Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P16: Focus Session: Cytoskeletal Dynamics and Cell Motility I |
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Sponsoring Units: DBP DPOLY DFD Chair: Greg Huber, University of Connecticut Health Center Room: Morial Convention Center 208 |
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P16.00001: Actin Disassembly Mediated by Severing, Debranching, and Hydrolysis Invited Speaker: For cells to respond effectively to their environment, the actin cytoskeleton must both assemble and disassemble rapidly in the presence of external cues. A great deal of theory has been focused on assembly, but disassembly has so far received less attention. The talk will describe two theoretical treatments of actin disassembly resulting from debranching, severing, and ATP hydrolysis. 1) The dynamics of \textit{in vitro} actin polymerization caused by filament branching or severing. Via a combination of stochastic-growth simulation and analytic theory, we show that highly branched structures such as those found near the edges of cells cannot persist in steady state. Early in polymerization, highly branched structures form, but disassemble over time leaving very few branched filaments. This causes an overshoot in light scattering intensity as a function of time. Inclusion of the effects of ATP hydrolysis shows that hydrolysis causes an overshoot in the amount of polymerized actin which can be observed in pyrene fluorescence experiments. 2) The interaction between severing and annealing in disassembling a model lamellipodial actin network. The network is treated as a periodic array of crosslinked actin filaments which sever randomly. The lamellipodial actin density drops abruptly as a function of distance from the membrane in the absence of annealing. When annealing is included, the drop is more gradual, and at a critical value of the annealing rate the thickness becomes infinite. It is shown that lamellipodial disassembly is controlled by two characteristic times: the time that a single subunit remains in the network, and the time that it takes for actin polymerized at the membrane to move to the edge of the lamellipodium. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P16.00002: A Possible Role for a Viscous Fingering-Type Instability in Cell Motility Andrew Callan-Jones, Jean-Francois Joanny, Jacques Prost We present a novel flow instability that can arise in thin films of cytoskeletal fluids if the friction with the substrate on which the film lies is sufficiently strong. The motivation for this work are the experiments of Verkhovsky et al. (Verkhovsky et al, Curr. Biol., 9: 11-20 (1999)) in which flat, circular, stationary cell fragments on a substrate, containing only actin and myosin motors, can either spontaneously or under applied force change shape and start moving. In the stationary state in our model, actin polymerizes at the fragment edge and depolymerizes uniformly in the bulk. The initial velocity profile is radial and is imposed by mass conservation for constant polymer density. The radius of the fragment is fixed by conservation of total --- monomer and filamentous---actin. Performing a linear stability analysis of the actin velocity due to perturbations of the fragment boundary, we find that as the dimensionless parameter $\frac{\eta}{\xi R_0^2}\rightarrow 0$, where $\xi$ is the actin-substrate friction, $\eta$ is the viscosity, and $R_0$ is the initial fragment radius, the perturbed velocity obeys a Darcy Law, and combined with the force-free condition at the fragment boundary, this leads identically to a viscous fingering instability. This asymptotic limit should be achievable since $R_0$ can be tuned by making a fragment with enough actin. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P16.00003: The Stochastic Dynamics of Filopodial Growth Garegin A. Papoian, Yueheng Lan, Pavel Zhuravlev A filopodium is a cytoplasmic projection, exquisitely built and regulated, which extends from the leading edge of the migrating cell, exploring the cell's neighborhood. Commonly, filopodia grow and retract after their initiation, exhibiting rich dynamical behaviors. We model the growth of a filopodium based on a stochastic description which incorporates mechanical, physical and biochemical components. Our model provides a full stochastic treatment of the actin monomer diffusion and polymerization of each individual actin filament under stress of the fluctuating membrane. We have investigated the length distribution of individual filaments in a growing filopodium and studied how it depends on various physical parameters. The distribution of filament lengths turned out to be narrow, which we explained by the negative feedback created by the membrane load and monomeric G-actin gradient. We also discovered that filopodial growth is strongly diminished upon increasing retrograde flow, suggesting that regulating the retrograde flow rate would be a highly efficient way to control filopodial extension dynamics. The filopodial length increases as the membrane fluctuations decrease, which we attributed to the unequal loading of the mem- brane force among individual filaments, which, in turn, results in larger average polymerization rates. We also observed significant diffusional noise of G-actin monomers, which leads to smaller G-actin flux along the filopodial tube compared with the prediction using the diffusion equation. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P16.00004: Mechanics of Lamellipodia D. A. Quint, J. M. Schwarz The actin cytoskeleton is a morphologically-complex assembly of cross-linked F-actin filaments. The cytoskeleton provides rigidity for the cell within appropriate time scales so that it can change its shape to, for example, crawl along surfaces. In addition to cross-linking proteins, many other proteins are involved in the assembly of the actin cytoskeleton such as branching proteins, capping proteins, and severing proteins. Presumably these proteins work cooperatively toward the dynamic formation of rigidity. We will initially focus on the role of branching proteins. The F-actin filaments in lamellipodia---protrusions of the mobile edge of a crawling cell---have some overall orientation due to the branching. Branched filaments emerge at a 70 degree angle from the mother filament's growing end.$^1$ This overall orientation is modelled as an anisotropy in an effective medium theory determining the cytoskeleton's elasticity in the static regime. The potential for a splay rigid phase, in addition to a rigid phase, is also investigated. \\ $^1$T. M. Svitkina and G. G. Borisy, {\it J. Cell Biol.} {\bf 145}, 1009 (1999). [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P16.00005: Assembly Mechanism of the Contractile Ring for Cytokinesis by Fission Yeast Dimitrios Vavylonis, Jian-Qiu Wu, Xiaolei Huang, Ben O'Shaughnessy, Thomas Pollard Animals and fungi assemble a contractile ring of actin filaments and the motor protein myosin to separate into individual daughter cells during cytokinesis. We studied the mechanism of contractile ring assembly in fission yeast with high time resolution confocal microscopy, computational image analysis methods, and numerical simulations. Approximately 63 nodes containing myosin, broadly distributed around the cell equator, assembled into a ring through stochastic motions, making many starts, stops, and changes of direction as they condense into a ring. Estimates of node friction coefficients from the mean square displacement of stationary nodes imply forces for node movement are greater than $\sim $ 4 pN, similarly to forces by a few molecular motors. Skeletonization and topology analysis of images of cells expressing fluorescent actin filament markers showed transient linear elements extending in all directions from myosin nodes and establishing connections among them. We propose a model with traction between nodes depending on transient connections established by stochastic search and capture (``search, capture, pull and release''). Numerical simulations of the model using parameter values obtained from experiment succesfully condense nodes into a continuous ring. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P16.00006: Nonlinear elasticity of composite networks of stiff biopolymers with flexible linkers Chase Broedersz, C. Storm, F.C. MacKintosh Motivated by recent experiments showing novel rheological properties of biopolymer networks, we develop an effective medium theory for rigid filaments cross-linked by flexible linkers. Specifically, we treat such a network as a collection of randomly oriented stiff polymers mechanically connected by highly compliant cross-linkers to an elastic continuum, which effectively represents the surrounding network. For cross-links with a finite compliance, we find a smooth cross-over between two distinct elastic regimes. Starting from a linear elastic regime dominated by cross-link elasticity, the network begins to stiffen significantly as the cross-links reach full compliance. We extend this model to a self-consistent one, in which the effective medium reflects the non-linear elastic properties of the cross-linked network. This model yields a cross-over to a nonlinear regime that is consistent with recent experimental studies of the cellular cytoskeletal polymer F-actinwith filamin cross-links$^1$. \par\vspace*{0.2cm} $1.$ \hspace*{0.25cm} ML Gardel, F Nakamura, J Hartwig JC Crocker, TP Stossel, DA Weitz, \textbf{103}, 1762 Proc. Nat. Ac. Sci. (2006). [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P16.00007: Effects of Osmotic Force and Torque on Microtubule Bundling and Pattern Formation Yongxing Guo, Yifeng Liu, Rudolf Oldenbourg, Jay Tang, James Valles We report the effect of Polyethylene Glycol (PEG, MW=35kd) on microtubule bundling and pattern formation. Without PEG, polymerizing microtubule (MT) solutions of a few mg/ml [1,2] can spontaneously form striated birefringence patterns through MT alignment, bundling and buckling in coordination. With PEG, bundles become more distinct and the birefringence pattern weakens. Using quantitative birefringence measurements, the average number of MTs in the cross section of a bundle induced by 1{\%} w/w PEG 35kd is determined to be around 26, with a wide spread in size. The amplitude of the buckling is reduced with increased PEG concentration. At sufficiently high PEG concentration ($\sim $0.5{\%} w/w), the pattern is totally suppressed and the sample contracts laterally during the development of a microtubule bundle network. We propose that the decrease of the buckling amplitude is due to the depletion of the dispersed MT network, which is essential for the pattern formation. We attribute the anisotropic contraction to an osmotic torque that drives bundles that cross to align. [1] Y. Liu, \textit{et al}., PNAS 103, 10654 (2006). [2] Y. Guo, \textit{et al}., PRL 98, 198103 (2007). [Supported by NASA (NNA04CC57G, NAG3-2882) and NSF (DMR 0405156)] [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P16.00008: Buckling and force propagation in intracellular microtubules Moumita Das, Alex J. Levine, F.C. MacKintosh Motivated by recent experiments [1] showing the buckling of microtubules in cells, we study theoretically the mechanical response of, and force propagation along elastic filaments embedded in a non-linear elastic medium. We find that embedded microtubules buckle when their compressive load exceeds a critical value $f_c$ which is two orders of magnitude larger than for an isolated MT as found earlier [1], and that the resulting deformation is restricted to a penetration depth that depends on both the non-linear material properties of the surrounding cytoskeleton, as well as the direct coupling of the microtubule to the cytoskeleton possibly through MT-associating proteins (MAPS). The deformation amplitude depends on the applied load $f > f_c$ as $(f-f_c)^{1/2}$. This work shows how the range of compressive force transmission by microtubules can be as large as tens of microns, and is governed by the mechanical coupling to the surrounding cytoskeleton. \newline References: \newline [1] CP Brangwynne et al., J. Cell Biology, 173, 733 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P16.00009: Hydrodynamic tether extrusion from ``gelly'' vesicles Karine Guevorkian, Sebastien Kremer, Francoise Brochard-Wyart Extrusion of cell tethers requires the detachment of the plasma membrane and can be used to probe the strength of membrane-cytoskeleton adhesion. We have studied the hydrodynamic extrusion of tethers from red blood cells [1] and developed a theoretical model based on permeation of lipids through the network of membrane proteins linked to the cytoskeleton [2]. Our aim here is to probe the model on biomimetic systems, namely lipid vesicles filled with artificial cytoskeleton made of synthetic or biological gels, where we can adjust the membrane-cytoskeleton coupling. The properties of tubes extruded from these ``gelly'' vesicles will be compared to simple vesicles on one hand, and to red blood cells or human carcinoid BON cells on the other. [1] N. Borghi et al, Biophys. J. 93 (2007) [2] F. Brochard-Wyart, et al, Proc. Natl. Acad. Sci. USA, 103 (2006) [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P16.00010: Living Microlens Arrays Jessica Zimberlin, Patricia Wadsworth, Alfred Crosby Using the properties of living cells and early tissue formation, we define adaptable surface structures of three-dimensional, hexagonal arrays of microlenses. These ``living'' microlenses are achieved by growing a monolayer cell sheet on a thin film of polystyrene [PS] attached to a substrate of crosslinked poly(dimethyl siloxane) [PDMS] microwells. The contractile nature of the cells attached to the surface and the compliance of the PDMS surface geometry allows the PS thin film to buckle, forming arrays of convex microlenses. The curvature of the microlens structures is related to the strain applied by monolayer cell sheets to the PS surface. We use this measurement to differentiate the strains applied by two different cell types and relate these strains to differences in the intercellular coupling of the different cell types. We also show that by adding different chemical triggers to the system, the contractile nature of the cells changes, modifying the focal length of the microlenses. This design introduces a new paradigm for advanced materials and offers great promise for a range of applications. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P16.00011: Local viscoelasticity of the surfaces of individual Gram-negative bacterial cells measured using atomic force microscopy Virginia Vadillo-Rodriguez, Terry Beveridge, John Dutcher The cell wall of Gram-negative bacteria performs many important biological functions: it plays a structural role, it allows the selective movement of molecules across itself, and it allows for growth and division. These functions not only suggest that the cell wall is dynamic, but that its mechanical properties are very important. We have used a novel, AFM-based approach to probe the mechanical properties of single bacterial cells by applying a constant compressive force to the cell under physiological conditions while measuring the time-dependent displacement (creep) of the AFM tip due to the viscoelastic properties of the cell. For these experiments, we chose a representative Gram-negative bacterium, \textit{P. aeruginosa} PAO1, and we used AFM tips of different size and geometry. We find that the cell response is well described by a three element mechanical model with an effective cell spring constant $k$ and an effective time constant $\tau $ for the creep motion. Adding glutaraldehyde, which increases the covalent bonding of the cell surface, produced a significant increase in $k$ and a significant decrease in $\tau $. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P16.00012: Stall Force and Response of Lung Cilia Richard Superfine, David Hill, Vinay Swaminathan, E. Timothy O'Brien, Ric Boucher, Brian Button, Ashley Estes We report on the response of lung cilia to applied forces. We have applied magnetic forces to magnetic beads attached to individual human lung cilia in cell cultures. Our magnetic system is capable of generating large forces ($\sim $1nanoNewton on 1 micron beads) with a 3kHz bandwidth. We record the cilia beat motion using video microscopy to record beat frequency and amplitude as a function of applied force. We present three major findings. First, the stall force is approximately 150 pN. Second the frequency is unchanged by the application of forces up to the stall point. Third, the speed of the beat motion slows down according to the diminution of the beat amplitude while maintaining a constant frequency and the speed of the motion is the same whether the beat direction is in the same direction as the applied force or against the applied force. [Preview Abstract] |
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