Laser-light scattering investigation of the micellar properties of gangliosides

Differential effects of ganglioside lipids on the conformation and aggregation of islet amyloid polypeptide

Despite causing over 1 million deaths annually, Type 2 Diabetes (T2D) currently has no curative treatments. Aggregation of the islet amyloid polypeptide (hIAPP) into amyloid plaques plays an important role in the pathophysiology of T2D and thus presents a target for therapeutic intervention. The mechanism by which hIAPP aggregates contribute to the development of T2D is unclear, but it is proposed to involve disruption of cellular membranes. However, nearly all research on hIAPP-lipid interactions has focused on anionic phospholipids, which are primarily present in the cytosolic face of plasma membranes. We seek here to characterize the effects of three gangliosides, the dominant anionic lipids in the outer leaflet of the plasma membrane, on the aggregation, structure, and toxicity of hIAPP. Our results show a dual behavior that depends on the molar ratio between the gangliosides and hIAPP. For each ganglioside, a low-lipid:peptide ratio enhances hIAPP aggregation and alters the morphology of hIAPP fibrils, while a high ratio eliminates aggregation and stabilizes an α-helix-rich hIAPP conformation. A more negative lipid charge more efficiently promotes aggregation, and a larger lipid headgroup improves inhibition of aggregation. hIAPP also alters the phase transitions of the lipids, favoring spherical micelles over larger tubular micelles. We discuss our results in the context of the available lipid surface area for hIAPP binding and speculate on a role for gangliosides in facilitating toxic hIAPP aggregation.

Ganglioside Micelles Affect Amyloid β Aggregation by Coassembly

Amyloid β peptide (Aβ) is the crucial protein component of extracellular plaques in Alzheimer's disease. The plaques also contain gangliosides lipids, which are abundant in membranes of neuronal cells and in cell-derived vesicles and exosomes. When present at concentrations above its critical micelle concentration (cmc), gangliosides can occur as mixed micelles. Here, we study the coassembly of the ganglioside GM1 and the Aβ peptides Aβ40 and 42 by means of microfluidic diffusional sizing, confocal microscopy, and cryogenic transmission electron microscopy. We also study the effects of lipid-peptide interactions on the amyloid aggregation process by fluorescence spectroscopy. Our results reveal coassembly of GM1 lipids with both Aβ monomers and Aβ fibrils. The results of the nonseeded kinetics experiments show that Aβ40 aggregation is delayed with increasing GM1 concentration, while that of Aβ42 is accelerated. In seeded aggregation reactions, the addition of GM1 leads to a retardation of the aggregation process of both peptides. Thus, while the effect on nucleation differs between the two peptides, GM1 may inhibit the elongation of both types of fibrils. These results shed light on glycolipid-peptide interactions that may play an important role in Alzheimer's pathology.

GM1 structural requirements to mediate neuronal functions

Since the 1980s, it has been known that the administration of ganglioside GM1 to cultured cells induced or enhanced neuronal differentiation. GM1 mechanism of action relies on its direct interaction and subsequent activation of the membrane tyrosine kinase receptor, TrkA, which naturally serves as NGF receptor. This process is mediated by the sole oligosaccharide portion of GM1, the pentasaccharide β-Gal-(1-3)-β-GalNAc-(1-4)-[α-Neu5Ac-(2-3)]-β-Gal-(1-4)-β-Glc. Here we detailed the minimum structural requirements of the oligosaccharide portion of GM1 for mediating the TrkA dependent neuritogenic processing. By in vitro and in silico biochemical approaches, we demonstrated that the minimal portion of GM1 required for the TrkA activation is the inner core of the ganglioside's oligosaccharide β-Gal-(1-3)-β-GalNAc-(1-4)-[α-Neu5Ac-(2-3)]-β-Gal. The addition of a sialic acid residue at position 3 of the outer galactose of the GM1 oligosaccharide, which forms the oligosaccharide of GD1a, prevented the interaction with TrkA and the resulting neuritogenesis. On the contrary, the addition of a fucose residue at position 2 of the outer galactose, forming the Fucosyl-GM1 oligosaccharide, did not prevent the TrkA-mediated neuritogenesis.

Novel insights on GM1 and Parkinson's disease: A critical review

GM1 is a crucial component of neuronal membrane residing both in the soma and nerve terminals. As reported in Parkinson’s disease patients, the reduction of GM1 determines the failure of fundamental functional processes leading to cumulative cell distress up to neuron death. This review reports on the role of GM1 in the pathogenesis of the disease, illustrating the current data available but also hypotheses on the additional mechanisms in which GM1 could be involved and which require further study. In the manuscript we discuss these points trying to explain the role of diminished content of brain GM1, particularly in the nigro-striatal system, in Parkinson’s disease etiology and progression.

Turning the spotlight on the oligosaccharide chain of GM1 ganglioside

It is well over a century that glycosphingolipids are matter of interest in different fields of research. The hydrophilic oligosaccharide and the lipid moiety, the ceramide, both or separately have been considered in different moments as the crucial portion of the molecule, responsible for the role played by the glycosphingolipids associated to the plasma-membranes or to any other subcellular fraction. Glycosphingolipids are a family of compounds characterized by thousands of structures differing in both the oligosaccharide and the ceramide moieties, but among them, the nervous system monosialylated glycosphingolipid GM1, belonging to the group of gangliosides, has gained particular attention by a multitude of Scientists. In recent years, a series of studies have been conducted on the functional roles played by the hydrophilic part of GM1, its oligosaccharide, that we have named “OligoGM1”. These studies allowed to shed new light on the mechanisms underlying the properties of GM1 defining the role of the OligoGM1 in determining precise interactions with membrane proteins instrumental for the neuronal functions, leaving to the ceramide the role of correctly positioning the GM1 in the membrane crucial for the oligosaccharide-protein interactions. In this review we aim to report the recent studies on the cascade of events modulated by OligoGM1, as the bioactive portion of GM1, to support neuronal differentiation and trophism together with preclinical studies on its potential to modify the progression of Parkinson’s disease.

GM1 Ganglioside Is A Key Factor in Maintaining the Mammalian Neuronal Functions Avoiding Neurodegeneration

Many species of ganglioside GM1, differing for the sialic acid and ceramide content, have been characterized and their physico-chemical properties have been studied in detail since 1963. Scientists were immediately attracted to the GM1 molecule and have carried on an ever-increasing number of studies to understand its binding properties and its neurotrophic and neuroprotective role. GM1 displays a well balanced amphiphilic behavior that allows to establish strong both hydrophobic and hydrophilic interactions. The peculiar structure of GM1 reduces the fluidity of the plasma membrane which implies a retention and enrichment of the ganglioside in specific membrane domains called lipid rafts. The dynamism of the GM1 oligosaccharide head allows it to assume different conformations and, in this way, to interact through hydrogen or ionic bonds with a wide range of membrane receptors as well as with extracellular ligands. After more than 60 years of studies, it is a milestone that GM1 is one of the main actors in determining the neuronal functions that allows humans to have an intellectual life. The progressive reduction of its biosynthesis along the lifespan is being considered as one of the causes underlying neuronal loss in aged people and severe neuronal decline in neurodegenerative diseases. In this review, we report on the main knowledge on ganglioside GM1, with an emphasis on the recent discoveries about its bioactive component.

Self-Assembly in Ganglioside‒Phospholipid Systems: The Co-Existence of Vesicles, Micelles, and Discs

Ganglioside lipids have been associated with several physiological processes, including cell signaling. They have also been associated with amyloid aggregation in Parkinson’s and Alzheimer’s disease. In biological systems, gangliosides are present in a mix with other lipid species, and the structure and properties of these mixtures strongly depend on the proportions of the different components. Here, we study self-assembly in model mixtures composed of ganglioside GM1 and a zwitterionic phospholipid, 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC). We characterize the structure and molecular dynamics using a range of complementary techniques, including cryo-TEM, polarization transfer solid state NMR, diffusion NMR, small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and calorimetry. The main findings are: (1) The lipid acyl chains are more rigid in mixtures containing both lipid species compared to systems that only contain one of the lipids. (2) The system containing DOPC with 10 mol % GM1 contains both vesicles and micelles. (3) At higher GM1 concentrations, the sample is more heterogenous and also contains small disc-like or rod-like structures. Such a co-existence of structures can have a strong impact on the overall properties of the lipid system, including transport, solubilization, and partitioning, which can be crucial to the understanding of the role of gangliosides in biological systems.

PIP2 Reshapes Membranes through Asymmetric Desorption

Phosphatidylinositol-4,5-bisphosphate (PIP2) is an important signaling lipid in eukaryotic cell plasma membranes, playing an essential role in diverse cellular processes. The headgroup of PIP2 is highly negatively charged, and this lipid displays a high critical micellar concentration compared to housekeeping phospholipid analogs. Given the crucial role of PIP2, it is imperative to study its localization, interaction with proteins, and membrane-shaping properties. Biomimetic membranes have served extensively to elucidate structural and functional aspects of cell membranes including protein-lipid and lipid-lipid interactions, as well as membrane mechanics. Incorporation of PIP2 into biomimetic membranes, however, has at times resulted in discrepant findings described in the literature. With the goal to elucidate the mechanical consequences of PIP2 incorporation, we studied the desorption of PIP2 from biomimetic giant unilamellar vesicles by means of a fluorescent marker. A decrease in fluorescence intensity with the age of the vesicles suggested that PIP2 lipids were being desorbed from the outer leaflet of the membrane. To evaluate whether this desorption was asymmetric, the vesicles were systematically diluted. This resulted in an increase in the number of internally tubulated vesicles within minutes after dilution, suggesting that the desorption was asymmetric and also generated membrane curvature. By means of a saturated chain homolog of PIP2, we showed that the fast desorption of PIP2 is facilitated by presence of an arachidonic lipid tail and is possibly due to its oxidation. Through measurements of the pulling force of membrane tethers, we quantified the effect of this asymmetric desorption on the spontaneous membrane curvature. Furthermore, we found that the spontaneous curvature could be modulated by externally increasing the concentration of PIP2 micelles. Given that the local concentration of PIP2 in biological membranes is variable, spontaneous curvature generated by PIP2 may affect the formation of highly curved structures that can serve as initiators for signaling events.

GM1 Softens POPC Membranes and Induces the Formation of Micron-Sized Domains

The influence of the glycolipid GM1 on the physical properties of POPC membranes was studied systematically by using different methods applied to giant and large unilamellar vesicles. The charge per GM1 molecule in the membrane was estimated from electrophoretic mobility measurements. Optical microscopy and differential scanning calorimetry were employed to construct a partial phase diagram of the GM1/POPC system. At room temperature, phase separation in the membrane was detected for GM1 fractions at and above ∼5 mol %, whereby GM1-rich gel-like domains were observed by fluorescent microscopy. Fluctuation analysis, vesicle electrodeformation, and micropipette aspiration were used to assess the bending rigidity of the membrane as a function of GM1 content. In the fluid phase, GM1 was shown to strongly soften the bilayer. In the region of coexistence of fluid and gel-like domains, the micropipette aspiration technique allowed measurements of the bending rigidity of the fluid phase only, whereas electrodeformation and fluctuation analysis were affected by the presence of the gel-phase domains. The observation that GM1 decreased the bilayer bending rigidity is important for understanding the role of this ganglioside in the flexibility of neuronal membranes.

Chemical properties of lipids strongly affect the kinetics of the membrane-induced aggregation of α-synuclein

Intracellular α-synuclein deposits, known as Lewy bodies, have been linked to a range of neurodegenerative disorders, including Parkinson's disease. α-Synuclein binds to synthetic and biological lipids, and this interaction has been shown to play a crucial role for both α-synuclein's native function, including synaptic plasticity, and the initiation of its aggregation. Here, we describe the interplay between the lipid properties and the lipid binding and aggregation propensity of α-synuclein. In particular, we have observed that the binding of α-synuclein to model membranes is much stronger when the latter is in the fluid rather than the gel phase, and that this binding induces a segregation of the lipids into protein-poor and protein-rich populations. In addition, α-synuclein was found to aggregate at detectable rates only when interacting with membranes composed of the most soluble lipids investigated here. Overall, our results show that the chemical properties of lipids determine whether or not the lipids can trigger the aggregation of α-synuclein, thus affecting the balance between functional and aberrant behavior of the protein.

Inhibition of influenza-virus-induced cytopathy by sialylglycoconjugates

The anti-viral activity of gangliosides such as SPG (sialylparagloboside), GD1a, GM3, and GM4 was assessed by inhibition of the cytopathy of MDCK cells due to infection with the influenza virus A/PR/8/34. The inhibitory effect was in the following sequence: SPG>GD1a>GM3>GM4. The IC50 of SPG and GD1a was 7 and 70 microM, respectively, indicating that they are more effective than the representative inhibitor amantadine. Although 3'-sialyllactose (3'-SL) and 3'-sialyllactosamine (3'-SLN), which are identical to the terminal trisaccharides of GM3 and SPG, respectively, did not show any inhibitory effect, introduction of an amino group to the reducing end of 3'-SL following amidation with lauroyl chloride gave the inhibitory potency, which was comparable to that of GM3. These results suggest that the viral hemagglutinin recognizes exogenous sialyloligosaccharides rather than inherent sialyloligosaccharides expressed on MDCK cells, since introduction of the hydrophobic moiety to oligosaccharides might cause micelle formation.

Point mutational analysis of the liganding site in human glycolipid transfer protein. Functionality of the complex

Mammalian glycolipid transfer proteins (GLTPs) facilitate the selective transfer of glycolipids between lipid vesicles in vitro. Recent structural determinations of the apo- and glycolipid-liganded forms of human GLTP have provided the first insights into the molecular architecture of the protein and its glycolipid binding site (Malinina, L., Malakhova, M. L., Brown, R. E., and Patel, D. J. (2004) Nature 430, 1048-1053). In the present study, we have evaluated the functional consequences of point mutation of the glycolipid liganding site of human GLTP within the context of a carrier-based mechanism of glycolipid intermembrane transfer. Different approaches were developed to rapidly and efficiently assess the uptake and release of glycolipid by GLTP. They included the use of glass-immobilized, glycolipid films to load GLTP with glycolipid and separation of GLTP/glycolipid complexes from vesicles containing glycolipid (galactosylceramide or lactosylceramide) or from monosialoganglioside dispersions by employing nickel-nitrilotriacetic acid-based affinity or gel filtration strategies. Point mutants of the sugar headgroup recognition center (Trp-96, Asp-48, Asn-52) and of the ceramide-accommodating hydrophobic tunnel (Phe-148, Phe-183, Leu-136) were analyzed for their ability to acquire and release glycolipid ligand. Two manifestations of point mutation within the liganding site were apparent: (i) impaired formation of the GLTP/glycolipid complex; (ii) impaired acquisition and release of bound glycolipid by GLTP. The results are consistent with a carrier-based mode of GLTP action to accomplish the intermembrane transfer of glycolipid. Also noteworthy was the inefficient release of glycolipid by wtGLTP into phosphatidylcholine acceptor vesicles, raising the possibility of a function other than intermembrane glycolipid transfer in vivo.

Cooperative behavior of ganglioside molecules in model systems

A concise discussion of the role of different geometrical conformational states in the process of self-assembling of gangliosides is given. The report focuses on the effects of the geometrical variations occurring in the head group region of gangliosides as reflected on the geometrical properties of the whole assembly. Collective phenomena happening at the water interfacial region are found to be coupled to the phase transition of the lipid moiety, that is, to the well-known order-disorder conformational transition involving the hydrophobic tails. The possible biological relevance of the head group bistability is envisaged.

Uptake and metabolism of exogenous glycosphingolipids by cultured cells

Exogenous glycosphingolipids, especially gangliosides, are used to study transport and metabolism of their endogenous counterparts as well as their role in cell adhesion, cell recognition and signal transduction. Unlike monodispersed solutes, in aqueous media ganglioside molecules aggregate into micelles (or bilayer structures) with a very low critical micellar concentration. Upon addition to cells in culture, exogenous gangliosides bind to the cell surface in three operationally defined modes: loosely associated micelles removable by serum; tightly attached micelles removable by proteases such as trypsin; and ganglioside molecules inserted into the outer leaflet of the plasma membrane. As shown by a biotin-labeled derivative of the ganglioside GM1 these inserted molecules are endocytosed and transported to intralysosomal membranes for catabolism. The benefit from using (partially) nondegradable as well as semi-truncated glycosphingolipids in transport studies is discussed.

Inhibitory activity of sulphoglycolipid derivatives towards pancreatic trypsin

Amphipathic sulpholipids have been shown to inhibit pancreatic serine proteases due to their detergent-like properties. To evaluate the structural requirement for this inhibitory activity, we examined the effects of various derivatives of sulphoglycolipids, some of which were prepared by deacylation with sphingolipid ceramide N-deacylase, followed by acylation with acyl chloride, on the activity of pancreatic trypsin. Both deacylated sulphatides and seminolipids exhibited inhibitory activity towards trypsin without any requirement for solubilisation and preincubation. On the other hand, stronger inhibition was observed for acylated sulphatides than for deacylated ones, but increasing the chain length of the fatty acid moiety resulted in the need for a solubilisation agent and preincubation in order to achieve maximal inhibitory activity. The structural isomers of sulphoglycolipids, such as I(6)SO(3)-GalCer, and phytosphingosine- and diglyceride-containing sulphoglycolipids, showed similar inhibitory activity, indicating the involvement of sulphate and hydrophobic groups, irrespective of the fine structure, in the inhibition. Among the sulphoglycolipids examined, II(3)SO(3)-LacCer was found to exhibit the highest inhibitory activity.

Cryoelectron microscopy of protein-lipid complexes of human myelin basic protein charge isomers differing in degree of citrullination

Myelin basic protein (MBP) is considered to be essential for the maintenance of stability of the myelin sheath. Reduction in cationicity of MBP, especially due to conversion of positively charged arginine residues to uncharged citrulline (Cit), has been found to be associated with multiple sclerosis (MS). Here, the interactions of an anionic phosphatidylserine/monosialoganglioside-G(M1) (4:1, w:w) lipid monolayer with 18.5-kDa MBP preparations from age-matched adult humans without MS (no Cit residues), with chronic MS (6 Cit), and with acute Marburg-type MS (18 Cit) were studied by transmission and ultralow dose scanning transmission electron microscopy under cryogenic conditions. Immunogold labeling and single particle electron crystallography were used to define the nature of the complexes visualized. These electron microscopical analyses showed that the three different MBP charge isomers all formed uniformly sized and regularly shaped protein-lipid complexes with G(M1), probably as hexamers, but exhibited differential association with and organization of the lipid. The least cationic Marburg MBP-Cit(18) formed the most open protein-lipid complex. The data show a disturbance in lipid-MBP interactions at the ultrastructural level that is related to degree of citrullination, and which may be involved in myelin degeneration in multiple sclerosis.

Conformation of the oligosaccharide chain of G(M1) ganglioside in a carbohydrate-enriched surface

The solution structure of ganglioside G(M1) carbohydrate moiety at the surface of a 102-kDa lipid-modified-G(M1) micelle is investigated by high-resolution 1H-NMR in H2O. The micellar surface can be considered a cluster-like lateral distribution of the gangliosides, each single monomer being anchored in a carbohydrate-enriched model membrane matrix. 1H NOESY measurements at short mixing times reveal a rigid trisaccharide core -beta-GalNAc-(1-4)-[alpha-Neu5Ac-(2-3)]-beta-Gal- and a more flexible beta-Gal-(1-3)-beta-GalNAc- terminal glycosidic bond. In the lipid-modified G(M1) ganglioside micellar system, there is no evidence that intermolecular side-by-side carbohydrate interactions modulate, or alter in any way, the head-group spatial arrangement. Possible intermonomer interactions at the level of the branched trisaccharide portion were further investigated on mixed micelles of natural N-glycolyl- and N-acetylneuraminic acid containing G(M1) in D2O, taking advantage of the different NMR features of N-glycolyl- and N-acetylneuraminic acids, which allow discrimination between sialic acid ring proton signals. Measurements of the water/ganglioside-OH proton chemical exchange rates suggest hydroxyl group involvement at position 8 of sialic acid in strong intramolecular interaction processes.

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)

Geometrical and conformational properties of ganglioside GalNAc-GD1a, IV4GalNAcIV3Neu5AcII3Neu5AcGgOse4Cer

The aggregative properties of GalNAc-GD1a ganglioside, in comparison with those of GD1a, have been investigated and correlated to the intrinsic conformation and mobility of the oligosaccharide chain of the molecules. Micellar parameters in aqueous solution (molecular mass, hydrodynamic radius as well as the surface area at the lipid/water interface and the packing parameter of the monomer inserted in the aggregate) are measured by the laser light-scattering technique. The presence of a further GalNAc residue causes a 22% increase in molecular mass, contrary to expectation. Oligosaccharide moiety three-dimensional structures have been modeled using molecular mechanics and dynamics calculations, based on NOE interactions observed for native gangliosides dissolved in deuterated dimethylsulfoxide or, as mixed micelles with fully deuterated dodecylphosphocholine, in D2O. Compared with GD1a the GalNAc-GD1a is less mobile, thus influencing the surface area, this lower mobility together with the GalNAc-GD1a conformation leads to a larger number of monomers participating in the formation of the micelle. The results further substantiate the model in which the three-dimensional structure and the intrinsic dynamic properties of the oligosaccharide chain affect the geometrical properties of the aggregate.

Aggregative properties of gangliosides in solution

The aggregative properties of gangliosides in diluted aqueous solutions are discussed on the basis of simple and well-established thermodynamic concepts. Theoretical assumptions are compared with experimental data obtained, mainly by scattering techniques, on GM3, GM2, GM1, GD1a, GalNAc-GD1a, GD1b, GD1b lactone and GT1b gangliosides, all containing ceramide portions of similar composition, and on GM1 molecular species containing different well-defined ceramide structures. We also report on mixed aggregates with amphiphilic compounds and on the ganglioside aggregate-soluble protein interaction effects which give rise to very stable lipoproteic complexes of well-defined ganglioside-protein composition.

Alteration of the passive electrical properties of lymphocyte membranes induced by GM1 and GM3 glycolipids

The electrical conductivity of normal human lymphocyte suspensions has been measured in the frequency range from 10 kHz to 100 MHz, where a well-pronounced conductivity dispersion occurs, caused by the surface polarization at the interface between the cell membrane and the extracellular solution. We have investigated the alteration of the passive electrical properties of the cytoplasmatic cell membrane induced by two different gangliosides (GM1 and GM3) inserted, at various concentrations, into the outer leaflet of membrane double layer. The alterations observed in the dielectric parameters (the membrane conductivity and the membrane permittivity) derived on the basis of a 'double-shell' model, result in an overall increase of the ion permeation across the membrane and an enhanced polarizability of its hydrophilic region for both gangliosides investigated. The relevance of these alterations is discussed.

The characteristic negative Cotton effect of ganglioside lactones observed by circular dichroism spectrometry

Circular dichroism spectrometry was applied to gangliosides and their lactones and revealed that the lactones have a characteristic strong negative Cotton effect around 235 nm. Four monolactones and two dilactones, which were formed from GM4, GD3, and GD1b, gave molar ellipticities at the wavelength in magnitude of 10(4), while their parent gangliosides, along with other gangliosides such as GM3, GM1, GD1a, GT1b, and GQ1b, showed no distinct feature. Two ganglioside esters, GM4-methyl ester and O-Ac-GT1b did not show the Cotton effect. The molar ellipticities had an additivity with respect to the number of lactone rings. The Cotton effect was attributed to the carbonyl group on the lactone ring.

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.

High-performance capillary electrophoresis of gangliosides

Gangliosides are sialic acid-containing glycosphingolipids. In aqueous media, these glycolipids have been shown to exist as stable micelles. Ganglioside micelles could be analyzed by high-performance zonal capillary electrophoresis in uncoated fused-silica capillaries within 10 min. The mass sensitivity determined by monitoring the absorption of ultraviolet light at 195 nm was in the order of 10(-11) mol. Increasing the pH of the running buffer from 3.0 to 7.4 or the voltage from 10 to 30 kV increased the relative mobilities of gangliosides. By contrast, increasing the ionic strength of the buffer decreased the migration and broadened the elution peak widths of gangliosides. Ganglioside* micelles including GM1, GD1b, and GT1b were resolved into separate peaks by capillary electrophoresis at physiological pH shortly after mixing. Upon prolonged incubation, the ganglioside peaks merged to form a single species. The fusion process was temperature-dependent. At 50 degrees C, formation of mixed micelles between polysialogangliosides GD1b and GT1b was complete within 30 min. In contrast, no fusion of the ganglioside peaks was observed at 0 degrees C even after 75 h. Formation of mixed micelles between GD1b and other polysialogangliosides including GD1a, GT1b, and GQ1b at 37 degrees C required 1.5, 3.0, and 2.0 h, respectively. Formation of mixed micelles between monosialoganglioside GM1 and polysialogangliosides were 6- to 36-fold slower. No fusion was observed between monosialogangliosides GM1 and GM2 after 2 days of incubation. These findings indicate that polysialogangliosides may have higher propensities than monosialoganglioside to form mixed micelles.

Aggregation properties of semisynthetic GM1 ganglioside (II3Neu5AcGgOse4Cer) containing an acetyl group as acyl moiety

The aggregative properties of GM1 ganglioside containing an acetyl group as acyl moiety [GM1(acetyl)] in aqueous solution have been studied by static and dynamic light scattering measurements and surface tension experiments. GM1 (acetyl) spontaneously aggregates as small micelles showing a hydrodynamic radius and molecular weight of 34 A and 102 kDa, respectively, down to a concentration of 2.0 x 10(-5) M.

Interleukin-2 binds to gangliosides in micelles and lipid bilayers

Gangliosides shed from the surface of tumour cells may be involved in tumour-induced immunosuppression. These anionic sialoglycolipids are known to be potent inhibitors of lymphocyte proliferation, and it has been suggested that they interfere with processes mediated by the growth factor interleukin-2 (IL-2). We have thus investigated the interaction of IL-2 with gangliosides in micelles and lipid bilayers. Gel filtration FPLC showed that 125I-IL-2 can bind to micellar gangliosides in aqueous solution, and this interaction was strongly promoted by low concentrations of serum. Binding to ganglioside micelles was specific in that it required a native IL-2 molecule. IL-2 binding remained unchanged in the presence of 40% ethylene glycol, suggesting that it was not due to hydrophobic interactions. Ganglioside oligosaccharides alone were not able to bind to IL-2. Direct binding studies and gel filtration chromatography indicated that both multilamellar liposomes and 100 nm unilamellar vesicles containing gangliosides were able to interact with IL-2. Bilayers of lipid alone showed no binding. The interaction of IL-2 with bilayer gangliosides was highly dependent on the bilayer lipid composition, but appeared independent of lipid phase state. These results suggest that gangliosides may be a physiologically relevant target for IL-2 binding.

Effect of the different supramolecular organization on the uptake and metabolization of exogenous GM1 ganglioside by human fibroblasts

In this report we have investigated the differences in the uptake and metabolization of exogenous GM1 by human fibroblasts, as a function of its supramolecular organization in solution. For this we used a tritium labelled GM1, given alone or inserted in dispersions of phosphatidylcholine (PC) or sulphatide. The addition of fetal calf serum (FCS) to these dispersions was also studied. With respect to GM1 pure micelles, the presence in the medium of a sulphatide/GM1, 10:1 molar ratio, greatly increased the incorporation of GM1-associated radioactivity by the cultured cells. Conversely, the presence of PC dramatically diminished the GM1 incorporation values. The metabolization of exogenous GM1 was favoured by the presence of FCS, regardless of the presence of sulphatide. The obtained data provide useful information on the appropriate procedure for feeding cultured fibroblasts with gangliosides.

Application of scattering techniques in the domain of amphiphiles

Light and neutron scattering techniques are discussed in connection with experiments on micellar solutions of gangliosides--amphiphilic molecules of biological origin. Micellar properties, like molecular weight, hydrodynamic radius and shape, are obtained with precise scattering measurements in conditions of negligible intermicellar interactions. It has also been verified that the GM1 ganglioside micelle does not change molecular weight and shape on addition of salt. Scattering techniques are also shown to be useful in studying intermicellar interactions, like the Coulomb repulsion between GM1 micelles, in the regime fo the long-range interactions obtainable at very low ionic strength.

Aggregation properties of GM3 ganglioside (II3Neu5AcLacCer) in aqueous solutions

The aggregative properties of highly pure GM3 ganglioside in aqueous solution have been studied by static and dynamic laser light scattering measurements and fluorescence spectroscopy experiments performed by the use of a GM3 derivative containing the pyrene group at the end of the fatty acid moiety. GM3 ganglioside spontaneously aggregates as unilamellar vesicles, down to a concentration of 1.25 x 10(-8) M, showing molecular weight and hydrodynamic radius ranging from 15,000 to 30,000 kDa and from 350 to 470 A, respectively. GM3 vesicles are stable with dilution and can be stored at room temperature for some weeks without appreciable change.

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.

Effect of exogenous gangliosides on the lipid composition of chick neurons in culture

When exogenous gangliosides are added to the growth medium of neuronal cell cultures they are inserted into their plasma membranes and are afterwards metabolized in the cytoplasmic interior. The action of exogenous gangliosides brings important morphological and biochemical changes to neurons in culture. The present report shows that the treatment with exogenous gangliosides of a primary culture of chick neurons modified the distribution of fatty acids in phosphatidylinositol (PI), mainly that of arachidonic acid and the fatty acids of the (n - 3) series without affecting the other phospholipids. The composition of neutral lipids did not change but their content was increased up to 2-3-fold depending upon the concentration of gangliosides. The change of the growth medium from one containing fetal calf serum to a chemically defined one reduced dramatically the content of free fatty acids while the addition of gangliosides raised this content to normal levels. The increase in the amount of diacylglycerol (DG) confirmed the finding that gangliosides stimulate phosphoinositide degradation. Finally the fatty acid composition of DG suggests indirectly that this compound might be produced also by degradation of phosphatidylcholine and not only of PI.

Thermodynamic and phase characterization of phosphatidylethanolamine and ganglioside GD1a mixtures

By employing diphenylhexatriene steady-state fluorescence anisotropy, pyrenedecanoic acid excimer formation, and high sensitivity scanning calorimetry we have demonstrated that the liposomes containing phosphatidylethanolamine (PE) and various mole fractions of ganglioside GD1a had a gel-liquid crystalline phase transition between 15 and 25 degrees C. Calorimetric measurements indicated that these phase transitions were broad and centered between 17 and 21 degrees C. The enthalpy change of the transition was linearly dependent on the ganglioside concentration up to 10.0 mol% and plateaued between 11.4-16.2 mol%. The high enthalpy change (37 kcal/mol of GD1a added into the PE bilayer) indicates the existence of PE-GD1a complex structure in the liposomal membrane. It is proposed that semi-fluid domains containing six PE and one ganglioside molecule are present in the PE-GD1a membranes at temperatures above gel-liquid crystalline phase transition. The Sendai virus induced leakage of PE-GD1a liposomes has been investigated by using an entrapped, self-quenching fluorescent dye, calcein. The leakage rate was dependent on the mole fraction of ganglioside GD1a and was maximal at 6.3 mol%. Arrhenius plots of the leakage rates showed breaks in the 20-25 degrees C temperature range, which correspond to the gel-liquid crystalline phase transition of the target liposomes. These data suggest that the rate of Sendai virus-induced leakage can be regulated via fluidity modulation by changing the PE to GD1a ratio at constant temperatures.

Light scattering measurements on gangliosides: dependence of micellar properties on molecular structure and temperature

Static and dynamic laser light scattering measurements on micellar aqueous solutions of gangliosides GM2, GM1, GD1a are reported. The aggregation number, the hydrodynamic radius and the micellar shape depend on the type of ganglioside and the unsaturation degree of the hydrocarbon chains. At a temperature of 25 degrees C the molecular weights of GM2, GM1 and GD1a are 740,000, 470,000 and 418,000 DA respectively. A significant decrease of micellar size with temperature has been found for saturated GM1 in the region 25 degrees-40 degrees C. The strong sensitivity of the micellar parameters to the ganglioside structure is explained by making reference to some simple model which takes into account geometrical packing considerations. By measuring the scattered light intensity at low ionic strength of the solution (0.1-30 mE) it was possible to evaluate the ganglioside micellar charge, 100 electronic units for GM2, 48 for GM1 and 60 for GD1a.