Single-Cell Analysis of the Liver Epithelium Reveals Dynamic Heterogeneity and an Essential Role for YAP in Homeostasis and Regeneration

Single-Cell Analysis of the Liver Epithelium Reveals Dynamic Heterogeneity and an Essential Role for YAP in Homeostasis and Regeneration The liver is an essential organ with compartmentalized metabolic processes and significant regenerative capabilities. Repopulation of the liver parenchyma can transpire from both main epithelial cell types, hepatocytes and biliary epithelial cells (BECs). Here, we harness high-throughput single-cell RNA sequencing (scRNA-seq) to dissect the transcriptional heterogeneity and cellular diversity of these epithelial compartments in homeostasis and injury. Our data argue against the idea of a rigidly defined liver progenitor cell in BECs, finding instead that heterogeneity in homeostatic BECs is principally distinguished by a YAP-dependent program that defines a dynamic cellular state. We report that this cellular state dynamically fluctuates between BECs and can be induced in the majority of BECs in response to environmental stimuli and injury. Functional studies demonstrate that YAP is distinctly required for BEC survival in homeostasis, uncovering a tight physiological necessity for YAP signaling in BECs compared to other tissues. YAP is also essential for hepatocyte reprogramming towards a ductal progenitor fate upon injury. Finally, our data demonstrate that this YAP-driven cellular state is highly responsive to injury by physiological exposure to bile acids (BAs) via apical sodium-bile acid transporter, and that sequestration of endogenous BAs rescues the cell loss phenotype associated with homeostatic Yap deletion. Together, our findings uncover previously undescribed molecular heterogeneity within the ductal epithelium and highlight a distinct and potent role for YAP as a protective rheostat and regenerative regulator in the mammalian liver. Overall design: RNA sequencing data from single biliary epithelial cells isolated from mouse livers