HOW AND WHEN DO OOCYTE CHROMOSOMES FALL APART DURING FEMALE AGEING?

Abstract

Mammalian female meiosis commences in utero and is marked by reciprocal exchange of DNA between parental homologues to form bivalent chromosomes. Thereafter, oocytes remain arrested in prophase of meiosis I and are recruited for growth and ovulation throughout reproductive life. Oocytes ovulated later in life are derived from a depleted stock and are characterised by an extended period of arrest in prophase of meiosis I. Extending this beyond ∼35 years in humans is associated with an increased incidence of meiotic segregation errors giving rise to aneuploid oocytes, most of which are not compatible with life. Notable exceptions are trisomies of chromosome 21 and the sex chromosomes. Bivalent chromosomes are stabilised by cohesin complexes containing the meiosis-specific alpha-kleisin subunit, Rec8. Cleavage of Rec8 by separase results in resolution of bivalents to their four constituent chromatids. This occurs in two steps: Rec8 is first cleaved on arms during anaphase I, giving rise to dyads chromosomes, and then on centromeres during anaphase II, giving rise to single chromatids. In female mammals, anaphase I occurs shortly before ovulation while/whereas anaphase II occurs after the fertilising sperm enters the egg. Studies on human oocytes indicate a strong correlation between female ageing and premature resolution of centromeric cohesin. Consistent with this, we and others have previously reported that female ageing in mice is characterised by depletion of Rec8 from oocyte chromosomes. This is accompanied by reduced recruitment of SgoL2, which protects centromeric Rec8 from cleavage by separase until anaphase II. Together, these events are sufficient to explain the prevalence of premature loss of centromeric cohesin in oocytes from older females. We are therefore interested in understanding the timing and mechanisms of cohesin depletion during female ageing. Our recent work indicates that cohesin is gradually depleted from oocyte chromosomes during the prolonged arrest at prophase of meiosis I, before oocytes are recruited for growth. Interestingly, the age-related decline in chromosome-associated Rec8 exceeds that of Smc3, which is a subunit of mitotic as well as meiotic cohesin complexes. This suggests that cohesin complexes containing alpha-kleisin subunits other than Rec8 are present on oocyte chromosomes and might be less vulnerable to depletion during female ageing. In addition, we have tested the idea that leaky inhibition of separase during the prolonged arrest at prophase of meiosis I contributes to gradual loss of Rec8. In support of this possibility, we can detect separase in oocytes before they are recruited for growth. We find, however, that deletion of separase specifically in oocytes does not prevent the decline in Rec8 levels observed during female ageing. Together, these findings indicate that cohesin loss occurs gradually during the prolonged prophase arrest and that this occurs by a separase-independent mechanism.