The small GTPase Rap1b regrulates the cross talk between platelet integrin α2β1 and integrin αIIbβ3

The involvement of the small GTPase Rap1b in platelet integrin alpha(2)beta(1)-dependent outside-in signaling was investigated. Platelet adhesion to 4 different specific ligands for integrin alpha(2)beta(1), monomeric collagen, decorin, and collagen-derived peptides CB8(II) and CB11(II), induced a robust and rapid activation of Rap1b. This process did not require secreted ADP or thromboxane A(2) production but was critically regulated by phospholipase C (PLC)derived second messengers. Both Ca2+ and protein kinase C were found to organize independent but additive pathways for Rap1b activation downstream of integrin-alpha(2)beta(1), which were completely blocked by inhibition of PLC with U73122. Moreover, integrin alpha(2)beta(1) engagement failed to trigger Rap1b activation in murine platelets lacking CaIDAG-GEFI, a guanine nucleotide exchange factor regulated by Ca2+ and diacylglycerol, despite normal phosphorylation and activation of PLC gamma 2. In addition, CaIDAG-GEFI-deficient platelets showed defective integrin alpha(2)beta(1)-dependent adhesion and spreading. We found that outside-in signaling through integrin alpha(2)beta(1) triggered inside-out activation of integrin alpha(IIb)beta(3) and promoted fibrinogen binding. Similarly to Rap1b stimulation, this process occurred downstream of PLC activation and was dramatically impaired in murine platelets lacking the Rap1 exchange factor CaIDAGGEFI. These results demonstrate that Rap1b is an important element in integrin-dependent outside-in signaling during platelet adhesion and regulates the cross talk between adhesive receptors.