Dual roles of fucoidan-GPIbα interaction in thrombosis and hemostasis: implications for drug development targeting GPIbα

Abstract

BackgroundPlatelet GPIbα–von Willebrand factor (VWF) interaction initiates platelet adhesion, activation, and thrombus growth, especially under high shear conditions. Therefore, the GPIb–VWF axis has been suggested as a promising target against arterial thrombosis. The polysaccharide fucoidan has been reported to have opposing prothrombotic and antithrombotic effects; however, its binding mechanism with platelets has not been adequately studied. ObjectiveThe objective of this study was to explore the mechanism of fucoidan and its hydrolyzed products in thrombosis and hemostasis. MethodsNatural fucoidan was hydrolyzed by using hydrochloric acid and was characterized by using size-exclusion chromatography, UV-visible spectroscopy, and fluorometry techniques. The effects of natural and hydrolyzed fucoidan on platelet aggregation were examined by using platelets from wild-type, VWF and fibrinogen-deficient, GPIbα-deficient, and IL4Rα/GPIbα-transgenic and αIIb-deficient mice and from human beings. Platelet activation markers (P-selectin expression, PAC-1, and fibrinogen binding) and platelet-VWF A1 interaction were measured by using flow cytometry. GPIbα–VWF A1 interaction was evaluated by using enzyme-linked immunosorbent assay. GPIb-IX–induced signal transduction was detected by using western blot. Heparinized whole blood from healthy donors was used to test thrombus formation and growth in a perfusion chamber. ResultsWe found that GPIbα is critical for fucoidan-induced platelet activation. Fucoidan interacted with the extracellular domain of GPIbα and blocked its interaction with VWF but itself could lead to GPIbα-mediated signal transduction and, subsequently, αIIbβ3 activation and platelet aggregation. Conversely, low-molecular weight fucoidan inhibited GPIb-VWF–mediated platelet aggregation, spreading, and thrombus growth at high shear. ConclusionFucoidan-GPIbα interaction may have unique therapeutic potential against bleeding disorders in its high-molecular weight state and protection against arterial thrombosis by blocking GPIb–VWF interaction after fucoidan is hydrolyzed.