94 Oleic acid dose-dependently compensates for the negative effect of saturated stearic acid on early bovine embryos

Previous studies in our group showed a dose-dependent negative impact of saturated stearic acid (SA) on maturing cumulus–oocyte complexes (COCs), while monounsaturated oleic acid (OA) was harmless. Cumulus cells protect the oocyte against SA with stearoyl-CoA-desaturase 1 (SCD1) by converting SA into OA. As cumulus cell protection is lost after ovulation, this study investigated the effect of OA and SA on early embryonic development and determined if SCD1 is expressed in embryos. COCs, collected from 2- to 8-mm follicles of bovine slaughterhouse ovaries, were 23 h in vitro matured and fertilized (n = 50/group). Embryo culture conditions included (1) culture in synthetic-oviductal-fluid (SOF) without (control) or with 25 or 50 µM SA or 25 or 50 µM OA during the first 5 days, followed by standard SOF from Days 5 to 8 (n = 8); and (2) culture from Day 5 to Day 8 in SOF without (control) or with 25 or 50 µM SA or 25 or 50 µM OA, preceded by standard SOF during the first 5 days (n = 4). The number of blastocysts was scored on Day 8. SCD1 mRNA expression in oocytes, zygotes, and embryos at Days 5 and 8 was analyzed by RT-qPCR. The SCD1 protein expression in Day 8 blastocysts, COCs (positive control), and oocytes (negative control) was detected by confocal microscopy after immunostaining with a primary SCD1 antibody and a second goat anti-rabbit Alexa^TM Fluor 647 antibody. Statistical analysis was performed using a binomial logistic regression model in RStudio version 4.2.3. Exposure to 25 or 50 µM SA during the first 5 days of culture resulted in lower blastocyst rates of 18.9 ± 1.6% and 2.6 ± 4.6%, compared with the control at 28.7 ± 6.3% (P < 0.05). Interestingly, after exposure to 50 µM OA, blastocyst rates were higher (34.6 ± 7.8%) than in the control (P < 0.05). Adding equimolar levels of OA to 25 µM SA or 50 µM SA fully compensated for the negative effect of SA, resulting in blastocyst rates of 26.0 ± 3.7% and 27.8 ± 6.4%, respectively. Exposure to 25 or 50 µM SA from Day 5 until Day 8 resulted, in comparison with Days 1 to 5 exposure, in a milder impact on embryo development, with a blastocyst rate of 18.0 ± 4.0% (nonsignificant) and 12.8 ± 2.3% (P < 0.05), respectively, compared with the control (24.1 ± 2.8%). SCD1 gene expression in presumed zygotes was lower (P < 0.01) than in Day 5 morulae and Day 8 blastocysts, independent of FFA conditions during the first 5 days of culture. Embryos exposed to 50 µM SA from Day 5 to Day 8 showed higher SCD1 gene expression (P < 0.05). SCD1 protein was detected in all embryo groups but not in oocytes, and it was the most abundantly expressed in Day 8 blastocysts. These data suggest that OA compensates for the negative effect of SA during early embryo development when OA concentrations are equal to or higher than those of SA. Furthermore, from Day 5 onward embryos appeared to be less sensitive than before, simultaneous with a higher expression level of SCD1 in Day 5 and Day 8 embryos. The higher SCD1 expression in embryos from Day 5 suggests that blastocysts may gain the possibility to convert potentially toxic SA into harmless OA after the embryonic genome activation.