Sperm induce a secondary increase in ATP levels in mouse eggs that is independent of Ca2+ oscillations

Abstract

Egg activation at fertilization in mouse eggs is caused by a series of cytosolic Ca2+ oscillations that are associated with an increase in ATP concentrations driven by increased mitochondrial activity. We have investigated the role of Ca2+ oscillations in these changes in ATP at fertilization by measuring the dynamics of ATP and Ca2+ in mouse eggs. An initial ATP increase started with the first Ca2+ transient at fertilization and then a secondary increase in ATP occurred ∼1 h later and this preceded a small and temporary increase in the frequency of Ca2+ oscillations. Other stimuli that caused Ca2+ oscillations such as PLCz1 or thimerosal, caused smaller or slower changes in ATP that failed to show the distinct secondary rise. Sperm-induced Ca2+ oscillations in the egg also triggered changes in the fluorescence of NADH which followed the pattern of Ca2+ spikes in a similar pattern to oscillations triggered by PLCz1 or thimerosal. When eggs were loaded with low concentrations of the Ca2+ chelator BAPTA, sperm triggered one small Ca2+ increase, but there were still extra phases of ATP increase that were similar to control fertilized eggs. Singular Ca2+ increases caused by thapsigargin were much less effective in elevating ATP levels. Together these data suggest that the secondary ATP increase at fertilization in mouse eggs is not caused by increases in cytosolic Ca2+. The fertilizing sperm may stimulate ATP production in eggs via both Ca2+ and by another mechanism that is independent of PLCz1 or Ca2+ oscillations.