Wnt-responsive element controls Lef-1 promoter expression during submucosal gland morphogenesis

Balancing WNT signalling in early forebrain development: The role of LRP4 as a modulator of LRP6 function

The specification of the forebrain relies on the precise regulation of WNT/ß-catenin signalling to support neuronal progenitor cell expansion, patterning, and morphogenesis. Imbalances in WNT signalling activity in the early neuroepithelium lead to congenital disorders, such as neural tube defects (NTDs). LDL receptor-related protein (LRP) family members, including the well-studied receptors LRP5 and LRP6, play critical roles in modulating WNT signalling capacity through tightly regulated interactions with their co-receptor Frizzled, WNT ligands, inhibitors and intracellular WNT pathway components. However, little is known about the function of LRP4 as a potential modulator of WNT signalling in the central nervous system. In this study, we investigated the role of LRP4 in the regulation of WNT signalling during early mouse forebrain development. Our results demonstrate that LRP4 can modulate LRP5- and LRP6-mediated WNT signalling in the developing forebrain prior to the onset of neurogenesis at embryonic stage 9.5 and is therefore essential for accurate neural tube morphogenesis. Specifically, LRP4 functions as a genetic modifier for impaired mitotic activity and forebrain hypoplasia, but not for NTDs in LRP6-deficient mutants. In vivo and in vitro data provide evidence that LRP4 is a key player in fine-tuning WNT signalling capacity and mitotic activity of mouse neuronal progenitors and of human retinal pigment epithelial (hTERT RPE-1) cells. Our data demonstrate the crucial roles of LRP4 and LRP6 in regulating WNT signalling and forebrain development and highlight the need to consider the interaction between different signalling pathways to understand the underlying mechanisms of disease. The findings have significant implications for our mechanistic understanding of how LRPs participate in controlling WNT signalling.

Expression dynamics of lymphoid enhancer-binding factor 1 in terminal Schwann cells, dermal papilla, and interfollicular epidermis

BACKGROUND

Transcription factor lymphoid enhancer-binding factor 1 (LEF1) is a downstream mediator of the Wnt/β-catenin signaling pathway. It is expressed in dermal papilla and surrounding cells in the hair follicle, promoting cell proliferation, and differentiation.

RESULTS

Here, we report that LEF1 is also expressed all through the hair cycle in the terminal Schwann cells (TSCs), a component of the lanceolate complex located at the isthmus. The timing of LEF1 appearance at the isthmus coincides with that of hair follicle innervation. LEF1 is not found at the isthmus in the aberrant hair follicles in nude mice. Instead, LEF1 in TSCs is found in the de novo hair follicles reconstituted on nude mice by stem cells chamber graft assay. Cutaneous denervation experiment demonstrates that the LEF1 expression in TSCs is independent of nerve endings. At last, LEF1 expression in the interfollicular epidermis during the early stage of skin development is significantly suppressed in transgenic mice with T-cell factor 3 (TCF3) overexpression.

CONCLUSION

We reveal the expression dynamics of LEF1 in skin during development and hair cycle. LEF1 expression in TSCs indicates that the LEF1/Wnt signal might help to establish a niche at the isthmus region for the lanceolate complex, the bulge stem cells and other neighboring cells.

Signaling Control of Mucociliary Epithelia: Stem Cells, Cell Fates, and the Plasticity of Cell Identity in Development and Disease

Mucociliary epithelia are composed of multiciliated, secretory, and stem cells and line various organs in vertebrates such as the respiratory tract. By means of mucociliary clearance, those epithelia provide a first line of defense against inhaled particles and pathogens. Mucociliary clearance relies on the correct composition of cell types, that is, the proper balance of ciliated and secretory cells. A failure to generate and to maintain correct cell type composition and function results in impaired clearance and high risk to infections, such as in congenital diseases (e.g., ciliopathies) as well as in acquired diseases, including asthma, chronic obstructive pulmonary disease (COPD), and idiopathic pulmonary fibrosis (IPF). While it remains incompletely resolved how precisely cell types are specified and maintained in development and disease, many studies have revealed important mechanisms regarding the signaling control in mucociliary cell types in various species. Those studies not only provided insights into the signaling contribution to organ development and regeneration but also highlighted the remarkable plasticity of cell identity encountered in mucociliary maintenance, including frequent trans-differentiation events during homeostasis and specifically in disease. This review will summarize major findings and provide perspectives regarding the future of mucociliary research and the treatment of chronic airway diseases associated with tissue remodeling.

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.

ΔN-Tp63 Mediates Wnt/β-Catenin-Induced Inhibition of Differentiation in Basal Stem Cells of Mucociliary Epithelia

Mucociliary epithelia provide a first line of defense against pathogens. Impaired regeneration and remodeling of mucociliary epithelia are associated with dysregulated Wnt/β-catenin signaling in chronic airway diseases, but underlying mechanisms remain elusive, and studies yield seemingly contradicting results. Employing the Xenopus mucociliary epidermis, the mouse airway, and human airway Basal cells, we characterize the evolutionarily conserved roles of Wnt/β-catenin signaling in vertebrates. In multiciliated cells, Wnt is required for cilia formation during differentiation. In Basal cells, Wnt prevents specification of epithelial cell types by activating ΔN-TP63, a master transcription factor, which is necessary and sufficient to mediate the Wnt-induced inhibition of specification and is required to retain Basal cells during development. Chronic Wnt activation leads to remodeling and Basal cell hyperplasia, which are reversible in vivo and in vitro, suggesting Wnt inhibition as a treatment option in chronic lung diseases. Our work provides important insights into mucociliary signaling, development, and disease.

Impaired (re-)generation of lung epithelia is associated with Wnt signaling changes in animals and human lung disease patients. Haas et al. demonstrate that ΔN-TP63 is a Wnt-regulated master transcription factor inhibiting (re-) generation of new epithelial cells from stem cells. These findings are equally important for understanding animal development and disease mechanisms.

Submucosal Gland Myoepithelial Cells Are Reserve Stem Cells That Can Regenerate Mouse Tracheal Epithelium

The mouse trachea is thought to contain two distinct stem cell compartments that contribute to airway repair-basal cells in the surface airway epithelium (SAE) and an unknown submucosal gland (SMG) cell type. Whether a lineage relationship exists between these two stem cell compartments remains unclear. Using lineage tracing of glandular myoepithelial cells (MECs), we demonstrate that MECs can give rise to seven cell types of the SAE and SMGs following severe airway injury. MECs progressively adopted a basal cell phenotype on the SAE and established lasting progenitors capable of further regeneration following reinjury. MECs activate Wnt-regulated transcription factors (Lef-1/TCF7) following injury and Lef-1 induction in cultured MECs promoted transition to a basal cell phenotype. Surprisingly, dose-dependent MEC conditional activation of Lef-1 in vivo promoted self-limited airway regeneration in the absence of injury. Thus, modulating the Lef-1 transcriptional program in MEC-derived progenitors may have regenerative medicine applications for lung diseases.

Multipotent Myoepithelial Progenitor Cells Are Born Early during Airway Submucosal Gland Development

Airway submucosal glands (SMGs) are facultative stem cell niches for the surface epithelium, but the phenotype of the SMG-derived progenitor cells remains unclear. In other organs, glandular myoepithelial cells (MECs) have been proposed to be multipotent progenitors for luminal cells. We sought to determine the developmental phase during which mouse tracheal glandular MECs are born and whether these MECs are progenitors for other cell phenotypes during SMG morphogenesis. To approach this question, we localized two MEC protein markers (α-smooth muscle actin [αSMA/ACTA2] and smooth muscle myosin heavy chain 11 [SMMHC/MYH11]) during various stages of SMG development (placode, elongation, branching, and differentiation) and used ACTA2-Cre^ERT2 and MYH11-Cre^ERT2 transgenic mice to fate map MEC-derived lineages during SMG morphogenesis. Both αSMA- and SMMHC-expressing cells emerged early after placode formation and during the elongation phase of SMG development. Lineage tracing in newborn mice demonstrated that lineage-positive MECs are born at the tips of invading tubules during the elongation phase of gland development. Lineage-positive MECs born within the first 7 days after birth gave rise to the largest percentage of multipotent progenitors capable of contributing to myoepithelial, serous, mucous, and ductal cell lineages. Serial tamoxifen-induction of both Cre-driver lines demonstrated that lineage-positive multipotent MECs contribute to ∼ 60% of glandular cells by 21 days after birth. In contrast, lineage-traced MECs did not contribute to cell types in the surface airway epithelium. These findings demonstrate that MECs born early during SMG morphogenesis are multipotent progenitors with the capacity to differentiate into other glandular cell types.

Wnt Signaling Regulates Airway Epithelial Stem Cells in Adult Murine Submucosal Glands

Wnt signaling is required for lineage commitment of glandular stem cells (SCs) during tracheal submucosal gland (SMG) morphogenesis from the surface airway epithelium (SAE). Whether similar Wnt-dependent processes coordinate SC expansion in adult SMGs following airway injury remains unknown. We found that two Wnt-reporters in mice (BAT-gal and TCF/Lef:H2B-GFP) are coexpressed in actively cycling SCs of primordial glandular placodes and in a small subset of adult SMG progenitor cells that enter the cell cycle 24 hours following airway injury. At homeostasis, these Wnt reporters showed nonoverlapping cellular patterns of expression in the SAE and SMGs. Following tracheal injury, proliferation was accompanied by dynamic changes in Wnt-reporter activity and the analysis of 56 Wnt-related signaling genes revealed unique temporal changes in expression within proximal (gland-containing) and distal (gland-free) portions of the trachea. Wnt stimulation in vivo and in vitro promoted epithelial proliferation in both SMGs and the SAE. Interestingly, slowly cycling nucleotide label-retaining cells (LRCs) of SMGs were spatially positioned near clusters of BAT-gal positive serous tubules. Isolation and culture of tet-inducible H2B-GFP LRCs demonstrated that SMG LRCs were more proliferative than SAE LRCs and culture expanded SMG-derived progenitor cells outcompeted SAE-derived progenitors in regeneration of tracheal xenograft epithelium using a clonal analysis competition assay. SMG-derived progenitors were also multipotent for cell types in the SAE and formed gland-like structures in xenografts. These studies demonstrate the importance of Wnt signals in modulating SC phenotypes within tracheal niches and provide new insight into phenotypic differences of SMG and SAE SCs. Stem Cells 2016;34:2758-2771.

Understanding multicellular function and disease with human tissue-specific networks

Tissue and cell-type identity lie at the core of human physiology and disease. Understanding the genetic underpinnings of complex tissues and individual cell lineages is crucial for developing improved diagnostics and therapeutics. We present genome-wide functional interaction networks for 144 human tissues and cell types developed using a data-driven Bayesian methodology that integrates thousands of diverse experiments spanning tissue and disease states. Tissue-specific networks predict lineage-specific responses to perturbation, reveal genes’ changing functional roles across tissues, and illuminate disease-disease relationships. We introduce NetWAS, which combines genes with nominally significant GWAS p-values and tissue-specific networks to identify disease-gene associations more accurately than GWAS alone. Our webserver, GIANT, provides an interface to human tissue networks through multi-gene queries, network visualization, analysis tools including NetWAS, and downloadable networks. GIANT enables systematic exploration of the landscape of interacting genes that shape specialized cellular functions across more than one hundred human tissues and cell types.

Understanding the development of the respiratory glands

BACKGROUND

The submucosal glands (SMGs) of the respiratory system are specialized structures essential for maintaining airway homeostasis. The significance of SMGs is highlighted by their involvement in respiratory diseases such as cystic fibrosis, asthma and chronic bronchitis, where their phenotype and function are severely altered. Uncovering the normal development of the airway SMGs is essential to elucidate their role in these disorders, however, very little is known about the cellular mechanisms and intracellular signals involved in their morphogenesis.

RESULTS

This review describes in detail the embryonic developmental journey of the nasal SMGs and the postnatal development of the tracheal SMGs in the mouse. Current knowledge of the genes and signalling molecules involved in SMG organogenesis is also explored.

CONCLUSION

Here we review the temporal localisation and development of the murine respiratory glands in the hope of stimulating further research into the mechanisms required for successful SMG patterning and function.

Sox2 modulates Lef-1 expression during airway submucosal gland development

Tracheobronchial submucosal glands (SMGs) are derived from one or more multipotent glandular stem cells that coalesce to form a placode in surface airway epithelium (SAE). Wnt/β-catenin-dependent induction of lymphoid enhancer factor (Lef-1) gene expression during placode formation is an early event required for SMG morphogenesis. We discovered that Sox2 expression is repressed as Lef-1 is induced within airway SMG placodes. Deletion of Lef-1 did not activate Sox2 expression in SMG placodes, demonstrating that Lef-1 activation does not directly inhibit Sox2 expression. Repression of Sox2 protein in SMG placodes occurred posttranscriptionally, since the activity of its endogenous promoter remained unchanged in SMG placodes. Thus we hypothesized that Sox2 transcriptionally represses Lef-1 expression in the SAE and that suppression of Sox2 in SMG placodes activates Wnt/β-catenin-dependent induction of Lef-1 during SMG morphogenesis. Consistent with this hypothesis, transcriptional reporter assays, ChIP analyses, and DNA-protein binding studies revealed a functional Sox2 DNA binding site in the Lef-1 promoter that is required for suppressing β-catenin-dependent transcription. In polarized primary airway epithelium, Wnt induction enhanced Lef-1 expression while also inhibiting Sox2 expression. Conditional deletion of Sox2 also enhanced Lef-1 expression in polarized primary airway epithelium, but this induction was significantly augmented by Wnt stimulation. Our findings provide the first evidence that Sox2 acts as a repressor to directly modulate Wnt-responsive transcription of the Lef-1 gene promoter. These studies support a model whereby Wnt signals and Sox2 dynamically regulate the expression of Lef-1 in airway epithelia and potentially also during SMG development.

Wnt signaling in lung organogenesis

Reporter transgene, knockout, and misexpression studies support the notion that Wnt/beta-catenin signaling regulates aspects of branching morphogenesis, regional specialization of the epithelium and mesenchyme, and establishment of progenitor cell pools. As demonstrated for other foregut endoderm-derived organs, beta-catenin and the Wnt/beta-catenin signaling pathway contribute to control of cellular proliferation, differentiation and migration. However, the contribution of Wnt/beta-catenin signaling to these processes is shaped by other signals impinging on target tissues. In this review, we will concentrate on roles for Wnt/beta-catenin in respiratory system development, including segregation of the conducting airway and alveolar compartments, specialization of the mesenchyme, and establishment of tracheal asymmetries and tracheal glands.

Atlas of Wnt and R-spondin gene expression in the developing male mouse lower urogenital tract

Prostate development is influenced by β-catenin signaling, but it is unclear which β-catenin activators are involved, where they are synthesized, and whether their mRNA abundance is influenced by androgens. We identified WNT/β-catenin-responsive β-galactosidase activity in the lower urogenital tract (LUT) of transgenic reporter mice, but β-galactosidase activity differed among the four mouse strains we examined. We used in situ hybridization to compare patterns of Wnts, r-spondins (Rspos, co-activators of β-catenin signaling), β-catenin-responsive mRNAs, and an androgen receptor-responsive mRNA in wild type fetal male, fetal female, and neonatal male LUT. Most Wnt and Rspo mRNAs were present in LUT during prostate development. Sexually dimorphic expression patterns were observed for WNT/β-catenin-responsive genes, and for Wnt2b, Wnt4, Wnt7a, Wnt9b, Wnt10b, Wnt11, Wnt16, and Rspo3 mRNAs. These results reveal sexual differences in WNT/β-catenin signaling in fetal LUT, supporting the idea that this pathway may be directly or indirectly responsive to androgens during prostate ductal development.

Glandular gene expression of sinus mucosa in chronic rhinosinusitis with and without cystic fibrosis

Secretory cells in submucosal glands (SMGs) secrete antibacterial proteins and mucin glycoproteins into the apical lumen of the respiratory tract, and these are critical for innate immune mucosal integrity. Glandular hyperplasia is manifested in diseases with obstructive respiratory pathologies associated with mucous hypersecretion, and is predominant in the sinus mucosa of patients with chronic rhinosinusitis (CRS), cystic fibrosis (CF), and clinical symptoms of CRS. To gain insights into the molecular basis of SMG hyperplasia in CRS, gene expression microarray analyses were performed to identify the differences in global and specific gene expression in the sinus mucosa of control, CRS, and CRS/CF patients. A marked up-regulation of 11 glandular-associated genes in CRS and CRS/CF sinus mucosa was evident. The RNA and protein expressions of the four most highly up-regulated genes (DSG3, KRT14, PTHLH, and OTX2) were evaluated. An increased expression of DSG3, KRT14, and PTHLH was demonstrated at the mRNA and protein levels in both CRS and CRS/CF sinus mucosa, whereas the increased expression of OTX2 was evident only for CRS/CF sinus mucosa, implicating OTX2 as a CF-specific gene. Immunofluorescence analysis localized DSG3, PTHLH, and OTX2 to serous cells, and KRT14 to myoepithelial cells, in SMGs. Because glandular hyperplasia is a central histologic feature of CRS, the identification of overexpressed glandular genes in the sinus mucosa lays the groundwork for future studies of glandular hyperplasia, and may ultimately lead to the development of novel treatments for mucous hypersecretion in patients with CRS.

Cell-autonomous integrin control of Wnt and Notch signalling during somitogenesis

Integrins act at signalling crossroads, and their interactions with other signal transduction pathways are key to the regulation of normal and pathological cell cytoarchitecture and behaviour. Here, we describe a signalling cascade that acts during the formation of the defining segmental features of the vertebrate body – the somites – in which β1-integrin activity regulates epithelialisation by controlling downstream Wnt and Notch activity crucial for somite border formation. Using in vivo transcriptional inhibition in the developing chick embryo, we show that β1-integrin in the anterior presomitic mesoderm activates canonical Wnt signalling in a cell-autonomous, `outside-inside' manner. Signalling is mediated by integrin-linked kinase (ILK), leading to modulation of glycogen synthase kinase 3β (GSK3β) phosphorylation, and activates Notch signalling in the anterior presomitic mesoderm. The two signalling pathways then cooperate to promote somite formation via cMESO1/Mesp2. Our results show that β1-integrin can regulate cell shape and tissue morphogenesis indirectly, by regulation of downstream signalling cascades.

Sox17 modulates Wnt3A/beta-catenin-mediated transcriptional activation of the Lef-1 promoter

Wnt/β-catenin-dependent activation of lymphoid enhancer factor 1 (Lef-1) plays an important role in numerous developmental processes. In this context, transcription of the Lef-1 gene is increased by Wnt-mediated TCF4/β-catenin activation on the Lef-1 promoter through mechanisms that remain poorly defined. In mouse airway submucosal gland progenitor cells, Wnt3A transiently induces Lef-1 gene expression, and this process is required for epithelial cell proliferation and glandular morphogenesis. In the present study, we sought to identify additional candidate transcriptional regulators of the Lef-1 gene during glandular morphogenesis. To this end, we found that Sox17 expression is dramatically downregulated in early glandular progenitor cells that induce Lef-1 expression. Wnt stimulation of undifferentiated primary airway epithelial cells induced similar changes in Sox17 and Lef-1 expression. Reporter assays revealed that ectopic expression of Sox17 suppresses Wnt3A/β-catenin activation of the Lef-1 promoter in cell lines. EMSA and ChIP analyses defined several Sox17- and TCF4-binding sites that collaborate in transcriptional control of the Lef-1 promoter. More specifically, Sox17 bound to four sites in the Lef-1 promoter, either directly or indirectly through TCF complexes. The DNA- or β-catenin-binding domains of Sox17 controlled context-specific binding of Sox17/TCF complexes on the Lef-1 promoter. Combinatorial site-directed mutagenesis of Sox17- or TCF-binding sites in the Lef-1 promoter demonstrated that these sites control Wnt/β-catenin-mediated induction and/or repression. These findings demonstrate for the first time that Sox17 can directly regulate Wnt/β-catenin-dependent transcription of the Lef-1 promoter and reveal new context-dependent binding sites in the Lef-1 promoter that facilitate protein-protein interactions between Sox17 and TCF4.

High-resolution gene expression analysis of the developing mouse kidney defines novel cellular compartments within the nephron progenitor population

The functional unit of the kidney is the nephron. During its organogenesis, the mammalian metanephric kidney generates thousands of nephrons over a protracted period of fetal life. All nephrons are derived from a population of self-renewing multi-potent progenitor cells, termed the cap mesenchyme. However, our understanding of the molecular and cellular mechanisms underlying nephron development is at an early stage. In order to identify factors involved in nephrogenesis, we performed a high-resolution, spatial profiling of a number of transcriptional regulators expressed within the cap mesenchyme and early developing nephron. Our results demonstrate novel, stereotypic, spatially defined cellular sub-domains within the cap mesenchyme, which may, in part, reflect induction of nephron precursors. These results suggest a hitherto unappreciated complexity of cell states that accompany the assembly of the metanephric kidney, likely reflecting diverse regulatory actions such as the maintenance and induction of nephron progenitors.

The glandular stem/progenitor cell niche in airway development and repair

Airway submucosal glands (SMGs) are major secretory structures that lie beneath the epithelium of the cartilaginous airway. These glands are believed to play important roles in normal lung function and airway innate immunity by secreting antibacterial factors, mucus, and fluid into the airway lumen. Recent studies have suggested that SMGs may additionally serve as a protective niche for adult epithelial stem/progenitor cells of the proximal airways. As in the case of other adult stem cell niches, SMGs are believed to provide the localized environmental signals required to both maintain and mobilize stem/progenitor cells, in the setting of normal cellular turnover or injury. Aberrant proliferation and differentiation of glandular stem/progenitor cells may be associated with several hypersecretory lung diseases, including chronic bronchitis, asthma, and cystic fibrosis. To better understand the molecular mechanisms that regulate the specification and proliferation of glandular stem/progenitor cells in lung diseases associated with SMG hypertrophy and hyperplasia, researchers have begun to search for the molecular signals and cell types responsible for establishing the glandular stem/progenitor cell niche, and to dissect how these determinants of the niche change in the setting of proximal airway injury and repair. Such studies have revealed certain similarities between stem/progenitor cell niches of the distal conducting airways and the SMGs of the proximal airways.

Formation and differentiation of multiple mesenchymal lineages during lung development is regulated by beta-catenin signaling

Background

The role of ß-catenin signaling in mesodermal lineage formation and differentiation has been elusive.

Methodology

To define the role of ß-catenin signaling in these processes, we used a Dermo1(Twist2)^Cre/+ line to target a floxed β-catenin allele, throughout the embryonic mesenchyme. Strikingly, the Dermo1^Cre/+; β-catenin^f/− conditional Knock Out embryos largely phenocopy Pitx1^−/−/Pitx2^−/− double knockout embryos, suggesting that ß-catenin signaling in the mesenchyme depends mostly on the PITX family of transcription factors. We have dissected this relationship further in the developing lungs and find that mesenchymal deletion of β-catenin differentially affects two major mesenchymal lineages. The amplification but not differentiation of Fgf10-expressing parabronchial smooth muscle progenitor cells is drastically reduced. In the angioblast-endothelial lineage, however, only differentiation into mature endothelial cells is impaired.

Conclusion

Taken together these findings reveal a hierarchy of gene activity involving ß-catenin and PITX, as important regulators of mesenchymal cell proliferation and differentiation.

PITX2 and beta-catenin interactions regulate Lef-1 isoform expression

Lef-1 and PITX2 function in the Wnt signaling pathway by recruiting and interacting with beta-catenin to activate target genes. Chromatin immunoprecipitation (ChIP) assays identified the Lef-1 promoter as a PITX2 downstream target. Transgenic mice expressing LacZ driven by the 2.5-kb LEF-1 promoter demonstrated expression in the tooth epithelium correlated with endogenous Lef-1 FL epithelial expression. PITX2 isoforms regulate the LEF-1 promoter, and beta-catenin synergistically enhanced activation of the LEF-1 promoter in combination with PITX2 and Lef-1 isoforms. PITX2 enhances endogenous expression of the full-length beta-catenin-dependent Lef-1 isoform (Lef-1 FL) while decreasing expression of the N-terminally truncated beta-catenin-independent isoform. Our research revealed a novel interaction between PITX2, Lef-1, and beta-catenin in which the Lef-1 beta-catenin binding domain is dispensable for its interaction with PITX2. PITX2 interacts with two sites within the Lef-1 protein. Furthermore, beta-catenin interacts with the PITX2 homeodomain and Lef-1 interacts with the PITX2 C-terminal tail. Lef-1 and beta-catenin interact simultaneously and independently with PITX2 through two different sites to regulate PITX2 transcriptional activity. These data support a role for PITX2 in cell proliferation, migration, and cell division through differential Lef-1 isoform expression and interactions with Lef-1 and beta-catenin.

Glycogen synthase kinase-3 inactivation and stabilization of beta-catenin induce nephron differentiation in isolated mouse and rat kidney mesenchymes

Wnt proteins are required for induction of nephrons in mouse metanephric kidneys, but the downstream pathways that mediate tubule induction and epithelial differentiation have remained obscure. The intracellular mechanisms by which Wnt signaling mediates nephron induction in embryonic kidney mesenchymes were studied. First is shown that transient exposure of isolated kidney mesenchymes to structurally different glycogen synthase kinase-3 (GSK3) inhibitors lithium or 6-bromoindirubin-3'-oxime results in abundant epithelial differentiation and full segregation of nephrons. Shown further by mice with genetically disrupted ureteric bud or Wolffian duct development is that this nephrogenic competence arises independent of the influence of Wolffian duct-derived epithelia. Analysis of the intracellular signaling cascades downstream of GSK3 inhibition revealed stabilization of beta-catenin and upregulation of Lef1 and Tcf1, both events that are associated with the active canonical Wnt signaling. Last, genetic evidence that metanephric mesenchyme-specific stabilization of beta-catenin is sufficient to induce nephron differentiation in isolated kidney mesenchymes, similar to that induced by GSK3 inhibitors, is provided. These data show that activation of canonical Wnt pathway is sufficient to induce nephrogenesis and suggest that this pathway mediates the nephron induction in murine kidney mesenchymes.

Wnt3a regulates Lef-1 expression during airway submucosal gland morphogenesis

Regulation of the lymphoid enhancer factor 1 (Lef-1) transcription factor is important for the inductive formation of many epithelial-derived appendages including airway submucosal glands (SMGs). Although Wnts have been linked to developmental processes involving transcriptional activation of the Lef-1 protein, there is little in vivo information directly linking Wnts with the transcriptional regulation of the Lef-1 promoter. In the present study, we hypothesized that Wnt3a directly regulates Lef-1 gene expression required for SMG morphogenesis in mice. In support of this hypothesis, TOPGAL reporter mice demonstrated activation of beta-catenin/Tcf complexes during early phases of SMG development and immunolocalization studies confirmed abundant expression of Tcf4, but not Tcf1 or Tcf3, at this stage. ChIP analysis in primary airway epithelial cells revealed that Tcf4 associates with a known Wnt Responsive Region in the Lef-1 promoter and transfection of Cos-1 cells with dominant active beta-catenin and Tcf4 synergistically activated the Lef-1 promoter. Using Wnt3a deficient and Lef-1 promoter-GFP reporter mice, we also demonstrate that Wnt3a induces Lef-1 gene expression in newly forming SMG buds of mice and is required for the maintenance of gland bud growth. These findings provide the first in vivo evidence that Wnt3a can transcriptionally regulate the Lef-1 gene.

Wnt activation and alternative promoter repression of LEF1 in colon cancer

Alternative promoters within the LEF1 locus produce polypeptides of opposing biological activities. Promoter 1 produces full-length LEF-1 protein, which recruits beta-catenin to Wnt target genes. Promoter 2 produces a truncated form that cannot interact with beta-catenin and instead suppresses Wnt regulation of target genes. Here we show that promoter 1 is aberrantly activated in colon cancers because it is a direct target of the Wnt pathway. T-cell factor (TCF)-beta-catenin complexes bind to Wnt response elements in exon 1 and dynamically regulate chromatin acetylation and promoter 1 activity. Promoter 2 is delimited to the intron 2/exon 3 boundary and, like promoter 1, is also directly regulated by TCF-beta-catenin complexes. Promoter 2 is nevertheless silent in colon cancer because an upstream repressor selectively targets the basal promoter leading to destabilized TCF-beta-catenin binding. We conclude that the biological outcome of aberrant LEF1 activation in colon cancer is directed by differential promoter activation and repression.

Developmental pathways and specification of intrapulmonary stem cells

Tissues have the capacity to maintain a homeostatic balance between wear-and-tear and regeneration. Repair of non-lethal injury also activates cell proliferation to repopulate the injured sites with appropriate cell types and to restore function. Although controversial, the source of the material appears to be at least partly from pools of unique, multipotent stem cells that reside in specialized locations referred to as "niches." Molecular interactions between the niche and the intracellular factors within stem cells are crucial in maintaining stem cell functions, particularly the balance between self-renewal and differentiation. Many of the mediators of the stem cell-niche interactions are similar or identical to those that control developmental pathways during organogenesis. In this review, we present a systematic discussion and evaluation of the relevant literature with a focused emphasis on three primary signaling pathways, WNT, SHH and BMP with potentially overlapping roles during both development and stem cell maintenance.

β-Catenin/T-cell factor-mediated transcription is modulated by cell density in human bronchial epithelial cells

The embryonic Wnt/beta-catenin ('canonical') pathway has been implicated in epithelial regeneration. To investigate the role of Wnt signal transduction in the airways, we characterised the expression of key pathway components in human bronchial epithelial cells (HBEC) and studied the influence of cell density on pathway activity, using sub-confluent cells in log-phase growth as a simple model of repairing epithelium. Primary HBEC and H292 bronchial epithelial cells were found to express TCF-4, TCF-3 and isoforms of LEF-1, transcription factors that are regulated by Wnt signalling. The cells also had the potential to respond to Wnt signalling through expression of several members of the Frizzled receptor family, including FZD-5 and -6. In confluent H292 cells, 20 mM lithium and 25% v/v Wnt-3a conditioned medium induced 4.5-fold (p = 0.008) and 1.4-fold (p = 0.006) increases in TOPflash activity, respectively. Under conditions of reduced cell density, TOPflash activity increased 1.8-fold (p = 0.002) in association with increased nuclear localisation of hypophosphorylated (active) beta-catenin and increased cell proliferation. This up-regulation in reporter activity occurred independently of EGF receptor activation and could not be recapitulated by use of low-calcium medium to disrupt cadherin-mediated cell-cell adhesion, but was associated with changes in FZD-6 expression. We conclude that reactivation of this embryonic pathway may play an important role in bronchial epithelial regeneration, and that modulation of Fzd-6 receptors may regulate Wnt signalling at confluence. Recognising that many chronic inflammatory disorders of the airways involve epithelial damage and repair, altered Wnt signalling might contribute to disease pathogenesis or progression.

Airway glandular development and stem cells

Submucosal glands in the lung play important roles in several hypersecretory lung disease processes, including chronic bronchitis, asthma, and cystic fibrosis. In this context, submucosal glands undergo abnormal growth and differentiation through processes that are poorly understood. To better understand the pathophysiological mechanisms that lead to submucosal gland hypertrophy and hyperplasia in the adult human lung, efforts have been made to dissect the molecular signals and cell types responsible for normal submucosal gland development in the airway. Such studies have revealed a close relationship between progenitor?stem cell phenotypes in the surface airway epithelia and submucosal glands, and thus it has been suggested that submucosal glands serve as a protective niche for surface airway epithelial stem cells. Furthermore, the pluripotent progenitor cells that exist in the surface airway epithelium, which have the capacity to differentiate into ciliated, secretory, intermediate, and basal cells, also have a developmental capacity for submucosal glands. This putative adult stem cell compartment of the airway epithelium has been the focus of research attempting to identify molecular markers for signaling pathways that control stem cell phenotypes and their capacity for proliferation and differentiation following airway injury.

Intercellular growth factor signaling and the development of mouse tracheal submucosal glands

To provide a genetic framework for investigating changes in airway submucosal gland function in human respiratory disease, we have investigated their counterparts in normal and mutant mice. We describe their morphogenesis in relation to the expression of genes encoding conserved intercellular signaling pathways. Submucosal glands are severely reduced in number and size in mice heterozygous for Fgf10. Glands are completely absent in mice lacking Ectodysplasin (Eda) and Edaradd (Eda receptor adaptor protein), members of the tumor necrosis (TNF) superfamily of signaling factors. Furthermore, components of the Eda and closely related pathways are transcribed throughout the respiratory system in the adult mouse. Finally, the temporal and spatial pattern of Bmp4 expression suggests that it may control submucosal gland development and homeostasis. Taken together, our observations have important implications for the better understanding of the submucosal gland remodeling that occurs in human respiratory disease.