Ovulated human and mouse oocytes are stalled in meiosis II. They are transcriptionally quiescent but have a maternally-loaded transcriptome and proteome. Fertilization triggers “egg activation”, in which a rise(s) of calcium in the oocyte induces several key events that allow transition to embryonic development, namely, meiotic resumption and completion, changes to the egg’s proteome, and genome activation after the first zygotic division. Therefore, egg activation is required for the oocyte to become a totipotent zygote. Despite the essential nature of this process for female fertility, the molecular events of egg activation are not well understood, primarily for technical reasons. Egg activation occurs without new transcription; thus nucleic acidsbased ‘omics comparisons are uninformative. The macromolecules that transduce the calcium signal to effect downstream cellular events are not known in humans or any other mammal.

Recent studies in MW’s lab exploited technical advantages of the Drosophila model system to show that there is large phospho-modulation of the maternally-provided proteome during egg activation (this also occurs in frogs and sea urchins). We hypothesized, and our genetic data supported, that this posttranslational modification regulated the activity of stored proteins to permit transition of an arrested mature oocyte to a cell that can undertake embryogenesis. We then showed that a calcium-regulated phospho-regulatory enzyme mediates these phospho-changes in the cell cycle machinery, translation factors and other proteins needed to
transition the egg to an embryo.

In this R21 we propose to test this model for mammalian oocytes, using mouse as a model. Following procedures analogous to those used for Drosophila, we will determine whether there are phosphoproteome changes during mouse egg activation, and which proteins undergo these changes. We will then test the role of CamKII, a calcium-regulated kinase that has been shown to be required for egg activation in mouse, in making these phospho-changes.

The results of our studies will lay the groundwork for the field in several ways, including developing phosphoproteomics for mouse oocytes and determining proteins that are phospho-regulated during egg activation. The results will provide information essential for future studies into the roles of the regulated proteins that we identify here, and the effects of specific phosphomodulations during this critical developmental transition. Such fundamental studies will be important for identifying the molecular and genetic bases of human infertilities associated with defective egg activation, providing biomarkers to monitor this process, and potentially for optimizing conditions for assisted reproductive technologies.