An oocyte-specific ELAVL2 isoform is a translational repressor ablated from meiotically competent antral oocytes.

Report Report Cell Cycle 13:7, 1187–1200; April 1, 2014; © 2014 Landes Bioscience An oocyte-specific ELAVL2 isoform is a translational repressor ablated from meiotically competent antral oocytes Katerina Chalupnikova1, Petr Solc2, Vadym Sulimenko1, Radislav Sedlacek1, and Petr Svoboda1,* Institute of Molecular Genetics AS CR; Prague, Czech Republic; 2Institute of Animal Physiology and Genetics AS CR; Libechov, Czech Republic 1 Keywords: ELAVL2, NSN, SN, chromatin, oocyte, ARE Abbreviations: ELAVL2, embryonic lethal abnormal vision like 2 protein; NSN, non-surrounded nucleolus; SN, surrounded nucleolus; ARE, AU-rich element; OET, oocyte-to-embryo transition; GV, germinal vesicle; CPE, cytoplasmic polyadenylation element; CPEB, CPE binding protein; SCMC, subcortical maternal complex; AUBP, ARE binding protein; MII, metaphase II; WT, wild type; TG, transgenic mouse; RRM, RNA recognition motif; RL, Renilla luciferase; FL, firefly luciferase At the end of the growth phase, mouse antral follicle oocytes acquire full developmental competence. In the mouse, this event is marked by the transition from the so-called non-surrounded nucleolus (NSN) chromatin configuration into the transcriptionally quiescent surrounded nucleolus (SN) configuration, which is named after a prominent perinucleolar condensed chromatin ring. However, the SN chromatin configuration alone is not sufficient for determining the developmental competence of the SN oocyte. There are additional nuclear and cytoplamic factors involved, while a little is known about the changes occurring in the cytoplasm during the NSN/SN transition. Here, we report functional analysis of maternal ELAVL2 an AU-rich element binding protein. Elavl2 gene encodes an oocyte-specific protein isoform (denoted ELAVL2°), which acts as a translational repressor. ELAVL2° is abundant in fully grown NSN oocytes, is ablated during the NSN/SN transition and remains low during the oocyte-to-embryo transition (OET). ELAVL2° overexpression during meiotic maturation causes errors in chromosome segregation, indicating the significance of naturally reduced ELAVL2° levels in SN oocytes. On the other hand, during oocyte growth, prematurely reduced Elavl2 expression results in lower yields of fully grown and meiotically matured oocytes, suggesting that Elavl2 is necessary for proper oocyte maturation. Moreover, Elavl2 knockdown showed stimulating effects on translation in fully grown oocytes. We propose that ELAVL2 has an ambivalent role in oocytes: it functions as a pleiotropic translational repressor in efficient production of fully grown oocytes, while its disposal during the NSN/SN transition contributes to the acquisition of full developmental competence. Introduction During life, cohorts of mammalian oocytes resting in follicles resume growth in response to hormones and other signaling cues while staying arrested at the prophase of the first meiotic division (reviewed in ref. 1). Growing oocytes increase their volume and accumulate factors that support later development. The end of the growth phase is marked by an acquisition of meiotic competence and, subsequently, full developmental competence of fully grown germinal vesicle (GV) oocytes. A common mark of the acquisition of developmental competence is transcriptionally silenced condensed chromatin that in the mouse transits gradually from the non-surrounded nucleolus (NSN) to the surrounded nucleolus (SN) chromatin configuration.2-4 While NSN oocytes show reduced rate of meiotic maturation and developmental arrest at the 2-cell stage, SN oocytes can develop to the blastocyst stage in vitro.5,6 Chromatin remodeling during the NSN/SN transition and meiotic maturation involves global changes in histone acetylation and methylation (as reviewed in ref. 7). Nuclear transplantation experiments suggest that both nuclear and cytoplasmic factors (but not the chromatin remodeling itself) determine the meiotic and developmental competence of fully grown SN oocytes in antral follicles.5 Nuclear factors associated with the NSN/SN transition include nucleophosphin 2 (NPM2), a nuclear factor required for establishment of the SN configuration,8,9 and plu(rC)-binding protein 1 (PCBP1), which is necessary for the maintenance of transcriptionally silent state, and knockdown of which causes higher proportion of the NSN configuration.10 In addition, nuclear factors POU5F1 (OCT-4) and DPPA3 (Stella) become upregulated during the NSN/SN transition and were suggested to contribute to the developmental competence of SN oocytes.11,12 Cytoplasmic factors contributing *Correspondence to: Petr Svoboda; Email: Submitted: 12/17/2013; Revised: 02/03/2014; Accepted: 02/04/2014; Published Online: 02/11/2014 http://dx.doi.org/10.4161/cc.28107 www.landesbioscience.com Cell Cycle 1187 to the acquisition of developmental competence during the NSN/ SN transition are poorly understood. The whole transcriptome analysis of NSN and SN oocytes revealed that oocytes with NSN and SN configurations differ in several metabolic pathways, and that during the NSN/SN transition oocytes accumulate factors implicated in meiotic maturation and early development.13 The NSN/SN transition has been linked with changes in the distribution of cytoplasmic subcortical maternal complex (SCMC)14 and in formation of subcortical aggregates from RNA binding proteins in the subcortical domain.15 Recently, a proteomic study of oocytes showed that FILIA and MATER, components of SCMC, become upregulated during the NSN/ SN transition, and that the loss of MATER reduces formation of SN chromatin.16 The absence of mRNA production between the fully grown SN oocyte and zygotic genome activation at the 2-cell stage means that during the OET, gene expression is largely dependent on post-transcriptional regulation of maternal mRNAs. The post-transcriptional control employs cis-acting elements in 3′-untranslated regions (3′UTRs) of maternal mRNAs and trans-acting factors that bind to them. The number, position, and combination of cis-acting elements and the presence of transacting factors offer a complex combinatorial system controlling mRNA localization, stability, and translation.17-20 In vertebrate oocytes, the most studied cis-acting 3′UTR element is the cytoplasmic polyadenylation element (CPE) and its binding protein (CPEB) (reviewed in ref. 21). CPEs and CPEBs play a major role in the meiotic maturation that is driven by cytoplasmic polyadenylation and sequential translational activation of dormant maternal mRNAs.17,22,23 Other highly studied cis-acting motifs are AU-rich elements (AREs). In somatic cells, they regulate stability and translation of up to 8% of mammalian mRNAs.24 At least 24 ARE-binding proteins (AUBPs) have been identified (reviewed in ref. 25), and 13 of them were shown to regulate the ARE-mediated mRNA decay or translation (reviewed in ref. 26). Interestingly, AREs can direct mRNA deadenylation without triggering ARE-mediated mRNA dec (...truncated)