Abstract
Von Willebrand factor (VWF) plays a key role in coagulation by tethering platelets to injured subendothelium via binding sites for platelet glycoprotein Ib and collagen. The binding sites for types I and III collagen in the VWF A3 domain are well characterized, and defects in this region have been implicated in von Willebrand disease (VWD). Additional collagens present in the vasculature may also be involved in interactions with VWF. A VWF A1 sequence variation, p.R1399H, has been associated with decreased binding to type VI collagen, but the clinical significance of this observation remains unclear. Type IV collagen is a common component of the basement membrane and as such may be an important ligand for VWF. While some VWD testing utilizes types I or III collagen, current clinical testing does not include collagen IV or VI.
To characterize the role of the VWF A1 domain in VWF-type IV collagen interactions, we generated several A1 domain variant human and/or murine recombinant VWF (rVWF) constructs including R1399H and several type 2M VWD variants localized to the same region (S1387I, Q1402P, and an 11 amino acid deletion mutant encompassing amino acids 1392-1402). These constructs were then expressed in HEK 293T cells. To further assess the role of the A1 domain, scanning alanine mutagenesis (SAM) of residues 1387 through 1412 was conducted. VWF antigen levels (VWF:Ag), collagen binding with type III (VWF:CB3), IV (VWF:CB4), or VI (VWF:CB6) collagen were determined, and multimer distribution was assessed for all recombinant VWF variants.
The role of R1399H in the context of human rVWF was characterized initially. Although VWF:Ag, VWF:CB3, and multimer distribution were normal for R1399H compared to wild-type (WT VWF), VWF:CB4 was undetectable. To examine this effect in a mouse model, the R1399H variant was expressed in the context of murine rVWF and collagen binding was determined. Similar to the human variant, murine R1399H rVWF demonstrated significantly reduced binding to murine type IV collagen, at only 7% of the binding seen with WT murine rVWF. In order to examine the behavior of R1399H under shear conditions, either WT or R1399H murine rVWF DNA was hydrodynamically injected into the tail veins of VWF -/- mice to induce expression of the proteins; blood was drawn from the vena cava 24 hours later and then examined on the VenaFlux flow apparatus. VWF expression levels and multimer distribution were similar for the R1399H- and WT-injected mice. Under static conditions, the murine plasma-derived R1399H demonstrated decreased VWF:CB4, at only 16% of the levels seen with WT VWF. No defect was seen in VWF:CB3. Furthermore, when binding to type IV collagen was assessed under flow conditions by VenaFlux, platelet adhesion was significantly decreased in mice expressing R1399H VWF as compared to mice expressing WT VWF.
When examining other A1 domain variants, Q1402P and del1392-1402 demonstrated absent VWF:CB4 while S1387I demonstrated a significant reduction in VWF:CB4 compared to WT VWF. All SAM VWF A1 domain variants demonstrated normal expression, multimerization, and VWF:CB3. However, type IV collagen binding was absent for R1392A, R1395A, R1399A, and K1406A and was reduced to less than 50% of WT VWF for Q1402A, K1405A, and K1407A. These residues map to an outside face of the VWF A1 domain crystal structure, and are likely the critical residues for VWF binding to type IV collagen.
Taken together, these data demonstrate that the type IV collagen binding site localizes to a specific region of the VWF A1 domain. Mutations in this region of VWF may be clinically significant due to a defect in the ability of VWF to attract platelets to exposed type IV collagen which may contribute to bleeding symptoms seen in VWD.
Disclosures:
No relevant conflicts of interest to declare.
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