The mitotic spindle in the one-cell C. elegans embryo is positioned with high precision and stability

Precise positioning of the mitotic spindle is important for specifying the plane of cell division, which in turn determines how the cytoplasmic contents are partitioned into the daughter cells, and how the daughters are positioned within the tissue. During metaphase in the early C. elegans embryo, the spindle is aligned and centered on the anterior-posterior axis by a microtubule-dependent machinery that exerts restoring forces when the spindle is displaced from the center. To investigate the accuracy and stability of centering, we tracked the position and orientation of the mitotic spindle during the first cell division with high temporal and spatial resolution. We found that the precision is remarkably high: the cell-to-cell variation in the transverse position of the center of the spindle during metaphase, as measured by the standard deviation, was only 1.5% of the length of the short axis of the cell. Spindle position is also very stable: the standard deviation of the fluctuations in transverse spindle position during metaphase was only 0.5% of the short axis of the cell. Assuming that stability is limited by fluctuations in the number of independent motor elements such as microtubules or dyneins underlying the centering machinery, we infer that the number is on the order of one thousand, consistent with the several thousand of astral microtubules in these cells. Astral microtubules grow out from the two spindle poles, make contact with the cell cortex, and then shrink back shortly thereafter. The high stability of centering can be accounted for quantitatively if, while making contact with the cortex, the astral microtubules buckle as they exert compressive, pushing forces. We thus propose that the large number of microtubules in the asters provides a highly precise mechanism for positioning the spindle during metaphase while assembly is completed prior to the onset of anaphase.Comment: Accepted in Biophysical Journal (2016

Evolutionary comparisons reveal a positional switch for spindle pole oscillations in Caenorhabditis embryos.

International audienceDuring the first embryonic division in Caenorhabditis elegans, the mitotic spindle is pulled toward the posterior pole of the cell and undergoes vigorous transverse oscillations. We identified variations in spindle trajectories by analyzing the outwardly similar one-cell stage embryo of its close relative Caenorhabditis briggsae. Compared with C. elegans, C. briggsae embryos exhibit an anterior shifting of nuclei in prophase and reduced anaphase spindle oscillations. By combining physical perturbations and mutant analysis in both species, we show that differences can be explained by interspecies changes in the regulation of the cortical Gα-GPR-LIN-5 complex. However, we found that in both species (1) a conserved positional switch controls the onset of spindle oscillations, (2) GPR posterior localization may set this positional switch, and (3) the maximum amplitude of spindle oscillations is determined by the time spent in the oscillating phase. By investigating microevolution of a subcellular process, we identify new mechanisms that are instrumental to decipher spindle positioning

Membrane Invaginations Reveal Cortical Sites that Pull on Mitotic Spindles in One-Cell C. elegans Embryos

Asymmetric positioning of the mitotic spindle in C. elegans embryos is mediated by force-generating complexes that are anchored at the plasma membrane and that pull on microtubules growing out from the spindle poles. Although asymmetric distribution of the force generators is thought to underlie asymmetric positioning of the spindle, the number and location of the force generators has not been well defined. In particular, it has not been possible to visualize individual force generating events at the cortex. We discovered that perturbation of the acto-myosin cortex leads to the formation of long membrane invaginations that are pulled from the plasma membrane toward the spindle poles. Several lines of evidence show that the invaginations, which also occur in unperturbed embryos though at lower frequency, are pulled by the same force generators responsible for spindle positioning. Thus, the invaginations serve as a tool to localize the sites of force generation at the cortex and allow us to estimate a lower limit on the number of cortical force generators within the cell

Mesure du spectre de fluctuations de vésicules géantes par analyse de contours (application aux membranes passives et actives)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Modelling oscillatory behavior in asymmetric division of C.elegans embryo

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Wnt-regulated dynamics of positional information in zebrafish somitogenesis

How signaling gradients supply positional information in a field of moving cells is an unsolved question in patterning and morphogenesis. Here, we ask how a Wnt signaling gradient regulates the dynamics of a wavefront of cellular change in a flow of cells during somitogenesis. Using time-controlled perturbations of Wnt signaling in the zebrafish embryo, we changed segment length without altering the rate of somite formation or embryonic elongation. This result implies specific Wnt regulation of the wavefront velocity. The observed Wnt signaling gradient dynamics and timing of downstream events support a model for wavefront regulation in which cell flow plays a dominant role in transporting positional information.This work was supported by the Max Planck Society; L.B. by Human Frontier Science Program (HFSP) and Marie Curie; A.C.O., L.G.M. and J.P. by the European Research Council (ERC) under the European Communities 7th Framework Programme [FP7/2007–2013]/[ERC grant 207634]; J.P. by German Research Foundation Normalverfaren [OA 53/2-1]; S.A. by Spanish Ministry of Economy and Competitiveness (MINECO) grant PHYSDEV [FIS2012-32349]; F.J. by the Max Planck Society; and A.C.O. by the Wellcome Trust [WT098025MA] and the Medical Research Council (MRC) [MC_UP_1202/3.European Community's Seventh Framework ProgramPublicad

Studying oscillatory behavior in asymmetric division of Caenorhabditis elegans embryo with fluorescence microscopy

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Astral microtubule dynamics regulate anaphase oscillation onset and set a robust final position of the C. elegans zygote spindle

Background: The correct positioning of the mitotic spindle during the asymmetric division of the nematode C. elegans zygote relies on the combination of centering and cortical–pulling forces. These forces, revealed by centrosome anaphase oscillations, are regulated through the dynamics of force generators, related to mitosis progression. Recently, we reported the control of oscillation onset by the posterior spindle pole position in related species C. briggsae, necessitating a re-evaluation of the role of astral microtubules dynamics. Results: After exhibiting such a positional switch in C. elegans, we mapped the microtubule ends at the cortex and observed a correlation between the proximity of the centrosomes and the density of microtubule contacts. To explore the functional consequences, we extended the “tug–of–war” model and successfully accounted for the positional switch. We predicted and experimentally validated that the control of oscillation onset was robust to changes in cell geometry or maximum number of attached force generators. We also predicted that the final position of the posterior centrosome and thus the spindle has a reduced dependence upon the force generator dynamics or number. Conclusion: The outburst of forces responsible of spindle anaphase oscillations and positioning is regulated by the spindle position through the spatial modulation of microtubule contacts at the cortex. This regulation superimposes that of force generator processivity putatively linked to the cell cycle. This novel control provides robustness to variations in zygote geometry or detailed properties of cortical force generators