Calcium Signals for Egg Activation in Mammals
Shunichi Miyazaki1,* and Masahiko Ito1
1Department of Physiology, Tokyo Women’s Medical University School of Medicine, 8-1 Kawada-cho, Shinjuku-ku, Tokyo 162-8666, Japan
Received February 21, 2006
Abstract. 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 meiotic 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 spermegg fusion. A current strong candidate of the sperm factor is a novel sperm-specific isozyme of phospholipase C (IP3-producing enzyme), PLCζ. Recent extensive research has reveled characteristics of PLCζ such as the Ca2+ oscillation-inducing activity after injection of PLCζ- encoding RNA or recombinant PLCζ into mouse eggs, extremely high Ca2+-sensitivity of the enzymatic activity in vitro, and nuclear translocation ability possibly related to cell-cycledependent regulation of Ca2+ oscillations. [Ca2+]i rises cause successive activation of calmodulindependent 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.
Keywords: intracellular Ca2+, fertilization, mammalian egg, sperm factor, egg activation
Source: J Pharmacol Sci 100, 545 – 552 (2006).