A study of an egg-activating molecule in the sperm cytoplasm

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In our study, we have found the following major results. Unfertilized eggs of the newt, Cynops pyrrhogaster, are arrested at the second meiotic metaphase. A transient increase of [Ca^<2+>]_i, which is triggered by fertilizing sperm and propagates over the egg cortex as Ca^<2+> wave, is a primary signal to cause egg activation. The injection of an extract of Cynops sperm (SE) into the unfertilized eggs induced the wave-like [Ca^<2+>]_i increase, and then the eggs underwent activation and resumption of meiosis. In the SE-injected eggs, not only degradation of cyclin B1, but also DNA replication was confirmed. When SE had been boiled or treated with Proteinase K before injection, the activity to cause egg activation was lost. Preinjection of Ca^<2+>-chelator BAPTA before SE-injection inhibited egg activation. These results indicate that a heat-labile and proteinous factor in sperm cytoplasm induces a transient increase of [Ca^<2+>]_i indispensable for egg activation. Injection of IP_3 into unfertilized eggs caused [Ca^<2+>]_i increase and egg activation, but injection of cADP-ribose did not. In addition, preinjection of heparin, an inhibitor of IP_3 receptors, prevented egg activation by fertilizing sperm or by SE-injection. These results indicate that not only fertilization, but also SE-injection induces [Ca^<2+>]_i increase through the IP_3 receptors, rather titan rianodine receptors.