Three different calcium wave pacemakers in ascidian eggs

Abstract

Calcium wave pacemakers in fertilized eggs of ascidians and mouse are associated with accumulations of cortical endoplasmic reticulum in the vegetal hemisphere. In ascidians, two distinct pacemakers (PM1 and PM2) generate two series of calcium waves necessary to drive meiosis I and II. Pacemaker PM2 is stably localized in a cortical ER accumulation situated in the vegetal contraction pole. We now find that pacemaker PM1 is situated in a cortical ER-rich domain that forms around the sperm aster and moves with it during the calcium-dependant cortical contraction triggered by the fertilizing sperm. Global elevations of inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3) levels produced by caged Ins(1,4,5)P3 or caged glycero-myo-PtdIns(4,5)P2 photolysis reveal that the cortex of the animal hemisphere, also rich in ER-clusters, is the cellular region most sensitive to Ins(1,4,5)P3 and acts as a third type of pacemaker (PM3). Surprisingly, the artificial pacemaker PM3 predominates over the natural pacemaker PM2, located at the opposite pole. Microtubule depolymerization does not alter the activity nor the location of the three pacemakers. By contrast, blocking the acto-myosin driven cortical contraction with cytochalasin B prevents PM1 migration and inhibits PM2 activity. PM3, however, is insensitive to cytochalasin B. Our experiments suggest that the three distinct calcium wave pacemakers are probably regulated by different spatiotemporal variations in Ins(1,4,5)P3 concentration. In particular, the activity of the natural calcium wave pacemakers PM1 and PM2 depends on the apposition of a cortical ER-rich domain to a source of Ins(1,4,5)P3 production in the cortex. Movies available on-line