ABSTRACT
Recent studies have shown that endothelial protein C receptor (EPCR) polymorphisms and soluble EPCR levels are associated with thrombotic diseases. It is unknown whether membrane EPCR (mEPCR) heterozygosity and/or physiologically elevated sEPCR levels directly impact the hemostatic balance and the outcome of endotoxemia. In these studies, thrombin infusion experiments revealed that EPCR heterozygosity (Procr+/-) impaired protein C activation by approximately 30%. Infusion of factor Xa with phospholipid demonstrated that the Procr+/- genotype increased the coagulant response relative to wild-type mice. Challenge of the Procr+/- mice with lipopolysaccharide (LPS) did not significantly exaggerate their response compared with wild-type mice. We also generated mice in which one allele of full-length EPCR was replaced by sEPCR (Procrs/+). Compared with Procr+/- mice, Procrs/+ mice had 5-fold higher sEPCR and similar mEPCR levels. Procr+/- and Procrs/+ mice generated similar levels of activated protein C (APC) upon thrombin infusion. They also exhibited a similar coagulant response upon factor Xa/phospholipid infusion. Only supraphysiologic levels of sEPCR could influence protein C activation and exaggerate the coagulant response. In conclusion, mEPCR, but not physiologically elevated sEPCR, regulated protein C activation. Procr heterozygosity results in a mild increase of thrombosis tendency and little influence on the response to endotoxin.
Subject(s)
Endotoxemia/etiology , Glycoproteins/physiology , Hemostasis , Animals , Endothelial Protein C Receptor , Heterozygote , Lipopolysaccharides/pharmacology , Membrane Proteins , Mice , Mice, Mutant Strains , Protein C/metabolism , Receptors, Cell Surface , Solubility , Thrombin/pharmacology , Thrombosis/etiologyABSTRACT
The endothelial cell protein C receptor (EPCR) augments protein C activation by the thrombin-thrombomodulin complex. Deletion of the EPCR gene (Procr) in mice leads to embryonic lethality before embryonic day 10 (E10.0). EPCR is detected in the giant trophoblast cells at the feto-maternal boundary from E7.5 and weakly in embryonic aortic endothelial cells from E13.5, suggesting that extraembryonic EPCR expression may be essential for embryonic viability. Using conditional knock-out strategies, we demonstrate that Procr-deficient embryos with EPCR expression on placenta giant trophoblasts can be carried to term and then develop normally. Conversely, EPCR expression in the embryo, without expression in the giant trophoblast cells, does not rescue the mice. In genetically modified mice with low tissue factor activity, Procr deficiency is not lethal to the embryo. As adults, Procr-deficient mice generate more thrombin and activate less protein C in response to procoagulant stimuli. Spontaneous thrombin formation in the deficient animals increases with age. These findings show that extraembryonic EPCR expression is critical for embryo development.
