Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session B11: Physics of the Cytoskeleton IFocus
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Sponsoring Units: DBIO Chair: Andrei Gasic, Rice University Room: Room 203 |
Monday, March 6, 2023 11:30AM - 12:06PM |
B11.00001: Mechanics of nuclear deformation in cells Invited Speaker: Tanmay Lele The cell nucleus is commonly considered to be a stiff organelle that mechanically resists changes in shape, and this resistance is thought to limit the ability of cells to migrate through pores or spread on surfaces. Generation of cytoskeletal stresses on the cell nucleus during migration and nuclear response to these stresses is fundamental to cell migration and mechano-transduction. I will discuss our experimental and computational evidence that supports a dynamic model, in which the soft nucleus is irreversibly shaped by viscous stresses generated by the motion of cell boundaries and transmitted through the intervening cytoskeletal network. While the nucleus is commonly modeled as a stiff elastic body, we review how nuclear shape changes on the timescale of migration can be explained by simple geometric constraints of constant nuclear volume and constant surface area of the nuclear lamina. Because the lamina surface area is in excess of that of a sphere of the same volume, these constraints permit dynamic transitions between a wide range of shapes during spreading and migration. The excess surface area allows the nuclear shape changes to mirror those of the cell with little mechanical resistance. Thus, the nucleus can be easily shaped by the moving cell boundaries over a wide range of shape changes and only becomes stiff to more extreme deformations that would require the lamina to stretch or the volume to compress. This model explains how nuclei can easily flatten on surfaces during cell spreading or elongate as cells move through pores until the lamina smooths out and becomes tense. |
Monday, March 6, 2023 12:06PM - 12:18PM |
B11.00002: Active mechanics of sea star oocytes Peter J Foster, Jinghui Liu, Alexandra Zampetaki, 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 Patiria miniata oocytes. Using pharmacological treatments targeting actin polymerization, we find that the cellular deformation rate is a nonmonotonic function of cortical actin density 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 deformation rate on the concentration of passive actin crosslinkers and motor proteins, including the surprising prediction that deformation rate decreases with increasing motor concentration. We test these predictions through protein overexpression and find quantitative agreement. Taken together, this work is an important step for bridging the molecular and cellular length scales for cytoskeletal networks in vivo. |
Monday, March 6, 2023 12:18PM - 12:30PM |
B11.00003: Investigating the control principles behind formation of stress fibers by probing their response to external forces in silico Alexandra Lamtyugina, Yuqing Qiu, Suriyanarayanan Vaikuntanathan Cytoskeletal stress fibers (SFs) are important for stabilizing cell structure against external forces. While many insightful studies of SFs have been performed both in vivo and in vitro, comprehensive understanding of the underlying mechanisms for SF force generation and nucleation is still lacking. One of the obstacles to developing a theoretical framework to describe these processes is the experimental inaccessibility of informative microscopic quantities such as forces experienced and exerted by individual SFs. In our work, we modify the molecular dynamics simulation engine Cytosim [Nedelec et al. N. J. Phys. 2007] to apply external forces to parts of SFs. This customized computational tool allows us to investigate the effects of the applied forces on the nucleation rate and force response of SF-like structures. Our results provide insight into the possible control principles governing these processes and how cytoskeletal systems may take advantage of them to achieve adaptive force generation. |
Monday, March 6, 2023 12:30PM - 12:42PM |
B11.00004: Ultrafast contraction: Investigating the structures underlying myoneme force generating networks in Spirostomum sp. Joseph Lannan, Jerry E Honts, Saad Bhamla, Mary W Elting Spirostomum sp. is a unicellular ciliate capable of contracting to a quarter of its body length in less than five milliseconds. When measured as fractional shortening, this is an order of magnitude faster than motion powered by actomyosin. Myonemes, which are networks of proteins found near the cortex of many protists, are thought to power this contraction. Fast contraction, slow elongation, and calcium triggering are hallmarks of myoneme-based motion. The biochemical basis of this motion, and the molecular mechanism that allows motion to occur at such fast speeds, are not well understood. Previous work suggests myoneme structures in some protists are rich in centrin, a protein that may underlie contraction. Centrin undergoes a significant conformational change in the presence of calcium allowing it to bind to other centrin molecules. We use transmission electron microscopy to image Spirostomum in both contracted and uncontracted states to observe corresponding structural changes in the myonemes. We identify the presence of centrin in myoneme fibers via immunogold labeling. We find that the appearance of these fibers changes from a loose network in uncontracted cells to a denser, more well-defined fiber when contracted. These structural changes shed a new perspective on how myonemes may generate force. In the long term, mechanical insight into how nature generates this ultrafast contraction may find applications in both understanding and controlling biological force generation. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B11.00005: Roles of Catch Bond Dynamics in Cytoskeletal Networks Md Foysal Rabbi, Taeyoon Kim The cytoskeleton plays an important role in various physiological processes. For example, actin networks which consist of F-actins inter-connected via cross-linking proteins are known to serve as a scaffolding structure in cells. The cross-linking proteins can be dissociated from F-actins as either a slip bond or a catch bond. We recently demonstrated that an actin network comprised of weak catch bonds exhibits higher yielding stress/strain than those with strong slip bonds. In this study, we employed an agent-based computational model to understand better how and when weak bonds can make a stronger material by exploring parametric spaces consisting of F-actin length, actin concentration, and the density and kinetics of cross-linking proteins. Further, we investigated how catch bonds affect the propagation of cracks formed in the network. It was found that weak catch bonds unbind from low-stress areas and eventually move to higher-stress areas, whereas slip bonds remain trapped in their initial positions. This difference leads to different yielding stress/strain and distinct crack propagation. Our study provides further insights into understanding potential roles of catch and slip bonds of various proteins in physiological processes. |
Monday, March 6, 2023 12:54PM - 1:30PM |
B11.00006: Force transmission via dynamic stretching of Talin as revealed by live-cell single-molecule imagingSawako Yamashiro (Kyoto University) Invited Speaker: Sawako Yamashiro Focal adhesions (FAs) are responsible for transmitting intracellular forces to the extracellular matrix. It is widely recognized that Talin is an essential FA protein that links actin filaments (F-actin) to integrins. However, we previously revealed that the actin network constantly moves at ~20 nm/sec over FAs as a single-unit by direct single-molecule observation of F-actin movement in Xenopus XTC cells (Yamashiro et al. MBoC 25:1010, 2014). How can FA molecules simultaneously maintain the connection to moving F-actin and transmit forces to integrins? In this study, we address this issue by performing Single-Molecule Speckle (SiMS) microscopy which elucidates true functions and kinetics of individual Talin molecules in live cells. Our SiMS data show that the majority of Talin molecules are bound only to either the moving F-actin network or the substrate whereas a small portion of Talin is liked to both structures via elastic transient clutch in FAs. By reconstituting Talin knockdown cells with Talin chimeric mutants, in which the Talin rod subdomains are replaced with the stretchable β-spectrin repeats, we show that the stretchable property is critical for force transmission. We carried out simulations to test whether unfolding of the Talin rod subdomains increases work at FAs. Our findings provide new insights into coupling and force transmission of the retrograde actin flow to the extracellular matrix. |
Monday, March 6, 2023 1:30PM - 1:42PM |
B11.00007: A universal form for heavy-tailed fluctuations in the actomyosin cortex Shankar N Sivarajan, Yu Shi, Katherine M Xiang, John C Crocker, Daniel H Reich The fluctuations of the actomyosin cortex still defy a fully satisfying explanation, despite long study. Arrays of flexible microposts allow for high-resolution measurements of cortical activity across a wide range of length scales [1][2]. Here we report that, after accounting for static heterogeneity in cell–micropost contacts, the displacement distribution of the fluctuations displays a universal form across time, cell type, and substrate stiffness, resembling an exponentially truncated stable distribution (ETSD). The parameters describing the ETSD, including the stability parameter (exponent) and characteristic lengths exhibit clear dependence on lag time. The relationship of these heavy-tailed fluctuations to the previously reported cytoquakes—rearrangements with large step-like displacements—will be discussed. The appearance of such an ETSD descriptor underscores the cortex’s similarity to soft glasses, where similar distributions have been observed. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B11.00008: Myosin-I modulates the dynamics and architecture of motile actin ‘comet tails’ Mengqi Xu, Luther Pollard, Grzegorz Rebowski, Malgorzata Boczkowska, Roberto Dominguez, Michael Ostap Actin and myosin are molecular machineries that convert free energy released from ATP hydrolysis into mechanical force. Polymerizing actin filaments generate force that powers membrane deformation and drive many important cellular processes. Myosin-Is are single-headed, membrane associated members of the myosin superfamily that have force-generating working strokes directed to actin's barbed end. Recent studies show that myosin-I isoforms frequently colocalize in areas of Arp2/3 complex-mediated actin polymerization at the leading edge of cell membrane, which implies an association of the two molecular complexes. To further investigate how myosin-Is affect the actin assembly, we reconstituted an in vitro actin-based motility system, where branched actin networks were nucleated by Arp2/3 complex from a micron-sized bead surface-coated with Arp2/3 activating factors. Actin filaments first formed a symmetric shell around the bead, which transitioned into a polarized comet tail after symmetry breaking and propelled the bead forward. We site-specifically coupled a range of densities of myosin-Is to the bead surface and assessed their effects on actin polymerization, network architecture, and symmetry breaking. We found that myosin changed the growth rate and the architecture of actin networks in comet tails, and also facilitated symmetry breaking. These studies show synergy between myosin activity and actin polymerization to power morphological changes at the cell membrane. |
Monday, March 6, 2023 1:54PM - 2:06PM |
B11.00009: Thermodynamic framework for nonequilibrium self-assembly of branched actin networks Elisabeth Rennert Branched actin networks are involved in a variety of cellular processes, most notably the formation of lamellipodia in the leading edge of the cell. These systems adapt to varying loads though changes in the network density and interaction angle. Recent experimental work has described growth and force feedback mechanisms in these systems. We seek to create a minimal model of this nonequilibrium self assembly process incorporating these mechanisms. Our results will show how constraints from stochastic thermodynamics and non-equilibrium thermodynamics may bound or constrain the structures that result in such processes. This will allow us to develop a minimal yet predictive thermodynamic framework for these kinds of force generating processes. |
Monday, March 6, 2023 2:06PM - 2:18PM |
B11.00010: Bio-inspired fiber networks using peptide-DNA nanotechnology Kengo Nishi, Margaret Daly, Stephen J Klawa, Kameryn Y Hinton, Yuan Gao, Ronit Freeman Cytoskeletal proteins such as actin and microtubule build multiscale architectures from filaments and bundles to networks. These architectures are mainly controlled by crosslinking proteins, which orchestrate mesoscale functional structures such as filopodia, lamellipodia, and stress fibers. Such natural self-assembly inspires the engineering of synthetic materials mimicking their structural organization. Numerous studies showed that various synthetic peptides could be self-assembled into well-defined nanostructures including fibers. However, bridging the hierarchical assembly of peptides from nano- to mesoscale remains challenging. Here, we introduce DNA crosslinkers into peptide-based fibrous networks to construct the mesoscale's structural hierarchy. DNA nanotechnologies allow us to systematically vary DNA crosslinkers' physical/chemical properties, such as flexibility, melting temperature, and the number of arms, which cannot be accomplished by conventional chemical crosslinkers. Our multiscale imaging reveals that the structural hierarchy from nanoscale fibers to tactoid-shaped thin bundles to large mesoscale bundles can be successfully introduced and finely controlled by DNA crosslinkers. We found that the structural hierarchy introduced by DNA crosslinkers regulates the mechanics of peptide-DNA networks for more than one order of magnitude. Lastly, we will discuss the application of our peptide-DNA technology to more biologically-relevant systems such as in vitro cytoskeletal filaments. |
Monday, March 6, 2023 2:18PM - 2:30PM |
B11.00011: Impacts of biosurfactants on bacterial spreading in soil Judy Yang, Yuan Li, Joe Sanfilippo, Daniel Kearns, Howard A Stone, Bonnie L Bassler, Niki Abbasi, Zemer Gitai Mechanistic understanding of bacterial spreading in soil, which has both air and water in angular pore space, is critical to control pathogenic contamination of soil and design bioremediation projects. A recent study (Yang et al., PNAS, 2021) shows that Pseudomonas aeruginosa can self-generate flows along sharp corners by producing rhamnolipids, a type of biosurfactants that change the hydrophobicity of solid surfaces. We hypothesize that other types of biosurfactants and biosurfactant-producing bacteria can also generate corner flows. Here, we first demonstrate that rhamnolipids and surfactin, biosurfactants with different chemical structures, can generate corner flows. We identified the critical concentrations of these two biosurfactants to generate corner flow. Second, we demonstrate that two common soil bacteria, P. fluorescens and B. subtilis (which produce rhamnolipids and surfactin, respectively), can generate corner flows along sharp corners at the speed of several millimeters per hour. We further show that a surfactin-deficient mutant of B. subtilis cannot generate corner flow. Third, we show that similar to P. aeruginosa, the critical corner angle for P. fluorescens and B. subtilis to generate corner flows can be predicted from classic corner flow theories. Finally, we show that the height of corner flows is limited by the roundness of corners. Our results suggest that biosurfactant-induced corner flows are prevalent in soil and should be considered in the modeling and prediction of bacterial spreading in soil. The critical biosurfactant concentrations we identify and the mathematical models we propose will provide a theoretical foundation for future predictions of bacterial spreading in soil. |
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