Ectopic overexpression of c-mpl by retroviral-mediated gene transfer suppressed megakaryopoiesis but enhanced erythropoiesis in mice

In this report, we tested whether ectopic overexpression of a cell surface receptor cDNA could be used to explore the physiological roles of that rece ptor. We generated c-mpl overexpressing animals by reconstituting mice with retroviral vector-transduced bone marrow (BM) cells, We observed that plat elet counts in the c-mpl overexpressing mice failed to recover to normal le vels and remained at <200 X 10(6)/mL post-transplantation, while platelet n umbers in the control mice returned to >800 x 10(6)/mL by 4 weeks post-tran splantation. However, platelet Counts in the c-mpl overexpressing mice coul d be stimulated to normal levels after administration of rhMGDF, No signifi cant changes in peripheral leukocyte counts were observed, although the num ber of CFU-E, GM CFC, and CFC-multi were reduced two- to threefold in the B M of the c-mpl overexpressing mice. In addition, enhanced erythropoiesis wa s observed in the c-mpl overexpressing mice, The mpl receptors on erythroid cells were functional as demonstrated by tyrosine-phosphorylation of mpl r eceptor on RBC and by in vitro erythroid colony-formation in response to MG DF stimulation, respectively. These results suggested that ectopically expr essed mpl receptors competed for ligand in vivo leading to an insufficient amount of circulating thrombopoietin (Tpo) for the development of megakaryo cytic lineage. These results further suggest that, in addition to sequester ing circulating Tpo, overexpression of the mpl receptor on erythroid progen itors may directly contribute to enhanced erythropoiesis in vivo, Our studi es demonstrate that ectopic overexpression of a receptor by retroviral-medi ated gene transfer provides an approach to explore the biological roles of novel receptors, (C) 1999 International Society for Experimental Hematology . Published by Elsevier Science Inc.