Synchronous chromosome segregation in mouse oocytes is ensured by biphasic securin destruction and cyclin B1-Cdk1

Abstract

Successful cell division relies on the faithful segregation of chromosomes. 13 If chromosomes segregate prematurely the cell is at risk of aneuploidy. Alternatively, 14 if cell division is attempted in the absence of complete chromosome segregation, non-15 segregated chromosomes can become trapped within the cleavage furrow and the cell 16 can lose viability. Securin plays a key role in this process, acting as a pseudosubstrate 17 to inhibit the protease separase that functions to cleave the cohesin rings that hold 18 chromosomes together. Consequently, securin must be depleted ahead of anaphase, 19 ensuring chromosome segregation occurs in time with the anaphase trigger. Here we 20 find that MI mouse oocytes contain a large excess of securin over separase and reveal 21 the existence of a novel mechanism that functions to promote the destruction of 22 excess securin in prometaphase. Critically, this mechanism relies on key residues that 23 are only exposed when securin is not bound to separase. We suggest that the majority 24 of non-separase bound securin is removed by this mechanism, allowing for separase 25 activity to be protected until just before anaphase. In addition, we further demonstrate 26 the importance of complementary mechanisms of separase inhibition by directly 27 measuring cleavage activity in live oocytes, confirming that both securin and 28 inhibition by cyclin B1-Cdk1 are independently sufficient to prevent premature 29 separase activation. 30 31 32 33 34 timed correctly. In situations where these events become disconnected, anaphase is 66 defective 23-25 . 67 In mitosis, the synchronous loss of cyclin B1 and securin is ensured by the similarity 68 of their destruction mechanisms. Both are ubiquitylated by the Anaphase Promoting 69 Complex/Cyclosome (APC/C) in metaphase 26,27 . Critically, this ubiquitylation relies 70 on both the availability of the APC/C activator protein Cdc20 and on a short linear 71 motif known as the D-box present in the N-terminus of both securin and cyclin B1 28-72 30 . Once all chromosomes are properly attached to spindle microtubules in metaphase, 73 Cdc20 and the APC/C form a bipartite receptor for D-box docking, triggering securin 74 and cyclin B1 destruction by the proteasome 31,32 . Prior to this, Cdc20 is sequestered 75 by the spindle assembly checkpoint (SAC), a diffusible signal generated at each 76 unattached kinetochore. The SAC functions to prevent docking of D-box motifs to the 77 APC/C and thereby inhibit anaphase until all chromosomes are attached to spindle 78 microtubules 33,34 . 79In contrast, securin and cyclin B1 destruction in mouse oocyte meiosis I is initiated in 80 prometaphase, prior to full chromosome alignment 35,36 . Initially this might seem like a 81 failure to ensure accurate chromosome segregation. However, our recent work 82 demonstrates that degradation of cyclin B1 in prometaphase is in fact a key feature of 83 a mechanism that functions to prevent aneuploidy in mouse oocytes 36 . At this time 84 point, destruction represents...

Accumulation of Securin on Spindle During Female Meiosis I

“…It was shown that securin is required for separase localization to the spindle (Kumada et al, 1998;Jensen et al, 2001). Also measuring the separase activity by a FRET sensor showed that the separase is activated in oocyte meiosis I tens of minutes before anaphase (Nikalayevich et al, 2018;Karasu et al, 2019;Thomas et al, 2019), which is in accordance with the loss of the securin signal on the spindle observed in our experiments. To evaluate whether the accumulation of securin on the spindle has functional significance will require additional experimental work.…”

“…It is conceivable that there is also an excess of separase in meiosis I, and then the extra securin is required to equilibrate this. It is also possible that securin is required as a substrate of APC/C in meiosis I, as a part of APC/C activity control ( Thomas et al, 2019 ). Administration of fresh cRNA into meiosis II oocytes increases the securin levels on average three times ( Figure 2C ), indicating that the lower expression of securin in meiosis II might be due to a lack of mRNA rather than subject of specific regulation.…”

“…Similar to mitosis, the ubiquitination of securin during mammalian female meiosis I is facilitated by APC/C and initiated only after silencing of SAC ( McGuinness et al, 2009 ). In mouse oocytes, the depletion of securin is not sufficient for separase premature activation, which emphasizes the role of maturation promoting factor (MPF) in the control of separase activity in these cells ( Thomas et al, 2019 ). However, in case of separase resistance to MPF phosphorylation, the securin becomes essential and sufficient for accurate timing of separase activation.…”

A PP2A-B56—Centered View on Metaphase-to-Anaphase Transition in Mouse Oocyte Meiosis I

“…In mouse oocytes, the APC/C ubiquitylates free Cyclin B1 and Securin before targeting the proteins in complexes (Securin with Separase and Cyclin B1 with Cdk1). Also, their degradation happens prior to all kinetochores being perfectly attached and SAC proteins being completely removed [61,62]. This means that some APC/C activity towards Cyclin B1 and Securin is already present in oocytes before the SAC is silenced [63].…”

“…However, the protection of the bound fraction of Securin depends on the dephosphorylation of Securin by PP2A-B56, that protects it from the APC/C, when bound to Separase [64]. However, the preferential degradation of free Securin and Cyclin B1 in mouse oocytes is regulated in a distinct manner-this activity is independent of SAC control and phosphoregulation [62], and specific to oocytes.…”