Despite the remarkable regeneration capacity of hepatocytes1, which are the major parenchymal cell type of the liver, chronic liver failure impairs this ability, leading to hepatic progenitor cell (HPC)-dependent regeneration2.
Chronic liver damage is mainly derived from alcohol abuse, chronic hepatitis C virus (HCV) infection3 and non-alcoholic fatty liver disease (NAFLD)4. It leads to sustained liver fibrosis, which is associated with the accumulation of extracellular matrix (ECM) proteins. Persisting ECM accumulation distorts intact hepatic architecture by forming a fibrous scar tissue5, subsequently resulting in cirrhosis with high morbidity and mortality. Many attempts have been made to mitigate the fibrotic response mainly by focusing on inhibiting profibrogenic cytokines and activated myofibroblasts6. The latter is primarily derived from hepatic stellate cells (HSCs), the principle hepatic non-parenchymal cells responsible for liver scar formation4. Nevertheless, regenerative therapies that stimulate endogenous cellular sources including HPCs to regenerate hepatocytes in the presence of sustained fibrogenic insults await further investigation.
Many experimental models of hepatic fibrosis have been described in mammals. Repetitive injection of carbon tetrachloride (CCl4) has been widely used to induce liver fibrosis in murine and rat models7. When combined with a high-fat (HF) diet, alcohol led to a substantial upregulation of profibrogenic gene expression and hepatic fibrosis8. While steatosis (lipid accumulation) results from acute alcohol exposure, it makes the liver susceptible to more severe hepatic injury9.
The zebrafish, Danio rerio, has emerged as an invaluable vertebrate model system for studying regeneration. Though other lower vertebrates such as newts and axolotls have a remarkable capacity for regeneration, the zebrafish has advantages over other model systems in regards to the gene manipulation and visualization strategies needed to manipulate potential regenerative factors10. The zebrafish also represents an attractive vertebrate model for studying alcoholic liver disease (ALD) by simply adding ethanol (EtOH) to their water. Acute EtOH exposure to larval and adult zebrafish caused hepatic steatosis11-13. When adult zebrafish received extended EtOH exposure, collagen deposition was observed with upregulation of fibrosis-related genes14. However, a need exists for developing models to study liver regeneration in response to EtOH as a fibrogenic stimulus.
Recently, we developed an EtOH-induced fibrotic liver model in zebrafish15. We combined a hepatocyte-specific genetic ablation system with EtOH treatment in larval and adult zebrafish. We generated two transgenic lines, Tg(fabp10a:CFP-NTR)gt1 and Tg(fabp10a:mCherry-NTR)gt2, in which E.coli nitroreductase (NTR) are fused to the cyan and mCherry fluorescent protein, respectively, under the control of the hepatocyte-specific fatty acid binding protein 10a, liver basic (fabp10a) promoter. In this system, NTR converts a nontoxic prodrug metronidazole (MTZ) into a DNA inter-strand cross-linking agent16, inducing explicit death of hepatocytes. Using this model, we demonstrated that a population of hepatic cells, which are responsive to Notch signaling, converted into hepatocytes in the near absence of hepatocytes and in the excess of ECM. We designated these cells as HPCs. Furthermore, through chemical screens, we identified small molecule activators of Wnt signaling and inhibitors of Notch signaling that augment hepatocyte regeneration in the fibrotic liver. Therefore, our fibrotic liver model in zebrafish represents a superb chemical screening system compared to cell culture- or mammalian-based screening system. It is an in vivo system with significant cost- and time-saving benefits. Here we describe the detailed procedures for establishing an EtOH-induced fibrotic liver model and for performing chemical screens using this model in zebrafish. Furthermore, time-course analyses were performed to investigate how hepatocyte regeneration occurs in the fibrotic liver. This protocol will provide an invaluable tool to study the mechanisms and strategies of enhancing hepatocyte regeneration in the fibrotic liver.