OC 28.4 - The flexible alphaC-connector is important for fibrin fibre integrity but not for fibre strength at low strains

Background: Fibrin fibres are highly extensible yet have relatively low stiffness, both of which are essential characteristics to fulfil its mechanical role as scaffold of blood clots. It is currently unknown how the inner structure of fibrin fibres underpins mechanical load-bearing elements.

Aims: To elucidate the load-bearing role of the protofibril backbone and the fibrin αC-region and develop a structural model underpinning mechanical behaviour.

Methods: Recombinant human fibrinogen variants were created by selectively truncating either the αC-domain (a390) or the complete αC-region (a220). Clots of WT, a390 and a220 fibrinogen were made on striated surface. Individual fibrin fibres were stretched using fluorescent microscopy-combined lateral force sensing atomic force microscopy and the measured stress-strain behaviour was analysed.

Results: At strains <~1.5, fibre stiffness was similar in all variants (p>0.9999, figure) indicating that low-strain response is independent of the αC-region. At intermediate strains (~1.8-2.8), fibre stiffness was lower in a390 and a220 clots compared with WT, suggesting some load bearing function of the globular αC-domain at these strains. The fibre rupture strain was only decreased when the complete αC-region (p=0.007) but not αC-domain alone (p=0.8384) was truncated, showing a role of the flexible αC-region in fibre integrity. Rupture stress and fibre toughness decreased gradually with gradual removal of the αC-region.

Conclusion(s): Even though absence of the αC-region leads to apparent differences in network structure, it does not lead to major alterations in the low-strain mechanical behaviour of fibrin fibres. Consequently, other structural elements are load bearing at low strain. We propose a novel model where the protofibril backbone is the main load bearing element yet branching of protofibrils enables their selective loading. Meanwhile the αC-region acts as a security latch at high strains thus contributing to fibrin’s high extensibility.