Sequential requirements of the N-terminal palmitoylation site and SH2 domain of Src family kinases in the initiation and progression of FcepsilonRI signaling

Regulation of T cell function by protein S-acylation

S-acylation, the reversible lipidation of free cysteine residues with long-chain fatty acids, is a highly dynamic post-translational protein modification that has recently emerged as an important regulator of the T cell function. The reversible nature of S-acylation sets this modification apart from other forms of protein lipidation and allows it to play a unique role in intracellular signal transduction. In recent years, a significant number of T cell proteins, including receptors, enzymes, ion channels, and adaptor proteins, were identified as S-acylated. It has been shown that S-acylation critically contributes to their function by regulating protein localization, stability and protein-protein interactions. Furthermore, it has been demonstrated that zDHHC protein acyltransferases, the family of enzymes mediating this modification, also play a prominent role in T cell activation and differentiation. In this review, we aim to highlight the diversity of proteins undergoing S-acylation in T cells, elucidate the mechanisms by which reversible lipidation can impact protein function, and introduce protein acyltransferases as a novel class of regulatory T cell proteins.

Function of Protein S-Palmitoylation in Immunity and Immune-Related Diseases

Protein S-palmitoylation is a covalent and reversible lipid modification that specifically targets cysteine residues within many eukaryotic proteins. In mammalian cells, the ubiquitous palmitoyltransferases (PATs) and serine hydrolases, including acyl protein thioesterases (APTs), catalyze the addition and removal of palmitate, respectively. The attachment of palmitoyl groups alters the membrane affinity of the substrate protein changing its subcellular localization, stability, and protein-protein interactions. Forty years of research has led to the understanding of the role of protein palmitoylation in significantly regulating protein function in a variety of biological processes. Recent global profiling of immune cells has identified a large body of S-palmitoylated immunity-associated proteins. Localization of many immune molecules to the cellular membrane is required for the proper activation of innate and adaptive immune signaling. Emerging evidence has unveiled the crucial roles that palmitoylation plays to immune function, especially in partitioning immune signaling proteins to the membrane as well as to lipid rafts. More importantly, aberrant PAT activity and fluctuations in palmitoylation levels are strongly correlated with human immunologic diseases, such as sensory incompetence or over-response to pathogens. Therefore, targeting palmitoylation is a novel therapeutic approach for treating human immunologic diseases. In this review, we discuss the role that palmitoylation plays in both immunity and immunologic diseases as well as the significant potential of targeting palmitoylation in disease treatment.

Protein palmitoylation in signal transduction of hematopoietic cells

The covalent attachment of palmitate to proteins can alter protein-lipid and protein-protein interactions thereby influencing protein function. Palmitoylation is a reversible post-translational modification. Thus, like protein phosphorylation, protein palmitoylation can function in activation-dependent signaling pathways. This review will provide an overview of the mechanisms and regulation of protein palmitoylation and focus on the role of palmitoylation in signal transduction pathways of lymphocytes and platelets.

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

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

Molecular underpinning of B-cell anergy

A byproduct of the largely stochastic generation of a diverse B-cell specificity repertoire is production of cells that recognize autoantigens. Indeed, recent studies indicate that more than half of the primary repertoire consists of autoreactive B cells that must be silenced to prevent autoimmunity. While this silencing can occur by multiple mechanisms, it appears that most autoreactive B cells are silenced by anergy, wherein they populate peripheral lymphoid organs and continue to express unoccupied antigen receptors yet are unresponsive to antigen stimulation. Here we review molecular mechanisms that appear operative in maintaining the antigen unresponsiveness of anergic B cells. In addition, we present new data indicating that the failure of anergic B cells to mobilize calcium in response to antigen stimulation is not mediated by inactivation of stromal interacting molecule 1, a critical intermediary in intracellular store depletion-induced calcium influx.

The limited contribution of Fyn and Gab2 to the high affinity IgE receptor signaling in mast cells

Several studies with mast cells from knock-out mice have suggested that the tyrosine kinase Fyn and its downstream substrate Gab2 may play a role in high affinity IgE receptor (FcepsilonRI)-mediated mast cell activation. To better understand the role of these two molecules and of Syk, we transiently transfected mast cells with small interference RNA (siRNA) targeted to Fyn, Gab2, or Syk to specifically decrease their expression. The siRNA suppression of Gab2 but not Fyn reduced activation of the phosphoinositide-3-kinase (PI3K) pathway as demonstrated by the change in phosphorylation of Akt; this indicates that Gab2 but not Fyn regulates this pathway. The decreased expression of Gab2 and Fyn had minor effects on degranulation. There were also some minor changes in activation of the NFAT or NFkappaB transcription factors in cells with reduced expression of Fyn or Gab2. Decreased Gab2 but not Fyn reduced the FcepsilonRI-induced activation of the Erk, Jnk, and p38 MAP kinases and the release of TNF-alpha. In contrast, decreased expression of Syk dramatically reduced FcepsilonRI-induced degranulation, activation of NFAT and NFkappaB. Therefore, the reduction in expression of these proteins in mast cells indicates that Syk is the major regulator of FcepsilonRI-mediated reactions, whereas Fyn has minor if any effects and Gab2 regulates primarily late events including MAP kinase activation and release of cytokines.

Roles for SH2 and SH3 domains in Lyn kinase association with activated FcepsilonRI in RBL mast cells revealed by patterned surface analysis

In mast cells, antigen-mediated cross-linking of IgE bound to its high-affinity surface receptor, FcepsilonRI, initiates a signaling cascade that culminates in degranulation and release of allergic mediators. Antigen-patterned surfaces, in which the antigen is deposited in micron-sized features on a silicon substrate, were used to examine the spatial relationship between clustered IgE-FcepsilonRI complexes and Lyn, the signal-initiating tyrosine kinase. RBL mast cells expressing wild-type Lyn-EGFP showed co-redistribution of this protein with clustered IgE receptors on antigen-patterned surfaces, whereas Lyn-EGFP containing an inhibitory point mutation in its SH2 domain did not significantly accumulate with the patterned antigen, and Lyn-EGFP with an inhibitory point mutation in its SH3 domain exhibited reduced interactions. Our results using antigen-patterned surfaces and quantitative cross-correlation image analysis reveal that both the SH2 and SH3 domains contribute to interactions between Lyn kinase and cross-linked IgE receptors in stimulated mast cells.

Regulation of the mast cell response to the type 1 Fc epsilon receptor

The type I Fc epsilon receptor (Fc epsilon RI) is one of the better understood members of its class and is central to the immunological activation of mast cells and basophils, the key players in immunoglobulin E (IgE)-dependent immediate hypersensitivity. This review provides background information on several distinct regulatory mechanisms controlling this receptor's stimulus-response coupling network. First, we review the current understanding of this network's operation, and then we focus on the inhibitory regulatory mechanisms. In particular, we discuss the different known cytosolic molecules (e.g. kinases, phosphatases, and adapters) as well as cell membrane proteins involved in negatively regulating the Fc epsilon RI-induced secretory responses. Knowledge of this field is developing at a fast rate, as new proteins endowed with regulatory functions are still being discovered. Our understanding of the complex networks by which these proteins exert regulation is limited. Although the scope of this review does not include addressing several important biochemical and biophysical aspects of the regulatory mechanisms, it does provide general insights into a central field in immunology.

Cellular palmitoylation and trafficking of lipidated peptides

Many important signaling proteins require the posttranslational addition of fatty acid chains for their proper subcellular localization and function. One such modification is the addition of palmitoyl moieties by enzymes known as palmitoyl acyltransferases (PATs). Substrates for PATs include C-terminally farnesylated proteins, such as H- and N-Ras, as well as N-terminally myristoylated proteins, such as many Src-related tyrosine kinases. The molecular and biochemical characterization of PATs has been hindered by difficulties in developing effective methods for the analysis of PAT activity. In this study, we describe the use of cell-permeable, fluorescently labeled lipidated peptides that mimic the PAT recognition domains of farnesylated and myristoylated proteins. These PAT substrate mimetics are accumulated by SKOV3 cells in a saturable and time-dependent manner. Although both peptides are rapidly palmitoylated, the SKOV3 cells have a greater capacity to palmitoylate the myristoylated peptide than the farnesylated peptide. Confocal microscopy indicated that the palmitoylated peptides colocalized with Golgi and plasma membrane markers, whereas the corresponding nonpalmitoylatable peptides accumulated in the Golgi but did not traffic to the plasma membrane. Overall, these studies indicate that the lipidated peptides provide useful cellular probes for quantitative and compartmentalization studies of protein palmitoylation in intact cells.

Role of Src-family kinases in formation and trafficking of macropinosomes

Src-family kinases that localize to the cytoplasmic side of cellular membranes through lipid modification play a role in signaling events including membrane trafficking. Macropinocytosis is an endocytic process for solute uptake by large vesicles called macropinosomes. Although macropinosomes can be visualized following uptake of fluorescent macromolecules, little is known about the dynamics of macropinosomes in living cells. Here, we show that constitutive c-Src expression generates macropinosomes in a kinase-dependent manner. Live-cell imaging of GFP-tagged c-Src (Src-GFP) reveals that c-Src associates with macropinosomes via its N-terminus continuously from their generation at membrane ruffles, through their centripetal trafficking, to fusion with late endosomes and lysosomes. Fluorescence recovery after photobleaching (FRAP) of Src-GFP shows that Src-GFP is rapidly recruited to macropinosomal membranes from the plasma membrane and intracellular organelles through vesicle transport even in the presence of a protein synthesis inhibitor. Furthermore, using a HeLa cell line overexpressing inducible c-Src, we show that following stimulation with epidermal growth factor (EGF), high levels of c-Src kinase activity promote formation of macropinosomes associated with the lysosomal compartment. Unlike c-Src, Lyn and Fyn, which are palmitoylated Src kinases, only minimally induce macropinosomes, although a Lyn mutant in which the palmitoylation site is mutated efficiently induces macropinocytosis. We conclude that kinase activity of nonpalmitoylated Src kinases including c-Src may play an important role in the biogenesis and trafficking of macropinosomes.

Fcε- and Fcγ-receptor signaling in diseases

It has become increasingly clear that receptors for the immunoglobulin Fc region play pivotal roles in immune homeostasis and disease. This review describes the fine regulation of the high-affinity IgE-receptor (FcepsilonRI) signaling, especially focusing on the early events that are coordinately regulated by Src family protein tyrosine kinases (PTKs), FcepsilonRI beta-subunit, and membrane lipid rafts. Because allergen-mediated FcepsilonRI cross-linking leads to the synthesis and release of a variety of proinflammatory mediators and cytokines, the duration and amplitude of the signal need to be strictly controlled, and the counterbalancing signaling is provided by specialized inhibitory receptors and molecules. However, recent work have revealed that Src family PTKs and FcepsilonRI beta-subunit transduce both positive and negative signaling with unexpectedly complex mechanisms. FcgammaRIIB exerts a unique inhibitory function on cell activation processes after the engagement of Fcgamma, FcepsilonRI and B cell receptors. Recent work has shown that FcgammaRIIB polymorphisms are associated with systemic lupus erythematosus, and that a transmembrane polymorphism in FcgammaRIIB results in an impaired distribution to lipid rafts and a reduced inhibitory function. Studies addressing the functions of disease-associated polymorphisms in the FcepsilonRI beta-subunit and low-affinity FcgammaRs are also considered.

Palmitoylation of ligands, receptors, and intracellular signaling molecules

Palmitate, a 16-carbon saturated fatty acid, is attached to more than 100 proteins. Modification of proteins by palmitate has pleiotropic effects on protein function. Palmitoylation can influence membrane binding and membrane targeting of the modified proteins. In particular, many palmitoylated proteins concentrate in lipid rafts, and enrichment in rafts is required for efficient signal transduction. This Review focuses on the multiple effects of palmitoylation on the localization and function of ligands, receptors, and intracellular signaling proteins. Palmitoylation regulates the trafficking and function of transmembrane proteins such as ion channels, neurotransmitter receptors, heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors, and integrins. In addition, immune receptor signaling relies on protein palmitoylation at many levels, including palmitoylated co-receptors, Src family kinases, and adaptor or scaffolding proteins. The localization and signaling capacities of Ras and G proteins are modulated by dynamic protein palmitoylation. Cycles of palmitoylation and depalmitoylation allow H-Ras and G protein alpha subunits to reversibly bind to and signal from different intracellular cell membranes. Moreover, secreted ligands such as Hedgehog, Wingless, and Spitz use palmitoylation to regulate the extent of long- or short-range signaling. Finally, palmitoylation can alter signaling protein function by direct effects on enzymatic activity and substrate specificity. The identification of the palmitoyl acyltransferases has provided new insights into the biochemistry of this posttranslational process and permitted new substrates to be identified.

Role of B cell inhibitory receptor polymorphisms in systemic lupus erythematosus: a negative times a negative makes a positive

B lymphocytes play a pivotal role in the pathogenesis of systemic lupus erythematosus (SLE). Here, we will review our studies on the role of polymorphisms of two genes coding for B cell inhibitory receptors, FCGR2B and CD72. In FCGR2B, a single nucleotide polymorphism leading to a nonsynonymous substitution, Ile232Thr, within the transmembrane domain was identified, and a significant association of the 232Thr/Thr genotype with SLE was observed in Japanese, Thai and Chinese populations, while this allele was found to be rare in Caucasians. On the other hand, the association of FCGR2B promoter polymorphism with SLE in Caucasians has been reported by two independent groups, but this allele was not found to be present in Japanese. These observations demonstrate that the association of FCGR2B polymorphisms with SLE is common to multiple populations, but the alleles associated with SLE depend upon the genetic background of each population. Functional analyses using a human B cell line lacking endogenous FcgammaRIIb revealed that SLE-associated 232Thr allele product was partially excluded from membrane lipid rafts under resting conditions and after coligation with B cell receptor, and was significantly less potent at inhibiting B cell activation. Two haplotypes were identified in CD72, one of which was associated with increased production of an alternative splicing isoform that substantially alters the extracellular region of CD72. Interestingly, the presence of the haplotype significantly decreased the risk of SLE conferred by FCGR2B-232Thr in an epistatic manner. These observations emphasize the need to understand human immune system diversity if we are to improve our understanding of the pathogenesis of autoimmune diseases.

Temporally resolved interactions between antigen-stimulated IgE receptors and Lyn kinase on living cells

Upon cross-linking by antigen, the high affinity receptor for immunoglobulin E (IgE), FcɛRI, is phosphorylated by the Src family tyrosine kinase Lyn to initiate mast cell signaling, leading to degranulation. Using fluorescence correlation spectroscopy (FCS), we observe stimulation-dependent associations between fluorescently labeled IgE-FcɛRI and Lyn-EGFP on individual cells. We also simultaneously measure temporal variations in the lateral diffusion of these proteins. Antigen-stimulated interactions between these proteins detected subsequent to the initiation of receptor phosphorylation exhibit time-dependent changes, suggesting multiple associations between FcɛRI and Lyn-EGFP. During this period, we also observe a persistent decrease in Lyn-EGFP lateral diffusion that is dependent on Src family kinase activity. These stimulated interactions are not observed between FcɛRI and a chimeric EGFP that contains only the membrane-targeting sequence from Lyn. Our results reveal real-time interactions between Lyn and cross-linked FcɛRI implicated in downstream signaling events. They demonstrate the capacity of FCS cross-correlation analysis to investigate the mechanism of signaling-dependent protein–protein interactions in intact, living cells.

A live cell, image-based approach to understanding the enzymology and pharmacology of 2-bromopalmitate and palmitoylation

The addition of a lipid moiety to a protein increases its hydrophobicity and subsequently its attraction to lipophilic environments like membranes. Indeed most lipid-modified proteins are localized to membranes where they associate with multiprotein signaling complexes. Acylation and prenylation are the two common categories of lipidation. The enzymology and pharmacology of prenylation are well understood but relatively very little is known about palmitoylation, the most common form of acylation. One distinguishing characteristic of palmitoylation is that it is a dynamic modification. To understand more about how palmitoylation is regulated, we fused palmitoylation substrates to fluorescent proteins and reported their subcellular distribution and trafficking. We used automated high-throughput fluorescence microscopy and a specialized computer algorithm to image and measure the fraction of palmitoylation reporter on the plasma membrane versus the cytoplasm. Using this system we determined the residence half-life of palmitate on the dipalmitoyl substrate peptide from GAP43 as well as the EC(50) for 2-bromopalmitate, a common inhibitor of palmitoylation.

FcγRIIB Ile232Thr transmembrane polymorphism associated with human systemic lupus erythematosus decreases affinity to lipid rafts and attenuates inhibitory effects on B cell receptor signaling

The B cell inhibitory receptor FcgammaRIIB plays crucial roles in the maintenance of self-tolerance. We have identified a polymorphism FCGR2B c.695T>C that results in the non-conservative replacement of 232Ile at the transmembrane helix to Thr and demonstrated the association of the polymorphism with susceptibility to systemic lupus erythematosus (SLE) in Asians. In this study, we examined the impact of FCGR2B c.695T>C on the functional properties of FcgammaRIIB by expressing each allele product in a human B cell line ST486 lacking endogenous FcgammaRIIB. FcgammaRIIB 232Thr was found to be significantly less potent than wild-type 232Ile in inhibiting B cell receptor (BCR)-mediated phosphatidylinositol-3,4,5-trisphosphate accumulation, Akt and PLCgamma2 activation and calcium mobilization, and to display decreased levels of tyrosine phosphorylation and SH2-containing 5'-inositolphosphate phosphatase recruitment compared with 232Ile after IgG Fc-mediated coligation with BCR. Notably, a quantitative analysis of the subcellular distribution of FcgammaRIIB using 125I-labeled anti-FcgammaRIIB revealed that FcgammaRIIB 232Thr is less effectively distributed to detergent-insoluble lipid rafts than 232Ile, findings in accordance with the importance of the transmembrane amino acid residues, in particular large hydrophobic amino acids including Ile, in the association of membrane proteins with lipid rafts. Given the crucial roles of lipid rafts in integrating BCR signaling, decreased association of FcgammaRIIB 232Thr could contribute to its impaired inhibitory potential. Collectively, the present findings indicate that the Ile232Thr substitution affects the localization and function of FcgammaRIIB and that the molecular mechanism may link the polymorphism and susceptibility to SLE.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Dysregulated NK receptor expression in patients with lymphoproliferative disease of granular lymphocytes

The natural killer (NK) type of lymphoproliferative disease of granular lymphocytes (LDGL) is associated with the expansion of CD3-, CD16+, and/or CD56+ lymphocytes. We have examined the repertoire of NK receptors expressed on these cells and delineated the functional activity. We found skewed NK receptor expression on patient NK cells. Reactivity to a single anti-killer cell immunoglobulin-like receptor (anti-KIR) antibody was noted in 7 of 13 patients. LDGL patients variably expressed NKp30, NKp44, and NKp46 RNA. In contrast, CD94 and its inhibitory heterodimerization partner NKG2A were homogenously expressed at high levels on these NK cells. Interestingly, these patients expressed a large number of activating KIR receptors by genotype analysis. Semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) demonstrated that lower than normal levels of RNA of the inhibitory KIR was present in some patients in contrast to normal NK cells. Consistent with a high level of activating receptors, we found the NK-LDGL cells have potent cytolytic function in both direct and redirected cytotoxicity assays. These results demonstrate that patients with NK-LDGL have an increased activating-to-inhibitory KIR ratio. This altered ratio might induce inappropriate lysis or cytokine production and impact the disease pathogenesis. (Blood. 2004;103:3431-3439)

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

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

A lipid raft environment enhances Lyn kinase activity by protecting the active site tyrosine from dephosphorylation

The plasma membrane contains ordered lipid domains, commonly called lipid rafts, enriched in cholesterol, sphingolipids, and certain signaling proteins. Lipid rafts play a structural role in signal initiation by the high affinity receptor for IgE. Cross-linking of IgE-receptor complexes by antigen causes their coalescence with lipid rafts, where they are phosphorylated by the Src family tyrosine kinase, Lyn. To understand how lipid rafts participate in functional coupling between Lyn and FcepsilonRI, we investigated whether the lipid raft environment influences the specific activity of Lyn. We used differential detergent solubility and sucrose gradient fractionation to isolate Lyn from raft and nonraft regions of the plasma membrane in the presence or absence of tyrosine phosphatase inhibitors. We show that Lyn recovered from lipid rafts has a substantially higher specific activity than Lyn from nonraft environments. Furthermore, this higher specific activity correlates with increased tyrosine phosphorylation at the active site loop of the kinase domain. Based on these results, we propose that lipid rafts exclude a phosphatase that negatively regulates Lyn kinase activity by constitutive dephosphorylation of the kinase domain tyrosine residue of Lyn. In this model, cross-linking of FcepsilonRI promotes its proximity to active Lyn in a lipid raft environment.

Investigation of early events in Fc epsilon RI-mediated signaling using a detailed mathematical model

Aggregation of Fc epsilon RI on mast cells and basophils leads to autophosphorylation and increased activity of the cytosolic protein tyrosine kinase Syk. We investigated the roles of the Src kinase Lyn, the immunoreceptor tyrosine-based activation motifs (ITAMs) on the beta and gamma subunits of Fc epsilon RI, and Syk itself in the activation of Syk. Our approach was to build a detailed mathematical model of reactions involving Fc epsilon RI, Lyn, Syk, and a bivalent ligand that aggregates Fc(epsilon)RI. We applied the model to experiments in which covalently cross-linked IgE dimers stimulate rat basophilic leukemia cells. The model makes it possible to test the consistency of mechanistic assumptions with data that alone provide limited mechanistic insight. For example, the model helps sort out mechanisms that jointly control dephosphorylation of receptor subunits. In addition, interpreted in the context of the model, experimentally observed differences between the beta- and gamma-chains with respect to levels of phosphorylation and rates of dephosphorylation indicate that most cellular Syk, but only a small fraction of Lyn, is available to interact with receptors. We also show that although the beta ITAM acts to amplify signaling in experimental systems where its role has been investigated, there are conditions under which the beta ITAM will act as an inhibitor.

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

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

Spatial raft coalescence represents an initial step in Fc gamma R signaling

Characterization of lipid rafts as separated membrane microdomains consist of heterogeneous proteins suggesting that lateral assembly of rafts after Ag receptor cross-linking represents the earliest signal generating process. In line with the concept, cross-linked Ag receptors have been shown to associate with detergent-insoluble raft fraction without the aid of Src family kinases. However, it has not been established whether spatial raft coalescence could also precede Src family kinase activation. In this study, we showed that spatial raft coalescence after low-affinity FcgammaR cross-linking in RAW264.7 macrophages is independent of Src family kinase activity. The lateral raft assembly was found to be ascribed to the action of ligand-binding subunits, rather than to immunoreceptor tyrosine-based activation motif-bearing signal subunits, because monomeric murine FcgammaRIIb expressed in rat basophilic leukemia cells successfully induced spatial raft reorganization after cross-linking. We also showed that extracellular and transmembrane region of FcgammaRIIb is sufficient for raft stabilization. Moreover, this receptor fragment triggers rapid calcium mobilization and linker for activation of T cells phosphorylation, in a manner sensitive to Src family kinase inhibition and to cholesterol depletion. Presence of immunoreceptor tyrosine-based inhibitory motif and addition of immunoreceptor tyrosine-based activation motif to the receptor fragment abolished and enhanced the responses, respectively, but did not affect raft stabilization. These findings support the concept that ligand-binding subunit is responsible for raft coalescence, and that this event triggers initial biochemical signaling.

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.

The transmembrane adapter LAT plays a central role in immune receptor signalling

The transmembrane adapter LAT (linker for activation of T cells) plays a central role in signalling by ITAM bearing receptors expressed on T cells, natural killer cells, mast cells and platelets. Receptor engagement leads to the phosphorylation of tyrosine residues present in the intracellular domain of LAT and formation of a multiprotein complex with other adapter molecules and enzymes including Grb2, Gads/SLP-76 and PLCgamma isoforms. These signalling events predominantly take place in glycolipid-enriched membrane domains. The constitutive presence of LAT in GEMs enables its function as the main scaffolding protein for the organization of GEM-localized signalling. The study of LAT-deficient mice and LAT-deficient cell lines further emphasizes the importance of LAT for these signalling cascades but also defines the existence of LAT-independent events downstream of the Syk-family kinase-ITAM complex.

Fc(epsilon)RI as a paradigm for a lipid raft-dependent receptor in hematopoietic cells

Lipid domains or rafts are currently embraced by immunologists as critical participants in receptor-mediated signaling events occurring at the plasma membrane. This view of membrane heterogeneity and its functional importance is supported by many years of different experimental approaches. We can now refine our investigations, moving beyond the simple models to ask more detailed questions about structural properties and mechanistic interactions. As highlighted for the IgE receptor (Fc(epsilon)RI), new information about initial engagement with src family kinases, cytoskeletal regulation, and coupling with downstream signaling is beginning to emerge.

SH2 domain-mediated targeting, but not localization, of Syk in the plasma membrane is critical for FcepsilonRI signaling

Aggregation of the high-affinity IgE receptor induces the tyrosine phosphorylation of subunits of the receptor and the subsequent association with the receptor of the cytosolic protein tyrosine kinase Syk. The current experiments examined the functional importance of membrane association of Syk and the role of the SH2 domain in receptor-mediated signal transduction. Wild-type Syk and chimeric Syk molecules with the c-Src myristylation sequence at the amino-terminus were expressed in a Syk-negative mast cell line. Chimeric Syk with the myristylation sequence was membrane associated, and a small fraction was constitutively colocalized with FcepsilonRI, Lyn, and LAT (linker for T-cell activation) in the glycolipid-enriched microdomains or rafts. However, even under these conditions, the tyrosine phosphorylation of Syk and the downstream propagation of signals required FcepsilonRI aggregation. This chimeric Syk was less active than wild-type Syk in FcepsilonRI-mediated signal transduction. In contrast, a truncated membrane-associated form of Syk that lacked the SH2 domains was not tyrosine phosphorylated by receptor aggregation and failed to transduce intracellular signals. These findings suggest that SH2 domain-mediated membrane translocation of Syk is essential for the FcepsilonRI-mediated activation of Syk for downstream signaling events leading to histamine release. Furthermore, the localization of Syk in glycolipid-enriched microdomains by itself is not enough to generate or enhance signaling events.

Fyn and Lyn phosphorylate the Fc receptor γ chain downstream of glycoprotein VI in murine platelets, and Lyn regulates a novel feedback pathway

Activation of platelets by collagen is mediated by the complex glycoprotein VI (GPVI)/Fc receptor γ (FcRγ chain). In the current study, the role of 2 Src family kinases, Fyn and Lyn, in GPVI signaling has been examined using murine platelets deficient in one or both kinases. In the fyn−/−platelets, tyrosine phosphorylation of FcRγ chain, phopholipase C (PLC) activity, aggregation, and secretion are reduced, though the time of onset of response is unchanged. In the lyn−/−platelets, there is a delay of up to 30 seconds in the onset of tyrosine phosphorylation and functional responses, followed by recovery of phosphorylation and potentiation of aggregation and α-granule secretion. Tyrosine phosphorylation and aggregation in response to stimulation by collagen-related peptide is further attenuated and delayed in fyn−/−lyn−/−double-mutant platelets, and potentiation is not seen. This study provides the first genetic evidence that Fyn and Lyn mediate FcR immune receptor tyrosine-based activation motif phosphorylation and PLCγ2 activation after the ligation of GPVI. Lyn plays an additional role in inhibiting platelet activation through an uncharacterized inhibitory pathway.

Fyn and Lyn phosphorylate the Fc receptor γ chain downstream of glycoprotein VI in murine platelets, and Lyn regulates a novel feedback pathway

Activation of platelets by collagen is mediated by the complex glycoprotein VI (GPVI)/Fc receptor γ (FcRγ chain). In the current study, the role of 2 Src family kinases, Fyn and Lyn, in GPVI signaling has been examined using murine platelets deficient in one or both kinases. In the fyn−/−platelets, tyrosine phosphorylation of FcRγ chain, phopholipase C (PLC) activity, aggregation, and secretion are reduced, though the time of onset of response is unchanged. In the lyn−/−platelets, there is a delay of up to 30 seconds in the onset of tyrosine phosphorylation and functional responses, followed by recovery of phosphorylation and potentiation of aggregation and α-granule secretion. Tyrosine phosphorylation and aggregation in response to stimulation by collagen-related peptide is further attenuated and delayed in fyn−/−lyn−/−double-mutant platelets, and potentiation is not seen. This study provides the first genetic evidence that Fyn and Lyn mediate FcR immune receptor tyrosine-based activation motif phosphorylation and PLCγ2 activation after the ligation of GPVI. Lyn plays an additional role in inhibiting platelet activation through an uncharacterized inhibitory pathway.

Differential involvement of Src family kinases in Fc gamma receptor-mediated phagocytosis

The tyrosine phosphorylation cascade originated from Fc gamma receptors (Fc gamma Rs) is essential for macrophage functions including phagocytosis. Although the initial step is ascribed to Src family tyrosine kinases, the role of individual kinases in phagocytosis signaling is still to be determined. In reconstitution experiments, we first showed that expression in the RAW 264.7 cell line of C-terminal Src kinase (Csk) inhibited and that of a membrane-anchored, gain-of-function Csk abolished the Fc gamma R-mediated signaling that leads to phagocytosis in a kinase-dependent manner. We next tested reconstruction of the signaling in the membrane-anchored, gain-of-function Csk-expressing cells by introducing Src family kinases the C-terminal negative regulatory sequence of which was replaced with a c-myc epitope. Those constructs derived from Lyn and Hck (a-Lyn and a-Hck) that associated with detergent-resistant membranes successfully reconstructed Fc gamma R-mediated Syk activation, filamentous actin rearrangement, and phagocytosis. In contrast, c-Src-derived construct (a-Src), that was excluded from detergent-resistant membranes, could not restore the series of phagocytosis signaling. Tyrosine phosphorylation of Vav and c-Cbl was restored in common by a-Lyn, a-Hck, and a-Src, but Fc gamma RIIB tyrosine phosphorylation, which is implicated in negative signaling, was reconstituted solely by a-Lyn and a-Hck. These findings suggest that Src family kinases are differentially involved in Fc gamma R-signaling and that selective kinases including Lyn and Hck are able to fully transduce phagocytotic signaling.