Incorporation of Coenzyme Q10 into Bovine Oocytes Improves Mitochondrial Features and Alleviates Effects of Summer Thermal Stress on Developmental Competence.

Mitochondrial function plays a key role throughout oogenesis and embryonic development. Environmental stress-induced alterations in oocyte mitochondria are suggested to deleteriously affect developmental competence of the ovarian pool of oocytes. The study examined seasonal effects on oocyte mitochondrial distribution, activity, mtDNA content and RNA expression, and whether incorporation of coenzyme Q10 (CoQ10), a component of the electron transport chain reaction, improves oocyte developmental competence. Bovine oocytes were collected during the summer (Jun–Aug), fall (Sep–Nov) and winter (Dec–May), in-vitro matured with or without 50 μM CoQ10, fertilized, and cultured for 8 days. Matured oocytes were classified according to mitochondrial distribution pattern (Mitotracker green) into four (I–IV) categories and examined for mitochondrial activity (JC-1 staining). The ratio between active and inactive mitochondria in the oocyte differed between categories (P<0.05), but was not affected by season or CoQ10 incorporation. In contrast, oocyte distribution into categories differed between seasons (P<0.05) and was affected by CoQ10, with an increased proportion of category I (i.e. putative MII-stage) oocytes in the fall. Oocyte mtDNA did not differ between seasons, but RNA expression of mitochondrial-associated genes ND2, SDHD, CYTB, COX3, B5ATP, TFAM and GAPDH was affected by seasons. For instance, ND2 and SDHD RNA expression increased while that of CYTB decreased in the fall compared to winter (P<0.05). Moreover, a season by treatment interaction was evident as CoQ10 increased the RNA expression of CYTB, COX3 and B5ATP only in the fall (P<0.05). In addition, the proportion of fertilized and cleaved oocytes did not differ between seasons, but blastocyst developmental rate was higher in the winter than in the summer, with an intermediate rate in the fall (P<0.05). CoQ10 incorporation did not affect blastocyst formation during the summer or winter, but it increased the proportion of developed blastocysts in the fall (P<0.05) to a level similar to that in winter. In summary, season-induced alterations in mitochondrial functions might explain, in part, reduced oocyte developmental competence upon environmental thermal stress. It seems that when the harm is moderate (i.e. in the fall), CoQ10 incorporation can alleviate, to a certain extent, these deleterious effects.