Differential gene expression in the distal tip endoderm of the embryonic mouse lung

Bud, branch, breathe! Building a mammalian lung over space and time

Many mammalian organs, such as the mammary and lachrymal glands, kidney and lungs develop by the process known as branching morphogenesis. An essential feature of this process is the reciprocal interaction between the inner branched tubular epithelium and the surrounding mesenchyme to optimize the final amount of epithelial tissue that is generated for specific functions. To achieve this expansion the initial epithelial population undergoes repeated rounds of bud formation, branch outgrowth and tip bifurcations, with each repertoire requiring dynamic changes in cell behavior. The process of branching morphogenesis was first studied experimentally by Grobstein and others who showed that the embryonic epithelium did not develop without so-called inductive signals from the mesenchyme. However, it was not known whether this activity was uniformly distributed throughout the mesoderm or localized to specific regions. The mouse lung was seen as a powerful system in which to investigate such questions since its early branching is highly stereotypic, both in vivo and in culture. This advantage was exploited by two young scientists, Alescio and Cassini, who used grafting techniques with explanted embryonic mouse lungs. They showed that mesenchyme from around distal buds could induce ectopic buds in the trachea and other non-branching regions of the epithelium. At the same time, distal regions denuded of their mesoderm failed to develop further. They speculated that inductive factors that promote bud formation and continued outgrowth in competent endoderm are specifically localized within the distal mesenchyme, establishing a conceptual framework for future experimentation. Since then, advances in many areas of biology and bioengineering have enabled the identification of gene regulatory networks, signaling pathways and biomechanical properties that mediate lung branching morphogenesis. However, a quantitative model of how these parameters are coordinated over space and time to control the pattern and scale of branching and the overall size of the lung, still remains elusive.

Discovering genes and microRNAs involved in human lung development unveils IGFBP3/miR-34a dynamics and their relevance for alveolar differentiation

BACKGROUND

During pseudoglandular stage of the human lung development the primitive bronchial buds are initially conformed by simple tubules lined by endoderm-derived epithelium surrounded by mesenchyme, which will progressively branch into airways and start to form distal epithelial saculles. For first time alveolar type II (AT2) pneumocytes appears. This study aims to characterize the genes and microRNAs involved in this differentiation process and decipher its role in the starting alveolar differentiation.

METHODS

Gene and microRNA profiling was performed in human embryonic lungs from 7 to 12 post conception weeks (pcw). Protein expression location of candidate genes were analyzed by immunofluorescense in embryonic lung tissue sections. mRNA/miRNA target pairs were identified using computational approaches and their expression was studied in purified epithelial/mesenchymal cell populations and in isolated tips and stalks from the bronchial tree. Additionally, silencing experiments in human embryonic lung mesenchymal cells and in human embryonic tip-derived lung organoids were performed, as well as organoid differentiation studies. AT2 cell markers were studied by qRT-PCR and by immunofluorescence. The TGFB-β phosphorylated pathways was analyzed with membrane protein arrays. Lung explants were cultured in air/liquid interface with/without peptides.

RESULTS

We identified 88 differentially expressed genes, including IGFBP3. Although IGFBP3 mRNA was detected in both epithelial and mesenchymal populations, the protein was restricted to the epithelium, indicating post-transcriptional regulation preventing IGFBP3 protein expression in the mesenchyme. MicroRNA profiling identified miR-34a as an IGFBP3 regulator. miR-34a was up-regulated in mesenchymal cells, and its silencing in human embryonic lung mesenchymal cells increased IGFBP3 levels. Additionally, IGFBP3 expression showed a marked downregulation from 7 to 12 pcw, suggesting its involvement in the differentiation process. The differentiation of human tip-derived lung embryonic organoids showed a drastic reduction in IGFBP3, supported by the scRNAseq data. IGFBP3 silencing in organoids activated an alveolar-like differentiation process characterized by stem cell markers downregulation and upregulation of AT2 markers. This process was mediated by TGFβ signalling inhibition and BMP pathway activation.

CONCLUSIONS

The IGFBP3/miR-34a axis restricts IGFBP3 expression in the embryonic undifferentiated lung epithelium, and the progressive downregulation of IGFBP3 during the pseudoglandular stage is required for alveolar differentiation.

The molecular and cellular choreography of early mammalian lung development

Mammalian lung development starts from a specific cluster of endodermal cells situated within the ventral foregut region. With the orchestrating of delicate choreography of transcription factors, signaling pathways, and cell-cell communications, the endodermal diverticulum extends into the surrounding mesenchyme, and builds the cellular and structural basis of the complex respiratory system. This review provides a comprehensive overview of the current molecular insights of mammalian lung development, with a particular focus on the early stage of lung cell fate differentiation and spatial patterning. Furthermore, we explore the implications of several congenital respiratory diseases and the relevance to early organogenesis. Finally, we summarize the unprecedented knowledge concerning lung cell compositions, regulatory networks as well as the promising prospect for gaining an unbiased understanding of lung development and lung malformations through state-of-the-art single-cell omics.

Small cells - big issues: biological implications and preclinical advancements in small cell lung cancer

Current treatment guidelines refer to small cell lung cancer (SCLC), one of the deadliest human malignancies, as a homogeneous disease. Accordingly, SCLC therapy comprises chemoradiation with or without immunotherapy. Meanwhile, recent studies have made significant advances in subclassifying SCLC based on the elevated expression of the transcription factors ASCL1, NEUROD1, and POU2F3, as well as on certain inflammatory characteristics. The role of the transcription regulator YAP1 in defining a unique SCLC subset remains to be established. Although preclinical analyses have described numerous subtype-specific characteristics and vulnerabilities, the so far non-existing clinical subtype distinction may be a contributor to negative clinical trial outcomes. This comprehensive review aims to provide a framework for the development of novel personalized therapeutic approaches by compiling the most recent discoveries achieved by preclinical SCLC research. We highlight the challenges faced due to limited access to patient material as well as the advances accomplished by implementing state-of-the-art models and methodologies.

Generation of rat-derived lung epithelial cells in Fgfr2b-deficient mice retains species-specific development

Regenerative medicine is a tool to compensate for the shortage of lungs for transplantation, but it remains difficult to construct a lung in vitro due to the complex three-dimensional structures and multiple cell types required. A blastocyst complementation method using interspecies chimeric animals has been attracting attention as a way to create complex organs in animals, although successful lung formation using interspecies chimeric animals has not yet been achieved. Here, we applied a reverse-blastocyst complementation method to clarify the conditions required to form lungs in an Fgfr2b-deficient mouse model. We then successfully formed a rat-derived lung in the mouse model by applying a tetraploid-based organ-complementation method. Importantly, rat lung epithelial cells retained their developmental timing even in the mouse body. These findings provide useful insights to overcome the barrier of species-specific developmental timing to generate functional lungs in interspecies chimeras.

Transcriptional responses to injury of regenerative lung alveolar epithelium

The significance of alveolar epithelial type 2 (AT2) cell proliferation for lung alveolar epithelial homeostasis and regeneration after injury has been widely accepted. However, the heterogeneity of AT2 cell population for cell proliferation capacity remains disputed. By single-cell RNA sequencing and genetic lineage labeling using the Ki67 knock-in mouse model, we map all proliferative AT2 cells in homeostatic and regenerating murine lungs after injury induced by Streptococcus pneumoniae infection. The proliferative AT2 cell population displays a unique transcriptional program, which is regulated by activating transcription factor 3 (ATF3) and thyroid hormone receptor alpha (THRA) transcription factors. Overexpression of these two transcription factors in AT2 cells promoted AT2 cell proliferation and improved lung function after injury. These results indicate that increased expression of ATF3 and THRA at the onset of lung epithelial regeneration is required to permit rapid AT2 cell proliferation and hence progression through the recovery of lung epithelium.

Distinct Epithelial Cell Profiles in Normal Versus Induced-Congenital Diaphragmatic Hernia Fetal Lungs

BACKGROUND

Recent studies identified a great diversity of cell types in precise number and position to create the architectural features of the lung that ventilation and respiration at birth depend on. With damaged respiratory function at birth, congenital diaphragmatic hernia (CDH) is one of the more severe causes of fetal lung hypoplasia with unspecified cellular dynamics.

OBJECTIVES

 To characterize the epithelial cell tissue in hypoplastic lungs, a careful analysis regarding pulmonary morphology and epithelial cell profile was conducted from pseudoglandular-to-saccular phases in normal versus nitrofen-induced CDH rat lungs.

DESIGN

Our analysis comprises three experimental groups, control, nitrofen (NF) and CDH, in which the relative expression levels (western blot) by group and developmental stage were analyzed in whole lung. Spatiotemporal distribution (immunohistochemistry) was revealed by pulmonary structure during normal and hypoplastic fetal lung development. Surfactant protein-C (SP-C), calcitonin gene-related peptide (CGRP), clara cell secretory protein (CCSP), and forkhead box J1 (FOXJ1) were the used molecular markers for alveolar epithelial cell type 2 (AEC2), pulmonary neuroendocrine, clara, and ciliated cell profiles, respectively.

RESULTS

Generally, we identified an aberrant expression of SP-C, CGRP, CCSP, and FOXJ1 in nitrofen-exposed lungs. For instance, the overexpression of FOXJ1 and CGRP in primordia of bronchiole defined the pseudoglandular stage in CDH lungs, whereas the increased expression of CGRP in bronchi; FOXJ1 and CGRP in terminal bronchiole; and SP-C in BADJ classified the canalicular and saccular stages in hypoplastic lungs. We also described higher expression levels in NF than CDH or control groups for both FOXJ1 in bronchi, terminal bronchiole and BADJ at canalicular stage, and SP-C in bronchi and terminal bronchiole at canalicular and saccular stages. Finally, we report an unexpected expression of FOXJ1 in BADJ at canalicular and saccular stages, whereas the multi cilia observed in bronchi were notably absent at embryonic day 21.5 in induced-CDH lungs.

CONCLUSION

The recognized alterations in the epithelial cell profile contribute to a better understanding of neonatal respiratory insufficiency in induced-CDH lungs and indicate a problem in the epithelial cell differentiation in hypoplastic lungs.

Derivation of self-renewing lung alveolar epithelial type II cells from human pluripotent stem cells

Alveolar epithelial type II cells (AEC2s) are the facultative progenitors of lung alveoli and serve as the surfactant-producing cells of air-breathing organisms. Although primary human AEC2s are difficult to maintain stably in cell cultures, recent advances have facilitated the derivation of AEC2-like cells from human pluripotent stem cells (hPSCs) in vitro. Here, we provide a detailed protocol for the directed differentiation of hPSCs into self-renewing AEC2-like cells that can be maintained for up to 1 year in culture as epithelial-only spheres without the need for supporting mesenchymal feeder cells. The month-long protocol requires recapitulation of the sequence of milestones associated with in vivo development of the distal lung, beginning with differentiation of cells into anterior foregut endoderm, which is followed by their lineage specification into NKX2-1+ lung progenitors and then distal/alveolar differentiation to produce progeny that express transcripts and possess functional properties associated with AEC2s.

Histone arginine methylation by Prmt5 is required for lung branching morphogenesis through repression of BMP signaling

Branching morphogenesis is essential for the successful development of a functional lung to accomplish its gas exchange function. Although many studies have highlighted requirements for the bone morphogenetic protein (BMP) signaling pathway during branching morphogenesis, little is known about how BMP signaling is regulated. Here, we report that the protein arginine methyltransferase 5 (Prmt5) and symmetric dimethylation at histone H4 arginine 3 (H4R3sme2) directly associate with chromatin of Bmp4 to suppress its transcription. Inactivation of Prmt5 in the lung epithelium results in halted branching morphogenesis, altered epithelial cell differentiation and neonatal lethality. These defects are accompanied by increased apoptosis and reduced proliferation of lung epithelium, as a consequence of elevated canonical BMP-Smad1/5/9 signaling. Inhibition of BMP signaling by Noggin rescues the lung branching defects of Prmt5 mutant in vitro Taken together, our results identify a novel mechanism through which Prmt5-mediated histone arginine methylation represses canonical BMP signaling to regulate lung branching morphogenesis.

Retinoic acid regulates avian lung branching through a molecular network

Retinoic acid (RA) is of major importance during vertebrate embryonic development and its levels need to be strictly regulated otherwise congenital malformations will develop. Through the action of specific nuclear receptors, named RAR/RXR, RA regulates the expression of genes that eventually influence proliferation and tissue patterning. RA has been described as crucial for different stages of mammalian lung morphogenesis, and as part of a complex molecular network that contributes to precise organogenesis; nonetheless, nothing is known about its role in avian lung development. The current report characterizes, for the first time, the expression pattern of RA signaling members (stra6, raldh2, raldh3, cyp26a1, rarα, and rarβ) and potential RA downstream targets (sox2, sox9, meis1, meis2, tgfβ2, and id2) by in situ hybridization. In the attempt of unveiling the role of RA in chick lung branching, in vitro lung explants were performed. Supplementation studies revealed that RA stimulates lung branching in a dose-dependent manner. Moreover, the expression levels of cyp26a1, sox2, sox9, rarβ, meis2, hoxb5, tgfβ2, id2, fgf10, fgfr2, and shh were evaluated after RA treatment to disclose a putative molecular network underlying RA effect. In situ hybridization analysis showed that RA is able to alter cyp26a1, sox9, tgfβ2, and id2 spatial distribution; to increase rarβ, meis2, and hoxb5 expression levels; and has a very modest effect on sox2, fgf10, fgfr2, and shh expression levels. Overall, these findings support a role for RA in the proximal-distal patterning and branching morphogenesis of the avian lung and reveal intricate molecular interactions that ultimately orchestrate branching morphogenesis.

Differentiation of Human Pluripotent Stem Cells into Functional Lung Alveolar Epithelial Cells

Lung alveoli, which are unique to air-breathing organisms, have been challenging to generate from pluripotent stem cells (PSCs) in part because there are limited model systems available to provide the necessary developmental roadmaps for in vitro differentiation. Here we report the generation of alveolar epithelial type 2 cells (AEC2s), the facultative progenitors of lung alveoli, from human PSCs. Using multicolored fluorescent reporter lines, we track and purify human SFTPC+ alveolar progenitors as they emerge from endodermal precursors in response to stimulation of Wnt and FGF signaling. Purified PSC-derived SFTPC+ cells form monolayered epithelial "alveolospheres" in 3D cultures without the need for mesenchymal support, exhibit self-renewal capacity, and display additional AEC2 functional capacities. Footprint-free CRISPR-based gene correction of PSCs derived from patients carrying a homozygous surfactant mutation (SFTPB121ins2) restores surfactant processing in AEC2s. Thus, PSC-derived AEC2s provide a platform for disease modeling and future functional regeneration of the distal lung.

Smoking-Dependent Distal-to-Proximal Repatterning of the Adult Human Small Airway Epithelium

RATIONALE

Small airways are the primary site of pathologic changes in chronic obstructive pulmonary disease (COPD), the major smoking-induced lung disorder.

OBJECTIVES

On the basis of the concept of proximal-distal patterning that determines regional specialization of the airway epithelium during lung development, we hypothesized that a similar program operates in the adult human lung being altered by smoking, leading to decreased regional identity of the small airway epithelium (SAE).

METHODS

The proximal and distal airway signatures were identified by comparing the transcriptomes of large and small airway epithelium samples obtained by bronchoscopy from healthy nonsmokers. The expression of these signatures was evaluated in the SAE of healthy smokers and smokers with COPD compared with that of healthy nonsmokers. The capacity of airway basal stem cells (BCs) to maintain region-associated phenotypes was evaluated using the air-liquid interface model.

MEASUREMENTS AND MAIN RESULTS

The distal and proximal airway signatures, containing 134 and 233 genes, respectively, were identified. These signatures included known developmental regulators of airway patterning, as well as novel regulators such as epidermal growth factor receptor, which was associated with the proximal airway phenotype. In the SAE of smokers with COPD, there was a dramatic smoking-dependent loss of the regional transcriptome identity with concomitant proximalization. This repatterning phenotype was reproduced by stimulating SAE BCs with epidermal growth factor, which was up-regulated in the SAE of smokers, during differentiation of SAE BCs in vitro.

CONCLUSIONS

Smoking-induced global distal-to-proximal reprogramming of the SAE represents a novel pathologic feature of COPD and is mediated by exaggerated epidermal growth factor/epidermal growth factor receptor signaling in SAE BCs.

A three-dimensional model of human lung development and disease from pluripotent stem cells

Recapitulation of lung development from human pluripotent stem cells (hPSCs) in three dimensions (3D) would allow deeper insight into human development, as well as the development of innovative strategies for disease modelling, drug discovery and regenerative medicine. We report here the generation from hPSCs of lung bud organoids (LBOs) that contain mesoderm and pulmonary endoderm and develop into branching airway and early alveolar structures after xenotransplantation and in Matrigel 3D culture. Expression analysis and structural features indicated that the branching structures reached the second trimester of human gestation. Infection in vitro with respiratory syncytial virus, which causes small airway obstruction and bronchiolitis in infants, led to swelling, detachment and shedding of infected cells into the organoid lumens, similar to what has been observed in human lungs. Introduction of mutation in HPS1, which causes an early-onset form of intractable pulmonary fibrosis, led to accumulation of extracellular matrix and mesenchymal cells, suggesting the potential use of this model to recapitulate fibrotic lung disease in vitro. LBOs therefore recapitulate lung development and may provide a useful tool to model lung disease.

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

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

Tbx2 and Tbx3 Act Downstream of Shh to Maintain Canonical Wnt Signaling during Branching Morphogenesis of the Murine Lung

Numerous signals drive the proliferative expansion of the distal endoderm and the underlying mesenchyme during lung branching morphogenesis, but little is known about how these signals are integrated. Here, we show by analysis of conditional double mutants that the two T-box transcription factor genes Tbx2 and Tbx3 act together in the lung mesenchyme to maintain branching morphogenesis. Expression of both genes depends on epithelially derived Shh signaling, with additional modulation by Bmp, Wnt, and Tgfβ signaling. Genetic rescue experiments reveal that Tbx2 and Tbx3 function downstream of Shh to maintain pro-proliferative mesenchymal Wnt signaling, in part by direct repression of the Wnt antagonists Frzb and Shisa3. In combination with our previous finding that Tbx2 and Tbx3 repress the cell-cycle inhibitors Cdkn1a and Cdkn1b, we conclude that Tbx2 and Tbx3 maintain proliferation of the lung mesenchyme by way of at least two molecular mechanisms: regulating cell-cycle regulation and integrating the activity of multiple signaling pathways.

Incorporation of non-canonical amino acids into the developing murine proteome

Analysis of the developing proteome has been complicated by a lack of tools that can be easily employed to label and identify newly synthesized proteins within complex biological mixtures. Here, we demonstrate that the methionine analogs azidohomoalanine and homopropargylglycine can be globally incorporated into the proteome of mice through facile intraperitoneal injections. These analogs contain bio-orthogonal chemical handles to which fluorescent tags can be conjugated to identify newly synthesized proteins. We show these non-canonical amino acids are incorporated into various tissues in juvenile mice and in a concentration dependent manner. Furthermore, administration of these methionine analogs to pregnant dams during a critical stage of murine development, E10.5-12.5 when many tissues are assembling, does not overtly disrupt development as assessed by proteomic analysis and normal parturition and growth of pups. This successful demonstration that non-canonical amino acids can be directly administered in vivo will enable future studies that seek to characterize the murine proteome during growth, disease and repair.

Overexpression of inhibitor of DNA-binding 2 attenuates pulmonary fibrosis through regulation of c-Abl and Twist

Fibrosis is a multicellular process leading to excessive extracellular matrix deposition. Factors that affect lung epithelial cell proliferation and activation may be important regulators of the extent of fibrosis after injury. We and others have shown that activated alveolar epithelial cells (AECs) directly contribute to fibrogenesis by secreting mesenchymal proteins, such as type I collagen. Recent evidence suggests that epithelial cell acquisition of mesenchymal features during carcinogenesis and fibrogenesis is regulated by several mesenchymal transcription factors. Induced expression of direct inhibitors to these mesenchymal transcription factors offers a potentially novel therapeutic strategy. Inhibitor of DNA-binding 2 (Id2) is an inhibitory helix-loop-helix transcription factor that is highly expressed by lung epithelial cells during development and has been shown to coordinate cell proliferation and differentiation of cancer cells. We found that overexpression of Id2 in primary AECs promotes proliferation by inhibiting a retinoblastoma protein/c-Abl interaction leading to greater c-Abl activity. Id2 also blocks transforming growth factor β1-mediated expression of type I collagen by inhibiting Twist, a prominent mesenchymal basic helix-loop-helix transcription factor. In vivo, Id2 induced AEC proliferation and protected mice from lung fibrosis. By using a high-throughput screen, we found that histone deacetylase inhibitors induce Id2 expression by adult AECs. Collectively, these findings suggest that Id2 expression by AECs can be induced, and overexpression of Id2 affects AEC phenotype, leading to protection from fibrosis.

Developmental programs of lung epithelial progenitors: a balanced progenitor model

The daunting task of lung epithelium development is to transform a cluster of foregut progenitors into a three-dimensional (3D) tubular network with distinct cell types distributed at their appropriate locations. A complete understanding of lung development needs to address not only how, but also where, different cell types form. We propose that the lung epithelium forms through regulated deployment of three developmental programs: branching morphogenesis to expand progenitors and build a tree-like tubular network, airway differentiation to specify cells for the proximal conducting airways, and alveolar differentiation to specify cells for the peripheral gas exchange region. Each developmental program has its unique morphological features and molecular control mechanisms; their spatiotemporal coordination can be accounted for in a balanced progenitor model where progenitors balance between alternative developmental programs in response to spatiotemporal cues. This model integrates progenitor morphogenesis and differentiation, and provides new insights to lung immaturity in preterm birth and lung evolution. Advanced gene targeting and 3D imaging tools are needed to achieve a comprehensive understanding of lung epithelial progenitors on molecular, cellular, and morphological levels. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Sox9: a master regulator of the pancreatic program

Over the last decade, it has been discovered that the transcription factor Sox9 plays several critical roles in governing the development of the embryonic pancreas and the homeostasis of the mature organ. While analysis of pancreata from patients affected by the Sox9 haploinsufficiency syndrome campomelic dysplasia initially alluded to a functional role of Sox9 in pancreatic morphogenesis, transgenic mouse models have been instrumental in mechanistically dissecting such roles. Although initially defined as a marker and maintenance factor for pancreatic progenitors, Sox9 is now considered to fulfill additional indispensable functions during pancreogenesis and in the postnatal organ through its interactions with other transcription factors and signaling pathways such as Fgf and Notch. In addition to maintaining both multipotent and bipotent pancreatic progenitors, Sox9 is also required for initiating endocrine differentiation and maintaining pancreatic ductal identity, and it has recently been unveiled as a key player in the initiation of pancreatic cancer. These functions of Sox9 are discussed in this article, with special emphasis on the knowledge gained from various loss-of-function and lineage tracing mouse models. Also, current controversies regarding Sox9 function in healthy and injured adult pancreas and unanswered questions and avenues of future study are discussed.

Lung epithelial branching program antagonizes alveolar differentiation

Mammalian organs, including the lung and kidney, often adopt a branched structure to achieve high efficiency and capacity of their physiological functions. Formation of a functional lung requires two developmental processes: branching morphogenesis, which builds a tree-like tubular network, and alveolar differentiation, which generates specialized epithelial cells for gas exchange. Much progress has been made to understand each of the two processes individually; however, it is not clear whether the two processes are coordinated and how they are deployed at the correct time and location. Here we show that an epithelial branching morphogenesis program antagonizes alveolar differentiation in the mouse lung. We find a negative correlation between branching morphogenesis and alveolar differentiation temporally, spatially, and evolutionarily. Gain-of-function experiments show that hyperactive small GTPase Kras expands the branching program and also suppresses molecular and cellular differentiation of alveolar cells. Loss-of-function experiments show that SRY-box containing gene 9 (Sox9) functions downstream of Fibroblast growth factor (Fgf)/Kras to promote branching and also suppresses premature initiation of alveolar differentiation. We thus propose that lung epithelial progenitors continuously balance between branching morphogenesis and alveolar differentiation, and such a balance is mediated by dual-function regulators, including Kras and Sox9. The resulting temporal delay of differentiation by the branching program may provide new insights to lung immaturity in preterm neonates and the increase in organ complexity during evolution.

Systems biology approaches to identify developmental bases for lung diseases

A greater understanding of the regulatory processes contributing to lung development could be helpful to identify strategies to ameliorate morbidity and mortality in premature infants and to identify individuals at risk for congenital and/or chronic lung diseases. Over the past decade, genomics technologies have enabled the production of rich gene expression databases providing information for all genes across developmental time or in diseased tissue. These data sets facilitate systems biology approaches for identifying underlying biological modules and programs contributing to the complex processes of normal development and those that may be associated with disease states. The next decade will undoubtedly see rapid and significant advances in redefining both lung development and disease at the systems level.

Tbx2 controls lung growth by direct repression of the cell cycle inhibitor genes Cdkn1a and Cdkn1b

Vertebrate organ development relies on the precise spatiotemporal orchestration of proliferation rates and differentiation patterns in adjacent tissue compartments. The underlying integration of patterning and cell cycle control during organogenesis is insufficiently understood. Here, we have investigated the function of the patterning T-box transcription factor gene Tbx2 in lung development. We show that lungs of Tbx2-deficient mice are markedly hypoplastic and exhibit reduced branching morphogenesis. Mesenchymal proliferation was severely decreased, while mesenchymal differentiation into fibrocytes was prematurely induced. In the epithelial compartment, proliferation was reduced and differentiation of alveolar epithelial cells type 1 was compromised. Prior to the observed cellular changes, canonical Wnt signaling was downregulated, and Cdkn1a (p21) and Cdkn1b (p27) (two members of the Cip/Kip family of cell cycle inhibitors) were strongly induced in the Tbx2-deficient lung mesenchyme. Deletion of both Cdkn1a and Cdkn1b rescued, to a large degree, the growth deficits of Tbx2-deficient lungs. Prolongation of Tbx2 expression into adulthood led to hyperproliferation and maintenance of mesenchymal progenitor cells, with branching morphogenesis remaining unaffected. Expression of Cdkn1a and Cdkn1b was ablated from the lung mesenchyme in this gain-of-function setting. We further show by ChIP experiments that Tbx2 directly binds to Cdkn1a and Cdkn1b loci in vivo, defining these two genes as direct targets of Tbx2 repressive activity in the lung mesenchyme. We conclude that Tbx2-mediated regulation of Cdkn1a and Cdkn1b represents a crucial node in the network integrating patterning information and cell cycle regulation that underlies growth, differentiation, and branching morphogenesis of this organ.

During organ formation, proliferation rates and differentiation patterns vary widely between different stages and tissue compartments. It is poorly understood how cell cycle progression is locally controlled and integrated with patterning processes in these developmental programs. Here, we used the mouse lung as a model to study how growth and differentiation are controlled on a transcriptional level. Combining genetic loss- and gain-of-function approaches, we show that the T-box transcription factor gene Tbx2 is required and sufficient to direct appropriate lung growth by maintaining proliferation and inhibiting differentiation in the mesenchymal compartment of the lung. We found that expression of the cell cycle inhibitor genes Cdkn1a (p21) and Cdkn1b (p27) inversely correlates with expression of Tbx2 and that deletion of both genes rescues, to a large degree, the growth deficits of Tbx2-mutant lungs. We further show by biochemical assays that Tbx2 directly binds to Cdkn1a and Cdkn1b loci in vivo, defining these two genes as direct targets of Tbx2 repressive activity in the lung mesenchyme. We conclude that Tbx2-mediated regulation of Cdkn1a and Cdkn1b represents a crucial module for the tissue-specific control of cell cycle progression that underlies growth, differentiation, and branching morphogenesis of this organ.

Genome-wide joint meta-analysis of SNP and SNP-by-smoking interaction identifies novel loci for pulmonary function

Genome-wide association studies have identified numerous genetic loci for spirometic measures of pulmonary function, forced expiratory volume in one second (FEV(1)), and its ratio to forced vital capacity (FEV(1)/FVC). Given that cigarette smoking adversely affects pulmonary function, we conducted genome-wide joint meta-analyses (JMA) of single nucleotide polymorphism (SNP) and SNP-by-smoking (ever-smoking or pack-years) associations on FEV(1) and FEV(1)/FVC across 19 studies (total N = 50,047). We identified three novel loci not previously associated with pulmonary function. SNPs in or near DNER (smallest P(JMA = )5.00×10(-11)), HLA-DQB1 and HLA-DQA2 (smallest P(JMA = )4.35×10(-9)), and KCNJ2 and SOX9 (smallest P(JMA = )1.28×10(-8)) were associated with FEV(1)/FVC or FEV(1) in meta-analysis models including SNP main effects, smoking main effects, and SNP-by-smoking (ever-smoking or pack-years) interaction. The HLA region has been widely implicated for autoimmune and lung phenotypes, unlike the other novel loci, which have not been widely implicated. We evaluated DNER, KCNJ2, and SOX9 and found them to be expressed in human lung tissue. DNER and SOX9 further showed evidence of differential expression in human airway epithelium in smokers compared to non-smokers. Our findings demonstrated that joint testing of SNP and SNP-by-environment interaction identified novel loci associated with complex traits that are missed when considering only the genetic main effects.

Genome-scale study of transcription factor expression in the branching mouse lung

BACKGROUND

Mammalian lung development consists of a series of precisely choreographed events that drive the progression from simple lung buds to the elaborately branched organ that fulfills the vital function of gas exchange. Strict transcriptional control is essential for lung development. Among the large number of transcription factors encoded in the mouse genome, only a small portion of them are known to be expressed and function in the developing lung. Thus a systematic investigation of transcription factors expressed in the lung is warranted.

RESULTS

To enrich for genes that may be responsible for regional growth and patterning, we performed a screen using RNA in situ hybridization to identify genes that show restricted expression patterns in the embryonic lung. We focused on the pseudoglandular stage during which the lung undergoes branching morphogenesis, a cardinal event of lung development. Using a genome-scale probe set that represents over 90% of the transcription factors encoded in the mouse genome, we identified 62 transcription factor genes with localized expression in the epithelium, mesenchyme, or both. Many of these genes have not been previously implicated in lung development.

CONCLUSIONS

Our findings provide new starting points for the elucidation of the transcriptional circuitry that controls lung development.

Numb expression and asymmetric versus symmetric cell division in distal embryonic lung epithelium

Proper balance between self-renewal and differentiation of lung-specific progenitors is absolutely required for normal lung morphogenesis/regeneration. Therefore, understanding the behavior of lung epithelial stem/progenitor cells could identify innovative solutions for restoring normal lung morphogenesis and/or regeneration. The Notch inhibitor Numb is a key determinant of asymmetric or symmetric cell division and hence cell fate. Yet Numb proximal-distal expression pattern and symmetric versus asymmetric division are uncharacterized during lung epithelial development. Herein, the authors find that the cell fate determinant Numb is highly expressed and asymmetrically distributed at the apical side of distal epithelial progenitors and segregated to one daughter cell in most mitotic cells. Knocking down Numb in MLE15 epithelial cells significantly increased the number of cells expressing the progenitor cell markers Sox9/Id2. Furthermore, cadherin hole analysis revealed that most distal epithelial stem/progenitor cells in embryonic lungs divide asymmetrically; with their cleavage, planes are predicted to bypass the cadherin hole, resulting in asymmetric distribution of the cadherin hole to the daughter cells. These novel findings provide evidence for asymmetric cell division in distal epithelial stem/progenitor cells of embryonic lungs and a framework for future translationally oriented studies in this area.

Multipotent capacity of immortalized human bronchial epithelial cells

While the adult murine lung utilizes multiple compartmentally restricted progenitor cells during homeostasis and repair, much less is known about the progenitor cells from the human lung. Translating the murine stem cell model to humans is hindered by anatomical differences between species. Here we show that human bronchial epithelial cells (HBECs) display characteristics of multipotent stem cells of the lung. These HBECs express markers indicative of several epithelial types of the adult lung when experimentally tested in cell culture. When cultured in three different three-dimensional (3D) systems, subtle changes in the microenvironment result in unique responses including the ability of HBECs to differentiate into multiple central and peripheral lung cell types. These new findings indicate that the adult human lung contains a multipotent progenitor cell whose differentiation potential is primarily dictated by the microenvironment. The HBEC system is not only important in understanding mechanisms for specific cell lineage differentiation, but also for examining changes that correlate with human lung diseases including lung cancer.

FoxM1 mediates the progenitor function of type II epithelial cells in repairing alveolar injury induced by Pseudomonas aeruginosa

The alveolar epithelium is composed of the flat type I cells comprising 95% of the gas-exchange surface area and cuboidal type II cells comprising the rest. Type II cells are described as facultative progenitor cells based on their ability to proliferate and trans-differentiate into type I cells. In this study, we observed that pneumonia induced by intratracheal instillation of Pseudomonas aeruginosa (PA) in mice increased the expression of the forkhead transcription factor FoxM1 in type II cells coincidentally with the induction of alveolar epithelial barrier repair. FoxM1 was preferentially expressed in the Sca-1(+) subpopulation of progenitor type II cells. In mice lacking FoxM1 specifically in type II cells, type II cells showed decreased proliferation and impaired trans-differentiation into type I cells. Lungs of these mice also displayed defective alveolar barrier repair after injury. Expression of FoxM1 in the knockout mouse lungs partially rescued the defective trans-differentiation phenotype. Thus, expression of FoxM1 in type II cells is essential for their proliferation and transition into type I cells and for restoring alveolar barrier homeostasis after PA-induced lung injury.

Epithelial progenitor cells in lung development, maintenance, repair, and disease

The vertebrate lung is elegantly patterned to carry out gas exchange and host defense. Similar to other organ systems, endogenous stem and progenitor cells fuel the organogenesis of the lung and maintain homeostasis in the face of normal wear and tear. In the context of acute injury, these progenitor populations are capable of effecting efficient repair. However, chronic injury, inflammation, and immune rejection frequently result in pathological airway remodeling and serious impairment of lung function. Here, we review the development, maintenance, and repair of the vertebrate respiratory system with an emphasis on the roles of epithelial stem and progenitor cells. We discuss what is currently known about their identities, lineage relationships, and the mechanisms that regulate their differentiation along various lineages. A deeper understanding of these progenitor populations will undoubtedly accelerate the discovery of improved cellular, genetic, molecular, and bioengineered therapies for lung disease.

Cell polarity and spindle orientation in the distal epithelium of embryonic lung

A proper balance between self-renewal and differentiation of lung-specific progenitors at the distal epithelial tips is absolutely required for normal lung morphogenesis. Cell polarity and mitotic spindle orientation play a critical role in the self-renewal/differentiation of epithelial cells and can impact normal physiological processes, including epithelial tissue branching and differentiation. Therefore, understanding the behavior of lung distal epithelial progenitors could identify innovative solutions to restoring normal lung morphogenesis. Yet little is known about cell polarity, spindle orientation, and segregation of cell fate determinant in the embryonic lung epithelium, which contains progenitor cells. Herein, we provide the first evidence that embryonic lung distal epithelium is polarized and highly mitotic with characteristic perpendicular cell divisions. Consistent with these findings, mInsc, LGN, and NuMA polarity proteins, which control spindle orientation, are asymmetrically localized in mitotic distal epithelial progenitors of embryonic lungs. Furthermore, the cell fate determinant Numb is asymmetrically distributed at the apical side of distal epithelial progenitors and segregated to one daughter cell in most mitotic cells. These findings provide evidence for polarity in distal epithelial progenitors of embryonic lungs and provide a framework for future translationally oriented studies in this area.

cAMP response element binding protein is required for differentiation of respiratory epithelium during murine development

The cAMP response element binding protein 1 (Creb1) transcription factor regulates cellular gene expression in response to elevated levels of intracellular cAMP. Creb1^−/− fetal mice are phenotypically smaller than wildtype littermates, predominantly die in utero and do not survive after birth due to respiratory failure. We have further investigated the respiratory defect of Creb1^−/− fetal mice during development. Lungs of Creb1^−/− fetal mice were pale in colour and smaller than wildtype controls in proportion to their reduced body size. Creb1^−/− lungs also did not mature morphologically beyond E16.5 with little or no expansion of airway luminal spaces, a phenotype also observed with the Creb1^−/− lung on a Crem^−/− genetic background. Creb1 was highly expressed throughout the lung at all stages examined, however activation of Creb1 was detected primarily in distal lung epithelium. Cell differentiation of E17.5 Creb1^−/− lung distal epithelium was analysed by electron microscopy and showed markedly reduced numbers of type-I and type-II alveolar epithelial cells. Furthermore, immunomarkers for specific lineages of proximal epithelium including ciliated, non-ciliated (Clara), and neuroendocrine cells showed delayed onset of expression in the Creb1^−/− lung. Finally, gene expression analyses of the E17.5 Creb1^−/− lung using whole genome microarray and qPCR collectively identified respiratory marker gene profiles and provide potential novel Creb1-regulated genes. Together, these results demonstrate a crucial role for Creb1 activity for the development and differentiation of the conducting and distal lung epithelium.

Six1 transcription factor is critical for coordination of epithelial, mesenchymal and vascular morphogenesis in the mammalian lung

Six1 is a member of the six-homeodomain family of transcription factors. Six1 is expressed in multiple embryonic cell types and plays important roles in proliferation, differentiation and survival of precursor cells of different organs, yet its function during lung development was hitherto unknown. Herein we show that Six1(-/-) lungs are severely hypoplastic with greatly reduced epithelial branching and increased mesenchymal cellularity. Six1 is expressed at the distal epithelial tips of branching tubules as well as in the surrounding distal mesenchyme. Six1(-/-) lung epithelial cells show increased expression of differentiation markers, but loss of progenitor cell markers. Six1 overexpression in MLE15 lung epithelial cells in vitro inhibited cell differentiation, but increases the expression of progenitor cell markers. In addition, Six1(-/-) embryos and newborn mice exhibit mesenchymal overproliferation, decreased Fgf10 expression and severe defects in the smooth muscle component of the bronchi and major pulmonary vessels. These defects lead to rupture of major vessels in mutant lungs after birth. Treatment of Six1(-/-) epithelial explants in culture with recombinant Fgf10 protein restores epithelial branching. As Shh expression is abnormally increased in Six1(-/-) lungs, we also treated mutant mesenchymal explants with recombinant Shh protein and found that these explants were competent to respond to Shh and continued to grow in culture. Furthermore, inhibition of Shh signaling with cyclopamine stimulated Six1(-/-) lungs to grow and branch in culture. This study provides the first evidence for the requirement of Six1 in coordinating Shh-Fgf10 signaling in embryonic lung to ensure proper levels of proliferation and differentiation along the proximodistal axis of epithelial, mesenchymal and endothelial cells. These findings uncover novel and essential functions for Six1 as a critical coordinator of Shh-Fgf10 signaling during embryonic lung development. We propose that Six1 is hence critical for coordination of proper lung epithelial, mesenchymal and vascular development.

Patterning and plasticity in development of the respiratory lineage

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

Eyes absent 1 (Eya1) is a critical coordinator of epithelial, mesenchymal and vascular morphogenesis in the mammalian lung

The proper level of proliferation and differentiation along the proximodistal axis is crucial for lung organogenesis. Elucidation of the factors that control these processes will therefore provide important insights into embryonic lung development and regeneration. Eya1 is a transcription factor/protein phosphatase that regulates cell lineage specification and proliferation. Yet its functions during lung development are unknown. In this paper we show that Eya1(-/-) lungs are severely hypoplastic with reduced epithelial branching and increased mesenchymal cellularity. Eya1 is expressed at the distal epithelial tips of branching tubules as well as in the surrounding distal mesenchyme. Eya1(-/-) lung epithelial cells show loss of progenitor cell markers with increased expression of differentiation markers and cell cycle exit. In addition, Eya1(-/-) embryos and newborn mice exhibit severe defects in the smooth muscle component of the bronchi and major pulmonary vessels with decreased Fgf10 expression. These defects lead to rupture of the major vessels and hemorrhage into the lungs after birth. Treatment of Eya1(-/-) epithelial explants in culture with recombinant Fgf10 stimulates epithelial branching. Since Shh expression and activity are abnormally increased in Eya1(-/-) lungs, we tested whether genetically lowering Shh activity could rescue the Eya1(-/-) lung phenotype. Indeed, genetic reduction of Shh partially rescues Eya1(-/-) lung defects while restoring Fgf10 expression. This study provides the first evidence that Eya1 regulates Shh signaling in embryonic lung, thus ensuring the proper level of proliferation and differentiation along the proximodistal axis of epithelial, mesenchymal and endothelial cells. These findings uncover novel functions for Eya1 as a critical upstream coordinator of Shh-Fgf10 signaling during embryonic lung development. We conclude, therefore, that Eya1 function is critical for proper coordination of lung epithelial, mesenchymal and vascular development.

Lung organogenesis

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

Connexin43 plays an important role in lung development

OBJECTIVES

Connexin43 (Cx43) is one of the proteins associated with gap junction. Connexin43 knockout mice die after birth owing to hypoplastic lungs. The purpose of this study was to analyze the hypoplastic lung of Cx43 knockout mice to clarify the role of the Cx43 during lung development.

METHODS

Adult hetero Cx43 mice were mated. Newborn mice were divided into the following groups: wild, hetero, and knockout. Total RNA was extracted from the right lung, and the left lung was fixed for immunohistochemical staining. The mRNA expression of surfactant protein C, aquaporin-5, and alpha-smooth muscle actin were analyzed by reverse transcriptase polymerase chain reaction. H&E and immunohistochemical staining for those markers were performed.

RESULTS

The mRNA expression of aquaporin-5, surfactant protein C, and alpha-smooth muscle actin was significantly lower in knockout mice than that in the wild and hetero mice. H&E staining in the knockout mice showed narrow airspaces and thicker interalveolar septae. Immunohistochemical staining in all markers showed the formation of alveoli to be delayed in the knockout mice.

CONCLUSION

Based on these findings, Cx43 is closely related to alveolar and vascular formation during lung development.

The Id2+ distal tip lung epithelium contains individual multipotent embryonic progenitor cells

The conducting airways (bronchi and bronchioles) and peripheral gas exchange (alveolar) regions of the mammalian lung are generated by a process of branching morphogenesis. Evidence suggests that during embryonic development, the undifferentiated epithelial progenitors are located at the distal tips of the branching epithelium. To test this hypothesis, we used an Id2-CreER(T2) knock-in mouse strain to lineage trace the distal epithelial tip cells during either the pseudoglandular or canalicular phases of development. During the pseudoglandular stage, the tip cells both self-renew and contribute descendents to all epithelial cell lineages, including neuroendocrine cells. In addition, individual Id2(+) tip cells can self-renew and contribute descendents to both the bronchiolar and alveolar compartments. By contrast, during the later canalicular stage, the distal epithelial tip cells only contribute descendents to the alveoli. Taken together, this evidence supports a model in which the distal tip of the developing lung contains a multipotent epithelial population, the fate of which changes during development.

Conditional gene inactivation reveals roles for Fgf10 and Fgfr2 in establishing a normal pattern of epithelial branching in the mouse lung

Fibroblast growth factor 10 (FGF10) signaling through FGF receptor 2 (FGFR2) is required for lung initiation. While studies indicate that Fgf10 and Fgfr2 are also important at later stages of lung development, their roles in early branching events remain unclear. We addressed this question through conditional inactivation of both genes in mouse subsequent to lung initiation. Inactivation of Fgf10 in lung mesenchyme resulted in smaller lobes with a reduced number of branches. Inactivation of Fgfr2 in lung epithelium resulted in disruption of lobes and small epithelial outgrowths that arose arbitrarily along the main bronchi. In both mutants, there was an increase in cell death. Also, the expression patterns of key signaling molecules implicated in branching morphogenesis were altered and a proximal lung marker was expanded distally. Our results indicate that both Fgf10 and Fgfr2 are required for a normal branching program and for proper proximal-distal patterning of the lung.

Proteomic analysis of an alpha7 nicotinic acetylcholine receptor interactome

The alpha7 nicotinic acetylcholine receptor (nAChR) is well established as the principal high-affinity alpha-bungarotoxin-binding protein in the mammalian brain. We isolated carbachol-sensitive alpha-bungarotoxin-binding complexes from total mouse brain tissue by affinity immobilization followed by selective elution, and these proteins were fractionated by SDS-PAGE. The proteins in subdivided gel lane segments were tryptically digested, and the resulting peptides were analyzed by standard mass spectrometry. We identified 55 proteins in wild-type samples that were not present in comparable brain samples from alpha7 nAChR knockout mice that had been processed in a parallel fashion. Many of these 55 proteins are novel proteomic candidates for interaction partners of the alpha7 nAChR, and many are associated with multiple signaling pathways that may be implicated in alpha7 function in the central nervous system. The newly identified potential protein interactions, together with the general methodology that we introduce for alpha-bungarotoxin-binding protein complexes, form a new platform for many interesting follow-up studies aimed at elucidating the physiological role of neuronal alpha7 nAChRs.

Lung epithelial progenitor cells: lessons from development

The current enthusiasm for stem cell research stems from the hope that damaged or diseased tissues may one day be repaired through the manipulation of endogenous or exogenous stem cells. The postnatal human respiratory system is highly accessible and provides unique opportunities for the application of such techniques. Several putative adult lung epithelial stem cells have been identified in the mouse model system. However, their in vivo capabilities to contribute to different lineages, and their control mechanisms, remain unclear. If stem cell-based therapies are to be successful in the lung, it is vitally important that we understand the normal behavior of adult lung stem cells, and how this is regulated. Lung embryonic progenitor cells are much better defined and characterized than their adult counterparts. Moreover, experiments on a variety of developing tissues are beginning to uncover general mechanisms by which embryonic progenitors influence final organ size and structure. This provides a framework for the study of lung embryonic progenitor cells, facilitating experimental design and interpretation. A similar approach to investigating adult lung stem cells could produce rapid advances in the field.

Wnt7b stimulates embryonic lung growth by coordinately increasing the replication of epithelium and mesenchyme

The effects of Wnt7b on lung development were examined using a conditional Wnt7b-null mouse. Wnt7b-null lungs are markedly hypoplastic, yet display largely normal patterning and cell differentiation. In contrast to findings in prior hypomorphic Wnt7b models, we find decreased replication of both developing epithelium and mesenchyme, without abnormalities of vascular smooth muscle development. We further demonstrate that Wnt7b signals to neighboring cells to activate both autocrine and paracrine canonical Wnt signaling cascades. In contrast to results from hypomorphic models, we show that Wnt7b modulates several important signaling pathways in the lung. Together, these cascades result in the coordinated proliferation of adjacent epithelial and mesenchymal cells to stimulate organ growth with few alterations in differentiation and patterning.

Role for mitogen-activated protein kinase p38 alpha in lung epithelial branching morphogenesis

In the early stages of lung development, the endoderm undergoes extensive and stereotypic branching morphogenesis. During this process, a simple epithelial bud develops into a complex tree-like system of tubes specialized for the transport and exchange of gas with blood. The endodermal cells in the distal tips of the developing lung express a special set of genes, have a higher proliferation rate than proximal part, undergo shape change and initiate branching morphogenesis. In this study, we found that of the four p38 genes, only p38 alpha mRNA is localized specifically to the distal endoderm suggesting a role in the regulation of budding morphogenesis. Chemical inhibitors specific for the p38 alpha and p38 beta isoforms suppress budding of embryonic mouse lung explants and isolated endoderm in vitro. Specific knockdown of p38 alpha in cultured lung endoderm using shRNA also inhibited budding morphogenesis, consistent with the chemical inhibition of the p38 signaling pathway. Disruption of p38 alpha did not affect proliferation or expression of the distal cell markers, Sox9 and Erm. However, the amount of E-cadherin protein increased significantly and ectopic expression of E-cadherin also impaired budding of endoderm in vitro. These results suggest that p38 alpha modulates epithelial cell-cell interactions and possibly cell rearrangement during branching morphogenesis. This study provides the first evidence that p38 alpha is involved in the morphogenesis of an epithelial organ.

Transgenic over-expression of the microRNA miR-17-92 cluster promotes proliferation and inhibits differentiation of lung epithelial progenitor cells

The miR-17-92 locus encodes a cluster of 7 microRNAs transcribed as a single primary transcript. It can accelerate c-Myc induced B cell lymphoma development and is highly expressed in many tumors, including lung tumors. However, the role of miR-17-92 in development has not been well studied. From analysis of microRNAs during lung development, expression of the miR-17-92 cluster is high at early stages, but declines as development proceeds. We used the mouse surfactant protein C (Sftpc) promoter to over-express the cluster in embryonic lung epithelium. Transgenic lungs have a very abnormal lethal phenotype. They contain numerous proliferative epithelial cells that retain high levels of Sox9, a marker of distal progenitors. The differentiation of proximal epithelial cells was also inhibited. Furthermore, a significant increase in the number of neuroendocrine cell clusters was observed in the lungs of dead transgenic pups. We identify a tumor suppressor, Rbl2 which belongs to the Rb family, as a new target for miR-17-5p. Together, these studies suggest that mir-17-92 normally promotes the high proliferation and undifferentiated phenotype of lung epithelial progenitor cells.

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

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

Effect of nitric oxide on fibroblast growth factor-10 and bone morphogenetic protein 4 expressions in the branching morphogenesis of fetal rat lung explants

PURPOSE

Nitric oxide (NO) can accelerate branching morphogenesis of fetal rat lung explants in vitro, whereas its exact mechanism remains unclear. In this study, we investigate the effect of NO on the expression of fibroblast growth factor-10 (FGF10) and bone morphogenetic protein-4 (BMP4), which plays an important role in bud formation.

METHODS

Fetal rat lungs harvested on day 13.5 of gestation were cultured in serum-free medium for 72 hours with 0, 50, 100, and 200 micromol/L of an NO donor, DETA NONOate (DETA/NO) (n = 4, 3, 6, and 5). The ratio of bud increment of each cultured lung was calculated, and the FGF10 and BMP4 mRNA expression levels were analyzed by real-time reverse transcription polymerase chain reaction.

RESULTS

Bud increment ratio was significantly increased in 50, 100, and 200 micromol/L DETA/NO (3.3 +/- 0.2, 3.0 +/- 0.3, and 3.5 +/- 0.5) compared to controls (1.9 +/- 0.3) (P < .05). There was a significant increase in BMP4 mRNA expression in 100 micromol/L DETA/NO (190% +/- 20%) compared to controls (100% +/- 30%) (P < .05), whereas FGF10 mRNA expression was not significantly different between each DETA/NO group and controls.

CONCLUSION

The NO donor not only promotes branching of fetal lung explants but also upregulates expression of BMP4, which is an important regulator of branching morphogenesis.

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

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

Global analysis of genes differentially expressed in branching and non-branching regions of the mouse embryonic lung

During development, the proximal and distal regions of respiratory tract undergo distinct processes that ultimately give rise to conducting airways and alveoli. To gain insights into the genetic pathways differentially activated in these regions when branching morphogenesis is initiating, we characterized their transcriptional profiles in murine rudiments isolated at embryonic (E) day 11.5. By using oligonucleotide microarrays, we identified 83 and 128 genes preferentially expressed in branching and non-branching regions, respectively. The majority of these genes (85%) had not been previously described in the lung, or in other organs. We report restricted expression patterns of 22 of these genes were by in situ hybridization. Among them in the lung potential components of the Wnt, TGF beta, FGF and retinoid pathways identified in other systems, and uncharacterized genes, such as translocases, small GTPases and splicing factors. In addition, we provide a more detailed analysis of the expression pattern and regulation of a representative gene from the distal (transforming growth factor, beta induced) and proximal (WW domain-containing protein 2) regions. Our data suggest that these genes may regulate focal developmental events specific of each of these regions during respiratory tract formation.

Novel role for Netrins in regulating epithelial behavior during lung branching morphogenesis

The development of many organs, including the lung, depends upon a process known as branching morphogenesis, in which a simple epithelial bud gives rise to a complex tree-like system of tubes specialized for the transport of gas or fluids. Previous studies on lung development have highlighted a role for fibroblast growth factors (FGFs), made by the mesodermal cells, in promoting the proliferation, budding, and chemotaxis of the epithelial endoderm. Here, by using a three-dimensional culture system, we provide evidence for a novel role for Netrins, best known as axonal guidance molecules, in modulating the morphogenetic response of lung endoderm to exogenous FGFs. This effect involves inhibition of localized changes in cell shape and phosphorylation of the intracellular mitogen-activated protein kinase(s) (ERK1/2, for extracellular signal-regulated kinase-1 and -2), elicited by exogenous FGFs. The temporal and spatial expression of netrin 1, netrin 4, and Unc5b genes and the localization of Netrin-4 protein in vivo suggest a model in which Netrins in the basal lamina locally modulate and fine-tune the outgrowth and shape of emergent epithelial buds.

Epithelial-mesenchymal interactions in the developing lung

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

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

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

Growth factor signaling in lung morphogenetic centers: automaticity, stereotypy and symmetry

Lung morphogenesis is stereotypic, both for lobation and for the first several generations of airways, implying mechanistic control by a well conserved, genetically hardwired developmental program. This program is not only directed by transcriptional factors and peptide growth factor signaling, but also co-opts and is modulated by physical forces. Peptide growth factors signal within repeating epithelial-mesenchymal temporospatial patterns that constitute morphogenetic centers, automatically directing millions of repetitive events during both stereotypic branching and nonstereotypic branching as well as alveolar surface expansion phases of lung development. Transduction of peptide growth factor signaling within these centers is finely regulated at multiple levels. These may include ligand expression, proteolytic activation of latent ligand, ligand bioavailability, ligand binding proteins and receptor affinity and presentation, receptor complex assembly and kinase activation, phosphorylation and activation of adapter and messenger protein complexes as well as downstream events and cross-talk both inside and outside the nucleus. Herein we review the critical Sonic Hedgehog, Fibroblast Growth Factor, Bone Morphogenetic Protein, Vascular Endothelial Growth Factor and Transforming Growth Factorbeta signaling pathways and propose how they may be functionally coordinated within compound, highly regulated morphogenetic gradients that drive first stereotypic and then non-stereotypic, automatically repetitive, symmetrical as well as asymmetrical branching events in the lung.