Oxana Galanina

Structural Basis for the Interaction between Human Milk Oligosaccharides and the Bacterial Lectin Pa-Iil of Pseudomonas Aeruginosa.

One of the mechanisms contributing to the protection by breast-feeding of the newborn against enteric diseases is related to the ability of human milk oligosaccharides to prevent the attachment of pathogenic bacteria to the duodenual epithelium. Indeed, a variety of fucosylated oligosaccharides, specific to human milk, form part of the innate immune system. In the present study, we demonstrate the specific blocking of PA-IIL, a fucose-binding lectin of the human pathogen Pseudomonas aeruginosa, by milk oligosaccharides. Two fucosylated epitopes, Lewis a and 3-fucosyl-lactose (Lewis x glucose analogue) bind to the lectin with dissociation constants of 2.2x10(-7) M and 3.6x10(-7) M respectively. Thermodynamic studies indicate that these interactions are dominated by enthalpy. The entropy contribution is slightly favourable when binding to fucose and to the highest-affinity ligand, Lewis a. The high-resolution X-ray structures of two complexes of PA-IIL with milk oligosaccharides allow the precise determination of the conformation of a trisaccharide and a pentasaccharide. The different types of interaction between the oligosaccharides and the protein involve not only hydrogen bonding, but also calcium- and water-bridged contacts, allowing a rationalization of the thermodynamic data. This study provides important structural information about compounds that could be of general application in new therapeutic strategies against bacterial infections.

High molecular weight neoglycoconjugates for solid phase assays

Adsorption of a carbohydrate on solid phase is the necessary stage of the immunosorbent assay (ELISA) and analogous methods of the study of carbohydrate-protein interaction. Usually physical adsorption on polystyrene requires a high concentration of conjugated carbohydrate and, thus, enormous consumption of it. In this study, we explored two approaches allowing more rational use of oligosaccharide (Glyc). The first of them is based on the covalent immobilization of neoglycoconjugates on the NH2-modified polystyrene; the second one is based on the elevated adherence of high m.w. neoglycoconjugates to polystyrene. Covalent immobilization of polyacrylamide conjugates, Glyc-PAA, provided a possibility to solve the problem, but the nonspecific binding of antibodies in ELISA proved to be unacceptably high. At the same time, the increase of the Glyc-PAA m.w. from 30 kDa to 2,000 kDa allowed a 10–20 fold decrease of its consumption, when using physical adsorption, whereas the assay background remained at the low level. The amount of 2,000 kDa Glyc-PAA that is sufficient for the coating of a standard 96-well plate corresponds to the nanomole level of oligosaccharide, this providing a possibility to use saccharides that are available in a very limited amount when studying the carbohydrate-protein interaction with solid-phase techniques. Published in 2005.

Fluorescent carbohydrate probes for cell lectins.

Fluorescein labeled carbohydrate (Glyc) probes were synthesized as analytical tools for the study of cellular lectins, i.e. SiaLe(x)-PAA-flu, Sia2-PAA-flu, GlcNAc2-PAA-flu, LacNAc-PAA-flu and a number of similar ones, with PAA a soluble polyacrylamide carrier. The binding of SiaLe(x)-PAA-flu was assessed using CHO cells transfected with E-selectin, and the binding of Sia2-PAA-flu was assessed by COS cells transfected with siglec-9. In flow cytometry assays, the fluorescein probes demonstrated a specific binding to the lectin-transfected cells that was inhibited by unlabeled carbohydrate ligands. The intense binding of SiaLe(x)-PAA-3H to the E-selectin transfected cells and the lack of binding to both native and permeabilized control cells lead to the conclusion that the polyacrylamide carrier itself and the spacer arm connecting the carbohydrate moiety with PAA did not contribute anymore to the binding. Tumors were obtained from nude mice by injection of CHO E-selectin or mock transfected cells. The fluorescent SiaLe(x)-PAA-flu probe could bind to the tumor sections from E-selectin positive CHO cells, but not from the control ones. Thus, these probes can be used to reveal specifically the carbohydrate binding sites on cells in culture as well as cells in tissue sections. The use of the confocal spectral imaging technique with Glyc-PAA-flu probes offered the unique possibility to detect lectins in different cells, even when the level of lectin expression was rather low. The confocal mode of spectrum recording provided an analysis of the probe localization with 3D submicron resolution. The spectral analysis (as a constituent part of the confocal spectral imaging technique) enabled interfering signals of the probe and intrinsic cellular fluorescence to be accurately separated, the distribution of the probe to be revealed and its local concentration to be measured.

Monomeric and Multimeric Blockers of Selectins: Comparison of in vitro and in vivo Activity

The potency of the oligosaccharides SiaLex, SiaLea, HSO3Lex, and HSO3Lea, their conjugates with polyacrylamide (PAA, 40 kD), and other monomeric and polymeric selectin inhibitors has been compared with that of the polysaccharide fucoidan. The following assay systems were used: 1) a 96-well assay based either on the use of recombinant E-, P-, and L- selectins or an analogous assay with natural P-selectin isolated from human platelets; 2) a platelet-based P-selectin cell assay; and 3) a rat model of peritoneal inflammation. IC50 values for the neoglycoconjugate SiaLea-PAA were 6, 40, and 85 µM for recombinant E-, P-, and L-selectins, respectively; all monomeric inhibitors were about two orders of magnitude weaker. PAA-conjugates, containing as a ligand tyrosine-O-sulfate (sTyr) in addition to one of the sialylated oligosaccharides, were the most potent synthetic blockers in vitro. Compared with fucoidan, the most potent known P- and L-selectin blocker, the bi-ligand glycoconjugate HSO3Lea-PAA-sTyr displayed similar inhibitory activity in vitro towards L-selectin and about ten times lower activity towards P-selectin. All of the tested synthetic polymers displayed a similar ability to inhibit neutrophil extravasation in the peritonitis model (in vivo) at 10 mg/kg. The data provide evidence that monomeric SiaLex is considerably more effective as a selectin blocker in vivo than in vitro, whereas the opposite is true for fucoidan and the bi-ligand neoglycoconjugate HSO3Lea-PAA-sTyr.

Biotinylated Multivalent Glycoconjugates for Surface Coating

Systematic studying of biological processes driven by multipoint high-cooperative carbohydrate recognition requires application of multivalent carbohydrates as tools. In this regard polyacrylamides with various pendant carbohydrate residues and labels are probably the most well advanced class of carbohydrate multimerics. Here we describe a synthetic approach to polyacrylamide-based glycoconjugates with biotin tag, with special emphasis to development of carbohydrate biosensors and arrays.

Detection of a potential receptor for the H-blood-group antigen on rat colon-carcinoma cells and normal tissues.

Up‐regulation of the synthesis of carbohydrate tumor‐associated antigens terminated by the disaccharide Fucα1‐2Gal is frequent in colon carcinoma and associated with poor prognosis. There is evidence that Fucα1‐2Gal (H‐disaccharide) structures increase cancer‐cell motility and tumorigenicity by as‐yet unknown mechanisms. Using polyacrylamide‐based neoglycoconjugates, we looked for a potential receptor for this disaccharide, and observed that a neoglycoconjugate probe containing the H‐disaccharide could bind rat colon‐carcinoma cells in a dose‐dependent manner, whereas very little binding was evidenced when a probe containing glucose was used. Binding of the H‐disaccharide probe could be inhibited by the free H‐disaccharide as well as by unlabeled neoglycoconjugates containing a terminal H‐disaccharide. The best inhibitor was the H‐type‐1 trisaccharide neoglycoconjugate. Histochemical detection of the potential H‐receptor was performed on rat normal tissues and in situ 1,2‐dimethylhydrazine‐induced colon carcinomas. A strong binding of the H‐disaccharide probe was evidenced on most tumors that could be partly inhibited by the trisaccharide Fucα1‐2Galβ1‐4Glc and by the unlabeled H‐disaccharide neoglycoconjugate, indicating carbohydrate specificity of the binding. Staining of normal colonic mucosa was much weaker. Strong staining was also observed on some normal tissues, such as the spleen or lymph nodes, while others, such as lungs or liver, were negative. Probes containing glucose or the Lewis‐a trisaccharide did not stain tumors or normal tissues. These results provide preliminary evidence for the existence of H‐specific binding sites, the number of which increases in colon carcinoma. Int. J. Cancer 76:136–140, 1998.© 1998 Wiley‐Liss, Inc.

Uncharged P-selectin blockers

The blocking potency of P- and L-selectin was studied for certain small molecule mannosides and their polyacrylamide (PAA, 30 kDa) conjugates in comparison to SiaLex and fucoidan. Two experimental systems were used: (1) solid phase static assay based on recombinant selectins, and (2) P-selectin dependent rat peritoneal inflammation. βMan-SC6H4NO2-p was four times more potent P-selectin inhibitor as compared to SiaLex. Docking of this molecule onto the P-selectin carbohydrate-binding site demonstrated that a nitro group enabled an electrostatic interaction with residue Lys 84, while the phenyl ring and the CH2 at C-6 contacted the CH2 groups of the same Lys residue. In vivo, βMan-SC6H4NO2-p blocked experimental inflammation better than SiaLex, but significantly lower than fucoidan. In vitro Man-polyacrylic acid conjugates appeared to be very potent inhibitors comparable to fucoidan, uncharged Man-PAA proved rather active, comparable to SiaLex-PAA both in vitro, and in vivo, whereas mannan did not display any P-selectin blocking effect. Published in 2004.

2-Aminopyridine—a label for bridging of oligosaccharides HPLC profiling and glycoarray printing

Abstract2-Aminopyridine derivatives of oligosaccharides (OS-AP) were printed onto microchips by two different ways. The first method is based on direct covalent insertion of OS-AP in polyacrylamide gel 3D chip. The second method is based on conversion of OS-AP into more reactive OS-aminoalditol followed by covalent printing onto NHS-activated glass slides. This approach extends the range of saccharides suitable for covalent printing due to availability of commercial OS-AP and easy high-performance liquid chromatography separation of glycoprotein N-chains in form of AP derivatives.

Assembly of P-Selectin Ligands on a Polymeric Template

High-affinity receptor-ligand interactions frequently involve molecular interactions at two distinct sites. A derivatized polyacrylic-based polymer was synthesized to allow substitution with multiple ligands (e.g., L(1) and L(2)) on the backbone. Two-site P-selectin-ligand interactions were first studied with SiaLe(x) (L(1)) and tyrosine sulfate (L(2)) covalently incorporated onto the flexible polymer. In competition assays, a marked synergistic inhibitory effect was observed when the polymer presented both L(1) and L(2) as opposed to either ligand alone. In a second approach, the SiaLe(X) ligand was reduced in complexity so that L(1) was fixed as Le(x) or Le(a), and alternative L(2) groups (to mimic sialic acid) were investigated. Certain combinations of L(1) and L(2) were better antagonists of P-selectin than SiaLe(x) itself. These approaches offer the potential of facilitating the discovery of novel inhibitors of receptors or enzymes.

Determination of Carbohydrate Specificity of Monoclonal Antibodies against MUC1

The carbohydrate specificity of 57 MAbs submitted to the ISOBM TD-4 Workshop on MUC1 were investigated by two versions of ELISA, direct binding and inhibition of binding. The following free saccharides and their polyacrylamide conjugates (Sug-PAA) were used: tetrasaccharides – SiaLe^x , SiaLe^a; trisaccharides – Le^x, 3′HSO₃Le^x, Le^a, 3′HSO₃Le^a, 3′SiaLac, A_tri, B_tri; a number of disaccharides including TF, H_di, SiaT_n, LactNAc, and monosaccharides. It was shown that MAbs 143 and 167 interacted only with SiaLe^x, MAbs 127 and 128 only with Le^x. Antibodies 123 and 164 interacted preferably with Le^a but also recognized Le^c. Antibody 151 recognized αGalNAc (T_n) and cross-reacted with βGalNAc. Antibody 157 displayed high affinity to A_tri and A_tetr (type 1). Neither anti-TF nor anti-SiaT_n antibodies were revealed.