Studies of adhesion-dependent platelet activation - Distinct roles for different participating receptors can be dissociated by proteolysis of collagen

The molecular differences between native-type collagen type I fibrils (NC) and their pepsinated monomers (PC) were used to uncover receptors involved in platelet-collagen interaction along the adhesion-activation axis. The pl atelet-depositing capacity of NC and PC under blood flow and their adhesive properties and respective morphologies, aggregation, procoagulant capacity , and tyrosine phosphorylation were compared under different cationic milie us, including or excluding the glycoprotein (GP) Ia/IIa, NC was consistentl y a more preferable and activating substrate than PC during flow (5 minutes ) and in platelet aggregation. In PPACK-treated blood, both NC (3.3-fold) a nd PC (2.7-fold) increased platelet attachment on elevation of the shear ra te from 500 to 1640 s(-1), whereas in citrated blood, adhesion and thrombus growth on PC were negligible under the high shear rate, unlike on NC (1.9- fold increase). The complete lack of platelet deposition on PC in citrated blood could be overcome by restoring physiological Mg2+ concentration, and in contrast to NC, platelets interacting with PC were highly dependent on M g2+ during adhesion, aggregation, and procoagulant response. Monoclonal ant ibody (mAb 131.7) against GP IV inhibited platelet deposition to NC in citr ated blood (2 minutes) by 49%, which was not further increased by coincubat ion with mAb against GP Ia (6F1). These results stress the importance of GP Ia/IIa in shear-resistant platelet deposition on collagen monomers. In nat ive fibers, however, the preserved quaternary structure with telopeptides a ctivates additional platelet receptors capable of substituting GP Ia/IIa an d GP IV.