Gonadotropin-mediated dynamic alterations during bovine oocyte maturation in vitro

Gonadotropins have been widely used in human-assisted reproduction and animal science for the past four decades. However, the effects of gonadotropins on oocyte maturation at the molecular and biochemical levels are poorly understood. To determine the effects of gonadotropins (recombinant follicle stimulating hormone and urinary human menopausal gonadotropin) on oocyte maturation, we used the bovine oocyte in vitro maturation model. First, we studied the effects of increasing gonadotropin concentrations on nuclear maturation and mitochondrial function in oocytes. Gonadotropins at concentrations of 0.075 and 0.75 IU/ml improved nuclear maturation and increased inner mitochondrial membrane potential and ATP levels; however, there were no beneficial effects at concentrations of 7.5 and 75 IU/ml. Second, we studied the effects of increasing gonadotropin concentrations on the status of methylation in matured (MII) oocytes. Aberrant methylation and demethylation of H19, SNRPN, and PEG3 genes were observed in MII oocytes at all concentrations except 0.075 IU/ml. The expression of genes that function in spindle formation, cell cycle control, and methylation was also downregulated by high gonadotropin concentrations. In conclusion, we established the optimal gonadotropin concentration (i.e., 0.075 IU/ml) to be used for bovine oocyte in vitro maturation studies. These results may provide a guide for clinical stimulation protocols and help to reduce the risks associated with gonadotropin administration during in vitro fertilization treatment.