Timing and regulation of cohesion depletion during mammalian oogenesis

Abstract

Sexual reproduction depends on the transmission of exactly one copy of each chromosome by the maternal and paternal gametes. This is accomplished during meiosis when diploid progenitors undergo two consecutive rounds of chromosome segregation following a single round of DNA replication. In most organisms, this relies on the establishment of bivalent chromosomes consisting of replicated parental homologues physically linked at sites of meiotic recombination. In female mammals, bivalents are formed during fetal development when the lifetime stock of primordial-stage oocytes is established. However, they are not resolved until shortly before ovulation. Extending this period beyond ~35 years results in a dramatic increase in embryo aneuploidy. Depletion of the lifetime stock of oocytes during ageing culminates in menopause. Our previous studies indicate that Rec8-containing cohesin complexes also become depleted from oocyte chromosomes during female ageing. Consistent with cohesin’s role in maintaining chromosome structure, depletion of Rec8 is associated with destabilisation of bivalents chromosomes. Nevertheless, the mechanisms and timing of cohesin depletion remain unknown. Here, I investigated the possibility that age-related cohesin depletion is a consequence of leaky inhibition of the protease separase, which cleaves Rec8 during anaphase. I found that oocyte-specific deletion of separase did not prevent depletion of oocyte cohesin during female ageing. I, therefore, conclude that age-related depletion occurs by a separase-independent mechanism. I next investigated the timing, during oogenesis, at which cohesin loss occurs. I found that cohesin is predominantly lost at the primordial stage before oocytes are recruited for growth. In addition, using an oocyte-specific deletion of Pten, I determined that this occurs independently of the decline in the ovarian stock of primordial-stage oocytes. Together, these results indicate that age-related cohesin depletion occurs at the primordial stage by a separase-independent mechanism. Other possible of mechanisms of cohesin depletion include protein damage and/or age-related deterioration of chromatin structure. From a clinical perspective, my work suggests that “rejuvenation’ of fertility by activating the residual pool of primordial oocytes is unlikely to be successful in older women.