Molecular regulation of lung development

Early Atf4 activity drives airway club and goblet cell differentiation

Activating transcription factor 4 (Atf4), which is modulated by the protein kinase RNA-like ER kinase (PERK), is a stress-induced transcription factor responsible for controlling the expression of a wide range of adaptive genes, enabling cells to withstand stressful conditions. However, the impact of the Atf4 signaling pathway on airway regeneration remains poorly understood. In this study, we used mouse airway epithelial cell culture models to investigate the role of PERK/Atf4 in respiratory tract differentiation. Through pharmacological inhibition and silencing of ATF4, we uncovered the crucial involvement of PERK/Atf4 in the differentiation of basal stem cells, leading to a reduction in the number of secretory cells. ChIP-seq analysis revealed direct binding of ATF4 to regulatory elements of genes associated with osteoblast differentiation and secretory cell function. Our findings provide valuable insights into the role of ATF4 in airway epithelial differentiation and its potential involvement in innate immune responses and cellular adaptation to stress.

Early-life pulmonary viral infection leads to long-term functional and lower airway structural changes in the lungs

Early-life respiratory virus infections have been correlated with enhanced development of childhood asthma. In particular, significant numbers of respiratory syncytial virus (RSV)-hospitalized infants go on to develop lung disease. It has been suggested that early-life viral infections may lead to altered lung development or repair that negatively impacts lung function later in life. Our data demonstrate that early-life RSV infection modifies lung structure, leading to decreased lung function. At 5 wk postneonatal RSV infection, significant defects are observed in baseline pulmonary function test (PFT) parameters consistent with decreased lung function as well as enlarged alveolar spaces. Lung function changes in the early-life RSV-infected group continue at 3 mo of age. The altered PFT and structural changes induced by early-life RSV were mitigated in TSLPR-/- mice that have previously been shown to have reduced immune cell accumulation associated with a persistent Th2 environment. Importantly, long-term effects were demonstrated using a secondary RSV infection 3 mo following the initial early-life RSV infection and led to significant additional defects in lung function, with severe mucus deposition within the airways, and consolidation of the alveolar spaces. These studies suggest that early-life respiratory viral infection leads to alterations in lung structure/repair that predispose to diminished lung function later in life.NEW & NOTEWORTHY These studies outline a novel finding that early-life respiratory virus infection can alter lung structure and function long-term. Importantly, the data also indicate that there are critical links between inflammatory responses and subsequent events that produce a more severe pathogenic response later in life. The findings provide additional data to support that early-life infections during lung development can alter the trajectory of airway function.

Pulmonary Vascular Regulation in the Fetal and Transitional Lung

Fetal lungs have fewer and smaller arteries with higher pulmonary vascular resistance (PVR) than a newborn. As gestation advances, the pulmonary circulation becomes more sensitive to changes in pulmonary arterial oxygen tension, which prepares them for the dramatic drop in PVR and increase in pulmonary blood flow (PBF) that occur when the baby takes its first few breaths of air, thus driving the transition from fetal to postnatal circulation. Dynamic and intricate regulatory mechanisms control PBF throughout development and are essential in supporting gas exchange after birth. Understanding these concepts is crucial given the role the pulmonary vasculature plays in the development of complications with transition, such as in the setting of persistent pulmonary hypertension of the newborn and congenital heart disease. An improved understanding of pulmonary vascular regulation may reveal opportunities for better clinical management.

Mechanics of Lung Development

We summarize how skeletal muscle and lung developmental biology fields have been bridged to benefit from mouse genetic engineering technologies and to explore the role of fetal breathing-like movements (FBMs) in lung development, by using skeletal muscle-specific mutant mice. It has been known for a long time that FBMs are essential for the lung to develop properly. However, the cellular and molecular mechanisms transducing the mechanical forces of muscular activity into specific genetic programs that propel lung morphogenesis (development of the shape, form and size of the lung, its airways, and gas exchange surface) as well as its differentiation (acquisition of specialized cell structural and functional features from their progenitor cells) are only starting to be revealed. This chapter is a brief synopsis of the cumulative findings from that ongoing quest. An update on and the rationale for our recent International Mouse Phenotyping Consortium (IMPC) search is also provided.

Polarity in respiratory development, homeostasis and disease

The respiratory system is composed of a multitude of cells that organize to form complex branched airways that end in alveoli, which respectively function to guide air flow and mediate gas exchange with the bloodstream. The organization of the respiratory sytem relies on distinct forms of cell polarity, which guide lung morphogenesis and patterning in development and provide homeostatic barrier protection from microbes and toxins. The stability of lung alveoli, the luminal secretion of surfactants and mucus in the airways, and the coordinated motion of multiciliated cells that generate proximal fluid flow, are all critical functions regulated by cell polarity, with defects in polarity contributing to respiratory disease etiology. Here, we summarize the current knowledge of cell polarity in lung development and homeostasis, highlighting key roles for polarity in alveolar and airway epithelial function and outlining relationships with microbial infections and diseases, such as cancer.

Activation of PI3K/p110α in the Lung Mesenchyme Affects Branching Morphogenesis and Club Cell Differentiation

Epithelial–mesenchymal interaction is required for normal growth, morphogenetic patterning, and cellular differentiation in developing lungs. Various signaling pathways have been defined in establishing the patterning of this branched organ. The phosphoinositide-3-kinase (PI3K) signaling plays an important role in disease pathogenesis but remains largely uncharacterized in embryonic development. In this study, we activated a specific catalytic subunit of PI3K catalytic enzymes, Class IA p110α (p110α), in the embryonic lung mesenchyme using the Dermo1-Cre mouse. Activation of p110α promoted branching morphogenesis and blocked club cell differentiation in both proximal and distal airways. Mechanistically, the LIM homeodomain gene Islet-1 (Isl1), fibroblast growth factor 10 (Fgf10), and SRY (sex-determining region Y)-box9 (Sox9) were found to be downstream targets of p110α. The significantly increased expressions of Isl1, Fgf10, and Sox9 resulted in the stimulation of branching in mutant lungs. Activation of p110α-mediated signaling also increased the expression of phosphatase and tensin homolog deleted on chromosome 10 (Pten) and hairy/enhancer of split 1 (Hes1), which in turn blocked club cell differentiation. Thus, the signaling pathway by which PI3K/p110α-regulated epithelial–mesenchymal interactions may entail Isl1–Fgf10–Sox9 and Pten–Hes1 networks, which consequently regulate branching morphogenesis and club cell differentiation, respectively.

Developmental Pathways Underlying Lung Development and Congenital Lung Disorders

Lung organogenesis is a highly coordinated process governed by a network of conserved signaling pathways that ultimately control patterning, growth, and differentiation. This rigorously regulated developmental process culminates with the formation of a fully functional organ. Conversely, failure to correctly regulate this intricate series of events results in severe abnormalities that may compromise postnatal survival or affect/disrupt lung function through early life and adulthood. Conditions like congenital pulmonary airway malformation, bronchopulmonary sequestration, bronchogenic cysts, and congenital diaphragmatic hernia display unique forms of lung abnormalities. The etiology of these disorders is not yet completely understood; however, specific developmental pathways have already been reported as deregulated. In this sense, this review focuses on the molecular mechanisms that contribute to normal/abnormal lung growth and development and their impact on postnatal survival.

A Developmental Role of the Cystic Fibrosis Transmembrane Conductance Regulator in Cystic Fibrosis Lung Disease Pathogenesis

The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein is a cAMP-activated anion channel that is critical for regulating fluid and ion transport across the epithelium. This process is disrupted in CF epithelia, and patients harbouring CF-causing mutations experience reduced lung function as a result, associated with the increased rate of mortality. Much progress has been made in CF research leading to treatments that improve CFTR function, including small molecule modulators. However, clinical outcomes are not necessarily mutation-specific as individuals harboring the same genetic mutation may present with varying disease manifestations and responses to therapy. This suggests that the CFTR protein may have alternative functions that remain under-appreciated and yet can impact disease. In this mini review, we highlight some notable research implicating an important role of CFTR protein during early lung development and how mutant CFTR proteins may impact CF airway disease pathogenesis. We also discuss recent novel cell and animal models that can now be used to identify a developmental cause of CF lung disease.

YAP regulates alveolar epithelial cell differentiation and AGER via NFIB/KLF5/NKX2-1

Ventilation is dependent upon pulmonary alveoli lined by two major epithelial cell types, alveolar type-1 (AT1) and 2 (AT2) cells. AT1 cells mediate gas exchange while AT2 cells synthesize and secrete pulmonary surfactants and serve as progenitor cells which repair the alveoli. We developed transgenic mice in which YAP was activated or deleted to determine its roles in alveolar epithelial cell differentiation. Postnatal YAP activation increased epithelial cell proliferation, increased AT1 cell numbers, and caused indeterminate differentiation of subsets of alveolar cells expressing atypical genes normally restricted to airway epithelial cells. YAP deletion increased expression of genes associated with mature AT2 cells. YAP activation enhanced DNA accessibility in promoters of transcription factors and motif enrichment analysis predicted target genes associated with alveolar cell differentiation. YAP participated with KLF5, NFIB, and NKX2-1 to regulate AGER. YAP plays a central role in a transcriptional network that regulates alveolar epithelial differentiation.

p53 regulation by MDM2 contributes to self-renewal and differentiation of basal stem cells in mouse and human airway epithelium

Proper physiological function of mammalian airways requires the differentiation of basal stem cells into secretory or multiciliated cells, among others. In addition, the self-renewal ability of these basal stem cells is crucial for developing a quick response to toxic agents in order to re-establish the epithelial barrier function of the airways. Although these epithelial missions are vital, little is known about those mechanism controlling airway epithelial regeneration in health and disease. p53 has been recently proposed as the guardian of homeostasis, promoting differentiation programs, and antagonizing a de-differentiation program. Here, we exploit mouse and human tracheal epithelial cell culture models to study the role of MDM2-p53 signaling in self-renewal and differentiation in the airway epithelium. We show that p53 protein regulation by MDM2 is crucial for basal stem cell differentiation and to keep proper cell proliferation. Therefore, we suggest that MDM2/p53 interaction modulation is a potential target to control regeneration of the mammalian airway epithelia without massively affecting the epithelium integrity and differentiation potential.

Junctional Adhesion Molecule 3 Expression in the Mouse Airway Epithelium Is Linked to Multiciliated Cells

Tight-junction (TJ) proteins are essential for establishing the barrier function between neighbor epithelial cells, but also for recognition of pathogens or cell migration. Establishing the expression pattern and localization of different TJ proteins will help to understand the development and physiology of the airway. Here we identify that the junctional adhesion molecule 3 (Jam3) expression is restricted to multiciliated cells (MCCs) in the airway epithelium. In vitro, Jam3 expression varies along airway basal stem cell (BSC) differentiation and upon DAPT treatment or IL6 exposure. However, Jam3 is not required for BSC differentiation to specific cell types. In addition, we found that MCC lacking Jam3 display normal cilia morphology and cilia beating frequency with a delay in BB assembly/positioning in MCCs during differentiation. Remarkably, Jam3 in MCC is mostly localized to subapical organelles, which are negative for the apical recycling endosome marker Rab11 and positive for EEA1. Our data show that Jam3 expression is connected to mature MCC in the airway epithelium and suggest a Jam3 role unrelated to its known barrier function.

Blastocyst complementation reveals that NKX2-1 establishes the proximal-peripheral boundary of the airway epithelium

BACKGROUND

Distinct boundaries between the proximal conducting airways and more peripheral-bronchial regions of the lung are established early in foregut embryogenesis, demarcated in part by the distribution of SOX family and NKX2-1 transcription factors along the cephalo-caudal axis of the lung. We used blastocyst complementation to identify the role of NKX2-1 in the formation of the proximal-peripheral boundary of the airways in mouse chimeric embryos.

RESULTS

While Nkx2-1-/- mouse embryos form primordial tracheal cysts, peripheral pulmonary structures are entirely lacking in Nkx2-1-/- mice. Complementation of Nkx2-1-/- embryos with NKX2-1-sufficient embryonic stem cells (ESCs) enabled the formation of all tissue components of the peripheral lung but did not enhance ESC colonization of the most proximal regions of the airways. In chimeric mice, a precise boundary was formed between NKX2-1-deficient basal cells co-expressing SOX2 and SOX9 in large airways and ESC-derived NKX2-1+ SOX9+ epithelial cells of smaller airways. NKX2-1-sufficient ESCs were able to selectively complement peripheral, rather than most proximal regions of the airways. ESC complementation did not prevent ectopic expression of SOX9 but restored β-catenin signaling in Nkx2-1-/- basal cells of large airways.

CONCLUSIONS

NKX2-1 and β-catenin function in an epithelial cell-autonomous manner to establish the proximal-peripheral boundary along developing airways.

Airway organoids as models of human disease

Studies developing and applying organoid technology have greatly increased in volume and visibility over the past decade. Organoids are three-dimensional structures that are established from pluripotent stem cells (PSCs) or adult tissue stem cells (ASCs). They consist of organ-specific cell types that self-organize through cell sorting and spatially restricted lineage commitment to generate architectural and functional characteristics of the tissue of interest. The field of respiratory development and disease has been particularly productive in this regard. Starting from human cells (PSCs or ASCs), models of the two segments of the lung, the airways and the alveoli, can be built. Such organoids allow the study of development, physiology and disease and thus bridge the gap between animal models and clinical studies. This review discusses current developments in the pulmonary organoid field, highlighting the potential and limitations of current models.

Correlation between sestrin2 expression and airway remodeling in COPD

BACKGROUND

Airway remodeling is a major pathological characteristic of chronic obstructive pulmonary disease (COPD), and has been shown to be associated with oxidative stress. Sestrin2 has recently drawn attention as an important antioxidant protein. However, the underlying correlation between sestrin2 and airway remodeling in COPD has yet to be clarified.

METHODS

A total of 124 subjects were enrolled in this study, including 62 control subjects and 62 COPD patients. The pathological changes in airway tissues were assessed by different staining methods. The expression of sestrin2 and matrix metalloproteinase 9 (MMP9) in airway tissues was monitored by immunohistochemistry. Enzyme-linked immunosorbent assays (ELISAs) were used to detect the serum concentrations of sestrin2 and MMP9. The airway parameters on computed tomography (CT) from all participants were measured for evaluating airway remodeling. The relationship between serum sestrin2 and MMP9 concentration and airway parameters in chest CT was also analyzed.

RESULTS

In patients with COPD, staining of airway structures showed distinct pathological changes of remodeling, including cilia cluttered, subepithelial fibrosis, and reticular basement membrane (Rbm) fragmentation. Compared with control subjects, the expression of sestrin2 and MMP9 was significantly increased in both human airway tissues and serum. Typical imaging characteristics of airway remodeling and increased airway parameters were also found by chest CT. Additionally, serum sestrin2 concentration was positively correlated with serum MMP9 concentration and airway parameters in chest CT.

CONCLUSION

Increased expression of sestrin2 is related to airway remodeling in COPD. We demonstrated for the first time that sestrin2 may be a novel biomarker for airway remodeling in patients with COPD.

Bioengineering of Pulmonary Epithelium With Preservation of the Vascular Niche

The shortage of transplantable donor organs directly affects patients with end-stage lung disease, for which transplantation remains the only definitive treatment. With the current acceptance rate of donor lungs of only 20%, rescuing even one half of the rejected donor lungs would increase the number of transplantable lungs threefold, to 60%. We review recent advances in lung bioengineering that have potential to repair the epithelial and vascular compartments of the lung. Our focus is on the long-term support and recovery of the lung ex vivo, and the replacement of defective epithelium with healthy therapeutic cells. To this end, we first review the roles of the lung epithelium and vasculature, with focus on the alveolar-capillary membrane, and then discuss the available and emerging technologies for ex vivo bioengineering of the lung by decellularization and recellularization. While there have been many meritorious advances in these technologies for recovering marginal quality lungs to the levels needed to meet the standards for transplantation – many challenges remain, motivating further studies of the extended ex vivo support and interventions in the lung. We propose that the repair of injured epithelium with preservation of quiescent vasculature will be critical for the immediate blood supply to the lung and the lung survival and function following transplantation.

Molecular, cellular, and bioengineering approaches to stimulate lung regeneration after injury

The lung is susceptible to damage from a variety of sources throughout development and in adulthood. As a result, the lung has great capacities for repair and regeneration, directed by precisely controlled sequences of molecular and signaling pathways. Impairments or alterations in these signaling events can have deleterious effects on lung structure and function, ultimately leading to chronic lung disorders. When lung injury is too severe for the normal pathways to repair, or if those pathways do not function properly, lung regenerative medicine is needed to restore adequate structure and function. Great progress has been made in recent years in the number of regenerative techniques and their efficacy. This review will address recent progress in lung regenerative medicine focusing on pharmacotherapy including the expanding role of nanotechnology, stem cell-based therapies, and bioengineering techniques. The use of these techniques individually and collectively has the potential to significantly improve morbidity and mortality associated with congenital and acquired lung disorders.

Autophagy is required for lung development and morphogenesis

Bronchopulmonary dysplasia (BPD) remains a major respiratory illness in extremely premature infants. The biological mechanisms leading to BPD are not fully understood, although an arrest in lung development has been implicated. The current study aimed to investigate the occurrence of autophagy in the developing mouse lung and its regulatory role in airway branching and terminal sacculi formation. We found 2 windows of epithelial autophagy activation in the developing mouse lung, both resulting from AMPK activation. Inhibition of AMPK-mediated autophagy led to reduced lung branching in vitro. Conditional deletion of beclin 1 (Becn1) in mouse lung epithelial cells (Becn1Epi-KO), either at early (E10.5) or late (E16.5) gestation, resulted in lethal respiratory distress at birth or shortly after. E10.5 Becn1Epi-KO lungs displayed reduced airway branching and sacculi formation accompanied by impaired vascularization, excessive epithelial cell death, reduced mesenchymal thinning of the interstitial walls, and delayed epithelial maturation. E16.5 Becn1Epi-KO lungs had reduced terminal air sac formation and vascularization and delayed distal epithelial differentiation, a pathology similar to that seen in infants with BPD. Taken together, our findings demonstrate that intrinsic autophagy is an important regulator of lung development and morphogenesis and may contribute to the BPD phenotype when impaired.

Myh10 deficiency leads to defective extracellular matrix remodeling and pulmonary disease

Impaired alveolar formation and maintenance are features of many pulmonary diseases that are associated with significant morbidity and mortality. In a forward genetic screen for modulators of mouse lung development, we identified the non-muscle myosin II heavy chain gene, Myh10. Myh10 mutant pups exhibit cyanosis and respiratory distress, and die shortly after birth from differentiation defects in alveolar epithelium and mesenchyme. From omics analyses and follow up studies, we find decreased Thrombospondin expression accompanied with increased matrix metalloproteinase activity in both mutant lungs and cultured mutant fibroblasts, as well as disrupted extracellular matrix (ECM) remodeling. Loss of Myh10 specifically in mesenchymal cells results in ECM deposition defects and alveolar simplification. Notably, MYH10 expression is downregulated in the lung of emphysema patients. Altogether, our findings reveal critical roles for Myh10 in alveologenesis at least in part via the regulation of ECM remodeling, which may contribute to the pathogenesis of emphysema.

Upregulated EFNB2 and EPHB4 promotes lung development in a nitrofen-induced congenital diaphragmatic hernia rat model

Congenital diaphragmatic hernia (CDH) is a common congenital malformation associated with high mortality rates, mainly due to pulmonary hypoplasia and persistent pulmonary hypertension following birth. The present study aimed to investigate abnormal lung development in a rat CDH model, and examine temporal and spatial changes in the expression of ephrin type‑B receptor 4 (EPHB4) and ephrin‑B2 (EFNB2) during fetal lung development, to elucidate the role of these factors during lung morphogenesis. Pregnant rats received nitrofen on embryonic day (E) 8.5 to induce CDH, and fetal lungs were collected on E13.5, E15.5, E17.5, E19.5, and E21.5. The mean linear intercept (MLI) and mean alveolar number (MAN) were observed in fetal lung tissue at E21.5 following hematoxylin and eosin staining. E13.5 fetal lungs were cultured for 96 h in serum‑free medium and branch development was observed under a microscope. The gene and protein expression levels of EPHB4 and EFNB2 were assessed by reverse transcription‑quantitative polymerase chain reaction analysis, and immunoblotting and immunohistochemistry, respectively. The fetal rat lungs were treated with EFNB2 and the activity of key signaling pathways was assessed. The lung index (lung weight/body weight) at E21.5 was significantly lower in the CDH rats, compared with that in the control fetal rats. The MLI and MAN were also lower in the CDH group. The number of lung terminal buds at E13.5 (embryonic stage), and the lung‑explant perimeter and surface were all smaller in the CDH group rats than in the control group at the same age. Pulmonary hypoplasia was observed following 96 h of in vitro culture. No significant differences were found in the expression levels of EFNB2 and EPHB4 between the CDH and control groups at E13.5 (embryonic stage) or E15.5 (pseudoglandular stage), however, EFNB2 and EPHB4 were significantly upregulated at E17.5 (canalicular stage), and at E19.5 and E21.5 (saccular/alveolar stages). EFNB2 stimulated pulmonary branching and EFNB2 supplementation decreased the activity of p38, c‑Jun NH2‑terminal kinase, extracellular signal‑regulated kinase, and signal transducer and activator of transcription. The CDH fetal rats developed pulmonary dysplasia at an early stage of fetal pulmonary development. Upregulated expression of EFNB2 and EPHB4 was observed in the rat lung of nitrofen‑induced CDH, and the increased expression of EFNB2 promoted rat lung development in the nitrofen‑induced CDH model.

FOXF1 transcription factor promotes lung morphogenesis by inducing cellular proliferation in fetal lung mesenchyme

Organogenesis is regulated by mesenchymal-epithelial signaling events that induce expression of cell-type specific transcription factors critical for cellular proliferation, differentiation and appropriate tissue patterning. While mesenchymal transcription factors play a key role in mesenchymal-epithelial interactions, transcriptional networks in septum transversum and splanchnic mesenchyme remain poorly characterized. Forkhead Box F1 (FOXF1) transcription factor is expressed in mesenchymal cell lineages; however, its role in organogenesis remains uncharacterized due to early embryonic lethality of Foxf1^-/- mice. In the present study, we generated mesenchyme-specific Foxf1 knockout mice (Dermo1-Cre Foxf1^-/-) and demonstrated that FOXF1 is required for development of respiratory, cardiovascular and gastrointestinal organ systems. Deletion of Foxf1 from mesenchyme caused embryonic lethality in the middle of gestation due to multiple developmental defects in the heart, lung, liver and esophagus. Deletion of Foxf1 inhibited mesenchyme proliferation and delayed branching lung morphogenesis. Gene expression profiling of micro-dissected distal lung mesenchyme and ChIP sequencing of fetal lung tissue identified multiple target genes activated by FOXF1, including Wnt2, Wnt11, Wnt5A and Hoxb7. FOXF1 decreased expression of the Wnt inhibitor Wif1 through direct transcriptional repression. Furthermore, using a global Foxf1 knockout mouse line (Foxf1^-/-) we demonstrated that FOXF1-deficiency disrupts the formation of the lung bud in foregut tissue explants. Finally, deletion of Foxf1 from smooth muscle cell lineage (smMHC-Cre Foxf1^-/-) caused hyper-extension of esophagus and trachea, loss of tracheal and esophageal muscle, mispatterning of esophageal epithelium and decreased proliferation of smooth muscle cells. Altogether, FOXF1 promotes lung morphogenesis by regulating mesenchymal-epithelial signaling and stimulating cellular proliferation in fetal lung mesenchyme.

Multiple roles of epithelial heparan sulfate in stomach morphogenesis

Heparan sulfate proteoglycans (HSPGs) have been shown to regulate various developmental processes. However, the function of heparan sulfate (HS) during the development of mammalian stomach has not been characterized yet. Here, we investigate the role of epithelial HS in embryonic stomach by examining mice deficient in the glycosyltransferase gene Ext1 We show that HS exhibits a specific and dynamic expression pattern in mouse embryonic stomach. Depletion of the epithelial HS leads to stomach hypoplasia, with phenotypic differences in the gastric mucosa between the forestomach and hindstomach. In the posterior stomach, HS depletion disrupts glandular stomach patterning and cytodifferentiation via attenuation of Fgf signaling activity. Inhibition of Fgf signaling in vitro recapitulates the patterning defect. Ligand and carbohydrate engagement assay (LACE) reveals a diminished assembly of Fgf10 and Fgfr2b in the mutant. In the anterior stomach, loss of epithelial HS leads to stratification and differentiation defects of the multilayered squamous epithelium, along with reduced Hh and Bmp signaling activity. Our data demonstrate that epithelial HS plays multiple roles in regulating mammalian stomach morphogenesis in a regional-specific manner.

In Vitro Models to Study Human Lung Development, Disease and Homeostasis

The main function of the lung is to support gas exchange, and defects in lung development or diseases affecting the structure and function of the lung can have fatal consequences. Most of what we currently understand about human lung development and disease has come from animal models. However, animal models are not always fully able to recapitulate human lung development and disease, highlighting an area where in vitro models of the human lung can compliment animal models to further understanding of critical developmental and pathological mechanisms. This review will discuss current advances in generating in vitro human lung models using primary human tissue, cell lines, and human pluripotent stem cell derived lung tissue, and will discuss crucial next steps in the field.

α5β1 integrin mediates pulmonary epithelial cyst formation

BACKGROUND

Formation of the epithelial cyst involves the establishment of apical-basolateral polarity through a series of cellular interactions that are in part mediated by the extracellular matrix (ECM). We report that in a three-dimensional multi-cellular self-assembly model of lung development, α5 integrin regulates epithelial cyst formation through organization of soluble fibronectin matrix into insoluble fibrils through a process called fibrillogenesis.

RESULTS

Dissociated murine embryonic lung cells self-assemble into three-dimensional pulmonary bodies that are dependent on α5β1 integrin mediated fibrillogenesis for cell-cell mediated self-assembly: compaction and epithelial cyst formation. Knockdown of α5 integrin resulted in a significant increase in another mediator of fibrillogenesis, αV integrin. Compensatory increased expression of another mediator of fibrillogenesis, αV integrin, was not sufficient to normalize epithelial cyst formation. Loss of α5 integrin-mediated fibrillogenesis perturbed the ability of clustered epithelial cells to establish clear polarity, loss of epithelial cell pyramidal shape, and disrupted apical F-actin-rich deposition. Lack of rich central epithelial localization of F-actin cytoskeleton and Podocalyxin suggests that loss of α5 integrin-mediated fibrillogenesis interferes with the normal cytoskeleton organization that facilitates epithelial cysts polarization.

CONCLUSIONS

We conclude that lung epithelial cyst formation in development is mediated in part by α5β1 integrin dependent fibrillogenesis. Developmental Dynamics 246:475-484, 2016. © 2017 Wiley Periodicals, Inc.

Mediastinal pulmonary artery is associated with greater artery diameter and lingular division volume

Pulmonary vessels have numerous variation and aberrant branching patterns. Mediastinal lingular artery (MLA), the most common aberrant branch, might contribute to greater blood flow to lingular division. Hence, we investigated a correlation between lingular division volume and MLA using three-dimensional CT volumetry. We included 199 consecutive patients who underwent surveillance chest CT to detect possible malignancies in April 2015. We measured lingular division volume and cross-sectional area of lingular arteries using three-dimensional CT volumetry. MLA was identified in 58 cases (29.1%). The MLA group had significantly greater lingular division volume (median ± quartile deviation: 378.3 ± 75.5 mL vs. 330.0 ± 87.5 mL; p = 0.021) and percentage lingular division to left lung volume (19.0 ± 2.62% vs. 16.6 ± 2.39%; p < 0.001) than the non-MLA group. Total cross-sectional area of lingular arteries of the MLA group was significantly larger than that of the non-MLA group (46.1 ± 9.46 vs. 40.2 ± 5.76 mm²; p = 0.003). The total cross-sectional area of the lingular arteries strongly correlated to the percentage of lingular division to left lung volume (r = 0.689, p < 0.001). This is the first report demonstrating a positive correlation between branching pattern of pulmonary artery and lung volume.

Apical Secretion of FSTL1 in the Respiratory Epithelium for Normal Lung Development

Follistatin-like 1 (FSTL1) is a secreted bone morphogenetic protein (BMP) antagonist, and it plays a crucial role in normal lung development. Deletion of Fstl1 leads to postnatal death in mice due to respiratory failure. To further explore the role of FSTL1 in mouse lung development, we created a transgene SFTPC-Fstl1 allele mouse displaying significant epithelial overexpression of Fstl1 in all stages of lung development. However, epithelial overexpression of Fstl1 did not alter lung morphogenesis, epithelial differentiation and lung function. Moreover, we found that FSTL1 function was blocked by the epithelial polarization, which was reflected by the remarkable apical secretion of FSTL1 and the basolateral BMP signaling. Taken together, this study demonstrates that tightly spatial interaction of FSTL1 and BMP signaling plays an essential role in lung development.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

A paracrine network regulates the cross-talk between human lung stem cells and the stroma

The signals that regulate stem cell self-renewal and differentiation in the lung remain elusive. Lung stem cells undergo self-renewal or lineage commitment to replenish tissue, depending on cross-talk with their environment. This environment, also known as the niche, includes mesenchymal and endothelial tissues. Here we define molecular mechanisms involved in the interaction between human lung Lgr6+ stem cells (LSCs) and fibroblasts in a functional microenvironment. We reveal a central role for p38α MAPK in establishing and maintaining such cross-talk, acting in both cell types. In LSCs, p38α induces the expression of SDF-1, which activates the stroma. p38α is essential for fibroblast activation and induction of cytokine expression, in particular TNFα. This paracrine network induces a hierarchical activation leading to the recruitment of endothelium, establishing a functional microenvironment. Disruption of this cross-talk abrogates proper LSC differentiation in vivo and may lead to lung dysfunction and disease.

The human lung contains Lgr6-positive progenitor cells, which have the potential to generate multiple epithelial lineages in cell explant studies. Here, the authors present a role of p38a MAPK signalling in regulating the cross-talk between this lung progenitor cell population and the stromal and endothelial cells comprising their niche microenvironment.

Crosstalk of dynamic functional modules in lung development of rhesus macaques

Lung development follows a complex series of dynamic histogenic events that depend on the fluctuation of gene expression, and the disruption of gene regulation could lead to devastating consequences, such as diseases in adulthood. In order to investigate the mechanism of lung development, we performed RNA sequencing by Illumina HiSeq™ 2000 to measure mRNA expression in lung tissues of nine rhesus macaques spanning from foetuses at gestation of 45 days to postnatal at 7 days. This development period was divided into three developmental stages, including the early stage (45-100 gestational days), the middle stage (137-163 gestational days) and the late stage (after birth at 4-7 days). Firstly, we identified stage-specific genes, based on which we found that the principle biological processes of the early stage were mainly associated with internal growth signalling, while the middle and late stage-specific genes controlled the external stress signalling. Then, we constructed a stage-specific protein-protein interaction (PPI) subnetwork, extracted dynamic modules, and identified crosstalk between modules. Moreover, we found four active pathways that could mediate the crosstalk, including the Notch signalling pathway, cell cycle, NOD-like receptor signalling pathway, and Toll-like receptor signalling pathway. These pathways not only played crucial roles in lung development, but also were implicated in lung diseases. Finally, some important bridgers, such as PSEN2, HSP90AA1 and CASP8, were discovered to explain the potential mechanism of crosstalk. Therefore, our study presents the landscape of gene expression of lung development of rhesus macaques, and provides an extended insight into the lung development mechanism.

Endogenous and Exogenous Stem/Progenitor Cells in the Lung and Their Role in the Pathogenesis and Treatment of Pediatric Lung Disease

The lung is a complex organ with a vast surface area whose main function is to release cellular waste to be exhaled and to replenish the supply of oxygen to the tissues of the body. The conduction of air from the external environment is not without risks, and the lung contains many specialized epithelial cell subtypes that are protecting the lung from foreign material and injury. Specialized cell subtypes are produced during lung development in the fetus as well as postnatally and injury to them due to genetic disease, premature birth, or postnatal environmental injury may lead to devastating disease. Chronic diseases, such as bronchopulmonary dysplasia, cystic fibrosis, and pulmonary arterial hypertension, contribute significantly to morbidity and mortality worldwide, yet successful interventions are often limited. Stem/progenitor cells have emerged as a potentially new preventative or therapeutic option. They are generally defined by the ability to undergo self-renewal and give rise to more differentiated cells. They are important in the early development of embryonic structures and organ differentiation in utero. Postnatally, they function in continued growth, maintenance, and regeneration. Clinically, the immunomodulatory properties of some classes of stem/progenitor cells avoid the major obstacle of immunological rejection seen in organ transplantation and other cell therapies. This review highlights some known human progenitor/stem cells and the most recent advances in stem cell therapies both in vivo and in vitro to prevent and treat pediatric lung disease.

Lung growth factors in the amniotic fluid of normal pregnancies and with congenital diaphragmatic hernia

AIM

Respiratory failure secondary to pulmonary hypoplasia is the main cause of death in congenital diaphragmatic hernia (CDH). Lung growth is regulated by growth factors (GFs), whose imbalances are reported in pathological conditions. We measured amniotic fluid levels of GFs, regulating lung development, in pregnancies with CDH and compared them with normal gestations.

METHODS

Amniotic fluid was collected at amniocentesis and delivery from 4 women carrying fetuses with CDH and 12 with normal pregnancy. GFs were isolated and quantified. Same GFs were measured in lung biopsies collected during autopsy of three newborns dead of CDH.

RESULTS

Impairment expression of lung GFs in the amniotic fluid of CDH pregnancies in comparison with normal was found. Fibroblast growth factor 10 (FGF10), fibroblast growth factor 7, vascular endothelial growth factor and transforming growth factor β (TGFβ) were decreased at amniocentesis, while platelet-derived growth factor (PDGF) increased. While FGF10 and PDGF tended to normalize at delivery, epidermal growth factor increased and TGFβ was still decreased. Same GFs were similarly expressed in both lungs of babies dead of CDH.

CONCLUSION

Anomalies in lung GFs expression of embryos and fetuses with CDH can be detected by measuring their levels in the amniotic fluid during pregnancy. Further investigation would help to correlate prenatal expression of GFs and clinical outcome of babies with CDH after birth.

Regulation of lung development and regeneration by the vascular system

Blood vessels have been described a long time ago as passive circuits providing sufficient blood supply to ensure proper distribution of oxygen and nutrition. Blood vessels are mainly formed during embryonic development and in the early postnatal period. In the adult, blood vessels are quiescent, but can be activated and subsequently induced under pathophysiological conditions, such as ischemia and tumor growth. Surprisingly, recent data have suggested an active function for blood vessels, named angiocrine signaling, releasing trophogens which regulate organ development and organ regeneration including in the pancreas, lung, tumor cells, liver and bone. Lung development is driven by hypoxia as well as an intense endothelial-epithelial interaction, and important mechanisms contributing to these processes have recently been identified. This review aims to summarize recent developments and concepts about embryonic pulmonary vascular development and lung regeneration. We discuss hypoxia-inducible factor HIF-2α and vascular endothelial growth factor VEGF as important mediators in lung development and focus on endothelial-epithelial interactions and angiocrine signaling mechanisms.

Soy Isoflavone Glycitin (4'-Hydroxy-6-Methoxyisoflavone-7-D-Glucoside) Promotes Human Dermal Fibroblast Cell Proliferation and Migration via TGF-β Signaling

Glycitin is a soy isoflavone that exhibits antioxidant, antiallergic, and anti-osteoporosis activities. We investigated the effects of glycitin on dermal fibroblast proliferation and migration. Treatment of primary dermal fibroblasts with glycitin increased cell proliferation and migration. In addition, treatment with 20 μM glycitin for 24 h induced the synthesis of collagen type I and type III at both the mRNA and protein levels. Fibronectin was also increased by 20% after treatment. Matrix metalloproteinase-1 collagenase was decreased in the media after 24-h incubation with glycitin, and the synthesis of transforming growth factor-beta (TGF-β) mRNA increased approximately twofold in cells following glycitin treatment. Phosphorylation of Smad2 and Smad3 increased after 1 h of glycitin treatment, and phosphorylation continued for 24 h. Furthermore, the phosphorylated form of AKT was increased in glycitin-treated cells after 3 h and remained higher for 24 h. Thus, glycitin treatment produces anti-aging effects including increased total collagen in the culture media, decreased elastase, and decreased β-galactosidase. Together, these results indicate that glycitin stimulates TGF-β secretion, and the subsequent autocrine actions of TGF-β induce proliferation of fibroblasts, ultimately protecting skin cells from aging and wrinkling.

Heterochrony and early left-right asymmetry in the development of the cardiorespiratory system of snakes

Snake lungs show a remarkable diversity of organ asymmetries. The right lung is always fully developed, while the left lung is either absent, vestigial, or well-developed (but smaller than the right). A 'tracheal lung' is present in some taxa. These asymmetries are reflected in the pulmonary arteries. Lung asymmetry is known to appear at early stages of development in Thamnophis radix and Natrix natrix. Unfortunately, there is no developmental data on snakes with a well-developed or absent left lung. We examine the adult and developmental morphology of the lung and pulmonary arteries in the snakes Python curtus breitensteini, Pantherophis guttata guttata, Elaphe obsoleta spiloides, Calloselasma rhodostoma and Causus rhombeatus using gross dissection, MicroCT scanning and 3D reconstruction. We find that the right and tracheal lung develop similarly in these species. By contrast, the left lung either: (1) fails to develop; (2) elongates more slowly and aborts early without (2a) or with (2b) subsequent development of faveoli; (3) or develops normally. A right pulmonary artery always develops, but the left develops only if the left lung develops. No pulmonary artery develops in relation to the tracheal lung. We conclude that heterochrony in lung bud development contributes to lung asymmetry in several snake taxa. Secondly, the development of the pulmonary arteries is asymmetric at early stages, possibly because the splanchnic plexus fails to develop when the left lung is reduced. Finally, some changes in the topography of the pulmonary arteries are consequent on ontogenetic displacement of the heart down the body. Our findings show that the left-right asymmetry in the cardiorespiratory system of snakes is expressed early in development and may become phenotypically expressed through heterochronic shifts in growth, and changes in axial relations of organs and vessels. We propose a step-wise model for reduction of the left lung during snake evolution.

Stem cells: a recapitulation of development

Pluripotent stem cells are cells that can differentiate into any tissue from all germ layers and include embryonic stem cells and induced pluripotent cells (iPS). Embryonic stem cells are derived from 8-day blastocysts obtained from unutilized embryos following in vitro fertilization, while iPS is obtained following transfection of dermal fibroblasts with pluripotent genes (sex determining region Y-binding, Kruppel-like factor 4, octamer-binding transcription factor 4 and c-Myc). The major challenge is to differentiate these cells into lung epithelium for therapeutic applications as well as to model lung diseases such as cystic fibrosis. In this review, the developmental pathways of the lung and how these pathways have been recapitulated in vitro to induce differentiation of pluripotent cells to lung epithelium were examined.

Angiogenesis and vascular remodeling in chronic airway diseases

Asthma and chronic obstructive pulmonary disease remain a global health problem, with increasing morbidity and mortality. Despite differences in the causal agents, both diseases exhibit various degrees of inflammatory changes, structural alterations of the airways leading to airflow limitation. The existence of transient disease phenotypes which overlap both diseases and which progressively decline the lung function has complicated the search for an effective therapy. Important characteristics of chronic airway diseases include airway and vascular remodeling, of which the molecular mechanisms are complex and poorly understood. Recently, we and others have shown that airway smooth muscle (ASM) cells are not only structural and contractile components of airways, rather they bear capabilities of producing large number of pro-inflammatory and mitogenic factors. Increase in size and number of blood vessels both inside and outside the smooth muscle layer as well as hyperemia of bronchial vasculature are contributing factors in airway wall remodeling in patients with chronic airway diseases, proposing for the ongoing mechanisms like angiogenesis and vascular dilatation. We believe that vascular changes directly add to the airway narrowing and hyper-responsiveness by exudation and transudation of proinflammatory mediators, cytokines and growth factors; facilitating trafficking of inflammatory cells; causing oedema of the airway wall and promoting ASM accumulation. One of the key regulators of angiogenesis, vascular endothelial growth factor in concerted action with other endothelial mitogens play pivotal role in regulating bronchial angiogenesis. In this review article we address recent advances in pulmonary angiogenesis and remodelling that contribute in the pathogenesis of chronic airway diseases.

Pbx1 activates Fgf10 in the mesenchyme of developing lungs

Insufficiency of surfactants is a core factor in respiratory distress syndrome, which causes apnea and neonatal death, particularly in preterm infants. Surfactant proteins are secreted by alveolar type II cells in the lung epithelium, the differentiation of which is regulated by Fgf10 elaborated by the adjacent mesenchyme. However, the molecular regulation of mesenchymal Fgf10 during lung development has not been fully understood. Here, we show that Pbx1, a homeodomain transcription factor, is required in the lung mesenchyme for the expression of Fgf10. Mouse embryos lacking Pbx1 in the lung mesenchyme show compact terminal saccules and perinatal lethality with failure of postnatal alveolar expansion. Mutant embryos had severely reduced expression of Fgf10 and surfactant genes (Spa, Spb, Spc, and Spd) that are essential for alveolar expansion for gas exchange at birth. Molecularly, Pbx1 directly binds to the Fgf10 promoter and cooperates with Meis and Hox proteins to transcriptionally activate Fgf10. Our results thus show how Pbx1 controls Fgf10 in the developing lung.

Impaired elastin deposition in Fstl1-/- lung allograft under the renal capsule

Lung alveolar development in late gestation is a process important to postnatal survival. Follistatin-like 1 (Fstl1) is a matricellular protein of the Bmp antagonist class, which is involved in the differentiation/maturation of alveolar epithelial cells during saccular stage of lung development. This study investigates the role of Fstl1 on elastin deposition in mesenchyme and subsequent secondary septation in the late gestation stage of terminal saccular formation. To this aim, we modified the renal capsule allograft model for lung organ culture by grafting diced E15.5 distal lung underneath the renal capsule of syngeneic host and cultured up to 7 days. The saccular development of the diced lung allografts, as indicated by the morphology, epithelial and vascular developments, occurred in a manner similar to that in utero. Fstl1 deficiency caused atelectatic phenotype companied by impaired epithelial differentiation in D3 Fstl1^−/− lung allografts, which is similar to that of E18.5 Fstl1^−/− lungs, supporting the role of Fstl1 during saccular stage. Inhibition of Bmp signaling by intraperitoneal injection of dorsomorphin in the host mice rescued the pulmonary atelectasis of D3 Fstl1^−/− allografts. Furthermore, a marked reduction in elastin expression and deposition was observed in walls of air sacs of E18.5 Fstl1^−/− lungs and at the tips of the developing alveolar septae of D7 Fstl1^−/− allografts. Thus, in addition to its role on alveolar epithelium, Fstl1 is crucial for elastin expression and deposition in mesenchyme during lung alveologenesis. Our data demonstrates that the modified renal capsule allograft model for lung organ culture is a robust and efficient technique to increase our understanding of saccular stage of lung development.

Patchy deletion of Bmpr1a potentiates proximal pulmonary artery remodeling in mice exposed to chronic hypoxia

Reduced vascular expression of bone morphogenetic protein type IA receptor (Bmpr1a) has been found in patients with pulmonary arterial hypertension. Our previous studies in mice with patchy deletion of Bmpr1a in vascular smooth muscle cells and cardiac myocytes showed decreased distal vascular remodeling despite a similar severity of hypoxic pulmonary hypertension (HPH). We speculate increased stiffness from ectopic deposition of collagen in proximal pulmonary arteries might account for HPH. Pulsatile pressure-flow relationships were measured in isolated, ventilated, perfused lungs of SM22α;TRE-Cre;R26R;Bmpr1a(flox/flox) (KO) mice and wild-type littermates, following 21 days (hypoxia) and 0 days (control) of chronic hypoxia. Pulmonary vascular impedance, which yields insight into proximal and distal arterial remodeling, was calculated. Reduced Bmpr1a expression had no effect on input impedance Z(0) (P = 0.52) or characteristic impedance Z(C) (P = 0.18) under control conditions; it also had no effect on the decrease in Z(0) via acute rho kinase inhibition. However, following chronic hypoxia, reduced Bmpr1a expression increased Z(C) (P < 0.001) without affecting Z(0) (P = 0.72). These results demonstrate that Bmpr1a deficiency does not significantly alter the hemodynamic function of the distal vasculature or its response to chronic hypoxia but larger, more proximal arteries are affected. In particular, reduced Bmpr1a expression likely decreased dilatation and increased stiffening in response to hypoxia, probably by collagen accumulation. Increased PA stiffness can have a significant impact on right ventricular function. This study illustrates for the first time how proximal pulmonary artery changes in the absence of distal pulmonary artery changes contribute to pulmonary arterial hypertension.

Adult lung stem cells and their contribution to lung tumourigenesis

The isolation and characterization of lung stem and progenitor cells represent an important step towards the understanding of lung repair after injury, lung disease pathogenesis and the identification of the target cells of transformation in lung carcinogenesis. Different approaches using prospective isolation of progenitor cells by flow cytometry or lineage-tracing experiments in mouse models of lung injury have led to the identification of distinct progenitor subpopulations in different morphological regions of the adult lung. Genetically defined mouse models of lung cancer are offering new perspectives on the cells of origin of different subtypes of lung cancer. These mouse models pave the way to further investigate human lung progenitor cells at the origin of lung cancers, as well as to define the nature of the lung cancer stem cells. It will be critical to establish the link between oncogenic driver mutations recently discovered in lung cancers, target cells of transformation and subtypes of lung cancers to enable better stratification of patients for improved therapeutic strategies.

Wt1-expressing progenitors contribute to multiple tissues in the developing lung

Lungs develop from paired endodermal outgrowths surrounded by a mesodermal mesenchyme. Part of this mesenchyme arises from epithelial-mesenchymal transition of the mesothelium that lines the pulmonary buds. Previous studies have shown that this mesothelium-derived mesenchyme contributes to the smooth muscle of the pulmonary vessels, but its significance for lung morphogenesis and its developmental fate are still little known. We have studied this issue using the transgenic mouse model mWt1/IRES/GFP-Cre (Wt1cre) crossed with the Rosa26R-EYFP reporter mouse. In the developing lungs, Wt1, the Wilms' tumor suppressor gene, is specifically expressed in the embryonic mesothelium. In the embryos obtained from the crossbreeding, the Wt1-expressing cell lineage produces the yellow fluorescent protein (YFP), allowing for colocalization with differentiation markers. Wt1cre-YFP cells were very abundant from the origin of the lung buds to postnatal stages, contributing significantly to pulmonary endothelial and smooth muscle cells, bronchial musculature, tracheal and bronchial cartilage, as well as CD34⁺ fibroblast-like interstitial cells. Thus Wt1cre-YFP mesenchymal cells show the very same differentiation potential as the splanchnopleural mesenchyme surrounding the lung buds. FSP1⁺ fibroblast-like cells were always YFP⁻; they expressed the common leukocyte antigen CD45 and were apparently recruited from circulating progenitors. We have also found defects in pulmonary development in Wt1-/- embryos, which showed abnormally fused lung lobes, round-shaped and reduced pleural cavities, and diaphragmatic hernia. Our results suggest a novel role for the embryonic mesothelium-derived cells in lung morphogenesis and involve the Wilms' tumor suppressor gene in the development of this organ.

Generation of Lung Epithelium from Pluripotent Stem Cells

The understanding of key processes and signaling mechanisms in lung development has been mainly demonstrated through gain and loss of function studies in mice, while human lung development remains largely unexplored due to inaccessibility. Several recent reports have exploited the identification of key signaling mechanisms that regulate lineage commitment and restriction in mouse lung development, to direct differentiation of both mouse and human pluripotent stem cells towards lung epithelial cells. In this review, we discuss the recent advances in the generation of respiratory epithelia from pluripotent stem cells and the potential of these engineered cells for novel scientific discoveries in lung diseases and future translation into regenerative therapies.

Abnormal control of lung branching in experimental esophageal atresia

PURPOSE

Esophageal atresia and tracheo-esophageal fistula (EA-TEF) result from abnormal division of the foregut into esophagus and trachea thus, it may influence airway branching and lung development. The present study examined lung morphogenesis in fetuses with EA-TEF focusing in the expression of FGF10 and its receptor FGFR2 IIIb.

METHODS

Pregnant rats received either 1.75 mg/kg i.p. adriamycin or vehicle on E7, E8 and E9. Embryos were recovered at E15, E18 and E21 and lungs processed for immunohistochemistry and RT-PCR. Three groups were studied: control, adriamycin-exposed with EA-TEF, and adriamycin-exposed without EA-TEF. Comparisons were performed with Mann-Whitney or t tests (significance level, 5 %).

RESULTS

Lung weight at E15 and E18 were significantly lower in adriaEA fetuses in which the relative mRNA levels of FGF10 were significantly higher. These differences disappeared near term. The receptor FGFR2 IIIb messenger was only significantly increased in adria noEA fetuses at E15. Immunohistochemical study was consistent with these findings.

CONCLUSIONS

Abnormal expression of FGF10 during earlier stages of development, when the lungs are smaller than controls, suggests a compensatory response aimed at "catching up" delayed tracheobronchial branching. Whether similar changes take place in the human condition and influence respiratory physiology remain to be determined.

Keratinocyte growth factor and dexamethasone plus elevated cAMP levels synergistically support pluripotent stem cell differentiation into alveolar epithelial type II cells

Alveolar epithelial type II (ATII)-like cells can be generated from murine embryonic stem cells (ESCs), although to date, no robust protocols applying specific differentiation factors are established. We hypothesized that the keratinocyte growth factor (KGF), an important mediator of lung organogenesis and primary ATII cell maturation and proliferation, together with dexamethasone, 8-bromoadenosine-cAMP, and isobutylmethylxanthine (DCI), which induce maturation of primary fetal ATII cells, also support the alveolar differentiation of murine ESCs. Here we demonstrate that the above stimuli synergistically potentiate the alveolar differentiation of ESCs as indicated by increased expression of the surfactant proteins (SP-) C and SP-B. This effect is most profound if KGF is supplied not only in the late stage, but at least also during the intermediate stage of differentiation. Our results indicate that KGF most likely does not enhance the generation of (mes)endodermal or NK2 homeobox 1 (Nkx2.1) expressing progenitor cells but rather, supported by DCI, accelerates further differentiation/maturation of respiratory progeny in the intermediate phase and maturation/proliferation of emerging ATII cells in the late stage of differentiation. Ultrastructural analyses confirmed the presence of ATII-like cells with intracellular composite and lamellar bodies. Finally, induced pluripotent stem cells (iPSCs) were generated from transgenic mice with ATII cell-specific lacZ reporter expression. Again, KGF and DCI synergistically increased SP-C and SP-B expression in iPSC cultures, and lacZ expressing ATII-like cells developed. In conclusion, ATII cell-specific reporter expression enabled the first reliable proof for the generation of murine iPSC-derived ATII cells. In addition, we have shown KGF and DCI to synergistically support the generation of ATII-like cells from ESCs and iPSCs. Combined application of these factors will facilitate more efficient generation of stem cell-derived ATII cells for future basic research and potential therapeutic application.

Deletion of STK40 protein in mice causes respiratory failure and death at birth

STK40 is a putative serine/threonine kinase and was shown to induce extraembryonic endoderm differentiation from mouse embryonic stem cells. However, little is known about its physiological function in vivo. Here, we generate Stk40 knock-out mice and demonstrate that loss of the Stk40 gene causes neonatal lethality at birth. Further examination reveals that the respiratory distress and atelectasis occur in the homozygous mutants. The maturation of lung and alveolar epithelium is delayed in the mutant, as indicated by narrowed air spaces, thickened interstitial septa, and increased glycogen content in the lungs of Stk40(-/-) mice. The reduction in levels of T1-α, SP-B, and SP-C indicates delayed maturation of both type I and type II respiratory epithelial cells in Stk40(-/-) lungs. Moreover, Stk40 is found to be most highly expressed in lungs of both fetal and adult mice among all organs tested. Mechanistically, a genome-wide RNA microarray analysis reveals significantly altered expression of multiple genes known to participate in lung development. The expression of some genes involved in lipid metabolism, immune response, and glycogen metabolism is also disrupted in the lung of Stk40(-/-) mice. Protein affinity purification identifies RCN2, an activator of ERK/MAPK signaling, as an STK40-associated protein. Consistently, Stk40 deficiency attenuates the ERK/MAPK activation, and inhibition of ERK/MAPK activities reduces surfactant protein gene expression in lung epithelial cells. Collectively, this study uncovers an important role of STK40 for lung maturation and neonatal survival. STK40 may associate with RCN2 to activate ERK/MAPK signaling and control the expression of multiple key regulators of lung development.