DIFFERENTIAL-EFFECTS OF AN ERYTHROPOIETIN RECEPTOR GENE DISRUPTION ONPRIMITIVE AND DEFINITIVE ERYTHROPOIESIS

Although the hormone erythropoietin (Epo) and its receptor (EpoR) are known to play important roles in the regulation of erythropoiesis, sev eral questions remain concerning the developmental role of Epo/EpoR si gnaling. As the functions of Epo have been defined primarily through s tudies of definitive erythroid cells, its importance for primitive, em bryonic erythropoiesis remains uncertain, as does the significance of EpoR expression in several nonerythroid cell types. To address these q uestions, mouse embryonic stem cells and embryos lacking a functional EpoR gene were produced by gene targeting. The effects of the mutation were examined in embryos developing in vivo, in chimeric adult mice p roduced with homozygous mutant embryonic stem cells, and in hemopoieti c cells cultured in vitro. No defects were apparent in nonerythroid ce ll lineages in which the EpoR normally is expressed, including megakar yocytes and endothelial cells. In the mutant yolk sac, primitive eryth rocytes were produced in normal numbers, they underwent terminal diffe rentiation, and expressed near normal levels of embryonic globins, alt hough they were reduced in size and their proliferation was severely r etarded after E9.5. In contrast, in the fetal liver, definitive erythr opoiesis beyond the late progenitor (CFU-E) stage was drastically inhi bited by the EpoR mutation, and virtually no definitive erythrocytes w ere produced in vivo, leading to embryonic death by E13.5. Thus, our r esults suggest a fundamental difference in the molecular mechanisms st imulating primitive and definitive erythropoiesis. It was also observe d that a few mutant definitive erythroid cells could terminally differ entiate when cultured with additional cytokines, demonstrating that al though Epo/EpoR signaling is important for definitive erythroid cell s urvival and proliferation, it is not an obligatory step in differentia tion.