Destabilization of spindle assembly checkpoint causes aneuploidy during meiosis II in murine post-ovulatory aged oocytes

Abstract

Mammalian oocyte quality degrades over time after ovulation in vitro, which can cause fatal defects such as chromosomal aneuploidy. As various oocyte manipulations employed in assisted reproductive technology are time consuming, post-ovulatory aging is a serious problem to overcome in reproductive medicine or ova research. In this study, we investigated the effects of postovulatory aging on the incidence of chromosome aneuploidy during meiosis II, with a focus on the expression of functional proteins from the spindle assembly checkpoint (SAC). Chromosome analysis was used to assess the rate of aneuploidy in in vitro aged oocytes, or in early embryos derived from aged oocytes. Immunofluorescent staining was used to detect the localization of MAD2, which is a SAC signal that monitors the correct segregation of sister chromatids. Immunoblotting was used to quantify cohesin subunits, which are adhesion factors connecting sister chromatids at the metaphase II (MII) centromere. It was shown that post-ovulatory oocyte aging inhibits MAD2 localization to the sister kinetochore. Furthermore, oocyte aging prevented cohesin subunits from being maintained or degraded at the appropriate time. These data suggest that the destabilization of SAC signaling causes sister chromatid segregation errors in MII oocytes, and consequently increases the incidence of aneuploidy in early embryos. Our findings have provided distinct evidence that the post-ovulatory aging of oocytes might also be a risk factor for aneuploidy, irrespective of maternal age.

Effects of post‐ovulatory aging on centromeric cohesin protection in murine MII oocytes

“… 10 , 11 Our previous study showed that post‐ovulatory aging also produces a high frequency of aneuploidy during meiosis II, irrespective of maternal age. 12 The spindle assembly checkpoint (SAC), which functions during cell division, is one of the checkpoint mechanisms that function during the cell cycle. SAC is a monitoring system that equally distributes chromosomes by correctly attaching spindle microtubules to the chromosome kinetochore.…”

Single‐cell transcriptome analysis of human oocyte ageing

“…Oocyte maturation depends on the regulation of proteins that determine the correct assembly of meiotic spindles and precise separation of chromosomes. Chromosomal aneuploidy is caused by chromosomal separation errors during meiosis I and II which increases in aged oocytes 37,38 . In our study, three hub genes for ‘oocyte meiosis’ and ‘cell cycle’ SKP1, CCNB1, MAD2L1 were downregulated, but ESPL1 was upregulated.…”

Melatonin improves the first cleavage of parthenogenetic embryos from vitrified–warmed mouse oocytes potentially by promoting cell cycle progression

“…It can be envisaged that mRNAs (transcripts) of these genes might be translated to maintain the spindle and chromosome morphology and led to delay of the chromosome segregation and subsequent pronuclear formation, as these two MAPK substrates are reportedly implicated in either maintaining the spindle integrity or controlling the microtubule organization of MII mouse oocytes [15]. Similarly, in previous studies it has been reported that post-ovulatory aging of mouse oocytes leads to decreased level of MAD2 (a component of spindle assembly checkpoint) transcripts [74] but increases its (MAD2) protein level [75]. It is likely that the decreased mRNA levels of Sedt2 and Ythdf2 following oocyte vitrification might affect the early embryonic zygote-genome activation process, thus resulting in the lower development of embryos to blastocysts stage, as these two proteins are implicated in either regulation of the trimethylation modification of H3K36 [26][27][28] or degradation of methylated adenine maternal mRNA [24,25].…”