Fibrinogen αC-region acts as a functional safety latch: Implications for a fibrin biomechanical behaviour model.

Fibrin has unique biomechanical properties which are essential for its role as a scaffold for blood clots. Fibrin is highly extensible and demonstrates significant strain stiffening behaviour, which is essential for stress-distribution in the network. Yet the exact structures of fibrin at the sub-fibre level that contribute to its unique biomechanical characteristic are unknown. Here we show how truncations of the fibrinogen αC-region impact the biomechanical properties of fibrin fibres. Surprisingly, absence of the complete αC-region did not influence the low strain modulus of fibrin fibres but led to premature fibre rupture and decreased extensibility. Intermediate effects were observed with partial deletion of the αC-region, reflected by intermediate rupture stress and toughness. However, overall strain-stiffening behaviour remained even in absence of the αC-region, indicating that strain stiffening is not due to stress being transferred from the αC-region to the protofibril backbone. Upon stress-relaxation, decay constants and their relative contribution to the total relaxation remained similar at all strains, showing that a distinct relaxation process is present until fibre rupture. However, relative contribution of fast relaxation was maximal only in crosslinked fibres if the flexible αC-connector was present. These data show that the αC-region is not the main load-bearing structure within fibrin fibres and point to a critical role for the protofibril backbone instead. We present a revised structural model based on protofibril branching that fully explains the unique biomechanical behaviour of fibrin fibres, while the αC-region primarily acts as a safety latch at the highest of strains. STATEMENT OF SIGNIFICANCE: The findings presented in this paper reveal critically important details about how the molecular structure of fibrin contributes to its unique mechanical properties which are essential to fulfil its function as the scaffold of blood clots. In this work we used engineered proteins with alterations in an important but highly disordered area of the molecule called αC-region and we provide direct evidence for the first time for how the absence of either the globular αC-domain, or the complete αC-region impacts the mechanical behaviour of individual fibrin fibres. Using these results we developed a new structural model of protofibril organisation within fibrin fibres that fully explains their strain stiffening, relatively low modulus and their high, largely variable, extensibility.