Maternal control of early mouse development

Chromosome remodeling and cytoplasmic distribution during embryonic development in fused pair embryos

Cell fusion is now widely employed as an in vitro model for inducing and investigating cell fate changes. In early embryo research, fusion of embryos is also a way to probe the mechanisms of mammalian oogenesis and preimplantation development. To establish a novel model for porcine early embryo studies and investigate its developmental mechanisms, pair of zona pellucida (ZP)-free oocytes were electro fused to produce the fused pair (FP) embryos, which were further in vitro cultured to the blastocyst stage. Firstly, developmental competence was assessed, revealing a cleavage rate of 92.27 ± 5.59 % and a blastocyst rate of 26.12 ± 6.61 %, which were similar to the parthenogenetic activation (PA) embryos (p > 0.05). Subsequently, nuclear and spindle staining was performed on FP embryos collected at 14-22 h, with 67.18 ± 3.18 % of the embryos spindle reorganization occurred and nuclei fusion, whereas a few displayed independent division of the two nuclei or tripolar spindle. Lastly, the distribution of lipid droplets (LDs) and mitochondria in FP embryos was assessed via fluorescent staining. Results showed that the even distribution of LDs from one oocyte was observed in each blastomere of 4-cell to blastocysts. A similar distribution pattern was observed for mitochondria, which was being observed at the 2-cell stage, a relatively earlier developmental stage than that of LDs. Results suggested that cytoplasm including mitochondria and LDs could redistribute once two oocytes fused into a single embryo. More studies are needed for the underlying mechanism and potential impact on the developmental ability of FP embryos.

UBE2V1 governs aging induced protein aggregation and developmental defects in oocytes and embryos

While protein aggregation is a well-documented factor in various age-related diseases, its specific impact on oocyte aging and the molecular mechanisms responsible remain poorly understood. In a mouse model of advanced maternal age, we observe that aging promotes ubiquitinated protein aggregation in oocytes and embryos. Starting with this clue, we identify that the expression of ubiquitin-conjugating enzyme (E2) UBE2V1 in oocyte increases with age and correlates with aggresome formation. We further provide evidence that UBE2V1 positively regulates protein aggregates formation in oocyte under both physiological and stress conditions. Moreover, enhanced UBE2V1 expression mimics the phenotypes observed in aged oocytes. Notably, restoring UBE2V1 expression in aged oocytes and embryos not only alleviates aggresome formation but also partly ameliorates the age-related defects in oocyte maturation and embryo development. Thus, our findings provide a mechanistic link between UBE2V1 expression, protein aggregation and developmental defects in aged oocytes and embryos.

Mir-218-5p from Extracellular Vesicles of Endometrium in Patients with Recurrent Implantation Failure Impairs Pre-Implantation Embryo Development

Background

Recurrent implantation failure (RIF) presents a crucial obstacle to in vitro fertilization success. Previous research has shown that small extracellular vesicles (EVs) from endometrial RIF patients hinder embryo development, yet the underlying mechanism and potential solutions remain largely unexplored. In this study, we aimed to investigate the effectiveness of miR-218-5p as a molecular factor in RIF-EVs. Our findings revealed that miR-218-5p disrupted mouse embryo development, and this effect could be reversed by engineered extracellular vesicles (E-EVs) containing anti-miR-218-5p.

Methods

The percentage of blastocyst development and hatching rates, embryo morphology, and the total cell number were measured. RNA-sequencing was used to analyze transcriptional changes in embryos post miR-218-5p agomir treatment. The abnormal segregation genes of trophectoderm (TE) and inner cell mass (ICM) were visualized via qRT-PCR and immunofluorescence staining. The E-EVs were using the EVs derived from Human Umbilical Cord Mesenchymal Stem Cells (HUMSCs). Characteristics of the EVs were measured using Western blotting, nanoparticle tracking analysis, and transmission electron microscopy. EVs internalization was visualized using BODIPY TR ceramide staining.

Results

Mouse embryos were arrested at the morula stage and demonstrated reduced blastocyst and hatching rates following miR-218-5p agomir treatment (P < 0.001). Essential transcription factors for TE and ICM, such as Cdx2, Yap1, Sox2, Nanog, Tead4, were reduced at the mRNA level in the miR-218-5p treated morula. This was accompanied by decreased Cdx2 protein levels at the 8-16-cell stage (P < 0.001) and disruption of co-localization of Yap1 and Cdx2. The blastocyte rate was increased by anti-miR-218-5p-encapsulated E-EVs compared with miR-218-5p group (P < 0.001).

Conclusion

This study offers valuable insights into the potential role of miR-218-5p in RIF and presents. The utilization of engineered vesicles containing anti-miR-218-5p may present a promising avenue for patients facing challenges with RIF.

Putrescine promotes maturation of oocytes from reproductively old mice via mitochondrial autophagy

RESEARCH QUESTION

Does putrescine (PUT) improve oocytes from reproductively old mice by promoting mitochondrial autophagy?

DESIGN

Germinal vesicle stage cumulus-oocyte complexes (COCs) were obtained from 9-month old female C57BL/6N mice and divided into control, PUT and difluoromethylornithine, inhibitor (DFMO) groups. These germinal vesicle COCs underwent mouse in-vitro maturation (IVM) culture to observe the extrusion of the first polar body in each group. Using JC-1, dichloro-dihydro-fluorescein diacetate fluorescent probes and a confocal microscope, the mitochondrial membrane potential integrity and reactive oxygen species levels were measured in metaphase II stage oocytes. The expression and cellular localization of the p53 protein were examined by immunofluorescence. Reverse transcription quantitative polymerase chain reaction was used to detect the activation of mitochondrial autophagy pathways. The potential mechanisms through which PUT improves oocytes from reproductively old mice were explored by single-cell transcriptomic analysis. Autophagosomes, autolysosomes and mitochondria in different groups were directly observed using transmission electron microscopy.

RESULTS

The addition of exogenous PUT can promote IVM of oocytes from reproductively old mice. It reduces oxidative stress by promoting the autophagy of damaged mitochondria, decreasing the levels of reactive oxygen species and increasing mitochondrial membrane potential. It affects the expression and subcellular localization of the p53 protein, and increases the expression of transcription factor EB, which may be the potential mechanism behind its promotion of autophagy.

CONCLUSION

The target and regulatory pathway of PUT in oocytes was clarified. Putrescine is an effective small molecule compound with significant potential for non-invasively improving the fertility of elderly women.

Mitofusin 1 Drives Preimplantation Development by Enhancing Chromatin Incorporation of Histone H3.3

Mitofusin 1 (MFN1) plays a crucial role in mitochondrial fusion and oocyte development. However, its function in preimplantation embryonic development and its potential involvement in epigenetic regulation remain poorly understood. In this study, it is shown that MFN1 interacts with PADI6, a key component of the cytoplasmic lattice in oocytes and early embryos. MFN1 deficiency in mice results in reduced PADI6 levels and decreased expression of translational machinery components, which suppress protein synthesis activity and lower histone H3.3 abundance. These disruptions lead to the failure of male pronucleus formation, aberrant zygotic genome activation, and impaired embryonic development. It is further demonstrated that the MFN1 activator S89 promotes H3.3 incorporation and rescues early development in maternally aged embryos with low MFN1 levels. Additionally, a positive correlation between MFN1 and H3.3 protein levels in early human embryos is observed. Together, these findings provide new insights into MFN1's role in regulating epigenetic reprogramming during preimplantation embryo development.

Epigenetic regulation in oogenesis and fetal development: insights into m6A modifications

The unique physiological structure of women has led to a variety of diseases that have attracted the attention of many people in recent years. Disturbances in the reproductive system microenvironment lead to the progression of various female tumours and pregnancy disorders. Numerous studies have shown that epigenetic modifications crucially influence both oogenesis and foetal development. m6A, a modification at the mRNA level, consists of three parts, namely, writers, erasers, and readers, which are involved in several biological functions, such as the nucleation and stabilisation of mRNAs, thereby regulating the development of reproductive system diseases. In this manuscript, we delineate the constituents of m6A, their biological roles, and advancements in understanding m6A within the maternal-foetal immunological context. In addition, we summarise the mechanism of m6A in gynaecological diseases and provide a new perspective for targeting m6A to delay the progression of reproductive system diseases in clinical practice.

The subcortical maternal complex safeguards mouse oocyte-to-embryo transition by preventing nuclear entry of SPIN1

How cytoplasmic regulators control nuclear events in mammalian oocytes and early embryos remains largely enigmatic. We previously identified a subcortical maternal complex (SCMC) that specifically resides in the cytoplasm of mammalian oocytes and early embryos but is also involved in nuclear events. Nevertheless, how the cytoplasmic SCMC exerts its role in nuclear processes remains unknown. In this study, we unveil SPIN1, a histone methylation reader, as a novel member of the SCMC. The SCMC component FILIA tightly regulates the expression and cytoplasmic localization of SPIN1 through direct interaction. When the expression of FILIA is decreased because of genetic mutations of SCMC genes, SPIN1 expression is dramatically reduced but the residual SPIN1 translocates into the nucleus. The abnormal nuclear presence of SPIN1 impairs H3K4me3 reprogramming, zygotic genome activation and physiological embryonic development. Inhibiting the interaction between SPIN1 and H3K4me3 partially rescues the abnormal phenotype in FILIA-null embryos. Mechanistically, SPIN1 partially perturbs the demethylation process by competing with KDM5B for binding to H3K4me3. Collectively, our work highlights the complexity of the mammalian SCMC and oocyte-to-embryo transition, revealing an intricate regulatory mechanism that facilitates the smooth progression of this process.

Endosomal-lysosomal organellar assembly (ELYSA) structures coordinate lysosomal degradation systems through mammalian oocyte-to-embryo transition

Mouse oocytes undergo drastic changes in organellar composition and their activities during maturation from the germinal vesicle (GV) to metaphase II (MII) stage. After fertilization, the embryo degrades parts of the maternal components via lysosomal degradation systems, including autophagy and endocytosis, as zygotic gene expression begins during embryogenesis. Here, we demonstrate that endosomal-lysosomal organelles form large spherical assembly structures, termed endosomal-lysosomal organellar assemblies (ELYSAs), in mouse oocytes. ELYSAs are observed in GV oocytes, attaining sizes up to 7-8 μm in diameter in MII oocytes. ELYSAs comprise tubular-vesicular structures containing endosomes and lysosomes along with cytosolic components. Most ELYSAs are also positive for an autophagy regulator, LC3. These characteristics of ELYSA resemble those of ELVA (endolysosomal vesicular assemblies) identified independently. The signals of V1-subunit of vacuolar ATPase tends to be detected on the periphery of ELYSAs in MII oocytes. After fertilization, the localization of the V1-subunit on endosomes and lysosomes increase as ELYSAs gradually disassemble at the 2-cell stage, leading to further acidification of endosomal-lysosomal organelles. These findings suggest that the ELYSA/ELVA maintain endosomal-lysosomal activity in a static state in oocytes for timely activation during early development.

The establishment of the 3D genome structure during zygotic genome activation

During zygotic genome activation (ZGA) and early development, hierarchical levels of chromatin structure undergo remarkable perturbations: changes in the nuclear-to-cytoplasmic ratio of various components; changes in chromatin accessibility; histone exchange; and the formation of 3D structures such as loops, topologically associated domains, and compartments. Here, we review the peculiarities, variability, and emergence of the chromatin structural features during ZGA in different organisms. Focusing on newly found structures called fountains, we describe the prerequisites for cohesin loading on DNA and possible mechanisms of genome organization in early development. Fountains resulting from asymmetric bidirectional cohesin extrusion spread from cohesin-loading points in a CTCF-independent manner. We discuss that fountains may not possess specific functions, unlike conventional chromatin structures, and could be found in other biological processes where cohesin loading occurs.

Mammalian oocytes receive maternal-effect RNAs from granulosa cells

It is currently thought that growing mammalian oocytes receive only small molecules via gap junctions from surrounding support cells, the granulosa cells. From the study of chimeric preantral oocyte and granulosa cell reaggregations, we provide evidence that growing mouse oocytes receive mRNAs from granulosa cells. Among the >1,000 granulosa-transcribed RNAs we identified in the oocyte, those that contribute to proper oocyte maturation and early embryo development were highly enriched. Predicted motifs for two RNA-binding proteins that function in RNA trafficking, FMRP and TDP43, were abundant in the UTRs of the granulosa-derived transcripts. Immunostaining demonstrated that both FMRP and TDP43 co-localize with the actin-rich granulosa cell protrusions that span the zone pellucida and connect to the oocyte, suggesting their role in importing mRNAs. Our results offer the possibility that oocyte failure may not always reflect an intrinsic oocyte deficiency but could arise from insufficient supply of maternal transcripts by granulosa cells during oocyte growth.

Two Novel Protein-Truncating Variants in NLRP2 and Their Functional Impacts on the Subcortical Maternal Complex

Female infertility is a prevalent reproductive disorder with high genetic heterogeneity. Previous reports have demonstrated the causal role of biallelic pathogenic variants in the Subcortical Maternal Complex (SCMC) genes in female reproductive failure with some leading to infertility, early embryonic loss, and molar pregnancies, while others are compatible with live birth with and without multilocus imprinting disorders (MLID). Here, we report two deleterious protein-truncating variants, c.1326delG, p.Leu443Phefs*78 and c.2802_2803del, p.Arg935Metfs*15, in heterozygous state in the NLRP2 gene of a patient with primary infertility, four early miscarriages, and one failed attempt of intracytoplasmic sperm injection. We show that the two variants mediate mRNA decay in EBV-transformed lymphoblastoid cells from the patient, lead to decreased NLRP2 protein levels, and alter NLRP2 interactions with other members of the SCMC in vitro. This study emphasizes the importance of performing clinical exomes for patients with recurrent reproductive failure and reporting their variants and reproductive histories to improve patient counseling.

Transcriptome and DNA methylation profiling during the NSN to SN transition in mouse oocytes

BACKGROUND

During the latter stages of their development, mammalian oocytes under dramatic chromatin reconfiguration, transitioning from a non-surrounded nucleolus (NSN) to a surrounded nucleolus (SN) stage, and concomitant transcriptional silencing. Although the NSN-SN transition is known to be essential for developmental competence of the oocyte, less is known about the accompanying molecular changes. Here we examine the changes in the transcriptome and DNA methylation during the NSN to SN transition in mouse oocytes.

RESULTS

To study the transcriptome and DNA methylation dynamics during the NSN to SN transition, we used single-cell (sc)M&T-seq to generate scRNA-seq and sc-bisulphite-seq (scBS-seq) data from GV oocytes classified as NSN or SN by Hoechst staining of their nuclei. Transcriptome analysis showed a lower number of detected transcripts in SN compared with NSN oocytes as well as downregulation of 576 genes, which were enriched for processes related to mRNA processing. We used the transcriptome data to generate a classifier that can infer chromatin stage in scRNA-seq datasets. The classifier was successfully tested in multiple published datasets of mouse models with a known skew in NSN: SN ratios. Analysis of the scBS-seq data showed increased DNA methylation in SN compared to NSN oocytes, which was most pronounced in regions with intermediate levels of methylation. Overlap with chromatin immunoprecipitation and sequencing (ChIP-seq) data for the histone modifications H3K36me3, H3K4me3 and H3K27me3 showed that regions gaining methylation in SN oocytes are enriched for overlapping H3K36me3 and H3K27me3, which is an unusual combination, as these marks do not typically coincide.

CONCLUSIONS

We characterise the transcriptome and DNA methylation changes accompanying the NSN-SN transition in mouse oocytes. We develop a classifier that can be used to infer chromatin status in single-cell or bulk RNA-seq data, enabling identification of altered chromatin transition in genetic knock-outs, and a quality control to identify skewed NSN-SN proportions that could otherwise confound differential gene expression analysis. We identify late-methylating regions in SN oocytes that are associated with an unusual combination of chromatin modifications, which may be regions with high chromatin plasticity and transitioning between H3K27me3 and H3K36me3, or reflect heterogeneity on a single-cell level.

PGC7 regulates maternal mRNA translation via AKT1-YBX1 interactions in mouse oocytes

Timely and accurate translation of maternal mRNA is essential for oocyte maturation and early embryonic development. Previous studies have highlighted the importance of Primordial Germ cell 7 (PGC7) as a maternal factor in maintaining DNA methylation of maternally imprinted loci in zygotes. However, it is still unknown whether PGC7 is involved in the regulation of Maternal mRNA Translation. In this study, we have identified that PGC7-AKT1-YBX1 axis is involved in promoting the translation of maternal mRNAs. PGC7 not only sustains AKT1 activity by counteracting PP2A dephosphorylation and facilitating PDK1-AKT1 binding but also assists AKT1 in phosphorylating the translation inhibitor YBX1. In the absence of PGC7, despite increased PIK3CA expression and AKT1 phosphorylation, AKT1 is unable to phosphorylate YBX1. PGC7 facilitates the interaction between AKT1 and YBX1, enhancing YBX1-Serine 100 phosphorylation, which leads to YBX1 dissociation from eIF4E, thereby activating the translation of maternal Cyclin B1 and YAP1. The findings demonstrate the indispensability of PGC7 for translation activation in mammalian oocytes and provide a potential network regulated by PGC7 in early oogenesis.

A novel homozygous mutation in the NLRP2 gene causes early embryonic arrest

PURPOSE

Successful reproduction in humans requires maturation and fertilization of gametes as well as early embryonic development. Any deviation from these processes leads to infertility. Early embryonic arrest (EEA) is common in female infertility and is primarily attributed to genetic factors. Mutations in the NLRP2 gene have been identified as the causative factors for EEA. In the present study, a novel mutation identified in NLRP2 underscored the novel homozygous variant and phenotypes that might contribute to its inclusion in the genetic counseling of infertile patients.

METHODS

We recruited a proband from a consanguineous family with a diagnosis of EEA. Peripheral blood samples were collected from the proband and family members for whole-exome sequencing to identify the genes and inheritance patterns associated with infertility; the results were substantiated by Sanger sequencing. All genetic variants and protein structures were analyzed based on computational predictions. Wild-type and mutant plasmids were constructed and transfected into HeLa cells. Subsequent in vitro analyses elucidated the functional impact of the variant.

RESULTS

A novel homozygous mutation in NLRP2 was identified in the proband. The patient harbored a frameshift deletion mutation (c.195delC: p.Tyr66Thrfs*32) in the pyrin structural domain. This genetic alteration resulted in the down-regulation of NLRP2 mRNA expression, truncation of the protein structure, and altered protein localization in cells.

CONCLUSIONS

The current findings broaden the spectra of NLRP2 variants, especially concerning EEA. Also, potential diagnostic markers for patients experiencing recurrent IVF/ICSI failure were identified, and a solid foundation was laid for genetic counseling for female infertility.

N6-Methyladenosine Modification on the Function of Female Reproductive Development and Related Diseases

BACKGROUND

N6-methyladenosine (m6A) modification is a widespread and reversible epigenetic alteration in eukaryotic mRNA, playing a pivotal role in various biological functions. Its significance in female reproductive development and associated diseases has recently become a focal point of research.

OBJECTIVE

This review aims to consolidate current knowledge of the role of m6A modification in female reproductive tissues, emphasizing its regulatory dynamics, functional significance, and implications in reproductive health and disease.

METHODS

A comprehensive analysis of recent studies focusing on m6A modification in ovarian development, oocyte maturation, embryo development, and the pathogenesis of reproductive diseases.

RESULTS

m6A modification exhibits dynamic regulation in female reproductive tissues, influencing key developmental stages and processes. It plays critical roles in ovarian development, oocyte maturation, and embryo development, underpinning essential aspects of reproductive health. m6A modification is intricately involved in the pathogenesis of several reproductive diseases, including polycystic ovary syndrome (PCOS), premature ovarian failure (POF), and endometriosis, offering insights into potential molecular mechanisms and therapeutic targets.

CONCLUSION

The review highlights the crucial role of m6A modification in female reproductive development and related diseases. It underscores the need for further research to explore innovative diagnostic and therapeutic strategies for reproductive disorders, leveraging the insights gained from understanding m6A modification's impact on reproductive health.

Two mechanisms repress cyclin B1 translation to maintain prophase arrest in mouse oocytes

In mammals, oocytes are arrested in prophase of meiosis I for long periods of time. Prophase arrest is critical for reproduction because it allows oocytes to grow to their full size to support meiotic maturation and embryonic development. Prophase arrest requires the inhibitory phosphorylation of the mitotic kinase CDK1. Whether prophase arrest is also regulated at the translational level is unknown. Here, we show that prophase arrest is regulated by translational control of dormant cyclin B1 mRNAs. Using Trim-Away, we identify two mechanisms that maintain cyclin B1 dormancy and thus prophase arrest. First, a complex of the RNA-binding proteins DDX6, LSM14B and CPEB1 directly represses cyclin B1 translation through interacting with its 3'UTR. Second, cytoplasmic poly(A)-binding proteins (PABPCs) indirectly repress the translation of cyclin B1 and other poly(A)-tail-less or short-tailed mRNAs by sequestering the translation machinery on long-tailed mRNAs. Together, we demonstrate how RNA-binding proteins coordinately regulate prophase arrest, and reveal an unexpected role for PABPCs in controlling mRNA dormancy.

RNAi epimutations conferring antifungal drug resistance are inheritable

Epimutations modify gene expression and lead to phenotypic variation while the encoding DNA sequence remains unchanged. Epimutations mediated by RNA interference (RNAi) and/or chromatin modifications can confer antifungal drug resistance and may impact virulence traits in fungi. However, whether these epigenetic modifications can be transmitted across generations following sexual reproduction was unclear. This study demonstrates that RNAi epimutations conferring antifungal drug resistance are transgenerationally inherited in the human fungal pathogen Mucor circinelloides . Our research revealed that RNAi-based antifungal resistance follows a DNA sequenceindependent, non-Mendelian inheritance pattern. Small RNAs (sRNAs) are the exclusive determinants of inheritance, transmitting drug resistance independently of other known repressive epigenetic modifications. Unique sRNA signature patterns can be traced through inheritance from parent to progeny, further supporting RNA as an alternative molecule for transmitting information across generations. Understanding how epimutations occur, propagate, and confer resistance may enable their detection in other eukaryotic pathogens, provide solutions for challenges posed by rising antimicrobial drug resistance (AMR), and also advance research on phenotypic adaptability and its evolutionary implications.

Advances in PIWI-piRNA function in female reproduction in mammals

PIWI-interacting RNAs (piRNAs), which associate with PIWI clade Argonaute proteins to form piRNA-induced silencing complexes (piRISCs) in germline cells, are responsible for maintaining genomic integrity and reproductive function through transcriptional or post-transcriptional suppression of transposable elements and regulation of protein-coding genes. Recent discoveries of crucial PIWI-piRNA functions in oogenesis and embryogenesis in golden hamsters suggest an indispensable role in female fertility that has been obscured in the predominant mouse model of PIWI-piRNA pathway regulation. In particular, studies of piRNA expression dynamics, functional redundancies, and compositional variations across mammal species have advanced our understanding of piRNA functions in male and, especially, female reproduction. These findings further support the use of hamsters as a more representative model of piRNA biology in mammals. In addition to discussing these new perspectives, the current review also covers emerging directions for piRNA research, its implications for female fertility, and our fundamental understanding of reproductive mechanisms.

AI-powered simulation-based inference of a genuinely spatial-stochastic gene regulation model of early mouse embryogenesis

Understanding how multicellular organisms reliably orchestrate cell-fate decisions is a central challenge in developmental biology, particularly in early mammalian development, where tissue-level differentiation arises from seemingly cell-autonomous mechanisms. In this study, we present a multi-scale, spatial-stochastic simulation framework for mouse embryogenesis, focusing on inner cell mass (ICM) differentiation into epiblast (EPI) and primitive endoderm (PRE) at the blastocyst stage. Our framework models key regulatory and tissue-scale interactions in a biophysically realistic fashion, capturing the inherent stochasticity of intracellular gene expression and intercellular signaling, while efficiently simulating these processes by advancing event-driven simulation techniques. Leveraging the power of Simulation-Based Inference (SBI) through the AI-driven Sequential Neural Posterior Estimation (SNPE) algorithm, we conduct a large-scale Bayesian inferential analysis to identify parameter sets that faithfully reproduce experimentally observed features of ICM specification. Our results reveal mechanistic insights into how the combined action of autocrine and paracrine FGF4 signaling coordinates stochastic gene expression at the cellular scale to achieve robust and reproducible ICM patterning at the tissue scale. We further demonstrate that the ICM exhibits a specific time window of sensitivity to exogenous FGF4, enabling lineage proportions to be adjusted based on timing and dosage, thereby extending current experimental findings and providing quantitative predictions for both mutant and wild-type ICM systems. Notably, FGF4 signaling not only ensures correct EPI-PRE lineage proportions but also enhances ICM resilience to perturbations, reducing fate-proportioning errors by 10-20% compared to a purely cell-autonomous system. Additionally, we uncover a surprising role for variability in intracellular initial conditions, showing that high gene-expression heterogeneity can improve both the accuracy and precision of cell-fate proportioning, which remains robust when fewer than 25% of the ICM population experiences perturbed initial conditions. Our work offers a comprehensive, spatial-stochastic description of the biochemical processes driving ICM differentiation and identifies the necessary conditions for its robust unfolding. It also provides a framework for future exploration of similar spatial-stochastic systems in developmental biology.

Advances in understanding the regulation of pluripotency fate transition in embryonic stem cells

Embryonic stem cells (ESCs) sourced from the inner cell mass of blastocysts, are akin to this tissue in function but lack the capacity to form all extraembryonic structures. mESCs are transient cell populations that express high levels of transcripts characteristic of 2-cell (2C) embryos and are identified as "2-cell-like cells" (2CLCs). Previous studies have shown that 2CLCs can contribute to both embryonic and extraembryonic tissues upon reintroduction into early embryos. Approximately 1% of mESCs dynamically transition from pluripotent mESCs into 2CLCs. Nevertheless, the scarcity of mammalian embryos presents a significant challenge to the molecular characterization of totipotent cells. To date, Previous studies have explored various methods for reprogramming pluripotent cells into totipotent cells. While there is a good understanding of the molecular regulatory network maintaining ES pluripotency, the process by which pluripotent ESCs reprogram into totipotent cells and the associated molecular mechanisms of totipotent regulation remain poorly understood. This review synthesizes recent insights into the regulatory pathways of ESC reprogramming into 2CLC, exploring molecular mechanisms modulated by transcriptional regulators, small molecules, and epigenetic changes. The objective is to construct a theoretical framework for the field of researchers.

The subcortical maternal complex modulates the cell cycle during early mammalian embryogenesis via 14-3-3

The subcortical maternal complex (SCMC) is essential for safeguarding female fertility in mammals. Assembled in oocytes, the SCMC maintains the cleavage of early embryos, but the underlying mechanism remains unclear. Here, we report that 14-3-3, a multifunctional protein, is a component of the SCMC. By resolving the structure of the 14-3-3-containing SCMC, we discover that phosphorylation of TLE6 contributes to the recruitment of 14-3-3. Mechanistically, during maternal-to-embryo transition, the SCMC stabilizes 14-3-3 protein and contributes to the proper control of CDC25B, thus ensuring the activation of the maturation-promoting factor and mitotic entry in mouse zygotes. Notably, the SCMC establishes a conserved molecular link with 14-3-3 and CDC25B in human oocytes/embryos. This study discloses the molecular mechanism through which the SCMC regulates the cell cycle in early embryos and elucidates the function of the SCMC in mammalian early embryogenesis.

Taf1 knockout is lethal in embryonic male mice and heterozygous females show weight and movement disorders

The TATA box-binding protein-associated factor 1 (TAF1) is a ubiquitously expressed protein and the largest subunit of the basal transcription factor TFIID, which plays a key role in initiation of RNA polymerase II-dependent transcription. TAF1 missense variants in human males cause X-linked intellectual disability, a neurodevelopmental disorder, and TAF1 is dysregulated in X-linked dystonia-parkinsonism, a neurodegenerative disorder. However, this field has lacked a genetic mouse model of TAF1 disease to explore its mechanism in mammals and treatments. Here, we generated and validated a conditional cre-lox allele and the first ubiquitous Taf1 knockout mouse. We discovered that Taf1 deletion in male mice was embryonically lethal, which may explain why no null variants have been identified in humans. In the brains of Taf1 heterozygous female mice, no differences were found in gross structure, overall expression and protein localisation, suggesting extreme skewed X inactivation towards the non-mutant chromosome. Nevertheless, these female mice exhibited a significant increase in weight, weight with age, and reduced movement, suggesting that a small subset of neurons was negatively impacted by Taf1 loss. Finally, this new mouse model may be a future platform for the development of TAF1 disease therapeutics.

LPS-Induced Mitochondrial Dysfunction Reduces Oocyte Maturation and Developmental Competence of Buffalo Embryos via ROS Mediated TLR4 Signalling

PROBLEM

Lipopolysaccharide (LPS) from gram-negative bacteria has reportedly been associated with infectious diseases like metritis, which has a substantial adverse effect on animal reproductive performance and causes serious financial losses for the dairy sector. The current work aimed to establish the impact of LPS on in vitro oocyte maturation and subsequent in vitro developmental competence of oocytes, as well as to investigate the explanatory molecular mechanism underlying this effect.

METHOD OF STUDY

Buffalo cumulus-oocyte complexes (COCs) were challenged with 0, 5, 10 and 20 µg/mL LPS during IVM followed by IVF and IVC. Cytoplasmic and nuclear maturation, cleavage and blastocyst rate, intracellular reactive oxygen species (ROS), mitochondrial membrane potential (MMP, ΔΨm) and transcript abundance of genes related to inflammation, antioxidation and apoptosis were evaluated.

RESULTS

The maturation and subsequent embryonic development competency were found to be significantly (p ≤ 0.05) reduced with the addition of 10 and 20 µg/mL LPS to IVM media. ROS production accompanied by a decreased ΔΨm was recorded in LPS-treated oocytes in comparison to the control group (p ≤ 0.05). Our results were further supported by the transcriptional expression of proinflammatory (TLR4, CD14 and RPS27A) and apoptotic gene (Caspase 3) which were found to be significantly increased while antioxidant genes (SOD2 and GPX1) were decreased significantly in matured oocytes and blastocyst after LPS exposure.

CONCLUSIONS

The deleterious effects of LPS are mediated through ROS generation, which triggers inflammatory processes via the TLR4 pathway and impairs oocyte maturation and subsequent embryonic development.

Maternal loss-of-function of Nlrp2 results in failure of epigenetic reprogramming in mouse oocytes

Background

NLRP2 belongs to the subcortical maternal complex (SCMC) of mammalian oocytes and preimplantation embryos. This multiprotein complex, encoded by maternal-effect genes, plays a pivotal role in the zygote-to-embryo transition, early embryogenesis, and epigenetic (re)programming. The maternal inactivation of genes encoding SCMC proteins has been linked to infertility and subfertility in mice and humans. However, the underlying molecular mechanisms for the diverse functions of the SCMC, particularly how this cytoplasmic structure influences DNA methylation, which is a nuclear process, are not fully understood.

Results

We undertook joint transcriptome and DNA methylome profiling of pre-ovulatory germinal-vesicle oocytes from Nlrp2-null, heterozygous (Het), and wild-type (WT) female mice. We identified numerous differentially expressed genes (DEGs) in Het and Nlrp2-null when compared to WT oocytes. The genes for several crucial factors involved in oocyte transcriptome modulation and epigenetic reprogramming, such as DNMT1, UHRF1, KDM1B and ZFP57 were overexpressed in Het and Nlrp2-null oocytes. Absence or reduction of Nlrp2, did not alter the distinctive global DNA methylation landscape of oocytes, including the bimodal pattern of the oocyte methylome. Additionally, although the methylation profile of germline differentially methylated regions (gDMRs) of imprinted genes was preserved in oocytes of Het and Nlrp2-null mice, we found altered methylation in oocytes of both genotypes at a small percentage of the oocyte-characteristic hyper- and hypomethylated domains. Through a tiling approach, we identified specific DNA methylation differences between the genotypes, with approximately 1.3% of examined tiles exhibiting differential methylation in Het and Nlrp2-null compared to WT oocytes.

Conclusions

Surprisingly, considering the well-known correlation between transcription and DNA methylation in developing oocytes, we observed no correlation between gene expression differences and gene-body DNA methylation differences in Nlrp2-null versus WT oocytes or Het versus WT oocytes. We therefore conclude that post-transcriptional changes in the stability of transcripts rather than altered transcription is primarily responsible for transcriptome differences in Nlrp2-null and Het oocytes.

Oocyte Maturation and miRNAs: Studying a Complicate Interaction to Reveal Possible Biomarkers for Female Infertility

Cellular metabolism, apoptosis, fertilization, and proliferation of granulosa cells belong to a battery of processes where microRNAs can be detected and associated with infertility. The aim of the present review is to focus on mammalian oocyte maturation events and the association between oocyte growth and miRNA expression. PubMed/Medline, Google Scholar and Scopus databases were searched, and 33 studies were included. Regarding the correlation among miRNA expression and the regulation of granulosa cells and cumulus cells, the most important miRNAs were let-7b, let-7c and miR-21. Additionally, the loss of Dicer, an enzyme involved in miRNA biogenesis, is probably a crucial factor in oogenesis, oocyte maturation and embryogenesis. Furthermore, miRNAs interfere with different cellular mechanisms like apoptosis, steroidogenesis, genome integrity, angiogenesis, antioxidative response and, consequently, oocyte maturation. Hence, it is of major importance to clarify the role and mechanism of each miRNA as understanding its action may develop new tools and establish new diagnostic and treatment approaches for infertility and ovarian disorders.

Control of poly(A)-tail length and translation in vertebrate oocytes and early embryos

During oocyte maturation and early embryogenesis, changes in mRNA poly(A)-tail lengths strongly influence translation, but how these tail-length changes are orchestrated has been unclear. Here, we performed tail-length and translational profiling of mRNA reporter libraries (each with millions of 3' UTR sequence variants) in frog oocytes and embryos and in fish embryos. Contrasting to previously proposed cytoplasmic polyadenylation elements (CPEs), we found that a shorter element, UUUUA, together with the polyadenylation signal (PAS), specify cytoplasmic polyadenylation, and we identified contextual features that modulate the activity of both elements. In maturing oocytes, this tail lengthening occurs against a backdrop of global deadenylation and the action of C-rich elements that specify tail-length-independent translational repression. In embryos, cytoplasmic polyadenylation becomes more permissive, and additional elements specify waves of stage-specific deadenylation. Together, these findings largely explain the complex tapestry of tail-length changes observed in early frog and fish development, with strong evidence of conservation in both mice and humans.

The identification and classification of candidate genes during the zygotic genome activation in the mammals

Zygotic genome activation (ZGA) is a critical event in early embryonic development, and thousands of genes are involved in this delicate and sophisticated biological process. To date, however, only a handful of these genes have revealed their core functions in this special process, and therefore the roles of other genes still remain unclear. In the present study, we used previously published transcriptome profiling to identify potential key genes (candidate genes) in minor ZGA and major ZGA in both human and mouse specimens, and further identified the conserved genes across species. Our results showed that 887 and 760 genes, respectively, were thought to be specific to human and mouse in major ZGA, and the other 135 genes were considered to be orthologous genes. Moreover, the conserved genes were most enriched in rRNA processing in the nucleus and cytosol, ribonucleoprotein complex biogenesis, ribonucleoprotein complex assembly and ribosome large subunit biogenesis. The findings of this first comprehensive identification and characterization of candidate genes in minor and major ZGA provide relevant insights for future studies on ZGA.

How great thou ART: biomechanical properties of oocytes and embryos as indicators of quality in assisted reproductive technologies

Assisted Reproductive Technologies (ART) have revolutionized infertility treatment and animal breeding, but their success largely depends on selecting high-quality oocytes for fertilization and embryos for transfer. During preimplantation development, embryos undergo complex morphogenetic processes, such as compaction and cavitation, driven by cellular forces dependent on cytoskeletal dynamics and cell-cell interactions. These processes are pivotal in dictating an embryo's capacity to implant and progress to full-term development. Hence, a comprehensive grasp of the biomechanical attributes characterizing healthy oocytes and embryos is essential for selecting those with higher developmental potential. Various noninvasive techniques have emerged as valuable tools for assessing biomechanical properties without disturbing the oocyte or embryo physiological state, including morphokinetics, analysis of cytoplasmic movement velocity, or quantification of cortical tension and elasticity using microaspiration. By shedding light on the cytoskeletal processes involved in chromosome segregation, cytokinesis, cellular trafficking, and cell adhesion, underlying oogenesis, and embryonic development, this review explores the significance of embryo biomechanics in ART and its potential implications for improving clinical IVF outcomes, offering valuable insights and research directions to enhance oocyte and embryo selection procedures.

Low-input lipidomics reveals lipid metabolism remodelling during early mammalian embryo development

Lipids are indispensable for energy storage, membrane structure and cell signalling. However, dynamic changes in various categories of endogenous lipids in mammalian early embryonic development have not been systematically characterized. Here we comprehensively investigated the dynamic lipid landscape during mouse and human early embryo development. Lipid signatures of different developmental stages are distinct, particularly for the phospholipid classes. We highlight that the high degree of phospholipid unsaturation is a conserved feature as embryos develop to the blastocyst stage. Moreover, we show that lipid desaturases such as SCD1 are required for in vitro blastocyst development and blastocyst implantation. One of the mechanisms is through the regulation of unsaturated fatty-acid-mediated fluidity of the plasma membrane and apical proteins and the establishment of apical-basal polarity during development of the eight-cell embryo to the blastocyst. Overall, our study provides an invaluable resource about the remodelling of the endogenous lipidome in mammalian preimplantation embryo development and mechanistic insights into the regulation of embryogenesis and implantation by lipid unsaturation.

The translational oscillation in oocyte and early embryo development

Translation is critical for development as transcription in the oocyte and early embryo is silenced. To illustrate the translational changes during meiosis and consecutive two mitoses of the oocyte and early embryo, we performed a genome-wide translatome analysis. Acquired data showed significant and uniform activation of key translational initiation and elongation axes specific to M-phases. Although global protein synthesis decreases in M-phases, translation initiation and elongation activity increases in a uniformly fluctuating manner, leading to qualitative changes in translation regulation via the mTOR1/4F/eEF2 axis. Overall, we have uncovered a highly dynamic and oscillatory pattern of translational reprogramming that contributes to the translational regulation of specific mRNAs with different modes of polysomal occupancy/translation that are important for oocyte and embryo developmental competence. Our results provide new insights into the regulation of gene expression during oocyte meiosis as well as the first two embryonic mitoses and show how temporal translation can be optimized. This study is the first step towards a comprehensive analysis of the molecular mechanisms that not only control translation during early development, but also regulate translation-related networks employed in the oocyte-to-embryo transition and embryonic genome activation.

Mammalian oocytes store proteins for the early embryo on cytoplasmic lattices

Mammalian oocytes are filled with poorly understood structures called cytoplasmic lattices. First discovered in the 1960s and speculated to correspond to mammalian yolk, ribosomal arrays, or intermediate filaments, their function has remained enigmatic to date. Here, we show that cytoplasmic lattices are sites where oocytes store essential proteins for early embryonic development. Using super-resolution light microscopy and cryoelectron tomography, we show that cytoplasmic lattices are composed of filaments with a high surface area, which contain PADI6 and subcortical maternal complex proteins. The lattices associate with many proteins critical for embryonic development, including proteins that control epigenetic reprogramming of the preimplantation embryo. Loss of cytoplasmic lattices by knocking out PADI6 or the subcortical maternal complex prevents the accumulation of these proteins and results in early embryonic arrest. Our work suggests that cytoplasmic lattices enrich maternally provided proteins to prevent their premature degradation and cellular activity, thereby enabling early mammalian development.

PADI6: What we know about the elusive fifth member of the peptidyl arginine deiminase family

Peptidyl arginine deiminase 6 (PADI6) is a maternal factor that is vital for early embryonic development. Deletion and mutations of its encoding gene in female mice or women lead to early embryonic developmental arrest, female infertility, maternal imprinting defects and hyperproliferation of the trophoblast. PADI6 is the fifth and least well-characterized member of the peptidyl arginine deiminases (PADIs), which catalyse the post-translational conversion of arginine to citrulline. It is less conserved than the other PADIs, and currently has no reported catalytic activity. While there are many suggested functions of PADI6 in the early mouse embryo, including in embryonic genome activation, cytoplasmic lattice formation, maternal mRNA and ribosome regulation, and organelle distribution, the molecular mechanisms of its function remain unknown. In this review, we discuss what is known about the function of PADI6 and highlight key outstanding questions that must be answered if we are to understand the crucial role it plays in early embryo development and female fertility. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Maternal RNA binding protein with multiple splicing 2 (RBPMS2) is involved in mouse blastocyst formation through the bone morphogenetic protein pathway

RESEARCH QUESTION

Is early embryo development in mice influenced by RNA binding protein with multiple splicing 2 (RBPMS2), a maternal factor that accumulates and is stored in the cytoplasm of mature oocytes?

DESIGN

The expression patterns of RBPMS2 in mouse were analysed using quantitative real-time PCR (qRT PCR) and immunofluorescence staining. The effect of knockdown of RBPMS2 on embryo development was evaluated through a microinjection of specific morpholino or small interfering RNA. RNA sequencing was performed for mechanistic analysis. The interaction between RBPMS2 and the bone morphogenetic protein (BMP) pathway was studied using BMP inhibitor and activator. The effect on the localization of E-cadherin was determined by immunofluorescence staining.

RESULTS

Maternal protein RBPMS2 is highly expressed in mouse oocytes, and knockdown of RBPMS2 inhibits embryo development from the morula to the blastocyst stage. Mechanistically, RNA sequencing showed that the differentially expressed genes were enriched in the transforming growth factor-β (TGF-β) signalling pathway. BMPs are members of the TGF-β superfamily of growth factors. It was found that the addition of BMP inhibitor to the culture medium led to a morula-stage arrest, similar to that seen in RBPMS2 knockdown embryos. This morula-stage arrest defect caused by RBPMS2 knockdown was partially rescued by BMP activator. Furthermore, the localization of E-cadherin to the membrane was impaired in response to a knockdown of RBPMS2 or inhibition of the BMP pathway.

CONCLUSION

This study suggests that RBPMS2 activates the BMP pathway and thus influences the localization of E-cadherin, which is important for early mouse embryo development during blastocyst formation.

MYC-MAX heterodimerization is essential for the induction of major zygotic genome activation and subsequent preimplantation development

In mouse preimplantation development, zygotic genome activation (ZGA), which synthesizes new transcripts in the embryo, begins in the S phase at the one-cell stage, with major ZGA occurring especially at the late two-cell stage. Myc is a transcription factor expressed in parallel with ZGA, but its direct association with major ZGA has not been clarified. In this study, we found that developmental arrest occurs at the two-cell stage when mouse embryos were treated with antisense oligonucleotides targeting Myc or MYC-specific inhibitors from the one-cell stage. To identify when MYC inhibition affects development, we applied time-limited inhibitor treatment and found that inhibition of MYC at the one-cell, four-cell, and morula stages had no effect on preimplantation development, whereas inhibitor treatment at the two-cell stage arrested development at the two-cell stage. Furthermore, transcriptome analysis revealed that when MYC function was inhibited, genes expressed in the major ZGA phase were suppressed. These results suggest that MYC is essential for the induction of major ZGA and subsequent preimplantation development. Revealing the function of MYC in preimplantation development is expected to contribute to advances in assisted reproductive technology.

PARP1-catalyzed PARylation of YY1 mediates endoplasmic reticulum stress in granulosa cells to determine primordial follicle activation

Although only a small number of primordial follicles are known to be selectively activated during female reproductive cycles, the mechanisms that trigger this recruitment remain largely uncharacterized. Misregulated activation of primordial follicles may lead to the exhaustion of the non-renewable pool of primordial follicles, resulting in premature ovarian insufficiency. Here, we found that poly(ADP-ribose) polymerase 1 (PARP1) enzymatic activity in the surrounding granulosa cells (GCs) in follicles determines the subpopulation of the dormant primordial follicles to be awakened. Conversely, specifically inhibiting PARP1 in oocytes in an in vitro mouse follicle reconstitution model does not affect primordial follicle activation. Further analysis revealed that PARP1-catalyzed transcription factor YY1 PARylation at Y185 residue facilitates YY1 occupancy at Grp78 promoter, a key molecular chaperone of endoplasmic reticulum stress (ERS), and promotes Grp78 transcription in GCs, which is required for GCs maintaining proper ERS during primordial follicle activation. Inhibiting PARP1 prevents the loss of primordial follicle pool by attenuating the excessive ERS in GCs under fetal bisphenol A exposure. Together, we demonstrate that PARP1 in GCs acts as a pivotal modulator to determine the fate of the primordial follicles and may represent a novel therapeutic target for the retention of primordial follicle pool in females.

LSM14B controls oocyte mRNA storage and stability to ensure female fertility

Controlled mRNA storage and stability is essential for oocyte meiosis and early embryonic development. However, how to regulate mRNA storage and stability in mammalian oogenesis remains elusive. Here we showed that LSM14B, a component of membraneless compartments including P-body-like granules and mitochondria-associated ribonucleoprotein domain (MARDO) in germ cell, is indispensable for female fertility. To reveal loss of LSM14B disrupted primordial follicle assembly and caused mRNA reduction in non-growing oocytes, which was concomitant with the impaired assembly of P-body-like granules. 10× Genomics single-cell RNA-sequencing and immunostaining were performed. Meanwhile, we conducted RNA-seq analysis of GV-stage oocytes and found that Lsm14b deficiency not only impaired the maternal mRNA accumulation but also disrupted the translation in fully grown oocytes, which was closely associated with dissolution of MARDO components. Moreover, Lsm14b-deficient oocytes reassembled a pronucleus containing decondensed chromatin after extrusion of the first polar body, through compromising the activation of maturation promoting factor, while the defects were restored via WEE1/2 inhibitor. Together, our findings reveal that Lsm14b plays a pivotal role in mammalian oogenesis by specifically controlling of oocyte mRNA storage and stability.

Photo-dependent cytosolic delivery of shRNA into a single blastomere in a mouse embryo

Single-cell-specific delivery of small RNAs, such as short hairpin RNA (shRNA) and small noncoding RNAs, allows us to elucidate the roles of specific upregulation of RNA expression and RNAi-mediated gene suppression in early embryo development. The photoinduced cytosolic dispersion of RNA (PCDR) method that we previously reported can introduce small RNAs into the cytosol of photoirradiated cells and enable RNA delivery into a single-cell in a spatiotemporally specific manner. However, the PCDR method has only been applied to planer cultured cells and not to embryos. This study demonstrated that the PCDR method can be utilized for photo-dependent cytosolic shRNA delivery into a single blastomere and for single blastomere-specific RNA interference in mouse embryos. Our results indicate that PCDR is a promising approach for studying the developmental process of early embryogenesis.

Genetic variants underlying developmental arrests in human preimplantation embryos

Developmental arrest in preimplantation embryos is one of the major causes of assisted reproduction failure. It is briefly defined as a delay or a failure of embryonic development in producing viable embryos during ART cycles. Permanent or partial developmental arrest can be observed in the human embryos from one-cell to blastocyst stages. These arrests mainly arise from different molecular biological defects, including epigenetic disturbances, ART processes, and genetic variants. Embryonic arrests were found to be associated with a number of variants in the genes playing key roles in embryonic genome activation, mitotic divisions, subcortical maternal complex formation, maternal mRNA clearance, repairing DNA damage, transcriptional, and translational controls. In this review, the biological impacts of these variants are comprehensively evaluated in the light of existing studies. The creation of diagnostic gene panels and potential ways of preventing developmental arrests to obtain competent embryos are also discussed.

Stable maternal proteins underlie distinct transcriptome, translatome, and proteome reprogramming during mouse oocyte-to-embryo transition

BACKGROUND

The oocyte-to-embryo transition (OET) converts terminally differentiated gametes into a totipotent embryo and is critically controlled by maternal mRNAs and proteins, while the genome is silent until zygotic genome activation. How the transcriptome, translatome, and proteome are coordinated during this critical developmental window remains poorly understood.

RESULTS

Utilizing a highly sensitive and quantitative mass spectrometry approach, we obtain high-quality proteome data spanning seven mouse stages, from full-grown oocyte (FGO) to blastocyst, using 100 oocytes/embryos at each stage. Integrative analyses reveal distinct proteome reprogramming compared to that of the transcriptome or translatome. FGO to 8-cell proteomes are dominated by FGO-stockpiled proteins, while the transcriptome and translatome are more dynamic. FGO-originated proteins frequently persist to blastocyst while corresponding transcripts are already downregulated or decayed. Improved concordance between protein and translation or transcription is observed for genes starting translation upon meiotic resumption, as well as those transcribed and translated only in embryos. Concordance between protein and transcription/translation is also observed for proteins with short half-lives. We built a kinetic model that predicts protein dynamics by incorporating both initial protein abundance in FGOs and translation kinetics across developmental stages.

CONCLUSIONS

Through integrative analyses of datasets generated by ultrasensitive methods, our study reveals that the proteome shows distinct dynamics compared to the translatome and transcriptome during mouse OET. We propose that the remarkably stable oocyte-originated proteome may help save resources to accommodate the demanding needs of growing embryos. This study will advance our understanding of mammalian OET and the fundamental principles governing gene expression.

Haploid Parthenogenetic Embryos Exhibit Unique Stress Response to pH, Osmotic and Oxidative Stress

Preimplantation-stage embryos are susceptible to various types of stress when cultured in vitro. Parthenogenetic embryos that lack spermatozoa contribution exhibit aberrant developmental dynamics due to their uniparental origin. Herein, we assessed whether the absence of paternal genome affects the susceptibility of the embryos to pH, osmotic and oxidative stress. Haploid parthenogenetic embryos (HPE) (activated oocytes with 1 pronucleus and 2 polar bodies) were generated by incubating cumulus oocyte complexes of Swiss albino mice with 10 mM strontium chloride for 3 h. Normally fertilized embryos (NFE) (fertilized oocytes with 2 pronuclei and 2 polar bodies) were derived using in vitro fertilization. At 2-cell stage, both HPE and NFE were exposed to various stressors including pH (6.8 to 8.2), osmotic (isotonic, hypotonic, and hypertonic), and peroxidatic oxidative (H2O2, 25 µM) stress. Endoplasmic reticulum stress response, mitochondrial membrane potential, and the rate of blastocyst development were assessed. HPE were susceptible to alteration in the pH that was well tolerated by NFE. Similarly, HPE displayed remarkable difference in sensitivity to hypertonic stress and oxidative stress compared to NFE. The results clearly indicate that the oocytes that develop into embryos in the absence of paternal contribution are more vulnerable to environmental stressors, further highlighting the importance of spermatozoa contribution and/or the ploidy status in mitigating these stressors and towards healthy early embryo development.

Single-cell quantification of ribosome occupancy in early mouse development

Translation regulation is critical for early mammalian embryonic development¹. However, previous studies had been restricted to bulk measurements², precluding precise determination of translation regulation including allele-specific analyses. Here, to address this challenge, we developed a novel microfluidic isotachophoresis (ITP) approach, named RIBOsome profiling via ITP (Ribo-ITP), and characterized translation in single oocytes and embryos during early mouse development. We identified differential translation efficiency as a key mechanism regulating genes involved in centrosome organization and N⁶-methyladenosine modification of RNAs. Our high-coverage measurements enabled, to our knowledge, the first analysis of allele-specific ribosome engagement in early development. These led to the discovery of stage-specific differential engagement of zygotic RNAs with ribosomes and reduced translation efficiency of transcripts exhibiting allele-biased expression. By integrating our measurements with proteomics data, we discovered that ribosome occupancy in germinal vesicle-stage oocytes is the predominant determinant of protein abundance in the zygote. The Ribo-ITP approach will enable numerous applications by providing high-coverage and high-resolution ribosome occupancy measurements from ultra-low input samples including single cells.

Sperm-borne microRNA-34c regulates maternal mRNA degradation and preimplantation embryonic development in mice

BACKGROUND

Studies have shown that sperm-borne microRNAs (miRNAs) are involved in mammalian preimplantation embryonic development. In humans, spermatozoan miR-34c levels are correlated with in vitro fertilization outcomes, such as embryo quality and the clinical pregnancy and live birth rates. In rabbits and cows, miR-34c improves the developmental competence of embryos generated by somatic cell nuclear transfer. However, the mechanisms underlying the regulation of embryonic development by miR-34c remain unknown.

METHODS

Female C57BL/6 mice (6-8 weeks old) were superovulated, and pronucleated zygotes were collected and microinjected with an miR-34c inhibitor or a negative-control RNA. The embryonic development of the microinjected zygotes was evaluated, and the messenger RNA (mRNA) expression profiles of the embryos at the two-cell, four-cell and blastocyst stages (five embryos per group) were determined by RNA sequencing analysis. Gene expression levels were verified by reverse transcription-quantitative polymerase chain reaction. Cluster analysis and heat map visualization were performed to detect differentially expressed mRNAs. Pathway and process enrichment analyses were performed using ontology resources. Differentially expressed mRNAs were systematically analyzed using the Search Tool for the Retrieval of Interacting Genes/Proteins database to determine their biological functions.

RESULTS

Embryonic developmental potential was significantly reduced in zygotes microinjected with the miR-34c inhibitor compared with those microinjected with a negative-control RNA. Two-cell stage embryos microinjected with an miR-34c inhibitor presented altered transcriptomic profiles, with upregulated expression of maternal miR-34c target mRNAs and classical maternal mRNAs. Differentially expressed transcripts were mainly of genes associated with lipid metabolism and cellular membrane function at the two-cell stage, with cell-cycle phase transition and energy metabolism at the four-cell stage; and with vesicle organization, lipid biosynthetic process and endomembrane system organization at the blastocyst stage. We also showed that genes related to preimplantation embryonic development, including Alkbh4, Sp1, Mapk14, Sin3a, Sdc1 and Laptm4b, were significantly downregulated after microinjection of an miR-34c inhibitor.

CONCLUSIONS

Sperm-borne miR-34c may regulate preimplantation embryonic development by affecting multiple biological processes, such as maternal mRNA degradation, cellular metabolism, cell proliferation and blastocyst implantation. Our data demonstrate the importance of sperm-derived miRNAs in the development of preimplantation embryos.

Maternal patterns of inheritance alter transcript expression in eggs

BACKGROUND

Modifications to early development can lead to evolutionary diversification. The early stages of development are under maternal control, as mothers produce eggs loaded with nutrients, proteins and mRNAs that direct early embryogenesis. Maternally provided mRNAs are the only expressed genes in initial stages of development and are tightly regulated. Differences in maternal mRNA provisioning could lead to phenotypic changes in embryogenesis and ultimately evolutionary changes in development. However, the extent that maternal mRNA expression in eggs can vary is unknown for most developmental models. Here, we use a species with dimorphic development- where females make eggs and larvae of different sizes and life-history modes-to investigate the extent of variation in maternal mRNA provisioning to the egg.

RESULTS

We find that there is significant variation in gene expression across eggs of different development modes, and that there are both qualitative and quantitative differences in mRNA expression. We separate parental effects from allelic effects, and find that both mechanisms contribute to mRNA expression differences. We also find that offspring of intraspecific crosses differentially provision their eggs based on the parental cross direction (a parental effect), which has not been previously demonstrated in reproductive traits like oogenesis.

CONCLUSION

We find that maternally controlled initiation of development is functionally distinct between eggs of different sizes and maternal genotypes. Both allele-specific effects and parent-of-origin effects contribute to gene expression differences in eggs. The latter indicates an intergenerational effect where a parent's genotype can affect gene expression in an egg made by the next generation.

RNA N6-methyladenosine modification in female reproductive biology and pathophysiology

Gene expression and posttranscriptional regulation can be strongly influenced by epigenetic modifications. N⁶-methyladenosine, the most extensive RNA modification, has been revealed to participate in many human diseases. Recently, the role of RNA epigenetic modifications in the pathophysiological mechanism of female reproductive diseases has been intensively studied. RNA m⁶A modification is involved in oogenesis, embryonic growth, and foetal development, as well as preeclampsia, miscarriage, endometriosis and adenomyosis, polycystic ovary syndrome, premature ovarian failure, and common gynaecological tumours such as cervical cancer, endometrial cancer, and ovarian cancer. In this review, we provide a summary of the research results of m⁶A on the female reproductive biology and pathophysiology in recent years and aim to discuss future research directions and clinical applications of m⁶A-related targets. Hopefully, this review will add to our understanding of the cellular mechanisms, diagnostic biomarkers, and underlying therapeutic strategies of female reproductive system diseases. Video Abstract.

HucMSC-EVs Facilitate In Vitro Development of Maternally Aged Preantral Follicles and Oocytes

Follicle developmental capacity and oocyte quality decline with advanced maternal age. Extracellular vesicles from human umbilical cord mesenchymal stem cells (HucMSC-EVs) act as a potential therapeutic product in the treatment of age-related ovarian dysfunction. In vitro culture (IVC) of preantral follicles is a useful method for understanding the mechanism of follicle development and is a promising means for improving female fertility. However, whether HucMSC-EVs have beneficial effects on aged follicle development during IVC has not yet been reported. Our research demonstrated that follicular development with single-addition withdrawal of HucMSC-EVs was better than that with continuous treatment with HucMSC-EVs. HucMSC-EVs facilitated the survival and growth of follicles, promoted the proliferation of granulosa cells (GCs), and improved the steroid hormone secretion of GCs during IVC of aged follicles. Both GCs and oocytes could uptake HucMSC-EVs. Moreover, we observed elevated cellular transcription in GCs and oocytes after treatment with HucMSC-EVs. The RNA sequencing (RNA-seq) results further validated that the differentially expressed genes are related to the promotion of GC proliferation, cell communication, and oocyte spindle organization. Additionally, the aged oocytes displayed a higher maturation rate, presented less aberrant spindle morphology, and expressed a higher level of the antioxidant protein Sirtuin 1 (SIRT1) after treatment with HucMSC-EVs. Our findings suggested that HucMSC-EVs can improve the growth and quality of aged follicles and oocytes in vitro through the regulation of gene transcription, which provides evidence for HucMSC-EVs as potential therapeutic reagents to restore female fertility with advanced age.

Polo-like kinase 1 promotes Cdc42-induced actin polymerization for asymmetric division in oocytes

Polo-like kinase I (Plk1) is a highly conserved seronine/threonine kinase essential in meiosis and mitosis for spindle formation and cytokinesis. Here, through temporal application of Plk1 inhibitors, we identify a new role for Plk1 in the establishment of cortical polarity essential for highly asymmetric cell divisions of oocyte meiosis. Application of Plk1 inhibitors in late metaphase I abolishes pPlk1 from spindle poles and prevents the induction of actin polymerization at the cortex through inhibition of local recruitment of Cdc42 and Neuronal Wiskott-Aldrich Syndrome protein (N-WASP). By contrast, an already established polar actin cortex is insensitive to Plk1 inhibitors, but if the polar cortex is first depolymerized, Plk1 inhibitors completely prevent its restoration. Thus, Plk1 is essential for establishment but not maintenance of cortical actin polarity. These findings indicate that Plk1 regulates recruitment of Cdc42 and N-Wasp to coordinate cortical polarity and asymmetric cell division.

The pyruvate dehydrogenase complex: Life's essential, vulnerable and druggable energy homeostat

Found in all organisms, pyruvate dehydrogenase complexes (PDC) are the keystones of prokaryotic and eukaryotic energy metabolism. In eukaryotic organisms these multi-component megacomplexes provide a crucial mechanistic link between cytoplasmic glycolysis and the mitochondrial tricarboxylic acid (TCA) cycle. As a consequence, PDCs also influence the metabolism of branched chain amino acids, lipids and, ultimately, oxidative phosphorylation (OXPHOS). PDC activity is an essential determinant of the metabolic and bioenergetic flexibility of metazoan organisms in adapting to changes in development, nutrient availability and various stresses that challenge maintenance of homeostasis. This canonical role of the PDC has been extensively probed over the past decades by multidisciplinary investigations into its causal association with diverse physiological and pathological conditions, the latter making the PDC an increasingly viable therapeutic target. Here we review the biology of the remarkable PDC and its emerging importance in the pathobiology and treatment of diverse congenital and acquired disorders of metabolic integration.

- Invited Review - Reproductive technologies needed for the generation of precise gene-edited pigs in the pathways from laboratory to farm

Gene editing (GE) offers a new breeding technique (NBT) of sustainable value to animal agriculture. There are 3 GE working sites covering 5 feasible pathways to generate GE pigs along with the crucial intervals of GE/genotyping, microinjection/electroporation, induced pluripotent stem cells, somatic cell nuclear transfer, cryopreservation, and nonsurgical embryo transfer. The extension of NBT in the new era of pig breeding depends on the synergistic effect of GE and reproductive biotechnologies; the outcome relies not only on scientific due diligence and operational excellence but also on the feasibility of application on farms to improve sustainability.

Karyopherin α deficiency contributes to human preimplantation embryo arrest

Preimplantation embryo arrest (PREMBA) is a common cause of female infertility and recurrent failure of assisted reproductive technology. However, the genetic basis of PREMBA is largely unrevealed. Here, using whole-exome sequencing data from 606 women experiencing PREMBA compared with 2,813 controls, we performed a population and gene-based burden test and identified a candidate gene, karyopherin subunit α7 (KPNA7). In vitro studies showed that identified sequence variants reduced KPNA7 protein levels, impaired KPNA7 capacity for binding to its substrate ribosomal L1 domain-containing protein 1 (RSL1D1), and affected KPNA7 nuclear transport activity. Comparison between humans and mice suggested that mouse KPNA2, rather than mouse KPNA7, acts as an essential karyopherin in embryonic development. Kpna2-/- female mice showed embryo arrest due to zygotic genome activation defects, recapitulating the phenotype of human PREMBA. In addition, female mice with an oocyte-specific knockout of Rsl1d1 recapitulated the phenotype of Kpna2-/- mice, demonstrating the vital role of substrate RSL1D1. Finally, complementary RNA (cRNA) microinjection of human KPNA7, but not mouse Kpna7, was able to rescue the embryo arrest phenotype in Kpna2-/- mice, suggesting mouse KPNA2 might be a homologue of human KPNA7. Our findings uncovered a mechanistic understanding for the pathogenesis of PREMBA, which acts by impairing nuclear protein transport, and provide a diagnostic marker for PREMBA patients.

A complex heterozygous mutation in PADI6 causes early embryo arrest: A case report

Background: The PADI6 gene is a component of the subcortical maternal effect complex (SCMC). Mutations in the PADI6 gene, which was the first gene discovered to impact the activation process of the human embryonic genome, have been shown to induce early embryo arrest. Case: A 29-year-old lady with primary infertility underwent in vitro fertilization embryo transfer (IVF-ET) for tubal reasons, who had normal hormone levels and ovarian reserve. A Progestin-Primed Ovarian Stimulation (PPOS) protocol of Ovarian stimulation with IVF was performed. The total of Gonadotropin (Gn) stimulation with u-FSH was 2100 IU, which lasted for 10 days. When three follicles measuring less than 18 mm in diameter were seen, r-hCG 250 ug and triptorelin acetate 0.2 mg were injected to trigger oocyte maturation. Nineteen oocytes (including thirteen MII oocytes) were picked up 37 h after the trigger, and seven of these were normal fertilized. Unfortunately, these many embryos were stopped at the 1- or 2-cell stage, hence this infertile patient's IVF treatment won't result in an embryo transfer. Using whole-exome sequencing, a complex heterozygous mutation in PADI6 was discovered: c. 1247T>C [p.Ile416Thr] in exon 12 of PADI6, and c. 2009_2010del [p.Glu670GlyfsTer48] in exon 17 of PADI6. Conclusion: We found a complex heterozygous mutation in the PADI6 gene (c. 1247T>C; c. 2009_2010del) that caused embryos were arrested at the 1- or 2- cell stage. The discovery in this patient adds to the evidence showing the PADI6 gene mutation causes early embryo arrest in humans.