Beta1-integrins mediate Ca2+-signalling and T cell spreading via divergent pathways

Adhesion to laminin-1 and collagen IV induces the formation of Ca2+ microdomains that sensitize mouse T cells for activation

During an immune response, T cells migrate from blood vessel walls into inflamed tissues by migrating across the endothelium and through extracellular matrix (ECM). Integrins facilitate T cell binding to endothelial cells and ECM proteins. Here, we report that Ca²⁺ microdomains observed in the absence of T cell receptor (TCR)/CD3 stimulation are initial signaling events triggered by adhesion to ECM proteins that increase the sensitivity of primary murine T cells to activation. Adhesion to the ECM proteins collagen IV and laminin-1 increased the number of Ca²⁺ microdomains in a manner dependent on the kinase FAK, phospholipase C (PLC), and all three inositol 1,4,5-trisphosphate receptor (IP₃R) subtypes and promoted the nuclear translocation of the transcription factor NFAT-1. Mathematical modeling predicted that the formation of adhesion-dependent Ca²⁺ microdomains required the concerted activity of two to six IP₃Rs and ORAI1 channels to achieve the increase in the Ca²⁺ concentration in the ER-plasma membrane junction that was observed experimentally and that required SOCE. Further, adhesion-dependent Ca²⁺ microdomains were important for the magnitude of the TCR-induced activation of T cells on collagen IV as assessed by the global Ca²⁺ response and NFAT-1 nuclear translocation. Thus, adhesion to collagen IV and laminin-1 sensitizes T cells through a mechanism involving the formation of Ca²⁺ microdomains, and blocking this low-level sensitization decreases T cell activation upon TCR engagement.

β1 Integrin induces adhesion and migration of human Th17 cells via Pyk2-dependent activation of P2X4 receptor

Integrin-mediated T-cell adhesion and migration is a crucial step in immune response and autoimmune diseases. However, the underlying signalling mechanisms are not fully elucidated. In this study, we examined the implication of purinergic signalling, which has been associated with T-cell activation, in the adhesion and migration of human Th17 cells across fibronectin, a major matrix protein associated with inflammatory diseases. We showed that the adhesion of human Th17 cells to fibronectin induces, via β1 integrin, a sustained release of adenosine triphosphate (ATP) from the mitochondria through the pannexin-1 hemichannels. Inhibition of ATP release or its degradation with apyrase impaired the capacity of the cells to attach and migrate across fibronectin. Inhibition studies identified a major role for the purinergic receptor P2X4 in T-cell adhesion and migration but not for P2X7 or P2Y₁₁ receptors. Blockade of P2X4 but not P2X7 or P2Y₁₁ receptors reduced cell adhesion and migration by inhibiting activation of β1 integrins, which is essential for ligand binding. Furthermore, we found that β1 integrin-induced ATP release, P2X4 receptor transactivation, cell adhesion and migration were dependent on the focal adhesion kinase Pyk2 but not FAK. Finally, P2X4 receptor inhibition also blocked fibronectin-induced Pyk2 activation suggesting the existence of a positive feedback loop of activation between β1 integrin/Pyk2 and P2X4 purinergic signalling pathways. Our findings uncovered an unrecognized link between β1 integrin and P2X4 receptor signalling pathways for promoting T-cell adhesion and migration across the extracellular matrix.

Paxillin regulates cell polarization and anterograde vesicle trafficking during cell migration

Cell polarization and directed migration play pivotal roles in diverse physiological and pathological processes. Herein, we identify new roles for paxillin-mediated HDAC6 inhibition in regulating key aspects of cell polarization in both two-dimensional and one-dimensional matrix environments. Paxillin, by modulating microtubule acetylation through HDAC6 regulation, was shown to control centrosome and Golgi reorientation toward the leading edge, a hallmark of cell polarization to ensure directed trafficking of promigratory factors. Paxillin was also required for pericentrosomal Golgi localization and centrosome cohesion, independent of its localization to, and role in, focal adhesion signaling. In addition, we provide evidence of an accumulation of paxillin at the centrosome that is dependent on focal adhesion kinase (FAK) and identify an important collaboration between paxillin and FAK signaling in the modulation of microtubule acetylation, as well as centrosome and Golgi organization and polarization. Finally, paxillin was also shown to be required for optimal anterograde vesicular trafficking to the plasma membrane.

Silencing of β1 integrin regulates airway remodeling by regulating the transcription of SOCE‑associated genes in asthmatic mice

The incidence of asthma is increasing globally; however, current treatments are only able to cure a certain proportion of patients. There is an urgent need to develop novel therapies. β1 integrin serves a role in the pathophysiology of asthma through the development of airway remodeling. The aim of the present study was to investigate silencing of the β1 integrin gene in pre-clinical models of allergic asthma. BALB/c mice were sensitized with ovalbumin through intraperitoneal injection and repeated aerosolized ovalbumin. A short hairpin RNA of the β1 integrin gene was designed and transfected into mouse models of asthma in vivo, in order to evaluate whether silencing of the β1 integrin gene affects airway smooth muscle cell proliferation and inflammation by regulating the mRNA expression of store-operated Ca²⁺ entry (SOCE)-associated genes. Silencing the β1 integrin gene may downregulate β1 integrin mRNA while not statistically decreasing α-smooth muscle actin gene expression and airway smooth muscle thickness. β1 integrin silencing was able to downregulate the transcription of SOCE-associated genes to normal levels, including calcium release-activated calcium modulator 1 and short transient receptor potential channel member 1, but not stromal interaction molecule 1, in asthma. Silencing of the β1 integrin gene additionally maintained nuclear factor of activated T-cells cytoplasmic 1 gene expression, and inflammatory cytokines interleukin-4 and interferon-γ at normal levels. The results of the present study provide evidence to suggest that silencing of the β1 integrin gene may be of therapeutic benefit for patients with asthma.

Integrin β1 mediates vaccinia virus entry through activation of PI3K/Akt signaling

Vaccinia virus has a broad range of infectivity in many cell lines and animals. Although it is known that the vaccinia mature virus binds to cell surface glycosaminoglycans and extracellular matrix proteins, whether additional cellular receptors are required for virus entry remains unclear. Our previous studies showed that the vaccinia mature virus enters through lipid rafts, suggesting the involvement of raft-associated cellular proteins. Here we demonstrate that one lipid raft-associated protein, integrin β1, is important for vaccinia mature virus entry into HeLa cells. Vaccinia virus associates with integrin β1 in lipid rafts on the cell surface, and the knockdown of integrin β1 in HeLa cells reduces vaccinia mature virus entry. Additionally, vaccinia mature virus infection is reduced in a mouse cell line, GD25, that is deficient in integrin β1 expression. Vaccinia mature virus infection triggers the activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling, and the treatment of cells with inhibitors to block P13K activation reduces virus entry in an integrin β1-dependent manner, suggesting that integrin β1-mediates PI3K/Akt activation induced by vaccinia virus and that this signaling pathway is essential for virus endocytosis. The inhibition of integrin β1-mediated cell adhesion results in a reduction of vaccinia virus entry and the disruption of focal adhesion and PI3K/Akt activation. In summary, our results show that the binding of vaccinia mature virus to cells mimics the outside-in activation process of integrin functions to facilitate vaccinia virus entry into HeLa cells.

Quantitative proteomics of the integrin adhesome show a myosin II-dependent recruitment of LIM domain proteins

A characteristic of integrins is their ability to transfer chemical and mechanical signals across the plasma membrane. Force generated by myosin II makes cells able to sense substrate stiffness and induce maturation of nascent adhesions into focal adhesions. In this paper, we present a comprehensive proteomic analysis of nascent and mature adhesions. The purification of integrin adhesion complexes combined with quantitative mass spectrometry enabled the identification and quantification of known and new adhesion-associated proteins. Furthermore, blocking adhesion maturation with the myosin II inhibitor blebbistatin markedly impaired the recruitment of LIM domain proteins to integrin adhesion sites. This suggests a common recruitment mechanism for a whole class of adhesion-associated proteins, involving myosin II and the zinc-finger-type LIM domain.

Integrin regulation of cytoplasmic calcium in excitatory neurons depends upon glutamate receptors and release from intracellular stores

Integrins regulate cytoplasmic calcium levels ([Ca(2+)]i) in various cell types but information on activities in neurons is limited. The issue is of current interest because of the evidence that both integrins and changes in [Ca(2+)]i are required for Long-Term Potentiation. Accordingly, the present studies evaluated integrin ligand effects in cortical neurons. Integrin ligands or alpha5beta1 integrin activating antisera rapidly increased [Ca(2+)]i with effects greater in glutamatergic than GABAergic neurons, absent in astroglia, and blocked by beta1 integrin neutralizing antisera and the tyrosine kinase antagonist genistein. Increases depended upon extracellular calcium and intracellular store release. Ligand-induced effects were reduced by voltage-sensitive calcium channel and NMDA receptor antagonists, but blocked by tetrodotoxin or AMPA receptor antagonists. These results indicate that integrin ligation triggers AMPA receptor/depolarization-dependent calcium influx followed by intracellular store release and suggest the possibility that integrin modulation of activity-induced changes in [Ca(2+)]i contributes importantly to lasting synaptic plasticity in forebrain neurons.

Integrin adhesion in regulation of lacrimal gland acinar cell secretion

The extracellular microenvironment regulates lacrimal gland acinar cell secretion. Culturing isolated rabbit lacrimal gland acinar cells on different extracellular matrix proteins revealed that laminin enhances carbachol-stimulated secretion to a greater extent than other extracellular matrix proteins investigated. Furthermore, immunofluorescence indicated that integrin subunits, potentially functioning as laminin receptors are present in acinar cells. Among these, the integrin alpha6 and beta1 subunit mRNA expression was also confirmed by RT-PCR and sequence analysis. Secretion assays, which measured beta-hexosaminidase activity released in the culture media, demonstrated that function-blocking integrin alpha6 and beta1 monoclonal antibodies (mAbs) induce a rapid, transient and dose-dependent secretory response in cultured cells. To determine the intracellular pathways by which integrin alpha6 and beta1 mAbs could induce secretion, selected second messenger molecules were inhibited. Although inhibitors of protein kinase C and IP(3)-induced Ca(2+) mobilization attenuated carbachol-stimulated secretion, no effect on integrin mAb-induced release was observed. In addition, protein tyrosine kinases do not appear to have a role in transducing signals arising from mAb interactions. Our data clearly demonstrate, though, that cell adhesion through integrins regulates secretion from lacrimal gland acinar cells. The fact that the integrin mAbs affect the cholinergic response differently and that the integrin beta1 mAb secretion, but not the alpha6, was attenuated by the phosphatase inhibitor, sodium orthovanadate, suggests that each subunit utilizes separate intracellular signaling pathways to induce exocytosis. The results also indicate that the secretory response triggered by the beta1 integrin mAb is generated through dephosphorylation events.

Integrin-dependent PLC-γ1 phosphorylation mediates fibronectin-dependent adhesion

Although integrin engagement initiates signaling events such as focal-adhesion kinase (FAK) and Src kinase activation, the role of phosphoinositide turnover in cell adhesion is less clear. To assess PLC-γ1 function in this process, Plcg1-/- fibroblasts (Null) were compared with the same fibroblasts in which PLC-γ1 was re-expressed (Null+). Following plating on fibronectin, Null cells displayed a significantly impaired rate of adhesion compared with Null+ cells. This defect was detected at low concentrations of fibronectin; at high fibronectin concentrations, the Null and Null+ cells displayed equivalent adhesion characteristics. The differences were not due to PLC-γ1-dependent changes in integrin subunit expression, nor was integrin receptor clustering impaired with the absence of PLC-γ1. Experiments with site-specific antibodies and PLC-γ1 mutants showed that fibronectin selectively increased phosphorylation of Tyr783 and that mutagenesis of this residue, but not Tyr771 or Tyr1253, abrogated fibronectin-dependent adhesion. The SH2 domains of PLC-γ1 were also required for maximal adhesion on fibronectin. Adhesion to fibronectin induced PLC-γ1 tyrosine phosphorylation that was inhibited by a Src-kinase inhibitor, but not an epidermal-growth-factor-receptor kinase inhibitor. Moreover, in cells null for Src family members, but not in cells null for FAK family members, integrin-dependent PLC-γ1 tyrosine phosphorylation was greatly reduced. Finally, the data demonstrated that PLC-γ1 co-immunoprecipitated with Src following fibronectin-induced integrin activation, and this association did not depend on FAK expression.

Adhesion of B cell lines to endothelial cells from human lymphoid tissue modulates tyrosine phosphorylation and endothelial cell activation

Through the production of cytokines and growth factors the endothelium of secondary lymphoid organs plays a crucial role in controlling lymphocyte migration to the lymphoid microenvironment, an essential step in the initiation of the immune response. Here we demonstrate that direct contact of B cell lines with tonsil-derived human endothelial cells resulted in changes in the phosphorylation state of endothelial cells, causing their functional activation. We found a rapid (<15-s) and transient dephosphorylation, followed by a rapid rephosphorylation of tyrosine residues of the focal adhesion kinase, paxillin, and ERK2. Maximal rephosphorylation occurred after 15-30 min of B cell contact. Preincubation of lymphoid B cells with an adhesion-blocking Ab directed against alpha(4)beta(1) integrin abrogated adhesion-mediated changes of endothelial cell tyrosine phosphorylation, suggesting that cell contact was essential. Similar patterns of tyrosine phosphorylation, but with slightly different kinetics were induced after cross-linking of beta(1) integrin or CD40 on endothelial cells. Functional activation of endothelial cells by B cell adhesion was confirmed by the production of IL-6, IL-8, monocyte chemoattractant protein-1, M-CSF, and macrophage inflammatory protein-1beta mRNA. However, direct cross-linking of beta(1) integrin and CD40 failed to accomplish the same functional activation. These data indicate that direct contact of lymphoid B cells with the endothelium from lymphoid tissue induce endothelial cell signaling, resulting in chemokine and cytokine production. This phenomenon may provide a mechanism for the remodeling of the endothelium from lymphoid tissues, thus contributing to the free migration of lymphocytes and other cells into the lymphoid organs.

Mechanisms involved in alpha6beta1-integrin-mediated Ca(2+) signalling

Contact of Jurkat T-lymphocytes with the extracellular matrix (ECM) protein laminin resulted in long-lasting alpha6beta1-integrin-mediated Ca(2+) signalling. Both Ca(2+) release from thapsigargin-sensitive Ca(2+) stores and capacitative Ca(2+) entry via Ca(2+) channels sensitive to SKF 96365 constitute important parts of this process. Inhibition of alpha6beta1-integrin-mediated Ca(2+) signalling by (1) the src kinase inhibitor PP2, (2) the PLC inhibitor U73122, and (3) the cyclic adenosine diphosphoribose (cADPR) antagonist 7-deaza-8-Br-cADPR indicate the involvement of src tyrosine kinases and the Ca(2+)-releasing second messengers D-myo-inositol 1,4,5-trisphosphate (InsP3) and cADPR.