Sialic Acid Deficiency Impairs GPVI-Mediated Platelet Activation in ST3Gal-IV^−/− Mice.

Abstract 3005

Poster Board II-972

Lack of terminal sialic acid residues on platelet surface glycoproteins results in rapid platelet clearance. Using null mice for the ST3Gal-IV sialyltransferase gene (ST3Gal-IV^−/− mice), we have recently identified galactose residues on the N-terminus of the platelet Von Willebrand Factor receptor GPIbαa as a major counter receptor for the lectin domain of the asialoglycoprotein receptor on both hepatocytes and liver Kupffer cells (Sørensen et al., Blood 2009). ST3Gal-IV^−/− mice have increased tail bleeding time. However, the role of terminal sialic acid residues on platelet activation is unclear. We investigated here whether loss of sialylation affects platelet activation mediated through the collagen receptor GPVI or by thrombin. Platelets were isolated from ST3Gal-IV^−/− and ST3Gal-IV^+/+ mouse littermates, stimulated with collagen-related peptide (CRP), convulxin (CVX), or thrombin, and platelet activation was evaluated by flow cytometry using P-selectin expression, as a marker for αa-granule secretion, and fibrinogen binding, as a marker for integrin αaIIbβ3 activation. Stimulation of ST3Gal-IV^−/− platelets with CRP and CVX revealed a profound activation defect, compared to ST3Gal-IV^+/+ platelets. The defect was not due to loss of surface receptor expression since ST3Gal-IV^−/− and ST3Gal-IV^+/+ platelets had comparable GPVI expression, as evidenced by flow cytometry. By contrast, activation of ST3Gal-IV^−/− platelets with thrombin was normal. The data show that terminal sialic acid residues on GPVI are required for maximal platelet activation by CRP and CVX.