title: Actomyosin-dependent cortical dynamics contributes to the prophase force-balance in the early Drosophila embryo.
abstract: BACKGROUND
  The assembly of the Drosophila embryo mitotic spindle during prophase depends upon a balance of outward forces generated by cortical dynein and inward forces generated by kinesin-14 and nuclear elasticity. Myosin II is known to contribute to the dynamics of the cell cortex but how this influences the prophase force-balance is unclear.
  
  
  RESULTS
  Here we investigated this question by injecting the myosin II inhibitor, Y27632, into early Drosophila embryos. We observed a significant increase in both the area of the dense cortical actin caps and in the spacing of the spindle poles. Tracking of microtubule plus ends marked by EB1-GFP and of actin at the cortex revealed that astral microtubules can interact with all regions of these expanded caps, presumably via their interaction with cortical dynein. In Scrambled mutants displaying abnormally small actin caps but normal prophase spindle length in late prophase, myosin II inhibition produced very short spindles.
  
  
  CONCLUSIONS
  These results suggest that two complementary outward forces are exerted on the prophase spindle by the overlying cortex. Specifically, dynein localized on the mechanically firm actin caps and the actomyosin-driven contraction of the deformable soft patches of the actin cortex, cooperate to pull astral microtubules outward. Thus, myosin II controls the size and dynamic properties of the actin-based cortex to influence the spacing of the poles of the underlying spindle during prophase.
authors.affiliation: Human Physiology Section, Department of Physiology, University of Pavia, Pavia, Italy.
authors.forename: Patrizia
authors.initials: P
authors.lastname: Sommi
authors.forename: Dhanya
authors.initials: D
authors.lastname: Cheerambathur
authors.forename: Ingrid
authors.initials: I
authors.lastname: Brust-Mascher
authors.forename: Alex
authors.initials: A
authors.lastname: Mogilner
mesh_terms: D000205:Actomyosin / Q000378:metabolism*; D000577:Amides / Q000494:pharmacology; D000818:Animals; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D004330:Drosophila; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism; D004398:Dyneins / Q000235:genetics / Q000378:metabolism; D004625:Embryo, Nonmammalian / Q000187:drug effects / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism; D008870:Microtubules / Q000378:metabolism; D008954:Models, Biological; D024462:Myosin Type II / Q000037:antagonists & inhibitors / Q000378:metabolism; D011418:Prophase / Q000235:genetics; D011725:Pyridines / Q000494:pharmacology; D008941:Spindle Apparatus / Q000378:metabolism
chemical_list: D000577:Amides; D018797:Cell Cycle Proteins; D029721:Drosophila Proteins; D004791:Enzyme Inhibitors; D008869:Microtubule-Associated Proteins; D011725:Pyridines; C421006:sced protein, Drosophila; C108830:Y 27632; D000205:Actomyosin; D024462:Myosin Type II; D004398:Dyneins