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 N06: Cytoskeletal and Cytoplasmic Dynamics and TransportFocus
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Sponsoring Units: DBIO Chair: Ajay Gopinathan, University of California Merced Room: Room 129 |
Wednesday, March 8, 2023 11:30AM - 12:06PM |
N06.00001: Theoretical and experimental investigation of dynein-mediated transport and positioning of the nucleus prior to first division in the early C. elegans embryo Invited Speaker: Adriana Dawes Centrosomes are nucleus-associated organelles that serve as the nucleation site for microtubule arrays. Microtubules nucleated from these arrays interact with motor proteins such as dynein at the periphery of the cell, which act to transport the nucleus and position it prior to division. In polarized cells, where specific factors are segregated to opposite ends of the cell as seen in early embryos of the nematode worm C. elegans, proper centrosome positioning is particularly important, determining whether the division process is symmetric or asymmetric. Using a combination of stochastic and continuum models with experimental validation in early C. elegans embryos, we demonstrate that centrosome asymmetry and the geometry of the early embryo are both critical for proper centrosome positioning in the early C. elegans embryo. |
Wednesday, March 8, 2023 12:06PM - 12:18PM |
N06.00002: Length-dependent poleward flux of sister kinetochore fibers promotes chromosome alignment Nenad Pavin Chromosome alignment at the spindle equator promotes proper chromosome segregation and depends on pulling forces exerted at kinetochore fiber tips together with polar ejection forces. However, kinetochore fibers are also subjected to forces driving their poleward flux. Here we introduce a flux-driven centering model that relies on flux generated by forces within the overlaps of bridging and kinetochore fibers. This centering mechanism works so that the longer kinetochore fiber fluxes faster than the shorter one, moving the kinetochores toward the center. We develop speckle microscopy in human spindles and confirm the key prediction that kinetochore fiber flux is length dependent. Kinetochores are better centered when overlaps are shorter and the kinetochore fiber flux slower than the bridging fiber flux. We identify Kif18A and Kif4A as overlap and flux regulators and NuMA as a fiber coupler. Thus, length-dependent sliding forces exerted by the bridging fiber onto kinetochore fibers support chromosome alignment. |
Wednesday, March 8, 2023 12:18PM - 12:30PM |
N06.00003: A model for microtubule-mediated deformation of a nucleus membrane Yuan-Nan Young, Reza Farhadifar, Michael J Shelley The cellular nucleus is enclosed by a permeable membrane mechanically supported by a meshwork of lamin fibers. The morphology and integrity of the nucleus are essential for the cell's function. Recent experiments show that loss of the lamin network results in nuclear deformations and rupture. To understand the mechanistic basis of this phenomenon, we developed a mathematical model that accounts for, and couples, the fluid flows around and through the permeable membrane, and the pulling on the membrane by membrane-bound, but mobile, molecular motors attached to impinging microtubules. Here the microtubules are assumed to nucleate from a cellular centrosome. We found that this model predicts the formation of a sharp corner in the vicinity of the centrosome, rather reminiscent of the Taylor cone for a surfactant-laden drop in an elongational flow. We analyze the equilibrium shape of the membrane in terms of the total number of motors and their mobility in the nuclear membrane. Our model provides a more mechanistic understanding of nuclear deformation in cells and can give insights into the correspondence between motor forces and membrane deformation leading to nuclear rupture, which has been observed in some cancer cells. Using numerical simulations we further investigate how the distribution of molecular motors couples to the deformation and shape dynamics of the nucleus. |
Wednesday, March 8, 2023 12:30PM - 12:42PM |
N06.00004: Microtubule organization into tactoids with active and passive crosslinkers Prashali Chauhan, J. M Schwarz, Jennifer L Ross The microtubule cytoskeleton provides significant structural support for cells, much like bones in the human body. This support is especially important for processes where the cell undergoes major structural changes, such as mitosis with the formation of the mitotic spindle. Biophysical mechanisms regulating the characteristic shape of the mitotic spindle involve microtubule associated proteins (MAP's), crosslinkers, and motor proteins, etc. We have been studying microtubule interactions with MAP65, a plant-derived, antiparallel crosslinking protein, from the MAP65/PRC1/Ase1 family. We have observed in vitro formation of microtubule tactoids, whose shape is reminiscent of the mitotic spindle. However, these tactoids are rigid and thin, unlike a mitotic spindle, which is more liquid crystal-like. To create more fluid-like tactoids, we introduce K401, a dimeric kinesin construct with the N-terminal 401 amino acid motor domain, as a dynamic microtubule crosslinker. We can modulate the relative concentrations of crosslinking MAP65 and translocating K401motors to determine how the competition influences the formation and mechanics of microtubule tactoids. |
Wednesday, March 8, 2023 12:42PM - 12:54PM |
N06.00005: Effects of the GTPs concentration and variation length on the dynamics of microtubules NGANFO YIFOUE WILLY ANISET In this work, we investigate the effects of the variable length of the microtubule and the GTPs concentration during the growth and shrink regimes, the cap length variations and the catastrophe phenomenon. The master equation and generating function were used to evaluate the mean length, the growth speed, the cap length probability and catastrophe probability of the microtubule. Results show that the microtubule growth with the GTPs concentration and exhibit two critical points 1.5 mol.l-1 M and 8 mol.l-1 M. After the critical concentration 8 mol.l-1 M, the microtubule attains the maximum length. The microtubule loses its cap when the length tends towards a maximum length and then, microtubule undergoes catastrophe phenomenon. The catastrophe probability increases with the microtubule length and decreases with the GTPs concentration. |
Wednesday, March 8, 2023 12:54PM - 1:06PM |
N06.00006: Emergent dynamics in motor-free active contractile networks Xiangting Lei, Tuhin Chakrabortty, Jerry E Honts, Saad Bhamla Living cells exploit contractile structures in the cytoskeleton to achieve various functions such as cell division and motility. Here we show experimental and computational studies on a Ca2+-triggered contractile cytoskeletal network composed of cortical proteins from ciliates (Tetrahymena). |
Wednesday, March 8, 2023 1:06PM - 1:18PM |
N06.00007: Identifying the Control Knobs of Cytoskeletal Flow using Simulations and Representation Learning Yuqing Qiu, Elizabeth White, Suriyanarayanan Vaikuntanathan, Aaron R Dinner
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Wednesday, March 8, 2023 1:18PM - 1:30PM |
N06.00008: DNA transport within confined motor-driven cytoskeletal networks Mehdi Shafiei Aporvari, Philip D Neill, Daisy H Achiriloaie, Ryan J McGorty, Rae M Robertson-Anderson Transport of macromolecules through the cytoskeleton is dictated by an interplay between steric obstacles and viscoelasticity from filamentous protein networks, advection and restructuring of the cytoskeleton via motor proteins, and confinement by the cell membrane. In vitro reconstitution of cytoskeletal networks have facilitated studies on how transport dynamics depend on each of these factors independently. Previously, we showed how macromolecular crowding and confinement synergistically couple to dramatically slow the diffusion of crowded DNA molecules that are confined by lipid membranes to cell-sized volumes. Here, we build on this work by incorporating motor-driven cytoskeletal composites into this platform. Specifically, we use differential dynamic microscopy and single-particle tracking to measure the active and thermal transport of particles in motor-driven composites of actin and microtubules confined in cell-sized droplets. We elucidate how anomalous subdiffusion due to caging and steric interactions competes with superdiffusive advection due to motor activity and how the dynamics depend on motor concentration and confinement size. |
Wednesday, March 8, 2023 1:30PM - 1:42PM |
N06.00009: Biodynamic Monitoring of Bacterial invasion of Living Tissue Zhen Hua, John Turek, Fernanda Cunha, Michael Ladisch, David D Nolte Light scattered from living tissue displays a broad range of Doppler frequency shifts related to complex cellular processes and their associated dynamic motion. The Doppler fingerprint of living tissue is extremely sensitive to subtle changes in intracellular dynamics, and BDI provides a powerful new technique for monitoring the response of 3D living tissue to xenobiotic challenges. In this work, we describe the use of BDI to monitor the infection of 3D living tissue by bacteria. Bacteria affect many of the dynamic processes within the living host, allowing the cellular response to perform the role of a living sentinel, reporting on the effects of the bacterial infection as well as monitoring the efficacy of antibiotic treatments. To illustrate the infection-induced power spectral responses, tumor spheroids of the DLD-1 colon adenocarcinoma cell lines were used to highlight different characteristics caused by infection by different bacterial strains. The spectral enhancements represent changes in dynamics with different frequency ranges associated with different types of intracellular motion. This work demonstrates the potential to translate BDI to the clinic to test for antibiotic-resistant infections. |
Wednesday, March 8, 2023 1:42PM - 1:54PM |
N06.00010: Revisiting the Temperature Dependence of Protein Diffusion inside Bacteria: Validity of the Stokes-Einstein Equation Yong Wang, Asmaa A Sadoon, William F Oliver Although the transport and mixing of proteins and other molecules inside bacteria rely on the diffusion of molecules, many aspects of the molecular diffusion in bacterial cytoplasm remain unclear or controversial, including how the diffusion-temperature relation follows the Stokes-Einstein equation. In this study, we applied single-particle tracking photoactivated localization microscopy (sptPALM) to investigate the diffusion of histone-like nucleoid structuring (H-NS) proteins and free dyes in bacterial cytoplasm at different temperatures. Although the diffusion of H-NS proteins in both live and dead bacteria increased at higher temperatures and appeared to follow the Arrhenius equation, the diffusion of free dyes decreased at higher temperatures, questioning the previously proposed theories based on superthermal fluctuations. To understand the measured diffusion-temperature relations, we developed an alternative model, in which the bacterial cytoplasm is considered as a polymeric network / mesh. In our model, the Stokes-Einstein equation remains valid, while the polymeric network contributes a significant term to the viscosity experienced by the molecules diffusing in bacterial cytoplasm. Our model was successful in predicting the diffusion-temperature relations for both H-NS proteins and free dyes in bacteria. In addition, we systematically examined the predicted diffusion-temperature relations with different parameters in the model, and predicted the possible existence of phase transitions. |
Wednesday, March 8, 2023 1:54PM - 2:06PM |
N06.00011: The effect of nanoparticle charge and size on 3D diffusion in live Escherichia coli cells. Diana S Mendez, Alp M Sunol, Benjamin P Bratton, Joseph P Sheehan, Liam J Holt, Zemer Gitai, Roseanna N Zia, Joshua W Shaevitz
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Wednesday, March 8, 2023 2:06PM - 2:18PM |
N06.00012: Hydrodynamic Modelling and Experiment of Spiroplasma's Cytoskeletal Driven Motility Paul M Ryan Nearly all swimming bacteria have evolved to utilize external, helical, rotating appendages called flagella to achieve motility. These flagella are anchored into a cell wall which provides a structural rigidity during the swimming process. Contrary to this, a unique helical bacterium called spiroplasma has neither a cell wall nor flagella, yet still swims in water with a set of unique internal cytoskeletal filaments. These filaments deform thereby deforming the cell’s external barrier, a viscoelastic bilayer lipid membrane. This mechanism allows for motility, yet the interaction between the filaments and spiroplasma’s membrane is unclear. We model this motility using a regularized stokeslet approximation to hydrodynamic interactions and investigate the cell’s ability to deform its membrane using torsion and bend-based modalities. We then use microscopy to compare which model is best suits spiroplasma’s motion. |
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