Expression of Endothelial Protein C Receptor Confines Hematopoietic Stem Cell in Murine Fetal Liver.

Endothelial protein C receptor (EPCR or CD201) is expressed in the endothelial cells of large blood vessels, such as aorta, and contributes to the natural anticoagulant and anti-inflammatory system by activating protein C. Recent study showed that EPCR is also expressed in murine bone marrow, and that the majority of the EPCR-positive cells are the hematopoietic stem cells (HSC) in quiescent state, defined as the side population (sp) fraction in flow cytometry, suggesting the possibility of EPCR involvement in the hematopoiesis. In the murine mid-embryonic stage, on the other hand, the main location of hematopoiesis is fetal liver (FL), where the HSCs are highly proliferative. In this period of development, there are no quiescent HSCs, which is different from adult bone marrow. It is not clarified whether EPCR has any relevance to the fetal hematopoiesis. Here, we demonstrate that EPCR is involved in the hematopoiesis in FL. Immunostaining of murine FL (E12) clearly showed the EPCR-positive cells, and they were located adjacent or connected to the Lyve-1-positive sinusoidal endothelial cells. Some of those were endothelial-like, and others were in round shape, co-expressing c-Kit. In the HSC-enriched Lineage⁻Sca-1⁺c-Kit⁺ (LSK) fraction, the proportion of the EPCR-positive cells was approximately 30%. Sorted EPCR⁺/LSK and EPCR⁻/LSK cells were examined for the capability of bone marrow reconstitution and multilineage differentiation. In colony assay, the EPCR⁺/LSK cells exhibited higher proliferation and differentiation capability into the full range of the hematopoietic cells, including the megakaryocytes and erythroids, while most of the EPCR⁻/LSK-derived colonies were granulocytes and macrophages. Furthermore, the EPCR⁺/LSK and EPCR⁻/LSK cells isolated from donor FL were transplanted with the bone marrow-derived competitor cells into the lethally irradiated recipient mice, resulting in the striking difference between two fractions, i.e., robust long-term reconstitution of EPCR⁺/LSK cells and almost no reconstitution of EPCR⁻/LSK cells. The EPCR⁺/LSK cells were serially transplantable. These data collectively conclude that EPCR-expressing cells in LSK fraction of E12.5 FL exclusively exhibit the capability for long-term bone marrow reconstitution and self-renewal, as well as multilineage differentiation. Thus, the EPCR⁺/LSK fraction is the highly HSC-enriched population. The function of the endothelial molecule in HSCs will be discussed.