The meiosis I-to-meiosis II transition in mouse oocytes requires separase activity

Faithful segregation of homologous chromosomes during the first meiotic division is essential for further embryo development. The question at issue is whether the same mechanisms ensuring correct separation of sister chromatids in mitosis are at work during the first meiotic division. In mitosis, sister chromatids are linked by a cohesin complex holding them together until their disjunction at anaphase [1-6]. Their disjunction is mediated by Separase, which cleaves the cohesin [7, 8]. The activation of Separase requires prior degradation of its associated inhibitor, called securin [9, 10]. Securin is a target of the APC/C (Anaphase Promoting Complex/Cyclosome), a cell cycle-regulated ubiquitin ligase that ubiquitinates securin at the metaphase-to-anaphase transition and thereby targets it for degradation by the 26S proteasome [11-14]. After securin degradation, Separase cleaves the cohesins and triggers chromatid separation, a prerequisite for anaphase. In yeast and worms, the segregation of homologous chromosomes in meiosis I depends on the APC/C and Separase activity. Yet, it is unclear if Separase is required for the first meiotic division in vertebrates because APC/C activity is thought to be dispensable in frog oocytes. We therefore investigated if Separase activity is required for correct chromosome segregation in meiosis I in mouse oocytes.