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 S11: Physics of Cytoskeleton Across Scales IVFocus Live
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Sponsoring Units: DBIO Chair: James Liman, Rice Univ; Jing Xu, University of California, Merced |
Thursday, March 18, 2021 11:30AM - 11:42AM Live |
S11.00001: Mechanisms of actin force production in clathrin-mediated endocytosis revealed by integrating computational modeling with in situ cryo-electron tomography Matthew Akamatsu, Daniel Serwas, Amir Moayed, Ritvik Vasan, Karthik Vegesna, Jennifer Hill, Johannes Schoneberg, Karen Davies, PADMINI RANGAMANI, David Drubin During clathrin-mediated endoytosis, the plasma membrane is deformed, forming clathrin-coated vesicles (CCVs) containing cargo. Membrane remodeling is supported by actin filament assembly, but its mode of function remains elusive. We previously used an experimentally constrained mathematical model to find that a minimal endocytic actin network can self-organize, bend, and produce sufficient force at sites of CME for pit internalization (Akamatsu et al., eLife 2020). Here we used cryo-electron tomography (cryo-ET) of intact mammalian cells to directly visualize networks of individual actin filaments at CME sites and CCVs, and used mathematical modeling to identify their mechanistic functions. Surprisingly, actin networks at CME sites consisted of both branched and unbranched filaments. Finally, we identified long proteins ~60 nm in length resembling the actin-CME linker Hip1R, both in the clathrin-coated area and in the neck of the pit. Mathematical modeling showed that this Hip1R neck localization not only directs filament growth toward the neck of CME sites, it also results in increased internalization efficiency. By combining mathematical modeling and in situ cryo-ET, deeper insights into actin mechanism during CME were achieved than either approach alone. |
Thursday, March 18, 2021 11:42AM - 11:54AM Live |
S11.00002: Direct measurements of interactions between intermediate filaments Anna V Schepers, Charlotta Lorenz, Peter Nietmann, Andreas Janshoff, Stefan Klumpp, Sarah Köster The cytoskeleton consists of F-actin, microtublues and intermediate filaments (IFs), which together form a complex composite network. This composite network provides stability and flexibility for cells and enables them to adapt to a variety of external conditions. F-actin and microtubule networks have been studied extensively and a large number of different cross-linkers are known. By contrast, the interactions within reconstituted IF networks are less well understood. Microrheological measurements on vimentin networks reveal slow network dynamics, and allow us to separate the filament and network formation time scales. By amplifying electrostatic attractions or diminishing hydrophobic interactions between filaments, we are able to study the impact of the respective effect on intra- and inter-filament interactions. Combining optical trapping and fluorescence microscopy enables us to bring two single vimentin IFs in contact and directly study the interactions between them. These results, in combination with studies of the mechanical properties of single IFs, allow us to model the interactions with Monte-Carlo simulations, thereby gaining a deeper understanding of cytoskeletal structures. |
Thursday, March 18, 2021 11:54AM - 12:30PM Live |
S11.00003: Cytoskeletal heavy tails Invited Speaker: Brian Camley The eukaryotic cell's cytoskeleton is a prototypical example of an active material, with objects embedded within it driven by force dipoles exerted by molecular motors. Experiments tracking the behavior of cell-attached objects have observed anomalous diffusion with a distribution of displacements that is non-Gaussian, with heavy tails. This has been attributed to "cytoquakes" or other spatially extended collective effects. We present a simple model that naturally creates heavy power-law tails in cytoskeletal displacements. We predict that this power law exponent should depend on the geometry of where active motors are distributed through the cell, but is likely not strongly dependent on the rheology of the cytoskeleton. We then discuss potential tests of this model both in cells and in synthetic active gels. |
Thursday, March 18, 2021 12:30PM - 12:42PM Live |
S11.00004: Nonequilibrium simulation of cytoskeletal proteins: assembly, bundling and gelation. Valerio Sorichetti, Martin Lenz
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Thursday, March 18, 2021 12:42PM - 12:54PM Live |
S11.00005: Studying in vitro the effect of actin dynamics on membrane tubes Antoine Allard, Clément Campillo, Cécile Sykes Living cells change their shape in biological processes like division, motility or intracellular trafficking. These morphological changes rely on a dynamic network of biopolymers, the actin cytoskeleton that interacts with membranes. In particular, intracellular transport implies the formation of tubular membrane intermediates, with a radius of 10-100nm, finally destabilised into vesicles. In vivo examples of actin-dependent formation, stabilisation or scission of such tubes have been reported, but the underlying physical mechanisms remain unclear. |
Thursday, March 18, 2021 12:54PM - 1:06PM Live |
S11.00006: Generic stress rectification in nonlinear elastic materials Félix Benoist, Guglielmo Saggiorato, Martin Lenz Beyond their response to stresses applied at their boundaries, elastic materials also deform in response to internally exerted stresses. In living systems, such stresses typically originate from molecular motors embedded in a fibrous matrix, which rectifies internal force dipoles of any sign towards a biologically crucial isotropic contraction. Here we show that rectification is a more general effect, present in any nonlinear elastic material regardless of the geometry of the applied forces, which results in contraction or expansion depending on the material's nonlinearities. |
Thursday, March 18, 2021 1:06PM - 1:18PM Live |
S11.00007: Coarse-grained simulations of actin polymerization Aaron R Hall, Brandon G Horan, Dimitrios Vavylonis The polymerization of actin, which plays a central role in maintaining cell structure and motility, is regulated by multiple monomer and filament binding proteins containing flexible disordered regions. Computational studies of actin polymerization require the use of coarse-grained models, since the timescales to simulate polymerization kinetics using all-atom models are too long. We show the promise of previously developed coarse-grained models with each residue represented by a single interaction site. We used such models to reproduce the polymerization of actin monomers to the barbed and pointed ends of actin filaments, the binding of profilin to actin, and the delivery of profilin-actin to the barbed end through flexible disordered regions of actin filament regulators. |
Thursday, March 18, 2021 1:18PM - 1:30PM Live |
S11.00008: Nonlinear Viscoelasticity of Semiflexible Polymer Networks Sihan Chen, Tomer Markovich, Frederick MacKintosh Semiflexible polymers exhibit amazing nonlinear viscoelastic response, as a result of the competition between the bending resistance and the entropic fluctuation. Most analytical studies of semiflexible polymer networks focus on the affine deformation of permanent-crosslinked networks, while the non-affine deformation of the networks and the stress relaxation introduced by transient crosslinks remain poorly understood. In this talk, we present an analytical framework for the viscoelasticity of semiflexible polymer networks. Our model predicts the nonlinear viscoelastic modulus of transient-crosslinked networks, as well as the non-affine deformation of isotropic polymer networks. |
Thursday, March 18, 2021 1:30PM - 1:42PM Live |
S11.00009: Structural actin analysis in primary rat cortical astrocytes predicts culturing nanotopography Nicholas Mennona, Barbara Barile, Kate O'Neill, Emanuela Saracino, Spandan Pathak, Maria Grazia Mola, Grazia Paola Nicchia, Valentina Benfenati, Wolfgang Losert Astrocytes, long regarded as merely passive supporting cells of the brain, have been proven vital in brain informational processing. The ‘Tripartite Synapse’ concept has recently emerged whereby astrocytes modulate neuronal activity. For this to occur, astrocytes must be able to physically interact with both neurons and other astrocytes. Therefore, the morphology of astrocytes is vital to understand brain health and function. Actin, a cellular structural protein, has not been studied in astrocytes extensively. To this end, we grow astrocytes on different nanotopographic surfaces, and then fix and stain the cells with Phallodin-488. These fixed cells are then imaged using STED microscopy to obtain detailed images of the actin structure in astrocytes. We then use image processing methods to extract the filament organization in the astrocytes. We demonstrate that there exists a significant angle organization difference between astrocytic actin on different nanotopographies. These results illustrate the importance of actin as a predictor for environmental influences. We propose that additional co-culture experiments (neurons cultured with astrocytes) will reveal similar disparities at this actin level and may predict astrocytic modulation of neuronal activity. |
Thursday, March 18, 2021 1:42PM - 1:54PM Live |
S11.00010: A design framework for highly crosslinked cytoskeletal networks Sebastian Fuerthauer, Daniel Needleman, Michael Shelley Living cells move, deform and divide. The engine of these behaviors is the cytoskeleton, a highly crosslinked network of polymer filaments and molecular scale motors that use chemical energy to do work. We develop a theory that predicts how the micro-scale properties of molecular motors and crosslinks tune the networks emergent material properties and generate predictable, and possibly controllable, behaviors. I will present how this theory is constructed, and discuss its implications for cytoskeletal networks in vitro and in vivo, highlighting how it has helped to quantitatively understand motor driven microtubule fluxes in a system made from XCTK2 motors and stabilized microtubules, and how it resolved long-standing puzzles about the motion of microtubules in spindles. |
Thursday, March 18, 2021 1:54PM - 2:30PM Live |
S11.00011: Non-equilibrium fluctuations in cells report on driving forces and organelle mechanics Invited Speaker: Kengo Nishi Cells are complex active materials. Dispersed motor proteins drive in more or less organized manners transport, organelle dynamics, shape changes and cell movements. Forces and local material properties are difficult to measure. We have here developed a method that takes advantage of motor-generated forces deforming rod shaped elastic objects, to measure both, the driving forces and the elastic properties of the rods. Examples of such rods are microtubules, intermediate filaments or externally introduced probes, such as carbon nanotubes. Their shapes are determined by the active forces, the response characteristics of the cytoplasm and the material properties of the rods. If the response of the cytoplasm is measured independently by microrheology, the other two quantities can be determined. We present the theory describing the non-equilibrium dynamics of a probe filament embedded in the cytoplasm and show how the method can be applied in HeLa cells. |
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