Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z44: Focus Session: Cell Mechanics III |
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Sponsoring Units: DBIO Chair: Eric Dufresne, Yale University Room: Hilton Baltimore Holiday Ballroom 1 |
Friday, March 22, 2013 11:15AM - 11:27AM |
Z44.00001: Energy barriers for cellular rearrangements in tissues Dapeng Bi, J.H. Lopez, J.M. Schwarz, M. Lisa Manning The behavior of cellular aggregates strongly influences morphogenesis, cancer growth and wound healing. While single cell mechanics has been extensively studied, the collective dynamics of cells inside a tissue is not well understood. Recent experiments have shown cells in tissues behave like fluids on long timescales and solids on shorter timescales, and exhibit caging behavior at intermediate timescales as they are more tightly packed. These observations are reminiscent of dynamic slowing down and dynamical heterogeneities due to mutual confinement and crowding of particles glassy systems. A common and crucial feature of glassy systems is the existence of a Potential Energy Landscape (PEL) for local rearrangements. For thermal glassy materials, when these barriers are large compared to thermal fluctuations, its rheology is dependent on the PEL and external mechanical driving. In contrast, cells in a tissue are non-thermal and overcome energy barriers in the PEL mainly through local active processes, i.e. making new adhesions and cell shape changes. We numerically map the PEL of a confluent tissue as functions of different transition pathways and single cell properties. Analytical calculations are also performed to find the minimal energy shapes for 2-D confluent cell packings. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z44.00002: Motion of individual and coupled amoebae during collective migration Chenlu Wang, Meghan Driscoll, Sagar Chowdhury, Satyandra K. Gupta, Carole Parent, Wolfgang Losert Collective migration is a ubiquitous natural phenomenon. We analyzed the migration of Dictyostelium Discoideum amoebae, which migrate both individually and collectively. We previously found that individually and collectively migrating cells have similar speed and straightness. We analyzed the effects of cell-cell contact and cell-surface contact on cell characteristics, such as adhesion, speed, and shape. We found that in the absence of cell-surface contact, cells form irregular clumps, yet are still able to migrate collectively in response to an external signal. Individually migrating cells exhibit waves of high boundary curvature that travel from the fronts to the backs of cells. By comparing the shape dynamics of individual cells and groups of cells, we found that these boundary curvature waves can be transmitted from one cell to another. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z44.00003: Cellular Particle Dynamics simulation of biomechanical relaxation processes of multi-cellular systems Matthew McCune, Ioan Kosztin Cellular Particle Dynamics (CPD) is a theoretical-computational-experimental framework for describing and predicting the time evolution of biomechanical relaxation processes of multi-cellular systems, such as fusion, sorting and compression. In CPD, cells are modeled as an ensemble of cellular particles (CPs) that interact via short range contact interactions, characterized by an attractive (adhesive interaction) and a repulsive (excluded volume interaction) component. The time evolution of the spatial conformation of the multicellular system is determined by following the trajectories of all CPs through numerical integration of their equations of motion. Here we present CPD simulation results for the fusion of both spherical and cylindrical multi-cellular aggregates. First, we calibrate the relevant CPD model parameters for a given cell type by comparing the CPD simulation results for the fusion of two spherical aggregates to the corresponding experimental results. Next, CPD simulations are used to predict the time evolution of the fusion of cylindrical aggregates. The latter is relevant for the formation of tubular multi-cellular structures (i.e., primitive blood vessels) created by the novel bioprinting technology. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z44.00004: Effects of TNF-alpha on Endothelial Cell Collective Migration Desu Chen, Di Wu, Jose Helim Aranda-Espinoza, Wolfgang Losert Tumor necrosis factor (TNF-alpha) is a small cell-signaling protein usually released by monocytes and macrophages during an inflammatory response. Previous work had shown the effects of TNF-alpha on single cell morphology, migration, and biomechanical properties. However, the effect on collective migrations remains unexplored. In this work, we have created scratches on monolayers of human umbilical endothelial cells (HUVECs) treated with 25ng/mL TNF-alpha on glass substrates. The wound healing like processes were imaged with phase contrast microscopy. Quantitative analysis of the collective migration of cells treated with TNF-alpha indicates that these cells maintain their persistent motion and alignment better than untreated cells. In addition, the collective migration was characterized by measuring the amount of non-affine deformations of the wound healing monolayer. We found a lower mean non-affinity and narrower distribution of non-affinities upon TNF-alpha stimulation. These results suggest that TNF-alpha introduces a higher degree of organized cell collective migration. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z44.00005: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z44.00006: Mitotic wavefronts mediated by mechanical signaling in early Drosophila embryos Louis Kang, Timon Idema, Andrea Liu, Tom Lubensky Mitosis in the early Drosophila embryo demonstrates spatial and temporal correlations in the form of wavefronts that travel across the embryo in each cell cycle. This coordinated phenomenon requires a signaling mechanism, which we suggest is mechanical in origin. We have constructed a theoretical model that supports nonlinear wavefront propagation in a mechanically-excitable medium. Previously, we have shown that this model captures quantitatively the wavefront speed as it varies with cell cycle number, for reasonable values of the elastic moduli and damping coefficient of the medium. Now we show that our model also captures the displacements of cell nuclei in the embryo in response to the traveling wavefront. This new result further supports that mechanical signaling may play an important role in mediating mitotic wavefronts. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z44.00007: Cells as Drops and Drops as Cells Invited Speaker: Eric R. Dufresne How do the mechanical properties of tissues emerge from the interactions of individual cells? To shed some light on this fundamental biological question, we consider a model system of clusters of cohesive cells adherent to soft substrates. We quantify traction forces over a wide range of cluster sizes. The scaling of traction stresses with cluster size suggests the emergence of an apparent surface tension for large colonies. To explore the possible impact of cellular surface tension on physiology, we consider the behavior of liquid droplets on soft substrates. In this case, we find that the competition of surface tension and substrate elasticity can lead to rich phenomenology, mimicking certain aspects of the physiology of cells and tissues. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z44.00008: Control Parameter Description of Eukaryotic Chemotaxis Eberhard Bodenschatz, Gabriel Amselem, Albert Bae, Mathias Theves, Carsten Beta The chemotaxis of eukaryotic cells depends both on the average concentration of the chemoattractant and on the steepness of its gradient. For the social amoeba Dictyostelium discoideum, we test quantitatively the prediction by Ueda and Shibata [ Biophys. J. 93 11 (2007)] that the efficacy of chemotaxis depends on a single control parameter only, namely, the signal-to-noise ratio (SNR), determined by the stochastic fluctuations of (i)~the binding of the chemoattractant molecule to the transmembrane receptor and (ii)~the intracellular activation of the effector of the signaling cascade. For SNR 1, the theory captures the experimental findings well, while for larger SNR noise sources further downstream in the signaling pathway need to be taken into account. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z44.00009: Mathematical Modeling of Bacterial Growth Samina Masood We develop a mathematical model for the study of bacterial growth. This model reproduces the growth curve from one equation as well as we can fit it to the experimental data. All the parameters of the model are discussed and compared with the already existing models. Experimental data for the bacterial growth is shown to fit this model. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z44.00010: Measuring the correlation between cell mechanics and myofibroblastic differentiation during maturation of 3D microtissues Ruogang Zhao, Weigang Wang, Thomas Boudou, Christopher Chen, Daniel Reich Tissue stiffness and cellular contractility are two of the most important biomechanical factors regulating pathological transitions of encapsulated cells, such as the differentiation of fibroblasts into myofibroblasts - a key event contributing to tissue fibrosis. However, a quantitative correlation between tissue stiffness and cellular contraction and myofibroblast differentiation has not yet been established in 3D environments, mainly due to the lack of suitable 3D tissue culture models that allow both tissue remodeling and simultaneous measurement of the cell/tissue mechanics. To address this, we have developed a magnetic microtissue tester system that allows the remodeling of arrays of cell-laden 3D collagen microtissues and the measurement of cell and tissue mechanics using magnetically actuated elastomeric microcantilevers. By measuring the development of cell/tissue mechanical properties and the expression level of $\alpha $-smooth muscle actin ($\alpha $-SMA, a marker for myofibroblast differentiation) during a 6 day culture period, we found microtissue stiffness increased by 45{\%} and $\alpha $-SMA expression increased by 38{\%}, but tissue contraction forces only increased by 10{\%}, indicating that tissue stiffness may be the predominant mechanical factor for regulation of myofibroblast differentiation. This study provides new quantitative insight into the regulatory effect of cell and tissue mechanics on cellular function. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z44.00011: Effects of Polymer Surfaces on Proliferation and Differentiation of Embryonic Stem Cells and Bone Marrow Stem Cells Sisi Qin, Wenbin Liao, Yupo Ma, Marcia Simon, Miriam Rafailovich Currently, proliferation and differentiation of stem cell is usually accomplished either \textit{in vivo}, or on chemical coated tissue culture petri dish with the presence of feeder cells. Here we investigated whether they can be directly cultured on polymeric substrates, in the absence of additional factors. We found that mouse embryonic stem cells did not require gelatin and could remain in the undifferentiated state without feeder cells at least for four passages on partially sulfonated polystyrene. The modulii of cells was measured and found to be higher for cells plated directly on the polymer surface than for those on the same surface covered with gelatin and feeder cells. When plated with feeder cells, the modulii was not sensitive to gelatin. Whereas the differentiation properties of human bone marrow stem cells, which are not adherent, are less dependent on either chemical or mechanical properties of the substrate. However, they behave differently on different toughness hydrogels as oppose to on polymer coated thin films. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z44.00012: An Elastic Model of Blebbing in Nuclear Lamin Meshworks Chloe Funkhouser, Rastko Sknepnek, Takeshi Shimi, Anne Goldman, Robert Goldman, Monica Olvera de la Cruz A two-component continuum elastic model is introduced to analyze a nuclear lamin meshwork, a structural element of the lamina of the nuclear envelope. The main component of the lamina is a meshwork of lamin protein filaments providing mechanical support to the nucleus and also playing a role in gene expression. Abnormalities in nuclear shape are associated with a variety of pathologies, including some forms of cancer and Hutchinson-Gilford progeria syndrome, and are often characterized by protruding structures termed nuclear blebs. Nuclear blebs are rich in A-type lamins and may be related to pathological gene expression. We apply the two-dimensional elastic shell model to determine which characteristics of the meshwork could be responsible for blebbing, including heterogeneities in the meshwork thickness and mesh size. We find that if one component of the lamin meshwork, rich in A-type lamins, has a tendency to form a larger mesh size than that rich in B-type lamins, this is sufficient to cause segregation of the lamin components and also to form blebs rich in A-type lamins. The model produces structures with comparable morphologies and mesh size distributions as the lamin meshworks of real, pathological nuclei. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z44.00013: The actin cytoskeleton of chemotactic amoebae operates close to the onset of oscillations Christian Westendorf, Jose Negrete Jr., Albert Bae, Rabea Sandmann, Eberhard Bodenschatz, Carsten Beta We report evidence that the actin machinery of chemotactic Dictyostelium cells operates close to an oscillatory instability. The averaged F-actin response of many cells to a short-time pulse of cAMP is reminiscent of a damped oscillation. At the single-cell level, however, the response dynamics ranged from short, strongly damped responses to slowly decaying, weakly damped oscillations. Furthermore, in a small subpopulation, we observed self-sustained oscillations in the cortical F-actin concentration. We systematically exposed a large number of cells to periodic pulse trains. The results indicate a resonance peak at periodic inputs of around 20 s. We propose a delayed feedback model that explains our experimental findings based on a time-delay in the actin regulatory network. To quantitatively test the model, we performed stimulation experiments with cells that express GFP-tagged fusion proteins of Coronin and Aip1. These served as markers of the F-actin disassembly process and thus allow us to estimate the delay time. Based on this independent estimate, our model predicts an intrinsic period of 20 s, which agrees with the resonance observed experimentally. [Preview Abstract] |
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