The role of neuraminidase 1 and 2 in glycoprotein Ibα-mediated integrin αIIbβ3 activation.

Upon vascular injury, platelets adhere to von Willebrand Factor (VWF) glycoprotein Ibα (GPIbα). GPIbα contains many glycans, capped by sialic acid. Sialic acid cleavage (desialylation) triggers clearance of platelets. Neuraminidases (NEU) are responsible for desialylation and so far, NEU1-4 have been identified. However, the role of NEU in healthy platelets is currently unknown. Aim of the study was to study the role of NEU1 and NEU2 in platelet signalling. Membrane association of platelet attached glycans, NEU1 and NEU2 was measured following activation with agonists using flow cytometry. Adhesion on fibrinogen, aggregation and fibrinogen-binding were assessed with/without the NEU-inhibitor, 2-deoxy-2-3-dide-hydro--acetylneuraminic acid. Cellular localisation of NEU1 and NEU2 was examined by fluorescence microscopy. Desialylation occurred following GPIbα-clustering by VWF. Basal levels of membrane NEU1 were low; glycoprotein Ibα-clustering induced a four-fold increase (n=3, <0.05). Inhibition of αβ-integrin prevented the increase in NEU1 membrane-association by ~60%. Membrane associated NEU2 increased two-fold (n=3, <0.05) upon VWF-binding, while inhibition/removal of GPIbα reduced the majority of membrane associated NEU1 and NEU2 (n=3, <0.05). High shear and addition of fibrinogen increased membrane NEU1 and NEU2. NEU-inhibitior prevented VWF-induced αIIbβ3-integrin activation by 50% (n=3, <0.05), however, promoted VWF-mediated agglutination, indicating a negative feedback mechanism for NEU activity. NEU1 or NEU2 were partially co-localised with mitochondria and α-granules respectively. Neither NEU1 nor NEU2 co-localised with lysosomal-associated membrane protein 1. These findings demonstrate a previously unrecognised role for NEU1 and NEU2 in GPIbα-mediated and αβ-integrin signalling.