Functionality of the spindle checkpoint during the first meiotic division of mammalian oocytes.

The spindle checkpoint ensures accurate chromosome segregation by delaying anaphase until all chromosomes are correctly aligned on the microtubule spindle. Although this mechanism is conserved throughout eukaryotic evolution, it is unclear whether it operates during meiosis in female mammals. The results of the present study show that in mouse oocytes spindle alterations prevent both chromosome segregation and MPF (M phase promoting factor) inactivation during the first meiotic M phase. Moreover, the spindle checkpoint component budding uninhibited by benzimidazole 1 (BUB1) localizes to kinetochores and is phosphorylated until anaphase of both meiotic M phases. Both localization and phosphorylation are similar to those observed in oocytes at microtubule depolymerization. In addition, the kinetochore localization and phosphorylation of BUB1 do not depend on the MOS/.../MAPK pathway. These data indicate that the spindle checkpoint is probably active during meiotic maturation in mouse oocytes. BUB1 remains associated with kinetochores and is phosphorylated during the metaphase arrest of the second meiotic M phase, indicating that this protein may also play a role in the natural metaphase II arrest in mammalian oocytes.