Bone morphogenetic protein signaling governs biliary‐driven liver regeneration in zebrafish through tbx2b and id2a

Abstract

Upon mild liver injury, new hepatocytes originate from pre-existing hepatocytes. However, if hepatocyte proliferation is impaired, a manifestation of severe liver injury, biliary epithelial cells (BECs) contribute to new hepatocytes through BEC dedifferentiation into liver progenitor cells (LPCs), also termed oval cells or hepatoblast-like cells (HB-LCs), and subsequent differentiation into hepatocytes. Despite the identification of several factors regulating BEC dedifferentiation and activation, little is known about factors involved in the regulation of LPC differentiation into hepatocytes during liver regeneration. Using a zebrafish model of near-complete hepatocyte ablation, we here show that Bmp signaling is required for BEC conversion to hepatocytes, particularly for LPC differentiation into hepatocytes. We found that severe liver injury led to the upregulation of genes involved in Bmp signaling, including smad5, tbx2b, and id2a, in the liver. Bmp suppression did not block BEC dedifferentiation into HB-LCs; however, the differentiation of HB-LCs into hepatocytes was impaired due to the maintenance of HB-LCs in an undifferentiated state. Later Bmp suppression did not affect HB-LC differentiation, but increased BEC number through proliferation. Notably, smad5, tbx2b, and id2a mutants exhibited similar liver regeneration defects as those observed in Bmp-suppressed livers. Moreover, BMP2 addition promoted the differentiation of a murine LPC cell line into hepatocytes in vitro. Conclusions: Bmp signaling regulates BEC-driven liver regeneration via smad5, tbx2b and id2a: it regulates HB-LC differentiation into hepatocytes via tbx2b and BEC proliferation via id2a. Our findings provide insights into promoting innate liver regeneration as a novel therapy.

Cell Plasticity in Liver Regeneration

“…In addition, mTORC1 and the BMP pathway were found to regulate the conversion of biliary epithelial cells to hepatocytes in zebrafish [55,56]. Inhibition of the mTORC1 signaling pathway abolished the conversion from biliary epithelial cells to hepatoblast-like cells via decreased Urb2-mediated protein synthesis [56], whereas the BMP pathway controlled the conversion of biliary epithelial cells through Tbx2b expression [55]. These studies together show several important signaling and epigenetic factors underlying biliary epithelial cell plasticity in zebrafish.…”

“…Inhibition of the bromodomain and extraterminal (BET) protein blocked the dedifferentiation of biliary epithelial cells to hepatoblastlike cells via reduced Prox1 and Hnf4a induction [54]. In addition, mTORC1 and the BMP pathway were found to regulate the conversion of biliary epithelial cells to hepatocytes in zebrafish [55,56]. Inhibition of the mTORC1 signaling pathway abolished the conversion from biliary epithelial cells to hepatoblast-like cells via decreased Urb2-mediated protein synthesis [56], whereas the BMP pathway controlled the conversion of biliary epithelial cells through Tbx2b expression [55].…”

Mammalian Target of Rapamycin Complex 1 Signaling Is Required for the Dedifferentiation From Biliary Cell to Bipotential Progenitor Cell in Zebrafish Liver Regeneration

“…Although the liver was smaller in mTOR and raptor mutants, the intrahepatic duct system has relatively normal morphology. Besides, many developmental factors and mechanisms can be reactivated to regulate liver regeneration such as Wnt, Bmp, and Notch, which have been reported to regulate BEC dedifferentiation, BP‐PC proliferation, and redifferentiation in zebrafish . Consistent with chemical inhibition, the dedifferentiation of BECs and redifferentiation were defective after liver injury in the mTOR and raptor mutants.…”

“…The kinetics of the zebrafish extensive hepatocyte ablation and the mouse severe liver injury models allow evaluation in vivo of BEC dedifferentiation, BP‐PC proliferation, redifferentiation, and the mechanisms involved in these cellular responses . The mTORC1 inhibitor rapamycin is known to significantly delay S‐phase entry and liver regeneration after PH, indicating important roles of mTORC1 signaling .…”

BMP Signalling at the Crossroad of Liver Fibrosis and Regeneration

“…A key role for BMP signalling in LPC-mediated liver regeneration has been uncovered using a zebrafish model [ 91 ]. Interestingly, hepatic expression of BMP-related genes, such as Smad5, is upregulated during LPC-driven liver regeneration.…”