Phosphorylation of the activation loop tyrosines is required for sustained Syk signaling and growth factor-independent B-cell proliferation

Differential regulatory effects of the N-terminal region in SYK-fusion kinases reveal unique activation-inducible nuclear translocation of ITK-SYK

ITK-SYK and TEL-SYK (also known as ETV6-SYK) are human tumor-causing chimeric proteins containing the kinase region of SYK, and the membrane-targeting, N-terminal, PH-TH domain-doublet of ITK or the dimerizing SAM-PNT domain of TEL, respectively. ITK-SYK causes peripheral T cell lymphoma, while TEL-SYK was reported in myelodysplastic syndrome. BTK is a kinase highly related to ITK and to further delineate the role of the N-terminus, we generated the corresponding fusion-kinase BTK-SYK. By generating and analyzing these fusion kinases, we aim to understand the contribution of N-terminal domains to their distinct cellular behavior and oncogenic properties. The fusion kinases were found to behave differently. TEL-SYK showed stronger oncogenic capacity when compared with ITK-SYK and BTK-SYK. Furthermore, ITK-SYK and BTK-SYK triggered IL-3-independent growth of BAF3 pro-B cells. In contrast to BTK-SYK and TEL-SYK, which predominantly localized in perinuclear region and cytoplasm respectively, ITK-SYK exhibits a more diverse cellular distribution, being present in the nucleus, cytoplasm and membrane-bound compartments. Notably, we observed that ITK-SYK undergoes activation-mediated nuclear translocation, a phenomenon that is uncommon among kinases. This unique feature of ITK-SYK is therefore of particular interest due to its potential connection to its transforming capability.

Asebogenin suppresses thrombus formation via inhibition of Syk phosphorylation

BACKGROUND AND PURPOSE

Thrombosis is a major cause of morbidity and mortality worldwide. Platelet activation by exposed collagen through glycoprotein VI (GPVI) and formation of neutrophil extracellular traps (NETs) are critical pathogenic factors for arterial and venous thrombosis. Both events are regulated by spleen tyrosine kinase (Syk)-mediated signalling events. Asebogenin is a dihydrochalcone whose pharmacological effects remain largely unknown. This study aims to investigate the antithrombotic effects of asebogenin and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Platelet aggregation was assessed using an aggregometer. Platelet P-selectin exposure, integrin activation and calcium mobilization were determined by flow cytometry. NETs formation was assessed by SYTOX Green staining and immunohistochemistry. Quantitative phosphoproteomics, microscale thermophoresis, in vitro kinase assay and molecular docking combined with dynamics simulation were performed to characterize the targets of asebogenin. The in vivo effects of asebogenin on arterial thrombosis were investigated using FeCl₃ -induced and laser-induced injury models, whereas those of venous thrombosis were induced by stenosis of the inferior vena cava.

KEY RESULTS

Asebogenin inhibited a series of GPVI-induced platelet responses and suppressed NETs formation induced by proinflammatory stimuli. Mechanistically, asebogenin directly interfered with the phosphorylation of Syk at Tyr525/526, which is important for its activation. Further, asebogenin suppressed arterial thrombosis demonstrated by decreased platelet accumulation and fibrin generation and attenuated venous thrombosis determined by reduced neutrophil accumulation and NETs formation, without increasing bleeding risk.

CONCLUSION AND IMPLICATIONS

Asebogenin exhibits potent antithrombotic effects by targeting Syk and is a potential lead compound for the development of efficient and safe antithrombotic agents.

Phosphorylation on Syk Y342 is important for both ITAM and hemITAM signaling in platelets

Immune cells express receptors bearing an immune tyrosine activation motif (ITAM) containing two YXXL motifs or hemITAMs containing only one YXXL motif. Phosphorylation of the ITAM/hemITAM is mediated by Src family kinases allowing for the binding and activation of spleen tyrosine kinase (Syk). It is believed that Syk must be phosphorylated on tyrosine residues for activation, and Tyr342, а conserved tyrosine in the interdomain B region, has been shown to be critical for regulating Syk in FcεR1-activated mast cells. Syk is a key mediator of signaling pathways downstream of several platelet pathways including the ITAM bearing glycoprotein VI (GPVI)/Fc receptor gamma chain collagen receptor and the hemITAM containing C-type lectin-like receptor-2 (CLEC-2). Since platelet activation is a crucial step in both hemostasis and thrombosis, we evaluated the importance of Syk Y342 in these processes by producing an Syk Y342F knock-in mouse. When using a CLEC-2 antibody as an agonist, reduced aggregation and secretion were observed in Syk Y342F mouse platelets when compared with control mouse platelets. Platelet reactivity was also reduced in response to the GPVI agonist collagen-related peptide. Signaling initiated by either GPVI or CLEC-2 was also greatly inhibited, including Syk Y519/520 phosphorylation. Hemostasis, as measured by tail bleeding time, was not altered in Syk Y342F mice, but thrombus formation in response to FeCl₃ injury was prolonged in Syk Y342F mice. These data demonstrate that phosphorylation of Y342 on Syk following stimulation of either GPVI or CLEC-2 receptors is important for the ability of Syk to transduce a signal.

A SYK/SHC1 pathway regulates the amount of CFTR in the plasma membrane

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause the recessive genetic disease cystic fibrosis, where the chloride transport across the apical membrane of epithelial cells mediated by the CFTR protein is impaired. CFTR protein trafficking to the plasma membrane (PM) is the result of a complex interplay between the secretory and membrane recycling pathways that control the number of channels present at the membrane. In addition, the ion transport activity of CFTR at the PM is modulated through post-translational protein modifications. Previously we described that spleen tyrosine kinase (SYK) phosphorylates a specific tyrosine residue in the nucleotide-binding domain 1 domain and this modification can regulate the PM abundance of CFTR. Here we identified the underlying biochemical mechanism using peptide pull-down assays followed by mass spectrometry. We identified in bronchial epithelial cells that the adaptor protein SHC1 recognizes tyrosine-phosphorylated CFTR through its phosphotyrosine-binding domain and that the formation of a complex between SHC1 and CFTR is induced at the PM in the presence of activated SYK. The depletion of endogenous SHC1 expression was sufficient to promote an increase in CFTR at the PM of these cells. The results identify a SYK/SHC1 pathway that regulates the PM levels of CFTR channels, contributing to a better understanding of how CFTR-mediated chloride secretion is regulated.

Mechanisms of B Cell Receptor Activation and Responses to B Cell Receptor Inhibitors in B Cell Malignancies

The B cell receptor (BCR) pathway has been identified as a potential therapeutic target in a number of common B cell malignancies, including chronic lymphocytic leukemia, diffuse large B cell lymphoma, Burkitt lymphoma, follicular lymphoma, mantle cell lymphoma, marginal zone B cell lymphoma, and Waldenstrom's macroglobulinemia. This finding has resulted in the development of numerous drugs that target this pathway, including various inhibitors of the kinases BTK, PI3K, and SYK. Several of these drugs have been approved in recent years for clinical use, resulting in a profound change in the way these diseases are currently being treated. However, the response rates and durability of responses vary largely across the different disease entities, suggesting a different proportion of patients with an activated BCR pathway and different mechanisms of BCR pathway activation. Indeed, several antigen-dependent and antigen-independent mechanisms have recently been described and shown to result in the activation of distinct downstream signaling pathways. The purpose of this review is to provide an overview of the mechanisms responsible for the activation of the BCR pathway in different B cell malignancies and to correlate these mechanisms with clinical responses to treatment with BCR inhibitors.

SKAP2 is required for defense against K. pneumoniae infection and neutrophil respiratory burst

Klebsiella pneumoniae is a respiratory, blood, liver, and bladder pathogen of significant clinical concern. We show that the adaptor protein, SKAP2, is required for protection against K. pneumoniae (ATCC 43816) pulmonary infections. Skap2-/- mice had 100-fold higher bacterial burden when compared to wild-type and burden was controlled by SKAP2 expression in innate immune cells. Skap2-/- neutrophils and monocytes were present in infected lungs, and the neutrophils degranulated normally in response to K. pneumoniae infection in mice; however, K. pneumoniae-stimulated reactive oxygen species (ROS) production in vitro was abolished. K. pneumoniae-induced neutrophil ROS response required the activity of SFKs, Syk, Btk, PLCγ2, and PKC. The loss of SKAP2 significantly hindered the K. pneumoniae-induced phosphorylation of SFKs, Syk, and Pyk2 implicating SKAP2 as proximal to their activation in pathogen-signaling pathways. In conclusion, SKAP2-dependent signaling in neutrophils is essential for K. pneumoniae-activated ROS production and for promoting bacterial clearance during infection.

Klebsiella pneumoniae is a type of bacteria that can cause life-threatening infections – including pneumonia, blood stream infections, and urinary tract infections – in hospitalized patients. These infections can be difficult to treat because some K. pneumoniae are resistant to antibiotics. The bacteria are normally found in the human intestine, and they do not usually cause infections in healthy people. This implies that healthy people’s immune systems are better able to fend off K. pneumoniae infections; learning how could help scientists develop new ways to treat or prevent infections in hospitalized patients.

In healthy people, a type of immune cell called neutrophils are the first line of defense against bacterial infections. Several different proteins are needed to activate neutrophils, including a protein called SKAP2. But the role of this protein in fighting K. pneumoniae infections is not clear.

To find out what role SKAP2 plays in the defense against pneumonia caused by K. pneumoniae, Nguyen et al. compared infections in mice with and without the protein. Mice lacking SKAP2 in their white blood cells had more bacteria in their lungs than normal mice. The experiments showed that neutrophils from mice with SKAP2 produce a burst of chemicals called “reactive oxygen species”, which can kill bacteria. But neutrophils without the protein do not. Without SKAP2, several proteins that help produce reactive oxygen species do not work.

Understanding the role of SKAP2 in fighting infections may help scientists better understand the immune system. This could help clinicians to treat conditions that cause it to be hyperactive or ineffective. More studies are needed to determine if SKAP2 works the same way in human neutrophils and if it works against all types of K. pneumoniae. If it does, then scientists might be able use this information to develop therapies that help the immune system fight infections.

Selective recruitment of Nck and Syk contribute to distinct leukocyte immune-type receptor-initiated target interactions

The ability of phagocytes to recognize, immobilize, and engulf extracellular targets are fundamental immune cell processes that allow for the destruction of a variety of microbial intruders. The phagocytic process depends onsignalling events that initiate dynamic changes in the plasma membrane architecture that are required to accommodate the internalization of large particulate targets. To better understand fundamental molecular mechanisms responsible for facilitating phagocytic receptor-mediated regulation of cytoskeletal networks, our research has focused on investigating representative immunoregulatory proteins from the channel catfish (Ictalurus punctatus) leukocyte immune-type receptor family (IpLITRs). Specifically, we have shown that a specific IpLITR-type can regulate the constitutive deployment of filopodial-like structures to actively capture and secure targets to the phagocyte surface, which is followed by F-actin mediated membrane dynamics that are associated with the formation of phagocytic cup-like structures that precede target engulfment. In the present study, we use confocal imaging to examine the recruitment of mediators of the F-actin cytoskeleton during IpLITR-mediated regulation of membrane dynamics. Our results provide novel details regarding the dynamic recruitment of the signaling effectors Nck and Syk during classical as well as atypical IpLITR-induced phagocytic processes.

Low intensity pulsed ultrasound (LIPUS) maintains osteogenic potency by the increased expression and stability of Nanog through spleen tyrosine kinase (Syk) activation

Mesenchymal stem cells (MSCs) are a powerful tool for cell-based, clinical therapies like bone regeneration. Therapeutic use of cell transplantation requires many cells, however, the expansion process needed to produce large quantities of cells reduces the differentiation potential of MSCs. Here, we examined the protective effects of low intensity pulsed ultrasound (LIPUS) on the maintenance of osteogenic potency. Primary osteoblastic cells were serially passaged between 2 and 12 times with daily LIPUS treatment. We found that LIPUS stimulation maintains osteogenic differentiation capacity in serially passaged cells, as characterized by improved matrix mineralization and Osteocalcin mRNA expression. Decreased expression of Nanog, Sox2, and Msx2, and increased expression of Pparg2 from serial passaging was recovered in LIPUS-stimulated cells. We found that LIPUS stimulation not only increased but also sustained expression of Nanog in primary osteoblasts and ST2 cells, a mouse mesenchymal stromal cell line. Nanog overexpression in serially passaged cells mimicked the recuperative effects of LIPUS on osteogenic potency, highlighting the important role of Nanog in LIPUS stimulation. Additionally, we found that spleen tyrosine kinase (Syk) is an important signaling molecule to induce Nanog expression in LIPUS-stimulated cells. Syk activation was regulated by both Rho-associated kinase 1 (ROCK1) and extracellular ATP in a paracrine manner. Interestingly, the LIPUS-induced increase in Nanog mRNA expression was regulated by ATP-P2X4-Syk Y323 activation, while the improvement of Nanog protein stability was controlled by the ROCK1-Syk Y525/526 pathway. Taken together, these results indicate that LIPUS stimulation recovers and maintains the osteogenic potency of serially passaged cells through a Syk-Nanog axis.

Tyrosine phosphorylation modulates cell surface expression of chloride cotransporters NKCC2 and KCC3

Cellular chloride transport has a fundamental role in cell volume regulation and renal salt handling. Cellular chloride entry or exit are mediated at the plasma membrane by cotransporter proteins of the solute carrier 12 family. For example, NKCC2 resorbs chloride with sodium and potassium ions at the apical membrane of epithelial cells in the kidney, whereas KCC3 releases chloride with potassium ions at the basolateral membrane. Their ion transport activity is regulated by protein phosphorylation in response to signaling pathways. An additional regulatory mechanism concerns the amount of cotransporter molecules inserted into the plasma membrane. Here we describe that tyrosine phosphorylation of NKCC2 and KCC3 regulates their plasma membrane expression levels. We identified that spleen tyrosine kinase (SYK) phosphorylates a specific N-terminal tyrosine residue in each cotransporter. Experimental depletion of endogenous SYK or pharmacological inhibition of its kinase activity increased the abundance of NKCC2 at the plasma membrane of human embryonic kidney cells. In contrast, overexpression of a constitutively active SYK mutant decreased NKCC2 membrane abundance. Intriguingly, the same experimental approaches revealed the opposite effect on KCC3 abundance at the plasma membrane, compatible with the known antagonistic roles of NKCC and KCC cotransporters in cell volume regulation. Thus, we identified a novel pathway modulating the cell surface expression of NKCC2 and KCC3 and show that this same pathway has opposite functional outcomes for these two cotransporters. The findings have several biomedical implications considering the role of these cotransporters in regulating blood pressure and cell volume.

Inhibition of SYK or BTK augments venetoclax sensitivity in SHP1-negative/BCL-2-positive diffuse large B-cell lymphoma

The BCL-2 inhibitor venetoclax has only limited activity in DLBCL despite frequent BCL-2 overexpression. Since constitutive activation of the B cell receptor (BCR) pathway has been reported in both ABC and GCB DLBCL, we investigated whether targeting SYK or BTK will increase sensitivity of DLBCL cells to venetoclax. We report that pharmacological inhibition of SYK or BTK synergistically enhances venetoclax sensitivity in BCL-2-positive DLBCL cell lines with an activated BCR pathway in vitro and in a xenograft model in vivo, despite the only modest direct cytotoxic effect. We further show that these sensitizing effects are associated with inhibition of the downstream PI3K/AKT pathway and changes in the expression of MCL-1, BIM, and HRK. In addition, we show that BCR-dependent GCB DLBCL cells are characterized by deficiency of the phosphatase SHP1, a key negative regulator of the BCR pathway. Re-expression of SHP1 in GCB DBLCL cells reduces SYK, BLNK, and GSK3 phosphorylation and induces corresponding changes in MCL1, BIM, and HRK expression. Together, these findings suggest that SHP1 deficiency is responsible for the constitutive activation of the BCR pathway in GCB DLBCL and identify SHP1 and BCL-2 as potential predictive markers for response to treatment with a venetoclax/BCR inhibitor combination.

SYK Inhibition Potentiates the Effect of Chemotherapeutic Drugs on Neuroblastoma Cells in Vitro

Neuroblastoma is a malignancy arising from the developing sympathetic nervous system and the most common and deadly cancer of infancy. New therapies are needed to improve the prognosis for high-risk patients and to reduce toxicity and late effects. Spleen tyrosine kinase (SYK) has previously been identified as a promising drug target in various inflammatory diseases and cancers but has so far not been extensively studied as a potential therapeutic target in neuroblastoma. In this study, we observed elevated SYK gene expression in neuroblastoma compared to neural crest and benign neurofibroma. While SYK protein was detected in the majority of examined neuroblastoma tissues it was less frequently observed in neuroblastoma cell lines. Depletion of SYK by siRNA and the use of small molecule SYK inhibitors significantly reduced the cell viability of neuroblastoma cell lines expressing SYK protein. Moreover, SYK inhibition decreased ERK1/2 and Akt phosphorylation. The SYK inhibitor BAY 61-3606 enhanced the effect of different chemotherapeutic drugs. Transient expression of a constitutive active SYK variant increased the viability of neuroblastoma cells independent of endogenous SYK levels. Collectively, our findings suggest that targeting SYK in combination with conventional chemotherapy should be further evaluated as a treatment option in neuroblastoma.

Significant Hypo-Responsiveness to GPVI and CLEC-2 Agonists in Pre-Term and Full-Term Neonatal Platelets and following Immune Thrombocytopenia

Neonatal platelets are hypo-reactive to the tyrosine kinase-linked receptor agonist collagen. Here, we have investigated whether the hypo-responsiveness is related to altered levels of glycoprotein VI (GPVI) and integrin α2β1, or to defects in downstream signalling events by comparison to platelet activation by C-type lectin-like receptor 2 (CLEC-2). GPVI and CLEC-2 activate a Src- and Syk-dependent signalling pathway upstream of phospholipase C (PLC) γ2. Phosphorylation of a conserved YxxL sequence known as a (hemi) immunotyrosine-based-activation-motif (ITAM) in both receptors is critical for Syk activation. Platelets from human pre-term and full-term neonates display mildly reduced expression of GPVI and CLEC-2, as well as integrin αIIbβ3, accounted for at the transcriptional level. They are also hypo-responsive to the two ITAM receptors, as shown by measurement of integrin αIIbβ3 activation, P-selectin expression and Syk and PLCγ2 phosphorylation. Mouse platelets are also hypo-responsive to GPVI and CLEC-2 from late gestation to 2 weeks of age, as determined by measurement of integrin αIIbβ3 activation. In contrast, the response to G protein-coupled receptor agonists was only mildly reduced and in some cases not altered in neonatal platelets of both species. A reduction in response to GPVI and CLEC-2, but not protease-activated receptor 4 (PAR-4) peptide, was also observed in adult mouse platelets following immune thrombocytopenia, whereas receptor expression was not impaired. Our results demonstrate developmental differences in platelet responsiveness to GPVI and CLEC-2, and also following immune platelet depletion leading to reduced Syk activation. The rapid generation of platelets during development or following platelet depletion is achieved at the expense of signalling by ITAM-coupled receptors.

Spleen tyrosine kinase (SYK) is a potential target for the treatment of cutaneous lupus erythematosus patients

Spleen tyrosine kinase (SYK) is a protein kinase involved in cell proliferation and the regulation of inflammatory pathways. Due to the increasing evidence that kinase inhibitors have potential as specific anti-inflammatory drugs, we have investigated the potential for SYK inhibition as a therapeutic target in autoimmune diseases, particularly cutaneous lupus erythematosus (CLE). Skin samples of patients with different CLE subtypes and appropriate controls were analysed for the expression of SYK and SYK-associated pro-inflammatory mediators via gene expression analysis and immunohistochemistry. The functional role of SYK in keratinocytes was investigated in vitro, using LE-typical pro-inflammatory stimuli and a selective inhibitor of SYK. SYK-associated genes are strongly upregulated in CLE skin lesions. Importantly, phosphorylated SYK (pSYK) is strongly expressed by several immune cell types and also keratinocytes in CLE skin. In vitro, immunostimulatory nucleic acids are capable of inducing SYK phosphorylation in keratinocytes leading to the induction of pro-inflammatory cytokines, while small-molecule SYK inhibition decreases the expression of these proteins. The results demonstrate that pSYK is expressed by immune cells and keratinocytes in skin lesions of CLE patients. LE-typical stimuli induce the expression of pSYK in vitro. Small-molecule SYK inhibition leads to a reduction of pSYK expression and downregulation of pro-inflammatory cytokines in keratinocytes. We therefore believe that pSYK provides a potential future drug target for the treatment of patients who suffer from CLE and related skin disorders. Specifically, our study reveals evidence supporting the use of topical SYK inhibitors in treating lupus.

Modulation of BCR Signaling by the Induced Dimerization of Receptor-Associated SYK

Clustering of the B cell antigen receptor (BCR) by polyvalent antigens is transmitted through the SYK tyrosine kinase to the activation of multiple intracellular pathways that determine the physiological consequences of receptor engagement. To explore factors that modulate the quantity and quality of signals sent by the crosslinked BCR, we developed a novel chemical mediator of dimerization to induce clustering of receptor-associated SYK. To accomplish this, we fused SYK with E. coli dihydrofolate reductase (eDHFR), which binds the small molecule trimethoprim (TMP) with high affinity and selectivity and synthesized a dimer of TMP with a flexible linker. The TMP dimer is able to induce the aggregation of eDHFR-linked SYK in live cells. The induced dimerization of SYK bound to the BCR differentially regulates the activation of downstream transcription factors, promoting the activation of Nuclear Factor of Activated T cells (NFAT) without affecting the activation of NFκB. The dimerization of SYK enhances the duration but not the amplitude of calcium mobilization by enhancing the extent and duration of its interaction with the crosslinked BCR at the plasma membrane.

Ectopic ILT3 controls BCR-dependent activation of Akt in B-cell chronic lymphocytic leukemia

The high proportion of long-term nonprogressors among chronic lymphocytic leukemia (CLL) patients suggests the existence of a regulatory network that restrains the proliferation of tumor B cells. The identification of molecular determinants composing such network is hence fundamental for our understanding of CLL pathogenesis. Based on our previous finding establishing a deficiency in the signaling adaptor p66Shc in CLL cells, we undertook to identify unique phenotypic traits caused by this defect. Here we show that a lack of p66Shc shapes the transcriptional profile of CLL cells and leads to an upregulation of the surface receptor ILT3, the immunoglobulin-like transcript 3 that is normally found on myeloid cells. The ectopic expression of ILT3 in CLL was a distinctive feature of neoplastic B cells and hematopoietic stem cells, thus identifying ILT3 as a selective marker of malignancy in CLL and the first example of phenotypic continuity between mature CLL cells and their progenitors in the bone marrow. ILT3 expression in CLL was found to be driven by Deltex1, a suppressor of antigen receptor signaling in lymphocytes. Triggering of ILT3 inhibited the activation of Akt kinase upon B-cell receptor (BCR) stimulation. This effect was achieved through the dynamic coalescence of ILT3, BCRs, and phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase 1 into inhibitory clusters at the cell surface. Collectively, our findings identify ILT3 as a signature molecule of p66Shc deficiency in CLL and indicate that ILT3 may functionally contribute to a regulatory network controlling tumor progression by suppressing the Akt pathway.

Differential mast cell outcomes are sensitive to FcεRI-Syk binding kinetics

Cross-linking of immunoglobulin E-bound FcεRI triggers multiple cellular responses, including degranulation and cytokine production. Signaling is dependent on recruitment of Syk via docking of its dual SH2 domains to phosphorylated tyrosines within the FcεRI immunoreceptor tyrosine-based activation motifs. Using single-molecule imaging in live cells, we directly visualized and quantified the binding of individual mNeonGreen-tagged Syk molecules as they associated with the plasma membrane after FcεRI activation. We found that Syk colocalizes transiently to FcεRI and that Syk-FcεRI binding dynamics are independent of receptor aggregate size. Substitution of glutamic acid for tyrosine between the Syk SH2 domains (Syk-Y130E) led to an increased Syk-FcεRI off-rate, loss of site-specific Syk autophosphorylation, and impaired downstream signaling. Genome edited cells expressing only Syk-Y130E were deficient in antigen-stimulated calcium release, degranulation, and production of some cytokines (TNF-a, IL-3) but not others (MCP-1, IL-4). We propose that kinetic discrimination along the FcεRI signaling pathway occurs at the level of Syk-FcεRI interactions, with key outcomes dependent upon sufficiently long-lived Syk binding events.

Mercury alters endogenous phosphorylation profiles of SYK in murine B cells

BACKGROUND

Epidemiological evidence and animal models suggest that exposure to low and non-neurotoxic concentrations of mercury may contribute to idiosyncratic autoimmune disease. Since defects in function and signaling in B cells are often associated with autoimmunity, we investigated whether mercury exposure might alter B cell responsiveness to self-antigens by interfering with B cell receptor (BCR) signal transduction. In this study we determined the effects of mercury on the protein tyrosine kinase SYK, a critical protein involved in regulation of the BCR signaling pathway.

METHODS

Phosphorylation sites of murine SYK were mapped before and after treatment of WEHI cell cultures with mercury, or with anti-IgM antibody (positive control) or pervanadate (a potent phosphatase inhibitor). Phosphopeptides were enriched by either titanium dioxide chromatography or anti-phosphotyrosine immunoaffinity, and analyzed by liquid chromatography-mass spectrometry. Select SYK phosphosite cluster regions were profiled for responsiveness to treatments using multiple reaction monitoring (MRM) methodology.

RESULTS

A total of 23 phosphosites were identified with high probability in endogenous SYK, including 19 tyrosine and 4 serine residues. For 10 of these sites phosphorylation levels were increased following BCR activation. Using MRM to profile changes in phosphorylation status we found that 4 cluster regions, encompassing 8 phosphosites, were activated by mercury and differentially responsive to all 3 treatments. Phosphorylation of tyrosine-342 and -346 residues were most sensitive to mercury exposure. This cluster is known to propagate normal BCR signal transduction by recruiting adaptor proteins such as PLC-γ and Vav-1 to SYK during formation of the BCR signalosome.

CONCLUSIONS

Our data shows that mercury alters the phosphorylation status of SYK on tyrosine sites known to have a role in promoting BCR signals. Considering the importance of SYK in the BCR signaling pathway, these data suggest that mercury can alter BCR signaling in B cells, which might affect B cell responsiveness to self-antigen and have implications with respect to autoimmunity and autoimmune disease.

Protein kinase B (AKT) regulates SYK activity and shuttling through 14-3-3 and importin 7

The Protein kinase B (AKT) regulates a plethora of intracellular signaling proteins to fine-tune signaling of multiple pathways. Here, we found that following B-cell receptor (BCR)-induced tyrosine phosphorylation of the cytoplasmic tyrosine kinase SYK and the adaptor BLNK, the AKT/PKB enzyme strongly induced BLNK (>100-fold) and SYK (>100-fold) serine/threonine phosphorylation (pS/pT). Increased phosphorylation promoted 14-3-3 binding to BLNK (37-fold) and SYK (2.5-fold) in a pS/pT-concentration dependent manner. We also demonstrated that the AKT inhibitor MK2206 reduced pS/pT of both BLNK (3-fold) and SYK (2.5-fold). Notably, the AKT phosphatase, PHLPP2 maintained the activating phosphorylation of BLNK at Y84 and increased protein stability (8.5-fold). In addition, 14-3-3 was required for the regulation SYK's interaction with BLNK and attenuated SYK binding to Importin 7 (5-fold), thereby perturbing shuttling to the nucleus. Moreover, 14-3-3 proteins also sustained tyrosine phosphorylation of SYK and BLNK. Furthermore, substitution of S295 or S297 for alanine abrogated SYK's binding to Importin 7. SYK with S295A or S297A replacements showed intense pY525/526 phosphorylation, and BLNK pY84 phosphorylation correlated with the SYK pY525/526 phosphorylation level. Conversely, the corresponding mutations to aspartic acid in SYK reduced pY525/526 phosphorylation. Collectively, these and previous results suggest that AKT and 14-3-3 proteins down-regulate the activity of several BCR-associated components, including BTK, BLNK and SYK and also inhibit SYK's interaction with Importin 7.

Dual Role of the Tyrosine Kinase Syk in Regulation of Toll-Like Receptor Signaling in Plasmacytoid Dendritic Cells

Crosslinking of regulatory immunoreceptors (RR), such as BDCA-2 (CD303) or ILT7 (CD85g), of plasmacytoid dendritic cells (pDCs) efficiently suppresses production of type-I interferon (IFN)-α/β and other cytokines in response to Toll-like receptor (TLR) 7/9 ligands. This cytokine-inhibitory pathway is mediated by spleen tyrosine kinase (Syk) associated with the ITAM-containing adapter of RR. Here we demonstrate by pharmacological targeting of Syk that in addition to the negative regulation of TLR7/9 signaling via RR, Syk also positively regulates the TLR7/9 pathway in human pDCs. Novel highly specific Syk inhibitor AB8779 suppressed IFN-α, TNF-α and IL-6 production induced by TLR7/9 agonists in primary pDCs and in the pDC cell line GEN2.2. Triggering of TLR9 or RR signaling induced a differential kinetics of phosphorylation at Y352 and Y525/526 of Syk and a differential sensitivity to AB8779. Consistent with the different roles of Syk in TLR7/9 and RR signaling, a concentration of AB8779 insufficient to block TLR7/9 signaling still released the block of IFN-α production triggered via the RR pathway, including that induced by hepatitis B and C viruses. Thus, pharmacological targeting of Syk partially restored the main pDC function—IFN-α production. Opposing roles of Syk in TLR7/9 and RR pathways may regulate the innate immune response to weaken inflammation reaction.

B-cell receptor signaling in the pathogenesis of lymphoid malignancies

B-cell receptor (BCR) signaling pathway plays a central role in B-lymphocyte development and initiation of humoral immunity. Recently, BCR signaling pathway has been shown as a major driver in the pathogenesis of B-cell malignancies. As a result, a vast array of BCR-associated kinases has emerged as rational therapeutic targets changing treatment paradigms in B cell malignancies. Based on high efficacy in early-stage clinical trials, there is rapid clinical development of inhibitors targeting BCR signaling pathway. Here, we describe the essential components of BCR signaling, their function in normal and pathogenic signaling and molecular effects of their inhibition in vitro and in vivo.

Microenvironment dependency in Chronic Lymphocytic Leukemia: The basis for new targeted therapies

Chronic Lymphocytic Leukemia (CLL) is a prototype microenvironment-dependent B-cell malignancy, in which the neoplastic B cells co-evolve together with a supportive tissue microenvironment, which promotes leukemia cell survival, growth, and drug-resistance. Chemo-immunotherapy is an established treatment modality for CLL patients, resulting in high rates of responses and improved survival, especially in low-risk CLL. New, alternative treatments target B-cell receptor (BCR) signaling and the Chemokine (C-X-C motif) Receptor 4 (CXCR4)-Chemokine (C-X-C motif) Ligand 12 (CXCL12) axis, which are key pathways of CLL-microenvironment cross talk. The remarkable clinical efficacy of inhibitors targeting the BCR-associated kinases Bruton's tyrosine kinase (BTK) and phosphoinositide 3-kinase delta (PI3Kδ) challenges established therapeutic paradigms and corroborates the central role of BCR signaling in CLL pathogenesis. In this review, we discuss the cellular and molecular components of the CLL microenvironment. We also describe the emerging therapeutic options for CLL patients, with a focus on inhibitors of CXCR4-CXCL12 and BCR signaling.

Depletion of STAT5 blocks TEL-SYK-induced APMF-type leukemia with myelofibrosis and myelodysplasia in mice

The spleen tyrosine kinase (SYK) was identified as an oncogenic driver in a broad spectrum of hematologic malignancies. The in vivo comparison of three SYK containing oncogenes, SYK(wt), TEL-SYK and IL-2-inducible T-cell kinase (ITK)-SYK revealed a general myeloexpansion and the establishment of three different hematologic (pre)diseases. SYK(wt) enhanced the myeloid and T-cell compartment, without leukemia/lymphoma development. ITK-SYK caused lethal T-cell lymphomas and the cytoplasmic TEL-SYK fusion induced an acute panmyelosis with myelofibrosis-type acute myeloid leukemia (AML) with up to 50% immature megakaryoblasts infiltrating bone marrow, spleen and liver, additional MPN features (myelofibrosis and granulocyte expansion) and MDS stigmata with megakaryocytic and erythroid dysplasia. LKS cells were reduced and all subsets (LT/ST/MPP) showed reduced proliferation rates. SYK inhibitor treatment (R788) of diseased TEL-SYK mice reduced leukocytosis, spleen and liver infiltration, enhanced the hematocrit and prolonged survival time, but could not significantly reduce myelofibrosis. Stat5 was identified as a major downstream mediator of TEL-SYK in vitro as well as in vivo. Consequently, targeted deletion of Stat5 in vivo completely abrogated TEL-SYK-induced AML and myelofibrosis development, proving Stat5 as a major driver of SYK-induced transformation. Our experiments highlight the important role of SYK in AML and myelofibrosis and prove SYK and STAT5 inhibitors as potent treatment options for those diseases.

Identifying Yersinia YopH-targeted signal transduction pathways that impair neutrophil responses during in vivo murine infection

Identifying molecular targets of Yersinia virulence effectors, or Yops, during animal infection is challenging because few cells are targeted by Yops in an infected organ, and isolating these sparse effector-containing cells is difficult. YopH, a tyrosine phosphatase, is essential for full virulence of Yersinia. Investigating the YopH-targeted signal transduction pathway(s) in neutrophils during infection of a murine host, we find that several host proteins, including the essential signaling adaptor SLP-76, are dephosphorylated in the presence of YopH in neutrophils isolated from infected tissues. YopH inactivated PRAM-1/SKAP-HOM and the SLP-76/Vav/PLCγ2 signal transduction axes, leading to an inhibition of calcium response in isolated neutrophils. Consistent with a failure to mount a calcium response, IL-10 production was reduced in neutrophils containing YopH from infected tissues. Finally, a yopH mutant survived better in the absence of neutrophils, indicating that neutrophil inactivation by YopH by targeting PRAM-1/SKAP-HOM and SLP-76/Vav/PLCγ2 signaling hubs may be critical for Yersinia survival.

Role of spleen tyrosine kinase in the pathogenesis of chronic lymphocytic leukemia

The antigen-dependent B-cell receptor (BCR) is triggered by binding to external antigens and transmits signals in normal B lymphocytes. Tonic signaling through the BCR plays a crucial role in the pathogenesis and progression of chronic lymphocytic leukemia (CLL). Spleen tyrosine kinase (Syk) is a key component of both BCR signals, and regulates multiple physiological functions of B lymphocytes. Studies have defined enhanced gene expression and protein expression of Syk in CLL cells which are closely related to the status of the immunoglobulin heavy chain variable region genes (IgVH). Recently, abrogating the BCR-induced signaling pathway by Syk inhibitors has represented a novel and active therapeutic approach for CLL. Studies of the correlation between Syk and ZAP-70 expression in CLL cells have brought a new perspective to determining the value of Syk in evaluating the effect of therapy and the prognosis of CLL. Therefore, we here review the role of Syk in the pathogenesis of CLL and provide an update of progress in the clinical development of Syk inhibitors.

Phosphorylation of the activation loop tyrosine 823 in c-Kit is crucial for cell survival and proliferation

The receptor tyrosine kinase c-Kit, also known as the stem cell factor receptor, plays a key role in several developmental processes. Activating mutations in c-Kit lead to alteration of these cellular processes and have been implicated in many human cancers such as gastrointestinal stromal tumors, acute myeloid leukemia, testicular seminomas and mastocytosis. Regulation of the catalytic activity of several kinases is known to be governed by phosphorylation of tyrosine residues in the activation loop of the kinase domain. However, in the case of c-Kit phosphorylation of Tyr-823 has been demonstrated to be a late event that is not required for kinase activation. However, because phosphorylation of Tyr-823 is a ligand-activated event, we sought to investigate the functional consequences of Tyr-823 phosphorylation. By using a tyrosine-to-phenylalanine mutant of tyrosine 823, we investigated the impact of Tyr-823 on c-Kit signaling. We demonstrate here that Tyr-823 is crucial for cell survival and proliferation and that mutation of Tyr-823 to phenylalanine leads to decreased sustained phosphorylation and ubiquitination of c-Kit as compared with the wild-type receptor. Furthermore, the mutated receptor was, upon ligand-stimulation, quickly internalized and degraded. Phosphorylation of the E3 ubiquitin ligase Cbl was transient, followed by a substantial reduction in phosphorylation of downstream signaling molecules such as Akt, Erk, p38, Shc, and Gab2. Thus, we propose that activation loop tyrosine 823 is crucial for activation of both the MAPK and PI3K pathways and that its disruption leads to a destabilization of the c-Kit receptor and decreased survival of cells.

LCK is an important mediator of B-cell receptor signaling in chronic lymphocytic leukemia cells

B-cell receptor (BCR) signals promote survival of chronic lymphocytic leukemia (CLL) cells, and it is believed that overexpressed and constitutively active Lyn mediates this signaling. Here, we show that CLL cells express lymphocyte-specific protein tyrosine kinase (LCK) and that inhibition of this Src family tyrosine kinase with the specific inhibitor [4-amino-5-(4-phenoxyphenyl)-7H-pyrrolo[3,2-d]pyrimidin-7-yl-cyclopentane (Lck-i)], or reduction of its expression with siRNA, blocks the induction of CD79a, Syk, inhibitor of IκB kinase (IKK), Akt, and extracellular signal-regulated kinase (ERK) phosphorylation by BCR cross-linking in these cells. Furthermore, we show that CLL cells with high levels of LCK expression have higher levels of BCR-mediated IKK, Akt, and ERK phosphorylation as well as cell survival than CLL cells with low levels of LCK expression. We also show that treatment of CLL cells with Lck-i inhibits BCR cross-linking-induced cell survival. Taken together, these data show a major role for LCK in proximal and distal BCR-mediated signaling in CLL cells and suggest that LCK expression is important in the pathogenesis of this disease. On a clinical level, these studies advocate the use of specific LCK inhibitors in the treatment of progressive CLL.

Signaling of the ITK (interleukin 2-inducible T cell kinase)-SYK (spleen tyrosine kinase) fusion kinase is dependent on adapter SLP-76 and on the adapter function of the kinases SYK and ZAP70

The inducible T cell kinase-spleen tyrosine kinase (ITK-SYK) oncogene consists of the Tec homology-pleckstrin homology domain of ITK and the kinase domain of SYK, and it is believed to be the cause of peripheral T cell lymphoma. We and others have recently demonstrated that this fusion protein is constitutively tyrosine-phosphorylated and is transforming both in vitro and in vivo. To gain a deeper insight into the molecular mechanism(s) underlying its activation and signaling, we mutated a total of eight tyrosines located in the SYK portion of the chimera into either phenylalanine or to the negatively charged glutamic acid. Although mutations in the interdomain-B region affected ITK-SYK kinase activity, they only modestly altered downstream signaling events. In contrast, mutations that were introduced in the kinase domain triggered severe impairment of downstream signaling. Moreover, we show here that SLP-76 is critical for ITK-SYK activation and is particularly required for the ITK-SYK-dependent phosphorylation of SYK activation loop tyrosines. In Jurkat cell lines, we demonstrate that expression of ITK-SYK fusion requires an intact SLP-76 function and significantly induces IL-2 secretion and CD69 expression. Furthermore, the SLP-76-mediated induction of IL-2 and CD69 could be further enhanced by SYK or ZAP-70, but it was independent of their kinase activity. Notably, ITK-SYK expression in SYF cells phosphorylates SLP-76 in the absence of SRC family kinases. Altogether, our data suggest that ITK-SYK exists in the active conformation state and is therefore capable of signaling without SRC family kinases or stimulation of the T cell receptor.

Chronic lymphocytic leukemia: a tale of one or two signals?

The significant correlation between disease aggressiveness and the gene and protein structures of the B-cell receptors (BCRs) expressed on chronic lymphocytic leukemia (CLL) cells, together with the evidence for chronic activation of the BCR pathway, have led to the hypothesis that this leukemia initiates and progresses by selecting normal B lymphocytes reactive with a restricted set of (auto)antigens. A study recently published in Nature identified a novel signal-initiating interaction between the third complementary determining region of the IG heavy chain variable domain (HCDR3) and an epitope in the second framework region (FR2) that appears to be unique to CLL B cells and that calls into question the need for classical antigen binding in the activation and expansion of the leukemic cells. These findings are discussed in the context of available information about the antigen reactivity of CLL B cells and its potential role in clonal survival and drive.

Novel Agents and Emerging Strategies for Targeting the B-Cell Receptor Pathway in CLL

Chronic lymphocytic leukemia (CLL) is a disease of malignant CD5+ B lymphocytes that are characterized by frequent expression of autoreactive B-cell receptors (BCRs) and marked dependence on microenvironmental signals for proliferation and survival. Among the latter, signals propagated through the BCR are believed to play a key role in leukemia initiation, maintenance and evolution. Drugs that can disrupt these signals have recently emerged as potential therapeutic agents in CLL and several of them are currently being evaluated in clinical trials. Particularly promising clinical responses have been obtained with inhibitors of the kinases SYK, BTK, and PI3Kδ, which function by blocking BCR signal transduction. In addition, recent studies focusing on the phosphatase PTPN22, which is involved in the pathogenesis of multiple autoimmune diseases and is markedly overexpressed in CLL cells, suggest that it may be possible in the future to develop strategies that will selectively reprogram BCR survival signals into signals that induce leukemic cell death. This review focuses on the biological basis behind these strategies and highlights some of the most promising BCR-targeting agents in ongoing preclinical and clinical studies.

The degree of BCR and NFAT activation predicts clinical outcomes in chronic lymphocytic leukemia

B-cell antigen receptor (BCR)-mediated signaling plays a critical role in chronic lymphocytic leukemia (CLL) pathogenesis and gives an in vitro survival advantage to B cells isolated from patients with unfavorable prognostic factors. In this study, we undertook to elucidate the signaling intermediates responsible for this biologic alteration. In responding cells only, in vitro BCR engagement triggers global phosphorylation of Syk, activation of phospholipase Cγ2, and intracellular calcium mobilization, reflecting competency of BCR signaling. The calcium-calcineurin-dependent transcription factor NFAT2 is up-regulated and to some extent constitutively activated in all CLL B cells. In contrast, its DNA-binding capacity is enhanced on IgM stimulation in responding cells only. NFAT inhibition using the VIVIT peptide prevents induction of CD23 target gene and IgM-induced survival, converting responding cells to unresponsive status. At the opposite, ionomycin-induced NFAT activity allows survival of nonresponding cells. These results demonstrate that the functional heterogeneity relies on variability of protein levels establishing BCR-dependent thresholds and NFAT-dependent activation. Finally, status of the BCR-NFAT pathway for each patient reveals its relevance for CLL clinical outcome and points out to BCR-NFAT intermediates as promising functional therapeutic targets.

Autoimmune regulator (AIRE) contributes to Dectin-1-induced TNF-α production and complexes with caspase recruitment domain-containing protein 9 (CARD9), spleen tyrosine kinase (Syk), and Dectin-1

BACKGROUND

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) syndrome is a complex immunologic disease caused by mutation of the autoimmune regulator (AIRE) gene. Autoimmunity in patients with APECED syndrome has been shown to result from deficiency of AIRE function in transcriptional regulation of thymic peripheral tissue antigens, which leads to defective T-cell negative selection. Candidal susceptibility in patients with APECED syndrome is thought to result from aberrant adaptive immunity.

OBJECTIVE

To determine whether AIRE could function in anticandidal innate immune signaling, we investigated an extrathymic role for AIRE in the immune recognition of β-glucan through the Dectin-1 pathway, which is required for defense against Candida species.

METHODS

Innate immune signaling through the Dectin-1 pathway was assessed in both PBMCs from patients with APECED syndrome and a monocytic cell line. Subcellular localization of AIRE was assessed by using confocal microscopy.

RESULTS

PBMCs from patients with APECED syndrome had reduced TNF-α responses after Dectin-1 ligation but in part used a Raf-1-mediated pathway to preserve function. In the THP-1 human monocytic cell line, reducing AIRE expression resulted in significantly decreased TNF-α release after Dectin-1 ligation. AIRE formed a transient complex with the known Dectin-1 pathway components phosphorylated spleen tyrosine kinase and caspase recruitment domain-containing protein 9 after receptor ligation and localized with Dectin-1 at the cell membrane.

CONCLUSION

AIRE can participate in the Dectin-1 signaling pathway, indicating a novel extrathymic role for AIRE and a defect that likely contributes to fungal susceptibility in patients with APECED syndrome.

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.

Schistosoma mansoni: signal transduction processes during the development of the reproductive organs

Among the topics of considerable interest concerning our understanding of the unusual biology of schistosomes is the sexual maturation of the female. The identification of genes coding for signal transduction proteins controlling essential steps of the pairing-dependent differentiation of the reproductive organs, vitellarium and ovary will help to substantiate our knowledge about this unique parasite. Furthermore, such signalling proteins could be potential targets to interfere with the development of this parasite to combat schistosomiasis since its pathology is caused by the eggs. This review summarises first post-genomic steps to elucidate the function of gonad-specific signalling molecules which were identified by homology-based cloning strategies, by in silico identification or by yeast two-hybrid interaction analyses, using a combination of novel techniques. These include the in vitro culture of adult schistosomes, their treatment with chemical inhibitors to block enzyme activity, the use of RNAi to silence gene function post-transcriptionally, and confocal laser scanning microscopy to study the morphological consequences of these experimental approaches. Finally, we propose a first model of protein networks that are active in the ovary regulating mitogenic activity and differentiation. Some of these molecules are also active in the testes of males, probably fulfilling similar roles as in the ovary.