Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C13: Physics of Biological Active Matter I: Cell ColoniesFocus Live
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Sponsoring Units: DBIO DPOLY DSOFT Chair: Ricard Alert, Princeton University; Joshua Shaevitz, Princeton University |
Monday, March 15, 2021 3:00PM - 3:36PM Live |
C13.00001: Mechanical behavior of a migrating cell monolayer. Invited Speaker: Helene Delanoe-Ayari Collective cell migration contributes to morphogenesis, wound healing or tumor metastasis. Culturing epithelial monolayers on a substrate is an in vitro configuration suitable to quantitatively characterize such |
Monday, March 15, 2021 3:36PM - 3:48PM Live |
C13.00002: Rigidity sensing regulates epithelial monolayer integrity Shaozhen LIN, Surabhi Sonam, Lakshmi Balasubramaniam, Jacques Roger Prost, Jean-François Rupprecht, René-Marc Mège, Benoît Ladoux Loss of epithelial integrity is known to play a pathological role in cancer metastasis while serving a physiological role in several developmental processes. Yet the physical mechanisms leading to hole formations within epithelial monolayers remains to clarified. Here, we observed the spontaneous formation of holes within epithelial monolayers in a substrate rigidity dependent manner which predominantly occurred on soft substrates. To decipher the physical mechanism of hole opening both at the cell and tissue levels, we design a cell-based computational framework (called vertex model) whereby we evaluate two possible models where holes may either form along junctions experiencing high tensile normal stresses (model 1) or along junctions of fast shear strain (model 2). We find that model 1 is more successful to describe the experimental observations on the location of hole opening events. In addition, based on a continuum hydrodynamic theory of 2D epithelial tissues (called active nematic theory), we propose that the forces driving the hole formation may be non-conservative (i.e. strain-rate related) with an overall dynamics displaying an effective inertia related to an expectedly long cell-shape relaxation time (in the 100 min range). |
Monday, March 15, 2021 3:48PM - 4:00PM Live |
C13.00003: Clustering and ordering in cell assemblies with generic asymmetric aligning interactions Thibault Bertrand, Joseph d'Alessandro, Ananyo Maitra, René-Marc Mège, Benoît Ladoux, Raphaël Voituriez Collective cell migration plays an essential role in various biological processes, such as development or cancer proliferation. While cell-cell interactions are clearly key determinants of collective cell migration, the physical mechanisms that control the emergence of cell clustering and collective cell migration are still poorly understood. Binary cell-cell collisions generally lead to anti-alignment of cell polarities and separation of pairs -- a process called contact inhibition of locomotion (CIL), which is expected to hinder the formation of coherent large scale cell clusters. Here, we report on a joint experimental and theoretical approach in which we determine the large scale dynamics of cell assemblies from elementary pairwise cell-cell interaction rules. We develop a generic equilibrium-like pairwise asymmetric aligning interaction potential that reproduces the CIL phenomenology. We build the corresponding active hydrodynamic theory and show that such asymmetric aligning interaction generically destroys large scale clustering and ordering. We conclude that CIL-like asymmetric interactions in general active systems control cluster sizes and polarity, and can prevent large-scale coarsening and long-range polarity, except in the singular regime of dense confluent systems. |
Monday, March 15, 2021 4:00PM - 4:12PM Live |
C13.00004: Boundary conditions determine emergence and dynamics of density patterns in microbial suspensions Nicole Drewes, Alexandros Fragkopoulos, Oliver Baeumchen Motility is vital for many microorganisms to explore and identify favorable habitats and living conditions. Self-propelled microbes exhibit a range of unique phenomena, including the separation into phases of high and low cell densities. We showed that dense suspensions of motile Chlamydomonas reinhardtii cells aggregate in 2D confinement in response to a reduction of the light intensity. At low light conditions the cells photosynthetic activity is suppressed and a metabolic switch from photosynthesis to respiration creates an inhomogeneous oxygen field in the compartment. The oxygen concentration is directly linked to the motility of the cells, thus creating localized patterns of low and high density. |
Monday, March 15, 2021 4:12PM - 4:24PM Live |
C13.00005: Deciphering how forces pull the nucleus during confined cell motility. Patricia Davidson, Sirine Amiri, Cécile Sykes The ability of cells to squeeze through constrictions is affected by the stiffness of the large and rigid nucleus, and is a hallmark of metastases and cancer progression. During this process, a sufficient force needs to be applied by the cytoskeleton to the nucleus. What is the mechanism of nucleus pulling during cell translocation through openings smaller than the nuclear diameter is what we address here.We recently showed that nuclear envelope proteins, such as nesprins, which mechanically link the actin cytoskeleton to the nuclear membrane, move towards the front at the surface of the nucleus1. SUN proteins follow the movement of nesprins. Moreover, we demonstrated that nesprin movement is initiated by the actin cytoskeleton which redistributes nesprins on the nuclear membrane and contributes to pulling. Myosin have a complementary role. We thus reveal one gear of the “mechanostransduction” chain between the cytoskeleton and the nucleoskeleton. |
Monday, March 15, 2021 4:24PM - 4:36PM Live |
C13.00006: Collective migration of cells in confinement - contact following and contact inhibition of locomotion Pedrom Zadeh, Brian Camley Collective cell migration, in which cells crawl in a coordinated, coherent way, is crucial in embryonic development and cancer metastasis. The extent of collective migration is controlled by how cells interact, which is often studied in cell pairs. When crawling cells collide head-on, some perform contact inhibition of locomotion (CIL), repolarizing away from contact and separating. However, recent work has established that cells may induce cells in contact with their back to follow them - "contact following of locomotion" (CFL). We build a model for CIL and CFL within the phase-field approach, which treats cells as continuously deformable objects. We hypothesize that cells can sense their velocity and repolarize to align to it over a finite timescale. With this assumption, we are able to reproduce the observed CIL within narrow linear confinement. We show the probability of CIL outcomes depends on the strength of alignment between cell velocity and polarity. We can also capture the experimental CFL behavior of cells migrating into Y-junctions. We show that the outcome of a train of cells meeting a Y-junction depends on junction geometry, strength of alignment and cell-cell adhesions. |
Monday, March 15, 2021 4:36PM - 4:48PM Live |
C13.00007: Collective chemotaxis in a vertex model for confluent clusters Elizabeth Lawson-Keister, M Lisa Manning Many cell types exhibit chemotaxis, where they sense and climb biochemical signaling gradients in their microenvironments. Other cell types, like neural crest cells in lymphocytes and border cells in drosophila melanogaster, cannot climb gradients individually, but are able to chemotax in small clusters. Previous attempts at modeling collective chemotaxis have focused on particle-based models for cells, which may miss important interactions between cells in a confluent cluster, where there are no gaps or overlaps between cells. Therefore, we construct a 2D Voronoi model for a confluent tissue that incorporates feedback between individual cell properties and a biochemical signaling gradient. In our model, cells can sense the local concentration and adapt their individual surface tensions in response. This simple coupling of signaling and cell properties allows a cluster of cells to climb a gradient, while an individual cell is unable to do so. |
Monday, March 15, 2021 4:48PM - 5:00PM Live |
C13.00008: A Mechano-chemical cell cycle for growing cell colonies Jintao Li, Simon Schnyder, Matthew Turner, Ryoichi Yamamoto Living cells coexist in colonies or tissues. We develop a simplified model of the cell cycle, the fundamental regulatory network controlling growth and division, and couple this to physical stress. In its simplest form this model involves three characteristic times (for adaptation of the cell cycle and volume, as well as a division time), two cell volumes (unstressed and quiescent) and a pressure at which the cell cycle stalls. We employ both particle-based computer simulations and a continuum theory to study quasi-2D colonies growing in a channel. These exhibit moving growth fronts with a profile and speed that are non-trivially related to the substrate friction and the mechano-chemical cell cycle parameters, providing a possible approach to measure such parameters in experiments. |
Monday, March 15, 2021 5:00PM - 5:12PM Live |
C13.00009: Fluctuations can induce local nematic order and extensile stress in motile cell monolayers Farzan Vafa, Mark J Bowick, Boris I Shraiman, M. Cristina Marchetti Recent experiments in various cell types have shown that two-dimensional tissues often display local nematic order, with evidence of extensile stresses manifest in the dynamics of topological defects. Using a mesoscopic model where tissue flow is generated by fluctuating traction forces coupled to the nematic order parameter, we show that the resulting tissue dynamics can spontaneously produce local nematic order and an extensile internal stress. A key element of the model is the assumption that in the presence of local nematic alignment, cells preferentially crawl along the nematic axis, resulting in anisotropy of fluctuations. Our work shows that activity can drive either extensile or contractile stresses in tissue, depending on the relative strength of the contractility of the cortical cytoskeleton and tractions by cells on the extracellular matrix. |
Monday, March 15, 2021 5:12PM - 5:24PM Live |
C13.00010: Hydrodynamic self-assembly of living chiral crystals Hugh Higinbotham, Tzer Han Tan, Alexander Mietke, Yuchao Chen, Peter Foster, Shreyas Gokhale, Jorn Dunkel, Nikta Fakhri
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Monday, March 15, 2021 5:24PM - 5:36PM Live |
C13.00011: Active wrinkling in viscoelastic thin sheets Rastko Sknepnek, Daniel Matoz Fernandez, Nicola R. Stanley-Wall, Fordyce A. Davidson Despite significant progress in understanding the behavior of active fluids, much less is known about how activity affects the behavior of solid and viscoelastic materials, such as epithelial tissues or biofilms. In this talk, we will show that a viscoelastic thin sheet driven out of equilibrium by active structural remodeling (e.g., fast growth) develops a wide variety of shapes as a result of a competition between viscous relaxation and activity. In the regime where active processes are faster than viscoelastic relaxation, shapes that are formed due to remodeling are inherently out of equilibrium. The later regime is of particular interest in developing a physical understanding of morphogenesis, where the embryo has to undergo a series of carefully orchestrated shape changes to establish the functioning organism. Our study suggests that keeping a growing system out of equilibrium increases the range of available morphologies. These observations point to a robust mechanism by which a system that is kept out of equilibrium could be steered toward the desired shape by chemical regulation of remodeling, relaxation, and mechanical parameters. |
Monday, March 15, 2021 5:36PM - 5:48PM Live |
C13.00012: Unravelling the photoreceptor underlying light-switchable adhesion of Chlamydomonas by micropipette force measurements Antoine Girot, Rodrigo Catalan, Luiza Vargas, Simon Kelterborn, Peter Hegemann, Oliver Baeumchen Bioadhesion is a ubiquitous phenomenon for many living systems such as mussels, bacteria or microalgae. In this presentation, we focus on the adhesion of the biflagellated unicellular model organism Chlamydomonas reinhardtii. We discovered that Chlamydomonas exhibits light-switchable adhesion [Kreis et al., Nature Physics, 2018], in which its flagella adhere to any type of surface [Kreis et al., Soft Matter, 2019] under blue but not under red light. Our goal is to unravel the blue-light sensitive photoreceptor that triggers this highly specific light-regulated behaviour. We employ in vivo micropipette force measurements [Backholm & Bäumchen, Nature Protocols, 2019] on single wild-type cells to precisely identify the full spectral response of flagellar adhesion using narrow bandpass filters and thereby detect the characteristic action spectrum of the photoreceptor associated to cell adhesion. We also present adhesion force measurements of single mutant cells, for which a specific photoreceptor was deleted by means of modern gene editing tools. |
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