Computer modeling of fibrin polymerization kinetics correlated with electron microscope and turbidity observations: clot structure and assembly are kinetically controlled

Abstract

Although much is known about fibrin polymerization, because it is complex, the effects of various modifications are not intuitively obvious and many experimental observations remain unexplained. A kinetic model presented here that is based on information about mechanisms of assembly accounts for most experimental observations and allows hypotheses about the effects of various factors to be tested. Differential equations describing the kinetics of polymerization were written and then solved numerically. The results have been related to turbidity profiles and electron microscope observations. The concentrations of intermediates in fibrin polymerization, and fiber diameters, fiber and protofibril lengths have been calculated from these models. The simplest model considered has three steps; fibrinopeptide A cleavage, protofibril formation, and lateral aggregation of protofibrils to form fibers. The average number of protofibrils per fiber, which is directly related to turbidity, can be calculated and plotted as a function of time. The lag period observed in turbidity profiles cannot be accurately simulated by such a model, but can be simulated by modifying the model such that oligomers must reach a minimum length before they aggregate. Many observations, reported here and elsewhere, can be accounted for by this model; the basic model may be modified to account for other experimental observations. Modeling predicts effects of changes in the rate of fibrinopeptide cleavage consistent with electron microscope and turbidity observations. Changes only in the rate constants for initiation of fiber growth or for addition of protofibrils to fibers are sufficient to account for a wide variety of other observations, e.g., the effects of ionic strength or fibrinopeptide B removal or thrombospondin. The effects of lateral aggregation of fibers has also been modeled: such behavior has been observed in turbidity curves and electron micrographs of clots formed in the presence of platelet factor 4. Thus, many aspects of clot structure and factors that influence structure are directly related to the rates of these steps of polymerization, even though these effects are often not obvious. Thus, to a large extent, clot structure is kinetically determined.

Citrullinated fibrinogen shows defects in FPA and FPB release and fibrin polymerization catalyzed by thrombin

“…It is well known that the diameter of fibrin fibers increases proportionally to fibrinogen concentration [12]. Since the turbidity change caused by thrombin-catalyzed fibrin polymerization of a mixture of NC-Fbg (0.09 mg/ml) and C-Fbg (0.09 mg/ml) was larger than that for control non-citrullinated fibrinogen alone (0.09 mg/ml) (Fig 2A), we observed (Fig 5).…”

The influence of type 2 diabetes on fibrin structure and function

“…Weisel and Nagaswami [27] have used computer modelling to predict how variation in the different parameters involved in fibrin polymerisation might influence clot structure. In their model, an increased FPB cleavage rate shortens the lag phase and increases the maximum rate of fibre assembly, resulting in the formation of shorter, thicker fibres, consistent with our results.…”

“…time zero reading]), and maximum absorbency at full polymerisation (calculated as the absorbency value at 30 min minus the baseline [time zero] absorbency reading). The lag phase represents the length of time required for fibrin protofibrils to grow to sufficient length to allow lateral aggregation to occur and is sensitive to a variety of factors, including fibrinogen concentration and rate of fibrinopeptide A (FPA) cleavage [27]. Clot turbidity is directly proportional to the average cross-sectional area of the fibres comprising it, and the maximum absorbency reflects the average size of the fibrin fibres within the clot [27].…”

Fibrin structural and diffusional analysis suggests that fibers are permeable to solute transport

“…However, protofibril formation happens more rapidly than lateral aggregation at high thrombin concentrations, while lateral aggregation is favored over protofibril formation at low thrombin concentrations [53]. Overall, values…”