Actin cytoskeleton modulates calcium signaling during maturation of starfish oocytes

Abstract

Before successful fertilization can occur, oocytes must undergo meiotic maturation. In starfish, this can be achieved in vitro by applying 1-methyladenine (1-MA). The immediate response to 1-MA is the fast Ca 2+ release in the cell cortex. Here, we show that this Ca 2+ wave always initiates in the vegetal hemisphere and propagates through the cortex, which is the space immediately under the plasma membrane. We have observed that alteration of the cortical actin cytoskeleton by latrunculin-A and jasplakinolide can potently affect the Ca 2+ waves triggered by 1-MA. This indicates that the cortical actin cytoskeleton modulates Ca 2+ release during meiotic maturation. The Ca 2+ wave was inhibited by the classical antagonists of the InsP 3-linked Ca 2+ signaling pathway, U73122 and heparin. To our surprise, however, these two inhibitors induced remarkable actin hyper-polymerization in the cell cortex, suggesting that their inhibitory effect on Ca 2+ release may be attributed to the perturbation of the cortical actin cytoskeleton. In post-meiotic eggs, U73122 and jasplakinolide blocked the elevation of the vitelline layer by uncaged InsP 3, despite the massive release of Ca 2+, implying that exocytosis of the cortical granules requires not only a Ca 2+ rise, but also regulation of the cortical actin cytoskeleton. Our results suggest that the cortical actin cytoskeleton of starfish oocytes plays critical roles both in generating Ca 2+ signals and in regulating cortical granule exocytosis.