Laminin expression in the mouse lung increases with development and stimulates spontaneous organotypic rearrangement of mixed lung cells

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.

A nonhuman primate model of lung regeneration: detergent-mediated decellularization and initial in vitro recellularization with mesenchymal stem cells

Currently, patients with end-stage lung disease are limited to lung transplantation as their only treatment option. Unfortunately, the lungs available for transplantation are few. Moreover, transplant recipients require life-long immune suppression to tolerate the transplanted lung. A promising alternative therapeutic strategy is decellularization of whole lungs, which permits the isolation of an intact scaffold comprised of innate extracellular matrix (ECM) that can theoretically be recellularized with autologous stem or progenitor cells to yield a functional lung. Nonhuman primates (NHP) provide a highly relevant preclinical model with which to assess the feasibility of recellularized lung scaffolds for human lung transplantation. Our laboratory has successfully accomplished lung decellularization and initial stem cell inoculation of the resulting ECM scaffold in an NHP model. Decellularization of normal adult rhesus macaque lungs as well as the biology of the resulting acellular matrix have been extensively characterized. Acellular NHP matrices retained the anatomical and ultrastructural properties of native lungs with minimal effect on the content, organization, and appearance of ECM components, including collagen types I and IV, laminin, fibronectin, and sulfated glycosaminoglycans (GAG), due to decellularization. Proteomics analysis showed enrichment of ECM proteins in total tissue extracts due to the removal of cells and cellular proteins by decellularization. Cellular DNA was effectively removed after decellularization (∼92% reduction), and the remaining nuclear material was found to be highly disorganized, very-low-molecular-weight fragments. Both bone marrow- and adipose-derived mesenchymal stem cells (MSC) attach to the decellularized lung matrix and can be maintained within this environment in vitro, suggesting that these cells may be promising candidates and useful tools for lung regeneration. Analysis of decellularized lung slice cultures to which MSC were seeded showed that the cells attached to the decellularized matrix, elongated, and proliferated in culture. Future investigations will focus on optimizing the recellularization of NHP lung scaffolds toward the goal of regenerating pulmonary tissue. Bringing this technology to eventual human clinical application will provide patients with an alternative therapeutic strategy as well as significantly reduce the demand for transplantable organs and patient wait-list time.

Lung self-assembly is modulated by tissue surface tensions

To identify cell-intrinsic properties that facilitate interaction between epithelial endodermal and mesenchymal mesodermal cells during lung morphogenesis, we developed a model of lung self-assembly that mimics fetal lung formation in structure, polarity, vasculature, and extracellular matrix expression. Three-dimensional pulmonary bodies (PBs) spontaneously self-assemble from single-cell suspensions and exhibit liquid-like properties that allow measurements of compaction rate and cohesion, and that may help to specify cellular self-organization. We hypothesized that changes in one or more of these parameters could potentially explain the lung hypoplasia associated with abnormal lung development. We examined the impact of endothelial/monocyte-activating polypeptide (EMAP) II in PBs, because EMAPII is highly expressed in lung hypoplasia. EMAPII significantly increased compaction rate and decreased overall cohesion of PBs composed of both epithelial and mesenchymal cells. Moreover, the effects of EMAPII on compaction and cohesion act exclusively through the mesenchymal cell population by interfering with fibronectin matrix assembly. We also show that EMAPII alters epithelial cell polarity and surfactant protein C expression. Our findings demonstrate, for the first time, that PBs possess liquid-like properties that can help to guide the self-assembly of fetal lungs, and that EMAPII expression can influence both mesenchymal and epithelial cells but through different molecular mechanisms.

Lung organogenesis

Developmental lung biology is a field that has the potential for significant human impact: lung disease at the extremes of age continues to cause major morbidity and mortality worldwide. Understanding how the lung develops holds the promise that investigators can use this knowledge to aid lung repair and regeneration. In the decade since the "molecular embryology" of the lung was first comprehensively reviewed, new challenges have emerged-and it is on these that we focus the current review. Firstly, there is a critical need to understand the progenitor cell biology of the lung in order to exploit the potential of stem cells for the treatment of lung disease. Secondly, the current familiar descriptions of lung morphogenesis governed by growth and transcription factors need to be elaborated upon with the reinclusion and reconsideration of other factors, such as mechanics, in lung growth. Thirdly, efforts to parse the finer detail of lung bud signaling may need to be combined with broader consideration of overarching mechanisms that may be therapeutically easier to target: in this arena, we advance the proposal that looking at the lung in general (and branching in particular) in terms of clocks may yield unexpected benefits.

Embryological origin of airway smooth muscle

Airway smooth muscle (SM) develops from local mesenchymal cells located around the tips of growing epithelial buds. These cells gradually displace from distal to proximal position alongside the bronchial tree, elongate, and begin to synthesize SM-specific proteins. Mechanical tension (either generated by cell spreading/elongation or stretch), as well as epithelial paracrine factors, regulates the process of bronchial myogenesis. The specific roles of many of these paracrine factors during normal lung development are currently unknown. It is also unknown how and if mechanical and paracrine signals integrate into a common myogenic pathway. Furthermore, as with vascular SM and other types of visceral SM, we are just beginning to elucidate the intracellular signaling pathways and the genetic program that controls lung myogenesis. Here we present what we have learned so far about the embryogenesis of bronchial muscle.

Integrins β1, α6, and α3contribute to mechanical strain-induced differentiation of fetal lung type II epithelial cells via distinct mechanisms

Mechanical forces regulate lung maturation in the fetus by promoting type II epithelial differentiation. However, the cell surface receptors that transduce these mechanical cues into cellular responses remain largely unknown. When distal lung type II epithelial cells isolated from embryonic day 19 rat fetuses were cultured on flexible plates coated with laminin, fibronectin, vitronectin, collagen, or elastin and exposed to a level of mechanical strain (5%) similar to that observed in utero, transmembrane signaling responses were induced under all conditions, as measured by ERK activation. However, mechanical stress maximally increased expression of the type II cell differentiation marker surfactant protein C when cells were cultured on laminin substrates. Strain-induced alveolar epithelial differentiation was inhibited by interfering with cell binding to laminin using soluble laminin peptides (IKVIV or YIGSR) or blocking antibodies against integrin β1, α3, or α6. Additional studies were carried out with substrates coated directly with different nonactivating anti-integrin antibodies. Blocking integrin β1and α6binding sites inhibited both cell adhesion and differentiation, whereas inhibition of α3prevented differentiation without altering cell attachment. These data demonstrate that various integrins contribute to mechanical control of type II lung epithelial cell differentiation on laminin substrates. However, they may act via distinct mechanisms, including some that are independent of their cell anchoring role.

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.

Distribution of laminin 5, integrin receptors, and branching morphogenesis during human fetal lung development

The role of the epithelial adhesion ligand laminin 5 (LN5) in lung development has been poorly investigated. To determine its potential involvement in lung organogenesis, we used immunofluorescence microscopy to investigate the distribution of LN5 and its integrin (Int) receptors alpha2beta1, alpha3beta1, alpha6beta1, and alpha6beta4 during human fetal airway branching morphogenesis and respiratory epithelium differentiation. At the pseudoglandular and canalicular stages of airway development, LN5 and its constituent chains were localized in the basement membrane (BM) of the proximal respiratory tubules and in the cytoplasm of the epithelial cells forming the growing epithelial buds, which expressed Int alpha2beta1, alpha3beta1, and, transiently, alpha6beta1. At the alveolar and adult stages, LN5 and its constituent chains were localized both in the BM of evolving and differentiated bronchioles and in the alveolar parenchyma. The bronchiolar epithelium markedly expressed Int alpha2beta1 and alpha3beta1, whereas the alveolar parenchyma strongly expressed Int alpha2beta1, alpha3beta1, and alpha6beta1. Throughout fetal development and in the adult, LN5 and its constituent chains were detected both in the tracheal BM, regardless of the degree of epithelial differentiation, and in the cytoplasm of the cells at the invading front of the growing glandular ducts. Ultrastructural studies showed that nucleation of the hemidesmosomes (HDs) correlated with the differentiation of the tracheal epithelium. These results suggest that LN5 may play multiple roles during branching morphogenesis, by modulating proliferation and/or migration of the epithelial cells in the respiratory buds and by establishing branch points, through interaction initially with Int alpha6beta1 and later with Int alpha2beta1 and alpha3beta1. We also propose that LN5 may regulate the differentiation of the tracheal epithelium by means of Int-beta4, which governs HD nucleation.

Laminin alpha 5 is required for lobar septation and visceral pleural basement membrane formation in the developing mouse lung

Laminin alpha/beta/gamma heterotrimers are the major noncollagenous components of all basement membranes. To date, five alpha, three beta, and three gamma chains have been identified. Laminin alpha 5 is expressed early in lung development and colocalizes with laminin alpha1. While laminin alpha1 expression in the lung is restricted to the embryonic period, laminin alpha 5 expression persists throughout embryogenesis and adulthood. Targeted mutation of the mouse laminin alpha 5 gene Lama5 causes embryonic lethality at E14-E17 associated with exencephaly, syndactyly, placentopathy, and kidney defects, all attributable to abnormal basement membranes. In this investigation, lung development in Lama5(-/-) mice up to E16.5 was examined. We observed normal lung branching morphogenesis and vasculogenesis, but incomplete lobar septation and absence of the visceral pleura basement membrane. Preservation of branching morphogenesis was associated with ectopic deposition of laminin alpha 4 in the airway basement membrane. Perturbation of pleural basement membrane formation and right lung septation correlated with absence of laminin alpha 5, which was found to be the only laminin alpha chain present in the normal visceral pleura basement membrane. Our finding of normal lung branching morphogenesis with abnormal lobar septation demonstrates that these processes are not obligatorily linked.

Heterogeneous nuclear ribonucleoprotein-H plays a suppressive role in visceral myogenesis

Mouse embryonic mesenchymal cells undergo spontaneous smooth muscle (SM) differentiation upon spreading/elongation in culture (Relan et al., J. Cell Biol. 147 (1999) 1341; Yang et al., Development 125 (1998) 2621; Yang et al., Development 126 (1999) 3027). Using these cells we generated a subtracted cDNA library to identify potential suppressors of SM myogenesis. One of the differentially expressed genes was heterogeneous nuclear ribonucleoprotein-H (hnRNP-H), which is involved in pre-mRNA alternative splicing. hnRNP-H was highly expressed in mesenchymal cells prior to the onset of SM differentiation, but its expression rapidly decreased in mesenchymal cells undergoing SM myogenesis. In vivo, the drop in hnRNP-H expression was restricted to visceral SM cells. Antisense oligodeoxynucleotide and antisense RNA were used to inhibit hnRNP-H synthesis in SM-differentiating mesenchymal cells and in embryonic lung explants. A decrease in hnRNP-H levels resulted in upregulation of SM-specific gene expression and increased bronchial SM development in lung explants. hnRNP-H overexpression in cell cultures had the opposite effect. These studies, therefore, indicate a novel role for hnRNP-H in the control of visceral myogenesis.

The molecular basis of lung morphogenesis

To form a diffusible interface large enough to conduct respiratory gas exchange with the circulation, the lung endoderm undergoes extensive branching morphogenesis and alveolization, coupled with angiogenesis and vasculogenesis. It is becoming clear that many of the key factors determining the process of branching morphogenesis, particularly of the respiratory organs, are highly conserved through evolution. Synthesis of information from null mutations in Drosophila and mouse indicates that members of the sonic hedgehog/patched/smoothened/Gli/FGF/FGFR/sprouty pathway are functionally conserved and extremely important in determining respiratory organogenesis through mesenchymal-epithelial inductive signaling, which induces epithelial proliferation, chemotaxis and organ-specific gene expression. Transcriptional factors including Nkx2.1, HNF family forkhead homologues, GATA family zinc finger factors, pou and hox, helix-loop-helix (HLH) factors, Id factors, glucocorticoid and retinoic acid receptors mediate and integrate the developmental genetic instruction of lung morphogenesis and cell lineage determination. Signaling by the IGF, EGF and TGF-beta/BMP pathways, extracellular matrix components and integrin signaling pathways also directs lung morphogenesis as well as proximo-distal lung epithelial cell lineage differentiation. Soluble factors secreted by lung mesenchyme comprise a 'compleat' inducer of lung morphogenesis. In general, peptide growth factors signaling through cognate receptors with tyrosine kinase intracellular signaling domains such as FGFR, EGFR, IGFR, PDGFR and c-met stimulate lung morphogenesis. On the other hand, cognate receptors with serine/threonine kinase intracellular signaling domains, such as the TGF-beta receptor family are inhibitory, although BMP4 and BMPR also play key inductive roles. Pulmonary neuroendocrine cells differentiate earliest in gestation from among multipotential lung epithelial cells. MASH1 null mutant mice do not develop PNE cells. Proximal and distal airway epithelial phenotypes differentiate under distinct transcriptional control mechanisms. It is becoming clear that angiogenesis and vasculogenesis of the pulmonary circulation and capillary network are closely linked with and may be necessary for lung epithelial morphogenesis. Like epithelial morphogenesis, pulmonary vascularization is subject to a fine balance between positive and negative factors. Angiogenic and vasculogenic factors include VEGF, which signals through cognate receptors flk and flt, while novel anti-angiogenic factors include EMAP II.

Bronchial smooth muscle hypoplasia in mouse embryonic lungs exposed to a laminin beta1 chain antisense oligonucleotide

We used an antisense oligonucleotide (ODN) to inhibit laminin (LM) beta1 chain synthesis in mouse embryonic lung explants and cell cultures. The ODN spanned 17 bases located 13 bases downstream the initiation codon and contained phosphorothioate and C-5 propynyl pyrimidine modifications. Penetration of the ODN into the lung explants was confirmed by fluorescein isothiocyanate (FITC) tagging. 50 microM of antisense ODN decreased LM beta1 chain synthesis by 82+/-6.9% with no significant changes in the synthesis of other LM chains. The same antisense probe but without C-5 propynyl pyrimidine modification, another 17-mer ODN complementary to the LM beta1 initiation codon, and a 17-mer ODN complementary to the LM alpha1 initiation codon had no antisense activity. Lung explants exposed to the active LM beta1 antisense ODN showed decreased LM-1 and collagen type IV deposition at the epithelial-mesenchymal interface and an arrest in bronchial smooth muscle (SM) development. Histological examination and cell motility assays suggested that this arrest was due to impaired spreading and migration of SM cell precursors over the defective basement membrane (BM). Our studies indicate that beta1-chain containing LMs play a role in bronchial myogenesis.

Developmental characterization of the gene for laminin alpha-chain in sea urchin embryos

We describe the isolation and characterization of a cDNA clone encoding a region of the carboxy terminal globular domain (G domain) of the alpha-1 chain of laminin from the sea urchin, Strongylocentrotus purpuratus. Sequence analysis indicates that the 1.3 kb cDNA (spLAM-alpha) encodes the complete G2 and G3 subdomains of sea urchin a-laminin. The 11 kb spLAM-alpha mRNA is present in the egg and declines slightly in abundance during development to the pluteus larva. The spLAM-alpha gene is also expressed in a variety of adult tissues. Whole mount in situ hybridization of gastrula stage embryos indicates that ectodermal and endodermal epithelia and mesenchyme cells contain the spLAM-alpha mRNA. Immunoprecipitation experiments using an antibody made to a recombinant fusion protein indicates spLAM-alpha protein is synthesized continuously from fertilization as a 420 kDa protein which accumulates from low levels in the egg to elevated levels in the pluteus larva. Light and electron microscopy identify spLAM-alpha as a component of the basal lamina. Blastocoelic microinjection of an antibody to recombinant spLAM-alpha perturbs gastrulation and skeleton formation by primary mesenchyme cells suggesting an important role for laminin in endodermal and mesodermal morphogenesis.

Basement membranes in development

The aim of this review is to introduce the reader to the main ECM constituents and to some of their roles in development. The main functions of the ECM during embryogenesis are the production, promotion, and regulation of normal tissue structure. Among the ECM components, LMs have been the most extensively studied in relation to embryo-genesis. Skin and skeletal muscle disorders have been shown to be caused by LM alterations. Additional experiments, e.g., with knockout mice, will help enormously to elucidate the functional significance of many ECM constituents and their involvement in development and disease.

Commitment and differentiation of lung cell lineages

To form a large diffusible interface capable of conducting respiratory gases to and from the circulation, the lung must undergo extensive cell proliferation, branching morphogenesis, and alveolar saccule formation, to generate sufficient surface area. In addition, the cells must differentiate into at least 40 distinct lung cell lineages. Specific transcriptional factors, peptide growth factor receptor-mediated signaling pathways, extracelluar matrix components, and integrin-signaling pathways interact to direct lung morphogenesis and lung cell lineage differentiation. Branching mutants of the respiratory tracheae in Drosophila have identified several functionally conserved genes in the fibroblast growth factor signaling pathway that also regulate pulmonary organogenesis in mice and probably also in man. Key transcriptional factors including Nkx2.1, hepatocyte nuclear factor family forkhead homologues, GATA family zinc finger factors, pou and homeodomain proteins, as well as basic helix-loop-helix factors, serve as master genes to integrate the developmental genetic instruction of lung morphogenesis and cell lineage determination. Key words: lung branching morphogenesis, lung cell proliferation, lung cell differentiation, alveolization, master genes, peptide growth factor signaling, extracellular matrix signaling, mesenchyme induction, alveolar epithelial cells, pulmonary neuroendocrine cells, stem cells, retinoic acid.

Laminin 2 attachment selects myofibroblasts from fetal mouse lung

Laminins (LNs) are extracellular matrix glycoproteins that are involved in cell adhesion, proliferation, and differentiation. So far, 11 LN variants (LN1 to LN11) have been described. In the lung, at least six LN variants have been identified. However, only the role of LN1 has been characterized to any extent. In this study, we hypothesized that the LN2 variant may play a role during lung development. We identified, by RT-PCR analysis, that the alpha2-chain mRNA of LN2 is expressed during mouse lung development. LN2 adhesion assays were then performed with cells from fetal mouse lung primary cultures. Our results showed that a specific subpopulation of fetal lung cells that expressed vimentin, alpha-smooth muscle actin, and desmin attached onto LN2, whereas the cells that did not adhere to LN2 as well as the total cell population were able to adhere readily on fibronectin. Cell attachment onto LN2 was inhibited by EDTA. In addition, we demonstrated, by RT-PCR and Western analysis, that the LN2-adherent cells autoexpressed the alpha2-chain of LN2. In the late pseudoglandular period, LN2 was localized by immunohistochemistry in the basement membrane of airways and blood vessels and around mesenchymal cells. We conclude that LN2 is expressed during lung development and that a specific subpopulation of fetal lung mesenchymal cells expressing a myofibroblastic phenotype can be selected by attachment to LN2 in primary culture. These findings lead us to speculate that LN2 may play a key role in the cell biology of myofibroblasts during lung development.

Modulation of laminin integrin receptors in the postnatal and adult rat lung

Recent studies have shown that type II pneumocytes, at birth and day 3 postnatally, have a diffuse distribution and localize at alveolar 'corners' between 3 and 7 days. Since alpha 3 beta 1 and alpha 6 beta 1 are laminin-binding receptors that are well expressed by rat type II alveolar epithelial cells, we postulated that they may play a role in the localization of the cells in the alveolus. To begin the evaluation of this hypothesis, we studied the temporal and spatial expression of the alpha 3, alpha 6, and beta 1 integrin subunit protein and mRNA in whole rat lungs during postnatal development by immunofluorescence, confocal microscopy, and Northern blot analysis. The temporal expression of proteins analyzed by immunochemistry, with integrin subunit specific antibodies, increased during the 3- to 7-day postnatal period and in adult lungs. Densitometric values of the alpha 3, alpha 6, and beta 1 mRNA expression, normalized to 28S rRNA, quadrupled from day 1 to day 3 postnatally. The mRNA expression of different integrin chains was elevated 1.5- to threefold from days 5 to 7 postnatally compared to day 1 levels. The alpha 3 and alpha 6 integrin subunit mRNA decreased to newborn levels in adult lungs, whereas the beta 1 integrin mRNA in adult lungs was expressed at approximately 50% of its level in newborn lungs. We postulate that the increases in alpha 3, alpha 6, and beta 1 integrin mRNA expression during the early neonatal period may be important for the spatial distribution of type II pneumocytes.

Expression of laminin alpha3, alpha4, and alpha5 chains by alveolar epithelial cells and fibroblasts

Laminins are principal components of basement membranes. Eleven laminin isoforms are known, each a heterotrimer composed of polypeptide chains designated alpha, beta, and gamma. Five alpha chains have been identified to date: alpha1, alpha2, alpha3, alpha4, and alpha5. Recent studies of fetal and adult mouse lung show prominence of alpha3, alpha4, and alpha5 in alveolar tissue, and point to differences in the cellular expression of these alpha chains in the developing alveolus. We examined isolated rat alveolar type II cells and lung fibroblasts for expression of laminins alpha3, alpha4, and alpha5. We found that laminin alpha3 was expressed only by alveolar epithelial cells, that laminin alpha4 was expressed only by lung fibroblasts, and that laminin alpha5 was expressed primarily by alveolar epithelial cells. Metabolic labeling and immunoprecipitation confirmed the production of laminin alpha4 by fibroblasts and laminin alpha5 by alveolar epithelial cells in culture. These studies indicate that different alveolar cell types contribute different laminin alpha chains to the laminin isoforms in alveolar basement membranes. Immunohistochemistry showed colocalization of these laminin alpha chains with the laminin beta1, beta2, and gamma1 chains, indicating the likelihood that laminins 6 to 11 are present in alveolar basement membranes.

Expression and potential role of the extracellular matrix in hepatic ontogenesis: a review

Studies from a number of laboratories have provided information on the temporal and spatial expression of a variety of extracellular matrix (ECM) components in the developing liver and insight into their potential roles in hepatogenesis. Collagen type IV and laminin are present in the basement membranes of the capsular mesothelium, vascular structures of the portal and hepatic vein branches, and the ductular elements of the developing liver. The mesothelial, vascular, and ductular epithelial cells synthesize laminin and type IV collagen. In contrast, fibronectin and type I collagen are restricted to the adjacent or surrounding interstitium of those ductal and vascular elements, but are not within the basement membrane proper. The hepatic perisinusoidal space (Space of Disse) of the fetal rat develops a delicate extracellular matrix by 12.5 days of gestation, which is characterized by banded collagen fibrils and bundles associated with filamentous and flocculent material. Fibronectin, laminin, and collagen types I, III, and IV are present in the developing perisinusoidal space by this early gestational date, with laminin being the most prevalent component detected. The laminin chains localized to that region in the fetal/neonatal period are alpha 2, beta 1, beta 2, and gamma 1, whereas the alpha 1 chain of laminin is absent from the developing Space of Disse. Similar data have been reported on the laminin phenotype in the perisinusoidal space during hepatic regeneration. Electron microscopy immunohistochemistry studies have demonstrated that the sinusoidal lining cells and hepatocytes synthesize these ECM proteins during hepatogenesis. By 6 to 8 weeks of postnatal life, laminin is not detectable in the perisinusoidal space. Both the transient expression of laminin and the similarity of the laminin chain phenotype expressed in the perisinusoidal space in the developing and regenerating liver suggests a role for this protein in the organization of the hepatic lobule in those forms of hepatic morphogenesis.

Laminin alpha1 chain synthesis in the mouse developing lung: requirement for epithelial-mesenchymal contact and possible role in bronchial smooth muscle development

Laminins, the main components of basement membranes, are heterotrimers consisting of alpha, beta, and gamma polypeptide chains linked together by disulfide bonds. Laminins-1 and -2 are both composed of beta1 and gamma1 chains and differ from each other on their alpha chain, which is alpha1 and alpha2 for laminin-1 and -2, respectively. The present study shows that whereas laminins-1 and -2 are synthesized in the mouse developing lung and in epithelial-mesenchymal cocultures derived from it, epithelial and mesenchymal monocultures lose their ability to synthesize the laminin alpha1 chain. Synthesis of laminin alpha1 chain however returns upon re-establishment of epithelial-mesenchymal contact. Cell-cell contact is critical, since laminin alpha1 chain is not detected in monocultures exposed to coculture-conditioned medium or in epithelial-mesenchymal cocultures in which heterotypic cell-cell contact is prevented by an interposing filter. Immunohistochemical studies on cocultures treated with brefeldin A, an inhibitor of protein secretion, indicated both epithelial and mesenchymal cells synthesize laminin alpha1 chain upon heterotypic cell- cell contact. In a set of functional studies, embryonic lung explants were cultured in the presence of monoclonal antibodies to laminin alpha1, alpha2, and beta/gamma chains. Lung explants exposed to monoclonal antibodies to laminin alpha1 chain exhibited alterations in peribronchial cell shape and decreased smooth muscle development, as indicated by low levels of smooth muscle alpha actin and desmin. Taken together, our studies suggest that laminin alpha1 chain synthesis is regulated by epithelial-mesenchymal interaction and may play a role in airway smooth muscle development.

Regulation of the beta 2 subunit chain of laminin in developing rabbit fetal lung tissue

Laminins are a family of basement membrane-associated heterotrimeric proteins that are important in mediating the growth, migration, and differentiation of a variety of cell types. The beta 2 subunit chain is a component of several laminin isoforms, e.g., laminin-3, laminin-4, laminin-7, and possibly other, as yet uncharacterized laminin isoforms. Utilizing monoclonal antibodies directed against the beta 2 subunit chain of laminin, we detected this protein in fetal, neonatal, and adult lung tissues. The relative amount of laminin beta 2 subunit chain in fetal lung tissue increased as gestation proceeded, reaching its peak around the time of alveolar type II cell differentiation in the rabbit. The laminin beta 2 subunit chain was localized in early gestational age rabbit fetal lung tissue primarily in basement membranes of prealveolar ducts, airways, and smooth muscle cells of airways and arterial blood vessels. A rabbit laminin beta 2 cDNA was generated using RT-PCR and utilized as a probe in northern blot analysis to characterize the levels of laminin beta 2 mRNA in developing rabbit lung tissue. Similar to the pattern of laminin beta 2 protein induction observed in fetal lung tissue, laminin beta 2 mRNA levels were maximal late in gestation. Utilizing a laminin beta 2 chain cRNA probe and in situ hybridization, we detected laminin beta 2 mRNA in the epithelial cells of prealveolar ducts, the alveolar wall, and airways, as well as in connective tissue cells, and the smooth muscle cells of airways and blood vessels in fetal and adult lung tissues. In addition, using an in vitro explant model, we determined that alveolar type II cells are capable of synthesizing laminin beta 2 subunit mRNA and depositing this laminin subunit chain in the basement membrane beneath type II cells. The results of this study are suggestive that the laminin beta 2 chain may be involved in alveolar epithelial cell differentiation.

Differential expression of integrin alpha subunits supports distinct roles during lung branching morphogenesis

Epithelial branching morphogenesis is a process by which a continuous epithelium, embedded in mesenchyme, forms tubules that extend and branch into the surrounding mesenchyme. The morphogenetic process is responsible for the architecture of many organs including the lung. Proper expression and function of extracellular matrix (ECM) molecules, such as collagens and laminins, are necessary for branching to occur normally. However, little is known about the role of epithelial cell surface molecules that mediate epithelial-matrix interactions during this process. We have studied the expression patterns of cell surface collagen and laminin integrin receptor alpha subunits, alpha 1, alpha 2, alpha 3, and alpha 6, in relation to that of collagen and laminin during lung branching morphogenesis. The alpha 1 integrin subunit was present on endothelia and smooth muscles around airways and large blood vessels. The mesenchyme expressed high levels of alpha 2 and alpha 6 but not alpha 3, whereas the epithelium expressed all three integrin subunits. In contrast to the widespread epithelial expression of alpha 3 and alpha 6, the epithelial expression of alpha 2 was restricted to branch tips. By performing in situ hybridization and immunofluorescence on serial sections, we found that alpha 2 protein expression on the epithelium correlated spatially and temporally with high level expression of collagen IV and laminin-1 mRNAs, suggesting that the alpha 2-expressing epithelial cells were in the process of producing and assembling their collagen and laminin matrices. While the expression of alpha 3 and alpha 6 on all lung epithelia suggests that these integrins may be important to lung epithelial development, the unique expression pattern of the alpha 2 subunit suggests that the alpha 2 beta 1 integrin may be important at branch tips either in the process of collagen/laminin synthesis and assembly or extension of the epithelial tubules into the mesenchyme.

Characterization of alpha 1, beta 1, and gamma 1 laminin subunits during rabbit fetal lung development

Laminin-1 is an extracellular matrix protein composed of three polypeptide chains that are designated alpha 1, beta 1, and gamma 1. We investigated the expression of laminin alpha 1, beta 1, and gamma 1 subunit chains during several stages of rabbit fetal lung development. Utilizing polyclonal antibodies directed against human placental laminin and immunoblot analysis, we found that the highest levels of laminin alpha 1, beta 1, and gamma 1 subunit chains in the fetal lung were present on day 26 of gestation (term = 31 days), coincident with the initiation of alveolar epithelial cell differentiation. Levels of the laminin chains were approximately five times higher in fetal lung at day 26 of gestation than in adult lung tissue. Different temporal patterns of laminin alpha 1, beta 1, and gamma 1 subunit chain expression were observed, data suggestive that the chains are independently regulated during lung development. Laminin was localized to the basement membranes of bronchi, bronchioles, prealveolar ducts, and blood vessels in fetal lung tissue, as shown by immunostaining with polyclonal laminin antibodies. A similar staining pattern was observed in adult lung tissue, but the alveolar wall was also stained. Laminin was also observed surrounding a few mesenchymal cells in fetal lung on day 19 of gestation; the number of positive mesenchymal cells increased with lung development. Laminin alpha 1 subunit chains, detected using a monoclonal antibody, were present in the basement membranes of bronchi, bronchioles, prealveolar ducts, and blood vessels in fetal lung tissue. No laminin alpha 1 chain staining was observed in the mesenchyme of early fetal lung tissue. Using a monoclonal antibody, laminin beta 1 subunit chains were immunolocalized in the basement membranes of bronchi, bronchioles, in prealveolar ducts, and surrounding some mesenchymal cells in fetal lung tissue. Laminin alpha 1 and beta 1 subunit chains in adult lung tissue were present in basement membranes of airways, blood vessels, and alveoli. Thus, changes in the localization and accumulation of laminin near the time of alveolar type I and type II epithelial cell differentiation suggest that laminin may play a role in mediating the differentiation of these cell types during rabbit fetal lung development.

Synthesis and expression of laminin during human foetal lung development

BACKGROUND

The lung develops by epithelial tubes budding and branching into a flexible mesenchyme. This growth is associated with the remodelling of the epithelial basement membrane, of which laminin is a major component.

METHODS

Both the synthesis and expression of laminin were studied in the human lung between 10 and 31 weeks of gestation, using in situ hybridization and immunohistochemistry.

RESULTS

The synthesis of the beta chain was active in the epithelial and surrounding mesenchymal cells. The mRNAs coding for the gamma chain were less abundant and mainly found in the epithelium. The synthesis of these two chains continued throughout gestation, and no significant difference in the density of hybridization grains could be detected between the tips of the expanding buds and the proximal portions. Immunohistochemical localization of laminin showed important modifications of the basement membrane during gestation. In the first part of the pseudoglandular stage the epithelial basement membrane stained continuously for laminin. Later, the basement membrane was labelled in a graded fashion: at the apex of the growing buds the staining became weak with focal disruptions. Both epithelial and mesenchymal synthesis of laminin remained active, while the polypeptide was undetectable using immunohistochemistry.

CONCLUSIONS

These findings suggest that the remodelling of the basement membrane during human lung morphogenesis is probably not related to a decreasing synthesis of laminin, but to either a proteolytic degradation or the assembly of an inadequate complex undetectable with the polyclonal antibody antilaminin.

Differential expression of laminin isoforms and alpha 6-beta 4 integrin subunits in the developing human and mouse intestine

The intestinal tissue is characterized by important morphogenetic movements during development as well as by a continuous dynamic crypt to villus epithelial cell migration leading to differentiation of specialized cells. In this study, we have examined the spatio-temporal distribution of laminin A and M chains as well as of alpha 6 and beta 4 integrin subunits in adult and developing human and mouse intestine by indirect immunofluorescence. Selective expression of the constituent polypeptides of laminin isoforms (A and M chains) was demonstrated. In the mature human intestine, A and M chains were found to be complementary, the M chain being restricted to the base of crypts and the A chain lining the villus basement membrane. In the developing human intestine, M chain expression was delayed as compared to that of A chain; as soon as the M chain was visualized, it exhibited the typical localization in the crypt basement membrane. A somewhat different situation was found in the adult mouse intestine, since both M and A chains were found in the crypts. During mouse intestinal development the delayed expression of the M chain as compared to that of the A chain was also obvious. The absence of M chain expression in mutant dy mouse did not impair intestinal morphogenesis nor cell differentiation. The expression of alpha 6 and beta 4 subunits was not coordinated. In both species the alpha 6 expression preceded that of beta 4. Furthermore, while beta 4 staining in adult mouse intestine was detected at the basal surface of all cells lining the crypt-villus, that of alpha 6 was mainly confined to the crypt cell compartment. An overall similarity of location between alpha 6 integrin subunit and laminin A chain at the epithelial/stromal interface was noted. These data indicate that the spatial and temporal distribution of laminin variants in the developing intestine may be characteristic for each species and that interactions of laminin variants with particular receptors may be important for induction and/or maintenance of differentiated cells.

Role of mesenchymal nidogen for epithelial morphogenesis in vitro

Recent biochemical studies suggested that the extracellular matrix protein nidogen is a binding molecule linking together basement membrane components. We studied its expression and role during development. By immunofluorescence and northern blotting, nidogen was found early during epithelial cell development of kidney and lung. Yet, in situ hybridization revealed that nidogen was not produced by epithelium but by the adjacent mesenchyme in both organs. Binding of mesenchymal nidogen to epithelial laminin may thus be a key event during epithelial development. This is supported by antibody perturbation experiments. Antibodies against the nidogen binding site on laminin B2 chain perturbed epithelial development in vitro in embryonic kidney and lung. Mesenchymal nidogen could be important for early stages of epithelial morphogenesis.

A novel laminin E8 cell adhesion site required for lung alveolar formation in vitro

Basement membrane-adherent type II alveolar cells isolated from lung assemble into lumen-containing cellular spheres which retain the correct polarity and thereby approximate the earliest fetal stage of alveolar morphogenesis. The molecular basis of this process, determined in initial experiments to be attributable mainly to the large heterotrimeric glycoprotein laminin, was probed with laminin proteolytic fragments, antibodies, and synthetic peptides. The carboxy-terminal fragment E8, but not equimolar amounts of fragment P1, blocked alveolar formation. To pursue this observation, we used several anti-E8 antibodies and identified one, prepared against A chain residues 2179-2198 ("SN-peptide") from the first loop of the G domain, as inhibitory. These results were confirmed by use of SN-peptide alone and further defined by trypsin digestion of SN-peptide to the sequence SINNNR. This conserved site promoted divalent cation dependent adhesion of both type II alveolar and HT1080 cells, was inhibitable with equimolar amounts of fragment E8 but not P1, and derives from a form of laminin present in fetal alveolar basement membranes. These studies point to an important novel cell adhesion site in the laminin E8 region with a key role in lung alveolar morphogenesis.