First meiotic anaphase requires Cep55-dependent inhibitory cyclin-dependent kinase 1 phosphorylation

Area of Science:

• Cell Biology
• Molecular Biology
• Reproductive Biology

Background:

• Anaphase onset relies on anaphase-promoting complex (APC)-driven degradation of securin and cyclin B1, inactivating cyclin-dependent kinase 1 (Cdk1).
• The spindle assembly checkpoint (SAC) regulates APC activity, controlling anaphase timing and preventing chromosome mis-segregation.
• Mammalian oocytes exhibit aneuploidy, but the underlying mechanisms remain unclear.

Purpose of the Study:

• To investigate the role of Cep55 in regulating anaphase I progression in mouse oocytes.
• To elucidate the mechanisms by which Cep55 influences Cdk1 inactivation and anaphase initiation.
• To understand the SAC's control over anaphase I in oocytes and identify potential sources of error.

Main Methods:

• Depletion of Cep55 in mouse oocytes.
• Analysis of meiotic progression, Cdk1 activity, and phosphorylation status.
• Assessment of SAC silencing and APC-mediated proteolysis.
• Evaluation of Cdc25 phosphatase activity.

Main Results:

• Cep55-depleted oocytes initiated anaphase I normally but failed to progress due to persistent Cdk1 activity.
• Cdk1 inactivation was impaired in Cep55-depleted oocytes, despite timely SAC silencing and functional APC.
• This impairment resulted from insufficient inhibitory Cdk1 phosphorylation due to unsuppressed Cdc25 phosphatase activity.
• A proteolysis-independent mechanism involving Cdc25 regulation is crucial for anaphase I initiation.

Conclusions:

• Cep55 is essential for Cdk1 inactivation and anaphase I progression in mouse oocytes.
• Oocyte anaphase I initiation is not solely controlled by the SAC, highlighting a proteolysis-independent regulatory step.
• These findings offer new insights into the mechanisms underlying aneuploidy vulnerability in mammalian oocytes.