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Cell Mechanics and Cross-linked Actin Networks |
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Cell motility, force generation, shape, and stiffness are largely mediated by the cytoskeleton: a dynamic biopolymer network, composed of actin, intermediate filaments, microtubules, and associated binding proteins that cross-link and regulate the network. We investigate the mechanics of actin networks and how the properties of these networks can be modulated and regulated by cross-linking proteins. We hope to connect the behavior of such reconstituted cytoskeletal model systems to the mechanics of living cells. In the process, we also learn strategies for designing strong and highly tuneable soft materials.
RECONSTITUTED ACTIN NETWORKS: The actin cytoskeleton is a viscoelastic material. The material properties of reconstitued actin networks depend strongly on the associated proteins that cross-link and bundle the actin filaments. Small, non-compliant cross-links yield networks whose elasticity arises primarily from thermal fluctuations in the actin filaments themselves. Many cross-linking proteins found in cells are large and flexible. The material properties of actin filaments connected by flexible cross-links are qualitatively different from those of non-compliant cross-links. Actin networks cross-linked with one such protein, filamin, form very soft elastic gels that can support large shear stresses at strains of more than 100%. We hope to elucidate the origins of elasticity in these networks.
Filamin: Filamin is an abundant actin binding protein that cross-links actin filaments into orthogonal networks, bundles actin filaments, and connects the actin network to specific transmembrane proteins. Filamin is a compliant and dynamic cross-linker. It is implicated in many highly mechanical cellular processes such as motility, membrane stability, and mechanoprotection.
Rheology: To investigate nonlinear properties of materials of reconstituted networks, we use a stress-controlled rheometer to apply both steady and oscillatory shear stresses. We find that the actin-filamin networks stiffen dramatically when large stresses and strains are applied. The differential elastic modulus, K', can increase by over two orders of magnitude before the network ruptures.
We are interested in understanding how this nonlinear strain-stiffening behavior depends upon actin filament length, actin concentration, and filamin concentration, and how such highly nonlinear material properties may be utilized by the cell.
MECHANICS OF CELLS: Cells are known to be in a state of prestress created by the contractile actin-myosin machinery of the cytoskeleton. It has been observed that as the contractile prestress in the cell increases, so does the stiffness. We would like to understand if the physics underlying this measured nonlinear response is the same as that in the reconstituted prestressed actin-filamin networks. We investigate the mechanics of filamin expressing and filamin null cell lines in a collaboration with the labs of Ning Wang and Tom Stossel. We probe the level of contractile prestress in the cytoskeleton by measuring the traction forces that the cell exerts on a deformable substrate. We measure cell stiffness by attaching magnetic beads through integrins in the cell membrane to the actin cytoskeleton inside the cell. We twist these beads by applying an oscillatory magnetic field and measure the resulting bead displacement. We hope to understand how the level of filamin expression in living cells affects both cell material properties and behavior. Website maintained by Karen Kasza. Collaborators: Fumihiko Nakamura and Tom Stossel, Hematology Division, Brigham and Women's Hospital, Department of Medicine, Harvard Medical School, Boston, Massachusetts. Ning Wang, Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois. Fred MacKintosh, Theoretical Physics, Vrije Universiteit, Amsterdam, the Netherlands. Gijsje Koenderink, AMOLF-FOM, Amsterdam, the Netherlands. Other people in the lab involved in this project: Yi-Chia Lin, Jiayu Liu. Previous work on this project: Margaret Gardel (The University of Chicago, Dept. of Physics) Last updated |
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