Calpain Cleavage of Focal Adhesion Proteins Regulates the Cytoskeletal Attachment of Integrin AlphaIIbbeta3 (platelet Glycoprotein IIb/IIIa) and the Cellular Retraction of Fibrin Clots

The intracellular thiol protease calpain catalyzes the limited proteolysis of various focal adhesion structural proteins and signaling enzymes in adherent cells. In human platelets, calpain activation is dependent on fibrinogen binding to integrin alphaIIbbeta3 and subsequent platelet aggregation, suggesting a potential role for this protease in the regulation of postaggregation responses. In this study, we have examined the effects of calpain activation on several postaggregation events in human platelets, including the cytoskeletal attachment of integrin alphaIIbbeta3, the tyrosine phosphorylation of cytoskeletal proteins, and the cellular retraction of fibrin clots. We demonstrate that calpain activation in either washed platelets or platelet-rich plasma is associated with a marked reduction in platelet-mediated fibrin clot retraction. This relaxation of clot retraction was observed in both thrombin and ionophore A23187-stimulated platelets. Calcium dose-response studies (extracellular calcium concentrations between 0.1 microM and 1 M) revealed a strong correlation between calpain activation and relaxed clot retraction. Furthermore, pretreating platelets with the calpain inhibitors calpeptin and calpain inhibitor I prevented the calpain-mediated reduction in clot retraction. Relaxed fibrin clot retraction was associated with the cleavage of several platelet focal adhesion structural proteins and signaling enzymes, resulting in the dissociation of talin, pp60(c-)src, and integrin alphaIIbbeta3 from the contractile cytoskeleton and the tyrosine dephosphorylation of multiple cytoskeletal proteins. These studies suggest an important role for calpain in the regulation of multiple postaggregation events in human platelets. The ability of calpain to inhibit clot retraction is likely to be due to the cleavage of both structural and signaling proteins involved in modulating integrin-cytoskeletal interactions.