Total inhibition of high shear stress induced platelet aggregation by homodimeric von Willebrand factor A1-loop fragments

Under high shear stress, the binding of von Willebrand factor (VWF) A1- loop domain to platelet glycoprotein (GP) Ibα occurs as the earliest event in thrombus formation. Therefore recombinant VWF A1-loop fragments could be of therapeutic use in blocking this interaction as competing ligands. We have prepared three homodimeric VWF A1-loop fragments [315 kD Fr III (a homodimer of amino acid [aa] residues 1-1365 of the subunit), 220 kD Fr (a homodimer of aa residues 1-708 of the subunit), and 116 kD Fr (a homodimer of aa residues 449-728 of the subunit) and two monomeric fragments [39/34 kD Fr (a monomer of aa residues 480/481-718 of the subunit] and His-rVWF465-728 (a monomer of aa residues 465-728 of the subunit)], and assessed their potency as inhibitors of botrocetin-induced VWF binding to GPIbα and high shear stress induced platelet aggregation mediated by intact VWF. All these fragments completely inhibited botrocetin-induced VWF binding to GPIbα at a final concentration of 40-200 μM. The homodimeric A1-loop fragments also totally inhibited high shear stress induced platelet aggregation at a final concentration of 0.45-2.0 μM in the following order: 315 kD Fr ≥ 220 kD Fr >> 116kD Fr. In contrast, the monomeric A1-loop fragments were only partial inhibitors of high shear stress induced platelet aggregation even at a final concentration of 20 μM, and their IC₅₀s were 13-39 times higher than those of the homodimers. These results indicate that the homodimeric structure of the A1-loop fragment is important for optimal molecular interaction with GPIbα under high shear stress.