Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A41: Focus Session: Active Biopolymers and Biomaterials |
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Sponsoring Units: DPOLY Chair: jennifer Ross, University of Massachusetts--Amherst Room: A115/117 |
Monday, March 21, 2011 8:00AM - 8:36AM |
A41.00001: Connecting Atomic Structures with Continuum Mechanics in Cytoskeletal Polymers Invited Speaker: The mechanics of the cytoskeleton, namely actin filaments and microtubules, are key to many of their cellular functions. These polymers have been extensively studied using a wide range of biophysical techniques, and we have sought to connect the dynamics we observe in all-atom molecular dynamics simulations with continuum mechanics properties. We have developed coarse-graining techniques that allow us calculate mechanical properties of these polymers using a simple mesoscopic description. Our findings match very well with experimental measurements and allow us to probe how the atomic level effects of small molecules and/or point mutations manifest themselves at the level of the polymer. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A41.00002: The Interplay of Nonlinearity and Architecture and Nonequilibrium Dynamics in Cytoskeletal Mechanics Shenshen Wang, Tongye Shen, Peter Wolynes The interplay between cytoskeletal architecture and the nonlinearity of the interactions due to bucklable filaments plays a key role in modulating the cell's mechanical stability and its structural rearrangements. We first study a model of cytoskeletal structure treating it as an amorphous network of hard centers rigidly cross-linked by nonlinear elastic strings, neglecting the effects of motorization. Using simulations along with a self-consistent phonon method, we show that this minimal model exhibits diverse thermodynamically stable mechanical phases that depend on excluded volume, crosslink concentration, filament length and stiffness. Within the framework set by the free energy functional formulation and making use of the random first order transition theory of structural glasses, we further estimate the characteristic densities for a kinetic glass transition to occur in this model system. Network connectivity strongly modulates the transition boundaries between various equilibrium phases, as well as the kinetic glass transition density. We further study the effects of motorization and polymerization upon the stability and dynamics of this model system. [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A41.00003: Buckling of Branched Cytoskeletal Filaments D.A. Quint, J.M. Schwarz {\it In vitro} experiments of growing dendritic actin networks demonstrate reversible stress-softening at high loads, above some critical load. The transition to the stress-softening regime has been attributed to the elastic buckling of individual actin filaments. To estimate the critical load above which softening should occur, we extend the elastic theory of buckling of individual filaments embedded in a network to include the buckling of branched filaments, a signature trait of growing dendritic actin networks. Under certain assumptions, there will be approximately a seven-fold increase in the classical critical bucking load, when compared to the unbranched filament, which is entirely due to the presence of a branch. Moreover, we go beyond the classical buckling regime to investigate the effect of entropic fluctuations. The result of compressing the filament in this case leads to an increase in these fluctuations and eventually the harmonic approximation breaks down signifying the onset of the buckling transition. We compute corrections to the classical critical buckling load near this breakdown. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A41.00004: ``Twist-state" transitions in parallel actin bundles induced by crosslinking proteins Homin Shin, Gregory Grason Parallel actin bundles are common structural motifs in many crucial cellular specializations, from filopodia to mechanosensory bundles of the inner ear. Here, we study a model of actin bundles, crosslinked by compact globular bundling proteins, known to modify the torsional state of filaments due to frustration between helical structure of the filaments and in-plane ordering of the bundle. Our coarse-grained model of parallel bundles maps the linker-induced ``twist-state'' transition of actin filament onto a {\it commensurate-incommensurate} phase transition, described by an effective Frenkel-Kontorowa model. We predict that the transition from the uncrosslinked, incommensurate helical symmetry to fully crosslinked, commensurate symmetry is highly sensitive to linker flexibility: flexible crosslinking smoothly distorts the twist state of bundled filaments, while rigidly crosslinked bundles undergo a phase transition, rapidly overtwisting filaments over a narrow range of free crosslinker concentrations. Additionally, we predict a rich spectrum of intermediate structures, composed of alternating domains of sparsely bound (untwisted) and strongly bound (overtwisted) filaments. This model reveals that subtle differences in crosslinking agents themselves modify not only the detailed structure of parallel actin bundles, but also the thermodynamic pathway by which they form. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A41.00005: Elasticity of a cross-linked active bundle Silke Henkes, Tanniemola B. Liverpool, M. Cristina Marchetti, A. Alan Middleton, Jennifer M. Schwarz Understanding the effect of motor proteins, such as myosins, on the elasticity of crosslinked actin networks is essential to our understanding of cell mechanics. Both in vivo and in vitro, these active networks have radically different mechanical properties from their equilibrium counterparts, including contractile behavior and higher elastic moduli. Existing theoretical models do not address the relative role of passive and active crosslinkers in controlling the network contractility and stiffening. We construct a one dimensional lattice model with minimal ingredients, that is, rigid polar filaments, spring-like passive crosslinks and active crosslinks with on/ off dynamics implemented through non-equilibrium Monte Carlo solution of the corresponding master equations. We find, consistent with experiments, that the network needs to be percolated through the passive crosslinks to be mechanically stable. Contractile behavior is observed for all concentrations of active crosslinks. We study the mechanical properties of the gel in the phase space of motor processivity, crosslink stiffness, and concentration of active crosslinks. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A41.00006: Bursts of active transport in living cells Bo Wang, James Kuo, Sung Chul Bae, Steve Granick This study of cargo motion in living cells, performed with nm resolution and an unprecedented large database, shows that the instantaneous speed of active transport deviates pervasively from the average speed yet with striking statistical regularity over several decades of time and space. The experimental approach involves single-particle tracking and special wavelet-based methods to discriminate active transport from passive diffusion, thus quantifying the instantaneous speed of endosomal and lysosomal active transport in living cells at times just longer than the motor stepping time. Pervasive bursts of acceleration stem from viscoelastic relaxation of the cytoplasm, the individual bursts displaying a time-averaged shape that we interpret to reflect stress buildup followed by rapid release. These statistical regularities did not change in response to changing the experimental conditions, specifically to changing the cell line and motor type, or to overexpressing microtubule binding proteins, thus indicating redundancy in regulation of cellular active transport. The power law of scaling is the same as seen in driven jammed colloids, powders, and magnetic systems, and is consistent with a simple heuristic argument. The implied regulation of active transport by environmental obstruction in the cytoplasm extends the classical notion of ``molecular crowding.'' [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A41.00007: Mechanically Activated Motion of a Single Self-Propelled Polymeric Microcapsule German Kolmakov, Alexander Schaefer, Igor Aranson, Anna Balazs Using a hybrid computational approach, we demonstrate that a single nanoparticle-filled microcapsule on a rigid substrate can undergo self-sustained motion in response to initial mechanical deformation. Nanoparticles released from the capsule modify the underlying substrate and the adhesion gradients of the nanoparticle concentration formed at the surface sustain the motion of the capsule. The permeability of the microcapsule's shell increases with its deformation and therefore, more deformed microcapsules release nanoparticles at higher rates. An initial, non-uniform mechanical deformation of the capsule by an applied force causes an asymmetry in the nanoparticle distribution on the substrate that initiates the microcapsule motion. We also develop a two-dimensional model of the phenomenon within the phase-field approximation and compare the results of the two approaches. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A41.00008: Coarse-grained models for biological simulations Zhe Wu, Qiang Cui, Arun Yethiraj The large timescales and length-scales of interest in biophysics preclude atomistic study of many systems and processes. One appealing approach is to use coarse-grained (CG) models where several atoms are grouped into a single CG site. In this work we describe a new CG force field for lipids, surfactants, and amino acids. The topology of CG sites is the same as in the MARTINI force field, but the new model is compatible with a recently developed CG electrostatic water (Big Multiple Water, BMW) model. The model not only gives correct structural, elastic properties and phase behavior for lipid and surfactants, but also reproduces electrostatic properties at water-membrane interface that agree with experiment and atomistic simulations, including the potential of mean force for charged amino acid residuals at membrane. Consequently, the model predicts stable attachment of cationic peptides (i.e., poly-Arg) on lipid bilayer surface, which is not shown in previous models with non-electrostatic water. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A41.00009: Guided Transport of a Transmembrane Nanochannel Meenakshi Dutt, Olga Kuksenok, Anna Balazs Via the Dissipative Particle Dynamics approach, we design a system that allows transport of a nanochannel to a desired location by applying an external force. Each nanochannel encompasses an ABA architecture, with a hydrophobic shaft (B) with two hydrophilic ends (A). One of the hydrophilic ends of the nanochannel is functionalized with hydrophilic functional groups, or hairs. The hydrophilic hairs serve a dual role: (1) control transport across the membrane barrier when the channel diffuses freely in the membrane, and (2) enable the channel relocation to a specific membrane site. Our system comprises a transmembrane hairy nanochannel with the hairs extending into solution. In our earlier work, we demonstrated the spontaneous insertion of such a hairy nanochannel into a lipid bilayer (Nanoscale DOI: 10.1039/C0NR00578A). First, we hold a suitably functionalized pipette stationary above the membrane while the nanochannel freely diffuses within the membrane. For an optimal range of parameters, we demonstrate that the hairs find the pipette and spontaneously anchor onto it. We then show that by moving the pipette for a range of velocities, we can effectively transport the channel to any location within the membrane. This prototype system can provide guidelines for designing a number of biomimetic applications. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:48AM |
A41.00010: Accidental Interactions and Purposeful Flaws in Polymer Brushes: Variations in Bioadhesive Mechanisms Invited Speaker: Water soluble brushes, such as polyethylene glycol and poly(hydroxyethyl methacrylate) are grafted, in many applications, to or from surfaces to prevent protein and cell adhesion. When brushes fail, as can be frequent in practice, protein adsorption becomes aggressive. Failure can occur if the brush architecture is simply too thin such that proteins can experience van der Waals and electrostatic attractions with the underlying substrate, or if the brush has flaws or holes, where the grafting procedure did not succeed. This talk compares the interactions of proteins and bacteria with nearly uniform brushes to the interactions of proteins and bacteria with patchy brushes. The latter are deliberately flawed by the inclusion of nanoscale adhesive elements (polymer coils and nanoparticles) at their base, which prevent local brush formation. The adhesive elements are smaller than the proteins themselves, but sufficient to perturb local brush structure. This talk demonstrates that large quantities of the appropriate types of random-coil (denatured) and globular (native) proteins can penetrate a brush and even displace it (if it is otherwise held in place by adsorbing anchor groups), while other proteins can be entirely repelled. The same is also true of the patchy brushes, but with patchy brushes, but the mechanism is different. With uniform brushes, small proteins and random coils sometimes penetrate sufficiently to experience electrostatic attractions once inside the brush. With patchy brushes, all proteins have the opportunity to interact electrostatically with the adhesive elements, but because large proteins can interact with greater numbers of adhesive elements, their capture is preferred. The result is different rankings of proteins which can ultimately adhere to thin uniform brushes or thicker patchy ones. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A41.00011: Nanoparticle Self-Lighting Photodynamic Therapy For Cancer Treatment Wei Chen Photodynamic therapy has been designated as a ``promising new modality in the treatment of cancer'' since the early 1980s. Light must be delivered in order to activate photodynamic therapy. Most photosensitizers have strong absorption in the ultraviolet -- blue range, therefore, UV -blue light is needed for their activation. Unfortunately, UV-blue light has minimal penetration into tissue and its application for \textit{in vivo} activation is a problem. To solve the problem and to enhance the PDT treatment for deep cancers, we introduce a new PDT system in which the light is generated by afterglow nanoparticles with attached photosensitizers. When the nanoparticle-photosensitizer conjugates are targeted to tumor, the light from afterglow nanoparticles will activate the photosensitizers for photodynamic therapy. Therefore, no external light is required for treatment. More importantly, it can be used to treat deep tumor such as breast cancer because the light source is attached to the photosensitizers and are delivered to the tumor cells all together. This modality is referred as nanoparticle self-lighting photodynamic therapy. [Preview Abstract] |
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