Studies of the c-Mpl thrombopoietin receptor through gene disruption and activation

The c-mpl gene encodes a receptor for thrombopoietin (TPO), a cytokine that potently stimulates megakaryocytopoiesis. To study the mechanisms of c-Mpl activation, we generated constitutively active receptor mutants. Substitution of cysteine residues into a dimer interface homology domain of c-Mpl forced ligand-independent homodimerization and constitutive receptor activation. In factor-dependent cells, mutant receptors induced autonomous growth and tumorigenicity. The receptors were constitutively phosphorylated in these cells, as were signal transduction molecules implicated in Mpl function. These data suggest that the normal process of ligand-induced Mpl activation involves receptor homodimerization and that mutated forms of the cellular mpl gene can contribute to tumorigenicity. We have also examined the biological role of c-Mpl in mpl-deficient mice generated via homologous recombination in embryonic stem cells. Homozygous mutant animals were deficient in megakaryocytes and severely thrombocytopenic. Mature cells from all other hemopoietic lineages were unaffected. Bone marrow cells from mpl-/- mice were incapable of binding to TPO or responding to the cytokine in clonogenic assays, and further displayed a marked deficiency in progenitor cells capable of megakaryocyte colony formation in response to other stimuli. Moreover, total progenitor numbers were also deficient and included significant reductions in colony-forming cells of multiple hemopoietic lineages. Unexpectedly, the numbers of progenitor cells of all lineages were not perturbed in mid-gestation mpl-/- fetal liver. Our analyses suggest an indispensable role for c-Mpl in megakaryocyte development and reveal that the function of TPO and its receptor is not confined solely to activities in megakaryocytopoiesis.