Mitochondrial modulation of calcium signaling at the initiation of development

Abstract

Fertilization triggers cytosolic Ca2+ oscillations that activate mammalian eggs and initiate development. Extensive evidence demonstrates that Ca2+ is released from endoplasmic reticulum stores; however, less is known about how the increased Ca2+ is restored to its resting level, forming the Ca2+ oscillations. We investigated whether mitochondria also play a role in activation-associated Ca2+ signaling. Mitochondrial dysfunction induced by the mitochondrial uncoupler FCCP or antimycin A disrupted cytosolic Ca2+ oscillations, resulting in sustained increase in cytosolic Ca2+, followed by apoptotic cell death. This suggests that functional mitochondria may participate in sequestering the released Ca2+, contributing to cytosolic Ca2+ oscillations and preventing cell death. By centrifugation, mouse eggs were stratified and separated into fractions containing both endoplasmic reticulum and mitochondria and fractions containing endoplasmic reticulum with no mitochondria. The former showed Ca2+ oscillations by activation, whereasthe latter exhibited sustained elevation in cytosolic Ca2+ but no Ca2+ oscillations, suggesting that mitochondria take up released cytosolic Ca2+. Further, using Rhod-2 for detection of mitochondrial Ca2+, we found that mitochondria exhibited Ca2+ oscillations, the frequency of which was not different from that of cytosolic Ca2+ oscillations, indicatingthat mitochondria are involved in Ca2+ signaling during egg activation. Therefore, we propose that mitochondria play a crucial role in Ca2+ signaling that mediates egg activation and development, and apoptotic cell death.