Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A10: Focus Session: Mechanics of Cells and Biological Networks I |
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Sponsoring Units: DBIO DPOLY GSNP Chair: Daniel Blair, Georgetown University Room: 201 |
Monday, March 3, 2014 8:00AM - 8:36AM |
A10.00001: Mechanics of composite cytoskeletal and extracellular networks Invited Speaker: Moumita Das Living cells sense and respond to mechanical forces in their surroundings. This mechanical response is mainly due to the cell cytoskeleton, and its interaction with the extracellular matrix (ECM). The cell cytoskeleton is a composite polymeric scaffold made of many different types of protein filaments and crosslinking proteins. Two major filament systems in the cytoskeleton are actin filaments (F-actin) and microtubules (MTs). Actin filaments are semiflexible, while the much stiffer MTs behave as rigid rods. I shall discuss theories that help understand how the direct coupling to the surrounding F-actin matrix allows intracellular MTs to bear large compressive forces and controls the range of force transmission along the MTs, and how the MTs not only enhance the stiffness of the cell cytoskeleton, but can also dramatically endow an initially nearly incompressible F-actin matrix with enhanced compressibility relative to its shear compliance. A second source of compositeness in the cytoskeleton is the presences of different types of crosslinkers that can interact cooperatively leading to enhanced mechanical rigidity and tunable response. Like the cytoskeleton, the ECM is also a polymeric composite. It is primarily composed of a mesh of fibrous proteins, mainly stiff collagen filaments, and a comparatively flexible gel of proteoglycans and hyaluronan. I shall discuss a model that shows how the interplay between the collagen network and the background elastic gel leads to a mechanically robust ECM. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A10.00002: Modeling the role of nuclear mechanics in determining cell shape and motility through microfluidic channels Jake Shechter, Kara Maki, Moumita Das Cell mechanics and migration through tight spaces are critical to life processes such as immune response and fertilization, in several diseases, and in diagnostics and drug delivery. For example, breast cancer cells have been shown to deform more easily and transit more rapidly through microfluidic channels than healthy breast cells. In this computational biophysics project, we simulate a cell moving through a microfluidic channel. We calculate the deformation energy of a model cell, which includes contributions from the cell cytoskeleton and the cell nucleus. We study how the model cell deforms in response to external forces, focusing on the deformability of the cell as it squeezes into and through a microfluidic channel and how the nucleus plays a part in this. Recent experiments suggest that the nucleus can be up to an order of magnitude stiffer than the rest of the cell and our results may provide insights into how the nucleus influences cell mechanics and migration. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A10.00003: Dynamic contact guidance of migrating cells Wolfgang Losert, Xiaoyu Sun, Can Guven, Meghan Driscoll, John Fourkas We investigate the effects of nanotopographical surfaces on the cell migration and cell shape dynamics of the amoeba Dictyostelium discoideum. Amoeboid motion exhibits significant contact guidance along surfaces with nanoscale ridges or grooves. We show quantitatively that nanoridges spaced 1.5 $\mu $m apart exhibit the greatest contact guidance efficiency. Using principal component analysis, we characterize the dynamics of the cell shape modulated by the coupling between the cell membrane and ridges. We show that motion parallel to the ridges is enhanced, while the turning, at the largest spatial scales, is suppressed. Since protrusion dynamics are principally governed by actin dynamics, we imaged the actin polymerization of cells on ridges. We found that actin polymerization occurs preferentially along nanoridges in a ``monorail'' like fashion. The ridges then provide us with a tool to study actin dynamics in an effectively reduced dimensional system. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A10.00004: Fc-receptor induced cell spreading during frustrated phagocytosis in J774A.1 macrophages Daniel Kovari, Jennifer Curtis, Wenbin Wei Phagocytosis is the process where by cells engulf foreign particles. It is the primary mechanism through which macrophages and neutrophils (white blood cells) eliminate pathogens and debris from the body. The behavior is the result of a cascade of chemical and mechanical cues, which result in the actin-driven expansion of the cell's membrane around its target. For macrophages undergoing Fc-mediated phagocytosis, we show that above a minimum threshold the spreading rate and maximum cell-target contact area are independent of the target's opsonin density. Qualitatively, macrophage phagocytic spreading is similar to the spreading of other cell types (e.g. fibroblasts, lymphocytes, and Dict.d.). Early spreading is most likely the result of ``passive'' alignment of the cell to the target surface. This is followed by an active expansion period driven by actin. Finally upon reaching a maximum contact area, typically 2-3 times the size of ``non-activated'' cells, macrophages often undergo a period of rapid contraction not reported in other cell types. We hypothesize that this, as yet unexplained, transition may be specific to the chemical and mechanical machinery associated with phagocytosis. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A10.00005: Silk Electrogel Rheology A.P. Tabatabai, J.S. Urbach, D.L. Blair, D.L. Kaplan We present experimental results on the rheology on electrogels derived from aqueous solutions of reconstituted \textit{Bombyx Mori} silk fibroin protein. Through electrochemistry, the silk protein solution develops local pH changes resulting in the assembly of protein into a weak gel. We determine the physical properties of the electrogels by performing rheology and observe that they exhibit the characteristics of a crosslinked biopolymer network. Interestingly, we find that these silk gels exhibit linear elasticity over a range of up to two orders of magnitude larger than most crosslinked biopolymer networks. Moreover, the nonlinear rheology exhibits a strain-stiffening behavior that is fundamentally different than the strain-stiffening observed in crosslinked biopolymers. Through rheological techniques we aim to understand this distinctive material that cannot be explained by current polymeric models. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A10.00006: Nonlinear intracellular elasticity controlled by myosin-generated fluctuating stress Ming-Tzo Wei, H. Daniel Ou-Yang The mechanics of biological cells are governed by a network of cytoskeletal filaments and molecular motors forming a dynamic mechanical entity. It has been found that local elasticity of in vitro active polymer networks, a synthesized cytoskeletal network, increase as a result of myosin-generated stresses. It is unknown this also holds in the local intracellular stress. We study the intracellular stress by the combination of the approaches of active and passive microrheology to measure the myosin-generated fluctuating stress and intracellular elasticity. Our experimental data show an increase in the fluctuations of the cellular elasticity with increasing motor-generated fluctuating local stress inside living cells. In addition, we found a direct correlation between the mean intracellular elasticity and steady-state intracellular stress. Our study provides a link between in vitro active polymer networks and in vivo cell experiments. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A10.00007: Structure-function relations in cartilage under shear: Does fiber organization matter? Moumita Das, Jesse Silverberg, Aliyah Barrett, Poul Peterson, Lawrence Bonassar, Itai Cohen Confocal elastography have enabled spatially resolved measurements of soft biological tissues such as articular cartilage (AC). With this technique it was discovered that the AC shear modulus has a compliant region near the tissue surface that is 10-100 times smaller than the bulk. This region also dissipates $\sim$ 90{\%} of the energy absorbed during shear, suggesting a functional role protecting the underlying tissue. Though the mechanical properties have depth-dependent trends that parallel the stereotypical collagen fiber organization, we explore this observation with structural, compositional, and shear mechanical data. We show the fiber-reinforced interpretation of the collagen network is inconsistent with experiments at small strains. Instead, we find the shear modulus strongly correlates with cartilage matrix density leading to the result that a 50{\%} variation in matrix density leads to a 10,000{\%} variation in shear modulus. We interpret these results in terms of a biopolymer rheology model that is known to produce such trends. This scaling arises from a second-order mechanical phase transition known as rigidity percolation, and with the inclusion of a reinforcing medium to more closely mimic cartilage, the empirical trends are reproduced. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A10.00008: Strain stiffening and stress heterogeneities in sheared collagen networks Invited Speaker: Jeffrey Urbach Disordered networks of stiff or semi-flexible filaments display unusual mechanical properties, including dramatic stiffening when sheared, but little is known about the spatial distribution of stresses. This talk will introduce the technique of \textit{Boundary Stress Microscopy}, which adapts the approach of traction force microscopy to rheological measurements in order to quantify the non-uniform surface stresses in sheared soft materials. Our results on networks of the biopolymer collagen, a major component of the extracellular matrix, show stress variations over length scales much larger than the network mesh size. We find that the heterogeneity increases with strain stiffening, with stresses at high strains exceeding average stresses by an order of magnitude. The strain stiffening behavior over a wide range of mesh sizes can be parameterized by a single characteristic strain and associated stress, which describes both the strain stiffening regime and network yielding. The characteristic stress is approximately proportional to network density, but the peak stress at both the characteristic strain and at yielding are remarkably insensitive to concentration. These results show the power of Boundary Stress Microscopy to reveal the nature of stress propagation in disordered soft materials, which is critical for understanding many important mechanical properties, including the ultimate strength of a material and the nature of appropriate microscopic constitutive equations. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A10.00009: Longitudinal fluctuations and higher cumulants of biopolymers in the strong stretching limit Lipeng Lai, Jianshu Cao Biopolymers, such as actins and spectrins, are important constituents in cytoskeletons. Previous studies revealed that the mechanical properties of cytoskeletons, which are essential to the functions of living organisms, are largely dictated by the elastic properties of individual polymers. Here we studied the fluctuations of individual biopolymers when they are strongly stretched with very small transverse deformations. Based on the Worm-like chain (WLC) model, general formulae for the fluctuations and higher cumulants of the end-to-end distance along the stretching direction (longitudinal) are obtained when the energy of the semi-flexible chains retains a quadratic form (e.g., when the polymer is subject to a point force at the end or a constant plug flow with the other end fixed). Our results are consistent with previous theoretical and experimental work. Besides providing additional criteria to check the region of validity of the WLC model, the results may also provide more insights into the study of the elastic properties of polymers and cytoskeletal networks. Furthermore, our results can also be generalized to other situations when the polymers are rod-like. A good example is the actin network, where the actin segments are stiff due to their large persistence length. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A10.00010: Universal Crossover Dynamics of a Semi-Flexible Polymer in Two Dimensions Aniket Bhattacharya, Aiqun Huang, Ramesh Adhikari, Kurt Binder We present a unified scaling theory for the dynamics of monomers for dilute solutions of semiflexible polymers under good solvent conditions in the free draining limit. Our theory encompasses the well-known regime of mean square displacements (MSDs) of stiff chains growing like $t^{3/4}$ with time (R. Granek, J. Phys. II (Paris) {\bf 7}, 1767 (1997); E. Farge and A. C. Maggs, Macromolecules {\bf 26}, 5041 (1993)) due to bending motions, and the Rouse regime $t^{2 \nu / (1+ 2\nu)}$ where $\nu$ is the Flory exponent describing the radius $R$ of a swollen flexible coil. We identify how the prefactors of these laws scale with the persistence length $\ell_p$, and show that a crossover from stiff to flexible behavior occurs at a MSD of order $\ell^2_p$ (at a time proportional to $\ell^3_p$), a second crossover (to diffusive motion) occurs when the MSD is of order $R^2$. We also provide compelling evidence for the theory by carrying out large scale Molecular Dynamics simulations in $d=2$ dimensions. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A10.00011: Conformations and Transverse Fluctuations of a Semi-Flexible Chain in Two Dimensions Aiqun Huang, Aniket Bhattacharya, Kurt Binder We study conformations and transverse fluctuations of a semi-flexible polymer using Langevin Dynamics simulation in two dimensions(2D). By showing that the end-to-end distance $\langle R_N^2 \rangle $ for a semiflexible chain characterized by its contour length $L$ and the persistence length $\ell_p$ follows the scaling relation $\langle R_N^2 \rangle \sim L^{1.5}\ell_p^{0.5}$, as proposed by Schaefer {\em et al.} and Nakanishi, we verify the absence of the Gaussian regime, thus disprove the validity of the worm-like chain (WLC) theory in 2D. We also verify that the bond autocorrelation function exhibits a power law $\langle \vec{b}_i\cdot \vec{b}_{i+s} \rangle \sim s^{-\beta}$ instead of an exponential decay as predicted by the WLC model. We further show that the normalized transverse fluctuations $\sqrt{\langle l_{\bot}^2\rangle}/L$ for the semiflexible chains of different persistence length and contour length collapse onto the same master plot as a function of $L/\ell_p$, which exhibits $\sqrt{\langle l_{\bot}^2\rangle}/L \sim (L/\ell_p)^{0.5}$ and $\sqrt{\langle l_{\bot}^2\rangle}/L \sim (L/\ell_p)^{-0.25}$ at two extreme limits $L/\ell_p \rightarrow 0$ and $L/\ell_p \rightarrow \infty$, respectively and exhibits a maximum for $L/\ell_p \sim 1.0$. [Preview Abstract] |
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