Melatonin improves the fertilization ability of post-ovulatory aged mouse oocytes by stabilizing ovastacin and Juno to promote sperm binding and fusion.

STUDY QUESTION: What are the underlying mechanisms of the decline in the fertilization ability of post-ovulatory aged oocytes?

SUMMARY ANSWER: Melatonin improves the fertilization ability of post-ovulatory aged oocytes by reducing aging-induced reactive oxygen species (ROS) levels and inhibiting apoptosis and by maintaining the levels and localization of the fertilization proteins, ovastacin and Juno.

WHAT IS KNOWN ALREADY: Following ovulation, the quality of mammalian metaphase II oocytes irreversibly deteriorates over time with a concomitant loss of fertilization ability. Melatonin has been found to prevent post-ovulatory oocyte aging and extend the window for optimal fertilization in mice.

STUDY DESIGN, SIZE, DURATION: Mouse oocytes were randomly assigned to three groups and aged in vitro for 0, 6, 12 and 24 h, respectively. Increasing concentrations of melatonin (10-9 M, 10-7 M, 10-5 M and 10-3 M) were added to the 24 h aging group.

PARTICIPANTS/MATERIALS, SETTING, METHODS: Sperm binding assays, in-vitro fertilization, immunofluorescent staining and western blotting were performed to investigate key regulators and events during fertilization of post-ovulatory aged mouse oocytes.

MAIN RESULTS AND THE ROLE OF CHANCE: We found that the actin cap which promotes a cortical granule (CG) free domain is disrupted with a re-distribution of CGs in the subcortex of aged oocytes. Ovastacin, a CG metalloendoprotease, is mis-located and prematurely exocytosed in aged oocytes with subsequent cleavage of the zona pellucida protein ZP2. This disrupts the sperm recognition domain and dramatically reduces the number of sperm binding to the zona pellucida. The abundance of Juno, the sperm receptor on the oocyte membrane, also is reduced in aged oocytes. Exposure of aged oocytes to melatonin significantly elevates in-vitro fertilization rates potentially by rescuing the above age-associated defects of fertilization, and reducing ROS and inhibiting apoptosis.

LARGE SCALE DATA: N/A.

LIMITATIONS, REASONS FOR CAUTION: We explored the mechanisms of the decline in fertilization ability decline in aged mouse oocytes, in vitro but not in vivo.

WIDER IMPLICATIONS OF THE FINDINGS: Our findings may contribute to the development a more efficient method, involving melatonin, for improving IVF success rates.

STUDY FUNDING/COMPETING INTEREST(S): This study was supported by the National Natural Science Foundation (31571545) and the Natural Science Foundation of Jiangsu Province (BK20150677). The authors have no conflict of interest to disclose.