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Jawahar A, Vermeil J, Heuvingh J, du Roure O, Piel M. The third dimension of the actin cortex. Curr Opin Cell Biol 2024; 89:102381. [PMID: 38905917 DOI: 10.1016/j.ceb.2024.102381] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2024] [Revised: 05/15/2024] [Accepted: 05/24/2024] [Indexed: 06/23/2024]
Abstract
The actin cortex, commonly described as a thin 2-dimensional layer of actin filaments beneath the plasma membrane, is beginning to be recognized as part of a more dynamic and three-dimensional composite material. In this review, we focus on the elements that contribute to the three-dimensional architecture of the actin cortex. We also argue that actin-rich structures such as filopodia and stress fibers can be viewed as specialized integral parts of the 3D actin cortex. This broadens our definition of the cortex, shifting from its simplified characterization as a thin, two-dimensional layer of actin filaments.
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Affiliation(s)
- Anumita Jawahar
- Physique et Mécanique des Milieux Hétérogènes, ESPCI Paris, PSL University, CNRS, Université Paris Cité, Sorbonne Université, Paris, France; Institut Curie and Institut Pierre Gilles de Gennes, PSL University, CNRS, Paris, France.
| | - Joseph Vermeil
- Physique et Mécanique des Milieux Hétérogènes, ESPCI Paris, PSL University, CNRS, Université Paris Cité, Sorbonne Université, Paris, France; Institut Curie and Institut Pierre Gilles de Gennes, PSL University, CNRS, Paris, France
| | - Julien Heuvingh
- Physique et Mécanique des Milieux Hétérogènes, ESPCI Paris, PSL University, CNRS, Université Paris Cité, Sorbonne Université, Paris, France
| | - Olivia du Roure
- Physique et Mécanique des Milieux Hétérogènes, ESPCI Paris, PSL University, CNRS, Université Paris Cité, Sorbonne Université, Paris, France
| | - Matthieu Piel
- Institut Curie and Institut Pierre Gilles de Gennes, PSL University, CNRS, Paris, France
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2
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Wang X, Zhou Y, Wang L, Haseeb A, Li H, Zheng X, Guo J, Cheng X, Yin W, Sun N, Sun P, Zhang Z, Yang H, Fan K. Fascin-1 Promotes Cell Metastasis through Epithelial-Mesenchymal Transition in Canine Mammary Tumor Cell Lines. Vet Sci 2024; 11:238. [PMID: 38921985 PMCID: PMC11209228 DOI: 10.3390/vetsci11060238] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2024] [Revised: 05/15/2024] [Accepted: 05/23/2024] [Indexed: 06/27/2024] Open
Abstract
Canine mammary tumors (CMTs) are the most common type of tumor in female dogs. In this study, we obtained a metastatic key protein, Fascin-1, by comparing the proteomics data of in situ tumor and metastatic cell lines from the same individual. However, the role of Fascin-1 in the CMT cell line is still unclear. Firstly, proteomics was used to analyze the differential expression of Fascin-1 between the CMT cell lines CHMm and CHMp. Then, the overexpression (CHMm-OE and CHMp-OE) and knockdown (CHMm-KD and CHMp-KD) cell lines were established by lentivirus transduction. Finally, the differentially expressed proteins (DEPs) in CHMm and CHMm-OE cells were identified through proteomics. The results showed that the CHMm cells isolated from CMT abdominal metastases exhibited minimal expression of Fascin-1. The migration, adhesion, and invasion ability of CHMm-OE and CHMp-OE cells increased, while the migration, adhesion, and invasion ability of CHMm-KD and CHMp-KD cells decreased. The overexpression of Fascin-1 can upregulate the Tetraspanin 4 (TSPAN4) protein in CHMm cells and increase the number of migrations. In conclusion, re-expressed Fascin-1 could promote cell EMT and increase lamellipodia formation, resulting in the enhancement of CHMm cell migration, adhesion, and invasion in vitro. This may be beneficial to improve female dogs' prognosis of CMT.
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Affiliation(s)
- Xin Wang
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Ye Zhou
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Linhao Wang
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Abdul Haseeb
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Hongquan Li
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Xiaozhong Zheng
- Medical Research Council (MRC) Centre for Inflammation Research, Queen’s Medical Research Institute, The University of Edinburgh, Edinburgh EH16 4TJ, UK
| | - Jianhua Guo
- Department of Veterinary Pathobiology, Schubot Exotic Bird Health Center, Texas A&M University, College Station, TX 77843, USA
| | - Xiaoliang Cheng
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Wei Yin
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Na Sun
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Panpan Sun
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Zhenbiao Zhang
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Huizhen Yang
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
| | - Kuohai Fan
- Shanxi Key Laboratory for Modernization of TCVM, College of Veterinary Medicine, Shanxi Agricultural University, Jinzhong 030801, China; (X.W.)
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Aydin F, Katkar HH, Morganthaler A, Harker AJ, Kovar DR, Voth GA. Prediction of the essential intermolecular contacts for side-binding of VASP on F-actin. Cytoskeleton (Hoboken) 2024. [PMID: 38647032 DOI: 10.1002/cm.21864] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2023] [Revised: 04/08/2024] [Accepted: 04/09/2024] [Indexed: 04/25/2024]
Abstract
Vasodilator-stimulated phosphoprotein (VASP) family proteins play a crucial role in mediating the actin network architecture in the cytoskeleton. The Ena/VASP homology 2 (EVH2) domain in each of the four identical arms of the tetrameric VASP consists of a loading poly-Pro region, a G-actin-binding domain (GAB), and an F-actin-binding domain (FAB). Together, the poly-Pro, GAB, and FAB domains allow VASP to bind to sides of actin filaments in a bundle, and recruit profilin-G-actin to processively elongate the filaments. The atomic resolution structure of the ternary complex, consisting of the loading poly-Pro region and GAB domain of VASP with profilin-actin, has been solved over a decade ago; however, a detailed structure of the FAB-F-actin complex has not been resolved to date. Experimental insights, based on homology of the FAB domain with the C region of WASP, have been used to hypothesize that the FAB domain binds to the cleft between subdomains 1 and 3 of F-actin. Here, in order to develop our understanding of the VASP-actin complex, we first augment known structural information about the GAB domain binding to actin with the missing FAB domain-actin structure, which we predict using homology modeling and docking simulations. In earlier work, we used mutagenesis and kinetic modeling to study the role of domain-level binding-unbinding kinetics of Ena/VASP on actin filaments in a bundle, specifically on the side of actin filaments. We further look at the nature of the side-binding of the FAB domain of VASP at the atomistic level using our predicted structure, and tabulate effective mutation sites on the FAB domain that would disrupt the VASP-actin complex. We test the binding affinity of Ena with mutated FAB domain using total internal reflection fluorescence microscopy experiments. The binding affinity of VASP is affected significantly for the mutant, providing additional support for our predicted structure.
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Affiliation(s)
- Fikret Aydin
- Department of Chemistry, The James Franck Institute, and Institute for Biophysical Dynamics, University of Chicago, Chicago, Illinois, USA
| | - Harshwardhan H Katkar
- Department of Chemistry, The James Franck Institute, and Institute for Biophysical Dynamics, University of Chicago, Chicago, Illinois, USA
| | - Alisha Morganthaler
- Department of Biochemistry and Molecular Biology and Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois, USA
| | - Alyssa J Harker
- Department of Biochemistry and Molecular Biology and Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois, USA
| | - David R Kovar
- Department of Biochemistry and Molecular Biology and Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois, USA
| | - Gregory A Voth
- Department of Chemistry, The James Franck Institute, and Institute for Biophysical Dynamics, University of Chicago, Chicago, Illinois, USA
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Eddington C, Schwartz JK, Titus MA. filoVision - using deep learning and tip markers to automate filopodia analysis. J Cell Sci 2024; 137:jcs261274. [PMID: 38264939 PMCID: PMC10941656 DOI: 10.1242/jcs.261274] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2023] [Accepted: 01/09/2024] [Indexed: 01/25/2024] Open
Abstract
Filopodia are slender, actin-filled membrane projections used by various cell types for environment exploration. Analyzing filopodia often involves visualizing them using actin, filopodia tip or membrane markers. Due to the diversity of cell types that extend filopodia, from amoeboid to mammalian, it can be challenging for some to find a reliable filopodia analysis workflow suited for their cell type and preferred visualization method. The lack of an automated workflow capable of analyzing amoeboid filopodia with only a filopodia tip label prompted the development of filoVision. filoVision is an adaptable deep learning platform featuring the tools filoTips and filoSkeleton. filoTips labels filopodia tips and the cytosol using a single tip marker, allowing information extraction without actin or membrane markers. In contrast, filoSkeleton combines tip marker signals with actin labeling for a more comprehensive analysis of filopodia shafts in addition to tip protein analysis. The ZeroCostDL4Mic deep learning framework facilitates accessibility and customization for different datasets and cell types, making filoVision a flexible tool for automated analysis of tip-marked filopodia across various cell types and user data.
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Affiliation(s)
- Casey Eddington
- Department of Genetics, Cell Biology, and Development, University of Minnesota, Minneapolis, MN 55455, USA
- Graduate Program in Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA
| | - Jessica K. Schwartz
- Department of Genetics, Cell Biology, and Development, University of Minnesota, Minneapolis, MN 55455, USA
| | - Margaret A. Titus
- Department of Genetics, Cell Biology, and Development, University of Minnesota, Minneapolis, MN 55455, USA
- Graduate Program in Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA
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Ena/VASP clustering at microspike tips involves lamellipodin but not I-BAR proteins, and absolutely requires unconventional myosin-X. Proc Natl Acad Sci U S A 2023; 120:e2217437120. [PMID: 36598940 PMCID: PMC9926217 DOI: 10.1073/pnas.2217437120] [Citation(s) in RCA: 7] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Sheet-like membrane protrusions at the leading edge, termed lamellipodia, drive 2D-cell migration using active actin polymerization. Microspikes comprise actin-filament bundles embedded within lamellipodia, but the molecular mechanisms driving their formation and their potential functional relevance have remained elusive. Microspike formation requires the specific activity of clustered Ena/VASP proteins at their tips to enable processive actin assembly in the presence of capping protein, but the factors and mechanisms mediating Ena/VASP clustering are poorly understood. Systematic analyses of B16-F1 melanoma mutants lacking potential candidate proteins revealed that neither inverse BAR-domain proteins, nor lamellipodin or Abi is essential for clustering, although they differentially contribute to lamellipodial VASP accumulation. In contrast, unconventional myosin-X (MyoX) identified here as proximal to VASP was obligatory for Ena/VASP clustering and microspike formation. Interestingly, and despite the invariable distribution of other relevant marker proteins, the width of lamellipodia in MyoX-KO mutants was significantly reduced as compared with B16-F1 control, suggesting that microspikes contribute to lamellipodium stability. Consistently, MyoX removal caused marked defects in protrusion and random 2D-cell migration. Strikingly, Ena/VASP-deficiency also uncoupled MyoX cluster dynamics from actin assembly in lamellipodia, establishing their tight functional association in microspike formation.
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Pokrant T, Hein JI, Körber S, Disanza A, Pich A, Scita G, Rottner K, Faix J. Ena/VASP clustering at microspike tips involves lamellipodin but not I-BAR proteins, and absolutely requires unconventional myosin-X. Proc Natl Acad Sci U S A 2023. [PMID: 36598940 DOI: 10.1101/2022.05.12.491613] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/16/2023] Open
Abstract
Sheet-like membrane protrusions at the leading edge, termed lamellipodia, drive 2D-cell migration using active actin polymerization. Microspikes comprise actin-filament bundles embedded within lamellipodia, but the molecular mechanisms driving their formation and their potential functional relevance have remained elusive. Microspike formation requires the specific activity of clustered Ena/VASP proteins at their tips to enable processive actin assembly in the presence of capping protein, but the factors and mechanisms mediating Ena/VASP clustering are poorly understood. Systematic analyses of B16-F1 melanoma mutants lacking potential candidate proteins revealed that neither inverse BAR-domain proteins, nor lamellipodin or Abi is essential for clustering, although they differentially contribute to lamellipodial VASP accumulation. In contrast, unconventional myosin-X (MyoX) identified here as proximal to VASP was obligatory for Ena/VASP clustering and microspike formation. Interestingly, and despite the invariable distribution of other relevant marker proteins, the width of lamellipodia in MyoX-KO mutants was significantly reduced as compared with B16-F1 control, suggesting that microspikes contribute to lamellipodium stability. Consistently, MyoX removal caused marked defects in protrusion and random 2D-cell migration. Strikingly, Ena/VASP-deficiency also uncoupled MyoX cluster dynamics from actin assembly in lamellipodia, establishing their tight functional association in microspike formation.
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Affiliation(s)
- Thomas Pokrant
- Institute for Biophysical Chemistry, Hannover Medical School, 30625 Hannover, Germany
| | - Jens Ingo Hein
- Institute for Biophysical Chemistry, Hannover Medical School, 30625 Hannover, Germany
| | - Sarah Körber
- Institute for Biophysical Chemistry, Hannover Medical School, 30625 Hannover, Germany
| | - Andrea Disanza
- IFOM ETS (Istituto Fondazione di Oncologia Molecolare ETS), - The AIRC (Italian Association for Cancer Research) Institute of Molecular Oncology, 20139 Milan, Italy
| | - Andreas Pich
- Research Core Unit Proteomics, Hannover Medical School, 30625 Hannover, Germany
| | - Giorgio Scita
- IFOM ETS (Istituto Fondazione di Oncologia Molecolare ETS), - The AIRC (Italian Association for Cancer Research) Institute of Molecular Oncology, 20139 Milan, Italy
- Department of Oncology and Haemato-Oncology, University of Milan, 20139 Milan, Italy
| | - Klemens Rottner
- Division of Molecular Cell Biology, Zoological Institute, Technische Universität Braunschweig, 38106 Braunschweig, Germany
- Molecular Cell Biology Group, Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany
| | - Jan Faix
- Institute for Biophysical Chemistry, Hannover Medical School, 30625 Hannover, Germany
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Fitz GN, Weck ML, Bodnya C, Perkins OL, Tyska MJ. Protrusion growth driven by myosin-generated force. Dev Cell 2023; 58:18-33.e6. [PMID: 36626869 PMCID: PMC9940483 DOI: 10.1016/j.devcel.2022.12.001] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2022] [Revised: 10/10/2022] [Accepted: 11/29/2022] [Indexed: 01/11/2023]
Abstract
Actin-based protrusions extend from the surface of all eukaryotic cells, where they support diverse activities essential for life. Models of protrusion growth hypothesize that actin filament assembly exerts force for pushing the plasma membrane outward. However, membrane-associated myosin motors are also abundant in protrusions, although their potential for contributing, growth-promoting force remains unexplored. Using an inducible system that docks myosin motor domains to membrane-binding modules with temporal control, we found that application of myosin-generated force to the membrane is sufficient for driving robust protrusion elongation in human, mouse, and pig cell culture models. Protrusion growth scaled with motor accumulation, required barbed-end-directed force, and was independent of cargo delivery or recruitment of canonical elongation factors. Application of growth-promoting force was also supported by structurally distinct myosin motors and membrane-binding modules. Thus, myosin-generated force can drive protrusion growth, and this mechanism is likely active in diverse biological contexts.
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Affiliation(s)
- Gillian N Fitz
- Department of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA
| | - Meredith L Weck
- Department of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA
| | - Caroline Bodnya
- Department of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA
| | - Olivia L Perkins
- Department of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA
| | - Matthew J Tyska
- Department of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
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Tsai FC, Henderson JM, Jarin Z, Kremneva E, Senju Y, Pernier J, Mikhajlov O, Manzi J, Kogan K, Le Clainche C, Voth GA, Lappalainen P, Bassereau P. Activated I-BAR IRSp53 clustering controls the formation of VASP-actin-based membrane protrusions. SCIENCE ADVANCES 2022; 8:eabp8677. [PMID: 36240267 DOI: 10.1101/2022.03.04.483020] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
Filopodia are actin-rich membrane protrusions essential for cell morphogenesis, motility, and cancer invasion. How cells control filopodium initiation on the plasma membrane remains elusive. We performed experiments in cellulo, in vitro, and in silico to unravel the mechanism of filopodium initiation driven by the membrane curvature sensor IRSp53 (insulin receptor substrate protein of 53 kDa). We showed that full-length IRSp53 self-assembles into clusters on membranes depending on PIP2. Using well-controlled in vitro reconstitution systems, we demonstrated that IRSp53 clusters recruit the actin polymerase VASP (vasodilator-stimulated phosphoprotein) to assemble actin filaments locally on membranes, leading to the generation of actin-filled membrane protrusions reminiscent of filopodia. By pulling membrane nanotubes from live cells, we observed that IRSp53 can only be enriched and trigger actin assembly in nanotubes at highly dynamic membrane regions. Our work supports a regulation mechanism of IRSp53 in its attributes of curvature sensation and partner recruitment to ensure a precise spatial-temporal control of filopodium initiation.
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Affiliation(s)
- Feng-Ching Tsai
- Institut Curie, Université PSL, Sorbonne Université, CNRS UMR168, Laboratoire Physico-Chimie Curie, 75005 Paris, France
| | - J Michael Henderson
- Institut Curie, Université PSL, Sorbonne Université, CNRS UMR168, Laboratoire Physico-Chimie Curie, 75005 Paris, France
- Unité de Trafic Membranaire et Pathogénèse, Département de Biologie Cellulaire et Infection, Institut Pasteur, Université de Paris, CNRS UMR 3691, 75015 Paris, France
| | - Zack Jarin
- Pritzker School for Molecular Engineering, University of Chicago, Chicago, IL 60637, USA
| | - Elena Kremneva
- Institute of Biotechnology and Helsinki Institute of Life Sciences, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland
| | - Yosuke Senju
- Research Institute for Interdisciplinary Science (RIIS), Okayama University, Okayama, Japan
| | - Julien Pernier
- Université Paris-Saclay, CEA, CNRS, Institute for Integrative Biology of the Cell (I2BC), 91198 Gif-sur-Yvette, France
| | - Oleg Mikhajlov
- Institut Curie, Université PSL, Sorbonne Université, CNRS UMR168, Laboratoire Physico-Chimie Curie, 75005 Paris, France
| | - John Manzi
- Institut Curie, Université PSL, Sorbonne Université, CNRS UMR168, Laboratoire Physico-Chimie Curie, 75005 Paris, France
| | - Konstantin Kogan
- Institute of Biotechnology and Helsinki Institute of Life Sciences, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland
| | - Christophe Le Clainche
- Université Paris-Saclay, CEA, CNRS, Institute for Integrative Biology of the Cell (I2BC), 91198 Gif-sur-Yvette, France
| | - Gregory A Voth
- Department of Chemistry, Chicago Center for Theoretical Chemistry, James Franck Institute, and Institute for Biophysical Dynamics, University of Chicago, Chicago, IL 60637, USA
| | - Pekka Lappalainen
- Institute of Biotechnology and Helsinki Institute of Life Sciences, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland
| | - Patricia Bassereau
- Institut Curie, Université PSL, Sorbonne Université, CNRS UMR168, Laboratoire Physico-Chimie Curie, 75005 Paris, France
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Sherer LA, Courtemanche N. Cooperative bundling by fascin generates actin structures with architectures that depend on filament length. Front Cell Dev Biol 2022; 10:974047. [PMID: 36120572 PMCID: PMC9479110 DOI: 10.3389/fcell.2022.974047] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2022] [Accepted: 08/12/2022] [Indexed: 11/30/2022] Open
Abstract
The assembly of actin-based structures with precisely defined architectures supports essential cellular functions, including motility, intracellular transport, and division. The geometric arrangements of the filaments within actin structures are stabilized via the association of crosslinking proteins, which bind two filaments simultaneously. Because actin polymerization and crosslinking occur concurrently within the dynamic environment of the cell, these processes likely play interdependent roles in shaping the architectures of actin-based structures. To dissect the contribution of polymerization to the construction of higher-order actin structures, we investigated how filament elongation affects the formation of simple, polarized actin bundles by the crosslinking protein fascin. Using populations of actin filaments to represent distinct stages of elongation, we found that the rate of bundle assembly increases with filament length. Fascin assembles short filaments into discrete bundles, whereas bundles of long filaments merge with one another to form interconnected networks. Although filament elongation promotes bundle coalescence, many connections formed between elongating bundles are short-lived and are followed by filament breakage. Our data suggest that initiation of crosslinking early in elongation aligns growing filaments, creating a template for continued bundle assembly as elongation proceeds. This initial alignment promotes the assembly of bundles that are resistant to large changes in curvature that are required for coalescence into interconnected networks. As a result, bundles of short filaments remain straighter and more topologically discrete as elongation proceeds than bundles assembled from long filaments. Thus, uncoordinated filament elongation and crosslinking can alter the architecture of bundled actin networks, highlighting the importance of maintaining precise control over filament length during the assembly of specialized actin structures.
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