Point mutation of a tyrosine in the linker region of Syk results in a gain of function

TULA-Family Regulators of Platelet Activation

The two members of the UBASH3/TULA/STS-protein family have been shown to critically regulate cellular processes in multiple biological systems. The regulatory function of TULA-2 (also known as UBASH3B or STS-1) in platelets is one of the best examples of the involvement of UBASH3/TULA/STS proteins in cellular regulation. TULA-2 negatively regulates platelet signaling mediated by ITAM- and hemITAM-containing membrane receptors that are dependent on the protein tyrosine kinase Syk, which currently represents the best-known dephosphorylation target of TULA-2. The biological responses of platelets to collagen and other physiological agonists are significantly downregulated as a result. The protein structure, enzymatic activity and regulatory functions of UBASH3/TULA/STS proteins in the context of platelet responses and their regulation are discussed in this review.

Gain-of-function variants in SYK cause immune dysregulation and systemic inflammation in humans and mice

Spleen tyrosine kinase (SYK) is a critical immune signaling molecule and therapeutic target. We identified damaging monoallelic SYK variants in six patients with immune deficiency, multi-organ inflammatory disease such as colitis, arthritis and dermatitis, and diffuse large B cell lymphomas. The SYK variants increased phosphorylation and enhanced downstream signaling, indicating gain of function. A knock-in (SYK-Ser544Tyr) mouse model of a patient variant (p.Ser550Tyr) recapitulated aspects of the human disease that could be partially treated with a SYK inhibitor or transplantation of bone marrow from wild-type mice. Our studies demonstrate that SYK gain-of-function variants result in a potentially treatable form of inflammatory disease.

TULA-family proteins: Jacks of many trades and then some

UBASH3/STS/TULA is a novel two-member family, which exerts several key regulatory effects in multiple cell types. UBASH3B/STS-1/TULA-2 is a highly active protein tyrosine phosphatase; its major target appears to be a specific regulatory site of protein tyrosine kinases of the Syk family, dephosphorylation of which inhibits Syk and Zap-70 kinases and suppresses receptor signaling mediated by these kinases. UBASH3A/STS-2/TULA exhibits substantial homology to UBASH3B/STS-1/TULA-2, but possesses only a small fraction of phosphatase activity of UBASH3B/STS-1/TULA-2, and thus, its regulatory effect may be based also on the phosphatase-independent mechanisms. Critical physiologic effects of these proteins have been demonstrated in T lymphocytes, platelets, stem cells, and other important cell types. These proteins have also been shown to play a key role in such pathologic conditions as autoimmunity, cancer, and thrombosis. The review focuses on the recent studies of this important family of cellular regulators.

Sirt1 negatively regulates FcεRI-mediated mast cell activation through AMPK- and PTP1B-dependent processes

Sirt1, a key regulator of metabolism and longevity, has recently been implicated in the regulation of allergic reactions, although the underlying mechanism remains unclear. Here we show that Sirt1 negatively regulates FcεRI-stimulated mast cell activation and anaphylaxis through two mutually regulated pathways involving AMP-activated protein kinase (AMPK) and protein tyrosine phosphatase 1B (PTP1B). Mast cell-specific knockout of Sirt1 dampened AMPK-dependent suppression of FcεRI signaling, thereby augmenting mast cell activation both in vitro and in vivo. Sirt1 inhibition of FcεRI signaling also involved an alternative component, PTP1B, which attenuated the inhibitory AMPK pathway and conversely enhanced the stimulatory Syk pathway, uncovering a novel role of this phosphatase. Moreover, a Sirt1 activator resveratrol stimulated the inhibitory AMPK axis, with reciprocal suppression of the stimulatory PTP1B/Syk axis, thus potently inhibiting anaphylaxis. Overall, our results provide a molecular explanation for the beneficial role of Sirt1 in allergy and underscore a potential application of Sirt1 activators as a new class of anti-allergic agents.

Mast cell activation is enhanced by Tim1:Tim4 interaction but not by Tim-1 antibodies

Polymorphisms in the T cell (or transmembrane) immunoglobulin and mucin domain 1 ( TIM-1) gene, particularly in the mucin domain, have been associated with atopy and allergic diseases in mice and human. Genetic- and antibody-mediated studies revealed that Tim-1 functions as a positive regulator of Th2 responses, while certain antibodies to Tim-1 can exacerbate or reduce allergic lung inflammation. Tim-1 can also positively regulate the function of B cells, NKT cells, dendritic cells and mast cells. However, the precise molecular mechanisms by which Tim-1 modulates immune cell function are currently unknown. In this study, we have focused on defining Tim-1-mediated signaling pathways that enhance mast cell activation through the high affinity IgE receptor (FceRI). Using a Tim-1 mouse model lacking the mucin domain (Tim-1 ^Dmucin), we show for the first time that the polymorphic Tim-1 mucin region is dispensable for normal mast cell activation. We further show that Tim-4 cross-linking of Tim-1 enhances select signaling pathways downstream of FceRI in mast cells, including mTOR-dependent signaling, leading to increased cytokine production but without affecting degranulation.

Tetraspanin CD151 Is a Negative Regulator of FcεRI-Mediated Mast Cell Activation

Mast cells are critical in the pathogenesis of allergic disease due to the release of preformed and newly synthesized mediators, yet the mechanisms controlling mast cell activation are not well understood. Members of the tetraspanin family are recently emerging as modulators of FcεRI-mediated mast cell activation; however, mechanistic understanding of their function is currently lacking. The tetraspanin CD151 is a poorly understood member of this family and is specifically induced on mouse and human mast cells upon FcεRI aggregation but its functional effects are unknown. In this study, we show that CD151 deficiency significantly exacerbates the IgE-mediated late phase inflammation in a murine model of passive cutaneous anaphylaxis. Ex vivo, FcεRI stimulation of bone marrow-derived mast cells from CD151(-/-) mice resulted in significantly enhanced expression of proinflammatory cytokines IL-4, IL-13, and TNF-α compared with wild-type controls. However, FcεRI-induced mast cell degranulation was unaffected. At the molecular signaling level, CD151 selectively regulated IgE-induced activation of ERK1/2 and PI3K, associated with cytokine production, but had no effect on the phospholipase Cγ1 signaling, associated with degranulation. Collectively, our data indicate that CD151 exerts negative regulation over IgE-induced late phase responses and cytokine production in mast cells.

Spleen Tyrosine Kinase (Syk) Mediates IL-1β Induction by Primary Human Monocytes during Antibody-enhanced Dengue Virus Infection

Approximately 500,000 people are hospitalized with severe dengue illness annually. Antibody-dependent enhancement (ADE) of dengue virus (DENV) infection is believed to contribute to the pathogenic cytokine storm described in severe dengue patients, but the precise signaling pathways contributing to elevated cytokine production are not elucidated. IL-1β is a potent inflammatory cytokine that is frequently elevated during severe dengue, and the unique dual regulation of IL-1β provides an informative model to study ADE-induced cytokines. This work utilizes patient-derived anti-DENV mAbs and primary human monocytes to study ADE-induced IL-1β and other cytokines. ADE of DENV serotype 2 (DENV-2) elevates mature IL-1β secretion by monocytes independent of DENV replication by 4 h postinoculation (hpi). Prior to this, DENV immune complexes activate spleen tyrosine kinase (Syk) within 1 hpi. Syk induces elevated IL1B, TNF, and IL6 mRNA by 2 hpi. Syk mediates elevated IL-1β secretion by activating ERK1/2, and both Syk and ERK1/2 inhibitors ablated ADE-induced IL-1β secretion. Maturation of pro-IL-1β during ADE requires caspase-1 and NLRP3, but caspase-1 is suboptimally increased by ADE and can be significantly enhanced by a typical inflammasome agonist, ATP. Importantly, this inflammatory Syk-ERK signaling axis requires DENV immune complexes, because DENV-2 in the presence of serotype-matched anti-DENV-2 mAb, but not anti-DENV-1 mAb, activates Syk, ERK, and IL-1β secretion. This study provides evidence that DENV-2 immune complexes activate Syk to mediate elevated expression of inflammatory cytokines. Syk and ERK may serve as new therapeutic targets for interfering with ADE-induced cytokine expression during severe dengue.

Inhibitory effect of açaí (Euterpe oleracea Mart.) pulp on IgE-mediated mast cell activation

The palm fruit açaí is known to have potential health benefits due to its antioxidant scavenging capacities. Pretreatment of IgE-sensitized mouse primary cultured mast cells with açaí pulp resulted in the dramatic suppression of antigen-induced degranulation in a dose-dependent manner. Similarly, açaí suppressed IgE-mediated degranulation and transcription of the cytokine genes from a cultured mast cell line of rat basophilic leukemia (RBL)-2H3 cells. Açaí could selectively inhibit FcεRI signaling pathways. Furthermore, the FcεRI-mediated complementary signaling pathway was also suppressed by açaí. These results demonstrate that açaí is a potent inhibitor of IgE-mediated mast cell activation.

Regulation and function of syk tyrosine kinase in mast cell signaling and beyond

The protein tyrosine kinase Syk plays a critical role in FcεRI signaling in mast cells. Binding of Syk to phosphorylated immunoreceptor tyrosine-based activation motifs (p-ITAM) of the receptor subunits results in conformational changes and tyrosine phosphorylation at multiple sites that leads to activation of Syk. The phosphorylated tyrosines throughout the molecule play an important role in the regulation of Syk-mediated signaling. Reconstitution of receptor-mediated signaling in Syk^−/− cells by wild-type Syk or mutants which have substitution of these tyrosines with phenylalanine together with in vitro assays has been useful strategies to understand the regulation and function of Syk.

Mast cell signaling: the role of protein tyrosine kinase Syk, its activation and screening methods for new pathway participants

The aggregation by antigen of the IgE bound to its high affinity receptor on mast cells initiates a complex series of biochemical events that result in the release of inflammatory mediators. The essential role of the protein tyrosine kinase Syk has been appreciated for some time, and newer results have defined the mechanism of its activation. The use of siRNA has defined the relative contribution of Syk, Fyn and Gab2 to signaling and has made possible a screening study to identify previously unrecognized molecules that are involved in these pathways.

Autoinhibition and adapter function of Syk

Development, survival, and activation of B lymphocytes are controlled by signals emanating from the B-cell antigen receptor (BCR). The BCR has an autonomous signaling function also known as tonic signaling that allows for long-term survival of B cells in the immune system. Upon binding of antigen to the BCR, the tonic signal is amplified and diversified, leading to alteration in gene expression and B-cell activation. The spleen tyrosine kinase (Syk) intimately cooperates with the signaling subunits of the BCR and plays a central role in the amplification and diversification of BCR signals. In this review, we discuss the molecular mechanisms by which Syk activity is inhibited and activated at the BCR. Importantly, Syk acts not only as a kinase that phosphorylates downstream substrates but also as an adapter that can bind to a diverse set of signaling proteins. Depending on its interactions and localization, Syk can signal opposing cell fate decisions such as proliferation or differentiation of B cells.

Tyrosines in the carboxyl terminus regulate Syk kinase activity and function

The Syk tyrosine kinase family plays an essential role in immunoreceptor tyrosine-based activation motif (ITAM) signaling. The binding of Syk to tyrosine-phosphorylated ITAM subunits of immunoreceptors, such as FcepsilonRI on mast cells, results in a conformational change, with an increase of enzymatic activity of Syk. This conformational change exposes the COOH-terminal tail of Syk, which has three conserved Tyr residues (Tyr-623, Tyr-624, and Tyr-625 of rat Syk). To understand the role of these residues in signaling, wild-type and mutant Syk with these three Tyr mutated to Phe was expressed in Syk-deficient mast cells. There was decreased FcepsilonRI-induced degranulation, nuclear factor for T cell activation and NFkappaB activation with the mutated Syk together with reduced phosphorylation of MAP kinases p38 and p42/44 ERK. In non-stimulated cells, the mutated Syk was more tyrosine phosphorylated predominantly as a result of autophosphorylation. In vitro, there was reduced binding of mutated Syk to phosphorylated ITAM due to this increased phosphorylation. This mutated Syk from non-stimulated cells had significantly reduced kinase activity toward an exogenous substrate, whereas its autophosphorylation capacity was not affected. However, the kinase activity and the autophosphorylation capacity of this mutated Syk were dramatically decreased when the protein was dephosphorylated before the in vitro kinase reaction. Furthermore, mutation of these tyrosines in the COOH-terminal region of Syk transforms it to an enzyme, similar to its homolog ZAP-70, which depends on other tyrosine kinases for optimal activation. In testing Syk mutated singly at each one of the tyrosines, Tyr-624 but especially Tyr-625 had the major role in these reactions. Therefore, these results indicate that these tyrosines in the tail region play a critical role in regulating the kinase activity and function of Syk.

The SYK tyrosine kinase: a crucial player in diverse biological functions

Spleen tyrosine kinase (SYK) is known to have a crucial role in adaptive immune receptor signalling. However, recent reports indicate that SYK also mediates other, unexpectedly diverse biological functions, including cellular adhesion, innate immune recognition, osteoclast maturation, platelet activation and vascular development. SYK is activated by C-type lectins and integrins, and activates new targets, including the CARD9-BCL-10-MALT1 pathway and the NLRP3 inflammasome. Studies using Drosophila melanogaster suggest that there is an evolutionarily ancient origin of SYK-mediated signalling. Moreover, SYK has a crucial role in autoimmune diseases and haematological malignancies. This Review summarizes our current understanding of the diverse functions of SYK and how this is being translated for therapeutic purposes.

Spleen tyrosine kinase: an Src family of non-receptor kinase has multiple functions and represents a valuable therapeutic target in the treatment of autoimmune and inflammatory diseases

Spleen tyrosine kinase (Syk) is involved in the development and function of B and T cells, the Fc receptor-mediated degranulation of basophils and mast cells. Recent work has assigned important roles for Syk in the aberrant function of T cells in patients with systemic lupus erythematosus (SLE), osteoclasts, and urate crystal-induced neutrophil stimulation. Preclinical and early clinical studies have urged Syk inhibition for the treatment of patients with rheumatoid arthritis, whereas ex vivo experiments and preclinical studies point to a therapeutic potential of Syk inhibition in patients with SLE and crystal-induced arthritides.

Syk tyrosine 317 negatively regulates osteoclast function via the ubiquitin-protein isopeptide ligase activity of Cbl

Cytoskeletal organization of the osteoclast (OC), which is central to the capacity of the cell to resorb bone, is induced by occupancy of the alphavbeta3 integrin or the macrophage colony-stimulating factor (M-CSF) receptor c-Fms. In both circumstances, the tyrosine kinase Syk is an essential signaling intermediary. We demonstrate that Cbl negatively regulates OC function by interacting with Syk(Y317). Expression of nonphosphorylatable Syk(Y317F) in primary Syk(-/-) OCs enhances M-CSF- and alphavbeta3-induced phosphorylation of the cytoskeleton-organizing molecules, SLP76, Vav3, and PLCgamma2, to levels greater than wild type, thereby accelerating the resorptive capacity of the cell. Syk(Y317) suppresses cytoskeletal organization and function while binding the ubiquitin-protein isopeptide ligase Cbl. Consequently, Syk(Y317F) abolishes M-CSF- and integrin-stimulated Syk ubiquitination. Thus, Cbl/Syk(Y317) association negatively regulates OC function and therefore is essential for maintenance of skeletal homeostasis.

The tyrosine kinase network regulating mast cell activation

Mast cell mediator release represents a pivotal event in the initiation of inflammatory reactions associated with allergic disorders. These responses follow antigen-mediated aggregation of immunoglobulin E (IgE)-occupied high-affinity receptors for IgE (Fc epsilon RI) on the mast cell surface, a response which can be further enhanced following stem cell factor-induced ligation of the mast cell growth factor receptor KIT (CD117). Activation of tyrosine kinases is central to the ability of both Fc epsilon RI and KIT to transmit downstream signaling events required for the regulation of mast cell activation. Whereas KIT possesses inherent tyrosine kinase activity, Fc epsilon RI requires the recruitment of Src family tyrosine kinases and Syk to control the early receptor-proximal signaling events. The signaling pathways propagated by these tyrosine kinases can be further upregulated by the Tec kinase Bruton's tyrosine kinase and downregulated by the actions of the tyrosine Src homology 2 domain-containing phosphatase 1 (SHP-1) and SHP-2. In this review, we discuss the regulation and role of specific members of this tyrosine kinase network in KIT and Fc epsilon RI-mediated mast cell activation.

Syk and pTyr'd: Signaling through the B cell antigen receptor

The B cell receptor (BCR) transduces antigen binding into alterations in the activity of intracellular signaling pathways through its ability to recruit and activate the cytoplasmic protein-tyrosine kinase Syk. The recruitment of Syk to the receptor, its activation and its subsequent interactions with downstream effectors are all regulated by its phosphorylation on tyrosine. This review discusses our current understanding of how this phosphorylation regulates the activity of Syk and its participation in signaling through the BCR.

Differential dephosphorylation of the FcRgamma immunoreceptor tyrosine-based activation motif tyrosines with dissimilar potential for activating Syk

The cell surface-expressed gamma chain of the high affinity receptor for IgE (FcepsilonRI) can be phosphorylated on two tyrosine residues of the immunoreceptor tyrosine-based activation motif (ITAM), leading to recruitment and activation of spleen tyrosine kinase (Syk), a kinase that is essential for mast cell signaling and allergic responses. However, it is not known whether preferential phosphorylation or dephosphorylation of the two individual FcRgamma tyrosines (the N-terminal Tyr47 and C-terminal Tyr58) could regulate Syk activation. Herein we report that phosphorylation of only Tyr58 was able to elicit Syk phosphorylation and a weak rise in intracellular calcium, suggesting that Tyr58 phosphorylation may be distinctively important for Syk activation. In vitro and in vivo studies revealed that both Tyr47 and Tyr58 could be similarly phosphorylated. However, mass spectrometric analysis of the phosphorylated FcepsilonRgamma from bone marrow-derived mast cells showed that phosphorylation at Tyr47 was at least 2-fold greater than at Tyr58. This suggested that, once phosphorylated, Tyr58 is preferentially dephosphorylated. In vitro studies demonstrated more efficient dephosphorylation of Tyr58 (by the receptor-associated phosphatases SHP-1 and SHP-2) than of Tyr47. Analysis of Syk binding to wild type and mutant phosphorylated FcepsilonRI revealed that mutation at Tyr58 almost completely ablated Syk binding, whereas mutation at Tyr47 moderately reduced Syk binding. The findings argue for a novel regulatory mechanism, where dephosphorylation of phospho-Tyr58 is likely to promote the down-regulation of Syk activation and suppression of mast cell responses.

Hepatitis C virus NS5A protein interacts with and negatively regulates the non-receptor protein tyrosine kinase Syk

Hepatitis C virus (HCV) is the major causative agent of hepatocellular carcinoma. However, the precise mechanism underlying the carcinogenesis is yet to be elucidated. It has recently been reported that Syk, a non-receptor protein tyrosine kinase, functions as a potent tumour suppressor in human breast carcinoma. This study first examined the possible effect of HCV infection on expression of Syk in vivo. Immunohistochemical analysis revealed that endogenous Syk, which otherwise was expressed diffusely in the cytoplasm of normal hepatocytes, was localized near the cell membrane with a patchy pattern in HCV-infected hepatocytes. The possible interaction between HCV proteins and Syk in human hepatoma-derived Huh-7 cells was then examined. Immunoprecipitation analysis revealed that NS5A interacted strongly with Syk. Deletion-mutation analysis revealed that an N-terminal portion of NS5A (aa 1-175) was involved in the physical interaction with Syk. An in vitro kinase assay demonstrated that NS5A inhibited the enzymic activity of Syk and that, in addition to the N-terminal 175 residues, a central portion of NS5A (aa 237-302) was required for inhibition of Syk. Moreover, Syk-mediated phosphorylation of phospholipase C-gamma1 was downregulated by NS5A. An interaction of NS5A with Syk was also detected in Huh-7.5 cells harbouring an HCV RNA replicon or infected with HCV. In conclusion, these results demonstrated that NS5A interacts with Syk resulting in negative regulation of its kinase activity. The results indicate that NS5A may be involved in the carcinogenesis of hepatocytes through the suppression of Syk kinase activities.

Syk-mediated translocation of PI3Kdelta to the leading edge controls lamellipodium formation and migration of leukocytes

The non-receptor tyrosine kinase Syk is mainly expressed in the hematopoietic system and plays an essential role in beta(2) integrin-mediated leukocyte activation. To elucidate the signaling pathway downstream of Syk during beta2 integrin (CD11/CD18)-mediated migration and extravasation of polymorphonuclear neutrophils (PMN), we generated neutrophil-like differentiated HL-60 (dHL-60) cells expressing a fluorescently tagged Syk mutant lacking the tyrosine residue at the position 323 (Syk-Tyr323) that is known to be required for the binding of the regulatory subunit p85 of the phosphatidylinositol 3-kinase (PI3K) class I(A). Syk-Tyr323 was found to be critical for the enrichment of the catalytic subunit p110delta of PI3K class I(A) as well as for the generation of PI3K products at the leading edge of the majority of polarized cells. In accordance, the translocation of PI3K p110delta to the leading edge was diminished in Syk deficient murine PMN. Moreover, the expression of EGFP-Syk Y323F interfered with proper cell polarization and it impaired efficient migration of dHL-60 cells. In agreement with a major role of beta2 integrins in the recruitment of phagocytic cells to sites of lesion, mice with a Syk-deficient hematopoietic system demonstrated impaired PMN infiltration into the wounded tissue that was associated with prolonged cutaneous wound healing. These data imply a novel role of Syk via PI3K p110delta signaling for beta2 integrin-mediated migration which is a prerequisite for efficient PMN recruitment in vivo.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

Phospholipase D2 acts as an essential adaptor protein in the activation of Syk in antigen-stimulated mast cells

Mast cells are responsible for IgE-mediated allergic reactions. Phospholipase D1 (PLD1) and PLD2 regulate mast cell activation, but the mechanisms remain unclear. Here we show that PLD2 associates with and promotes activation of Syk, a key enzyme in mast cell activation. Antigen stimulation resulted in increased association and colocalization of Syk with PLD2 on the plasma membrane as indicated by coimmunoprecipitation and confocal microscopy. This association was dependent on tyrosine phosphorylation of Syk but not on PLD2 activity. In vitro, PLD2 interacted via its Phox homology (PX) domain with recombinant Syk to induce phosphorylation and activation of Syk. Furthermore, overexpression of PLD2 or catalytically inactive PLD2K758R enhanced antigen-induced phosphorylations of Syk and its downstream targets, the adaptor proteins LAT and SLP-76, while expression of a PLD2 siRNA blocked these phosphorylations. Apparently, the interaction of PLD2 with Syk is an early critical event in the activation of mast cells.

ITAM-mediated tonic signalling through pre-BCR and BCR complexes

Studies carried out over the past few years provide strong support for the idea that Ig alpha-Ig beta-containing complexes such as the pre-B-cell receptor and the B-cell receptor can signal independently of ligand engagement, and this has been termed tonic signalling. In this Review, I discuss recent literature that is relevant to the potential mechanisms by which tonic signals are initiated and regulated, and discuss views on how tonic and ligand-dependent (aggregation-mediated) signalling differ. These mechanisms are relevant to the possibility that tonic signals generated through immunoreceptor tyrosine-based activation motif (ITAM)-containing proteins that are expressed by oncogenic viruses induce transformation in non-haematopoietic cells.

Regulation of rat basophilic leukemia-2H3 mast cell secretion by a constitutive Lyn kinase interaction with the high affinity IgE receptor (Fc epsilon RI)

Signaling through the high affinity IgE receptor is initiated by noncovalently associated Lyn kinase, resulting in the secretion of inflammatory mediators from mast cells. A fraction of the total cellular Lyn is associated via its N-terminal unique domain with the cytoplasmic domain of the Fc epsilonRI beta subunit before receptor aggregation. In the current study, we stably transfected the unique domain of Lyn into rat basophilic leukemia-2H3 mast cells and examined the consequences on Fc epsilonRI-induced signal transduction and mediator secretion to further define the role of the unique domain of Lyn in mast cell secretion. Tyrosine phosphorylation of Fc epsilonRI beta and gamma subunits was partially inhibited in the Lyn unique domain transfectants after Ag stimulation. Ag stimulation of Lyn unique domain transfectants was accompanied by enhanced phosphorylation of MEK and ERK-2, which are required for leukotriene C4 (LTC4) release, and production of LTC4 was increased 3- to 5-fold, compared with cells transfected with vector alone. Conversely, tyrosine phosphorylation of the adaptor protein Gab2, which is essential for mast cell degranulation, was inhibited after Ag stimulation of Lyn unique domain transfectants, and Ag-induced release of histamine was inhibited up to 48%. In rat basophilic leukemia-2H3 cells, Lyn thus plays a dual role by positively regulating Fc epsilonRI phosphorylation and degranulation while negatively regulating LTC4 production. This study provides further evidence that the constitutive interaction between the unique domain of Lyn and the Fc epsilonRI beta subunit is a crucial step in the initiation of Fc epsilonRI signaling and that Lyn is limiting for Fc epsilonRI-induced secretion of inflammatory mediators.

B cell signaling and tumorigenesis

The proliferation and differentiation of lymphocytes are regulated by receptors localized on the cell surface. Engagement of these receptors induces the activation of intracellular signaling proteins that transmit the receptor signals to distinct targets and control the cellular responses. The first signaling proteins to be discovered in higher organisms were the products of oncogenes. For example, the kinases Src and Abelson (Abl) were originally identified as oncogenes and were later characterized as important proteins for signal transduction in various cell types, including lymphocytes. Now, as many cellular signaling molecules have been discovered and ordered into certain pathways, we can better understand why particular signaling proteins are associated with tumorigenesis. In this review, we discuss recent progress in unraveling the molecular mechanisms of signaling pathways that control the proliferation and differentiation of early B cells. We point out the concepts of auto-inhibition and subcellular localization as crucial aspects in the regulation of B cell signaling.

Tyrosine phosphorylation of adaptor protein 3BP2 induces T cell receptor-mediated activation of transcription factor

Molecular adaptors/scaffolds have indispensable roles in the activation of lymphocytes. In this report, we have demonstrated the role of tyrosine phosphorylation of an adaptor protein 3BP2 (c-Abl-SH3 domain binding protein-2, also known as SH3BP2) in T cell receptor (TCR)-mediated activation of transcription factor. Short interfering RNA for 3BP2 suppresses the expression level of endogenous 3BP2 and inhibits TCR-mediated activation of interleukin (IL)-2 promoter and nuclear factor of activated T cells (NFAT) element. Engagement of TCR induces tyrosine phosphorylation and lipid raft translocation of 3BP2. The overexpression studies reveal that substitution of 3BP2-Tyr(183), Tyr(446), or Arg(486) in the SH2 domain suppresses TCR-mediated activation of NFAT. Point mutations of 3BP2 cannot affect the translocation of 3BP2 into the lipid raft. Phosphorylation of Tyr(183) is required for the interaction with Vav1, the guanine nucleotide exchanging factor of Rac1. In fact, overexpression of dominant-negative form of Rac1 inhibits TCR-mediated activation of NFAT. Phosphorylation of Tyr(446) recruits the SH2 domain of Lck for the optimal activation of transcription factors. Furthermore, point mutation of Arg(486) in the 3BP2-SH2 domain that couples ZAP-70 to LAT dramatically reduces NFAT activation. These results suggest that the site-directed functions of 3BP2 induce the activation of transcription factors.

Selective Inhibition of FcεRI-Mediated Mast Cell Activation by a Truncated Variant of Cbl-b Related to the Rat Model of Type 1 Diabetes Mellitus

Ubiquitin-protein ligase Cbl-b negatively regulates high affinity IgE receptor (FcepsilonRI)-mediated degranulation and cytokine gene transcription in mast cells. In this study, we have examined the role of a truncated variant of Cbl-b related to the rat model of type 1 diabetes mellitus using the mast cell signaling model. Overexpression of the truncated Cbl-b that lacks the C-terminal region did not suppress the activation of proximal and distal signaling molecules leading to degranulation. FcepsilonRI-mediated tyrosine phosphorylation of Syk, Gab2, and phospholipase C-gamma1, and activation of c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAP kinase), and inhibitor of nuclear factor kappaB kinase (IKK), and generation of Rac1 are unaffected in cells overexpressing the truncated Cbl-b in the lipid raft. On the other hand, FcepsilonRI-mediated transcriptional activation of nuclear factor of activated T cells (NFAT), and transcription of interleukin-3 (IL-3) and IL-4 mRNA are inhibited by overexpression of the truncated variant of Cbl-b. This suppression parallels the re-compartmentalization of specific effector molecules in the lipid raft. These structural and functional analyses reveal the mechanism underlying the selective inhibition of cellular signaling by the truncated variant of Cbl-b related to insulin-dependent diabetes mellitus.

Distinct roles for the linker region tyrosines of Syk in FcepsilonRI signaling in primary mast cells

Stimulation of FcepsilonRI, the high affinity IgE receptor of mast cells results in the rapid binding of the Syk tyrosine kinase to cytoplasmic domains of FcepsilonRI and to its subsequent activation. Syk plays an essential role in signal transduction from FcepsilonRI as shown by Syk-deficient mast cells, which are defective in receptor-induced degranulation, cytokine synthesis, and intracellular pathways. However the mechanism by which Syk activates these pathways remains unclear. Activation of Syk is associated with its phosphorylation on several tyrosine residues, including the linker tyrosines Tyr317, Tyr342, and Tyr346. These residues have been proposed to play important roles in the transduction of signals by binding to other signaling proteins. To test these hypotheses in primary murine mast cells, we used retroviral infection of Syk-deficient mast cells to generate cells expressing Syk proteins bearing mutations in the linker tyrosines. We show that Tyr342 and Tyr346 contribute positively to the function of Syk and have both overlapping as well as distinct functions. Mutations in either Tyr342 or Tyr346 alone had no effect on FcepsilonRI-induced degranulation or calcium flux, whereas mutation of both residues caused a significant reduction in both pathways. In contrast, phosphorylation of PLCgamma1, PLCgamma2, and Vav1 was strongly decreased by a mutation in Tyr342 alone, whereas phosphorylation of ERK and Akt was more dependent on Tyr346. Finally we show that Tyr317 functions as a negative regulatory site and that its mutation can partially compensate for the loss of both Tyr342 and Tyr346.

Molecular basis for a direct interaction between the Syk protein-tyrosine kinase and phosphoinositide 3-kinase

After engagement of the B cell receptor for antigen, the Syk protein-tyrosine kinase becomes phosphorylated on multiple tyrosines, some of which serve as docking sites for downstream effectors with SH2 or other phosphotyrosine binding domains. The most frequently identified binding partner for catalytically active Syk identified in a yeast two-hybrid screen was the p85 regulatory subunit of phosphoinositide 3-kinase. The C-terminal SH2 domain of p85 was sufficient for mediating an interaction with tyrosine-phosphorylated Syk. Interestingly, this domain interacted with Syk at phosphotyrosine 317, a site phosphorylated in trans by the Src family kinase, Lyn, and identified previously as a binding site for c-Cbl. This site interacted preferentially with the p85 C-terminal SH2 domain compared with the c-Cbl tyrosine kinase binding domain. Molecular modeling studies showed a good fit between the p85 SH2 domain and a peptide containing phosphotyrosine 317. Tyr-317 was found to be essential for Syk to support phagocytosis mediated by FcgammaRIIA receptors expressed in a heterologous system. These studies establish a new type of p85 binding site that can exist on proteins that serve as substrates for Src family kinases and provide a molecular explanation for observations on direct interactions between Syk and phosphoinositide 3-kinase.

Inactivation of c-Cbl or Cbl-b differentially affects signaling from the high affinity IgE receptor

The Cbl family of proteins negatively regulate signaling from tyrosine kinase-coupled receptors. Among the three members of this family, only c-Cbl and Cbl-b are expressed in hemopoietic cells. To examine the role of c-Cbl and Cbl-b in Fc epsilon RI signaling, mast cell cultures from wild-type, c-Cbl(-/-), and Cbl-b(-/-) mice were generated. Cell growth rates and cell surface expression of Fc epsilon RI were similar in the different cell populations. Compared with control cells, Cbl-b inactivation resulted in increases in Fc epsilon RI-induced Ca(2+) response and histamine release. Fc epsilon RI-induced tyrosine phosphorylation of total cellular proteins, Syk, and phospholipase C-gamma was also enhanced by Cbl-b deficiency, whereas receptor-initiated phosphorylation of Vav, JNK, and p38 kinases was not changed in these cells. In contrast to Cbl-b, c-Cbl deficiency had no detectable effect on Fc epsilon RI-induced histamine release or on the phosphorylation of total cellular proteins or Syk. The absence of c-Cbl increased the phosphorylation of ERK after receptor stimulation, but resulted in slightly reduced p38 phosphorylation and Ca(2+) response. These results suggest that Cbl-b and c-Cbl have divergent effects on Fc epsilon RI signal transduction and that Cbl-b, but not c-Cbl, functions as a negative regulator of Fc epsilon RI-induced degranulation.

Point mutations of 3BP2 identified in human-inherited disease cherubism result in the loss of function

Adaptor protein 3BP2 positively regulates the high affinity IgE receptor (FcepsilonRI)-mediated activation of degranulation in mast cells. Genetic study identified the point mutations of 3BP2 gene in human-inherited disease cherubism. The multiple cysts in cherubism lesion of jaw bones are filled with the activated osteoclasts and stromal cells, including mast cells. By over-expression study using rat basophilic leukaemia RBL-2H3 mast cells, we have analysed the effect of the point mutations on the function of 3BP2 protein, which plays a positive regulatory role on FcepsilonRI-mediated mast cell activation. Over-expression of 3BP2 mutants suppressed the antigen-induced degranulation and cytokine gene transcription. Antigen-induced phosphorylation of Vav1, activation of Rac1, extracellular signal regulated kinase (ERK), c-Jun N-terminal kinase (JNK), p38 mitogen activated protein kinase (MAPK), inhibitor of nuclear factor kappaB kinase (IKK) and nuclear factor of activated T cells (NFAT) were all impaired in the cells over-expressing the cherubism mutants of 3BP2. Furthermore, cherubism mutations of 3BP2 may abrogate the binding ability to interact with chaperone protein 14-3-3. These results demonstrate that over-expression of the mutant form of 3BP2 inhibits the antigen-induced mast cell activation. It suggests that point mutations of 3BP2 gene cause the dysfunction of 3BP2 in vivo.

Dysregulated FcepsilonRI signaling and altered Fyn and SHIP activities in Lyn-deficient mast cells

Studies in B cells from Lyn-deficient mice have identified Lyn as both a kinetic accelerator and negative regulator of signaling through the BCR. The signaling properties of bone marrow-derived mast cells from Lyn(-/-) mice (Lyn(-/-) BMMCs) have also been explored, but their signaling phenotype remains controversial. We confirm that Lyn(-/-) BMMCs release more beta-hexosaminidase than wild-type BMMCs following FcepsilonRI cross-linking and show that multiple mast cell responses to FcepsilonRI cross-linking (the phosphorylation of receptor subunits and other proteins, the activation of phospholipase Cgamma isoforms, the mobilization of Ca(2+), the synthesis of phosphatidylinositol 3,4,5-trisphosphate, the activation of the alpha(4)beta(1) integrin, VLA-4) are slow to initiate in Lyn(-/-) BMMCs, but persist far longer than in wild-type cells. Mechanistic studies revealed increased basal as well as stimulated phosphorylation of the Src kinase, Fyn, in Lyn(-/-) BMMCs. Conversely, there was very little basal or stimulated tyrosine phosphorylation or activity of the inositol phosphatase, SHIP, in Lyn(-/-) BMMCs. We speculate that Fyn may substitute (inefficiently) for Lyn in signal initiation in Lyn(-/-) BMMCs. The loss of SHIP phosphorylation and activity very likely contributes to the increased levels of phosphatidylinositol 3,4,5-trisphosphate and the excess FcepsilonRI signaling in Lyn(-/-) BMMCs. The unexpected absence of the transient receptor potential channel, Trpc4, from Lyn(-/-) BMMCs may additionally contribute to their altered signaling properties.

The Cbl Family and Other Ubiquitin Ligases

Regulation of tyrosine kinase-mediated cellular activation through antigen receptors is of great biological and practical significance. The evolutionarily conserved Cbl family ubiquitin ligases have emerged as key negative regulators of activated tyrosine kinase-coupled receptors, and their impaired function switches a normal immune response into autoimmunity. Cbl proteins facilitate the ubiquitinylation of activated tyrosine kinases and other signaling proteins and of the signaling chains of receptors themselves; monoubiquitin tag promotes sorting of activated receptors and associated proteins into internal vesicles of the multivesicular body, facilitating their lysosomal degradation, whereas polyubiquitin tag promotes proteasomal degradation. Notably, increased expression of Cbl proteins and other ubiquitin ligases is a component of anergic signaling program in T cells. Thus, controlled destruction of the signaling apparatus has emerged as a key to fine-tuning antigen receptor signaling. Further studies of this pathway are likely to elucidate the pathogenesis of autoimmune diseases and offer new therapeutic targets.

Negative regulation of Lyn protein-tyrosine kinase by c-Cbl ubiquitin-protein ligase in Fc epsilon RI-mediated mast cell activation

BACKGROUND

Recent studies have demonstrated that c-Cbl functions as a ubiquitin-protein ligase toward immune receptors and non-receptor protein-tyrosine kinase Syk by facilitating their ubiquitination and subsequent targeting to proteasomes. However, it was not clear whether Src family kinase Lyn is regulated by the Cbl family of ubiquitin-protein ligases.

RESULTS

Aggregation of the high affinity IgE receptor (Fc epsilon RI) induces the rapid ubiquitination of Lyn in rat basophilic leukaemia RBL-2H3 cells. Treatment of cells with a proteasome inhibitor enhances the ubiquitination of Lyn. Stimulation of Fc epsilon RI results in the association of Lyn with c-Cbl and Cbl-b, both of which then become tyrosine phosphorylated. Co-transfection study shows that both c-Cbl and Cbl-b could induce the ubiquitination of activated Lyn in COS cells. Furthermore, over-expression of membrane-anchored form of c-Cbl inhibits the Fc epsilon RI-mediated degranulation and cytokine gene production in RBL-2H3 cells by the down-regulation of the kinase activity of Lyn through the enhanced ubiquitination.

CONCLUSIONS

These results demonstrate that Lyn is down-regulated by c-Cbl-mediated ubiquitination and subsequent degradation in proteasome after Fc epsilon RI stimulation in mast cells. Targeting of c-Cbl in the lipid raft results in the inhibition of Fc epsilon RI-mediated mast cell activation.

Negative regulation of FcϵRI-mediated mast cell activation by a ubiquitin-protein ligase Cbl-b

Aggregation of the high-affinity immunoglobulin E (IgE) receptor (FcϵRI) on mast cells induces a number of biochemical events, including protein-tyrosine phosphorylation leading to degranulation and multiple cytokine gene transcription. Here, we have demonstrated that a second member of the Cbl family of ubiquitin-protein ligase Cbl-b translocates into the lipid raft after FcϵRI engagement. Overexpression of Cbl-b in the lipid raft inhibits FcϵRI-mediated degranulation and cytokine gene transcription through the distinct mechanism. A point mutation of Cys373 in the RING finger domain of Cbl-b abrogates the suppression of FcϵRI-mediated degranulation but not cytokine gene transcription. The antigen-induced tyrosine phosphorylation of FcϵRI, Syk, phospholipase C-γ (PLC-γ), activation of c-Jun N-terminal kinase (JNK), extracellular signal regulated kinase (ERK), inhibitor of nuclear factor κB kinase (IKK), and Ca++ influx were all suppressed in the cells overexpressing Cbl-b in the lipid raft. In particular, the expression amount of Gab2 protein and thereby its FcϵRI-mediated tyrosine phosphorylation were dramatically down-regulated by ubiquitin-protein ligase activity of Cbl-b. These results suggest that Cbl-b is a negative regulator of both Lyn-Syk-LAT and Gab2mediated complementary signaling pathways in FcϵRI-mediated mast cell activation.

Activation of Syk protein tyrosine kinase in response to osmotic stress requires interaction with p21-activated protein kinase Pak2/gamma-PAK

The p21-activated serine/threonine protein kinase Pak2/gamma-PAK and the nonreceptor type of protein tyrosine kinase Syk are known to be activated when the cells are exposed to osmotic stress. The purpose of the present study was to examine whether Pak2 and Syk functionally cooperate in cellular signaling. Cotransfection studies revealed that Pak2 associates with Syk in COS cells. The constitutively active form of Cdc42 increases the association of Pak2 with Syk. Pak2 coexpressed with an inactive form of Cdc42 or kinase-inactive Pak2 interacts to a lesser extent with Syk, suggesting that Pak2-Syk association is enhanced by Pak2 activation. Interaction with Pak2 enhances the intrinsic kinase activity of Syk. This is supported by in vitro studies showing that Pak2 phosphorylates and activates Syk. Treatment of cells with sorbitol to induce hyperosmolarity results in the translocation of Pak2 and Syk to the region surrounding the nucleus and in dramatic enhancement of their association. Furthermore, cotransfection of Pak2 and Syk leads to the activation of c-Jun N-terminal kinase (JNK) under hyperosmotic conditions. Pak2 short interfering RNA suppresses sorbitol-mediated activation of endogenous Syk and JNK, thus identifying a novel pathway for JNK activation by Cdc42. These results demonstrate that Pak2 and Syk positively cooperate to regulate cellular responses to stress.

Phosphorylation of Tyr342 in the linker region of Syk is critical for Fc epsilon RI signaling in mast cells

The linker region of Syk and ZAP70 tyrosine kinases plays an important role in regulating their function. There are three conserved tyrosines in this linker region; Tyr317 of Syk and its equivalent residue in ZAP70 were previously shown to negatively regulate the function of Syk and ZAP70. Here we studied the roles of the other two tyrosines, Tyr342 and Tyr346 of Syk, in Fc epsilon RI-mediated signaling. Antigen stimulation resulted in Tyr342 phosphorylation in mast cells. Syk with Y342F mutation failed to reconstitute Fc epsilon RI-initiated histamine release. In the Syk Y342F-expressing cells there was dramatically impaired receptor-induced phosphorylation of multiple signaling molecules, including LAT, SLP-76, phospholipase C-gamma2, but not Vav. Compared to wild-type Syk, Y342F Syk had decreased binding to phosphorylated immunoreceptor tyrosine-based activation motifs and reduced kinase activity. Surprisingly, mutation of Tyr346 had much less effect on Fc epsilon RI-dependent mast cell degranulation. An anti-Syk-phospho-346 tyrosine antibody indicated that antigen stimulation induced only a very minor increase in the phosphorylation of this tyrosine. Therefore, Tyr342, but not Tyr346, is critical for regulating Syk in mast cells and the function of these tyrosines in immune receptor signaling appears to be different from what has been previously reported for the equivalent residues of ZAP70.

Activation of Syk tyrosine kinase is required for c-Cbl-mediated ubiquitination of Fcepsilon RI and Syk in RBL cells

Engagement of the high affinity receptor for IgE (FcepsilonRI) on mast cells and basophils results in FcepsilonRI beta and gamma subunits ubiquitination by an as yet undefined mechanism. Here we show that, upon FcepsilonRI engagement on RBL-2H3 cells Syk undergoes ubiquitination and Syk kinase activity is required for its own ubiquitination and that of FcepsilonRI beta and gamma chains. This requirement was demonstrated by overexpression of Syk wild-type or its kinase-dead mutant in RBL cells or using an Syk-deficient RBL-derived cell line transfected with wild-type or a kinase inactive form of Syk. We also identify c-Cbl as the E3 ligase responsible for both Syk and receptor ubiquitination. Furthermore, we demonstrate that Syk controls tyrosine phosphorylation of Syk-associated Cbl induced after receptor engagement. These data suggest a mutual regulation between Syk and Cbl activities. Finally, we show that a selective inhibitor of proteasome degradation induces persistence of tyrosine-phosphorylated receptor complexes, of activated Syk, and of FcepsilonRI-triggered degranulation. Our results provide a molecular mechanism for down-regulation of engaged receptor complexes by targeting ubiquitinated FcepsilonRI and activated Syk to the proteasome for degradation.

Regulation of FcepsilonRI-mediated degranulation by an adaptor protein 3BP2 in rat basophilic leukemia RBL-2H3 cells

Aggregation of high-affinity IgE receptor FcepsilonRI induces sequential activation of nonreceptor-type protein-tyrosine kinases and subsequent tyrosine phosphorylation of cellular proteins, leading to degranulation in mast cells. A hematopoietic cell-specific adaptor protein, 3BP2, that was originally identified as an Abl SH3-binding protein was rapidly tyrosine phosphorylated by the aggregation of FcepsilonRI on rat basophilic leukemia RBL-2H3 cells. Tyrosine phosphorylation of 3BP2 did not depend on calcium influx from external sources. To examine the role of 3BP2 in mast cells, we overexpressed the SH2 domain of 3BP2 in the RBL-2H3 cells. Overexpression of 3BP2-SH2 domain resulted in a suppression of antigen-induced degranulation as assessed by beta-hexosaminidase release. Even though overall tyrosine phosphorylation of cellular protein was not altered, antigen-mediated tyrosine phosphorylation of phospholipase C-gamma (PLC-gamma) and calcium mobilization were significantly suppressed in the cells overexpressing the 3BP2-SH2 domain. Furthermore, antigen stimulation induced the association of 3BP2-SH2 domain with LAT and other signaling molecule complexes in the RBL-2H3 cells. FcepsilonRI-mediated phosphorylation of JNK and ERK was not affected by the overexpression of 3BP2-SH2 domain. These data indicate that 3BP2 functions to positively regulate the FcepsilonRI-mediated tyrosine phosphorylation of PLC-gamma and thereby the signals leading to degranulation.

Regulation of ionomycin-mediated granule release from rat basophil leukemia cells

Cross-linking the high affinity IgE receptor on the rat basophil leukemia clone 2H3 (RBL-2H3) cell line, an vitro model for mast cell signaling, results in granule release. A great deal of research has focused on the earliest steps in this signaling cascade resulting in models which include the participation of lyn, syk, phospholipase C (PLC), protein kinase C (PKC) and intracellular calcium mobilization. In an effort to look at pathways downstream of calcium mobilization, ionomycin-mediated granule release was studied. The kinase inhibitors PP1 (src family), GF109203X (PKC), PD98059 (MEK1/2), and U0126 (MEK1/2) substantially inhibited ionomycin-mediated granule release, while the p38 kinase inhibitor SB203580 did not. Both p38 and erk were phosphorylated upon ionomycin treatment, but only extracellular regulated kinase (erk) activation was completely inhibited by PP1 treatment and partially inhibited by the MEK inhibitors, thus, correlating with the granule release data. Interestingly, while GF109203X alone had no affect on erk activation, combining it with U0126 completely blocked this response. This suggests the existence an alternate pathway for erk activation that is MEK independent and PKC dependent. Experiments in which ionomycin and PP1 were titrated (independently) demonstrated a correlation between erk phosphorylation and granule release, implicating erk in a PP1-inhibitable pathway operating downstream of calcium and controlling mast cell degranulation.

Protein tyrosine kinase Syk in mast cell signaling

The tyrosine kinase Syk is essential for signaling from FcrepsilonRI in mast cells. The Src homology domain mediated binding of Syk to the phosphorylated immunoreceptor tyrosine-based motif (ITAM) of the receptor subunits results in a conformational change and activation. Studies in Syk deficient mast cells have defined the pathways that are activated upstream and downstream of Syk and have demonstrated the functional importance of the linker region of Syk in signaling in mast cells.

Adhesion via CD43 induces Syk activation and cell proliferation in TF-1 cells

The effect of adhesion via CD43 (leukosialin, sialophorin) on cell proliferation and phosphorylation signaling were examined in a growth factor-dependent hematopoietic progenitor cell line, TF-1. TF-1 cells promptly resulted in death after withdrawal of growth factors. However, the viable cell number increased when TF-1 cells were cultured on anti-CD43 monoclonal antibody-coated plates. In this case, sustained activation of protein tyrosine kinase Syk and extracellular signal-regulated kinase (Erk) 1/2 were detected. Overexpression of exogenous Syk on TF-1 cells by the adenovirus vector system induced enhancement of the cell proliferation accompanied with enhancement of the Erk activation by a dominant-positive effect. The signal transducer and activator of transcription (STAT) 5 seemed not to be associated with the CD43-mediated cell proliferation. These results indicated that adhesion via CD43 induces the proliferation of TF-1 cells in the absence of growth factors in part by Syk-dependent Erk 1/2 signaling.

A critical role for Syk in endothelial cell proliferation and migration

Syk is a protein-tyrosine kinase that is widely expressed in haematopoietic cells and involved in coupling activated immunoreceptors to downstream signaling. On the other hand, Syk-deficient mice showed severe petechiae in utero and died shortly after birth. Recently we have shown the expression of Syk in endothelial cells and morphological defects of these cells in embryonic Syk-deficient mice. Here we report that both proliferation and migration of human umbilical vein endothelial cells were severely impaired by adenovirus-mediated expression of Syk dominant negative mutants. Furthermore, a close relationship between Syk kinase activity and extracellular signal-regulated kinase activation was suggested. Our results indicate that Syk plays a critical role in endothelial cell functions, including morphogenesis, cell growth, migration, and survival, and contributes to maintaining vascular integrity in vivo.

CrkL is an adapter for Wiskott-Aldrich syndrome protein and Syk

Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia are caused by mutations of the WAS protein (WASP) gene. WASP may be involved in the regulation of podosome, an actin-rich dynamic cell adhesion structure formed by various types of cells. The molecular links between WASP and podosomes or other cell adhesion structures are unknown. Platelets express an SH2-SH3 adapter molecule, CrkL, that can directly associate with paxillin, which is localized in podosomes. The hypothesis that CrkL binds to WASP was, therefore, tested. Results from coprecipitation experiments using anti-CrkL and GST-fusion proteins suggest that CrkL binds to WASP through its SH3 domain and that the binding was not affected by WASP tyrosine phosphorylation. The binding of GST-fusion SH3 domain of PSTPIP1 in vitro was also not affected by WASP tyrosine phosphorylation, suggesting that the binding of the SH3 domains to WASP is not inhibited by tyrosine phosphorylation of WASP. Anti-CrkL also coprecipitates a 72-kd protein, which was identified as syk tyrosine kinase, critical for collagen induced-platelet activation. CrkL immunoprecipitates contain kinase-active syk, as evidenced by an in vitro kinase assay. Coprecipitation experiments using GST-fusion CrkL proteins suggest that both SH2 and SH3 domains of CrkL are involved in the binding of CrkL to syk. WASP, CrkL, syk, and paxillin-like Hic-5 incorporated to platelet cytoskeleton after platelet aggregation. Thus, CrkL is a novel molecular adapter for WASP and syk and may potentially transfer these molecules to the cytoskeleton through association with cytoskeletal proteins such as Hic-5.

Phosphorylation of Syk activation loop tyrosines is essential for Syk function. An in vivo study using a specific anti-Syk activation loop phosphotyrosine antibody

Syk is an important protein-tyrosine kinase in immunoreceptor signaling. FcepsilonRI aggregation in mast cells induces tyrosine phosphorylation and increased enzymatic activity of Syk. The two adjacent tyrosines in the Syk activation loop are thought to be important for the propagation of FcepsilonRI signaling. To evaluate the phosphorylation of these tyrosines in vivo and further understand the relationship of Syk tyrosine phosphorylation with its function, an antibody was developed specific for phosphorylated tyrosines in the activation loop of Syk. FcepsilonRI aggregation on mast cells induced the phosphorylation of both tyrosine residues of the activation loop. The kinase activity of Syk played the major role in phosphorylating its activation loop tyrosines both in vivo and in vitro. In FcepsilonRI-stimulated mast cells, the total Syk tyrosine phosphorylation paralleled the phosphorylation of its activation loop tyrosines and downstream propagation of signals for histamine release. In contrast, the cell surface binding of anti-ganglioside monoclonal antibody AA4 induced only strong general tyrosine phosphorylation of Syk and minimal histamine release and weak phosphorylation of activation loop tyrosines. These results demonstrate that phosphorylation of the activation loop tyrosines is important for mediating receptor signaling and is a better marker of Syk function than is total Syk tyrosine phosphorylation.