Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R51: Self Organization in the Cytoskeleton IIFocus
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Sponsoring Units: DBIO Chair: Jennifer Ross, University of Massachusetts Amherst Room: LACC 511C |
Thursday, March 8, 2018 8:00AM - 8:12AM |
R51.00001: Self-Organization of Polymerizing Microtubules Jennifer Ross, Bianca Edozie, Sumon Sahu The cell is a complex autonomous machine taking in information, performing computations, and responding to the environment. To enable agile read/write capabilities, much of the molecular biochemistry that performs these computations must be transient and weak, allowing signals to be carried as a function of the concentration of numerous and coupled interactions. Traditionally, biochemical experiments can only measure strongly interacting systems that can last for long times in dilute concentrations. We have previously shown that many weak, transient interactions can have strong repercussions on the overall activity and can, in fact, overpower strongly interacting systems. Here, we will present new results on microtubule self-organization in the presence of these weak crosslinkers during polymerization. We find that the filament length and relative crosslinker concentration dictate the organization of the filaments from a percolated network to a non-connected set of tactoid-like structures. |
Thursday, March 8, 2018 8:12AM - 8:24AM |
R51.00002: Measuring and modeling polymer gradients argues that spindle microtubules regulate their own nucleation Bryan Kaye, olivia steihl, Peter Foster, Michael Shelley, Daniel Needleman, Sebastian Fuerthauer Spindle microtubules are nucleated by accessory proteins whose activity is |
Thursday, March 8, 2018 8:24AM - 8:36AM |
R51.00003: Microtubules and Motor Proteins for Synthetic Beating Cilia Isabella Guido, Smrithika Subramani, Christian Westendorf, Eberhard Bodenschatz In nature, many vital processes rely on fluid flow based transport of cargo in order to overcome the time constraint of diffusive transport. The most common transport motif involves flows driven by cilia or flagella, from removal of pollutants in the trachea to the movement of microscopic organisms in viscous fluid environments. Cilia are microscopic hair-like structures, flexible membrane extensions of the cell that present a rhythmic waving or beating motion. Each cilium comprises microtubule-doublet bundles held together by several proteins. |
Thursday, March 8, 2018 8:36AM - 8:48AM |
R51.00004: Non-canonical Actomyosin Interactions Couple a Broad Spectrum of F-actin Bending Modes Daniel Seara, Ian Linsmeier, Alan Tabatabai, Patrick Oakes, Ali Tabei, Shiladitya Banerjee, Michael Murrell The cell cortex is a 2-dimensional network made of semi-flexible F-actin polymers driven out-of-equilibrium by molecular motors. Myosin is a molecular motor protein whose canonical interactions include the relative sliding of F-actin, leading to tensile loading and contraction of the polymer network. Here, we report non-canonical actomyosin interactions in in vitro quasi-2D, stable and non-contractile actomyosin networks that result in non-equilibrium fluctuations of the endogenous polymers. Below the concentration of myosin required to contract, myosin motors induce large and anomalous deformations of F-actin polymers transverse to the F-actin long axis. These enhanced fluctuations are found to break detailed balance, indicating that myosin activity couples F-actin bending modes across a broad spectrum. These correlations cannot arise from an increased effective temperature, and the resulting active stresses are dissipated across all length scales in stable actomyosin networks. Filament curvature is thus implicated in network stability while it has previously only been associated with contractility. |
Thursday, March 8, 2018 8:48AM - 9:24AM |
R51.00005: Non-equilibrium dynamics in the actin cortex Invited Speaker: Christoph Schmidt The actin cortex of cells is a mechanically resilient but also highly active material that provides a protective shell for cells and, at the same time, drives many crucial dynamic processes. Myosin motors locally exert contractile force on the actin network which is kept in a dynamic steady state with the help of a multitude of regulatory proteins. It is not well understood how the cortex self-organizes to form complex structures and perform collective dynamic functions. |
Thursday, March 8, 2018 9:24AM - 9:36AM |
R51.00006: The Mechanical Interplay between Cell Shape and Actin Cytoskeleton Organization Koen Schakenraad, Jeremy Ernst, Wim Pomp, Thomas Schmidt, Roeland Merks, Luca Giomi The shape of adhering cells is determined by the interplay between contractile forces, arising from the cytoskeleton, and the resistance of the underlying substrate. Experiments with fibroblasts on elastic micro-pillar arrays show that fibroblasts possess a high degree of orientational order of the actin stress fibers. This anisotropy causes the shape of the cell edge and the distribution of adhesion forces to deviate from that of cells with an isotropic cytoskeleton, while at the same time this cellular shape influences the orientation of the actin stress fibers. We study this interplay between stress fiber alignment and cell shape and contractility. We theoretically describe the contractility of the cytoskeleton as a combination of directed forces, in the direction of stress fibers, and isotropic forces. We model the stress fiber organization as a competition between alignment within the cytoskeleton and alignment of the cytoskeleton with the cell edge. We compare our results to experimental data on fibroblast shape and actin cytoskeleton organization. Our work illustrates the strong coupling between anisotropy on the inside of the cell to anisotropy in shape and forces on the outside of the cell. |
Thursday, March 8, 2018 9:36AM - 9:48AM |
R51.00007: Regulation of Cytoskeletal Dynamics during T Cell Activation by Substrate Stiffness Altug Ozcelikkale, Arpita Upadhyaya T lymphocytes are an integral part of the adaptive immune response. The detection of infectious agents critically depends on the interaction of T cells with antigen presenting cells, which have varying mechanical stiffness and complex topological features. It has been recently recognized that T cell activation is regulated both by stiffness of the antigen presenting surface and by cytoskeletal forces which partially arise from actomyosin contractility. However, the relationship between stiffness and the force generating machinery driving T cell activation is not well understood. To address this problem, we characterized actin and myosin dynamics during the activation of Jurkat T cells on stimulatory elastic substrates with variable stiffness using total internal reflection and confocal fluorescence microscopy. Activated T cells exhibited lamellipodial actin and myosin flows at the cell periphery as well as lamellar rings of actomyosin bundles. We have explored the stiffness-dependent organization of these distinct actomyosin structures, flows, and their correlation with the spatiotemporal variation of traction stresses. This study brings insight into the potential role of stiffness in regulating cytoskeletal organization and force generation during T cell activation. |
Thursday, March 8, 2018 9:48AM - 10:00AM |
R51.00008: Polyelectrolyte Nature of Cytoskeleton Filaments and Their Properties Ernesto Alva Sevilla, Chris Hunley, Marcelo Marucho A variety of dynamic functions of cytoskeletal filaments, such as cellular transport, motility, etc, involve interactions with other proteins and filaments, which are often dominated by electrostatic interactions, and many times mediated and modulated by the biological environment. Little is known about how these electrostatic property differences may affect our molecular understanding in not only the function, and behavior of F-actin, but also the electrostatics governing the interaction with other proteins and membranes. We describe our preliminary theoretical and experimental findings on the biological environment effects on the electrostatic properties under a variety of electrolyte solution conditions. We implemented a full atomistic description of the molecular structure of F-actin to study the surface electric potential and the iso-surface charge density distribution. We also used a molecular solvation electrical double layer theory, developed by our group (J. Chem. Phys. 141,225103, 2014), to study the corresponding hydration and layering formation effects. We found a strong correlation between pH, electrolyte concentration, and water crowding effects, which were observed to highly impact the formation and properties of the electrical double layer around the filaments. |
Thursday, March 8, 2018 10:00AM - 10:12AM |
R51.00009: Measuring and Simulating Cellular Flows during Spindle Positioning Ehssan Nazockdast, Hai Yin Wu, Daniel Needleman, Michael Shelley
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Thursday, March 8, 2018 10:12AM - 10:24AM |
R51.00010: Catastrophic Depolymerization of Microtubules Driven by Tubulin Subunit Shape Change Jonathan Bollinger, Mark Stevens Microtubules exhibit a distinctive dynamic instability between growth and catastrophically-fast depolymerization. GTP-tubulin (a dimer bound to GTP) self-assembles, but dephosphorylation of GTP- to GDP-tubulin within the tubule results in destabilization. The molecular origins of this instability have remained unclear despite their importance for bioengineering strategies. One hypothesis is that dephosphorylation causes tubulin to change shape, frustrating bonds and generating stress. To test this idea, we perform molecular dynamics simulations of microtubules built from a new coarse-grained model of tubulin, implementing conformational changes in tubulin thought to drive the instability. We find that this shape change induces depolymerization via unpeeling ''ram's horns'' characteristic of microtubules. Depolymerization can be averted by caps with undeformed dimers, i.e., GTP-rich regions, and model microtubules exhibit mechanical responses consistent with experiments. |
Thursday, March 8, 2018 10:24AM - 10:36AM |
R51.00011: Liquid behavior and motor-induced division of actin droplets: a theoretical model Kinjal Dasbiswas, Kimberly Weirich, Thomas Witten, Margaret Gardel, Suriyanarayanan Vaikuntanathan Self-organized liquid droplets of biomaterial that grow and divide are intriguing models for cell behavior besides being novel instances of active matter. Recent experiments at the Gardel laboratory show that cross-linked, short actin filaments form elongated liquid-like domains in vitro, which are susceptible to shape change and division under myosin motor activity. We attribute the characteristic spindle shape of these droplets to the anisotropic interfacial tension originating from the underlying nematic ordering of the constituent actin filaments. Based on such a continuum picture and available experimental evidence, we propose a simple explanation for the observed droplet division by modeling self-organized clusters of motors as defect-inducing centers within the droplet of actin. This “colloid-in-nematic” type of model incorporates the interplay of droplet geometry and the binding affinity of myosin to actin. Consistent with experimental observation, the model indicates a critical strength and size of motor clusters for droplet division. We next indicate how the shape instabilities can arise from motor-induced active stresses and explore connections to a more complete hydrodynamic theory. |
Thursday, March 8, 2018 10:36AM - 10:48AM |
R51.00012: Diffusion and conformational dynamics of single DNA molecules crowded by cytoskeletal proteins Kathryn Regan, Rachel Dotterweich, Shea Ricketts, Rae Anderson The high concentrations of proteins crowding cells greatly influence intracellular DNA dynamics. These crowders, ranging from small mobile proteins to large cytoskeletal filaments such as semiflexible actin and rigid microtubules, can hinder diffusion and induce conformational changes in DNA. While previous studies have mainly focused on the effect of small mobile crowders on DNA transport, we focus instead on the role of the cytoskeleton. Specifically, we use fluorescence microscopy and custom single-molecule tracking algorithms to measure center-of-mass transport and time-varying conformational changes of single DNA molecules diffusing in in vitro networks of actin and microtubules. To determine the roles that cytoskeletal filament rigidity and size have on DNA dynamics, we vary the relative concentrations and polymerization states of actin and microtubules crowding DNA and quantify resulting DNA diffusion coefficients, degrees of anomalous diffusion, and changes to conformational size and shape. |
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