The role of the LIM-homeobox gene Lhx2 in stem cell function, organ development and disease, implications for regenerative medicine

Expression of Lhx2 in the hematopoietic system led to the generation of hematopoietic stem cell (HSC)-like cell lines. These cell lines are similar to normal HSCs since they share many basic characteristics with normal HSCs. We have also defined the signaling pathways that are required for this expansion of Lhx2-expressing HSC-like cells. Since these cell lines represents an unlimited source of HSC-like cells, they are also used as a model system for analysing basic molecular, cellular and biochemical properties of HSCs, such as intracellular signaling and gene expression pattern.

Lhx2 is expressed in neural progenitor cells in the olfactory epithelium. We have shown that Lhx2 is required for the formation of olfactory neurons since Lhx2-/- mouse embryos show a block in differentiation of olfactory neurons at the precursor level. Similar to the hematopoietic system, the olfactory epithelium is a continuously regenerating tissue.

Hair is formed in specialized mini-organs in the skin called hair follicles (HFs). Most HFs develop during embryogenesis and after birth hair is continuously regenerated since the HFs undergo cyclic phases of regression (catagen), inactivity (telogen) and active growth (anagen), where the anagen phase reveal several similarities with embryonic development of the HF. We have shown the Lhx2 is expressed in the HFs during embryogenesis and during the active phase (anagen) in postnatal HF cycling. Lhx2 is expressed primarily in the areas where stem cells are proliferating and differentiating into the different parts of the HF, e.g. the hair shaft and the supporting layers. These observations suggest that Lhx2 play an important role in the generation and regeneration of hair. In support of this hypothesis we could show that Lhx2 is required for efficient hair formation, and transgenic expression of Lhx2 in telogen HFs was sufficient to induce anagen. Lhx2 is periodically expressed in the HF and is an essential positive regulator of hair growth. Thus, in at least three different organ systems, Lhx2 can regulate stem/progenitor cell function and molecular elucidation of the role of Lhx2 function would therefore give general insights into stem/progenitor cell biology.

Lhx2 is expressed in developing hepatic stellate cells in fetal liver and this expression is maintained in hepatic stellate cells in adult liver, a cell type critically involved in the development of hepatic fibrosis/cirrhosis. We have shown that Lhx2-/- mouse embryos develop progressive liver fibrosis and the liver in these embryos shows all signs of the wound repair process usually found in adult fibrotic/cirrhotic livers. Consequently the liver architecture in Lhx2-/- embryos is severely disrupted. Thus, Lhx2 expression in developing stellate cells is important for proper expansion and differentiation of most cell types in the fetal liver and Lhx2 appears to function as an inhibitor of the fibrogenic process.

To elucidate the molecular basis of Lhx2 function, we have generated HSC-like cell lines where Lhx2 expression is regulated by a Tet-on system and hence dependent on the presence of doxycycline. These cell lines efficiently down-regulate Lhx2 expression upon doxycycline withdrawal leading to rapid differentiation into various myeloid cell types. Global gene expression analyses of these cell lines comparing Lhx2+ HSC-like cells to their Lhx2- progeny have identified genes putatively linked to self-renewal/differentiation of HSCs, function of Lhx2 in organ development, and function of Lhx2 in stem/progenitor cells of non-hematopoietic origin.

We also have developed in vivo models where it is possible to study the function of genes in progenitor/stem cell populations in the formation of various organs/tissues. In one of these in vivo models we have been able to define the first progenitor cells committed to eye development in the forebrain. Lhx2 is essential for the development of both the forebrain and the eyes, but with this in vivo model we can study the function of various genes that are linked to Lhx2 function during eye development. Moreover, we can also distinguish the function of genes in the formation of the forebrain to their function in eye development.

Thus, Lhx2 is involved in stem/progenitor cell function in vitro and in vivo, organ development in vivo and in the development of a common and devastating disease. We are currently analysing the molecular basis for Lhx2 function in these processes.