Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A35: Active Matter: Collective Phenomena in Living Systems IFocus
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Sponsoring Units: DBIO GSOFT GSNP Chair: Karsten Kruse, Saarland University, Germany Room: 338 |
Monday, March 14, 2016 8:00AM - 8:36AM |
A35.00001: Self-organization of stress patterns drives state transition in actin cortices Invited Speaker: Nikta Fakhri |
Monday, March 14, 2016 8:36AM - 8:48AM |
A35.00002: Spontaneous actin dynamics in contractile rings Karsten Kruse, Viktoria Wollrab, Raghavan Thiagarajan, Anne Wald, Daniel Riveline Networks of polymerizing actin filaments are known to be capable to self-organize into a variety of structures. For example, spontaneous actin polymerization waves have been observed in living cells in a number of circumstances, notably, in crawling neutrophils and slime molds. During later stages of cell division, they can also spontaneously form a contractile ring that will eventually cleave the cell into two daughter cells. We present a framework for describing networks of polymerizing actin filaments, where assembly is regulated by various proteins. It can also include the effects of molecular motors. We show that the molecular processes driven by these proteins can generate various structures that have been observed in contractile rings of fission yeast and mammalian cells. We discuss a possible functional role of each of these patterns. [Preview Abstract] |
Monday, March 14, 2016 8:48AM - 9:00AM |
A35.00003: Fluctuations and nematic order in collective motion of filamentous bacteria Daiki Nishiguchi, Ken H. Nagai, Masaki Sano Although there are many numerical and theoretical studies on Vicsek-like models, there have been no convincing experiments that clearly observe predicted properties of collective motion such as giant number fluctuations. To realize such experiments with a biological system, we used filamentous bacteria, which are 20 times as long as usual bacteria. Due to strong alignment interactions arising from their elongated shapes, these bacteria exhibit a nematic state when their dense suspensions are confined in a quasi-two-dimensional plane. We have quantitatively evaluated the nematic order parameter in this ordered state and concluded that it has true long-range order, and we have obtained giant number fluctuations in this true long-range ordered state. All the obtained experimental results are consistent with a Vicsek-like model with the same symmetry as our experiments, namely, the Vicsek-like self-propelled rods model, in which each particle has polarity and their interactions are nematic. [Preview Abstract] |
Monday, March 14, 2016 9:00AM - 9:12AM |
A35.00004: Modeling flexible active nematics Michael Varga, Robin Selinger We study active nematic phases of self-propelled flexible chains in two dimensions using computer simulation, to investigate effects of chain flexibility. In a ``dry'' phase of self-propelled flexible chains, we find that increasing chain stiffness enhances orientational order and correlation length, narrows the distribution of turning angles, increases persistence length, and increases the magnitude of giant density fluctuations. We further adapt the simulation model to describe behavior of microtubules driven by kinesin molecular motors in two different environments: on a rigid substrate with kinesin immobilized on the surface; and on a lipid membrane where kinesin is bonded to lipid head groups and can diffuse. Results are compared to experiments by L. Hirst and J. Xu. Lastly, we consider active nematics of flexible particles enclosed in soft, deformable encapsulation in two dimensions, and demonstrate novel mechanisms of pattern formation that are fundamentally different from those observed in bulk. [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:24AM |
A35.00005: Active nematics confined within a shell Rui Zhang, Ye Zhou, Mohammad Rahimi, Juan de Pablo Active fluids exhibit many striking flow patterns when confined within complex geometries. For example, recent work has demonstrated that when a thin film of extensile microtubules is confined within a vesicle, the four $+1/2$ defects periodically oscillate between a tetrahedral and a planar configuration (Keber, {\it et al}. Science (2014)). Here we employ hybrid lattice Boltzmann simulations to study the dynamics of active nematics confined between two concentric spherical surfaces. We find that in both extensile and contractile systems, the four defects are coupled with noticeable macroscopic velocities and they move along their symmetry axes, eventhough in different patterns. We observe that in extensile systems with moderate activity, defects repel each other due to elastic forces, and their collective motion leads to the same patterned dynamics as observed in the above experiment. We further show that this periodic dynamics is accompanied by oscillations of the defect velocity, system's elastic energy, and the emergence and annihilation of vortices. We also observe that with stronger activity, the extensile system evolves to chaos. In contrast, the contractile system remains passive for the entire activity range, with defects being attracted to each other in pairs. [Preview Abstract] |
Monday, March 14, 2016 9:24AM - 9:36AM |
A35.00006: Active Cellular Nematics Guillaume Duclos, Christoph Erlenkaemper, Simon Garcia, Hannah Yevick, Jean-François Joanny, Pascal Silberzan We study the emergence of a nematic order in a two-dimensional tissue of apolar elongated fibroblast cells. Initially, these cells are very motile and the monolayer is characterized by giant density fluctuations, a signature of far-from-equilibrium systems. As the cell density increases because of proliferation, the cells align with each other forming large perfectly oriented domains while the cellular movements slow down and eventually freeze. Therefore topological defects characteristic of nematic phases remain trapped at long times, preventing the development of infinite domains. By analogy with classical non-active nematics, we have investigated the role of boundaries and we have shown that cells confined in stripes of width smaller than typically 500 \textmu m are perfectly aligned in the stripe direction. Experiments performed in cross-shaped patterns show that both the number of cells and the degree of alignment impact the final orientation. \textit{Reference}: Duclos G., Garcia S., Yevick H.G. and Silberzan P., "Perfect nematic order in confined monolayers of spindle-shaped cells", Soft Matter, 10, 14, 2014 [Preview Abstract] |
Monday, March 14, 2016 9:36AM - 9:48AM |
A35.00007: Defect dynamics and ordering in compressible active nematics. Prashant Mishra, Pragya Srivastava, M. Cristina Marchetti Active nematics, such as suspensions of biopolymers activated by molecular motors or bacteria swimming in passive liquid crystals, exhibit complex self-sustained flow, excitability and defect generation. Activity renders the defect themselves self-propelled particles, capable of organizing in emergent ordered structures. We have developed a minimal model of compressible active nematics on a substrate. We eliminate the flow velocity in favor of the nematic order parameter via the balance of frictional dissipation and active driving to obtain a dynamical description entirely in terms of the nematic alignment order parameter. Activity renormalizes the bend and splay elastic constants rendering them anisotropic and driving them to zero or even negative, resulting in the appearance of modulated states and defective structures. Using linear stability analysis and numerics we organize the various regimes into a phase diagram and discuss the relation to experiments. [Preview Abstract] |
Monday, March 14, 2016 9:48AM - 10:00AM |
A35.00008: Directed and persistent movement arises from mechanochemistry of the ParA/ParB system Longhua Hu, Anthony G. Vecchiarelli, Kiyoshi Mizuuchi, Keir C. Neuman, Jian Liu The segregation of DNA prior to cell division is essential for faithful genetic inheritance. In many bacteria, segregation of the low-copy-number plasmids involves an active partition system composed of ParA ATPase and its stimulator protein ParB. Recent experiments suggest that ParA/ParB system motility is driven by a diffusion-ratchet mechanism in which ParB-coated plasmid both creates and follows a ParA gradient on the nucleoid surface. However, the detailed mechanism of ParA/ParB-mediated directed and persistent movement remains unknown. We develop a theoretical model describing ParA/ParB-mediated motility. We show that the ParA/ParB system can work as a Brownian ratchet, which effectively couples the ATPase-dependent cycling of ParA-nucleoid affinity to the motion of the ParB bound cargo. Paradoxically, the resulting processive motion relies on quenching diffusive plasmid motion through a large number of transient ParA/ParB-mediated tethers to the nucleoid surface. Our work sheds light on a new emergent phenomenon in which non-motor proteins work collectively via mechanochemical coupling to propel cargos --- an ingenious solution shaped by evolution to cope with the lack of processive motor proteins in bacteria. [Preview Abstract] |
Monday, March 14, 2016 10:00AM - 10:12AM |
A35.00009: Decoupling tissue and cell scale stresses using embedded oil microdroplets Elijah Shelton, Friedhelm Serwane, Alessandro Mongera, Adam Lucio, Otger Camp\`{a}s Embryonic development and organ morphogenesis require mechanical stresses to be patterned in space and time over length scales ranging from cellular to tissue level. While several approaches use 4D live-imaging to infer forces from the observed flow fields, few techniques allow direct measurements of stress in vivo and in situ. We use oil microdroplets injected in between cells as direct stress sensors. Through confocal imaging and custom software for high resolution 3D droplet surface reconstruction, we can directly measure the patterns of stress by looking at the deformations of the drop. This analysis allows us to decouple the stresses at the tissue scale from those generated at cellular scales by disentangling ellipsoidal drop deformation modes from higher order drop deformations. Using this technique we measure both tissue and cell scale stresses within aggregates of mesenchymal cells as well as within developing zebrafish embryonic tissues. The decoupling of mechanical stresses at cell and tissue scales makes our technique uniquely suited for understanding how tissue scale reorganizations emerge from cell scale interactions. [Preview Abstract] |
Monday, March 14, 2016 10:12AM - 10:24AM |
A35.00010: Volume Changes During Active Shape Fluctuations in Cells Caterina A. M. La Porta, Alessandro Taloni, Elena Kardash, Oguz Umut Salman, Lev Truskinovsky, Stefano Zapperi Cells modify their volume in response to changes in osmotic pressure but it is usually assumed that other active shape variations do not involve significant volume fluctuations. Here we report experiments demonstrating that water transport in and out of the cell is needed for the formation of blebs, commonly observed protrusions in the plasma membrane driven by cortex contraction. We develop and simulate a model of fluid-mediated membrane-cortex deformations and show that a permeable membrane is necessary for bleb formation which is otherwise impaired. Taken together, our experimental and theoretical results emphasize the subtle balance between hydrodynamics and elasticity in actively driven cell morphological changes \footnote{A. Taloni et al. Phys. Rev. Lett. 114, 208101 (2015)}. [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 10:36AM |
A35.00011: Probing the Dynamics of the Cellular Actomyosin Network with Magnetic Microposts Yu Shi, Steven Henry, John Crocker, Daniel Reich The actomyosin network in living cells is commonly accepted as an archetypal example of an active matter system. To characterize the dynamic properties and the effects of non-thermal motion of such a system requires simultaneously measuring the fluctuation spectrum of internal stresses as well as its local viscoelasticity. Via use of PDMS micropost arrays with magnetic nanowires embedded in selected posts, we measure the local complex modulus of cells through mechanical actuation of the magnetic microposts using a dual magnetic tweezer system. The microposts are also used as passive probes to measure the force fluctuations inside the cytoskeleton. The active and passive responses of fibroblasts will be presented, together with measurements of correlations between different subcellular regions, and the influence of cytoskeletal and myosin inhibitors. Results on the anisotropy of internal stress fluctuations and their response to chemical perturbations will also be discussed. [Preview Abstract] |
Monday, March 14, 2016 10:36AM - 10:48AM |
A35.00012: Active Contraction of Microtubule Networks Peter Foster, Sebastian Fürthauer, Michael Shelley, Daniel Needleman Many cellular processes are driven by cytoskeletal assemblies. It remains unclear how cytoskeletal filaments and motor proteins organize into cellular scale structures and how molecular properties of cytoskeletal components affect the large scale behaviors of these systems. Here we investigate the self-organization of stabilized microtubules in \emph{Xenopus} oocyte extracts and find that they can form macroscopic networks that spontaneously contract. We propose that these contractions are driven by the clustering of microtubule minus ends by dynein. Based on this idea, we construct an active fluid theory of network contractions which predicts a dependence of the timescale of contraction on initial network geometry, a development of density inhomogeneities during contraction, a constant final network density, and a strong influence of dynein inhibition on the rate of contraction, all in quantitative agreement with experiments. These results demonstrate that the motor-driven clustering of filament ends is a generic mechanism leading to contraction. [Preview Abstract] |
Monday, March 14, 2016 10:48AM - 11:00AM |
A35.00013: Collective dynamics during cell division Stefano Zapperi, Zsolt Bertalan, Zoe Budrikis, Caterina A. M. La Porta In order to correctly divide, cells have to move all their chromosomes at the center, a process known as congression. This task is performed by the combined action of molecular motors and randomly growing and shrinking microtubules. Chromosomes are captured by growing microtubules and transported by motors using the same microtubules as tracks\footnote{Z. Bertalan et al. Navigation Strategies of Motor Proteins on Decorated Tracks PLoS One 10 e0136945 (2015)}. Coherent motion occurs as a result of a large collection of random and deterministic dynamical events. Understanding this process is important since a failure in chromosome segregation can lead to chromosomal instability one of the hallmarks of cancer. We describe this complex process in a three dimensional computational model involving thousands of microtubules. The results show that coherent and robust chromosome congression can only happen if the total number of microtubules is neither too small, nor too large. Our results allow for a coherent interpretation a variety of biological factors already associated in the past with chromosomal instability and related pathological conditions\footnote{Z. Bertalan et al. Role of the Number of Microtubules in Chromosome Segregation during Cell Divison, PLoS One, 10 e0141305 (2015)}. [Preview Abstract] |
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