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

Oxidative stress-triggered Wnt signaling perturbation characterizes the tipping point of lung adeno-to-squamous transdifferentiation

Lkb1 deficiency confers the Kras-mutant lung cancer with strong plasticity and the potential for adeno-to-squamous transdifferentiation (AST). However, it remains largely unknown how Lkb1 deficiency dynamically regulates AST. Using the classical AST mouse model (Kras ^LSL-G12D/+;Lkb1^flox/flox, KL), we here comprehensively analyze the temporal transcriptomic dynamics of lung tumors at different stages by dynamic network biomarker (DNB) and identify the tipping point at which the Wnt signaling is abruptly suppressed by the excessive accumulation of reactive oxygen species (ROS) through its downstream effector FOXO3A. Bidirectional genetic perturbation of the Wnt pathway using two different Ctnnb1 conditional knockout mouse strains confirms its essential role in the negative regulation of AST. Importantly, pharmacological activation of the Wnt pathway before but not after the tipping point inhibits squamous transdifferentiation, highlighting the irreversibility of AST after crossing the tipping point. Through comparative transcriptomic analyses of mouse and human tumors, we find that the lineage-specific transcription factors (TFs) of adenocarcinoma and squamous cell carcinoma form a "Yin-Yang" counteracting network. Interestingly, inactivation of the Wnt pathway preferentially suppresses the adenomatous lineage TF network and thus disrupts the "Yin-Yang" homeostasis to lean towards the squamous lineage, whereas ectopic expression of NKX2-1, an adenomatous lineage TF, significantly dampens such phenotypic transition accelerated by the Wnt pathway inactivation. The negative correlation between the Wnt pathway and AST is further observed in a large cohort of human lung adenosquamous carcinoma. Collectively, our study identifies the tipping point of AST and highlights an essential role of the ROS-Wnt axis in dynamically orchestrating the homeostasis between adeno- and squamous-specific TF networks at the AST tipping point.

Wnt signaling in lung development, regeneration, and disease progression

The respiratory tract is a vital, intricate system for several important biological processes including mucociliary clearance, airway conductance, and gas exchange. The Wnt signaling pathway plays several crucial and indispensable roles across lung biology in multiple contexts. This review highlights the progress made in characterizing the role of Wnt signaling across several disciplines in lung biology, including development, homeostasis, regeneration following injury, in vitro directed differentiation efforts, and disease progression. We further note uncharted directions in the field that may illuminate important biology. The discoveries made collectively advance our understanding of Wnt signaling in lung biology and have the potential to inform therapeutic advancements for lung diseases.

Cody Aros, Carla Pantoja, and Brigitte Gomperts review the key role of Wnt signaling in all aspects of lung development, repair, and disease progression. They provide an overview of recent research findings and highlight where research is needed to further elucidate mechanisms of action, with the aim of improving disease treatments.

WNT/RYK signaling restricts goblet cell differentiation during lung development and repair

Goblet cell metaplasia and mucus hypersecretion are observed in many pulmonary diseases, including asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis. However, the regulation of goblet cell differentiation remains unclear. Here, we identify a regulator of this process in an N-ethyl-N-nitrosourea (ENU) screen for modulators of postnatal lung development; Ryk mutant mice exhibit lung inflammation, goblet cell hyperplasia, and mucus hypersecretion. RYK functions as a WNT coreceptor, and, in the developing lung, we observed high RYK expression in airway epithelial cells and moderate expression in mesenchymal cells as well as in alveolar epithelial cells. From transcriptomic analyses and follow-up studies, we found decreased WNT/β-catenin signaling activity in the mutant lung epithelium. Epithelial-specific Ryk deletion causes goblet cell hyperplasia and mucus hypersecretion but not inflammation, while club cell-specific Ryk deletion in adult stages leads to goblet cell hyperplasia and mucus hypersecretion during regeneration. We also found that the airway epithelium of COPD patients often displays goblet cell metaplastic foci, as well as reduced RYK expression. Altogether, our findings reveal that RYK plays important roles in maintaining the balance between airway epithelial cell populations during development and repair, and that defects in RYK expression or function may contribute to the pathogenesis of human lung diseases.

Complete Tracheal Ring Deformity. A Translational Genomics Approach to Pathogenesis

Rationale: Complete tracheal ring deformity (CTRD) is a rare congenital abnormality of unknown etiology characterized by circumferentially continuous or nearly continuous cartilaginous tracheal rings, variable degrees of tracheal stenosis and/or shortening, and/or pulmonary arterial sling anomaly.Objectives: To test the hypothesis that CTRD is caused by inherited or de novo mutations in genes required for normal tracheal development.Methods: CTRD and normal tracheal tissues were examined microscopically to define the tracheal abnormalities present in CTRD. Whole-exome sequencing was performed in children with CTRD and their biological parents ("trio analysis") to identify gene variants in patients with CTRD. Mutations were confirmed by Sanger sequencing, and their potential impact on structure and/or function of encoded proteins was examined using human gene mutation databases. Relevance was further examined by comparison with the effects of targeted deletion of murine homologs important to tracheal development in mice.Measurements and Main Results: The trachealis muscle was absent in all of five patients with CTRD. Exome analysis identified six de novo, three recessive, and multiple compound-heterozygous or rare hemizygous variants in children with CTRD. De novo variants were identified in SHH (Sonic Hedgehog), and inherited variants were identified in HSPG2 (perlecan), ROR2 (receptor tyrosine kinase-like orphan receptor 2), and WLS (Wntless), genes involved in morphogenetic pathways known to mediate tracheoesophageal development in mice.Conclusions: The results of the present study demonstrate that absence of the trachealis muscle is associated with CTRD. Variants predicted to cause disease were identified in genes encoding Hedgehog and Wnt signaling pathway molecules, which are critical to cartilage formation and normal upper airway development in mice.

Secondary crest myofibroblast PDGFRα controls the elastogenesis pathway via a secondary tier of signaling networks during alveologenesis

Postnatal alveolar formation is the most important and the least understood phase of lung development. Alveolar pathologies are prominent in neonatal and adult lung diseases. The mechanisms of alveologenesis remain largely unknown. We inactivated Pdgfra postnatally in secondary crest myofibroblasts (SCMF), a subpopulation of lung mesenchymal cells. Lack of Pdgfra arrested alveologenesis akin to bronchopulmonary dysplasia (BPD), a neonatal chronic lung disease. The transcriptome of mutant SCMF revealed 1808 altered genes encoding transcription factors, signaling and extracellular matrix molecules. Elastin mRNA was reduced, and its distribution was abnormal. Absence of Pdgfra disrupted expression of elastogenic genes, including members of the Lox, Fbn and Fbln families. Expression of EGF family members increased when Tgfb1 was repressed in mouse. Similar, but not identical, results were found in human BPD lung samples. In vitro, blocking PDGF signaling decreased elastogenic gene expression associated with increased Egf and decreased Tgfb family mRNAs. The effect was reversible by inhibiting EGF or activating TGFβ signaling. These observations demonstrate the previously unappreciated postnatal role of PDGFA/PDGFRα in controlling elastogenic gene expression via a secondary tier of signaling networks composed of EGF and TGFβ.

Hippo signaling promotes lung epithelial lineage commitment by curbing Fgf10 and β-catenin signaling

Organ growth and tissue homeostasis rely on the proliferation and differentiation of progenitor cell populations. In the developing lung, localized Fgf10 expression maintains distal Sox9-expressing epithelial progenitors and promotes basal cell differentiation in the cartilaginous airways. Mesenchymal Fgf10 expression is induced by Wnt signaling but inhibited by Shh signaling, and epithelial Fgf10 signaling activates β-catenin signaling. The Hippo pathway is a well-conserved signaling cascade that regulates organ size and stem/progenitor cell behavior. Here, we show that Hippo signaling promotes lineage commitment of lung epithelial progenitors by curbing Fgf10 and β-catenin signaling. Our findings show that both inactivation of the Hippo pathway (nuclear Yap) or ablation of Yap result in increased β-catenin and Fgf10 signaling, suggesting a cytoplasmic role for Yap in epithelial lineage commitment. We further demonstrate redundant and non-redundant functions for the two nuclear effectors of the Hippo pathway, Yap and Taz, during lung development.

Modulation of Wnt signaling is essential for the differentiation of ciliated epithelial cells in human airways

Wnt signaling is essential for the differentiation of airway epithelial cells during development. Here, we examined the role of Wnt signaling during redifferentiation of ciliated airway epithelial cells in vitro at the air liquid interface as a model of airway epithelial repair. Phases of proliferation and differentiation were defined. Markers of squamous metaplasia and epithelial ciliation were followed while enhancing β‐catenin signaling by blocking glycogen synthase kinase 3β with SB216763 and shRNA as well as inhibiting canonical WNT signaling with apical application of Dickkopf 1 (Dkk1). Our findings indicate that enhanced β‐catenin signaling decreases the number of ciliated cells and causes squamous changes in the epithelium, whereas treatment with DDk1 leads to an increased number of ciliated cells.

Wnt/β-catenin signaling links embryonic lung development and asthmatic airway remodeling

Embryonic lung development requires reciprocal endodermal-mesodermal interactions; mediated by various signaling proteins. Wnt/β-catenin is a signaling protein that exhibits the pivotal role in lung development, injury and repair while aberrant expression of Wnt/β-catenin signaling leads to asthmatic airway remodeling: characterized by hyperplasia and hypertrophy of airway smooth muscle cells, alveolar and vascular damage goblet cells metaplasia, and deposition of extracellular matrix; resulting in decreased lung compliance and increased airway resistance. The substantial evidence suggests that Wnt/β-catenin signaling links embryonic lung development and asthmatic airway remodeling. Here, we summarized the recent advances related to the mechanistic role of Wnt/β-catenin signaling in lung development, consequences of aberrant expression or deletion of Wnt/β-catenin signaling in expansion and progression of asthmatic airway remodeling, and linking early-impaired pulmonary development and airway remodeling later in life. Finally, we emphasized all possible recent potential therapeutic significance and future prospectives, that are adaptable for therapeutic intervention to treat asthmatic airway remodeling.

Thymus neuroendocrine tumors with CTNNB1 gene mutations, disarrayed ß-catenin expression, and dual intra-tumor Ki-67 labeling index compartmentalization challenge the concept of secondary high-grade neuroendocrine tumor: a paradigm shift

We herein report an uncommon association of intimately admixed atypical carcinoid (AC) and large cell neuroendocrine (NE) carcinoma (LCNEC) of the thymus, occurring in two 20- and 39-year-old Caucasian males. Both tumors were treated by maximal thymectomy. The younger patient presented with a synchronous lesion and died of disease after 9 months, while the other patient was associated with a recurrent ectopic adrenocorticotropic hormone Cushing's syndrome and is alive with disease at the 2-year follow-up. MEN1 syndrome was excluded in either case. Immunohistochemically, disarrayed cytoplasmic and nuclear ß-catenin expression was seen alongside an intra-tumor Ki-67 antigen labeling index (LI) ranging from 2 to 80% in the younger patient's tumor and from 3 to 45% in the other. Both exhibited upregulated cyclin D1 and retinoblastoma, while vimentin was overexpressed in the recurrent LCNEC only. Next-generation sequencing revealed CTNNB1, TP53, and JAK3 mutations in the synchronous tumor and CTNNB1 mutation alone in the metachronous tumor (the latter with the same mutation as the first tumor of 17 years prior). None of the 23 T-NET controls exhibited this hallmarking triple alteration (p = 0.003). These findings suggested that LCNEC components developed from pre-existing CTNNB1-mutated AC upon loss-of-function TP53 and gain-of-function JAK3 mutations in one case and an epithelial-mesenchymal transition upon vimentin overexpression in the other case. Both tumors maintained intact cyclin D1-retinoblastoma machinery. Our report challenges the concept of secondary LCNEC as an entity that develops from pre-existing AC as a result of tumor progression, suggesting a paradigm shift to the current pathogenesis of NET.

Efficient Derivation of Functional Human Airway Epithelium from Pluripotent Stem Cells via Temporal Regulation of Wnt Signaling

Effective derivation of functional airway organoids from induced pluripotent stem cells (iPSCs) would provide valuable models of lung disease and facilitate precision therapies for airway disorders such as cystic fibrosis. However, limited understanding of human airway patterning has made this goal challenging. Here, we show that cyclical modulation of the canonical Wnt signaling pathway enables rapid directed differentiation of human iPSCs via an NKX2-1⁺ progenitor intermediate into functional proximal airway organoids. We find that human NKX2-1⁺ progenitors have high levels of Wnt activation but respond intrinsically to decreases in Wnt signaling by rapidly patterning into proximal airway lineages at the expense of distal fates. Using this directed approach, we were able to generate cystic fibrosis patient-specific iPSC-derived airway organoids with a defect in forskolin-induced swelling that is rescued by gene editing to correct the disease mutation. Our approach has many potential applications in modeling and drug screening for airway diseases.

YAP is essential for mechanical force production and epithelial cell proliferation during lung branching morphogenesis

Branching morphogenesis is a fundamental program for tissue patterning. We show that active YAP, a key mediator of Hippo signaling, is distributed throughout the murine lung epithelium and loss of epithelial YAP severely disrupts branching. Failure to branch is restricted to regions where YAP activity is removed. This suggests that YAP controls local epithelial cell properties. In support of this model, mechanical force production is compromised and cell proliferation is reduced in Yap mutant lungs. We propose that defective force generation and insufficient epithelial cell number underlie the branching defects. Through genomic analysis, we also uncovered a feedback control of pMLC levels, which is critical for mechanical force production, likely through the direct induction of multiple regulators by YAP. Our work provides a molecular pathway that could control epithelial cell properties required for proper morphogenetic movement and pattern formation.

DOI:http://dx.doi.org/10.7554/eLife.21130.001

Air enters our lungs through a system of airways that spread outwards from the windpipe like the branches of a tree. Before we are born, each branch is shaped by the organization and movement of cells that form the walls of the airways, called epithelial cells. This process requires the cells to communicate and coordinate with each other by receiving and/or sending chemical signals.

One important system that epithelial cells use to communicate is called the Hippo pathway, which uses a molecule called YAP to execute received messages. Exactly how YAP helps airways in the lungs to develop was not well understood.

By studying developing mouse lungs, Lin et al. have now found that YAP is present in the epithelial cells of all developing airways. Inactivating YAP in specific parts of the lungs prevented the formation of new branches of the airway in just those regions that lacked YAP. This suggests that YAP is needed for airways to branch properly and form the extensive network present in healthy lungs.

Further investigation using genomics approaches revealed that YAP regulates the activity of genes that control how epithelial cells divide and contract. Without YAP, fewer cells were produced and they were unable to produce the forces required to change shape and move to form airways. In particular, YAP controls the production of a modified form of a protein called phosphorylated myosin light chain (pMLC) through a regulatory pathway. The pMLC protein is critical for the cells to produce the mechanical forces that they need to be able to contract correctly.

Overall, the results presented by Lin et al. suggest that YAP controls the properties of the epithelial cells to enable them to form new airway branches. Branched structures also form in a number of other organs, and the mechanisms that cause these structures to form are thought to be similar to those that form the airways. Lin et al.’s work could therefore help us to understand how organs develop more generally.

DOI:http://dx.doi.org/10.7554/eLife.21130.002

Rac1 modulates mammalian lung branching morphogenesis in part through canonical Wnt signaling

Lung branching morphogenesis relies on a number of factors, including proper epithelial cell proliferation and differentiation, cell polarity, and migration. Rac1, a small Rho GTPase, orchestrates a number of these cellular processes, including cell proliferation and differentiation, cellular alignment, and polarization. Furthermore, Rac1 modulates both noncanonical and canonical Wnt signaling, important pathways in lung branching morphogenesis. Culture of embryonic mouse lung explants in the presence of the Rac1 inhibitor (NSC23766) resulted in a dose-dependent decrease in branching. Increased cell death and BrdU uptake were notably seen in the mesenchyme, while no direct effect on the epithelium was observed. Moreover, vasculogenesis was impaired following Rac1 inhibition as shown by decreased Vegfa expression and impaired LacZ staining in Flk1-Lacz reporter mice. Rac1 inhibition decreased Fgf10 expression in conjunction with many of its associated factors. Moreover, using the reporter lines TOPGAL and Axin2-LacZ, there was an evident decrease in canonical Wnt signaling in the explants treated with the Rac1 inhibitor. Activation of canonical Wnt pathway using WNT3a or WNT7b only partially rescued the branching inhibition. Moreover, these results were validated on human explants, where Rac1 inhibition resulted in impaired branching and decreased AXIN2 and FGFR2b expression. We therefore conclude that Rac1 regulates lung branching morphogenesis, in part through canonical Wnt signaling. However, the exact mechanisms by which Rac1 interacts with canonical Wnt in human and mouse lung requires further investigation.

Mesodermal ALK5 controls lung myofibroblast versus lipofibroblast cell fate

Background

Epithelial-mesenchymal cross talk is centerpiece in the development of many branched organs, including the lungs. The embryonic lung mesoderm provides instructional information not only for lung architectural development, but also for patterning, commitment and differentiation of its many highly specialized cell types. The mesoderm also serves as a reservoir of progenitors for generation of differentiated mesenchymal cell types that include αSMA-expressing fibroblasts, lipofibroblasts, endothelial cells and others. Transforming Growth Factor β (TGFβ) is a key signaling pathway in epithelial-mesenchymal cross talk. Using a cre-loxP approach we have elucidated the role of the TGFβ type I receptor tyrosine kinase, ALK5, in epithelial-mesenchymal cross talk during lung morphogenesis.

Results

Targeted early inactivation of Alk5 in mesodermal progenitors caused abnormal development and maturation of the lung that included reduced physical size of the sub-mesothelial mesoderm, an established source of specific mesodermal progenitors. Abrogation of mesodermal ALK5-mediated signaling also inhibited differentiation of cell populations in the epithelial and endothelial lineages. Importantly, Alk5 mutant lungs contained a reduced number of αSMA^pos cells and correspondingly increased lipofibroblasts. Elucidation of the underlying mechanisms revealed that through direct and indirect modulation of target signaling pathways and transcription factors, including PDGFRα, PPARγ, PRRX1, and ZFP423, ALK5-mediated TGFβ controls a process that regulates the commitment and differentiation of αSMA^pos versus lipofibroblast cell populations during lung development.

Conclusion

ALK5-mediated TGFβ signaling controls an early pathway that regulates the commitment and differentiation of αSMA^pos versus LIF cell lineages during lung development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0242-9) contains supplementary material, which is available to authorized users.

β-catenin and Kras/Foxm1 signaling pathway are critical to restrict Sox9 in basal cells during pulmonary branching morphogenesis

BACKGROUND

Lung morphogenesis is regulated by interactions between the canonical Wnt/β-catenin and Kras/ERK/Foxm1 signaling pathways that establish proximal-peripheral patterning of lung tubules. How these interactions influence the development of respiratory epithelial progenitors to acquire airway as compared to alveolar epithelial cell fate is unknown. During branching morphogenesis, SOX9 transcription factor is normally restricted from conducting airway epithelial cells and is highly expressed in peripheral, acinar progenitor cells that serve as precursors of alveolar type 2 (AT2) and AT1 cells as the lung matures.

RESULTS

To identify signaling pathways that determine proximal-peripheral cell fate decisions, we used the SFTPC gene promoter to delete or overexpress key members of Wnt/β-catenin and Kras/ERK/Foxm1 pathways in fetal respiratory epithelial progenitor cells. Activation of β-catenin enhanced SOX9 expression in peripheral epithelial progenitors, whereas deletion of β-catenin inhibited SOX9. Surprisingly, deletion of β-catenin caused accumulation of atypical SOX9-positive basal cells in conducting airways. Inhibition of Wnt/β-catenin signaling by Kras(G12D) or its downstream target Foxm1 stimulated SOX9 expression in basal cells. Genetic inactivation of Foxm1 from Kras(G12D) -expressing epithelial cells prevented the accumulation of SOX9-positive basal cells in developing airways.

CONCLUSIONS

Interactions between the Wnt/β-catenin and the Kras/ERK/Foxm1 pathways are essential to restrict SOX9 expression in basal cells. Developmental Dynamics 245:590-604, 2016. © 2016 Wiley Periodicals, Inc.

Progenitors of secondary crest myofibroblasts are developmentally committed in early lung mesoderm

Development of the mammalian lung is predicated on cross-communications between two highly interactive tissues, the endodermally derived epithelium and the mesodermally derived pulmonary mesenchyme. While much attention has been paid for the lung epithelium, the pulmonary mesenchyme, partly due to lack of specific tractable markers remains under-investigated. The lung mesenchyme is derived from the lateral plate mesoderm and is the principal recipient of Hedgehog (Hh) signaling, a morphogenetic network that regulates multiple aspects of embryonic development. Using the Hh-responsive Gli1-cre(ERT2) mouse line, we identified the mesodermal targets of Hh signaling at various time points during embryonic and postnatal lung development. Cell lineage analysis showed these cells serve as progenitors to contribute to multiple lineages of mesodermally derived differentiated cell types that include parenchymal or interstitial myofibroblasts, peribronchial and perivascular smooth muscle as well as rare populations of cells within the mesothelium. Most importantly, Gli1-cre(ERT2) identified the progenitors of secondary crest myofibroblasts, a hitherto intractable cell type that plays a key role in alveolar formation, a vital process about which little is currently known. Transcriptome analysis of Hh-targeted progenitor cells transitioning from the pseudoglandular to the saccular phase of lung development revealed important modulations of key signaling pathways. Among these, there was significant downregulation of canonical WNT signaling. Ectopic stabilization of β-catenin via inactivation of Apc by Gli1-cre(ERT2) expanded the Hh-targeted progenitor pools, which caused the formation of fibroblastic masses within the lung parenchyma. The Gli1-cre(ERT2) mouse line represents a novel tool in the analysis of mesenchymal cell biology and alveolar formation during lung development.

Wnt and FGF mediated epithelial-mesenchymal crosstalk during lung development

BACKGROUND

The adaptation to terrestrial life required the development of an organ capable of efficient air-blood gas exchange. To meet the metabolic load of cellular respiration, the mammalian respiratory system has evolved from a relatively simple structure, similar to the two-tube amphibian lung, to a highly complex tree-like system of branched epithelial airways connected to a vast network of gas exchanging units called alveoli. The development of such an elaborate organ in a relatively short time window is therefore an extraordinary feat and involves an intimate crosstalk between mesodermal and endodermal cell lineages.

RESULTS

This review describes the molecular processes governing lung development with an emphasis on the current knowledge on the role of Wnt and FGF signaling in lung epithelial differentiation.

CONCLUSIONS

The Wnt and FGF signaling pathways are crucial for the dynamic and reciprocal communication between epithelium and mesenchyme during lung development. In addition, some of this developmental crosstalk is reemployed in the adult lung after injury to drive regeneration, and may, when aberrantly or chronically activated, result in chronic lung diseases. Novel insights into how the Wnt and FGF pathways interact and are integrated into a complex gene regulatory network will not only provide us with essential information about how the lung regenerates itself, but also enhance our understanding of the pathogenesis of chronic lung diseases, as well as improve the controlled differentiation of lung epithelium from pluripotent stem cells.

Cognitive flexibility and long-term depression (LTD) are impaired following β-catenin stabilization in vivo

The cadherin/β-catenin adhesion complex is a key mediator of the bidirectional changes in synapse strength which are believed to underlie complex learning and memory. In the present study, we demonstrate that stabilization of β-catenin in the hippocampus of adult mice results in significant impairments in cognitive flexibility and spatial reversal learning, including impaired extinction during the reversal phase of the Morris water maze and deficits in a delayed nonmatch to place T-maze task. In accordance with these deficits, β-catenin stabilization was found to abolish long-term depression by stabilizing cadherin at the synaptic membrane and impairing AMPA receptor endocytosis, while leaving basal synaptic transmission and long-term potentiation unaffected. These results demonstrate that the β-catenin/cadherin adhesion complex plays an important role in learning and memory and that aberrant increases in synaptic adhesion can have deleterious effects on cognitive function.

The p66Shc adapter protein regulates the morphogenesis and epithelial maturation of fetal mouse lungs

Many signaling pathways are mediated by Shc adapter proteins that, in turn, are expressed as three isoforms with distinct functions. The p66(Shc) isoform antagonizes proliferation, regulates oxidative stress, and mediates apoptosis. It is highly expressed in the canalicular but not the later stages of mouse lung development, and its expression persists in bronchopulmonary dysplasia, a chronic disease associated with premature birth. These observations suggest that p66(Shc) has a developmental function. However, constitutive p66(Shc) deletion yields no morphological phenotype, and the structure of the Shc gene precludes its inducible deletion. To elucidate its function in lung development, we transfected p66(Shc) or nonsilencing small-interfering RNA (siRNA) into the epithelia of embryonic day 11 mouse lungs that were then cultured for 3 days and analyzed morphometrically. To assess cellular proliferation and epithelial differentiation, lung explants were immunostained and immunoblotted for p66(Shc), proliferating cell nuclear antigen (PCNA), the proximal airway differentiation antigens Clara cell 10-kDa protein (CC10) and thyroid transcription factor (TTF)-1, and the alveolar surfactant proteins (SP)-A, -B, and -C. Explants transfected with nonsilencing siRNA demonstrated specific epithelial uptake and normal morphological development relative to uninjected controls. In contrast, transfection with p66(Shc) siRNA significantly increased lumenal cross-sectional areas, decreased branching, and increased epithelial proliferation (P < 0.05 for all). Relative to controls, the expression of SP-B, SP-C, CC10, and TTF-1 was decreased by p66(Shc) knockdown. SP-A was not expressed in either control or treated lungs. These data suggest that p66(Shc) attenuates epithelial proliferation while promoting both distal and proximal epithelial maturation.

E-cadherin controls bronchiolar progenitor cells and onset of preneoplastic lesions in mice

Although progenitor cells of the conducting airway have been spatially localized and some insights have been gained regarding their molecular phenotype, relatively little is known about the mechanisms regulating their maintenance, activation, and differentiation. This study investigates the potential roles of E-cadherin in mouse Clara cells, as these cells were shown to represent the progenitor/stem cells of the conducting airways and have been implicated as the cell of origin of human non-small cell lung cancer. Postnatal inactivation of E-cadherin affected Clara cell differentiation and compromised airway regeneration under injury conditions. In steady-state adult lung, overexpression of the dominant negative E-cadherin led to an expansion of the bronchiolar stem cells and decreased differentiation concomitant with canonical Wnt signaling activation. Expansion of the bronchiolar stem cell pool was associated with an incessant proliferation of neuroepithelial body.associated Clara cells that ultimately gave rise to bronchiolar hyperplasia. Despite progressive hyperplasia, only a minority of the mice developed pulmonary solid tumors, suggesting that the loss of E-cadherin function leads to tumor formation when additional mutations are sustained. The present study reveals that E-cadherin plays a critical role in the regulation of proliferation and homeostasis of the epithelial cells lining the conducting airways.

Tissue-dependent consequences of Apc inactivation on proliferation and differentiation of ciliated cell progenitors via Wnt and notch signaling

The molecular signals that control decisions regarding progenitor/stem cell proliferation versus differentiation are not fully understood. Differentiation of motile cilia from progenitor/stem cells may offer a simple tractable model to investigate this process. Wnt and Notch represent two key signaling pathways in progenitor/stem cell behavior in a number of tissues. Adenomatous Polyposis Coli, Apc is a negative regulator of the Wnt pathway and a well known multifunctional protein. Using the cre-LoxP system we inactivated the Apc locus via Foxj1-cre, which is expressed in cells committed to ciliated cell lineage. We then characterized the consequent phenotype in two select tissues that bear motile cilia, the lung and the testis. In the lung, Apc deletion induced β-catenin accumulation and Jag1 expression in ciliated cells and by lateral induction, triggered Notch signaling in adjacent Clara cells. In the bronchiolar epithelium, absence of Apc blocked the differentiation of a subpopulation of cells committed to the ciliogenesis program. In the human pulmonary adenocarcinoma cells, Apc over-expression inhibited Jag1 expression and promoted motile ciliogenic gene expression program including Foxj1, revealing the potential mechanism. In the testis, Apc inactivation induced β-catenin accumulation in the spermatogonia, but silenced Notch signaling and depleted spermatogonial stem cells, associated with reduced proliferation, resulting in male infertility. In sum, the present comparative analysis reveals the tissue-dependent consequences of Apc inactivation on proliferation and differentiation of ciliated cell progenitors by coordinating Wnt and Notch signaling.

Gene expression signatures: a new approach to understanding the pathophysiology of chronic rhinosinusitis

Chronic rhinosinusitis (CRS) is a complex inflammatory disease with variable disease manifestation. Though external risk factors are associated with development and/or persistence of CRS, the host mucosal response is also important, as nasal epithelium acts as a physical and immune barrier. Under inflammatory stress, the nasal epithelium can undergo injury, followed by a rapid remodeling response ranging from epithelial hyperplasia, to goblet-cell metaplasia, to denudation, loss of cilia, fibrosis, and basement membrane thickening. Identification of gene expression signatures and molecular pathways in CRS pathogenesis have now begun to contribute significantly to a better understanding of the genetic and molecular alterations underlying CRS development and progression. Genetic studies are especially illuminating when multiple gene variants synergize within a permissive environmental context, and are expected to guide development of more effective therapeutic targets for CRS treatment.

Apc deficiency alters pulmonary epithelial cell fate and inhibits Nkx2.1 via triggering TGF-beta signaling

Wnt signaling is critical for cell fate specification and cell differentiation in many organs, but its function in pulmonary neuroendocrine cell (PNEC) differentiation has not been fully addressed. In this study, we examined the role of canonical Wnt signaling by targeting the gene for Adenomatous Polyposis Coli (Apc), which controls Wnt signaling activity via mediating phosphorylation of beta-catenin (Ctnnb). Targeting the Apc gene in lung epithelial progenitors by Nkx2.1-cre stabilized Ctnnb and activated canonical Wnt signaling. Apc deficiency altered lung epithelial cell fate by inhibiting Clara and ciliated cell differentiation and activating Uchl1, a marker of neuroendocrine cells. Similar to PNEC in normal lung, Uchl1(positive) cells were innervated. In mice with targeted inactivation of Ctnnb by Nkx2.1-cre, PNEC differentiation was not interrupted. These indicate that, after lung primordium formation, Wnt signaling is not essential for PNEC differentiation; however, its over-activation promotes PNEC features. Interestingly, Nkx2.1 was extinguished in Apc deficient epithelial progenitors before activation of Uchl1. Examination of Nkx2.1 null lungs suggested that early deletion of Nkx2.1 inhibits PNEC differentiation, while late repression does not. Nkx2.1 was specifically inhibited in Apc deficient lungs but not in Ctnnb gain-of-function lungs indicating a functional difference between Apc deletion and Ctnnb stabilization, both of which activate Wnt signaling. Further analysis revealed that Apc deficiency led to increased TGF-beta signaling, which inhibited Nkx2.1 in cultured lung endodermal explants. In contrast, TGF-beta activity was not increased in Ctnnb gain-of-function lungs. Therefore, our studies revealed an important mechanism involving Apc and TGF-beta signaling in regulating the key transcriptional factor, Nkx2.1, for lung epithelial progenitor cell fate determination.

Interventional and intrinsic airway homeostasis and repair

Human airways are a paragon of intrinsic engineering. They experience 7,000-10,000 liters of airflow/day, have a 70-m(2) surface area, and undergo complete renewal every 100-400 days. Despite this, airways are susceptible to aging, injury, and diseases that are major causes of mortality. Current airway regeneration research is focused both on understanding the cells and strategies responsible for maintaining intrinsic tissue homeostasis as well as on establishing clinical interventions for improving repair.

Wnt-responsive Lgr5-expressing stem cells are hair cell progenitors in the cochlea

Auditory hair cells are surrounded on their basolateral aspects by supporting cells, and these two cell types together constitute the sensory epithelium of the organ of Corti, which is the hearing apparatus of the ear. We show here that Lgr5, a marker for adult stem cells, was expressed in a subset of supporting cells in the newborn and adult murine cochlea. Lgr5-expressing supporting cells, sorted by flow cytometry and cultured in a single-cell suspension, compared with unsorted cells, displayed an enhanced capacity for self-renewing neurosphere formation in response to Wnt and were converted to hair cells at a higher (>10-fold) rate. The greater differentiation of hair cells in the neurosphere assay showed that Lgr5-positive cells had the capacity to act as cochlear progenitor cells, and lineage tracing confirmed that Lgr5-expressing cells accounted for the cells that formed neurospheres and differentiated to hair cells. The responsiveness to Wnt of cells with a capacity for division and sensory cell formation suggests a potential route to new hair cell generation in the adult cochlea.

Airway stem cells: review of potential impact on understanding of upper airway diseases

Epithelial remodeling is a part of our natural defense mechanisms, and includes migration, proliferation, and differentiation of epithelial cells, as well as the interactions between epithelial and stromal cells. It is not yet possible to distinguish between cause and effect during epithelium remodeling, and are there no clear roles for the many factors involved in respiratory infectious and inflammatory diseases due to a lack of critical information about epithelial cell responses. Most reported data are from lower airway studies or animal models. Therefore, research based on human nasal epithelial stem/progenitor cells can illuminate the pathophysiology of nasal airway disease from a different, more specific perspective. In this review, we discuss epithelial stem/progenitor cell research throughout the airway, with special attention to phenotypes and characterization of these cells from the nasal airway. Recently, we have isolated and cultured P63-positive human epithelial stem/progenitor cells from turbinate biopsies of healthy volunteers and from inflamed mucosa of patients with chronic rhinosinusitis with and without nasal polyposis. These cells propagate in serum-free, growth factor-supplemented, Dulbecco's modified Eagle's medium/F12 media, on either human fibroblast or 3T3 feeder layers. Self-renewal, proliferation, and differentiation potential at an air-liquid interface are being investigated to understand the molecular pathways underlying nasal inflammation. This in vitro culture system for nasal epithelial regeneration will allow molecular studies of human nasal epithelial cell interactions, differentiation, and repair, as well as responses to both environmental agents and to potential anti-inflammatory treatments.

Roles for β-catenin and doxycycline in the regulation of respiratory epithelial cell frequency and function

The expression of β-catenin-dependent genes can be increased through the Cre recombinase (Cre)-mediated elimination of the exon 3-encoded sequence. This mutant β-catenin is termed DE3, and promotes the expression of β-catenin-dependent genes. Our previous study used the DE3 model to demonstrate that persistent β-catenin activity inhibited bronchiolar Clara-to-ciliated cell differentiation. The present study was designed to evaluate the roles of β-catenin in regulating the tracheal progenitor cell hierarchy. However, initial experiments demonstrated that the tetracycline-responsive element-Cre transgene (TRE-Cre) was active in the absence of a reverse tetracycline transactivator driver or inducer, doxycycline (Dox). This spurious TRE-Cre transgene activity was not detected using the ROSA26-floxed STOP-LacZ reporter. To determine if the phenotype was a consequence of genotype or treatment with Dox, tracheal and lung specimens were evaluated using quantitative histomorphometric techniques. Analyses of uninduced mice demonstrated a significant effect of genotype on tracheal epithelial cell mass, involving basal, Clara-like cell types. The bronchial and bronchiolar Clara cell mass was also decreased. Paradoxically, an effect on ciliated cell mass was not detected. Activation of the β-catenin reporter transgene TOPGal demonstrated that β-catenin-dependent gene expression led to the genotype-dependent tracheal and bronchiolar phenotype. Comparative analyses of wild-type or keratin 14-rtTA(+/0)/TRE-cre(+/0)/DE3(+/+) mice receiving standard or Dox chow demonstrated an effect of treatment with Dox on basal, Clara-like, and Clara cell masses. We discuss these results in terms of cautionary notes and with regard to alterations of progenitor cell hierarchies in response to low-level injury.

Airway epithelial cell differentiation during lung organogenesis requires C/EBPα and C/EBPβ

BACKGROUND

CCAAT/enhancer-binding protein (C/EBP)α is crucial for lung development and differentiation of the pulmonary epithelium. Conversely, no lung defects have been observed in C/EBPβ-deficient mice, although C/EBPβ trans-activate pulmonary genes by binding to virtually identical DNA-sequences as C/EBPα. Thus, the pulmonary phenotype of mice lacking C/EBPβ could be explained by functional replacement with C/EBPα. We investigated whether C/EBPα and C/EBPβ have overlapping functions in regulating lung epithelial differentiation during organogenesis. Epithelial differentiation was assessed in mice with a lung epithelial-specific (SFTPC-Cre-mediated) deletion of C/EBPα (Cebpa(ΔLE) ), C/EBPβ (Cebpb(ΔLE) ), or both genes (Cebpa(ΔLE) ; Cebpb(ΔLE) ).

RESULTS

Both Cebpa(ΔLE) mice and Cebpa(ΔLE) ; Cebpb(ΔLE) mice demonstrated severe pulmonary immaturity compared to wild-type littermates, while no differences in lung histology or epithelial differentiation were observed in Cebpb(ΔLE) mice. In contrast to Cebpa(ΔLE) mice, Cebpa(ΔLE) ; Cebpb(ΔLE) mice also displayed undifferentiated Clara cells with markedly impaired protein and mRNA expression of Clara cell secretory protein (SCGB1A1), compared to wild-type littermates. In addition, ectopic mucus-producing cells were observed in the conducting airways of Cebpa(ΔLE) ; Cebpb(ΔLE) mice.

CONCLUSIONS

Our findings demonstrate that C/EBPα and C/EBPβ play pivotal, and partly overlapping roles in determining airway epithelial differentiation, with possible implications for tissue regeneration in lung homeostasis and disease.

β-Catenin-SOX2 signaling regulates the fate of developing airway epithelium

Wnt-β-catenin signaling regulates cell fate during organ development and postnatal tissue maintenance, but its contribution to specification of distinct lung epithelial lineages is still unclear. To address this question, we used a Cre recombinase (Cre)-LoxP approach to activate canonical Wnt signaling ectopically in developing lung endoderm. We found that persistent activation of canonical Wnt signaling within distal lung endoderm was permissive for normal development of alveolar epithelium, yet led to the loss of developing bronchiolar epithelium and ectasis of distal conducting airways. Activation of canonical Wnt led to ectopic expression of a lymphoid-enhancing factor and a T-cell factor (LEF and TCF, respectively) and absence of SRY (sex-determining region Y)-box 2 (SOX2) and tumor protein p63 (p63) expression in proximal derivatives. Conditional loss of SOX2 in airways phenocopied epithelial differentiation defects observed with ectopic activation of canonical Wnt. Our data suggest that Wnt negatively regulates a SOX2-dependent signaling program required for developmental progression of the bronchiolar lineage.

Wnt and Kras signaling-dark siblings in lung cancer

Aberrant Kras signaling is observed in a high percentage of human lung cancers while activating mutations in the Wnt/β-catenin signaling pathway are only rarely found. Our recent work has shown that the combined activation of both Kras and Wnt/β-catenin signaling leads to a dramatic increase in both tumor incidence and size. Moreover, lung tumors generated by the combined activation of both of these pathways exhibit a distinct phenotype similar to embryonic progenitors found in the developing lung. Thus, combinatorial activation of Kras and Wnt/β-catenin pathways leads to a significant increase in lung tumor formation characterized by a more progenitor like phenotype.

β-Catenin determines upper airway progenitor cell fate and preinvasive squamous lung cancer progression by modulating epithelial-mesenchymal transition

Human lung cancers, including squamous cell carcinoma (SCC) are a leading cause of death and, whilst evidence suggests that basal stem cells drive SCC initiation and progression, the mechanisms regulating these processes remain unknown. In this study we show that β-catenin signalling regulates basal progenitor cell fate and subsequent SCC progression. In a cohort of preinvasive SCCs we established that elevated basal cell β-catenin signalling is positively associated with increased disease severity, epithelial proliferation and reduced intercellular adhesiveness. We demonstrate that transgene-mediated β-catenin inhibition within keratin 14-expressing basal cells delayed normal airway repair while basal cell-specific β-catenin activation increased cell proliferation, directed differentiation and promoted elements of early epithelial-mesenchymal transition (EMT), including increased Snail transcription and reduced E-cadherin expression. These observations are recapitulated in normal human bronchial epithelial cells in vitro following both pharmacological β-catenin activation and E-cadherin inhibition, and mirrored our findings in preinvasive SCCs. Overall, the data show that airway basal cell β-catenin determines cell fate and its mis-expression is associated with the development of human lung cancer. Copyright © 2012 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Direct and indirect roles for β-catenin in facultative basal progenitor cell differentiation

The conducting airway epithelium is maintained and repaired by endogenous progenitor cells. Dysregulated progenitor cell proliferation and differentiation is thought to contribute to epithelial dysplasia in chronic lung disease. Thus modification of progenitor cell function is an attractive therapeutic goal and one that would be facilitated by knowledge of the molecular pathways that regulate their behavior. We modeled the human tracheobronchial epithelium using primary mouse tracheal epithelial cell cultures that were differentiated by exposure to the air-liquid-interface (ALI). A basal cell subset, termed facultative basal cell progenitors (FBP), initiate these cultures and are the progenitor for tracheal-specific secretory cells, the Clara-like cell, and ciliated cells. To test the hypothesis that β-catenin is necessary for FBP function, ALI cultures were generated from mice homozygous for the Ctnb(flox(E2-6)) allele. In this model, exons 2-6 of the β-catenin gene are flanked by LoxP sites, allowing conditional knockout of β-catenin. The β-catenin locus was modified through transduction with Adenovirus-5-encoding Cre recombinase. This approach generated a mosaic epithelium, comprised of β-catenin wild-type and β-catenin knockout cells. Dual immunostaining and quantitative histomorphometric analyses demonstrated that β-catenin played a direct role in FBP-to-ciliated cell differentiation and that it regulated cell-cell interactions that were necessary for FBP-to-Clara-like cell differentiation. β-catenin was also necessary for FBP proliferation and long-term FBP viability. We conclude that β-catenin is a critical determinant of FBP function and suggest that dysregulation of the β-catenin signaling pathway may contribute to disease pathology.

The a"MAZE"ing world of lung-specific transgenic mice

The purpose of this review is to give a comprehensive overview of transgenic mouse lines suitable for studying gene function and cellular lineage relationships in lung development, homeostasis, injury, and repair. Many of the mouse strains reviewed in this Perspective have been widely shared within the lung research community, and new strains are continuously being developed. There are many transgenic lines that target subsets of lung cells, but it remains a challenge for investigators to select the correct transgenic modules for their experiment. This review covers the tetracycline- and tamoxifen-inducible systems and focuses on conditional lines that target the epithelial cells. We point out the limitations of each strain so investigators can choose the system that will work best for their scientific question. Current mesenchymal and endothelial lines are limited by the fact that they are not lung specific. These lines are summarized in a brief overview. In addition, useful transgenic reporter mice for studying lineage relationships, promoter activity, and signaling pathways will complete our lung-specific conditional transgenic mouse shopping list.

β-catenin dosage is a critical determinant of tracheal basal cell fate determination

The purpose of this study was to determine whether β-catenin regulates basal cell fate determination in the mouse trachea. Analysis of TOPGal transgene reporter activity and Wnt/β-catenin pathway gene expression suggested a role for β-catenin in basal cell proliferation and differentiation after naphthalene-mediated Clara-like and ciliated cell depletion. However, these basal cell activities occurred simultaneously, limiting precise determination of the role(s) played by β-catenin. This issue was overcome by analysis of β-catenin signaling in tracheal air-liquid interface cultures. The cultures could be divided into two phases: basal cell proliferation and basal cell differentiation. A role for β-catenin in basal cell proliferation was indicated by activation of the TOPGal transgene on proliferation days 3 to 5 and by transient expression of Myc (alias c-myc). Another peak of TOPGal transgene activity was detected on differentiation days 2 to 10 and was associated with the expression of Axin 2. These results suggest a role for β-catenin in basal to ciliated and basal to Clara-like cell differentiation. Genetic stabilization of β-catenin in basal cells shortened the period of basal cell proliferation but had a minor effect on this process. Persistent β-catenin signaling regulated basal cell fate by driving the generation of ciliated cells and preventing the production of Clara-like cells.

Cell type-specific expression of adenomatous polyposis coli in lung development, injury, and repair

Adenomatous polyposis coli (Apc) is critical for Wnt signaling and cell migration. The current study examined Apc expression during lung development, injury, and repair. Apc was first detectable in smooth muscle layers in early lung morphogenesis, and was highly expressed in ciliated and neuroendocrine cells in the advanced stages. No Apc immunoreactivity was detected in Clara or basal cells, which function as stem/progenitor cell in adult lung. In ciliated cells, Apc is associated mainly with apical cytoplasmic domain. In response to naphthalene-induced injury, Apc(positive) cells underwent squamous metaplasia, accompanied by changes in Apc subcellular distribution. In conclusion, both spatial and temporal expression of Apc is dynamically regulated during lung development and injury repair. Differential expression of Apc in progenitor vs. nonprogenitor cells suggests a functional role in cell-type specification. Subcellular localization changes of Apc in response to naphthalene injury suggest a role in cell shape and cell migration.

Cross-talk between transforming growth factor-beta and Wingless/Int pathways in lung development and disease

Lung development depends on accurate and precise patterning of a pulmonary anlagen, consisting of both endodermally and mesodermally derived progenitor cells. In this process, the need to establish communication and control among individual cells is paramount. Transforming growth factor-beta (TGFbeta) and Wingless/int (Wnt) signaling pathways serve this need. The individual functional repertoire of the two pathways is further expanded by cross-talk and integration of signaling at multiple levels taking advantage of their hard-wired multi-component signal transduction platforms. Cross-talk creates the possibility for both specificity and versatility in signaling during development and during repair of injured tissue. Understanding the mechanics and the physiological implications of this cross-talk is necessary for therapeutic or preventive targeting of either TGFbeta or Wnt signaling pathways.

Lung organogenesis

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