Biorelevant polyanions stabilize fibrin against mechanical and proteolytic decomposition: Effects of polymer size and electric charge

The release of neutrophil extracellular traps (NETs) containing DNA and histones is an essential mechanism in the neutrophil-mediated innate immunity. In thrombi the polyanionic DNA confers mechanical and lytic resistance to fibrin and heparin interfere with the effects of NET components. Heparins are polyanions used not only as therapeutic agents, but they are also released by mast cells at entry sites of pathogens. Platelets and microorganisms release a different type of polyanions (polyphosphates) of various size (in the range 60-1000 phosphate monomers). With the current study we aimed to evaluate if the stability of fibrin is influenced by the type of polyanion, its molecular size or relative electric charge. Fibrin structure was approached with scanning electron microscopy (SEM) and pressure-driven permeation. An oscillation rheometer was used to investigate viscoelastic properties. Kinetic turbidimetric assays for the generation and dissolution of composite fibrin clots containing unfractionated heparin (UFH), and its partially or fully desulfated derivatives, as well as low molecular-weight heparin (LMWH), pentasaccharide (S5), and polyphosphates composed of 45 (P45), 100 (P100) or 700 (P700) monomers at average. The smaller polyanions P45, P100, LMWH, and S5 accelerated, whereas P700 and UFH retarded clot formation. All polyanions altered the fibrin structure: SEM and clot permeation showed thicker fibers with smaller (LMWH, S5, P700) or larger (UFH, P100) pores. All polyanions stabilized the clots mechanically, but the smaller P45, P100 and LMWH decreased the deformability of fibrin, whereas the large UFH and P700 increased the maximal bearable deformation of clots. Despite the size-dependent structural changes, all heparin caused a 10-15% prolongation of lysis-times with plasmin, and UFH-effects depended on sulfation patterns. The 20-35% prolongation of lysis-times caused by all polyphosphates was a kringle-dependent phenomenon, and was dampened in the presence of 6-aminohexanoate blocking the lysine-binding sites of plasmin. In summary, we found that polyanions of different chemical structure stabilize fibrin clots via size-dependent modulation of fibrin structure and kringle-dependent inhibition of plasmin-mediated fibrinolysis.