LS-13-P-3279 Oocyte Spindle Assembly Checkpoint in Space and Time

Mammalian oocytes and embryos frequently suffer from aneuploidy, caused by chromosome segregation errors. Most of the time, the aneuploidy originated in meiosis or in first mitotic cycles in developing embryos would prevent further development. However, in several cases, the extra chromosomes are tolerated, which leads into severe mental and developmental disorders, such as Down syndrome. The reason, why chromosome segregation errors are so frequent in germ cells and embryos, is unknown. It seems that the problem lies in less stringent control mechanisms operating in these cells. In our study we have focused on a surveillance checkpoint mechanism called Spindle Assembly Checkpoint (SAC) during female meiosis I. Using live cell imaging multichannel microscopy we have tested, whether mouse oocytes are capable of detecting univalent chromosomes and single chromatids in meiosis I. We have also monitored the activity of SAC on every single kinetochore within individual cells throughout meiosis I. These events were detected together with chromosome movements, spindle formation, Anaphase Promoting Complex (APC) activation and polar body extrusion (PBE) simultaneously in individual oocytes at various time points during first meiotic division. Our results showed that SAC in mammalian oocytes works differently, compared to the somatic cells. In contrast to the somatic cells, single chromatids, univalents and unaligned chromosomes are unable to prolong anaphase onset. Moreover, in oocytes from aged individuals, SAC proteins are displaced from individual kinetochores with different dynamics then in young oocytes. This indicates that checkpoint mechanisms operating in oocytes, which are involved in monitoring chromosome segregation, are insufficient in prevention of propagating the aneuploidy to the embryo.