The role of Src family kinases in egg activation

The Src family kinases (SFKs) are believed to mediate some of the early events of egg activation at fertilization - intracellular Ca²⁺ increase and resumption of the second meiotic division (RMII). SFKs are both necessary and sufficient for triggering intracellular Ca²⁺ increase in eggs of sea urchin, sea star, Xenopus etc, but their role in mammalian eggs is not entirely determined. In this study we examined the involvement of SFKs in the events leading to Ca²⁺ increase in rat eggs and demonstrated their involvement in RMII. Microinjecting mRNAs of active forms of Fyn or c-Yes but not of c-Src, into ovulated eggs, triggered RMII without evoking Ca²⁺ increase. A specific SFKs inhibitor (SU6656) or dominant-negative (DN) forms of Fyn or c-Yes were unable to block Ca²⁺ oscillations rather, modulated them, in fertilized eggs or in parthenogenetically activated eggs. Moreover, inhibiting SFKs activity blocked RMII and decreased the level of cyclin B1 degradation. Our results imply participation of SFKs in the signal transduction pathway leading to egg activation, but not in the one leading to Ca²⁺ increase. We propose that SFKs act downstream to Ca²⁺ increase at the level of M-phase promoting factor (MPF).