Tumor Necrosis Factor-α-Converting Enzyme (ADAM17) Mediates GPIbα Shedding from Platelets In Vitro and In Vivo.

Interaction of the platelet receptor GPIb-V-IX with von Willebrand factor exposed at a site of vascular injury is an essential step in the initiation of a hemostatic plug. Proteolytic cleavage (shedding) of the GPIbα subunit was first described more than 25 years ago, the protease mediating this event as well as its physiological function, however, have not been elucidated. We have recently reported that shedding of GPIbα and platelet clearance induced by platelet storage or mitochondrial injury involves a platelet-derived metalloproteinase(s). Here we show that GPIbα shedding in response to mitochondrial injury or physiological activation is inhibited in platelets obtained from chimeric mice, which express inactive tumor necrosis factor-α converting enzyme (TACEΔZn/ΔZn) in blood cells only. Shedding was also inhibited in mouse and human platelets in the presence of two potent TACE inhibitors, TAPI and TMI-1. Our data further suggest that TACE is important in the regulation of GPIbα expression in vivo, as we observed an ~ 90% reduction in soluble GPIbα (glycocalicin) in plasma of TACEΔZn/ΔZn chimeras as well as significantly increased levels of GPIbα on circulating platelets. In contrast, shedding of P-selectin from activated platelets was not affected by the mutation in TACE. Mitochondrial damage in TACEΔZn/ΔZn platelets did not result in reduced posttransfusion recovery or impaired hemostatic function of such platelets in mice, indicating that TACE not only mediates GPIbα shedding but that it may also be crucial for the development of cellular changes leading to platelet clearance. In conclusion, our data demonstrate that TACE is expressed in platelets and that it is the key enzyme mediating shedding of GPIbα. Inhibition of TACE during platelet storage could be a powerful means to improve the effectiveness of platelet transfusions.