Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D49: Focus Session: Active Living Matter I |
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Sponsoring Units: GSOFT DBIO Chair: Vernita Gordon, University of Texas at Austin Room: 217D |
Monday, March 2, 2015 2:30PM - 3:06PM |
D49.00001: Rheology of Active Gels Invited Speaker: Daniel Chen Active networks drive a diverse range of critical processes ranging from motility to division in living cells, yet a full picture of their rheological capabilities in non-cellular contexts is still emerging, e.g., How does the rheological response of a network capable of remodeling under internally-generated stresses differ from that of a passive biopolymer network? In order to address this and other basic questions, we have engineered an active gel composed of microtubules, bidirectional kinesin motors, and molecular depletant that self-organizes into a highly dynamic network of active bundles. The network continually remodels itself under ATP-tunable cycles of extension, buckling, fracturing, and self-healing. Using confocal rheometry we have simultaneously characterized the network's linear and non-linear rheological responses to shear deformation along with its dynamic morphology. We find several surprising and unique material properties for these active gels; most notably, rheological cloaking, the ability of the active gel to drive large-scale fluid mixing over several orders of flow magnitude while maintaining an invariant, solid-like rheological profile and spontaneous flow under confinement, the ability to exert micro-Newton forces to drive persistent directed motion of the rheometer tool. Taken together, these results and others to be discussed highlight the rich stress-structure-dynamics relationships in this class of biologically-derived active gels. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D49.00002: Collective dynamics of sperm in viscoelastic fluid Chih-kuan Tung, Alyssa G. Fiore, Florencia Ardon, Susan S. Suarez, Mingming Wu Collective dynamics of artificial swimmers has gathered a lot of attention from physicists, in part because of its close relations to emergent behaviors in condensed matter, such as phase transitions. However, the emergence of order tends to be less drastic in the systems composed of real living cells, sometimes due to the natural variability in individual organisms. Here, using bull sperm as a model system, we demonstrate that the local orientation order of sperm spontaneously emerges in viscoelastic fluids, migrating collectively in clusters in high cell concentrations, or pairs in low cell concentrations. This collectiveness is similar to a liquid-gas phase transition, as both phases coexist simultaneously in our system. Unlike bacterial swarming, this collectiveness does not require the cells to be in a different phenotype than the regular swimming one, providing further simplicity to the physicists. We will discuss the underlying interaction mechanism, and the potential influence in biology. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D49.00003: Large-scale violation of detailed balance in biological systems Chase Broedersz, Christopher Battle, Nikta Fakhri, Fred MacKintosh, Christopher Schmidt Living systems are out of equilibrium. A fundamental manifestation of non-equilibrium dynamics in biological systems is the violation of detailed balance: at the microscopic level, enzymatic processes such as kinetic proofreading or molecular motor activity clearly violate detailed balance. We study how such non-equilibrium dynamics emerge at macroscopic scales in cellular assemblies. We measure the steady-state dynamics of two systems, beating flagella of \emph{Chlamydomonas reinhardtii} and mechanosensitive primary cilia protruding from epithelial kidney cells. The flagellum exhibits clear non-equilibrium driving, whereas fluctuations in the primary cilium are difficult to differentiate from Brownian motion. We parameterize the shapes of the flagellum and primary cilium using a low-dimensional representation of their configuration phase space, and use the measured dynamics to infer the steady-state probability distributions and probability currents. For both the flagellum and the primary cilium we find significant, coherent circulating probability currents, demonstrating that these systems violate detailed balance at the mesoscopic scale. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D49.00004: Molecular motor driven transportation on microtubule loops Aurelien Sikora, Filippo Federici, Kyongwan Kim, Hikaru Nakazawa, Mitsuo Umetsu, Wonmuk Hwang, Winfried Teizer Molecular motors such as kinesin are naturally fitted for the transport of cargo. By offering an unlimited path, microtubule loops allow the study of kinesin motility on distances exceeding that offered by a single microtubule. Moreover, the periodicity of the path allows the comparisons of trajectories between laps. Here we study the motility of quantum dot labeled kinesin on microtubule loops. Motility of kinesins over multiple laps is observed and their trajectories are extracted from kymograph using a custom algorithm. Distribution of velocities at given locations do not vary randomly but show a correlation with the presence of obstacles. Possible mechanisms responsible for the long range transport are discussed in the context of available theories. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D49.00005: Active stochastic stress fluctuations in the cell cytoskeleton stir the cell and activate primary cilia Christoph F. Schmidt, Nikta Fakhri, Christopher Battle, Carolyn M. Ott, Alok D. Wessel, Jennifer Lippincott-Schwartz, Frederick C. MacKintosh Cells are active systems with molecular force generation that drives complex dynamics at the supramolecular scale. Much of cellular dynamics is driven by myosin motors interacting with the actin cytoskeleton. We discovered active random ``stirring'' driven by cytoplasmic myosin as an intermediate mode of transport, different from both thermal diffusion and directed motor activity. We found a further manifestation of cytoskeletal dynamics in the active motion patterns of primary cilia generated by epithelial cells. These cilia were thought to be immotile due to the absence of dynein motors, but it turns out that their anchoring deeper inside the cell in combination with the strongly fluctuating cortex results in clearly measurable non-equilibrium fluctuations. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D49.00006: Biofilm-forming bacteria can self-attract by chemotaxis, but only part of the population gets the message Qiuxian Cai, Qi Ouyang, Vernita Gordon Chemotaxis has been shown to be important for the formation of \textit{P. aeruginosa} biofilms, but the specific role of chemotaxis in the biofilm-formation process has been unknown. Using a recently-developed microfluidic device for assaying chemotaxis, we show that \textit{P. aeruginosa} will chemotax towards its own cellular products. This could act to magnify small heterogeneities in density and promote the accumulation of a high density of bacteria, as in a biofilm. The paradigmatic model organism for chemotaxis is \textit{E. coli}. \textit{E. coli} has multiple flagella and uses these to swim with a run-and-tumble random walk, biasing its runs towards chemoattractant. However, \textit{P. aeruginosa} has only a single polar flagellum and therefore in a bulk fluid can only go forward and backward (with small changes in angle possible). This would seem to pose a significant barrier to efficient chemotaxis. We find that the efficiency of \textit{P. aeruginosa} chemotaxis depends strongly on the initial swimming direction as well as the steepness of the sensed gradient of chemoattractant. Cells swimming up a sufficiently-steep gradient continue going up and do not reverse direction; the remainder show no chemotactally-directed motion. Thus, populations of P. aeruginosa show bimodal response to chemoattractant. Higher levels of chemoattractant increase overall chemotaxis not by increasing swimming speed but by increasing the proportion of bacteria that are in the chemotaxing sub-population. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D49.00007: Non-equilibrium phase transition in reconstituted acto-myosin cortices Nikta Fakhri, Enas Abu Shah, Maya Malik-Garbi, Fred C. MacKintosh, Kinneret Keren, Christoph F. Schmidt The cortical actin cytoskeleton is a quasi 2-D active material in which dynamics are dominated by rapid actin turnover and myosin-driven contractility. Here we present a reconstituted model system that emulates these processes in artificial cell-like compartments. By tuning physical and chemical parameters, we induce a non-equilibrium phase transition. We characterize the local dynamics of these reconstituted cortices by tracking embedded single-walled carbon nanotubes (SWNTs). We create high-resolution maps of the contractile actomyosin flows in a homogenous and during transition to an inhomogeneous steady state. We find evidence that connectivity percolation drives the non-equilibrium phase transition. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D49.00008: Regulation of myosin II activity by actin architecture Kimberly Weirich, Samantha Stam, Patrick McCall, Edwin Munro, Margaret Gardel Networks of actin filaments containing myosin II motors generate forces and motions that promote biological processes such as cell division, motility, and cargo transport. In cells, actin filaments are arranged in various structures from disordered meshworks to tight bundles. Clusters of myosin II motors, known as myosin filaments, crosslink and generate force on neighboring actin filaments. We hypothesized that the local actin architecture controls the magnitude and duration of force generated by myosin II motors. We used fluorescence imaging to directly measure the mobility of myosin II filaments on actin networks and bundles with varying actin filament polarity, orientation, spacing, and length. On unipolar bundles, myosin exhibits fast, unidirectional motion consistent with their unloaded gliding speed. On mixed polarity bundles, myosin speed is reduced by one order of magnitude and marked by direction switching and trapping. Increasing filament spacing and bundle flexibility reduces the duration of trapping and enhances the mobility of motors. Simulations indicate that stable trapping is a signature of large generated forces while increased mobility indicates force release. Our data underscore that the efficiency of force generation by myosin motors in an actin network depends sensitively on its architecture and suggests actin crosslinking proteins are tuned to optimize actomyosin contractility. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D49.00009: Chiral symmetry breaking in model bacterial suspensions: mechanism and system size dependence Robin Selinger, Rebekka Breier, Giovanni Ciccotti, Stephan Herminghaus, Marco Mazza We investigate the mechanism by which chiral structures spontaneously emerge in a model bacterial suspension with achiral interactions. Simulation studies demonstrate that the probability to nucleate a chiral velocity profile depends strongly on system size. To understand this dependence, we consider a classical 1-d rotor model with nearest-neighbor Lebwohl-Lasher interactions and periodic boundary conditions. In a dynamics simulation with a Langevin thermostat, we repeatedly quench the 1-d rotor model from high T to near T$=$0. In each trial we find either a metastable final state with one or more half twists, or else the untwisted ground state. We find that for a given quench rate, the mean square number of twists grows linearly with system size. For short chains, the untwisted ground state is the most probable outcome, but for chains beyond a threshold length, the most probable final state has exactly one half twist. We discuss the implications for understanding chiral symmetry breaking in bacterial suspensions. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D49.00010: Spontaneous 1 chiral symmetry breaking in model bacterial suspensions Rebekka Breier, Robin Selinger, Giovanni Ciccotti, Stephan Herminghaus, Marco G. Mazza Chiral symmetry breaking is ubiquitous in biological systems, from DNA to bacterial suspensions. A key unresolved problem is how chiral structures may spontaneously emerge from achiral interactions. We study a simple model of bacterial suspensions in three dimensions that effectively incorporates active motion and hydrodynamic interactions. We perform large-scale molecular dynamics simulations (up to $10^6$ particles) and describe stable (or long-lived metastable) collective states that exhibit chiral organization although the interactions are achiral. We elucidate under which conditions these chiral states will emerge and grow to large scales. We also study a related equilibrium model that clarifies the role of orientational fluctuations. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D49.00011: Sperm Cell Dynamics in Shallow Chambers Carlos Condat, Veronica Marconi, Alejandro Guidobaldi, Laura Giojalas, Alejandro Silhanek, Yogesh Jeyaram, Victor Moshchalkov Self-propelled microorganisms are attracted to surfaces. This makes their dynamic behavior in restricted geometries very different from that observed in the bulk. Here we analyze the motion of spermatozoids confined to shallow chambers, investigating the nature of the cell trajectories and their accumulation near the side boundaries. Observed cell trajectories are composed of a succession of quasi-circular and quasi-linear segments. This suggests that the cells follow a path of intermittent trappings near the top and down surfaces separated by stretches of quasi-free motion near the center of the gap. Use of microstructured petal-shaped edges limits accumulation near the borders and contributes to increase the concentration in the chamber interior. System stabilization occurs over times of the order of minutes, which agrees well with a theoretical estimate that assumes that the cell mean-square displacement is largely due to the quasi-linear segments. Pure quasi-circular trajectories would require several hours to stabilize. Our estimates also indicate that stabilization proceeds 2.5 times faster in the rosette geometries than in the smooth-edged chambers, which is another practical reason to prefer the former. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D49.00012: Shape-Conserved Dynamic Condensation in the Process of Aster Formation from a System of Microtubules and Cross-Linked Kinesin Motors K. Kim, A. Sikora, H. Nakazawa, M. Umetsu, W. Hwang, W. Teizer We report fluorescence microscopy studies of a cellular element-based active system that is composed of rhodamine-labeled microtubules and functionalized kinesin motor proteins, cross-linked via streptavidin-coated quantum dots. The motor proteins organize microtubules into aster-like structures containing core aggregations of the quantum dot-motor protein complexes. The cores result from the dynamic condensation of sub-clusters that are connected to each other randomly. The inter-cluster distance decays exponentially with time during the condensation. Intriguingly, the shape defined by lines connecting the clusters is well conserved while the dynamic process reduces the size. This shape conservation is governed by a scaling behavior during the condensation, following a power law with respect to the distance between sub-clusters. We explain this isomorphic contraction during the aster formation process using a simple mechanistic model. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D49.00013: Broken detailed balance in active fluctuations of semiflexible filaments Jannes Gladrow, Nikta Fakhri, Fred C. MacKintosh, Christoph F. Schmidt, Chase P. Broedersz Non-equilibrium microscopic force generation in cells often results in stochastic steady-state fluctuations. In the cell cytoskeleton, for example, cytoplasmic myosins can drive vigorous conformational fluctuations of actin filaments and microtubules. We here present an analytical and numerical analysis of randomly driven shape fluctuations of semiflexible filaments in a viscoelastic environment. To detect and quantify non-equilibrium dynamics, we focus on the breaking of detailed balance in a conformational phase space subtended by eigenmodes of the beam equation. Molecular dynamics simulations reveal a non-zero circulatory flux in phase space induced by motor activity. Furthermore, we derived an analytical expression of nonequilibrium mode correlations that allows us to predict temporal effects of active molecular motors. [Preview Abstract] |
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