Defining the mechanistic basis of the airway metaplastic response: the roles of stem cell heterogeneity, Yap, and EGFR signaling

About This Grant

Although squamous and mucous metaplasia are the two cardinal forms of pathologic epithelial injury response in airway disease, the cellular source and molecular mechanisms governing their formation have not been clearly defined. It has long been assumed that both forms of metaplasia arise from a common basal stem cell population biased by specific signaling factors, however, both metaplasias occur together in the same patient in the same signaling milieu. Furthermore, we have previously reported that the distal murine tracheal epithelium is predisposed to mucous metaplasia, while squamous metaplasia tends to form in the dorsal murine and human airway. In parallel, we have reported that specific hillock basal stem cells are found in dorsally located stratified squamous epithelial structures that we named hillocks. In aggregate, these findings suggest the hypothesis that squamous and mucous metaplasia arise from regionally distinct stem cell populations and that these heterogeneous stem cell populations respond differently to common pathologic signaling cascades. With regard to mechanism, high Yap signaling activity has been associated with squamous metaplasia while mucous cell differentiation requires a suppression of Yap activity. As such, we will establish the propensity of anatomically regionalized basal stem cell populations of the mouse and human airway to undergo either squamous or mucous metaplasia including (1) dorsally located murine and human hillock basal stem cells, (2) proximal and (3) distal pseudostratified murine and human basal stem cells populations. Since Yap has been directly associated with mouse and human metaplasia, we will assess the effect of temporally regulated Yap overexpression on the above stem cell populations and the consequences on both squamous and mucous metaplasia. Additionally, using ATAC-Seq and RNA-seq, we will determine the accessibility and expression of the Yap target genes that underpin the differential metaplastic propensities of the above stem cell populations. In contrast to Yap signaling, EGFR signaling activation causes both pathologic mucous and squamous metaplasia. Therefore, we will define the effects of EGFR modulation on both hillock and non-hillock pseudostratified mouse and human basal stem cells. We will also assess whether Yap overexpression will prevent EGFR-induced mucous metaplasia in distal basal stem cell populations and whether suppressing Yap will lead to diminished EGFR-induced squamous metaplasia arising from hillock basal stem cells. Finally, we provide evidence that Yap activity is dramatically upregulated following injury, but this activity subsides as injury resolves. We will define the effects of Yap modulation on the formation of the early post-injury squamous barrier epithelium and injury-associated squamous metaplasia. Understanding how heterogenous stem cell populations of the airway contribute to both squamous and mucous metaplasia and establishing how these stem cells respond to disease-associated signaling pathways will inform strategies to control pathologic metaplasia.