The Effect of Acidic and Alkaline Seawater on the F-Actin-Dependent Ca2+ Signals Following Insemination of Immature Starfish Oocytes and Mature Eggs

The Effect of Synthetic Polyamine BPA-C8 on the Fertilization Process of Intact and Denuded Sea Urchin Eggs

Sea urchin eggs are covered with layers of extracellular matrix, namely, the vitelline layer (VL) and jelly coat (JC). It has been shown that sea urchin eggs' JC components serve as chemoattractants or ligands for the receptor on the fertilizing sperm to promote the acrosome reaction. Moreover, the egg's VL provides receptors for conspecific sperm to bind, and, to date, at least two sperm receptors have been identified on the surface of sea urchin eggs. Interestingly, however, according to our previous work, denuded sea urchin eggs devoid of the JC and VL do not fail to become fertilized by sperm. Instead, they are bound and penetratedby multiple sperm, raising the possibility that an alternative pathway independent of the VL-residing sperm receptor may be at work. In this research, we studied the roles of the JC and VL using intact and denuded eggs and the synthetic polyamine BPA-C8. BPA-C8 is known to bind to the negatively charged macromolecular complexes in the cells, such as the JC, VL, and the plasma membrane of echinoderm eggs, as well as to the actin filaments in fibroblasts. Our results showed that, when added to seawater, BPA-C8 significantly repressed the Ca2+ wave in the intact P. lividus eggs at fertilization. In eggs deprived of the VL and JC, BPA-C8 binds to the plasma membrane and increases fibrous structures connecting microvilli, thereby allowing the denuded eggs to revert towards monospermy at fertilization. However, the reduced Ca2+ signal in denuded eggs was nullified compared to the intact eggs because removing the JC and VL already decreased the Ca2+ wave. BPA-C8 does not cross the VL and the cell membrane of unfertilized sea urchin eggs to diffuse into the cytoplasm at variance with the fibroblasts. Indeed, the jasplakinolide-induced polymerization of subplasmalemmal actin filaments was inhibited in the eggs microinjected with BPA-C8, but not in the ones bath-incubated with the same dose of BPA-C8.

Application of Raman spectroscopy to the evaluation of F-actin changes in sea urchin eggs at fertilization

The actin filaments on the surface of echinoderm oocytes and eggs readily undergo massive reorganization during meiotic maturation and fertilization. In sea urchin eggs, the actin cytoskeletal response to the fertilizing sperm is fast enough to accompany Ca2+ signals and to guide sperm's entry into the egg. Although recent work using live cell imaging technology confirmed changes in the actin polymerization status in fertilized eggs, as was previously shown using light and electron microscopy, it failed to provide experimental evidence of F-actin depolymerization a few seconds after insemination, which is concurrent with the sperm-induced Ca2+ release. In the present study, we applied Raman microspectroscopy to tackle this issue by examining the spectral profiles of the egg's subplasmalemmal regions before and after treating the eggs with actin drugs or fertilizing sperm. At both early (15 s) and late (15 min) time points after fertilization, specific peak shifts in the Raman spectra revealed change in the actin structure, and Raman imaging detected the cytoskeletal changes corresponding to the F-actin reorganization visualized with LifeAct-GFP in confocal microscopy. Our observation suggests that the application of Raman spectroscopy, which does not require microinjection of fluorescent probes and exogenous gene expression, may serve as an alternative or even advantageous method in disclosing rapid subtle changes in the subplasmalemmal actin cytoskeleton that are difficult to resolve.

Dithiothreitol Affects the Fertilization Response in Immature and Maturing Starfish Oocytes

Immature starfish oocytes isolated from the ovary are susceptible to polyspermy due to the structural organization of the vitelline layer covering the oocyte plasma membrane, as well as the distribution and biochemical properties of the actin cytoskeleton of the oocyte cortex. After the resumption of the meiotic cycle of the oocyte triggered by the hormone 1-methyladenine, the maturing oocyte reaches fertilizable conditions to be stimulated by only one sperm with a normal Ca2+ response and cortical reaction. This cytoplasmic ripening of the oocyte, resulting in normal fertilization and development, is due to the remodeling of the cortical actin cytoskeleton and germinal vesicle breakdown (GVBD). Since disulfide-reducing agents such as dithiothreitol (DTT) are known to induce the maturation and GVBD of oocytes in many species of starfish, we analyzed the pattern of the fertilization response displayed by Astropecten aranciacus oocytes pre-exposed to DTT with or without 1-MA stimulation. Short treatment of A. aranciacus immature oocytes with DTT reduced the rate of polyspermic fertilization and altered the sperm-induced Ca2+ response by changing the morphology of microvilli, cortical granules, and biochemical properties of the cortical F-actin. At variance with 1-MA, the DTT treatment of immature starfish oocytes for 70 min did not induce GVBD. On the other hand, the DTT treatment caused an alteration in microvilli morphology and a drastic depolymerization of the cortical F-actin, which impaired the sperm-induced Ca2+ response at fertilization and the subsequent embryonic development.