Summary
Integrin αIIbβ3 plays a pivotal role in platelet aggregation by binding to fibrinogen.
The β3 cytoplasmic domain of αIIbβ3 interacts with cytoskeletal and signaling proteins
and is cleaved by µ-calpain, a calcium regulated cysteine protease. In the present
study, we have investigated in more detail the cleavage of the β3 cytoplasmic domain
during platelet aggregation induced by thrombin, TRAP-1 and TRAP-4. Our data show
that β3 is cleaved in all three cases. The time course of β3 cleavage and the amount
of cleaved β3 depends on the way platelets are activated and on the complete activation
of µ-calpain, with a maximum of 90% of cleaved β3 obtained when thrombin is used.
Furthermore, our results also show that the cleaved αIIbβ3 is mainly distributed in
the Triton soluble fraction, indicating its inability to bind to the cytoskeleton.
Interestingly, in the absence of GPIbα or following inhibition of thrombin binding
to GPIbα, there is a reduction in the thrombin-induced calcium flux, β3 cleavage and
µ-calpain activation. These results suggest that cleavage of the β3 cytoplasmic domain
by µ-calpain might be an important step regulating the link between the cytoskeleton
and αIIbβ3 during platelet aggregation, and that GPIbα could function as a cofactor
for the complete activation of platelets by thrombin.
Keywords Integrin αIIbβ3 - β3 cytoplasmic domain - PAR - GPIbα