202 Endoplasmic reticulum stress inhibitor supports in vitro growth and maturation of bovine oocytes

In vitro grown oocytes are highly susceptible to stresses causing poor developmental competence. Accumulating evidence indicates that endoplasmic reticulum (ER) stress interferes with developmental processes in oocyte maturation and embryo development. In vitro growth (IVG) is associated with low developmental competence and ER stress may play a role. Therefore, this study aims to investigate the effect of tauroursodeoxycholic acid (TUDCA), an ER stress inhibitor, on in vitro growth of bovine oocytes. Oocyte granulosa cell complexes (OGCs) were collected from early antral follicles (1.5–1.8 mm) and allowed to grow in growth culture medium for 5 days at 38.5°C in a humidified atmosphere at 5% CO₂ in the air. The OGCs from antral follicles (4–6 mm) were allowed to IVM and IVF and served as in vivo groups. Basic growth culture medium was α-MEM supplemented with polyvinylpyrrolidone (4% w/v), hypoxanthine (4 mM), fetal bovine serum (5%; v/v), dexamethasone (0.05 µM), IBMX (50 µM), 17β-oestradiol (100 ng mL⁻¹), androstenedione (10 ng mL⁻¹), and gentamicin (10 µg mL⁻¹). TUDCA (T) was added to the basic growth culture medium at various concentrations (0, 50, 100, 250, and 500 µM). Oocyte diameter was measured before growth (Day 0) and after growth (Day 5). Oocyte survivability and antrum formation was recorded during growth. Oxidative stress was assessed by intracellular reactive oxygen species (ROS) at Day 5 of IVG. Surviving oocytes were allowed to IVM for 22 h and the maturation rate was assessed by aceto-orcein staining. The IVG grown oocytes (control and T 100 µM) were allowed to IVF after IVM to assess the developmental competences. Data were analysed using R studio and subjected to one-way analysis of variance followed by Tukey’s HSD test. After in vitro growth, oocyte diameters were significantly increased from initial diameter (at Day 0) in all groups (P < 0.05). T 50 µM showed higher survivability than T 500 µM (P < 0.05). At Day 5, a higher number of oocytes showed antrum formation in the T 100 µM group than T 500 µM (P < 0.05). Relative ROS levels were lower in the TUDCA group than control (P < 0.05). After IVM of in vitro grown oocytes, T 100 µM showed higher maturation rate (MII) than control (P < 0.05). In the cleavage and blastocyst rate, no significant changes were observed between control and TUDCA group, whereas in vivo grown oocytes showed higher cleavage and blastocyst rate than in vitro (P < 0.05). Total cell number and TUNEL positive cells in blastocysts did not differ significantly in in vitro and in vivo grown oocytes. TUDCA at higher concentrations (500 µM) in growth culture is detrimental, whereas 100 µM is found to be beneficial to improve the growth environment. These results indicate that ER stress could impair IVG and maturation rate of bovine oocytes and these detrimental effects could be alleviated by TUDCA.