CHANGES IN CALPAIN DURING MEIOSIS IN THE RAT EGG

Resumption of meiosis at fertilization is mediated by increased levels of calcium which activate several calcium-dependent enzymes. Calpain, a neutral calcium-activated thiol protease, is present in the cytopla sm of many cells. Its activation is associated with limited autolysis and relocalization in the cell. Calpain is thought to participate in t he regulation of mitosis and resumption of meiosis in Xenopus oocytes. In this study we followed the activation and localization of calpain during maturation and fertilization in rat eggs using a polyclonal ant ibody raised against chicken muscle calpain. A band of 80 kDa was dete cted in GV oocytes and its level increased in unfertilized MII eggs. A t the early stages of fertilization, we observed a transient decrease in the level of calpain which was regained at the pronuclear stage. Ad ding Ca2+ to lysate of MII eggs resulted in an additional band, repres enting the de graded fragment of the activated protein. In eggs activa ted by ionomycin, calpain level decreased, followed by an increase in a dynamic similar to that observed in fertilized eggs. Egg activation also led to changes in calpain localization. A homogenous distribution was observed in GV and in MII eggs, while in activated eggs it was lo calized predominantly overlying the metaphase plate. In the current st udy we demonstrate the presence of calpain in the rat egg. During matu ration, calpain level increases; however, during egg activation, in re sponse to [Ca2+](i) changes, calpain undergoes autolysis, translocatio n, and fluctuation in its level. We therefore suggest a correlation be tween calpain activation and fertilization. (C) 1997 Wiley-Liss, Inc.