Regulation of primary and secondary alveologenesis by FGF signaling pathways

Washington University, Saint Louis MO

Investigators

Abstract

Title: Regulation of primary and secondary alveologenesis by FGF signaling pathways Summary: Alveologenesis is the final phase of lung development where the surface area of the lung is increased by subdividing alveolar saccules through the formation of secondary septae (septal ridges), followed by thinning of the septal walls to generate an efficient air/blood gas exchange organ. Bronchopulmonary dysplasia (BPD) is a common complication of preterm birth in which alveologenesis is impaired. BPD often results in chronic respiratory disease. However, besides Vitamin A and supportive care, no therapies exist to promote lung alveolar and vascular development to improve the outcomes of infants with BPD. Alveologenesis occurs in several stages in mice and humans. The initial expansion of alveolar surface area occurs in the absence of alveolar myofibroblasts (MyoFB) in the saccular stage. In the classical or first stage of alveologenesis, MyoFB and other mesenchymal and epithelial cells regulate the formation of secondary septae. During the second stage of alveologenesis, some additional alveoli are formed and, importantly, the septal walls thin through loss of mesenchymal cells and maturation of the microvasculature to a single layer capillary network juxtaposed with AT1 cells. The mature alveolar wall ensures efficient air/blood gas exchange in the adult lung. An in-depth understanding the mechanisms that regulate alveolar septation and septal wall maturation will be required to develop therapies for premature infants with BPD and for developing potential therapies for adult lung regeneration. However, there are specific knowledge gaps about the identity and regulation of progenitors that give rise to MyoFB, the functions of MyoFB and other mesenchymal cell types, and the mechanisms that terminate and clear MyoFB from the lung at the completion of secondary septation. Fibroblast Growth Factor 18 (Fgf18) is expressed at high levels in MyoFB and AT1 cells during alveologenesis. In preliminary data, we show that conditional inactivation of Fgf18 in the neonatal lung results in impaired alveologenesis and increased expression of Fgf9 in mature MyoFB. Through lineage tracing of Fgf18- expressing cells, we find that MyoFB are cleared from the lung after the first stage of alveologenesis but that Fgf18-expressing AT1 cells are retained, suggesting that FGF18 could be involved in septal wall maturation. In this proposal, we use a combination of unique genetic tools to target specific cell populations in the neonatal lung, single nucleus RNA sequencing, and cell sorting coupled with single nucleus ATAC sequencing, to identify the cellular mechanisms by which the FGF signaling pathway regulates cellular interactions, cell proliferation, differentiation, and death, during the first and second stages of alveologenesis. Our research goals include the identification of novel transcriptional mechanisms that regulate MyoFB function during primary alveologenesis, and cellular mechanisms by which FGF signaling regulates septal maturation during secondary alveologenesis. This research will impact our understanding of pathogenic mechanisms affecting lung maturation in BPD and mechanisms that could regulate differentiation and expansion of MyoFB in interstitial lung disease.