Studying the central spindle-associated Rho guanine nucleotide exchange factor ECT2 during meiotic cytokinesis

Abstract

Polar body formation has been of great interest both as a fundamental process in sexual reproduction and as an extreme example of asymmetric cell division (cytokinesis) in cell biology. The guanine nucleotide exchange factor ECT2 regulates cytokinesis in all metazoans analyzed to date. Although it has been known that ECT2 is required for the localization of active RhoA to the equatorial cortex, its role in vertebrate polar body emission is not explored yet. It is believed that ECT2 is the GEF for RhoA during mitotic cytokinesis. However, there is no direct evidence supporting co-localization of ECT2 with Rho contractile ring, at least during the early stage of cytokinesis. To investigate this hypothesis, we employed fluorescence confocal microscopy to reveal for the first time that ECT2 never overlaps with the active RhoA zone in the oocytes during polar body emission. These results suggest that, although ECT2 function is required for Rho contractile ring formation and constriction, ECT2 is unlikely the direct activator of RhoA during polar body emission. More importantly, our time-lapse imaging experiments with ECT2 and active Rho reveals an exciting mechanism by which Cdc42, a small Rho GTPase, regulates polar body emission: during Rho contractile ring constriction, Cdc42 promotes membrane protrusion to "pull" one spindle pale and the associated polar body chromosomes into the forming polar body enclosure.