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Kandiyoth FB, Michelot A. Reconstitution of actin-based cellular processes: Why encapsulation changes the rules. Eur J Cell Biol 2023; 102:151368. [PMID: 37922812 DOI: 10.1016/j.ejcb.2023.151368] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2023] [Revised: 10/02/2023] [Accepted: 10/20/2023] [Indexed: 11/07/2023] Open
Abstract
While in vitro reconstitution of cellular processes is progressing rapidly, the encapsulation of biomimetic systems to reproduce the cellular environment is a major challenge. Here we review the difficulties, using reconstitution of processes dependent on actin polymerization as an example. Some of the problems are purely technical, due to the need for engineering strategies to encapsulate concentrated solutions in micrometer-sized compartments. However, other significant issues arise from the reduction of experimental volumes, which alters the chemical evolution of these non-equilibrium systems. Important parameters to consider for successful reconstitutions are the amount of each component, their consumption and renewal rates to guarantee their continuous availability.
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Affiliation(s)
| | - Alphée Michelot
- Aix Marseille Univ, CNRS, IBDM, Turing Centre for Living Systems, Marseille, France.
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2
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Abstract
Accurate decoding of spatial chemical landscapes is critical for many cell functions. Eukaryotic cells decode local chemical gradients to orient growth or movement in productive directions. Recent work on yeast model systems, whose gradient sensing pathways display much less complexity than those in animal cells, has suggested new paradigms for how these very small cells successfully exploit information in noisy and dynamic pheromone gradients to identify their mates. Pheromone receptors regulate a polarity circuit centered on the conserved Rho-family GTPase, Cdc42. The polarity circuit contains both positive and negative feedback pathways, allowing spontaneous symmetry breaking and also polarity site disassembly and relocation. Cdc42 orients the actin cytoskeleton, leading to focused vesicle traffic that promotes movement of the polarity site and also reshapes the cortical distribution of receptors at the cell surface. In this article, we review the advances from work on yeasts and compare them with the excitable signaling pathways that have been revealed in chemotactic animal cells. Expected final online publication date for the Annual Review of Biophysics, Volume 51 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
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Affiliation(s)
- Debraj Ghose
- Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina, USA;
| | - Timothy Elston
- Department of Pharmacology, University of North Carolina at Chapel Hill, North Carolina, USA
| | - Daniel Lew
- Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina, USA;
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3
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Dupraz S, Hilton BJ, Husch A, Santos TE, Coles CH, Stern S, Brakebusch C, Bradke F. RhoA Controls Axon Extension Independent of Specification in the Developing Brain. Curr Biol 2019; 29:3874-3886.e9. [PMID: 31679934 DOI: 10.1016/j.cub.2019.09.040] [Citation(s) in RCA: 53] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2019] [Revised: 08/22/2019] [Accepted: 09/16/2019] [Indexed: 12/22/2022]
Abstract
The specification of an axon and its subsequent outgrowth are key steps during neuronal polarization, a prerequisite to wire the brain. The Rho-guanosine triphosphatase (GTPase) RhoA is believed to be a central player in these processes. However, its physiological role has remained undefined. Here, genetic loss- and gain-of-function experiments combined with time-lapse microscopy, cell culture, and in vivo analysis show that RhoA is not involved in axon specification but confines the initiation of neuronal polarization and axon outgrowth during development. Biochemical analysis and super-resolution microscopy together with molecular and pharmacological manipulations reveal that RhoA restrains axon growth by activating myosin-II-mediated actin arc formation in the growth cone to prevent microtubules from protruding toward the leading edge. Through this mechanism, RhoA regulates the duration of axon growth and pause phases, thus controlling the tightly timed extension of developing axons. Thereby, this work unravels physiologically relevant players coordinating actin-microtubule interactions during axon growth.
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Affiliation(s)
- Sebastian Dupraz
- Axonal Growth and Regeneration Group, German Center for Neurodegenerative Diseases (DZNE), Venusberg-Campus 1, Building 99, 53127 Bonn, Germany
| | - Brett J Hilton
- Axonal Growth and Regeneration Group, German Center for Neurodegenerative Diseases (DZNE), Venusberg-Campus 1, Building 99, 53127 Bonn, Germany
| | - Andreas Husch
- Axonal Growth and Regeneration Group, German Center for Neurodegenerative Diseases (DZNE), Venusberg-Campus 1, Building 99, 53127 Bonn, Germany
| | - Telma E Santos
- Axonal Growth and Regeneration Group, German Center for Neurodegenerative Diseases (DZNE), Venusberg-Campus 1, Building 99, 53127 Bonn, Germany
| | - Charlotte H Coles
- Axonal Growth and Regeneration Group, German Center for Neurodegenerative Diseases (DZNE), Venusberg-Campus 1, Building 99, 53127 Bonn, Germany
| | - Sina Stern
- Axonal Growth and Regeneration Group, German Center for Neurodegenerative Diseases (DZNE), Venusberg-Campus 1, Building 99, 53127 Bonn, Germany
| | - Cord Brakebusch
- Biotech Research & Innovation Centre, University of Copenhagen, Ole Maaløes Vej 5, 2200 Copenhagen, Denmark
| | - Frank Bradke
- Axonal Growth and Regeneration Group, German Center for Neurodegenerative Diseases (DZNE), Venusberg-Campus 1, Building 99, 53127 Bonn, Germany.
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4
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The Cytoskeleton-A Complex Interacting Meshwork. Cells 2019; 8:cells8040362. [PMID: 31003495 PMCID: PMC6523135 DOI: 10.3390/cells8040362] [Citation(s) in RCA: 181] [Impact Index Per Article: 36.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2019] [Revised: 04/15/2019] [Accepted: 04/15/2019] [Indexed: 12/22/2022] Open
Abstract
The cytoskeleton of animal cells is one of the most complicated and functionally versatile structures, involved in processes such as endocytosis, cell division, intra-cellular transport, motility, force transmission, reaction to external forces, adhesion and preservation, and adaptation of cell shape. These functions are mediated by three classical cytoskeletal filament types, as follows: Actin, microtubules, and intermediate filaments. The named filaments form a network that is highly structured and dynamic, responding to external and internal cues with a quick reorganization that is orchestrated on the time scale of minutes and has to be tightly regulated. Especially in brain tumors, the cytoskeleton plays an important role in spreading and migration of tumor cells. As the cytoskeletal organization and regulation is complex and many-faceted, this review aims to summarize the findings about cytoskeletal filament types, including substructures formed by them, such as lamellipodia, stress fibers, and interactions between intermediate filaments, microtubules and actin. Additionally, crucial regulatory aspects of the cytoskeletal filaments and the formed substructures are discussed and integrated into the concepts of cell motility. Even though little is known about the impact of cytoskeletal alterations on the progress of glioma, a final point discussed will be the impact of established cytoskeletal alterations in the cellular behavior and invasion of glioma.
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5
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Dekraker C, Boucher E, Mandato CA. Regulation and Assembly of Actomyosin Contractile Rings in Cytokinesis and Cell Repair. Anat Rec (Hoboken) 2018; 301:2051-2066. [PMID: 30312008 DOI: 10.1002/ar.23962] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2018] [Revised: 08/24/2018] [Accepted: 08/27/2018] [Indexed: 01/17/2023]
Abstract
Cytokinesis and single-cell wound repair both involve contractile assemblies of filamentous actin (F-actin) and myosin II organized into characteristic ring-like arrays. The assembly of these actomyosin contractile rings (CRs) is specified spatially and temporally by small Rho GTPases, which trigger local actin polymerization and myosin II contractility via a variety of downstream effectors. We now have a much clearer view of the Rho GTPase signaling cascade that leads to the formation of CRs, but some factors involved in CR positioning, assembly, and function remain poorly understood. Recent studies show that this regulation is multifactorial and goes beyond the long-established Ca2+ -dependent processes. There is substantial evidence that the Ca2+ -independent changes in cell shape, tension, and plasma membrane composition that characterize cytokinesis and single-cell wound repair also regulate CR formation. Elucidating the regulation and mechanistic properties of CRs is important to our understanding of basic cell biology and holds potential for therapeutic applications in human disease. In this review, we present a primer on the factors influencing and regulating CR positioning, assembly, and contraction as they occur in a variety of cytokinetic and single-cell wound repair models. Anat Rec, 301:2051-2066, 2018. © 2018 Wiley Periodicals, Inc.
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Affiliation(s)
- Corina Dekraker
- Department of Anatomy and Cell Biology, Faculty of Medicine, McGill University, Montreal, Quebec, Canada
| | - Eric Boucher
- Department of Anatomy and Cell Biology, Faculty of Medicine, McGill University, Montreal, Quebec, Canada
| | - Craig A Mandato
- Department of Anatomy and Cell Biology, Faculty of Medicine, McGill University, Montreal, Quebec, Canada
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6
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Fang C, Hui TH, Wei X, Yan Z, Qian J, Lin Y. Interaction and fusion dynamics between cellular blebs. J Biomech 2018; 81:113-121. [PMID: 30366658 DOI: 10.1016/j.jbiomech.2018.10.004] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2018] [Revised: 09/26/2018] [Accepted: 10/03/2018] [Indexed: 11/17/2022]
Abstract
Membrane blebbing, as a mechanism for cells to regulate their internal pressure and membrane tension, is believed to play important roles in processes such as cell migration, spreading and apoptosis. However, the fundamental question of how different blebs interact with each other during their life cycles remains largely unclear. Here, we report a combined theoretical and experimental investigation to examine how the growth and retraction of a cellular bleb are influenced by neighboring blebs as well as the fusion dynamics between them. Specifically, a boundary integral model was developed to describe the shape evolution of cell membrane during the blebbing/retracting process. We showed that a drop in the intracellular pressure will be induced by the formation of a bleb whose retraction then restores the pressure level. Consequently, the volume that a second bleb can reach was predicted to heavily depend on its initial weakened size and the time lag with respect to the first bleb, all in quantitative agreement with our experimental observations. In addition, it was found that as the strength of membrane-cortex adhesion increases, the possible coalescence of two neighboring blebs changes from smooth fusion to abrupt coalescence and eventually to no fusion at all. Phase diagrams summarizing the dependence of such transition on key physical factors, such as the intracellular pressure and bleb separation, were also obtained.
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Affiliation(s)
- Chao Fang
- Department of Mechanical Engineering, The University of Hong Kong, Hong Kong; HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), Shenzhen, Guangdong, China
| | - Tsz Hin Hui
- Department of Mechanical Engineering, The University of Hong Kong, Hong Kong; HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), Shenzhen, Guangdong, China
| | - Xi Wei
- Department of Mechanical Engineering, The University of Hong Kong, Hong Kong; HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), Shenzhen, Guangdong, China
| | - Zishen Yan
- Department of Mechanical Engineering, The University of Hong Kong, Hong Kong; HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), Shenzhen, Guangdong, China
| | - Jin Qian
- Department of Engineering Mechanics, Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Hangzhou, Zhejiang, China.
| | - Yuan Lin
- Department of Mechanical Engineering, The University of Hong Kong, Hong Kong; HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), Shenzhen, Guangdong, China.
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7
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Ding Y, Wang J, Xu GK, Wang GF. Are elastic moduli of biological cells depth dependent or not? Another explanation using a contact mechanics model with surface tension. SOFT MATTER 2018; 14:7534-7541. [PMID: 30152838 DOI: 10.1039/c8sm01216d] [Citation(s) in RCA: 31] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Atomic force microscopy (AFM) has become the most commonly used tool to measure the mechanical properties of biological cells. In AFM indentation experiments, the Hertz and Sneddon models of contact mechanics are usually adopted to extract the elastic modulus by analyzing the load-indent depth curves for spherical and conical tips, respectively. However, the effects of surface tension, neglected in existing contact models, become more significant in indentation responses due to the lower elastic moduli of living cells. Here, we present two simple yet robust relations between load and indent depth considering surface tension effects for spherical and conical indentations, through dimensional analysis and finite element simulations. When the indent depth is smaller than the intrinsic length defined as the ratio of surface tension to elastic modulus, the elastic modulus obtained by classical contact mechanics theories would be overestimated. Contrary to the majority of reported results, we find that the elastic modulus of a cell could be independent of indent depths if surface tension is taken into account. Our model seems to be in agreement with experimental data available. A comprehensive comparison will be done in the future.
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Affiliation(s)
- Yue Ding
- Department of Engineering Mechanics, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an 710049, China.
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8
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Lymphocyte-specific protein 1 regulates mechanosensory oscillation of podosomes and actin isoform-based actomyosin symmetry breaking. Nat Commun 2018; 9:515. [PMID: 29410425 PMCID: PMC5802837 DOI: 10.1038/s41467-018-02904-x] [Citation(s) in RCA: 41] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2016] [Accepted: 01/05/2018] [Indexed: 01/06/2023] Open
Abstract
Subcellular fine-tuning of the actomyosin cytoskeleton is a prerequisite for polarized cell migration. We identify LSP (lymphocyte-specific protein) 1 as a critical regulator of actomyosin contractility in primary macrophages. LSP1 regulates adhesion and migration, including the parameters cell area and speed, and also podosome turnover, oscillation and protrusive force. LSP1 recruits myosin IIA and its regulators, including myosin light chain kinase and calmodulin, and competes with supervillin, a myosin hyperactivator, for myosin regulators, and for actin isoforms, notably β-actin. Actin isoforms are anisotropically distributed in myosin IIA-expressing macrophages, and contribute to the differential recruitment of LSP1 and supervillin, thus enabling an actomyosin symmetry break, analogous to the situation in cells expressing two myosin II isoforms. Collectively, these results show that the cellular pattern of actin isoforms builds the basis for the differential distribution of two actomyosin machineries with distinct properties, leading to the establishment of discrete zones of actomyosin contractility. The actomyosin cytoskeleton plays an important role in polarised cell migration. Here the authors identify lymphocyte-specific protein (LSP)-1 as a regulator of actomyosin contractility in macrophages, by competing with supervillin for myosin IIA activators acting specifically on the β-actin isoform.
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9
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Jevtić P, Milunović-Jevtić A, Dilsaver MR, Gatlin JC, Levy DL. Use of Xenopus cell-free extracts to study size regulation of subcellular structures. THE INTERNATIONAL JOURNAL OF DEVELOPMENTAL BIOLOGY 2018; 60:277-288. [PMID: 27759156 DOI: 10.1387/ijdb.160158dl] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Striking size variations are prominent throughout biology, at the organismal, cellular, and subcellular levels. Important fundamental questions concern organelle size regulation and how organelle size is regulated relative to cell size, also known as scaling. Uncovering mechanisms of organelle size regulation will inform the functional significance of size as well as the implications of misregulated size, for instance in the case of nuclear enlargement in cancer. Xenopus egg and embryo extracts are powerful cell-free systems that have been utilized extensively for mechanistic and functional studies of various organelles and subcellular structures. The open biochemical nature of the extract permits facile manipulation of its composition, and in recent years extract approaches have illuminated mechanisms of organelle size regulation. This review largely focuses on in vitro Xenopus studies that have identified regulators of nuclear and spindle size. We also discuss potential relationships between size scaling of the nucleus and spindle, size regulation of other subcellular structures, and extract experiments that have clarified developmental timing mechanisms. We conclude by offering some future prospects, notably the integration of Xenopus extract with microfluidic technology.
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Affiliation(s)
- Predrag Jevtić
- Department of Molecular Biology, University of Wyoming, Laramie, WY, USA
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10
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Drak Is Required for Actomyosin Organization During Drosophila Cellularization. G3-GENES GENOMES GENETICS 2016; 6:819-28. [PMID: 26818071 PMCID: PMC4825652 DOI: 10.1534/g3.115.026401] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
The generation of force by actomyosin contraction is critical for a variety of cellular and developmental processes. Nonmuscle myosin II is the motor that drives actomyosin contraction, and its activity is largely regulated by phosphorylation of the myosin regulatory light chain. During the formation of the Drosophila cellular blastoderm, actomyosin contraction drives constriction of microfilament rings, modified cytokinesis rings. Here, we find that Drak is necessary for most of the phosphorylation of the myosin regulatory light chain during cellularization. We show that Drak is required for organization of myosin II within the microfilament rings. Proper actomyosin contraction of the microfilament rings during cellularization also requires Drak activity. Constitutive activation of myosin regulatory light chain bypasses the requirement for Drak, suggesting that actomyosin organization and contraction are mediated through Drak's regulation of myosin activity. Drak is also involved in the maintenance of furrow canal structure and lateral plasma membrane integrity during cellularization. Together, our observations suggest that Drak is the primary regulator of actomyosin dynamics during cellularization.
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11
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Ito H, Nishigami Y, Sonobe S, Ichikawa M. Wrinkling of a spherical lipid interface induced by actomyosin cortex. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 92:062711. [PMID: 26764731 DOI: 10.1103/physreve.92.062711] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/03/2015] [Indexed: 06/05/2023]
Abstract
Actomyosin actively generates contractile forces that provide the plasma membrane with the deformation stresses essential to carry out biological processes. Although the contractile property of purified actomyosin has been extensively studied, to understand the physical contribution of the actomyosin contractile force on a deformable membrane is still a challenging problem and of great interest in the field of biophysics. Here, we reconstitute a model system with a cell-sized deformable interface that exhibits anomalous curvature-dependent wrinkling caused by the actomyosin cortex underneath the spherical closed interface. Through a shape analysis of the wrinkling deformation, we find that the dominant contributor to the wrinkled shape changes from bending elasticity to stretching elasticity of the reconstituted cortex upon increasing the droplet curvature radius of the order of the cell size, i.e., tens of micrometers. The observed curvature dependence is explained by the theoretical description of the cortex elasticity and contractility. Our present results provide a fundamental insight into the deformation of a curved membrane induced by the actomyosin cortex.
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Affiliation(s)
- Hiroaki Ito
- Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
| | - Yukinori Nishigami
- Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
| | - Seiji Sonobe
- Department of Life Science, Graduate School of Life Science, University of Hyogo, Harima Science Park City, Hyogo 678-1297, Japan
| | - Masatoshi Ichikawa
- Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
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12
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Goins LM, Mullins RD. A novel tropomyosin isoform functions at the mitotic spindle and Golgi in Drosophila. Mol Biol Cell 2015; 26:2491-504. [PMID: 25971803 PMCID: PMC4571303 DOI: 10.1091/mbc.e14-12-1619] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2014] [Accepted: 05/05/2015] [Indexed: 12/28/2022] Open
Abstract
Most eukaryotic cells express multiple isoforms of the actin-binding protein tropomyosin that help construct a variety of cytoskeletal networks. Only one nonmuscle tropomyosin (Tm1A) has previously been described in Drosophila, but developmental defects caused by insertion of P-elements near tropomyosin genes imply the existence of additional, nonmuscle isoforms. Using biochemical and molecular genetic approaches, we identified three tropomyosins expressed in Drosophila S2 cells: Tm1A, Tm1J, and Tm2A. The Tm1A isoform localizes to the cell cortex, lamellar actin networks, and the cleavage furrow of dividing cells--always together with myosin-II. Isoforms Tm1J and Tm2A colocalize around the Golgi apparatus with the formin-family protein Diaphanous, and loss of either isoform perturbs cell cycle progression. During mitosis, Tm1J localizes to the mitotic spindle, where it promotes chromosome segregation. Using chimeras, we identified the determinants of tropomyosin localization near the C-terminus. This work 1) identifies and characterizes previously unknown nonmuscle tropomyosins in Drosophila, 2) reveals a function for tropomyosin in the mitotic spindle, and 3) uncovers sequence elements that specify isoform-specific localizations and functions of tropomyosin.
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Affiliation(s)
- Lauren M Goins
- Department of Cellular and Molecular Pharmacology, School of Medicine, University of California, San Francisco, San Francisco, CA 94158
| | - R Dyche Mullins
- Department of Cellular and Molecular Pharmacology, School of Medicine, University of California, San Francisco, San Francisco, CA 94158
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Bussonnier M, Carvalho K, Lemière J, Joanny JF, Sykes C, Betz T. Mechanical detection of a long-range actin network emanating from a biomimetic cortex. Biophys J 2014; 107:854-62. [PMID: 25140420 PMCID: PMC4142247 DOI: 10.1016/j.bpj.2014.07.008] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2014] [Revised: 06/30/2014] [Accepted: 07/07/2014] [Indexed: 10/24/2022] Open
Abstract
Actin is ubiquitous globular protein that polymerizes into filaments and forms networks that participate in the force generation of eukaryotic cells. Such forces are used for cell motility, cytokinesis, and tissue remodeling. Among those actin networks, we focus on the actin cortex, a dense branched network beneath the plasma membrane that is of particular importance for the mechanical properties of the cell. Here we reproduce the cellular cortex by activating actin filament growth on a solid surface. We unveil the existence of a sparse actin network that emanates from the surface and extends over a distance that is at least 10 times larger than the cortex itself. We call this sparse actin network the "actin cloud" and characterize its mechanical properties with optical tweezers. We show, both experimentally and theoretically, that the actin cloud is mechanically relevant and that it should be taken into account because it can sustain forces as high as several picoNewtons (pN). In particular, it is known that in plant cells, actin networks similar to the actin cloud have a role in positioning the nucleus; in large oocytes, they play a role in driving chromosome movement. Recent evidence shows that such networks even prevent granule condensation in large cells.
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Affiliation(s)
- Matthias Bussonnier
- Institut Curie, Centre de Recherche, Paris, France; Université Paris Diderot, Sorbonne Paris Cité, Paris, France; Sorbonne Universités, Université Pierre et Marie Curie, Université Paris 06, UMR 168, Paris, France; Centre National de la Recherche Scientifique, UMR168, Paris, France
| | - Kevin Carvalho
- Institut Curie, Centre de Recherche, Paris, France; Sorbonne Universités, Université Pierre et Marie Curie, Université Paris 06, UMR 168, Paris, France; Centre National de la Recherche Scientifique, UMR168, Paris, France
| | - Joël Lemière
- Institut Curie, Centre de Recherche, Paris, France; Université Paris Diderot, Sorbonne Paris Cité, Paris, France; Sorbonne Universités, Université Pierre et Marie Curie, Université Paris 06, UMR 168, Paris, France; Centre National de la Recherche Scientifique, UMR168, Paris, France
| | - Jean-François Joanny
- Institut Curie, Centre de Recherche, Paris, France; Sorbonne Universités, Université Pierre et Marie Curie, Université Paris 06, UMR 168, Paris, France; Centre National de la Recherche Scientifique, UMR168, Paris, France
| | - Cécile Sykes
- Institut Curie, Centre de Recherche, Paris, France; Sorbonne Universités, Université Pierre et Marie Curie, Université Paris 06, UMR 168, Paris, France; Centre National de la Recherche Scientifique, UMR168, Paris, France
| | - Timo Betz
- Institut Curie, Centre de Recherche, Paris, France; Sorbonne Universités, Université Pierre et Marie Curie, Université Paris 06, UMR 168, Paris, France; Centre National de la Recherche Scientifique, UMR168, Paris, France.
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14
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Symmetry breaking in spore germination relies on an interplay between polar cap stability and spore wall mechanics. Dev Cell 2014; 28:534-46. [PMID: 24636258 DOI: 10.1016/j.devcel.2014.01.023] [Citation(s) in RCA: 60] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2013] [Revised: 01/06/2014] [Accepted: 01/23/2014] [Indexed: 11/23/2022]
Abstract
The morphogenesis of single cells depends on their ability to coordinate surface mechanics and polarity. During germination, spores of many species develop a polar tube that hatches out of a rigid outer spore wall (OSW) in a process termed outgrowth. However, how these awakening cells reorganize to stabilize this first growth axis remains unknown. Here, using quantitative experiments and modeling, we reveal the mechanisms underlying outgrowth in fission yeast. We find that, following an isotropic growth phase during which a single polarity cap wanders around the surface, outgrowth occurs when spores have doubled their volume, concomitantly with the stabilization of the cap and a singular rupture in the OSW. This rupture happens when OSW mechanical stress exceeds a threshold, releases the constraints of the OSW on growth, and stabilizes polarity. Thus, outgrowth exemplifies a self-organizing morphogenetic process in which reinforcements between growth and polarity coordinate mechanics and internal organization.
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15
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Blanchoin L, Boujemaa-Paterski R, Sykes C, Plastino J. Actin dynamics, architecture, and mechanics in cell motility. Physiol Rev 2014; 94:235-63. [PMID: 24382887 DOI: 10.1152/physrev.00018.2013] [Citation(s) in RCA: 857] [Impact Index Per Article: 85.7] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Tight coupling between biochemical and mechanical properties of the actin cytoskeleton drives a large range of cellular processes including polarity establishment, morphogenesis, and motility. This is possible because actin filaments are semi-flexible polymers that, in conjunction with the molecular motor myosin, can act as biological active springs or "dashpots" (in laymen's terms, shock absorbers or fluidizers) able to exert or resist against force in a cellular environment. To modulate their mechanical properties, actin filaments can organize into a variety of architectures generating a diversity of cellular organizations including branched or crosslinked networks in the lamellipodium, parallel bundles in filopodia, and antiparallel structures in contractile fibers. In this review we describe the feedback loop between biochemical and mechanical properties of actin organization at the molecular level in vitro, then we integrate this knowledge into our current understanding of cellular actin organization and its physiological roles.
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16
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Lim FY, Koon YL, Chiam KH. A computational model of amoeboid cell migration. Comput Methods Biomech Biomed Engin 2013; 16:1085-95. [DOI: 10.1080/10255842.2012.757598] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
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17
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Martinelli R, Kamei M, Sage PT, Massol R, Varghese L, Sciuto T, Toporsian M, Dvorak AM, Kirchhausen T, Springer TA, Carman CV. Release of cellular tension signals self-restorative ventral lamellipodia to heal barrier micro-wounds. ACTA ACUST UNITED AC 2013; 201:449-65. [PMID: 23629967 PMCID: PMC3639391 DOI: 10.1083/jcb.201209077] [Citation(s) in RCA: 68] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Endothelial and epithelial barrier disruptions are detected via local decrease in cellular tension, which are coupled to reactive oxygen species–dependent self-restorative actin remodeling dynamics. Basic mechanisms by which cellular barriers sense and respond to integrity disruptions remain poorly understood. Despite its tenuous structure and constitutive exposure to disruptive strains, the vascular endothelium exhibits robust barrier function. We show that in response to micrometer-scale disruptions induced by transmigrating leukocytes, endothelial cells generate unique ventral lamellipodia that propagate via integrins toward and across these “micro-wounds” to close them. This novel actin remodeling activity progressively healed multiple micro-wounds in succession and changed direction during this process. Mechanical probe-induced micro-wounding of both endothelia and epithelia suggests that ventral lamellipodia formed as a response to force imbalance and specifically loss of isometric tension. Ventral lamellipodia were enriched in the Rac1 effectors cortactin, IQGAP, and p47Phox and exhibited localized production of hydrogen peroxide. Together with Apr2/3, these were functionally required for effective micro-wound healing. We propose that barrier disruptions are detected as local release of isometric tension/force unloading, which is directly coupled to reactive oxygen species–dependent self-restorative actin remodeling dynamics.
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Affiliation(s)
- Roberta Martinelli
- Department of Medicine, Center for Vascular Biology Research, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA
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Huber F, Schnauß J, Rönicke S, Rauch P, Müller K, Fütterer C, Käs J. Emergent complexity of the cytoskeleton: from single filaments to tissue. ADVANCES IN PHYSICS 2013; 62:1-112. [PMID: 24748680 PMCID: PMC3985726 DOI: 10.1080/00018732.2013.771509] [Citation(s) in RCA: 122] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/21/2011] [Revised: 01/11/2013] [Indexed: 05/17/2023]
Abstract
Despite their overwhelming complexity, living cells display a high degree of internal mechanical and functional organization which can largely be attributed to the intracellular biopolymer scaffold, the cytoskeleton. Being a very complex system far from thermodynamic equilibrium, the cytoskeleton's ability to organize is at the same time challenging and fascinating. The extensive amounts of frequently interacting cellular building blocks and their inherent multifunctionality permits highly adaptive behavior and obstructs a purely reductionist approach. Nevertheless (and despite the field's relative novelty), the physics approach has already proved to be extremely successful in revealing very fundamental concepts of cytoskeleton organization and behavior. This review aims at introducing the physics of the cytoskeleton ranging from single biopolymer filaments to multicellular organisms. Throughout this wide range of phenomena, the focus is set on the intertwined nature of the different physical scales (levels of complexity) that give rise to numerous emergent properties by means of self-organization or self-assembly.
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Affiliation(s)
- F. Huber
- Institute for Experimental Physics I, University of Leipzig, Leipzig, Germany
| | - J. Schnauß
- Institute for Experimental Physics I, University of Leipzig, Leipzig, Germany
| | - S. Rönicke
- Institute for Experimental Physics I, University of Leipzig, Leipzig, Germany
| | - P. Rauch
- Institute for Experimental Physics I, University of Leipzig, Leipzig, Germany
| | - K. Müller
- Institute for Experimental Physics I, University of Leipzig, Leipzig, Germany
| | - C. Fütterer
- Institute for Experimental Physics I, University of Leipzig, Leipzig, Germany
| | - J. Käs
- Institute for Experimental Physics I, University of Leipzig, Leipzig, Germany
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19
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Zeng F, Yang W, Huang J, Chen Y, Chen Y. Determination of the lowest concentrations of aldehyde fixatives for completely fixing various cellular structures by real-time imaging and quantification. Histochem Cell Biol 2012. [DOI: 10.1007/s00418-012-1058-5] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
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20
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MacQueen LA, Thibault M, Buschmann MD, Wertheimer MR. Electromechanical deformation of mammalian cells in suspension depends on their cortical actin thicknesses. J Biomech 2012; 45:2797-803. [DOI: 10.1016/j.jbiomech.2012.08.040] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2012] [Revised: 08/28/2012] [Accepted: 08/30/2012] [Indexed: 01/07/2023]
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21
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Rodríguez-Fraticelli AE, Auzan M, Alonso MA, Bornens M, Martín-Belmonte F. Cell confinement controls centrosome positioning and lumen initiation during epithelial morphogenesis. ACTA ACUST UNITED AC 2012; 198:1011-23. [PMID: 22965908 PMCID: PMC3444774 DOI: 10.1083/jcb.201203075] [Citation(s) in RCA: 83] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
Epithelial organ morphogenesis involves sequential acquisition of apicobasal polarity by epithelial cells and development of a functional lumen. In vivo, cells perceive signals from components of the extracellular matrix (ECM), such as laminin and collagens, as well as sense physical conditions, such as matrix stiffness and cell confinement. Alteration of the mechanical properties of the ECM has been shown to promote cell migration and invasion in cancer cells, but the effects on epithelial morphogenesis have not been characterized. We analyzed the effects of cell confinement on lumen morphogenesis using a novel, micropatterned, three-dimensional (3D) Madin-Darby canine kidney cell culture method. We show that cell confinement, by controlling cell spreading, limits peripheral actin contractility and promotes centrosome positioning and lumen initiation after the first cell division. In addition, peripheral actin contractility is mediated by master kinase Par-4/LKB1 via the RhoA-Rho kinase-myosin II pathway, and inhibition of this pathway restores lumen initiation in minimally confined cells. We conclude that cell confinement controls nuclear-centrosomal orientation and lumen initiation during 3D epithelial morphogenesis.
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Affiliation(s)
- Alejo E Rodríguez-Fraticelli
- Centro de Biología Molecular Severo Ochoa, Consejo Superior de Investigaciones Científicas-Universidad Autónoma de Madrid, 28049 Madrid, Spain
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22
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Abstract
In vivo, F-actin flows are observed at different cell life stages and participate in various developmental processes during asymmetric divisions in vertebrate oocytes, cell migration, or wound healing. Here, we show that confinement has a dramatic effect on F-actin spatiotemporal organization. We reconstitute in vitro the spontaneous generation of F-actin flow using Xenopus meiotic extracts artificially confined within a geometry mimicking the cell boundary. Perturbations of actin polymerization kinetics or F-actin nucleation sites strongly modify the network flow dynamics. A combination of quantitative image analysis and biochemical perturbations shows that both spatial localization of F-actin nucleators and actin turnover play a decisive role in generating flow. Interestingly, our in vitro assay recapitulates several symmetry-breaking processes observed in oocytes and early embryonic cells.
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23
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Huergo MAC, Pasquale MA, González PH, Bolzán AE, Arvia AJ. Growth dynamics of cancer cell colonies and their comparison with noncancerous cells. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2012; 85:011918. [PMID: 22400602 DOI: 10.1103/physreve.85.011918] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/27/2011] [Revised: 12/08/2011] [Indexed: 05/31/2023]
Abstract
The two-dimensional (2D) growth dynamics of HeLa (cervix cancer) cell colonies was studied following both their growth front and the pattern morphology evolutions utilizing large population colonies exhibiting linearly and radially spreading fronts. In both cases, the colony profile fractal dimension was d(f)=1.20±0.05 and the growth fronts displaced at the constant velocity 0.90±0.05 μm min(-1). Colonies showed changes in both cell morphology and average size. As time increased, the formation of large cells at the colony front was observed. Accordingly, the heterogeneity of the colony increased and local driving forces that set in began to influence the dynamics of the colony front. The dynamic scaling analysis of rough colony fronts resulted in a roughness exponent α = 0.50±0.05, a growth exponent β = 0.32±0.04, and a dynamic exponent z=1.5±0.2. The validity of this set of scaling exponents extended from a lower cutoff l(c)≈60 μm upward, and the exponents agreed with those predicted by the standard Kardar-Parisi-Zhang continuous equation. HeLa data were compared with those previously reported for Vero cell colonies. The value of d(f) and the Kardar-Parisi-Zhang-type 2D front growth dynamics were similar for colonies of both cell lines. This indicates that the cell colony growth dynamics is independent of the genetic background and the tumorigenic nature of the cells. However, one can distinguish some differences between both cell lines during the growth of colonies that may result from specific cooperative effects and the nature of each biosystem.
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Affiliation(s)
- M A C Huergo
- Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), (UNLP, CONICET), Sucursal 4, Casilla de Correo 16, (1900) La Plata, Argentina
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24
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Biomechanical regulation of contractility: spatial control and dynamics. Trends Cell Biol 2011; 22:61-81. [PMID: 22119497 DOI: 10.1016/j.tcb.2011.10.001] [Citation(s) in RCA: 212] [Impact Index Per Article: 16.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2011] [Revised: 09/28/2011] [Accepted: 10/13/2011] [Indexed: 11/21/2022]
Abstract
Cells are active materials; they can change shape using internal energy to build contractile networks of actin filaments and myosin motors. Contractility of the actomyosin cortex is tightly regulated in space and time to orchestrate cell shape changes. Conserved biochemical pathways regulate actomyosin networks in subcellular domains which drive cell shape changes. Actomyosin networks display complex dynamics, such as flows and pulses, which participate in myosin distribution and provide a more realistic description of the spatial distribution and evolution of forces during morphogenesis. Such dynamics are influenced by the mechanical properties of actomyosin networks. Moreover, actomyosin can self-organize and respond to mechanical stimuli through multiple types of biomechanical feedback. In this review we propose a framework encapsulating spatiotemporal regulation of contractility from established pathways with the dynamics and mechanics of actomyosin networks. Through the comparison of cytokinesis, cell migration and epithelial morphogenesis, we delineate emergent properties of contractile activity, including self-organization, adaptability and robustness.
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25
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Huergo MAC, Pasquale MA, González PH, Bolzán AE, Arvia AJ. Dynamics and morphology characteristics of cell colonies with radially spreading growth fronts. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 84:021917. [PMID: 21929029 DOI: 10.1103/physreve.84.021917] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/04/2011] [Revised: 05/23/2011] [Indexed: 05/31/2023]
Abstract
The dynamics of two-dimensional (2D) radially spreading growth fronts of Vero cell colonies was investigated utilizing two types of colonies, namely type I starting from clusters with a small number of cells, which initially exhibited arbitrary-shaped rough growth fronts and progressively approached quasicircular ones as the cell population increased; and type II colonies, starting from a relatively large circular three-dimensional (3D) cell cluster. For large cell population colonies, the fractal dimension of the fronts was D(F) = 1.20±0.05. For low cell populations, the mean colony radius increased exponentially with time, but for large ones the constant radial front velocity 0.20±0.02 μm min(-1) was reached. Colony spreading was accompanied by changes in both cell morphology and average size, and by the formation of very large cells, some of them multinuclear. Therefore the heterogeneity of colonies increased and local driving forces that set in began to influence the 2D growth front kinetics. The retardation effect related to the exponential to constant radial front velocity transition was assigned to a number of possible interferences including the cell duplication and 3D growth in the bulk of the colony. The dynamic scaling analysis of overhang-corrected rough colony fronts, after arc-radius coordinate system transformation, resulted in roughness exponent α = 0.50±0.05 and growth exponent β = 0.32±0.04, for arc lengths greater than 100 μm. This set of scaling exponents agreed with that predicted by the Kardar, Parisi, and Zhang continuous equation. For arc lengths shorter than 2-3 cell diameters, the value α = 0.85±0.05 would be related to a cell front roughening caused by temporarily membrane deformations occasionally interfered by cell proliferation.
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Affiliation(s)
- M A C Huergo
- Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA) (UNLP, CONICET), Sucursal 4, Casilla de Correo 16, (1900) La Plata, Argentina
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26
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Lecuit T, Lenne PF, Munro E. Force generation, transmission, and integration during cell and tissue morphogenesis. Annu Rev Cell Dev Biol 2011; 27:157-84. [PMID: 21740231 DOI: 10.1146/annurev-cellbio-100109-104027] [Citation(s) in RCA: 386] [Impact Index Per Article: 29.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Cell shape changes underlie a large set of biological processes ranging from cell division to cell motility. Stereotyped patterns of cell shape changes also determine tissue remodeling events such as extension or invagination. In vitro and cell culture systems have been essential to understanding the fundamental physical principles of subcellular mechanics. These are now complemented by studies in developing organisms that emphasize how cell and tissue morphogenesis emerge from the interplay between force-generating machines, such as actomyosin networks, and adhesive clusters that transmit tensile forces at the cell cortex and stabilize cell-cell and cell-substrate interfaces. Both force production and transmission are self-organizing phenomena whose adaptive features are essential during tissue morphogenesis. A new era is opening that emphasizes the similarities of and allows comparisons between distant dynamic biological phenomena because they rely on core machineries that control universal features of cytomechanics.
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Affiliation(s)
- Thomas Lecuit
- Developmental Biology Institute of Marseilles-Luminy, Centre National de la Recherche Scientifique, Université de la Méditerranée, 13288 Marseille Cedex 9, France.
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27
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Integrin adhesion drives the emergent polarization of active cytoskeletal stresses to pattern cell delamination. Proc Natl Acad Sci U S A 2011; 108:9107-12. [PMID: 21571643 DOI: 10.1073/pnas.1018652108] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Tissue patterning relies on cellular reorganization through the interplay between signaling pathways and mechanical stresses. Their integration and spatiotemporal coordination remain poorly understood. Here we investigate the mechanisms driving the dynamics of cell delamination, diversely deployed to extrude dead cells or specify distinct cell fates. We show that a local mechanical stimulus (subcellular laser perturbation) releases cellular prestress and triggers cell delamination in the amnioserosa during Drosophila dorsal closure, which, like spontaneous delamination, results in the rearrangement of nearest neighbors around the delaminating cell into a rosette. We demonstrate that a sequence of "emergent cytoskeletal polarities" in the nearest neighbors (directed myosin flows, lamellipodial growth, polarized actomyosin collars, microtubule asters), triggered by the mechanical stimulus and dependent on integrin adhesion, generate active stresses that drive delamination. We interpret these patterns in the language of active gels as asters formed by active force dipoles involving surface and body stresses generated by each cell and liken delamination to mechanical yielding that ensues when these stresses exceed a threshold. We suggest that differential contributions of adhesion, cytoskeletal, and external stresses must underlie differences in spatial pattern.
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28
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Newman SA. Animal egg as evolutionary innovation: a solution to the “embryonic hourglass” puzzle. JOURNAL OF EXPERIMENTAL ZOOLOGY PART B-MOLECULAR AND DEVELOPMENTAL EVOLUTION 2011; 316:467-83. [DOI: 10.1002/jez.b.21417] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/06/2011] [Revised: 04/01/2011] [Accepted: 04/07/2011] [Indexed: 12/26/2022]
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29
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Norman LL, Brugués J, Brugés J, Sengupta K, Sens P, Aranda-Espinoza H. Cell blebbing and membrane area homeostasis in spreading and retracting cells. Biophys J 2011; 99:1726-33. [PMID: 20858416 DOI: 10.1016/j.bpj.2010.07.031] [Citation(s) in RCA: 70] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/21/2010] [Revised: 07/15/2010] [Accepted: 07/16/2010] [Indexed: 10/19/2022] Open
Abstract
Cells remodel their plasma membrane and cytoskeleton during numerous physiological processes, including spreading and motility. Morphological changes require the cell to adjust its membrane tension on different timescales. While it is known that endo- and exocytosis regulate the cell membrane area in a timescale of 1 h, faster processes, such as abrupt cell detachment, require faster regulation of the plasma membrane tension. In this article, we demonstrate that cell blebbing plays a critical role in the global mechanical homeostasis of the cell through regulation of membrane tension. Abrupt cell detachment leads to pronounced blebbing (which slow detachment does not), and blebbing decreases with time in a dynamin-dependent fashion. Cells only start spreading after a lag period whose duration depends on the cell's blebbing activity. Our model quantitatively reproduces the monotonic decay of the blebbing activity and accounts for the lag phase in the spreading of blebbing cells.
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Affiliation(s)
- Leann L Norman
- Fischell Department of Bioengineering, University of Maryland, College Park, Maryland, USA
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30
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Guck J, Lautenschläger F, Paschke S, Beil M. Critical review: cellular mechanobiology and amoeboid migration. Integr Biol (Camb) 2010; 2:575-83. [PMID: 20871906 DOI: 10.1039/c0ib00050g] [Citation(s) in RCA: 65] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
Cell motility is important for tissue homeostasis and plays a central role in various pathologies, notably inflammation and cancer. Research into the critical processes involved in cell migration has so far mostly focused on cell adhesion and proteolytic degradation of the extracellular matrix. However, pharmacological interference with these processes only partially blocks cell motility in vivo. In this review we summarize the arising evidence that the mechanical properties of the cell body have a major role to play in cell motility--especially in a low-adhesion, amoeboid-like migration mode in three-dimensional tissue structures. We summarize the processes determining cell mechanics and discuss relevant measurement technologies including their applications in medical cell biology.
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Affiliation(s)
- Jochen Guck
- Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Ave, Cambridge CB3 0HE, UK.
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31
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Renkawitz J, Sixt M. Mechanisms of force generation and force transmission during interstitial leukocyte migration. EMBO Rep 2010; 11:744-50. [PMID: 20865016 DOI: 10.1038/embor.2010.147] [Citation(s) in RCA: 121] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2010] [Accepted: 08/27/2010] [Indexed: 01/12/2023] Open
Abstract
For innate and adaptive immune responses it is essential that inflammatory cells use quick and flexible locomotion strategies. Accordingly, most leukocytes can efficiently infiltrate and traverse almost every physiological or artificial environment. Here, we review how leukocytes might achieve this task mechanistically, and summarize recent findings on the principles of cytoskeletal force generation and transduction at the leading edge of leukocytes. We propose a model in which the cells switch between adhesion-receptor-mediated force transmission and locomotion modes that are based on cellular deformations, but independent of adhesion receptors. This plasticity in migration strategies allows leukocytes to adapt to the geometry and molecular composition of their environment.
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Affiliation(s)
- Jörg Renkawitz
- Department of Molecular Cell Biology, Max Planck Institute of Biochemistry, 82152 Martinsried, Germany
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32
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Huergo MAC, Pasquale MA, Bolzán AE, Arvia AJ, González PH. Morphology and dynamic scaling analysis of cell colonies with linear growth fronts. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 82:031903. [PMID: 21230104 DOI: 10.1103/physreve.82.031903] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/23/2010] [Revised: 07/28/2010] [Indexed: 05/30/2023]
Abstract
The growth of linear cell colony fronts is investigated from the morphology of cell monolayer colonies, the cell size and shape distribution, the front displacement velocity, and the dynamic scaling analysis of front roughness fluctuations. At the early growth stages, colony patterns consist of rather ordered compact domains of small cells, whereas at advanced stages, an uneven distribution of cells sets in, and some large cells and cells exhibiting large filopodia are produced. Colony front profiles exhibit overhangs and behave as fractals with the dimension D(F)=1.25±0.05. The colony fronts shift at 0.22±0.02 μm min(-1) average constant linear velocity and their roughness (w) increases with time (t). Dynamic scaling analysis of experimental and overhang-corrected growth profile data shows that w versus system width l log-log plots collapse to a single curve when l exceeds a certain threshold value l(o), a width corresponding to the average diameter of few cells. Then, the influence of overhangs on the roughness dynamics becomes negligible, and a growth exponent β=0.33±0.02 is derived. From the structure factor analysis of overhang-corrected profiles, a global roughness exponent α(s)=0.50±0.05 is obtained. For l>200 μm, this set of exponents fulfills the Family-Vicsek relationship. It is consistent with the predictions of the continuous Kardar-Parisi-Zhang model.
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Affiliation(s)
- M A C Huergo
- Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas, UNLP, CONICET, Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina
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33
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Cepeda-García C, Delgehyr N, Juanes Ortiz MA, ten Hoopen R, Zhiteneva A, Segal M. Actin-mediated delivery of astral microtubules instructs Kar9p asymmetric loading to the bud-ward spindle pole. Mol Biol Cell 2010; 21:2685-95. [PMID: 20534809 PMCID: PMC2912354 DOI: 10.1091/mbc.e10-03-0197] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
We show that Kar9p polarity is instructed by a feedback loop that requires astral microtubules, actin cables, and Myo2p-based transport to enforce Kar9p loading to the bud-ward pole. This novel mechanism also provides the basis for a model unifying Kar9p polarity and the stereotyped pattern of spindle pole inheritance known to occur in yeast. In Saccharomyces cerevisiae, Kar9p, one player in spindle alignment, guides the bud-ward spindle pole by linking astral microtubule plus ends to Myo2p-based transport along actin cables generated by the formins Bni1p and Bnr1p and the polarity determinant Bud6p. Initially, Kar9p labels both poles but progressively singles out the bud-ward pole. Here, we show that this polarization requires cell polarity determinants, actin cables, and microtubules. Indeed, in a bud6Δ bni1Δ mutant or upon direct depolymerization of actin cables Kar9p symmetry increased. Furthermore, symmetry was selectively induced by myo2 alleles, preventing Kar9p binding to the Myo2p cargo domain. Kar9p polarity was rebuilt after transient disruption of microtubules, dependent on cell polarity and actin cables. Symmetry breaking also occurred after transient depolymerization of actin cables, with Kar9p increasing at the spindle pole engaging in repeated cycles of Kar9p-mediated transport. Kar9p returning to the spindle pole on shrinking astral microtubules may contribute toward this bias. Thus, Myo2p transport along actin cables may support a feedback loop by which delivery of astral microtubule plus ends sustains Kar9p polarized recruitment to the bud-ward spindle pole. Our findings also explain the link between Kar9p polarity and the choice setting aside the old spindle pole for daughter-bound fate.
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34
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Schmidt S, Friedl P. Interstitial cell migration: integrin-dependent and alternative adhesion mechanisms. Cell Tissue Res 2010; 339:83-92. [PMID: 19921267 PMCID: PMC2784868 DOI: 10.1007/s00441-009-0892-9] [Citation(s) in RCA: 137] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2009] [Accepted: 09/28/2009] [Indexed: 12/27/2022]
Abstract
Adhesion and migration are integrated cell functions that build, maintain and remodel the multicellular organism. In migrating cells, integrins are the main transmembrane receptors that provide dynamic interactions between extracellular ligands and actin cytoskeleton and signalling machineries. In parallel to integrins, other adhesion systems mediate adhesion and cytoskeletal coupling to the extracellular matrix (ECM). These include multifunctional cell surface receptors (syndecans and CD44) and discoidin domain receptors, which together coordinate ligand binding with direct or indirect cytoskeletal coupling and intracellular signalling. We review the way that the different adhesion systems for ECM components impact cell migration in two- and three-dimensional migration models. We further discuss the hierarchy of these concurrent adhesion systems, their specific tasks in cell migration and their contribution to migration in three-dimensional multi-ligand tissue environments.
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Affiliation(s)
- Samuel Schmidt
- Microscopical Imaging of the Cell, Department of Cell Biology (283), Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands
| | - Peter Friedl
- Microscopical Imaging of the Cell, Department of Cell Biology (283), Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands
- Rudolf Virchow Zentrum and Department for Dermatology, University of Würzburg, Josef-Schneider-Strasse 2, 97080 Würzburg, Germany
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35
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Abstract
Cell migration underlies tissue formation, maintenance, and regeneration as well as pathological conditions such as cancer invasion. Structural and molecular determinants of both tissue environment and cell behavior define whether cells migrate individually (through amoeboid or mesenchymal modes) or collectively. Using a multiparameter tuning model, we describe how dimension, density, stiffness, and orientation of the extracellular matrix together with cell determinants—including cell–cell and cell–matrix adhesion, cytoskeletal polarity and stiffness, and pericellular proteolysis—interdependently control migration mode and efficiency. Motile cells integrate variable inputs to adjust interactions among themselves and with the matrix to dictate the migration mode. The tuning model provides a matrix of parameters that control cell movement as an adaptive and interconvertible process with relevance to different physiological and pathological contexts.
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Affiliation(s)
- Peter Friedl
- Department of Cell Biology, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen, 6500 HB Nijmegen, Netherlands.
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36
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Interstitial cell migration: integrin-dependent and alternative adhesion mechanisms. Cell Tissue Res 2009. [DOI: 10.1007/s00441-009-0892-9 doi:dx.doi.org] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/29/2022]
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37
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Estecha A, Sánchez-Martín L, Puig-Kröger A, Bartolomé RA, Teixidó J, Samaniego R, Sánchez-Mateos P. Moesin orchestrates cortical polarity of melanoma tumour cells to initiate 3D invasion. J Cell Sci 2009; 122:3492-501. [DOI: 10.1242/jcs.053157] [Citation(s) in RCA: 84] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Tumour cell dissemination through corporal fluids (blood, lymph and body cavity fluids) is a distinctive feature of the metastatic process. Tumour cell transition from fluid to adhesive conditions involves an early polarization event and major rearrangements of the submembrane cytoskeleton that remain poorly understood. As regulation of cortical actin-membrane binding might be important in this process, we investigated the role of ezrin and moesin, which are key crosslinking proteins of the ERM (ezrin, radixin, moesin) family. We used short interfering RNA (siRNA) to show that moesin is crucial for invasion by melanoma cells in 3D matrices and in early lung colonization. Using live imaging, we show that following initial adhesion to the endothelium or 3D matrices, moesin is redistributed away from the region of adhesion, thereby generating a polarized cortex: a stable cortical actin dome enriched in moesin and an invasive membrane domain full of blebs. Using Lifeact-GFP, a 17-amino-acid peptide that binds F-actin, we show the initial symmetry breaking of cortical actin cytoskeleton during early attachment of round cells. We also demonstrated that ezrin and moesin are differentially distributed during initial invasion of 3D matrices, and, specifically, that moesin controls adhesion-dependent activation of Rho and subsequent myosin II contractility. Our results reveal that polarized moesin plays a role in orienting Rho activation, myosin II contractility, and cortical actin stability, which is crucial for driving directional vertical migration instead of superficial spreading on the fluid-to-solid tissue interface. We propose that this mechanism of cortical polarization could sustain extravasation of fluid-borne tumour cells during the process of metastasis.
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Affiliation(s)
- Ana Estecha
- Laboratorio de Inmuno-oncología, Hospital General Universitario Gregorio Marañón, 28007 Madrid, Spain
| | - Lorena Sánchez-Martín
- Laboratorio de Inmuno-oncología, Hospital General Universitario Gregorio Marañón, 28007 Madrid, Spain
| | - Amaya Puig-Kröger
- Laboratorio de Inmuno-oncología, Hospital General Universitario Gregorio Marañón, 28007 Madrid, Spain
| | - Rubén A. Bartolomé
- Department of Cellular and Molecular Physiopathology, Centro de Investigaciones Biológicas, 28040 Madrid, Spain
| | - Joaquín Teixidó
- Department of Cellular and Molecular Physiopathology, Centro de Investigaciones Biológicas, 28040 Madrid, Spain
| | - Rafael Samaniego
- Unidad de Microscopía Confocal, Hospital General Universitario Gregorio Marañón, 28007 Madrid, Spain
| | - Paloma Sánchez-Mateos
- Laboratorio de Inmuno-oncología, Hospital General Universitario Gregorio Marañón, 28007 Madrid, Spain
- Unidad de Microscopía Confocal, Hospital General Universitario Gregorio Marañón, 28007 Madrid, Spain
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Liebl D, Griffiths G. Transient assembly of F-actin by phagosomes delays phagosome fusion with lysosomes in cargo-overloaded macrophages. J Cell Sci 2009; 122:2935-45. [PMID: 19638408 DOI: 10.1242/jcs.048355] [Citation(s) in RCA: 67] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Dynamic remodelling of the cortical actin cytoskeleton is required for phagocytic uptake of pathogens and other particles by macrophages. Actin can also be nucleated de novo on membranes of nascent phagosomes, a process that can stimulate or inhibit phagosome fusion with lysosomes. Recently, phagosomes were shown to polymerize actin in transient pulses, called actin ;flashing', whose function remains unexplained. Here, we investigated phagosomal actin dynamics in live macrophages expressing actin tagged with green fluorescent protein (GFP). We show that only immature phagosomes can transiently induce assembly of actin coat, which forms a barrier preventing phagosome-lysosome docking and fusion. The capacity of phagosomes to assemble actin is enhanced in cells exposed to increased phagocytic load, which also exhibit a delay in phagosome maturation. Parallel analysis indicated that polymerization of actin on macropinosomes also induces compression and propulsion. We show that dynamic interactions between membrane elastic tension and compression forces of polymerizing actin can also lead to macropinosome constriction and scission - a process that is obstructed on rigid phagosomes. We hypothesize that the rate of individual phagosome maturation, as well as the biogenesis and remodelling of macropinosomes, can be regulated by the extent and manner of actin assembly on their membrane.
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Affiliation(s)
- David Liebl
- Cell Biology and Biophysics Unit, EMBL Heidelberg, Meyerhofstrasse 1, 69117 Heidelberg, Germany.
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Pulsed-laser creation and characterization of giant plasma membrane vesicles from cells. J Biol Phys 2009; 35:279-95. [PMID: 19669579 DOI: 10.1007/s10867-009-9167-7] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2009] [Accepted: 05/21/2009] [Indexed: 10/20/2022] Open
Abstract
Femtosecond-pulsed laser irradiation was found to initiate giant plasma membrane vesicle (GPMV) formation on individual cells. Laser-induced GPMV formation resulted from intracellular cavitation and did not require the addition of chemical stressors to the cellular environment. The viscosity, structure, and contents of laser-induced GPMVs were measured with fluorescence microscopy and single-particle tracking. These GPMVs exhibit the following properties: (1) GPMVs grow fastest immediately after laser irradiation; (2) GPMVs contain barriers to free diffusion of incorporated fluorescent beads; (3) materials from both the cytoplasm and surrounding media flow into the growing GPMVs; (4) the GPMVs are surrounded by phospholipids, including phosphatidylserine; (5) F-actin is incorporated into the vesicles; and (6) caspase activity is not essential for GPMV formation. The effective viscosity of 65 nm polystyrene nanoparticles within GPMVs ranged from 32 to 434 cP. The nanoparticle diffusion was commonly affected by relatively large, macromolecular structures within the bleb.
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Affiliation(s)
- Jonathon Howard
- Max Planck Institute of Molecular Cell Biology and Genetics, 01307 Dresden, Germany
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41
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Abstract
Blebs are spherical membrane protrusions that are produced by contractions of the actomyosin cortex. Blebs are often considered to be a hallmark of apoptosis; however, blebs are also frequently observed during cytokinesis and during migration in three-dimensional cultures and in vivo. For tumour cells and a number of embryonic cells, blebbing migration seems to be a common alternative to the more extensively studied lamellipodium-based motility. We argue that blebs should be promoted to a more prominent place in the world of cellular protrusions.
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Murthy K, Wadsworth P. Dual role for microtubules in regulating cortical contractility during cytokinesis. J Cell Sci 2008; 121:2350-9. [PMID: 18559890 DOI: 10.1242/jcs.027052] [Citation(s) in RCA: 73] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Microtubules stimulate contractile-ring formation in the equatorial cortex and simultaneously suppress contractility in the polar cortex; how they accomplish these differing activities is incompletely understood. We measured the behavior of GFP-actin in mammalian cells treated with nocodazole under conditions that either completely eliminate microtubules or selectively disassemble astral microtubules. Selective disassembly of astral microtubules resulted in functional contractile rings that were wider than controls and had altered dynamic activity, as measured by FRAP. Complete microtubule disassembly or selective loss of astral microtubules resulted in wave-like contractile behavior of actin in the non-equatorial cortex, and mislocalization of myosin II and Rho. FRAP experiments showed that both contractility and actin polymerization contributed to the wave-like behavior of actin. Wave-like contractile behavior in anaphase cells was Rho-dependent. We conclude that dynamic astral microtubules function to suppress Rho activation in the non-equatorial cortex, limiting the contractile activity of the polar cortex.
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Affiliation(s)
- Kausalya Murthy
- Department of Biology and Program in Molecular and Cellular Biology, University of Massachusetts at Amherst, Amherst, MA 01003, USA
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43
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Mogilner A. Mathematics of cell motility: have we got its number? J Math Biol 2008; 58:105-34. [PMID: 18461331 DOI: 10.1007/s00285-008-0182-2] [Citation(s) in RCA: 188] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2007] [Revised: 04/15/2008] [Indexed: 02/06/2023]
Abstract
Mathematical and computational modeling is rapidly becoming an essential research technique complementing traditional experimental biological methods. However, lack of standard modeling methods, difficulties of translating biological phenomena into mathematical language, and differences in biological and mathematical mentalities continue to hinder the scientific progress. Here we focus on one area-cell motility-characterized by an unusually high modeling activity, largely due to a vast amount of quantitative, biophysical data, 'modular' character of motility, and pioneering vision of the area's experimental leaders. In this review, after brief introduction to biology of cell movements, we discuss quantitative models of actin dynamics, protrusion, adhesion, contraction, and cell shape and movement that made an impact on the process of biological discovery. We also comment on modeling approaches and open questions.
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Affiliation(s)
- Alex Mogilner
- Department of Neurobiology, Physiology and Behavior, University of California, Davis, CA, 95618, USA.
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Rogers SS, Waigh TA, Lu JR. Intracellular microrheology of motile Amoeba proteus. Biophys J 2008; 94:3313-22. [PMID: 18192370 PMCID: PMC2275677 DOI: 10.1529/biophysj.107.123851] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2007] [Accepted: 11/16/2007] [Indexed: 11/18/2022] Open
Abstract
The motility of Amoeba proteus was examined using the technique of passive particle tracking microrheology, with the aid of newly developed particle tracking software, a fast digital camera, and an optical microscope. We tracked large numbers of endogeneous particles in the amoebae, which displayed subdiffusive motion at short timescales, corresponding to thermal motion in a viscoelastic medium, and superdiffusive motion at long timescales due to the convection of the cytoplasm. Subdiffusive motion was characterized by a rheological scaling exponent of 3/4 in the cortex, indicative of the semiflexible dynamics of the actin fibers. We observed shear-thinning in the flowing endoplasm, where exponents increased with increasing flow rate; i.e., the endoplasm became more fluid-like. The rheology of the cortex is found to be isotropic, reflecting an isotropic actin gel. A clear difference was seen between cortical and endoplasmic layers in terms of both viscoelasticity and flow velocity, where the profile of the latter is close to a Poiseuille flow for a Newtonian fluid.
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Affiliation(s)
- Salman S Rogers
- Biological Physics Group, School of Physics and Astronomy, University of Manchester, Manchester M60 1QD, United Kingdom
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45
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Gardel ML, Kasza KE, Brangwynne CP, Liu J, Weitz DA. Chapter 19: Mechanical response of cytoskeletal networks. Methods Cell Biol 2008; 89:487-519. [PMID: 19118688 DOI: 10.1016/s0091-679x(08)00619-5] [Citation(s) in RCA: 158] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
The cellular cytoskeleton is a dynamic network of filamentous proteins, consisting of filamentous actin (F-actin), microtubules, and intermediate filaments. However, these networks are not simple linear, elastic solids; they can exhibit highly nonlinear elasticity and a thermal dynamics driven by ATP-dependent processes. To build quantitative mechanical models describing complex cellular behaviors, it is necessary to understand the underlying physical principles that regulate force transmission and dynamics within these networks. In this chapter, we review our current understanding of the physics of networks of cytoskeletal proteins formed in vitro. We introduce rheology, the technique used to measure mechanical response. We discuss our current understanding of the mechanical response of F-actin networks, and how the biophysical properties of F-actin and actin cross-linking proteins can dramatically impact the network mechanical response. We discuss how incorporating dynamic and rigid microtubules into F-actin networks can affect the contours of growing microtubules and composite network rigidity. Finally, we discuss the mechanical behaviors of intermediate filaments.
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Affiliation(s)
- Margaret L Gardel
- Department of Physics and Institute for Biophysical Dynamics, University of Chicago, Illinois 60637, USA
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46
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Lecuit T, Lenne PF. Cell surface mechanics and the control of cell shape, tissue patterns and morphogenesis. Nat Rev Mol Cell Biol 2007; 8:633-44. [PMID: 17643125 DOI: 10.1038/nrm2222] [Citation(s) in RCA: 830] [Impact Index Per Article: 48.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Embryonic morphogenesis requires the execution of complex mechanisms that regulate the local behaviour of groups of cells. The orchestration of such mechanisms has been mainly deciphered through the identification of conserved families of signalling pathways that spatially and temporally control cell behaviour. However, how this information is processed to control cell shape and cell dynamics is an open area of investigation. The framework that emerges from diverse disciplines such as cell biology, physics and developmental biology points to adhesion and cortical actin networks as regulators of cell surface mechanics. In this context, a range of developmental phenomena can be explained by the regulation of cell surface tension.
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Affiliation(s)
- Thomas Lecuit
- Institute of Developmental Biology of Marseille-Luminy, UMR6216 CNRS-Université de la Méditerranée, Campus de Luminy case 907, 13288 Marseille Cedex 09, France.
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47
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Abstract
Directed cell motility is preceded by cell polarization-development of a front-rear asymmetry of the cytoskeleton and the cell shape. Extensive studies implicated complex spatial-temporal feedbacks between multiple signaling pathways in establishing cell polarity, yet physical mechanisms of this phenomenon remain elusive. Based on observations of lamellipodial fragments of fish keratocyte cells, we suggest a purely thermodynamic (not involving signaling) quantitative model of the cell polarization and bistability. The model is based on the interplay between pushing force exerted by F-actin polymerization on the cell edges, contractile force powered by myosin II across the cell, and elastic tension in the cell membrane. We calculate the thermodynamic work produced by these intracellular forces, and show that on the short timescale, the cell mechanics can be characterized by an effective energy profile with two minima that describe two stable states separated by an energy barrier and corresponding to the nonpolarized and polarized cells. Cell dynamics implied by this energy profile is bistable-the cell is either disk-shaped and stationary, or crescent-shaped and motile-with a possible transition between them upon a finite external stimulus able to drive the system over the macroscopic energy barrier. The model accounts for the observations of the keratocyte fragments' behavior and generates quantitative predictions about relations between the intracellular forces' magnitudes and the cell geometry and motility.
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Affiliation(s)
- Michael M Kozlov
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel.
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