Transitional Alveolar Epithelial Cells in Normal Lung Development - Minoo, Parviz ABSTRACT: In the face of injury, the adult lung mobilizes AT2 facultative stem cells to repair and reestablish homeostasis and respiratory function. What about immature lungs undergoing alveologenesis in prematurely born infants? To our knowledge, there is virtually no information regarding the existence or potential regenerative capacity in immature, AT2 cells in lungs undergoing alveologenesis. This is important information with significant bearing on pathogenesis of BPD which results from injuries in immature lungs during alveologenesis. The scientific premise of this project is grounded in our surprising findings of defined intermediate/transitional alveolar epithelial cells (AEC) in the process of AT2→AT1 differentiation in murine neonatal lungs undergoing alveologenesis. We show for the first time that these “normal” intermediate/transitional cells share several transcriptional features with recently reported transitional/intermediate AECs that accumulate in human IPF and murine bleomycin injury models and thought to be cells “caught” in the AT2→AT1 differentiation process as part of the regenerative response to injury. This suggests that the biological mechanisms of AT2→AT1 process during normal alveologenesis may have molecular and functional overlapping similarities to those of AT2→AT1 differentiation during regenerative repair in response to lung injury. The biology & functional significance of these cells in the adult or the neonate remain largely unknown. In preliminary data we demonstrate that the neonatal intermediate/transitional AECs in immature lungs during alveologenesis reside within alveolar niches in close proximity to Hedgehog-responsive, Pdgfra(+), mesenchymal cells we first identified as Secondary Crest Myofibroblasts or SCMF. Genetic loss of function studies show that SCMF control the ontogeny of intermediate/transitional AECs and in particular self-renewal of a unique proliferative AEC subpopulation during alveologenesis. SCMF are in turn maintained by TGF-beta signaling. Therefore, SCMF~AEC axis constitutes an alveolar intra-niche communication network. In this application, combining our results in normal neonatal lung, with findings from adult injury and disease models, we will use a multiplex technical approach that combines single cell RNA sequencing, snRNAse+sc/ATACseq Multi-Omic, in vivo lineage tracing, hyperoxia injury model and 3D organoid cultures to conduct “deep-dive” studies into the biology of neonatal intermediate transitional AECs and their regulation in neonatal and adult mouse lungs. We project that the deliverables from these studies will impact not only our understanding of pathogenesis of neonatal chronic lung disease BPD, but also may provide significant insight into the biology of the recently found “aberrant” transitional epithelial cells in adult pulmonary diseases such as IPF.
