Glycolipid modulation of membrane adhesion

Molecular dynamics simulation of GM1 in phospholipid bilayer

We report molecular dynamics simulation of fully hydrated lipid bilayer of dimyristoyl phosphatidyl choline (DMPC) at room temperature with ganglioside GM1 attached to it in the upper layer under periodic boundary conditions. The simulation results indicate that the presence of a single GM1 molecule has local effects on the bilayer. Three sugar residues (GalNAc-Gal-Glc) of the pentasaccharide head group of GM1 remain on the lipid surface where as the NeuNAc residue extends out in the aqueous layer. The radial distribution functions suggest ordering of water molecules near the glycerol and carboxyl group of the sialic acid in the upper layer. One of the ceramide chains of GM1, the sphingosine chain, folds up and is stacked under the sugar residues lying on the surface. The other ceramide chain is inserted into the lipid bilayer. The arrangement of the polar head group as well as the acyl chains of the lipids which are immediate neighbours of the GM1 are modified compared to the non-neighbour ones and others at the lower layer. The time average conformation of GM1-pentasaccharide is stabilized by a number of inter residue hydrogen bonds that were observed experimentally. The trajectory average conformation of GM1-pentasaccharide was docked on to the cholera toxin molecule and the minimized complex reveals alternative binding modes between the toxin and the GM1-pentasaccharide moiety. The results of these simulation studies might help to understand the structure and nature of the effects of GM1 on the membrane at atomic resolution.

Long- and short-range interactions between phospholipid/ganglioside GM1 bilayers

The structure and interactive properties of liquid-crystalline egg phosphatidylcholine (EPC) bilayers containing the ganglioside GM1 and its uncharged analogue, asialoGM1 (AGM1), have been obtained by X-ray diffraction analysis of osmotically stressed liposomes. Both electron density profiles and reciprocal space modeling indicate that (1) the incorporation of up to 30 mol % GM1 into EPC bilayers has little effect on bilayer organization and (2) the oligosaccharide portion of the GM1 molecule extends at least 12 A beyond the EPC head group into the fluid space, implying that the GM1 head group is nearly fully extended from the bilayer surface. Pressure-distance relations for EPC:GM1 bilayers in 100 mM ionic strength buffer show that, for large bilayer separations, the interbilayer repulsive pressure decays exponentially with a decay length and magnitude expected for electrostatic repulsion arising from the charged GM1. However, at interbilayer separations of < or = 30 A for 7:3 and 8:2 EPC:GM1 and < or = 22 A for 9:1 EPC:GM1, the pressure-distance curves have distinct upward breaks, with the sharpness of this break depending strongly on the amount of GM1 in the bilayer. For 7:3 EPC:GM1 bilayers, the break is quite sharp so that the distance between bilayers does not decrease below 28 A with further increases in applied pressure. For EPC:GM1 8:2 and 9:1 bilayers, the upward break becomes softer with decreasing GM1 concentration. For uncharged EPC:AGM1 bilayers, the repulsive pressure extends only to an equilibrium fluid separation of about 36 A, but has a similar behavior to the pressure-distance data for EPC:GM1 for separations below 20 A. We argue that the nonelectrostatic repulsive pressures arise primarily from the steric interactions between the hydrated oligosaccharide head groups that protrude from the bilayer surface.

Congregation of gangliosides at the junction between two model membranes

The diversity and distribution of gangliosides in vertebrate tissue suggests an important role in cellular recognition. Two types of experiments are reported to test the hypothesis that gangliosides can congregate to form an adhesive junction between two membranes. First, to monitor ganglioside distribution and mobility in different regions of two large spherical bilayer membranes, fluorescent derivatives of natural gangliosides were synthesized. Second, the cation carrier nonactin was used as a conductance probe to measure the membrane surface potential, which would be altered if there were a redistribution of the charged gangliosides. These studies were conducted in large spherical artificial membranes made from egg phosphatidylcholine or oleoylpalmitoylphosphatidylcholine with 0-12 mol % bovine brain gangliosides dissolved in n-decane. The fluorescent gangliosides utilized were lucifer yellow adducts to the sialic acids (LY-gangliosides) or a cis-paranaric acid substitution of the N-acyl moiety in the ceramide portion of gangliosides GM1 and GD1a (paranaryl-GM1 and paranaryl-GD1a). The polarized fluorescence from the adhesive junction between two membranes containing LY-gangliosides or either paranarylganglioside was compared to that in nonadhesive regions. For LY-gangliosides, total fluorescence in the junction decreased with time, possibly due to electrostatic repulsion of this highly charged derivative. For paranarylgangliosides, fluorescence in the junction increased 7-fold with time, suggesting congregation of this ganglioside. In both cases, a measure of rotational mobility, fluorescence anisotropy, increased dramatically, about 2-fold, as expected for restricted mobility of adhesive compounds. Independent evidence for congregation of charge-bearing gangliosides was found with the conductance probe nonactin.(ABSTRACT TRUNCATED AT 250 WORDS)

Polarity decrease at the adhesive junction between two model membranes containing gangliosides

The increased electrical conductance previously observed between two model membranes containing gangliosides suggests the creation of a new environment in the adhesive junction [Brewer, G. J., & Thomas, P.D. (1984) Biochim. Biophys. Acta 776, 279]. In order to provide a mechanism for this novel finding, we now report an investigation of the micropolarity in the adhesive junction. Emission from the fluorescent probe PRODAN is a sensitive measure of polarity of the probe environment. A bimodal linear relationship correlates the emission wavelength from PRODAN with the inverse of solvent dielectric constant (1/epsilon). A better single linear relationship is obtained using Reichardt's relative polarity measure (RPM). Creation of two macroscopic spherical lipid bilayers from phosphatidylcholine, brain gangliosides, and PRODAN allowed selective excitation and observation of fluorescence from either a single bilayer or the double bilayer in the adhesive junction. The reported PRODAN polarity of -0.57 in a single ganglioside-containing membrane was midway between the polarity of water and n-hexane, suggesting PRODAN localization near the lipid carbonyls. The adhesive junctional region exhibited two new less polar environments of PRODAN fluorescence, RPM = -0.45 and -0.29. These measures are consistent with a relatively dehydrated immobilized phase. These changes were not observed in the adhesion zone between two membranes made with phosphatidylcholine without gangliosides. The changes in molecular structure in the junction that could be responsible for the altered PRODAN emission are discussed. A decrease in the hydrocarbon thickness of junctional membranes or a decrease in the aqueous junctional polarity could be responsible for the polarity decrease reported by PRODAN.

Complex carbohydrate-lectin interaction at the interface: a model for cellular adhesion. I. Effect of vesicle size on the kinetics of aggregation between a fatty acid conjugate of lectin and a liposomal asialoganglioside

Two types of phospholipid vesicles capable of mutual recognition have been tailor-made to serve as a model system for the study of carbohydrate-mediated cellular adhesion. One of the vesicles contained a fatty acid conjugate of a galactose specific lectin (lectin vesicle) and the other an asialoganglioside with a reactive terminal galactose residue (galactose vesicle). The kinetics of aggregation of these two types of vesicles was followed by monitoring time-dependent change in turbidity. A 10-100-fold enhancement in the forward rate constant (kf ranging from 7.1 x 10(5) to 4.5 x 10(7) M-1.s-1 at 27 degrees C) was observed when compared with that for the lectin-galactose system in solution (kf being 4.5 x 10(5) M-1.s-1), reported in the literature. A study of the influence of vesicle size on the rate of aggregation showed that enhancement depended on the curvature of the galactose vesicle rather than the density of asialoganglioside suggesting a possible diffusion in the plane of the membrane. The ratio, kf/kd is found to be approx. 10(10) M-1 indicating that the formation of multiple bonds plays a role for stable adhesion.

Curvature and composition-dependent lipid asymmetry in phosphatidylcholine vesicles containing phosphatidylethanolamine and gangliosides

The effect of curvature on transbilayer lipid asymmetry in vesicles is investigated using vesicles of different sizes (30-140 nm) prepared by sonication and polycarbonate filter extrusion techniques. The transbilayer distributions of phosphatidylethanolamine and gangliosides are measured using 2,4,6-trinitrobenzenesulphonic acid and Clostridium perfringens neuraminidase as non-penetrating probes, respectively. The distribution of phosphatidylethanolamine in a phosphatidylcholine/phosphatidylethanolamine (4:1, molar ratio) system is more or less symmetric and curvature seems to have little effect. However, the distribution of gangliosides in a phosphatidylcholine/ganglioside (10:1, molar ratio) system is asymmetric in favour of the outer layer in smaller vesicles, the asymmetry disappearing as the degree of curvature decreases. In a phosphatidylcholine/phosphatidylethanolamine/ganglioside (8:2:1, molar ratio) system, both phosphatidylethanolamine and gangliosides distribute asymmetrically, indicating a composition-dependent asymmetric distribution of phosphatidylethanolamine. In this system asymmetry also increases with increasing curvature. The asymmetric distribution of gangliosides in vesicles of low curvature may be due to their long headgroup and larger headgroup surface area in accordance with the theoretical predictions of Israelachvili et al. (Biochim. Biophys. Acta 470 (1977) 185-201).