Generation and Initial Characterization of Three Murine Models with Altered Platelet TFPI Expression

Introduction: Tissue factor pathway inhibitor (TFPI) is an anticoagulant protein that inhibits tissue factor and prothrombinase during early stages of the procoagulant response. It is produced in alternatively spliced isoforms. In humans, TFPIα is within platelets and released upon activation and also is present in plasma, while TFPIβ is bound to the endothelial surface. Mice differ from humans in that they do not have TFPIα in plasma, making them an optimal model system for study of the function of platelet TFPIα. Studies of murine hematopoietic cell TFPI using fetal liver transplantation models have found that platelet TFPI dampens platelet accumulation in a growing thrombus following vascular injury and alters bleeding severity in mice with hemophilia. However, results of these studies are potentially confounded by altered TFPI expression on other hematopoietic cells, such as monocytes. To better understand the function of platelet TFPI, murine models of total TFPIα deletion (TFPIα KO), of platelet-specific TFPIα deletion (pTFPIα KO) and platelet-specific TFPIα over expression (pTFPIα high) were produced and characterized.

Methods: A TFPIα deleted mouse was generated by inserting a stop codon within exon 10 of the TFPI gene effectively blocking translation of the highly conserved basic C-terminal region of TFPIα, but not altering TFPIβ. The PF4-Cre transgene was bred into the floxed-TFPI(K1) mouse to produce mice specifically lacking platelet TFPIα. A transgenic mouse over expressing platelet TFPIα under control of the GP1b promoter was produced.  Mice were characterized for platelet and plasma TFPI antigen by ELISA, for activity by TF-FVIIa inhibition and whole blood platelet aggregation, and for bleeding by tail clip.

Results: All 3 strains were viable and reproduced. Platelet TFPI antigen was decreased approximately 25-fold in the TFPIα KO and pTFPIα KO mice and elevated approximately 5-fold in the pTFPIα high mice with corresponding changes in TF-FVIIa inhibitory activity. Plasma TFPI antigen was no different between pTFPIα KO (20.6±4.4 nM) and wild type (21.5±6.2 nM) littermates. Plasma thrombin-antithrombin complex levels were not elevated in any strain. In tail clip bleeding time assays, there was no difference between pTFPIα KO (0.23±0.16 mg/ml Hb) and wild type (0.23±0.37 mg/ml Hb), but there was a trend towards increased blood loss in pTFPIα high mice (1.25±1.8 mg/ml Hb). The pTFPIα high mice also had altered whole blood platelet aggregation that was evident primarily in a prolonged time for initiation of aggregation with 6.50uM ADP (81±12 sec vs. 6.1±5.6 sec for control) or 0.8ug/ml collagen (35±17 sec vs. 16±21 sec for control). Finally, the pTFPIα high transgene was bred into TFPI heterozygous mice which were then bred to determine if over expression of platelet TFPIα would rescue the TFPI null embryonic lethal phenotype. However, no live TFPI null pups were observed following genotyping of over 50 pups from these matings.

Conclusions: Three new mouse models of altered TFPI expression have been made providing valuable tools to study the role of platelet TFPI in bleeding and clotting disorders. Plasma TFPI is unchanged in mice lacking platelet TFPI demonstrating that platelet TFPI does not contribute to plasma TFPI in mice. Platelet TFPIα antigen and activity correlate in all three models. Deletion of the basic C-terminal region in TFPIα in the TFPIα KO mice produces a TFPIα hypomorphic mouse, with only trace amounts of truncated TFPIα present in platelets. This lack of TFPIα does not produce a generalized procoagulant state. The 5-fold excess TFPIα antigen in the pTFPIα high mice substantially delays the onset of platelet aggregation in whole blood assays initiated with collagen or ADP. Results from tail clip bleeding assays suggest that over expression of platelet TFPIα may prolong bleeding in adult mice, but it is not sufficient to rescue the embryonic lethality of TFPI null mice.