Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V5: Physics of Cellular OrganizationFocus
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Sponsoring Units: DBIO GSNP Chair: Michael Gramlich, Washington University School of Medicin Room: 264 |
Thursday, March 16, 2017 2:30PM - 3:06PM |
V5.00001: Sharing is Caring: The Role of Actin/Myosin-V in Synaptic Vesicle Transport between Synapses in vivo Invited Speaker: Michael Gramlich Inter-synaptic vesicle sharing is an important but not well understood process of pre-synaptic function. Further, the molecular mechanisms that underlie this inter-synaptic exchange are not well known, and whether this inter-synaptic vesicle sharing is regulated by neural activity remains largely unexplored. I address these questions by studying CA1/CA3 Hippocampal neurons at the single synaptic vesicle level. Using high-resolution tracking of individual vesicles that have recently undergone endocytosis, I observe long-distance axonal transport of synaptic vesicles is partly mediated by the actin network. Further, the actin-dependent transport is predominantly carried out by Myosin-V. I develop a correlated-motion analysis to characterize the mechanics of how actin and Myosin-V affect vesicle transport. Lastly, I also observe that vesicle exit rates from the synapse to the axon and long-distance vesicle transport are both regulated by activity, but Myosin-V does not appear to mediate the activity dependence. These observations highlight the roles of the axonal actin network, and Myosin-V in particular, in regulating inter-synaptic vesicle exchange. [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:18PM |
V5.00002: Cytoplasmic Flow Enhances Organelle Dispersion in Eukaryotic Cells Elena Koslover, Saurabh Mogre, Caleb Chan, Julie Theriot The cytoplasm of a living cell is an active environment through which intracellular components move and mix. We explore, using theoretical modeling coupled with microrheological measurements, the efficiency of particle dispersion via different modes of transport within this active environment. In particular, we focus on the role of cytoplasmic flow over different scales in contributing to organelle transport within two different cell types. In motile neutrophil cells, we show that bulk fluid flow associated with rapid cell deformation enhances particle transport to and from the cell periphery. In narrow fungal hyphae, localized flows due to hydrodynamic entrainment are shown to contribute to optimally efficient organelle dispersion. Our results highlight the importance of non-traditional modes of transport associated with flow of the cytoplasmic fluid in the distribution of organelles throughout eukaryotic cells. [Preview Abstract] |
Thursday, March 16, 2017 3:18PM - 3:30PM |
V5.00003: Cell Proliferation on Planar and Curved Substrates Michelle Gaines, Ya Wen Chang, Ricardo Cruz, Alexandros Fragkopoulos, Andres Garcia, Alberto Fernandez-Nieves Aberrant epithelial collective cell growth is one of the major challenges to be addressed in order to treat diseases such as cancer and organ fibrosis. The conditions of the extracellular microenvironment, properties of the cells' cytoskeleton, and interfacial properties of the substratum (the surface in contact with epithelial cells) have a significant influence on the migratory behavior of epithelial cells, cell proliferation and migration. This work focuses on understanding the impact the substratum curvature has on cell behavior. We focus on cell proliferation first and study MDCK cells on both planar and curved hydrogel substrates. The curved hydrogels are based on polyacrylamide and have toroidal shape, with tube radius \textasciitilde 200 um and an aspect ratio in the rage between 2 and 9. Proliferation is measured using the Click-it EDU assay (Invitrogen), which measures cells that are synthesizing DNA. [Preview Abstract] |
Thursday, March 16, 2017 3:30PM - 3:42PM |
V5.00004: Active Tension Network model reveals an exotic mechanical state realized in epithelial tissues Nicholas Noll, Madhav Mani, Idse Heemskerk, Sebastian Streicha, Boris Shraiman Mechanical interactions play a crucial role in epithelial morphogenesis, yet understanding the complex mechanisms through which stress and deformation affect cell behavior remains an open problem. Here we formulate and analyze the Active Tension Network (ATN) model, which assumes that mechanical balance of cells is dominated by cortical tension and introduces tension dependent active remodeling of the cortex. We find that ATNs exhibit unusual mechanical properties: i) ATN behaves as a fluid at short times, but at long times it supports external tension, like a solid; ii) its mechanical equilibrium state has extensive degeneracy associated with a discrete conformal - "isogonal" - deformation of cells. ATN model predicts a constraint on equilibrium cell geometry, which we demonstrate to hold in certain epithelial tissues. We further show that isogonal modes are observed in a fruit fly embryo, accounting for the striking variability of apical area of ventral cells and helping understand the early phase of gastrulation. Living matter realizes new and exotic mechanical states, understanding which helps understand biological phenomena. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 4:18PM |
V5.00005: Microtubule defects influence kinesin-based transport in vitro. Invited Speaker: Jing Xu Microtubules are protein polymers that form ``molecular highways'' for long-range transport within living cells. Molecular motors actively step along microtubules to shuttle cellular materials between the nucleus and the cell periphery; this transport is critical for the survival and health of all eukaryotic cells. Structural defects in microtubules exist, but whether these defects impact molecular motor-based transport remains unknown. Here, we report a new, to our knowledge, approach that allowed us to directly investigate the impact of such defects. Using a modified optical-trapping method, we examined the group function of a major molecular motor, conventional kinesin, when transporting cargos along individual microtubules. We found that microtubule defects influence kinesin-based transport in vitro. The effects depend on motor number: cargos driven by a few motors tended to unbind prematurely from the microtubule, whereas cargos driven by more motors tended to pause. To our knowledge, our study provides the first direct link between microtubule defects and kinesin function. The effects uncovered in our study may have physiological relevance in vivo. [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:30PM |
V5.00006: Disorder Measures and Non-equilibrium States of Cellular Matter Sascha Hilgenfeldt, Sangwoo Kim, Yiliang Wang Cellular materials such as foams, emulsions, or biological tissues in general have a plethora of configurations in mechanical equilibrium. Identifying a global minimum (ground state) in a disordered domain system is a formidable task. However, protocols for lowering total energy through successive topological transitions have been suggested. Through modeling and simulations, we investigate systematic energy variation through a sequence of local equilibrium states, and the parallel changes in various measures of disorder and size-topology correlation in the structure. Statistical measures are identified that allow for quantification of the distance of the current structure from the ground state. This work can be applied as a tool to assess the mechanical state of foam or tissue structures from visual information only, with applications ranging from tissue diagnostics to regenerative medicine. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V5.00007: Phase field models of Dictyostelium discoideum migration Yunsong Zhang, Yanxiang Zhao, Brian Camley, Wouter-Jan Rappel, Herbert Levine The migration of eukaryotic cells is a result of the interplay between quite a few different factors, including cell mechanics and biochemistry. Such complexity has brought great challenges in the modeling of individual moving cells. Coupling biochemistry, cellular mechanics together with changing cell morphology, phase field models have been successful in explaining some behaviors of moving cells, such as periodic movements, rotations and turning. Here, we extend our current phase field to the situations with membrane-bound biochemical processes, to provide a framework for studying Dictyostelium discoideum, which exhibit extremely irregular morphology in migration. The phase field model will not only implement the tracking of cell shape, but also enable the studies of traction force patterns the cell may exert on the substrate. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V5.00008: Chemotaxis: A Multi-Scale Modeling Approach. Arpan Bhowmik We are attempting to build a working simulation of population level self-organization in dictyostelium discoideum cells by combining existing models for chemo-attractant production and detection, along with phenomenological motility models. Our goal is to create a computationally-viable model-framework within which a population of cells can self-generate chemo-attractant waves and self-organize based on the directional cues of those waves. The work is a direct continuation of our previous work published in Physical Biology titled ``Excitable waves and direction-sensing in Dictyostelium Discoideum: steps towards a chemotaxis model''. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V5.00009: Bursts of activity in collective cell migration Caterina La Porta, Oleksandr Chepizhko, Costanza Giampietro, Eleonora Mastrapasqua, Mehdi Nourazar, Miriam Ascagni, Michela Sugni, Umberto Fascio, Livio Leggio, Chiara Malinverno, Giorgio Scita, Stephane Santucci, Mikko Alava, Stefano Zapperi Dense monolayers of living cells display intriguing relaxation dynamics, reminiscent of soft and glassy materials close to the jamming transition, and migrate collectively when space is available, as in wound healing or in cancer invasion. Here we show that collective cell migration occurs in bursts that are similar to those recorded in the propagation of cracks, fluid fronts in porous media and ferromagnetic domain walls. In analogy with these systems, the distribution of activity bursts displays scaling laws that are universal in different cell types and for cells moving on different substrates. The main features of the invasion dynamics are quantitatively captured by a model of interacting active particles moving in a disordered landscape. Our results illustrate that collective motion of living cells is analogous to the corresponding dynamics in driven, but inanimate, systems. [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V5.00010: Collective Cellular Decision-Making Gives Developmental Plasticity: A Model of Signaling in Branching Roots W. Tyler McCleery, Nadiatul A. Mohd-Radzman, Veronica A. Grieneisen Cells within tissues can be regarded as autonomous entities that respond to their local environment and signaling from neighbors. Cell coordination is particularly important in plants, where root architecture must strategically invest resources for growth to optimize nutrient acquisition. Thus, root cells are constantly adapting to environmental cues and neighbor communication in a non-linear manner. To explain such plasticity, we view the root as a swarm of coupled multi-cellular structures, "metamers", rather than as a continuum of identical cells. These metamers are individually programmed to achieve a local objective - developing a lateral root primordia, which aids in local foraging of nutrients. Collectively, such individual attempts may be halted, structuring root architecture as an emergent behavior. Each metamer's decision to branch is coordinated locally and globally through hormone signaling, including processes of controlled diffusion, active polar transport, and dynamic feedback. We present a physical model of the signaling mechanism that coordinates branching decisions in response to the environment. [Preview Abstract] |
Thursday, March 16, 2017 5:18PM - 5:30PM |
V5.00011: Cell morphology, budding propensity and cell death of \emph{ Saccharomyces cerevisiae} at high hydrostatic pressure Khanh Nguyen, Jeffrey Lewis, Pradeep Kumar A large biomass on earth thrives in extremes of physical and chemical conditions including high pressure and temperature. Budding yeast, \emph{S. cerevisiae}, is a eukaryotic model organism due to its amenability to molecular biology tools. To understand the effects of hydrostatic pressure on a eukaryotic cell, we have performed quantitative experiments of the growth, the propensity of budding, and cell death of \emph{S. cerevisiae} in a wide range of pressures. An automated image analysis method for the quantification of the budding index was developed and applied along with a continuum model of budding to investigate the effects of pressure on cell division and cell morphology. We find that the growth, the budding propensity, the average cell size, and the ellipticity of the cells decrease with increasing pressure. Furthermore, large hydrostatic pressure led to the small but finite probability of cell death. Our experiments suggest that the decrease of budding propensity arises from cellular arrest at the cell cycle checkpoints during different stages of cell division. [Preview Abstract] |
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