Pericrine regulation of physiological and pathological liver functions via hepatic stellate cell-derived RSPO3

The liver exerts multiple essential functions of our body, such as digestion, detoxification, biosynthesis, and storage of various nutrients. These functions are executed via a complex crosstalk among different cell types within the liver, including hepatocytes, the parenchymal cells of the liver, liver sinusoidal endothelial cells (LSEC), Kupffer cells (KC), and hepatic stellate cells (HSC). Given the diverse tasks performed by the liver, liver functions are spatially distributed along the lobule for efficiently coordinating and performing metabolic processes. This spatial division of labor in the liver microenvironment is termed “liver zonation”. Liver zonation is maintained via a complex regulation of various microenvironmental factors. Perturbed liver zonation drives pathological changes and causes liver diseases. Previous studies have identified LSEC-derived Wnt signaling and Rspo3 (a Wnt signaling enhancer) as primary environmental factors for maintaining liver metabolic zonation. Indeed, loss of LSEC-derived Wnt ligands or Rspo3 perturbs zonation of glutamine synthetase (GS), a critical enzyme that maintains liver metabolic stability. Although LSEC are the major source of Wnt ligands in the liver, other non-parenchymal cells of the liver also produce Wnt ligands that may, in turn, regulate metabolic architecture and function of the liver. Notably, a recent study revealed the zone-specific expression of Rspo3 in HSC. This suggests that HSC-derived Rspo3/Wnt signaling may play specific roles in regulating liver functions in a zone-specific manner. Despite these correlative findings, functions of HSC-derived Wnt signaling in the liver have not been systematically addressed. This thesis aimed to study the role of HSC-derived Wnt signaling in regulating liver functions. To this end, I generated HSC-specific Rspo3 and Wnt transporter Evi/Wls (Wnt ligand secretion factor) knockout mice. Lack of Rspo3 from HSC did not lead to changes in GS zoantion, as shown in LSEC-specific Rspo3 knockout mice. Instead, an altered zonation of cytochrome P450, enzymes that are involved in the metabolism of toxic products, was revealed. Furthermore, loss of Rspo3 in HSC led to impaired liver regeneration in a 2/3 partial hepatectomy model. Pathologically, HSC-specific Rspo3 deletion protected the liver from CCl4- and APAP-induced, but not from cholestatic liver damage induced by bile duct ligation. In contrast, this protective phenotype was not seen in LSEC-specific Rspo3 or HSC-specific Evi/Wls knockout mice, indicating HSC and Rspo3 specificity in this regulation. To further elucidate underlying mechanisms of the protective phenotype upon HSC-specific Rspo3 deletion, I performed RNA-sequencing on HSC and hepatocytes isolated from CCl4-treated mice. Gene expression analyses suggested that HSC-specific Rspo3 deletion primed activated HSC to a pro-apoptotic/anti-proliferative signature, and inhibited apoptotic pathways in hepatocytes.