von Willebrand factor A1 domain can adequately substitute for A3 domain in recruitment of flowing platelets to collagen

Background: Binding of von Willebrand factor (VWF) to platelet GPIb alpha and to collagen is attributed to VWF A1 and A3 domains, respectively. Objectives: Using VWF, VWF lacking A1 (Delta A1-VWF) or A3 (Delta A3-VWF) and VWF with defective A3 (H1786A-VWF), in combination with recombinant A1 (residues 1262-1492) or A3 (residues 1671-1878), fused to glutathione-S-transferase (GST-A1 and GST-A3), we have re-investigated the role of A1 in platelet recruitment to surfaces of collagen. Methods and Results: In flow, measurable binding of Delta A3-VWF occurred to horse tendon, but also to human type III collagen. GST-A1 and GST-A3 both competed for binding of Delta A1-VWF and Delta A3-VWF to horse tendon collagen fibrils in static conditions and to human collagen III during plasmon surface resonance studies, substantiating overlapping binding sites on both collagens for A1 and A3. Heparin did not affect A3-mediated binding of VWF and Delta A1-VWF, but inhibited binding to horse tendon collagen of GST-A1 and Delta A3-VWF. Furthermore, A1-mediated binding to type III collagen of Delta A3-VWF binding was strongly salt-sensitive. During perfusions at wall shear rate 2500 s(-1) of calcein-labeled platelets in reconstituted blood, Delta A3-VWF and H1786A-VWF triggered platelet binding to horse tendon collagen comparably and as potently as VWF, and to human type III collagen, only fivefold less potently, Delta A1-VWF being inactive. Additional flow-controlled interaction studies with Delta A3-VWF, H1786A-VWF, the collagen-VWF antagonist saratin, heparin and the VWF neutralizing antibody 82D6A3 confirmed that H1786A-VWF binds to collagen exclusively via A1. Conclusion: Hence, in shear forces the VWF A1 domain can assume the role of A3 to trigger substantial platelet recruitment to human collagen fibres.