Differential in vivo roles of Mpl cytoplasmic tyrosine residues in murine hematopoiesis and myeloproliferative disease

Thrombopoietin (Tpo), which binds to its specific receptor, the Mpl protein, is the major cytokine regulator of megakaryopoiesis and circulating platelet number. Produced primarily in the liver, Tpo binds to Mpl on the surface of target cells triggering activation of Janus kinase 2 (Jak2) and phosphorylation of the receptor, as well as activation of several intracellular signalling cascades that mediate cellular responses. Three tyrosine (Y) residues in the C-terminal region of the Mpl intracellular domain have been implicated as sites of phosphorylation required for the activation and regulation of major Tpo-stimulated signalling pathways: Mpl-Y565, Mpl-Y599 and Mpl-Y604. Here, we have introduced mutations in the mouse germline to define the roles of Mpl receptor intracellular domains and tyrosine residues in steady-state and pathophysiological actions of Tpo/Mpl signalling. We report a consistent physiological requirement for Mpl-Y599, absence of which resulted in thrombocytopenia, deficient megakaryopoiesis, low hematopoietic stem cell (HSC) number and function, and attenuated responses to myelosuppression caused by administration of 5-fluorouracil (5-FU). We further show that in models of myeloproliferative neoplasms (MPN) driven by mutations in Jak2 or hCalr, where Mpl is required for pathogenesis, thrombocytosis was dependent on intact Mpl-Y599. In contrast, Mpl-Y565 was required for negative regulation of Tpo responses; mutation of this residue resulted in excess megakaryopoiesis at steady-state and in response to 5-FU, and exacerbated mutant Jak2 and hCalr-driven thrombocytosis.