The platelet collagen receptor glycoprotein VI (GPVI) and the fibrinogen receptor integrin αIIbβ3 trigger intracellular signalling cascades involving the tyrosine kinase Syk, the adapter SLP-76 and phospholipase Cγ2 (PLCγ2). Similar pathways are activated downstream of immune receptors in lymphocytes, where they have been localized in part to glycolipid-enriched membrane domains (GEMs). Here we provide several lines of evidence that GPVI-mediated tyrosine phosphorylation of PLCγ2 in platelets is dependent on GEM-organized signalling and utilizes the GEM resident adapter protein LAT (linker for activation of T cells). In sharp contrast, although fibrinogen binding to platelets stimulates αIIbβ3-dependent activation of Syk and tyrosine phosphorylation of SLP-76 and PLCγ2, it does not utilize GEMs to promote these responses or to support platelet aggregation. These results establish that GPVI and αIIbβ3 trigger distinct patterns of receptor signalling in platelets, leading to tyrosine phosphorylation of PLCγ2, and they highlight the role of GEMs in compartmentalizing signalling reactions involved in haemostasis.
Abbreviations used: GPVI, platelet collagen receptor glycoprotein VI; PLCγ2, phospholipase Cγ2; GEMs, glycolipid-enriched membrane domains; LAT, linker for activation of T cells; GPI, glycosyl-phosphatidylinositol; TCR, T-cell antigen receptor; ITAM, immunoreceptor tyrosine-based activation motif; FcR γ-chain, Fc receptor γ-chain; CRP, collagen-related peptide [YGKP∗(GPP∗)10GKP∗G, where P∗ represents hydroxyproline]; mAb, monoclonal antibody; ERK, extracellular-signal-regulated protein kinase; 5-HT, 5-hydroxytryptamine; NP-40, Nonidet P40.