Interaction of porcine von Willebrand factor (platelet aggregating factor) with human platelets

Activation of Human Platelets by the Membrane-Expressed A1 Domain of von Willebrand Factor

Platelet activation and microthrombus formation are invariable features of xenograft rejection and the vascular injury observed when porcine organs are transplanted into primates. This pathological process could be mediated, at least in part, by aberrant interactions of von Willebrand Factor (vWF) associated with the donor vasculature with host platelets. Unlike human vWF, native porcine vWF (pvWF) interacts with human GPIb independently of shear stress or nonphysiological stimuli, eg, ristocetin. We therefore contrasted the potential of isolated human and porcine vWF–A1-domains to interact with human platelets in vitro. Both human and porcine vWF–A1-domains expressed as glycosyl phosphatidylinositol–linked FLAG fusion proteins on COS-7 cells induced GPIb-dependent aggregation and intracellular Ca++ uptake of platelets, independent of both the remainder of the vWF protein and additional modifying factors. Porcine A1-domains were more potent than human homologues, and in addition ristocetin could boost platelet aggregation only with the human A1-domain. Putative conformational changes in the porcine A1-domain could result in the heightened, ristocetin-independent interactions observed with human platelets and may be of importance for xenograft survival.

Activation of Human Platelets by the Membrane-Expressed A1 Domain of von Willebrand Factor

Platelet activation and microthrombus formation are invariable features of xenograft rejection and the vascular injury observed when porcine organs are transplanted into primates. This pathological process could be mediated, at least in part, by aberrant interactions of von Willebrand Factor (vWF) associated with the donor vasculature with host platelets. Unlike human vWF, native porcine vWF (pvWF) interacts with human GPIb independently of shear stress or nonphysiological stimuli, eg, ristocetin. We therefore contrasted the potential of isolated human and porcine vWF–A1-domains to interact with human platelets in vitro. Both human and porcine vWF–A1-domains expressed as glycosyl phosphatidylinositol–linked FLAG fusion proteins on COS-7 cells induced GPIb-dependent aggregation and intracellular Ca++ uptake of platelets, independent of both the remainder of the vWF protein and additional modifying factors. Porcine A1-domains were more potent than human homologues, and in addition ristocetin could boost platelet aggregation only with the human A1-domain. Putative conformational changes in the porcine A1-domain could result in the heightened, ristocetin-independent interactions observed with human platelets and may be of importance for xenograft survival.

Glycocalicin binding to von Willebrand factor adsorbed onto collagen-coated or polystyrene surfaces

In order to analyze the interaction of platelets with von Willebrand factor (vWF) and collagen, we studied the binding of glycocalicin (GC) and formalin-fixed platelets to vWF adsorbed onto uncoated or collagen-coated polystyrene surfaces. These studies show that three-fold more vWF binds to collagen-coated polystyrene than to polystyrene coated with fibrin monomer or fibrinogen. At saturation, 37 +/- 2.9 ng vWF bound to the collagen-coated wells, compared to 12.8 +/- 5.4 ng, and 10.9 +/- 2.7 ng of vWF bound to wells coated with fibrin monomer and fibrinogen, respectively. GC also bound significantly more to collagen-coated wells than to wells coated with fibrinogen, and this binding was increased approximately two-fold (from 7 +/- 0.65 ng to 14 +/- 1.1 ng) in the presence of vWF adsorbed to the collagen-coated surface. Only 2 ng of GC was bound to 3000 ng of vWF when the latter was adsorbed directly onto a polystyrene surface. In contrast, GC binding to vWF adsorbed onto a collagen-coated surface was enhanced 600-fold with 7.0 ng of GC bound to 18 ng of immobilized vWF. Formalin-fixed platelets showed little binding to vWF adsorbed onto the microtiter wells. At saturation, 7 x 10(4) platelets bound to 3000 ng of vWF; a 6-fold increase in platelet binding was observed using collagen-coated wells and this binding was increased even further in the presence of vWF, resulting in 250-fold increase in platelet binding to vWF when the latter was adsorbed onto a collagen surface. These studies suggest that (1) GC is involved in platelet binding to collagen and this binding is increased by vWF; (2) GC binding to vWF is enhanced by the collagen-coated surface; (3) the adsorption of vWF onto a collagen surface may induce conformational changes in vWF that promote its interaction with GC or glycoprotein Ib.

Binding of porcine von Willebrand factor to human platelets in the presence of ristocetin

The interaction of porcine von Willebrand factor (vWF) with human platelets in the presence of ristocetin was examined. Binding was rapid, specific, saturable and dependent on ristocetin concentration with half-maximal stimulation occurring at a ristocetin concentration of 0.5 mM. Assuming vWF to be a tetramer with a MW of 9.5 x 10(5), approximately 94,000 vWF binding sites per platelet were found with an average binding constant of 2.1 x 10(-8)M. Human vWF competed with the porcine protein for this site only in the presence of ristocetin. Binding of porcine vWF to platelets was found to be less sensitive to reduction with dithiothreitol than platelet agglutinating activity.