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Activator-inhibitor coupling between Rho signalling and actin assembly makes the cell cortex an excitable medium. Bement WM et al. (2015) Animal cell cytokinesis results from patterned activation of the small GTPase Rho, which directs assembly of actomyosin in the equatorial cortex. Cytokinesis is restricted to a portion of the cell cycle following anaphase onset in which the cortex is responsive to signals from the spindle. We show that shortly after anaphase onset oocytes and embryonic cells of frogs and echinoderms exhibit cortical waves of Rho activity and F-actin polymerization. The waves are modulated by cyclin-dependent kinase 1 (Cdk1) activity and require the Rho GEF (guanine nucleotide exchange factor), Ect2. Surprisingly, during wave propagation, although Rho activity elicits F-actin assembly, F-actin subsequently inactivates Rho. Experimental and modelling results show that waves represent excitable dynamics of a reaction-diffusion system with Rho as the activator and F-actin the inhibitor. We propose that cortical excitability explains fundamental features of cytokinesis including its cell cycle regulation.//////////////////
Epithelial Cell Transforming Protein 2 (ECT2) Depletion Blocks Polar Body Extrusion and Generates Mouse Oocytes Containing Two Metaphase II Spindles. Elbaz J et al. Completion of the first meiosis in oocytes is achieved by the extrusion of the first polar body (PBI), a particular example of cell division. In mitosis, the small GTPase RhoA, which is activated by epithelial cell transforming protein 2 (ECT2), orchestrates contractile ring constriction, thus enabling cytokinesis. However, the involvement of this pathway in mammalian oocytes has not been established. To characterize the role of ECT2 in PBI emission in mouse oocytes, the small interfering RNA approach was employed. We found that ECT2 depletion significantly reduces PBI emission, induces first metaphase arrest, and generates oocytes containing two properly formed spindles of the second metaphase. Moreover, we describe, for the first time, that before PBI emission, RhoA forms a ring that is preceded by a dome-like accumulation at the oocyte cortex, next to the spindle. This unique mode of RhoA translocation failed to occur in the absence of ECT2. We further found that the Rho-dependent kinase, a main RhoA effector, is essential for PBI emission. In addition, we demonstrate herein that ECT2 is subjected to phosphorylation/dephosphorylation throughout meiosis in oocytes and further reveal that PBI emission is temporally associated with ECT2 dephosphorylation. Our data provide the first demonstration that an active cyclin-dependent kinase 1, the catalytic subunit of the maturation-promoting factor, phosphorylates ECT2 during the first meiotic metaphase and that cyclin-dependent kinase 1 inactivation at anaphase allows ECT2 dephosphorylation. In conclusion, our study demonstrates the indispensable role of the maturation-promoting factor/ECT2/RhoA pathway in PBI extrusion in mouse oocytes.
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