Rap1 is a potent activation signal for leukocyte function-associated antigen 1 distinct from protein kinase C and phosphatidylinositol-3-OH kinase

RasGRP3 limits Toll-like receptor-triggered inflammatory response in macrophages by activating Rap1 small GTPase

Host immune cells can detect and destruct invading pathogens via pattern-recognition receptors. Small Rap GTPases act as conserved molecular switches coupling extracellular signals to various cellular responses, but their roles as regulators in Toll-like receptor (TLR) signalling have not been fully elucidated. Here we report that Ras guanine nucleotide-releasing protein 3 (RasGRP3), a guanine nucleotide-exchange factor activating Ras and Rap1, limits production of proinflammatory cytokines (especially IL-6) in macrophages by activating Rap1 on activation by low levels of TLR agonists. We demonstrate that RasGRP3, a dominant member of RasGRPs in macrophages, impairs TLR3/4/9-induced IL-6 production and relieves dextrane sulphate sodium-induced colitis and collagen-induced arthritis. In RasGRP3-deficient RAW264.7 cells obtained by CRISPR-Cas9 genome editing, TLR3/4/9-induced activation of Rap1 was inhibited while ERK1/2 activation was enhanced. Our study suggests that RasGRP3 limits inflammatory response by activating Rap1 on low-intensity pathogen infection, setting a threshold for preventing excessive inflammatory response.

Toll like receptors (TLRs) couple microbial sensing to initiation of immune responses, which are essential for defense against pathogens but may cause immunopathology when activated excessively. Here the authors show that RasGRP3 sets a threshold of TLR activation to prevent immunopathology.

Rab13 acts downstream of the kinase Mst1 to deliver the integrin LFA-1 to the cell surface for lymphocyte trafficking

In lymphocytes, the kinase Mst1 is required for the proper organization of integrins in the plasma membrane at the leading edge of migrating cells, which is critical for lymphocyte trafficking. We found a functional link between the small G protein Rab13 and Mst1 in lymphocyte adhesion and migration. In response to stimulation of T lymphocytes with chemokine, Mst1 promoted phosphorylation of the guanine nucleotide exchange factor DENND1C (differentially expressed in normal and neoplastic cells domain 1C), which activated Rab13. Active Rab13 associated with Mst1 to facilitate the delivery of the integrin LFA-1 (lymphocyte function-associated antigen 1) to the leading edge of lymphocytes. Delivery of LFA-1 involved the recruitment of myosin Va along actin filaments, which extended as a result of the localization of the actin regulatory protein VASP to the cell periphery through phosphorylation of VASP at Ser(157) by Mst1. Inhibition of Rab13 function reduced the adhesion and migration of lymphocytes on ICAM-1 (intercellular adhesion molecule-1), the ligand for LFA-1, and inhibited the formation of a ring-like arrangement of LFA-1 at the contact sites between T cells and antigen-presenting cells. The lymphoid tissues of Rab13-deficient mice had reduced numbers of lymphocytes because of the defective trafficking capability of these cells. These results suggest that Rab13 acts with Mst1 to regulate the spatial distribution of LFA-1 and the motility and trafficking of lymphocytes.

PI3K is involved in β1 integrin clustering by PSGL-1 and promotes β1 integrin-mediated Jurkat cell adhesion to fibronectin

P-selectin glycoprotein ligand-1 (PSGL-1) is involved in the initial step of lymphocyte homing by interacting with P- or E-selectins expressed on activated endothelium cells. Besides, it also functions as a receptor to induce signals that increase integrin affinity to ligands and mediate cell adhesion to endothelium. Integrin is required for the second step of lymphocyte homing, whose activation has been reported tightly regulated by inside-out signals triggered by chemokines or the shear-stress generated during lymphocyte rolling on endothelium. However, the relationship between PSGL-1-triggered signals and integrin activation is not clear. In this study, we demonstrated that PSGL-1 ligation induces β1 integrin-mediated adhesion to fibronectin via regulation of both β1 subunit clustering and conformation changes. Phosphoinositide 3-kinase (PI3K) is required for PSGL-1-induced β1 integrin clustering which ultimately regulates β1 integrin-mediated Jurkat cell adhesion to fibronectin. However, PI3K is not involved in the conformation changes or increases in the total expression of β1 integrin. Taken together, we found a novel signal pathway, PSGL-1-PI3K-β1 integrin, demonstrating the cooperation between initial adhesion and subsequent arrest and stable adhesion.

Coxsackievirus B3 regulates T-cell infiltration into the heart by lymphocyte function-associated antigen-1 activation via the cAMP/Rap1 axis

Coxsackievirus B3 (CVB3) infection can trigger myocarditis and can ultimately lead to dilated cardiomyopathy. It is known that CVB3-induced T-cell infiltration into cardiac tissues is one of the pathological factors causing cardiomyocyte injury by inflammation. However, the underlying mechanism for this remains unclear. We investigated the mechanism of T-cell infiltration by two types of CVB3: the H3 WT strain and the YYFF attenuated strain. T-cell activation was confirmed by changes in the distribution of lymphocyte function-associated antigen-1 (LFA-1). Finally, we identified which viral gene was responsible for LFA-1 activation. CVB3 could infect and activate T-cells in vivo and in vitro, and activated T-cells were detected in CVB3-infected mouse hearts. LFA-1 expressed on the surface of these T-cells had been activated through the cAMP/Rap1 pathway. Recombinant lentiviruses expressing VP2 of CVB3 could also induce LFA-1 activation via an increase in cAMP, whilst VP2 of YYFF did not. These results indicated that CVB3 infection increased cAMP levels and then activated Rap1 in T-cells. In particular, VP2, among the CVB3 proteins, might be critical for this activation. This VP2-cAMP-Rap1-LFA-1 axis could be a potential therapeutic target for treating CVB3-induced myocarditis.

A talin mutant that impairs talin-integrin binding in platelets decelerates αIIbβ3 activation without pathological bleeding

Tight regulation of integrin affinity is critical for hemostasis. A final step of integrin activation is talin binding to 2 sites within the integrin β cytoplasmic domain. Binding of talin to a membrane-distal NPxY sequence facilitates a second, weaker interaction of talin with an integrin membrane-proximal region (MPR) that is critical for integrin activation. To test the functional significance of these distinct interactions on platelet function in vivo, we generated knock-in mice expressing talin1 mutants with impaired capacity to interact with the β3 integrin MPR (L325R) or NPLY sequence (W359A). Both talin1(L325R) and talin1(W359A) mice were protected from experimental thrombosis. Talin1(L325R) mice, but not talin(W359A) mice, exhibited a severe bleeding phenotype. Activation of αIIbβ3 was completely blocked in talin1(L325R) platelets, whereas activation was reduced by approximately 50% in talin1(W359A) platelets. Quantitative biochemical measurements detected talin1(W359A) binding to β3 integrin, albeit with a 2.9-fold lower affinity than wild-type talin1. The rate of αIIbβ3 activation was slower in talin1(W359A) platelets, which consequently delayed aggregation under static conditions and reduced thrombus formation under physiological flow conditions. Together our data indicate that reduction of talin-β3 integrin binding affinity results in decelerated αIIbβ3 integrin activation and protection from arterial thrombosis without pathological bleeding.

Control of epithelial cell migration and invasion by the IKKβ- and CK1α-mediated degradation of RAPGEF2

Epithelial cell migration is crucial for the development and regeneration of epithelial tissues. Aberrant regulation of epithelial cell migration has a major role in pathological processes such as the development of cancer metastasis and tissue fibrosis. Here, we report that in response to factors that promote cell motility, the Rap guanine exchange factor RAPGEF2 is rapidly phosphorylated by I-kappa-B-kinase-β and casein kinase-1α and consequently degraded by the proteasome via the SCF(βTrCP) ubiquitin ligase. Failure to degrade RAPGEF2 in epithelial cells results in sustained activity of Rap1 and inhibition of cell migration induced by HGF, a potent metastatic factor. Furthermore, expression of a degradation-resistant RAPGEF2 mutant greatly suppresses dissemination and metastasis of human breast cancer cells. These findings reveal a molecular mechanism regulating migration and invasion of epithelial cells and establish a key direct link between IKKβ and cell motility controlled by Rap-integrin signaling.

Ezrin and moesin are required for efficient T cell adhesion and homing to lymphoid organs

T cell trafficking between the blood and lymphoid organs is a complex, multistep process that requires several highly dynamic and coordinated changes in cyto-architecture. Members of the ezrin, radixin and moesin (ERM) family of actin-binding proteins have been implicated in several aspects of this process, but studies have yielded conflicting results. Using mice with a conditional deletion of ezrin in CD4+ cells and moesin-specific siRNA, we generated T cells lacking ERM proteins, and investigated the effect on specific events required for T cell trafficking. ERM-deficient T cells migrated normally in multiple in vitro and in vivo assays, and could undergo efficient diapedesis in vitro. However, these cells were impaired in their ability to adhere to the β1 integrin ligand fibronectin, and to polarize appropriately in response to fibronectin and VCAM-1 binding. This defect was specific for β1 integrins, as adhesion and polarization in response to ICAM-1 were normal. In vivo, ERM-deficient T cells showed defects in homing to lymphoid organs. Taken together, these results show that ERM proteins are largely dispensable for T cell chemotaxis, but are important for β1 integrin function and homing to lymphoid organs.

RIAM (Rap1-interacting adaptor molecule) regulates complement-dependent phagocytosis

Phagocytosis mediated by the complement receptor CR3 (also known as integrin αMß2 or Mac-1) is regulated by the recruitment of talin to the cytoplasmic tail of the ß2 integrin subunit. Talin recruitment to this integrin is dependent on Rap1 activation. However, the mechanism by which Rap1 regulates this event and CR3-dependent phagocytosis remains largely unknown. In the present work, we examined the role of the Rap1 effector RIAM, a talin-binding protein, in the regulation of complement-mediated phagocytosis. Using the human myeloid cell lines HL-60 and THP-1, we determined that knockdown of RIAM impaired αMß2 integrin affinity changes induced by stimuli fMLP and LPS. Phagocytosis of complement-opsonized RBC particles, but not of IgG-opsonized RBC particles, was impaired in RIAM knockdown cells. Rap1 activation via EPAC induced by 8-pCPT-2'-O-Me-cAMP resulted in an increase of complement-mediated phagocytosis that was abrogated by knockdown of RIAM in HL-60 and THP-1 cell lines and in macrophages derived from primary monocytes. Furthermore, recruitment of talin to ß2 integrin during complement-mediated phagocytosis was reduced in RIAM knockdown cells. These results indicate that RIAM is a critical component of the phagocytosis machinery downstream of Rap1 and mediates its function by recruiting talin to the phagocytic complement receptors.

Association of SIPA1 545 C > T polymorphism with survival in Chinese women with metastatic breast cancer

It has been demonstrated that single nucleotide polymorphisms (SNPs) of SIPA1 (signal-induced proliferation associated gene 1) are associated with metastatic efficiency in both human and rodents. The purpose of this study was to determine whether SIPA1 545 C > T polymorphism was associated with overall survival in patients with metastatic breast cancer. In this study, SIPA1 545 C > T polymorphism was detected in 185 metastatic breast cancer patients using polymerase chain reaction-restriction fragment length polymorphism assay (PCR-RFLP). Survival curves for patients with SIPA1 545 C > T polymorphism was compared using the Kaplan-Meier method with log-rank tests. We found that SIPA1 545 C > T polymorphism was significantly associated with survival in 185 patients with metastatic breast cancer. Patients with SIPA1 545 T/T genotype had a significantly worse overall survival (OS) than did patients with C/T or C/C genotype (50.0% vs. 62.9%, P = 0.042). Moreover, in multivariate analysis, as compared with the C/C or C/T genotype, the T/T genotype remained an independent unfavorable prognostic marker of OS in this cohort (hazard ratio [HR] = 2.16; 95% CI = 1.12-4.15; P = 0.022). Our findings indicate that metastatic breast cancer patients with SIPA1 545 T/T genotype have a poorer survival compared to patients with C/C or C/T genotype.

Rap1 can bypass the FAK-Src-Paxillin cascade to induce cell spreading and focal adhesion formation

We developed new image analysis tools to analyse quantitatively the extracellular-matrix-dependent cell spreading process imaged by live-cell epifluorescence microscopy. Using these tools, we investigated cell spreading induced by activation of the small GTPase, Rap1. After replating and initial adhesion, unstimulated cells exhibited extensive protrusion and retraction as their spread area increased, and displayed an angular shape that was remodelled over time. In contrast, activation of endogenous Rap1, via 007-mediated stimulation of Epac1, induced protrusion along the entire cell periphery, resulting in a rounder spread surface, an accelerated spreading rate and an increased spread area compared to control cells. Whereas basal, anisotropic, spreading was completely dependent on Src activity, Rap1-induced spreading was refractory to Src inhibition. Under Src inhibited conditions, the characteristic Src-induced tyrosine phosphorylations of FAK and paxillin did not occur, but Rap1 could induce the formation of actomyosin-connected adhesions, which contained vinculin at levels comparable to that found in unperturbed focal adhesions. From these results, we conclude that Rap1 can induce cell adhesion and stimulate an accelerated rate of cell spreading through mechanisms that bypass the canonical FAK-Src-Paxillin signalling cascade.

Phosphoinositide 3-OH kinase regulates integrin-dependent processes in neutrophils by signaling through its effector ARAP3

ARAP3, a GTPase activating protein for Rho and Arf family GTPases, is one of many phosphoinositide 3-OH kinase (PI3K) effectors. In this study, we investigate the regulatory input of PI3K upstream of ARAP3 by analyzing neutrophils from an ARAP3 pleckstrin homology (PH) domain point mutation knock-in mouse (R302, 303A), in which ARAP3 is uncoupled from activation by PI3K. ARAP3 PH domain point mutant neutrophils are characterized by disturbed responses linked to stimulation by either integrin ligands or immobilized immune complexes. These cells exhibit increased β2 integrin inside-out signaling (binding affinity and avidity), and our work suggests the disturbed responses to immobilized immune complexes are secondary to this. In vitro, neutrophil chemotaxis is affected in the mutant. In vivo, ARAP3 PH domain point mutant bone marrow chimeras exhibit reduced neutrophil recruitment to the peritoneum on induction of sterile peritonitis and also reduced inflammation in a model for rheumatoid arthritis. The current work suggests a dramatic regulatory input of PI3K into the regulation of β2 integrin activity, and processes dependent on this, by signaling through its effector ARAP3.

Radil controls neutrophil adhesion and motility through β2-integrin activation

Various agonists trigger β2-integrin activation in neutrophils, yet the mechanisms that regulate β2-integrin inside-out signaling remain obscure. Radil, a novel Rap downstream effector, is an important adapter in the pathway that links G protein–coupled chemoattractant receptors to adhesion complexes during neutrophil chemotaxis.

Integrin activation is required to facilitate multiple adhesion-dependent functions of neutrophils, such as chemotaxis, which is critical for inflammatory responses to injury and pathogens. However, little is known about the mechanisms that mediate integrin activation in neutrophils. We show that Radil, a novel Rap1 effector, regulates β1- and β2-integrin activation and controls neutrophil chemotaxis. On activation and chemotactic migration of neutrophils, Radil quickly translocates from the cytoplasm to the plasma membrane in a Rap1a-GTP–dependent manner. Cells overexpressing Radil show a substantial increase in cell adhesion, as well as in integrin/focal adhesion kinase (FAK) activation, and exhibit an elongated morphology, with severe tail retraction defects. This phenotype is effectively rescued by treatment with either β2-integrin inhibitory antibodies or FAK inhibitors. Conversely, knockdown of Radil causes severe inhibition of cell adhesion, β2-integrin activation, and chemotaxis. Furthermore, we found that inhibition of Rap activity by RapGAP coexpression inhibits Radil-mediated integrin and FAK activation, decreases cell adhesion, and abrogates the long-tail phenotype of Radil cells. Overall, these studies establish that Radil regulates neutrophil adhesion and motility by linking Rap1 to β2-integrin activation.

Caffeic acid inhibits acute hyperhomocysteinemia-induced leukocyte rolling and adhesion in mouse cerebral venules

OBJECTIVE

To investigate the effects and possible mechanisms of CA on acute HHcy-induced leukocyte rolling and adhesion in mouse cerebral venules.

METHODS

Male C57 BL/6J mice were injected with DL-Hcy (50 mg/kg) and CA (10 mg/kg). The effect of CA on HHcy-induced leukocyte rolling and adhesion in cerebral vessels was assessed using intravital microscopy. Plasma cytokines and chemokines were evaluated by cytometric bead array. ROS production in HUVECs and adhesion molecule expression on leukocytes were determined by flow cytometry. E-selectin and ICAM-1 expression in cerebrovascular endothelium was detected by immunohistochemistry. CD18 phosphorylation and the Src/PI3K/Akt pathway in leukocytes were determined by confocal microscopy and Western blot.

RESULTS

CA inhibited HHcy-elicited leukocyte rolling and adhesion, decreased ROS production in HUVECs, and reduced plasma KC, MIP-2, and MCP-1 levels. CA reduced the E-selectin and ICAM-1 expression on cerebrovascular endothelium and CD11b/CD18 on leukocytes caused by HHcy. Of notice, CA depressed CD18 phosphorylation and the Src/PI3K/Akt pathway in leukocytes.

CONCLUSIONS

CA inhibited HHcy-provoked leukocyte rolling and adhesion in cerebral venules, ameliorating adhesion molecule expression and activation, which is related to the suppression of the Src/PI3K/Akt pathway in leukocytes.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

The Small GTPase Rap1b: A Bidirectional Regulator of Platelet Adhesion Receptors

Integrins and other families of cell adhesion receptors are responsible for platelet adhesion and aggregation, which are essential steps for physiological haemostasis, as well as for the development of thrombosis. The modulation of platelet adhesive properties is the result of a complex pattern of inside-out and outside-in signaling pathways, in which the members of the Rap family of small GTPases are bidirectionally involved. This paper focuses on the regulation of the main Rap GTPase expressed in circulating platelets, Rap1b, downstream of adhesion receptors, and summarizes the most recent achievements in the investigation of the function of this protein as regulator of platelet adhesion and thrombus formation.

Neoplastic leptomeningitis presenting in a melanoma patient treated with dabrafenib (a V600EBRAF inhibitor): a case report

INTRODUCTION

Leptomeningeal metastases are occurring at higher frequency in cancer patients. The prognosis of leptomeningeal metastases is poor and standard treatment, which includes radiotherapy and chemotherapy, is mostly ineffective. Melanoma represents one of the tumors with the highest incidence of leptomeningeal metastases. For such a disease, the BRAF inhibitors have recently been demonstrated to be effective on melanoma brain metastases harboring the V600EBRAF mutation.

CASE PRESENTATION

We report a case of a 39-year-old Italian woman with advanced melanoma with brain, lung and peritoneum metastases harboring the V600EBRAF mutation. In August 2010 she was enrolled into the BRIM3 trial and after the randomization process she received dacarbazine. After two cycles, there was evidence of disease progression in her peritoneum and lung. For this reason, she was enrolled into another clinical trial with the GSK2118436 BRAF inhibitor, dabrafenib, as a second line of therapy. She had a partial response that was maintained until 13 weeks of treatment. In January 2011 she developed symptoms typical for brain metastases and received a diagnosis of leptomeningeal involvement of melanoma cells after an examination of her cerebral spinal fluid; magnetic resonance imaging was negative for meningitis or brain metastases. Analysis of her cerebral spinal fluid sample confirmed that the melanoma cells still carried the V600EBRAF mutation. After a few days, our patient went into a coma and died.

CONCLUSION

Starting with a clinical case, we discuss the pathogenesis of leptomeningeal metastases and whether the leptomeninges may represent a sanctuary where melanoma cells may generate resistance and/or BRAF inhibitors cannot reach an adequate concentration for significant activity. We assess whether treatment with BRAF inhibitors in melanoma patients should be interrupted as soon as disease progression appears or continued beyond progression, through the administration of additional compounds.

Central role of paxillin phosphorylation in regulation of LFA-1 integrins activity and lymphocyte migration

Coordinated changes of actin cytoskeleton and cell adhesion accompany maturation of lymphoid cells, their migration through lymphoid organs and to sites of inflammation, as well as metastasis of transformed cells. Here we discuss the central role of the actin-regulating adaptor protein, paxillin, during lymphocyte transition from a polarized, motile cell phenotype with partially active LFA-1 integrins to a round and immobile one with fully active LFA-1. In Baf3 murine pro-B lymphocytes, the former phenotype is induced by IL-3 that stimulates a FAK-mediated phosphorylation of paxillin at tyrosines (Y) 31 and 118 and a consequent Rac1 activation. Rearrangements of actin cytoskeleton that lead to the cell's acquisition of a spherical shape and LFA-1 activation are achieved upon activation of PKC-δ that binds and directly phosphorylates paxillin at threonine (T) 538 with consequent RhoA activation. This is accompanied by dephosphorylation of paxillin Y31/118 and by Rac1 inactivation. We propose a model of signaling cascades that reflects the interplay between the IL-3- and PKC-δ-mediated pathways.

Binding properties and stability of the Ras-association domain of Rap1-GTP interacting adapter molecule (RIAM)

The Rap1-GTP interacting adapter protein (RIAM) is an important protein in Rap1-mediated integrin activation. By binding to both Rap1 GTPase and talin, RIAM recruits talin to the cell membrane, thus facilitating talin-dependent integrin activation. In this article, we studied the role of the RIAM Ras-association (RA) and pleckstrin-homology (PH) domains in the interaction with Rap1. We found that the RA domain was sufficient for GTP-dependent interaction with Rap1B, and the addition of the PH domain did not change the binding affinity. We also detected GTP-independent interaction of Rap1B with the N-terminus of RIAM. In addition, we found that the PH domain stabilized the RA domain both in vitro and in cells.

Ras family of small GTPases in immunity and inflammation

The Ras superfamily of small GTPases is a group of more than 150 small G proteins, all of which share some degree of homology to the founding member Ras. These small GTPases function as molecular switches within cells, impacting nearly all cellular processes. The Ras superfamily can be further divided into several smaller subfamilies, with those proteins that most closely resemble Ras belonging to the Ras subfamily. While heavily studied within the field of cancer biology, the Ras family of proteins also plays cardinal roles in immunity and inflammation. Here we review the roles of these molecular switches in regulating immune cell homeostasis and functions.

Role of Epac proteins in mechanisms of cAMP-dependent immunoregulation

This review presents observations on the role of Epac proteins (exchange protein directly activated by cAMP) in immunoregulation mechanisms. Signaling pathways that involve Epac proteins and their domain organization and functions are considered. The role of Epac1 protein expressed in the immune system cells is especially emphasized. Molecular mechanisms of the cAMP-dependent signal via Epac1 are analyzed in monocytes/macrophages, T-cells, and B-lymphocytes. The role of Epac1 is shown in the regulation of adhesion, leukocyte chemotaxis, as well as in phagocytosis and bacterial killing. The molecular cascade initiated by Epac1 is examined under conditions of antigen activation of T-cells and immature B-lymphocytes.

An integrated stochastic model of "inside-out" integrin activation and selective T-lymphocyte recruitment

The pattern of T-lymphocyte homing is hypothesized to be controlled by combinations of chemokine receptors and complementary chemokines. Here, we use numerical simulation to explore the relationship among chemokine potency and concentration, signal transduction, and adhesion. We have developed a form of adhesive dynamics-a mechanically accurate stochastic simulation of adhesion-that incorporates stochastic signal transduction using the next subvolume method. We show that using measurable parameter estimates derived from a variety of sources, including signaling measurements that allow us to test parameter values, we can readily simulate approximate time scales for T-lymphocyte arrest. We find that adhesion correlates with total chemokine receptor occupancy, not the frequency of occupation, when multiple chemokine receptors feed through a single G-protein. A general strategy for selective T-lymphocyte recruitment appears to require low affinity chemokine receptors. For a single chemokine receptor, increases in multiple cross-reactive chemokines can lead to an overwhelming increase in adhesion. Overall, the methods presented here provide a predictive framework for understanding chemokine control of T-lymphocyte recruitment.

Overview of integrin signaling in the immune system

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

Ras-related GTPases Rap1 and RhoA collectively induce the phagocytosis of serum-opsonized zymosan particles in macrophages

Phagocytosis occurs primarily through two main processes in macrophages: the Fcγ receptor- and the integrin αMβ2-mediated processes. Complement C3bi-opsonized particles are known to be engulfed through integrin αMβ2-mediated process, which is regulated by RhoA GTPase. C3 toxin fused with Tat-peptide (Tat-C3 toxin), an inhibitor of the Rho GTPases, was shown to markedly inhibit the phagocytosis of serum (C3bi)-opsonized zymosans (SOZs). However, 8CPT-2Me-cAMP, an activator of exchange protein directly activated by cAMP (Epac, Rap1 guanine nucleotide exchange factor), restored the phagocytosis of the SOZs that was previously inhibited by the Tat-C3 toxin. In addition, a constitutively active form of Rap1 GTPase (CA-Rap1) also restored the phagocytosis that was previously reduced by a dominant negative form of RhoA GTPase (DN-RhoA). This suggests that Rap1 can replace the function of RhoA in the phagocytosis. Inversely, CA-RhoA rescued the phagocytosis that was suppressed by DN-Rap1. These findings suggest that both RhoA and Rap1 GTPases collectively regulate the phagocytosis of SOZs. In addition, filamentous actin was reduced by the Tat-C3 toxin, which was again restored by 8CPT-2Me-cAMP. Small interfering profilin suppressed the phagocytosis, suggesting that profilin is essential for the phagocytosis of SOZs. Furthermore, 8CPT-2Me-cAMP increased the co-immunoprecipitation of profilin with Rap1, whereas Tat-C3 toxin decreased that of profilin with RhoA. Co-immunoprecipitations of profilin with actin, Rap1, and RhoA GTPases were augmented in the presence of GTPγS rather than GDP. Therefore, we propose that both Rap1 and RhoA GTPases regulate the formation of filamentous actin through the interaction between actin and profilin, thereby collectively inducing the phagocytosis of SOZs in macrophages.

Rho GTPases: masters of T lymphocyte migration and activation

Rho GTPases are key signal transducer elements activated in T cells by both chemokine and antigen receptors. These two signalling pathways control the two main functions of T lymphocytes: motility and activation. Rho GTPases are thus crucial for the development of an adequate immune response. In this review, we mostly focus on the roles of RhoA, Rac1 and Cdc42 in T cells. We show their importance in phenomena such as adhesion, morphological polarization, migration and antigen recognition.

Homing of progenitor cells to ischemic tissues

Progenitor cells mobilized from the bone marrow are recruited to ischemic tissues and increase neovascularization. Cell therapy is a promising new therapeutic option for treating patients with ischemic disorders. The efficiency of cell therapy to augment recovery after ischemia depends on the sufficient recruitment and engraftment of the cells to the target tissue. Homing to sites of active neovascularization is a complex process depending on a timely and spatially orchestrated interplay between chemokines, chemokine receptors, adhesion molecules (selectins and integrins), and intracellular signaling cascades, including also oxidative signaling. This review will focus on the homing mechanisms of progenitor and stem cells to ischemic tissues. Specifically, we discuss the role of chemokines and adhesion molecules such as selectins and integrins and the crosstalk between chemokines and integrins in progenitor cell homing.

SKAP1 protein PH domain determines RapL membrane localization and Rap1 protein complex formation for T cell receptor (TCR) activation of LFA-1

Although essential for T cell function, the identity of the T cell receptor (TCR) "inside-out" pathway for the activation of lymphocyte function-associated antigen 1 (LFA-1) is unclear. SKAP1 (SKAP-55) is the upstream regulator needed for TCR-induced RapL-Rap1 complex formation and LFA-1 activation. In this paper, we show that SKAP1 is needed for RapL binding to membranes in a manner dependent on the PH domain of SKAP1 and the PI3K pathway. A SKAP1 PH domain-inactivating mutation (i.e. R131M) markedly impaired RapL translocation to membranes for Rap1 and LFA-1 binding and the up-regulation of LFA-1-intercellular adhesion molecule 1 (ICAM-1) binding. Further, N-terminal myr-tagged SKAP1 for membrane binding facilitated constitutive RapL membrane and Rap1 binding and effectively substituted for PI3K and TCR ligation in the activation of LFA-1 in T cells.

Priming of eosinophils by GM-CSF is mediated by protein kinase CbetaII-phosphorylated L-plastin

The priming of eosinophils by cytokines leading to augmented response to chemoattractants and degranulating stimuli is a characteristic feature of eosinophils in the course of allergic inflammation and asthma. Actin reorganization and integrin activation are implicated in eosinophil priming by GM-CSF, but their molecular mechanism of action is unknown. In this regard, we investigated the role of L-plastin, an eosinophil phosphoprotein that we identified from eosinophil proteome analysis. Phosphoproteomic analysis demonstrated the upregulation of phosphorylated L-plastin after eosinophil stimulation with GM-CSF. Additionally, coimmunoprecipitation studies demonstrated a complex formation of phosphorylated L-plastin with protein kinase CβII (PKCβII), GM-CSF receptor α-chain, and two actin-associated proteins, paxilin and cofilin. Inhibition of PKCβII with 4,5-bis(4-fluoroanilino)phtalimide or PKCβII-specific small interfering RNA blocked GM-CSF-induced phosphorylation of L-plastin. Furthermore, flow cytometric analysis also showed an upregulation of α(M)β(2) integrin, which was sensitive to PKCβII inhibition. In chemotaxis assay, GM-CSF treatment allowed eosinophils to respond to lower concentrations of eotaxin, which was abrogated by the above-mentioned PKCβII inhibitors. Similarly, inhibition of PKCβII blocked GM-CSF induced priming for degranulation as assessed by release of eosinophil cationic protein and eosinophil peroxidase in response to eotaxin. Importantly, eosinophil stimulation with a synthetic L-plastin peptide (residues 2-19) phosphorylated on Ser(5) upregulated α(M)β(2) integrin expression and increased eosinophil migration in response to eotaxin independent of GM-CSF stimulation. Our results establish a causative role for PKCβII and L-plastin in linking GM-CSF-induced eosinophil priming for chemotaxis and degranulation to signaling events associated with integrin activation via induction of PKCβII-mediated L-plastin phosphorylation.

Ezrin is required for efficient Rap1-induced cell spreading

The Rap family of small GTPases regulate the adhesion of cells to extracellular matrices. Several Rap-binding proteins have been shown to function as effectors that mediate Rap-induced adhesion. However, little is known regarding the relationships between these effectors, or about other proteins that are downstream of or act in parallel to the effectors. To establish whether an array of effectors was required for Rap-induced cell adhesion and spreading, and to find new components involved in Rap-signal transduction, we performed a small-scale siRNA screen in A549 lung epithelial cells. Of the Rap effectors tested, only Radil blocked Rap-induced spreading. Additionally, we identified a novel role for Ezrin downstream of Rap1. Ezrin was necessary for Rap-induced cell spreading, but not Rap-induced cell adhesion or basal adhesion processes. Furthermore, Ezrin depletion inhibited Rap-induced cell spreading in several cell lines, including primary human umbilical vein endothelial cells. Interestingly, Radixin and Moesin, two proteins with high homology to Ezrin, are not required for Rap-induced cell spreading and cannot compensate for loss of Ezrin to rescue Rap-induced cell spreading. Here, we present a novel function for Ezrin in Rap1-induced cell spreading and evidence of a non-redundant role of an ERM family member.

RGMa modulates T cell responses and is involved in autoimmune encephalomyelitis

In multiple sclerosis, activated CD4(+) T cells initiate an immune response in the brain and spinal cord, resulting in demyelination, degeneration and progressive paralysis. Repulsive guidance molecule-a (RGMa) is an axon guidance molecule that has a role in the visual system and in neural tube closure. Our study shows that RGMa is expressed in bone marrow-derived dendritic cells (BMDCs) and that CD4(+) T cells express neogenin, a receptor for RGMa. Binding of RGMa to CD4(+) T cells led to activation of the small GTPase Rap1 and increased adhesion of T cells to intracellular adhesion molecule-1 (ICAM-1). Neutralizing antibodies to RGMa attenuated clinical symptoms of mouse myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE) and reduced invasion of inflammatory cells into the CNS. Silencing of RGMa in MOG-pulsed BMDCs reduced their capacity to induce EAE following adoptive transfer to naive C57BL/6 mice. CD4(+) T cells isolated from mice treated with an RGMa-specific antibody showed diminished proliferative responses and reduced interferon-γ (IFN-γ), interleukin-2 (IL-2), IL-4 and IL-17 secretion. Incubation of PBMCs from patients with multiple sclerosis with an RGMa-specific antibody reduced proliferative responses and pro-inflammatory cytokine expression. These results demonstrate that an RGMa-specific antibody suppresses T cell responses, and suggest that RGMa could be a promising molecular target for the treatment of multiple sclerosis.

Significance of talin in cancer progression and metastasis

Upon detachment from the extracellular matrix, tumor epithelial cells and tumor-associated endothelial cells are capable of overcoming anoikis, gain survival benefits, and hence contribute to the process of metastasis. The focal-adhesion complex formation recruits the association of key adaptor proteins such as FAK (focal-adhesion kinase). Vimentin, paxillin, and talin are responsible for mediating the interaction between the actin cytoskeleton and integrins. Talin is an early-recruited focal-adhesion player that is of structural and functional significance in mediating interactions with integrin cytoplasmic tails leading to destabilization of the transmembrane complex and resulting in rearrangements in the extracellular integrin compartments that mediate integrin activation. Talin-mediated integrin activation plays a definitive role in integrin-mediated signaling and induction of downstream survival pathways leading to protection from anoikis and consequently resulting in cancer progression to metastasis. We recently reported that talin expression is significantly increased in prostate cancer compared with benign and normal prostate tissue and that this overexpression correlates with progression to metastatic disease implicating a prognostic value for talin during tumor progression. At the molecular level, talin is functionally associated with enhanced survival and proliferation pathways and confers anoikis resistance and metastatic spread of primary tumor cells via activation of the Akt survival pathway. In this review, we discuss the growing evidence surrounding the value of talin as a prognostic marker of cancer progression to metastasis and as therapeutic target in advanced prostate cancer, as well as the current understanding of mechanisms regulating its signaling activity in cancer.

Rap1 and integrin inside-out signaling

In leukocytes, integrins play important roles in adhesive interactions with endothelium, antigen-presenting cells, and effector functions such as cytotoxicity. This chapter describes methods to study Ras proximity 1 (Rap1), a signaling molecule that has been increasingly recognized as an important regulator of integrin-mediated cell adhesion in the immune system as well as hemostasis. Rap1 is activated by a wide variety of external stimuli including chemokines and antigens. Signaling via Rap1 transmits an inside-out signal to the integrins, thereby increasing adhesiveness to ligands such as immunoglobulin superfamily proteins as well as extracellular matrix proteins and plasma proteins. This process induces leukocyte cell adhesion to the endothelium and antigen-presenting cells. In addition to integrin regulation, activated Rap1 induces cell polarity of lymphocytes, which is coordinated with LFA-1 redistribution to the leading edge.

Integrin activation in the immune system

Modulation of leukocyte adhesiveness is critical to leukocyte function during the immune response. A central paradigm in this phenomenon is represented by integrin activation, which is controlled by inside-out signal transduction mechanisms triggered by selectins, chemoattractants and TcR-bound Ag and facilitated by mechanochemical forces. Integrins are heterodimeric adhesive receptors differently expressed on all leukocyte subtypes. At least two distinct modalities of integrin activation are known, namely conformational changes, leading to increased affinity, and lateral mobility leading to increased valency, both enhancing cell avidity (adhesiveness). Several signal transduction events have been correlated to integrin activation in leukocytes. The complexity of intracellular signaling networks leading to leukocyte integrin activation is likely functional to generate robustness and fine tuning of integrin activation allowing integration of qualitative and quantitative variations of extracellular signals leading to leukocyte-, agonist- and integrin-specific control of adhesion. In this context, the recent modular abstraction proposed for the functional architecture of biological networks may provide a powerful paradigm to understand regulation and specificity of signaling events. Accordingly, pro-adhesive intracellular signaling networks may be organized in regulatory signalosomes, or modules, corresponding to discrete clusters of interacting signaling proteins, with some devoted to context-dependent regulation of specificity and dynamics of integrin activation. The principles and technologies of systems biology, and more specifically of network theory, may help to address this complexity and unveil the inner logic governing leukocyte recruitment during the immune response.

Adhesion shapes T cells for prompt and sustained T-cell receptor signalling

During T-cell migration, cell polarity is orchestrated by chemokine receptors and adhesion molecules and involves the functional redistribution of molecules and organelles towards specific cell compartments. In contrast, it is generally believed that the cell polarity established when T cells meet antigen-presenting cells (APCs) is controlled by the triggered T-cell receptor (TCR). Here, we show that, during activation of human T lymphocytes by APCs, chemokines and LFA-1 establish cell polarity independently of TCR triggering. Chemokine-induced LFA-1 activation results in fast recruitment of MTOC and mitochondria towards the potential APC, a process required to amplify TCR Ca(2+) signalling at the upcoming immunological synapse, to promote nuclear translocation of transcriptional factor NFATc2 and boost CD25 expression. Our data show that the initial adhesive signals delivered by chemokines and LFA-1 shape and prepare T cells for antigen recognition.

Sevoflurane binds and allosterically blocks integrin lymphocyte function-associated antigen-1

BACKGROUND

Volatile anesthetics have been shown to modify immune cell functions via several mechanisms, some of which have been only partially elucidated. We demonstrated that isoflurane inhibits primary leukocyte integrin lymphocyte function-associated antigen-1 (LFA-1) by binding to the allosteric cavity critical for conformational activation to its high-affinity form. It remains to be determined whether the allosteric inhibition of LFA-1 by isoflurane can be generalized to other anesthetics such as sevoflurane.

METHODS

The effects of sevoflurane on the ability of LFA-1 to bind to its counter-ligand, intercellular adhesion molecule-1, was studied in leukocytes by flow cytometry. To examine whether sevoflurane acts directly on LFA-1, we measured ligand-binding using beads coated with purified LFA-1 protein. To distinguish between competitive versus allosteric inhibition, we analyzed the effects of sevoflurane on both wild-type and mutant-locked high-affinity LFA-1. One-way analysis of variance was employed for statistical analysis of the data. Nuclear magnetic resonance spectroscopy was used to identify sevoflurane binding site(s).

RESULTS

Sevoflurane at clinically relevant concentrations inhibited the ligand-binding function of LFA-1 in leukocytes as well as in cell-free assays (P<0.05). Sevoflurane blocked wild-type but not locked high-affinity LFA-1, thereby demonstrating an allosteric mode of inhibition. Nuclear magnetic resonance spectroscopy revealed that sevoflurane bound to the allosteric cavity, to which LFA-1 allosteric antagonists and isoflurane also bind.

CONCLUSIONS

This study suggests that sevoflurane also blocks the activation-dependent conformational changes of LFA-1 to the high-affinity form. The allosteric mode of action exemplified by sevoflurane and isoflurane via LFA-1 might represent one of the underlying mechanisms of anesthetic-mediated immunomodulation.

Rap1A regulates generation of T regulatory cells via LFA-1-dependent and LFA-1-independent mechanisms

The small GTPase Rap1A has a critical role in regulating cell-matrix and cell-cell adhesion. In T lymphocytes, Rap1A mediates LFA-1 activation and LFA-1-mediated adhesion. LFA-1 reduces the threshold of TCR signals for low affinity ligands. Previously, we determined that mice expressing constitutively active Rap1A on T cells have increased frequency of CD103(+) T regulatory cells (Treg). We hypothesized that Rap1A-GTP might affect the differentiation of Treg by regulating LFA-1 activation. Using Foxp3-GFP-KI, LFA-1-KO and Rap1A-GTP-Tg mice we determined that Rap1A has an active role in the development of thymic Treg but LFA-1 is not mandatory for this function. Rap1A is also involved in the generation of peripheral Treg and this effect is mediated via LFA-1-dependent and LFA-1-independent mechanisms. Identification of the signaling pathways via which Rap1-GTP contributes to the differentiation of Treg will provide new insights to the function of Rap1A and to designing targeted approaches for generation of Treg for therapeutic applications.

Rolling on E- or P-selectin induces the extended but not high-affinity conformation of LFA-1 in neutrophils

Human blood neutrophils rolling on E- or P-selectin reduced their rolling velocity when intercellular adhesion molecule (ICAM)-1 was available. Similar to mouse neutrophils, this was dependent on P-selectin glycoprotein ligand 1 (PSGL1), alpha(L)beta(2) integrin, the Src family tyrosine kinase FGR and spleen tyrosine kinase SYK. Blocking phospholipase C or p38 MAP kinase attenuated, but did not abolish the velocity reduction. To test expression of integrin activation epitopes, we adapted an immobilized reporter assay and developed a new homogeneous microfluidics-based reporter antibody binding assay. Rolling on E- or P-selectin induced the extension reporter epitopes KIM127 and NKI-L16, but not the high affinity reporter epitope monoclonal antibody (mAb) 24. This enabled rolling neutrophils to bind to immobilized extension reporter, but not activation reporter antibodies and allowed binding of soluble KIM127 during rolling. We conclude that human neutrophil rolling on E- or P-selectin induces the extended alpha(L)beta(2) integrin conformation through signaling triggered by PSGL-1 engagement.

Phosphorylation of paxillin at threonine 538 by PKCdelta regulates LFA1-mediated adhesion of lymphoid cells

We investigated the PKCdelta-mediated phosphorylation of paxillin within its LIM4 domain and the involvement of this phosphorylation in activation of LFA-1 integrins of the Baf3 pro-B lymphocytic cell line. Using phosphorylated-threonine-specific antibodies, phosphorylated amino acid analysis and paxillin phosphorylation mutants, we demonstrated that TPA, the pharmacological analog of the endogenous second messenger diacyl glycerol, stimulates paxillin phosphorylation at threonine 538 (T538). The TPA-responsive PKC isoform PKCdelta directly binds paxillin in a yeast two-hybrid assay and phosphorylates paxillin at T538 in vitro and also co-immunoprecipitates with paxillin and mediates phosphorylation of this residue in vivo. Recombinant wild-type paxillin, its phospho-inhibitory T538A or phospho-mimetic T538E mutants were expressed in the cells simultaneously with siRNA silencing of the endogenous paxillin. These experiments suggest that phosphorylation of paxillin T538 contributes to dissolution of the actin cytoskeleton, redistribution of LFA-1 integrins and an increase in their affinity. We also show that phosphorylation of T538 is involved in the activation of LFA-1 integrins by TPA.

Pharmacological activation of guanine nucleotide exchange factors for the small GTPase Rap1 recruits high-affinity beta1 integrins as coreceptors for parvovirus B19: improved ex vivo gene transfer to human erythroid progenitor cells

Parvovirus B19 has potential as a gene therapy vector because of its restricted tropism for human erythroid progenitor cells in the bone marrow. B19 binds to the cell surface through P antigen and we identified activated beta(1) integrins as coreceptors for internalization. Because differentiation with phorbol ester induces beta(1) integrin coreceptor activity, but cell differentiation is not desirable in gene transfer to human progenitor cells and one of the downstream effectors of phorbol esters is the small GTPase Rap1, the role of Rap1 in the recruitment of beta(1) integrins on hematopoietic cells was examined. Expression of a constitutively active Rap1 (63E) was sufficient to recruit beta(1) integrin coreceptors in erythroleukemic K562 cells by inducing high-affinity integrin conformation. A crucial role of actin polymerization in Rap1-mediated beta(1) integrin recruitment was documented by complete inhibition of the 63E Rap1 effect with low-dose cytochalasin D and by the ability of a constitutively active mutant of the actin cytoskeleton regulator Rac1 to sensitize K562 cells to the pharmacological activation of endogenous Rap1, using the Rap1 exchange factor-specific 8-pCPT-2'-O-Me-cAMP [8-(4-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate]. Interestingly, in primary human erythroid progenitor cells, 8-pCPT-2'-O-Me-cAMP was sufficient to significantly increase B19-mediated gene transfer, suggesting that these cells possess the cytoskeleton organization capacity required for efficient recruitment of beta(1) integrins by brief pharmacological stimulation of Rap1 GTP loading. Because 8-pCPT-2'-O-Me-cAMP has been implicated in enhanced homing of progenitor cells, these results identify a novel tool with which to optimize ex vivo B19-mediated gene transfer and potentially improve homing of transduced cells by Rap1-beta(1) integrin activation with 8-pCPT-2'-O-Me-cAMP.

Rap signaling in normal lymphocyte development and leukemia genesis

Although Rap GTPases of the Ras family remained enigmatic for years, extensive studies in this decade have revealed diverse functions of Rap signaling in the control of cell proliferation, differentiation, survival, adhesion, and movement. With the use of gene-engineered mice, we have uncovered essential roles of endogenous Rap signaling in normal lymphocyte development of both T- and B-lineage cells. Deregulation of Rap signaling, on the other hand, results in the development of characteristic leukemia in manners highly dependent on the contexts of cell lineages. These results highlight crucial roles of Rap signaling in the physiology and pathology of lymphocyte development.

Rap1, a mercenary among the Ras-like GTPases

The small Ras-like GTPase Rap1 is an evolutionary conserved protein that originally gained interest because of its capacity to revert the morphological phenotype of Ras-transformed fibroblasts. Rap1 is regulated by a large number of stimuli that include growth factors and cytokines, but also physical force and osmotic stress. Downstream of Rap1, a plethora of effector molecules has been proposed on the basis of biochemical studies. Here, we present an overview of genetic studies on Rap1 in various model organisms and relate the observed phenotypes to in vitro studies. The picture that emerges is one in which Rap1 is a versatile regulator of morphogenesis, by regulating diverse processes that include establishment of cellular polarity, cell-matrix interactions and cell-cell adhesion. Surprisingly, genetic experiments indicate that in the various model organisms, Rap1 uses distinct effector molecules that impinge upon the actin cytoskeleton and adhesion molecules.

Novel aspects in the regulation of the leukocyte adhesion cascade

Leukocyte recruitment plays a major role in the immune response to infectious pathogens and during inflammatory and autoimmune disorders. The process of leukocyte extravasation from the blood into the inflamed tissue requires a complex cascade of adhesive events between the leukocytes and the endothelium including leukocyte rolling, adhesion and transendothelial migration. Leukocyte-endothelial interactions are mediated by tightly regulated binding interactions between adhesion receptors on both cells. In this regard, leukocyte adhesion onto the endothelium is governed by leukocyte integrins and their endothelial counter-receptors of the immunoglobulin superfamily. The present review will focus on novel aspects with respect to the modulation of the leukocyte adhesion cascade.

Activated integrin VLA-4 localizes to the lamellipodia and mediates T cell migration on VCAM-1

Lymphocyte migration from blood into lymphoid tissues or to sites of inflammation occurs through interactions between cell surface integrins and their ligands expressed on the vascular endothelium and the extracellular matrix. VLA-4 (alpha(4)beta(1)) is a key integrin in the effective trafficking of lymphocytes. Although it has been well established that integrins undergo functionally significant conformational changes to mediate cell adhesion, there is no mechanistic information that explains how these are dynamically and spatially regulated during lymphocyte polarization and migration. Using dynamic fluorescence resonance energy transfer analysis of a novel VLA-4 FRET sensor under total internal reflection fluorescence microscopy, we show that VLA-4 activation localizes to the lamellipodium in living cells. During T cell migration on VCAM-1, VLA-4 activation concurs with spatial redistribution of chemokine receptor and active Rap1 at the leading edge. Selective inhibition of the activated VLA-4 at the leading edge with a small molecule inhibitor is sufficient to block T cell migration. These data suggest that a subpopulation of activated VLA-4 is mainly localized to the leading edge of polarized human T cells and is critical for T cell migration on VCAM-1.

Dynamic changes in Rap1 activity are required for cell retraction and spreading during mitosis

At the onset of mitosis, most adherent cells undergo cell retraction characterised by the disassembly of focal adhesions and actin stress fibres. Mitosis takes place in rounded cells, and the two daughter cells spread again after cytokinesis. Because of the well-documented ability of the small GTPase Rap1 to stimulate integrin-dependent adhesion and spreading, we assessed its role during mitosis. We show that Rap1 activity is regulated during this process. Changes in Rap1 activity play an essential role in regulating cell retraction and spreading, respectively, before and after mitosis of HeLa cells. Indeed, endogenous Rap1 is inhibited at the onset of mitosis; conversely, constitutive activation of Rap1 inhibits the disassembly of premitotic focal adhesions and of the actin cytoskeleton, leading to delayed mitosis and to cytokinesis defects. Rap1 activity slowly increases after mitosis ends; inhibition of Rap1 activation by the ectopic expression of the dominant-negative Rap1[S17A] mutant prevents the rounded cells from spreading after mitosis. For the first time, we provide evidence for the direct regulation of adhesion processes during mitosis via the activity of the Rap1 GTPase.

SLP-76-ADAP adaptor module regulates LFA-1 mediated costimulation and T cell motility

Although adaptor ADAP (FYB) and its binding to SLP-76 has been implicated in TcR-induced "inside-out" signaling for LFA-1 activation in T cells, little is known regarding its role in LFA-1-mediated "outside-in" signaling. In this study, we demonstrate that ADAP and SLP-76-ADAP binding are coupled to LFA-1 costimulation of IL-2 production, F-actin clustering, cell polarization, and T cell motility. LFA-1 enhancement of anti-CD3-induced IL-2 production was completely dependent on SLP-76-ADAP binding. Further, anti-CD3 was found to require CD11a ligation by antibody or ICAM1 to cause T cell polarization. ADAP augmented this polarization induced by anti-CD3/CD11a, but not by anti-CD3 alone. ADAP expression with LFA-1 ligation alone was sufficient to polarize T cells directly and to increase T cell motility whereas the loss of ADAP in ADAP-/- primary T cells reduced motility. A mutant lacking SLP-76-binding sites (M12) blocked LFA-1 costimulation of IL-2 production, polarization, and motility. LFA-1-ADAP polarization was also dependent on src kinases, Rho GTPases, phospholipase C, and phosphoinositol 3-kinase. Our findings provide evidence of an obligatory role for the SLP-76-ADAP module in LFA-1-mediated costimulation in T cells.

The Role of SH2 Domain-containing Leukocyte Phosphoprotein of 76 kDa in the Regulation of Immune Cell Development and Function

Recent years have seen an explosion of new knowledge defining the molecular events that are critical for development and activation of immune cells. Much of this new information has come from a careful molecular dissection of key signal transduction pathways that are initiated when immune cell receptors are engaged. In addition to the receptors themselves and critical effector molecules, these signaling pathways depend on adapters, proteins that have no intrinsic effector function but serve instead as scaffolds to nucleate multimolecular complexes. This review summarizes some of what has been learned about one such adapter protein, SH2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76), and how it regulates and integrates signals after engagement of immunoreceptors and integrins on various immune cell lineages.

T cell activation

This year marks the 25th anniversary of the first Annual Review of Immunology article to describe features of the T cell antigen receptor (TCR). In celebration of this anniversary, we begin with a brief introduction outlining the chronology of the earliest studies that established the basic paradigm for how the engaged TCR transduces its signals. This review continues with a description of the current state of our understanding of TCR signaling, as well as a summary of recent findings examining other key aspects of T cell activation, including cross talk between the TCR and integrins, the role of costimulatory molecules, and how signals may negatively regulate T cell function.Acronyms and DefinitionsAdapter protein: cellular protein that functions to bridge molecular interactions via characteristic domains able to mediate protein/protein or protein/lipid interactions Costimulation: signals delivered to T cells by cell surface receptors other than the TCR itself that potentiate T cell activation cSMAC: central supramolecular activation cluster Immunoreceptor tyrosine-based activation motif (ITAM): a short peptide sequence in the cytoplasmic tails of key surface receptors on hematopoietic cells that is characterized by tyrosine residues that are phosphorylated by Src family PTKs, enabling the ITAM to recruit activated Syk family kinases Inside-out signaling: signals initiated by engagement of immunoreceptors that lead to conformational changes and clustering of integrins, thereby increasing the affinity and avidity of the integrins for their ligands NFAT: nuclear factor of activated T cells PI3K: phosphoinositide 3-kinase PKC: protein kinase C PLC: phospholipase C pMHC: peptide major histocompatibility complex (MHC) complex pSMAC: peripheral supramolecular activation cluster PTK: protein tyrosine kinase Signal transduction: biochemical events linking surface receptor engagement to cellular responses TCR: T cell antigen receptor

The ins and outs of leukocyte integrin signaling

Integrins are the principal cell adhesion receptors that mediate leukocyte migration and activation in the immune system. These receptors signal bidirectionally through the plasma membrane in pathways referred to as inside-out and outside-in signaling. Each of these pathways is mediated by conformational changes in the integrin structure. Such changes allow high-affinity binding of the receptor with counter-adhesion molecules on the vascular endothelium or extracellular matrix and lead to association of the cytoplasmic tails of the integrins with intracellular signaling molecules. Leukocyte functional responses resulting from outside-in signaling include migration, proliferation, cytokine secretion, and degranulation. Here, we review the key signaling events that occur in the inside-out versus outside-in pathways, highlighting recent advances in our understanding of how integrins are activated by a variety of stimuli and how they mediate a diverse array of cellular responses.