Glycoprotein Ib-V-IX, a receptor for von Willebrand factor, couples physically and functionally to the Fc receptor gamma-chain, Fyn, and Lyn to activate human platelets.
The adhesion molecule von Willebrand factor (vWF) activates platelets upon binding 2 surface receptors, glycoprotein (GP) Ib-V-IX and integrin alpha(IIb)beta(3). We have used 2 approaches to selectively activate GP Ib using either the snake venom lectin alboaggregin-A or mutant recombinant forms of vWF (triangle upA1-vWF and RGGS-vWF) with selective binding properties to its 2 receptors. We show that activation of GP Ib induces platelet aggregation, secretion of 5-hydroxy tryptamine (5-HT), and an increase in cytosolic calcium. Syk becomes tyrosine phosphorylated and activated downstream of GP Ib, and associates with several tyrosine-phosphorylated proteins including the Fc receptor gamma-chain through interaction with Syk SH2 domains. GP Ib physically associates with the gamma-chain in GST-Syk-SH2 precipitates from platelets stimulated through GP Ib, and 2 Src family kinases, Lyn and Fyn, also associate with this signaling complex. In addition, GP Ib stimulation couples to tyrosine phosphorylation of phospholipase Cgamma2. The Src family-specific inhibitor PP1 dose-dependently inhibits phosphorylation of Syk, its association with tyrosine-phosphorylated gamma-chain, phosphorylation of PLCgamma2, platelet aggregation, and 5-HT release. The results indicate that, upon activation, GP Ib is physically associated with FcR gamma-chain and members of the Src family kinases, leading to phosphorylation of the gamma-chain, recruitment, and activation of Syk. Phosphorylation of PLCgamma2 also lies downstream of Src kinase activation and may critically couple early signaling events to functional platelet responses.
Falati
S
S
Department of Pharmacology, University of Bristol, School of Medical Sciences, Bristol, UK.
Edmead
C E
CE
Poole
A W
AW
D001792:Blood Platelets / Q000378:metabolism*; D006801:Humans; D015539:Platelet Activation; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000737:chemistry / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051076:Proto-Oncogene Proteins c-fyn; D011940:Receptor Aggregation; D011961:Receptors, Fc / Q000378:metabolism*; D015398:Signal Transduction; D019061:src-Family Kinases / Q000378:metabolism*; D014841:von Willebrand Factor / Q000378:metabolism
D019038:Platelet Glycoprotein GPIb-IX Complex; D011518:Proto-Oncogene Proteins; D011961:Receptors, Fc; D014841:von Willebrand Factor; C494957:FYN protein, human; D051076:Proto-Oncogene Proteins c-fyn; D019061:src-Family Kinases