Spatiotemporally restricted Hippo signalings instruct the fate and maturation of hepatobiliary cells

The Hippo pathway is an evolutionary conserved signaling cascade involved in organ size control and tumorigenesis, but its cellular functions and regulatory mechanisms are not fully understood. Recently, we have defined two independent modules (HPO1 and HPO2) in the Hippo signaling network. Here, by using spatially resolved transcriptomic and imaging analysis of mouse livers with defective Hippo signaling, we show that HPO1 and HPO2 operate in distinct cells at different developmental stages to regulate the fate and maturation of liver parenchymal cells. HPO1 controls the maturation of hepatocytes postnatally, and its perturbation leads to the expansion of immature hepatocytes (imHep). HPO2, on the other hand, regulates the maturation of cholangiocytes perinatally, and its ablation results in the accumulation of immature cholangiocytes (imCho2) identical to developing ductal plate cells. Moreover, the inactivation of HPO1 or HPO2 causes the conversion of hepatocytes into immature cholangiocytes (imCho1). These immature cells are also observed in regenerating livers following different damages. In contrast, deletion of Yap/Taz encoding downstream effectors of the Hippo pathway accelerates liver maturation and promotes cell death. These findings suggest that the spatiotemporally restricted Hippo signaling modules act as checkpoints in liver development and may coordinate cell proliferation and maturation to ensure proper liver size and function.