Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B39: Cellular Biomechanics II |
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Sponsoring Units: DBP Chair: Arpita Upadhyaya, University of Maryland Room: 411 |
Monday, March 16, 2009 11:15AM - 11:27AM |
B39.00001: Fibroblast motility on substrates with different rigidities: modeling approach Maria Gracheva, Irina Dokukina We develop a discrete model for cell locomotion on substrates with different rigidities and simulate experiments described in Lo, Wang, Dembo, Wang (2000) ``Cell movement is guided by the rigidity of the substrate'', Biophys. J. 79: 144-152. In these experiments fibroblasts were planted on a substrate with a step rigidity and showed preference for locomotion over stiffer side of the substrate when approaches the boundary between the soft and the stiff sides of the substrate. The model reproduces experimentally observed behavior of fibroblasts. In particular, we are able to show with our model how cell characteristics (such as cell length, shape, area and speed) change during cell crawling through the ``soft-stiff'' substrate boundary. Also, our model suggests the temporary increase of both cell speed and area in that very moment when cell leaves soft side of substrate. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B39.00002: Locomotion of C. elegans through jammed granular media Kevin Lu, Paulo E. Arratia It is quantitatively demonstrated in this experiment on the undulatory swimming of $C.$ (\textit{Caenorhabditis}) elegans that, in a highly-resistive media, the animal only executes beating frequencies and amplitudes in discrete values. This behavior of $C.$ elegans is inferred from the peaks in the particle velocity distributions where the most probable velocities match the transverse velocities of the nematode body. The behavior in the velocity distribution is more pronounced for particles in denser arrangements and for those closer to the thrashing gait of the worm. These results contribute to the existing data on the worm locomotion and further facilitate the identification of the endogenous genes and neural circuitry to the exogenous behavioral responses of $C. $elegans. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B39.00003: Mechanics of an Ultrafast Cellular Contraction Gaurav Misra, Richard B. Dickinson, Tony Ladd \textit{Vorticella Convallaria} is one of a class of fast-moving organisms, traversing its body size in less than a millisecond. It has two main parts, the cell body and a stalk, which attaches the cell body to the substrate. The stalk houses a slender, elastic structure called Spasmoneme, which winds helically inside the stalk and generates a strong tensile force in response to Calcium signaling. We are developing numerical simulations of the collapsing stalk to quantify the magnitude and time scale of the force generation. We have coupled a Kirchhoff model of an elastic rod (representing the stalk) with an embedded helically wound filament (representing the Spasmoneme). Contraction of this assembly is driven by a constant velocity Calcium signal that induces a state of tension in the Spasmoneme. Depending on the speed of the Calcium signal, we observe different mechanical responses from the contracting stalk, which we compare with experimental observations. We follow the interplay of contraction, twist and bend to explain some unexpected features of the retraction process. Two different macroscopic models have been proposed to explain the time-dependent velocity of the cell body; we compare the predictions of these models with the dynamics revealed by our filament model. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B39.00004: Polymer Microlenses for Quantifying Cell Sheet Mechanics Guillaume Miquelard-Garnier, Jessica Zimberlin, Patricia Wadsworth, Alfred Crosby Mechanical interactions between individual cells and their substrate have been studied extensively over the past decade; however, our understanding of how these interactions change as cells interact with neighboring cells in the development of a cell sheet, or early stage tissue, is less developed. We present a recently developed experimental technique for quantifying the mechanics of confluent cell sheets (Zimberlin J.A., et al., Cell Motility and the Cytoskeleton, 65, 9, 762). Living cells are cultured on a thin film of polystyrene [PS], which is attached to a patterned substrate of crosslinked poly(dimethyl siloxane) microwells. As the cell sheet grows, cells apply sufficient force to buckle the PS film over individual microwells to form a microlens array. The curvature for each microlens is measured by confocal microscopy and can be related to the strain and stress applied by the cell sheet. We demonstrate that this technique can be used to decouple mechanical contributions of intercellular junctions and focal adhesions while also providing insight into the important materials properties and length scales that govern cell sheet responses. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B39.00005: Why is Actin Patchy? Anders Carlsson The intracellular protein actin, by reversibly polymerizing into filaments, generates forces for motion and shape changes of many types of biological cells. Fluorescence imaging studies show that actin often occurs in the form of localized patches of size roughly one micrometer at the cell membrane. Patch formation is most prevalent when the free-actin concentration is low. I investigate possible mechanisms for the formation of actin patches by numerically simulating the ``dendritic nucleation'' model of actin network growth. The simulations include filament growth, capping, branching, severing, and debranching. The attachment of membrane-bound activators to actin filaments, and subsequent membrane diffusion of unattached activators, are also included. It is found that as the actin concentration increases from zero, the actin occurs in patches at lower actin concentrations, and the size of the patches increases with increasing actin concentration. At a critical value of the actin concentration, the system undergoes a transition to complete coverage. The results are interpreted within the framework of reaction-diffusion equations in two dimensions. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B39.00006: Direct dynamical measurement of the cytoskeletal contribution to the adhesion and mechanics of living cells Marie-Josee Colbert, Cecile Fradin, Kari Dalnoki-Veress The cytoskeleton is involved in the interaction of the cell with its surroundings through adhesion and the elastic response of the cell. To dynamically probe these properties, we have developed a new tool that takes advantage of an `L' shaped micropipette to micromanipulate a single cell and put it in contact with an adhesive surface mounted on a translation stage. The spring constant of the micropipette is carefully measured and its deflection is used to apply a calibrated force. This technique gives access to real time monitoring of the cell response to an applied deformation, thus exploring the relaxation processes of the cell when subjected to an external load. The polymerization of actin and microtubules is prevented to explore the cytoskeletal contribution to the processes involved in the interaction with the substrate, such as the elastic response and adhesion. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B39.00007: Force generated by polymerization of actin filaments: an entropic role? Jean Baudry, Coraline Brangbour, Olivia du Roure, Emmanu\`ele Helfer, Marc Fermigier, Paul M. Chaikin, Marie-France Carlier, Jerome Bibette Actin polymerization drives protrusions at the cell surface and leads to cell motility. Using magnetic colloids, we measure how the chemical reaction of polymerization generates mechanical forces. Rapid force- distance measurement gives us access to the filaments organisation between colloids, whereas long experiments at constant forces give the force- velocity relation of growing actin filaments. A simple model based on entropic forces seems to explain our observations. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B39.00008: Cytoskeleton mediated spreading dynamics of immune cells King-Lam Hui, Jessica Wayt, Brian Grooman, Arpita Upadhyaya We have studied the spreading of Jurkat T-cells on anti-CD3 antibody-coated substrates as a model of immune synapse formation. Cell adhesion area versus time was measured via interference reflection contrast microscopy. We found that the spread area exhibited a sigmoidal growth as a function of time in contrast to the previously proposed universal power-law growth for spreading cells. We used high-resolution total internal reflection fluorescence microscopy of these cells transfected with GFP-actin to study cytoskeletal dynamics during the spreading process. Actin filaments spontaneously organized into a variety of structures including traveling waves, target patterns, and mobile clusters emanating from an organizing center. We quantify these dynamic structures and relate them to the spreading rates. We propose that the spreading kinetics are determined by active rearrangements of the cytoskeleton initiated by signaling events upon antibody binding by T-cell receptors. Membrane deformations induced by such wavelike organization of the cytoskeleton may be a general phenomenon that underlies cell movement and cell-substrate interactions. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B39.00009: How deep cells feel: Mean-field Computations and Experiments Amnon Buxboim, Shamik Sen, Dennis E. Discher Most cells in solid tissues exert contractile forces that mechanically couple them to elastic surroundings and that significantly influence cell adhesion, cytoskeletal organization and differentiation. However, strains within the depths of matrices are often unclear and are likely relevant to thin matrices, such as basement membranes, relative to cell size as well as to defining how far cells can ``feel.'' We present experimental results for cell spreading on thin, ligand- coated gels and for prestress in stem cells in relation to gel stiffness. Matrix thickness affects cell spread area, focal adhesions and cytoskeleton organization in stem cells, which we will compare to differentiated cells. We introduce a finite element computation to estimate the elastostatic deformations within the matrix on which a cell is placed. Interfacial strains between cell and matrix show large deviations only when soft matrices are a fraction of cell dimensions, proving consistent with experiments. 3-D cell morphologies that model stem cell-derived neurons, myoblasts, and osteoblasts show that a cylinder-shaped myoblast induces the highest strains, consistent with the prominent contractility of muscle. Groups of such cells show a weak crosstalk via matrix strains only when cells are much closer than a cell-width. Cells thus feel on length scales closer to that of adhesions than on cellular scales. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B39.00010: Tether extrusion from biomimetic cells Karine Guevorkian, L\'ea Laetitia Pontani, C\'ecile Sykes, Fran\c{c}oise Brochard-Wyart The plasma membrane of a cell is coupled to its underlying cytoskeleton through membrane binding proteins. By pulling membrane tethers, one can measure the strength of these attachments and also probe the rheology of the membrane. In the past, we have used the hydrodynamic tether extrusion technique to study tether dynamics of Red Blood Cells [1]. To describe the non-linear force-velocity behavior at high extrusion forces, we have developed a theoretical model based on lipid permeation through the network of membrane binding proteins [2]. To test this model, we use a biomimetic system consisting of liposomes encapsulating an actin cortex in which the density of membrane-cytoskeleton linkers can be controlled. Here we will present our recent experimental results and compare them to the theoretical predictions. [1] N. Borghi et al, Biophys. J. 93 (2007) [2] F. Brochard-Wyart, et al, Proc. Natl. Acad. Sci. USA, 103 (2006) [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B39.00011: Mechanics of Nascent Cell Adhesions Cecile O. Mejean, Andrew W. Schaefer, Paul Forscher, Eric R. Dufresne Cells have the ability to sense and respond to mechanical and biochemical cues from their environment. In neurons, the binding and restraint of transmembrane cell adhesion molecules (CAMs) can trigger acute periods of axon growth. Preceding growth, the cell must create a stiff mechanical linkage between the CAM and the cytoskeleton. Using holographic optical tweezers, we manipulate CAM-coated beads on the membrane of the cell. We investigate the dynamics of the mechanical properties of this linkage as a function of time, applied force, and CAM density. We find that CAM-coated beads exhibit stochastic intermittent binding to the cytoskeleton. In time, we observed that the adhesions stiffen and their mechanical properties depend on the applied force. Treatment of cells with small molecules that alter cytoskeletal dynamics are used to probe the roles of actin filament assembly and myosin motor activity in adhesion formation. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B39.00012: Epithelial Mechanics during Germband Retraction in Fruit Fly Embryogenesis Xiaoyan Ma, Holley E. Lynch, M. Shane Hutson During germband retraction in the early embryonic development of fruit fly embryos, the epithelial cells of the amnioserosa (AS) undergo a dramatic change in cell shape. The average cell aspect ratio reduces from $\alpha $ $\sim $10 to $\sim $1 within three hours. We performed laser hole-drilling and confocal microscopy to investigate the mechanics of this process in live fly embryos. We find that the laser-induced recoil dynamics of AS cells during germband retraction (when $\alpha \quad \sim $10) is dramatically different from that during the later dorsal closure stage (when $\alpha $ $\sim $1). First, in the earliest stage of germband retraction, some AS cells actually shrink instead of expand in the first one second after ablation. After this point, the cells do slowly expand. Second, in either phase, the cell speeds were much slower, in the range of $\pm $ 1 $\mu $m/s (compared with speeds in excess of 10 $\mu $m/s during dorsal closure). Theses results suggest a much smaller tensile (and in some cases, compressive) stress in the whole cell sheet in early germband retraction. As retraction proceeds towards dorsal closure, the stresses increase. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B39.00013: Matrix elasticity directs stem cell differentiation in 3D too Allison Zajac, Florian Rehfeldt, Dennis Discher Microenvironments appear important in stem cell lineage specification but can be difficult to adequately characterize or control with soft tissues. Naive mesenchymal stem cells (MSCs) are shown here to specify lineage andcommit to phenotypes with extreme sensitivity to tissue level elasticity. Soft matrices that mimic brain are neurogenic, stiffer matrices that mimic muscle are myogenic, and comparatively rigid matrices that mimic collagenous bone prove osteogenic. During the initial week in culture, reprogramming of these lineages is possible with addition of soluble induction factors, but after several weeks in culture, the cells commit to the lineage specified by matrix elasticity, consistent with the elasticity-insensitive commitment of differentiated cell types. Inhibition of nonmuscle myosin II blocks all elasticitydirected lineage specification--without strongly perturbing many other aspects of cell function and shape. The results have significant implications for understanding physical effects of the in vivo microenvironment and also for therapeutic uses of stem cells. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B39.00014: Cell response to long term mechanical interaction with nanopipettes Zulfiya Orynbayeva, Riju Singhal, Elina Vitol, Michael Bouchard, Jane Azizkhan-Clifford, Bradley Layton, Gary Friedman, Yury Gogotsi Traditional microinjection into cells is performed over a relatively short term. Pipettes are typically withdrawn following any kind of injection. On the other hand, there is growing interest in using nanopipettes for cellular and subcellular probing. This interest is partly due to new developments in nanopipette technology which employ carbon nanotubes and provide robustness, flexibility, and biocompatibility. However, as far as we know, no systematic study of physiological, biochemical, and biophysical processes associated with cell response to lengthy mechanical stimulations by nanopipette probing have been performed so far. We present a detailed investigation of a wide range of effects of long term pipette insertion into a cell. Both traditional glass micropipettes and the novel carbon nanotube-tipped probes were involved in this study. The mechanism of Ca2+ response to the mechanical stimuli introduced by the nanopipette, and the role of different organelles in this mechanism were studied. We hypothesize that the calcium response is a function of cytoskeleton integrity and the mode of coupling between the cytoskeleton and the plasma membrane domains. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B39.00015: Gold Nanoparticles effect on Human Dermal Fibroblast Tatsiana Mironava, Nadine Pernodet, Miriam Rafailovich Recently many researchers brought to the light the fact that due to high surface/bulk ratio nanoparticles can penetrate unusually deep human organs and case health problems. Gold nanoparticles are widely used nowadays, however, their effects on cells are still under investigation. Here, we studied the effect of inert citrate/gold nanoparticles as a function of size (13 nm and 45 nm), concentration and time exposure (from 1 to 6 days) on human dermal fibroblasts, since skin is one of the major routs to exposure to nanoparticles. We measured apoptosis rate as a function of nanoparticles size, time exposure and concentration. We found that the presence of 45-nm gold particles had more severe effects on these cells when compared to 13-nm nanoparticles, as the nanoparticles entry use 2 different pathways. In addition the question of cells recovery as a function of time exposure and concentration was investigated. [Preview Abstract] |
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