Reducing state attenuates ectodomain shedding of GPVI while restoring adhesion capacities of stored platelets: evidence addressing the controversy around the effects of redox condition on thrombosis

Thrombosis involves different stages including platelet adhesion to the site of injury, aggregatory events governed by integrin activation, pro-inflammatory responses recruiting leukocytes and finally, pro-coagulant activity which results in fibrin generation and clot formation. As important signaling agents, reactive oxygen species (ROS) reduce thrombus volume and growth, however given such a multistage mechanism, it is not well-elucidated how ROS inhibition modulates thrombosis. PRP-platelet concentrates (PCs) were either treated with ROS-reducing agents (1 mM NAC or 30 μM NOX inhibitor, VAS2870) or kept untreated during storage. Shedding and expression of platelet adhesion receptors in presence of inhibitors, agonists and CCCP (as controls) were analyzed by flow cytometery and western blot respectively. Besides above parameters, platelet adhesion to collagen in stored platelets was examined in presence of ROS inhibitors using fluorescence-microscopy. Highest levels of adhesion receptors shedding were achieved by ionophore and CCCP while collagen induces much more GPVI shedding than that of GPIbα. ROS inhibition reduced receptors shedding from day 3 of storage while enhanced their expressions. ROS inhibition not only did not reduce platelet adhesion capacity but it also enhanced platelets adhesion (in presence of NAC) or spreading (in presence of VAS2870) in 5 days-stored PCs. While reducing state significantly inhibits platelet aggregation and thrombus growth, our results indicated that as a first stage of thrombosis, platelet adhesion is resistance to such inhibitory effects. These findings highlight the fact that integrin-dependent platelet activation is much more vulnerable to the inhibition of ROS generation than GPVI-dependent platelet adhesion. Presumably, inhibition of platelet activating signals by ROS inhibitors preserves platelet adhesiveness to collagen due to lessening GPVI shedding.