Protection of porcine oocytes against apoptotic cell death caused by oxidative stress during in vitro maturation: Role of cumulus cells

The present study was conducted to examine the protective effect of cumulus cells on oocyte damage caused by reactive oxygen species (ROS), generated by the hypoxanthine-xanthine oxidase (XOD) system, during in vitro maturation of porcine oocytes. Cumulus-oocyte complexes (COCs) and cumulus-denuded oocytes (DOs) were cultured for 44 h in NCSU37 supplemented with cysteine, gonadotropins, 10% porcine follicular fluid, and hypoxanthine in the presence or absence of XOD. DNA cleavage and damage were analyzed using the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) method and single cell microgel electrophoresis (comet) assay, respectively, and caspase-3 activity and glutathione (GSH) content were measured in each experimental group. Exposure of DOs to ROS resulted in meiotic arrest and the increase of degenerated oocytes. These degenerated DOs underwent apoptosis, as shown by the TUNEL-positive reaction within their germinal vesicles and the activation of caspase-3. The length of DNA migration in DOs treated with XOD was significantly longer than that of untreated DOs (P < 0.05). However, irreparable cell damage caused by ROS was not observed in COCs, and no difference was observed in the caspase-3 activity of both COCs treated with and without XOD, A significantly (P < 0.05) high level of GSH was found in COCs after 44 h of culture, compared with that of oocytes freshly isolated from their follicles, whereas GSH content in Dos markedly decreased after treatment with or without XOD. These findings suggest that cumulus cells have a critical role in protecting oocytes against oxidative stress-induced apoptosis through the enhancement of GSH content in oocytes.