Effects of folic acid on the development and oxidative stress of mouse embryos exposed to heat stress

The development of mammalian pre-implantation embryos is inhibited by heat stress, and the inhibitory effect is associated with excess reactive oxygen species (ROS). Folate is a nutrient with various physiological functions including antioxidative effects. We first investigated the transcript expression for 10 enzymes in the cycle of folate metabolism (folate-methionine cycle) in mouse embryos at the 1-cell, 2-cell, 4- to 8-cell, morula and blastocyst stages using reverse transcription-polymerase chain reaction. All of the transcripts were consistently expressed, except for Mat1a, which was not detected from the 4- to 8-cell stage onward. Next, the effects of folic acid (the synthetic form of folate) on the development and ROS levels of heat-stressed embryos were investigated. One-cell mouse embryos were cultured with or without 1000 ng/ml folic acid basically at 38°C, and in the heat-stressed groups, embryos were exposed to 39.5°C/10 h/day on the first two days of culture. The heat stress significantly (p < 0.05) decreased blastocyst development and cell number and increased ROS levels compared to those in the group not subjected to heat stress; however, among the heat-stressed groups, blastocyst development and cell number were increased and the ROS level was decreased by the addition of folic acid. These results indicate that the mRNA of folate-methionine cycle enzymes are expressed in mouse pre-implantation embryos, suggesting they can independently utilize folate, and the inhibitory effects of heat stress on the development of mouse pre-implantation embryos are ameliorated by folic acid. The ameliorating effects of folic acid may be partly due to its antioxidative property.