Calcium signals for egg activation in mammals

A dramatic increase in intracellular Ca2+ concentration ([Ca2+](i)) occurs in eggs at fertilization common to all animal species examined to date, and this serves as a pivotal signal for egg activation characterized by resumption of melotic cell division and formation of the pronuclei. In mammalian eggs, repetitive [Ca2+](i) rises (Ca2+ oscillations) each of which accompanies a propagating wave across the egg occur due to release of Ca2+ from the endoplasmic reticulum mainly through type 1 inositol 1,4,5-trisphosphate (IP3) receptor. Ca2+ oscillations are induced by a cytosolic sperm factor driven into the egg cytoplasm upon sperm-egg fusion. A current strong candidate of the sperm factor is a novel sperm-specific isozyme of phospholipase C (IP3-producing enzyme), PLC zeta. Recent extensive research has reveled characteristics of PLC zeta such as the Ca2+ oscillation-inducing activity after injection of PLC alpha-encoding RNA or recombinant PLCC into mouse eggs, extremely high Ca2+-sensitivity of the enzymatic activity in vitro, and nuclear translocation ability possibly related to cell-cycle-dependent regulation of Ca2+ oscillations. [Ca2+](i) rises cause successive activation of calmodulin-dependent kinase II and E3 ubiquitin ligase, lead to proteolysis of ubiquitinated cyclin B1 and inactivation of metaphase-promoting factor (Cdk1/cyclin B1 complex), and result in the release of eggs from meiotic arrest.