Identification of a SOX2/SOX9 Double Positive Cells Progenitor Cell Population Required for Branching Morphogenesis in Human Lung

Abstract

BackgroundTo date, the proximal‐distal (PD) axis of the lung has been distinguished by the localization of two main transcription factors: Sox2 and Sox9. In the developing mouse lung, distal tip epithelial progenitors express Sox9, whereas proximal progenitors in the stalk express Sox2. Moreover, in the mouse lung, Fgf10 controls the PD axis by preventing the differentiation of epithelial Sox9+ progenitors into Sox2‐expressing airway cells. Whereas lung branching has been extensively studied in animal models little is known about the mechanisms underlying human lung branching and whether findings from mouse apply to human. We aimed to determine the role of FGF signaling in human fetal lung.MethodsFresh human fetal lung tissue between gestational ages 10–16 weeks were collected and either stained or used for culture. Distal lung segments (1–3 mm3) with intact edges were cultured in air‐liquid interface for up to 72 hours. Explants were treated with different fibroblast growth factors (FGFs) known to alter PD epithelial cell differentiation as well as the tyrosine kinase inhibitor (SU5402).ResultsCareful examination of the human fetal lung showed that, similar to mouse, proximal progenitors express SOX2. However, the distal tip progenitors co‐expressed SOX2 and SOX9, a pattern never seen in mouse. This SOX2/SOX9+ progenitor population persists during the pseudoglandular stage up to around 16 weeks gestation. Moreover, we showed that in response to FGF7 and FGF9, branching was impaired accompanied by cysting of distal buds, decreased SOX2 expression, expansion of SOX9 with decreased of SOX2/SOX9 double positive cells. FGF10 did not promote branching in whole explant cultures or in epithelial bud cultures. A decline in SOX2/SOX9 double positive cells was also observed. In contrast, treatment of human lung explants with SU5402, a tyrosine kinase inhibitor, resulted in expansion of SOX2 and decrease of SOX9, resulting in the decline of SOX2/SOX9+ progenitor cell population.ConclusionIn conclusion, we identified the presence of a new progenitor cell population in the human lung co‐expressing SOX2 and SOX9, never seen in mouse. While FGF7 and FGF9 have similar effects on human and mouse lung branching, the effect of Fgf10 was different in that it failed to induce branching of human lung. We believe that this is mainly due to the alteration of the cells co‐expressing SOX2/SOX9+. We therefore conclude that the maintenance of this progenitor population is required for proper branching morphogenesis in human lung. However, we still need to identify the signals required for the maintenance of SOX2/SOX9 progenitor cell identity. Understanding the mechanisms driving airway branching will transform our concepts of human lung development and may allow for the discovery of possible therapeutic avenues to restore or enhance lung development for severely premature infants at increased risk for lung diseases.Support or Funding InformationAmerican Heart Association, Saban Research Institute