Beta-catenin regulates differentiation of respiratory epithelial cells in vivo

Beta-catenin/T-cell factor signaling is activated during lung injury and promotes the survival and migration of alveolar epithelial cells

The Wnt/beta-catenin signaling cascade activates genes that allow cells to adopt particular identities throughout development. In adult self-renewing tissues like intestine and blood, activation of the Wnt pathway maintains a progenitor phenotype, whereas forced inhibition of this pathway promotes differentiation. In the lung alveolus, type 2 epithelial cells (AT2) have been described as progenitors for the type 1 cell (AT1), but whether AT2 progenitors use the same signaling mechanisms to control differentiation as rapidly renewing tissues is not known. We show that adult AT2 cells do not exhibit constitutive beta-catenin signaling in vivo, using the AXIN2(+/LacZ) reporter mouse, or after fresh isolation of an enriched population of AT2 cells. Rather, this pathway is activated in lungs subjected to bleomycin-induced injury, as well as upon placement of AT2 cells in culture. Forced inhibition of beta-catenin/T-cell factor signaling in AT2 cultures leads to increased cell death. Cells that survive show reduced migration after wounding and reduced expression of AT1 cell markers (T1alpha and RAGE). These results suggest that AT2 cells may function as facultative progenitors, where activation of Wnt/beta-catenin signaling during lung injury promotes alveolar epithelial survival, migration, and differentiation toward an AT1-like phenotype.

A role for Wnt signaling genes in the pathogenesis of impaired lung function in asthma

RATIONALE

Animal models demonstrate that aberrant gene expression in utero can result in abnormal pulmonary phenotypes.

OBJECTIVES

We sought to identify genes that are differentially expressed during in utero airway development and test the hypothesis that variants in these genes influence lung function in patients with asthma.

METHODS

Stage 1 (Gene Expression): Differential gene expression analysis across the pseudoglandular (n = 27) and canalicular (n = 9) stages of human lung development was performed using regularized t tests with multiple comparison adjustments. Stage 2 (Genetic Association): Genetic association analyses of lung function (FEV(1), FVC, and FEV(1)/FVC) for variants in five differentially expressed genes were conducted in 403 parent-child trios from the Childhood Asthma Management Program (CAMP). Associations were replicated in 583 parent-child trios from the Genetics of Asthma in Costa Rica study.

MEASUREMENTS AND MAIN RESULTS

Of the 1,776 differentially expressed genes between the pseudoglandular (gestational age: 7-16 wk) and the canalicular (gestational age: 17-26 wk) stages, we selected 5 genes in the Wnt pathway for association testing. Thirteen single nucleotide polymorphisms in three genes demonstrated association with lung function in CAMP (P < 0.05), and associations for two of these genes were replicated in the Costa Ricans: Wnt1-inducible signaling pathway protein 1 with FEV(1) (combined P = 0.0005) and FVC (combined P = 0.0004), and Wnt inhibitory factor 1 with FVC (combined P = 0.003) and FEV(1)/FVC (combined P = 0.003).

CONCLUSIONS

Wnt signaling genes are associated with impaired lung function in two childhood asthma cohorts. Furthermore, gene expression profiling of human fetal lung development can be used to identify genes implicated in the pathogenesis of lung function impairment in individuals with asthma.

Wnt signaling regulates smooth muscle precursor development in the mouse lung via a tenascin C/PDGFR pathway

Paracrine signaling from lung epithelium to the surrounding mesenchyme is important for lung SMC development and function and is a contributing factor in an array of pulmonary diseases such as bronchopulmonary dysplasia, pulmonary hypertension, and asthma. Wnt7b, which is exclusively expressed in the lung epithelium, is important for lung vascular smooth muscle integrity, but the underlying mechanism by which Wnt signaling regulates lung SMC development is unclear. In this report, we have demonstrated that Wnt7b regulates a program of mesenchymal differentiation in the mouse lung that is essential for SMC development. Genetic loss-of-function studies showed that Wnt7b and beta-catenin were required for expression of Pdgfralpha and Pdgfrbeta and proliferation in pulmonary SMC precursors. In contrast, gain-of-function studies showed that activation of Wnt signaling increased the expression of both Pdgfralpha and Pdgfrbeta as well as the proliferation of SMC precursors. We further showed that the effect on Pdgfr expression was, in part, mediated by direct transcriptional regulation of the ECM protein tenascin C (Tnc), which was necessary and sufficient for Pdgfralpha/beta expression in lung explants. Moreover, this pathway was highly upregulated in a mouse model of asthma and in lung tissue from patients with pulmonary hypertension. Together, these data define a Wnt/Tnc/Pdgfr signaling axis that is critical for smooth muscle development and disease progression in the lung.

Pleiotrophin regulates lung epithelial cell proliferation and differentiation during fetal lung development via beta-catenin and Dlk1

The role of pleiotrophin in fetal lung development was investigated. We found that pleiotrophin and its receptor, protein-tyrosine phosphatase receptor beta/zeta, were highly expressed in mesenchymal and epithelial cells of the fetal lungs, respectively. Using isolated fetal alveolar epithelial type II cells, we demonstrated that pleiotrophin promoted fetal type II cell proliferation and arrested type II cell trans-differentiation into alveolar epithelial type I cells. Pleiotrophin also increased wound healing of injured type II cell monolayer. Knockdown of pleiotrophin influenced lung branching morphogenesis in a fetal lung organ culture model. Pleiotrophin increased the tyrosine phosphorylation of beta-catenin, promoted beta-catenin translocation into the nucleus, and activated T cell factor/lymphoid enhancer factor transcription factors. Dlk1, a membrane ligand that initiates the Notch signaling pathway, was identified as a downstream target of the pleiotrophin/beta-catenin pathway by endogenous dlk1 expression, promoter assay, and chromatin immunoprecipitation. These results provide evidence that pleiotrophin regulates fetal type II cell proliferation and differentiation via integration of multiple signaling pathways including pleiotrophin, beta-catenin, and Notch pathways.

Stabilized beta-catenin in lung epithelial cells changes cell fate and leads to tracheal and bronchial polyposis

The precise mechanisms by which beta-catenin controls morphogenesis and cell differentiation remain largely unknown. Using embryonic lung development as a model, we deleted exon 3 of beta-catenin via Nkx2.1-cre in the Catnb[+/lox(ex3)] mice and studied its impact on epithelial morphogenesis. Robust selective accumulation of truncated, stabilized beta-catenin was found in Nkx2.1-cre;Catnb[+/lox(ex3)] lungs that were associated with the formation of polyp-like structures in the trachea and main-stem bronchi. Characterization of polyps suggests that accumulated beta-catenin impacts epithelial morphogenesis in at least two ways. "Intracellular" accumulation of beta-catenin blocked differentiation of spatially-appropriate airway epithelial cell types, Clara cells, ciliated cells and basal cells, and activated UCHL1, a marker for pulmonary neuroendocrine cells. There was also evidence for a "paracrine" impact of beta-catenin accumulation, potentially mediated via activation of Bmp4 that inhibited Clara and ciliated, but not basal cell differentiation. Thus, excess beta-catenin can alter cell fate determination by both direct and paracrine mechanisms.

Functional cross-talk between beta-catenin and NFkappaB signaling pathways in colonic crypts of mice in response to progastrin

We recently reported a critical role of NFkappaB in mediating hyperproliferative and anti-apoptotic effects of progastrin on proximal colonic crypts of transgenic mice overexpressing progastrin (Fabp-PG mice). We now report activation of beta-catenin in colonic crypts of mice in response to chronic (Fabp-PG mice) and acute (wild type FVB/N mice) progastrin stimulation. Significant increases were measured in relative levels of cellular and nuclear beta-catenin and pbeta-cat45 in proximal colonic crypts of Fabp-PG mice compared with that in wild type littermates. Distal colonic crypts were less responsive. Interestingly, beta-catenin activation was downstream of IKKalpha,beta/NFkappaB, because treatment of Fabp-PG mice with the NFkappaB essential modulator (NEMO) peptide (inhibitor of IKKalpha,beta/NFkappaB activation) significantly blocked increases in cellular/nuclear levels of total beta-catenin/pbeta-cat45/and pbeta-cat552 in proximal colons. Cellular levels of pbeta-cat33,37,41, however, increased in proximal colons in response to NEMO, probably because of a significant increase in pGSK-3betaTyr216, facilitating degradation of beta-catenin. NEMO peptide significantly blocked increases in cyclin D1 expression, thereby, abrogating hyperplasia of proximal crypts. Goblet cell hyperplasia in colonic crypts of Fabp-PG mice was abrogated by NEMO treatment, suggesting a cross-talk between the NFkappaB/beta-catenin and Notch pathways. Cellular proliferation and crypt lengths increased significantly in proximal but not distal crypts of FVB/N mice injected with 1 nM progastrin associated with a significant increase in cellular/nuclear levels of total beta-catenin and cyclin D1. Thus, intracellular signals, activated in response to acute and chronic stimulation with progastrin, were similar and specific to proximal colons. Our studies suggest a novel possibility that activation of beta-catenin, downstream to the IKKalpha,beta/NFkappaB pathway, may be integral to the hyperproliferative effects of progastrin on proximal colonic crypts.

Global transcriptional characterization of a mouse pulmonary epithelial cell line for use in genetic toxicology

Prior to its application for in vitro toxicological assays, thorough characterization of a cell line is essential. The present study uses global transcriptional profiling to characterize a lung epithelial cell line (FE1) derived from MutaMouse [White, P.A., Douglas, G.R., Gingerich, J., Parfett, C., Shwed, P., Seligy, V., Soper, L., Berndt, L., Bayley, J., Wagner, S., Pound, K., Blakey, D., 2003. Development and characterization of a stable epithelial cell line from Muta Mouse lung. Environmental and Molecular Mutagenesis 42, 166-184]. Results presented here demonstrate the origin of the FE1 lung cell line as epithelial, presenting both type I and type II alveolar phenotype. An assessment of toxicologically-relevant genes, including those involved in the response to stress and stimuli, DNA repair, cellular metabolism, and programmed cell death, revealed changes in expression of 22-27% of genes in one or more culture type (proliferating and static FE1 cultures, primary epithelial cultures) compared with whole lung isolates. Gene expression analysis at 4 and 24h following benzo(a)pyrene exposure revealed the induction of cyp1a1, cyp1a2, and cyp1b1 in FE1 cells and lung isolates. The use of DNA microarrays for gene expression profiling allows an improved understanding of global, coordinated cellular events arising in cells under different physiological conditions. Taken together, these data indicate that the FE1 cell line is derived from a cell type relevant to toxic responses in vivo, and shows some similarity in response to chemical insult as the original tissue.

Multiple roles for Sox2 in the developing and adult mouse trachea

The esophagus, trachea and lung develop from the embryonic foregut, yet acquire and maintain distinct tissue phenotypes. Previously, we demonstrated that the transcription factor Sox2 is necessary for foregut morphogenesis and esophagus development. We show that Sox2 is also required for the normal development of the trachea and lung. In both the embryo and adult, Sox2 is exclusively expressed in the epithelium of the trachea and airways. We use an Nkx2.5-Cre transgene and a Sox2 floxed allele to conditionally delete Sox2 in the ventral epithelial domain of the early anterior foregut, which gives rise to the future trachea and lung buds. All conditional mutants die of respiratory distress at birth, probably due to abnormal differentiation of the laryngeal and tracheal cartilage as a result of defective epithelial-mesenchymal interaction. About 60% of the mutants have a short trachea, suggesting that the primary budding site of the lung shifts anteriorly. In the tracheal epithelium of all conditional mutants there are significantly more mucus-producing cells compared with wild type, and fewer basal stem cells, ciliated and Clara cells. Differentiation of the epithelium lining the conducting airways in the lung is abnormal, suggesting that Sox2 also plays a role in the differentiation of embryonic airway progenitors into specific lineages. Conditional deletion of Sox2 was then used to test its role in adult epithelium maintenance. We found that epithelial cells, including basal stem cells, lacking Sox2 show a reduced capacity to proliferate in culture and to repair after injury in vivo. Taken together, these results define multiple roles for Sox2 in the developing and adult trachea.

Wnt pathway aberrations including autocrine Wnt activation occur at high frequency in human non-small-cell lung carcinoma

Lung cancer is the most common cause of cancer mortality worldwide. Non-small-cell lung carcinomas (NSCLCs), which represent around 80% of lung tumors, exhibit poor prognosis and are usually refractory to conventional chemotherapy. Elucidating the molecular and cellular mechanisms that are dysregulated in NSCLCs may lead to new possibilities for targeted therapy or enhanced efficacy of current therapies. Here we demonstrate Wnt pathway activation in around 50% of human NSCLC cell lines and primary tumors, through different mechanisms, including autocrine Wnt pathway activation involving upregulation of specific Wnt ligands. Downregulation of activated Wnt signaling inhibited NSCLC proliferation and induced a more differentiated phenotype. Together, our findings establish importance of activated Wnt signaling in human NSCLCs and offer the possibility of targeting upregulated Wnt signaling as a new therapeutic modality for this disease.

WNT signaling in lung disease: a failure or a regeneration signal?

The WNT family of signaling proteins is essential to organ development in general and lung morphogenesis in particular. Originally identified as a developmentally active signaling pathway, the WNT pathway has recently been linked to the pathogenesis of important lung diseases, in particular lung cancer and pulmonary fibrosis. This review summarizes our current understanding about WNT signaling in lung development and disease, and is structured into three chapters. The first chapter presents an introduction to WNT signaling, outlining WNT proteins, their receptors and signaling intermediates, as well as the regulation of this complex pathway. The second chapter focuses on the role of WNT signaling in the normal embryonic and adult lung, and highlights recent findings of altered WNT signaling in lung diseases, such as lung cancer, pulmonary fibrosis, or pulmonary arterial hypertension. In the last chapter, we will discuss novel data and ideas about the biological effects of WNT signaling on the cellular level, highlighting pleiotropic effects induced by WNT ligands on distinct cell types, and how these cellular effects may be relevant to the pathogenesis of the aforementioned diseases.

Lung cancer: developmental networks gone awry?

High-throughput genomic data for both lung development and lung cancer continue to accumulate. Significant molecular intersection between these two processes has been hypothesized due to overlap in phenotypes and genomic variation. Examining the network biology of both cancer and development of the lung may shed functional light on the individual signaling modules involved. Stem cell biology may explain a portion of this network intersection and consequently studying lung organogenesis may have relevance for understanding lung cancer. This review summarizes our understanding of the potential overlapping mechanisms involved in lung development and lung tumorigenesis.

Airway mucus: the good, the bad, the sticky

Mucus production is a primary defense mechanism for maintaining lung health. However, the overproduction of mucin (the chief glycoprotein component of mucus) is a common pathological feature in asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis (CF), and lung cancer. Although it is associated with disease progression, effective therapies that directly target mucin overproduction and hypersecretion are lacking. Recent advances in our understanding of the control of mucin gene expression in the lungs, the cells that produce airway mucins, and the mechanisms used for releasing them into the airways have provided new potentials for the development of efficacious interventions that will be discussed in this review.

Deciphering the function of canonical Wnt signals in development and disease: conditional loss- and gain-of-function mutations of beta-catenin in mice

Wnt signaling is one of a handful of powerful signaling pathways that play crucial roles in the animal life by controlling the genetic programs of embryonic development and adult homeostasis. When disrupted, these signaling pathways cause developmental defects, or diseases, among them cancer. The gateway of the canonical Wnt pathway, which contains >100 genes, is an essential molecule called beta-catenin (Armadillo in Drosophila). Conditional loss- and gain-of-function mutations of beta-catenin in mice provided powerful tools for the functional analysis of canonical Wnt signaling in many tissues and organs. Such studies revealed roles of Wnt signaling that were previously not accessible to genetic analysis due to the early embryonic lethality of conventional beta-catenin knockout mice, as well as the redundancy of Wnt ligands, receptors, and transcription factors. Analysis of conditional beta-catenin loss- and gain-of-function mutant mice demonstrated that canonical Wnt signals control progenitor cell expansion and lineage decisions both in the early embryo and in many organs. Canonical Wnt signaling also plays important roles in the maintenance of various embryonic or adult stem cells, and as recent findings demonstrated, in cancer stem cell types. This has opened new opportunities to model numerous human diseases, which have been associated with deregulated Wnt signaling. Our review summarizes what has been learned from genetic studies of the Wnt pathway by the analysis of conditional beta-catenin loss- and gain-of-function mice.

A Gata6-Wnt pathway required for epithelial stem cell development and airway regeneration

Epithelial organs, including the lung, are known to possess regenerative abilities through activation of endogenous stem cell populations, but the molecular pathways regulating stem cell expansion and regeneration are not well understood. Here we show that Gata6 regulates the temporal appearance and number of bronchioalveolar stem cells (BASCs) in the lung, its absence in Gata6-null lung epithelium leading to the precocious appearance of BASCs and concurrent loss in epithelial differentiation. This expansion of BASCs was the result of a pronounced increase in canonical Wnt signaling in lung epithelium upon loss of Gata6. Expression of the noncanonical Wnt receptor Fzd2 was downregulated in Gata6 mutants and increased Fzd2 or decreased beta-catenin expression rescued, in part, the lung epithelial defects in Gata6 mutants. During lung epithelial regeneration, canonical Wnt signaling was activated in the niche containing BASCs and forced activation of Wnt signaling led to a large increase in BASC numbers. Moreover, Gata6 was required for proper lung epithelial regeneration, and postnatal loss of Gata6 led to increased BASC expansion and decreased differentiation. Together, these data demonstrate that Gata6-regulated Wnt signaling controls the balance between progenitor expansion and epithelial differentiation required for both lung development and regeneration.

Functional Wnt signaling is increased in idiopathic pulmonary fibrosis

Background

Idiopathic pulmonary fibrosis (IPF) is a fatal lung disease, characterized by distorted lung architecture and loss of respiratory function. Alveolar epithelial cell injury and hyperplasia, enhanced extracellular matrix deposition, and (myo)fibroblast activation are features of IPF. Wnt/β-catenin signaling has been shown to determine epithelial cell fate during development. As aberrant reactivation of developmental signaling pathways has been suggested to contribute to IPF pathogenesis, we hypothesized that Wnt/β-catenin signaling is activated in epithelial cells in IPF. Thus, we quantified and localized the expression and activity of the Wnt/β-catenin pathway in IPF.

Methodology/Principal Findings

The expression of Wnt1, 3a, 7b, and 10b, the Wnt receptors Fzd1-4, Lrp5-6, as well as the intracellular signal transducers Gsk-3β, β-catenin, Tcf1, 3, 4, and Lef1 was analyzed in IPF and transplant donor lungs by quantitative real-time (q)RT-PCR. Wnt1, 7b and 10b, Fzd2 and 3, β-catenin, and Lef1 expression was significantly increased in IPF. Immunohistochemical analysis localized Wnt1, Wnt3a, β-catenin, and Gsk-3β expression largely to alveolar and bronchial epithelium. This was confirmed by qRT-PCR of primary alveolar epithelial type II (ATII) cells, demonstrating a significant increase of Wnt signaling in ATII cells derived from IPF patients. In addition, Western blot analysis of phospho-Gsk-3β, phospho-Lrp6, and β-catenin, and qRT-PCR of the Wnt target genes cyclin D1, Mmp 7, or Fibronectin 1 demonstrated increased functional Wnt/β-catenin signaling in IPF compared with controls. Functional in vitro studies further revealed that Wnt ligands induced lung epithelial cell proliferation and (myo)fibroblast activation and collagen synthesis.

Conclusions/Significance

Our study demonstrates that the Wnt/β-catenin pathway is expressed and operative in adult lung epithelium. Increased Wnt/β-catenin signaling may be involved in epithelial cell injury and hyperplasia, as well as impaired epithelial-mesenchymal cross-talk in IPF. Thus, modification of Wnt signaling may represent a therapeutic option in IPF.

Wnt signaling pathway and lung disease

The Wnt pathway plays an important role in development and in regulating adult stem cell systems. A variety of cellular processes is mediated by Wnt signaling, which includes cellular proliferation, differentiation, survival, apoptosis, and cell motility. Loss of regulation of these pathways can lead to tumorigenesis, and the Wnt pathway has been implicated in the development of several types of cancers, including colon, lung, leukemia, breast, thyroid, and prostate. The Wnt pathway has also been associated with other lung diseases such as interstitial lung disease (ILD) and asthma. Our increasing understanding of the Wnt pathway offers great hope that new molecular-based screening tests and pharmaceutical agents that selectively target this pathway will be developed to diagnose and treat these diseases in the future.

Conditional stabilization of beta-catenin expands the pool of lung stem cells

Maintenance of classic stem cell hierarchies is dependent upon stem cell self-renewal mediated in part by Wnt/beta-catenin regulation of the cell cycle. This function is critical in rapidly renewing tissues due to the obligate role played by the tissue stem cell. However, the stem cell hierarchy responsible for maintenance of the conducting airway epithelium is distinct from classic stem cell hierarchies. The epithelium of conducting airways is maintained by transit-amplifying cells in the steady state; rare bronchiolar stem cells are activated to participate in epithelial repair only following depletion of transit-amplifying cells. Here, we investigate how signaling through beta-catenin affects establishment and maintenance of the stem cell hierarchy within the slowly renewing epithelium of the lung. Conditional potentiation of beta-catenin signaling in the embryonic lung results in amplification of airway stem cells through attenuated differentiation rather than augmented proliferation. Our data demonstrate that the differentiation-modulating activities of stabilized beta-catenin account for expansion of tissue stem cells.

Idiopathic pulmonary fibrosis: aberrant recapitulation of developmental programs?

The authors discuss evidence suggesting that embryonic signaling pathways involved in epithelium/mesenchymal communication and epithelial cell plasticity may be aberrantly switched on in idiopathic pulmonary fibrosis.

Enhanced expression of 70-kilodalton heat shock protein limits cell division in a sepsis-induced model of acute respiratory distress syndrome

OBJECTIVE

Fibrotic changes are initiated early in acute respiratory distress syndrome. This may involve overproliferation of alveolar type II cells. In an animal model of acute respiratory distress syndrome, we have shown that the administration of an adenoviral vector overexpressing the 70-kd heat shock protein (AdHSP) limited pathophysiological changes. We hypothesized that this improvement may be modulated, in part, by an early AdHSP-induced attenuation of alveolar type II cell proliferation.

DESIGN

Laboratory investigation.

SETTING

Hadassah-Hebrew University and University of Pennsylvania animal laboratories.

SUBJECTS

Sprague-Dawley Rats (250 g).

INTERVENTIONS

Lung injury was induced in male Sprague-Dawley rats via cecal ligation and double puncture. At the time of cecal ligation and double puncture, we injected phosphate-buffered saline, AdHSP, or AdGFP (an adenoviral vector expressing the marker green fluorescent protein) into the trachea. Rats then received subcutaneous bromodeoxyuridine. In separate experiments, A549 cells were incubated with medium, AdHSP, or AdGFP. Some cells were also stimulated with tumor necrosis factor-alpha. After 48 hrs, cytosolic and nuclear proteins from rat lungs or cell cultures were isolated. These were subjected to immunoblotting, immunoprecipitation, electrophoretic mobility shift assay, fluorescent immunohistochemistry, and Northern blot analysis.

MEASUREMENTS AND MAIN RESULTS

Alveolar type I cells were lost within 48 hrs of inducing acute respiratory distress syndrome. This was accompanied by alveolar type II cell proliferation. Treatment with AdHSP preserved alveolar type I cells and limited alveolar type II cell proliferation. Heat shock protein 70 prevented overexuberant cell division, in part, by inhibiting hyperphosphorylation of the regulatory retinoblastoma protein. This prevented retinoblastoma protein ubiquitination and degradation and, thus, stabilized the interaction of retinoblastoma protein with E2F1, a key cell division transcription factor.

CONCLUSIONS

: Heat shock protein 70-induced attenuation of cell proliferation may be a useful strategy for limiting lung injury when treating acute respiratory distress syndrome if consistent in later time points.

Ror2 modulates the canonical Wnt signaling in lung epithelial cells through cooperation with Fzd2

Background

Wnt signaling is mediated through 1) the beta-catenin dependent canonical pathway and, 2) the beta-catenin independent pathways. Multiple receptors, including Fzds, Lrps, Ror2 and Ryk, are involved in Wnt signaling. Ror2 is a single-span transmembrane receptor-tyrosine kinase (RTK). The functions of Ror2 in mediating the non-canonical Wnt signaling have been well established. The role of Ror2 in canonical Wnt signaling is not fully understood.

Results

Here we report that Ror2 also positively modulates Wnt3a-activated canonical signaling in a lung carcinoma, H441 cell line. This activity of Ror2 is dependent on cooperative interactions with Fzd2 but not Fzd7. In addition, Ror2-mediated enhancement of canonical signaling requires the extracellular CRD, but not the intracellular PRD domain of Ror2. We further provide evidence that the positive effect of Ror2 on canonical Wnt signaling is inhibited by Dkk1 and Krm1 suggesting that Ror2 enhances an Lrp-dependent STF response.

Conclusion

The current study demonstrates the function of Ror2 in modulating canonical Wnt signaling. These findings support a functional scheme whereby regulation of Wnt signaling is achieved by cooperative functions of multiple mediators.

FGF10 maintains distal lung bud epithelium and excessive signaling leads to progenitor state arrest, distalization, and goblet cell metaplasia

BACKGROUND

Interaction with the surrounding mesenchyme is necessary for development of endodermal organs, and Fibroblast growth factors have recently emerged as mesenchymal-expressed morphogens that direct endodermal morphogenesis. The fibroblast growth factor 10 (Fgf10) null mouse is characterized by the absence of lung bud development. Previous studies have shown that this requirement for Fgf10 is due in part to its role as a chemotactic factor during branching morphogenesis. In other endodermal organs Fgf10 also plays a role in regulating differentiation.

RESULTS

Through gain-of-function analysis, we here find that FGF10 inhibits differentiation of the lung epithelium and promotes distalization of the embryonic lung. Ectopic expression of FGF10 in the lung epithelium caused impaired lung development and perinatal lethality in a transgenic mouse model. Lung lobes were enlarged due to increased interlobular distance and hyperplasia of the airway epithelium. Differentiation of bronchial and alveolar cell lineages was inhibited. The transgenic epithelium consisted predominantly of proliferating progenitor-like cells expressing Pro-surfactant protein C, TTF1, PEA3 and Clusterin similarly to immature distal tip cells. Strikingly, goblet cells developed within this arrested epithelium leading to goblet cell hyperplasia.

CONCLUSION

We conclude that FGF10 inhibits terminal differentiation in the embryonic lung and maintains the distal epithelium, and that excessive levels of FGF10 leads to metaplastic differentiation of goblet cells similar to that seen in chronic inflammatory diseases.

Tissue-specific transgenic, conditional knockout and knock-in mice of genes in the canonical Wnt signaling pathway

The Wnt signaling pathway plays key roles in the development and homeostasis of a number of organs such as the brain, lung, liver, heart, gastrointestinal tract, mammary gland, skin, and bone, as well as of the immune system. Studies on conventional knockout mice of the genes in the Wnt signaling pathway have revealed its essential roles in these tissues; however, most of these knockout mice die during embryogenesis or soon after birth. Through more advanced techniques such as Cre/loxP and tetracycline-inducible systems, a gene of interest can be expressed or inactivated in a tissue-specific and time-controlled manner. Here we review recent papers on the tissue-specific transgenic, conditional knockout and knock-in mice of the genes in the Wnt signaling pathway In addition to such engineered mice, we also list reporter mice that have been generated to determine the activity of the canonical Wnt signaling pathway in mouse tissues.

SERKAL syndrome: an autosomal-recessive disorder caused by a loss-of-function mutation in WNT4

The WNT-signaling pathway plays a major role during mammalian embryogenesis. We report a novel autosomal-recessive syndrome that consists of female to male sex reversal and renal, adrenal, and lung dysgenesis and is associated with additional developmental defects. Using a candidate-gene approach, we identified a disease-causing homozygous missense mutation in the human WNT4 gene. The mutation was found to result in markedly reduced WNT4 mRNA levels in vivo and in vitro and to downregulate WNT4-dependent inhibition of beta-catenin degradation. Taken together with previous observations in animal models, the present data attribute a pivotal role to WNT4 signaling during organogenesis in humans.

Determination of the role of target tissue metabolism in lung carcinogenesis using conditional cytochrome P450 reductase-null mice

Critical to mechanisms of chemical carcinogenesis and the design of chemopreventive strategies is whether procarcinogen bioactivation in an extrahepatic target tissue (e.g., the lung) is essential for tumor formation. This study aims to develop a mouse model capable of revealing the role of pulmonary microsomal cytochrome P450 (P450)-mediated metabolic activation in xenobiotic-induced lung cancer. A novel triple transgenic mouse model, with the NADPH-P450 reductase (Cpr) gene deleted in a lung-specific and doxycycline-inducible fashion (lung-Cpr-null), was generated. CPR, the obligate electron donor for microsomal P450 enzymes, is essential for the bioactivation of many procarcinogens. The lung-Cpr-null mouse was studied to resolve whether pulmonary P450 plays a major role in 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung cancer by producing carcinogenic metabolites in the target tissue. A liver-Cpr-null mouse was also studied to test whether hepatic P450 contributes predominantly to systemic clearance of NNK, thereby decreasing NNK-induced lung cancer. The numbers of NNK-induced lung tumors were reduced in the lung-Cpr-null mice but were increased in the liver-Cpr-null mice, relative to wild-type control mice. Decreased lung tumor multiplicity in the lung-Cpr-null mice correlated with reduced lung O6-methylguanine adduct levels, without decreases in NNK bioavailability, consistent with decreased NNK bioactivation in the lung. Moreover, lung tumors in lung-Cpr-null mice were positive for CPR expression, indicating that the tumors did not originate from Cpr-null cells. Thus, we have confirmed the essential role of pulmonary P450-mediated metabolic activation in NNK-induced lung cancer, and our mouse models should be applicable to studies on other procarcinogens that require P450-mediated metabolic activation.

Lung Epithelial Cells Induce Endodermal Differentiation in Mouse Mesenchymal Bone Marrow Stem Cells by Paracrine Mechanism

Mesenchymal stem cells (MSCs) from bone marrow are a potential source for reconstructive therapy. In vitro, MSCs differentiate into cells of mesodermal and ectodermal lineages but rarely into cells of endodermal lineage. We developed an in vitro model to study the endodermal differentiation of MSCs using co-culture of MSCs and transformed lung epithelial (A-549) cells. The cells were separated using a cell-impermeable membrane to eliminate the possibility of cell fusion. Under these conditions, MSCs expressed several lung epithelial markers (cytokeratins 5, 8, 14, 18, 19, pro-surfactant protein C, zonula occludens-1), detected using quantitative reverse transcriptase polymerase chain reaction and Western blot, and beta-catenin signaling was activated in MSCs. Treatment of MSCs with 10 to 20 mM lithium chloride activated the beta-catenin pathway and enhanced expression of epithelial markers, although this activation was transient. We conclude that A-549 cells can trigger epithelial differentiation of MSCs by a paracrine mechanism that may include activation of beta-catenin signaling.

Key mechanisms of early lung development

Lung morphogenesis requires the integration of multiple regulatory factors, which results in a functional air-blood interface required for gas exchange at birth. The respiratory tract is composed of endodermally derived epithelium surrounded by cells of mesodermal origin. Inductive signaling between these 2 tissue compartments plays a critical role in formation and differentiation of the lung, which is mediated by evolutionarily conserved signaling families used reiteratively during lung formation, including the fibroblast growth factor, hedgehog, retinoic acid, bone morphogenetic protein, and Wnt signaling pathways. Cells coordinate their response to these signaling proteins largely through transcription factors, which determine respiratory cell fate and pattern formation via the activation and repression of downstream target genes. Gain- and loss-of-function studies in null mutant and transgenic mice models have greatly facilitated the identification and hierarchical classification of these molecular programs. In this review, we highlight select molecular events that drive key phases of pulmonary development, including specification of a lung cell fate, primary lung bud formation, tracheoesophageal septation, branching morphogenesis, and proximal-distal epithelial patterning. Understanding the genetic pathways that regulate respiratory tract development is essential to provide insight into the pathogenesis of congenital anomalies and to develop innovative strategies to treat inherited and acquired lung disease.

Central role of Muc5ac expression in mucous metaplasia and its regulation by conserved 5' elements

Mucus hypersecretion contributes to morbidity and mortality in many obstructive lung diseases. Gel-forming mucins are the chief glycoprotein components of airway mucus, and elevated expression of these during mucous metaplasia precedes the hypersecretory phenotype. Five orthologous genes (MUC2, MUC5AC, MUC5B, MUC6, and MUC19) encode the mammalian gel-forming mucin family, and several have been implicated in asthma, cystic fibrosis, and chronic obstructive pulmonary disease pathologies. However, in the absence of a comprehensive analysis, their relative contributions remain unclear. Here, we assess the expression of the entire gel-forming mucin gene family in allergic mouse airways and show that Muc5ac is the predominant gel-forming mucin induced. We previously showed that the induction of mucous metaplasia in ovalbumin-sensitized and -challenged mouse lungs occurs within bronchial Clara cells. The temporal induction and localization of Muc5ac transcripts correlate with the induced expression and localization of mucin glycoproteins in bronchial airways. To better understand the tight regulation of Muc5ac expression, we analyzed all available 5'-flanking sequences of mammalian MUC5AC orthologs and identified evolutionarily conserved regions within domains proximal to the mRNA coding region. Analysis of luciferase reporter gene activity in a mouse transformed Clara cell line demonstrates that this region possesses strong promoter activity and harbors multiple conserved transcription factor-binding motifs. In particular, SMAD4 and HIF-1alpha bind to the promoter, and mutation of their recognition motifs abolishes promoter function. In conclusion, Muc5ac expression is the central event in antigen-induced mucous metaplasia, and phylogenetically conserved 5' noncoding domains control its regulation.

Transcriptional profiling of mucociliary differentiation in human airway epithelial cells

When cultured at an air-liquid interface (ALI) in the appropriate medium, primary human airway epithelial cells form a polarized, pseudostratified epithelium composed of ciliated and mucus-secreting cells. This culture system provides a useful tool for the in vitro study of airway epithelial biology and differentiation. We have performed microarray analysis on ALI cultures of human bronchial epithelial cells (HBECs) grown over a 28-d period to identify genes involved in mucociliary differentiation. We identified over 2,000 genes that displayed statistically significant 2-fold or greater changes in expression during the time course. Of the genes showing the largest increases, many are involved in processes associated with airway epithelial biology, such as cell adhesion, immunity, transport, and cilia formation; however, many novel genes were also identified. We compared our results with data from proteomic analyses of the ciliary axoneme and identified candidate genes that may have roles in cilia formation or function. Gene networks were generated using Ingenuity Pathways Analysis (Ingenuity Systems, Redwood City, CA) to identify signaling pathways involved in mucociliary cell differentiation or function. Networks containing genes involved in TGF-beta, WNT/beta-catenin, and epidermal growth factor receptor (EGFR) pathways were identified, suggesting potential roles for these families in airway epithelia. Microarray results were validated by real-time RT-PCR for a number of representative genes. This work has provided extensive information about gene expression changes during differentiation of airway epithelial cells, and will be a useful resource for researchers interested in respiratory function, pathology, and toxicology.

SPDEF regulates goblet cell hyperplasia in the airway epithelium

Goblet cell hyperplasia and mucous hypersecretion contribute to the pathogenesis of chronic pulmonary diseases including cystic fibrosis, asthma, and chronic obstructive pulmonary disease. In the present work, mouse SAM pointed domain-containing ETS transcription factor (SPDEF) mRNA and protein were detected in subsets of epithelial cells lining the trachea, bronchi, and tracheal glands. SPDEF interacted with the C-terminal domain of thyroid transcription factor 1, activating transcription of genes expressed selectively in airway epithelial cells, including Sftpa, Scgb1a1, Foxj1, and Sox17. Expression of Spdef in the respiratory epithelium of adult transgenic mice caused goblet cell hyperplasia, inducing both acidic and neutral mucins in vivo, and stainined for both acidic and neutral mucins in vivo. SPDEF expression was increased at sites of goblet cell hyperplasia caused by IL-13 and dust mite allergen in a process that was dependent upon STAT-6. SPDEF was induced following intratracheal allergen exposure and after Th2 cytokine stimulation and was sufficient to cause goblet cell differentiation of Clara cells in vivo.

Transcriptional control of lung morphogenesis

The vertebrate lung consists of multiple cell types that are derived primarily from endodermal and mesodermal compartments of the early embryo. The process of pulmonary organogenesis requires the generation of precise signaling centers that are linked to transcriptional programs that, in turn, regulate cell numbers, differentiation, and behavior, as branching morphogenesis and alveolarization proceed. This review summarizes knowledge regarding the expression and proposed roles of transcription factors influencing lung formation and function with particular focus on knowledge derived from the study of the mouse. A group of transcription factors active in the endodermally derived cells of the developing lung tubules, including thyroid transcription factor-1 (TTF-1), beta-catenin, Forkhead orthologs (FOX), GATA, SOX, and ETS family members are required for normal lung morphogenesis and function. In contrast, a group of distinct proteins, including FOXF1, POD1, GLI, and HOX family members, play important roles in the developing lung mesenchyme, from which pulmonary vessels and bronchial smooth muscle develop. Lung formation is dependent on reciprocal signaling among cells of both endodermal and mesenchymal compartments that instruct transcriptional processes mediating lung formation and adaptation to breathing after birth.

Cell Regeneration in Lung Injury

The acute respiratory distress syndrome (ARDS) is a lethal inflammatory disorder of the lung. Its incidence is estimated at 75 cases per 100,000 population and appears to be increasing [1]. Even with optimal treatment, mortality is about 30% [1–3]. As such, ARDS represents a major public health problem. The effects of two recent crises created by unusual viral infections of the respiratory tract — the severe acute respiratory syndrome (SARS) epidemic caused by the novel SARS coronavirus [4, 5] and the bird flu [6] highlight the importance of research into ARDS. Both viruses cause an ARDS-like picture. Because lung repair and regeneration contribute substantially to the pathophysiology of ARDS, understanding these processes is essential [7]. This chapter focuses on specific cell populations and markers involved in cell division and regeneration. In addition, a brief review of two pathways intimately associated with cell division is provided because of their potential for pharmacologic manipulation.

Cell Regeneration in Lung Injury

The acute respiratory distress syndrome (ARDS) is a lethal inflammatory disorder of the lung. Its incidence is estimated at 75 cases per 100,000 population and appears to be increasing [1]. Even with optimal treatment, mortality is about 30% [1–3]. As such, ARDS represents a major public health problem. The effects of two recent crises created by unusual viral infections of the respiratory tract — the severe acute respiratory syndrome (SARS) epidemic caused by the novel SARS coronavirus [4], [5] and the bird flu [6] highlight the importance of research into ARDS. Both viruses cause an ARDS-like picture. Because lung repair and regeneration contribute substantially to the pathophysiology of ARDS, understanding these processes is essential [7]. This chapter focuses on specific cell populations and markers involved in cell division and regeneration. In addition, a brief review of two pathways intimately associated with cell division is provided because of their potential for pharmacologic manipulation.

Gene expression profiling identifies regulatory pathways involved in the late stage of rat fetal lung development

Fetal lung development is a complex biological process that involves temporal and spatial regulations of many genes. To understand the molecular mechanisms of this process, we investigated gene expression profiles of fetal lungs on gestational days 18, 19, 20, and 21, as well as newborn and adult rat lungs. For this analysis, we used an in-house rat DNA microarray containing 6,000 known genes and 4,000 expressed sequence tags (ESTs). Of these, 1,512 genes passed the statistical significance analysis of microarray (SAM) test; an at least twofold change was shown for 583 genes (402 known genes and 181 ESTs) between at least two time points. K-means cluster analysis revealed seven major expression patterns. In one of the clusters, gene expression increased from day 18 to day 20 and then decreased. In this cluster, which contained 10 known genes and 5 ESTs, 8 genes are associated with development. These genes can be integrated into regulatory pathways, including growth factors, plasma membrane receptors, adhesion molecules, intracellular signaling molecules, and transcription factors. Real-time PCR analysis of these 10 genes showed an 88% consistency with the microarray data. The mRNA of LIM homeodomain protein 3a (Lhx3), a transcription factor, was enriched in fetal type II cells. In contrast, pleiotrophin, a growth factor, had a much higher expression in fetal lung tissues than in fetal type II cells. Immunohistochemistry revealed that Lhx3 was localized in fetal lung epithelial cells and pleiotrophin in the mesenchymal cells adjacent to the developing epithelium and blood vessel. Using GenMAPP, we identified four regulatory pathways: transforming growth factor-beta signaling, inflammatory response, cell cycle, and G protein signaling. We also identified two metabolic pathways: glycolysis-gluconeogenesis and proteasome degradation. Our results may provide new insights into the complex regulatory pathways that control fetal lung development.