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 J12: Self-Organization in Biological Systems: Subcellular to Tissue ScalesFocus Live
|
Hide Abstracts |
Sponsoring Units: DBIO DSOFT Chair: Moumita Das, Rochester Institute of Technology; Wolfgang Losert, University of Maryland, College Park |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J12.00001: Decision making behaviors in a brainless organism (Physarum polycephalum) can emerge from self-organized physical interactions within a single cell. Abid Haque, Jason Graham, Andrew Edwards, Subash Ray, Simon Garnier Decision-making is traditionally studied in the context of neurobiology. However, little is known about complex decision-making abilities that arise even in the absence of neurons. In this study, we use a brainless unicellular protist Physarum polycephalum (slime mold) to examine the mechanisms it employs to “choose” one food source over another. Previous studies show that an underlying network of actin fibers drives membrane contractions, leading to pressure gradients, fluid flow and ultimately locomotion. Our observations suggest that the local disintegration of the actin network may cause the slime mold to bias its movement towards one food source. We use a dynamical systems approach to model pressure interactions, volume exchange and actin disintegration to explore the emergence of a system-wide decision. Through this model, we demonstrate that stable contraction patterns similar to empirical observations can emerge through purely physical interactions. Furthermore, the “choice” of one food source over another can also be explained by locally reducing the contractile response of the actin network. These results indicate that complex decision-making behaviors can arise from purely physical interactions within a single cell, bypassing the need for a nervous system. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J12.00002: Cooperatively enhanced reactivity and "stabilitaxis" of dissociating oligomeric proteins Jaime Agudo-Canalejo, Pierre Illien, Ramin Golestanian Many functional units in biology, such as enzymes or molecular motors, are composed of several subunits that can reversibly assemble and disassemble. This includes oligomeric proteins composed of several smaller monomers, as well as protein complexes assembled from a few proteins. By studying the generic spatial transport properties of such proteins, we investigate here whether their ability to reversibly associate and dissociate may confer them a functional advantage with respect to non-dissociating proteins [1]. In uniform environments with position-independent association-dissociation, we find that enhanced diffusion in the monomeric state coupled to reassociation into the functional oligomeric form leads to enhanced reactivity with distant targets. In non-uniform environments with position-dependent association-dissociation, caused e.g. by spatial gradients of an inhibiting chemical, we find that dissociating proteins generically tend to accumulate in regions where they are most stable, a process that we term "stabilitaxis". |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J12.00003: Uni- and bidirectional forcing in Dictyostelium discoideum streaming Abby Bull, Molly Mosher, Matt Hourwitz, John Fourkas, Wolfgang Losert The head-to-tail collective motion of cells, called streaming, is an important method of collective migration utilized by a wide range of cells including metastatic cancer cells. Streaming has been extensively studied in the simple model organism D. discoideum in the context of collective guidance by chemical signals. Separately, our group has investigated bidirectional guidance of D. discoideum cells by nano-ridged surfaces. Here we investigate the combination of chemical and topographic guidance cues. We find that nano-ridges alter streaming of cells in a density dependent manner. To gain insights into how the streaming phenotype can be disturbed by physical cues, we supplement our experiments with coarse-grained simulations that model the cells as self-propelled disks guided by chemical signals and directional forcing. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J12.00004: Geometric signatures of tissue surface tension in a three-dimensional model of confluent tissue Preeti Sahu, J M Schwarz, M Lisa Manning Dense biological tissues ensure that cell types performing different roles remain segregated by maintaining sharp interfaces. To better understand the mechanisms for such sharp compartmentalization, we study the effect of an imposed heterotypic tension at the interface between two distinct cell types in a fully 3D model for confluent tissues. We find that cells rapidly sort and self-organize to generate a tissue-scale interface between cell types, and cells adjacent to this interface exhibit signature geometric features including nematic-like ordering, bimodal facet areas, and registration of cell centers on either side of the two-tissue interface. The magnitude of these features scale directly with the magnitude of imposed tension, suggesting that experiments might estimate the magnitude of tissue surface tension between two tissue types simply by segmenting a 3D tissue. To uncover the underlying physical mechanisms driving these geometric features, we develop two simple toy models that identify an energy competition between bulk cell shapes and two-tissue interface area. When the area term dominates, changes to neighbor topology are costly, pinning neighbor topologies and generating the observed geometric features. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J12.00005: The Physical Basis of Curvaure Sensing by Septins Wenzheng Shi, Kevin Cannon, Amy Gladfelter, Ehssan Nazockdast The ability of the cell to sense and change its shape is key to many processes, including membrane trafficking, endocytosis and morphogenesis. Septins are GTP-binding nanoscopic proteins that localize to sites of micrometer-scale membrane curvature. Upon binding, the septins diffuse and anneal (polymerize) to form longer filaments and assemblies that span the membrane surface. The relationship between the curvature-dependent binding of septins and their self-assembly remains unclear. Here we use a combination of biophysical modeling and simulations, single molecule imaging and scanning electron microscopy to study the interplay between membrane’s curvature and the different processes involved in septin assembly, including septins’ membrane association/dissociation, diffusion and polymerization rates. Our modeling and experimental results suggest that curvature sensing by septins is qualitatively changed with the structure and density of the bound septins and, thus, operates at multiple length- and time-scales. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J12.00006: Unraveling cytoplasmic streaming using a coarse-grained model of microtubule hydrodynamics David Stein, Gabriele De Canio, Eric Lauga, Raymond E Goldstein, Michael Shelley During the development of the fruit fly oocyte, flows with short-ranged correlations transition to a dramatic cell-spanning vortex, accompanied by coherent deformations in the microtubule cytoskeleton. Using a coarse-grained model for the hydrodynamics of ordered fibers, we show that sufficiently dense microtubule arrays, forced only by molecular motors transporting cargo, undergo a "swirling transition" that is fundamentally different than the buckling transition which leads to the flapping motion of isolated filaments. Our model produces streaming velocities consistent with in vivo measurements, and allows us to place bounds on the number density of kinesin-1 motors transporting cargo within the microtubule array. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J12.00007: Chimeras as a way to model anatomical reentry in cardiac models Andrea Welsh, Flavio Fenton The FitzHugh-Nagumo (FHN) model is a two variable dynamical system that adequately describes many phenomena in excitable biological tissue. When used to model coupled oscillators in 2D space, solutions look like traveling waves, phase synchronization, or spiral waves. These solutions can be useful particularly when describing membrane potentials of cardiac tissue cells. Of interest are the dynamics of membrane potentials in tissue when there are unresponsive cells, causing waves to travel around and cause reentry leading to more deadly behavior like arrythmias and fibrillation. We consider chimera states, where there are spatial-temporal patterns of coupled oscillators that are made from two or more patterns. The appearance of chimeras is thought to require non-local coupling among oscillators and have exhibited regions of coordinated oscillations and uncoordinated oscillations. For past studies of chimera states in FHN, coupling exists among both the membrane potential variable and the relaxation variable, which is unphysical when comparing to a cardiac system. We present an extension to a FHN system with purely local coupling that models waves and unresponsive cells and explore the rich dynamics that result. |
Tuesday, March 16, 2021 4:24PM - 5:00PM Live |
J12.00008: Irwin Oppenheim Award (2021): Design of conditions for self-replication Invited Speaker: Sumantra Sarkar A “self-replicator” is usually understood to be an object of definite form that promotes the conversion of materials in its environment into a nearly identical copy of itself. The challenge of engineering novel, micro- or nanoscale self-replicators has attracted keen interest in recent years, both because exponential amplification is an attractive method for generating high yields of specific products and, also, because self-reproducing entities have the potential to be optimized or adapted through rounds of iterative selection. Substantial steps forward have been achieved both in the engineering of particular self-replicating molecules and in the characterization of the physical basis for possible mechanisms of self-replication. At present, however, there is a need for a theoretical treatment of what physical conditions are most conducive to the emergence of novel self-replicating structures from a reservoir of building blocks on a desired time scale. In this talk, I shall report progress in addressing this need. By analyzing the kinetics of a toy chemical model, we demonstrate that the emergence of self-replication can be controlled by coarse, tunable features of the chemical system, such as the fraction of fast reactions and the width of the rate constant distribution. We also find that the typical mechanism is dominated by the cooperation of multiple interconnected reaction cycles as opposed to a single isolated cycle. The quantitative treatment presented here may prove useful for designing novel self-replicating chemical systems. |
Tuesday, March 16, 2021 5:00PM - 5:36PM Live |
J12.00009: Self-organization in composite biopolymer liquid crystals Invited Speaker: Kimberly Weirich Complex mixtures of macromolecules self-organize to form the soft and active biological materials that structure the cellular cytoplasm. Ordered assemblies of cytoskeletal filaments, such as stress fibers and mitotic spindles, orchestrate the complex mechanical behavior of cells. Key to understanding these exquisite mechanics is elucidating the physical principles of self-organization in these systems. We recently reported dense condensates of cytoskeletal filaments that form liquid crystal condensed phases, where structure arises from the anisotropy of the filaments. Here, we discuss composite biological liquid crystals, formed form filaments mixed with biological polymers of different rigidities. We investigate emergent self-organization in these composite liquid crystals, and the shape changes that result from confinement. Our results highlight the role of anisotropy in the self-organization of biological materials and suggest physical mechanisms of controlling shape change in bio-inspired, soft materials. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J12.00010: How motors shape their roads: self-organisation in dynamic filament networks Moritz Striebel, Fridtjof Brauns, Erwin Frey The active remodelling of the cytoskeleton is essential for fundamental cellular processes such as cell division. Different molecular motors that move cytoskeletal filaments, bundle them, and control their length play key roles in the restructuring of the cytoskeleton. Here we propose a microscopic model for the interplay between the spatial microtubule network architecture and molecular motors that control filament length by catalysing their depolymerization. In particular, we investigate how the transport of resources (free motors and tubulin monomers) impacts the dynamics by causing heterogeneous distribution of resources. Strikingly, we find that the interplay between resource redistribution and microtubule length regulation alone, without active contraction, is sufficient to organize the filament network into polar bundles. This polarity sorting in turn influences active transport of motors in the network. We show that the interaction between active motor transport, diffusive mass redistribution and length regulation leads to self-organization of the filament network in large-scale polar structures. |
Tuesday, March 16, 2021 5:48PM - 6:00PM On Demand |
J12.00011: Interacting associative memory networks as a model for tissue self-organization Matthew Smart, Anton Zilman The rapid development of single-cell RNA sequencing has led to widespread interest in dynamical modelling of cell state. Cell types are defined by stable patterns of gene expression in the transcriptomic data, which has led theorists to search for dynamical models capable of encoding identified cell types as stable fixed points. Hopfield networks offer an elegant solution and have been used to describe reprogramming in individual cells. To capture cell-cell interactions we propose to model interacting cells using a lattice of interacting Hopfield networks. We consider cell-cell interactions mediated by paracrine (ligand-receptor) signalling and exosomes, and use a single parameter to tune the relative strength of intra-cell and inter-cell gene regulation. We investigate under what conditions the single-cell attractors remain stable, and whether cell-cell interactions can facilitate the emergence of new stable single-cell states. This approach captures multiple levels of biological organization and displays self-organizing properties which resemble multicellular development and homeostasis. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700