Abstract
Eukaryotic cells advance in phases of protrusion, pause and withdrawal1. Protrusion occurs in lamellipodia, which are composed of diagonal networks of actin filaments, and withdrawal terminates with the formation of actin bundles parallel to the cell edge. Using correlated live-cell imaging and electron microscopy, we have shown that actin filaments in protruding lamellipodia subtend angles from 15–90° to the front, and that transitions from protrusion to pause are associated with a proportional increase in filaments oriented more parallel to the cell edge. Microspike bundles of actin filaments also showed a wide angular distribution and correspondingly variable bilateral polymerization rates along the cell front. We propose that the angular shift of filaments in lamellipodia serves in adapting to slower protrusion rates while maintaining the filament densities required for structural support; further, we suggest that single filaments and microspike bundles contribute to the construction of the lamella behind and to the formation of the cell edge when protrusion ceases. Our findings provide an explanation for the variable turnover dynamics of actin filaments in lamellipodia observed by fluorescence speckle microscopy2 and are inconsistent with a current model of lamellipodia structure that features actin filaments branching at 70° in a dendritic array3.
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Acknowledgements
The authors thank the Human Frontier Science Program Organization (HFSPO), The Austrian Science Research Council (FWF) and the Vienna Science Research and Technology Fund (WWTF) as well as the City of Vienna/Zentrum für Innovation und Technologie via the Spot of Excellence grant 'Center of Molecular and Cellular Nanostructure' for financial support. K.R. was supported in part by grants from the Deutsche Forschungsgemeinschaft (SPP1150 and FOR629). We also thank Guenter Resch for the electron microscope facility management and advice with image processing, Tibor Kulcsar and Hannes Tkadletz for graphics and Natalia Andreyeva for helpful comments. The authors thank Roger Tsien, Annette Muller-Taubenberger, Malgorzata Szczodrak, George Patterson, Jennifer Lippincott-Schwarz and Rex Chisholm for probes, and Jeff Segall and Bob van de Water for MTLn3 cells.
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Koestler, S., Auinger, S., Vinzenz, M. et al. Differentially oriented populations of actin filaments generated in lamellipodia collaborate in pushing and pausing at the cell front. Nat Cell Biol 10, 306–313 (2008). https://doi.org/10.1038/ncb1692
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DOI: https://doi.org/10.1038/ncb1692
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