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Vol. 21, Issue 2, 323-333, January 15, 2010

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Fast Microtubule Dynamics in Meiotic Spindles Measured by Single Molecule Imaging: Evidence That the Spindle Environment Does Not Stabilize Microtubules

Daniel J. Needleman^*, Aaron Groen, Ryoma Ohi, Tom Maresca, Leonid Mirny, and Tim Mitchison

*School of Engineering and Applied Sciences, Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138; Systems Biology, Harvard Medical School, Boston, MA 02445; Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; and Harvard-MIT Division of Health Sciences and Technology, and Physics, Massachusetts Institute of Technology, Cambridge, MA 02139

Submitted September 23, 2009; Revised November 10, 2009; Accepted November 17, 2009
Monitoring Editor: Kerry S. Bloom

Metaphase spindles are steady-state ensembles of microtubules that turn over rapidly and slide poleward in some systems. Since the discovery of dynamic instability in the mid-1980s, models for spindle morphogenesis have proposed that microtubules are stabilized by the spindle environment. We used single molecule imaging to measure tubulin turnover in spindles, and nonspindle assemblies, in Xenopus laevis egg extracts. We observed many events where tubulin molecules spend only a few seconds in polymer and thus are difficult to reconcile with standard models of polymerization dynamics. Our data can be quantitatively explained by a simple, phenomenological model—with only one adjustable parameter—in which the growing and shrinking of microtubule ends is approximated as a biased random walk. Microtubule turnover kinetics did not vary with position in the spindle and were the same in spindles and nonspindle ensembles nucleated by Tetrahymena pellicles. These results argue that the high density of microtubules in spindles compared with bulk cytoplasm is caused by local enhancement of nucleation and not by local stabilization. It follows that the key to understanding spindle morphogenesis will be to elucidate how nucleation is spatially controlled.

Address correspondence to: Daniel J. Needleman (dan_needleman{at}harvard.edu).