Abstract
Upon severe acute or chronic liver injury, hepatic progenitor cells (HPCs) become activated. HPCs are adult stem cells of the liver and are considered a reserve population acting as second line of defense in liver regeneration. However, in many cases of severe liver disease this repair mechanism falls short and
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symptoms of liver failure develop. Insight into activation mechanisms of HPCs may provide novel cues for liver regenerative medicine strategies. HPC activation mechanisms have been studied in experimental rodent models and in human liver pathology, but to a far lesser extent in companion animals. In the Netherlands alone, millions of dogs and cats are kept as pet animals which can suffer from spontaneously occurring liver disease similar to human patients. Knowledge of similarities and differences in biology of liver disease may provide new, clinically relevant translational models of liver disease. In part one of the thesis fundamental studies were performed into HPC activation mechanisms. Activated HPCs and their micro-environment (or niche) were molecularly characterized in canine samples of several types of hepatitis and biliary disease. A severe liver disease that occurs specifically in dogs is lobular dissecting hepatitis (LDH). LDH has a rapid clinical course and is associated with prominent HPC activation. Samples of canine LDH were used to study the involvement of the Wnt/β-catenin and Notch signaling pathways in canine HPC activation. To further pinpoint intracellular signals that convey external activation signals into a functional proliferative response, a high throughput siRNA kinome screen was performed in an HPC-like cell line. One kinase was identified that had a inhibitory effect of S phase entry in HPCs. This finding was validated in primary HPCs cultured as liver organoids and in liver organoids expressing one extra allele of the kinase. Exact gene dosage turned out to be essential for balanced S phase entry of HPCs. In part two of the thesis applied studies of growth factors and HPCs in models of liver disease are described. We tested the regenerative potential of Hepatocyte Growth Factor (HGF) treatment in dogs with liver hypoplasia secondary to a congenital portosystemic shunt. It was possible to induce liver growth with HGF, but only for the duration of the treatment. The therapeutic potential of HPCs was investigated in a transplantation study of autologous canine liver organoids. Organoids were cultured from COMMD1-deficient dogs that develop copper toxicosis similar to human Wilson’s disease. HPCs were isolated from biopsies, genetically corrected, massively expanded as organoids in vitro and transplanted either via the portal vein or by intrahepatic injections. Under specific circumstances it was possible to find transplanted cells back in the liver with evidence for in vivo proliferation. In the last chapter of this thesis HPCs cultured as liver organoids were tested for their potential to model fatty liver disease (steatosis), a disease seen in cats and in humans. To this end a feline liver organoid culture was established and characterized. Feline organoids incubated with fatty acids accumulated more fat than human organoids, which is in line with the more severe phenotype of feline hepatic steatosis. In conclusion, this thesis translates fundamental findings in liver regenerative medicine to applications in (pre)clinical models of liver disease to benefit both human and veterinary patients.
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