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von Willebrand factor. A protein which binds at the cell surface interface between platelets
Abstract:
von Willebrand factor (VWF) functions in platelet aggregation, a form of cellular interaction. In vitro analysis of platelet aggregation, as measured by the platelet aggregometer, requires addition of a promoter such as the glycopeptide antibiotic ristocetin. Native multimeric VWF (Mr = 1-20 X 10(6)) can be reduced with sulfhydryl reagents to a monomeric state (Mr = 2 X 10(5)). In this study, the binding of bovine VWF and ristocetin to bovine platelets was investigated using fluorescence anisotropy of derivatized monomer protein and ristocetin and also by radioisotope methods using 125I-labeled monomer and native protein. Ristocetin bound to bovine platelets but not to VWF. VWF binding to formaldehyde-fixed platelets was dependent on the presence of a promoter such as ristocetin. The monomer and multimer VWF bound equally well in the presence of low ristocetin concentrations. Under these conditions, plots of VWF binding versus platelet concentration were sigmoidal, indicating positive cooperativity with respect to platelets. At higher (100 micrograms/ml) ristocetin concentrations, the binding curve was no longer sigmoidal. Ristocetin promoted the formation of small platelet aggregates, an effect that was amplified by the presence of VWF. In fact, all conditions which resulted in monomer or multimer VWF binding to platelets also caused formation of platelet aggregates observed by light microscopy. These combined results were consistent with VWF binding only to the interface between proximal platelets. High affinity binding could be provided by the presence of two cell surfaces and the resulting multiple binding interactions. Polycations, such as poly(L-lysine) and Polybrene, also promoted the formation of platelet aggregates and facilitated the binding of VWF to platelets. Physiological platelet activators such as thrombin, ADP, and collagen also facilitated VWF binding to native platelets and caused platelet aggregation. It appears possible that any process which causes the surface membranes of platelets to become spatially close will allow expression of VWF activity.
Insights
Von Willebrand factor (VWF) binds to platelets when they are close together, a process promoted by ristocetin and other agents. This interaction is crucial for VWF
Area of Science:
- Hematology
- Biochemistry
- Cell Biology
Background:
- Platelet aggregation is a key cellular interaction in hemostasis.
- Von Willebrand factor (VWF) plays a critical role in platelet aggregation.
- Ristocetin is a commonly used promoter for in vitro platelet aggregation studies.
Purpose of the Study:
- To investigate the binding of von Willebrand factor (VWF) and ristocetin to bovine platelets.
- To elucidate the mechanism of VWF-mediated platelet aggregation.
- To determine the role of ristocetin concentration in VWF binding and platelet aggregation.
Main Methods:
- Utilized fluorescence anisotropy and radioisotope labeling (125I) to study VWF and ristocetin binding.
- Investigated binding kinetics using varying concentrations of ristocetin and platelet density.
- Observed platelet aggregation via light microscopy.
Main Results:
- Ristocetin bound to platelets, but not VWF; VWF binding to platelets required a promoter like ristocetin.
- VWF binding exhibited positive cooperativity with respect to platelet concentration at low ristocetin levels.
- Ristocetin and VWF synergistically promoted platelet aggregation, with VWF binding occurring at the interface between adjacent platelets.
Conclusions:
- VWF binding to platelets is facilitated by agents that bring platelets into close proximity, such as ristocetin, polycations, and physiological activators.
- The binding of VWF to platelets appears to occur at the interface between closely apposed platelet surfaces.
- These findings suggest that spatial proximity of platelet membranes is a key factor in expressing VWF activity.