The APC/C activator FZR1 coordinates the timing of meiotic resumption during prophase I arrest in mammalian oocytes

Abstract

FZR1, an activator of the anaphase-promoting complex/cyclosome (APC/C), is recognized for its roles in the mitotic cell cycle. To examine its meiotic function in females we generated an oocyte-specific knockout of the Fzr1 gene (Fzr1Δ/Δ). The total number of fully grown oocytes enclosed in cumulus complexes was 35-40% lower in oocytes from Fzr1Δ/Δ mice and there was a commensurate rise in denuded, meiotically advanced and/or fragmented oocytes. The ability of Fzr1Δ/Δ oocytes to remain prophase I/germinal vesicle (GV) arrested in vitro was also compromised, despite the addition of the phosphodiesterase milrinone. Meiotic competency of smaller diameter oocytes was also accelerated by Fzr1 loss. Cyclin B1 levels were elevated ~5-fold in Fzr1Δ/Δ oocytes, whereas securin and CDC25B, two other APC/CFZR1 substrates, were unchanged. Cyclin B1 overexpression can mimic the effects of Fzr1 loss on GV arrest and here we show that cyclin B1 knockdown in Fzr1Δ/Δ oocytes affects the timing of meiotic resumption. Therefore, the effects of Fzr1 loss are mediated, at least in part, by raised cyclin B1. Thus, APC/CFZR1 activity is required to repress cyclin B1 levels in oocytes during prophase I arrest in the ovary, thereby maintaining meiotic quiescence until hormonal cues trigger resumption.

HECT-E3 ligase ETC-1 regulates securin and cyclin B1 cytoplasmic abundance to promote timely anaphase during meiosis inC. elegans

“…Reduction of activity of ETC-1 and APC/C also caused the premature NEBD phenotype with low-penetrance during prophase I. In mammalian oocytes, APC/C activity is required to maintain low cyclin B1 levels during prophase I arrest (Reis et al, 2007;Holt et al, 2011). CYB-1-dependent phosphorylation of the nuclear pore component NPP-12 (Gp210) is required for timely NEBD during mitosis (Galy et al, 2008).…”

The APC/C in female mammalian meiosis I

“…Significantly, even within the inhibitory cAMP-rich follicular environment within the ovary, Fzr1 D/D oocytes exhibited an increased tendency to undergo GVBD that was related to elevated cyclin B1 levels. Importantly, in Fzr1 D/D oocytes, there was no change in the levels of Cdc25B, a potential APC/C Cdh1 substrate (Holt et al 2011), altogether underlining the importance of the APC/ C-cyclin B1 pathway for regulating G2/prophase arrest independent of Cdk1 phosphorylation status. Interestingly, even though many aspects of follicular physiology remained intact in Fzr1 D/D mutant mice, there was nevertheless a 35-40% reduction in the numbers of cumulus-enclosed oocytes (Holt et al 2011), suggesting that defects in oocyte-centred control brought about through APC/C de-regulation were detrimental to overall follicular health and integrity.…”

“…It was found that such Cdh1-depleted oocytes exhibited an increased susceptibility to undergoing GVBD even under conditions in which cAMP levels were chemically sustained using the phosphodiesterase inhibitor, milrinone. More recently, this mechanism has R62 H Homer been shown to be physiologically relevant for sustaining the reservoir of G2/prophase-arrested oocytes using an in vivo mouse model in which Fzr1 was specifically deleted from growing and mature oocytes using Cre-LoxP technology (Fzr1 D/D mice; Holt et al 2011). Significantly, even within the inhibitory cAMP-rich follicular environment within the ovary, Fzr1 D/D oocytes exhibited an increased tendency to undergo GVBD that was related to elevated cyclin B1 levels.…”

Reduced ability to recover from spindle disruption and loss of kinetochore spindle assembly checkpoint proteins in oocytes from aged mice

“…Oocytes were handled in M2 media containing 2.5 µM milrinone (Sigma-Aldrich) under mineral oil at 37 °C, and maturation was stimulated by milrinone washout, timed relative to extrusion of the first polar body. 70 Microinjections were performed on the heated stage of a Nikon TE300 inverted microscope as previously described.…”