Measurement of intracellular IP3 during Ca2+ oscillations in mouse eggs with GFP-based FRET probe

Intracellular Ca2+ oscillations in fertilized mammalian eggs, the key signal that stimulates egg activation and early embryonic development, are regulated by inositol 1,4,5-trisphosphate (IP3) signaling pathway. We investigated temporal changes in intracellular IP3 concentration ([IP3]i) in mouse eggs, using a fluorescent probe based on fluorescence resonance energy transfer between two green fluorescent protein variants, during Ca2+ oscillations induced by fertilization or expression of phospholipase Czeta (PLCzeta), an egg-activating sperm factor candidate. Fluorescence measurements suggested the elevation of [IP3]i in fertilized eggs, and the enhancement of PLCzeta-mediated IP3 production by cytoplasmic Ca2+ was observed during Ca2+ oscillations or in response to CaCl2 microinjection. The results supported the view that PLCzeta is the sperm factor to stimulate IP3 pathway, and suggested that high Ca2+ sensitivity of PLCzeta activity and positive feedback from released Ca2+ are important for triggering and maintaining Ca2+ oscillations.