Friday, May 9, 2025
5/9/2025
Regulation of Overall Cell Numbers During Epithelial Tissue Homeostasis and Pathogenesis - PROJECT SUMMARY/ABSTRACT Cellular turnover is essential for the form and function of epithelial tissues. The rate of cell turnover slows during aging, can be accelerated during injury and repair, and is precociously stimulated during carcinogenesis; but the mechanisms guiding it in living tissues is not well understood. We recently discovered that elimination of unfit cells by extrusion from epithelia stimulates proliferation of nearby stem cells to replace the lost cells and maintain overall cell numbers in the tissue. This intriguing finding suggests extrusion provides a key link between cell loss and proliferation, and thereby controls the rate of cell turnover. Thus, identification of the mechanisms that underlie extrusion may provide new insights into endogenous processes that can be leveraged to promote cellular replacement or prevent the unwanted addition of new cells. Our long-term goal is to define the cellular and molecular mechanisms underlying the rate of cellular turnover in epithelial tissues. Using the developing zebrafish to study cell extrusion in a living epithelial tissue, we have found that cells fated to extrude alter their mechanical properties in the form of pulsatile actomyosin contractions that are controlled by enrichment of the bioactive lipid sphingosine-1-phosphate (S1P). We have also interrogated the cell loss-induced signaling events and cellular responses, including inflammatory cell recruitment and epidermal cell proliferation, that drive turnover. We identified a significant upregulated expression of the epidermal growth factor receptor ligand epigen (EPGN) upon induced cell extrusion, suggesting that transient increases in EPGN may aid in sustaining epithelial form and function during cell loss. Consistent with this idea, we found that treatment with recombinant human EPGN (hrEPGN) suppressed epithelial cell extrusion after receiving damage stimuli, which in turn reduced the compensatory stem cell proliferation. These data led to the hypothesis that EPGN regulates extrusion to dictate the rate of cellular turnover in epithelial tissues. One formidable challenge to studying cellular turnover and testing this hypothesis in a living organism involves visualizing and perturbing the complex interplay between extruding cells, the surrounding stem cells that replace the lost cells and immune cells to sense and respond to disruptions in integrity. Therefore, we created tools to manipulate different cellular and molecular components individually or in combination in living epithelial tissues of developing zebrafish and analyze changes to turnover in the presence of an innate immune system. Our work over the next five years we utilize this new approach and will focus on three essential areas that emerged from our ongoing studies and address key gaps in our knowledge of cellular turnover. First, we will determine the mechanisms regulating the localized changes in physical forces that are required to remove defective cells by extrusion. Second, we will determine how cell loss promotes changes in the epigenetic and transcriptional states in surrounding stem cells to stimulate proliferation and replace the lost cells. Third, we will determine the role of the innate immune system in promoting cell turnover and maintenance of epithelial tissue homeostasis.
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