PROJECT ABSTRACT The lung is an architecturally complex organ, comprised of specialized epithelial cells surrounded by a dense network of mesenchyme. Recent work from our lab as well as others has shown that spatially discreet epithelial to mesenchymal niche signaling is required for proper lung development as well as adult tissue quiescence and regeneration. These studies prompted us to develop an in vivo pathway reporter system to monitor mesenchymal signaling pathway activity. Using this reporter system, we have deconstructed the mesenchyme and classified lineages using innovative methods; Single-Cell and Lineage-Seq transcriptome profiling, three-dimensional spatial orientation, in vivo response to injury and ex vivo niche support capacity. We have characterized a mesenchymal alveolar niche cell (MANC) as Wnt responsive, expresses platelet- derived growth factor receptor alpha (PDGFRa), and is critical for alveolar epithelial cell growth and self- renewal. In contrast, the Axin2+ myofibrogenic progenitor (AMP) cell generates pathologically deleterious myofibroblasts after injury. These studies provide a platform for defining the cellular and molecular framework of the lung mesenchymal niches. Moving forward, in Aim 1 of this proposal, I will examine the lineage ontogeny of Axin2-positive mesenchyme and elucidate the role of PDGFRa-signaling in controlling lineage allocation and development of the alveolar niche during the postnatal period of alveologenesis. In the independent phase outlined in Aim 2, I will define mechanisms controlling the pro-regenerative versus myofibrogenic responses in the mesenchyme after acute lung injury. Based on bioinformatic analyses using multicellular alignment of ligand and cognate receptor pairs from the alveolar niche, I will define the impact of Notch and Cxcr4 pathways in provoking the myofibrogenic response from the AMP lineage, in vivo. Importantly, this proposal outlines a rigorous training plan that will establish the foundation to advance my career in biomedical research.