title: FlnA-null megakaryocytes prematurely release large and fragile platelets that circulate poorly.
abstract: Filamin A (FlnA) is a large cytoplasmic protein that crosslinks actin filaments and anchors membrane receptors and signaling intermediates. FlnA(loxP) PF4-Cre mice that lack FlnA in the megakaryocyte (MK) lineage have a severe macrothrombocytopenia because of accelerated platelet clearance. Macrophage ablation by injection of clodronate-encapsulated liposomes increases blood platelet counts in FlnA(loxP) PF4-Cre mice and reveals the desintegration of FlnA-null platelets into microvesicles, a process that occurs spontaneously during storage. FlnA(loxP) PF4-Cre bone marrows and spleens have a 2.5- to 5-fold increase in MK numbers, indicating increased thrombopoiesis in vivo. Analysis of platelet production in vitro reveals that FlnA-null MKs prematurely convert their cytoplasm into large CD61(+) platelet-sized particles, reminiscent of the large platelets observed in vivo. FlnA stabilizes the platelet von Willebrand factor receptor, as surface expression of von Willebrand factor receptor components is normal on FlnA-null MKs but decreased on FlnA-null platelets. Further, FlnA-null platelets contain multiple GPIbα degradation products and have increased expression of the ADAM17 and MMP9 metalloproteinases. Together, the findings indicate that FlnA-null MKs prematurely release large and fragile platelets that are removed rapidly from the circulation by macrophages.
authors.affiliation: Division of Translational Medicine, Brigham and Women's Hospital, and Department of Medicine, Harvard Medical School, Boston, MA, USA.
authors.forename: Antonija
authors.initials: A
authors.lastname: Jurak Begonja
authors.forename: Karin M
authors.initials: KM
authors.lastname: Hoffmeister
authors.forename: John H
authors.initials: JH
authors.lastname: Hartwig
authors.forename: Hervé
authors.initials: H
authors.lastname: Falet
mesh_terms: D051722:ADAM Proteins / Q000097:blood; D000072198:ADAM17 Protein; D000818:Animals; D001792:Blood Platelets / Q000166:cytology* / Q000378:metabolism / Q000648:ultrastructure; D002454:Cell Differentiation / Q000235:genetics / Q000502:physiology; D048429:Cell Size; D005260:Female; D064448:Filamins; D039661:Integrin beta3 / Q000097:blood; D008297:Male; D020780:Matrix Metalloproteinase 9 / Q000097:blood; D008533:Megakaryocytes / Q000166:cytology* / Q000378:metabolism / Q000648:ultrastructure; D051379:Mice; D018345:Mice, Knockout; D046529:Microscopy, Electron, Transmission; D008870:Microtubules / Q000648:ultrastructure; D009419:Nerve Tissue Proteins / Q000097:blood / Q000172:deficiency* / Q000235:genetics; D010976:Platelet Count; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000378:metabolism; D010980:Platelet Membrane Glycoproteins / Q000502:physiology; D011247:Pregnancy; D055550:Protein Stability; D013921:Thrombocytopenia / Q000097:blood / Q000209:etiology; D034061:Thrombopoiesis / Q000235:genetics / Q000502:physiology
chemical_list: D064448:Filamins; C529499:FlnA protein, mouse; D039661:Integrin beta3; D009419:Nerve Tissue Proteins; D019038:Platelet Glycoprotein GPIb-IX Complex; D010980:Platelet Membrane Glycoproteins; C028254:von Willebrand factor receptor; D051722:ADAM Proteins; D020780:Matrix Metalloproteinase 9; C501830:Mmp9 protein, mouse; D000072198:ADAM17 Protein; C000606463:Adam17 protein, mouse