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

Germinal center kinases in immune regulation

Germinal center kinases (GCKs) participate in a variety of signaling pathways needed to regulate cellular functions including apoptosis, cell proliferation, polarity and migration. Recent studies have shown that GCKs are participants in both adaptive and innate immune regulation. However, the differential activation and regulatory mechanisms of GCKs, as well as upstream and downstream signaling molecules, remain to be fully defined. It remains unresolved whether and how GCKs may cross-talk with existing signaling pathways. This review stresses the progresses in research of GCKs relevant to the immune system.

Signal transduction pathways in chronic inflammatory autoimmune disease: small GTPases

Ras superfamily small GTPases represent a wide and diverse class of intracellular signaling proteins that are highly conserved during evolution. These enzymes serve as key checkpoints in coupling antigen receptor, growth factor, cytokine and chemokine stimulation to cellular responses. Once activated, via their ability to regulate multiple downstream signaling pathways, small GTPases amplify and diversify signaling cascades which regulate cellular proliferation, survival, cytokine expression, trafficking and retention. Small GTPases, particularly members of the Ras, Rap, and Rho family, critically coordinate the function and interplay of immune and stromal cells during inflammatory respones, and increasing evidence indicates that alterations in small GTPase signaling contribute to the pathological behavior of these cell populations in human chronic inflammatory diseases such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Here, we review how Ras, Rap, and Rho family GTPases contribute to the biology of cell populations relevant to human chronic inflammatory disease, highlight recent advances in understanding how alterations in these pathways contribute to pathology in RA and SLE, and discuss new therapeutic strategies that may allow specific targeting of small GTPases in the clinic.

Mammalian sterile 20-like kinase 1 suppresses lymphoma development by promoting faithful chromosome segregation

The mammalian Hippo signaling pathway has been implicated in oncogenesis in the context of solid tumors such as hepatocellular carcinoma. Mammalian sterile 20-like kinase 1 (MST1), the core component of the Hippo signaling pathway, is highly expressed in hematopoietic cells. However, its possible impact on tumorigenesis in this setting is unknown. In this study, we provide evidence that Mst1 loss in the mouse enhances chemically and genetically induced lymphoma development by inducing chromosomal instability. Mst1 deficiency increased susceptibility to T-cell acute lymphoblastic leukemia induced by mutagen exposure. Notably, before transformation Mst1(-/-) normal thymocytes showed no changes in proliferation or apoptosis in vitro and in vivo, but they displayed elevated levels of abnormal mitotic chromosomes and aneuploidy, conditions known to promote tumorigenesis. Mst1(-/-) mice also showed accelerated formation of spontaneous lymphomas in a p53-deficient background, accompanied by severe aneuploidy. In clinical specimens of lymphoma and leukemia, we documented frequent downregulation of MST1 expression, consistent with our findings. Taken together, our findings reveal a tumor suppressive function of Mst1 based on its ability to prevent chromosomal instability in lymphocytes.

Protein kinases of the Hippo pathway: regulation and substrates

The "Hippo" signaling pathway has emerged as a major regulator of cell proliferation and survival in metazoans. The pathway, as delineated by genetic and biochemical studies in Drosophila, consists of a kinase cascade regulated by cell-cell contact and cell polarity that inhibits the transcriptional coactivator Yorkie and its proliferative, anti-differentiation, antiapoptotic transcriptional program. The core pathway components are the GC kinase Hippo, which phosphorylates the noncatalytic polypeptide Mats/Mob1 and, with the assistance of the scaffold protein Salvador, phosphorylates the ndr-family kinase Lats. In turn phospho-Lats, after binding to phospho-Mats, autoactivates and phosphorylates Yorkie, resulting in its nuclear exit. Hippo also uses the scaffold protein Furry and a different Mob protein to control another ndr-like kinase, the morphogenetic regulator Tricornered. Architecturally homologous kinase cascades consisting of a GC kinase, a Mob protein, a scaffolding polypeptide and an ndr-like kinase are well described in yeast; in Saccharomyces cerevisiae, e.g., the MEN pathway promotes mitotic exit whereas the RAM network, using a different GC kinase, Mob protein, scaffold and ndr-like kinase, regulates cell polarity and morphogenesis. In mammals, the Hippo orthologs Mst1 and Mst2 utilize the Salvador ortholog WW45/Sav1 and other scaffolds to regulate the kinases Lats1/Lats2 and ndr1/ndr2. As in Drosophila, murine Mst1/Mst2, in a redundant manner, negatively regulate the Yorkie ortholog YAP in the epithelial cells of the liver and gut; loss of both Mst1 and Mst2 results in hyperproliferation and tumorigenesis that can be largely negated by reduction or elimination of YAP. Despite this conservation, considerable diversification in pathway composition and regulation is already evident; in skin, e.g., YAP phosphorylation is independent of Mst1Mst2 and Lats1Lats2. Moreover, in lymphoid cells, Mst1/Mst2, under the control of the Rap1 GTPase and independent of YAP, promotes integrin clustering, actin remodeling and motility while restraining the proliferation of naïve T cells. This review will summarize current knowledge of the structure and regulation of the kinases Hippo/Mst1&2, their noncatalytic binding partners, Salvador and the Rassf polypeptides, and their major substrates Warts/Lats1&2, Trc/ndr1&2, Mats/Mob1 and FOXO.

Tumor suppressor Hippo/MST1 kinase mediates chemotaxis by regulating spreading and adhesion

Chemotaxis depends on a network of parallel pathways that coordinate cytoskeletal events to bias cell movement along a chemoattractant gradient. Using a forward genetic screen in Dictyostelium discoideum, we identified the Ste20 kinase KrsB, a homolog of tumor suppressors Hippo and MST1/2, as a negative regulator of cell spreading and substrate attachment. The excessive adhesion of krsB(-) cells reduced directional movement and prolonged the streaming phase of multicellular aggregation. These phenotypes depended on an intact kinase domain and phosphorylation of a conserved threonine (T176) within the activation loop. Chemoattractants triggered a rapid, transient autophosphorylation of T176 in a heterotrimeric G protein-dependent and PI3K- and TorC2-independent manner. The active phosphorylated form of KrsB acts to decrease adhesion to the substrate. Taken together these studies suggest that cycling between active and inactive forms of KrsB may provide the dynamic regulation of cell adhesion needed for proper cell migration and chemotaxis. KrsB interacts genetically with another D. discoideum Hippo/MST homolog, KrsA, but the two genes are not functionally redundant. These studies show that Hippo/MST proteins, like the tumor suppressor PTEN and oncogenes Ras and PI3K, play a key role in cell morphological events in addition to their role in regulating cell growth.

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.

First insight into the kinome of human regulatory T cells

Regulatory T cells (Tregs) are essential for controlling peripheral tolerance by the active suppression of various immune cells including conventional T effector cells (Teffs). Downstream of the T cell receptor (TCR), more than 500 protein kinases encoded by the human genome have to be considered in signaling cascades regulating the activation of Tregs and Teffs, respectively. Following TCR engagement, Tregs posses a number of unique attributes, such as constitutive expression of Foxp3, hyporesponsiveness and poor cytokine production. Furthermore, recent studies showed that altered regulation of protein kinases is important for Treg function. These data indicate that signaling pathways in Tregs are distinctly organized and alterations at the level of protein kinases contribute to the unique Treg phenotype. However, kinase-based signaling networks in Tregs are poorly understood and necessitate further systematic characterization. In this study, we analyzed the differential expression of kinases in Tregs and Teffs by using a kinase-selective proteome strategy. In total, we revealed quantitative information on 185 kinases expressed in the human CD4(+) T cell subsets. The majority of kinases was equally abundant in both T cell subsets, but 11 kinases were differentially expressed in Tregs. Most strikingly, Tregs showed an altered expression of cell cycle kinases including CDK6. Quantitative proteomics generates first comparative insight into the kinase complements of the CD4(+) Teff and Treg subset. Treg-specific expression pattern of 11 protein kinases substantiate the current opinion that TCR-mediated signaling cascades are altered in Tregs and further suggests that Tregs exhibit significant specificities in cell-cycle control and progression.

Activation of the Hippo pathway by CTLA-4 regulates the expression of Blimp-1 in the CD8+ T cell

During the primary response, the commitment of the CD8(+) T cell to Blimp-1 expression and the terminal differentiation that Blimp-1 induces must be timed so as not to impair the process of clonal expansion. We determined whether the Hippo pathway, which links cell-cell contact to differentiation in other cell lineages, controls Blimp-1 expression. Activating the CD8(+) T cell with antigen and IL-2 causes expression of the core Hippo pathway components, including the pivotal transcriptional cofactor Yap. Contact between activated CD8(+) T cells induces Hippo pathway-mediated Yap degradation and Blimp-1 expression; a Hippo-resistant, stable form of Yap suppresses Blimp-1 expression. Cytotoxic T lymphocyte antigen 4 (CTLA-4) and CD80 comprise the receptor-ligand pair that mediates contact-dependent Hippo pathway activation. In vivo, CD8(+) T cells expressing Hippo resistant-Yap or lacking CTLA-4 have diminished expression of the senescence marker, KLRG1, during a viral infection. The CTLA-4/Hippo pathway/Blimp-1 system may couple terminal differentiation of CD8(+) T cell with the magnitude of clonal expansion.

A loss of naivete

Monogenic disorders leading to primary immunodeficiency have fascinated scientists and clinicians alike by their capacity to reveal the complexities of intracellular signaling pathways. Two articles in this issue of Blood by Abdollahpour et al and Nehme et al illustrate this point vividly, describing for the first time the clinical and immunologic phenotype associated with genetic mutations in STK4, manifested largely by a loss of T-cell naiveté.

The Gαq/11 proteins contribute to T lymphocyte migration by promoting turnover of integrin LFA-1 through recycling

The role of Gαi proteins coupled to chemokine receptors in directed migration of immune cells is well understood. In this study we show that the separate class of Gαq/11 proteins is required for the underlying ability of T cells to migrate both randomly and in a directed chemokine-dependent manner. Interfering with Gαq or Gα11 using dominant negative cDNA constructs or siRNA for Gαq causes accumulation of LFA-1 adhesions and stalled migration. Gαq/11 has an impact on LFA-1 expression at plasma membrane level and also on its internalization. Additionally Gαq co-localizes with LFA-1- and EEA1-expressing intracellular vesicles and partially with Rap1- but not Rab11-expressing vesicles. However the influence of Gαq is not confined to the vesicles that express it, as its reduction alters intracellular trafficking of other vesicles involved in recycling. In summary vesicle-associated Gαq/11 is required for the turnover of LFA-1 adhesion that is necessary for migration. These G proteins participate directly in the initial phase of recycling and this has an impact on later stages of the endo-exocytic pathway.

Sustained Rap1 activation in autoantigen-specific T lymphocytes attenuates experimental autoimmune encephalomyelitis

Altered Ras superfamily guanine nucleotide triphosphatase signaling may contribute to the activation of autoreactive T cells in diseases such as rheumatoid arthritis and systemic lupus erythematosus. Here, we show that transgenic expression of activated Rap1, a Ras-related protein which is protective in murine arthritis, in both wildtype (WT) and 2D2 mice, enhances autoreactive T cell activation by myelin oligodendrocyte glycoprotein peptide in vitro and in vivo. However, RapV12 reduces the number of autoreactive T cells in both WT and 2D2 mice, and increases murine survival in experimental autoimmune encephalitis, suggesting Rap1 activation restricts autoimmune T cell-mediated pathology through enhancing tolerance.

Chemokines and the signaling modules regulating integrin affinity

Integrin-mediated adhesion is a general concept referring to a series of adhesive phenomena including tethering–rolling, affinity, valency, and binding stabilization altogether controlling cell avidity (adhesiveness) for the substrate. Arrest chemokines modulate each aspect of integrin activation, although integrin affinity regulation has been recognized as the prominent event in rapid leukocyte arrest induced by chemokines. A variety of inside-out and outside-in signaling mechanisms have been related to the process of integrin-mediated adhesion in different cellular models, but only few of them have been clearly contextualized to rapid integrin affinity modulation by arrest chemokines in primary leukocytes. Complex signaling processes triggered by arrest chemokines and controlling leukocyte integrin activation have been described for ras-related rap and for rho-related small GTPases. We summarize the role of rap and rho small GTPases in the regulation of rapid integrin affinity in primary leukocytes and provide a modular view of these pro-adhesive signaling events. A potential, albeit still speculative, mechanism of rho-mediated regulation of cytoskeletal proteins controlling the last step of integrin activation is also discussed. We also discuss data suggesting a functional integration between the rho- and rap-modules of integrin activation. Finally we examine the universality of signaling mechanisms regulating integrin triggering by arrest chemokines.

Distinct roles for talin-1 and kindlin-3 in LFA-1 extension and affinity regulation

In inflammation, neutrophils and other leukocytes roll along the microvascular endothelium before arresting and transmigrating into inflamed tissues. Arrest requires conformational activation of the integrin lymphocyte function-associated antigen-1 (LFA-1). Mutations of the FERMT3 gene encoding kindlin-3 underlie the human immune deficiency known as leukocyte adhesion deficiency-III. Both kindlin-3 and talin-1, another FERM domain-containing cytoskeletal protein, are required for integrin activation, but their individual roles in the induction of specific integrin conformers are unclear. Here, we induce differential LFA-1 activation in neutrophils through engagement of the selectin ligand P-selectin glycoprotein ligand-1 or the chemokine receptor CXCR2. We find that talin-1 is required for inducing LFA-1 extension, which corresponds to intermediate affinity and induces neutrophil slow rolling, whereas both talin-1 and kindlin-3 are required for induction of the high-affinity conformation of LFA-1 with an open headpiece, which results in neutrophil arrest. In vivo, both slow rolling and arrest are defective in talin-1-deficient neutrophils, whereas only arrest is defective in kindlin-3-deficient neutrophils. We conclude that talin-1 and kindlin-3 serve distinct functions in LFA-1 activation.

The Mst1 and Mst2 kinases control activation of rho family GTPases and thymic egress of mature thymocytes

In mice lacking both Mst1 and Mst2 in the lymphoid compartment, thymocyte development is normal, but single-positive thymocytes exhibit excessive apoptosis and greatly diminished thymic egress, accompanied by loss of chemokine activation of RhoA and Rac1.

The Mst1 kinase is an important regulator of murine T cell adhesion, migration, proliferation, and apoptosis. In this study, we analyze mice lacking both Mst1 and Mst2 in hematopoietic cells. Compared with wild-type mice, these double knockout (DKO) mice exhibit a severe reduction in the number of mature T cells in the circulation and in secondary lymphoid organs (SLOs). CD4⁺CD8⁻ and CD4⁻CD8⁺ single-positive (SP) thymocytes in DKO mice resemble mature T cells of wild-type mice but undergo excessive apoptosis, and their egress from the thymus is reduced by >90%. Even when placed directly in the circulation, DKO SP thymocytes failed to enter SLOs. In SP thymocytes, deficiency of Mst1 and Mst2 abolished sphingosine-1 phosphate– and CCL21-induced Mob1 phosphorylation, Rac1 and RhoA GTP charging, and subsequent cell migration. When phosphorylated by Mst1 or Mst2, Mob1 binds and activates the Rac1 guanyl nucleotide exchanger Dock8, which is abundant in the thymus. Thus, the Mst1 and Mst2 kinases control Rho GTPase activation and the migratory responses of SP thymocytes.

TLR signaling paralyzes monocyte chemotaxis through synergized effects of p38 MAPK and global Rap-1 activation

Toll-like receptors (TLRs) that recognize pathogen associated molecular patterns and chemoattractant receptors (CKRs) that orchestrate leukocyte migration to infected tissue are two arms of host innate immunity. Although TLR signaling induces synthesis and secretion of proinflammatory cytokines and chemokines, which recruit leukocytes, many studies have reported the paradoxical observation that TLR stimulation inhibits leukocyte chemotaxis in vitro and impairs their recruitment to tissues during sepsis. There is consensus that physical loss of chemokine receptor (CKR) at the RNA or protein level or receptor usage switching are the mechanisms underlying this effect. We show here that a brief (<15 min) stimulation with LPS (lipopolysaccharide) at ~0.2 ng/ml inhibited chemotactic response from CCR2, CXCR4 and FPR receptors in monocytes without downmodulation of receptors. A 3 min LPS pre-treatment abolished the polarized accumulation of F-actin, integrins and PIP(3) (phosphatidylinositol-3,4,5-trisphosphate) in response to chemokines in monocytes, but not in polymorphonuclear neutrophils (PMNs). If chemoattractants were added before or simultaneously with LPS, chemotactic polarization was preserved. LPS did not alter the initial G-protein signaling, or endocytosis kinetics of agonist-occupied chemoattractant receptors (CKRs). The chemotaxis arrest did not result from downmodulation of receptors or from inordinate increase in adhesion. LPS induced rapid p38 MAPK activation, global redistribution of activated Rap1 (Ras-proximate-1 or Ras-related protein 1) GTPase and Rap1GEF (guanylate exchange factor) Epac1 (exchange proteins activated by cyclic AMP) and disruption of intracellular gradient. Co-inhibition of p38 MAPK and Rap1 GTPase reversed the LPS induced breakdown of chemotaxis suggesting that LPS effect requires the combined function of p38 MAPK and Rap1 GTPase.

The phenotype of human STK4 deficiency

We describe a novel clinical phenotype associating T- and B-cell lymphopenia, intermittent neutropenia, and atrial septal defects in 3 members of a consanguineous kindred. Their clinical histories included recurrent bacterial infections, viral infections, mucocutaneous candidiasis, cutaneous warts, and skin abscesses. Homozygosity mapping and candidate gene sequencing revealed a homozygous premature termination mutation in the gene STK4 (serine threonine kinase 4, formerly having the symbol MST1). STK4 is the human ortholog of Drosophila Hippo, the central constituent of a highly conserved pathway controlling cell growth and apoptosis. STK4-deficient lymphocytes and neutrophils exhibit enhanced loss of mitochondrial membrane potential and increased susceptibility to apoptosis. STK4 deficiency is a novel human primary immunodeficiency syndrome.

Overview of integrin signaling in the immune system

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

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

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

Mst1 and Mst2 protein kinases restrain intestinal stem cell proliferation and colonic tumorigenesis by inhibition of Yes-associated protein (Yap) overabundance

Ablation of the kinases Mst1 and Mst2, orthologs of the Drosophila antiproliferative kinase Hippo, from mouse intestinal epithelium caused marked expansion of an undifferentiated stem cell compartment and loss of secretory cells throughout the small and large intestine. Although median survival of mice lacking intestinal Mst1/Mst2 is 13 wk, adenomas of the distal colon are common by this age. Diminished phosphorylation, enhanced abundance, and nuclear localization of the transcriptional coactivator Yes-associated protein 1 (Yap1) is evident in Mst1/Mst2-deficient intestinal epithelium, as is strong activation of β-catenin and Notch signaling. Although biallelic deletion of Yap1 from intestinal epithelium has little effect on intestinal development, inactivation of a single Yap1 allele reduces Yap1 polypeptide abundance to nearly wild-type levels and, despite the continued Yap hypophosphorylation and preferential nuclear localization, normalizes epithelial structure. Thus, supraphysiologic Yap polypeptide levels are necessary to drive intestinal stem cell proliferation. Yap is overexpressed in 68 of 71 human colon cancers and in at least 30 of 36 colon cancer-derived cell lines. In colon-derived cell lines where Yap is overabundant, its depletion strongly reduces β-catenin and Notch signaling and inhibits proliferation and survival. These findings demonstrate that Mst1 and Mst2 actively suppress Yap1 abundance and action in normal intestinal epithelium, an antiproliferative function that frequently is overcome in colon cancer through Yap1 polypeptide overabundance. The dispensability of Yap1 in normal intestinal homeostasis and its potent proliferative and prosurvival actions when overexpressed in colon cancer make it an attractive therapeutic target.

Why integrin as a primary target for imaging and therapy

Integrin-mediated cell adhesion is involved in many essential normal cellular and pathological functions including cell survival, growth, differentiation, migration, inflammatory responses, platelet aggregation, tissue repair and tumor invasion. 24 different heterodimerized transmembrane integrin receptors are combined from 18 different α and 8 different β subunits. Each integrin subunit contains a large extracellular domain, a single transmembrane domain and a usually short cytoplasmic domain. Integrins bind extracellular matrix (ECM) proteins through their large extracellular domain, and engage the cytoskeleton via their short cytoplasmic tails. These integrin-mediated linkages on either side of the plasma membrane are dynamically linked. Thus, integrins communicate over the plasma membrane in both directions, i.e., outside-in and inside-out signaling. In outside-in signaling through integrins, conformational changes of integrin induced by ligand binding on the extracellular domain altered the cytoplasmic domain structures to elicit various intracellular signaling pathways. Inside-out signaling originates from non-integrin cell surface receptors or cytoplasmic molecules and it activates signaling pathways inside the cells, ultimately resulting in the activation/deactivation of integrins. Integrins are one of key family proteins for cell adhesion regulation through binding to a large number of ECM molecules and cell membrane proteins. Lack of expression of integrins may result in a wide variety of effects ranging from blockage in pre-implantation to embryonic or perinatal lethality and developmental defects. Based on both the key role they played in angiogenesis, leukocytes function and tumor development and easy accessibility as cell surface receptors interacting with extracellular ligands, the integrin superfamily represents the best opportunity of targeting both antibodies and small-molecule antagonists for both therapeutic and diagnostic utility in various key diseases so far.

The gap junction protein Cx43 regulates B-lymphocyte spreading and adhesion

The gap junction protein connexin43 (Cx43) is widely expressed in mammalian cells and forms intercellular channels for the transfer of small molecules between adjacent cells, as well as hemichannels that mediate bidirectional transport of molecules between the cell and the surrounding environment. Cx43 regulates cell adhesion and migration in neurons and glioma cells, and we now show that Cx43 influences BCR-, LFA-1- and CXCL12-mediated activation of the Rap1 GTPase. Using shRNA knockdown of Cx43 in WEHI 231 cells, we show that Cx43 is required for sustained Rap1 activation and BCR-mediated spreading. To determine the domains of Cx43 that are important for this effect, Cx43-null J558 μm3 B cells (which express a wild-type IgM BCR) were transfected with wild-type Cx43-GFP or a C-terminal-truncated Cx43 (Cx43ΔT-GFP). Expression of wild-type Cx43-GFP, but not Cx43ΔT-GFP, was sufficient to restore sustained, BCR-mediated Rap1 activation and cell spreading. Cx43, and specifically the C-terminal domain, was also important for LFA-1- and CXCL12-mediated Rap1 activation, spreading and adhesion to an endothelial cell monolayer. These data show that Cx43 has an important and previously unreported role in B-cell processes that are essential to normal B-cell development and immune responses.

Impairment of immunological synapse formation in adaptively tolerant T cells

Adaptive tolerance is a hyporesponsive state in which lymphocyte Ag receptor signaling becomes desensitized after prolonged in vivo encounter with Ag. The molecular mechanisms underlying this hyporesponsive state in T cells are not fully understood, although a major signaling block has been shown to be present at the level of ZAP70 phosphorylation of linker for activation of T cells (LAT). In this study, we investigated the ability of adaptively tolerant mouse T cells to form conjugates with Ag-bearing APCs and to translocate signaling molecules into the interface between the T cells and APCs. Compared with naive or preactivated T cells, adaptively tolerant T cells showed no dramatic impairment in their formation of conjugates with APCs. In contrast, there was a large impairment in immunological synapse formation. Adaptively tolerant T cells were defective in their translocation of signaling molecules, such as ZAP70, LAT, and phospholipase C γ1, into the T cell-APC contact sites. Although Ag-induced activation of VAV1 was normal, VAV's recruitment into the synapse was also impaired. Interestingly, expressions of both IL-2-inducible T cell kinase and growth factor receptor-bound protein 2-related adaptor downstream of SHC were decreased by 60-80% in adaptively tolerant T cells. These decreases, in addition to the impairment in LAT phosphorylation by ZAP70, appear to be the major impediments to the phosphorylation of SLP76 (SRC homology 2 domain-containing leukocyte protein of 76 kDa) and the recruitment of VAV1, which are important for stable immunological synapse formation.

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.

Chemokine triggered integrin activation and actin remodeling events guiding lymphocyte migration across vascular barriers

Chemokine signals activate leukocyte integrins and actin remodeling machineries critical for leukocyte adhesion and motility across vascular barriers. The arrest of leukocytes at target blood vessel sites depends on rapid conformational activation of their α4 and β2 integrins by the binding of endothelial-displayed chemokines to leukocyte Gi-protein coupled receptors (GPCRs). A universal regulator of this event is the integrin-actin adaptor, talin1. Chemokine-stimulated GPCRs can transmit within fractions of seconds signals via multiple Rho GTPases, which locally raise plasma membrane levels of the talin activating phosphatidyl inositol, PtdIns(4,5)P2 (PIP2). Additional pools of GPCR stimulated Rac-1 and Rap-1 GTPases together with GPCR stimulated PLC and PI3K family members regulate the turnover of focal contacts of leukocyte integrins, induce the collapse of leukocyte microvilli, and promote polarized leukocyte crawling in search of exit cues. Concomitantly, other leukocyte GTPases trigger invasive protrusions into and between endothelial cells in search of basolateral chemokine exit cues. We will review here major findings and open questions related to these sequential guiding activities of endothelial presented chemokines, focusing mainly on lymphocyte-endothelial interactions as a paradigm for other leukocytes.

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.

The tumor suppressor RASSF1A is a novel effector of small G protein Rap1A

Rap1A is a small G protein implicated in a spectrum of biological processes such as cell proliferation, adhesion, differentiation, and embryogenesis. The downstream effectors through which Rap1A mediates its diverse effects are largely unknown. Here we show that Rap1A, but not the related small G proteins Rap2 or Ras, binds the tumor suppressor Ras association domain family 1A (RASSF1A) in a manner that is regulated by phosphorylation of RASSF1A. Interaction with Rap1A is shown to influence the effect of RASSF1A on microtubule behavior.

Mammalian Hippo pathway: from development to cancer and beyond

The Hippo pathway was discovered as a signal transduction pathway that regulates organ size in Drosophila melanogaster. It is composed of three components: cell surface upstream regulators including cell adhesion molecules and cell polarity complexes; a kinase cascade comprising two serine-threonine kinases with regulators and adaptors; and a downstream target, a transcription coactivator. The coactivator mediates the transcription of cell proliferation-promoting and anti-apoptotic genes. The pathway negatively regulates the coactivator to restrict cell proliferation and to promote cell death. Thus, the pathway prevents tissue overgrowth and tumourigenesis. The framework of the pathway is conserved in mammals. A dysfunction of the pathway is frequently detected in human cancers and correlates with a poor prognosis. Recent works indicated that the Hippo pathway plays an important role in tissue homoeostasis through the regulation of stem cells, cell differentiation and tissue regeneration.

Mst1/2 signalling to Yap: gatekeeper for liver size and tumour development

The mechanisms controlling mammalian organ size have long been a source of fascination for biologists. These controls are needed to both ensure the integrity of the body plan and to restrict inappropriate proliferation that could lead to cancer. Regulation of liver size is of particular interest inasmuch as this organ maintains the capacity for regeneration throughout life, and is able to regain precisely its original mass after partial surgical resection. Recent studies using genetically engineered mouse strains have shed new light on this problem; the Hippo signalling pathway, first elucidated as a regulator of organ size in Drosophila, has been identified as dominant determinant of liver growth. Defects in this pathway in mouse liver lead to sustained liver overgrowth and the eventual development of both major types of liver cancer, hepatocellular carcinoma and cholangiocarcinoma. In this review, we discuss the role of Hippo signalling in liver biology and the contribution of this pathway to liver cancer in humans.

PIPKI gamma 90 negatively regulates LFA-1-mediated adhesion and activation in antigen-induced CD4+ T cells

T cell activation requires the formation and maintenance of stable interactions between T cells and APCs. The formation of stable T cell-APC contacts depends on the activation of the integrin LFA-1 (CD11aCD18). Several positive regulators of LFA-1 activation downstream of proximal TCR signaling have been identified, including talin; however, negative regulators of LFA-1 activity remain largely unexplored. Extended isoform of phosphatidylinositol phosphate kinase type I γ (PIPKIγ90) is a member of the type I phosphatidylinositol phosphate kinase family that has been shown previously to modulate talin activation of integrins through production of phosphatidylinositol 4,5-bisphosphate and direct binding to talin. In this study, we show that PIPKIγ90 negatively regulates LFA-1-mediated adhesion and activation of T cells. Using CD4(+) T cells from PIPKIγ90-deficient mice, we show that CD4(+) T cells exhibit increased LFA-1-dependent adhesion to ICAM-1 and increased rates of T cell-APC conjugate formation with enhanced LFA-1 polarization at the synapse. In addition to increased adhesiveness, PIPKIγ90-deficient T cells exhibit increased proliferation both in vitro and in vivo and increased production of IFN-γ and IL-2. Together, these results demonstrate that PIPKIγ90 is a negative regulator of Ag-induced T cell adhesion and activation.

Proapoptotic Rassf1A/Mst1 signaling in cardiac fibroblasts is protective against pressure overload in mice

Mammalian sterile 20-like kinase 1 (Mst1) is a mammalian homolog of Drosophila Hippo, the master regulator of cell death, proliferation, and organ size in flies. It is the chief component of the mammalian Hippo pathway and promotes apoptosis and inhibits compensatory cardiac hypertrophy, playing a critical role in mediating heart failure. How Mst1 is regulated, however, remains unclear. Using genetically altered mice in which expression of the tumor suppressor Ras-association domain family 1 isoform A (Rassf1A) was modulated in a cell type-specific manner, we demonstrate here that Rassf1A is an endogenous activator of Mst1 in the heart. Although the Rassf1A/Mst1 pathway promoted apoptosis in cardiomyocytes, thereby playing a detrimental role, the same pathway surprisingly inhibited fibroblast proliferation and cardiac hypertrophy through both cell-autonomous and autocrine/paracrine mechanisms, playing a protective role during pressure overload. In cardiac fibroblasts, the Rassf1A/Mst1 pathway negatively regulated TNF-α, a key mediator of hypertrophy, fibrosis, and resulting cardiac dysfunction. These results suggest that the functional consequence of activating the proapoptotic Rassf1A/Mst1 pathway during pressure overload is cell type dependent in the heart and that suppressing this mechanism in cardiac fibroblasts could be detrimental.

Tumor suppressor ras association domain family 5 (RASSF5/NORE1) mediates death receptor ligand-induced apoptosis

Epigenetic silencing of RASSF (Ras association domain family) genes RASSF1 and RASSF5 (also called NORE1) by CpG hypermethylation is found frequently in many cancers. Although the physiological roles of RASSF1 have been studied in some detail, the exact functions of RASSF5 are not well understood. Here, we show that RASSF5 plays an important role in mediating apoptosis in response to death receptor ligands, TNF-α and TNF-related apoptosis-inducing ligand. Depletion of RASSF5 by siRNA significantly reduced TNF-α-mediated apoptosis, likely through its interaction with proapoptotic kinase MST1, a mammalian homolog of Hippo. Consistent with this, siRNA knockdown of MST1 also resulted in resistance to TNF-α-induced apoptosis. To further study the role of Rassf5 in vivo, we generated Rassf5-deficient mouse. Inactivation of Rassf5 in mouse embryonic fibroblasts (MEFs) resulted in resistance to TNF-α- and TNF-related apoptosis-inducing ligand-mediated apoptosis. Importantly, Rassf5-null mice were significantly more resistant to TNF-α-induced apoptosis and failed to activate Mst1. Loss of Rassf5 also resulted in spontaneous immortalization of MEFs at earlier passages than the control MEFs, and Rassf5-null immortalized MEFs, but not the immortalized wild type MEFs, were fully transformed by K-RasG12V. Together, our results demonstrate a direct role for RASSF5 in death receptor ligand-mediated apoptosis and provide further evidence for RASSF5 as a tumor suppressor.

Two-photon microscopy analysis of leukocyte trafficking and motility

During the last several years, live tissue imaging, in particular using two-photon laser microscopy, has advanced our understanding of leukocyte trafficking mechanisms. Studies using this technique are revealing distinct molecular requirements for leukocyte migration in different tissue environments. Also emerging from the studies are the ingenious infrastructures for leukocyte trafficking, which are produced by stromal cells. This review summarizes the recent imaging studies that provided novel mechanistic insights into in vivo leukocyte migration essential for immunosurveillance.

Rap GTPase-mediated adhesion and migration: A target for limiting the dissemination of B-cell lymphomas?

B-cell lymphomas, which arise in lymphoid organs, can spread rapidly via the circulatory system and form solid tumors within multiple organs. Rate-limiting steps in this metastatic process may be the adhesion of lymphoma cells to vascular endothelial cells, their exit from the vasculature and their migration to tissue sites that will support tumor growth. Thus proteins that control B cell adhesion and migration are likely to be key factors in lymphoma dissemination, and hence potential targets for therapeutic intervention. The Rap GTPases are master regulators of integrin activation, cell motility and the underlying cytoskeletal, adhesion and membrane dynamics. We have recently shown that Rap activation is critical for B-lymphoma cells to undergo transendothelial migration in vitro and in vivo. As a consequence, suppressing Rap activation impairs the ability of intravenously injected B-lymphoma cells to form solid tumors in the liver and other organs. We discuss this work in the context of targeting Rap, its downstream effectors, or other regulators of B cell adhesion and migration as an approach for limiting the dissemination of B-lymphoma cells and the development of secondary tumors.

RhoL controls invasion and Rap1 localization during immune cell transmigration in Drosophila

Human immune cells penetrate an endothelial barrier during their beneficial pursuit of infection and their destructive infiltration in autoimmune diseases. This transmigration requires Rap1 GTPase to activate Integrin affinity1. We define a new model system for this process by demonstrating with live imaging and genetics that during embryonic development, Drosophila melanogaster immune cells penetrate an epithelial, DE-Cadherin-based tissue barrier. A mutant in RhoL, a GTPase homolog that is specifically expressed in hemocytes, blocks this invasive step but not other aspects of guided migration. RhoL mediates Integrin adhesion caused by Drosophila Rap1 over-expression and moves Rap1 away from a cytoplasmic concentration to the leading edge during invasive migration. These findings indicate that a programmed migratory step during Drosophila development bears striking molecular similarities to vertebrate immune cell transmigration during inflammation and identify RhoL as a new regulator of invasion, adhesion and Rap1 localization. Our work establishes the utility of Drosophila for identifying novel components of immune cell transmigration and for understanding the in vivo interplay of immune cells with the barriers they penetrate.

The final steps of integrin activation: the end game

Cell-directed changes in the ligand-binding affinity ('activation') of integrins regulate cell adhesion and migration, extracellular matrix assembly and mechanotransduction, thereby contributing to embryonic development and diseases such as atherothrombosis and cancer. Integrin activation comprises triggering events, intermediate signalling events and, finally, the interaction of integrins with cytoplasmic regulators, which changes an integrin's affinity for its ligands. The first two events involve diverse interacting signalling pathways, whereas the final steps are immediately proximal to integrins, thus enabling integrin-focused therapeutic strategies. Recent progress provides insight into the structure of integrin transmembrane domains, and reveals how the final steps of integrin activation are mediated by integrin-binding proteins such as talins and kindlins.

T cell receptor "inside-out" pathway via signaling module SKAP1-RapL regulates T cell motility and interactions in lymph nodes

Although essential for T cell function, the identity of the T cell receptor "inside-out" pathway for lymphocyte function-associated antigen 1 (LFA-1) adhesion has proved elusive. Here, we define the "inside-out" pathway mediated by N-terminal SKAP1 (SKAP-55) domain binding to the C-terminal SARAH domain of RapL. TcR induced Rap1-RapL complex formation and LFA-1 binding failed to occur in Skap1(-/-) primary T cells. SKAP1 generated a SKAP1-RapL-Rap1 complex that bound to LFA-1, whereas a RapL mutation (L224A) that abrogated SKAP1 binding without affecting MST1 disrupted component colocalization in vesicles as well as T cell-dendritic cell (DC) conjugation. RapL expression also "slowed" T cell motility in D011.10 transgenic T cells in lymph nodes (LNs), an effect reversed by the L224A mutation with reduced dwell times between T cells and DCs. Overall, our findings define a TCR "inside-out" pathway via N-SKAP1-C-RapL that regulates T cell adhesion, motility, and arrest times with DCs in LNs.

Molecular regulators of leucocyte chemotaxis during inflammation

A fundamental feature of any immune response is the movement of leucocytes from one site in the body to another to provide effector functions. Therefore, elucidating the molecular mechanisms underlying the migration of leucocytes from the blood to tissues is critical to our understanding of immune function during inflammation. The classic steps of leucocyte trafficking involve leucocyte tethering and rolling on vessel walls of the vasculature, followed by firm adhesion to the endothelium. Recent evidence suggests that upon adhering, leucocytes crawl within the vessels before transmigrating across vessel walls and crawling into targeted tissues. The directed nature of the crawling events is orchestrated by a complex array of soluble factors and molecular regulators in combination with the local intravascular and extracellular environment. In fact, this process is known as chemotaxis and orientates cell movement in relation to the ligand gradient. Several signalling pathways have been proposed to be involved in this gradient-sensing and amplification process, but the best studied, discussed in detail here, is the phosphatidylinositol 3-kinase pathway. Substantial progress has been made in understanding how cells roll and adhere in blood vessels; however, how cells crawl in blood vessels, emigrate, and then crawl in tissues has received much less attention. Therefore, the focus of this review is to provide recent insights into molecular mechanisms and cellular processes that mediate leucocyte crawling in blood vessels and tissues during the inflammatory response.

Recent insights into cerebral cavernous malformations: a complex jigsaw puzzle under construction

Cerebral cavernous malformations (CCM) are common vascular malformations with an unpredictable risk of hemorrhage, the consequences of which range from headache to stroke or death. Three genes, CCM1, CCM2 and CCM3, have been linked to the disease. The encoded CCM proteins interact with each other within a large protein complex. Within the past 2 years, a plethora of new data has emerged on the signaling pathways in which CCM proteins are involved. CCM proteins regulate diverse aspects of endothelial cell morphogenesis and blood vessel stability such as cell-cell junctions, cell shape and polarity, or cell adhesion to the extracellular matrix. Although fascinating, a global picture is hard to depict because little is known about how these pathways coordinate to orchestrate angiogenesis. Here we present what is known about the structural domain organization of CCM proteins, their association as a ternary complex and their subcellular localization. Numerous CCM partners have been identified using two-hybrid screens, genetic analyses or proteomic studies. We focus on the best-characterized partners and review data on the signaling pathways they regulate as a step towards a better understanding of the etiology of CCM disease.

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.

Mst1-FoxO signaling protects Naïve T lymphocytes from cellular oxidative stress in mice

Background

The Ste-20 family kinase Hippo restricts cell proliferation and promotes apoptosis for proper organ development in Drosophila. In C. elegans, Hippo homolog also regulates longevity. The mammalian Ste20-like protein kinase, Mst1, plays a role in apoptosis induced by various types of apoptotic stress. Mst1 also regulates peripheral naïve T cell trafficking and proliferation in mice. However, its functions in mammals are not fully understood.

Methodology/Principal Findings

Here, we report that the Mst1-FoxO signaling pathway plays a crucial role in survival, but not apoptosis, of naïve T cells. In Mst1^−/− mice, peripheral T cells showed impaired FoxO1/3 activation and decreased FoxO protein levels. Consistently, the FoxO targets, Sod2 and catalase, were significantly down-regulated in Mst1^−/− T cells, thereby resulting in elevated levels of intracellular reactive oxygen species (ROS) and induction of apoptosis. Expression of constitutively active FoxO3a restored Mst1^−/− T cell survival. Crossing Mst1 transgenic mice (Mst1 Tg) with Mst1^−/− mice reduced ROS levels and restored normal numbers of peripheral naïve T cells in Mst1 Tg;Mst1^−/− progeny. Interestingly, peripheral T cells from Mst1^−/− mice were hypersensitive to γ-irradiation and paraquat-induced oxidative stresses, whereas those from Mst1 Tg mice were resistant.

Conclusions/Significance

These data support the hypothesis that tolerance to increased levels of intracellular ROS provided by the Mst1-FoxOs signaling pathway is crucial for the maintenance of naïve T cell homeostasis in the periphery.

Mst1 and Mst2 maintain hepatocyte quiescence and suppress hepatocellular carcinoma development through inactivation of the Yap1 oncogene

Hippo-Lats-Yorkie signaling regulates tissue overgrowth and tumorigenesis in Drosophila. We show that the Mst1 and Mst2 protein kinases, the mammalian Hippo orthologs, are cleaved and constitutively activated in the mouse liver. Combined Mst1/2 deficiency in the liver results in loss of inhibitory Ser127 phosphorylation of the Yorkie ortholog, Yap1, massive overgrowth, and hepatocellular carcinoma (HCC). Reexpression of Mst1 in HCC-derived cell lines promotes Yap1 Ser127 phosphorylation and inactivation and abrogates their tumorigenicity. Notably, Mst1/2 inactivates Yap1 in liver through an intermediary kinase distinct from Lats1/2. Approximately 30% of human HCCs show low Yap1(Ser127) phosphorylation and a majority exhibit loss of cleaved, activated Mst1. Mst1/2 inhibition of Yap1 is an important pathway for tumor suppression in liver relevant to human HCC.

Src homology 2-domain containing leukocyte-specific phosphoprotein of 76 kDa is mandatory for TCR-mediated inside-out signaling, but dispensable for CXCR4-mediated LFA-1 activation, adhesion, and migration of T cells

Engagement of the TCR or of chemokine receptors such as CXCR4 induces adhesion and migration of T cells via so-called inside-out signaling pathways. The molecular processes underlying inside-out signaling events are as yet not completely understood. In this study, we show that TCR- and CXCR4-mediated activation of integrins critically depends on the membrane recruitment of the adhesion- and degranulation-promoting adapter protein (ADAP)/Src kinase-associated phosphoprotein of 55 kDa (SKAP55)/Rap1-interacting adapter protein (RIAM)/Rap1 module. We further demonstrate that the Src homology 2 domain containing leukocyte-specific phosphoprotein of 76 kDa (SLP76) is crucial for TCR-mediated inside-out signaling and T cell/APC interaction. Besides facilitating membrane recruitment of ADAP, SKAP55, and RIAM, SLP76 regulates TCR-mediated inside-out signaling by controlling the activation of Rap1 as well as Rac-mediated actin polymerization. Surprisingly, however, SLP76 is not mandatory for CXCR4-mediated inside-out signaling. Indeed, both CXCR4-induced T cell adhesion and migration are not affected by loss of SLP76. Moreover, after CXCR4 stimulation, the ADAP/SKAP55/RIAM/Rap1 module is recruited to the plasma membrane independently of SLP76. Collectively, our data indicate a differential requirement for SLP76 in TCR- vs CXCR4-mediated inside-out signaling pathways regulating T cell adhesion and migration.