Polarized Cdc42 activation promotes polar body protrusion and asymmetric division in mouse oocytes

Benoit Dehapiot,Virginie Carrière,John Carroll,Guillaume Halet

doi:10.1016/j.ydbio.2013.01.029

Abstract

Asymmetric meiotic divisions in mammalian oocytes rely on the eccentric positioning of the spindle and the remodeling of the overlying cortex, resulting in the formation of small polar bodies. The mechanism of this cortical polarization, exemplified by the formation of a thick F-actin cap, is poorly understood. Cdc42 is a major player in cell polarization in many systems; however, the spatio-temporal dynamics of Cdc42 activation during oocyte meiosis, and its contribution to mammalian oocyte polarization, have remained elusive. In this study, we investigated Cdc42 activation (Cdc42–GTP), dynamics and role during mouse oocyte meiotic divisions. We show that Cdc42–GTP accumulates in restricted cortical regions overlying meiotic chromosomes or chromatids, in a Ran–GTP-dependent manner. This polarized activation of Cdc42 is required for the recruitment of N-WASP and the formation of F-actin-rich protrusions during polar body formation. Cdc42 inhibition in MII oocytes resulted in the release of N-WASP into the cytosol, a loss of the polarized F-actin cap, and a failure to protrude the second polar body. Cdc42 inhibition also resulted in central spindle defects in activated MII oocytes. In contrast, emission of the first polar body during oocyte maturation could occur in the absence of a functional Cdc42/N-WASP pathway. Therefore, Cdc42 is a new protagonist in chromatin-induced cortical polarization in mammalian oocytes, with an essential role in meiosis II completion, through the recruitment and activation of N-WASP, downstream of the chromatin-centered Ran–GTP gradient.

Highlights

To become functional gametes competent for fertilization and preimplantation embryonic development, mammalian oocytes execute two rounds of asymmetric meiotic divisions, resulting in the formation of a large oocyte, which retains most of the maternal stores, and two small polar bodies
We found that the polarized activation of Cdc42 is driven by the Ran–GTP gradient generated by meiotic chromosomes, from metaphase through anaphase
We further show that this Cdc42/N-WASP/ F-actin cascade is required for membrane protrusion and cytokinesis during polar body formation in meiosis II, but seems dispensable for emission of the first polar body

Summary

Introduction

To become functional gametes competent for fertilization and preimplantation embryonic development, mammalian oocytes execute two rounds of asymmetric meiotic divisions, resulting in the formation of a large oocyte, which retains most of the maternal stores, and two small polar bodies. Studies have revealed the unique ability of meiotic chromosomes to induce cortical remodeling in a microtubule-independent manner (Longo and Chen 1985; Maro et al 1986), and a recent report has uncovered the instrumental role of the GTPase Ran in this process (Deng et al, 2007). Ran is activated (Ran–GTP) by the GTP/GDP exchange factor RCC1, which is bound to chromatin, resulting in the generation of a Ran–GTP gradient centered on the meiotic chromosomes (Dumont et al, 2007) This diffusible Ran–GTP signal is necessary for meiotic chromosomes to induce, at a distance, remodeling of the nearby cortex, such as formation of the F-actin cap (Deng et al, 2007). The molecular cascade initiated by Ran to polarize the oocyte is poorly understood

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Conclusion