PABPN1 functions as a hub in the assembly of nuclear poly(A) domains that are essential for mouse oocyte development.

Growing oocytes store a large amount of maternal mRNA to support the subsequent "maternal-zygotic transition" process. At present, it is not clear how the growing oocytes store and process the newly transcribed mRNA under physiological conditions. In this study, we report non-membrane-bound compartments, nuclear poly(A) domains (NPADs), as the hub for newly transcribed mRNA, in developing mouse oocytes. The RNA binding protein PABPN1 promotes the formation of NPAD through its N-terminal disordered domain and RNA-recognized motif by means of liquid phase separation. Pabpn1-null growing oocytes cannot form NPAD normally in vivo and have defects in stability of oocyte growing-related transcripts and formation of long 3' untranslated region isoform transcripts. Ultimately, Pabpn1fl/fl;Gdf9-Cre mice are completely sterile with primary ovarian insufficiency. These results demonstrate that NPAD formed by the phase separation properties of PABPN1-mRNA are the hub of the newly transcribed mRNA and essential for the development of oocytes and female reproduction.

Nuclear poly(A) binding protein 1 (PABPN1) mediates zygotic genome activation-dependent maternal mRNA clearance during mouse early embryonic development.

An embryo starts its life with maternal mRNA clearance, which is crucial for embryonic development. The elimination of maternal transcripts occurs by the joint action of two pathways: the maternally encoded mRNA decay pathway (M-decay) and the zygotic genome activation (ZGA)-dependent pathway (Z-decay). However, zygotic factors triggering maternal mRNA decay in early mammalian embryos remain largely unknown. In this study, we identified the zygotically encoded nuclear poly(A) binding protein 1 (PABPN1) as a factor required for maternal mRNA turnover, with a previously undescribed cytoplasmic function. Cytoplasmic PABPN1 docks on 3'-uridylated transcripts, downstream of terminal uridylyl transferases TUT4 and TUT7, and recruits 3'-5' exoribonuclease DIS3L2 to its targets, facilitating maternal mRNA decay. Pabpn1-knockout in mice resulted in preimplantation stage mortality due to early developmental arrest at the morula stage. Maternal mRNAs to be eliminated via the Z-decay pathway failed to be removed from Pabpn1-depleted embryos. Furthermore, PABPN1-mediated Z-decay is essential for major ZGA and regulates the expression of cell fate-determining factors in mouse preimplantation embryos. This study revealed an unforeseen cytoplasmic function of PABPN1 coupled with early embryonic development, characterized the presence of a zygotic destabilizer of maternal mRNA, and elucidated the Z-decay process mechanisms, which potentially contribute to human fertility.

Oocyte meiosis-coupled poly(A) polymerase α phosphorylation and activation trigger maternal mRNA translation in mice.

Mammalian oocyte maturation is driven by strictly regulated polyadenylation and translational activation of maternal mRNA stored in the cytoplasm. However, the poly(A) polymerase (PAP) that directly mediates cytoplasmic polyadenylation in mammalian oocytes has not been determined. In this study, we identified PAPα as the elusive enzyme that catalyzes cytoplasmic mRNA polyadenylation implicated in mouse oocyte maturation. PAPα was mainly localized in the germinal vesicle (GV) of fully grown oocytes but was distributed to the ooplasm after GV breakdown. Inhibition of PAPα activity impaired cytoplasmic polyadenylation and translation of maternal transcripts, thus blocking meiotic cell cycle progression. Once an oocyte resumes meiosis, activated CDK1 and ERK1/2 cooperatively mediate the phosphorylation of three serine residues of PAPα, 537, 545 and 558, thereby leading to increased activity. This mechanism is responsible for translational activation of transcripts lacking cytoplasmic polyadenylation elements in their 3'-untranslated region (3'-UTR). In turn, activated PAPα stimulated polyadenylation and translation of the mRNA encoding its own (Papola) through a positive feedback circuit. ERK1/2 promoted Papola mRNA translation in a 3'-UTR polyadenylation signal-dependent manner. Through these mechanisms, PAPα activity and levels were significantly amplified, improving the levels of global mRNA polyadenylation and translation, thus, benefiting meiotic cell cycle progression.

Revisiting poly(A)-binding proteins: Multifaceted regulators during gametogenesis and early embryogenesis.

Post-transcriptional regulation faces a distinctive challenge in gametes. Transcription is limited when the germ cells enter the division phase due to condensed chromatin, while gene expression during gamete maturation, fertilization, and early cleavage depends on existing mRNA post-transcriptional coordination. The dynamics of the 3'-poly(A) tail play crucial roles in defining mRNA fate. The 3'-poly(A) tail is covered with poly(A)-binding proteins (PABPs) that help to mediate mRNA metabolism and recent work has shed light on the number and function of germ cell-specific expressed PABPs. There are two structurally different PABP groups distinguished by their cytoplasmic and nuclear localization. Both lack catalytic activity but are coupled with various roles through their interaction with multifunctional partners during mRNA metabolism. Here, we present a synopsis of PABP function during gametogenesis and early embryogenesis and describe both conventional and current models of the functions and regulation of PABPs, with an emphasis on the physiological significance of how germ cell-specific PABPs potentially affect human fertility.

PABPN1L mediates cytoplasmic mRNA decay as a placeholder during the maternal-to-zygotic transition.

Maternal mRNA degradation is a critical event of the maternal-to-zygotic transition (MZT) that determines the developmental potential of early embryos. Nuclear Poly(A)-binding proteins (PABPNs) are extensively involved in mRNA post-transcriptional regulation, but their function in the MZT has not been investigated. In this study, we find that the maternally expressed PABPN1-like (PABPN1L), rather than its ubiquitously expressed homolog PABPN1, acts as an mRNA-binding adapter of the mammalian MZT licensing factor BTG4, which mediates maternal mRNA clearance. Female Pabpn1l null mice produce morphologically normal oocytes but are infertile owing to early developmental arrest of the resultant embryos at the 1- to 2-cell stage. Deletion of Pabpn1l impairs the deadenylation and degradation of a subset of BTG4-targeted maternal mRNAs during the MZT. In addition to recruiting BTG4 to the mRNA 3'-poly(A) tails, PABPN1L is also required for BTG4 protein accumulation in maturing oocytes by protecting BTG4 from SCF-ßTrCP1 E3 ubiquitin ligase-mediated polyubiquitination and degradation. This study highlights a noncanonical cytoplasmic function of nuclear poly(A)-binding protein in mRNA turnover, as well as its physiological importance during the MZT.

The CRL4-DCAF13 ubiquitin E3 ligase supports oocyte meiotic resumption by targeting PTEN degradation.

Cullin ring-finger ubiquitin ligase 4 (CRL4) has multiple functions in the maintenance of oocyte survival and meiotic cell cycle progression. DCAF13, a novel CRL4 adaptor, is essential for oocyte development. But the mechanisms by which CRL4-DCAF13 supports meiotic maturation remained unclear. In this study, we demonstrated that DCAF13 stimulates the meiotic resumption-coupled activation of protein synthesis in oocytes, partially by maintaining the activity of PI3K signaling pathway. CRL4-DCAF13 targets the polyubiquitination and degradation of PTEN, a lipid phosphatase that inhibits PI3K pathway as well as oocyte growth and maturation. Dcaf13 knockout in oocytes caused decreased CDK1 activity and impaired meiotic cell cycle progression and chromosome condensation defects. As a result, chromosomes fail to be aligned at the spindle equatorial plate, the spindle assembly checkpoint is activated, and most Dcaf13 null oocytes are arrested at the prometaphase I. The DCAF13-dependent PTEN degradation mechanism fits in as a missing link between CRL4 ubiquitin E3 ligase and PI3K pathway, both of which are crucial for translational activation during oocyte GV-MII transition.

Maternal DCAF13 Regulates Chromatin Tightness to Contribute to Embryonic Development.

Maternal-zygotic transition (MZT) is critical for the developmental control handed from maternal products to newly synthesized zygotic genome in the earliest stage of embryogenesis. However, the spatiotemporal dynamic regulation of MZT by maternal factors is largely unknown. Here, we reported a novel maternal factor, DCAF13, which was highly expressed in growing oocyte nucleolus and had key maternal effects on oocyte and zygotic chromatin tightness during maternal to zygotic transition. DCAF13 specifically deleted in oocytes resulted in loose chromatin structure in fully grown germinal vesicle oocytes. Despite normal nuclear maturation in maternal DCAF13-deleted oocytes, the chromosomes at MII stage were not properly condensed. Consequently, the nuclear and nucleolar structure reorganized abnormally, and transcription was inactive in zygotic embryos. RNA-seq analysis of MII oocytes and 2-cell embryos demonstrated that the transcriptomes between knockout and control oocyte were similar, but the maternal DCAF13 deleted two-cell embryos showed a significant decrease in transcription. In addition, the maternal DCAF13-deleted embryos displayed arrest at the two-cell stage, which could not be rescued by injecting flag-Dcaf13 mRNA in the zygote. This revealed that DCAF13 was a unique maternal effect factor regulating the nucleolus.

Mammalian nucleolar protein DCAF13 is essential for ovarian follicle maintenance and oocyte growth by mediating rRNA processing.

During mammalian oocyte growth, chromatin configuration transition from the nonsurrounded nucleolus (NSN) to surrounded nucleolus (SN) type plays a key role in the regulation of gene expression and acquisition of meiotic and developmental competence by the oocyte. Nonetheless, the mechanism underlying chromatin configuration maturation in oocytes is poorly understood. Here we show that nucleolar protein DCAF13 is an important component of the ribosomal RNA (rRNA)-processing complex and is essential for oocyte NSN-SN transition in mice. A conditional knockout of Dcaf13 in oocytes led to the arrest of oocyte development in the NSN configuration, follicular atresia, premature ovarian failure, and female sterility. The DCAF13 deficiency resulted in pre-rRNA accumulation in oocytes, whereas the total mRNA level was not altered. Further exploration showed that DCAF13 participated in the 18S rRNA processing in growing oocytes. The lack of 18S rRNA because of DCAF13 deletion caused a ribosome assembly disorder and then reduced global protein synthesis. DCAF13 interacted with a protein of the core box C/D ribonucleoprotein, fibrillarin, i.e., a factor of early pre-rRNA processing. When fibrillarin was knocked down in the oocytes from primary follicles, follicle development was inhibited as well, indicating that an rRNA processing defect in the oocyte indeed stunts chromatin configuration transition and follicle development. Taken together, these results elucidated the in vivo function of novel nucleolar protein DCAF13 in maintaining mammalian oogenesis.

DCAF13 promotes pluripotency by negatively regulating SUV39H1 stability during early embryonic development.

Mammalian oocytes and zygotes have the unique ability to reprogram a somatic cell nucleus into a totipotent state. SUV39H1/2-mediated histone H3 lysine-9 trimethylation (H3K9me3) is a major barrier to efficient reprogramming. How SUV39H1/2 activities are regulated in early embryos and during generation of induced pluripotent stem cells (iPSCs) remains unclear. Since expression of the CRL4 E3 ubiquitin ligase in oocytes is crucial for female fertility, we analyzed putative CRL4 adaptors (DCAFs) and identified DCAF13 as a novel CRL4 adaptor that is essential for preimplantation embryonic development. Dcaf13 is expressed from eight-cell to morula stages in both murine and human embryos, and Dcaf13 knockout in mice causes preimplantation-stage mortality. Dcaf13 knockout embryos are arrested at the eight- to sixteen-cell stage before compaction, and this arrest is accompanied by high levels of H3K9me3. Mechanistically, CRL4-DCAF13 targets SUV39H1 for polyubiquitination and proteasomal degradation and therefore facilitates H3K9me3 removal and zygotic gene expression. Taken together, CRL4-DCAF13-mediated SUV39H1 degradation is an essential step for progressive genome reprogramming during preimplantation embryonic development.

Mitochondrial Function Regulated by Mitoguardin-1/2 Is Crucial for Ovarian Endocrine Functions and Ovulation.

The balances of mitochondrial dynamic changes, mitochondrial morphology, and mitochondrial number are critical in cell metabolism. Once they are disturbed, disorders in these processes generally cause diseases or even death in animals. We performed large-scale genetic screenings in fruit flies and discovered the mitoguardin gene (Miga) that encodes for a mitochondrial outer membrane protein. To examine the physiological functions of its mammalian homologs Miga1 and Miga2, we generated Miga1 and Miga2 single- and double-knockout mouse strains and found that the knockout mice were viable, but the females were subfertile. The ovarian phenotypes of these mice suggested that the MIGA1/2 proteins play an important role in ovulation and ovarian steroidogenesis. In vivo and in vitro analyses of Miga1/2-knockout granulosa cells showed severe defects in luteinization and steroidogenesis and disordered mitochondrial morphology and function in response to gonadotropins. This is a report of genes involved in mitochondrial fusion and morphology-regulating mitochondrial functions during ovulation and luteinization. These results suggest a mechanism of gonadotropin-regulated ovarian endocrine functions and provide clues for therapeutic treatments of infertile females.