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1.
Cell ; 175(3): 796-808.e14, 2018 10 18.
Artigo em Inglês | MEDLINE | ID: mdl-30340043

RESUMO

During cell division, mitotic motors organize microtubules in the bipolar spindle into either polar arrays at the spindle poles or a "nematic" network of aligned microtubules at the spindle center. The reasons for the distinct self-organizing capacities of dynamic microtubules and different motors are not understood. Using in vitro reconstitution experiments and computer simulations, we show that the human mitotic motors kinesin-5 KIF11 and kinesin-14 HSET, despite opposite directionalities, can both organize dynamic microtubules into either polar or nematic networks. We show that in addition to the motor properties the natural asymmetry between microtubule plus- and minus-end growth critically contributes to the organizational potential of the motors. We identify two control parameters that capture system composition and kinetic properties and predict the outcome of microtubule network organization. These results elucidate a fundamental design principle of spindle bipolarity and establish general rules for active filament network organization.


Assuntos
Cinesinas/metabolismo , Microtúbulos/metabolismo , Simulação de Dinâmica Molecular , Fuso Acromático/metabolismo , Animais , Humanos , Cinesinas/química , Microtúbulos/química , Células Sf9 , Fuso Acromático/química , Spodoptera
2.
Cell ; 174(4): 884-896.e17, 2018 08 09.
Artigo em Inglês | MEDLINE | ID: mdl-30057119

RESUMO

Clathrin-mediated endocytosis is an essential cellular function in all eukaryotes that is driven by a self-assembled macromolecular machine of over 50 different proteins in tens to hundreds of copies. How these proteins are organized to produce endocytic vesicles with high precision and efficiency is not understood. Here, we developed high-throughput superresolution microscopy to reconstruct the nanoscale structural organization of 23 endocytic proteins from over 100,000 endocytic sites in yeast. We found that proteins assemble by radially ordered recruitment according to function. WASP family proteins form a circular nanoscale template on the membrane to spatially control actin nucleation during vesicle formation. Mathematical modeling of actin polymerization showed that this WASP nano-template optimizes force generation for membrane invagination and substantially increases the efficiency of endocytosis. Such nanoscale pre-patterning of actin nucleation may represent a general design principle for directional force generation in membrane remodeling processes such as during cell migration and division.


Assuntos
Citoesqueleto de Actina/metabolismo , Actinas/metabolismo , Endocitose/fisiologia , Proteínas de Saccharomyces cerevisiae/metabolismo , Saccharomyces cerevisiae/metabolismo , Vesículas Secretórias/metabolismo , Família de Proteínas da Síndrome de Wiskott-Aldrich/metabolismo , Actinas/química , Membrana Celular/metabolismo , Microscopia de Fluorescência , Modelos Teóricos , Conformação Proteica , Família de Proteínas da Síndrome de Wiskott-Aldrich/química
3.
Development ; 150(6)2023 03 15.
Artigo em Inglês | MEDLINE | ID: mdl-36897576

RESUMO

Actin dynamics play an important role in tissue morphogenesis, yet the control of actin filament growth takes place at the molecular level. A challenge in the field is to link the molecular function of actin regulators with their physiological function. Here, we report an in vivo role of the actin-capping protein CAP-1 in the Caenorhabditis elegans germline. We show that CAP-1 is associated with actomyosin structures in the cortex and rachis, and its depletion or overexpression led to severe structural defects in the syncytial germline and oocytes. A 60% reduction in the level of CAP-1 caused a twofold increase in F-actin and non-muscle myosin II activity, and laser incision experiments revealed an increase in rachis contractility. Cytosim simulations pointed to increased myosin as the main driver of increased contractility following loss of actin-capping protein. Double depletion of CAP-1 and myosin or Rho kinase demonstrated that the rachis architecture defects associated with CAP-1 depletion require contractility of the rachis actomyosin corset. Thus, we uncovered a physiological role for actin-capping protein in regulating actomyosin contractility to maintain reproductive tissue architecture.


Assuntos
Actomiosina , Caenorhabditis elegans , Animais , Actomiosina/metabolismo , Caenorhabditis elegans/genética , Caenorhabditis elegans/metabolismo , Actinas/metabolismo , Proteínas de Capeamento de Actina/metabolismo , Citoesqueleto de Actina/metabolismo , Miosinas/metabolismo , Células Germinativas/metabolismo
4.
Cell ; 147(6): 1397-407, 2011 Dec 09.
Artigo em Inglês | MEDLINE | ID: mdl-22153081

RESUMO

Bipolar spindles must separate chromosomes by the appropriate distance during cell division, but mechanisms determining spindle length are poorly understood. Based on a 2D model of meiotic spindle assembly, we predicted that higher localized microtubule (MT) depolymerization rates could generate the shorter spindles observed in egg extracts of X. tropicalis compared to X. laevis. We found that katanin-dependent MT severing was increased in X. tropicalis, which, unlike X. laevis, lacks an inhibitory phosphorylation site in the katanin p60 catalytic subunit. Katanin inhibition lengthened spindles in both species. In X. tropicalis, k-fiber MT bundles that connect to chromosomes at their kinetochores extended through spindle poles, disrupting them. In both X. tropicalis extracts and the spindle simulation, a balance between k-fiber number and MT depolymerization is required to maintain spindle morphology. Thus, mechanisms have evolved in different species to scale spindle size and coordinate regulation of multiple MT populations in order to generate a robust steady-state structure.


Assuntos
Adenosina Trifosfatases/metabolismo , Fuso Acromático/metabolismo , Xenopus laevis/fisiologia , Xenopus/fisiologia , Adenosina Trifosfatases/química , Sequência de Aminoácidos , Animais , Extratos Celulares , Humanos , Katanina , Microtúbulos/metabolismo , Dados de Sequência Molecular , Tamanho das Organelas , Fosforilação , Alinhamento de Sequência , Especificidade da Espécie
5.
Nat Rev Mol Cell Biol ; 14(7): 452-9, 2013 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-23778971

RESUMO

The mechanisms underlying the appearance of asymmetry between cells in the early embryo and consequently the specification of distinct cell lineages during mammalian development remain elusive. Recent experimental advances have revealed unexpected dynamics of and new complexity in this process. These findings can be integrated in a new unified framework that regards the early mammalian embryo as a self-organizing system.


Assuntos
Linhagem da Célula , Embrião de Mamíferos/citologia , Animais , Padronização Corporal , Polaridade Celular , Simulação por Computador , Desenvolvimento Embrionário , Regulação da Expressão Gênica no Desenvolvimento , Humanos , Modelos Biológicos
6.
Proc Natl Acad Sci U S A ; 119(33): e2206398119, 2022 08 16.
Artigo em Inglês | MEDLINE | ID: mdl-35960844

RESUMO

During cell division, cross-linking motors determine the architecture of the spindle, a dynamic microtubule network that segregates the chromosomes in eukaryotes. It is unclear how motors with opposite directionality coordinate to drive both contractile and extensile behaviors in the spindle. Particularly, the impact of different cross-linker designs on network self-organization is not understood, limiting our understanding of self-organizing structures in cells but also our ability to engineer new active materials. Here, we use experiment and theory to examine active microtubule networks driven by mixtures of motors with opposite directionality and different cross-linker design. We find that although the kinesin-14 HSET causes network contraction when dominant, it can also assist the opposing kinesin-5 KIF11 to generate extensile networks. This bifunctionality results from HSET's asymmetric design, distinct from symmetric KIF11. These findings expand the set of rules underlying patterning of active microtubule assemblies and allow a better understanding of motor cooperation in the spindle.


Assuntos
Cinesinas , Microtúbulos , Proteínas Oncogênicas , Fuso Acromático , Divisão Celular , Humanos , Cinesinas/química , Cinesinas/fisiologia , Microtúbulos/química , Microtúbulos/fisiologia , Proteínas Oncogênicas/química , Proteínas Oncogênicas/fisiologia , Fuso Acromático/química , Fuso Acromático/fisiologia
7.
Cell ; 138(3): 502-13, 2009 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-19665972

RESUMO

In animal and plant cells, mitotic chromatin locally generates microtubules that self-organize into a mitotic spindle, and its dimensions and bipolar symmetry are essential for accurate chromosome segregation. By immobilizing microscopic chromatin-coated beads on slide surfaces using a microprinting technique, we have examined the effect of chromatin on the dimensions and symmetry of spindles in Xenopus laevis cytoplasmic extracts. While circular spots with diameters around 14-18 microm trigger bipolar spindle formation, larger spots generate an incorrect number of poles. We also examined lines of chromatin with various dimensions. Their length determined the number of poles that formed, with a 6 x 18 microm rectangular patch generating normal spindle morphology. Around longer lines, multiple poles formed and the structures were disorganized. While lines thinner than 10 mum generated symmetric structures, thicker lines induced the formation of asymmetric structures where all microtubules are on the same side of the line. Our results show that chromatin defines spindle shape and orientation. For a video summary of this article, see the PaperFlick file available with the online Supplemental Data.


Assuntos
Cromatina/química , Fuso Acromático/química , Animais , Extratos Celulares , Dineínas/metabolismo , Cinesinas/metabolismo , Proteínas de Xenopus/metabolismo , Xenopus laevis
8.
Biophys J ; 122(18): 3611-3629, 2023 09 19.
Artigo em Inglês | MEDLINE | ID: mdl-36540027

RESUMO

Constriction kinetics of the cytokinetic ring are expected to depend on dynamic adjustment of contractile ring composition, but the impact of ring component abundance dynamics on ring constriction is understudied. Computational models generally assume that contractile networks maintain constant total amounts of components, which is not always true. To test how compositional dynamics affect constriction kinetics, we first measured F-actin, non-muscle myosin II, septin, and anillin during Caenorhabditis elegans zygotic mitosis. A custom microfluidic device that positioned the cell with the division plane parallel to a light sheet allowed even illumination of the cytokinetic ring. Measured component abundances were implemented in a three-dimensional agent-based model of a membrane-associated contractile ring. With constant network component amounts, constriction completed with biologically unrealistic kinetics. However, imposing the measured changes in component quantities allowed this model to elicit realistic constriction kinetics. Simulated networks were more sensitive to changes in motor and filament amounts than those of crosslinkers and tethers. Our findings highlight the importance of network composition for actomyosin contraction kinetics.


Assuntos
Citoesqueleto de Actina , Citocinese , Animais , Cinética , Citocinese/fisiologia , Citoesqueleto de Actina/metabolismo , Actinas/metabolismo , Citoesqueleto/metabolismo , Actomiosina/metabolismo , Caenorhabditis elegans
9.
Nature ; 536(7616): 344-348, 2016 08 18.
Artigo em Inglês | MEDLINE | ID: mdl-27487217

RESUMO

During pre-implantation development, the mammalian embryo self-organizes into the blastocyst, which consists of an epithelial layer encapsulating the inner-cell mass (ICM) giving rise to all embryonic tissues. In mice, oriented cell division, apicobasal polarity and actomyosin contractility are thought to contribute to the formation of the ICM. However, how these processes work together remains unclear. Here we show that asymmetric segregation of the apical domain generates blastomeres with different contractilities, which triggers their sorting into inner and outer positions. Three-dimensional physical modelling of embryo morphogenesis reveals that cells internalize only when differences in surface contractility exceed a predictable threshold. We validate this prediction using biophysical measurements, and successfully redirect cell sorting within the developing blastocyst using maternal myosin (Myh9)-knockout chimaeric embryos. Finally, we find that loss of contractility causes blastomeres to show ICM-like markers, regardless of their position. In particular, contractility controls Yap subcellular localization, raising the possibility that mechanosensing occurs during blastocyst lineage specification. We conclude that contractility couples the positioning and fate specification of blastomeres. We propose that this ensures the robust self-organization of blastomeres into the blastocyst, which confers remarkable regulative capacities to mammalian embryos.


Assuntos
Massa Celular Interna do Blastocisto/citologia , Diferenciação Celular , Divisão Celular , Movimento Celular , Embrião de Mamíferos/citologia , Embrião de Mamíferos/embriologia , Proteínas Adaptadoras de Transdução de Sinal/metabolismo , Animais , Blastocisto/citologia , Blastômeros/citologia , Proteínas de Ciclo Celular , Linhagem da Célula , Polaridade Celular , Desenvolvimento Embrionário , Feminino , Masculino , Camundongos , Fosfoproteínas/metabolismo , Transporte Proteico , Reprodutibilidade dos Testes , Proteínas de Sinalização YAP
10.
Genes Dev ; 27(3): 335-49, 2013 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-23388829

RESUMO

Nuclear migration during yeast karyogamy, termed nuclear congression, is required to initiate nuclear fusion. Congression involves a specific regulation of the microtubule minus end-directed kinesin-14 motor Kar3 and a rearrangement of the cytoplasmic microtubule attachment sites at the spindle pole bodies (SPBs). However, how these elements interact to produce the forces necessary for nuclear migration is less clear. We used electron tomography, molecular genetics, quantitative imaging, and first principles modeling to investigate how cytoplasmic microtubules are organized during nuclear congression. We found that Kar3, with the help of its light chain, Cik1, is anchored during mating to the SPB component Spc72 that also serves as a nucleator and anchor for microtubules via their minus ends. Moreover, we show that no direct microtubule-microtubule interactions are required for nuclear migration. Instead, SPB-anchored Kar3 exerts the necessary pulling forces laterally on microtubules emanating from the SPB of the mating partner nucleus. Therefore, a twofold symmetrical application of the core principle that drives nuclear migration in higher cells is used in yeast to drive nuclei toward each other before nuclear fusion.


Assuntos
Núcleo Celular/metabolismo , Proteínas Associadas aos Microtúbulos/metabolismo , Microtúbulos/metabolismo , Proteínas de Saccharomyces cerevisiae/metabolismo , Saccharomyces cerevisiae/fisiologia , Fuso Acromático/metabolismo , Núcleo Celular/ultraestrutura , Simulação por Computador , Proteínas Nucleares/metabolismo , Ligação Proteica , Saccharomyces cerevisiae/metabolismo , Saccharomyces cerevisiae/ultraestrutura
11.
Biophys J ; 118(11): 2703-2717, 2020 06 02.
Artigo em Inglês | MEDLINE | ID: mdl-32365328

RESUMO

Molecular motors drive cytoskeletal rearrangements to change cell shape. Myosins are the motors that move, cross-link, and modify the actin cytoskeleton. The primary force generator in contractile actomyosin networks is nonmuscle myosin II (NMMII), a molecular motor that assembles into ensembles that bind, slide, and cross-link actin filaments (F-actin). The multivalence of NMMII ensembles and their multiple roles have confounded the resolution of crucial questions, including how the number of NMMII subunits affects dynamics and what affects the relative contribution of ensembles' cross-linking versus motoring activities. Because biophysical measurements of ensembles are sparse, modeling of actomyosin networks has aided in discovering the complex behaviors of NMMII ensembles. Myosin ensembles have been modeled via several strategies with variable discretization or coarse graining and unbinding dynamics, and although general assumptions that simplify motor ensembles result in global contractile behaviors, it remains unclear which strategies most accurately depict cellular activity. Here, we used an agent-based platform, Cytosim, to implement several models of NMMII ensembles. Comparing the effects of bond type, we found that ensembles of catch-slip and catch motors were the best force generators and binders of filaments. Slip motor ensembles were capable of generating force but unbound frequently, resulting in slower contractile rates of contractile networks. Coarse graining of these ensemble types from two sets of 16 motors on opposite ends of a stiff rod to two binders, each representing 16 motors, reduced force generation, contractility, and the total connectivity of filament networks for all ensemble types. A parallel cluster model, previously used to describe ensemble dynamics via statistical mechanics, allowed better contractility with coarse graining, though connectivity was still markedly reduced for this ensemble type with coarse graining. Together, our results reveal substantial tradeoffs associated with the process of coarse graining NMMII ensembles and highlight the robustness of discretized catch-slip ensembles in modeling actomyosin networks.


Assuntos
Actomiosina , Miosina Tipo II , Citoesqueleto de Actina , Actinas , Contração Muscular , Miosinas
12.
J Cell Sci ; 132(4)2018 12 13.
Artigo em Inglês | MEDLINE | ID: mdl-30404824

RESUMO

Cytoskeletal networks of actin filaments and myosin motors drive many dynamic cell processes. A key characteristic of these networks is their contractility. Despite intense experimental and theoretical efforts, it is not clear what mechanism favors network contraction over expansion. Recent work points to a dominant role for the nonlinear mechanical response of actin filaments, which can withstand stretching but buckle upon compression. Here, we present an alternative mechanism. We study how interactions between actin and myosin-2 at the single-filament level translate into contraction at the network scale by performing time-lapse imaging on reconstituted quasi-2D networks mimicking the cell cortex. We observe myosin end-dwelling after it runs processively along actin filaments. This leads to transport and clustering of actin filament ends and the formation of transiently stable bipolar structures. Further, we show that myosin-driven polarity sorting produces polar actin asters, which act as contractile nodes that drive contraction in crosslinked networks. Computer simulations comparing the roles of the end-dwelling mechanism and a buckling-dependent mechanism show that the relative contribution of end-dwelling contraction increases as the network mesh-size decreases.


Assuntos
Actinas/fisiologia , Simulação por Computador , Citoesqueleto/fisiologia , Miosinas/fisiologia , Citoesqueleto de Actina/química , Actomiosina/fisiologia , Movimento Celular/fisiologia , Proteínas do Citoesqueleto/fisiologia , Modelos Biológicos , Contração Muscular/fisiologia
13.
Proc Natl Acad Sci U S A ; 114(17): 4418-4423, 2017 04 25.
Artigo em Inglês | MEDLINE | ID: mdl-28400519

RESUMO

The fast bloodstream of animals is associated with large shear stresses. To withstand these conditions, blood cells have evolved a special morphology and a specific internal architecture to maintain their integrity over several weeks. For instance, nonmammalian red blood cells, mammalian erythroblasts, and platelets have a peripheral ring of microtubules, called the marginal band, that flattens the overall cell morphology by pushing on the cell cortex. In this work, we model how the shape of these cells stems from the balance between marginal band rigidity and cortical tension. We predict that the diameter of the cell scales with the total microtubule polymer and verify the predicted law across a wide range of species. Our analysis also shows that the combination of the marginal band rigidity and cortical tension increases the ability of the cell to withstand forces without deformation. Finally, we model the marginal band coiling that occurs during the disk-to-sphere transition observed, for instance, at the onset of blood platelet activation. We show that when cortical tension increases faster than cross-linkers can unbind, the marginal band will coil, whereas if the tension increases more slowly, the marginal band may shorten as microtubules slide relative to each other.


Assuntos
Plaquetas/citologia , Simulação por Computador , Eritrócitos/citologia , Microtúbulos/fisiologia , Modelos Biológicos , Animais , Fenômenos Biomecânicos , Plaquetas/fisiologia , Eritrócitos/fisiologia , Especificidade da Espécie
14.
Phys Biol ; 16(4): 046004, 2019 04 23.
Artigo em Inglês | MEDLINE | ID: mdl-31013252

RESUMO

Active networks composed of filaments and motor proteins can self-organize into a variety of architectures. Computer simulations in two or three spatial dimensions and including or omitting steric interactions between filaments can be used to model active networks. Here we examine how these modelling choices affect the state space of network self-organization. We compare the networks generated by different models of a system of dynamic microtubules and microtubule-crosslinking motors. We find that a thin 3D model that includes steric interactions between filaments is the most versatile, capturing a variety of network states observed in recent experiments. In contrast, 2D models either with or without steric interactions which prohibit microtubule crossings can produce some, but not all, observed network states. Our results provide guidelines for the most appropriate choice of model for the study of different network types and elucidate mechanisms of active network organization.


Assuntos
Microtúbulos/química , Proteínas Motores Moleculares/química , Simulação por Computador , Reagentes de Ligações Cruzadas/química , Citoesqueleto/metabolismo , Multimerização Proteica , Transdução de Sinais
15.
Mol Syst Biol ; 13(9): 941, 2017 09 27.
Artigo em Inglês | MEDLINE | ID: mdl-28954810

RESUMO

Morphogenesis in animal tissues is largely driven by actomyosin networks, through tensions generated by an active contractile process. Although the network components and their properties are known, and networks can be reconstituted in vitro, the requirements for contractility are still poorly understood. Here, we describe a theory that predicts whether an isotropic network will contract, expand, or conserve its dimensions. This analytical theory correctly predicts the behavior of simulated networks, consisting of filaments with varying combinations of connectors, and reveals conditions under which networks of rigid filaments are either contractile or expansile. Our results suggest that pulsatility is an intrinsic behavior of contractile networks if the filaments are not stable but turn over. The theory offers a unifying framework to think about mechanisms of contractions or expansion. It provides the foundation for studying a broad range of processes involving cytoskeletal networks and a basis for designing synthetic networks.


Assuntos
Actomiosina/química , Citoesqueleto/metabolismo , Biologia de Sistemas/métodos , Algoritmos , Animais , Citoesqueleto/química , Modelos Biológicos
16.
PLoS Comput Biol ; 11(10): e1004538, 2015 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-26517669

RESUMO

Endocytosis is an essential process by which cells internalize a piece of plasma membrane and material from the outside. In cells with turgor, pressure opposes membrane deformations, and increases the amount of force that has to be generated by the endocytic machinery. To determine this force, and calculate the shape of the membrane, we used physical theory to model an elastic surface under pressure. Accurate fits of experimental profiles are obtained assuming that the coated membrane is highly rigid and preferentially curved at the endocytic site. The forces required from the actin machinery peaks at the onset of deformation, indicating that once invagination has been initiated, endocytosis is unlikely to stall before completion. Coat proteins do not lower the initiation force but may affect the process by the curvature they induce. In the presence of isotropic curvature inducers, pulling the tip of the invagination can trigger the formation of a neck at the base of the invagination. Hence direct neck constriction by actin may not be required, while its pulling role is essential. Finally, the theory shows that anisotropic curvature effectors stabilize membrane invaginations, and the loss of crescent-shaped BAR domain proteins such as Rvs167 could therefore trigger membrane scission.


Assuntos
Actinas/metabolismo , Membrana Celular/fisiologia , Endocitose/fisiologia , Mecanotransdução Celular/fisiologia , Fluidez de Membrana/fisiologia , Modelos Biológicos , Membrana Celular/ultraestrutura , Tamanho Celular , Simulação por Computador , Módulo de Elasticidade/fisiologia , Pressão
17.
PLoS Comput Biol ; 11(5): e1004245, 2015 May.
Artigo em Inglês | MEDLINE | ID: mdl-26016478

RESUMO

The different actin structures governing eukaryotic cell shape and movement are not only determined by the properties of the actin filaments and associated proteins, but also by geometrical constraints. We recently demonstrated that limiting nucleation to specific regions was sufficient to obtain actin networks with different organization. To further investigate how spatially constrained actin nucleation determines the emergent actin organization, we performed detailed simulations of the actin filament system using Cytosim. We first calibrated the steric interaction between filaments, by matching, in simulations and experiments, the bundled actin organization observed with a rectangular bar of nucleating factor. We then studied the overall organization of actin filaments generated by more complex pattern geometries used experimentally. We found that the fraction of parallel versus antiparallel bundles is determined by the mechanical properties of actin filament or bundles and the efficiency of nucleation. Thus nucleation geometry, actin filaments local interactions, bundle rigidity, and nucleation efficiency are the key parameters controlling the emergent actin architecture. We finally simulated more complex nucleation patterns and performed the corresponding experiments to confirm the predictive capabilities of the model.


Assuntos
Citoesqueleto de Actina/química , Citoesqueleto de Actina/metabolismo , Modelos Moleculares , Multimerização Proteica , Citoesqueleto de Actina/ultraestrutura , Animais , Fenômenos Biomecânicos , Biologia Computacional , Simulação por Computador , Humanos , Técnicas In Vitro
18.
Phys Biol ; 11(1): 016008, 2014 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-24476749

RESUMO

Microtubules (MTs) nucleated by centrosomes form star-shaped structures referred to as asters. Aster motility and dynamics is vital for genome stability, cell division, polarization and differentiation. Asters move either toward the cell center or away from it. Here, we focus on the centering mechanism in a membrane independent system of Xenopus cytoplasmic egg extracts. Using live microscopy and single particle tracking, we find that asters move toward chromatinized DNA structures. The velocity and directionality profiles suggest a random-walk with drift directed toward DNA. We have developed a theoretical model that can explain this movement as a result of a gradient of MT length dynamics and MT gliding on immobilized dynein motors. In simulations, the antagonistic action of the motor species on the radial array of MTs leads to a tug-of-war purely due to geometric considerations and aster motility resembles a directed random-walk. Additionally, our model predicts that aster velocities do not change greatly with varying initial distance from DNA. The movement of asymmetric asters becomes increasingly super-diffusive with increasing motor density, but for symmetric asters it becomes less super-diffusive. The transition of symmetric asters from superdiffusive to diffusive mobility is the result of number fluctuations in bound motors in the tug-of-war. Overall, our model is in good agreement with experimental data in Xenopus cytoplasmic extracts and predicts novel features of the collective effects of motor-MT interactions.


Assuntos
DNA/metabolismo , Dineínas/metabolismo , Microtúbulos/química , Microtúbulos/metabolismo , Mitose , Animais , Extratos Celulares , Citoplasma , Oócitos/citologia , Xenopus
19.
Nature ; 456(7220): 395-9, 2008 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-19020621

RESUMO

The simplest animal eyes are eyespots composed of two cells only: a photoreceptor and a shading pigment cell. They resemble Darwin's 'proto-eyes', considered to be the first eyes to appear in animal evolution. Eyespots cannot form images but enable the animal to sense the direction of light. They are characteristic for the zooplankton larvae of marine invertebrates and are thought to mediate larval swimming towards the light. Phototaxis of invertebrate larvae contributes to the vertical migration of marine plankton, which is thought to represent the biggest biomass transport on Earth. Yet, despite its ecological and evolutionary importance, the mechanism by which eyespots regulate phototaxis is poorly understood. Here we show how simple eyespots in marine zooplankton mediate phototactic swimming, using the marine annelid Platynereis dumerilii as a model. We find that the selective illumination of one eyespot changes the beating of adjacent cilia by direct cholinergic innervation resulting in locally reduced water flow. Computer simulations of larval swimming show that these local effects are sufficient to direct the helical swimming trajectories towards the light. The computer model also shows that axial rotation of the larval body is essential for phototaxis and that helical swimming increases the precision of navigation. These results provide, to our knowledge, the first mechanistic understanding of phototaxis in a marine zooplankton larva and show how simple eyespots regulate it. We propose that the underlying direct coupling of light sensing and ciliary locomotor control was a principal feature of the proto-eye and an important landmark in the evolution of animal eyes.


Assuntos
Anelídeos/fisiologia , Anelídeos/efeitos da radiação , Luz , Locomoção/efeitos da radiação , Visão Ocular/fisiologia , Visão Ocular/efeitos da radiação , Zooplâncton/fisiologia , Zooplâncton/efeitos da radiação , Animais , Anelídeos/citologia , Anelídeos/crescimento & desenvolvimento , Cílios/fisiologia , Cílios/efeitos da radiação , Simulação por Computador , Olho/citologia , Olho/efeitos da radiação , Larva/citologia , Larva/fisiologia , Larva/efeitos da radiação , Células Fotorreceptoras de Invertebrados/fisiologia , Células Fotorreceptoras de Invertebrados/efeitos da radiação , Receptores Nicotínicos/metabolismo , Natação/fisiologia , Zooplâncton/citologia , Zooplâncton/crescimento & desenvolvimento
20.
Proc Natl Acad Sci U S A ; 108(35): 14473-8, 2011 Aug 30.
Artigo em Inglês | MEDLINE | ID: mdl-21844347

RESUMO

Female meiotic spindles in many organisms form in the absence of centrosomes, the organelle typically associated with microtubule (MT) nucleation. Previous studies have proposed that these meiotic spindles arise from RanGTP-mediated MT nucleation in the vicinity of chromatin; however, whether this process is sufficient for spindle formation is unknown. Here, we investigated whether a recently proposed spindle-based MT nucleation pathway that involves augmin, an 8-subunit protein complex, also contributes to spindle morphogenesis. We used an assay system in which hundreds of meiotic spindles can be observed forming around chromatin-coated beads after introduction of Xenopus egg extracts. Spindles forming in augmin-depleted extracts showed reduced rates of MT formation and were predominantly multipolar, revealing a function of augmin in stabilizing the bipolar shape of the acentrosomal meiotic spindle. Our studies also have uncovered an apparent augmin-independent MT nucleation process from acentrosomal poles, which becomes increasingly active over time and appears to partially rescue the spindle defects that arise from augmin depletion. Our studies reveal that spatially and temporally distinct MT generation pathways from chromatin, spindle MTs, and acentrosomal poles all contribute to robust bipolar spindle formation in meiotic extracts.


Assuntos
Centrossomo/metabolismo , Meiose/fisiologia , Proteínas Associadas aos Microtúbulos/fisiologia , Complexos Multiproteicos/fisiologia , Fuso Acromático/fisiologia , Proteínas de Xenopus/fisiologia , Animais , Feminino , Microtúbulos/metabolismo , Óvulo/metabolismo , Subunidades Proteicas , Xenopus laevis
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