Interplay between Spatial Confinement of Thrombin Activity and Dense Granule Secretion Regulates Dynamics of Thrombus Formation

Background

Thrombi formed in healthy murine microvessels have a heterogeneous structure, comprising a thrombin-dependent core and a shell supplied by platelet-derived agonists, such as ADP from dense granules. After the injury is inflicted, the core steadily grows until it reaches a plateau in size, while the shell follows three stage dynamics of rapid growth, shrinkage and stabilization. However, it is not clear how the complex dynamics of the overall thrombus structure is orchestrated by various cues.

Aim

We aimed to analyze whether spatiotemporal kinetics of thrombin-induced dense granule secretion is responsible for the observed dynamics of thrombus formation in microvessels.

Methods

We developed a multiscale computational model of thrombogenesis in response to a vessel wall injury. Platelets were modeled as two-dimensional particles, while platelet agonists were described as virtual particles, which induced platelet activation through affecting mechanical interactions between platelets and triggering platelet degranulation. As a platelet aggregate grew, the blood flow was recalculated. The model was built in a bottom-up manner and its description of single-cell processes was based on the experimental findings.

Results

The model showed that thrombin produced at the injury site was mostly confined to the innermost layers of the thrombus due to competing intrathrombus diffusion and flow-mediated dilution. Such spatial confinement of thombin resulted in the formation of a thrombus with a pronounced thrombin-activated core where platelets were able to secrete their dense granules. Released ADP recruited new platelets and triggered the formation of a thrombin-free thrombus shell. The thrombus grew until it ruptured under the hydrodynamic force and the growth repeated in cycles. However, our results predicted that if the propagation of thrombin along the flow was limited either by volumetric or surface inhibition, the cycles of thrombus formation stopped once the dense granule pool in the core was depleted. Without the supply of newly secreted ADP, the thrombus shell disassembled and the thrombus size was stabilized, in line with experimental findings.

Conclusion

Computational modelling predicts that thrombin-induced dense granule secretion is responsible for the rapid formation of the thrombus shell, which disintegrates once the granule pool is depleted due to intense secretion in the confined thrombus core. We suggest that this mechanism is responsible for the three-stage dynamics of thrombus growth in vivo.