Hepatic ductular reaction: a double-edged sword

enabled by the plasticity of its epithelial cell compartments. Hepatocytes can re-enter the cell cycle to repopulate the hepatocyte pool or transdifferentiate into biliary epithelial cells (BECs; also termed cholangiocytes) in response to injury [1]. BECs form the bile ducts responsible for bile excretion as well as peripheral ductules and canals of Hering, where bile is collected. Upon injury activated BECs expand around the portal vein and form a transient luminal epithelium establishing an auxiliary biliary system in a process referred to as ductular reaction (DR) [2]. When hepatocyte-mediated regeneration is impaired (e.g. due to hepatocyte senescence), BECs can transdifferentiate into functional hepatocytes restoring injured liver parenchyma [3]. Despite the relevance of a DR for promoting liver regeneration, the mechanisms regulating this process were not fully understood. Recently, we have investigated signaling pathways controlling BEC expansion using a focused CRISPRbased loss-of-function screen in mouse BEC-organoids. MTORC1, YAP signaling and WNT/β-Catenin signaling were the top hits that we identified in vitro and further studied in vivo during a DR, induced by DDC-mediated liver injury. While mTORC1 and YAP signaling promoted BEC expansion, and YAP induced hepatocyte-to-BEC transdifferentiation, LGR4/5-mediated WNT/β-Catenin signaling was dispensable for these processes during a DR in vivo. Detailed assessment of YAP and WNT/β-Catenin signaling during different steps of a DR and recovery from DDC-induced injury confirmed that YAP but not WNT/β-Catenin signaling is activated in BECs. It remains unclear why the dependency of BEC organoids on WNT/β-Catenin signaling did not translate in vivo. We cannot exclude that WNT/β-Catenin signaling is activated in BECs during certain conditions that our injury models did not mimic despite availability of RSPO3 and WNT ligands within the DR. However, it is also plausible that the unphysiological niche provided by Matrigel in which BEC-organoids were embedded biased their signaling in our experiments. Nevertheless, besides revealing mTORC1 and YAP as key regulators of a DR, our CRISPR screen also retrieved many common tumor suppressors in cholangiocarcinoma, which is believed to originate from malignant transformed BECs. This suggests that BEC organoids are a robust research tool Editorial