Antibody against the actin-binding protein depactin attenuates Ca2+ signaling in starfish eggs

Being present in starfish oocytes, the cofilin/ADF (actin-depolymerizing factor) family protein depactin severs actin filaments. Previously, we reported that exogenous cofilin microinjected into starfish eggs significantly augmented the Ca2+ release in response to inositol 1,4,5-trisphosphate (InsP3) or fertilizing sperm, raising the possibility that intracellular Ca2+ signaling could be modulated by the actin cytoskeleton. In this communication, we have targeted the endogenous depactin by use of the specific antibody that was raised against its actin-binding domain. The anti-depactin antibody microinjected into the starfish oocytes and eggs effectively altered the structure of the actin cytoskeleton, and significantly delayed the meiotic progression induced by 1-methyladenine. When microinjected into the mature eggs, the anti-depactin antibody markedly reduced the amplitude of the Ca2+ response in a dose-dependent manner, corroborating the results of our previous study with cofilin. In addition, the eggs microinjected with the anti-depactin antibody displayed reduced rate of successful elevation of the fertilization envelope and an elevated tendency of polyspermic interaction. Taken together, our data suggest that the actin cytoskeleton is implicated not only in meiotic maturation and intracellular Ca2+ signaling, but also in the fine regulation of gametes interaction and cortical granules exocytosis.