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

Transmural pressure signals through retinoic acid to regulate lung branching

During development, the mammalian lung undergoes several rounds of branching, the rate of which is tuned by the relative pressure of the fluid within the lumen of the lung. We carried out bioinformatics analysis of RNA-sequencing of embryonic mouse lungs cultured under physiologic or sub-physiologic transmural pressure and identified transcription factor-binding motifs near genes whose expression changes in response to pressure. Surprisingly, we found retinoic acid (RA) receptor binding sites significantly overrepresented in the promoters and enhancers of pressure-responsive genes. Consistently, increasing transmural pressure activates RA signaling, and pharmacologically inhibiting RA signaling decreases airway epithelial branching and smooth muscle wrapping. We found that pressure activates RA signaling through the mechanosensor Yap. A computational model predicts that mechanical signaling through Yap and RA affects lung branching by altering the balance between epithelial proliferation and smooth muscle wrapping, which we test experimentally. Our results reveal that transmural pressure signals through RA to balance the relative rates of epithelial growth and smooth muscle differentiation in the developing mouse lung and identify RA as a previously unreported component in the mechanotransduction machinery of embryonic tissues.

Fgf10/Fgfr2b Signaling Orchestrates the Symphony of Molecular, Cellular, and Physical Processes Required for Harmonious Airway Branching Morphogenesis

Airway branching morphogenesis depends on the intricate orchestration of numerous biological and physical factors connected across different spatial scales. One of the key regulatory pathways controlling airway branching is fibroblast growth factor 10 (Fgf10) signaling via its epithelial fibroblast growth factor receptor 2b (Fgfr2b). Fine reviews have been published on the molecular mechanisms, in general, involved in branching morphogenesis, including those mechanisms, in particular, connected to Fgf10/Fgfr2b signaling. However, a comprehensive review looking at all the major biological and physical factors involved in branching, at the different scales at which branching operates, and the known role of Fgf10/Fgfr2b therein, is missing. In the current review, we attempt to summarize the existing literature on airway branching morphogenesis by taking a broad approach. We focus on the biophysical and mechanical forces directly shaping epithelial bud initiation, branch elongation, and branch tip bifurcation. We then shift focus to more passive means by which branching proceeds, via extracellular matrix remodeling and the influence of the other pulmonary arborized networks: the vasculature and nerves. We end the review by briefly discussing work in computational modeling of airway branching. Throughout, we emphasize the known or speculative effects of Fgfr2b signaling at each point of discussion. It is our aim to promote an understanding of branching morphogenesis that captures the multi-scalar biological and physical nature of the phenomenon, and the interdisciplinary approach to its study.

Retinoic acid in developmental toxicology: Teratogen, morphogen and biomarker

This review explores the usefulness retinoic acid (RA) related physiological factors as possible biomarkers of embryotoxicity. RA is involved in the morphogenesis of the early embryo as well as in the development and maturation of a wide variety of organ anlagen. The region-specific homeostasis of RA in the embryo is in many ways the driving force determining developmental cell proliferation versus differentiation. As a consequence, RA concentrations are carefully controlled in time and space in the developing embryo. RA deficiency and overdosing both result in characteristic patterns of malformations that may involve many different organ systems. The central role of RA in embryo development provides us with a set of sensitive biomarkers that may be employed in developmental toxicity testing. This includes the synthesizing and metabolizing enzymes of RA, but also a myriad of related morphogenetic factors and their genes, of which the expression may be affected by changes in RA balance. Several examples of embryotoxicants interfering with the homeostasis of RA and related parameters have been described. A preliminary adverse outcome pathway framework for RA mediated malformations has been published. Expansion of this framework and its application in developmental toxicity testing may allow the detection of a large variety of embryotoxicants with diverse modes of action. RA homeostasis therefore provides a promising set of molecular tools that may be employed in the advancement of mode of action driven animal-free developmental toxicity testing.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSION

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

Defective parasympathetic innervation is associated with airway branching abnormalities in experimental CDH

Developmental mechanisms leading to lung hypoplasia in congenital diaphragmatic hernia (CDH) remain poorly defined. Pulmonary innervation is defective in the human disease and in the rodent models of CDH. We hypothesize that defective parasympathetic innervation may contribute to airway branching abnormalities and, therefore, lung hypoplasia, during lung development in CDH. The murine nitrofen model of CDH was utilized to study the effect of the cholinergic agonist carbachol on embryonic day 11.5 (E11.5) lung explant cultures. Airway branching and contractions were quantified. In a subset of experiments, verapamil was added to inhibit airway contractions. Sox9 immunostaining and 5-bromo-2-deoxyuridine incorporation were used to identify and quantify the number and proliferation of distal airway epithelial progenitor cells. Intra-amniotic injections were used to determine the in vivo effect of carbachol. Airway branching and airway contractions were significantly decreased in nitrofen-treated lungs compared with controls. Carbachol resulted in increased airway contractions and branching in nitrofen-treated lungs. Nitrofen-treated lungs exhibited an increased number of proliferating Sox9-positive distal epithelial progenitor cells, which were decreased and normalized by treatment with carbachol. Verapamil inhibited the carbachol-induced airway contractions in nitrofen-treated lungs but had no effect on the carbachol-induced increase in airway branching, suggesting a direct carbachol effect independent of airway contractions. In vivo treatment of nitrofen-treated embryos via amniotic injection of carbachol at E10.5 resulted in modest increases in lung size and branching at E17.5. These results suggest that defective parasympathetic innervation may contribute to airway branching abnormalities in CDH.

A novel in vitro model to study alveologenesis

Many pediatric pulmonary diseases are associated with significant morbidity and mortality due to impairment of alveolar development. The lack of an appropriate in vitro model system limits the identification of therapies aimed at improving alveolarization. Herein, we characterize an ex vivo lung culture model that facilitates investigation of signaling pathways that influence alveolar septation. Postnatal Day 4 (P4) mouse pup lungs were inflated with 0.4% agarose, sliced, and cultured within a collagen matrix in medium that was optimized to support cell proliferation and promote septation. Lung slices were grown with and without 1D11, an active transforming growth factor-β-neutralizing antibody. After 4 days, the lung sections (designated P4 + 4) and noncultured lung sections were examined using quantitative morphometry to assess alveolar septation and immunohistochemistry to evaluate cell proliferation and differentiation. We observed that the P4 + 4 lung sections exhibited ex vivo alveolarization, as evidenced by an increase in septal density, thinning of septal walls, and a decrease in mean linear intercept comparable to P8, age-matched, uncultured lungs. Moreover, immunostaining showed ongoing cell proliferation and differentiation in cultured lungs that were similar to P8 controls. Cultured lungs exposed to 1D11 had a distinct phenotype of decreased septal density when compared with untreated P4 + 4 lungs, indicating the utility of investigating signaling in these lung slices. These results indicate that this novel lung culture system is optimized to permit the investigation of pathways involved in septation, and potentially the identification of therapeutic targets that enhance alveolarization.

Amniotic fluid stem cells rescue both in vitro and in vivo growth, innervation, and motility in nitrofen-exposed hypoplastic rat lungs through paracrine effects

Lung hypoplasia can be prevented in vitro by retinoic acid (RA). Recent evidence suggests that amniotic fluid stem (AFS) cells may integrate injured lungs and influence their recovery. We tested the hypothesis that AFS cells might improve lung growth and motility by paracrine mechanisms. Pregnant rats received either nitrofen or vehicle on E9.5. In vitro E13 embryonic lungs were cultured in the presence of culture medium alone or with RA, basophils, or AFS cells. In vivo green fluorescent protein-expressing (GFP(+)) rat AFS cells were transplanted in nitrofen-exposed rats on E10.5. E13 lung explants were cultured before analysis. The surface, the number of terminal buds, and the frequency of bronchial contractions were assessed. Protein gene product 9.5 (PGP 9.5) and α-actin protein levels were measured. The lung explants transplanted with AFS cells were stained for α-actin, PGP 9.5, and TTF-1. The levels of FGF-10, VEGFα, and TGF-β1 secreted by the AFS cells in the culture medium were measured. Comparison between groups was made by ANOVA. In vitro, the surface, the number of terminal buds, and the bronchial peristalsis were increased in nitrofen+AFS cell explants in comparison with nitrofen-exposed lungs. While nitrofen+RA lungs were similar to nitrofen+AFS ones, basophils did not normalize these measurements. PGP 9.5 protein was decreased in nitrofen lungs, but after adding AFS cells, the value was similar to controls. No differences were found in the expression of α-actin. In vivo, the surface, number of terminal buds, and peristalsis were similar to control after injection of AFS cells in nitrofen-exposed rats. Colocalization with TTF-1-positive cells was found. The levels of FGF-10 and VEGFα were increased in nitrofen+AFS cell explants, while the levels of TGF-β1 were similar to controls. Lung growth, bronchial motility, and innervation were decreased in nitrofen explants and rescued by AFS cells both in vitro and in vivo, similarly to that observed before with RA. The AFS cell beneficial effect was probably related to paracrine action of growth factor secretion.