title: Regulation of actin polymerization by tropomodulin-3 controls megakaryocyte actin organization and platelet biogenesis.
abstract: The actin cytoskeleton is important for platelet biogenesis. Tropomodulin-3 (Tmod3), the only Tmod isoform detected in platelets and megakaryocytes (MKs), caps actin filament (F-actin) pointed ends and binds tropomyosins (TMs), regulating actin polymerization and stability. To determine the function of Tmod3 in platelet biogenesis, we studied Tmod3(-/-) embryos, which are embryonic lethal by E18.5. Tmod3(-/-) embryos often show hemorrhaging at E14.5 with fewer and larger platelets, indicating impaired platelet biogenesis. MK numbers are moderately increased in Tmod3(-/-) fetal livers, with only a slight increase in the 8N population, suggesting that MK differentiation is not significantly affected. However, Tmod3(-/-) MKs fail to develop a normal demarcation membrane system (DMS), and cytoplasmic organelle distribution is abnormal. Moreover, cultured Tmod3(-/-) MKs exhibit impaired proplatelet formation with a wide range of proplatelet bud sizes, including abnormally large proplatelet buds containing incorrect numbers of von Willebrand factor-positive granules. Tmod3(-/-) MKs exhibit F-actin disturbances, and Tmod3(-/-) MKs spreading on collagen fail to polymerize F-actin into actomyosin contractile bundles. Tmod3 associates with TM4 and the F-actin cytoskeleton in wild-type MKs, and confocal microscopy reveals that Tmod3, TM4, and F-actin partially colocalize near the membrane of proplatelet buds. In contrast, the abnormally large proplatelets from Tmod3(-/-) MKs show increased F-actin and redistribution of F-actin and TM4 from the cortex to the cytoplasm, but normal microtubule coil organization. We conclude that F-actin capping by Tmod3 regulates F-actin organization in mouse fetal liver-derived MKs, thereby controlling MK cytoplasmic morphogenesis, including DMS formation and organelle distribution, as well as proplatelet formation and sizing.
authors.affiliation: Department of Cell and Molecular Biology, The Scripps Research Institute, La Jolla, CA;
authors.forename: Zhenhua
authors.initials: Z
authors.lastname: Sui
authors.affiliation: Department of Cell and Molecular Biology, The Scripps Research Institute, La Jolla, CA;
authors.forename: Roberta B
authors.initials: RB
authors.lastname: Nowak
authors.affiliation: Department of Laboratory Medicine, Yale Stem Cell Center, and.
authors.forename: Chad
authors.initials: C
authors.lastname: Sanada
authors.affiliation: Division of Hematology, Department of Internal Medicine, Yale School of Medicine, New Haven, CT.
authors.forename: Stephanie
authors.initials: S
authors.lastname: Halene
authors.affiliation: Department of Laboratory Medicine, Yale Stem Cell Center, and.
authors.forename: Diane S
authors.initials: DS
authors.lastname: Krause
authors.affiliation: Department of Cell and Molecular Biology, The Scripps Research Institute, La Jolla, CA;
authors.forename: Velia M
authors.initials: VM
authors.lastname: Fowler
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism / Q000473:pathology*; D000818:Animals; D017209:Apoptosis; D001792:Blood Platelets / Q000378:metabolism / Q000473:pathology*; D015153:Blotting, Western; D002462:Cell Membrane / Q000378:metabolism / Q000473:pathology*; D049109:Cell Proliferation; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D004622:Embryo, Mammalian / Q000378:metabolism / Q000473:pathology*; D005260:Female; D005455:Fluorescent Antibody Technique; D006410:Hematopoiesis / Q000502:physiology; D006470:Hemorrhage / Q000209:etiology* / Q000378:metabolism / Q000473:pathology; D047468:Immunoprecipitation; D008533:Megakaryocytes / Q000378:metabolism / Q000473:pathology*; D051379:Mice; D018345:Mice, Knockout; D018613:Microscopy, Confocal; D046529:Microscopy, Electron, Transmission; D008856:Microscopy, Fluorescence; D011003:Ploidies; D058105:Polymerization; D051348:Tropomodulin / Q000502:physiology*
chemical_list: C495837:Tmod3 protein, mouse; D051348:Tropomodulin