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

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.

Actin-capping proteins play essential roles in asymmetric division of maturing mouse oocytes

“…The cortical actin cap, which forms near to the approaching spindle, contains a dense actin network (Yi and Li, 2012;Yi et al, 2013a), and its formation is dependent on the Arp2/3 complex, Rho family GTPases and various nucleation-promoting factors (Dehapiot et al, 2013;Deng et al, 2007;Leblanc et al, 2011;Wang et al, 2013). Capping protein is an essential factor in an Arp2/3-mediated actin polymerization and depolymerization model called the 'dendritic treadmilling model' (Blanchoin et al, 2000a;Blanchoin et al, 2000b;Pollard and Borisy, 2003); therefore, this difference between the localization of capping protein and that of the actin-rich cortical cap is unexpected.…”

Roles of actin binding proteins in mammalian oocyte maturation and beyond

“…23 Moreover, the ARP2/3 complex is responsible for the formation of a thicker cortex region called the 'subcortex'; the formation of the subcortex by ARP2/3 may be a factor mediating the exclusion of non-muscle myosin II and can cause changes in cortex softening during oocyte maturation. 12,13 Nucleation-promoting factor (NPF) family proteins are necessary for the activation of the ARP2/3 complex, and several NPFs, including Wiskott-Aldrich syndrome protein family member 2 (WAVE2), 66 neuronal Wiskott-Aldrich syndrome protein (N-WASP), 67 junction-mediated regulatory protein (JMY), 68 WASP homolog associated with actin, membranes, and microtubules (WHAMM), 69 and WAS protein family homolog 1 (WASH), 70 are implicated in oocyte maturation (Fig. 2D).…”

Spindle asymmetry drives non-Mendelian chromosome segregation

“…2A). This polarized cortex is enriched in multiple signaling factors, including active CDC42 and RAC GTPases, and in polymerized actin (called the actin cap) (6,23,24) (Fig. 2A).…”

“…S7). Because CDC42 activity is required for actin cap formation at the polarized cortex (24) (Fig. 2A), we tested whether the actin cap contributes to spindle asymmetry.…”