Abstract
Endocytosis is key to fibrinogen (Fg) uptake, trafficking of integrins (αIIbβ(3), α(v)β(3)), and purinergic receptors (P2Y(1), P2Y(12)), and thus normal platelet function. However, the molecular machinery required and possible trafficking routes are still ill-defined. To further identify elements of the platelet endocytic machinery, we examined the role of a vesicle-residing, soluble N-ethylmaleimide factor attachment protein receptor (v-SNARE) called cellubrevin/vesicle-associated membrane protein-3 (VAMP-3) in platelet function. Although not required for normal platelet exocytosis or hemostasis, VAMP-3(-/-) mice had less platelet-associated Fg, indicating a defect in Fg uptake/storage. Other granule markers were unaffected. Direct experiments, both in vitro and in vivo, showed that loss of VAMP-3 led to a robust defect in uptake/storage of Fg in platelets and cultured megakaryocytes. Uptake of the fluid-phase marker, dextran, was only modestly affected. Time-dependent uptake and endocytic trafficking of Fg and dextran were followed using 3-dimensional-structured illumination microscopy. Dextran uptake was rapid compared with Fg, but both cargoes progressed through Rab4(+), Rab11(+), and von Willebrand factor (VWF)(+) compartments in wild-type platelets in a time-dependent manner. In VAMP-3(-/-) platelets, the 2 cargoes showed limited colocalization with Rab4, Rab11, or VWF. Loss of VAMP-3 also affected some acute platelet functions, causing enhanced spreading on Fg and fibronectin and faster clot retraction compared with wild-type. In addition, the rate of Janus kinase 2 phosphorylation, initiated through the thrombopoietin receptor (TPOR/Mpl) activation, was affected in VAMP-3(-/-) platelets. Collectively, our studies show that platelets are capable of a range of endocytosis steps, with VAMP-3 being pivotal in these processes.