T cell receptor engagement leads to the recruitment of IBP, a novel guanine nucleotide exchange factor, to the immunological synapse

Detection and Analysis of Genes Affecting the Number of Thoracic Vertebrae in Licha Black Pigs

The number of thoracic vertebrae (NTV) in pigs is an important economic trait that significantly influences pork production. While the Licha black pig is a well-known Chinese pig breed with multiple thoracic vertebrae, the genetic mechanism is still unknown. Here, we performed a selective signal analysis on the genome of Licha black pigs, comparing individuals with 15 NTV versus those with 16 NTV to better understand functional genes associated with NTV. A total of 2265 selection signal regions were detected across the genome, including 527 genes and 1073 QTL that overlapped with the selection signal regions. Functional enrichment analysis revealed that LRP5 and SP5 genes were involved in biological processes such as bone morphogenesis and Wnt protein binding. Furthermore, three genes, LRP8, DEF6, and SCUBE3, associated with osteoblast differentiation and bone formation, were located within or close to the QTL related to bone development and vertebrae number. These five genes were hypothesized to be potential candidates for regulating the NTV trait in Licha black pigs. Our findings revealed several candidate genes that play crucial roles in NTV regulation and provide a theoretical foundation to understand the genetic mechanism of the NTV trait in pig breeding.

Human iPSC-Derived Proinflammatory Macrophages cause Insulin Resistance in an Isogenic White Adipose Tissue Microphysiological System

Chronic white adipose tissue (WAT) inflammation has been recognized as a critical early event in the pathogenesis of obesity-related disorders. This process is characterized by the increased residency of proinflammatory M1 macrophages in WAT. However, the lack of an isogenic human macrophage-adipocyte model has limited biological studies and drug discovery efforts, highlighting the need for human stem cell-based approaches. Here, human induced pluripotent stem cell (iPSC) derived macrophages (iMACs) and adipocytes (iADIPOs) are cocultured in a microphysiological system (MPS). iMACs migrate toward and infiltrate into the 3D iADIPOs cluster to form crown-like structures (CLSs)-like morphology around damaged iADIPOs, recreating classic histological features of WAT inflammation seen in obesity. Significantly more CLS-like morphologies formed in aged and palmitic acid-treated iMAC-iADIPO-MPS, showing the ability to mimic inflammatory severity. Importantly, M1 (proinflammatory) but not M2 (tissue repair) iMACs induced insulin resistance and dysregulated lipolysis in iADIPOs. Both RNAseq and cytokines analyses revealed a reciprocal proinflammatory loop in the interactions of M1 iMACs and iADIPOs. This iMAC-iADIPO-MPS thus successfully recreates pathological conditions of chronically inflamed human WAT, opening a door to study the dynamic inflammatory progression and identify clinically relevant therapies.

DEF6(differentially exprehomolog) exacerbates pathological cardiac hypertrophy via RAC1

Pathological cardiac hypertrophy involves multiple regulators and several signal transduction pathways. Currently, the mechanisms of it are not well understood. Differentially expressed in FDCP 6 homolog (DEF6) was reported to participate in immunity, bone remodeling, and cancers. The effects of DEF6 on pathological cardiac hypertrophy, however, have not yet been fully characterized. We initially determined the expression profile of DEF6 and found that DEF6 was upregulated in hypertrophic hearts and cardiomyocytes. Our in vivo results revealed that DEF6 deficiency in mice alleviated transverse aortic constriction (TAC)-induced cardiac hypertrophy, fibrosis, dilation and dysfunction of left ventricle. Conversely, cardiomyocyte-specific DEF6-overexpression aggravated the hypertrophic phenotype in mice under chronic pressure overload. Similar to the animal experiments, the in vitro data showed that adenovirus-mediated knockdown of DEF6 remarkably inhibited phenylephrine (PE)-induced cardiomyocyte hypertrophy, whereas DEF6 overexpression exerted the opposite effects. Mechanistically, exploration of the signal pathways showed that the mitogen-activated extracellular signal-regulated kinase 1/2 (MEK1/2)-extracellular signal-regulated kinase 1/2 (ERK1/2) cascade might be involved in the prohypertrophic effect of DEF6. Coimmunoprecipitation and GST (glutathione S-transferase) pulldown analyses demonstrated that DEF6 can directly interact with small GTPase Ras-related C3 botulinum toxin substrate 1 (Rac1), and the Rac1 activity assay revealed that the activity of Rac1 is altered with DEF6 expression in TAC-cardiac hypertrophy and PE-triggered cardiomyocyte hypertrophy. In the end, western blot and rescue experiments using Rac1 inhibitor NSC23766 and the constitutively active mutant Rac1(G12V) verified the requirement of Rac1 and MEK1/2-ERK1/2 activation for DEF6-mediated pathological cardiac hypertrophy. Our study substantiates that DEF6 acts as a deleterious regulator of cardiac hypertrophy by activating the Rac1 and MEK1/2-ERK1/2 signaling pathways, and suggests that DEF6 may be a potential treatment target for heart failure.

Molecular mechanisms controlling age-associated B cells in autoimmunity

Age-associated B cells (ABCs) have emerged as critical components of immune responses. Their inappropriate expansion and differentiation have increasingly been linked to the pathogenesis of autoimmune disorders, aging-associated diseases, and infections. ABCs exhibit a distinctive phenotype and, in addition to classical B cell markers, often express the transcription factor T-bet and myeloid markers like CD11c; hence, these cells are also commonly known as CD11c+ T-bet+ B cells. Formation of ABCs is promoted by distinctive combinations of innate and adaptive signals. In addition to producing antibodies, these cells display antigen-presenting and proinflammatory capabilities. It is becoming increasingly appreciated that the ABC compartment exhibits a high degree of heterogeneity, plasticity, and sex-specific regulation and that ABCs can differentiate into effector progeny via several routes particularly in autoimmune settings. In this review, we will discuss the initial insights that have been obtained on the molecular machinery that controls ABCs and we will highlight some of the unique aspects of this control system that may enable ABCs to fulfill their distinctive role in immune responses. Given the expanding array of autoimmune disorders and pathophysiological settings in which ABCs are being implicated, a deeper understanding of this machinery could have important and broad therapeutic implications for the successful, albeit daunting, task of targeting these cells.

Def6 regulates endogenous type-I interferon responses in osteoblasts and suppresses osteogenesis

Bone remodeling involves a balance between bone resorption and formation. The mechanisms underlying bone remodeling are not well understood. DEF6 is recently identified as a novel loci associated with bone mineral density. However, it is unclear how Def6 impacts bone remodeling. We identify Def6 as a novel osteoblastic regulator that suppresses osteoblastogenesis and bone formation. Def6 deficiency enhances both bone resorption and osteogenesis. The enhanced bone resorption in Def6-/- mice dominates, leading to osteoporosis. Mechanistically, Def6 inhibits the differentiation of both osteoclasts and osteoblasts via a common mechanism through endogenous type-I IFN-mediated feedback inhibition. RNAseq analysis shows expression of a group of IFN stimulated genes (ISGs) during osteoblastogenesis. Furthermore, we found that Def6 is a key upstream regulator of IFNβ and ISG expression in osteoblasts. Collectively, our results identify a novel immunoregulatory function of Def6 in bone remodeling, and shed insights into the interaction between immune system and bone.

MicroRNA Regulation of the Small Rho GTPase Regulators-Complexities and Opportunities in Targeting Cancer Metastasis

The small Rho GTPases regulate important cellular processes that affect cancer metastasis, such as cell survival and proliferation, actin dynamics, adhesion, migration, invasion and transcriptional activation. The Rho GTPases function as molecular switches cycling between an active GTP-bound and inactive guanosine diphosphate (GDP)-bound conformation. It is known that Rho GTPase activities are mainly regulated by guanine nucleotide exchange factors (RhoGEFs), GTPase-activating proteins (RhoGAPs), GDP dissociation inhibitors (RhoGDIs) and guanine nucleotide exchange modifiers (GEMs). These Rho GTPase regulators are often dysregulated in cancer; however, the underlying mechanisms are not well understood. MicroRNAs (miRNAs), a large family of small non-coding RNAs that negatively regulate protein-coding gene expression, have been shown to play important roles in cancer metastasis. Recent studies showed that miRNAs are capable of directly targeting RhoGAPs, RhoGEFs, and RhoGDIs, and regulate the activities of Rho GTPases. This not only provides new evidence for the critical role of miRNA dysregulation in cancer metastasis, it also reveals novel mechanisms for Rho GTPase regulation. This review summarizes recent exciting findings showing that miRNAs play important roles in regulating Rho GTPase regulators (RhoGEFs, RhoGAPs, RhoGDIs), thus affecting Rho GTPase activities and cancer metastasis. The potential opportunities and challenges for targeting miRNAs and Rho GTPase regulators in treating cancer metastasis are also discussed. A comprehensive list of the currently validated miRNA-targeting of small Rho GTPase regulators is presented as a reference resource.

Human DEF6 deficiency underlies an immunodeficiency syndrome with systemic autoimmunity and aberrant CTLA-4 homeostasis

Immune responses need to be controlled tightly to prevent autoimmune diseases, yet underlying molecular mechanisms remain partially understood. Here, we identify biallelic mutations in three patients from two unrelated families in differentially expressed in FDCP6 homolog (DEF6) as the molecular cause of an inborn error of immunity with systemic autoimmunity. Patient T cells exhibit impaired regulation of CTLA-4 surface trafficking associated with reduced functional CTLA-4 availability, which is replicated in DEF6-knockout Jurkat cells. Mechanistically, we identify the small GTPase RAB11 as an interactor of the guanine nucleotide exchange factor DEF6, and find disrupted binding of mutant DEF6 to RAB11 as well as reduced RAB11⁺CTLA-4⁺ vesicles in DEF6-mutated cells. One of the patients has been treated with CTLA-4-Ig and achieved sustained remission. Collectively, we uncover DEF6 as player in immune homeostasis ensuring availability of the checkpoint protein CTLA-4 at T-cell surface, identifying a potential target for autoimmune and/or cancer therapy.

CTLA-4 is critical for balancing protective immunity with self-tolerance. Here the authors identify homozygous DEF6 mutations in patients with severe autoimmunity, one of which received and responds to CTLA-4-Ig, and show that DEF6 is crucial for CTLA-4 cell surface trafficking and immune regulatory function.

Overexpression and clinical significance of IBP in epithelial ovarian carcinoma

Interferon regulatory factor-4 binding protein (IBP) is as a type of ρ GTPase suggested to serve an important role in tumor occurrence and development through the effects of cytoskeletal remodeling, and cell conduction mechanism. IBP is widely expressed in the immune system and expressed in several types of tumors. However, its expression and prognostic value in epithelial ovarian carcinoma (EOC) remain unclear. The present study aimed to investigate the expression of IBP in EOC, and its effect on clinicopathological variables and prognosis. A total of 107 and 30 cases of epithelial ovarian carcinoma and benign ovarian disease tissue sections, respectively, were examined using immunohistochemistry. The results indicated that the IBP expression status was negative or markedly weak in normal tissues, but highly expressed in EOC tissues. A significant association was observed between IBP overexpression and various clinicopathological factors, including advanced International Federation of Gynecology and Obstetrics stage (P<0.001), poor histologic grade (P=0.002), peritoneal carcinomatosis (P<0.001), lymph-node metastasis (P=0.023) and recurrence (P<0.001). Multivariate Cox regression analysis additionally suggested that IBP overexpression was an independent factor affecting recurrence-free survival [hazard ratio (HR)=4.099; 95% confidence interval (CI), 2.209-7.606; P<0.001) and overall survival (HR=2.317; 95% CI, 1.484-3.617; P<0.001) in patients with EOC. The results of the present study demonstrated that IBP overexpression may be associated with tumor development and progression in EOC, and therefore may serve as a novel target for treating this disease.

Phosphoproteomics Reveals Regulatory T Cell-Mediated DEF6 Dephosphorylation That Affects Cytokine Expression in Human Conventional T Cells

Regulatory T cells (Tregs) control key events of immune tolerance, primarily by suppression of effector T cells. We previously revealed that Tregs rapidly suppress T cell receptor (TCR)-induced calcium store depletion in conventional CD4⁺CD25⁻ T cells (Tcons) independently of IP₃ levels, consequently inhibiting NFAT signaling and effector cytokine expression. Here, we study Treg suppression mechanisms through unbiased phosphoproteomics of primary human Tcons upon TCR stimulation and Treg-mediated suppression, respectively. Tregs induced a state of overall decreased phosphorylation as opposed to TCR stimulation. We discovered novel phosphosites (T595_S597) in the DEF6 (SLAT) protein that were phosphorylated upon TCR stimulation and conversely dephosphorylated upon coculture with Tregs. Mutation of these DEF6 phosphosites abrogated interaction of DEF6 with the IP₃ receptor and affected NFAT activation and cytokine transcription in primary Tcons. This novel mechanism and phosphoproteomics data resource may aid in modifying sensitivity of Tcons to Treg-mediated suppression in autoimmune disease or cancer.

Regulation of systemic autoimmunity and CD11c+ Tbet+ B cells by SWEF proteins

Recent studies have revealed the existence of a T-bet dependent subset of B cells, which expresses unique phenotypic and functional characteristics including high levels of CD11c and CD11b. In the murine system this B cell subset has been termed Age/autoimmune-associated B cells (ABCs) since it expands with age in non-autoimmune mice and it prematurely accumulates in autoimmune-prone strains. The molecular mechanisms that promote the expansion and function of ABCs are largely unknown. This review will focus on the SWEF proteins, a small family of Rho GEFs comprised of SWAP-70 and its homolog DEF6, a newly identified risk variant for human SLE. We will first provide an overview of the SWEF proteins and then discuss the complex array of biological processes that they control and the autoimmune phenotypes that spontaneously develop in their absence, highlighting the emerging involvement of these proteins in regulating ABCs. A better understanding of the pathways controlled by the SWEF proteins could help provide new insights into the mechanisms responsible for the expansion of ABCs in autoimmunity and potentially guide the design of novel therapeutic approaches.

Regulation of Effector Treg Cells in Murine Lupus

OBJECTIVE

Treg cells need to acquire an effector phenotype to function in settings of inflammation. Whether effector Treg cells can limit disease severity in lupus is unknown. Interferon regulatory factor 4 (IRF-4) is an essential controller of effector Treg cells and regulates their ability to express interleukin-10 (IL-10). In non-Treg cells, IRF-4 activity is modulated by interactions with DEF-6 and its homolog switch-associated protein 70 (SWAP-70). Although mice lacking both DEF-6 and SWAP-70 (double-knockout [DKO] mice) develop lupus, they display normal survival, suggesting that in DKO mice, Treg cells can moderate disease development. The purpose of this study was to investigate whether Treg cells from DKO mice have an increased capacity to become effector Treg cells due to the ability of DEF-6 and SWAP-70 to restrain IRF-4 activity.

METHODS

Treg cells were evaluated by fluorescence-activated cell sorting. The B lymphocyte-induced maturation protein 1 (BLIMP-1)/IL-10 axis was assessed by crossing DKO mice with BLIMP-1-YFP-10BiT dual-reporter mice. Deletion of IRF-4 in Treg cells from DKO mice was achieved by generating FoxP3(Cre) IRF-4(fl/fl) DKO mice.

RESULTS

The concomitant absence of DEF-6 and SWAP-70 led to increased numbers of Treg cells, which acquired an effector phenotype in a cell-intrinsic manner. In addition, Treg cells from DKO mice exhibited enhanced expression of the BLIMP-1/IL-10 axis. Notably, DKO effector Treg cells survived and expanded as disease progressed. The accumulation of Treg cells from DKO mice was associated with the up-regulation of genes controlling autophagy. IRF-4 was required for the expansion and function of effector Treg cells from DKO mice.

CONCLUSION

This study revealed the existence of mechanisms that, by acting on IRF-4, can fine-tune the function and survival of effector Treg cells in lupus. These findings suggest that the existence of a powerful effector Treg cell compartment that successfully survives in an unfavorable inflammatory environment could limit disease development.

Role of methionine adenosyltransferase 2A in bovine preimplantation development and its associated genomic regions

Methionine adenosyltransferase (MAT) is involved in folate-mediated one-carbon metabolism, which is essential for preimplantation embryos in terms of both short-term periconceptional development and long-term phenotypic programming beyond the periconceptional period. Here, our immunofluorescence analysis of bovine oocytes and preimplantation embryos revealed the consistent expression of MAT2A (the catalytic subunit of the ubiquitously expressed-type of MAT isozyme) during this period. Addition of the MAT2A inhibitor FIDAS to the culture media of bovine preimplantation embryos reduced their blastocyst development, revealing the particular importance of MAT2A in successful blastocyst development. Exploration of MAT2A-associated genomic regions in bovine blastocysts using chromatin immunoprecipitation and sequencing (ChIP-seq) identified candidate MAT2A-associated genes implicated not only in short-term periconceptional embryo development, but also in long-term phenotypic programming during this period in terms of growth, metabolism, and immune functions. These results suggest the critical involvement of MAT2A in the periconceptional period in life-long programming of health and disease as well as successful preimplantation development.

Identification of a Novel Alternatively Spliced Form of Inflammatory Regulator SWAP-70-Like Adapter of T Cells

Activation of naive CD4⁺ T cells results in the development of several distinct subsets of effector Th cells, including Th2 cells that play a pivotal role in allergic inflammation and helminthic infections. SWAP-70-like adapter of T cells (SLAT), also known as Def6 or IBP, is a guanine nucleotide exchange factor for small GTPases, which regulates CD4⁺ T cell inflammatory responses by controlling Ca²⁺/NFAT signaling. In this study, we have identified a novel alternatively spliced isoform of SLAT, named SLAT2, which lacks the region encoded by exons 2-7 of the Def6 gene. SLAT2 was selectively expressed in differentiated Th2 cells after the second round of in vitro stimulation, but not in differentiated Th1, Th17, or regulatory T (Treg) cells. Functional assays revealed that SLAT2 shared with SLAT the ability to enhance T cell receptor- (TCR-) mediated activation of NFAT and production of IL-4 but was unable to enhance TCR-induced adhesion to ICAM-1. Ectopic expression of SLAT2 or SLAT in Jurkat T cells resulted in the expression of distinct forms of filopodia, namely, short versus long ones, respectively. These results demonstrate that modulating either SLAT2 or SLAT protein expression could play critical roles in cytokine production and actin reorganization during inflammatory immune responses.

Def6 Restrains Osteoclastogenesis and Inflammatory Bone Resorption

Inflammatory bone resorption mediated by osteoclasts is a major cause of morbidity and disability in many inflammatory disorders, including rheumatoid arthritis (RA). The mechanisms that regulate osteoclastogenesis and bone resorption in inflammatory settings are complex and have not been well elucidated. In this study, we identify the immunoregulator differentially expressed in FDCP 6 homolog (Def6) as a novel inhibitor of osteoclastogenesis in physiological and inflammatory conditions. Def6 deficiency in Def6^-/- mice enhanced the sensitivity of osteoclast precursors to the physiological osteoclastogenic inducer receptor activator for NF-κB ligand, and Def6^-/- osteoclasts formed actin rings. Furthermore, Def6 deficiency markedly increased TNF-α-induced osteoclastogenesis in vitro and in vivo and enhanced bone resorption in an inflammatory osteolysis mouse model. TNF-α serum levels correlated negatively with Def6 expression levels in osteoclast precursors obtained from RA patients, and the osteoclastogenic capacity of the osteoclast precursors was significantly inversely correlated with their Def6 expression levels, indicating that Def6 functions as an inhibitor of excessive osteoclast formation and bone destruction in RA. Mechanistically, Def6 suppressed osteoclastogenesis and the expression of key osteoclastogenic factors NFATc1, B lymphocyte-induced maturation protein-1, and c-Fos by regulating an endogenous IFN-β-mediated autocrine feedback loop. The Def6-dependent pathway may represent a novel therapeutic target to prevent pathological bone destruction.

The F-actin modulator SWAP-70 controls podosome patterning in osteoclasts

Osteoclasts are bone resorbing cells acting as key mediators of bone disorders. Upon adhesion to bone, osteoclasts polarize and reorganize their cytoskeleton to generate a ring-like F-actin-rich structure, the sealing zone, wherein the osteoclast's resorptive organelle, the ruffled border, is formed. The dynamic self-organization of actin-rich adhesive structures, the podosomes, from clusters to belts is crucial for osteoclast-mediated bone degradation. Mice lacking the protein SWAP-70 display an osteopetrotic phenotype due to defective bone resorption caused by impaired actin ring formation in Swap-70^−/− osteoclasts. To further elucidate the mechanisms underlying this defect, we investigated the specific function of SWAP-70 in the organization and dynamics of podosomes. These detailed studies show that the transition from podosome clusters to rings is impaired in Swap-70^−/− osteoclasts. Live cell imaging of dynamic F-actin turnover and SWAP-70 localization during podosome patterning indicate that SWAP-70 is dispensable for cluster formation but plays a key role in F-actin ring generation. Our data provide insights in the role of SWAP-70's F-actin binding domain and pleckstrin homology (PH) domain in the proper localization of SWAP-70 and formation of a peripheral podosome belt, respectively. Ex vivo bone analyses revealed that SWAP-70-deficient osteoclasts exhibit defective ruffled border formation and V-ATPase expression. Our findings suggest an important role of membrane binding of SWAP-70 for the regulation of actin dynamics, which is essential for podosome patterning, and thus for the resorptive activity of osteoclasts.

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SWAP-70 controls dynamic podosome patterning but not assembly of podosomes.

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PIP3 and F-actin binding are required for proper subcellular localization of SWAP-70.

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SWAP-70-deficient osteoclasts are impaired in ruffled border formation.

High-density genotyping of immune-related loci identifies new SLE risk variants in individuals with Asian ancestry

Systemic lupus erythematosus (SLE) has a strong but incompletely understood genetic architecture. We conducted an association study with replication in 4,478 SLE cases and 12,656 controls from six East Asian cohorts to identify new SLE susceptibility loci and better localize known loci. We identified ten new loci and confirmed 20 known loci with genome-wide significance. Among the new loci, the most significant locus was GTF2IRD1-GTF2I at 7q11.23 (rs73366469, Pmeta = 3.75 × 10(-117), odds ratio (OR) = 2.38), followed by DEF6, IL12B, TCF7, TERT, CD226, PCNXL3, RASGRP1, SYNGR1 and SIGLEC6. We identified the most likely functional variants at each locus by analyzing epigenetic marks and gene expression data. Ten candidate variants are known to alter gene expression in cis or in trans. Enrichment analysis highlights the importance of these loci in B cell and T cell biology. The new loci, together with previously known loci, increase the explained heritability of SLE to 24%. The new loci share functional and ontological characteristics with previously reported loci and are possible drug targets for SLE therapeutics.

F-actin remodeling defects as revealed in primary immunodeficiency disorders

Primary immunodeficiencies (PIDs) are a heterogeneous group of immune-related diseases. PIDs develop due to defects in gene-products that have consequences to immune cell function. A number of PID-proteins is involved in the remodeling of filamentous actin (f-actin) to support the generation of a contact zone between the antigen-specific T cell and antigen presenting cell (APC): the immunological synapse (IS). IS formation is the first step towards T-cell activation and essential for clonal expansion and acquisition of effector function. We here evaluated PIDs in which aberrant f-actin-driven IS formation may contribute to the PID disease phenotypes as seen in patients. We review examples of such contributions to PID phenotypes from literature, and highlight cases in which PID-proteins were evaluated for a role in f-actin polymerization and IS formation. We conclude with the proposition that patient groups might benefit from stratifying them in distinct functional groups in regard to their f-actin remodeling phenotypes in lymphocytes.

SLAT promotes TCR-mediated, Rap1-dependent LFA-1 activation and adhesion through interaction of its PH domain with Rap1

SLAT (also known as DEF6) promotes T cell activation and differentiation by regulating NFAT-Ca(2+) signaling. However, its role in TCR-mediated inside-out signaling, which induces integrin activation and T cell adhesion, a central process in T cell immunity and inflammation, has not been explored. Here, we show that SLAT is crucial for TCR-induced adhesion to ICAM-1 and affinity maturation of LFA-1 in CD4(+) T cells. Mechanistic studies revealed that SLAT interacts, through its PH domain, with a key component of inside-out signaling, namely the active form of the small GTPase Rap1 (which has two isoforms, Rap1A and Rap1B). This interaction has been further shown to facilitate the interdependent recruitment of Rap1 and SLAT to the T cell immunological synapse upon TCR engagement. Furthermore, a SLAT mutant lacking its PH domain drastically inhibited LFA-1 activation and CD4(+) T cell adhesion. Finally, we established that a constitutively active form of Rap1, which is present at the plasma membrane, rescues the defective LFA-1 activation and ICAM-1 adhesion in SLAT-deficient (Def6(-/-)) T cells. These findings ascribe a new function to SLAT, and identify Rap1 as a target of SLAT function in TCR-mediated inside-out signaling.

Association of the EF-hand and PH domains of the guanine nucleotide exchange factor SLAT with IP₃ receptor 1 promotes Ca²⁺ signaling in T cells

The guanine nucleotide exchange factor SLAT (SWAP-70-like adaptor of T cells) regulates T cell activation and differentiation by enabling Ca(2+) release from intracellular stores in response to stimulation of the T cell receptor (TCR). We found a TCR-induced association between SLAT and inositol 1,4,5-trisphosphate (IP3) receptor type 1 (IP3R1). The N-terminal region of SLAT, which contains two EF-hand motifs that we determined bound Ca(2+), and the SLAT pleckstrin homology (PH) domain independently bound to IP3R1 by associating with a conserved motif within the IP3R1 ligand-binding domain. Disruption of the SLAT-IP3R1 interaction with cell-permeable, IP3R1-based fusion peptides inhibited TCR-stimulated Ca(2+) signaling, activation of the transcription factor NFAT (nuclear factor of activated T cells), and production of cytokines, suggesting that this interaction is required for optimal T cell activation. The finding that SLAT is an IP3R1-interacting protein required for T cell activation suggests that this interaction could be a potential target for a selective immunosuppressive drug.

Role of guanine nucleotide exchange factor-H1 in complement-mediated RhoA activation in glomerular epithelial cells

Visceral glomerular epithelial cells (GEC), also known as podocytes, are vital for the structural and functional integrity of the glomerulus. The actin cytoskeleton plays a central role in maintaining GEC morphology. In a rat model of experimental membranous nephropathy (passive Heymann nephritis (PHN)), complement C5b-9-induced proteinuria was associated with the activation of the actin regulator small GTPase, RhoA. The mechanisms of RhoA activation, however, remained unknown. In this study, we explored the role of the epithelial guanine nucleotide exchange factor, GEF-H1, in complement-induced RhoA activation. Using affinity precipitation to monitor GEF activity, we found that GEF-H1 was activated in glomeruli isolated from rats with PHN. Complement C5b-9 also induced parallel activation of GEF-H1 and RhoA in cultured GEC. In GEC in which GEF-H1 was knocked down, both basal and complement-induced RhoA activity was reduced. On the other hand, GEF-H1 knockdown augmented complement-mediated cytolysis, suggesting a role for GEF-H1 and RhoA in protecting GEC from cell death. The MEK1/2 inhibitor, U0126, and mutation of the ERK-dependent phosphorylation site (T678A) prevented complement-induced GEF-H1 activation, indicating a role for the ERK pathway. Further, complement induced GEF-H1 and microtubule accumulation in the perinuclear region. However, both the perinuclear accumulation and the activation of GEF-H1 were independent of microtubules and myosin-mediated contractility, as shown using drugs that interfere with microtubule dynamics and myosin II activity. In summary, we have identified complement-induced ERK-dependent GEF-H1 activation as the upstream mechanism of RhoA stimulation, and this pathway has a protective role against cell death.

Def-6, a novel regulator of small GTPases in podocytes, acts downstream of atypical protein kinase C (aPKC) λ/ι

The atypical protein kinase C (aPKC) isotypes PKCλ/ι and PKCζ are both expressed in podocytes; however, little is known about differences in their function. Previous studies in mice have demonstrated that podocyte-specific loss of PKCλ/ι leads to a severe glomerular phenotype, whereas mice deficient in PKCζ develop no renal phenotype. We analyzed various effects caused by PKCλ/ι and PKCζ deficiency in cultured murine podocytes. In contrast to PKCζ-deficient podocytes, PKCλ/ι-deficient podocytes exhibited a severe actin cytoskeletal phenotype, reduced cell size, decreased number of focal adhesions, and increased activation of small GTPases. Comparative microarray analysis revealed that the guanine nucleotide exchange factor Def-6 was specifically up-regulated in PKCλ/ι-deficient podocytes. In vivo Def-6 expression is significantly increased in podocytes of PKCλ/ι-deficient mice. Cultured PKCλ/ι-deficient podocytes exhibited an enhanced membrane association of Def-6, indicating enhanced activation. Overexpression of aPKCλ/ι in PKCλ/ι-deficient podocytes could reduce the membrane-associated expression of Def-6 and rescue the actin phenotype. In the present study, PKCλ/ι was identified as an important factor for actin cytoskeletal regulation in podocytes and Def-6 as a specific downstream target of PKCλ/ι that regulates the activity of small GTPases and subsequently the actin cytoskeleton of podocytes.

SLAT negatively regulates RANKL-induced osteoclast differentiation

RANKL induces the formation of osteoclasts, which are responsible for bone resorption. Herein, we investigated the role of SWAP-70-like adapter of T cells (SLAT) in RANKL-induced osteoclastogenesis. Expression levels of SLAT were reduced during RANKL-induced osteoclastogenesis. Overexpression of SLAT in BMMs inhibited TRAP-positive multinuclear osteoclast formation and attenuated the expression of NFATc1, which is an important modulator in osteoclastogenesis. Furthermore, silencing of SLAT by RNA interference enhanced osteoclast formation as well as NFATc1 expression. In addition, SLAT was involved in RANKL-induced JNK activation in osteoclasts. Taken together, our data suggest that SLAT acts as a negative modulator of RANKL-induced osteoclastogenesis.

Dendritic cell podosome dynamics does not depend on the F-actin regulator SWAP-70

In addition to classical adhesion structures like filopodia or focal adhesions, dendritic cells similar to macrophages and osteoclasts assemble highly dynamic F-actin structures called podosomes. They are involved in cellular processes such as extracellular matrix degradation, bone resorption by osteoclasts, and trans-cellular diapedesis of lymphocytes. Besides adhesion and migration, podosomes enable dendritic cells to degrade connective tissue by matrix metalloproteinases. SWAP-70 interacts with RhoGTPases and F-actin and regulates migration of dendritic cells. SWAP-70 deficient osteoclasts are impaired in F-actin-ring formation and bone resorption. In the present study, we demonstrate that SWAP-70 is not required for podosome formation and F-actin turnover in dendritic cells. Furthermore, we found that toll-like receptor 4 ligand induced podosome disassembly and podosome-mediated matrix degradation is not affected by SWAP-70 in dendritic cells. Thus, podosome formation and function in dendritic cells is independent of SWAP-70.

SLAT regulates CD8+ T cell clonal expansion in a Cdc42- and NFAT1-dependent manner

After antigenic stimulation, CD8(+) T cells undergo clonal expansion and differentiation into CTLs that can mount a strong defense against intracellular pathogens and tumors. SWAP-70-like adapter of T cells (SLAT), also known as Def6, is a novel guanine nucleotide exchange factor for the Cdc42 GTPase and plays a role in CD4(+) T cell activation and Th cell differentiation by controlling Ca(2+)/NFAT signaling, but its requirement in CD8(+) T cell response has not been explored. Using a range of transgenic and knockout in vivo systems, we show that SLAT is required for efficient expansion of CD8(+) T cells during the primary response but is not necessary for CTL differentiation. The reduced clonal expansion observed in the absence of SLAT resulted from a CD8(+) T cell-intrinsic proliferation defect and a reduced IL-2-dependent cell survival. On a molecular level, we show that Def6 deficiency resulted in defective TCR/CD28-induced NFAT translocation to the nucleus in CD8(+) T cells. Constitutively active Cdc42 or NFAT1 mutants fully restored the impaired expansion of Def6(-/-) CD8(+) T cells. Taken together, these data describe a new and pivotal role of SLAT-mediated NFAT activation in CD8(+) T cells, providing new insight into the signaling pathways involved in CD8(+) T cell proliferation.

Ectopically expressed IBP promotes cell proliferation in oral squamous cell carcinoma

IFN regulatory factor 4 binding protein (IBP) has been shown to play an important role in the progression of malignant tumors such as breast cancer cells, but its function in oral squamous cell carcinoma (OSCC) remains unclear. We found that IBP ectopically expressed in some OSCC specimens but not in normal oral mucosa epithelium tissues. IBP expression was significantly correlated with tumor size, differentiation, clinical stage, and distant metastasis. Furthermore, IBP markedly promoted OSCC cell proliferation, shortened the G1 interval in the cell cycle, and increased cyclin D1 expression. These findings suggest that IBP may be a potential therapeutic target for OSCC.

Structure analysis between the SWAP-70 RHO-GEF and the newly described PLD2-GEF

Small GTPases like Rac2 are crucial regulators of many cell functions central to life itself. Our laboratory has recently found that phospholipase D2 (PLD2) can act as a guanine nucleotide exchange factor (GEF) for Rac2. PLD2 has a Pleckstrin Homology (PH) domain but does not bear a Dbl homology (DH) or DOCK homology region (DHR) domain. It has, however, a Phox (PX) domain upstream of its PH domain. To better understand the novel finding of PLD2 as an enhancer of GDP/GTP exchange, we modeled the N-terminal portion of PLD2 (as the crystal structure of this protein has not as of yet been resolved), and studied the correlation with two known GEFs, SWAP-70 and the Leukemic Associated RhoGEF (LARG). Structural similarities between PLD2's PH and SWAP-70s or LARG's PH domain are very extensive, while similarities between PLD2's PX and SWAP-70s or LARG's DH domains are less evident. This indicates that PLD functions as a GEF utilizing its PH domain and part of its PX domain and possibly other regions. All this makes PLD unique, and an entirely new class of GEF. By bearing two enzymatic activities (break down of PC and GDP/GTP exchange), it is realistic to assume that PLD is an important signaling node for several intracellular pathways. Future experiments will ascertain how the newly described PLD2's GEF is regulated in the context of cell activation.

Interferon regulatory factor 4 binding protein is a novel p53 target gene and suppresses cisplatin-induced apoptosis of breast cancer cells

Background

Our previous work demonstrated that ectopic expression of interferon regulatory factor 4 binding protein (IBP) was correlated with the malignant behaviour of human breast cancer cells. The mechanisms controlling differential expression of IBP in breast cancer still remain unknown.

Results

To investigate the mechanism of IBP dysregulation in breast cancer, we identified IBP was a novel p53 target gene. IBP expression was negatively regulated by wild-type p53 and was p53 dependently suppressed by DNA damage agent cisplatin. Furthermore, high levels of IBP were found to decrease cisplatin-induced growth suppression and apoptotic cell death, which was associated with decreased p53 activity and imbalanced Bcl-2 family member expression.

Conclusions

IBP is a novel p53 target gene which suppresses cisplatin-mediated apoptosis of breast cancer cells via negative feedback regulation of the p53 signalling pathway, suggesting IBP may serve as a target for pharmacologic intervention of breast cancer resistant to cisplatin therapy.

DEF6, a novel substrate for the Tec kinase ITK, contains a glutamine-rich aggregation-prone region and forms cytoplasmic granules that co-localize with P-bodies

Localization of DEF6 (SLAT/IBP), a Rho-family guanine nucleotide exchange factor, to the center of the immune synapse is dependent upon ITK, a Tec-family kinase that regulates the spatiotemporal organization of components of T cell signaling pathways and Cdc42-dependent actin polymerization. Here we demonstrate that ITK both interacts with DEF6 and phosphorylates DEF6 at tyrosine residues Tyr(210) and Tyr(222). Expression of a GFP-tagged Y210E-Y222E phosphomimic resulted in the formation of DEF6 cytoplasmic granules that co-localized with decapping enzyme 1 (DCP1), a marker of P-bodies; sites of mRNA degradation. Similarly treatment of cells with puromycin or sodium arsenite, reagents that arrest translation, also resulted in the accumulation of DEF6 in cytoplasmic granules. Bioinformatics analysis identified a glutamine-rich, heptad-repeat region; a feature of aggregating proteins, within the C-terminal region of DEF6 with the potential to promote granule formation through a phosphorylation-dependent unmasking of this region. These data suggest that in addition to its role as a GEF, DEF6 may also function in regulating mRNA translation.

SWAP70 functions as a Rac/Rop guanine nucleotide-exchange factor in rice

Rho family small GTPases are involved in diverse signaling processes including immunity, growth, and development. The activity of Rho GTPases is regulated by cycling between guanosine diphosphate (GDP)-bound inactive and guanosine triphosphate (GTP)-bound active forms, in which guanine nucleotide exchange factors (GEFs) predominantly function to promote activation of the GTPases. In animals, most Rho GEFs possess a Dbl (diffuse B-cell lymphoma) homology (DH) domain which functions as a GEF-catalytic domain. However, no proteins with the DH domain have been identified in plants so far. Instead, plant-specific Rho GEFs with the PRONE domain responsible for GEF activity have been found to constitute a large family in plants. In this study, we found rice homologs of human SWAP70, Oryza sativa (Os) SWAP70A and SWAP70B, containing the DH domain. OsSWAP70A interacted with rice Rho GTPase OsRac1, an important signaling factor for immune responses. The DH domain of OsSWAP70A exhibited the GEF-catalytic activity toward OsRac1 as found in animal Rho GEFs, indicating that plants have the functional DH domains. Transient expression of OsSWAP70A enhanced OsRac1-mediated production of reactive oxygen species in planta. Reduction of OsSWAP70A and OsSWAP70B mRNA levels by RNA interference resulted in the suppression of chitin elicitor-induced defense gene expression and ROS production. Thus, it is likely that OsSWAP70 regulates immune responses through activation of OsRac1.

Dual regulation of IRF4 function in T and B cells is required for the coordination of T-B cell interactions and the prevention of autoimmunity

Effective humoral responses to protein antigens require the precise execution of carefully timed differentiation programs in both T and B cell compartments. Disturbances in this process underlie the pathogenesis of many autoimmune disorders, including systemic lupus erythematosus (SLE). Interferon regulatory factor 4 (IRF4) is induced upon the activation of T and B cells and serves critical functions. In CD4(+) T helper cells, IRF4 plays an essential role in the regulation of IL-21 production, whereas in B cells it controls class switch recombination and plasma cell differentiation. IRF4 function in T helper cells can be modulated by its interaction with regulatory protein DEF6, a molecule that shares a high degree of homology with only one other protein, SWAP-70. Here, we demonstrate that on a C57BL/6 background the absence of both DEF6 and SWAP-70 leads to the development of a lupus-like disease in female mice, marked by simultaneous deregulation of CD4(+) T cell IL-21 production and increased IL-21 B cell responsiveness. We furthermore show that DEF6 and SWAP-70 are differentially used at distinct stages of B cell differentiation to selectively control the ability of IRF4 to regulate IL-21 responsiveness in a stage-specific manner. Collectively, these data provide novel insights into the mechanisms that normally couple and coordinately regulate T and B cell responses to ensure tight control of productive T-B cell interactions.

IRF-4 suppresses BCR/ABL transformation of myeloid cells in a DNA binding-independent manner

Interferon regulatory factor 4 (IRF-4) is essential for B and T cell development and immune response regulation, and has both nuclear and cytoplasmic functions. IRF-4 was originally identified as a proto-oncogene resulting from a t(6;14) chromosomal translocation in multiple myeloma and its expression was shown to be essential for multiple myeloma cell survival. However, we have previously shown that IRF-4 functions as a tumor suppressor in the myeloid lineage and in early stages of B cell development. In this study, we found that IRF-4 suppresses BCR/ABL transformation of myeloid cells. To gain insight into the molecular pathways that mediate IRF-4 tumor suppressor function, we performed a structure-function analysis of IRF-4 as a suppressor of BCR/ABL transformation. We found that the DNA binding domain deletion mutant of IRF-4, which is localized only in the cytoplasm, is still able to inhibit BCR/ABL transformation of myeloid cells. IRF-4 also functions as a tumor suppressor in bone marrow cells deficient in MyD88, an IRF-4-interacting protein found in the cytoplasm. However, IRF-4 tumor suppressor activity is lost in IRF association domain (IAD) deletion mutants. These results demonstrate that IRF-4 suppresses BCR/ABL transformation by a novel cytoplasmic function involving its IAD domain.

Def6 is required for convergent extension movements during zebrafish gastrulation downstream of Wnt5b signaling

During gastrulation, convergent extension (CE) cell movements are regulated through the non-canonical Wnt signaling pathway. Wnt signaling results in downstream activation of Rho GTPases that in turn regulate actin cytoskeleton rearrangements essential for co-ordinated CE cell movement. Rho GTPases are bi-molecular switches that are inactive in their GDP-bound stage but can be activated to bind GTP through guanine nucleotide exchange factors (GEFs). Here we show that def6, a novel GEF, regulates CE cell movement during zebrafish gastrulation. Def6 morphants exhibit broadened and shortened body axis with normal cell fate specification, reminiscent of the zebrafish mutants silberblick and pipetail that lack Wnt11 or Wnt5b, respectively. Indeed, def6 morphants phenocopy Wnt5b mutants and ectopic overexpression of def6 essentially rescues Wnt5b morphants, indicating a novel role for def6 as a central GEF downstream of Wnt5b signaling. In addition, by knocking down both def6 and Wnt11, we show that def6 synergises with the Wnt11 signaling pathway.

Fine tuning of IRF-4 expression by SWAP-70 controls the initiation of plasma cell development

The generation of plasma cells (PCs) is key for proper humoral immune responses. The transcription factors IRF-4 and BLIMP-1 (B-lymphocyte induce maturation protein-1) control PC commitment, but the underlying regulatory mechanisms are incompletely understood. Here we have identified SWAP-70 as being critically involved in Toll-like receptor (TLR)-triggered PC differentiation. Upon activation through various TLRs, Swap-70(-/-) B cells were activated and proliferated normally. However, expression of BLIMP-1 was markedly reduced and PC differentiation was impaired. Four hours of LPS stimulation were sufficient to drive PC differentiation, and SWAP-70 was required during this initial period. Swap-70(-/-) B cells pre-activated in vitro failed to efficiently differentiate into PCs upon adoptive transfer into recipient mice. Re-introduction of SWAP-70 into Swap-70(-/-) B cells rescued their development into PCs, and SWAP-70 over-expression in wild-type (WT) B cells increased PC generation. In the absence of SWAP-70, IRF-4 protein levels were reduced and the IRF-4(high) B220(+) CD138(-) compartment, including PC precursors, was strongly diminished. Ectopic expression of SWAP-70 increases IRF-4 protein levels and PC differentiation in WT and Swap-70(-/-) B cells, and IRF-4 over-expression in Swap-70(-/-) B cells elevates PC differentiation to WT levels. Thus, in a dose-dependent manner, SWAP-70 controls IRF-4 protein expression and thereby regulates the initiation of PC differentiation.

Phosphorylation of IRF4 by ROCK2 regulates IL-17 and IL-21 production and the development of autoimmunity in mice

Deregulated production of IL-17 and IL-21 plays a key pathogenic role in many autoimmune disorders. A delineation of the mechanisms that underlie the inappropriate synthesis of IL-17 and IL-21 in autoimmune diseases can thus provide important insights into potential therapies for these disorders. Here we have shown that the serine-threonine kinase Rho-associated, coiled-coil-containing protein kinase 2 (ROCK2) becomes activated in mouse T cells under Th17 skewing conditions and phosphorylates interferon regulatory factor 4 (IRF4), a transcription factor that is absolutely required for the production of IL-17 and IL-21. We furthermore demonstrated that ROCK2-mediated phosphorylation of IRF4 regulated the synthesis of IL-17 and IL-21 and the differentiation of Th17 cells. Whereas CD4+ T cells from WT mice activated ROCK2 physiologically under Th17 conditions, CD4+ T cells from 2 different mouse models of spontaneous autoimmunity aberrantly activated ROCK2 under neutral conditions. Moreover, administration of ROCK inhibitors ameliorated the deregulated production of IL-17 and IL-21 and the inflammatory and autoantibody responses observed in these autoimmune mice. Our findings thus uncover a crucial link among ROCK2, IRF4, and the production of IL-17 and IL-21 and support the idea that selective inhibition of ROCK2 could represent an important therapeutic regimen for the treatment of autoimmune disorders.

IRF4 and its regulators: evolving insights into the pathogenesis of inflammatory arthritis?

Accumulating evidence from murine and human studies supports a key role for interleukin-17 (IL-17) and IL-21 in the pathogenesis of inflammatory arthritis. The pathways and molecular mechanisms that underlie the production of IL-17 and IL-21 are being rapidly elucidated. This review focuses on interferon regulatory factor 4 (IRF4), a member of the IRF family of transcription factors, which has emerged as a crucial controller of both IL-17 and IL-21 production. We first outline the complex role of IRF4 in the function of CD4(+) T cells and then discuss recent studies from our laboratory that have revealed a surprising role for components of Rho GTPase-mediated pathways in controlling the activity of IRF4. A better understanding of these novel pathways will hopefully provide new insights into mechanisms responsible for the development of inflammatory arthritis and potentially guide the design of novel therapeutic approaches.

SLAT/Def6 plays a critical role in the development of Th17 cell-mediated experimental autoimmune encephalomyelitis

SWAP-70-like adapter of T cells (SLAT; also known as Def6) is a novel guanine nucleotide exchange factor for Rho GTPases that has been previously shown to play a role in CD4+ T cell activation and Th1/Th2 differentiation. However, the role of SLAT/Def6 in autoimmunity and its associated Th1- and Th17-specific responses has not yet been clearly elucidated. We used a prototypical and pathologically relevant Th1/Th17-mediated autoimmune model, that is, experimental autoimmune encephalomyelitis, to assess the role of SLAT/Def6 in autoantigen-specific T cell response. We found that T cell-expressed SLAT/Def6 was critical for experimental autoimmune encephalomyelitis development and pathogenesis, as evidenced by the resistance of Def6-deficient (Def6(-/-)) mice to clinical signs of the disease associated with a lack of CNS inflammation and demyelination in myelin oligodendrocyte glycoprotein-immunized Def6(-/-) mice. Moreover, Def6 deficiency resulted in a severely diminished myelin oligodendrocyte glycoprotein-specific CD4+ T cell proliferation as well as a defect in IFN-gamma and IL-17 production in secondary lymphoid organs and the CNS. Lastly, Def6(-/-) CD4+ T cells were grossly deficient in their ability to differentiate into Th17 cells both in vitro and in vivo in a T cell-intrinsic manner. Therefore, our study establishes T cell-expressed SLAT/Def6 as a pivotal positive regulator of Th17 inflammatory responses and, thus, essential in controlling autoimmune and inflammatory diseases.

SWAP-70-like adapter of T cells: a novel Lck-regulated guanine nucleotide exchange factor coordinating actin cytoskeleton reorganization and Ca2+ signaling in T cells

SWAP-70-like adapter of T cells (SLAT) is a recently identified guanine nucleotide exchange factor (GEF) for Cdc42 and Rac1, which is highly expressed in both thymocytes and peripheral T cells. Here, we present and discuss findings resulting from biochemical and genetic analyses aimed at unveiling the role of SLAT in CD4+ T-cell development, activation, and T-helper (Th) cell differentiation. Slat(-/-) mice display a developmental defect at one of the earliest stages of thymocyte differentiation, the double negative 1 (DN1) stage, leading to decreased peripheral T-cell numbers. Slat(-/-) peripheral CD4+ T cells demonstrate impaired T-cell receptor/CD28-induced proliferation and IL-2 production. Moreover, SLAT positively regulates the development of Th1 and Th2 inflammatory responses by controlling Ca2+/NFAT signaling. SLAT is also a positive regulator of the recently emerging Th subset, i.e., Th17 cells, as evidenced by its critical role in Th17 cell-mediated central nervous system inflammation. Furthermore, TCR engagement induces SLAT translocation to the immunological synapse, a process mediated by its Lck-dependent phosphorylation, which thereafter facilitates the triggering of SLAT GEF activity towards Cdc42 and Rac1, leading to NFAT activation and Th1/Th2 differentiation. Future work will aim to dissect the interacting partners of SLAT and may thus shed light on the poorly understood events that coordinate and link actin cytoskeleton reorganization to Ca2+ signaling and gene transcription in T cells.

Rho family GTPases and their regulators in lymphocytes

Rho family GTPases, and the proteins that regulate them, have important roles in many cellular processes, including cell division, survival, migration and adhesion. Although most of our understanding of these proteins has come from studies using cell lines, more recent gene targeting studies in mice are providing insights into the in vivo function of these proteins. Here we review recent progress revealing crucial roles for these proteins in lymphocyte development, activation, differentiation and migration. The emerging picture shows that Rho family GTPases transduce signals from receptors for antigens, chemokines and cytokines, as well as adhesion molecules and pattern recognition receptors, and that they function as focal points for crosstalk between different signalling pathways.

Overexpression of the Interferon regulatory factor 4-binding protein in human colorectal cancer and its clinical significance

BACKGROUND

IFN regulatory factor 4-binding protein (IBP) is a novel type of activator of Rho GTPases. It has been linked with differentiation and apoptosis of lymphocytes, but its function in oncogenesis remains unclear. Here we studied the expression of endogenous IBP in four human colorectal cancer cell lines, normal, adenoma and tumor colorectal tissues.

METHODS

Molecular (Western blot and RT-PCR), and confocal analyses were used to investigate IBP expression in human colorectal cancer cell lines. Matched normal and tumor tissue sections of 63 patients and 15 adenoma tissue sections were analyzed for IBP expression by immunohistochemistry (IHC).

RESULTS

IBP was ubiquitely expressed in human colorectal cancer cell lines. The expression of IBP can be detected at both the mRNA and protein level in SW480, SW620 and HT29 cells. Clinically, IBP were elevated in human colorectal cancer specimens in comparison to normal colorectal tissues. Substantial high expression of IBP was observed in colorectal cancer tissues (67%), whereas corresponding normal tissues and 15 adenoma tissues showed consistently absent immunoreactivity of IBP. Moreover, IBP expression is correlated with the differentiation level of colorectal cancer cells (p<0.05) and clinical stage of patients (p<0.01).

CONCLUSIONS

Our data show, for the first time, a dysregulated expression of IBP in human colorectal cancer, offering new perspectives for its role in cancer development and progression. IBP may be a novel tumor marker and a therapeutic target for colorectal cancer.

Adaptor protein SLAT modulates Fcgamma receptor-mediated phagocytosis in murine macrophages

SLAT (SWAP-70-like adaptor protein of T cells) is an adaptor protein expressed in cells of the hematopoietic system. SLAT interacts with and alters the function of small GTPase Rac1 in fibroblasts. In these nonhematopoietic models, the SLAT-Rac interaction leads to changes in F-actin and causes cytoskeletal reorganization. In T cells, SLAT expression regulates the development of T helper cells through Cdc42- and Rac1-mediated activation of the NF-AT transcription factor. Here we show that SLAT is expressed in macrophages. Overexpression of SLAT in a macrophage cell line inhibits the IgG Fcgamma receptor-mediated phagocytic ability of THP1 cells. In bone marrow-derived macrophages, SLAT protein is recruited to the early phagosomes formed via Fcgamma receptor engagement. SLAT recruitment to the phagosome was most efficient when the macrophages express at least one isoform of Rac (Rac1 or Rac2), because SLAT recruitment was reduced in macrophages of Rac-deficient mice. Macrophages derived from animals lacking SLAT show an elevation in the rate of Fcgamma receptor-mediated phagocytosis. The absence of SLAT is associated with an increase in the amount of F-actin formed around these phagosomes as well as an increase in the amount of Rac1 protein recruited to the phagosome. Our results suggest that SLAT acts as a gatekeeper for the amount of Rac recruited to the phagosomes formed by Fcgamma receptor engagement and thus is able to regulate F-actin re-organization and consequently phagocytosis.

The ectopic expression of IFN regulatory factor 4-binding protein is correlated with the malignant behavior of human breast cancer cells

Many proteins that are aberrantly expressed in malignant tumors play important roles in promoting tumorigenesis, metastasis and immune escape. IFN regulatory factor 4-binding protein (IBP), which is a novel PH-DH-like protein related to SWAP-70, and functions as an upstream activator of Rho GTPases. It is widely expressed in cells of the immune system and is involved in coupling activated cell receptors to downstream signaling events that mediate cell proliferation, differentiation and polarization. Although IBP was detected in human chondrosarcoma, its function in tumor cells remains unknown. In this study, newly generated monoclonal anti-IBP antibodies were employed and they detected higher level expression of IBP in some human invasive breast carcinoma tissues and in two breast cancer cell lines that form highly invasive tumors in nude mice. In contrast, the levels of IBP mRNA and protein were low or undetectable in normal human breast tissues, benign breast lesions or low-tumorigenic breast cancer cell lines. Over-expression of wild-type IBP in an IBP-negative breast cancer cell line markedly increased its proliferation and invasiveness in vitro. Conversely, RNA interference-mediated knockdown of IBP expression in an IBP-positive breast cancer cell line significantly reduced cell growth and invasiveness. Our results indicate that IBP is expressed in more highly invasive human breast cancer cells, such as MCF-7 and MDA-MB-231, with lower expression in normal breast tissue, benign tumors and less aggressive breast cancer cells, such as SKBR3 and MDA-MB-453. Thus, expression of IBP is correlated with the degree of malignant breast tumors. Nevertheless, it should be pointed our that further study with more tumor types is required to fully elucidate the role of IBP in tumorigenesis and the potential of IBP as a marker for more highly malignant tumors.

BALB/c mice genetically susceptible to proteoglycan-induced arthritis and spondylitis show colony-dependent differences in disease penetrance

Introduction

The major histocompatibility complex (H-2d) and non-major histocompatibility complex genetic backgrounds make the BALB/c strain highly susceptible to inflammatory arthritis and spondylitis. Although different BALB/c colonies develop proteoglycan-induced arthritis and proteoglycan-induced spondylitis in response to immunization with human cartilage proteoglycan, they show significant differences in disease penetrance despite being maintained by the same vendor at either the same or a different location.

Methods

BALB/c female mice (24 to 26 weeks old after 4 weeks of acclimatization) were immunized with a suboptimal dose of cartilage proteoglycan to explore even minute differences among 11 subcolonies purchased from five different vendors. In vitro-measured T-cell responses, and serum cytokines and (auto)antibodies were correlated with arthritis (and spondylitis) phenotypic scores. cDNA microarrays were also performed using spleen cells of naïve and immunized BALB/cJ and BALB/cByJ mice (both colonies from The Jackson Laboratory, Bar Harbor, ME, USA), which represent the two major BALB/c sublines.

Results

The 11 BALB/c colonies could be separated into high (n = 3), average (n = 6), and low (n = 2) responder groups based upon their arthritis scores. While the clinical phenotypes showed significant differences, only a few immune parameters correlated with clinical or histopathological abnormalities, and seemingly none of them affected differences found in altered clinical phenotypes (onset time, severity or incidence of arthritis, or severity and progression of spondylitis). Affymetrix assay (Affymetrix, Santa Clara, CA, USA) explored 77 differentially expressed genes (at a significant level, P < 0.05) between The Jackson Laboratory's BALB/cJ (original) and BALB/cByJ (transferred from the National Institutes of Health, Bethesda, MD, USA). Fourteen of the 77 differentially expressed genes had unknown function; 24 of 77 genes showed over twofold differences, and only 8 genes were induced by immunization, some in both colonies.

Conclusions

Using different subcolonies of the BALB/c strain, we can detect significant differences in arthritis phenotypes, single-nucleotide polymorphisms (SNPs), and a large number of differentially expressed genes, even in non-immunized animals. A number of the known genes (and SNPs) are associated with immune responses and/or arthritis in this genetically arthritis-prone murine strain, and a number of genes of as-yet-unknown function may affect or modify clinical phenotypes of arthritis and/or spondylitis.

Keeping autoimmunity in check: how to control a Th17 cell controller

The transcription factor IRF-4 is necessary for Th17 cell differentiation. In this issue of Immunity, Chen et al. (2008) show that IRF-4-binding protein is a critical negative regulator of IRF-4 function, regulating production of the cytokines IL-21 and IL-17.

Rho GTPase-mediated pathways in mature CD4+ T cells

Effective immune responses require the appropriate activation and differentiation of peripheral CD4(+) T cells. These processes need to be followed by the timely elimination of the responding T cells in order to restore T cell homeostasis. Defects in the appropriate regulation of T cell activation, expansion, and survival underlie the pathogenesis of many autoimmune disorders including SLE. The molecular machinery employed by T cells to properly control these processes and prevent the onset of autoimmunity has not been fully elucidated. Rho GTPases (which include the Rac, Cdc42, and Rho subfamilies) are molecular switches that control a wide range of cellular processes. Their fundamental role in biology is due to their ability to regulate both cytoskeletal dynamics and a large number of signal transduction pathways. Activation of Rho GTPases is now recognized as a key event in the coordination of immune responses and, particularly, in the activation of T cells. In this review, we will first provide an overview of the role of Rho GTPase-mediated pathways in mature CD4(+) T cells and then we will discuss recent studies, which suggest that deregulation of these pathways may play a role in the pathogenesis of SLE.

Regulation of TLR4-mediated signaling by IBP/Def6, a novel activator of Rho GTPases

TLRs play a fundamental role in innate immune responses. Although Rho GTPases have been implicated in TLR-mediated signaling pathways, the molecules that control their activation in response to TLR engagement are largely unknown. IFN regulatory factor-4-binding protein (IBP; which is encoded by the gene Def6) is a unique type of activator for Rac that plays a crucial role in TCR-mediated signaling and adaptive immune responses. Here, we demonstrate that IBP/Def6 also controls innate immune responses by modulating TLR-induced signaling events. Mice deficient in IBP/Def6 are protected from LPS-induced septic shock. This protection is associated with a decrease in the production of proinflammatory cytokines and is accompanied by diminished activation of MAPKs and NF-kappaB. Our results thus identify IBP/Def6 as a novel component of the TLR4-induced signaling cascade that controls the production of proinflammatory cytokines.

IRF-4-binding protein inhibits interleukin-17 and interleukin-21 production by controlling the activity of IRF-4 transcription factor

The T helper 17 (Th17) cell lineage is important in inflammatory and autoimmune responses, via its ability to produce interleukin-17 (IL-17) and IL-21. Given the potentially deleterious effects of Th17 cells, their generation needs to be strictly controlled. IRF-4 is a transcription factor that has recently emerged as a key regulator of Th17 cell differentiation. Here, we showed that mice deficient in a previously isolated protein, IBP (IRF-4-binding protein), rapidly developed rheumatoid arthritis-like joint disease and large-vessel vasculitis. The pathology was associated with an enhanced responsiveness of T cells to low levels of stimulation and with the inappropriate synthesis of IL-17 and IL-21. IBP sequestered IRF-4 and prevented it from targeting the transcriptional regulatory regions of the genes that encode IL-17 and IL-21. Thus, IBP appears to be important in preventing T cell-mediated autoimmunity by ensuring that the production of IL-17 and IL-21 does not occur in response to self-antigens.

Tyrosine-phosphorylation-dependent translocation of the SLAT protein to the immunological synapse is required for NFAT transcription factor activation

SWAP-70-like adaptor of T cells (SLAT) is a guanine nucleotide exchange factor for Rho GTPases that regulates the development of T helper 1 (Th1) and Th2 cell inflammatory responses by controlling the Ca(2+)-NFAT signaling pathway. However, the mechanism used by SLAT to regulate these events is unknown. Here, we report that the T cell receptor (TCR)-induced translocation of SLAT to the immunological synapse required Lck-mediated phosphorylation of two tyrosine residues located in an immunoreceptor tyrosine-based activation motif-like sequence but was independent of the SLAT PH domain. This subcellular relocalization was coupled to, and necessary for, activation of the NFAT pathway. Furthermore, membrane targeting of the SLAT Dbl-homology (catalytic) domain was sufficient to trigger TCR-mediated NFAT activation and Th1 and Th2 differentiation in a Cdc42-dependent manner. Therefore, tyrosine-phosphorylation-mediated relocalization of SLAT to the site of antigen recognition is required for SLAT to exert its pivotal role in NFAT-dependent CD4(+) T cell differentiation.

Stimulatory and suppressive signal transduction regulates vasoactive intestinal peptide receptor-1 (VPAC-1) in primary mouse CD4 T cells

Vasoactive intestinal peptide receptor-1 (VPAC-1) is an anti-proliferative, G-protein coupled receptor that is highly expressed on naïve T cells, and has been reported to be downregulated upon T cell activation. The T cell signaling molecules involved in mediating low VPAC-1 levels have not been identified. Therefore, to gain a greater understanding into this regulation, this study investigated the signaling pathways that regulate (VPAC-1) in murine, primary CD4 T cells. To this end, murine, splenic CD4 T cells were pretreated separately with 10 different pharmacological inhibitors and incubated +/- anti-CD3 for 24h. Total RNA was isolated, and VPAC-1 mRNA levels were measured by qPCR. Our results support that JNK kinases, downstream from the protein kinase, Zap70, are involved in suppressive regulation of VPAC-1 steady-state mRNA levels after anti-CD3 treatment. In contrast, inhibitors against PKC, ERK, p38, Zap70 and Rac1 supported a stimulatory influence in VPAC-1 regulation in the absence of T cell signaling. By studying the signaling pathways that regulate VPAC-1 in T cells, we can gain greater insight into the role of this anti-inflammatory receptor in autoimmunity and infectious diseases.

SLAT regulates Th1 and Th2 inflammatory responses by controlling Ca2+/NFAT signaling

SWAP-70-like adapter of T cells (SLAT) is a novel guanine nucleotide exchange factor for Rho GTPases that is upregulated in Th2 cells, but whose physiological function is unclear. We show that SLAT(-/-) mice displayed a developmental defect at one of the earliest stages of thymocyte differentiation, the double-negative 1 (DN1) stage, leading to decreased peripheral T cell numbers. SLAT(-/-) peripheral CD4(+) T cells demonstrated impaired TCR/CD28-induced proliferation and IL-2 production, which was rescued by the addition of exogenous IL-2. Importantly, SLAT(-/-) mice were grossly impaired in their ability to mount not only Th2, but also Th1-mediated lung inflammatory responses, as evidenced by reduced airway neutrophilia and eosinophilia, respectively. Levels of Th1 and Th2 cytokine in the lungs were also markedly reduced, paralleling the reduction in pulmonary inflammation. This defect in mounting Th1/Th2 responses, which was also evident in vitro, was traced to a severe reduction in Ca(2+) mobilization from ER stores, which consequently led to defective TCR/CD28-induced translocation of nuclear factor of activated T cells 1/2 (NFATc1/2). Thus, SLAT is required for thymic DN1 cell expansion, T cell activation, and Th1 and Th2 inflammatory responses.