Abstract
The molecular mechanisms regulating the expansion of the hematopoietic system including hematopoietic stem cells (HSCs) in the fetal liver during embryonic development are largely unknown. The LIM-homeobox gene Lhx2 is a candidate regulator of fetal hematopoiesis since it is expressed in the fetal liver and Lhx2−/− mice die in utero due to severe anemia. Moreover, expression of Lhx2 in embryonic stem (ES) cell-derived embryoid bodies (EBs) can lead to the generation of HSC-like cell lines. To further define the role of this transcription factor in hematopoietic regulation, we generated ES cell lines that enabled tet-inducible expression of Lhx2. Using this approach we observed that Lhx2 expression synergises with specific signalling pathways, resulting in increased frequency of colony forming cells in developing EB cells. The increase in growth factor-responsive progenitor cells directly correlates to the efficiency in generating HSC-like cell lines, suggesting that Lhx2 expression induce self-renewal of a distinct multipotential hematopoietic progenitor cell in EBs. Signalling via the c-kit tyrosine kinase receptor and the gp130 signal transducer by IL-6 is necessary and sufficient for the Lhx2 induced self-renewal. While inducing self-renewal of multipotential progenitor cells, expression of Lhx2 inhibited proliferation of primitive erythroid precursor cells and interfered with early ES cell commitment, indicating striking lineage specificity of this effect.
Highlights
The mammalian hematopoietic system continuously generate large numbers of functional erythroid, myeloid and lymphoid cells throughout life
We have previously shown that Lhx2 expression in hematopoietic progenitor cells derived from embryonic stem (ES) cells differentiated in vitro and from adult bone marrow could lead to the generation of hematopoietic stem cells (HSCs)-like cell lines [41,42,43,46]
By using an efficient system to express Lhx2 during ES cell differentiation in vitro we have shown that Lhx2 expression cause three different phenotypes summarised in Figure 7: 1) self-renewal of a distinct definitive multipotential hematopoietic progenitor cell in embryoid bodies (EBs) in a growth factor-specific manner, 2) complete block in proliferation and differentiation of committed EryP precursor cells and to some extent EryD precursor cells, and 3) interference with the initial steps in ES cell differentiation in vitro
Summary
The mammalian hematopoietic system continuously generate large numbers of functional erythroid, myeloid and lymphoid cells throughout life. Intraembryonic hematopoiesis is established approximately at E10.5, initially in the so-called aorta-gonadmesonephros (AGM) region and shortly thereafter the fetal liver becomes colonised by progenitor cells [7,13,14,15,16,17,18,19], marking the switch from primitive to definitive hematopoiesis as the formation of the whole spectrum of hematopoietic lineages including the definitive erythroid (EryD) lineage commences at this stage The latter cell type differs from the EryP lineage in that they are smaller, lack nucleus and produce adult hemoglobin [8,9]
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