Van Gogh-like 2 is essential for the architectural patterning of the mammalian biliary tree

Abstract

In the developing liver, bipotent epithelial progenitor cells known as hepatoblasts undergo lineage segregation to form the two major epithelial cell types, hepatocytes that constitute the bulk of the liver parenchyma and biliary epithelial cells (cholangiocytes) which comprise the bile duct, a complex tubular network which is critical for normal liver function. Notch and TGFβ signalling promote the formation of a sheet of biliary epithelial cells, the ductal plate that organises into discontinuous tubular structures. How these structures elongate and connect to form a continuous duct remains undefined. We aimed to define the mechanisms by which the ductal plate transitions from simple sheet of epithelial cells to a complex and connected bile duct. By combining single cell RNA sequencing from embryonic mouse livers with genetic tools and organoid models we functionally dissected the role of planar cell polarity in duct patterning. We show that the planar cell polarity protein, VANGL2 is expressed late in intrahepatic bile duct development and patterns the formation of cell-cell contacts between biliary cells. The patterning of these cell contacts regulates the normal polarisation of the actin cytoskeleton within biliary cells and loss of Vangl2-function results in the abnormal distribution of cortical actin remodelling resulting in the failure of bile duct formation. Planar cell polarity is a critical step in the post-specification sculpture of the bile duct and is essential for establishing normal tissue architecture. Human disease and mouse models have allowed us to define how the mammalian biliary lineage is specified during liver development. Once this relatively simple epithelium has formed though, how it undergoes morphogenesis to form a complex and branched structure is not clear. Similar to other branched tissues such as the liver and kidney the bile ducts use planar cell polarity signalling to coordinate cell movements; however how these biochemical signals are linked to ductular patterning remains unclear. Here we show that the core planar cell polarity protein, VANGL2 patterns how cell-cell contacts form in the mammalian bile duct and how ductular cells transmit confluent mechanical changes along the length of a duct. This work sheds light on how biological tubes are pattered across mammalian tissues (including within the liver) and will be important in how we promote ductular growth in patients where the duct is mis-patterned or poorly formed.