Expression of inositol 1,4,5-trisphosphate receptors in mouse oocytes and early embryos: The type I isoform is upregulated in oocytes and downregulated after fertilization

A fertilization-induced increase in intracellular Ca2+ is responsible for initiating all of the events of egg activation. In mammals, the Ca2+ increase takes the form of a series of Ca2+ oscillations showing complex temporal and spatial properties. To understand the nature of these changes, we have investigated the expression patterns of the three isoforms of the inositol trisphosphate receptor (InsP(3)R) during oocyte maturation and preimplantation development. We find that mouse oocytes express mRNAs for all three InsP(3)R subtypes. Semiquantitative ratio reverse-transcriptase polymerase chain reaction shows that the type II isoform is the predominant message in mature oocytes, representing 67% of the InsP(3)R mRNA. In contrast, protein analysis reveals that the type I isoform accounts for all of the detectable InsP(3)R protein, despite representing only 20% of the InsP(3)R mRNA. The levels of InsP(3)R protein were examined to determine whether they correlated with the Ca2+ signaling events surrounding the fertilization process. Type I InsP(3)R protein increased during oocyte maturation and, in addition, within 8 h of fertilization underwent a dramatic decrease. During development to the blastocyst the level of type I InsP(3)R protein did not return to prefertilization levels and types II and III remained below our detection limit. The decrease in InsP(3)R protein after fertilization was found to correlate with a decrease in the sensitivity of InsP(3)-induced Ca2+ release. These studies show that the expression of InsP(3)R mRNA is developmentally regulated, that Ca2+ signaling at fertilization is mediated exclusively through the type I InsP(3)R, and that the InsP(3)R is downregulated after fertilization. (C) 1998 Academic Press.