Glycosphingolipids: 2H NMR study of the influence of carbohydrate headgroup structure on ceramide acyl chain behavior in glycolipid-phospholipid bilayers

Molecular Dynamics of Glycolipids in Liposomes

Glycosphingolipids (GSLs) in the mammalian plasma membrane are essential for various biological events as they form glycolipid-rich membrane domains, such as lipid rafts. GSLs consist of a certain oligosaccharide head group and a ceramide tail with various lengths of acyl chains. The structure of the head group as well as the carbon number and degree of the unsaturation of the acyl chain are known to regulate the membrane distributions and interleaflet couplings of GSLs by altering physicochemical properties, such as dynamics, interactions, and cluster sizes. This chapter provides the detailed use of time-resolved fluorescence measurement for investigating the membrane properties of lactosylceramide (LacCer)-enriched domains in bilayer membranes. LacCer belongs to the neutral GSLs and is believed in forming a highly ordered phase in model membranes and biological membranes, while the details of the domain remain unclear. Here, we suggest using trans-parinaric acid (tPA) and tPA-LacCer fluorescent probes to reveal the dynamics and size of the GSL domains since they prefer to be distributed in the GSL-rich ordered phase. The fluorescence lifetime in the nanosecond timescale reveals the difference in the surrounding membrane environments, which relates to hydrocarbon chain ordering, membrane hydration, and submicrometer domain size. The fluorescence lifetime of these probes can thus provide important information on submicron- to nano-scale small GSL domains not only in model membranes but also in biological membranes.

Effect of cholesterol on the lactosylceramide domains in phospholipid bilayers

Lactosylceramide (LacCer) in the plasma membranes of immune cells is an important lipid for signaling in innate immunity through the formation of LacCer-rich domains together with cholesterol (Cho). However, the properties of the LacCer domains formed in multicomponent membranes remain unclear. In this study, we examined the properties of the LacCer domains formed in Cho containing 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) membranes by deuterium solid-state NMR and fluorescence lifetimes. The potent affinity of LacCer-LacCer (homophilic interaction) is known to induce a thermally stable gel phase in the unitary LacCer bilayer. In LacCer/Cho binary membranes, Cho gradually destabilized the LacCer gel phase to form the liquid-ordered (Lo) phase by its potent order effect. In the LacCer/POPC binary systems without Cho, the ²H NMR spectra of 10',10'-d₂-LacCer and 18',18',18'-d₃-LacCer probes revealed that LacCer was poorly miscible with POPC in the membranes and formed stable gel phases without being distributed in the liquid crystalline (Ld) domain. The lamellar structure of the LacCer/POPC membrane was gradually disrupted at around 60 °C, while the addition of Cho increased the thermal stability of the lamellarity. Furthermore, the area of the LacCer gel phase and its chain order were decreased in the LacCer/POPC/Cho ternary membranes, while the Lo domain, which was observed in the LacCer/Cho binary membrane, was not observed. Cho surrounding the LacCer gel domain liberated LacCer and facilitated forming the submicron- to nano-scale small domains in the Ld domain of the LacCer/POPC/Cho membranes, as revealed by the fluorescence lifetimes of trans-parinaric acid (tPA) and tPA-LacCer. Our findings on the membrane properties of the LacCer domains, particularly in the presence of Cho, would help elucidate the properties of the LacCer domains in biological membranes.

Membrane Lipid Nanodomains

Lipid membranes can spontaneously organize their components into domains of different sizes and properties. The organization of membrane lipids into nanodomains might potentially play a role in vital functions of cells and organisms. Model membranes represent attractive systems to study lipid nanodomains, which cannot be directly addressed in living cells with the currently available methods. This review summarizes the knowledge on lipid nanodomains in model membranes and exposes how their specific character contrasts with large-scale phase separation. The overview on lipid nanodomains in membranes composed of diverse lipids (e.g., zwitterionic and anionic glycerophospholipids, ceramides, glycosphingolipids) and cholesterol aims to evidence the impact of chemical, electrostatic, and geometric properties of lipids on nanodomain formation. Furthermore, the effects of curvature, asymmetry, and ions on membrane nanodomains are shown to be highly relevant aspects that may also modulate lipid nanodomains in cellular membranes. Potential mechanisms responsible for the formation and dynamics of nanodomains are discussed with support from available theories and computational studies. A brief description of current fluorescence techniques and analytical tools that enabled progress in lipid nanodomain studies is also included. Further directions are proposed to successfully extend this research to cells.

An NMR database for simulations of membrane dynamics

Computational methods are powerful in capturing the results of experimental studies in terms of force fields that both explain and predict biological structures. Validation of molecular simulations requires comparison with experimental data to test and confirm computational predictions. Here we report a comprehensive database of NMR results for membrane phospholipids with interpretations intended to be accessible by non-NMR specialists. Experimental ¹³C-¹H and ²H NMR segmental order parameters (S(CH) or S(CD)) and spin-lattice (Zeeman) relaxation times (T(1Z)) are summarized in convenient tabular form for various saturated, unsaturated, and biological membrane phospholipids. Segmental order parameters give direct information about bilayer structural properties, including the area per lipid and volumetric hydrocarbon thickness. In addition, relaxation rates provide complementary information about molecular dynamics. Particular attention is paid to the magnetic field dependence (frequency dispersion) of the NMR relaxation rates in terms of various simplified power laws. Model-free reduction of the T(1Z) studies in terms of a power-law formalism shows that the relaxation rates for saturated phosphatidylcholines follow a single frequency-dispersive trend within the MHz regime. We show how analytical models can guide the continued development of atomistic and coarse-grained force fields. Our interpretation suggests that lipid diffusion and collective order fluctuations are implicitly governed by the viscoelastic nature of the liquid-crystalline ensemble. Collective bilayer excitations are emergent over mesoscopic length scales that fall between the molecular and bilayer dimensions, and are important for lipid organization and lipid-protein interactions. Future conceptual advances and theoretical reductions will foster understanding of biomembrane structural dynamics through a synergy of NMR measurements and molecular simulations.

Searching for the molecular arrangement of transmembrane ceramide channels

Ceramides have been implicated in the initiation of apoptosis by permeabilizing the mitochondrial outer membrane to small proteins, including cytochrome c. In addition, ceramides were shown to form large metastable channels in planar membranes and liposomes, indicating that these lipids permeabilize membranes directly. Here we analyze molecular models of ceramide channels and test their stability in molecular dynamics simulations. The structural units are columns of four to six ceramides H-bonded via amide groups and arranged as staves in either a parallel or antiparallel manner. Two cylindrical assemblies of 14 columns (four or six molecules per column) were embedded in a fully hydrated palmitoyloleoyl-phosphatidylcholine phospholipid bilayer, and simulated for 24 ns in total. After equilibration, the water-filled pore adopted an hourglass-like shape as headgroups of ceramides and phospholipids formed a smooth continuous interface. The structure-stabilizing interactions were both hydrogen bonds between the headgroups (including water-mediated interactions) and packing of the hydrocarbon tails. Ceramide's essential double bond reduced the mobility of the hydrocarbon tails and stabilized their packing. The six-column assembly remained stable throughout a 10-ns simulation. During simulations of four-column assemblies, pairs of columns displayed the tendency of splitting out from the channels, consistent with the previously proposed mechanism of channel disassembly.

Extraction of lipid membrane incorporated vitamin E by sucrose polyesters

2H and 31P solid state NMR have been used to probe, at the molecular level, the interaction between structurally different sucrose polyesters and a phospholipid membrane into which alpha-tocopherol and specifically deuterated alpha-[5,7-(2)H(6)] tocopherol has been incorporated. Our results show that at high concentration (>or=10 mol%) sucrose octapalmitate (SOP) and sucrose hexapalmitate (SHxP) deplete bilayer-associated alpha-tocopherol in dipalmitoyl phosphatidalcholine (DPPC) multilamellar dispersions and preferentially sequester the alpha-tocopherol into a fluid sucrose polyesters (SPE) phase located proximal to the membrane surface. It is demonstrated that the ability of SPEs to function as a 'lipophilic sink' depends strongly on sucrose polyester concentration and degree of esterification.

Membrane perturbing properties of sucrose polyesters

Sucrose polyester (SPE), in the form of sucrose octaesters and sucrose hexaesters of palmitic (16:0), stearic (18:0), oleic (18:1cis), and linoleic (18:2cis) acids, have many uses. Applications include: a non-caloric fat substitute, detoxification agent, and oral contrast agent for human abdominal (MRI) magnetic resonance imaging. However, it has been shown that the ingestion of SPE was shown to generate a depletion of physiologically important lipidic vitamins and other lipophilic molecules. In order to better understand, at the molecular level, the type of interaction between SPE and lipid membrane, we have, first synthesized different type of labelled and non-labelled SPEs. Secondly, we have studied the effect of SPEs on multilamellar dispersions of dielaidoylphosphatidylethanolamine (DEPE) and dipalmitoylphosphocholine (DPPC) as a function of temperature, SPE composition and concentration. The effects of SPEs were studied by differential scanning calorimetry (DSC), X-ray diffraction, 2H and 31P NMR spectroscopy. At low concentration (< 1 mol%) all of the SPEs lowered the bilayer to the inverted hexagonal phase transition temperature of DEPE and induced the formation of a cubic phase in a composition dependent manner. At the same low concentration, SPEs in DPPC induce the formation of a non-bilayer phase as seen by 31P NMR. Order parameter measurements of DPPC-d62/SPE mixtures show that the SPE effect on the DPPC monolayer thickness is dependent on the SPE, concentration, chains length and saturation level. At higher concentration (> or = 10 mol%) SPE are very potent DEPE bilayer to HII phase transition promoters, although at that concentration the SPE have lost the ability to form cubic phases. SPEs have profound effects on the phase behaviour of model membrane systems, and may be important to consider when developing current and potential industrial and medical applications.

The lactosylceramide binding specificity of Helicobacter pylori

The possible role of glycosphingolipids as adhesion receptors for the human gastric pathogen Helicobacter pylori was examined by use of radiolabeled bacteria, or protein extracts from the bacterial cell surface, in the thin-layer chromatogram binding assay. Of several binding specificities found, the binding to lactosylceramide is described in detail here, the others being reported elsewhere. By autoradiography a preferential binding to lactosylceramide having sphingosine/phytosphingosine and 2-D hydroxy fatty acids was detected, whereas lactosylceramide having sphingosine and nonhydroxy fatty acids was consistently nonbinding. A selective binding of H. pylori to lactosylceramide with phytosphingosine and 2-D hydroxy fatty acid was obtained when the different lactosylceramide species were incorporated into liposomes, but only in the presence of cholesterol, suggesting that this selectivity may be present also in vivo . Importantly, lactosylceramide with sphingosine and hydroxy fatty acids does not bind in this assay. Furthermore, a lactosylceramide-based binding pattern obtained for different trisaccharide glycosphingolipids is consistent with the assumption that this selectivity is due to binding of a conformation of lactosylceramide in which the oxygen of the 2-D fatty acid hydroxyl group forms a hydrogen bond with the Glc hydroxy methyl group, yielding an epitope presentation different from other possible conformers. An alternative conformation that may come into consideration corresponds to the crystal structure found for cerebroside, in which the fatty acid hydroxyl group is free to interact directly with the adhesin. By isolating glycosphingolipids from epithelial cells of human stomach from seven individuals, a binding of H.pylori to the diglycosylceramide region of the non-acid fraction could be demonstrated in one of these cases. Mass spectrometry showed that the binding-active sample contained diglycosylceramides with phytosphingosine and 2-D hydroxy fatty acids with 16-24 carbon atoms in agreement with the results related above.

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.

Carbohydrate dynamics at a micellar surface: GD1a headgroup transformations revealed by NMR spectroscopy

The conformational dynamics of the carbohydrate headgroup of ganglioside GD1a, NeuAc alpha 2-->3Gal beta 1-->3GalNAc beta 1-->4[NeuAc alpha 2-->3]Gal beta 1-->4Glc beta 1-->1Cer, anchored in a perdeuterated dodecylphosphocholine micelle in aqueous solution, were probed by high resolution NMR spectroscopy. The observed 1H/1H NOE interactions revealed conformational averaging of the terminal NeuAc alpha 2-->3Gal and Gal beta 1-->3GalNAc glycosidic linkages. The pronounced flexibility of this trisaccharide moiety was substantiated further by two-dimensional proton-detected 13C T1, T1 rho and 1H/13C NOE measurements. The anchoring effect of the micelle allowed the detection of conformational fluctuations of the headgroup on the time scale of a few hundred picoseconds. NMR experiments performed on the GD1a/DPC micelles in H2O at low temperatures permitted the observation of hydroxyl proton resonances, contributing valuable conformational information.

Behaviour of complex oligosaccharides at a bilayer membrane surface: probed by 2H-NMR

Deuterium wideline NMR was used in an attempt to directly assess oligosaccharide arrangement and motional characteristics of complex glycosphingolipids dispersed as minor components in phospholipid membranes. A convenient, general synthetic approach was developed which involved replacement of the acetate group of amido sugars with deuteroacetate (-COCD3). This provided excellent signal-to-noise when applied to the terminal GalNAc residue of globoside, and the terminal NANA residue of GM1. Simultaneously, globoside and GM1 fatty acids were replaced with stearic acid deuterated at C-2- a probe location sensitive to glycolipid hydrophobic backbone orientation and rigid body motion. Deuterated GM1 and globoside were studied by 2H-NMR in bilayers of 1-palmitoyl-2-oleoyl phosphatidylcholine, in the presence and absence of physiological quantities of cholesterol. The monoglycosyl glycosphingolipid, glucosyl ceramide, which is the common skeleton of many complex glycosphingolipids including those studied here, was also deuterated at fatty acid C-2 for comparative study in the same matrices. Correlation with spectra of the complex glycolipids demonstrated that, for a given temperature and membrane composition, ceramide backbone conformation was very similar amongst the species studied. Spectral features of GM1 deuterated on terminal NANA and assembled at a membrane surface, were found to be highly consistent with the oligosaccharide conformation determined in studies of GM1 in solution. In contrast, globoside deuterated in the terminal GalNAc residue gave spectra very different from those predicted on the basis of the conformation considered to exist in solution. It seems likely that this result reflects a combination of greater oligosaccharide chain flexibility relative to GM1, and the presence of the membrane environment. Interestingly, although there was highly significant spatial geometry associated with the complex oligosaccharide chains, and although temperature and the presence of cholesterol exert measurable effects on the membrane-inserted portion, these factors had very little impact on the measured spectral parameters associated with the NANA residue of GM1 or the terminal GalNAc residue of globoside. This seems to indicate lack of sensitivity of the complex oligosaccharide chains to conformation and internal motions of the hydrophobic chain segments in these fluid and semi-fluid membranes; and has important implications for mechanisms of crypticity.

Acyl chain length effects related to glycosphingolipid crypticity in phospholipid membranes: probed by 2H-NMR

Wideline 2H-NMR was used to consider the relationships amongst glycosphingolipid and phospholipid fatty acid chain length and glycosphingolipid receptor function, in a system classically associated with crypticity. Galactosyl ceramide (GalCer), having 18- or 24-carbon fatty acid, was deuterium labelled at the conformationally-restricted fatty acid alpha-carbon (C-2). 2H-NMR spectra of N-[2,2-2H2]stearoyl and N-[2,2-2H2]lignoceroyl GalCer (GalCer with 18-vs. 24-carbon selectively deuterated fatty acid) were then compared over a range of temperatures in phosphatidylcholine/cholesterol membranes in which the host phospholipid had dimyristoyl, dipalmitoyl, or distearoyl fatty acid composition. Findings were evaluated in the light of known sensitivity of antibody interaction with GalCer to temperature and to both glycolipid fatty acid chain length and host matrix fatty acid chain length. Under the conditions of experimentation, spectra were not obtainable for glycolipids having rigid body motions that were slow on the NMR timescale (10(-4)-10(-5) s)-i.e.. motions typical of non-fluid (gel phase) membranes. The systems, DPPC/cholesterol and DSPC/cholesterol, in which the original observation was made of increased antibody binding to GalCer with long fatty acid, proved to be characterised by receptor motions that were in this slow timescale for both 18:0 and 24:0 GalCer at 22-24 degrees C. Under conditions for which spectra could be obtained, those for GalCer with [2,2-2H2]lignoceroyl (24-carbon alpha-deuterated) fatty acid were qualitatively similar to those of its 18-carbon analogue in all (fluid) membranes examined. However, spectral splittings differed quantitatively between deuterated 18:0 and 24:0 GalCer at a given temperature, dependent upon host matrix. These differences were most marked at lower temperatures and in the longer chain (more ordered) matrices, DPPC/cholesterol and DSPC/cholesterol. This suggests that maximum effects of glycolipid chain length on glycolipid receptor function may be expected to occur in spatially and motionally constrained lipid environments. There was little effect of temperature on spectral splittings seen for a given sample containing deuterated 18:0 GalCer. The small differences seen could be adequately accounted for by relatively minor alterations in glycolipid order and backbone conformation. In contrast, 24:0 GalCer in DPPC/cholesterol and DSPC/cholesterol displayed significant variation in its spectral splittings as the temperature was reduced; and these proved to be the source of the quantitative differences between 18:0 and 24:0 GalCer referred to above.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycosphingolipid backbone conformation and behavior in cholesterol-containing phospholipid bilayers

2H NMR spectroscopy was used to consider correspondence between existing single-crystal X-ray data for glycosphingolipids and their ceramide backbone conformation in fluid phospholipid membranes. A monoglycosylated sphingolipid, glucosylceramide (GlcCer), which represents the core structure of many important glycosphingolipids, was derived by partial synthesis through replacement of all native fatty acids with the 18-carbon species, stearic acid, deuterated at C2. N-[2,2-2H2]stearoyl-GlcCer was used to probe glycosphingolipid orientation and motion at low concentration in "fluid" phospholipid bilayers composed of dimyristoylphosphatidylcholine (DMPC), with and without physiological amounts of cholesterol. Spectral analysis, aided by stereoselective monodeuteration of the GlcCer fatty acid at C2, demonstrated that glycosphingolipid average acyl chain backbone conformation in fluid phospholipid membranes, with or without cholesterol, is likely closely related to that predicted from single crystal X-ray studies [Pascher, I. (1976) Biochim. Biophys. Acta 455, 433-451; Pascher, I., & Sundell, S. (1977) Chem. Phys. Lipids 20, 175-191]. To test the generality of this observation, specific comparisons were made involving galactosylceramide (GalCer) and globoside. GalCer provided a glycolipid differing only in monosaccharide stereochemistry (galactose vs glucose). Globoside permitted isolation of the effect of headgroup size, since it is derived from GlcCer via extension of the carbohydrate portion by the oligosaccharide, GalNAc beta 1-->3Gal alpha 1-->4Gal attached in beta 1-->4 linkage to the Glc residue.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of synthetic glycophospholipids with phospholipid bilayer membranes

A series of glycophospholipids synthesized by coupling mono-, di-, or tri-saccharides to dioleoylphosphatidylethanolamine (DOPE) by reductive amination was used to investigate the interaction of glycophospholipids with phospholipid bilayer membranes. These synthetic glycophospholipids functioned as a stabilizer for the formation of DOPE bilayer vesicles. The minimal mol% of glycophospholipid needed to stabilize the DOPE vesicles were as follows: 8% N-neuraminlactosyl-DOPE (NANL-DOPE), 20% N-maltotriosyl-DOPE (MAT-DOPE), 30% N-lactosyl-DOPE (Lac-DOPE), and 42% N-galactosyl-DOPE (Gal-DOPE). The estimated hydration number of glycophospholipid in reverse micelles was 87, 73, 46, and 14 for NANL-DOPE, MAT-DOPE, Lac-DOPE, and Gal-DOPE, respectively. Thus, the hydration intensity of the glycophospholipid was directly related to the ability to stabilize the DOPE bilayer phase for vesicle formation. Glycophospholipids also reduced the transition temperature from gel to liquid-crystalline phase (Tm) of dipalmitoylphosphatidylcholine (DPPC) bilayers. Interestingly, incorporation of NANL-DOPE induced a decrease of membrane fluidity of DPPC bilayers in the gel phase while other glycophospholipids had no effect. Also, low level of NANL-DOPE but not other glycophospholipids increased the transition temperature (TH) from liquid-crystalline to hexagonal phase of dielaidoylphosphatidylethanolamine bilayers. These results showed that NANL-DOPE with a highly hydratable headgroup which provides a strong stabilization activity for the L alpha phase of phospholipid membranes, may also be involved in specific interactions with neighboring phospholipids via its saccharide moiety.

Behaviour of a glycosphingolipid with unsaturated fatty acid in phosphatidylcholine bilayers

N-(Oleoyl)galactosylceramide with perdeuterated acyl chain was prepared by partial synthesis, and studied by wide line 2H-NMR in phospholipid liposomes. Spectra were obtained for low glycolipid concentrations in bilayers of dimyristoyl-, distearoyl-, and 1-palmitoyl-2-oleoylphosphatidylcholines. In an attempt to isolate the effects of glycosphingolipid fatty acid cis unsaturation on glycolipid behaviour in membranes, spectral findings related to the above species were compared to literature NMR data for pure 1-palmitoyl-2-oleoylphosphatidylcholine bilayers in which the oleoyl chain of the phospholipid had been deuterated, and to analogously deuterated glycerol based lipids in Acholeplasma laidlawii membranes. The results for N-(oleoyl-d33)galactosylceramide proved to be qualitatively and quantitatively very similar to published data dealing with glycerol based lipids at comparable temperatures. In addition, the results were strikingly similar for glycolipids dispersed in saturated and unsaturated phospholipid host matrices. It would appear that the primary effects of cis 9,10 fatty acid unsaturation in glycosphingolipids (at low concentration in fluid phospholipid membranes) are the same as those of fatty acid cis unsaturation in glycerolipids. It further appears that the overall dynamic behaviour of N-(oleoyl)galactosylceramide in fluid phospholipid membranes is very similar to that of glycerolipids with comparable acyl chains.