title: Ultralarge multimers of von Willebrand factor form spontaneous high-strength bonds with the platelet glycoprotein Ib-IX complex: studies using optical tweezers.
abstract: Ultralarge von Willebrand factor (ULVWF) multimers have been implicated in the pathogenesis of the catastrophic microangiopathic disorder, thrombotic thrombocytopenic purpura. Spontaneous ULVWF binding to platelets has been ascribed to increased avidity due to the greatly increased number of binding sites for platelets (the A1 domain) per molecule. To address the mechanism of enhanced ULVWF binding to platelets, we used optical tweezers to study the unbinding forces from the glycoprotein Ib-IX (GP Ib-IX) complex of plasma VWF, ULVWF, and isolated A1 domain. The unbinding force was defined as the minimum force required to pull ligand-coated beads away from their attachment with GP Ib-IX-expressing cells. Beads coated with plasma VWF did not bind to the cells spontaneously, requiring the modulators ristocetin or botrocetin. The force required to break the ristocetin- and botrocetin-induced plasma VWF-GP Ib-IX bonds occurred in integer multiples of 6.5 pN and 8.8 pN, respectively, depending on the number of bonds formed. In contrast, beads coated with either ULVWF or A1 domain bound the cells in the absence of modulators, with bond strengths in integer multiples of approximately 11.4 pN for both. Thus, in the absence of shear stress, ULVWF multimers form spontaneous high-strength bonds with GP Ib-IX, while plasma VWF requires exogenous modulators. The strength of individual bonds formed with GP Ib-IX was similar for both ULVWF and the isolated A1 domain and greater than those of plasma VWF induced by either modulator. Therefore, we suggest that the conformational state of ULVWF multimers is more critical than their size for interaction with platelets.
authors: Department of Bioengineering, Rice University, Houston, TX, USA.
authors: Maneesh
authors: M
authors: Arya
authors: Bahman
authors: B
authors: Anvari
authors: Gabriel M
authors: GM
authors: Romo
authors: Miguel A
authors: MA
authors: Cruz
authors: Jing-Fei
authors: JF
authors: Dong
authors: Larry V
authors: LV
authors: McIntire
authors: Joel L
authors: JL
authors: Moake
authors: José A
authors: JA
authors: López
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D001792:Blood Platelets / Q000502:physiology; D006224:Cricetinae; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000737:chemistry* / Q000187:drug effects; D011485:Protein Binding; D011994:Recombinant Proteins / Q000737:chemistry; D012310:Ristocetin / Q000494:pharmacology; D014162:Transfection; D014841:von Willebrand Factor / Q000737:chemistry* / Q000187:drug effects
chemical_list: D000911:Antibodies, Monoclonal; D046911:Macromolecular Substances; D019038:Platelet Glycoprotein GPIb-IX Complex; D011994:Recombinant Proteins; D014841:von Willebrand Factor; D012310:Ristocetin