O-glycan determinants regulate VWF trafficking to Weibel-Palade bodies.

Von Willebrand factor (VWF) undergoes complex post-translational modification within endothelial cells (EC) prior to secretion. This includes significant N- and O-linked glycosylation. Previous studies have demonstrated that changes in N-linked glycan structures significantly influence VWF biosynthesis. In contrast, although abnormalities in VWF O-linked glycans (OLG) have been associated with enhanced VWF clearance, their effect on VWF biosynthesis remains poorly explored. Herein, we report a novel role for OLG determinants in regulating VWF biosynthesis and trafficking within EC. We demonstrate that alterations in OLG (notably reduced terminal sialylation) lead to activation of the A1 domain of VWF within EC. In the presence of altered OLG, VWF multimerization is reduced and Weibel-Palade body (WPB) formation significantly impaired. Consistently, the amount of VWF secreted from WPB following EC activation was significantly reduced in the context of O-glycosylation inhibition. Finally, altered OLG on VWF not only reduced the amount of VWF secreted following EC activation, but also affected its hemostatic efficacy. Notably, VWF secreted following WPB exocytosis consisted predominantly of low molecular weight multimers and the length of tethered VWF string formation on the surface of activated ECs was significantly reduced. In conclusion, our data therefore support the hypothesis that alterations in O-glycosylation pathways directly impact VWF trafficking within human EC. These findings are interesting given that previous studies have reported altered OLG on plasma VWF (notably increased T antigen expression) in patients with von Willebrand disease.