Src and its downstream signaling molecule PLC gamma 2 are implicated in GPIb-IX-V (GPIbR) signalling. GPIbR-mediated platelet activation correlates with cytoskeletal association of Src, activation of PI3K and the appearance of multiple tyrosine-phosphorylated proteins (Jackson et al. 1994). von Willebrand Factor (vWF) and the vWF modulator botrocetin induce tyrosine phosphorylation of FceRIgamma, Syk, LAT and PLCgamma2. Src kinase inhibition markedly suppresses these events (Wu et al. 2001). Src and Lyn form a complex with FceRIgamma and Syk upon GPIbR/vWF interaction (Wu et al. 2003). FcgammaRIIa was tyrosine phosphorylated upon vWF and ristocetin-induced-platelet activation, followed by Syk and PLCgamma2 activation. A selective Src kinase inhibitor inhibited these events (Torti et al. 1994).
Though a considerable body of evidence suggests Src as a signaling molecule downstream of GPIbR the mechanism that connects Src to GPIbR is not clear. There are obvious similarities with the GPIV signal transduction pathway but also important differences: Src appears to be recruited to GPIbR upon platelet activation, while Lyn and Fyn constitutively associate with GPVI; GPVI activation induces a robust level of inositol phosphate production and PLCgamma2 activity, while GPIbRactivation PLCgamma2 activation is modest and the tyrosine phosphorylation sites of PLCgamma2 are distinct from those of GPVI stimulation (Suzuki-Inoue et al. 2004). GPVI signalling requires the FCeRIgamma chain while mouse knockouts suggest it is not required for GPIbR signalling (Kaiser-Friede et al. 2004).
Studies on GPIbalpha transgenic mice suggested that GPIbR activates AlphaIIbBeta3 Integrin through Src and PLC gamma2 activation (Kaiser-Friede et al. 2004). An alternative suggested mechansim is indirect association via 14-3-3-zeta and the p85 subunit of PI3K; the p85 subunit of PI3K constitutively associates with GPIbR so upon vWF/GPIb-IX-V interaction can bind Src via its SH3 domain (Wu et al. 2003).
Although many studies support a role for Src signaling in vWF/GPIb induced platelet activation, Src-independent platelet activation has been reported for platelets spreading on surfaces coated with echicetin, a GPIb-cross-linking component of snake venom (Navdaev & Clemetson, 2002).