Cytoskeletal self-organisation at fluid interfaces
Final Report Abstract
The actin cytoskeleton is a network of filaments formed by the protein actin and a host of actin-binding proteins. The latter control, for example, actin filament assembly. Myosin motor proteins that cross-link several actin filaments can use the chemical energy that is liberated during hydrolysis of Adenosine-Triphosphate and generate mechanical stresses in the actin network. The actin cytoskeleton is an essential structure that is involved in many vital cellular processes like division and motility. In animal cells it forms a well-defined layer beneath the cytoplasmic membrane called the actin cortex. The cortex determines the mechanical properties of animal cells and can generate various substructures, e.g., contractile rings that physically cleave cells during division. In our project, we aimed at understanding the possibility of the actin cytoskeleton to self-organize in the vicinity of fluid membranes. In a theoretical study we found that sufficiently active motors are indispensible for generating a well-defined cortical layer. For this study we described the actin network as an actively contracting continuum material. We checked this assumption by characterizing experimentally the effects of myosin motors on reconstituted actin networks in vitro. We found that for sufficiently weak motor activity, the network indeed contracted. Strikingly, the contraction dynamics depended on the size and geometry of the actin network. Furthermore, we observed acto-myosin sheets that had a height of several tens of micrometers and lateral extensions of several hundreds of micrometers to buckle in the course of contraction. This generated patterns similar to the undulated form of plant leaves. We decided to follow up this hitherto unknown behavior. Our detailed study that combined in vitro reconstitution experiments and theoretical analysis revealed that the dependence of the contraction velocity on the initial size of the acto-myosin sheet could results from 'stress-activation' of the myosin: the more they are exposed to stress, the higher their activity. Buckling then resulted from the fact that at some point the interior of an acto-myosin sheet contracts faster than the periphery of the sheet would need to contract in order to geometrically allow for a planar contraction. Currently, we are exploring possible ways to use the system we have developed in this work to study mechanical mechanisms that could induce morphological transformations during embryonic development.
Publications
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Myosin II does it all: Assembly, Remodeling, and Disassembly of actin networks are governed by myosin II activity. Soft Matter 9, 7127 (2013)
Y. Ideses, A. Sonn-Segev, Y. Roichman, and A. Bernheim
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The actin cortex as an active wetting layer. Eur. Phys. J. E 36, 52 (2013)
J.-F. Joanny, K. Kruse, J. Prost, and S. Ramaswamy
