Rapid tyrosine phosphorylation and activation of Bruton's tyrosine/Tec kinases in platelets induced by collagen binding or CD32 cross-linking

Stimulation of the platelet nonintegrin collagen receptor, glycoprotein VI, evokes a signaling response similar to that induced by antigen receptor ac tivation in B and T lymphocytes, A key transducer of the lymphocyte signali ng pathways is the Bruton's tyrosine kinase (Btk)/Tec kinase family, which connects receptors to the elevation of intracellular-free calcium levels. A n important signaling function for Btk in collagen-induced platelet activat ion in vitro was recently demonstrated by other researchers using Btk-defic ient platelets from patients with X-linked agammaglobulinemia (XLA), Since Btk-deficiency does not induce an overt platelet-based bleeding disorder in vivo, collagen receptor responses may include other Btk/Tec kinase family members in normal platelets, Both Btk and Tec had increased tyrosine follow ing stimulation of collagen receptors or CD32 cross-linking, Data from kine tic analyses and inhibitor studies and the use of phosphopeptide-specific a ntibodies recognizing 2 Btk regulatory phosphorylated tyrosine residues sug gest a mechanism for coordinate recruitment of Btk and iec through the immu noreceptor tyrosine-based activation motif, Src family kinases, and phospha tidylinositol 3-kinase, In XLA platelets, collagen treatment increased tyro sine phosphorylation of Tec and several other signaling proteins, including Lyn, Fyb, Slp-76, and the Wiskott-Aldrich syndrome protein. This indicates that important elements of the collagen signaling pathway proximal and dis tal to Btk and Tec are preserved despite the lack of functional Btk, The re sults are consistent with the conclusion that activation of Tec may sustain XLA platelet function in vivo, while some in vitro assays of nonintegrin c ollagen receptor signaling through the Btk/Tec kinase family reflect the ad ditive dosage of the transducers, (C) 2000 by The American Society of Hemat ology.