Sperm-triggered Ca2+ oscillations and Ca2+homeostasis in the mouse egg have an absolute requirement for mitochondrial ATP production

At fertilisation, repetitive increases in the intracellular Ca2+ concentration, Ca2+i, drive the completion of meiosis and initiate the development of the quiescent egg into an embryo. Although the requirement for an ATP supply is evident, the relative roles of potential ATP sources remains unclear in the mammalian egg, and the specific role of mitochondria in Ca2+i regulation as well as in the sperm-triggered Ca2+ oscillations is unknown. We have used fluorescence and luminescence imaging to investigate mitochondrial activity in single mouse eggs. Simultaneous imaging of mitochondrial redox state (NADH and flavoprotein autofluorescence) and Ca2+i revealed that sperm-triggered Ca2+ oscillations are transmitted to the mitochondria where they directly stimulate mitochondrial activity. Inhibition of mitochondrial oxidative phosphorylation caused release of Ca2+ from the endoplasmic reticulum because of local ATP depletion. Mitochondrial ATP production is an absolute requirement for maintaining a low resting Ca2+i and for sustaining sperm-triggered Ca2+ oscillations. Luminescence measurements of intracellular ATP from single eggs confirmed that mitochondrial oxidative phosphorylation is the major source of ATP synthesis in the dormant unfertilised egg. These observations show that a high local ATP consumption is balanced by mitochondrial ATP production, and that balance is critically poised. Mitochondrial ATP supply and demand are thus closely coupled in mouse eggs. As mitochondrial ATP generation is essential to sustain the Ca2+ signals that are crucial to initiate development, mitochondrial integrity is clearly fundamental in sustaining fertility in mammalian eggs.