Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H7: Complex Active Biomaterials: Mechanics and Microrheology |
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Sponsoring Units: DBP DPOLY Chair: John Crocker, University of Pennsylvania Room: Morial Convention Center RO5 |
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H7.00001: Non-equilibrium mechanics of motor-driven cytoskeletal polymer networks Invited Speaker: Cells both actively generate and sensitively react to forces using their mechanical framework, the cytoskeleton, which is a non-equilibrium, composite material including polymers and motor proteins. We have measured the dynamics and mechanical properties of a simple three-component model system, consisting of myosin II, actin filaments, and crosslinkers. Stresses arising from motor activity control network mechanics: both increasing stiffness by a factor of nearly 100 and qualitatively changing the viscoleastic response of the network in an ATP-dependent manner. We have quantified the mechanical properties as well as the active fluctuations in these networks by a combination of passive and active microrheology. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H7.00002: Non-equilibrium mechanics and dynamics of active gels and living cells Invited Speaker: Much like the bones in our bodies, the \textit{cytoskeleton} consisting of filamentous proteins largely determines the mechanical response and stability of cells. Such important cellular processes as locomotion, cell division, and mechanosensing are largely governed by complex networks of cytoskeletal biopolymers and 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. In the cell, however, these polymer networks or gels are far from equilibrium in a way unique to biology: they are subject to active internal force generation by molecular motors. We describe recent theoretical and experimental results on active in vitro networks that demonstrate significant stiffening and non-equilibrium fluctuations due to motor activity [1]. We show how this activity leads generically to a colored, 1/$\omega ^{2}$ spectrum of force fluctuations, which can account for surprisingly Brownian-like motion in elastic networks. We also discuss how the fluctuations of individual cytoskeletal filaments can be used to probe both mechanical properties and non-equilibrium activity in living cells [2]. \par [1] D Mizuno, C Tardin, CF Schmidt, FC MacKintosh, \textit{Science}, 315:370 (2007). \par [2] CP Brangwynne, FC MacKintosh, DA Weitz, \textit{PNAS}, 104:16128 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H7.00003: Cytoskeletal mechanics: Structure and Dynamics Invited Speaker: The actin cytoskeleton, a dynamic network of semiflexible filaments and associated regulatory proteins, is responsible for the extraordinary viscoelastic properties of cells. Especially for cellular motility the controlled self assembly to defined structures and the dynamic reorganization on different time scales are of outstanding importance. A prominent example for the controlled self assembly are actin bundles: in many cytoskeletal processes cells rely on the tight control of the structural and mechanical properties of the actin bundles. Using an\textit{ in vitro} model system we show that size control relies on a mismatch between the helical structure of individual actin filaments and the packing symmetry within bundles. While such self assembled structure may evoke the picture of a static network the contrary is the case: the cytoskeleton is highly dynamic and a constant remodeling takes place in vivo. Such dynamic reorganization of the cytoskeleton relies on the non-static nature of single actin/ABP bonds. Here, we study the thermal and forced unbinding events of individual ABP in such \textit{in vitro} networks. The binding kinetics of the transient crosslinkers determines the mechanical response of such networks -- in the linear as well in the non-linear regime. These effects are important prerequisites for the high adaptability of cells and at the same time might be the molecular mechanism employed by them for mechanosensing. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H7.00004: Microrheology in Active Cytoskeletal Networks Invited Speaker: The mechanics of the in vivo cytoskeleton is controlled in part by the details of its non-equilibrium steady-state. In this ``active'' material, molecular motors (e.g. myosin) exert transient contractile stresses on the F-actin filament network, driving it into a particular non-equilibrium state. Since microrheology traditionally relies of the linear response properties of the soft materials in thermal equilibrium, this departure from equilibrium has profound implications for the interpretation of microrheological data from the interior of living cells and in vitro active networks. In active networks, such as the in vitro systems of Mizuno et al. [Science 315 (5810) pp. 370-373 (2007).] and in living cells, the underlying theoretical foundation of the interpretation of microrheology -- the Fluctuation-Dissipation theorem -- does not apply. New ideas are needed. In this talk, I review microrheology, and then discuss a new theoretical interpretation of microrheology in active (i.e. molecular motor driven) networks. To develop this new theory, I introduce a motor-driven, two-fluid model of the active network and background (aqueous) solvent. Using this model and knowledge of the statistical properties of the molecular-motor induced forces, I calculate the non-equilibrium fluctuation spectrum expected for one- and two-particle microrheology in the driven system. I then compare these results to the data of Mizuno et al.. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 11:00AM |
H7.00005: Force fluctuations and polymerization dynamics of intracellular microtubules Invited Speaker: Microtubules are dynamic biopolymers within the cytoskeleton of living cells. They play a central role in many biological processes including cell division, migration, and cargo transport. Microtubules are significantly more rigid than other cytoskeletal biopolymers, such as actin filaments, and are insensitive to thermal fluctuations on cellular length scales. However, we show that intracellular microtubules exhibit bending amplitudes with a surprisingly thermal-like wavevector dependence, but with an apparent persistence length about 100 times smaller than that measured \textit{in vitro}. By studying the time-dependent bending fluctuations of individual filaments, we find that the thermal-like bends are fluctuating significantly only on short length scales, while they are frozen-in on longer length scales [1], reminiscent of non-ergodic behavior seen in systems far from equilibrium. Long wavelength bends are suppressed by the surrounding elastic cytoskeleton, which confines bending to short length scales on the order of a few microns [2]. These short wavelength bending fluctuations naturally cause fluctuations in the orientation of the microtubule tip. Tip fluctuations result in a persistent random walk trajectory of microtubule growth, but with a small non-equilibrium persistence length, explaining the origin of quenched thermal-like bends. These results suggest that intracellular motor activity has a highly fluctuating character that dominates over thermal fluctuations, with important consequences for fundamental biological processes. \newline \newline [1] CP Brangwynne, FC MacKintosh, DA Weitz, \textit{PNAS}, 104:16128 (2007). \newline [2] CP Brangwynne, FC MacKintosh, S Kumar, NA Geisse, J Talbot, L. Mahadevan, KK Parker, DE Ingber, DA Weitz, \textit{JCB}, 173:733 (2006). [Preview Abstract] |
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