Expression of the integrin subunit alpha8 in murine lung development

Nephronectin is required to maintain right lung lobar separation during embryonic development

Nephronectin (NPNT) is a basement membrane (BM) protein and high-affinity ligand of integrin α8β1 that is required for kidney morphogenesis in mice. In the lung, NPNT also localizes to BMs, but its potential role in pulmonary development has not been investigated. Mice with a floxed Npnt allele were used to generate global knockouts (KOs). Staged embryos were obtained by timed matings of heterozygotes and lungs were isolated for analysis. Although primary and secondary lung bud formation was normal in KO embryos, fusion of right lung lobes, primarily the medial and caudal, was first detected at E13.5 and persisted into adulthood. The lung parenchyma of KO mice was indistinguishable from wild-type (WT) and lobe fusion did not alter respiratory mechanics in adult KO mice. Interrogation of an existing single-cell RNA-seq atlas of embryonic and adult mouse lungs identified Npnt transcripts in mesothelial cells at E12.5 and into the early postnatal period, but not in adult lungs. KO embryonic lungs exhibited increased expression of laminin α5 and deposition of collagen IV in the mesothelial BM, accompanied by abnormalities in collagen fibrils in the adjacent stroma. Cranial and accessory lobes extracted from KO embryonic lungs fused ex vivo when cultured in juxtaposition, with the area of fusion showing loss of the mesothelial marker Wilms tumor 1. Because a similar pattern of lobe fusion was previously observed in integrin α8 KO embryos, our results suggest that NPNT signaling through integrin α8, likely in the visceral pleura, maintains right lung lobe separation during embryogenesis.

Immunohistochemical Study of Smooth Muscle Cells and Elastin in Goose Lungs

There are many differences (morphological, physiological and mechanical) between the lungs of birds and the lungs of mammals. Birds have a more efficient exchange of oxygen and carbon dioxide than mammals. In this article, we studied the presence of four antibodies (actin, α-smooth muscle actin, desmin and elastin) in the lungs of geese. Smooth muscle cells (SMCs) immunoreactive to actin, α-SMA and desmin were observed in the primary and secondary bronchi and arranged as a continuous layer. In the tertiary bronchus (parabronchus), immunoreactive cells on α-SMA and desmin were observed as aggregations of smooth muscle cells in the septum tips in atrial opening. A small number of α-SMA and desmin-positive cells were observed on the periphery of the parabronchi and between the air and blood capillaries. The elastic fibres were found in the large bronchi in connection with smooth muscle bands. In the parabronchi the elastic fibres form an elastic membrane lining the parabronchial lumen. In the blood vessels, the elastic fibres form the inner and outer elastic membrane. The individual elastic membranes connect neighbouring blood vessels.

Endotoxin-induced acute lung injury in mice with postnatal deletion of nephronectin

Acute injury of the lung involves damage to the epithelium and its underlying extracellular matrix (ECM), the basement membrane (BM). How BMs contribute to injury resolution is poorly understood. Nephronectin (NPNT) is a high-affinity ligand for integrin α8β1 and, although first identified in the mouse kidney, is prominently expressed in the lung, where it localizes to BMs in the alveoli. To determine if NPNT plays a role in acute injury and inflammation of the lung, we developed a model for postnatal deletion of NPNT using mice with a floxed allele of Npnt in combination with a tamoxifen-inducible Cre recombinase expressed at the ROSA locus. Expression of NPNT was substantially reduced in lungs from tamoxifen-treated Cre+ animals. Cre+ mice and Cre- controls were given E. coli LPS by oropharyngeal aspiration to induce injury and inflammation. In Cre- lungs, although both Npnt and Itga8 (integrin α8) transcripts were downregulated at the peak of inflammation, NPNT protein was still detectable. While the onset of inflammation was similar for Cre+ and Cre-, NPNT-deficient lungs still had thickened alveolar septa and there were increased macrophages in the bronchoalveolar lavage fluid (BALF) in the resolution phase. BALF from Cre+ lungs was more chemotactic for bone marrow-derived macrophages than Cre- in in vitro experiments, but there were no differences in the elaboration of chemokines in vivo. We speculate that absence of NPNT in BMs of the alveoli impairs or delays inflammatory and injury resolution in this model, but further studies are needed to establish the precise role of NPNT in tissue repair.

The Development of Integrin Alpha-8 Deficient Lungs Shows Reduced and Altered Branching and a Correction of the Phenotype During Alveolarization

Lung development involves epithelial–mesenchymal interactions and integrins represent one of the key elements. These extracellular matrix receptors form hetero-dimers of alpha and beta subunits. The integrin α8β1 is highly expressed in mouse tissues, including lung. It forms a cellular receptor for fibronectin, vitronectin, osteopontin, nephronectin, and tenascin-C. This study aims to investigate the role of the integrin α8-subunit (α8) during lung development. Wild type and α8-deficient lungs were explanted at embryonic days 11.5/12.5. After 24–73 h in culture α8-deficient lung explants displayed reduced growth, reduced branching, enlarged endbuds, altered branching patterns, and faster spontaneous contractions of the airways as compared to wild type. Postnatally, a stereological investigation revealed that lung volume, alveolar surface area, and the length of the free septal edge were significantly reduced in α8-deficient lungs at postnatal days P4 and P7. An increased formation of new septa in α8-deficient lungs rescued the phenotype. At day P90 α8-deficient lungs were comparable to wild type. We conclude that α8β1 takes not only part in the control of branching, but also possesses a morphogenic effect on the pattern and size of the future airways. Furthermore, we conclude that the phenotype observed at day P4 is caused by reduced branching and is rescued by a pronounced formation of the new septa throughout alveolarization. More studies are needed to understand the mechanism responsible for the formation of new septa in the absence of α8β1 in order to be of potential therapeutic benefit for patients suffering from structural lung diseases.

CD248 and integrin alpha-8 are candidate markers for differentiating lung fibroblast subtypes

Background

Lung fibrosis is a serious life-threatening condition whose manifestation varies according to the localization and characteristics of fibroblasts, which are considered heterogeneous. Therefore, to better understand the pathology and improve diagnosis and treatment of this disease, it is necessary to elucidate the nature of this heterogeneity and identify markers for the accurate classification of human lung fibroblast subtypes.

Methods

We characterized distinct mouse lung fibroblast subpopulations isolated by fluorescence-activated cell sorting (FACS) and performed microarray analysis to identify molecular markers that could be useful for human lung fibroblast classification. Based on the expression of these markers, we evaluated the fibroblast-like cell subtype localization in normal human lung samples and lung samples from patients with idiopathic pulmonary fibrosis (IPF).

Results

Mouse lung fibroblasts were classified into Sca-1^high fibroblasts and Sca-1^low fibroblasts by in vitro biological analyses. Through microarray analysis, we demonstrated CD248 and integrin alpha-8 (ITGA8) as cell surface markers for Sca-1^high fibroblasts and Sca-1^low fibroblasts, respectively. In mouse lungs, Sca-1^high fibroblasts and Sca-1^low fibroblasts were localized in the collagen fiber-rich connective tissue and elastic fiber-rich connective tissue, respectively. In normal human lungs and IPF lungs, two corresponding major fibroblast-like cell subtypes were identified: CD248^highITGA8^low fibroblast-like cells and CD248^lowITGA8^high fibroblast-like cells, localized in the collagen fiber-rich connective tissue and in the elastic fiber-rich connective tissue, respectively.

Conclusion

CD248^highITGA8^low fibroblast-like cells and CD248^lowITGA8^high fibroblast-like cells were localized in an almost exclusive manner in human lung specimens. This human lung fibroblast classification using two cell surface markers may be helpful for further detailed investigations of the functions of lung fibroblast subtypes, which can provide new insights into lung development and the pathological processes underlying fibrotic lung diseases.

Integrins as Therapeutic Targets for Respiratory Diseases

Integrins are a large family of transmembrane heterodimeric proteins that constitute the main receptors for extracellular matrix components. Integrins were initially thought to be primarily involved in the maintenance of cell adhesion and tissue integrity. However, it is now appreciated that integrins play important roles in many other biological processes such as cell survival, proliferation, differentiation, migration, cell shape and polarity. Lung cells express numerous combinations and permutations of integrin heterodimers. The complexity and diversity of different integrin heterodimers being implicated in different lung diseases present a major challenge for drug development. Here we provide a comprehensive overview of the current knowledge of integrins from studies in cell culture to integrin knockout mouse models and provide an update of results from clinical trials for which integrins are therapeutic targets with a focus on respiratory diseases (asthma, emphysema, pneumonia, lung cancer, pulmonary fibrosis and sarcoidosis).

Temporal Expression of Alpha–Smooth Muscle Actin and Drebrin in Septal Interstitial Cells during Alveolar Maturation

In rat lung, the definitive alveoli are established during development by the outgrowth of secondary septa from the primary septa present in newborn; however, the mechanism of alveolar formation has not yet been fully clarified. In this study, we characterize the septal interstitial cells in developing alveoli. During the perinatal period, alpha-SMA–containing slender cells were found in the primitive alveolar septa. Alpha-SMA–containing cells were detected at the tips of the septa until postnatal day 21, when the alveolar formation was almost completed, but disappeared in adult. Immunoelectron microscopy demonstrated that alpha-SMA is localized mainly in the cellular protrusions, which are connected with the elastic fibers around the interstitial cells. Developmentally regulated brain protein (drebrin) is also located in the cell extensions containing alpha-SMA in immature alveolar interstitial cells. In adult lung, alpha-SMA–positive cells are located only at the alveolar ducts but are not found in the secondary septa. Desmin is expressed only in alpha-SMA–containing cells at the alveolar ducts but not in those at the tip of alveolar septa. These results suggest that a part of the septal interstitial cells are temporarily alpha-SMA– and drebrin-positive during maturation. Alpha-SMA– and drebrin-containing septal interstitial cells (termed septal myofibroblast-like cells) may play an important role in alveolar formation.

Reduction of microRNA-206 contributes to the development of bronchopulmonary dysplasia through up-regulation of fibronectin 1

OBJECTIVE

To characterize microRNA-206 (miR-206) in the development of bronchopulmonary dysplasia (BPD).

DESIGN/METHODS

We assessed the expression of miR-206 in BPD mouse lung tissues and blood samples of BPD patients by quantitative real-time PCR. Then, the role of miR-206 in regulating cell biology were examined by XTT assay, flow cytometry, transwell invasion assay, wound healing assay and adhesion assay in vitro. Furthermore, luciferase reporter assay, real-time PCR, western blot and Immunofluorescence staining were performed to figure out the target gene of miR-206.

RESULTS

A reduction in expression of miR-206 was observed in BPD mice compared with controls and in BPD patients compared with controls. miR-206 overexpression significantly induced cell apoptosis, reduced cell proliferation, migration and adhesion abilities, whereas the inhibition of miR-206 expression had the opposite effect. Fibronectin 1 (FN1) is a direct target of miR-206, and fn 1 can be transcriptionally and translationally regulated by miR-206. Down-regulation of miR-206 modulates biological functions of the cells, at least in part, by increasing the level of fn 1. Furthermore, fn 1 expression levels were increased in the BPD mice and BPD patients.

CONCLUSIONS

The expression of miR-206 and its target gene, fn 1, may contribute to the progression of BPD.

Transcription factor Klf4, induced in the lung by oxygen at birth, regulates perinatal fibroblast and myofibroblast differentiation

The fluid-filled lung exists in relative hypoxia in utero (∼25 mm Hg), but at birth fills with ambient air where the partial pressure of oxygen is ∼150 mm Hg. The impact of this change was studied in mouse lung with microarrays to analyze gene expression one day before, and 2, 6, 12 and 24 hours after birth into room air or 10% O₂. The expression levels of >150 genes, representing transcriptional regulation, structure, apoptosis and antioxidants were altered 2 hrs after birth in room air but blunted or absent with birth in 10% O₂. Kruppel-like factor 4 (Klf4), a regulator of cell growth arrest and differentiation, was the most significantly altered lung gene at birth. Its protein product was expressed in fibroblasts and airway epithelial cells. Klf4 mRNA was induced in lung fibroblasts exposed to hyperoxia and constitutive expression of Klf4 mRNA in Klf4-null fibroblasts induced mRNAs for p21^cip1/Waf1, smooth muscle actin, type 1 collagen, fibronectin and tenascin C. In Klf4 perinatal null lung, p21^cip1/Waf1mRNA expression was deficient prior to birth and associated with ongoing cell proliferation after birth; connective tissue gene expression was deficient around birth and smooth muscle actin protein expression was absent from myofibroblasts at tips of developing alveoli; p53, p21^cip1/Waf1 and caspase-3 protein expression were widespread at birth suggesting excess apoptosis compared to normal lung. We propose that the changing oxygen environment at birth acts as a physiologic signal to induce lung Klf4 mRNA expression, which then regulates proliferation and apoptosis in fibroblasts and airway epithelial cells, and connective tissue gene expression and myofibroblast differentiation at the tips of developing alveoli.

Tensegrin in context: Dual role of α8 integrin in the migration of different cell types

α8β1 integrin is highly expressed in cells with contractile function, such as mesangial cells of the kidneys and vascular smooth muscle cells (VSMCs). Although it promotes migration of neural crest cells and breast cancer cells, recent studies suggest that α8 integrin has a negative regulatory role in VSMC migration. In this review, the question of why α8β1 integrin plays a dual role in cell migration is raised and discussed. It seems that cells require optimum contractility and balanced tensile forces for migration. α8β1 integrin promotes migration of cells that are initially in a less than optimal contractile state (e.g. neural cells) and reduces the migration of cells known as contractile cells. α8β1 integrin can be called “Tensegrin” as it fits perfectly into the tensegrity model (tensional integrity) and seems to play a prominent role in the integration of the tensile forces.

Integrin alpha8beta1 regulates adhesion, migration and proliferation of human intestinal crypt cells via a predominant RhoA/ROCK-dependent mechanism

Background. Integrins are transmembrane αβ heterodimer receptors that function as structural and functional bridges between the cytoskeleton and ECM (extracellular matrix) molecules. The RGD (arginine-glycine-aspartate tripeptide motif)-dependent integrin α8β1 has been shown to be involved in various cell functions in neuronal and mesenchymal-derived cell types. Its role in epithelial cells remains unknown.

Results. Integrin α8β1 was found to be expressed in the crypt cell population of the human intestine but was absent from differentiating and mature epithelial cells of the villus. The function of α8β1 in epithelial crypt cells was investigated at the cellular level using normal HIECs (human intestinal epithelial cells). Specific knockdown of α8 subunit expression using an shRNA (small-hairpin RNA) approach showed that α8β1 plays important roles in RGD-dependent cell adhesion, migration and proliferation via a RhoA/ROCK (Rho-associated kinase)-dependent mechanism as demonstrated by active RhoA quantification and pharmacological inhibition of ROCK. Moreover, loss of α8β1, through RhoA/ROCK, impairs FA (focal adhesion) complex integrity as demonstrated by faulty vinculin recruitment.

Conclusions. Integrin α8β1 is expressed in epithelial cells. In intestinal crypt cells, α8β1 is closely involved in the regulation of adhesion, migration and cell proliferation via a predominant RhoA/ROCK-dependent mechanism. These results suggest an important role for this integrin in intestinal crypt cell homoeostasis.

Molecular basis of the recognition of nephronectin by integrin alpha8beta1

Integrin alpha8beta1 interacts with a variety of Arg-Gly-Asp (RGD)-containing ligands in the extracellular matrix. Here, we examined the binding activities of alpha8beta1 integrin toward a panel of RGD-containing ligands. Integrin alpha8beta1 bound specifically to nephronectin with an apparent dissociation constant of 0.28 +/- 0.01 nm, but showed only marginal affinities for fibronectin and other RGD-containing ligands. The high-affinity binding to alpha8beta1 integrin was fully reproduced with a recombinant nephronectin fragment derived from the RGD-containing central "linker" segment. A series of deletion mutants of the recombinant fragment identified the LFEIFEIER sequence on the C-terminal side of the RGD motif as an auxiliary site required for high-affinity binding to alpha8beta1 integrin. Alanine scanning mutagenesis within the LFEIFEIER sequence defined the EIE sequence as a critical motif ensuring the high-affinity integrin-ligand interaction. Although a synthetic LFEIFEIER peptide failed to inhibit the binding of alpha8beta1 integrin to nephronectin, a longer peptide containing both the RGD motif and the LFEIFEIER sequence was strongly inhibitory, and was approximately 2,000-fold more potent than a peptide containing only the RGD motif. Furthermore, trans-complementation assays using recombinant fragments containing either the RGD motif or LFEIFEIER sequence revealed a clear synergism in the binding to alpha8beta1 integrin. Taken together, these results indicate that the specific high-affinity binding of nephronectin to alpha8beta1 integrin is achieved by bipartite interaction of the integrin with the RGD motif and LFEIFEIER sequence, with the latter serving as a synergy site that greatly potentiates the RGD-driven integrin-ligand interaction but has only marginal activity to secure the interaction by itself.

Anti-alpha8 integrin immunoliposomes in glomeruli of lupus-susceptible mice: a novel system for delivery of therapeutic agents to the renal glomerulus in systemic lupus erythematosus

OBJECTIVE

Glomerular mesangial cells are active participants in the pathogenesis of lupus glomerulonephritis (GN). Thus, targeted delivery of therapeutic agents to mesangial cells would be an attractive approach to treatment. However, lack of known unique mesangial cell surface markers has hampered this process. This study was undertaken in a mouse model of lupus GN to identify mesangial markers and to develop a system for targeted drug delivery to the glomerulus.

METHODS

Based on previous observations, alpha8 integrin expressed on the surface of glomerular mesangial cells was selected as a target molecule for delivery. Two mouse strains susceptible to lupus GN, NZM2328 and (NZM2328 x NOD)F1, were studied. Glomerular expression of alpha8 integrin in normal and nephritic mice was confirmed by immunofluorescence and quantitative polymerase chain reaction analysis. Liposomes were formulated and conjugated with an anti-alpha8 integrin antibody. These immunoliposomes were loaded with DiI, a red fluorescent dye, to allow tracking in vivo, and injected into the tail vein of female mice at different ages. Specificity of targeting was studied by fluorescence microscopy and flow cytometry.

RESULTS

Expression of alpha8 integrin was observed in the glomeruli of normal and nephritic mice. Anti-alpha8 integrin immunoliposomes were detected in the glomerulus and glomerular mesangial cells after tail vein injection in normal and nephritic mice. Delivery of DiI by anti-alpha8 integrin immunoliposomes was tissue specific, being observed predominantly in the glomeruli, with some nonspecific uptake by CD11b cells.

CONCLUSION

These findings are the first demonstration of specific delivery of anti-alpha8 integrin immunoliposomes to the mesangium following tail vein injection in mice. Anti-alpha8 integrin immunoliposomes thus offer a novel approach for targeted drug therapy in lupus and other glomerular diseases.

Role for ephrinB2 in postnatal lung alveolar development and elastic matrix integrity

Alveoli are formed in the lung by the insertion of secondary tissue folds, termed septa, which are subsequently remodeled to form the mature alveolar wall. Secondary septation requires interplay between three cell types: endothelial cells forming capillaries, contractile interstitial myofibroblasts, and epithelial cells. Here, we report that postnatal lung alveolization critically requires ephrinB2, a ligand for Eph receptor tyrosine kinases expressed by the microvasculature. Mice homozygous for the hypomorphic knockin allele ephrinB2DeltaV/DeltaV, encoding mutant ephrinB2 with a disrupted C-terminal PDZ interaction motif, show severe postnatal lung defects including an almost complete absence of lung alveoli and abnormal and disorganized elastic matrix. Lung alveolar formation is not sensitive to loss of ephrinB2 cytoplasmic tyrosine phosphorylation sites. Postnatal day 1 mutant lungs show extracellular matrix alterations without differences in proportions of major distal cell populations. We conclude that lung alveolar formation relies on endothelial ephrinB2 function.

Developmental alveolarization of the mouse lung

Postnatal lung development is not well characterized in mice, especially the time point when alveolarization is completed. Using the total length and the length density of the free septal edge as measured for the formation of new septa, we followed alveolarization throughout postnatal lung development (days 2-125). Furthermore, the alveolar surface area was estimated. The formation of new septa was observed until day 36. Approximately 10% of the septa present in adult mice were formed prenatally by branching morphogenesis, approximately 50% were generated postnatally before and approximately 40% after maturation of the alveolar microvasculature. Approximately 5% of the alveolar surface area present during adulthood was present before alveolarization started, approximately 55% was formed during alveolarization (days 4-36) and approximately 40% afterward due to growth processes. We conclude that alveolarization continues until young adulthood and that the maturation of the alveolar microvasculature does not preclude further alveolarization.

Regulation of alveologenesis: clinical implications of impaired growth

During its development that begins in intrauterine life, the lung is transformed from a simple epithelial lined sac that emerges from the foregut into a complex arrangement of blood vessels, airways, and alveoli that make up the mature lung structure. This remarkable transformation that continues for several years postnatally, is achieved by the influence of several genes, transcription factors, growth factors and hormones upon the cells and proteins of the lung bud. A seminal event in this process is the formation of the air-blood barrier within the alveolar wall, an evolutionary modification that permits independent air-breathing existence in mammals. Molecular biological techniques have enabled elucidation of the mechanistic pathways contributing to alveologenesis and have provided probable molecular bases for examples of impaired alveologenesis encountered by the paediatric pathologist.

Similarities and dissimilarities of branching and septation during lung development

The lungs of small premature babies are at a developmental stage of finalizing their airway tree by a process called branching morphogenesis, and of creating terminal gas exchange units by a mechanism called septation. If the branching process is disturbed, the lung has a propensity to be hypoplastic. If septation is impaired, the terminal gas exchange units, the alveoli, tend to be enlarged and reduced in number, an entity known as bronchopulmonary dysplasia. Here, we review current knowledge of key molecules influencing branching and septation. In particular, we discuss the molecular similarities and dissimilarities between the two processes of airspace enlargement. Understanding of the molecular mechanisms regulating branching and septation may provide perinatologists with targets for improving lung growth and maturation.