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

Abstract

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 (pHi ) 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 pHi (<7.0) in metaphase-I-arrested oocytes. The increase in pHi (>7.0) that occurs after spawning triggers polyadenylation of cyclin B mRNA and progression into meiosis II.

Intracellular pH Regulates Cancer and Stem Cell Behaviors: A Protein Dynamics Perspective

“…We now know, however, that pHi is dynamic in normal cells and clearly dysregulated in a number of diseases. In normal cells, pHi changes during cell cycle progression, increasing ∼0.3-0.4 pH units at the end of S phase and if this increase is blocked, G2/M is delayed with increased inhibitory phosphorylation of Cdk1-Tyr15 and suppressed cyclin B1 expression (1)(2)(3). Additionally, pHi dynamics regulates cell-substrate adhesion remodeling and migration, with increased pHi enabling both behaviors (4)(5)(6)(7).…”

SGK regulates pH increase and cyclin B–Cdk1 activation to resume meiosis in starfish ovarian oocytes

“…Soon after stimulation by the hormone 1-methyladenine (1-MA; Kanatani et al, 1969), pH i increases to ∼6.9 (Moriwaki et al, 2013), after which the oocytes undergo GVBD and ultimately arrest at MI (Harada et al, 2003;Moriwaki et al, 2013). To maintain the MI arrest, pH i is maintained at ∼6.9 until spawning (Harada et al, 2003(Harada et al, , 2010Oita et al, 2004;Usui et al, 2008;Moriwaki et al, 2013;Ochi et al, 2016). The pH i increase induced by 1-MA requires starfish NHE3 (sfNHE3; Harada et al, 2003Harada et al, , 2010Moriwaki et al, 2013).…”

Recycling of uridylated mRNAs in starfish embryos

“…Although the mechanisms underlying the decision-making processes of degradation or re-polyadenylation of uridylated mRNAs in starfish embryos remain to be fully elucidated, our results indicate that the 3 UTR is likely involved in the choice of fate and uridylation, because the 3 UTRs of ribosomal protein mRNAs and cyclin B mRNA reproduced the pattern of degradation or re-polyadenylation of endogenous mRNAs at the morula, blastula, and gastrula stages (Figure 1 and 4). Upon resumption of meiosis in Xenopus, mouse, and starfish oocytes, cytoplasmic polyadenylation of many maternal mRNAs, including cyclin B, is regulated by 3 UTRs containing the PAS AAUAAA and the CPE, which are bound by CPSF and the CPEB, respectively (16,(39)(40)(41). Subsequently, in Xenopus oocytes, the terminal nucleotidyl transferase 2 Gld2, which interacts with CPSF and CPEB, elongates the poly(A) tails (18,42,43).…”