Fully-mature antral mouse oocytes are transcriptionally silent but their heterochromatin maintains a transcriptional permissive histone acetylation profile

The chromatin accessibility landscape of mouse oocytes during configuration transition

The transition of chromatin configuration in mammalian oocytes from a non-surrounded nucleolus (NSN) to a surrounded nucleolus (SN) is critical for acquiring the developmental competence. However, the genomic and epigenomic features underlying this process remain poorly understood. In the present study, we first establish the chromatin accessibility landscape of mouse oocytes from NSN to SN stage. Through the integrative analysis of multi-omics, we find that the establishment of DNA methylation in oocytes is independent of the dynamics of chromatin accessibility. In contrast, histone H3K4me3 status is closely associated with the dynamics of accessible regions during configuration transition. Furthermore, by focusing on the actively transcribed genes in NSN and SN oocytes, we discover that chromatin accessibility coupled with histone methylation (H3K4me3 and H3K27me3) participates in the transcriptional control during phase transition. In sum, our data provide a comprehensive resource for probing configuration transition in oocytes, and offer insights into the mechanisms determining chromatin dynamics and oocyte quality.

DOR activation in mature oligodendrocytes regulates α-ketoglutarate metabolism leading to enhanced remyelination in aged mice

The decreased ability of mature oligodendrocytes to produce myelin negatively affects remyelination in demyelinating diseases and aging, but the underlying mechanisms are incompletely understood. In the present study, we identify a mature oligodendrocyte-enriched transcriptional coregulator diabetes- and obesity-related gene (DOR)/tumor protein p53-inducible nuclear protein 2 (TP53INP2), downregulated in demyelinated lesions of donors with multiple sclerosis and in aged oligodendrocyte-lineage cells. Dor ablation in mice of both sexes results in defective myelinogenesis and remyelination. Genomic occupancy in oligodendrocytes and transcriptome profiling of the optic nerves of wild-type and Dor conditional knockout mice reveal that DOR and SOX10 co-occupy enhancers of critical myelinogenesis-associated genes including Prr18, encoding an oligodendrocyte-enriched, proline-rich factor. We show that DOR targets regulatory elements of genes responsible for α-ketoglutarate biosynthesis in mature oligodendrocytes and is essential for α-ketoglutarate production and lipid biosynthesis. Supplementation with α-ketoglutarate restores oligodendrocyte-maturation defects in Dor-deficient adult mice and improves remyelination after lysolecithin-induced demyelination and cognitive function in 17-month-old wild-type mice. Our data suggest that activation of α-ketoglutarate metabolism in mature oligodendrocytes can promote myelin production during demyelination and aging.

Comprehensive analysis of nonsurrounded nucleolus and surrounded nucleolus oocytes on chromatin accessibility using ATAC-seq

Mouse germinal vesicle (GV) oocytes are divided into surrounded nucleolus (SN) and nonsurrounded nucleolus (NSN) oocytes based on chromatin morphology. NSN oocytes spontaneously transform into SN oocytes after accumulating enough maternal transcripts. SN oocytes show transcriptional silencing. When oocyte maturation is abnormal or takes place in vitro, NSN oocytes do not go through SN stage before proceeding to MII. Nontransitive oocytes show developmental retardation, a low fertilization rate, and arrest at the two-cell embryo stage in mice. Here, chromatin-binding ribonucleic acid polymerase II (RNAP II) activity, newly synthesized RNA, and chromatin accessibility in GV oocytes were examined. In SN oocytes, RNAP II did not bind to DNA, neo-RNA was not generated in nuclei, and the phosphorylation state of RNAP II did not affect the chromatin-binding activity. The number of accessible genes in SN oocytes was remarkably lower than that in NSN oocytes. The accessibility of different functional genes was also different between the two types of oocytes. Thus, low chromatin accessibility leads to transcriptional silencing in SN oocytes.

Overexpression of Tfap2a in Mouse Oocytes Impaired Spindle and Chromosome Organization

Transcription factor AP-2-alpha (Tfap2a) is an important sequence-specific DNA-binding protein that can regulate the transcription of multiple genes by collaborating with inducible viral and cellular enhancer elements. In this experiment, the expression, localization, and functions of Tfap2a were investigated in mouse oocytes during maturation. Overexpression via microinjection of Myc-Tfap2a mRNA into the ooplasm, immunofluorescence, and immunoblotting were used to study the role of Tfap2a in mouse oocyte meiosis. According to our results, Tfap2a plays a vital role in mouse oocyte maturation. Levels of Tfap2a in GV oocytes of mice suffering from type 2 diabetes increased considerably. Tfap2a was distributed in both the ooplasm and nucleoplasm, and its level gradually increased as meiosis resumption progressed. The overexpression of Tfap2a loosened the chromatin, accelerated germinal vesicle breakdown (GVBD), and blocked the first polar body extrusion 14 h after maturation in vitro. The width of the metaphase plate at metaphase I stage increased, and the spindle and chromosome organization at metaphase II stage were disrupted in the oocytes by overexpressed Tfap2a. Furthermore, Tfap2a overexpression dramatically boosted the expression of p300 in mouse GV oocytes. Additionally, the levels of pan histone lysine acetylation (Pan Kac), histone H4 lysine 12 acetylation (H4K12ac), and H4 lysine 16 acetylation (H4K16ac), as well as pan histone lysine lactylation (Pan Kla), histone H3 lysine18 lactylation (H3K18la), and H4 lysine12 lactylation (H4K12la), were all increased in GV oocytes after Tfap2a overexpression. Collectively, Tfap2a overexpression upregulated p300, increased the levels of histone acetylation and lactylation, impeded spindle assembly and chromosome alignment, and ultimately hindered mouse oocyte meiosis.

Maternal effect factors that contribute to oocytes developmental competence: an update

Oocyte developmental competence is defined as the capacity of the female gamete to be fertilized and sustain development to the blastocyst stage. Epigenetic reprogramming, a correct cell division pattern, and an efficient DNA damage response are all critical events that, before embryonic genome activation, are governed by maternally inherited factors such as maternal-effect gene (MEG) products. Although these molecules are stored inside the oocyte until ovulation and exert their main role during fertilization and preimplantation development, some of them are already functioning during folliculogenesis and oocyte meiosis resumption. This mini review summarizes the crucial roles played by MEGs during oocyte maturation, fertilization, and preimplantation development with a direct/indirect effect on the acquisition or maintenance of oocyte competence. Our aim is to inspire future research on a topic with potential clinical perspectives for the prediction and treatment of female infertility.

Redox Biology of Human Cumulus Cells: Basic Concepts, Impact on Oocyte Quality, and Potential Clinical Use

Significance: Four decades have passed since the first successful human embryo conceived from a fertilization in vitro. Despite all advances, success rates in assisted reproduction techniques still remain unsatisfactory and it is well established that oxidative stress can be one of the major factors causing failure in in vitro fertilization (IVF) techniques. Recent Advances: In the past years, researchers have been shown details of the supportive role CCs play along oocyte maturation, development, and fertilization processes. Regarding redox metabolism, it is now evident that the synergism between gamete and somatic CCs is fundamental to further support a healthy embryo, since the oocyte lacks several defense mechanisms that are provided by the CCs. Critical Issues: There are many sources of reactive oxygen species (ROS) in the female reproductive tract in vivo that can be exacerbated (or aggravated) by pathological features. While an imbalance between ROS and antioxidants can result in oxidative damage, physiological levels of ROS are essential for oocyte maturation, ovulation, and early embryonic growth where they act as signaling molecules. At the event of an assisted reproduction procedure, the cumulus/oophorus complex is exposed to additional sources of oxidative stress in vitro. The cumulus cells (CCs) play essential roles in protecting the oocytes from oxidative damage. Future Directions: More studies are needed to elucidate redox biology in human CCs and oocyte. Also, randomized controlled trials will identify possible benefits of in vivo or in vitro administration of antioxidants for patients seeking IVF procedure.

Chromatin Modification and Global Transcriptional Silencing in the Oocyte Mediated by the mRNA Decay Activator ZFP36L2

Global transcriptional silencing is a highly conserved mechanism central to the oocyte-to-embryo transition. We report the unexpected discovery that global transcriptional silencing in oocytes depends on an mRNA decay activator. Oocyte-specific loss of ZFP36L2 an RNA-binding protein that promotes AU-rich element-dependent mRNA decay prevents global transcriptional silencing and causes oocyte maturation and fertilization defects, as well as complete female infertility in the mouse. Single-cell RNA sequencing revealed that ZFP36L2 downregulates mRNAs encoding transcription and chromatin modification regulators, including a large group of mRNAs for histone demethylases targeting H3K4 and H3K9, which we show are bound and degraded by ZFP36L2. Oocytes lacking Zfp36l2 fail to accumulate histone methylation at H3K4 and H3K9, marks associated with the transcriptionally silent, developmentally competent oocyte state. Our results uncover a ZFP36L2-dependent mRNA decay mechanism that acts as a developmental switch during oocyte growth, triggering wide-spread shifts in chromatin modification and global transcription.

Specialize and Divide (Twice): Functions of Three Aurora Kinase Homologs in Mammalian Oocyte Meiotic Maturation

The aurora kinases (AURKs) comprise an evolutionarily conserved family of serine/threonine kinases involved in mitosis and meiosis. While most mitotic cells express two AURK isoforms (AURKA and AURKB), mammalian germ cells also express a third, AURKC. Although much is known about the functions of the kinases in mitosis, less is known about how the three isoforms function to coordinate meiosis. This review is aimed at describing what is known about the three isoforms in female meiosis, the similarities and differences between kinase functions, and speculates as to why mammalian germ cells require expression of three AURKs instead of two.

Kdm5/Lid Regulates Chromosome Architecture in Meiotic Prophase I Independently of Its Histone Demethylase Activity

During prophase of the first meiotic division (prophase I), chromatin dynamically reorganises to recombine and prepare for chromosome segregation. Histone modifying enzymes are major regulators of chromatin structure, but our knowledge of their roles in prophase I is still limited. Here we report on crucial roles of Kdm5/Lid, one of two histone demethylases in Drosophila that remove one of the trimethyl groups at Lys4 of Histone 3 (H3K4me3). In the absence of Kdm5/Lid, the synaptonemal complex was only partially formed and failed to be maintained along chromosome arms, while localisation of its components at centromeres was unaffected. Kdm5/Lid was also required for karyosome formation and homologous centromere pairing in prophase I. Although loss of Kdm5/Lid dramatically increased the level of H3K4me3 in oocytes, catalytically inactive Kdm5/Lid can rescue the above cytological defects. Therefore Kdm5/Lid controls chromatin architecture in meiotic prophase I oocytes independently of its demethylase activity.

Accurate transmission of chromosomes carrying genetic materials from generation to generation is essential for life. Cell divisions that generate gametes, such as eggs and sperm, are critical, as chromosomes inherited from both parents recombine and are accurately sorted into gametes. Errors in these cell divisions often result in infertility, miscarriages or birth defects such as Down syndrome in humans. During these divisions, chromosomes undergo dramatic reorganisation but the molecular mechanisms are not well understood. Chromosome organisation is known to be regulated by various epigenetic marks, which are chemical marks on chromatin crucial for regulating gene expression. We found that an enzyme (Kdm5/Lid) that erases a mark linked to active gene expression regulates multiple aspects of meiotic chromatin organisation in oocytes, including stability of the recombination machinery. Unexpectedly, this function does not require its enzymatic activity. Our findings provide novel insights into how chromosomes are reorganised during reproduction and prompt re-evaluation of the role of this eraser enzyme.

Nucleolar Translocation of Histone Deacetylase 2 Is Involved in Regulation of Transcriptional Silencing in the Cat Germinal Vesicle

Histone deacetylase 2 (HDAC2) is a key transcriptional coregulator that is suspected to play a role during oogenesis. It is known that RNA transcription in the cat germinal vesicle (GV) stops during folliculogenesis at the late antral follicle stage and is unrelated to histone deacetylation or chromatin condensation. The objective of the present study was to determine if and how HDAC2 participates in transcription regulation in the cat GV. Spatiotemporal HDAC2 protein expression was examined by immunostaining oocytes from primary to large antral follicles. HDAC2 was detected in the majority of GVs within oocytes from early, small, and large antral follicles. At early and small antral stages, HDAC2 was found primarily in the GV's nucleoplasm. There then was a significant shift in HDAC2 localization into the nucleolus, mostly in oocytes from large antral follicles. Assessments revealed that transcription was active in oocytes that contained nucleoplasm-localized HDAC2, whereas nucleolar-bound HDAC2 was associated with loss of both global transcription and ribosomal RNA presence at all antral stages. When oocytes were exposed to the HDAC inhibitor valproic acid, results indicated that HDAC regulated transcriptional activity in the nucleoplasm, but not in the nucleolus. Collective results suggest that nucleolar translocation of HDAC2 is associated with transcriptional silencing in the GV, thereby likely contributing to an oocyte's acquisition of competence.

A requirement for ERK-dependent Dicer phosphorylation in coordinating oocyte-to-embryo transition in C. elegans

Signaling pathways and small RNAs direct diverse cellular events, but few examples are known of defined signaling pathways directly regulating small RNA biogenesis. We show that ERK phosphorylates Dicer on two conserved residues in its RNase IIIb and double-stranded RNA (dsRNA)-binding domains and that phosphorylation of these residues is necessary and sufficient to trigger Dicer's nuclear translocation in worms, mice, and human cells. Phosphorylation of Dicer on either site inhibits Dicer function in the female germline and dampens small RNA repertoire. Our data demonstrate that ERK phosphorylates and inhibits Dicer during meiosis I for oogenesis to proceed normally in Caenorhabditis elegans and that this inhibition is released before fertilization for embryogenesis to proceed normally. The conserved Dicer residues, their phosphorylation by ERK, and the consequences of the resulting modifications implicate an ERK-Dicer nexus as a fundamental component of the oocyte-to-embryo transition and an underlying mechanism coupling extracellular cues to small RNA production.

Nucleolus-like bodies of fully-grown mouse oocytes contain key nucleolar proteins but are impoverished for rRNA

It is well known that fully-grown mammalian oocytes, rather than typical nucleoli, contain prominent but structurally homogenous bodies called "nucleolus-like bodies" (NLBs). NLBs accumulate a vast amount of material, but their biochemical composition and functions remain uncertain. To clarify the composition of the NLB material in mouse GV oocytes, we devised an assay to detect internal oocyte proteins with fluorescein-5-isothiocyanate (FITC) and applied the fluorescent RNA-binding dye acridine orange to examine whether NLBs contain RNA. Our results unequivocally show that, similarly to typical nucleoli, proteins and RNA are major constituents of transcriptionally active (or non-surrounded) NLBs as well as of transcriptionally silent (or surrounded) NLBs. We also show, by exposing fixed oocytes to a mild proteinase K treatment, that the NLB mass in oocytes of both types contains nucleolar proteins that are involved in all major steps of ribosome biogenesis, including rDNA transcription (UBF), early rRNA processing (fibrillarin), and late rRNA processing (NPM1/nucleophosmin/B23, nucleolin/C23), but none of the nuclear proteins tested, including SC35, NOBOX, topoisomerase II beta, HP1α, and H3. The ribosomal RPL26 protein was detected within the NLBs of NSN-type oocytes but is virtually absent from NLBs of SN-type oocytes. Taking into account that the major class of nucleolar RNA is ribosomal RNA (rRNA), we applied fluorescence in situ hybridization with oligonucleotide probes targeting 18S and 28S rRNAs. The results show that, in contrast to active nucleoli, NLBs of fully-grown oocytes are impoverished for the rRNAs, which is consistent with the absence of transcribed ribosomal genes in the NLB mass. Overall, the results of this study suggest that NLBs of fully-grown mammalian oocytes serve for storing major nucleolar proteins but not rRNA.

Restraint stress on female mice diminishes the developmental potential of oocytes: roles of chromatin configuration and histone modification in germinal vesicle stage oocytes

The mechanisms by which restraint stress impairs oocyte developmental potential are unclear. Factors causing differences between the developmental potential of oocytes with surrounded nucleolus (SN) and that of oocytes with nonsurrounded nucleolus (NSN) are not fully characterized. Furthermore, the relationship between increased histone acetylation and methylation and the increased developmental competence in SN oocytes is particularly worth exploring using a system where the SN configuration can be uncoupled (dissociated) from increased histone modifications. In this study, female mice were subjected to restraint for 24 or 48 h or for 23 days before being examined for oocyte chromatin configuration, histone modification, and development in vitro and in vivo. Results showed that restraint for 48 h or 23 days impaired NSN-to-SN transition, histone acetylation and methylation in SN oocytes, and oocyte developmental potential. However, whereas the percentage of stressed SN oocytes returned to normal after a 48-h postrestraint recovery, neither histone acetylation/methylation in SN oocytes nor developmental competence recovered following postrestraint recovery with equine chorionic gonadotropin (eCG) injection. Priming unstressed mice with eCG expedited oocyte histone modification to an early completion. Contrary to the levels of acetylated and methylated histones, the level of phosphorylated H3S10 increased significantly in the stressed SN oocytes. Together, the results suggest that 1) restraint stress impaired oocyte potential with disturbed histone modifications; 2) SN configuration was uncoupled from increased histone acetylation/methylation in the restraint-stressed oocytes; and 3) the developmental potential of SN oocytes is more closely correlated with epigenetic histone modification than with chromatin configuration.