Vav1: a key signal transducer downstream of the TCR

TIAM-1 regulates polarized protrusions during dorsal intercalation in the Caenorhabditis elegans embryo through both its GEF and N-terminal domains

Mediolateral cell intercalation is a morphogenetic strategy used throughout animal development to reshape tissues. Dorsal intercalation in the Caenorhabditis elegans embryo involves the mediolateral intercalation of two rows of dorsal epidermal cells to create a single row that straddles the dorsal midline, and thus is a simple model to study cell intercalation. Polarized protrusive activity during dorsal intercalation requires the C. elegans Rac and RhoG orthologs CED-10 and MIG-2, but how these GTPases are regulated during intercalation has not been thoroughly investigated. In this study, we characterized the role of the Rac-specific guanine nucleotide exchange factor (GEF) TIAM-1 in regulating actin-based protrusive dynamics during dorsal intercalation. We found that TIAM-1 can promote formation of the main medial lamellipodial protrusion extended by intercalating cells through its canonical GEF function, whereas its N-terminal domains function to negatively regulate the generation of ectopic filiform protrusions around the periphery of intercalating cells. We also show that the guidance receptor UNC-5 inhibits these ectopic filiform protrusions in dorsal epidermal cells and that this effect is in part mediated via TIAM-1. These results expand the network of proteins that regulate basolateral protrusive activity during directed rearrangement of epithelial cells in animal embryos.

TIAM-1 regulates polarized protrusions during dorsal intercalation in the C. elegans embryo through both its GEF and N-terminal domains

Mediolateral cell intercalation is a morphogenetic strategy used throughout animal development to reshape tissues. Dorsal intercalation in the C. elegans embryo involves the mediolateral intercalation of two rows of dorsal epidermal cells to create a single row that straddles the dorsal midline, and so is a simple model to study cell intercalation. Polarized protrusive activity during dorsal intercalation requires the C. elegans Rac and RhoG orthologs CED-10 and MIG-2, but how these GTPases are regulated during intercalation has not been thoroughly investigated. In this study, we characterize the role of the Rac-specific guanine nucleotide exchange factor (GEF), TIAM-1, in regulating actin-based protrusive dynamics during dorsal intercalation. We find that TIAM-1 can promote protrusion formation through its canonical GEF function, while its N-terminal domains function to negatively regulate this activity, preventing the generation of ectopic protrusions in intercalating cells. We also show that the guidance receptor UNC-5 inhibits ectopic protrusive activity in dorsal epidermal cells, and that this effect is in part mediated via TIAM-1. These results expand the network of proteins that regulate basolateral protrusive activity during directed cell rearrangement.

Summary statement

TIAM-1 activates the Rac pathway to promote protrusion formation via its GEF domain, while its N-terminal domains suppress ectopic protrusions during dorsal intercalation in the C. elegans embryo.

Characterization of Novel Derivatives of MBQ-167, an inhibitor of the GTP-binding proteins Rac/Cdc42

Rac and Cdc42, are homologous GTPases that regulate cell migration, invasion, and cell cycle progression; thus, representing key targets for metastasis therapy. We previously reported on the efficacy of MBQ-167, which blocks both Rac1 and Cdc42 in breast cancer cells and mouse models of metastasis. To identify compounds with increased activity, a panel of MBQ-167 derivatives was synthesized, maintaining its 9-ethyl-3-(1H-1,2,3-triazol-1-yl)-9H-carbazole core. Similar to MBQ-167, MBQ-168 and EHop-097, inhibit activation of Rac and Rac1B splice variant and breast cancer cell viability, and induce apoptosis. MBQ-167 and MBQ-168 inhibit Rac and Cdc42 by interfering with guanine nucleotide binding, and MBQ-168 is a more effective inhibitor of PAK (1,2,3) activation. EHop-097 acts via a different mechanism by inhibiting the interaction of the guanine nucleotide exchange factor (GEF) Vav with Rac. MBQ-168 and EHop-097 inhibit metastatic breast cancer cell migration, and MBQ-168 promotes loss of cancer cell polarity to result in disorganization of the actin cytoskeleton and detachment from the substratum. In lung cancer cells, MBQ-168 is more effective than MBQ-167 or EHop-097 at reducing ruffle formation in response to EGF. Comparable to MBQ-167, MBQ-168 significantly inhibits HER2+ tumor growth and metastasis to lung, liver, and spleen. Both MBQ-167 and MBQ-168 inhibit the cytochrome P450 (CYP) enzymes 3A4, 2C9, and 2C19. However, MBQ-168 is ~10X less potent than MBQ-167 at inhibiting CYP3A4, thus demonstrating its utility in relevant combination therapies. In conclusion, the MBQ-167 derivatives MBQ-168 and EHop-097 are additional promising anti metastatic cancer compounds with similar and distinct mechanisms.

Hematopoietic Cells Influence Vascular Development in the Retina

Hematopoietic cells play a crucial role in the adult retina in health and disease. Monocytes, macrophages, microglia and myeloid angiogenic cells (MACs) have all been implicated in retinal pathology. However, the role that hematopoietic cells play in retinal development is understudied. The temporal changes in recruitment of hematopoietic cells into the developing retina and the phenotype of the recruited cells are not well understood. In this study, we used the hematopoietic cell-specific protein Vav1 to track and investigate hematopoietic cells in the developing retina. By flow cytometry and immunohistochemistry, we show that hematopoietic cells are present in the retina as early as P0, and include microglia, monocytes and MACs. Even before the formation of retinal blood vessels, hematopoietic cells localize to the inner retina where they eventually form networks that intimately associate with the developing vasculature. Loss of Vav1 lead to a reduction in the density of medium-sized vessels and an increased inflammatory response in retinal astrocytes. When pups were subjected to oxygen-induced retinopathy, hematopoietic cells maintained a close association with the vasculature and occasionally formed 'frameworks' for the generation of new vessels. Our study provides further evidence for the underappreciated role of hematopoietic cells in retinal vasculogenesis and the formation of a healthy retina.

Local delivery of FTY720 induces neutrophil activation through chemokine signaling in an oronasal fistula model

Purpose

Cleft palate repair surgeries lack a regenerative reconstructive option and, in many cases, develop complications including oronasal fistula (ONF). Our group has developed a novel murine phenocopy of ONF to study the oral cavity wound healing program. Using this model, our team previously identified that delivery of FTY720 on a nanofiber scaffold had a unique immunomodulatory effect directing macrophages and monocytes into a pro-regenerative state during ONF healing. Here, the objective of this study was to determine the effects of local biomaterial-based FTY720 delivery in the ONF model on the early bulk gene expression and neutrophil phenotypic response within the regenerating tissue.

Methods

Using a mouse model of ONF formation, a palate defect was created and was treated with FTY720 nanofiber scaffolds or (blank) vehicle control nanofibers. At 1 and 3 days post-implantation, ONF oral mucosal tissue from the defect region was collected for RNA sequencing analysis or flow cytometry. For the RNA-seq expression profiling, intracellular pathways were assessed using the KEGG Pathway database and Gene Ontology (GO) Terms enrichment interactive graph. To assess the effects of FTY720 on different neutrophil subpopulations, flow cytometry data was analyzed using pseudotime analysis based on Spanning-tree Progression Analysis of Density-normalized Events (SPADE).

Results

RNA sequencing analysis of palate mucosa injured tissue identified 669 genes that were differentially expressed (DE) during the first 3 days of ONF wound healing after local delivery of FTY720, including multiple genes in the sphingolipid signaling pathway. Evaluation of the DE genes at the KEGG Pathway database also identified the inflammatory immune response pathways (chemokine signaling, cytokine-cytokine receptor interaction, and leukocyte transendothelial migration), and the Gene Ontology enrichment analysis identified neutrophil chemotaxis and migration terms. SPADE dendrograms of CD11b⁺Ly6G⁺ neutrophils at both day 1 and day 3 post-injury showed significantly distinct subpopulations of neutrophils in oral mucosal defect tissue from the FTY720 scaffold treatment group compared to the vehicle control group (blank). Increased expression of CD88 and Vav1, among other genes, were found and staining of the ONF area demonstrated increased VAV1 staining in FTY720‐treated healing oral mucosa.

Conclusion

Treatment of oral mucosal defects using FTY720 scaffolds is a promising new immunotherapy to improve healing outcomes and reducing ONF formation during cleft palate surgical repair. Local delivery of FTY720 nanofiber scaffolds during ONF healing significantly shifted early gene transcription associated with immune cell recruitment and modulation of the immune microenvironment results in distinct neutrophil subpopulations in the oral mucosal defect tissue that provides a critical shift toward pro-regenerative immune signaling.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40883-021-00208-z.

Distinct Roles of Vav Family Members in Adaptive and Innate Immune Models of Arthritis

Genetic evidence suggests that three members of the VAV family (VAV1, VAV2 and VAV3) of signal transduction proteins could play important roles in rheumatoid arthritis. However, it is not known currently whether the inhibition of these proteins protects against this disease and, if so, the number of family members that must be eliminated to get a therapeutic impact. To address this issue, we have used a collection of single and compound Vav family knockout mice in experimental models for antigen-dependent (methylated bovine serum albumin injections) and neutrophil-dependent (Zymosan A injections) rheumatoid arthritis in mice. We show here that the specific elimination of Vav1 is sufficient to block the development of antigen-induced arthritis. This protection is likely associated with the roles of this Vav family member in the development and selection of immature T cells within the thymus as well as in the subsequent proliferation and differentiation of effector T cells. By contrast, we have found that depletion of Vav2 reduces the number of neutrophils present in the joints of Zymosan A-treated mice. Despite this, the elimination of Vav2 does not protect against the joint degeneration triggered by this experimental model. These findings indicate that Vav1 is the most important pharmacological target within this family, although its main role is limited to the protection against antigen-induced rheumatoid arthritis. They also indicate that the three Vav family proteins do not play redundant roles in these pathobiological processes.

The protooncogene Vav1 regulates murine leukemia virus-induced T-cell leukemogenesis

Vav1 is expressed exclusively in hematopoietic cells and is required for T cell development and activation. Vav1-deficient mice show thymic hypocellularity due to a partial block during thymocyte development at the DN3 stage and between the double positive (DP) and single positive (SP) transition. Vav1 has been shown to play a significant role in several non-hematopoietic tumors but its role in leukemogenesis is unknown. To address this question, we investigated the role of Vav1 in retrovirus-induced T cell leukemogenesis. Infection of Vav1-deficient mice with the Moloney strain of murine leukemia virus (M-MuLV) significantly affected tumor phenotype without modulating tumor incidence or latency. M-MuLV-infected Vav1-deficient mice showed reduced splenomegaly, higher hematocrit levels and hypertrophic thymi. Notably, Vav1-deficient mice with M-MuLV leukemias presented with markedly lower TCRβ/CD3 levels, indicating that transformation occurred at an earlier stage of T cell development than in WT mice. Thus, impaired T cell development modulates the outcome of retrovirus-induced T cell leukemias, demonstrating a link between T cell development and T cell leukemogenesis.

Vav1 is Essential for HIF-1α Activation via a Lysosomal VEGFR1-Mediated Degradation Mechanism in Endothelial Cells

The vascular response to hypoxia and ischemia is essential for maintaining homeostasis during stressful conditions and is particularly critical for vital organs such as the heart. Hypoxia-inducible factor-1 (HIF-1) is a central regulator of the response to hypoxia by activating transcription of numerous target genes, including vascular endothelial growth factor (VEGF). Here we identify the guanine nucleotide exchange factor (GEF) Vav1, a regulator of the small Rho-GTPase and cell signaling in endothelial cells, as a key vascular regulator of hypoxia. We show that Vav1 is present in the vascular endothelium and is essential for HIF-1 activation under hypoxia. So, we hypothesized that Vav1 could be a key regulator of HIF-1 signaling. In our findings, Vav1 regulates HIF-1α stabilization through the p38/Siah2/PHD3 pathway. In normoxia, Vav1 binds to vascular endothelial growth factor receptor 1 (VEGFR1), which directs Vav1 to lysosomes for degradation. In contrast, hypoxia upregulates Vav1 protein levels by inhibiting lysosomal degradation, which is analogous to HIF-1α regulation by hypoxia: both proteins are constitutively produced and degraded in normoxia allowing for a rapid response when stress occurs. Consequently, hypoxia rapidly stabilizes Vav1, which is required for HIF-1α accumulation. This shows that Vav1 is the key mediator controlling the stabilization of HIF1α in hypoxic conditions. With this finding, we report a novel pathway to stabilize HIF-1, which shows a possible reason why clinical trials targeting HIF-1 has not been effective. Targeting Vav1 can be the new approach to overcome hypoxic tumors.

Vav1 inhibits RANKL-induced osteoclast differentiation and bone resorption

Vav1 is a Rho/Rac guanine nucleotide exchange factor primarily expressed in hematopoietic cells. In this study, we investigated the potential role of Vav1 in osteoclast (OC) differentiation by comparing the ability of bone marrow mononuclear cells (BMMCs) obtained from Vav1-deficient (Vav1-/-) and wild-type (WT) mice to differentiate into mature OCs upon stimulation with macrophage colony stimulating factor and receptor activator of nuclear kappa B ligand in vitro. Our results suggested that Vav1 deficiency promoted the differentiation of BMMCs into OCs, as indicated by the increased expression of tartrate-resistant acid phosphatase, cathepsin K, and calcitonin receptor. Therefore, Vav1 may play a negative role in OC differentiation. This hypothesis was supported by the observation of more OCs in the femurs of Vav1-/- mice than in WT mice. Furthermore, the bone status of Vav1-/- mice was analyzed in situ and the femurs of Vav1-/- mice appeared abnormal, with poor bone density and fewer number of trabeculae. In addition, Vav1-deficient OCs showed stronger adhesion to vitronectin, an αvβ3 integrin ligand important in bone resorption. Thus, Vav1 may inhibit OC differentiation and protect against bone resorption. [BMB Reports 2019; 52(11): 659-664].

Lysine Acetylation Reshapes the Downstream Signaling Landscape of Vav1 in Lymphocytes

Vav1 works both as a catalytic Rho GTPase activator and an adaptor molecule. These functions, which are critical for T cell development and antigenic responses, are tyrosine phosphorylation-dependent. However, it is not known whether other posttranslational modifications can contribute to the regulation of the biological activity of this protein. Here, we show that Vav1 becomes acetylated on lysine residues in a stimulation- and SH2 domain-dependent manner. Using a collection of both acetylation- and deacetylation-mimicking mutants, we show that the acetylation of four lysine residues (Lys²²², Lys²⁵², Lys⁵⁸⁷, and Lys⁷¹⁶) leads to the downmodulation of the adaptor function of Vav1 that triggers the stimulation of the nuclear factor of activated T cells (NFAT). These sites belong to two functional subclasses according to mechanistic criteria. We have also unveiled additional acetylation sites potentially involved in either the stimulation (Lys⁷⁸²) or the downmodulation (Lys³³⁵, Lys³⁷⁴) of specific Vav1-dependent downstream responses. Collectively, these results indicate that Nε-lysine acetylation can play variegated roles in the regulation of Vav1 signaling. Unlike the case of the tyrosine phosphorylation step, this new regulatory layer is not conserved in other Vav family paralogs.

The Rho/Rac Guanine Nucleotide Exchange Factor Vav1 Regulates Hif-1α and Glut-1 Expression and Glucose Uptake in the Brain

Vav1 is a Rho/Rac (Ras-related C3 botulinum toxin substrate) guanine nucleotide exchange factor expressed in hematopoietic and endothelial cells that are involved in a wide range of cellular functions. It is also stabilized under hypoxic conditions when it regulates the accumulation of the transcription factor HIF (Hypoxia Inducible Factor)-1α, which activates the transcription of target genes to orchestrate a cellular response to low oxygen. One of the genes induced by HIF-1α is GLUT (Glucose Transporter)-1, which is the major glucose transporter expressed in vessels that supply energy to the brain. Here, we identify a role for Vav1 in providing glucose to the brain. We found that Vav1 deficiency downregulates HIF-1α and GLUT-1 levels in endothelial cells, including blood-brain barrier cells. This downregulation of GLUT-1, in turn, reduced glucose uptake to endothelial cells both in vitro and in vivo, and reduced glucose levels in the brain. Furthermore, endothelial cell-specific Vav1 knock-out in mice, which caused glucose uptake deficiency, also led to a learning delay in fear conditioning experiments. Our results suggest that Vav1 promotes learning by activating HIF-1α and GLUT-1 and thereby distributing glucose to the brain. We further demonstrate the importance of glucose transport by endothelial cells in brain functioning and reveal a potential new axis for targeting GLUT-1 deficiency syndromes and other related brain diseases.

Protective effects of microRNA-330 on amyloid β-protein production, oxidative stress, and mitochondrial dysfunction in Alzheimer's disease by targeting VAV1 via the MAPK signaling pathway

This study aims to explore the effect of miR-330 targeting VAV1 on amyloid β-protein (Aβ) production, oxidative stress (OS), and mitochondrial dysfunction in Alzheimer's disease (AD) mice through the MAPK signaling pathway. Putative targeted gene of miR-330 was performed by a miRNA target prediction website and dual-luciferase reporter gene assay. AD mouse model was successfully established. Fourteen C57 mice were randomized into AD and control groups. The positive protein expression rate of VAV1 was measured by immunohistochemistry. Neuron cells were assigned into control, blank, negative control (NC), miR-330 mimics, miR-330 inhibitors, siRNA-VAV1, and miR-330 inhibitors + siRNA-VAV1 groups. Expression of miR-330, VAV1, ERK1, JNK1, P38MAPK, Aβ, COX, and lipoprotein receptor-related protein-1 (LRP-1) were determined using RT-qPCR and Western blotting. Colorimetry was applied to measure the levels of OS parameters of superoxide dismutase (SOD) and malondialdehyde (MDA). Aβ production in brain tissue was detected using ELISA, while that in neuron cell was measured by radioimmunoassay. MiR-330 was down-regulated in neuron cells of AD mice and VAV1 was negatively regulated by miR-330. Compared with the control group, the positive protein expression rate of VAV1 was significantly elevated in the AD group. Overexpression of miR-330 decreased the expression of VAV1, ERK1, JNK1, P38MAPK, and Aβ, but increased the expression of COX and LRP-1. AD mice revealed elevated Aβ production and MDA with decreased SOD level. The result indicates that overexpressed miR-330 targeting VAV1 through the MAPK signaling pathway reduces Aβ production and alleviates OS and mitochondrial dysfunction in AD.

Signaling networks that regulate cell migration

SUMMARY

Stimuli that promote cell migration, such as chemokines, cytokines, and growth factors in metazoans and cyclic AMP in Dictyostelium, activate signaling pathways that control organization of the actin cytoskeleton and adhesion complexes. The Rho-family GTPases are a key convergence point of these pathways. Their effectors include actin regulators such as formins, members of the WASP/WAVE family and the Arp2/3 complex, and the myosin II motor protein. Pathways that link to the Rho GTPases include Ras GTPases, TorC2, and PI3K. Many of the molecules involved form gradients within cells, which define the front and rear of migrating cells, and are also established in related cellular behaviors such as neuronal growth cone extension and cytokinesis. The signaling molecules that regulate migration can be integrated to provide a model of network function. The network displays biochemical excitability seen as spontaneous waves of activation that propagate along the cell cortex. These events coordinate cell movement and can be biased by external cues to bring about directed migration.

Involvement of the Rac1-IRSp53-Wave2-Arp2/3 Signaling Pathway in HIV-1 Gag Particle Release in CD4 T Cells

During HIV-1 assembly, the Gag viral proteins are targeted and assemble at the inner leaflet of the cell plasma membrane. This process could modulate the cortical actin cytoskeleton, located underneath the plasma membrane, since actin dynamics are able to promote localized membrane reorganization. In addition, activated small Rho GTPases are known for regulating actin dynamics and membrane remodeling. Therefore, the modulation of such Rho GTPase activity and of F-actin by the Gag protein during virus particle formation was considered. Here, we studied the implication of the main Rac1, Cdc42, and RhoA small GTPases, and some of their effectors, in this process. The effect of small interfering RNA (siRNA)-mediated Rho GTPases and silencing of their effectors on Gag localization, Gag membrane attachment, and virus-like particle production was analyzed by immunofluorescence coupled to confocal microscopy, membrane flotation assays, and immunoblot assays, respectively. In parallel, the effect of Gag expression on the Rac1 activation level was monitored by G-LISA, and the intracellular F-actin content in T cells was monitored by flow cytometry and fluorescence microscopy. Our results revealed the involvement of activated Rac1 and of the IRSp53-Wave2-Arp2/3 signaling pathway in HIV-1 Gag membrane localization and particle release in T cells as well as a role for actin branching and polymerization, and this was solely dependent on the Gag viral protein. In conclusion, our results highlight a new role for the Rac1-IRSp53-Wave2-Arp2/3 signaling pathway in the late steps of HIV-1 replication in CD4 T lymphocytes.

IMPORTANCE

During HIV-1 assembly, the Gag proteins are targeted and assembled at the inner leaflet of the host cell plasma membrane. Gag interacts with specific membrane phospholipids that can also modulate the regulation of cortical actin cytoskeleton dynamics. Actin dynamics can promote localized membrane reorganization and thus can be involved in facilitating Gag assembly and particle formation. Activated small Rho GTPases and effectors are regulators of actin dynamics and membrane remodeling. We thus studied the effects of the Rac1, Cdc42, and RhoA GTPases and their specific effectors on HIV-1 Gag membrane localization and viral particle release in T cells. Our results show that activated Rac1 and the IRSp53-Wave2-Arp2/3 signaling pathway are involved in Gag plasma membrane localization and viral particle production. This work uncovers a role for cortical actin through the activation of Rac1 and the IRSp53/Wave2 signaling pathway in HIV-1 particle formation in CD4 T lymphocytes.

Coordinate control of cytoskeletal remodeling and calcium mobilization during T-cell activation

Ca(2+) mobilization and cytoskeletal reorganization are key hallmarks of T-cell activation, and their interdependence has long been recognized. Recent advances in the field have elucidated the molecular pathways that underlie these events and have revealed several points of intersection. Ca(2+) signaling can be divided into two phases: initial events leading to release of Ca(2+) from endoplasmic reticulum stores, and a second phase involving STIM 1 (stromal interaction molecule 1) clustering and CRAC (calcium release activated calcium) channel activation. Cytoskeletal dynamics promote both phases. During the first phase, the actin cytoskeleton promotes mechanotransduction and serves as a dynamic scaffold for microcluster assembly. Proteins that drive actin polymerization such as WASp (Wiskott-Aldrich syndrome protein) and HS1 (hematopoietic lineage cell-specific protein 1) promote signaling through PLCγ1 (phospholipase Cγ1) and release of Ca(2+) from endoplasmic reticulum stores. During the second phase, the WAVE (WASP-family verprolin homologous protein) complex and the microtubule cytoskeleton promote STIM 1 clustering at sites of plasma membrane apposition, opening Orai channels. In addition, gross cell shape changes and organelle movements buffer local Ca(2+) levels, leading to sustained Ca(2+) mobilization. Conversely, elevated intracellular Ca(2+) activates cytoskeletal remodeling. This can occur indirectly, via calpain activity, and directly, via Ca(2+) -dependent cytoskeletal regulatory proteins such as myosin II and L-plastin. While it is true that the cytoskeleton regulates Ca(2+) responses and vice versa, interdependence between Ca(2+) and the cytoskeleton also encompasses signaling events that occur in parallel, downstream of shared intermediates. Inositol cleavage by PLCγ1 simultaneously triggers both endoplasmic reticulum store release and diacylglycerol-dependent microtubule organizing center reorientation, while depleting the pool of phosphatidylinositol-4,5-bisphosphate, an activator of multiple actin-regulatory proteins. The close interdependence of Ca(2+) signaling and cytoskeletal dynamics in T cells provides positive feedback mechanisms for T-cell activation and allows for finely tuned responses to extracellular cues.

VAV1 and BAFF, via NFκB pathway, are genetic risk factors for myasthenia gravis

Objective

To identify novel genetic loci that predispose to early-onset myasthenia gravis (EOMG) applying a two-stage association study, exploration, and replication strategy.

Methods

Thirty-four loci and one confirmation loci, human leukocyte antigen (HLA)-DRA, were selected as candidate genes by team members of groups involved in different research aspects of MG. In the exploration step, these candidate genes were genotyped in 384 EOMG and 384 matched controls and significant difference in allele frequency were found in eight genes. In the replication step, eight candidate genes and one confirmation loci were genotyped in 1177 EOMG patients and 814 controls, from nine European centres.

Results

Allele frequency differences were found in four novel loci: CD86, AKAP12, VAV1, B-cell activating factor (BAFF), and tumor necrosis factor-alpha (TNF-α), and these differences were consistent in all nine cohorts. Haplotype trend test supported the differences in allele frequencies between cases and controls. In addition, allele frequency difference in female versus male patients at HLA-DRA and TNF-α loci were observed.

Interpretation

The genetic associations to EOMG outside the HLA complex are novel and of interest as VAV1 is a key signal transducer essential for T- and B-cell activation, and BAFF is a cytokine that plays important roles in the proliferation and differentiation of B-cells. Moreover, we noted striking epistasis between the predisposing VAV1 and BAFF haplotypes; they conferred a greater risk in combination than alone. These, and CD86, share the same signaling pathway, namely nuclear factor-kappaB (NFκB), thus implicating dysregulation of proinflammatory signaling in predisposition to EOMG.

c-Abl tyrosine kinase plays a critical role in β2 integrin-dependent neutrophil migration by regulating Vav1 activity

The recruitment and migration of neutrophils are critical for innate immunity and acute inflammatory responses. However, the mechanism that regulates the recruitment and migration of neutrophils has not been well characterized. We here reveal a novel function of c-Abl kinase in regulating neutrophil migration. Our results demonstrate that c-Abl kinase is required for neutrophil recruitment in vivo and migration in vitro, and the inhibition of c-Abl kinase activity has a significant impact on neutrophil migratory behavior. Moreover, c-Abl kinase activation depends on β2 integrin engagement, and the activated c-Abl kinase further regulates actin polymerization and membrane protrusion dynamics at the extended leading edges during neutrophil migration. In addition, we identify the Rho GEF Vav1 as a major downstream effector of c-Abl kinase. The C-terminal SH3-SH2-SH3 domain and proline-rich region of Vav1 are required for its interaction with c-Abl kinase, and c-Abl kinase probably regulates the activity of Vav1 by direct phosphorylation at Tyr-267 in the DH domain. Together, these results indicate that c-Abl kinase plays a critical role in β2 integrin-dependent neutrophil migration by regulating Vav1 activity.

The N-terminal 20-amino acid region of guanine nucleotide exchange factor Vav1 plays a distinguished role in T cell receptor-mediated calcium signaling

Vav1 is a guanine nucleotide exchange factor (GEF) specifically expressed in hematopoietic cells. It consists of multiple structural domains and plays important roles in T cell activation. The other highly conserved isoforms of Vav family, Vav2 and Vav3, are ubiquitously expressed in human tissues including lymphocytes. All three Vav proteins activate Rho family small GTPases, which are involved in a variety of biological processes during T cell activation. Intensive studies have demonstrated that Vav1 is indispensable for T cell receptor (TCR)-mediated signal transduction, whereas Vav2 and Vav3 function as GEFs that overlap with Vav1 on TCR-induced cytoskeleton reorganization. T cells lacking Vav1 exhibited severe defect in TCR-mediated calcium elevation, indicating that the co-existing Vav2 and Vav3 did not compensate Vav1 in calcium signaling. What is the functional particularity of Vav1 in lymphocytes? In this study, we identified the N-terminal 20 amino acids of Vav1 in the calponin homology (CH) domain to be essential for its interaction with calmodulin (CaM) that leads to TCR-induced calcium mobilization. Substitution of the 1-20 amino acids of Vav1 with those of Vav2 or Vav3 abolished the association with CaM, and the N-terminal mutations of Vav1 failed to potentiate normal TCR-induced calcium mobilization, that in turn, suspended nuclear factor of activated T cells (NFAT) activation and IL-2 production. This study highlights the importance of the N-terminal 20 aa of Vav1 for CaM binding, and provides new insights into the distinguished and irreplaceable role of Vav1 in T cell activation and signal transduction.

The emerging role of protein kinase Cθ in cytoskeletal signaling

Cytoskeletal rearrangements often occur as the result of transduction of signals from the extracellular environment. Efficient awakening of this powerful machinery requires multiple activation and deactivation steps, which usually involve phosphorylation or dephosphorylation of different signaling units by kinases and phosphatases, respectively. In this review, we discuss the signaling characteristics of one of the nPKC isoforms, PKCθ, focusing on PKCθ-mediated signal transduction to cytoskeletal elements, which results in cellular rearrangements critical for cell type-specific responses to stimuli. PKCθ is the major PKC isoform present in hematopoietic and skeletal muscle cells. PKCθ plays roles in T cell signaling through the IS, survival responses in adult T cells, and T cell FasL-mediated apoptosis, all of which involve cytoskeletal rearrangements and relocation of this enzyme. PKCθ has been linked to the regulation of cell migration, lymphoid cell motility, and insulin signaling and resistance in skeletal muscle cells. Additional roles were suggested for PKCθ in mitosis and cell-cycle regulation. Comprehensive understanding of cytoskeletal regulation and the cellular "modus operandi" of PKCθ holds promise for improving current therapeutic applications aimed at autoimmune diseases.

Vav protein guanine nucleotide exchange factor regulates CD36 protein-mediated macrophage foam cell formation via calcium and dynamin-dependent processes

Atherosclerosis, a chronic inflammatory disease, results in part from the accumulation of modified lipoproteins in the arterial wall and formation of lipid-laden macrophages, known as "foam cells." Recently, we reported that CD36, a scavenger receptor, contributes to activation of Vav-family guanine nucleotide exchange factors by oxidatively modified LDL in macrophages. We also discovered that CD36-dependent uptake of oxidized LDL (oxLDL) in vitro and foam cell formation in vitro and in vivo was significantly reduced in macrophages deficient of Vav proteins. The goal of the present study was to identify the mechanisms by which Vav proteins regulate CD36-dependent foam cell formation. We now show that a Vav-dynamin signaling axis plays a critical role in generating calcium signals in mouse macrophages exposed to CD36-specific oxidized phospholipid ligands. Chelation of intracellular Ca(2+) or inhibition of phospholipase C-γ (PLC-γ) inhibited Vav activation (85 and 70%, respectively, compared with vehicle control) and reduced foam cell formation (approximately 75%). Knockdown of expression by siRNA or inhibition of GTPase activity of dynamin 2, a Vav-interacting protein involved in endocytic vesicle fission, significantly blocked oxLDL uptake and inhibited foam cell formation. Immunofluorescence microscopy studies showed that Vav1 and dynamin 2 colocalized with internalized oxLDL in macrophages and that activation and mobilization of dynamin 2 by oxLDL was impaired in vav null cells. These studies identified previously unknown components of the CD36 signaling pathway, demonstrating that Vav proteins regulate oxLDL uptake and foam cell formation via calcium- and dynamin 2-dependent processes and thus represent novel therapeutic targets for atherosclerosis.

Notch3 and canonical NF-kappaB signaling pathways cooperatively regulate Foxp3 transcription

Notch3 overexpression has been previously shown to positively regulate the generation and function of naturally occurring regulatory T cells and the expression of Foxp3, in cooperation with the pTα/pre-TCR pathway. In this study, we show that Notch3 triggers the trans activation of Foxp3 promoter depending on the T cell developmental stage. Moreover, we discovered a novel CSL/NF-κB overlapping binding site within the Foxp3 promoter, and we demonstrate that the activation of NF-κB, mainly represented by p65-dependent canonical pathway, plays a positive role in Notch3-dependent regulation of Foxp3 transcription. Accordingly, the deletion of protein kinase C, which mediates canonical NF-κB activation, markedly reduces regulatory T cell number and per cell Foxp3 expression in transgenic mice with a constitutive activation of Notch3 signaling. Collectively, our data indicate that the cooperation among Notch3, protein kinase C, and p65/NF-κB subunit modulates Foxp3 expression, adding new insights in the understanding of the molecular mechanisms involved in regulatory T cell homeostasis and function.

Age-related defects in the cytoskeleton signaling pathways of CD4 T cells

It has been postulated that the cytoskeleton controls many aspects of T cell function, including activation, proliferation and apoptosis. Recent advances in our understanding of F-actin polymerization and the Ezrin-Radixin-Moesin (ERM) family of cytoskeleton signal proteins have provided new insights into immunological synapse formation during T cell activation. During aging there is a significant decline of T cell function largely attributable to declines in activation of CD4 T cells and defects in the formation of the immunological synapse. Here we discuss recent progress in the understanding of how aging alters F-actin and ERM proteins in mouse CD4 T cells, and the implications of these changes for the T cell activation process.

Signaling microdomains in T cells

Sub-micron scale signaling domains induced in the plasma membrane of cells are thought to play important roles in signal transduction. In T cells, agonist MHC-peptide complexes induce small diffraction-limited domains enriched in T cell receptor (TCR) and signaling molecules. These microclusters serve as transient platforms for signal initiation and are required for sustained signaling in T cells, although each microcluster functions for only a couple of minutes. How they are formed, and what mechanisms promote and regulate signaling within TCR microclusters is largely unknown, although it is clear that TCR engagement and dynamic reorganization of cortical actin are involved. Here, we review current understanding of signaling within microclusters in T cells, and speculate on how these structures may form, initiate biochemical signals, and serve as sites of both signal integration and amplification, while also facilitating appropriate termination of TCR and related signaling.

A role for VAV1 in experimental autoimmune encephalomyelitis and multiple sclerosis

Multiple sclerosis, the most common cause of progressive neurological disability in young adults, is a chronic inflammatory disease. There is solid evidence for a genetic influence in multiple sclerosis, and deciphering the causative genes could reveal key pathways influencing the disease. A genome region on rat chromosome 9 regulates experimental autoimmune encephalomyelitis, a model for multiple sclerosis. Using interval-specific congenic rat lines and association of single-nucleotide polymorphisms with inflammatory phenotypes, we localized the gene of influence to Vav1, which codes for a signal-transducing protein in leukocytes. Analysis of seven human cohorts (12,735 individuals) demonstrated an association of rs2546133-rs2617822 haplotypes in the first VAV1 intron with multiple sclerosis (CA: odds ratio, 1.18; CG: odds ratio, 0.86; TG: odds ratio, 0.90). The risk CA haplotype also predisposed for higher VAV1 messenger RNA expression. VAV1 expression was increased in individuals with multiple sclerosis and correlated with tumor necrosis factor and interferon-gamma expression in peripheral blood and cerebrospinal fluid cells. We conclude that VAV1 plays a central role in controlling central nervous system immune-mediated disease and proinflammatory cytokine production critical for disease pathogenesis.

Balanced Vav2 GEF activity regulates neurite outgrowth and branching in vitro and in vivo

We have investigated the role of Vav2, a reported Rac1/Cdc42 GEF, on the development of Xenopus spinal neurons in vitro and in vivo. Both gain and loss of Vav2 function inhibited the rate neurite extension on laminin (LN), while only GFP-Vav2 over-expression enhanced process formation and branching. Vav2 over-expression protected neurons from RhoA-mediated growth cone collapse, similar to constitutively active Rac1, suggesting that Vav2 activates Rac1 in spinal neurons. Enhanced branching on LN required both Vav2 GEF activity and N-terminal tyrosine residues, but protection from RhoA-mediated collapse only required GEF activity. Interestingly, wild-type spinal neurons exhibited increased branching on the cell adhesion molecule L1, which required Vav2 GEF function, but not N-terminal tyrosine residues. Finally, we find that Vav2 differentially affects the Rohon-Beard peripheral and central process extension but promotes neurite branching of commissural interneurons near the ventral midline. Together, we suggest that balanced Vav2 activity is necessary for optimal neurite outgrowth and promotes branching by targeting GEF activity to branch points.

VAV2 and VAV3 as candidate disease genes for spontaneous glaucoma in mice and humans

Background

Glaucoma is a leading cause of blindness worldwide. Nonetheless, the mechanism of its pathogenesis has not been well-elucidated, particularly at the molecular level, because of insufficient availability of experimental genetic animal models.

Methodology/Principal Findings

Here we demonstrate that deficiency of Vav2 and Vav3, guanine nucleotides exchange factors for Rho guanosine triphosphatases, leads to an ocular phenotype similar to human glaucoma. Vav2/Vav3-deficient mice, and to a lesser degree Vav2-deficient mice, show early onset of iridocorneal angle changes and elevated intraocular pressure, with subsequent selective loss of retinal ganglion cells and optic nerve head cupping, which are the hallmarks of glaucoma. The expression of Vav2 and Vav3 tissues was demonstrated in the iridocorneal angle and retina in both mouse and human eyes. In addition, a genome-wide association study screening glaucoma susceptibility loci using single nucleotide polymorphisms analysis identified VAV2 and VAV3 as candidates for associated genes in Japanese open-angle glaucoma patients.

Conclusions/Significance

Vav2/Vav3-deficient mice should serve not only as a useful murine model of spontaneous glaucoma, but may also provide a valuable tool in understanding of the pathogenesis of glaucoma in humans, particularly the determinants of altered aqueous outflow and subsequent elevated intraocular pressure.

Heterozygous deficiency of delta-catenin impairs pathological angiogenesis

Vascular and neuronal networks share a similar branching morphology, and emerging evidence implicates common mechanisms in the formation of both systems. δ-Catenin is considered a neuronal catenin regulating neuron cell–cell adhesion and cell motility. Here, we report expression of δ-catenin in vascular endothelium, and show that deletion of only one allele of δ-catenin is sufficient to impair endothelial cell motility and vascular assembly in vitro and pathological angiogenesis in vivo, thereby inhibiting tumor growth and wound healing. In contrast, deletion of one or both allele of δ-catenin had no effects on hormone-induced physiological angiogenesis in the uterus. Molecular analysis confirmed a gene dosage effect of δ-catenin on Rho GTPase activity. Moreover, we show that inflammatory cytokines, but not angiogenic factors, regulate δ-catenin expression, and the levels of δ-catenin positively correlate to human lung cancers. Collectively, our data suggest that inflammation, commonly associated with disease conditions, induces δ-catenin expression that specifically regulates pathological, and not physiological, angiogenesis. Because only pathological angiogenesis is sensitive to decreased levels of δ-catenin, this may provide a good target for antiangiogenic therapy.

Modulation of CTLA-4 and GITR for cancer immunotherapy

The rational manipulation of antigen-specific T cells to reignite a tumor-specific immune response in cancer patients is a challenge for cancer immunotherapy. Targeting coinhibitory and costimulatory T cell receptors with specific antibodies in cancer patients is an emerging approach to T cell manipulation, namely "immune modulation." Cytotoxic T-lymphocyte antigen-4 (CTLA-4) and glucocorticoid-induced tumor necrosis factor family receptor (GITR) are potential targets for immune modulation through anti-CTLA-4 blocking antibodies and anti-GITR agonistic antibodies, respectively. In this review, we first discuss preclinical findings key to the understanding of the mechanisms of action of these immunomodulatory antibodies and the preclinical evidence of antitumor activity which preceded translation into the clinic. We next describe the outcomes and immune related adverse effects associated with anti-CTLA-4 based clinical trials with particular emphasis on specific biomarkers used to elucidate the mechanisms of tumor immunity in patients. The experience with anti-CTLA-4 therapy and the durable clinical benefit observed provide proof of principle to effective antitumor immune modulation and the promise of future clinical immune modulatory antibodies.

Function of the nucleotide exchange activity of vav1 in T cell development and activation

The guanine nucleotide exchange factor (GEF) Vav1 is essential for transducing T cell antigen receptor (TCR) signals and therefore plays a critical role in the development and activation of T cells. It has been presumed that the GEF activity of Vav1 is important for its function; however, there has been no direct demonstration of this. Here, we generated mice expressing enzymatically inactive, but normally folded, Vav1 protein. Analysis of these mice showed that the GEF activity of Vav1 was necessary for the selection of thymocytes and for the optimal activation of T cells, including signal transduction to Rac1, Akt, and integrins. In contrast, the GEF activity of Vav1 was not required for TCR-induced calcium flux, activation of extracellular signal-regulated kinase and protein kinase D1, and cell polarization. Thus, in T cells, the GEF activity of Vav1 is essential for some, but not all, of its functions.

The Rho/Rac exchange factor Vav2 controls nitric oxide-dependent responses in mouse vascular smooth muscle cells

The regulation of arterial contractility is essential for blood pressure control. The GTPase RhoA promotes vasoconstriction by modulating the cytoskeleton of vascular smooth muscle cells. Whether other Rho/Rac pathways contribute to blood pressure regulation remains unknown. By studying a hypertensive knockout mouse lacking the Rho/Rac activator Vav2, we have discovered a new signaling pathway involving Vav2, the GTPase Rac1, and the serine/threonine kinase Pak that contributes to nitric oxide-triggered blood vessel relaxation and normotensia. This pathway mediated the Pak-dependent inhibition of phosphodiesterase type 5, a process that favored RhoA inactivation and the subsequent depolymerization of the F-actin cytoskeleton in vascular smooth muscle cells. The inhibition of phosphodiesterase type 5 required its physical interaction with autophosphorylated Pak1 but, unexpectedly, occurred without detectable transphosphorylation events between those 2 proteins. The administration of phosphodiesterase type 5 inhibitors prevented the development of hypertension and cardiovascular disease in Vav2-deficient animals, demonstrating the involvement of this new pathway in blood pressure regulation. Taken together, these results unveil one cause of the cardiovascular phenotype of Vav2-knockout mice, identify a new Rac1/Pak1 signaling pathway, and provide a mechanistic framework for better understanding blood pressure control in physiological and pathological states.

The use of knockout mice reveals a synergistic role of the Vav1 and Rasgrf2 gene deficiencies in lymphomagenesis and metastasis

Background

Vav1 and RasGRF2 are GDP/GTP exchange factors for Ras superfamily GTPases with roles in the development and/or effector functions of T–lymphocytes.

Methodology/Principal Findings

Given that the phenotype of Vav1^–/–, Rasgrf2^–/– and Vav1^–/–;Rasgrf2^–/– mice has been studied so far in young animals, we decided to explore the long–term consequences of the inactivation of those loci in the immune system. Unexpectedly, our studies revealed that the inactivation of the Vav1 proto–oncogene favors the formation of lymphoblastic lymphoma–like tumors in aging mice. Those tumors, that can be found either localized exclusively inside the thymus or widely disseminated in hematopoietic and non–hematopoietic tissues, are composed of CD3⁺ lymphoblasts that display heterogeneous combinations of CD4 and CD8 surface markers. Interestingly, the additional deletion of the Rasgrf2 gene induces a shortening in the latency period for the development of those tumors, an increase in the percentage of disseminated tumors outside the thymus and, as a result, higher mortality rates.

Conclusions/Significance

These data reveal unexpected negative roles for Vav1 and RasGRF2 in different stages of T–cell lymphoma progression. They also suggest that the inactivation of Vav1 function may represent an inadequate strategy to treat T–cell lymphomas, especially those associated with low levels of Rasgrf2 gene expression.

RTKN2 Induces NF-KappaB Dependent Resistance to Intrinsic Apoptosis in HEK Cells and Regulates BCL-2 Genes in Human CD4(+) Lymphocytes

The gene for Rhotekin 2 (RTKN2) was originally identified in a promyelocytic cell line resistant to oxysterol-induced apoptosis. It is differentially expressed in freshly isolated CD4⁺ T-cells compared with other hematopoietic cells and is down-regulated following activation of the T-cell receptor. However, very little is known about the function of RTKN2 other than its homology to Rho-GTPase effector, rhotekin, and the possibility that they may have similar roles. Here we show that stable expression of RTKN2 in HEK cells enhanced survival in response to intrinsic apoptotic agents; 25-hydroxy cholesterol and camptothecin, but not the extrinsic agent, TNFα. Inhibitors of NF-KappaB, but not MAPK, reversed the resistance and mitochondrial pro-apoptotic genes, Bax and Bim, were down regulated. In these cells, there was no evidence of RTKN2 binding to the GTPases, RhoA or Rac2. Consistent with the role of RTKN2 in HEK over-expressing cells, suppression of RTKN2 in primary human CD4⁺ T-cells reduced viability and increased sensitivity to 25-OHC. The expression of the pro-apoptotic genes, Bax and Bim were increased while BCL-2 was decreased. In both cell models RTKN2 played a role in the process of intrinsic apoptosis and this was dependent on either NF-KappaB signaling or expression of downstream BCL-2 genes. As RTKN2 is a highly expressed in CD4⁺ T-cells it may play a role as a key signaling switch for regulation of genes involved in T-cell survival.

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.

Age-related changes in lck-Vav signaling pathways in mouse CD4 T cells

Activation of lck-fyn kinases during T cell receptor signaling leads to Vav phosphorylation, activation of downstream targets including Rac1, and a transient decline in ezrin and moesin phosphorylation. We have shown that age increases Rac1 activity and lowers ezrin and moesin phosphorylation in resting mouse CD4 cells, changes that could be the results of alterations in lck-Vav signaling. Analysis of Vav in CD4 cells from old mice shows increases in the phosphorylation of two key regulatory residues, Tyr160 and Tyr174, suggesting enhancement of Vav GTPase activity. In addition, analysis of lck status also shows age-related increases in phosphorylation of two key residues, Tyr394 and Tyr505, which have opposite effects on lck function. These changes in lck-Vav signals in resting CD4 cells may contribute in turn to age-related increases in Rac1 activity and declines in phosphorylation of cytoskeletal proteins including Ezrin and Moesin.

A Pyk2-Vav1 complex is recruited to beta3-adhesion sites to initiate Rho activation

Integrin alphavbeta3-mediated adhesion of haemopoietic cells to vitronectin results in beta3 tyrosine phosphorylation and Rho activation which is necessary for adhesion. Previously, we have shown that the RhoGEF (Rho guanine-nucleotide-exchange factor) Vav1 could associate indirectly with alphavbeta3 during leucocyte adhesion to vitronectin. In the present study, we have identified the non-receptor tyrosine kinase Pyk2 (proline-rich tyrosine kinase 2) as the adaptor protein that links Vav1 with alphavbeta3. The association of Pyk2 and Vav1 with beta3 relies on the presence of Tyr747 in beta3, the primary site of beta3 phosphorylation. However, association of Pyk2 with Vav1 is independent of beta3 tyrosine phosphorylation. Formation of a Pyk2-Vav1 complex occurs upon cell adhesion and Pro717 of Pyk2 plays a key role in Pyk2 interaction with Vav1. Utilizing purified recombinant proteins, we confirmed the direct interaction between Pyk2 and Vav1 In vitro. Cells transfected with GFP (green fluorescent protein)-Pyk2-P717A demonstrated severely suppressed cytoskeletal reorganization, impaired Vav1 recruitment, decreased Rho GTPase activation and loss of cell adhesion. Using siRNA (small interfering RNA) to specifically reduce Pyk2 levels in cells resulted in disrupted association between Vav1 and beta3 and impaired cell adhesion. These results indicate that Pyk2 is a critical signalling molecule downstream of beta3 integrin tyrosine phosphorylation and mediates Vav1 recruitment to accomplish actin reorganization necessary for adhesion.

Rac GTPases play critical roles in early T-cell development

The Rac1 and Rac2 GTPases play important roles in many processes including cytoskeletal reorganization, proliferation, and survival, and are required for B-cell development. Previous studies had shown that deficiency in Rac2 did not affect T-cell development, whereas the function of Rac1 in this process has not been investigated. We now show that simultaneous absence of both GTPases resulted in a very strong developmental block at the pre-TCR checkpoint and in defective positive selection. Unexpectedly, deficiency of Rac1 and Rac2 also resulted in the aberrant survival of thymocytes lacking expression of TCR beta, showing hallmarks of hyperactive Notch signaling. Furthermore, we found a similar novel phenotype in the absence of Vav1, Vav2, and Vav3, which function as guanine nucleotide exchange factors for Rac1 and Rac2. These results show that a pathway containing Vav and Rac proteins may negatively regulate Notch signaling during early thymic development.

Lymphocyte signaling: beyond knockouts

The analysis of lymphocyte signaling was greatly enhanced by the advent of gene targeting, which allows the selective inactivation of a single gene. Although this gene 'knockout' approach is often informative, in many cases, the phenotype resulting from gene ablation might not provide a complete picture of the function of the corresponding protein. If a protein has multiple functions within a single or several signaling pathways, or stabilizes other proteins in a complex, the phenotypic consequences of a gene knockout may manifest as a combination of several different perturbations. In these cases, gene targeting to 'knock in' subtle point mutations might provide more accurate insight into protein function. However, to be informative, such mutations must be carefully based on structural and biophysical data.

T-cell receptor- and CD28-induced Vav1 activity is required for the accumulation of primed T cells into antigenic tissue

Localization of primed T cells to antigenic tissue is essential for the development of effective immunity. Together with tissue-selective homing molecules, T-cell receptor (TCR)- and CD28-mediated signals have been shown to promote transendothelial migration of specific T cells into nonlymphoid antigen-rich tissue. However, the cellular and molecular requirements for T-cell accumulation to target tissue following their recruitment are largely undefined. The guanine nucleotide exchange factor (GEF) Vav1 has an integral role in coupling TCR and CD28 to signaling pathways that regulate T-cell activation and migration. Here, we have investigated the contribution of TCR- and CD28-induced Vav1 activity to the trafficking and localization of primed HY-specific CD4(+) T cells to antigenic sites. Severe migratory defects displayed by Vav1(-/-) T cells in vitro were fully compensated by a combination of shear flow and chemokines, leading to normal recruitment of Vav1(-/-) T cells in vivo. In contrast, Vav1(-/-) T-cell retention into antigen-rich tissue was severely impaired, reflecting T cells' inability to engage in sustained TCR- and CD28-mediated interactions with tissue-resident antigen-presenting cells (APCs). This novel function of APC-induced, and TCR- and CD28-mediated Vav1 activity in the regulation of effector T-cell immunity highlights its potential as a therapeutic target in T cell-mediated tissue damage.

Targeted cleavage of signaling proteins by caspase 3 inhibits T cell receptor signaling in anergic T cells

T cell receptor (TCR) engagement in the absence of costimulation induces the calcium-dependent upregulation of a program of gene expression that leads to the establishment of T cell anergy. Casp3 is one of the genes activated during anergy induction. Here we show that caspase 3 is required for the induction of T cell unresponsiveness. Suboptimal T cell stimulation induced caspase 3 activation, which did not result in cell death. Furthermore, caspase 3-deficient T cells showed impaired responses to anergizing stimuli. In anergic T cells, activated caspase 3 associated to the plasma membrane, where it cleaved and inactivated proteins such as the Grb2-related adaptor downstream of shc (GADS) and the guanine-nucleotide exchange factor Vav1, causing a blockade in TCR signaling. Our results identify a role for caspase 3 in nonapoptotic T cells and support that caspase 3-dependent proteolytic inactivation of signaling proteins is essential to maintain T cell tolerance.

Regulatory roles for Tiam1, a guanine nucleotide exchange factor for Rac1, in glucose-stimulated insulin secretion in pancreatic beta-cells

Using various biochemical, pharmacological and molecular biological approaches, we have recently reported regulatory roles for Rac1, a small G-protein, in glucose-stimulated insulin secretion (GSIS). However, little is understood with respect to localization of, and regulation by, specific regulatory factors of Rac1 in GSIS. Herein, we investigated regulatory roles for Tiam1, a specific nucleotide exchange factor (GEF) for Rac1, in GSIS in pancreatic beta-cells. Western blot analysis indicated that Tiam1 is predominantly cytosolic in distribution. NSC23766, a specific inhibitor of Tiam1-mediated activation of Rac1, markedly attenuated glucose-induced, but not KCl-induced insulin secretion in INS 832/13 cells and normal rat islets. Further, NSC23766 significantly reduced glucose-induced activation (i.e. GTP-bound form) and membrane association of Rac1 in INS 832/13 cells and rat islets. Moreover, siRNA-mediated knock-down of Tiam1 markedly inhibited glucose-induced membrane trafficking and activation of Rac1 in INS 832/13 cells. Interestingly, however, in contrast to the inhibitory effects of NSC23766, Tiam1 gene depletion potentiated GSIS in these cells; such a potentiation of GSIS was sensitive to extracellular calcium. Together, our studies present the first evidence for a regulatory role for Tiam1/Rac1-sensitive signaling step in GSIS. They also provide evidence for the existence of a potential Rac1/Tiam1-independent, but calcium-sensitive component for GSIS in these cells.

Anti-inflammatory effects of an inflammatory chemokine: CCL2 inhibits lymphocyte homing by modulation of CCL21-triggered integrin-mediated adhesions

Our studies focus on the pathways that restrict homing of specific subsets of immune cells, and thereby fine-tune the immune response at specific lymphoid and peripheral tissues. Here, we report that CCL2 (at picomolar [pM] levels) renders both murine and human T cells defective in their ability to develop CCR7-triggered activation of LFA-1- and LFA-1-mediated adhesion strengthening to endothelial ICAM-1 both in vitro and in vivo. CCL2 also attenuated lymphocyte chemotaxis toward lymph node chemokines. Consequently, low-dose CCL2 inhibited lymphocyte homing to peripheral lymph nodes but did not affect lymphocyte trafficking through the spleen. Impaired homing of lymphocytes to peripheral lymph nodes resulted in attenuated progression of both asthma and adjuvant arthritis. Thus, pM levels of circulating CCL2 can exert global suppressive effects on T-cell trafficking and differentiation within peripheral lymph nodes, and may be clinically beneficial as an anti-inflammatory agent.

Genomic expression profiling of TNF-alpha-treated BDC2.5 diabetogenic CD4+ T cells

TNF-alpha plays an important role in immune regulation, inflammation, and autoimmunity. Chronic TNF exposure has been shown to down-modulate T cell responses. In a mouse T cell hybridoma model, TNF attenuated T cell receptor (TCR) signaling. We have confirmed that chronic TNF and anti-TNF exposure suppressed and increased T cell responses, respectively. In adult TCR (BDC2.5) transgenic nonobese diabetic mice, DNA microarray analysis of global gene expression in BDC2.5 CD4(+) T cells in response to chronic TNF or anti-TNF exposure showed that genes involved in functional categories including T cell signaling, cell cycle, proliferation, ubiquitination, cytokine synthesis, calcium signaling, and apoptosis were modulated. Genes such as ubiquitin family genes, cytokine inducible Src homology 2-containing genes, cyclin-dependent kinase inhibitors p21, p57, calmodulin family genes (calmodulin-1, -2, and -3) and calcium channel voltage-dependent, N type alpha1B subunit (CaV2.2) were induced by TNF, whereas Vav2, Rho GTPase-activating protein, calcium channel voltage-dependent, L type alpha1C subunit (CaV1.2), IL-1 receptor-associated kinase-1 and -2, and IL enhancer binding factor 3 were reduced by TNF. Genes such as CaV1.2 and proliferating cell nuclear antigen, repressed by TNF, were induced by anti-TNF treatment. Further, we showed that chronic TNF exposure impaired NF-kappaB and adaptor protein 1 transactivation activity, leading to T cell unresponsiveness. Thus, our results present a detailed picture of transcriptional programs affected by chronic TNF exposure and provide candidate target genes that may function to mediate TNF-induced T cell unresponsiveness.

A fundamental role for interleukin-21 in the generation of T follicular helper cells

T cell help to B cells is a fundamental property of adaptive immunity, yet only recently have many of the cellular and molecular mechanisms of T cell help emerged. T follicular helper (Tfh) cells are the CD4(+) T helper cells that provide cognate help to B cells for high-affinity antibody production in germinal centers (GC). Tfh cells produce interleukin-21 (IL-21), and we show that IL-21 was necessary for GC formation. However, the central role of IL-21 in GC formation reflected its effects on Tfh cell generation rather than on B cells. Expression of the inducible costimulator (ICOS) was necessary for optimal production of IL-21, indicative of interplay between these two Tfh cell-expressed molecules. Finally, we demonstrate that IL-21's costimulatory capacity for T helper cell differentiation operated at the level of the T cell receptor signalosome through Vav1, a signaling molecule that controls T cell helper function. This study reveals a previously unappreciated role for Tfh cells in the formation of the GC and isotype switching through a CD4(+) T cell-intrinsic requirement for IL-21.

Crucial structural role for the PH and C1 domains of the Vav1 exchange factor

The Vav family of proteins are guanine nucleotide exchange factors (GEFs) for the Rho family of GTPases, which regulate various cellular functions, including T-cell activation. They contain a catalytic Dbl homology (DH) domain that is invariably followed by a pleckstrin homology (PH) domain, which is often required for catalytic activity. Vav proteins are the first GEFs for which an additional C1 domain is required for full biological activity. Here, we present the structure of a Vav1 fragment comprising the DH-PH-C1 domains bound to Rac1. This structure shows that the PH and C1 domains form a single structural unit that packs against the carboxy-terminal helix of the DH domain to stabilize its conformation and to promote nucleotide exchange. In contrast to previous reports, this structure shows that there are no direct contacts between the GTPase and C1 domain but instead suggests new mechanisms for the regulation of Vav1 activity.

Endogenous RhoG is dispensable for integrin-mediated cell spreading but contributes to Rac-independent migration

Rac activation by integrins is essential for cell spreading, migration, growth and survival. Based mainly on overexpression of dominant-negative mutants, RhoG has been proposed to mediate integrin-dependent Rac activation upstream of ELMO and Dock180. RhoG-knockout mice, however, display no significant developmental or functional abnormalities. To clarify the role of RhoG in integrin-mediated signaling, we developed a RhoG-specific antibody, which, together with shRNA-mediated knockdown, allowed analysis of the endogenous protein. Despite dramatic effects of dominant-negative constructs, nearly complete RhoG depletion did not substantially inhibit cell adhesion, spreading, migration or Rac activation. Additionally, RhoG was not detectably activated by adhesion to fibronectin. Using Rac1(-/-) cells, we found that constitutively active RhoG induced membrane ruffling via both Rac-dependent and -independent pathways. Additionally, endogenous RhoG was important for Rac-independent cell migration. However, RhoG did not significantly contribute to cell spreading even in these cells. These data therefore clarify the role of RhoG in integrin signaling and cell motility.

Tec kinases, actin, and cell adhesion

The Tec family non-receptor tyrosine kinases have been recognized for their roles in the regulation of phospholipase C-gamma and Ca(2+) mobilization downstream from antigen receptors on lymphocytes. Recent data, however, show that the Tec family kinase interleukin-2-inducible T-cell kinase (Itk) also participates in pathways regulating the actin cytoskeleton and 'inside-out' signaling to integrins downstream from the T-cell antigen receptor. Data suggest that Itk may function in a kinase-independent fashion to regulate proper recruitment of the Vav1 guanine nucleotide exchange factor. By enhancing actin cytoskeleton reorganization, recruitment of signaling molecules to the immune synapse, and integrin clustering in response to both antigen and chemokine receptors, the Tec kinases serve as modulators or amplifiers that can increase the duration of T-cell signaling and regulate T-cell functional responses.