Repetitive sperm-induced Ca2+ transients in mouse oocytes are cell cycle dependent

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.