Megakaryocytes package contents into separate α-granules that are differentially distributed in platelets

In addition to their primary roles in hemostasis and thrombosis, platelets participate in many other physiological and pathological processes, including, but not limited to inflammation, wound healing, tumor metastasis, and angiogenesis. Among their most interesting properties is the large number of bioactive proteins stored in their alpha-granules, the major storage granule of platelets. We previously showed that platelets differentially package pro- and antiangiogenic proteins in distinct alpha-granules that undergo differential release upon platelet activation. Nevertheless, how megakaryocytes achieve differential packaging is not fully understood. In this study, we use a mouse megakaryocyte culture system and endocytosis assay to establish when and where differential packaging occurs during platelet production. Live cell microscopy of primary mouse megakaryocytes incubated with fluorescently conjugated fibrinogen and endostatin showed differential endocytosis and packaging of the labeled proteins into distinct alpha-granule subpopulations. Super-resolution microscopy of mouse proplatelets and human whole-blood platelet alpha-granules simultaneously probed for 2 different membrane proteins (VAMP-3 and VAMP-8), and multiple granular content proteins (bFGF, ENDO, TSP, VEGF) confirmed differential packaging of protein contents into alpha-granules. These data suggest that megakaryocytes differentially sort and package alpha-granule contents, which are preserved as alpha-granule subpopulations during proplatelet extension and platelet production.