47 Inhibition of endoplasmic reticulum stress during in vitro maturation improves the developmental competence of bovine cumulus-oocyte complexes

Abstract

Endoplasmic reticulum (ER) stress, a dysfunction in protein-folding capacity of ER, is involved in many physiological responses including embryonic development. Evidence shows that the ER stress-induced unfolded protein response signaling pathway is associated with impairment of oocyte maturation and pre-implantation embryonic development; supplementation of culture medium with tauroursodeoxycholic acid (TUDCA), an ER stress inhibitor, improved the developmental process of oocytes and embryos by attenuating ER stress. However, no reports are available on the role of TUDCA in reducing ER stress during IVM of bovine oocytes. Therefore, the aim of this study was to examine the mechanism of TUDCA on reducing ER stress in maturation of bovine cumulus-oocyte complexes (COCs) and whether inhibition of ER stress during maturation can promote subsequent embryonic development. Bovine ovaries were collected from a local slaughterhouse, and after aspiration COCs were matured with/without TUDCA (50, 100, and 200 µM) for 22 h at 38.5°C in a humidified atmosphere of 5% CO₂. After IVM, we examined the maturation rate, reactive oxygen species, apoptosis, protein/mRNA expression levels, and subsequent embryonic development after IVF. The data were analysed using analysis of variance followed by the Tukey-Kramer multiple comparison test. As a result, the dose-dependent experiment shows that a 100 μM concentration of TUDCA significantly increased the maturation rate and decreased the percentage of apoptotic cells in COCs and reactive oxygen species levels in denuded oocytes. Subsequently, the expression of ER stress inducible protein GRP78/BIP significantly decreased in COCs treated with 100 µM TUDCA compared with the control COCs. In addition, the mRNA expression of ER stress and pro-apoptotic markers (GRP78/BIP, PERK, IER1, ATF4, XBP1, CHOP, and BAX) in COCs were significantly decreased by TUDCA (100 µM) treatment, whereas it increased anti-apoptotic BCL2 expression. Moreover, we show that TUDCA (100 µM) supplementation enhances embryonic development by significantly increasing the blastocyst formation rate (43.6 ± 1.8% vs. 49.7 ± 1.3%) and decreasing the number of apoptotic cells (7.7 ± 1.1% vs. 5.03 ± 0.6%) in blastocysts. These findings suggest that existence of ER stress during maturation alters the developmental competence of bovine COCs. Therefore, for the first time, we demonstrate that application of TUDCA during IVM plays a crucial role in reducing ER stress and improves the meiotic maturation, oocyte quality, and subsequent embryonic development in vitro.