Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F61: Active Matter IIIFocus
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Sponsoring Units: GSOFT DBIO GSNP Chair: Karsten Kruse, Max Planck Institute for Chemical Physics of Solids Room: BCEC 258B |
Tuesday, March 5, 2019 11:15AM - 11:51AM |
F61.00001: Self-Organizing Microtubule Spindles Invited Speaker: Jennifer Ross The cell is a complex autonomous machine taking in information, performing computations, and responding to the environment. Many of the internal structures and architecture is transient and created through active processes. Recent advances in active matter physics with biological elements are opening new insights into the physics behind how cellular organizations are generated, maintained, and destroyed. I will present several stories about how microtubules can be self-organized into cellular structures using molecular motors (kinesin-1), crosslinking proteins (MAP65), and the inherent microtubule polymerization. These works illustrate the importance of fundamental physics to build the structures inside living cells while informing on new physics we can learn from biological elements and materials. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F61.00002: Alignment and controlled formation of topological defects in living fibroblast cells by liquid crystals Taras Turiv, Jess Krieger, Hao Yu, Irakli Chaganava, Qi-Huo Wei, Min-Ho Kim, O D Lavrentovich Arrays of living tissue-forming cells behave as orientationally ordered active nematics and create topological defects of strength +1/2 and -1/2. These defects play an important role in compressive-dilative stresses in tissues and facilitate effects such as apoptosis and cell migration. The challenge is to design orientational patterns of cells with predetermined spatial locations of topological defects in them. We propose an approach to control the alignment of human dermal fibroblast (HDF) cells by substrates with photoaligned liquid crystal polymers (LCPs). With a plasmonic metamask alignment method, we patterned the director orientation of the LCPs with topological defects of integer (+1, -1) and semi-integer (+1/2, -1/2) strength. Combination of polarized, phase contrast and fluorescent microscopies proves that the HDF cells align along the patterned director of the LCP substrate. The patterns cause a modulation of cell density, as the cells accumulate near the cores of the defects with positive topological charge. The approach could be used to control the locations of defect formation in tissues of living cells and potentially control the extrusion of undesirable cells. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F61.00003: Bioenergetics of cell jamming Stephen J DeCamp, Nicolas Chiu Ogassavara, Jennifer Mitchel, Jeffrey Fredberg Cellular jamming is a ubiquitous phenomenon in epithelial biology that has been shown to govern processes ranging from development in the drosophila gastrulation and zebrafish embryo vertebrate axis elongation, to disease pathophysiology including carcinoma metastasis and asthmatic airway remodeling. A jammed epithelial monolayer remains quiescent and solid-like, whereas unjamming of the monolayer leads to a solid-to-fluid transition in which the cellular collective gains a migratory phenotype, elongated cell shapes, and accompanying mechanical softening. Although jamming has been demonstrated to play a fundamental role in the dynamics of confluent epithelial tissues, the metabolic requirements that fuel the jamming transition and its associated far-from-equilibrium cellular mechanics remain unexplored. Here we measure the metabolic state of individual cells within a confluent monolayer using the cytosolic redox ratio (NAD+/NADH), a reporter of overall cellular bioenergetic potential. Surprisingly, we find that jammed cells near a PDMS barrier have a significantly increased metabolic demand. After barrier removal, and subsequent unjamming, the redox potential of cells near the leading edge of an advancing monolayer decreases to a baseline value consistent with the jammed bulk. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F61.00004: Spatial heterogeneity of cilia contributes to directed flow generation Guillermina Ramirez-San Juan, Mu He, Arnold Mathijssen, Lily Jan, Wallace Marshall, Manu Prakash In living organisms arrays of thousands of micrometer-scale motile cilia coordinate over centimeters to transport fluid. Tissues need to accommodate a variety of specialized cell types, thus cilia do not cover surfaces uniformly. However, how the density and localization of cilia impact flow generation is unknown. Here we combine measurements of cilia organization in the mouse airway with a reduced order hydrodynamic model, to study how spatial organization of cilia integrates across scales to produce long-range flows. Our measurements show that ciliated cells are uniformly distributed but occupy only a fraction of the total surface of the tissue. Furthermore, we measure basal body alignment and tissue-scale cilia orientation (from nm to cm) and find large variations in the local and global orientation of cilia. Despite this spatial heterogeneity, flow measurements show that ciliated cells produce large-scale directed steady flows. Using our model we explore the robustness of the flow to changes in density and orientation of cilia. We find that a fractional coverage of the area by ciliated cells allows the flow to be robust to changes in cilia orientation. Altogether our results highlight the importance of collective cilia properties for flow generation by cilia arrays. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F61.00005: Cellular herding: learning from swarming dynamics to experimentally control collective cell migration Daniel Cohen Multicellular life is driven by collective cell migration spanning morphogenesis, growth, wound healing, and even cancer invasion. Our lab works to interactively manipulate collective cell migration in living tissues, akin to how a shepherd herds sheep. Our first approach relies on ‘Outside-In’ perturbations to direct epithelial collective migration in real-time by exposing cells to programmable electric fields (electrotaxis). Crucially, we have shown that collective migration patterns within a tissue will track even complex 2D field commands (e.g. diverging fields)—a fact that we are exploiting to interactively ‘sculpt’ living tissues via collective migration control. In parallel, we have developed an ‘Inside-Out’ swarm control approach based on introducing ‘cellular mimics’--3D microstructures mimicking the geometry and cadherin presentation of native cell-cell junctions—to tissues in order to recapitulate cell-cell recognition and adhesion between a living tissue and a cellular mimic. By linking into the endogenous coupling network (cell-cell adhesion), these cellular mimics are allowing us to manipulate and program collective cell migration from within a tissue. Together, our swarm control approaches offer new tools to interactively control the behavior of living tissues. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F61.00006: Collective mechanical properties of insect swarms Kasper Van der Vaart, Michael Sinhuber, Nicholas Ouellette Social animals routinely form groups, which are thought to display emergent, collective behavior. This suggests that animal groups should have properties at the group scale that are not directly linked to the individuals, much as bulk materials have properties distinct from those of their constituent atoms. We show that laboratory insect swarms possess emergent mechanical properties, displaying a collective viscoelastic response to applied oscillatory visual stimuli. We find that the swarms strongly damp perturbations. Thus, unlike bird flocks, which appear to use collective behavior to promote lossless information flow through the group, our results suggest that insect swarms use it to stabilize themselves against environmental perturbations. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F61.00007: Adhesion Strategies of Chlamydomonas in Heterogeneous Habitats Christian Titus Kreis, Alexandros Fragkopoulos, Marine Le Blay, Christine Linne, Alice Grangier, Marcin M Makowski, Oliver Baeumchen In contrast to marine phytoplankton, many photoactive microbes live in complex environments, such as liquid-infused soil and moist rocks, where they encounter and colonize a plethora of surfaces. We discovered that the adhesion of Chlamydomonas to surfaces can be reversibly switched on and off by light [1]. Our in vivo single-cell micropipette force spectroscopy experiments suggest that light-switchable adhesiveness is a natural functionality to actively regulate the transition between freely-swimming (planktonic) and surface-associated state, which yields an adhesive adaptation to optimize the photosynthetic efficiency of the cells in variable and inhomogeneous light conditions. Probing the adhesion forces on model substrates with tailored properties and dissecting the contributions from different intermolecular interactions reveal a universal protein-mediated adhesion mechanism that allows the cells to effectively colonize any type of abiotic surface in their heterogeneous natural habitats. Complementary to our single-cell force measurements, we characterize the surface colonization by cell adsorption assays from which we extract population level morphological and dynamical characteristics. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F61.00008: Out-of-plane beating components of active axonemes isolated from Chlamydomonas reinhardtii Azam Gholami, Soheil Mojiri, Albert Johann Bae, Jörg Enderlein, Eberhard Bodenschatz Cilia and flagella are ubiquitous in the living world. They are essential for micro-scale driven transport of fluids or cells by cilia/flagellar beating. Their slender bodies are composed of a microtubule/molecular motor structure that when taken independently are called an axoneme. Axonemes move by bending waves that emerge from the interplay between internal stresses generated by dynein motor proteins . Here we use the novel multi-plane phase contrast imaging technique to record the three dimensional beating pattern of isolated axonemes from Chlamydomonas reinhardtii that beat in the vicinity of a substrate. We measure the torsion of the axoneme along the contour length with high spatiotemporal resolution. High precision information on out-of-plane beating component of axonemes allows us to check the validity of the resistive-force theory. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F61.00009: Fly larvae align under compression Olga Shishkov, Michael MacAlino, David L Hu Black soldier fly larvae are a non-pest insect under consideration as a method of recycling food waste to sustainable protein. These larvae live in large groups in rotting food waste, where they experience hydrostatic forces from the dirt they are in and active forces from their bodies colliding. We investigate how larvae react to these forces by compressing larva aggregations and measuring their response. Larvae align in a container as they follow their instinct to dig downwards, and thus increase their packing fraction and elastic modulus. This study will benefit how larvae are raised in industry by ensuring that larvae are comfortable with their environment and level of substrate, do not expend energy on hiding, and eat and grow quickly. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F61.00010: Insect Swarms under External Perturbations Michael Sinhuber, Kasper van der Vaart, Nicholas Ouellette In the wild, many animal species form aggregations that behave collectively. Unavoidably, these systems are subject to ubiquitous environmental perturbations such as wind, acoustic and visual stimuli. The way these environmental perturbations influence the animals’ collective behavior, however, is poorly understood, in part because conducting controlled quantitative perturbation experiments in the wild is challenging. To circumvent the need for controlling environmental conditions in the field, we study collective swarms of the midge Chironomus riparius in a laboratory experiment where we have control over external perturbations. In this talk, we consider the effect of laboratory-generated perturbations like air flow or variable light exposure on the swarming behavior. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F61.00011: Effects of Social Relationships on the Collective Motion of Bird Flock Hangjian Ling, Guillam E McIvor, Kasper van der Vaart, Richard T Vaughan, Alex Thornton, Nicholas Ouellette Collective animal motion has long been modeled using self-propelled particles that are assumed to be identical and to follow same interaction rules. In nature, however, group members can be quite different, and such differences may shape the group behavior. Here, we study how social relationships in bird flocks affect the local interactions and group dynamics. We used 3D optical tracking to study flocks of jackdaws (Corvus monedula), a highly social corvid species that forms lifelong, monogamous pair bonds. We show that jackdaw flocks contain discrete pairs that are likely to reflect the pair bonds. We find that paired birds interact with fewer neighbors than unpaired birds and use less energy when flying. Pairing thus appears to grant energetic benefits. However, we also find that flocks with more pairs have shorter velocity correlation lengths, in agreement with a generic self-propelled particle model, indicating that social relationships may inhibit efficient information transfer through the group. Our results reveal a critical tension between individual- and group-level benefits during collective behavior in species with differentiated social relationships, and have many evolutionary and cognitive implications. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F61.00012: Active flow of low density pedestrian crowds Alessandro Corbetta, Chung-min Lee, Joris Willems, roberto benzi, Federico Toschi The dynamics of pedestrian crowds share deep connections with the statistical physics of active matter. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F61.00013: WITHDRAWN ABSTRACT
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