Cell cycle-dependent calcium oscillations in mouse embryonic stem cells

During cell cycle progression, somatic cells exhibit different patterns of intracellular Ca(2+) signals during the G(0) phase, the transition from G(1) to S, and from G(2) to M. Because pluripotent embryonic stem (ES) cells progress through cell cycle without the gap phases G(1) and G(2), we aimed to determine whether mouse ES (mES) cells still exhibit characteristic changes of intracellular Ca(2+) concentration during cell cycle progression. With confocal imaging of the Ca(2+)-sensitive dye fluo-4 AM, we identified that undifferentiated mES cells exhibit spontaneous Ca(2+) oscillations. In control cultures where 50.4% of the cells reside in the S phase of the cell cycle, oscillations appeared in 36% of the cells within a colony. Oscillations were not initiated by Ca(2+) influx but depended on inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release and the refilling of intracellular stores by a store-operated Ca(2+) influx (SOC) mechanism. Using cell cycle synchronization, we determined that Ca(2+) oscillations were confined to the G(1)/S phase ( approximately 70% oscillating cells vs. G(2)/M with approximately 15% oscillating cells) of the cell cycle. ATP induced Ca(2+) oscillations, and activation of SOC could be induced in G(1)/S and G(2)/M synchronized cells. Intracellular Ca(2+) stores were not depleted, and all three IP(3) receptor isoforms were present throughout the cell cycle. Cell cycle analysis after EGTA, BAPTA-AM, 2-aminoethoxydiphenyl borate, thapsigargin, or U-73122 treatment emphasized that IP(3)-mediated Ca(2+) release is necessary for cell cycle progression through G(1)/S. Because the IP(3) receptor sensitizer thimerosal induced Ca(2+) oscillations only in G(1)/S, we propose that changes in IP(3) receptor sensitivity or basal levels of IP(3) could be the basis for the G(1)/S-confined Ca(2+) oscillations.