Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S21: Cellular Biomechanics |
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Sponsoring Units: DBP Chair: Alexander Neiman, Ohio University Room: LACC 409A |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S21.00001: Simulation of Actin-Polymerization-Mediated Propulsion Kun-Chun Lee, Andrea Liu An important component of the cellular cytoskeleton is F-actin, a biopolymer whose self-assembly is key to the process of cell crawling. The polymerization and branching of F-actin near the cell membrane is known to drive cell crawling, but the precise mechanism by which these processes lead to the generation of a mechanical force is still controversial. We have constructed a Brownian dynamics simulation of F-actin polymerizing near a surface, which includes all known important processes, including polymerization, depolymerization, branching, crosslinking and capping. Using this model, we study the dynamics of the moving surface in conjunction with the stresses in the system. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S21.00002: Hyaluronan-mediated cellular adhesion Jennifer Curtis, Christian Schmitz, Joachim Spatz Many cells surround themselves with a cushioning halo of polysaccharides that is further strengthened and organized by proteins. In fibroblasts and chrondrocytes, the primary component of this pericellular matrix is hyaluronan, a large linear polyanion. Hyaluronan production is linked to a variety of disease, developmental, and physiological processes. Cells manipulate the concentration of hyaluronan and hyaluronan receptors for numerous activities including modulation of cell adhesion, cell motility, and differentiation. Recent investigations by identify hyaluronan's role in mediating early-stage cell adhesion. An open question is how the cell removes the 0.5-10 micron thick pericellular matrix to allow for further mature adhesion events requiring nanometer scale separations. In this investigation, holographic optical tweezers are used to study the adhesion and viscoelastic properties of chondrocytes' pericellular matrix. Ultimately, we aim to shed further light on the spatial and temporal details of the dramatic transition from micron to nanometer gaps between the cell and its adhesive substrate. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S21.00003: A 3-D Biophysical Model of Mitotic Spindle Formation Stuart Schaffner, Jorge Jose The mitotic spindle is the scaffolding on which plant and animal cell division occurs. It is known that under certain circumstances the spindle self-assembles, using only a few functional elements. We have been developing increasingly realistic biophysical models of spindle self-assembly. Our earlier 2-D model produced spindle patterns under certain conditions. Our more realistic 3-D model is defined by coupled Langevin equations that mimic the mechanical and thermal interactions between microtubules and molecular motors. Microtubules pivot on fixed kinesin motors and are drawn into poles by dynein motors. The distribution of pivot points form boundary conditions whose spherical asymmetry guides spindle formation. Unlike the 2-D model, the 3-D model correctly handles microtubule entanglement. Initial runs of the 3-D model show that spindle self-assembly is indeed possible under certain conditions. We are currently performing calculations to determine how parameter changes affect spindle formation and pattern morphology. In particular, we are varying dynein motor processivity, the degree of spherical asymmetry, and dynein motor concentrations. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S21.00004: Self-organization of cytoskeletal systems: formation and dynamics of bundles, rings, and spindles Alexander Zumdieck, Karsten Kruse, Frank J\"ulicher The cytoskeleton is a complex network of protein filaments. Driven by active processes such as filament polymerization and depolymerization and the action of molecular motors, it represents an active system which by self-organization can form dynamic patterns and exhibit active mechanical properties. Starting from a microscopic picture, we develop a coarse grained description for the dynamics of bundles of filaments and motors in the presence of filament polymerization and depolymerization. We show that filament treadmilling in the presence of passive cross-linkers can, similarly to motor proteins, generate tensile stresses that may result in bundle contraction. Motivated by contractile rings that cleave cells during cell division we extend our description to cylindrical geometries and show that filament rings with contractile properties can form by self-organization phenomena. Furthermore we discuss the stability of bipolar spindles, taking into account the simultaneous action of several types of motor proteins. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S21.00005: MSP dynamics and retraction in nematode sperm Charles Wolgemuth, Long Miao, Orion Vanderlinde, Tom Roberts, George Oster Most eukaryotic cells can crawl over surfaces. In general, this motility requires three distinct actions: polymerization at the leading edge, adhesion to the substrate, and retraction at the rear. Recent \textit{in vitro }experiments with extracts from spermatozoa from the nematode \textit{Ascaris suum} suggest that retraction forces are generated by depolymerization of the Major Sperm Protein (MSP) cytoskeleton. Combining polymer entropy with a simple kinetic model for disassembly I propose a model for disassembly-induced retraction that fit the \textit{in vitro }experimental data. This model explains the mechanism by which deconstruction of the cytoskeleton produces the force necessary to pull the cell body forward and suggest further experiments that can test the validity of the model. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S21.00006: Untwisting the mystery of supercoiling: Mbl configuration in growing bacterial filaments Sulav Mukherjee, Barbara Setlow, Peter Setlow, Charles Wolgemuth \textit{Bacillus subtilis}, a commonly studied prokaryote form long filaments, or chains of cells, when the cells fail to separate upon replication. These mutants undergo supercoiling where the bacterial filament buckles and wraps about itself like an over-twisted phone cord. It has long been supposed that twisting stress is generated in the cell wall during growth and causes this coiling. But, the twisting mechanism has remained an enigma. A recently discovered actin-like protein, Mbl, forms helical structures under the cell wall and controls cell morphogenesis in \textit{B. subtilis}. Based on these findings, a new model suggests how these helical structures could lead to supercoiling. We report here experiments connecting growth, Mbl structure, and supercoiling. We have studied the helical pitch of the Mbl under regular growth conditions, various concentrations of xylose, and under the influence of different concentrations of ammonium and magnesium. These experiments demonstrate how growth effects the configuration of the Mbl cables and suggest that growth induced deformation of the Mbl cables generate twist in the filaments, which eventually leads to supercoiling in bacterial filaments. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S21.00007: Stochastic description of pilus retraction dynamics Martin Lind\'en, Emil Johansson, Mats Wallin, Ann-Beth Jonsson Motility of certain gram-negative bacteria is mediated by retraction of type IV pili surface filaments, which are essential for infectivity. Type IV pili are helical filaments with 4 nm periodicity and 5 subunits per turn. The retraction is powered by a strong molecular motor protein, PilT, producing very high forces in excess of 100 pN[1]. One possible explanation for the high forces are that several ATP are hydrolyzed to retract each subunit.\\ We consider a widely used class of discrete hopping models, which has been used to describe well-known motor proteins such as kinesin[2] and myosin[3]. The model describes recent experimental measurements[1] on \emph{Neisseria gonorrhoeae} well, and makes several interesting predictions for the randomness of the retraction dynamics.\medskip\\ 1. Maier et al, PNAS 101:10961 (2004)\\ 2. M. E. Fisher and A. B. Kolomeisky, PNAS 98:7748 (2001)\\ 3. A. B. Kolomeisky and M. E. Fisher, Biophys. J. 84:1650 (2003) [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S21.00008: Why nozzles are required for bacterial gliding? Junhwan Jeon, Andrey Dobrynin Many microorganisms transduce an energy stored during polymerization reactions into mechanical force propelling them over surfaces. For example, cyanobacteria has nozzles-like organelles secreting a polysaccharide gel. On the other hand, listeria propels itself through the cell by polymerizing a network of actin filaments from its surface. The actin filaments have a persistence length of the order of $2-10\mu m$ leading to the high value of Young's modulus ($10^3-10^4$Pa). The polysaccharide gel has lower shear modulus ($10^2$Pa) than that of the actin gel and does not have sufficient strength to support compression and propel bacteria. In this case nozzles play a role of the compression chambers that improve the elastic properties of a gel resulting in bacteria translocation. To elucidate the effect of chain rigidity on bacterial motility, we performed molecular dynamics simulations of crosslinked semiflexible polymers growing inside a nozzle-like orgenelle and from the surface of a bacteria. We have analyzed the correlation between object's velocity and the chain stiffness. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S21.00009: Force Regulation in Living Tissue Yusuke Toyama, Xomalin G. Peralta, Stephanos Venakides, Daniel P. Kiehart, Glenn S. Edwards Forces within tissue are involved in the shaping of an embryo. Measuring the net forces exerted by groups of cells and tissues in live organisms is a challenging endeavor. Quantitative physical modeling based on experimental results has the potential for increasing our understanding of this question. We use a developmental stage in Drosophila embryo known as dorsal closure, which to a large extent occurs in two dimensions and is a consequence of four biological processes that are synchronized in time and coordinated in space [1]. To reveal the interaction of forces and tensions within the components of these tissues, we use a steerable UV-laser microbeam to cut them and monitor the resulting behavior with confocal microscopy. We present experimental evidence for an increase in the force that could be a consequence of the laser cut supported by a quantitative model and a biological mutant. [1] M. S. Hutson, et al. Science, 300, 145 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S21.00010: The TITS Algorithm: A Simple and Robust Method for Calculating Stable Shapes of Axisymmetric Vesicles Gerald Lim I have implemented a simple and robust numerical technique for calculating axisymmetric equilibrium shapes of one-component lipid bilayer vesicles. This so-called Tethered Infinitesimal Tori and Spheres (TITS) Algorithm gives shapes that are automatically stable with respect to axisymmetric perturbations. The latest version of this algorithm can, but is not restricted to, impose constraints on any of three geometrical quantities: the area, volume and pole-to-pole distance (in the case of tether formation). In this talk, I will introduce the basic principles of the TITS Algorithm and demonstrate its versatility through a few example shape calculations involving the Helfrich and Area Difference Elasticity bending free energies. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S21.00011: Microinterferometric demonstration of actively driven bending excitations of the cell envelope of mouse macrophages. Alexandra Zidovska, Erich Sackmann We observed pronounced undulation excitations of cell envelope of weakly adhering macrophages. This so called flickering of the cell membrane gives rise to strong entropic disjoining pressures which are consequence of freezing in long wavelength undulations of adhering cells. Membrane fluctuations were analyzed by Reflection Interference Contrast Microscopy (RICM) with $\sim $ 0.3 micron lateral and $\sim $ 1 nm vertical resolution. Under physiological conditions we observe amplitudes of 8-10 nm corresponding to apparent bending moduli of the order of $\kappa \quad \sim $ 1000 k$_{B}$T. This anomously small bending stiffness very strongly suggests that the excitations are driven by fluctuating biochemical forces. Latrunculin (an actin polymerization blocker) causes softening of the cell envelope resulting in an increase of the membrane undulations amplitude up to 20 nm. Sequestering of intracellular Ca$^{++}$ by the chelator BAPTA leads to a lowering of the membrane fluctuation amplitude to 5-7 nm. The apoptosis inducing agent camptothecin induced strong reductions of the fluctuating amplitudes to 3-4 nm. The capillary length and elastic moduli were determined using the discrete Fourier Transformation. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S21.00012: Dynamics of Caveolae in Endothelial Cells Meron Mengistu, Linda Lowe-Krentz, H. Daniel Ou-Yang The blood flow subjects endothelial cells to various shear stress conditions, regulating the formation and localization of caveolae for macromolecular transport and potentially mechanosensing. We simulate this condition by exposing cultured bovine endothelial cells to various flow conditions in flow chambers. Using GFP-constructs of caveolar markers such as caveolin-1, dynamin II, and intersectin, we targeted caveolae with optical tweezers laser as probes to measure changes in viscoelastic properties that the cell undergoes in the different flow conditions. We also tracked the transport of fluorescently labeled Bovine serum albumin (BSA) through caveolae using confocal microscopy. This technique allows us to study the transport dynamics of caveolae once they are internalized in endothelial cells. Integrating optical tweezers and confocal fluorescence microscopy will allow us to measure the micro-mechanical properties of caveolae and give us insights into its function as a mechanosensor as well as its role in transcytosis. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S21.00013: Cellular tolerance to pulsed heating Dmitri Simanovskii, Daniel Palanker, Alan Schwettman, Mainak Sarkar, Afraz Irani, Caitlin C. O'Connell-Rodwell, Christopher Contag In many medical applications knowledge about the threshold temperature leading to irreversible cellular damage is critically important. We study the dependence of the threshold temperature on duration of the heat exposure in the range of 0.3 ms to 1 second. Thin layer of cells cultured in a Petri dish was exposed to a pulsed CO$_{2}$ laser radiation. Laser beam was focused onto a surface of Petri dish providing Gaussian intensity distribution in the focal plane with a typical beam diameter (2w) 10 mm. Surface temperature in the central part of the focal spot (1mm in diameter) was measured by thermal IR emission from the sample recorded with a fast (ns) MCT detector. For pulses shorter than 1 s the temperature profile across the focal spot was found to closely correspond to the radial distribution of the laser beam, thus allowing for accurate determination of temperature at any given distance from the center of the spot. Immediate cellular damage was assessed using vital staining with the live/dead fluorescent assay. Threshold temperatures were found to vary from 55 $^{o}$C at 1 s of heating to 160 $^{o}$C at pulses of 0.3 ms in duration. The shorter end of this range was limited by vaporization which occurs during the laser pulse and results in mechanical damage to cells. [Preview Abstract] |
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S21.00014: A physical basis for actin filament lengths in cytoskeletal networks: Dennis Discher, Paul Dalhaimer, Tom Lubensky Actin filaments are typically crosslinked in cells by highly flexible proteins, leading to a wide range of cytoskeletal structures or microphases. Two-dimensional or membrane networks composed of actin and spectrin family proteins have been particularly well-characterized in both the red blood cell (RBC) -- where actin filaments are short -- and the outer hair cell (OHC) of the inner ear, where the filaments are long. General aspects of the phase behavior and anisotropic elasticity of these two systems are addressed here by simulating actin-like rods in two-dimensions crosslinked by a soft overlying network of spectrin-like chains. With short rods (per RBC), networks become glassy with compression whereas longer rods transition to a nematic phase. At zero applied pressure, a locked-in or quenched nematic emerges when actin length equals or exceeds crosslinker length (per OHC). Applying tension to quenched nematic states further reveals a soft response in the direction perpendicular to the director -- the direction of sound propagation through the OHC. Properties such as isotropic surface elasticity of RBC and directional elasticity in OHC would seem respectively useful in flow through blood capillaries and sound propagation in the ear and thus follow from disparate actin filament lengths crosslinked at suitable densities. [Preview Abstract] |
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