A conditional form of Bruton's tyrosine kinase is sufficient to activate multiple downstream signaling pathways via PLC Gamma 2 in B cells

Activation of Btk:ER was sufficient to induce multiple B cell signaling pathways in PLCγ2-sufficient DT40 cells. These included tyrosine phosphorylation of PLCγ2, mobilization of intracellular calcium, activation of extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) mitogen-activated protein kinase (MAPK) pathways, and apoptosis. In DT40 B cells deficient for PLCγ2, Btk:ER activation failed to induce the signaling events described above with the consequence that the cells failed to undergo apoptosis.These data suggest that Btk:ER regulates downstream signaling pathways primarily via PLCγ2 in B cells. While it is not known whether activated Btk:ER precisely mimics activated Btk, this conditional system will likely facilitate the dissection of the role of Btk and its family members in a variety of biological processes in many different cell types.Mutations in Bruton's tyrosine kinase (Btk) are responsible for the human disease termed X-linked agammaglobulinemia (XLA) (reviewed in reference [1]). The B cell antigen receptor (BCR) signaling defect is very severe such that XLA patients have a block in the pro-B to pre-B cell transition and consequently have no mature B cells. The xid mouse, in which Btk is mutated, and the Btk knockout mouse display similar, although somewhat less severe, phenotypes [1]. Btk is the prototypical member of the Tec family of non-receptor protein tyrosine kinases (PTKs) that includes Bmx, Itk, Tec and Txk [2, 3]. In addition to a COOH-terminal PTK domain, Btk has an NH2- terminal pleckstrin homology (PH) domain, a proline-rich Tec homology domain, a Src-homology 3 (SH3) domain and a Src-homology 2 (SH2) domain. Btk was originally identified in B cells but is now known to be expressed in most leukocytes with the exception of T cells and NK cells.Btk is thought to be activated upon BCR cross-linking by a two-step mechanism involving phosphatidylinositol (PI) 3-kinase and the Src family PTK Lyn (reviewed in referenc