Requirement for a negative charge at threonine 60 of the FcRgamma for complete activation of Syk

Trioxidized cysteine in the aging proteome mimics the structural dynamics and interactome of phosphorylated serine

Aging is the primary risk factor for the development of numerous human chronic diseases. On a molecular level, it significantly impacts the regulation of protein modifications, leading to the accumulation of degenerative protein modifications (DPMs) such as aberrant serine phosphorylation (p-Ser) and trioxidized cysteine (t-Cys) within the proteome. The altered p-Ser is linked to abnormal cell signaling, while the accumulation of t-Cys is associated with chronic diseases induced by oxidative stress. Despite this, the potential cross-effects and functional interplay between these two critical molecular factors of aging remain undisclosed. This study analyzes the aging proteome of wild-type C57BL/6NTac mice over 2 years using advanced proteomics and bioinformatics. Our objective is to provide a comprehensive analysis of how t-Cys affects cell signaling and protein structure in the aging process. The results obtained indicate that t-Cys residues accumulate in the aging proteome, interact with p-Ser interacting enzymes, as validated in vitro, and alter their structures similarly to p-Ser. These findings have significant implications for understanding the interplay of oxidative stress and phosphorylation in the aging process. Additionally, they open new venues for further research on the role(s) of these protein modifications in various human chronic diseases and aging, wherein exacerbated oxidation and aberrant phosphorylation are implicated.

Regulation of FcRγ function by site-specific serine phosphorylation

The common FcRγ, an immunoreceptor tyrosine-based activation motif (ITAM)- containing adaptor protein, associates with multiple leukocyte receptor complexes and mediates signal transduction through the ITAM in the cytoplasmic domain. The presence of multiple serine and threonine residues within this motif suggests the potential for serine/threonine phosphorylation in modulating signaling events. Single-site mutational analysis of these residues in RBL-2H3 cells indicates that each may contribute to net FcRγ-mediated signaling, and mass spectrometry of WT human FcRγ from receptor-stimulated cells shows consistent preferential phosphorylation of the serine residue at position 51. Immunoblot analysis, mass spectrometry, and mutational analyses showed that phosphorylation of serine 51 in the 7-residue spacer between the 2 YxxL sequences regulates FcRγ signaling by inhibiting tyrosine phosphorylation at the membrane proximal Y47 position of the ITAM, but not phosphorylation at position Y58. This inhibition results in reduced Syk recruitment and activation. With in vitro kinase assays, PKC-δ and PKA show preferential phosphorylation of S51. Serine/threonine phosphorylation of the FcRγ ITAM, which functions as an integrator of multiple signaling elements, may explain in part the contribution of variants in PKC-δ and other PKC isoforms to some autoimmune phenotypes.

Regulation of FcεRI signaling by lipid phosphatases

Mast cells (MCs) are tissue-resident sentinels of hematopoietic origin that play a prominent role in allergic diseases. They express the high-affinity receptor for IgE (FcεRI), which when cross-linked by multivalent antigens triggers the release of preformed mediators, generation of arachidonic acid metabolites, and the synthesis of cytokines and chemokines. Stimulation of the FcεRI with increasing antigen concentrations follows a characteristic bell-shaped dose-responses curve. At high antigen concentrations, the so-called supra-optimal conditions, repression of FcεRI-induced responses is facilitated by activation and incorporation of negative signaling regulators. In this context, the SH2-containing inositol-5'-phosphatase, SHIP1, has been demonstrated to be of particular importance. SHIP1 with its catalytic and multiple protein interaction sites provides several layers of control for FcεRI signaling. Regulation of SHIP1 function occurs on various levels, e.g., protein expression, receptor and membrane recruitment, competition for protein-protein interaction sites, and activating modifications enhancing the phosphatase function. Apart from FcεRI-mediated signaling, SHIP1 can be activated by diverse unrelated receptor systems indicating its involvement in the regulation of antigen-dependent cellular responses by autocrine feedback mechanisms or tissue-specific and/or (patho-) physiologically determined factors. Thus, pharmacologic engagement of SHIP1 may represent a beneficial strategy for patients suffering from acute or chronic inflammation or allergies.

Fc receptor-γ subunits with both polar or non-polar amino acids at position of T22 are capable of restoring surface expression of the high-affinity IgE receptor and degranulation in γ subunit-deficient rat basophilic leukemia cells

The high-affinity IgE receptor (FcɛRI) is formed by the IgE-binding α subunit, β subunit and γ subunits homodimer. All three subunits are required for proper expression of the receptor on the plasma membrane of mast cells and basophils. However, the exact molecular mechanism of inter-subunit interactions required for correct expression and function of the FcɛRI complex remains to be identified. A recent study suggested that polar aspartate at position 194 within the transmembrane domain of the α subunit could interact by hydrogen bonding with polar threonine at position 22 in the transmembrane domains of the γ subunits. To verify this, we used previously isolated rat basophilic leukemia (RBL)-2H3 variant cells deficient in the expression of the FcɛRI-γ subunit (FcR-γ), and transfected them with DNA vectors coding for FcR-γ of the wild-type or mutants in which T22 was substituted for nonpolar alanine (T22A mutant) or polar serine (T22S mutant). Analysis of the transfectants showed that both T22A and T22S mutants were capable to restore surface expression of the FcɛRI similar to wild-type FcR-γ. Furthermore, cells transfected with wild-type, T22A or T22S FcR-γ showed comparably enhanced FcɛRI-mediated degranulation. Our data indicate that substitution of FcR-γ T22 with non-polar amino acid does not interfere with surface expression of the FcɛRI and its signaling capacity.

CLEC-2 activates Syk through dimerization

The C-type lectin receptor CLEC-2 activates platelets through Src and Syk tyrosine kinases, leading to tyrosine phosphorylation of downstream adapter proteins and effector enzymes, including phospholipase-C gamma2. Signaling is initiated through phosphorylation of a single conserved tyrosine located in a YxxL sequence in the CLEC-2 cytosolic tail. The signaling pathway used by CLEC-2 shares many similarities with that used by receptors that have 1 or more copies of an immunoreceptor tyrosine-based activation motif, defined by the sequence Yxx(L/I)x(6-12)Yxx(L/I), in their cytosolic tails or associated receptor chains. Phosphorylation of the conserved immunoreceptor tyrosine-based activation motif tyrosines promotes Syk binding and activation through binding of the Syk tandem SH2 domains. In this report, we present evidence using peptide pull-down studies, surface plasmon resonance, quantitative Western blotting, tryptophan fluorescence measurements, and competition experiments that Syk activation by CLEC-2 is mediated by the cross-linking through the tandem SH2 domains with a stoichiometry of 2:1. In support of this model, cross-linking and electron microscopy demonstrate that CLEC-2 is present as a dimer in resting platelets and converted to larger complexes on activation. This is a unique mode of activation of Syk by a single YxxL-containing receptor.

Differential roles of the PKC novel isoforms, PKCdelta and PKCepsilon, in mouse and human platelets

BACKGROUND

Increasing evidence suggests that individual isoforms of protein kinase C (PKC) play distinct roles in regulating platelet activation.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we focus on the role of two novel PKC isoforms, PKCdelta and PKCepsilon, in both mouse and human platelets. PKCdelta is robustly expressed in human platelets and undergoes transient tyrosine phosphorylation upon stimulation by thrombin or the collagen receptor, GPVI, which becomes sustained in the presence of the pan-PKC inhibitor, Ro 31-8220. In mouse platelets, however, PKCdelta undergoes sustained tyrosine phosphorylation upon activation. In contrast the related isoform, PKCepsilon, is expressed at high levels in mouse but not human platelets. There is a marked inhibition in aggregation and dense granule secretion to low concentrations of GPVI agonists in mouse platelets lacking PKCepsilon in contrast to a minor inhibition in response to G protein-coupled receptor agonists. This reduction is mediated by inhibition of tyrosine phosphorylation of the FcRgamma-chain and downstream proteins, an effect also observed in wild-type mouse platelets in the presence of a PKC inhibitor.

CONCLUSIONS

These results demonstrate a reciprocal relationship in levels of the novel PKC isoforms delta and epsilon in human and mouse platelets and a selective role for PKCepsilon in signalling through GPVI.

Src family kinases and lipid mediators in control of allergic inflammation

The Src family kinases Fyn and Lyn are important modulators of the molecular events initiated by engagement of the high-affinity IgE receptor (Fc epsilon RI). These kinases control many of the early signaling events and initiate the production of several lipid metabolites that have an important role in regulating mast cell responses. The intracellular level of phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), which is produced by phosphatidylinositol 3-OH kinase, plays an important role in determining the extent of a mast cells response to a stimulus. Enhanced levels lead to a hyperdegranulating phenotype (as seen in SHIP-1(-/-) and Lyn(-/-) mast cells), whereas decreased levels cause hypodegranulation (as seen in Fyn(-/-) mast cells). Downregulation of mast cell phosphatase and tensin homologue deleted on chromosone 10 expression, a phosphatase that reduces cellular levels of PIP(3), caused constitutive cytokine production, demonstrating that this response is particularly sensitive to PIP(3) levels. Lyn and Fyn are also intimately linked to other lipid kinases, like sphingosine kinases (SphK). By producing sphingosine-1-phosphate (S1P), SphKs contribute to mast cell chemotaxis and degranulation. In vivo studies now reveal that circulating S1P as well as that found within the mast cell is important in determining mast cell responsiveness. These studies demonstrate the connection between Src protein tyrosine kinases and lipid second messengers that control mast cell function and allergic responses.

Association of protein kinase C-δ with the B cell antigen receptor complex

Protein kinase C (PKC)-delta is a diacylglycerol-dependent, calcium-independent novel PKC isoform and has been demonstrated to exert negative regulatory functions in B lymphocytes as well as in mast cells. Whereas in mast cells PKC-delta functionally interacts with the high-affinity receptor for IgE, FcepsilonR1, no such association has been described for the B cell antigen receptor (BCR). In this report, for the first time, we demonstrate the interaction of PKC-delta with different classes of BCR by means of affinity purification and native protein complex analysis. Using a C-terminally truncated Ig-alpha as well as non-phosphorylated and phosphorylated peptides representing C-terminal regions of Ig-alpha, the dependence of this BCR/PKC-delta interaction on tyrosine-phosphorylated Ig-alpha is shown. Finally, splenocytes from PKC-delta-deficient mice are found to exert reduced phosphorylation of PKD (a.k.a. PKC-mu) in response to BCR engagement, suggesting the early, membrane-proximal activation of an attenuating kinase complex including PKC-delta and PKD.

Negative Signaling in Fc Receptor Complexes

Cell activation results from the transient displacement of an active balance between positive and negative signaling. This displacement depends in part on the engagement of cell surface receptors by extracellular ligands. Among these are receptors for the Fc portion of immunoglobulins (FcRs). FcRs are widely expressed by cells of hematopoietic origin. When binding antibodies, FcRs provide these cells with immunoreceptors capable of triggering numerous biological responses in response to a specific antigen. FcR-dependent cell activation is regulated by negative signals which are generated together with positive signals within signalosomes that form upon FcR engagement. Many molecules involved in positive signaling, including the FcRbeta subunit, the src kinase lyn, the cytosolic adapter Grb2, and the transmembrane adapters LAT and NTAL, are indeed also involved in negative signaling. A major player in negative regulation of FcR signaling is the inositol 5-phosphatase SHIP1. Several layers of negative regulation operate sequentially as FcRs are engaged by extracellular ligands with an increasing valency. A background protein tyrosine phosphatase-dependent negative regulation maintains cells in a "resting" state. SHIP1-dependent negative regulation can be detected as soon as high-affinity FcRs are occupied by antibodies in the absence of antigen. It increases when activating FcRs are engaged by multivalent ligands and, further, when FcR aggregation increases, accounting for the bell-shaped dose-response curve observed in excess of ligand. Finally, F-actin skeleton-associated high-molecular weight SHIP1, recruited to phosphorylated ITIMs, concentrates in signaling complexes when activating FcRs are coengaged with inhibitory FcRs by immune complexes. Based on these data, activating and inhibitory FcRs could be used for new therapeutic approaches to immune disorders.

An evolutionary and structural perspective on T cell antigen receptor function

After a brief overview of the themes and variations that occur in the family of receptors containing immunoreceptor tyrosine-based activation motifs (ITAMs), and of recent structural data on the ligand-binding subunits of these receptors, we use these data to revisit how information on the state and quality of occupancy of the binding site of the T cell antigen receptor (TCR) is conveyed to the proximal components of the TCR transduction cassette.

A novel lipid raft-associated glycoprotein, TEC-21, activates rat basophilic leukemia cells independently of the type 1 Fc epsilon receptor

Recent data suggest that initiation of signal transduction via type 1 Fc epsilon receptor (Fc epsilon RI) and other immunoreceptors is spatially constrained to lipid rafts. In order to better understand the complexity and function of these structures, we prepared mAb against lipid rafts from the rat basophilic leukemia cell line, RBL-2H3, which is extensively used for analysis of Fc epsilon RI-mediated activation. One of the antibodies was found to recognize a novel glycosylphosphatidylinositol-anchored plasma membrane glycoprotein of 250 amino acids, designated TEC-21, containing a cysteine-rich domain homologous to those found in the urokinase plasminogen activator receptor/Ly-6/snake neurotoxin family. TEC-21 is abundant on the surface of RBL-2H3 cells (>10 (6) molecules/cell), but is absent in numerous rat tissues except for testes. Aggregation of TEC-21 on RBL-2H3 cells induced a rapid increase in tyrosine phosphorylation of several substrates including Syk kinase and LAT adaptor, calcium flux, and release of secretory components. Similar but more profound activation events were observed in cells activated via Fc epsilon RI. However, aggregation of TEC-21 did not induce changes in density of IgE-Fc epsilon RI complexes, tyrosine phosphorylation of Fc epsilon RI beta and gamma subunits, and co-aggregation of Lyn kinase. TEC-21-induced activation events were also observed in Fc epsilon RI(-) mutants of RBL-2H3 cells. Thus, TEC-21 is a novel lipid raft component of RBL-2H3 cells whose aggregation induces activation independently of Fc epsilon RI.

Structure-function analysis of Lyn kinase association with lipid rafts and initiation of early signaling events after Fcepsilon receptor I aggregation

The first step in immunoreceptor signaling is represented by ligand-dependent receptor aggregation, followed by receptor phosphorylation mediated by tyrosine kinases of the Src family. Recently, sphingolipid- and cholesterol-rich plasma membrane microdomains, called lipid rafts, have been identified and proposed to function as platforms where signal transduction molecules may interact with the aggregated immunoreceptors. Here we show that aggregation of the receptors with high affinity for immunoglobulin E (FcepsilonRI) in mast cells is accompanied by a co-redistribution of the Src family kinase Lyn. The co-redistribution requires Lyn dual fatty acylation, Src homology 2 (SH2) and/or SH3 domains, and Lyn kinase activity, in cis or in trans. Palmitoylation site-mutated Lyn, which is anchored to the plasma membrane but exhibits reduced sublocalization into lipid rafts, initiates the tyrosine phosphorylation of FcepsilonRI subunits, Syk protein tyrosine kinase, and the linker for activation of T cells, along with an increase in the concentration of intracellular Ca(2+). However, Lyn mutated in both the palmitoylation and myristoylation sites does not anchor to the plasma membrane and is incapable of initiating FcepsilonRI phosphorylation and early signaling events. These data, together with our finding that a constitutively tyrosine-phosphorylated FcepsilonRI does not exhibit an increased association with lipid rafts, suggest that FcepsilonRI phosphorylation and early activation events can be initiated outside of lipid rafts.

Rottlerin-independent attenuation of pervanadate-induced tyrosine phosphorylation events by protein kinase C-delta in hemopoietic cells

The understanding and control of many pathophysiological conditions is based on knowledge of subtly regulated intracellular signaling networks. We have found that in pervanadate (PV)-treated J558L myeloma cells, amongst other signaling proteins, protein kinase C (PKC)-delta and src homology 2-containing inositol phosphatase (SHIP) are tyrosine phosphorylated on expression of the B cell receptor, suggesting a role for these proteins in the preformed B cell receptor transducer complex. Rottlerin, a widely used PKC-delta-specific inhibitor, efficiently blocks these PV-induced tyrosine phosphorylation events. Furthermore, PV treatment of bone marrow-derived mast cells (BMMC) also results in tyrosine phosphorylation of PKC-delta, SHIP, and additional proteins. Rottlerin also inhibits these responses, indicating that PKC-delta might play an important enhancing role in the propagation of phosphotyrosine signals in B cells and mast cells and hence in the regulation of function of both cell types. Therefore, BMMC from PKC-delta -/- mice were generated by in vitro differentiation and assayed for tyrosine phosphorylation events in response to PV. Intriguingly, and opposite to the Rottlerin data, PKC-delta -/- BMMC show a stronger response to PV than wild-type cells, suggesting an attenuating role for PKC-delta. This response can be inhibited equally well by Rottlerin, indicating clearly that Rottlerin is not specific for PKC-delta in vivo. A comparison between Rottlerin and the panspecific PKC inhibitor bisindolylmaleimide suggests that Rottlerin also targets kinases beyond the PKC family. Moreover, Ser473 phosphorylation of protein kinase B (PKB) after PV treatment is blocked by Rottlerin as efficiently as by the phosphatidylinositol 3-kinase inhibitor LY294002. In this report, we provide evidence that PKC-delta constitutes a crucial attenuating factor in B cell and mast cell signal transduction and suggest that PKC-delta is important for the regulation of physiological B and mast cell functions as well as for their pathophysiology. Furthermore, dominant PKC-delta-independent effects of Rottlerin are presented, indicating restrictions of this inhibitor for use in signal transduction research.

Vav1 regulates phospholipase cgamma activation and calcium responses in mast cells

The hematopoietic cell-specific protein Vav1 is a substrate of tyrosine kinases activated following engagement of many receptors, including FcepsilonRI. Vav1-deficient mice contain normal numbers of mast cells but respond more weakly than their normal counterparts to a passive systemic anaphylaxis challenge. Vav1-deficient bone marrow-derived mast cells also exhibited reduced degranulation and cytokine production, although tyrosine phosphorylation of FcepsilonRI, Syk, and LAT (linker for activation of T cells) was normal. In contrast, tyrosine phosphorylation of phospholipase Cgamma1 (PLCgamma1) and PLCgamma2 and calcium mobilization were markedly inhibited. Reconstitution of deficient mast cells with Vav1 restored normal tyrosine phosphorylation of PLCgamma1 and PLCgamma2 and calcium responses. Thus, Vav1 is essential to FcepsilonRI-mediated activation of PLCgamma and calcium mobilization in mast cells. In addition to its known role as an activator of Rac1 GTPases, these findings demonstrate a novel function for Vav1 as a regulator of PLCgamma-activated calcium signals.

Spontaneous phosphorylation of the receptor with high affinity for IgE in transfected fibroblasts

Receptors with high affinity for IgE, FcepsilonRI, which had been transfected into Chinese hamster ovary fibroblasts exhibit an over 20-fold greater spontaneous phosphorylation at physiological temperatures than the same receptors on the widely studied rat mucosal mast cell line, RBL-2H3. This enhanced phosphorylation was not accounted for either by changes in the src-family kinase responsible for the phosphorylation, by reduced activity of phosphatases, or by spontaneous association of the receptors with microdomains. A variety of approaches failed to detect evidence for stable spontaneous aggregates of the receptor. Whereas the altered posttranslational glycosylation of the receptor's principal ectodomain we detected could promote transient spontaneous aggregation and explain the observed effect, other changes in the membrane milieu cannot be excluded. The functional consequences of such spontaneous phosphorylation are considered.