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Published online September 24, 2007
doi:10.1083/jcb.200706012
The Journal of Cell Biology, Vol. 178, No. 7, 1207-1221
The Rockefeller University Press, 0021-9525 $30.00
© 2007 Yam et al.
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Article

 Actin–myosin network reorganization breaks symmetry at the cell rear to spontaneously initiate polarized cell motility

Patricia T. Yam1, Cyrus A. Wilson1, Lin Ji3, Benedict Hebert4, Erin L. Barnhart1, Natalie A. Dye1, Paul W. Wiseman4,5, Gaudenz Danuser3, and Julie A. Theriot1,2

1 Department of Biochemistry and 2 Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305
3 Laboratory for Computational Cell Biology, Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037
4 Department of Physics and 5 Department of Chemistry, McGill University, Montreal, Quebec H3A 2T8, Canada

Correspondence to Julie A. Theriot: theriot{at}stanford.edu

We have analyzed the spontaneous symmetry breaking and initiation of actin-based motility in keratocytes (fish epithelial cells). In stationary keratocytes, the actin network flow was inwards and radially symmetric. Immediately before motility initiation, the actin network flow increased at the prospective cell rear and reoriented in the perinuclear region, aligning with the prospective axis of movement. Changes in actin network flow at the cell front were detectable only after cell polarization. Inhibition of myosin II or Rho kinase disrupted actin network organization and flow in the perinuclear region and decreased the motility initiation frequency, whereas increasing myosin II activity with calyculin A increased the motility initiation frequency. Local stimulation of myosin activity in stationary cells by the local application of calyculin A induced directed motility initiation away from the site of stimulation. Together, these results indicate that large-scale actin–myosin network reorganization and contractility at the cell rear initiate spontaneous symmetry breaking and polarized motility of keratocytes.

P.T. Yam's present address is Institut de recherches cliniques de Montreal, Montreal, Quebec H2W 1R7, Canada.

Abbreviations used in this paper: F-actin, filamentous actin; FSM, fluorescent speckle microscopy; MLCK, myosin light chain kinase; PH, pleckstrin homology; PI 3-kinase, phosphatidylinositol phosphate 3-kinase; STICS, spatio-temporal image correlation spectroscopy.


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