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Rac1 Temporarily Suppresses Fertilization Envelope Formation Immediately After 1-Methyladenine Stimulation
Sakurako Aida1, Takako Matsumoto1, Yuna Yamazaki1
1Department of Biological Sciences, Ochanomizu University, Tokyo 112-8610, Japan.
Abstract:
In starfish oocytes, the hormone 1-methyladenine (1-MA) induces germinal vesicle breakdown (GVBD) through a signaling cascade involving PI3K, SGK, Cdc25, and Cdk1/cyclin via G-proteinβγ subunit. Following GVBD, fertilization triggers an intracellular calcium increase, leading to the formation of the fertilization envelope (FE) via cortical granule exocytosis. While transient calcium elevations are known to occur after 1-MA stimulation even without fertilization, the inability of these calcium elevations to induce cortical granule exocytosis and FE formation remained unexplained. In this study, we found that co-treatment with 1-MA and calcium ionophore A23187 prevents FE formation, revealing a transient period termed the "no FE phase" persisting for several minutes. After no FE phase, the oocytes regain full competence to form the FE. Furthermore, we identified that the GEF/Rac1 signaling cascade is activated during the no FE phase. Notably, constitutively active Rac1 expressed in oocytes reproduces this inhibition even in the absence of 1-MA stimulation. These findings suggest that the GEF/Rac1 cascade, triggered by 1-MA, initiates the no FE phase and plays a critical role in coordinating the progression of subsequent fertilization events.
Insights
Hormone 1-methyladenine (1-MA) in starfish oocytes triggers a "no fertilization envelope (FE) phase" via the GEF/Rac1 cascade, preventing early FE formation. This phase is crucial for coordinating subsequent fertilization events.
Area of Science:
- Cellular and Molecular Biology
- Reproductive Biology
- Developmental Biology
Background:
- 1-methyladenine (1-MA) induces germinal vesicle breakdown (GVBD) in starfish oocytes, initiating a signaling cascade.
- Fertilization triggers calcium increase and cortical granule exocytosis for fertilization envelope (FE) formation.
- Transient calcium elevations after 1-MA stimulation do not induce FE formation, a phenomenon lacking explanation.
Purpose of the Study:
- To investigate the mechanism behind the inability of early calcium elevations to induce FE formation after 1-MA stimulation.
- To identify signaling pathways involved in the period following 1-MA stimulation but preceding fertilization competence.
Main Methods:
- Starfish oocytes were co-treated with 1-MA and calcium ionophore A23187.
- The timing of FE formation was monitored.
- Activation of the GEF/Rac1 signaling cascade was assessed.
- Constitutively active Rac1 was expressed in oocytes to evaluate its effect.
Main Results:
- Co-treatment with 1-MA and A23187 revealed a transient
- no FE phase
- where FE formation is inhibited.
- This
- no FE phase
- lasts for several minutes, after which oocytes regain FE formation competence.
- The GEF/Rac1 signaling cascade was found to be activated during the
- no FE phase
- .
- Constitutively active Rac1 expression mimicked the inhibitory effect of the
- no FE phase
- even without 1-MA stimulation.
Conclusions:
- 1-MA stimulation initiates a
- no FE phase
- in starfish oocytes mediated by the GEF/Rac1 signaling cascade.
- This phase temporarily inhibits cortical granule exocytosis and FE formation.
- The GEF/Rac1 cascade plays a critical role in coordinating the temporal progression of fertilization events following 1-MA-induced maturation.
