Female meiotic divisions are extremely asymmetric, producing large oocytes and small polar bodies (PBs). In mouse oocytes, the spindle relocates to the cortex before anaphase of meiosis I (MI). It is presumed that by displacing the future midzone, pre-anaphase spindle repositioning alone ensures asymmetry. But how subsequent anaphase events might contribute to asymmetric PB extrusion (PBE) is unknown. Here, we find that inactivation of cyclin-dependent kinase 1 (Cdk1) induces anaphase and simultaneously triggers cytoplasmic formin-mediated F-actin polymerisation that propels the spindle into the cortex causing it to protrude while anaphase progresses. Significantly, if post-anaphase-onset spindle migration fails, protrusion and asymmetry are severely threatened even with intact pre-anaphase migration. Conversely, post-anaphase migration can completely compensate for failed pre-anaphase migration. These data identify a cell-cycle-triggered phase of spindle displacement occurring after anaphase-onset, which, by inducing protrusion, is necessary for extreme asymmetry in mouse oocytes and uncover a pathway for maximising unequal division.

Research in the mouse model has identified a number of candidate genes and pathways that are important for chromosome segregation in this species. However, many of these have not yet been investigated in human oocytes so it is uncertain at this stage to what extent they apply to women. The challenge for the future involves applying emerging knowledge of female meiotic molecular regulation towards improving clinical fertility management.

In mitotic cells, DNA damage induces temporary G2 arrest via inhibitory Cdk1 phosphorylation. In contrast, fully grown G2-stage oocytes readily enter M phase immediately following chemical induction of DNA damage in vitro, indicating that the canonical immediate-response G2/M DNA damage response (DDR) may be deficient. Senataxin (Setx) is involved in RNA/DNA processing and maintaining genome integrity. Here we find that mouse oocytes deleted of Setx accumulate DNA damage when exposed to oxidative stress in vitro and during aging in vivo, after which, surprisingly, they undergo G2 arrest. Moreover, fully grown wild-type oocytes undergo G2 arrest after chemotherapy-induced in vitro damage if an overnight delay is imposed following damage induction. Unexpectedly, this slow-evolving DDR is not mediated by inhibitory Cdk1 phosphorylation but by APC-Cdh1–mediated proteolysis of the Cdk1 activator, cyclin B1, secondary to increased Cdc14B-dependent APC-Cdh1 activation and reduced Emi1-dependent inhibition. Thus, oocytes are unable to respond immediately to DNA damage, but instead mount a G2/M DDR that evolves slowly and involves a phosphorylation-independent proteolytic pathway.

Meiotic divisions in oocytes are extremely asymmetric and require pre-and post-anaphaseonset phases of spindle migration. The latter induces membrane protrusion that is moulded around the spindle thereby reducing cytoplasmic loss. Here, we find that depleting the NAD biosynthetic enzyme, nicotinamide phosphoribosyl-transferase (Nampt), in mouse oocytes results in markedly longer spindles and compromises asymmetry. By analysing spindle speed in live oocytes, we identify a striking and transient acceleration after anaphase-onset that is severely blunted following Nampt-depletion. Slow-moving midzones of elongated spindles induce cortical furrowing deep within the oocyte before protrusions can form, altogether resulting in larger oocyte fragments being cleaved off. Additionally, we find that Namptdepletion lowers NAD and ATP levels and that reducing NAD using small molecule Nampt inhibitors also compromises asymmetry. These data show that rapid midzone displacement is critical for extreme asymmetry by delaying furrowing to enable protrusions to form and link metabolic status to asymmetric division.