Retinoic acid selectively regulates Fgf10 expression and maintains cell identity in the prospective lung field of the developing foregut

Retinoic acid rescues deficient airway innervation and peristalsis of hypoplastic rat lung explants

BACKGROUND

Bronchial peristalsis modulates lung growth and is deficient in hypoplastic nitrofen-exposed rat lung explants. Retinoic acid (RA) rescues lung hypoplasia. This study examines whether decreased bronchial innervation contributes to this developmental deficiency and if RA is able to recover bronchial innervation and motility.

MATERIAL AND METHODS

After IRB approval, pregnant rats received either 100 mg nitrofen or vehicle on gestational day 9.5 (E9.5). Embryonic lung primordia harvested on E13 were cultured for 72 h and RA was added daily to the medium when appropriate. Lung growth was assessed by counting the number of terminal buds and measuring explant surface, total DNA and protein in control, control + RA, nitrofen and nitrofen + RA groups. Peristaltic contractions were recorded for 10 min under an inverted microscope. Lung explants stained for anti-protein gene product 9.5 (PGP 9.5) and smooth muscle α-actin were examined under a confocal microscope for depicting the specific relationship between neural and smooth muscle cells. PGP 9.5 and smooth muscle α-actin levels were quantified by Western blot analysis for assessing the neural and muscle cell expressions. Comparisons between groups were made with non-parametric tests.

RESULTS

The number of terminal buds, the explants' surface and the DNA and protein contents were significantly decreased in nitrofen-exposed lungs in comparison with controls. In contrast, these measurements were normal in explants exposed to both nitrofen and RA. Bronchial peristalsis (contractions/min) was significantly decreased in nitrofen-exposed lungs in comparison with controls; in contrast, in nitrofen + RA lungs it was similar to controls. In all study groups, the airways were surrounded by smooth muscle and ensheathed in a plexus of nerve fibers containing ganglia. PGP 9.5 protein levels were decreased in nitrofen-exposed lungs, but they normalized when RA was added. No differences were found in α-actin protein levels. Explants exposed only to RA were similar to control.

CONCLUSIONS

Lung growth, bronchial innervation and peristalsis are decreased in nitrofen-exposed lung explants and are rescued by RA. If deficient airway innervation contributing to dysmotility and pulmonary hypoplasia can be pharmacologically rescued, new relatively simple prenatal interventions could be envisioned.

Retinoic acid is a key regulatory switch determining the difference between lung and thyroid fates in Xenopus laevis

Background

The lung and thyroid are derived from the anterior endoderm. Retinoic acid and Fgf signalling are known to be essential for development of the lung in mouse but little is known on how the lung and thyroid are specified in Xenopus.

Results

If either retinoic acid or Fgf signalling is inhibited, there is no differentiation of the lung as assayed by expression of sftpb. There is no change in expression of thyroid gland markers when retinoic acid signalling is blocked after gastrulation and when Fgf signalling is inhibited there is a short window of time where pax2 expression is inhibited but expression of other markers is unaffected. If exogenous retinoic acid is given to the embryo between embryonic stages 20 and 26, the presumptive thyroid expresses sftpb and sftpc, specific markers of lung differentiation and expression of key thyroid transcription factors is lost. When the presumptive thyroid is transplanted into the posterior embryo, it also expresses sftpb, although pax2 expression is not blocked.

Conclusions

After gastrulation, retinoic acid is required for lung but not thyroid differentiation in Xenopus while Fgf signalling is needed for lung but only for early expression of pax2 in the thyroid. Exposure to retinoic acid can cause the presumptive thyroid to switch to a lung developmental program.

Minireview: Intrinsic and extrinsic factors in thyroid gland development: an update

In vertebrates the portion of the thyroid gland synthesizing the thyroid hormones develops from a small group of endodermal cells in the foregut. The nature of the signals that lead to the biochemical and morphogenetic events responsible for the organization of these cells into the adult thyroid gland has only recently become evident. In this review we summarize recent developments in the understanding of these processes, derived from evidence collected in several organisms.

[A study of the metabolic pathways of vitamin A in the fetal human lung]

INTRODUCTION

Retinoic acid plays an essential role in lung development and is involved in all stages of embryogenesis and morphogenesis. We aimed to determine whether the human foetal lung is able to synthesize retinoic acid.

METHODS

ADH3, RALDH1 and CYP26B1 RNA were studied by qualitative and semi-quantitative RT-PCR in human lungs at different stages of development. In human alveolar epithelial cells (lines A549), RAR beta (induced by retinoic acid) was quantified, after treatment by retinol, by q-PCR at 24h and 48 h.

RESULTS

The expression of the RNA of ADH3, RALDH1 and CYP26B1 was detected for each of the four stages studied and in the A549 cell line. Only the level of RALDH1 RNA changed during the course of lung development. In the A549 cell line, treatment by retinol induced transcription of the RAR beta gene at 24 and 48 hours.

CONCLUSION

The presence of ADH3, RALDH1 and CYP26B1 during the four stages of normal lung development and in the A549 cell line, as well as the capacity to convert retinol to retinoic acid in these cells, indicate that foetal human lung has the ability to regulate the supply of vitamin A from the pseudoglandular stage.

Patterning and plasticity in development of the respiratory lineage

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

Impact of environmental chemicals on lung development

BACKGROUND

Disruption of fundamental biologic processes and associated signaling events may result in clinically significant alterations in lung development.

OBJECTIVES

We reviewed evidence on the impact of environmental chemicals on lung development and key signaling events in lung morphogenesis, and the relevance of potential outcomes to public health and regulatory science .

DATA SOURCES

We evaluated the peer-reviewed literature on developmental lung biology and toxicology, mechanistic studies, and supporting epidemiology.

DATA SYNTHESIS

Lung function in infancy predicts pulmonary function throughout life. In utero and early postnatal exposures influence both childhood and adult lung structure and function and may predispose individuals to chronic obstructive lung disease and other disorders. The nutritional and endogenous chemical environment affects development of the lung and can result in altered function in the adult. Studies now suggest that similar adverse impacts may occur in animals and humans after exposure to environmentally relevant doses of certain xenobiotics during critical windows in early life. Potential mechanisms include interference with highly conserved factors in developmental processes such as gene regulation, molecular signaling, and growth factors involved in branching morphogenesis and alveolarization.

CONCLUSIONS

Assessment of environmental chemical impacts on the lung requires studies that evaluate specific alterations in structure or function-end points not regularly assessed in standard toxicity tests. Identifying effects on important signaling events may inform protocols of developmental toxicology studies. Such knowledge may enable policies promoting true primary prevention of lung diseases. Evidence of relevant signaling disruption in the absence of adequate developmental toxicology data should influence the size of the uncertainty factors used in risk assessments.

The role of pleiotrophin and beta-catenin in fetal lung development

Mammalian lung development is a complex biological process, which is temporally and spatially regulated by growth factors, hormones, and extracellular matrix proteins. Abnormal changes of these molecules often lead to impaired lung development, and thus pulmonary diseases. Epithelial-mesenchymal interactions are crucial for fetal lung development. This paper reviews two interconnected pathways, pleiotrophin and Wnt/β-catenin, which are involved in fibroblast and epithelial cell communication during fetal lung development.

A retinoic acid-dependent network in the foregut controls formation of the mouse lung primordium

The developmental abnormalities associated with disruption of signaling by retinoic acid (RA), the biologically active form of vitamin A, have been known for decades from studies in animal models and humans. These include defects in the respiratory system, such as lung hypoplasia and agenesis. However, the molecular events controlled by RA that lead to formation of the lung primordium from the primitive foregut remain unclear. Here, we present evidence that endogenous RA acts as a major regulatory signal integrating Wnt and Tgfbeta pathways in the control of Fgf10 expression during induction of the mouse primordial lung. We demonstrated that activation of Wnt signaling required for lung formation was dependent on local repression of its antagonist, Dickkopf homolog 1 (Dkk1), by endogenous RA. Moreover, we showed that simultaneously activating Wnt and repressing Tgfbeta allowed induction of both lung buds in RA-deficient foreguts. The data in this study suggest that disruption of Wnt/Tgfbeta/Fgf10 interactions represents the molecular basis for the classically reported failure to form lung buds in vitamin A deficiency.

The role of mesodermal signals during liver organogenesis in zebrafish

Three germ cell layers, the ectoderm, mesoderm and endoderm, are established during the gastrulation stage. All cell types in different organs and tissues are derived from these 3 germ cell layers at later stages. For example, skin epithelial cells and neuronal cells are derived from the ectoderm, while endothelial cells and muscle cells from the mesoderm and lung, and intestine epithelial cells from the endoderm. While in a normal situation different germ cells are destined to specific cell fates in different organs and tissues, each type of germ cells or its derivatives also produce extracellular signaling molecules to direct and facilitate the specification and differentiation of other germ cells during organogenesis. Liver is derived from the endoderm, but completion of liver organogenesis is regulated at different levels. While the pan-endoderm factors (e.g. FoxA and Gata families) and liver specific factors (e.g. Prox1 and Hhex) are essential intrinsic factors for endoderm cells to be differentiated into hepatoblasts, the role of signals produced by neighboring mesoderm cells for liver organogenesis is equally important. This review summarizes recent progress in studying the role of Bone morphogenetic proteins (Bmp), Fibroblast growth factors (Fgf), retinoic acid (RA) and Wingless and Int (Wnt), the 4 types of signaling molecules produced by the mesoderm cells, in liver organogenesis in zebrafish.

Mechanisms of lung development: contribution to adult lung disease and relevance to chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) results in major remodeling of the distal airspaces and changes in the differentiation profile of the airway epithelium. The cellular and molecular mechanisms involved in initiation and progression of this disease are little understood. Although environmental factors, including cigarette smoke, have been directly implicated in the pathogenesis of COPD, genetic risk factors also appear to play a fundamental role in the individual's susceptibility to this disease. Lung development depends on precise coordination of signals, such as fibroblast growth factors (Fgf), Sonic Hedgehog (Shh), retinoic acid, Notch, and Tgf beta. Dramatic changes in the pattern of branching and differentiation of the lung epithelium results from disruption of these signals in genetically altered mice. Recent studies, including whole-genome expression and genome-wide association analyses, suggest that some molecular regulators originally described in developmental processes may be altered in patients with COPD. Whether disturbances in the molecular and cellular events mediated by these genes during development participate in the initiation or exacerbation of COPD, needs further investigation. The role of selected pathways, including Sonic hedgehog, Notch, retinoid, and Tgf beta in the developing lung and the potential association with COPD are discussed.

Morphogenesis of the thyroid gland

Congenital hypothyroidism is mainly due to structural defects of the thyroid gland, collectively known as thyroid dysgenesis. The two most prevalent forms of this condition are abnormal localization of differentiated thyroid tissue (thyroid ectopia) and total absence of the gland (athyreosis). The clinical picture of thyroid dysgenesis suggests that impaired specification, proliferation and survival of thyroid precursor cells and loss of concerted movement of these cells in a distinct spatiotemporal pattern are major causes of malformation. In normal development the thyroid primordium is first distinguished as a thickening of the anterior foregut endoderm at the base of the prospective tongue. Subsequently, this group of progenitors detaches from the endoderm, moves caudally and ultimately differentiates into hormone-producing units, the thyroid follicles, at a distant location from the site of specification. In higher vertebrates later stages of thyroid morphogenesis are characterized by shape remodeling into a bilobed organ and the integration of a second type of progenitors derived from the caudal-most pharyngeal pouches that will differentiate into C-cells. The present knowledge of thyroid developmental dynamics has emerged from embryonic studies mainly in chicken, mouse and more recently also in zebrafish. This review will highlight the key morphogenetic steps of thyroid organogenesis and pinpoint which crucial regulatory mechanisms are yet to be uncovered. Considering the co-incidence of thyroid dysgenesis and congenital heart malformations the possible interactions between thyroid and cardiovascular development will also be discussed.

Epigenetic mechanisms modulate thyroid transcription factor 1-mediated transcription of the surfactant protein B gene

Epigenetic regulation of transcription plays an important role in cell-specific gene expression by altering chromatin structure and access of transcriptional regulators to DNA binding sites. Surfactant protein B (Sftpb) is a developmentally regulated lung epithelial gene critical for lung function. Thyroid transcription factor 1 (Nkx2-1) regulates Sftpb gene expression in various species. We show that Nkx2-1 binds to the mouse Sftpb (mSftpb) promoter in the lung. In a mouse lung epithelial cell line (MLE-15), Nkx2-1 knockdown reduces Sftpb expression, and mutation of Nkx2-1 cis-elements significantly reduces mSftpb promoter activity. Whether chromatin structure modulates Nkx2-1 regulation of Sftpb transcription is unknown. We found that DNA methylation of the mSftpb promoter inversely correlates with known patterns of Sftpb expression in vivo. The mSftpb promoter activity can be manipulated by altering its cytosine methylation status in vitro. Nkx2-1 activation of the mSftpb promoter is impaired by DNA methylation. The unmethylated Sftpb promoter shows an active chromatin structure enriched in the histone modification H3K4me3 (histone 3-lysine 4 trimethylated). The ATP-dependent chromatin remodeling protein Brg1 is recruited to the Sftpb promoter in Sftpb-expressing, but not in non-expressing tissues and cell lines. Brg1 knockdown in MLE-15 cells greatly decreases H3K4me3 levels at the Sftpb promoter region and expression of the Sftpb gene. Brg1 can be co-immunoprecipitated with Nkx2-1 protein. Last, Nkx2-1 and Brg1 with intact ATPase activity are required for mSftpb promoter activation in vitro. Our findings suggest that DNA methylation and chromatin modifications cooperate with Nkx2-1 to regulate Sftpb gene cell specific expression.

Multiple roles for Sox2 in the developing and adult mouse trachea

The esophagus, trachea and lung develop from the embryonic foregut, yet acquire and maintain distinct tissue phenotypes. Previously, we demonstrated that the transcription factor Sox2 is necessary for foregut morphogenesis and esophagus development. We show that Sox2 is also required for the normal development of the trachea and lung. In both the embryo and adult, Sox2 is exclusively expressed in the epithelium of the trachea and airways. We use an Nkx2.5-Cre transgene and a Sox2 floxed allele to conditionally delete Sox2 in the ventral epithelial domain of the early anterior foregut, which gives rise to the future trachea and lung buds. All conditional mutants die of respiratory distress at birth, probably due to abnormal differentiation of the laryngeal and tracheal cartilage as a result of defective epithelial-mesenchymal interaction. About 60% of the mutants have a short trachea, suggesting that the primary budding site of the lung shifts anteriorly. In the tracheal epithelium of all conditional mutants there are significantly more mucus-producing cells compared with wild type, and fewer basal stem cells, ciliated and Clara cells. Differentiation of the epithelium lining the conducting airways in the lung is abnormal, suggesting that Sox2 also plays a role in the differentiation of embryonic airway progenitors into specific lineages. Conditional deletion of Sox2 was then used to test its role in adult epithelium maintenance. We found that epithelial cells, including basal stem cells, lacking Sox2 show a reduced capacity to proliferate in culture and to repair after injury in vivo. Taken together, these results define multiple roles for Sox2 in the developing and adult trachea.

Gamma-secretase activation of notch signaling regulates the balance of proximal and distal fates in progenitor cells of the developing lung

Little is known about the mechanisms by which the lung epithelial progenitors are initially patterned and how proximal-distal boundaries are established and maintained when the lung primordium forms and starts to branch. Here we identified a number of Notch pathway components in respiratory progenitors of the early lung, and we investigated the role of Notch in lung pattern formation. By preventing gamma-secretase cleavage of Notch receptors, we have disrupted global Notch signaling in the foregut and in the lung during the initial stages of murine lung morphogenesis. We demonstrate that Notch signaling is not necessary for lung bud initiation; however, Notch is required to maintain a balance of proximal-distal cell fates at these early stages. Disruption of Notch signaling dramatically expands the population of distal progenitors, altering morphogenetic boundaries and preventing formation of proximal structures. Our data suggest a novel mechanism in which Notch and fibroblast growth factor signaling interact to control the proximal-distal pattern of forming airways in the mammalian lung.

Retinoic acid induces prostatic bud formation

Formation of prostatic buds from the urogenital sinus (UGS) to initiate prostate development requires localized action of several morphogenetic factors. This report reveals all-trans-retinoic acid (RA) to be a powerful inducer of mouse prostatic budding that is associated with reciprocal changes in expression of two regulators of budding: sonic hedgehog (Shh) and bone morphogenetic protein 4 (Bmp4). Localization of retinoid signaling and expression of RA synthesis, metabolism, and receptor genes in the UGS on embryonic days 14.5-17.5 implicate RA in the mechanism of bud initiation. In UGS organ culture, RA increased prostatic budding, increased Shh expression, and decreased Bmp4. Prostatic budding was stimulated in the absence of RA by recombinant SHH, by blocking BMP4 signaling with NOGGIN, or by combined treatment with SHH and NOGGIN in UGS organ culture media. These observations suggest that reciprocal changes in hedgehog and BMP signaling by RA may regulate bud initiation.

Congenital diaphragmatic hernia and retinoids: searching for an etiology

Congenital diaphragmatic hernia (CDH) is a major life-threatening cause of respiratory failure in the newborn. Recent data reveal the role of a retinoid-signaling pathway disruption in the pathogenesis of CDH. We describe the epidemiology and pathophysiology of human CDH, the metabolism of retinoids and the implications of retinoids in the development of the diaphragm and lung. Finally, we describe the existing evidence of a disruption of the retinoid-signaling pathway in CDH.

Effects of retinoic acid on compensatory lung growth

Background

We investigated the effect of Retinoic acid in the growth of contralateral lung after pneumonectomy.

Methods

Twentyone adult male Wistar albino rats from the same colony were used. They were divided into three groups (Group A, B and C). Group A undergone only left posterolateral thoracotomy. In Group B and C, the rats were subjected to left posterolateral thoracotomy and left pneumonectomy. In Group C, rats were given intraperitoneal Retinoic acid during the operation and continued to be given everyday postoperatively. Rats were sacrificed on the 10^th day and their total body, right lung weights and right lung volumes were measured.

Results

The volume and weight indices of the lung were found to be higher in Group C. In histopathological examination, there was a reduction in the mean number of alveoli in Group B and C. A significant rise in the mean dimension and average wall thickness of the alveolar structure were determined in Group C.

Conclusion

Retinoic acid contributes to the compensatory growth of the residual lung tissue.

Prenatal treatment with retinoic acid promotes pulmonary alveologenesis in the nitrofen model of congenital diaphragmatic hernia

BACKGROUND/PURPOSE

Severe pulmonary hypoplasia remains the main cause of the high mortality in newborn infants with congenital diaphragmatic hernia (CDH). Retinoids are a family of molecules derived from vitamin A, which play an important role in lung development. We hypothesized that retinoids promote alveologenesis at the end of gestation and therefore designed this study to investigate the effects of retinoid acid on nitrofen-induced hypoplastic lungs in CDH.

METHODS

Pregnant rats were exposed to either olive oil or 100 mg nitrofen on day 9 of gestation. Retinoic acid 5 mg/kg was given intraperitoneally on days 18, 19, and 20 of gestation and fetuses were recovered on day 21. We had 4 study groups: control (n = 24), control + retinoic acid (n = 22), CDH (n = 24), and CDH + retinoic acid (n = 19). Lungs from the 4 study groups were fixed, and the following stereological measurements were performed on vertical random sections: total lung volume, volume density of airspaces, volume density of air walls, gas exchange surface area, alveolar volume, and total number of alveoli per lung. Total DNA content of each lung was measured using a spectrophotometer.

RESULTS

Total lung volume increased in CDH lungs after the addition of retinoic acid but remained the same in the control group. Gas exchange surface area was larger in CDH lungs after the addition of retinoic acid but remained unchanged in the control group. The total number of alveoli per lung was higher after the addition of retinoic acid. Total DNA content as well as total DNA content-lung weight ratio of the left lung increased significantly in the CDH group after the addition of retinoic acid compared with CDH without retinoic acid.

CONCLUSIONS

Our results demonstrate that prenatal treatment with retinoic acid stimulates alveologenesis in hypoplastic lungs in CDH.

Linking animal models to human congenital diaphragmatic hernia

BACKGROUND

Congenital diaphragmatic hernia (CDH) is a major life-threatening malformation, occurring in approximately 1 in 3,000 live births. Over the years, different animal models have been used to gain insight into the etiology of this complex congenital anomaly and to develop treatment strategies. However, to date the pathogenic mechanism is still not understood, and treatment remains difficult because of the associated pulmonary hypoplasia and pulmonary hypertension.

METHODS

In this review, data available from several animal models will be discussed. The retinoic acid signaling pathway (RA pathway, retinoid pathway) will be addressed as a developmental pathway that is potentially disrupted in the pathogenesis of CDH. Furthermore, genetic factors involved in diaphragm and lung development will be discussed.

CONCLUSIONS

With this review article, we aim to provide a concise overview of the current most important experimental genetic data available in the field of CDH.

Characterization of the mid-foregut transcriptome identifies genes regulated during lung bud induction

To identify genes expressed during initiation of lung organogenesis, we generated transcriptional profiles of the prospective lung region of the mouse foregut (mid-foregut) microdissected from embryos at three developmental stages between embryonic day 8.5 (E8.5) and E9.5. This period spans from lung specification of foregut cells to the emergence of the primary lung buds. We identified a number of known and novel genes that are temporally regulated as the lung bud forms. Genes that regulate transcription, including DNA binding factors, co-factors, and chromatin remodeling genes, are the main functional groups that change during lung bud formation. Members of key developmental transcription and growth factor families, not previously described to participate in lung organogenesis, are expressed in the mid-foregut during lung bud induction. These studies also show early expression in the mid-foregut of genes that participate in later stages of lung development. This characterization of the mid-foregut transcriptome provides new insights into molecular events leading to lung organogenesis.

Wt1 and retinoic acid signaling are essential for stellate cell development and liver morphogenesis

Previous studies of knock-out mouse embryos have shown that the Wilms' tumor suppressor gene (Wt1) is indispensable for the development of kidneys, gonads, heart, adrenals and spleen. Using OPT (Optical Projection Tomography) we have found a new role for Wt1 in mouse liver development. In the absence of Wt1, the liver is reduced in size, and shows lobing abnormalities. In normal embryos, coelomic cells expressing Wt1, GATA-4, RALDH2 and RXRalpha delaminate from the surface of the liver, intermingle with the hepatoblasts and incorporate to the sinusoidal walls. Some of these cells express desmin, suggesting a contribution to the stellate cell population. Other cells, keeping high levels of RXRalpha immunoreactivity, are negative for stellate or smooth muscle cell markers. However, coelomic cells lining the liver of Wt1-null embryos show decreased or absent RALDH2 expression, the population of cells expressing high levels of RXRalpha is much reduced and the proliferation of hepatoblasts and RXRalpha-positive cells is significantly decreased. On the other hand, the expression of smooth muscle cell specific alpha-actin increases throughout the liver, suggesting an accelerated and probably anomalous differentiation of stellate cell progenitors. We describe a similar retardation of liver growth in RXRalpha-null mice as well as in chick embryos after inhibition of retinoic acid synthesis. We propose that Wt1 expression in cells delaminating from the coelomic epithelium is essential for the expansion of the progenitor population of liver stellate cells and for liver morphogenesis. Mechanistically, at least part of this effect is mediated via the retinoic acid signaling pathway.

Key mechanisms of early lung development

Lung morphogenesis requires the integration of multiple regulatory factors, which results in a functional air-blood interface required for gas exchange at birth. The respiratory tract is composed of endodermally derived epithelium surrounded by cells of mesodermal origin. Inductive signaling between these 2 tissue compartments plays a critical role in formation and differentiation of the lung, which is mediated by evolutionarily conserved signaling families used reiteratively during lung formation, including the fibroblast growth factor, hedgehog, retinoic acid, bone morphogenetic protein, and Wnt signaling pathways. Cells coordinate their response to these signaling proteins largely through transcription factors, which determine respiratory cell fate and pattern formation via the activation and repression of downstream target genes. Gain- and loss-of-function studies in null mutant and transgenic mice models have greatly facilitated the identification and hierarchical classification of these molecular programs. In this review, we highlight select molecular events that drive key phases of pulmonary development, including specification of a lung cell fate, primary lung bud formation, tracheoesophageal septation, branching morphogenesis, and proximal-distal epithelial patterning. Understanding the genetic pathways that regulate respiratory tract development is essential to provide insight into the pathogenesis of congenital anomalies and to develop innovative strategies to treat inherited and acquired lung disease.

Neural crest contribution to the cardiovascular system

Normal cardiovascular development requires complex remodeling of the outflow tract and pharyngeal arch arteries to create the separate pulmonic and systemic circulations. During remodeling, the outflow tract is septated to form the ascending aorta and the pulmonary trunk. The initially symmetrical pharyngeal arch arteries are remodeled to form the aortic arch, subclavian and carotid arteries. Remodeling is mediated by a population of neural crest cells arising between the mid-otic placode and somite four called the cardiac neural crest. Cardiac neural crest cells form smooth muscle and pericytes in the great arteries, and the neurons of cardiac innervation. In addition to the physical contribution of smooth muscle to the cardiovascular system, cardiac neural crest cells also provide signals required for the maintenance and differentiation of the other cell layers in the pharyngeal apparatus. Reciprocal signaling between the cardiac neural crest cells and cardiogenic mesoderm of the secondary heart field is required for elaboration of the conotruncus and disruption in this signaling results in primary myocardial dysfunction. Cardiovascular defects attributed to the cardiac neural crest cells may reflect either cell autonomous defects in the neural crest or defects in signaling between the neural crest and adjacent cell layers.

Inhibition of Tgf beta signaling by endogenous retinoic acid is essential for primary lung bud induction

Disruption of retinoic acid (RA) signaling during early development results in severe respiratory tract abnormalities, including lung agenesis. Previous studies suggest that this might result from failure to selectively induce fibroblast growth factor 10 (Fgf10) in the prospective lung region of the foregut. Little is known about the RA-dependent pathways present in the foregut that may be crucial for lung formation. By performing global gene expression analysis of RA-deficient foreguts from a genetic [retinaldehyde dehydrogenase 2 (Raldh2)-null] and a pharmacological (BMS493-treated) mouse model, we found upregulation of a large number of Tgfbeta targets. Increased Smad2 phosphorylation further suggested that Tgfbeta signaling was hyperactive in these foreguts when lung agenesis was observed. RA rescue of the lung phenotype was associated with low levels of Smad2 phosphorylation and downregulation of Tgfbeta targets in Raldh2-null foreguts. Interestingly, the lung defect that resulted from RA-deficiency could be reproduced in RA-sufficient foreguts by hyperactivating Tgfbeta signaling with exogenous TGF beta 1. Preventing activation of endogenous Tgfbeta signaling with a pan-specific TGFbeta-blocking antibody allowed bud formation and gene expression in the lung field of both Raldh2-null and BMS493-treated foreguts. Our data support a novel mechanism of RA-Tgfbeta-Fgf10 interactions in the developing foregut, in which endogenous RA controls Tgfbeta activity in the prospective lung field to allow local expression of Fgf10 and induction of lung buds.

Matthew-Wood syndrome is caused by truncating mutations in the retinol-binding protein receptor gene STRA6

Retinoic acid (RA) is a potent teratogen in all vertebrates when tight homeostatic controls on its endogenous dose, location, or timing are perturbed during early embryogenesis. STRA6 encodes an integral cell-membrane protein that favors RA uptake from soluble retinol-binding protein; its transcription is directly regulated by RA levels. Molecular analysis of STRA6 was undertaken in two human fetuses from consanguineous families we previously described with Matthew-Wood syndrome in a context of severe microphthalmia, pulmonary agenesis, bilateral diaphragmatic eventration, duodenal stenosis, pancreatic malformations, and intrauterine growth retardation. The fetuses had either a homozygous insertion/deletion in exon 2 or a homozygous insertion in exon 7 predicting a premature stop codon in STRA6 transcripts. Five other fetuses presenting at least one of the two major signs of clinical anophthalmia or pulmonary hypoplasia with at least one of the two associated signs of diaphragmatic closure defect or cardiopathy had no STRA6 mutations. These findings suggest a molecular basis for the prenatal manifestations of Matthew-Wood syndrome and suggest that phenotypic overlap with other associations may be due to genetic heterogeneity of elements common to the RA- and fibroblast growth factor-signaling cascades.

Cathespin H is an Fgf10 target involved in Bmp4 degradation during lung branching morphogenesis

During lung development, signaling by Fgf10 (fibroblast growth factor 10) and its receptor Fgfr2b is critical for induction of a gene network that controls proliferation, differentiation, and branching of the epithelial tubules. The downstream events triggered by Fgf10-Fgfr2b signaling during this process are still poorly understood. In a global screen for transcriptional targets of Fgf10, we identified Ctsh (cathepsin H), a gene encoding a lysosomal cysteine protease of the papain family, highly up-regulated in the developing lung epithelium. Here we show that among other cathepsin genes present in the lung, Ctsh is the only family member selectively induced by Fgf10 in the lung epithelium. We provide evidence that, during branching morphogenesis, epithelial expression of Ctsh overlaps temporally and spatially with that of Bmp4 (bone morphogenetic protein 4), another target of Fgf10. Moreover, we show that Ctsh controls the availability of mature Bmp4 protein in the embryonic lung and that inhibiting Ctsh activity leads to a marked accumulation of Bmp4 protein and disruption of branching morphogenesis. Tightly controlled levels of Bmp4 signaling are critical for patterning of the distal lung epithelium. Our study suggests a potentially novel posttranscriptional mechanism in which Ctsh rapidly removes Bmp4 from forming buds to limit Bmp4 action. The presence of both Ctsh and Bmp4 or Bmp4 signaling activity in other developing structures, such as the kidney, yolk sac, and choroid plexus, suggests a possible general role of Ctsh in regulating Bmp4 proteolysis in different morphogenetic events.

Rescue of morphogenetic defects and of retinoic acid signaling in retinaldehyde dehydrogenase 2 (Raldh2) mouse mutants by chimerism with wild-type cells

Retinoic acid (RA), the active vitamin A derivative, is an important developmental signaling molecule in vertebrates. In this study, we have assessed whether minimal numbers and/or specific distributions of RA-producing cells can support normal mouse embryonic development. Retinaldehyde dehydrogenase 2 (RALDH2) is the main RA-synthesizing enzyme acting during development. We have generated an embryonic stem (ES) cell line homozygous for an Raldh2 gene disruption, and have analyzed chimeric embryos with various contributions of wild-type cells. Whereas embryos almost completely derived from Raldh2(-/-) cells phenocopy the corresponding germline null mutants, the presence of even small numbers (<10%) of wild-type cells can rescue most of the morphogenetic defects, including embryonic turning and axial elongation, and left-right looping of the heart tube. No consistent bias in the distribution of wild-type cells was observed in the phenotypically rescued Raldh2(-/-) chimeras. Analysis of an RA-sensitive transgene indicates that RA can diffuse from wild-type cells and elicit a widespread transcriptional response in Raldh2-deficient cells. Our results show that few wild-type RA-producing cells, even when present in apparent random distributions, can support early morphogenesis of the mouse embryo. However, the Raldh2(-/-) chimeric fetuses display lung abnormalities, persistent truncus arteriosus, and abnormal myocardial differentiation, showing that subsequent RA-dependent events cannot be fully rescued by the mosaic presence of wild-type cells.

Retinoic acid is required for endodermal pouch morphogenesis and not for pharyngeal endoderm specification

Because tissues from all three germ layers contribute to the pharyngeal arches, it is not surprising that all major signaling pathways are involved in their development. We focus on the role of retinoic acid (RA) signaling because it has been recognized for quite some time that alterations in this pathway lead to craniofacial malformations. Several studies exist that describe phenotypes observed upon RA perturbations in pharyngeal arch development; however, these studies did not address whether RA plays multiple roles at distinct time points during development. Here, we report the resulting phenotypes in the hindbrain, the neural crest-derived tissues, and the pharyngeal endoderm when RA synthesis is disrupted during zebrafish gastrulation and pharyngeal arch morphogenesis. Our results demonstrate that RA is required for the post-gastrulation morphogenesis and segmentation of endodermal pouches, and that loss of RA does not affect the length of the pharyngeal ectoderm or medial endoderm along the anterior-posterior axis. We also provide evidence that RA is not required for the specification of pharyngeal pouch endoderm and that the pharyngeal endoderm consists of at least two different cell populations, of which the pouch endoderm is sensitive to RA and the more medial pharyngeal endoderm is not. These results demonstrate that the developmental processes underlying pharyngeal arch defects differ depending on when RA signaling is disturbed during development.

Morphogenesis of the trachea and esophagus: current players and new roles for noggin and Bmps

The development of the anterior foregut of the mammalian embryo involves changes in the behavior of both the epithelial endoderm and the adjacent mesoderm. Morphogenetic processes that occur include the extrusion of midline notochord cells from the epithelial definitive endoderm, the folding of the endoderm into a foregut tube, and the subsequent separation of the foregut tube into trachea and esophagus. Defects in foregut morphogenesis underlie the constellation of human birth defects known as esophageal atresia (EA) and tracheoesophageal fistula (TEF). Here, we review what is known about the cellular events in foregut morphogenesis and the gene mutations associated with EA and TEF in mice and humans. We present new evidence that about 70% of mouse embryos homozygous null for Nog, the gene encoding noggin, a bone morphogenetic protein (Bmp) antagonist, have EA/TEF as well as defects in lung branching. This phenotype appears to correlate with abnormal morphogenesis of the notochord and defects in its separation from the definitive endoderm. The abnormalities in foregut and lung morphogenesis of Nog null mutant can be rescued by reducing the gene dose of Bmp4 by 50%. This suggests that normal foregut morphogenesis requires that the level of Bmp4 activity is carefully controlled by means of antagonists such as noggin. Several mechanisms are suggested for how Bmps normally function, including by regulating the intercellular adhesion and behavior of notochord and foregut endoderm cells. Future research must determine how Noggin/Bmp antagonism fits into the network of other factors known to regulate tracheal and esophagus development, both in mouse or humans.

Regulation of early lung morphogenesis: questions, facts and controversies

During early respiratory system development, the foregut endoderm gives rise to the tracheal and lung cell progenitors. Through branching morphogenesis, and in coordination with vascular development, a tree-like structure of epithelial tubules forms and differentiates to produce the airways and alveoli. Recent studies have implicated the fibroblast growth factor, sonic hedgehog, bone morphogenetic protein, retinoic acid and Wnt signaling pathways, and various transcription factors in regulating the initial stages of lung development. However, the precise roles of these molecules and how they interact in the developing lung is subject to debate. Here, we review early stages in lung development and highlight questions and controversies regarding their molecular regulation.

Retinoic acid regulates morphogenesis and patterning of posterior foregut derivatives

Retinoic acid (RA) is an embryonic signaling molecule regulating a wide array of target genes, thereby being a master regulator of patterning and differentiation in a variety of organs. Here we show that mouse embryos deficient for the RA-synthesizing enzyme retinaldehyde dehydrogenase 2 (RALDH2), if rescued from early lethality by maternal RA supplementation between E7.5 and E8.5, lack active RA signaling in the foregut region. The resulting mutants completely fail to develop lungs. Development of more posterior foregut derivatives (stomach and duodenum), as well as liver growth, is also severely affected. A primary lung bud is specified in the RA-deficient embryos, which fails to outgrow due to defective FGF10 signaling and lack of activation of FGF-target genes, such as Pea3 and Bmp4 in the epithelium. Specific Hox and Tbx genes may mediate these RA regulatory effects. Development of foregut derivatives can be partly restored in mutants by extending the RA supplementation until at least E10.5, but lung growth and branching remain defective and a hypoplastic lung develops on the right side only. Such conditions poorly restore FGF10 signaling in the lung buds. Explant culture of RALDH2-deficient foreguts show a capacity to undergo lung budding and early branching in the presence of RA or FGF10. Our data implicate RA as a regulator of gene expression in the early embryonic lung and stomach region upstream of Hox, Tbx and FGF10 signaling.

Distinct roles for retinoic acid receptors alpha and beta in early lung morphogenesis

Retinoic acid (RA) signaling is required for normal development of multiple organs. However, little is known about how RA influences the initial stages of lung development. Here, we used a combination of genetic, pharmacological and explant culture approaches to address this issue, and to investigate how signaling by different RA receptors (RAR) mediates the RA effects. We analyzed initiation of lung development in retinaldehyde dehydrogenase-2 (Raldh2) null mice, a model in which RA signaling is absent from the foregut from its earliest developmental stages. We provide evidence that RA is dispensable for specification of lung cell fate in the endoderm. By using synthetic retinoids to selectively activate RAR alpha or beta signaling in this model, we demonstrate novel and unique functions of these receptors in the early lung. We show that activation of RAR beta, but not alpha, induces expression of the fibroblast growth factor Fgf10 and bud morphogenesis in the lung field. Similar analysis of wild type foregut shows that endogenous RAR alpha activity is required to maintain overall RA signaling, and to refine the RAR beta effects in the lung field. Our data support the idea that balanced activation of RAR alpha and beta is critical for proper lung bud initiation and endodermal differentiation.

Growth factors in lung development

Organized and coordinated lung development follows transcriptional regulation of a complex set of cell-cell and cell-matrix interactions resulting in a blood-gas interface ready for physiologic gas exchange at birth. Transcription factors, growth factors, and various other signaling molecules regulate epithelial-mesenchymal interactions by paracrine and autocrine mechanisms. Transcriptional control at the earliest stages of lung development results in cell differentiation and cell commitment in the primitive lung bud, in essence setting up a framework for pattern formation and branching morphogenesis. Branching morphogenesis results in the formation of the conductive airway system, which is critical for alveolization. Lung development is influenced at all stages by spatial and temporal distribution of various signaling molecules and their receptors and also by the positive and negative control of signaling by paracrine, autocrine, and endocrine mechanisms. Lung bud formation, cell differentiation, and its interaction with the splanchnic mesoderm are regulated by HNF-3beta, Shh, Nkx2.1, HNF-3/Forkhead homolog-8 (HFH-8), Gli, and GATA transcription factors. HNF-3beta regulates Nkx2.1, a transcription factor critical to the formation of distal pulmonary structures. Nkx2.1 regulates surfactant protein genes that are important for the development of alveolar stability at birth. Shh, produced by the foregut endoderm, regulates lung morphogenesis signaling through Gli genes expressed in the mesenchyme. FGF10, produced by the mesoderm, regulates branching morphogenesis via its receptors on the lung epithelium. Alveolization and formation of the capillary network are influenced by various factors that include PDGF, vascular endothelial growth factor (VEGF), and retinoic acid. Epithelial-endothelial interactions during lung development are important in establishing a functional blood-gas interface. The effects of various growth factors on lung development have been demonstrated by gain- or loss-of-function studies in null mutant and transgenic mice models. Understanding the role of growth factors and various other signaling molecules and their cellular interactions in lung development will provide us with new insights into the pathogenesis of bronchopulmonary dysplasia and disorders of lung morphogenesis.

Retinoic acid rescues lung hypoplasia in nitrofen-induced hypoplastic foetal rat lung explants

There is increasing evidence to suggest that the retinoid pathway is involved in the pathogenesis of congenital diaphragmatic hernia (CDH). We hypothesised that retinoids are involved in the pathogenesis of associated pulmonary hypoplasia in CDH and therefore designed this study to investigate the effects of retinoid acid on nitrofen-induced hypoplastic lungs. Pregnant rats were exposed to either olive oil or 100 mg nitrofen on day 9.5 of gestation. Foetal lungs were harvested on embryonic day 13.5 and were cultured for 96 h with or without exogenous retinoic acid (RA) (1 muM) added daily to the culture medium. Lungs were divided into four study groups: control (n=31); control + RA (n=19); nitrofen (n=19); and nitrofen + RA (n=12). Lung growth was assessed in each group by measuring branching morphogenesis, total DNA content and the proportion of proliferating cells stained by immunohistochemistry. One-way ANOVA test was used for statistical analysis. Retinoic acid significantly increased the growth of nitrofen-induced hypoplastic lungs, whilst growth of control lungs did not change. The number of lung buds and lung area of nitrofen-exposed hypoplastic lungs after 96 h of culture significantly increased after the addition of RA compared to the non-treated hypoplastic lungs (25.75+/-6.47 vs 15.11+/-3.29 and 0.98+/-0.18 mm(2) vs 0.65+/-0.13 mm(2), respectively; P<0.0001). Lung perimeter was also higher when RA was added to hypoplastic lungs compared to the non-treated ones, although it did not reach significance (12.51+/-2.53 mm vs 11.19+/-2.56 mm; P=0.17). Conversely, the addition of RA to control lungs did not affect the number of lung buds, lung area or lung perimeter after 96 h in culture compared to the non-treated ones (31.28+/-4.66 vs 31.81+/-6.67; 1.29+/-0.18(2) vs 1.29+/-0.23 mm(2) and 18.47+/-3.47 mm vs 17.89+/-2.94 mm, respectively; P=NS). Retinoic acid also increased the total DNA content and the proportion of proliferating cells in hypoplastic lungs compared to the non-treated ones (2.59+/-0.58 mug vs 1.96+/-0.31 mug and 57.89+/-9.46% vs 36.76+/-8.15%, respectively; P<0.001). The addition of RA did not affect either total DNA content or the proportion of proliferating cells in control lungs compared to the non-treated ones (4.04+/-0.64 mug vs 3.79+/-0.85 mug and 58.67+/-11.23% vs 56.03+/-10.36%, respectively; P=NS). This study demonstrates for the first time that RA rescues lung hypoplasia in nitrofen-induced hypoplastic lungs. These results suggest that retinoid pathway may be involved in the pathogenesis of associated pulmonary hypoplasia in CDH.

Molecular mechanisms of early lung specification and branching morphogenesis

The "hard wiring" encoded within the genome that determines the emergence of the laryngotracheal groove and subsequently early lung branching morphogenesis is mediated by finely regulated, interactive growth factor signaling mechanisms that determine the automaticity of branching, interbranch length, stereotypy of branching, left-right asymmetry, and finally gas diffusion surface area. The extracellular matrix is an important regulator as well as a target for growth factor signaling in lung branching morphogenesis and alveolarization. Coordination not only of epithelial but also endothelial branching morphogenesis determines bronchial branching and the eventual alveolar-capillary interface. Improved prospects for lung protection, repair, regeneration, and engineering will depend on more detailed understanding of these processes. Herein, we concisely review the functionally integrated morphogenetic signaling network comprising the critical bone morphogenetic protein, fibroblast growth factor, Sonic hedgehog, transforming growth factor-beta, vascular endothelial growth factor, and Wnt signaling pathways that specify and drive early embryonic lung morphogenesis.

The role of Fgf10 signaling in branching morphogenesis and gene expression of the rat prostate gland: lobe-specific suppression by neonatal estrogens

Brief exposure of rats to high-dose estrogen during the neonatal period interrupts prostate development in a lobe-specific manner and predisposes the gland to dysplasia with aging, a phenomenon referred to as developmental estrogenization. Our previous studies have revealed that these effects are initiated through altered steroid receptor expression; however, the immediate downstream targets remain unclear. We have recently shown that developmental expression of Shh-ptc-gli is downregulated in the dorsolateral prostate following estrogenization, and this is responsible, in part, for branching deficits observed in that prostatic region specifically. In the present study, we examine the role of Fgf10 signaling during rat prostate development and as a mediator of the developmental estrogenized phenotype. Fgf10 and FgfR2iiib localize to the distal signaling center of elongating and branching ducts in separate prostate lobes where they regulate the expression of multiple morphoregulatory genes including Shh, ptc, Bmp7, Bmp4, Hoxb13, and Nkx3.1. Ventral and lateral lobe organ cultures and mesenchyme-free ductal cultures demonstrate a direct role for Fgf10/FgfR2iiib in ductal elongation, branching, epithelial proliferation, and differentiation. Based on these findings, a model is proposed depicting the localized expression and feedback loops between several morphoregulatory factors in the developing prostate that contribute to tightly regulated branching morphogenesis. Similar to Shh-ptc-gli, neonatal estrogen exposure downregulates Fgf10, FgfR2iiib, and Bmp7 expression in the dorsolateral prostate while ventral lobe expression of these genes is unaffected. Lateral prostate organ culture experiments demonstrate that growth and branching inhibition as well as Fgf10/FgfR2iiib suppression are mediated directly at the prostatic level. Furthermore, exogenous Fgf10 fully rescues the growth and branching deficits due to estrogen exposure. Together, these studies demonstrate that alterations in Fgf10 signaling are a proximate cause of Shh-ptc-gli and Bmp7 downregulation that together result in branching inhibition of the dorsolateral prostate following neonatal estrogen exposure.

Engineering stem cells into organs: topobiological transformations demonstrated by beak, feather, and other ectodermal organ morphogenesis

To accomplish regenerative medicine, several critical issues in stem cell biology have to be solved, including the identification of sources, the expanding population, building them into organs, and assimilating them to the host. Although many stem cells can now differentiate along certain lineages, knowledge on how to use them to build organs lags behind. Here we focus on topobiological events that bridge this gap, for example, the regulation of number, size, axes, shape, arrangement, and architecture during organogenesis. Rather than reviewing detail molecular pathways known to disrupt organogenesis when perturbed, we highlight conceptual questions at the topobiological level and ask how cellular and molecular mechanisms can work to explain these phenomena. The avian integument is used as the Rosetta stone because the molecular activities are linked to organ forms that are visually apparent and have functional consequences during evolution with fossil records and extant diversity. For example, we show that feather pattern formation is the equilibrium of stochastic interactions among multiple activators and inhibitors. Although morphogens and receptors are coded by the genome, the result is based on the summed physical-chemical properties on the whole cell's surface and is self-organizing. For another example, we show that developing chicken and duck beaks contain differently configured localized growth zones (LoGZs) and can modulate chicken beaks to phenocopy diverse avian beaks in nature by altering the position, number, size, and duration of LoGZs. Different organs have their unique topology and we also discuss shaping mechanisms of liver and different ways of branching morphogenesis. Multi-primordium organs (e.g., feathers, hairs, and teeth) have additional topographic specificities across the body surface, an appendage field, or within an appendage. Promises and problems in reconstitute feather/hair follicles and other organs are discussed. Finally, simple modification at the topobiological level may lead to novel morphology for natural selection at the evolution level.