NAADP⁺ initiates the Ca²⁺ response during fertilization of starfish oocytes

We have explored the role of the recently discovered second messenger nicotinic acid adenine nucleotide phosphate (NAADP⁺) in Ca²⁺ swings that accompany the fertilization process in starfish oocytes. The injection of NAADP⁺ deep into the cytoplasm of oocytes matured by the hormone 1-methyladenine (1-MA), mobilized Ca²⁺ exclusively in the cortical layer, showing that the NAADP⁺-sensitive Ca²⁺ pool is restricted to the subplasma membrane region of the cell. At variance with this, InsP₃ initiated the liberation of Ca²⁺ next to the point of injection in the center of the cell. The initial cortical Ca²⁺ liberation induced by NAADP⁺ was followed by a spreading of the Ca²⁺ wave to the remainder of the cell and by a massive cortical granule exocytosis similar to that routinely observed on injection of InsP₃. A striking difference in the responses to NAADP⁺ and InsP₃ was revealed by the removal of the nucleus from immature oocytes, i.e., from oocytes not treated with 1-MA. Whereas the Ca²⁺ response and the cortical granule exocytosis induced by NAADP⁺ were unaffected by the removal of the nucleus, the Ca²⁺ response promoted by InsP₃ was significantly slowed. In addition, the cortical granule exocytosis was completely abolished. When enucleated oocytes were fertilized, the spermatozoon still promoted the Ca²⁺ wave and normal cortical exocytosis, strongly suggesting that the Ca²⁺ response was mediated by NAADP⁺ and not by InsP₃. InsP₃-sensitive Ca²⁺ stores may mediate the propagation of the wave initiated by NAADP⁺ since its spreading was strongly affected by removal of the nucleus.