Regulation of cohesin and Plk1 function in mouse oocytes

Abstract

Faithful segregation of homologous chromosomes during the first meiotic division (MI) is essential for the formation of haploid gametes. Recent research in our lab and others has shown that female ageing is associated with depletion of chromosomal cohesin and is accompanied by a marked decline in the ability of oocytes to segregate chromosomes synchronously during MI. This mechanism likely underlies the dramatic increase in infertility, miscarriage and birth defects that accompany female reproductive ageing. Here, I have used a mouse model and the transgenic Rec8-myc mouse strain to investigate mechanisms regulating chromosome segregation in oocytes and to identify pathways leading to missegregation during MI. I show for the first time that Plk1 is required for stepwise removal of cohesin, which is essential for normal segregation of chromosomes during meiosis. I found that a highly specific small-molecule inhibitor of Plk1 (BI 2536) inhibited APC/C-mediated degradation of securin, thereby preventing cleavage of cohesin by separase, which in turn prevents anaphase onset during MI. By using a lower concentration of BI 2536, which was permissive for securin degradation, I unmasked a function for Plk1 in protecting centromeric cohesin during anaphase of MI. My findings indicate that loss of centromeric cohesin in the presence of BI 2536 is due to mislocalisation of the cohesin protector Sgo2. These data indicate that PLK1 kinase activity is essential for two key events required for normal segregation of chromosomes during MI. In somatic cells, Plk1 is involved in removal of cohesin by the separase-independent prophase pathway, which removes the bulk of arm cohesin during prophase/prometaphase. My findings indicate that removal of cohesin during prophase is particularly relevant to the problem of female reproductive ageing. I show that age-related depletion of chromosomal cohesin occurs during the prolonged period of prophase arrest experienced by oocytes from older ii females. I also find that the cohesin protector Sgo2 is not recruited to chromosomes until the transition from prophase to prometaphase of MI and that its recruitment is impaired in cohesin-deficient oocytes. Moreover, I present data indicating that removal of cohesin by a Plk1-mediated mechanism is unlikely to contribute to the age-related loss of cohesin during progression through prometaphase of MI. Taken together the data indicate that Plk1 is essential for stepwise removal of cohesin in oocytes, but is not likely to be a major contributor to its loss during mammalian female ageing. Overall, these findings advance our understanding of the molecular mechanisms controlling chromosome segregation during meiosis I in mammalian oocytes, and how these are influenced by female reproductive ageing.