Mechanisms of fibrin polymerization and clinical implications

Abstract

Research on all stages of fibrin polymerization, using a variety of approaches including naturally occurring and recombinant variants of fibrinogen, x-ray crystallography, electron and light microscopy, and other biophysical approaches, has revealed aspects of the molecular mechanisms involved. The ordered sequence of fibrinopeptide release is essential for the knob-hole interactions that initiate oligomer formation and the subsequent formation of 2-stranded protofibrils. Calcium ions bound both strongly and weakly to fibrin(ogen) have been localized, and some aspects of their roles are beginning to be discovered. Much less is known about the mechanisms of the lateral aggregation of protofibrils and the subsequent branching to yield a 3-dimensional network, although the αC region and B:b knob-hole binding seem to enhance lateral aggregation. Much information now exists about variations in clot structure and properties because of genetic and acquired molecular variants, environmental factors, effects of various intravascular and extravascular cells, hydrodynamic flow, and some functional consequences. The mechanical and chemical stability of clots and thrombi are affected by both the structure of the fibrin network and cross-linking by plasma transglutaminase. There are important clinical consequences to all of these new findings that are relevant for the pathogenesis of diseases, prophylaxis, diagnosis, and treatment.

Figure 1

Schematic representation of a short oligomer formed by 3 fibrin monomers based on the x-ray crystallographic structure of fibrinogen (Protein Data Bank entry: 3GHG). Shown for each monomer are the central nodule (blue), coiled-coil connectors (red), the γ- and β-nodules (green), which make up the main part of the lateral D region, and the αC regions (orange). The molecules are shown with the addition of the missing parts of the crystal structure reconstructed from molecular dynamics simulations, namely the amino terminal ends of the α-chains in the central nodule and the αC regions. A:a knob-hole bonds that are the major basis of fibrin polymerization maintain the third (lower) monomer in a half-staggered arrangement. The intermolecular noncovalent coupling and the covalent cross-linking at the D-D interface hold the two (upper) monomers in a linear arrangement. Created by and published with permission from A. Zhmurov, O. Kononova, and V. Barsegov, University of Massachusetts–Lowell.

Figure 2

Schematic representation of the consecutive steps of fibrin polymerization. The figure shows the following steps: (1) the enzymatic release of fibrinopeptides from fibrinogen, the formation of monomeric fibrin-containing exposed knobs, and the partial dissociation of the αC regions; (2) the self-assembly of monomeric fibrin via knob-hole interactions and the formation of half-staggered 2-stranded fibrin oligomers; (3) the lateral aggregation of protofibrils (fibrin oligomers made of 20 to 25 monomers) promoted by homophilic αC-αC-interactions within and between protofibrils, including the formation of αC-polymers; (4) the packing of protofibrils into a fiber with a 22.5-nm periodic cross-striation due to the half-staggered molecular structure and regular paracrystalline arrangement; and (5) the fibrin network formation due to the branching of fibers by either of 2 mechanisms.

Figure 3

Scanning electron microscope images of clots and thrombi. In vitro clots made from human (A) platelet-poor plasma and (B) platelet-rich plasma. (C) The fibrin formed on a surface coated with collagen-adherent platelets is made up of many fibers aligned along the direction of flow. (D) The image shows an ex vivo human coronary artery thrombus obtained by aspiration from a patient with ST-elevation myocardial infarction. Magnification bars represent 10 μm.