Btk/Tec kinases regulate sustained increases in intracellular Ca2+ following B-cell receptor activation

Posttranscriptional regulation of Bruton's tyrosine kinase expression in antigen receptor-stimulated splenic B cells

Mutation of Bruton's tyrosine kinase (Btk) causes human X-linked agammaglobulinemia and murine X-linked immunodeficiency syndrome (xid). Quantitative aspects of B lymphocyte development and function have been demonstrated to depend on Btk level in vivo by using a murine transgenic model system. A sensitive intracellular immunofluorescent assay was developed to measure Btk protein on a per cell basis to test the hypothesis that its dosage is dynamically regulated during B cell development or functional responses. Marrow-derived hematopoietic stem cells, common lymphoid progenitor cells, and developing B and myeloid lineages expressed Btk protein at comparable levels. Resting peripheral B lineage cells had a significantly lower amount of Btk than marrow-derived cells in both wild-type and xid mice. Activation of the B cell antigen receptor up-regulated Btk protein level 10-fold within several hours by a phosphatidylinositol 3-kinase-dependent, posttranscriptional mechanism. In contrast, the protein level of Btk R28C in activated B lymphocytes from xid mice remained low. Bypass of the antigen receptor signaling pathways by treatment of cells with phorbol myristic acid and ionomycin rescued up-regulation of Btk protein in xid splenic B cells. These combined results suggest that certain receptor signals mediated by Btk regulate the level of expression of Btk protein in responding B lymphocytes to potentiate signal transduction. Dynamic regulation of Btk protein dosage is an additional mechanism to modulate B lymphocyte immune functions.

Sequential requirements of the N-terminal palmitoylation site and SH2 domain of Src family kinases in the initiation and progression of FcepsilonRI signaling

Initial biochemical signaling originating from high-affinity immunoglobulin E receptor (FcepsilonRI) has been ascribed to Src family kinases. To understand the mechanisms by which individual kinases drive the signaling, we conducted reconstitution experiments: FcepsilonRI signaling in RBL2H3 cells was first suppressed by a membrane-anchored, gain-of-function C-terminal Src kinase and then reconstructed with Src family kinases whose C-terminal negative regulatory sequence was replaced with a c-myc epitope. Those constructs derived from Lyn and Fyn, which are associated with detergent-resistant membranes (DRMs), physically interacted with resting FcepsilonRI and reconstructed clustering-induced signaling that leads to calcium mobilization and ERK1 and -2 activation. c-Src-derived construct, which was excluded from DRMs, failed to interact with FcepsilonRI and to restore the signaling, whereas creation of palmitoylatable Cys3 enabled it to interact with DRMs and with FcepsilonRI and to restore the signaling. Deletion of Src homology 3 (SH3) domain from the Lyn-derived construct did not alter its ability to transduce the series of signaling. Deletion of SH2 domain did not affect its association with DRMs and with FcepsilonRI nor clustering-induced tyrosine phosphorylation of FcepsilonRI beta and gamma subunits, but it almost abrogated the next step of tyrosine phosphorylation of Syk and its recruitment to FcepsilonRI. These findings suggest that Lyn and Fyn could, but c-Src could not, drive FcepsilonRI signaling and that N-terminal palmitoylation and SH2 domain are required in sequence for the initial interaction with FcepsilonRI and for the signal progression to the molecular assembly.

The B-cell-specific Src-family kinase Blk is dispensable for B-cell development and activation

The B-cell lymphocyte kinase (Blk) is a src-family protein tyrosine kinase specifically expressed in B-lineage cells of mice. The early onset of Blk expression during B-cell development in the bone marrow and the high expression levels of Blk in mature B cells suggest a possible important role of Blk in B-cell physiology. To study the in vivo function of Blk, mice homozygous for the targeted disruption of the blk gene were generated. In homozygous mutant mice, neither blk mRNA nor Blk protein is expressed. Despite the absence of Blk, the development, in vitro activation, and humoral immune responses of B cells to T-cell-dependent and -independent antigens are unaltered. These data are consistent with functional redundancy of Blk in B-cell development and immune responses.

Effects of Src homology domain 2 (SH2)-containing inositol phosphatase (SHIP), SH2-containing phosphotyrosine phosphatase (SHP)-1, and SHP-2 SH2 decoy proteins on Fc gamma RIIB1-effector interactions and inhibitory functions

Coaggregation of Fc gamma RIIB1 with B cell Ag receptors (BCR) leads to inhibition of BCR-mediated signaling via recruitment of Src homology domain 2 (SH2)-containing phosphatases. In vitro peptide binding experiments using phosphotyrosine-containing sequences derived from the immunoreceptor tyrosine-based inhibitory motif (ITIM) known to mediate Fc gamma RIIB1 effects suggest that the receptor uses SH2-containing inositol phosphatase (SHIP) and SH2-containing phosphotyrosine phosphatase (SHP)-1, as well as SHP-2 as effectors. In contrast, coimmunoprecipitation studies of receptor-effector associations suggest that the predominant Fc gamma RIIB1 effector protein is SHIP. However, biologically significant interactions may be lost in such studies if reactants' dissociation rates (Kd) are high. Thus, it is unclear to what extent these assays reflect the relative recruitment of SHIP, SHP-1, and SHP-2 to the receptor in vivo. As an alternative approach to this question, we have studied the effects of ectopically expressed SHIP, SHP-1, or SHP-2 SH2-containing decoy proteins on Fc gamma RIIB1 signaling. Results demonstrate the SHIP is the predominant intracellular ligand for the phosphorylated Fc gamma RIIB1 ITIM, although the SHP-2 decoy exhibits some ability to bind Fc gamma RIIB1 and block Fc receptor function. The SHIP SH2, while not affecting Fc gamma RIIB1 tyrosyl phosphorylation, blocks receptor-mediated recruitment of SHIP, SHIP phosphorylation, recruitment of p52 Shc, phosphatidylinositol 3,4,5-trisphosphate hydrolysis, inhibition of mitogen-activated protein kinase activation, and, albeit more modestly, Fc gamma RIIB1 inhibition of Ca2+ mobilization. Taken together, results implicate ITIM interactions with SHIP as a major mechanism of Fc gamma RIIB1-mediated inhibitory signaling.

Regulated Association Between the Tyrosine Kinase Emt/Itk/Tsk and Phospholipase-Cγ1 in Human T Lymphocytes

The Emt/Itk/Tsk tyrosine kinase is involved in intracellular signaling events induced by several lymphocyte surface receptors. Modulation of TCR/CD3-induced phospholipase-Cγ1 (PLCγ1) activity by the tyrosine kinase Emt/Itk/Tsk has been demonstrated based on studies of Itk-deficient murine T lymphocytes. Here we report a TCR/CD3-regulated association between Emt and PLCγ1 in both normal and leukemic T cells. In addition, this association was enhanced following independent ligation of the CD2, CD4, or CD28 costimulatory molecules, but not of CD5 or CD6 surface receptors, correlating to the induced tyrosine phosphorylation of Emt. Before Ab-induced T cell activation, we found that the Emt-SH3 domain was crucial for the constitutive Emt/PLCγ1 association; however, upon TCR/CD3 engagement, the Emt-SH2 domain was more efficient in mediating the enhanced Emt/PLCγ1 interaction. Furthermore, the PLCγ1-SH3 domain, but not the two PLCγ1-SH2 domains, contributed to formation of the protein complex. Thus, we describe a regulated interaction between Emt and PLCγ1, and based on our studies with individual Emt and PLCγ1 SH2/SH3 domains, we propose a mechanism for this association.

Phospholipase C-gamma as a signal-transducing element

A ubiquitous signaling event in hormonal responses is the phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4, 5-bisphosphate to produce the metabolite second messenger molecules inositol 1,4,5-trisphosphate and diacylglycerol. The former provokes a transient increase in intracellular free Ca(2+), while the latter serves as a direct activator of protein kinase C. In tyrosine kinase-dependent signaling pathways this reaction is mediated by the PLC-gamma isozymes. These are direct substrates of many tyrosine kinases in a wide variety of cell types. The mechanism of PLC-gamma activation involves its association with and phosphorylation by receptor and non-receptor tyrosine kinases, as well as interaction with specialized adaptor molecules and, perhaps, other second messenger molecules. However, the biochemistry of PLC-gamma is at a more advanced state than a clear understanding of exactly how this signaling element functions in the generation of a mitogenic response.

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.

SYK is upstream of phosphoinositide 3-kinase in B cell receptor signaling

We have recently demonstrated that the D3-phosphoinositide phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P(3)) is critical for producing sustained calcium signals through its role in promoting the function of TEC family tyrosine kinases such as Bruton's tyrosine kinase. Although PtdIns-3,4,5-P(3) can potentially be synthesized by any of several types of phosphoinositide 3-kinases (PI3Ks), B cell receptor (BCR)-induced PtdIns-3,4,5-P(3) production is thought to occur primarily through the activation of the class Ia (p85/p110) PI3Ks. This process has been proposed to be mediated by an interaction between the Src family kinase LYN and the p85 subunit of PI3K and/or through p85 membrane recruitment mediated by CBL and/or CD19. However, calcium signaling and other PI3K-dependent signals are relatively preserved in a LYN kinase-deficient B lymphocyte cell line, suggesting that an alternative pathway for PI3K activation exists. As SYK/ZAP70 kinases are upstream from many BCR-initiated signaling events, we directly analyzed SYK-dependent accumulation of both PtdIns-3,4,5-P(3) and PtdIns-3,4-P(2) in B cell receptor signaling using both dominant negative and genetic knockout approaches. Both methods indicate that SYK is upstream of, and necessary for, a significant portion of BCR-induced PtdIns-3,4, 5-P(3) production. Whereas CD19 does not appear to be involved in this SYK-dependent pathway, the SYK substrate CBL is likely involved as the dominant negative SYK markedly attenuates CBL tyrosine phosphorylation and completely blocks the BCR-dependent association of CBL with p85 PI3K.

Bruton's tyrosine kinase (Btk) associates with protein kinase C mu

Bruton's tyrosine kinase (Btk) is considered an essential signal transducer in B-cells. Mutational defects are associated with a severe immunodeficiency syndrome, X-chromosome linked agammaglobulinemia (XLA). Here we show by coimmunoprecipitation that a member of the protein kinase C (PKC) family, PKCmu, is constitutively associated with Btk. Neither antigen receptor (Ig) crosslinking nor stimulation of B-cells with phorbol ester or H(2)O(2) affected Btk/PKCmu interaction. GST precipitation analysis revealed association of the Btk pleckstrin/Tec homology domain with PKCmu. Transient overexpression of PKCmu deletion mutants as well as expression of selected PKCmu domains in 293T cells revealed that both the kinase domain and the regulatory C1 region are independently capable of binding to the Btk PH-TH domain. These data show the existence of a PKCmu/Btk complex in vivo and identify two PKCmu domains that participate in Btk interaction.

Functional independence and interdependence of the Src homology domains of phospholipase C-gamma1 in B-cell receptor signal transduction

B-cell receptor (BCR)-induced activation of phospholipase C-gamma1 (PLCgamma1) and PLCgamma2 is crucial for B-cell function. While several signaling molecules have been implicated in PLCgamma activation, the mechanism coupling PLCgamma to the BCR remains undefined. The role of PLCgamma1 SH2 and SH3 domains at different steps of BCR-induced PLCgamma1 activation was examined by reconstitution in a PLCgamma-negative B-cell line. PLCgamma1 membrane translocation required a functional SH2 N-terminal [SH2(N)] domain, was decreased by mutation of the SH3 domain, but was unaffected by mutation of the SH2(C) domain. Tyrosine phosphorylation did not require the SH2(C) or SH3 domains but depended exclusively on a functional SH2(N) domain, which mediated the association of PLCgamma1 with the adapter protein, BLNK. Forcing PLCgamma1 to the membrane via a myristoylation signal did not bypass the SH2(N) domain requirement for phosphorylation, indicating that the phosphorylation mediated by this domain is not due to membrane anchoring alone. Mutation of the SH2(N) or the SH2(C) domain abrogated BCR-stimulated phosphoinositide hydrolysis and signaling events, while mutation of the SH3 domain partially decreased signaling. PLCgamma1 SH domains, therefore, have interrelated but distinct roles in BCR-induced PLCgamma1 activation.

Interaction of SLP adaptors with the SH2 domain of Tec family kinases

Activation of lymphocytes through their antigen receptors leads to mobilization of intracellular Ca(2+) ions. This process requires expression of SLP adaptors and involves phosphorylation of phospholipase C-gamma isoforms by the Tec-related protein tyrosine kinase Btk in B cells and Itk in T cells. The SH2 domain of Btk and Itk is essential for phospholipase C-gamma phosphorylation and mutations in this domain lead to the X-linked agammaglobulinemia immuno deficiency in humans. Here we show that, in contrast to SH2 domains from other signaling proteins, the Btk and Itk SH2 domains exhibit a restricted binding specificity. They bind selectively to tyrosine-phosphorylated SLP-65 and SLP-76 in activated B and T cells, respectively. Our findings suggest that Btk/Itk and phospholipase C-gamma both bind via their SH2 domain to phosphorylated SLP adaptors, and that this association is required for the activation of phospholipase C-gamma.

Syk and Bruton's tyrosine kinase are required for B cell antigen receptor-mediated activation of the kinase Akt

Activation of Akt by multiple stimuli including B cell antigen receptor (BCR) engagement requires phosphatidylinositol 3-kinase and regulates processes including cell survival, proliferation, and metabolism. BCR cross-linking activates three families of non-receptor protein tyrosine kinases (PTKs) and these are transducers of signaling events including phospholipase C and mitogen-activated protein kinase activation; however, the relative roles of PTKs in BCR-mediated Akt activation are unknown. We examined Akt activation in Lyn-, Syk- and Btk-deficient DT40 cells and B cells from Lyn(-/-) mice. BCR-mediated Akt activation required Syk and was partially dependent upon Btk. Increased BCR-induced Akt phosphorylation was observed in Lyn-deficient DT40 cells and Lyn(-/-) mice compared with wild-type cells suggesting that Lyn may negatively regulate Akt function. BCR-induced tyrosine phosphorylation of the phosphatidylinositol 3-kinase catalytic subunit was abolished in Syk-deficient cells consistent with a receptor-proximal role for Syk in BCR-mediated phosphatidylinositol 3-kinase activation; in contrast, it was maintained in Btk-deficient cells, suggesting Btk functions downstream of phosphatidylinositol 3-kinase. Calcium depletion did not influence BCR-induced Akt phosphorylation/activation, showing that neither Syk nor Btk mediates its effects via changes in calcium levels. Thus, BCR-mediated Akt stimulation is regulated by multiple non-receptor PTK families which regulate Akt both proximal and distal to phosphatidylinositol 3-kinase activation.

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.

Impaired NFATc translocation and failure of Th2 development in Itk-deficient CD4+ T cells

Naive Itk-deficient CD4+ T cells were unable to establish stable IL-4 production, even when primed in Th2-inducing conditions. In contrast, IFNgamma production was little affected. Failure to express IL-4 occurred even among cells that had gone through multiple cell divisions and was associated with a delay in the kinetics and magnitude of NFATc nuclear localization. IL-4 production was restored genetically by retroviral reconstitution of Itk or biochemically by augmenting the calcium flux with ionomycin. In vivo, Itk-deficient mice were unable to establish functional Th2 cells. Development of protective Th1 cells was unimpeded. These data define a nonredundant role for Itk in modulating signals from the TCR/CD28 pathways that are specific for the establishment of stable IL-4 but not IFNgamma expression.

Identification of the SH2 Domain Binding Protein of Bruton’s Tyrosine Kinase as BLNK—Functional Significance of Btk-SH2 Domain in B-Cell Antigen Receptor-Coupled Calcium Signaling

Bruton’s tyrosine kinase (Btk) is a critical component in the B-cell antigen receptor (BCR)-coupled signaling pathway. Its deficiency in B cells leads to loss or marked reduction in the BCR-induced calcium signaling. It is known that this BCR-induced calcium signaling depends on the activation of phospholipase Cγ (PLCγ), which is mediated by Btk and another tyrosine kinase Syk and that the SH2 and pleckstrin homology (PH) domains of Btk play important roles in this activation process. Although the importance of the PH domain of Btk has been explained by its role in the membrane targeting of Btk, the functional significance of the SH2 domain in the calcium signaling has remained merely a matter of speculation. In this report, we identify that one of the major Btk-SH2 domain-binding proteins in B cells is BLNK (B-cell linker protein) and present evidences that the interaction of BLNK and the SH2 domain of Btk contributes to the complete tyrosine phosphorylation of PLCγ.

Distinct signal thresholds for the unique antigen receptor-linked gene expression programs in mature and immature B cells

Although it is well established that immature B lymphocytes are exquisitely sensitive to tolerance induction compared with their mature counterparts, the molecular basis for this difference is unknown. We demonstrate that signaling by B cell antigen receptors leads to distinct and mutually exclusive biologic responses in mature and immature B cells: upregulation of CD86, CD69, and MHC class II in mature cells and receptor editing in immature cells. These responses can be induced simply by elevation of intracellular free calcium levels, as occurs after receptor aggregation. Importantly, induction of immature B cell responses requires much smaller increases in intracellular free calcium than does induction of mature B cell responses. These differences in biologic response and sensitivity to intracellular free calcium likely contributes to selective elimination at the immature stage of even those B cells that express low affinity for self-antigens.

Cutting Edge: Association of Phospholipase C-γ2 Src Homology 2 Domains with BLNK Is Critical for B Cell Antigen Receptor Signaling

To explore the mechanism(s) by which phospholipase C (PLC)-γ2 participates in B cell Ag receptor (BCR) signaling, we have studied the function of PLC-γ2 mutants in B cells deficient in PLC-γ2. Mutation of the N-terminal Src homology 2 domain [SH2(N)] resulted in the complete loss of inositol 1,4,5-trisphosphate generation upon BCR engagement. A possible explanation for the SH2(N) requirement was provided by findings that this mutation abrogates the association of PLC-γ2 with an adaptor protein BLNK. Moreover, expression of a membrane-associated form (CD16/PLC-γ2) with SH2(N) mutation required coligation of BCR and CD16 for inositol 1,4,5-trisphosphate generation. Together, our results suggest a central role for the SH2(N) domain in directing PLC-γ2 into the close proximity of BCR signaling complex by its association with BLNK, whereby PLC-γ2 becomes tyrosine phosphorylated and thereby activated.

The B Cell Antigen Receptor Activates the Akt (Protein Kinase B)/Glycogen Synthase Kinase-3 Signaling Pathway Via Phosphatidylinositol 3-Kinase

We have previously shown that the B cell Ag receptor (BCR) activates phosphatidylinositol (PI) 3-kinase. We now show that a serine/threonine kinase called Akt or protein kinase B is a downstream target of PI 3-kinase in B cells. Akt has been shown to promote cell survival as well as the transcription and translation of proteins involved in cell cycle progression. Using an Ab that specifically recognizes the activated form of Akt that is phosphorylated on serine 473, we show that BCR engagement activates Akt in a PI 3-kinase-dependent manner. These results were confirmed using in vitro kinase assays. Moreover, BCR ligation also induced phosphorylation of Akt of threonine 308, another modification that is required for activation of Akt. In the DT40 chicken B cell line, phosphorylation of Akt on serine 473 was completely dependent on the Lyn tyrosine kinase, while the Syk tyrosine kinase was required for sustained phosphorylation of Akt. Complementary experiments in BCR-expressing AtT20 endocrine cells confirmed that Src kinases are sufficient for BCR-induced Akt phosphorylation, but that Syk is required for sustained phosphorylation of Akt on both serine 473 and threonine 308. In insulin-responsive cells, Akt phosphorylates and inactivates the serine/threonine kinase glycogen synthase kinase-3 (GSK-3). Inactivation of GSK-3 may promote nuclear accumulation of several transcription factors, including NF-ATc. We found that BCR engagement induced GSK-3 phosphorylation and decreased GSK-3 enzyme activity. Thus, BCR ligation initiates a PI 3-kinase/Akt/GSK-3 signaling pathway.

The human high-affinity immunoglobulin G receptor activates SH2-containing inositol phosphatase (SHIP)

On cytokine-primed U937 cells, aggregation of the human high-affinity immunoglobulin receptor, FcgammaRI, initiates signal transduction cascades which lead to the release of calcium from intracellular stores and no significant calcium influx. In these cells, aggregation of FcgammaRI results in no significant increase in inositol trisphosphate production, but rather phospholipase D is activated. Here we show that, in interferon-gamma (IFN-gamma)-primed cells, the SH2 containing inositol 5' phosphatase, SHIP, is constitutively associated with the membrane fraction. Following aggregation of FcgammaRI, SHIP is rapidly and transiently tyrosine phosphorylated and becomes associated with the adapter molecule Shc. Shc also becomes tyrosine phosphorylated and translocates from the cytoplasm to the membrane fraction concomitant with the association between Shc and SHIP. Further, SHIP and Shc appear to be recruited to membrane-associated immune complexes following FcgammaRI aggregation. As no immunoreceptor inhibitory motif has been demonstrated to associate with FcgammaRI, these data suggest that SHIP may be recruited to the receptor through an SH2 domain interaction with Shc.

Tyrosine Phosphorylation of Vav Stimulates IL-6 Production in Mast Cells by a Rac/c-Jun N-Terminal Kinase-Dependent Pathway

This study investigates whether the guanine nucleotide exchange activity of Vav is linked to cytokine production in mast cells. Overexpression of Vav in the RBL-2H3 mast cell line resulted in the constitutive tyrosine phosphorylation and activation of Vav. We analyzed the functional effect of Vav overexpression on cytokine production. IL-2 and IL-6 mRNA levels were dramatically increased in Vav-overexpressing cells and correlated with increased NF-AT activity. Little or no effect was observed on the mRNA levels of IL-3, IL-4, GM-CSF, TNF-α, and TGF-β. FcεRI engagement did not further enhance IL-2 and IL-6 mRNA levels and only slightly enhanced NF-AT activity, but dramatically increased the mRNA levels of other tested cytokines. To understand the signal transduction required, we focused primarily on IL-6 induction by measuring mitogen-activated protein kinase activity and analyzing the effects of mutant or dominant negative forms of Vav, Rac1, and c-Jun N-terminal kinase-1 (JNK1). Vav overexpression resulted in the constitutive activation of JNK1 with little or no effect on p38 mitogen-activated protein kinase and ERK2. This was dependent on Vav-mediated activation of Rac1 as a Dbl domain-mutated Vav, inactive Rac N17, and inactive JNK1 down-regulated the Vav-induced JNK1 or IL-6 responses. Vav expression, but not expression of domain-mutated Vav, increased IL-6 secretion from nonimmortalized bone marrow-derived mast cells upon FcεRI engagement. We conclude that Vav phosphorylation contributes to IL-6 induction in mast cells.

Pleckstrin homology domains interact with filamentous actin

A fraction of Bruton's tyrosine kinase (Btk) co-localizes with actin fibers upon stimulation of mast cells via the high affinity IgE receptor (FcepsilonRI). In this study, a molecular basis of the Btk co-localization with actin fibers is presented. Btk and other Tec family tyrosine kinases have a pleckstrin homology (PH) domain at their N termini. The PH domain is a short peptide module frequently found in signal-transducing proteins and cytoskeletal proteins. Filamentous actin (F-actin) is shown to be a novel ligand for a subset of PH domains, including that of Btk. The actin-binding site was mapped to a 10-residue region of the N-terminal region of Btk. Basic residues in this short stretch are demonstrated to be involved in actin binding. Isolated PH domains induced actin filament bundle formation. Consistent with these observations, Btk binds F-actin in vitro and in vivo. Wild-type Btk protein is in part translocated to the cytoskeleton upon FcepsilonRI cross-linking, whereas Btk containing a mutated PH domain is not. Phosphatidylinositol 3,4, 5-trisphosphate-mediated membrane translocation of Btk was enhanced in cytochalasin D-pretreated, FcepsilonRI-stimulated mast cells. These data indicate that PH domain-mediated F-actin binding plays a role in Btk co-localization with actin filaments.

Regulation of nuclear localization and transcriptional activity of TFII-I by Bruton's tyrosine kinase

Bruton's tyrosine kinase (Btk) is required for normal B-cell development, as defects in Btk lead to X-linked immunodeficiency (xid) in mice and X-linked agammaglobulinemia (XLA) in humans. Here we demonstrate a functional interaction between the multifunctional transcription factor TFII-I and Btk. Ectopic expression of wild-type Btk enhances TFII-I-mediated transcriptional activation and its tyrosine phosphorylation in transient-transfection assays. Mutation of Btk in either the PH domain (R28C, as in the murine xid mutation) or the kinase domain (K430E) compromises its ability to enhance both the tyrosine phosphorylation and the transcriptional activity of TFII-I. TFII-I associates constitutively in vivo with wild-type Btk and kinase-inactive Btk but not xid Btk. However, membrane immunoglobulin M cross-linking in B cells leads to dissociation of TFII-I from Btk. We further show that while TFII-I is found in both the nucleus and cytoplasm of wild-type and xid primary resting B cells, nuclear TFII-I is greater in xid B cells. Most strikingly, receptor cross-linking of wild-type (but not xid) B cells results in increased nuclear import of TFII-I. Taken together, these data suggest that although the PH domain of Btk is primarily responsible for its physical interaction with TFII-I, an intact kinase domain of Btk is required to enhance transcriptional activity of TFII-I in the nucleus. Thus, mutations impairing the physical and/or functional association between TFII-I and Btk may result in diminished TFII-I-dependent transcription and contribute to defective B-cell development and/or function.

Physiological requirement for both SH2 domains for phospholipase C-gamma1 function and interaction with platelet-derived growth factor receptors

Two approaches have been utilized to investigate the role of individual SH2 domains in growth factor activation of phospholipase C-gamma1 (PLC-gamma1). Surface plasmon resonance analysis indicates that the individual N-SH2 and C-SH2 domains are able to specifically recognize a phosphotyrosine-containing peptide corresponding to Tyr 1021 of the platelet-derived growth factor (PDGF) beta receptor. To assess SH2 function in the context of the full-length PLC-gamma1 molecule as well as within the intact cell, PLC-gamma1 SH2 domain mutants, disabled by site-directed mutagenesis of the N-SH2 and/or C-SH2 domain(s), were expressed in Plcg1(-/-) fibroblasts. Under equilibrium incubation conditions (4 degrees C, 40 min), the N-SH2 domain, but not the C-SH2 domain, was sufficient to mediate significant PLC-gamma1 association with the activated PDGF receptor and PLC-gamma1 tyrosine phosphorylation. When both SH2 domains in PLC-gamma1 were disabled, the double mutant did not associate with activated PDGF receptors and was not tyrosine phosphorylated. However, no single SH2 mutant was able to mediate growth factor activation of Ca2+ mobilization or inositol 1,4,5-trisphosphate (IP3) formation. Subsequent kinetic experiments demonstrated that each single SH2 domain mutant was significantly impaired in its capacity to mediate rapid association with activated PDGF receptors and become tyrosine phosphorylated. Hence, when assayed under physiological conditions necessary to achieve a rapid biological response (Ca2+ mobilization and IP3 formation), both SH2 domains of PLC-gamma1 are essential to growth factor responsiveness.

Genetic analysis of B cell antigen receptor signaling

In B lymphocytes, a signaling complex that contributes to cell fate decisions is the B cell antigen receptor (BCR). Data from knockout experiments in cell lines and mice have revealed distinct functions for the intracellular protein tyrosine kinases (Lyn, Syk, Btk) in BCR signaling and B cell development. Combinations of intracellular signaling pathways downstream of these PTKs determine the quality and quantity of BCR signaling. For example, concerted actions of the PLC-gamma 2 and PI3-K pathways are required for proper calcium responses. Similarly, the regulation of ERK and JNK responses involves both PLC-gamma 2 and GTPases pathways. Since the immune response in vivo is regulated by alteration of these signaling outcomes, achieving a precise understanding of intracellular molecular events leading to B lymphocyte proliferation, deletion, anergy, receptor editing, and survival still remains a challenge for the future.

The high-affinity IgE receptor (Fc epsilon RI): from physiology to pathology

The high affinity receptor for immunoglobulin E (designated Fc epsilon RI) is the member of the antigen (Ag) receptor superfamily responsible for linking pathogen-or allergen-specific IgEs with cellular immunologic effector functions. This review provides background information on Fc epsilon RI function combined with more detailed summaries of recent progress in understanding specific aspects of Fc epsilon RI biology and biochemistry. Topics covered include the coordination and function of the large multiprotein signaling complexes that are assembled when Fc epsilon RI and other Ag receptors are engaged, new information on human receptor structures and tissue distribution, and the role of the FcR beta chain in signaling and its potential contribution to atopic phenotypes.

The role of SHIP in growth factor induced signalling

The recently cloned, hemopoietic-specific, src homology 2 (SH2)-containing inositol phosphatase, SHIP, is rapidly gaining prominence as a potential regulator of all phosphatidylinositol (PI)-3 kinase mediated events since it has been shown both in vitro and in vivo to hydrolyze the 5' phosphate from phosphatidylinositol-3,4,5-trisphosphate (PI-3,4,5-P3). Thus SHIP, and its more widely expressed counterpart, SHIP2, could play a central role in determining PI-3,4,5-P3 and PI-3,4-P2 levels in many cell types. To explore the in vivo function of SHIP further we recently generated a SHIP knock out mouse and in this review we discuss experiments carried out with bone marrow derived mast cells (BMMCs) from these animals.

Early Arrest in B Cell Development in Transgenic Mice That Express the E41K Bruton’s Tyrosine Kinase Mutant Under the Control of the CD19 Promoter Region

Bruton’s tyrosine kinase (Btk) is a nonreceptor protein kinase that is defective in X-linked agammaglobulinemia in humans and in X-linked immunodeficiency in mice. To study the effect of Btk activation in early B cell development in vivo, we have created transgenic mouse strains expressing Btk under the control of the human CD19 promoter region. The transgenic expression of wild-type human Btk corrected all X-linked immunodeficiency features in mice carrying a targeted disruption of the Btk gene. In contrast, expression of an activated form of Btk, the E41K mutant, resulted in an almost complete arrest of B cell development in the immature IgM+IgD− B cell stage in the bone marrow, irrespective of the presence of the endogenous intact Btk gene. Immature B cells were arrested at the progression from IgMlow into IgMhigh cells, which reflects the first immune tolerance checkpoint at which autoreactive B cells become susceptible to apoptosis. As the constitutive activation of Btk is likely to mimic B cell receptor occupancy by autoantigens in the bone marrow, our findings are consistent with a role for Btk as a mediator of B cell receptor-induced apoptotic signals in the immature B cell stage. Whereas the peripheral mature B cell pool was reduced to <1% of the normal size, significant numbers of IgM-secreting plasma cells were present in the spleen. Serum IgM levels were substantial and increased with age, but specific Ab responses in vivo were lacking. We conclude that the residual peripheral B cells were efficiently driven into IgM+ plasma cell differentiation, apparently without functional selection.

Bruton's tyrosine kinase controls a sustained calcium signal essential for B lineage development and function

Genetic data support a role for Btk during the B lineage development transitions regulated by signaling through both the pre-B and the B cell antigen receptors. Dysregulated signaling at each of these transitions can result in failure of these cell populations to proliferate and subsequent cell death. Btk-dependent IP3 production is crucial for maintaining the sustained calcium signal in response to BCR engagement and is likely to regulate a subset of transcriptional events essential for B lineage growth or survival. Identification of these Btk-dependent signals will be important in understanding B cell activation, differentiation, and cell death. This information may lead to therapies specifically targeting these events in B cell autoimmunity or malignancy and provide a fuller understanding of the appropriate target populations and potential negative consequences of Btk gene therapy in XLA. Identification of Btk/Tec family kinases in an increasing number of vertebrate and invertebrate cell lineages suggests that the link between Btk and the PLC gamma/IP3/calcium signaling pathways may be broadly conserved.

Signal transduction from the B cell antigen-receptor

Ligation of the B cell antigen-receptor triggers an intricate maze of intercalated biochemical events that ultimately affect B cell biological responses. Recent advances have helped to connect many loose ends by identifying key adaptor proteins, such as BLNK/SLP-65, defining crucial roles for phosphatidylinositol-3-kinase and mapping pathways controlling the mitogen-activated protein kinases (ERK, JNK and p38).

CDw150 Associates with Src-Homology 2-Containing Inositol Phosphatase and Modulates CD95-Mediated Apoptosis

CDw150, a receptor up-regulated on activated T or B lymphocytes, has a key role in regulating B cell proliferation. Patients with X-linked lymphoproliferative disease have mutations in a gene encoding a protein, DSHP/SAP, which interacts with CDw150 and is expressed in B cells. Here we show that CDw150 on B cells associates with two tyrosine-phosphorylated proteins, 59 kDa and 145 kDa in size. The 59-kDa protein was identified as the Src-family kinase Fgr. The 145-kDa protein is the inositol polyphosphate 5′-phosphatase, SH2-containing inositol phosphatase (SHIP). Both Fgr and SHIP interact with phosphorylated tyrosines in CDw150’s cytoplasmic tail. Ligation of CDw150 induces the rapid dephosphorylation of both SHIP and CDw150 as well as the association of Lyn and Fgr with SHIP. CD95/Fas-mediated apoptosis is enhanced by signaling via CDw150, and CDw150 ligation can override CD40-induced rescue of CD95-mediated cell death. The ability of CDw150 to regulate cell death does not correlate with serine phosphorylation of the Akt kinase, but does correlate with SHIP tyrosine dephosphorylation. Thus, the CDw150 receptor may function to regulate the fate of activated B cells via SHIP as well as via the DSHP/SAP protein defective in X-linked lymphoproliferative disease patients.

Reconstitution of Btk signaling by the atypical tec family tyrosine kinases Bmx and Txk

Bruton's tyrosine kinase (Btk) is mutated in X-linked agammaglobulinemia patients and plays an essential role in B cell receptor signal transduction. Btk is a member of the Tec family of nonreceptor protein-tyrosine kinases that includes Bmx, Itk, Tec, and Txk. Cell lines deficient for Btk are impaired in phospholipase C-gamma2 (PLCgamma2)-dependent signaling. Itk and Tec have recently been shown to reconstitute PLCgamma2-dependent signaling in Btk-deficient human cells, but it is not known whether the atypical Tec family members, Bmx and Txk, can reconstitute function. Here we reconstitute Btk-deficient DT40 B cells with Bmx and Txk to compare their function with other Tec kinases. We show that in common with Itk and Tec, Bmx reconstituted PLCgamma2-dependent responses including calcium mobilization, extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) activation, and apoptosis. Txk also restored PLCgamma2/calcium signaling but, unlike other Tec kinases, functioned in a phosphatidylinositol 3-kinase-independent manner and failed to reconstitute apoptosis. These results are consistent with a common role for Tec kinases as amplifiers of PLCgamma2-dependent signal transduction, but suggest that the pleckstrin homology domain of Tec kinases, absent in Txk, is essential for apoptosis.

Immunoreceptor tyrosine-based inhibition motif-bearing receptors regulate the immunoreceptor tyrosine-based activation motif-induced activation of immune competent cells

ITIM-bearing receptors, a family which only recently has been recognized, play a key role in the regulation of the ITAM-induced activation of immune competent cells. The mechanism of ITM-mediated regulation in various cells was recently clarified. The present review focuses on ITIM bearing membrane proteins that negatively regulate the activation of cells when co-crosslinked with ITAM containing receptors, illustrates the inhibitory processes by the negative regulation of B-, NK-, T-cells and mast cells and summarizes current views on the mechanism of ITIM-mediated inhibition.

Phosphatidylinositol 3-kinase-dependent membrane association of the Bruton's tyrosine kinase pleckstrin homology domain visualized in single living cells

Phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) has been proposed to act as a second messenger to recruit regulatory proteins to the plasma membrane via their pleckstrin homology (PH) domains. The PH domain of Bruton's tyrosine kinase (Btk), which is mutated in the human disease X-linked agammaglobulinemia, has been shown to interact with PI(3,4,5)P3 in vitro. In this study, a fusion protein containing the PH domain of Btk and the enhanced green fluorescent protein (BtkPH-GFP) was constructed and utilized to study the ability of this PH domain to interact with membrane inositol phospholipids inside living cells. The localization of expressed BtkPH-GFP in quiescent NIH 3T3 cells was indistinguishable from that of GFP alone, both being cytosolic as assessed by confocal microscopy. In NIH 3T3 cells coexpressing BtkPH-GFP and the epidermal growth factor receptor, activation of epidermal growth factor or endogenous platelet-derived growth factor receptors caused a rapid (<3 min) translocation of the cytosolic fluorescence to ruffle-like membrane structures. This response was not observed in cells expressing GFP only and was completely inhibited by treatment with the PI 3-kinase inhibitors wortmannin and LY 292004. Membrane-targeted PI 3-kinase also caused membrane localization of BtkPH-GFP that was slowly reversed by wortmannin. When the R28C mutation of the Btk PH domain, which causes X-linked agammaglobulinemia, was introduced into the fluorescent construct, no translocation was observed after stimulation. In contrast, the E41K mutation, which confers transforming activity to native Btk, caused significant membrane localization of BtkPH-GFP with characteristics indicating its possible binding to PI(4,5)P2. This mutant, but not wild-type BtkPH-GFP, interfered with agonist-induced PI(4,5)P2 hydrolysis in COS-7 cells. These results show in intact cells that the PH domain of Btk binds selectively to 3-phosphorylated lipids after activation of PI 3-kinase enzymes and that losing such binding ability or specificity results in gross abnormalities in the function of the enzyme. Therefore, the interaction with PI(3,4,5)P3 is likely to be an important determinant of the physiological regulation of Btk and can be utilized to visualize the dynamics and spatiotemporal organization of changes in this phospholipid in living cells.

Inhibitory modulation of B cell receptor-mediated Ca2+ mobilization by Src homology 2 domain-containing inositol 5'-phosphatase (SHIP)

Src homology 2 domain-containing inositol 5'-phosphatase (SHIP) mediates inhibitory signals that attenuate intracellular Ca2+ mobilization in B cells upon B cell receptor (BCR) stimulation. To clarify the mechanisms affected by SHIP, we analyzed Ca2+ mobilization in the DT40 B cell line in which the SHIP gene was disrupted. In SHIP-deficient cells, Ca2+ transient elicited by BCR stimulation was more prolonged than that in control cells both in the presence and absence of extracellular Ca2+. Inositol 1,4, 5-trisphosphate production following BCR stimulation was enhanced in SHIP-deficient cells. In SHIP-deficient cells in comparison with the control cells, BCR stimulation in the absence of extracellular Ca2+ induced a greater degree of Ca2+ store depletion and the Ca2+ influx upon re-addition of extracellular Ca2+ was also greater. However, store-operated Ca2+ influx (SOC) elicited by thapsigargin-induced store depletion was not affected by SHIP. These results indicate that the primary target pathway of SHIP is the Ca2+ release from the stores, and that Ca2+ influx by the SOC mechanism is secondarily controlled by the level of Ca2+ in the stores without direct inhibition of SOC. In this way, SHIP may play an important role in ensuring the robust tuning of Ca2+ signaling in B cells.

Phosphorylation of CD19 Y484 and Y515, and Linked Activation of Phosphatidylinositol 3-Kinase, Are Required for B Cell Antigen Receptor-Mediated Activation of Bruton’s Tyrosine Kinase

Bruton’s tyrosine kinase (Btk) plays a critical role in B cell Ag receptor (BCR) signaling, as indicated by the X-linked immunodeficiency and X-linked agammaglobulinemia phenotypes of mice and men that express mutant forms of the kinase. Although Btk activity can be regulated by Src-family and Syk tyrosine kinases, and perhaps by phosphatidylinositol 3,4,5-trisphosphate, BCR-coupled signaling pathways leading to Btk activation are poorly understood. In view of previous findings that CD19 is involved in BCR-mediated phosphatidylinositol 3-kinase (PI3-K) activation, we assessed its role in Btk activation. Using a CD19 reconstituted myeloma model and CD19 gene-ablated animals we found that BCR-mediated Btk activation and phosphorylation are dependent on the expression of CD19, while BCR-mediated activation of Lyn and Syk is not. Wortmannin preincubation inhibited the BCR-mediated activation and phosphorylation of Btk. Btk activation was not rescued in the myeloma by expression of a CD19 mutant in which tyrosine residues previously shown to mediate CD19 interaction with PI3-K, Y484 and Y515, were changed to phenylalanine. Taken together, the data presented indicate that BCR aggregation-driven CD19 phosphorylation functions to promote Btk activation via recruitment and activation of PI3-K. Resultant phosphatidylinositol 3,4,5-trisphosphate probably functions to localize Btk for subsequent phosphorylation and activation by Src and Syk family kinases.

Structure of the PH domain from Bruton's tyrosine kinase in complex with inositol 1,3,4,5-tetrakisphosphate

BACKGROUND

The activity of Bruton's tyrosine kinase (Btk) is important for the maturation of B cells. A variety of point mutations in this enzyme result in a severe human immunodeficiency known as X-linked agammaglobulinemia (XLA). Btk contains a pleckstrin-homology (PH) domain that specifically binds phosphatidylinositol 3,4,5-trisphosphate and, hence, responds to signalling via phosphatidylinositol 3-kinase. Point mutations in the PH domain might abolish membrane binding, preventing signalling via Btk.

RESULTS

We have determined the crystal structures of the wild-type PH domain and a gain-of-function mutant E41K in complex with D-myo-inositol 1,3,4,5-tetra-kisphosphate (Ins (1,3,4,5)P4). The inositol Ins (1,3,4,5)P4 binds to a site that is similar to the inositol 1,4,5-trisphosphate binding site in the PH domain of phospholipase C-delta. A second Ins (1,3,4,5)P4 molecule is associated with the domain of the E41K mutant, suggesting a mechanism for its constitutive interaction with membrane. The affinities of Ins (1,3,4,5)P4 to the wild type (Kd = 40 nM), and several XLA-causing mutants have been measured using isothermal titration calorimetry.

CONCLUSIONS

Our data provide an explanation for the specificity and high affinity of the interaction with phosphatidylinositol 3,4,5-trisphosphate and lead to a classification of the XLA mutations that reside in the Btk PH domain. Mis-sense mutations that do not simply destabilize the PH fold either directly affect the interaction with the phosphates of the lipid head group or change electrostatic properties of the lipid-binding site. One point mutation (Q127H) cannot be explained by these facts, suggesting that the PH domain of Btk carries an additional function such as interaction with a Galpha protein.

Paired immunoglobulin-like receptor B (PIR-B) inhibits BCR-induced activation of Syk and Btk by SHP-1

Coligation of paired immunoglobulin-like receptor B (PIR-B) with B cell antigen receptor (BCR) blocks antigen-induced B cell activation. This inhibition is mediated in part by recruitment of SHP-1 and SHP-2 to the phosphorylated ITIMs in the cytoplasmic domain of PIR-B; however the molecular target(s) of these phosphatases remain elusive. Here we show that PIR-B ligation inhibits the BCR-induced tyrosine phosphorylation of Igalpha/Igbeta, Syk, Btk and phospholipase C (PLC)-gamma2. Overexpression of a catalytically inactive form of SHP-1 prevents the PIR-B-mediated inhibition of tyrosine phosphorylation of Syk, Btk, and PLC-gamma2. Dephosphorylation of Syk and Btk mediated by SHP-1 leads to a decrease of their kinase activity, which in turn inhibits tyrosine phosphorylation of PLC-gamma2. Furthermore, we define a requirement for Lyn in mediating tyrosine phosphorylation of PIR-B. Based on these results, we propose a model of PIR-B-mediated inhibitory signaling in which coligation of PIR-B and BCR results in phosphorylation of ITIMs by Lyn, subsequent recruitment of SHP-1, and a resulting inhibition of the BCR-induced inositol 1,4,5-trisphosphate generation by dephosphorylation of Syk and Btk.

B cell development: signal transduction by antigen receptors and their surrogates

Constitutive signal transduction by B cell antigen-receptors and/or their surrogates appears to be critical for progression through multiple developmental checkpoints and for survival of mature B cells in the periphery. Antigen-induced signaling via the B cell receptor can compensate for defects in constitutive signaling and initiates receptor editing, apoptosis and anergy in normal mice - purging the repertoire of autoreactive cells. Thus development and survival of mature B cells seem to require continuous receptor signaling of a defined amplitude.

Involvement of Wiskott-Aldrich Syndrome Protein in B-Cell Cytoplasmic Tyrosine Kinase Pathway

Bruton’s tyrosine kinase (Btk) has been shown to play a role in normal B-lymphocyte development. Defective expression of Btk leads to human and murine immunodeficiencies. However, the exact role of Btk in the cytoplasmic signal transduction in B cells is still unclear. This study represents a search for the substrate for Btk in vivo. We identified one of the major phosphoproteins associated with Btk in the preB cell line NALM6 as the Wiskott-Aldrich syndrome protein (WASP), the gene product responsible for Wiskott-Aldrich syndrome, which is another hereditary immunodeficiency with distinct abnormalities in hematopoietic cells. We demonstrated that WASP was transiently tyrosine-phosphorylated after B-cell antigen receptor cross-linking on B cells, suggesting that WASP is located downstream of cytoplasmic tyrosine kinases. An in vivo reconstitution system demonstrated that WASP is physically associated with Btk and can serve as the substrate for Btk. A protein binding assay suggested that the tyrosine-phosphorylation of WASP alters the association between WASP and a cellular protein. Furthermore, identification of the phosphorylation site of WASP in reconstituted cells allowed us to evaluate the catalytic specificity of Btk, the exact nature of which is still unknown.

The SH2 domain-containing inositol 5'-phosphatase (SHIP) recruits the p85 subunit of phosphoinositide 3-kinase during FcgammaRIIb1-mediated inhibition of B cell receptor signaling

Coligation of FcgammaRIIb1 with the B cell receptor (BCR) or FcepsilonRI on mast cells inhibits B cell or mast cell activation. Activity of the inositol phosphatase SHIP is required for this negative signal. In vitro, SHIP catalyzes the conversion of the phosphoinositide 3-kinase (PI3K) product phosphatidylinositol 3,4, 5-trisphosphate (PIP3) into phosphatidylinositol 3,4-bisphosphate. Recent data demonstrate that coligation of FcgammaRIIb1 with BCR inhibits PIP3-dependent Btk (Bruton's tyrosine kinase) activation and the Btk-dependent generation of inositol trisphosphate that regulates sustained calcium influx. In this study, we provide evidence that coligation of FcgammaRIIb1 with BCR induces binding of PI3K to SHIP. This interaction is mediated by the binding of the SH2 domains of the p85 subunit of PI3K to a tyrosine-based motif in the C-terminal region of SHIP. Furthermore, the generation of phosphatidylinositol 3,4-bisphosphate was only partially reduced during coligation of BCR with FcgammaRIIb1 despite a drastic reduction in PIP3. In contrast to the complete inhibition of Tec kinase-dependent calcium signaling, activation of the serine/threonine kinase Akt was partially preserved during BCR and FcgammaRIIb1 coligation. The association of PI3K with SHIP may serve to activate PI3K and to regulate downstream events such as B cell activation-induced apoptosis.

In situ detection of activated Bruton's tyrosine kinase in the Ig signaling complex by phosphopeptide-specific monoclonal antibodies

Bruton's tyrosine kinase (Btk) is a critical transducer of signals originating from the B cell antigen receptor (BCR). Dosage, sequential phosphorylation, and protein interactions are interdependent mechanisms influencing Btk function. Phosphopeptide-specific mAbs recognizing two distinct phosphotyrosine modifications were used to quantify Btk activation by immunofluorescent techniques during B cell stimulation. In a population of cultured B cells stimulated by BCR crosslinking and analyzed by flow cytometry, transient phosphorylation of the regulatory Btk tyrosine residues (551Y and 223Y) was detected. The kinetics of phosphorylation of the residues were temporally distinct. Tyrosine 551, a transactivating substrate site for Src-family kinases, was maximally phosphorylated within approximately 30 seconds of stimulation as monitored by flow cytometry. Tyrosine 223, an autophosphorylation site within the SH3 domain, was maximally phosphorylated at approximately 5 minutes. Btk returned to a low tyrosine phosphorylation level within 30 minutes, despite persistent elevation of global tyrosine phosphorylation. Colocalization of activated Btk molecules with the crosslinked BCR signaling complex was observed to coincide with the period of maximal Btk tyrosine phosphorylation when stimulated B cells were analyzed with confocal microscopy. The results of these in situ temporal and spatial analyses imply that Btk signaling occurs in the region of the Ig receptor signaling complex, suggesting a similar location for downstream targets of its activity.

Terreic acid, a quinone epoxide inhibitor of Bruton's tyrosine kinase

Bruton's tyrosine kinase (Btk) plays pivotal roles in mast cell activation as well as in B cell development. Btk mutations lead to severe impairments in proinflammatory cytokine production induced by cross-linking of high-affinity IgE receptor on mast cells. By using an in vitro assay to measure the activity that blocks the interaction between protein kinase C and the pleckstrin homology domain of Btk, terreic acid (TA) was identified and characterized in this study. This quinone epoxide specifically inhibited the enzymatic activity of Btk in mast cells and cell-free assays. TA faithfully recapitulated the phenotypic defects of btk mutant mast cells in high-affinity IgE receptor-stimulated wild-type mast cells without affecting the enzymatic activities and expressions of many other signaling molecules, including those of protein kinase C. Therefore, this study confirmed the important roles of Btk in mast cell functions and showed the usefulness of TA in probing into the functions of Btk in mast cells and other immune cell systems. Another insight obtained from this study is that the screening method used to identify TA is a useful approach to finding more efficacious Btk inhibitors.

How to extinguish lymphocyte activation, immunotyrosine-based inhibition motif (ITIM)-bearing molecules a solution?

ITIM-bearing molecules represent a novel family of inhibitory receptors expressed widely through the hematopoietic compartment. These molecules share certain features such as the presence in their intracytoplasmic domain of the so-called motif ITIM (ImmunoTyrosine-based Inhibition Motif). These molecules are able to recruit phosphatases on their phosphorylated ITIM and thus mediate a localized inhibition of the transduction pathways. The molecular basis of this inhibitory pathway is discussed below.