Affinity of low molecular weight fucoidan for P-selectin triggers its binding to activated human platelets

Background: P-selectin is an adhesion receptor expressed on activated platelets and endothelial cells. Its natural ligand, P-selectin glycoprotein ligand-1, is expressed on leucocytes and the P-selectin/PSGL-1 interaction is involved in leukocyte rolling. We have compared the interaction of P-selectin with several low molecular weight polysaccharides: fucoidan, heparin and dextran sulfate.Methods: Binding assays were obtained from the interaction of the polysaccharides with Sialyl Lewis X and PSGL-1 based constructs onto microtiter plates coated with P-selectin. SELDI TOF mass spectrometry was performed with anionic chips arrays coated with P-selectin in the absence or in the presence of polysaccharides. Kd were obtained from surface plasmon resonance experiments with immobilized P-selectin constructs, polysaccharides being injected in the mobile phase. Human whole blood flow cytometry experiments were performed with fluorescein isothiocyanate labelled polysaccharides with or without platelets activators.Results: The fucoidan prevented P-selectin binding to Sialyl Lewis X with an IC50 of 20 nM as compared to 400 nM for heparin and < 25000 nM for dextran sulfate. It exhibited the highest affinity for immobilized P-selectin with a KD of 1.2 nM, two orders of magnitude greater than the KD of the other polysaccharides. Mass spectrometry evidenced the formation of a complex between P-selectin and fucoidan. The intensity of the fucoidan binding to platelets was dependent on the level of platelet activation. Competition between fucoidan and an anti P-selectin antibody demonstrated the specificity of the interaction.General significance: Low molecular weight fucoidan is a promising therapeutic agent of natural origin for biomedical applications.