Sperm-induced calcium oscillations at fertilisation in ascidians are controlled by cyclin B1-dependent kinase activity

The generation of calcium oscillations at fertilisation and during mitosis appears to be controlled by the cell cycle machinery. For example, the calc ium oscillations in oocytes and embryos occur during metaphase and terminat e upon entry into interphase. Here we report the manipulation of sperm-trig gered calcium oscillations by cyclin-dependent kinase (CDK) activity, the m ajor component of maturation/M phase promoting factor (MPF). To control the CDK activity we microinjected mRNAs encoding full-length GFP-tagged cyclin pi or a truncated and therefore stabilised form of cyclin B1 (Delta 90) in to unfertilised oocytes, In the presence of full-length cyclin B1, the calc ium oscillations terminate when cyclin B1 levels fall along with the concom itant fall in the associated CDK activity. In addition, when the CDK activi ty is elevated indefinitely with Delta 90 cyclin B1, the calcium oscillatio ns also continue indefinitely. Finally, in oocytes that contain low mitogen -activated protein (MAP) kinase activity and elevated CDK activity, the spe rm-triggered calcium oscillations are again prolonged. We conclude that the CDK activity of the ascidian oocyte can be regarded as a positive regulato r of sperm-triggered calcium oscillations, a finding that may apply to othe r oocytes that display sperm-triggered calcium oscillations at fertilisatio n. Furthermore, these findings may have a bearing upon the mitotic calcium signals of early embryos.