Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F07: Biological Active Matter II: Eukaryotic Cells and TissuesRecordings Available
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Sponsoring Units: DBIO Chair: Sujit Datta, Princeton University Room: McCormick Place W-179A |
Tuesday, March 15, 2022 8:00AM - 8:12AM |
F07.00001: Non-liquid behavior of coalescing droplets of liquid-like cellular aggregates Haicen Yue, Daniel M Sussman, Justin C Burton The fusion of cellular aggregates, such as droplets composed of thousands of epithelial cells, is often modeled by treating each droplet as a liquid-like collection of interacting particles. We conduct numerical simulations of droplet coalescence using both standard particulate models and vertex models meant to more accurately represent the dynamics and mechanics of dense cellular matter. We find that even in the liquid-like regime, the fusion process of model biological droplets can be very different from that of standard liquid coalescence. These differences appear both at the early stages of coalescence (where "thermal" capillary effects are important) and even more markedly at intermediate and late stages (where the scaling laws describing the overall fusion process are characterized by anomalous exponents). We discuss how the character of the model used leads to these unusual dynamical properties. By comparing both our particulate and cellular simulations with experimental results on cell droplet fusion we highlight ongoing challenges in understanding the physics of these mesoscale biological systems. |
Tuesday, March 15, 2022 8:12AM - 8:24AM |
F07.00002: Learning developmental mode dynamics from single-cell trajectories Nicolas Romeo, Alasdair Hastewell, Alexander Mietke, Jorn Dunkel Over the last years, progress in high-resolution in-vivo imaging has provided unprecedented insight into the collective cell dynamics at different stages of embryogenesis. These rapid experimental advances pose the theoretical challenge of translating high-dimensional imaging data into predictive low-dimensional dynamical models that capture the essential principles governing developmental cell migration. Here, we have combined mode decomposition ideas that have proved successful in condensed matter and fluid physics with sparse dynamical systems inference to learn interpretable biophysical models from single-cell imaging data. Using zebrafish embryos as an example, we discuss how cell trajectory data can be coarse-grained and compressed. The resulting low-dimensional representation allows for quantitative comparisons of cellular and active Brownian particle dynamics, revealing similarities between their short term behaviors and emerging differences at longer time-scales. |
Tuesday, March 15, 2022 8:24AM - 8:36AM |
F07.00003: Models of Galvanotaxis: Coupling Cell Migration and Shape Ifunanya Nwogbaga, Brian A Camley During wound healing, keratocyte cells undergo galvanotaxis where they follow a wound-induced electric field (EF). In addition to their directed motion, keratocytes can also exhibit oscillatory and circular motion. We developed a coarse-grained phenomenological model that qualitatively captures these keratocyte behaviors and fits experimental data on response to a field being turned on. A critical element of our model is a tendency for cells to turn toward their long axis, arising from a coupling between cell shape and velocity, which gives rise to oscillatory and circular motion. The parameters that most affect galvanotaxis are cell speed, cell shape relaxation rate, and rate of polarization to the field. When the cell reacts to an EF being turned on, our model predicts that stiff, slow cells react slowly but follow the signal more reliably. Cells that polarize and align to the field at a faster rate react more quickly and follow the signal more reliably. When cells are exposed to a field that switches direction rapidly, cells follow the average of field directions, while if the field is switched more slowly, cells follow a “staircase” pattern. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F07.00004: Dynamics of an expanding cell monolayer Evgeniy Khain In this talk, we consider an expansion of a dense monolayer of cells: a collective multicellular phenomenon, where cells divide, grow, and maintain contacts with their neighbors. During migration, cells display complex behavior, adjusting both their division rate and their growth after division to the local mechanical environment. Experimental observations show that cells near the edge of the expanding monolayer are larger and move faster than cells deep inside the colony. To explain these observations and describe cell migration patterns, we formulate a spatio-temporal theoretical model for multicellular dynamics in terms of the cell area distribution; the model includes cell growth after division and effective pressure. Numerical simulations of the model predict both the speed of invasion and the width of the outer proliferative rim; these predictions are in a good agreement with experimental observations. Theoretical analysis yields the equation for density of cells and reveals a novel type of propagating front with compact support. The velocity of front propagation (monolayer expansion) is obtained analytically and its dependence on all the relevant parameters is determined. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F07.00005: How Short-Range Forces Generate Long Range Order in Extreme Tissue Deformations Tapan Goel, Ellen Adams, Cassidy M Tran, Trevor Rowe, Johanna Schubert, Patrick H Diamond, Eva-Maria S Collins Extreme deformations in epithelial tissues play a key role in animal development and physiology. Tissue deformations have long range order and often arise from stochastic, short-range forces that must be coordinated in the absence of a master regulator. We use Hydra mouth opening as an in vivo biomechanics model for extreme tissue deformations. The mouth opening is tens of cell diameters wide, radially symmetric and occurs within 1-2 minutes. This extreme deformation is produced by stochastic contractile forces (tugs) acting over 2-3 cell diameters. Using Hydra’s regenerative capabilities, we employ tissue excision, grafting, and live imaging to identify the organizing “feature” that coordinates the tugs. Our experiments suggest that mechanical cell-cell coupling coordinate the tugs that pull the mouth open and that the boundary conditions control symmetry. Modelling the epithelial tissue as an active viscoelastic continuum, we identify how the stochastic force and tissue mechanical properties must be tuned to produce the observed opening behavior. We then relax the parameter space to identify regions that permit global order to arise from stochastic local forces more generally. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F07.00006: Scaling entangled active matter locomotion Chantal Nguyen, Saad Bhamla, Orit Peleg Many worms, nematodes, and arthropods form dense aggregations in which constant physical contact between constituent individuals can give rise to emergent macroscale behavior. For instance, centimeter-sized, high-aspect ratio California blackworms (Lumbriculus variegatus) collectively move as a blob using physical entanglements, while millimeter-sized low-aspect ratio C. elegans swarm together using motility-induced phase separation. Motivated by these observations, we model individual organisms as active polymers that attract each other to form a collective. Using this model, we investigate the scaling dynamics and functional trade-offs based on geometry, size and aspect ratio of individuals. By experimentally testing these physical principles on living systems from different biological taxa, we explore the consequences of morphological forms of the individual on the emergent properties of the entangled living collective. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F07.00007: Topological defects reveal signatures of odd elasticity in a living chiral crystal Yuchao Chen, Yu-Chen Chao, Junang Li, Alexander Mietke, Tzer Han Tan, Jorn Dunkel, Nikta Fakhri Materials with nonreciprocal microscopic interactions can exhibit unusual properties such as odd elasticity. Recently, we have discovered a naturally occurring chiral crystal of starfish embryos which exhibits signatures of odd elasticity. Here, we investigate the mechanics and dynamics of topological defects in these living chiral crystals. We analyze the strain field around a dislocation, the motion of isolated dislocations in relation to their Burgers vector as well as pair dislocation interactions. Our results are in agreement with recently developed theories of odd elasticity and further reveal the odd elastic nature of the living chiral crystal |
Tuesday, March 15, 2022 9:24AM - 9:36AM |
F07.00008: Gradients in Surface Tension Drive Global Flows in Cell Aggregates Vikrant Yadav, Sulaiman Yousafzai, Sorosh Amiri, Robert Styles, Eric R Dufresne, Michael P Murrell The surface tension of living cells and tissues originates from active stresses within the cytoskeleton. We perturb surface tension in model tissues using laser ablation. Upon sudden release of surface tension at a point, cells move rapidly throughout the entire sample. Subsequently, cells reverse and slowly return to their original positions as tensional homeostasis is re-established. These movements resemble viscous flow, even for flow durations that are significantly smaller than the viscoelastic timescales. Analysis of deformation fields allows us to evaluate the magnitude of the surface tension perturbation, which we find to be proportional to the aggregate size and correlated to the pre-strain of cells at the surface. Together these analyses suggest that the existence of a size-dependent tension and requirement of maintaining tensional homeostasis determines the magnitude of global flow velocities and their temporal orientation, respectively. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F07.00009: Active mechanics of starfish oocytes Peter J Foster, Sebastian Fürthauer, Nikta Fakhri Actomyosin is a canonical example of an active material, driven out of equilibrium in part through the injection of energy by myosin motors. This influx of energy allows actomyosin networks to generate cellular-scale contractility, which underlies cellular processes ranging from division to migration. While the molecular players underlying actomyosin contractility have been well characterized, how cellular-scale deformation in disordered actomyosin networks emerges from filament-scale interactions is not well understood. Here, we address this question in vivo using the meiotic surface contraction wave of starfish oocytes. Using pharmacological treatments targeting actin polymerization, we find that the rate of cellular deformation is not a monotonic function of cortical actin density, but is instead peaked near the wild type density. To understand this, we develop an active fluid model coarse-grained from filament-scale interactions and find quantitative agreement with the measured data. This model further predicts the dependence of the strain rate on the concentrations of active motors and passive actin crosslinkers, which we experimentally verify. Taken together, this work is an important step towards bridging the molecular and cellular length scales for cytoskeletal networks. |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F07.00010: Non-equilibrium shape fluctuations in living cells report driving forces and organelle mechanics Kengo Nishi, Sufi Raja, An Pham, Ronit Freeman, Antonina Roll-Mecak, Frederick C MacKintosh, Christoph F Schmidt The cytoplasm of cells is an active composite material containing a high concentration of macromolecules, including motor proteins, as well as cytoskeletal filaments and a multitude of organelles. The mechanical properties of cytoskeletal filaments are important for their dynamics and functions. Microtubules (MTs) are relatively rigid and form extended networks. Their mechanical properties in the cell are believed to be regulated by post-translational modifications, but in vivo measurements of MT mechanics are rather difficult. We here report a method using motor-generated forces that deform MTs in living cells (which can also be applied to other rod-shaped organelles), to measure both, the driving forces and the elastic properties of the MTs. MT bending dynamics are governed by their material properties, the active forces, and the response characteristics of the cytoplasm. We probe the cytoplasm by active magnetic-bead microrheology, and can thus determine the other two quantities. Reversing this logic, we show that the cytoplasmic response can alternatively be measured from localized large-amplitude bends of MTs of known stiffness. We present a theoretical description and measure the bending stiffness of MTs in vivo, finding general agreement with published in vitro data. We then study the effect of polyglutamylation, a post-translational modification, of MTs on their stiffness. We find that polyglutamylation significantly stiffens microtubules. |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F07.00011: Principles of cellular behavior: a unicellular walker controlled by a microtubule-based finite state machine Ben T Larson, Jack Garbus, Jordan B Pollack, Wallace F Marshall
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Tuesday, March 15, 2022 10:12AM - 10:24AM |
F07.00012: Active nematics on deformable surfaces Waleed A Mirza, Marino Arroyo, Alejandro Torres-Sánchez, Guillermo Vilanova Morphogenesis in biological surfaces resulting from an interplay among nematic order, geometric properties, topological constraints and hydrodynamic interaction has been a subject of experimental investigation. From the computational standpoint, this interplay has been widely explored on rigid surfaces using coarse-grained or agent-based active nematic models. However, shape change in deformable surfaces resulting from this tight coupling has been explored in fewer studies. In this study, we propose an active gel model that accounts for shape changes in a deformable surface resulting from an interplay between the evolution of orientational order, Newtonian Rheology as well as active flows. We describe the numerical solution of the resulting Euler-Lagrange equations for biologically relevant case studies such as for shape changes in the manifold of contractile cytoskeleton network undergoing cell division and for shape changes in a monolayer of microtubules and kinesin enclosed in a lipid vesicle. The trends obtained from numerical studies are compared and verified against the experimental data. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F07.00013: Curvature induces active matter velocity waves in rotating multicellular spheroids Tom Brandstätter, David B Brückner, Yulong Han, Ricard Alert, Ming Guo, Chase P Broedersz Collective behavior of cells in 3D curved environements determine the multicellular organization of diverse systems, such as embryos, intestines and tumors. In these settings, cells establish supracellular patterns of motion, including collective rotation and invasion. While such collective modes are increasingly well understood in 2D flat systems, the consequences of geometrical and topological constraints on collective cell migration in 3D curved tissues are largely unknown. We study 3D collective migration in a mammary cell spheroid, which represents a common and conceptually simple curved geometry. We discover that these rotating spheroids exhibit a collective mode of cell migration in the form of a velocity wave propagating along the equator with a wavelength equal to the spheroid perimeter. This wave is accompanied by a pattern of incompressible cellular flow across the spheroid surface featuring topological defects and motion along geodesics. Using a minimal active particle model, we reveal that this collective mode originates from the active flocking behaviour of an incompressible cell layer confined to a curved surface. Our results identify curvature-induced velocity waves as a generic active matter mode, which could manifest in a wide range of 3D curved active systems. |
Tuesday, March 15, 2022 10:36AM - 10:48AM |
F07.00014: A competitive advantage through fast dead matter elimination in confined cellular aggregates Yoav G Pollack, Yoav G Pollack, Philip Bittihn, Ramin Golestanian Competition of different cell types for limited space is relevant in biological processes such as tissue morphogenesis and tumor growth. Predicting the outcome for non-adversarial competition of such growing active matter is non-trivial, as it depends on how processes like growth, proliferation and the degradation of cellular matter are regulated in confinement; regulation that happens even in the absence of competition to achieve homeostasis. We show that passive by-products of the processes maintaining homeostasis can significantly alter fitness, enabling cell types with lower homeostatic pressure to outcompete those with higher homeostatic pressure. Using both a theoretical toy model and an agent-based computational model that include finite-time mechanical persistence of dead cells, we reveal that interfaces play a critical role in the competition: There, growing matter with a higher proportion of active cells can better exploit local growth opportunities that continuously arise as the active processes keep the system out of mechanical equilibrium. Our results show that optimizing the proportion of growing (active) cells can be as important to survival as sensitivity to mechanical cues. |
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