Calpain activation in shear-induced platelet aggregation

Cleavage of platelet endothelial cell adhesion molecule-1 (PECAM-1) in platelets exposed to high shear stress

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a 130 kDa transmembrane glycoprotein that belongs to the immunoglobulin superfamily and is expressed on the surface of endothelial cells, platelets, and other blood cells. Although the importance of this adhesion molecule in various cell-cell interactions is established, its functional role in platelets remains to be elucidated. In this study, we examined whether PECAM-1 underwent changes in platelets exposed to high shear stress. Platelet PECAM-1 was cleaved under high shear stress and was released into the extracellular fluid as a fragment with an approximate molecular weight of 118 kDa. The cleavage was inhibited by an anti-VWF MoAb, but not by recombinant VWF A1 domains. These findings suggest that the GPIb-VWF interaction is involved in PECAM-1 cleavage under high shear stress, and that the cleavage is independent of GPIb clustering by VWF multimers. Furthermore, EGTA or calpeptin inhibited PECAM-1 cleavage. This finding provides evidence for the involvement of calpain in PECAM-1 cleavage. Flow-cytometric analysis revealed that PECAM-1 expression on the platelet surface was decreased under high shear stress. This reduction occurred exclusively in a specific population of platelets, which corresponded to platelet-derived microparticles (PMP). In conclusion, PECAM-1 cleavage under high shear stress is closely related to the activation of calpain and the process of PMP formation mediated by the GPIb-VWF interaction.

Shear stress-induced aggregation of oxidized platelets

Although shear stress-induced platelet aggregation (SIPA) has been noted, the shear stress-induced aggregation of oxidized platelets (SIOPA) has not been investigated. To investigate SIOPA, small quantity of hemoglobin (Hb) solution was first added to plasma to induce oxidation stress of platelets and then the plasma was sheared by a cone/plate viscometer at shear stress of 66 dyn/cm2. In addition, to investigate the change in expression of the membrane receptors of glycoprotein Ib (GpIb) for oxidized platelets, mean fluorescence intensity (MFI) for those platelets was detected by flow cytometric technique. The results showed that the level of the oxidation stress of platelets, as presented in malondialdehyde (MDA) values, was well correlated with the quantities of added Hb. Moreover, the aggregation of SIOPA corresponded with the levels of oxidation stress of platelets. Platelets pretreated with aspirin could only partially reduce the aggregation of SIOPA. Similarly, aspirin pretreatment could partially reduce the lowering of MFI value that indicated downexpression of GpIb receptors. Furthermore, the aggregations of SIOPA corresponded with the lowered percentage of those MFI values for studied cases. We thereby suggested that the aggregation of SIOPA is related to both the level of oxidation stress and the downexpression of GpIb receptors for oxidized platelets.