Cdc42 Activation Couples Spindle Positioning to First Polar Body Formation in Oocyte Maturation

Department of Biochemistry, Microbiology and Immunology, University of Ottawa, Ottawa, Ontario, Canada
Current Biology (Impact Factor: 9.57). 02/2006; 16(2):214-20. DOI: 10.1016/j.cub.2005.11.067
Source: PubMed


During vertebrate egg maturation, cytokinesis initiates after one pole of the bipolar metaphase I spindle attaches to the oocyte cortex, resulting in the formation of a polar body and the mature egg. It is not known what signal couples the spindle pole positioning to polar body formation. We approached this question by drawing an analogy to mitotic exit in budding yeast, as asymmetric spindle attachment to the appropriate cortical region is the common regulatory cue. In budding yeast, the small G protein Cdc42 plays an important role in mitotic exit following the spindle pole attachment . We show here that inhibition of Cdc42 activation blocks polar body formation. The oocytes initiate anaphase but fail to properly form and direct a contractile ring. Endogenous Cdc42 is activated at the spindle pole-cortical contact site immediately prior to polar body formation. The cortical Cdc42 activity zone, which directly overlays the spindle pole, is circumscribed by a cortical RhoA activity zone; the latter defines the cytokinetic contractile furrow . As the RhoA ring contracts during cytokinesis, the Cdc42 zone expands, maintaining its complementary relationship with the RhoA ring. Cdc42 signaling may thus be an evolutionarily conserved mechanism that couples spindle positioning to asymmetric cytokinesis.

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Available from: Hélène Benink
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    • "Actin also regulates polar body extrusion in mouse oocytes, and several molecules play an important role in cytokinesis via their effects on the actin cytoskeleton. For example, Cdc42 and Rac, which activate Arp2/3, are necessary for polar body extrusion and spindle migration during meiotic maturation [28]–[30]. Ran GTPase, which functions upstream of the Arp2/3 complex, also plays an important role in controlling cortical polarity during polar body extrusion in mouse oocytes by mediating chromatin signaling [31]. Our results show that the regulatory mechanism for the Arp2/3 complex in porcine oocytes was also mediated through an actin-dependent pathway. "
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    • "Although individual oocytes vary, often considerably, in the timing of GVBD after the addition of progesterone, we found that they are remarkably synchronized from GVBD to first polar body emission (Ma et al., 2006; Zhang et al., 2008). Typically, 120 min after GVBD, the oocyte initiates anaphase I. "
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    • "After plasmid linearization, cRNAs were prepared in vitro using the mMessage mMachine T7 Ultra kit (Ambion) and pressure-injected in GV or MII oocytes. Expression of the EGFP–wGBD probe was not associated with significant dominant-negative phenotype in previous studies (Kim et al., 2000; Cannon et al., 2001; Benink and Bement, 2005; Ma et al., 2006; Zhang et al., 2008), nor in the present study. To inhibit Ran function, oocytes were injected with purified RanT24N protein (Cytoskeleton, Denver, CO, USA; 0.5 mg/ml in the pipette). "
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    ABSTRACT: Asymmetric meiotic divisions in mammalian oocytes rely on the eccentric positioning of the spindle and the remodelling 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.
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