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Projekt Druckansicht

Der Notch-Signalweg in Plastizität, Entwicklung und Regeneration der Leber

Fachliche Zuordnung Gastroenterologie
Förderung Förderung von 2011 bis 2017
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 194010126
 
Erstellungsjahr 2017

Zusammenfassung der Projektergebnisse

After acute loss of liver mass (i.e. after resection) a sequence of well-orchestrated cellular events is initiated which leads to proliferation of mature hepatocytes and biliary cells of the normally quiescent organ to ultimately restore liver function and size. However, in chronic liver disease, when hepatocyte and cholangiocyte proliferation is impaired, ductular reactions (DRs) emerge. DRs comprise cellular reactions of ductular (biliary) phenotype at the portalparenchymal interface. Mainly derived from 2D histological and in vitro studies, it has been widely accepted as a basic principle of adult liver repair that these DRCs act as bipotential progenitors and serve as an adult cellular emergency compartment. DRCs are believed to be capable of producing both cholangiocytes and hepatocytes in case acute or chronic liver injury compromise self-duplication of these mature epithelial cells. Hence, DRCs are frequently synonymously denominated “liver progenitor cells (LPC)” or “adult liver stem cells”. Beyond liver repair, LPC have also come into focus as a likely candidate cellular compartment susceptive to malignant transformation and that can give rise to hepatocellular carcinoma (HCC). The functional relevance of this putative progenitor response to injury repair is controversially debated. Moreover, the molecular mechanisms driving expansion, morphogenesis and lineage allocation of adult progenitors to hepatocytes or mature biliary cells are not well defined. However, emerging concepts propose that liver progenitor cellmediated repair mechanisms in adult livers may recapitulate developmental processes using identical molecular pathways, morphogens, and growth factors usually active during embryonic development. In our first funding period we have established the decisive role of Notchsignaling in cell fate decisions and morphogenesis during embryonic liver development as well as in mediating cellular reprogramming of adult liver cells. In our second funding period, in order to characterize the functional relevance of the Notch signaling pathway in expansion and lineage allocation of the putative liver progenitor compartment in adult liver repair and carcinogenesis we have established a novel mouse strain that allows highly effective and compartment-specific inducible genetic labeling of the adult biliary/progenitor compartment with a fluorescent dye in order to trace the cellular destiny of these cells in mice subjected to multiple exogenous and endogenous genetic regeneration protocols. By simultaneously combining this lineage tracing approach with conditional gain- and loss-offunction mouse models for the Notch- and Notch-related (Wnt/β-Catenin) pathways we sought to determine not only the prospective cellular potency of DRs in liver regeneration but also to systematically characterize the role of Notch in regulating progenitor cell expansion, morphogenesis, and lineage allocation. By utilizing our novel lineage tracing mouse model we could demonstrate that biliary cells are the predominant precursors of DRs found over a wide range of experimental and genetic liver injury models but, surprisingly, lack significant capacity to produce new hepatocytes, even when liver injuries were prolonged up to 12 months. Genetic modulation of Notch and/or Wnt/β-catenin signaling within lineage-tagged DRs impaired DR expansion but failed to deliberate DRs from their biliary fate towards the hepatocyte lineage. Further, lineage-labeled DRs did not produce tumors in genetic and chemical HCC mouse models. In summary, we found no evidence that mouse biliary-derived DRs significantly act as LPC pool to replenish hepatocytes in injury or give rise to HCCs. Ongoing analysis of injury-induced DR architecture by novel imaging methods and specific DR-specific loss-of-function mouse models suggest that DRs are mounted rather for providing a structural network for proper bile flow rather than acting as progenitor pool to provide new hepatocytes.

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