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N-linked glycan sites within the A2 domain of von Willebrand factor modulate macrophage-mediated clearance

Alain Chion, Jamie M. O'Sullivan, Clive Drakeford, Gudmundur Bergsson, Niall Dalton, Sonia Aguila, Soracha Ward, Padraic G. Fallon, Teresa M. Brophy, Roger J.S. Preston, Lauren Brady, Orla Sheils, Michael Laffan, Thomas A.J. McKinnon and James S. O'Donnell

Key points

  • The A1 domain of VWF contains a cryptic binding site that plays a key role in regulating macrophage-binding and clearance.

  • The N-linked glycans presented at N1515 and N1574 within the A2 domain of VWF modulate macrophage-mediated clearance.

Abstract

Enhanced von Willebrand factor (VWF) clearance is important in the etiology of von Willebrand disease. However the molecular mechanisms underlying VWF clearance remain poorly understood. In this study, we have investigated the role of VWF domains and specific glycan moieties in regulating in vivo clearance. Our findings demonstrate that the A1 domain of VWF contains a receptor-recognition site that plays a key role in regulating the interaction of VWF with macrophages. In A1-A2-A3 and full-length VWF, this macrophage-binding site is cryptic but becomes exposed following exposure to shear or ristocetin. Previous studies have demonstrated that the N-linked glycans within the A2 domain play an important role in modulating susceptibility to ADAMTS13 proteolysis. We further demonstrate that these glycans presented at N1515 and N1574 also play a critical role in protecting VWF against macrophage-binding and clearance. Indeed, loss of the N-glycan at N1515 resulted in markedly enhanced VWF clearance that was significantly faster than that observed with any previously described VWF mutations. In addition, A1-A2-A3 fragments containing the N1515Q or N1574Q substitutions also demonstrated significantly enhanced clearance. Importantly, clodronate-induced macrophage depletion significantly attenuated the increased clearance observed with N1515Q and N1574Q in both full-length VWF and in A1-A2-A3. Finally, we further demonstrate that loss of these N-linked glycans does not enhance clearance in VWF in the presence of a structurally constrained A2 domain. Collectively, these novel findings support the hypothesis that conformation of the VWF A domains plays a critical role in modulating macrophage-mediated clearance of VWF in vivo.

  • Submitted April 18, 2016.
  • Accepted August 16, 2016.