title: The platelet interior revisited: electron tomography reveals tubular alpha-granule subtypes.
abstract: We have used (cryo) electron tomography to provide a 3-dimensional (3D) map of the intracellular membrane organization of human platelets at high spatial resolution. Our study shows that the open canalicular system and dense tubular system are highly intertwined and form close associations in specialized membrane regions. 3D reconstructions of individual alpha-granules revealed large heterogeneity in their membrane organization. On the basis of their divergent morphology, we categorized alpha-granules into the following subtypes: spherical granules with electron-dense and electron-lucent zone containing 12-nm von Willebrand factor tubules, subtypes containing a multitude of luminal vesicles, 50-nm-wide tubular organelles, and a population with 18.4-nm crystalline cross-striations. Low-dose (cryo) electron tomography and 3D reconstruction of whole vitrified platelets confirmed the existence of long tubular granules with a remarkably curved architecture. Immunoelectron microscopy confirmed that these extended structures represent alpha-granule subtypes. Tubular alpha-granules represent approximately 16% of the total alpha-granule population and are detected in approximately half of the platelet population. They express membrane-bound proteins GLUT3 and alphaIIb-beta3 integrin and contain abundant fibrinogen and albumin but low levels of beta-thromboglobulin and no von Willebrand factor. Our 3D study demonstrates that, besides the existence of morphologically different alpha-granule subtypes, high spatial segregation of cargo exists within individual alpha-granules.
authors.affiliation: Cell Microscopy Center and Department of Cell Biology, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Hezder
authors.initials: H
authors.lastname: van Nispen tot Pannerden
authors.forename: Felix
authors.initials: F
authors.lastname: de Haas
authors.forename: Willie
authors.initials: W
authors.lastname: Geerts
authors.forename: George
authors.initials: G
authors.lastname: Posthuma
authors.forename: Suzanne
authors.initials: S
authors.lastname: van Dijk
authors.forename: Harry F G
authors.initials: HF
authors.lastname: Heijnen
mesh_terms: D001792:Blood Platelets / Q000378:metabolism* / Q000648:ultrastructure*; D003594:Cytoplasmic Granules / Q000145:classification* / Q000648:ultrastructure*; D055032:Electron Microscope Tomography; D005340:Fibrinogen / Q000378:metabolism; D051274:Glucose Transporter Type 3 / Q000378:metabolism; D006801:Humans; D016253:Microscopy, Immunoelectron; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism; D001620:beta-Thromboglobulin / Q000378:metabolism; D014841:von Willebrand Factor / Q000378:metabolism
chemical_list: D051274:Glucose Transporter Type 3; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; C495687:SLC2A3 protein, human; D001620:beta-Thromboglobulin; D014841:von Willebrand Factor; D005340:Fibrinogen