Investigation of bipotent differentiation of hepatoblasts using inducible diphtheria toxin receptor-transgenic mice

The default and directed pathways of hepatoblast differentiation involve distinct epigenomic mechanisms

The effectiveness of multiomics analyses in defining cell differentiation pathways during development is ambiguous. During liver development, hepatoblasts follow a default or directed pathway to differentiate into hepatocytes or cholangiocytes, respectively, and this provides a practical model to address this issue. Our study discovered that promoter-associated histone modifications and chromatin accessibility dynamics, rather than enhancer-associated histone modifications, effectively delineated the "default vs. directed" process of hepatoblast differentiation. Histone H3K27me3 on bivalent promoters is associated with this asymmetric differentiation strategy in mice and humans. We demonstrated that Ezh2 and Jmjd3 exert opposing regulatory roles in hepatoblast-cholangiocyte differentiation. Additionally, active enhancers, regulated by P300, correlate with the development of both hepatocytes and cholangiocytes. This research proposes a model highlighting the division of labor between promoters and enhancers, with promoter-associated chromatin modifications governing the "default vs. directed" differentiation mode of hepatoblasts, whereas enhancer-associated modifications primarily dictate the progressive development processes of hepatobiliary lineages.

Hepatobiliary Differentiation: Principles from Embryonic Liver Development

Hepatocytes and biliary epithelial cells (BECs), the two endodermal cell types of the liver, originate from progenitor cells called hepatoblasts. Based principally on in vitro data, hepatoblasts are thought to be bipotent stem cells with the potential to produce both hepatocytes and BECs. However, robust in vivo evidence for this model has only recently emerged. We examine the molecular mechanisms that stimulate hepatoblast differentiation into hepatocytes or BECs. In the absence of extrinsic cues, the default fate of hepatoblasts is hepatocyte differentiation. Inductive cues from the hepatic portal vein, however, initiate transcription factor expression in hepatoblasts, driving biliary specification. Defining the mechanisms of hepatobiliary differentiation provides important insights into congenital disorders, such as Alagille syndrome, and may help to better characterize the poorly understood hepatic lineage relationships observed during regeneration from liver injury.

Lgr5+ stem and progenitor cells reside at the apex of a heterogeneous embryonic hepatoblast pool

During mouse embryogenesis, progenitors within the liver known as hepatoblasts give rise to adult hepatocytes and cholangiocytes. Hepatoblasts, which are specified at E8.5-E9.0, have been regarded as a homogeneous progenitor population that initiate differentiation from E13.5. Recently, scRNA-seq analysis has identified sub-populations of transcriptionally distinct hepatoblasts at E11.5. Here, we show that hepatoblasts are not only transcriptionally but also functionally heterogeneous, and that a subpopulation of E9.5-E10.0 hepatoblasts exhibit a previously unidentified early commitment to cholangiocyte fate. Importantly, we also identify a subpopulation constituting 2% of E9.5-E10.0 hepatoblasts that express the adult stem cell marker Lgr5, and generate both hepatocyte and cholangiocyte progeny that persist for the lifespan of the mouse. Combining lineage tracing and scRNA-seq, we show that Lgr5 marks E9.5-E10.0 bipotent liver progenitors residing at the apex of a hepatoblast hierarchy. Furthermore, isolated Lgr5⁺ hepatoblasts can be clonally expanded in vitro into embryonic liver organoids, which can commit to either hepatocyte or cholangiocyte fates. Our study demonstrates functional heterogeneity within E9.5 hepatoblasts and identifies Lgr5 as a marker for a subpopulation of bipotent liver progenitors.