Interaction of neutral polysaccharides with human erythrocyte membrane: involvement of phospholipid bilayer

Separation methods for isolation of human polymorphonuclear leukocytes affect their motile activity

Five commonly used methods for the isolation of human polymorphonuclear leukocytes (PMNLs) from blood and their subsequent effect on cell motile activity were compared. Although all methods (isolation from blood clots, hemolysis of erythrocytes in hypotonic solutions, and sedimentation with the use of Percoll, Ficoll 400 or Dextran T 500 solutions) preserved cell viability, they demonstrated different effects on cell spreading and the speed of spontaneous cell movement. The highest motile activity was shown by PMNLs separated from blood clots and Percoll solutions. In the presence of formylated peptides, N-FMLP-mediated movement was markedly stimulated in PMNLs separated by all five methods, but cells isolated with the use of Dextran T 500 or Ficoll 400 were relatively slower than those isolated with other methods. This suggests that the cells had preserved the sensitivity of their receptors specific for ligands stimulating chemokinesis (chemotaxis) after all five methods of separation. Immunofluorescence observations showed that PMNLs isolated in the presence of FITC-Dextran exhibited polysaccharide-coated surfaces with receptor proteins extended above that coat - an observation which explains why cell coating with polysaccharides does not disturb cell phenotyping with flow cytometry and FACS methods.

Assembly and topography of the prepore complex in cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins are a family of poreforming proteins that have been shown to be virulence factors for a large number of pathogenic bacteria. The mechanism of pore formation for these toxins involves a complex series of events that are known to include binding, oligomerization, and insertion of a transmembrane beta-barrel. Several features of this mechanism remain poorly understood and controversial. Whereas a prepore mechanism has been proposed for perfringolysin O, a very different mechanism has been proposed for the homologous member of the family, streptolysin O. To distinguish between the two models, a novel approach that directly measures the dimension of transmembranes pores was used. Pore formation itself was examined for both cytolysins by encapsulating fluorescein-labeled peptides and proteins of different sizes into liposomes. When these liposomes were re-suspended in a solution containing anti-fluorescein antibodies, toxin-mediated pore formation was monitored directly by the quenching of fluorescein emission as the encapsulated molecules were released, and the dyes were bound by the antibodies. The analysis of pore formation determined using this approach reveals that only large pores are produced by perfringolysin O and streptolysin O during insertion (and not small pores that grow in size). These results are consistent only with the formation of a prepore complex intermediate prior to insertion of the transmembrane beta-barrel into the bilayer. Fluorescence quenching experiments also revealed that PFO in the prepore complex contacts the membrane via domain 4, and that the individual transmembrane beta-hairpins in domain 3 are not exposed to the nonpolar core of the bilayer at this intermediate stage.

Syntheses and biological evaluations of alpha-D-mannosyl [60]fullerenols

[60]Fullerenols carrying mono- and bis-alpha-D-mannosyl linkages on the surface were prepared via a [3+2]-cycloaddition reaction between 2-azidoethyl alpha-D-mannoside and C(60) followed by polyhydroxylation with aqueous NaOH. Their biological activity was evaluated in terms of binding affinity to lectins by hemagglutination assay and surface plasmon resonance. [60]Fullerenols without the mannosyl linkage caused aggregation of erythrocytes and binding to a beta-D-galactopyranoside specific lectin (RCA(120)). In contrast, mono- and bis-mannosyl fullerenols were found to decrease the activity for both aggregating erythrocytes and binding to RCA(120), and mono-mannosyl fullerenols turned to binding to alpha-D-mannose specific lectin (Con A).

Characterization and phase behaviour of phospholipid bilayers adsorbed on spherical polysaccharidic nanoparticles

In this paper a new drug carrier, the Light-biovector, is described. These biovectors are composed of a neutral, anionic or cationic polysaccharidic core surrounded by phospholipids. They can be prepared with high yield and in a nearly pure form as determined by density analysis on sucrose gradients. These particles showed great stability with no sedimentation being observed after more than one year of storage. Physicochemical studies carried out with dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylglycerol mixtures showed that in Light-biovectors, the lipids are organized in bilayer surrounding the polysaccharidic core. In presence of a neutral polysaccharidic core, the gel to liquid phase transition temperature Tm of DPPC was only slightly affected as compared to liposomal dispersions of the lipid. In contrast, for cationic and anionic Light-biovectors, the Tm of the lipids was affected by the electric charge born by the polysaccharidic core, indicating that electrostatic interactions contribute to the organization of the lipid bilayer in these systems. It was also found that the association of anionic membrane to anionic polysaccharidic cores and the association of cationic membrane to cationic polysaccharidic cores was possible.

Dextran protection of erythrocytes from low-pH-induced hemolysis

Low-pH-induced hemolysis of erythrocytes is inhibited by dextrans. The protective effect was observed with dextrans larger than 40 kDa. Electron microscopy showed dextrans of 150 kDa in a tight association with the erythrocyte membrane. These results indicate that dextrans stop the low-pH-induced hemolysis by interacting with the acid-induced defects in the erythrocyte membrane [(1989) Biochim. Biophys. Acta, in press].

Specific interaction of arabinose residue in ginsenoside with egg phosphatidylcholine vesicles

The interaction of the specific sugar residue in ginsenosides with egg phosphatidylcholine vesicles was investigated by ESR spectrometry using phosphatidic acid spin-labeled at the polar head groups. Ginsenoside-Rc, which has an alpha-L-arabinofuranose residue and agglutinability toward egg yolk phosphatidylcholine vesicles (Fukuda, K. et al. (1985) Biochim. Biophys. Acta 820, 199-206), caused the restriction of the segmental motion of spin-labeled phosphatidic acid in egg phosphatidylcholine vesicles, indicating that the saponin interacted with the polar head groups of vesicles. Other ginsenosides-Rb2, Rb1, Rd and p-nitrophenyl glycoside derivatives which have less or no agglutinability were also investigated in the same manner. Only ginsenoside-Rb2 and p-nitrophenyl alpha-L-arabinofuranoside which have the specific sugar residue (arabinose) showed a strong interaction with the polar head groups of vesicles. To gain an insight into the mechanism of agglutination by ginsenoside-Rc, the interaction with the fatty acyl groups was also studied by using phosphatidylcholine spin-labeled at the fatty acyl groups. Ginsenoside-Rc increased the order parameter of the spin-labeled phosphatidylcholine, indicating that the saponin was inserted into lipid bilayers. In other saponins investigated, only ginsenoside-Rb2 interacted with the fatty acyl part of vesicles. The process of expression of agglutination by ginsenoside-Rc was discussed on the basis of the ESR studies.

Sodium-induced aggregation of phosphatidic acid and mixed phospholipid vesicles

Sodium-induced aggregations of sonicated vesicles prepared from synthetic phosphatidic acid and from its 1:1 mixtures with synthetic phosphatidylethanolamine and phosphatidylcholine were studied by turbidimetric measurements. The aggregation reactions were almost completely reversible on change in the Na+ concentration, pH or temperature. The threshold concentrations of Na+ for aggregations of pure dipalmitoylphosphatidic acid vesicles and mixed dipalmitoylphosphatidylethanolamine- and dimyristoylphosphatidylcholine-dipalmitoylphosphatidic acid vesicles were found to be 200, 310 and 550 mM, respectively, at 25 degrees C and pH 7.2. The hydrocarbon chain lengths of phosphatidic acid and phosphatidylethanolamine had little effect on the threshold concentrations. The threshold concentrations for phospholipid vesicles composed of phosphatidic acid alone or its 1:1 mixture with phosphatidylethanolamine were changed by varying either the pH or temperature, while that for phosphatidylcholine-phosphatidic acid vesicles was almost independent of the pH and temperature, implying that aggregation of the latter vesicles is induced by a somewhat different mechanism.

Glycolipid crypticity in membranes--not a simple shielding effect of macromolecules

The potential of membrane-bound macromolecules for shielding glycolipids from involvement in specific binding events was considered in model membranes. Serum albumin and several Dextrans were covalently derivatized with oleic acid so that they adsorbed irreversibly to lipid bilayers. This provided a means of generating bilayer membranes with a considerable layer of attached material. Gangliosides dispersed in such membranes were subjected to attack by the enzyme, neuraminidase, in order to assess their "accessibility'. We were surprised to find that we could not demonstrate any significant reduction in ganglioside hydrolysis in phosphatidylcholine bilayers bearing extensive surface coats of protein or polysaccharide. We conclude that non-specific, physical shielding by macromolecules is an unlikely source of the often-observed "crypticity' of glycolipids at the cell surface. Consistent with this interpretation was a relative lack of headgroup motional restriction seen for spin-labelled ganglioside headgroups in the same bilayers and in cell membranes.

Co-operative binding of concanavalin A to a glycoprotein in lipid bilayers

Lectin-binding curves are reported for a concanavalin A receptor glycoprotein in lipid bilayers and intact cells. The results are consistent with previous studies of the structurally dissimilar transmembrane glycoprotein, glycophorin. High-affinity lectin binding to model membranes was influenced by the presence of apparently unrelated macromolecules, which we suggest is an example of receptor modulation by local interactions. Furthermore, high-affinity binding to the model membranes displayed characteristics, including positive cooperativity, similar to those seen with intact cells.

Protein-dependent lipid lateral phase separation as a mechanism of human erythrocyte ghost resealing

The hypothesis of a correlation between a 10 degrees-20 degrees C lipid phase transition and the resealing process of human erythrocyte membrane has been investigated. The conditions required to reseal human erythrocyte ghosts have been studied by measuring the amount of fluorescein-labeled dextran (FD) that is trapped into the membrane. Temperature per se was sufficient to induce membrane resealing: (1) at 5 mM sodium phosphate, pH 7.8 (5P8), resealing began at 12 degrees C; (2) at salt concentrations above 8 mM sodium phosphate, it occurred at lower temperature; and (3) in isotonic saline was detected just above 5 degrees C. The removal of peripheral membrane proteins from unsealed membranes by chymotrypsin at 0 degree C in 5P8 was followed by membrane resealing. This seems to imply that the presence of proteins is necessary to maintain the membrane unsealed. Protein-induced lateral phase separation of lipids may be a reasonable mechanism for the observed phenomena. In fact, the permeability of phosphatidylserine-phosphatidylcholine mixed liposomes to FD is modified by lipid lateral phase separation induced by pH or poly-L-lysine. Electron spin resonance studies of membrane fluidity by a spin labeled stearic acid showed a fluidity break around 11 degrees C, which may be due to a gel-liquid phase transition. Fluidity changes are abolished by chymotrypsin treatment. It is suggested that a lateral phase separation is responsible for the permeability of open ghosts to FD. Accordingly, disruption of phase separation apparently produces membrane reconstitution. In this respect peripheral proteins and particularly the spectrin-actin network, may play a major role in membrane resealing.