Development of calcium releasing activity induced by inositol trisphosphate and cyclic ADP-ribose during in vitro maturation of sea urchin oocytes

Abstract

During fertilization of sea urchin eggs, the cytoplasmic Ca2+ concentration ([Ca2+]i) transiently increases (Ca 2+ transient). Increased [Ca2+]i results from a rapid release from intracellular stores, mediated by one or both of two signaling pathways; inositol 1,4,5-trisphosphate (IP3) and IP 3 receptor (IP3R) or cyclic GMP (cGMP), cyclic ADP-ribose (cADPR) and ryanodine receptor (RyR). During fertilization, cGMP and cADPR increase preceding the Ca2+ transient, suggesting their contribution to this. If the RyR pathway contributed to the Ca2+ transient, its Ca2+ releasing activity would develop in parallel with that of the IP3 system during maturation of oocytes. Sea urchin oocytes were cultivated in vitro and Ca2+ transients induced by photolysis of caged IP3 or caged cADPR were measured during maturation. Oocytes spontaneously began to maturate in seawater. More than 50% of oocytes underwent germinal vesicle breakdown within 25 h and the second meiosis within 35 h, but it took more than 24 h until they became functionally identical to in vivo-matured eggs. Both IP3 and cADPR induced Ca2+ transients comparable to those of in vivo-matured eggs later than 24 h from the second meiosis. However, cADPR induced a small Ca2+ transient even before meiosis, whereas IP3 and sperm almost did not. © 2006 The Authors.