O-039 Mitochondrial ATP generation in mammalian eggs and Ca2+ signaling

Abstract

Abstract text In metaphase II arrested mammalian oocytes (eggs) and cleavage stage embryos the mitochondria are responsible for nearly all ATP production because glycolysis is inactivated. Luciferase assays show that ATP levels in eggs are strictly dependent upon pyruvate and fatty acid oxidation. The level of ATP in eggs appears to be maximal in conventional medium because the addition of extra mitochondrial substrates to eggs does not increase cytosolic ATP. The only clear elevation of ATP is seen at fertilization and is associated with sperm induced Ca2+ oscillations. Our recent findings suggest that the level of ATP modulates events at fertilization. At fertilization, the egg is activated by sperm derived PLCzeta which triggers a series of Ca2+ oscillations, with each Ca2+ release event causes by inositol trisphosphate (InsP3). Previous studies have shown that mouse eggs are more sensitive to PLCzeta, and generate higher frequency Ca2+ oscillations, than human eggs. Mouse eggs also generate Ca2+ oscillations and activate in response to Sr2+ that directly stimulates InsP3 receptors. In contrast, human eggs that contain the same type of InsP3 receptors do not generate Ca2+ oscillations in response to Sr2+. The difference in sensitivity of Ca2+ release between species can be explained by the fact that mouse eggs are about ten times more sensitive to InsP3 than human eggs. The reason for this difference appears to be due to ATP. The ATP level in unfertilized mouse eggs is about twice that in human eggs. Furthermore, the ability of mouse eggs to Sr2+ medium can be abolished by removing the mitochondrial substrate pyruvate, which reduces the ATP level. Adding back pyruvate to such eggs restores ATP levels promotes Sr2+ induced Ca2+ levels in mouse eggs. These data suggest that the level of ATP, possibly as ATP4-, modulates the sensitivity of the InsP3 receptor and the ability of eggs to generate Ca2+ oscillations. The level of cytosolic ATP may represent a significant ‘egg factor’ in determining the success of fertilization in humans. Enhancing mitochondrial ATP production could be useful in improving activation and embryo development after fertilization, or after artificial egg activation.