Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U26: Focus Session: Cytoskeletal Dynamics |
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Sponsoring Units: GSNP DBP DPOLY Chair: Christina Marchetti, Syracuse University Room: Baltimore Convention Center 323 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U26.00001: Isotropic, nematic and polarized states in active motor-filament solutions Aphrodite Ahmadi, M. Cristina Marchetti, Tanniemola B. Liverpool We characterize the phase diagram of interacting polar biofilaments and motor proteins in terms of experimentally accessible parameters. The active filament solution is described by a set of hydrodynamic equations. These in turn are obtained by coarse-graining the Smoluchowski equation for rods coupled by active crosslinkers that mediate the exchange of forces among the filaments. We find that motor activity and the polarity of motor clusters play a key role in the formation of homogeneous isotropic, nematic and polarized states. We also investigate the stability of such homogeneous states against spatially varying fluctuations in the hydrodynamic fields. Motor-induced bundling can destabilize each homogeneous state at high filament and motor density, albeit via different mechanisms (diffusive versus oscillatory). Our analysis suggests that spatially inhomogeneous oscillatory structures, such as vortices, can be formed in the polarized state. [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U26.00002: Rheology of active polymer solutions M. Cristina Marchetti, Tanniemola B. Liverpool In vitro solutions of biopolymers and associated motor proteins have been used to probe cytoskeletal dynamics under controlled conditions. Experiments have shown that motor-induced filament sliding competes with the slow Brownian polymer dynamics, driving the organization of the polymer network into complex patterns and altering its rheological and mechanical properties. Starting from a semi-microscopic model of the motor-mediated interaction among filaments, we have obtained continuum equations for the coarse-grained fields (filament and motor densities, polarization, alignment tensor) describing the response of the active solution to an externally imposed flow. After deriving an expression for the active contribution to the stress tensor, we have evaluated the linear viscoelastic response of a dilute solution to a shear stress. In the isotropic state motor activity strongly enhances the viscoelasticity of the system, especially near the transition to an orientationally ordered state. Activity also increases the high-frequency shear modulus of the solution. -- MCM was support by the National Science Foundation, grants DMR-0305407 and DMR-0219292. TBL acknowledges the support of the Royal Society. [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U26.00003: Thermally Controlling the Polymeric Cytoskeleton in Living Cells Chao-Min Cheng, Philip LeDuc Cell structure is controlled to a large degree by the cytoskeleton, which is an intracellular polymer network. This cytoskeleton is critical as it strongly influences many cellular functions such as motility, organelle transport, mechanotransduction and mitosis. In our studies, we controlled the thermal environment of living cells and after applying an increase in temperature of only 5 $^{o}$C, we observed a change in the polymer network as the actin filaments depolymerized. Interestingly, when we then lowered the temperature, the actin repolymerized indicating a reversible phase that is controlled by the thermal environment. We characterized the presence of F-actin and G-actin for these phases through analyzing the intensity from immunofluorescent studies for these proteins. The F-actin concentration decreased when increasing the temperature from the initial state and then increased when decreasing the temperature. Although the cell is known to be affected by heat shock responses, this is not a function of just the polymers as they do not exhibit these polymerization characteristics when we probed them as single filaments in vitro. These studies suggest that the cell has distinct phases or patterns while maintaining a reversible equilibrium due to the thermal environment for these networked polymers. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 8:48AM |
U26.00004: How to detect single cancer cell? Nadine Pernodet, Jessica Fields, Lenny Slutsky, Taylor Bernheim, Kaustabh Ghosh, Shouren Ge, Miriam Rafailovich Cell mechanics is now considered as a critical parameter closely related to cell functions. Moreover, we know that cytoplasm of cancer cells compared to normal cells is disorganized; therefore this should be directly translated to their mechanical properties. Here we show that we are able to distinguish cancer cells from normal cells in a cell mixture through their mechanical properties. Advantage of our method is that we measure single cell mechanical response where any cells, cancer or normal, are attached on a surface \textit{in situ}. When imaged in the mixture of cells, through usual microscopic imaging, cancer and normal cells did not show obvious differences and could not be identified with certitude unless using specific biochemical markers. In a mixture of cancer and normal cells, mechanical measurements were done randomly on six different cells. The relative modulus gave a bimodal distribution. These moduli were compared to the known modulus obtained from normal or cancer cell and assigned to each group of cells very precisely. This method, which is directly related to the intrinsic cytoskeleton cell mechanical properties, is a sensitive and reliable tool to detect cancer cells from a culture or a tissue at a single cell level. [Preview Abstract] |
Thursday, March 16, 2006 8:48AM - 9:00AM |
U26.00005: The Effects of Chronological Age on the Cellular Mechanics of Human Dermal Fibroblasts Z. Pan, V. Hung, S. Kambhampati, S.R. Ge, M. Rafailovich, K. Ghosh, R. Clark, Y.J. Liu, T. Nakamura, X.Z. Shu, G. Prestwich It is often observed that older people display diminished wound healing abilities. Understanding of this phenomenon is important for many in vivo applications of tissue engineering. In this study, the cell mechanics of dermal fibroblasts from 25, 40 and 84 years old female subjects were compared. These cells were cultured on functionalized hyaluronic acid hydrogel substrates which emulated physiological conditions in dermal tissue. The deformation of the substrate caused by cellular traction forces was detected by tracing the displacement of fluorescent beads embedded in the substrate using Digital Image Speckle Correlation. Then cellular traction forces were quantitatively determined by Finite Element Method in a linear elastic model with a high spatial resolution. These results were correlated with auxiliary measurements of substrate modulus, cell modulus and migration. We found that with increasing age, the magnitude of the cellular traction forces diminished. Similarly, the ability of the cells to adapt to changes in the mechanical properties of their environment and migrate was also impaired. The interrelationship between these factors and wound healing will be discussed. This work is supported by NSF- MRSEC program. [Preview Abstract] |
Thursday, March 16, 2006 9:00AM - 9:12AM |
U26.00006: Interplay between crosslinkers and dynamic molecular motor-induced instabilities in the moderation of biopolymer organization David Smith, David Humphrey, Falko Ziebert, Walter Zimmermann, Josef K\"{a}s Structure and function of biological cells rely on the highly-dynamic self-organization of protein filaments to an intracellular cytoskeleton responsive to mechanical and chemical stimuli. While dissolving these complex cellular structures through Brownian motion is inherently slow (tens of minutes), changes in the activity of the molecular motor myosin II cause rapid order-disorder transitions within 1-2 minutes in reconstituted cytoskeletal actin networks. When motor-induced filament sliding decreases, actin network structure rapidly and reversibly self-organizes into various assemblies triggered by a nonlinear instability. Modulation of static crosslinker concentrations allow for a wide phase space of order ranging from nematics to compact asters \& dense packing of motor-filament clusters. The observed isothermal transitions between disorder and self-organization illustrate that molecular motors can substantially contribute to dynamic cellular organization. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:48AM |
U26.00007: The mechanics of cell protrusion Invited Speaker: The protrusion of the cell edge is the first step in a cycle of molecular processes that drive cell movements during development, immune responses, wound healing and many other physiological functions. It is also the earliest pathological event observed during metastasis of cancer. Textbook models associate protrusion with the assembly of an actin polymer network subadjacent to the cell plasma membrane. However, for this process to be transformed into edge advancement, polymerization-induced forces need to be balanced by adhesion complexes that link the actin network to the extracellular domain. Also, the effectiveness of network assembly in mediating forward movement of the cell edge depends on how contraction forces pull the network in the cell front retrogradly towards the cell center. Thus, what is observed in a microscope as cell protrusion reflects the kinematic output of at least three space- and time-modulated mechanisms of force generation. The coordination of these machineries is thought to be regulated by a complex network of mechano-chemical signals. Our goal is to establish the contributions of each those mechanisms and their control by reconstructing the spatiotemporal distribution of intracellular forces via inverse dynamics and molecular intervention with the relevant signalling pathways. To this end, we have developed quantitative Fluorescent Speckle Microscopy (qFSM) which provides high-resolution spatiotemporal measurements of actin network deformation and material properties in migrating cells. In addition, qFSM delivers maps of cytoskeleton assembly and disassembly, so that we can infer the plasticity of the material in situ. Together, this data allows us to deduce intracellular force distributions from the constitutive laws of strain and stress in the actin polymer network. Using this approach we discovered that unperturbed cells protrude in a dynamic steady state where periodic patterns of network assembly, adhesion formation, and cytoskeleton transport are tightly connected to protrusion waves. We exploited the sub-cellular heterogeneity of these patterns to identify the causality and timing between dynamic events in the actin network, leading towards a first integral view of the mechano-chemical process interaction in the protrusion machinery. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U26.00008: Tensile Force Generation by Actin-Myosin Networks Anders Carlsson Tensile force generation by the actin-myosin system is a crucial factor in many cellular processes, including the function of the contractile ring in cytokinesis. Calculations of such tensile forces have often been based on specific one-dimensional models of the structure based on parallel overlapping filaments, sometimes in sarcomere-like structures. However, the detailed arrangement of the actin filaments is not known in general, and it is likely to be disordered. For this reason we have developed a general theory of force generation by myosin in actin networks, based on treating the myosin motors as external forces in a viscoelastic medium. The analysis is based on two ingredients: the strain field of a force dipole in a homogeneous medium, and a correction for the inhomogeneity of the actin network. We obtain a simple expression for the tensile stress induced by the myosin motors in terms of the density of motors and the average actin filament length. This formula is used to relate the force that can be generated by a contractile ring to the actin network structure. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U26.00009: Simulation of Actin-Polymerization Near Moving Surface 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, severing and capping. Using this model, we study the dynamics of the moving surface in conjunction with the morphology of the resulting actin network. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U26.00010: The Fysics of Filopodia (or The Physics of Philopodia) Jen Schwarz, Ajay Gopinathan, Kun-Chun Lee, Andrea Liu, Louise Yang Cell motility is driven by the dynamic reorganization of the cellular cytoskeleton which is composed of actin. Monomeric actin assembles into filaments that grow, shrink, branch and bundle. Branching generates new filaments that form a mesh-like structure that protrudes outward allowing the cell to move somewhere. But how does it know where to move? It has been proposed that filopodia serve as scouts for the cell. Filopodia are bundles of actin filaments that extend out ahead of the rest of the cell to probe its upcoming environment. Recent in vitro experiments [Vignjevic {\it et al.}, J. Ce ll Bio. {\bf 160}, 951 (2003)] determine the minimal ingredients required for such a process. We model these experiments analytically and via Monte Carlo simulations to estimate the typical bundle size and length. We also estimate the size of the mesh-like structure from which the filopodia emerge and explain the observed nonmonotonicity of this size as a function of capping protein concentration, which inhibits filament growth. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U26.00011: Lipid-Protein Nanotubes with Open or Closed Ends, Microtubules Bundles and Inverted Tubulin Nanotubes Uri Raviv, Daniel J. Needleman, Miguel A. Ojeda-Lopez, Youli Li, Herb P. Miller, Leslie Wilson, Cyrus R. Safinya We describe synchrotron x-ray diffraction, electron microscopy, and optical imaging data of the self-assembly of microtubules (MTs) with various cationic agents. We established the conditions under which cationic liposomes can coat MTs and form lipid-protein nanotubes (LPNs). The LPNs exhibit a rather remarkable architecture with the cylindrical lipid bilayer sandwiched between a MT and outer tubulin oligomers forming rings or spirals. By controlling the cationic lipid/tubulin stoichiometry it is possible to switch between two states of nanotubes with either open ends or closed ends with lipid caps, a process which forms the basis for controlled chemical and drug encapsulation and release (Raviv et al, PNAS, 2005). Multivalent (3+,4+ and 5+) cations can form three dimensional MT bundles that in some cases become tubulin based inverted nanotubules. Divalent cations form two dimensional MT necklaces (Needleman et al, PNAS, 2004). [Preview Abstract] |
Thursday, March 16, 2006 10:36AM - 10:48AM |
U26.00012: The mechanics of cell crawling over a flat surface Baldomero Alonso-Latorre, Javier Rodriguez-Rodriguez, Alberto Aliseda, Rudolf Meili, Richard Firtel, Juan Lasheras The chemotaxis of different strains of the amoeba \textit{Dictyostelium Dicoideum} when exposed to a wide range of concentrations and gradients of chemoattractant has been studied experimentally. First, the time evolution of the velocity as well as the shape of the cell have been measured from microscopy images for a large number of individuals. Secondly, the force that the amoebas exert over the substrate in order to propel themselves has also been measured. Some insights into the physical mechanism by which cells crawl over the surface are obtained by comparing the time evolution of those magnitudes for the different strains under study. [Preview Abstract] |
Thursday, March 16, 2006 10:48AM - 11:00AM |
U26.00013: Micromechanical Properties of Endothelial Cell Cytoskeleton Meron Mengistu, Linda Lowe-Krentz, H. Daniel Ou-Yang Atherosclerotic plaques occur in regions of arterial curvature, where there is blood re-circulation and physiologically low shear stress conditions. This phenotype may be related to flow-induced shear stress on the monolayer of endothelial cells that make up the endothelium. When endothelial cells in static culture are exposed to laminar flow, they respond by rearranging their cytoskeleton, and aligning their actin filaments in the direction of flow. The changes in cytoskeletal structures induced by flow are different from region to region of the same cell. We employ the optical tweezers technique to obtain very local mechanical properties of endothelial cell cytoskeleton. We used endocytosed polystyrene beads, as well as intrinsic granular structures, as probes for our measurements. Endothelial cells were also treated with Cytochalasin B and Nocadozole, which are drugs that de-polymerize actin filaments and microtubules respectively, to measure the visco-elastic moduli, and obtain the contribution of each cytoskeletal structure in the cells' micro-mechanical properties. [Preview Abstract] |
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