Once phosphorylated, tyrosines in carboxyl terminus of protein-tyrosine kinase Syk interact with signaling proteins, including TULA-2, a negative regulator of mast cell degranulation

Role of Tula-Family Proteins in Cell Signaling and Activation: Advances and Challenges

This Special Issue entitled "Role of Tula-Family Proteins in Cell Signaling and Activation: Advances and Challenges" is focused on a relatively novel vertebrate gene/protein family termed alternatively TULA, UBASH3, or STS [...].

Inactivation of the Sts enzymes promotes resistance to lethal Staphylococcus aureus infection

Staphylococcus aureus is a highly infective Gram-positive bacterial pathogen that causes a wide range of diseases in both healthy and immunocompromised individuals. It can evade host immune defenses by expressing numerous virulence factors and toxins. Coupled with the inability of the human host to develop protective immunity against S. aureus, the emergence of antibiotic-resistant strains complicates treatment options. The non-canonical Sts phosphatases negatively regulate signaling pathways in varied immune cell types. To determine the role of the Sts proteins in regulating host responses to a Gram-positive microorganism, we investigated the response of mice lacking Sts expression to S. aureus infection. Herein, we demonstrate that Sts -/- animals are significantly resistant to lethal intravenous doses of S. aureus strain USA300. Resistance is characterized by significantly enhanced survival and accelerated bacterial clearance in multiple peripheral organs. Infected Sts -/- animals do not display increased levels of cytokines TNFα, IFNγ, and IL-6 in the spleen, liver, and kidney during the early stages of the infection, suggesting that a heightened pro-inflammatory response does not underlie the resistance phenotype. In vivo ablation of mononuclear phagocytes compromises the Sts -/- enhanced CFU clearance phenotype. Additionally, Sts -/- bone marrow-derived macrophages demonstrate significantly enhanced restriction of intracellular S. aureus following ex vivo infection. These results reveal the Sts enzymes to be critical regulators of host immunity to a virulent Gram-positive pathogen and identify them as therapeutic targets for optimizing host anti-microbial responses.

Phosphorylation of spleen tyrosine kinase at Y346 negatively regulates ITAM-mediated signaling and function in platelets

Spleen tyrosine kinase (Syk) is expressed in a variety of hemopoietic cells. Upon phosphorylation of the platelet immunoreceptor-based activation motif of the glycoprotein VI (GPVI)/Fc receptor gamma chain collagen receptor, both the tyrosine phosphorylation and activity of Syk are increased leading to downstream signaling events. Although it has been established that the activity of Syk is regulated by tyrosine phosphorylation, the specific roles of individual phosphorylation sites remain to be elucidated. We observed that Syk Y346 in mouse platelets was still phosphorylated when GPVI-induced Syk activity was inhibited. We then generated Syk Y346F mice and analyzed the effect this mutation exerts on platelet responses. Syk Y346F mice bred normally, and their blood cell count was unaltered. We did observe potentiation of GPVI-induced platelet aggregation and ATP secretion as well as increased phosphorylation of other tyrosines on Syk in the Syk Y346F mouse platelets when compared to WT littermates. This phenotype was specific for GPVI-dependent activation, since it was not seen when AYPGKF, a PAR4 agonist, or 2-MeSADP, a purinergic receptor agonist, was used to activate platelets. Despite a clear effect of Syk Y346F on GPVI-mediated signaling and cellular responses, there was no effect of this mutation on hemostasis as measured by tail-bleeding times, although the time to thrombus formation determined using the ferric chloride injury model was reduced. Thus, our results indicate a significant effect of Syk Y346F on platelet activation and responses in vitro and reveal its complex nature manifesting itself by the diversified translation of platelet activation into physiological responses.

TULA Proteins in Men, Mice, Hens, and Lice: Welcome to the Family

The two members of the UBASH3/STS/TULA protein family have been shown to critically regulate key biological functions, including immunity and hemostasis, in mammalian biological systems. Negative regulation of signaling through immune receptor tyrosine-based activation motif (ITAM)- and hemITAM-bearing receptors mediated by Syk-family protein tyrosine kinases appears to be a major molecular mechanism of the down-regulatory effect of TULA-family proteins, which possess protein tyrosine phosphatase (PTP) activity. However, these proteins are likely to carry out some PTP-independent functions as well. Whereas the effects of TULA-family proteins overlap, their characteristics and their individual contributions to cellular regulation also demonstrate clearly distinct features. Protein structure, enzymatic activity, molecular mechanisms of regulation, and biological functions of TULA-family proteins are discussed in this review. In particular, the usefulness of the comparative analysis of TULA proteins in various metazoan taxa, for identifying potential roles of TULA-family proteins outside of their functions already established in mammalian systems, is examined.

The Sts Proteins: Modulators of Host Immunity

The suppressor of TCR signaling (Sts) proteins, Sts-1 and Sts-2, are a pair of closely related signaling molecules that belong to the histidine phosphatase (HP) family of enzymes by virtue of an evolutionarily conserved C-terminal phosphatase domain. HPs derive their name from a conserved histidine that is important for catalytic activity and the current evidence indicates that the Sts HP domain plays a critical functional role. Sts-1HP has been shown to possess a readily measurable protein tyrosine phosphatase activity that regulates a number of important tyrosine-kinase-mediated signaling pathways. The in vitro catalytic activity of Sts-2HP is significantly lower than that of Sts-1HP, and its signaling role is less characterized. The highly conserved unique structure of the Sts proteins, in which additional domains, including one that exhibits a novel phosphodiesterase activity, are juxtaposed together with the phosphatase domain, suggesting that Sts-1 and -2 occupy a specialized intracellular signaling niche. To date, the analysis of Sts function has centered predominately around the role of Sts-1 and -2 in regulating host immunity and other responses associated with cells of hematopoietic origin. This includes their negative regulatory role in T cells, platelets, mast cells and other cell types, as well as their less defined roles in regulating host responses to microbial infection. Regarding the latter, the use of a mouse model lacking Sts expression has been used to demonstrate that Sts contributes non-redundantly to the regulation of host immunity toward a fungal pathogen (C. albicans) and a Gram-negative bacterial pathogen (F. tularensis). In particular, Sts-/- animals demonstrate significant resistance to lethal infections of both pathogens, a phenotype that is correlated with some heightened anti-microbial responses of phagocytes derived from mutant mice. Altogether, the past several years have seen steady progress in our understanding of Sts biology.

STS1 and STS2 Phosphatase Inhibitor Baicalein Enhances the Expansion of Hematopoietic and Progenitor Stem Cells and Alleviates 5-Fluorouracil-Induced Myelosuppression

STS1 and STS2, as the protein phosphatases that dephosphorylate FLT3 and cKIT, negatively regulate the self-renewal and differentiation of hematopoietic stem and progenitor cells (HSPCs). To obtain the small molecule inhibitors of STS1/STS2 phosphatase activity used to expand HSPCs both in vitro and in vivo, we establish an in vitro phosphatase assay using the recombinant proteins of the STS1/STS2 histidine phosphatase (HP) domain, by which we screened out baicalein (BC) as one of the effective inhibitors targeting STS1 and STS2. Then, we further demonstrate the direct binding of BC with STS1/STS2 using molecular docking and capillary electrophoresis and verify that BC can restore the phosphorylation of FLT3 and cKIT from STS1/STS2 inhibition. In a short-term in vitro culture, BC promotes profound expansion and enhances the colony-forming capacity of both human and mouse HSPCs along with the elevation of phospho-FLT3 and phospho-cKIT levels. Likewise, in vivo administration with BC significantly increases the proportions of short-term hematopoietic stem cells (ST-HSCs), multipotent progenitors (MPPs) and especially long-term HSCs (LT-HSCs) in healthy mouse bone marrow and increases the numbers of colony-forming units (CFU) formed by HSPCs as well. More importantly, pre-administration of BC significantly enhances the survival of mice with lethal 5-fluorouracil (5-FU) injection due to the alleviation of 5-FU-induced myelosuppression, as evidenced by the recovery of bone marrow histologic injury, the increased proportions of LT-HSCs, ST-HSCs and MPPs, and enhanced colony-forming capacity. Collectively, our study not only suggests BC as one of the small molecule candidates to stimulate HSPC expansion both in vitro and in vivo when needed in either physiologic or pathologic conditions, but also supports STS1/STS2 as potential therapeutic drug targets for HSPC expansion and hematopoietic injury recovery.

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.

Siglec-8 Signals Through a Non-Canonical Pathway to Cause Human Eosinophil Death In Vitro

Sialic acid-binding immunoglobulin-like lectin (Siglec)-8 is a glycan-binding receptor bearing immunoreceptor tyrosine-based inhibitory and switch motifs (ITIM and ITSM, respectively) that is selectively expressed on eosinophils, mast cells, and, to a lesser extent, basophils. Previous work has shown that engagement of Siglec-8 on IL-5-primed eosinophils causes cell death via CD11b/CD18 integrin-mediated adhesion and NADPH oxidase activity and identified signaling molecules linking adhesion, reactive oxygen species (ROS) production, and cell death. However, the proximal signaling cascade activated directly by Siglec-8 engagement has remained elusive. Most members of the Siglec family possess similar cytoplasmic signaling motifs and recruit the protein tyrosine phosphatases SHP-1/2, consistent with ITIM-mediated signaling, to dampen cellular activation. However, the dependence of Siglec-8 function in eosinophils on these phosphatases has not been studied. Using Siglec-8 antibody engagement and pharmacological inhibition in conjunction with assays to measure cell-surface upregulation and conformational activation of CD11b integrin, ROS production, and cell death, we sought to identify molecules involved in Siglec-8 signaling and determine the stage of the process in which each molecule plays a role. We demonstrate here that the enzymatic activities of Src family kinases (SFKs), Syk, SHIP1, PAK1, MEK1, ERK1/2, PLC, PKC, acid sphingomyelinase/ceramidase, and Btk are all necessary for Siglec-8-induced eosinophil cell death, with no apparent role for SHP-1/2, SHIP2, or c-Raf. While most of these signaling molecules are necessary for Siglec-8-induced upregulation of CD11b integrin at the eosinophil cell surface, Btk is phosphorylated and activated later in the signaling cascade and is instead necessary for CD11b activation. In contrast, SFKs and ERK1/2 are phosphorylated far earlier in the process, consistent with their role in augmenting cell-surface levels of CD11b. In addition, pretreatment of eosinophils with latrunculin B or jasplakinolide revealed that actin filament disassembly is necessary and sufficient for surface CD11b integrin upregulation and that actin polymerization is necessary for downstream ROS production. These results show that Siglec-8 signals through an unanticipated set of signaling molecules in IL-5-primed eosinophils to induce cell death and challenges the expectation that ITIM-bearing Siglecs signal through inhibitory pathways involving protein tyrosine phosphatases to achieve their downstream functions.

The role of PLCγ2 in immunological disorders, cancer, and neurodegeneration

Phosphatidylinositol-specific phospholipase Cγ2 (PLCγ2) is a critical signaling molecule activated downstream from a variety of cell surface receptors that contain an intracellular immunoreceptor tyrosine-based activation motif. These receptors recruit kinases such as Syk, BTK, and BLNK to phosphorylate and activate PLCγ2, which then generates 1D-myo-inositol 1,4,5-trisphosphate and diacylglycerol. These well-known second messengers are required for diverse membrane functionality including cellular proliferation, endocytosis, and calcium flux. As a result, PLCγ2 dysfunction is associated with a variety of diseases including cancer, neurodegeneration, and immune disorders. The diverse pathologies associated with PLCγ2 are exemplified by distinct genetic variants. Inherited mutations at this locus cause PLCγ2-associated antibody deficiency and immune dysregulation, in some cases with autoinflammation. Acquired mutations at this locus, which often arise as a result of BTK inhibition to treat chronic lymphocytic leukemia, result in constitutive downstream signaling and lymphocyte proliferation. Finally, a third group of PLCγ2 variants actually has a protective effect in a variety of neurodegenerative disorders, presumably by increased uptake and degradation of deleterious neurological aggregates. Therefore, manipulating PLCγ2 activity either up or down could have therapeutic benefit; however, we require a better understanding of the signaling pathways propagated by these variants before such clinical utility can be realized. Here, we review the signaling roles of PLCγ2 in hematopoietic cells to help understand the effect of mutations driving immune disorders and cancer and extrapolate from this to roles which may relate to protection against neurodegeneration.

RACK1 plays a critical role in mast cell secretion and Ca2+ mobilization by modulating F-actin dynamics

Although RACK1 is known to act as a signaling hub in immune cells, its presence and role in mast cells (MCs) is undetermined. MC activation via antigen stimulation results in mediator release and is preceded by cytoskeleton reorganization and Ca2+ mobilization. In this study, we found that RACK1 was distributed throughout the MC cytoplasm both in vivo and in vitro. After RACK1 knockdown (KD), MCs were rounded, and the cortical F-actin was fragmented. Following antigen stimulation, in RACK1 KD MCs, there was a reduction in cortical F-actin, an increase in monomeric G-actin and a failure to organize F-actin. RACK1 KD also increased and accelerated degranulation. CD63+ secretory granules were localized in F-actin-free cortical regions in non-stimulated RACK1 KD MCs. Additionally, RACK1 KD increased antigen-stimulated Ca2+ mobilization, but attenuated antigen-stimulated depletion of ER Ca2+ stores and thapsigargin-induced Ca2+ entry. Following MC activation there was also an increase in interaction of RACK1 with Orai1 Ca2+-channels, β-actin and the actin-binding proteins vinculin and MyoVa. These results show that RACK1 is a critical regulator of actin dynamics, affecting mediator secretion and Ca2+ signaling in MCs. This article has an associated First Person interview with the first author of the paper.

A role for the Sts phosphatases in negatively regulating IFNγ-mediated production of nitric oxide in monocytes

Introduction

The atypical Sts phosphatases negatively regulate signaling pathways in diverse immune cell types, with two of their molecular targets being the related kinases Syk and Zap‐70. Mice lacking Sts expression (Sts^−/−) are resistant to infection by the live vaccine strain (LVS) of Francisella tularensis. Although the mechanisms underlying the enhanced resistance of Sts^−/− mice have not been definitively established, Sts^−/− bone marrow‐derived monocytes (BMMs) demonstrate greater clearance of intracellular LVS following ex vivo infection, relative to wild type cells. To determine how the Sts proteins regulate monocyte bactericidal properties, we analyzed responses of infected cells.

Methods

Monocyte bacterial clearance was assayed using ex vivo coculture infections followed by colony‐forming unit analysis of intracellular bacteria. Levels of gene expression were quantified by quantitative reverse‐transcription polymerase chain reaction, levels of Nos2 protein levels were quantified by Western blot analysis, and levels of nitric oxide (NO) were quantified directly using the Griess reagent. We characterized monocyte cytokine production via enzyme‐linked immunosorbent assay.

Results

We demonstrate that Sts^−/− monocyte cultures produce elevated levels of interferon‐γ (IFNγ) after infection, relative to wild type cultures. Sts^−/− monocytes also demonstrate heightened responsiveness to IFNγ. Specifically, Sts^−/− monocytes produce elevated levels of antimicrobial NO following IFNγ stimulation, and this NO plays an important role in LVS restriction. Additional IFNγ‐stimulated genes, including Ip10 and members of the Gbp gene family, also display heightened upregulation in Sts^−/− cells. Both Sts‐1 and Sts‐2 contribute to the regulation of NO production, as evidenced by the responses of monocytes lacking each phosphatase individually. Finally, we demonstrate that the elevated production of IFNγ‐induced NO in Sts^−/− monocytes is abrogated following chemical inhibition of Syk kinase.

Conclusion

Our results indicate a novel role for the Sts enzymes in regulating monocyte antibacterial responses downstream of IFNγ.

Interplay between HGAL and Grb2 proteins regulates B-cell receptor signaling

Human germinal center (GC)-associated lymphoma (HGAL) is an adaptor protein expressed in GC B cells. HGAL regulates cell motility and B-cell receptor (BCR) signaling, processes that are central for the successful completion of the GC reaction. Herein, we demonstrate phosphorylation of HGAL by Syk and Lyn kinases at tyrosines Y80, Y86, Y106Y107, Y128, and Y148. The HGAL YEN motif (amino acids 107-109) is similar to the phosphopeptide motif pYXN used as a binding site to the growth factor receptor-bound protein 2 (Grb2). We demonstrate by biochemical and molecular methodologies that HGAL directly interacts with Grb2. Concordantly, microscopy studies demonstrate HGAL-Grb2 colocalization in the membrane central supramolecular activation clusters (cSMAC) following BCR activation. Mutation of the HGAL putative binding site to Grb2 abrogates the interaction between these proteins. Further, this HGAL mutant localizes exclusively in the peripheral SMAC and decreases the rate and intensity of BCR accumulation in the cSMAC. Furthermore, we demonstrate that Grb2, HGAL, and Syk interact in the same complex, but Grb2 does not modulate the effects of HGAL on Syk kinase activity. Overall, the interplay between the HGAL and Grb2 regulates the magnitude of BCR signaling and synapse formation.

An unexpected 2-histidine phosphoesterase activity of suppressor of T-cell receptor signaling protein 1 contributes to the suppression of cell signaling

The suppressor of T-cell receptor (TCR) signaling (Sts) proteins Sts-1 and Sts-2 suppress receptor-mediated signaling pathways in various immune cells, including the TCR pathway in T cells and the Dectin-1 signaling pathway in phagocytes. As multidomain enzymes, they contain an N-terminal ubiquitin-association domain, a central Src homology 3 domain, and a C-terminal histidine phosphatase domain. Recently, a 2-histidine (2H) phosphoesterase motif was identified within the N-terminal portion of Sts. The 2H phosphoesterase motif defines an evolutionarily ancient protein domain present in several enzymes that hydrolyze cyclic phosphate bonds on different substrates, including cyclic nucleotides. It is characterized by two invariant histidine residues that play a critical role in catalytic activity. Consistent with its assignment as a phosphoesterase, we demonstrate here that the Sts-1 2H phosphoesterase domain displays catalytic, saturable phosphodiesterase activity toward the dinucleotide 2',3'-cyclic NADP. The enzyme exhibited a high degree of substrate specificity and selectively generated the 3'-nucleotide as the sole product. Sts-1 also had phosphodiesterase catalytic activity toward a 5-mer RNA oligonucleotide containing a 2',3'-cyclic phosphate group at its 3' terminus. To investigate the functional significance of Sts-1 2H phosphoesterase activity, we generated His-to-Ala variants and examined their ability to negatively regulate cellular signaling pathways. Substitution of either conserved histidine compromised the ability of Sts-1 to suppress signaling pathways downstream of both the TCR and the Dectin-1 receptor. Our results identify a heretofore unknown cellular enzyme activity associated with Sts-1 and indicate that this catalytic activity is linked to specific cell-signaling outcomes.

SHIP interacts with adaptor protein Nck and restricts actin turnover in B cells

SH2 domain-containing inositol 5'-phosphatase (SHIP) has critical functions in regulating signal transduction. In additional to its lipid phosphatase activity, SHIP engages in multiple protein-protein interactions, which can serve to localize either SHIP or its binding partners to a particular subcellular domain. Knock-out and knock-down studies have elucidated that SHIP negatively regulates the accumulation of F-actin in leukocytes, usually resulting in inhibition of actin dependent cellular activities such as spreading and migration. Here, we demonstrate that overexpression of SHIP inhibits B cell antigen receptor (BCR)-mediated cell spreading in murine and human B cell lines. B cell stimulation via the BCR or pervanadate induces an interaction between SHIP and Nck, an adaptor protein known to promote actin polymerization. Using a fluorescence recovery after photobleaching (FRAP) assay, we demonstrate that overexpression of SHIP slows F-actin dynamics in BCR-stimulated B cells and this can be overcome by co-overexpression of Nck. Our data supports a role for SHIP in limiting actin turnover and suggests it may do so in part by sequestering Nck.

The lectin ArtinM activates RBL-2H3 mast cells without inducing degranulation

Mast cells are connective tissue resident cells with morphological and functional characteristics that contribute to their role in allergic and inflammatory processes, host defense and maintenance of tissue homeostasis. Mast cell activation results in the release of pro-inflammatory mediators which are largely responsible for the physiological functions of mast cells. The lectin ArtinM, extracted from Artocarpus heterophyllus (jackfruit), binds to D-manose, thus inducing degranulation of mast cells. ArtinM has several immunomodulatory properties including acceleration of wound healing, and induction of cytokine release. The aim of the present study was to investigate the role of ArtinM in the activation and proliferation of mast cells. The rat mast cell line RBL-2H3 was used throughout this study. At a low concentration (0.25μg/mL), ArtinM induced mast cell activation and the release of IL-6 without stimulating the release of pre-formed or newly formed mediators. Additionally, when the cells were activated by ArtinM protein tyrosine phosphorylation was stimulated. The low concentration of ArtinM also activated the transcription factor NFkB, but not NFAT. ArtinM also affected the cell cycle and stimulated cell proliferation. Therefore, ArtinM may have therapeutic applications by modulating immune responses due to its ability to activate mast cells and promote the release of newly synthesized mediators. Additionally, ArtinM could have beneficial effects at low concentrations without degranulating mast cells and inducing allergic reactions.

TULA proteins as signaling regulators

Two members of the UBASH3/STS/TULA family exhibit a unique protein domain structure, which includes a histidine phosphatase domain, and play a key role in regulating cellular signaling. UBASH3A/STS-2/TULA is mostly a lymphoid protein, while UBASH3B/STS-1/TULA-2 is expressed ubiquitously. Dephosphorylation of tyrosine-phosphorylated proteins by TULA-2 and, probably to a lesser extent, by TULA critically contribute to the molecular basis of their regulatory effect. The notable differences between the effects of the two family members on cellular signaling and activation are likely to be linked to the difference between their specific enzymatic activities. However, these differences might also be related to the functions of their domains other than the phosphatase domain and independent of their phosphatase activity. The down-regulation of the Syk/Zap-70-mediated signaling, which to-date appears to be the best-studied regulatory effect of TULA family, is discussed in detail in this publication.

Regulation of the pleiotropic effects of tissue-resident mast cells

Mast cells (MCs), which are best known for their detrimental role in patients with allergic diseases, act in a diverse array of physiologic and pathologic functions made possible by the plurality of MC types. Their various developmental avenues and distinct sensitivity to (micro-) environmental conditions convey extensive heterogeneity, resulting in diverse functions. We briefly summarize this heterogeneity, elaborate on molecular determinants that allow MCs to communicate with their environment to fulfill their tasks, discuss the protease repertoire stored in secretory lysosomes, and consider different aspects of MC signaling. Furthermore, we describe key MC governance mechanisms (ie, the high-affinity receptor for IgE [FcεRI]), the stem cell factor receptor KIT, the IL-4 system, and both Ca²⁺- and phosphatase-dependent mechanisms. Finally, we focus on distinct physiologic functions, such as chemotaxis, phagocytosis, host defense, and the regulation of MC functions at the mucosal barriers of the lung, gastrointestinal tract, and skin. A deeper knowledge of the pleiotropic functions of MC mediators, as well as the molecular processes of MC regulation and communication, should enable us to promote beneficial MC traits in physiology and suppress detrimental MC functions in patients with disease.

TULA-2 Deficiency Enhances Platelet Functional Responses to CLEC-2 Agonists

Platelet activation is essential for hemostasis. Central to platelet activation are the signals transmitted through surface receptors such as glycoprotein VI, the protease-activated receptors, and C-type lectin-like receptor 2 (CLEC-2). CLEC-2 is a HemITAM (hem-immunoreceptor tyrosine activation motif)-bearing receptor that binds podoplanin and signals through spleen tyrosine kinase (Syk). T-cell ubiquitin ligand-2 (TULA-2) is a protein tyrosine phosphatase that is highly expressed in platelets and targets phosphorylated Y352 of Syk. We wanted to determine whether TULA-2 regulates Syk phosphorylation and activity downstream of CLEC-2. To that end, we used TULA-2 knockout mice and wild-type (WT) littermate controls. We found that TULA-2 deficiency enhances the aggregation and secretion response following stimulation with an excitatory CLEC-2 antibody or the CLEC-2 agonist rhodocytin. Consistently, Syk phosphorylation of Y346 is enhanced, as well as phosphorylation of the downstream signaling molecule PLCγ2, in TULA-2 knockout platelets treated with either CLEC-2 antibody or rhodocytin, compared with WT control platelets. Furthermore, the kinetics of Syk phosphorylation, as well as that of PLCγ2 and SLP-76, is enhanced in TULA-2 knockout platelets treated with 2.5-μg/mL CLEC-2 antibody compared with WT platelets. Similarly, thromboxane production was enhanced, in both amount and kinetics, in TULA-2 ^−/− platelets treated with 2.5-μg/mL CLEC-2 antibody. TULA-2 acts as a negative regulator of CLEC-2 signaling by dephosphorylating Syk on Y346 and restraining subsequent Syk-mediated signaling.

Phagocytes from Mice Lacking the Sts Phosphatases Have an Enhanced Antifungal Response to Candida albicans

Mice lacking expression of the homologous phosphatases Sts-1 and Sts-2 (Sts^−/− mice) are resistant to disseminated candidiasis caused by the fungal pathogen Candida albicans. To better understand the immunological mechanisms underlying the enhanced resistance of Sts^−/− mice, we examined the kinetics of fungal clearance at early time points. In contrast to the rapid C. albicans growth seen in normal kidneys during the first 24 h postinfection, we observed a reduction in kidney fungal CFU within Sts^−/− mice beginning at 12 to 18 h postinfection. This corresponds to the time period when large numbers of innate leukocytes enter the renal environment to counter the infection. Because phagocytes of the innate immune system are important for host protection against pathogenic fungi, we evaluated responses of bone marrow leukocytes. Relative to wild-type cells, Sts^−/− marrow monocytes and bone marrow-derived dendritic cells (BMDCs) displayed a heightened ability to inhibit C. albicans growth ex vivo. This correlated with significantly enhanced production of reactive oxygen species (ROS) by Sts^−/− BMDCs downstream of Dectin-1, a C-type lectin receptor that plays a critical role in stimulating host responses to fungi. We observed no visible differences in the responses of other antifungal effector pathways, including cytokine production and inflammasome activation, despite enhanced activation of the Syk tyrosine kinase downstream of Dectin-1 in Sts^−/− cells. Our results highlight a novel mechanism regulating the immune response to fungal infections. Further understanding of this regulatory pathway could aid the development of therapeutic approaches to enhance protection against invasive candidiasis.

Systemic candidiasis caused by fungal Candida species is becoming an increasingly serious medical problem for which current treatment is inadequate. Recently, the Sts phosphatases were established as key regulators of the host antifungal immune response. In particular, genetic inactivation of Sts significantly enhanced survival of mice infected intravenously with Candida albicans. The Sts^−/−in vivo resistance phenotype is associated with reduced fungal burden and an absence of inflammatory lesions. To understand the underlying mechanisms, we studied phagocyte responses. Here, we demonstrate that Sts^−/− phagocytes have heightened responsiveness to C. albicans challenge relative to wild-type cells. Our data indicate the Sts proteins negatively regulate phagocyte activation via regulating selective elements of the Dectin-1–Syk tyrosine kinase signaling axis. These results suggest that phagocytes lacking Sts respond to fungal challenge more effectively and that this enhanced responsiveness partially underlies the profound resistance of Sts^−/− mice to systemic fungal challenge.

MicroRNA-1291 targets the FOXA2-AGR2 pathway to suppress pancreatic cancer cell proliferation and tumorigenesis

Pancreatic cancer is the fourth leading cause of cancer death in the United States. Better understanding of pancreatic cancer biology may help identify new oncotargets towards more effective therapies. This study investigated the mechanistic actions of microRNA-1291 (miR-1291) in the suppression of pancreatic tumorigenesis. Our data showed that miR-1291 was downregulated in a set of clinical pancreatic carcinoma specimens and human pancreatic cancer cell lines. Restoration of miR-1291 expression inhibited pancreatic cancer cell proliferation, which was associated with cell cycle arrest and enhanced apoptosis. Furthermore, miR-1291 sharply suppressed the tumorigenicity of PANC-1 cells in mouse models. A proteomic profiling study revealed 32 proteins altered over 2-fold in miR-1291-expressing PANC-1 cells that could be assembled into multiple critical pathways for cancer. Among them anterior gradient 2 (AGR2) was reduced to the greatest degree. Through computational and experimental studies we further identified that forkhead box protein A2 (FOXA2), a transcription factor governing AGR2 expression, was a direct target of miR-1291. These results connect miR-1291 to the FOXA2-AGR2 regulatory pathway in the suppression of pancreatic cancer cell proliferation and tumorigenesis, providing new insight into the development of miRNA-based therapy to combat pancreatic cancer.

Modulating Host Signaling Pathways to Promote Resistance to Infection by Candida albicans

Candida albicans is a common human fungal pathogen capable of causing serious systemic infections that can progress to become lethal. Current therapeutic approaches have limited effectiveness, especially once a systemic infection is established, in part due to the lack of an effective immune response. Boosting the immune response to C. albicans has been the goal of immunotherapy, but it has to be done selectively to prevent deleterious hyperinflammation (sepsis). Although an efficient inflammatory response is necessary to fight infection, the typical response to C. albicans results in collateral damage to tissues thereby exacerbating the pathological effects of infection. For this reason, identifying specific ways of modulating the immune system holds promise for development of new improved therapeutic approaches. This review will focus on recent studies that provide insight using mutant strains of mice that are more resistant to bloodstream infection by C. albicans. These mice are deficient in signal transduction proteins including the Jnk1 MAP kinase, the Cbl-b E3 ubiquitin ligase, or the Sts phosphatases. Interestingly, the mutant mice display a different response to C. albicans that results in faster clearance of infection without hyper-inflammation and collateral damage. A common underlying theme between the resistant mouse strains is loss of negative regulatory proteins that are known to restrain activation of cell surface receptor-initiated signaling cascades. Understanding the cellular and molecular mechanisms that promote resistance to C. albicans in mice will help to identify new approaches for improving antifungal therapy.

Unperturbed Immune Function despite Mutation of C-Terminal Tyrosines in Syk Previously Implicated in Signaling and Activity Regulation

The nonreceptor tyrosine kinase Syk, a central regulator of immune cell differentiation and activation, is a promising drug target for treatment of leukemia and allergic and inflammatory diseases. The clinical failure of Syk inhibitors underscores the importance of understanding the regulation of Syk function and activity. A series of previous studies emphasized the importance of three C-terminal tyrosines in Syk for kinase activity regulation, as docking sites for downstream effector molecules, and for Ca²⁺ mobilization. Here, we investigated the roles of these C-terminal tyrosines in the mouse. Surprisingly, expression of a triple tyrosine-to-phenylalanine human Syk mutant, SYK(Y3F), was not associated with discernible signaling defects either in reconstituted DT40 cells or in B or mast cells from mice expressing SYK(Y3F) instead of wild-type Syk. Remarkably, lymphocyte differentiation, calcium mobilization, and 2,4,6-trinitrophenyl (TNP)-specific immune responses were unperturbed in SYK(Y3F) mice. These results emphasize the capacity of immune cells to compensate for specific molecular defects, likely using redundant intermolecular interactions, and highlight the importance of in vivo analyses for understanding cellular signaling mechanisms.

Increased Resistance to Intradermal Francisella tularensis LVS Infection by Inactivation of the Sts Phosphatases

The Suppressor of TCR signaling proteins (Sts-1 and Sts-2) are two homologous phosphatases that negatively regulate signaling pathways in a number of hematopoietic lineages, including T lymphocytes. Mice lacking Sts expression are characterized by enhanced T cell responses. Additionally, a recent study demonstrated that Sts^-/- mice are profoundly resistant to systemic infection by Candida albicans, with resistance characterized by enhanced survival, more rapid fungal clearance in key peripheral organs, and an altered inflammatory response. To investigate the role of Sts in the primary host response to infection by a bacterial pathogen, we evaluated the response of Sts^-/- mice to infection by a Gram-negative bacterial pathogen. Francisella tularensis is a facultative bacterial pathogen that replicates intracellularly within a variety of cell types and is the causative agent of tularemia. Francisella infections are characterized by a delayed immune response, followed by an intense inflammatory reaction that causes widespread tissue damage and septic shock. Herein, we demonstrate that mice lacking Sts expression are significantly resistant to infection by the live vaccine strain (LVS) of F. tularensis Resistance is characterized by reduced lethality following high-dose intradermal infection, an altered cytokine response in the spleen, and enhanced bacterial clearance in multiple peripheral organs. Sts^-/- bone marrow-derived monocytes and neutrophils, infected with F. tularensis LVS ex vivo, display enhanced restriction of intracellular bacteria. These observations suggest the Sts proteins play an important regulatory role in the host response to bacterial infection, and they underscore a role for Sts in regulating functionally relevant immune response pathways.

Regulation of immune cell signaling by SHIP1: A phosphatase, scaffold protein, and potential therapeutic target

The phosphoinositide phosphatase SHIP is a critical regulator of immune cell activation. Despite considerable study, the mechanisms controlling SHIP activity to ensure balanced cell activation remain incompletely understood. SHIP dampens BCR signaling in part through its association with the inhibitory coreceptor Fc gamma receptor IIB, and serves as an effector for other inhibitory receptors in various immune cell types. The established paradigm emphasizes SHIP's inhibitory receptor-dependent function in regulating phosphoinositide 3-kinase signaling by dephosphorylating the phosphoinositide PI(3,4,5)P₃ ; however, substantial evidence indicates that SHIP can be activated independently of inhibitory receptors and can function as an intrinsic brake on activation signaling. Here, we integrate historical and recent reports addressing the regulation and function of SHIP in immune cells, which together indicate that SHIP acts as a multifunctional protein controlled by multiple regulatory inputs, and influences downstream signaling via both phosphatase-dependent and -independent means. We further summarize accumulated evidence regarding the functions of SHIP in B cells, T cells, NK cells, dendritic cells, mast cells, and macrophages, and data suggesting defective expression or activity of SHIP in autoimmune and malignant disorders. Lastly, we discuss the biological activities, therapeutic promise, and limitations of small molecule modulators of SHIP enzymatic activity.

TULA-2 (T-Cell Ubiquitin Ligand-2) Inhibits the Platelet Fc Receptor for IgG IIA (FcγRIIA) Signaling Pathway and Heparin-Induced Thrombocytopenia in Mice

OBJECTIVE

The objective of this study is to investigate the role of T-cell ubiquitin ligand-2 (TULA-2) in the platelet Fc receptor for IgG IIA (FcγRIIA) pathway and in the pathogenesis of heparin-induced thrombocytopenia (HIT).

APPROACH AND RESULTS

HIT is a life-threatening thrombotic disease in which IgG antibodies against the heparin-platelet factor 4 complex activate platelets via FcγRIIA. We reported previously differential expression of TULA-2 in human population was linked to FcγRIIA responsiveness. In this study, we investigated the role of TULA-2, a protein phosphatase, in the FcγRIIA pathway and HIT pathogenesis by crossing TULA-2^-/- mice with transgenic FcγRIIA ^+/+ mice. Ablation of TULA-2 resulted in hyperphosphorylation of spleen tyrosine kinase, linker for the activation of T cells, and phospholipase Cγ2 in platelets via FcγRIIA activation. Platelet integrin activation, granule secretion, phosphatidylserine exposure, and aggregation were also enhanced in TULA-2^-/- murine platelets. Compared with wild-type mice, TULA-2^-/- mice showed aggravated antibody-mediated thrombocytopenia, augmented thrombin generation, and shortened tail bleeding time. In contrast, there was no significant difference between TULA-2^-/- and TULA-2^+/+ platelets in platelet spreading and clot retraction. Of note, heterozygous TULA-2^+/- mice, whose platelets contained 50% as much protein as the TULA-2^+/+ platelets, showed significantly increased platelet reactivity and more severe thrombocytopenia in vivo compared with TULA-2^+/+ mice.

CONCLUSIONS

Together, the data demonstrate that not only the absence of TULA-2 but also the relative level of TULA-2 expression modulates FcγRIIA-mediated platelet reactivity and HIT in vivo. TULA-2 expression could be a valuable marker for HIT and inhibiting TULA-2 may serve as a potential therapy to reverse the bleeding adverse effect of anticoagulants.

TULA-2 Protein Phosphatase Suppresses Activation of Syk through the GPVI Platelet Receptor for Collagen by Dephosphorylating Tyr(P)346, a Regulatory Site of Syk

Protein-tyrosine phosphatase TULA-2 has been shown to regulate receptor signaling in several cell types, including platelets. Platelets are critical for maintaining vascular integrity; this function is mediated by platelet aggregation in response to recognition of the exposed basement membrane collagen by the GPVI receptor, which is non-covalently associated with the signal-transducing FcRγ polypeptide chain. Our previous studies suggested that TULA-2 plays an important role in negatively regulating signaling through GPVI-FcRγ and indicated that the tyrosine-protein kinase Syk is a key target of the regulatory action of TULA-2 in platelets. However, the molecular basis of the down-regulatory effect of TULA-2 on Syk activation via FcRγ remained unclear. In this study, we demonstrate that suppression of Syk activation by TULA-2 is mediated, to a substantial degree, by dephosphorylation of Tyr(P)³⁴⁶, a regulatory site of Syk, which becomes phosphorylated soon after receptor ligation and plays a critical role in initiating the process that yields fully activated Syk. TULA-2 is capable of dephosphorylating Tyr(P)³⁴⁶ with high efficiency, thus controlling the overall activation of Syk, but is less efficient in dephosphorylating other regulatory sites of this kinase. Therefore, dephosphorylation of Tyr(P)³⁴⁶ may be considered an important "checkpoint" in the regulation of Syk activation process. Putative biological functions of TULA-2-mediated dephosphorylation of Tyr(P)³⁴⁶ may include deactivation of receptor-activated Syk or suppression of Syk activation by suboptimal stimulation.

GWAS analysis implicates NF-κB-mediated induction of inflammatory T cells in multiple sclerosis

To identify genes and biologically relevant pathways associated with risk to develop multiple sclerosis (MS), the Genome-Wide Association Studies noise reduction method (GWAS-NR) was applied to MS genotyping data. Regions of association were defined based on the significance of linkage disequilibrium blocks. Candidate genes were cross-referenced based on a review of current literature, with attention to molecular function and directly interacting proteins. Supplementary annotations and pathway enrichment scores were generated using The Database for Annotation, Visualization and Integrated Discovery. The candidate set of 220 MS susceptibility genes prioritized by GWAS-NR was highly enriched with genes involved in biological pathways related to positive regulation of cell, lymphocyte and leukocyte activation (P=6.1E-15, 1.2E-14 and 5.0E-14, respectively). Novel gene candidates include key regulators of NF-κB signaling and CD4+ T helper type 1 (Th1) and T helper type 17 (Th17) lineages. A large subset of MS candidate genes prioritized by GWAS-NR were found to interact in a tractable pathway regulating the NF-κB-mediated induction and infiltration of pro-inflammatory Th1/Th17 T-cell lineages, and maintenance of immune tolerance by T-regulatory cells. This mechanism provides a biological context that potentially links clinical observations in MS to the underlying genetic landscape that may confer susceptibility.

Proteome analysis of mast cell releasates reveals a role for chymase in the regulation of coagulation factor XIIIA levels via proteolytic degradation

BACKGROUND

Mast cells are significantly involved in IgE-mediated allergic reactions; however, their roles in health and disease are incompletely understood.

OBJECTIVE

We aimed to define the proteome contained in mast cell releasates on activation to better understand the factors secreted by mast cells that are relevant to the contribution of mast cells in diseases.

METHODS

Bone marrow-derived cultured mast cells (BMCMCs) and peritoneal cell-derived mast cells were used as "surrogates" for mucosal and connective tissue mast cells, respectively, and their releasate proteomes were analyzed by mass spectrometry.

RESULTS

Our studies showed that BMCMCs and peritoneal cell-derived mast cells produced substantially different releasates following IgE-mediated activation. Moreover, we observed that the transglutaminase coagulation factor XIIIA (FXIIIA) was one of the most abundant proteins contained in the BMCMC releasates. Mast cell-deficient mice exhibited increased FXIIIA plasma and activity levels as well as reduced bleeding times, indicating that mast cells are more efficient in their ability to downregulate FXIIIA than in contributing to its amounts and functions in homeostatic conditions. We found that human chymase and mouse mast cell protease-4 (the mouse homologue of human chymase) had the ability to reduce FXIIIA levels and function via proteolytic degradation. Moreover, we found that chymase deficiency led to increased FXIIIA amounts and activity, as well as reduced bleeding times in homeostatic conditions and during sepsis.

CONCLUSIONS

Our study indicates that the mast cell protease content can shape its releasate proteome. Moreover, we found that chymase plays an important role in the regulation of FXIIIA via proteolytic degradation.

The extended human PTPome: a growing tyrosine phosphatase family

Tyr phosphatases are, by definition, enzymes that dephosphorylate phospho-Tyr (pTyr) from proteins. This activity is found in several structurally diverse protein families, including the protein Tyr phosphatase (PTP), arsenate reductase, rhodanese, haloacid dehalogenase (HAD) and His phosphatase (HP) families. Most of these families include members with substrate specificity for non-pTyr substrates, such as phospho-Ser/phospho-Thr, phosphoinositides, phosphorylated carbohydrates, mRNAs, or inorganic moieties. A Cys is essential for catalysis in PTPs, rhodanese and arsenate reductase enzymes, whereas this work is performed by an Asp in HAD phosphatases and by a His in HPs, via a catalytic mechanism shared by all of the different families. The category that contains most Tyr phosphatases is the PTP family, which, although it received its name from this activity, includes Ser, Thr, inositide, carbohydrate and RNA phosphatases, as well as some inactive pseudophosphatase proteins. Here, we propose an extended collection of human Tyr phosphatases, which we call the extended human PTPome. The addition of new members (SACs, paladin, INPP4s, TMEM55s, SSU72, and acid phosphatases) to the currently categorized PTP group of enzymes means that the extended human PTPome contains up to 125 proteins, of which ~ 40 are selective for pTyr. We set criteria to ascribe proteins to the extended PTPome, and summarize the more important features of the new PTPome members in the context of their phosphatase activity and their relationship with human disease.

Protection from systemic Candida albicans infection by inactivation of the Sts phosphatases

The human fungal pathogen Candida albicans causes invasive candidiasis, characterized by fatal organ failure due to disseminated fungal growth and inflammatory damage. The suppressor of TCR signaling 1 (Sts-1) and Sts-2 are two homologous phosphatases that negatively regulate signaling pathways in a number of hematopoietic cell lineages, including T lymphocytes, mast cells, and platelets. Functional inactivation of both Sts enzymes leads to profound resistance to systemic infection by C. albicans, such that greater than 80% of mice lacking Sts-1 and -2 survive a dose of C. albicans (2.5 × 10(5) CFU/mouse) that is uniformly lethal to wild-type mice within 10 days. Restriction of fungal growth within the kidney occurs by 24 h postinfection in the mutant mice. This occurs without induction of a hyperinflammatory response, as evidenced by the decreased presence of leukocytes and inflammatory cytokines that normally accompany the antifungal immune response. Instead, the absence of the Sts phosphatases leads to the rapid induction of a unique immunological environment within the kidney, as indicated by the early induction of a proinflammatory cytokine (CXL10). Mice lacking either Sts enzyme individually display an intermediate lethality phenotype. These observations identify an opportunity to optimize host immune responses toward a deadly fungal pathogen.

Mast cell activation: a complex interplay of positive and negative signaling pathways

Mast cells regulate the immunological responses causing allergy and autoimmunity, and contribute to the tumor microenvironment through generation and secretion of a broad array of preformed, granule-stored and de novo synthesized bioactive compounds. The release and production of mast cell mediators is the result of a coordinated signaling machinery, followed by the FcεRI and FcγR antigen ligation. In this review, we present the latest understanding of FcεRI and FcγR signaling, required for the canonical mast cell activation during allergic responses and anaphylaxis. We then describe the cooperation between the signaling of FcR and other recently characterized membrane-bound receptors (i.e., IL-33R and thymic stromal lymphopoietin receptor) and their role in the chronic settings, where mast cell activation is crucial for the development and the sustainment of chronic diseases, such as asthma or airway inflammation. Finally, we report how the FcR activation could be used as a therapeutic approach to treat allergic and atopic diseases by mast cell inactivation. Understanding the magnitude and the complexity of mast cell signaling is necessary to identify the mechanisms underlying the potential effector and regulatory roles of mast cells in the biology and pathology of those disease settings in which mast cells are activated.

Insights into the suppressor of T-cell receptor (TCR) signaling-1 (Sts-1)-mediated regulation of TCR signaling through the use of novel substrate-trapping Sts-1 phosphatase variants

High affinity substrate-trapping protein tyrosine phosphatases have been widely used both to investigate the endogenous targets of many phosphatases and to address questions of substrate specificity. Herein, we extend the concept of a substrate-trapping phosphatase to include an enzyme of the histidine phosphatase superfamily. This is the first description of substrate-trapping technology applied to a member of the histidine phosphatase family. The phosphatase suppressor of T-cell receptor signaling (Sts)-1 has recently been reported to negatively regulate signaling downstream of the T-cell receptor. We generated high-affinity substrate-trapping variants of Sts-1 by mutagenesis of key active site residues within the phosphatase catalytic domain. Mutation of both the nucleophilic His380 and the general acid Glu490 yielded Sts-1 enzymes that were catalytically inactive but showed high affinity for an important tyrosine kinase in T cells that Sts-1 is known to regulate, Zap-70. Sts-1 substrate-trapping mutants isolated tyrosine-phosphorylated Zap-70 from lysates of activated T cells, validating Zap-70 as a possible substrate for Sts-1 and highlighting the efficacy of the mutants as substrate-trapping agents. Inhibition of the Zap-70 interaction by vanadate suggests that the substrate-trapping effect occurred via the Sts-1 phosphatase active site. Finally, overexpression of Sts-1 substrate-trapping mutants in T cells blocked T-cell receptor signaling, confirming the inhibitory effect of Sts-1 on Zap-70.

Phosphatase regulation of immunoreceptor signaling in T cells, B cells and mast cells

Recent progress has begun to reveal the often complex and changing roles of phosphotyrosine and phosphoinositide phosphatases in regulation of immunoreceptor signaling. The resultant confusion has been further increased by discoveries of new players. Here we provide a review of recent progress in defining the roles of these enzymes in immunoreceptor-dependent mast cell, T cell and B cell activation.

Atypical protein phosphatases: emerging players in cellular signaling

It has generally been considered that protein phosphatases have more diverse catalytic domain structures and mechanisms than protein kinases; however, gene annotation efforts following the human genome project appeared to have completed the whole array of protein phosphatases. Ser/Thr phosphatases are divided into three subfamilies that have different structures from each other, whereas Tyr phosphatases and dual-specificity phosphatases targeting Tyr, Ser and Thr belong to a single large family based on their common structural features. Several years of research have revealed, however, the existence of unexpected proteins, designated here as “atypical protein phosphatases”, that have structural and enzymatic features different from those of the known protein phosphatases and are involved in important biological processes. In this review, we focus on the identification and functional characterization of atypical protein phosphatases, represented by eyes absent (EYA), suppressor of T-cell receptor signaling (Sts) and phosphoglycerate mutase family member 5 (PGAM5) and discuss their biological significance in cellular signaling.

TULA-2, a novel histidine phosphatase, regulates bone remodeling by modulating osteoclast function

Bone is a dynamic tissue that depends on the intricate relationship between protein tyrosine kinases (PTK) and protein tyrosine phosphatases (PTP) for maintaining homeostasis. PTKs and PTPs act like molecular on and off switches and help modulate differentiation and the attachment of osteoclasts to bone matrix regulating bone resorption. The protein T cell ubiquitin ligand-2 (TULA-2), which is abundantly expressed in osteoclasts, is a novel histidine phosphatase. Our results show that of the two family members, only TULA-2 is expressed in osteoclasts and that its expression is sustained throughout the course of osteoclast differentiation, suggesting that TULA-2 may play a role during early as well late stages of osteoclast differentiation. Skeletal analysis of mice that do not express TULA or TULA-2 proteins (DKO mice) revealed that there was a decrease in bone volume due to increased osteoclast numbers and function. Furthermore, in vitro experiments indicated that bone marrow precursor cells from DKO mice have an increased potential to form osteoclasts. At the molecular level, the absence of TULA-2 in osteoclasts results in increased Syk phosphorylation at the Y352 and Y525/526 residues and activation of phospholipase C gamma 2 (PLCγ2) upon engagement of immune-receptor-tyrosine-based-activation-motif (ITAM)-mediated signaling. Furthermore, expression of a phosphatase-dead TULA-2 leads to increased osteoclast function. Taken together, these results suggest that TULA-2 negatively regulates osteoclast differentiation and function.