The Leech Product Saratin Is a Potent Inhibitor of Both VWF and Integrin α²β¹ Binding to Collagen.

Background: Subendothelial collagen plays an important role, via both direct and indirect mechanisms, in the initiation of thrombus formation at sites of vascular injury. Collagen binds plasma von Willebrand factor (VWF), which mediates platelet recruitment to collagen under high shear. Subsequently, the direct binding of the platelet receptors GPVI and α₂β₁ to collagen is critical for platelet activation and stable adhesion. Hematophages, such as leeches, have evolved a number of inhibitors directed toward platelet-colagen interactions so as to prevent haemostasis in the host during haematophagy.

Objective: To elucidate the molecular mechanisms underlying the ability of the leech product saratin to inhibit platelet binding to collagen.

Results: In the presence of inhibitors of adenosine diphosphate (ADP) and thromboxane A₂ (TxA₂), both saratin and 6D1, a blocking α₂β₁ mAb, abrogated platelet adhesion to fibrillar and soluble collagen. Additionally, saratin eliminated α₂β₁-dependent platelet adhesion to soluble collagen in the presence of a Src kinase inhibitor. Moreover, saratin prevented platelet-rich-plasma adhesion to fibrillar collagen, a process dependent upon both α₂β₁ and VWF binding to collagen. Furthermore, saratin specifically inhibited the binding of the α₂I-domain to collagen, and prevented platelet adhesion to collagen under flow to the same extent as observed in the presence of an anti-GPIb and anti-α₂β₁ mAb.

Conclusions: The present study demonstrates that saratin interferes with integrin α₂β₁ binding to collagen in addition to inhibiting VWF-collagen binding, presumably by binding to an overlapping epitope on collagen. This has significant implications for the use of saratin as a tool to inhibit platelet-collagen interactions.