Important Regulatory Roles of Erythrocytes in Platelet Adhesion to the von Willebrand Factor on the Wall under Blood Flow Conditions.

The role of erythrocytes in platelet adhesion to von Willebrand factor (VWF) on the vessel wall through their membrane glycoprotein (GP)Ibα under blood flow conditions has not yet been elucidated. Blood specimens containing fluorescent-labeled platelets and native, biochemically fixed, or artificial erythrocytes at various hematocrits were perfused on the surface of VWF immobilized on the wall at a shear rate of 1,500 s-1. The rates of platelet adhesion were measured under each condition. The computer simulation of platelet adhesion to the VWF on the wall at the same shear rate was conducted by solving the governing equations with a finite-difference method on a K computer. The rates of platelet adhesion were calculated at various hematocrit conditions in the computational domain of 100 µm (x-axis) × 400 µm (y-axis) × 100 µm (z-axis). Biological experiments demonstrated a positive correlation between the rates of platelet adhesion and hematocrit values in native, fixed, and artificial erythrocytes. (r = 0.992, 0.934, and 0.825 respectively, p < 0.05 for all). The computer simulation results supported the hematocrit-dependent increase in platelet adhesion rates on VWF (94.3/second at 10%, 185.2/second at 20%, and 327.9/second at 30%). These results suggest that erythrocytes play an important role in platelet adhesion to VWF. The augmented z-axis fluctuation of flowing platelets caused by the physical presence of erythrocytes is speculated to be the cause of the hematocrit-dependent increase in platelet adhesion.

Prediction of Molecular Interaction between Platelet Glycoprotein Ibα and von Willebrand Factor using Molecular Dynamics Simulations.

AIM: The molecular mechanism of the unique interaction between platelet membrane glycoprotein Ibα (GPIbα) and von Willebrand Factor (VWF), necessary for platelet adhesion under high shear stress, is yet to be clarified. METHODS: The molecular dynamics simulation using NAMD (Nanoscale Molecular Dynamics) package with the CHARMM 22 (Chemistry at Harvard Macromolecular Mechanics) force field were used to predict dynamic structural changes occurring in the binding site of A1 domain of VWF and N terminus domain of GPIbα under water soluble condition. RESULTS: The mean distance between the mass center of A1 domain of VWF and GPIbα in the stable form was predicted as 27.3 Å. The potential of mean force between the A1 domain of VWF and GPIbα were calculated in conditions of various distances of the mass center between them. All the calculated values were fitted to the Morse potential energy function curve. The maximum adhesive force between A1 domain of VWF and GPIbα was predicted as 62.3 pN by differentiating the potential of mean force with respect to the molecular distance. CONCLUSIONS: The molecular dynamics simulation is useful for predicting the dynamic structure changes of protein bonds involved in platelet adhesion and for predicting the adhesive forces generated between their interactions.

Development of virtual platelets implementing the functions of three platelet membrane proteins with different adhesive characteristics.

AIM: Computer simulation is a new method for understanding biological phenomena. In this report, we developed a simple platelet simulator representing platelet adhesion under blood flow conditions. METHODS: We generated virtual platelets based on the functions of three key adhesive proteins: glycoprotein (GP) Ibα, GPIIb/IIIa and collagen receptors. The adhesive force between GPIbα and von Willebrand factor (VWF) was set to increase in association with increments in the fluid shear stress. GPIIb/IIIa acquires an adhesive force to bind with ligands only when platelets are activated following multiple GPIbα stimulation by VWF or collagen receptors. RESULTS: Upon perfusion over the area of virtual endothelial injury, the virtual platelets adhered and became activated to form platelet thrombi. A total of 286/mm(2) of activated platelets was found to have accumulated downstream of the flow obstacle within 30 seconds, with 59/mm(2) platelets adhering upstream. The results obtained with the virtual model were consistent with those for real platelets in human blood in the presence of similarly shaped flow obstacles. CONCLUSIONS: Our computer platelet simulator, which employs the functions of three key platelet membrane proteins, shows similar findings for adhesion in the presence and absence of blood flow obstacles.