Influence of temperature on complement-dependent immune damage to liposomes

Activation of complement by therapeutic liposomes and other lipid excipient-based therapeutic products: prediction and prevention

Some therapeutic liposomes and lipid excipient-based anticancer drugs are recognized by the immune system as foreign, leading to a variety of adverse immune phenomena. One of them is complement (C) activation, the cause, or major contributing factor to a hypersensitivity syndrome called C activation-related pseudoallergy (CARPA). CARPA represents a novel subcategory of acute (type I) hypersensitivity reactions (HSR), which is mostly mild, transient, and preventable by appropriate precautions. However, in an occasional patient, it can be severe or even lethal. Because a main manifestation of C activation is cardiopulmonary distress, CARPA may be a safety issue primarily in cardiac patients. Along with an overview of the various types of liposome-immune system interactions, this review updates the experimental and clinical information on CARPA to different therapeutic liposomes and lipid excipient-based (micellar) anticancer drugs, including PEGylated liposomal doxorubicin sulfate (PLD, Doxil®) and paclitaxel (Taxol®). The substantial individual variation of in vitro and in vivo findings reflects an extremely complex immune phenomenon involving multiple, redundant pathways of C activation, signal transduction in allergy-mediating cells and vasoactive mediator actions at the effector cell level. The latest advances in this field include the proposal of doxorubicin-induced shape changes and aggregation of liposomes in Doxil as possible contributing factors to CARPA caused by PLD, and the finding that Doxil-induced immune suppression prevents HSR to co-administered carboplatin, a significant benefit of Doxil in combination chemotherapy with carboplatin. The review evaluates the use of in vitro C assays and the porcine liposome-induced cardiopulmonary distress model for predicting CARPA. It is concluded that CARPA may become a frequent safety issue in the upcoming era of nanomedicines, necessitating its prevention at an early stage of nanomedicine R&D.

Liposome Opsonization

Adsorption of serum proteins to the liposomal surface plays a critical role in the clearance of liposomes from the blood circulation. In this review, we will discuss the role of the liposomal opsonins proposed so far in liposome clearance. Additional, related topics that will be addressed are the cell-surface receptors that might be involved in liposome elimination from the blood compartment and the effect of poly(ethylene glycol) (PEG) modification on prevention of liposome opsonization.

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.

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.

Glycosphingolipid acyl chain order profiles: substituent effects

Fatty acid order parameter profiles were determined by 2H-NMR in order to characterize the arrangement and behaviour of the hydrophobic region of glycosphingolipids (GSLs) dispersed as minor components in phosphatidylcholine/cholesterol membranes. Direct comparison was made amongst species with important fatty acid structural features found in natural glycosphingolipids. Galactosyl ceramides (GalCer) were prepared by partial synthesis having 18:0[d35], D-alpha-OH 18:0[d34], 18:1[d33], and 24:0[d47] fatty acids. Unsonicated multilamellar liposomes of the common natural phospholipid, 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), containing 23% cholesterol, were employed as host matrix. Smoothed profiles of the order parameter, SCD, for 18:0[d35] GalCer proved to be very similar to profiles known for 16:0 and 18:0 fatty acids of glycerolipids in cholesterol-containing bilayers. In general, order along the GSL chain was slightly higher than anticipated for equivalent chain segments in phospholipids. Order parameter profiles for the GSL 18-carbon saturated fatty acids were strikingly similar. However, small quantitative differences were found for glycolipids having D- and L-alpha-hydroxylation at C-2 - the D-stereoisomer being marginally more ordered in the plateau region. Although order profiles have not been reported for unsaturated glycerolipid fatty acids in cholesterol-rich membranes, spectra of 18:1[d33] GalCer appeared to be assignable by applying known ordering effects of cholesterol to existing data for unsaturated glycerolipids. The unsaturated chain was found to be less ordered than saturated 18-carbon chains toward the membrane surface, but more ordered in the region of the bilayer midplane. The ordering may result from cholesterol-induced restriction of isomerisation at the cis-double bond, and represents an apparent exaggeration of a phenomenon known for glycerolipids. Addition of an 'extra' 6 carbons to the fatty acid (24:0[d47] GalCer) produced no significant effect on the order profile to a membrane depth of C-12-C-13. These results suggest that fluid membrane area requirements for GSLs with saturated fatty acids are not strongly influenced by the nature of that fatty acid when the GSL is a minor component. Order parameter profiles for the very long chain GSL deviated to higher order below this point, and formed a second 'plateau' of reduced negative slope toward the methyl terminus: this is characteristic of profiles for very long chain GSLs. These features were essentially unchanged over a range of temperatures providing different degrees of spatial constraint.

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)

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)

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.

Glycosphingolipids: 2H NMR study of the influence of ceramide fatty acid characteristics on the carbohydrate headgroup in phospholipid bilayers

Galactosylceramides bearing a variety of different pure fatty acid chains were 2H labeled in the carbohydrate headgroup at C6 of the terminal galactose residue, for study by 2H NMR. Fatty acids investigated included the 24-carbon saturated lignoceric acid, 18-carbon saturated stearic acid, cis-9,10-unsaturated oleic acid, and D- and L-stereoisomers of alpha-hydroxystearic acid. Headgroup-deuterated glycolipids were incorporated at 10 mol % into unsonicated bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine, and 2H NMR spectra were recorded at 65 and 40 degrees C. Under these experimental conditions, the membranes studied were primarily in the liquid-crystalline phase. At a given temperature, spectra for deuterated galactosylceramides dispersed in the fluid phase were remarkably similar, regardless of the nature of the fatty acid attached to the glycolipid sphingosine backbone. In each case, the spectrum consisted of a superposition of two quadrupolar powder patterns of approximately equal intensity. The spectra may be interpreted as arising from equal populations of two stereoisomers (pro-R and pro-S) of the deuterated galactose hydroxymethyl function, which is undergoing rapid (greater than 10(6) s-1) interconversion among the possible rotamers about the C5-C6 bond of the sugar ring. Within experimental error, the only fatty-acid-induced spectral difference detected among these glycosphingolipids deuterated in the carbohydrate headgroup was in the species with alpha-hydroxy-substituted fatty acids. At 65 degrees C, N-(D-alpha-hydroxy)stearoyl- and N-(L-alpha-hydroxy)stearoylgalactosylceramide gave rise to the same quadrupole splittings, but these differed marginally from the splittings observed for the other glycolipids studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Oligosaccharide order in a membrane-incorporated complex glycosphingolipid

Galactosylceramide (GalCer) and the ganglioside, GM1, were 2H-labelled at C-6 (the hydroxymethyl moiety) of their single terminal galactosyl residues. Each deuterated glycosphingolipid was incorporated at a biologically relevant low concentration into multibilayers of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC). 2H-NMR spectra of aqueous dispersions of GalCer-POPC in the liquid crystal phase were characteristic of restricted headgroup motion (ordering) with effective axial symmetry. The degree of headgroup ordering was analogous to that of GalCer in pure aqueous multibilayers (Skarjune, R. and Oldfield, E. (1979) Biochim. Biophys. Acta 556, 208-218). In the case of GM1, 2H-labelled in the terminal galactose residue of the pentasaccharide headgroup, the 2H-NMR spectra were remarkably like those of the simple glycolipid, GalCer. This suggests substantial restriction of motion about the glycosidic and sugar-ceramide bonds of the complex GM1 headgroup, and that both lipids have comparable degrees of orientational averaging (fluctuation) about the bilayer normal. The result is the first direct demonstration that headgroup orientational order can exist for a complex glycolipid incorporated into 'fluid' bilayer membranes. Such behaviour argues for the possibility of modulation of membrane receptor properties through surface effects on average headgroup orientation and conformation.

Glycosphingolipid interdigitation in phospholipid bilayers examined by deuterium NMR and EPR

Glycosphingolipid fatty acids commonly have up to eight methylene carbons more than do their surrounding phospholipid-attached counterparts. The resultant 'extra' segment may very well modulate glycosphingolipid function as receptor and structural element. As part of an investigation of this phenomenon, galactosylceramide was prepared with a deuterated 18-carbon fatty acid chain. Deuterium-labelled galactosylceramide was assembled at 10 mol% into unsonicated phosphatidylcholine bilayers having all 14-carbon or all 18-carbon saturated fatty acid chains (DMPC and DSPC, respectively). The systems were studied by 2H-NMR spectroscopy above and below the phase transition temperatures, Tm, of the host matrices. At comparable reduced temperatures in fluid membranes the degree of motional order exhibited by the glycolipid fatty acid was significantly higher in the phospholipid host matrix that was four carbons shorter. The fatty acid chain segment least affected by the change from long to short chain host matrix was the terminal (deutero)methyl group (an increase of 8% in quadrupolar splitting for the terminal methyl vs. 16% for deuterons at C17 and 23-28% for the remainder of the chain). Order parameter profiles for galactosylceramide were qualitatively very similar in the two host membranes, arguing against any major conformational difference between the arrangement of the 18-carbon glycolipid fatty acid in the 18-carbon vs. 14-carbon host matrices. Similarly a nitroxide spin probe covalently attached to carbon-12 of the galactosylceramide fatty acid gave clear indication of greater order in the fluid 14-carbon fatty acid phospholipid bilayer. These results are consistent with 'tethering' of the extra length of fatty acid via interdigitation into the opposing monolayer. There was no spectroscopic evidence of any intrinsic difference in glycolipid behaviour in the two fluid host matrices. 2H-NMR spectra of galactosylceramide at comparable reduced temperatures below Tm of the phospholipid bilayer were very different for 14-carbon vs. 18-carbon host matrices. The glycolipid fatty acid showed evidence of relatively reduced mobility in the shorter chain matrix.

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)

Analysis of vesicle immunolysis assays. The direct binding model

Assays based on lysis of lipid vesicles have shown high sensitivity. However, little as yet is known about the quantitative relationships among the various assay parameters, due in part to the lack of a predictive theoretical model. This paper presents the derivation of the equations that describe a simple model assay system in terms of the total fraction of vesicles with bound antibodies and the distribution of vesicles with one, two, or more antibodies bound. The equations show how the binding of antibodies to vesicles is affected by such variables as: vesicle concentration, antigen density on vesicle surfaces, antibody concentration, and antibody affinity. With the distribution functions, experiments can be designed to determine the minimum number of antibodies needed to lyse a vesicle. In addition, it is shown how estimations of the ultimate sensitivity of lipid vesicle lytic assays can be made. The model can be used to optimize vesicle lysis assay systems.

Dynamic properties of the haptenic site of lipid haptens in phosphatidylcholine membranes. Their relation to the phase transition of the host lattice

The relation between the dynamic properties of the haptenic site of lipid haptens and the phase transition of the host lattice was investigated using head group spin-labeled phosphatidylethanolamines, that is, spin-label lipid haptens (Brûlet, P., and H. M. McConnell, 1976, Proc. Natl. Acad. Sci. USA., 73:2977-2981; Brûlet, P., and H. M. McConnell, 1977, Biochemistry, 16:1209-1217). The electron spin resonance (ESR) spectra of the lipid haptens in liposomal membranes showed three narrow resonance lines, whose widths and hyperfine splitting values suggested that the haptenic site, i.e., the spin-label moiety, should be exposed in the water phase. The line width of each peak depended on the host lipid species and on the incubation temperature. A temperature study using dipalmitoylphosphatidylcholine (DPPC) liposomes showed that the dynamic properties of the haptenic site were related to the main phase transition and the subphase transition of the host lattice but not to the prephase transition. The angular amplitudes of the tumbling motion of the haptenic site were estimated using oriented multibilayer systems. The angular amplitude of dipalmitoyl-phosphatidyl-N-[[N-(1-oxyl-2,2,6, 6-tetramethyl-4-piperidinyl)-carbamoyl]-methyl]-ethanolamine in DPPC membranes was 63 degrees at 2 degrees C, and it increased slightly with an increase in temperature regardless of the phase transition of the host lattice. The value for egg phosphatidylcholine (PC) at 25 degrees C was the same as for DPPC above its main phase transition temperature. Rotational correlation time analysis showed that the axial rotation of the haptenic site was preferable to the tumbling motion of the rotational axis, and the predominance depended on the phase transition, Lc----L beta' and P beta'----L alpha. Elongation of the spacer arm between the haptenic site and phosphate increased the angular amplitude of the tumbling motion but reduced the effect of the host lattice. Spin-label lipid haptens with unsaturated fatty acyl chains were distributed heterogeneously in DPPC membranes, whereas those with the same fatty acyl chain as the host lattice were distributed randomly. The ESR spectrum of a lipid hapten under its prephase transition temperature showed two components, broad and narrow. This suggests that at least two different domains, a hapten-rich domain and a hapten-poor one, may coexist in membranes. ESR measurements at various temperatures suggested that the haptenic site fraction in the hapten-rich domain decreased in part during the phase transition from L beta' to P beta', and disappeared completely in the La phase. The spatial mobility and lateral diffusion of lipid haptens will be discussed in greater detail.

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)

Toxicity of perfluorinated fatty acids for human and murine B cell lines

The toxicity of two perfluorinated fatty acids, penta decafluoro-n-octanoic acid (PFOA) and nonadecafluoro-n-decanoic acid (NDFDA), on three mammalian B cell lines was evaluated. Cells were exposed to the perfluorinated molecules for either 24 or 48 hr under a variety of culture conditions. Immunoglobulin secretion and surface membrane expression were unaffected by both PFOA and NDFDA at sublethal concentrations. Lethal effects of PFOA and NDFDA are diminished by either lowering culture temperature (37 to 20 degrees C) or including fetal bovine serum or human serum albumin in media. At lethal concentrations, both PFOA and NDFDA possess detergent activity since they can release IgM in soluble form from a cell line that does not secrete immunoglobulins, and brief exposure (15 min) to 1-2 mM of both perfluorinated fatty acids results in solubilization of F4 cells equivalent to the anionic detergent deoxycholic acid. Our data suggest that at subtoxic concentrations, neither PFOA nor NDFDA alters expression or secretion of a differentiated gene product (IgM). At lethal levels, both chemicals cause increased solubilization of proteins from lymphoblastoid cell lines.

Inhibitory effects of gangliosides on immune reactions of antibodies to neutral glycolipids in liposomes

Specific immune damage to liposomes containing Forssman or globoside glycolipid was inhibited when the liposomes also contained ganglioside. The activity of a human monoclonal Waldenström macroglobulin antibody to Forssman glycolipid was inhibited by each of three gangliosides tested, GM3, GD1a and GD1b. Inhibition of the monoclonal antibody was dependent on the amount of ganglioside in the liposomes, and was diminished by reducing the relative amount of ganglioside. Inhibition also correlated positively with the number of ganglioside sialic acid groups, with inhibition by GT1b greater than GD1a greater than GM3. Naturally occurring human antibodies to globoside glycolipid were detected in 18% (9 out of 50) of normal human sera tested. Immune damage to liposomes induced by each of the three highest-reacting human anti-globoside sera was blocked by liposomal GM3. We conclude that gangliosides can strongly influence immune damage to membranes induced by antibody interactions with adjacent neutral glycolipids.

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.

Complement protein C9 labeled with fluorescein isothiocyanate can be used to monitor C9 polymerization and formation of the cytolytic membrane lesion

Human complement protein C9 was covalently labeled with the fluorescent chromophore fluorescein isothiocyanate (FITC) with only a small reduction in the cytolytic activity of the protein. Polymerization of the labeled protein--either by incubating with lipid vesicles treated with complement proteins C5b-8 (activating the C5b-9 membrane lesion) or by heating the protein [Tschopp, J., Muller-Eberhard, H.J., & Podack, E.R. (1982) Nature (London) 298, 534]--resulted in a 40-60% decrease in the fluorescence emission from FITC. The decrease in total fluorescence was accompanied by an increase in the steady-state anisotropy following activation and polymerization of FITC-C9 by C5b-8 membranes, while heat-induced aggregation of the protein resulted in a dramatic depolarization of fluorescence. Only small changes in either the absorbance spectrum or fluorescence lifetime of the chromophore were detected upon FITC-C9 polymerization. Evidence is presented that the measured changes in FITC fluorescence upon C9 activation are due to self energy transfer between closely apposed fluorescein chromophores which occur in the polymerized form of the protein. The significance of these observations to the molecular structure of the assembled C5b-9 complex is discussed, as are the potential applications of this fluorescent derivative of C9.

Binding of choleragen and anti-ganglioside antibodies to gangliosides incorporated into preformed liposomes

Exogenously added gangliosides were taken up and incorporated into liposomes just as they are incorporated into cells. Ganglioside GM1 was rapidly taken up by liposomes containing dimyristoyl- or dipalmitoylphosphatidylcholine, cholesterol and dicetyl phosphate. When incubated with a wide range of GM1 concentrations for 18 h, the liposomes incorporated about 10% of the added ganglioside. The rate of GM1 uptake by preformed liposomes was both time- and temperature-dependent. The liposomes also incorporated other gangliosides to a similar extent. The GM1 taken up by preformed liposomes was predominantly located on the outer surface of the liposomes and did not appear to be internalized into the inner half of the lipid bilayer. Liposomes containing GM1 added after liposome formation bound as many anti-GM1 antibodies and as much choleragen as liposomes having GM1 added during the formation of the lipid bilayers. Thus, preformed liposomes sensitized by incubation with GM1 are a good model system for studying the interactions of antibodies and toxins with membrane-associated gangliosides.