Differential distribution of inositol trisphosphate receptor isoforms in mouse oocytes

In mammalian fertilization, inositol 1,4,5-trisphosphate receptor (IP3R)-de pendent Ca2+ release is a crucial signaling event that originates from the vicinity of sperm-egg interaction and spreads as a wave throughout the egg cytoplasm. While it is known that Ca2+ is released by the type 1 IP3R in th e egg cortex, the potential involvement of other isoform types responsible for the Ca2+ rise in the mouse egg (interior) and their spatial distributio n are not known. In addition, the biochemical basis has not been definitive ly established for the development of increased sensitivity to inositol 1,4 ,5-trisphosphate (IP3) during meiotic maturation. Using specific antibodies to the type 1, 2, and 3 IP3R, we tested the hypotheses that different IP3R isoforms are responsible for the internal Ca2+ elevation and that they con tribute to the maturation-associated acquisition of IP3 sensitivity. In bot h preovulatory oocytes and ovulated eggs of CF-1 mice, immunofluorescence r evealed that types 1 and 2 isoforms were present in the cell cortex and int erior. Type 1 was observed throughout the cytoplasm, and Western analysis i ndicated a 1.9-fold maturation-associated increase. In contrast, the signal s detected for the type 2 (high-affinity) isoform and type 3 were present t o a lesser extent, with type 2 restricted to isolated islands (similar to a ggregates of vesicles detected by electron microscopy), which, in the corte x, may amplify early sperm-egg signaling events. The cortical-to-perinuclea r localization of the receptor and cortical vesicle aggregates imply an eff icient mechanism for propagating Ca2+ release from the cortex into the inte rior of the egg to activate development, and the isoform localization analy sis indicates a clear spatial and biochemical heterogeneity. Types 1 and 2 isoforms were also present in granulosa cells.