Above the Matrix: Functional Roles for Apically Localized Integrins

EspP2 Regulates the Adhesion of Glaesserella parasuis via Rap1 Signaling Pathway

Different levels of EspP2 expression are seen in strains of Glaesserella parasuis with high and low pathogenicity. As a potential virulence factor for G. parasuis, the pathogenic mechanism of EspP2 in infection of host cells is not clear. To begin to elucidate the effect of EspP2 on virulence, we used G. parasuis SC1401 in its wild-type form and SC1401, which was made EspP2-deficient. We demonstrated that EspP2 causes up-regulation of claudin-1 and occludin expression, thereby promoting the adhesion of G. parasuis to host cells; EspP2-deficiency resulted in significantly reduced adhesion of G. parasuis to cells. Transcriptome sequencing analysis of EspP2-treated PK15 cells revealed that the Rap1 signaling pathway is stimulated by EspP2. Blocking this pathway diminished occludin expression and adhesion. These results indicated that EspP2 regulates the adhesion of Glaesserella parasuis via Rap1 signaling pathway.

Sarcoma Cells Secrete Hypoxia-Modified Collagen VI to Weaken the Lung Endothelial Barrier and Promote Metastasis

Intratumoral hypoxia correlates with metastasis and poor survival in patients with sarcoma. Using an impedance sensing assay and a zebrafish intravital microinjection model, we demonstrated here that the hypoxia-inducible collagen-modifying enzyme lysyl hydroxylase PLOD2 and its substrate collagen type VI (COLVI) weaken the lung endothelial barrier and promote transendothelial migration. Mechanistically, hypoxia-induced PLOD2 in sarcoma cells modified COLVI, which was then secreted into the vasculature. Upon reaching the apical surface of lung endothelial cells, modified COLVI from tumor cells activated integrin β1 (ITGβ1). Furthermore, activated ITGβ1 colocalized with Kindlin2, initiating their interaction with F-actin and prompting its polymerization. Polymerized F-actin disrupted endothelial adherens junctions and induced barrier dysfunction. Consistently, modified and secreted COLVI was required for the late stages of lung metastasis in vivo. Analysis of patient gene expression and survival data from The Cancer Genome Atlas (TCGA) revealed an association between the expression of both PLOD2 and COLVI and patient survival. Furthermore, high levels of COLVI were detected in surgically resected sarcoma metastases from patient lungs and in the blood of tumor-bearing mice. Together, these data identify a mechanism of sarcoma lung metastasis, revealing opportunities for therapeutic intervention.

SIGNIFICANCE

Collagen type VI modified by hypoxia-induced PLOD2 is secreted by sarcoma cells and binds to integrin β1 on endothelial cells to induce barrier dysfunction, which promotes sarcoma vascular dissemination and metastasis.

Apical dehydration impairs the cystic fibrosis airway epithelium barrier via a β1-integrin/YAP1 pathway

Defective hydration of airway surface mucosa is associated with lung infection in cystic fibrosis (CF), partly caused by disruption of the epithelial barrier integrity. Although rehydration of the CF airway surface liquid (ASL) alleviates epithelium vulnerability to infection by junctional protein expression, the mechanisms linking ASL to barrier integrity are unknown. We show here the strong degradation of YAP1 and TAZ proteins in well-polarized CF human airway epithelial cells (HAECs), a process that was prevented by ASL rehydration. Conditional silencing of YAP1 in rehydrated CF HAECs indicated that YAP1 expression was necessary for the maintenance of junctional complexes. A higher plasma membrane tension in CF HAECs reduced endocytosis, concurrent with the maintenance of active β1-integrin ectopically located at the apical membrane. Pharmacological inhibition of β1-integrin accumulation restored YAP1 expression in CF HAECs. These results indicate that dehydration of the CF ASL affects epithelial plasma membrane tension, resulting in ectopic activation of a β1-integrin/YAP1 signaling pathway associated with degradation of junctional proteins.

Upregulation of occludin by cytolethal distending toxin facilitates Glaesserella parasuis adhesion to respiratory tract cells

Virulent Glaesserella parasuis may engender systemic infection characterized by fibrinous polyserositis and pneumonia. G. parasuis causes systemic disease through upper respiratory tract infection, but the mechanism has not been fully characterized. Tight junction (TJ) proteins maintain the integrity and impermeability of the epithelial barriers. In this work, we applied the recombinant cytolethal distending toxin (CDT) holotoxin and cdt-deficient mutants to assess whether CDT interacted with TJ proteins of airway tract cells. Our results indicated that CDT induced the TJ occludin (OCLN) expression in newborn pig tracheal epithelial cells within the first 3 hours of bacterial infection, followed by a significant decrease. Overexpression of OCLN in target cells made them more susceptible to G. parasuis adhesion, whereas ablation of OCLN expression by CRISPR/Cas 9 gene editing technology in target cells decreased their susceptibility to bacterial adhesion. In addition, CDT treatment could upregulate the OCLN levels in the lung tissue of C57/BL6 mice. In summary, highly virulent G. parasuis strain SC1401 stimulated the tight junction expression, resulting in higher bacterial adhesion to respiratory tract cells, and this process is closely related to CDT. Our results may provide novel insights into G. parasuis infection and CDT-mediated pathogenesis.

Stepwise modulation of apical orientational cell adhesions for vertebrate neurulation

Neurulation transforms the neuroectoderm into the neural tube. This transformation relies on reorganising the configurational relationships between the orientations of intrinsic polarities of neighbouring cells. These orientational intercellular relationships are established, maintained, and modulated by orientational cell adhesions (OCAs). Here, using zebrafish (Danio rerio) neurulation as a major model, we propose a new perspective on how OCAs contribute to the parallel, antiparallel, and opposing intercellular relationships that underlie the neural plate-keel-rod-tube transformation, a stepwise process of cell aggregation followed by cord hollowing. We also discuss how OCAs in neurulation may be regulated by various adhesion molecules, including cadherins, Eph/Ephrins, Claudins, Occludins, Crumbs, Na+ /K+ -ATPase, and integrins. By comparing neurulation among species, we reveal that antiparallel OCAs represent a conserved mechanism for the fusion of the neural tube. Throughout, we highlight some outstanding questions regarding OCAs in neurulation. Answers to these questions will help us understand better the mechanisms of tubulogenesis of many tissues.

CC16 drives VLA-2-dependent SPLUNC1 expression

Rationale

CC16 (Club Cell Secretory Protein) is a protein produced by club cells and other non-ciliated epithelial cells within the lungs. CC16 has been shown to protect against the development of obstructive lung diseases and attenuate pulmonary pathogen burden. Despite recent advances in understanding CC16 effects in circulation, the biological mechanisms of CC16 in pulmonary epithelial responses have not been elucidated.

Objectives

We sought to determine if CC16 deficiency impairs epithelial-driven host responses and identify novel receptors expressed within the pulmonary epithelium through which CC16 imparts activity.

Methods

We utilized mass spectrometry and quantitative proteomics to investigate how CC16 deficiency impacts apically secreted pulmonary epithelial proteins. Mouse tracheal epithelial cells (MTECS), human nasal epithelial cells (HNECs) and mice were studied in naïve conditions and after Mp challenge.

Measurements and main results

We identified 8 antimicrobial proteins significantly decreased by CC16-/- MTECS, 6 of which were validated by mRNA expression in Severe Asthma Research Program (SARP) cohorts. Short Palate Lung and Nasal Epithelial Clone 1 (SPLUNC1) was the most differentially expressed protein (66-fold) and was the focus of this study. Using a combination of MTECs and HNECs, we found that CC16 enhances pulmonary epithelial-driven SPLUNC1 expression via signaling through the receptor complex Very Late Antigen-2 (VLA-2) and that rCC16 given to mice enhances pulmonary SPLUNC1 production and decreases Mycoplasma pneumoniae (Mp) burden. Likewise, rSPLUNC1 results in decreased Mp burden in mice lacking CC16 mice. The VLA-2 integrin binding site within rCC16 is necessary for induction of SPLUNC1 and the reduction in Mp burden.

Conclusion

Our findings demonstrate a novel role for CC16 in epithelial-driven host defense by up-regulating antimicrobials and define a novel epithelial receptor for CC16, VLA-2, through which signaling is necessary for enhanced SPLUNC1 production.

Storming the gate: New approaches for targeting the dynamic tight junction for improved drug delivery

As biologics used in the clinic outpace the number of new small molecule drugs, an important challenge for their efficacy and widespread use has emerged, namely tissue penetrance. Macromolecular drugs - bulky, high-molecular weight, hydrophilic agents - exhibit low permeability across biological barriers. Epithelial and endothelial layers, for example within the gastrointestinal tract or at the blood-brain barrier, present the most significant obstacle to drug transport. Within epithelium, two subcellular structures are responsible for limiting absorption: cell membranes and intercellular tight junctions. Previously considered impenetrable to macromolecular drugs, tight junctions control paracellular flux and dictate drug transport between cells. Recent work, however, has shown tight junctions to be dynamic, anisotropic structures that can be targeted for delivery. This review aims to summarize new approaches for targeting tight junctions, both directly and indirectly, and to highlight how manipulation of tight junction interactions may help usher in a new era of precision drug delivery.

Naturally Occurring N-Terminal Fragments of Bovine Milk Osteopontin Are Transported across Models of the Intestinal Barrier

Osteopontin (OPN) is a bioactive integrin-binding protein found in high concentrations in milk, where it is present both as a full-length protein and as several N-terminally derived fragments. OPN resists gastric digestion, and via interaction with receptors in the gut or by crossing the intestinal barrier into circulation, ingested milk OPN may influence physiological processes. The aim of this study was to investigate OPN interaction with intestinal cells and its transport across models of the intestinal barrier. Immunodetection of OPN incubated with Caco-2 cells at 4 °C and 37 °C showed that OPN binds to the intestinal cells, but it is not internalised. Transepithelial transport was studied using mono- and co-cultures of Caco-2 cells and mucus-producing HT29-MTX cells in transwell membranes. OPN was shown to cross the barrier models in a time-, temperature-, and energy-dependent process inhibited by wortmannin, indicating that the transport takes place via the transcytosis pathway. Analyses of the naturally occurring milk mixture of full-length and N-terminal fragments showed that the N-terminal fragments of OPN bound intestinal cells most effectively and that the fragments were transported across the intestinal membrane models. This suggests that proteolytic processing of OPN increases its biological activity after ingestion.

The Effect of Human and Bovine Milk Osteopontin on Intestinal Caco-2 Cells: A Transcriptome Comparison

Osteopontin (OPN) is a multifunctional protein abundantly present in human milk, whereas the concentration is significantly lower in bovine milk. Human and bovine milk OPN are structurally similar and both proteins resist gastric digestion and reach the intestines in a bioactive form. Intervention studies have indicated the beneficial effects of supplementing infant formula with bovine milk OPN and several in vivo and in vitro studies have shown that bovine milk OPN positively influences intestinal development. To investigate the functional relationship, we compared the effect of simulated gastrointestinal digested human and bovine milk OPN on gene expression in Caco-2 cells. After incubation, total RNA was extracted and sequenced and transcripts were mapped to the human genome. Human and bovine milk OPN regulated the expression of 239 and 322 genes, respectively. A total of 131 genes were similarly regulated by the OPNs. As a control, a whey protein fraction with a high content of alpha-lactalbumin had a very limited transcriptional impact on the cells. Enrichment data analysis showed that biological processes related to the ubiquitin system, DNA binding, and genes associated with transcription and transcription control pathways were affected by the OPNs. Collectively, this study shows that human and bovine milk OPN have a significant and highly comparable effect on the intestinal transcriptome.

Nanoarchitecture and molecular interactions of epithelial cell junction proteins revealed by super-resolution microscopy

Epithelial cells are polarized with defined apical tight junctions (TJs), lateral adherens junctions (AJs), and basal integrin-matrix interactions. However, it is increasingly recognized that resident cell junction proteins can be found in varying locations and with previously unrecognized functions. Our study here presents the nanoarchitecture and nanocolocalization of cell junction proteins in culture and tissue by stochastic optical reconstruction microscopy (STORM). The Z-axial view of noncancerous MDCK-II and PZ-HPV-7 cell-cell junctions resolved β-catenin and p120^ctn localizations to TJs and AJs, with p120^ctn apical to β-catenin and colocalizing with TJ protein claudin-7. More basally, p120^ctn and β-catenin become colocalized. This topography was lost in isogenic Ras-transformed MDCK cells and cancerous PC3 cells, where p120^ctn becomes basally localized in relation to β-catenin. Claudin-7 gene conditional knockout (cKO) in mice also have altered polarity of p120^ctn relative to β-catenin, like that seen in normal-to-cancer cell phenotypic transformation. Additionally, claudin-7 cKO resulted in redistribution and relocalization of other cell junction proteins, including claudin-1, zonula occludens-1, integrin α2, epithelial cell adhesion molecule, and focal adhesion kinase (FAK); specifically, integrin α2 and FAK were observed at the apical-lateral compartment. Our data show that STORM reveals regional cellular junction nanoarchitecture previously uncharacterized, providing new insight into potential trans-compartmental modulation of protein functions.

An innovative strategy to identify new targets for delivering antibodies to the brain has led to the exploration of the integrin family

Background

Increasing brain exposure of biotherapeutics is key to success in central nervous system disease drug discovery. Accessing the brain parenchyma is especially difficult for large polar molecules such as biotherapeutics and antibodies because of the blood-brain barrier. We investigated a new immunization strategy to identify novel receptors mediating transcytosis across the blood-brain barrier.

Method

We immunized mice with primary non-human primate brain microvascular endothelial cells to obtain antibodies. These antibodies were screened for their capacity to bind and to be internalized by primary non-human primate brain microvascular endothelial cells and Human Cerebral Microvascular Endothelial Cell clone D3. They were further evaluated for their transcytosis capabilities in three in vitro blood-brain barrier models. In parallel, their targets were identified by two different methods and their pattern of binding to human tissue was investigated using immunohistochemistry.

Results

12 antibodies with unique sequence and internalization capacities were selected amongst more than six hundred. Aside from one antibody targeting Activated Leukocyte Cell Adhesion Molecule and one targeting Striatin3, most of the other antibodies recognized β1 integrin and its heterodimers. The antibody with the best transcytosis capabilities in all blood-brain barrier in vitro models and with the best binding capacity was an anti-αnβ1 integrin. In comparison, commercial anti-integrin antibodies performed poorly in transcytosis assays, emphasizing the originality of the antibodies derived here. Immunohistochemistry studies showed specific vascular staining on human and non-human primate tissues.

Conclusions

This transcytotic behavior has not previously been reported for anti-integrin antibodies. Further studies should be undertaken to validate this new mechanism in vivo and to evaluate its potential in brain delivery.

Cytostatic Effects of Polyethyleneimine Surfaces on the Mesenchymal Stromal Cell Cycle

Polyelectrolytes assembled layer-by-layer (PEMs) are commonly used as functional coatings to build-up biological interfaces, particularly suitable as compatible layers for the interaction with a biological medium, providing suitable conditions to promote or prevent cell seeding while maintaining the phenotype. The proper assessment of the biocompatibility of PEMs and the elucidation of the related mechanisms are therefore of paramount importance. In this study, we report in detail the effect of two different PEM endings, polystyrene sulfonate (PSS) and polyethylenimine (PEI), respectively, on the cell adhesion, growth, and viability of human bone mesenchymal stromal cells (MSCs). The results have shown that PSS-ended substrates appear to be the most suitable to drive the cell adhesion and phenotype maintenance of MSCs, showing good biocompatibility. On the contrary, while the cells seem to adhere more quickly and strongly on the PEI-ended surfaces, the interaction with PEI significantly affects the growth and viability, reducing the cell spreading capability, by sequestering the adhesion molecules already in the very early steps of cell–substrate contact. These results point to the promotion of a cytostatic effect of PEI, rather than the often-claimed cytotoxicity.

The potential role of integrin alpha 6 in human mesenchymal stem cells

Human mesenchymal stem cells (MSCs) are isolated from various adult and perinatal tissues. Although mesenchymal stem cells from multiple sources exhibit similar morphology and cell surface markers, they differ in their properties. In this study, we determined that the expression of integrin alpha 6 (ITGA6) and ITGA6 antisense RNA (ITGA6-AS1) correlates with the proliferation, cell size, and differentiation potential. The expression of ITGA6 was inversely correlated with ITGA6-AS1 in MSCs. The expression of ITGA6 was higher, but ITGA6-AS1 was lower in MSCs from cord placenta junction, cord tissue, and Wharton's jelly. In contrast, ITGA6 expression was lower, while ITGA6-AS1 was higher in MSCs from the placenta. The bioinformatic analysis showed that ITGA6 genomic DNA transcribes ITGA6-AS1 from the reverse strand, overlapping ITGA6 exon-2. Additionally, we identify several putative promoters (P1-P10) of ITGA6. ITGA6-P10 is CG rich and contains CGI. EMBOSS Cpgplot software revealed a CGI length of 180 bp that extends from nucleotide 125 to 304 of the P10 sequence. We suggest that the post-transcriptional regulation of the ITGA6 in mesenchymal stem cells is controlled by the ITGA6-AS1, which could be a critical factor responsible for the heterogeneity in function and cell fate of human MSCs. These results may provide further impetus for investigations to unravel the mechanisms of ITGA6 regulation that could help maintain or improve the properties of mesenchymal stem cells.