Induction of alveolar type II cell differentiation in embryonic tracheal epithelium in mesenchyme-free culture

Decoding FGF/FGFR Signaling: Insights into Biological Functions and Disease Relevance

Fibroblast Growth Factors (FGFs) and their cognate receptors, FGFRs, play pivotal roles in a plethora of biological processes, including cell proliferation, differentiation, tissue repair, and metabolic homeostasis. This review provides a comprehensive overview of FGF-FGFR signaling pathways while highlighting their complex regulatory mechanisms and interconnections with other signaling networks. Further, we briefly discuss the FGFs involvement in developmental, metabolic, and housekeeping functions. By complementing current knowledge and emerging research, this review aims to enhance the understanding of FGF-FGFR-mediated signaling and its implications for health and disease, which will be crucial for therapeutic development against FGF-related pathological conditions.

Differentiation of embryonic stem cells into lung-like cells using lung-derived matrix sheets

Various extracellular matrix (ECM) in the lungs regulate tissue development and homeostasis, as well as provide support for cell structures. However, few studies regarding the effects of lung cell differentiation using lung-derived ECM (LM) alone have been reported. The present study investigated the capability of lung-derived matrix sheets (LMSs) to induce lung cell differentiation using mouse embryonic stem (ES) cells. Expressions of lung-related cell markers were significantly upregulated in ES-derived embryoid bodies (EBs) cultured on an LMS for two weeks. Moreover, immunohistochemical analysis of EBs grown on LMSs revealed differentiation of various lung-related cells. These results suggest that an LMS can be used to promote differentiation of stem cells into lung cells.

Establishment of Intestinal Organoid from Rousettus leschenaultii and the Susceptibility to Bat-Associated Viruses, SARS-CoV-2 and Pteropine Orthoreovirus

Various pathogens, such as Ebola virus, Marburg virus, Nipah virus, Hendra virus, Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV), Middle East Respiratory Syndrome Coronavirus (MERS-CoV), and SARS-CoV-2, are threatening human health worldwide. The natural hosts of these pathogens are thought to be bats. The rousette bat, a megabat, is thought to be a natural reservoir of filoviruses, including Ebola and Marburg viruses. Additionally, the rousette bat showed a transient infection in the experimental inoculation of SARS-CoV-2. In the current study, we established and characterized intestinal organoids from Leschenault's rousette, Rousettus leschenaultii. The established organoids successfully recapitulated the characteristics of intestinal epithelial structure and morphology, and the appropriate supplements necessary for long-term stable culture were identified. The organoid showed susceptibility to Pteropine orthoreovirus (PRV) but not to SARS-CoV-2 in experimental inoculation. This is the first report of the establishment of an expandable organoid culture system of the rousette bat intestinal organoid and its sensitivity to bat-associated viruses, PRV and SARS-CoV-2. This organoid is a useful tool for the elucidation of tolerance mechanisms of the emerging rousette bat-associated viruses such as Ebola and Marburg virus.

Induction of lung-like cells from mouse embryonic stem cells by decellularized lung matrix

Decellularization of tissues is a recently developed technique mostly used to provide a 3-dimensional matrix structure of the original organ, including decellularized lung tissues for lung transplantation. Based on the results of the present study, we propose new utilization of decellularized tissues as inducers of stem cell differentiation. Decellularized lung matrix (L-Mat) samples were prepared from mouse lungs by SDS treatment, then the effects of L-Mat on differentiation of ES cells into lung cells were investigated. ES cell derived-embryoid bodies (EBs) were transplanted into L-Mat samples and cultured for 2 weeks. At the end of the culture, expressions of lung cell-related markers, such as TTF-1 and SP-C (alveolar type II cells), AQP5 (alveolar type I cells), and CC10 (club cells), were detected in EB outgrowths in L-Mat, while those were not found in EB outgrowths attached to the dish. Our results demonstrated that L-Mat has an ability to induce differentiation of ES cells into lung-like cells.

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Differentiation of ES cells by decellularized lung matrix (L-Mat) was investigated.

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L-Mat induced differentiation of various lung cell-like cells from ES cells.

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L-Mat plays an important role for inducing differentiation of lung cells.

Timing is everything: Reiterative Wnt, BMP and RA signaling regulate developmental competence during endoderm organogenesis

A small number of signaling pathways are used repeatedly during organogenesis, and they can have drastically different effects on the same population of cells depending on the embryonic stage. How cellular competence changes over developmental time is not well understood. Here we used Xenopus, mouse, and human pluripotent stem cells to investigate how the temporal sequence of Wnt, BMP, and retinoic acid (RA) signals regulates endoderm developmental competence and organ induction, focusing on respiratory fate. While Nkx2-1+ lung fate is not induced until late somitogenesis stages, here we show that lung competence is restricted by the gastrula stage as a result of Wnt and BMP-dependent anterior-posterior (A-P) patterning. These early Wnt and BMP signals make posterior endoderm refractory to subsequent RA/Wnt/BMP-dependent lung induction. We further mapped how RA modulates the response to Wnt and BMP in a temporal specific manner. In the gastrula RA promotes posterior identity, however in early somite stages of development RA regulates respiratory versus pharyngeal potential in anterior endoderm and midgut versus hindgut potential in posterior endoderm. Together our data suggest a dynamic and conserved response of vertebrate endoderm during organogenesis, wherein early Wnt/BMP/RA impacts how cells respond to later Wnt/BMP/RA signals, illustrating how reiterative combinatorial signaling can regulate both developmental competence and subsequent fate specification.

Lung Regeneration: Endogenous and Exogenous Stem Cell Mediated Therapeutic Approaches

The tissue turnover of unperturbed adult lung is remarkably slow. However, after injury or insult, a specialised group of facultative lung progenitors become activated to replenish damaged tissue through a reparative process called regeneration. Disruption in this process results in healing by fibrosis causing aberrant lung remodelling and organ dysfunction. Post-insult failure of regeneration leads to various incurable lung diseases including chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis. Therefore, identification of true endogenous lung progenitors/stem cells, and their regenerative pathway are crucial for next-generation therapeutic development. Recent studies provide exciting and novel insights into postnatal lung development and post-injury lung regeneration by native lung progenitors. Furthermore, exogenous application of bone marrow stem cells, embryonic stem cells and inducible pluripotent stem cells (iPSC) show evidences of their regenerative capacity in the repair of injured and diseased lungs. With the advent of modern tissue engineering techniques, whole lung regeneration in the lab using de-cellularised tissue scaffold and stem cells is now becoming reality. In this review, we will highlight the advancement of our understanding in lung regeneration and development of stem cell mediated therapeutic strategies in combating incurable lung diseases.

Branching of lung epithelium in vitro occurs in the absence of endothelial cells

BACKGROUND

Early lung morphogenesis is driven by tissue interactions. Signals from the lung mesenchyme drive epithelial morphogenesis, but which individual mesenchymal cell types are influencing early epithelial branching and differentiation remains unclear. It has been shown that endothelial cells are involved in epithelial repair and regeneration in the adult lung, and they may also play a role in driving early lung epithelial branching. These data, in combination with evidence that endothelial cells influence early morphogenetic events in the liver and pancreas, led us to hypothesize that endothelial cells are necessary for early lung epithelial branching.

RESULTS

We blocked vascular endothelial growth factor (VEGF) signaling in embryonic day (E) 12.5 lung explants with three different VEGF receptor inhibitors (SU5416, Ki8751, and KRN633) and found that in all cases the epithelium was able to branch despite the loss of endothelial cells. Furthermore, we found that distal lung mesenchyme depleted of endothelial cells retained its ability to induce terminal branching when recombined with isolated distal lung epithelium (LgE). Additionally, isolated E12.5 primary mouse lung endothelial cells, or human lung microvascular endothelial cells (HMVEC-L), were not able to induce branching when recombined with LgE.

CONCLUSIONS

Our observations support the conclusion that endothelial cells are not required for early lung branching.

Identification of a proximal progenitor population from murine fetal lungs with clonogenic and multilineage differentiation potential

Lung development-associated diseases are major causes of morbidity and lethality in preterm infants and children. Access to the lung progenitor/stem cell populations controlling pulmonary development during embryogenesis and early postnatal years is essential to understand the molecular basis of such diseases. Using a Nkx2-1^mCherry reporter mouse, we have identified and captured Nkx2-1-expressing lung progenitor cells from the proximal lung epithelium during fetal development. These cells formed clonal spheres in semisolid culture that could be maintained in vitro and demonstrated self-renewal and expansion capabilities over multiple passages. In-vitro-derived Nkx2-1-expressing clonal spheres differentiated into a polarized epithelium comprised of multiple cell lineages, including basal and secretory cells, that could repopulate decellularized lung scaffolds. Nkx2-1 expression thus defines a fetal lung epithelial progenitor cell population that can be used as a model system to study pulmonary development and associated pediatric diseases.

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Nkx2-1 expression can be used to isolate proximal lung progenitors

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The fetal proximal lung progenitors are distinct from currently known progenitors

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The fetal proximal lung progenitors are clonogenic and self-renewing

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The fetal proximal lung progenitors are multipotent for airway lineages

Airway epithelial progenitors are region specific and show differential responses to bleomycin-induced lung injury

Mechanisms that regulate regional epithelial cell diversity and pathologic remodeling in airways are poorly understood. We hypothesized that regional differences in cell composition and injury-related tissue remodeling result from the type and composition of local progenitors. We used surface markers and the spatial expression pattern of an SFTPC-GFP transgene to subset epithelial progenitors by airway region. Green fluorescent protein (GFP) expression ranged from undetectable to high in a proximal-to-distal gradient. GFP(hi) cells were subdivided by CD24 staining into alveolar (CD24(neg)) and conducting airway (CD24(low)) populations. This allowed for the segregation of three types of progenitors displaying distinct clonal behavior in vitro. GFP(neg) and GFP(low) progenitors both yielded lumen containing colonies but displayed transcriptomes reflective of pseudostratified and distal conducting airways, respectively. CD24(low)GFP(hi) progenitors were present in an overlapping distribution with GFP(low) progenitors in distal airways, yet expressed lower levels of Sox2 and expanded in culture to yield undifferentiated self-renewing progeny. Colony-forming ability was reduced for each progenitor cell type after in vivo bleomycin exposure, but only CD24(low) GFP(hi) progenitors showed robust expansion during tissue remodeling. These data reveal intrinsic differences in the properties of regional progenitors and suggest that their unique responses to tissue damage drive local tissue remodeling.

The three R's of lung health and disease: repair, remodeling, and regeneration

All tissues and organs can be classified according to their ability to repair and regenerate during adult homeostasis and after injury. Some exhibit a high rate of constant cell turnover, while others, such as the lung, exhibit only low-level cell regeneration during normal adult homeostasis but have the ability to rapidly regenerate new cells after injury. Lung regeneration likely involves both activation of progenitor cells as well as cell replacement through proliferation of remaining undamaged cells. The pathways and factors that control this process and its role in disease are only now being explored. In this Review, we will discuss the connection between pathways required for lung development and how the lung responds to injury and disease, with a particular emphasis on recent studies describing the role for the epithelium in repair and regeneration.

Mesenchymal nuclear factor I B regulates cell proliferation and epithelial differentiation during lung maturation

The Nuclear factor I (NFI) transcription factor family consists of four genes (Nfia, Nfib, Nfic and Nfix) that regulate the development of multiple organ systems in mice and humans. Nfib is expressed in both lung mesenchyme and epithelium and mice lacking Nfib have severe lung maturation defects and die at birth. Here we continue our analysis of the phenotype of Nfib⁻/⁻ lungs and show that Nfib specifically in lung mesenchyme controls late epithelial and mesenchymal cell proliferation and differentiation. There are more PCNA, BrdU, PHH3 and Ki67 positive cells in Nfib⁻/⁻ lungs than in wild type lungs at E18.5 and this increase in proliferation marker expression is seen in both epithelial and mesenchymal cells. The loss of Nfib in all lung cells decreases the expression of markers for alveolar epithelial cells (Aqp5 and Sftpc), Clara cells (Scgb1a1) and ciliated cells (Foxj1) in E18.5 lungs. To test for a specific role of Nfib in lung mesenchyme we generated and analyzed Nfib(flox/flox), Dermo1-Cre mice. Loss of Nfib only in mesenchyme results in decreased Aqp5, Sftpc and Foxj1 expression, increased cell proliferation, and a defect in sacculation similar to that seen in Nfib⁻/⁻ mice. In contrast, mesenchyme specific loss of Nfib had no effect on the expression of Scgb1a1 in the airway. Microarray and QPCR analyses indicate that the loss of Nfib in lung mesenchyme affects the expression of genes associated with extracellular matrix, cell adhesion and FGF signaling which could affect distal lung maturation. Our data indicate that mesenchymal Nfib regulates both mesenchymal and epithelial cell proliferation through multiple pathways and that mesenchymal NFI-B-mediated signals are essential for the maturation of distal lung epithelium.

Patterning and plasticity in development of the respiratory lineage

The mammalian respiratory lineage, consisting of the trachea and lung, originates from the ventral foregut in an early embryo. Reciprocal signaling interactions between the foregut epithelium and its associated mesenchyme guide development of the respiratory endoderm, from a naive sheet of cells to multiple cell types that line a functional organ. This review synthesizes current understanding of the early events in respiratory system development, focusing on three main topics: (1) specification of the respiratory system as a distinct organ of the endoderm, (2) patterning and differentiation of the nascent respiratory epithelium along its proximal-distal axis, and (3) plasticity of the respiratory cells during the process of development. This review also highlights areas in need of further study, including determining how early endoderm cells rapidly switch their responses to the same signaling cues during development, and how the general proximal-distal pattern of the lung is converted to fine-scale organization of multiple cell types along this axis.

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.

Heparan sulfate in lung morphogenesis: The elephant in the room

Heparan sulfate (HS) is a structurally complex polysaccharide located on the cell surface and in the extracellular matrix, where it participates in numerous biological processes through interactions with a vast number of regulatory proteins such as growth factors and morphogens. HS is crucial for lung development; disruption of HS synthesis in flies and mice results in a major aberration of airway branching, and in mice, it results in neonatal death as a consequence of malformed lungs and respiratory distress. Epithelial-mesenchymal interactions governing lung morphogenesis are directed by various diffusible proteins, many of which bind to, and are regulated by HS, including fibroblast growth factors, sonic hedgehog, and bone morphogenetic proteins. The majority of research into the molecular mechanisms underlying defective lung morphogenesis and pulmonary pathologies, such as bronchopulmonary dysplasia and pulmonary hypoplasia associated with congenital diaphragmatic hernia (CDH), has focused on abnormal protein expression. The potential contribution of HS to abnormalities of lung development has yet to be explored to any significant extent, which is somewhat surprising given the abnormal lung phenotype exhibited by mutant mice synthesizing abnormal HS. This review summarizes our current understanding of the role of HS and HS-binding proteins in lung morphogenesis and will present in vitro and in vivo evidence for the fundamental importance of HS in airway development. Finally, we will discuss the future possibility of HS-based therapeutics for ameliorating insufficient lung growth associated with lung diseases such as CDH.

Serum-free differentiation of murine embryonic stem cells into alveolar type II epithelial cells

Alveolar type II (AT2) epithelial cells have important functions including the production of surfactant and regeneration of lost alveolar type I epithelial cells. The ability of in vitro production of AT2 cells would offer new therapeutic options in treating pulmonary injuries and disorders including genetically based surfactant deficiencies. Aiming at the generation of AT2-like cells, the differentiation of murine embryonic stem cells (mESCs) toward mesendodermal progenitors (MEPs) was optimized using a "Brachyury-eGFP-knock in" mESC line. eGFP expression demonstrated generation of up to 65% MEPs at day 4 after formation of embryoid bodies (EBs) under serum-free conditions. Plated EBs were further differentiated into AT2-like cells for a total of 25 days in serum-free media resulting in the expression of endodermal marker genes (FoxA2, Sox17, TTR, TTF-1) and of markers for distal lung epithelium (surfactant proteins (SP-) A, B, C, and D, CCSP, aquaporin 5). Notably, expression of SP-C as the only known AT2 cell specific marker could be detected after serum-induction as well as under serum-free conditions. Cytoplasmic localization of SP-C was demonstrated by confocal microscopy. The presence of AT2-like cells was confirmed by electron microscopy providing evidence for polarized cells with apical microvilli and lamellar body-like structures. Our results demonstrate the differentiation of AT2-like cells from mESCs after serum-induction and under serum-free conditions. The established serum-free differentiation protocol will facilitate the identification of key differentiation factors leading to a more specific and effective generation of AT2-like cells from ESCs.

Misexpression of MIA disrupts lung morphogenesis and causes neonatal death

Microarray experiments designed to identify genes differentially expressed in the E11.5 lung and trachea showed that melanoma inhibitory activity (Mia1) was expressed only in the lung. Mia1 was abundantly expressed during early lung development, but was virtually absent by the end of gestation. Distal embryonic lung epithelium showed high levels of Mia1 expression, which was suppressed by treatment with either retinoic acid or the FGF signaling antagonist SU5402. Late-gestation fetuses in which lung epithelial hyperplasia was induced by misexpression of FGF7 or FGF10 showed continued expression of Mia1 in areas of aberrant morphogenesis. Mia1 expression was also significantly increased in urethane-induced lung adenomas. Treatment of E18.5 lung explants with exogenous MIA caused significant reductions in the expression of the lung differentiation markers Sftpa, Sftpb, Sftpc, and Abca3. Bitransgenic mice expressing MIA under the control of the SFTPC promoter after E16.5, the age when Mia1 is normally silenced, died from respiratory failure at birth with morphologically immature lungs associated with reduced levels of saturated phosphatidylcholine and mature SP-B. Microarray analysis showed significant reductions in the expression of Sftpa, Sftpb, Abca3, Aqp5, Lzp-s, Scd2, and Aytl2 in lungs misexpressing MIA. These results suggest that the silencing of Mia1 that occurs in late gestation may be required for maturation of the surfactant system.

Elf5 is an epithelium-specific, fibroblast growth factor-sensitive transcription factor in the embryonic lung

Fibroblast growth factor (FGF) signaling has been shown to be essential for many aspects of normal lung development. To determine epithelial targets of FGF signaling, we cultured embryonic day (E) 11.5 mouse lungs for 24 hr in the presence or absence of the FGF receptor antagonist SU5402, which inhibited branching morphogenesis. Affymetrix gene chip analysis of treated and control epithelia identified several genes regulated by FGF signaling, including Elf5, a member of the Epithelial-specific Ets family of transcription factors. SU5402 reduced Elf5 expression in mesenchyme-free cultures of E12.5 epithelium, demonstrating that the inhibition was direct. In situ hybridization revealed that Elf5 had a dynamic pattern of expression during lung development. We found that expression of Elf5 was induced by FGF7 and FGF10, ligands that primarily bind FGFR2b. To further define the pathways by which FGFs activate Elf5 expression, we cultured E11.5 lung tips in the presence of compounds to inhibit FGF receptors (SU5402), PI3-Kinase/Akt-mediated signaling (LY294002), and MAP Kinase/Erk-mediated signaling (U0126). We found that SU5402 and LY294002 significantly reduced Elf5 expression, whereas U0126 had no effect. LY294002 also reduced Elf5 expression in cultures of purified epithelium. Finally, pAkt was coexpressed with Elf5 in the proximal epithelial airways of E17.5 lungs. These results demonstrate that Elf5 is an FGF-sensitive transcription factor in the lung with a dynamic pattern of expression and that FGF regulation of Elf5 by means of FGFR2b occurs through the PI3-Kinase/Akt pathway.

A murine respiratory-inducing niche displays variable efficiency across human and mouse embryonic stem cell species

Human embryonic stemlike cells (hESCs) are pluripotent cells derived from blastocysts. Differentiating hESCs into respiratory lineages may benefit respiratory therapeutic programs. We previously demonstrated that 24% of all mouse embryonic stem cell (mESC) derivatives cocultured with embryonic day 11.5 (E11.5) mouse lung rudiments display immunoreactivity to the pneumonocyte II specific marker surfactant-associated protein C (Sftpc). Here we further investigate the effects of this inductive niche in terms of its competence to induce hESC derivative SFTPC immunoreactivity and the expression of other markers of terminal lung secretory units. When hESCs were cocultured as single cells, clumps of approximately 10 cells or embryoid bodies (EBs), hESC derivatives formed pan-keratin-positive epithelial tubules at high frequency (>30% of all hESC derivatives). However, human-specific SFTPC immunoreactivity associated with tubule formation only at low frequency (<0.1% of all hESC derivatives). Human-specific SFTPD and secretoglobin family 1A member 1 (SCGB1A1, also known as CC10) transcripts were detected by PCR after prolonged culture. Expression of other terminal lung secretory unit markers (TITF1, SFTPA, and SFTPB) was not detected at any time point analyzed. On the other hand, hESC derivatives cultured as plated EBs in media previously demonstrated to induce Sftpc expression in isolated mouse fetal tracheal epithelium expressed all terminal lung secretory unit markers examined. mESCs and hESCs thus display fundamental differences in their response to the E11.5 mouse lung inductive niche, and these data provide an important step in the delineation of signaling mechanisms capable of efficiently inducing hESC differentiation into terminal secretory units of the lung.

Stage-specific effects of cAMP signaling during distal lung epithelial development

cAMP signaling is postulated to play a role in distal lung epithelial differentiation based on several observations. First, it enhances fibroblast growth factor-induced transdifferentiation of early tracheal epithelium into respiratory epithelium. Second, there are cAMP-responsive elements in the heterologous promoters of Sftpb and Sftpa genes. Third, cAMP augments the effect of dexamethasone in maintaining differentiation of human fetal type II pneumocyte culture. However, this concept has not been thoroughly tested in vivo. In the current study, we modulated cAMP signaling in developing distal lung epithelium in vivo using an inducible transgenic system that expressed a mutant form of Galpha(s) (Galpha(s)Q227L). We failed to demonstrate the ability of cAMP to promote distal epithelial maturation during embryonic stages. The results argue against its physiological role in this process. In addition, induction of cAMP signaling at the late pseudoglandular stage but not during the canalicular or saccular stage surprisingly delayed distal differentiation by suppressing the expression of Sftpc, Sftpa, and Aquaporin5 as well as the formation of lamellar bodies. This stage-specific inhibitory effect was observed in the absence of cellular toxicity or changes in branching. Transgenic lungs did not show significant changes in the known pathways that are important for distal differentiation. Therefore, we propose the existence of yet-to-be identified cAMP-sensitive novel regulators of early distal lung epithelial differentiation. Although the delay of differentiation seemed to be reversible at later stages, it still led to pronounced permanent postnatal airspace enlargement due to impaired paracrine function of distal epithelium in regulating alveolar myofibroblast development.

Derivation of distal airway epithelium from human embryonic stem cells

The pluripotency of embryonic stem cells (ESC) is offering new opportunities in tissue engineering and cell therapy. We have shown previously that alveolar epithelial cells, specifically type II pneumocytes, can be derived from murine ESC and hypothesized that a similar protocol could be used successfully on human ESC. Undifferentiated human ESC were induced to form embryoid bodies that were transferred into adherent culture conditions and grown in a medium designed for the maintenance of mature small airway epithelium. On inverted microscopy, the generated cells showed the cobblestone-like morphology of epithelium. The presence of surfactant protein C, a specific marker of type II pneumocytes, and its corresponding RNA were demonstrated by immunostaining and reverse transcription polymerase chain reaction, respectively. Electron microscopy revealed frequent cells with the typical ultrastructure of type II pneumocytes. This study provides evidence for in vitro induction of the differentiation from human ESC of alveolar type II cells, which have the potential for therapeutic use or construction of an in vitro model of human lung.

Embryonic stem cells form glandular structures and express surfactant protein C following culture with dissociated fetal respiratory tissue

Mouse embryonic stem cells (MESCs) are pluripotent, theoretically immortal cells derived from the inner cell mass of developing blastocysts. The respiratory epithelium develops from the primitive foregut endoderm as a result of inductive morphogenetic interactions with the surrounding visceral mesoderm. After dissociation of the explanted fetal lung into single cells, these morphogenetic signaling pathways instruct reconstitution of the developing lung according to a process known as organotypic regeneration. Data presented here demonstrate that such fetal lung morphogenetic cues induce MESC derivatives to incorporate into the reforming pseudoglandular-like tubular ducts, display pan-keratin and surfactant protein C (Sftpc) immunoreactivity, and express Sftpc transcripts while displaying a normal diploid karyotype in coculture. The Sftpc inductive capacity of dissociated fetal lung tissue shows stage specificity with 24% of all MESC derivatives displaying Sftpc immunoreactivity after coculture with embryonic day 11.5 (E11.5) lung buds compared with 6% and 0.02% following coculture with E12.5 and E13.5 lung buds, respectively. MESC derivative Sftpc immunoreactivity follows a spatial and temporal specific maturation profile with an initially ubiquitous cellular Sftpc immunostaining pattern becoming apically polarized with time. Directing differentiation of MESCs into respiratory lineages has important implications for cell replacement therapeutics aimed at treating respiratory-specific diseases such as cystic fibrosis and idiopathic pulmonary fibrosis.

Erm/thyroid transcription factor 1 interactions modulate surfactant protein C transcription

Expression of surfactant protein C (SP-C), which is restricted to alveolar type II epithelial cells of the adult lung, is critically dependent on thyroid transcription factor 1 (TTF-1). In the present study we have demonstrated that Erm, a member of the Ets family of transcription factors, is expressed in the distal lung epithelium during development and is also restricted to alveolar type II cells in the adult. Erm was up-regulated by fibroblast growth factors (FGFs) in culture, and blocking FGF signaling inhibited Erm expression both in vivo and in vitro. The SP-C minimal promoter was found to contain two potential Ets binding sites, and electrophoretic mobility shift assays showed that two 20-bp wild-type oligonucleotides containing the 5'-GGA(A/T)-3' Ets consensus binding motif were shifted by nuclear extracts from MLE15 cells. Co-transfection assays showed that Erm by itself had little effect on SP-C promoter activity but that Erm significantly enhanced TTF-1-mediated SP-C transcription. Mutation of one of the Ets binding sites reduced SP-C transcription to background levels, whereas mutation of the other site resulted in increased SP-C transcription. Protein-protein interactions between Erm and TTF-1 were demonstrated by mammalian two-hybrid assays and by co-immunoprecipitation assays. Mapping studies showed that the Ets domain of Erm and the combined N terminus and homeodomain of TTF-1 were critical for this interaction. Treatment of primary cultures of adult alveolar type II cells with siRNA targeting Erm diminished expression of both Erm and SP-C but had no effect on beta-actin or GAPDH (glyceraldehyde-3-phosphate dehydrogenase). Taken together, these results demonstrate that Erm is involved in SP-C regulation, which results from an interaction with TTF-1.

Differential response of the epithelium and interstitium in developing human fetal lung explants to hyperoxia

Hyperoxia is closely linked with the development of chronic lung disease of prematurity (CLD), but the exact mechanisms whereby hyperoxia alters the lung architecture in the developing lung remain largely unknown. We developed a fetal human lung organ culture model to investigate (a) the morphologic changes induced by hyperoxia and (b) whether hyperoxia resulted in differential cellular responses in the epithelium and interstitium. The effects of hyperoxia on lung morphometry were analyzed using computer-assisted image analysis. The lung architecture remained largely unchanged in normoxia lasting as long as 4 d. In contrast, hyperoxic culture of pseudoglandular fetal lungs resulted in significant dilatation of airways, thinning of the epithelium, and regression of the interstitium including the pulmonary vasculature. Although there were no significant differences in Ki67 between normoxic and hyperoxic lungs, activated caspase-3 was significantly increased in interstitial cells, but not epithelial cells, under hyperoxic conditions. These changes show that exposure of pseudoglandular lungs to hyperoxia modulates the lung architecture to resemble saccular lungs.

Iroquois genes influence proximo-distal morphogenesis during rat lung development

Lung development is a highly regulated process directed by mesenchymal-epithelial interactions, which coordinate the temporal and spatial expression of multiple regulatory factors required for proper lung formation. The Iroquois homeobox (Irx) genes have been implicated in the patterning and specification of several Drosophila and vertebrate organs, including the heart. Herein, we investigated whether the Irx genes play a role in lung morphogenesis. We found that Irx1-3 and Irx5 expression was confined to the branching lung epithelium, whereas Irx4 was not expressed in the developing lung. Antisense knockdown of all pulmonary Irx genes together dramatically decreased distal branching morphogenesis and increased distention of the proximal tubules in vitro, which was accompanied by a reduction in surfactant protein C-positive epithelial cells and an increase in beta-tubulin IV and Clara cell secretory protein positive epithelial structures. Transmission electron microscopy confirmed the proximal phenotype of the epithelial structures. Furthermore, antisense Irx knockdown resulted in loss of lung mesenchyme and abnormal smooth muscle cell formation. Expression of fibroblast growth factors (FGF) 1, 7, and 10, FGF receptor 2, bone morphogenetic protein 4, and Sonic hedgehog (Shh) were not altered in lung explants treated with antisense Irx oligonucleotides. All four Irx genes were expressed in Shh- and Gli(2)-deficient murine lungs. Collectively, these results suggest that Irx genes are involved in the regulation of proximo-distal morphogenesis of the developing lung but are likely not linked to the FGF, BMP, or Shh signaling pathways.

Epithelial laminin alpha5 is necessary for distal epithelial cell maturation, VEGF production, and alveolization in the developing murine lung

Laminin alpha5 is prominent in the basement membrane of alveolar walls, airways, and pleura in developing and adult lung. Targeted deletion of laminin alpha5 in mice causes developmental defects in multiple organs, but embryonic lethality has precluded examination of the latter stages of lung development. To identify roles for laminin alpha5 in lung development, we have generated an inducible lung epithelial cell-specific Lama5 null (SP-CLama5(fl/-)) mouse through use of the Cre/loxP system, the human surfactant protein C promoter, and the reverse tetracycline transactivator. SP-CLama5(fl/-) embryos exposed to doxycycline from E6.5 died a few hours after birth. Compared to control littermates, SP-CLama5(fl/-) lungs had dilated, enlarged distal airspaces, but basement membrane ultrastructure was preserved. Distal epithelial cell differentiation was perturbed, with a marked reduction of alveolar type II cells and a virtual absence of type I cells. Cell proliferation was reduced and apoptosis was increased. Capillary density was diminished, and this was associated with a decrease in total lung VEGF production. Overall, these findings indicate that epithelial laminin alpha5, independent of its structural function, is necessary for murine lung development, and suggest a role for laminin alpha5 in signaling pathways that promote alveolar epithelial cell differentiation and VEGF expression.

Identification of novel resident pulmonary stem cells: form and function of the lung side population

Resident lung stem cells function to replace all lineages of pulmonary tissue, including mesenchyme, epithelium, and vasculature. The phenotype of the lung side population (SP) cells is currently under investigation; their function is currently unknown. Recent data suggest lung SP cells are an enriched tissue-specific source of organ-specific pulmonary precursors and, therefore, a source of adult stem cells. The adult lung SP cell population has been isolated and characterized for expression of markers indicative of stem cell, epithelial, and mesenchymal lineages. These studies determined that the adult mouse lung SP has epithelial and mesenchymal potential that resides within a CD45- mesenchymal subpopulation, as well as limited hematopoietic ability, which resides in the bone marrow-derived CD45+ subpopulation. The ability to identify these adult lung precursor cells allows us to further study the potential of these cells and their role in the regulation of tissue homeostasis and response to injury. The identification of this target population will potentially allow earlier treatment and, long term, a functional restoration of injured pulmonary tissue and lung health.

FGF-10 induces SP-C and Bmp4 and regulates proximal-distal patterning in embryonic tracheal epithelium

The induction, growth, and differentiation of epithelial lung buds are regulated by the interaction of signals between the lung epithelium and its surrounding mesenchyme. Fibroblast growth factor-10 (FGF-10), which is expressed in the mesenchyme near the distal tips, and bone morphogenetic protein 4 (BMP4), which is expressed in the most distal regions of the epithelium, are important molecules in lung morphogenesis. In the present study, we used two in vitro systems to examine the induction, growth, and differentiation of lung epithelium. Transfilter cultures were used to determine the effect of diffusible factors from the distal lung mesenchyme (LgM) on epithelial branching, and FGF-10 bead cultures were used to ascertain the effect of a high local concentration of a single diffusible molecule on the epithelium. Embryonic tracheal epithelium (TrE) was induced to grow in both culture systems and to express the distal epithelial marker surfactant protein C at the tips nearest the diffusible protein source. TrE cultured on the opposite side of a filter to LgM branched in a pattern resembling intact lungs, whereas TrE cultured in apposition to an FGF-10 bead resembled a single elongating epithelial bud. Examination of the role of BMP4 on lung bud morphogenesis revealed that BMP4 signaling suppressed expression of the proximal epithelial genes Ccsp and Foxj1 in both types of culture and upregulated the expression of Sprouty 2 in TrE cultured with an FGF-10 bead. Antagonizing BMP signaling with Noggin, however, increased expression of both Ccsp and Foxj1.

FGF-18 is upregulated in the postnatal rat lung and enhances elastogenesis in myofibroblasts

The fibroblast growth factors (FGFs) are key players in fetal lung development, but little is known about their status in postnatal lung. Here, we investigated the expression pattern of FGF-18 transcripts through the perinatal period and evidenced a sevenfold increase after birth that paralleled changes in elastin expression. In vitro, recombinant human (rh)FGF-18 had a mitogenic activity on day 21 fetal rat lung fibroblasts and stimulated its own expression in the latter, whereas FGF-2 inhibited it. At 50 or 100 ng/ml, rhFGF-18 increased the expression of α-smooth muscle actin (α-SMA; 2.5-fold), a characteristic marker of myofibroblasts, of tropoelastin (6.5-fold), of lysyl oxidase (2-fold), and of fibulins 1 and 5 (8- and 2.2-fold) in confluent fibroblasts isolated from fetal day 21 lung; similar results were obtained with fibroblasts from day 3 postnatal lungs. Elastin protein expression was also slightly increased in fetal fibroblasts. Lung analysis on day 4 in rat pups that had received rhFGF-18 (3 μg) on days 0 and 1 showed a 1.7-fold increase of tropoelastin transcripts, whereas α-SMA transcripts were unchanged. In contrast, rhFGF-2 markedly decreased expression of elastin in vitro and in vivo and of fibulin 5 in vitro. In addition, vitamin A, which is known to enhance alveolar development, elevated FGF-18 and elastin expressions in day 2 lungs, thus advancing the biological increase. We postulate that FGF-18 is involved in postnatal lung development through stimulating myofibroblast proliferation and differentiation.

Epithelial-mesenchymal interactions in the developing lung

Classical experiments in embryology have shown that normal growth, morphogenetic patterning, and cellular differentiation in the developing lung depend on interactive signaling between the endodermal epithelium and mesenchyme derived from splanchnic mesoderm. These interactions are mediated by a myriad of diffusible factors that are precisely regulated in their temporal and spatial expression. In this review we first describe factors regulating formation of the embryonic foregut. We then discuss the experiments demonstrating the importance of tissue interactions in lung patterning and differentiation. Finally, we detail the roles that a few key signaling systems-fibroblast growth factors and their receptors, sonic hedgehog and Gli genes, Wnt genes and beta-catenin, and BMP4-play as mediators of epithelial-mesenchymal interactions in the developing lung.

Effects of vascular endothelial growth factor on isolated fetal alveolar type II cells

Previous investigations gained from in vivo or lung explant studies suggested that VEGF is an autocrine proliferation and maturation factor for developing alveolar type II cells. The objective of this work was to determine whether VEGF exerted its growth and maturation effects directly on isolated type II cells. These were isolated from 19-day fetal rat lung and cultured in defined medium. The presence of VEGF receptor-2 was assessed in cultured cells at the pre- and posttranslational levels. Recombinant VEGF(165), formerly found to be active on lung explants, failed to enhance type II cell proliferation estimated by thymidine and 5-bromo-2'-deoxy-uridine incorporation. It increased choline incorporation in saturated phosphatidylcholine by 27% but did not increase phospholipid surfactant pool size. VEGF (100 ng/ml) left unchanged the transcript level of surfactant proteins (SP)-A, SP-C, and SP-D but increased SP-B transcripts to four times the control steady-state level. VEGF slightly retarded, but did not prevent, the in vitro transdifferentiation of type II into type I cells, as assessed by immunolabeling of the type I cell marker T1alpha. We conclude that, with the exception of SP-B expression, which appears to be controlled directly, the previously observed effects of this VEGF isoform on type II cells are likely to be exerted indirectly through reciprocal paracrine interactions involving other lung cell types.

Chondroitin sulfate proteoglycans are required for lung growth and morphogenesis in vitro

Proteoglycans (PGs) have been shown to play a key role in the development of many tissues. We have investigated the role of sulfated PGs in early rat lung development by treating cultured tissues with 30 mM sodium chlorate, a global inhibitor of PG sulfation. Chlorate treatment disrupted growth and branching of embryonic day 13 lung explants. Isolated lung epithelium (LgE) migrated toward and invaded lung mesenchyme (LgM), and chlorate irreversibly suppressed this response. Chlorate also inhibited migration of LgE toward beads soaked in FGF10. Chlorate severely decreased branching morphogenesis in tissue recombinants consisting of LgM plus either LgE or tracheal epithelium (TrE) and decreased expression of surfactant protein C gene (SP-C). Chlorate also reduced bone morphogenetic protein-4 expression in cultured tips and recombinants but had no effect on the expression of clara cell 10-kDa protein (CC10), sonic hedgehog (Shh), FGF10, and FGF receptor 2IIIb. Chlorate reduced the growth of LgE in mesenchyme-free culture but did not affect SP-C expression. In contrast, chlorate inhibited both rudiment growth and the induction of SP-C in mesenchyme-free cultured TrE. Treatment of lung tips and tissue recombinants with chondroitinase ABC abolished branching morphogenesis. Chondroitinase also suppressed growth of TrE in mesenchyme-free culture. Chondroitinase treatment, however, had no effect on the induction of SP-C expression in any of these cultures. These results demonstrate the overall importance of sulfated PGs to normal lung development and demonstrate a dynamic role for chondroitin sulfate PGs in embryonic lung growth and morphogenesis.

Role for ETS domain transcription factors Pea3/Erm in mouse lung development

During the development of the mouse lung, the expression of a number of genes, including those encoding growth factors and components of their downstream signaling pathways, is enriched in the epithelium and/or mesenchyme of the distal buds. In this location, they regulate processes such as cell proliferation, branching morphogenesis, and the differentiation of specialized cell types. Here, we report that the expression of Pea3 and Erm (or Etv5, Ets variant gene 5), which encode Pea3 subfamily ETS domain transcription factors, is initially restricted to the distal buds of the developing mouse lung. Erm is transcribed exclusively in the epithelium, while Pea3 is expressed in both epithelium and mesenchyme. Erm/Pea3 are downstream of FGF signaling from the mesenchyme, but their responses toward different FGFs are not the same. The functions of the two proteins were investigated by transgenic expression of a repressor form of Erm specifically in the embryonic lung epithelium. When examined at E18.5, the distal epithelium of transgenic lungs is composed predominantly of immature type II cells, while no mature type I cells are observed. In contrast, the differentiation of proximal epithelial cells, including ciliated cells and Clara cells, appears to be unaffected. A model is proposed for the role of Pea3/Erm during the dynamic process of lung bud outgrowth and proximal-distal differentiation, in response to FGF signaling. Our results provide the first functional evidence that Pea3 subfamily members play a role in epithelial-mesenchymal interactions during lung organogenesis.

Differentiation of human pulmonary type II cells in vitro by glucocorticoid plus cAMP

Mature alveolar type II cells that produce pulmonary surfactant are essential for adaptation to extrauterine life and prevention of infant respiratory distress syndrome. We have developed a new in vitro model to further investigate regulation of type II cell differentiation. Epithelial cells isolated from human fetal lung were cultured in serum-free medium on plastic. Cells treated with dexamethasone + cAMP analog and isobutylmethylxanthine for 4 days exhibited increased phosphatidylcholine synthesis and content of disaturated phosphatidylcholine species, manyfold increases in all surfactant proteins with processing to mature forms, and abundant lamellar bodies. DNA microarray analysis identified approximately 3,100 expressed genes, including subsets of genes induced 2- to >100-fold (approximately 2.5%) or repressed 2- to 18-fold (approximately 1.2%) by hormone treatment. Of the highly regulated genes, most were coregulated in an additive or synergistic manner by dexamethasone and cAMP agents. Approximately 90% of the regulated genes identified by this initial microarray analysis have not been previously recognized as hormone responsive. One newly identified hormone-induced gene is Nkx2.1 (thyroid transcription factor-1), which has a critical role in surfactant protein gene expression. Our findings indicate that glucocorticoid + cAMP is sufficient and necessary for precocious induction of functional type II cells in this in vitro system and that these hormones act primarily in combination to regulate expression of a subset of specific genes.

Regulation of thymic epithelium by keratinocyte growth factor

Here we demonstrate that keratinocyte growth factor (KGF) and FGFR2IIIb signaling can affect development and function of thymic epithelium (TE) and that alphabeta-lineage thymocytes contribute to intrathymic levels of KGF. Thymocyte expression of KGF is developmentally regulated, being undetectable in CD3-4-8- thymocytes and expressed at highest levels by mature CD4 or CD8 thymocytes. Exposure of thymocyte-depleted fetal thymic lobes to KGF resulted in reduced thymic epithelial expression of class II major histocompatibility complex (MHC), invariant chain (Ii), and cathepsin L (CatL) molecules involved in thymocyte-positive selection and also stimulated expression of the cytokines interleukin 6 (IL-6) and thymic stromal-derived lymphopoietin (TSLP), while having little effect on IL-7 or stem cell factor expression. Within intact fetal thymic organ culture (FTOC), exogenous KGF impairs the generation of CD4 thymocytes. Two lines of evidence point to responsiveness of the medullary TE compartment to KGF and FGFR2IIIb signaling. First, the medullary compartment is expanded in intact FTOC exposed to KGF in vitro. Second, in the RAG-deficient thymus, where the thymocytes do not express detectable levels of KGF message, the hypoplastic medullary TE compartment can be expanded by administration of recombinant KGF in vivo. This expansion is accompanied by restoration of the normal profile of medullary TE-associated chemokine expression in the RAG2(-/-) thymus. Collectively, these findings point to a role for KGF and FGFR signaling in the development and function of thymic epithelium.

BMP4 modulates fibroblast growth factor-mediated induction of proximal and distal lung differentiation in mouse embryonic tracheal epithelium in mesenchyme-free culture

Lung morphogenesis and differentiation require interaction between the epithelium and mesenchyme, which is mediated by diffusible molecules such as fibroblast growth factors (FGFs), bone morphogenetic protein 4 (BMP4), and Shh. We have used mesenchyme-free culture to study the effects of these molecules on lung epithelial differentiation. We have tested the individual abilities of FGF1, FGF2, FGF7, FGF9, FGF10, and FGF18, as well as BMP4 and Shh to promote growth and specify distal lung differentiation in mouse tracheal epithelium. The different FGFs exhibited distinct abilities to induce epithelial growth and the expression of the distal lung epithelial marker, surfactant protein C (SP-C), although all FGFs were able to induce expression of BMP4. Tracheal epithelium treated with FGF10 showed little growth and failed to express SP-C as measured by whole-mount in situ hybridization and quantitative real-time polymerase chain reaction. FGF1 treatment resulted in the strongest induction of SP-C. Treatment with BMP4 inhibited epithelial growth and differentiation and antagonized the stimulatory effects of FGF1. In contrast, inhibition of endogenous BMP4 signaling with Noggin protein did not inhibit growth or expression of SP-C but did increase the expression of the proximal lung markers CCSP and HFH4. Expression of Shh was not affected by any of the conditions tested. These results suggest that BMP4 does not signal epithelial cells to adopt a distal fate but may regulate the expansion of proximal epithelial cells in the lung.

Derivation of type II alveolar epithelial cells from murine embryonic stem cells

Embryonic stem (ES) cell pluripotency is being investigated increasingly to obtain specific cell lineages for tissue engineering. However, the possibility that ES cells can give rise to lung tissue has not been tested. We hypothesized that lung epithelial cells (type II pneumocytes) can be derived in vitro from murine ES cells. After withdrawal of leukemia inhibitory factor (LIF) and formation of embryoid bodies in maintenance medium for 10, 20, and 30 days, differentiating ES cells were kept in the same medium or transferred to serum-free small airway growth medium (SAGM) for a further 3 or 14 days of culture. The presence of type II pneumocytes in the resulting mixed cultures was demonstrated by reverse transcriptase-polymerase chain reaction (RT-PCR) of surfactant protein C (SPC) mRNA, immunostaining of SPC, and electron microscopy of osmiophilic lamellar bodies only at 30 days sampling time. SAGM appeared to be more favorable for type II cell formation than ES medium. No SPC transcripts were found in differentiating cells grown under the same conditions without formation of embryoid bodies. These findings could form the basis for the enrichment of ES cell-derived cultures with type II pneumocytes, and provide an in vitro system for investigating mechanisms of lung repair and regeneration.

Maintenance of the differentiated type II cell characteristics by culture on an acellular human amnion membrane

We have developed a Culture system for guinea pig alveolar type II cells using an epithelium-denuded human amnion membrane as a substratum. The differentiated morphology was maintained for 3 wk by both air-interface feeding and immersion feeding when type II cells were cultured on the basement membrane side of the amnion with fibroblasts on the opposite side (coculture). Functionally high levels of surfactant protein B (SP-B) and C (SP-C) messenger ribonucleic acids (mRNAs) were expressed even after the 3-wk cultivation and surfactant protein A mRNA was detected on day 10 of the culture. The differentiation was also maintained when fibroblasts were cultured on lower chambers of the culture plates (separate culture). In contrast, culture of type II cells without fibroblasts (monoculture) could not preserve the mature morphology. When the monoculture was supplemented with keratinocyte growth factor or hepatocyte growth factor, a monolayer of rather cuboidal type II cells with apical microvilli was maintained. However, the percent area of lamellar bodies in these cells was significantly less than that in freshly isolated type II cells, and mRNA expressions of SP-B and SP-C were also considerably suppressed. These findings suggest that other growth factors or combinations of these factors are necessary for the maintenance of the differentiated phenotype. As substratum, a permeable collagen membrane or a thin gel layer of Engelbreth-Holm-Swarm mouse sarcoma extracts did not preserve the mature characteristics. This culture system using an acellular human amnion membrane may provide novel models for research in type II cells.

Maintenance of differentiated function of the surfactant system in human fetal lung type II epithelial cells cultured on plastic

We report a simplified culture system for human fetal lung type II cells that maintains surfactant expression. Type II cells isolated from explant cultures of hormone-treated lungs (18-22 wk gestation) by collagenase + trypsin digestion were cultured on plastic for 4 days in serum-free medium containing dexamethasone (Dex, 10 nM) + 8-bromo-cAMP (0.1 mM + isobutylmethylxanthine (0.1 mM) or were untreated (control). Surfactant protein (SP) mRNAs decreased markedly in control cells between days 1 and 4 of culture, but mRNA levels were high in treated cells on day) 4 (SP-A, SP-B, SP-C, SP-D; 600%, 100%, 85%, 130% of day 0 content, respectively). Dex or cAMP alone increased SP-B, SP-C, and SP-D mRNAs and together had additive effects. The greatest increase in SP-A mRNA occurred with cAMP alone. Treated cells processed pro-SP-B and pro-SP-C proteins to mature forms and had a higher rate of phosphatidylcholine (PC) synthesis (2-fold) and higher saturation of PC (approximately 34% versus 27%) than controls. Only treated cells maintained secretagogue-responsive phospholipid synthesis. By electron microscopy, the treated cells retained lamellar bodies and extensive microvilli. We conclude that Dex and cAMP additively stimulate expression of surfactant components in isolated fetal type II cells, providing a simplified culture system for investigation of surfactant-related, and perhaps other, type II cell functions.

Molecular regulation of lung development

There is increasing evidence suggesting that formation of the tracheobronchial tree and alveoli results from heterogeneity of the epithelial-mesenchymal interactions along the developing respiratory tract. Recent genetic data support this idea and show that this heterogeneity is likely the result of activation of distinct networks of signaling molecules along the proximal-distal axis. Among these signals, fibroblast growth factors, retinoids, Sonic hedgehog, and transforming growth factors appear to play prominent roles. We discuss how these and other pattern regulators may be involved in initiation, branching, and differentiation of the respiratory system.

Phenotypic consequences of lung-specific inducible expression of FGF-3

Members of the fibroblast growth factor (FGF) family play a critical role in embryonic lung development and adult lung physiology. The in vivo investigation of the role FGFs play in the adult lung has been hampered because the constitutive pulmonary expression of these factors often has deleterious effects and frequently results in neonatal lethality. To circumvent these shortcomings, we expressed FGF-3 in the lungs under the control of the progesterone antagonist-responsive binary transgenic system. Four binary transgenic lines were obtained that showed ligand-dependent induction of FGF-3 with induced levels of FGF-3 expression dependent on the levels of expression of the GLp65 regulator as well as the dose of the progesterone antagonist, RU486, administered. FGF-3 expression in the adult mouse lung resulted in two phenotypes depending on the levels of induction of FGF-3. Low levels of FGF-3 expression resulted in massive free alveolar macrophage infiltration. High levels of FGF-3 expression resulted in diffuse alveolar type II cell hyperplasia. Both phenotypes were reversible after the withdrawal of RU486. This system will be a valuable means of investigating the diverse roles of FGFs in the adult lung.

Role of keratinocyte growth factor in the control of surfactant synthesis by fetal lung mesenchyme

Fetal lung maturation is regulated by mesenchymal-epithelial cell communication, which plays a major role in the control of surfactant synthesis by alveolar type II cells. We have recently shown that keratinocyte growth factor (KGF), also called fibroblast growth factor-7, enhances the maturation of fetal alveolar epithelial type II cells. Here, we investigated, among the factors produced by lung mesenchyme, the part attributable to KGF in the control of surfactant synthesis. Using a KGF-neutralizing antibody, we assessed surfactant phospholipid synthesis by measuring choline incorporation into disaturated phosphatidylcholine of isolated fetal type II cells. We found that KGF accounts for about half of the stimulating activity present in fetal lung fibroblast-conditioned medium (FCM). By contrast, the use of an epidermal growth factor-neutralizing antibody did not alter the FCM-stimulating activity. To further delineate KGF properties as a mesenchymal mediator, we wondered about its possibility to relay glucocorticoid-stimulating activity on the synthesis of the phospholipid moiety of surfactant in fetal lung fibroblasts. A 24-h exposure to dexamethasone led us to detect a 50% increase in the level of KGF messenger RNA (mRNA) in isolated fetal lung fibroblasts. Moreover, anti-KGF antibody totally abolished the further increase of FCM-stimulating activity induced by dexamethasone. Thus, KGF seems to be a major player in mediating glucocorticoid stimulation of fetal lung maturation.

Lung fibroblasts improve differentiation of rat type II cells in primary culture

Epithelial-mesenchymal interactions mediate prenatal lung morphogenesis and differentiation, yet little is known about their effects in the adult. In this study we have examined the influence of cocultured lung fibroblasts on rat alveolar type II cell differentiation in primary culture. Type II cells that were co-cultured with lung fibroblasts showed significant increases in messenger RNA (mRNA) levels of surfactant protein (SP)-A, SP-B, SP-C, and SP-D. Metabolic labeling and immunohistochemistry demonstrated that these mRNAs were translated and processed. Addition of 10(-7) M dexamethasone (DEX) to cocultures antagonized the effects of the fibroblasts on SP-A and SP-C, but significantly augmented the effects on SP-B; expression of SP-D was unaffected. Coculture of type II cells with lung fibroblasts also increased acetate incorporation into phospholipids 10-fold, which was antagonized by DEX. Keratinocyte growth factor (KGF) mimicked the effects of lung fibroblasts on SP gene expression, but KGF neutralizing antibodies only partially reduced the effects of lung fibroblasts. KGF increased acetate incorporation into surfactant phospholipids, and the addition of DEX augmented this response. Together, our observations suggest that epithelial--mesenchymal interactions affect type II cell differentiation in the adult lung, and that these effects are partially mediated by KGF.

Coordinated expression of Hoxb genes and signaling molecules during development of the chick respiratory tract

To elucidate the molecular mechanism for regulating the region-specific morphogenesis of the chicken respiratory tract, we analyzed the spatiotemporal expression patterns of the Hoxb genes, Bmp-2, Bmp-4, Wnt-5a, and Wnt-11 in the developing respiratory tract. We found region-specific expression of these genes in the mesenchymal layer of the respiratory tract. Before bronchial branching proceeds, Hoxb genes show nested expression patterns around the ventral-distal tip of the lung bud. As morphogenesis proceeds, these expression domains correspond to the morphological subdivisions of the chick respiratory tract. Hoxb-5 and Hoxb-6 expression domains demarcate the trachea, bronchial tree, and air sacs. Particularly the expression domains of Hoxb-6 to -9 correspond to the morphological subdivisions of the air sacs along the proximodistal axis. Bmp-4 and Bmp-2 are expressed throughout the entire pulmonary mesenchyme and its dorsal half, respectively. Wnt-5a and Wnt-11 are expressed in the tracheal mesenchyme. Interestingly, the expression domain of Bmp-2 is complementary to the Hoxb-6 domain. The respiratory mesenchyme influences the process of epithelial branching during morphogenesis. By tissue recombination experiments, we found that the dorsal and the ventral pulmonary mesenchyme, demarcated by Hoxb-6 expression, have different inductive capacities toward the tracheal epithelium. These observations suggest the possibility that Hoxb genes are involved in the system specifying regional differences in morphogenesis and cytodifferentiation of respiratory tract. In addition, it is possible that BMPs and WNTs mediate region-specific epithelial-mesenchymal interaction in this system.

FGF10 is an inducer and Pax6 a competence factor for lacrimal gland development

We investigated the mechanism of tissue induction and specification using the lacrimal gland as a model system. This structure begins its morphogenesis as a bud-like outgrowth of the conjunctival epithelium and ultimately forms a branched structure with secretory function. Using a reporter transgene as a specific marker for gland epithelium, we show that the transcription factor Pax6 is required for normal development of the gland and is probably an important competence factor. In investigating the cell-cell signaling required, we show that fibroblast growth factor (FGF) 10 is sufficient to stimulate ectopic lacrimal bud formation in ocular explants. Expression of FGF10 in the mesenchyme adjacent to the presumptive lacrimal bud and absence of lacrimal gland development in FGF10-null mice strongly suggest that it is an endogenous inducer. This was supported by the observation that inhibition of signaling by a receptor for FGF10 (receptor 2 IIIb) suppressed development of the endogenous lacrimal bud. In explants of mesenchyme-free gland epithelium, FGF10 stimulated growth but not branching morphogenesis. This suggested that its role in induction is to stimulate proliferation and, in turn, that FGF10 combines with other factors to provide the instructive signals required for lacrimal gland development.

In vitro effects of growth factors on lung hypoplasia in a model of congenital diaphragmatic hernia

BACKGROUND/PURPOSE

Pulmonary hypoplasia, a leading contributor to the lethality of congenital diaphragmatic hernia (CDH), precedes diaphragmatic malformation in the nitrofen model and persists to allow experimental manipulations in organ culture. Fibroblast growth factors (FGFs) are crucial to early lung development. Acidic FGF (FGF-1) binds to all FGF receptors and enhances in vitro branching morphogenesis. Basic FGF (FGF-2) is localized to developing airway epithelium, basement membrane, and extracellular matrix. Heparin (HEP) modulates FGF kinetics and inhibits smooth muscle proliferation in lung primordia. The aim of this study was to examine the morphological effects of fibroblast growth factors and heparin on lung hypoplasia in an organ culture model.

METHODS

Sprague-Dawley rats were fed nitrofen on day 9.5 of pregnancy to induce lung hypoplasia and CDH in newborns. Control rats received olive oil. Normal and hypoplastic lung primordia were microdissected on day 13.5 of gestation and cultured up to 78 hours in plain media with or without FGF-1 or FGF-2, with or without HEP. In vitro morphological development was studied by serial measurements of terminal bud count, lung area, and lung perimeter.

RESULTS

Over 120 fetal lung specimens were studied (n > or = 4 per group). Significant increases in area, perimeter, and bud count were seen in normal lungs cultured with FGF-1 plus HEP compared with control media (P < .05). In the nitrofen lungs, FGF1 plus HEP yielded reductions in all parameters compared with those in control media (P < .05), whereas FGF-2 produced significant expansion in lung area but marked reductions in bud count and lung perimeter divided by square root of area (P < .05). Heparin did not produce substantial or sustained alteration of morphology in normal or hypoplastic lungs.

CONCLUSIONS

These observations may indicate an intrinsic abnormality of FGF processing in the hypoplastic nitrofen lung before diaphragmatic malformation. Heparin did not rescue abnormal lung development. Mechanisms underlying the differential effects of these agents now need to be explored to target fetal lung growth and improve the dismal prognosis of human CDH.