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The forces that position a mitotic spindle asymmetrically are tethered until after the time of spindle assembly.

Labbé JC, McCarthy EK, Goldstein B - J. Cell Biol. (2004)

Bottom Line: The spindle does not shift asymmetrically during these early phases due to a tethering force, mediated by astral microtubules that reach the anterior cell cortex.Monitoring microtubule dynamics by photobleaching segments of microtubules during anaphase revealed that spindle microtubules do not undergo significant poleward flux in C. elegans.We propose that the forces positioning the mitotic spindle asymmetrically are tethered until after the time of spindle assembly and that these same forces are used later to drive chromosome segregation at anaphase.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. jc.labbe@umontreal.ca

ABSTRACT
Regulation of the mitotic spindle's position is important for cells to divide asymmetrically. Here, we use Caenorhabditis elegans embryos to provide the first analysis of the temporal regulation of forces that asymmetrically position a mitotic spindle. We find that asymmetric pulling forces, regulated by cortical PAR proteins, begin to act as early as prophase and prometaphase, even before the spindle forms and shifts to a posterior position. The spindle does not shift asymmetrically during these early phases due to a tethering force, mediated by astral microtubules that reach the anterior cell cortex. We show that this tether is normally released after spindle assembly and independently of anaphase entry. Monitoring microtubule dynamics by photobleaching segments of microtubules during anaphase revealed that spindle microtubules do not undergo significant poleward flux in C. elegans. Together with the known absence of anaphase A, these data suggest that the major forces contributing to chromosome separation during anaphase originate outside the spindle. We propose that the forces positioning the mitotic spindle asymmetrically are tethered until after the time of spindle assembly and that these same forces are used later to drive chromosome segregation at anaphase.

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Model depicting the transient tethers that act during mitosis in cells that divide symmetrically and early C. elegans embryos, which divide asymmetrically. In this model, the net pulling vectorial forces are depicted as arrows and tethers are depicted in dark. In cells that divide symmetrically, tension is present at the kinetochores during metaphase and forces are kept at equilibrium by cohesins that link sister chromatids (McNeill and Berns, 1981; Hays and Salmon, 1990). Cohesins are degraded at anaphase onset, allowing chromosome segregation to occur. In asymmetrically dividing C. elegans embryos, a posterior pulling force is present during late prophase and prometaphase, and this force is kept at equilibrium by the tethering of astral microtubules at the anterior of the embryo. This tether is released during metaphase, allowing posterior movement of the spindle. MT, microtubule.
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fig7: Model depicting the transient tethers that act during mitosis in cells that divide symmetrically and early C. elegans embryos, which divide asymmetrically. In this model, the net pulling vectorial forces are depicted as arrows and tethers are depicted in dark. In cells that divide symmetrically, tension is present at the kinetochores during metaphase and forces are kept at equilibrium by cohesins that link sister chromatids (McNeill and Berns, 1981; Hays and Salmon, 1990). Cohesins are degraded at anaphase onset, allowing chromosome segregation to occur. In asymmetrically dividing C. elegans embryos, a posterior pulling force is present during late prophase and prometaphase, and this force is kept at equilibrium by the tethering of astral microtubules at the anterior of the embryo. This tether is released during metaphase, allowing posterior movement of the spindle. MT, microtubule.

Mentions: Finally, our results provide an interesting parallel between cells that divide symmetrically and those that divide asymmetrically (Fig. 7). In symmetrically dividing cells, poleward microtubule flux and microtubule plus end dynamics generate forces at the kinetochores before anaphase onset (Mitchison and Salmon, 1992; Waters et al., 1996), and cohesion between sister chromatids contributes to a tethering force that temporarily prevents chromosome segregation. Likewise, in C. elegans embryos, we observed an asymmetry in the spindle-positioning pulling forces before anaphase, and anterior astral microtubules contribute to a tethering force that temporarily prevents posterior movement of the spindle. Although chromosome segregation and anaphase entry are regulated by components of the spindle checkpoint, it is not clear whether such a checkpoint exists to regulate the timing of posterior spindle displacement. Such a spindle-positioning checkpoint could be required to ensure that posterior spindle displacement does not initiate before all kinetochores have made microtubule attachments, thus ensuring fidelity of chromosome segregation. The difference between symmetric and asymmetric cell divisions lies in a polarization of the cell before entry into mitosis. Therefore, it is possible that the components required to establish and maintain cell polarity, which are active in asymmetrically dividing cells, are impeding on the cell cycle machinery in ways that have yet to be described. In support of this possibility, it should be noted that disrupting the function of the polarity genes par-2 and par-3 in C. elegans affects the progression of embryos through mitosis, prolonging the time between anaphase onset and cytokinetic furrow ingression (Kirby et al., 1990; unpublished data). It will be of interest to study in more detail links between polarity and cell cycle in the future.


The forces that position a mitotic spindle asymmetrically are tethered until after the time of spindle assembly.

Labbé JC, McCarthy EK, Goldstein B - J. Cell Biol. (2004)

Model depicting the transient tethers that act during mitosis in cells that divide symmetrically and early C. elegans embryos, which divide asymmetrically. In this model, the net pulling vectorial forces are depicted as arrows and tethers are depicted in dark. In cells that divide symmetrically, tension is present at the kinetochores during metaphase and forces are kept at equilibrium by cohesins that link sister chromatids (McNeill and Berns, 1981; Hays and Salmon, 1990). Cohesins are degraded at anaphase onset, allowing chromosome segregation to occur. In asymmetrically dividing C. elegans embryos, a posterior pulling force is present during late prophase and prometaphase, and this force is kept at equilibrium by the tethering of astral microtubules at the anterior of the embryo. This tether is released during metaphase, allowing posterior movement of the spindle. MT, microtubule.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2172534&req=5

fig7: Model depicting the transient tethers that act during mitosis in cells that divide symmetrically and early C. elegans embryos, which divide asymmetrically. In this model, the net pulling vectorial forces are depicted as arrows and tethers are depicted in dark. In cells that divide symmetrically, tension is present at the kinetochores during metaphase and forces are kept at equilibrium by cohesins that link sister chromatids (McNeill and Berns, 1981; Hays and Salmon, 1990). Cohesins are degraded at anaphase onset, allowing chromosome segregation to occur. In asymmetrically dividing C. elegans embryos, a posterior pulling force is present during late prophase and prometaphase, and this force is kept at equilibrium by the tethering of astral microtubules at the anterior of the embryo. This tether is released during metaphase, allowing posterior movement of the spindle. MT, microtubule.
Mentions: Finally, our results provide an interesting parallel between cells that divide symmetrically and those that divide asymmetrically (Fig. 7). In symmetrically dividing cells, poleward microtubule flux and microtubule plus end dynamics generate forces at the kinetochores before anaphase onset (Mitchison and Salmon, 1992; Waters et al., 1996), and cohesion between sister chromatids contributes to a tethering force that temporarily prevents chromosome segregation. Likewise, in C. elegans embryos, we observed an asymmetry in the spindle-positioning pulling forces before anaphase, and anterior astral microtubules contribute to a tethering force that temporarily prevents posterior movement of the spindle. Although chromosome segregation and anaphase entry are regulated by components of the spindle checkpoint, it is not clear whether such a checkpoint exists to regulate the timing of posterior spindle displacement. Such a spindle-positioning checkpoint could be required to ensure that posterior spindle displacement does not initiate before all kinetochores have made microtubule attachments, thus ensuring fidelity of chromosome segregation. The difference between symmetric and asymmetric cell divisions lies in a polarization of the cell before entry into mitosis. Therefore, it is possible that the components required to establish and maintain cell polarity, which are active in asymmetrically dividing cells, are impeding on the cell cycle machinery in ways that have yet to be described. In support of this possibility, it should be noted that disrupting the function of the polarity genes par-2 and par-3 in C. elegans affects the progression of embryos through mitosis, prolonging the time between anaphase onset and cytokinetic furrow ingression (Kirby et al., 1990; unpublished data). It will be of interest to study in more detail links between polarity and cell cycle in the future.

Bottom Line: The spindle does not shift asymmetrically during these early phases due to a tethering force, mediated by astral microtubules that reach the anterior cell cortex.Monitoring microtubule dynamics by photobleaching segments of microtubules during anaphase revealed that spindle microtubules do not undergo significant poleward flux in C. elegans.We propose that the forces positioning the mitotic spindle asymmetrically are tethered until after the time of spindle assembly and that these same forces are used later to drive chromosome segregation at anaphase.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. jc.labbe@umontreal.ca

ABSTRACT
Regulation of the mitotic spindle's position is important for cells to divide asymmetrically. Here, we use Caenorhabditis elegans embryos to provide the first analysis of the temporal regulation of forces that asymmetrically position a mitotic spindle. We find that asymmetric pulling forces, regulated by cortical PAR proteins, begin to act as early as prophase and prometaphase, even before the spindle forms and shifts to a posterior position. The spindle does not shift asymmetrically during these early phases due to a tethering force, mediated by astral microtubules that reach the anterior cell cortex. We show that this tether is normally released after spindle assembly and independently of anaphase entry. Monitoring microtubule dynamics by photobleaching segments of microtubules during anaphase revealed that spindle microtubules do not undergo significant poleward flux in C. elegans. Together with the known absence of anaphase A, these data suggest that the major forces contributing to chromosome separation during anaphase originate outside the spindle. We propose that the forces positioning the mitotic spindle asymmetrically are tethered until after the time of spindle assembly and that these same forces are used later to drive chromosome segregation at anaphase.

Show MeSH
Related in: MedlinePlus