title: Identification of a novel, actin-rich structure, the actin nodule, in the early stages of platelet spreading.
abstract: BACKGROUND
  During platelet spreading, the actin cytoskeleton undergoes marked changes, forming filopodia, lamellipodia and stress fibres. In the present study, we report the identification of a novel actin-rich structure, termed an actin nodule, which appears prior to lamellipodia and stress fibre formation.
  
  
  METHODS
  Platelet spreading was monitored using human platelets and mouse GFP-actin platelets using real-time and end-point DIC, and fluorescent and electron microscopy (EM).
  
  
  RESULTS
  We identified a small, novel actin structure, the actin nodule, in the early stages of adhesion and spreading, which we hypothesize to be a precursor of lamellipodia and stress fibres. Nodule formation shows an inverse correlation to Rho kinase and myosin-II activity, is independent of PI3-kinase, but dependent on Src kinase activity. Actin nodules contain multiple proteins, including Arp2/3, Fyn, Rac, and beta1- and beta3- integrins, but not Src. EM analysis revealed that actin filaments extend in all directions from the nodules. Actin nodules are present on multiple matrices, including fibrinogen, laminin and VWF + botrocetin.
  
  
  CONCLUSIONS
  This work identifies a novel platelet actin structure, which we propose is a precursor to both lamellipodia and stress fibres and acts to drive platelet spreading.
authors.affiliation: Centre for Cardiovascular Sciences, Institute of Biomedical Research, The Medical School, University of Birmingham, Birmingham, UK. s.calaminus@beatson.gla.ac.uk
authors.forename: S D J
authors.initials: SD
authors.lastname: Calaminus
authors.forename: S
authors.initials: S
authors.lastname: Thomas
authors.forename: O J T
authors.initials: OJ
authors.lastname: McCarty
authors.forename: L M
authors.initials: LM
authors.lastname: Machesky
authors.forename: S P
authors.initials: SP
authors.lastname: Watson