BTG4, a maternal mRNA cleaner

Neddylation inhibition affects early embryonic development by disrupting maternal-to-zygotic transition and mitochondrial function in mice

Post-translational modifications (PTMs) are critical for early development in mice because early cleavage-stage embryos are characterized by transcriptional inactivity. Neddylation is an important ubiquitin-like PTM that regulates multiple biophysical processes. However, the exact roles of neddylation in regulating early embryonic development remain largely unknown. In the present study, we found that inhibition of neddylation by specific inhibitor MLN4924 led to severe arrest of early embryonic development. Transcriptomic analysis showed that neddylation inhibition changed the expression of 3959 genes at the 2-cell stage. Importantly, neddylation inhibition blocked zygotic genome activation and maternal mRNA degradation, thus disrupting the maternal-to-zygotic transition. Moreover, inhibition of neddylation induced mitochondrial dysfunction including aberrant mitochondrial distribution, decreased mitochondrial membrane potential, and reduced ATP content. Further analysis showed that inhibition of neddylation resulted in the accumulation of reactive oxygen species and superoxide anion, thereby resulting in oxidative stress and severe DNA damage at the 2-cell stage. Overall, this study demonstrates that neddylation is vital for early embryonic development in mice. Our findings suggest that proper neddylation regulation is essential for the timely inter-stage transition during early embryonic development.

LSM14B is essential for oocyte meiotic maturation by regulating maternal mRNA storage and clearance

Fully grown oocytes remain transcriptionally quiescent, yet many maternal mRNAs are synthesized and retained in growing oocytes. We now know that maternal mRNAs are stored in a structure called the mitochondria-associated ribonucleoprotein domain (MARDO). However, the components and functions of MARDO remain elusive. Here, we found that LSM14B knockout prevents the proper storage and timely clearance of mRNAs (including Cyclin B1, Btg4 and other mRNAs that are translationally activated during meiotic maturation), specifically by disrupting MARDO assembly during oocyte growth and meiotic maturation. With decreased levels of storage and clearance, the LSM14B knockout oocytes failed to enter meiosis II, ultimately resulting in female infertility. Our results demonstrate the function of LSM14B in MARDO assembly, and couple the MARDO with mRNA clearance and oocyte meiotic maturation.

Acute exposure of triclocarban affects early embryo development in mouse through disrupting maternal-to-zygotic transition and epigenetic modifications

Triclocarban (TCC) is a broad-spectrum antibacterial agent used globally, and high concentrations of this harmful chemical exist in the environment. The human body is directly exposed to TCC through skin contact. Moreover, TCC is also absorbed through diet and inhaled through breathing, which results in its accumulation in the body. The safety profile of TCC and its potential impact on human health are still not completely clear; therefore, it becomes imperative to evaluate the reproductive toxicity of TCC. Here, we explored the effect of TCC on the early embryonic development of mice and its associated mechanisms. We found that acute exposure of TCC affected the early embryonic development of mice in a dose-dependent manner. Approximately 7600 differentially expressed genes (DEGs) were obtained by sequencing the transcriptome of 2-cell mouse embryos; of these, 3157 genes were upregulated and 4443 genes were downregulated in the TCC-treated embryos. GO and KEGG analysis revealed that the enriched genes were mainly involved in redox processes, RNA synthesis, DNA damage, apoptosis, mitochondria, endoplasmic reticulum, Golgi apparatus, cytoskeleton, peroxisome, RNA polymerase, and other components or processes. Moreover, the Venn analysis showed that the zygotic genome activation (ZGA) was affected and the degradation of maternal effector genes was inhibited. TCC induced changes in the epigenetic modification of 2-cell embryos. The level of DNA methylation increased significantly. Further, the levels of H3K27ac, H3K9ac, and H3K27me3 histone modifications decreased significantly, whereas those of H3K4me3 and H3K9me3 modifications increased significantly. Additionally, TCC induced oxidative stress and DNA damage in the 2-cell embryos. In conclusion, acute exposure of TCC affected early embryo development, destroyed early embryo gene expression, interfered with ZGA and maternal gene degradation, induced changes in epigenetic modification of early embryos, and led to oxidative stress and DNA damage in mouse early embryos.

A novel homozygous C-terminal deletion in BTG4 causes zygotic cleavage failure and female infertility

PURPOSE

We aimed to identify pathogenic variants in a female patient with primary infertility and recurrent failure of in vitro fertilization with zygotic cleavage failure.

METHODS

The genomic DNA from the affected individual was subjected to whole-exome sequencing and the variant was confirmed by Sanger sequencing. The functional effect of the identified variant was further investigated in 293 T cells.

RESULTS

We identified a novel homozygous deletion in BTG4 (c.580_616del) in the affected individual. The deletion results in frameshift and replacement of the last 29 residues (aa195-223) with 66 random amino acids. The mutated amino acid residues are highly conserved among mammalian species. Co-immunoprecipitation in 293 T cells showed that the mutation abolished the interaction between BTG4 and PABPN1L.

CONCLUSION

This study conforms previous studies and expands the mutational spectrum of BTG4. Our findings prove the functional importance of the C-terminal of BTG4. BTG4 is a potential diagnostic and therapeutic target for patients suffering from zygotic cleavage failure.

Cnot8 eliminates naïve regulation networks and is essential for naïve-to-formative pluripotency transition

Mammalian early epiblasts at different phases are characterized by naïve, formative, and primed pluripotency states, involving extensive transcriptome changes. Here, we report that deadenylase Cnot8 of Ccr4-Not complex plays essential roles during the transition from naïve to formative state. Knock out (KO) Cnot8 resulted in early embryonic lethality in mice, but Cnot8 KO embryonic stem cells (ESCs) could be established. Compared with the cells differentiated from normal ESCs, Cnot8 KO cells highly expressed a great many genes during their differentiation into the formative state, including several hundred naïve-like genes enriched in lipid metabolic process and gene expression regulation that may form the naïve regulation networks. Knockdown expression of the selected genes of naïve regulation networks partially rescued the differentiation defects of Cnot8 KO ESCs. Cnot8 depletion led to the deadenylation defects of its targets, increasing their poly(A) tail lengths and half-life, eventually elevating their expression levels. We further found that Cnot8 was involved in the clearance of targets through its deadenylase activity and the binding of Ccr4-Not complex, as well as the interacting with Tob1 and Pabpc1. Our results suggest that Cnot8 eliminates naïve regulation networks through mRNA clearance, and is essential for naïve-to-formative pluripotency transition.

Maternal RNF114-mediated target substrate degradation regulates zygotic genome activation in mouse embryos

Zygotic genomic activation (ZGA) is a landmark event in the maternal-to-zygotic transition (MZT), and the regulation of ZGA by maternal factors remains to be elucidated. In this study, the depletion of maternal ring finger protein 114 (RNF114), a ubiquitin E3 ligase, led to developmental arrest of two-cell mouse embryos. Using immunofluorescence and transcriptome analysis, RNF114 was proven to play a crucial role in major ZGA. To study the underlying mechanism, we performed protein profiling in mature oocytes and found a potential substrate for RNF114, chromobox 5 (CBX5), ubiquitylation and degradation of which was regulated by RNF114. The overexpression of CBX5 prevented embryonic development and impeded major ZGA. Furthermore, TAB1 was abnormally accumulated in mutant two-cell embryos, which was consistent with the result of in vitro knockdown of Rnf114. Knockdown of Cbx5 or Tab1 in maternal RNF114-depleted embryos partially rescued developmental arrest and the defect of major ZGA. In summary, our study reveals that maternal RNF114 plays a precise role in degrading some important substrates during the MZT, the misregulation of which may impede the appropriate activation of major ZGA in mouse embryos.

Oocyte Spontaneous Activation: An Overlooked Cellular Event That Impairs Female Fertility in Mammals

In mammals, including humans, mature oocytes are ovulated into the oviduct for fertilization. Normally, these oocytes are arrested at metaphase of the second meiosis (MII), and this arrest can be maintained for a certain period, which is essential for fertilization in vivo and oocyte manipulations in vitro, such as assisted reproduction in clinics and nuclear/spindle transfer in laboratories. However, in some species and under certain circumstances, exit from MII occurs spontaneously without any obvious stimulation or morphological signs, which is so-called oocyte spontaneous activation (OSA). This mini-review summarizes two types of OSA. In the first type (e.g., most rat strains), oocytes can maintain MII arrest in vivo, but once removed out, oocytes undergo OSA with sister chromatids separated and eventually scattered in the cytoplasm. Because the stimulation is minimal (oocyte collection itself), this OSA is incomplete and cannot force oocytes into interphase. Notably, once re-activated by sperm or chemicals, those scattered chromatids will form multiple pronuclei (MPN), which may recapitulate certain MPN and aneuploidy cases observed in fertility clinics. The second type of OSA occurs in ovarian oocytes (e.g., certain mouse strains and dromedary camel). Without ovulation or fertilization, these OSA-oocytes can initiate intrafollicular development, but these parthenotes cannot develop to term due to aberrant genomic imprinting. Instead, they either degrade or give rise to ovarian teratomas, which have also been reported in female patients. Last but not the least, genetic models displaying OSA phenotypes and the lessons we can learn from animal OSA for human reproduction are also discussed.

Loss of Cnot6l Impairs Inosine RNA Modifications in Mouse Oocytes

Mammalian oocytes must degrade maternal transcripts through a process called translational mRNA decay, in which maternal mRNA undergoes translational activation, followed by deadenylation and mRNA decay. Once a transcript is translationally activated, it becomes deadenylated by the CCR4-NOT complex. Knockout of CCR4-NOT Transcription Complex Subunit 6 Like (Cnot6l), a deadenylase within the CCR4-NOT complex, results in mRNA decay defects during metaphase I (MI) entry. Knockout of B-cell translocation gene-4 (Btg4), an adaptor protein of the CCR4-NOT complex, results in mRNA decay defects following fertilization. Therefore, mechanisms controlling mRNA turnover have significant impacts on oocyte competence and early embryonic development. Post-transcriptional inosine RNA modifications can impact mRNA stability, possibly through a translation mechanism. Here, we assessed inosine RNA modifications in oocytes, eggs, and embryos from Cnot6l^-/- and Btg4^-/- mice, which display stabilization of mRNA and over-translation of the stabilized transcripts. If inosine modifications have a role in modulating RNA stability, we hypothesize that in these mutant backgrounds, we would observe changes or a disruption in inosine mRNA modifications. To test this, we used a computational approach to identify inosine RNA modifications in total and polysomal RNA-seq data during meiotic maturation (GV, MI, and MII stages). We observed pronounced depletion of inosine mRNA modifications in samples from Cnot6l^-/-, but not in Btg4^-/- mice. Additionally, analysis of ribosome-associated RNA revealed clearance of inosine modified mRNA. These observations suggest a novel mechanism of mRNA clearance during oocyte maturation, in which inosine-containing transcripts decay in an independent, but parallel mechanism to CCR4-NOT deadenylation.

A story of birth and death: mRNA translation and clearance at the onset of maternal-to-zygotic transition in mammals†

In mammals, maternal-to-zygotic transition (MZT), or oocyte-to-embryo transition, begins with oocyte meiotic resumption due to the sequential translational activation and destabilization of dormant maternal transcripts stored in the ooplasm. It then continues with the elimination of maternal transcripts during oocyte maturation and fertilization and ends with the full transcriptional activation of the zygotic genome during embryonic development. A hallmark of MZT in mammals is its reliance on translation and the utilization of stored RNAs and proteins, rather than de novo transcription of genes, to sustain meiotic maturation and early development. Impaired maternal mRNA clearance at the onset of MZT prevents zygotic genome activation and causes early arrest of developing embryos. In this review, we discuss recent advances in our knowledge of the mechanisms whereby mRNA translation and degradation are controlled by cytoplasmic polyadenylation and deadenylation which set up the competence of maturing oocyte to accomplish MZT. The emphasis of this review is on the mouse as a model organism for mammals and BTG4 as a licensing factor of MZT under the translational control of the MAPK cascade.

BTG4 is A Novel p53 Target Gene That Inhibits Cell Growth and Induces Apoptosis

BTG4 is the last cloned and poorly studied member of BTG/Tob family. Studies have suggested that BTG4 is critical for the degradation of maternal mRNAs in mice during the process of maternal-to-zygotic transition, and downregulated in cancers, such as gastric cancer. However, the regulatory mechanism of BTG4 and its function in cancers remain elusive. In this study, we have for the first time identified the promoter region of the human BTG4 gene. Serial luciferase reporter assay demonstrated that the core promoter of BTG4 is mainly located within the 388 bp region near its transcription initiation site. Transcription factor binding site analysis revealed that the BTG4 promoter contains binding sites for canonical transcription factors, such as Sp1, whereas its first intron contains two overlapped consensus p53 binding sites. However, overexpression of Sp1 has negligible effects on BTG4 promoter activity, and site-directed mutagenesis assay further suggested that Sp1 is not a critical transcription factor for the transcriptional regulation of BTG4. Of note, luciferase assay revealed that one of the intronic p53 binding sites is highly responsive to p53. Both exogenous p53 overexpression and adriamycin-mediated endogenous p53 activation result in the transcriptional upregulation of BTG4. In addition, BTG4 is downregulated in lung and colorectal cancers, and overexpression of BTG4 inhibits cell growth and induces apoptosis in cancer cells. Taken together, our results strongly suggest that BTG4 is a novel p53-regulated gene and probably functions as a tumor suppressor in lung and colorectal cancers.

Maternal factors regulating preimplantation development in mice

Mammalian embryogenesis depends on maternal factors accumulated in eggs prior to fertilization and on placental transfers later in gestation. In this review, we focus on initial events when the organism has insufficient newly synthesized embryonic factors to sustain development. These maternal factors regulate preimplantation embryogenesis both uniquely in pronuclear formation, genome reprogramming and cell fate determination and more universally in regulating cell division, transcription and RNA metabolism. Depletion, disruption or inappropriate persistence of maternal factors can result in developmental defects in early embryos. To better understand the origins of these maternal effects, we include oocyte maturation processes that are responsible for their production. We focus on recent publications and reference comprehensive reviews that include earlier scientific literature of early mouse development.

CNOT6L couples the selective degradation of maternal transcripts to meiotic cell cycle progression in mouse oocyte

Meiotic resumption-coupled degradation of maternal transcripts occurs during oocyte maturation in the absence of mRNA transcription. The CCR4-NOT complex has been identified as the main eukaryotic mRNA deadenylase. In vivo functional and mechanistic information regarding its multiple subunits remains insufficient. Cnot6l, one of four genes encoding CCR4-NOT catalytic subunits, is preferentially expressed in mouse oocytes. Genetic deletion of Cnot6l impaired deadenylation and degradation of a subset of maternal mRNAs during oocyte maturation. Overtranslation of these undegraded mRNAs caused microtubule-chromosome organization defects, which led to activation of spindle assembly checkpoint and meiotic cell cycle arrest at prometaphase. Consequently, Cnot6l ^-/- female mice were severely subfertile. The function of CNOT6L in maturing oocytes is mediated by RNA-binding protein ZFP36L2, not maternal-to-zygotic transition licensing factor BTG4, which interacts with catalytic subunits CNOT7 and CNOT8 of CCR4-NOT Thus, recruitment of different adaptors by different catalytic subunits ensures stage-specific degradation of maternal mRNAs by CCR4-NOT This study provides the first direct genetic evidence that CCR4-NOT-dependent and particularly CNOT6L-dependent decay of selective maternal mRNAs is a prerequisite for meiotic maturation of oocytes.

CFP1 Regulates Histone H3K4 Trimethylation and Developmental Potential in Mouse Oocytes

Trimethylation of histone H3 at lysine-4 (H3K4me3) is associated with eukaryotic gene promoters and poises their transcriptional activation during development. To examine the in vivo function of H3K4me3 in the absence of DNA replication, we deleted CXXC finger protein 1 (CFP1), the DNA-binding subunit of the SETD1 histone H3K4 methyltransferase, in developing oocytes. We find that CFP1 is required for H3K4me3 accumulation and the deposition of histone variants onto chromatin during oocyte maturation. Decreased H3K4me3 in oocytes caused global downregulation of transcription activity. Oocytes lacking CFP1 failed to complete maturation and were unable to gain developmental competence after fertilization, due to defects in cytoplasmic lattice formation, meiotic division, and maternal-zygotic transition. Our study highlights the importance of H3K4me3 in continuous histone replacement for transcriptional regulation, chromatin remodeling, and normal developmental progression in a non-replicative system.