The role of Nogo-B signaling in zebrafish liver development

Nogo-B, also known as Reticulon 4B, plays important roles in vascular injuries. In mammals, Nogo-B is expressed specifically in cholangiocytes and non-parenchymal cells in the liver. Nogo-B expression is down-regulated with the progression of liver fibrosis and in intrahepatic cholangiocarcinoma (ICC), a relatively rare type of primary liver cancer with highly malignant behavior. Circulating Nogo-B may be an effective indicator for liver cirrhosis. Nogo-B inhibition could diminish hepatic stellate cell (HSC) activation. Nogo-B expression in hepatocytes facilitates hepatocyte proliferation and liver regeneration. These data prompt us to consider whether Nogo-B/Nogo receptor signaling participates in the development of the zebrafish liver. So far, its function in the liver development is not understood. In this study, we characterized the roles of Nogo-B/Nogo receptor in hepatic formation and differentiation by morpholino oligonucleotide (MO) knockdown and whole-mount in situ hybridization with several markers in zebrafish embryos from 24 to 72 hpf. Knockdown of Nogo-B inhibited the proliferation of liver by pHH3 immunostaining. In addition, Nogo-b morphants showed lower expression of foxa3, prox1a and lfabp compared to control MO injected embryos by in situ hybridization. On the other hand, Nogo signaling in zebrafish, like that in mammals, involves three ligands and four receptors. We identified that knockdown of Nogo-B and its receptor-NgR and two coreceptors, p75 and TROY, all decreased liver size during early development. Thus, we hypothesized that Nogo-B/NgR, p75, and TROY will form a complex to be involved in zebrafish liver formation.