Oocyte aging: looking beyond chromosome segregation errors

Unveiling the role of chronic inflammation in ovarian aging: insights into mechanisms and clinical implications

Ovarian aging, a natural process in women and various other female mammals as they age, is characterized by a decline in ovarian function and fertility due to a reduction in oocyte reserve and quality. This phenomenon is believed to result from a combination of genetic, hormonal, and environmental factors. While these factors collectively contribute to the shaping of ovarian aging, the substantial impact and intricate interplay of chronic inflammation in this process have been somewhat overlooked in discussions. Chronic inflammation, a prolonged and sustained inflammatory response persisting over an extended period, can exert detrimental effects on tissues and organs. This review delves into the novel hallmark of aging-chronic inflammation-to further emphasize the primary characteristics of ovarian aging. It endeavors to explore not only the clinical symptoms but also the underlying mechanisms associated with this complex process. By shining a spotlight on chronic inflammation, the aim is to broaden our understanding of the multifaceted aspects of ovarian aging and its potential clinical implications.

Time-lapse imaging of morula compaction for selecting high-quality blastocysts: a retrospective cohort study

PURPOSE

Before blastocyst development, embryos undergo morphological and metabolic changes crucial for their subsequent growth. This study aimed to investigate the relationship between morula compaction and blastocyst formation and the subsequent chromosomal status of the embryos.

METHODS

This retrospective cohort study evaluated embryo development (n = 371) using time-lapse imaging; 94 blastocysts underwent preimplantation genetic testing for aneuploidy (PGT-A).

RESULTS

The embryos were classified as fully (Group 1, n = 194) or partially (Group 2, n = 177) compacted. Group 1 had significantly higher proportions of good- and average-quality blastocysts than Group 2 (21.6% vs. 3.4%, p = 0.001; 47.9% vs. 26.6%, p = 0.001, respectively). The time from the morula stage to the beginning and completion of compaction and blastocyst formation was significantly shorter in Group 1 than in Group 2 (78.6 vs. 82.4 h, p = 0.001; 87.0 vs. 92.2 h, p = 0.001; 100.2 vs. 103.7 h, p = 0.017, respectively). Group 1 embryos had larger surface areas than Group 2 embryos at various time points following blastocyst formation. Group 1 blastocysts had significantly higher average expansion rates than Group 2 blastocysts (653.6 vs. 499.2 μm2/h, p = 0.001). PGT-A revealed a higher proportion of euploid embryos in Group 1 than in Group 2 (47.2% vs. 36.6%, p = 0.303).

CONCLUSION

Time-lapse microscopy uncovered a positive relationship between compaction and blastocyst quality and its association with embryo ploidy. Hence, compaction evaluation should be prioritized before blastocyst selection for transfer or cryopreservation.

The Role of One-Carbon Metabolism and Methyl Donors in Medically Assisted Reproduction: A Narrative Review of the Literature

One-carbon (1-C) metabolic deficiency impairs homeostasis, driving disease development, including infertility. It is of importance to summarize the current evidence regarding the clinical utility of 1-C metabolism-related biomolecules and methyl donors, namely, folate, betaine, choline, vitamin B12, homocysteine (Hcy), and zinc, as potential biomarkers, dietary supplements, and culture media supplements in the context of medically assisted reproduction (MAR). A narrative review of the literature was conducted in the PubMed/Medline database. Diet, ageing, and the endocrine milieu of individuals affect both 1-C metabolism and fertility status. In vitro fertilization (IVF) techniques, and culture conditions in particular, have a direct impact on 1-C metabolic activity in gametes and embryos. Critical analysis indicated that zinc supplementation in cryopreservation media may be a promising approach to reducing oxidative damage, while female serum homocysteine levels may be employed as a possible biomarker for predicting IVF outcomes. Nonetheless, the level of evidence is low, and future studies are needed to verify these data. One-carbon metabolism-related processes, including redox defense and epigenetic regulation, may be compromised in IVF-derived embryos. The study of 1-C metabolism may lead the way towards improving MAR efficiency and safety and ensuring the lifelong health of MAR infants.

NADase CD38 is a key determinant of ovarian aging

The ovary ages earlier than most other tissues, yet the underlying mechanisms remain elusive. Here a comprehensive analysis of transcriptomic landscapes in different organs in young and middle-aged mice revealed that the ovaries showed earlier expression of age-associated genes, identifying increased NADase CD38 expression and decreased NAD+ levels in the ovary of middle-aged mice. Bulk and single-cell RNA sequencing revealed that CD38 deletion mitigated ovarian aging, preserving fertility and follicle reserve in aged mice by countering age-related gene expression changes and intercellular communication alterations. Mechanistically, the earlier onset of inflammation induced higher expression levels of CD38 and decreased NAD+ levels in the ovary, thereby accelerating ovarian aging. Consistently, pharmacological inhibition of CD38 enhanced fertility in middle-aged mice. Our findings revealed the mechanisms underlying the earlier aging of the ovary relative to other organs, providing a potential therapeutic target for ameliorating age-related female infertility.

Human follicular fluid elicits select dose- and age-dependent effects on mouse oocytes and cumulus-oocyte complexes in a heterologous in vitro maturation assay

Follicular fluid (FF) is a primary microenvironment of the oocyte within an antral follicle. Although several studies have defined the composition of human FF in normal physiology and determined how it is altered in disease states, the direct impacts of human FF on the oocyte are not well understood. The difficulty of obtaining suitable numbers of human oocytes for research makes addressing such a question challenging. Therefore, we used a heterologous model in which we cultured mouse oocytes in human FF. To determine whether FF has dose-dependent effects on gamete quality, we performed in vitro maturation of denuded oocytes from reproductively young mice (6-12 weeks) in 10%, 50%, or 100% FF from participants of mid-reproductive age (32-36 years). FF impacted meiotic competence in a dose-dependent manner, with concentrations >10% inhibiting meiotic progression and resulting in spindle and chromosome alignment defects. We previously demonstrated that human FF acquires a fibro-inflammatory cytokine signature with age. Thus, to determine whether exposure to an aging FF microenvironment contributes to the age-dependent decrease in gamete quality, we matured denuded oocytes and cumulus-oocyte complexes (COCs) in FF from reproductively young (28-30 years) and old (40-42 years) participants. FF decreased meiotic progression of COCs, but not oocytes, from reproductively young and old (9-12 months) mice in an age-dependent manner. Moreover, FF had modest age-dependent impacts on mitochondrial aggregation in denuded oocytes and cumulus layer expansion dynamics in COCs, which may influence fertilization or early embryo development. Overall, these findings demonstrate that acute human FF exposure can impact select markers of mouse oocyte quality in both dose- and age-dependent manners.

Proteomic quantification of native and ECM-enriched mouse ovaries reveals an age-dependent fibro-inflammatory signature

The ovarian microenvironment becomes fibrotic and stiff with age, in part due to increased collagen and decreased hyaluronan. However, the extracellular matrix (ECM) is a complex network of hundreds of proteins, glycoproteins, and glycans which are highly tissue specific and undergo pronounced changes with age. To obtain an unbiased and comprehensive profile of age-associated alterations to the murine ovarian proteome and ECM, we used a label-free quantitative proteomic methodology. We validated conditions to enrich for the ECM prior to proteomic analysis. Following analysis by data-independent acquisition (DIA) and quantitative data processing, we observed that both native and ECM-enriched ovaries clustered separately based on age, indicating distinct age-dependent proteomic signatures. We identified a total of 4,721 proteins from both native and ECM-enriched ovaries, of which 383 proteins were significantly altered with advanced age, including 58 ECM proteins. Several ECM proteins upregulated with age have been associated with fibrosis in other organs, but to date their roles in ovarian fibrosis are unknown. Pathways regulating DNA metabolism and translation were downregulated with age, whereas pathways involved in ECM remodeling and immune response were upregulated. Interestingly, immune-related pathways were upregulated with age even in ECM-enriched ovaries, suggesting a novel interplay between the ECM and the immune system. Moreover, we identified putative markers of unique immune cell populations present in the ovary with age. These findings provide evidence from a proteomic perspective that the aging ovary provides a fibroinflammatory milieu, and our study suggests target proteins which may drive these age-associated phenotypes for future investigation.

Opening the black box: why do euploid blastocysts fail to implant? A systematic review and meta-analysis

BACKGROUND

A normal chromosomal constitution defined through PGT-A assessing all chromosomes on trophectoderm (TE) biopsies represents the strongest predictor of embryo implantation. Yet, its positive predictive value is not higher than 50-60%. This gap of knowledge on the causes of euploid blastocysts' reproductive failure is known as 'the black box of implantation'.

OBJECTIVE AND RATIONALE

Several embryonic, maternal, paternal, clinical, and IVF laboratory features were scrutinized for their putative association with reproductive success or implantation failure of euploid blastocysts.

SEARCH METHODS

A systematic bibliographical search was conducted without temporal limits up to August 2021. The keywords were '(blastocyst OR day5 embryo OR day6 embryo OR day7 embryo) AND (euploid OR chromosomally normal OR preimplantation genetic testing) AND (implantation OR implantation failure OR miscarriage OR abortion OR live birth OR biochemical pregnancy OR recurrent implantation failure)'. Overall, 1608 items were identified and screened. We included all prospective or retrospective clinical studies and randomized-controlled-trials (RCTs) that assessed any feature associated with live-birth rates (LBR) and/or miscarriage rates (MR) among non-mosaic euploid blastocyst transfer after TE biopsy and PGT-A. In total, 41 reviews and 372 papers were selected, clustered according to a common focus, and thoroughly reviewed. The PRISMA guideline was followed, the PICO model was adopted, and ROBINS-I and ROB 2.0 scoring were used to assess putative bias. Bias across studies regarding the LBR was also assessed using visual inspection of funnel plots and the trim and fill method. Categorical data were combined with a pooled-OR. The random-effect model was used to conduct the meta-analysis. Between-study heterogeneity was addressed using I2. Whenever not suitable for the meta-analysis, the included studies were simply described for their results. The study protocol was registered at http://www.crd.york.ac.uk/PROSPERO/ (registration number CRD42021275329).

OUTCOMES

We included 372 original papers (335 retrospective studies, 30 prospective studies and 7 RCTs) and 41 reviews. However, most of the studies were retrospective, or characterized by small sample sizes, thus prone to bias, which reduces the quality of the evidence to low or very low. Reduced inner cell mass (7 studies, OR: 0.37, 95% CI: 0.27-0.52, I2 = 53%), or TE quality (9 studies, OR: 0.53, 95% CI: 0.43-0.67, I2 = 70%), overall blastocyst quality worse than Gardner's BB-grade (8 studies, OR: 0.40, 95% CI: 0.24-0.67, I2 = 83%), developmental delay (18 studies, OR: 0.56, 95% CI: 0.49-0.63, I2 = 47%), and (by qualitative analysis) some morphodynamic abnormalities pinpointed through time-lapse microscopy (abnormal cleavage patterns, spontaneous blastocyst collapse, longer time of morula formation I, time of blastulation (tB), and duration of blastulation) were all associated with poorer reproductive outcomes. Slightly lower LBR, even in the context of PGT-A, was reported among women ≥38 years (7 studies, OR: 0.87, 95% CI: 0.75-1.00, I2 = 31%), while obesity was associated with both lower LBR (2 studies, OR: 0.66, 95% CI: 0.55-0.79, I2 = 0%) and higher MR (2 studies, OR: 1.8, 95% CI: 1.08-2.99, I2 = 52%). The experience of previous repeated implantation failures (RIF) was also associated with lower LBR (3 studies, OR: 0.72, 95% CI: 0.55-0.93, I2 = 0%). By qualitative analysis, among hormonal assessments, only abnormal progesterone levels prior to transfer were associated with LBR and MR after PGT-A. Among the clinical protocols used, vitrified-warmed embryo transfer was more effective than fresh transfer (2 studies, OR: 1.56, 95% CI: 1.05-2.33, I2 = 23%) after PGT-A. Lastly, multiple vitrification-warming cycles (2 studies, OR: 0.41, 95% CI: 0.22-0.77, I2 = 50%) or (by qualitative analysis) a high number of cells biopsied may slightly reduce the LBR, while simultaneous zona-pellucida opening and TE biopsy allowed better results than the Day 3 hatching-based protocol (3 studies, OR: 1.41, 95% CI: 1.18-1.69, I2 = 0%).

WIDER IMPLICATIONS

Embryo selection aims at shortening the time-to-pregnancy, while minimizing the reproductive risks. Knowing which features are associated with the reproductive competence of euploid blastocysts is therefore critical to define, implement, and validate safer and more efficient clinical workflows. Future research should be directed towards: (i) systematic investigations of the mechanisms involved in reproductive aging beyond de novo chromosomal abnormalities, and how lifestyle and nutrition may accelerate or exacerbate their consequences; (ii) improved evaluation of the uterine and blastocyst-endometrial dialogue, both of which represent black boxes themselves; (iii) standardization/automation of embryo assessment and IVF protocols; (iv) additional invasive or preferably non-invasive tools for embryo selection. Only by filling these gaps we may finally crack the riddle behind 'the black box of implantation'.

DNA methylation abnormalities induced by advanced maternal age in villi prime a high-risk state for spontaneous abortion

BACKGROUND

Advanced maternal age (AMA) has increased in many high-income countries in recent decades. AMA is generally associated with a higher risk of various pregnancy complications, and the underlying molecular mechanisms are largely unknown. In the current study, we profiled the DNA methylome of 24 human chorionic villi samples (CVSs) from early pregnancies in AMA and young maternal age (YMA), 11 CVSs from early spontaneous abortion (SA) cases using reduced representation bisulfite sequencing (RRBS), and the transcriptome of 10 CVSs from AMA and YMA pregnancies with mRNA sequencing(mRNA-seq). Single-cell villous transcriptional atlas presented expression patterns of targeted AMA-/SA-related genes. Trophoblast cellular impairment was investigated through the knockdown of GNE expression in HTR8-S/Vneo cells.

RESULTS

AMA-induced local DNA methylation changes, defined as AMA-related differentially methylated regions (DMRs), may be derived from the abnormal expression of genes involved in DNA demethylation, such as GADD45B. These DNA methylation changes were significantly enriched in the processes involved in NOTCH signaling and extracellular matrix organization and were reflected in the transcriptional alterations in the corresponding biological processes and specific genes. Furthermore, the DNA methylation level of special AMA-related DMRs not only significantly changed in AMA but also showed more excessive defects in CVS from spontaneous abortion (SA), including four AMA-related DMRs whose nearby genes overlapped with AMA-related differentially expressed genes (DEGs) (CDK11A, C19orf71, COL5A1, and GNE). The decreased DNA methylation level of DMR near GNE was positively correlated with the downregulated expression of GNE in AMA. Single-cell atlas further revealed comparatively high expression of GNE in the trophoblast lineage, and knockdown of GNE in HTR8-S/Vneo cells significantly impaired cellular proliferation and migration.

CONCLUSION

Our study provides valuable resources for investigating AMA-induced epigenetic abnormalities and provides new insights for explaining the increased risks of pregnancy complications in AMA pregnancies.

Maternal age affects pronuclear and chromatin dynamics, morula compaction and cell polarity, and blastulation of human embryos

STUDY QUESTION

Does maternal ageing impact early and late morphokinetic and cellular processes of human blastocyst formation?

SUMMARY ANSWER

Maternal ageing significantly affects pronuclear size and intra- and extra-nuclear dynamics during fertilization, dysregulates cell polarity during compaction, and reduces blastocoel expansion.

WHAT IS KNOWN ALREADY

In ART, advanced maternal age (AMA) affects oocyte yield, fertilization, and overall developmental competence. However, with the exception of chromosome segregation errors occurring during oocyte meiosis, the molecular and biochemical mechanisms responsible for AMA-related subfertility and reduced embryo developmental competence remain unclear. In particular, studies reporting morphokinetics and cellular alterations during the fertilization and pre-implantation period in women of AMA remain limited.

STUDY DESIGN, SIZE, DURATION

A total of 2058 fertilized oocytes were stratified by maternal age according to the Society for Assisted Reproductive Technology classification (<35, 35-37, 38-40, 41-42, and >42 years) and retrospectively analysed. AMA effects were assessed in relation to: embryo morphokinetics and morphological alterations; and the presence and distribution of cell polarity markers-Yes-associated protein (YAP) and protein kinase C-ζ (PKC-ζ)-involved in blastocyst morphogenesis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

A total of 1050 cycles from 1050 patients met the inclusion criteria and were analysed. Microinjected oocytes were assessed using a time-lapse culture system. Immature oocytes at oocyte retrieval and mature oocytes not suitable for time-lapse monitoring, owing to an excess of residual corona cells or inadequate orientation for correct observation, were not analysed. Phenomena relevant to meiotic resumption, pronuclear dynamics, cytoplasmic/cortical modifications, cleavage patterns and embryo quality were annotated and compared among groups. Furthermore, 20 human embryos donated for research by consenting couples were used for immunofluorescence.

MAIN RESULTS AND THE ROLE OF CHANCE

Static microscopic observation revealed that blastocyst formation and expansion were impaired in the 41-42 and >42-year groups (P < 0.0001). The morphological grades of the inner cell mass and trophectoderm were poorer in the >42-year group than those in the <35-year group (P = 0.0022 and P < 0.0001, respectively). Time-lapse microscopic observation revealed a reduction in nucleolus precursor body alignment in female pronuclei in the 41-42 and >42-year groups (P = 0.0010). Female pronuclear area decreased and asynchronous pronuclear breakdown increased in the >42-year group (P = 0.0027 and P < 0.0122, respectively). Developmental speed at cleavage stage, incidence of irregularity of first cleavage, type and duration of blastomere movement, and number of multinucleated cells were comparable among age groups. Delayed embryonic compaction and an increased number of extruded blastomeres were observed in the >42-year group (P = 0.0002 and P = 0.0047, respectively). Blastulation and blastocyst expansion were also delayed in the 41-42 and >42-year groups (P < 0.0001 for both). YAP positivity rate in the outer cells of morulae and embryo PKC-ζ immunoflourescence decreased in the >42-year group (P < 0.0001 for both).

LIMITATIONS, REASONS FOR CAUTION

At the cellular level, the investigation was limited to cell polarity markers. Cell components of other developmental pathways should be studied in relation to AMA.

WIDER IMPLICATIONS OF THE FINDINGS

The study indicates that maternal ageing affects the key functions of embryo morphogenesis, irrespective of the well-established influence on the fidelity of oocyte meiosis.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the participating institutions. The authors have no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.