The B-lymphoid Grb2 interaction code

Modulating PAK1: Accessory Proteins as Promising Therapeutic Targets

The p21-activated kinase (PAK1), a serine/threonine protein kinase, is critical in regulating various cellular processes, including muscle contraction, neutrophil chemotaxis, neuronal polarization, and endothelial barrier function. Aberrant PAK1 activity has been implicated in the progression of several human diseases, including cancer, heart disease, and neurological disorders. Increased PAK1 expression is often associated with poor clinical prognosis, invasive tumor characteristics, and therapeutic resistance. Despite its importance, the cellular mechanisms that modulate PAK1 function remain poorly understood. Accessory proteins, essential for the precise assembly and temporal regulation of signaling pathways, offer unique advantages as therapeutic targets. Unlike core signaling components, these modulators can attenuate aberrant signaling without completely abolishing it, potentially restoring signaling to physiological levels. This review highlights PAK1 accessory proteins as promising and novel therapeutic targets, opening new horizons for disease treatment.

GRB2: A dynamic adaptor protein orchestrating cellular signaling in health and disease

GRB2, or Growth Factor Receptor-Bound Protein 2, is a pivotal adaptor protein in intracellular signal transduction pathways, particularly within receptor tyrosine kinase (RTK) signaling cascades. Its crystal structure reveals a modular architecture comprising a single Src homology 2 (SH2) domain flanked by two Src homology 3 (SH3) domains, facilitating dynamic interactions critical for cellular signaling. While SH2 domains recognize phosphorylated tyrosines, SH3 domains bind proline-rich sequences, enabling GRB2 to engage with various downstream effectors. Folding and binding studies of GRB2 in its full-length form and isolated domains highlight a complex interplay between its protein-protein interaction domains on the folding energy landscape and in driving its function. Being at the crosslink of many key molecular pathways in the cell, GRB2 possesses a role in cancer pathogenesis, particularly in mediating the Ras-mitogen activated protein kinase (MAPK) pathway. Thus, pharmacological targeting of GRB2 domains is a promising field in cancer therapy, with efforts focused on disrupting protein-protein interactions. However, the dynamic interplay driving GRB2 function suggests the presence of allosteric sites at the interface between domains that could be targeted to modulate the binding properties of its constituent domains. We propose that the analysis of GRB2 proteins from other species may provide additional insights to make the allosteric pharmacological targeting of GRB2 a more feasible strategy.

Role of Sam68 as an adaptor protein in inflammatory signaling

Sam68 is a ubiquitously expressed KH-domain containing RNA-binding protein highly studied for its involvement in regulating multiple steps of RNA metabolism. Sam68 also contains multiple protein-protein interaction regions such as proline-rich regions, tyrosine phosphorylation sites, and arginine methylation sites, all of which facilitate its participation as an adaptor protein in multiple signaling pathways, likely independent of its RNA-binding role. This review focuses on providing a comprehensive report on the adaptor roles of Sam68 in inflammatory signaling and inflammatory diseases. The insights presented here have the potential to open new avenues in inflammation research and justify targeting Sam68 to control aberrant inflammatory responses.

GRB2 dimerization mediated by SH2 domain-swapping is critical for T cell signaling and cytokine production

GRB2 is an adaptor protein required for facilitating cytoplasmic signaling complexes from a wide array of binding partners. GRB2 has been reported to exist in either a monomeric or dimeric state in crystal and solution. GRB2 dimers are formed by the exchange of protein segments between domains, otherwise known as "domain-swapping". Swapping has been described between SH2 and C-terminal SH3 domains in the full-length structure of GRB2 (SH2/C-SH3 domain-swapped dimer), as well as between α-helixes in isolated GRB2 SH2 domains (SH2/SH2 domain-swapped dimer). Interestingly, SH2/SH2 domain-swapping has not been observed within the full-length protein, nor have the functional influences of this novel oligomeric conformation been explored. We herein generated a model of full-length GRB2 dimer with an SH2/SH2 domain-swapped conformation supported by in-line SEC-MALS-SAXS analyses. This conformation is consistent with the previously reported truncated GRB2 SH2/SH2 domain-swapped dimer but different from the previously reported, full-length SH2/C-terminal SH3 (C-SH3) domain-swapped dimer. Our model is also validated by several novel full-length GRB2 mutants that favor either a monomeric or a dimeric state through mutations within the SH2 domain that abrogate or promote SH2/SH2 domain-swapping. GRB2 knockdown and re-expression of selected monomeric and dimeric mutants in a T cell lymphoma cell line led to notable defects in clustering of the adaptor protein LAT and IL-2 release in response to TCR stimulation. These results mirrored similarly-impaired IL-2 release in GRB2-deficient cells. These studies show that a novel dimeric GRB2 conformation with domain-swapping between SH2 domains and monomer/dimer transitions are critical for GRB2 to facilitate early signaling complexes in human T cells.

Integrin β1 regulates marginal zone B cell differentiation and PI3K signaling

Marginal zone (MZ) B cells represent innate-like B cells that mediate a fast immune response. The adhesion of MZ B cells to the marginal sinus of the spleen is governed by integrins. Here, we address the question of whether β1-integrin has additional functions by analyzing Itgb1fl/flCD21Cre mice in which the β1-integrin gene is deleted in mature B cells. We find that integrin β1-deficient mice have a defect in the differentiation of MZ B cells and plasma cells. We show that integrin β1-deficient transitional B cells, representing the precursors of MZ B cells, have enhanced B cell receptor (BCR) signaling, altered PI3K and Ras/ERK pathways, and an enhanced interaction of integrin-linked kinase (ILK) with the adaptor protein Grb2. Moreover, the MZ B cell defect of integrin β1-deficient mice could, at least in part, be restored by a pharmacological inhibition of the PI3K pathway. Thus, β1-integrin has an unexpected function in the differentiation and function of MZ B cells.

Identification of RHEX as a novel biomarker related to progression and immunity of non-small cell lung carcinoma

Background

The therapeutic response and prognosis of patients with non-small cell lung carcinoma (NSCLC) are widely related to immunity. To improve the prognosis of patients and provide reliable information to guide appropriate personalized treatment strategies, it is necessary to identify reliable prognostic or predictive indicators closely related to tumor phenotype and immune traits in NSCLC.

Methods

Based on The Cancer Genome Atlas (TCGA)-NSCLC mRNA expression profile data, a novel approach combining differential gene expression analysis, single-sample gene set enrichment analysis (ssGSEA), and weighted gene co-expression network analysis (WGCNA) was used to screen hub genes. Subsequently, the regulator of hemoglobinization and erythroid cell expansion (RHEX) was identified as a key gene using the log-rank test and confirmed in the ArrayExpress database. The relationship between RHEX and clinicopathological parameters was analyzed using the Wilcoxon rank-sum test. More importantly, through gene set enrichment analysis (GSEA) and cell-type identification by estimating relative subsets of RNA transcripts (CIBERSORT) algorithms, and with reference to the Tumor IMmune Estimation Resource (TIMER) database, we explored the relevant pathways of RHEX and its relationship with tumor-infiltrating immune cells (TICs). Finally, we depicted the association between RHEX and immunomodulators in the TCGA and a web portal TISIDB.

Results

The RHEX mRNA expression levels in tumor tissues were lower than those in normal tissues and declined with the progression of NSCLC. Meanwhile, RHEX overexpression was associated with high immune infiltration levels and a favorable clinical prognosis. RHEX may participate in tumor microenvironment (TME) regulation through multiple tumor-immune related pathways, especially the JAK-STAT signaling pathway. Furthermore, RHEX expression affected the infiltrating abundance of multiple TICs and positively correlated with most of the immunomodulators in NSCLC.

Conclusions

Our study is the first to propose that RHEX is an immune-related gene with prognostic value in NSCLC and reveals the underlying mechanism between RHEX and tumor-immune system interactions. These results ultimately provide guidance for prognosis and immunotherapy for NSCLC patients.

Allosteric regulation of GRB2 modulates RAS activation

RAS activation is a multiple-step process in which linkage of the extracellular stimuli to the RAS activator SOS1 is the main step in RAS activation. GRB2 adaptor protein is the main modulator in SOS1 recruitment to the plasma membrane and its activation. This interaction is well studied but the exact mechanism of GRB2-SOS1 complex formation and SOS1 activation has yet remained obscure. Here, a new allosteric mechanism for the GRB2 regulation is described as a prerequisite for the modulation of SOS1 activation. This regulatory mechanism comprises a series of intramolecular interactions which are potentiated by GRB2 interaction with upstream ligands.Abbreviations: GRB2, growth factor receptor-bound protein 2; SOS1, son of sevenless 1; RAS, Rat Sarcoma; GEF, guanine nucleotide exchange factor; GAP, GTPase-activating protein; HER2, human epidermal growth factor receptor; SH3, SRC Homology 3; SH2, SRC Homology 2; PRD, proline-rich domain; PRM, proline-rich motif; PRP, proline-rich peptide; RTK, receptor tyrosine kinases.

Transcriptome profiling based on protein-protein interaction networks provides a set of core genes for understanding the immune response mechanisms of the egg-protecting behavior in Octopus ocellatus

Protection via of the immune system is indispensable to the life of organisms. Within an immune network, problems with a given link will affect the normal life activities of the organism. Octopus ocellatus is cephalopod widely distributed throughout the world's oceans. Because of its unique nervous system and locomotive organs, research on this species has gradually increased in recent years. Many immune response mechanisms associated with behaviors of O. ocellatus are still unclear. Moreover, as a factor affecting the normal growth of O. ocellatus, egg protection has rarely been considered in previous behavioral studies. In this study, we analyzed the transcriptome profile of gene expression in O. ocellatus larvae, and identified 5936 differentially expressed genes (DEGs). GO and KEGG enrichment analyses were used to search for immune-related DEGs. Protein-protein interaction networks were constructed to examine the interactions between immune-related genes. Fifteen hub genes involved in multiple KEGG signaling pathways or with multiple protein-protein interaction relationships were obtained and verified by quantitative RT-PCR. We first studied the effects of egg protection on the immunity of O. ocellatus larvae by means of protein-protein interaction networks, and the results provide valuable genetic resources for understanding the immunity of invertebrate larvae. The data serve as a foundation for further research on the egg-protecting behavior of invertebrates.

The intramolecular allostery of GRB2 governing its interaction with SOS1 is modulated by phosphotyrosine ligands

Growth factor receptor-bound protein 2 (GRB2) is a trivalent adaptor protein and a key element in signal transduction. It interacts via its flanking nSH3 and cSH3 domains with the proline-rich domain (PRD) of the RAS activator SOS1 and via its central SH2 domain with phosphorylated tyrosine residues of receptor tyrosine kinases (RTKs; e.g. HER2). The elucidation of structural organization and mechanistic insights into GRB2 interactions, however, remain challenging due to their inherent flexibility. This study represents an important advance in our mechanistic understanding of how GRB2 links RTKs to SOS1. Accordingly, it can be proposed that (1) HER2 pYP-bound SH2 potentiates GRB2 SH3 domain interactions with SOS1 (an allosteric mechanism); (2) the SH2 domain blocks cSH3, enabling nSH3 to bind SOS1 first before cSH3 follows (an avidity-based mechanism); and (3) the allosteric behavior of cSH3 to other domains appears to be unidirectional, although there is an allosteric effect between the SH2 and SH3 domains.

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.

The regulators of BCR signaling during B cell activation

B lymphocytes produce antibodies under the stimulation of specific antigens, thereby exerting an immune effect. B cells identify antigens by their surface B cell receptor (BCR), which upon stimulation, directs the cell to activate and differentiate into antibody generating plasma cells. Activation of B cells via their BCRs involves signaling pathways that are tightly controlled by various regulators. In this review, we will discuss three major BCR mediated signaling pathways (the PLC-γ2 pathway, PI3K pathway and MAPK pathway) and related regulators, which were roughly divided into positive, negative and mutual-balanced regulators, and the specific regulators of the specific signaling pathway based on regulatory effects.

Epstein-Barr virus microRNAs regulate B cell receptor signal transduction and lytic reactivation

MicroRNAs (miRNAs) are post-transcriptional regulatory RNAs that can modulate cell signaling and play key roles in cell state transitions. Epstein-Barr virus (EBV) expresses >40 viral miRNAs that manipulate both viral and cellular gene expression patterns and contribute to reprogramming of the host environment during infection. Here, we identified a subset of EBV miRNAs that desensitize cells to B cell receptor (BCR) stimuli, and attenuate the downstream activation of NF-kappaB or AP1-dependent transcription. Bioinformatics and pathway analysis of Ago PAR-CLIP datasets identified multiple EBV miRNA targets related to BCR signal transduction, including GRB2, SOS1, MALT1, RAC1, and INPP5D, which we validated in reporter assays. BCR signaling is critical for B cell activation, proliferation, and differentiation, and for EBV, is linked to reactivation. In functional assays, we demonstrate that EBV miR-BHRF1-2-5p contributes to the growth of latently infected B cells through GRB2 regulation. We further determined that activities of EBV miR-BHRF1-2-5p, EBV miR-BART2-5p, and a cellular miRNA, miR-17-5p, directly regulate virus reactivation triggered by BCR engagement. Our findings provide mechanistic insight into some of the key miRNA interactions impacting the proliferation of latently infected B cells and importantly, governing the latent to lytic switch.

Grb2 and GRAP connect the B cell antigen receptor to Erk MAP kinase activation in human B cells

The B cell antigen receptor (BCR) employs enzymatically inactive adaptor proteins to facilitate activation of intracellular signaling pathways. In animal model systems, adaptor proteins of the growth factor receptor-bound 2 (Grb2) family have been shown to serve critical functions in lymphocytes. However, the roles of Grb2 and the Grb2-related adaptor protein (GRAP) in human B lymphocytes remain unclear. Using TALEN-mediated gene targeting, we show that in human B cells Grb2 and GRAP amplify signaling by the immunoglobulin tail tyrosine (ITT) motif of mIgE-containing BCRs and furthermore connect immunoreceptor tyrosine-based activation motif (ITAM) signaling to activation of the Ras-controlled Erk MAP kinase pathway. In contrast to mouse B cells, BCR-induced activation of Erk in human B cells is largely independent of phospholipase C-ɣ activity and diacylglycerol-responsive members of Ras guanine nucleotide releasing proteins. Together, our results demonstrate that Grb2 family adaptors are critical regulators of ITAM and ITT signaling in naïve and IgE-switched human B cells.

Grb2 carboxyl-terminal SH3 domain can bivalently associate with two ligands, in an SH3 dependent manner

Src homology domain containing leukocyte protein of 65 kDa (SLP65), the growth factor receptor binding protein 2 (Grb2), and the guanine nucleotide exchange factor for the Rho family GTPases (Vav), self associate in unstimulated B cells as components of the preformed B cell receptor transducer module, in an SH3-dependent manner. The complex enables the B cell to promptly respond to BCR aggregation, resulting in signal amplification. It also facilitates Vav translocation to the membrane rafts, for activation. Here we uncover the molecular mechanism by which the complex may be formed in the B cell. The C-terminal SH3 domain (SH3C) of Grb2 bivalently interacts with the atypical non-PxxP proline rich region of SLP65, and the N-terminal SH3 domain (SH3N) of Vav, both the interactions crucial for the proper functioning of the B cell. Most surprisingly, the two ligands bind the same ligand binding site on the surface of Grb2 SH3C. Addition of SLP65 peptide to the Grb2-Vav complex abrogates the interaction completely, displacing Vav. However, the addition of Vav SH3N to the SLP65-Grb2 binary complex, results in a trimeric complex. Extrapolating these results to the in vivo conditions, Grb2 should bind the SLP65 transducer module first, and then Vav should associate.

The Dok-3/Grb2 adaptor module promotes inducible association of the lipid phosphatase SHIP with the BCR in a coreceptor-independent manner

The SH2 domain-containing inositol 5'-phosphatase (SHIP) plays a key role in preventing autoimmune phenomena by limiting antigen-mediated B cell activation. SHIP function is thought to require the dual engagement of the BCR and negative regulatory coreceptors as only the latter appear capable of recruiting SHIP from the cytosol to the plasma membrane by the virtue of phosphorylated immunoreceptor tyrosine-based inhibitory motifs. Here, we demonstrate a coreceptor-independent membrane recruitment and function of SHIP in B cells. In the absence of coreceptor ligation, SHIP translocates to sites of BCR activation through a concerted action of the protein adaptor unit Dok-3/Grb2 and phosphorylated BCR signaling components. Our data reveal auto-inhibitory SHIP activation by the activated BCR and suggest an unexpected negative-regulatory capacity of immunoreceptor tyrosine-based activation motifs in Igα and Igβ.

Themis1 enhances T cell receptor signaling during thymocyte development by promoting Vav1 activity and Grb2 stability

The T cell signaling protein Themis1 is essential for the positive and negative selection of thymocytes in the thymus. Although the developmental defect that results from the loss of Themis1 suggests that it enhances T cell receptor (TCR) signaling, Themis1 also recruits Src homology 2 domain-containing phosphatase-1 (SHP-1) to the vicinity of TCR signaling complexes, suggesting that it has an inhibitory role in TCR signaling. We used TCR signaling reporter mice and quantitative proteomics to explore the role of Themis1 in developing T cells. We found that Themis1 acted mostly as a positive regulator of TCR signaling in vivo when receptors were activated by positively selecting ligands. Proteomic analysis of the Themis1 interactome identified SHP-1, the TCR-associated adaptor protein Grb2, and the guanine nucleotide exchange factor Vav1 as the principal interacting partners of Themis1 in isolated mouse thymocytes. Analysis of TCR signaling in Themis1-deficient and Themis1-overexpressing mouse thymocytes demonstrated that Themis1 promoted Vav1 activity both in vitro and in vivo. The reduced activity of Vav1 and the impaired T cell development in Themis1(-/-) mice were due in part to increased degradation of Grb2, which suggests that Themis1 is required to maintain the steady-state abundance of Grb2 in thymocytes. Together, these data suggest that Themis1 acts as a positive regulator of TCR signaling in developing T cells, and identify a mechanism by which Themis1 regulates thymic selection.

Role of Calcium Signaling in B Cell Activation and Biology

Increase in intracellular levels of calcium ions (Ca2+) is one of the key triggering signals for the development of B cell response to the antigen. The diverse Ca2+ signals finely controlled by multiple factors participate in the regulation of gene expression, B cell development, and effector functions. B cell receptor (BCR)-initiated Ca2+ mobilization is sourced from two pathways: one is the release of Ca2+ from the intracellular stores, endoplasmic reticulum (ER), and other is the prolonged influx of extracellular Ca2+ induced by depleting the stores via store-operated calcium entry (SOCE) and calcium release-activated calcium (CRAC) channels. The identification of stromal interaction molecule 1(STIM1), the ER Ca2+ sensor, and Orai1, a key subunit of the CRAC channel pore, has now provided the tools to understand the mode of Ca2+ influx regulation and physiological relevance. Herein, we discuss our current understanding of the molecular mechanisms underlying BCR-triggered Ca2+ signaling as well as its contribution to the B cell biological processes and diseases.

The Strep-tag system for one-step affinity purification of proteins from mammalian cell culture

The Strep-tag-or its improved version Strep-tagII-is an eight amino acid sequence that can be easily fused or conjugated to any protein or peptide of interest and that was engineered for high affinity toward streptavidin, which otherwise is widely known as a tight biotin-binding reagent. Especially in combination with immobilized Strep-Tactin, a mutant streptavidin specifically optimized toward the Strep-tagII, this system enables the facile one-step affinity purification of various biomolecules, including oligomeric and even membrane proteins. The Strep-tagII/Strep-Tactin interaction shows exquisite specificity, thus allowing efficient separation from host cell proteins, and it can be reversed simply by addition of biotin (or a suitable derivative thereof, such as desthiobiotin). Therefore, this system has become very popular for the highly efficient affinity chromatography under biochemically mild conditions. Here, we describe the purification of Strep-tagged proteins from mammalian cell lysates and cell culture supernatants.

The immunoglobulin tail tyrosine motif upgrades memory-type BCRs by incorporating a Grb2-Btk signalling module

The vigorous response of IgG-switched memory B cells to recurring pathogens involves enhanced signalling from their B-cell antigen receptors (BCRs). However, the molecular signal amplification mechanisms of memory-type BCRs remained unclear. Here, we identify the immunoglobulin tail tyrosine (ITT) motif in the cytoplasmic segments of membrane-bound IgGs (mIgGs) as the principle signal amplification device of memory-type BCRs in higher vertebrates and decipher its signalling microanatomy. We show that different families of protein tyrosine kinases act upstream and downstream of the ITT. Spleen tyrosine kinase (Syk) activity is required for ITT phosphorylation followed by recruitment of the adaptor protein Grb2 into the mIgG-BCR signalosome. Grb2 in turn recruits Bruton's tyrosine kinase (Btk) to amplify BCR-induced Ca(2+) mobilization. This molecular interplay of kinases and adaptors increases the antigen sensitivity of memory-type BCRs, which provides a cell-intrinsic trigger mechanism for the rapid reactivation of IgG-switched memory B cells on antigen recall.

RHEX, a novel regulator of human erythroid progenitor cell expansion and erythroblast development

RHEX is a novel target of the human erythropoietin receptor, and modulator of EPO-dependent red cell production.

Ligation of erythropoietin (EPO) receptor (EPOR) JAK2 kinase complexes propagates signals within erythroid progenitor cells (EPCs) that are essential for red blood cell production. To reveal hypothesized novel EPOR/JAK2 targets, a phosphotyrosine (PY) phosphoproteomics approach was applied. Beyond known signal transduction factors, 32 new targets of EPO-modulated tyrosine phosphorylation were defined. Molecular adaptors comprised one major set including growth factor receptor-bound protein 2 (GRB2)–associated binding proteins 1–3 (GAB1-3), insulin receptor substrate 2 (IRS2), docking protein 1 (DOK1), Src homology 2 domain containing transforming protein 1 (SHC1), and sprouty homologue 1 (SPRY1) as validating targets, and SPRY2, SH2 domain containing 2A (SH2D2A), and signal transducing adaptor molecule 2 (STAM2) as novel candidate adaptors together with an ORF factor designated as regulator of human erythroid cell expansion (RHEX). RHEX is well conserved in Homo sapiens and primates but absent from mouse, rat, and lower vertebrate genomes. Among tissues and lineages, RHEX was elevated in EPCs, occurred as a plasma membrane protein, was rapidly PY-phosphorylated >20-fold upon EPO exposure, and coimmunoprecipitated with the EPOR. In UT7epo cells, knockdown of RHEX inhibited EPO-dependent growth. This was associated with extracellular signal-regulated kinase 1,2 (ERK1,2) modulation, and RHEX coupling to GRB2. In primary human EPCs, shRNA knockdown studies confirmed RHEX regulation of erythroid progenitor expansion and further revealed roles in promoting the formation of hemoglobinizing erythroblasts. RHEX therefore comprises a new EPO/EPOR target and regulator of human erythroid cell expansion that additionally acts to support late-stage erythroblast development.

Growth factor receptor-bound protein 2 contributes to (hem)immunoreceptor tyrosine-based activation motif-mediated signaling in platelets

RATIONALE

Platelets are anuclear cell fragments derived from bone marrow megakaryocytes (MKs) that safeguard vascular integrity but may also cause pathological vessel occlusion. One major pathway of platelet activation is triggered by 2 receptors that signal through an (hem)immunoreceptor tyrosine-based activation motif (ITAM), the activating collagen receptor glycoprotein (GP) VI and the C-type lectin-like receptor 2 (CLEC-2). Growth factor receptor-bound protein 2 (Grb2) is a ubiquitously expressed adapter molecule involved in signaling processes of numerous receptors in different cell types, but its function in platelets and MKs is unknown.

OBJECTIVE

We tested the hypothesis that Grb2 is a crucial adapter protein in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets.

METHODS AND RESULTS

Here, we show that genetic ablation of Grb2 in MKs and platelets did not interfere with MK differentiation or platelet production. However, Grb2-deficiency severely impaired glycoprotein VI-mediated platelet activation because of defective stabilization of the linker of activated T-cell (LAT) signalosome and activation of downstream signaling proteins that resulted in reduced adhesion, aggregation, and coagulant activity on collagen in vitro. Similarly, CLEC-2-mediated signaling was impaired in Grb2-deficient platelets, whereas the cells responded normally to stimulation of G protein-coupled receptors. In vivo, this selective (hem)immunoreceptor tyrosine-based activation motif signaling defect resulted in prolonged bleeding times but affected arterial thrombus formation only after concomitant treatment with acetylsalicylic acid, indicating that defective glycoprotein VI signaling in the absence of Grb2 can be compensated through thromboxane A2-induced G protein-coupled receptor signaling pathways.

CONCLUSIONS

These results reveal an important contribution of Grb2 in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets in hemostasis and thrombosis by stabilizing the LAT signalosome.

The Dok-3/Grb2 protein signal module attenuates Lyn kinase-dependent activation of Syk kinase in B cell antigen receptor microclusters

Recruitment of the growth factor receptor-bound protein 2 (Grb2) by the plasma membrane-associated adapter protein downstream of kinase 3 (Dok-3) attenuates signals transduced by the B cell antigen receptor (BCR). Here we describe molecular details of Dok-3/Grb2 signal integration and function, showing that the Lyn-dependent activation of the BCR transducer kinase Syk is attenuated by Dok-3/Grb2 in a site-specific manner. This process is associated with the SH3 domain-dependent translocation of Dok-3/Grb2 complexes into BCR microsignalosomes and augmented phosphorylation of the inhibitory Lyn target SH2 domain-containing inositol 5' phosphatase. Hence, our findings imply that Dok-3/Grb2 modulates the balance between activatory and inhibitory Lyn functions with the aim to adjust BCR signaling efficiency.

Recruitment of Grb2 and SHIP1 by the ITT-like motif of TIGIT suppresses granule polarization and cytotoxicity of NK cells

Activating and inhibitory receptors control natural killer (NK) cell activity. T-cell immunoglobulin and ITIM (immunoreceptor tyrosine-based inhibition motif) domain (TIGIT) was recently identified as a new inhibitory receptor on T and NK cells that suppressed their effector functions. TIGIT harbors the immunoreceptor tail tyrosine (ITT)-like and ITIM motifs in its cytoplasmic tail. However, how its ITT-like motif functions in TIGIT-mediated negative signaling is still unclear. Here, we show that TIGIT/PVR (poliovirus receptor) engagement disrupts granule polarization leading to loss of killing activity of NK cells. The ITT-like motif of TIGIT has a major role in its negative signaling. After TIGIT/PVR ligation, the ITT-like motif is phosphorylated at Tyr225 and binds to cytosolic adapter Grb2, which can recruit SHIP1 to prematurely terminate phosphatidylinositol 3-kinase (PI3K) and MAPK signaling, leading to downregulation of NK cell function. In support of this, Tyr225 or Asn227 mutation leads to restoration of TIGIT/PVR-mediated cytotoxicity, and SHIP1 silencing can dramatically abolish TIGIT/PVR-mediated killing inhibition.

B cell receptor-mediated antigen gathering requires ubiquitin ligase Cbl and adaptors Grb2 and Dok-3 to recruit dynein to the signaling microcluster

The B cell receptor (BCR) mediates B cell antigen gathering and acquisition for presentation to T cells. Although the amount of antigen presentation to T cells determines the extent of B cell activation, the molecular mechanisms underlying antigen gathering remain unexplored. Here, through a combination of high-resolution imaging, genetics and quantitative mass spectrometry, we demonstrate that adaptors Grb2 and Dok-3, and ubiquitin ligase Cbl in signaling BCR microclusters mediate association with the microtubule motor dynein. Furthermore, we visualize the localization and movement of these microclusters on the underlying microtubule network. Importantly, disruption of this network or diminished dynein recruitment in Grb2-, Dok-3-, or Cbl-deficient B cells, does not influence microcluster formation or actin-dependent spreading, but abrogates directed movement of microclusters and antigen accumulation. Thus we identify a surprising but pivotal role for dynein and the microtubule network alongside Grb2, Dok-3, and Cbl in antigen gathering during B cell activation.

The B-cell antigen receptor signals through a preformed transducer module of SLP65 and CIN85

Spleen tyrosine kinase Syk and its substrate SLP65 (also called BLNK) are proximal signal transducer elements of the B-cell antigen receptor (BCR). Yet, our understanding of signal initiation and processing is limited owing to the incomplete list of SLP65 interaction partners and our ignorance of their association kinetics. We have now determined and quantified the in vivo interactomes of SLP65 in resting and stimulated B cells by mass spectrometry. SLP65 orchestrated a complex signal network of about 30 proteins that was predominantly based on dynamic interactions. However, a stimulation-independent and constant association of SLP65 with the Cbl-interacting protein of 85 kDa (CIN85) was requisite for SLP65 phosphorylation and its inducible plasma membrane translocation. In the absence of a steady SLP65/CIN85 complex, BCR-induced Ca(2+) and NF-κB responses were abrogated. Finally, live cell imaging and co-immunoprecipitation experiments further confirmed that both SLP65 and CIN85 are key components of the BCR-associated primary transducer module required for the onset and progression phases of BCR signal transduction.

Complex phosphorylation dynamics control the composition of the Syk interactome in B cells

Spleen tyrosine kinase Syk provides critical transducer functions for a number of immune cell receptors and has been implicated in the generation of several forms of leukemias. Catalytic activity and the ability of Syk to interact with other signaling elements depend on the phosphorylation status of Syk. We have now identified and quantified the full spectrum of phosphoacceptor sites in human Syk as well as the interactome of Syk in resting and activated B cells by high-resolution mass spectrometry. While the majority of inducible phosphorylations occurred on tyrosine residues, one of the most frequently detected phosphosites encompassed serine 297 located within the linker insert distinguishing the long and short isoforms of Syk. Full-length Syk can associate with more than 25 distinct ligands including the 14-3-3γ adaptor protein, which binds directly to phosphoserine 297. The latter complex attenuates inducible plasma membrane recruitment of Syk, thereby limiting antigen receptor-proximal signaling pathways. Collectively, the established ligand library provides a basis to understand the complexity of the Syk signaling network.

Growth-factor receptor-bound protein-2 (Grb2) signaling in B cells controls lymphoid follicle organization and germinal center reaction

Grb2 (growth-factor receptor-bound protein-2) is a signaling adaptor that interacts with numerous receptors and intracellular signaling molecules. However, its role in B-cell development and function remains unknown. Here we show that ablation of Grb2 in B cells results in enhanced B-cell receptor signaling; however, mutant B cells do not form germinal centers in the spleen after antigen stimulation. Furthermore, mutant mice exhibit defects in splenic architecture resembling that observed in B-cell-specific lymphotoxin-β-deficient mice, including disruption of marginal zone and follicular dendritic cell networks. We find that grb2(-/-) B cells are defective in lymphotoxin-β expression. Although lymphotoxin can be up-regulated by chemokine CXCL13 and CD40 ligand stimulation in wild-type B cells, elevation of lymphotoxin expression in grb2(-/-) B cells is only induced by anti-CD40 but not by CXCL13. Our results thus define Grb2 as a nonredundant regulator that controls lymphoid follicle organization and germinal center reaction. Loss of Grb2 has no effect on B-cell chemotaxis to CXCL13, indicating that Grb2 executes this function by connecting the CXCR5 signaling pathway to lymphotoxin expression but not to chemotaxis.

Grb2 regulates B-cell maturation, B-cell memory responses and inhibits B-cell Ca2+ signalling

Grb2 is a ubiquitously expressed adaptor protein, which activates Ras and MAP kinases in growth factor receptor signalling, while in B-cell receptor (BCR) signalling this role is controversial. In B cell lines it was shown that Grb2 can inhibit BCR-induced Ca(2+) signalling. Nonetheless, the physiological role of Grb2 in primary B cells is still unknown. We generated a B-cell-specific Grb2-deficient mouse line, which had a severe reduction of mature follicular B cells in the periphery due to a differentiation block and decreased B-cell survival. Moreover, we found several changes in important signalling pathways: enhanced BCR-induced Ca(2+) signalling, alterations in mitogen-activated protein kinase activation patterns and strongly impaired Akt activation, the latter pointing towards a defect in PI3K signalling. Interestingly, B-cell-specific Grb2-deficient mice showed impaired IgG and B-cell memory responses, and impaired germinal centre formation. Thus, Grb2-dependent signalling pathways are crucial for lymphocyte differentiation processes, as well as for control of secondary humoral immune responses.

The signaling tool box for tyrosine-based costimulation of lymphocytes

Triggering lymphocyte effector functions is controlled by a diverse array of immune cell coreceptors that dampen or potentiate the primary activation signal from antigen receptors. Attenuation of lymphocyte activation has been shown to be accomplished by immunoreceptor tyrosine-based inhibition motifs that upon phosphorylation recruit protein or lipid phosphatases. By contrast, a general concept of signal amplification and/or diversification is still out. However, the recent discovery of antigen receptor-intrinsic costimulation by membrane-bound immunoglobulins in class-switched memory B cells identified a consensus phosphorylation motif that can boost antigen-induced signal chains and is also employed by costimulatory receptors on T and Natural Killer cells to provide secondary signals for cellular activation. Here we define a common basis of tyrosine-based lymphocyte costimulation comprising immunoglobulin tail tyrosine (ITT)-like phosphorylation motifs and their proximal effectors, growth factor receptor-bound protein (Grb) 2 and phosphatidylinositol-3 kinase (PI3K) enzymes of class IA.