Proteasome and APC/C inhibitors synergistically suppress spontaneous oocyte activation in vitro and maintain developmental competence

In most mammals, the development of ovulated oocytes is temporarily arrested during metaphase II until fertilization. However, after retrieval, oocytes can spontaneously activate. This phenomenon is common in rats and has been reported in humans. In rats, spontaneously activated oocytes extrude a second polar body or form membrane protrusions. Spontaneous activation causes chromosomal abnormalities and impairs developmental competence; therefore, suppression of spontaneous activation is required. Here, we demonstrate that the proteasome inhibitor MG132 and the anaphase-promoting complex/cyclosome (APC/C) inhibitor apcin synergistically suppress spontaneous activation of rat oocytes in culture, preventing second polar body extrusion and membrane protrusion while maintaining the correct chromosomal alignment. Our data suggest that this suppression results from inhibition of APC/C-mediated, proteasome-dependent degradation of cyclin B1 and securin. Furthermore, this combination promotes embryonic development after intracytoplasmic sperm injection. In addition, using the non-invasive chromosome observation method, we clearly showed that abnormal chromosome alignment in oocytes treated with the inhibitor alone adversely affects embryonic development. Our findings will help advance reproductive technologies for species prone to spontaneous activation and may offer insights into future treatment strategies for spontaneous activation-induced infertility in mammals, including humans.