Glucocorticoid regulation of lung development: lessons learned from conditional GR knockout mice

Abstract

Glucocorticoid (GC) steroid hormones have well-characterized roles in the regulation of systemic homeostasis, yet less understood is their known role in utero to mature the developing respiratory system in preparation for birth. During late gestation, endogenously produced GCs thin the interstitial tissue of the lung, causing the vasculature and future airspaces to come into close alignment, allowing for efficient gas exchange at birth. More potent synthetic GCs are also used worldwide to reduce the severity of respiratory distress suffered by preterm infants; however, their clinical benefits are somewhat offset by potential detrimental long-term effects on health and development. Here, we review the recent literature studying both global and conditional gene-targeted respiratory mouse models of either GC deficiency or glucocorticoid receptor ablation. Although some discrepancies exist between these transgenic mouse strains, these models have revealed specific roles for GCs in particular tissue compartments of the developing lung and identify the mesenchyme as the critical site for glucocorticoid receptor-mediated lung maturation, particularly for the inhibition of cell proliferation and epithelial cell differentiation. Specific mesenchymal and epithelial cell-expressed gene targets that may potentially mediate the effect of GCs have also been identified in these studies and imply a GC-regulated system of cross talk between compartments during lung development. A better understanding of the specific roles of GCs in specific cell types and compartments of the fetal lung will allow the development of a new generation of selective GC ligands, enabling better therapeutic treatments with fewer side effects for lung immaturity at birth in preterm infants.

Figure 1.. An overview of the stages of lung development.

A, The embryonic phase (E9.5–E12.5) begins with the appearance of 2 primordial buds, which evaginate ventrally from the foregut endoderm. Separation of the trachea and esophagus is complete by ∼E11.5, by which time airway branching has begun. B, The pseudoglandular stage is characterized by extensive dichotomous branching of the newly formed buds to form the respiratory tree. C, During the pseudoglandular phase, the epithelium begins to differentiate into the various cell lineages of the conducting airway including mucosal, neuroendocrine, ciliated, and secretory cells. Differentiation of the epithelium progresses in a proximal to distal manner, with proximal progenitors losing expression of the distal multipotent cell marker Sox9, but gaining expression of the proximal marker Sox2 as well as specific markers of conducting airway lineages. D, During the canalicular /saccular phase (E16.5–PN5), the distal lung epithelium expands to form “sacculi” while the mesenchyme thins. Differentiation of distal epithelial progenitors leads to formation of the BADJ (the boundary between the conducting airway and the alveolar epithelium), as well as type I and type II AECs. E, In the alveolar phase (PN5–PN30), elastin fiber deposition by myofibroblasts drives the formation of secondary septae, which subdivide sacculi into alveoli (i). Secondary septae initially include dual capillary layers (ii), which finally fuse via microvascularization to form a single capillary layer (iii). A, anterior; D, dorsal; L, left; P, posterior; R, right; V, ventral. Epithelial markers used the following: Sox2, SRY-box containing gene 2; Scgb1a1, secretoglobin, family 1A, member 1 (uteroglobin); FoxJ1, forkhead box J1; Spdef, SAM pointed domain containing ets transcription factor; CGRP, calcitonin/calcitonin-related polypeptide, α; Sox9, SRY-box containing gene 9; Aqp5, aquaporin 5; Sftpb, surfactant associated protein B; Sftpc, surfactant-associated protein C; αSMA, actin, α 2, smooth muscle, aorta; CD31, platelet/endothelial cell adhesion molecule 1.

Figure 2.. Endogenous and exogenous GC effects on murine lung maturation.

A, Lung morphology in E16.5 and E18.5 GR^−/− fetal mice. B, Endogenous GC function during late lung development. From E16.5 to E18.5, GC-mediated remodeling of the distal lung results in thinner saccular walls and expansion of the airway lumen. In mouse models of GC deficiency, saccular walls are thicker with less luminal expansion. C, Left lung lobe morphology in E16.5 wild-type mice treated with saline (left) or Dex (right). Insets show magnified regions of the distal lung parenchyma. D, Lung morphology in PN7 mice treated with saline (left) or Dex (right).

Figure 3.. Variation in respiratory phenotype between conditional GR knockout mice.

A, Representation of GR deletion in the developing lung epithelium and mesenchyme and globally in all compartments. Black layers depict GR deletion. B, E18.5 lung morphology in the respective GR knockout mice. Note that representative lung images shown are from the mouse strains described in Bird et al (40). C, Summary of the neonatal survival and morphological lung phenotypes observed in (1) Manwani et al (42), (2) Bird et al (40), (3) Habermehl et al (32), and (4) Li et al (41).

Figure 4.. GR-mediated mechanisms in the distal mouse lung during late gestation (late pseudoglandular to saccular phases).

A, Mesenchymal GR restrains cell proliferation in the lung via inhibition of Vcan expression and promotes elastin fiber synthesis via induction of Eln and Lox expression. GR-mediated inhibition of Tnc and Fbn2 expression may also participate in correct elastin fiber synthesis. B, Model of mesenchymal GR-regulated epithelial cell differentiation in the distal lung. The distal lung epithelium contains a pool of Sox9⁺ progenitors, which are stimulated by mesenchymal GR signaling to become either bronchoalveolar stem cells (BASCs), forming the BADJ, or more distally, type I AECs. Type II AEC differentiation is not affected by GR signaling. C, Model of GR-mediated type II AEC maturation by induction of surfactant-associated factors. Mesenchymal GR induces expression of genes including Nrg1β, Fgf7, and leptin, which act on type II AECs via paracrine signaling. Epithelial GR induces expression of genes including Fasn and Abca3 via autocrine signaling.