Essential role for Gab2 in the allergic response

Gab1 Overexpression Attenuates Susceptibility to Ventricular Arrhythmias in Pressure Overloaded Heart Failure Mouse Hearts

PURPOSE

Grb2 associated binding protein 1 (Gab1) is an adaptor protein that is important for intracellular signal transduction which involved in several pathological process. However, the role of Gab1 in pressure overload-induced ventricular arrhythmias (VAs) remain poorly understood. In the current study, we aimed to test the role of Gab1 in VA susceptibility induced by pressure overload.

METHODS

We overexpressed Gab1 in the hearts using an adeno-associated virus 9 (AAV9) system through tail vein injection. Aortic banding (AB) surgery was performed in C57BL6/J mice to induce heart failure (HF). Four weeks following AB, histology, echocardiography, and biochemical analysis were conducted to investigate cardiac structural remodeling and electrophysiological studies were performed to check the electrical remodeling. Western blot analysis was used to explore the underlying mechanisms.

RESULTS

The mRNA and protein expression were downregulated in AB hearts compared to sham hearts. Gab1 overexpression significantly reversed AB-induced cardiac structural remodeling including ameliorated AB-induced cardiac dysfunction, cardiac fibrosis, and inflammatory response. Moreover, Gab1 overexpression also markedly alleviated AB-induced electrical remodeling including ion channel alterations and VA susceptibility. Mechanistically, we found that TLR4/MyD88/NF-κB contributes to the cardio protective effect of Gab1 overexpression on AB-induced VAs.

CONCLUSIONS

Our study manifested that Gab1 may serve as a promising anti-arrhythmic target via inhibiting TLR4/MyD88/NF-κB signaling pathway induced by AB.

Mast cell deficiency prevents BCR::ABL1 induced splenomegaly and cytokine elevation in a CML mouse model

The persistence of leukemic stem cells (LSCs) represents a problem in the therapy of chronic myeloid leukemia (CML). Hence, it is of utmost importance to explore the underlying mechanisms to develop new therapeutic approaches to cure CML. Using the genetically engineered ScltTA/TRE-BCR::ABL1 mouse model for chronic phase CML, we previously demonstrated that the loss of the docking protein GAB2 counteracts the infiltration of mast cells (MCs) in the bone marrow (BM) of BCR::ABL1 positive mice. Here, we show for the first time that BCR::ABL1 drives the cytokine independent expansion of BM derived MCs and sensitizes them for FcεRI triggered degranulation. Importantly, we demonstrate that genetic mast cell deficiency conferred by the Cpa3^Cre allele prevents BCR::ABL1 induced splenomegaly and impairs the production of pro-inflammatory cytokines. Furthermore, we show in CML patients that splenomegaly is associated with high BM MC counts and that upregulation of pro-inflammatory cytokines in patient serum samples correlates with tryptase levels. Finally, MC-associated transcripts were elevated in human CML BM samples. Thus, our study identifies MCs as essential contributors to disease progression and suggests considering them as an additional target in CML therapy. Mast cells play a key role in the pro-inflammatory tumor microenvironment of the bone marrow. Shown is a cartoon summarizing our results from the mouse model. BCR::ABL1 transformed MCs, as part of the malignant clone, are essential for the elevation of pro-inflammatory cytokines, known to be important in disease initiation and progression.

Potential Anti-allergic Effects of Bibenzyl Derivatives from Liverworts, Radula perrottetii

The liverwort Radula perrottetii contains various bibenzyl derivatives which are known to possess various biological activities, such as anti-inflammatory effects. Mast cells (MC) play crucial roles in allergic and inflammatory diseases; thus, inhibition of MC activation is pivotal for the treatment of allergic and inflammatory disorders. We investigated the effects of perrottetin D (perD), isolated from Radula perrottetii, and perD diacetate (Ac-perD) on antigen-induced activation of MCs. Bone marrow-derived MCs (BMMCs) were generated from C57BL/6 mice. The degranulation ratio, histamine release, and the interleukin (IL)-4 and leukotriene B₄ productions on antigen-triggered BMMC were investigated. Additionally, the effects of the bibenzyls on binding of IgE to FcεRI were observed by flow cytometry, and signal transduction proteins was examined by Western blot. Furthermore, binding of the bibenzyls to the Fyn kinase domain was calculated. At 10 µM, perD decreased the degranulation ratio (p < 0.01), whereas 10 µM Ac-perD down-regulated IL-4 production (p < 0.05) in addition to decreasing the degranulation ratio (p < 0.01). Both compounds tended to decrease histamine release at a concentration of 10 µM. Although 10 µM perD reduced only Syk phosphorylation, 10 µM Ac-perD diminished phosphorylation of Syk, Gab2, PLC-γ, and p38. PerD appeared to selectively bind Fyn, whereas Ac-perD appeared to act as a weak but broad-spectrum inhibitor of kinases, including Fyn. In conclusion, perD and Ac-perD suppressed the phosphorylation of signal transduction molecules downstream of the FcεRI and consequently inhibited degranulation, and/or IL-4 production. These may be beneficial potential lead compounds for the development of novel anti-allergic and anti-inflammatory drugs.

Gab2 deficiency prevents Flt3-ITD driven acute myeloid leukemia in vivo

Internal tandem duplications (ITD) of the FMS-like tyrosine kinase 3 (FLT3) predict poor prognosis in acute myeloid leukemia (AML) and often co-exist with inactivating DNMT3A mutations. In vitro studies implicated Grb2-associated binder 2 (GAB2) as FLT3-ITD effector. Utilizing a Flt3-ITD knock-in, Dnmt3a haploinsufficient mouse model, we demonstrate that Gab2 is essential for the development of Flt3-ITD driven AML in vivo, as Gab2 deficient mice displayed prolonged survival, presented with attenuated liver and spleen pathology and reduced blast counts. Furthermore, leukemic bone marrow from Gab2 deficient mice exhibited reduced colony-forming unit capacity and increased FLT3 inhibitor sensitivity. Using transcriptomics, we identify the genes encoding for Axl and the Ret co-receptor Gfra2 as targets of the Flt3-ITD/Gab2/Stat5 axis. We propose a pathomechanism in which Gab2 increases signaling of these receptors by inducing their expression and by serving as downstream effector. Thereby, Gab2 promotes AML aggressiveness and drug resistance as it incorporates these receptor tyrosine kinases into the Flt3-ITD signaling network. Consequently, our data identify GAB2 as a promising biomarker and therapeutic target in human AML.

Scaffolding protein Gab2 is involved in postnatal development and lipopolysaccharide-induced activation of microglia in the mouse brain

Grb2-associated-binding protein-2 (Gab2) is a member of the Gab/DOS family and functions as an adapter protein downstream of several growth factor signaling pathways. Gab2 is considered an Alzheimer's disease susceptibility gene. However, the role of Gab2 in the brain is still largely unknown. Herein, we report that Gab2 is involved in the postnatal development of microglia in mice. The Gab2 expression in the brain was detected at postnatal day 1 (P1) and increased until P14 but decreased thereafter. The tyrosine phosphorylation of Gab2 (pGab2) was also detected at P1 and increased until P14. Next, we focused on microglial development in Gab2 knockout and heterozygous mice. Although differences were not detected in the cytoplasmic area of Iba1-labeled microglia between Gab2(±) and Gab2(-/-) mice, the analysis of CD68 and cathepsin D (indicators of microglial lysosomal activation) immunolabeling within Iba1+ cells revealed significant underdevelopment of microglial lysosomes in Gab2(-/-) mice at P60. In addition to the developmental abnormality of microglia in Gab2(-/-) mice, lipopolysaccharide-induced lysosomal activation was selectively suppressed in Gab2(-/-) mice compared to that in Gab2(±) mice. Our findings suggest that Gab2 is involved not only in postnatal development but also in lysosomal activation of microglia, therefore Gab2 dysfunction in microglia might potentially contribute to the development of neurodegenerative diseases.

Gab2 (Grb2-Associated Binder2) Plays a Crucial Role in Inflammatory Signaling and Endothelial Dysfunction

[Figure: see text].

Cytoskeletal Transport, Reorganization, and Fusion Regulation in Mast Cell-Stimulus Secretion Coupling

Mast cells are well known for their role in allergies and many chronic inflammatory diseases. They release upon stimulation, e.g., via the IgE receptor, numerous bioactive compounds from cytoplasmic secretory granules. The regulation of granule secretion and its interaction with the cytoskeleton and transport mechanisms has only recently begun to be understood. These studies have provided new insight into the interaction between the secretory machinery and cytoskeletal elements in the regulation of the degranulation process. They suggest a tight coupling of these two systems, implying a series of specific signaling effectors and adaptor molecules. Here we review recent knowledge describing the signaling events regulating cytoskeletal reorganization and secretory granule transport machinery in conjunction with the membrane fusion machinery that occur during mast cell degranulation. The new insight into MC biology offers novel strategies to treat human allergic and inflammatory diseases targeting the late steps that affect harmful release from granular stores leaving regulatory cytokine secretion intact.

Gab2 deficiency suppresses high-fat diet-induced obesity by reducing adipose tissue inflammation and increasing brown adipose function in mice

Obesity is caused by a long-term imbalance between energy intake and consumption and is regulated by multiple signals. This study investigated the effect of signaling scaffolding protein Gab2 on obesity and its relevant regulation mechanism. Gab2 knockout (KO) and wild-type (WT) mice were fed with a standard diet (SD) or high-fat diet (HFD) for 12 weeks. The results showed that the a high-fat diet-induced Gab2 expression in adipose tissues, but deletion of Gab2 attenuated weight gain and improved glucose tolerance in mice fed with a high-fat diet. White adipose tissue and systemic inflammations were reduced in HFD-fed Gab2 deficiency mice. Gab2 deficiency increased the expression of Ucp1 and other thermogenic genes in brown adipose tissue. Furthermore, the regulation of Gab2 on the mature differentiation and function of adipocytes was investigated in vitro using primary or immortalized brown preadipocytes. The expression of brown fat-selective genes was found to be elevated in differentiated adipocytes without Gab2. The mechanism of Gab2 regulating Ucp1 expression in brown adipocytes involved with its downstream PI3K (p85)-Akt-FoxO1 signaling pathway. Our research suggests that deletion of Gab2 suppresses diet-induced obesity by multiple pathways and Gab2 may be a novel therapeutic target for the treatment of obesity and associated complications.

Elevated expressions of SHP2 and GAB2 correlated with VEGF in eutopic and ectopic endometrium of women with ovarian endometriosis

Aims: Protein tyrosine phosphatase Src-homology-2-domain-containing phosphatase 2 (SHP2) and adaptor protein Grb2-associated binding protein 2 (GAB2) can bind to each other in various signal transduction. However, the expression of SHP2 and GAB2 have not been investigated in endometriosis. The aim of the study was to evaluate the expressions of SHP2 and GAB2, and explore the correlation with Ki67 and VEGF in ovarian endometriosis.Materials and methods: The protein expressions and localizations were assessed immunohistochemically in ectopic, eutopic endometrium and normal endometrium from patients with (n = 30) and without (n = 30) ovarian endometriosis.Results: SHP2 was mainly present in the endometrial glandular epithelium, with increased expression in eutopic endometrium and even higher expression in ectopic endometrium compared to control endometrium (p < .05). GAB2 was immunolocalized in endometrial epithelium and stroma, increasing its expression from control endometrium to eutopic and ectopic endometrium (p < .05). Positive correlation was found between SHP2 and GAB2 in endometrium (p < .01). SHP2 and GAB2 both positively correlated with VEGF (p < .05), but not Ki67 in endometrium.Conclusions: We provide the first evidence that the protein expressions of SHP2 and GAB2 were elevated in ectopic and eutopic endometrium, suggesting GAB2-SHP2 axis regulating VEGF might contribute to the pathomechanism of endometriosis.

G-CSF-dependent neutrophil differentiation requires downregulation of MAPK activities through the Gab2 signaling pathway

Granulocyte colony-stimulating factor (G-CSF) stimulation of myeloid cells induced tyrosine-phosphorylation of cellular proteins. One of the tyrosine-phosphorylated proteins was found to be a scaffold protein, Grb2-associated binding protein 2 (Gab2). Another member of Gab family protein, Gab3, was exogenously overexpressed in neutrophil progenitor cells to make the Gab3 protein to compete with the endogenous Gab2 for the G-CSF-dependent signaling. In Gab3-overexpressed cells, the level of tyrosine phosphorylation of endogenous Gab2 by G-CSF stimulation was markedly downregulated, while the phosphorylation of Gab3 was significantly enhanced. The Gab3-overexpressed cells continuously proliferated in the medium containing G-CSF and lost the ability to differentiate to the mature neutrophil, characterized by the lobulated nucleus. The G-CSF stimulation-dependent tyrosine phosphorylation of Gab3, the association of SHP2 to Gab3 and the following mitogen-activated protein kinase (MAPK) activation were prolonged in the Gab3-overexpressed cells, compared to the parental cells, where the binding of SHP2 to Gab2 protein and thereby the activation of MAPK were not sustained after G-CSF stimulation. Inhibition of MAPK by pharmaceutical inhibitor restored the Gab3-overexpressed cells to the ability to differentiate to mature neutrophil. Therefore, G-CSF-dependent Gab2 phosphorylation and following its downregulation led the short-term MAPK activation. The downregulation of MAPK after transient Gab2 phosphorylation was necessary for the consequent neutrophil differentiation induced by G-CSF stimulation.

Involvement of intracellular caveolin-1 distribution in the suppression of antigen-induced mast cell activation by cationic liposomes

Cationic liposomes are commonly used as vectors to effectively introduce foreign genes into target cells. In another function, we recently showed that cationic liposomes bound to the mast cell surface suppress the degranulation induced by the cross-linking of high-affinity immunoglobulin E receptor in a time- and dose-dependent manner. This suppression is mediated by the impairment of the sustained level of intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) via the inhibition of store-operated Ca²⁺ entry. Further, we revealed that the mechanism underlying an impaired [Ca²⁺ ]_i increase is the inhibition of the activation of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway. Yet, how cationic liposomes inhibit the PI3K-Akt pathway is still unclear. Here, we focused on caveolin-1, a major component of caveolae, which is reported to be involved in the activation of the PI3K-Akt pathway in various cell lines. In this study, we showed that caveolin-1 translocated from the cytoplasm to the plasma membrane after the activation of mast cells and colocalized with the p85 subunit of PI3K, which seemed to be essential for PI3K activity. Meanwhile, cationic liposomes suppressed the translocation of caveolin-1 to the plasma membrane and the colocalization of caveolin-1 with PI3K p85 also at the plasma membrane. This finding provides new information for the development of therapies using cationic liposomes against allergies.

The Putatively Specific Synthetic REV-ERB Agonist SR9009 Inhibits IgE- and IL-33-Mediated Mast Cell Activation Independently of the Circadian Clock

The cell-autonomous circadian clock regulates IgE- and IL-33-mediated mast cell activation, both of which are key events in the development of allergic diseases. Accordingly, clock modifiers could be used to treat allergic diseases, as well as many other circadian-related diseases, such as sleep and metabolic disorders. The nuclear receptors REV-ERB-α and -β (REV-ERBs) are crucial components of the circadian clockwork. Efforts to pharmacologically target REV-ERBs using putatively specific synthetic agonists, particularly SR9009, have yielded beneficial effects on sleep and metabolism. Here, we sought to determine whether REV-ERBs are functional in the circadian clockwork in mast cells and, if so, whether SR9009 affects IgE- and IL-33-mediated mast cell activation. Bone marrow-derived mast cells (BMMCs) obtained from wild-type mice expressed REV-ERBs, and SR9009 or other synthetic REV-ERBs agonists affected the mast cell clockwork. SR9009 inhibited IgE- and IL-33-mediated mast cell activation in wild-type BMMCs in association with inhibition of Gab2/PI3K and NF-κB activation. Unexpectedly, these suppressive effects of SR9009 were observed in BMMCs following mutation of the core circadian gene Clock. These findings suggest that SR9009 inhibits IgE- and IL-33-mediated mast cell activation independently of the functional circadian clock activity. Thus, SR9009 or other synthetic REV-ERB agonists may have potential for anti-allergic agents.

A phenotypic and genomics approach in a multi-ethnic cohort to subtype systemic lupus erythematosus

Systemic lupus erythematous (SLE) is a heterogeneous autoimmune disease in which outcomes vary among different racial groups. Here, we aim to identify SLE subgroups within a multiethnic cohort using an unsupervised clustering approach based on the American College of Rheumatology (ACR) classification criteria. We identify three patient clusters that vary according to disease severity. Methylation association analysis identifies a set of 256 differentially methylated CpGs across clusters, including 101 CpGs in genes in the Type I Interferon pathway, and we validate these associations in an external cohort. A cis-methylation quantitative trait loci analysis identifies 744 significant CpG-SNP pairs. The methylation signature is enriched for ethnic-associated CpGs suggesting that genetic and non-genetic factors may drive outcomes and ethnic-associated methylation differences. Our computational approach highlights molecular differences associated with clusters rather than single outcome measures. This work demonstrates the utility of applying integrative methods to address clinical heterogeneity in multifactorial multi-ethnic disease settings.

Inhibitory effect of aqueous extract of Cuminum cyminum L. seed on degranulation of RBL-2H3 cells and passive cutaneous anaphylaxis reaction in mice

Cuminum cyminum L. (cumin) seed is used as a spice in various countries. Although several functions of the components in cumin seed have been reported, the anti-allergic effect of the water-soluble component in cumin seed has not been reported yet. In this study, we focused on the suppressive effect of cumin seed aqueous extract on degranulation in order to reveal the anti-allergic effect of cumin. Cumin seed aqueous extract significantly suppressed the antigen-induced degranulation of rat basophilic leukemia cell line RBL-2H3 cells in a dose-dependent manner without cytotoxicity. The extract also inhibited the elevation of the intracellular calcium ion concentration induced by antigen. Immunoblot analysis revealed that the extract suppresses phosphorylation of phosphatidylinositol 3-kinase, Bruton's tyrosine kinase, phospholipase C-γ1/2, and Akt in the signaling pathways activated by antigen induction via FcεRI. Furthermore, the extract suppressed microtubule formation induced by antigen. In addition, oral administration of cumin seed aqueous extract significantly suppressed the passive cutaneous anaphylaxis reaction in BALB/c mice. Our findings suggest that cumin seed contains water-soluble components with the anti-allergic effect. Therefore, cumin seed has potential as anti-allergic functional food.

Gab2 and Gab3 Redundantly Suppress Colitis by Modulating Macrophage and CD8+ T-Cell Activation

Inflammatory Bowel Disease (IBD) is a multi-factorial chronic inflammation of the gastrointestinal tract prognostically linked to CD8⁺ T-cells, but little is known about their mechanism of activation during initiation of colitis. Here, Grb2-associated binding 2/3 adaptor protein double knockout mice (Gab2/3^−/−) were generated. Gab2/3^−/− mice, but not single knockout mice, developed spontaneous colitis. To analyze the cellular mechanism, reciprocal bone marrow (BM) transplantation demonstrated a Gab2/3^−/− hematopoietic disease-initiating process. Adoptive transfer showed individual roles for macrophages and T-cells in promoting colitis development in vivo. In spontaneous disease, intestinal intraepithelial CD8⁺ but much fewer CD4⁺, T-cells from Gab2/3^−/− mice with rectal prolapse were more proliferative. To analyze the molecular mechanism, reduced PI3-kinase/Akt/mTORC1 was observed in macrophages and T-cells, with interleukin (IL)-2 stimulated T-cells showing increased pSTAT5. These results illustrate the importance of Gab2/3 collectively in signaling responses required to control macrophage and CD8⁺ T-cell activation and suppress chronic colitis.

LIF-dependent survival of embryonic stem cells is regulated by a novel palmitoylated Gab1 signalling protein

The cytokine leukaemia inhibitory factor (LIF) promotes self-renewal of mouse embryonic stem cells (ESCs) through activation of the transcription factor Stat3. However, the contribution of other ancillary pathways stimulated by LIF in ESCs, such as the MAPK and PI3K pathways, is less well understood. We show here that naive-type mouse ESCs express high levels of a novel effector of the MAPK and PI3K pathways. This effector is an isoform of the Gab1 (Grb2-associated binder protein 1) adaptor protein that lacks the N-terminal pleckstrin homology (PH) membrane-binding domain. Although not essential for rapid unrestricted growth of ESCs under optimal conditions, the novel Gab1 variant (Gab1β) is required for LIF-mediated cell survival under conditions of limited nutrient availability. This enhanced survival is absolutely dependent upon a latent palmitoylation site that targets Gab1β directly to ESC membranes. These results show that constitutive association of Gab1 with membranes through a novel mechanism promotes LIF-dependent survival of murine ESCs in nutrient-poor conditions.

A ERK/RSK-mediated negative feedback loop regulates M-CSF-evoked PI3K/AKT activation in macrophages

Activation of the RAS/ERK and its downstream signaling components is essential for growth factor-induced cell survival, proliferation, and differentiation. The Src homology-2 domain containing protein tyrosine phosphatase 2 (SHP2), encoded by protein tyrosine phosphatase, non-receptor type 11 ( Ptpn11), is a positive mediator required for most, if not all, receptor tyrosine kinase-evoked RAS/ERK activation, but differentially regulates the PI3K/AKT signaling cascade in various cellular contexts. The precise mechanisms underlying the differential effects of SHP2 deficiency on the PI3K pathway remain unclear. We found that mice with myelomonocytic cell-specific [ Tg(LysM-Cre); Ptpn11fl/fl mice] Ptpn11 deficiency exhibit mild osteopetrosis. SHP2-deficient bone marrow macrophages (BMMs) showed decreased proliferation in response to M-CSF and decreased osteoclast generation. M-CSF-evoked ERK1/2 activation was decreased, whereas AKT activation was enhanced in SHP2-deficient BMMs. ERK1/2, via its downstream target RSK2, mediates this negative feedback by negatively regulating phosphorylation of M-CSF receptor at Tyr721 and, consequently, its binding to p85 subunit of PI3K and PI3K activation. Pharmacologic inhibition of RSK or ERK phenotypically mimics the signaling defects observed in SHP2-deficient BMMs. Furthermore, this increase in PI3K/AKT activation enables BMM survival in the setting of SHP2 deficiency.-Wang, L., Iorio, C., Yan, K., Yang, H., Takeshita, S., Kang, S., Neel, B.G., Yang, W. An ERK/RSK-mediated negative feedback loop regulates M-CSF-evoked PI3K/AKT activation in macrophages.

Critical Role for GAB2 in Neuroblastoma Pathogenesis through the Promotion of SHP2/MYCN Cooperation

Growing evidence suggests a major role for Src-homology-2-domain-containing phosphatase 2 (SHP2/PTPN11) in MYCN-driven high-risk neuroblastoma, although biologic confirmation and a plausible mechanism for this contribution are lacking. Using a zebrafish model of MYCN-overexpressing neuroblastoma, we demonstrate that mutant ptpn11 expression in the adrenal gland analog of MYCN transgenic fish promotes the proliferation of hyperplastic neuroblasts, accelerates neuroblastomagenesis, and increases tumor penetrance. We identify a similar mechanism in tumors with wild-type ptpn11 and dysregulated Gab2, which encodes a Shp2 activator that is overexpressed in human neuroblastomas. In MYCN transgenic fish, Gab2 overexpression activated the Shp2-Ras-Erk pathway, enhanced neuroblastoma induction, and increased tumor penetrance. We conclude that MYCN cooperates with either GAB2-activated or mutant SHP2 in human neuroblastomagenesis. Our findings further suggest that combined inhibition of MYCN and the SHP2-RAS-ERK pathway could provide effective targeted therapy for high-risk neuroblastoma patients with MYCN amplification and aberrant SHP2 activation.

Cooperative interaction of hepatocyte growth factor and neuregulin regulates Schwann cell migration and proliferation through Grb2-associated binder-2 in peripheral nerve repair

The sequential reactive changes in Schwann cell phenotypes in transected peripheral nerves, including dedifferentiation, proliferation and migration, are essential for nerve repair. Even though the injury-induced migratory and proliferative behaviors of Schwann cells resemble epithelial and mesenchymal transition (EMT) in tumors, the molecular mechanisms underlying this phenotypic change of Schwann cells are still unclear. Here we show that the reactive Schwann cells exhibit migratory features dependent on the expression of a scaffolding oncoprotein Grb2-associated binder-2 (Gab2), which was transcriptionally induced by neuregulin 1-ErbB2 signaling following nerve injury. Injury-induced Gab2 expression was dependent on c-Jun, a transcription factor critical to a Schwann cell reprograming into a repair-type cell. Interestingly, the injury-induced activation (tyrosine phosphorylation) of Gab2 in Schwann cells was regulated by an EMT signal, the hepatocyte growth factor-c-Met signaling, but not by neuregulin 1. Gab2 knockout mice exhibited a deficit in nerve repair after nerve transection due to limited Schwann cell migration. Furthermore, Gab2 was required for the proliferation of Schwann cells following nerve injury and in vitro, and was over-expressed in human Schwann cell-derived tumors. In contrast, the tyrosine phosphorylation of Gab1 after nerve injury was principally regulated by the neuregulin 1-ErbB2 signaling and was indispensable for remyelination after crush injury, but not for the proliferation and migration of Schwann cells. Our findings indicate that Gab1 and Gab2 in Schwann cells are nonredundant and play a crucial role in peripheral nerve repair.

Cationic liposomes suppress intracellular calcium ion concentration increase via inhibition of PI3 kinase pathway in mast cells

Cationic liposomes are commonly used as vectors to effectively introduce foreign genes (antisense DNA, plasmid DNA, siRNA, etc.) into target cells. Cationic liposomes are also known to affect cellular immunocompetences such as the mast cell function in allergic reactions. In particular, we previously showed that the cationic liposomes bound to the mast cell surface suppress the degranulation induced by cross-linking of high affinity IgE receptors in a time- and dose-dependent manner. This suppression is mediated by impairment of the sustained level of intracellular Ca²⁺ concentration ([Ca²⁺]_i) via inhibition of store-operated Ca²⁺ entry (SOCE). Here we study the mechanism underlying an impaired [Ca²⁺]_i increase by cationic liposomes in mast cells. We show that cationic liposomes inhibit the phosphorylation of Akt and PI3 kinases but not Syk and LAT. As a consequence, SOCE is suppressed but Ca²⁺ release from endoplasmic reticulum (ER) is not. Cationic liposomes inhibit the formation of STIM1 puncta, which is essential to SOCE by interacting with Orai1 following the Ca²⁺ concentration decrease in the ER. These data suggest that cationic liposomes suppress SOCE by inhibiting the phosphorylation of PI3 and Akt kinases in mast cells.

Understanding the mechanism of binding between Gab2 and the C terminal SH3 domain from Grb2

Gab2 is a large disordered protein that regulates several cellular signalling pathways and is overexpressed in different forms of cancer. Because of its disordered nature, a detailed characterization of the mechanisms of recognition between Gab2 and its physiological partners is particularly difficult. Here we provide a detailed kinetic characterization of the binding reaction between Gab2 and the C-terminal SH3 domain of the growth factor receptor-bound protein 2 (Grb2). We demonstrate that Gab2 folds upon binding following an induced fit type mechanism, whereby recognition is characterized by the formation of an intermediate, in which Gab2 is primarily disordered. In this scenario, folding of Gab2 into the bound conformation occurs only after binding. However, an alanine scanning of the proline residues of Gab2 suggests that the intermediate contains some degree of native-like structure, which might play a role for the recognition event to take place. The results, which represent a fundamental step forward in the understanding of this functional protein-protein interaction, are discussed on the light of previous structural works on these proteins.

Increased levels of Gab1 and Gab2 adaptor proteins skew interleukin-4 (IL-4) signaling toward M2 macrophage-driven pulmonary fibrosis in mice

M2-polarized macrophages, also known as alternatively activated macrophages, have long been associated with pulmonary fibrosis; however, the mechanism has not been fully defined. Gab1 and Gab2 proteins belong to the Gab family of adaptors and are integral components of the signal specificity in response to various extracellular stimuli. In this report, we found that levels of both Gab1 and Gab2 were elevated in M2-polarized macrophages isolated from bleomycin-induced fibrotic lungs. In vitro Gab1/2 deficiency in bone marrow-derived macrophages abrogated IL-4-mediated M2 polarization. Furthermore, in vivo conditional removal of Gab1 (Gab1^MyKO) and germ line knock-out of Gab2 (Gab2^-/-) in macrophages prevented a bias toward the M2 phenotype and attenuated bleomycin-induced fibrotic lung remodeling. In support of these observations, Gab1/2 were involved in responses predominated by IL-4 signaling, an essential determinant for macrophage M2 polarization. Further investigation revealed that both Gab1 and -2 are recruited to the IL-4 receptor, synergistically enhancing downstream signal amplification but conferring IL-4 signal preference. Mechanistically, the loss of Gab1 attenuated AKT activation, whereas the absence of Gab2 suppressed STAT6 activation in response to IL-4 stimulation, both of which are commonly attributed to M2-driven pulmonary fibrosis in mice. Taken together, these observations define a non-redundant role of Gab docking proteins in M2 polarization, adding critical insights into the pathogenesis of idiopathic pulmonary fibrosis.

Mecp2 Mediates Experience-Dependent Transcriptional Upregulation of Ryanodine Receptor Type-3

Mecp2 is a DNA methylation reader that plays a critical role in experience-dependent plasticity. Increasing evidence supports a role for epigenetic modifications in activity-induced gene expression. Hence, candidate genes related to such phenomena are of great interest. Ryanodine receptors are intracellular calcium channels that contribute to hippocampal synaptic plasticity, dendritic spine remodeling, and participate in learning and memory processes. Here we exposed mice to the enriched environment (EE) paradigm, which through increased stimulation induces experience dependent-plasticity, to explore a role for methyl-cytosines, and Mecp2 in directing Ryanodine receptor 3 (Ryr3) transcriptional activity. EE induced a hippocampal-specific increase in the methylation of discrete cytosines located at a Ryr3 isoform promoter; chromatin immunoprecipitation experiments revealed that EE increased Mecp2 binding to this Ryr3 isoform promoter. Interestingly, the experimental paradigm induced robust Ryr3 upregulation, accompanied by miR132-dependent suppression of p250GAP, a pathway driving synaptogenesis. In contrast to WT mice, Mecp2-null mice showed diminished levels of Ryr3 and displayed impaired EE-induced Ryr3 upregulation, compromising miR132 dependent suppression of p250GAP and experience-dependent structural plasticity. Based on these results, we propose that Mecp2 acts as a transcriptional activator of Ryr3, contributing to experience-dependent plasticity.

The Scaffolding Protein, Grb2-associated Binder-1, in Skeletal Muscles and Terminal Schwann Cells Regulates Postnatal Neuromuscular Synapse Maturation

The vertebrate neuromuscular junction (NMJ) is considered as a "tripartite synapse" consisting of a motor axon terminal, a muscle endplate, and terminal Schwann cells that envelope the motor axon terminal. The neuregulin 1 (NRG1)-ErbB2 signaling pathway plays an important role in the development of the NMJ. We previously showed that Grb2-associated binder 1 (Gab1), a scaffolding mediator of receptor tyrosine kinase signaling, is required for NRG1-induced peripheral nerve myelination. Here, we determined the role of Gab1 in the development of the NMJ using muscle-specific conditional Gab1 knockout mice. The mutant mice showed delayed postnatal maturation of the NMJ. Furthermore, the selective loss of the gab1 gene in terminal Schwann cells produced delayed synaptic elimination with abnormal morphology of the motor endplate, suggesting that Gab1 in both muscles and terminal Schwann cells is required for proper NMJ development. Gab1 in terminal Schwann cells appeared to regulate the number and process elongation of terminal Schwann cells during synaptic elimination. However, Gab2 knockout mice did not show any defects in the development of the NMJ. Considering the role of Gab1 in postnatal peripheral nerve myelination, our findings suggest that Gab1 is a pleiotropic and important component of NRG1 signals during postnatal development of the peripheral neuromuscular system.

Phosphoprotein profiles of candidate markers for early cellular responses to low-dose γ-radiation in normal human fibroblast cells

Ionizing radiation causes biological damage that leads to severe health effects. However, the effects and subsequent health implications caused by exposure to low-dose radiation are unclear. The objective of this study was to determine phosphoprotein profiles in normal human fibroblast cell lines in response to low-dose and high-dose γ-radiation. We examined the cellular response in MRC-5 cells 0.5 h after exposure to 0.05 or 2 Gy. Using 1318 antibodies by antibody array, we observed ≥1.3-fold increases in a number of identified phosphoproteins in cells subjected to low-dose (0.05 Gy) and high-dose (2 Gy) radiation, suggesting that both radiation levels stimulate distinct signaling pathways. Low-dose radiation induced nucleic acid-binding transcription factor activity, developmental processes, and multicellular organismal processes. By contrast, high-dose radiation stimulated apoptotic processes, cell adhesion and regulation, and cellular organization and biogenesis. We found that phospho-BTK (Tyr550) and phospho-Gab2 (Tyr643) protein levels at 0.5 h after treatment were higher in cells subjected to low-dose radiation than in cells treated with high-dose radiation. We also determined that the phosphorylation of BTK and Gab2 in response to ionizing radiation was regulated in a dose-dependent manner in MRC-5 and NHDF cells. Our study provides new insights into the biological responses to low-dose γ-radiation and identifies potential candidate markers for monitoring exposure to low-dose ionizing radiation.

Network-based analysis reveals novel gene signatures in peripheral blood of patients with chronic obstructive pulmonary disease

BACKGROUND

Chronic obstructive pulmonary disease (COPD) is currently the third leading cause of death and there is a huge unmet clinical need to identify disease biomarkers in peripheral blood. Compared to gene level differential expression approaches to identify gene signatures, network analyses provide a biologically intuitive approach which leverages the co-expression patterns in the transcriptome to identify modules of co-expressed genes.

METHODS

A weighted gene co-expression network analysis (WGCNA) was applied to peripheral blood transcriptome from 238 COPD subjects to discover co-expressed gene modules. We then determined the relationship between these modules and forced expiratory volume in 1 s (FEV1). In a second, independent cohort of 381 subjects, we determined the preservation of these modules and their relationship with FEV1. For those modules that were significantly related to FEV1, we determined the biological processes as well as the blood cell-specific gene expression that were over-represented using additional external datasets.

RESULTS

Using WGCNA, we identified 17 modules of co-expressed genes in the discovery cohort. Three of these modules were significantly correlated with FEV1 (FDR < 0.1). In the replication cohort, these modules were highly preserved and their FEV1 associations were reproducible (P < 0.05). Two of the three modules were negatively related to FEV1 and were enriched in IL8 and IL10 pathways and correlated with neutrophil-specific gene expression. The positively related module, on the other hand, was enriched in DNA transcription and translation and was strongly correlated to CD4+, CD8+ T cell-specific gene expression.

CONCLUSIONS

Network based approaches are promising tools to identify potential biomarkers for COPD.

TRIAL REGISTRATION

The ECLIPSE study was funded by GlaxoSmithKline, under ClinicalTrials.gov identifier NCT00292552 and GSK No. SCO104960.

Extracellular Signal-Regulated Kinases 1 and 2 Phosphorylate Gab2 To Promote a Negative-Feedback Loop That Attenuates Phosphoinositide 3-Kinase/Akt Signaling

The scaffolding adapter protein Gab2 (Grb2-associated binder) promotes cell proliferation, survival, and motility by engaging several signaling pathways downstream of growth factor and cytokine receptors. In particular, Gab2 plays essential roles in mast cells, as it is required for phosphoinositide 3-kinase (PI3K) activation in response to Kit and the high-affinity IgE receptor. While the positive role of Gab2 in PI3K signaling is well documented, very little is known about the mechanisms that attenuate its function. Here we show that Gab2 becomes phosphorylated on multiple proline-directed sites upon stimulation of the Ras/extracellular signal-regulated kinase (ERK) signaling pathway. We demonstrate that ERK1 and ERK2 interact with Gab2 via a novel docking motif, which is required for subsequent Gab2 phosphorylation in response to ERK1/2 activation. We identified four ERK1/2-dependent phosphorylation sites in Gab2 that prevent the recruitment of the p85 regulatory subunit of PI3K. Using bone marrow-derived mast cells to study Gab2-dependent signaling, we found that the inhibition of ERK1/2 activity promotes Akt signaling in response to Kit and the high-affinity IgE receptor. Together, our results indicate that ERK1/2 participates in a negative-feedback loop that attenuates PI3K/Akt signaling in response to various agonists.

Scaffolding protein Gab1 regulates myeloid dendritic cell migration in allergic asthma

Asthma is a common allergic disorder involving a complex interplay among multiple genetic and environmental factors. Recent studies identified genetic variants of human GAB1 as a novel asthma susceptibility factor. However, the functions of Gab1 in lung remain largely unexplored. In this study, we first observed an elevation of Gab1 level in peripheral blood mononuclear cells from asthmatic patients during acute exacerbation compared with convalescence. Mice with a selectively disrupted Gab1 in myeloid dendritic cells (mDCs) considerably attenuated allergic inflammation in experimental models of asthma. Further investigations revealed a prominent reduction in CCL19-mediated migration of Gab1-deficient mDCs to draining lymph nodes and subsequent impairment of Th2-driven adaptive activation. Mechanistically, Gab1 is an essential component of the CCL19/CCR7 chemokine axis that regulates mDC migration during asthmatic responses. Together, these findings provide the first evidence for the roles of Gab1 in lung, giving us deeper understanding of asthmatic pathogenesis.

Cell-based phenotypic screening of mast cell degranulation unveils kinetic perturbations of agents targeting phosphorylation

Mast cells play an essential role in initiating allergic diseases. The activation of mast cells are controlled by a complicated signal network of reversible phosphorylation, and finding the key regulators involved in this network has been the focus of the pharmaceutical industry. In this work, we used a method named Time-dependent cell responding profile (TCRP) to track the process of mast cell degranulation under various perturbations caused by agents targeting phosphorylation. To test the feasibility of this high-throughput cell-based phenotypic screening method, a variety of biological techniques were used. We further screened 145 inhibitors and clustered them based on the similarities of their TCRPs. Stat3 phosphorylation has been widely reported as a key step in mast cell degranulation. Interestingly, our TCRP results showed that a Stat3 inhibitor JSI124 did not inhibit degranulation like other Stat3 inhibitors, such as Stattic, clearly inhibited degranulation. Regular endpoint assays demonstrated that the distinctive TCRP of JSI124 potentially correlated with the ability to induce apoptosis. Consequently, different agents possibly have disparate functions, which can be conveniently detected by TCRP. From this perspective, our TCRP screening method is reliable and sensitive when it comes to discovering and selecting novel compounds for new drug developments.

Deletion of Gab2 in mice protects against hepatic steatosis and steatohepatitis: a novel therapeutic target for fatty liver disease

Fatty liver disease is a serious health problem worldwide and is the most common cause for chronic liver disease and metabolic disorders. The major challenge in the prevention and intervention of this disease is the incomplete understanding of the underlying mechanism and thus lack of potent therapeutic targets due to multifaceted and interdependent disease factors. In this study, we investigated the role of a signaling adaptor protein, GRB2-associated-binding protein 2 (Gab2), in fatty liver using an animal disease model. Gab2 expression in hepatocytes responded to various disease factor stimulations, and Gab2 knockout mice exhibited resistance to fat-induced obesity, fat- or alcohol-stimulated hepatic steatosis, as well as methionine and choline deficiency-induced steatohepatitis. Concordantly, the forced expression or knockdown of Gab2 enhanced or diminished oleic acid (OA)- or ethanol-induced lipid production in hepatocytes in vitro, respectively. During lipid accumulation in hepatocytes, both fat and alcohol induced the recruitment of PI3K or Socs3 by Gab2 and the activation of their downstream signaling proteins AKT, ERK, and Stat3. Therefore, Gab2 may be a disease-associated protein that is induced by pathogenic factors to amplify and coordinate multifactor-induced signals to govern disease development in the liver. Our research provides a novel potential target for the prevention and intervention of fatty liver disease.

Protein tyrosine phosphatase 1B (PTP1B) is dispensable for IgE-mediated cutaneous reaction in vivo

Mast cells play a critical role in allergic reactions. The cross-linking of FcεRI-bound IgE with multivalent antigen initiates a cascade of signaling events leading to mast cell activation. It has been well-recognized that cross linking of FcεRI mediates tyrosine phosphorylation. However, the mechanism involved in tyrosine dephosphorylation in mast cells is less clear. Here we demonstrated that protein tyrosine phosphatase 1B (PTP1B)-deficient mast cells showed increased IgE-mediated phosphorylation of the signal transducer and activator of transcription 5 (STAT5) and enhanced production of CCL9 (MIP-1γ) and IL-6 in IgE-mediated mast cells activation in vitro. However, IgE-mediated calcium mobilization, β-hexaosaminidase release (degranulation), and phosphorylation of IκB and MAP kinases were not affected by PTP1B deficiency. Furthermore, PTP1B deficient mice showed normal IgE-dependent passive cutaneous anaphylaxis and late phase cutaneous reactions in vivo. Thus, PTP1B specifically regulates IgE-mediated STAT5 pathway, but is redundant in influencing mast cell function in vivo.

The Inhibition of Mast Cell Activation of Radix Paeoniae alba Extraction Identified by TCRP Based and Conventional Cell Function Assay Systems

Chinese herbs have long been used to treat allergic disease, but recently the development was greatly impeded by the lack of good methods to explore the mechanism of action. Here, we showed the effects of Chinese herb Radix Paeoniae alba were identified and characterized by a mast cell activation assay that involves electronic impedance readouts for dynamic monitoring of cellular responses to produce time-dependent cell responding profiles (TCRPs), and the anti-allergic activities were further confirmed with various conventional molecular and cell biology tools. We found Radix P. alba can dose-dependently inhibit TCPRs, and have anti-allergic function in vitro and in vivo. Radix P. alba suppressed mast cell degranulation not only inhibiting the translocation of granules to the plasma membrane, but also blocking membrane fusion and exocytosis; and that there may be other anti-allergic components in addition to paeoniflorin. Our results suggest that Radix P. alba regulated mast cell activation with multiple targets, and this approach is also suitable for discovering other mast cell degranulation-targeting Chinese herbs and their potential multi-target mechanisms.

Biochemical and Functional Insights into the Integrated Regulation of Innate Immune Cell Responses by Teleost Leukocyte Immune-Type Receptors

Across vertebrates, innate immunity consists of a complex assortment of highly specialized cells capable of unleashing potent effector responses designed to destroy or mitigate foreign pathogens. The execution of various innate cellular behaviors such as phagocytosis, degranulation, or cell-mediated cytotoxicity are functionally indistinguishable when being performed by immune cells isolated from humans or teleost fishes; vertebrates that diverged from one another more than 450 million years ago. This suggests that vital components of the vertebrate innate defense machinery are conserved and investigating such processes in a range of model systems provides an important opportunity to identify fundamental features of vertebrate immunity. One characteristic that is highly conserved across vertebrate systems is that cellular immune responses are dependent on specialized immunoregulatory receptors that sense environmental stimuli and initiate intracellular cascades that can elicit appropriate effector responses. A wide variety of immunoregulatory receptor families have been extensively studied in mammals, and many have been identified as cell- and function-specific regulators of a range of innate responses. Although much less is known in fish, the growing database of genomic information has recently allowed for the identification of several immunoregulatory receptor gene families in teleosts. Many of these putative immunoregulatory receptors have yet to be assigned any specific role(s), and much of what is known has been based solely on structural and/or phylogenetic relationships with mammalian receptor families. As an attempt to address some of these shortcomings, this review will focus on our growing understanding of the functional roles played by specific members of the channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs), which appear to be important regulators of several innate cellular responses via classical as well as unique biochemical signaling networks.

Distinct GAB2 signaling pathways are essential for myeloid and lymphoid transformation and leukemogenesis by BCR-ABL1

Tyrosine kinase inhibitors (TKIs) directed against BCR-ABL1, the product of the Philadelphia (Ph) chromosome, have revolutionized treatment of patients with chronic myeloid leukemia (CML). However, acquired resistance to TKIs is a significant clinical problem in CML, and TKI therapy is much less effective against Ph(+)B-cell acute lymphoblastic leukemia (B-ALL). BCR-ABL1, via phosphorylated Tyr177, recruits the adapter GRB2-associated binding protein 2 (GAB2) as part of a GRB2/GAB2 complex. We showed previously that GAB2 is essential for BCR-ABL1-evoked myeloid transformation in vitro. Using a genetic strategy and mouse models of CML and B-ALL, we show here that GAB2 is essential for myeloid and lymphoid leukemogenesis by BCR-ABL1. In the mouse model, recipients of BCR-ABL1-transducedGab2(-/-)bone marrow failed to develop CML-like myeloproliferative neoplasia. Leukemogenesis was restored by expression of GAB2 but not by GAB2 mutants lacking binding sites for its effectors phosphatidylinositol 3-kinase (PI3K) or SRC homology 2-containing phosphotyrosine phosphatase 2 (SHP2). GAB2 deficiency also attenuated BCR-ABL1-induced B-ALL, but only the SHP2 binding site was required. The SHP2 and PI3K binding sites were differentially required for signaling downstream of GAB2. Hence, GAB2 transmits critical transforming signals from Tyr177 to PI3K and SHP2 for CML pathogenesis, whereas only the GAB2-SHP2 pathway is essential for lymphoid leukemogenesis. Given that GAB2 is dispensable for normal hematopoiesis, GAB2 and its effectors PI3K and SHP2 represent promising targets for therapy in Ph(+)hematologic neoplasms.

Differential Roles of Phospholipase D Proteins in FcεRI-Mediated Signaling and Mast Cell Function

Phospholipase D (PLD) proteins are enzymes that catalyze the hydrolysis of phosphatidylcholine to generate an important signaling lipid, phosphatidic acid. Phosphatidic acid is a putative second messenger implicated in the regulation of vesicular trafficking and cytoskeletal reorganization. Previous studies using inhibitors and overexpression of PLD proteins indicate that PLD1 and PLD2 play positive roles in FcεRI-mediated signaling and mast cell function. We used mice deficient in PLD1, PLD2, or both to study the function of these enzymes in mast cells. In contrast to published studies, we found that PLD1 deficiency impaired FcεRI-mediated mast cell degranulation; however, PLD2 deficiency enhanced it. Biochemical analysis showed that PLD deficiency affected activation of the PI3K pathway and RhoA. Furthermore, our data indicated that, although PLD1 deficiency impaired F-actin disassembly, PLD2 deficiency enhanced microtubule formation. Together, our results suggested that PLD1 and PLD2, two proteins that catalyze the same enzymatic reaction, regulate different steps in mast cell degranulation.

PI3K signaling in arterial diseases: Non redundant functions of the PI3K isoforms

Cardiovascular diseases are the most common cause of death around the world. This includes atherosclerosis and the adverse effects of its treatment, such as restenosis and thrombotic complications. The development of these arterial pathologies requires a series of highly-intertwined interactions between immune and arterial cells, leading to specific inflammatory and fibroproliferative cellular responses. In the last few years, the study of phosphoinositide 3-kinase (PI3K) functions has become an attractive area of investigation in the field of arterial diseases, especially since inhibitors of specific PI3K isoforms have been developed. The PI3K family includes 8 members divided into classes I, II or III depending on their substrate specificity. Although some of the different isoforms are responsible for the production of the same 3-phosphoinositides, they each have specific, non-redundant functions as a result of differences in expression levels in different cell types, activation mechanisms and specific subcellular locations. This review will focus on the functions of the different PI3K isoforms that are suspected as having protective or deleterious effects in both the various immune cells and types of cell found in the arterial wall. It will also discuss our current understanding in the context of which PI3K isoform(s) should be targeted for future therapeutic interventions to prevent or treat arterial diseases.

Structure and function of Gab2 and its role in cancer (Review)

The docking proteins of the Grb-associated binder (Gab) family transduce cellular signals between receptors and intracellular downstream effectors, and provide a platform for protein-protein interactions. Gab2, a key member of the Gab family of proteins, is involved in the amplification and integration of signal transduction, evoked by a variety of extracellular stimuli, including growth factors, cytokines and antigen receptors. Gab2 protein lacks intrinsic catalytic activity; however, when phosphorylated by protein-tyrosine kinases (PTKs), Gab2 recruits several Src homology-2 (SH2) domain-containing proteins, including the SH2-containing protein tyrosine phosphatase 2 (SHP2), the p85 subunit of phosphoinositide-3 kinase (PI3K), phospholipase C-γ (PLCγ)1, Crk, and GC-GAP. Through these interactions, the Gab2 protein triggers various downstream signal effectors, including SHP2/rat sarcoma viral oncogene/RAF/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase and PI3K/AKT, involved in cell growth, differentiation, migration and apoptosis. It has been previously reported that aberrant Gab2 and/or Gab2 signaling is closely associated with human tumorigenesis, particularly in breast cancer, leukemia and melanoma. The present review aimed to focus on the structure and effector function of Gab2, its role in cancer and its potential for use as an effective therapeutic target.

Signal transduction and chemotaxis in mast cells

Mast cells play crucial roles in both innate and adaptive arms of the immune system. Along with basophils, mast cells are essential effector cells for allergic inflammation that causes asthma, allergic rhinitis, food allergy and atopic dermatitis. Mast cells are usually increased in inflammatory sites of allergy and, upon activation, release various chemical, lipid, peptide and protein mediators of allergic reactions. Since antigen/immunoglobulin E (IgE)-mediated activation of these cells is a central event to trigger allergic reactions, innumerable studies have been conducted on how these cells are activated through cross-linking of the high-affinity IgE receptor (FcεRI). Development of mature mast cells from their progenitor cells is under the influence of several growth factors, of which the stem cell factor (SCF) seems to be the most important. Therefore, how SCF induces mast cell development and activation via its receptor, KIT, has been studied extensively, including a cross-talk between KIT and FcεRI signaling pathways. Although our understanding of the signaling mechanisms of the FcεRI and KIT pathways is far from complete, pharmaceutical applications of the knowledge about these pathways are underway. This review will focus on recent progresses in FcεRI and KIT signaling and chemotaxis.

PI3K signalling in inflammation

PI3Ks regulate several key events in the inflammatory response to damage and infection. There are four Class I PI3K isoforms (PI3Kα,β,γ,δ), three Class II PI3K isoforms (PI3KC2α, C2β, C2γ) and a single Class III PI3K. The four Class I isoforms synthesise the phospholipid 'PIP3'. PIP3 is a 'second messenger' used by many different cell surface receptors to control cell movement, growth, survival and differentiation. These four isoforms have overlapping functions but each is adapted to receive efficient stimulation by particular receptor sub-types. PI3Kγ is highly expressed in leukocytes and plays a particularly important role in chemokine-mediated recruitment and activation of innate immune cells at sites of inflammation. PI3Kδ is also highly expressed in leukocytes and plays a key role in antigen receptor and cytokine-mediated B and T cell development, differentiation and function. Class III PI3K synthesises the phospholipid PI3P, which regulates endosome-lysosome trafficking and the induction of autophagy, pathways involved in pathogen killing, antigen processing and immune cell survival. Much less is known about the function of Class II PI3Ks, but emerging evidence indicates they can synthesise PI3P and PI34P2 and are involved in the regulation of endocytosis. The creation of genetically-modified mice with altered PI3K signalling, together with the development of isoform-selective, small-molecule PI3K inhibitors, has allowed the evaluation of the individual roles of Class I PI3K isoforms in several mouse models of chronic inflammation. Selective inhibition of PI3Kδ, γ or β has each been shown to reduce the severity of inflammation in one or more models of autoimmune disease, respiratory disease or allergic inflammation, with dual γ/δ or β/δ inhibition generally proving more effective. The inhibition of Class I PI3Ks may therefore offer a therapeutic opportunity to treat non-resolving inflammatory pathologies in humans. This article is part of a Special Issue entitled Phosphoinositides.

Tespa1 negatively regulates FcεRI-mediated signaling and the mast cell-mediated allergic response

Antigen-mediated cross-linking of IgE on mast cells triggers a signaling cascade that results in their degranulation and proinflammatory cytokine production, which are key effectors in allergic reactions. We show that the activation of mast cells is negatively regulated by the newly identified adaptor protein Tespa1. Loss of Tespa1 in mouse mast cells led to hyper-responsiveness to stimulation via FcεRI. Mice lacking Tespa1 also displayed increased sensitivity to IgE-mediated allergic responses. The dysregulated signaling in KO mast cells was associated with increased activation of Grb2-PLC-γ1-SLP-76 signaling within the LAT1 (linker for activation of T cells family, member 1) signalosome versus the LAT2 signalosome. Collectively, these findings show that Tespa1 orchestrates mast cell activation by tuning the balance of LAT1 and LAT2 signalosome assembly.

Essential roles of Gab1 tyrosine phosphorylation in growth factor-mediated signaling and angiogenesis

Growth factors and their downstream receptor tyrosine kinases (RTKs) mediate a number of biological processes controlling cell function. Adaptor (docking) proteins, which consist exclusively of domains and motifs that mediate molecular interactions, link receptor activation to downstream effectors. Recent studies have revealed that Grb2-associated-binders (Gab) family members (including Gab1, Gab2, and Gab3), when phosphorylated on tyrosine residues, provide binding sites for multiple effector proteins, such as Src homology-2 (SH2)-containing protein tyrosine phosphatase 2 (SHP2) and phosphatidylinositol 3-kinase (PI3K) regulatory subunit p85, thereby playing important roles in transducing RTKs-mediated signals into pathways with diversified biological functions. Here, we provide an up-to-date overview on the domain structure and biological functions of Gab1, the most intensively studied Gab family protein, in growth factor signaling and biological functions, with a special focus on angiogenesis.

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

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

Combined detection of Gab1 and Gab2 expression predicts clinical outcome of patients with glioma

Grb2-associated binder 1 (Gab1) and Gab2 play important roles in cancer cell signaling. In particular, it has been demonstrated that the upregulation of Gab2 may be correlated with the World Health Organization (WHO) grade of gliomas and that patients with high Gab2 expression levels exhibited shorter survival time. However, the prognostic value of combined expression of Gab1 and Gab2 has not been explored. Gab1 and Gab2 expression in human gliomas and non-neoplastic brain tissues was measured by immunohistochemistry. Both the expression levels of Gab1 and Gab2 proteins in glioma tissues were significantly higher than those in non-neoplastic brain tissues (both P < 0.001). In addition, the overexpression of Gab1 and Gab2 proteins were both significantly associated with advanced WHO grades (both P < 0.001) and low KPS (both P = 0.01). Moreover, the overall survival of patients with high Gab1 protein expression or high Gab2 protein expression was obviously lower than those with low expressions (both P < 0.001). Notably, glioma patients with combined overexpression of Gab1 and Gab2 proteins (Gab1-high/Gab2-high) had shortest overall survival (P < 0.001). Furthermore, multivariate analysis showed that Gab1 expression (P = 0.01), Gab2 expression (P = 0.02), and combined expression of Gab1 and Gab2 (Gab1/Gab2, P = 0.006) were all independent prognostic factors for overall survival in glioma patients. Gab1 and Gab2 proteins are differentially expressed in glioma patients and closely correlated with the biological behavior of this malignancy. Combination of Gab1 and Gab2 expression may represent a promising biomarker for prognostication of human gliomas.

Grb2-associated binder 1 is essential for cardioprotection against ischemia/reperfusion injury

We have shown recently that endothelial Grb-2-associated binder 1 (Gab1), an intracellular scaffolding adaptor, has a protective effect against limb ischemia via mediating angiogenic signaling pathways. However, the role of Gab1 in cardiac ischemia/reperfusion (I/R) injury remains unknown. In this study, we show that Gab1 is required for cardioprotection against I/R injury. I/R injury led to remarkable phosphorylation of Gab1 in cardiomyocytes. Compared with controls, the mice with cardiomyocyte-specific deletion of Gab1 gene (CGKO mice) exhibited an increase in infarct size and a decrease in cardiac function after I/R injury. Consistently, in hearts of CGKO mice subjected to I/R, the activation of caspase 3 and myocardial apoptosis was markedly enhanced whereas the activation of protein kinase B (Akt) and mitogen-activated protein kinase (MAPK), which are critical for cardiomyocyte survival, was attenuated. Oxidative stress is regarded as a major contributor to myocardial I/R injury. To examine the role of Gab1 in oxidative stress directly, isolated adult cardiomyocytes were subject to oxidant hydrogen peroxide and the cardioprotective effects of Gab1 were confirmed. Furthermore, we found that the phosphorylation of Gab1 and Gab1-mediated activation of Akt and MAPK by oxidative stress was suppressed by ErbB receptor and Src kinase inhibitors, accompanied by an increase in apoptotic cell death. In conclusion, our results suggest that Gab1 is essential for cardioprotection against I/R oxidative injury via mediating survival signaling.

DOCK5 functions as a key signaling adaptor that links FcεRI signals to microtubule dynamics during mast cell degranulation

Mast cells play a key role in the induction of anaphylaxis, a life-threatening IgE-dependent allergic reaction, by secreting chemical mediators that are stored in secretory granules. Degranulation of mast cells is triggered by aggregation of the high-affinity IgE receptor, FcεRI, and involves dynamic rearrangement of microtubules. Although much is known about proximal signals downstream of FcεRI, the distal signaling events controlling microtubule dynamics remain elusive. Here we report that DOCK5, an atypical guanine nucleotide exchange factor (GEF) for Rac, is essential for mast cell degranulation. As such, we found that DOCK5-deficient mice exhibit resistance to systemic and cutaneous anaphylaxis. The Rac GEF activity of DOCK5 is surprisingly not required for mast cell degranulation. Instead, DOCK5 associated with Nck2 and Akt to regulate microtubule dynamics through phosphorylation and inactivation of GSK3β. When DOCK5-Nck2-Akt interactions were disrupted, microtubule formation and degranulation response were severely impaired. Our results thus identify DOCK5 as a key signaling adaptor that orchestrates remodeling of the microtubule network essential for mast cell degranulation.

Signal-transducing adaptor protein-2 controls the IgE-mediated, mast cell-mediated anaphylactic responses

Signal-transducing adaptor protein-2 (STAP-2) is a recently identified adaptor protein that regulates immune and inflammatory responses through interactions with a variety of signaling and transcriptional molecules. In the current study, we clarified the physiological role of STAP-2 in mast cell function, a key mediator of IgE-associated allergic responses. STAP-2 is constitutively expressed in mast cells. STAP-2 deficiency in mast cells greatly enhances FcεRI-mediated signals, resulting in the increased tyrosine phosphorylation of the phospholipase C-γ isoform, calcium mobilization, and degranulation. Of importance, STAP-2-deficient mice challenged with DNP-BSA after passive sensitization with anti-DNP IgE show more severe rectal temperature decrease than do wild-type mice. STAP-2-deficient mice also show increased vascular permeability and more severe cutaneous anaphylaxis after DNP-BSA injection. These regulatory functions performed by STAP-2 indicate that there is an interaction between STAP-2 and FcεRI. In addition, our previous data indicate that STAP-2 binds to the phospholipase C-γ isoform and IκB kinase-β. Therefore, our data described in this article strongly suggest that manipulation of STAP-2 expression in mast cells may control the pathogenesis of allergic diseases and have the potential for treating patients with allergy.