Regulation of actin polymerization by tropomodulin-3 controls megakaryocyte actin organization and platelet biogenesis

Blood. 2015 Jul 23;126(4):520-30. doi: 10.1182/blood-2014-09-601484. Epub 2015 May 11.


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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Actin Cytoskeleton / metabolism
  • Actin Cytoskeleton / pathology*
  • Animals
  • Apoptosis
  • Blood Platelets / metabolism
  • Blood Platelets / pathology*
  • Blotting, Western
  • Cell Membrane / metabolism
  • Cell Membrane / pathology*
  • Cell Proliferation
  • Cells, Cultured
  • Cytoplasm / metabolism
  • Embryo, Mammalian / metabolism
  • Embryo, Mammalian / pathology*
  • Female
  • Fluorescent Antibody Technique
  • Hematopoiesis / physiology
  • Hemorrhage / etiology*
  • Hemorrhage / metabolism
  • Hemorrhage / pathology
  • Immunoprecipitation
  • Megakaryocytes / metabolism
  • Megakaryocytes / pathology*
  • Mice
  • Mice, Knockout
  • Microscopy, Confocal
  • Microscopy, Electron, Transmission
  • Microscopy, Fluorescence
  • Ploidies
  • Polymerization
  • Tropomodulin / physiology*


  • Tmod3 protein, mouse
  • Tropomodulin