Hematopoietic stem cell deficiencies in mice lacking c-Mpl, the receptor for thrombopoietin

Thrombopoietin (TPO) acts as a lineage-specific late-acting factor to stimulate megakaryocyte and platelet formation. However, analysis of mice lacking either the cytokine or its receptor, c-Mpl, also revealed deficiencies in progenitor cells of multiple hematopoietic lineages, suggesting that TPO signaling may play an important role in the regulation of the hematopoietic stem cell compartment. To investigate this hypothesis, we determined preprogenitor and colony forming unit-spleen (CFU-S) numbers and analyzed the long-term hematopoietic repopulating capacity of bone marrow cells from mpl(−/−) mice. mpl(−/−) mice had 4- to 12-fold fewer preprogenitor cells than wild-type mice. In irradiated normal recipients, mpl(−/−) bone marrow generated 8- to 10-fold fewer spleen colonies than wild-type marrow at both 8 and 12 days after transplantation. This defect was intrinsic to the transplanted hematopoietic cells, as the microenvironment of mpl(−/−) recipients supported similar CFU-S growth to that observed in wild-type recipients. In definitive assays of stem cell function, bone marrow cells from mpl(−/−) mice failed to compete effectively with normal cells for long-term reconstitution of the hematopoietic organs of irradiated recipients, even when transplanted in 10-fold excess. Serial transplantation studies further suggested that stem cell self-renewal also may be compromised in mpl(−/−) mice. These data imply that TPO, signaling through c-Mpl, plays a vital physiological role in the regulation of hematopoietic stem cell production and function.