Tec regulates platelet activation by GPVI in the absence of Btk

The Tec family kinase Btk plays an important role in the regulation of phospholipase Cgamma2 (PLCgamma2) downstream of the collagen receptor glycoprotein VI (GPVI) in human platelets. Platelets also express a second member of this family, Tec; however, its function has not been analyzed. To address the role of Tec, we analyzed Btk(-/-), Tec(-/-), and Btk/Tec double-deficient (Btk(-/-)/Tec(-/-)) platelets. Tec(-/-) platelets exhibit a minor reduction in aggregation to threshold concentrations of collagen or the GPVI-specific agonist collagen-related peptide (CRP), whereas responses to higher concentrations are normal. Tyrosine phosphorylation of PLCgamma2 by collagen and CRP is not altered in Tec(-/-) platelets. However, Btk(-/-)/Tec(-/-) platelets exhibit a greater reduction in PLCgamma2 phosphorylation than is seen in the absence of Btk, thus revealing an important role for Tec in this situation. Furthermore, Btk(-/-)/Tec(-/-) platelets fall to undergo an increase in Ca2+, aggregation, secretion, and spreading in response to collagen or CRP, whereas they aggregate normally to adenosine diphosphate (ADP) and spread on fibrinogen. A residual GPVI signal exists in the Btk(-/-)/Tec(-/-) platelets as CRP synergizes with ADP to mediate aggregation. These results demonstrate an essential requirement for Tec and Btk in platelet activation by GPVI and reveal a functional role for Tec in the regulation of PLCgamma2 in the absence of Btk. (C) 2003 by The American Society of Hematology.