Notch/Wnt signalling and the hepatic progenitor response in hepatocellular regeneration
Minnis-Lyons, Sarah Elizabeth
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Chronic liver disease remains a significant cause of morbidity and mortality globally. Transplantation is the only effective treatment for end-stage disease but is limited by organ availability, surgical complications and risks of long term immunosuppression. Novel therapies for advanced disease are therefore required. The liver has a remarkable capacity to regenerate through division of mature hepatocytes, however in chronic or severe disease hepatocyte replication fails, senescence occurs and liver failure ensues. Ductular reactions (DRs), containing hepatic progenitor cells capable of repopulating the parenchyma, arise in chronic liver injury when hepatocyte regeneration is impaired. Enhancing this endogenous repair mechanism is a key therapeutic goal. Notch and Wnt are key signals required for liver regeneration, however to date they have principally been characterised in end-point disease and the temporal kinetics of these signalling pathways not known. I sought to identify if these signals control expansion of DRs after hepatocyte injury and whether they can be therapeutically manipulated. I examined the dynamics of Notch and Wnt activity using a genetic model of hepatocellular injury and ductular-mediated regeneration whereby induction of injury could be timed, synchronising the regenerative response. Using lineage tracing, small molecules, blocking antibodies and genetic loss of function experiments I defined distinct time-sensitive Notch and Wnt signatures where early regeneration is driven by Notch and the later response by Wnt. I demonstrated that inhibition of Notch1 and Notch3 but not Notch2 reduces the generation of DRs. I identified that DRs were a source of potent growth hormone IGF1 and this production was Wnt driven. Notch driven expression of IGF1-receptor within DRs identified this axis as a node for cooperation between Notch and Wnt signals. Blocking the IGF1 axis prevented DR expansion, which conversely could be enhanced by administration of recombinant IGF1. Here, I functionally defined complex temporal dynamics controlling of DRs and identified therapeutic pathways to enhance liver regeneration.