title: Spindle fusion requires dynein-mediated sliding of oppositely oriented microtubules.
abstract: BACKGROUND
  Bipolar spindle assembly is critical for achieving accurate segregation of chromosomes. In the absence of centrosomes, meiotic spindles achieve bipolarity by a combination of chromosome-initiated microtubule nucleation and stabilization and motor-driven organization of microtubules. Once assembled, the spindle structure is maintained on a relatively long time scale despite the high turnover of the microtubules that comprise it. To study the underlying mechanisms responsible for spindle assembly and steady-state maintenance, we used microneedle manipulation of preassembled spindles in Xenopus egg extracts.
  
  
  RESULTS
  When two meiotic spindles were brought close enough together, they interacted, creating an interconnected microtubule structure with supernumerary poles. Without exception, the perturbed system eventually re-established bipolarity, forming a single spindle of normal shape and size. Bipolar spindle fusion was blocked when cytoplasmic dynein function was perturbed, suggesting a critical role for the motor in this process. The fusion of Eg5-inhibited monopoles also required dynein function but only occurred if the initial interpolar separation was less than twice the microtubule radius of a typical monopole.
  
  
  CONCLUSIONS
  Our experiments uniquely illustrate the architectural plasticity of the spindle and reveal a robust ability of the system to attain a bipolar morphology. We hypothesize that a major mechanism driving spindle fusion is dynein-mediated sliding of oppositely oriented microtubules, a novel function for the motor, and posit that this same mechanism might also be involved in normal spindle assembly and homeostasis.
authors.affiliation: Marine Biological Laboratory, Woods Hole, MA 02543, USA. jgatlin@email.unc.edu
authors.forename: Jesse C
authors.initials: JC
authors.lastname: Gatlin
authors.forename: Alexandre
authors.initials: A
authors.lastname: Matov
authors.forename: Aaron C
authors.initials: AC
authors.lastname: Groen
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Needleman
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Maresca
authors.forename: Gaudenz
authors.initials: G
authors.lastname: Danuser
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Mitchison
authors.forename: E D
authors.initials: ED
authors.lastname: Salmon
mesh_terms: D000818:Animals; D016764:Cell Polarity; D020090:Chromosome Segregation; D004398:Dyneins / Q000378:metabolism*; D016547:Kinesins / Q000235:genetics / Q000378:metabolism; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008870:Microtubules / Q000378:metabolism*; D009865:Oocytes / Q000166:cytology / Q000502:physiology; D008941:Spindle Apparatus / Q000378:metabolism*; D029867:Xenopus Proteins / Q000235:genetics / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: C088335:KIF11 protein, Xenopus; D008869:Microtubule-Associated Proteins; D029867:Xenopus Proteins; D004398:Dyneins; D016547:Kinesins