FSAP and altered fibrin clot structure: Is there a link to FXIII?

M. Etscheid, N. Beer, R. Seitz, J. Dodt (Langen, Germany)

Bleeding disorders, coagulation and fibrinolytic factors
Date: 17.02.2017,
Time: 17:15 - 18:15

Objective: Factor VII activating protease (FSAP) is a plasma protease affecting both coagulation and fibrinolysis, but a physiologic role in haemostasis is still under debate. Fibrinogen (Fbg) is the clot forming protein during coagulation and is composed of 3 pairs of polypeptide chains arranged in two identical subunits (AαBßy)2. Recently we showed that FSAP cleaves Fbg in the Aα and Bß chains, altering the fibrin clot structure and facilitating fibrinolysis. FXIII is a plasma transglutaminase that forms crosslinks between Fbg α and ß chains, and is an important determinant in clot structure. Here we investigated whether FXIII is involved in the altered clot structure observed with FSAP.

Methods: FXIII as a potential target substrate for FSAP was studied in SDS-PAGE/WB and FXIII activity assays. The fibrin structure of normal plasma or FXIII-depleted plasma upon treatment with FSAP was studied by confocal laser scanning microscopy (LSM). The influence of FSAP on clot pore size was analyzed in permeability experiments

Results: We found no evidence that FSAP could directly activate or inactivate FXIII. However, when FXIII-depleted plasma (FXIII-DP) was treated with FSAP, no clear change in the clot structure was seen in LSM, in contrast to normal plasma. This effect was partially reversible, when FXIII-DP was supplemented with FXIII. Moreover, FSAP-treated normal plasma -but not FXIII-DP- showed a significantly reduced clot pore size (permeability). Also this effect could be reversed when FXIII-DP was supplemented with FXIII.

Conclusion: FSAP does not directly activate or inactivate FXIII, but there is a link between FSAP, FXIII and fibrin clot structure. Cleavage of Fbg by FSAP apparently involves regions in the Aα chain crucial for FXIIIa function and contributing to overall clot structure. Most likely these are the FXIIIa Gln donor and Lys acceptor sites in the αC-region of Fbg, which are partially released by FSAP. The denser fibrin clot structure with thinner fibers seen in FSAP-treated normal plasma after clotting is obviously not only caused by N-terminal truncation of the Bß chain of Fbg (release of BßN1-53), which destabilizes protofibrils and fiber formation. Also the truncation of the αC-region and, thus, the limited formation of α-α and α-y crosslinks by FXIIIa seem to contribute to the altered clot structure of fibrin clots of FSAP-treated Fbg.