Inositol 1,4,5-trisphosphate receptors are downregulated in mouse oocytes in response to sperm or adenophostin A but not to increases in intracellular Ca²⁺ or egg activation

Fertilization in mammals stimulates a series of Ca²⁺ oscillations that continue for 3-4 h. Cell-cycle-dependent changes in the ability to release Ca²⁺ are one mechanism that leads to the inhibition of Ca²⁺ transients after fertilization. The downregulation of InsP₃Rs at fertilization may be an additional mechanism for inhibiting Ca²⁺ transients. In the present study we examine the mechanism of this InsP₃R downregulation. We find that neither egg activation nor Ca²⁺ transients are necessary or sufficient for the stimulation of InsP₃R downregulation. First, parthenogenetic activation fails to stimulate downregulation. Second, downregulation persists when fertilization-induced Ca²⁺ transients and egg activation are inhibited using BAPTA. Third, downregulation can be induced in immature oocytes that do not undergo egg activation. Other than fertilization, the only stimulus that downregulated InsP₃Rs was microinjection of the potent InsP₃R agonist adenophostin A. InsP₃R downregulation was inhibited by the cysteine protease inhibitor ALLN but MG132 and lactacystin were not effective. Finally, we have injected maturing oocytes with adenophostin A and produced MII eggs depleted of InsP₃Rs. We show that sperm-induced Ca²⁺ signaling is inhibited in such InsP₃R-depleted eggs. These data show that InsP₃R binding is sufficient for downregulation and that Ca²⁺ signaling at fertilization is mediated via the InsP₃R. (C) 2000 Academic Press.