The role of Lhx2 in the hematopoietic stem cell function, liver development and disease

During embryonic development, generation of functional organs is dependent on proper interactions between different cell types. Elucidation of the mechanisms operating during organ formation might provide insights into the origin of many pathological disorders in the adult. Gene inactivation studies in mice have provided invaluable tool to study the function of genes critical for morphogenesis of distinct organs. A LIM-homeodomain transcription factor Lhx2 has previously been reported to play a role in fetal liver development and hematopoiesis, as its inactivation leads to lethal anemia due to underdeveloped liver. This thesis focuses on the function of Lhx2 in the development of these two organ systems. Reciprocal signaling between ventral foregut endoderm and mesenchyme of the septum transversum regulates the liver formation, expansion and differentiation. A fully formed liver is composed of endoderm-derived hepatocytes and cholangiocytes and a variety of mesenchyme-derived cell types, such as endothelial cells and hepatic stellate cells. In early stages of liver development Lhx2 is expressed in the liver-associated septum transversum mesenchyme, a part of which becomes integrated into the liver organ and develops into hepatic stellate cells. Functional Lhx2 expression in the hepatic mesenchyme is necessary for normal liver outgrowth and differentiation. Loss of Lhx2 from developing hepatic stellate cells leads to their activation and excessive deposition of collagen fibres, resulting in hepatic fibrosis and severely distorted liver architecture. Transfection of Lhx2 to human stellate cell line downregulates genes associated with stellate cell activation and fibrogenesis. Thus, Lhx2 is the first gene identified to negatively regulate events leading to hepatic fibrosis. Elucidation of the molecular mechanisms involved in this process might therefore be instrumental for the development of novel therapies useful in treatment of this disorder. Fetal liver is also a major site of hematopoiesis in the embryo and provides physiological conditions necessary for the efficient expansion of hematopoietic stem cells (HSCs). The hematopoietic defect observed in Lhx2-deficient embryos is cell-nonautonomous, indicating that Lhx2 might control secreted factors involved in the self-renewal of HSCs. This putative second role of Lhx2 has been investigated by analyzing the mechanism whereby Lhx2 expression generates in vitro self-renewing HSC-like cell lines. Interestingly, in agreement with the cell nonautonomous phenotype of the lethal anemia in Lhx2-/- embryos, the mechanism of self-renewal is dependent on Lhx2 expression and occurs via secreted factor(s). Identification of these factor(s) might potentially allow ex vivo expansion of HSCs for therapeutic purposes. The Lhx2-immortalized HSC-like cell lines share many basic features with HSCs and self-renew in vitro in presence of Steel factor (SF). SF/c-Kit signaling mediates a wide variety of biological activities in cells at many different levels in the hematopoietic hierarchy. We used the HSC-like cell lines as an in vitro model system to compare signal transduction pathways from c-Kit receptor in stem cells versus differentiated hematopoietic cells. HSCs require PI-3K dependent activation of Raf1-Mek-Erk cascade for their survival and self-renewal in response to SF, whereas activation of Erk is PI-3K independent in committed myeloid and mast cells. Thus, the mode of SF/c-Kit signaling is dependent on the differentiation status of the cells.