Block of CDK1‐dependent polyadenosine elongation of Cyclin B mRNA in metaphase‐i‐arrested starfish oocytes is released by intracellular pH elevation upon spawning

SUMMARY Meiotic progression requires the translation of maternal mRNAs in a strict temporal order. In isolated animal oocytes, translation of maternal mRNAs containing a cytoplasmic polyadenylation element (CPE), such as cyclin B , is activated by in vitro stimulation of meiotic resumption which induces phosphorylation of CPEB (CPE ‐ binding protein) and elongation of their polyadenosine (poly(A)) tails; whether or not this model can be applied in vivo to oocytes arrested at metaphase of meiosis I in ovaries is unknown. In this study, we found that active CDK1 (cyclin‐dependent kinase 1) phosphorylated CPEB in ovarian oocytes arrested at metphase I in the starfish body cavity, but phosphorylation of CPEB was not sufficient for elongation of cyclin B poly(A) tails. Immediately after spawning, however, mRNA was polyadenylated, suggesting that an increase in intracellular pH (pH i ) upon spawning triggers the elongation of poly(A) tails. Using a cell‐free system made from maturing oocytes at metaphase I, we demonstrated that polyadenylation was indeed suppressed at pH below 7.0. These results suggest that a pH‐sensitive process, functioning after CPEB phosphorylation, is blocked under physiologically low pH i (<7.0) in metaphase‐I‐arrested oocytes. The increase in pH i (>7.0) that occurs after spawning triggers polyadenylation of cyclin B mRNA and progression into meiosis II. Mol. Reprod. Dev. 83: 79–87, 2016. © 2015 Wiley Periodicals, Inc .