Polyphosphate as a haemostatic modulator

Platelets are small anuclear cells that play a central role in haemostasis. Platelets become activated in response to various stimuli triggering release of their granular contents into the surrounding milieu. One of these types of granules, termed dense granules, have been found to contain polyphosphate (polyP) in addition to other inorganic biomolecules, such as serotonin, ADP, ATP, PPi. Individuals deficient in dense granules exhibit bleeding tendencies, emphasizing their importance in haemostasis. Platelet polyP is of a relatively defined size, approximately 60-100 phosphate monomers in length. These linear polymers act at various points in the coagulation and fibrinolytic systems thereby modulating the haemostatic response. Due to its highly anionic nature, polyP lends itself to being a natural activator of the contact system. The contact system functions in multiple pathways including coagulation, fibrinolysis, inflammation and complement. Activation of the contact system accelerates thrombin generation, the terminal enzyme in the coagulation cascade. PolyP also modulates factors further downstream in the coagulation cascade to augment thrombin generation. The net effect is increased fibrin formation and platelet activation resulting in faster clot formation. PolyP is incorporated into the forming clot thereby modifying the structure of the resulting fibrin network and its susceptibility to degradation by certain plasminogen activators. In conclusion, release of platelet polyP at the site of injury may facilitate clot formation and augment clot stability thereby promoting wound healing.