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

Cytohesin-1 controls the activation of RhoA and modulates integrin-dependent adhesion and migration of dendritic cells

Adhesion and motility of mammalian leukocytes are essential requirements for innate and adaptive immune defense mechanisms. We show here that the guanine nucleotide exchange factor cytohesin-1, which had previously been demonstrated to be an important component of beta-2 integrin activation in lymphocytes, regulates the activation of the small GTPase RhoA in primary dendritic cells (DCs). Cytohesin-1 and RhoA are both required for the induction of chemokine-dependent conformational changes of the integrin beta-2 subunit of DCs during adhesion under physiological flow conditions. Furthermore, use of RNAi in murine bone marrow DCs (BM-DCs) revealed that interference with cytohesin-1 signaling impairs migration of wild-type dendritic cells in complex 3D environments and in vivo. This phenotype was not observed in the complete absence of integrins. We thus demonstrate an essential role of cytohesin-1/RhoA during ameboid migration in the presence of integrins and further suggest that DCs without integrins switch to a different migration mode.

Mst1 controls lymphocyte trafficking and interstitial motility within lymph nodes

The regulation of lymphocyte adhesion and migration plays crucial roles in lymphocyte trafficking during immunosurveillance. However, our understanding of the intracellular signalling that regulates these processes is still limited. Here, we show that the Ste20-like kinase Mst1 plays crucial roles in lymphocyte trafficking in vivo. Mst1(-/-) lymphocytes exhibited an impairment of firm adhesion to high endothelial venules, resulting in an inefficient homing capacity. In vitro lymphocyte adhesion cascade assays under physiological shear flow revealed that the stopping time of Mst1(-/-) lymphocytes on endothelium was markedly reduced, whereas their L-selectin-dependent rolling/tethering and transition to LFA-1-mediated arrest were not affected. Mst1(-/-) lymphocytes were also defective in the stabilization of adhesion through alpha4 integrins. Consequently, Mst1(-/-) mice had hypotrophic peripheral lymphoid tissues and reduced marginal zone B cells and dendritic cells in the spleen, and defective emigration of single positive thymocytes. Furthermore, Mst1(-/-) lymphocytes had impaired motility over lymph node-derived stromal cells and within lymph nodes. Thus, our data indicate that Mst1 is a key enzyme involved in lymphocyte entry and interstitial migration.

Regulation of T cell integrin function by adapter proteins

Integrins are cell surface heterodimers that bind adhesion molecules expressed on other cells or in the extracellular matrix. Integrin-mediated interactions are critical for T cell development in the thymus, migration of T cells in the periphery, and induction of T cell effector functions. In resting T cells, integrins are maintained in a low affinity state. Engagement of the T cell receptor or chemokine receptors increases integrin affinity, enabling integrins to bind their ligands and initiate a signaling cascade resulting in altered cell morphology and motility. Our laboratory is interested how adapter proteins, mediators of intracellular signal transduction, regulate both signals from the T cell receptor to integrins (inside-out signaling) and (outside-in) signals from integrins into the cell.

EPAC proteins transduce diverse cellular actions of cAMP

It has now been over 10 years since efforts to completely understand the signalling actions of cAMP (3'-5'-cyclic adenosine monophosphate) led to the discovery of exchange protein directly activated by cAMP (EPAC) proteins. In the current review we will highlight important advances in the understanding of EPAC structure and function and demonstrate that EPAC proteins mediate multiple actions of cAMP in cells, revealing future targets for pharmaceutical intervention. It has been known for some time that drugs that elevate intracellular cAMP levels have proven therapeutic benefit for diseases ranging from depression to inflammation. The challenge now is to determine which of these positive actions of cAMP involve activation of EPAC-regulated signal transduction pathways. EPACs are specific guanine nucleotide exchange factors for the Ras GTPase homologues, Rap1 and Rap2, which they activate independently of the classical routes for cAMP signalling, cyclic nucleotide-gated ion channels and protein kinase A. Rather, EPAC activation is triggered by internal conformational changes induced by direct interaction with cAMP. Leading from this has been the development of EPAC-specific agonists, which has helped to delineate numerous cellular actions of cAMP that rely on subsequent activation of EPAC. These include regulation of exocytosis and the control of cell adhesion, growth, division and differentiation. Recent work also implicates EPAC in the regulation of anti-inflammatory signalling in the vascular endothelium, namely negative regulation of pro-inflammatory cytokine signalling and positive support of barrier function. Further elucidation of these important signalling mechanisms will no doubt support the development of the next generation of anti-inflammatory drugs.

Integrins in immunity

A successful immune response depends on the capacity of immune cells to travel from one location in the body to another–these cells are rapid migrators, travelling at speeds of μm/minute. Their ability to penetrate into tissues and to make contacts with other cells depends chiefly on the β2 integrin known as LFA-1. For this reason, we describe the control of its activity in some detail. For the non-immunologist, the fine details of an immune response often seem difficult to fathom. However, the behaviour of immune cells, known as leukocytes (Box 1), is subject to the same biological rules as many other cell types, and this holds true particularly for the functioning of the integrins on these cells. In this Commentary, we highlight, from a cell-biology point of view, the integrin-mediated immune-cell migration and cell-cell interactions that occur during the course of an immune response.

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

Mechanisms of neutrophil transendothelial migration

Neutrophil recruitment is an integral part of the immune response to infection as well as of inflammatory disorders. The process of neutrophil extravasation comprises a complex multistep cascade that is orchestrated by a tightly coordinated sequence of adhesive interactions with vessel wall endothelial cells. Adhesion receptors as well as signaling molecules in both neutrophils and endothelial cells regulate the recruitment of neutrophils into the site of inflammation or infection. The present review will focus on novel aspects with regards to the last step of neutrophil recruitment, namely the transmigration of neutrophils through endothelial cells.

Spa-1 (Sipa1) and Rap signaling in leukemia and cancer metastasis

Although Rap GTPases of the Ras family remained enigmatic for years, extensive studies in this decade have revealed diverse functions of Rap in the control of cell proliferation, differentiation, survival, adhesion, and movement. With the use of genetic engineering strategies, we have uncovered essential roles of Rap signaling in normal lymphohematopoietic cell development as well as its crucial involvement in the development of a wide spectrum of leukemia in manners highly dependent on the contexts of cell lineages. Incidentally, recent results also indicate an important role of Spa-1, a Rap GTPase-activating protein, in invasion and metastasis in human cancers. While it is unlikely that Rap can function as a classic oncogene by itself, like Ras, emerging findings unveil crucial involvements of Rap GTPases in the distinct aspects of malignancy, including leukemia genesis and cancer metastasis.

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.

The Nore1B/Mst1 complex restrains antigen receptor-induced proliferation of naïve T cells

The Mst1 and Mst2 protein kinases are the mammalian homologs of hippo, a major inhibitor of cell proliferation in Drosophila. Mst1 is most abundant in lymphoid tissues. Mice lacking Mst1 exhibit markedly reduced levels of the Mst1 regulatory protein Nore1B/RAPL in lymphoid cells, whereas Mst2 abundance is unaltered. Mst1-null mice exhibit normal T cell development but low numbers of mature naïve T cells with relatively normal numbers of effector/memory T cells. In vitro, the Mst1-deficient naïve T cells exhibit markedly greater proliferation in response to stimulation of the T cell receptor whereas the proliferative responses of the Mst1-null effector/memory T cell cohort is similar to wild type. Thus, elimination of Mst1 removes a barrier to the activation and proliferative response of naïve T cells. The levels of Mst1 and Nore1B/RAPL in wild-type effector/memory T cells are approximately 10% those seen in wild-type naïve T cells, which may contribute to the enhanced proliferative responses of the former. Freshly isolated Mst1-null T cells exhibit high rates of ongoing apoptosis, a likely basis for their low numbers in vivo; they also exhibit defective clustering of LFA-1, as previously observed for Nore1B/RAPL-deficient T cells. Among known Mst1 substrates, only the phosphorylation of the cell cycle inhibitory proteins MOBKL1A/B is lost entirely in TCR-stimulated, Mst1-deficient T cells. Mst1/2-catalyzed MOBKL1A/B phosphorylation slows proliferation and is therefore a likely contributor to the anti-proliferative action of Mst1 in naïve T cells. The Nore1B/RAPL-Mst1 complex is a negative regulator of naïve T cell proliferation.

PKC-theta selectively controls the adhesion-stimulating molecule Rap1

The antigen-specific interaction of a T cell with an antigen-presenting cell (APC) results in the formation of an immunologic synapse (IS) between the membranes of the 2 cells. beta(2) integrins on the T cell, namely, leukocyte function-associated antigen 1 (LFA-1) and its counter ligand, namely, immunoglobulin-like cell adhesion molecule 1 (ICAM-1) on the APC, critically stabilize this intercellular interaction. The small GTPase Rap1 controls T-cell adhesion through modulating the affinity and/or spatial organization of LFA-1; however, the upstream regulatory components triggered by the T-cell receptor (TCR) have not been resolved. In the present study, we identified a previously unknown function of a protein kinase C- theta (PKC-theta)/RapGEF2 complex in LFA-1 avidity regulation in T lymphocytes. After T-cell activation, the direct phosphorylation of RapGEF2 at Ser960 by PKC- theta regulates Rap1 activation as well as LFA-1 adhesiveness to ICAM-1. In OT-II TCR-transgenic CD4(+) T cells, clustering of LFA-1 after antigen activation was impaired in the absence of PKC- theta. These data define that, among other pathways acting on LFA-1 regulation, PKC- theta and its effector RapGEF2 are critical factors in TCR signaling to Rap1. Taken together, PKC- theta sets the threshold for T-cell activation by positively regulating both the cytokine responses and the adhesive capacities of T lymphocytes.

Induction of cell retraction by the combined actions of Abl-CrkII and Rho-ROCK1 signaling

Dynamic modulation of cell adhesion is integral to a wide range of biological processes. The small guanosine triphosphatase (GTPase) Rap1 is an important regulator of cell-cell and cell-matrix adhesions. We show here that induced expression of activated Abl tyrosine kinase reduces Rap1-GTP levels through phosphorylation of Tyr221 of CrkII, which disrupts interaction of CrkII with C3G, a guanine nucleotide exchange factor for Rap1. Abl-dependent down-regulation of Rap1-GTP causes cell rounding and detachment only when the Rho-ROCK1 pathway is also activated, for example, by lysophosphatidic acid (LPA). During ephrin-A1-induced retraction of PC3 prostate cancer cells, we show that endogenous Abl is activated and disrupts the CrkII-C3G complex to reduce Rap1-GTP. Interestingly, ephrin-A1-induced PC3 cell retraction also requires LPA, which stimulates Rho to a much higher level than that is activated by ephrin-A1. Our results establish Rap1 as another downstream target of the Abl-CrkII signaling module and show that Abl-CrkII collaborates with Rho-ROCK1 to stimulate cell retraction.

Cell signaling and transcription factor genes expressed during whole body regeneration in a colonial chordate

BACKGROUND

The restoration of adults from fragments of blood vessels in botryllid ascidians (termed whole body regeneration [WBR]) represents an inimitable event in the chordates, which is poorly understood on the mechanistic level.

RESULTS

To elucidate mechanisms underlying this phenomenon, a subtracted EST library for early WBR stages was previously assembled, revealing 76 putative genes belonging to major signaling pathways, including Notch/Delta, JAK/STAT, protein kinases, nuclear receptors, Ras oncogene family members, G-Protein coupled receptor (GPCR) and transforming growth factor beta (TGF-beta) signaling. RT-PCR on selected transcripts documented specific up-regulation in only regenerating fragments, pointing to a broad activation of these signaling pathways at onset of WBR. The followed-up expression pattern of seven representative transcripts from JAK/STAT signaling (Bl-STAT), the Ras oncogene family (Bl-Rap1A, Bl-Rab-33), the protein kinase family (Bl-Mnk), Bl-Cnot, Bl-Slit and Bl-Bax inhibitor, revealed systemic and site specific activations during WBR in a sub-population of circulatory cells.

CONCLUSION

WBR in the non-vertebrate chordate Botrylloides leachi is a multifaceted phenomenon, presided by a complex array of cell signaling and transcription factors. Above results, provide a first insight into the whole genome molecular machinery of this unique regeneration process, and reveal the broad participation of cell signaling and transcription factors in the process. While regeneration involves the participation of specific cell populations, WBR signals are systemically expressed at the organism level.

Mammalian Rho GTPases: new insights into their functions from in vivo studies

Rho GTPases are key regulators of cytoskeletal dynamics and affect many cellular processes, including cell polarity, migration, vesicle trafficking and cytokinesis. These proteins are conserved from plants and yeast to mammals, and function by interacting with and stimulating various downstream targets, including actin nucleators, protein kinases and phospholipases. The roles of Rho GTPases have been extensively studied in different mammalian cell types using mainly dominant negative and constitutively active mutants. The recent availability of knockout mice for several members of the Rho family reveals new information about their roles in signalling to the cytoskeleton and in development.

Constitutively active Rap2 transgenic mice display fewer dendritic spines, reduced extracellular signal-regulated kinase signaling, enhanced long-term depression, and impaired spatial learning and fear extinction

Within the Ras superfamily of GTPases, Rap1 and Rap2 are the closest homologs to Ras. In non-neural cells, Rap signaling can antagonize Ras signaling. In neurons, Rap also seems to oppose Ras in terms of synaptic function. Whereas Ras is critical for long-term potentiation (LTP), Rap1 has been shown to be required for long-term depression (LTD), and Rap2 has been implicated in depotentiation. Moreover, active Rap1 and Rap2 cause loss of surface AMPA receptors and reduced miniature EPSC amplitude and frequency in cultured neurons. The role of Rap signaling in vivo, however, remains poorly understood. To study the function of Rap2 in the brain and in behavior, we created transgenic mice expressing either constitutively active (Rap2V12) or dominant-negative (Rap2N17) mutants of Rap2 in postnatal forebrain. Multiple lines of Rap2N17 mice showed only weak expression of the transgenic protein, and no phenotype was observed. Rap2V12 mice displayed fewer and shorter dendritic spines in CA1 hippocampal neurons, and enhanced LTD at CA3-CA1 synapses. Behaviorally, Rap2V12 mice showed impaired spatial learning and defective extinction of contextual fear, which correlated with reduced basal phosphorylation of extracellular signal-regulated kinase (ERK) and blunted activation of ERK during fear extinction training. Our data support the idea that Rap2 opposes Ras-ERK signaling in the brain, thereby inhibiting dendritic spine development/maintenance, promoting synaptic depression rather than LTP, and impairing learning. The findings also implicate Rap2 signaling in fear extinction mechanisms, which are thought to be aberrant in anxiety disorders and posttraumatic stress disorder.

The vessel wall and its interactions

Blood cell interactions with the vessel wall were first documented almost 170 years ago. Modern advances have revealed that leukocyte and platelet interactions with the endothelium are at the nexus of complex, dynamic cellular and molecular networks that, when dysregulated, may lead to pathological inflammation and thrombosis, which are major sources of morbidity and mortality in the Western world. In this review, we relate the history of blood cell interactions with the vasculature, discuss recent progress, and raise some unresolved questions awaiting the field.

Integrin activation

Agonist stimulation of integrin receptors, composed of transmembrane alpha and beta subunits, leads cells to regulate integrin affinity ('activation'), a process that controls cell adhesion and migration, and extracellular matrix assembly. A final step in integrin activation is the binding of talin to integrin beta cytoplasmic domains. We used forward, reverse and synthetic genetics to engineer and order integrin activation pathways of a prototypic integrin, platelet alphaIIbbeta3. PMA activated alphaIIbbeta3 only after expression of both PKCalpha (protein kinase Calpha) and talin at levels approximating those in platelets. Inhibition of Rap1 GTPase reduced alphaIIbbeta3 activation, whereas expression of constitutively active Rap1A(G12V) bypassed the requirement for PKCalpha. Overexpression of a Rap effector, RIAM (Rap1-GTP-interacting adaptor molecule), activated alphaIIbbeta3 and bypassed the requirement for PKCalpha and Rap1. In addition, shRNA (short hairpin RNA)-mediated knockdown of RIAM blocked talin interaction with and activation of integrin alphaIIbbeta3. Rap1 activation caused the formation of an 'activation complex' containing talin and RIAM that redistributed to the plasma membrane and activated alphaIIbbeta3. The central finding was that this Rap1-induced formation of an 'integrin activation complex' leads to the unmasking of the integrin-binding site on talin, resulting in integrin activation.

Phosphatidylinositol-3-kinase-gamma is integral to homing functions of progenitor cells

Endothelial progenitor cells (EPCs) and hematopoietic progenitor cells are recruited to ischemic regions, improving neovascularization. beta1 and beta2 integrins play a crucial role for progenitor cell homing to ischemic tissues. Integrin activity is regulated by chemokines and their respective G protein-coupled receptors. The phosphatidylinositol-3-kinase catalytic subunit gamma (PI3Kgamma) is the PI3K isoform that selectively transduces signals from G protein-coupled receptors. Here, we investigated the role of PI3Kgamma as a signaling intermediate in the chemokine-induced integrin-dependent homing functions of progenitor cells. A pharmacological PI3Kgamma inhibitor significantly reduced chemokine-induced chemotaxis and stromal cell-derived factor (SDF)1alpha-induced transmigration of human EPCs. Moreover, the PI3Kgamma inhibitor significantly reduced SDF1alpha-induced adhesion of EPCs to intercellular adhesion molecule-1 and human umbilical vein endothelial cell monolayers. These findings were corroborated with Lin(-) bone marrow-derived progenitor cells from PI3Kgamma-deficient mice that displayed reduced SDF1alpha-induced migration and intercellular adhesion molecule-1 adhesion as compared with wild-type cells. Pharmacological inhibition or genetic ablation of PI3Kgamma reduced SDF1alpha-induced integrin activation in human EPCs and in murine Lin(-) BM-derived progenitor cells, respectively. In vivo, the homing of PI3Kgamma-deficient Lin(-) progenitor cells to ischemic muscles after intravenous infusion in the model of hindlimb ischemia and their neovascularization-promoting capacity was reduced as compared with wild-type cells. In conclusion, PI3Kgamma is integral to the integrin-dependent homing of progenitor cells.

The reverse stop-signal model for CTLA4 function

Activation of the T-cell co-receptor cytotoxic T-lymphocyte antigen 4 (CTLA4) has a pivotal role in adjusting the threshold for T-cell activation and in preventing autoimmunity and massive tissue infiltration by T cells. Although many mechanistic models have been postulated, no single model has yet accounted for its overall function. In this Opinion article, I outline the strengths and weaknesses of the current models, and present a new 'reverse stop-signal model' to account for CTLA4 function.

cAMP-induced Epac-Rap activation inhibits epithelial cell migration by modulating focal adhesion and leading edge dynamics

Epithelial cell migration is a complex process crucial for embryonic development, wound healing and tumor metastasis. It depends on alterations in cell-cell adhesion and integrin-extracellular matrix interactions and on actomyosin-driven, polarized leading edge protrusion. The small GTPase Rap is a known regulator of integrins and cadherins that has also been implicated in the regulation of actin and myosin, but a direct role in cell migration has not been investigated. Here, we report that activation of endogenous Rap by cAMP results in an inhibition of HGF- and TGFbeta-induced epithelial cell migration in several model systems, irrespective of the presence of E-cadherin adhesion. We show that Rap activation slows the dynamics of focal adhesions and inhibits polarized membrane protrusion. Importantly, forced integrin activation by antibodies does not mimic these effects of Rap on cell motility, even though it does mimic Rap effects in short-term cell adhesion assays. From these results, we conclude that Rap inhibits epithelial cell migration, by modulating focal adhesion dynamics and leading edge activity. This extends beyond the effect of integrin affinity modulation and argues for an additional function of Rap in controlling the migration machinery of epithelial cells.

Rap1a null mice have altered myeloid cell functions suggesting distinct roles for the closely related Rap1a and 1b proteins

The Ras-related GTPases Rap1a and 1b have been implicated in multiple biological events including cell adhesion, free radical production, and cancer. To gain a better understanding of Rap1 function in mammalian physiology, we deleted the Rap1a gene. Although loss of Rap1a expression did not initially affect mouse size or viability, upon backcross into C57BL/6J mice some Rap1a-/- embryos died in utero. T cell, B cell, or myeloid cell development was not disrupted in Rap1a-/- mice. However, macrophages from Rap1a null mice exhibited increased haptotaxis on fibronectin and vitronectin matrices that correlated with decreased adhesion. Chemotaxis of lymphoid and myeloid cells in response to CXCL12 or CCL21 was significantly reduced. In contrast, an increase in FcR-mediated phagocytosis was observed. Because Rap1a was previously copurified with the human neutrophil NADPH oxidase, we addressed whether GTPase loss affected superoxide production. Neutrophils from Rap1a-/- mice had reduced fMLP-stimulated superoxide production as well as a weaker initial response to phorbol ester. These results suggest that, despite 95% amino acid sequence identity, similar intracellular distribution, and broad tissue distribution, Rap1a and 1b are not functionally redundant but rather differentially regulate certain cellular events.

RAP-1 and the RAL-1/exocyst pathway coordinate hypodermal cell organization in Caenorhabditis elegans

The small Ras-like GTPase Rap1 has been identified as a regulator of integrin activation and cadherin-mediated cell-cell contacts. Surprisingly, null mutants of RAP-1 in Caenorhabditis elegans are viable and fertile. In a synthetic lethal RNAi screen with C. elegans rap-1 mutants, the Ras-like GTPase ral-1 emerged as one of seven genes specifically required for viability. Depletion of exoc-8 and sec-5, encoding two putative RAL-1 effectors and members of the exocyst complex, also caused lethality of rap-1 mutants, but did not affect wild-type worms. The RAP-1 and the RAL-1/exocyst pathway appear to coordinate hypodermal cell movement and elongation during embryonic development. They mediate their effect in part through targeting the alpha-catenin homologue HMP-1 to the lateral membrane. Genetic interactions show that the RAP-1 and RAL-1/exocyst pathway also act in parallel during larval stages. Together these data provide in vivo evidence for the exocyst complex as a downstream RAL-1 effector in cell migration.

Active Rap1, a small GTPase that induces malignant transformation of hematopoietic progenitors, localizes in the nucleus and regulates protein expression

Rap1, a member of the Ras superfamily, regulates cytoskeletal changes in lower eukaryots and integrin-mediated adhesion in hematopoietic cells. Sustained activation of Rap1 in mouse hematopoietic stem cells causes expansion of hematopoietic progenitors, followed by a myeloproliferative disorder mimicking chronic myeloid leukemia. Moreover, these mice develop a B-cell lymphoproliferative disorder resembling chronic lymphocytic leukemia. Here, we used HEK 293 cells as a tool to examine the molecular effects of Rap1. We observed that a constitutively active Rap1 mutant localized predominantly in the nucleus. Nuclear localization of endogenous Rap1-GTP was also detected upon physiologic activation. A potential consequence of nuclear localization of Rap1-GTP is the regulation of gene expression. We used a high throughput proteomic approach to identify gene products potentially modulated by Rap1-GTP. Out of 1000 proteins examined, 64 proteins were upregulated and 66 proteins were downregulated. The differentially expressed gene products belong to cytoskeletal regulator proteins, signaling molecules, transcription factors, viability regulators, and protein transporters. This analysis provides the first fingerprint of gene product expression regulated by Rap1 and may contribute to our understanding of malignant transformation mechanisms regulated by this small GTPase.

In vivo disruption of T cell development by expression of a dominant-negative polypeptide designed to abolish the SLP-76/Gads interaction

Multi-molecular complexes nucleated by adaptor proteins play a central role in signal transduction. In T cells, one central axis consists of the assembly of several signaling proteins linked together by the adaptors linker of activated T cells (LAT), Src homology 2 domain-containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76), and Grb2-related adaptor downstream of Shc (Gads). Each of these adaptors has been shown to be important for normal T cell development, and their proper sub-cellular localization is critical for optimal function in cell lines. We previously demonstrated in Jurkat T cells and a rat basophilic leukemic cell line that expression of a 50-amino acid polypeptide identical to the site on SLP-76 that binds to Gads blocks proper localization of SLP-76 and SLP-76-dependent signaling events. Here we extend these studies to investigate the ability of this polypeptide to inhibit TCR-induced integrin activity in Jurkat cells and to inhibit in vivo thymocyte development and primary T cell function. These data provide evidence for the in vivo function of a dominant-negative peptide based upon the biology of SLP-76 action and suggest the possibility of therapeutic potential of targeting the SLP-76/Gads interaction.

Compartmentalization of ITAM and integrin signaling by adapter molecules

Adapters are multidomain molecules that recruit effector proteins during signal transduction by immunoreceptors and integrins. The absence of these scaffolding molecules profoundly affects development and function of various hematopoietic lineages, underscoring their importance as regulators of signaling cascades. An emerging aspect of the mechanism by which engaged immunoreceptors and integrins transmit signals within the cell is by differential usage of various adapters that function to nucleate formation of distinct signaling complexes in a specific location within the cell. In this review, we discuss the mechanisms by which adapter proteins coordinate signal transduction with an emphasis on the role of subcellular compartmentalization in adapter function.

Guanine nucleotide exchange factors of the cytohesin family and their roles in signal transduction

Members of the cytohesin protein family, a group of guanine nucleotide exchange factors for adenosine diphosphate ribosylation factor (ARF) guanosine triphosphatases, have recently emerged as important regulators of signal transduction in vertebrate and invertebrate biology. These proteins share a modular domain structure, comprising carboxy-terminal membrane recruitment elements, a Sec7 homology effector domain, and an amino-terminal coiled-coil domain that serve as a platform for their integration into larger signaling complexes. Although these proteins have a highly similar overall build, their individual biological functions appear to be at least partly specific. Cytohesin-1 had been identified as a regulator of beta2 integrin inside-out regulation in immune cells and was subsequently shown to be involved in mitogen-associated protein kinase signaling in tumor cell proliferation as well as in T-helper cell activation and differentiation. Cytohesin-3, which had been discovered to be strongly associated with T-cell anergy, was very recently described as an essential component of insulin signal transduction in Drosophila and in human and murine liver cells. Future work will aim to dissect the mechanistic details of the modes of action of the cytohesins as well as to define the precise roles of these versatile proteins in vertebrates at the genetic level.

T-cell receptor signaling to integrins

Integrin adhesion receptors are critical for antigen recognition by T cells and for regulated recirculation and trafficking into and through various tissues in the body. T-cell receptor (TCR) signaling induces rapid increases in integrin function that facilitate T-cell activation by promoting stable contact with antigen-presenting cells and extracellular proteins in the environment. In this review, we outline the molecular mechanisms by which the TCR signals to integrins and present a model that highlights four key events: (i) initiation of proximal TCR signals nucleated by the linker for activated T cells (LAT) adapter protein and involving Itk, phospholipase C-gamma1, Vav1, and Src homology 2 domain-containing leukocyte-specific phosphoprotein of 76 kDa; (ii) transmission of integrin activation signals from the LAT signalosome to integrins by protein kinase (PK) C and the adapter protein, adhesion and degranulation-promoting adapter protein; (iii) assembly of integrin-associated signaling complexes that include PKD, the guanosine triphosphatase Rap1 and its effectors, and talin; and (iv) reorganization of the actin cytoskeleton by WAVE2 and other actin-remodeling proteins. These events coordinate changes in integrin conformation and clustering that result in enhanced integrin functional activity following TCR stimulation.

Regulation of T-cell antigen receptor-mediated inside-out signaling by cytosolic adapter proteins and Rap1 effector molecules

Integrins are critical for the migration of T cells to lymphoid organs and to sites of inflammation and are also necessary for productive interactions between T cells and antigen-presenting cells. Integrin activation is enhanced following T-cell receptor (TCR) engagement, as signals initiated by the TCR increase affinity and avidity of integrins for their ligands. This process, known as inside-out signaling, has been shown to require several molecular components including the cytosolic adapter proteins adhesion and degranulation-promoting adapter protein and Src homology 2 domain-containing adapter protein of 55 kDa, the low molecular weight guanosine triphosphatase Rap1, and the Rap1 effector proteins Rap1 guanosine triphosphate-interacting adapter molecule, regulator of adhesion and cell polarization enriched in lymphoid tissues, and protein kinase D1. Herein, we review recent findings about how the TCR is linked to integrin activation through inside-out signaling.

A LAD-III syndrome is associated with defective expression of the Rap-1 activator CalDAG-GEFI in lymphocytes, neutrophils, and platelets

Leukocyte and platelet integrins rapidly alter their affinity and adhesiveness in response to various activation (inside-out) signals. A rare leukocyte adhesion deficiency (LAD), LAD-III, is associated with severe defects in leukocyte and platelet integrin activation. We report two new LAD cases in which lymphocytes, neutrophils, and platelets share severe defects in β₁, β₂, and β₃ integrin activation. Patients were both homozygous for a splice junction mutation in their CalDAG-GEFI gene, which is a key Rap-1/2 guanine exchange factor (GEF). Both mRNA and protein levels of the GEF were diminished in LAD lymphocytes, neutrophils, and platelets. Consequently, LAD-III platelets failed to aggregate because of an impaired α_IIbβ₃ activation by key agonists. β₂ integrins on LAD-III neutrophils were unable to mediate leukocyte arrest on TNFα-stimulated endothelium, despite normal selectin-mediated rolling. In situ subsecond activation of neutrophil β₂ integrin adhesiveness by surface-bound chemoattractants and of primary T lymphocyte LFA-1 by the CXCL12 chemokine was abolished. Chemokine inside-out signals also failed to stimulate lymphocyte LFA-1 extension and high affinity epitopes. Chemokine-triggered VLA-4 adhesiveness in T lymphocytes was partially defective as well. These studies identify CalDAG-GEFI as a critical regulator of inside-out integrin activation in human T lymphocytes, neutrophils, and platelets.

The M-Ras-RA-GEF-2-Rap1 pathway mediates tumor necrosis factor-alpha dependent regulation of integrin activation in splenocytes

The Rap1 small GTPase has been implicated in regulation of integrin-mediated leukocyte adhesion downstream of various chemokines and cytokines in many aspects of inflammatory and immune responses. However, the mechanism for Rap1 regulation in the adhesion signaling remains unclear. RA-GEF-2 is a member of the multiple-member family of guanine nucleotide exchange factors (GEFs) for Rap1 and characterized by the possession of a Ras/Rap1-associating domain, interacting with M-Ras-GTP as an effector, in addition to the GEF catalytic domain. Here, we show that RA-GEF-2 is specifically responsible for the activation of Rap1 that mediates tumor necrosis factor-alpha (TNF-alpha)-triggered integrin activation. In BAF3 hematopoietic cells, activated M-Ras potently induced lymphocyte function-associated antigen 1 (LFA-1)-mediated cell aggregation. This activation was totally abrogated by knockdown of RA-GEF-2 or Rap1. TNF-alpha treatment activated LFA-1 in a manner dependent on M-Ras, RA-GEF-2, and Rap1 and induced activation of M-Ras and Rap1 in the plasma membrane, which was accompanied by recruitment of RA-GEF-2. Finally, we demonstrated that M-Ras and RA-GEF-2 were indeed involved in TNF-alpha-stimulated and Rap1-mediated LFA-1 activation in splenocytes by using mice deficient in RA-GEF-2. These findings proved a crucial role of the cross-talk between two Ras-family GTPases M-Ras and Rap1, mediated by RA-GEF-2, in adhesion signaling.

RIAM links the ADAP/SKAP-55 signaling module to Rap1, facilitating T-cell-receptor-mediated integrin activation

One outcome of T-cell receptor (TCR) signaling is increased affinity and avidity of integrins for their ligands. This occurs through a process known as inside-out signaling, which has been shown to require several molecular components including the adapter proteins ADAP (adhesion and degranulation-promoting adapter protein) and SKAP-55 (55-kDa src kinase-associated phosphoprotein) and the small GTPase Rap1. Herein, we provide evidence linking ADAP and SKAP-55 to RIAM, a recently described adapter protein that binds selectively to active Rap1. We identified RIAM as a key component linking the ADAP/SKAP-55 module to the small GTPase Rap1, facilitating TCR-mediated integrin activation. We show that RIAM constitutively interacts with SKAP-55 in both a heterologous transfection system and primary T cells and map the region essential for this interaction. Additionally, we find that the SKAP-55/RIAM complex is essential both for TCR-mediated adhesion and for efficient conjugate formation between T cells and antigen-presenting cells. Mechanistic studies revealed that the ADAP/SKAP-55 module relocalized RIAM and Rap1 to the plasma membrane following TCR activation to facilitate integrin activation. These results describe for the first time a link between ADAP/SKAP-55 and the Rap1/RIAM complex and provide a potential new mechanism for TCR-mediated integrin activation.

Integrin Regulation of Lymphocyte Trafficking: Lessons from Structural and Signaling Studies

High trafficking capability of lymphocytes is crucial in immune surveillance and antigen responses. Central to this regulatory process is a dynamic control of lymphocyte adhesion behavior regulated by chemokines and adhesion receptors such as integrins. Modulation of lymphocyte adhesive responses occurs in a wide range of time window from less than a second to hours, enabling rolling lymphocyte to attach to and migrate through endothelium and interact with antigen-presenting cells. While there has been a rapid progress in the understanding of integrin structure, elucidation of signaling events to relay extracellular signaling to integrins in physiological contexts has recently emerged from studies using gene-targeting and gene-silencing technique. Regulatory molecules critical for integrin activity control distribution of integrins, polarized cell morphology and motility, suggesting a signaling network that coordinates integrin function with lymphocyte migration. Here, I review recent studies of integrin structural changes and intracellular signal molecules that trigger integrin activation (inside-out signals), and discuss molecular mechanisms that control lymphocyte integrins and how inside-out signals coordinately modulate adhesive reactions and cell shape and migration.

Regulation of immune responses and hematopoiesis by the Rap1 signal

Rap1 (Ras-proximity 1), a member of the Ras family of small guanine triphosphatases (GTPases), is activated by diverse extracellular stimuli. While Rap1 has been discovered originally as a potential Ras antagonist, accumulating evidence indicates that Rap1 per se mediates unique signals and exerts biological functions distinctly different from Ras. Rap1 plays a dominant role in the control of cell-cell and cell-matrix interactions by regulating the function of integrins and other adhesion molecules in various cell types. Rap1 also regulates MAP kinase (MAPK) activity in a manner highly dependent on the context of cell types. Recent studies (including gene-targeting analysis) have uncovered that the Rap1 signal is integrated crucially and unpredictably in the diverse aspects of comprehensive biological systems. This review summarizes the role of the Rap1 signal in developments and functions of the immune and hematopoietic systems as well as in malignancy. Importantly, Rap1 activation is tightly regulated in tissue cells, and dysregulations of the Rap1 signal in specific tissues result in certain disorders, including myeloproliferative disorders and leukemia, platelet dysfunction with defective hemostasis, leukocyte adhesion-deficiency syndrome, lupus-like systemic autoimmune disease, and T cell anergy. Many of these disorders resemble human diseases, and the Rap1 signal with its regulators may provide rational molecular targets for controlling certain human diseases including malignancy.

Phosphorylation of the LFA-1 Integrin β2-Chain on Thr-758 Leads to Adhesion, Rac-1/Cdc42 Activation, and Stimulation of CD69 Expression in Human T Cells

Phosphorylation of the leukocyte function-associated antigen-1 (LFA-1) integrin beta2-chain on Thr-758 occurs after T cell receptor stimulation and leads to 14-3-3 recruitment to the integrin, actin cytoskeleton reorganization, and increased adhesion. Here, we have investigated the signaling effects of beta2 integrin Thr-758 phosphorylation. A penetratin-coupled phospho-Thr-758-beta2 peptide (mimicking the part of the integrin beta-chain surrounding Thr-758) stimulated adhesion of human T cells to the LFA-1 ligand intercellular adhesion molecule-1 (ICAM-1). Additionally, the peptide activated the small GTPases Rac-1 and Cdc42 in T cells. Constitutively active forms of Rac-1 and Cdc42, but not Rho, could compensate for the reduction of cell adhesion to ICAM-1 caused by the T758A mutation in the beta2 integrin. Additionally, the active GTPases salvaged the cell-spreading defect of T758A integrin-transfected cells on coated ICAM-1. A dominant negative form of Cdc42, on the other hand, significantly reduced wild-type beta2 integrin-mediated cell adhesion and spreading. In a T cell stimulation system, the pThr-758 penetratin peptide acted in a similar manner to coated ICAM-1 to increase T cell receptor-induced CD69 expression. These results show that Thr-758-phosphorylated LFA-1 is upstream of Rac-1/Cdc42, cell adhesion, and costimulatory activation of human T cells, thus identifying phosphorylation of Thr-758 in beta2 as a proximal element in LFA-1 signaling.

Reconstructing and deconstructing agonist-induced activation of integrin alphaIIbbeta3

BACKGROUND

Integrin receptors, composed of transmembrane alpha and beta subunits, are essential for the development and functioning of multicellular animals. Agonist stimulation leads cells to regulate integrin affinity ("activation"), thus controlling cell adhesion and migration, controlling extracellular-matrix assembly, and contributing to angiogenesis, tumor cell metastasis, inflammation, the immune response, and hemostasis. A final step in integrin activation is the binding of talin, a cytoskeletal protein, to integrin beta cytoplasmic domains. Many different signaling molecules that regulate integrin affinity have been described, but a pathway that connects agonist stimulation to talin binding and activation has not been mapped.

RESULTS

We used forward, reverse, and synthetic genetics to engineer and order an integrin activation pathway in cells expressing a prototype activatable integrin, platelet alphaIIbbeta3. Phorbol myristate acetate (PMA) activated alphaIIbbeta3 only after the increased expression of both recombinant protein kinase Calpha (PKCalpha) and talin to levels approximating those in platelets. Inhibition of Rap1 GTPase reduced alphaIIbbeta3 activation, whereas activated Rap1A(G12V) bypassed the requirement for PKC, establishing that Rap1 is downstream of PKC. Talin binding to integrins mediates Rap1-induced activation because Rap1A(G12V) failed to activate alphaIIbbeta3 in cells expressing integrin binding-defective talin (W359A). Rap1 activated integrins by forming an integrin-associated complex containing talin in combination with the Rap effector, RIAM. Furthermore, siRNA-mediated knockdown of RIAM blocked integrin activation.

CONCLUSIONS

We have, for the first time, ordered a pathway from agonist stimulation to integrin activation and established the Rap1-induced formation of an "integrin activation complex," containing RIAM and talin, that binds to and activates the integrin.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

P-selectin activates integrin-mediated colon carcinoma cell adhesion to fibronectin

During hematogenous cancer metastasis, tumor cells separate from a primary mass, enter the bloodstream, disperse throughout the body, migrate across vessel walls, and generate distant colonies. The later steps of metastasis superficially resemble leukocyte extravasation, a process initiated by selectin-mediated cell tethering to the blood vessel wall followed by integrin-mediated arrest and transendothelial migration. Some cancer cells express P-selectin ligands and attach to immobilized P-selectin, suggesting that these cells can arrest in blood vessels using sequential selectin- and integrin-mediated adhesion, as do leukocytes. We hypothesize that selectin binding may regulate subsequent integrin-mediated steps in metastasis. Using a model system of cultured Colo 320 human colon adenocarcinoma cells incubated with soluble P-selectin-IgG chimeric protein, we have found that P-selectin can stimulate activation of the alpha(5)beta(1) integrin resulting in a specific increase of adhesion and spreading of these cells on fibronectin substrates. P-selectin binding also induced activation of p38 mitogen-activated protein kinase (p38 MAPK) and phosphatidylinositol 3-kinase (PI3-K). PI3-K inhibitors blocked P-selectin-mediated integrin activation, cell attachment, and cell spreading. Inhibition of p38 MAPK activation blocked cell spreading, but not cell attachment. P-selectin binding also resulted in formation of a signaling complex containing PI3-K and p38 MAPK. These results suggest that P-selectin binding to tumor cells can activate alpha(5)beta(1) integrin via PI3-K and p38 MAPK signaling pathways leading to increased cell adhesion. We propose that P-selectin ligands are important tumor cell signaling molecules that modulate integrin-mediated cell adhesion in the metastatic process.

The Rap GTPases mediate CXCL13- and sphingosine1-phosphate-induced chemotaxis, adhesion, and Pyk2 tyrosine phosphorylation in B lymphocytes

The localization of B cells to lymphoid organs where they can become activated and differentiate into antibody-secreting plasma cells is controlled by multiple chemoattractants that promote cell migration and integrin-mediated adhesion. CXCL13 and sphingosine 1-phosphate (S1P) are two important chemoattractants that control the trafficking of B cells. CXCL13 directs B lymphocytes to lymphoid follicles where they receive survival signals and, if activated, undergo a germinal center response. In contrast, S1P allows B cells and plasma cells to exit lymphoid organs and re-enter the circulation. The Rap1 GTPase is a key regulator of cell adhesion and cell migration in a number of systems. We now show that Rap activation is required for CXCL13 and S1P to induce B cell migration as well as adhesion to ICAM-1 and VCAM-1. We also show that Pyk2, a tyrosine kinase involved in cytoskeleton rearrangements and B cell migration, is a downstream target of both CXCL13 and S1P signaling and that Rap activation is important for CXCL13 and S1P to stimulate tyrosine phosphorylation of Pyk2, a modification that increases Pyk2 kinase activity. This suggests that the ability of CXCL13 and S1P to direct the trafficking and localization of B cells in vivo may be dependent on Rap activation.

Spatiotemporal regulation of the kinase Mst1 by binding protein RAPL is critical for lymphocyte polarity and adhesion

RAPL, a protein that binds the small GTPase Rap1, is required for efficient immune cell trafficking. Here we have identified the kinase Mst1 as a critical effector of RAPL. RAPL regulated the localization and kinase activity of Mst1. 'Knockdown' of the gene encoding Mst1 demonstrated its requirement for the induction of both a polarized morphology and integrin LFA-1 clustering and adhesion triggered by chemokines and T cell receptor ligation. RAPL and Mst1 localized to vesicular compartments and dynamically translocated with LFA-1 to the leading edge upon Rap1 activation, suggesting a regulatory function for the RAPL-Mst1 complex in intracellular transport of LFA-1. Our study demonstrates a previously unknown function for Mst1 of relaying the Rap1-RAPL signal to induce cell polarity and adhesion of lymphocytes.

The cyclic AMP-Epac1-Rap1 pathway is dissociated from regulation of effector functions in monocytes but acquires immunoregulatory function in mature macrophages

cAMP mediates its intracellular effects through activation of protein kinase A (PKA), nucleotide-gated ion channels, or exchange protein directly activated by cAMP (Epac). Although elevation of cAMP in lymphocytes leads to suppression of immune functions by a PKA-dependent mechanism, the effector mechanisms for cAMP regulation of immune functions in monocytes and macrophages are not fully understood. In this study, we demonstrate the presence of Epac1 in human peripheral blood monocytes and activation of Rap1 in response to cAMP. However, by using an Epac-specific cAMP analog (8-CPT-2'-O-Me-cAMP), we show that monocyte activation parameters such as synthesis and release of cytokines, stimulation of cell adhesion, chemotaxis, phagocytosis, and respiratory burst are not regulated by the Epac1-Rap1 pathway. In contrast, activation of PKA by a PKA-specific compound (6-Bnz-cAMP) or physiological cAMP-elevating stimuli like PGE(2) inhibits monocyte immune functions. Furthermore, we show that the level of Epac1 increases 3-fold during differentiation of monocytes into macrophages, and in monocyte-derived macrophages cAMP inhibits FcR-mediated phagocytosis via both PKA and the Epac1-Rap1 pathway. However, LPS-induced TNF-alpha production is only inhibited through the PKA pathway in these cells. In conclusion, the Epac1-Rap1 pathway is present in both monocytes and macrophages, but only regulates specific immune effector functions in macrophages.

The PDZ-GEF dizzy regulates cell shape of migrating macrophages via Rap1 and integrins in the Drosophila embryo

In Drosophila embryos, macrophages originate from the cephalic mesoderm and perform a complex migration throughout the entire embryo. The molecular mechanisms regulating this cell migration remain largely unknown. We identified the Drosophila PDZ G-nucleotide exchange factor (PDZ-GEF) Dizzy as a component essential for normal macrophage migration. In mutants lacking Dizzy, macrophages have smaller cellular protrusions, and their migration is slowed down significantly. This phenotype appears to be cell-autonomous, as it is also observed in embryos with a dsRNA-induced reduction of dizzy function in macrophages. In a complementary fashion, macrophages overexpressing Dizzy are vastly extended and form very long protrusions. These cell shape changes depend on the function of the small GTPase Rap1: in rap1 mutants, Dizzy is unable to induce the large protrusions. Furthermore, forced expression of a dominant-active form of Rap1, but not of the wild-type form, induces similar cell shape changes as Dizzy does overexpression. These findings suggest that Dizzy acts through Rap1. We propose that integrin-dependent adhesion is a Rap1-mediated target of Dizzy activity: in integrin mutants, neither Dizzy nor Rap1 can induce cell shape changes in macrophages. These data provide the first link between a PDZ-GEF, the corresponding small GTPase and integrin-dependent cell adhesion during cell migration in embryonic development.

CpG DNA enhances macrophage cell spreading by promoting the Src-family kinase-mediated phosphorylation of paxillin

Macrophages are an important component of the innate immune response to infection by microbial pathogens. The activation of macrophages by pathogens is largely mediated by Toll-like receptors (TLRs). Bacterial DNA, which contains unmethylated CpG dinucleotide motifs, is specifically recognised by TLR9 and triggers the activation of a complex network of intracellular signalling pathways that orchestrates the ensuing inflammatory responses of macrophages to the pathogen. Here, we have established that CpG DNA promotes reorganisation of the actin cytoskeleton and enhances cell spreading by primary mouse bone marrow macrophages. CpG DNA stimulation resulted in an approximately 70% increase in cell size. Notably, CpG DNA-induced cell spreading was dependent on the activity of Src-family kinases. Tyrosine phosphorylation of several proteins was increased in a Src-family kinase-dependent manner following CpG DNA stimulation of bone marrow macrophages, including the cytoskeletal protein paxillin. Paxillin was phosphorylated both in vitro and in vivo by the Src-family kinase Hck. Significantly, paxillin from CpG DNA-stimulated bone marrow macrophages had a greater capacity to bind the SH2 domain of the adapter protein Crk than did paxillin from unstimulated bone marrow macrophages. Furthermore, phosphorylation of paxillin by Hck created a binding site for Crk. We propose that the formation of paxillin-Crk complexes may mediate the cytoskeletal changes that underlie the increased cell spreading of macrophages following their activation by CpG DNA.

Enhancement of LFA-1-mediated T cell adhesion by human T lymphotropic virus type 1 p12I1

Cell-to-cell transmission of retroviruses, such as human T lymphotropic virus type 1 (HTLV-1), is well documented, but the roles of viral regulatory or other nonstructural proteins in the modulation of T cell adhesion are incompletely understood. In this study we tested the role of the HTLV-1 accessory protein, p12(I), on LFA-1-mediated cell adhesion. p12(I) is critical for early HTLV-1 infection by causing the release of calcium from the endoplasmic reticulum to activate NFAT-mediated transcription. We tested the role of this novel viral protein in mediating LFA-1-dependent cell adhesion. Our data indicated that T cells expressing a mutant HTLV-1 provirus that does not produce p12(I) mRNA (ACH.p12(I)) exhibited reduced LFA-1-mediated adhesion compared with wild-type HTLV-1-expressing cells (ACH). Furthermore, the expression of p12(I) in Jurkat T cells using lentiviral vectors enhanced LFA-1-mediated cell adhesion, which was inhibited by the calcium chelator BAPTA-AM, the calcium channel blocker SK&F 96365, and calpeptin, an inhibitor of the calcium-dependent protease calpain. Similar to the intracellular calcium mobilizer, thapsigargin, the expression of p12(I) in Jurkat T cells induced cell surface clustering of LFA-1 without changing the level of integrin expression. Our data are the first to indicate that HTLV-1 p12(I), in addition to enhancing T cell activation, promotes cell-to-cell spread by inducing LFA-1 clustering on T cells via calcium-dependent signaling.

Constitutive integrin activation on tumor cells contributes to progression of leptomeningeal metastases

Leptomeningeal metastases are a serious neurological complication in cancer patients and associated with a dismal prognosis. Tumor cells that enter the subarachnoid space adhere to the leptomeninges and form tumor deposits. It is largely unknown which adhesion molecules mediate tumor cell adhesion to leptomeninges. We studied the role of integrin expression and activation in the progression of leptomeningeal metastases. For this study, we used a mouse acute lymphocytic leukemic cell line that was grown in suspension (L1210-S cell line) to develop an adherent L1210 cell line (L1210-A) by selectively culturing the few adherent cells in the cell culture. beta1, beta2, and beta3 integrins were in a constitutively high active state on L1210-A cells and in a low, but inducible, active state on L1210-S cells. Expression levels of these integrins were comparable in the two cell lines. Static adhesion levels of L1210-A cells on a leptomeningeal cell layer were significantly higher than those of L1210-S cells. All mice that were injected intrathecally with L1210-A cells died rapidly of leptomeningeal leukemia. In contrast, 45% long-term survival was seen after intrathecal injection of mice with L1210-S cells. Our data indicate that constitutive integrin activation on leukemic cells promotes progression of leptomeningeal leukemia by increased tumor cell adhesion to the leptomeninges. We argue that an aberrantly regulated inside-out signaling pathway underlies constitutive integrin activation on the adherent leukemic cell population.

Rap1A-deficient T and B cells show impaired integrin-mediated cell adhesion

Studies in tissue culture cells have demonstrated a role for the Ras-like GTPase Rap1 in the regulation of integrin-mediated cell-matrix and cadherin-mediated cell-cell contacts. To analyze the function of Rap1 in vivo, we have disrupted the Rap1A gene by homologous recombination. Mice homozygous for the deletion allele are viable and fertile. However, primary hematopoietic cells isolated from spleen or thymus have a diminished adhesive capacity on ICAM and fibronectin substrates. In addition, polarization of T cells from Rap1-/- cells after CD3 stimulation was impaired compared to that of wild-type cells. Despite this, these defects did not result in hematopoietic or cell homing abnormalities. Although it is possible that the relatively mild phenotype is a consequence of functional complementation by the Rap1B gene, our genetic studies confirm a role for Rap1A in the regulation of integrins.

Specific integrin alpha and beta chain phosphorylations regulate LFA-1 activation through affinity-dependent and -independent mechanisms

Integrins are adhesion receptors that are crucial to the functions of multicellular organisms. Integrin-mediated adhesion is a complex process that involves both affinity regulation and cytoskeletal coupling, but the molecular mechanisms behind this process have remained incompletely understood. In this study, we report that the phosphorylation of each cytoplasmic domain of the leukocyte function-associated antigen-1 integrin mediates different modes of integrin activation. α Chain phosphorylation on Ser1140 is needed for conformational changes in the integrin after chemokine- or integrin ligand–induced activation or after activation induced by active Rap1 (Rap1V12). In contrast, the β chain Thr758 phosphorylation mediates selective binding to 14-3-3 proteins in response to inside-out activation through the T cell receptor, resulting in cytoskeletal rearrangements. Thus, site-specific phosphorylation of the integrin cytoplasmic domains is important for the dynamic regulation of these complex receptors in cells.

Membrane-proximal signaling events in beta-2 integrin activation

In the immune system, integrins have essential roles in leukocyte trafficking and function. These include immune cell attachment to endothelial and antigen-presenting cells, cytotoxicity, and extravasation into tissues. The integrin leukocyte function-associated antigen-1 (LFA-1), which is exclusively expressed on hematopoietic cells, has been intensely studied since this receptor is important for many functions of the immune system. LFA-1 is involved in a) the interaction between T-cells and antigen presenting cells, b) the adhesion of cells to post-capillary high endothelial venules or to activated endothelium at sites of inflammation (extravasation), c) the control of cell differentiation and proliferation, and d) the regulation of T-cell effector functions. Therefore, a precise understanding of the spatial and temporal control of LFA-1 interaction with its cellular counter-receptors, the intercellular adhesion molecules (ICAM) -1, -2 and -3, in the various contexts, is of high interest. LFA-1 mediated adhesion is induced by several extracellular stimuli in different cell types. In T-cells, LFA-1 becomes activated upon signaling from the T-cell receptor (TCR), and upon cytokine and chemokine sensing. Adhesion of monocytes to ICAM-1 is induced by lipopolysaccharide (LPS), a component of the bacterial cell wall. To investigate the regulation of LFA-1 adhesiveness, research has focused on the identification of interaction partners of the intracellular portions of the integrin alpha and beta subunits. This review will highlight recent developments on transmembrane and intracellular signaling proteins, which have been implicated in beta-2 integrin activation.

Rap1 regulation of RIAM and cell adhesion

The small GTPase Rap1 has been involved in different cellular processes. Rap1 is known to increase cell adhesion by means of integrin activation, to induce cell spreading, and to regulate adherent junctions at cell-cell contacts. How Rap1 mediates these cell responses is poorly known, but currently developing evidence points to the involvement of different effector pathways. Recently, we described RIAM, a Rap1 interacting adaptor protein that regulates integrin activation and hence cell adhesion. RIAM is required for Rap1-induced adhesion and seems to control Rap1 localization at the plasma membrane, where Rap1 regulates integrin activation. In this chapter, we focus in the role of RIAM in regulating Rap1-mediated cell adhesion. We describe the method for studying the Rap1-RIAM interaction using in vitro and in vivo approaches such as yeast two hybrids, pull-down assays. and coimmunoprecipitation. The role of Rap1 and RIAM in integrin-mediated adhesion is studied by cell adhesion assays to immobilized integrin substrates and by changes in integrin activation as determined by activation epitope exposure. Finally, we describe an approach to determine the role of RIAM in regulating intracellular localization of active Rap1.