Reactive oxygen species and oocyte aging: Role of superoxide, hydrogen peroxide, and hypochlorous acid

Aging of the unfertilized oocyte inevitably occurs following ovulation, limiting its fertilizable life span. However, the mechanisms that regulate oocyte aging are still unclear. We hypothesize that reactive oxygen species such as superoxide (O₂{radical dot}^-), hydrogen peroxide (H₂O₂), and hypochlorous acid (HOCl) are likely candidates that may initiate these changes in the oocyte. In order to test this hypothesis, we investigated direct effects of O₂{radical dot}^- [hypoxanthine/xanthine oxidase system generating 0.12 (n = 42) and 0.25 (n = 45) μM O₂{radical dot}^-/min], H₂O₂ (20 or 100 μM, n = 60), and HOCl, (1, 10, and 100 μM, n = 50) on freshly ovulated or relatively old mouse oocytes, while their sibling oocytes were fixed immediately or cultured under physiological conditions (n = 96). The aging process was assessed by the zona pellucida dissolution time (ZPDT), ooplasm microtubule dynamics (OMD), and cortical granule (CG) status. The ZPDT increased 2-fold in relatively old, compared to young, untreated oocytes (P < 0.0001). Exposure to O₂{radical dot}^- increased it even further (P < 0.0001). Similarly, more O₂{radical dot}^- exposed oocytes exhibited increased OMD and major CG loss, with fewer having normal OMD and intact CG compared to untreated controls. Interestingly, young oocytes resisted "aging," when exposed to 20 μM H₂O₂, while the same enhanced the aging phenomena in relatively old oocytes (P < 0.05). Exposure to even very low levels of HOCl induced the aging phenomena in young and relatively old oocytes, and higher concentrations of HOCl compromised oocyte viability. Overall, O₂{radical dot}^-, H₂O₂, and HOCl each augment oocyte aging, more so in relatively old oocytes, suggesting compromised antioxidant capacity in aging oocytes.