PROJECT SUMMARY Biliary atresia (BA) results in severe disruption of bile acid homeostasis and is the predominant disease causing neonatal obstructive cholestasis. Bile acid homeostasis in regulated by the nuclear hormone receptor Farnesoid X Receptor (FXR) by activation of the intestinal and hepatic hormone, fibroblast growth factor 19 (FGF19). FGF19 suppresses bile acid synthesis of hepatic cytochrome P450 7a1 (Cyp7a1), the key enzyme for classical bile acid synthesis. In healthy infants, the FGF19 expression is mainly in the intestine and low in the liver. In contrast, hepatic FGF19 is elevated in BA infants to compensate for loss of intestinal bile acid-induced FGF19 secretion. The increase in hepatic FGF19 and bile acid accumulation in BA infants suggests an uncoupling of FGF19 signaling that normally suppresses hepatic Cyp7a1 and bile acid synthesis. A secondary source of bile acid synthesis in hepatocytes and cholangiocytes is the alternative pathway via Cyp27a1. There is a gap in our knowledge of how the regulation of Cyp7a1 and Cyp27a1 via FXR-FGF19 signaling contributes to bile acid homeostasis during neonatal obstructive cholestasis. This proposal aims to understand the adaptive, time- dependent mechanism by which the liver compensates for altered intestinal bile acid signaling and how this compensation effects bile acid synthesis via the classical and alternative pathways. Our central hypothesis is that hepatic adaptation to obstructive cholestasis leads to increased hepatocyte FXR-FGF19 signaling, but with impaired suppression of Cyp7a1 or compensation by Cyp27a1 to sustain bile acid accumulation. In Aim 1a we will characterize the ontogeny of cholestatic liver injury, bile acid homeostasis, and cell-specific hepatocyte and cholangiocyte gene expression signatures in a newly developed neonatal pig model of bile duct ligation (BDL). In Aim 1b, we will quantify the molecular mechanisms of FXR and FGF19 signaling pathways involved in bile acid synthesis during long-term bile acid exposure using human cultured hepatocyte and cholangiocyte organoids. In Aim 2a, we will test whether FGF19 infusion dose-dependently suppresses Cyp7a1 expression at early and late time points after BDL. In Aim 2b, we will test if early activation of FXR-FGF19 signaling protects against BDL induced cholestasis. This study will provide understanding of the mechanism that regulate bile acid homeostasis in the context of neonatal obstructive cholestasis and test potential therapeutic interventions. Career development and environment: To enhance my potential for future success as an independent researcher I will receive mentorship from Dr. Douglas Burrin, PhD and training in use of stable isotope kinetics modeling, mass spectrometry analysis, RNA-seq pipeline analysis and visualization, and hepatic organoid culturing. This training will be conducted in an outstanding research environment of the Children’s Nutrition Research Center (CNRC) with support of NIDDK Diges...