Mitochondrial malate-aspartate shuttle regulates mouse embryo nutrient consumption

Pyruvate has been considered the sole substrate that can support development of the mouse zygote to the two-cell stage, with lactate able to support development from the two-cell stage. This study has determined for the first time that mitochondrial reducing equivalent shuttles regulate metabolism in the early embryo. Activity of the malate-aspartate shuttle was found to be essential for the metabolism of lactate in the two-cell embryo. Furthermore, the inability of the mouse zygote to use lactate as an energy source was a result of a lack of malate-aspartate shuttle activity. The mRNA for the four enzymes for shuttle activity were detected at all stages of development. It was determined that aspartate was a rate-limiting factor in the activity of the malate-aspartate shuttle in mouse zygotes probably due to the high Km of the cytoplasmic aspartate aminotransferase. Addition of high concentrations of exogenous aspartate to the culture medium enabled mouse zygotes to utilize lactate in the absence of pyruvate and develop normally to the blastocyst stage as well as produce normal viable offspring. This study determined that the malate-aspartate shuttle is a key regulator of embryo metabolism and therefore viability and is the first report that mouse zygotes can develop normally to term in the absence of pyruvate.