Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P34: Focus Session: Cytoskeletal Dynamics and Cell Migration I |
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Sponsoring Units: DBP GSNP DPOLY Chair: Margaret Gardel, Scripps Institute Room: Colorado Convention Center 404 |
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P34.00001: Integration of actin dynamics and adhesion in cell migration Invited Speaker: Cell migration requires transmission of motion generated in the actin cytoskeleton to the extracellular environment through a complex assembly of proteins in focal adhesions. We developed Correlational Fluorescent Speckle Microscopy to measure the coupling of focal adhesion proteins to actin filaments. Different classes of focal adhesion structural and regulatory molecules exhibited varying degrees of correlated motions with actin filaments, indicating hierarchical transmission of actin motion through focal adhesions. Interactions between vinculin, talin and actin filaments appear to constitute a slippage interface between the cytoskeleton and integrins, generating a molecular clutch that is regulated during the morphodynamic transitions of cell migration. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P34.00002: The Translation of Actin Dynamics into Traction Force via Focal Adhesions in Migrating Cells Margaret Gardel, Benedikt Sabass, Lin Ji, Ulrich Schwarz, Clare Waterman Forces are generated in the actin cytoskeleton by myosin-II motors and transmitted to the extracellular matrix (ECM) via dynamic macromolecular assemblies called focal adhesions (FA). To explore how forces are transmitted from the contractile actomyosin network to the ECM, we combine traction force microscopy and fluorescent speckle microscopy (FSM) of FAs and actin cytoskeleton in Ptk1 epithelial cells. We find that the relationship between intracellular actin flow and traction force is spatially segregated within individual focal adhesions. Near the leading edge, actin flow is inversely related to force, while towards the cell center, there is a positive correlation. This change is regulated by small GTPase signal transduction pathways and myosin II motor based contraction. Thus, the FA is a molecular clutch that exhibits regulatory switching between different coupling mechanisms. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P34.00003: Modeling and imaging the topography of nascent adhesions. Erdinc Altigan, David Entenberg, Ben Ovryn We have developed a model to explain the initiation of adhesions on the ventral surface of a cell. An analysis of the energetics of membrane bending and the effects of a composite system of freely diffusing repellers and receptors and a fixed network of ligands on the extracellular matrix demonstrates that a small bundle of actin filaments is able to push the membrane down to the extracellular matrix and nucleate a nascent adhesion. This model is consistent with experiments which demonstrate that cell motility requires cycles of actin polymerization and depolymerization at the leading edge of cell protrusions; the leading lamella adheres to the extracellular matrix and stable focal contacts form which can resist strong contractile forces. Although several of the mechanisms responsible for focal contact formation have been elucidated, the detailed processes leading to the formation of the earliest adhesions have remained elusive. Based upon the energetics of adhesion formation, our model predicts the shape of the membrane at the nucleated adhesion. We have developed a novel form of confocal interference microscopy to measure the distance between the ventral surface of a cell and the substratum with several nanometer precision and we have measured the topography of focal adhesions. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P34.00004: Actin-Filamin Networks and Cell Mechanics Karen Kasza, Fumihiko Nakamura, Thomas Stossel, Ning Wang, David Weitz We seek to elucidate the mechanisms underlying stress dependent stiffening of the cellular cytoskeleton. Filamin A (FLNa) is a protein that cross-links and bundles actin filaments into soft gels that stiffen dramatically with applied mechanical stress. Living cells show similar stiffening behavior, but the underlying physical mechanism is poorly understood. While it is known that FLNa plays an important \textit{biological} role in some very mechanical cellular processes, it is still unclear whether FLNa plays such a dominant \textit{mechanical} role in the cell as it does in simple reconstituted actin networks. Here, we work with a human melanoma cell line deficient in FLNa and a transfected subline expressing FLNa. For both cell lines, we probe cell stiffness measured by magnetic twisting cytometry as we increase the stress supported by the actin cytoskeleton to determine the contribution of FLNa to both the linear and nonlinear material properties of the cell cytoskeleton. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P34.00005: Critical state enhances cross-linker denaturation under stress in biopolymer networks Brian DiDonna, Alex J. Levine We report on the statistical behavior of cross-linker molecules containing numerous unfolding domains when they are used to bind a random semiflexible polymer network. Cross-linkers with unfolding domains are ubiquitous in the F-actin component of the cytoskeleton - examples include filamin and a-actinin. We show, through mean field calculations and simulations, that under tension the cross-linkers naturally organize into a critical state which greatly enhances their propensity to unfold. Unfolding of cross-links could play a role in stress-regulation and mechanotransduction. The critical state is characterized by an exponential or faster growth in the population of cross-linkers as a function of tension up to a characteristic unfolding tension. This critical state should occur at physiologically relevant stress levels in any open random network built with such cross-linkers. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P34.00006: Molecular motor-induced instabilities and crosslinkers determine biopolymer organization David Smith, Falko Ziebert, David Humphrey, Cynthia Duggan, Walter Zimmermann, Josef Kaes All eukaryotic cells rely on the active self-organization of protein filaments to form a responsive intracellular cytoskeleton. The need for motility and reaction to stimuli additionally requires pathways that quickly and reversibly change cytoskeletal organization. While thermally-driven order-disorder transitions are, from the viewpoint of physics, the most obvious method for controlling such organization, the timescales necessary for effective cellular dynamics would require temperatures exceeding the physiologically viable temperature range. We report a mechanism whereby myosin II can cause near-instantaneous order-disorder transitions in reconstituted cytoskeletal actin solutions. When motor-induced filament sliding diminishes, the actin network structure rapidly and reversibly self-organizes into various assemblies. Addition of stable crosslinkers was found to alter the architecture of ordered assemblies. These isothermal transitions between dynamic disorder and self-assembled ordered states illustrate that the interplay between passive crosslinking and molecular motor activity plays a substantial role in dynamic cellular organization. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P34.00007: Instabilities in filament-motor solutions with crosslinkers. Falko Ziebert, Ronny Peter, Walter Zimmermann Filament-motor systems are in nonequilibrium due to the energy consumption during motor movement (via ATP hydrolysis), and thus display pattern and structure formation. We report on simple mesoscopic modeling based on conservation laws with active filament currents. We discuss instabilities in a recent experiment on actomyosin, where ATP is depleted in the presence of a small amount of crosslinker proteins. In the limit of high density of crosslinkers, we propose a model where transported filaments are coupled to an elastic crosslinked network, leading to oscillatory behavior. \newline References: \newline D. Smith, F. Ziebert, D. Humphrey, C. Duggan, W. Zimmermann and J. Kaes, submitted to Biophys. J. ; R. Peter, F. Ziebert and W. Zimmermann, submitted to Europhys. Lett. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P34.00008: Interaction of Semi-flexible Filaments and Molecular Motors Dmitry Karpeev, Igor Aronson, Lev Tsimring, Hans Kaper We consider effects of finite flexibility on interaction of two microtubules with molecular motor. On the basis of numerical solution to nonlinear elasticity equation we show that the flexibility enhances tendency of microtubules to align, which, in turn, favors formation of large-scale structures in the multi-tubules system. Moreover, for much softer filaments, like actin, we observed that the action of the motor may result in formation of multiple loops due to buckling of the filaments. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P34.00009: Effective medium theory of semiflexible filamentous networks Moumita Das, Alex J. Levine, F.C. MacKintosh We develop an effective medium approach to the mechanics of disordered, semiflexible polymer networks such as those forming the cytoskeleton and study their response to both spatially uniform and nonuniform strain. We identify distinct elastic regimes in which the effective filament bending stiffness or stretch modulus vanishes. We also show that our effective medium theory predicts a crossover between affine and non-affine strain, consistent with both prior numerical studies and scaling theory. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P34.00010: Forced shape deformations of interfaces and biopolymer networks Wolfgang Losert, Andrew Pomerance, Cory Poole, Erin Rericha What sets the characteristic length and timescale of shape deformations of motile cells? To investigate possible contributions to these scales, we investigate shape deformations of biopolymer networks and lipid bilayers, two key components of motile cells. Controlled deformations are generated with holographic optical tweezers and detected optically. We observe that for small deformation lengths of up to 4 microns (for cage sizes less than one micron) and short time deformations of order seconds, actin networks respond mostly elastically. We see evidence of coupling between two nearby deformation fields in an actin network. Relaxations of directly forced giant unilamellar vesicles reveal that -during free relaxation- apparent membrane stresses remain localized on micron scales. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P34.00011: Viscoelasticity and rheology of a suspension of active filaments M. Cristina Marchetti, Tanniemola B. Liverpool We study the viscoelasticity of an active solution of polar biofilaments and motor proteins under an externally imposed stress. Adapting methods from polymer physics, we derive the constitutive equations for the stress tensor in the isotropic phase and in phases with liquid crystalline order (nematic and polarized). The stress relaxation in the various phases is discussed. Activity is responsible for a strong enhancement (a divergence in 2d) of the viscosity at the isotropic-nematic transition. This behavior is reminiscent of an equilibrium liquid-solid transition rather than a liquid-liquid transition, and is a direct consequence of contractile bundling. A second signature of activity is found in the nematic phase, where the stress tensor acquires a nonequilibrium contribution proportional to ATP (Adenosine Tri-Phosphate) consumption rate that remains finite in the absence of imposed mechanical deformation. The role of boundaries on these phenomena will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P34.00012: Dynamics and statistical mechanics of semiflexible polymer bundles Claus Heussinger, Mark Bathe, Erwin Frey Bundles formed from semiflexible polymers are ubiquitous in nature (e.g. filopodia) and many areas of technology (e.g. carbon nanotube bundles). Despite their simple structure, their mechanical and dynamical properties are only poorly understood. We set up an elastic energy functional that allows characterizing the dynamical and statistical mechanical properties of polymer bundles, in much the same way as the standard worm-like chain model (WLC) does for single polymers. The key result of our analysis is that bundles must be characterized by a wave-number dependent persistence length $l_p(q)$ instead of just a single $q$-independent value. This finding is shown to have dramatic consequences not only on the static and dynamic fluctuation spectrum of an isolated bundle but also on the scaling behaviour of their entangled solutions as well as their cross-linked networks. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P34.00013: Dynamic Control of F-actin Polymerization Using Electrical Interfaces Ian Y. Wong, Matthew J. Footer, Nicholas A. Melosh The cytoskeletal biopolymer F-actin plays a crucial role in the mechanics and motility of eukaryotic cells and is also a model system for the investigation of the physics of semiflexible polymers. Historically, the polymerization of reconstituted F-actin has been initiated in vitro by increasing the bulk ion concentration from reduced to physiological levels. In this work, nanoscale electrodes are used to achieve spatial and temporal control of F-actin polymerization. The application of a low-frequency AC voltage alternately concentrates divalent cations and negatively charged G-actin monomers at the electrode surface, promoting highly localized polymerization. Unlike bulk polymerization, the kinetics of this electronically activated polymerization are governed by two competing mechanisms: ionic activation through Mg$^{2+}$ binding and nucleation of actin trimers. Additional control can be achieved through the superposition of a high-frequency AC signal to align and trap filaments through dielectrophoresis. This combination of low and high frequency AC voltages may allow for the dynamic assembly of nanostructures with precisely controlled size and registry. [Preview Abstract] |
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