Influence of phospholipid chain length on verotoxin/globotriaosyl ceramide binding in model membranes: comparison of a supported bilayer film and liposomes

Synthesis of an artificial glycoconjugate polymer carrying Pk-antigenic trisaccharide and its potent neutralization activity against Shiga-like toxin

Fluorescence-labeled glycoconjugate polymers carrying carbohydrate segments of a globotriaosyl ceramide (Gb3) were synthesized and subjected to biological assays using Escherichia coli O-157 strains and Shiga-like toxins (Stx-I and Stx-II). For the fluorescence labeling, a new polymerizable fluorescent monomer with a TBMB carbonyl chromophore (Ex. 325 nm, Em. 410 nm) was designed. A glycosyl monomer of the trisaccharide segment of Gb3 was prepared from p-nitrophenyl beta-lactoside and copolymerized with acrylamide and the fluorescent monomer to prepare a fluorescence-labeled glycoconjugate copolymer carrying [alpha-D-galactopyranosyl-(1-->4)-beta-D-galactopyranosyl]-(1-->4)-beta- D-glucopyranoside. The polymer showed potent neutralization activity against Stx-I and also binding activity onto E. coli O-157 strains.

A FTIR spectroscopy evidence of the interactions between wheat germ agglutinin and N-acetylglucosamine residues

Wheat germ agglutinin (WGA), a lectin binding a N-acetyl-D-neuraminic acid (NeuNAc) and/or N-acetyl-D-glucosamine (GlcNAc) group, was studied by Fourier transform infrared (FTIR) spectroscopy. Deconvolution of the FTIR spectrum of WGA alone indicated the presence of few alpha-helices and beta-sheets, in contrast to many other lectins. These results agree with previous WGA crystal data. The WGA conformational changes, induced by GlcNAc-bearing liposomes or GlcNAc oligomers, were studied by infrared differential spectroscopy. The GlcNAc binding to WGA resulted in a decrease of turns and alpha-helices and a concomitant appearance of beta-sheets, inducing more or less peptidic N-H deuteration.

Adamantyl globotriaosyl ceramide: a monovalent soluble mimic which inhibits verotoxin binding to its glycolipid receptor

The globotriaosylceramide (Gb3) verotoxin (VT) interaction is one of several examples of glycolipid receptors where the ceramide (or lipid) free oligosaccharides fail to show the expected binding parameters. We present a novel, yet simple strategy to synthesize monovalent, water soluble glycosphingolipid mimics which retain receptor function. Replacing the fatty acid chain with rigid, three dimensional hydrocarbon frames, such as adamantane, gives a novel class of neohydrocarbon glycoconjugates. Such adamantyl conjugates derived from Gb3 showed significantly enhanced solubility in water compared to natural Gb3. Adamantyl-Gb3 showed a thousand fold enhanced inhibitory activity (IC50 = 1 microM) for VT-Gb3 binding as compared to a lipid free Gb3 oligosaccharide derivative, alphaGal1-4betaGal1-4betaGlc1-O-CH2CH(CH2SO2C 4H9)2 (IC50 > 2 mM). This represents a new approach to the generation of antagonists of glycolipid receptors.

Structure and function of a ganglioside receptor for porcine rotavirus

A ganglioside fraction isolated from pooled intestines from newborn to 4-week-old piglets, which we previously partially characterized and showed to specifically inhibit the binding of porcine rotavirus (OSU strain) to host cells (M. D. Rolsma, H. B. Gelberg, and M. S. Kuhlenschmidt, J. Virol. 68:258-268, 1994), was further purified and found to contain two major monosialogangliosides. Each ganglioside was purified to apparent homogeneity, and their carbohydrate structure was examined by high-pH anion-exchange chromatography coupled with pulsed amperometric detection and fast atom bombardment mass spectroscopy. Both gangliosides possessed a sialyllactose oligosaccharide moiety characteristic of GM3 gangliosides. Compositional analyses indicated that each ganglioside was composed of sialic acid, galactose, glucose, and sphingosine in approximately a 1:1:1:1 molar ratio. Each ganglioside differed, however, in the type of sialic acid residue it contained. An N-glycolylneuraminic acid (NeuGc) moiety was found in the more polar porcine GM3, whereas the less polar GM3 species contained N-acetylneuraminic acid (NeuAc). Both NeuGcGM3 and NeuAcGM3 displayed dose-dependent inhibition of virus binding to host cells. NeuGcGM3 was approximately two to three times more effective than NeuAcGM3 in blocking virus binding. Inhibition of binding occurred with as little as 400 pmol of NeuGcGM3/50 ng of virus (approximately 2 x 10(7) virions) and 2 x 10(6) cells/ml. Fifty percent inhibition of binding was achieved with 0.64 and 1.5 microM NeuGcGM3 and NeuAcGM3, respectively. The free oligosaccharides 3'- and 6'-sialyllactose inhibited binding 50% at millimolar concentrations, which were nearly 1,000 times the concentration of intact gangliosides required for the same degree of inhibition. Direct binding of infectious, triple-layer rotavirus particles, but not noninfectious, double-layered rotavirus particles, to NeuGcGM3 and NeuAcGM3 was demonstrated by using a thin-layer chromatographic overlay assay. NeuGcGM3 and NeuAcGM3 inhibited virus infectivity of MA-104 cells by 50% at concentrations of 3.97 and 9. 84 microM, respectively. NeuGcGM3 (700 nmol/g [dry weight] of intestine) was found to be the predominant enterocyte ganglioside (comprising 75% of the total lipid-bound sialic acid) in neonatal piglets, followed by NeuAcGM3 (200 nmol/g [dry weight] of intestine). NeuGcGM3 and NeuAcGM3 together comprised nearly 100% of the lipid-bound sialic acid in the neonatal intestine, but their quantities rapidly diminished during the first 5 weeks of life. These data support the hypothesis that porcine NeuGcGM3 and NeuAcGM3 are physiologically relevant receptors for porcine rotavirus (OSU strain). Further support for this hypothesis was obtained from virus binding studies using mutant or neuraminidase-treated cell lines. Lec-2 cells, a mutant clone of CHO cells characterized by a 90% reduction in sialyllation of its glycoconjugates, bound less than 5% of the virus compared to control cell binding. In contrast, Lec-1 cells, a mutant CHO clone characterized by a deficiency in glycosylation of N-linked oligosaccharides, still bound rotavirus. Furthermore, exogenous addition of NeuGcGM3 to the Lec-2 mutant cells restored their ability to bind rotavirus in amounts equivalent to that of their parent (CHO) cell line. In the virus-permissive MA-104 cell line, NeuGcGM3 was also able to partially restore rotavirus infectivity in neuraminidase-treated cells. These data suggest that gangliosides play a major role in recognition of host cells by porcine rotavirus (OSU strain).

Ligand-targeted liposomes

Liposomes have gained increased attention as systemic drug delivery vehicles following recent regulatory approvals of several vesicle-formulated drugs. These products have demonstrated improved therapeutic indices over their corresponding conventional drugs by avoiding sensitive tissues and/or increasing delivery to specific targets in vivo. They have achieved these improvements primarily through physical means: (1) by retaining drug within vesicles while in the circulation, thus avoiding or minimizing uptake by sensitive normal tissues; and (2) by selectively extravasating into target tissues, releasing active drug. In order to improve upon these therapies in the future, clinically active liposome delivery systems most likely will need to include site-directed surface ligands to further enhance their selective delivery. This may be crucial for the in vivo transport and delivery of macromolecules, including antisense, oligonucleotide aptamers, and genes, which-unlike most conventional drugs-do not circulate well and often require cellular uptake by fusion, endocytosis, or other processes to reach their active sites. This manuscript reviews technologies applicable to directing liposomes and their contents to selected in vivo targets using surface-bound, site-specific ligands. Presented are the biological barriers to be overcome, criteria for selecting the determinants to be targeted, various targeting ligands and overall delivery system design considerations. Several novel targets as well as novel ligand constructs for site-directed therapy are reviewed and discussed. Systemic liposome therapy, which currently must be administered by the intravenous route, is the principal focus of this analysis.

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.

Two distinct binding sites for globotriaosyl ceramide on verotoxins: identification by molecular modelling and confirmation using deoxy analogues and a new glycolipid receptor for all verotoxins

BACKGROUND

The Escherichia coli verotoxins (VTs) can initiate human vascular disease via the specific recognition of globotriaosyl-ceramide (Gb3) on target endothelial cells. To explore the structural basis for receptor recognition by different VTs we used molecular modelling based on the crystal structure of VT1, mutational data and binding data for deoxy galabiosyl receptors.

RESULTS

We propose a model for the verotoxin 'cleft-site complex' with Gb3. Energy minimizations of Gb3 within the 'cleft site' of verotoxins VT1, VT2, VT2c and VT2e resulted in stable complexes with hydrogen-bonding systems that were in agreement with binding data obtained for mono-deoxy analogues of Gb3. N-deacetylated globoside (aminoGb4), which was found to be a new, efficient receptor for all verotoxins, can be favourably accommodated in the cleft site of the VTs by formation of a salt bridge between the galactosamine and a cluster of aspartates in the site. The model is further extended to explain the binding of globoside by VT2e. Docking data support the possibility of an additional binding site for Gb3 on VT1.

CONCLUSIONS

The proposed models for the complexes of verotoxins with their globoglycolipid receptors are consistent with receptor analogue binding data and explain previously published mutational studies. The results provide a first approach to the design of specific inhibitors of VT-receptor binding.