Sperm-triggered calcium oscillations during meiosis in ascidian oocytes first pause, restart, then stop: correlations with cell cycle kinase activity

We have investigated the relationship between the sperm-triggered Ca2+ osci llations and the activities of two cell cycle kinases (MPF activity and MAP kinase activity) at fertilisation of Ascidiella aspersa oocytes. Maturatio n Promoting Factor (MPF) activity is elevated in the metaphase I (MI)-arres ted unfertilised oocyte (as measured by phosphorylation of exogenous histon e H1) and falls 5 minutes after fertilisation to remain at low levels for 5 minutes. The first polar body (pbl) is extruded when the MPF activity is l ow. The MPF activity is elevated again 15 minutes after fertilisation and f inally becomes inactivated 25 minutes after fertilisation when the pb2 is e xtruded. MAP kinase activity increases from an initially elevated level to reach maximal activity 10 minutes after fertilisation and subsequently fall s to reach low levels 25 minutes after fertilisation, Sperm trigger a serie s of Ca2+ oscillations that pause for 5 minutes while only the MPF activity is low and are present when both MPF and MAP kinase activity are elevated. We next attempted to determine whether the second phase of calcium oscillat ions is required to reactivate the MPF activity that precedes extrusion of the second polar body. To do this, we triggered a monotonic Ca2+ signal. Th is leads to the inactivation of MPF followed by MPF reactivation. The MPF a ctivity then remains elevated for an extended period of time. During this p eriod, the chromatin remains condensed and a metaphase II (MII) spindle for ms. Fertilisation of these MII oocytes triggers extrusion of pb2 in 7 minut es. Interestingly, the second phase of Ca2+ oscillations is completely abse nt when MII oocytes are fertilised, Thus, in both MI and MII oocytes, the s perm-triggered Ca2+ oscillations follow the MPF activity. Finally we discuss our finding that the Ca2+ release system remains sensiti ve during the metaphase-like state (including the period when the Ca2+ osci llations pause).