Tissue Factor Pathway Inhibitor Modulates Fibrin Deposition In The Brain In a Manner That Is Independent Of Thrombomodulin and Activated Protein C

Fibrin deposition in cerebral vessels contributes to the pathogenesis of stroke and vascular dementia. Under quiescent conditions fibrin deposition is prevented or limited by natural anticoagulants of the endothelium. Tissue factor pathway inhibitor (TFPI) and thrombomodulin (TM) are the two major endothelial anticoagulants and act on different portions of the clotting cascade. TFPI inhibits the initiation of coagulation by inhibiting TF-FVIIa and FXa. TM slows the propagation of coagulation by promoting the activation of protein C (APC), which inactivates FVa and FVIIIa. Both proteins are reported to have low expression in the brain and their function in prevention of cerebral vascular fibrin deposition is unclear. This is emphasized by findings from murine models of decreased TFPI and TM (TM^pro/pro) activity. TFPI null embryos have fibrin deposition in the brain and liver, while TM^pro/pro mice have fibrin deposition in the lungs, heart, spleen and liver, but the brain is protected from fibrin deposition even following lethal LPS doses. Interestingly, TFPI heterozygosity combined with TM^pro/pro results in cerebral vascular fibrin. Here, we report a detailed analysis of TFPI expression in the brain vasculature, the consequences of vascular TFPI deficiency in adult mice, and the capacity of APC to compensate for TFPI deficiency.

Real time PCR studies demonstrated that TFPI expression in total brain is lower than in other tissues in comparison to the RPL-19 housekeeping gene. However, when the endothelial-specific genes, VE-cadherin or CD31 were used as housekeeping genes, brain TFPI expression was higher than that of lung endothelium (TFPIa: ∼9 fold higher; TFPIb: ∼5-fold higher). In situ hybridization studies revealed that TFPI and TM are expressed on endothelium throughout the brain parenchyma. In addition, TFPI was expressed within the granule layer of the cerebellum. TFPI null embryos succumb to embryonic lethality with necrotic lesions and fibrin deposition in the brain. To determine if increased TM-APC anticoagulant activity would rescue TFPI null embryos, a transgene over expressing human activated protein C (hHPC) was bred into heterozygous TFPI mice, which were then bred to determine if TFPI null mice expressing hHPC survive embryogenesis. However, the hHPC transgene did not rescue TFPI null embryos. Furthermore, TFPI null embryos had similar brain lesions and brain fibrin deposition regardless of the presence of the hHPC transgene. A hematopoietic cell transplant model system was used to examine the consequences of TFPI deficiency in the adult mouse brain. Mice transplanted with hematopoietic cells lacking TFPI had cerebral vascular fibrin deposition that was not observed in mice transplanted with hematopoietic cells producing normal amounts of TFPI. Interestingly, the fibrin was often found at sites of endothelial injury as demonstrated by co-localization with anti-C1qA, which binds to phosphatidylserine on damaged endothelium.

TFPI prevents necrotic brain lesions and cerebral fibrin deposition in developing mouse embryos and over expression of hHPC is not sufficient to compensate for the absence of TFPI. Adult mice lacking TFPI in hematopoietic cells have increased amounts of fibrin deposited at sites of cerebral vascular injury despite having normal endothelial TM. These data indicate that the distinct function of TFPI in the cerebral vasculature is not compensated by the anticoagulant activity of TM in regulating cerebral vascular thrombosis.

Mast:Novo Nordisk: Honoraria, Research Funding.