Dynamics of protein phosphorylation during meiotic maturation.

PURPOSE: To ask whether distinct kinase signaling pathways mediate cytoplasmic or nuclear maturation of mouse oocytes and if in vitro maturation influences the distribution and timing of these phosphorylation events.
METHODS: Mouse cumulus oocyte complexes (COCs) were matured under conditions known to influence oocyte quality (basal or supplemented media) and assayed with epitope specific antibodies that would distinguish between Cdk1 or tyrosine kinase targets at 0, 2, 4, 8, and 16 hrs. Semi-quantitative image analysis was used to assess the topographical patterns of protein phosphorylation during in vitro maturation. In vitro fertilization and embryo culture were used to examine the effects of culture conditions on developmental potential.
RESULTS: Protein tyrosine phosphorylation increased during meiotic progression from methaphase-I to metaphase-II. Levels were significantly higher in the oocyte cortex. Levels of cortical staining are enhanced in oocytes matured in supplemented media that displayed higher developmental competence. In contrast, bulk substrates for Cdk1 kinase localize to the meiotic spindle while cytoplasmic levels of kinase activity increase throughout meiotic progression; culture media had no measurable effect. Ablation of the tyrosine kinase Fyn significantly reduced cortical levels of tyrosine phosphorylation.
CONCLUSIONS: The findings indicate that distinct signaling pathways mediate nuclear and cytoplasmic maturation during in vitro maturation in a fashion consistent with a role for tyrosine kinases in cortical maturation and oocyte quality.