Cyclin A2 Is Required for Sister Chromatid Segregation, But Not Separase Control, in Mouse Oocyte Meiosis

Abstract

In meiosis, two specialized cell divisions allow the separation of paired chromosomes first, then of sister chromatids. Separase removes the cohesin complex holding sister chromatids together in a stepwise manner from chromosome arms in meiosis I, then from the centromere region in meiosis II. Using mouse oocytes, our study reveals that cyclin A2 promotes entry into meiosis, as well as an additional unexpected role; namely, its requirement for separase-dependent sister chromatid separation in meiosis II. Untimely cyclin A2-associated kinase activity in meiosis I leads to precocious sister separation, whereas inhibition of cyclin A2 in meiosis II prevents it. Accordingly, endogenous cyclin A is localized to kinetochores throughout meiosis II, but not in anaphase I. Additionally, we found that cyclin B1, but not cyclin A2, inhibits separase in meiosis I. These findings indicate that separase-dependent cohesin removal is differentially regulated by cyclin B1 and A2 in mammalian meiosis.

The APC/C in female mammalian meiosis I

“…Recent data also indicate that cyclin B2 is also important for Cdk1 activation at GVBD and that Hec1 sustains cyclin B2 levels by sequestering it away from APC/C Cdh1 (Gui & Homer 2013). It also appears that cyclin A2 makes a contribution to Cdk1 activation (Touati et al 2012). Note that although the effect of exogenous cyclin A2 cRNA constructs bearing D-box mutations on GVBD in mouse oocytes is indicative of regulation by APC/C Cdh1 (Touati et al 2012), this remains to be formally proven.…”

“…It also appears that cyclin A2 makes a contribution to Cdk1 activation (Touati et al 2012). Note that although the effect of exogenous cyclin A2 cRNA constructs bearing D-box mutations on GVBD in mouse oocytes is indicative of regulation by APC/C Cdh1 (Touati et al 2012), this remains to be formally proven.…”

“…The cyclin-Cdk1 saga relevant to the APC/C does not end there as recent data also implicate cyclin A2, another known APC/C substrate, as being important for the G2-M transition in mouse oocytes (Touati et al 2012). Mammals express two A-type cyclins capable of binding and activating Cdk1, cyclin A1 and A2 (Satyanarayana & Kaldis 2009).…”

“…Although TPX2 and HURP are known to promote acentrosomal spindle assembly (Brunet et al 2008, Breuer et al 2010 and are potential APC/C Cdh1 substrates, in mouse oocytes, they may not be subject to direct APC/C Cdh1 regulation (Holt et al 2012). Following the establishment of kMt attachments, SAC-mediated inhibition dissipates, thereby allowing activation of APC/C Cdc20 , which could then be tasked with degrading not only cyclin B1 and securin but also cyclin A2 and cyclin B2 (Touati et al 2012, Gui & Homer 2013. It should be noted that APC/C Cdh1 -mediated regulation of APC/C Cdc20 through Cdc20 proteolysis (Reis et al 2007) is not depicted here.…”

“…The role of cyclin A2 in mouse oocytes was recently examined using two commercially available anti-cyclin A antibodies, one raised in mouse (Abcam, Cambridge, UK) and the other in rabbit (Santa Cruz Biotechnology), along with a variety of exogenous cyclin A2 cRNA constructs (Touati et al 2012). Microinjection of both the mouse and rabbit antibodies into GV-stage oocytes inhibited GVBD, an effect that could be negated using epitope-blocked antibody or by expressing exogenous cyclin A2, indicating a role for cyclin A2 at the G2-M transition (Fig.…”