Lung epithelial branching program antagonizes alveolar differentiation

Abstract

Mammalian organs, including the lung and kidney, often adopt a branched structure to achieve high efficiency and capacity of their physiological functions. Formation of a functional lung requires two developmental processes: branching morphogenesis, which builds a tree-like tubular network, and alveolar differentiation, which generates specialized epithelial cells for gas exchange. Much progress has been made to understand each of the two processes individually; however, it is not clear whether the two processes are coordinated and how they are deployed at the correct time and location. Here we show that an epithelial branching morphogenesis program antagonizes alveolar differentiation in the mouse lung. We find a negative correlation between branching morphogenesis and alveolar differentiation temporally, spatially, and evolutionarily. Gain-of-function experiments show that hyperactive small GTPase Kras expands the branching program and also suppresses molecular and cellular differentiation of alveolar cells. Loss-of-function experiments show that SRYbox containing gene 9 (Sox9) functions downstream of Fibroblast growth factor (Fgf)/Kras to promote branching and also suppresses premature initiation of alveolar differentiation. We thus propose that lung epithelial progenitors continuously balance between branching morphogenesis and alveolar differentiation, and such a balance is mediated by dual-function regulators, including Kras and Sox9. The resulting temporal delay of differentiation by the branching program may provide new insights to lung immaturity in preterm neonates and the increase in organ complexity during evolution.

Epithelial inactivation of Yy1 abrogates lung branching morphogenesis

“…Yy1 function is thus required in both mesenchymal and epithelial cell layers for the correct development of the respiratory tract. Epithelial inactivation of the Yy1 gene inhibited lung branching but maintained distal epithelial cell differentiation, which agrees with the notion of negative correlation between branching and alveolar differentiation (Chang et al, 2013). The Yy1 mutation also caused the formation of large cysts that mimicked the Shh lung phenotype (Litingtung et al, 1998;Pepicelli et al, 1998).…”

“…Recent data indicate that the transition from branching morphogenesis to epithelial cell differentiation depends on control mechanisms involving key players of lung development such as Fgf10 and SOX9. These latter act by regulating the delicate balance between distal and proximal endodermal progenitors, promoting branching and preventing precocious alveolar differentiation (Chang et al, 2013;Volckaert et al, 2013;Yang and Chen, 2014).…”

FGF signaling activates a Sox9-Sox10 pathway for the formation and branching morphogenesis of mouse ocular glands

“…Sox9 regulates Mia1 expression through binding to a SOX consensus sequence in the Mia1 promoter (Xie et al, 1999). Mia1 is also an FGF-regulated protein during lung branching morphogenesis (Lin et al, 2008) and is decreased in expression in the Sox9 mutant lung (Chang et al, 2013). Our data showed that Mia1 mRNA was strongly expressed in primary lacrimal bud tip cells at E13.5 (Fig.…”

DA-Raf–dependent inhibition of the Ras-ERK signaling pathway in type 2 alveolar epithelial cells controls alveolar formation

“…During tracheal morphogenesis, the distribution of the activated ERK1/2 in epithelial cells at terminal lung buds is regulated by Sprouty1/2, which determines mitotic spindle orientation and airway shape (14,15). Furthermore, the Ras-ERK signaling pathway in Sox9 + cells at the branch tips of developing lungs regulates the balance between branching and alveolar differentiation (16). Our findings demonstrate that DA-Raf predominantly inhibits the Ras-ERK signaling pathway in AEC2s during alveolar formation.…”

“…The Ras-ERK signaling pathway is one of the key regulators for not only tracheal branching but also alveolar formation in developing lungs (14)(15)(16). The Ras-ERK signaling pathway transmits extracellular stimuli from the plasma membrane to the cytoplasm or the nucleus (17)(18)(19).…”