While foregut endoderm research has primarily focused on the development and regeneration of the liver, pancreas, and intestine, relatively little attention have been placed on the nexus by which these organs are connected. The mammals and zebrafish the extrahepatopancreatic duct (EHPD) system functions as a tubular network joining the liver, pancreas, and gall bladder together with the duodenum. Extensive genetic and morphogenetic conservation between human and zebrafish EHPDs not only qualifies the zebrafish as a practical genetic model for this tissue, but also suggests more critical roles for the EHPDs beyond simply transporting hepatic bile and pancreatic enzymes. We propose here to leverage the zebrafish vertebrate model to rigorously explore the function of EHPD cells as a novel and significant source of progenitors for liver development, homeostasis, and regeneration. Using a zebrafish model that phenocopies hepatic biliary cell loss in Alagille Syndrome (ALGS), we found that agenesis of intrahepatic duct (IHD) cells, due to transient loss of the Notch ligand Jagged, can lead to a robust cellular response outside the liver, within the EHPD. Specifically, the EHPD cells react to the liver duct cell loss by proliferating excessively and contributing primarily to the regenerated IHD cells and hepatocytes also. These unexpected findings suggest that the EHPD system harbors multipotent progenitors that can contribute to the liver. Our discovery of multipotent progenitors residing in the EHPD leads to fundamental new questions: To what extent and by what mechanisms do the EHPD cells contribute to the liver during development, homeostasis, and regeneration? And how are these progenitors formed and maintained? We hypothesize that the EHPDs act as the initial source of nearly all progenitors for the liver. We posit that EHDP progenitors have high proliferative capacity and are maintained in an undifferentiated, quiescent state by FGF signaling. Further, we hypothesize that these EHPD cells are poised for expansion and migration to become distributed throughout the liver, ultimately adopting cholangiocyte or hepatocyte identity during development and homeostasis, and to a greater degree, in response to specific types of liver damage. Insufficient EHPD progenitors may lead to alternative mechanisms of regeneration such as transdifferentiation. Testing these hypotheses may lead to a unifying, dynamic liver regeneration model, whereby the availability of EHPDs progenitors is a significant determinant of which regenerative mechanism predominates. Uncovering a more fundamental role for the EHPD cells in liver regenerative biology will advance our understanding and treatment of liver diseases including ALGS and other cholangiopathies, as well as hepatocyte disorders. A central role for the EHPD in liver regeneration would also implicate it as a novel target tissue for liver therapies. We propose to investigate the lineage contribution of EHPD cells during liver development, homeostasis, and regeneration, and the genetic and cellular mechanisms driving this complex process.
We have identified a potential new stem cell population that surprisingly resides outside of the liver, in the ductal structure that connects the liver to the intestine. Our research proposal to investigate how these cells contribute to the complex and dynamic mechanisms of liver regenerative biology will advance our understanding and treatment of developmental and degenerative liver diseases. Uncovering a fundamental role for these cells in liver regeneration would implicate them as a novel target tissue for liver therapies.
