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The hepatic progenitor cell niche under experimental conditions and in human liver disease

Research project P6/36 (Research action P6)

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The purpose of this research proposal is to study the morphology, function and cellular physiology of the progenitor cell niche in the normal and diseased liver.
Local adult progenitor cell niches are special microenvironments that contain somatic progenitor cells (ASPCs). The latter are generally slow-cycling cells which give rise to transit amplifying cells (TACs) that leave the niche. TACs cycle faster than progenitor cells but have a limited proliferation potential. The niche also contains non-stem niche cells, axonal processes of neurons and specialized extracellular matrix. A complex molecular crosstalk between the progenitor cells and the non-stem niche cells ensures that progenitor cells divide asymmetrically, producing one daughter progenitor cell and one daughter cell that becomes a TAC.

In the liver, canals of Hering and bile ductuli function as minute progenitor cell niches which contain both endodermal and mesenchymal ASPCs. Cells of endodermal origin are at least bipotent: they can give rise to hepatocytes and to bile duct epithelial cells. Mesenchymal ASPCs could give rise to mature stellate cells, sinusoidal endothelial cells or other hepatic mesenchymal cells, but formal proof for such differentiation is lacking. Apart from their presence in the hepatic niche, progenitor cells may also be recruited to the liver from more distant locations such as the bone marrow.

Work package (WP) 1 aims at studying the ontogeny and the structure of the hepatic progenitor cell niche. These studies will be undertaken in embryonic and adult murine and human liver.

WP2 will be carried out with a view to develop new strategies to isolate, enrich and purify the various cell types that make up the murine hepatic progenitor cell niche. This new strategy will then be adapted for human liver tissue.

In WP3 we will analyse the phenotype of isolated and purified ASPCs under various experimental conditions. Which factors drive the cells into proliferation and into differentiation? Is transdifferentiation between endodermal and mesenchymal ASPCs possible?

In WP4, we will transplant isolated and purified ASPCs into urokinase plasminogen activator (uPA+/+) and uPA+/+-scid transgenic mice. These animals offer an ideal model to study engraftment of cells into the liver, as well as their subsequent survival, proliferation and differentiation.

WP5 aims at studying ASPCs in animal models of human liver disease. ASPCs are activated and proliferate in nearly every hepatic disease. The humanised uPA+/+-scid model will be used as a model for viral hepatitis B and C. Hepatocellular carcinoma will be studied in a carcinogen-induced and two transgenic models. Non-alcoholic fatty liver disease (NAFLD) will be investigated in a nutritional model, in the ob/ob leptin-deficient model, and in the new NEMOΔhepa conditional knockout model.

In WP6, we will analyse the ASPC compartment in diseased human liver: chronic hepatitis B and C, biliary diseases, alcoholic liver disease, non-alcoholic fatty liver diseases, and hepatocellular and cholangiocarcinoma. These analyses will rely on samples retrieved from the large tissue bank held at KULeuven and on tissue samples prospectively collected in the frame of this research network.

WP7 envisages to characterize the phenotype and plasticity of established progenitor cells (human mesenchymal stem cells and rat liver epithelial cells). This study will be undertaken to elucidate the transitional processes that lead to the differentiation of progenitors into hepatocyte-like or mesenchymal liver cells with a view to generate alternative sources of functional hepatocytes for in vitro pharmacological and toxicological research.

WP8 will further document recruitment of mesenchymal progenitor cells of bone marrow origin to the liver. The recruitment of such cells, their differentiation into sinusoidal liver cells or hepatic myofibroblasts, and their role in feeding and activating the hepatic progenitor niche will be examined. Moreover, we will also investigate whether these recruited myofibroblasts contribute to fibrotic scar formation in chronically damaged liver.

In WP9, we plan to perform cell lineage tracing studies using existing Cre mice that will be crossed with floxed eGFP reporter mice. This powerful technology will permit cell-lineage tracing at specific stages of embryonic or postnatal development. It will enable us to determine unequivocally the endodermal versus mesodermal origin of cells, the occurrence of transdifferentiation, and recruitment to the liver of cells from the various “side populations”.

This project will add significantly to our understanding of the structure and functions of the highly specialized cellular environment that constitutes the hepatic progenitor cell niche. New possibilities for autologous and allogeneic cell transplantation, and novel strategies for the treatment of chronic liver diseases, aiming at stimulating the proliferation of local progenitor cells, may stem from this research.

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