REGULATION OF THE SERUM CONCENTRATION OF THROMBOPOIETIN IN THROMBOCYTOPENIC NF-E2 KNOCKOUT MICE

The mechanisms that regulate circulating levels of thrombopoietin (Tpo ) are incompletely understood, According to one favored model, the rat e of Tpo synthesis is constant, whereas the serum concentration of fre e Tpo is modulated through binding to c-Mpl receptor expressed on bloo d platelets. Additionally, a role for c-Mpl expressed on megakaryocyte s is suggested, particularly by the observation that serum Tpo levels are not elevated in human immune thrombocytopenic purpura. Whereas dir ect binding of Tpo to platelets has been demonstrated in vitro and in vivo, the role of megakaryocytes in modulating serum Tpo levels has no t been addressed experimentally. The profoundly thrombocytopenic mice lacking transcription factor p45 NF-EP do not show the predicted incre ase in serum Tpo concentration. To evaluate the fate of the ligand in these animals, we injected I-125-Tpo intravenously into mutant and con trol mice. In contrast to normal littermates, NF-E2 knockout mice show negligible association of radioactivity with blood cellular component s, consistent with an absence of platelets. There is no corresponding increase in plasma-associated radioactivity to suggest persistence in the circulation. However, a greater fraction of the radioligand is bou nd to hematopoietic tissues. In the bone marrow this is detected virtu ally exclusively in association with megakaryocytes, whereas in the sp leen it is associated with megakaryocytes and small, abnormal, platele t-like particles or megakaryocyte fragments that are found within or i n close contact with macrophages. These findings implicate the combina tion of megakaryocytes and the latter particles as a sink for circulat ing Tpo in NF-EP knockout mice, and provide an explanation for the lac k of elevated serum Tpo levels in this unique animal model of thromboc ytopenia. (C) 1997 by The American Society of Hematology.