximum is observed in the kinetics at a temperature of approx. 25 degrees C lower than the phase transition temperature of lipids. Higher rates of vesicle size increase are obtained as the lipid chain length increases. The process is accompanied by a release of internal vesicle content and by membrane lipid mixing.

Novel crystalline sheets of Na,K-ATPase induced by phospholipase A2

Treatment of purified preparations of Na,K-ATPase by phospholipase A2 has led to the formation of two-dimensional crystals of the protein. Control tests with another phospholipase and two detergents have shown that crystallization occurs as the result of hydrolysis and/or solubilization of the phospholipids in the enzyme vesicles. Experimentation with various buffer systems has indicated that reduction in the amount of phospholipids alone is sufficient for inducing the formation of crystalline sheets. Inclusion of crystal inducing ions in the buffer facilitates the crystallization process, resulting in more extensive arrays. The new crystalline sheets are exclusively dimeric with average unit cell dimensions: a = 15.8 +/- 0.4 nm, b = 4.9 +/- 0.2 nm, and gamma = 64 +/- 3 degrees. Examination of the micrographs shows that the initial intermolecular interaction leading to the formation of sheets is between the alpha subunits. Results from this study suggest that removal and/or modification of phospholipids by phospholipases could prove successful in crystallizing those membrane proteins in which excess lipid is the main barrier to the formation of two-dimensional arrays.

Electric pulse-induced fusion of mouse lymphoma cells: roles of divalent cations and membrane lipid domains

Mouse leukemic lymphoblasts (L5178Y) brought into close contact by dielectrophoresis underwent cell fusion following the application of electrical pulses in the presence of electrolytes. The electrically fused cells became spherical after switching off the dielectrophoretic field. Fusion between a cell vitally stained with Janus Green and that with Neutral Red resulted in the homokaryon with a mixed color. Intracellular potentials simultaneously recorded from the two cells located on both sides of the homokaryon were identical. The fusion efficiency was remarkably dependent upon temperature, displaying a discontinuity at about 11 degrees C in the Arrhenius plot. The extracellular application of phospholipase-A2 or -C suppressed the fusion yield. Thus, it appears that the phospholipid domains play a crucial role in the electric pulse-induced cell fusion. Treatment of the cells with proteolytic enzymes markedly enhanced the fusion yield, presumably due to removing the glycocalix and/or giving rise to fusion-potent, protein-free lipid domains. The presence of millimolar concentrations of divalent cations (irrespective of Mg2+ or Ca2+) as well as of micromolar concentrations of Ca2+ (but not Mg2+) was prerequisite to the resealing of membranes suffered from electrical breakdown upon exposure to electric pulses. In addition, extracellular Ca2+ (but not Mg2+) ions at more than micromolar concentrations were indispensable for the cell fusion.

Fusion of phospholipid vesicles induced by Zn2+, Cd2+, and Hg2+

The capacity of Zn2+, Cd2+, or Hg2+ to induce fusion of phospholipid vesicles composed of 50%/50%, 60%/40%, or 80%/20% dipalmitoyl phosphatidylcholine (DPPC)/bovine brain phosphatidylserine (PS) was investigated and compared to that of Ca2+ and Mg2+. In vesicles composed of 50%/50% or 60%/40% DPPC/PS, Zn2+ and Cd2+-induced fusion at concentrations considerably lower than were required for Ca2+-induced fusion. Only limited fusion of 80%/20% DPPC/PS vesicles occurred and Zn2+ was more effective than Ca2+ or Cd2+ in inducing fusion of these vesicles. Mg2+ and Hg2+ did not induce fusion in any of the vesicle systems.

Verapamil enhances receptor-mediated endocytosis of low density lipoproteins by aortic cells in culture

Bovine aortic endothelial and smooth muscle cells (SMC) and human skin fibroblasts (HSF) were used to study the effect of verapamil on cellular interactions with human low density lipoprotein (LDL). Verapamil, 10 to 50 microM, increased 125I-LDL uptake and degradation by 70% to 200% in the various cells after 24 to 48 hours of incubation. The increase in the total amount of LDL endocytosed, labeled with 3H-cholesteryl linoleate (3H-CL), was comparable to that determined with 125I-LDL. In HSF and SMC, a delay in 125I-LDL degradation and hydrolysis of 3H-CL was seen in cells treated for 3 to 24 hours with verapamil. Pretreatment of HSF with 50 microM verapamil for 24 hours and incubation with 2 to 50 micrograms 125I-LDL protein/ml for 1 hour resulted in a 50% to 200% increase in heparin releasable and in a 40% to 130% increase in cellular 125I-LDL. Thus, the increase in 125I-LDL binding and uptake in verapamil-treated cells was apparently due to an increase in receptor number, rather than in receptor affinity. The effect of verapamil on LDL uptake and degradation was also seen in cells that were pretreated for 24 hours and incubated with 125I-LDL in the absence of verapamil. The effect of verapamil was not apparent in LDL receptor-negative cells. Cycloheximide blocked the verapamil effect. The Na+ channel blocker, tetrodotoxin x 10(-6) M, caused a 30% to 50% increase in the total amount of LDL endocytosed, but no delay in LDL degradation; amiloride 2 x 10(-3) M was not effective. If the presently described effect of verapamil also occurs in vivo, this might contribute to the reported beneficial effects of Ca++ channel blockers in experimental atherosclerosis by promoting transfer of LDL cholesteryl ester from the aortic interstitium to a catabolic compartment.

Cytochemical localization of ouabain-sensitive, K+ -dependent p-nitrophenylphosphatase and Ca++-stimulated adenosine triphosphatase activities in human parotid and submandibular glands

K+ -dependent p-nitrophenylphosphatase (pNPPase) and Ca++ -stimulated adenosine triphosphatase (ATPase) activities were studied in human parotid and submandibular glands using cytochemical methods at the ultrastructural level. In both glands, only the striated-duct epithelium showed K+ -pNPPase reaction product, thereby indicating the localization of Na+, K+ -ATPase. The precipitate was concentrated on the deep invaginations of the basolateral plasma membranes, in close association with their cytoplasmic surface. Ca++ -ATPase activity was also found on the basolateral plasma membranes, but two striking differences from the K+ -pNPPase distribution were observed: firstly, Ca++ -ATPase appeared in both acinar and ductal cells, and secondly, it was localized on the outer side of the plasma membranes.

Effect of ethanol on hepatic secretory proteins

Both acute and chronic ethanol administration inhibit the secretion of albumin and glycoproteins from the liver. Impairment of posttranslational steps of the secretory process are mainly involved in this secretory defect, although in some instances altered synthesis of the protein moiety may be a factor. Decreased secretion following ethanol administration results in the intrahepatic retention of export proteins. The secretory defect is a consequence of the metabolism of ethanol and is likely mediated via acetaldehyde, although more conclusive proof is still required. The manner by which acetaldehyde impairs the secretory process is unknown, but may be related to its high reactivity with hepatocellular proteins. The specific posttranslational steps or processes involved in the secretory defect are still unclear; however, it appears that the final steps of secretion (post-Golgi events) may be the primary site of impairment. Impaired secretion of proteins from the liver could contribute to altered levels of plasma proteins and hepatomegaly as well as to the liver injury observed in the alcoholic.

3',5'-cyclic adenosine monophosphate- and Ca2+-calmodulin-dependent endogenous protein phosphorylation activity in membranes of the bovine chromaffin secretory vesicles: identification of two phosphorylated components as tyrosine hydroxylase and protein kinase regulatory subunit type II

Membranes of the secretory vesicles from bovine adrenal medulla were investigated for the presence of the endogenous protein phosphorylation activity. Seven phosphoprotein bands in the molecular weight range of 250,000 to 30,000 were observed by means of the sodium dodecyl sulphate electrophoresis and autoradiography. On the basis of the criteria of molecular weight, selective stimulation of the phosphorylation by cyclic AMP (as compared with cyclic GMP) and immunoprecipitation by specific antibodies, band 5 (molecular weight 60,300) was found to represent the phosphorylated form of the secretory vesicle-bound tyrosine hydroxylase. The electrophoretic mobility, the stimulatory and inhibitory effects of cyclic AMP in presence of Mg2+ and Zn,2+ respectively, and immunoreactivity toward antibodies showed band 6 to contain two forms of the regulatory subunits of the type II cyclic AMP-dependent protein kinase, distinguishable by their molecular weights (56,000 and 52,000, respectively). Phosphorylation of band 7 (molecular weight 29,800) was stimulated about 2 to 3 times by Ca2+ and calmodulin in the concentration range of both agents believed to occur in the secretory tissues under physiological conditions.

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.

Studies on the mechanism of membrane fusion. Role of head-group composition in calcium- and magnesium-induced fusion of mixed phospholipid vesicles

We have investigated the contribution of various phospholipids to membrane fusion induced by divalent cations. Fusion was followed by means of a new fluorescence assay monitoring the mixing of internal aqueous contents of large (0.1 micrometer diameter) unilamellar liposomes. The rate and extent of fusion induced by Ca2+ in mixed phosphatidylserine/phosphatidylcholine vesicles were lower compared to those in pure phosphatidylserine vesicles. The presence of 50% phosphatidylcholine completely inhibited fusion, although the vesicles aggregated upon Ca2+ addition. When phosphatidylserine was mixed with phosphatidylethanolamine, however, rapid fusion could be induced by Ca2+ even in mixtures that contained only 25% phosphatidylserine. Phosphatidylethanolamine also facilitated fusion by Mg2+ which could not fuse pure phosphatidylserine vesicles. In phosphatidylserine/phosphatidylethanolamine/phosphatidylcholine mixtures, in which the phosphatidylcholine content was kept at 25%, phosphatidylethanolamine could not substitute for phosphatidylserine, and the fusogenic capacity of Mg2+ was abolished by the presence of merely 10% phosphatidylcholine. The initial rate of release of vesicle contents was slower than the rate of fusion in all the mixtures used. The presence of phosphate effected a considerable decrease in the threshold concentration of Ca2+ and also enhanced the rate and extent of fusion. Mg2+ had a synergistic effect on Ca2+-induced fusion of phosphatidylserine/phosphatidylethanolamine vesicles. We suggest that the role of phospholipids in membrane fusion is related to their ability to form dehydrated intermembrane complexes with divalent cations.

The extraction from maize (Zea mays) root cells of membrane-bound protein with Ca2+-dependent ATPase activity and its possible role in membrane fusion in vitro

Membrane fusion in vitro between Golgi apparatus- and plasma-membrane-rich fractions isolated from maize (Zea mays) roots was found to be dependent on Ca2+ and the membrane proteins. Trypsin treatment of mixed membrane fractions before the addition of Ca2+ inhibited their ability to fuse. It resulted also in a selective and progressive elimination of a characteristic intense polypeptide band (B1) on gel electrophoresis. This polypeptide was not removed by chymotrypsin or thermolysin. B1 is an integral membrane protein with an exposed portion to the outside. Sodium deoxycholate was used to solubilize the proteins of mixed membrane fractions. Extracted proteins analysed by non-SDS (sodium dodecyl sulphate) polyacrylamide-gel electrophoresis revealed the presence of four isolated bands. When re-electrophoresed in the presence of SDS, one of these bands exhibited the same mobility as polypeptide B1. Enzymic staining of non-SDS-polyacrylamide gels showed that this protein has Ca2+- and Mg2+-dependent ATPase activity. Its possible role in membrane fusion is discussed.

Lipids of synaptic vesicles: relevance to the mechanism of membrane fusion

Synaptic vesicles from the electric organ of the marine ray Narcine brasiliensis, purified to at least 90% homogeneity, were analyzed for the lipid and fatty acid content of their membranes. The major lipids (mol %) were phosphatidylcholine (32.3%), phosphatidylethanolamine (20.5%), phosphatidylserine (6.1%), sphingomyelin (3.0%), and cholesterol (33.3%), a composition which did not differ greatly from that of the parent electric organ. While the number of double bonds per fatty acid molecule was similar for both synaptic vesicle and whole electric organ phospholipids, the vesicles were highly enriched in docosahexenoic acid (22:6). Reaction with the amine labeling reagents isethionylacetimidate and trinitrobenzenesulfonic acid indicated that 40% of the phosphatidylserine and 60% of the phosphatidylethanolamine are present on the external (cytoplasmic) surface of the synaptic vesicle. These data on a natural fusing membrane have relevance to models of membrane fusion, which have been based largely on studies of in vitro fusion using synthetic membranes.

Isolation and characterization of secretory vesicles from bovine neurohypophyses

A procedure is described for the isolation of secretory vesicles from bovine neurohypophyses by differential centrifugation followed by density gradient centrifugation on iso-osmolal gradients of percoll/sucrose. Only negligible contamination of the secretory vesicle fraction with markers for mitochondria, microsomes and plasma membranes could be detected. The amount of Ca2-ATPase in the isolated neurohypophysial secretory vesicles was of the same low order of magnitude as that of (Na, K)-ATPase. Thin-section electromicrographs confirmed the high purity of the isolated secretory vesicle fractions, In freeze-fracture electronmicrographs, vesicle fusion was demonstrated after incubation with Ca2. As shown in dodecyl sulfate-gel electrophoresis and subsequent autoradiography secretory vesicles exhibited an endogenous phosphorylation activity. The secretory vesicles contained an average of 23.1 microgram vasopressin/mg of protein. On incubation in media differing in ionic strength, pH and Ca2 concentration the vesicles were stable for at least 1 h.

Fusion of isolated myoblast plasma membranes. An approach to the mechanism

Fusion of plasma membranes isolated from myoblasts grown in culture has been investigated. 1. Membrane fusion was specifically dependent of Ca2+ at physiological concentrations. However, at higher concentrations of cations, fusion could be triggered not only by Ca2+, but by Mg2+ and Sr2+ as well. 2. The amount of fusion was directly proportional to temperature. 3. Fusion was found to depend on the state of maturation of the myoblast membranes. 4. Experiments with chemically and enzymatically modified membranes and with membranes derived from myoblasts grown in the presence of inhibitors of protein biosynthesis suggest the participation of proteinaceous membrane components in the fusion mechanism.