Effect of a low ethanol concentration during in vitro maturation of bovine oocytes and subsequent embryo development

Effect of low ethanol concentration in maturation medium on developmental ability, mitochondria, and gene expression profile in mouse oocytes

Ethanol affects pre-conceptional oocyte quality in women. In this study, we examined the effect of low ethanol concentrations on mouse oocytes. Oocytes were collected from the ovaries of 9-10 week old mice and allowed to mature in vitro in the presence of low concentrations of ethanol (0.1% and 0.2% v/v) for 24 h. Treatment of oocytes with ethanol (0.2%) during maturation decreased the mitochondrial DNA content and membrane potential compared to that in untreated ones, whereas the ATP content did not differ between the groups. Both 0.1% and 0.2% ethanol reduced the lipid content in the oocytes. In addition, immunostaining revealed that oocytes cultured in maturation medium containing ethanol (0.2%) had reduced levels of global DNA methylation and DNMT3A compared with untreated oocytes, and decreased rate of blastocyst development with low mitochondrial protein levels (TOMM40) in embryo. RNA-sequencing of the ethanol-treated (0.2%) and untreated oocytes revealed that mitochondria were a major target of ethanol. In conclusion, treatment of oocytes with low concentration of ethanol reduces the developmental rate to the blastocyst stage, with a lower total cell number and global DNA methylation. In addition, ethanol affected mitochondrial function and mitochondria-related gene expression.

Effect of lipotoxicity on mitochondrial function and epigenetic programming during bovine in vitro embryo production

Maternal metabolic disorders may cause lipotoxic effects on the developing oocyte. Understanding the timing at which this might disrupt embryo epigenetic programming and how this is linked with mitochondrial dysfunction is crucial for improving assisted reproductive treatments, but has not been investigated before. Therefore, we used a bovine in vitro model to investigate if pathophysiological palmitic acid (PA) concentrations during in vitro oocyte maturation and in vitro embryo culture alter embryo epigenetic patterns (DNA methylation (5mC) and histone acetylation/methylation (H3K9ac/H3K9me2)) compared to control (CONT) and solvent control (SCONT), at the zygote and morula stage. Secondly, we investigated if these epigenetic alterations are associated with mitochondrial dysfunction and changes in ATP production rate, or altered expression of epigenetic regulatory genes. Compared to SCONT, H3K9ac and H3K9me2 levels were increased in PA-derived zygotes. Also, 5mC and H3K9me2 levels were increased in PA-exposed morulae compared to SCONT. This was associated with complete inhibition of glycolytic ATP production in oocytes, increased mitochondrial membrane potential and complete inhibition of glycolytic ATP production in 4-cell embryos and reduced SOD2 expression in PA-exposed zygotes and morulae. For the first time, epigenetic alterations in metabolically compromised zygotes and morulae have been observed in parallel with mitochondrial dysfunction in the same study.