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Integrins, the extracellular matrix receptors that facilitate cell adhesion and migration, are necessary for organ morphogenesis; however, their role in maintaining adult tissue homeostasis is poorly understood. To define the functional importance of β1 integrin in adult mouse lung, we deleted it after completion of development in type 2 alveolar epithelial cells (AECs). Aged β1 integrin-deficient mice exhibited chronic obstructive pulmonary disease-like (COPD-like) pathology characterized by emphysema, lymphoid aggregates, and increased macrophage infiltration. These histopathological abnormalities were preceded by β1 integrin-deficient AEC dysfunction such as excessive ROS production and upregulation of NF-κB-dependent chemokines, including CCL2. Genetic deletion of the CCL2 receptor, Ccr2, in mice with β1 integrin-deficient type 2 AECs impaired recruitment of monocyte-derived macrophages and resulted in accelerated inflammation and severe premature emphysematous destruction. The lungs exhibited reduced AEC efferocytosis and excessive numbers of inflamed type 2 AECs, demonstrating the requirement for recruited monocytes/macrophages in limiting lung injury and remodeling in the setting of a chronically inflamed epithelium. These studies support a critical role for β1 integrin in alveolar homeostasis in the adult lung.
BACKGROUND - Systemic inflammation and muscle wasting are highly prevalent and coexist in patients on maintenance hemodialysis (MHD). We aimed to determine the effects of systemic inflammation on skeletal muscle protein metabolism in MHD patients.
METHODS - Whole body and skeletal muscle protein turnover were assessed by stable isotope kinetic studies. We incorporated expressions of E1, E214K, E3αI, E3αII, MuRF-1, and atrogin-1 in skeletal muscle tissue from integrin β1 gene KO CKD mice models.
RESULTS - Among 129 patients with mean (± SD) age 47 ± 12 years, 74% were African American, 73% were male, and 22% had diabetes mellitus. Median high-sensitivity C-reactive protein (hs-CRP) concentration was 13 (interquartile range 0.8, 33) mg/l. There were statistically significant associations between hs-CRP and forearm skeletal muscle protein synthesis, degradation, and net forearm skeletal muscle protein balance (P < 0.001 for all). The associations remained statistically significant after adjustment for clinical and demographic confounders, as well as in sensitivity analysis, excluding patients with diabetes mellitus. In attempting to identify potential mechanisms involved in this correlation, we show increased expressions of E1, E214K, E3αI, E3αII, MuRF-1, and atrogin-1 in skeletal muscle tissue obtained from an animal model of chronic kidney disease.
CONCLUSION - These data suggest that systemic inflammation is a strong and independent determinant of skeletal muscle protein homeostasis in MHD patients, providing rationale for further studies using anticytokine therapies in patients with underlying systemic inflammation.
FUNDING - This study was in part supported by NIH grants R01 DK45604 and 1K24 DK62849, the Clinical Translational Science Award UL1-TR000445 from the National Center for Advancing Translational Sciences, the Veterans Administration Merit Award I01 CX000414, the SatelliteHealth Normon Coplon Extramural Grant Program, and the FDA grant 000943.
Kidney collecting system development requires integrin-dependent cell-extracellular matrix interactions. Integrins are heterodimeric transmembrane receptors consisting of α and β subunits; crucial integrins in the kidney collecting system express the β1 subunit. The β1 cytoplasmic tail has two NPxY motifs that mediate functions by binding to cytoplasmic signaling and scaffolding molecules. Talins, scaffolding proteins that bind to the membrane proximal NPxY motif, are proposed to activate integrins and to link them to the actin cytoskeleton. We have defined the role of talin binding to the β1 proximal NPxY motif in the developing kidney collecting system in mice that selectively express a Y-to-A mutation in this motif. The mice developed a hypoplastic dysplastic collecting system. Collecting duct cells expressing this mutation had moderate abnormalities in cell adhesion, migration, proliferation and growth factor-dependent signaling. In contrast, mice lacking talins in the developing ureteric bud developed kidney agenesis and collecting duct cells had severe cytoskeletal, adhesion and polarity defects. Thus, talins are essential for kidney collecting duct development through mechanisms that extend beyond those requiring binding to the β1 integrin subunit NPxY motif.
© 2017. Published by The Company of Biologists Ltd.
Apicobasolateral polarity is a fundamental property of epithelial cells, and its loss is a hallmark of cancer. Integrin-mediated contact with the extracellular matrix defines the basal surface, setting in motion E-cadherin-mediated cell-cell contact, which establishes apicobasolateral polarity. Role(s) for lateral integrins in this polarization process and the consequences of their disruption are incompletely understood. We show that addition of an integrin β1-activating monoclonal antibody, P4G11, to invasive colorectal cancer cells in three-dimensional type 1 collagen reverts the invasive phenotype and restores apicobasolateral polarity. P4G11 induces clustering of integrin α5β1 at lateral, intercellular surfaces. This leads to deposition and polymerization of fibronectin and recruitment of paxillin to sites of lateral integrin α5β1 clustering and is followed by tight junction formation, as determined by ZO-1 localization. Inducible elimination of integrin α5 abrogates the epithelial-organizing effects of P4G11. In addition, polymerization of fibronectin is required for the effects of P4G11, and addition of polymerized superfibronectin is sufficient to induce tight junction formation and apicobasolateral polarization. In the normal human colon, we show that integrin α5 localizes to the lateral membrane of terminally differentiated colonocytes and that integrin α5 staining may be reduced in colorectal cancer. Thus we propose a novel role for integrin α5β1 in regulating epithelial morphogenesis.
© 2017 Starchenko et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).
Integrins are transmembrane receptors composed of α and β subunits. Although most integrins contain β1, canonical activation mechanisms are based on studies of the platelet integrin, αIIbβ3. Its inactive conformation is characterized by the association of the αIIb transmembrane and cytosolic domain (TM/CT) with a tilted β3 TM/CT that leads to activation when disrupted. We show significant structural differences between β1 and β3 TM/CT in bicelles. Moreover, the 'snorkeling' lysine at the TM/CT interface of β subunits, previously proposed to regulate αIIbβ3 activation by ion pairing with nearby lipids, plays opposite roles in β1 and β3 integrin function and in neither case is responsible for TM tilt. A range of affinities from almost no interaction to the relatively high avidity that characterizes αIIbβ3 is seen between various α subunits and β1 TM/CTs. The αIIbβ3-based canonical model for the roles of the TM/CT in integrin activation and function clearly does not extend to all mammalian integrins.
Laminin peptides influence tumor behavior. In this study, we addressed whether laminin peptide C16 (KAFDITYVRLKF, γ1 chain) would increase invadopodia activity of cells from squamous cell carcinoma (CAL27) and fibrosarcoma (HT1080). We found that C16 stimulates invadopodia activity over time in both cell lines. Rhodamine-conjugated C16 decorates the edge of cells, suggesting a possible binding to membrane receptors. Flow cytometry showed that C16 increases activated β1 integrin, and β1 integrin miRNA-mediated depletion diminishes C16-induced invadopodia activity in both cell lines. C16 stimulates Src and ERK 1/2 phosphorylation, and ERK 1/2 inhibition decreases peptide-induced invadopodia activity. C16 also increases cortactin phosphorylation in both cells lines. Based on our findings, we propose that C16 regulates invadopodia activity over time of squamous carcinoma and fibrosarcoma cells, probably through β1 integrin, Src and ERK 1/2 signaling pathways.
Integrins require an activation step prior to ligand binding and signaling. How talin and kindlin contribute to these events in non-hematopoietic cells is poorly understood. Here we report that fibroblasts lacking either talin or kindlin failed to activate β1 integrins, adhere to fibronectin (FN) or maintain their integrins in a high affinity conformation induced by Mn(2+). Despite compromised integrin activation and adhesion, Mn(2+) enabled talin- but not kindlin-deficient cells to initiate spreading on FN. This isotropic spreading was induced by the ability of kindlin to directly bind paxillin, which in turn bound focal adhesion kinase (FAK) resulting in FAK activation and the formation of lamellipodia. Our findings show that talin and kindlin cooperatively activate integrins leading to FN binding and adhesion, and that kindlin subsequently assembles an essential signaling node at newly formed adhesion sites in a talin-independent manner.
The current paradigm suggests that Yersinia enterocolitica (Ye) adheres to host cells via the outer membrane proteins Yersinia adhesin A (YadA) or invasin (Inv) to facilitate injection of Yops by the type III secretion system. In this process Inv binds directly to β1 integrins of host cells while YadA may bind indirectly via extracellular matrix proteins to β1 integrins. Here we challenged this paradigm and investigated the requirements for Yop injection. We demonstrate that Inv- but not YadA-mediated adhesion depends on β1 integrin binding and activation, and that tight adhesion is a prerequisite for Yop injection. By means of novel transgenic cell lines, shRNA approaches and RGD peptides, we found that YadA, in contrast to Inv, may use a broad host cell receptor repertoire for host cell adhesion. In the absence of β1 integrins, YadA mediates Yop injection by interaction with αV integrins in cooperation with yet unknown cofactors expressed by epithelial cells, but not fibroblasts. Electron microscopic and flow chamber studies revealed that a defined intimate contact area between Ye and host cells resulting in adhesion forces resisting shear stress is required for Yop injection. Thus, the indirect binding of YadA to a broad extracellular matrix (ECM) binding host cell receptor repertoire of different cell types makes YadA a versatile tool to ensure Yop injection. In conclusion, given the differential expression of the outer membrane proteins Inv and YadA in the course of Ye infection and differential expression of integrins by various host cell populations, the data demonstrate that Ye is flexibly armed to accomplish Yop injection in different host cell types, a central event in its immune evasion strategy.
© 2015 John Wiley & Sons Ltd.
Integrin-dependent interactions between cells and extracellular matrix regulate lung development; however, specific roles for β1-containing integrins in individual cell types, including epithelial cells, remain incompletely understood. In this study, the functional importance of β1 integrin in lung epithelium during mouse lung development was investigated by deleting the integrin from E10.5 onwards using surfactant protein C promoter-driven Cre. These mutant mice appeared normal at birth but failed to gain weight appropriately and died by 4 months of age with severe hypoxemia. Defects in airway branching morphogenesis in association with impaired epithelial cell adhesion and migration, as well as alveolarization defects and persistent macrophage-mediated inflammation were identified. Using an inducible system to delete β1 integrin after completion of airway branching, we showed that alveolarization defects, characterized by disrupted secondary septation, abnormal alveolar epithelial cell differentiation, excessive collagen I and elastin deposition, and hypercellularity of the mesenchyme occurred independently of airway branching defects. By depleting macrophages using liposomal clodronate, we found that alveolarization defects were secondary to persistent alveolar inflammation. β1 integrin-deficient alveolar epithelial cells produced excessive monocyte chemoattractant protein 1 and reactive oxygen species, suggesting a direct role for β1 integrin in regulating alveolar homeostasis. Taken together, these studies define distinct functions of epithelial β1 integrin during both early and late lung development that affect airway branching morphogenesis, epithelial cell differentiation, alveolar septation and regulation of alveolar homeostasis.
© 2014. Published by The Company of Biologists Ltd.
Stimulation of endogenous β-cell expansion could facilitate regeneration in patients with diabetes. In mice, connective tissue growth factor (CTGF) is expressed in embryonic β-cells and in adult β-cells during periods of expansion. We discovered that in embryos CTGF is necessary for β-cell proliferation, and increased CTGF in β-cells promotes proliferation of immature (MafA(-)) insulin-positive cells. CTGF overexpression, under nonstimulatory conditions, does not increase adult β-cell proliferation. In this study, we tested the ability of CTGF to promote β-cell proliferation and regeneration after partial β-cell destruction. β-Cell mass reaches 50% recovery after 4 weeks of CTGF treatment, primarily via increased β-cell proliferation, which is enhanced as early as 2 days of treatment. CTGF treatment increases the number of immature β-cells but promotes proliferation of both mature and immature β-cells. A shortened β-cell replication refractory period is also observed. CTGF treatment upregulates positive cell-cycle regulators and factors involved in β-cell proliferation, including hepatocyte growth factor, serotonin synthesis, and integrin β1. Ex vivo treatment of whole islets with recombinant human CTGF induces β-cell replication and gene expression changes consistent with those observed in vivo, demonstrating that CTGF acts directly on islets to promote β-cell replication. Thus, CTGF can induce replication of adult mouse β-cells given a permissive microenvironment.
© 2015 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered.