Establishment of smooth muscle and cartilage juxtaposition in the developing mouse upper airways

The T-Type Calcium Channel CACNA1H is Required for Smooth Muscle Cytoskeletal Organization During Tracheal Tubulogenesis

Abnormalities of tracheal smooth muscle (SM) formation are associated with several clinical disorders including tracheal stenosis and tracheomalacia. However, the cellular and molecular mechanisms underlying tracheal SM formation remain poorly understood. Here, it is shown that the T-type calcium channel CACNA1H is a novel regulator of tracheal SM formation and contraction. Cacna1h in an ethylnitrosourea forward genetic screen for regulators of respiratory disease using the mouse as a model is identified. Cacna1h mutants exhibit tracheal stenosis, disorganized SM and compromised tracheal contraction. CACNA1H is essential to maintain actin polymerization, which is required for tracheal SM organization and tube formation. This process appears to be partially mediated through activation of the actin regulator RhoA, as pharmacological increase of RhoA activity ameliorates the Cacna1h-mutant trachea phenotypes. Analysis of human tracheal tissues indicates that a decrease in CACNA1H protein levels is associated with congenital tracheostenosis. These results provide insight into the role for the T-type calcium channel in cytoskeletal organization and SM formation during tracheal tube formation and suggest novel targets for congenital tracheostenosis intervention.

Wnt signaling regulates ion channel expression to promote smooth muscle and cartilage formation in developing mouse trachea

Ion channels play critical roles in the physiology and function of the nervous system and contractile tissue; however, their role in noncontractile tissue and embryonic development has yet to be understood. Tracheobronchomalacia (TBM) and complete tracheal rings (CTR) are disorders affecting the muscle and cartilage of the trachea and bronchi, whose etiology remains poorly understood. We demonstrated that trachealis muscle organization and polarity are disrupted after epithelial ablation of Wntless (Wls), a cargo receptor critical for the Wnt signaling pathway, in developing trachea. The phenotype resembles the anomalous trachealis muscle observed after deletion of ion channel encoding genes in developing mouse trachea. We sought to investigate whether and how the deletion of Wls affects ion channels during tracheal development. We hypothesize that Wnt signaling influences the expression of ion channels to promote trachealis muscle cell assembly and patterning. Deleting Wls in developing trachea causes differential regulation of genes mediating actin binding, cytoskeleton organization, and potassium ion channel activity. Wnt signaling regulates the expression of Kcnj13, Kcnd3, Kcnj8, and Abcc9 as demonstrated by in vitro studies and in vivo analysis in Wnt5a and β-catenin-deficient tracheas. Pharmacological inhibition of potassium ion channels and Wnt signaling impaired contractility of developing trachealis smooth muscle and formation of cartilaginous mesenchymal condensation. Thus, in mice, epithelial-induced Wnt/β-catenin signaling mediates trachealis muscle and cartilage development via modulation of ion channel expression, promoting trachealis muscle architecture, contractility, and cartilaginous extracellular matrix. In turn, ion channel activity may influence tracheal morphogenesis underlying TBM and CTR.NEW & NOTEWORTHY Ion channels play critical roles in the physiology and function of the nervous system and contractile tissue; however, their role in noncontractile tissue and embryonic development has yet to be understood. In this study, we focused on the role of ion channels in the differentiation and patterning of the large airways of the developing respiratory tract. We identify a mechanism by which Wnt-beta-catenin signaling controls levels of ion channel-encoding genes to promote tracheal differentiation.

Bronchial epithelial and airway smooth muscle cell interactions in health and disease

Chronic pulmonary diseases such as asthma, COPD, and Idiopathic pulmonary fibrosis are significant causes of mortality and morbidity worldwide. Currently, there is no radical treatment for many chronic pulmonary diseases, and the treatment options focus on relieving the symptoms and improving lung function. Therefore, efficient therapeutic agents are highly needed. Bronchial epithelial cells and airway smooth muscle cells and their crosstalk play a significant role in the pathogenesis of these diseases. Thus, targeting the interactions of these two cell types could open the door to a new generation of effective therapeutic options. However, the studies on how these two cell types interact and how their crosstalk adds up to respiratory diseases are not well established. With the rise of modern research tools and technology, such as lab-on-a-chip, organoids, co-culture techniques, and advanced immunofluorescence imaging, a substantial degree of evidence about these cell interactions emerged. Hence, this contribution aims to summarize the growing evidence of bronchial epithelial cells and airway smooth muscle cells crosstalk under normal and pathophysiological conditions. The review first discusses the impact of airway smooth muscle cells on the epithelium in inflammatory settings. Later, it examines the role of airway smooth muscle cells in the early development of bronchial epithelial cells and their recovery after injury. Then, it deliberates the effects of both healthy and stressed epithelial cells on airway smooth muscle cells, taking into account three themes; contraction, migration, and proliferation.

Wnt signaling regulates ion channel expression to promote smooth muscle and cartilage formation in developing mouse trachea

Ion channels play critical roles in the physiology and function of the nervous system and contractile tissue; however, their role in non-contractile tissue and embryonic development has yet to be understood. Tracheobronchomalacia (TBM) and complete tracheal rings (CTR) are disorders affecting the muscle and cartilage of the trachea and bronchi, whose etiology remains poorly understood. We demonstrated that trachealis muscle organization and polarity are disrupted after epithelial ablation of Wls, a cargo receptor critical for the Wnt signaling pathway, in developing trachea. The phenotype resembles the anomalous trachealis muscle observed after deletion of ion channel encoding genes in developing mouse trachea. We sought to investigate whether and how the deletion of Wls affects ion channels during tracheal development. We hypothesize that Wnt signaling influences the expression of ion channels to promote trachealis muscle cell assembly and patterning. Deleting Wls in developing trachea causes differential regulation of genes mediating actin binding, cytoskeleton organization, and potassium ion channel activity. Wnt signaling regulated expression of Kcnj13, Kcnd3, Kcnj8, and Abcc9 as demonstrated by in vitro studies and in vivo analysis in Wnt5a and β-catenin deficient tracheas. Pharmacological inhibition of potassium ion channels and Wnt signaling impaired contractility of developing trachealis smooth muscle and formation of cartilaginous mesenchymal condensation. Thus, in mice, epithelial-induced Wnt/β-catenin signaling mediates trachealis muscle and cartilage development via modulation of ion channel expression, promoting trachealis muscle architecture, contractility, and cartilaginous extracellular matrix. In turn, ion channel activity may influence tracheal morphogenesis underlying TBM and CTR.

Systematic Analysis of Smooth Muscle and Cartilage Ring Formation during Mouse Tracheal Tubulogenesis

The trachea tube is the exclusive route to allow gas exchange between the external environment and the lungs. Recent studies have shown the critical role of mesenchymal cells in tracheal tubulogenesis. Improved methods for studying the dynamics of the tracheal mesenchyme development are needed to investigate the cellular and molecular mechanisms during tracheal tubulogenesis. Here, we describe a detailed protocol for a systematic analysis of tracheal tube development to enable observing tracheal smooth muscle (SM) and cartilage ring formation. We describe immunostaining, confocal and stereomicroscopy imaging, and quantitative methods to study the process of tracheal SM and cartilage ring development, including SM cell alignment, polarization, and changes in cell shape as well as mesenchymal condensation. The technologies and approaches described here not only improve analysis of the patterning of the developing trachea but also help uncover the mechanisms underlying airway disease. This protocol also provides a useful technique to analyze cell organization, polarity, and nuclear shape in other organ systems.

The upper airway parameters: the potential diagnostic clues for congenital intrathoracic lesions

BACKGROUND

The diagnosis of congenital intrathoracic lesions still has limitations. The airway development was influenced by intrathoracic factors. Whether the diagnostic value of the upper airway parameters in congenital intrathoracic lesions has not been confirmed.

OBJECTIVES

We aimed to compare fetal upper airway parameters between normal fetuses and fetuses with intrathoracic lesions, and we tried to verify its diagnostic value in intrathoracic lesions.

METHODS

This was an observational case-control study. In the control group, 77 women were screened at 20-24 weeks' gestational age, 23 were screened at 24-28 weeks' gestational age, and 27 were screened at 28-34 weeks' gestational age. In the case group, 41 cases were enrolled (6 cases of intrathoracic bronchopulmonary sequestration, 22 of congenital pulmonary airway malformations, and 13 of congenital diaphragmatic hernia). Fetal upper airway parameters (tracheal width, the narrowest lumen width, and width of the subglottic cavity and laryngeal vestibule) were measured using ultrasound equipment. The correlations between fetal upper airway parameters and gestational age, and the differences in fetal upper airway parameters between cases and controls, were analyzed. The standardized airway paraments were acquired, and their potential diagnostic value for congenital intrathoracic lesions were analyzed.

RESULTS

The fetal upper airway parameters of both groups were positively correlated with the gestational age: The control group, tracheal width (R² = 0.569, p < 0.001), narrowest lumen width (R² = 0.429, p < 0.001), subglottic cavity width (R² = 0.551, p < 0.001), laryngeal vestibule width (R² = 0.349, p < 0.001). The case group (tracheal width R² = 0.474, p < 0.001) narrowest lumen width (R² = 0.425, p < 0.001), subglottic cavity width (R² = 0.623, p < 0.001), laryngeal vestibule width (R² = 0.347, p < 0.001). Fetal upper airway parameters of the cases group were smaller than those of the controls group. The tracheal width in fetuses with congenital diaphragmatic hernia was the smallest among the other case groups studied. The standardized tracheal width has the best diagnostic value for congenital intrathoracic lesions in the standardized airway paraments (the area under the ROC curve was 0.894), and has a high diagnostic value for congenital pulmonary airway malformations and congenital diaphragmatic hernia (the area under the ROC curve was 0.911 and 0.992, respectively).

CONCLUSION

Fetal upper airway parameters differ between normal fetuses and fetuses with intrathoracic lesions, and might offer potential diagnostic clues for congenital intrathoracic lesions.

Airway basal cells show regionally distinct potential to undergo metaplastic differentiation

Basal cells are multipotent stem cells of a variety of organs, including the respiratory tract, where they are major components of the airway epithelium. However, it remains unclear how diverse basal cells are and how distinct subpopulations respond to airway challenges. Using single cell RNA-sequencing and functional approaches, we report a significant and previously underappreciated degree of heterogeneity in the basal cell pool, leading to identification of six subpopulations in the adult murine trachea. Among these, we found two major subpopulations, collectively comprising the most uncommitted of all the pools, but with distinct gene expression signatures. Notably, these occupy distinct ventral and dorsal tracheal niches and differ in their ability to self-renew and initiate a program of differentiation in response to environmental perturbations in primary cultures and in mouse injury models in vivo. We found that such heterogeneity is acquired prenatally, when the basal cell pool and local niches are still being established, and depends on the integrity of these niches, as supported by the altered basal cell phenotype of tracheal cartilage-deficient mouse mutants. Finally, we show that features that distinguish these progenitor subpopulations in murine airways are conserved in humans. Together, the data provide novel insights into the origin and impact of basal cell heterogeneity on the establishment of regionally distinct responses of the airway epithelium during injury-repair and in disease conditions.

Tracheal Ring Formation

The trachea is a long tube that enables air passage between the larynx and the bronchi. C-shaped cartilage rings on the ventral side stabilise the structure. On its esophagus-facing dorsal side, deformable smooth muscle facilitates the passage of food in the esophagus. While the symmetry break along the dorsal-ventral axis is well understood, the molecular mechanism that results in the periodic Sox9 expression pattern that translates into the cartilage rings has remained elusive. Here, we review the molecular regulatory interactions that have been elucidated, and discuss possible patterning mechanisms. Understanding the principles of self-organisation is important, both to define biomedical interventions and to enable tissue engineering.

Malfunction of airway basal stem cells plays a crucial role in pathophysiology of tracheobronchopathia osteoplastica

Understanding disease-associated stem cell abnormality has major clinical implications for prevention and treatment of human disorders, as well as for regenerative medicine. Here we report a multifaceted study on airway epithelial stem cells in Tracheobronchopathia Osteochondroplastica (TO), an under-detected tracheobronchial disorder of unknown etiology and lack of specific treatment. Epithelial squamous metaplasia and heterotopic bone formation with abnormal cartilage proliferation and calcium deposits are key pathological hallmarks of this disorder, but it is unknown whether they are coincident or share certain pathogenic mechanisms in common. By functional evaluation and genome-wide profiling at both transcriptional and epigenetic levels, we reveal a role of airway basal cells in TO progression by acting as a repository of inflammatory and TGFβ-BMP signals, which contributes to both epithelial metaplasia and mesenchymal osteo-chondrogenesis via extracellular signaling and matrix remodeling. Restoration of microenvironment by cell correction or local pathway intervention may provide therapeutic benefits.

Tracheobronchopathia osteoplastica (TO), is an underreported affliction characterized by squamous metaplasia and heterotopic bone formation in trachea and bronchi. Here the authors apply functional, as well as genome-wide transcriptional and epigenetic profiling to identify airway basal cells dysfunction underlying TO.

An essential function for autocrine hedgehog signaling in epithelial proliferation and differentiation in the trachea

The tracheal epithelium is a primary target for pulmonary diseases as it provides a conduit for air flow between the environment and the lung lobes. The cellular and molecular mechanisms underlying airway epithelial cell proliferation and differentiation remain poorly understood. Hedgehog (HH) signaling orchestrates communication between epithelial and mesenchymal cells in the lung, where it modulates stromal cell proliferation, differentiation and signaling back to the epithelium. Here, we reveal a previously unreported autocrine function of HH signaling in airway epithelial cells. Epithelial cell depletion of the ligand sonic hedgehog (SHH) or its effector smoothened (SMO) causes defects in both epithelial cell proliferation and differentiation. In cultured primary human airway epithelial cells, HH signaling inhibition also hampers cell proliferation and differentiation. Epithelial HH function is mediated, at least in part, through transcriptional activation, as HH signaling inhibition leads to downregulation of cell type-specific transcription factor genes in both the mouse trachea and human airway epithelial cells. These results provide new insights into the role of HH signaling in epithelial cell proliferation and differentiation during airway development.

Summary: A conserved autocrine role for HH signaling in tracheal epithelial cell proliferation and differentiation is revealed, suggesting potential new interventions for airway epithelial proliferation and differentiation defects.

BMP4 and Wnt signaling interact to promote mouse tracheal mesenchyme morphogenesis

Tracheobronchomalacia and complete tracheal rings are congenital malformations of the trachea associated with morbidity and mortality for which the etiology remains poorly understood. Epithelial expression of Wls (a cargo receptor mediating Wnt ligand secretion) by tracheal cells is essential for patterning the embryonic mouse trachea's cartilage and muscle. RNA sequencing indicated that Wls differentially modulated the expression of BMP signaling molecules. We tested whether BMP signaling, induced by epithelial Wnt ligands, mediates cartilage formation. Deletion of Bmp4 from respiratory tract mesenchyme impaired tracheal cartilage formation that was replaced by ectopic smooth muscle, recapitulating the phenotype observed after epithelial deletion of Wls in the embryonic trachea. Ectopic muscle was caused in part by anomalous differentiation and proliferation of smooth muscle progenitors rather than tracheal cartilage progenitors. Mesenchymal deletion of Bmp4 impaired expression of Wnt/β-catenin target genes, including targets of WNT signaling: Notum and Axin2. In vitro, recombinant (r)BMP4 rescued the expression of Notum in Bmp4-deficient tracheal mesenchymal cells and induced Notum promoter activity via SMAD1/5. RNA sequencing of Bmp4-deficient tracheas identified genes essential for chondrogenesis and muscle development coregulated by BMP and WNT signaling. During tracheal morphogenesis, WNT signaling induces Bmp4 in mesenchymal progenitors to promote cartilage differentiation and restrict trachealis muscle. In turn, Bmp4 differentially regulates the expression of Wnt/β-catenin targets to attenuate mesenchymal WNT signaling and to further support chondrogenesis.

Live imaging approach of dynamic multicellular responses in ERK signaling during vertebrate tissue development

The chemical and mechanical responses of cells via the exchange of information during growth and development result in the formation of biological tissues. Information processing within the cells through the signaling pathways and networks inherent to the constituent cells has been well-studied. However, the cell signaling mechanisms responsible for generating dynamic multicellular responses in developing tissues remain unclear. Here, I review the dynamic multicellular response systems during the development and growth of vertebrate tissues based on the extracellular signal-regulated kinase (ERK) pathway. First, an overview of the function of the ERK signaling network in cells is provided, followed by descriptions of biosensors essential for live imaging of the quantification of ERK activity in tissues. Then adducing four examples, I highlight the contribution of live imaging techniques for studying the involvement of spatio-temporal patterns of ERK activity change in tissue development and growth. In addition, theoretical implications of ERK signaling are also discussed from the viewpoint of dynamic systems. This review might help in understanding ERK-mediated dynamic multicellular responses and tissue morphogenesis.

A census of the lung: CellCards from LungMAP

The human lung plays vital roles in respiration, host defense, and basic physiology. Recent technological advancements such as single-cell RNA sequencing and genetic lineage tracing have revealed novel cell types and enriched functional properties of existing cell types in lung. The time has come to take a new census. Initiated by members of the NHLBI-funded LungMAP Consortium and aided by experts in the lung biology community, we synthesized current data into a comprehensive and practical cellular census of the lung. Identities of cell types in the normal lung are captured in individual cell cards with delineation of function, markers, developmental lineages, heterogeneity, regenerative potential, disease links, and key experimental tools. This publication will serve as the starting point of a live, up-to-date guide for lung research at https://www.lungmap.net/cell-cards/. We hope that Lung CellCards will promote the community-wide effort to establish, maintain, and restore respiratory health.

E3 ubiquitin ligase FBXW7 balances airway cell fates

The airway epithelium is composed of multiple cell types each with designated roles. A stereotyped ratio of these cells is essential for proper airway function. Imbalance of airway cell types underlies many lung diseases, including chronic obstructive pulmonary disease (COPD) and asthma. While a number of signals and transcription factors have been implicated in airway cell specification, how cell numbers are coordinated, especially at the protein level is poorly understood. Here we show that in the mouse trachea which contain epithelial cell types similar to human airway, epithelium-specific inactivation of Fbxw7, which encodes an E3 ubiquitin ligase, led to reduced club and ciliated cells, increased goblet cells, and ectopic P63-negative, Keratin5-positive transitory basal cells in the luminal layer. The protein levels of FBXW7 targets including NOTCH1, KLF5 and TGIF were increased. Inactivation of either Notch1, Klf5 but not Tgif genes in the mutant background led to attenuation of selected aspects of the phenotypes, suggesting that FBXW7 acts through different targets to control different cell fates. These findings demonstrate that protein-level regulation by the ubiquitin proteasome system is critical for balancing airway cell fates.

Molecular crosstalk in tracheal development and its recurrence in adult tissue regeneration

The trachea is a rigid air duct with some mobility, which comprises the upper region of the respiratory tract and delivers inhaled air to alveoli for gas exchange. During development, the tracheal primordium is first established at the ventral anterior foregut by interactions between the epithelium and mesenchyme through various signaling pathways, such as Wnt, Bmp, retinoic acid, Shh, and Fgf, and then segregates from digestive organs. Abnormalities in this crosstalk result in lethal congenital diseases, such as tracheal agenesis. Interestingly, these molecular mechanisms also play roles in tissue regeneration in adulthood, although it remains less understood compared with their roles in embryonic development. In this review, we discuss cellular and molecular mechanisms of trachea development that regulate the morphogenesis of this simple tubular structure and identities of individual differentiated cells. We also discuss how the facultative regeneration capacity of the epithelium is established during development and maintained in adulthood.

Dorso-ventral heterogeneity in tracheal basal stem cells

The tracheal basal cells (BCs) function as stem cells to maintain the epithelium in steady state and repair it after injury. The airway is surrounded by cartilage ventrolaterally and smooth muscle dorsally. Lineage tracing using Krt5-CreER shows dorsal BCs produce more, larger, clones than ventral BCs. Large clones were found between cartilage and smooth muscle where subpopulation of dorsal BCs exists. Three-dimensional organoid culture of BCs demonstrated that dorsal BCs show higher colony forming efficacy to ventral BCs. Gene ontology analysis revealed that genes expressed in dorsal BCs are enriched in wound healing while ventral BCs are enriched in response to external stimulus and immune response. Significantly, ventral BCs express Myostatin, which inhibits the growth of smooth muscle cells, and HGF, which facilitates cartilage repair. The results support the hypothesis that BCs from the dorso-ventral airways have intrinsic molecular and behavioural differences relevant to their in vivo function.

Summary: Spatial difference of tracheal epithelium, especially focused on the heterogeneity of basal stem cells, is elucidated by lineage tracing in vivo, histological analysis, tracheosphere culture, and gene ontology analysis.

SOX21 modulates SOX2-initiated differentiation of epithelial cells in the extrapulmonary airways

SOX2 expression levels are crucial for the balance between maintenance and differentiation of airway progenitor cells during development and regeneration. Here, we describe patterning of the mouse proximal airway epithelium by SOX21, which coincides with high levels of SOX2 during development. Airway progenitor cells in this SOX2+/SOX21+ zone show differentiation to basal cells, specifying cells for the extrapulmonary airways. Loss of SOX21 showed an increased differentiation of SOX2+ progenitor cells to basal and ciliated cells during mouse lung development. We propose a mechanism where SOX21 inhibits differentiation of airway progenitors by antagonizing SOX2-induced expression of specific genes involved in airway differentiation. Additionally, in the adult tracheal epithelium, SOX21 inhibits basal to ciliated cell differentiation. This suppressing function of SOX21 on differentiation contrasts SOX2, which mainly drives differentiation of epithelial cells during development and regeneration after injury. Furthermore, using human fetal lung organoids and adult bronchial epithelial cells, we show that SOX2+/SOX21+ regionalization is conserved. Lastly, we show that the interplay between SOX2 and SOX21 is context and concentration dependent leading to regulation of differentiation of the airway epithelium.

Mammalian tracheal development and reconstruction: insights from in vivo and in vitro studies

The trachea delivers inhaled air into the lungs for gas exchange. Anomalies in tracheal development can result in life-threatening malformations, such as tracheoesophageal fistula and tracheomalacia. Given the limitations of current therapeutic approaches, development of technologies for the reconstitution of a three-dimensional trachea from stem cells is urgently required. Recently, single-cell sequencing technologies and quantitative analyses from cell to tissue scale have been employed to decipher the cellular basis of tracheal morphogenesis. In this Review, recent advances in mammalian tracheal development and the generation of tracheal tissues from pluripotent stem cells are summarized.

The lower respiratory airway wall in children in health and disease

Alone or in association with other lung or thorax component disorders, the airway wall (AWW) remains one of the most frequently involved elements in paediatric lung diseases. A myriad of AWW disorders will present with similar symptomatology. It is thus important for the clinician to reappraise the normal development and structure of the AWW to better understand the underlying disease patterns. We herein provide an overview of the structure of the AWW and a description of its development from the fetal period to adulthood. We also detail the most common AWW changes observed in several acute and chronic respiratory disorders as well as after cigarette smoke or chronic pollution exposure. We then describe the relationship between the AWW structure and lung function. In addition, we present the different ways of investigating the AWW structure, from biopsies and histological analyses to the most recent noninvasive airway (AW) imaging techniques. Understanding the pathophysiological processes involved in an individual patient will lead to the judicious choice of nonspecific or specific personalised treatments, in order to prevent irreversible AW damage.

Congenital tracheal defects: embryonic development and animal models

Tracheal anomalies are potentially catastrophic congenital defects. As a newborn begins to breathe, the trachea needs to maintain an appropriate balance of elasticity and rigidity. If the tracheal cartilages are disorganized or structurally weak, the airways can collapse, obstructing breathing. Cartilage rings that are too small or too rigid can also obstruct breathing. These anomalies are frequently associated with craniofacial syndromes, and, despite the importance, are poorly understood. In this review, we summarize the spectrum of pathological phenotypes of the trachea and correlate them with the molecular events uncovered in mouse models.

The biomechanical basis of biased epithelial tube elongation in lung and kidney development

During lung development, epithelial branches expand preferentially in a longitudinal direction. This bias in outgrowth has been linked to a bias in cell shape and in the cell division plane. How this bias arises is unknown. Here, we show that biased epithelial outgrowth occurs independent of the surrounding mesenchyme, of preferential turnover of the extracellular matrix at the bud tips and of FGF signalling. There is also no evidence for actin-rich filopodia at the bud tips. Rather, we find epithelial tubes to be collapsed during early lung and kidney development, and we observe fluid flow in the narrow tubes. By simulating the measured fluid flow inside segmented narrow epithelial tubes, we show that the shear stress levels on the apical surface are sufficient to explain the reported bias in cell shape and outgrowth. We use a cell-based vertex model to confirm that apical shear forces, unlike constricting forces, can give rise to both the observed bias in cell shapes and tube elongation. We conclude that shear stress may be a more general driver of biased tube elongation beyond its established role in angiogenesis. This article has an associated 'The people behind the papers' interview.

The control of lung branching morphogenesis

Branching morphogenesis generates epithelial trees which facilitate gas exchange, filtering, as well as secretion processes with their large surface to volume ratio. In this review, we focus on the developmental mechanisms that control the early stages of lung branching morphogenesis. Lung branching morphogenesis involves the stereotypic, recurrent definition of new branch points, subsequent epithelial budding, and lung tube elongation. We discuss current models and experimental evidence for each of these steps. Finally, we discuss the role of the mesenchyme in determining the organ-specific shape.

Disruption of a hedgehog-foxf1-rspo2 signaling axis leads to tracheomalacia and a loss of sox9+ tracheal chondrocytes

Congenital tracheomalacia, resulting from incomplete tracheal cartilage development, is a relatively common birth defect that severely impairs breathing in neonates. Mutations in the Hedgehog (HH) pathway and downstream Gli transcription factors are associated with tracheomalacia in patients and mouse models; however, the underlying molecular mechanisms are unclear. Using multiple HH/Gli mouse mutants including one that mimics Pallister-Hall Syndrome, we show that excessive Gli repressor activity prevents specification of tracheal chondrocytes. Lineage tracing experiments show that Sox9+ chondrocytes arise from HH-responsive splanchnic mesoderm in the fetal foregut that expresses the transcription factor Foxf1. Disrupted HH/Gli signaling results in 1) loss of Foxf1 which in turn is required to support Sox9+ chondrocyte progenitors and 2) a dramatic reduction in Rspo2, a secreted ligand that potentiates Wnt signaling known to be required for chondrogenesis. These results reveal a HH-Foxf1-Rspo2 signaling axis that governs tracheal cartilage development and informs the etiology of tracheomalacia.

Incoherent Feedforward Regulation via Sox9 and ERK Underpins Mouse Tracheal Cartilage Development

Tracheal cartilage provides architectural integrity to the respiratory airway, and defects in this structure during embryonic development cause severe congenital anomalies. Previous genetic studies have revealed genes that are critical for the development of tracheal cartilage. However, it is still unclear how crosstalk between these proteins regulates tracheal cartilage formation. Here we show a core regulatory network underlying murine tracheal chondrogenesis from embryonic day (E) 12.5 to E15.5, by combining volumetric imaging of fluorescence reporters, inhibitor assays, and mathematical modeling. We focused on SRY-box transcription factor 9 (Sox9) and extracellular signal-regulated kinase (ERK) in the tracheal mesenchyme, and observed a synchronous, inverted U-shaped temporal change in both Sox9 expression and ERK activity with a peak at E14.5, whereas the expression level of downstream cartilage matrix genes, such as collagen II alpha 1 (Col2a1) and aggrecan (Agc1), monotonically increased. Inhibitor assays revealed that the ERK signaling pathway functions as an inhibitory regulator of tracheal cartilage differentiation during this period. These results suggest that expression of the cartilage matrix genes is controlled by an incoherent feedforward loop via Sox9 and ERK, which is supported by a mathematical model. Furthermore, the modeling analysis suggests that a Sox9-ERK incoherent feedforward regulation augments the robustness against the variation of upstream factors. The present study provides a better understanding of the regulatory network underlying the tracheal development and will be helpful for efficient induction of tracheal organoids.

Maldevelopment of intrapulmonary bronchial cartilage in congenital diaphragmatic hernia

BACKGROUND

Pulmonary hypoplasia is an important cause of morbidity and mortality in infants with congenital diaphragmatic hernia (CDH). This study aimed to verify our hypothesis that the abnormal development of bronchial cartilage as well as alveolar immaturity, might play a central role in hypoplasia of the lung in human CDH.

METHOD

We retrospectively analyzed autopsied lungs from 10 CDH cases and compared with nine age-matched controls to assess the bronchial cartilage and alveolar maturity using morphological techniques.

RESULT

Ki-67 and thyroid transcription factor-1 (TTF-1) expression in the alveoli significantly increased in bilateral lungs with CDH. The shortest distance from the bronchial cartilage to the pleura was significantly shorter in ipsilateral (left) lungs with CDH, showing a positive correlation with the radial alveolar count (RAC). Regarding the small bronchial cartilages less than 20 000 μm² , the average cartilage area significantly decreased in left lungs with CDH, and tended to decrease in right lungs with CDH. In addition, cartilage around the bronchi less than 200 μm in diameter tended to be smaller in left lungs with CDH. In contrast, regarding the cartilage around the bronchi 200 to 400 μm in diameter, the ratio of the total cartilage area relative to the bronchial diameter tended to be higher in left lungs with CDH, although there was a large variation.

CONCLUSIONS

These opposite directional cartilage abnormalities around the distal and more proximal bronchi support our hypothesis that abnormal development of bronchial cartilage might play an important role in the hypoplastic lung in CDH.

Diagnosis and management of complete tracheal rings with concurrent tracheoesophageal fistula

OBJECTIVE

Characterize patients with complete tracheal rings and tracheoesophageal fistula (TEF) and summarize management options.

METHODS

A systematic review of patients under 18 years of age with complete tracheal rings and TEF was conducted. Authors were contacted for additional patient information and new cases were added. Patients with iatrogenic TEF and tracheal stenosis due to other causes were excluded.

RESULTS

Sixteen patients with a median (IQR) follow-up of 10 months (3-12 months) were identified. All had a distal TEF with complete tracheal rings distal to the TEF. There were 10 (63%) type C esophageal atresia + TEF (EA/TEF), and 1 (6%) type D (5 missing data). Median (IQR) airway diameter was 2 mm (1.5-2.2 mm). Complete tracheal rings were diagnosed prior to TEF repair in 5 (31.3%) patients, after ≥1 failed extubation in 3 (12.5%) patients, and intra-operatively during respiratory distress in 1 patient. Ten patients (62.5%) were intubated with an endotracheal tube and one with a 6 Fr flexible aortic canula (5 missing data). Four patients with an endotracheal tube for TEF repair developed ventilatory problems. Complete tracheal rings were repaired in 9 (56%) patients (8 slide tracheoplasty, 1 pericardial patch) and followed conservatively in 3 (19%). One patient required tracheotomy. Four patients died.

CONCLUSIONS

Complete tracheal rings with concurrent TEF is a rare entity that pose challenges for ventilatory management during operative repair. Bronchoscopy prior to TEF repair is critical to allow for proper preoperative planning.

Smooth Muscle Differentiation Is Essential for Airway Size, Tracheal Cartilage Segmentation, but Dispensable for Epithelial Branching

Airway smooth muscle is best known for its role as an airway constrictor in diseases such as asthma. However, its function in lung development is debated. A prevalent model, supported by in vitro data, posits that airway smooth muscle promotes lung branching through peristalsis and pushing intraluminal fluid to branching tips. Here, we test this model in vivo by inactivating Myocardin, which prevented airway smooth muscle differentiation. We found that Myocardin mutants show normal branching, despite the absence of peristalsis. In contrast, tracheal cartilage, vasculature, and neural innervation patterns were all disrupted. Furthermore, airway diameter is reduced in the mutant, counter to the expectation that the absence of smooth muscle constriction would lead to a more relaxed and thereby wider airway. These findings together demonstrate that during development, while airway smooth muscle is dispensable for epithelial branching, it is integral for building the tracheal architecture and promoting airway growth.

Isl1 Regulation of Nkx2.1 in the Early Foregut Epithelium Is Required for Trachea-Esophageal Separation and Lung Lobation

The esophagus and trachea arise from the dorsal and ventral aspects of the anterior foregut, respectively. Abnormal trachea-esophageal separation leads to the common birth defect esophageal atresia with or without trachea-esophageal fistula (EA/TEF). Yet the underlying cellular mechanisms remain unknown. Here, we combine Xenopus and mouse genetic models to identify that the transcription factor Isl1 orchestrates trachea-esophageal separation through modulating a specific epithelial progenitor cell population (midline epithelial cells [MECs], Isl1+ Nkx2.1+ Sox2+) located at the dorsal-ventral boundary of the foregut. Lineage tracing experiments show that MECs contribute to both tracheal and esophageal epithelium, and Isl1 is required for Nkx2.1 transcription in MECs. Deletion of the chromosomal region spanning the ISL1 gene has been found in patients with abnormal trachea-esophageal separation. Our studies thus provide definitive evidence that ISL1 is a critical player in the process of foregut morphogenesis, acting in a small progenitor population of boundary cells.

Complete Tracheal Ring Deformity. A Translational Genomics Approach to Pathogenesis

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

Cellular crosstalk in the development and regeneration of the respiratory system

The respiratory system, including the peripheral lungs, large airways and trachea, is one of the most recently evolved adaptations to terrestrial life. To support the exchange of respiratory gases, the respiratory system is interconnected with the cardiovascular system, and this interconnective nature requires a complex interplay between a myriad of cell types. Until recently, this complexity has hampered our understanding of how the respiratory system develops and responds to postnatal injury to maintain homeostasis. The advent of new single-cell sequencing technologies, developments in cellular and tissue imaging and advances in cell lineage tracing have begun to fill this gap. The view that emerges from these studies is that cellular and functional heterogeneity of the respiratory system is even greater than expected and also highly adaptive. In this Review, we explore the cellular crosstalk that coordinates the development and regeneration of the respiratory system. We discuss both the classic cell and developmental biology studies and recent single-cell analysis to provide an integrated understanding of the cellular niches that control how the respiratory system develops, interacts with the external environment and responds to injury.

Fibrillin-2 is a key mediator of smooth muscle extracellular matrix homeostasis during mouse tracheal tubulogenesis

Epithelial tubes, comprised of polarised epithelial cells around a lumen, are crucial for organ function. However, the molecular mechanisms underlying tube formation remain largely unknown. Here, we report on the function of fibrillin (FBN)2, an extracellular matrix (ECM) glycoprotein, as a critical regulator of tracheal tube formation.We performed a large-scale forward genetic screen in mouse to identify regulators of respiratory organ development and disease. We identified Fbn2 mutants which exhibit shorter and narrowed tracheas as well as defects in tracheal smooth muscle cell alignment and polarity.We found that FBN2 is essential for elastic fibre formation and Fibronectin accumulation around tracheal smooth muscle cells. These processes appear to be regulated at least in part through inhibition of p38-mediated upregulation of matrix metalloproteinases (MMPs), as pharmacological decrease of p38 phosphorylation or MMP activity partially attenuated the Fbn2 mutant tracheal phenotypes. Analysis of human tracheal tissues indicates that a decrease in ECM proteins, including FBN2 and Fibronectin, is associated with tracheomalacia.Our findings provide novel insights into the role of ECM homeostasis in mesenchymal cell polarisation during tracheal tubulogenesis.

Wnt/Fgf crosstalk is required for the specification of basal cells in the mouse trachea

Basal progenitor cells are crucial for the establishment and maintenance of the tracheal epithelium. However, it remains unclear how these progenitor cells are specified during foregut development. Here, we found that ablation of the Wnt chaperone protein Gpr177 (also known as Wntless) in mouse tracheal epithelium causes a significant reduction in the number of basal progenitor cells accompanied by cartilage loss in Shh-Cre;Gpr177^loxp/loxp mutants. Consistent with the association between cartilage and basal cell development, Nkx2.1⁺p63⁺ basal cells are co-present with cartilage nodules in Shh-Cre;Ctnnb1^DM/loxp mutants, which maintain partial cell-cell adhesion but not the transcription regulation function of β-catenin. More importantly, deletion of Ctnnb1 in the mesenchyme leads to the loss of basal cells and cartilage, concomitant with reduced transcript levels of Fgf10 in Dermo1-Cre;Ctnnb1^loxp/loxp mutants. Furthermore, deletion of Fgf receptor 2 (Fgfr2) in the epithelium also leads to significantly reduced numbers of basal cells, supporting the importance of Wnt/Fgf crosstalk in early tracheal development.

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.

The potassium channel KCNJ13 is essential for smooth muscle cytoskeletal organization during mouse tracheal tubulogenesis

Tubulogenesis is essential for the formation and function of internal organs. One such organ is the trachea, which allows gas exchange between the external environment and the lungs. However, the cellular and molecular mechanisms underlying tracheal tube development remain poorly understood. Here, we show that the potassium channel KCNJ13 is a critical modulator of tracheal tubulogenesis. We identify Kcnj13 in an ethylnitrosourea forward genetic screen for regulators of mouse respiratory organ development. Kcnj13 mutants exhibit a shorter trachea as well as defective smooth muscle (SM) cell alignment and polarity. KCNJ13 is essential to maintain ion homeostasis in tracheal SM cells, which is required for actin polymerization. This process appears to be mediated, at least in part, through activation of the actin regulator AKT, as pharmacological increase of AKT phosphorylation ameliorates the Kcnj13-mutant trachea phenotypes. These results provide insight into the role of ion homeostasis in cytoskeletal organization during tubulogenesis.

Synchronized mesenchymal cell polarization and differentiation shape the formation of the murine trachea and esophagus

Tube morphogenesis is essential for internal-organ development, yet the mechanisms regulating tube shape remain unknown. Here, we show that different mechanisms regulate the length and diameter of the murine trachea. First, we found that trachea development progresses via sequential elongation and expansion processes. This starts with a synchronized radial polarization of smooth muscle (SM) progenitor cells with inward Golgi-apparatus displacement regulates tube elongation, controlled by mesenchymal Wnt5a-Ror2 signaling. This radial polarization directs SM progenitor cell migration toward the epithelium, and the resulting subepithelial morphogenesis supports tube elongation to the anteroposterior axis. This radial polarization also regulates esophageal elongation. Subsequently, cartilage development helps expand the tube diameter, which drives epithelial-cell reshaping to determine the optimal lumen shape for efficient respiration. These findings suggest a strategy in which straight-organ tubulogenesis is driven by subepithelial cell polarization and ring cartilage development.

[Regulatory role of Shh signaling pathway in lung development in fetal mice]

OBJECTIVE

To investigate the regulatory role of classical Shh signaling pathway in the development of the epithelium and mesenchyme (bronchial cartilage and smooth muscles) during lung development in fetal mice.

METHODS

Immunohistochemical technique was used to detect the expression of Shh signaling pathway receptor Smo and Pdgfr-α in murine fetal lungs to explore the spatial and temporal characteristics of their expression. Based on the interstitial specificity of Pdgfr-α expression, we constructed a Pdgfr-α-cre to establish a E12.5 - E16.5 transgenic mice with specific knockout of the key Shh signaling molecule Smo in the pulmonary interstitium with tamoxifen induction. Immunofluorescence technique was used to observe the epithelium and mesenchyme (bronchial cartilage and smooth muscle) during fetal lung development in the transgenic mice to assess the role of Shh signaling pathway in the epithelial-to-mesenchymal (EMT) transition during the lung development.

RESULTS

Smo was highly expressed in the epithelial and stromal lung tissues in the pseudoglandular stage and was gradually lowered over time with its distribution mainly in the interstitial tissues. Pdgfr-α was enriched in the distal lung epithelial and mesenchy tissues in early embryonic lungs and gradually migrated to the proximal stroma until becoming concentrated around the main bronchial proximal stroma. We successfully specific established mouse models of specific mesenchymal Smo knockout. Compared with the control group, the transgenic mice during E12.5-E16.5 showed significantly reduced lung the volume and bronchial branching with also decreased expression of the proximal epithelial P63 (P<0.05). The transgenic mice exhibited alterations in the expression of α-smooth muscle actin with delayed bronchial cartilage development and decreased expression of mucoprotein.

CONCLUSION

The temporospatial specific expression of Shh signaling pathway plays an important role in developmental regulation of mouse embryonic lung epithelium and mesenchyme (bronchial cartilage and smooth muscle).

Follistatin like-1 (Fstl1) is required for the normal formation of lung airway and vascular smooth muscle at birth

Fstl1, a secreted protein of the BMP antagonist class, has been implicated in the regulation of lung development and alveolar maturation. Here we generated a Fstl1-lacZ reporter mouse line as well as a Fstl1 knockout allele. We localized Fstl1 transcript in lung smooth muscle cells and identified Fstl1 as essential regulator of lung smooth muscle formation. Deletion of Fstl1 in mice led to postnatal death as a result of respiratory failure due to multiple defects in lung development. Analysis of the mutant phenotype showed impaired airway smooth muscle (SM) manifested as smaller SM line in trachea and discontinued SM surrounding bronchi, which were associated with decreased transcriptional factors myocardin/serum response factor (SRF) and impaired differentiation of SM cells. Fstl1 knockout mice also displayed abnormal vasculature SM manifested as hyperplasia SM in pulmonary artery. This study indicates a pivotal role for Fstl1 in early stage of lung airway smooth muscle development.

Cartilage rings contribute to the proper embryonic tracheal epithelial differentiation, metabolism, and expression of inflammatory genes

The signaling cross talk between the tracheal mesenchyme and epithelium has not been researched extensively, leaving a substantial gap of knowledge in the mechanisms dictating embryonic development of the proximal airways by the adjacent mesenchyme. Recently, we reported that embryos lacking mesenchymal expression of Sox9 did not develop tracheal cartilage rings and showed aberrant differentiation of the tracheal epithelium. Here, we propose that tracheal cartilage provides local inductive signals responsible for the proper differentiation, metabolism, and inflammatory status regulation of the tracheal epithelium. The tracheal epithelium of mesenchyme-specific Sox9^Δ/Δ mutant embryos showed altered mRNA expression of various epithelial markers such as Pb1fa1, surfactant protein B (Sftpb), secretoglobulin, family 1A, member 1 (Scgb1a1), and trefoil factor 1 (Tff1). In vitro tracheal epithelial cell cultures confirmed that tracheal chondrocytes secrete factors that inhibit club cell differentiation. Whole gene expression profiling and ingenuity pathway analysis showed that the tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), and transforming growth factor-β (TGF-β) signaling pathways were significantly altered in the Sox9 mutant trachea. TNF-α and IFN-γ interfered with the differentiation of tracheal epithelial progenitor cells into mature epithelial cell types in vitro. Mesenchymal knockout of Tgf-β1 in vivo resulted in altered differentiation of the tracheal epithelium. Finally, mitochondrial enzymes involved in fat and glycogen metabolism, cytochrome c oxidase subunit VIIIb (Cox8b) and cytochrome c oxidase subunit VIIa polypeptide 1 (Cox7a1), were strongly upregulated in the Sox9 mutant trachea, resulting in increases in the number and size of glycogen storage vacuoles. Our results support a role for tracheal cartilage in modulation of the differentiation and metabolism and the expression of inflammatory-related genes in the tracheal epithelium by feeding into the TNF-α, IFN-γ, and TGF-β signaling pathways.

Mesodermal lineages in the developing respiratory system

The life-sustaining air-blood interface of the respiratory system requires the exquisite integration of the epithelial lining with the mesenchymal capillary network, all supported by elastic smooth muscle and rigid cartilage keeping the expandable airways open. These intimate tissue interactions originate in the early embryo, where bidirectional paracrine signaling between the endoderm epithelium and adjacent mesoderm orchestrates lung and trachea development and controls the stereotypical branching morphogenesis. Although much attention has focused on how these interactions impact the differentiation of the respiratory epithelium, relatively less is known about the patterning and differentiation of the mesenchyme. Endothelial cells, smooth muscle cells, and chondrocytes together with other types of mesenchymal cells are essential components of a functional respiratory system, and malformation of these cells can lead to various congenital defects. In this review, we summarize the current understanding of mesenchymal development in the fetal trachea and lung, focusing on recent findings from animal models that have begun to shed light on the poorly understood respiratory mesenchyme lineages.

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

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

Tissue crosstalk in lung development

Lung development follows a stereotypic program orchestrated by key interactions among epithelial and mesenchymal tissues. Deviations from this developmental program can lead to pulmonary diseases including bronchopulmonary dysplasia and pulmonary hypertension. Significant efforts have been made to examine the cellular and molecular basis of the tissue interactions underlying these stereotypic developmental processes. Genetically engineered mouse models, lung organ culture, and advanced imaging techniques are a few of the tools that have expanded our understanding of the tissue interactions that drive lung development. Intimate crosstalk has been identified between the epithelium and mesenchyme, distinct mesenchymal tissues, and individual epithelial cells types. For interactions such as the epithelial-mesenchymal crosstalk regulating lung specification and branching morphogenesis, the key molecular players, FGF, BMP, WNT, and SHH, are well established. Additionally, VEGF regulation underlies the epithelial-endothelial crosstalk that coordinates airway branching with angiogenesis. Recent work also discovered a novel role for SHH in the epithelial-to-mesenchymal (EMT) transition of the mesothelium. In contrast, the molecular basis for the crosstalk between upper airway cartilage and smooth muscle is not yet known. In this review we examine current evidence of the tissue interactions and molecular crosstalk that underlie the stereotypic patterning of the developing lung and mediate injury repair.

Localized Smooth Muscle Differentiation Is Essential for Epithelial Bifurcation during Branching Morphogenesis of the Mammalian Lung

The airway epithelium develops into a tree-like structure via branching morphogenesis. Here, we show a critical role for localized differentiation of airway smooth muscle during epithelial bifurcation in the embryonic mouse lung. We found that during terminal bifurcation, changes in the geometry of nascent buds coincided with patterned smooth muscle differentiation. Evaluating spatiotemporal dynamics of α-smooth muscle actin (αSMA) in reporter mice revealed that αSMA-expressing cells appear at the basal surface of the future epithelial cleft prior to bifurcation and then increase in density as they wrap around the bifurcating bud. Disrupting this stereotyped pattern of smooth muscle differentiation prevents terminal bifurcation. Our results reveal stereotyped differentiation of airway smooth muscle adjacent to nascent epithelial buds and suggest that localized smooth muscle wrapping at the cleft site is required for terminal bifurcation during airway branching morphogenesis.

Endodermal Wnt signaling is required for tracheal cartilage formation

Tracheobronchomalacia is a common congenital defect in which the walls of the trachea and bronchi lack of adequate cartilage required for support of the airways. Deletion of Wls, a cargo receptor mediating Wnt ligand secretion, in the embryonic endoderm using ShhCre mice inhibited formation of tracheal-bronchial cartilaginous rings. The normal dorsal-ventral patterning of tracheal mesenchyme was lost. Smooth muscle cells, identified by Acta2 staining, were aberrantly located in ventral mesenchyme of the trachea, normally the region of Sox9 expression in cartilage progenitors. Wnt/β-catenin activity, indicated by Axin2 LacZ reporter, was decreased in tracheal mesenchyme of Wls(f/f);Shh(Cre/+) embryos. Proliferation of chondroblasts was decreased and reciprocally, proliferation of smooth muscle cells was increased in Wls(f/f);Shh(Cre/+) tracheal tissue. Expression of Tbx4, Tbx5, Msx1 and Msx2, known to mediate cartilage and muscle patterning, were decreased in tracheal mesenchyme of Wls(f/f);Shh(Cre/+) embryos. Ex vivo studies demonstrated that Wnt7b and Wnt5a, expressed by the epithelium of developing trachea, and active Wnt/β-catenin signaling are required for tracheal chondrogenesis before formation of mesenchymal condensations. In conclusion, Wnt ligands produced by the tracheal epithelium pattern the tracheal mesenchyme via modulation of gene expression and cell proliferation required for proper tracheal cartilage and smooth muscle differentiation.

A breath of fresh air on the mesenchyme: impact of impaired mesenchymal development on the pathogenesis of bronchopulmonary dysplasia

The early mouse embryonic lung, with its robust and apparently reproducible branching pattern, has always fascinated developmental biologists. They have extensively used this embryonic organ to decipher the role of mammalian orthologs of Drosophila genes in controlling the process of branching morphogenesis. During the early pseudoglandular stage, the embryonic lung is formed mostly of tubes that keep on branching. As the branching takes place, progenitor cells located in niches are also amplified and progressively differentiate along the proximo-distal and dorso-ventral axes of the lung. Such elaborate processes require coordinated interactions between signaling molecules arising from and acting on four functional domains: the epithelium, the endothelium, the mesenchyme, and the mesothelium. These interactions, quite well characterized in a relatively simple lung tubular structure remain elusive in the successive developmental and postnatal phases of lung development. In particular, a better understanding of the process underlying the formation of secondary septa, key structural units characteristic of the alveologenesis phase, is still missing. This structure is critical for the formation of a mature lung as it allows the subdivision of saccules in the early neonatal lung into alveoli, thereby considerably expanding the respiratory surface. Interruption of alveologenesis in preterm neonates underlies the pathogenesis of chronic neonatal lung disease known as bronchopulmonary dysplasia. De novo formation of secondary septae appears also to be the limiting factor for lung regeneration in human patients with emphysema. In this review, we will therefore focus on what is known in terms of interactions between the different lung compartments and discuss the current understanding of mesenchymal cell lineage formation in the lung, focusing on secondary septae formation.

The pulmonary mesenchyme directs lung development

Each of the steps of respiratory system development relies on intricate interactions and coordinated development of the lung epithelium and mesenchyme. In the past, more attention has been paid to the epithelium than the mesenchyme. The mesenchyme is a source of specification and morphogenetic signals as well as a host of surprisingly complex cell lineages that are critical for normal lung development and function. This review highlights recent research focusing on the mesenchyme that has revealed genetic and epigenetic mechanisms of its development in the context of other cell layers during respiratory lineage specification, branching morphogenesis, epithelial differentiation, lineage distinction, vascular development, and alveolar maturation.

Ezh2 represses the basal cell lineage during lung endoderm development

The development of the lung epithelium is regulated in a stepwise fashion to generate numerous differentiated and stem cell lineages in the adult lung. How these different lineages are generated in a spatially and temporally restricted fashion remains poorly understood, although epigenetic regulation probably plays an important role. We show that the Polycomb repressive complex 2 component Ezh2 is highly expressed in early lung development but is gradually downregulated by late gestation. Deletion of Ezh2 in early lung endoderm progenitors leads to the ectopic and premature appearance of Trp63+ basal cells that extend the entire length of the airway. Loss of Ezh2 also leads to reduced secretory cell differentiation. In their place, morphologically similar cells develop that express a subset of basal cell genes, including keratin 5, but no longer express high levels of either Trp63 or of standard secretory cell markers. This suggests that Ezh2 regulates the phenotypic switch between basal cells and secretory cells. Together, these findings show that Ezh2 restricts the basal cell lineage during normal lung endoderm development to allow the proper patterning of epithelial lineages during lung formation.

Wnt and FGF mediated epithelial-mesenchymal crosstalk during lung development

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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