The pathogenesis of fibrotic lung disease involves the inability of proliferating alveolar type II cells (AT2) to differentiate effectively into type I (AT1) cells, leading to faulty epithelial repair, irreversible damage, loss of function, and fibrosis. The mechanisms that normally control this process are not fully understood, so potential regulatory molecules or pathways that may be altered in fibrotic pulmonary diseases have not been elucidated. We propose that the key to normal alveolar cell differentiation is the relative sulfation of the extracellular matrix (ECM) microenvironment underlying alveolar cell types. This in turn controls expression of two important differentiation factors: a member of the forkhead (Fox) family of transcription factors and specific wingless (Wnt) signaling pathways. These factors act in conjunction with expression and signaling of transforming growth factor ? (TGF?), which enhances Wnt signaling targets, to collectively drive the cell differentiation process and establish stable alveolar phenotypes. In this proposal, we will show that following proliferative events associated with re-epithelialization in the alveolus, there is a critical, dynamic balance between alveolar epithelial cells and their ECM microenvironment. This is significantly modulated by both fixed and soluble sulfated ECMs, whose downstream effects are to specifically enhance Wnt7a and Foxa1 expression, which act together with TGF? to regulate the shift from the AT2 phenotype and control AT1 cell differentiation. The hypothesis to be addressed is: Following DNA synthesis and cytokinesis, exposure of the daughter AT2 cell to high levels of sulfated ECMs triggers enhanced expression of Foxa1 and Wnt7a in parallel with increased TGF? expression and signaling, which converge to effectively drive differentiation of AT2 to AT1 cells. To address this hypothesis, we will utilize isolated AT2 cells from humans and normal as well as conditional gene knockouts and overexpressors from rodents in traditional and modified co-culture with human and rodent fibroblasts - important regulators of the AT2 cell microenvironment. Cells and ECMs will be selectively modified with specific enhancers or inhibitors of Fox expression, and TGF? and Wnt expression and signaling, and ECM composition. These results will serve as a contextural backdrop for examination of targeted molecules by protein and/or gene expression methods in a whole animal model of alveolar injury and fibrosis. Results of these studies are expected to provide essential information needed to better understand basic cell-cell and cell-ECM relationships in alveolar epithelial homeostasis as well as the mechanisms that steer the pathogenesis of fibrogenic change in the lung as a consequence of alveolar injury and/or disease.
Disease or environmentally-based toxic agents can damage or injure cells that line the internal surfaces of the lung, and how these cell repopulate themselves in large part determines whether the lung heals or not. To accomplish this, specific cells must divide and differentiate into other cells that carry out critically important lung-specific functions. Interruption of this process has serious consequences resulting in faulty repair and permanent damage to the lung and compromises function. This grant will define the specific mechanisms that control this process, and in doing so, enable the development of new and innovative ways to promote the healing process in injured lungs.
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