The Fc receptor γ-chain and the tyrosine kinase Syk are essential for activation of mouse platelets by collagen

Abstract

(FcR γ-chain), the non-receptor tyrosine kinase Syk and Bristol BS8 1TD, UK phospholipase Cγ2 (PLCγ2) (Blake et al., 1994; Daniel et al., 1994a; Fujii et al., 1994;Yanaga et al., 1995; 5 Corresponding author Asazuma et al., 1996;Gibbins et al., 1996). Syk assembles A.Poole, J.M.Gibbins and M.Turner contributed equally to this work into signalling complexes at the plasma membrane via interaction between its tandem Src homology 2 (SH2) Activation of mouse platelets by collagen is associated domains and a tyrosine-phosphorylated immunoreceptor with tyrosine phosphorylation of multiple proteins tyrosine-based activation motif (ITAM). We recently proincluding the Fc receptor γ-chain, the tyrosine kinase posed a model for collagen-induced signalling in human Syk and phospholipase Cγ2, suggesting that collagen platelets in which receptor clustering induced by collagen signals in a manner similar to that of immune receptors.leads to tyrosine phosphorylation of the FcR γ-chain, This hypothesis has been tested using platelets from possibly by a Src family kinase, allowing binding of Syk, mice lacking the Fc receptor γ-chain or Syk. Tyrosine which becomes tyrosine phosphorylated and activated phosphorylation of Syk and phospholipase Cγ2 by (Gibbins et al., 1996). This initiates a series of events collagen stimulation is absent in mice lacking the Fc which may involve other kinases and adapter proteins receptor γ-chain. Tyrosine phosphorylation of phospholeading to tyrosine phosphorylation and activation of lipase Cγ2 by collagen stimulation is also absent in mice PLCγ2. This model has been evaluated in the present platelets which lack Syk, although phosphorylation of study using platelets from genetically modified mice which the Fc receptor γ-chain is maintained. In contrast, lack the FcR γ-chain or Syk. tyrosine phosphorylation of platelet proteins by the G protein-coupled receptor agonist thrombin is maintained in mouse platelets deficient in Fc receptor Results γ-chain or Syk. The absence of Fc receptor γ-chain or Syk is accompanied by a loss of secretion and aggrega-Tyrosine phosphorylation in collagen-and tion responses in collagen-but not thrombin-stimulated thrombin-stimulated platelets platelets. These observations provide the first direct Collagen and thrombin stimulated distinct but overlapping evidence of an essential role for the immunoreceptor increases in whole cell tyrosine phosphorylation in platetyrosine-based activation motif (ITAM) in signalling lets (Figures 1 and 2A). Both agonists stimulated increases by a non-immune receptor stimulus.in tyrosine phosphorylation of proteins of~42, 70-75, Keywords: collagen/Fc receptor γ-chain/ITAM/platelets/ 100, 110 and 130 kDa in platelets from B6 mice; the 42 Syk and 110 kDa proteins were more heavily phosphorylated in thrombin-stimulated cells. The largest increase in tyrosine phosphorylation was in the 70-75 kDa proteins. Collagen but not thrombin also stimulated marked tyrosine phos-

A novel interaction between FlnA and Syk regulates platelet ITAM-mediated receptor signaling and function

“…After ligation of GPVI with collagen, CRP, or convulxin, the two YXXL motifs of the associated FcR chain ITAM become phosphorylated by Src family kinases, i.e., Fyn and Lyn (Ezumi et al, 1998). Syk recognizes and binds the phosphorylated ITAM, leading to Syk autophosphorylation and activation (Poole et al, 1997). Downstream signals through adapter proteins, such as LAT and SLP-76, lead to PLC-2 phosphorylation and activation (Watson et al, 2005).…”

Variation in Human Platelet Glycoprotein VI Content Modulates Glycoprotein VI–Specific Prothrombinase Activity

“…18 In addition, in genetically engineered mice that do not express FcR ␥-chain, there is a reproducible impairment of collagen-induced platelet activation. 19,20 CVX is a protein from the venom of C durissus terrificus that binds uniquely to GPVI. Because it is polymeric, CVX readily induces platelet activation.…”

Expression and Function of the Collagen Receptor GPVI during Megakaryocyte Maturation

“…In platelets, FcR ␥ chain is associated as a noncovalent complex with GPVI and acts as the signaling chain (8,11). We checked whether the FcR ␥ chain is also expressed by adult or neonate CD34…”

“…GPVI has recently been characterized as a new member of the immunoglobulin superfamily of cell-associated receptors, in which expression is restricted to the hematopoietic lineage (5)(6)(7). It is coexpressed as a noncovalent complex with the common immunoglobulin receptor ␥ (FcR␥) chain, which acts as the signaling subunit of the complex (8, 9) As a consequence, signaling pathways coupled to GPVI dimerization are identical to those coupled to other immune receptors; tyrosine phosphorylation of the FcR ␥ chain on its immunoreceptor tyrosine-based motif (ITAM) by a Src familly kinase (8,10,11) allows recruitment by the SH2 (Src homology 2) domain of P72 Syk that is in turn phosphorylated and activated. Downstream of Syk, PLC ␥2 is activated, leading to platelet aggregation and dense granule secretion (12).…”