Poor embryo development in mouse oocytes aged in vitro is associated with impaired calcium homeostasis

We examined whether impairment of intracellular Ca(2+) homeostasis is related to poor embryo development in in vitro-aged oocytes. We found that in vitro aging of mouse oocytes affected the patterns of Ca(2+) oscillations at fertilization: these Ca(2+) oscillations were lower in amplitude and higher in frequency compared with oocytes without in vitro aging. We also observed that the intracellular Ca(2+) store was decreased in in vitro-aged oocytes. A decrease in the Ca(2+) store induced by thapsigargin, a specific endoplasmic reticulum (ER) membrane Ca(2+)-ATPase inhibitor, resulted in a lower fertilization rate and in poorer embryo development. The frequency of Ca(2+) oscillations was significantly increased at fertilization, whereas their amplitude was decreased in thapsigargin-treated oocytes. These results suggest that impairment of intracellular Ca(2+) homeostasis (such as a decrease in the ER Ca(2+) store) caused an alteration in Ca(2+) oscillations and the poor embryo development in in vitro-aged oocytes. Because embryo fragmentation is closely related to apoptosis, we examined expression of BAX (a proapototic protein) and BCL2 (an antiapoptotic protein) in in vitro-aged oocytes. Although BCL2 was strongly expressed in oocytes without in vitro aging, expression of BCL2 was significantly reduced in oocytes of other culture conditions and treatments such as those in in vitro aging and those that were pretreated with H(2)O(2) or thapsigargin. Acting together, alteration in Ca(2+) oscillations and decrease in BCL2 expression in in vitro-aged oocytes may lead to poor embryo development.