Project Summary - The role of liver progenitor cells in liver regeneration Acute and chronic liver disease causes liver failure and is a growing cause for morbidity and mortality. The COVID-19 pandemic has further increased the magnitude of this problem. The liver is a highly regenerative organ, yet the presence of a dedicated stem cell population remains controversial. In our preliminary work, we describe a near-total hepatocyte ablation model in zebrafish where organ regeneration is derived from biliary epithelial cells. Our objective here is to characterize the functional implications and molecular mechanisms of EAE to affect organ development. Our central hypothesis is that severe hepatocyte injury will reveal the facultative stem cell potential of biliary epithelial cells. This process appears to be driven by EGFR, PI3K, and mTOR signaling. Two Specific Aims are proposed to define the generation and contribution of facultative hepatic stem cells. In Specific Aim 1 we will utilize single-cell transcriptomic and high-resolution imaging analyses after near-total hepatocyte ablation to investigate whether and when biliary epithelial cells undergo transcriptional and morphological changes to become hepatocytes. In the process, we will generate an inventory of all liver cells in the zebrafish and determine evolutionary conservation of cell identity and function by comparison to murine and human liver datasets. Specific Aim2 will determine the degree of biliary epithelial cell proliferation hepatoblast transcription factors gene expression prior to hepatocyte differentiation and define the signaling pathways involved in the generation, function and proliferation of this facultative stem cell population. Identification of a signal like EGFR that can cause biliary epithelial cells to become hepatocytes has significant therapeutic potential for patients with liver failure.