Molecular Structural Origins of Clot and Thrombus Mechanical Properties

Abstract 2257

Although the mechanical function of blood clots to stem bleeding is obvious and higher clot stiffness has been associated with premature coronary thrombosis, we are only now beginning to understand the origins of these vital mechanical characteristics. Fibrinogen, the precursor of fibrin, provides building blocks for the fibrin polymer, the scaffold of blood clots and thrombi. The mechanical properties of fibrin molecules are essential for the ability of clots to accomplish hemostasis and are an important determinant of the pathological properties of thrombi, as they control how clots and thrombi respond to mechanical deformation. For example, the mechanical properties of a thrombus affect whether it can be deformed or become obstructive, whether embolization occurs, and the responses to treatments such as angioplasty, thrombolysis, and vascular surgery. Despite such critical importance, the molecular structural basis of clot mechanics is not well understood, even though it is essential to integrate molecular data with information at the fiber, network, and macroscopic levels. We carried out combined experimental and theoretical studies of the mechanical properties of fibrin(ogen) at the nanometer scale to resolve the sub-molecular mechanisms underlying forced elongation of fibrin. The results of atomic force microscopy-induced unfolding of fibrinogen monomers and oligomers were directly compared with force-extension curves obtained using biomolecular simulations on graphics processing units on the centi-second timescale. This novel means of computational acceleration makes direct comparison of the experimental and simulation results of force measurements possible. Hence, dynamic signatures for unfolding transitions observed in the simulations can be used to provide meaningful interpretation and modeling of the force peaks obtained experimentally and to unmask unfolding mechanisms that are likely to be important in vivo. We demonstrated that the mechanical unraveling of fibrin(ogen) repeats is determined by microscopic molecular transitions that couple reversible extension-contraction of the α-helical coiled-coil regions and unfolding of the terminal γC-modules, with the central E region playing a minor role. The coiled-coils can store mechanical energy to amortize an external mechanical perturbation, and to transmit and distribute force among the γC-modules. Unfolding of the γC-modules, stabilized by strong inter-domain interactions with the neighboring βC-modules, results in three distinct force signals. The unfolding transitions are characterized by an average force of ∼90 pN and peak-to-peak distance of ∼25 nm. The C-terminal β-strand insert supports the integral multidomain structure of the γC-modules. Because interdomain interactions at the D-D junction are weak, individual fibrin(ogen) monomers undergo independent unfolding transitions in fibrin polymers. To conclude, these studies have revealed important information and structural characteristics of the dynamic mechanical behavior of fibrin(ogen) at different spatial scales. The results obtained provide important qualitative and quantitative characteristics of fibrin(ogen) nanomechanics necessary to understand fibrin mechanical properties at the microscopic and macroscopic scales, the relationships with thrombosis and embolization, and suggest new approaches for modulation of these properties as potential therapeutic interventions.