Platelet α‐granule cargo packaging and release are affected by the luminal proteoglycan, serglycin

Abstract

BackgroundSerglycin (SRGN) is an intragranular, sulfated proteoglycan in hematopoietic cells that affects granule composition and function. ObjectiveTo understand how SRGN affects platelet granule packaging, cargo release, and extra‐platelet microenvironments. MethodsPlatelets and megakaryocytes from SRGN−/− mice were assayed for secretion kinetics, cargo levels, granule morphology upon activation, and receptor shedding. ResultsMetabolic, 35SO4 labeling identified SRGN as a major sulfated macromolecule in megakaryocytes. SRGN colocalized with α‐granule markers (platelet factor 4 [PF4], von Willebrand factor [VWF], and P‐selectin), but its deletion did not affect α‐granule morphology or number. Platelet α‐granule composition was altered, with a reduction in basic proteins (pI ≥8; e.g., PF4, SDF‐1, angiogenin) and constitutive release of PF4 from SRGN−/− megakaryocytes. P‐Selectin, VWF, and fibrinogen were unaffected. Serotonin (5‐HT) uptake and β‐hexosaminidase (HEXB) were slightly elevated. Thrombin‐induced exocytosis of PF4 from platelets was defective; however, release of RANTES/CCL5 was normal and osteopontin secretion was more rapid. Release of 5‐HT and HEXB (from dense granules and lysosomes, respectively) were unaffected. Ultrastructural studies showed distinct morphologies in activated platelets. The α‐granule lumen of SRGN−/− platelet had a grainy staining pattern, whereas that of wild‐type granules had only fibrous material remaining. α‐Granule swelling and decondensation were reduced in SRGN−/− platelets. Upon stimulation of platelets, a SRGN/PF4 complex was released in a time‐ and agonist‐dependent manner. Shedding of GPVI from SRGN−/− platelets was modestly enhanced. Shedding of GP1b was unaffected. ConclusionThe polyanionic proteoglycan SRGN influences α‐granule packaging, cargo release, and shedding of platelet membrane proteins.