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Functional Manipulation of Maternal Gene Products Using In Vitro Oocyte Maturation in Zebrafish
Published on: April 22, 2017
Oocyte Maturation in Starfish
1Department of Biological Sciences, Ochanomizu University, Tokyo 112-8610, Japan.
Abstract:
Oocyte maturation is a process that occurs in the ovaries, where an immature oocyte resumes meiosis to attain competence for normal fertilization after ovulation/spawning. In starfish, the hormone 1-methyladenine binds to an unidentified receptor on the plasma membrane of oocytes, inducing a conformational change in the heterotrimeric GTP-binding protein α-subunit (Gα), so that the α-subunit binds GTP in exchange of GDP on the plasma membrane. The GTP-binding protein βγ-subunit (Gβγ) is released from Gα, and the released Gβγ activates phosphatidylinositol-3 kinase (PI3K), followed by the target of rapamycin kinase complex2 (TORC2) and 3-phosphoinositide-dependent protein kinase 1 (PDK1)-dependent phosphorylation of serum- and glucocorticoid-regulated kinase (SGK) of ovarian oocytes. Thereafter, SGK activates Na+/H+ exchanger (NHE) to increase the intracellular pH (pHi) from ~6.7 to ~6.9. Moreover, SGK phosphorylates Cdc25 and Myt1, thereby inducing the de-phosphorylation and activation of cyclin B-Cdk1, causing germinal vesicle breakdown (GVBD). Both pHi increase and GVBD are required for spindle assembly at metaphase I, followed by MI arrest at pHi 6.9 until spawning. Due to MI arrest or SGK-dependent pHi control, spawned oocytes can be fertilized normally.
Insights
Hormone-induced oocyte maturation involves signaling pathways that increase intracellular pH and trigger germinal vesicle breakdown (GVBD). This ensures proper spindle assembly and fertilization competence in spawned oocytes.
Area of Science:
- Reproductive Biology
- Cell Signaling
- Molecular Endocrinology
Background:
- Oocyte maturation is crucial for fertilization competence.
- Hormonal signals initiate meiotic resumption in immature oocytes.
- Starfish oocytes provide a model for studying oocyte maturation.
Purpose of the Study:
- To elucidate the molecular mechanisms of 1-methyladenine-induced oocyte maturation in starfish.
- To identify key signaling pathways regulating intracellular pH and meiotic progression.
- To understand the role of SGK in controlling oocyte maturation events.
Main Methods:
- Hormone stimulation (1-methyladenine) of starfish oocytes.
- Analysis of G-protein activation and downstream signaling cascades.
- Measurement of intracellular pH (pHi) and assessment of germinal vesicle breakdown (GVBD).
Main Results:
- 1-methyladenine triggers Gα-GTP formation and Gβγ release.
- Gβγ activates PI3K, TORC2, and PDK1, leading to SGK phosphorylation.
- SGK activation increases pHi and phosphorylates Cdc25/Myt1, inducing cyclin B-Cdk1 activation and GVBD.
Conclusions:
- A signaling cascade involving G-proteins, PI3K, TORC2, PDK1, and SGK mediates 1-methyladenine-induced oocyte maturation.
- Increased pHi and GVBD, regulated by SGK, are essential for meiotic arrest at metaphase I and fertilization competence.
- This pathway highlights conserved mechanisms in oocyte maturation across species.
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