Laminins in lung development

α5β1 integrin mediates pulmonary epithelial cyst formation

BACKGROUND

Formation of the epithelial cyst involves the establishment of apical-basolateral polarity through a series of cellular interactions that are in part mediated by the extracellular matrix (ECM). We report that in a three-dimensional multi-cellular self-assembly model of lung development, α5 integrin regulates epithelial cyst formation through organization of soluble fibronectin matrix into insoluble fibrils through a process called fibrillogenesis.

RESULTS

Dissociated murine embryonic lung cells self-assemble into three-dimensional pulmonary bodies that are dependent on α5β1 integrin mediated fibrillogenesis for cell-cell mediated self-assembly: compaction and epithelial cyst formation. Knockdown of α5 integrin resulted in a significant increase in another mediator of fibrillogenesis, αV integrin. Compensatory increased expression of another mediator of fibrillogenesis, αV integrin, was not sufficient to normalize epithelial cyst formation. Loss of α5 integrin-mediated fibrillogenesis perturbed the ability of clustered epithelial cells to establish clear polarity, loss of epithelial cell pyramidal shape, and disrupted apical F-actin-rich deposition. Lack of rich central epithelial localization of F-actin cytoskeleton and Podocalyxin suggests that loss of α5 integrin-mediated fibrillogenesis interferes with the normal cytoskeleton organization that facilitates epithelial cysts polarization.

CONCLUSIONS

We conclude that lung epithelial cyst formation in development is mediated in part by α5β1 integrin dependent fibrillogenesis. Developmental Dynamics 246:475-484, 2016. © 2017 Wiley Periodicals, Inc.

Membrane mediated development of the vertebrate blood-gas-barrier

During embryonic lung development, establishment of the gas-exchanging units is guided by epithelial tubes lined by columnar cells. Ultimately, a thin blood-gas barrier (BGB) is established and forms the interface for efficient gas exchange. This thin BGB is achieved through processes, which entail lowering of tight junctions, stretching, and thinning in mammals. In birds the processes are termed peremerecytosis, if they involve cell squeezing and constriction, or secarecytosis, if they entail cutting cells to size. In peremerecytosis, cells constrict at a point below the protruding apical part, resulting in fusion of the opposing membranes and discharge of the aposome, or the cell may be squeezed by the more endowed cognate neighbors. Secarecytosis may entail formation of double membranes below the aposome, subsequent unzipping and discharge of the aposome, or vesicles form below the aposome, fuse in a bilateral manner, and release the aposome. These processes occur within limited developmental windows, and are mediated through cell membranes that appear to be of intracellular in origin. In addition, basement membranes (BM) play pivotal roles in differentiation of the epithelial and endothelial layers of the BGB. Laminins found in the BM are particularly important in the signaling pathways that result in formation of squamous pneumocytes and pulmonary capillaries, the two major components of the BGB. Some information exists on the contribution by BM to BGB formation, but little is known regarding the molecules that drive peremerecytosis, or even the origins and composition of the double and vesicular membranes involved in secarecytosis.

Effects of fibroblast growth factors on the differentiation of the pulmonary progenitors from murine embryonic stem cells

The fibroblast growth factors (FGFs) play an important role in the development of embryonic lung. In this study, we investigated the effects of mainly FGF 1, 2, and 10 at concentrations selected on the basis of data obtained from previous in vitro culture on the derivation of the pulmonary progenitors from murine embryonic stem cells cultured on gelatin or Matrigel-coated plates. For cells cultured on a gelatin-coated plate, high concentrations of FGF1 were found to enhance the expression of mRNAs for SPC and CC10, markers of distal airway epithelium, while high levels of FGF2 decreased the expression of RNAs for not only SPC, CC10 but also for the additional markers SPD and aquaporin 5. FGF10 at all tested concentrations was found to have no effect on the differentiation of pneumocytes when ESCs were grown on gelatin-coated plates. However, when differentiation was performed on Matrigel-coated plates, the addition of 60 ng/ml FGF10 enhanced the expression of pneumocyte markers, suggesting a synergic effect of FGF10 and extracellular matrix. In conclusion, growth factors were proven to be effective in the differentiation of pulmonary progenitors from mESCs. The need of signals from extracellular matrix proteins depends on the growth factors supplemented.

Laminin-binding integrin alpha7 is required for contractile phenotype expression by human airway myocytes

Contractile airway smooth muscle (ASM) cells retain the ability for phenotype plasticity in response to multiple stimuli, which equips them with capacity to direct modeling and remodeling during development, and in disease states such as asthma. We have shown that endogenously expressed laminin is required for maturation of human ASM cells to a contractile phenotype, as occurs during ASM thickening in asthma. In this study, we profiled the expression of laminin-binding integrins alpha3beta1, alpha6beta1, and alpha7beta1, and tested whether they are required for laminin-induced myocyte maturation. Immunoblotting revealed that myocyte maturation induced by prolonged serum withdrawal, which was marked by the accumulation of contractile phenotype marker protein desmin, was also associated with the accumulation of alpha3A, alpha6A, and alpha7B. Flow cytometry revealed that alpha7B expression was a distinct feature of individual myocytes that acquired a contractile phenotype. siRNA knockdown of alpha7, but not alpha3 or alpha6, suppressed myocyte maturation. Thus, alpha7B is a novel marker of the contractile phenotype, and alpha7 expression is essential for human ASM cell maturation, which is a laminin-dependent process. These observations provide new insight into mechanisms that likely underpin normal development and remodeling associated with airways disease.

Endogenous laminin is required for human airway smooth muscle cell maturation

Background

Airway smooth muscle (ASM) contraction underlies acute bronchospasm in asthma. ASM cells can switch between a synthetic-proliferative phenotype and a contractile phenotype. While the effects of extracellular matrix (ECM) components on modulation of ASM cells to a synthetic phenotype have been reported, the role of ECM components on maturation of ASM cells to a contractile phenotype in adult lung is unclear. As both changes in ECM components and accumulation of contractile ASM are features of airway wall remodelling in asthma, we examined the role of the ECM protein, laminin, in the maturation of contractile phenotype in human ASM cells.

Methods

Human ASM cells were made senescence-resistant by stable expression of human telomerase reverse transcriptase. Maturation to a contractile phenotype was induced by 7-day serum deprivation, as assessed by immunoblotting for desmin and calponin. The role of laminin on ASM maturation was investigated by comparing the effects of exogenous laminin coated on culture plates, and of soluble laminin peptide competitors. Endogenous expression of laminin chains during ASM maturation was also measured.

Results

Myocyte binding to endogenously expressed laminin was required for ASM phenotype maturation, as laminin competing peptides (YIGSR or GRGDSP) significantly reduced desmin and calponin protein accumulation that otherwise occurs with prolonged serum deprivation. Coating of plastic cell culture dishes with different purified laminin preparations was not sufficient to further promote accumulation of desmin or calponin during 7-day serum deprivation. Expression of α2, β1 and γ1 laminin chains by ASM cells was specifically up-regulated during myocyte maturation, suggesting a key role for laminin-2 in the development of the contractile phenotype.

Conclusion

While earlier reports suggest exogenously applied laminin slows the spontaneous modulation of ASM to a synthetic phenotype, we show for the first time that endogenously expressed laminin is required for ASM maturation to the contractile phenotype. As endogenously expressed laminin chains α2, β1 and γ1 are uniquely increased during myocyte maturation, these laminin chains may be key in this process. Thus, human ASM maturation appears to involve regulated endogenous expression of a select set of laminin chains that are essential for accumulation of contractile phenotype myocytes.

Tissue inhibitor of metalloproteinases 3 regulates extracellular matrix--cell signaling during bronchiole branching morphogenesis

Tissue inhibitors of metalloproteinases (TIMPs) regulate extracellular matrix (ECM) degradation by matrix metalloproteinases (MMPs) throughout embryogenesis. We examined lungs from TIMP3 null mice and found decreased bronchiole branching, enhanced activity of MMPs and enhanced fibronectin degradation throughout lung development compared to controls. Activation of focal adhesion kinase (FAK) was also reduced from embryonic days 12.5 through 14.5 in TIMP3 null lungs. Treatment with a synthetic MMP inhibitor, GM6001, in utero enhanced the branching pattern in both wild type and null lungs accompanied by a restoration of fibronectin localization, signaling through FAK and epithelial cell proliferation in null lungs. Direct down-regulation of FAK abundance in WT lung organ culture by siRNA targeting resulted in reduced bronchiole branching, phenocopying the TIMP3 defect. We propose that enhanced MMP activity in the absence of TIMP3 interferes with focal ECM proteolysis, perturbing the intracellular signaling necessary for correct pattern formation of the bronchiole tree during bronchiole branching morphogenesis. Thus, TIMP3 can indirectly regulate epithelial cell proliferation via MMP inhibitory activity. While others have demonstrated this function for MMPs, and there is in vitro evidence that TIMP3 controls proliferation, to our knowledge this is the first evidence of TIMP3 regulating proliferation in vivo.

Generation of a conditionally null allele of the laminin alpha1 gene

Laminins are heterotrimeric glycoproteins of the basement membranes. Laminin 1 (alpha1, beta1, gamma1) is the major laminin expressed during early mouse embryogenesis. To gain access to the physiological function of laminin alpha1 chain, we developed a conditionally null allele of its encoding gene (Lama1) using the cre/loxP system. Floxed-allele-carrying mice (Lama1(flox/flox)) display no overt phenotype. Lama1(flox/flox) mice were crossed with transgenic deleter mice (CMV-Cre) to generate Lama1-deficient mice (Lama1(Delta/Delta)). Lama1(Delta/Delta) embryos die during the early postimplantation period after embryonic day 6.5. They lack Reichert's membrane, an extraembryonic basement membrane in which laminin alpha1 is normally highly expressed. In parallel, Lama1(Delta/Delta) embryos display 1) parietal and visceral endoderm differentiation defects with altered expression of cytokeratin 19 and GATA4, respectively, and 2) an induction of apoptosis. This new mouse model is of particular interest as it will allow time- and tissue-specific inactivation of the Lama1 gene in various organs.

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.

Epithelial–Mesenchymal Interactions Are Linked to Neovascularization

Lung morphogenesis is dependent on interactions between mesenchymal and epithelial cells. We have previously demonstrated that inhibition of neovascularization by endothelial monocyte-activating polypeptide (EMAP) II also attenuates fetal lung morphogenesis in vivo, and hypothesized that epithelial-mesenchymal interactions are regulated by vascular signals. To address this postulate, we evaluated the formation of epithelial cysts in vitro and assessed this complex interaction through: (i) identification of vascular formation in vitro; (ii) assessment of the effect of selective vascular inhibition on cell viability, proliferation, and cellular interactions as measured by epithelial cyst formation; and (iii) examination of whether there is an interdependent relationship between epithelial and mesenchymal cells and a vascular mediator's protein expression. Vascular networks in vitro formed in direct relationship to the presence of epithelial cysts. Disruption of the vasculature by delivery of a selective antiangiogenic protein EMAP II was associated with disruption of epithelial cyst formation. Lastly, control of the vascular formation regulatory protein EMAP II is a direct result of epithelial-mesenchymal cell interactions. These findings suggest that vascular formation modulates and is modulated by the normal cellular communication and interactions that direct lung morphology.

Immunization of naïve mice with mouse laminin-1 affected pregnancy outcome in a mouse model

PROBLEM

Laminins have important roles during placental and embryonic development. The aim of our study was to determine if active immunization of mice with laminin-1 could elicit an autoimmune response, and induce features of reproductive failure.

METHOD OF STUDY

BALB/c mice were immunized with mouse laminin-1. Autoantibodies to laminin-1 were measured by enzyme-linked immunosorbent assay. Pregnant mice were killed on day 14 of pregnancy and examined for pregnancy outcome.

RESULTS

Mice immunized with laminin-1 developed elevated levels of anti-laminin-1 auto-antibodies contrary to the control group. A higher fetal resorption rate was found in the laminin-1 immunized group (23.8%) compared with that of the control group (12.2%), and was even higher in the subgroup of those animals with very high levels of anti-laminin-1 (P < 0.01). Laminin-1 immunized mice also had lower fetal and placental weights.

CONCLUSIONS

Active immunization with laminin-1 followed by elevated circulating anti-laminin-1 antibodies results in reproductive failure manifested by a higher fetal resorption rate.

The laminins in the murine inner ear: developmental transitions and expression in cochlear basement membranes

The laminins are a family of heterotrimeric extracellular matrix molecules that form suprastructural networks in basement membranes and elsewhere. They interact with integrin receptors, playing key roles in modulating programs of cytodifferentiation and maintaining tissue homeostasis in animals. Earlier studies have demonstrated an extensive laminin network in both the developing and adult cochlea, primarily associated with the basement membranes. These studies, however, did not address the laminin chain composition of these networks. In this study, we used antibodies specific for the known laminin chains to examine the composition of laminins in both the developing and adult murine cochlea. The results illustrate a complex and dynamic postnatal developmental regulation pattern for most of these chains, and suggest that an unusually large number of laminin heterotrimers are present in both the developing and adult cochlea. The laminin composition at postnatal day 2 is relatively simple. By postnatal day 7, however, activation of several laminin chains results in a very complex laminin composition. In the basement membrane underlying the region of the basilar membrane under the developing organ of Corti, eight of the 11 known basement membrane laminins are possible by co-localization inference. Dynamic changes in expression continue through day 14, but simplify by adulthood. Thus, the most dynamic period for laminin expression in the mouse cochlea coincides with terminal cytodifferentiation of the cochlear epithelial structures. Considering the well established role of laminins in regulating both embryonic and organ development in other systems, these data suggest a closer look at the role of the laminins in cochlear development and function may be warranted.

Human bronchial epithelial cells secrete laminin 5, express hemidesmosomal proteins, and assemble hemidesmosomes

Epithelial cells attach to the basement membrane through adhesive contacts between the basal cells of the epithelium and the proteins of the extracellular matrix (ECM). The hemidesmosome (HD) is a specialized cell-ECM contact, that mediates the attachment of the epithelial cell basal surface to the ECM. In bronchial epithelial cells, the protein components that constitute the HD have not been demonstrated. Using immunohistochemical techniques, we determined that normal human bronchial epithelial (NHBE) cells express the HD cell surface integrin alpha6beta4 and produce laminin 5, the ECM protein associated with HDs. Furthermore, expression of the HD-associated structural proteins, bullous pemphigoid antigens 1 (BPAG 1) and 2 (BPAG 2), was demonstrated in NHBE cells by immunofluorescence microscopy and immunoblot analyses. In addition, we confirmed the presence of laminin 5 in the basement membrane (BM) of bronchial epithelial biopsy specimens and of BP230, BP180, and the alpha6beta4 integrin heterodimer at the site of bronchial epithelial cell-ECM interaction in vivo. Finally, using electron microscopy, we were able to demonstrate intact HDs in a glutaraldehyde-fixed NHBE cell monolayer. These findings suggest that bronchial epithelium forms HDs and that the laminin 5-alpha6beta4 integrin interaction may be important in stabilizing epithelial cell adhesion to the BM in the lung.

Cloning of rat fibrillin-2 cDNA and its role in branching morphogenesis of embryonic lung

Fibrillin-2 is an extracellular matrix protein. It is associated with elastic fibers in several tissues and is believed to serve as a ligand for alphavbeta3 integrin, the latter being a known morphogen. In this study, the role of fibrillin-2 in lung development was investigated. Also, rat fibrillin-2 cDNA was isolated and sequenced and its spatiotemporal expression determined. It had approximately 88% homology with human fibrillin-2 and had Ca(2+) binding epidermal growth factor-like domains, transforming growth factor-beta binding protein motifs, and two RGD binding sites. Northern blot analysis revealed an approximately 10-kb transcript, and fibrillin-2 expression was developmentally regulated, and it paralleled that of tropoelastin. At day 13 of gestation, fibrillin-2 was expressed in the mesenchyme and at the epithelial:mesenchymal interface. From day 13 to 19 of gestation, its expression intensified and was confined around the tracheobronchial airways, while it lessened during the postnatal period. Immunoprecipitation revealed an approximately 350-kDa band by SDS-PAGE. Treatment with fibrillin-2 antisense oligodeoxynucleotide induced dysmorphogenesis of the lung explants. They were smaller and had rudimentary lung bud branches, collapsed conducting airways, and loose expanded mesenchyme. Concomitantly, fibrillin-2 mRNA, antibody reactivity in the explants, and fibrillin-2-specific radioincorporation were reduced. Anti-alphav and -laminin antibody reactivity and their respective incorporated specific radioactivities were unaltered. These data indicate that fibrillin-2 modulates organogenesis of the lung in the context of epithelial:mesenchymal interactions. Conceivably, the collapse of the conducting airways may also be related to the perturbed biology of the fibrillin-2 interacting protein, i.e., elastin, the latter being critical for the normal biophysiology of the lungs.

Sulfation of extracellular matrices modifies growth factor effects on type II cells on laminin substrata

The alveolar basement membrane contains a variety of extracellular matrix (ECM) molecules, including laminin and sulfated glycosaminoglycans of proteoglycans. These mixtures exist within microdomains of differing levels of sulfate, which may specifically interact to be key determinants of the known capacity of the type II cell to respond to certain growth factors. Isolated type II cells were exposed to either acidic fibroblast growth factor (FGF-1), basic fibroblast growth factor (FGF-2), or keratinocyte growth factor (KGF; FGF-7) on culture wells precoated with laminin alone or in combination with chondroitin sulfate (CS), high-molecular-weight heparin, or their desulfated forms. Desulfated heparin significantly elevated FGF-1- and FGF-2-stimulated DNA synthesis, whereas desulfated CS and N-desulfated heparin elevated FGF-7-stimulated DNA synthesis by type II cells on laminin substrata. When FGF-1 was mixed into the various test matrix substrata, DNA synthesis was significantly increased in all cases. These results demonstrated that decreased levels of sulfate in ECM substrata act to upregulate responses to heparin-binding growth factors by alveolar epithelial cells on laminin substrata.