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Non-equilibrium fluctuations and mechanics of active gels and living cells

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Much like the bones in our bodies, filamentous proteins make up the cytoskeleton that largely determines the mechanical response and stability of cells. Such important cellular processes as locomotion, cell division and the sensing of mechanical stimuli are largely governed by complex networks of cytoskeletal biopolymers and the associated proteins that cross-link these and/or generate forces within the network. In addition to their important role in cell mechanics, cytoskeletal biopolymers have also provided new insights and challenges for polymer physics and rheology. Biopolymer networks, for instance, exhibit strongly nonlinear rheology—in many cases stiffening by orders of magnitude when subject to shear strains of less than unity. In the cell, these polymer networks or gels are far from equilibrium in a way unique to biology: they are subject to active, non-thermal internal forces generated by molecular motors. We describe recent theoretical and experimental results on active networks in vitro that demonstrate significant non-equilibrium fluctuations due to motor activity [1,2]. Furthermore, such gels hold out the promise of active materials, whose stiffness can be controlled by enzymatic activity. We also show how fluctuations and dynamics of individual cytoskeletal filaments can be used to probe both mechanical properties and non-equilibrium activity in living cells [3,4].

1. D Mizuno, C Tardin, CF Schmidt, FC MacKintosh, Science, 315:370 (2007). 2. FC MacKintosh and AJ Levine, Phys Rev Lett, 100:018104 (2008). 3. CP Brangwynne, FC MacKintosh, DA Weitz, PNAS , 104:16128 (2007). 4. CP Brangwynne, GH Koenderink, FC MacKintosh, DA Weitz, J Cell Biology, 183: 583-587 (2008).

This talk is part of the BSS Formal Seminars series.

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