Cell-sized liposomes reveal how actomyosin cortical tension drives shape change
Edited by James A. Spudich, Stanford University School of Medicine, Stanford, CA, and approved August 29, 2013 (received for review December 11, 2012)
Significance
Animal cells continuously move, divide, and transmit forces by actively reorganizing their internal scaffold or cytoskeleton. Molecular motors pull actin filaments together and generate contraction of the cytoskeleton underneath the cell membrane. We address the detailed mechanism of contraction by using a minimal in vitro assay: a liposome membrane to which we attach actin filaments and molecular motors in a controlled manner. We reproduce contraction like in cells. We show that the scaffold needs to be tightly attached to the membrane for efficient contraction, but, under some conditions, efficient contraction can lead to liposome destruction. These results suggest that cells must precisely control their contractility to remain intact during cellular events.
Abstract
Animal cells actively generate contractile stress in the actin cortex, a thin actin network beneath the cell membrane, to facilitate shape changes during processes like cytokinesis and motility. On the microscopic scale, this stress is generated by myosin molecular motors, which bind to actin cytoskeletal filaments and use chemical energy to exert pulling forces. To decipher the physical basis for the regulation of cell shape changes, here, we use a cell-like system with a cortex anchored to the outside or inside of a liposome membrane. This system enables us to dissect the interplay between motor pulling forces, cortex–membrane anchoring, and network connectivity. We show that cortices on the outside of liposomes either spontaneously rupture and relax built-up mechanical stress by peeling away around the liposome or actively compress and crush the liposome. The decision between peeling and crushing depends on the cortical tension determined by the amount of motors and also on the connectivity of the cortex and its attachment to the membrane. Membrane anchoring strongly affects the morphology of cortex contraction inside liposomes: cortices contract inward when weakly attached, whereas they contract toward the membrane when strongly attached. We propose a physical model based on a balance of active tension and mechanical resistance to rupture. Our findings show how membrane attachment and network connectivity are able to regulate actin cortex remodeling and membrane-shape changes for cell polarization.
Acknowledgments
We thank D. Levy and A. Di Cicco for electron microscopy; J. LeeTinWah for myosin 2 functionality tests; A. Yamada for the experiments using ADP-Myosin; and J. Lemiere, M. Bussionnier, and M. Kuit-Vinkenoog for protein preparation. We also thank B. Alonso Latorre and N. Becker for discussions, F. van der Linden and B. Gentry for actin-length distribution, S. Roth for 3D detection and image analysis, and C. Marques for advice on the precursor method. This work was supported by French Agence Nationale de la Recherche (ANR) Grants ANR 09BLAN0283 and ANR 12BSV5001401, Fondation pour la Recherche Médicale Grant DEQ20120323737, a Vidi grant from the Netherlands Organization for Scientific Research (NWO), and the Foundation for Fundamental Research on Matter, which is part of NWO. K.C. acknowledges a fellowship from the ARC.
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Published online: September 24, 2013
Published in issue: October 8, 2013
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Acknowledgments
We thank D. Levy and A. Di Cicco for electron microscopy; J. LeeTinWah for myosin 2 functionality tests; A. Yamada for the experiments using ADP-Myosin; and J. Lemiere, M. Bussionnier, and M. Kuit-Vinkenoog for protein preparation. We also thank B. Alonso Latorre and N. Becker for discussions, F. van der Linden and B. Gentry for actin-length distribution, S. Roth for 3D detection and image analysis, and C. Marques for advice on the precursor method. This work was supported by French Agence Nationale de la Recherche (ANR) Grants ANR 09BLAN0283 and ANR 12BSV5001401, Fondation pour la Recherche Médicale Grant DEQ20120323737, a Vidi grant from the Netherlands Organization for Scientific Research (NWO), and the Foundation for Fundamental Research on Matter, which is part of NWO. K.C. acknowledges a fellowship from the ARC.
Notes
This article is a PNAS Direct Submission.
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The authors declare no conflict of interest.
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Cell-sized liposomes reveal how actomyosin cortical tension drives shape change, Proc. Natl. Acad. Sci. U.S.A.
110 (41) 16456-16461,
https://doi.org/10.1073/pnas.1221524110
(2013).
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