Regulation of Fibrin Clot Structure by the Fibrinogen α- and γ-chains and its Relationship to Thrombosis

Cardiovascular disease (CVD) is a group of conditions that primarily affect the circulatory system. CVD is the leading cause of death worldwide, with thrombosis being a key contributor. Fibrin is a central component of the thrombus, and its structural and functional properties can influence thrombi function and impact the success of treatment. Fibrinogen is a protein composed of 3 pairs of polypeptide chains (Aα,Bβ,γ)2, and the conversion to fibrin by thrombin, releasing the fibrinopeptides, forms a fibrous network supporting the clot.
Both the α- and γ-chains of fibrinogen are critical to protein function, but the exact functional regions are only partially understood. This requires further research, particularly as α- and γ chain variants, including y’, are associated with bleeding and thrombotic conditions. To delineate the roles of these chains on fibrin network characteristics, recombinant fibrinogen variants were produced in Chinese hamster ovary cells. Five γ’-chain truncations were produced, each displaying sequentially four less residues, together with full-length homodimer. Two αC-region truncations were produced; α390 (lacking the αC-domain) and α220 (missing the entire αC-region). Fibrinogens were studied in purified protein, plasma, and whole blood systems.
αC-region truncations altered clot structure, with α390 producing a denser clot composed of thinner fibres, while α220 resulted in a porous structure with stunted fibres and limited longitudinal fibre growth. Complete loss of the αC-region prevented whole blood clot contraction. Fibrinogen with γ’-chain residues showed fibre curvature and was mechanically weaker compared to wild-type.
The addition of γ’-fibrinogen to fibrinogen-deficient plasma produced clots with reduced maximum optical density without hindering whole blood clot contraction or incorporation of RBC or platelets.
This thesis highlights the distinct roles of the αC-domain and αC-connector in fibrin formation, fibre growth, clot structure and stability and shows that γ’-chain residues influence clot structure and decrease mechanical stability.