A novel mechanism controls the Ca2+ oscillations triggered by activation of ascidian eggs and has an absolute requirement for Cdk1 activity

Fertilisation in ascidians triggers a series of periodic rises in cytosolic Ca2+ that are essential for release from metaphase I arrest and progression through meiosis II. These sperm-triggered Ca2+ oscillations are switched off at exit from meiosis II. Ascidian zygotes provided the first demonstration of the positive feedback loop whereby elevated Cdk1 activity maintained these Ca2+ oscillations. Since then it has been reported that Cdk1 sensitises the type I inositol trisphosphate [Ins( 1,4,5) P-3] receptor in somatic cells, and that sperm-triggered Ca2+ oscillations in mouse zygotes stop because the forming pronuclei sequester phospholipase C zeta that was delivered to the egg by the fertilising sperm. Here, using enucleation, we demonstrate in ascidian eggs that Ca2+ spiking stops at the correct time in the absence of pronuclei. Sequestration of sperm factor is therefore not involved in terminating Ca2+ spiking for these eggs. Instead we found that microinjection of the Cdk1 inhibitor p21 blocked Ca2+ spiking induced by ascidian sperm extract (ASE). However, such eggs were still capable of releasing Ca2+ in response to Ins(1,4,5) P3 receptor agonists, indicating that ASE-triggered Ca2+ oscillations can stop even though the response to Ins(1,4,5) P-3 remained elevated. These data suggest that Cdk1 activity promotes Ins(1,4,5) P-3 production in the presence of the sperm factor, rather than sensitising the Ca2+ releasing machinery to Ins(1,4,5) P-3. These findings suggest a new link between this cell cycle kinase and the Ins(1,4,5) P-3 pathway.