The contribution of mesothelial cells to lung development
Dixit, Radhika Nagaraj
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Mesothelium-derived progenitors have been demonstrated to contribute to differentiated mesenchymal components of the heart, liver, and gut during organogenesis. The precise contribution of the mesothelium to lung development, however, has not been fully clarified and the key signals regulating mesothelial cell entry have not been identified. To rigorously address this issue, we employed mice with an inducible Cre expressed from the Wilm's tumor-1 (WT1) locus for high fidelity lineage tracing after confirming that Cre-recombinase was mesothelial-specific and faithfully recapitulated endogenous WT1 gene expression. We visualized WT1+ mesothelial cell entry into the fetal lung by live imaging and identified their progenies in subpopulations of bronchial smooth muscle cells, vascular smooth muscle cells, and desmin+ fibroblasts by lineage tagging. In view of the role of Sonic Hedgehog (Hh) signaling in regulating mesenchymal cell differentiation and epithelial-mesenchymal transition, we hypothesized that this pathway regulates events associated with migration of mesothelial cells into the developing lung. To examine for this, we first used two independent reporter mice to show that Hh signaling is active within the lung mesothelium at time points coinciding with the appearance of mesothelium-derived cells in the lung parenchyma. Using loss-of-function assays in organ cultures, and targeted mesothelial-restricted loss-of hedgehog function mice, we demonstrated that mesothelial cell movement into the lung requires the direct action of Hh signaling. In order to examine whether WT1 interacts with Hh pathway, we conducted ChIP assays on fetal lung mesothelial cells, and found that WT1 directly binds and regulates promoter elements of downstream targets of Hh pathway. Consistent with this observation, Hh pathway gene expression was down-regulated in isolated WT1 deficient fetal lung mesothelial cells. Taken together, these findings lend further support to a paradigm in which mesothelial cells are an important source of progenitors for mesenchymal structures. Our findings also reveal a role for Hh pathway in the early events associated with mesothelial cell entry and indicate that WT1 likely acts upstream of Hh signaling.
Thesis (Ph.D.)--Boston University