Increased expression of ERp57 in rat oocytes during meiotic maturation is associated with sperm–egg fusion

Oocyte meiotic maturation is a developmental transition that starts during germinal‐vesicle breakdown and ends at the arrest in metaphase of meiosis II. This transition is associated with changes to both the proteins that are synthesized and the abundance/distribution of post‐translational modifications that are crucial for subsequent fertilization and embryogenesis. Here, we isolated and cultured rat oocytes in vitro during both metaphase of meiosis I (MI) and meiosis II (MII) stages, respectively, and then compared their proteomic profiles by high‐resolution, two‐dimensional gel electrophoresis (2DE) followed by mass spectrometry. We found that the expression of five proteins was up‐regulated while six proteins were down‐regulated when comparing MI to MII oocytes. The expression of ERp57, an endoplasmic reticulum chaperone, underwent a dramatic increase between MI and MII oocytes, and became concentrated in a dome‐shaped area of the cell surface within the microvillar region. A similar profile was observed during spermatogenesis, and sperm ERp57 eventually localized to the head and flagellum surfaces, finally ending in the equatorial region of acrosome‐reacted sperm. Given the localization pattern, we tested and found that a polyclonal antiserum created against recombinant rat ERp57 significantly inhibited spermatozoa from penetrating zona pellucida‐free oocytes without affecting either sperm motility or the acrosome reaction. These results indicate that ERp57 expression on oocytes, and possibly sperm, plays an important physiological role during sperm–egg fusion. Mol. Reprod. Dev. 81: 315–325, 2014. © 2014 Wiley Periodicals, Inc.