Steady state kinetics of spleen tyrosine kinase investigated by a real time fluorescence assay

A reevaluation of the spleen tyrosine kinase (SYK) activation mechanism

Spleen tyrosine kinase (SYK) is a signaling node in many immune pathways and comprises two tandem Src homology (SH) 2 domains, an SH2-kinase linker, and a C-terminal tyrosine kinase domain. Two prevalent models of SYK activation exist. The "OR-gate" model contends that SYK can be fully activated by phosphorylation or binding of its SH2 domains to a dual-phosphorylated immune-receptor tyrosine-based activation motif (ppITAM). An alternative model proposes that SYK activation requires ppITAM binding and phosphorylation of the SH2-kinase linker by a SRC family kinase such as LYN proto-oncogene, SRC family tyrosine kinase (LYN). To evaluate these two models, we generated directly comparable unphosphorylated (upSYK) and phosphorylated (pSYK) proteins with or without an N-terminal glutathione S-transferase (GST) tag, resulting in monomeric or obligatory dimeric SYK, respectively. We assessed the ability of a ppITAM peptide and LYN to activate these SYK proteins. The ppITAM peptide strongly activated GST-SYK but was less effective in activating upSYK untagged with GST. LYN alone activated untagged upSYK to a greater extent than did ppITAM, and inclusion of both proteins rapidly and fully activated upSYK. Using immunoblot and phosphoproteomic approaches, we correlated the kinetics and order of site-specific SYK phosphorylation. Our results are consistent with the alternative model, indicating that ppITAM binding primes SYK for rapid LYN-mediated phosphorylation of Tyr-352 and then Tyr-348 of the SH2-kinase linker, which facilitates activation loop phosphorylation and full SYK activation. This gradual activation mechanism may also explain how SYK maintains ligand-independent tonic signaling, important for B-cell development and survival.

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.

Phosphorylated immunoreceptor tyrosine-based activation motifs and integrin cytoplasmic domains activate spleen tyrosine kinase via distinct mechanisms

Spleen tyrosine kinase (Syk) is involved in cellular adhesion and also in the activation and development of hematopoietic cells. Syk activation induced by genomic rearrangement has been linked to certain T-cell lymphomas, and Syk inhibitors have been shown to prolong survival of patients with B-cell lineage malignancies. Syk is activated either by its interaction with a double-phosphorylated immunoreceptor tyrosine-based activation motif (pITAM), which induces rearrangements in the Syk structure, or by the phosphorylation of specific tyrosine residues. In addition to its immunoreceptor function, Syk is activated downstream of integrin pathways, and integrins bind to the same region in Syk as does pITAM. However, it is unknown whether integrins and pITAM use the same mechanism to activate Syk. Here, using purified Syk protein and fluorescence-based enzyme assay we investigated whether interaction of the integrin β₃ cytoplasmic domain with the Syk regulatory domain causes changes in Syk activity similar to those induced by pITAM peptides. We observed no direct Syk activation by soluble integrin peptide, and integrin did not compete with pITAM-induced activation even though at high concentrations, the integrin cytoplasmic domain peptide competed with Syk's substrate. However, clustered integrin peptides induced Syk activation, presumably via a transphosphorylation mechanism. Moreover, the clustered integrins also activated a Syk variant in which tyrosines were replaced with phenylalanine (Y348F/Y352F), indicating that clustered integrin-induced Syk activation involved other phosphorylation sites. In conclusion, integrin cytoplasmic domains do not directly induce Syk conformational changes and do not activate Syk via the same mechanism as pITAM.

Quantification of Cell Signaling Networks Using Kinase Activity Chemosensors

The ability to directly determine endogenous kinase activity in tissue homogenates provides valuable insights into signaling aberrations that underlie disease phenotypes. When activity data is collected across a panel of kinases, a unique "signaling fingerprint" is generated that allows for discrimination between diseased and normal tissue. Here we describe the use of peptide-based kinase activity sensors to fingerprint the signaling changes associated with disease states. This approach leverages the phosphorylation-sensitive sulfonamido-oxine (Sox) fluorophore to provide a direct readout of kinase enzymatic activity in unfractionated tissue homogenates from animal models or clinical samples. To demonstrate the application of this technology, we focus on a rat model of nonalcoholic fatty liver disease (NAFLD). Sox-based activity probes allow for the rapid and straightforward analysis of changes in kinase enzymatic activity associated with disease states, providing leads for further investigation using traditional biochemical approaches.

Selective inhibition of spleen tyrosine kinase (SYK) with a novel orally bioavailable small molecule inhibitor, RO9021, impinges on various innate and adaptive immune responses: implications for SYK inhibitors in autoimmune disease therapy

Introduction

Spleen tyrosine kinase (SYK) is a key integrator of intracellular signals triggered by activated immunoreceptors, including Bcell receptors (BCR) and Fc receptors, which are important for the development and function of lymphoid cells. Given the clinical efficacy of Bcell depletion in the treatment of rheumatoid arthritis and multiple sclerosis, pharmacological modulation of Bcells using orally active small molecules that selectively target SYK presents an attractive alternative therapeutic strategy.

Methods

A SYK inhibitor was developed and assayed in various in vitro systems and in the mouse model of collagen-induced arthritis (mCIA).

Results

A novel ATP-competitive inhibitor of SYK, 6-[(1R,2S)-2-Amino-cyclohexylamino]-4-(5,6-dimethyl-pyridin-2-ylamino)-pyridazine-3-carboxylic acid amide, designated RO9021, with an adequate kinase selectivity profile and oral bioavailability, was developed. In addition to suppression of BCR signaling in human peripheral blood mononuclear cells (PBMC) and whole blood, FcγR signaling in human monocytes, and FcϵR signaling in human mast cells, RO9021 blocked osteoclastogenesis from mouse bone marrow macrophages in vitro. Interestingly, Toll-like Receptor (TLR) 9 signaling in human Bcells was inhibited by RO9021, resulting in decreased levels of plasmablasts, immunoglobulin (Ig) M and IgG upon B-cell differentiation. RO9021 also potently inhibited type I interferon production by human plasmacytoid dendritic cells (pDC) upon TLR9 activation. This effect is specific to TLR9 as RO9021 did not inhibit TLR4- or JAK-STAT-mediated signaling. Finally, oral administration of RO9021 inhibited arthritis progression in the mCIA model, with observable pharmacokinetics (PK)-pharmacodynamic (PD) correlation.

Conclusions

Inhibition of SYK kinase activity impinges on various innate and adaptive immune responses. RO9021 could serve as a starting point for the development of selective SYK inhibitors for the treatment of inflammation-related and autoimmune-related disorders.

Inhibitors of switch kinase 'spleen tyrosine kinase' in inflammation and immune-mediated disorders: a review

Spleen tyrosine kinase (Syk), a member of Syk family of non-receptor protein tyrosine kinases plays a significant role in the immune cell signaling in B cells, mast cells, macrophages and neutrophils. Anomalous regulation of this kinase can lead to different allergic disorders and antibody-mediated autoimmune diseases such as rheumatoid arthritis, asthma, psoriasis and allergic rhinitis. Being involved in the growth and survive mechanism of B cells, its inhibition can be beneficial in B-cell lymphoma. Thus, Syk can be sited as a therapeutically relevant target for various allergic and autoimmune disorders. This review article describes the structure of Syk and its role in B-cell signaling. In addition to this, data regarding small molecule inhibitors of Syk has also been reviewed from different papers and patents published.

Structural and biophysical characterization of the Syk activation switch

Syk is an essential non-receptor tyrosine kinase in intracellular immunological signaling, and the control of Syk kinase function is considered as a valuable target for pharmacological intervention in autoimmune or inflammation diseases. Upon immune receptor stimulation, the kinase activity of Syk is regulated by binding of phosphorylated immune receptor tyrosine-based activating motifs (pITAMs) to the N-terminal tandem Src homology 2 (tSH2) domain and by autophosphorylation with consequences for the molecular structure of the Syk protein. Here, we present the first crystal structures of full-length Syk (fl-Syk) as wild type and as Y348F,Y352F mutant forms in complex with AMP-PNP revealing an autoinhibited conformation. The comparison with the crystal structure of the truncated Syk kinase domain in complex with AMP-PNP taken together with ligand binding studies by surface plasmon resonance (SPR) suggests conformational differences in the ATP sites of autoinhibited and activated Syk forms. This hypothesis was corroborated by studying the thermodynamic and kinetic interaction of three published Syk inhibitors with isothermal titration calorimetry and SPR, respectively. We further demonstrate the modulation of inhibitor binding affinities in the presence of pITAM and discuss the observed differences of thermodynamic and kinetic signatures. The functional relevance of pITAM binding to fl-Syk was confirmed by a strong stimulation of in vitro autophosphorylation. A structural feedback mechanism on the kinase domain upon pITAM binding to the tSH2 domain is discussed in analogy of the related family kinase ZAP-70 (Zeta-chain-associated protein kinase 70). Surprisingly, we observed distinct conformations of the tSH2 domain and the activation switch including Tyr348 and Tyr352 in the interdomain linker of Syk in comparison to ZAP-70.

A peptide-based biosensor assay to detect intracellular Syk kinase activation and inhibition

Spleen tyrosine kinase (Syk) has been implicated in a number of pathologies including cancer and rheumatoid arthritis and thus has been pursued as a novel therapeutic target. Because of the complex relationship between Syk's auto- and other internal phosphorylation sites, scaffolding proteins, enzymatic activation state and sites of phosphorylation on its known substrates, the role of Syk's activity in these diseases has not been completely clear. To approach such analyses, we developed a Syk-specific artificial peptide biosensor (SAStide) to use in a cell-based assay for direct detection of intracellular Syk activity and inhibition in response to physiologically relevant stimuli in both laboratory cell lines and primary splenic B cells. This peptide contains a sequence derived from known Syk substrate preference motifs linked to a cell permeable peptide, resulting in a biosensor that is phosphorylated in live cells in a Syk-dependent manner, thus serving as a reporter of Syk catalytic activity in intact cells. Because the assay is compatible with live, primary cells and can report pharmacodynamics for drug action on an intended target, this methodology could be used to facilitate a better understanding of Syk's function and the effect of its inhibition in disease.

Online electrospray ionization mass spectrometric monitoring of protease-catalyzed reactions in real time

Although there are a lot of well established methods for monitoring enzyme-catalyzed reactions, most of them are based on changes in spectroscopic properties during the conversion of substrates to products. However, reactions without optical changes are common, which are inapplicable to these spectroscopic methods. As an alternative technique for enzymologic research, mass spectrometry (MS) is favored due to its specificity, sensitivity, and the ability to obtain stoichiometric information. In this work, probe electrospray ionization (PESI) source coupled with a time of flight mass spectrometer was employed to monitor some typical protease-catalyzed reactions, including pepsinolysis and trypsinolysis of cytochrome c in real time. Due to the high electrical conductivity of each reaction system, corona discharges are likely to occur, which would decrease intensities of mass spectrometric signals. An ultra-fine sampling probe and an auxiliary vapor spray were adopted to prevent corona discharges. Experimental results from peptic and tryptic digestions of cytochrome c showed different and characteristic catalytic pathways. With the data presented in this study, PESI-MS can be considered as a potential tool for real-time monitoring of enzymatic reactions because of its simplicity in instrumental configuration, wide applicability under harsh conditions, and flexibility in combination with other techniques.

Biophysical and mechanistic insights into novel allosteric inhibitor of spleen tyrosine kinase

Extracellular stimulation of the B cell receptor or mast cell FcεRI receptor activates a cascade of protein kinases, ultimately leading to antigenic or inflammation immune responses, respectively. Syk is a soluble kinase responsible for transmission of the receptor activation signal from the membrane to cytosolic targets. Control of Syk function is, therefore, critical to the human antigenic and inflammation immune response, and an inhibitor of Syk could provide therapy for autoimmune or inflammation diseases. We report here a novel allosteric Syk inhibitor, X1, that is noncompetitive against ATP (K(i) 4 ± 1 μM) and substrate peptide (K(i) 5 ± 1 μM), and competitive against activation of Syk by its upstream regulatory kinase LynB (K(i) 4 ± 1 μM). The inhibition mechanism was interrogated using a combination of structural, biophysical, and kinetic methods, which suggest the compound inhibits Syk by reinforcing the natural regulatory interactions between the SH2 and kinase domains. This novel mode of inhibition provides a new opportunity to improve the selectivity profile of Syk inhibitors for the development of safer drug candidates.

Curcumin inhibits GPVI-mediated platelet activation by interfering with the kinase activity of Syk and the subsequent activation of PLCgamma2

Turmeric (Curcuma longa), a herbal remedy and culinary spice, has been used in traditional Indian culture for millennia. An active ingredient found in turmeric is curcumin (diferuloylmethane). In the current study, we investigated the antiplatelet properties of this naturally occurring compound. Curcumin inhibited human platelet aggregation and dense granule secretion induced by GPVI agonist convulxin in a concentration-dependent manner. At 50 microM, it effectively inhibited the maximal extent of aggregation and dense granule secretion to as much as 75%. It also dramatically inhibited the activation-dependent tyrosine phosphorylation of Y753 and Y759 on PLCgamma2, but did not affect the phosphorylation of Y145 residue on the cytosolic adaptor protein SLP-76. Interestingly, curcumin had no significant effect on the phosphorylation of Y525/Y526 present on the activation loop of Syk (spleen tyrosine kinase), but had a significant inhibitory effect on in vitro Syk kinase activity. Moreover, the inhibitory action of curcumin is not due to an inhibition of thromboxane generation because all our studies were performed using aspirin-treated platelets. We conclude that curcumin inhibits platelet activation induced by GPVI agonists through interfering with the kinase activity of Syk and the subsequent activation of PLCgamma2.

Regulation of Syk by phosphorylation on serine in the linker insert

The Syk protein-tyrosine kinase is phosphorylated on multiple tyrosines after the aggregation of the B cell antigen receptor. However, metabolic labeling experiments indicate that Syk is inducibly phosphorylated to an even greater extent on serine after receptor ligation. A combination of phosphopeptide mapping and mass spectrometric analyses indicates that serine 291 is a major site of phosphorylation. Serine 291 lies within a 23-amino acid insert located within the linker B region that distinguishes Syk from SykB and Zap-70. The phosphorylation of serine-291 by protein kinase C enhances the ability of Syk to couple the antigen receptor to the activation of the transcription factors NFAT and Elk-1. Protein interaction studies indicate a role for the phosphorylated linker insert in promoting an interaction between Syk and the chaperone protein, prohibitin.

The Src, Syk, and Tec family kinases: distinct types of molecular switches

The Src, Syk, and Tec family kinases are three of the most well characterized tyrosine kinase families found in the human genome. Members of these kinase families function downstream of antigen and F(c) receptors in hematopoietic cells and transduce signals leading to calcium mobilization, altered gene expression, cytokine production, and cell proliferation. Over the last several years, structural and biochemical studies have begun to uncover the molecular mechanisms regulating activation of these kinases. It appears that each kinase family functions as a distinct type of molecular switch. This review discusses the activation of the Src, Syk, and Tec kinases from the perspective of structure, phosphorylation, allosteric regulation, and kinetics. The multiple factors that regulate the Src, Syk, and Tec families illustrate the important role played by each of these kinases in immune cell signaling.

Therapeutic prospect of Syk inhibitors

BACKGROUND

The non-receptor spleen tyrosine kinase (Syk; EC 2.7.10.2) is involved in signal transduction in a variety of cell types. In particular, it is a key mediator of immune receptors signaling in host inflammatory cells (B cells, mast cells, macrophages and neutrophils), important for both allergic and antibody-mediated autoimmune diseases. Deregulated Syk kinase activity also allows growth factor-independent proliferation and transforms bone marrow-derived pre-B cells that are able to induce leukemia. Consequently, the development of Syk kinase inhibitors could conceivably treat these disorders and so they have became a major focus in the pharmaceutical and biotech industry.

OBJECTIVE

In this review, we analyze the structure and role of Syk kinase, the use of small molecules, interacting with ATP-binding site, as inhibitors of kinase activity and finally the potential of using inhibitors of Syk kinase expression to attenuate pathological conditions.

CONCLUSION

Syk kinase inhibition is suggested as a powerful tool for the therapy of different pathologies.

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.

Molecular mechanism of the Syk activation switch

Many immune signaling pathways require activation of the Syk tyrosine kinase to link ligation of surface receptors to changes in gene expression. Despite the central role of Syk in these pathways, the Syk activation process remains poorly understood. In this work we quantitatively characterized the molecular mechanism of Syk activation in vitro using a real time fluorescence kinase assay, mutagenesis, and other biochemical techniques. We found that dephosphorylated full-length Syk demonstrates a low initial rate of substrate phosphorylation that increases during the kinase reaction due to autophosphorylation. The initial rate of Syk activity was strongly increased by either pre-autophosphorylation or binding of phosphorylated immune tyrosine activation motif peptides, and each of these factors independently fully activated Syk. Deletion mutagenesis was used to identify regions of Syk important for regulation, and residues 340-356 of the SH2 kinase linker region were identified to be important for suppression of activity before activation. Comparison of the activation processes of Syk and Zap-70 revealed that Syk is more readily activated by autophosphorylation than Zap-70, although both kinases are rapidly activated by Src family kinases. We also studied Syk activity in B cell lysates and found endogenous Syk is also activated by phosphorylation and immune tyrosine activation motif binding. Together these experiments show that Syk functions as an "OR-gate" type of molecular switch. This mechanism of switch-like activation helps explain how Syk is both rapidly activated after receptor binding but also sustains activity over time to facilitate longer term changes in gene expression.