Single-cell quantitative proteomic analysis of human oocyte maturation revealed high heterogeneity in in vitro matured oocytes

DNA methylation landscapes of in vitro matured oocytes retrieved during endoscopic gynaecological procedures

In vitro maturation (IVM) of human oocytes offers cost efficiency and minimal invasiveness, serving as a valuable supplementary tool in assisted reproduction for fertility preservation, ovarian hyperstimulation syndrome prevention, and other reproductive strategies. Despite its availability for three decades, the clinical use of IVM remains limited due to efficacy and safety concerns. This study examines the DNA methylation profile of IVM oocytes collected during laparoscopic/hysteroscopic surgeries compared to in vivo matured oocytes via reduced representation bisulfite sequencing. Results indicate IVM oocytes exhibit a higher global methylation level. Differentially methylated regions (DMR) analysis reveals that the in vitro group displays more hypermethylated and fewer hypomethylated DMRs compared to the in vivo group. Additionally, the in vitro group exhibits a higher level of non-CpG methylation than the in vivo group. However, no significant correlation between methylation levels and transcriptional activity in these oocytes is found, especially for those specific imprinted genes or genes related to embryonic development. These findings shed light on the epigenetic landscape of IVM oocytes, contributing to the ongoing assessment of their clinical feasibility and safety in assisted reproduction.

Single-Cell Proteomics Using the One-Step Droplet-in-Oil Digestion Method Reveals Proteins Important for Male Meiotic Progression

Different from DNA or RNA, proteins cannot be amplified. The performance of single-cell proteomics is limited by sample loss during sample preparation. Here, we present a one-step droplet-in-oil digestion (OSDO) method that involves one-step water-in-oil processing using cyclohexane or n-heptane, which can reduce the sample adsorption loss and sample volume change due to evaporation and increase the sensitivity and stability of single-cell proteomics. The OSDO is demonstrated to be compatible with tandem mass tag (TMT) labeling to improve the throughput. The OSDO, followed by data-independent acquisition (DIA), quantified more than 3700 proteins per cell during meiotic progression from pachytene to metaphase I, with no correlation between protein and mRNA levels. Inhibition of VPS34 and DNMT1, two proteins up-regulated in metaphase I, both affected metaphase I formation. The OSDO is an easy-to-operate method compatible with both subsequent labeled and unlabeled quantification to expand the depth, throughput, and applicability in single-cell proteomics.

Single-cell proteomics analysis of human oocytes during GV-to-MI transition

STUDY QUESTION

Which proteins are involved in the transition of human oocytes from the germinal vesicle (GV) to metaphase I (MI) phase?

SUMMARY ANSWER

A total of 2369 proteins were identified, including 149 with significantly differential expression, 79 with upregulated expression in MI oocytes and 70 with downregulated expression.

WHAT IS KNOWN ALREADY

During oocyte maturation, maternal proteins and RNA are stored to support early embryo development. However, GV oocytes matured in vitro have a lower chance of developing into blastocysts than MI oocytes. Therefore, identifying key differentially expressed proteins between the GV and MI stages can provide a better understanding of human oocyte development and maturation mechanisms and improve the utilization of oocytes.

STUDY DESIGN, SIZE, DURATION

In total, 16 oocytes at the GV and MI stages were collected from female patients who underwent ovulation induction due to male factor infertility requiring embryo retrieval for ICSI. Differential proteins were identified in 16 oocytes using liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, and the expression of several differential proteins was verified by immunofluorescence (IF). RNA interference was employed to identify the functions of specific proteins during oocyte maturation.

PARTICIPANTS/MATERIALS, SETTING, METHODS

16 immature human oocytes discarded during ICSI cycles (eight GV oocytes and eight MI oocytes) were collected from 10 female patients. Two cohorts of oocytes underwent zona pellucida removal, lysis, and enzymatic digestion prior to peptide detection using LC-MS/MS methodology. Peptide detection outcomes were subjected to differential protein screening and functional annotation employing distinct analytical algorithms and datasets. To corroborate the sequencing findings, proteins exhibiting notable differential expression were authenticated via IF. Concerning protein functionality, siRNA was introduced during the GV phase, and oocyte maturation was evaluated through observation of polar body extrusion, alongside assessment of siRNA interference efficacy via IF analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

A total of 2369 proteins were identified, including 149 with significantly differential expression, 79 with upregulated expression in MI oocytes and 70 with downregulated expression. Gene ontology functional annotation and functional analysis revealed that these differentially expressed proteins are involved mainly in organic matter and cell metabolism, biological regulation, primary metabolism, nitrogen compound metabolism, and other biological processes. Kyoto Encyclopedia of Genes and Genomes analysis revealed that the differentially expressed genes were involved mainly in the following pathways: transport and catabolism, signal transduction, protein folding, and energy and amino acid metabolism. The differentially expressed proteins included actin-related protein 2 (ACTR2), NADH: Ubiquinone Oxidoreductase Core Subunit S1 (NDUFS1), Tubulin Gamma Complex Component 3 (TUBGCP3), Heat Shock Protein Family B (Small) Member 1 (HSPB1), and Eukaryotic Translation Initiation Factor 3 Subunit B, which are involved mainly in mitochondrial function, cell division, and signal transduction. ACTR2, HSPB1, NDUFS1, and TUBGCP3 were selected for IF staining, and the difference in fluorescence intensity between GV and MI oocytes was consistent with the sequencing results. Three pairs of primers were designed for each gene corresponding to the top 10 differentially upregulated and downregulated proteins (with siRNAs successfully designed for eight upregulated and seven downregulated proteins) to study their function, and the results revealed that the protein expression of TUBGCP3 was downregulated after RNA interference.

LARGE SCALE DATA

See supplementary tables.

LIMITATIONS, REASONS FOR CAUTION

Although we have identified some differentially expressed proteins during the transition from human oocyte GV to MI stage, their crucial roles in oocyte maturation remain elusive. To elucidate the functions of these proteins in oocyte maturation, we have generated conditional knockout mice targeting selected proteins.

WIDER IMPLICATIONS OF THE FINDINGS

We conducted single-cell level analysis to identify differentially expressed proteins between the human oocyte GV and MI stages. Our objective is to ascertain the potential of supplementing these proteins in the in vitro maturation culture medium to augment both oocyte maturation rates and quality.

STUDY FUNDING/COMPETING INTEREST(S)

This research was supported by the National Natural Science Foundation of China (82171599 and 82471657, B.X., 82301871, L.L.); China Postdoctoral Science Foundation (2024M763169, S.B.); and the National Key Research and Development Project of China (2029YFA0802600, B.X.). None of the authors has any conflict of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Human oocyte quality and reproductive health

Declining female fertility is a health issue that has received broad global attention. Oocyte quality is the key limiting factor of female fertility, and key processes affecting oocyte quality involve the secretion of and response to hormones, ovarian function, oogenesis, oocyte maturation, and meiosis. However, compared with other species, the research and understanding of human oocyte quality and human reproductive health is limited. This review highlights our current understanding of the physiological factors and pathological factors related to human oocyte quality and discusses potential treatments. In terms of physiology, we discuss the regulation of the hypothalamic-pituitary-gonadal axis, granulosa cells, key subcellular structures, maternal mRNA homeostasis, the extracellular matrix, the maternal microenvironment, and multi-omics resources related to human oocyte quality. In terms of pathology, we review hypothalamic-pituitary-gonadal defects, ovarian dysfunction (including premature ovarian insufficiency and polycystic ovary syndrome), human oocyte development defects, and aging. In terms of the pathological aspects of human oocyte development and quality defects, nearly half of the reported pathogenic genes are involved in meiosis, while the remainder are involved in maternal mRNA regulation, the subcortical maternal complex, zona pellucida formation, ion channels, protein transport, and mitochondrial function. Furthermore, we outline the emerging scientific prospects and challenges for future explorations of the biological mechanisms behind infertility and the development of clinical treatments. This review seeks to deepen our understanding of the mechanisms regulating human oocyte quality and to provide novel insights into clinical female infertility characterized by defects in oocyte quality and oocyte development.

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.

The adult ovary at single cell resolution: an expert review

The ovaries play a crucial role in both the endocrine health and fertility of adult women. The fundamental functional units of the ovaries, primordial follicles, form during fetal development and constitute the ovarian reserve. Ovaries age prematurely in comparison to other organs, with the quality of oocytes declining steeply prior to the entire reserve becoming depleted, usually around age 50. Despite the pivotal role of ovaries in women's overall health, surprisingly little is known about the mechanisms controlling follicle dormancy, growth activation, atresia, maturation, and oocyte quality. Understanding ovarian function on a cellular and molecular level is increasingly important for several reasons. First, the global trend of women delaying childbirth creates a growing population of patients wishing to conceive when the quality and quantity of their oocytes are already critically low. Second, conditions affecting the ovaries, such as polycystic ovary syndrome and endometriosis, are widespread, yet diagnosis and treatment still present challenges. Lastly, advancements in cancer therapies have increased the number of cancer survivors who contend with late complications affecting fertility and hormonal balance. Clearly, a better understanding of diseases, aging, and toxicity in ovaries is needed for the development of novel treatments, preventive therapies, and safer pharmaceuticals. Human ovaries are notoriously difficult to obtain for research due to their pivotal role in women's health, and the highly heterogeneous distribution of follicles within the tissue combined with monthly cyclical changes present further challenges. Single-cell profiling techniques are creating new opportunities, enabling the characterization of small amounts of tissue with unprecedented resolution. Here, we review the literature on single-cell characterization of adult, reproductive-age ovaries. The majority of the 46 identified studies have focused on oocytes discarded during assisted reproduction, with only a handful focusing on ovarian tissue. The overwhelming focus of the studies is on follicles and oocytes, although the somatic cell niche in the ovary undoubtedly plays an important role in endocrine function and follicle biology. Altogether, the studies reveal unexpected diversity and heterogeneity among ovarian somatic and germ cells, highlighting the prevailing knowledge gaps in basic ovarian biology. As the most common outcome for a follicle is atresia, it is possible that part of the cell diversity relates to the biology of follicles destined to degenerate. The absence of spatial coordinates in single-cell studies further complicates the interpretation of the roles and significance of the various reported cell clusters. Accomplishing a representative ovarian single-cell atlas will require merging these studies. However, direct comparisons are challenging due to nonuniform nomenclature, differing tissue sources, varying meta-data reporting, and lack of gold standards in technical approaches. Although these reports establish a single-cell draft of adult-fertile age human ovaries, more detailed metadata and better quality reporting will be essential for the development of a robust ovarian cell atlas in health and disease.

Comparative proteomic landscapes elucidate human preimplantation development and failure

Understanding mammalian preimplantation development, particularly in humans, at the proteomic level remains limited. Here, we applied our comprehensive solution of ultrasensitive proteomic technology to measure the proteomic profiles of oocytes and early embryos and identified nearly 8,000 proteins in humans and over 6,300 proteins in mice. We observed distinct proteomic dynamics before and around zygotic genome activation (ZGA) between the two species. Integrative analysis with translatomic data revealed extensive divergence between translation activation and protein accumulation. Multi-omic analysis indicated that ZGA transcripts often contribute to protein accumulation in blastocysts. Using mouse embryos, we identified several transcriptional regulators critical for early development, thereby linking ZGA to the first lineage specification. Furthermore, single-embryo proteomics of poor-quality embryos from over 100 patient couples provided insights into preimplantation development failure. Our study may contribute to reshaping the framework of mammalian preimplantation development and opening avenues for addressing human infertility.

Single-Cell Multi-Omics Analysis of In Vitro Post-Ovulatory-Aged Oocytes Revealed Aging-Dependent Protein Degradation

Once ovulated, the oocyte has to be fertilized in a short time window or it will undergo post-ovulation aging (POA), whose underlying mechanisms are still not elucidated. Here, we optimized single-cell proteomics methods and performed single-cell transcriptomic, proteomic, and phosphoproteomic analysis of fresh, POA, and melatonin-treated POA oocytes. POA oocytes showed downregulation of most differentially expressed proteins, with little correlation with mRNA expression, and the protein changes can be rescued by melatonin treatment. MG132 treatment rescued the decreased fertilization and polyspermy rates and upregulated fragmentation and parthenogenesis rates of POA oocytes. MG132-treated oocytes displayed health status at proteome, phosphoproteome, and fertilization ability similar to fresh oocytes, suggesting that protein stabilization might be the underlying mechanism for melatonin to rescue POA. The important roles of proteasome-mediated protein degradation during oocyte POA revealed by single-cell multi-omics analyses offer new perspectives for increasing oocyte quality during POA and improving assisted reproduction technologies.

Environmental exposure to lead and cadmium only minimally affects the redox system of the follicular fluid and the outcome of intracytoplasmic sperm injection

The purpose of our study was to determine the influence of lead and cadmium in concentrations commonly found in the environment on the redox system of the follicular fluid (FF) and on the results of assisted reproduction. A prospective study of 113 patients with unexplained infertility who qualified for intracytoplasmic sperm injection (ICSI). Patients with moderate or severe endometriosis or poor ovarian reserve were excluded from the study. Biochemical analyses and heavy metal assays of follicular fluid and serum (blood) were followed by statistical analyses of dependencies between lead and cadmium and the components of redox system and results of assisted reproduction. A highly significant linear correlation of lead (Pb) and cadmium (Cd) concentrations in serum and in FF was stated. The number of retrieved oocytes and MII (metaphase II stage) oocytes depended on the malondialdehyde (MDA), catalase (CAT), catalase/g of protein (CAT/g of protein), and glutathione reductase (GR) concentrations. Among biochemical factors, MDA was the only factor that correlated negatively with cadmium concentration in serum and FF and simultaneously influenced the number of retrieved oocytes and MII oocytes. The fertilization rate of MII oocytes was influenced by thiol groups-SH, SH/g of protein, CAT, CAT/g of protein, and glutathione peroxidase/g of protein (GPx/g of protein). The Pb and Cd concentrations in FF did not significantly influence the fertilization rates. Lead as well as cadmium at concentrations commonly found in women of reproductive age despite some adaptive changes in the redox system in follicular fluid do not cause large changes in the ovarian follicular environment as a whole and do not significantly worsen the final results of assisted reproduction.

Quantitative proteomics and phosphoproteomics profiling of meiotic divisions in the fission yeast Schizosaccharomyces pombe

In eukaryotes, chromosomal DNA is equally distributed to daughter cells during mitosis, whereas the number of chromosomes is halved during meiosis. Despite considerable progress in understanding the molecular mechanisms that regulate mitosis, there is currently a lack of complete understanding of the molecular mechanisms regulating meiosis. Here, we took advantage of the fission yeast Schizosaccharomyces pombe, for which highly synchronous meiosis can be induced, and performed quantitative proteomics and phosphoproteomics analyses to track changes in protein expression and phosphorylation during meiotic divisions. We compared the proteomes and phosphoproteomes of exponentially growing mitotic cells with cells harvested around meiosis I, or meiosis II in strains bearing either the temperature-sensitive pat1-114 allele or conditional ATP analog-sensitive pat1-as2 allele of the Pat1 kinase. Comparing pat1-114 with pat1-as2 also allowed us to investigate the impact of elevated temperature (25 °C versus 34 °C) on meiosis, an issue that sexually reproducing organisms face due to climate change. Using TMTpro 18plex labeling and phosphopeptide enrichment strategies, we performed quantification of a total of 4673 proteins and 7172 phosphosites in S. pombe. We found that the protein level of 2680 proteins and the rate of phosphorylation of 4005 phosphosites significantly changed during progression of S. pombe cells through meiosis. The proteins exhibiting changes in expression and phosphorylation during meiotic divisions were represented mainly by those involved in the meiotic cell cycle, meiotic recombination, meiotic nuclear division, meiosis I, centromere clustering, microtubule cytoskeleton organization, ascospore formation, organonitrogen compound biosynthetic process, carboxylic acid metabolic process, gene expression, and ncRNA processing, among others. In summary, our findings provide global overview of changes in the levels and phosphorylation of proteins during progression of S. pombe cells through meiosis at normal and elevated temperatures, laying the groundwork for further elucidation of the functions and importance of specific proteins and their phosphorylation in regulating meiotic divisions in this yeast.

Assessment of active translation in cumulus-enclosed and denuded oocytes during standard in vitro maturation and early embryo development

STUDY QUESTION

Which actively translated maternal transcripts are differentially regulated between clinically relevant in vitro and in vivo maturation (IVM) conditions in mouse oocytes and zygotes?

SUMMARY ANSWER

Our findings uncovered significant differences in the global transcriptome as well as alterations in the translation of specific transcripts encoding components of energy production, cell cycle regulation, and protein synthesis in oocytes and RNA metabolism in zygotes.

WHAT IS KNOWN ALREADY

Properly regulated translation of stored maternal transcripts is a crucial factor for successful development of oocytes and early embryos, particularly due to the transcriptionally silent phase of meiosis.

STUDY DESIGN, SIZE, DURATION

This is a basic science study utilizing an ICR mouse model, best suited for studying in vivo maturation. In the treatment group, fully grown germinal vesicle oocytes from stimulated ovaries were in vitro matured to the metaphase II (MII) stage either as denuded without gonadotropins (IVM DO), or as cumulus-oocyte complexes (IVM COC) in the presence of 0.075 IU/ml recombinant FSH (rFSH) and 0.075 IU/ml recombinant hCG (rhCG). To account for changes in developmental competence, IVM COC from non-stimulated ovaries (IVM COC-) were included. In vivo matured MII oocytes (IVO) from stimulated ovaries were used as a control after ovulation triggering with rhCG. To simulate standard IVM conditions, we supplemented media with amino acids, vitamins, and bovine serum albumin. Accordingly, in vitro pronuclear zygotes (IMZ) were generated by IVF from IVM DO, and were compared to in vivo pronuclear zygotes (IVZ). All experiments were performed in quadruplicates with samples collected for both polyribosome fractionation and total transcriptome analysis. Samples were collected over three consecutive months.

PARTICIPANTS/MATERIALS, SETTING, METHODS

All ICR mice were bred under legal permission for animal experimentation (no. MZE-24154/2021-18134) obtained from the Ministry of Agriculture of the Czech Republic. Actively translated (polyribosome occupied) maternal transcripts were detected in in vitro and in vivo matured mouse oocytes and zygotes by density gradient ultracentrifugation, followed by RNA isolation and high-throughput RNA sequencing. Bioinformatic analysis was performed and subsequent data validation was done by western blotting, radioactive isotope, and mitotracker dye labelling.

MAIN RESULTS AND THE ROLE OF CHANCE

Gene expression analysis of acquired polysome-derived high-throughput RNA sequencing data revealed significant changes (RPKM ≥ 0.2; P ≤ 0.005) in translation between in vitro and in vivo matured oocytes and respectively produced pronuclear zygotes. Surprisingly, the comparison between IVM DO and IVM COC RNA-seq data of both fractionated and total transcriptome showed very few transcripts with more than a 2-fold difference. Data validation by radioactive isotope labelling revealed a decrease in global translation bof20% in IVM DO and COC samples in comparison to IVO samples. Moreover, IVM conditions compromised oocyte energy metabolism, which was demonstrated by both changes in polysome recruitment of each of 13 mt-protein-coding transcripts as well as by validation using mitotracker red staining.

LARGE SCALE DATA

The data discussed in this publication have been deposited in NCBI's Gene Expression Omnibus and are accessible through GEO Series accession number GSE241633 (https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE241633).

LIMITATIONS, REASONS FOR CAUTION

It is extremely complicated to achieve in vivo consistency in animal model systems such as porcine or bovine. To achieve a high reproducibility of in vivo stimulations, the ICR mouse model was selected. However, careful interpretation of our findings with regard to assisted reproductive techniques has to be made by taking into consideration intra-species differences between the mouse model and humans. Also, the sole effect of the cumulus cells' contribution could not be adequately addressed by comparing IVM COC and IVM DO, because the IVM DO were matured without gonadotropin supplementation.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings confirmed the inferiority of standard IVM technology compared with the in vivo approach. It also pointed at compromised biological processes employed in the critical translational regulation of in vitro matured MII oocytes and pronuclear zygotes. By highlighting the importance of proper translational regulation during in vitro oocyte maturation, this study should prompt further clinical investigations in the context of translation.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the Czech Grant Agency (22-27301S), Charles University Grant Agency (372621), Ministry of Education, Youth and Sports (EXCELLENCE CZ.02.1.01/0.0/0.0/15_003/0000460 OP RDE), and Institutional Research Concept RVO67985904. No competing interest is declared.

Single-cell proteomics reveals decreased abundance of proteostasis and meiosis proteins in advanced maternal age oocytes

Advanced maternal age is associated with a decline in oocyte quality, which often leads to reproductive failure in humans. However, the mechanisms behind this age-related decline remain unclear. To gain insights into this phenomenon, we applied plexDIA, a multiplexed data-independent acquisition, single-cell mass spectrometry method, to analyze the proteome of oocytes from both young women and women of advanced maternal age. Our findings primarily revealed distinct proteomic profiles between immature fully grown germinal vesicle and mature metaphase II oocytes. Importantly, we further show that a woman's age is associated with changes in her oocyte proteome. Specifically, when compared to oocytes obtained from young women, advanced maternal age oocytes exhibited lower levels of the proteasome and TRiC complex, as well as other key regulators of proteostasis and meiosis. This suggests that aging adversely affects the proteostasis and meiosis networks in human oocytes. The proteins identified in this study hold potential as targets for improving oocyte quality and may guide future studies into the molecular processes underlying oocyte aging.

Single-cell proteomics reveals decreased abundance of proteostasis and meiosis proteins in advanced maternal age oocytes

Advanced maternal age is associated with a decline in oocyte quality, which often leads to reproductive failure in humans. However, the mechanisms behind this age-related decline remain unclear. To gain insights into this phenomenon, we applied plexDIA, a multiplexed, single-cell mass spectrometry method, to analyze the proteome of oocytes from both young women and women of advanced maternal age. Our findings primarily revealed distinct proteomic profiles between immature fully grown germinal vesicle and mature metaphase II oocytes. Importantly, we further show that a woman's age is associated with changes in her oocyte proteome. Specifically, when compared to oocytes obtained from young women, advanced maternal age oocytes exhibited lower levels of the proteasome and TRiC complex, as well as other key regulators of proteostasis and meiosis. This suggests that aging adversely affects the proteostasis and meiosis networks in human oocytes. The proteins identified in this study hold potential as targets for improving oocyte quality and may guide future studies into the molecular processes underlying oocyte aging.

EPAS1 expression contributes to maintenance of the primordial follicle pool in the mouse ovary

Oxygen availability can have profound effects on cell fate decisions and survival, in part by regulating expression of hypoxia-inducible factors (HIFs). In the ovary, HIF expression has been characterised in granulosa cells, however, any requirement in oocytes remains relatively undefined. Here we developed a Hif2a/Epas1 germline-specific knockout mouse line in which females were fertile, however produced 40% fewer pups than controls. No defects in follicle development were detected, and quality of MII oocytes was normal, as per assessments of viability, intracellular reactive oxygen species, and spindle parameters. However, a significant diminishment of the primordial follicle pool was evident in cKO females that was attributed to accelerated follicle loss from postnatal day 6 onwards, potentially via disruption of the autophagy pathway. These data demonstrate the importance of HIF signalling in oocytes, particularly at the primordial follicle stage, and lend to the importance of controlling oxygen tension in the development of in vitro growth and maturation approaches for assisted reproduction.

Single-Cell Transcriptome Analysis Reveals Development-Specific Networks at Distinct Synchronized Antral Follicle Sizes in Sheep Oocytes

The development of the ovarian antral follicle is a complex, highly regulated process. Oocytes orchestrate and coordinate the development of mammalian ovarian follicles, and the rate of follicular development is governed by a developmental program intrinsic to the oocyte. Characterizing oocyte signatures during this dynamic process is critical for understanding oocyte maturation and follicular development. Although the transcriptional signature of sheep oocytes matured in vitro and preovulatory oocytes have been previously described, the transcriptional changes of oocytes in antral follicles have not. Here, we used single-cell transcriptomics (SmartSeq2) to characterize sheep oocytes from small, medium, and large antral follicles. We characterized the transcriptomic landscape of sheep oocytes during antral follicle development, identifying unique features in the transcriptional atlas, stage-specific molecular signatures, oocyte-secreted factors, and transcription factor networks. Notably, we identified the specific expression of 222 genes in the LO, 8 and 6 genes that were stage-specific in the MO and SO, respectively. We also elucidated signaling pathways in each antral follicle size that may reflect oocyte quality and in vitro maturation competency. Additionally, we discovered key biological processes that drive the transition from small to large antral follicles, revealing hub genes involved in follicle recruitment and selection. Thus, our work provides a comprehensive characterization of the single-oocyte transcriptome, filling a gap in the mapping of the molecular landscape of sheep oogenesis. We also provide key insights into the transcriptional regulation of the critical sizes of antral follicular development, which is essential for understanding how the oocyte orchestrates follicular development.

Revolutionizing the female reproductive system research using microfluidic chip platform

Comprehensively understanding the female reproductive system is crucial for safeguarding fertility and preventing diseases concerning women's health. With the capacity to simulate the intricate physio- and patho-conditions, and provide diagnostic platforms, microfluidic chips have fundamentally transformed the knowledge and management of female reproductive health, which will ultimately promote the development of more effective assisted reproductive technologies, treatments, and drug screening approaches. This review elucidates diverse microfluidic systems in mimicking the ovary, fallopian tube, uterus, placenta and cervix, and we delve into the culture of follicles and oocytes, gametes' manipulation, cryopreservation, and permeability especially. We investigate the role of microfluidics in endometriosis and hysteromyoma, and explore their applications in ovarian cancer, endometrial cancer and cervical cancer. At last, the current status of assisted reproductive technology and integrated microfluidic devices are introduced briefly. Through delineating the multifarious advantages and challenges of the microfluidic technology, we chart a definitive course for future research in the woman health field. As the microfluidic technology continues to evolve and advance, it holds great promise for revolutionizing the diagnosis and treatment of female reproductive health issues, thus propelling us into a future where we can ultimately optimize the overall wellbeing and health of women everywhere.

Ultrasensitive proteomics depicted an in-depth landscape for the very early stage of mouse maternal-to-zygotic transition

Single-cell or low-input multi-omics techniques have revolutionized the study of pre-implantation embryo development. However, the single-cell or low-input proteomic research in this field is relatively underdeveloped because of the higher threshold of the starting material for mammalian embryo samples and the lack of hypersensitive proteome technology. In this study, a comprehensive solution of ultrasensitive proteome technology (CS-UPT) was developed for single-cell or low-input mouse oocyte/embryo samples. The deep coverage and high-throughput routes significantly reduced the starting material and were selected by investigators based on their demands. Using the deep coverage route, we provided the first large-scale snapshot of the very early stage of mouse maternal-to-zygotic transition, including almost 5,500 protein groups from 20 mouse oocytes or zygotes for each sample. Moreover, significant protein regulatory networks centered on transcription factors and kinases between the MII oocyte and 1-cell embryo provided rich insights into minor zygotic genome activation.

Single-cell proteomics: quantifying post-transcriptional regulation during development with mass-spectrometry

Many developmental processes are regulated post-transcriptionally. Such post-transcriptional regulatory mechanisms can now be analyzed by robust single-cell mass spectrometry methods that allow accurate quantification of proteins and their modification in single cells. These methods can enable quantitative exploration of protein synthesis and degradation mechanisms that contribute to developmental cell fate specification. Furthermore, they may support functional analysis of protein conformations and activities in single cells, and thus link protein functions to developmental processes. This Spotlight provides an accessible introduction to single-cell mass spectrometry methods and suggests initial biological questions that are ripe for investigation.

Label-free single cell proteomics utilizing ultrafast LC and MS instrumentation: A valuable complementary technique to multiplexing

The ability to map a proteomic fingerprint to transcriptomic data would master the understanding of how gene expression translates into actual phenotype. In contrast to nucleic acid sequencing, in vitro protein amplification is impossible and no single cell proteomic workflow has been established as gold standard yet. Advances in microfluidic sample preparation, multi-dimensional sample separation, sophisticated data acquisition strategies, and intelligent data analysis algorithms have resulted in major improvements to successfully analyze such tiny sample amounts with steadily boosted performance. However, among the broad variation of published approaches, it is commonly accepted that highest possible sensitivity, robustness, and throughput are still the most urgent needs for the field. While many labs have focused on multiplexing to achieve these goals, label-free SCP is a highly promising strategy as well whenever high dynamic range and unbiased accurate quantification are needed. We here focus on recent advances in label-free single-cell mass spectrometry workflows and try to guide our readers to choose the best method or combinations of methods for their specific applications. We further highlight which techniques are most propitious in the future and which applications but also limitations we foresee for the field.

Dynamic Changes in Proteome during Yak Oocyte Maturation Analyzed Using iTRAQ Technology

The aim of this study was to investigate protein regulation at different time points during the in vitro maturation of yak oocytes. Yak oocytes at GV, MI, and MII stages were collected during in vitro maturation, and differential proteomics sequencing was performed using iTRAQ technology. GO functional classification indicated that the differential proteins were closely associated with biological processes such as "metabolic processes", and molecular events such as "binding" molecular-function-related categories were active. KOG analysis showed that energy-metabolism-related activities were vigorous during oocyte development from the GV phase to MI phase, and genetic material preparation activities were more active when oocytes developed from the MI stage to MII stage. KEGG pathway analysis showed that the PPAR metabolic pathway, Hippo signaling pathway, and ECM-receptor interaction and metabolic pathway were enriched from the GV to the MI stages. The PI3K-Akt, TGF-β, and phagosome pathways were enriched from the MI stage to the MII stage. These results indicate that transient dynamic changes occurred in the proteome during the maturation of yak oocytes, and the physiological functions mediated by these were also different. The accurate identification of the differential proteins in the three stages of GV, MI, and MII was helpful in further analyzing the molecular regulatory mechanism of yak oocyte maturation.

Porcine oocytes matured in a chemically defined medium are transcriptionally active

The statement that fully-grown porcine oocytes (oocytes from follicles with diameter from 3 to 6 mm) are transcriptionally quiescent is not as strongly supported as it was before. Currently, we know that there is a difference between the transcription profile of germinal vesicle (GV) and metaphase II (MII) oocytes. The goal of our study was to compare the transcription profile of GV, germinal vesicle breakdown (GVBD), metaphase I (MI), and MII oocytes matured in the chemically defined medium FLI. Oocytes were sequenced, and the results were subsequently validated using quantitative reverse transcription polymerase chain reaction (RT-qPCR). We detected multiple differentially transcribed mRNAs, of which many were upregulated. Among them we found mRNAs necessary for protein production, mitochondrial functions and cytoplasmic maturation. Collectively, these data support the hypothesis that transcription activity in fully-grown porcine oocytes is necessary for key processes during their successful maturation in vitro in a chemically defined maturation medium.

Discovery of Oogenesis Biomarkers from Mouse Oocytes Using a Single-Cell Proteomics Approach

We established an efficient and simplified single-cell proteomics (ES-SCP) workflow to realize proteomics profiling at the single-oocyte level. With the ES-SCP workflow, we constructed a deep coverage proteome library during oocyte maturation, which contained more than 6000 protein groups, and identified and quantified more than 4000 protein groups from a pool of only 15 oocytes at germinal vesicle (GV), GV breakdown (GVBD), and metaphase II (MII) stages. More than 1500 protein groups can be identified from single oocytes. We found that marker proteins including maternal factors and mRNA regulators, such as ZAR1, TLE6, and BTG4, showed significant variations in abundance during oocyte maturation, and it was discovered that maternal mRNA degradation was indispensable during oocyte maturation. Proteomics analysis from single oocytes revealed that changes in antioxidant factors, maternal factors, mRNA stabilization, and energy metabolism were the factors that affect the oocyte quality during ovary aging. Our data laid the foundation for future innovations in assisted reproduction.

In vivo and in vitro matured bovine oocytes present a distinct pattern of single-cell gene expression

Oocyte gene expression is a well controlled event that promotes gamete competence to undergo maturation, fertilization, and to support early embryo development, directly affecting reproductive outcomes. Considering that in vivo controlled ovarian stimulation or in vitro maturation (IVM) for the acquisition of mature oocytes has distinct implications for gene expression, we sought to evaluate the effects of these procedures on the expression of competence-related genes in single-cell oocytes. Healthy Nelore cows of reproductive age were synchronized to harvest in vivo matured oocytes; ovaries from slaughtered animals were used to obtain cumulus-oocyte complexes that were in vitro matured. Single-cell gene expression was performed using TaqMan Low-Density Arrays and 42 genes were evaluated. In silico analysis of protein interactions and Gene Ontology (GO) analysis was performed. Reduced gene expression was observed for 24 targets in IVM oocytes when compared with those of in vivo matured oocytes (P < 0.05). Differences ranged from 1.5-fold to 4.8-fold higher in in vivo oocytes and the BMP15 (5.28), GDF9 (6.23), NOBOX (7.25), HSPA8 (7.85) and MSX1 (11.00) showed the greatest fold increases. The strongest score of functional interactions was observed between the CDC20 and CKS2, with the differentially expressed gene CDC20 being the main marker behind GO enrichment. IVM negatively affected the expression of important genes related to oocyte competency, and showed higher expression levels in in vivo matured oocytes. In vivo controlled ovarian stimulation may be a better strategy to achieve proper oocyte competence and increase the success of assisted reproductive technologies.