Stem cell function and organ development : analysis of Lhx2 function in hematopoietic stem cells and eye development

Abstract: When a multicellular organism suffers damages to tissues/organs it heals itself by either substituting the lost cellular matrix by scar formation or by regenerating the lost tissue. Regeneration likely occurs by a recapitulation of the developmental process that formed the organ. Many processes regulating organ development are based on epithelial-mesenchymal interactions and a strict control of organ specific stem/progenitor cells. Elucidation of the molecular basis of these processes is therefore vital in order to develop novel therapies in regenerative medicine. The LIM homebox gene Lhx2 is interesting in this context since Lhx2 has been shown to be important for the formation of several organs by regulating epithelial-mesenchymal interactions and progenitor cell function. Targeted inactivation of Lhx2 leads to a lethal anemia due to malformed liver and severe neural abnormalities such as hypoplasia of the forebrain and anophtalmia. Thus, elucidation of the mechanisms of the function of Lhx2 in different organ systems would give important insights into the molecular mechanisms regulating epithelial-mesenchymal interactions and stem/progenitor cell function. To elucidate the function of Lhx2 in the hematopoietic system Lhx2 was initially expressed in hematopoietic progenitor cells derived from ES cells differentiated in vitro using retroviral vectors. This approach led to the generation of hematopoietic stem cell (HSC)-like cell lines suggesting that Lhx2 could impact HSC function. However neither the specificity nor the efficiency of the Lhx2-induced phenotype could be determined using this approach. To be able to elucidate the function of Lhx2 in the hematopoietic system, an ES cell line with inducible Lhx2 expression was generated. Lhx2 expression induces self-renewal of a distinct hematopoietic progenitor cell from which HSC-like cell lines were established. Down-regulation of Lhx2 in these HSC-like cell lines leads to a rapid loss of stem cell character, providing a good model to study the molecular function of Lhx2 in hematopoietic stem/progenitor cells. A global gene expression analysis was performed comparing the Lhx2+ stem cell population to the Lhx2- differentiated progeny. This approach identified genes putatively linked to self-renewal/differentiation of HSCs. A considerable proportion of the genes showed an overlapping gene expression pattern with Lhx2 expression in tissue of non-hematopoietic origin suggesting that Lhx2 function in stem/progenitor cells partly overlap with Lhx2 function during organ development. In order to define other Lhx2-dependent progenitor cell populations and to generate a tool to analyze the function of Lhx2 in organ development a new transgenic mouse model was generated. By using a specific part of the Lhx2 promoter to drive expression of Cre recombinase in vivo (Lhx2-Cre mice) we have been able to define the first eye committed progenitor cells in the forebrain. By using the Lhx2-Cre mice it will be possible to distinguish the function of genes during eye development from their function in the patterning of the forebrain e.g. the eye field transcription factors. Conditional inactivation of Lhx2 in these eye specific progenitor cells causes an immediate developmental arrest. The transgene is also active in Lhx2-/- embryonic forebrain, but re-expression of Lhx2 in Lhx2-/- progenitor cells only promote formation of retinal pigment epithelium cells. Analysis of genes expressed by the Lhx2+ stem cell population allowed us to define novel genes putatively linked to Lhx2 function in eye development. Thus, we have defined the progenitor cells in the forebrain committed to eye development and the expansion and patterning of these progenitors are dependent on Lhx2. Although commitment to eye development is Lhx2-independent, Lhx2 might be important for the acquisition of the oligopotent fate of these progenitor cells.