An astral simulacrum of the central spindle accounts for normal, spindle-less, and anucleate cytokinesis in echinoderm embryos.
Bottom Line: Although the nature, origin, and variety of such cues have long been obscure, one component is certainly the Rho activator Ect2.Here we describe the behavior and function of Ect2 in echinoderm embryos, showing that Ect2 migrates from spindle midzone to astral microtubules in anaphase and that Ect2 shapes the pattern of Rho activation in incipient furrows.In all these cases, the cell assembles essentially the same cytokinetic signaling ensemble—opposed astral microtubules decorated with Ect2 and Cyk4.
Affiliation: Oregon Institute of Marine Biology, Charleston, OR 97420 Cancer Research UK London Research Institute, Clare Hall Laboratories, South Mimms EN6 3LD, United Kingdom.Show MeSH
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Mentions: The apparent dependence of normal furrowing on central spindle–localized proteins, plus the well-documented propensity for diverse cells to develop furrows between pairs of asters in various experimental circumstances, suggests the possibility that overlapping, mechanistically distinct signals contribute to furrow induction (as in Murata-Hori and Wang, 2002; Bringmann and Hyman, 2005; Baruni et al., 2008; von Dassow, 2009). At least some animal cells, such as C. elegans embryos, possess spindle- and centralspindlin-independent means to signal furrow initiation (Dechant and Glotzer, 2003; Werner et al., 2007; Tse et al., 2012). Alternatively, it may be that the same signaling mechanism can interpret diverse geometric arrangements of microtubules into a similar outcome. To test this, we used several approaches to disrupt astral microtubules or separate them physically from the spindle. We showed previously that trichostatin A strongly inhibits astral microtubule growth while leaving the spindle functional for anaphase and that such asterless cells cleave accurately despite a much broader than normal zone of Rho activity (von Dassow et al., 2009). Accordingly, we found that in trichostatin A–treated cells, Ect2 remained concentrated at the central spindle and on tips of short astral microtubules (Figure 6) and was only faintly detectable over a broad region around the equatorial cortex, corresponding to the wider than normal furrow. Cyk4 behaved similarly (unpublished data). This is consistent with the idea that astral microtubules serve to focus a central spindle–derived signal—Ect2—that would otherwise reach the cell surface by diffusion.
Affiliation: Oregon Institute of Marine Biology, Charleston, OR 97420 Cancer Research UK London Research Institute, Clare Hall Laboratories, South Mimms EN6 3LD, United Kingdom.