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Published on: June 19, 2018
Incompetence of preovulatory mouse oocytes to undergo cortical granule exocytosis following induced calcium
A L Abbott1, R A Fissore, T Ducibella
1Sackler School of Biomedical Sciences, Tufts University School of Medicine and New England Medical Center, Boston, Massachusetts, 02111, USA.
Abstract:
Immature oocytes of many species are incompetent to undergo cortical granule (CG) exocytosis upon fertilization. In mouse eggs, CG exocytosis is dependent primarily on an inositol 1,4,5-trisphosphate (IP3)-mediated elevation of intracellular calcium ([Ca2+]i). While deficiencies upstream of [Ca2+]i release are known, this study examined whether downstream deficiencies also contribute to the incompetence of preovulatory mouse oocytes to release CGs. The experimental strategy was to bypass upstream deficiencies by inducing normal, fertilization-like [Ca2+]i oscillations in fully grown, germinal vesicle (GV) stage oocytes and determine if the extent of CG exocytosis was restored to levels observed in mature, metaphase II (MII)-stage eggs. Because IP3 does not stimulate a normal Ca2+ response in GV-stage oocytes, three alternate methods were used to induce oscillations: thimerosal treatment, electroporation, and sperm factor injection. Long-lasting oscillations from thimerosal treatment resulted in 64 and 10% mean CG release at the MII and GV stages, respectively (P < 0.001). Three electrical pulses induced mean [Ca2+]i elevations of approximately 730 and 650 nM in MII- and GV-stage oocytes, respectively, and 31% CG release in MII-stage eggs and 9% in GV-stage oocytes (P < 0.001). Sperm factor microinjection resulted in 86% CG release in MII-stage eggs, while similarly treated GV-stage oocytes exhibited < 1% CG release (P < 0.001). Taken together, these results demonstrate a deficiency downstream of [Ca2+]i release which is developmentally regulated in the 12 h prior to ovulation.
Insights
Immature mouse oocytes are unable to release cortical granules (CGs) due to a deficiency downstream of calcium release. This defect is developmentally regulated and corrected before ovulation, impacting fertilization competence.
Area of Science:
- Developmental Biology
- Cellular Biology
- Reproductive Science
Background:
- Cortical granule (CG) exocytosis is crucial for fertilization competence in oocytes.
- In mouse eggs, CG exocytosis relies on inositol 1,4,5-trisphosphate (IP3)-mediated intracellular calcium ([Ca2+]i) release.
- Immature oocytes often lack CG exocytosis competence, with known upstream deficiencies in calcium signaling.
Purpose of the Study:
- To investigate potential deficiencies downstream of [Ca2+]i release in immature mouse oocytes.
- To determine if inducing fertilization-like [Ca2+]i oscillations can restore CG exocytosis in germinal vesicle (GV)-stage oocytes.
- To assess the developmental regulation of CG exocytosis competence prior to ovulation.
Main Methods:
- Induced [Ca2+]i oscillations in GV-stage oocytes using thimerosal, electroporation, and sperm factor injection, bypassing upstream signaling.
- Compared CG exocytosis extent in GV-stage oocytes with mature metaphase II (MII)-stage oocytes.
- Quantified CG release percentages following induced calcium oscillations.
Main Results:
- Thimerosal treatment induced 10% CG release in GV-stage oocytes versus 64% in MII-stage oocytes.
- Electroporation resulted in 9% CG release in GV-stage oocytes compared to 31% in MII-stage oocytes.
- Sperm factor injection led to <1% CG release in GV-stage oocytes versus 86% in MII-stage oocytes.
Conclusions:
- A deficiency downstream of [Ca2+]i release contributes to the incompetence of immature mouse oocytes to undergo CG exocytosis.
- This downstream deficiency is developmentally regulated during the 12 hours preceding ovulation.
- Restoring calcium oscillations alone does not fully rescue CG exocytosis in immature oocytes, highlighting a post-calcium signaling defect.
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