von Willebrand Factor-ADAMTS13 Interactions

Abstract SCI-18

ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats-13) is a plasma metalloenzyme that cleaves von Willebrand factor (vWF), thereby limiting excessive platelet adhesion and aggregation and attenuating the rate of thrombus formation at the site of injury. Severe deficiency of plasma ADAMTS13 activity results in thrombotic thrombocytopenic purpura (TTP), a potentially fatal syndrome, while mild to moderate deficiency of plasma ADAMTS13 activity is associated with an increased risk for myocardial infarction and stroke. Adamts13 null mice in the ApoE null background exhibit a dramatically increased systemic inflammation and early atherosclerosis. Thus, ADAMTS13 plays a crucial role in pathogenesis of arterial thrombotic disorders such as TTP and chronic inflammatory diseases such as atherosclerosis. vWF is the only known substrate for ADAMTS13 to date. Structure-function analysis demonstrates that the N-terminal portion of ADAMTS13 (MDTCS) is sufficient for proteolytic cleavage of vWF, while the C-terminal portion of ADAMTS13 (T2–8 and CUB) inhibits shear-dependent platelet aggregation via free thiols (-SH). The proteolytic cleavage of vWF by ADAMTS13 is dramatically accelerated by binding of Factor VIII (FVIII) and/or platelets under shear stress. A B-domainless FVIII (FVIII-SQ) or a light chain of FVIII (FVIII-LC) appears to be sufficient for the enhanced proteolysis of vWF by ADAMTS13 in vitro and in vivo using a murine model. In contrast, a heavy chain of FVIII (FVIII-HC) or a light chain lacking an acidic region (a3) (FVIII-LC#x2610;¢a3) exhibits no rate enhancing effect under the same conditions. These results suggest that the light chain, particularly the acidic (a3) region that contains a high-affinity vWF binding site is required for the cofactor activity in accelerating vWF proteolysis. Consistent with this hypothesis, mutations in the D'D3 domain of vWF naturally occurring in patients with type 2N von Willebrand disease that reduces FVIII binding also impair the FVIII-dependent proteolysis by ADAMTS13 under fluid shear stress. To develop novel therapeutics, we have constructed, expressed, and screened for a panel of ADAMTS13 variants with a modification of the exosites (motif A and motif B) in the spacer domain. We identified several ADAMTS13 variants that have enhanced specific activity and are also resistant to inhibition by autoantibodies against ADAMTS13 in patients with acquired idiopathic TTP. These gain-of-function and autoantibody-resistant ADAMTS13 variants may be further developed as potential therapeutics for acquired TTP with inhibitors.