Dickkopf-1 (DKK1) reveals that fibronectin is a major target of Wnt signaling in branching morphogenesis of the mouse embryonic lung

Effects of Genetic Loci Associated with Central Obesity on Adipocyte Lipolysis

OBJECTIVES

Numerous genetic loci have been associated with measures of central fat accumulation, such as waist-to-hip ratio adjusted for body mass index (WHRadjBMI). However the mechanisms by which genetic variations influence obesity remain largely elusive. Lipolysis is a key process for regulation of lipid storage in adipocytes, thus is implicated in obesity and its metabolic complications. Here, genetic variants at 36 WHRadjBMI-associated loci were examined for their influence on abdominal subcutaneous adipocyte lipolysis.

SUBJECTS AND METHODS

Fasting subcutaneous adipose tissue biopsies were collected from 789 volunteers (587 women and 202 men, body mass index (BMI) range 17.7-62.3 kg/m2). We quantified subcutaneous adipocyte lipolysis, both spontaneous and stimulated by the catecholamine isoprenaline or a cyclic AMP analogue. DNA was extracted from peripheral blood mononuclear cells and genotyping of SNPs associated with WHRadjBMI conducted. The effects on adipocyte lipolysis measures were assessed for SNPs individually and combined in a SNP score.

RESULTS

The WHRadjBMI-associated loci CMIP, PLXND1, VEGFA and ZNRF3-KREMEN1 demonstrated nominal associations with spontaneous and/or stimulated lipolysis. Candidate genes in these loci have been reported to influence NFκB-signaling, fat cell size and Wnt signalling, all of which may influence lipolysis.

SIGNIFICANCE

This report provides evidence for specific WHRadjBMI-associated loci as candidates to modulate adipocyte lipolysis. Additionally, our data suggests that genetically increased central fat accumulation is unlikely to be a major cause of altered lipolysis in abdominal adipocytes.

Role of Wnt/β-Catenin Signaling in Epithelial Differentiation of Lung Resident Mesenchymal Stem Cells

Accumulating evidence has demonstrated that stem cells have the ability to repair the lung tissue injuries following either injection of cultured cells or bone marrow transplantation. As a result, increasing attention has focused on the lung resident mesenchymal stem cells (LR-MSCs) for repairing damaged lung tissues. Meanwhile, some studies have revealed that Wnt/β-catenin signaling plays an important role in the epithelial differentiation of mesenchymal stem cells (MSCs). In the current study, our aim was to explore the roles of Wnt/β-catenin signaling on cell proliferation and epithelial differentiation of LR-MSCs. We have successfully isolated the stem cell antigen (Sca)-1(+) CD45(-) CD31(-) cells which were proposed to be LR-MSCs by magnetic-activated cell sorting (MACS). Furthermore, we demonstrated the expression of epithelial markers on LR-MSCs following indirect co-culture of these cells with alveolar epithelial type II (ATII) cells, confirming the epithelial phenotype of LR-MSCs following co-culture. In order to clarify the regulatory mechanisms of Wnt/β-catenin signaling in epithelial differentiation of LR-MSCs, we measured the protein levels of several important members involved in Wnt/β-catenin signaling in the presence or absence of some canonical activators and inhibitors of the β-catenin pathways. In conclusion, our study demonstrated that Wnt/β-catenin signaling may be an essential mechanism underlying the regulation of epithelial differentiation of LR-MSCs.

Fluorescence-based gene reporter plasmid to track canonical Wnt signaling in ENS inflammation

In several gut inflammatory or cancer diseases, cell-cell interactions are compromised, and an increased cytoplasmic expression of β-catenin is observed. Over the last decade, numerous studies provided compelling experimental evidence that the loss of cadherin-mediated cell adhesion can promote β-catenin release and signaling without any specific activation of the canonical Wnt pathway. In the present work, we took advantage of the ability of lipofectamine-like reagent to cause a synchronous dissociation of adherent junctions in cells isolated from the rat enteric nervous system (ENS) for obtaining an in vitro model of deregulated β-catenin signaling. Under these experimental conditions, a green fluorescent protein Wnt reporter plasmid called ΔTop_EGFP3a was successfully tested to screen β-catenin stabilization at resting and primed conditions with exogenous Wnt3a or lipopolysaccharide (LPS). ΔTop_EGFP3a provided a reliable and strong fluorescent signal that was easily measurable and at the same time highly sensitive to modulations of Wnt signaling following Wnt3a and LPS stimulation. The reporter gene was useful to demonstrate that Wnt3a exerts a protective activity in the ENS from overstimulated Wnt signaling by promoting a downregulation of the total β-catenin level. Based on this evidence, the use of ΔTop_EGFP3a reporter plasmid could represent a more reliable tool for the investigation of Wnt and cross-talking pathways in ENS inflammation.

The extracellular calcium-sensing receptor regulates human fetal lung development via CFTR

Optimal fetal lung growth requires anion-driven fluid secretion into the lumen of the developing organ. The fetus is hypercalcemic compared to the mother and here we show that in the developing human lung this hypercalcaemia acts on the extracellular calcium-sensing receptor, CaSR, to promote fluid-driven lung expansion through activation of the cystic fibrosis transmembrane conductance regulator, CFTR. Several chloride channels including TMEM16, bestrophin, CFTR, CLCN2 and CLCA1, are also expressed in the developing human fetal lung at gestational stages when CaSR expression is maximal. Measurements of Cl⁻-driven fluid secretion in organ explant cultures show that pharmacological CaSR activation by calcimimetics stimulates lung fluid secretion through CFTR, an effect which in humans, but not mice, was also mimicked by fetal hypercalcemic conditions, demonstrating that the physiological relevance of such a mechanism appears to be species-specific. Calcimimetics promote CFTR opening by activating adenylate cyclase and we show that Ca²⁺-stimulated type I adenylate cyclase is expressed in the developing human lung. Together, these observations suggest that physiological fetal hypercalcemia, acting on the CaSR, promotes human fetal lung development via cAMP-dependent opening of CFTR. Disturbances in this process would be expected to permanently impact lung structure and might predispose to certain postnatal respiratory diseases.

Wnt/β-catenin signaling in kidney injury and repair: a double-edged sword

The Wnt/β-catenin signaling cascade is an evolutionarily conserved, highly complex pathway that is known to be involved in kidney injury and repair after a wide variety of insults. Although the kidney displays an impressive ability to repair and recover after injury, these repair mechanisms can be overwhelmed, leading to maladaptive responses and eventual development of chronic kidney disease (CKD). Emerging evidence demonstrates that Wnt/β-catenin signaling possesses dual roles in promoting repair/regeneration or facilitating progression to CKD after acute kidney injury (AKI), depending on the magnitude and duration of its activation. In this review, we summarize the expression, intracellular modification, and secretion of Wnt family proteins and their regulation in a variety of kidney diseases. We also explore our current understanding of the potential mechanisms by which transient Wnt/β-catenin activation positively regulates adaptive responses of the kidney after AKI, and discuss how sustained activation of this signaling triggers maladaptive responses and causes destructive outcomes. A better understanding of these mechanisms may offer important opportunities for designing targeted therapy to promote adaptive kidney repair/recovery and prevent progression to CKD in patients.

Modulation of apical constriction by Wnt signaling is required for lung epithelial shape transition

In lung development the apically constricted columnar epithelium forms numerous buds during the pseudoglandular stage and subsequently changes the shape into flat or cuboidal pneumocytes that compose the air sacs during the canalicular and saccular (canalicular-saccular) stages, yet the impact of cell shapes on tissue morphogenesis remains unclear. The expression of Wnt components were decreased in the canalicular-saccular stages, and genetically constitutive activation of Wnt signaling impaired air sac formation by inducing apical constriction in the epithelium as seen in the pseudoglandular stage. Organ culture models also demonstrated that Wnt signaling induces apical constriction through the apical actomyosin cytoskeletal organization. Mathematical modeling revealed that apical constriction induces bud formation and loss of apical constriction is required for the formation of an air sac-like structure. MAP/Microtubule affinity-regulating kinase (MARK1) was identified as a downstream molecule of Wnt signaling and required for the apical cytoskeletal organization and bud formation. These results suggest that Wnt signaling is required for bud formation by inducing apical constriction during the pseudoglandular stage, while loss of Wnt signaling is for air sac formation in the canalicular-saccular stages.

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.

Lung endoderm morphogenesis: gasping for form and function

The respiratory endoderm develops from a small cluster of cells located on the ventral anterior foregut. This population of progenitors generates the myriad epithelial lineages required for proper lung function in adults through a complex and delicately balanced series of developmental events controlled by many critical signaling and transcription factor pathways. In the past decade, understanding of this process has grown enormously, helped in part by cell lineage fate analysis and deep sequencing of the transcriptomes of various progenitors and differentiated cell types. This review explores how these new techniques, coupled with more traditional approaches, have provided a detailed picture of development of the epithelial lineages in the lung and insight into how aberrant development can lead to lung disease.

Transcriptome Analysis of the Preterm Rabbit Lung after Seven Days of Hyperoxic Exposure

The neonatal management of preterm born infants often results in damage to the developing lung and subsequent morbidity, referred to as bronchopulmonary dysplasia (BPD). Animal models may help in understanding the molecular processes involved in this condition and define therapeutic targets. Our goal was to identify molecular pathways using the earlier described preterm rabbit model of hyperoxia induced lung-injury. Transcriptome analysis by mRNA-sequencing was performed on lungs from preterm rabbit pups born at day 28 of gestation (term: 31 days) and kept in hyperoxia (95% O₂) for 7 days. Controls were preterm pups kept in normoxia. Transcriptomic data were analyzed using Array Studio and Ingenuity Pathway Analysis (IPA), in order to identify the central molecules responsible for the observed transcriptional changes. We detected 2217 significantly dysregulated transcripts following hyperoxia, of which 90% could be identified. Major pathophysiological dysregulations were found in inflammation, lung development, vascular development and reactive oxygen species (ROS) metabolism. To conclude, amongst the many dysregulated transcripts, major changes were found in the inflammatory, oxidative stress and lung developmental pathways. This information may be used for the generation of new treatment hypotheses for hyperoxia-induced lung injury and BPD.

Nanoparticle modulation of the tumor microenvironment enhances therapeutic efficacy of cisplatin

The tumor microenvironment (TME) serves as a multidrug resistant center for tumors under the assault of chemotherapy and a physiological barrier against the penetration of therapeutic nanoparticles (NPs). Previous studies have indicated the ability for therapeutic NP to distribute into, and deplete tumor-associated fibroblasts (TAFs) for improved therapeutic outcomes. However, a drug resistant phenotype gradually arises after repeated doses of chemotherapeutic NP. Herein, the acquisition of drug resistant phenotypes in the TME after repeated cisplatin NP treatment was examined. Particularly, this study was aimed at investigating the effects of NP damaged TAFs on neighboring cells and alteration of stromal structure after cisplatin treatment. Findings suggested that while off-targeted NP damaged TAFs and inhibited tumor growth after an initial dose, chronic exposure to cisplatin NP led to elevated secretion of Wnt16 in a paracrine manner in TAFs. Wnt16 upregulation was then attributed to heightened tumor cell resistance and stroma reconstruction. Results attest to the efficacy of Wnt16 knockdown in damaged TAFs as a promising combinatory strategy to improve efficacy of cisplatin NP in a stroma-rich bladder cancer model.

Kremen1 and Dickkopf1 control cell survival in a Wnt-independent manner

In multicellular organisms, a tight control of cell death is required to ensure normal development and tissue homeostasis. Improper function of apoptotic or survival pathways can not only affect developmental programs but also favor cancer progression. Here we describe a novel apoptotic signaling pathway involving the transmembrane receptor Kremen1 and its ligand, the Wnt-antagonist Dickkopf1. Using a whole embryo culture system, we first show that Dickkopf1 treatment promotes cell survival in a mouse model exhibiting increased apoptosis in the developing neural plate. Remarkably, this effect was not recapitulated by chemical Wnt inhibition. We then show that Dickkopf1 receptor Kremen1 is a bona fide dependence receptor, triggering cell death unless bound to its ligand. We performed Wnt-activity assays to demonstrate that the pro-apoptotic and anti-Wnt functions mediated by Kremen1 are strictly independent. Furthermore, we combined phylogenetic and mutagenesis approaches to identify a specific motif in the cytoplasmic tail of Kremen1, which is (i) specifically conserved in the lineage of placental mammals and (ii) strictly required for apoptosis induction. Finally, we show that somatic mutations of kremen1 found in human cancers can affect its pro-apoptotic activity, supporting a tumor suppressor function. Our findings thus reveal a new Wnt-independent function for Kremen1 and Dickkopf1 in the regulation of cell survival with potential implications in cancer therapies.

Heterotypic control of basement membrane dynamics during branching morphogenesis

Many mammalian organs undergo branching morphogenesis to create highly arborized structures with maximized surface area for specialized organ function. Cooperative cell-cell and cell-matrix adhesions that sculpt the emerging tissue architecture are guided by dynamic basement membranes. Properties of the basement membrane are reciprocally controlled by the interacting epithelial and mesenchymal cell populations. Here we discuss how basement membrane remodeling is required for branching morphogenesis to regulate cell-matrix and cell-cell adhesions that are required for cell patterning during morphogenesis and how basement membrane impacts morphogenesis by stimulation of cell patterning, force generation, and mechanotransduction. We suggest that in addition to creating mature epithelial architecture, remodeling of the epithelial basement membrane during branching morphogenesis is also essential to promote maturation of the stromal mesenchyme to create mature organ structure. Recapitulation of developmental cell-matrix and cell-cell interactions are of critical importance in tissue engineering and regeneration strategies that seek to restore organ function.

In vitro reconstruction of branched tubular structures from lung epithelial cells in high cell concentration gradient environment

We have succeeded in developing hollow branching structure in vitro commonly observed in lung airway using primary lung airway epithelial cells. Cell concentration gradient is the key factor that determines production of the branching cellular structures, as optimization of this component removes the need for heterotypic culture. The higher cell concentration leads to the more production of morphogens and increases the growth rate of cells. However, homogeneous high cell concentration does not make a branching structure. Branching requires sufficient space in which cells can grow from a high concentration toward a low concentration. Simulation performed using a reaction-diffusion model revealed that long-range inhibition prevents cells from branching when they are homogeneously spread in culture environments, while short-range activation from neighboring cells leads to positive feedback. Thus, a high cell concentration gradient is required to make branching structures. Spatial distributions of morphogens, such as BMP-4, play important roles in the pattern formation. This simple yet robust system provides an optimal platform for the further study and understanding of branching mechanisms in the lung airway, and will facilitate chemical and genetic studies of lung morphogenesis programs.

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.

Β-catenin is involved in Bex2 down-regulation induced glioma cell invasion/migration inhibition

Previously, we found that brain expressed X-linked gene 2 (Bex2) regulates the invasion/migration ability of glioma cells. However, the mechanism of this effect remains unknown. In current study, we reported that Bex2 down-regulation inhibited glioma cell migration and invasion by decreasing the nucleus and cytoplasm protein level of β-catenin. We found that the protein levels of Bex2 and β-catenin were up-regulated and showed direct correlation in glioma tissues. Bex2 down-regulation significantly decreased β-catenin protein levels but not its mRNA levels. Furthermore, the decreased protein level of β-catenin was located in the nucleus and cytoplasm but not in the cell membrane. Further study found that the effects of Bex2 down-regulation on the invasion and migration of glioma cell could be reversed by β-catenin over-expression. Taken together, Bex2 affects the invasion and migration ability of glioma cells by regulating β-catenin.

Canonical Wnt signaling activity in early stages of chick lung development

Wnt signaling pathway is an essential player during vertebrate embryonic development which has been associated with several developmental processes such as gastrulation, body axis formation and morphogenesis of numerous organs, namely the lung. Wnt proteins act through specific transmembrane receptors, which activate intracellular pathways that regulate cellular processes such as cell proliferation, differentiation and death. Morphogenesis of the fetal lung depends on epithelial-mesenchymal interactions that are governed by several growth and transcription factors that regulate cell proliferation, fate, migration and differentiation. This process is controlled by different signaling pathways such as FGF, Shh and Wnt among others. Wnt signaling is recognized as a key molecular player in mammalian pulmonary development but little is known about its function in avian lung development. The present work characterizes, for the first time, the expression pattern of several Wnt signaling members, such as wnt-1, wnt-2b, wnt-3a, wnt-5a, wnt-7b, wnt-8b, wnt-9a, lrp5, lrp6, sfrp1, dkk1, β-catenin and axin2 at early stages of chick lung development. In general, their expression is similar to their mammalian counterparts. By assessing protein expression levels of active/total β-catenin and phospho-LRP6/LRP6 it is revealed that canonical Wnt signaling is active in this embryonic tissue. In vitro inhibition studies were performed in order to evaluate the function of Wnt signaling pathway in lung branching. Lung explants treated with canonical Wnt signaling inhibitors (FH535 and PK115-584) presented an impairment of secondary branch formation after 48 h of culture along with a decrease in axin2 expression levels. Branching analysis confirmed this inhibition. Wnt-FGF crosstalk assessment revealed that this interaction is preserved in the chick lung. This study demonstrates that Wnt signaling is crucial for precise chick lung branching and further supports the avian lung as a good model for branching studies since it recapitulates early mammalian pulmonary development.

Automated image analysis of lung branching morphogenesis from microscopic images of fetal rat explants

Background. Regulating mechanisms of branching morphogenesis of fetal lung rat explants have been an essential tool for molecular research. This work presents a new methodology to accurately quantify the epithelial, outer contour, and peripheral airway buds of lung explants during cellular development from microscopic images. Methods. The outer contour was defined using an adaptive and multiscale threshold algorithm whose level was automatically calculated based on an entropy maximization criterion. The inner lung epithelium was defined by a clustering procedure that groups small image regions according to the minimum description length principle and local statistical properties. Finally, the number of peripheral buds was counted as the skeleton branched ends from a skeletonized image of the lung inner epithelia. Results. The time for lung branching morphometric analysis was reduced in 98% in contrast to the manual method. Best results were obtained in the first two days of cellular development, with lesser standard deviations. Nonsignificant differences were found between the automatic and manual results in all culture days. Conclusions. The proposed method introduces a series of advantages related to its intuitive use and accuracy, making the technique suitable to images with different lighting characteristics and allowing a reliable comparison between different researchers.

Inhibition of β-catenin/p300 interaction proximalizes mouse embryonic lung epithelium

BACKGROUND

Wnt/β-catenin signaling has been suggested to regulate proximal-distal determination of embryonic lung epithelium based upon genetically modified mouse models. The previously identified and characterized small molecule inhibitor IQ1 can pharmacologically decrease the interaction between β-catenin and its transcriptional coactivator p300, thereby enhancing the β-catenin/CBP interaction. Inhibition of the β-catenin/p300 interaction by IQ1 blocks the differentiation of embryonic stem cells and epicardial progenitor cells; however, whether differential coactivator usage by β-catenin plays a role in proximal-distal determination of lung epithelium is unknown.

METHODS

We examined the effects of inhibiting the β-catenin/p300 interaction with IQ1 on lung branching morphogenesis in mouse embryos in utero and mouse embryonic lung organ culture ex vivo. The phenotype of IQ1 treated lungs was analyzed by epithelial staining, histology, quantitative PCR and in situ hybridization.

RESULTS

Inhibition of the β-catenin/p300 interaction by IQ1 disrupted the distal branching of mouse lung epithelium both in utero and ex vivo. IQ1 proximalized lung epithelium with decreased expression of the genes Bmp4 and Fgf10, hallmarks of distal lung determination, and increased expression of the proximal genes Sox2 and Scgb1a1 (CC10) as shown by quantitative PCR and in situ hybridization. The disruption of branching was reversible ex vivo as branching was reinitiated after removal of IQ1 from the media.

CONCLUSIONS

The results demonstrate that the β-catenin/p300 interaction plays a critical role in proximal-distal determination of the epithelium in mouse lung branching morphogenesis and β-catenin/p300 inhibition pharmacologically proximalizes lung epithelium.

Inhibition of β-catenin/p300 interaction proximalizes mouse embryonic lung epithelium

Background

Wnt/β-catenin signaling has been suggested to regulate proximal-distal determination of embryonic lung epithelium based upon genetically modified mouse models. The previously identified and characterized small molecule inhibitor IQ1 can pharmacologically decrease the interaction between β-catenin and its transcriptional coactivator p300, thereby enhancing the β-catenin/CBP interaction. Inhibition of the β-catenin/p300 interaction by IQ1 blocks the differentiation of embryonic stem cells and epicardial progenitor cells; however, whether differential coactivator usage by β-catenin plays a role in proximal-distal determination of lung epithelium is unknown.

Methods

We examined the effects of inhibiting the β-catenin/p300 interaction with IQ1 on lung branching morphogenesis in mouse embryos in utero and mouse embryonic lung organ culture ex vivo. The phenotype of IQ1 treated lungs was analyzed by epithelial staining, histology, quantitative PCR and in situ hybridization.

Results

Inhibition of the β-catenin/p300 interaction by IQ1 disrupted the distal branching of mouse lung epithelium both in utero and ex vivo. IQ1 proximalized lung epithelium with decreased expression of the genes Bmp4 and Fgf10, hallmarks of distal lung determination, and increased expression of the proximal genes Sox2 and Scgb1a1 (CC10) as shown by quantitative PCR and in situ hybridization. The disruption of branching was reversible ex vivo as branching was reinitiated after removal of IQ1 from the media.

Conclusions

The results demonstrate that the β-catenin/p300 interaction plays a critical role in proximal-distal determination of the epithelium in mouse lung branching morphogenesis and β-catenin/p300 inhibition pharmacologically proximalizes lung epithelium.

Cellular and physical mechanisms of branching morphogenesis

Branching morphogenesis is the developmental program that builds the ramified epithelial trees of various organs, including the airways of the lung, the collecting ducts of the kidney, and the ducts of the mammary and salivary glands. Even though the final geometries of epithelial trees are distinct, the molecular signaling pathways that control branching morphogenesis appear to be conserved across organs and species. However, despite this molecular homology, recent advances in cell lineage analysis and real-time imaging have uncovered surprising differences in the mechanisms that build these diverse tissues. Here, we review these studies and discuss the cellular and physical mechanisms that can contribute to branching morphogenesis.

R-spondin1 arguments hepatic fibrogenesis in vivo and in vitro

BACKGROUND

The development of liver fibrosis is the key stage toward a number of mortal complications of liver diseases, including cirrhosis and hepatocellular carcinoma. Canonical Wnt pathway is crucial in diverse biological processes and mediates the progression and regression of liver fibrosis. As a potent Wnt pathway agonist, roof plate-specific spondin-1 (R-spondin1) protein's role in the hepatic fibrosis has not been well elucidated. The purpose of this study was to investigate whether R-spondin1 contributed to hepatic stellate cells (HSC) activation, the key event in liver fibrogenesis.

MATERIALS AND METHODS

Tissue microarrays of human fibrotic liver samples, hepatocellular carcinoma samples, and normal hepatic tissue samples were constructed and immunostained for R-spondin1. Protein expression and transcriptional level of freshly isolated mice HSC were analyzed by Western blot assay and real-time polymerase chain reaction, respectively. Exogenous stimulation with recombinant R-spondin1 and Dickkopf-1 was performed to investigate the functionality. Nuclear β-catenin level and T-cell specific transcription factor activity were analyzed, and HSC proliferation was tested by Methyl-Thiazol-Tetrazolium bromide assay.

RESULTS

Overexpression of R-spondin1 was observed in both fibrotic liver tissues and culture-activated HSC. Coculture with recombinant R-spondin1 induced a dose-dependent increase in both the transcription factor activity and the protein level of α-smooth muscle actin, collagen I, and nuclear β-catenin. Additionally, Dickkopf-1 repressed R-spondin1's effect on HSC.

CONCLUSIONS

These findings suggested that R-spondin1 might argument liver fibrogenesis by enhancing the canonical Wnt pathway.

The hippo pathway effector Yap controls patterning and differentiation of airway epithelial progenitors

How epithelial progenitor cells integrate local signals to balance expansion with differentiation during organogenesis is still little understood. Here, we provide evidence that the Hippo pathway effector Yap is a key regulator of this process in the developing lung. We show that when epithelial tubules are forming and branching, a nucleocytoplasmic shift in Yap localization marks the boundary between the airway and the distal lung compartments. At this transition zone, Yap specifies a transcriptional program that controls Sox2 expression and ultimately generates the airway epithelium. Without Yap, epithelial progenitors are unable to properly respond to local TGF-β-induced cues and control levels and distribution of Sox2 to form airways. Yap levels and subcellular localization also markedly influence Sox2 expression and differentiation of adult airway progenitors. Our data reveal a role for the Hippo-Yap pathway in integrating growth-factor-induced cues in the developing and adult lung potentially key for homeostasis and regeneration repair.

β-catenin as a regulator and therapeutic target for asthmatic airway remodeling

INTRODUCTION

Pathological alteration in the airway structure, termed as airway remodeling, is a hallmark feature of individuals with asthma and has been described to negatively impact lung function in asthmatics. Recent studies have raised considerable interest in the regulatory role of β-catenin in remodeling asthmatic airways. The WNT/β-catenin signaling pathway is the key to normal lung development and tightly coordinates the maintenance of tissue homeostasis under steady-state conditions. Several studies indicate the crucial role of β-catenin signaling in airway remodeling in asthma and suggest that this pathway may be activated by both the growth factors and mechanical stimuli such as bronchoconstriction.

AREAS COVERED

In this review, we discuss recent literature regarding the mechanisms of β-catenin signaling activation and its mechanistic role in asthmatic airway remodeling. Further, we discuss the possibilities of therapeutic targeting of β-catenin.

EXPERT OPINION

The aberrant activation of β-catenin signaling by both WNT-dependent and -independent mechanisms in asthmatic airways plays a key role in remodeling the airways, including cell proliferation, differentiation, tissue repair and extracellular matrix production. These findings are interesting from both a mechanistic and therapeutic perspective, as several drug classes have now been described that target β-catenin signaling directly.

Engineering de novo assembly of fetal pulmonary organoids

Induction of morphogenesis by competent lung progenitor cells in a 3D environment is a central goal of pulmonary tissue engineering, yet little is known about the microenvironmental signals required to induce de novo assembly of alveolar-like tissue in vitro. In extending our previous reports of alveolar-like tissue formation by fetal pulmonary cells stimulated by exogenous fibroblast growth factors (FGFs), we identified some of the key endogenous mediators of FGF-driven morphogenesis (organoid assembly), for example, epithelial sacculation, endothelial network assembly, and epithelial-endothelial interfacing. Sequestration of endogenously secreted vascular endothelial growth factor-A (VEGF-A) potently inhibited endothelial network formation, with little or no effect on epithelial morphogenesis. Inhibition of endogenous sonic hedgehog (SHH) partially attenuated FGF-driven endothelial network formation, while the addition of exogenous SHH in the absence of FGFs was able to induce epithelial and endothelial morphogenesis, although with distinct morphological characteristics. Notably, SHH-induced endothelial networks exhibited fewer branch points, reduced sprouting behavior, and a periendothelial extracellular matrix (ECM) virtually devoid of tenascin-C (TN-C). By contrast, focal deposition of endogenous TN-C was observed in the ECM-surrounding endothelial networks of FGF-induced organoids, especially around sprouting tips. In the FGF-induced organoids, TN-C was also observed in the clefts of sacculated epithelium and at the epithelial-endothelial interface. In support of a critical role in the formation of alveolar-like tissue in vitro, TN-C blocking inhibited endothelial network formation and epithelial sacculation. Upon engraftment of in-vitro-generated pulmonary organoids beneath the renal capsule of syngeneic mice, robust neovascularization occurred in 5 days with a large contribution of patent vessels from engrafted organoids, providing proof of principle for exploring intrapulmonary engraftment of prevascularized hydrogel constructs. Expression of proSpC, VEGF-A, and TN-C following 1 week in vivo mirrored the patterns observed in vitro. Taken together, these findings advance our understanding of endogenous growth factor and ECM signals important for de novo formation of pulmonary tissue structures in vitro and demonstrate the potential of an organoid-based approach to lung tissue augmentation.

CTSB promotes porcine preadipocytes differentiation by degrading fibronectin and attenuating the Wnt/β-catenin signaling pathway

The process of preadipocytes differentiation plays a vital role in adipose tissue expansion and many factors are involved in this event. Cathepsin B (CTSB), secreted from lysosome, has been reported in regulating a variety of physiological processes. In this study, we demonstrated CTSB promotes lipid accumulation and adipogenic genes expression in porcine primary preadipocytes by degrading fibronectin (Fn), a key component of extracellular matrix. Lithium chloride (LiCl) is an activator of Wnt/β-catenin signaling through stabilizing β-catenin. We found that CTSB can relieve the anti-adipogenic effects of LiCl, indicating that CTSB could impact Wnt/β-catenin signaling pathway. Interestingly, Fn is an important target gene of Wnt/β-catenin. So we considered that CTSB promote preadipocytes differentiation by suppressing these two pathways.

BMP and activin membrane-bound inhibitor (BAMBI) inhibits the adipogenesis of porcine preadipocytes through Wnt/β-catenin signaling pathway

The process of differentiation from preadipocytes to adipocytes contributes to adipose tissue expansion in obesity. Blocking adipogenesis may be conducive to the etiology of obesity-related diseases. BMP and activin membrane-bound inhibitor (BAMBI) is a transmembrane protein, which was identified as a target of β-catenin in colorectal and hepatocellular tumor cells. However, whether BAMBI affects adipogenesis by Wnt/β-catenin signaling remains to be explored. In this study, we distinguish BAMBI as an inhibitor of preadipocytes differentiation. We found that BAMBI was downregulated during preadipocytes differentiation. Knockdown of BAMBI increased adipogenesis and blocked Wnt/β-catenin signaling by repressing β-catenin accumulation. In BAMBI overexpression cells, lipid accumulation was reduced by promoting nuclear translocation of β-catenin. Lithium chloride (LiCl) is an activator of Wnt/β-catenin signaling, which is an inhibitor of glycogen synthetase kinase-3 (GSK-3), maintaining the stability of β-catenin in cytosolic. We showed BAMBI strengthened the anti-adipogenic effects of LiCl. In addition, the results indicated that BAMBI was upregulated by β-catenin. These observations illuminated that BAMBI inhibits adipogenesis by a feedback loop (BAMBI→β-catenin nuclear translocation→BAMBI), which forms with Wnt/β-catenin signaling.

Hmga2 is required for canonical WNT signaling during lung development

Background

The high-mobility-group (HMG) proteins are the most abundant non-histone chromatin-associated proteins. HMG proteins are present at high levels in various undifferentiated tissues during embryonic development and their levels are strongly reduced in the corresponding adult tissues, where they have been implicated in maintaining and activating stem/progenitor cells. Here we deciphered the role of the high-mobility-group AT-hook protein 2 (HMGA2) during lung development by analyzing the lung of Hmga2-deficient mice (Hmga2^−/−).

Results

We found that Hmga2 is expressed in the mouse embryonic lung at the distal airways. Analysis of Hmga2^−/− mice showed that Hmga2 is required for proper cell proliferation and distal epithelium differentiation during embryonic lung development. Hmga2 knockout led to enhanced canonical WNT signaling due to an increased expression of secreted WNT glycoproteins Wnt2b, Wnt7b and Wnt11 as well as a reduction of the WNT signaling antagonizing proteins GATA-binding protein 6 and frizzled homolog 2. Analysis of siRNA-mediated loss-of-function experiments in embryonic lung explant culture confirmed the role of Hmga2 as a key regulator of distal lung epithelium differentiation and supported the causal involvement of enhanced canonical WNT signaling in mediating the effect of Hmga2-loss-of-fuction. Finally, we found that HMGA2 directly regulates Gata6 and thereby modulates Fzd2 expression.

Conclusions

Our results support that Hmga2 regulates canonical WNT signaling at different points of the pathway. Increased expression of the secreted WNT glycoproteins might explain a paracrine effect by which Hmga2-knockout enhanced cell proliferation in the mesenchyme of the developing lung. In addition, HMGA2-mediated direct regulation of Gata6 is crucial for fine-tuning the activity of WNT signaling in the airway epithelium. Our results are the starting point for future studies investigating the relevance of Hmga2-mediated regulation of WNT signaling in the adult lung within the context of proper balance between differentiation and self-renewal of lung stem/progenitor cells during lung regeneration in both homeostatic turnover and repair after injury.

Transciptome analysis of the gill and swimbladder of Takifugu rubripes by RNA-Seq

The fish gill, as one of the mucosal barriers, plays an important role in mucosal immune response. The fish swimbladder functions for regulating buoyancy. The fish swimbladder has long been postulated as a homologous organ of the tetrapod lung, but the molecular evidence is scarce. In order to provide new information that is complementary to gill immune genes, initiate new research directions concerning the genetic basis of the gill immune response and understand the molecular function of swimbladder as well as its relationship with lungs, transcriptome analysis of the fugu Takifugu rubripes gill and swimbladder was carried out by RNA-Seq. Approximately 55,061,524 and 44,736,850 raw sequence reads from gill and swimbladder were generated, respectively. Gene ontology (GO) and KEGG pathway analysis revealed diverse biological functions and processes. Transcriptome comparison between gill and swimbladder resulted in 3,790 differentially expressed genes, of which 1,520 were up-regulated in the swimbladder while 2,270 were down-regulated. In addition, 406 up regulated isoforms and 296 down regulated isoforms were observed in swimbladder in comparison to gill. By the gene enrichment analysis, the three immune-related pathways and 32 immune-related genes in gill were identified. In swimbladder, five pathways including 43 swimbladder-enriched genes were identified. This work should set the foundation for studying immune-related genes for the mucosal immunity and provide genomic resources to study the relatedness of the fish swimbladder and mammalian lung.

Fetal calcium regulates branching morphogenesis in the developing human and mouse lung: involvement of voltage-gated calcium channels

Airway branching morphogenesis in utero is essential for optimal postnatal lung function. In the fetus, branching morphogenesis occurs during the pseudoglandular stage (weeks 9–17 of human gestation, embryonic days (E)11.5–16.5 in mouse) in a hypercalcaemic environment (∼1.7 in the fetus vs. ∼1.1–1.3 mM for an adult). Previously we have shown that fetal hypercalcemia exerts an inhibitory brake on branching morphogenesis via the calcium-sensing receptor. In addition, earlier studies have shown that nifedipine, a selective blocker of L-type voltage-gated Ca²⁺ channels (VGCC), inhibits fetal lung growth, suggesting a role for VGCC in lung development. The aim of this work was to investigate the expression of VGCC in the pseudoglandular human and mouse lung, and their role in branching morphogenesis. Expression of L-type (Ca_V1.2 and Ca_V1.3), P/Q type (Ca_V2.1), N-type (Ca_V2.2), R-type (Ca_V2.3), and T-type (Ca_V3.2 and Ca_V3.3) VGCC was investigated in paraffin sections from week 9 human fetal lungs and E12.5 mouse embryos. Here we show, for the first time, that Ca_v1.2 and Ca_v1.3 are expressed in both the smooth muscle and epithelium of the developing human and mouse lung. Additionally, Ca_v2.3 was expressed in the lung epithelium of both species. Incubating E12.5 mouse lung rudiments in the presence of nifedipine doubled the amount of branching, an effect which was partly mimicked by the Ca_v2.3 inhibitor, SNX-482. Direct measurements of changes in epithelial cell membrane potential, using the voltage-sensitive fluorescent dye DiSBAC₂(3), demonstrated that cyclic depolarisations occur within the developing epithelium and coincide with rhythmic occlusions of the lumen, driven by the naturally occurring airway peristalsis. We conclude that VGCC are expressed and functional in the fetal human and mouse lung, where they play a role in branching morphogenesis. Furthermore, rhythmic epithelial depolarisations evoked by airway peristalsis would allow for branching to match growth and distension within the developing lung.

Cancer stem cell radioresistance and enrichment: where frontline radiation therapy may fail in lung and esophageal cancers

Many studies have highlighted the role cancer stem cells (CSC) play in the development and progression of various types of cancer including lung and esophageal cancer. More recently, it has been proposed that the presence of CSCs affects treatment efficacy and patient prognosis. In reviewing this new area of cancer biology, we will give an overview of the current literature regarding lung and esophageal CSCs and radioresistance of CSC, and discuss the potential therapeutic applications of these findings.

Pharmacological inhibition of GSK-3 in a guinea pig model of LPS-induced pulmonary inflammation: I. Effects on lung remodeling and pathology

Background

Glycogen synthase kinase-3 (GSK-3) is a constitutively active kinase that regulates multiple signalling proteins and transcription factors involved in a myriad of cellular processes. The kinase acts as a negative regulator in β-catenin signalling and is critically involved in the smad pathway. Activation of both pathways may contribute to pulmonary features of chronic obstructive pulmonary disease (COPD).

Methods

In the present study, we investigated the effect of the selective GSK-3 inhibitor SB216763 on pulmonary pathology in a guinea pig model of lipopolysaccharide (LPS)-induced COPD. Guinea pigs were instilled intranasally with LPS or saline twice weekly for 12 weeks and pre-treated with either intranasally instilled SB216763 or corresponding vehicle 30 min prior to each LPS/saline challenge.

Results

Repeated LPS exposures activated β-catenin signalling, primarily in the airway epithelium and submucosa. LPS also induced pulmonary inflammation and tissue remodelling as indicated by inflammatory cell influx, increased pulmonary fibronectin expression and enhanced small airway collagen content. Inhibition of GSK-3 by SB216763 did not affect LPS-induced inflammatory cell influx, but prevented the small airway remodelling and, unexpectedly, inhibited the activation of β-catenin in vivo. LPS or SB216763 treatment had no effect on the airway smooth muscle content and alveolar airspace size. However, GSK-3 inhibition prevented LPS-induced right ventricle hypertrophy.

Conclusions

Our findings indicate that GSK-3 inhibition prevents LPS-induced pulmonary pathology in guinea pigs, and that locally reduced LPS-induced β-catenin activation appears in part to underlie this effect.

Tenascin-C downregulates wnt inhibitor dickkopf-1, promoting tumorigenesis in a neuroendocrine tumor model

The extracellular matrix molecule tenascin-C (TNC) is a major component of the cancer-specific matrix, and high TNC expression is linked to poor prognosis in several cancers. To provide a comprehensive understanding of TNC's functions in cancer, we established an immune-competent transgenic mouse model of pancreatic β-cell carcinogenesis with varying levels of TNC expression and compared stochastic neuroendocrine tumor formation in abundance or absence of TNC. We show that TNC promotes tumor cell survival, the angiogenic switch, more and leaky vessels, carcinoma progression, and lung micrometastasis. TNC downregulates Dickkopf-1 (DKK1) promoter activity through the blocking of actin stress fiber formation, activates Wnt signaling, and induces Wnt target genes in tumor and endothelial cells. Our results implicate DKK1 downregulation as an important mechanism underlying TNC-enhanced tumor progression through the provision of a proangiogenic tumor microenvironment.

Localized Fgf10 expression is not required for lung branching morphogenesis but prevents differentiation of epithelial progenitors

Localized Fgf10 expression in the distal mesenchyme adjacent to sites of lung bud formation has long been thought to drive stereotypic branching morphogenesis even though isolated lung epithelium branches in the presence of non-directional exogenous Fgf10 in Matrigel. Here, we show that lung agenesis in Fgf10 knockout mice can be rescued by ubiquitous overexpression of Fgf10, indicating that precisely localized Fgf10 expression is not required for lung branching morphogenesis in vivo. Fgf10 expression in the mesenchyme itself is regulated by Wnt signaling. Nevertheless, we found that during lung initiation simultaneous overexpression of Fgf10 is not sufficient to rescue the absence of primary lung field specification in embryos overexpressing Dkk1, a secreted inhibitor of Wnt signaling. However, after lung initiation, simultaneous overexpression of Fgf10 in lungs overexpressing Dkk1 is able to rescue defects in branching and proximal-distal differentiation. We also show that Fgf10 prevents the differentiation of distal epithelial progenitors into Sox2-expressing airway epithelial cells in part by activating epithelial β-catenin signaling, which negatively regulates Sox2 expression. As such, these findings support a model in which the main function of Fgf10 during lung development is to regulate proximal-distal differentiation. As the lung buds grow out, proximal epithelial cells become further and further displaced from the distal source of Fgf10 and differentiate into bronchial epithelial cells. Interestingly, our data presented here show that once epithelial cells are committed to the Sox2-positive airway epithelial cell fate, Fgf10 prevents ciliated cell differentiation and promotes basal cell differentiation.

Lgr4-mediated Wnt/β-catenin signaling in peritubular myoid cells is essential for spermatogenesis

Peritubular myoid cells (PMCs) are myofibroblast-like cells that surround the seminiferous tubules and play essential roles in male fertility. How these cells modulate spermatogenesis and the signaling pathways that are involved are largely unknown. Here we report that Lgr4 is selectively expressed in mouse PMCs in the testes, and loss of Lgr4 leads to germ cells arresting at meiosis I and then undergoing apoptosis. In PMCs of Lgr4 mutant mice, the expression of androgen receptor, alpha-smooth muscle actin and extracellular matrix proteins was dramatically reduced. Malfunctioning PMCs further affected Sertoli cell nuclear localization and functional protein expression in Lgr4(-/-) mice. In addition, Wnt/β-catenin signaling was activated in wild-type PMCs but attenuated in those of Lgr4(-/-) mice. When Wnt/β-catenin signaling was reactivated by crossing with Apc(min/+) mice or by Gsk3β inhibitor treatment, the Lgr4 deficiency phenotype in testis was partially rescued. Together, these data demonstrate that Lgr4 signaling through Wnt/β-catenin regulates PMCs and is essential for spermatogenesis.

The extracellular calcium-sensing receptor, CaSR, in fetal development

In fetal mammals, serum levels of both total and ionized calcium significantly exceed those in the adult. This relative fetal hypercalcemia is crucial for skeletal development and is maintained irrespectively of maternal serum calcium levels. Elegant studies by Kovacs and Kronenberg have previously addressed the role of the CaSR in creating and maintaining this relative fetal hypercalcemia, through the regulation of parathyroid hormone-related peptide secretion. More recently we have shown that the CaSR is widely distributed throughout the developing fetus, where the receptor plays major, unexpected roles in ensuring growth and maturation of several organs. In this article, we present evidence for a role of the CaSR in the control of skeletal development, and how fetal hypercalcemia, acting through the CaSR, regulates lung development.

miR-375 regulates rat alveolar epithelial cell trans-differentiation by inhibiting Wnt/β-catenin pathway

Alveolar epithelial cell (AEC) trans-differentiation is a process where type II alveolar epithelial cells (AEC II) trans-differentiate into type I alveolar epithelial cells (AEC I) during lung recovery after various injuries, in which AEC I are damaged. This process is critical for lung tissue repair. MicroRNAs are a group of small RNAs that regulate gene expression at the post-transcriptional level. They have the potential to regulate almost every aspect of cell physiology. However, whether AEC trans-differentiation is regulated by microRNAs is completely unknown. In this study, we found that miR-375 was downregulated during AEC trans-differentiation. The overexpression of miR-375 with an adenoviral vector inhibited alveolar epithelial trans-differentiation as indicated by an increase in the AEC II marker, surfactant protein C, and decreases in the AEC I markers, T1α and advanced glycosylation end product-specific receptor. miR-375 also inhibited the Wnt/β-catenin pathway. The constitutively activation of Wnt/β-catenin signaling with a stabilized form of β-catenin blocked the miR-375 effects. Frizzled 8 was identified as a target of miR-375. In summary, our results demonstrate that miR-375 regulates AEC trans-differentiation through the Wnt/β-catenin pathway. This discovery may provide new targets for therapeutic intervention to benefit lung recovery from injuries.

Sustained Wnt/β-catenin signaling rescues high glucose induction of transforming growth factor-β1-mediated renal fibrosis

INTRODUCTION

Although diabetic nephropathy is attributable to transforming growth factor-β1 (TGF-β1) overproduction in glomer-ular mesangial cells, the biological role of Wnt/β-catenin signaling in controlling high glucose-induced TGF-β1 has not yet been elucidated.

METHODS

This study found that sustained Wnt/β-catenin signaling was required to protect glomerular mesangial cells from high glucose induction of TGF-β1-mediated fibrosis using in vitro and in vivo diabetic models.

RESULTS

High glucose down-regulated the Wnt signaling associated with increased TGF-β1 and fibronectin messenger RNA expression in glomerular mesangial cells. Restoring Wnt4, Wnt5a and cytosolic β-catenin levels by transfecting Wnt4, Wnt5a and stable β-catenin alleviated the stimulatory effect of high glucose on c-Jun mediated TGF-β1 fibrosis. Transfection of kinase-active glycogen synthase kinase-3β (GSK-3β) also abrogated high glucose promotion of nuclear c-Jun levels, TGF-β1 and fibronectin messenger RNA expression in mesangial cells. Pharmacological modulation of GSK-3ββ and ββ-catenin signaling by recombinant Wnt5a or GSK-3β inhibitor (BIO or LiCl) suppressed high glucose promotion of TGF-β1-mediated fibrosis. Exogenous BIO and SB216763 alleviated TGF-β1-mediated fibrogenic expression in the kidneys of diabetic rats. Immunohistochemistry showed that GSK-3β inhibitor significantly reversed the diabetic attenuation of TGF-β1 and c-Jun coinciding with fibronectin immunoreactivity within glomeruli. Immunofluorescence demonstrated that cells within the glomeruli restored β-catenin expression after BIO and SB216763 treatment in cells within diabetic glomeruli colocalized with fragmented nuclei by 4',6-diamidino-2-phenylindole staining.

CONCLUSIONS

Sustained Wnt signaling reduced c-Jun-dependent TGF-β1-mediated fibronectin accumulation in mesangial cells. These findings suggest that modulation of Wnt signaling is a viable alternative strategy to rescue the TGF-β1-mediated fibrotic signaling pathway in diabetic renal injury.

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.

Wnt ligands signal in a cooperative manner to promote foregut organogenesis

Endoderm-mesenchyme cross-talk is a central process in the development of foregut-derived organs. How signaling pathways integrate the activity of multiple ligands to guide organ development is poorly understood. We show that two Wnt ligands, Wnt2 and Wnt7b, cooperatively induce Wnt signaling without affecting the stabilization of the Wnt canonical effector β-catenin despite it being necessary for Wnt2-Wnt7b cooperativity. Wnt2-Wnt7b cooperation is specific for mesenchymal cell lineages and the combined loss of Wnt2 and Wnt7b leads to more severe developmental defects in the lung than loss of Wnt2 or Wnt7b alone. High-throughput small-molecule screens and biochemical assays reveal that the Pdgf pathway is required for cooperative Wnt2-Wnt7b signaling. Inhibition of Pdgf signaling in cell culture reduces Wnt2-Wnt7b cooperative signaling. Moreover, inhibition of Pdgf signaling in lung explant cultures results in decreased Wnt signaling and lung smooth-muscle development. These data suggest a model in which Pdgf signaling potentiates Wnt2-Wnt7b signaling to promote high levels of Wnt activity in mesenchymal progenitors that is required for proper development of endoderm-derived organs, such as the lung.

Comparative molecular developmental aspects of the mammalian- and the avian lungs, and the insectan tracheal system by branching morphogenesis: recent advances and future directions

Gas exchangers fundamentally form by branching morphogenesis (BM), a mechanistically profoundly complex process which derives from coherent expression and regulation of multiple genes that direct cell-to-cell interactions, differentiation, and movements by signaling of various molecular morphogenetic cues at specific times and particular places in the developing organ. Coordinated expression of growth-instructing factors determines sizes and sites where bifurcation occurs, by how much a part elongates before it divides, and the angle at which branching occurs. BM is essentially induced by dualities of factors where through feedback- or feed forward loops agonists/antagonists are activated or repressed. The intricate transactions between the development orchestrating molecular factors determine the ultimate phenotype. From the primeval time when the transformation of unicellular organisms to multicellular ones occurred by systematic accretion of cells, BM has been perpetually conserved. Canonical signalling, transcriptional pathways, and other instructive molecular factors are commonly employed within and across species, tissues, and stages of development. While much still remain to be elucidated and some of what has been reported corroborated and reconciled with rest of existing data, notable progress has in recent times been made in understanding the mechanism of BM. By identifying and characterizing the morphogenetic drivers, and markers and their regulatory dynamics, the elemental underpinnings of BM have been more precisely explained. Broadening these insights will allow more effective diagnostic and therapeutic interventions of developmental abnormalities and pathologies in pre- and postnatal lungs. Conservation of the molecular factors which are involved in the development of the lung (and other branched organs) is a classic example of nature's astuteness in economically utilizing finite resources. Once purposefully formed, well-tested and tried ways and means are adopted, preserved, and widely used to engineer the most optimal phenotypes. The material and time costs of developing utterly new instruments and routines with every drastic biological change (e.g. adaptation and speciation) are circumvented. This should assure the best possible structures and therefore functions, ensuring survival and evolutionary success.

Foxm1 mediates cross talk between Kras/mitogen-activated protein kinase and canonical Wnt pathways during development of respiratory epithelium

While Kras/mitogen-activated protein kinase (MAPK) and canonical Wnt/β-catenin are critical for lung morphogenesis, mechanisms integrating these important signaling pathways during lung development are unknown. Herein, we demonstrate that the Foxm1 transcription factor is a key downstream target of activated Kras(G12D). Deletion of Foxm1 from respiratory epithelial cells during lung formation prevented structural abnormalities caused by activated Kras(G12D). Kras/Foxm1 signaling inhibited the activity of canonical Wnt signaling in the developing lung in vivo. Foxm1 decreased T-cell factor (TCF) transcriptional activity induced by activated β-catenin in vitro. Depletion of Foxm1 by short interfering RNA (siRNA) increased nuclear localization of β-catenin, increased expression of β-catenin target genes, and decreased mRNA and protein levels of the β-catenin inhibitor Axin2. Axin2 mRNA was reduced in distal lung epithelium of Foxm1-deficient mice. Foxm1 directly bound to and increased transcriptional activity of the Axin2 promoter region. Foxm1 is required for Kras signaling in distal lung epithelium and provides a mechanism integrating Kras and canonical Wnt/β-catenin signaling during lung development.

Simulations demonstrate a simple network to be sufficient to control branch point selection, smooth muscle and vasculature formation during lung branching morphogenesis

Proper lung functioning requires not only a correct structure of the conducting airway tree, but also the simultaneous development of smooth muscles and vasculature. Lung branching morphogenesis is strongly stereotyped and involves the recursive use of only three modes of branching. We have previously shown that the experimentally described interactions between Fibroblast growth factor (FGF)10, Sonic hedgehog (SHH) and Patched (Ptc) can give rise to a Turing mechanism that not only reproduces the experimentally observed wildtype branching pattern but also, in part counterintuitive, patterns in mutant mice. Here we show that, even though many proteins affect smooth muscle formation and the expression of Vegfa, an inducer of blood vessel formation, it is sufficient to add FGF9 to the FGF10/SHH/Ptc module to successfully predict simultaneously the emergence of smooth muscles in the clefts between growing lung buds, and Vegfa expression in the distal sub-epithelial mesenchyme. Our model reproduces the phenotype of both wildtype and relevant mutant mice, as well as the results of most culture conditions described in the literature.

Signaling networks regulating development of the lower respiratory tract

The lungs serve the primary function of air-blood gas exchange in all mammals and in terrestrial vertebrates. Efficient gas exchange requires a large surface area that provides intimate contact between the atmosphere and the circulatory system. To achieve this, the lung contains a branched conducting system (the bronchial tree) and specialized air-blood gas exchange units (the alveoli). The conducting system brings air from the external environment to the alveoli and functions to protect the lung from debris that could obstruct airways, from entry of pathogens, and from excessive loss of fluids. The distal lung enables efficient exchange of gas between the alveoli and the conducting system and between the alveoli and the circulatory system. In this article, we highlight developmental and physiological mechanisms that specify, pattern, and regulate morphogenesis of this complex and essential organ. Recent advances have begun to define molecular mechanisms that control many of the important processes required for lung organogenesis; however, many questions remain. A deeper understanding of these molecular mechanisms will aid in the diagnosis and treatment of congenital lung disease and in the development of strategies to enhance the reparative response of the lung to injury and eventually permit regeneration of functional lung tissue.

Porous membrane substrates offer better niches to enhance the Wnt signaling and promote human embryonic stem cell growth and differentiation

Human embryonic stem cells (hESCs) require specific niches for adhesion, expansion, and lineage-specific differentiation. In this study, we showed that a membrane substrate offers better tissue niches for hESC attachment, spreading, proliferation, and differentiation. The cell doubling time was shortened from 46.3±5.7 h for hESCs grown on solid substrates to 25.6±2.6 h for those on polyester (PE) membrane substrates with pore size of 0.4 μm. In addition, we observed an increase of approximately five- to ninefold of definitive endoderm marker gene expression in hESCs differentiated on PE or polyethylene terephthalate membrane substrates. Global gene expression analysis revealed upregulated expressions of a number of extracellular matrix and cell adhesion molecules in hESCs grown on membrane substrates. Further, an enhanced nuclear translocation of β-catenin was detected in these cells. These observations suggested the augmentation of Wnt signaling in hESCs grown on membrane substrates. These results also demonstrated that a membrane substrate can offer better physicochemical cues for enhancing in vitro hESC attachment, proliferation, and differentiation.

Parabronchial smooth muscle constitutes an airway epithelial stem cell niche in the mouse lung after injury

During lung development, parabronchial SMC (PSMC) progenitors in the distal mesenchyme secrete fibroblast growth factor 10 (Fgf10), which acts on distal epithelial progenitors to promote their proliferation. β-catenin signaling within PSMC progenitors is essential for their maintenance, proliferation, and expression of Fgf10. Here, we report that this Wnt/Fgf10 embryonic signaling cascade is reactivated in mature PSMCs after naphthalene-induced injury to airway epithelium. Furthermore, we found that this paracrine Fgf10 action was essential for activating surviving variant Clara cells (the cells in the airway epithelium from which replacement epithelial cells originate) located at the bronchoalveolar duct junctions and adjacent to neuroendocrine bodies. After naphthalene injury, PSMCs secreted Fgf10 to activate Notch signaling and induce Snai1 expression in surviving variant Clara cells, which subsequently underwent a transient epithelial to mesenchymal transition to initiate the repair process. Epithelial Snai1 expression was important for regeneration after injury. We have therefore identified PSMCs as a stem cell niche for the variant Clara cells in the lung and established that paracrine Fgf10 signaling from the niche is critical for epithelial repair after naphthalene injury. These findings also have implications for understanding the misregulation of lung repair in asthma and cancer.

Perturbation of zebrafish swimbladder development by enhancing Wnt signaling in Wif1 morphants

Wnt signaling plays critical roles in development of both tetrapod lung and fish swimbladder, which are the two evolutionary homologous organs. Our previous data reveal that down-regulation of Wnt signaling leads to defective swimbladder development. However, the effects of up-regulation of Wnt signaling on swimbladder development remain unclear. By knockdown of the Wnt protein inhibitory gene wif1, we demonstrated that up-regulation of Wnt signaling also resulted in perturbed development of the swimbladder. Specifically, the growth of epithelium and mesenchyme was greatly inhibited, the smooth muscle differentiation was abolished, and the organization of mesothelium was disturbed. Furthermore, our data reveal that it is the reduced cell proliferation, but not enhanced apoptosis, that contributes to the disturbance of swimbladder development in wif1 morphants. Blocking Wnt signaling by the Wnt antagonist IWR-1 did not affect wif1 expression in the swimbladder, but complete suppression of Hedgehog signaling in smo-/- mutants abolished wif expression, consistent with our earlier report of a negative feedback regulation of Wnt signaling in the swimbladder by the Hedgehog signaling. Our works established the importance of proper level of Wnt signaling for normal development of swimbladder in zebrafish.

Activation of WNT/β-catenin signaling in pulmonary fibroblasts by TGF-β₁ is increased in chronic obstructive pulmonary disease

Background

Chronic obstructive pulmonary disease (COPD) is characterized by abnormal extracellular matrix (ECM) turnover. Recently, activation of the WNT/β-catenin pathway has been associated with abnormal ECM turnover in various chronic diseases. We determined WNT-pathway gene expression in pulmonary fibroblasts of individuals with and without COPD and disentangled the role of β-catenin in fibroblast phenotype and function.

Methods

We assessed the expression of WNT-pathway genes and the functional role of β-catenin, using MRC-5 human lung fibroblasts and primary pulmonary fibroblasts of individuals with and without COPD.

Results

Pulmonary fibroblasts expressed mRNA of genes required for WNT signaling. Stimulation of fibroblasts with TGF-β₁, a growth factor important in COPD pathogenesis, induced WNT-5B, FZD₈, DVL3 and β-catenin mRNA expression. The induction of WNT-5B, FZD₆, FZD₈ and DVL3 mRNA by TGF-β₁ was higher in fibroblasts of individuals with COPD than without COPD, whilst basal expression was similar. Accordingly, TGF-β₁ activated β-catenin signaling, as shown by an increase in transcriptionally active and total β-catenin protein expression. Furthermore, TGF-β₁ induced the expression of collagen1α1, α-sm-actin and fibronectin, which was attenuated by β-catenin specific siRNA and by pharmacological inhibition of β-catenin, whereas the TGF-β₁-induced expression of PAI-1 was not affected. The induction of transcriptionally active β-catenin and subsequent fibronectin deposition induced by TGF-β₁ were enhanced in pulmonary fibroblasts from individuals with COPD.

Conclusions

β-catenin signaling contributes to ECM production by pulmonary fibroblasts and contributes to myofibroblasts differentiation. WNT/β-catenin pathway expression and activation by TGF-β₁ is enhanced in pulmonary fibroblasts from individuals with COPD. This suggests an important role of the WNT/β-catenin pathway in regulating fibroblast phenotype and function in COPD.

β-Catenin signaling is required for TGF-β1-induced extracellular matrix production by airway smooth muscle cells

Chronic inflammatory airway diseases like asthma and chronic obstructive pulmonary disease (COPD) are characterized by airway remodeling with altered extracellular matrix (ECM) deposition. Transforming growth factor-β(1) (TGF-β(1)) is upregulated in asthma and COPD and contributes to tissue remodeling in the airways by driving ECM production by structural cells, including airway smooth muscle. In this study, we investigated the activation of β-catenin signaling and its contribution to ECM production by airway smooth muscle cells in response to TGF-β(1). Stimulation of airway smooth muscle cells with TGF-β(1) resulted in a time-dependent increase of total and nonphosphorylated β-catenin protein expression via induction of β-catenin mRNA and inhibition of GSK-3. In addition, the TGF-β(1)-induced β-catenin activated TCF/LEF-dependent gene transcription, as determined by the β-catenin sensitive TOP-flash luciferase reporter assay. Furthermore, TGF-β(1) stimulation increased mRNA expression of collagen Iα1, fibronectin, versican, and PAI-1. Pharmacological inhibition of β-catenin by PKF115-584 or downregulation of β-catenin expression by specific small interfering RNA (siRNA) substantially inhibited TGF-β(1)-induced expression of the ECM genes. Fibronectin protein deposition by airway smooth muscle cells in response to TGF-β(1) was also inhibited by PKF115-584 and β-catenin siRNA. Moreover, transfection of airway smooth muscle cells with a nondegradable β-catenin mutant (S33Y β-catenin) was sufficient for inducing fibronectin protein expression. Collectively, these findings indicate that β-catenin signaling is activated in response to TGF-β(1) in airway smooth muscle cells, which is required and sufficient for the regulation of ECM protein production. Targeting β-catenin-dependent gene transcription may therefore hold promise as a therapeutic intervention in airway remodeling in both asthma and COPD.