• ADAP;
  • collagen;
  • GPVI;
  • integrin α2β1;
  • platelets

Summary.  Background: Collagen-induced platelet activation is a key step in the development of arterial thrombosis via its interaction with the receptors glycoprotein (GP)VI and integrin α2β1. Adhesion and degranulation-promoting adapter protein (ADAP) regulates αIIbβ3 in platelets and αLβ2 in T cells, and is phosphorylated in GPVI-deficient platelets activated by collagen. Objectives: To determine whether ADAP plays a role in collagen-induced platelet activation and in the regulation and function of α2β1. Methods: Using ADAP−/− mice and synthetic collagen peptides, we investigated the role of ADAP in platelet aggregation, adhesion, spreading, thromboxane synthesis, and tyrosine phosphorylation. Results and Conclusions: Platelet aggregation and phosphorylation of phospholipase Cγ2 induced by collagen were attenuated in ADAP−/− platelets. However, aggregation and signaling induced by collagen-related peptide (CRP), a GPVI-selective agonist, were largely unaffected. Platelet adhesion to CRP was also unaffected by ADAP deficiency. Adhesion to the α2β1-selective ligand GFOGER and to a peptide (III-04), which supports adhesion that is dependent on both GPVI and α2β1, was reduced in ADAP−/− platelets. An impedance-based label-free detection technique, which measures adhesion and spreading of platelets, indicated that, in the absence of ADAP, spreading on GFOGER was also reduced. This was confirmed with non-fluorescent differential-interference contrast microscopy, which revealed reduced filpodia formation in ADAP−/− platelets adherent to GFOGER. This indicates that ADAP plays a role in mediating platelet activation via the collagen-binding integrin α2β1. In addition, we found that ADAP−/− mice, which are mildly thrombocytopenic, have enlarged spleens as compared with wild-type animals. This may reflect increased removal of platelets from the circulation.