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Repetitive sperm-induced Ca2+ transients in mouse oocytes are cell cycle dependent
K T Jones1, J Carroll, J A Merriman
1Medical Research Council Experimental Embryology and Teratology Unit, St. George's Hospital Medical School, London, UK.
Abstract:
Mature mouse oocytes are arrested at metaphase of the second meiotic division. Completion of meiosis and a block to polyspermy is caused by a series of repetitive Ca2+ transients triggered by the sperm at fertilization. These Ca2+ transients have been widely reported to last for a number of hours but when, or why, they cease is not known. Here we show that Ca2+ transients cease during entry into interphase, at the time when pronuclei are forming. In fertilized oocytes arrested at metaphase using colcemid, Ca2+ transients continued for as long as measurements were made, up to 18 hours after fertilization. Therefore sperm is able to induce Ca2+ transients during metaphase but not during interphase. In addition metaphase II oocytes, but not pronuclear stage 1-cell embryos showed highly repetitive Ca2+ oscillations in response to microinjection of inositol trisphosphate. This was explored further by treating in vitro maturing oocytes at metaphase I for 4-5 hours with cycloheximide, which induced nuclear progression to interphase (nucleus formation) and subsequent re-entry to metaphase (nuclear envelope breakdown). Fertilization of cycloheximide-treated oocytes revealed that continuous Ca2+ oscillations in response to sperm were observed after nuclear envelope breakdown but not during interphase. However interphase oocytes were able to generate Ca2+ transients in response to thimerosal. This data suggests that the ability of the sperm to trigger repetitive Ca2+ transients in oocytes is modulated in a cell cycle-dependent manner.
Insights
Sperm-induced calcium (Ca2+) transients in mouse oocytes stop during interphase, not metaphase. This cell cycle-dependent regulation impacts fertilization and meiosis completion.
Area of Science:
- Reproductive Biology
- Cellular Signaling
- Developmental Biology
Background:
- Mature mouse oocytes arrest at metaphase II, with fertilization triggering calcium (Ca2+) transients essential for meiosis completion and polyspermy block.
- The duration and cessation mechanisms of these sperm-induced Ca2+ transients, lasting hours, remain poorly understood.
Purpose of the Study:
- To investigate the cell cycle-dependent regulation of sperm-induced Ca2+ transients in mouse oocytes.
- To determine when and why Ca2+ transients cease during fertilization and early embryonic development.
Main Methods:
- Utilized calcium imaging in fertilized mouse oocytes arrested at metaphase with colcemid.
- Assessed Ca2+ responses to sperm and inositol trisphosphate (IP3) in metaphase II oocytes and pronuclear embryos.
- Investigated Ca2+ signaling in cycloheximide-treated oocytes progressing through interphase and re-entering metaphase.
Main Results:
- Ca2+ transients ceased during interphase when pronuclei formed, but persisted for up to 18 hours in metaphase-arrested oocytes.
- Metaphase II oocytes, unlike interphase embryos, exhibited robust Ca2+ oscillations upon IP3 microinjection.
- Sperm-induced Ca2+ oscillations were observed after nuclear envelope breakdown but not during induced interphase in treated oocytes.
Conclusions:
- The ability of sperm to elicit repetitive Ca2+ transients in oocytes is regulated by the cell cycle stage.
- Oocyte interphase represents a refractory period for sperm-induced Ca2+ signaling, distinct from metaphase arrest.
- Thimerosal can induce Ca2+ transients in interphase oocytes, suggesting specific signaling pathways are cell cycle-modulated.