Disruption of the endothelial cell protein C receptor gene in mice causes placental thrombosis and early embryonic lethality

The endothelial cell protein C receptor (EPCR) is a type 1 transmembrane protein found primarily on endothelium that binds both protein C and activated protein C with similar affinity. EPCR augments the activation of protein C by the thrombin-thrombomodulin complex. To determine the physiological importance of EPCR, we generated EPCR-deficient mice by homologous targeting in embryonic stem cells. Genotyping of progeny obtained from EPCR+/- interbreeding indicated that EPCR-/- embryos died on or before embryonic day 10.5 (E10.5). Reverse transcriptase-PCR confirmed the absence of EPCR mRNA in EPCR-/- embryos. EPCR-/- embryos removed from extra-embryonic membranes and tissues at day E7.5 and cultured in vitro developed beyond E10.5, suggesting a role for EPCR in the normal function of the placenta and/or at the materno-embryonic interface. Immunohistochemistry revealed the lack of EPCR in trophoblast giant cells of EPCR-/- embryos. These cells, which normally express EPCR, are in direct contact with the maternal circulation and its clotting factors. In EPCR-/- embryos, greatly increased fibrin deposition was detected around these cells. To prevent this fibrin deposition, EPCR+/--crossed female mice received a daily subcutaneous injection of enoxaparin through pregnancy. Although some EPCR-/- embryos were rescued from midgestational lethality, this regimen yielded no EPCR-/- pups. We conclude that EPCR is essential for normal embryonic development. Moreover, EPCR plays a key role in preventing thrombosis at the maternal-embryonic interface.