Biochemical characterization of segreated membrane vesicles from human erythrocytes with increased intracellular Ca2+

Intraerythrocytic development of Plasmodium knowlesi: structure, temperature- and Ca2+-response of the host and parasite membranes

Thin-sectioning and freeze-etching electron microscopy were applied to explore the structure and the temperature- and Ca2+-response of the different host and parasite membranes during intraerythrocytic development of Plasmodium knowlesi in Macaca mulatta. The plasma membrane of uninfected erythrocytes is temperature- and Ca2+-responsive: chilling to 4 degrees C and exposure to 5 mM Ca2+ induces a slight decrease in IMP-frequency and the emergence of small IMP-devoid patches on P-faces. On parasite infection, the erythrocyte membrane becomes modified as indicated by an enhanced temperature-response and the appearance of caveolae, ca. 70-90 nm in diameter. The frequency of these caveolae is increased in schizont-infected erythrocytes. Moreover, electron dense plaques, ca. 40 nm in width, appear just beneath the erythrocyte membrane in late trophozoites and schizonts, thus indicating a further modification of the host cell membrane during parasite development. The membrane of the parasitophorous vacuole, derived from the host plasma membrane, dramatically reduces the IMP-frequency especially on the P-face upon parasite infection. This leads to an apparent reversal of the IMP-distribution persisting throughout the whole infection cycle. The parasite plasma membrane forms local compaction domains with the nuclear envelope in ca. 30% of the ring-stages and trophozoites, which disappear in late trophozoites and schizonts. Moreover, the IMP-frequency on plasma membrane fracture faces almost doubles during parasite development. Chilling induces a decrease in the IMP-frequency on P-faces of the plasma membrane. Surprisingly, however, the parasite plasma membrane and the vacuolar membrane respond to externally applied Ca2+ with almost a doubling of the IMP-frequency. The different parasite endomembranes also undergo characteristic changes during parasite development.

Chemical composition of the plasma membrane from epimastigote forms of Trypanosoma cruzi

The chemical composition of two plasma membrane fractions from epimastigote forms of Trypanosoma cruzi is reported. Fraction M, a preparation obtained by conventional methods of cell fractionation is composed of 31% proteins, 34% lipids, 16% carbohydrates and 3% of the lipopeptidophosphoglycan. Phospholipids and sterols account for 7.5 and 9%, respectively, of the total mass. Phosphatidylethanolamine is the major phospholipid in fraction M, representing 45% of the total membrane phospholipids. The other fraction, fraction V (vesicles), was obtained by treatment of the cell with a vesiculating agent. This fraction contains 42% lipids, 20% carbohydrates, 13% proteins and 21% of the lipopeptidophosphoglycan. Phospholipids and sterols make up 17 and 8%, respectively, of the total mass of this fraction. Phosphatidylcholine and phosphatidylethanolamine are the main phospholipids found in fraction V. Phosphonolipids and sialic acid have not been detected in either membrane fraction. Sodium dodecyl sulfate polyacrylamide gel electrophoretic analysis show that the glycoproteins ABC and the lipopeptidophosphoglycan are 50- and 10-times more concentrated, respectively, in fractions V and M than in the whole cell homogenate. The high molar sterol/phospholipid ratio found in fraction M suggests that this fraction is less fluid than fraction V, perhaps reflecting a migration of certain membrane components in the presence of the vesiculating agent. Hence, fraction M is, probably, more representative of the epimastigote plasma membrane as a whole than fraction V.

Plasmodial modifications of erythrocyte surfaces

The maturation of malarial parasites in red blood cells produces major alterations in the composition and properties of the host cell surfaces. Existing surface-exposed proteins are modified and new antigenic glycoproteins are synthesized by the parasite and inserted into the membrane. Some of the neoproteins are associated with surface excrescences on the host cells and in some cases these foster adhesion of those cells to capillary endothelium. The erythrocyte endoskeleton is degraded and in association the infected cells become deformed and lose pliability. An increase in intracellular Ca2+ may contribute to the changes in host cells surfaces.

Calcium binding to plasma membranes from normal and SV40 transformed hamster lymphocytes

The calcium binding properties of isolated plasma membranes from normal and SV40 transformed hamster lymphocytes were compared over the Ca2+ concentration range of 10(-5)M to 5 x 10(-3)M and at physiological ionic strength. At all Ca2+ concentrations, normal membranes bound more Ca2+ than tumor membranes; at blood Ca2+ levels (1-2 mM) plasma membranes of normal cells bind twice as much as membranes from tumor cells. Normal plasma membranes demonstrated positive cooperative Ca2+ binding whereas tumor membranes displayed non-interacting Ca2+ binding sites. Ca2+ binding to both membranes was insensitive to Mg2+ (0.1 to 2.5 mM). A pH shift from 7 to 6 resulted in a 70% decrease of normal membrane-bound Ca2+ compared to a 40% decrease observed with tumor membranes. Extracellular surface Ca2+ binding to intact cells was also studied after a 72-hour equilibration of cells with 45 Ca2+ and with ethyleneglycol-bis-(beta-amino-ethyl ether) N,N'-tetraacetate chelation as marker for surface Ca2+. Tumor cell surface Ca2+ binding was only 10% of that observed with quiescent lymphocytes. Normal lymphocytes stimulated to divide with phytohemagglutinin also showed a decreased level of surface Ca2+ (50%). However, plasma membranes isolated from non-dividing and phytohemagglutinin-stimulated lymphocytes exhibited equivalent Ca2+ binding.

The lateral distribution of intramembrane particles in the erythrocyte membrane and recombinant vesicles

Triton X-100 (in concentrations which did not cause a significant solubilization of membrane material) caused aggregation of the intramembrane particles of human erythrocyte ghosts. Ghosts from which the extrinsic proteins had been removed by alkali treatment showed a temperature-induced aggregation of the particles. With virtually no spectrin present, the particles in these stripped ghosts could still be aggregated by manipulations with ionic strength and pH, or by the addition of calcium. Recombinant vesicles were made from a Triton X-100 extract and a mixture of phospholipids with a composition which resembled that of the inner monolayer of erythrocyte membrane. In these recombinants the same manipulations with ionic strength and pH and the addition of calcium caused a rearrangement of the particles, resulting in the appearance of particle-free areas. In recombinants prepared from a Triton X-100 extract and egg phosphatidylcholine the lateral distribution of the particles was not altered by these manipulations. It is concluded that in the erythrocyte membrane the intramembrane particles can be aggregated by effects of external agents on lipid components. In this light the role of spectrin in stabilizing the membrane by interactions with lipids in the inner monolayer is discussed.

Plasma membrane vesicles isolated from epimastigote forms of Trypanosoma cruzi

Membrane vesicles can be obtained from epimastigote forms of Trypansoma cruzi by incubating cells with either cross-linking reagents or acid pH. Acetate, phtalate or citrate, at pH 4.0, but not at higher pH values, were able to induce plasma membrane vesiculation. Vesicles have been purified by sucrose density centrifugation and their membrane origin was demonstrated by the following criteria: (a) Vesicles are 5--10 times richer in protein-bound iodine when they are prepared from cells previously labeled with 131I by the lactoperoxidase catalyzed reaction. (b) Electron microscopy of vesiculating cells shows physical continuity between cell plasma membrane and vesicle membrane. (c) Antibodies prepared against purified vesicles are able to agglutinate epimastigote forms of T. cruzi with sera dilutions up to 1 : 256 to 1 : 512. (d) Freeze-fracture studies of the purified vesicles have shown images of faces P and E compatible with known images of the intact cell plasma membrane. Typical preparations of acetate vesicles present the following characteristics: total carbohydrate : protein=1.5--2.0; orcinol : protein-0.07 and absence of diphenylamine reaction. Vesicles contain 0.2--0.5% and 0.3--1.0% of the total homogenate protein and carbohydrate, respectively. The presence of 10 major protein bands and 30--50-fold enrichment of the four sugar-containing macromolecules present in epimastigote forms of T. cruzi have been demonstrated in these preparations.

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.

Membrane proteins in human erythrocytes during cell fusion induced by oleoylglycerol

1. The fusion of human erythrocytes into multicellular bodies that is induced by microdroplets of oleoylglycerol was investigated by optical and electron microscopy, and by gel electrophoresis of membrane proteins. 2. At the highest concentrations of oleoylglycerol and Ca(2+) used, at least 80% of the cells fused after 30min at 37 degrees C and only about 5% of the cells had completely lysed; the shapes of fused multicellular bodies were usually retained in ;ghosts' prepared by hypo-osmotic lysis. 3. The rate of cell fusion was related to the concentration of Ca(2+), although some cells fused when no exogenous Ca(2+) was present. 4. Interactions of microdroplets of oleoylglycerol with the cells led to abnormalities in the structural appearance of the erythrocyte membrane; subsequent membrane fusion occurred, at least in some instances, at the sites of the microdroplets. 5. The intramembranous particles on the P-fracture face of the treated cells were more randomly distributed, but not significantly increased in number by comparison with the control cells. 6. Gel electrophoresis of the proteins of ;ghosts' prepared from fused human erythrocytes showed a production of material of very high molecular weight, the development of a new component in the band-3 region, an increased staining of bands 4.3 and 4.5, and a new component moving slightly faster than band 6. 7. Bands 2.1-2.3 were altered, band 3 was decreased and band 4.1 was lost. 8. Most, but not all, of the changes in the membrane proteins appeared to result from the entry of Ca(2+) into the cell. 9. 1-Chloro-4-phenyl-3-l-toluene-p-sulphonamidobutan-2-one partially inhibited both cell fusion and the associated decrease in band-3 protein. 10. The possibility that proteolytic degradation of membrane proteins may be involved in cell fusion induced by oleoylglycerol is considered, and some implications of this possibility are discussed.