An embryonic poly(A)-binding protein (ePAB) is expressed in mouse oocytes and early preimplantation embryos

Abstract

Gene expression during oocyte maturation, fertilization, and early embryo development until zygotic gene activation is regulated mainly by translational activation of maternally derived mRNAs. This process requires the presence of a poly(A)-binding protein. However, the cytoplasmic somatic cell poly(A)-binding protein (PABP1) is not expressed until later in embryogenesis. We recently identified an embryonic poly(A)-binding protein (ePAB) in Xenopus. ePAB is the predominant cytoplasmic PABP in Xenopus oocytes and early embryos and prevents deadenylation of mRNAs, suggesting its importance in the regulation of gene expression during early Xenopus development. Here we report the identification of the mouse ortholog of Xenopus ePAB. The mouse ePAB gene on chromosome 2 contains 14 exons that specify an alternatively spliced mRNA encoding a protein of 608 or 561 aa with Ϸ65% identity to Xenopus ePAB. Mouse ePAB mRNA is expressed in ovaries and testis but not in somatic tissues. In situ hybridization localizes ePAB RNA to oocytes and confirms its absence from surrounding somatic cells in the mouse ovary. During early development, mouse ePAB is expressed in prophase I and metaphase II oocytes and one-cell and two-cell embryos and then becomes undetectable in four-or-more-cell embryos. In contrast, PABP1 mRNA expression is minimal in oocytes and early embryos until the eight-cell stage when it increases, becoming predominant at the blastocyst stage. The expression of mouse ePAB before zygotic gene activation argues for its importance in translational activation of maternally derived mRNAs during mammalian oocyte and early preimplantation embryo development.translational activation ͉ embryogenesis ͉ oogenesis M echanisms for establishing the germline and carrying out oogenesis in evolutionarily distant animals exhibit common themes. Gametes develop from primordial germ cells that are set aside during early embryogenesis (1). In most metazoans, primordial germ cells have an extragonadal origin and migrate to reach the somatic gonad, where they proliferate by mitosis to form oocytes (1). Oocytes, in turn, enter meiosis only to be arrested at the prophase of the first meiotic division (2, 3). This first meiotic arrest may last up to a few years in Xenopus and several decades in humans and is characterized by synthesis and storage of large quantities of dormant mRNA (4, 5). The resumption of meiosis is stimulated by progesterone in Xenopus (6, 7) or by gonadotropins in mouse and human (8, 9) and marks the onset of oocyte maturation.Oocyte maturation is accompanied by a complex network of translational activation and repression of dormant maternal mRNAs (10-13), whereas transcription is limited at best. These maternal mRNAs drive the oocyte's reentry into meiosis and control the rate of mitosis during the early cleavage divisions of the embryo (13-16). The transcriptional silencing that begins with oocyte maturation persists during the initial mitotic divisions of the embryonic cells. In Xenopus, after 12 rapid synchronous cle...

Metazoan oocyte and early embryo development program: a progression through translation regulatory cascades

“…Cold Spring Harbor Laboratory Press on March 23, 2019 -Published by genesdev.cshlp.org Downloaded from ePABP has been characterized in Xenopus and mouse (Voeltz et al 2001;Seli et al 2005) and is ∼70% identical at the amino acid level to classical PABP, with the most marked differences near the C terminus. Since this region is essential for interaction with PABP-specific regulators that can modulate function, truncation as well as variations in the C-terminal region of ePABP relative to PABP may define novel roles.…”

“…Yet, PABP and ePABP have been demonstrated to be interchangeable for many PABP functions, including rescue of a pab1⌬ lethality in Saccharomyces cerevisiae, interactions with the cap-binding eIF4G complex, and with termination factor eRF3, poly(A), and AU-rich element binding, as well as preventing deadenylation of the mRNA (Voeltz et al 2001;Cao and Richter 2002;Cosson et al 2002). A feature that distinguishes ePABP from the normal cytoplasmic PABP is its expression pattern: ePABP has so far been found to be expressed exclusively in an oocyte-and embryo-specific manner (Voeltz et al 2001;Seli et al 2005). The realization that ePABP is the predominant PABP present during oocyte maturation and early embryogenesis in both Xenopus and mouse begs the question of how it may function differently from cytoplasmic PABP, which replaces it later in development.…”

The poly(A)-binding protein genes, EPAB, PABPC1, and PABPC3 are differentially expressed in infertile men with non-obstructive azoospermia

“…Notably, in a recently published study, the EPAB mRNA being transcribed in the human somatic cells and 8-cell and blastocyst stage early embryos includes a premature stop codon in the exon 8 [30]. In marked contrast, mouse Epab is found to be transcribed only in the ovary and testis tissues, but not in the somatic tissues [25]. Further analysis on the postnatal mouse testes and isolated spermatogenic cells showed that it is expressed solely in the spermatogenic cells, and exhibits prominent expressional differences during testicular development [26], and in the postnatal mouse ovaries [32].…”

“…The embryonic poly(A)-binding protein (EPAB, also known as ePABP and PABPc1-like), is expressed in mouse [25,26] and human [27] gonads. In the postnatal mouse testes, Epab mRNA expression is localized to only spermatogenic cell types, but it is not expressed in the somatic cells [26].…”

“…Conversely, the Epab −/− female mice are infertile due to impairments in oocyte maturation and early embryo development processes [29]. The poly(A)-binding protein, cytoplasmic 1 (PABPC1, also known as PABP1) defined as a somatic poly(A)-binding protein is generally expressed in mouse [25,26] and human [27,30] gonads. On the other hand, the poly(A)-binding protein, cytoplasmic 3 (PABPC3; also known as tPABP, and PABPC2 in mouse) is synthesized only in spermatocytes and round spermatids in mouse [21,22], but only in round spermatids in human testis [24].…”