Expression of genes encoding antioxidant enzymes in human and mouse oocytes during the final stages of maturation

The mRNA expression of five enzymes: catalase, Cu-Zn-superoxide dismutase ( Cu-Zn-SOD), Mn-superoxide dismutase (Mn-SOD), glutathione peroxidase (GPX), and gamma-glutamylcysteine synthetase (GCS) each involved in protection ag ainst free radicals was studied in human and mouse oocytes. In the mouse, o ocytes were collected at different stages of maturation in order to determi ne the storage of these transcripts. For the human, germinal vesicle (GV) o ocytes harvested during intracytoplasmic sperm injection (ICSI) procedures and failed fertilized metaphase II (MII) oocytes were analysed. Human and m ouse were compared in order to determine whether the differential developme ntal capacity of mouse and human preimplantation embryos in culture could b e explained by the variations in the patterns of expression for these enzym es, mRNA expression for these enzymes was examined using reverse transcript ion-polymerase chain reaction (RT-PCR). In the mouse, all transcripts (exce pt for catalase) were detected, whatever the maturation stage. No qualitati ve differences were detected between GV and MII oocytes. In human, all the enzymes (except for catalase) were expressed in MII oocytes and Cu-Zn-SOD w as particularly highly expressed. Transcripts corresponding to GPX and Mn-S OD were not detected at GV stage but only at Mil stage, suggesting that sto rage could occur between GV and MII stages. However, using 3' end-specific primers for GPX and Mn-SOD, instead of the oligo(dT)(12-18) primer, for the reverse transcription reaction, the transcripts for these antioxidants enz ymes have been detected in human oocytes at the GV stage. This suggests the presence of maturation-specific polyadenylation of these transcripts. Thes e enzymes can be considered as markers of cytoplasmic maturation.