Role of Two Series of Ca2+Oscillations in Activation of Ascidian Eggs

AbstractChanges in [Ca2+]iare an essential factor regulating egg activation. Matured ascidian eggs are arrested at metaphase I, and two series of [Ca2+]itransients have been observed after fertilization: Ca2+waves just after fertilization (Series I) and [Ca2+]ioscillation between the first and second polar body extrusion (Series II). We investigated mechanisms involved in the elevation of [Ca2+]iand the role of the [Ca2+]itransients during egg activation inCiona savignyi.The monoclonal antibody 18A10 against IP3receptor type 1, which inhibits IP3-induced Ca2+release in hamster and mouse eggs, did not show substantial inhibitory effects on series I or egg deformation, whereas Series II and the first cell division were inhibited by the antibody. Ruthenium red, an inhibitor of ryanodine receptor-mediated Ca2+release, had no apparent effect of [Ca2+]itransients and other events related to the egg activation. Microinjection of IP3into unfertilized eggs induced [Ca2+]itransients similar to those seen in Series I, whereas injection of cyclic ADP ribose, an agonist of ryanodine receptors, rarely induced [Ca2+]itransient. Adenophostin B, a potent nonmetabolizable agonist of IP3receptors, induced [Ca2+]ioscillations which continued after first polar body extrusion, without separation to two series, and led to extrusion of first and second polar bodies. These results suggest that Series II is driven by the mouse type 1-like IP3receptor while Series I seems to be mediated by another type of IP3receptor. Injection of IP3only induced the first polar body extrusion and the egg was arrested at metaphase II even when a higher amount of IP3was injected. On the other hand, reinjection of IP3after the first polar body extrusion led to emission of the second polar body. Thus, Series I and II of [Ca2+]itransients are likely to be required for metaphase–anaphase transition in meiosis