New Advances Along the Common Coagulation Pathway

Hemostasis is achieved through spatially and temporally regulated thrombin generation following vascular injury. Blood coagulation factor V plays an important role in this process as it has a major impact on thrombin production. FV is a large, heavily glycosylated protein sharing homology and domain organization with FVIII (A1-A2-B-A3-C1-C2). It is an inactive procofactor and must be cleaved by thrombin or other proteases to remove the B-domain to yield activated FV (FVa). FVa is a cofactor for FXa in the prothrombinase complex, the enzyme that activates prothrombin. Given the profound effect FVa has on thrombin formation, FV activation represents an important step in hemostasis. Mechanistic studies have revealed that proteolysis within the FV B-domain serves to remove key autoinhibitory sequences that serve to conceal/block FXa binding site(s). These sequences consist of an evolutionary conserved basic region (BR) in the middle of the B-domain and flanking acidic regions (AR1 and AR2) which define the minimal sequences necessary to maintain FV as a procofactor. Removal of either of these regions results in a molecule that can function in prothrombinase. These sequences are intact and present in plasma FV; however, FV released by activated platelets and FV cleaved by FXa represent active forms FV(a) that lack the BR. Recent work indicates these forms of FV(a) may be regulated by TFPIathrough molecular mimicry. TFPIahas a C-terminal basic tail which shares remarkable homology to the FV BR suggesting TFPIamay be an important regulator at the level of prothrombinase. Remarkably, another form of FV missing the BR, resulting from alternative splicing, has been recently described. The FV-East Texas, FV-Amsterdam, and FV-Atlanta alterations all result in the enhanced production of an alternatively spliced form of FV (FV-short) that has most of the B-domain removed including the BR. While these forms of FV should be constitutively active, studies have shown that FV-short circulates in complex with TFPIa. Further, in each of the clinical cases, TFPIalevels are substantially elevated resulting in a bleeding phenotype. While there are several remaining unanswered questions, the FV(a)-TFPIα interaction as well as the identification of FV-short, has the potential to fundamentally alter our understanding of the regulation of the initiation of coagulation.