Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W20: Physics of the Cytoskeleton Across Scales IV: In vivoFocus
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Sponsoring Units: DBIO Chair: Loren Hough Room: 301 |
Friday, March 6, 2020 8:00AM - 8:36AM |
W20.00001: Harmonic oscillation frequencies of cellular contractility support a wave shape model Invited Speaker: Amy Shaub Madox Animal cell shape changes such as cytokinesis are driven by poorly understood rearrangements of the actomyosin cortical cytoskeleton. To gain novel insights into cell-autonomous cytokinetic contractility, we used the C. elegans zygote as a model cell and imaged cytokinesis with unprecedented temporal resolution. Cytokinetic ring closure underwent cycles of acceleration and deceleration. We quantified contractile oscillations via continuous wavelet transform and mode decomposition. Ring inward displacement dynamics were the composite of co-existing amplitude- and frequency-modulated wave modes with ~ 18, 36 and 72-second periodicity. The periodicities of speed oscillations were only subtly changed by depletion of a panel of conserved actomyosin regulators and structural components, but oscillation amplitudes were suppressed by reduction of force generation, and enhanced by reduction of network crosslinking. As suggested by the relationship of the modes’ periodicities as a fundamental frequency, a harmonic and a subharmonic, the range of speed oscillations was well described by a wave-shape model with a single time-varying amplitude, a single time-varying frequency, and a shape factor. Finally, to retain the spatial relationships among contracting segments of the cytokinetic ring, we performed mode decomposition in three dimensions on a space-time-frequency kymocube of our wavelet transform output. We found three major classes of frequency surfaces varying little over space and time. Principal component analysis of these two-dimensional modes confirmed that the frequencies of contractile oscillations are related as a harmonic and sub-harmonic around a fundamental frequency. We propose that the latter reflects the Rho pacemaker driving contractility, and that the harmonic is emergent due to non-linearities in the system. Dissipation of contractility within the network may explain the slower, sub-harmonic contractile oscillations. |
Friday, March 6, 2020 8:36AM - 8:48AM |
W20.00002: Actomyosin-driven mechanics of starfish oocytes Peter Foster, Nikta Fakhri Actomyosin networks underlie most force generation by eukaryotic cells. These networks are driven out of equilibrium in part by myosin which crosslinks and exerts forces on actin filaments. While myosin’s role in force generation is well studied, how actin structure and dynamics influence the active mechanics of the networks during force generation is not understood. Here, we address this issue using oocytes from the starfish Patiria miniate. During maturation, the oocytes undergo surface contraction waves driven by the actomyosin cortex. Using pharmacological inhibitions, which target actin polymerization dynamics, we find that cellular deformation during the contraction wave is not a monotonic function of cortical actin density and is peaked near the wild type. This is reminiscent of in vitro actomyosin networks, which have been shown to have maximal contractility at intermediate levels of network connectivity. To test if this is also the case in starfish oocytes, we probe the oocyte’s mechanical properties and how these change under targeted molecular perturbations. |
Friday, March 6, 2020 8:48AM - 9:00AM |
W20.00003: Spatiotemporal dynamics of the neuronal cytoskeleton across scales during development Kate M O'Neill, Emily K Robinson, Wolfgang Losert Neuronal structure is intrinsically tied to neuronal function because the spatial arrangement of the output (axon) and inputs (dendrites) of a neuron determines how it integrates into the neuronal network. A critical component of the cytoskeleton that drives neuronal morphology is actin, which is necessary for establishing connections (synapses) early in development to refining connections as networks mature. Using live confocal imaging, we study how the dynamics of actin change as cultures of in vitro primary rat cortical neurons develop. We employ two-dimensional Laplacian of Gaussian (LoG) filtering to identify neuronal processes and a pixel-based optical flow algorithm to track actin dynamics in developing neurons. We observe micron-scale actin dynamics in younger neurons (1-5 days in vitro) that clearly drive the pathfinding of growth cones, whereas older neurons display actin dynamics on a much smaller scale, confined to small regions of dendrites and to synaptic spines. Future work will further characterize these dynamics at key developmental timepoints. |
Friday, March 6, 2020 9:00AM - 9:12AM |
W20.00004: Computational model of mitotic spindle positioning in polarized cells Jeffrey M Moore, Adam R Lamson, Matthew A. Glaser, Meredith Betterton The microtubule cytoskeleton plays important roles during the cell life cycle, in events including mitotic spindle assembly, chromosome segregation, nuclear elongation, and spindle positioning. During cell division in budding yeast, interactions between astral microtubules and proteins at the cell cortex position the spindle and nucleus at the bud neck before cytokinesis. The spindle moves due to pulling forces on the microtubules generated by the minus-end-directed motor dynein, which are activated by binding to protein domains localized at the cell cortex. Both the asymmetric distribution of cortical dynein binding domains and microtubule buckling and stabilization are thought to be important for regulating spindle positioning, but how the interplay between microtubule dynamics and motor-driven pulling forces lead to proper positioning at the site of cytokinesis is not well understood. We present results from simulations of a minimal spindle positioning model in a polarized cell driven by interactions between astral filaments, motorized tethers, and cortical binding domains. Our results show how binding domain localization and filament dynamics affect spindle positioning in polarized cells. |
Friday, March 6, 2020 9:12AM - 9:24AM |
W20.00005: Flagellar length control in biflagellate eukaryotes : Cooperative phenomena of generating and regenerating the flagellum Swayamshree Patra, Frank Julicher, Debashish Chowdhury Chlamydomonas Reinhardtii is a biflagellate eukaryote which generates its own flagella of correct length (in a controlled manner) after cell division (ciliogenesis), regenerates both of them after deflagellation and regenerates the amputated flagellum. A pool of proteins needed to form, maintain and regenerate the flagella is synthesized in the cell body. These proteins are transported from the pool to the distal tips for flagellar assembly and those released from the flagellar tip due to ongoing turnover are shuttled back to the pool by intraflagellar transport trains. Combining the dynamics of trafficking intraflagellar transport trains, the pool and the kinetics of flagellar assembly and disassembly, we have developed a stochastic model for understanding the collective phenomena of flagellar length control. Our model accounts for all key features of experimentally known phenomena which include ciliogenesis, resorption, deflagellation as well as regeneration after selective amputation of one of the two flagella. Moreover, we show how the communication among both the flagella through the common pool influences the nature, duration and extent of regeneration under different circumstances and governs the correlations between the fluctuating lengths of the two flagella. |
Friday, March 6, 2020 9:24AM - 9:36AM |
W20.00006: Bridging microtubules promote centering of kinetochores by length-dependent pulling forces Agneza Bosilj, Iva Tolić, Nenad Pavin The mitotic spindle, by exerting forces, segregates chromosomes into two daughter cells during cell division. During metaphase, chromosome are positioned in the equatorial plane of the mitotic spindle, which is necessary to prevent lagging chromosomes and abnormal nuclear envelope reformation. It has been proposed that two centering mechanisms play a key role here, microtubule catastrophe promoted by kinesin-8 motors and pushing forces exerted by chromokinesins. Here we show, by combining a theoretical model and quantitative experiments, that kinetochore microtubules cross-linked by bridging microtubules exert length-dependent centering pulling forces. Our model also shows that length-dependent catastrophe and rescue regulated by motor proteins and passive cross-linkers are necessary for well defined length of microtubules and their antiparallel overlap, respectively. We predict that stable antiparallel overlaps exert length-dependent forces on kinetochores to navigate their positioning in the center of the metaphase plate. |
Friday, March 6, 2020 9:36AM - 9:48AM |
W20.00007: Effects of rapid impact loading on neural progenitor cells Delany Rodriguez, Luke H. C. Patterson, Jennifer L. Walker, Evelyn Rodriguez-Mesa, Kevin Shields, John S. Foster, Adele M. Doyle, Kimberly L. Foster, Megan Valentine We recently developed a high throughput microfluidic MEMS device, the μHammer, to subject individual cells to rapid impact loading, at strains of up to 40% over typical impact durations of ~10 μs. With the μHammer, we can subject >100,000 cells to controlled impacts per experiment, allowing measurement of both single cell properties and statistically-relevant population averages. Cells are collected post-impact and cultured, allowing the time course of damage and recovery to be determined through subsequent analysis. Here, we report the range of cytoskeletal structures and dynamics observed in neural progenitor cells (NPC) before and after sub-lethal impacts, as assessed by high-resolution confocal microscopy. These studies establish important baseline properties of the NPC cytoskeleton, while providing insight into the effects of traumatic injuries on cells and tissues. |
Friday, March 6, 2020 9:48AM - 10:00AM |
W20.00008: Assemblies of Calcium/Calmodulin Dependent Kinase II with Actin and Their Dynamic Regulation by Calmodulin in Dendritic Spines Qian Wang, Mingchen Chen, Nicholas Schafer, Carlos Bueno, Sarah Song, Andy Hudmon, Peter G Wolynes, Neal Waxham, Margaret Cheung Calcium-calmodulin dependent kinase II (CaMKII) plays a key role in the plasticity of dendritic spines. Calcium signals cause calcium-calmodulin to activate CaMKII, which leads to remodeling of the actin filament (F-actin) network in the spine. We elucidate the mechanism of the remodeling by combining computer simulations with protein array experiments and electron microscopic imaging, to arrive at a structural model for the dodecameric complex of CaMKII with F-actin. The binding interface involves multiple domains of CaMKII. This structure explains the architecture of the micron-scale CaMKII/F-actin bundles arising from the multivalence of CaMKII. We also show that the regulatory domain of CaMKII may either bind calmodulin or F-actin, but not both. This frustration along with the multipartite nature of the binding interface allows calmodulin transiently to strip CaMKII from actin assemblies so that they can reorganize. This observation therefore provides a simple mechanism by which the structural dynamics of CaMKII establishes the link between calcium signaling and the morphological plasticity of dendritic spines. |
Friday, March 6, 2020 10:00AM - 10:12AM |
W20.00009: A generalized clutch model to explain cell adhesion mechanics Chiara Venturini, Pablo Saez Integrin-based cell adhesion is a key mechanism in a large number of physiological processes and diseases. The composition and nanoscale organization of adhesion complexes have shown that the integrin-talin-actin chain plays a central role in the formation of small nascent adhesion and further maturation into focal adhesions. The role of ligand spacing and substrate rigidity has been also clearly demonstrated. Clutch models have been widely used to describe how cell adhesion works. However, they have not been capable of rationalizing many of the aspects described above; probably, because current clutch models are built under a number of simplifications of the adhesion mechanisms. Here, we extend the classical clutch model with a detail description of the talin rod as well as a space-dependent ligand distribution. We will show the minimal building block and adhesion length of focal adhesion. Following the same computational model, we will also show that focal adhesion forms for stiff substrates for low spacing while they form in soft substrates for large spacing. In summary, we proposed a model that unifies our current understanding of cell adhesion architecture and turnover while replicating previous experimental results. |
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