Differential integrin activity mediated by platelet collagen receptor engagement under flow conditions.

The platelet receptors glycoprotein (Gp)VI, integrin αβ and GpIb/V/IX mediate platelet adhesion and activation during thrombogenesis. Increases of intracellular Ca ([Ca]) are key signals during platelet activation; however, their relative importance in coupling different collagen receptors to functional responses under shear conditions remains unclear. To study shear-dependent, receptor-specific platelet responses, we used collagen or combinations of receptor-specific collagen-mimetic peptides as substrates for platelet adhesion and activation in whole human blood under arterial flow conditions and compared real-time and endpoint parameters of thrombus formation alongside [Ca] measurements using confocal imaging. All three collagen receptors coupled to [Ca] signals, but these varied in amplitude and temporal pattern alongside variable integrin activation. GpVI engagement produced large, sustained [Ca] signals leading to real-time increases in integrins αβ- and αβ-mediated platelet adhesion. αβ-dependent platelet aggregation was dependent on PY signalling. Co-engagement of αβ and GpIb/V/IX generated transient [Ca] spikes and low amplitude [Ca] responses that potentiated GpVI-dependent [Ca] signalling. Therefore αβ, GpIb/V/IX and GpVI synergise to generate [Ca] signals that regulate platelet behaviour and thrombus formation. Antagonism of secondary signalling pathways reveals distinct, separate roles for αβ in stable platelet adhesion and aggregation.