Regulation of in vitro and in vivo immune functions by the cytosolic adaptor protein SKAP-HOM

Generalized machine learning based on multi-omics data to profile the effect of ferroptosis pathway on prognosis and immunotherapy response in patients with bladder cancer

INTRODUCTION

Bladder cancer (BLCA) affects millions of people worldwide, with high rates of incidence and mortality. Ferroptosis proves to be a novel form of cell death process that is triggered by oxidative stress.

METHODS

We procured a total of 25 single nuclear RNA-seq (snRNA-seq) samples from GSE169379 in GEO database. We obtained different cohorts of BLCA patients from the TCGA and GEO databases for model training and validation. A total of 369 ferroptosis-related genes (FRGs) were selected from the FerrDb database. AUCell analysis was performed to assign ferroptosis scores to all the cell types. Weighted Gene Co-Expression Network Analysis (WGCNA), COX, and LASSO regression analysis were conducted to retain and finalize the genes of prognostic values. Various bioinformatic approaches were utilized to depict immune infiltration profile. We conducted a series of colony formation analysis, flow cytometry and western blot (WB) analysis to determine the role of SKAP1 in BLCA.

RESULTS

We divided the cells into high ferroptosis group and low ferroptosis group according to ferroptosis activity score, and then screened 2150 genes most associated with ferroptosis by differential expression analysis, which are related to UV-induced DNA damage, male hormone response, fatty acid metabolism and hypoxia. Subsequently, WGCNA algorithm further screened 741 ferroptosis related genes from the 2150 genes for the construction of prognostic model. Lasso-Cox regression analysis was used to construct the prognostic model, and the prognostic model consisting of 6 genes was obtained, namely JUN, SYT1, MAP3K8, GALNT14, TCIRG1, and SKAP1. Next, we constructed a nomogram model that integrated clinical factors to improving the accuracy. In addition, we performed drug sensitivity analyses in different subgroups and found that Staurosporine, Rapamycin, Gemcitabine, and BI-2536 may be candidates for the drugs treatment in high-risk populations. The ESTIMATE results showed higher stromal scores, immune scores, and ESTIMATE scores in the low-risk group, indicating a higher overall immunity level and immunogenicity of tumor microenvironment (TME) in this group, and tumor immune dysfunction and exclusion (TIDE) analysis confirmed a better response to immunotherapy in the low-risk group. Finally, we selected the oncogene SKAP1 in the prognostic gene for in vitro validation, and found that SKAP1 directly regulated BLCA cell proliferation and apoptosis.

CONCLUSION

We identified a set of six genes, JUN, SYT1, MAP3K8, GALNT14, TCIRG1, and SKAP1, that exhibited significant potential in stratification of BLCA patients with varying prognosis. In addition, we uncovered the direct regulatory effect of SKAP1 on BLCA cell proliferation and apoptosis, shedding some light on the role of FRGs in pathogenesis of BLCA.

SKAP2-A Molecule at the Crossroads for Integrin Signalling and Immune Cell Migration and Function

Src-kinase associated protein 2 (SKAP2) is an intracellular scaffolding protein that is broadly expressed in immune cells and is involved in various downstream signalling pathways, including, but not limited to, integrin signalling. SKAP2 has a wide range of binding partners and fine-tunes the rearrangement of the cytoskeleton, thereby regulating cell migration and immune cell function. Mutations in SKAP2 have been associated with several inflammatory disorders such as Type 1 Diabetes and Crohn's disease. Rodent studies showed that SKAP2 deficient immune cells have diminished pathogen clearance due to impaired ROS production and/or phagocytosis. However, there is currently no in-depth understanding of the functioning of SKAP2. Nevertheless, this review summarises the existing knowledge with a focus of its role in signalling cascades involved in cell migration, tissue infiltration and immune cell function.

SKAP2 Modular Organization Differently Recognizes SRC Kinases Depending on Their Activation Status and Localization

Dimerization of SRC kinase adaptor phosphoprotein 2 (SKAP2) induces an increase of binding for most SRC kinases suggesting a fine-tuning with transphosphorylation for kinase activation. This work addresses the molecular basis of SKAP2-mediated SRC kinase regulation through the lens of their interaction capacities. By combining a luciferase complementation assay and extensive site-directed mutagenesis, we demonstrated that SKAP2 interacts with SRC kinases through a modular organization depending both on their phosphorylation-dependent activation and subcellular localization. SKAP2 contains three interacting modules consisting in the dimerization domain, the SRC homology 3 (SH3) domain, and the second interdomain located between the Pleckstrin homology and the SH3 domains. Functionally, the dimerization domain is necessary and sufficient to bind to most activated and myristyl SRC kinases. In contrast, the three modules are necessary to bind SRC kinases at their steady state. The Pleckstrin homology and SH3 domains of SKAP2 as well as tyrosines located in the interdomains modulate these interactions. Analysis of mutants of the SRC kinase family member hematopoietic cell kinase supports this model and shows the role of two residues, Y390 and K7, on its degradation following activation. In this article, we show that a modular architecture of SKAP2 drives its interaction with SRC kinases, with the binding capacity of each module depending on both their localization and phosphorylation state activation. This work opens new perspectives on the molecular mechanisms of SRC kinases activation, which could have significant therapeutic impact.

SKAP2 is downregulated in the villous tissues of patients with missed abortion and regulates growth and migration in trophoblasts through the WAVE2-ARP2/3 signaling pathway

INTRODUCTION

Abnormal placental trophoblast function is the main cause of missed abortion (MA). Src kinase-associated phosphoprotein 2 (SKAP2) indirectly affects actin reunion, which is significantly associated with cell migration.

METHODS

Twenty women with MA and 20 healthy women who underwent voluntarily induced abortion were included in this study. Immunohistochemistry, qRT-PCR, and western blotting were used to determine SKAP2, WAVE2, and ARP2 expression in the villous tissues. We investigated the effects of SKAP2 and the W336K mutant (blocked SKAP2 Src homology 3 function) on growth and migration in HTR8/SVneo cells using the CCK8 assay, flow cytometry, and transwell assay. The effects of SKAP2 on the WAVE2-ARP2/3 signaling pathway in HTR8/SVneo cells were evaluated by western blotting and immunofluorescence.

RESULTS

Compared to the women in the voluntary abortion group, SKAP2 and WAVE2 expression levels were downregulated in the villous of patients with MA. In HTR8/SVneo cells, SKAP2 siRNA silencing regulated the growth and migration, while SKAP2 overexpression promoted growth and migration, and inhibited apoptosis. Additionally, SKAP2 regulated the expression of WAVE2 and ARP2, as well as the colocalization of actin with WAVE2. The SKAP2 W336K mutant could not alter WAVE2 and ARP2 expression, nor HTR8/SVneo cell growth and migration, with or without SKAP2 siRNA transfection.

DISCUSSION

SKAP2 could activate the WAVE2-ARP2/3 pathway resulting in an increase of growth and migration in trophoblasts. SKAP2 probably played an important role in MA by affecting the growth and migration of trophoblasts.

Single-cell eQTL mapping identifies cell type-specific genetic control of autoimmune disease

The human immune system displays substantial variation between individuals, leading to differences in susceptibility to autoimmune disease. We present single-cell RNA sequencing (scRNA-seq) data from 1,267,758 peripheral blood mononuclear cells from 982 healthy human subjects. For 14 cell types, we identified 26,597 independent cis-expression quantitative trait loci (eQTLs) and 990 trans-eQTLs, with most showing cell type-specific effects on gene expression. We subsequently show how eQTLs have dynamic allelic effects in B cells that are transitioning from naïve to memory states and demonstrate how commonly segregating alleles lead to interindividual variation in immune function. Finally, using a Mendelian randomization approach, we identify the causal route by which 305 risk loci contribute to autoimmune disease at the cellular level. This work brings together genetic epidemiology with scRNA-seq to uncover drivers of interindividual variation in the immune system.

Comprehensive bioinformatics analyses identified Homeobox B9 as a potential prognostic biomarker and therapeutic target for gastric cancer

Background

The Homeobox B (HOXB) family promotes tumor progression, but the mechanism of its action in gastric cancer (GC) is unclear. We sought to identify the HOXB family members that are critical to the prognosis of GC patients.

Methods

The Oncomine, Gene Expression Profiling Interactive Analysis (GEPIA), cBioPortal, UALCAN, Kaplan-Meier plotter, and the GeneMANIA databases were used to analyze the messenger RNA (mRNA) expression levels, prognostic value, and gene-gene interaction network of the HOXB9 family members in GC. The expression of HOXB9 in GC and its relationship with various clinicopathological parameters and the prognosis of patients were verified by immunohistochemistry.

Results

The expression of HOXB3, HOXB5, HOXB6, HOXB7, HOXB9, and HOXB13 mRNA was significantly upregulated in GC. There was a significant correlation between the upregulation of HOXB3, HOXB5, and HOXB9 mRNA and a low overall survival (OS) rate. The high expression of HOXB7, HOXB9, and HOXB13 mRNA was closely correlated to tumor grade and stage. HOXB9 was the HOXB family member most closely related to the occurrence and development of GC. A further analysis showed that HOXB9 might be involved in deoxyribonucleic acid repair and division regulation. A validation study showed that the advanced cancer group had a higher level of HOXB9 expression than the early cancer group. The high expression of HOXB9 in gastric tissue plays an important role in the survival and prognosis of GC patients.

Conclusions

HOXB family members have different degrees of abnormal expression in GC. High HOXB9 expression in GC tissues was significantly correlated with a worse prognosis. Thus, HOXB9 is a potential novel biomarker and therapeutic target for GC.

Neutrophils require SKAP2 for reactive oxygen species production following C-type lectin and Candida stimulation

Signaling cascades converting the recognition of pathogens to efficient inflammatory responses by neutrophils are critical for host survival. SKAP2, an adaptor protein, is required for reactive oxygen species (ROS) generation following neutrophil stimulation by integrins, formyl peptide receptors, and for host defense against the Gram-negative bacterial pathogens, Klebsiella pneumoniae and Yersinia pseudotuberculosis. Using neutrophils from murine HoxB8-immortalized progenitors, we show that SKAP2 in neutrophils is crucial for maximal ROS response to purified C-type lectin receptor agonists and to the fungal pathogens, Candida glabrata and Candida albicans, and for robust killing of C. glabrata. Inside-out signaling to integrin and Syk phosphorylation occurred independently of SKAP2 after Candida infection. However, Pyk2, ERK1/2, and p38 phosphorylation were significantly reduced after infection with C. glabrata and K. pneumoniae in Skap2-/- neutrophils. These data demonstrate the importance of SKAP2 in ROS generation and host defense beyond antibacterial immunity to include CLRs and Candida species.

Diabetes With Multiple Autoimmune and Inflammatory Conditions Linked to an Activating SKAP2 Mutation

OBJECTIVE

Multiple genome-wide association studies have identified a strong genetic linkage between the SKAP2 locus and type 1 diabetes (T1D), but how this leads to disease remains obscure. Here, we characterized the functional consequence of a novel SKAP2 coding mutation in a patient with T1D to gain further insight into how this impacts immune tolerance.

RESEARCH DESIGN AND METHODS

We identified a 24-year-old individual with T1D and other autoimmune and inflammatory conditions. The proband and first-degree relatives were recruited for whole-exome sequencing. Functional studies of the protein variant were performed using a cell line and primary myeloid immune cells collected from family members.

RESULTS

Sequencing identified a de novo SKAP2 variant (c.457G>A, p.Gly153Arg) in the proband. Assays using monocyte-derived macrophages from the individual revealed enhanced activity of integrin pathways and a migratory phenotype in the absence of chemokine stimulation, consistent with SKAP2 p.Gly153Arg being constitutively active. The p.Gly153Arg variant, located in the well-conserved lipid-binding loop, induced similar phenotypes when expressed in a human macrophage cell line. SKAP2 p.Gly153Arg is a gain-of-function, pathogenic mutation that disrupts myeloid immune cell function, likely resulting in a break in immune tolerance and T1D.

CONCLUSIONS

SKAP2 plays a key role in myeloid cell activation and migration. This particular mutation in a patient with T1D and multiple autoimmune conditions implicates a role for activating SKAP2 variants in autoimmune T1D.

SKAP2 as a new regulator of oligodendroglial migration and myelin sheath formation

Oligodendroglial progenitor cells (OPCs) are highly proliferative and migratory cells, which differentiate into complex myelin forming and axon ensheathing mature oligodendrocytes during myelination. Recent studies indicate that the oligodendroglial cell population is heterogeneous on transcriptional and functional level depending on the location in the central nervous system. Here, we compared intrinsic properties of OPC from spinal cord and brain on functional and transcriptional level. Spinal cord OPC demonstrated increased migration as well as differentiation capacity. Moreover, transcriptome analysis revealed differential expression of several genes between both OPC populations. In spinal cord OPC, we confirmed upregulation of SKAP2, a cytoplasmatic adaptor protein known for its implication in cytoskeletal remodeling and migration in other cell types. Recent findings suggest that actin dynamics determine not only oligodendroglial migration, but also differentiation: Whereas actin polymerization is important for process extension, actin destabilization and depolymerization is required for myelin sheath formation. Downregulation or complete lack of SKAP2 in OPC resulted in reduced migration and impaired morphological maturation in oligodendrocytes. In contrast, overexpression of SKAP2 as well as constitutively active SKAP2 increased OPC migration suggesting that SKAP2 function is dependent on activation by phosphorylation. Furthermore, lack of SKAP2 enhanced the positive effect on OPC migration after integrin activation suggesting that SKAP2 acts as modulator of integrin dependent migration. In summary, we demonstrate the presence of intrinsic differences between spinal cord and brain OPC and identified SKAP2 as a new regulator of oligodendroglial migration and sheath formation.

The Multiple Roles of the Cytosolic Adapter Proteins ADAP, SKAP1 and SKAP2 for TCR/CD3 -Mediated Signaling Events

T cells are the key players of the adaptive immune response. They coordinate the activation of other immune cells and kill malignant and virus-infected cells. For full activation T cells require at least two signals. Signal 1 is induced after recognition of MHC/peptide complexes presented on antigen presenting cells (APCs) by the clonotypic TCR (T-cell receptor)/CD3 complex whereas Signal 2 is mediated via the co-stimulatory receptor CD28, which binds to CD80/CD86 molecules that are present on APCs. These signaling events control the activation, proliferation and differentiation of T cells. In addition, triggering of the TCR/CD3 complex induces the activation of the integrin LFA-1 (leukocyte function associated antigen 1) leading to increased ligand binding (affinity regulation) and LFA-1 clustering (avidity regulation). This process is termed "inside-out signaling". Subsequently, ligand bound LFA-1 transmits a signal into the T cells ("outside-in signaling") which enhances T-cell interaction with APCs (adhesion), T-cell activation and T-cell proliferation. After triggering of signal transducing receptors, adapter proteins organize the proper processing of membrane proximal and intracellular signals as well as the activation of downstream effector molecules. Adapter proteins are molecules that lack enzymatic or transcriptional activity and are composed of protein-protein and protein-lipid interacting domains/motifs. They organize and assemble macromolecular complexes (signalosomes) in space and time. Here, we review recent findings regarding three cytosolic adapter proteins, ADAP (Adhesion and Degranulation-promoting Adapter Protein), SKAP1 and SKAP2 (Src Kinase Associated Protein 1 and 2) with respect to their role in TCR/CD3-mediated activation, proliferation and integrin regulation.

SKAP2, a Candidate Gene for Type 1 Diabetes, Regulates β-Cell Apoptosis and Glycemic Control in Newly Diagnosed Patients

The single nucleotide polymorphism rs7804356 located in the Src kinase-associated phosphoprotein 2 (SKAP2) gene is associated with type 1 diabetes (T1D), suggesting SKAP2 as a causal candidate gene. The objective of the study was to investigate if SKAP2 has a functional role in the β-cells in relation to T1D. In a cohort of children with newly diagnosed T1D, rs7804356 predicted glycemic control and residual β-cell function during the 1st year after diagnosis. In INS-1E cells and rat and human islets, proinflammatory cytokines reduced the content of SKAP2. Functional studies revealed that knockdown of SKAP2 aggravated cytokine-induced apoptosis in INS-1E cells and primary rat β-cells, suggesting an antiapoptotic function of SKAP2. In support of this, overexpression of SKAP2 afforded protection against cytokine-induced apoptosis, which correlated with reduced nuclear content of S536-phosphorylated nuclear factor-κB (NF-κB) subunit p65, lower nitric oxide production, and diminished CHOP expression indicative of decreased endoplasmic reticulum stress. Knockdown of CHOP partially counteracted the increase in cytokine-induced apoptosis caused by SKAP2 knockdown. In conclusion, our results suggest that SKAP2 controls β-cell sensitivity to cytokines possibly by affecting the NF-κB-inducible nitric oxide synthase-endoplasmic reticulum stress pathway.

Variant-to-Gene-Mapping Analyses Reveal a Role for the Hypothalamus in Genetic Susceptibility to Inflammatory Bowel Disease

BACKGROUND & AIMS

Inflammatory bowel disease (IBD) is a polygenic disorder characterized principally by dysregulated inflammation impacting the gastrointestinal tract. However, there also is increasing evidence for a clinical association with stress and depression. Given the role of the hypothalamus in stress responses and in the pathogenesis of depression, useful insights could be gleaned from understanding its genetic role in IBD.

METHODS

We conducted genetic correlation analyses on publicly available genome-wide association study summary statistics for depression and IBD traits to identify genetic commonalities. We used partitioned linkage disequilibrium score regression, leveraging our ATAC sequencing and promoter-focused Capture C data, to measure enrichment of IBD single-nucleotide polymorphisms within promoter-interacting open chromatin regions of human embryonic stem cell-derived hypothalamic-like neurons (HNs). Using the same data sets, we performed variant-to-gene mapping to implicate putative IBD effector genes in HNs. To contrast these results, we similarly analyzed 3-dimensional genomic data generated in epithelium-derived colonoids from rectal biopsy specimens from donors without pathologic disease noted at the time of colonoscopy. Finally, we conducted enrichment pathway analyses on the implicated genes to identify putative IBD dysfunctional pathways.

RESULTS

We found significant genetic correlations (rg) of 0.122 with an adjusted P (Padj) = 1.4 × 10-4 for IBD: rg = 0.122; Padj = 2.5 × 10-3 for ulcerative colitis and genetic correlation (rg) = 0.094; Padj = 2.5 × 10-3 for Crohn's disease, and significant approximately 4-fold (P = .005) and approximately 7-fold (P = .03) enrichment of IBD single-nucleotide polymorphisms in HNs and colonoids, respectively. We implicated 25 associated genes in HNs, among which CREM, CNTF, and RHOA encode key regulators of stress. Seven genes also additionally were implicated in the colonoids. We observed an overall enrichment for immune and hormonal signaling pathways, and a colonoid-specific enrichment for microbiota-relevant terms.

CONCLUSIONS

Our results suggest that the hypothalamus warrants further study in the context of IBD pathogenesis.

Yersinia pseudotuberculosis YopH targets SKAP2-dependent and independent signaling pathways to block neutrophil antimicrobial mechanisms during infection

Yersinia suppress neutrophil responses by using a type 3 secretion system (T3SS) to inject 6–7 Yersinia effector proteins (Yops) effectors into their cytoplasm. YopH is a tyrosine phosphatase that causes dephosphorylation of the adaptor protein SKAP2, among other targets in neutrophils. SKAP2 functions in reactive oxygen species (ROS) production, phagocytosis, and integrin-mediated migration by neutrophils. Here we identify essential neutrophil functions targeted by YopH, and investigate how the interaction between YopH and SKAP2 influence Yersinia pseudotuberculosis (Yptb) survival in tissues. The growth defect of a ΔyopH mutant was restored in mice defective in the NADPH oxidase complex, demonstrating that YopH is critical for protecting Yptb from ROS during infection. The growth of a ΔyopH mutant was partially restored in Skap2-deficient (Skap2KO) mice compared to wild-type (WT) mice, while induction of neutropenia further enhanced the growth of the ΔyopH mutant in both WT and Skap2KO mice. YopH inhibited both ROS production and degranulation triggered via integrin receptor, G-protein coupled receptor (GPCR), and Fcγ receptor (FcγR) stimulation. SKAP2 was required for integrin receptor and GPCR-mediated ROS production, but dispensable for degranulation under all conditions tested. YopH blocked SKAP2-independent FcγR-stimulated phosphorylation of the proximal signaling proteins Syk, SLP-76, and PLCγ2, and the more distal signaling protein ERK1/2, while only ERK1/2 phosphorylation was dependent on SKAP2 following integrin receptor activation. These findings reveal that YopH prevents activation of both SKAP2-dependent and -independent neutrophilic defenses, uncouple integrin- and GPCR-dependent ROS production from FcγR responses based on their SKAP2 dependency, and show that SKAP2 is not required for degranulation.

Pathogenic Yersinia species carry a virulence plasmid encoding a type 3 secretion system that translocates 6–7 effector Yops into host cells. We demonstrate that YopH protects Yersinia pseudotuberculosis from neutrophil-produced reactive oxygen species (ROS) and degranulation by interfering with signaling pathways downstream of three major receptor classes in neutrophils. We show that a previously identified target of YopH, SKAP2, controls some of the pathways essential for YopH to inactivate during infection. SKAP2 is essential in mediating ROS production downstream of two major receptors; however, it is dispensable for degranulation from the three major receptors tested. Our study illustrates that YopH protects Y. pseudotuberculosis by blocking both SKAP2-dependent and independent signaling pathways that regulate several neutrophil functions.

SKAP2 is required for defense against K. pneumoniae infection and neutrophil respiratory burst

Klebsiella pneumoniae is a respiratory, blood, liver, and bladder pathogen of significant clinical concern. We show that the adaptor protein, SKAP2, is required for protection against K. pneumoniae (ATCC 43816) pulmonary infections. Skap2-/- mice had 100-fold higher bacterial burden when compared to wild-type and burden was controlled by SKAP2 expression in innate immune cells. Skap2-/- neutrophils and monocytes were present in infected lungs, and the neutrophils degranulated normally in response to K. pneumoniae infection in mice; however, K. pneumoniae-stimulated reactive oxygen species (ROS) production in vitro was abolished. K. pneumoniae-induced neutrophil ROS response required the activity of SFKs, Syk, Btk, PLCγ2, and PKC. The loss of SKAP2 significantly hindered the K. pneumoniae-induced phosphorylation of SFKs, Syk, and Pyk2 implicating SKAP2 as proximal to their activation in pathogen-signaling pathways. In conclusion, SKAP2-dependent signaling in neutrophils is essential for K. pneumoniae-activated ROS production and for promoting bacterial clearance during infection.

Klebsiella pneumoniae is a type of bacteria that can cause life-threatening infections – including pneumonia, blood stream infections, and urinary tract infections – in hospitalized patients. These infections can be difficult to treat because some K. pneumoniae are resistant to antibiotics. The bacteria are normally found in the human intestine, and they do not usually cause infections in healthy people. This implies that healthy people’s immune systems are better able to fend off K. pneumoniae infections; learning how could help scientists develop new ways to treat or prevent infections in hospitalized patients.

In healthy people, a type of immune cell called neutrophils are the first line of defense against bacterial infections. Several different proteins are needed to activate neutrophils, including a protein called SKAP2. But the role of this protein in fighting K. pneumoniae infections is not clear.

To find out what role SKAP2 plays in the defense against pneumonia caused by K. pneumoniae, Nguyen et al. compared infections in mice with and without the protein. Mice lacking SKAP2 in their white blood cells had more bacteria in their lungs than normal mice. The experiments showed that neutrophils from mice with SKAP2 produce a burst of chemicals called “reactive oxygen species”, which can kill bacteria. But neutrophils without the protein do not. Without SKAP2, several proteins that help produce reactive oxygen species do not work.

Understanding the role of SKAP2 in fighting infections may help scientists better understand the immune system. This could help clinicians to treat conditions that cause it to be hyperactive or ineffective. More studies are needed to determine if SKAP2 works the same way in human neutrophils and if it works against all types of K. pneumoniae. If it does, then scientists might be able use this information to develop therapies that help the immune system fight infections.

Immune Cell-Type Specific Ablation of Adapter Protein ADAP Differentially Modulates EAE

The cytosolic adhesion and degranulation-promoting adapter protein ADAP is expressed in various hematopoietic cells including T cells, NK cells, myeloid cells, and platelets but absent in mature B cells. The role of ADAP in T cell activation, proliferation and integrin activation is well-accepted. We previously demonstrated that conventional ADAP knockout mice show a significantly attenuated course of experimental autoimmune encephalomyelitis (EAE). To dissect the impact of different ADAP expressing cell populations on the reduced EAE severity, here, we generated lineage-specific conditional knockout mice. ADAP was deleted in T cells, myeloid cells, NK cells and platelets, respectively. Specific loss of ADAP was confirmed on the protein level. Detailed immunophenotyping was performed to assess the consequence of deletion of ADAP with regard to the maturation and distribution of immune cells in primary and secondary lymphoid organs. The analysis showed equivalent results as for conventional ADAP knockout mice: impaired thymocyte development in ADAP^fl/fl Lck-Cre mice, normal NK cell and myeloid cell distribution in ADAP^fl/fl NKp46-Cre mice and ADAP^fl/fl LysM-Cre mice, respectively as well as thrombocytopenia in ADAP^fl/fl PF4-Cre mice. Active EAE was induced in these animals by immunization with the myelin oligodendrocyte glycoprotein₃₅₋₅₅ peptide. The clinical course of EAE was significantly milder in mice with loss of ADAP in T cells, myeloid cells and NK cells compared to ADAP-sufficient control littermates. Surprisingly, specific deletion of ADAP in platelets resulted in a more exacerbated disease. These data show that T cell-independent as well as T cell-dependent mechanisms are responsible for the complex phenotype observed in conventional ADAP knockout mice.

Characterization of Mice with a Platelet-Specific Deletion of the Adapter Molecule ADAP

The adhesion and degranulation-promoting adapter protein (ADAP) is expressed in T cells, NK cells, myeloid cells, and platelets. The involvement of ADAP in the regulation of receptor-mediated inside-out signaling leading to integrin activation is well characterized, especially in T cells and in platelets. Due to the fact that animal studies using conventional knockout mice are limited by the overlapping effects of the different ADAP-expressing cells, we generated conditional ADAP knockout mice (ADAP^fl/fl PF4-Cre^tg) (PF4, platelet factor 4). We observed that loss of ADAP restricted to the megakaryocytic lineage has no impact on other hematopoietic cells even under stimulation conditions. ADAP^fl/fl PF4-Cre^tg mice showed thrombocytopenia in combination with reduced plasma levels of PF4 and transforming growth factor β1 (TGF-β1). In vitro, platelets from these mice revealed reduced P-selectin expression, lower levels of TGF-β1 release, diminished integrin αIIbβ3 activation, and decreased fibrinogen binding after stimulation with podoplanin, the ligand of C-type lectin-like receptor 2 (CLEC-2). Furthermore, loss of ADAP was associated with impaired CLEC-2-mediated activation of phospholipase Cγ2 (PLCγ2) and extracellular signal-regulated kinase 1/2 (ERK1/2). Induction of experimental autoimmune encephalomyelitis (EAE) in mice lacking ADAP expression in platelets caused a more severe disease. In vivo administration of TGF-β1 early after T cell transfer reduced EAE severity in mice with loss of ADAP restricted to platelets. Our results reveal a regulatory function of ADAP in platelets in vitro and during autoimmune disease EAE in vivo.

The SRC-family tyrosine kinase HCK shapes the landscape of SKAP2 interactome

The SRC Kinase Adaptor Phosphoprotein 2 (SKAP2) is a broadly expressed adaptor associated with the control of actin-polymerization, cell migration, and oncogenesis. After activation of different receptors at the cell surface, this dimeric protein serves as a platform for assembling other adaptors such as FYB and some SRC family kinase members, although these mechanisms are still poorly understood. The goal of this study is to map the SKAP2 interactome and characterize which domains or binding motifs are involved in these interactions. This is a prerequisite to finely analyze how these pathways are integrated in the cell machinery and to study their role in cancer and other human diseases when this network of interactions is perturbed. In this work, the domain and the binding motif of fourteen proteins interacting with SKAP2 were precisely defined and a new interactor, FAM102A was discovered. Herein, a fine-tuning between the binding of SRC kinases and their activation was identified. This last process, which depends on SKAP2 dimerization, indirectly affects the binding of FYB protein. Analysis of conformational changes associated with activation/inhibition of SRC family members, presently limited to their effect on kinase activity, is extended to their interactive network, which paves the way for therapeutic development.

Neutrophils to the ROScue: Mechanisms of NADPH Oxidase Activation and Bacterial Resistance

Reactive oxygen species (ROS) generated by NADPH oxidase play an important role in antimicrobial host defense and inflammation. Their deficiency in humans results in recurrent and severe bacterial infections, while their unregulated release leads to pathology from excessive inflammation. The release of high concentrations of ROS aids in clearance of invading bacteria. Localization of ROS release to phagosomes containing pathogens limits tissue damage. Host immune cells, like neutrophils, also known as PMNs, will release large amounts of ROS at the site of infection following the activation of surface receptors. The binding of ligands to G-protein-coupled receptors (GPCRs), toll-like receptors, and cytokine receptors can prime PMNs for a more robust response if additional signals are encountered. Meanwhile, activation of Fc and integrin directly induces high levels of ROS production. Additionally, GPCRs that bind to the bacterial-peptide analog fMLP, a neutrophil chemoattractant, can both prime cells and trigger low levels of ROS production. Engagement of these receptors initiates intracellular signaling pathways, resulting in activation of downstream effector proteins, assembly of the NADPH oxidase complex, and ultimately, the production of ROS by this complex. Within PMNs, ROS released by the NADPH oxidase complex can activate granular proteases and induce the formation of neutrophil extracellular traps (NETs). Additionally, ROS can cross the membranes of bacterial pathogens and damage their nucleic acids, proteins, and cell membranes. Consequently, in order to establish infections, bacterial pathogens employ various strategies to prevent restriction by PMN-derived ROS or downstream consequences of ROS production. Some pathogens are able to directly prevent the oxidative burst of phagocytes using secreted effector proteins or toxins that interfere with translocation of the NADPH oxidase complex or signaling pathways needed for its activation. Nonetheless, these pathogens often rely on repair and detoxifying proteins in addition to these secreted effectors and toxins in order to resist mammalian sources of ROS. This suggests that pathogens have both intrinsic and extrinsic mechanisms to avoid restriction by PMN-derived ROS. Here, we review mechanisms of oxidative burst in PMNs in response to bacterial infections, as well as the mechanisms by which bacterial pathogens thwart restriction by ROS to survive under conditions of oxidative stress.

Genome-wide association analysis for chronic venous disease identifies EFEMP1 and KCNH8 as susceptibility loci

Chronic venous disease (CVD) is a multifactorial condition representing one of the most common disorders among populations of Western countries. The heritability of about 17% suggests genetic risk factors in CVD etiology. However, so far the genetic causes are unknown. We undertook the hitherto first genome-wide association study (GWAS) for CVD, analyzing more than 1.93 M SNPs in 4,942 German individuals, followed by replication in two independent German data sets. The combined analysis of discovery and replication stages (2,269 cases and 7,765 controls) yielded robust associations within the two genes EFEMP1 and KCNH8 (rs17278665, rs727139 with P < 5 × 10⁻⁸), and suggestive association within gene SKAP2 (rs2030136 with P < 5 × 10⁻⁷). Association signals of rs17278665 and rs727139 reside in regions of low linkage disequilibrium containing no other genes. Data from the ENCODE and Roadmap Epigenomics projects show that tissue specific marks overlap with the variants. SNPs rs17278665 and rs2030136 are known eQTLs. Our study demonstrates that GWAS are a valuable tool to study the genetic component of CVD. With our approach, we identified two novel genome-wide significant susceptibility loci for this common disease. Particularly, the extracellular matrix glycoprotein EFEMP1 is promising for future functional studies due to its antagonistic role in vessel development and angiogenesis.

D120 and K152 within the PH Domain of T Cell Adapter SKAP55 Regulate Plasma Membrane Targeting of SKAP55 and LFA-1 Affinity Modulation in Human T Lymphocytes

The β2-integrin lymphocyte function-associated antigen 1 (LFA-1) is needed for the T cell receptor (TCR)-induced activation of LFA-1 to promote T cell adhesion and interaction with antigen-presenting cells (APCs). LFA-1-mediated cell-cell interactions are critical for proper T cell differentiation and proliferation. The Src kinase-associated phosphoprotein of 55 kDa (SKAP55) is a key regulator of TCR-mediated LFA-1 signaling (inside-out/outside-in signaling). To gain an understanding of how SKAP55 controls TCR-mediated LFA-1 activation, we assessed the functional role of its pleckstrin homology (PH) domain. We identified two critical amino acid residues within the PH domain of SKAP55, aspartic acid 120 (D120) and lysine 152 (K152). D120 facilitates the retention of SKAP55 in the cytoplasm of nonstimulated T cells, while K152 promotes SKAP55 membrane recruitment via actin binding upon TCR triggering. Importantly, the K152-dependent interaction of the PH domain with actin promotes the binding of talin to LFA-1, thus facilitating LFA-1 activation. These data suggest that K152 and D120 within the PH domain of SKAP55 regulate plasma membrane targeting and TCR-mediated activation of LFA-1.

Skap2 is required for β2 integrin-mediated neutrophil recruitment and functions

Integrin activation is required for neutrophil functions. Impaired integrin activation on neutrophils is the hallmark of leukocyte adhesion deficiency (LAD) syndrome in humans, characterized by impaired leukocyte recruitment and recurrent infections. The Src kinase-associated phosphoprotein 2 (Skap2) is involved in integrin functions in different leukocyte subtypes. However, the role of Skap2 in β2 integrin activation and neutrophil recruitment is unknown. In this study, we demonstrate the crucial role of Skap2 in regulating actin polymerization and binding of talin-1 and kindlin-3 to the β2 integrin cytoplasmic domain, thereby being indispensable for β2 integrin activation and neutrophil recruitment. The direct interaction of Skap2 with the Wiskott-Aldrich syndrome protein via its SH3 domain is critical for integrin activation and neutrophil recruitment in vivo. Furthermore, Skap2 regulates integrin-mediated outside-in signaling events and neutrophil functions. Thus, Skap2 is essential to activate the β2 integrins, and loss of Skap2 function is sufficient to cause a LAD-like phenotype in mice.

Integrin Activation Through the Hematopoietic Adapter Molecule ADAP Regulates Dendritic Development of Hippocampal Neurons

Integrin-mediated cell adhesion and signaling is of critical importance for neuronal differentiation. Recent evidence suggests that an “inside-out” activation of β1-integrin, similar to that observed in hematopoietic cells, contributes to the growth and branching of dendrites. In this study, we investigated the role of the hematopoietic adaptor protein adhesion and degranulation promoting adapter protein (ADAP) in these processes. We demonstrate the expression of ADAP in the developing and adult nervous hippocampus, and in outgrowing dendrites of primary hippocampal neurons. We further show that ADAP occurs in a complex with another adaptor protein signal-transducing kinase-associated phosphoprotein-homolog (SKAP-HOM), with the Rap1 effector protein RAPL and the Hippo kinase macrophage-stimulating 1 (MST1), resembling an ADAP/SKAP module that has been previously described in T-cells and is critically involved in “inside-out” activation of integrins. Knock down of ADAP resulted in reduced expression of activated β1-integrin on dendrites. It furthermore reduced the differentiation of developing neurons, as indicated by reduced dendrite growth and decreased expression of the dendritic marker microtubule-associated protein 2 (MAP2). Our data suggest that an ADAP-dependent integrin-activation similar to that described in hematopoietic cells contributes to the differentiation of neuronal cells.

Translational utility of experimental autoimmune encephalomyelitis: recent developments

Multiple sclerosis (MS) is a complex autoimmune condition with firmly established genetic and environmental components. Genome-wide association studies (GWAS) have revealed a large number of genetic polymorphisms in the vicinity of, and within, genes that associate to disease. However, the significance of these single-nucleotide polymorphisms in disease and possible mechanisms of action remain, with a few exceptions, to be established. While the animal model for MS, experimental autoimmune encephalomyelitis (EAE), has been instrumental in understanding immunity in general and mechanisms of MS disease in particular, much of the translational information gathered from the model in terms of treatment development (glatiramer acetate and natalizumab) has been extensively summarized. In this review, we would thus like to cover the work done in EAE from a GWAS perspective, highlighting the research that has addressed the role of different GWAS genes and their pathways in EAE pathogenesis. Understanding the contribution of these pathways to disease might allow for the stratification of disease subphenotypes in patients and in turn open the possibility for new and individualized treatment approaches in the future.

Spatial and temporal expression of RA70/Scap2 in the developing neural tube

Src kinase-associated phosphoprotein 2 (Ra70/scap2), which was originally isolated as a retinoic acid (RA)-induced gene, associates with molecules that modulate integrin-survival signals. Although RA is essential for vertebrate organogenesis in the posterior region, little is known about the biological role of RA70/Scap2 during development. In the present study, we demonstrate that Ra70/scap2 mRNA is temporally expressed during the RA-induced neuronal differentiation of P19 embryonic carcinoma cells. Homozygous knockout mice in which the Ra70/scap2 gene was replaced with LacZ exhibited embryonic lethality, while heterozygous mice displayed preferential expression of LacZ in posterior neural tissues, including the neural tube and hindbrain during development (E7.5-11.5), but not the forebrain. Ra70/scap2 was expressed in the ependymal layer and ventricular zone in the neural tube, where neuroepithelial cells and neuroblasts with proliferation capacity are localized, respectively. Thus, RA70/Scap2 may be necessary for RA-induced neuronal differentiation from the posterior neuroectoderm.

Identifying Yersinia YopH-targeted signal transduction pathways that impair neutrophil responses during in vivo murine infection

Identifying molecular targets of Yersinia virulence effectors, or Yops, during animal infection is challenging because few cells are targeted by Yops in an infected organ, and isolating these sparse effector-containing cells is difficult. YopH, a tyrosine phosphatase, is essential for full virulence of Yersinia. Investigating the YopH-targeted signal transduction pathway(s) in neutrophils during infection of a murine host, we find that several host proteins, including the essential signaling adaptor SLP-76, are dephosphorylated in the presence of YopH in neutrophils isolated from infected tissues. YopH inactivated PRAM-1/SKAP-HOM and the SLP-76/Vav/PLCγ2 signal transduction axes, leading to an inhibition of calcium response in isolated neutrophils. Consistent with a failure to mount a calcium response, IL-10 production was reduced in neutrophils containing YopH from infected tissues. Finally, a yopH mutant survived better in the absence of neutrophils, indicating that neutrophil inactivation by YopH by targeting PRAM-1/SKAP-HOM and SLP-76/Vav/PLCγ2 signaling hubs may be critical for Yersinia survival.

T cell-independent modulation of experimental autoimmune encephalomyelitis in ADAP-deficient mice

The adhesion- and degranulation-promoting adaptor protein (ADAP), expressed in T cells, myeloid cells, and platelets, is known to regulate receptor-mediated inside-out signaling leading to integrin activation and adhesion. In this study, we demonstrate that, upon induction of active experimental autoimmune encephalomyelitis (EAE) by immunization with the myelin oligodendrocyte glycoprotein35-55 peptide, ADAP-deficient mice developed a significantly milder clinical course of EAE and showed markedly less inflammatory infiltrates in the CNS than wild-type mice. Moreover, ADAP-deficient recipients failed to induce EAE after adoptive transfer of myelin oligodendrocyte glycoprotein-specific TCR-transgenic T cells (2D2 T cells). In addition, ex vivo fully activated 2D2 T cells induced significantly less severe EAE in ADAP-deficient recipients. The ameliorated disease in the absence of ADAP was not due to expansion or deletion of a particular T cell subset but rather because of a strong reduction of all inflammatory leukocyte populations invading the CNS. Monitoring the adoptively transferred 2D2 T cells over time demonstrated that they accumulated within the lymph nodes of ADAP-deficient hosts. Importantly, transfer of complete wild-type bone marrow or even bone marrow of 2D2 TCR-transgenic mice was unable to reconstitute EAE in the ADAP-deficient animals, indicating that the milder EAE was dependent on (a) radio-resistant nonhematopoietic cell population(s). Two-photon microscopy of lymph node explants revealed that adoptively transferred lymphocytes accumulated at lymphatic vessels in the lymph nodes of ADAP-deficient mice. Thus, our data identify a T cell-independent mechanism of EAE modulation in ADAP-deficient mice.

Regulation of the Src kinase-associated phosphoprotein 55 homologue by the protein tyrosine phosphatase PTP-PEST in the control of cell motility

PTP-PEST is a cytosolic ubiquitous protein tyrosine phosphatase (PTP) that contains, in addition to its catalytic domain, several protein-protein interaction domains that allow it to interface with several signaling pathways. Among others, PTP-PEST is a key regulator of cellular motility and cytoskeleton dynamics. The complexity of the PTP-PEST interactome underscores the necessity to identify its interacting partners and physiological substrates in order to further understand its role in focal adhesion complex turnover and actin organization. Using a modified yeast substrate trapping two-hybrid system, we identified a cytosolic adaptor protein named Src kinase-associated phosphoprotein 55 homologue (SKAP-Hom) as a novel substrate of PTP-PEST. To confirm PTP-PEST interaction with SKAP-Hom, in vitro pull down assays were performed demonstrating that the PTP catalytic domain and Proline-rich 1 (P1) domain are respectively binding to the SKAP-Hom Y260 and Y297 residues and its SH3 domain. Subsequently, we generated and rescued SKAP-Hom-deficient mouse embryonic fibroblasts (MEFs) with WT SKAP-Hom, SKAP-Hom tyrosine mutants (Y260F, Y260F/Y297F), or SKAP-Hom SH3 domain mutant (W335K). Given the role of PTP-PEST, wound-healing and trans-well migration assays were performed using the generated lines. Indeed, SKAP-Hom-deficient MEFs showed a defect in migration compared with WT-rescued MEFs. Interestingly, the SH3 domain mutant-rescued MEFs showed an enhanced cell migration corresponding potentially with higher tyrosine phosphorylation levels of SKAP-Hom. These findings suggest a novel role of SKAP-Hom and its phosphorylation in the regulation of cellular motility. Moreover, these results open new avenues by which PTP-PEST regulates cellular migration, a hallmark of metastasis.

The Src substrate SKAP2 regulates actin assembly by interacting with WAVE2 and cortactin proteins

In our attempt to screen for substrates of Src family kinases in glioblastoma, Src kinase-associated phosphoprotein 2 (SKAP2) was identified. Although SKAP2 has been suggested to be associated with integrin-mediated adhesion of hematopoietic cells, little is known about its molecular function and the effects in other types of cells and tumors. Here, we demonstrate that SKAP2 physically associates with actin assembly factors WAVE2 and cortactin and inhibits their interaction. Cortactin is required for the membrane localization of WAVE2, and SKAP2 suppresses actin polymerization mediated by WAVE2 and cortactin in vitro. Knockdown of SKAP2 in NIH3T3 accelerated cell migration and enhanced translocation of WAVE2 to the cell membrane, and those effects of SKAP2 depend on the binding activity of SKAP2 to WAVE2. Furthermore, reduction of SKAP2 in the glioblastoma promoted tumor invasion both in ex vivo organotypic rat brain slices and immune-deficient mouse brains. These results suggest that SKAP2 negatively regulates cell migration and tumor invasion in fibroblasts and glioblastoma cells by suppressing actin assembly induced by the WAVE2-cortactin complex, indicating that SKAP2 may be a novel candidate for the suppressor of tumor progression.

Macrophages require Skap2 and Sirpα for integrin-stimulated cytoskeletal rearrangement

Macrophages migrate to sites of insult during normal inflammatory responses. Integrins guide such migration, but the transmission of signals from integrins into the requisite cytoskeletal changes is poorly understood. We have discovered that the hematopoietic adaptor protein Skap2 is necessary for macrophage migration, chemotaxis, global actin reorganization and local actin reorganization upon integrin engagement. Binding of phosphatidylinositol [3,4,5]-triphosphate to the Skap2 pleckstrin-homology (PH) domain, which relieves its conformational auto-inhibition, is critical for this integrin-driven cytoskeletal response. Skap2 enables integrin-induced tyrosyl phosphorylation of Src-family kinases (SFKs), Adap, and Sirpα, establishing their roles as signaling partners in this process. Furthermore, macrophages lacking functional Sirpα unexpectedly have impaired local integrin-induced responses identical to those of Skap2(-/-) macrophages, and Skap2 requires Sirpα for its recruitment to engaged integrins and for coordinating downstream actin rearrangement. By revealing the positive-regulatory role of Sirpα in a Skap2-mediated mechanism connecting integrin engagement with cytoskeletal rearrangement, these data demonstrate that Sirpα is not exclusively immunoinhibitory, and illuminate previously unexplained observations implicating Skap2 and Sirpα in mouse models of inflammatory disease.

The adapter protein ADAP is required for selected dendritic cell functions

Background

The cytosolic adaptor protein ADAP (adhesion and degranulation promoting adapter protein) is expressed by T cells, natural killer cells, myeloid cells and platelets. ADAP is involved in T-cell-receptor-mediated inside-out signaling, which leads to integrin activation, adhesion and reorganization of the actin cytoskeleton. However, little is known about the role of ADAP in myeloid cells. In the present study, we analyzed the function of ADAP in bone-marrow-derived dendritic cells (BMDCs) from ADAP-deficient mice.

Results

ADAP-deficient BMDCs showed almost normal levels of antigen uptake, adhesion, maturation, migration from the periphery to the draining lymph nodes, antigen-specific T-cell activation, and production of the proinflammatory cytokines IL-6 and TNF-∝. Furthermore, we provide evidence that the activation of signaling pathways after lipopolysaccharide (LPS) stimulation are not affected by the loss of ADAP. In contrast, ADAP-deficient BMDCs showed defects in CD11c-mediated cellular responses, with significantly diminished production of IL-6, TNF-∝ and IL-10. Actin polymerization was enhanced after CD11c integrin stimulation.

Conclusions

In summary, we propose that the adapter molecule ADAP is critical for selected CD11c integrin-mediated functions of dendritic cells.

Overview of integrin signaling in the immune system

It has been well established that integrins mediate cell-cell and cell-matrix adhesion and play crucial roles in the immune system such as leukocyte-endothelium interactions, immune synapse formation, and effector functions. Since the discovery that integrins undergo dynamic changes of adhesive activities in response to external stimuli, intensive studies have been conducted to elucidate the signaling events that control the activation of integrins (inside-out signaling) and signaling events from the induced integrin-dependent adhesion (outside-in signaling). The molecular characterization of these signaling pathways highlights the importance of integrins as bidirectional signaling receptors. The characteristics of integrin signaling are best exemplified in the immune system. This chapter highlights the recent studies of intracellular signaling pathways that regulate integrins in immunological contexts.

CCR7-mediated LFA-1 functions in T cells are regulated by 2 independent ADAP/SKAP55 modules

The β2-integrin lymphocyte function-associated antigen-1 (LFA-1) plays a crucial role within the immune system. It regulates the interaction between T cells and antigen-presenting cells and facilitates T-cell adhesion to the endothelium, a process that is important for lymphocyte extravasation and homing. Signals mediated via the T-cell receptor and the chemokine receptor CCR7 activate LFA-1 through processes known as inside-out signaling. The molecular mechanisms underlying inside-out signaling are not completely understood. Here, we have assessed the role of the ADAP/SKAP55 module for CCR7-mediated signaling. We show that loss of the module delays homing and reduces intranodal T-cell motility in vivo. This is probably because of a defect in CCR7-mediated adhesion that affects both affinity and avidity regulation of LFA-1. Further analysis of how the ADAP/SKAP55 module regulates CCR7-induced integrin activation revealed that 2 independent pools of the module are expressed in T cells. One pool interacts with a RAPL/Mst1 complex, whereas the other pool is linked to a RIAM/Mst1/Kindlin-3 complex. Importantly, both the RAPL/Mst1 and the RIAM/Mst1/Kindlin-3 complexes require ADAP/SKAP55 for binding to LFA-1 upon CCR7 stimulation. Hence, 2 independent ADAP/SKAP55 modules are essential components of the signaling machinery that regulates affinity and avidity of LFA-1 in response to CCR7.

SKAP2, a novel target of HSF4b, associates with NCK2/F-actin at membrane ruffles and regulates actin reorganization in lens cell

In addition to roles in stress response, heat shock factors (HSFs) play crucial roles in differentiation and development. Heat shock transcription factor 4 (HSF4) deficiency leads to defect in lens epithelial cell (LEC) differentiation and cataract formation. However, the mechanism remains obscure. Here, we identified Src kinase-associated phosphoprotein 2 (SKAP2) as a downstream target of HSF4b and it was highly expressed at the anterior tip of lens elongating fibre cells in vivo. The HSF4-deficient lenses showed reduced SKAP2 expression and defects in actin reorganization. The disassembly of stress fibres and formation of cortical actin fibres are critical for the initiation of LEC differentiation. SKAP2 localized at actin-rich ruffles in human LECs (SRA01/04 cells) and knockdown SKAP2 using RNA interference impaired the disassembly of cellular stress fibres in response to fibroblast growth factor (FGF)-b. Overexpression of SKAP2, but not the N-terminal deletion mutant of SKAP2, induced the actin remodelling. We further found that SKAP2 interacted with the SH2 domain of non-catalytic region of tyrosine kinase adaptor protein 2 (NCK2) via its N-terminus. The complex of SKAP2-NCK2-F-actin accumulated at the leading edge of the lamellipodium, where FGF receptors and focal adhesion were also recruited. These results revealed an essential role for HSF4-mediated SKAP2 expression in the regulation of actin reorganization during lens differentiation, likely through a mechanism that SKAP2 anchors the complex of NCK2/focal adhesion to FGF receptors at the lamellipodium in lens epithelial cells.

Substrate specificity of lymphoid-specific tyrosine phosphatase (Lyp) and identification of Src kinase-associated protein of 55 kDa homolog (SKAP-HOM) as a Lyp substrate

A missense single-nucleotide polymorphism in the gene encoding the lymphoid-specific tyrosine phosphatase (Lyp) has been identified as a causal factor in a wide spectrum of autoimmune diseases. Interestingly, the autoimmune-predisposing variant of Lyp appears to represent a gain-of-function mutation, implicating Lyp as an attractive target for the development of effective strategies for the treatment of many autoimmune disorders. Unfortunately, the precise biological functions of Lyp in signaling cascades and cellular physiology are poorly understood. Identification and characterization of Lyp substrates will help define the chain of molecular events coupling Lyp dysfunction to diseases. In the current study, we identified consensus sequence motifs for Lyp substrate recognition using an "inverse alanine scanning" combinatorial library approach. The intrinsic sequence specificity data led to the discovery and characterization of SKAP-HOM, a cytosolic adaptor protein required for proper activation of the immune system, as a bona fide Lyp substrate. To determine the molecular basis for Lyp substrate recognition, we solved crystal structures of Lyp in complex with the consensus peptide as well as the phosphopeptide derived from SKAP-HOM. Together with the biochemical data, the structures define the molecular determinants for Lyp substrate specificity and provide a solid foundation upon which novel therapeutics targeting Lyp can be developed for multiple autoimmune diseases.

The pleckstrin homology domain in the SKAP55 adapter protein defines the ability of the adapter protein ADAP to regulate integrin function and NF-kappaB activation

Adhesion and degranulation promoting adapter protein (ADAP) is a multifunctional hematopoietic adapter protein that regulates TCR-dependent increases in both integrin function and activation of the NF-κB transcription factor. Activation of integrin function requires both ADAP and the ADAP-associated adapter Src kinase-associated phosphoprotein of 55 kDa (SKAP55). In contrast, ADAP-mediated regulation of NF-κB involves distinct binding sites in ADAP that promote the inducible association of ADAP, but not SKAP55, with the CARMA1 adapter and the TAK1 kinase. This suggests that the presence or absence of associated SKAP55 defines functionally distinct pools of ADAP. To test this hypothesis, we developed a novel SKAP-ADAP chimeric fusion protein and demonstrated that physical association of ADAP with SKAP55 is both sufficient and necessary for the rescue of integrin function in ADAP-deficient T cells. Similar to wild-type ADAP, the SKAP-ADAP chimera associated with the LFA-1 integrin after TCR stimulation. Although the SKAP-ADAP chimera contains the CARMA1 and TAK1 binding sequences from ADAP, expression of the chimera does not restore NF-κB signaling in ADAP(-/-) T cells. A single point mutation in the pleckstrin homology domain of SKAP55 (R131M) blocks the ability of the SKAP-ADAP chimera to restore integrin function and to associate with LFA-1. However, the R131M mutant was now able to restore NF-κB signaling in ADAP-deficient T cells. We conclude that integrin regulation by ADAP involves the recruitment of ADAP to LFA-1 integrin complexes by the pleckstrin homology domain of SKAP55, and this recruitment restricts the ability of ADAP to interact with the NF-κB signalosome and regulate NF-κB activation.

HPK1 associates with SKAP-HOM to negatively regulate Rap1-mediated B-lymphocyte adhesion

Background

Hematopoietic progenitor kinase 1 (HPK1) is a Ste20-related serine/threonine kinase activated by a range of environmental stimuli including genotoxic stress, growth factors, inflammatory cytokines and antigen receptor triggering. Being inducibly recruited to membrane-proximal signalling scaffolds to regulate NFAT, AP-1 and NFκB-mediated gene transcription in T-cells, the function of HPK1 in B-cells to date remains rather ill-defined.

Methodology/Principal Findings

By using two loss of function models, we show that HPK1 displays a novel function in regulating B-cell integrin activity. Wehi 231 lymphoma cells lacking HPK1 after shRNA mediated knockdown exhibit increased basic activation levels of Ras-related protein 1 (Rap1), accompanied by a severe lymphocyte function-associated antigen-1 (LFA-1) dependent homotypic aggregation and increased adhesion to intercellular adhesion molecule 1 (ICAM-1). The observed phenotype of enhanced integrin activity is caused downstream of Src, by a signalling module independent of PI3K and PLC, involving HPK1, SKAP55 homologue (SKAP-HOM) and Rap1-GTP-interacting adaptor molecule (RIAM). This alters actin dynamics and renders focal adhesion kinase (FAK) constitutively phosphorylated. Bone marrow and splenic B-cell development of HPK1^−/− mice are largely unaffected, except age-related tendencies for increased splenic cellularity and BCR downregulation. In addition, naïve splenic knockout B-cells appear hyperresponsive to a range of stimuli applied ex vivo as recently demonstrated by others for T-cells.

Conclusions/Significance

We therefore conclude that HPK1 exhibits a dual function in B-cells by negatively regulating integrin activity and controlling cellular activation, which makes it an interesting candidate to study in pathological settings like autoimmunity and cancer.

Adapters in the organization of mast cell signaling

Mast cells are pivotal in innate immunity and play an important role in amplifying adaptive immunity. Nonetheless, they have long been known to be central to the initiation of allergic disorders. This results from the dysregulation of the immune response whereby normally innocuous substances are recognized as non-self, resulting in the production of IgE antibodies to these 'allergens'. Preformed and newly synthesized inflammatory (allergic) mediators are released from the mast cell following allergen-mediated aggregation of allergen-specific IgE bound to the high-affinity receptors for IgE (FcepsilonRI). Thus, the process by which the mast cell is able to interpret the engagement of FcepsilonRI into the molecular events necessary for release of their allergic mediators is of considerable therapeutic interest. Unraveling these molecular events has led to the discovery of a functional class of proteins that are essential in organizing activated signaling molecules and in coordinating and compartmentalizing their activity. These so-called 'adapters' bind multiple signaling proteins and localize them to specific cellular compartments, such as the plasma membrane. This organization is essential for normal mast cell responses. Here, we summarize the role of adapter proteins in mast cells focusing on the most recent advances toward understanding how these molecules work upon FcepsilonRI engagement.

Phosphoinositide 3-kinase-regulated adapters in lymphocyte activation

Signaling via phosphoinositide 3-kinases (PI3Ks) has emerged as a central component of lymphocyte activation via immunoreceptors, costimulatory receptors, cytokine receptors, and chemokine receptors. The discovery of phosphoinositide-binding pleckstrin homology (PH) domains has substantially increased understanding of how PI3Ks activate cellular responses. Accumulating evidence indicates that PH-domain containing adapter molecules provide important links between PI3K and lymphocyte function. Here, we review data on PI3K-regulated adapter proteins of the Grb-associated binder (GAB), Src kinase-associated phosphoprotein (SKAP), and B-lymphocyte adapter molecule of 32 kDa (Bam32)/ dual-adapter for phosphotyrosine and 3-phosphoinositides (DAPP)/TAPP families, with a focus on the latter group. Current data support the model that recruitment of these adapters to the plasma membrane of activated lymphocytes is driven by the phosphoinositides phosphatidylinositol-3,4,5-tris-phosphate and phosphatidylinositol-3,4-bisphosphate, generated through the action of PI3Ks and under the regulatory control of lipid phosphatases Src homology 2 domain-containing inositol phosphatase (SHIP), phosphatase and tensin homolog, and inositol polyphosphate 4-phosphatase. At the plasma membrane, these adapters serve to assemble distinct protein complexes. Bam32/DAPP1 and SKAPs function to promote activation of monomeric guanosine triphosphatases, including Rac and Rap, and promote integrin activation, lymphocyte adhesion to matrix proteins, and cell:cell interactions between B and T lymphocytes. GABs can provide feedforward amplification or feedback inhibition of PI3K signaling. Current work is further defining the molecular interactions driven by these molecules and identifying the functions of TAPP adapters, which also appear to be involved in lymphocyte adhesion and are specific effectors downstream of the SHIP product phosphatidylinositol-3,4-bisphosphate.

Bam32/DAPP1 promotes B cell adhesion and formation of polarized conjugates with T cells

B cell Ag receptors function in both signaling activation of Ag-specific cells and in collecting specific Ag for presentation to T lymphocytes. Signaling via PI3K is required for BCR-mediated activation and Ag presentation functions; however, the relevant downstream targets of PI3K in B cells are incompletely defined. In this study, we have investigated the roles of the PI3K effector molecule Bam32/DAPP1 in BCR signaling and BCR-mediated Ag presentation functions. In mouse primary B cells, Bam32 was required for efficient activation of the GTPase Rac1 and downstream signaling to JNK, but not activation of BLNK, phospholipase C gamma2, or calcium responses. Consistent with a role of this adaptor in Rac-mediated cytoskeletal rearrangement, Bam32 was required for BCR-induced cell adhesion and spreading responses on ICAM-1 or fibronectin-coated surfaces. The function of Bam32 in promoting Rac activation and adhesion required tyrosine 139, a known site of phosphorylation by Lyn kinase. After BCR crosslinking by Ag, Bam32-deficient B cells are able to carry out the initial steps of Ag endocytosis and processing, but show diminished ability to form Ag-specific conjugates with T cells and polarize F-actin at the B-T interface. As a result, Bam32-deficient B cells were unable to efficiently activate Ag-specific T cells. Together, these results indicate that Bam32 serves to integrate PI3K and Src kinase signaling to promote Rac-dependent B cell adhesive interactions important for Ag presentation function.

Role for ADAP in shear flow-induced platelet mechanotransduction

Binding of platelets to fibrinogen via integrin alphaIIbbeta3 stimulates cytoskeletal reorganization and spreading. These responses depend on tyrosine phosphorylation of multiple proteins by Src family members and Syk. Among Src substrates in platelets is adhesion- and degranulation-promoting adapter protein (ADAP), an adapter with potential binding partners: SLP-76, VASP, and SKAP-HOM. During studies of platelet function under shear flow, we discovered that ADAP(-/-) mouse platelets, unlike ADAP+/+ platelets, formed unstable thrombi in response to carotid artery injury. Moreover, fibrinogen-adherent ADAP(-/-) platelets in shear flow ex vivo showed reduced spreading and smaller zones of contact with the matrix. These abnormalities were not observed under static conditions, and they could not be rescued by stimulating platelets with a PAR4 receptor agonist or by direct alphaIIbbeta3 activation with MnCl2, consistent with a defect in outside-in alphaIIbbeta3 signaling. ADAP+/+ platelets subjected to shear flow assembled F-actin-rich structures that colocalized with SLP-76 and the Rac1 exchange factor, phospho-Vav1. In contrast, platelets deficient in ADAP, but not those deficient in VASP or SKAP-HOM, failed to form these structures. These results establish that ADAP is an essential component of alphaIIbbeta3-mediated platelet mechanotransduction that promotes F-actin assembly and enables platelet spreading and thrombus stabilization under fluid shear stress.

Multiple loci comprising immune-related genes regulate experimental neuroinflammation

A 58 Mb region on rat chromosome 4 known to regulate experimental autoimmune encephalomyelitis (EAE) was genetically dissected. High-resolution linkage analysis in an advanced intercross line (AIL) revealed four quantitative trait loci (QTLs), Eae24-Eae27. Both Eae24 and Eae25 regulated susceptibility and severity phenotypes, whereas Eae26 regulated severity and Eae27 regulated susceptibility. Analyses of the humoral immune response revealed that the levels of serum anti-myelin oligodendrocyte glycoprotein (MOG) immunoglobin G1 (IgG1) antibodies are linked to Eae24 and anti-MOG IgG2b antibodies are linked to both Eae24 and Eae26. We tested the parental DA strain and six recombinant congenic strains that include overlapping fragments of this region in MOG-EAE. Eae24 and Eae25 showed significant protection during the acute phase of EAE, whereas Eae25 and Eae26 significantly modified severity but not susceptibility. The smallest congenic fragment, which carries Eae25 alone, influenced both susceptibility and severity, and protected from the chronic phase of disease. These results support the multiple QTLs identified in the AIL. By demonstrating several QTLs comprising immune-related genes, which potentially interact, we provide a significant step toward elucidation of the polygenically regulated pathogenesis of MOG-EAE and possibly multiple sclerosis (MS), and opportunities for comparative genetics and testing in MS case-control cohorts.

Gene expression profiling of aging in multiple mouse strains: identification of aging biomarkers and impact of dietary antioxidants

We used DNA microarrays to identify panels of transcriptional markers of aging that are differentially expressed in young (5 month) and old (25 month) mice of multiple inbred strains (129sv, BALB/c, CBA, DBA, B6, C3H and B6C3F(1)). In the heart, age-related changes of five genes were studied throughout the mouse lifespan: complement component 4, chemokine ligand 14, component of Sp100-rs, phenylalanine hydroxylase and src family associated phosphoprotein 2. A similar analysis in the brain (cerebellum) involved complement component 1q (alpha polypeptide), complement component 4, P lysozyme structural, glial fibrillary acidic protein and cathepsin S. Caloric restriction (CR) inhibited age-related expression of these genes in both tissues. Parametric analysis of gene set enrichment identified several biological processes that are induced with aging in multiple mouse strains. We also tested the ability of dietary antioxidants to oppose these transcriptional markers of aging. Lycopene, resveratrol, acetyl-l-carnitine and tempol were as effective as CR in the heart, and alpha-lipoic acid and coenzyme Q(10) were as effective as CR in the cerebellum. These findings suggest that transcriptional biomarkers of aging in mice can be used to estimate the efficacy of aging interventions on a tissue-specific basis.

Expression of SKAP-HOM in DCs is required for an optimal immune response in vivo

The cytosolic adaptor molecule SKAP-HOM, similar to the T cell-specific homologue SKAP55, interacts directly with ADAP, and both molecules are involved in inside-out signaling. Previous studies have shown that in the absence of SKAP-HOM, antigen receptor-triggered integrin-mediated adhesion is impaired severely in B cells but not in T cells. In addition, loss of SKAP-HOM results in a less severe clinical course of EAE. DCs are the most potent APCs and express SKAP-HOM. However, the role of SKAP-HOM in DCs remains unknown. Here, we assessed whether the reduced severity of EAE observed in SKAP-HOM-deficient mice is at least partially a result of an impaired cooperation between APCs and T cells. We demonstrate that migration of LC in vivo and the spontaneous motility of BMDCs in vitro are increased in the absence of SKAP-HOM. In contrast, triggering of the integrin results in a drastic decrease of DC motility and in enhanced actin polymerization in SKAP-HOM-deficient DCs. Furthermore, the antigen-dependent conjugate formed between wild-type T cells and SKAP-HOM(-/-) DCs is delayed in comparison with wild-type DCs. Strikingly, fewer antigen-specific T cells are induced by immunization with SKAP-HOM(-/-) BMDCs as compared with wild-type BMDCs in vivo. Thus, these findings suggest that SKAP-HOM expression in DCs is required for the induction of an optimal immune response.

Identification and analysis of differentially expressed genes in immune tissues of Atlantic cod stimulated with formalin-killed, atypical Aeromonas salmonicida

Physiological changes, elicited in animal immune tissues by exposure to pathogens, may be studied using functional genomics approaches. We created and characterized reciprocal suppression subtractive hybridization (SSH) cDNA libraries to identify differentially expressed genes in spleen and head kidney tissues of Atlantic cod (Gadus morhua) challenged with intraperitoneal injections of formalin-killed, atypical Aeromonas salmonicida. Of 4,154 ESTs from four cDNA libraries, 10 genes with immune-relevant functional annotations were selected for QPCR studies using individual fish templates to assess biological variability. Genes confirmed by QPCR as upregulated by A. salmonicida included interleukin-1 beta, interleukin-8, a small inducible cytokine, interferon regulatory factor 1 (IRF1), ferritin heavy subunit, cathelicidin, and hepcidin. This study is the first large-scale discovery of bacteria-responsive genes in cod and the first to demonstrate upregulation of IRF1 in fish immune tissues as a result of bacterial antigen stimulation. Given the importance of IRF1 in vertebrate immune responses to viral and bacterial pathogens, the full-length cDNA sequence of Atlantic cod IRF1 was obtained and compared with putative orthologous sequences from other organisms. Functional annotations of assembled SSH library ESTs showed that bacterial antigen stimulation caused changes in many biological processes including chemotaxis, regulation of apoptosis, antimicrobial peptide production, and iron homeostasis. Moreover, differences in spleen and head kidney gene expression responses to the bacterial antigens pointed to a potential role for the cod spleen in blood-borne pathogen clearance. Our data show that Atlantic cod immune tissue responses to bacterial antigens are similar to those seen in other fish species and higher vertebrates.

The Skap-hom dimerization and PH domains comprise a 3'-phosphoinositide-gated molecular switch

PH domains, by binding to phosphoinositides, often serve as membrane-targeting modules. Using crystallographic, biochemical, and cell biological approaches, we have uncovered a mechanism that the integrin-signaling adaptor Skap-hom uses to mediate cytoskeletal interactions. Skap-hom is a homodimer containing an N-terminal four-helix bundle dimerization domain, against which its two PH domains pack in a conformation incompatible with phosphoinositide binding. The isolated PH domains bind PI[3,4,5]P(3), and mutations targeting the dimerization domain or the PH domain's PI[3,4,5]P(3)-binding pocket prevent Skap-hom localization to ruffles. Targeting is retained when the PH domain is deleted or by combined mutation of the PI[3,4,5]P(3)-binding pocket and the PH/dimerization domain interface. Thus, the dimerization and PH domain form a PI[3,4,5]P(3)-responsive molecular switch that controls Skap-hom function.

Distinct regulation of integrin-dependent T cell conjugate formation and NF-kappa B activation by the adapter protein ADAP

Following TCR stimulation, T cells utilize the hematopoietic specific adhesion and degranulation-promoting adapter protein (ADAP) to control both integrin adhesive function and NF-kappaB transcription factor activation. We have investigated the molecular basis by which ADAP controls these events in primary murine ADAP(-/-) T cells. Naive DO11.10/ADAP(-/-) T cells show impaired adhesion to OVAp (OVA aa 323-339)-bearing APCs that is restored following reconstitution with wild-type ADAP. Mutational analysis demonstrates that the central proline-rich domain and the C-terminal domain of ADAP are required for rescue of T:APC conjugate formation. The ADAP proline-rich domain is sufficient to bind and stabilize the expression of SKAP55 (Src kinase-associated phosphoprotein of 55 kDa), which is otherwise absent from ADAP(-/-) T cells. Interestingly, forced expression of SKAP55 in the absence of ADAP is insufficient to drive T:APC conjugate formation, demonstrating that both ADAP and SKAP55 are required for optimal LFA-1 function. Additionally, the ADAP proline-rich domain is required for optimal Ag-induced activation of CD69, CD25, and Bcl-x(L), but is not required for assembly of the CARMA1/Bcl10/Malt1 (caspase-recruitment domain (CARD) membrane-associated guanylate kinase (MAGUK) protein 1/B-cell CLL-lymphoma 10/mucosa-associated lymphoid tissue lymphoma translocation protein 1) signaling complex and subsequent TCR-dependent NF-kappaB activity. Our results indicate that ADAP is used downstream of TCR engagement to delineate two distinct molecular programs in which the ADAP/SKAP55 module is required for control of T:APC conjugate formation and functions independently of ADAP/CARMA1-mediated NF-kappaB activation.