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 α-granules, the major storage granule of platelets. We previously showed that platelets differentially package pro- and antiangiogenic proteins in distinct α-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 α-granule subpopulations. Super-resolution microscopy of mouse proplatelets and human whole-blood platelet α-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 α-granules. These data suggest that megakaryocytes differentially sort and package α-granule contents, which are preserved as α-granule subpopulations during proplatelet extension and platelet production.
© 2019 by The American Society of Hematology.
Conflict of interest statement
Conflict-of-interest disclosure: J.N.T. and J.E.I. have financial interest in and are founders of Platelet BioGenesis, a company that aims to produce donor-independent human platelets from human-induced pluripotent stem cells at scale. They are both inventors on this patent. The interests of J.N.T. and J.E.I. were reviewed and are managed by the Brigham and Women’s Hospital and Partners HealthCare in accordance with their conflict-of-interest policies. R.F. has a private equity interest in Platelet Diagnostics, and he is also a founder and consultant for that company. The interests of R.F. are reviewed and managed by the Beth Israel Deaconess Medical Center Office of Compliance and Business Conduct. The remaining authors declare no competing financial interests.
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