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Updated: Sep 24, 2025
Chromatin Spread Preparations for the Analysis of Mouse Oocyte Progression from Prophase to Metaphase II
Published on: February 26, 2018
The oocyte spindle midzone pauses Cdk1 inactivation during fertilization to enable male pronuclear formation and
1The Christopher Chen Oocyte Biology Research Laboratory, Centre for Clinical Research, The University of Queensland, Herston, 4029 QLD, Australia.
Abstract:
Inactivation of cyclin-dependent kinase 1 (Cdk1), controlled by cyclin B1 proteolysis, orders events during mitotic exit. Here, we used a FRET biosensor to study Cdk1 activity while simultaneously monitoring anaphase II and pronuclear (PN) formation in live mouse eggs throughout fertilization. We find that Cdk1 inactivation occurs over two phases separated by a 3-h pause, the first induces anaphase II and the second induces PN formation. Although both phases require the inhibitory Cdk1 kinase Wee1B, only the first involves cyclin B1 proteolysis. Enforcing the 3-h pause is critical for providing the delay required for male PN formation and is mediated by spindle midzone-dependent sequestration of Wee1B between the first and second phases. Thus, unlike continuous Cdk1 inactivation driven by cyclin B1 proteolysis during mitotic exit, MII oocytes engineer a physiologically important pause during fertilization involving two different pathways to inactivate Cdk1, only the first of which requires proteolysis.
Insights
Cyclin-dependent kinase 1 (Cdk1) inactivation in mouse eggs occurs in two phases, separated by a 3-hour pause, crucial for fertilization events like pronuclear formation.
Area of Science:
- Cell Biology
- Reproductive Biology
- Molecular Biology
Background:
- Cyclin-dependent kinase 1 (Cdk1) inactivation, regulated by cyclin B1 proteolysis, is essential for orchestrating mitotic exit.
- Understanding the precise regulation of Cdk1 activity during oocyte maturation and fertilization is critical for reproductive success.
Purpose of the Study:
- To investigate the spatiotemporal dynamics of Cdk1 activity during fertilization in live mouse eggs.
- To elucidate the mechanisms underlying Cdk1 inactivation and its role in regulating key events such as anaphase II and pronuclear formation.
Main Methods:
- Utilized a Förster Resonance Energy Transfer (FRET) biosensor to monitor Cdk1 activity in real-time.
- Simultaneously observed anaphase II progression and pronuclear (PN) formation in live mouse oocytes during fertilization.
Main Results:
- Cdk1 inactivation proceeds in two distinct phases, separated by a significant 3-hour pause.
- The first phase of Cdk1 inactivation triggers anaphase II, while the second phase is required for pronuclear (PN) formation.
- Both inactivation phases depend on the inhibitory kinase Wee1B, but only the first involves cyclin B1 proteolysis.
- A 3-hour pause, mediated by spindle midzone-dependent Wee1B sequestration, is essential for male PN formation.
Conclusions:
- Unlike mitotic exit, MII oocytes employ a unique, two-phase Cdk1 inactivation strategy during fertilization, incorporating a critical pause.
- This regulated pause, involving Wee1B sequestration, ensures proper timing for male PN development.
- Fertilization involves distinct Cdk1 inactivation pathways, with proteolysis only required for the initial phase.
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