A defective redox scavenger system negatively affects the in vitro embryo development of cow oocytes

BACKGROUND-AIM Mitochondria produce most of the ATP necessary for the cell via oxidative phosphorylation, and despite being constantly exposed to the generation of oxidant species, they remain functional thanks to the mitochondrial antioxidant defense system, comprising glutathione (GSH), glutaredoxin, and thioredoxin. Since mitochondrial dysfunctions are associated with low fertility, we carried preliminary investigations of mitochondria amount, activity and distribution, and redox buffering systems in bovine oocytes with low developmental competence. METHODS Cumulus–oocyte complexes (COCs) were retrieved from 2-8 mm follicles of abattoir-derived bovine ovaries with ≤10 (low developmental competence) or > 10 middle antral follicles (control) and processed either immediately as immature (GV) or upon in vitro maturation as metaphase II (MII). Total and active mitochondria were stained using MitoTracker Green and Orange, respectively, and GSH concentration was assessed using a spectrophotometric assay. Upon manipulation of the GSH content, some oocytes were in vitro fertilized and cultured for 8 days. Data were analyzed by Fisher’s exact test, Mann–Whitney test, T-test, or one-way ANOVA. RESULTS Oocytes with low developmental competence failed to allocate the mitochondria to the cortex and in accumulate redox scavengers during oocyte maturation. Notably, blastocyst formation significantly improved when the GSH content was experimentally increased in low developmental competence oocytes. Conversely, the ovarian type did not affect mitochondrial mass and activity, while we observed a general decrease in mitochondrial activity at the MII stage. CONCLUSIONS These preliminary findings indicate that defective redox scavengers negatively affect in vitro embryo development of cow oocytes, seemingly without perturbing mitochondrial amount and activity. Furthermore, in agreement with previous reports of polysomal-bound mRNAs, showing that energy production and oxidative phosphorylation are among the most down-regulated cellular functions in bovine MII oocytes, these findings seem to convey that ATP generation is not the primary function of mitochondria at this stage. Whether anomalies in mitochondria localization may impact embryo development remains to be assessed.