The roles of Ca2+, downstream protein kinases, and oscillatory signaling in regulating fertilization and the activation of development.

Reviews in Developmental Biology have covered the pathways that generate the all-important intracellular calcium (Ca(2+)) signal at fertilization [Miyazaki, S., Shirakawa, H., Nakada, K., Honda, Y., 1993a. Essential role of the inositol 1,4,5-trisphosphate receptor/Ca(2+) release channel in Ca(2+) waves and Ca(2+) oscillations at fertilization of mammalian eggs. Dev. Biol. 158, 62-78; Runft, L., Jaffe, L., Mehlmann, L., 2002. Egg activation at fertilization: where it all begins. Dev. Biol. 245, 237-254] and the different temporal responses of Ca(2+) in many organisms [Stricker, S., 1999. Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev. Biol. 211, 157-176]. Those reviews raise the importance of identifying how Ca(2+) causes the events of egg activation (EEA) and to what extent these temporal Ca(2+) responses encode developmental information. This review covers recent studies that have analyzed how these Ca(2+) signals are interpreted by specific proteins, and how these proteins regulate various EEA responsible for the onset of development. Many of these proteins are protein kinases (CaMKII, PKC, MPF, MAPK, MLCK) whose activity is directly or indirectly regulated by Ca(2+), and whose amount increases during late oocyte maturation. We cover biochemical progress in defining the signaling pathways between Ca(2+) and the EEA, as well as discuss how oscillatory or multiple Ca(2+) signals are likely to have specific advantages biochemically and/or developmentally. These emerging concepts are put into historical context, emphasizing that key contributions have come from many organisms. The intricate interdependence of Ca(2+), Ca(2+)-dependent proteins, and the EEA raise many new questions for future investigations that will provide insight into the extent to which fertilization-associated signaling has long-range implications for development. In addition, answers to these questions should be beneficial to establishing parameters of egg quality for human and animal IVF, as well as improving egg activation protocols for somatic cell nuclear transfer to generate stem cells and save endangered species.