Dual-FRET imaging of IP3 and Ca2+ revealed Ca2+-induced IP3 production maintains long lasting Ca2+ oscillations in fertilized mouse eggs

In most species, fertilization induces Ca2+ transients in the egg. In mammals, the Ca2+ rises are triggered by phospholipase C zeta (PLC zeta) released from the sperm; IP3 generated by PLC zeta induces Ca2+ release from the intracellular Ca2+ store through IP3 receptor, termed IP3-induced Ca2+ release. Here, we developed new fluorescent IP3 sensors (IRIS-2s) with the wider dynamic range and higher sensitivity (Kd = 0.047-1.7 mu M) than that we developed previously. IRIS-2s employed green fluorescent protein and Halo-protein conjugated with the tetramethylrhodamine ligand as fluorescence resonance energy transfer (FRET) donor and acceptor, respectively. For simultaneous imaging of Ca2+ and IP3, using IRIS-2s as the IP3 sensor, we developed a new single fluorophore Ca2+ sensor protein, DYC3.60. With IRIS-2s and DYC3.60, we found that, right after fertilization, IP3 concentration ([IP3]) starts to increase before the onset of the first Ca2+ wave. [IP3] stayed at the elevated level with small peaks followed after Ca2+ spikes through Ca2+ oscillations. We detected delays in the peak of [IP3] compared to the peak of each Ca2+ spike, suggesting that Ca2+-induced regenerative IP3 production through PLC produces small [IP3] rises to maintain [IP3] over the basal level, which results in long lasting Ca2+ oscillations in fertilized eggs.