The effects of cerebrosides on model membrane shape

Cellular uptake of liposome consisting mainly of glucocerebroside from the starfish Asterias amurensis into Caco-2 cells

Glucocerebroside (GlcCer) is a group of compounds consisting of β-linked glucose and ceramide with various chain lengths, some of which possess anti-tumor activity and improve skin barrier function for atopic patients when administered orally. The amphiphilic GlcCer molecules are generally easy to aggregate in aqueous solution and result in low absorption in the gut, which can be improved by forming a liposome. With a recognition that a relatively large amount of GlcCer is contained in the starfish and is being discarded, we prepared a liposome consisting mainly of GlcCer (over 95%) with 100 nm in diameter. The adsorption efficiency of the liposome into cultured Caco-2 cells was investigated by live-cell imaging using fluorescently labeled liposomes. We found an immediate internalization of GlcCer-liposome on exposure without significant accumulation on the plasma membrane. The membrane fluidity was transiently affected as evidenced by fluorescence recovery after photobleaching (FRAP) experiments without no significant cellular damage, which indicates a liposome with high content of GlcCer might be useful as the carrier of dietary and/or drug molecules.

Uncovering the Potential of Lipid Drugs: A Focus on Transient Membrane Microdomain-Targeted Lipid Therapeutics

Membrane lipids are generally viewed as inert physical barriers, but many vital cellular processes greatly rely on the interaction with these structures, as expressed by the membrane hypothesis that explain the genesis of schizophrenia, Alzheimer's and autoimmune diseases, chronic fatigue or cancer, among others. The concept that the cell membrane displays transient membrane microdomains with distinct lipid composition provide the basis for the development of selective lipid-targeted therapies, the membrane-lipid therapies (MLTs). In this concern, medicinal chemists may design therapeutically valuable compounds 1) with a higher affinity for the lipids in these microdomains to restore the normal physiological conditions, 2) that can directly or 3) indirectly (via enzyme inhibition/activation) replace damaged lipids or restore the regular lipid levels in the whole membrane or microdomain, 4) that alter the expression of genes related to lipid genesis/metabolism or 5) that modulate the pathways related to the membrane binding affinity of lipid-anchored proteins. In this context, this mini-review aims to explore the structural diversity and clinical applications of some of the main membrane and microdomain-targeted lipid drugs.

Wrapping axons in mammals and Drosophila: Different lipids, same principle

Plasma membranes of axon-wrapping glial cells develop specific cylindrical bilayer membranes that surround thin individual axons or axon bundles. Axons are wrapped with single layered glial cells in lower organisms whereas in the mammalian nervous system, axons are surrounded with a characteristic complex multilamellar myelin structure. The high content of lipids in myelin suggests that lipids play crucial roles in the structure and function of myelin. The most striking feature of myelin lipids is the high content of galactosylceramide (GalCer). Serological and genetic studies indicate that GalCer plays a key role in the formation and function of the myelin sheath in mammals. In contrast to mammals, Drosophila lacks GalCer. Instead of GalCer, ceramide phosphoethanolamine (CPE) has an important role to ensheath axons with glial cells in Drosophila. GalCer and CPE share similar physical properties: both lipids have a high phase transition temperature and high packing, are immiscible with cholesterol and form helical liposomes. These properties are caused by both the strong headgroup interactions and the tight packing resulting from the small size of the headgroup and the hydrogen bonds between lipid molecules. These results suggest that mammals and Drosophila wrap axons using different lipids but the same conserved principle.

Formation of tubules and helical ribbons by ceramide phosphoethanolamine-containing membranes

Ceramide phosphoethanolamine (CPE), a major sphingolipid in invertebrates, is crucial for axonal ensheathment in Drosophila. Darkfield microscopy revealed that an equimolar mixture of bovine buttermilk CPE (milk CPE) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (diC18:1 PC) tends to form tubules and helical ribbons, while pure milk CPE mainly exhibits amorphous aggregates and, at low frequency, straight needles. Negative staining electron microscopy indicated that helices and tubules were composed of multilayered 5–10 nm thick slab-like structures. Using different molecular species of PC and CPE, we demonstrated that the acyl chain length of CPE but not of PC is crucial for the formation of tubules and helices in equimolar mixtures. Incubation of the lipid suspensions at the respective phase transition temperature of CPE facilitated the formation of both tubules and helices, suggesting a dynamic lipid rearrangement during formation. Substituting diC18:1 PC with diC18:1 PE or diC18:1 PS failed to form tubules and helices. As hydrated galactosylceramide (GalCer), a major lipid in mammalian myelin, has been reported to spontaneously form tubules and helices, it is believed that the ensheathment of axons in mammals and Drosophila is based on similar physical processes with different lipids.

Disposition of axonal caspr with respect to glial cell membranes: Implications for the process of myelination

Neurofascin-155 (NF155) and caspr are transmembrane proteins found at discrete locations early during development of the nervous system. NF155 is present in the oligodendrocyte cell body and processes, whereas caspr is on the axonal surface. In mature nerves, these proteins are clustered at paranodes, flanking the node of Ranvier. To understand how NF155 and caspr become localized to the paranodal regions of myelinated nerves, we have studied their distribution over time in myelinating cultures. Our observations indicate that these two proteins are recruited to the cell surface at the contact zone between axons and oligodendrocytes, where they trans-interact. This association explains the early pattern of caspr distribution, a helical coil that winds around the axon, resembling the turns of the myelin sheath. Caspr, an axonal membrane protein, therefore seems to move in register with the overlying myelinating cell via its interactions with myelin proteins. We suggest that NF155 is the glial cell membrane protein responsible for caspr distribution. The pair act as interacting partners on either side of the axoglial contact area. Most likely, there are other proteins on the axonal surface whose distribution is equally influenced by interaction with the nascent myelin sheath. The fact that caspr follows the movement of the spiraling membrane has a direct affect on the interpretation of the way in which myelin is formed.

Direct visualization of the lateral structure of porcine brain cerebrosides/POPC mixtures in presence and absence of cholesterol

We studied the thermal behavior of membranes composed of mixtures of natural cerebrosides (from porcine brain) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with and without cholesterol, using differential scanning calorimetry, Fourier transform infrared spectroscopy, and confocal/multiphoton fluorescence microscopy. The POPC/cerebroside mixture display solid ordered/liquid disordered phase coexistence in a broad range of compositions and temperatures in agreement with previous results reported for POPC/(bovine brain)cerebrosides. The observed phase coexistence scenario consists of elongated, micrometer-sized cerebroside-rich solid ordered domains that span the bilayer, embedded in a POPC-rich liquid disordered phase. The data obtained from differential scanning calorimetry and Fourier transform infrared spectroscopy was in line with that obtained in the microscopy experiments for the binary mixture, except at very high cerebroside molar fractions (0.8-0.9) were some differences are observed. Cholesterol incorporation exerts strong changes on the lateral organization of POPC/porcine brain cerebroside membranes. At intermediate cholesterol concentrations (10-25 mol %) the solid ordered/liquid disordered phase coexistence scenario gradually transform to a solid ordered/liquid ordered one. Above 25 mol % of cholesterol two distinct regions with liquid ordered phase character are visualized in the membrane until a single liquid ordered phase forms at 40 mol % cholesterol. The observed cholesterol effect largely differs from that reported for POPC/porcine brain ceramide, reflecting the impact of the sphingolipids polar headgroup on the membrane lateral organization.

Heat shock-induced changes in lipid and protein metabolism in the endoplasmic reticulum of barley aleurone layers

Heat shock in barley aleurone layers induces heat shock protein synthesis and suppresses secretory protein synthesis by selectively destabilizing their mRNAs. In addition, the endoplasmic reticulum (ER) membranes upon which secretory protein mRNAs are translated become vesiculated during heat shock, leading to the hypothesis that ER dissociation and targeted mRNA destabilization are linked mechanistically. Supporting this, ER can be heat adapted, and heat-adapted ER has higher levels of fatty acid saturation in membrane phospholipids which do not vesiculate upon heat shock. Secretory protein mRNAs are also more stable in heat-adapted cells. To understand better heat shock-induced changes in ER membranes, we examined ER membrane proteins and enzymes involved in phosphatidylcholine biosynthesis and phospholipid turnover in heat-shocked aleurone cells. Heat shock significantly increased the activity of phospholipases A2 and D, and shortly thereafter significant but gradual increases in choline kinase and phosphocholine glyceride transferase activities and a sharp increase in phosphorylcholine citidyl transferase activity were observed. Only minor changes were observed in SDS-PAGE analyses of proteins from sonicated ER membranes fractionated on continuous sucrose gradients. Overall, heat shock reduced total lipid in ER membranes relative to protein, and in intact, ultracentrifuged aleurone cells examined by light and electron microscopy the ER band appeared to increase in density. The changes in phospholipid metabolism coupled with the suppression of secretory protein synthesis indicate that in addition to inducing a classic heat shock response, high temperature also induces a classic unfolded protein response in the ER of this secretory cell.

Structure of core domain of fibril-forming PHF/Tau fragments

Short peptide sequences within the microtubule binding domain of the protein Tau are proposed to be core nucleation sites for formation of amyloid fibrils displaying the paired helical filament (PHF) morphology characteristic of neurofibrillary tangles. To study the structure of these proposed nucleation sites, we analyzed the x-ray diffraction patterns from the assemblies formed by a variety of PHF/tau-related peptide constructs containing the motifs VQIINK (PHF6*) in the second repeat and VQIVYK (PHF6) in the third repeat of tau. Peptides included: tripeptide acetyl-VYK-amide (AcVYK), tetrapeptide acetyl-IVYK-amide (AcPHF4), hexapeptide acetyl-VQIVYK-amide (AcPHF6), and acetyl-GKVQIINKLDLSNVQKDNIKHGSVQIVYKPVDLSKVT-amide (AcTR4). All diffraction patterns showed reflections at spacings of 4.7 A, 3.8 A, and approximately 8-10 A, which are characteristic of an orthogonal unit cell of beta-sheets having dimensions a=9.4 A, b=6.6 A, and c=approximately 8-10 A (where a, b, and c are the lattice constants in the H-bonding, chain, and intersheet directions). The sharp 4.7 A reflections indicate that the beta-crystallites are likely to be elongated along the H-bonding direction and in a cross-beta conformation. The assembly of the AcTR4 peptide, which contains both the PHF6 and PHF6* motifs, consisted of twisted sheets, as indicated by a unique fanning of the diffuse equatorial scattering and meridional accentuation of the (210) reflection at 3.8 A spacing. The diffraction data for AcVYK, AcPHF4, and AcPHF6 all were consistent with approximately 50 A-wide tubular assemblies having double-walls, where beta-strands constitute the walls. In this structure, the peptides are H-bonded together in the fiber direction, and the intersheet direction is radial. The positive-charged lysine residues face the aqueous medium, and tyrosine-tyrosine aromatic interactions stabilize the intersheet (double-wall) layers. This particular contact, which may be involved in PHF fibril formation, is proposed here as a possible aromatic target for anti-tauopathy drugs.

Interactions between glucosylceramide and galactosylceramide I(3) sulfate and microstructures formed

The monohexoside glycosphingolipids (GSLs), galactosylceramide (GalC), glucosylceramide (GluC), and their sulfated forms are abundant in cell membranes from a number of tissues. Carbohydrate-carbohydrate interactions between the head groups of some GSLs can occur across apposed membranes and may be involved in cell-cell interactions. In the present study, the ability of GluC to participate in trans interactions with galactosylceramide I(3) sulfate (CBS) was investigated by transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy. Gaucher's spleen GluC had polymorphic phase behavior; in its metastable state, it formed large wrinkled vesicles. It transformed to a stable state via an intermediate state in which the surface of the vesicles consisted of narrow ribbons. In the stable state, the narrow ribbons split off from the surface to form membrane fragments and flat and helical ribbons. The strength of the intermolecular hydrogen bonding interactions between the carbonyls increased in the order metastable<intermediate<stable state. Aqueous dispersions of GluC and CBS were combined to allow trans carbohydrate-carbohydrate interaction across apposed bilayers, or the lipids were premixed in a solvent before hydration to allow lateral cis interactions. Premixed dispersions of NFA-GluC and CBS remained in the metastable state even when incubated under stable state conditions. When NFA-GluC dispersions were combined with CBS dispersions, they had a small effect on each other's amide groups in the metastable state. Furthermore, conversion of NFA-GluC to the stable state was inhibited, although it reached the intermediate state, suggesting that some degree of trans interaction between these two lipids occurred.

Induction of neuron-like tubes and liposome networks by cooperative effect of gangliosides and phospholipids

Although there is a rather large abundance of gangliosides in neurons, their functional role is still unclear. We focused on a physicochemical role of gangliosides in the formation of tubular structures, such as axons or dendrites in neurons. When a ganglioside, GM3, was added to cell-size liposomes that consisted of dioleoylphosphatidyl-choline, tubular structures were induced and liposome networks connected by the tubes were observed by differential interference microscopy and fluorescence microscopy. The potential for various gangliosides to induce tubes was dependent on the structures of their hydrophilic head group. With a large excess of gangliosides, the tubes are destabilized and small fragments, or micelles, are generated. The phenomenon was suggested by physical model calculation. Gangliosides may play a role as building material in neural unique tubular structures.

The formation of helical tubular vesicles by binary monolayers containing a nickel-chelating lipid and phosphoinositides in the presence of basic polypeptides

Binary lipid monolayers consisting of equimolar proportions of a phosphoinositide and a nickel-chelating lipid formed helical tubular vesicular structures, which appeared to be induced and/or stabilized by myelin basic protein (MBP). Another basic polypeptide, poly-L-lysine, had a similar effect but not to as great a degree as MBP; the proteins thus appeared to act as polycations. Although, the nickel-chelating lipid is a synthetic product, other endogenous divalent cations such as Zn(2+), as well as phosphoinositides, are integral and dynamic components of the myelin sheath in vivo. There, comparable helical tubular structures might represent a means for sequestration of these lipids into domains of high local concentration, perhaps in regions where the membrane is greatly curved.

Polymorphic phases of galactocerebrosides: spectroscopic evidence of lamellar crystalline structures

Fourier transform infrared spectroscopy was applied to study the structural and thermal properties of bovine brain galactocerebroside (GalCer) containing amide linked non-hydroxylated or alpha-hydroxy fatty acids (NFA- and HFA-GalCer, respectively). Over the temperature range 0-90 degrees C, both GalCer displayed complex thermal transitions, characteristic of polymorphic phase behavior. Upon heating, aqueous dispersions of NFA- and HFA-GalCer exhibited high order-disorder transition temperatures near 80 and 72 degrees C, respectively. En route to the chain melting transition, the patterns of the amide I band of NFA-GalCer were indicative of two different lamellar crystalline phases, whereas those of HFA-GalCer were suggestive of lamellar gel and crystalline bilayers. Cooling from the liquid-crystalline phase resulted in the formation of another crystalline phase of NFA-GalCer and a gel phase of HFA-GalCer, with a phase transition near 62 and 66 degrees C, respectively. Prolonged incubation of GalCer bilayers at 38 degrees C revealed conversions among lamellar crystalline phases (NFA-GalCer) or between lamellar gel and crystalline bilayer structures (HFA-GalCer). Spectral changes indicated that the temperature and/or time induced formation of the lamellar crystalline structures of NFA- and HFA-GalCer was accompanied by partial dehydration and by rearrangements of the hydrogen bonding network and bilayer packing mode of GalCer.

MAL, a proteolipid in glycosphingolipid enriched domains: functional implications in myelin and beyond

The myelin and lymphocyte protein MAL (VIP17/MVP17) is a proteolipid of 17 kD with a hydrophobicity pattern that indicates a four transmembrane domain structure. The MAL cDNA has been cloned from human T-cells, rat oligodendrocytes and the Madin-Darby canine kidney (MDCK) cell line. In the nervous system both myelinating cells, oligodendrocytes and Schwann cells, express MAL protein. MAL expression parallels myelin formation, and MAL is predominantly localized in compact myelin. Prior to myelin formation MAL is also found in immature Schwann cells. Outside the nervous system MAL expression is found in T-cells and in distinct epithelial cells, e.g. in kidney, stomach and thyroid gland, where MAL is localised in the apical plasma membrane. Specific glycosphingolipids, e.g. galactosylceramide and sulfatide, are enriched in such apical kidney and stomach membranes as well as in myelin. MAL copurifies with these glycosphingolipids in detergent insoluble domains, indicating a close association and possible functional interactions of MAL with glycosphingolipids in these tissues. Moreover, recent reports point to additional functions of MAL-glycosphingolipid complexes in signalling, cell differentiation and apical sorting. The role of MAL in the formation, stabilisation and maintenance of glycosphingolipid-enriched membrane microdomains and its contribution to specific membrane properties in myelin and epithelial cells are discussed.

Trans interactions between galactosylceramide and cerebroside sulfate across apposed bilayers

The two glycosphingolipids galactosylceramide (GalC) and its sulfated form, cerebroside sulfate (CBS), are present at high concentrations in the multilayered myelin sheath and are involved in carbohydrate-carbohydrate interactions between the lipid headgroups. In order to study the structure of the complex of these two glycolipids by Fourier transform infrared (FTIR) spectroscopy, GalC dispersions were combined with CBS dispersions in the presence and absence of Ca(2+). The FTIR spectra indicated that a strong interaction occurred between these glycolipids even in the absence of Ca(2+). The interaction resulted in dehydration of the sulfate, changes in the intermolecular hydrogen bonding interactions of the sugar and other oxygens, decreased intermolecular hydrogen bonding of the amide C==O of GalC and dehydration of the amide region of one or both of the lipids in the mixture, and disordering of the hydrocarbon chains of both lipids. The spectra also show that Ca(2+) interacts with the sulfate of CBS. Although they do not reveal which other groups of CBS and GalC interact with Ca(2+) or which groups participate in the interaction between the two lipids, they do show that the sulfate is not directly involved in interaction with GalC, since it can still bind to Ca(2+) in the mixture. The interaction between these two lipids could be either a lateral cis interaction in the same bilayer or a trans interaction between apposed bilayers. The type of interaction between the lipids, cis or trans, was investigated using fluorescent and spin-label probes and anti-glycolipid antibodies. The results confirmed a strong interaction between the GalC and the CBS microstructures. They suggested further that this interaction caused the CBS microstructures to be disrupted so that CBS formed a single bilayer around the GalC multilayered microstructures, thus sequestering GalC from the external aqueous phase. Thus the CBS and GalC interacted via a trans interaction across apposed bilayers, which resulted in dehydration of the headgroup and interface region of both lipid bilayers. The strong interaction between these lipids may be involved in stabilization of the myelin sheath.

Modulation of nanotube formation by structural modifications of sphingolipids

Galactosylceramides (GalCers) containing nervonoyl (24:1(Delta15(cis))) acyl chains have the capacity to assemble into nanotubular microstructures in excess water (. Biophys. J. 69:1976-1986). To define the structural parameters that modulate nanotube formation, GalCer derivatives were synthesized that contained cis monounsaturated acyl chains with the formula X:1((X-9)). X indicates the total acyl carbon number (24, 22, 20, or 18), and 1 indicates a single cis double bond, the location of which is designated by the superscript (X-9). Deep etching of freeze-fractured 24:1(Delta15(cis)) GalCer dispersions followed by replica production and transmission electron microscopic analysis confirmed nanotube morphology (25-30-nm diameter). Control experiments revealed that tubule formation was promoted by cooling through the main enthalpic phase transition coupled with repetitive freeze-thaw cycling. Imparting a negative charge to the sugar headgroup of 24:1(Delta15)GalCer via sulfate dramatically altered mesomorpholgy and resulted in myelinic-like, multilamellar structures. Removal of the sugar headgroup (24:1(Delta15)Cer) resulted in flattened cylindrical structures with a cochleate appearance. Compared to these large-scale changes in morphology, more subtle changes were induced by structural changes in the acyl chain of 24:1(Delta15)GalCer. 22:1(Delta13)GalCer dispersions consisted of long, smooth tubules (35-40-nm diameters) with a strong tendency to self-align into bundle-like aggregates. In contrast, the microstructures formed by 20:1(Delta11)GalCer resembled helical ribbons with a right-handed twist. Ribbon widths averaged 30-35 nm, with helical pitches of 80-90 nm. 18:1(Delta9)GalCer displayed a variety of morphologies, including large-diameter multilamellar cylinders and liposome-like structures, as well as stacked, plate-like arrays. The results are discussed within the context of current theories of lipid tubule formation.

Oligomerization of Vibrio cholerae cytolysin yields a pentameric pore and has a dual specificity for cholesterol and sphingolipids in the target membrane

Vibrio cholerae cytolysin permeabilizes animal cell membranes. Upon binding to the target lipid bilayer, the protein assembles into homo-oligomeric pores of an as yet unknown stoichiometry. Pore formation has been observed with model liposomes consisting of phosphatidylcholine and cholesterol, but the latter were much less susceptible to the cytolysin than were erythrocytes or intestinal epithelial cells. We here show that liposome permeabilization is strongly promoted if cholesterol is combined with sphingolipids, whereby the most pronounced effects are observed with monohexosylceramides and free ceramide. These two lipid species are prevalent in mammalian intestinal brush border membranes. We therefore propose that, on its natural target membranes, the cytolysin has a dual specificity for both cholesterol and ceramides. To assess the stoichiometry of the pore, we generated hybrid oligomers of two naturally occurring variants of the toxin that differ in molecular weight. On SDS-polyacrylamide gel electrophoresis, the mixed oligomers formed a pattern of six distinct bands. Ordered by decreasing electrophoretic mobility, the six oligomer species must comprise 0 to 5 subunits of the larger form; the pore thus is a pentamer. Due to both lipid specificity and pore stoichiometry, V. cholerae cytolysin represents a novel prototype in the class of bacterial pore-forming toxins.

Formation of high-axial-ratio-microstructures from natural and synthetic sphingolipids

Amphiphiles that form high-axial-ratio-microstructures (HARMs) are being considered as novel materials for controlled release of drugs and other biologically functional molecules. HARMs consisting of tubules, ribbons, solid rods and helices are formed from sphingolipids by addition of water to a solution of amphiphile in DMF. Single molecular species of galactocerebroside (GalCer) containing long unsaturated fatty acid chains or natural GalCer containing mixed-length, non-hydroxy fatty acids (NFA-GalCer) or alpha-hydroxy fatty acids (HFA-GalCer) form cylindrical structures. In contrast, single molecular species of GalCer containing long saturated fatty acids form ribbons and helices. GalCer HARMs are typically under 100 nm in diameter and have lengths of several microns. The importance of the amide of GalCer for HARM formation was evaluated using psychosine, which forms solid fibers, whereas sphingosine and an analog of GalCer in which the amide is reduced to a secondary amine form amorphous aggregates. Single molecular species of ceramide containing long unsaturated fatty acid chains form cylindrical structures, whereas those with long saturated fatty acids form ribbons and helices. Short chain saturated ceramide also forms cylindrical structures. GalCer analogs with N-acetyl-glycine in place of the galactose form fibers whereas those with N-acetyl-proline yield amorphous material. The N-acetyl-proline-containing amphiphile can de doped into pure GalCer or NFA-GalCer without perturbing tubule formation.

Globoside as a membrane receptor: a consideration of oligosaccharide communication with the hydrophobic domain

Recognition of macromolecules by glycosphingolipids is closely correlated with the nature of the glycolipid carbohydrate; however, it is also thought to be secondarily modulated by the structure of the single fatty acid. In the present work, we sought insight into what physical effect a change in this fatty acid has on the extramembranous portion of globosides at liposomal surfaces mimicking systems for which modulated receptor function has been recorded in the past. Protons of the exocyclic hydroxymethyl group on the terminal Gal residue of globotriaosylceramide (Gb3) were replaced with deuterium. In this location, the nonperturbing probe nuclei sampled cumulative conformational and orientational characteristics of the oligosaccharide chain at a sugar residue that is critical in specific binding of verotoxins. Deuterated Gb3 having 18:1 fatty acid was compared to the same species having 22:1 fatty acid, at 6.3 mol % in unsonicated bilayers of dipalmitoylphosphatidylcholine/cholesterol. Both produced narrow, apparently axially asymmetric 2H NMR spectra over a wide temperature range. Motional properties of the terminal sugar were measurably influenced by the fluidity of the host matrix; however, evidence was not found for conformational or orientational variation in this sugar brought about by the fatty acid alteration. In related experiments, acetate protons on the terminal N-acetyl galactosamine (GalNAc) residue of globotetraosylceramide (Gb4) were substituted with deuterium, and the natural fatty acid was replaced with 18:0 or 24:0 species deuterated at C2. Once again, species with short vs long fatty acid were examined for evidence of headgroup differences. Spectra of Gb4 were compared at 10 mol % in unsonicated fluid bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine, and at 5 mol % in membranes containing 33 mol% cholesterol. Spectral splittings reflecting cumulative effects on conformation and order at the terminal deuterated sugar remained unchanged between species having 18:0 vs 24:0 fatty acid in POPC/cholesterol. In a pure POPC host matrix, there was clear evidence of a motional difference between the two--the longer chain Gb4 demonstrating spectral asymmetry--but the spectral width was unchanged. Transverse relaxation times, T2, were measured. Our findings appear to help correlate the conclusions of a number of workers dealing with the molecular basis of crypticity. We suggest that changes in glycolipid receptor function based on ceramide fatty acid variation have a major origin in the fatty acid's ability to determine the thermodynamics of interaction with the host matrix, as reflected in such parameters as glycolipid motional properties, local membrane curvature, and likely glycolipid time-dependent lateral associations. The result at low concentrations of glycolipid may often be only a subtly altered collective surface epitope, best detected by a specific recognition event.

Effect of saposins A and C on the enzymatic hydrolysis of liposomal glucosylceramide

The degradation of glucosylceramide in lysosomes is accomplished by glucosylceramidase with the assistance of, at least, another protein, saposin C (Sap C), which is generated from a large precursor together with three other similar proteins, saposins A, B, and D. In the present study, we have examined the effects of saposins on the enzymatic hydrolysis of glucosylceramide inserted in large and small phospholipid liposomes. The glucosylceramide contained in large unilamellar vesicles (LUV) was degraded by glucosylceramidase at a rate 7-8-fold lower than glucosylceramide inserted in small unilamellar vesicles (SUV). The separate addition of either Sap A or Sap C to the LUV system partially stimulated the sphingolipid degradation while saposins B and D had no effect. In the presence of both Sap A and Sap C, the rate of sphingolipid degradation was higher than the sum of the rates with the two saposins individually, indicating synergism in their actions. The stimulatory effect of the two saposins depended on the incorporation of an acidic phospholipid such as phosphatidylserine (PS) into LUV. The characteristics of glucosylceramidase activation by Sap C were different from those of Sap A. Sap C increased the rate of hydrolysis of both the artificial water soluble substrate, 4-methylumbelliferyl-beta-D-glucopyranoside, and the lipid substrate, glucosylceramide, while Sap A only stimulated degradation of the sphingolipid. Also the binding properties of Saps A and C were markedly different. At acidic pH values, Sap C bound to PS-containing LUV and promoted the association of glucosylceramidase with the membrane. In contrast, Sap A had poor affinity for the membrane even in the presence of glucosylceramide; moreover, Sap A did not potentiate the capacity of Sap C to mediate glucosylceramidase binding. In conclusion, our results show that both Sap A and Sap C are required for maximal hydrolysis of glucosylceramide inserted in PS-containing LUV, that their effects are synergistic, and that their mode of action is different. Sap C is responsible for the membrane binding of glucosylceramidase, while Sap A stimulation is possibly related to its effect on the conformation of the enzyme. It can be envisaged that Sap A in conjunction with Sap C might have a physiological role in glucosylceramide degradation.

Sphingolipid-derived signalling modulators: interaction with phosphatidylserine

We previously described the synthesis of two deuterium-labelled sphingoid bases, which made it possible to perform NMR spectroscopy on this family of signalling modulators in membranes (Rigby, A.C, Barber, K.R and Grant, C.W.M. (1995) Biochim. Biophys. Acta 1240, 75-82). In the present work we sought to test the concept that such mediators may display altered physical behaviour through association with anionic lipids - as a possible mechanism of involvement in signal transduction. Lyso-dihydrogalactosylceramide with deuterium nuclei at C4 and C5 of the sphingosine backbone and at C'3 and C'4 of the galactose ring ([2H4]lyso-GalCer), and N,N-dimethylsphingosine with deuterated amino-methyl groups ([2H6]dimethylsphingosine), were assembled as minor components into unsonicated fluid bilayer membranes of 1-palmitoyl-2-oleoylphosphatidylcholine/cholesterol. The effect of (anionic) phosphatidylserine was considered in this zwitterionic host matrix. The results present a picture of rapidly reversible interaction. The (-) charged phosphatidylserine exerted readily-measurable control over the orientation of the carbohydrate residue of [2H4]lyso-GalCer. In contrast, surrounding (-) charges exerted little spectral influence at the level of C4 and C5 of the lyso-GalCer, membrane-inserted, backbone; or at the level of the amino group of dimethylsphingosine. It would appear that packing alterations induced by the phosphatidylserine/sphingoid base association can translate into sizeable spatial constraints in the neighbouring aqueous domain.

Minor influence of sialic acid on conformation of a membrane-bound oligosaccharide recognition site

Wideline 2H NMR spectroscopy was used to assess the conformational and orientational effects of N-acetylneuraminic acid (NeuAc) (sialic acid) as a component of a particular oligosaccharide chain at a bilayer membrane surface. For this purpose, three glycosphingolipids, sharing a neutral core tetrasaccharide and differing only in the number of sialic acid residues, were compared. The starting compound was GD1A, which has terminal sialic acid attached to the second and fourth sugars of its neutral tetrasaccharide core. GD1A was probe-labeled in a non-perturbing fashion on both of these sialic acid residues and on its single GalNAc residue by replacement of -COCH3 with -COCD3 giving [(d3NeuAc)2,d3-GalNAc]GA1a. This represents the most complex glycolipid to have been studied by 2H NMR spectroscopy at a bilayer membrane surface. The sialic acid residue on the fourth sugar from the membrane was subsequently removed to produce the glycolipid [d3NeuAc,d3GalNAc]GM1, deuterated at the two remaining amino sugars. The neutral glycolipid [d3GalNAc]asialo-GM1 was then generated by removal of the second sialic acid residue, leaving an uncharged species deuterated at one (internal) oligosaccharide chain site (GalNAc). The effect of sialic acid was futher examined by selective deuteration of GM1 and asialo-GM1 at C6 of the terminal Gal residue, giving [d2Gal]GM1 and [d2Gal]asialo-GM1. Spectra of the three glycosphingolipids were compared at 7.7 mol % in unsoncicated fluid bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine containing 23 mol % cholesterol. For liposomes suspend in buffered salt solutions with 2 mM Ca2+, 2H NMR spectra demonstrated the presence of well defined average conformation for each oligosaccharide chain. This preferred average conformation persisted over a wide temperature range, consistent with there being a single major oligosaccharide conformer in each case. Spectral features arising from both deuterated amino sugar (GalNAc) of asialo-GM1 could be identified, little changed, in spectra of GM1 and GD1A. Similarly, deuterons in the terminal Gal residue of asialo-Gm1 produced the same spectrum seen for this residue in GM1. Our findings indicate that certain major conformational and orientational features of this complex oligosaccharide recognition site are preserved, within maximum angular deviation + or -5 degrees or less upon addition or removal of a sialic acid residue.

Glucosylceramides of oat root plasma membranes--physicochemical behaviour in natural and in model systems

Glucosylceramides from oat root plasma membranes have been characterized using HPLC-particle beam-mass spectrometry, differential scanning calorimetry, low angle X-ray diffraction and surface balance technique. 24:1-OH was dominating fatty acid (90%) together with 24:0-OH and 22:1-OH. The sphingosine base was sphingadienine isomers and the monosaccharide alpha and beta glucose. Differential scanning calorimetry of an aqueous dispersion of glucosylceramide revealed during heating an endothermic gel-liquid crystalline transition with double peaks at 47 degrees and 51 degrees C, the lowest known for naturally occurring glucosylceramides. A cooling scan after the endothermic gel-liquid transition showed one exotherm at 15 degrees C and if this was followed by another heating scan a large exotherm appeared with a peak at 18 degrees C. During the second heating the matrix was hydrated and the exotherm at 18 degrees C reflects then the transition between the supercooled metastable gel phase and the corresponding hydrated form. The calorimetric data indicate a lamellar phase which during the cooling scan appeared as an supercooled liquid crystalline phase. Low angle X-ray diffraction confirmed these calorimetric data. The surface pressure-area-curves of pure oat glucosylceramides were more expanded than those of bovine origin. Mixtures of oat glucosylceramides and phosphatidylcholine species similar to those present in oat root plasma membranes showed molecular miscibility but no interaction.

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.

Self-assembled microstructures from 1,2-ethanediol suspensions of pure and binary mixtures of neutral and acidic biological galactosylceramides

Optical and electron microscopy were employed to characterize microstructures formed by thermal mechanical treatment of glycol suspensions of various pure and binary mixtures of the brain-derived galactosphingolipids hydroxy fatty acid cerebroside (HFA-Cer), non-hydroxy fatty acid cerebroside (NFA-Cer) and sulfatide (S-Cer). Negative staining indicated some new features of the neutral cerebroside suspensions in glycol. HFA-Cer formed a small fraction of both unilamellar cylinders (ULCs) (lumina ca. 27 nm) and giant multilamellar cochleates in addition to the typical nonhelical multilamellar cylinders (MLCs) (lumina ca. 10-30 nm). NFA-Cer formed a gel composed of a significant fraction of very long ULCs (lumina ca. 17 nm) without helical substructure, in addition to multilamellar helical structures such as ribbons and cylinders (lumina ca. 70 nm). Anisotropic lamellar micelle-shards of NFA-Cer were also detected by negative staining. S-Cer formed short ULCs (lumina ca. 44 nm) with no obvious helical substructure. Complex mixture data are thought to result from thermodynamic and kinetic factors. HFA-Cer is highly insoluble and promotes a network of rigid intralamellar hydrogen bonding that tends to exclude other lipids. NFA-Cer stabilizes helical defects in the lamellae, and S-Cer enhances disorder or micellization. The processes of microstructure nucleation and lipid phase separation were affected by mixtures such that metastable microstructures were trapped or the length of lamellar cylinders was altered.

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.

Thermotropic properties of dispersions of cholesterol with tetraether lipids from Thermoplasma acidophilum

The main glycophospholipid from Thermoplasma acidophilum is composed of a diisopranol-2,3-glycerotetraether. The fraction of pentane cyclizations of its hydrocarbon chains increases with the growth temperature of the source organism (39-59 degrees C). Hydrated mixtures of these lipids together with cholesterol have been studied by calorimetry. With the reduction of the phase transition temperatures and enthalpy changes of the transitions, cholesterol is readily incorporated into lipid monolayers in the liquid-crystalline and the (metastable) solid-analogue phase. Lipid samples with a high number of acyclic hydrocarbon chains form a stable and a metastable solid-analogue phase. With the increasing concentration of cholesterol the metastable solid-analogue phase is stabilized and the time constant for the formation of the stable solid-analogue phase is prolonged.

Characterization of glucosylceramide from plasma membranes of plant root cells

Plasma membranes of oat root cells were isolated from intracellular membranes by subfractionation of the microsomal fraction using an aqueous polymer two-phase system. The plasma membranes originated from oat plants which were acclimated to dehydration by exposure to a repeated water-deficit stress program. Glucosylceramides was a major component of the plasma membrane lipids and amounted to 9% of the lipid of control plants and 5% of the lipid of acclimated plants. Structural analysis using FAB-MS showed only one type of glucosylceramides. The constituent monosaccharide was exclusively glucose and the sphingosine base was 4,8-sphingadienine. The fatty acid composition was determined to 24:1-OH, with only trace levels of non-hydroxy acids. The decrease in the level of glucosylceramides during acclimation to dehydration was accompanied by a corresponding decrease in phospholipids and increase in free sterols.

Diacetylenic lipid microstructures: structural characterization by X-ray diffraction and comparison with the saturated phosphatidylcholine analogue

Thermotropic and lyotropic mesomorphism in the polymerizable lecithin 1,2-ditricosa-10,12-diynoyl-sn-glycero-3-phosphocholine and its saturated analogue, 1,2-ditricosanoyl-sn-glycero-3-phosphocholine, has been investigated by wide- and low-angle X-ray diffraction of both powder and oriented samples and by differential scanning calorimetry. Previous studies have shown that the hydrated diacetylenic lipid forms novel microstructures (tubules and stacked bilayer sheets) in its low-temperature phase. The diffraction results indicate that at low temperatures fully hydrated tubules and sheets have an identical lamellar repeat size (d001 = 66.4 A) and crystalline-like packing of the acyl chains. Chain packing in the lamellar crystalline phase is hydration independent. A model for the polymerizable lecithin with (1) fully extended all-trans methylene segments, (2) a long-axis tilt of 32 degrees, and (3) minimal chain interdigitation seems most reasonable on energetic grounds, is consistent with the diffraction data (to 3.93-A resolution), and is likely to support facile polymerization. Above the chain "melting" transition the lamellar repeat of the polymerizable lipid increases to 74 A. The conformational similarity between tubules, sheets, and the dry powder is corroborated by calorimetry, which reveals a cooling exotherm at the same temperature where tubules form upon cooling hydrated sheets. The data suggest that although a high degree of conformational order is a pertinent feature of tubules, this character alone is not sufficient to account for tubule formation. The conformation of the corresponding saturated phosphatidylcholine appears to be similar to that of other saturated phosphatidylcholines in the lamellar gel phase. Furthermore, above the main transition temperature, the dry, saturated lipid shows evidence of a P delta phase (112 degrees C), whereas the diacetylenic lipid appears to exhibit a centered rectangular phase, R alpha (55 degrees C).

Bilayer-stabilizing properties of myelin basic protein in dioleoylphosphatidylethanolamine systems

31P-NMR and X-ray diffraction techniques are used to study the comparative ability of myelin basic protein (MBP) vs. other basic proteins to convert hexagonal (HII) phases to stable lamellar (L alpha) structures. Pure dioleoylphosphatidylethanolamine (DOPE) at pH 9 and 7, and mixtures of DOPE/phosphatidylserine (PS) (95:5 and 80:20% w/w) at pH 7 were employed for this investigation. The polymorphic behavior of the lipid suspensions was evaluated in the presence and absence of several basic proteins (MBP, calf thymus histone, lysozyme, melittin) and the cationic polypeptide, polylysine (PL). Each of the proteins and PL was capable of binding the pure DOPE HII phase at pH 9 but with varying morphological consequences, i.e., lamellar stabilization (MBP, histone, PL), formation of new protein-DOPE HII phases (lysozyme) or lipid disordering/vesiculation (melittin). Reduction to pH 7 resulted in the dissociation of protein from DOPE - with the exception of melittin - and the reformation of a pure lipid HII phase. Additions of PS to DOPE at pH 7 facilitated protein binding, but among the proteins examined, only MBP was capable of converting the lipid suspension into a stable multilamellar form. Differences in the lipid morphology produced by each protein are discussed in terms of protein physicochemical characteristics. In addition, a possible relationship between MBP-lipid interactions and the stability of myelin sheath lipid multilayers is inferred from the significant bilayer-stabilizing capacity of MBP.

Freeze/thaw-induced destabilization of the plasma membrane and the effects of cold acclimation

Disruption of the plasma membrane is a primary cause of freezing injury. In this review, the mechanisms of injury resulting from freeze-induced cell dehydration are presented, including destabilization of the plasma membrane resulting from (a) freeze/thaw-induced osmotic excursions and (b) lyotropic phase transitions in the plasma membrane lipids. Cold acclimation dramatically alters the behavior of the plasma membrane during a freeze/thaw cycle--increasing the tolerance to osmotic excursions and decreasing the propensity for dehydration-induced lamellar to hexagonal-II phase transitions. Evidence for a casual relationship between the increased cryostability of the plasma membrane and alterations in the lipid composition is reviewed.

Myelin proteins in reverse micelles: tight lipid association required for insertion of the Folch-Pi proteolipid into a membrane-mimetic system

The solubility and reactivity of the Folch-Pi proteolipid from bovine CNS have been studied in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate, isooctane, and water. Such a membrane-mimetic system resembles the aqueous spaces of the native myelin sheath in terms of its physicochemical properties. Although the proteolipid is completely insoluble in water, it can be inserted into the water-containing micellar system. In contrast, the lipid-depleted protein failed to be incorporated into these organized assemblies. The lipid requirements for insertion of the proteolipid were studied, therefore, after delipidation by several precipitations with isooctane, a nondenaturing solvent. Novel extraction procedures and quantitative analyses by HPLC of the protein-bound lipids revealed the persistence of a lipid-protein complex (6 +/- 1 mol of lipid/mol of protein) displaying optimal micellar solubilization. Competition experiments carried out with brain lipids provide evidence for a preference of the myelin protein for sulfatide, phosphatidylinositol, and phosphatidylserine, in that order. The resulting proteolipid, although differing in relative composition, showed good solubility in the membrane-mimetic system. In contrast, reconstitution experiments carried out with the lipid-depleted protein resulted in weak lipid binding and poor micellar incorporation. These results suggest that the tightly bound acidic lipids may stabilize a protein conformation required for insertion into the micellar system.

Thermodynamic-geometric correlations for the morphology of self-assembled structures of glycosphingolipids and their mixtures with dipalmitoylphosphatidylcholine

The morphology of aqueous dispersions of five neutral glycosphingolipids (GalCer, GlcCer, LacCer, asialo-GM2, asialo-GM1), sulfatide, and five gangliosides (GM3, GM2, GM1, GD1a and GT1b) and their mixtures with dipalmitoylphosphatidylcholine was studied by negative staining electron microscopy. The morphological features are interpreted on the basis of thermodynamic and geometric constraints previously studied in these systems (Maggio, B (1985) Biochim. Biophys. Acta 815, 245-258). The correlation between the theoretical predictions and the experimental findings are in reasonable agreement. Small changes in the molecular parameters of the individual glycosphingolipids or in their proportion in mixtures with dipalmitoylphosphatidylcholine bring about remarkable variations on the type of structure formed, its radius of curvature and thermodynamic stability.

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)

Deuterium NMR studies of the interactions of polyhydroxyl compounds and of glycolipids with lipid model membranes

The physical properties of bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in the presence of four water-soluble polyhydroxyl compounds, trehalose, sorbitol, glycerol, and ethyleneglycol, and three neutral glycolipids - monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and nonhydroxy fattyacyl-cerebrosides (NHFA-Cer) - were investigated using 2H-NMR. All four polyhydroxyl compounds induced small, but comparable concentration-dependent changes in the choline headgroup conformation which were consistent with the presence of a small negative charge being conferred upon the bilayer surface. The latter may be explained by dipolar interactions brought about by changes in the long-range order of the water layer at the membrane surface. Trehalose had a small ordering effect on the hydrophobic interior of the membrane while ethyleneglycol induced a disordering, at both the head group level and in the hydrophobic interior. The presence of high amounts of carbohydrate at the membrane surface was ensured when POPC was mixed with various proportions of one of three glycolipids, MGDG, DGDG and NHFA-Cer. In these cases the conformation of the choline headgroup was only marginally altered when not masked by macroscopic phase changes. The headgroup conformational changes observed in the presence of any of the above-mentioned compounds were modest in comparison to the effects induced by charged substances.

Phase behavior of cerebroside and its fractions with phosphatidylcholines: calorimetric studies

Bovine brain cerebroside and its kerasin (beta-D-galactosyl-N-acyl-D-sphingosine) and phrenosin (beta-D-galactosyl-N-(2-D-hydroxyacyl)-D-sphingosine) fractions were mixed with diacylphosphatidylcholines (PCs) to form fully hydrated lamellar phases. These mixtures were examined by differential scanning calorimetry, and phase diagrams for cerebroside/diacylPC mixtures were constructed from the data. Cerebroside was found to be miscible with egg PC at low mole fractions X of cerebroside; the mixture behaves non-ideally for X greater than 0.25. The non-ideal behavior appears to be a superposition of separate interactions of kerasin and phrenosin with egg PC. Strikingly, phrenosin mixes nearly ideally with egg PC. Kerasin mixed with egg PC yields a peritectic phase diagram. Cerebroside and phrenosin were found to be immiscible with dimyristoylphosphatidylcholine (DMPC) in the gel state in low proportions. Both stable and metastable gel phases of kerasin were detected in different endotherms of kerasin/PC mixtures. Kerasin in the stable and metastable gel states exhibits discontinuous and continuous ranges of miscibility, respectively, with DMPC. The stable gel phase of kerasin does not segregate in natural cerebroside. Natural kerasin was found to act isomorphic to semi-synthetic (natural configuration) D-kerasins but not completely to synthetic DL-kerasins of single acyl chain lengths.

Surface topography of sulfatide and gangliosides in unilamellar vesicles of dipalmitoylphosphatidylcholine

The property of the dyes, acridine orange and methylene blue, to exhibit metachromatic changes upon binding to negatively charged groups that are within a defined spatial separation was employed to study the lateral and transverse topography of sulfatide and gangliosides GM1 and GD1a mixed with dipalmitoylphosphatidylcholine (DPPC) in unilamellar vesicles. The spectral changes of the dyes in the presence of liposomes containing anionic glycosphingolipids (GSLs) (hypochromism and frequency shift) are typical of polyanionic lattices while minor changes are found for neutral lipids. The metachromatic changes are abolished by the presence of Ca2+ in the external medium. The proportion of anionic GSLs accessible to the dyes on the external surface of the liposomes is greater as the GSLs are more complex (sulfatide less than GM1 less than GD1a) and as its proportion in the mixture decreases. The number of molecules of anionic GSLs that are laterally distributed on the external surface in a position favorable for the formation of dye dimers (at intermolecular distances not exceeding 1 nm) is greater for sulfatide than for ganglioside. This is correlated to the greater intermolecular distances and delocalization in ganglioside-, compared to sulfatide-containing interfaces. The experimental values indicate that the mixture with DPPC of any of the anionic GSLs studied behaves as if it was more enriched in the GSLs compared to the proportions of the whole mixture.