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, InsP(3) 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 InsP(3). A striking difference in the responses to NAADP(+) and InsP(3) 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 InsP(3) 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 InsP(3). InsP(3)-sensitive Ca2+ stores may mediate the propagation of the wave initiated by NAADP(+) since its spreading was strongly affected by removal of the nucleus.