title: Requirement of VPS33B, a member of the Sec1/Munc18 protein family, in megakaryocyte and platelet alpha-granule biogenesis.
abstract: Bleeding problems are associated with defects in platelet alpha-granules, yet little is known about how these granules are formed and released. Mutations affecting VPS33B, a novel Sec1/Munc18 protein, have recently been linked to arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome. We have characterized platelets from patients with ARC syndrome and observed reduced aggregation with arachidonate and adenosine diphosphate (ADP). Structural abnormalities seen in ARC platelets included increased platelet size, a pale appearance in blood films, elevated numbers of delta-granules, and completely absent alpha-granules. Soluble and membrane-bound alpha-granule proteins were significantly decreased or undetectable in ARC platelets, suggesting that both the releasable protein pools and membrane components of alpha-granules were absent. The role of VPS33B in platelet granule biogenesis was evaluated by immunofluorescence microscopy in normal human megakaryocytes. VPS33B colocalized appreciably with markers of alpha-granules, moderately with late endosomes/lysosomes, minimally with delta-granules/lysosomes, and not with cis-Golgi complexes. VPS33B protein expression determined by immunoblotting confirmed the presence of VPS33B in control fibroblasts but not in ARC fibroblasts, and in normal megakaryocytes but not in platelets. We conclude that like other Sec1/Munc18 proteins, VPS33B is involved in intracellular vesicle trafficking, being essential for the development of platelet alpha-granules but not for granule secretion.
authors.affiliation: Department of Paediatrics, Division of Clinical and Metabolic Genetics, Programme in Cell Biology, The Hospital for Sick Children, University of Toronto, 555 University Ave, Toronto, ON, M5G 1X8, Canada.
authors.forename: Bryan
authors.initials: B
authors.lastname: Lo
authors.forename: Ling
authors.initials: L
authors.lastname: Li
authors.forename: Paul
authors.initials: P
authors.lastname: Gissen
authors.forename: Hilary
authors.initials: H
authors.lastname: Christensen
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: McKiernan
authors.forename: Charles
authors.initials: C
authors.lastname: Ye
authors.forename: Mohamed
authors.initials: M
authors.lastname: Abdelhaleem
authors.forename: Jason A
authors.initials: JA
authors.lastname: Hayes
authors.forename: Michael D
authors.initials: MD
authors.lastname: Williams
authors.forename: David
authors.initials: D
authors.lastname: Chitayat
authors.forename: Walter H A
authors.initials: WH
authors.lastname: Kahr
mesh_terms: D001176:Arthrogryposis / Q000378:metabolism; D001792:Blood Platelets / Q000378:metabolism* / Q000648:ultrastructure; D002478:Cells, Cultured; D002779:Cholestasis / Q000378:metabolism; D006470:Hemorrhage; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007674:Kidney Diseases / Q000378:metabolism; D008533:Megakaryocytes / Q000378:metabolism*; D008565:Membrane Proteins / Q000145:classification / Q000378:metabolism*; D046529:Microscopy, Electron, Transmission; D051938:Munc18 Proteins / Q000145:classification / Q000378:metabolism*; D011506:Proteins / Q000145:classification / Q000378:metabolism*; D012995:Solubility; D013577:Syndrome; D033921:Vesicular Transport Proteins
chemical_list: D008565:Membrane Proteins; D051938:Munc18 Proteins; D011506:Proteins; C411584:VPS33B protein, human; D033921:Vesicular Transport Proteins