Paxillin binding to the alpha 4 integrin subunit stimulates LFA-1 (integrin alpha L beta 2)-dependent T cell migration by augmenting the activation of focal adhesion kinase/proline-rich tyrosine kinase-2

Noncanonical functions of adhesion proteins in inflammation

Cell adhesion proteins localize to epithelial and endothelial cell membranes to form junctional complexes between neighboring cells or between cells and the underlying basement membrane. The structural and functional integrities of these junctions are critical to establish cell polarity and maintain tissue barrier function, while also facilitating leukocyte migration and adhesion to sites of inflammation. In addition to their adhesive properties, however, junctional proteins can also serve important noncanonical functions in inflammatory signaling and transcriptional regulation. Intriguingly, recent work has unveiled novel roles for cell adhesion proteins as both signaling initiators and downstream targets during inflammation. In this review, we discuss both the traditional functions of junction proteins in cell adhesion and tissue barrier function as well as their noncanonical signaling roles that have been implicated in facilitating diverse inflammatory pathologies.

LFA-1 Activation in T-Cell Migration and Immunological Synapse Formation

Integrin LFA-1 plays a critical role in T-cell migration and in the formation of immunological synapses. LFA-1 functions through interacting with its ligands with differing affinities: low, intermediate, and high. Most prior research has studied how LFA-1 in the high-affinity state regulates the trafficking and functions of T cells. LFA-1 is also presented in the intermediate-affinity state on T cells, however, the signaling to activate LFA-1 to the intermediate-affinity state and the role of LFA-1 in this affinity state both remain largely elusive. This review briefly summarizes the activation and roles of LFA-1 with varied ligand-binding affinities in the regulation of T-cell migration and immunological synapse formation.

Identification of Hub Genes and Biological Mechanisms Associated with Non-Alcoholic Fatty Liver Disease and Triple-Negative Breast Cancer

The relationship between non-alcoholic fatty liver disease (NAFLD) and triple-negative breast cancer (TNBC) has been widely recognized, but the underlying mechanisms are still unknown. The objective of this study was to identify the hub genes associated with NAFLD and TNBC, and to explore the potential co-pathogenesis and prognostic linkage of these two diseases. We used GEO, TCGA, STRING, ssGSEA, and Rstudio to investigate the common differentially expressed genes (DEGs), conduct functional and signaling pathway enrichment analyses, and determine prognostic value between TNBC and NAFLD. GO and KEGG enrichment analyses of the common DEGs showed that they were enriched in leukocyte aggregation, migration and adhesion, apoptosis regulation, and the PPAR signaling pathway. Fourteen candidate hub genes most likely to mediate NAFLD and TNBC occurrence were identified and validation results in a new cohort showed that ITGB2, RAC2, ITGAM, and CYBA were upregulated in both diseases. A univariate Cox analysis suggested that high expression levels of ITGB2, RAC2, ITGAM, and CXCL10 were associated with a good prognosis in TNBC. Immune infiltration analysis of TNBC samples showed that NCF2, ICAM1, and CXCL10 were significantly associated with activated CD8 T cells and activated CD4 T cells. NCF2, CXCL10, and CYBB were correlated with regulatory T cells and myeloid-derived suppressor cells. This study demonstrated that the redox reactions regulated by the NADPH oxidase (NOX) subunit genes and the transport and activation of immune cells regulated by integrins may play a central role in the co-occurrence trend of NAFLD and TNBC. Additionally, ITGB2, RAC2, and ITGAM were upregulated in both diseases and were prognostic protective factors of TNBC; they may be potential therapeutic targets for treatment of TNBC patients with NAFLD, but further experimental studies are still needed.

The current landscape of CAR T-cell therapy for solid tumors: Mechanisms, research progress, challenges, and counterstrategies

The successful outcomes of chimeric antigen receptor (CAR) T-cell therapy in treating hematologic cancers have increased the previously unprecedented excitement to use this innovative approach in treating various forms of human cancers. Although researchers have put a lot of work into maximizing the effectiveness of these cells in the context of solid tumors, few studies have discussed challenges and potential strategies to overcome them. Restricted trafficking and infiltration into the tumor site, hypoxic and immunosuppressive tumor microenvironment (TME), antigen escape and heterogeneity, CAR T-cell exhaustion, and severe life-threatening toxicities are a few of the major obstacles facing CAR T-cells. CAR designs will need to go beyond the traditional architectures in order to get over these limitations and broaden their applicability to a larger range of malignancies. To enhance the safety, effectiveness, and applicability of this treatment modality, researchers are addressing the present challenges with a wide variety of engineering strategies as well as integrating several therapeutic tactics. In this study, we reviewed the antigens that CAR T-cells have been clinically trained to recognize, as well as counterstrategies to overcome the limitations of CAR T-cell therapy, such as recent advances in CAR T-cell engineering and the use of several therapies in combination to optimize their clinical efficacy in solid tumors.

Regulation of Dynamic Cell Adhesion by Integrin-Integrin Crosstalk

Most cells express several integrins. The integrins are able to respond to various cellular functions and needs by modifying their own activation state, but in addition by their ability to regulate each other by activation or inhibition. This crosstalk or transdominant regulation is strictly controlled. The mechanisms resulting in integrin crosstalk are incompletely understood, but they often involve intracellular signalling routes also used by other cell surface receptors. Several studies show that the integrin cytoplasmic tails bind to a number of cytoskeletal and adaptor molecules in a regulated manner. Recent work has shown that phosphorylations of integrins and key intracellular molecules are of pivotal importance in integrin-cytoplasmic interactions, and these in turn affect integrin activity and crosstalk. The integrin β-chains play a central role in regulating crosstalk. In addition to Integrin-integrin crosstalk, crosstalk may also occur between integrins and related receptors, including other adhesion receptors, growth factor and SARS-CoV-2 receptors.

LFA-1 activation enriches tumor-specific T cells in a cold tumor model and synergizes with CTLA-4 blockade

The inability of CD8⁺ effector T cells (Teffs) to reach tumor cells is an important aspect of tumor resistance to cancer immunotherapy. The recruitment of these cells to the tumor microenvironment (TME) is regulated by integrins, a family of adhesion molecules that are expressed on T cells. Here, we show that 7HP349, a small-molecule activator of lymphocyte function–associated antigen-1 (LFA-1) and very late activation antigen-4 (VLA-4) integrin cell-adhesion receptors, facilitated the preferential localization of tumor-specific T cells to the tumor and improved antitumor response. 7HP349 monotherapy had modest effects on anti–programmed death 1–resistant (anti–PD-1–resistant) tumors, whereas combinatorial treatment with anti–cytotoxic T lymphocyte–associated protein 4 (anti–CTLA-4) increased CD8⁺ Teff intratumoral sequestration and synergized in cooperation with neutrophils in inducing cancer regression. 7HP349 intratumoral CD8⁺ Teff enrichment activity depended on CXCL12. We analyzed gene expression profiles using RNA from baseline and on treatment tumor samples of 14 melanoma patients. We identified baseline CXCL12 gene expression as possibly improving the likelihood or response to anti–CTLA-4 therapies. Our results provide a proof-of-principle demonstration that LFA-1 activation could convert a T cell–exclusionary TME to a T cell–enriched TME through mechanisms involving cooperation with innate immune cells.

Syndecans and Their Synstatins: Targeting an Organizer of Receptor Tyrosine Kinase Signaling at the Cell-Matrix Interface

Receptor tyrosine kinases (RTKs) and integrin matrix receptors have well-established roles in tumor cell proliferation, invasion and survival, often functioning in a coordinated fashion at sites of cell-matrix adhesion. Central to this coordination are syndecans, another class of matrix receptor, that organize RTKs and integrins into functional units, relying on docking motifs in the syndecan extracellular domains to capture and localize RTKs (e.g., EGFR, IGF-1R, VEGFR2, HER2) and integrins (e.g., αvβ3, αvβ5, α4β1, α3β1, α6β4) to sites of adhesion. Peptide mimetics of the docking motifs in the syndecans, called “synstatins”, prevent assembly of these receptor complexes, block their signaling activities and are highly effective against tumor cell invasion and survival and angiogenesis. This review describes our current understanding of these four syndecan-coupled mechanisms and their inhibitory synstatins (SSTN_IGF1R, SSTN_VEGFR2, SSTN_VLA-4, SSTN_EGFR and SSTN_HER2).

Effects of Deoxynivalenol and Fumonisins Fed in Combination to Beef Cattle: Immunotoxicity and Gene Expression

We evaluated the effects of a treatment diet contaminated with 1.7 mg deoxynivalenol and 3.5 mg fumonisins (B1, B2 and B3) per kg ration on immune status and peripheral blood gene expression profiles in finishing-stage Angus steers. The mycotoxin treatment diet was fed for a period of 21 days followed by a two-week washout period during which time all animals consumed the control diet. Whole-blood leukocyte differentials were performed weekly throughout the experimental and washout period. Comparative profiles of CD4+ and CD8+ T cells, along with bactericidal capacity of circulating neutrophils and monocytes were evaluated at 0, 7, 14, 21 and 35 days. Peripheral blood gene expression was measured at 0, 7, 21 and 35 days via RNA sequencing. Significant increases in the percentage of CD4-CD8+ T cells were observed in treatment-fed steers after two weeks of treatment and were associated with decreased CD4:CD8 T-cell ratios at this same timepoint (p ≤ 0.10). No significant differences were observed as an effect of treatment in terms of bactericidal capacity at any timepoint. Dietary treatments induced major changes in transcripts associated with endocrine, metabolic and infectious diseases; protein digestion and absorption; and environmental information processing (inhibition of signaling and processing), as evaluated by dynamic impact analysis. DAVID analysis also suggested treatment effects on oxygen transport, extra-cellular signaling, cell membrane structure and immune system function. These results indicate that finishing-stage beef cattle are susceptible to the immunotoxic and transcript-inhibitory effects of deoxynivalenol and fumonisins at levels which may be realistically encountered in feedlot situations.

Acquisition of New Migratory Properties by Highly Differentiated CD4+CD28null T Lymphocytes in Rheumatoid Arthritis Disease

Expanded CD4+CD28^null T lymphocytes are found in the tissues and peripheral blood of patients with many autoimmune diseases, such as rheumatoid arthritis (RA). These highly differentiated cells present potent inflammatory activity and capability to induce tissue destruction, which has been suggested to predispose to the development of more aggressive disease. In fact, preferential migration to inflammatory sites has been proposed to be a contributing factor in the progression of autoimmune and cardiovascular diseases frequently found in these patients. The functional activity of CD4+CD28^null T lymphocytes is largely dependent on interleukin 15 (IL-15), and this cytokine may also act as a selective attractor of these cells to local inflammatory infiltrates in damaged tissues. We have analysed, in RA patients, the migratory properties and transcriptional motility profile of CD4+CD28^null T lymphocytes compared to their counterparts CD28+ T lymphocytes and the enhancing role of IL-15. Identification of the pathways involved in this process will allow us to design strategies directed to block effector functions that CD4+CD28^null T lymphocytes have in the target tissue, which may represent therapeutic approaches in this immune disorder.

Optical Control of CD8+ T Cell Metabolism and Effector Functions

Although cancer immunotherapy is effective against hematological malignancies, it is less effective against solid tumors due in part to significant metabolic challenges present in the tumor microenvironment (TME), where infiltrated CD8+ T cells face fierce competition with cancer cells for limited nutrients. Strong metabolic suppression in the TME is often associated with impaired T cell recruitment to the tumor site and hyporesponsive effector function via T cell exhaustion. Increasing evidence suggests that mitochondria play a key role in CD8+ T cell activation, effector function, and persistence in tumors. In this study, we showed that there was an increase in overall mitochondrial function, including mitochondrial mass and membrane potential, during both mouse and human CD8+ T cell activation. CD8+ T cell mitochondrial membrane potential was closely correlated with granzyme B and IFN-γ production, demonstrating the significance of mitochondria in effector T cell function. Additionally, activated CD8+ T cells that migrate on ICAM-1 and CXCL12 consumed significantly more oxygen than stationary CD8+ T cells. Inhibition of mitochondrial respiration decreased the velocity of CD8+ T cell migration, indicating the importance of mitochondrial metabolism in CD8+ T cell migration. Remote optical stimulation of CD8+ T cells that express our newly developed "OptoMito-On" successfully enhanced mitochondrial ATP production and improved overall CD8+ T cell migration and effector function. Our study provides new insight into the effect of the mitochondrial membrane potential on CD8+ T cell effector function and demonstrates the development of a novel optogenetic technique to remotely control T cell metabolism and effector function at the target tumor site with outstanding specificity and temporospatial resolution.

Integrin Crosstalk Contributes to the Complexity of Signalling and Unpredictable Cancer Cell Fates

Integrins are heterodimeric cell surface receptors composed of α and β subunits that control adhesion, proliferation and gene expression. The integrin heterodimer binding to ligand reorganises the cytoskeletal networks and triggers multiple signalling pathways that can cause changes in cell cycle, proliferation, differentiation, survival and motility. In addition, integrins have been identified as targets for many different diseases, including cancer. Integrin crosstalk is a mechanism by which a change in the expression of a certain integrin subunit or the activation of an integrin heterodimer may interfere with the expression and/or activation of other integrin subunit(s) in the very same cell. Here, we review the evidence for integrin crosstalk in a range of cellular systems, with a particular emphasis on cancer. We describe the molecular mechanisms of integrin crosstalk, the effects of cell fate determination, and the contribution of crosstalk to therapeutic outcomes. Our intention is to raise awareness of integrin crosstalk events such that the contribution of the phenomenon can be taken into account when researching the biological or pathophysiological roles of integrins.

Current challenges and emerging opportunities of CAR-T cell therapies

Infusion of chimeric antigen receptor (CAR)-genetically modified T cells (CAR-T cells) have led to remarkable clinical responses and cancer remission in patients suffering from relapsed or refractory B-cell malignancies. This is a new form of adoptive T cell therapy (ACT), whereby the artificial CAR enables the redirection of T cells endogenous antitumor activity towards a predefined tumor-associated antigen, leading to the elimination of a specific tumor. The early success in blood cancers has prompted the US Food and Drug Administration (FDA) to approve the first CAR-T cell therapies for the treatment of CD19-positive leukemias and lymphomas in 2017. Despite the emergence of CAR-T cells as one of the latest breakthroughs of cancer immunotherapies, their wider application has been hampered by specific life-threatening toxicities, and a substantial lack of efficacy in the treatment of solid tumors, owing to the strong immunosuppressive tumor microenvironment and the paucity of reliable tumor-specific targets. Herein, besides providing an overview of the emerging CAR-technologies and current clinical applications, the major hurdles of CAR-T cell therapies will be discussed, namely treatment-related life-threatening toxicities and the obstacles posed by the immunosupressive tumor-microenvironment of solid tumors, as well as the next-generation strategies currently designed to simultaneously improve safety and efficacy of CAR-T cell therapies in vivo.

VLA-4 phosphorylation during tumor and immune cell migration relies on its coupling to VEGFR2 and CXCR4 by syndecan-1

When targeted by the tumor-promoting enzyme heparanase, cleaved and shed syndecan-1 (Sdc1) then couples VEGFR2 (also known as KDR) to VLA-4, activating VEGFR2 and the directed migration of myeloma cells. But how VEGFR2 activates VLA-4-mediated motility has remained unknown. We now report that VEGFR2 causes PKA-mediated phosphorylation of VLA-4 on S988, an event known to stimulate tumor metastasis while suppressing cytotoxic immune cells. A key partner in this mechanism is the chemokine receptor CXCR4, a well-known mediator of cell motility in response to gradients of the chemokine SDF-1 (also known as CXCL12). The entire machinery necessary to phosphorylate VLA-4, consisting of CXCR4, AC7 (also known as ADCY7) and PKA, is constitutively associated with VEGFR2 and is localized to the integrin by Sdc1. VEGFR2 carries out the novel phosphorylation of Y135 within the DRY microswitch of CXCR4, sequentially activating Gα_iβγ, AC7 and PKA, which phosphorylates S988 on the integrin. This mechanism is blocked by a syndecan-mimetic peptide (SSTN_VEGFR2), which, by preventing VEGFR2 linkage to VLA-4, arrests tumor cell migration that depends on VLA-4 phosphorylation and stimulates the LFA-1-mediated migration of cytotoxic leukocytes.

Differential nanoscale organisation of LFA-1 modulates T-cell migration

Effector T-cells rely on integrins to drive adhesion and migration to facilitate their immune function. The heterodimeric transmembrane integrin LFA-1 (αLβ2 integrin) regulates adhesion and migration of effector T-cells through linkage of the extracellular matrix with the intracellular actin treadmill machinery. Here, we quantified the velocity and direction of F-actin flow in migrating T-cells alongside single-molecule localisation of transmembrane and intracellular LFA-1. Results showed that actin retrograde flow positively correlated and immobile actin negatively correlated with T-cell velocity. Plasma membrane-localised LFA-1 forms unique nano-clustering patterns in the leading edge, compared to the mid-focal zone, of migrating T-cells. Deleting the cytosolic phosphatase PTPN22, loss-of-function mutations of which have been linked to autoimmune disease, increased T-cell velocity, and leading-edge co-clustering of pY397 FAK, pY416 Src family kinases and LFA-1. These data suggest that differential nanoclustering patterns of LFA-1 in migrating T-cells may instruct intracellular signalling. Our data presents a paradigm where T-cells modulate the nanoscale organisation of adhesion and signalling molecules to fine tune their migration speed, with implications for the regulation of immune and inflammatory responses.This article has an associated First Person interview with the first author of the paper.

Cell Adhesion Molecules and Their Roles and Regulation in the Immune and Tumor Microenvironment

The immune system and cancer have a complex relationship with the immune system playing a dual role in tumor development. The effector cells of the immune system can recognize and kill malignant cells while immune system-mediated inflammation can also promote tumor growth and regulatory cells suppress the anti-tumor responses. In the center of all anti-tumor responses is the ability of the immune cells to migrate to the tumor site and to interact with each other and with the malignant cells. Cell adhesion molecules including receptors of the immunoglobulin superfamily and integrins are of crucial importance in mediating these processes. Particularly integrins play a vital role in regulating all aspects of immune cell function including immune cell trafficking into tissues, effector cell activation and proliferation and the formation of the immunological synapse between immune cells or between immune cell and the target cell both during homeostasis and during inflammation and cancer. In this review we discuss the molecular mechanisms regulating integrin function and the role of integrins and other cell adhesion molecules in immune responses and in the tumor microenvironment. We also describe how malignant cells can utilize cell adhesion molecules to promote tumor growth and metastases and how these molecules could be targeted in cancer immunotherapy.

Evaluation of therapeutic effects of FAK inhibition in murine models of atherosclerosis

Objective

Therapeutic effects of focal adhesion kinase (FAK) inhibition using a small molecule inhibitor was evaluated in apolipoprotein E (apoE) knockout (KO) and low-density lipoprotein receptor (LDLr) KO mouse atherosclerosis models.

Results

The prevention trial consisted of an 8-week treatment with an FAK inhibitor concurrent treatment with a high fat (HF)/high cholesterol (HC) diet. The intervention trial consisted of 6- and 8-week treatment after 6- and 8-week pre-loading, respectively, of a HF/HC diet in apoE KO and LDLr KO mice, respectively. The inhibitor was admixed with a HF/HC diet and mice were given free access to the admixture. The FAK inhibitor exhibited marked inhibition against the development of the atherosclerosis in both of prevention and intervention trials at a dose of 0.03% without showing any remarkable toxic properties in biochemical examinations. These results indicated that FAK inhibition might be a possible candidate for novel therapeutic targets against atherosclerosis.

Electronic supplementary material

The online version of this article (10.1186/s13104-019-4220-5) contains supplementary material, which is available to authorized users.

Teaching an old dog new tricks: next-generation CAR T cells

Adoptive T cell therapy (ACT) refers to the therapeutic use of T cells. T cells genetically engineered to express chimeric antigen receptors (CAR) constitute the most clinically advanced form of ACT approved to date for the treatment of CD19-positive leukaemias and lymphomas. CARs are synthetic receptors that are able to confer antigen-binding and activating functions on T cells with the aim of therapeutically targeting cancer cells. Several factors are essential for CAR T cell therapy to be effective, such as recruitment, activation, expansion and persistence of bioengineered T cells at the tumour site. Despite the advances made in CAR T cell therapy, however, most tumour entities still escape immune detection and elimination. A number of strategies counteracting these problems will need to be addressed in order to render T cell therapy effective in more situations than currently possible. Non-haematological tumours are also the subject of active investigation, but ACT has so far shown only marginal success rates in these cases. New approaches are needed to enhance the ability of ACT to target solid tumours without increasing toxicity, by improving recognition, infiltration, and persistence within tumours, as well as an enhanced resistance to the suppressive tumour microenvironment.

Soluble VCAM-1/soluble ICAM-1 ratio is a promising biomarker for diagnosing endometriosis

Study question

Do cell adhesion molecules play a role in endometriosis, and can they be used as a biomarker for diagnosing endometriosis?

Summary answer

Altered expression of vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) in the endometrium and peritoneum may play a key role in endometriosis and the soluble VCAM-1/soluble ICAM-1 ratio is a promising biomarker.

What is known already

Cell adhesion molecules are cell surface proteins that mediate cellular adherence, inflammatory and immune responses, and cancer-related biological processes. Altered expression of VCAM-1 and ICAM-1 in women with endometriosis has been investigated previously; however, gene expression levels in tissues and protein levels in the serum have not been investigated in the same patients.

Study design size, duration

We performed a prospective, longitudinal study (the Endometriosis Marker Austria) in patients who underwent a laparoscopy for benign gynecological pathology in a university-based tertiary referral center for endometriosis. From a total of 138 women who were included in the study from July 2013 through September 2014, 97 had not received hormonal treatment for at least 3 months prior to recruitment and were included in the analysis; 49 (50.5%) of these women had endometriosis, and the 48 (49.5%) who did not have endometriosis served as a control group.

Participants/materials setting methods

During laparoscopy, tissue samples were obtained from ectopic and eutopic endometrium, and from normal pelvic peritoneum. In addition, serum samples were collected immediately before and 6-10 weeks after surgery. The mRNA levels of VCAM-1, ICAM-1 and epithelial cell adhesion molecule (EpCAM) were measured using quantitative real-time PCR, and serum protein levels of soluble VCAM-1 (sVCAM-1), ICAM-1 (sICAM-1) and EpCAM (sEpCAM) were measured using ELISA and correlated with endometriosis status.

Main results and the role of chance

The mRNA levels of both VCAM-1 and ICAM-1 were higher in ectopic endometriotic lesions than in eutopic endometrium (P < 0.001). Moreover, the mRNA levels of both VCAM-1 and ICAM-1 were higher in normal peritoneum samples obtained from women with endometriosis compared to those from controls (P = 0.038 and P = 0.009). The mRNA levels of VCAM-1 were also higher in the eutopic endometrium samples obtained from women with endometriosis compared to controls (P = 0.018). With respect to serum protein levels, compared to controls, the women with endometriosis had lower serum levels of sICAM-1 (P = 0.042) and higher levels of sVCAM-1 (P < 0.001). Our analysis revealed that the serum levels of sVCAM-1 were not affected by lesion entity, menstrual cycle phase or disease severity. An receiver operating characteristics curve, calculated to determine whether preoperative serum sVCAM-1 concentration can be used to predict endometriosis, found an AUC of 0.868 with 80% specificity and 84% sensitivity at a cutoff value of 370 pg/ml. This predictive performance can be further improved by calculation of the sVCAM-1/sICAM-1 ratio, leading to an AUC of 0.929 with 86.7% specificity and 90.3% sensitivity at a cutoff ratio value of 1.55.

Large scale data

Not applicable.

Limitations reasons for caution

The relatively small sample size in the expression analyses is a possible limitation of this study.

Wider implications of the findings

Our findings could contribute to an improved understanding of the pathogenesis of endometriosis and the role of cell adhesion molecules. In addition, the results may lead to the development of new, non-invasive tools for diagnosing endometriosis. The ability to diagnose patients by measuring serum sVCAM-1 levels or the sVCAM-1/sICAM-1 ratio would have considerable clinical value.

Study funding/competing interest(s)

The Ingrid Flick Foundation (Grant no. FA751C0801), which played no role in the study design, data collection and analysis, decision to publish or preparation of the manuscript. The authors declare no competing interests.

Paxillin: a crossroad in pathological cell migration

Paxilllin is a multifunctional and multidomain focal adhesion adapter protein which serves an important scaffolding role at focal adhesions by recruiting structural and signaling molecules involved in cell movement and migration, when phosphorylated on specific Tyr and Ser residues. Upon integrin engagement with extracellular matrix, paxillin is phosphorylated at Tyr31, Tyr118, Ser188, and Ser190, activating numerous signaling cascades which promote cell migration, indicating that the regulation of adhesion dynamics is under the control of a complex display of signaling mechanisms. Among them, paxillin disassembly from focal adhesions induced by extracellular regulated kinase (ERK)-mediated phosphorylation of serines 106, 231, and 290 as well as the binding of the phosphatase PEST to paxillin have been shown to play a key role in cell migration. Paxillin also coordinates the spatiotemporal activation of signaling molecules, including Cdc42, Rac1, and RhoA GTPases, by recruiting GEFs, GAPs, and GITs to focal adhesions. As a major participant in the regulation of cell movement, paxillin plays distinct roles in specific tissues and developmental stages and is involved in immune response, epithelial morphogenesis, and embryonic development. Importantly, paxillin is also an essential player in pathological conditions including oxidative stress, inflammation, endothelial cell barrier dysfunction, and cancer development and metastasis.

A Small Molecule, Which Competes with MAdCAM-1, Activates Integrin α4β7 and Fails to Prevent Mucosal Transmission of SHIV-SF162P3

Mucosal HIV-1 transmission is inefficient. However, certain viral and host characteristics may play a role in facilitating HIV acquisition and systemic expansion. Cells expressing high levels of integrin α4β7 have been implicated in favoring the transmission process and the infusion of an anti-α4β7 mAb (RM-Act-1) prior to, and during a repeated low-dose vaginal challenge (RLDC) regimen with SIVmac251 reduced SIV acquisition and protected the gut-associated lymphoid tissues (GALT) in the macaques that acquired SIV. α4β7 expression is required for lymphocyte trafficking to the gut lamina propria and gut inductive sites. Several therapeutic strategies that target α4β7 have been shown to be effective in treating inflammatory conditions of the intestine, such as inflammatory bowel disease (IBD). To determine if blocking α4β7 with ELN, an orally available anti-α4 small molecule, would inhibit SHIV-SF162P3 acquisition, we tested its ability to block MAdCAM-1 (α4β7 natural ligand) and HIV-gp120 binding in vitro. We studied the pharmacokinetic profile of ELN after oral and vaginal delivery in macaques. Twenty-six macaques were divided into 3 groups: 9 animals were treated with ELN orally, 9 orally and vaginally and 8 were used as controls. All animals were challenged intra-vaginally with SHIV-SF162P3 using the RLDC regimen. We found that ELN did not protect macaques from SHIV acquisition although it reduced the SHIV-induced inflammatory status during the acute phase of infection. Notably, integrins can exist in different activation states and, comparing the effect of ELN and the anti-α4β7 mAb RM-Act-1 that reduced susceptibility to SIV infection, we determined that ELN induces the active conformation of α4β7, while RM-Act-1 inhibits its activation through an allosteric mechanism. These results suggest that inhibition of α4β7 activation may be necessary to reduce susceptibility to SIV/SHIV infection and highlight the complexity of anti-integrins therapeutic approach in HIV as well as in IBD and other autoimmune diseases.

Function and Dynamics of Tetraspanins during Antigen Recognition and Immunological Synapse Formation

Tetraspanin-enriched microdomains (TEMs) are specialized membrane platforms driven by protein–protein interactions that integrate membrane receptors and adhesion molecules. Tetraspanins participate in antigen recognition and presentation by antigen-­presenting cells (APCs) through the organization of pattern-recognition receptors (PRRs) and their downstream-induced signaling, as well as the regulation of MHC-II–peptide trafficking. T lymphocyte activation is triggered upon specific recognition of antigens present on the APC surface during immunological synapse (IS) formation. This dynamic process is characterized by a defined spatial organization involving the compartmentalization of receptors and adhesion molecules in specialized membrane domains that are connected to the underlying cytoskeleton and signaling molecules. Tetraspanins contribute to the spatial organization and maturation of the IS by controlling receptor clustering and local accumulation of adhesion receptors and integrins, their downstream signaling, and linkage to the actin cytoskeleton. This review offers a perspective on the important role of TEMs in the regulation of antigen recognition and presentation and in the dynamics of IS architectural organization.

T Lymphocyte Migration: An Action Movie Starring the Actin and Associated Actors

The actin cytoskeleton is composed of a dynamic filament meshwork that builds the architecture of the cell to sustain its fundamental properties. This physical structure is characterized by a continuous remodeling, which allows cells to accomplish complex motility steps such as directed migration, crossing of biological barriers, and interaction with other cells. T lymphocytes excel in these motility steps to ensure their immune surveillance duties. In particular, actin cytoskeleton remodeling is a key to facilitate the journey of T lymphocytes through distinct tissue environments and to tune their stop and go behavior during the scanning of antigen-presenting cells. The molecular mechanisms controlling actin cytoskeleton remodeling during T lymphocyte motility have been only partially unraveled, since the function of many actin regulators has not yet been assessed in these cells. Our review aims to integrate the current knowledge into a comprehensive picture of how the actin cytoskeleton drives T lymphocyte migration. We will present the molecular actors that control actin cytoskeleton remodeling, as well as their role in the different T lymphocyte motile steps. We will also highlight which challenges remain to be addressed experimentally and which approaches appear promising to tackle them.

CD11c/CD18 Signals Very Late Antigen-4 Activation To Initiate Foamy Monocyte Recruitment during the Onset of Hypercholesterolemia

Recruitment of foamy monocytes to inflamed endothelium expressing VCAM-1 contributes to the development of plaque during atherogenesis. Foamy CD11c(+) monocytes arise in the circulation during the onset of hypercholesterolemia and recruit to nascent plaque, but the mechanism of CD11c/CD18 and very late Ag-4 (VLA-4) activation and cooperation in shear-resistant cell arrest on VCAM-1 are ill defined. Within 1 wk of the onset of a Western high-fat diet (WD) in apolipoprotein E-deficient mice, an inflammatory subset of foamy monocytes emerged that made up one fourth of the circulating population. These cells expressed ∼3-fold more CD11c/CD18 and 50% higher chemokine receptors than nonfoamy monocytes. Recruitment from blood to a VCAM-1 substrate under shear stress was assessed ex vivo using a unique artery-on-a-chip microfluidic assay. It revealed that foamy monocytes from mice on a WD increased their adhesiveness over 5 wk, rising to twice that of mice on a normal diet or CD11c(-/-) mice fed a WD. Shear-resistant capture of foamy human or mouse monocytes was initiated by high-affinity CD11c, which directly activated VLA-4 adhesion via phosphorylated spleen tyrosine kinase and paxillin within focal adhesion complexes. Lipid uptake and activation of CD11c are early and critical events in signaling VLA-4 adhesive function on foamy monocytes competent to recruit to VCAM-1 on inflamed arterial endothelium.

Functions of the FAK family kinases in T cells: beyond actin cytoskeletal rearrangement

T cells control the focus and extent of adaptive immunity in infectious and pathological diseases. The activation of T cells occurs when the T cell antigen receptor (TCR) and costimulatory and/or adhesion receptors are engaged by their ligands. This process drives signaling that promotes cytoskeletal rearrangement and transcription factor activation, both of which regulate the quality and magnitude of the T cell response. However, it is not fully understood how different receptor-induced signals combine to alter T cell activation. The related non-receptor tyrosine kinases focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2) are phosphorylated downstream of the TCR and several costimulatory and adhesion receptors. FAK family proteins integrate receptor-mediated signals that influence actin cytoskeletal rearrangement and effector T cell responses. In this review, we summarize the receptor-specific roles that FAK and Pyk2 control to influence T cell development and activation.

Fine-tuning Tumor Immunity with Integrin Trans-regulation

Inefficient T-cell homing to tissues limits adoptive T-cell immunotherapy of solid tumors. αLβ2 and α4β1 integrins mediate trafficking of T cells into tissues via engagement of ICAM-1 and VCAM-1, respectively. Inhibiting protein kinase A (PKA)-mediated phosphorylation of α4 integrin in cells results in an increase in αLβ2-mediated migration on mixed ICAM-1-VCAM-1 substrates in vitro, a phenomenon termed "integrin trans-regulation." Here, we created an α4(S988A)-bearing mouse, which precludes PKA-mediated α4 phosphorylation, to examine the effect of integrin trans-regulation in vivo. The α4(S988A) mouse exhibited a dramatic and selective increase in migration of lymphocytes, but not myeloid cells, to sites of inflammation. Importantly, we found that the α4(S988A) mice exhibited a marked increase in T-cell entry into and reduced growth of B16 melanomas, consistent with antitumor roles of infiltrating T cells and progrowth functions of tumor-associated macrophages. Thus, increased α4 trans-regulation of αLβ2 integrin function biases leukocyte emigration toward lymphocytes relative to myeloid cells and enhances tumor immunity.

Adaptor regulation of LFA-1 signaling in T lymphocyte migration: Potential druggable targets for immunotherapies?

The integrin lymphocyte function associated antigen-1 (LFA-1) plays a key role in leukocyte trafficking and in adaptive immune responses through interactions with adhesive ligands, such as ICAM-1. Specific blockade of these interactions has validated LFA-1 as a therapeutic target in many chronic inflammatory diseases, however LFA-1 antagonists have not been clinically successful due to the development of a general immunosuppression, causing fatal side effects. Growing evidence has now established that LFA-1 mediates an array of intracellular signaling pathways by triggering a number of downstream molecules. In this context, a class of multimodular domain-containing proteins capable of recruiting two or more effector molecules, collectively known as "adaptor proteins," has emerged as important mediators in LFA-1 signal transduction. Here, we provide an overview of the adaptor proteins involved in the intracellular signaling cascades by which LFA-1 regulates T-cell motility and immune responses. The complexity of the LFA-1-associated signaling delineated in this review suggests that it may be an important and challenging focus for future research, enabling the identification of "tunable" targets for the development of immunotherapies.

Antifibrotic effects of focal adhesion kinase inhibitor in bleomycin-induced pulmonary fibrosis in mice

Focal adhesion kinase (FAK) is a nonreceptor tyrosine kinase involved in various biological functions, including cell survival, proliferation, migration, and adhesion. FAK is an essential factor for transforming growth factor β to induce myofibroblast differentiation. In the present study, we investigated whether the targeted inhibition of FAK by using a specific inhibitor, TAE226, has the potential to regulate pulmonary fibrosis. TAE226 showed inhibitory activity of autophosphorylation of FAK at tyrosine 397 in lung fibroblasts. The addition of TAE226 inhibited the proliferation of lung fibroblasts in response to various growth factors, including platelet-derived growth factor and insulin-like growth factor I, in vitro. TAE226 strongly suppressed the production of type I collagen by lung fibroblasts. Furthermore, treatment of fibroblasts with TAE226 reduced the expression of α-smooth muscle actin induced by transforming growth factor β, indicating the inhibition of differentiation of fibroblasts to myofibroblasts. Administration of TAE226 ameliorated the pulmonary fibrosis induced by bleomycin in mice even when used late in the treatment. The number of proliferating mesenchymal cells was reduced in the lungs of TAE226-treated mice. These data suggest that FAK signal plays a significant role in the progression of pulmonary fibrosis and that it can become a promising target for therapeutic approaches to pulmonary fibrosis.

Characterization of 14-3-3-ζ Interactions with integrin tails

Integrins are a family of heterodimeric (α+β) adhesion receptors that play key roles in many cellular processes. Integrins are unusual in that their functions can be modulated from both outside and inside the cell. Inside-out signaling is mediated by binding adaptor proteins to the flexible cytoplasmic tails of the α- and β-integrin subunits. Talin is one well-known intracellular activator, but various other adaptors bind to integrin tails, including 14-3-3-ζ, a member of the 14-3-3 family of dimeric proteins that have a preference for binding phosphorylated sequence motifs. Phosphorylation of a threonine in the β2 integrin tail has been shown to modulate β2/14-3-3-ζ interactions, and recently, the α4 integrin tail was reported to bind to 14-3-3-ζ and associate with paxillin in a ternary complex that is regulated by serine phosphorylation. Here, we use a range of biophysical techniques to characterize interactions between 14-3-3-ζ and the cytoplasmic tails of α4, β1, β2 and β3 integrins. The X-ray structure of the 14-3-3-ζ/α4 complex indicates a canonical binding mode for the α4 phospho-peptide, but unexpected features are also observed: residues outside the consensus 14-3-3-ζ binding motif are shown to be essential for an efficient interaction; in contrast, a short β2 phospho-peptide is sufficient for high-affinity binding to 14-3-3-ζ. In addition, we report novel 14-3-3-ζ/integrin tail interactions that are independent of phosphorylation. Of the integrin tails studied, the strongest interaction with 14-3-3-ζ is observed for the β1A variant. In summary, new insights about 14-3-3-ζ/integrin tail interactions that have implications for the role of these molecular associations in cells are described.

The membrane-proximal KXGFFKR motif of α-integrin mediates chemoresistance

Cell adhesion-mediated drug resistance contributes to minimal residual disease and relapse in hematological malignancies. Here, we show that adhesion of Jurkat T-acute lymphoblastic leukemia cells to substrates engaging α4β1-integrin or α5β1-integrin promotes chemoresistance to doxorubicin-induced apoptosis. Reconstituted expression of α4δ, a truncated α4-integrin with KXGFFKR as the cytoplasmic motif, in α4-deficient cells promoted chemoresistance to doxorubicin in a manner independent of α4-mediated adhesion. The adhesion-independent chemoresistance did not require β1-integrin as the heterodimeric pair, since expression of Tacδ, a monomeric nonintegrin transmembrane protein fused to the juxtamembrane KXGFFKR, was sufficient to reproduce the phenomenon. The requirement for integrin-mediated adhesion in stimulation of Akt phosphorylation and activation was bypassed for cells expressing α4δ and Tacδ. Cells expressing α4δ and Tacδ exhibited a high influx of extracellular Ca(2+), and inhibition of Ca(2+) channels with verapamil attenuated the adhesion-independent chemoresistance. Tacδ cells also exhibited greater rates of drug efflux. α4δ and Tacδ interacted with the Ca(2+)-binding protein calreticulin, in a manner dependent on the KXGFFKR motif. Adhesion-mediated engagement of α4-integrins promoted an increased calreticulin-α4 association and greater influx of extracellular Ca(2+) than in nonadherent cells. The α-integrin KXGFFKR motif is involved in adhesion-mediated control of chemoresistance in T cells.

Small molecule agonist of very late antigen-4 (VLA-4) integrin induces progenitor cell adhesion

Activation of the integrin family of cell adhesion receptors on progenitor cells may be a viable approach to enhance the effects of stem cell-based therapies by improving cell retention and engraftment. Here, we describe the synthesis and characterization of the first small molecule agonist identified for the integrin α4β1 (also known as very late antigen-4 or VLA-4). The agonist, THI0019, was generated via two structural modifications to a previously identified α4β1 antagonist. THI0019 greatly enhanced the adhesion of cultured cell lines and primary progenitor cells to α4β1 ligands VCAM-1 and CS1 under both static and flow conditions. Furthermore, THI0019 facilitated the rolling and spreading of cells on VCAM-1 and the migration of cells toward SDF-1α. Molecular modeling predicted that the compound binds at the α/β subunit interface overlapping the ligand-binding site thus indicating that the compound must be displaced upon ligand binding. In support of this model, an analog of THI0019 modified to contain a photoreactive group was used to demonstrate that when cross-linked to the integrin, the compound behaves as an antagonist instead of an agonist. In addition, THI0019 showed cross-reactivity with the related integrin α4β7 as well as α5β1 and αLβ2. When cross-linked to αLβ2, the photoreactive analog of THI0019 remained an agonist, consistent with it binding at the α/β subunit interface and not at the ligand-binding site in the inserted ("I") domain of the αL subunit. Co-administering progenitor cells with a compound such as THI0019 may provide a mechanism for enhancing stem cell therapy.

Paxillin phosphorylation by JNK and p38 is required for NFAT activation

Paxillin is an adaptor protein associated with focal adhesion complex, and is activated by tyrosine phosphorylation through focal adhesion kinase (FAK) and Src kinase. Recent studies reveal that serine phosphorylation of paxillin by JNK and p38 MAPK is essential for cell migration or neurite extension, but their cellular targets remain unclear. In this study, we examined the requirement of paxillin phosphorylation by p38 MAPK or JNK in T-cell motility and activation using paxillin mutants at the respective phosphorylation sites, Ser85, and Ser178. (S85A)-paxillin, (S178A)-paxillin, or (S85A/S178A)-paxillin inhibited the motility of NIH/3T3 fibroblasts, but did not interfere with T-cell migration and integrin-mediated T-cell adhesion. In contrast, activation of T cells was effectively suppressed by (S85A/S178A)-paxillin. Transgenic (S85A/S178A)-paxillin expression inhibited T-cell proliferation and reduced the production of IL-2, IFN-γ, and IL-4. In searching for signals modulated by (S85A/S178A)-paxillin, we found that NFAT activation was specifically blocked by (S85A/S178A)-paxillin. This could be partly attributed to diminished stromal interaction molecule 1 (STIM1) expression and attenuated TCR-induced Ca(2+) influx. Our results demonstrate that dual phosphorylation of paxillin by JNK and p38 MAPK is essential for T-cell activation and suggest that NFAT is a functional target of the JNK/p38 phosphorylated paxillin.

Abolished adherence alters signaling pathways in phorbol ester-induced human U937 cells

Phorbol ester (TPA) treatment of human U937 myeloid leukemia cells is associated with increasing adherence and monocyte-like maturation whereby the role of β₂ integrin-mediated attachment for subsequent growth properties and the differentiation program remains unclear. Here, stably-transfected U937 cells with a pMTH1 vector containing the β₂ integrin gene of CD11b in antisense orientation (asCD11b-U937) demonstrated a significantly reduced proliferative capacity in contrast to control vector transfectants (pMTH1-U937) or wild-type U937 cells. Phorbol ester exposure induced adherence and growth arrest in more than 90% of pMTH1-U937 and wild-type U937 cells after 72 h. In contrast, TPA-treated asCD11b-U937 failed to attach and the proliferation continued in more than 30% of the cells. Moreover, increased apoptosis appeared in asCD11b-U937 after TPA induction in contrast to pMTH1-U937 cells. In addition, non-specific inhibition of adherence on an agarose surface demonstrated internucleosomal DNA fragmentation in both, pMTH1-U937 and asCD11b-U937 after TPA treatment indicating a functional relationship between abolished adherence, regulation of proliferation and induction of apoptosis. Western blot analysis revealed differences in the expression levels and altered phosphorylation patterns of Pyk-2, pp60src and p42/p44 MAP kinases between pMTH1-U937 and asCD11b-U937 following TPA exposure which was also substantiated by Pyk-2 immunoprecipitation. These findings suggested that induced adherence predominantly mediated by a functional CD11b/CD18 integrin in U937 cells is involved in the activation of downstream signaling kinases and contributes to cell cycle regulation and apoptosis during monocytic maturation.

Association between α4 integrin cytoplasmic tail and non-muscle myosin IIA regulates cell migration

α4β1 integrin regulates cell migration via cytoplasmic interactions. Here, we report an association between the cytoplasmic tail of α4 integrin (α4 tail) and non-muscle myosin IIA (MIIA), demonstrated by co-immunoprecipitation of the MIIA heavy chain (HC) with anti-α4-integrin antibodies and pull-down of MIIA-HC with recombinant α4 tail from cell lysates. The association between the α4 tail and MIIA does not require paxillin binding or phosphorylation at Ser988 in the α4 tail. We found that substituting Glu982 in the α4 tail with alanine (E982A) disrupts the α4-MIIA association without interfering with the paxillin binding or Ser988 phosphorylation. By comparing stably transfected CHO cells, we show that the E982A mutation reduces the ability of α4β1 integrin to mediate cell spreading and to promote front-back polarization. In addition, we show that E982A impairs shear-flow-induced migration of the α4-integrin-expressing CHO cells by reducing their migration speed and directional persistence. The E982A mutation also leads to defects in the organization of MIIA filament bundles. Furthermore, when cells are plated on fibronectin and simulated with shear flow, α4β1 integrin forms filament-like patterns that co-align with MIIA filament bundles. These results provide a new mechanism for linking integrins to the actomyosin cytoskeleton and for regulating cell migration by integrins and non-muscle myosin II.

Talin and signaling through integrins

Integrin adhesion receptors are essential for the development and functioning of multicellular animals. Integrins mediate cell adhesion to the extracellular matrix and to counter-receptors on adjacent cells, and the ability of integrins to bind extracellular ligands is regulated in response to intracellular signals that act on the short cytoplasmic tails of integrin subunits. Integrin activation, the rapid conversion of integrin receptors from low to high affinity, requires binding of talin to integrin β tails and, once bound, talin provides a connection from activated integrins to the actin cytoskeleton. A wide range of experimental approaches have contributed to the current understanding of the importance of talin in integrin signaling. Here, we describe two methods that have been central to our investigations of talin; a biochemical assay that has allowed characterization of interactions between integrin cytoplasmic tails and talin, and a fluorescent-activated cell-sorting procedure to assess integrin activation in cultured cells expressing talin domains, mutants, dominant negative constructs, or shRNA.

A small molecule that inhibits the interaction of paxillin and alpha 4 integrin inhibits accumulation of mononuclear leukocytes at a site of inflammation

Extracellular antagonists of alpha 4 integrin are an effective therapy for several autoimmune and inflammatory diseases; however, these agents that directly block ligand binding may exhibit mechanism-based toxicities. Inhibition of alpha 4 integrin signaling by mutations of alpha 4 that block paxillin binding inhibits inflammation while limiting mechanism-based toxicities. Here, we test a pharmacological approach by identifying small molecules that inhibit the alpha 4 integrin-paxillin interaction. By screening a large (approximately 40,000-compound) chemical library, we identified a noncytotoxic inhibitor of this interaction that impaired integrin alpha 4-mediated but not alpha L beta 2-mediated Jurkat T cell migration. The identified compound had no effect on alpha 4-mediated migration in cells bearing the alpha 4(Y991A) mutation that disrupts the alpha 4-paxillin interaction, establishing the specificity of its action. Administration of this compound to mice led to impaired recruitment of mononuclear leukocytes to a site of inflammation in vivo, whereas an isomer that does not inhibit the alpha 4-paxillin interaction had no effect on alpha 4-mediated cell migration, cell spreading, or recruitment of leukocytes to an inflammatory site. Thus, a small molecule inhibitor that interferes with alpha 4 integrin signaling reduces alpha 4-mediated T cell migration in vivo, thus providing proof of principle for inhibition of alpha 4 integrin signaling as a target for the pharmacological reduction of inflammation.

Regulation of angiogenesis: apoptotic cues from the ECM

The extracellular matrix (ECM) acts both as a physical scaffold for cells and as a repository for growth factors. Moreover, ECM structure and physical-chemical properties convey precise information to cells that profoundly influences their biology by interactions with cell surface receptors termed integrins. During angiogenesis, the perivascular ECM plays a critical role in determining the proliferative, invasive and survival responses of the local vascular cells to the angiogenic growth factors. Dynamic changes in both the ECM and the local vascular cells act in concert to regulate new blood vessel growth. The digestion of ECM components by proteolysis is critical for the invasive capacity of endothelial cells, but also creates ECM fragments, which antagonize the mechanosensory function of integrins, and can be apoptogenic. Here, we discuss the roles of integrins in modulating cellular responses to a changing ECM, in particular the regulation of survival and invasion among invasive endothelial cells.

Cell motility in chronic lymphocytic leukemia: defective Rap1 and alphaLbeta2 activation by chemokine

Chemokine-induced activation of alpha4beta1 and alphaLbeta2 integrins (by conformational change and clustering) is required for lymphocyte transendothelial migration (TEM) and entry into lymph nodes. We have previously reported that chemokine-induced TEM is defective in chronic lymphocytic leukemia (CLL) and that this defect is a result of failure of the chemokine to induce polar clustering of alphaLbeta2; engagement of alpha4beta1 and autocrine vascular endothelial growth factor (VEGF) restore clustering and TEM. The aim of the present study was to characterize the nature of this defect in alphaLbeta2 activation and determine how it is corrected. We show here that the alphaLbeta2 of CLL cells is already in variably activated conformations, which are not further altered by chemokine treatment. Importantly, such treatment usually does not cause an increase in the GTP-loading of Rap1, a GTPase central to chemokine-induced activation of integrins. Furthermore, we show that this defect in Rap1 GTP-loading is at the level of the GTPase and is corrected in CLL cells cultured in the absence of exogenous stimuli, suggesting that the defect is the result of in vivo stimulation. Finally, we show that, because Rap1-induced activation of both alpha4beta1 and alphaLbeta2 is defective, autocrine VEGF and chemokine are necessary to activate alpha4beta1 for ligand binding. Subsequently, this binding and both VEGF and chemokine stimulation are all needed for alphaLbeta2 activation for motility and TEM. The present study not only clarifies the nature of the alphaLbeta2 defect of CLL cells but is the first to implicate activation of Rap1 in the pathophysiology of CLL.

Mixed extracellular matrix ligands synergistically modulate integrin adhesion and signaling

Cell adhesion to extracellular matrix (ECM) components through cell-surface integrin receptors is essential to the formation, maintenance and repair of numerous tissues, and therefore represents a central theme in the design of bioactive materials that successfully interface with the body. While the adhesive responses associated with a single ligand have been extensively analyzed, the effects of multiple integrin subtypes binding to multivalent ECM signals remain poorly understood. In the present study, we generated a high throughput platform of non-adhesive surfaces presenting well-defined, independent densities of two integrin-specific engineered ligands for the type I collagen (COL-I) receptor alpha(2)beta(1) and the fibronectin (FN) receptor alpha(5)beta(1) to evaluate the effects of integrin cross-talk on adhesive responses. Engineered surfaces displayed ligand density-dependent adhesive effects, and mixed ligand surfaces significantly enhanced cell adhesion strength and focal adhesion assembly compared to single FN and COL-I ligand surfaces. Moreover, surfaces presenting mixed COL-I/FN ligands synergistically enhanced FAK activation compared to the single ligand substrates. The enhanced adhesive activities of the mixed ligand surfaces also promoted elevated proliferation rates. Our results demonstrate interplay between multivalent ECM ligands in adhesive responses and downstream cellular signaling.

Transmembrane and cytoplasmic domains in integrin activation and protein-protein interactions (review)

Integrins are heterodimeric membrane-spanning adhesion receptors that are essential for a wide range of biological functions. Control of integrin conformational states is required for bidirectional signalling across the membrane. Key components of this control mechanism are the transmembrane and cytoplasmic domains of the alpha and beta subunits. These domains are believed to interact, holding the integrin in the inactive state, while inside-out integrin activation is accompanied by domain separation. Although there are strong indications for domain interactions, the majority of evidence is insufficient to precisely define the interaction interface. The current best model of the complex, derived from computational calculations with experimental restraints, suggests that integrin activation by the cytoplasmic protein talin is accomplished by steric disruption of the alpha/beta interface. Better atomic-level resolution structures of the alpha/beta transmembrane/cytoplasmic domain complex are still required for the resting state integrin to corroborate this. Integrin activation is also controlled by competitive interactions involving the cytoplasmic domains, particularly the beta-tails. The concept of the beta integrin tail as a focal adhesion interaction 'hub' for interactions and regulation is discussed. Current efforts to define the structure and affinity of the various complexes formed by integrin tails, and how these interactions are controlled, e.g. by phosphorylation and localization, are described.

Blockade of alpha4 integrin signaling ameliorates the metabolic consequences of high-fat diet-induced obesity

OBJECTIVE

Many prevalent diseases of advanced societies, such as obesity-induced type 2 diabetes, are linked to indolent mononuclear cell-dependent inflammation. We previously proposed that blockade of alpha4 integrin signaling can inhibit inflammation while limiting mechanism-based toxicities of loss of alpha4 function. Thus, we hypothesized that mice bearing an alpha4(Y991A) mutation, which blocks signaling, would be protected from development of high-fat diet-induced insulin resistance.

RESEARCH DESIGN AND METHODS

Six- to eight-week-old wild-type and alpha4(Y991A) C57Bl/6 male mice were placed on either a high-fat diet that derived 60% calories from lipids or a chow diet. Metabolic testing was performed after 16-22 weeks of diet.

RESULTS

Alpha4(Y991A) mice were protected from development of high-fat diet-induced insulin resistance. This protection was conferred on wild-type mice by alpha4(Y991A) bone marrow transplantation. In the reverse experiment, wild-type bone marrow renders high-fat diet-fed alpha4(Y991A) acceptor animals insulin resistant. Furthermore, fat-fed alpha4(Y991A) mice showed a dramatic reduction of monocyte/macrophages in adipose tissue. This reduction was due to reduced monocyte/macrophage migration rather than reduced monocyte chemoattractant protein-1 production.

CONCLUSIONS

Alpha4 integrins contribute to the development of HFD-induced insulin resistance by mediating the trafficking of monocytes into adipose tissue; hence, blockade of alpha4 integrin signaling can prevent the development of obesity-induced insulin resistance.

Treating multiple sclerosis with monoclonal antibodies

Therapeutic monoclonal antibodies (mAbs) are potent new tools for a molecular targeted approach to modify the course of multiple sclerosis (MS). Besides natalizumab, which was approved in 2006, three other mAbs (alemtuzumab, rituximab and daclizumab) were successfully tested in Phase II MS trials. In this review, introductory notes on the development and systematic nomenclature of therapeutic mAbs in general, set the stage for a detailed discussion of the four mAbs mentioned. We summarize non-MS indications, expression and function of target antigens, scientific rationales for MS therapy, putative modes of action and pharmacological aspects. Particularly, we provide a critical discussion of clinical MS trials, including protocols and interim analyses of trials currently underway. The natalizumab section pays special attention to the clinical handling of safety issues and the diagnostic use of neutralizing antibodies. We finally develop a scenario for how each of the four mAbs might evolve into the market of MS therapeutics within the coming years.

Soluble form of vascular cell adhesion molecule 1 induces migration and proliferation of vascular smooth muscle cells

BACKGROUND

Serum levels of soluble vascular cell adhesion molecule 1 (sVCAM-1) shed from its membrane-bound form are elevated in hypertension. This study clarified the effects of sVCAM-1 on vascular responses in rat aortic smooth muscle cells (RASMCs).

METHODS

Boyden chamber, 5-bromo-2'-deoxyuridine incorporation and ex vivo aortic ring assays for migration and proliferation, and Western blot for the kinase activity were used.

RESULTS

Spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) rats were compared functionally. sVCAM-1 increased RASMC migration and proliferation, which were greater in SHR compared with WKY rats. RASMCs expressed the very late antigen 4alpha receptor integrin with no difference between SHR and WKY rats. Inhibitors of phosphoinositide kinase 3 (PI3K) and spleen tyrosine kinase (Syk) and small interference RNA-Syk abolished the sVCAM-1-induced migration, proliferation and phosphorylation of focal adhesion kinase. The phosphorylation of Syk was significantly greater in RASMCs from SHR than from WKY rats. sVCAM-1 increased aortic sprout outgrowth, which was inhibited by inhibitors of PI3K and Syk.

CONCLUSIONS

This study suggests that sVCAM-1 promotes the RASMC migration and proliferation via the focal adhesion kinase pathway regulated by Syk and PI3K, and the altered sVCAM-1-induced responses during hypertension are closely associated with the increments in intracellular signal transmission.

Integrin modulation and signaling in leukocyte adhesion and migration

The movement of leukocytes from the blood into peripheral tissues plays a key role in immunity as well as chronic inflammatory and autoimmune diseases. The shear force of blood flow presents special challenges to leukocytes as they establish adhesion on the vascular endothelium and migrate into the underlying tissues. Integrins are a family of cell adhesion and signaling molecules, whose function can be regulated to meet these challenges. The affinity of integrins for their vascular ligands can be stimulated in subseconds by chemoattractant signaling. This aids in inducing leukocyte adhesion under flow conditions. Further, linkage of these integrins to the actin cytoskeleton also helps to establish adhesion to the endothelium under flow conditions. In the case of alpha4beta1 integrins, this linkage of the integrin to the cytoskeleton is mediated in part by the binding of paxillin to the alpha4 integrin subunit and the subsequent binding of paxillin to the cytoskeleton molecule talin. The movement of leukocytes along the vascular endothelium and in between endothelial cells requires the temporal and spatial regulation of small guanosine triphosphatases, such as Rac1. We describe mechanisms through which alpha4beta1 integrin signaling regulates appropriate Rac activation to drive leukocyte migration.

Lymph node chemokines promote sustained T lymphocyte motility without triggering stable integrin adhesiveness in the absence of shear forces

Lymphocyte motility in lymph nodes is regulated by chemokines, but the contribution of integrins to this motility remains obscure. Here we examined lymphocyte migration over CCR7-binding chemokines that 'decorate' lymph node stroma. In a shear-free environment, surface-bound lymph node chemokines but not their soluble counterparts promoted robust and sustained T lymphocyte motility. The chemokine CCL21 induced compartmentalized clustering of the integrins LFA-1 and VLA-4 in motile lymphocytes, but both integrins remained nonadhesive to ligands on lymphocytes, dendritic cells and stroma. The application of shear stress to lymphocytes interacting with CCL21 and integrin ligands promoted robust integrin-mediated adhesion. Thus, lymph node chemokines that promote motility and strongly activate lymphocyte integrins under shear forces fail to stimulate stable integrin adhesiveness in extravascular shear-free environments.

Talin 1 and Paxillin Facilitate Distinct Steps in Rapid VLA-4-mediated Adhesion Strengthening to Vascular Cell Adhesion Molecule 1

VLA-4 (alpha4beta1) is a key integrin in lymphocytes, interacting with endothelial vascular cell adhesion molecule 1 (VCAM-1) on blood vessels and stroma. To dissect the contribution of the two cytoskeletal VLA-4 adaptor partners paxillin and talin to VLA-4 adhesiveness, we transiently knocked them down in Jurkat T cells and primary resting human T cells by small interfering RNA silencing. Paxillin was required for VLA-4 adhesiveness to low density VCAM-1 under shear stress conditions and was found to control mechanical stability of bonds mediated by the alpha4 subunit but did not affect the integrin affinity or avidity to VCAM-1 in shear-free conditions. Talin 1 maintained VLA-4 in a high affinity conformation, thereby promoting rapid VLA-4 adhesion strengthening to VCAM-1 under both shear stress and shear-free conditions. Talin 1, but not paxillin, was required for VLA-4 to undergo optimal stimulation by the prototypic chemokine, CXCL12, under shear stress conditions. Interestingly, talin 1 and paxillin played the same distinct roles in VLA-4 adhesions of primary T lymphocytes, although VLA-4 affinity to VCAM-1 was at least 200-fold lower in these cells than in Jurkat cells. Collectively, our results suggest that whereas paxillin is a mechanical regulator of VLA-4 bonds generated in the absence of chemokine signals and low VCAM-1 occupancy, talin 1 is a versatile VLA-4 affinity regulator implicated in both spontaneous and chemokine-triggered rapid adhesions to VCAM-1.

The role of vascular cell adhesion molecule-1 in tumor immune evasion

Tumor immune escape is a critical trait of cancer but the mechanisms involved have yet to fully emerge. One recent study has shown that tumor cells can escape T-cell immunity by overexpressing the endothelial cell adhesion molecule vascular cell adhesion molecule-1 (VCAM-1), which normally mediates leukocyte extravasion to sites of tissue inflammation. Renal cell carcinoma (RCC) was identified as one tumor type where VCAM-1 is commonly highly overexpressed. Together, our findings suggest that RCCs might exploit VCAM-1 overexpression for immune escape.

Neurite outgrowth on a fibronectin isoform expressed during peripheral nerve regeneration is mediated by the interaction of paxillin with alpha4beta1 integrins

BACKGROUND

The regeneration of peripheral nerve is associated with a change in the alternative splicing of the fibronectin primary gene transcript to re-express embryonic isoforms containing a binding site for alpha4beta1 integrins that promote neurite outgrowth. Here we use PC12 cells to examine the role of the interaction between paxillin and the alpha4 integrin cytoplasmic domain in neurite outgrowth.

RESULTS

Expression of alpha4 with mutations in the paxillin-binding domain reduced neurite outgrowth on recombinant embryonic fibronectin fragments relative to wild type alpha4. Over-expression of paxillin promoted neurite outgrowth while a mutant isoform lacking the LD4 domain implicated in the regulation of ARF and Rac GTPases was less effective. Optimal alpha4-mediated migration in leucocytes requires spatial regulation of alpha4 phosphorylation at Ser988, a post-translational modification that blocks paxillin binding to the integrin cytoplasmic domain. In keeping with this alpha4(S988D), which mimics phosphorylated alpha4, did not promote neurite outgrowth. However, alpha4 was not phosphorylated in the PC12 cells, and a non-phosphorylatable alpha4(S988A) mutant promoted neurite outgrowth indistinguishably from the wild type integrin.

CONCLUSION

We establish the importance of the alpha4 integrin-paxillin interaction in a model of axonal regeneration and highlight differing dependence on phosphorylation of alpha4 for extension of neuronal growth cones and migration of non-neural cells.

Alpha4 integrins are type I cAMP-dependent protein kinase-anchoring proteins

A-kinase anchoring proteins (AKAPs) control the localization and substrate specificity of cAMP-dependent protein kinase (PKA), tetramers of regulatory (PKA-R) and catalytic (PKA-C) subunits, by binding to PKA-R subunits. Most mammalian AKAPs bind Type II PKA through PKA-RII (ref. 2), whereas dual specificity AKAPs bind both PKA-RI and PKA-RII (ref. 3). Inhibition of PKA-AKAP interactions modulates PKA signalling. Localized PKA activation in pseudopodia of migrating cells phosphorylates alpha4 integrins to provide spatial cues governing cell motility. Here, we report that the alpha4 cytoplasmic domain is a Type I PKA-specific AKAP that is distinct from canonical AKAPs in two ways: the alpha4 interaction requires the PKA holoenzyme, and is insensitive to amphipathic peptides that disrupt most PKA-AKAP interactions. We exploited type-specific PKA anchoring peptides to create genetically encoded baits that sequester specific PKA isoforms to the mitochondria and found that mislocalization of Type I, but not Type II, PKA disrupts alpha4 phosphorylation and markedly inhibits the velocity and directional persistence of cell migration.