RNA-Binding Protein IGF2BP2/IMP2 is a Critical Maternal Activator in Early Zygotic Genome Activation

Characterization of Metabolic Patterns in Mouse Oocytes during Meiotic Maturation

Well-balanced and timed metabolism is essential for making a high-quality egg. However, the metabolic framework that supports oocyte development remains poorly understood. Here, we obtained the temporal metabolome profiles of mouse oocytes during in vivo maturation by isolating large number of cells at key stages. In parallel, quantitative proteomic analyses were conducted to bolster the metabolomic data, synergistically depicting the global metabolic patterns in oocytes. In particular, we discovered the metabolic features during meiotic maturation, such as the fall in polyunsaturated fatty acids (PUFAs) level and the active serine-glycine-one-carbon (SGOC) pathway. Using functional approaches, we further identified the key targets mediating the action of PUFA arachidonic acid (ARA) on meiotic maturation and demonstrated the control of epigenetic marks in maturing oocytes by SGOC network. Our data serve as a broad resource on the dynamics occurring in metabolome and proteome during oocyte maturation.

Dynamics and clinical relevance of maternal mRNA clearance during the oocyte-to-embryo transition in humans

Maternal mRNA clearance is an essential process that occurs during maternal-to-zygotic transition (MZT). However, the dynamics, functional importance, and pathological relevance of maternal mRNA decay in human preimplantation embryos have not yet been analyzed. Here we report the zygotic genome activation (ZGA)-dependent and -independent maternal mRNA clearance processes during human MZT and demonstrate that subgroups of human maternal transcripts are sequentially removed by maternal (M)- and zygotic (Z)-decay pathways before and after ZGA. Key factors regulating M-decay and Z-decay pathways in mouse have similar expression pattern during human MZT, suggesting that YAP1-TEAD4 transcription activators, TUT4/7-mediated mRNA 3ʹ-oligouridylation, and BTG4/CCR4-NOT-induced mRNA deadenylation may also be involved in the regulation of human maternal mRNA stability. Decreased expression of these factors and abnormal accumulation of maternal transcripts are observed in the development-arrested embryos of patients who seek assisted reproduction. Defects of M-decay and Z-decay are detected with high incidence in embryos that are arrested at the zygote and 8-cell stages, respectively. In addition, M-decay is not found to be affected by maternal TUBB8 mutations, although these mutations cause meiotic cell division defects and zygotic arrest, which indicates that mRNA decay is regulated independent of meiotic spindle assembly. Considering the correlations between maternal mRNA decay defects and early developmental arrest of in vitro fertilized human embryos, M-decay and Z-decay pathway activities may contribute to the developmental potential of human preimplantation embryos.

How maternal RNA clearance is regulated in human preimplantation embryos is unclear. Here, the authors show there is a potential correlation between maternal mRNA decay defects and early developmental arrest from in vitro fertilized human embryos, suggesting that M-decay and Z-decay pathways may regulate such early development.

YTHDF2 Regulates Maternal Transcriptome Degradation and Embryo Development in Goat

Maternal mRNA clearance is critical for the early embryo development, which is under the tight control of RNA N6-methyladenosine (m⁶A). However, little information is known regarding the maternal mRNA clearance and mechanisms behind it in farm animals. In the present study, 3362 differentially expressed genes (DEGs) were found during the maternal-to-zygotic transition (MZT) and determined as maternal mRNAs in goat. Of which, 1961 was decreased at the 4-cell stage embryos, while 1401 was trigged down-regulation at the 8-cell stage embryos, which were termed as maternally encoded mRNA decay genes and zygotic genome activation (ZGA)-dependent maternal mRNAs, respectively. The expression of m⁶A reader YTHDF2 was increased during goat ZGA, and knockdown of YTHDF2 resulted in decreased blastocyst rate. In the 8-cell stage YTHDF2 knockdown embryos, the M-decay and Z-decay maternal mRNA clearance were impaired. Specifically, the expression of deadenylase (CNOT1 and CNOT11) and decapping enzymes (DCP1A and DCP2) was decreased. In conclusion, we ascertained maternal mRNAs and inferred that maternal mRNA clearance is also ZGA-dependent in goat. We reported that YTHDF2 is vital for goat early embryogenesis as it advances maternal mRNA clearance, which might through the recruitment of deadenylases and mRNA decapping enzymes. This work will be of great value for understanding the stochastic reprogramming events during MZT and achieving better development of goat embryos in vitro.

The Diverse Functions of IMP2/IGF2BP2 in Metabolism

The human insulin-like growth factor 2 (IGF2) mRNA binding protein family (IMPs/IGF2BPs) is involved in a spectrum of biological processes, including development, tumorigenesis, and stemness. IMPs play a major role in post-transcriptional regulation of RNAs through the ribonucleoprotein complex (RNP). They have emerged as direct mammalian target of rapamycin (mTOR) substrates that coordinate nutrient stimulation and RNA life cycle control. IMP2 is a human type 2 diabetes (T2D) gene associated with impaired insulin secretion. Recently, using murine models, the substantial progress in understanding disease mechanisms has highlighted the significance of IMP2 in metabolism. This new knowledge may have the potential for therapeutic benefit.

Roles of N6-Methyladenosine (m6A) in Stem Cell Fate Decisions and Early Embryonic Development in Mammals

N6-methyladenosine (m⁶A) is one of the most abundant internal mRNA modifications, and it affects multiple biological processes related to eukaryotic mRNA. The majority of m⁶A sites are located in stop codons and 3′UTR regions of mRNAs. m⁶A regulates RNA metabolism, including alternative splicing (AS), alternative polyadenylation (APA), mRNA export, decay, stabilization, and translation. The m⁶A metabolic pathway is regulated by a series of m⁶A writers, erasers and readers. Recent studies indicate that m⁶A is essential for the regulation of gene expression, tumor formation, stem cell fate, gametogenesis, and animal development. In this systematic review, we summarized the recent advances in newly identified m⁶A effectors and the effects of m⁶A on RNA metabolism. Subsequently, we reviewed the functional roles of RNA m⁶A modification in diverse cellular bioprocesses, such as stem cell fate decisions, cell reprogramming and early embryonic development, and we discussed the potential of m⁶A modification to be applied to regenerative medicine, disease treatment, organ transplantation, and animal reproduction.

LYPD4, mouse homolog of a human acrosome protein, is essential for sperm fertilizing ability and male fertility†

Fertilization is one of the fundamental biological processes, but so far, we still do not have a full understanding of the underlying molecular mechanism. We have identified a human acrosome protein, LY6/PLAUR domain containing 4 (LYPD4), expressed specifically in human testes and sperm, and conserved within mammals. Mouse Lypd4, also specific to the testis and sperm, is essential for male fertility. LYPD4 protein first appeared in round spermatids during acrosome biogenesis and became part of acrosomes during spermatogenesis and in mature sperm. Lypd4 knockout mice are infertile with normal sperm number and motility. Mutant sperm, however, failed to reach oviduct during sperm migration inside the female reproductive tract, leading to fertilization failure and infertility. In addition, Lypd4 mutant sperms were unable to fertilize denuded egg via IVF (in vitro fertilization) but could fertilize eggs within intact Cumulus-Oocyte Complex, supporting an additional role in sperm-zona interaction. Out of more than five thousand spermatozoa proteins identified by mass spectrometry analysis, only a small subset of proteins (26 proteins) was changed in the absence of LYPD4, revealing a whole proteome picture of mutant sperm defective in sperm migration and sperm-zona binding. ADAM3, a key component of fertilization complex, as well as other sperm ADAM proteins are significantly reduced. We hence propose that LYPD4 plays an essential role in mammalian fertilization, and further investigation of its function and its interaction with other sperm membrane complexes may yield insights into human fertilization and novel strategy to improve IVF success.

Inhibition of lysine-specific histone demethylase 1A results in meiotic aberration during oocyte maturation in vitro in goats

Histone methylation is associated with oocyte maturation in several species and is also expected in goat oocytes, while the mechanism is still unclear. Therefore, single-cell RNA sequencing (scRNA-seq) was performed on goat germinal vesicle (GV) and metaphase II (MII) oocytes, and the functions of lysine-specific histone demethylase 1A (LSD1), one of the differentially expressed genes (DEGs) were investigated during in vitro maturation (IVM) of goat oocytes. Through scRNA-seq, 4516 DEGs were identified from GV oocytes and MII oocytes in goats, among which there were 16 histone methyltransferase and demethylase DEGs (including LSD1). The functions of LSD1 during IVM of goat oocytes were investigated through its inhibitor, GSK-LSD1. We found that the first polar body extrusion rate of goat oocytes significantly reduced with an increase in GSK-LSD1 concentration supplemented into IVM medium (0 μM: 58.84 ± 0.95%; 2.5 μM: 52.14 ± 0.51%, P < 0.01; 50 μM: 41.22 ± 0.42%, P < 0.001; 100 μM: 29.78 ± 1.78%, P < 0.001). Moreover, compared with the control group, the level of H3K4me2 methylation and p-H2AX in goat oocytes significantly increased (P < 0.001 and P < 0.01, respectively) upon 50-μM GSK-LSD1 treatment for 12 h. Furthermore, abnormalities in spindle assembly (25.94 ± 1.02% vs. 71.15 ± 3.32%; P < 0.01) and chromosome alignment (22.93 ± 1.11% vs. 76.03 ± 3.25%; P < 0.01) were observed, and cytoskeletal organization (15.31 ± 1.60% vs. 67.50 ± 3.09%; P < 0.001) was disrupted upon treatment with 50-μM GSK-LSD1 for 12 h, which compared with that in the control group. Additionally, the ratio of BCL2:BAX significantly higher (P < 0.01) in oocytes with 50-μM GSK-LSD1 treatment than that in control group. Collectively, these results indicate the important role of LSD1 in meiotic maturation of goat oocytes. Our data not only clarify dynamic changes in mRNA during oocyte maturation but also provide a theoretical basis and technical means for further studies of meiotic maturation of goat oocytes.