The role of Lyn in VWF-induced activation of MAPK and a schematic of Lyn-dependent GPIb-IX signaling pathway leading to platelet adhesion and aggregation. (A) Wild-type or Lyn^−/− platelets were stimulated with botrocetin (1.2 μg/mL) in the absence or presence of 10 μg/mL of VWF for 8 minutes in a platelet aggregometer. Platelets were solubilized and immunoblotted with anti-phospho-p38 (Thr180/Tyr182) antibody (p-p38) or anti-p38 polyclonal antibody to indicate loading levels (p38; top panels), and anti–phospho-Erk1/2 (Thr202/Tyr204) antibody (p-ERK) or corresponding anti-Erk2 antibody to indicate loading levels (ERK; bottom panels). (B) VWF binding to GPIb–IX activates Lyn, which stimulates the PI3K-Akt-NO-cGMP-PKG-MAPK–dependent early GPIb-IX signaling pathway leading to integrin activation and stable platelet adhesion to VWF under shear stress. This signaling pathway together with integrin outside-in signaling causes TXA₂ synthesis and granule secretion of agonists, such as ADP, leading to amplification of integrin activation, increased stable platelet adhesion, and platelet aggregation.