Junctional Adhesion Molecule a Helps Maintain Integrin αIIbβ3 in Resting State

Under physiological conditions, fibrinogen receptor integrin αIIbβ3 on the circulating platelets is in a low-affinity, or resting state, unable to bind soluble ligands. During platelet activation by agonists, a cascade of signaling events induces a conformational change in the extracellular domain of αIIbβ3, thereby converting it into a high-affinity state capable of binding ligands through a process known as “inside-out signaling”. What maintains this integrin in a low-affinity state is not well understood. We have previously identified JAM-A, junctional adhesion molecule A, on the platelet surface. We have shown that an antibody blockade of JAM-A dose-dependently activates platelets. To understand the molecular mechanism through which JAM-A regulates platelet aggregation, we used Jam-A null mice. Interestingly, the mouse bleeding times were significantly shortened in Jam-A null mice compared to wildtype littermates. Furthermore, the majority of these mice showed a rebleeding phenotype. This phenotype was further confirmed by FeCl3-induced carotid artery occlusion, a well-accepted in vivo model for thrombosis. Platelets derived from Jam-A-null mice were used to evaluate the role of JAM-A in agonist-induced platelet aggregation. We found that Jam-A null platelets showed enhanced aggregation in response to physiological agonists such as PAR4 peptide, collagen, and ADP as compared to platelets from wildtype littermates. JAM-A was found to associate with αIIbβ3 in unactivated human platelets, but this association was disrupted by both agonist-induced platelet aggregation and during outside-in signaling initiated upon platelet spreading on immobilized Fg. We also found that in resting platelets, JAM-A is phosphorylated on a conserved tyrosine 280 in its cytoplasmic domain, which was dephosphorylated upon platelet activation. Furthermore, JAM-A is rapidly and transiently phosphorylated on serine 284 residue during platelet activation by agonists. Interestingly, JAM-A also formed a complex with Csk, a tyrosine kinase known to be inhibitory to Src activation, in resting platelets. This complex was dissociated upon activation of platelets by agonists. These results suggest that tyrosine-phosphorylated JAM-A recruits Csk to αIIbβ3 in resting platelets, thus maintaining a low-affinity state of integrin αIIbβ3. Agonist–induced activation of platelets results in rapid dephosphorylation of JAM-A on Y280 and phosphorylation on S284 residues. This causes dissociation of JAM-A from integrin αIIbβ3 facilitating platelet aggregation.