Effect of metabolic inhibitors on sucrose-induced metaphase spindle elongation and spindle recovery

Compression regulates mitotic spindle length by a mechanochemical switch at the poles

BACKGROUND

Although the molecules involved in mitosis are becoming better characterized, we still lack an understanding of the emergent mechanical properties of the mitotic spindle. For example, we cannot explain how spindle length is determined. To gain insight into how forces are generated and responded to in the spindle, we developed a method to apply controlled mechanical compression to metaphase mitotic spindles in living mammalian cells while monitoring microtubules and kinetochores by fluorescence microscopy.

RESULTS

Compression caused reversible spindle widening and lengthening to a new steady state. Widening was a passive mechanical response, and lengthening was an active mechanochemical process requiring microtubule polymerization but not kinesin-5 activity. Spindle morphology during lengthening and drug perturbations suggested that kinetochore fibers are pushed outward by pole-directed forces generated within the spindle. Lengthening of kinetochore fibers occurred by inhibition of microtubule depolymerization at poles, with no change in sliding velocity, interkinetochore stretching, or kinetochore dynamics.

CONCLUSIONS

We propose that spindle length is controlled by a mechanochemical switch at the poles that regulates the depolymerization rate of kinetochore fibers in response to compression and discuss models for how this switch is controlled. Poleward force appears to be exerted along kinetochore fibers by some mechanism other than kinesin-5 activity, and we speculate that it may arise from polymerization pressure from growing plus ends of interpolar microtubules whose minus ends are anchored in the fiber. These insights provide a framework for conceptualizing mechanical integration within the spindle.

Altered patterns of tubulin polymerization in dividing leaf cells of Chlorophyton comosum after a hyperosmotic treatment

•  Microtubule organization and tubulin polymerization in meristematic leaf cells of Chlorophyton comosum treated with an aqueous solution of 1 M mannitol, inducing plasmolysis, were examined with immunofluorescence and transmission electron microscopy. •  Hyperosmotic treatment induced disintegration of the interphase microtubule systems. Free tubulin, either liberated from the depolymerized microtubules or pre-existing as a nonassembled pool, was incorporated into a network of paracrystals. In most of the dividing cells, mitotic and cytokinetic microtubule systems were replaced by atypical spindle-like structures displaying bipolarity and atypical phragmoplasts, respectively. These atypical mitotic and cytokinetic structures consisted of large densely packed bundles of macrotubules (32 nm diameter) or macrotubules and paracrystals. Tubulin paracrystals also occurred in ectopic positions in plasmolysed mitotic and cytokinetic cells. Dividing cells displaying paracrystals only did not form atypical mitotic and cytokinetic apparatuses. •  Short hyperosmotic stress causes disintegration of all microtubule arrays in dividing cells of C. comosum. Free tubulin is incorporated into macrotubules and tubulin paracrystals. The latter exhibit definite periodicity and characteristic fine structure.

Selective reduction of anaphase B in quinacrine-treated PtK1 cells

Quinacrine, an acridine derivative which competitively binds to ATP binding sites, has previously been shown to cause the reorganization of metaphase spindle microtubules (MTs) due to changes in interactions of non-kinetochore microtubules (nkMTs) of opposite polarity (Armstrong and Snyder: Cell Motil. Cytoskeleton 7:10-19, 1987). In the study presented here, mitotic PtK1 cells were treated in early anaphase with concentrations of quinacrine ranging from 2 to 12 microM to determine energy requirements for chromosome motion. The rate and extent of chromosome-to-pole movements (anaphase A) were not affected by these quinacrine treatments. The extent of anaphase B (kinetochore-kinetochore separation) was reduced with increasing concentrations of quinacrine. Five micromolar quinacrine reduced the extent of kinetochore-kinetochore separation by 20%, and addition of 12 microM quinacrine reduced the kinetochore-kinetochore separation by 40%. To determine the role of nkMTs in anaphase spindle elongation, quinacrine-treated metaphase cells were treated with hyperosmotic sucrose concentrations, and spindle elongation was measured (Snyder et al.: Eur J. Cell Biol. 39:373-379, 1985). Metaphase cells treated with 2-10 microM concentrations of quinacrine for 2-5 min reduced spindle lengths by 10-50% prior to 0.5 M sucrose treatment for 5 min. This treatment showed a significant reduction in the ability of sucrose to induce spindle elongation in cells pretreated with quinacrine. As spindle length and birefringence was reduced by quinacrine treatment, sucrose-induced elongation was concomitantly diminished. These data suggest that quinacrine-sensitive linkages are necessary for anaphase B motions. Reduction in these linkages and/or MT length in the nkMT continuum may reduce the ability of the nkMTs to hold compression at metaphase. This form of energy is thought to drive a significant proportion of normal anaphase B in PtK1 cells and sucrose-induced metaphase spindle elongation.