Src family tyrosine kinase Lyn mediates VWF GPIb-IX induced platelet activation via the cGMP signaling pathway

The platelet receptor for von Willebrand factor (VWF), glycoprotein (GP) Ib-IX, mediates initial platelet adhesion and transmits signals leading to platelet activation. Src family tyrosine kinases (SFKs) play an important role in VWF-induced GPIb-IX signaling. However, the SFK-dependent downstream signaling pathway is unclear but is thought to involve thromboxane A₂ (TXA₂) synthesis. Here we show that, although platelets deficient in SFK members, Lyn or Fyn, were defective in the TXA₂-dependent second wave of platelet aggregation induced by botrocetin/VWF. only Lyn-knockout platelets were also defective in stable platelet adhesion to VWF under shear stress that is independent of the TXA₂ pathway. Lyn-knockout platelets also spread poorly on VWF but spread normally on fibrinogen, indicating an important role for Lyn in VWF-mediated GPIb signaling but not in inte-grin outside-in signaling. Importantly, Lyn knockout abrogated VWF-induced cGMP elevation. Addition of low concentrations of 8-bromo-cGMP, however, corrected the defective stable adhesion of Lyn-knockout platelets or PP2-treated platelets on VWF. These results demonstrate an important role for Lyn in VWF/GPIb-IX-induced integrin activation mediated via the cGMP signaling pathway independently of TXA₂ synthesis and also indicate that Lyn is critically important in GPIb-IX-mediated activation of the cGMP pathway.