Fibrinogen αC-subregions critically contribute blood clot fibre growth, mechanical stability, and resistance to fibrinolysis

Abstract

Fibrinogen is essential for blood coagulation. The C-terminus of the fibrinogen α-chain (αC-region) is composed of an αC-domain and αC-connector. Two recombinant fibrinogen variants (α390 and α220) were produced to investigate the role of subregions in modulating clot stability and resistance to lysis. The α390 variant, truncated before the αC-domain, produced clots with a denser structure and thinner fibres. In contrast, the α220 variant, truncated at the start of the αC-connector, produced clots that were porous with short, stunted fibres and visible fibre ends. These clots were mechanically weak and susceptible to lysis. Our data demonstrate differential effects for the αC-subregions in fibrin polymerisation, clot mechanical strength, and fibrinolytic susceptibility. Furthermore, we demonstrate that the αC-subregions are key for promoting longitudinal fibre growth. Together, these findings highlight critical functions of the αC-subregions in relation to clot structure and stability, with future implications for development of novel therapeutics for thrombosis.

The Story of the Fibrin(ogen) αC-Domains: Evolution of Our View on Their Structure and Interactions

“…57 Since chicken αC regions practically do not have αC-connectors, this observation suggests the important role of the αC-connectors in the fibrin assembly process. A recent study 58 confirmed the importance of the αC-connectors in fibrin assembly and revealed an additional role of the αC regions in this process. Namely, it was shown that the recombinant fibrinogen variant α390, truncated before the αC-domain, produced a clot consisting of thinner fibers making up a denser structural network.…”

“…The stiffness of fibrin clots is substantially increased by interactions between αC regions, and factor XIIIa-catalyzed covalent cross-linking of the α chains contributes further to increased fibrin clot stiffness. 60 A recent study with fibrinogen variant α390, devoid of the αC-domains, which revealed reduced mechanical stability of fibrin clots formed by this variant, 58 further demonstrates the important contribution of the αC-domains to mechanical properties of fibrin clots.…”

“…38,47 Such interactions accelerate the fibrin assembly process, promote longitudinal fiber growth, and increase the mechanical stability of fibrin. 30,58,60,61 Furthermore, formation of αC polymers in fibrin results in the exposure and/or formation of their multiple binding sites 41,[66][67][68]70,72 that provide interactions of the αC regions with different proteins and cell types and their participation in various physiological and pathological processes. Thus, these previously enigmatic parts of the fibrinogen molecule are finally revealing their true nature.…”

Why fibrin biomechanical properties matter for hemostasis and thrombosis

“…The presence of the αC‐region results in thicker fibers by reducing the threshold length of protofibrils for lateral aggregation but it does not change the basic structure of protofibrils, thus the αC‐region likely has a kinetic effect on the lateral assembly of protofibrils 21 . A recent study from our lab found that the absence of the αC‐domain (α390 truncation) leads to thinner, highly branched fibers resulting in reduced clot firmness, whereas abolishing the complete αC‐region including the globular domain and the connector region (α220 truncation) impairs longitudinal growth of protofibrils and results in clots with even lower firmness and a highly abnormal network structure with stunted fibers and a severely reduced resistance to fibrinolysis 29 . Thus, besides the kinetic effect on fibrin assembly and lateral aggregation, the αC‐region has important roles in clot formation including longitudinal fibrin fiber growth and clot stability.…”

“…However, bulk measurements show that fibers are not completely straight but straightened by up to 25% shear strain of the whole clot, and fibers only start to align in the direction of stress above 25% shear strain 54,57 . Fibers appear less straight on electron microscopy images than confocal microscopy 29 and fibers appear less straight in uncrosslinked clots 30 …”