Antifertility characteristics of the N-terminal region of mouse equatorial segment protein

To investigate antifertility characteristics of the equatorial segment protein (ESP) and its potential immunocontraceptive effect, three partially overlapping cDNA fragments P1/P2/P3, together covering the entire mouse ESP, were cloned, expressed, and purified. The roles of P1/P2/P3 in fertility were investigated through in vitro fertilization and mouse mating test. Antibodies against P1/P2 significantly reduced the rates of fertilization in vitro in the zona-intact experiments. Coincubation of zona-free mouse oocytes with capacitated mouse spermatozoa in the presence of antibodies against P1/P2 also inhibited sperm-oolemma binding and fusion, while anti-P3 antibody virtually had no effect on in vitro fertilization at the same concentration. Immunization of female BALB/c mice with N-terminal of mouse ESP (recombinant P1 and P2) resulted in a significant decrease in the fertility rate as well as the litter size. Double immunofluorescence staining showed that mouse ESP protein was localized to the equatorial segment of acrosome of mouse sperm, and was exposed and surface-accessible after acrosome reaction. Mouse ESP was also demonstrated to have complementary binding sites on the mouse egg plasma membrane by indirect immunofluorescence assay. These findings suggest that the N-terminal of mouse ESP could play an important role in fertility and might be a vaccine candidate for contraception.