Exposure of galactosylceramide to galactose oxidase in liposomes: dependence on lipid environment and ceramide composition

Phase Partitioning of GM1 and Its Bodipy-Labeled Analog Determine Their Different Binding to Cholera Toxin

Driven by interactions between lipids and proteins, biological membranes display lateral heterogeneity that manifests itself in a mosaic of liquid-ordered (Lo) or raft, and liquid-disordered (Ld) or non-raft domains with a wide range of different properties and compositions. In giant plasma membrane vesicles and giant unilamellar vesicles, specific binding of Cholera Toxin (CTxB) to GM1 glycolipids is a commonly used strategy to label raft domains or Lo membrane environments. However, these studies often use acyl-chain labeled bodipy-GM1 (bdGM1), whose headgroup accessibility and membrane order or phase partitioning may differ from those of GM1, rendering the interpretation of CTxB binding data quite problematic. To unravel the molecular basis of CTxB binding to GM1 and bdGM1, we explored the partitioning and the headgroup presentation of these gangliosides in the Lo and Ld phases using atomistic molecular dynamics simulations complemented by CTxB binding experiments. The conformation of both GM1 and bdGM1 was shown to be largely similar in the Lo and Ld phases. However, bdGM1 showed reduction in receptor availability when reconstituted into synthetic bilayer mixtures, highlighting that membrane phase partitioning of the gangliosides plays a considerable role in CTxB binding. Our results suggest that the CTxB binding is predominately modulated by the partitioning of the receptor to an appropriate membrane phase. Further, given that the Lo and Ld partitioning of bdGM1 differs from those of GM1, usage of bdGM1 for studying GM1 behavior in cells can lead to invalid interpretation of experimental data.

A major fraction of glycosphingolipids in model and cellular cholesterol-containing membranes is undetectable by their binding proteins

Glycosphingolipids (GSLs) accumulate in cholesterol-enriched cell membrane domains and provide receptors for protein ligands. Lipid-based "aglycone" interactions can influence GSL carbohydrate epitope presentation. To evaluate this relationship, Verotoxin binding its receptor GSL, globotriaosyl ceramide (Gb(3)), was analyzed in simple GSL/cholesterol, detergent-resistant membrane vesicles by equilibrium density gradient centrifugation. Vesicles separated into two Gb(3/)cholesterol-containing populations. The lighter, minor fraction (<5% total GSL), bound VT1, VT2, IgG/IgM mAb anti-Gb(3), HIVgp120 or Bandeiraea simplicifolia lectin. Only IgM anti-Gb(3), more tolerant of carbohydrate modification, bound both vesicle fractions. Post-embedding cryo-immuno-EM confirmed these results. This appears to be a general GSL-cholesterol property, because similar receptor-inactive vesicles were separated for other GSL-protein ligand systems; cholera toxin (CTx)-GM1, HIVgp120-galactosyl ceramide/sulfatide. Inclusion of galactosyl or glucosyl ceramide (GalCer and GlcCer) rendered VT1-unreactive Gb(3)/cholesterol vesicles, VT1-reactive. We found GalCer and GlcCer bind Gb(3), suggesting GSL-GSL interaction can counter cholesterol masking of Gb(3). The similar separation of Vero cell membrane-derived vesicles into minor "binding," and major "non-binding" fractions when probed with VT1, CTx, or anti-SSEA4 (a human GSL stem cell marker), demonstrates potential physiological relevance. Cell membrane GSL masking was cholesterol- and actin-dependent. Cholesterol depletion of Vero and HeLa cells enabled differential VT1B subunit labeling of "available" and "cholesterol-masked" plasma membrane Gb(3) pools by fluorescence microscopy. Thus, the model GSL/cholesterol vesicle studies predicted two distinct membrane GSL formats, which were demonstrated within the plasma membrane of cultured cells. Cholesterol masking of most cell membrane GSLs may impinge many GSL receptor functions.

Structural requirements of anti-GD1a antibodies determine their target specificity

The acute motor axonal neuropathy (AMAN) variant of Guillain-Barré syndrome (GBS) is associated with anti-GD1a and anti-GM1 IgG antibodies. The basis of preferential motor nerve injury in this disease is not clear, however, because biochemical studies demonstrate that sensory and motor nerves express similar quantities of GD1a and GM1 gangliosides. To elucidate the pathophysiology of AMAN, we have developed several monoclonal antibodies (mAbs) with GD1a reactivity and reported that one mAb, GD1a-1, preferentially stained motor axons in human and rodent nerves. To understand the basis of this preferential motor axon staining, several derivatives of GD1a were generated by various chemical modifications of N-acetylneuraminic (sialic) acid residues (GD1a NeuAc 1-amide, GD1a NeuAc ethyl ester, GD1a NeuAc 1-alcohol, GD1a NeuAc 1-methyl ester, GD1a NeuAc 7-alcohol, GD1a NeuAc 7-aldehyde) on this ganglioside. Binding of anti-GD1a mAbs and AMAN sera with anti-GD1a Abs to these derivatives was examined. Our results indicate that mAbs with selective motor axon staining had a distinct pattern of reactivity with GD1a-derivatives compared to mAbs that stain both motor and sensory axons. The fine specificity of the anti-GD1a antibodies determines their motor selectivity, which was validated by cloning a new mAb (GD1a-E6) with a chemical and immunocytochemical binding pattern similar to that of GD1a-1 but with two orders of magnitude higher affinity. Control studies indicate that selective binding of mAbs to motor nerves is not due to differences in antibody affinity or ceramide structural specificity. Since GD1a-reactive mAb with preferential motor axon staining showed similar binding to sensory- and motor nerve-derived GD1a in a solid phase assay, we generated computer models of GD1a based on binding patterns of different GD1a-reactive mAbs to different GD1a-derivatives. These modelling studies suggest that critical GD1a epitopes recognized by mAbs are differentially expressed in motor and sensory nerves. The GD1a-derivative binding patterns of AMAN sera resembled those with motor-specific mAbs. On the basis of these findings we postulate that both the fine specificity and ganglioside orientation/exposure in the tissues contribute to target recognition by anti-ganglioside antibodies and this observation provides one explanation for preferential motor axon injury in AMAN.

Sphingolipids and the formation of sterol-enriched ordered membrane domains

This review is focused on the formation of lateral domains in model bilayer membranes, with an emphasis on sphingolipids and their interaction with cholesterol. Sphingolipids in general show a preference for partitioning into ordered domains. One of the roles of cholesterol is apparently to modulate the fluidity of the sphingolipid domains and also to help segregate the domains for functional purposes. Cholesterol shows a preference for sphingomyelin over phosphatidylcholine with corresponding acyl chains. The interaction of cholesterol with different sphingolipids is largely dependent on the molecular properties of the particular sphingolipid in question. Small head group size clearly has a destabilizing effect on sphingolipid/cholesterol interaction, as exemplified by studies with ceramide and ceramide phosphoethanolamine. Ceramides actually displace sterol from ordered domains formed with saturated phosphatidylcholine or sphingomyelin. The N-linked acyl chain is known to be an important stabilizer of the sphingolipid/cholesterol interaction. However, N-acyl phosphatidylethanolamines failed to interact favorably with cholesterol and to form cholesterol-enriched lateral domains in bilayer membranes. Glycosphingolipids also form ordered domains in membranes but do not show a strong preference for interacting with cholesterol. It is clear from the studies reviewed here that small changes in the structure of sphingolipids alter their partitioning between lateral domains substantially.

Galactose oxidase action on galactose containing glycolipids--a fluorescence method

Features that alter the glycolipid sugar headgroup accessibility at the membrane interface have been studied in bilayer lipid model vesicles using a fluorescence technique with the enzyme galactose oxidase. The effects on oxidation caused by variation in the hydrophobic moiety of galactosylceramide or the membrane environment for galactosylceramide, monogalactosyldiacylglycerol and digalactosyldiacylglycerol were studied. For this study we combined the galactose oxidase method for determining the oxidizability of galactose containing glycolipids, and the fluorescence method for determining enzymatic hydrogen peroxide production. Exposed galactose residues with a free hydroxymethyl group at position 6 in the headgroup of glycolipids were oxidized with galactose oxidase and subsequently the resultant hydrogen peroxide was determined by a combination of horseradish peroxidase and 10-acetyl-3,7-dihydroxyphenoxazine (Amplex Red). Amplex Red reacts with hydrogen peroxide in the presence of horseradish peroxidase with a 1:1 stoichiometry to form resorufin. With this coupled enzyme approach it is also possible to determine the galactolipid transbilayer membrane distribution (inside-outside) in bilayer vesicles.

A glycosynapse in myelin?

Myelin, the multilayered membrane which surrounds nerve axons, is the only example of a membranous structure where contact between extracellular surfaces of membrane from the same cell occurs. The two major glycosphingolipids (GSLs) of myelin, galactosylceramide (GalC) and its sulfated form, galactosylceramide I(3)-sulfate (SGC), can interact with each other by trans carbohydrate-carbohydrate interactions across apposed membranes. They occur in detergent-insoluble lipid rafts containing kinases and thus may be located in membrane signaling domains. These signaling domains may contact each other across apposed extracellular membranes, thus forming glycosynapses in myelin. Multivalent forms of these carbohydrates, GalC/SGC-containing liposomes, or galactose conjugated to albumin, have been added to cultured oligodendrocytes (OLs) to mimic interactions which might occur between these signaling domains when OL membranes or the extracellular surfaces of myelin come into contact. These interactions between multivalent carbohydrate and the OL membrane cause co-clustering or redistribution of myelin GSLs, GPI-linked proteins, several transmembrane proteins, and signaling proteins to the same membrane domains. They also cause depolymerization of the cytoskeleton, indicating that they cause transmission of a signal across the membrane. Their effects have similarities to those of anti-GSL antibodies on OLs, shown by others, suggesting that the multivalent carbohydrate interacts with GalC/SGC in the OL membrane. Communication between the myelin sheath and the axon regulates both axonal and myelin function and is necessary to prevent neurodegeneration. Participation of transient GalC and SGC interactions in glycosynapses between the apposed extracellular surfaces of mature compact internodal myelin might allow transmission of signals throughout the myelin sheath and thus facilitate myelin-axonal communication.

Carbohydrate–protein interactions at interfaces: comparison of the binding of Ricinus communis lectin to two series of synthetic glycolipids using surface plasmon resonance studies

Two C-lactosyl lipids and the related C-galactosyl lipids have been synthesised and their binding to RCA120 plant lectin was compared with a second series of thiolactosylethoxyalkanes. The interactions were measured quantitatively in real time by surface plasmon resonance (BIAcore) at a range of concentrations and temperatures from 5 to 30 degrees C. The C-galactosyl lipid (1,3-dimethyl-5-[beta-D-galactopyranosyl]-5-(4-octadecyloxybenzyl)pyrimidine-2,4,6-trione) bound much more weakly with a K(A) = 8.86 x 10(5) than the corresponding C-lactosyl lipid (1,3-dimethyl-5-[beta-D-galactopyranosyl-(1 --> 4)-beta-D-glucopyranosyl]-5-(4-octadecyloxybenzyl)pyrimidine-2,4,6-trione) (K(A) = 2.31 x 10(7)). The influence of the linker region of the two different series of lactosyl lipids was clearly demonstrated by the differences in the binding to RCA120 lectin. The changes in kinetic values and in the enthalpic and entropic contribution to the free energy of binding reflected the importance of the linker and the hydrocarbon anchor holding the synthetic glycolipids in the neomembrane.

Catalytic effects of galactose oxidase on micelle-forming galactolipids

Catalytic effects of galactose oxidase on the oxidation of beta-D-galactose-carrying lipids with an oligo-ethylene glycol spacer (number of ethylene glycol units (n)=1, 2, 3, 6, 9, 13, and 20) were examined. The affinity of galactose oxidase for the galactose residue in the amphiphile (estimated by the inverse of the Michaelis constant, K(m)) was much higher than those for free D-galactose and small beta-D-galactopyranosides, and dependent on the length of the ethylene glycol spacer. That is, both below and above the critical micellar concentration, the 1/K(m) values decreased with an increase in the n value. The effectiveness of the enzyme, which can be estimated by the k(cat)/K(m) value, showed the same tendency as the 1/K(m) value. These results could be attributed to the role of the nonpolar environment around the galactose residue in the binding by the enzyme. A significant enhancement of the enzymatic oxidation of galactose residue on the liposome surface was also observed.

Probing for preferential interactions among sphingolipids in bilayer vesicles using the glycolipid transfer protein

We have investigated the intervesicular transfer of galactosylceramide between unilamellar bilayer vesicles composed of differing sphingomyelin and phosphatidylcholine molar ratios. To monitor glycolipid transfer from donor to acceptor vesicles, we used a fluorescence resonance energy transfer assay involving anthrylvinyl-labeled galactosylceramide (AV-GalCer) and perylenoyl-labeled triglyceride. The transfer was mediated by glycolipid transfer protein (GLTP), purified from bovine brain and specific for glycolipids. The initial transfer rate and the total accessible pool of glycolipid in the donor vesicles were both measured. An increase in the sphingomyelin content of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) vesicles decreased the transfer rate in a nonlinear fashion. Decreased transfer rates were clearly evident at sphingomyelin mole fractions of 0.22 or higher. The pool of AV-GalCer available for GLTP-mediated transfer also was smaller in vesicles containing high sphingomyelin content. In contrast, AV-GalCer was more readily transferred from vesicles composed of POPC and different disaturated phosphatidylcholines. Our results show that GLTP acts as a sensitive probe for detecting interactions of glycosphingolipids with neighboring lipids and that the lateral mixing of glycolipids is probably affected by the matrix lipid composition. The compositionally driven changes in lipid interactions, sensed by GLTP, occur in membranes that are either macroscopically fluid-phase or gel/fluid-phase mixtures. Gaining insights into how changes in membrane sphingolipid composition alter accessibility to soluble proteins with affinity for membrane glycolipids is likely to help increase our understanding of how sphingolipid-enriched microdomains (i.e., "rafts" and caveolae) are formed and maintained in cells.

Divalent cation-mediated interaction between cerebroside sulfate and cerebrosides: an investigation of the effect of structural variations of lipids by electrospray ionization mass spectrometry

Divalent cations mediate a carbohydrate-carbohydrate association between the two major glycolipids, galactosylceramide (GalCer) and its sulfated form, cerebroside sulfate (CBS), of the myelin sheath. We have suggested that interaction between these glycolipids on apposed extracellular surfaces of myelin may be involved in the stability or function of this multilayered structure. A mutant mouse lacking galactolipids because of a disruption in the gene that encodes a galactosyltransferase forms myelin that initially appears relatively normal but is unstable. This myelin contains glucosylceramide (GlcCer) instead of GalCer. To better understand the role of GlcCer in myelin in this mutant, we have compared the ability of divalent cations to complex CBS (galactosyl form) with GlcCer or GalCer in methanol solution by using positive ion electrospray ionization mass spectrometry. Because both the alpha-hydroxylated fatty acid species (HFA) and the nonhydroxylated fatty acid species (NFA) of these lipids occur in myelin, we have also compared the HFA and NFA species. In addition to monomeric Ca2+ complexes of all three lipids and oligomeric Ca2+ complexes of both GalCer and GlcCer, Ca2+ also caused heterotypic complexation of CBS to both GalCer and GlcCer. The heterotypic complexes had the greatest stability of all oligomers formed and survived better at high declustering potentials. Complexes of CBS with GlcCer were less stable than those with GalCer. This was confirmed by using the free sugars and glycosides making up the carbohydrate headgroups of these lipids. HFA species of CBS and GalCer formed more stable complexes than NFA species, but hydroxylation of the fatty acid of GlcCer had no effect. The ability of GlcCer to also complex with CBS, albeit with lower stability, may allow GlcCer to partially compensate for the absence of GalCer in the mouse mutant.

Characterization of protein-glycolipid recognition at the membrane bilayer

A growing number of important molecular recognition events are being shown to involve the interactions between proteins and glycolipids. Glycolipids are molecules in which one or more monosaccharides are glycosidically linked to a lipid moiety. The lipid moiety is generally buried in the cell membrane or other bilayer, leaving the oligosaccharide moiety exposed but in close proximity to the bilayer surface. This presents a unique environment for protein-carbohydrate interactions, and studies to determine the influence of the bilayer on these phenomena are in their infancy. One important property of the bilayer is the ability to orient and cluster glycolipid species, as strong interactions in biological systems are often achieved through multivalency arising from the simultaneous association of two or more proteins and receptors. This is especially true of protein-carbohydrate binding because of the unusually low affinities that characterize the monovalent interactions. More recent studies have also shown that the composition of the lipid bilayer is a critical parameter in protein-glycolipid recognition. The fluidity of the bilayer allows for correct geometric positioning of the oligosaccharide head group relative to the binding sites on the protein. In addition, there are activity-based and structural data demonstrating the impact of the bilayer microenvironment on the modulation of oligosaccharide presentation. The use of model membranes in biosensor-based methods has supplied decisive evidence of the importance of the membrane in receptor presentation. These data can be correlated with three-dimensional structural information from X-ray crystallography, NMR, and molecular mechanics to provide insight into specific protein-carbohydrate inter--actions at the bilayer.

Determination of galactose and galactocerebroside using a galactose oxidase column and electrochemical detector

A method has been developed to measure galactose and galactocerebroside using galactose oxidase immobilized on a solid resin. Galactose oxidase converts galactose and galactocerebroside to their corresponding aldehydes and hydrogen peroxide, the latter being electroactive and measurable by electrochemical detection using DC amperometric detection. The minimal detection limits of galactose and galactocerebroside were 1 and 2 microM, respectively. The linear response to galactose and galactocerebroside was to at least 300 microM. About 100 samples can be measured per hour using flow injection analysis. The activity of sulfatidase (cerebroside-3-sulfate-3-sulfohydrolase), which converts sulfatide (sulfogalactocerebroside) to galactocerebroside, was measured, and its inhibition by O-phospho-L-tyrosine was determined.

Porcine 987P glycolipid receptors on intestinal brush borders and their cognate bacterial ligands

Certain strains of enterotoxigenic Escherichia coli adhere to piglet intestinal epithelial cells by means of the 987P fimbriae. The 987P fimbrial structure consists of a helical arrangement of three fimbrial proteins, namely, the major subunit FasA and two minor subunits, FasF and FasG. FasG, which is located at the fimbrial tip and at various positions along the fimbriae, mediates 987P binding to glycoprotein receptors. In this study, we isolated and analyzed the structure of piglet glycolipid brush border receptors and characterized their cognate ligands on the 987P fimbriae. Two major glycolipid bands recognized by 987P fimbrial probes in thin-layer chromatography overlay assays were further purified by high-performance thin-layer chromatography and shown to comigrate with control galactosylceramide containing hydroxylated fatty acids and with sulfatide. Their structures were confirmed by fast atom bombardment mass spectrometry, which detected homologous series of ceramide monohexoside and sulfatide with hydroxylated fatty acyl chains ranging from h16:0 to h24:0. Assembled 987P fimbriae, pre- and postassembly dissociated fimbrial subunits, and Fab fragments of specific anti-FasG, -FasF, and -FasA were used to inhibit 987P-mediated bacterial binding to the two identified piglet glycolipids and corresponding isoreceptor controls. Only assembled fimbriae and anti-FasG Fab fragments were significantly able to inhibit bacterial binding to sulfatide, indicating that in addition to glycoproteins, FasG recognizes a specific glycolipid of piglet brush borders. In contrast, only anti-FasA Fab fragments were significantly able to inhibit bacterial binding to galactosylceramide with hydroxylated fatty acids and piglet hydroxylated ceramide monohexoside, indicating that FasA may determine a third type of ligand-receptor interaction in the piglet intestines. Since these bacterial adhesins recognize their respective glycolipid receptors only after being assembled in their final fimbrial quaternary structure, adhesin binding may involve cooperative interactions and the subunits by themselves may have very low binding affinities. Alternatively, conformation-sensitive domains of these subunits present in the assembled fimbriae may be required for glycolipid binding.

Aglycone modulation of glycolipid receptor function

The aglycone has been largely ignored in consideration of glycoconjugate function. Evidence is reviewed which suggests that the role of the lipid in glycolipid carbohydrate function may be particularly significant. The lipid moiety can promote or reduce carbohydrate exposure of membrane glycolipids. Theoretical calculation has indicated that the plane of the plasma membrane can restrict the permitted conformations of a given glycolipid oligosaccharide. Thus the lipid moiety may influence the relative conformation of such carbohydrate sequences. Evidence of ceramide regulation of glycolipid function can be found in studies of enzyme substrate specificity, antiglycolipid recognition and bacterial/host cell interactions. Studies of verotoxin binding to its glycolipid receptor globotriaosyl ceramide indicate that modulation of receptor function by glycolipid fatty acid content plays an important role in in vitro binding assays, cell cytotoxicity and intracellular routing.

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.

Age-related changes of the ganglioside long-chain base composition in rat cerebellum

The ganglioside mixture from the cerebellum of young, 6 month old and two years old rats, was fractionated by reversed phase high performance liquid chromatography, each ganglioside homogeneous in the oligosaccharide chain as well as in the long-chain base being subsequently quantified. Two long-chain bases, LCB, were components of the five major gangliosides GM1, GD1a, GD1b, GT1b and GQ1b, these being the C18:1 LCB and C20:1 LCB. The content of C20:1 ganglioside molecular species was lower than that of the C18:1 one. In very young animals, day 8, the C20:1 ganglioside species represented about 8% of the total ganglioside content, then they progressively increased and reached, at 2 years, about 42% of the total. C18:1 GD1a and C18:1 GT1b, were the major species in young animals and reached their highest content at day 29, being 1.45 and 1.28 nmol/mg protein, respectively. The content of these two species decreased in adult and old animals and at two years it was 0.71 and 0.82 nmol/mg protein, respectively.

Glycosphingolipid headgroup orientation in fluid phospholipid/cholesterol membranes: similarity for a range of glycolipid fatty acids

Galactosyl ceramide (GalCer) was labeled for nuclear magnetic resonance (NMR) spectroscopy by replacement of a hydrogen atom at C6 of the galactose residue with deuterium. Wideline 2H NMR of [d1]GalCer permitted consideration of a mechanism traditionally entertained for cell surface recognition site modulation: that the nature of the fatty acid attached to the sphingosine backbone of glycosphingolipids (GSLs) importantly influences carbohydrate headgroup orientation. Comparison was made among various glycolipid fatty acids by altering hydroxylation, saturation, and chain length. Studies were carried out in unsonicated bilayer membranes mimicking several important characteristics of cell plasma membranes: fluidity, low GSL content, predominant [sn-2]monounsaturated phosphatidylcholine (PC) (1-palmitoyl-2-oleoyl PC), and the presence of cholesterol. Spectroscopy was performed on samples over a range of temperatures, which included the physiological. 2H NMR spectra of [d1]GalCer having 18-carbon saturated fatty acid (stearic acid), cis-9-unsaturated fatty acid (oleic acid), D- and L-stereoisomers of alpha-OH stearic acid, or 24-carbon saturated fatty acid (lignoceric acid) were importantly similar. This argues that for GSLs dispersed as minor components in fluid membranes, variation of the glycolipid fatty acid does not provide as much potential for direct conformational modulation of the carbohydrate portion as has sometimes been assumed. However, there was some evidence of motional differences among the species studied. The 2H NMR spectra that were obtained proved to be more complex than was anticipated. Their features could be approximated by assuming a combination of axially symmetric and axially asymmetric glycolipid motions. Presuming the appropriateness of such a analysis, at a magnetic field of 3.54 T (23.215 MHz), the experimental spectra suggested predominantly asymmetric motional contributions. At the higher field of 11.7 T (76.7 MHz, equivalent to a proton frequency of 500 MHz), spectra indicated dominance by axially symmetric rotational modes. There was also evidence of some bilayer orientation in the stronger magnetic field. The unusual observation of spectral differences between the two magnetic field strengths may involve a diamagnetic response to high field on the part of some liposome physical characteristics.

Oligosaccharide behavior of complex natural glycosphingolipids in multicomponent model membranes

Wideline 2H NMR of model membranes was used to consider the molecular consequences of factors often suggested as modulators of complex glycosphingolipid oligosaccharide arrangement and motional characteristics at cell surfaces. GM1, asialo-GM1, and globoside were studied as examples of plasma membrane recognition sites. The experimental approach involved substitution of deuterons (D) for protons at specific locations within the carbohydrate chains. Deuterated glycolipids were then dispersed at 7-10 mol% in unsonicated bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine. Factors tested for their significance to carbohydrate chain conformation and dynamics included glycolipid natural alkyl and acyl chain variability, membrane fluidity, and the presence of cholesterol and a charged sugar residue (neuraminic acid). Effects of Ca2+ and membrane-associated protein were briefly considered. Two distinct strategies were employed in substituting deuterons for selected protons of carbohydrate residues. Neither approach necessitated alteration of the glycolipid natural fatty acid composition. (i) Protons of the exocyclic hydroxymethyl group on the terminal Gal residue of GM1 and asialo-GM1, and on the terminal N-acetylgalactosamine (GalNAc) residue of globoside, were replaced with deuterium (producing -CDHOH) by an enzymatic oxidation/reduction cycle. This represents the first application of such an approach to deuteration of complex neutral glycolipids. Spectral results were compared to those obtained for the similarly-deuterated monoglycosyl lipid, galactosylceramide (GalCer), with natural fatty acid composition. Efficacy of this labeling method may in principle be influenced by structural variations within a given glycolipid family. Also, asymmetric rotation of the deuterated group made it less attractive than the second method for relating spectral features to receptor geometry. (ii) A general synthetic, nonenzymatic method was investigated for replacing amino sugar N-acetyl groups with deuterated acetate (-COCD3). The acetate group of the GalNAc residue of globoside, GM1, and asialo-GM1, as well as that on neuraminic acid in GM1, was replaced with -COCD3. This second method afforded better signal-to-noise--an important consideration for 2H NMR. The NMR technique employed had the potential for detecting changes of as little as 10% in oligosaccharide orientation or motional order. Each glycolipid demonstrated clear evidence of preferred average oligosaccharide conformations in all (fluid) membrane environments examined. The most striking observation was that, in fluid matrices, conformation and motional order of the complex oligosaccharide chains were only modestly influenced by factors tested, including natural variation in the glycolipid hydrocarbon chains, membrane fluidity, temperature, and the presence of cholesterol or the N-acetylneuraminic acid (NeuAc) residue on GM1.(ABSTRACT TRUNCATED AT 400 WORDS)

Exposure to galactose oxidase of GM1 ganglioside molecular species embedded into phospholipid vesicles

The exposure of GM1 molecular species present in the native ganglioside, carrying C18:1 or C20:1 long-chain bases (LCB), to Dactylium dendroides galactose oxidase was studied. When native GM1 (49.3% C18:1 and 50.7% C20:1 LCB, respectively), was inserted in dipalmitoylphosphatidylcholine vesicles and partially oxidized (10%), the proportion of C18:1 and C20:1 species in the oxidized GM1 was 59.6% and 40.4%, respectively, suggesting a preferential action of the enzyme on the shorter species. The Vmax of the enzyme was higher on C18:1 GM1 than on C20:1 GM1. The molecular species were affected without any preference after partial (10%) oxidation of GM1 incorporated in egg phosphatidylcholine vesicles or in micellar form. These data indicate that the exposure of the terminal galactose moiety of GM1 ganglioside to galactose oxidase is affected by the ganglioside ceramide composition as well as the phospholipid environment, that presumably determine the distribution (molecular dispersion, segregation) of the ganglioside within the membrane.

Lipid modulation of glycolipid receptor function. Availability of Gal(alpha 1-4)Gal disaccharide for verotoxin binding in natural and synthetic glycolipids

Verotoxins bind to glycosphingolipids containing terminal Gal(alpha 1-4)Gal residues. Globotriaosylceramide is the most effective receptor for verotoxin-1 in vitro and is the functional plasma-membrane receptor which mediates cytopathology for most sensitive cells. Binding of verotoxin-1 to a series of galabiose-containing or globotriaose-containing synthetic glycolipids with monoalkylsulfides and bisalkylsulfides or sulfones as the lipid moiety, have been studied for toxin binding by TLC overlay and in solid phase in the presence of auxiliary lipids. The results demonstrate that for an identical carbohydrate, binding is dramatically altered according to the nature of the lipid moiety. The close proximity of the galabiose sequence and the hydrophobic species also compromised recognition. The lipid environment is also a major determinant of receptor function, since species that were effective, even preferred toxin receptors as monitored by TLC overlay, were not necessarily recognized in the presence of auxiliary lipids. Certain glycolipids, which were not recognized by TLC overlay, were nevertheless found to be effective receptors in an auxiliary lipid matrix. These results demonstrate the crucial role of the lipid moiety in verotoxin/glycolipid recognition and are discussed in relation to toxin pathogenesis and glycolipid receptor function.

Do the long fatty acid chains of sphingolipids interdigitate across the center of a bilayer of shorter chain symmetric phospholipids?

Novel cerebroside sulfate (CBS) spin labels containing long chain C24 or C26 fatty acids with a nitroxide spin label on the 22nd carbon were synthesized and used to investigate the ability of the long fatty acid chains of glycosphingolipids to interdigitate across the center of a non-interdigitated bilayer of phospholipids formed of symmetric saturated or unsaturated shorter fatty acid chain species, in the presence or absence of cholesterol. The motion of these long chain spin labels incorporated at 1 mole% in dimyristoylphosphatidylcholine (diC14-PC), dipalmitoylphosphatidylcholine (diC16-PC), distearoylphosphatidylcholine (diC18-PC), dibehenoylphosphatidylcholine (diC22-PC), spingomyelin (SM), 1-stearoyl-2-oleoylphosphatidylcholine (18:0.18:1-PC), and dimyristoylphosphatidylethanolamine (diC14-PE) was compared to that of CBS spin labels containing stearic acid spin labeled at the 5th carbon and at the 16th carbon. The results indicated that the C26 chain is interdigitated in the gel phase of diC14-PC, diC16-PC, SM, and possibly diC18-PC, but not diC14-PE, and the C24 chain may interdigitate in diC14-PC but not in the other phospholipids. Thus in order to interdigitate across the center of gel phase bilayers, the long acyl chain of the sphingolipid probably must be long enough to nearly span the phospholipid bilayer. The inability to interdigitate in diC14-PE is likely due to the close packing of this lipid in the gel phase. The C26 chain may also be interdigitated in these lipids in the presence of cholesterol at low temperatures. However, at physiological temperatures in the presence of cholesterol and in the liquid-crystalline phase of all the lipids, the results indicate that the long acyl chain of the glycosphingolipid is not interdigitated, but rather must terminate at the bilayer center. This may force the carbohydrate headgroup of the glycosphingolipid farther above the bilayer surface, allowing it to be recognized better by various carbohydrate binding ligands and proteins.

Thermotropic phase behavior of mixtures of long chain fatty acid species of cerebroside sulfate with different fatty acid chain length species of phospholipid

The thermotropic phase behavior of asymmetric, long fatty acid chain species of cerebroside sulfate, C24-CBS and C26-CBS, with symmetric species of phosphatidylcholine (PC) containing fatty acid chains of 14-18 carbons in length (diC14-PC, diC16-PC, diC18-PC) and dimyristoylphosphatidylethanolamine (diC14-PE) in 0.1 M KCl was studied by differential scanning calorimetry. Novel cerebroside sulfate (CBS) spin labels containing long chain C24 and C26 fatty acid spin labels with the nitroxide group on the twenty-second carbon were used to study the lipid organization of the gel phases of these mixtures. The phase diagrams of all the mixtures indicated the presence of two immiscible gel phases at low CBS concentrations. All except the C26-CBS/diC14-PC mixture had eutectic phase behavior at low CBS concentrations suggesting that the long fatty acid chain of the CBS species had a destabilizing effect on the gel phase of most of the phospholipids. The C26-CBS/diC14-PC mixture had peritectic phase behavior at low CBS concentrations indicating a stabilizing effect of the CBS C26 acyl chain on diC14-PC. These results are consistent with the relative compatibility of the CBS acyl chain length with the bilayer thickness of the PC; only in the case of the C26-CBS/diC14-PC mixture is the acyl chain of CBS long enough to span the PC bilayer. At intermediate to high CBS concentrations, the CBS and phospholipid (PL) were miscible with the exception of the C24-CBS/diC18-PC combination, which had eutectic phase behavior over a wide concentration range. Thus when the PL acyl chain length was similar to the sphingosine chain length of CBS, CBS bilayers could accommodate symmetric phospholipid molecules better than phospholipid bilayers could accommodate asymmetric molecules of CBS. Use of the spin labels indicated that, at low temperatures and at intermediate to high CBS concentrations, all of the mixtures were in a triple chain mixed interdigitated gel phase which immobilized the spin label. This gel phase slowly transformed over a wide temperature range to a double chain partially interdigitated gel phase in which the spin labels had much more motion. This transformation could be detected as a broad low enthalpy transition by differential scanning calorimetry. In all cases the presence of phospholipid destabilized the mixed interdigitated phase. Stabilization of the partially interdigitated bilayer by intermolecular hydrogen bonding interactions must outweigh the destabilizing forces caused by disruptions in packing and van der Waals interactions between CBS molecules resulting from insertion of molecules of phospholipid into this type of bilayer.(ABSTRACT TRUNCATED AT 400 WORDS)