Beta1 integrins: zip codes and signaling relay for blood cells

Integrin β1-mediated mast cell immune-surveillance of blood vessel content

BACKGROUND

IgE-mediated degranulation of mast cells (MCs) provides rapid protection against environmental hazards, including animal venoms. A fraction of tissue-resident MCs intimately associates with blood vessels. These perivascular MCs were reported to extend projections into the vessel lumen and to be the first MCs to acquire intravenously injected IgE, suggesting that IgE loading of MCs depends on their vascular association.

OBJECTIVE

We sought to elucidate the molecular basis of the MC-blood vessel interaction and to determine its relevance for IgE-mediated immune responses.

METHODS

We selectively inactivated the Itgb1 gene, encoding the β1 chain of integrin adhesion molecules (ITGB1), in MCs by conditional gene targeting in mice. We analyzed skin MCs for blood vessel association, surface IgE density, and capability to bind circulating antibody specific for MC surface molecules, as well as in vivo responses to antigen administered via different routes.

RESULTS

Lack of ITGB1 expression severely compromised MC-blood vessel association. ITGB1-deficient MCs showed normal densities of surface IgE but reduced binding of intravenously injected antibodies. While their capacity to degranulate in response to IgE ligation in vivo was unimpaired, anaphylactic responses to antigen circulating in the vasculature were largely abolished.

CONCLUSIONS

ITGB1-mediated association of MCs with blood vessels is key for MC immune surveillance of blood vessel content, but is dispensable for slow steady-state loading of endogenous IgE onto tissue-resident MCs.

Loss of Hem1 disrupts macrophage function and impacts migration, phagocytosis, and integrin-mediated adhesion

Hematopoietic-specific protein 1 (Hem1) is an essential subunit of the WAVE regulatory complex (WRC) in immune cells. WRC is crucial for Arp2/3 complex activation and the protrusion of branched actin filament networks. Moreover, Hem1 loss of function in immune cells causes autoimmune diseases in humans. Here, we show that genetic removal of Hem1 in macrophages diminishes frequency and efficacy of phagocytosis as well as phagocytic cup formation in addition to defects in lamellipodial protrusion and migration. Moreover, Hem1-null macrophages displayed strong defects in cell adhesion despite unaltered podosome formation and concomitant extracellular matrix degradation. Specifically, dynamics of both adhesion and de-adhesion as well as concomitant phosphorylation of paxillin and focal adhesion kinase (FAK) were significantly compromised. Accordingly, disruption of WRC function in non-hematopoietic cells coincided with both defects in adhesion turnover and altered FAK and paxillin phosphorylation. Consistently, platelets exhibited reduced adhesion and diminished integrin αIIbβ3 activation upon WRC removal. Interestingly, adhesion phenotypes, but not lamellipodia formation, were partially rescued by small molecule activation of FAK. A full rescue of the phenotype, including lamellipodia formation, required not only the presence of WRCs but also their binding to and activation by Rac. Collectively, our results uncover that WRC impacts on integrin-dependent processes in a FAK-dependent manner, controlling formation and dismantling of adhesions, relevant for properly grabbing onto extracellular surfaces and particles during cell edge expansion, like in migration or phagocytosis.

MALT1 as a promising target to treat lymphoma and other diseases related to MALT1 anomalies

Mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) is a key adaptor protein that regulates the NF-κB pathway, in which MALT1 functions as a scaffold protein and protease to trigger downstream signals. The abnormal expression of MALT1 is closely associated with lymphomagenesis and other diseases, including solid tumors and autoimmune diseases. MALT1 is the only protease in the underlying pathogenesis of these diseases, and its proteolytic activity can be pharmacologically regulated. Therefore, MALT1 is a potential and promising target for anti-lymphoma and other MALT1-related disease treatments. Currently, the development of MALT1 inhibitors is still in its early stages. This review presents an overview of MALT1, particularly its X-ray structures and biological functions, and elaborates on the pathogenesis of diseases associated with its dysregulation. We then summarize previously reported MALT1 inhibitors, focusing on their molecular structure, biological activity, structure-activity relationship, and limitations. Finally, we propose future research directions to accelerate the discovery of novel MALT1 inhibitors with clinical applications. Overall, this review provides a comprehensive and systematic overview of MALT1-related research advances and serves as a theoretical basis for drug discovery and research.

Visualization of T Cell Migration in the Spleen Reveals a Network of Perivascular Pathways that Guide Entry into T Zones

Lymphocyte homeostasis and immune surveillance require that T and B cells continuously recirculate between secondary lymphoid organs. Here, we used intravital microscopy to define lymphocyte trafficking routes within the spleen, an environment of open blood circulation and shear forces unlike other lymphoid organs. Upon release from arterioles into the red pulp sinuses, T cells latched onto perivascular stromal cells in a manner that was independent of the chemokine receptor CCR7 but sensitive to Gi protein-coupled receptor inhibitors. This latching sheltered T cells from blood flow and enabled unidirectional migration to the bridging channels and then to T zones, entry into which required CCR7. Inflammatory responses modified the chemotactic cues along the perivascular homing paths, leading to rapid block of entry. Our findings reveal a role for vascular structures in lymphocyte recirculation through the spleen, indicating the existence of separate entry and exit routes and that of a checkpoint located at the gate to the T zone.

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Perivascular pathways support T cell entry into splenic T zones, but not egress from them

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Attachment to the homing paths requires activation of GPCRs other than CCR7

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CCR7 mediates one-directional migration and entry into T zones

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Inflammation leads to modification of the homing paths and to rapid block of entry

Integrins Have Cell-Type-Specific Roles in the Development of Motor Neuron Connectivity

Formation of the nervous system requires a complex series of events including proper extension and guidance of neuronal axons and dendrites. Here we investigate the requirement for integrins, a class of transmembrane cell adhesion receptors, in regulating these processes across classes of C. elegans motor neurons. We show α integrin/ina-1 is expressed by both GABAergic and cholinergic motor neurons. Despite this, our analysis of hypomorphic ina-1(gm144) mutants indicates preferential involvement of α integrin/ina-1 in GABAergic commissural development, without obvious involvement in cholinergic commissural development. The defects in GABAergic commissures of ina-1(gm144) mutants included both premature termination and guidance errors and were reversed by expression of wild type ina-1 under control of the native ina-1 promoter. Our results also show that α integrin/ina-1 is important for proper outgrowth and guidance of commissures from both embryonic and post-embryonic born GABAergic motor neurons, indicating an ongoing requirement for integrin through two phases of GABAergic neuron development. Our findings provide insights into neuron-specific roles for integrin that would not be predicted based solely upon expression analysis.

Tissue linkage through adjoining basement membranes: The long and the short term of it

Basement membranes (BMs) are thin dense sheets of extracellular matrix that surround most tissues. When the BMs of neighboring tissues come into contact, they usually slide along one another and act to separate tissues and organs into distinct compartments. However, in certain specialized regions, the BMs of neighboring tissues link, helping to bring tissues together. These BM connections can be transient, such as during tissue fusion events in development, or long-term, as with adult tissues involved with filtration, including the blood brain barrier and kidney glomerulus. The transitory nature of these connections in development and the complexity of tissue filtration systems in adults have hindered the understanding of how juxtaposed BMs fasten together. The recent identification of a BM-BM adhesion system in C. elegans, termed B-LINK (BM linkage), however, is revealing cellular and extracellular matrix components of a nascent tissue adhesion system. We discuss insights gained from studying the B-LINK tissue adhesion system in C. elegans, compare this adhesion with other BM-BM connections in Drosophila and vertebrates, and outline important future directions towards elucidating this fascinating and poorly understood mode of adhesion that joins neighboring tissues.

Serosal Cavities Contain Two Populations of Innate-like integrin α4highCD4+ T Cells, Integrin α4β1+α6β1+α4β7- and α4β1+α6β1-α4β7+ Cells

We previously reported peritoneal innate-like integrin α4 (CD49d)^highCD4⁺ T cells that provided help for B-1a cells. Here we analyzed the expression of various integrin chains on the peritoneal and pleural integrin α4^highCD4⁺ T cells and investigated the functional heterogeneity of the subpopulations based on the integrin expression. Pleural cavity contained a lower ratio of integrin α4^highCD4⁺ T cells to integrin α4^lowCD4⁺ T cells than peritoneal cavity, but the pleural integrin α4^highCD4⁺ T cells have the same characteristics of the peritoneal integrin α4^highCD4⁺ T cells. Most of integrin α4^highCD4⁺ T cells were integrin β1^highβ7^-, but a minor population of integrin α4^highCD4⁺ T cells was integrin β1⁺β7⁺. Interestingly, the integrin α4^highβ1^highβ7^- CD4⁺ T cells expressed high levels of integrin α4β1 and α6β1, whereas integrin α4^highβ1⁺β7⁺ CD4⁺ T cells expressed high levels of integrin α4β1 and α4β7, suggesting an alternative expression of integrin α6β1 or α4β7 in combination with α4β1 in respective major and minor populations of integrin α4^highCD4⁺ T cells. The minor population, integrin α4^highβ1⁺β7⁺ CD4⁺ T cells, were different from the integrin α4^highβ1^highβ7^- CD4⁺ T cells in that they secreted a smaller amount of Th1 cytokines upon stimulation and expressed lower levels of Th1-related chemokine receptors CCR5 and CXCR3 than the integrin α4^highβ₁^highβ₇^- CD4⁺ T cells. In summary, the innate-like integrin α4^highCD4⁺ T cells could be divided into 2 populations, integrin α4β1⁺α6β1⁺α4β7^- and α4β1⁺α6β1^-α4β7⁺ cells. The functional significance of serosal integrin α4β7⁺ CD4⁺ T cells needed to be investigated especially in view of mucosal immunity.

Energy landscape differences among integrins establish the framework for understanding activation

Li and Springer demonstrate differences between integrins α₄β₁ and α₅β₁ in intrinsic affinities and relative free energies of three conformational states. Integrin conformational equilibria are both subunit and cell type specific. The energy landscapes of intact receptors on the cell surface provide a framework for understanding regulation of integrin adhesiveness.

Why do integrins differ in basal activity, and how does affinity for soluble ligand correlate with cellular adhesiveness? We show that basal conformational equilibrium set points for integrin α₄β₁ are cell type specific and differ from integrin α₅β₁ when the two integrins are coexpressed on the same cell. Although α₄β₁ is easier to activate, its high-affinity state binds vascular cell adhesion molecule and fibronectin 100- to 1,000-fold more weakly than α₅β₁ binds fibronectin. Furthermore, the difference in affinity between the high- and low-affinity states is more compressed in α₄β₁ (600- to 800-fold) than in α₅β₁ (4,000- to 6,000-fold). α₄β₁ basal conformational equilibria differ among three cell types, define affinity for soluble ligand and readiness for priming, and may reflect differences in interactions with intracellular adaptors but do not predict cellular adhesiveness for immobilized ligand. The measurements here provide a necessary framework for understanding integrin activation in intact cells, including activation of integrin adhesiveness by application of tensile force by the cytoskeleton, across ligand–integrin–adaptor complexes.

Bidirectional factors impact the migration of NK cells to draining lymph node in aged mice during influenza virus infection

Natural killer (NK) cells play an important role in controlling several viral diseases. Our previous studies demonstrated an age-dependent susceptibility to mousepox due to defective NK cell responses and trafficking. However, the mechanisms that underlie the age-related impairment in NK cell migration have yet to be identified. In the present study, we demonstrated that after influenza A virus (IAV) infection, NK cells from aged mice (17-19months old) failed to accumulate in draining lymph node (D-LN). We found that both environmental and intrinsic factors played roles for this defect. After infection, increase of chemokine transcripts, especially CXCL9, 10 and 11, which are important for NK cells homing to D-LN, was significantly lower in the D-LN of aged mice compared with those of young mice. Further, the expression levels of β2-integrins and β-actins, which play critical roles in NK cells homing to D-LN failed to be up-regulated in NK cells from aged mice. Finally, actin polymerization rates in NK cells from aged mice were also delayed compared to that of the young mice after IAV infection. Taken together, our data indicate that bi-directional factors play essential roles in the defective NK cell trafficking to the D-LN in the aged mice after IAV infection.

Identification of biomarkers to measure HIV-specific mucosal and systemic CD8(+) T-cell immunity using single cell Fluidigm 48.48 Dynamic arrays

Thirty genes composed of cytokines, chemokines, granzymes, perforin and integrins were evaluated in gut and splenic K(d)Gag197-205-specific single CD8(+) T cells using Fluidigm 48.48 Dynamic arrays, with the aim of identifying biomarkers to predict effective mucosal and systemic vaccine efficacy. The mRNA expression profiles were analyzed in three ways: (i) the "number" of K(d)Gag197-205-specific CD8(+) T cells expressing the biomarker, (ii) "level" of mRNA expression using principal component analysis (PCA) and (iii) poly-functionality in relation to RANTES expression. In total, 21 genes were found to be differentially expressed between the vaccine groups and the immune compartments tested. Overall, the PCA indicated that IL-13Rα2 or IL-4R antagonist adjuvanted vaccines that previously induced high-avidity mucosal/systemic CD8(+) T cells with better protective efficacy, the "level" of mRNA expression, specifically RANTES, MIP-1β, and integrin α4 in gut K(d)Gag197-205-specific single CD8(+) T cells, were significantly elevated compared to unadjuvanted vaccine. Furthermore, significantly elevated granzymes/perforin levels were detected in IL-13(-/-) mice given the unadjuvanted vaccine, indicating that the degree of IL-13 inhibition (total, transient or no inhibition) can considerably alter the level of T-cell activity/poly-functionality. When splenic- and gut-K(d)Gag197-205-specific CD8(+) T cells were compared, PC1 vs. PC2 scores revealed that not only RANTES, MIP-1β, and integrin α4 mRNA, but also perforin, granzymes A/B, and integrins β1 and β2 mRNA were elevated in spleen. Collectively, data suggest that RANTES, MIP-1β, perforin, and integrins α4, β1 and β7 mRNA in single HIV-specific CD8(+) T cells could be used as a measure of effective mucosal and systemic vaccine efficacy.

The activation of CD99 inhibits cell-extracellular matrix adhesion by suppressing β(1) integrin affinity

CD99 is known to be involved in the regulation of cell-cell adhesion. However, it remains unclear whether CD99 controls cell-extracellular matrix adhesion. In this study, the effects of CD99 activation on cell-extracellular matrix adhesion were investigated. It was found that engagement of CD99 with the stimulating antibody YG32 downregulated the adhesion of MCF-7 cells to fibronectin, laminin and collagen IV in a dose-dependent manner. The CD99 effect on cell-ECM adhesion was inhibited by overexpression of the dominant negative form of CD99 or CD99 siRNA transfection. Treatment of cells with Mn(2+) or by β(1) integrin-stimulating antibody restored the inhibitory effect of CD99 on cell-ECM adhesion. Cross-linking CD99 inactivated β(1) integrin through conformational change. CD99 activation caused dephosphorylation at Tyr-397 in FAK, which was restored by the β(1) stimulating antibody. Taken together, these results provide the first evidence that CD99 inhibits cell-extracellular matrix adhesion by suppressing β(1) integrin affinity. [BMB reports 2012; 45(3): 159-164].

VEGF-A isoforms differentially regulate ATF-2-dependent VCAM-1 gene expression and endothelial-leukocyte interactions

VEGF-A isoforms differentially stimulate endothelial VCAM-1 gene expression via an ERK1/2 protein kinase and ATF-2 transcription factor–dependent mechanism. Such signal transduction enables VEGF-A isoform–specific stimulation of leukocyte binding to endothelial cells, explaining how inflammation could be differentially regulated.

Vascular endothelial growth factor A (VEGF-A) regulates many aspects of vascular physiology. VEGF-A stimulates signal transduction pathways that modulate endothelial outputs such as cell migration, proliferation, tubulogenesis, and cell–cell interactions. Multiple VEGF-A isoforms exist, but the biological significance of this is unclear. Here we analyzed VEGF-A isoform–specific stimulation of VCAM-1 gene expression, which controls endothelial–leukocyte interactions, and show that this is dependent on both ERK1/2 and activating transcription factor-2 (ATF-2). VEGF-A isoforms showed differential ERK1/2 and p38 MAPK phosphorylation kinetics. A key feature of VEGF-A isoform–specific ERK1/2 activation and nuclear translocation was increased phosphorylation of ATF-2 on threonine residue 71 (T71). Using reverse genetics, we showed ATF-2 to be functionally required for VEGF-A–stimulated endothelial VCAM-1 gene expression. ATF-2 knockdown blocked VEGF-A–stimulated VCAM-1 expression and endothelial–leukocyte interactions. ATF-2 was also required for other endothelial cell outputs, such as cell migration and tubulogenesis. In contrast, VCAM-1 was essential only for promoting endothelial–leukocyte interactions. This work presents a new paradigm for understanding how soluble growth factor isoforms program complex cellular outputs and responses by modulating signal transduction pathways.

MIG-10 (lamellipodin) has netrin-independent functions and is a FOS-1A transcriptional target during anchor cell invasion in C. elegans

To transmigrate basement membrane, cells must coordinate distinct signaling activities to breach and pass through this dense extracellular matrix barrier. Netrin expression and activity are strongly associated with invasion in developmental and pathological processes, but how netrin signaling is coordinated with other pathways during invasion is poorly understood. Using the model of anchor cell (AC) invasion in C. elegans, we have previously shown that the integrin receptor heterodimer INA-1/PAT-3 promotes netrin receptor UNC-40 (DCC) localization to the invasive cell membrane of the AC. UNC-6 (netrin)/UNC-40 interactions generate an invasive protrusion that crosses the basement membrane. To understand how UNC-40 signals during invasion, we have used genetic, site of action and live-cell imaging studies to examine the roles of known effectors of UNC-40 signaling in axon outgrowth during AC invasion. UNC-34 (Ena/VASP), the Rac GTPases MIG-2 and CED-10 and the actin binding protein UNC-115 (abLIM) are dedicated UNC-40 effectors that are recruited to the invasive membrane by UNC-40 and generate F-actin. MIG-10 (lamellipodin), an effector of UNC-40 in neurons, however, has independent functions from UNC-6/UNC-40. Furthermore, unlike other UNC-40 effectors, its expression is regulated by FOS-1A, a transcription factor that promotes basement membrane breaching. Similar to UNC-40, however, MIG-10 localization to the invasive cell membrane is also dependent on the integrin INA-1/PAT-3. These studies indicate that MIG-10 has distinct functions from UNC-40 signaling in cell invasion, and demonstrate that integrin coordinates invasion by localizing these molecules to the cell-basement membrane interface.

Expression of integrin α2 receptor in human cord blood CD34+CD38-CD90+ stem cells engrafting long-term in NOD/SCID-IL2Rγ(c) null mice

Human hematopoietic stem cells reside in the CD34+CD38-CD90+ population in cord blood and bone marrow. However, this cell fraction is heterogeneous, and the phenotype of the rare primitive stem cells remains poorly defined. We here report that primitive cord blood CD34+CD38-CD90+ stem cells, with the ability to reconstitute NOD/SCID-IL2Rγ(c) null (NSG) mice long-term, at 24 weeks after transplantation, can be prospectively isolated at an increased purity by using integrin α2 receptor as an additional stem cell marker. Using a limiting dilution transplantation assay, we found a highly significant enrichment of multilineage reconstituting stem cells in the CD34+CD38-CD90+ cell fraction expressing the integrin α2 receptor, with a frequency of 1/29 cells, as compared to a frequency of 1/157 in the corresponding integrin α2- cells. In line with this, long-term reconstituting stem cells within the cord blood CD34+CD38- cell population were significantly enriched in the integrin α2+ fraction, while stem cells and progenitors reconstituting short-term, at 8-12 weeks, were heterogeneous in integrin α2 expression. Global gene expression profiling revealed that the lineage-marker negative (Lin-) CD34+CD38-CD90+CD45RA- integrin α2+ cell population was molecularly distinct from the integrin α2- cell population and the more mature Lin-CD34+CD38-CD90-CD45RA- cell population. Our findings identify integrin α2 as a novel stem cell marker, which improves prospective isolation of the primitive human hematopoietic stem cells within the CD34+CD38-CD90+ cell population for experimental and therapeutic stem cell applications.

T cell receptor signalling networks: branched, diversified and bounded

Engagement of antigen-specific T cell receptors (TCRs) is a prerequisite for T cell activation. Acquisition of appropriate effector T cell function requires the participation of multiple signals from the T cell microenvironment. Trying to understand how these signals integrate to achieve specific functional outcomes while maintaining tolerance to self is a major challenge in lymphocyte biology. Several recent publications have provided important insights into how dysregulation of T cell signalling and the development of autoreactivity can result if the branching and integration of signalling pathways are perturbed. We discuss how these findings highlight the importance of spatial segregation of individual signalling components as a way of regulating T cell responsiveness and immune tolerance.

Proteomic analysis of α4β1 integrin adhesion complexes reveals α-subunit-dependent protein recruitment

Integrin adhesion receptors mediate cell-cell and cell-extracellular matrix interactions, which control cell morphology and migration, differentiation, and tissue integrity. Integrins recruit multimolecular adhesion complexes to their cytoplasmic domains, which provide structural and mechanosensitive signaling connections between the extracellular and intracellular milieux. The different functions of specific integrin heterodimers, such as α4β1 and α5β1, have been attributed to distinct signal transduction mechanisms that are initiated by selective recruitment of adhesion complex components to integrin cytoplasmic tails. Here, we report the isolation of ligand-induced adhesion complexes associated with wild-type α4β1 integrin, an activated α4β1 variant in the absence of the α cytoplasmic domain (X4C0), and a chimeric α4β1 variant with α5 leg and cytoplasmic domains (α4Pα5L), and the cataloguing of their proteomes by MS. Using hierarchical clustering and interaction network analyses, we detail the differential recruitment of proteins and highlight enrichment patterns of proteins to distinct adhesion complexes. We identify previously unreported components of integrin adhesion complexes and observe receptor-specific enrichment of molecules with previously reported links to cell migration and cell signaling processes. Furthermore, we demonstrate colocalization of MYO18A with active integrin in migrating cells. These datasets provide a resource for future studies of integrin receptor-specific signaling events.

Activated lymphocytes and high liver expression of IFN-γ are associated with fulminant hepatic failure in patients

BACKGROUND & AIMS

To study immunological mechanisms of fulminant hepatic failure (FHF) derived from extensive liver lesions, 14 patients with FHF induced by different aetiologies were investigated by observance of both lymphocyte phenotyping and cytokine levels.

METHODS

Five patients bearing benign acute hepatitis B (AHB) and seven healthy liver donors (HC) were used as controls. Samples of liver and blood from both FHF patients and HC were obtained during transplantation procedures. Plasma levels of IL-1β, IL-4, IL-6, IL-8, IL-10, IFN-γ, TNF-α, MCP-1, RANTES and MIP-1α were quantified using a multiplex immunoassay. Cell characterization was carried out by flow cytometry. IFN-γ staining was performed on liver sections using immunofluorescence methods.

RESULTS

An increase of peripheral frequency of natural killer (NK) cells expressing early activation markers (CD69, HLA-DR and CD38) and adhesion molecule CD44 was observed in FHF patients. Elevated frequency of T lymphocytes CD4(+) and CD8(+) expressing CD38 and adhesion molecules CD29 and CD44 was also observed in FHF. Additionally, an increase of natural killer T cells (NKT) was detected in FHF patients. High plasma cytokine levels were not statistically different between FHF and AHB patients. In comparison to HC, a strong liver expression of IFN-γ was detected in FHF patients. The increased frequency of CD4(+) CD44(+) and IL-8 cytokine was found in patients with poor prognosis.

CONCLUSIONS

These findings indicate the involvement of NK and NKT cells as well as T lymphocytes CD4(+) and CD8(+) in the inflammatory process inducing FHF, confirmed by the high hepatic expression of IFN-γ.

Overview: studying integrins in vivo

Integrins are adhesive proteins that have evolved to mediate cell-cell and cell-matrix communication that is indispensable for development and postnatal physiology. Despite their widespread expression, the genetic deletion of specific integrin family members in lower organisms as well as mammals leads to relatively distinct abnormalities. Many of the processes in which integrins participate have a requirement for strong adhesion coincident with times of mechanical stress. In Drosophila, the absence of specific integrins leads to detachment of muscle from the gut and body wall and separation of the two epithelial layers in the wing. In mice and humans, a deletion of either subunit of the laminin-binding integrin, α6β4 leads to severe skin blistering and defects in other epithelial layers. In addition, integrins have also evolved to serve more subspecialized roles ranging from the establishment of a stem cell niche in Drosophila and mammals, to the regulation of pathogenic tumor vascularization, platelet adhesion, and leukocyte transmigration in mammalian systems. However, some cells seem to function normally in the absence of all integrins, as revealed by the very surprising finding that deletion of all the major integrin types on dendritic cells of mice has no effect on the ability of these cells to migrate within the interstitium of the skin and enter into lymphatics. In addition to serving as transmembrane mechanical links, integrins in vertebrates synergize with a number of receptors including growth factor receptors, to enhance responses. This leads to the activation of a large signaling network that affects cell proliferation and differentiation, as well as cell shape and migration. In vivo studies, in lower organisms, knockout mouse models as well as in inherited human diseases together have provided important insights into how this major, primordial family of adhesion receptors have remained true to their name "integrins" as their diverse functions have in common the ability to integrate extracellular stimuli into intracellular signals that affect cell behavior.

Evidence that growth factor independence 1b regulates dormancy and peripheral blood mobilization of hematopoietic stem cells

Donor-matched transplantation of hematopoietic stem cells (HSCs) is widely used to treat hematologic malignancies but is associated with high mortality. The expansion of HSC numbers and their mobilization into the bloodstream could significantly improve therapy. We report here that adult mice conditionally deficient for the transcription Growth factor independence 1b (Gfi1b) show a significant expansion of functional HSCs in the bone marrow and blood. Despite this expansion, Gfi1b(ko/ko) HSCs retain their ability to self-renew and to initiate multilineage differentiation but are no longer quiescent and contain elevated levels of reactive oxygen species. Treatment of Gfi1b(ko/ko) mice with N-acetyl-cystein significantly reduced HSC numbers indicating that increased reactive oxygen species levels are at least partially responsible for the expansion of Gfi1b-deficient HSCs. Moreover, Gfi1b(-/-) HSCs show decreased expression of CXCR4 and Vascular cell adhesion protein-1, which are required to retain dormant HSCs in the endosteal niche, suggesting that Gfi1b regulates HSC dormancy and pool size without affecting their function. Finally, the additional deletion of the related Gfi1 gene in Gfi1b(ko/ko) HSCs is incompatible with the maintenance of HSCs, suggesting that Gfi1b and Gfi1 have partially overlapping functions but that at least one Gfi gene is essential for the generation of HSCs.

Natalizumab treatment reduces endothelial activity in MS patients

Vascular cell adhesion molecule-1 a ligand for leukocyte very late activating antigen-4 is a key player in leukocyte extravasation in MS lesions. Natalizumab a monoclonal antibody against VLA-4 blocks this interaction. VCAM-1 and its soluble form are up-regulated during endothelial activation in MS. We investigated the effect of Natalizumab on sVCAM-1 and VLA-4 on circulating leukocytes in MS patients. Natalizumab reduced levels of sVCAM-1 compared to controls (256 vs. 597 ng/mL). This effect was sustained and only reversed in patients with neutralizing antibodies against Natalizumab. Correspondingly Natalizumab diminished VLA-4 on leukocyte subsets. Our findings indicate that Natalizumab reduces transmigration not only by blocking VLA-4 but also by down-regulating VCAM-1.

Chemerin contributes to inflammation by promoting macrophage adhesion to VCAM-1 and fibronectin through clustering of VLA-4 and VLA-5

Chemerin is a potent macrophage chemoattractant protein. We used murine peritoneal exudate cells (PECs) in adhesion, flow cytometry, and confocal microscopy assays to test the hypothesis that chemerin can also contribute to inflammation by promoting macrophage adhesion. Chemerin stimulated the adhesion of PECs to the extracellular matrix protein fibronectin and to the adhesion molecule VCAM-1 within a minute, with an EC(50) of 322 and 196 pM, respectively. Experiments using pertussis toxin and PECs from ChemR23(-/-) mice demonstrated that chemerin stimulated the adhesion of macrophages via the Gi protein-coupled receptor ChemR23. Blocking Abs against integrin subunits revealed that 89% of chemerin-stimulated adhesion to fibronectin was dependent on increased avidity of the integrin VLA-5 (alpha(5)beta(1)) and that 88% of adhesion to VCAM-1 was dependent on increased avidity of VLA-4 (alpha(4)beta(1)). Although chemerin was unable to induce an increase in integrin affinity as judged by the binding of soluble ligand, experiments using confocal microscopy revealed an increase in valency resulting from integrin clustering as the mechanism responsible for chemerin-stimulated macrophage adhesion. PI3K, Akt, and p38 were identified as key signaling mediators in chemerin-stimulated adhesion. The finding that chemerin can rapidly stimulate macrophage adhesion to extracellular matrix proteins and adhesion molecules, taken together with its ability to promote chemotaxis, suggests a novel role for chemerin in the recruitment and retention of macrophages at sites of inflammation.

Antigen receptor signaling to NF-kappaB via CARMA1, BCL10, and MALT1

The signaling pathway controlling antigen receptor-induced regulation of the transcription factor NF-kappaB plays a key role in lymphocyte activation and development and the generation of lymphomas. Work of the past decade has led to dramatic progress in the identification and characterization of new players in the pathway. Moreover, novel enzymatic activities relevant for this pathway have been discovered, which represent interesting drug targets for immuno-suppression or lymphoma treatment. Here, we summarize these findings and give an outlook on interesting open issues that need to be addressed in the future.

beta1 integrin expression increases susceptibility of memory B cells to Epstein-Barr virus infection

Epstein-Barr virus (EBV) uses nasal mucosa-associated lymphoid tissue (NALT) as a portal of entry to establish life-long persistence in memory B cells. We previously showed that naïve and memory B cells from NALT are equally susceptible to EBV infection. Here we show that memory B cells from NALT are significantly more susceptible to EBV infection than those from remote lymphatic organs. We identify beta(1) integrin, which is expressed the most by naïve B cells of distinct lymphoid origin and by memory B cells from NALT, as a mediator of increased susceptibility to infection by EBV. Furthermore, we show that BMRF-2-beta(1) integrin interaction and the downstream signal transduction pathway are critical for postbinding events. An increase of beta(1) integrin expression in peripheral blood memory B cells provoked by CD40 stimulation plus B-cell receptor cross-linking increased the susceptibility of non-NALT memory B cells to EBV infection. Thus, EBV seems to utilize the increased activation status of memory B cells residing in the NALT to establish and ensure persistence.

Peripheral facial nerve axotomy in mice causes sprouting of motor axons into perineuronal central white matter: time course and molecular characterization

Generation of new axonal sprouts plays an important role in neural repair. In the current study, we examined the appearance, composition and effects of gene deletions on intrabrainstem sprouts following peripheral facial nerve axotomy. Axotomy was followed by the appearance of galanin(+) and calcitonin gene-related peptide (CGRP)(+) sprouts peaking at day 14, matching both large, neuropeptide(+) subpopulations of axotomized facial motoneurons, but with CGRP(+) sprouts considerably rarer. Strong immunoreactivity for vesicular acetylcholine transporter (VAChT) and retrogradely transported MiniRuby following its application on freshly cut proximal facial nerve stump confirmed their axotomized motoneuron origin; the sprouts expressed CD44 and alpha7beta1 integrin adhesion molecules and grew apparently unhindered along neighboring central white matter tracts. Quantification of the galanin(+) sprouts revealed a stronger response following cut compared with crush (day 7-14) as well as enhanced sprouting after recut (day 8 + 6 vs. 14; 14 + 8 vs. 22), arguing against delayed appearance of sprouting being the result of the initial phase of reinnervation. Sprouting was strongly diminished in brain Jun-deficient mice but enhanced in alpha7 null animals that showed apparently compensatory up-regulation in beta1, suggesting important regulatory roles for transcription factors and the sprout-associated adhesion molecules. Analysis of inflammatory stimuli revealed a 50% reduction 12-48 hours following systemic endotoxin associated with neural inflammation and a tendency toward more sprouts in TNFR1/2 null mutants (P = 10%) with a reduced inflammatory response, indicating detrimental effects of excessive inflammation. Moreover, the study points to the usefulness of the facial axotomy model in exploring physiological and molecular stimuli regulating central sprouting.

Preclinical testing of strategies for therapeutic targeting of human T-cell trafficking in vivo

Naive T cells are migratory cells that continuously recirculate between blood and lymphoid tissues. Antigen-specific stimulation of T cells within the lymph nodes reprograms the trafficking properties of T cells by inducing a specific set of adhesion molecules and chemokine receptors on their surface which allow these activated and effector T cells to effectively and specifically home to extralymphoid organs. The observations of organ-specific homing of T cells initiated the development of therapeutic strategies targeting adhesion receptors for organ-specific inhibition of chronic inflammation. As most adhesion receptors have additional immune functions besides mediating leukocyte trafficking, these drugs may have additional immunomodulatory effects. Therapeutic targeting of T-cell trafficking to the central nervous system is the underlying concept of a novel treatment of relapsing remitting multiple sclerosis with the humanized anti-alpha-4-integrin antibody natalizumab. In this chapter, we describe a possible preclinical in vivo approach to directly visualize the therapeutic efficacy of a given drug in inhibiting T-cell homing to a certain organ at the example of the potential of natalizumab to inhibit the trafficking of human T cells to the inflamed central nervous system in an animal model of multiple sclerosis.

Integrin acts upstream of netrin signaling to regulate formation of the anchor cell's invasive membrane in C. elegans

Integrin expression and activity have been strongly correlated with developmental and pathological processes involving cell invasion through basement membranes. The role of integrins in mediating these invasions, however, remains unclear. Utilizing the genetically and visually accessible model of anchor cell (AC) invasion in C. elegans, we have recently shown that netrin signaling orients a specialized invasive cell membrane domain toward the basement membrane. Here, we demonstrate that the integrin heterodimer INA-1/PAT-3 plays a crucial role in AC invasion, in part by targeting the netrin receptor UNC-40 (DCC) to the AC's plasma membrane. Analyses of the invasive membrane components phosphatidylinositol 4,5-bisphosphate, the Rac GTPase MIG-2, and F-actin further indicate that INA-1/PAT-3 plays a broad role in promoting the plasma membrane association of these molecules. Taken together, these studies reveal a role for integrin in regulating the plasma membrane targeting and netrin-dependent orientation of a specialized invasive membrane domain.

ICAM-1 is necessary for epithelial recruitment of gammadelta T cells and efficient corneal wound healing

Wound healing and inflammation are both significantly reduced in mice that lack gammadelta T cells. Here, the role of epithelial intercellular adhesion molecule-1 (ICAM-1) in gammadelta T cell migration in corneal wound healing was assessed. Wild-type mice had an approximate fivefold increase in epithelial gammadelta T cells at 24 hours after epithelial abrasion. ICAM-1(-/-) mice had 50.9% (P < 0.01) fewer gammadelta T cells resident in unwounded corneal epithelium, which failed to increase in response to epithelial abrasion. Anti-ICAM-1 blocking antibody in wild-type mice reduced epithelial gammadelta T cells to a number comparable to that of ICAM-1(-/-) mice, and mice deficient in lymphocyte function-associated antigen-1 (CD11a/CD18), a principal leukocyte receptor for ICAM-1, exhibited a 48% reduction (P < 0.01) in peak epithelial gammadelta T cells. Re-epithelialization and epithelial cell division were both significantly reduced ( approximately 50% at 18 hours, P < 0.01) after abrasion in ICAM-1(-/-) mice versus wild-type, and at 96 hours, recovery of epithelial thickness was only 66% (P < 0.01) of wild-type. ICAM-1 expression by corneal epithelium in response to epithelial abrasion appears to be critical for accumulation of gammadelta T cells in the epithelium, and deficiency of ICAM-1 significantly delays wound healing. Since gammadelta T cells are necessary for efficient epithelial wound healing, ICAM-1 may contribute to wound healing by facilitating gammadelta T cell migration into the corneal epithelium.

Cutting edge: Natalizumab blocks adhesion but not initial contact of human T cells to the blood-brain barrier in vivo in an animal model of multiple sclerosis

The humanized anti-alpha(4) integrin Ab Natalizumab is an effective treatment for relapsing-remitting multiple sclerosis. Natalizumab is thought to exert its therapeutic efficacy by blocking the alpha(4) integrin-mediated binding of circulating immune cells to the blood-brain barrier (BBB). As alpha(4) integrins control other immunological processes, natalizumab may, however, execute its beneficial effects elsewhere. By means of intravital microscopy we demonstrate that natalizumab specifically inhibits the firm adhesion but not the rolling or capture of human T cells on the inflamed BBB in mice with acute experimental autoimmune encephalomyelitis (EAE). The efficiency of natalizumab to block T cell adhesion to the inflamed BBB was found to be more effective in EAE than in acute systemic TNF-alpha-induced inflammation. Our data demonstrate that alpha(4) integrin-mediated adhesion of human T cells to the inflamed BBB during EAE is efficiently blocked by natalizumab and thus provide the first direct in vivo proof of concept of this therapy in multiple sclerosis.

Beta1 integrin-mediated adhesion signalling is essential for epidermal progenitor cell expansion

BACKGROUND

There is a major discrepancy between the in vitro and in vivo results regarding the role of beta1 integrins in the maintenance of epidermal stem/progenitor cells. Studies of mice with skin-specific ablation of beta1 integrins suggested that epidermis can form and be maintained in their absence, while in vitro data have shown a fundamental role for these adhesion receptors in stem/progenitor cell expansion and differentiation.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate this discrepancy we generated hypomorphic mice expressing reduced beta1 integrin levels on keratinocytes that developed similar, but less severe defects than mice with beta1-deficient keratinocytes. Surprisingly we found that upon aging these abnormalities attenuated due to a rapid expansion of cells, which escaped or compensated for the down-regulation of beta1 integrin expression. A similar phenomenon was observed in aged mice with a complete, skin-specific ablation of the beta1 integrin gene, where cells that escaped Cre-mediated recombination repopulated the mutant skin in a very short time period. The expansion of beta1 integrin expressing keratinocytes was even further accelerated in situations of increased keratinocyte proliferation such as wound healing.

CONCLUSIONS/SIGNIFICANCE

These data demonstrate that expression of beta1 integrins is critically important for the expansion of epidermal progenitor cells to maintain epidermal homeostasis.

Barrier immunity and IL-17

CD4+ T(H)17 cells display a featured role in barrier immunity. This effector population of T cells is important for clearance of microorganisms but can also promote autoimmunity at barrier sites. Recent work has indicated that these effector cells share a pathway with CD4+ regulatory T cells (T(R) cells) that also have a critical function in barrier protection and immune regulation. The development and function of T(H)17 cells, and their relationship with T(R) cells are discussed.

Beta1 integrins differentially control extravasation of inflammatory cell subsets into the CNS during autoimmunity

Inhibiting the alpha(4) subunit of the integrin heterodimers alpha(4)beta(1) and alpha(4)beta(7) with the monoclonal antibody natalizumab is an effective treatment for multiple sclerosis (MS). However, the pharmacological action of natalizumab is not understood conclusively. Previous studies suggested that natalizumab inhibits activation, proliferation, or extravasation of inflammatory cells. To specify which mechanisms, cell types, and alpha(4) heterodimers are affected by the antibody treatment, we studied MS-like experimental autoimmune encephalomyelitis (EAE) in mice lacking the beta(1)-integrin gene either in all hematopoietic cells or selectively in T lymphocytes. Our results show that T cells critically rely on beta(1) integrins to accumulate in the central nervous system (CNS) during EAE, whereas CNS infiltration of beta(1)-deficient myeloid cells remains unaffected, suggesting that T cells are the main target of anti-alpha(4)-antibody blockade. We demonstrate that beta(1)-integrin expression on encephalitogenic T cells is critical for EAE development, and we therefore exclude alpha(4)beta(7) as a target integrin of the antibody treatment. T cells lacking beta(1) integrin are unable to firmly adhere to CNS endothelium in vivo, whereas their priming and expansion remain unaffected. Collectively, these results suggest that the primary action of natalizumab is interference with T cell extravasation via inhibition of alpha(4)beta(1) integrins.

T cell costimulation via the integrin VLA-4 inhibits the actin-dependent centralization of signaling microclusters containing the adaptor SLP-76

Antigen-dependent T cell activation drives the formation of signaling microclusters containing the adaptor SLP-76. Costimulatory integrins regulate SLP-76 phosphorylation and could influence SLP-76 microclusters in the integrin-rich periphery of the immune synapse. We report that costimulation by the integrin VLA-4 (alpha4beta1) required SLP-76 domains implicated in microcluster assembly. Pro-adhesive ligands enlarged the contact and increased the number of SLP-76 microclusters regardless of their costimulatory potential. Costimulatory VLA-4 ligands also prevented the centralization of SLP-76, promoted microcluster persistence, prolonged lateral interactions between SLP-76 and its upstream kinase, ZAP-70, and retained SLP-76 in tyrosine-phosphorylated peripheral structures. SLP-76 centralization was driven by dynamic actin polymerization and was correlated with inward actin flows. VLA-4 ligation retarded these flows, even in the absence of SLP-76. These data suggest a widely applicable model of costimulation, in which integrins promote sustained signaling by attenuating cytoskeletal movements that drive the centralization and inactivation of SLP-76 microclusters.

Culturing of human mesenchymal stem cells as three-dimensional aggregates induces functional expression of CXCR4 that regulates adhesion to endothelial cells

Culture-expanded human mesenchymal stem cells (hMSCs) are increasingly used in a variety of preclinical and clinical studies. However, these cells have a low rate of engraftment to bone marrow or damaged tissues. Several laboratories have shown that during isolation and subculturing mesenchymal stem cells quickly lose the expression of CXCR4, the key receptor responsible for lymphocytes and hematopoietic stem cell homing. Here we show that culturing of hMSCs as three-dimensional aggregates (hMSC spheroids) restores CXCR4 functional expression. Expression of CXCR4 inversely correlates with the secretion of SDF-1 by hMSCs. Cells from hMSC spheroids up-regulate expression of CD49b, the alpha2 integrin subunit, and suppress the expression of CD49d, the alpha4 integrin subunit. Transfer of cells from the spheroids back to a monolayer suppresses the expression of CXCR4 and CD49b and restores the expression of CD49d. Treatment of cells from the spheroids with SDF-1 leads to CXCR4 internalization and activation of ERK-1,2. Adhesion of hMSCs to human umbilical vein endothelial cells (HUVECs) was investigated. SDF-1, AMD-3100, or exposure of HUVECs to hypoxia did not affect adhesion of hMSCs from a monolayer to HUVECs. Adhesion of cells from hMSC spheroids to HUVECs was stimulated by SDF-1, AMD-3100, or by exposure of HUVECs to hypoxia. Stimulatory effects of hypoxia and addition of SDF-1 or AMD-3100 were not additive. Overall, our data indicate that the expression of CXCR4 by hMSCs regulates hMSC adhesion to endothelial cells.

Extracellular matrix regulation of drug resistance in small-cell lung cancer

PURPOSE

Lung cancer is the leading cause of cancer deaths in the developed world. Small cell lung cancer (SCLC) has the worst prognosis due to the emergence of resistance to chemotherapy. This article will review recent work that has defined mechanisms of chemo-resistance focusing on the role of integrins.

RESULTS

SCLC is surrounded by an extensive stroma of extracellular matrix (ECM) and high levels of expression correlate with poor prognosis. ECM protects SCLC cells against chemotherapy-induced cell death by activating beta1 integrins leading to activation of phosphoinositide-3-OH kinase (PI3-kinase), which prevents etoposide-induced caspase-3 activation and subsequent apoptosis. Engagement of ECM prevents etoposide and radiation induced G2/M cell cycle arrest in SCLC cells by blocking the up-regulation of p21Cip1/WAF1 and p27Kip1 and the down-regulation of cyclins E, A and B. These effects are abrogated by pharmacological and genetic inhibition of PI3-kinase signalling.

CONCLUSIONS

Thus, ECM via beta1 integrin-mediated PI3-kinase activation allows SCLC cells to survive treatment induced cell cycle arrest and apoptosis with persistent DNA damage, providing a model to account for the emergence of acquired drug resistance. Novel therapeutic strategies may therefore be directed at inhibiting integrin-mediated cell survival signals improving response rates and cure in this devastating cancer.

Forces and bond dynamics in cell adhesion

Adhesion of a biological cell to another cell or the extracellular matrix involves complex couplings between cell biochemistry, structural mechanics, and surface bonding. The interactions are dynamic and act through association and dissociation of bonds between very large molecules at rates that change considerably under stress. Combining molecular cell biology with single-molecule force spectroscopy provides a powerful tool for exploring the complexity of cell adhesion, that is, how cell signaling processes strengthen adhesion bonds and how forces applied to cell-surface bonds act on intracellular sites to catalyze chemical processes or switch molecular interactions on and off. Probing adhesion receptors on strategically engineered cells with force during functional stimulation can reveal key nodes of communication between the mechanical and chemical circuitry of a cell.

Evaluating integrin function in models of angiogenesis and vascular permeability

All vascular biological processes are influenced to some degree by integrins expressed on endothelial cells, vascular smooth muscle cells, fibroblasts, platelets, or other circulating cells. In particular, angiogenesis requires cells to process signals from their microenvironment and respond by altering their cell-cell and cell-matrix adhesion, events which allow migration and vascular remodeling over the period of days to weeks. On the other hand, endothelial cells can respond to a permeability stimulus and alter their junctional adhesion molecules or vesicular transport machinery within seconds or minutes. This chapter will discuss the current understanding of how integrins participate in these processes, and explore the in vitro and in vivo models available to study the role of integrin function during angiogenesis and vascular leak.

Analysis of integrin functions in peri-implantation embryos, hematopoietic system, and skin

Integrins mediate cell adhesion, permit traction forces important for cell migration, and cross-talk with growth factor receptors to regulate cell proliferation, cell survival, and cell differentiation. The plethora of functions explains their central role for development and disease. The progress in mouse genetics and the ease with which the mouse genome can be manipulated enormously contributed to our understanding of how integrins exert their functions at the molecular level. In the present chapter, we describe tests that are routinely used in our laboratory to investigate embryos, organs, and cells (peri-implantation embryos, hematopoietic system, epidermis, and hair follicles) that lack the expression of integrins or integrin-associated proteins.