Genetic Absence of Thrombin Receptor Par4 Overcomes the Obligate Requirement of EPCR in Mouse Placenta

Introduction: Murine models suggest that the Thrombomodulin-Protein C system plays a critical role in placentation and the maintenance of pregnancy. Severe Protein C deficiency in the mother results in pregnancy failure in early gestation. Thrombomodulin (Thbd) or the Endothelial Protein C Receptor (EPCR/ProcR) gene deletions result in embryonic death, secondary to developmental and functional abnormalities of the placenta. These molecules play multiple roles in coagulation and inflammation. The mechanisms governing their role in placental development and maintenance of placental function remain to be fully understood. The objective of this work is to identify the critical functions of EPCR and Thbd that are required for placental development. Both Thbd and EPCR augment activated protein C generation, albeit to different extents. We have examined if reduced activated Protein C generation mediates placental abnormalities of EPCR- and Thbd- null mice. Activation of thrombin receptors expressed on platelets and trophoblast cells can also contribute to placental failure. We examined the role of thrombin receptor Par4 in placental failure of EPCR-null mice.

Methods: To assess the role of aPC generation in placental phenotype of Thbd- and EPCR- null mice, we used a transgene to express a hyperactivatable form of murine protein C (hMPC) under the control of transthyretin promoter. Thrombin cleaves this mutant form of Protein C 30-fold more efficiently than wild type protein C, without requiring the cofactor function of thrombomodulin. Wild type mice expressing hMPC show 2-fold increase in PC and 3-fold increase in aPC levels. hMPC expression in PC-null mice restores their ability to carry pregnancies. Breeding strategies were used to generate hMPC^tg ProcR^+/- or hMPC^tg Thbd^+/- female mice. These were mated to ProcR^+/- or Thbd^+/- males, respectively, and survival of ProcR^-/- and Thbd^-/-embryos was analyzed. Similar genetic strategy was used to analyze the role of thrombin receptor Par4 in the demise of EPCR-null embryos. Placental phenotypes and embryonic survival was compared with experiments in which the mother was continuously infused with LMWH using a subcutaneous osmotic pump.

Results: As previously reported, EPCR-null mice die before 10.5 days post coitum (dpc) (ProcR^+/- intercrosses, out of 41 live embryos none were ProcR^-/-, 10 were expected, 21 aborted not genotyped, 7 pregnancies analyzed at 11.5 dpc) and none are found at wean (out of 30 live pups none were ProcR^-/-, 8 were expected, 5 litters analyzed). Transgenic expression of hMPC in the mother resulted in some live ProcR^-/- embryos at 11.5 dpc (4 ProcR^-/- out of 41 live embryos, 10 were expected, 15 aborted not genotyped, 7 pregnancies at 11.5 dpc) and pups at wean (2 ProcR^-/- out of 28 live, 7 litters analyzed). Despite transgenic hMPC expression ProcR^-/- embryos and pups were underrepresented (P=0.007, chi square GOF test). Surviving ProcR^-/- embryos showed normal placental histology grossly comparable to littermate controls. Expression of hMPC in the mother did not ameliorate fetal death of Thbd-null mice (out of 38 live embryos none were Thbd^-/-, 10 expected, 16 aborted not genotyped, 7 pregnancies at 9.5 dpc). Continuous infusion of LMWH also resulted in some live ProcR^-/- embryos at 11.5 dpc (3 ProcR^-/- out of 19 live embryos, 5 expected, 11 aborted not genotyped, 3 pregnancies analyzed), but two were growth retarded and all 3 placentae showed markedly reduced placental labyrinth formation. In contrast to transgenic expression of hMPC and treatment with LMWH, when Par4^-/-ProcR^+/- animals were intercrossed, ProcR^-/- animals were born at an expected Mendelian frequency (7 ProcR^-/-out of 35 live pups, 9 expected, 7 litters analyzed).          

Conclusions: Our results show that transgenic expression of hMPC allows normal placental development and rescues a fraction of EPCR-null embryos. Thus, placental defect of EPCR-null mice is in part mediated by reduced generation of aPC on placental cells. In contrast to the transgenic expression of hMPC and LMWH treatment, genetic absence of Par4 completely overcame the placental defect and allowed development of EPCR-null embryos. Further studies will clarify contributions of maternal versus fetal Par4 in this phenomenon.