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

Although the molecules involved in mitosis are becoming better characterized,we still lack an understanding of the emergent mechanical properties of the mitotic spindle. Forexample, we cannot explain how spindle length is determined. To gain insight into how forcesare generated and responded to in the spindle, we developed a method to apply controlledmechanical compression to metaphase mitotic spindles in living mammalian cells, whilemonitoring microtubules and kinetochores by fluorescence microscopy.Compression caused reversible spindle widening and lengthening to a new steadystate.Widening was a passive mechanical response, and lengthening an active mechanochemicalprocess requiring microtubule polymerization but not kinesin-5 activity. Spindle morphologyduring lengthening and drug perturbations suggested that kinetochore fibers are pushed outwardsby pole-directed forces generated within the spindle. Lengthening of kinetochore fibers occurredby inhibition of microtubule depolymerization at poles, with no change in sliding velocity, interkinetochorestretching, or kinetochore dynamics.We propose that spindle length is controlled by a mechanochemical switch at thepoles 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 alongkinetochore fibers by some mechanism other than kinesin-5 activity, and we speculate that itmay arise from polymerization pressure from growing plus-ends of interpolar microtubuleswhose minus-ends are anchored in the fiber. These insights provide a framework forconceptualizing mechanical integration within the spindle.

Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Current Biology 19 (2009): 1086-1095, doi:10.1016/j.cub.2009.05.056.

S.D. received support from a Milton Fund (Harvard University)and T.J.M. was supported by NIH grants GM039565 and P50 GM068763.