Down-regulation of the inositol 1,4,5-trisphosphate receptor in mouse eggsfollowing fertilization or parthenogenetic activation

Fertilization in mammalian eggs is characterized by the presence of intrace llular calcium ([Ca2+]i) oscillations. In mouse eggs, these oscillations ce ase after a variable period of time and this is accompanied by a decrease i n inositol 1,4,5-trisphosphate receptor (IP3R) responsiveness and down-regu lation of the IP3R type 1 (IP3R-1). To investigate the signaling pathway re sponsible for inducing IP3R-1 down-regulation during fertilization, mouse e ggs were exposed to or injected with several Ca2+-releasing agonists and th e amounts of IP3R-1 immunoreactivity evaluated by Western blotting. Exposur e to ethanol or ionomycin, which induce a single [Ca2+]i rise, failed to si gnal down-regulation of IP3R-1. However, [Ca2+]i oscillations induced by in jection of boar sperm fractions (SF), which presumably stimulate production of IP3, or adenophostin A, an IP3R agonist, both induced down-regulation o f IP3R-1 of a magnitude similar to or greater than that observed after fert ilization. Exposure to thimerosal, an oxidizing agent that modifies the IP3 R without stimulating production of IP3, also initiated down-regulation of IP3R-1, although oscillations initiated by SrCl2 failed to evoke down-regul ation of IP3R-1. The degradation of IP3R-1 in mouse eggs appears to be medi ated by the proteasome pathway because it was inhibited by preincubation wi th lactacystin, a very specific proteasome inhibitor. We therefore suggest that persistent stimulation of the phosphoinositide pathway in mouse eggs b y the sperm during fertilization or by injection of SF leads to down-regula tion of the IP3R-1. (C) 2000 Academic Press.