Tetraspanin CD9 is required for microparticle release from coated-platelets

CD9, a member of the tetraspanin superfamily, is the third most abundant protein on the platelet surface, but its function remains unknown. In this report, we demonstrate that CD9 is required for the release of microparticles from coated-platelets. Coated-platelets are formed as a result of dual agonist activation with collagen and thrombin, and each coated-platelet releases 15-25 microparticles averaging 0.4 mu m in diameter. We report here that four separate monoclonal antibodies against CD9 inhibited microparticle release from coated-platelets by 72-102% with an IC(50) of approximately 500 ng/mL for ALB6 and SN4. In addition, the anti-alpha(IIb)beta(3) monoclonal antibody AP2 also inhibited microparticle release although additional anti-alpha(IIb)beta(3) monoclonals did not. These data support participation of the tetraspanin CD9, together with the integrin alpha(IIb)beta(3), in the membrane vesiculation process associated with platelet microparticle release.