Signaling through GP Ib-IX-V activates αIIbβ3 independently of other receptors

Platelet adhesion to von Willebrand factor (VWF) activates alphaIIbbeta3, a prerequisite for thrombus formation. However, it is unclear whether the primary VWF receptor, glycoprotein (GP) Ib-IX-V, mediates alphaIIbbeta3 activation directly or through other signaling proteins physically associated with it (eg, FcR gamma-chain), possibly with the contribution of other agonist receptors and of VWF signaling through alphaIIbbeta3. To resolve this question, human and GP Ibalpha transgenic mouse platelets were plated on dimeric VWF A1 domain (dA1VWF), which engages only GP Ib-IX-V, in the presence of inhibitors of other agonist receptors. Platelet adhesion to dA1VWF induced Src kinase-dependent tyrosine phosphorylation of the FcR gamma-chain and the adapter molecule, ADAP, and triggered intracellular Ca2+ oscillations and alphaIIbbeta3 activation. Inhibition of Ca2+ oscillations with BAPTA-AM prevented alphaIIbbeta3 activation but not tyrosine phosphorylation. Pharmacologic inhibition of protein kinase C (PKC) or phosphatidylinositol 3-kinase (PI 3-kinase) prevented alphaIIbbeta3 activation but not Ca2+ oscillations. Inhibition of Src with 2 distinct compounds blocked all responses downstream of GP Ib-IX-V under static or flow conditions. However, dA1VWF-induced responses were reduced only slightly in GP Ibalpha transgenic platelets lacking FcR gamma-chain. These data establish that GP Ib-IX-V itself can signal to activate alphaIIbbeta3, through sequential actions of Src kinases, Ca2+ oscillations, and PI 3-kinase/PKC. (C) 2004 by The American Society of Hematology.