Epigenetic changes occur at decidualisation genes as a function of reproductive ageing in mice

Understanding epigenetic regulation in the endometrium - lessons from mouse models with implantation defects

Endometrial function, crucial for successful embryo implantation, is significantly influenced by epigenetic regulation. This review investigates the crucial roles of DNA methylation, histone modifications, chromatin remodeling, and RNA methylation in endometrial receptivity and implantation, based on a survey of recent literature on knockout mouse models with implantation defects. These models illuminate how epigenetic disruptions contribute to implantation failure, a significant human reproductive health concern. DNA methylation and histone modifications modulate endometrial receptivity by affecting gene silencing and chromatin structure, respectively. Chromatin remodeling factors also play a critical role in endometrial dynamics, influencing gene expression. Furthermore, RNA methylation emerges as critical in implantation through transcriptional and translational control. While human studies provide limited epigenetic snapshots, mouse models with suppressed epigenetic regulators reveal direct causal links between epigenetic alterations and implantation failure. Understanding these epigenetic interactions offers potential for novel therapies addressing reproductive disorders.

Rescue intracytoplasmic sperm injection improved cumulative live birth rate for cycles with second polar body extrusion rate <50% in young women: generalized addictive model

OBJECTIVE

To determine the indications for early rescue intracytoplasmic sperm injection (ICSI) application.

DESIGN

A retrospective cohort study.

SUBJECTS

The study included 19,808 patients who underwent conventional in vitro fertilization (IVF) or rescue ICSI for their first cycles between February 2017 and December 2021.

EXPOSURE

Rescue ICSI cycles constituted the study group, where oocytes that had not extruded the second polar body 4-6 hours after insemination were rescued by ICSI. The control group consisted of conventional IVF cycles with no interventions to rescue oocytes without the second polar body. Generalized additive models were constructed to describe the relationship between the second polar body extrusion rate and cumulative live birth rate in conventional IVF and rescue ICSI cycles, respectively. The cutoff value of the second polar body extrusion rate guiding rescue ICSI application was determined from the intersection point of generalized additive models. Maternal age range applicable to rescue ICSI was further analyzed using the same method. Clinical outcomes were compared between conventional IVF and rescue ICSI cycles across different second polar body extrusion rate and maternal age subgroups.

MAIN OUTCOME MEASURES

The second polar body extrusion rate and maternal age range for rescue ICSI application, normal fertilization rate, and cumulative live birth rate.

RESULTS

Generalized additive models showed that the cutoff value for the second polar body extrusion rate about rescue ICSI application was 50%. When the rate <50%, normal fertilization rate and cumulative live birth rate (63.7% vs. 46.1%; odds ratio, 1.609; 95% confidence interval, 1.276-2.030) were significantly higher in rescue ICSI cycles than conventional IVF cycles. When the rate ≥50%, rescue ICSI cycles had similar normal fertilization rate and cumulative live birth rate compared with conventional IVF cycles. Further analysis on maternal age in cycles with second polar body extrusion rate <50% released that rescue ICSI cycles showed a higher cumulative live birth rate (67.7% vs. 48.3%; odds ratio, 1.732; 95% confidence interval, 1.361-2.202) than conventional IVF cycles for women aged <38 years.

CONCLUSION

In vitro fertilization cycles with second polar body extrusion rate <50% in women aged <38 years was applicable to early rescue ICSI.

Hallmarks of female reproductive aging in physiologic aging mice

The female reproductive axis is one of the first organ systems to age, which has consequences for fertility and overall health. Here, we provide a comprehensive overview of the biological process of female reproductive aging across reproductive organs, tissues and cells based on research with widely used physiologic aging mouse models, and describe the mechanisms that underpin these phenotypes. Overall, aging is associated with dysregulation of the hypothalamic-pituitary-ovarian axis, perturbations of the ovarian stroma, reduced egg quantity and quality, and altered uterine morphology and function that contributes to reduced capacity for fertilization and impaired embryo development. Ultimately, these age-related phenotypes contribute to altered pregnancy outcomes and adverse consequences in offspring. Conserved mechanisms of aging, as well as those unique to the reproductive system, underlie these phenotypes. The knowledge of such mechanisms will lead to development of therapeutics to extend female reproductive longevity and support endocrine function and overall health.

Multiomics insights into the female reproductive aging

The human female reproductive lifespan significantly diminishes with age, leading to decreased fertility, reduced fertility quality and endocrine function disorders. While many aspects of aging in general have been extensively documented, the precise mechanisms governing programmed aging in the female reproductive system remain elusive. Recent advancements in omics technologies and computational capabilities have facilitated the emergence of multiomics deep phenotyping. Through the application and refinement of various high-throughput omics methods, a substantial volume of omics data has been generated, deepening our comprehension of the pathogenesis and molecular underpinnings of reproductive aging. This review highlights current and emerging multiomics approaches for investigating female reproductive aging, encompassing genomics, epigenomics, transcriptomics, proteomics, metabolomics, and microbiomics. We elucidate their influence on fundamental cell biology and translational research in the context of reproductive aging, address the limitations and current challenges associated with multiomics studies, and offer a glimpse into future prospects.

Endometrial DNA methylation signatures during the time of breeding in relation to the pregnancy outcome in postpartum dairy cows fed a control diet or supplemented with rumen-protected methionine

Post calving metabolic stress reduces the fertility of high producing dairy cows possibly by altering the expression of genes in the maternal environment via epigenetic modifications. Therefore, this study was conducted to identify endometrial DNA methylation marks that can be associated with pregnancy outcomes in postpartum cows at the time of breeding. For this, twelve days post-calving, cows were either offered a control diet or supplemented daily with rumen-protected methionine. Cows showing heat 50-64 days postpartum were artificially inseminated. Endometrial cytobrush samples were collected 4-8 h after artificial insemination and classified based on the pregnancy out comes as those derived from cows that resulted in pregnancy or resulted in no pregnancy. The DNAs isolated from endometrial samples were then subject to reduced representative bisulfite sequencing for DNA methylation analysis. Results showed that in the control diet group, 1,958 differentially methylated CpG sites (DMCGs) were identified between cows that resulted in pregnancy and those that resulted in no pregnancy of which 890 DMCGs were located on chr 27: 6217254-6225600 bp. A total of 537 DMCGs were overlapped with 313 annotated genes that were involved in various pathways including signal transduction, signalling by GPCR, aldosterone synthesis and secretion. Likewise, in methionine supplemented group, 3,430 CpG sites were differentially methylated between the two cow groups of which 18.7% were located on Chr27: 6217254-6225600 bp. A total of 1,781 DMCGS were overlapped with 890 genes which involved in developmental and signalling related pathways including WNT-signalling, focal adhesion and ECM receptor interaction. Interestingly, 149 genes involved in signal transduction, axon guidance and non-integrin membrane-ECM interactions were differentially methylated between the two cow groups irrespective of their feeding regime, while 453 genes involved in axon guidance, notch signalling and collagen formation were differentially methylated between cows that received rumen protected methionine and control diet irrespective of their fertility status. Overall, this study indicated that postpartum cows that could potentially become pregnant could be distinguishable based on their endometrial DNA methylation patterns at the time of breeding.

DNA Methylation of Window of Implantation Genes in Cervical Secretions Predicts Ongoing Pregnancy in Infertility Treatment

Window of implantation (WOI) genes have been comprehensively identified at the single cell level. DNA methylation changes in cervical secretions are associated with in vitro fertilization embryo transfer (IVF-ET) outcomes. Using a machine learning (ML) approach, we aimed to determine which methylation changes in WOI genes from cervical secretions best predict ongoing pregnancy during embryo transfer. A total of 2708 promoter probes were extracted from mid-secretory phase cervical secretion methylomic profiles for 158 WOI genes, and 152 differentially methylated probes (DMPs) were selected. Fifteen DMPs in 14 genes (BMP2, CTSA, DEFB1, GRN, MTF1, SERPINE1, SERPINE2, SFRP1, STAT3, TAGLN2, TCF4, THBS1, ZBTB20, ZNF292) were identified as the most relevant to ongoing pregnancy status. These 15 DMPs yielded accuracy rates of 83.53%, 85.26%, 85.78%, and 76.44%, and areas under the receiver operating characteristic curves (AUCs) of 0.90, 0.91, 0.89, and 0.86 for prediction by random forest (RF), naïve Bayes (NB), support vector machine (SVM), and k-nearest neighbors (KNN), respectively. SERPINE1, SERPINE2, and TAGLN2 maintained their methylation difference trends in an independent set of cervical secretion samples, resulting in accuracy rates of 71.46%, 80.06%, 80.72%, and 80.68%, and AUCs of 0.79, 0.84, 0.83, and 0.82 for prediction by RF, NB, SVM, and KNN, respectively. Our findings demonstrate that methylation changes in WOI genes detected noninvasively from cervical secretions are potential markers for predicting IVF-ET outcomes. Further studies of cervical secretion of DNA methylation markers may provide a novel approach for precision embryo transfer.

The emerging role of TET enzymes in the immune microenvironment at the maternal-fetal interface during decidualization and early pregnancy

A dysregulated immune microenvironment at the maternal-fetal interface in early pregnancy may lead to early pregnancy loss, fetal growth restriction, and preeclampsia. However, major questions about how epigenetic modifications regulate the immune microenvironment during the decidualization process and embryo implantation remain unanswered. DNA methylation, the main epigenetic mechanism involved in the endometrial cycle, is crucial for specific transcriptional networks associated with endometrial stromal cell (ESC) proliferation, hormone response, decidualization, and embryo implantation. Ten-eleven translocation (TET) enzymes, responsible for catalyzing the conversion of 5-methylcytosine to 5-hydroxymethylcyosine, 5-formylytosine, and 5-carboxylcyosine to achieve the DNA demethylation process, appear to play a critical role in decidualization and embryo implantation. Here, we provide a comprehensive view of their structural similarities and the common mechanism of regulation in the microenvironment at the maternal-fetal interface during decidualization and early pregnancy. We also discuss their physiological role in the decidual immune microenvironment. Finally, we propose a key hypothesis regarding TET enzymes at the maternal-fetal interface between decidual immune cells and ESCs. Future work is needed to elucidate their functional role and examine therapeutic strategies targeting these enzymes in pregnancy-related disease preclinical models, which would be of great value for future implications in disease diagnosis or treatment.

Epigenetic clocks provide clues to the mystery of uterine ageing

BACKGROUND

Rising maternal ages and age-related fertility decline are a global challenge for modern reproductive medicine. Clinicians and researchers pay specific attention to ovarian ageing and hormonal insufficiency in this regard. However, uterine ageing is often left out of the picture, with the majority of reproductive clinicians being close to unanimous on the absence of age-related functional decline in the uterine tissues. Therefore, most existing techniques to treat an age-related decline in implantation rates are based primarily on hormonal supplementation and oocyte donation. Solving the issue of uterine ageing might lead to an adjustment to these methods.

OBJECTIVE AND RATIONALE

A focus on uterine ageing and the possibility of slowing it emerged with the development of the information theory of ageing, which identifies genomic instability and erosion of the epigenetic landscape as important drivers of age-related decline in the functionality of most cells and tissues. Age-related smoothing of this landscape and a decline in tissue function can be assessed by measuring the ticking of epigenetic clocks. Within this review, we explore whether the uterus experiences age-related alterations using this elegant approach. We analyse existing data on epigenetic clocks in the endometrium, highlight approaches to improve the accuracy of the clocks in this cycling tissue, speculate on the endometrial pathologies whose progression might be predicted by the altered speed of epigenetic clocks and discuss the possibilities of slowing down the ticking of these clocks.

SEARCH METHODS

Data for this review were identified by searches of Medline, PubMed and Google Scholar. References from relevant articles using the search terms 'ageing', 'maternal age', 'female reproduction', 'uterus', 'endometrium', 'implantation', 'decidualization', 'epigenetic clock', 'biological age', 'DNA methylation', 'fertility' and 'infertility' were selected. A total of 95 articles published in English between 1985 and 2022 were included, six of which describe the use of the epigenetic clock to evaluate uterine/endometrium ageing.

OUTCOMES

Application of the Horvath and DNAm PhenoAge epigenetic clocks demonstrated a poor correlation with chronological age in the endometrium. Several approaches were suggested to enhance the predictive power of epigenetic clocks for the endometrium. The first was to increase the number of samples in the training dataset, as for the Zang clock, or to use more sophisticated clock-building algorithms, as for the AltumAge clock. The second method is to adjust the clocks according to the dynamic nature of the endometrium. Using either approach revealed a strong correlation with chronological age in the endometrium, providing solid evidence for age-related functional decline in this tissue. Furthermore, age acceleration/deceleration, as estimated by epigenetic clocks, might be a promising tool to predict or to gain insights into the origin of various endometrial pathologies, including recurrent implantation failure, cancer and endometriosis. Finally, there are several strategies to slow down or even reverse epigenetic clocks that might be applied to reduce the risk of age-related uterine impairments.

WIDER IMPLICATIONS

The uterine factor should be considered, along with ovarian issues, to correct for the decline in female fertility with age. Epigenetic clocks can be tested to gain a deeper understanding of various endometrial disorders.

Uterine-specific SIRT1 deficiency confers premature uterine aging and impairs invasion and spacing of blastocyst, and stromal cell decidualization, in mice

A distinct age-related alteration in the uterine environment has recently been identified as a prevalent cause of the reproductive decline in older female mice. However, the molecular mechanisms that underlie age-associated uterine adaptability to pregnancy are not known. Sirtuin 1 (SIRT1), a multifunctional NAD+-dependent deacetylase that regulates cell viability, senescence and inflammation during aging, is reduced in aged decidua. Thus, we hypothesize that SIRT1 plays a critical role in uterine adaptability to pregnancy and that uterine-specific ablation of Sirt1 gene accelerates premature uterine aging. Female mice with uterine ablation of Sirt1 gene using progesterone receptor Cre (PgrCre) exhibit subfertility and signs of premature uterine aging. These Sirt1-deficient mothers showed decreases in litter size from their 1st pregnancy and became sterile (25.1 ± 2.5 weeks of age) after giving birth to the third litter. We report that uterine-specific Sirt1 deficiency impairs invasion and spacing of blastocysts, and stromal cell decidualization, leading to abnormal placentation. We found that these problems traced back to the very early stages of hormonal priming of the uterus. During the window of receptivity, Sirt1 deficiency compromises uterine epithelial-stromal crosstalk, whereby estrogen, progesterone and Indian hedgehog signaling pathways are dysregulated, hampering stromal cell priming for decidualization. Uterine transcriptomic analyses also link these causes to perturbations of histone proteins and epigenetic modifiers, as well as adrenomedullin signaling, hyaluronic acid metabolism, and cell senescence. Strikingly, our results also identified genes with significant overlaps with the transcriptome of uteri from aged mice and transcriptomes related to master regulators of decidualization (e.g. Foxo1, Wnt4, Sox17, Bmp2, Egfr and Nr2f2). Our results also implicate accelerated deposition of aging-related fibrillar Type I and III collagens in Sirt1-deficient uteri. Collectively, SIRT1 is an important age-related regulator of invasion and spacing of blastocysts, as well as decidualization of stromal cells.

Uterine bleeding: how understanding endometrial physiology underpins menstrual health

Menstruation is a physiological process that is typically uncomplicated. However, up to one third of women globally will be affected by abnormal uterine bleeding (AUB) at some point in their reproductive years. Menstruation (that is, endometrial shedding) is a fine balance between proliferation, decidualization, inflammation, hypoxia, apoptosis, haemostasis, vasoconstriction and, finally, repair and regeneration. An imbalance in any one of these processes can lead to the abnormal endometrial phenotype of AUB. Poor menstrual health has a negative impact on a person's physical, mental, social, emotional and financial well-being. On a global scale, iron deficiency and iron deficiency anaemia are closely linked with AUB, and are often under-reported and under-recognized. The International Federation of Gynecology and Obstetrics have produced standardized terminology and a classification system for the causes of AUB. This standardization will facilitate future research endeavours, diagnosis and clinical management. In a field where no new medications have been developed for over 20 years, emerging technologies are paving the way for a deeper understanding of the biology of the endometrium in health and disease, as well as opening up novel diagnostic and management avenues.

The role of epigenetic mechanisms in the regulation of gene expression in the cyclical endometrium

BACKGROUND

The human endometrium is a highly dynamic tissue whose function is mainly regulated by the ovarian steroid hormones estradiol and progesterone. The serum levels of these and other hormones are associated with three specific phases that compose the endometrial cycle: menstrual, proliferative, and secretory. Throughout this cycle, the endometrium exhibits different transcriptional networks according to the genes expressed in each phase. Epigenetic mechanisms are crucial in the fine-tuning of gene expression to generate such transcriptional networks. The present review aims to provide an overview of current research focused on the epigenetic mechanisms that regulate gene expression in the cyclical endometrium and discuss the technical and clinical perspectives regarding this topic.

MAIN BODY

The main epigenetic mechanisms reported are DNA methylation, histone post-translational modifications, and non-coding RNAs. These epigenetic mechanisms induce the expression of genes associated with transcriptional regulation, endometrial epithelial growth, angiogenesis, and stromal cell proliferation during the proliferative phase. During the secretory phase, epigenetic mechanisms promote the expression of genes associated with hormone response, insulin signaling, decidualization, and embryo implantation. Furthermore, the global content of specific epigenetic modifications and the gene expression of non-coding RNAs and epigenetic modifiers vary according to the menstrual cycle phase. In vitro and cell type-specific studies have demonstrated that epithelial and stromal cells undergo particular epigenetic changes that modulate their transcriptional networks to accomplish their function during decidualization and implantation.

CONCLUSION AND PERSPECTIVES

Epigenetic mechanisms are emerging as key players in regulating transcriptional networks associated with key processes and functions of the cyclical endometrium. Further studies using next-generation sequencing and single-cell technology are warranted to explore the role of other epigenetic mechanisms in each cell type that composes the endometrium throughout the menstrual cycle. The application of this knowledge will definitively provide essential information to understand the pathological mechanisms of endometrial diseases, such as endometriosis and endometrial cancer, and to identify potential therapeutic targets and improve women's health.