Endothelial protein C receptor supports hematopoietic stem cell engraftment and expansion in Mpl-deficient mice

Thrombopoietin (Thpo)/myeloproliferative leukemia virus oncogene (Mpl) signaling controls hematopoietic stem cell (HSC) self-renewal and quiescence; however, how these 2 seemingly opposing functions are controlled is not well understood. By transplantation of lentiviral-transduced hematopoietic cells in the Mpl-deficient mouse model, we addressed whether known or predicted Thpo target genes were able to rescue the Mpl-deficient phenotype of the mice. Among the tested genes, we identified endothelial protein C receptor (Epcr) to expand HSCs with the long-term (LT)-HSC surface phenotype in Mpl(-/-) mice and to enable secondary transplantation of Mpl-deficient bone marrow (BM). Epcrtransduced Mpl(-/-) HSCs enter quiescence earlier after transplantation than controltransduced Mpl(-/-) cells, and upregulated expression of the anti-apoptotic gene Bcl-xL. Also, in the wild-type background, Epcr expression marked the engrafting population in the BM. Furthermore, Epcr expression in Mpl(-/-) hematopoiesis increased the number of megakaryocytes in the BM. In vitro Thpo supported the surface expression of Epcr on primary murine hematopoietic stem and progenitor cells. With these data, we add new insights into Thpo-dependent influence on HSC engraftment after transplantation. This may be of use for the in vitro manipulation of HSCs, also in the context of gene therapy.