Defective cerebrovascular development in mice lacking TFPI is restored by activated protein C

Mice lacking tissue factor pathway inhibitor (Tfpi-/-) succumb to embryonic lethality from excess thrombin production and associated cerebrovascular defects called glomeruloid bodies. A transgene-producing hyperactivatable mouse protein C (hMPC) was bred into Tfpi+/- mice to determine whether excess activated PC would correct the cerebrovascular defects in Tfpi-/- embryos. Tfpi-/-/hMPC+ embryos survived to adulthood. Despite the rescue of embryonic lethality, hMPC reduced glomeruloid body numbers by only 36% and did not prevent fibrin deposition or disruption of the blood-brain barrier within glomeruloid bodies. However, decreased hypoxia and cellular death in Tfpi-/-/hMPC+ brains suggest that cytoprotective effects of hMPC contributed to Tfpi-/- rescue. The glomeruloid bodies were completely resolved in Tfpi-/-/hMPC+ postnatal day 10 pups revealing a distinct temporal effect of TFPI on embryonic cerebrovascular development. Bulk RNA sequencing of embryonic day 15.5 brain tissue identified increased angiogenesis as the overwhelming biological process altered in Tfpi-/- brain. This included changes in genes encoding apelin, adrenomedulin, and UNC5b, which was consistent with abundant endothelial tip cells within glomeruloid bodies. The increased expression of these angiogenic genes was reversed by the hMPC transgene. These findings define TFPI as an essential inhibitor of thrombin generation during embryonic angiogenesis that acts temporally within or around developing cerebral vasculature in a manner that is not compensated for by other anticoagulant proteins. The findings emphasize the importance of blood coagulation proteases and regulation of their activity in diverse biological processes.