Melatonin maintains mitochondrial membrane potential and decreases excessive intracellular Ca2+ levels in immature human oocytes

Abstract Aims Previous reports have demonstrated that melatonin exists in multiple extrapineal sites, and higher amounts of melatonin are present in human follicular fluid than in serum, which indicates that it might play key roles in human oocyte maturation and subsequent embryonic development. Melatonin has been shown to be a potent antioxidant and might be beneficial to human oocytes during in vitro maturation (IVM). However, the underlying mechanisms of melatonin action during IVM have not been thoroughly investigated. Main methods Immunofluorescence staining, western blotting, and ELISA were applied to investigate whether melatoninergic components are expressed in the cultured human ovarian cumulus cells. TMRE staining and Fluo-4 AM staining were performed to detect the mitochondrial membrane potential and intracellular Ca2+ levels of immature human oocytes respectively. Key findings First, cultured human ovary cumulus cells synthesized melatonin in vitro, and it expressed serotonin (the precursor of melatonin) and the two key enzymes, i.e. N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT). Additionally, the results suggest that melatonin maintains the mitochondrial membrane potential and decrease excessive Ca2+ levels in immature human oocytes during IVM. Significance In conclusion, we provide evidence that the melatoninergic components were expressed in cultured human ovarian cumulus cells, and melatonin might reduce oxidative stress of human oocytes by ameliorating mitochondrial function. In view of the significant clinical value that immature human oocytes have in assisted reproductive technology (ART), our findings highlight a potential treatment strategy of using melatonin to improve mitochondrial function and to enhance the quality of human oocytes during IVM.