The murine form of TXK, a novel TEC kinase expressed in thymus maps to chromosome 5

TEC FAMILY KINASES IN T LYMPHOCYTE DEVELOPMENT AND FUNCTION

The Tec family tyrosine kinases are now recognized as important mediators of antigen receptor signaling in lymphocytes. Three members of this family, Itk, Rlk, and Tec, are expressed in T cells and activated in response to T cell receptor (TCR) engagement. Although initial studies demonstrated a role for these proteins in TCR-mediated activation of phospholipase C-gamma, recent data indicate that Tec family kinases also regulate actin cytoskeletal reorganization and cellular adhesion following TCR stimulation. In addition, Tec family kinases are activated downstream of G protein-coupled chemokine receptors, where they play parallel roles in the regulation of Rho GTPases, cell polarization, adhesion, and migration. In all these systems, however, Tec family kinases are not essential signaling components, but instead function to modulate or amplify signaling pathways. Although they quantitatively reduce proximal signaling, mutations that eliminate Tec family kinases in T cells nonetheless qualitatively alter T cell development and differentiation.

Mutual regulation of protein-tyrosine phosphatase 20 and protein-tyrosine kinase Tec activities by tyrosine phosphorylation and dephosphorylation

PTP20, also known as HSCF/protein-tyrosine phosphatase K1/fetal liver phosphatase 1/brain-derived phosphatase 1, is a cytosolic protein-tyrosine phosphatase with currently unknown biological relevance. We have identified that the nonreceptor protein-tyrosine kinase Tec-phosphorylated PTP20 on tyrosines and co-immunoprecipitated with the phosphatase in a phosphotyrosine-dependent manner. The interaction between the two proteins involved the Tec SH2 domain and the C-terminal tyrosine residues Tyr-281, Tyr-303, Tyr-354, and Tyr-381 of PTP20, which were also necessary for tyrosine phosphorylation/dephosphorylation. Association between endogenous PTP20 and Tec was also tyrosine phosphorylation-dependent in the immature B cell line Ramos. Finally, the Tyr-281 residue of PTP20 was shown to be critical for deactivating Tec in Ramos cells upon B cell receptor ligation as well as dephosphorylation and deactivation of Tec and PTP20 itself in transfected COS7 cells. Taken together, PTP20 appears to play a negative role in Tec-mediated signaling, and Tec-PTP20 interaction might represent a negative feedback mechanism.

Severe B cell deficiency in mice lacking the tec kinase family members Tec and Btk

The cytoplasmic protein tyrosine kinase Tec has been proposed to have important functions in hematopoiesis and lymphocyte signal transduction. Here we show that Tec-deficient mice developed normally and had no major phenotypic alterations of the immune system. To reveal potential compensatory roles of other Tec kinases such as Bruton's tyrosine kinase (Btk), Tec/Btk double-deficient mice were generated. These mice exhibited a block at the B220(+)CD43(+) stage of B cell development and displayed a severe reduction of peripheral B cell numbers, particularly immunoglobulin (Ig)M(lo)IgD(hi) B cells. Although Tec/Btk(null) mice were able to form germinal centers, the response to T cell-dependent antigens was impaired. Thus, Tec and Btk together have an important role both during B cell development and in the generation and/or function of the peripheral B cell pool. The ability of Tec to compensate for Btk may also explain phenotypic differences in X-linked immunodeficiency (xid) mice compared with human X-linked agammaglobulinemia (XLA) patients.

Mediation by the protein-tyrosine kinase Tec of signaling between the B cell antigen receptor and Dok-1

A variety of growth factor receptors induce the tyrosine phosphorylation of a nonreceptor protein-tyrosine kinase Tec as well as that of a Tec-binding protein of 62 kDa. Given the similarity in properties between this 62-kDa protein and p62(Dok-1), the possibility that these two proteins are identical was investigated. Overexpression of a constitutively active form of Tec in a pro-B cell line induced the hyperphosphorylation of endogenous Dok-1. Tec also associated with Dok-1 in a phosphorylation-dependent manner in 293 cells. Tec mediated marked phosphorylation of Dok-1 both in vivo and in vitro, and this effect required both the Tec homology and Src homology 2 domains of Tec in addition to its kinase activity. Expression of Dok-1 in 293 cells induced inhibition of Ras activity, suggesting that Dok-1 is a negative regulator of Ras. In the immature B cell line Ramos, cross-linking of the B cell antigen receptor (BCR) resulted in tyrosine phosphorylation of Dok-1, and this effect was markedly inhibited by expression of dominant negative mutants of Tec. Furthermore, overexpression of Dok-1 inhibited activation of the c-fos promoter induced by stimulation of the BCR. These results suggest that Tec is an important mediator of signaling from the BCR to Dok-1.

A role for the Tec family tyrosine kinase Txk in T cell activation and thymocyte selection

Recent data indicate that several members of the Tec family of protein tyrosine kinases function in antigen receptor signal transduction. Txk, a Tec family protein tyrosine kinase, is expressed in both immature and mature T cells and in mast cells. By overexpressing Txk in T cells throughout development, we found that Txk specifically augments the phospholipase C (PLC)-gamma1-mediated calcium signal transduction pathway upon T cell antigen receptor (TCR) engagement. Although Txk is structurally different from inducible T cell kinase (Itk), another Tec family member expressed in T cells, expression of the Txk transgene could partially rescue defects in positive selection and signaling in itk(-)(/)(-) mice. Conversely, in the itk(+/+) (wild-type) background, overexpression of Txk inhibited positive selection of TCR transgenic thymocytes, presumably due to induction of cell death. These results identify a role for Txk in TCR signal transduction, T cell development, and selection and suggest that the Tec family kinases Itk and Txk perform analogous functions.

Molecular cloning of a docking protein, BRDG1, that acts downstream of the Tec tyrosine kinase

Tec, Btk, Itk, Bmx, and Txk constitute the Tec family of protein tyrosine kinases (PTKs), a family with the distinct feature of containing a pleckstrin homology (PH) domain. Tec acts in signaling pathways triggered by the B cell antigen receptor (BCR), cytokine receptors, integrins, and receptor-type PTKs. Although upstream regulators of Tec family kinases are relatively well characterized, little is known of the downstream effectors of these enzymes. The yeast two-hybrid system has identified several proteins that interact with the kinase domain of Tec, one of which is now revealed to be a previously unknown docking protein termed BRDG1 (BCR downstream signaling 1). BRDG1 contains a proline-rich motif, a PH domain, and multiple tyrosine residues that are potential target sites for Src homology 2 domains. In 293 cells expressing recombinant BRDG1 and various PTKs, Tec and Pyk2, but not Btk, Bmx, Lyn, Syk, or c-Abl, induced marked phosphorylation of BRDG1 on tyrosine residues. BRDG1 was also phosphorylated by Tec directly in vitro. Efficient phosphorylation of BRDG1 by Tec required the PH and SH2 domains as well as the kinase domain of the latter. Furthermore, BRDG1 was shown to participate in a positive feedback loop by increasing the activity of Tec. BRDG1 transcripts are abundant in the human B cell line Ramos, and the endogenous protein underwent tyrosine phosphorylation in response to BCR stimulation. BRDG1 thus appears to function as a docking protein acting downstream of Tec in BCR signaling.

Tec family of protein-tyrosine kinases: an overview of their structure and function

The Tec family is a recently emerging subfamily of non-receptor protein-tyrosine kinases (PTKs) represented by its first member, Tec. This family is composed of five members, namely Tec, Btk. Itk/Emt/Tsk, Bmx and Txk/Rlk. The most characteristic feature of this family is the presence of a pleckstrin homology (PH) domain in their protein structure. The PH domain is known to bind phosphoinositides; on this basis, Tec family PTKs may act as merge points of phosphotyrosine-mediated and phospholipid-mediated signaling systems. Many Tec family proteins are abundantly expressed in hematopoietic tissues, and are presumed to play important roles in the growth and differentiation processes of blood cells. Supporting this, mutations in the Btk gene cause X chromosome-linked agammaglobulinemia (XLA) in humans and X chromosome-linked immunodeficiency (Xid) in mice, indicating that Btk activity is indispensable for B-cell ontogeny. In addition, Tec family kinases have been shown to be involved in the intracellular signaling mechanisms of cytokine receptors, lymphocyte surface antigens, heterotrimeric G-protein-coupled receptors and integrin molecules. Efforts are being made to identify molecules which interact with Tec kinases to transfer Tec-mediated signals in vivo. Candidates for such second messengers include PLC-gamma2, guanine nucleotide exchange factors for RhoA and TFII-I/BAP-135. This review summarizes current knowledge concerning the input and output factors affecting the Tec kinases.

Leukocyte protein tyrosine kinases: potential targets for drug discovery

Intracellular signal transduction following the extracellular ligation of a wide variety of different types of surface molecules on leukocytes involves the activation of protein tyrosine kinases. The dependence of successful intracellular signaling on the functions of the nontransmembrane class of protein tyrosine kinases coupled with the cell type-specific expression patterns for several of these enzymes makes them appealing targets for therapeutic intervention. Development of drugs that can interfere with the catalytic functions of the nontransmembrane protein tyrosine kinases or that can disrupt critical interactions with regulatory molecules and/or substrates should find clinical applications in the treatment of allergic diseases, autoimmunity, transplantation rejection, and cancer.