title: Mechanisms and implications of platelet discoid shape.
abstract: The platelet marginal band consists of a single peripheral microtubule (MT) that is wound in 8 to 12 coils and maintains discoid cell shape. About 90% of beta-tubulin in the marginal band is of the divergent, megakaryocyte (MK)/platelet-restricted beta1 isoform. beta1-tubulin-null mice show reduced proplatelet formation, thrombocytopenia, and platelet spherocytosis. Here, we show that structural abnormalities in resting beta1-tubulin-/- platelets include frequent kinks and breaks in the marginal band. Platelets derived from mice lacking the transcription factor GATA1 show similar defects, probably as a direct consequence of absent beta1-tubulin. beta1-tubulin+/- platelets have normal ratios of beta-tubulin isotypes but the marginal band is half the normal thickness, which is sufficient to maintain elliptical cell shape. Thus, a threshold 50% or less of the normal amount of beta1-tubulin is required to preserve marginal band integrity and cell shape. beta1-tubulin-/- platelets have normal size and contents and show no defects in serotonin release or aggregation. Accordingly, the apparently isolated spherocytosis allows investigation of the role of discoid platelet shape in hemostasis. On agonist stimulation, the disorganized MTs in beta1-tubulin-/- platelets fail to condense into central rings and instead are dispersed in short bundles and linear arrays. Nevertheless, intravital microscopy and flow chamber studies demonstrate full functionality of these spherocytic platelets under physiologic shear conditions. Together, these findings highlight the essential requirements of the MK/platelet-restricted beta1-tubulin isoform in platelet structure and suggest that spherocytosis does not impair many aspects of platelet function.
authors.affiliation: Division of Hematology and Department of Medicine, Brigham & Women's Hospital and Harvard Medical School, Boston, MA, USA.
authors.forename: Joseph E
authors.initials: JE
authors.lastname: Italiano
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Bergmeier
authors.forename: Sanjay
authors.initials: S
authors.lastname: Tiwari
authors.forename: Hervé
authors.initials: H
authors.lastname: Falet
authors.forename: John H
authors.initials: JH
authors.lastname: Hartwig
authors.forename: Karin M
authors.initials: KM
authors.lastname: Hoffmeister
authors.forename: Patrick
authors.initials: P
authors.lastname: André
authors.forename: Denisa D
authors.initials: DD
authors.lastname: Wagner
authors.forename: Ramesh A
authors.initials: RA
authors.lastname: Shivdasani
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000737:chemistry / Q000502:physiology / Q000648:ultrastructure*; D048429:Cell Size; D002470:Cell Survival; D004268:DNA-Binding Proteins / Q000172:deficiency / Q000502:physiology; D050981:Erythroid-Specific DNA-Binding Factors; D050982:GATA1 Transcription Factor; D018056:Hemorheology; D006487:Hemostasis; D008297:Male; D008533:Megakaryocytes / Q000737:chemistry / Q000502:physiology / Q000648:ultrastructure; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008870:Microtubules / Q000648:ultrastructure; D015539:Platelet Activation; D010974:Platelet Aggregation; D014157:Transcription Factors / Q000172:deficiency / Q000502:physiology; D014404:Tubulin / Q000737:chemistry / Q000172:deficiency / Q000502:physiology*
chemical_list: D004268:DNA-Binding Proteins; D050981:Erythroid-Specific DNA-Binding Factors; D050982:GATA1 Transcription Factor; C494689:Gata1 protein, mouse; D014157:Transcription Factors; D014404:Tubulin