The Kunitz-3 domain of TFPI-α is required for protein S–dependent enhancement of factor Xa inhibition

Protein S (PS) enhances the inhibition of factor Xa (FXa) by tissue factor pathway inhibitor-α (TFPI-α) in the presence of Ca2+ and phospholipids. Altered forms of recombinant TFPI-α were used to determine the structures within TFPI-α that may be involved in this PS-dependent effect. Wild-type TFPI-α (TFPIWT), TFPI-α lacking the K3 domain (TFPI-ΔK3), and TFPI-α containing a single amino acid change at the putative P1 residue of K3 (R199L, TFPIK3P1) produced equivalent FXa inhibition in the absence of PS, whereas the response in FXa inhibition produced by PS was reduced with TFPIK3P1 (EC50 61.8 ± 13.4nM vs 8.0 ± 0.4nM for TFPIWT) and not detectable with TFPI-ΔK3. Ligand blotting and surface plasmon resonance experiments demonstrated that FXa bound TFPIWT and TFPI-ΔK3 but not the isolated K3 domain, whereas PS bound TFPIWT and the K3 domain but not TFPI-ΔK3. Addition of TFPIWT, TFPIK3P1, or TFPI-ΔK3 produced comparable prolongation of FXa-induced coagulation in PS-deficient plasma, but the anticoagulant effect of TFPIWT was substantially greater than that of TFPIK3P1 > TFPI-ΔK3 in normal plasma and PS-deficient plasma reconstituted with PS. We conclude that the PS-mediated enhancement of FXa inhibition by TFPI-α involves an interaction between PS and TFPI-α, which requires the K3 domain of TFPI-α

Platelet-VWF complexes are preferred substrates of ADAMTS13 under fluid shear stress

Endothelial cells secrete prothrombotic ultralarge von Willebrand factor (VWF) multimers, and the metalloprotease ADAMTS13 cleaves them into smaller, less dangerous multimers. This reaction is stimulated by tensile force applied to the VWF substrate, which may occur on cell surfaces or in the circulating blood. The cleavage of soluble VWF by ADAMTS13 was accelerated dramatically by a combination of platelets and fluid shear stress applied in a cone-plate viscometer. Platelet-dependent cleavage of VWF was blocked by an anti-GPIbα monoclonal antibody or by a recombinant soluble fragment of GPIbα that prevents platelet-VWF binding. Multimeric gel analysis showed that shear and platelet-dependent cleavage consumed large VWF multimers. Therefore, ADAMTS13 preferentially acts on platelet-VWF complexes under fluid shear stress. This reaction is likely to account for a majority of VWF proteolysis after secretion and to determine the steady-state size distribution of circulating VWF multimers in vivo