Interaction of gangliosides with proteins depending on oligosaccharide chain and protein surface modification

Effect of Transmembrane Electric Field on GM1 Containing DMPC-Cholesterol Monolayer: A Computational Study

Transmembrane electric potentials and membrane curvature have always provided pathways to mediate different cellular processes. We present results of molecular dynamics (MD) simulations of lipid monolayer composed of 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol (CHOL) under a transverse electric field to monitor the effect of electric field on membrane containing ganglioside monosialo 1 (GM1). Four systems were studied with membrane monolayer in the presence and absence of GM1 with and without applying electric field along the normal of the monolayer. The applied transmembrane electric field was 0.4 mV/Å which corresponds to the action potential of animal cell. Our results indicate that the electric field induces a considerable lateral stress on the monolayer in the presence of GM1, which is evident from the lateral pressure profiles. It was found that due to the application of electric field major perturbation was caused to the system containing GM1, manifested by the bending of the monolayer. We believe this study provides correlation between electric field and spontaneous membrane bending, specially based on the membrane composition. The consequences of these MD simulations provide considerable insights to different biological phenomenon and lipid membrane models.

Phase Behavior of GM1-Containing DMPC-Cholesterol Monolayer: Experimental and Theoretical Study

Organization and distribution of lipids in cellular membranes play an important role in a diverse range of biological processes, such as membrane trafficking and signaling. Here, we present the combined experimental and simulated results to elucidate the phase behavioral features of ganglioside monosialo 1 (GM1)-containing mixed monolayer of the lipids 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) and cholesterol (CHOL). Two monolayers having compositions DMPC-CHOL and GM1-DMPC-CHOL are investigated at air-water and air-solid interfaces using Langmuir-Blodgett experiments and scanning electron microscopy (SEM), respectively, to ascertain the phase behavior change of the monolayers. Surface pressure isotherms and SEM imaging of domain formation indicate that addition of GM1 to the monolayer at low surface pressure causes a fluidization of the system but once the system attains the surface pressure corresponding to its liquid-condensed phase, the monolayer becomes more ordered than the system devoid of GM1 and interacts among each other more cooperatively. Besides, the condensing effect of cholesterol on the DMPC monolayer was also verified by our experiments. Apart from these, the effects induced by GM1 on the phase behavior of the binary mixture of DMPC-CHOL were studied with and without applying liquid-expanded (L_E)-liquid-condensed (L_C) equilibrium surface pressure using molecular dynamics (MD) simulation. Our molecular dynamics (MD) simulation results give an atomistic-level explanation of our experimental findings and furnish a similar conclusion.

Investigation on the structure of water/AOT/IPM/alcohols reverse micelles by conductivity, dynamic light scattering, and small angle X-ray scattering

We have systematically investigated the effect of alcohols (ethanol, propanol, butanol, and pentanol) on the structure of the water/AOT/IPM system using conductivity, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) techniques. The results show that no percolation phenomenon is observed in the water/AOT/IPM system, whereas the addition of ethanol (propanol and butanol) induces apparently percolation. The threshold water content (W(p)) depends closely on the alcohol type and concentration. The effect of alcohols on the conductance behavior is discussed from the physical properties of alcohols, the interfacial flexibility, and the attractive interactions between droplets. The hydrodynamic diameter of droplets (d(H)) obtained from DLS increases markedly with the increase in water content (W(0)); however, it decreases gradually with increasing alcohol chain length and concentration. SAXS measurements display distinctly the shoulder, the low hump peaks, and the heavy tail phenomenon in the pair distance distribution function p(r) profile, which rely strongly on the alcohol species and its concentration. The gyration radius (R(g)) increases with increasing W(0), and decreases with the increase of alcohol chain length and concentration. Schematic diagram of the conductance mechanism of water/AOT/IPM/alcohol systems is primarily depicted. Three different phases of the discrete droplets, the oligomers, and the isolated ellipsoidal droplets existed in the different W(0) ranges correspond to three different stages in the conductivity-W(0) curve. Coupling the structure characteristics of reverse micelles obtained from DLS and SAXS techniques with conductivity could be greatly helpful to deeply understand the percolation mechanism of water/AOT/IPM/alcohols systems.

Sphingolipid/cholesterol regulation of neurotransmitter receptor conformation and function

Like all other monomeric or multimeric transmembrane proteins, receptors for neurotransmitters are surrounded by a shell of lipids which form an interfacial boundary between the protein and the bulk membrane. Among these lipids, cholesterol and sphingolipids have attracted much attention because of their well-known propensity to segregate into ordered platform domains commonly referred to as lipid rafts. In this review we present a critical analysis of the molecular mechanisms involved in the interaction of cholesterol/sphingolipids with neurotransmitter receptors, in particular acetylcholine and serotonin receptors, chosen as representative members of ligand-gated ion channels and G protein-coupled receptors. Cholesterol and sphingolipids interact with these receptors through typical binding sites located in both the transmembrane helices and the extracellular loops. By altering the conformation of the receptors ("chaperone-like" effect), these lipids can regulate neurotransmitter binding, signal transducing functions, and, in the case of multimeric receptors, subunit assembly and subsequent receptor trafficking to the cell surface. Several sphingolipids (especially gangliosides) also exhibit low/moderate affinity for neurotransmitters. We suggest that such lipids could facilitate (i) the attachment of neurotransmitters to the post-synaptic membrane and in some cases (ii) their subsequent delivery to specific protein receptors. Overall, various experimental approaches provide converging evidence that the biological functions of neurotransmitters and their receptors are highly dependent upon sphingolipids and cholesterol, which are active partners of synaptic transmission. Several decades of research have been necessary to untangle the skein of a complex network of molecular interactions between neurotransmitters, their receptors, cholesterol and sphingolipids. This sophisticated crosstalk between all four distinctive partners may allow a fine biochemical tuning of synaptic transmission.

Curvature-dependent lateral distribution of raft markers in the human erythrocyte membrane

The distribution of raft markers in curved membrane exvaginations and invaginations, induced in human erythrocytes by amphiphile-treatment or increased cytosolic calcium level, was studied by fluorescence microscopy. Cholera toxin subunit B and antibodies were used to detect raft components. Ganglioside GM1 was enriched in membrane exvaginations (spiculae) induced by cytosolic calcium and amphiphiles. Stomatin and the cytosolic proteins synexin and sorcin were enriched in spiculae when induced by cytosolic calcium, but not in spiculae induced by amphiphiles. No enrichment of flotillin-1 was detected in spiculae. Analyses of the relative protein content of released exovesicles were in line with the microscopic observations. In invaginations induced by amphiphiles, the enrichment of ganglioside GM1, but not of the integral membrane proteins flotillin-1 and stomatin, was observed. Based on the experimental results and theoretical considerations we suggest that membrane skeleton-detached, laterally mobile rafts may sort into curved or flat membrane regions dependent on their intrinsic molecular shape and/or direct interactions between the raft elements.

Ganglioside GD1a increases the excitability of voltage-dependent sodium channels

The effect of the negatively charged ganglioside GD1a, one of the major brain gangliosides [H. Beitinger, W. Probst, R. Hilbig, H. Rahmann, Seasonal variability of sialo-glycoconjugates in the brain of the Djungarian hamster (Phodopus sungorus). Comp. Biochem. Physiol., B 86 (1987) 377-384] on the function of brain derived BTX-modified voltage-dependent sodium channel was studied using the planar lipid bilayer system. Bilayers were formed either with a mixture of neutral phospholipids (4 phosphoethanolamine (PE):1 phosphocholine (PC)) alone or with one containing 6% of the disialoganglioside GD1a. The permeation and activation properties of the channels were measured in the presence of symmetrical 200 mM NaCl. We found that the single channel conductance was not affected by GD1a, whereas the steady-state activation curve displayed a hyperpolarizing shift in the presence of GD1a. Since the lipid distribution in these membranes is symmetrical, then the GD1a effect on sodium channels may result either from an induction of channel conformational changes or from an asymmetrical interaction between the channel (extracellular vs. intracellular channel aspect) and GD1a. Regardless of the mechanism, the data indicate that differences in ganglioside content in neuronal cells may contribute to the previously observed sodium channel functional variability within (soma, dentritic, axon hillock) and between neuronal cells as well as to excitability changes in those physiological and pathological conditions where changes in the neuronal ganglioside content occur.

Disialogangliosides and Their Interaction with Cholera Toxin—Investigation by Molecular Modeling, Molecular Mechanics and Molecular Dynamics

Molecular mechanics and molecular dynamics studies are performed to investigate the conformational preference of cell surface disialogangliosides (GD1A, GD1B and GD3) in aqueous environment. The molecular mechanics calculation reveals that water mediated hydrogen bonding network plays a significant role in the structural stabilization of GD1A, GD1B and GD3. These water mediated hydrogen bonds not only exist between neighboring residues but also exist between residues that are separated by 2 to 3 residues in between. The conformational energy difference between different conformational states of gangliosides correlates very well with the number of water mediated and direct hydrogen bonds. The spatial flexibility of NeuNAc of gangliosides at the binding site of cholera toxin is worked out. The NeuNAc has a limited allowed eulerian space at the binding site of Cholera Toxin (2.4%). The molecular modeling, molecular mechanics and molecular dynamics of disialoganglioside-cholera toxin complex reveal that cholera toxin can accommodate the disialoganglioside GD1A in three different modes. A single mode of binding is permissible for GD1B and GD3. Direct and water mediated hydrogen bonding interactions stabilizes these binding modes and play an essential role in defining the order of specificity for different disialogangliosides towards cholera toxin. This study not only provides models for the disialoganglioside-cholera toxin complexes but also identifies the NeuNAc binding site as a site for design of inhibitors that can restrict the pathogenic activity of cholera toxin.

Determination of asymmetric structure of ganglioside-DPPC mixed vesicle using SANS, SAXS, and DLS

Functions of mammalian cell membrane microdomains being rich in glycosphingolipids, so-called rafts, are now one of the current hot topics in cell biology from the intimate relation to cell adhesion and signaling. However, little is known about the role of glycosphingolipids in the formation and stability of the domains. By the use of the inverse contrast variation method in small-angle neutron scattering (SANS), combined with small-angle x-ray scattering (SAXS) and dynamic light scattering (DLS), we have determined an asymmetric internal structure of the bilayer of the small unilamellar vesicle (SUV) of monosialoganglioside (G(M1))-dipalmitoylphosphatidylcholine (DPPC) mixture ([G(M1)]:[DPPC] = 0.1:1). A direct method using a shell-model fitting with a size distribution function describes consistently all experimental results of SANS, SAXS, and DLS. We have found that G(M1) molecules predominantly localize at SUV outer surface to form a highly hydrophilic layer which is dehydrated with the rise of temperature from 25 degrees C to 55 degrees C accompanied by the conformational change of the oligosaccharide chains. The average SUV size determined is approximately 200 A, which is comparable to the reported value 260 +/- 130 A of glycosphingolipids microdomains. The present results suggest that the preferential asymmetric distribution of gangliosides is essential to define the size and stability of the domains.

New EPR method for cellular surface characterization

An electron paramagnetic resonance (EPR)-based membrane surface characterization method is presented to detect the properties of the carbohydrate-rich part of membrane surfaces as well as carbohydrate interaction with other membrane constituents and water-soluble molecules. The proposed method relies on the spin-labeling and spectral decomposition based on spectral simulation and optimization with EPRSIM software. In order to increase the sensitivity of characterization to the carbohydrate-rich part of the membrane surface, the sucrose-contrasting approach is introduced. With this method, which was established on model membranes with glycolipids and tested on erythrocyte membrane, we were able to characterize the surface and lipid bilayer lateral heterogeneity. Additionally, some properties of the interaction between glycocalyx and lipid bilayer as well as between glycocalyx and sucrose molecules were determined. The experiments also provided some information about the anchoring and aggregation of the glycosylated molecules. According to the results, some functions of the glycosylated surface are discussed.

Intensive extrusion and occlusion of water in ganglioside micelles with thermal reversibility

By using a shell-modeling analysis for small-angle scattering data of ganglioside micellar dispersion, we recently reported that the elevation of temperature induces a significant shrinkage of the hydrophilic region of the ganglioside micelle, suggesting that the oligosaccharide chains with sialic acid residues of ganglioside molecules change the conformation, sensitively responding to a change in temperature (Hirai et al., 1996. Biophys. J. 70:1761-1768; J. Phys. Chem. 100:11675-11680). We have carried out further analyses of the temperature dependence of the structural parameters reported previously, and we have found clear evidence of reversible extrusion and occlusion of a large amount of water in the hydrophilic region of the ganglioside micelle in the physiological temperature range of 6-60 degrees C. The present results suggest a remarkable function of ganglioside molecules: they change the hydrophilicity of the cell surface locally as a response to variations in temperature. This phenomenon might be involved in various surface events, such as cell-cell interaction and cell surface-protein interaction.