Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W12: Invited Session: Active Matter and the Cytoskeleton |
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Sponsoring Units: DBIO Chair: Daniel Chen, Brandeis University Room: 205 |
Thursday, March 6, 2014 2:30PM - 3:06PM |
W12.00001: Active stresses and hydrodynamics of microtubule/motor-protein assemblies Invited Speaker: Meredith Betterton In biologically-inspired soft active materials, chemical energy (typically from ATP) is transduced to generate active stresses arising from reconiguration or forcing of the microstructure. This can lead to novel material organization, mechanical properties, and active flows. While much focus has been on active gels and motile suspensions, another important class are suspensions of microtubules (MTs) crosslinked by motile molecular motors. These are central actors in biological phenomena such as pronuclear transport and spindle formation. Here we develop a multi-scale theory for studying such systems. At the discrete level, we use Brownian dynamics of MTs with moving crosslinks to study microscopic organization and active stress development. We observe, surprisingly, that activity generated extensile stresses arise from both polarity sorting and crosslink relaxation. These simulations estimate polarity-dependent active stress coeffcients in a Doi-Onsager kinetic theory -- similar to those developed previously for motile suspensions -- that captures polarity sorting and induced hydrodynamic flows. In simulating recent experiments of active flows on immersed surfaces, the model exhibits turbulent-like dynamics, and the continous generation and annihilation of disclination defects associated with coherent flow structures. We can associate the system's coherent features with instabilities of aligned linear and nonlinear states. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W12.00002: Active Matter and the Spindle Invited Speaker: Daniel Needleman |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W12.00003: Defect Dynamics in Active Nematics Invited Speaker: M. Cristina Marchetti In vitro suspensions of cytoskeletal filaments and motor proteins can form active fluids and gels with liquid crystalline order and self-sustained flows characterized by evolving topological defects. While in passive nematics opposite-sign defect attract and ultimately annihilate, in active liquid crystals defect pairs are continuously generated by activity. Using a continuum model of a planar active nematic in two dimensions, we have demonstrated that activity results in a turbulent-like state with a steady concentration of defect-antidefect pairs, as observed in recent experiments in suspensions of active microtubules-kinesin bundles. We have shown that these ``active defects'' behave as self-propelled particles with equilibrium interactions and a self-propulsion speed proportional to activity. This particle model quantitatively describes the dynamics of the four required defects in active nematics confined to the surface of vesicles that oscillate between tetrahedral and planar configurations at a tunable frequency. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W12.00004: Dynamics of active actin networks Invited Speaker: Simone Koehler Local mechanical and structural properties of a eukaryotic cell are determined by its cytoskeleton. To adapt to their environment, cells rely on constant self-organized rearrangement processes of their actin cytoskeleton. To shed light on the principles underlying these dynamic self-organization processes we investigate a minimal reconstituted active system consisting of actin filaments, crosslinking molecules and molecular motor filaments. Using quantitative fluorescence microscopy and image analysis, we show, that these minimal model systems exhibit a generic structure formation mechanism. The competition between force generation by molecular motors and the stabilization of the network by crosslinking proteins results in a highly dynamic reorganization process which is characterized by anomalous transport dynamics with a superdiffusive behavior also found in intracellular dynamics. \textit{In vitro}, these dynamics are governed by chemical and physical parameters that alter the balance of motor and crosslinking proteins, such as pH. These findings can be expected to have broad implications in our understanding of cytoskeletal regulation \textit{in vivo}. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W12.00005: Active mechanics and geometry of adherent cells and cell colonies Invited Speaker: Shiladitya Banerjee Measurements of traction stresses exerted by adherent cells or cell colonies on elastic substrates have yielded new insight on how the mechanical and geometrical properties of the substrate regulate cellular force distribution, mechanical energy, spreading, morphology or stress ber architecture. We have developed a generic mechanical model of adherent cells as an active contractile gel mechanically coupled to an elastic substrate and to neighboring cells in a tissue. The contractile gel model accurately predicts the distribution of cellular and traction stresses as observed in single cell experiments, and captures the dependence of cell shape, traction stresses and stress ber polarization on the substrate's mechanical and geometrical properties. The model further predicts that the total strain energy of an adherent cell is solely regulated by its spread area, in agreement with recent experiments on micropatterned substrates with controlled geometry. When used to describe the behavior of colonies of adherent epithelial cells, the model demonstrates the crucial role of the mechanical cross-talk between intercellular and extracellular adhesion in regulating traction force distribution. Strong intercellular mechanical coupling organizes traction forces to the colony periphery, whereas weaker intercellular coupling leads to the build up of traction stresses at intercellular junctions. Furthermore, in agreement with experiments on large cohesive keratinocyte colonies, the model predicts a linear scaling of traction forces with the colony size. This scaling suggests the emergence of an effective surface tension as a scale-free material property of the adherent tissue, originating from actomyosin contractility. [Preview Abstract] |
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