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)P₃] receptor in somatic cells, and that sperm-triggered Ca²⁺ oscillations in mousezygotes 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)P₃ remained elevated. These data suggest that Cdk1 activity promotes Ins(1,4,5)P₃ production in the presence of the sperm factor,rather than sensitising the Ca²⁺ releasing machinery to Ins(1,4,5)P₃. These findings suggest a new link between this cell cycle kinase and the Ins(1,4,5)P₃ pathway.