Early events of fertilization in sea urchin eggs are sensitive to actin-binding organic molecules

Abstract

We previously demonstrated that many aspects of the intracellular Ca2+ increase in fertilized eggs of starfish are significantly influenced by the state of the actin cytoskeleton. In addition, the actin cytoskeleton appeared to play comprehensive roles in modulating cortical granules exocytosis and sperm entry during the early phase of fertilization. In the present communication, we have extended our work to sea urchin which is believed to have bifurcated from the common ancestor in the phylogenetic tree some 500 million years ago. To corroborate our earlier findings in starfish, we have tested how the early events of fertilization in sea urchin eggs are influenced by four different actin-binding drugs that promote either depolymerization or stabilization of actin filaments. We found that all the actin drugs commonly blocked sperm entry in high doses and significantly reduced the speed of the Ca2+ wave. At low doses, however, cytochalasin B and phalloidin increased the rate of polyspermy. Overall, certain aspects of Ca2+ signaling in these eggs were in line with the morphological changes induced by the actin drugs. That is, the time interval between the cortical flash and the first Ca2+ spot at the sperm interaction site (the latent period) was significantly prolonged in the eggs pretreated with cytochalasin B or latrunculin A, whereas the Ca2+ decay kinetics after the peak was specifically attenuated in the eggs pretreated with jasplakinolide or phalloidin. In addition, the sperm interacting with the eggs pretreated with actin drugs often generated multiple Ca2+ waves, but tended to fail to enter the egg. Thus, our results indicated that generation of massive Ca2+ waves is neither indicative of sperm entry nor sufficient for cortical granules exocytosis in the inseminated sea urchin eggs, whereas the structure and functionality of the actin cytoskeleton are the major determining factors in the two processes.