Modulation of neuraminidase activity by the physical state of phospholipid bilayers containing gangliosides Gd1a and Gt1b

Composition-driven surface domain structuring mediated by sphingolipids and membrane-active proteins. Above the nano- but under the micro-scale: mesoscopic biochemical/structural cross-talk in biomembranes

Biomembranes contain a wide variety of lipids and proteins within an essentially two-dimensional structure. The coexistence of such a large number of molecular species causes local tensions that frequently relax into a phase or compositional immiscibility along the lateral and transverse planes of the interface. As a consequence, a substantial microheterogeneity of the surface topography develops and that depends not only on the lipid-protein composition, but also on the lateral and transverse tensions generated as a consequence of molecular interactions. The presence of proteins, and immiscibility among lipids, constitute major perturbing factors for the membrane sculpturing both in terms of its surface topography and dynamics. In this work, we will summarize some recent evidences for the involvement of membrane-associated, both extrinsic and amphitropic, proteins as well as membrane-active phosphohydrolytic enzymes and sphingolipids in driving lateral segregation of phase domains thus determining long-range surface topography.

Biophysics of sphingolipids II. Glycosphingolipids: An assortment of multiple structural information transducers at the membrane surface

Glycosphingolipids are ubiquitous components of animal cell membranes. They are constituted by the basic structure of ceramide with its hydroxyl group linked to single carbohydrates or oligosaccharide chains of different complexity. The combination of the properties of their hydrocarbon moiety with those derived from the variety and complexity of their hydrophilic polar head groups confers to these lipids an extraordinary capacity for molecular-to-supramolecular transduction across the lateral/transverse planes in biomembranes and beyond. In our opinion, most of the advances made over the last decade on the biophysical behavior of glycosphingolipids can be organized into three related aspects of increasing structural complexity: (1) intrinsic codes: local molecular interactions of glycosphingolipids translated into structural self-organization. (2) Surface topography: projection of molecular shape and miscibility of glycosphingolipids into formation of coexisting membrane domains. (3) Beyond the membrane interface: glycosphingolipid as modulators of structural topology, bilayer recombination and surface biocatalysis.

Sphingolipids

A significant corpus of work over the last decade has firmly established an important role for sphingolipids in a variety of important biological processes. Such processes include signaling events related to cell growth, differentiation, programmed cell death, and stress responses. These processes not only involve those sphingolipids that accumulate as a result of a variety of inherited lysosomal storage disorders, but, in addition, sphingolipids associated with long-chain base metabolism. This article reviews the chemical properties, pathways, regulated metabolism, and signaling function of sphingolipids. In addition, the potential roles of sphingolipids in renal-specific processes are considered. While a variety of cellular functions have been ascribed to sphingolipids, in many cases proof of the concept has yet to be well established. Thus, a number of critical questions can be posed in interpreting these studies. Several of these questions are considered.

Influence of phospholipid chain length on verotoxin/globotriaosyl ceramide binding in model membranes: comparison of a supported bilayer film and liposomes

The importance of the surrounding lipid environment on the availability of glycolipid carbohydrate for ligand binding was demonstrated by studying the influence of phosphatidylcholine fatty acid chain length on binding of verotoxins (VT1 and VT2c) to their specific cell surface receptor, globotriaosylceramide (Gb3) in the presence of auxiliary lipids both in a microtitre plate surface bilayer film and in a liposome membrane model system. In the microtitre assay, both VT1 and VT2c binding to Gb3 was increased as a function of decreasing PC acyl chain length likely resulting in increased Gb3 exposure. In the liposome assay VT1 binding was similarly modulated, however the effect of VT2c binding was more complex and did not follow a simple function of increased carbohydrate exposure. Earlier work established that C22:1 and C18:1Gb3 fatty acid homologues were the preferred Gb3 receptor containing liposomes, but in C14PC liposomes, binding to C22:1Gb3 (but not C18:1Gb3) was elevated such that this Gb3 species now became the preferred receptor for both toxins. This change in verotoxin/Gb3 homologue binding selectivity in the presence of C14PC did not occur in the microtitre bilayer format. These results are consistent with our proposal that these toxins recognize different epitopes on the Gb3 oligosaccharide. We infer that relative availability of these epitopes for toxin binding in an artificial bilayer is influenced not only by the exposure due to the discrepancy between the fatty acyl chain lengths of Gb3 and PC, but by the physical mode of presentation of the bilayer structure. Such acyl chain length differences have a more marked effect in a supported bilayer film whereas only the largest discrepancies affect Gb3 receptor function in liposomes. The basis of phospholipid modulation of glycolipid carbohydrate accessibility for receptor function is likely complex and will involve phase separation, gel/liquid crystalline transition, packing and lateral mobility within the bilayer, suggesting that such parameters should be considered in the assessment of glycolipid receptor function in cells.

Properties of ganglioside GM1 in phosphatidylcholine bilayer membranes

Gangliosides have been shown to function as cell surface receptors, as well as participating in cell growth, differentiation, and transformation. In spite of their multiple biological functions, relatively little is known about their structure and physical properties in membrane systems. The thermotropic and structural properties of ganglioside GM1 alone and in a binary system with 1,2-dipalmitoyl phosphatidylcholine (DPPC) have been investigated by differential scanning calorimetry (DSC) and x-ray diffraction. By DSC hydrated GM1 undergoes a broad endothermic transition TM = 26 degrees C (delta H = 1.7 kcal/mol GM1). X-ray diffraction below (-2 degrees C) and above (51 degrees C) this transition indicates a micellar structure with changes occurring only in the wide angle region of the diffraction pattern (relatively sharp reflection at 1/4.12 A-1 at -2 degrees C; more diffuse reflection at 1/4.41 A-1 at 51 degrees C). In hydrated binary mixtures with DPPC, incorporation of GM1 (0-30 mol%; zone 1) decreases the enthalpy of the DPPC pretransition at low molar compositions while increasing the TM of both the pre- and main transitions (limiting values, 39 and 44 degrees C, respectively). X-ray diffraction studies indicate the presence of a single bilayer gel phase in zone 1 that can undergo chain melting to an L alpha bilayer phase. A detailed hydration study of GM1 (5.7 mol %)/DPPC indicated a conversion of the DPPC bilayer gel phase to an infinite swelling system in zone 1 due to the presence of the negatively charged sialic acid moiety of GM1. At 30-61 mol % GM1 (zone 2), two calorimetric transitions are observed at 44 and 47 degrees C, suggesting the presence of two phases. The lower transition reflects the bilayer gel --> L alpha transition (zone 1), whereas the upper transition appears to be a consequence of the formation of a nonbilayer, micellar or hexagonal phase, although the structure of this phase has not been defined by x-ray diffraction. At > 61 mol % GM1 (zone 3) the calorimetric and phase behavior is dominated by the micelle-forming properties of GM1; the presence of mixed GM1/DPPC micellar phases is predicted.

Modulation of the activity of Clostridium perfringens neuraminidase by the molecular organization of gangliosides in monolayers

The activity of Clostridium perfringens neuraminidase against gangliosides GM3, GD1a and GM1 was studied in lipid monolayers at the air-buffer solution interface. The enzyme activity assay against pure ganglioside monolayers is based on the markedly different molecular packing areas of the substrate gangliosides and the resulting product glycosphingolipids. This allows to control and monitor the surface pressure and the ganglioside intermolecular organization (cross-sectional packing areas and dipole potentials) in a continuous manner during the catalytic process. It was found that the rate and the extent of the enzymatic reaction depended markedly on the lateral surface pressure. In general, the activity of neuraminidase against GM3 and GD1a was higher at lower surface pressure. This corresponded to larger intermolecular spacings among the ganglioside molecules. Both the activity and the extent of the reaction against GM3 were higher than toward GD1a. GM1 could not be degraded by the enzyme, irrespective of the surface pressure but the enzyme could interact with this ganglioside. A latency period, longer for GM3 than for GD1a, was observed prior to the onset of rapid degradation; this indicates that pre-catalytic steps are occurring at the interface before effective ganglioside degradation takes place. The latency period, the total amount of ganglioside degraded, and the velocity of the reaction varied with the surface pressure in different manners. Our data indicate that the different steps of the catalytic reaction occurring at the surface (i.e., substrate recognition and interfacial adsorption, catalysis, maximum extent of substrate conversion) are independently regulated by the molecular organization of the substrate gangliosides.

Role of gangliosides in behavioral and biochemical actions of alcohol: cell membrane structure and function

Alcohol exerts its pharmacological effects in adult brain by altering the physicochemical properties of cellular plasma membranes. Although alcohol does induce changes in membrane lipid composition, studies to relate these alterations to the development of behavioral tolerance to alcohol and the withdrawal effects have been unsuccessful. Actions of alcohol on developing brain are even more complex. Some of the reported effects include inhibition of embryogenesis, cell migration, and differentiation, including synaptogenesis. Gangliosides have neuroprotective action against a variety of neural insults (e.g., mechanical injury, drug toxicity, or hypoxic insult). This review addresses the role and significance of gangliosides in the CNS pathophysiology of alcohol exposure, as well as the effect of changes in endogenous gangliosides on membrane structure and function. We also describe the role of exogenous gangliosides in prevention of alcohol (acute and/or chronic)-induced CNS (prenatal and postnatal) neurotoxicity through their action on cellular plasma membranes. We propose that ganglioside's neuroprotective effects against alcohol neurotoxicity involve protection and restoration of plasma membrane structure (proteins and lipids) and thereby its function (ionic homeostasis, neurotransmitter receptor-mediated signal transduction). Thus gangliosides may have potential therapeutic use in treatment of alcohol-related problems.

Liposomes with prolonged circulation times: factors affecting uptake by reticuloendothelial and other tissues

Many of the applications of liposomes drug-delivery systems have been limited by their short circulation half-lives as a result of rapid uptake into the reticuloendothelial (mononuclear phagocyte) system. We have recently described liposomes formulations with long circulation half-lives in mice (Allen, T.M. and Chonn, A. (1987) FEBS Lett. 223, 42-46). A study of the principal factors important to the attainment of liposomes with prolonged circulation half-lives is presented in this manuscript. Liposomes with the longest circulation half-lives, in mice, had compositions which mimicked the outer leaflet of red blood cell membranes (egg phosphatidylcholine/sphingomyelin/cholesterol/ganglioside GM1, molar ratio 1:1:1:0.14). Several other gangliosides and glycolipids were examined, but none could substitute for GM1 in their ability to prolong circulation half-lives. However, other negatively charged lipids with bulky headgroups, i.e., sulfatides and phosphatidylinositol, had some effect in prolonging circulation half-lives, but GM1 was clearly superior in this regard. Bilayer rigidity, imparted by sphingomyelin or other high-phase-transition lipids, acted synergistically with the negatively charged components, especially GM1, in extending circulation times. Circulation half-lives of liposomes increased with decreasing size, but even larger (0.2-0.4 microns) liposomes of the optimum formulations had significantly prolonged half-lives in circulation. Uptake of liposomes into tissues other than liver and spleen increased with increasing circulation times of the liposomes for i.v. and for i.p. injections. Liposomes appeared to move from the circulation into the carcass between 6 and 24 h post-injection. Our ability to achieve significant prolongation in circulation times of liposomes makes possible a number of therapeutic applications of liposomes which, until now, have not been achievable.

Globoside with spin-labelled fatty acid: bilayer lateral distribution and immune recognition

We have critically addressed the question of lateral distribution of glycolipids in bilayer membranes, and the effect of glycolipid fatty acid chain length upon such distribution. For this purpose we synthesised the complex neutral glycosphingolipid, globoside, with spin-labelled fatty acid. Base hydrolysis to remove the natural fatty acid was found to deacetylate the GalNAc residue concomitantly, necessitating application of the synthetic route described for gangliosides by Neuenhofer et al. (Biochemistry 24, 525-532 (1985)). Globosides were produced with 18-carbon and 24-carbon fatty acids bearing a spin label at the C-16 position. Spin-labelled globosides were incorporated at 2 and 10 mol% into rigid, highly cooperative bilayer matrices of 1,2-dipalmitoylglycerophosphocholine (DPPC) and also into semi-fluid, non-cooperative membranes of DPPC/cholesterol. Recorded electron paramagnetic resonance (EPR) spectra were analysed by comparison with a library of standards representing samples of known composition. Spectra were manipulated using a computer program which permitted linear combination of standards to stimulate coexistence of laterally separated domains of different composition. The most important conclusions were as follows: (1) at least 80% of the globoside was definitely not confined to domains highly enriched in glycolipid, although there was evidence of binary-phase separation in the rigid DPPC/globoside matrix; (2) the presence of 33 mol% cholesterol reduced the evidence of globoside phase separation; (3) there was remarkably little difference in results whether the globoside fatty acid chain length was similar to that of the phospholipid host matrix or eight carbons longer. Temperature profiles derived over the phase-transition region of DPPC using spin-labelled globoside or an unattached amphiphilic spin label were consistent with these findings. The same systems lent themselves to consideration of the role of glycolipid fatty acid chan length and cholesterol in determining glycolipid crypticity in membranes: (1) polyclonal anti-globoside IgG bound to globoside in DPPC liposomes without inducing agglutination. (2) The same antibodies did agglutinate DPPC/cholesterol liposomes bearing globoside. (3) The effect of cholesterol probably was upon glycolipid dynamics or attitude in the membrane, rather than upon distribution. (4) These observations were basically unaffected by the choice of 18-carbon vs. 24-carbon glycolipid fatty acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Thermal stability and intersubunit interactions of cholera toxin in solution and in association with its cell-surface receptor ganglioside GM1

The thermal stability of cholera toxin free in solution and in association with its cell-surface receptor ganglioside GM1 has been studied by using high-sensitivity differential scanning calorimetry and differential solubility thermal gel analysis. In the absence of ganglioside GM1, cholera toxin undergoes two distinct thermally induced transitions centered at 51 and 74 degrees C, respectively. The low-temperature transition has been assigned to the irreversible thermal denaturation of the active A subunit. The second transition has been assigned to the reversible unfolding of the B subunit pentamer. The isolated B subunit pentamer exhibits a single transition also centered at 74 degrees C, suggesting that the attachment of the A subunit does not contribute to the stability of the pentamer. In the intact toxin, the A subunit dissociates from the B subunit pentamer at a temperature that coincides with the onset of the B subunit thermal unfolding. In aqueous solution, the denatured A subunit precipitates after dissociation from the B subunit pentamer. This phenomenon can be detected calorimetrically by the appearance of an exothermic heat effect. In the presence of ganglioside GM1, the B subunit is greatly stabilized as indicated by an increase of 20 degrees C in the transition temperature. In addition, ganglioside GM1 greatly enhances the cooperative interactions between B subunits. In the absence of ganglioside, each monomer within the B pentamer unfolds in an independent fashion whereas the fully ganglioside-bound pentamer behaves as a single cooperative unit. On the contrary, the thermotropic behavior of the A subunit is only slightly affected by the presence of increasing concentrations of ganglioside GM1.(ABSTRACT TRUNCATED AT 250 WORDS)

A long chain spin label for glycosphingolipid studies: transbilayer fatty acid interdigitation of lactosyl ceramide

16-Carbon and 18-carbon fatty acids with covalently attached nitroxide free radicals have seen wide usage in membrane studies of phospholipid dynamics, orientation, and associations. However, they are inadequate for dealing with some very important questions that relate to glycosphingolipids. We report here the synthesis of a long chain (24-carbon) spin-labelled fatty acid designed for such problems. We have used both the new 24-carbon and the more conventional 18-carbon spin-labelled fatty acids to replace the natural fatty acid of lactosyl ceramide so that we may begin to compare short and long chain derivatives to analyse the molecular basis of their functional differences. Spectra seen are consistent with the view that in a bilayer host matrix the methyl end of the long fatty acid crosses the hydrophobic membrane center and interdigitates with fatty acids of phospholipids of the opposing monolayer.

Effect of calcium ions on the thermotropic behaviour of neutral and anionic glycosphingolipids

In the concentration range of 10(-5) to 10(-1) M Ca2+ modulates the thermotropic properties of several neutral and anionic glycosphingolipids (galactosylceramide, asialo-GM1, sulfatide, GM1, GD1a, GT1b) and of their mixtures with dipalmitoylphosphatidylcholine. The transition temperature of gangliosides is not appreciably changed while the transition enthalpy increases by 20% in the presence of Ca2+. The more marked effect of Ca2+ is on the thermotropic behavior of systems containing sulfatide. Increasing concentrations of Ca2+ between 10(-5) and 10(-3) M (up to a molar ratio of Ca2+/sulfatide 1:2) induce a progressive increase of both the transition temperature and enthalpy. Further increases up to 10(-1) M Ca2+ induce a new phase transition at a lower temperature. No evidence is found for induction of phase separation of pure glycosphingolipid-Ca2+ domains in mixtures of any of the glycosphingolipids with dipalmitoylphosphatidylcholine. The modification of the phase behavior of anionic glycosphingolipids by Ca2+ does not involve detectable variations of the intermolecular packing but is accompanied by marked modifications of the dipolar properties of the polar head group region.

Visualization of domains in rigid ganglioside/phosphatidylcholine bilayers: Ca2+ effects

We have considered the extent to which details of lectin binding directly visualized by freeze-etch electron microscopy are consistent with current concepts of ganglioside arrangement in phosphatidylcholine bilayer membranes. Native lectins in general seem appropriate labels for this type of study. Wheat germ agglutinin, Ricinus communis agglutinin, and peanut agglutinin are adequately resolved on membrane surfaces as spherical particles of diameters 6 nm, 10 nm, and 13 nm, respectively (uncorrected for platinum shadow thickness). The finite areas covered by these markers correspond to some 56, 157, and 265 lipid molecules, respectively, on the surfaces of the shadowed rigid phosphatidylcholine matrices employed here; and this constitutes a basic limitation to the precision with which one can localize a given glycolipid receptor. Ricinus communis agglutinin provides a marker whose size permits adequate quantitation of bound material while minimally obscuring detail. Using it we estimated the size limits of GM1-enriched domains, since this is the ganglioside which has shown the greatest evidence of discontinuous distribution in our hands (Peters, M.W., Mehlhorn, I.E., Barber, K.R. and Grant, C.W.M. (1984) Biochim. Biophys. Acta 778, 419-428). Results of such analyses indicate the probable existence of phase separated domains selectively enriched in GM1 up to 60 nm in extent (5600 lipid molecules) for rigid dipalmitoylphosphatidylcholine membranes bearing up to 14 mol% GM1. Similar observations were true of rigid bilayers of dimyristoylphosphatidylcholine; however, if domains enriched in GM1 exist in fluid dimyristoylphosphatidylcholine, they are on the order of 6 nm or less in diameter (or are dispersed by lectin binding). Employing the small lectin, wheat germ agglutinin, which binds to all gangliosides, we then examined the effect of exposure to Ca2+ ions (while in the fluid state) on the ganglioside 'domain structure' referred to above in rigid dipalmitoylphosphatidylcholine host matrices. GM1, GD1a and GT1b were studied at 0, 2 and 10 mM Ca2+ concentrations. It was demonstrated by spin label measurements that the dipalmitoylphosphatidylcholine matrix retained its basic melting characteristics in the presence of added Ca2+ and ganglioside under these conditions. Within the technique's functional resolution limit of some 6 nm we were unable to identify any effect of Ca2+ in physiological concentration on ganglioside topography as reflected by bound lectin distribution.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuritogenic and metabolic effects of individual gangliosides and their interaction with nerve growth factor in cultures of neuroblastoma and pheochromocytoma

The 4 major ganglioside species, GM1, GD1a, GD1b and GT1b (200 micrograms/ml), were tested individually for the ability to stimulate neuronal trophic responses. The growth parameters measured were: morphologic changes, quantitated by computer-assisted morphometry of neurite length and number per soma, and metabolic changes, indicated by alterations in ornithine decarboxylase activity (ODC). In addition, the interaction of each ganglioside with nerve growth factor (NGF) was investigated with an NGF-responsive pheochromocytoma PC12 cell line and NGF-insensitive neuroblastoma Neuro-2a cultures. PC12 cells responded to gangliosides only in the presence of NGF (20 micrograms/ml): GM1 produced the greatest morphologic response, but did not alter metabolic levels; GT1b increased both parameters. The presence (5 micrograms/ml) or absence of NGF did not have an effect on the ganglioside-mediated morphologic responses of Neuro-2a cells to each species: GD1b elicited the greatest increase in neurite length, while GD1a and GT1b stimulated both length and number. In contrast, while GT1b alone was able to elevate ODC activity independently of NGF, the simultaneous exposure of Neuro-2a cultures to NGF and GM1 or GD1a resulted in a stimulation of cellular metabolism. These results indicate that each ganglioside species has a specific target action in the stimulation of different trophic responses and that performance in one category is not a predictor of the result in another. In addition, it is possible to confer a sensitivity to NGF by simultaneous treatment with specific gangliosides. This indicates that membrane gangliosides may modulate the actions of neurotrophic factors.

Model membranes bearing glycolipids and glycoproteins

The forces that hold cell membrane components together are non-covalent and thermodynamically favoured in aqueous media. Hence virtually any glycolipid or membrane glycoprotein might be expected to be incorporable into lipid bilayer membranes and this expectation has been borne out. In addition methods have been developed for linking lipid fragments to species that would not otherwise be expected to associate with bilayers. Techniques that have been successfully used to generate bilayer structures bearing glycolipids and glycoproteins include hydration of films dried down from non-aqueous solutions of the components, detergent removal from aqueous component solutions, exogenous addition to preformed membranes, and various organic solvent injection or reverse phase approaches. Bilayer association of glycolipids and membrane glycoproteins, with preservation of specific receptor function, seem easy to achieve--in fact difficult not to achieve. Optimization of receptor function to accurately mimic that of cell membranes and efficient preservation of functions such as transport or second messenger activation, are typically more demanding, although still feasible. A systematic approach can give considerable insight into the processes involved via identification of minimal necessary factors. Unfortunately, the actual relative arrangement of components, so critical to subtleties of glycolipid and glycoprotein function, remains almost totally unknown for lack of morphological information in the size range of individual macromolecules. The latter problem has come to be the most critical limitation to many studies.

Gangliosides modulate glutamate receptor binding in rat brain synaptic plasma membranes

The influence of gangliosides on the binding of L-[3H]glutamate (Glu) to its receptor(s) on isolated rat brain synaptic plasma membranes was investigated in two different buffer systems using an in vitro filter binding assay. It was found that the tested pure monosialogangliosides GM1 and GM2 as well as the polysialogangliosides GT1b and GD1a enhanced binding by up to about 100%, but only in the presence of 5 mM calcium. The binding site involved was sensitive to quisqualate but insensitive to N-methyl-D-aspartate and kainate. GM1 did exert its stimulating effect by increasing the number of binding sites on the membranes whereas the receptor's affinity for L-Glu was unchanged. Regional differences in rat brain were found: while the hippocampus and cortex exhibited significant Ca/GM1-induced stimulation of Glu binding, the cerebellum was unaffected. Our results suggest modulation of at least one subtype of the Glu receptor by gangliosides.

Lateral diffusion of ganglioside GM1 in phospholipid bilayer membranes

The lateral diffusion coefficient of ganglioside GM1 incorporated into preformed dimyristoylphosphatidylcholine (DMPC) vesicles has been investigated under a variety of conditions using the technique of fluorescence photobleaching recovery. For these studies the fluorescent probe 5-(((2-Carbohydrazino)methyl)thio)acetyl) amino eosin was covalently attached to the periodate-oxidized sialic acid residue of ganglioside GM1. This labeled ganglioside exhibited a behavior similar to that of the intact ganglioside, and was able to bind cholera toxin. The lateral diffusion coefficient of the ganglioside was dependent upon the gel-liquid crystalline transition of DMPC. Above Tm the lateral diffusion coefficient of the ganglioside was 4.7 X 10(-9) cm2 s-1 (with greater than 80% fluorescence recovery). This diffusion coefficient is significantly slower than the one previously observed for phospholipids in DMPC bilayers. The addition of increasing amounts of ganglioside, up to a maximum of 10 mol %, did not have a significant effect on the lateral diffusion coefficient or in the percent recovery. At 30 degrees C, the lateral mobility of ganglioside GM1 was not affected by the presence of 5 mM Ca2+, suggesting that, at least above Tm, Ca2+ does not induce a major perturbation in the lateral organization of the ganglioside molecules. The addition of stoichiometric amounts of cholera toxin to samples containing either 1 or 10 mol % ganglioside GM1 produced only a small decrease in the measured diffusion coefficient. The fluorescence recovery after photobleaching experiments were complemented with excimer formation experiments using pyrene-phosphatidylcholine. Above the transition temperature the presence of 10 mol % ganglioside GMI induced a large decrease in the rate of excimer formation. These results also indicated that the addition of ganglioside GMI to phospholipid bilayer vesicles induces a significant restriction in the lateral mobility parameters of the lipid bilayer and that the presence of Ca2' does not have a further effect in the mobility of the probe molecules.

Electrokinetic and electrostatic properties of bilayers containing gangliosides GM1, GD1a, or GT1. Comparison with a nonlinear theory

We formed vesicles from mixtures of egg phosphatidylcholine (PC) and the gangliosides GM1, GD1a, or GT1 to model the electrokinetic properties of biological membranes. The electrophoretic mobilities of the vesicles are similar in NaCl, CsCl, and TMACl solutions, suggesting that monovalent cations do not bind significantly to these gangliosides. If we assume the sialic acid groups on the gangliosides are located some distance from the surface of the vesicle and the sugar moieties exert hydrodynamic drag, we can describe the mobility data in 1, 10, and 100 mM monovalent salt solutions with a combination of the Navier-Stokes and nonlinear Poisson-Boltzmann equations. The values we assume for the thickness of the ganglioside head group and the location of the charge affect the theoretical predictions markedly, but the Stokes radius of each sugar and the location of the hydrodynamic shear plane do not. We obtain a reasonable fit to the mobility data by assuming that all ganglioside head groups project 2.5 nm from the bilayer and all fixed charges are in a plane 1 nm from the bilayer surface. We tested the latter assumption by estimating the surface potentials of PC/ganglioside bilayers using four techniques: we made 31P nuclear magnetic resonance, fluorescence, electron spin resonance, and conductance measurements. The results are qualitatively consistent with our assumption.

The role of gangliosides in the interaction of a growth inhibitor with mouse LM cells

We have isolated and characterized glycopeptides, derived from mouse and bovine cerebral cortex cells, that inhibit protein synthesis and cell growth of normal but not transformed cells. The inhibitor binds to target cell surfaces, and gangliosides have previously been shown to influence cell sensitivity to the glycopeptides. Preincubation with 3.0 micrograms/ml ganglioside GM1 at 0 degrees C for 3 hr sensitized the mouse L-cell line to the inhibitor, as determined by protein synthesis assays. Preincubation of LM cells with ganglioside GM1 alone did not affect protein synthesis rates. In addition, the gangliosides GD1a and GM3 also sensitized the LM cells to the protein synthesis inhibitory effect of the glycopeptide inhibitor. Binding experiments were performed with 3T3 (sensitive) and LM (insensitive) cells to determine if sensitivity to the glycopeptide inhibitor was reflected in binding of the inhibitor to these cells. Binding of 125I-labeled inhibitor to 3T3 cells was maximal after 60 min at 0 degrees C and saturable at approximately 1 X 10(4) molecules/cell. Furthermore, binding of the inhibitor was dose-dependent, with half-maximal binding at 1.5-2.0 nM and saturation at 8.0-10.0 nM. Scatchard plot analysis indicated that the Kd was about 1 X 10(-9) M and that there are 1 X 10(4) receptors/cell. Binding of the inhibitor to LM cells was maximal after 30 min at 0 degrees C and saturation occurred at 5 X 10(3) molecules/cell. We then examined the possibility that gangliosides are the cellular receptor or co-receptor for the glycopeptide inhibitor. Binding of the inhibitor to ganglioside GM1 was first examined after the ganglioside had been preadsorbed to polystyrene tubes. These experiments indicated that the ganglioside did not bind the inhibitor. Ganglioside-containing liposomes from phosphatidylcholine or LM cell membrane components were also prepared; these artificial membranes did not bind appreciable amounts of the iodinated inhibitor. Competition experiments showed that the gangliosides GM1 and GD1a did not neutralize the protein synthesis inhibitory activity of the glycopeptides, indicating that gangliosides do not directly interact with the glycopeptide inhibitor. In addition, binding of the inhibitor to LM cells preincubated with ganglioside GM1 was studied. Although the binding of the inhibitor to LM cells was one-half that observed for 3T3 cells, incorporation of exogenous gangliosides into LM cells did not result in increased binding of the inhibitor.(ABSTRACT TRUNCATED AT 400 WORDS)

The interaction of calcium with gangliosides in bilayer membranes

We studied the binding of calcium to bilayer membranes formed from mixtures of phosphatidylcholine and mono-, di-, or trisialoganglioside by measuring its effect on the electrophoretic mobility of multilamellar vesicles and the conductance of planar bilayers. In 0.001 M monovalent salt solutions the surface potential of the membranes is large and micromolar concentrations of calcium have a significant effect on the mobility and conductance. In 0.1 M monovalent salt solutions the surface potential is small and millimolar concentrations of calcium are required to affect these parameters. The strong apparent binding of calcium we observed at low ionic strength could be due to the nonspecific accumulation of calcium in the electrical diffuse double layer. To distinguish between this nonspecific effect and binding of calcium to the membrane, we substituted dimethonium for calcium. Dimethonium is a divalent cation that screens negative charges but does not bind to lipids. We also examined the effect of replacing phosphatidylcholine by monoolein: calcium binds to phosphatidylcholine but not to monoolein. We describe our electrophoretic mobility results by combining the Poisson-Boltzmann and Navier-Stokes equations with the Langmuir adsorption isotherm. We conclude that calcium binds weakly to gangliosides with an intrinsic association constant of less than 100 M-1, which is similar to the association constant of calcium with phospholipids.

Thermotropic behavior of binary mixtures of dipalmitoylphosphatidylcholine and glycosphingolipids in aqueous dispersions

The thermotropic behavior of mixtures of dipalmitoylphosphatidylcholine (DPPC) with natural glycosphingolipids (galactosylceramide, phrenosine, kerasine, glucosylceramide, lactosylceramide, asialo-GM1, sulfatide, GM3, GM1, GD1a, GT1b) in dilute aqueous dispersions were studied by high sensitivity differential scanning calorimetry over the entire composition range. The pretransition of DPPC is abolished and the cooperativity of the main transition decreases sharply at mole fractions of glycosphingolipids below 0.2. All systems exhibit non-ideal temperature-composition phase diagrams. The mono- and di-hexosylceramides are easily miscible with DPPC when the proportion of glycosphingolipids in the system is high. A limited quantity (1-6 molecules of DPPC per molecule of glycosphingolipid (GSL) can be incorporated into a homogeneously mixed lipid phase. Domains of DPPC, immiscible with the rest of a mixed GSL-DPPC phase that shows no cooperative phase transition, are established as DPPC exceeds a certain proportion in the system. One negative charge (sulfatide) or four neutral carbohydrate residues (asialo-GM1) in the oligosaccharide chain of the glycosphingolipids results in phase diagrams exhibiting coexistence of gel and liquid phases over a broad temperature-composition range. Systems containing gangliosides show complex phase diagrams, with more than one phase transition. However, no evidence for phase-separated domains of pure ganglioside species is found. The thermotropic behavior of systems containing DPPC and glycosphingolipids correlates well with their interactions in mixed monolayers at the air/water interface.

Lipid phase separations induced by the association of cholera toxin to phospholipid membranes containing ganglioside GM1

The interactions of cholera toxin and their isolated binding and active subunits with phospholipid bilayers containing the toxin receptor ganglioside GM1 have been studied by using high-sensitivity differential scanning calorimetry and steady-state and time-resolved fluorescence and phosphorescence spectroscopy. The results of this investigation indicate that cholera toxin associates with phospholipid bilayers containing ganglioside GM1, independent of the physical state of the membrane. In the absence of Ca2+, calorimetric scans of intact cholera toxin bound to dipalmitoylphosphatidylcholine (DPPC) large unilamellar vesicles containing ganglioside GM1 result in a broadening of the lipid phase transition peak and a slight decrease (less than 5%) in the transition enthalpy. In the presence of Ca2+ concentrations sufficient to cause ganglioside phase separation, the association of the intact toxin to the membrane results in a significant decrease of enthalpy change for the lipid transition, indicating that under these conditions the toxin molecule perturbs the hydrophobic core of the bilayer. Calorimetric scans using isolated binding subunits lacking the hydrophobic toxic subunit did not exhibit a decrease in the phospholipid transition enthalpy even in the presence of Ca2+, indicating that the binding subunits per se do not perturb the hydrophobic core of the bilayer. On the other hand, the hydrophobic A1 subunit by itself was able to reduce the phospholipid transition enthalpy when reconstituted into DPPC vesicles. These calorimetric observations were confirmed by fluorescence experiments using pyrene phospholipids.(ABSTRACT TRUNCATED AT 250 WORDS)