Positioning and Elongation of the Fission Yeast Spindle by Microtubule-Based Pushing

Abstract

In eukaryotic cells, proper position of the mitotic spindle is necessary for successful cell division and development. We explored the nature of forces governing the positioning and elongation of the mitotic spindle in Schizosaccharomyces pombe. We hypothesized that astral microtubules exert mechanical force on the S. pombe spindle and thus help align the spindle with the major axis of the cell. Microtubules were tagged with green fluorescent protein (GFP) and visualized by two-photon microscopy. Forces were inferred both from time-lapse imaging of mitotic cells and, more directly, from mechanical perturbations induced by laser dissection of the spindle and astral microtubules. We found that astral microtubules push on the spindle poles in S. pombe, in contrast to the pulling forces observed in a number of other cell types. Further, laser dissection of the spindle midzone induced spindle collapse inward. This offers direct evidence in support of the hypothesis that spindle elongation is driven by the sliding apart of antiparallel microtubules in the spindle midzone. Broken spindles recovered and mitosis completed as usual. We propose a model of spindle centering and elongation by microtubule-based pushing forces.

Cell cycle control of spindle elongation

“…Whereas fungi Ustilago maydis and Nectria haematococca use the MT minus end directed motor protein dynein to pull on aMTs for pole separation, 10,11,28 in common fungal models S. pombe and S. cerevisiae anaphase B is mainly driven by kinesin motor activity at the midzone. 25,27,[50][51][52][53][54] Similar variations are seen in higher eukaryotes. C. elegans anaphase B predominantly depends on cortical pulling forces to extend the mitotic spindle, 13 while cultured mammalian cells on the other hand use the combination of both forces to separate their spindle poles.…”

Assembling the spindle midzone in the right place at the right time

“…Therefore, genetic or physical perturbations of the spindle midzone result in deficient separation of the spindle poles and collapse of the anaphase spindle. [3][4][5][6] In addition, in all eukaryotes the midzone regulates different aspects of cytokinesis. 5,[7][8][9][10] Ase1, a conserved MT bundler.…”

The perpetual movements of anaphase

“…separation of spindle poles by cortical mediated pulling forces on astral microtubules), also known as anaphase C [212]. The microtubule sliding mechanism proposed by McIntosh and colleagues (1969) explains at least part of the spindle elongation in many systems, including diatoms, yeasts and Drosophila embryos [96,[213][214][215][216]. At the molecular level, the players involved in each system might be different, although their modus operandi must be essentially the same: cross-linking, sliding and eventually growth/stabilization of interpolar microtubules of opposite polarity.…”