HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY 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 C 2 (PLC 2) 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 PLC 2 by collagen and CRP is not altered in Tec / platelets. However, Btk / /Tec / platelets exhibit a greater reduction in PLC 2 phosphorylation than is seen in the absence of Btk, thus revealing an important role for Tec in this situation. Furthermore, Btk / /Tec / platelets fail 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 PLC 2 in the absence of Btk. (Blood. 2003;102: 3592-3599)