PROJECT SUMMARY/ABSTRACT Gastric ulcer is increasing in the United States due to increasing use of non-steroidal anti-inflammatory drugs (NSAIDs) and Helicobacter pylori infection. It has been a major cause of gastrointestinal surgery with high morbidity and mortality. While molecular and cellular mechanisms associated with gastric ulcer repair have been heavily investigated, information on the cell of origin for regenerated epithelium is limited. Microscopically, gastric ulcer is characterized by mucosal injuries involving the muscularis mucosae and a cavity surrounded by acute and chronic inflammation. Numbers of acid secreting parietal cells are decreased in the ulcer edges. Parietal cells are also decreased in preneoplastic lesions such as atrophic gastritis and Ménétrier’s disease. We have previously demonstrated that parietal cells undergo apoptosis in an atrophic gastritis mouse model. Ménétrier’s disease is a rare, acquired, premalignant protein-losing hypertrophic gastropathy induced by EGF receptor (EGFR) signaling activation. Ménétrier’s disease also shows decreased number of parietal cells, however, apoptosis is not changed. This raises the possibility that parietal cells transdifferentiate into other cell types rather than dying in Ménétrier’s disease. As a part of my K08 project, we examined the fate of parietal cells in a Ménétrier’s disease mouse model and observed that parietal cells can give rise to other epithelial cell types. Our preliminary data shows that parietal cells can transdifferentiate into various other epithelial cell types in another EGFR activated condition gastric ulcer. Preliminary analysis on single cell RNA sequencing (scRNA-seq) comparing normal stomach and gastric ulcer at day 3 showed upregulated Sox4 expression and activated FoxO signaling pathway in parietal cells from gastric ulcer. Sox4 and FoxO signaling have been associated with stem cell maintenance, transdifferentiation, and cancer progression. It has been reported that spasmolytic polypeptide expressing metaplasia (SPEM) develops at the ulcer edges and plays an important role in gastric ulcer healing. We will investigate if parietal cells can transdifferentiate into SPEM in gastric ulcer and give rise to other epithelial cell types during gastric ulcer healing by lineage tracing followed by immunofluorescence microscopy and single cell RNA sequencing. The role of Sox4 in parietal cell plasticity will also be investigated by knocking out or overexpressing Sox4 in parietal cells. Successful completion of the studies proposed in this application will not only provide us with better understanding of the underlying mechanism of gastric epithelial plasticity but also with a foundation for effective treatment of gastric ulcer. Also, comparing results from this project with my K08 project will contribute to my long- term research goal of preventing gastric tumorigenesis and facilitating gastric mucosal regeneration.