Accumulation of Electrophilic Aldehydes During Postovulatory Aging of Mouse Oocytes Causes Reduced Fertility, Oxidative Stress, and Apoptosis

With increasing periods of time following ovulation, the metaphase II (MII)-stage oocyte experiences overproduction of reactive oxygen species and elevated levels of lipid peroxidation that are implicitly linked with functional deficiencies acquired during postovulatory oocyte aging. We have demonstrated that the electrophilic aldehydes 4-hydroxynonenal (4HNE), malondialdehyde, and acrolein are by-products of nonenzymatic lipid peroxidation in the murine MII-stage oocyte, adducting to multiple proteins within the cell. The covalent modification of oocyte proteins by these aldehydes increased with extended periods of time postovulation; the mitochondrial protein succinate dehydrogenase (SDHA) was identified as a primary target for 4HNE adduction. Time-and dose-dependent studies revealed that exposure to elevated levels of electrophilic aldehydes causes mitochondrial reactive oxygen species production, lipid peroxidation, loss of mitochondrial membrane potential, and eventual apoptosis within the MII oocyte, presumably as a consequence of electron transport chain collapse following SDHA adduction. Additionally, we have determined that short-term exposure to low doses of 4HNE dramatically impairs the oocyte's ability to participate in fertilization and support embryonic development; however, this loss of functionality can be prevented by supplementation with the antioxidant penicillamine. In conclusion, this study has revealed that the accumulation of electrophilic aldehydes is linked to postovulatory oocyte aging, causing reduced fertility, oxidative stress, and apoptosis of this highly specialized cell. These data highlight the importance of timely fertilization of the mammalian oocyte postovulation and emphasize the potential advantages associated with antioxidant supplementation of oocyte culture medium in circumstances where reinsemination of oocytes may be desirable (i.e., rescue intracytoplasmic sperm injection), or where in vitro fertilization may be delayed.