Defective PDI release from platelets and endothelial cells impairs thrombus formation in Hermansky-Pudlak syndrome

Protein disulfide isomerase (PDI), secreted from platelets and endothelial cells upon injury, is required for thrombus formation. The effect of platelet and endothelial cell granule contents on PDI-mediated thrombus formation was studied by intravital microscopy using a mouse model of Hermansky-Pudlak syndrome in which platelet dense granules are absent. Platelet deposition and fibrin generation were nearly absent and extracellular PDI was signficantly reduced in HPS6 -/- mice following vascular injury. HPS6 -/- platelets displayed impaired PDI secretion and impaired exocytosis of α-granules, lysosomes and T-granules due to decreased sensitivity to thrombin but these defects could be corrected by addition of subthreshold amounts of ADP. Human Hermansky-Pudlak syndrome platelets demonstrated similar characteristics. Infusion of WT platelets rescued thrombus formation in HPS6 -/- mice. HUVECs in which the HPS6 gene was silenced displayed impaired PDI secretion and exocytosis of Weibel-Palade bodies. Defective thrombus formation in Hermansky-Pudlak syndrome, associated with impaired exocytosis of residual granules in endothelial cells and platelets, the latter due to deficiency of ADP, is characterized by a defect in T-granule secretion, a deficiency in extracellular PDI secretion and impaired fibrin generation and platelet aggregation. Hermansky-Pudlak syndrome is an example of a hereditary disease where impaired PDI secretion contributes to a bleeding phenotype.