In the intestinal epithelium, the regulatory mechanisms that mediate the balancing act between proliferation and differentiation are incompletely defined, but are critical for maintaining normal homeostasis and its recovery from disease. Intestinal epithelial homoeostasis requires precise regulation: excess proliferation can lead to tumor formation, but insufficient proliferation would cause tissue collapse. The objective of this application is to bridge the gap in our understanding of intestinal homeostasis by investigating an unexpected transcription factor's essential role in this process. Preliminary studies show that inactivation of the transcription factor YY1 in the adult murine intestinal epithelium triggers massive imbalance of epithelial homeostasis: the progenitor population expands while the differentiated population contracts. Therefore, defining how YY1 controls epithelial turnover could provide a novel therapeutic entry point in restoring epithelial homeostasis in disease. This project's first Aim is to test 3 plausible, non-exclusive explanations for the increase in proliferating crypt cells in intestinal epithelia lacking YY1: 1) YY1 controls stem cell turnover, 2) YY1 controls crypt cell proliferation, or 3) YY1 mediates cell cycle arrest and terminal differentiation. Mouse genetic and histological approaches will measure epithelial cell populations and behaviors upon YY1 loss. Completion of this aim should define the specific process(es) by which YY1 maintains homeostatic balance. The second Aim of the project will define the transcriptional regulatory mechanisms underlying YY1's control of epithelial turnover. ChIP-seq will be employed to map YY1 interactions with the genome of isolated intestinal epithelial cell populations. Identification of YY1's genomic binding locations will allow us to identify its direct regulatory target genes, identify its regulatory partner transcription factors,and determine whether YY1 operates in active or repressed chromatin structure. These analyses will be integrated with gene expression analysis of the YY1 knockout, and complemented by chromatin ChIP-seq data collected from the current K01. The proposed research is innovative in that it is expected to reveal a novel, YY1-dependent regulatory mechanism governing the proliferation/differentiation decision by using cutting-edge epigenomic technologies. Completion of the proposed work will reveal new mechanisms underlying homeostatic cell turnover and should be of significant value to fields as diverse as wound healing and oncogenesis. Finally, as is intended for this R03 mechanism, funding this project should enhance the capability of the K01-awarded investigator to conduct research. Establishing new investigators to join the fight against intestinal disease is a priority for the NIDDK as part of a long-term goal to solve one of the Country's largest medical problems. The proposed research on the regulatory mechanisms of YY1 is perfectly suited for this purpose in that it can be completed in a short timeframe, yet i likely to open a new area of research on the mechanisms of intestinal epithelial renewal.